Synthesis of Methanol and Higher Alcohols from Syngas over K Promoted Cu Based 
Catalysts in Supercritical Solvent 
 
by 
 
Rui Xu 
 
 
 
 
A dissertation submitted to the Graduate Faculty of 
Auburn University 
in partial fulfillment of the 
requirements for the Degree of 
Doctor of Philosophy 
 
Auburn, Alabama 
May 5th, 2013 
 
 
 
 
Keywords: Supercritical solvent, Alcohol synthesis, Syngas utilization, Copper catalysts 
 
 
Copyright 2013 by Rui Xu 
 
 
Approved by 
 
Christopher B. Roberts, Chair, Professor of Chemical Engineering 
Mario Richard Eden, Professor of Chemical Engineering 
Yoon Young. Lee,  Professor of Chemical Engineering 
Maobing Tu, Assistant Professor of Forest Bioenergy and Bioproducts 
 
 
 
 
 
 
 ii
 
 
 
 
 
 
Abstract 
 
 
With today?s increasing crude oil prices and a desire to reduce our dependence on 
imported oil, the synthesis of methanol and higher alcohol represents a promising pathway for 
utilizing synthesis gas (a mixture of CO and H2, and usually called syngas) in order to create 
liquid fuels or fuel additives. The driving force behind the research that focuses on syngas as a 
fuel production platform is that syngas can be derived from various carbonaceous sources, such 
as coal and biomass, which are either abundant or renewable. Although the process of converting 
syngas to alcohols has been under development for nearly a century, from a practical point of 
view, it still suffers from low selectivity towards higher alcohols. In order to meet the practical 
requirements of industry, numerous studies have been devoted to the investigation of the higher 
alcohols synthesis (HAS) over the past thirty years, including the development of modified 
catalysts, the utilization of double bed reactors, etc.  
The objective of this work is to identify and demonstrate the benefits of introducing 
supercritical solvent into alcohol synthesis from syngas. In chapter 3, we have prepared a 
traditional low temperature methanol synthesis Cu based catalyst and evaluated its catalytic 
performance under gas phase conditions. Supercritical hexanes was introduced into the system as 
a reaction medium in order to evaluate the effect of the supercritical solvent on methanol and 
higher alcohol synthesis. The results illustrate a notable reduction in the CH4 selectivity due to 
the enhanced heat transfer in the supercritical hexanes medium while this solvent medium also 
 iii 
facilitated the extraction of alcohols, especially methanol, from the catalyst pores. As such, the 
formation of mixed alcohols is promoted by the presence of the supercritical hexanes medium.  
In chapter 4, this Cu-based methanol synthesis catalyst was promoted by cobalt, which is 
widely recognized to provide active sites for carbon chain growth reactions. The interaction 
between Cu and Co was assumed to enhance the formation of higher alcohols. We investigated 
higher alcohol synthesis under a series of reaction conditions that span the supercritical regime 
and compare these results with these obtained from gas phase operation. The results from 
experiments performed when using supercritical hexanes as the reaction media for this catalytic 
system illustrate that the presence of the SCF medium improves the heat transfer from the 
catalyst bed as evidenced by a significant reduction in the formation of CH4. In addition, the 
results of these catalytic investigations demonstrate that the presence of the supercritical medium 
has a significant effect on the selectivity and the productivity towards higher alcohols. 
In chapter 5, the effect of syngas composition on the formation of higher alcohols over 
the Cu-Co based catalyst has been evaluated under both gas phase and supercritical hexanes 
phase reaction conditions. Four different values of H2/CO ratio, 2.0, 1.75, 1.35, and 1.0, were 
utilized in order to represent the common syngas composition derived from various 
carbonaceous resources. The results demonstrate that under the presence of supercritical hexanes 
CO conversion was maintained at a stable value while decreasing the H2/CO ratio from 2 to 1. 
Additionally, lower H2/CO ratio effectively enhanced the carbon chain growth in supercritical 
higher alcohol synthesis.   
 
  
 iv
 
 
 
 
 
Acknowledgments 
 
 
I?d like to thank my advisor Dr. Christopher B. Roberts who has always provided 
encouragement, thoughtful discussions, and needed advice, whether it was scholarly or personal. 
I would like to thank all of my committee members for taking the time to make significant 
contributions to all portions of this dissertation. A special thanks to Brian Schweiker and 
Kenneth Hornsby who provided frequent help without which this work would have been 
impossible to be finished. I would also like to thank Steven Moore for his assistance in SEM and 
XRD catalyst characterization.  
I would also like to thank former members of Dr. Roberts? research group: Dr. Xiwen 
Huang, Dr. Nimir Elbashir and Dr. Ed Durham, Dr. Juncheng Liu, Dr. Kendall Hurst, Dr. Steve 
Saunders and Dr. Deborah Bacik for their help in broadening my academic knowledge and 
scientific techniques; as well as current group members Charlotte Stewart, David Roe, Rajeshwar 
Chinnawar, Jie Zhong, Jennifer Boice, Pranav Vengsarkar and former undergraduate research 
assistant Michael Columbus for their assistance. 
Finally, I would like to thank my wife, Sihe Zhang, my mother, Zhen li, and my father, 
Chongwen, Xu for their endless love, unconditional support and continuous encouragement.  
 
 
 
  
 v
 
 
 
 
 
Table of Contents 
 
 
Abstract  ........................................................................................................................................ ii 
Acknowledgments.......................................................................................................................... iv 
List of Tables ................................................................................................................................. ix 
List of Figures ..................................................................................................................................x 
Chapter 1 A Review of Mixed Alcohol Synthesis ...........................................................................1 
1.1 Introduction ..................................................................................................................... 1 
1.1.1 Background ........................................................................................................ 3 
1.2 Alcohol Synthesis from Syngas ...................................................................................... 5 
1.2.1 Methanol Synthesis ............................................................................................ 5 
1.2.1.1. Background ................................................................................................... 5 
1.2.1.2. Mechanism for methanol synthesis ............................................................... 7 
1.2.1.3. Catalysts for methanol synthesis ................................................................. 10 
1.2.2 Mixed Alcohols Synthesis (MAS) ................................................................... 11 
1.2.2.1. Background ................................................................................................. 11 
1.2.2.2. Catalysts for Mixed Alcohols Synthesis (MAS) ......................................... 15 
1.2.2.3. Chemistry and Reaction Mechanism of Mixed Alcohol Synthesis ............. 19 
1.2.2.4. Aspects affecting higher alcohols? selectivity ............................................. 24 
1.2.3 Advances in the synthesis of higher alcohols .................................................. 32 
1.2.3.1. Modified Catalysts ...................................................................................... 32 
1.2.3.2. Addition of Lower Alcohols ....................................................................... 34 
 vi
1.2.3.3. Dual-bed reactor .......................................................................................... 36 
1.2.3.4. Slurry Phase Reactor ................................................................................... 37 
1.2.3.5. Supercritical Phase Reaction ....................................................................... 38 
1.3 Summary ....................................................................................................................... 41 
Chapter 2 Supercritical Fluids and its Application in Heterogeneous Catalytic Reactions...........43 
2.1 Background on the application of supercritical fluids in heterogeneous reactions ...... 43 
2.2 Application of SCFs in heterogeneous catalytic reactions ........................................... 46 
2.2.1 Activity Enhancement ...................................................................................... 46 
2.2.2 Control of Selectivity ....................................................................................... 47 
2.2.3 Enhanced Mass and Heat Transfer ................................................................... 48 
2.2.4 Catalyst Lifetime and Regeneration ................................................................. 48 
Chapter 3 Supercritical Hexanes as a Reaction Medium for Mixed Alcohol Synthesis in a 
Continuous Fixed Bed Reactor .....................................................................................50 
3.1 Introduction ................................................................................................................... 50 
3.2 Experimental Section .................................................................................................... 53 
3.2.1 Catalyst preparation ......................................................................................... 53 
3.2.2 Catalyst characterization .................................................................................. 54 
3.2.3 Reaction procedure and product analysis ......................................................... 55 
3.3 Results and Discussion ................................................................................................. 58 
3.3.1 Effects of hexanes/syngas molar ratio on CO and H2 conversions .................. 59 
3.3.2 Effects of hexanes/syngas molar ratio on CO2 and CH4 selectivity ................. 62 
3.3.3 Effects of hexanes/syngas molar ratio on productivity and selectivity  ........... 65 
3.3.4 Effect of temperature on conversion and selectivity under both gas and SCF 
phase conditions ............................................................................................... 70 
3.4 Conclusion .................................................................................................................... 77 
 vii
Chapter 4 Investigation of Supercritical Fluids as Reaction Media for Higher Alcohol Synthesis 
from Syngas over a K Promoted Cu-Co-Zn Catalyst ...................................................78 
4.1 Introduction ................................................................................................................... 78 
4.2 Experimental Section .................................................................................................... 83 
4.2.1 Catalyst preparation ......................................................................................... 83 
4.2.2 Catalyst testing ................................................................................................. 84 
4.3 Results and Discussion ................................................................................................. 85 
4.3.1 The effect of hexanes/syngas molar ratio on the catalytic performance .......... 85 
4.3.2 Selectivity comparison for same conversion under gas phase and SCF phase 
conditions ......................................................................................................... 90 
4.3.3 The effect of GHSV ......................................................................................... 93 
4.3.4 Using Argon as balance gas ............................................................................. 96 
4.4 Conclusion .................................................................................................................. 102 
Chapter 5 Effect of H2/CO ratio on higher alcohol synthesis over a Cu-Co catalyst under 
supercritical hexanes conditions .................................................................................103 
5.1 Introduction ................................................................................................................. 103 
5.2 Experimental Section .................................................................................................. 106 
5.2.1 Catalyst preparation ....................................................................................... 106 
5.2.2 HAS Catalytic Testing ................................................................................... 107 
5.3 Results and Discussion ............................................................................................... 108 
5.4 Conclusion .................................................................................................................. 117 
Chapter 6 Future work .................................................................................................................119 
6.1 Catalyst characterization of the fresh and spent catalysts ........................................... 119 
6.2 Modified catalyst for supercritical higher alcohol synthesis from syngas .................. 119 
6.3 Comparison of supercritical phase and slurry phase higher alcohol synthesis ........... 120 
6.4 Dual catalyst bed reactor design ................................................................................. 120 
 viii 
6.5 Mechanistic study of higher alcohol synthesis over the Cu-Co catalysts ................... 121 
6.6 Phase behavior study of the reaction mixtures in HAS .............................................. 122 
Reference  .....................................................................................................................................123 
 
  
 ix
 
 
 
 
 
List of Tables 
 
 
Table 1.1 Elementary steps proposed in the ?redox? mechanism of methanol synthesis 
(Askgaard et al. 1995) ................................................................................................9 
Table 1.2 Properties of different types of fuels (Methanol institute, 2010) .............................14 
Table 1.3 Emission comparisons for different fuel types (Methanol institute, 2010) ..............14 
Table 1.4 Catalytic performances over different types of alcohol synthesis catalysts (Thao, 
2007) .........................................................................................................................16 
Table 2.1 Comparison of density and transport properties of CO2 in gas, liquid and 
supercritical phases (Taylor, 1996) ..........................................................................44 
Table 3.1 Physical-chemical properties of catalysts ................................................................54 
Table 3.2 Conversion, selectivity and yields of supercritical MAS obtained with a H2/CO = 
2.0 synthesis gas at 300 ?C and at various hexanes/syngas molar ratios with GHSV 
= 3000 L/kg cat/hr ....................................................................................................59 
Table 3.3 Effect of reaction temperature on CO conversion, CO2 and CH4 selectivity, and 
alcohol productivity obtaining under both gas phase and SCF phase conditions. ...76 
Table 4.1 The effect of GHSV on the conversion and selectivities under gas phase conditions96 
Table 4.2 The effect of reaction medium on the conversion and selectivity ............................98 
Table 5.1 Syngas inlet compositions with respect to different H2/CO ratio ..........................107 
Table 5.2 Catalytic performance as a function of H2/CO ratio in gas phase HAS .................111 
Table 5.3 Catalytic performance as a function of H2/CO ratio in SC-HAS ...........................112 
  
 x
 
 
 
 
 
List of Figures 
 
 
Figure 1.1 Crude oil prices history: 1947-2008 (www.wtrg.com) ............................................. 2 
Figure 1.2 Histogram from the indexing site Engineering Village of papers including both 
?alcohol synthesis? and ?synthesis gas? by year ....................................................... 3 
Figure 1.3 Pathways for syngas conversion to fuels, additives and chemicals (Stelmachowski 
& Nowicki 2003) ....................................................................................................... 4 
Figure 1.4 Proposed reaction network for methanol and higher alcohol synthesis (Hilmen et al. 
1998) ........................................................................................................................ 22 
Figure 1.5 Effect of reaction temperature on the alcohol distribution for MAS on Cu-based 
catalyst (Majocchi et al. 1998). ............................................................................... 25 
Figure 1.6 Olefin and paraffin yields versus carbon number for FTS reactions in various 
phases over Ru/Al2O3 catalyst (Fan and Fujimoto, 1999)....................................... 41 
Figure 2.1 Pressure-temperature schematic diagram for a pure fluid showing the supercritical 
region path ............................................................................................................... 44 
Figure 3.1 XRD patterns of the catalysts before reduction and after reaction.......................... 55 
Figure 3.2 Schematic diagram of the high-pressure MAS reactor and analysis system ........... 56 
Figure 3.3 Syngas conversion and selectivity to CO2 and CH4 as a function of hexane/syngas 
molar ratio. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, 
pressure 4.5 MPa-20 MPa ....................................................................................... 61 
Figure 3.4 Pressure effect on the conversion and selectivity. Reaction conditions were as 
follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 
sccm/gcat, H2/CO = 2.0, temperature 300 ?C, hexanes/syngas molar ratio = 1 ....... 64 
Figure 3.5 Pressure effect on syngas conversion and selectivity to CO2 and CH4 under gas 
phase conditions. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, 
pressure 4.5 MPa-20 MPa ....................................................................................... 65 
 xi
Figure 3.6 Effect of hexane/syngas molar ratio on the productivity of methanol (a) and ethanol 
(b). Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, 
pressure 4.5 MPa-18 MPa ....................................................................................... 67 
Figure 3.7 Effect of hexane/syngas molar ratio on the carbon selectivity towards C1-C4 
alcohols. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3; syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, 
pressure 4.5 MPa-18 MPa ....................................................................................... 68 
Figure 3.8 Pressure effect on alcohol selectivity under gas phase conditions. Reaction 
conditions were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas 
flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, pressure 6.9 MPa-15.4 
MPa ......................................................................................................................... 68 
Figure 3.9 Effect of temperature on the syngas conversion and selectivity to CH4 and CO2 
(filled icons represent the data obtained under gas phase conditions and the open 
icons represent the data obtained under SCF phase conditions) Reaction conditions 
were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 
sccm/gcat, H2/CO = 2.0, syngas partial pressure 6.2 MPa ....................................... 74 
Figure 3.10 The effect of temperature on the carbon selectivity to alcohol products under gas 
phase condition. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, pressure 6.2 MPa . 75 
Figure 3.11 The effect of temperature on the carbon selectivity to alcohol products under SCF 
condition. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, hexanes/syngas 
molar ratio 2, syngas partial pressure 6.2 MPa ....................................................... 75 
Figure 3.12 Changes in chain growth probability factor ? with reaction temperature under gas 
phase and supercritical hexanes phase conditions. Reaction conditions were as 
follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 
sccm/gcat, H2/CO = 2.0, syngas partial pressure 6.2 MPa ..................................... 76 
Figure 4.1 Selectivity as function of composition in the ternary system CuO-CoO-Cr2O3 
(Courty et al., 1982) ................................................................................................ 81 
Figure 4.2 CO and H2 conversion as a function of hexanes/syngas molar ratio. Reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, 
syngas flow rate = 50 sccm, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 
MPa-18MPa ............................................................................................................ 86 
Figure 4.3 CO2 and CH4 selectivity as a function of hexane/syngas molar ratio. Reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, 
syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 
MPa-18MPa ............................................................................................................ 88 
 xii
Figure 4.4 Higher alcohol productivity as a function of hexanes/syngas molar ratio. Reaction 
Conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, 
syngas flow rate = 50 sccm, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 
MPa-18MPa ............................................................................................................ 89 
Figure 4.5 C+OH wt% as a function of hexane/syngas molar ratio (C+OH wt% defined as the 
weight of CnOH divided by the total weight of alcohol products). Reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, 
syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 
MPa-18MPa ............................................................................................................ 90 
Figure 4.6 Comparison of CH4 and CO2 selectivities as a function of CO conversion under 
both gas phase and supercritical phase conditions. The reaction conditions were as 
follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 15-
100 sccm/gcat, H2/CO ratio = 2.0, temperature = 300 ?C; pressure = 4.5 MPa under 
gas phase conditions and pressure =18 MPa under supercritical phase conditions 
(where hexanes/syngas molar ratio = 3) .................................................................. 92 
Figure 4.7 Comparison of alcohol selectivities at the same CO conversion under both gas 
phase and supercritical phase conditions.  The reaction conditions were as follows: 
catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 15-100 
sccm/gcat, H2/CO ratio = 2.0, temperature - 300 ?C; pressure = 4.5 MPa under gas 
phase conditions and pressure = 18 MPa under supercritical phase conditions 
(where hexane/syngas molar ratio = 3), CO conversion = 12.6%........................... 93 
Figure 4.8 CO conversion, H2 conversion, CH4 selectivity, and CO2 selectivity as a function of 
GHSV in SC-HAS. The reaction conditions were as follows: catalyst = 0.5 wt% K 
promoted Cu/Co/ZnO/Al2O3, H2/CO ration = 2.0, temperature = 300 ?C, pressure = 
18 MPa, hexanes/syngas molar ratio = 3. GHSV was calculated based on the 
volumetric flow rate of syngas and the volume of catalyst bed. ............................. 95 
Figure 4.9 Selectivity of methanol, ethanol, propanol and butanol isomers as a function of 
syngas flow rate in SC-HAS. Reaction conditions were as follows: catalyst = 0.5 wt% 
K promoted Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, temperature = 300 ?C, pressure 
= 18 MPa, hexanes/syngas molar ratio = 3. ............................................................ 96 
Figure 4.10 Productivity of C1-C8 alcohols in gas-phase and supercritical-phase HAS. The 
reaction conditions were as follows: catalyst = 0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, temperature = 300 ?C, pressure = 13.5 MPa, 
hexanes/syngas molar ratio = 2 for the supercritical hexanes phase operation, 
argon/syngas molar ratio = 2 for the argon phase operation. ................................ 100 
Figure 4.11 Productivity of C8-C18 n-paraffin in gas phase and supercritical phase HAS. 
Reaction conditions were as follows: catalyst = 0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, temperature = 300 ?C, pressure = 13.5 MPa, 
hexanes/syngas molar ratio = 2 for the supercritical hexanes phase operation, 
argon/syngas molar ratio = 2 for the argon phase operation. ................................ 100 
 xiii 
Figure 4.12 The variation of log(wn/n) with respect to n (the number of carbon atoms) of 
alcohols (left) and hydrocarbons (right) obtained in SC-HAS. Reaction conditions 
were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO ratio = 
2.0, temperature = 300 ?C, pressure = 13.5 MPa, hexanes/syngas molar ratio = 2.
 ............................................................................................................................... 101 
Figure 4.13 The variation of log(wn/n) with respect to n (the number of carbon atoms) of 
alcohols (left) and hydrocarbons (right) obtained in gas phase HAS. Reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, 
H2/CO ratio = 2.0, temperature = 300 ?C, pressure = 13.5 MPa, argon/syngas molar 
ratio = 2. ................................................................................................................ 101 
Figure 5.1 Effect of H2/CO ratio on conversion and gas product selectivity in GP-HAS. The 
reaction conditions were as follows: catalyst = 0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 290 ?C, pressure = 
4.5 MPa ................................................................................................................. 109 
Figure 5.2 Effect of H2/CO ratio on conversion and gas product selectivity in SC-HAS. The 
reaction conditions were as follows: catalyst = 0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 290 ?C, pressure = 
13.5 MPa, syngas partial pressure = 4.5 MPa, Hexanes/syngas molar ratio = 2 .. 110 
Figure 5.3 Effect of H2/CO ratios on the productivity of higher alcohols under gas phase 
conditions. The reaction conditions were as follows: catalyst = 0.5 wt% K 
promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 290 ?C, 
pressure = 4.5 MPa ................................................................................................ 114 
Figure 5.4 Effect of H2/CO ratios on the productivity of higher alcohols under supercritical 
hexanes phase conditions. The reaction conditions were as follows: catalyst = 0.5 
wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 
290 ?C, pressure = 13.5 MPa, syngas partial pressure = 4.5 MPa, Hexanes/syngas 
molar ratio = 2 ....................................................................................................... 115 
 
 
 
 
 
 
 
 
 1
Chapter 1 A Review of Mixed Alcohol Synthesis 
1.1 Introduction 
Extensive research efforts in the past eight decades have been devoted to the utilization of C1-
chemistry as a possible pathway for fuels and chemicals. C1-chemistry refers to the utilization of 
single-carbon molecules; such as carbon monoxide, syngas (a mixture of CO and H2), carbon 
dioxide, methane etc. 
 The main resources of C1 molecules are natural gas, coal, biomass, and organic wastes. The 
driving force behind the increasing demand of alternative fuels is that the supplies of crude oil 
are expected to be less available, more expensive and that there will be more environmental 
concern in the coming century (Dresselhaus & Thomas, 2001). One pathway for converting 
syngas into fuels is the synthesis of mixed alcohols, which has been researched and utilized for 
nearly a century. Methanol and higher alcohols, together refered as mixed alcohols, can be used 
as transportation fuels or as fuel additives. Although the production of methanol from syngas has 
been industrialized, the production of higher alcohols suffers from low selectivity and has not 
advanced beyond the pilot scale. Research into higher alcohol synthesis includes the 
development of new catalysts as well as the utilization of innovative processes or technologies 
with the goal of achieving the criteria of selectivity and productivity in order to justify this 
process industrially.  
 2
 
Figure 1.1 Crude oil prices history: 1947-2008 (www.wtrg.com) 
 
With the crude oil price floating up and down, the investigation of higher alcohol synthesis 
developed rapidly at times while experiencing plateaus at other times. Figure 1.1 demonstrates 
that crude oil prices changed significantly over the years, where it depended not only on the 
demand but was also affected by political happenings around the world. Sharp increases in the 
price of crude oil called for the use of alternative fuels and produced a sudden driving force to 
this field of research, as evidenced by a summit in the number of papers published. Fig.1.2 shows 
a plot of the number of articles reported by ?Engineering Village? as a function of year, as 
indexed with keywords ?alcohol synthesis? and ?synthesis gas?. It can be observed that two 
summits in the number of papers produced were experienced in the year 1986 and 2009, which 
occurred a few years after the peaks in the global crude oil prices shown in Fig.1.1. 
As the reserves of crude oil are depleted and concerns over their environmental impacts 
continue to escalate, it could be expected that the conversion of syngas, as derived from various 
 3
carbonaceous resources, into alternative fuels would receive significant attention. Although, 
replacing gasoline or diesel fuel with alternative fuels of equivalent energy densities is going to 
take much more effort and greater control over chemical reactivity.  
 
Figure 1.2 Histogram from the indexing site Engineering Village of papers including both ?alcohol 
synthesis? and ?synthesis gas? by year 
1.1.1 Background 
Synthesis gas (syngas) is a gas mixture that consists of varying amounts of carbon monoxide 
(CO) and hydrogen (H2). Syngas can be derived from steam reforming or partial oxidation of 
natural gas (CH4), the gasification of coal or biomass and in some cases waste-to-energy 
gasification facilities. Syngas is combustible and is often used as a fuel source or as an 
intermediate for the production of fuels and other chemicals. These conversion processes include 
ammonia production, methanol synthesis, and Fischer-Tropsch synthesis (Beychok, 1974). 
The utilization of syngas started at the beginning of the 20th century by the production of 
methane via the hydrogenation of carbon monoxide in 1902. In the 1920s, the discovery of the 
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Fischer-Tropsch synthesis and its subsequent use in manufacturing methanol and higher alcohols 
made the utilization of syngas a more popular research topic (Wender, 1996). Various processes 
for converting syngas to generate fuels, fuel additives and chemicals are shown in Fig. 1.3.  
Depending on reaction conditions and the catalysts used, different chemicals can be produced on 
a large industrial scale using these synthesis routes. These conversion processes from syngas 
provide alternative pathways to produce fuels, fuel additives, and chemicals with lower sulfur 
concentration compared to conventional petroleum fuels. 
  
Figure 1.3 Pathways for syngas conversion to fuels, additives and chemicals (Stelmachowski & 
Nowicki 2003) 
One of the pathways in Figure 1.3 involves the synthesis of methanol and mixtures of higher 
alcohols (i.e. C2 to C6 alcohols). Our study is and will be mainly focused on this pathway of 
syngas conversion.  
Alcohols C1-C6 
Higher alcohol 
synthesis 
F-T 
synthesis 
1-butanol 
dehydration 
Diesel 
H2 + CO 1-butylene 
MTG 
MTBE 
synthesis 
Methanol 
synthesis 
F-T 
synthesis 
Gasoline Methanol
  
MTBE 
 5
1.2 Alcohol Synthesis from Syngas 
1.2.1 Methanol Synthesis 
1.2.1.1. Background 
Methanol is a basic chemical, a transportation fuel, and an energetic material for fuel cells. 11 
billion gallons of methanol are produced annually worldwide (Herman, 2000). According to a 
report from the Methanol Institute, the methanol industry generates $12 billion in economic 
activity each year around the world, while creating nearly 100,000 jobs (Dolan, 2010). 
The utilization of methanol as an alternative fuel began in 1965, when pure methanol was 
widely used in a USAC Indy car competition (Reed & Lerner, 1973).  During the OPEC 1973 oil 
crisis, Reed and Lerner (1973) advocated methanol from coal as a proven fuel with well 
established manufacturing technology and sufficient resources to replace gasoline. Using 
methanol as a fuel in spark ignition engines can offer increased thermal efficiency and increased 
power output (as compared to gasoline) due to its high octane rating and high heat of 
vaporization. However, due to its high hygroscopic properties, it may absorb water directly from 
the atmosphere, causing a phase separation of methanol-gasoline blends (Reed & Lerner, 1973). 
Methanol also presents some disadvantages, including low volatility, difficulty in ignition, and 
difficulty in blending with gasoline. Despite these disadvantages, methanol has been considered 
as a replacement for gasoline-based fuel and a number of attempts have been made towards 
switching from gasoline to methanol. In response to the oil price shocks, the state of California 
ran an experimental program from 1978 to 1990 which subsidize the conversion of gasoline 
fueled vehicle to 85% methanol (with 15% additives of choice) fueled vehicle (Bromberg & 
Cheng, 2010). Although the methanol transition program successfully reduced the emissions of 
smog, NOx, CO and particulate matter, the development of methanol vehicles faced considerable 
difficulties (Dolan, 2010). In 2005, California?s Governor, Arnold Schwarzenegger, stopped the 
 6
methanol conversion program after 25 years and 200,000,000 miles, to join the expanding use of 
ethanol (driven by producers of corn) (Dolan, 2010). In spite of this, 2006 Noble prize winner 
George A. Olah and colleagues proposed an entire methanol economy based on energy storage in 
synthetically produced methanol.   
China is currenly the largest user of methanol for transportation fuels in the world. Interest in 
China on the use of methanol as a transportation fuel is high (but local) as there is an abundance 
of readily available feedstocks, with coal being the main choice (Li et al., 2009). Presently, M5 
(5% methanol in gasoline), M10, M15, M85 and M100 methanol gasoline are sold on the market. 
The estimated volume of methanol consumption in the Chinese transportation sector is around 
1.5?2 billion gallons (Bromberg & Cheng, 2010). 
The production of methanol is typically from syngas via steam reforming of natural gas, 
although some is from coal (Bromberg & Cheng, 2010). Other feed stocks include wood, 
municipal solid wastes and sewage. Catalytic synthesis of methanol from syngas is one of the 
most active and selective industrial processes carried out on a commodity scale today (Herman, 
2000). Three conversions are involved in the process, which are given by the following 
stoichiometric relationships: 
OHCH2HCO 32 =+  (?H298K = -91 kJ/mol)                           (1-1) 
OHOHCH3HCO 2322 +=+  (?H298K = -50 kJ/mol)                      (1-2) 
222 HCOOHCO +?+  (?H298K = -41 kJ/mol)                        (1-3) 
Equation 1-1 suggests an ideal molar ratio of H2/CO = 2 for a single pass conversion, and 
these reactions are highly exothermic. Water-gas-shift and reverse water-gas-shift reactions (Eqs. 
1-3) co-exist, resulting in a change of the ideal syngas stoichiometric ratio. Accounting for both 
CO and CO2 hydrogenation, Lange (2001) adjusted the ideal stoichiometric ratio to (H2-
 7
CO2)/(CO+CO2) = 2. The effect of syngas molar ratio on the production of alcohols will be 
further discussed in 1.2.2.4.4. 
1.2.1.2. Mechanism for methanol synthesis 
Various mechanisms have been proposed for the synthesis of methanol from syngas. Four 
mechanisms are discussed as below: 
a) Xu et al.(1987) regarded the following mechanism proposed by Ellgen et al. (1979) as the 
most plausible one. 
      
The * denotes the absorbed surface species and (g) denotes gaseous species. This mechanism 
has been supported by kinetic studies with isotopic labeling and spectroscopic observation of the 
reaction intermediates (Chinchen, Waugh, & Whan, 1986). It has been proven that copper is 
present in the catalyst both in the metallic and ionic form (Mehta, Simmons, Klier, & Herman, 
1979). The formation of methanol and the dissociation of hydrogen occur at the metallic copper 
part of the catalyst surface, whereas the adsorption of CO takes places on the Cu ion site (Xu et 
al. 1987). Consequently, an active and selective methanol catalyst requires the homogenous 
distribution of both metallic and ionic Cu.  
b) Nakamura et al. (1990) have proposed the ?surface-redox? mechanism and ?formate 
mechanism? for the water-gas shift reaction. Strong evidence for the surface-redox mechanism 
has been found in Cu single-crystal studies (Campbell and Daube 1987). Based on the redox 
mechanism, Askgaard (1995) proposed a kinetic model for methanol synthesis, elementary steps 
of which are listed in Table 1.1. From steps 1 to 8 in table1.1, H2O(g) is completely dissociated 
to O* and H2(g), and the surface oxide O* is removed by CO* to produce CO2(g). Elementary 
steps 9-13 constitute the simplest reaction mechanism of the methanol synthesis reaction. The 
CH3OH(g)CO(g) CO* HCO* H2CO* CH3O*H* H*H*H*
 8
apparent rate determining step in this mechanism has been found to be the hydrogenation of 
H2COO* to methoxide and oxide (step 14) (Askgaard et al., 1995). 
c) Dissimilarly, the ?formate mechanism? suggested that H2O* dissociates into OH* and H*, 
followed by the formation of the formate HCOO* converting from CO* and OH*. HCOO* 
reacted with H* and eventually converted to methanol (Nakamura et al. 1990). 
d) It is worthy to note that alternative to the pathways above, carbonylation of methanol and 
hydrogenation of methyl formate were also considered as a pathway for the reaction (Palekar et 
al., 1993).  
33 HCOOCH OHCHCO =+                                              (1-4) 
OHCHHHCOOCH 323 22 =+                                             (1-5) 
 
 
 
 
 
 
 
 
 
 
 
 
  
 9
Table 1.1 Elementary steps proposed in the ?redox? mechanism of methanol synthesis (Askgaard et 
al. 1995) 
Steps Surface reactions 
1 H2O(g) + ?  ?  H 2O? 
2 H2O? + ?  ? OH? + H? 
3 2OH?  ? H2O? + O? 
4 OH? + ? ? O? +H? 
5 2H? ? H2 + 2? 
6 CO(g) +? ? CO? 
7 CO? +O? ? CO2? + ? 
8 CO2? ? CO2(g) + ? 
9 CO2? + H? ? HCOO? + ? 
10 HCOO? + H? ? H2COO? + ? 
11 H2COO? + H? ? H3CO? + O? 
12 H3CO? + H? ? CH3OH? + ? 
13 CH3OH? ? CH3OH(g) + ? 
14 H2COO? + ? ? HCHO? + O? 
15 HCHO? ? HCHO(g) + ? 
16 H2COO? + H? ? HCHO? + OH? 
 
 
 
 
 
 
 
 
 10
1.2.1.3. Catalysts for methanol synthesis 
The catalytic synthesis of methanol is typically carried out over two types of catalysts, 
a) The first type of methanol synthesis catalyst is copper-free zinc chromite (ZnO/Cr2O3) 
catalysts operating under high temperature (400 ?C) and pressure reaction conditions (10-20 MPa) 
(Herman, 2000).  The liquid product synthesized by this type of catalyst is essentially pure 
methanol with a small amount of water and higher alcohols. The maximum volume percentage 
of methanol in the liquid product is about 90%. A certain ZnO-Cr2O3 based catalyst that could 
hydrogenate CO to methanol at 24-30 MPa and 350-400 ?C was commercially adopted by BASF 
in 1923 (Satterfield, 1991).  However, the high selectivity and formation rate toward methanol 
for this catalyst is restricted to these rigorous reaction conditions. Bradford et al. (2003) found 
that when operating a ZnO/Cr2O3 catalyst at only 320 ?C and 3.41 MPa, the maximum allowable 
conversion of CO to CH3OH was restricted to 4.2%. 
b) The second type of methanol synthesis catalyst is copper-based catalysts that contain 
ZnO and oxide support and operate at a low temperature (250 ?C) and low pressure (5-10 MPa), 
e.g. Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3 (Herman, 2000). The low-temperature methanol synthesis 
process using these catalysts has entirely replaced the older high-temperature process that 
operated at 15 MPa and 400 ?C using a Cu-free ?zinc-chromite-type? (ZnO/Cr2O3) catalyst 
(Subramani & Gangwal, 2008). The high selectivity, long term performance, and resistance to 
oxidizing gases have resulted in this kind catalyst being widely used in the methanol synthesis 
industry (Xu et al. 1987). For example, a Cu/ZnO/Cr2O3 catalyst was shown to have a 
200g/kgcat?h methanol formation rate at the reaction conditions of P = 7.6 MPa, T =275 ?C, 
H2/CO = 0.45, and a space velocity of 3200 l/gcat?h (Campos-Mart?n, 1996). An interesting 
characteristic of these catalysts is that the mixed catalysts are at least three orders of magnitude 
more active than each of the separate catalyst components (Herman et al. 1979). Mehta et al. 
 11
(1979) proved that copper is indeed soluble in the zinc oxide, and that the electron transfer 
between copper metal and ZnO may play a contributory, but not an essential, role in promoting 
the synthesis of methanol. The disadvantages of these copper-based catalyst are few but 
significant: low resistance to thermal shocks, which does not allow the catalyst to be operated 
above 300 ?C and extreme sensitivity to sulfur and chlorine poisoning that requires a careful and 
expensive purification of the syngas (Herman et al. 1979). 
Overall, the Cu/ZnO/Al2O3 catalyst has been the preferred catalyst for methanol synthesis 
industry for many years, producing billions of gallons of methanol every year worldwide.  
1.2.2 Mixed Alcohols Synthesis (MAS) 
1.2.2.1. Background 
The phrase ?mixed alcohol? refers to the mixture of methanol and higher alcohols, which are 
usually C2-C4 alcohols (ethanol, propanol, butanol and isomers). The general chemical formula 
for higher alcohols is CnH2n+1OH (n ?2). They can be synthesized chemically or biologically, and 
they have characteristics allowing them to be used in current engines (Forzatti et al. 1991).  
The direct catalytic synthesis of higher alcohols from carbon monoxide and hydrogen was first 
reported in the 1920?s (Mahdavi et al. 2005). Synthesis of higher alcohols, isobutanol in 
particular, from coal, biomass or natural gas via syngas has been attracting research interest due 
to the potential for producing clean and high octane-value fuels.   
Higher alcohols can be used directly as fuels, as fuel additives, and can be further converted 
into valuable chemicals. The advantages and disadvantages are as follows. 
a) It has been confirmed that higher alcohols are clean burning fuels when used directly. 
Higher alcohols have comparable and some even have higher octane ratings, with ethanol at 130 
RON (Research Octane Number), 96 MON (Motor Octane Number), (which equates to 113 AKI) 
 12
and n-butanol at 96 RON, 78 MON (which equates to 87 AKI), as shown in Table 1.2. Note that 
AKI refers to ?Anti-Knock Index? which averages the RON and MON ratings and is reported on 
U.S. gas station pumps. It has been shown that a fuel with a higher octane rating is less prone to 
knocking (extremely rapid and spontaneous combustion by compression) and possesses 
improved energy efficiency, leading to a better fuel economy than the conventional gasoline. In 
spark ignition engines, alcohols can run at much higher exhaust gas recirculation rates and with 
higher compression ratios (Klier et al. 1997). Moreover, alcohols have been shown to reduce the 
emission of CO, hydrocarbons, ozone, and particulates, and may reduce the emission of NOx. A 
test with E85 (ethanol 85%) fueled Chevrolet Luminas showed that NMHC (non-methane 
hydrocarbons) went down by 20-22%, NOx by 25-32% and CO by 12-24% compared to 
reformulated gasoline (Kelly, Bailey, Coburn, Clark, & Lissiuk, 1996). It is important to note 
that using alcohols directly as fuels in current vehicle systems requires modifications to some 
components in the engine system. 
b) Higher alcohols, ethanol in particular, are often used as fuel additives. In order to reduce 
pollution, many states in the U.S. have mandated that all gasoline fuels be blended with 10 
percent alcohol (usually ethanol) during some or all of the year. Butanol has a higher energy 
density than ethanol and has been demonstrated to work in vehicles designed for use with 
gasoline without modification. The current butanol supply mainly comes from the fermentation 
of biomass and the refining of fossil fuels. With the development of bacteria modification, the 
number of commercial butanol plants has been growing, supplying increasing amounts of 
butanol to the fuel additive market. Characteristics of using the mixtures of alcohols as fuel 
additives include:  
 13
1. Loss of energy density. As shown in Table 1.2, one liter of ethanol contains 21.1 MJ, a 
liter of methanol 15.8 MJ and a liter of gasoline approximately 32.6 MJ. Consequently, for the 
same energy content as one liter of gasoline, one needs 1.6 liters of ethanol and 2.1 liters of 
methanol. However, actual fuel consumption doesn't increase as much as energy content 
numbers indicate, because the alcohol fuels are more energy efficient (Li et al. 2004).  
2. Enhancement of octane number. Octane number parameters such as RON and MON are 
important properties of fuel that reflect their propensity to resist ?knock? in spark-ignited engines. 
In general, RON and MON values of pure hydrocarbons decrease in the following order: 
aromatics, iso-paraffins, olefins and naphthenes, and n-paraffins (Anderson et al. 2010). Higher 
alcohols have smaller molecular weight and directly provide oxygen to gasoline, therefore a 
range of higher octane numbers could be achieved by changing the composition of alcohol 
additives. The properties of different types of fuels with or without alcohol additives are 
summarized in Table 1.2. Higher AKI could be observed from the Gasohol and E85, when 
comparing to the value of traditional gasoline. 
3. Improved combustion efficiency. As a fuel additive, mixed alcohols have been shown to 
exhibit a reduction in CO, HCs, and NOx emissions (Hohlein, Von Oer Decken, Holtje, & 
Mausbeck, 1991). Table 1.3 compares the emissions from combusting different types of fuels in 
terms of the amount of CO, HC, and in the exhaust gas. It is clear that adding methanol or DME 
into gasoline can reduce the emission of hydrocarbon and NOx. A study of the influence of 
ethanol on emissions and fuel consumption of automobiles showed that with increasing the 
percentage of ethanol in gasoline (from 5% to 25%) the emission of CO and NOx decreased 
continuously (Gautam et al. 2000) 
 14
4. Integration of use within the existing gasoline infrastructure and vehicles. Higher 
alcohols are preferred over methanol in this respect because of their lower water solubility, 
reducing the possibility of damaging current vehicles systems.  
Table 1.2 Properties of different types of fuels (Methanol institute, 2010) 
Fuels 
Energy 
density 
(MJ/L) 
Air-fuel 
ratio 
Specific 
energy 
(MJ/kg air) 
Heat of 
vaporization 
(MJ/kg) 
RON MON AKI 
Gasoline and biogasoline 32 14.6 2.9 0.36 91-99 81-89 86-94 
Butanol fuel 29.2 11.1 3.2 0.43 96 78 87 
Ethanol fuel 19.6 9.0 3.0 0.92 130 96 113 
Methanol 16 6.4 3.1 1.2 136 104 120 
Gasohol (10% ethanol) 28.06 n/a n/a n/a 96-101 83-92 89-97 
E85 (85% ethanol) 25.65 n/a n/a n/a 113 94 104 
 
Table 1.3 Emission comparisons for different fuel types (Methanol institute, 2010) 
Emission 
(g/km) H2 DME Methanol CNG
a LPGb Diesel 
Gasoline 
with  
carburetor 
Gasoline without 
carburetor 
CO 0 0.12 0.34 0.4 0.89 0.24 1.47 8.96 
HC 0 0.04 0.043 0.41 0.115 0.095 0.09 1.27 
NOx 0.037 0.034 0.102 0.125 0.155 0.67 0.315 2.64 
a Compressed natural gas (CNG) 
b Liquefied petroleum gas (LPG) 
 
c) Higher alcohols could be used as feed stock in the chemical process industry. First, 
ethanol is an important industrial ingredient and has widespread use as a base chemical for other 
organic compounds. These include ethyl halides, ethyl esters, diethyl ether, acetic acid, ethyl 
amines and to a lesser extent butadiene. Second, water-soluble alcohols (low-molecular weight 
products) are used as solvents for the manufacture of coatings, dyes and inks, plastics, flavorings, 
personal-care products, pharmaceuticals, and cleaners. Higher molecular weight alcohols (C6-C9 
 15
and C10-C16) and their derivatives are used as intermediates of plasticizers, detergents and 
lubricants (Mertzweiller, Rouge, Cull, & McCracken, 1958). 
Overall, higher alcohols present good energy efficiency and provide options for sustainable 
renewable transportation fuels. With the increasing demand for fuels, higher alcohols could 
potentially reduce the dependence on imported crude oil. In 2007, U.S government established a 
new Renewable Fuel Standard (RFS2) in the Energy Independence and Security act, as a result 
of which greater biofuel production and consumption is expected through 2022. 
1.2.2.2. Catalysts for Mixed Alcohols Synthesis (MAS) 
The synthesis of higher alcohols suffers from low selectivity, impeding the industrialization. 
Modification of catalysts as an effective means to enhance higher alcohols? selectivity has been 
widely developed and reviewed (Nunan et al. 1989; Majocchi et al. 1998; Hilmen et al. 1998; 
Herman 2000; Spath and Dayton 2003; Fang et al. 2009; Subramani and Gangwal 2008). For 
approximately the last 25 years, there has been intensive research centered on the development 
of active and selective catalysts for shifting alcohol synthesis away from methanol and toward 
higher alcohols (Herman 2000). These catalysts can be divided into four categories, with 
examples of each shown in Table 1.4. Spath & Dayton (2003) believed that the modified 
methanol and modified FT catalysts have been the most effective in the production of mixed 
alcohols. Other catalyst types that have been researched for higher alcohol synthesis include 
sulfide-based, oxide-based, and rhodium based. 
a) Modified methanol catalysts. The modification of methanol catalysts for higher alcohol 
formation is usually accomplished by the addition of alkali promoters, which leads to the 
formation of higher alcohols from H2/CO mixtures (Hilmen et al., 1998). Aldol condensation of 
alcohols occurs within the coordination sphere of the heavy alkali cation, which enhance the 
 16
basicity of the catalyst (Klier et al., 1993). Among the alkali-metal dopants, it has been shown 
that the order of promotion is Cs > Rb > K > Na using formate salt (Nunan et al. 1989) or 
hydroxide (Vedage, Himelfarb, Simmons, & Klier, 1985). Cs appears to be the best promoter, 
but K is often used because of its availability and low cost.  
b) Fischer Tropsch Synthesis (FTS) elements (Fe, Co, Ni, etc.) modified copper catalysts 
and group VI-VIII metal-based catalysts, e.g. Mg and Fe supported catalysts. Copper is known to 
be the major element for methanol synthesis, enabling the dissociative chemisorption of 
hydrogen and the associative adsorption of CO (Xu et al., 1987). FTS elements such as Co and 
Ni have been found to be effective promoters to enhance the alcohol and hydrocarbon yields. 
Sibillia et al. (1984) found that the addition of Fe to Cu/ZnO methanol synthesis catalysts led to a 
significant increase in hydrocarbons yields.  The use of Co-Cu based catalysts has been reported 
to give high catalytic activity and good selectivity towards higher alcohols (Courty, Durand, 
Freund, & Sugier, 1982). In addition, the carbon number distribution of products over the Co-Cu 
catalysts follows the Anderson-Schulz-Flory (ASF) distribution (Smith and Anderson, 1984). 
Table 1.4 Catalytic performances over different types of alcohol synthesis catalysts (Thao, 2007) 
No. Catalysts  
Conditions CO 
Conversion 
(%) 
Selectivity 
(carbon atom,%) 
Productivity 
(g/kgcat?h) 
T(K) P(MPa) H2/CO GHSV MeOH C2+OH MeOH C2+OH 
1 Cs/Zn/Cr 678 6.9 1 Not found 9.0 72.2 0.8 116 1 
2 Pd/Cu/Me/Ce 583 4.5 1 6000 l/kgcat/h 10.6 74.4 11.6 23.6 7.8 
3 Cs/Cu/Zn/Al 583 7.6 0.45 5330 l/kgcat/h 13.8 85.1 5.9 436.5 16.9 
4 Cs/Cu/Zn/Cr 583 7.6 0.45 5530 l/kgcat/h 21.1 48.7 29.8 202.0 51.9 
5 K/Cu/Co/Zn/Al 563 6.0 0.5-2 4000 h-1 21-24 20-27 30-50 60-100 NA 
6 Cu/Co/Zn/Al 573 6.0 1 3000 h-1 Not reported 44.0 30 85.0 NA 
7 Na/Cu/Co/Zn/Al 573 6.0 1 3000 h-1 Not reported 37.0 33.0 85.0 NA 
8 Cs/Co/Mo/Clay 573 13.8 1.1 4000 h-1 4.7 61.8 26.11 60-64 35-40 
9 LaRhO3 573 0.6 1 Not found Not reported 12.2 5.6 16 15 
 
 17
c) Sulfide-based catalyst. Alkali-promoted transition metal sulfide catalysts, particularly 
MoS2 require the similar reaction conditions as those employed with the low temperature copper-
based oxide catalysts (Fang et al., 2009). In contrast to the alkali-promoted Cu-based catalysts, it 
has been shown that the higher alcohols formed over the sulfide catalysts were linear and that C-
C bond formation proceeded via a CO-insertion into an ?-carbon mechanism (Smith et al. 
1991).The products formed over MoS2 catalysts contained 15%-20% hydrocarbons, principally 
consisting of methane. The carbon number distribution of both the alcohols and the hydrocarbons 
followed an ASF distribution (Spath & Dayton, 2003). Because of the ASF distribution, 
selectivity to higher alcohols is limited. Additionally, the sulfide-based catalysts are less active 
than oxide-based catalysts, especially the Cu/ZnO-based catalysts. 
It is important to note that alkali-doped molybdenum sulfide catalysts posses an excellent 
sulfur tolerance and high activity for the water-gas-shift reaction, making them promising 
catalysts for mixed alcohol synthesis. These catalysts can be operated at a relatively high 
reaction temperatures (270-330 ?C) and operation pressure 10-28 MPa with H2/CO ratio of 1-2. 
The product mixture usually comprises 70%-80% alcohols and 15%-30% hydrocarbons on CO2-
free basis. The total alcohol yield can reach 0.3 g/mlcat/h, but with more than 50% products as 
methanol (Forzatti et al. 1991). 
d) Rh based complexes. Rhodium-based catalysts are very selective toward methane (Nirula 
1994) and C2 oxygenated products, such as ethanol, acetaldehyde and acetic acid (Fukushima, 
Arakawa, & Ichikawa, 1985; Ichikawa, 1982). Li, Fu, and Jiang (1999) argued that a strong 
interaction occurs between the rhodium modifiers and the K-Mo-O support. They further 
concluded that the coexistence of cationic and metallic Rh stabilized by this interaction may be 
responsible for the increased selectivity toward higher alcohols. Foley et al. (1990) suggested 
 18
that the interaction between Rh and Mo leads to the formation of electron-poor sites where the 
formation of higher alcohols occurs. Decanio et al. (1991) believed that, depending on the status 
of the rhodium species, properties of alkali promoter, nature of the support, and the reaction 
conditions, the rhodium species are capable of catalyzing dissociation, insertion, and CO 
hydrogenation. The Rh-based catalysts, which are especially selective for C2 oxygenates, are 
interesting from both a theoretical and practical point of view. However, Rh is scarce and its 
price is very high compared to other elements used in the synthesis of higher alcohols. The 
availability of Rh is limited to about 20 ton/y at present and over 70% of the available Rh is 
already being consumed by the automobile industry for making three-way auto-exhaust catalysts 
(Subramani & Gangwal, 2008). The high cost and limited availability of rhodium for higher 
alcohol synthesis catalysts will impact any commercialization of these synthetic processes for 
converting syngas to higher alcohols (Xu et al. 1987). 
 
 
 
 
 
 
 
 
 
 
 19
1.2.2.3. Chemistry and Reaction Mechanism of Mixed Alcohol Synthesis 
1.2.2.3.1 Chemistry 
Mixed alcohol synthesis (MAS) is similar to methanol synthesis and Fisher-Tropsch synthesis, 
preferring to be operated at high pressure and relatively low temperature. Views both from the 
reaction stoichiometry and exothermic nature of the reaction lend evidence to these reaction 
conditions (Minahan, Epling, & Hoflund, 1998). In the MAS process, the major reaction is the 
alcohol formation, while hydrocarbon formation and the water-gas-shift reaction constitute side 
reactions: 
Alcohol formation 
nCO + 2nH2 ? CnH2n+1OH +(n-1)H2O                                        (1-6) 
Hydrocarbon formation 
nCO + (2n+1)H2 ? CnH2n+2 + nH2O                                           (1-7) 
Water-gas-shift reaction 
CO + H2O ? CO2 +H2                                                   (1-8) 
Meanwhile, the hydrogenation of CO over alkali-doped catalysts also leads to the formation 
of a small amount of other reaction products: aldehyde, ester, ketones etc. (Forzatti et al. 1991). 
This complexity of the product mixture is consequent to the wide variety of catalytic functions 
that are active under synthesis conditions. A discussion of different mechanism that has been 
proposed for MAS is presented below: 
The reaction pathways that link these products are then specific to each catalyst type. CO-
insertion mechanisms are believed to be dominant mechanism for modified Fischer Tropsch 
synthesis catalysts (Hindermann et al. 1993). Aldol-type condensation mechanisms of Cn with C1 
species are believed to prevail on the low temperature Cu-based catalysts (Nunan et al., 1988). 
Cn + C1 and Cn + Cm condensations and ketonizations are active over high-temperature alkali-
 20
modified methanol catalysts (Forzatti et al. 1991; Tronconi et al. 1992). It is not always clear 
whether a proposed mechanism on one type of catalyst is necessarily applicable to other catalysts. 
Furthermore, the effects of catalyst supports and promoters make the mechanism more 
complicated. 
1.2.2.3.2 Mechanism for the low temperature Cu-based catalysts  
In order to probe the mechanisms of the C-C bond formation process, various experimental 
methods have been used, including 13C NMR (Nunan et al., 1988; Nunan, Bogdan, Herman, et 
al., 1989), in situ FTIR (Yang et al. 2005), and cofeeding probe molecules in pairs (Beretta, 
Tronconi, et al., 1996). 
Figure 1.4 shows a general reaction network that has been proposed for the synthesis of higher 
oxygenates over the modified methanol synthesis catalysts (Cu based) (Hilmen et al., 1998). This 
mechanism is consistent with the ones proposed by Smith & Anderson (1983), Smith et al (1991) 
and (Breman et al. 1994; Breman et al. 1995). The individual steps of the mechanistic network 
can be grouped into several distinct reaction types illustrated as below, including linear and aldol 
condensation chain growth mechanisms.  
a) ? addition (linear chain growth): 
? addition occurs at the alcohol end of the growing chain to give linear primary alcohols such 
as ethanol, 1- propanol, 1-butanol. For example,  
12CH313CH2OH + 12CO/H2 ? 12CH213CH212CH2OH 
The alpha addition is a slow step with a large activation energy (140 kJ/mol) while beta 
addition is faster and has a smaller activation energy (66 kJ/mol) (Smith & Anderson, 1984). 
Based on the hydrogenation-base bifunctionality of the catalyst, a probable reaction mechanism 
utilized a nucleophilic attack at the ? carbon of an absorbed aldehydic intermediate by a formyl 
 21
species (HCO ? ) yields a deoxygenated species (Nunan et al. 1988, 1989a). This species 
subsequently undergoes hydrogenation/dehydration to yield the Cn+1 alcohol (Fox, Pesa, & 
Curatolo, 1984). The nature of the C1 reactive species has not been conclusively identified. Lietti 
et al.(1992) investigated different oxidation states with C1 probe molecules, indicating 
formaldehydes at catalyst surface as the most probable reactant. 
 
 
 
 22
 
Figure 1.4 Proposed reaction network for methanol and higher alcohol synthesis (Hilmen et al. 1998) 
 
 
  
 23
b) ?-addition 
?-additions include the major ? addition between C1 and Cn (n ? 2) surface species and the 
minor ? addition between Cm (m = 2,3) and Cn (n ? 2) surface species. The former results in 1-
propanol and branched primary alcohols such as 2-methyl-1-propanol, the latter gives rise to a 
variety of alcohols such as 1-butanol, 2-butanol, 1-pentanol (Nunan et al., 1988, 1989a). The 
mechanism of ? addition involves two paths, aldol coupling with oxygen retention reversal ?(R) 
and aldol condensation ?(N). Aldol-type coupling reactions of alcohols require both Cu and basic 
sites (Hilmen et al., 1998). 
Aldol coupling with oxygen retention reversal ?(R): this step involves the retention of 
oxygen associated with the C1 fragment and the rejection of oxygen associated with the Cn 
fragment. In this step, carbon chain growth is dominated by ?(R) addition wherein the adding C1 
intermediate retains its oxygen. A stoichiometric relationship is presented as below, 
12CH313CH2OH + 12CO/H2 ? 13CH212CH212CH2OH 
Aldol condensation ?(N): this step involves the retention of oxygen associated with the Cn 
fragment and the rejection of oxygen associated with the C1 fragment: 
12CH313CH2OH + 12CO/H2 ? 12CH212CH213CH2OH 
Due to the high C1 concentration (the methanol concentration is roughly one order of 
magnitude higher than the concentration of the other alcohols (Majocchi et al., 1998)), it is likely 
that the Cn + C1 ? Cn+1 processes dominate over the Cn + Cm ? Cn+m steps in the chain growth 
mechanisms, resulting in isobutanol as a terminal species due to no ? carbon in its precursor 
(Lietti et al., 1992). This has also been observed by Nunan et al. (1988), who claims that of the 
two types of ? additions ?(N) and ?(R), oxygen retention reversal ?(R) dominates the C2 to C3 
step over the Cs/Cu/ZnO catalyst at high temperature.  
 24
1-propanol can be formed both by aldol coupling and by linear chain growth (carbonylation) 
pathways. 1-butanol is predominatly formed by ethanol aldol self-condensation instead of linear 
chain growth pathways. Isobutanol is formed only by C1 addtion to 1-propanol or 
propionaldehyde (Nunan et al., 1989a, 1988). Aldol condensation pathways are about eight times 
faster than linear chain growth on 2.9 wt% Cs-Cu/ZnO/Al2O3 catalysts (Hilmen et al., 1998). 
Hydrocarbon formation may involve dehydration from the corresponding alcohol (with the 
corresponding alkyl group as intermediates) and/or a parallel Fisher-Tropsch pathway (Majocchi 
et al., 1998). A significant increase in ethane formation that has been observed upon ethanol 
injection in the feed stream confirms that the formation of hydrocarbons via alcohol dehydration 
is active over the Cu-based catalyst (Majocchi et al., 1998). 
It should be noted that before the aldol C-C bond formation can be utilized in the chain growth 
of higher alcohols from lower alcohols, the single carbon species must be converted to a C+ 
oxygenate. This can occur by various chemical mechanisms but one usually dominates over 
another when different catalysts are used (Klier et al., 1997).  
1.2.2.4. Aspects affecting higher alcohols? selectivity 
1.2.2.4.1 Temperature  
It is well known that the exothermic alcohol synthesis reaction is thermodynamically limited 
by high temperature. The increase of reaction temperature shift the methanol equilibrium 
backwards towards the reactants and increase temperature also tends to favor the formation of 
various side-products that include dimethyl ether (DME) and CH4 (Li et al. 2004). A strong 
effect of temperature on the product distribution has been widely observed, an example for a Cu-
based catalyst being shown in Figure 1.5. 
 25
 
Figure 1.5 Effect of reaction temperature on the alcohol distribution for MAS on Cu-based catalyst 
(Majocchi et al. 1998). 
Within the studied temperature range (225 ?C-325 ?C) the methanol and C2+ linear alcohols 
selectivities show maxima on increasing the reaction temperature. The formation of branched 
alcohols and methane (not listed in figure 1.5) also increase with increasing temperature but level 
off at high temperature. Methanol formation is believed to approach chemical equilibrium with 
increasing the reaction temperature. Because of the exothermic nature of the reaction, further 
increasing the reaction temperature can decrease the methanol formation.  
The effect of temperature on MAS is similar for different type of catalyst. Mahdavi et al.(2005) 
investigated the effect of temperature (270 ?C-355 ?C) on Co containing Cu-based catalyst?s 
activity at constant pressure (4.05 MPa). It was found that increasing temperature from 270 ?C to 
325 ?C significantly increased the activity. CH4 and C2+OH selectivity decreased with 
temperature slightly, and a maximum in the total alcohol selectivity was observed at 325 ?C. 
 26
285 ?C -300 ?C was regarded as a proper range of operation, because the values of the alcohols? 
selectivity remained constant.  
Temperature plays an important role in the selectivity of higher alcohols. It is believed that 
higher reaction temperature will shorten the catalysts? lifetime and diminish the active surface 
areas due to bulking of the catalyst. Meanwhile, some oxygenates are not be stable enough and 
will decompose at temperature as low as 450 ?C, which sets an upper temperature limit to the 
synthesis of alcohols (Xu et al. 1987). 
Additionally, when HAS is operated at lower temperatures smaller contents of ketones 
relative to primary alcohols are expected. Smaller quantities of aldehydes are typically reported 
among the products of HAS over modified low temperature methanol catalysts than over high 
temperature methanol catalysts (Forzatti et al. 1991). Majocchi et al.(1998) showed that for Cu 
based ZnO doped catalysts the lowest viable reaction temperature is 250 ?C corresponding to the 
temperature threshold for the formation of C2+ oxygenates. 
1.2.2.4.2 Pressure  
Generally, the selectivity to mixed alcohols increases with increasing operating pressure. In 
the normal operating ranges, the higher the pressure at a given temperature, the more selective 
the process will be to alcohols. For example, Burcham et al. (1998) studied the effect of reaction 
pressure (varied in the range of 7.6 MPa ? 12.4 MPa) on the synthesis rate of isobutanol for a 
Cs/Cu/ZnO/Cr2O3 catalyst. An Increase in reaction rate was found, but the effect was most 
pronounced for the formation of methanol. Mahdavi et al. (2005) concluded that an increase in 
pressure had a strong effect on the conversion rate, and therefore, led to an increased total liquid 
production rate. Additionally, with increasing pressure, the selectivity to higher alcohols 
increased and selectivity towards methane decreased. However, while high operating pressures 
 27
may be desirable, the pressure is often limited primarily by the cost of high pressure vessels, 
compressors and the energy costs needed to carry out the higher pressure reactions (Stevens and 
Conway, 1988). 
1.2.2.4.3 Syngas flow rate 
The syngas flow rate, represented by gas hourly space velocity (GHSV), is a measure of the 
volume of syngas at standard temperature and pressure passing a given volume of catalyst in an 
hour?s time. Higher space velocity means a shorter contact time. It is commonly observed that 
the selectivity towards higher alcohols can be enhanced by decreasing the GHSV (longer 
residence time, higher conversion) (Boz et al. 1994; Majocchi et al. 1998; Forzatti et al. 1991), 
indicating that the formation of higher alcohols takes place consecutively to the formation of 
methanol. 
Hilmen et al.(1998) compared the effects of space velocity on alcohol synthesis rates and 
selectivities for 1.2 wt% Cs-Cu/ZnO/Al2O3 and 1.0 wt% K-Cu0.5Mg5CeOx catalysts. CO 
conversions were found to increase linearly with decreasing syngas flow rate at low CO 
conversions, except for when the predominant methanol synthesis approaches thermodynamic 
equilibrium. C2-C3 alcohols? selectivity reached maximum values at intermediate residence times, 
indicating they are intermediate products that undergo further chain growth. The selectivity 
towards isobutanol increases monotonically with decreasing syngas space velocity, indicating 
they are formed in secondary chain growth reactions. 
Another example is illustrated by Mahdavi et al.(2005). Increasing the syngas flow rate was 
shown to have a negative effect on both the catalytic activity and the C2+OH selectivity. 
Majocchi et al.(1998) observed that with increasing GHSV the branched/linear alcohol ratio 
 28
slightly decreased. For the short contact times, C2-C3 production is favored with respect to the 
higher oxygenates. 
However, (Xu et al. 1987) believed that when the reaction is partially or totally mass transfer 
controlled, an increase in space velocity will be favorable for the mass transfer of reactants to the 
core of the catalyst particles and unfavorable for secondary reactions. Since the influence of 
mass transfer on various reactions is different (more severe for higher alcohols and higher 
hydrocarbons), an increase of space velocity may be favorable for the synthesis of higher 
alcohols. This phenomenon was observed with the MoS2-based catalyst (Xu et al. 1987), where 
at a high space velocity the selectivity for higher alcohols was higher. 
Hence, in order to achieve the optimum higher alcohol to methanol ratios, the optimal 
GHSV/conversion level will vary with the catalyst type. 
1.2.2.4.4 H2/CO ratio 
Overall, the formation of alcohols: 
nCO + 2nH2 ? CnH2n+1OH + nH2O 
Consequently, the ideal stoichiometrical consumption ratio (H2/CO) is 2.0. Additionally, 
formation of other products such as water-gas shift reactions, alkenes or alkanes formations can 
occur, making the actual ratio of H2/CO change along the reactor and differ from the one at the 
inlet value (Xu et al. 1987). Several investigators have reported that the optimum H2/CO ratio for 
HAS over various catalysts is below 2 (Forzatti et al. 1991; Smith and Anderson 1983; Herman 
2000; Boz et al. 1994). Forzatti et al. (1991) found that for a ZnCrO + 15% Cs2O catalyst a 
maximum in the yields of methanol occurs at the molar ratio of 2 and the maximum of higher 
alcohols is at the molar ratio of 1.  
 29
The H2/CO ratio plays an important role in the formation of higher alcohols. Under constant 
total pressure, a change in the H2/CO ratio will change the partial pressure of both H2 and CO. It 
is expected that an increase in the hydrogen partial pressure will favor the hydrogenation of the 
aldehydic intermediates to the corresponding alcohols, thus inhibiting the chain growth to higher 
oxygenates (Xu et al. 1987). As a result, the gradual enrichment of hydrogen in the reactant 
mixture causes a progressive increase in the formation of methanol and a decrease in the 
amounts of both linear and branched higher alcohols. The average carbon atom number of the 
C2+ oxygenates slightly decreases on increasing the H2/CO ratio. The rate of formation of 
methane and higher paraffins (for example: ethane) is largely unaffected by the feed composition 
in the investigated range (Majocchi et al., 1998). 
Moreover, it has been found that lower H2/CO ratio has a negative effect over the conversion 
of CO and a positive effect over the selectivity to C2+ alcohols. In addition, coke formation is 
unfavorable at higher H2 partial pressure, resulting in good activity maintenance (Xu et al. 1987). 
A H2/CO = 1 has usually been adopted as a reasonable ratio for the investigation of HAS 
(Mahdavi et al., 2005). 
1.2.2.4.5 Addition of alkali metal oxide 
It is well known that the catalytic activity of HAS can be promoted by adding an alkali metal 
oxide, such as Na, K, Cs, Sr, Ba, etc, into a methanol synthesis catalyst (Herman 2000; Forzatti 
et al. 1991; Fang et al. 2009; Subramani and Gangwal 2008). Vedage et al. (1985) and Sheffer & 
King (1989) found that catalytic activity upon the addition of group 1A elements increased in the 
order of Li to Cs. Potassium is regarded as the most cost effective additive, and most research 
has concentrated on its effect.  
 30
The addition of alkali promoters titrate the surface acidity of the catalyst and increase the 
density of basic sites (Hilmen et al., 1998), thereby facilitates various C-C and C-O bond-
forming reactions (Nunan et al., 1988) and suppressing unwanted side reactions such as 
isomerization, dehydration, coke deposition etc (Subramani & Gangwal, 2008). In CO 
hydrogenation reactions, it is generally believed that the CO molecules adsorbed dissociatively 
are responsible for hydrocarbon formation while those adsorbed associatively favor the 
formation of alcohols (Subramani & Gangwal, 2008).  The addition of an alkali promoter can 
reduce the active sites for dissociative adsorption of CO, thereby decreasing the formation of 
hydrocarbons.  
An optimum amount of alkali doping has been observed, depending on the kind of catalyst 
used. Methanol and higher alcohol synthesis rates were previously shown to reach a maximum 
with increasing alkali loading (Nunan et al. 1989; Smith & Anderson 1983; Vedage et al. 1985). 
Smith & Anderson (1983) and Calverley and Smith (1991) found that the activity for both 
methanol and higher alcohol production is promoted by the addition of alkali metal oxides up to 
around 1 wt% while further addition has an inhibiting effect. Smith & Anderson (1983) found 
that the selectivity to higher alcohols, in particular isobutanol, goes through a maximum as the K 
loading is increased. Furthermore, (Boz et al., 1994) observed that methanol selectivity increased 
continuously with increasing K loading from 0 to 5 wt%, mostly at the expense of hydrocarbons, 
and the selectivity of both higher alcohols and aldehydes reached maxima at a loading of 0.5 wt% 
K2O.  
Excess amount of alkali may block active sites on the catalyst surface and decrease the BET 
surface area, leading to activity loss (Smith & Anderson, 1983). At very high Cs concentrations, 
 31
alcohol synthesis rates may ultimately decrease, because Cu-metal sites, required for carbon 
chain growth, can also be titrated by Cs (Hilmen et al., 1998). 
To sum up, the amount of alkali required for optimum higher alcohol synthesis depends on the 
identity of the alkali and on the density of acid sits that require titration on a given catalyst 
(Nunan et al. 1989). 
1.2.2.4.6 Heat transfer rate  
As was discussed in section 1.2, reactions which produce alcohols, hydrocarbons and methane 
are highly exothermic. Excessive heat release from the active pore sites requires a rapid heat 
transfer rate to prevent temperature runaway, catalyst deactivation and other problems.   
Methanol formation is less exothermic than the formation of higher alcohols. Consequently, 
the equilibrium constant for methanol formation does not decrease as rapidly with increasing 
temperature as the equilibrium constants for higher alcohol formation. It was demonstrated in 
section 1.3.3.1 that high temperature is not favorable for the formation of higher alcohols. 
Thermodynamics constraints may affect the selectivity of the synthesis for many products, such 
as methanol, water, aldehydes, secondary alcohols, ketones, esters, as well as the overall 
production of CO2 (Forzatti et al. 1991). The low heat transfer capacity provided by vapor phase 
reaction medium may not effectively remove the reaction heat, resulting in decrease of 
selectivity towards higher alcohols. 
Methanol synthesis is frequently done by using a fixed bed reactor design. Most of the 
researches of HAS are also be investigated by using this kind of reactor. An alternative reactor 
design to improve upon heat management includes slurry phase and bubble column reactors; 
these have been utilized in methanol synthesis and the HAS researches (Brown et al. 1991; 
 32
Roberts et al.1997; Tijm et al. 2001). The advantages of slurry phase reactor applying for higher 
alcohol synthesis are summarized in the section 1.4.4. 
Overall, thermodynamic constraints limit the possible yield of HAS. As in other syngas-to-
liquids processes one of the most important issues to HAS is the removal of the considerable heat 
of reaction. The relationship between heat transfer rate and the synthesis of mixed alcohols has 
not been examined, although it is well known that these exothermic reactions could benefit from 
high heat transfer rate.   
1.2.3 Advances in the synthesis of higher alcohols 
As was discussed in the mechanism section above, there is no ?-carbon to assist in 
converting the C1 intermediates in order to form ethanol, so C1 to C2 step is the slow and rate 
determining step in the formation of the higher alcohols from synthesis gas (Tronconi et al. 
1990). In order to realize a commercial HAS process, substantial research efforts have been 
applied into developing catalysts and defining operating conditions that produce greater 
selectivity toward higher alcohols (Fierro, 1993). Approaches to achieve necessary higher space 
time yields of higher alcohols include developing modified catalysts, injection of lower alcohols 
into the reactant stream, using catalyst beds in series, and using a slurry phase reactor. 
1.2.3.1. Modified Catalysts 
The activating character of alkali metal promoters is a function of their basicity. Alkali metals 
provide a basic site to catalyze the aldol condensation reaction by activating surface adsorbed 
CO and enhancing the formation of the formate intermediate (Spath & Dayton, 2003).  
Cu is known to be the major element for alcohol synthesis, serving as the site for dissociative 
chemisorption of hydrogen and the associative adsorption of CO (Xu et al. 1987). Fe or Co 
element is the active site for Fischer-Tropsch (F-T) function of dissociative CO adsorption (C-C 
 33
chain growth) and hydrogenation. Fang et al. (2009) believed that the production of higher 
alcohols requires the synergetic functioning of both Fe (or Co) and Cu, which means that a 
metallic copper atom should be adjacent to an iron atom. Thus, the modification of this kind 
catalyst requires precise preparation conditions to achieve a homogeneous distribution of the 
metals.  
Another approach made to increase the productivity of isobutanol formation from synthesis 
gas is to combine ZnO with an ?isosynthesis? component, i.e. ZrO2, and promote the catalyst 
with a redox oxide and a strong base. Isosynthesis is a high pressure process directed toward 
converting synthesis gas to aliphatic branched hydrocarbons, especially C4 hydrocarbons. High 
productivities and selectivities for isobutanol were observed with these catalysts at a high 
reaction temperature and high pressure. For example, W. Falter (1994) achieved an isobutanol 
productivity of 630 g/L cat/h over a K/Pd promoted Zr/Zn/Mn catalyst at a temperature of 427 ?C 
and 25 MPa (Herman 2000). A disadvantage of these catalysts is that the high reaction 
temperature increases the productivity of hydrocarbons along with that of the alcohols, 
decreasing the alcohol selectivity. 
The 3d transition metals, in particular F-T elements such as Co and Ni, have also been found 
to be effective promoters for MoS2 catalysts in enhancing the alcohol yields and C2+OH 
selectivity. A Ni promoted MoS2-based catalyst showed very high yield of total alcohols (0.40 
g/ml cat/h), high yield of C2+ alcohols (0.16 g/ml cat/h) as well as a up to 2000 hours of stable 
performance (Fang et al., 2009). 
Based on the fact that molybdenum carbide exhibits some properties similar to MoS2, and 
the fact that the addition of 3-d transition metals (such as Co, Ni, Rh, Pd) is able to improve the 
catalytic activity and selectivity of C2+ alcohols over MoS2-based catalysts, (Xiang et al. 2007) 
 34
developed a series K/Co/?-Mo2C catalysts and investigated their catalytic performance. Their 
results showed that the activity and selectivity of higher alcohols is significantly influenced by 
the Co/Mo molar ratio, and that the cobalt promoter exerted strong promotion for carbon chain 
growth, especially for the C1OH to C2OH step. Mo2C catalysts promoted with K2CO3 have been 
found to greatly enhance the selectivity to linear C1-C7 alcohols. Compared to Cu-based 
methanol synthesis catalysts, molybdenum Carbide catalysts possess much higher catalytic 
activity, higher selectivity towards hydrocarbons (consisting of 80-100% olefins), higher 
selectivity towards CO2 (around 50%), as well as a high yield of higher alcohols (Fang et al., 
2009).  
Hilmen et al. (1998) used a number of K-CuyMg5CeOx catalysts for CO hydrogenation 
reactions to form higher alcohols. K promoted Cu0.5Mg5CeOx catalysts are active for isobutanol 
synthesis and give products with a high alcohol-to-hydrocarbon ratio at a relatively low 
temperature (583K) and pressures (4.5MPa). The addition of small amounts of Pd to K-
Cu0.5Mg5CeOx demonstrated that Pd promoted isobutanol synthesis selectivity and rates only at 
high CO2 concentrations, suggesting that Pd resisted oxidation, retained its hydrogenation 
activity and weakened the inhibitory effects of CO2. 
1.2.3.2. Addition of Lower Alcohols 
Since the C1 to C2 step that leads the conversion from methanol to ethanol is the bottleneck to 
forming isobutanol in high space time yields, injection of ethanol or n-propanol into the 
synthesis gas stream entering a fixed-bed continuous flow reactor has been studied in order to 
react directly with the C1 intermediate, leading to the formation of the higher alcohols (Herman 
2000). In principle, the addition of lower alcohols should increase the probability of the 
intermediates which further go through carbon chain growth thus eventually enhancing the 
 35
formation of higher alcohols (isobutanol) kinetically. The net formation of methanol may be 
reduced to zero by recycling a sufficient amount of methanol to satisfy the equilibrium 
relationship for its synthesis (Mawson, McCutchen, Lim, & Roberts, 1993). It has been shown 
through isotope labeling that at 280 ?C with the injection of ethanol nearly all of the 1-butanol, 
2-butanol, and 2-methyl-1-propanol observed in the product were formed. Additionally, it 
increased the yields of 1- propanol by a factor of 18.6 and of methyl acetate by a factor of 47 
(Nunan et al., 1988). These results clearly support the conclusion that the pathway for the 
formation of C3+ oxygenates includes ethanol. Similar research was conducted by Hilmen et 
al.(1998). Adding ethanol or 1-propanol was shown to significantly enhance the selectivity 
towards higher alcohols. Similarly, Majocchi et al.(1998) also found a general increase in the 
productivity of the various reaction products after adding ethanol into the feed gas mixture, 
indicating that ethanol plays a key-role in the chain growth process to higher oxygenates. An 
increase in the amount of ethane has also been observed during ethanol addition, indicating that 
ethanol may go through dehydration step leading to ethylene followed by a rapid hydrogenation 
of the olefinic intermediate. 
Additionally, Nunan et al.(1988) used 13C-NMR characterization method to investigate the 
reaction mechanism on a Cs doped Cu-based catalyst by injecting methanol and ethanol with 
abundance of 13C in specific positions respectively. They found that lower alcohols were 
incorporated into the synthesis of higher alcohols and proposed the ?aldol coupling with oxygen 
retention reversal? reaction pathway. Details of this pathway are discussed in the section 
1.2.2.3.2. 
The addition of alcohols to the MAS process is a promising pathway, although industrially it 
would require an external source or extensive recycling. 
 36
1.2.3.3. Dual-bed reactor 
In addition to catalyst development, reactor design also have potentials for increasing the HAS 
yield and enhanced selectivity towards higher alcohols. Methanol synthesis is an extremely 
exothermic reaction and the productivity of methanol is severely limited by increasing the 
reaction temperature. Herman (2000) argued that methanol has an unfavorable equilibrium 
constant for high temperature operation, and increasingly decomposes back to synthesis gas as 
temperature is increased. However, the productivity of C2+OH increases with increasing the 
reaction temperature. Based on these thermodynamic features, the concept of a dual-bed reactor 
has been proposed and has been investigated for different catalyst combinations and reaction 
conditions (Beretta et al. 1995). The most common scenario to date is to maximize the formation 
of methanol in the first bed by using Cu-based catalyst at lower temperature, followed by a 
second catalyst bed equipped with high temperature catalyst promoting the subsequent carbon 
chain growth step. 
Sequential beds of Cs-Cu/ZnO/Cr2O3 catalysts with the top bed at 325 ?C and lower bed at 
340 ?C were tested in a single down flow reactor under higher alcohol synthesis conditions 
(Burcham et al., 1998). The upper, low-temperature, bed provided oxygenates (i.e. methanol, 
ethanol, 1-propanol) to the second, higher temperature, bed where they were converted by a 
chain growth mechanisms to higher alcohols. This configuration produced a greater yield 
isobutanol (202 g/kg cat/h) than either catalyst in a single bed configuration. Beretta et al. (1995) 
also reported a doubled productivity of isobutanol by using the dual bed reactor configuration as 
compared with using the same Cs/Cu/ZnO/Cr2O3 catalyst in a single configuration. A kinetic 
model for methanol synthesis from H2/CO mixtures over Cu/Zn oxide-based catalysts was 
developed and was shown to be a useful predictive tool for optimizing the relative amounts of 
the catalyst in each bed to maximize C2+OH formation (Smith et al. 1990; 1991). They also argue 
 37
that kinetic models can be used to optimize the reaction conditions, such as the size of each bed, 
pressure and space velocity of syngas. 
1.2.3.4. Slurry Phase Reactor 
Fang et al. (2009) believed that the highly exothermic nature of mixed alcohol synthesis result 
in the generation of hotspots, facilitating the carbon deposition and the sintering of active 
materials. Therefore, rapid removal of the reaction heat is a major consideration for the design of 
HAS reactors. It has been shown that carrying out the synthesis of higher alcohols in a slurry 
phase reactor rather than a fixed bed configuration has several advantages: 
1. A high heat transfer rate can be achieved in a slurry phase reactor (Herman 2000). Syngas 
conversion and selectivity towards higher alcohols could be enhanced theoretically by 
using slurry phase reactor. 
2. Slurry phase reactor has good temperature control and greater thermal uniformity. This 
aspect effectively prevents hot spots that induce sintering and deactivation of catalysts 
(Herman, 2000). 
3. It is possible to add more catalysts into the slurry phase reactor on-line and withdraw 
portions of used catalyst, which can increase plant on-stream days and decrease downtime 
(Herman, 2000). 
4. Slurry phase reactor has a smaller pressure drop, compared to fixed bed reactor. 
5. Fine powder catalysts used in slurry phase reactor can enhance the diffusion and the 
reaction rate. 
The synthesis of higher alcohols using slurry phase reactor has been investigated at the pilot 
scale (Air Products and Chemicals, LaPorte, TX). A 0.45 m diameter and 12.2 height stainless-
steel reactor was designed to operate at up to 12.5 MPa and 350 ?C. The charge to the reactor 
 38
consisted of 502 kg of Cs-promoted Cu/ZnO/Al2O3 catalyst in a 40 wt% oil slurry was utilized. 
The productivity to isobutanol was found in the range of 17-26 g/kg cat/h. Moreover, the slurry 
phase reactor can also be utilized in combination of the injection of lower alcohols in order to 
further enhance the higher alcohol selectivity. 
1.2.3.5. Supercritical Phase Reaction 
A supercritical fluid (SCF) is any substance whose temperature and pressure is higher than its 
critical point. At the critical point, the two fluid phases (liquid and vapor) become 
indistinguishable. Above the critical point no phase change is possible. SCFs are widely accepted 
as a unique and sometimes superior medium for chemical reactions, facilitating single phase 
operation while possessing a high diffusivity and low viscosity. Utilizing SCFs as reaction media 
was considered shortly after their discovery. Interest in utilization of SCFs flourished in the 
1980s, spreading in industries like food, chemicals, oils, pharmaceuticals, etc (Johnston, 1994).  
The unique properties of SCFs offer enhancement in heat transfer and mass transfer for 
reactions, especially for heterogeneous reactions. By facilitating single phase operation and 
possessing gas-like viscosity and liquid-like diffusivity SCF can enhance the in situ extraction of 
non-volatility/low volatility products from catalysts pores, diminish the limitation of transport in 
reactions, and integrate desorption and separation processes (Baiker, 1999; Subramaniam, 2001; 
Elbashir et al., 2010). The advantage of being able to easily change the chemical reaction media 
properties (e.g. density) near the critical point by slightly tuning the operation temperature and/or 
pressure makes SCFs favorable. 
Supercritical fluids have been used in many heterogeneous catalysis reactions, including 
alkylation, amination, cracking, disproportionation, esterification and Fischer-Tropsch synthesis. 
Baiker (1999) summarized the benefits brought by proper use of supercritical fluids in 
 39
heterogeneous catalysis, including a) enhancing the reaction rate, b) enhancing mass and heat 
transfer, c) increasing catalyst lifetime and regeneration, d) tunability of solvent properties by 
pressure and co-solvent, e) facilitating easier separation, f) offering unique opportunities in 
catalyst preparation. 
In hydrocarbon synthesis with heterogeneous catalysis, Subramaniam (2001) demonstrated 
the ability of SCF as reaction medium to extract and dissolve the low-volatile hydrocarbons 
(heavy hydrocarbons) from catalysts pore surface. This property of SCFs gives rise to higher 
pore active site accessibility, better coking resistance, and increased primary product selectivity. 
Additionally, results in the hydrocarbon synthesis study with SCF media demonstrated that the 
selectivity to methane was significantly suppressed (Fan and Fujimoto, 1999; Huang et al., 2004). 
It is recognized that hot spots in the catalyst bed can result in enhancement of methane selectivity, 
and the dense supercritical media can offer better axial thermal uniformity than FTS in gas phase, 
which can help reduce hot spots (Van Der Laan and Beenackers, 1999; Huang,  2003; Fan and 
Fujimoto, 1999; Huang et al., 2004).  
Considerable and intensive effort has been expended in exploring the application of 
supercritical fluids in Fischer-Tropsch synthesis (Yokota and Fujimoto, 1989; 1991(a);  1991(b); 
Jacobs et al., 2003; Huang, 2003; Huang et al., 2004; Elbashir et al., 2005). Yokota and Fujimoto 
(1989) have reported that the F-T synthesis reaction in a supercritical fluid has unique 
characteristics for either the reaction rate or the product distribution. Heat transfer in the 
supercritical phase was also found more effective than in the gas phase and close to that in the 
liquid phase. The enhanced in situ extraction of primary products when using SCF as reaction 
media has also been shown by a number of studies (Huang, 2003; Bukur et al., 1997; Lang et al., 
1995; Yokota and Fujimoto, 1991; Fan et al., 1999). Figure 1.6 shows the olefin and paraffin 
 40
yields for each carbon number varies with different carbon number in SC-FTS, liquid phase FTS 
(LP-FTS) and GP-FTS (Fan et al., 1999). The decrease in the olefin content with increasing 
carbon number in SCF-FTS is attributed to the well balanced desorption and the diffusion of 
products, resulting in suppression of the hydrogenation of primary olefins. However, the chain 
growth probabilities were similar for gas phase, supercritical hexane phase and liquid phase?s 
operation, which suggest that the supercritical n-hexane had little effect on the reaction itself 
(Yokota and Fujimoto 1991). 
Similar to Fischer-Tropsch synthesis, both higher alcohol formation and methanation are all 
exothermic. Fang et al. (2009) believed that the use of supercritical synthesis fluid medium could 
solve most the problems for both methanol and higher alcohol synthesis, such as alleviating the 
?hotspot? phenomena. By using a suitable supercritical solvent, the equilibrium for methanol 
synthesis could shift beyond the thermodynamic limitation under traditional reaction conditions, 
and the selectivity for higher alcohol synthesis could also be improved.  
Higher selectivity toward C2+OH can be expected when applying a properly selected SCF 
medium into the process. Liu, Qin, and Wang (2001) calculated the effects of several solvents on 
the equilibrium alcohol yield in alcohol synthesis using the SRK equation of state. These 
calculated results showed that n-hexane and n-heptane enhanced the conversion to methanol at a 
temperature of 200 ?C to 210 ?C (lower than their critical points) and the pressure above 8 MPa. 
C10 to C16 alkanes as supercritical media promoted higher alcohols formation at higher 
temperatures. A experimental exploration was made by Jiang, Niu, and Zhong (2001); they 
utilized a mixture of C10 to C13 normal alkanes as their supercritical media and investigated the 
synthesis of higher alcohols over a Zn-Cr-K catalyst at 400 ?C, syngas pressure 7.5 MPa, and 
GHSV 1700 h-1. They claimed that the introduction of SCF increased the selectivity of ethanol 
 41
and n-propanol from 1-3% to 6-12%, and that methanol selectivity was decreased by 30%. 
However, in this study the partial pressure of reaction media was much lower than the critical 
point, and the molar ratio of the medium to the reaction is very low making the phase behavior of 
the reaction mixture very different from the reaction media. 
 
 
Figure 1.6 Olefin and paraffin yields versus carbon number for FTS reactions in various phases over 
Ru/Al2O3 catalyst (Fan and Fujimoto, 1999).    
 
1.3 Summary 
Generally, as one of the alternative routes to the petroleum pathway, the mixed alcohol 
synthesis offers a compelling alternative to petroleum based fuels and chemicals. However, the 
process suffers from the low selectivity to higher alcohol. The current focus of the higher alcohol 
synthesis is on the enhancement of higher alcohols? selectivity and productivity. Typically, 
 42
higher alcohol/methanol weight ratios in the range 30:70 ? 50:50 represent the industrial target 
(Forzatti et al. 1991). 
Our group has extensive experience in SC-FTS. Previous investigations into SCF-FTS have 
suggested the following advantages: (1) in situ extraction of heavy hydrocarbons from the 
catalyst due to high solubility in the supercritical phase (Elbashir et al., 2010); (2) elimination of 
interphase transport limitations, thus promoting reaction pathways toward the desired products 
(Elbashir et al., 2005); (3) enhanced desorption of primary products prior to their undergoing 
secondary reactions promotes a high ?-olefin selectivity (Huang and Roberts, 2003); (4) superior 
heat transfer compared to gas-phase reaction, resulting in more long chain products (Huang and 
Roberts, 2003). Studies conducted by other group (Ginosar & Subramaniam, 1994; Yokota & 
Fujimoto, 1989, 1991; Yokota et al., 1991) have consistently shown that supercritical reaction 
media enhance catalyst lifetime, promote heat and mass transfer and improve the selectivity to 
desired products. 
The purpose of this study is to investigate the effect of supercritical media phase on the 
synthesis of methanol and higher alcohols, and to develop novel reaction operating strategies for 
the catalytic conversion of syngas to higher alcohols. The project objectives are to investigate the 
effect of temperature, operating pressure, reaction media, and syngas ratio on the CO conversion, 
product distribution, selectivity to CH4 and higher alcohols as well as productivity to mixed 
alcohols in a fixed bed reactor. 
 
  
 43
Chapter 2 Supercritical Fluids and its Application in Heterogeneous Catalytic 
Reactions 
In 1869, Thomas Andrews first recognized the presence of the critical point of CO2, which 
gave birth to a new world of critical phenomena and supercritical fluid science (Huang, 2003). 
Later in 1879, Hannay and Hogarth found that supercritical ethanol was able to dissolve 
inorganic salts which are insoluble in liquid ethanol (Huang, 2003). More interestingly, they 
found that they were able to manipulate the solubility of the solutes by changing the pressure. By 
the mid 1980s, research on new applications of SCFs shifted towards more complex and valuable 
substances that undergo a much broader range of physical and chemical transformations 
(Elbashir, 2004). Since then, the tunable solvent characteristics of supercritical fluids has been 
widely recognized and utilized in various areas, including extraction, dry cleaning, gas 
chromatography and chemical reactions, etc. 
2.1 Background on the application of supercritical fluids in heterogeneous reactions 
Figure 2.1 presents a pressure versus temperature phase diagram of a pure substance, which 
illustrates the existence of a supercritical fluid region.  The areas where the substance exists as a 
single solid, liquid or gas phase are labeled, as is the triple point where the three phases coexist. 
At the critical point, the densities of the two phases become identical and the distinction between 
the gas and the liquid disappears. The critical point has pressure and temperature co-ordinates on 
the phase diagram, which are referred to as the critical temperature, Tcr, and the critical pressure, 
Pcr. The region, in which temperature and pressure are both above the critical point, is called the 
supercritical region and the fluid located in this supercritical region is called a supercritical fluid 
(Huang, 2003). 
 
    
Figure 2.1 Pressure-temperature schematic diagram for a pure fluid showing the supercritical region 
path  
 
Table 2.1 Comparison of density and transport properties of CO
phases (Taylor, 1996)
Property 
Density (kg m-3*10-3) 
Viscosity (kg m-1s-1*105) 
Diffusion coefficient (m2s-1)
 
Many physical properties of a supercritical fluid are intermediate between those of a liquid and 
a gas (Schmitt and Reid 1988; Sengers 1991; Huang 2003)
dependent properties of fluids, which influence mass and heat transfer (diffusivity, viscosity, 
thermal conductivity, heat capacity). Table 1.1 provid
44
 
2 in gas, liquid and supercritical 
 
Gas Liquid 
10-3 1.0 
10-1 10 
 10-5 10-9 
. This is particularly true for density 
es a rough comparison of the magnitude of 
Supercritical 
0.7 
5 
10-7 
 45
some of these properties for liquids, gases, and supercritical fluids in the near critical region. The 
unique property of an SCF is its pressure-dependent density, which can be continuously adjusted 
from that of a vapor to that of a liquid (Baiker, 1999; Huang, 2003). In the Vicinity of the critical 
point (1.05-1.2 Tc and 0.9-2.0 Pc), fluids are highly compressible, and their density and transport 
properties can be continuously varied from gas-like to liquid-like values via relatively small 
variations in pressure (Subramaniam, 2001). These unique properties of supercritical fluids can 
cause that compounds which are insoluble in a fluid at ambient conditions become soluble in the 
fluid at supercritical conditions (McHugh & Paulaitis, 1980), or conversely, compounds which 
are soluble at ambient conditions can become less soluble at supercritical conditions (Armellini 
& Tester, 1993).  
The utilization of supercritical fluids (SCF) have been thoroughly reviewed, including the use 
of SCF extraction technology in chemical separation processes (Subramaniam and McHugh 
1986; Penninger et al. 1985), and the use of a SCF solvent as a reaction medium (Baiker, 1999; 
Elbashir et al., 2010; Subramaniam & McHugh, 1986; Subramaniam, 2001). Supercritical fluids 
have been increasingly used as solvents for separations and reaction in the chemical, food, 
pharmaceutical and biochemical industries. They offer many advantages over traditional solvents: 
(1) the solvent power of SCFs for dissolving nonvolatile substances approach that of 
conventional solvents due to liquid-like densities; (2) the low viscosity and high diffusivity of 
SCFs result in superior mass transfer characteristics compared to liquids; (3) the surface tension 
of the SCFs is low enabling easy penetration into the pores of a solid matrix (catalyst) for 
extraction of nonvolatile materials from the pores; (4) gases are completely miscible with SCFs 
resulting in high concentrations compared to that in liquid solvents; (5) the high compressibility 
of SCFs near the critical point induces large changes in density with very small changes in 
 46
pressure and temperature enabling separation of dissolved material from the SCF easily and 
completely (Sengers 1991; Huang 2003).  
2.2 Application of SCFs in heterogeneous catalytic reactions 
As briefly mentioned in Chapter 1, SCFs possess great potential as reaction media in 
heterogeneous catalytic reactions. Several opportunities present themselves in SCFs including: 
the ability to dissolve gases and liquids in one phase; the elimination of external film resistances; 
the minimization of internal pore-diffusion limitations; enhanced heat transfer from the catalyst 
surface and in-situ extraction of coke precursors. As a result, proper use of supercritical fluids in 
heterogeneous catalysis can enhance the catalyst activity, control selectivity and increase catalyst 
lifetime (Baiker, 1999). 
2.2.1 Activity Enhancement 
A significant part of the effect of pressure on the reaction rate in the supercritical region is 
attributed to the thermodynamic pressure effect on the reaction rate. Other reasons such as 
enhanced mass transfer or higher reactant solubility, can also contribute to the increase in the 
reaction rate (Baiker, 1999). Another promoting factors of SCFs may be attributed to the fact that 
reactions can be performed homogeneously in the mixture critical regime and, thus, better 
promote intimate contact between the reactants and the catalyst (Subramaniam & McHugh, 
1986).  
An example coming from Fischer-Tropsch synthesis showed that SCFs help to balance the 
rate of coke formation and the rate of heavy product deposition, thereby stabilizing the catalyst 
activity (Huang, 2003). In conventional reaction media, the coke and heavy products 
accumulation results in progressively increasing diffusion limitations by pore-mouth plugging. 
Because the coke precursor compounds and heavy products have low volatilities in gas phase 
 47
reaction environment, very little of these oligomers is desorbed from the catalyst, thereby 
causing the oligomers to accumulate in the catalyst pores (Huang and Roberts 2003). The 
adsorbed oligomers can undergo further transformation to consolidated coke, leading to catalyst 
deactivation. When the reaction occurs in a liquid phase, the oligomers may be dissolved by the 
reaction medium. However, pore diffusion limitations hinder the transport of the extracted 
oligomers out of the catalyst pores. Hence, the optimal reaction medium would be one that has 
liquid-like densities to solubilize (i.e.desorb) the coke precursors and gas like transport properties 
to effectively transport these coke species out of the catalyst pores. Supercritical fluids media 
may be pressure-tuned to possess such an optimum combination of fluid properties. The effective 
in situ removal of the coke precursors by SCFs media would mitigate coke buildup and pore 
diffusion limitations, therefore improving the catalyst activity (Huang, 2003). 
2.2.2 Control of Selectivity 
For a network of parallel or competing reactions, the thermodynamic pressure effect on each 
of the individual rate constants may be different due to different activation volumes (Baiker, 
1999). Thus, increased pressure may favor one of the reactions over the others. This offers some 
potential to enhance the selectivity to the desired product by operating at the appropriate pressure 
and temperature, where the desired reaction is favored compared to the unwanted side reactions 
(Baiker, 1999; Huang, 2003). Under supercritical conditions the reaction mixture can be a single 
phase, which allows an independent control of the reaction parameters, such as temperature, 
pressure, and concentrations. Futher possibilities for selectivity control are linked with tunable 
solubility, tuning of solute-solvent interactions (change of local density, clustering) and the use 
of cosolvents which through specific interactions with a transition state or a product can alter 
rates and product distributions (Baiker, 1999; Dillow et al., 1997). 
 48
2.2.3 Enhanced Mass and Heat Transfer 
In heterogeneous catalysis, transport properties such as diffusivity, viscosity, and thermal 
conductivity are crucial for a reaction system. A well-known advantage of supercritical fluids 
compared to ordinary liquids is that overall diffusion coefficients in supercritical fluids are 
higher than that in liquids by the elimination of multiple phases, which may exist in 
multicomponent systems under ordinary conditions (Huang, 2003). This results in significantly 
enhanced mass transfer under supercritical conditions. The viscosity of a supercritical fluid is 
comparable to that of the gas at the same temperature, but the density is at least 2 orders of 
magnitude higher. Because of the greater diffusivities and lower viscosities than liquid, mass 
transfer controlled liquid reaction may be accelerated by working under supercritical conditions. 
Supercritical fluids possess significantly higher thermal conductivity than corresponding gases 
and consequently heat transfer is enhanced in these media. This effect can be utilized for better 
heat removal in highly exothermic gas-phase reactions, where careful temperature control is 
essential for selectivity and product stability (Baiker, 1999).  
2.2.4 Catalyst Lifetime and Regeneration 
As mentioned in section 2.2.2, supercritical fluids exhibit considerably higher solubility than 
corresponding gases for heavy products which may act as catalyst blocking agents and thereby 
deactivate catalyst. This deactivation may be suppressed by changing reaction conditions from 
gas phase to dense supercritical media (Ginosar & Subramaniam, 1994). Furthermore, enhanced 
diffusivity can accelerate the transfer of poisons from the internal and external catalyst surface 
(Baiker, 1999). Regeneration of catalyst deactivated by coking can be accomplished by 
extracting the carbonaceous deposits from the catalyst surface.  
 49
As emerges from the discussion above, the potential of supercritical fluids can be realized in 
various ways for heterogeneous catalytic reactions.  Many heterogeneous catalytic reactions have 
been investigated under supercritical fluid medium conditions, including alkylation, amination, 
cracking, hydrogenation and Fischer-Tropsch synthesis etc (Baiker, 1999).  
  
 50
Chapter 3 Supercritical Hexanes as a Reaction Medium for Mixed Alcohol 
Synthesis in a Continuous Fixed Bed Reactor 
Abstract 
Mixed alcohol synthesis (MAS) from syngas involves an overall reduction in the 
number of moles (i.e. volume reduction) and is highly exothermic. As such, the high-
pressure, dense-phase solvent conditions of a supercritical reaction medium affords 
opportunities to enhance the reactions via Le Chatelier?s principle and to improve 
heat transfer relative to gas phase operation due to the improved heat transfer 
capacity of the supercritical fluid (SCF). The effect of solvent on the reaction 
performance of mixed alcohol synthesis under supercritical conditions was studied in 
a fixed bed reactor. Hexanes was chosen as the solvent due to the fact that its critical 
temperature and pressure are close to the reaction conditions for mixed alcohol 
synthesis. The catalyst used was a 0.5 wt% K promoted Cu based mixed metal oxide 
catalyst. Investigations were performed in order to study the effect of the 
supercritical reaction medium on the production of C1 to C4 alcohols in the liquid 
product. The results illustrated a notable reduction in the CH4 selectivity due to the 
enhanced heat transfer in the supercritical hexanes medium while this solvent 
medium also facilitated the extraction of alcohols, especially methanol, from the 
catalyst pores. As such, the formation of mixed alcohols is promoted by the presence 
of the supercritical hexanes medium.  
 
Keywords: Mixed alcohol synthesis; supercritical fluid; supercritical reaction media; copper 
catalyst 
3.1 Introduction 
There remains considerable interest in the synthesis of methanol and higher alcohols (referred 
to mixed alcohols) from syngas, particularly give their potential in view of its applicability as an 
alternative fuel, fuel additives and especially as a raw material for various chemicals.  
The synthesis of methanol and higher alcohols from mixtures of hydrogen and carbon oxides 
using mixed metals and metal oxides has been investigated since 1923(Tijm et al., 2001). Cu 
based catalysts, such as Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3, have been widely used in the methanol 
synthesis industry(Fang et al., 2009; Gerber, White, & Stevens, 2007; Herman, 2000; 
Stelmachowski & Nowicki, 2003; Wender, 1996). When promoted with alkali, such as Li, K and 
 51
Cs, the Cu based catalysts demonstrate a certain ability to produce higher alcohols. Compared to 
pure methanol, mixtures of methanol and higher alcohols exhibit several advantages as fuel 
additives, including 1) low tendency towards phase separation in the presence of water 
(Bromberg & Cheng, 2010; Forzatti et al., 1991); 2) higher octane blending characteristics 
(Gautam et al., 2000; Gautam & Martin II, 2000); 3) good compatibility with the materials 
employed in the existing automobile fuel system (Bromberg & Cheng, 2010), etc. Given the 
Open Fuel Standard Act of 2011 which advocated the use of flexible fuel vehicles, mixed 
alcohols are receiving renewed attentions as a promising alternative fuel to gasoline, thereby 
bringing broader research interests to the synthesis and utilization of mixed alcohols. 
Similar to Fischer-Tropsch synthesis (FTS), which is another means of converting syngas to 
transportation fuels and other chemicals, mixed alcohol synthesis is highly exothermic and its 
performance is promoted by high pressure. Fujimoto?s group pioneered the study of FTS under 
supercritical fluid conditions and elucidated several characteristic features of this type of 
operation, including effective heat management, and mass transfer, and improved accessibility of 
the catalytic sites due to extraction of high boiling products (Fan & Fujimoto, 1999; Fan, Yokota, 
& Fujimoto, 1992; Yokota & Fujimoto, 1989, 1991; Yokota et al., 1991). These benefits have 
been attributed to the unique properties of the supercritical reaction media which are 
intermediate to those of a liquid or a gas. The density, while is less than that of a liquid, is 
remarkably higher than that of a gas. The diffusivity, while lower than that of a gas, is 
significantly higher than that of a liquid. These unique-characteristics of supercritical fluids 
provide several motivating reasons for considering SCF reaction media in fixed bed mixed 
alcohol synthesis. 
 52
Supercritical fluids have been previously employed in some early studies for methanol and 
mixed alcohols synthesis. Liu et al.(Liu et al., 2001) calculated the equilibrium conversions of 
methanol synthesis under different supercritical fluid reaction media conditions by using Soave-
Redlich-Kwong equation of state. Their results showed that the addition of proper solvents, such 
as n-hexane and n-heptane, could provide a solvent effect on the reaction beyond the simple 
dilution effect, therefore improving the CO conversion. Jiang et al.(Jiang et al., 2001) have tested 
a mixture of C10 ? C13 alkanes as a reaction medium for the mixed alcohol synthesis over a Zn-
Cr-K catalyst in a fixed bed reactor. The partial pressure of reaction medium was kept at 1.78 
MPa and a syngas partial pressure at 7.5 MPa. Although the partial pressure of reaction solvent is 
much lower than the critical points of reaction media, the effect of having added this solvent was 
observable despite the fact that the operation conditions are well below the critical point of the 
overall reaction medium. An enhancement in CO conversion was related to the pressure 
dependence of the reaction rate coefficient. The introduction of the reaction medium was 
believed to promote the carbon chain growth and change the product distribution. 
Reubroycharoen et al.(Reubroycharoen, Yoneyama, Vitidsant, & Tsubaki, 2003) utilized 2-
butanol as a reaction medium to improve the methanol synthesis. They found that the use of 2-
butanol as reaction medium improved the CO conversion and the product distribution. These 
improvements were attributed to not only the better heat and mass transfer but also the catalytic 
solvent effect that 2-butanol played in the formation of methanol as an intermediate. Zhang et 
al.(Zhang, Li, & Zhu, 2006) have investigated supercritical phase methanol synthesis in a three-
phase slurry reactor using n-hexane as a reaction medium. They believed that the use of n-hexane 
as a supercritical medium enhanced the rapid removal of products out of the reaction system, 
thereby overcoming the inherent reaction equilibrium and enhancing the conversion of reactants.  
 53
This paper presents the results of a series of investigations in which supercritical hexanes was 
employed as a reaction medium for MAS in a continuous, high pressure fixed-bed reactor using a 
K promoted Cu-based catalyst (0.5 wt% K doped Cu/ZnO/Al2O3). The reaction pressures (4.5-20 
Mpa) employed in this study provided the ability to ensure that the partial pressure of solvent 
was sufficiently high to provide liquid like densities in the supercritical reaction medium. The 
effect of supercritical hexanes on the CO conversion, CO2 and CH4 selectivity, and especially 
mixed alcohols productivity and distribution are discussed in detail. In addition, the effect of 
temperature (200 ?C to 300 ?C) on this MAS system was examined under both supercritical 
phase and gas phase conditions.  
3.2 Experimental Section 
3.2.1 Catalyst preparation 
The catalyst used in this study is a potassium promoted copper based zinc oxide and alumina 
catalyst (0.5 wt% K promoted Cu/ZnO/Al2O3). A Cu/Zn/Al mixed nitrate salt solution was first 
prepared at a mass ratio of 58:24:18. The single phase precursor was prepared by the 
simultaneous addition of the nitrate salt solution and a 1.0 M K2CO3 solution dropwise to a 1L 
flask, which was initially filled with 200 ml of deionized water at 80 ?C. Continuous stirring and 
a pH of 7 were maintained during the addition of the mixed metal salt solution. After subsequent 
aging for an hour, the precipitate was decanted, and then washed three times with deionized 
water at 80 ?C. The filter mass was dried at ambient pressure at 80 ?C for 24 hours. The dried 
precipitate was then ground, followed by stepwise calcinations to 350 ?C to give the 
corresponding mixed oxides. Before doping with potassium, the total pore volume was 
determined by adding deionized water into the mixed oxides until it formed a slurry paste, 
followed by drying. Equal amounts of 0.025 M K2CO3 solution was added into the dried mixed 
 54
oxides in order to disperse potassium uniformly within of the pore structure. The paste was then 
dried at ambient pressure at 80 ?C overnight. This process may be repeated several times in order 
to reach the required loading of K2O. Finally, the dried powder was recalcined in air by heating 
at a rate of 5 ?C/min up to 350 ?C and then held for 4 hours.  
3.2.2 Catalyst characterization 
BET surface area of  catalysts were determined by N2 adsorption at -196 ?C using a 
Quantachrome NOVA 2200e surface area analyzer, and powder X-ray diffraction (XRD) 
patterns of the catalysts were recorded on a Bruker D8 advanced diffractometer using Cu K? 
radiation. 
A summary of the BET measurement of the catalyst is shown in Table 3.1. The BET surface 
area of the used catalyst is lower than that of the unreduced catalyst (19.7 m2/g versus 26.0 m2/g). 
However, the pore volumes of the catalysts are nearly identical before reduction and after 
reaction. A decrease in pore radius has been observed after the reaction (40.0 ? versus 30.2 ?).  
The diffraction peaks in XRD patterns demonstrate the primary phase of the copper in the 
unused catalyst is CuO while the primary phase after reaction is Cu, as shown in Figure 3.1. 
Table 3.1 Physical-chemical properties of catalysts 
 0.5 wt% K promoted Cu/ZnO/Al2O3 catalyst Before reduction After reaction 
BET area (m2/g) 26.0 19.7 
Pore volume (cm3/g) 0.1 0.1 
Pore radius (?) 40.0 30.2 
Phase of copper detected by XRD CuO Cu 
 55
 
Figure 3.1 XRD patterns of the catalysts before reduction and after reaction 
3.2.3 Reaction procedure and product analysis 
The schematic diagram of the high pressure MAS reaction system is shown in Figure 3.2. 
This reactor design allows H2, syngas and helium to be delivered individually into the reaction 
zone at desired flow rates controlled by mass flow controllers (Brooks 5850E). A liquid solvent 
is delivered via an HPLC pump (Acuflow series III, Fisher) and is introduced into a pre-heater 
where it is heated under pressure in order to be converted to the desired supercritical state prior 
to the reactor. Before being mixed with the supercritical solvent, the syngas is passed through a 5 
inch long, ? inch diameter tubing that is packed with glass wool and heated to 250 ?C in order to 
remove iron carbonyls contained in the premixed syngas.  The solvent and syngas are combined 
just prior to entering the reactor, where the high temperature results in a supercritical phase 
mixture. The reactor is situated in a furnace with a programmed temperature controller system. 
The reaction pressure is controlled by a back pressure regulator (Straval-BPH) located between 
the hot trap and the cold trap. Two pressure indicators are located at the inlet of reactor and the 
outlet of the hot trap, respectively, to monitor the reaction pressure. After leaving the reactor, the 
Int
en
sit
y(C
ou
nts
)
45-0937> Tenorite - CuO
78-1605> Spinel (Ni - CuAl2O4
04-0836> Copper - Cu
10 20 30 40 50 60 70 80
2-Theta(??
 56
effluent passes through a hot trap that is maintained at 200 ?C to condense heavy components 
(heavy hydrocarbons, if any), which might cause blockage of the delivery lines. The effluent is 
then delivered into heat exchanger that is cooled externally with cold water before entering the 
cold trap (maintained at 5 ?C) where the effluent stream is then separated into two phases. The 
liquid phase products are collected and manually injected into a Varian 3380 gas chromatograph 
(GC) with a capillary column (DB-wax) equipped with a FID detector. The gas phase exiting the 
cold trap is analyzed by an online Varian GC with a packed column (Hayesep-DB 100/120) 
equipped with a TCD detector.  
 
Figure 3.2 Schematic diagram of the high-pressure MAS reactor and analysis system 
In a typical gas phase experiment, one gram of the catalyst mixed with 1.5 ml glass beads (45?
70 mesh) was loaded into the reactor, and held in place in the middle of the reactor by glass wool 
packed on both sides of the catalyst bed. Before initiating the reaction, the catalyst was 
pretreated using a flow of 5% H2 in 95% N2 at a flow rate of 50 sccm (standard cubic centimeter 
per min) at 300 ?C and ambient pressure for 12 hours. Helium was then delivered into the system 
until the desired reaction pressure was reached. After the temperature and pressure were 
stabilized, a predetermined syngas flow rate was initiated and controlled by a mass flow 
 57
controller. The reactions were allowed to run continuously until a steady state was achieved with 
respect to both the conversion and selectivity. The gas phase products were analyzed 
approximately every two hours and the liquid phase product was collected every 12 hours and 
injected into the GC-FID for analysis.  
Studies involving supercritical hexanes were performed with identical loadings of the catalyst 
as described in the paragraph above. The pretreatment of the catalyst was also performed as 
described above. Hexanes was preheated (250 ?C), delivered at flow rates controlled by a HPLC 
pump, and then mixed with the preheated syngas prior to entering the reactor. Under supercritical 
hexanes phase conditions, reaction temperatures and pressure ranged from 200 ?C to 300 ?C and 
4.5 MPa to 20 MPa, respectively. The supercritical solvent used in this study is a mixture of 
hexane isomers (ACS grade), and as such is referred to as ?hexanes?. The syngas concentration 
used in this study is 64.0% H2, 32.0% CO, and 4.0% N2 (internal standard). 
CO or H2 conversion was defined as the molar ratio of the consumed carbon monoxide or 
hydrogen to that which was fed to the reactor. Taking CO as an example, the CO conversion is 
defined as: 
CO
CO
CO n
nX '1?=
 
where COn'  is the molar flow rate of the outlet CO and COn  is the initial molar flowrate of CO 
in the syngas inlet. In addition, the selectivity of any species in the outlet stream is defined as: 
%100' ???=
COCO
ii
i nn
NnS
 
 58
Where in is the molar flow rate of species i in the outlet stream, and iN is carbon number of  
that species (e.g. iN for methane is 1). Similarly, the selectivity towards each alcohol product can 
be calculated by using the equation above.  
3.3 Results and Discussion 
The results of the MAS studies over a Cu/Zn/Al catalyst under both the gas phase and 
supercritical hexanes phase conditions at different hexanes/syngas ratios are presented in Table 
3.2. The molar ratio of the supercritical solvent to the reactants (i.e. hexanes/syngas molar ratio) 
is considered to be an important factor affecting catalytic performance (Elbashir et al., 2005). 
Elbashir et al. (2005) demonstrated that higher hexanes/syngas ratios in their SCF-FT studies 
resulted in phase behavior and mixture physical properties more closely resembled that of the 
pure solvent thereby making it easier to maintain single phase operation. However, in order to 
maintain the same syngas partial pressure and therefore similar reactivity at elevated 
hexanes/syngas molar ratios, higher pressures and higher solvent throughput will be needed 
(Elbashir et al., 2010). Increasing the solvent/syngas molar ratio affects the overall concentration 
of CO and H2 due to a simple solvent dilution effect thereby decreasing the mole fraction of the 
reactant gases in the feed mixture. In addition, if the solvent/syngas molar ratio is too low, the 
addition of this modest amount of solvent cannot provide enough density to enhance the heat 
transfer properties, as desired under SCF conditions. Ratios in the range of 3.0 to 3.5 have been 
commonly employed in the studies of FTS under supercritical solvent conditions (Bochniak & 
Subramaniam, 1998; Elbashir et al., 2010; Jacobs et al., 2003; Linghu, Li, Asami, & Fujimoto, 
2006). In the current MAS study, the syngas flow rate was kept constant at 50 sccm/gcat. The 
flow rate of hexanes was then adjusted to provide the required hexanes/syngas molar ratio. In 
order to keep the syngas partial pressure consistent and to focus on the effect of the supercritical 
 59
hexanes on the MAS, the reaction pressure was controlled as a function of hexane/syngas molar 
ratio. 
Table 3.2 Conversion, selectivity and yields of supercritical MAS obtained with a H2/CO = 2.0 
synthesis gas at 300 ?C and at various hexanes/syngas molar ratios with GHSV = 3000 
L/kg cat/hr 
 Hexanes/syngas molar ratio 
 0 1 1.5 2 3 
Partial pressure of sygnas (MPa) 4.5 4.5 4.5 4.5 4.5 
Reaction pressure (MPa) 4.5 9.0 11.2 13.5 18 
CO Conversion (%) 27.7 31.0 49.1 52.1 60.2 
H2 Conversion (%) 17.8 18.5 35.7 40.3 48.7 
Selectivity (%)      
CO2 25.1 20.8 15.8 12.7 10.2 
CH4 6.4 4.4 3.5 2.8 2.4 
Alcohols 37.5 45.0 56.3 66.7 65.5 
      
Alcohol yields (g/kgcat/h)      
Methanol  139.6 163.1 321.9 434.1 506.5 
Ethanol 11.1 15.7 32.0 12.1 10.8 
1-propanol 0.1 1.3 2.7 6.0 4.7 
1-butanol 0.1 0.8 1.5 3.01 2.24 
? value (C1-C4 linear alcohol) 0.05 0.10 0.10 0.13 0.11 
 
3.3.1 Effects of hexanes/syngas molar ratio on CO and H2 conversions 
Figure 3.3a illustrates the influence of the hexanes/syngas molar ratio on the activity of the 
catalyst as measured by CO and H2 conversion at 300 ?C and syngas partial pressure of 4.5 MPa. 
When the hexanes/syngas ratio = 0 (Ptotal = Psyngas = 4.5 MPa), the data represents the catalytic 
behavior under gas phase conditions. A CO conversion of 27.7% and a H2 conversion of 17.8% 
have been observed, as shown in Fig. 3.3a. A modest increase in CO conversion was obtained 
when shifting the process from gas-phase operation (hexanes/syngas = 0) to the operating 
condition where the molar ratio of hexanes/syngas = 1 (e.g. the CO conversion was changed 
from 27.7% to 31.0%) while holding the syngas partial pressure constant. As the hexanes/syngas 
molar ratio was further increased to the value of 2, the conversion of CO and H2 increased 
 60
sharply to values of 52.1% and 40.3%, respectively. The CO and H2 conversions continued to 
increase when the molar ratio was further elevated to the value of 3, where the CO conversion 
was 60.2% and the H2 conversion was 48.7%. 
Huang & Roberts (2003) have shown that activity in SCF-FTS can be higher than in the gas-
phase FTS over a cobalt catalyst. They attributed an observed increase in activity to higher 
solubility of FTS products and in situ extractability of the SCF solvent medium which resulted in 
more active sites being available for reaction. Bukur, Lang, & Nowicki (2005) studied FTS on an 
iron catalyst where they attributed an increase in activity under supercritical fluid conditions to 
higher diffusivities relative to the conventional mode of operation (stirred tank slurry phase). In 
the case of SCF methanol synthesis, Liu et al. (2001) described that the addition of a supercritical 
solvent into a system at a certain temperature and pressure could influence the CO conversion in 
two ways, namely a dilution effect and a solvent effect. In the current study, the experiments 
were well designed such that the partial pressure of syngas was maintained at a consistent value 
of 4.5 MPa at each total pressure studied, in order to be able to separate the inherent reaction rate 
(which is proportional to the reactant gas partial pressure) from the effect of the SCF solvent on 
reactivity. Figure 3a demonstrates that the addition of supercritical hexanes at a molar ratio from 
1 to 3 has a significant effect on the CO and H2 conversion. The increase in reactivity may come 
either from a thermodynamic pressure effect (total pressure) on the activity of alcohol 
synthesis(X. Xu et al., 1987) or from a solvent effect on the reaction due to the elevated amount 
of hexane present(Wu, Klein, & Sandler, 1991).  
 61
 
 
Figure 3.3 Syngas conversion and selectivity to CO2 and CH4 as a function of hexane/syngas molar 
ratio. Reaction conditions were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, 
syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, pressure 4.5 MPa-20 
MPa 
 
0%
10%
20%
30%
40%
50%
60%
70%
0 1 2 3 4
Co
nv
er
sio
n  
(%
)
Hexanes/syngas ratio
H2 conversion
CO conversion
0%
5%
10%
15%
20%
25%
30%
0 1 2 3 4
Se
lec
tiv
ity
 (%
)
Hexanes/syngas ratio
CH4 selectivity
CO2 selectivity
 62
3.3.2  Effects of hexanes/syngas molar ratio on CO2 and CH4 selectivity 
Figure 3.3b shows that upon switching the reaction from gas phase operation to SCF phase 
operations, the selectivity to CO2 decreased from 25.1% to 10.2%. CH4 selectivity was also 
reduced from 6.4% under gas phase conditions to around 2% under supercritical phase 
conditions (Hexane/syngas = 3). Due to the fact that two variables are changing at the same time, 
i.e. the total pressure and the hexane flow rate, more evidence is needed to prove that this is the 
result of a supercritical solvent effect. Study of MAS under gas phase conditions has shown that 
selectivity to CO2 and CH4 is relatively stable with increasing pressure, as shown in Figure 3.5. 
Therefore, the decrease in CO2 and CH4 selectivity in this case can be attributed to the elevated 
presence of the supercritical hexanes. This observation is consistent with previous research 
findings on SCF-FTS(Fan et al., 1992; Huang & Roberts, 2003; Yokota & Fujimoto, 1991; 
Yokota et al., 1991). They further attributed the lower selectivity towards methane to the more 
effective removal of reaction heat from the catalyst surface than in the gas-phase reaction. Huang 
and Roberts (2003) also reported a significant decrease in methane selectivity in SCF-FTS 
compared to gas phase FTS. The large amount of heat generated in the FTS reaction was 
believed to be resulting in the cracking of long chain hydrocarbons into smaller ones if this heat 
is not efficiently removed from the catalyst pores. In the supercritical phase, however, the heat 
transfer rate was believed to be improved due to the higher heat capacity and thermal 
conductivity as compared to gas phase. As in the process of FTS the rapid removal of reaction 
heat is also considered as an important issue in the mixed alcohol synthesis. The high heat 
transfer capacity provided by supercritical hexanes has significantly reduced the selectivity 
towards to CH4 and CO2. Combining with the fact that remarkable increase in CO conversion has 
been found under supercritical hexanes phase conditions (hexanes/syngas molar ratio >1), it is 
 63
reasonable to speculate that more carbon has been converted into liquid product under 
supercritical hexanes phase conditions. 
In order to examine the influence of pressure on this reaction, an experiment was performed 
where the overall pressure was increased at constant syngas and hexanes flow rate as shown in 
Figure 3.4 (both syngas and hexanes partial pressure were increased). This figure demonstrates 
that an increase in total pressure and syngas partial pressure results in an increase in the CO and 
H2 conversion to values that are roughly doubled that at lower pressure operation. It should be 
noted that the CO2 selectivity decreased with this increase in total pressure and the CH4 
selectivity remained relatively low and fairly constant at a value of around 4%. Unfortunately, 
this experiment does not allow determination as to whether the increase in activity with the 
increased addition of solvent (observed as in Figure 3.3a) is from a pressure effect or from the 
presence of the supercritical medium.  
To further examine the effect of pressure on this reaction, a set of experiments were performed 
in the absence of supercritical hexanes under gas phase conditions at a series of elevated 
pressures. This was done in order to determine the inherent effect of elevated pressure on the 
reactivity and selectivity of this particular catalyst. Figure 3.5 presents the effect of pressure on 
syngas conversion and selectivity at pressures ranging from the initial syngas partial pressure of 
4.5 MPa to 20 MPa. It demonstrates that there is a significant pressure effect on the CO and H2 
conversion where the CO conversion increased rapidly from 27.7% all the way to 75.3% over 
this pressure range, consistent with pressure effects observed previously in the synthesis of 
mixed alcohol(Burcham et al., 1998; Mahdavi et al., 2005), this figure also illustrates that there is 
little to no effect on both the CO2 and CH4 selectivity over the pressure range of 4.5 to 12 MPa. 
When pressure further increased from 12 MPa to 20 Mpa under gas phase conditions, CH4 
 64
selectivity increased from 8.7% to 18.9%. Recall that Figure 3.3b demonstrated that the presence 
of supercritical solvent affected a decrease in both the CO2 and CH4 selectivity to a significant 
degree, when the hexanes/syngas molar ratio was increased from 0 (gas phase condition, syngas 
partial pressure at 4.5 MPa) to 3 (total pressure at 18.5 MPa, syngas partial pressure at 4.5 MPa). 
These results demonstrate that the presence of supercritical solvent allows the activity to be 
positively affected by pressure while also providing suppression of undesired side reactions to 
CO2 and CH4.  
 
 
Figure 3.4 Pressure effect on the conversion and selectivity. Reaction conditions were as follows: 
catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, 
temperature 300 ?C, hexanes/syngas molar ratio = 1 
0%
10%
20%
30%
40%
50%
60%
70%
80%
5 10 15 20
Pressure (MPa)
Co
nv
er
sio
n 
an
d s
ele
ct
ivi
ty 
(%
)
H2 conversion
CO conversion
CH4 selectivity
CO2 selectivity
 65
 
Figure 3.5 Pressure effect on syngas conversion and selectivity to CO2 and CH4 under gas phase 
conditions. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, 
pressure 4.5 MPa-20 MPa 
3.3.3  Effects of hexanes/syngas molar ratio on alcohols? productivity and selectivity 
Figure 3.6 presents the effect of hexanes/syngas molar ratio on the productivity towards 
methanol and ethanol. It shows that methanol formation is significantly enhanced by the 
introduction of the supercritical fluid solvent. Of the conditions studied, the methanol formation 
rate reaches a maximum at a molar ratio equal to 3. The increase in productivity can be attributed 
to both the presence of supercritical hexanes and the enhanced reaction pressure. As shown in 
Figure 3.6b, the ethanol productivity increased first from 1.3 g/kg cat/h to 32.0 g/kg cat/h with 
the increase in hexanes/syngas molar ratio from 0 to 1.5 and then decreased to 10.8 g/kg cat/h 
with a further increase in hexanes/syngas molar ratio to 3. It suggested that a maximum in the 
formation of ethanol would appear somewhere between the hexanes/syngas molar ratio of 1 and 
2. The productivities of higher alcohol species observed over this particular catalyst were two 
orders of magnitudes less than the value for ethanol under gas phase conditions and were 
0%
10%
20%
30%
40%
50%
60%
70%
80%
0 5 10 15 20 25C
on
ve
rs
ion
 an
d s
ele
cti
vit
y (
%)
Pressure (MPa)
H? conversion CO conversion CO?selectivity CH? Selectivity
 66
positively affected by the presence of the SC hexanes. The productivities of the C1 to C4 linear 
alcohols under this set of elevated hexanes/syngas molar ratios are presented in Table 3.2, as 
well as the carbon chain growth factors for each of the hexanes/syngas molar ratios studied. The 
enhancement in the productivity towards higher alcohols by the introduction of supercritical 
hexanes into the system is apparent. For instance, a productivity of 6.03 g/kg cat/h towards 1-
propanol has been obtained under hexanes/syngas molar ratio of 2 reaction conditions, compared 
to that of 0.1 g/kg cat/h under gas phase conditions. An interesting observation has been found in 
terms of the ? values. The presence of supercritical hexanes seems to have a positive effect on 
the ? value, i.e. it doubled from 0.05 under gas phase condition to a value of 0.1 under 
hexanes/syngas molar ratio of 1 reaction conditions. However, further elevation in the amount of 
supercritical hexanes introduced into the system appears to have little to no effect on the carbon 
chain growth factor, indicating that the mechanism of alcohol formation may not be dependent 
on the reaction media.  
Furthermore, in order to better understand the impact of the SCF on the formation of mixed 
alcohols, the carbon selectivity was plotted as a function of hexane/syngas molar ratio. Figure 
3.7 shows that the selectivity towards methanol has been remarkably enhanced by the presence 
of supercritical hexanes, though it is only slightly increased at a molar ratio equal to one. Based 
on the combined results of productivity and selectivity, it is expected that a hexanes/syngas 
molar in the range of 1.5 to 2 would allow for maximal impact of the SCF on the formation of 
ethanol and other higher alcohols. Figure 3.7 also shows that the presence of SC hexanes did 
improve the carbon selectivity towards each higher alcohol compared to the results under gas 
phase conditions (where the molar ratio equals zero). Of the conditions studied, a maximum 
value for each higher alcohol was found at a hexanes/syngas molar ratio of 2. The results of the 
 67
liquid product analysis indicates that there is an optimum amount of hexanes needed to be added 
in order to get a maximum productivity towards different higher alcohol products over this 0.5 
wt% K promoted Cu/ZnO/Al2O3 catalyst. 
 
 
 
Figure 3.6 Effect of hexane/syngas molar ratio on the productivity of methanol (a) and ethanol (b). 
Reaction conditions were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas 
flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, pressure 4.5 MPa-18 MPa 
0
100
200
300
400
500
600
0 1 1.5 2 3
Pr
od
uc
tiv
ity
 (g
/kg
ca
t/h
)
Hexane/syngas molar ratio
Methanol
0
10
20
30
40
0 1 1.5 2 3
Pr
od
uc
tiv
ity
 (g
/kg
ca
t/h
)
Hexane/syngas molar ratio
Ethanol
 68
 
Figure 3.7 Effect of hexane/syngas molar ratio on the carbon selectivity towards C1-C4 alcohols. 
Reaction conditions were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3; syngas 
flow rate = 50 sccm/gcat, H2/CO = 2.0, temperature 300 ?C, pressure 4.5 MPa-18 MPa 
 
 
Figure 3.8 Pressure effect on alcohol selectivity under gas phase conditions. Reaction conditions 
were as follows: catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 
sccm/gcat, H2/CO = 2.0, temperature 300 ?C, pressure 6.9 MPa-15.4 MPa 
0%
1%
2%
3%
4%
5%
6%
7%
0 1 2 3 4
Ca
rb
on
  s
ele
cti
vit
y (
%)
Hexanes/syngas molar ratio
Methanol/10
Ethanol
1-propanol
1-butanol
2-butanol
iso-butanol
0%
1%
2%
3%
4%
5%
6%
7%
5 10 15 20
Ca
rb
on
  s
ele
cti
vit
y (
%)
Pressure (MPa)
Methanol/10
Ethanol
1-propanol
1-butanol
2-butanol
iso-butanol
 69
Figure 3.8 presents the effect of pressure on the selectivity towards each of the C1 to C4 
alcohols under gas phase conditions. It can be observed that selectivity of methanol initially 
increased from 37.4% to 49.3%, when pressure was increased from 6.9 to 12 Mpa. A decrease in 
methanol selectivity was found when the pressure was further increased to 15.4 MPa. Similar 
trends have been observed for the selectivity towards ethanol and higher alcohols over the same 
pressure range but at much lower values. It should be noted that an increase in CH4 selectivity 
has been detected when the pressure was increased from 12 MPa to 15.4 MPa in gas phase, as 
shown in Figure 3.5. Recall that pressure has a positive effect on the CO and H2 conversion over 
the whole 4.5 MPa to 20 MPa range. The decrease in selectivity towards mixed alcohols, and the 
corresponding increase in CH4 selectivity might indicate that when the reaction pressure is 
higher than 12 MPa in gas phase operation the excessive pressure favors the formation of 
hydrocarbons over the formation of alcohols.  
When the effect of pressure on the selectivity towards mixed alcohols under gas phase 
conditions (Figure 3.8) is compared with this effect under supercritical hexanes phase conditions 
(Figure 3.7), it can be found that instead of exhibiting a maximum at 12 MPa under gas phase, 
the methanol selectivity continued to increase when the total pressure increased from 11.25 MPa 
(hexanes/syngas molar raotio of 1.5) to 18.5 MPa (hexanes/syngas molar raotio of 3).  The 
highest value of ethanol selectivity was achieved at ca. 6.7% when the MAS was operated under 
hexanes/syngas molar of 1.5, much higher than the value of 2.3% achieved under gas phase. The 
increase in the C3 and C4 alcohols selectivity under supercritical hexanes phase conditions can be 
mainly attributed to the beneficial effect of increased system pressure.  
Overall, the results presented above suggest that the presence of the supercritical hexanes 
facilitates heat and product removal thereby resulting in a significant increase in the production 
 70
of methanol and ethanol and a remarkable reduction in the methane formation, while also 
sustaining the beneficial effects of the elevated pressure on the CO and H2 conversion and on the 
selectivity towards C3 and C4 alcohols. A value of hexanes/syngas molar ratio in the range of 1 to 
2 would be suggested for the subsequent studies in order to optimize the effect of supercritical 
hexanes on the MAS process. 
3.3.4 Effect of temperature on conversion and selectivity under both gas and SCF 
phase conditions 
The kinetics of MAS (the rate of the various reactions that occur on the catalyst surface) is 
strongly influenced by the reaction temperature (Park, 1997; X. Xu et al., 1987). The reaction 
temperature also affects the phase behavior of the reaction media, as well as the thermo-physical 
properties of the mixture (reactants, solvent, and products) (Subramaniam & McHugh, 1986). 
The effect of reaction temperature at 200-300 ?C on supercritical hexanes phase mixed alcohol 
synthesis was investigated, while maintaining the syngas partial pressure at 6.2 MPa and a mass 
space velocity of 3000 L(STP) kg?1 h?1 under reaction conditions both with and without hexanes 
as reaction media. 
Figure 3.9 presents the effect of temperature on the syngas conversion (Figure 3.9 a&b) and 
selectivity towards CO2 (Fig. 3.9c) and CH4 (Fig. 3.9d). In increasing the temperature from 
200 ?C to 300 ?C under gas phase reaction conditions, the CO conversion increased rapidly from 
5% to about 35%. A more rapid increase in CO conversion was observed when the temperature 
was increased under supercritical hexane phase conditions. At temperature of 300 ?C under 
supercritical hexanes phase conditions, a CO conversion of 66% was achieved. The effect of 
temperature on the H2 conversion follows a similar trend as on the CO conversion. Under gas 
phase conditions, the H2 conversion initially increased with an increase in temperature from 
200 ?C to 240 ?C, and then increased even more rapidly as the temperature was raised beyond 
 71
260 ?C. Under supercritical phase reaction conditions, the H2 conversion increased with the 
increase in temperature from 200 ?C to 240 ?C, and the highest value of 55% (among all the 
temperatures studied) was obtained at 290 ?C.  
Fig. 3.9c demonstrated that selectivity to CO2 was favored with the increase in reaction 
temperature for both gas phase and supercritical phase conditions. However, the presence of the 
supercritical hexane fluid media maintained the CO2 selectivity at a relatively low value. For 
instance, at the reaction temperature of 300 ?C, the CO2 selectivity was 11.5% under 
supercritical hexanes phase conditions, much lower than the CO2 selectivity of 23.5% that was 
obtained under gas phase conditions. Fig. 3.9d shows that the selectivity towards CH4 was 
remarkably reduced when the temperature was increased from 200 ?C to 240 ?C under gas phase 
conditions. It should be noted that the CO and H2 conversions were less than 10% in the 
temperature range of 200 ?C to 290 ?C in the gas phase. It should also be noted that no liquid 
product was detected in the gas phase in this same temperature range. Therefore, the high 
methane selectivity in this instance is the result of the negative effect of temperature on the 
conversion of CO to alcohols and not an increase in methanation since methanation is 
thermodynamically favored at higher temperatures. At these low temperatures in the gas phase, 
the conversions are low, the productivities of alcohols are low, and the CO that does react is 
converted to methane in high percentage. Fig. 3.9d also shows that the selectivity to CH4 was 
maintained at a relatively low value (around 2% ) as temperature increased from 200 ?C to 
300 ?C in the presence of the solvent. These significantly lower values of CO2 and CH4 
selectivity are attributed to the improved thermal uniformity afforded by the SCF solvent, 
consisted with similar observations in supercritical phase Fischer-Tropsch synthesis(Abbaslou, 
 72
Mohammadzadeh, & Dalai, 2009; Durham, Zhang, & Roberts, 2010; Elbashir et al., 2005; 
Huang & Roberts, 2003).  
Figure 3.10 shows the carbon selectivity towards C1-C4 alcohols as a function of temperature 
under gas phase reaction conditions. It can be observed that the selectivity towards methanol and 
ethanol went through a maximum at ea. 260 ?C as the temperature was increased from 240 ?C to 
300 ?C. At temperatures higher than 260 ?C, the selectivity towards C1-C4 alcohols decreased 
rapidly, especially methanol. Majocchi et al. (1998) studied the temperature effect on the mixed 
alcohol synthesis over a Cs-promoted Cu/ZnO/Cr2O3 catalyst. They found that within a certain 
temperature range (225 ?C to 325 ?C) the methanol and C2+ linear alcohols productivity show 
maxima upon increasing the reaction temperature, which is quite similar to what has found in 
this current study. Methanol formation is believed to approach chemical equilibrium with 
increasing the reaction temperature. Similarly, Boz et al. (1994) showed that the formation of 
methanol is less favored thermodynamically at higher temperatures when investigating MAS 
over a similar Cu/ZnO/Al2O3 catalyst under gas phase condition.  
The changes observed in the selectivity towards each of the C1 to C4 alcohols upon increasing 
the reaction temperature in the range of 240-300 ?C under supercritical hexanes phase conditions 
are shown in Figure 3.11.  At 240 ?C, methanol accounts for almost all of the overall liquid 
productivity. Only minor amounts of higher alcohols are formed, along with CO2 and CH4 
(shown in Fig. 3.9 c&d). Increasing the temperature has a positive effect on the formation of 
alcohols in the presence of supercritical hexanes. The selectivity towards methanol and higher 
alcohols exhibits maximum around 290 ?C, which is higher than that obtained under gas phase 
conditions where the maximum selectivity occurred around a temperature of 260 ?C. The 
presence of a maximum in the productivity of methanol under gas phase conditions is related to 
 73
kinetic and thermodynamic reasons (Majocchi et al., 1998). As the temperature is increased, the 
methanol synthesis reaction approaches chemical equilibrium such that a further increase in 
temperature results in a decrease in the methanol concentration since this is an exothermic 
reaction. The presence of supercritical hexanes will enhance the removal of the reaction heat and 
can therefore mitigate thermodynamic restriction. As such, the productivity of methanol is 
monotonically increased with the increase in temperature under supercritical hexanes conditions, 
as shown in Table 3.3.  
The carbon chain growth probability factor for alcohol as a function of temperature for gas 
phase and supercritical phase condition are summarized in Figure 3.11. It can be observed that 
alcohols have one constant chain growth factor (0.35 ? 0.05) over the temperature range 
investigated in this study.  The rate of methanol formation falls on the Schulz-Flory line used in 
chain growth probability factor calculations. This correspond well with the observations reported 
by Boz(Boz, 2003), in which only one mechanism for methanol and higher alcohols? formation 
was believed. Slightly changes have been observed when the chain growth factor in supercritical 
phase conditions was compared with the one obtained in gas phase. It indicates that the 
mechanism of alcohol formation may not be dependent on the reaction media. 
 
 
 
 74
 
(a)                                                                   (b) 
 
(c)                                                                (d) 
Figure 3.9 Effect of temperature on the syngas conversion and selectivity to CH4 and CO2 (filled 
icons represent the data obtained under gas phase conditions and the open icons represent 
the data obtained under SCF phase conditions) Reaction conditions were as follows: 
catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, 
syngas partial pressure 6.2 MPa 
 
 
0%
10%
20%
30%
40%
50%
60%
70%
200 220 240 260 280 300 320
CO
 co
nv
er
sio
n (
%)
Reaction temperature ( ?C)
0%
10%
20%
30%
40%
50%
60%
200 220 240 260 280 300 320
H 2
co
nv
er
sio
n (
%)
Reaction temperature ( ?C)
0%
5%
10%
15%
20%
25%
200 220 240 260 280 300 320
CO
2s
ele
cti
vit
y (
%)
Reaction temperature ( ?C)
0%
5%
10%
15%
20%
25%
200 220 240 260 280 300 320
CH
4s
ele
cti
vit
y (
%)
Reaction temperature ( ?C)
 75
 
Figure 3.10 The effect of temperature on the carbon selectivity to alcohol products under gas phase 
condition. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, pressure 6.2 MPa 
 
Figure 3.11 The effect of temperature on the carbon selectivity to alcohol products under SCF 
condition. Reaction conditions were as follows: catalyst = 0.5 wt% K doped 
Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, hexanes/syngas molar ratio 
2, syngas partial pressure 6.2 MPa 
 
 
0%
2%
4%
6%
8%
10%
220 240 260 280 300 320
Ca
rb
on
  s
ele
cti
vit
y (
%)
Temperature (?C)
Methanol/10
Ethanol
1-propanol
1-butanol
2-butanol
iso-butanol
0%
2%
4%
6%
8%
10%
12%
14%
220 240 260 280 300 320
Ca
rb
on
  s
ele
cti
vit
y (
%)
Temperature (?C)
Methanol/10
Ethanol
1-propanol
1-butanol
2-butanol
iso-butanol
 76
 
 
Figure 3.12 Changes in chain growth probability factor ? with reaction temperature under gas phase 
and supercritical hexanes phase conditions. Reaction conditions were as follows: catalyst 
= 0.5 wt% K doped Cu/ZnO/Al2O3, syngas flow rate = 50 sccm/gcat, H2/CO = 2.0, 
syngas partial pressure 6.2 MPa 
  
 
Table 3.3 Effect of reaction temperature on CO conversion, CO2 and CH4 selectivity, and alcohol 
productivity obtaining under both gas phase and SCF phase conditions.  
T (?C ) CO % Selectivity (%) Productivity (g/kg cat/h ) CO
2 CH4 Methanol Ethanol C3 alcohols C4 alcohols 
Gas 
phase 
220 7.2 5.9 14.1 57.6 Na Na Na 
240 9.2 7.7 10.1 84.1 1.6 2.1 2.4 
260 18.8 10.0 6.2 214.7 3.8 3.4 3.8 
280 28.3 14.5 5.3 244.0 3.2 3.0 3.5 
300 32.9 23.6 6.4 198.0 1.5 1.0 0.89 
SCF 
Phase 
240 37.2 3.5 1.3 247.0 Na 0.4 2.5 
260 51.1 4.2 1.2 533.1 4.6 3.5 4.3 
280 60.0 6.9 1.7 589.1 8.9 6.2 7.1 
300 66.0 11.6 2.8 630.8 14.1 8.6 8.3 
Reaction conditions were as follows: 
catalyst = 0.5 wt% K doped Cu/ZnO/Al2O3 
syngas flow rate = 50 sccm/gcat 
H2/CO = 2.0 
pressure 6.2 MPa for gas phase condition, 18.6 MPa for SCF condition. 
0
0.1
0.2
0.3
0.4
0.5
200 220 240 260 280 300 320
Ch
ain
 gr
wo
th
 pr
ob
ab
ilty
fac
to
r ?
Temperature (?C)
Gas phase
SCF
 77
3.4 Conclusion 
The 0.5 wt% K promoted Cu/ZnO/Al2O3 catalyst prepared in this study demonstrated stable 
catalytic activity for mixed alcohol synthesis. It was observed that syngas conversion, methanol 
productivity, and higher alcohol selectivity can be enhanced by introducing supercritical hexanes 
as a reaction medium. A set of hexanes/syngas molar ratios were investigated in order to 
determine  suitable reaction conditions, while maintaining a partial pressure of syngas of 4.5 
MPa in all cases. The introduction of supercritical hexanes into the reaction mixture results in a 
rapid increase in both CO and H2 conversion and has been attributed to the increase in reaction 
pressure that corresponds to the amount of hexanes introduced. A significant decrease in the 
selectivity towards CH4 has been observed under supercritical reaction conditions compared to 
gas phase and this has been attributed to the enhanced heat transfer provided by the supercritical 
hexanes. The presence of the supercritical hexanes allows the activity to be positively affected by 
pressure while also providing suppression of undesired side reactions to CO2 and CH4, thereby 
achieving an increase in the productivity of alcohols, especially methanol and ethanol. Of the 
conditions studied, a hexanes/syngas molar ratio in the range of 1 to 2 would be suggested for 
optimizing the effect of the supercritical solvent on the MAS studies. The effect of temperature 
has also been investigated on conversion, selectivity and alcohol productivities for operation 
under both gas phase and supercritical phase conditions. An optimum temperature was found to 
be ca. 290?C under supercritical hexanes phase conditions. In general, this work has shown that 
the use of supercritical hexanes as a reaction medium provides opportunities to enhance the 
performance of mixed alcohol synthesis and similar results would be expected on other related 
catalyst systems. 
  
 78
Chapter 4 Investigation of Supercritical Fluids as Reaction Media for Higher 
Alcohol Synthesis from Syngas over a K Promoted Cu-Co-Zn 
Catalyst 
Abstract 
Due to the unique properties of supercritical fluid, the use of it as reaction media 
has been demonstrated a lot benefits in Fischer-Tropsch synthesis and methanol 
synthesis from syngas. In this study, a Cu-Co based catalyst (0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3) has been investigated in a fixed bed reactor under supercritical 
hexanes phase conditions. A series of hexanes/syngas molar ratio has been studied, 
while the syngas partial pressure was maintained at 4.5 MPa. It has been found that 
the CO conversion was remained stable as increasing the hexanes/syngas molar ratio, 
while the CH4 selectivity decreased continuously. Higher alcohol productivity was 
found increased monotonically with the increase in hexanes/syngas molar ratio. A 
molar ratio higher than 2 was suggested for the subsequent higher alcohol synthesis 
study. Comparison of results in the gas phase with those in the supercritical phase 
demonstrated that CH4 selectivity has been reduced significantly under the 
supercritical hexanes conditions. At the same CO conversion of 12.6%, switching 
reaction conditions from gas phase to supercritical hexanes phase resulted in 
enhanced ethanol selectivity at the expense of methanol selectivity. In addition, the 
effect of GHSV in the range of 500 to 2000 h-1 has also been investigated. The 
introduction of supercritical hexanes provides a clear thought about the effect of 
GHSV in this low flow rate range on the higher alcohol synthesis. At last, argon was 
used as a reaction media to compare with supercritical hexanes. It has been found 
that the enhanced higher alcohol productivity in line with the improved extraction of 
alcohol products from the pores. The introduction of the supercritical media 
improved the heat removal from the active sites, resulting in a remarkably lower 
selectivity towards C1-C4 paraffin. 
 
Keywords: Supercritical fluid; Higher alcohol synthesis; Supercritical reaction media; Copper-
Cobalt catalyst 
4.1 Introduction 
The synthesis of higher alcohol represents a promising pathway for the utilization of synthesis 
gas (a mixture of CO and H2, refered to as syngas) in order to create liquid fuels and/or fuel 
additives. The driving force behind the research that focuses on the use of syngas as a fuel 
production platform is that syngas can be derived from various carbonaceous sources, such as 
coal and biomass, which are either abundant or renewable. Although the process of converting 
 79
syngas to alcohols has been under development for nearly a century, from a practical point of 
view, it still suffers from low selectivity towards higher alcohols (Fang et al., 2009; Feng, Wang, 
Jiang, & Ji, 2011; Forzatti et al., 1991; Subramani & Gangwal, 2008). In order to meet the 
practical requirements of industry, numerous studies have been devoted to the investigation of 
the higher alcohols synthesis (HAS) over the past thirty years (Boz, 2003; Burcham et al., 1998; 
Chaumette, Courty, Kiennemann, & Ernst, 1995; Dalmon, Chaumette, & Mirodatos, 1992; 
Herman, 2000; Kiennemann, Diagne, Hindermann, Chaumette, & Courty, 1989; Klier et al., 
1997; Ma, Li, Lin, & Zhang, 2010; Mawson et al., 1993; Nunan, Bogdan, Klier, et al., 1989; 
Smith & Anderson, 1983; Tronconi et al., 1990; Xiang et al., 2008; X. Xu et al., 1987), including 
the development of modified catalysts (Boz, 2003; Dong et al., 2009; D. Li et al., 2007; Nguyen 
Tien-Thao, Zahedi-Niaki, Alamdari, & Kaliaguine, 2007; Xiang et al., 2008), the utilization of 
double bed reactors (Beretta, Sun, Herman, & Klier, 1996; Burcham et al., 1998), etc.  
Various catalysts that have been used in HAS studies have been thoroughly discussed in the 
literature (Dalmon et al., 1992; Fang et al., 2009; Forzatti et al., 1991; Gerber et al., 2007; 
Herman, 2000; Spivey & Egbebi, 2007; Subramani & Gangwal, 2008; Vannice, 1976). These 
HAS catalysts are commonly divided into four types: i) modified low temperature methanol 
synthesis catalysts (Cu-based catalysts), ii) molybdenum sulfide catalysts, iii) Rh containing 
catalysts, and iv) modified Fischer-Tropsch catalysts (Ni, Fe or Co containing catalysts). The 
first use of a Cu-Co based catalyst was reported by IFP (Institute Fran?ais du P?trole), who then 
further claimed a number of patents on Cu-Co based catalysts (Courty et al., 1982; Sugier, 
Freund, & Page, 1982; Sugier & Freund, 1981). Typically, a suitable Cu-Co based catalyst 
contains Cu, Co, other trivalent metals (Al or Cr), Zn or Mg and an alkali metal (Chaumette et al., 
1995; Courty, Chaumette, Durand, & Verdon, 1988). It is believed that Co dissociates CO and 
 80
hydrogenates those resulting surface carbon species into hydrocarbons, and Cu assists in non-
dissociative activation of CO (Courty et al., 1988; X. Xu et al., 1987). The idea of combining Co 
and Cu is intended to promote adsorbtion of molecular CO on the Cu, which terminates the 
carbon chain growth that occurs on the Co, thereby leading to higher alcohols. 
Cu-Co based catalysts require similar reaction conditions to the modified low temperature 
methanol synthesis catalyst. The IFP process operates at 6-10 MPa, 260 ?C ? 320 ?C, and GHSV 
3000 ? 6000 with H2/CO ratios of 1.0 - 2.0 (Courty et al., 1982). It has been shown that Cu-Co 
based catalysts have high activity and selectivity towards higher alcohols (Chaumette et al., 
1995). With the CuO/CoO/ZnO/Al2O3 catalysts, productivity up to 0.2g/g cat/h was reached at 
moderate temperature (below 310 ?C). Straight chain alcohols have been observed as the main 
products on this type of catalyst, following an Anderson-Schulz-Flory (ASF) type distribution. A 
60%-80% selectivity towards C1-C6 alcohols and a C2+OH/(C2+OH + C1OH) ratio higher than 60% 
has been observed (Kiennemann et al., 1989). 
Cu-Co based catalysts contain at least four components. The composition of each element 
plays an important part in the catalytic activity and the product selectivity.  Dalmon et al.(1992) 
discuss that the complex and successful combination of Co and Cu ions in spinel-like structures 
is considered to be the key step leading to the tight interaction observed between the two 
elements in the activated catalyst, which provides selectivity towards alcohol synthesis (Dalmon 
et al., 1992).  
In the case of ternary Cu-Co-Cr oxide catalysts, the diagram in Figure 1 shows the 
composition of these catalysts that promote methanol synthesis, hydrocarbon synthesis or higher 
alcohol synthesis. For intermediate compositions, especially in the area defined by 1?Cu/Co?3, 
Co/Cr?0.5, a mixture of C1-C6 light alcohols is obtained with C1-C6 hydrocarbons as byproducts.  
 81
 
 
Figure 4.1 Selectivity as function of composition in the ternary system CuO-CoO-Cr2O3 (Courty et 
al., 1982) 
Cu-Co catalysts have been reported to suffer from deactivation caused by coke deposition, 
separation of Co from the originally homogeneously distributed Co and Cu phases, loss of cobalt 
as carbonyls, and formation of surface carbides (Courty et al., 1982). To further improve the 
catalytic performance of Cu-Co catalysts, nonconventional catalysts, such as Co/Cu based 
perovskites (Thao, 2007), Co/Cu supported on multiwall carbon nanotubes (Dong et al., 2009), 
and Co/Cu nanoparticles (Subramanian, Balaji, Kumar, & Spivey, 2009a) have been investigated. 
In general, these studies aimed to develop a catalyst that would have a good Cu and Co structural 
homogeneity in order to enhance the selectivity towards higher alcohols.  
 82
It has been found on CuO/CoO/ZnO/Al2O3 catalysts that the tendency for methanation was 
high when the reaction temperature was over 290 ?C (Courty et al., 1982). Difficulties were also 
found in thermally controlling start-up of the reactions (Courty et al., 1982). Furthermore, the 
selectivity towards alcohols has been found to be highly dependent upon the preparation and 
temperature-dependant activation procedures (Mahdavi et al., 2005). As discussed below, 
supercritical fluid reaction media can provide certain advantages in maintaining thermal 
uniformity, and therefore better temperature control during start-up and continuous operation of 
these Cu-Co catalysts systems, where they have been shown to improve the performance of 
similar exothermic catalytic reactions (Durham et al., 2010; Elbashir et al., 2010; Huang et al., 
2004). 
Supercritical fluids have been recognized as an unique medium for heterogeneous reactions, 
offering single phase operation, a tunable density between typical liquid-like and gas-like 
densities, as well as gas-like diffusivity. These properties can provide enhanced mass transfer 
and heat transfer, help to eliminate mass transport limitations, integrate reaction and product 
separation processes, and enhance in situ extraction of low volatility products from porous 
catalysts (Abbaslou et al., 2009; Fan & Fujimoto, 1999; Huang & Roberts, 2003; Subramaniam 
& McHugh, 1986; Wu et al., 1991). The utilization of supercritical media in Fischer-Tropsch 
Synthesis (FTS) has been developed for more than twenty years. It has been shown that the 
presence of supercritical media provides several advantages into FTS as listed below; 
1) SCFs increase the in situ extraction of heavy products in supercritical FTS (Fan & 
Fujimoto, 1999). 
2) The catalyst effectiveness factor and pore accessibility increases with pressure in SCF 
media (Bochniak & Subramaniam, 1998). 
 83
3) The selectivity towards ?-olefins is enhanced under SCF operation (Lang et al., 1995). 
4) Optimal operating conditions exist when there is a fine balance between the effects of 
SCF bulk diffusion vs. pore diffusivity within the SCF depending on the temperature and 
pressure employed (Huang & Roberts, 2003). 
Higher alcohol synthesis represents another highly exothermic reaction that could benefit 
from the advantages provided by supercritical reaction media. Jiang et al. (2001) investigated the 
influence of a mixture of C10-C13 alkanes as SC reaction media in higher alcohol synthesis over a 
Zn-Cr-K catalyst and found that the selectivity towards ethanol, n-propanol and isobutanol were 
raised and that of methanol was decreased. To the author?s knowledge, there is no literature to 
date that reports on the performance of Cu-Co catalysts under the influence of supercritical 
reaction media for higher alcohol synthesis. Herein, we investigate HAS under a series of 
reaction conditions that span the supercritical regime and compare these results with those 
obtained from analogous gas phase operation. The purpose of this study is to investigate the 
effect of supercritical hexanes on the formation of higher alcohols, including the specific reaction 
performance parameters of carbon chain growth, productivity and selectivity towards higher 
alcohols. 
4.2 Experimental Section 
4.2.1 Catalyst preparation 
The preparation of the Cu-Co based catalysts employed in this study consists of continuous 
coprecipitaion of nitrate solutions of respective metals under controlled conditions. A 
Cu/Co/Zn/Al mixed nitrate salt solution was first prepared at a mass ratio of 43.5:14.5:24:18. 
The precursor was prepared by co-precipitating this mixed nitrate salt solution with a 1.0 M 
K2CO3 solution in a 1L flask, which was initially filled with 200 ml deionized water at 80 ?C. 
 84
The pH value was strictly kept at 7 in order to get a homogeneous dispersion. After aging for an 
hour, the precipitate was decanted and then washed with sufficient deionized water at 80 ?C in 
order to eliminate the excess K introduced with the carbonate. The moisturized precursors were 
dried at 80 ?C and at ambient pressure for 24 hours. The dried precipitate was then ground, 
calcined at 350 ?C under continuous air flow to give the corresponding mixed oxides. The 
promotion of K was carried out through incipient wetness method using aqueous potassium 
carbonate solution. The slurry paste was then dried at 80 ?C overnight and calcined at 350 ?C for 
4 hours. 
4.2.2 Catalyst testing 
The activity and selectivity of the Cu-Co catalyst for the synthesis of higher alcohols from 
syngas were determined using a stainless steel fixed-bed reactor. Hexanes solvent was purchased 
from VWR and syngas (64% H2, 32% CO and 4% N2) was purchased and certified by Airgas. 
The flow rate of syngas and balancing gas argon was controlled by a mass flow controller 
(Brooks 5850 E), while the volumetric flow rate of hexanes was controlled by an HPLC pump 
(Acuflow Series III). Hexanes and syngas were heated separately and mixed together before 
entering the reactor. Reaction pressure was controlled by a back pressure regulator (Straval-BPH) 
installed between the hot trap and the cold trap. One gram of catalyst (45-70 mesh) was loaded 
into our high pressure reactor fixed into position by packing with glass wool. Effluent from the 
reactor passes through the hot trap (240 ?C), a heat exchanger, and cold trap (5 ?C) and was then 
separated into gas phase and liquid phase products. Reactants and products were analyzed by two 
gas chromatographs. The analyses of gas phase products were performed on a Varian CP-3380 
Gas Chromatograph equipped with a Haysep-DB column and a TCD detector. The analyses of 
 85
liquid phase product were performed on a Varian CP-3300 Gas Chromatograph equipped with a 
capillary DB-wax column and a FID detector.  
4.3 Results and Discussion 
4.3.1 The effect of hexanes/syngas molar ratio on the catalytic performance 
In order to perform the synthesis of higher alcohols from syngas under the supercritical 
reaction media conditions, hexanes solvent was continuously injected with the reactant gases into 
the fixed-bed reactor. A hexanes/syngas molar ratio is determined by the molar flow rate of 
hexanes and the molar flow rate of syngas. The higher the hexanes/syngas molar ratio, the closer 
the properties of the mixture are to that of the pure solvent, therefore more readily achieving 
single phase supercritical operation (Elbashir et al., 2005). Hexanes/syngas molar ratios in the 
range of 3.0 to 3.5 have been previously employed in the studies of FTS under supercritical 
solvent conditions (Bochniak & Subramaniam, 1998; Elbashir et al., 2010; Linghu et al., 2006). 
Xu et al (2012) has investigated supercritical mixed alcohol synthesis over a Cu-based catalyst at 
a series of hexanes/syngas molar ratios and found that the productivity of higher alcohols 
increased with the increase in the hexanes/syngas molar ratio. In the current work (this paper), 
cobalt was added into the Cu based catalyst in order to enhance the carbon chain growth of 
alcohol products. This paper presents a similar hexanes/syngas molar ratio study on this Cu-Co 
based catalyst under supercritical hexanes reaction conditions so as to determine the suitable 
hexanes/syngas molar ratio for the optimal formation of higher alcohols with additional interests 
on the fact that hydrocarbons can also form at the Co active sites. The syngas flow rate was kept 
constant at 50 sccm, the flow rate of hexanes was then adjusted to provide the required 
hexanes/syngas molar ratio. The reaction pressure was controlled as a function of the 
hexanes/syngas molar ratio, while the partial pressure of syngas was kept constant.  
 
Figure 4.2 presents the conversion of CO and H
ratio over the 0.5 wt% K promoted Cu/Co/ZnO/Al
molar ratio at 0) and supercritical hexane phase conditions (hexanes/syngas molar rat
and 3). Under gas phase conditions (hexanes/syngas molar ratio = 0) the conversion of H
CO are around 10%. In increasing the gas phase reaction pressure from 4.5 MPa to 9.0 MPa, the 
conversions increased rapidly to 38% in the case of H
conversion. A slight increase in CO and H
condition was changed from gas phase to supercritical hexanes phase. While the syngas partial 
pressure was kept at 4.5 MPa, the co
the hexanes/syngas molar ratio as shown in Figure 
Figure 4.2 CO and H2 conversion as a function of hexanes/syngas molar ratio. React
were as follows: catalyst
50 sccm, H2/CO = 2.0, temperature = 300 ?C, pressure = 
Figure 4.3 presents the selectivity towards CH
over the same catalyst under both gas phase and supercritical phase conditions. When the HAS 
86
2 as a function of the hexanes/syngas molar 
2O3 catalyst under gas phase (hexanes/syngas 
2 conversion and 46% in the case of CO 
2 conversion has been observed when the reaction 
nversion of CO and H2 remains stable with an increase in 
4.2.  
 = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 
4.5 MPa-18MPa
4 as a function of hexanes/syngas molar ratio 
ios of 1, 2, 
2 and 
 
ion conditions 
 
 87
was conducted under gas phase conditions (at hexanes/syngas molar ratio = 0), the selectivity 
towards CH4 was close to 35%. Increasing the pressure from 4.5 MPa to 9.0 MPa under the gas 
phase reaction conditions (at hexanes/syngas molar ratio = 0) increased the CH4 selectivity from 
35% to 59%, as shown in Figure 4.3. This observation is consistent with that of Courty et al. 
(1982), De Aquino & Gomez Cobo (2001) and Subramanian et al. (2009a), who observed high 
methane selectivity over Cu-Co-Al catalysts at elevated pressure. Subramanian et al. (2009a) 
reported that the thermodynamically favored methane formation must be kinetically limited in 
order to increase the ethanol yield and selectivity. Figure 4.3 also shows that CH4 selectivity 
decreased significantly when the reaction conditions were switched from gas phase (ca. 34.1%) 
to supercritical hexanes phase conditions. Furthermore, CH4 selectivity decreased from 12.6% at 
hexanes/syngas molar ratio of 1 to 5.2% at hexanes/syngas molar ratio of 3, due in part to the 
improved ability of the supercritical medium to manage the reaction heat effectively (Durham et 
al., 2010; Elbashir et al., 2010). 
Figure 4.3 also presents the selectivity towards CO2 as a function of hexanes/syngas molar 
ratio. Under gas phase reaction conditions (Hexanes/syngas molar ratio = 0), the increase in 
pressure had a slight positive effect on the CO2 selectivity, which increased from 22% to 30% 
when pressure increased from 4.5 MPa to 9 MPa. As also shown in Figure 4.3, CO2 selectivity 
was very stable with or without the presence of supercritical hexanes, where the CO2 selectivity 
remained ca. 20% at each of the hexanes/syngas ratios studied while the partial pressure of 
syngas was held constant at 4.5 MPa.  
 
 
Figure 4.3 CO2 and CH4 selectivity as a function of hexane/syng
were as follows: cata
50 sccm/gcat, H2/CO = 2.0, temperature
Figure 4.4 shows the distribution of higher alcohols as a funct
ratio. Linear alcohols, such as 1-
the gas phase reaction (hexanes/syngas molar ratio = 0) over this Cu
observation corresponds with previously
Tien-Thao, Hassanzahediniaki, Alamdari, & Kaliaguine, 2007)
supercritical hexanes phase conditions again yielded linear alcohols, though the enhancement in 
productivity of higher alcohols was quite significant.
increased from 0.5 g/kgcat./h to 1.4 g/kg
from 0 to 3, and the productivity of 1
important to note that the productivity of each higher alcohol increase monotonically with the 
increase in the hexanes/syngas molar ratio. The hexanes/syngas molar raito of 3 appears to be the 
most suitable reaction condition for HAS of all of the conditions studied. N
88
as molar ratio. Reaction conditions 
lyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 
 = 300 ?C, pressure = 4.5 MPa-18MPa
ion of hexanes/syngas molar 
propanol and 1-butanol, were observed as the main products in 
-Co based catalyst. This 
 reported literature (Boz, 2003; Gerber et
. Conducting the reaction under 
 For example, the productivity of 1
cat./h as the hexanes/syngas molar ratio was increased 
-propanol increased from 0.6 g/kgcat./h to 1.7 g/kg
ote that in order to 
 
 
 al., 2007; N 
-butanol 
cat./h. It is 
 89
maintain the syngas partial pressure at the value of 4.5 MPa a total pressure of 18 MPa was 
required in the case of the hexanes/syngas molar ratio of 3 (supercritical hexanes phase 
conditions). Unfortunately, higher hexanes/syngas molar ratios could not be examined due to 
pressure limitations in this particular reactor system. The hexanes/syngas molar ratio of 2 also 
showed appreciable increases in the production of higher alcohols compared to gas phase 
operation.   
 
Figure 4.4 Higher alcohol productivity as a function of hexanes/syngas molar ratio. Reaction 
Conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas 
flow rate = 50 sccm, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 MPa-18MPa 
In order to better understand the impact of the SCF solvent on the formation of higher 
alcohols, the C+OH wt% was plotted as a function of hexane/syngas molar ratio, as shown in 
Figure 4.5. The C+OH wt% was defined as the weight of a certain alcohol divided by the total 
weight of all the alcohol products collected in the liquid product stream. Figure 4.5 demonstrated 
that the selectivity of methanol has been remarkably enhanced by the presence of supercritical 
hexanes when the molar ratio is higher than 1. Interestingly, the productivity towards methanol 
was the lowest at a molar ratio of 1. Moreover, the selectivity towards ethanol is optimal at a 
 90
molar ratio of 1 among the conditions studied. Further studies on this topic are required before 
firm conclusions should be drawn about optimal molar ratio for ethanol. 
 
Figure 4.5 C+OH wt% as a function of hexane/syngas molar ratio (C+OH wt% defined as the weight 
of CnOH divided by the total weight of alcohol products). Reaction conditions were as 
follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 50 
sccm/gcat, H2/CO = 2.0, temperature = 300 ?C, pressure = 4.5 MPa-18MPa  
4.3.2 Selectivity comparison for same conversion under gas phase and SCF phase 
conditions 
In order to better evaluate the effect of the supercritical hexanes solvent on the reaction 
performance, CO2 and CH4 selectivities obtained under both gas phase and supercritical hexanes 
phase conditions were compared at the same CO conversions, as shown in Figure 4.6. The SC-
HAS reaction conditions were maintained at a hexanes/syngas molar ratio of 3, and the reaction 
pressure was held constant at 18 MPa. The CO conversion was varied by changing the gas 
hourly space velocity (GHSV) of syngas under gas phase conditions, while both the GHSV of 
syngas and the flow rate of hexanes were simultaneously adjusted under the supercritical phase 
conditions in order to make direct comparsions between the gas phase operation and the 
supercritical phase operation at the same levels of CO conversion. It can be observed from 
Figure 4.6 that the presence of the supercritical hexanes reaction medium significantly decreased 
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
C+ OH wt%
0 1 1.5 2 3
Hexane/syngas molar ratio
methanol
ethanol
C3 alcohols
C4 alchols
 91
the selectivity towards CH4 and had a negligible effect on the selectivity of CO2 at each of the 
conversions studied. In comparison, for a conversion of 24.9% under supercritical hexanes phase 
conditions, the CH4 selectivity was maintained at 9.7% which was significantly lower than the 
CH4 selectivity of 64.9% that was obtained at the same conversion in gas phase. The CH4 
selectivities at each of the three conversion levels in the gas phase were in the range of 60%, in 
good keeping with prior observations that methanation was high over this Co containing catalyst 
at similar reaction conditions (Forzatti et al., 1991; Subramanian, Balaji, Kumar, & Spivey, 
2009b). Previous studies have suggested that the high CH4 selectivity could be associated with 
the sintering of the active sites at high temperature which leads to higher selectivity towards 
hydrocarbons (Chaumette et al., 1995). In addition, the gas phase CO conversion steadily 
increased as a function of time on stream over the 250 hours period of experimentation in this 
study, which can be mainly attributed to the enhanced methanation activity.  
Under gas phase reaction conditions, the reaction heat removal is insufficient and local 
overheating of the catalyst can thereby result in a slow segregation of Co from the Cu-Co 
interactive structure (Chaumette et al., 1995; Courty et al., 1988, 1982). This inhomogeneity is 
believed to convert the catalyst from an alcohol synthesis catalyst to a Fischer-Tropsch synthesis 
catalyst thereby enhancing the selectivity towards CH4 and hydrocarbons as has been observed 
previously (Courty et al., 1982; Spath & Dayton, 2003). This potential segregation of the Co 
from the Cu-Co catalyst under gas phase conditions, as opposed to the supercritical phase 
conditions, is further revealed from a comparison of the respective alcohol productivity levels 
shown in Figure 4.7. At the same CO conversion (ca. 12.6%), ethanol accounts for 69% of the 
total alcohols produced under supercritical hexanes phase conditions while it constitutes only 7%  
of the total alcohols obtained in the gas phase. Methanol, in contrast, predominates under gas 
 92
phase conditions (ca. 90% of the total alcohol produced). Tien-Thao et al (2006) reported that 
copper located outside the supporting materials (perovskite in their case) leads to the formation 
of methanol and methane. In the current study, switching from supercritical phase to gas phase 
conditions reduces the ability to remove heat from the active sites, thereby resulting in gradual 
catalyst segregation. Therefore, methanol and methane were found to be the predominate 
products under gas phase conditions.  
 
Figure 4.6 Comparison of CH4 and CO2 selectivities as a function of CO conversion under both gas 
phase and supercritical phase conditions. The reaction conditions were as follows: 
catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 15-100 sccm/gcat, 
H2/CO ratio = 2.0, temperature = 300 ?C; pressure = 4.5 MPa under gas phase conditions 
and pressure =18 MPa under supercritical phase conditions (where hexanes/syngas molar 
ratio = 3) 
 93
 
Figure 4.7 Comparison of alcohol selectivities at the same CO conversion under both gas phase and 
supercritical phase conditions.  The reaction conditions were as follows: catalyst = 0.5 wt% 
K promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 15-100 sccm/gcat, H2/CO ratio = 2.0, 
temperature - 300 ?C; pressure = 4.5 MPa under gas phase conditions and pressure = 18 
MPa under supercritical phase conditions (where hexane/syngas molar ratio = 3), CO 
conversion = 12.6%. 
4.3.3 The effect of GHSV  
It is a common observation that the selectivity to higher alcohols over a modified methanol 
synthesis catalyst can be enhanced by lowering the space velocity under typical methanol 
synthesis conditions (Forzatti et al., 1991). The effect of GHSV on HAS over this catalyst has 
been investigated under supercritical hexanes phase conditions in this study. The use of 
supercritical hexanes allows for much smaller values of GHSV to be applied.  Specifically, the 
GHSV values under the supercritical conditions employed in this study range from 300 to 2000 
h-1, which are much smaller than most of the values previously reported in the literature.  
Figure 4.8 presents the CO conversion, the H2 conversion, and the selectivity towards CH4 
and CO2 when the GHSV was increased from 300 to 2000 h-1 under supercritical hexanes phase 
conditions. The CO conversion decreased from 24.9% at GHSV = 300 h-1 to 12.6% at GHSV = 
2000 h-1. H2 conversion follows a similar trend, but decreased more slowly from 17.1% to 8.1% 
 94
with the same increase in GHSV, as shown in Figure 4.8. The CH4 and CO2 selectivity is favored 
by the lower GHSV values (i.e. longer contact times) under the supercritical hexanes phase 
conditions.  
In Figure 4.9, the effect of GHSV on the selectivity towards methanol and higher alcohols is 
presented. It was found that the selectivity towards methanol reached a maximum at the GHSV 
of 500 h-1 when holding the total pressure at 18 MPa and the hexanes/syngas molar ratio at a 
value of 3. Ethanol selectivity reached its highest value somewhere around 1000 h-1 GHSV. The 
selectivity towards the C3 and C4 alcohols was consistently in the range of 1%-2% over the range 
of GHSV values studied under these supercritical hexanes phase conditions. These results 
suggest that moderate residence time (ca. 1000 h-1) favors higher alcohol formation, while 
additional contact time with the catalyst results in a significant increase in the formation of 
methanol and CH4 at the expense of higher alcohol selectivity. Table 4.1 shows the effect of 
GHSV on the CO conversion and alcohol selectivity under gas phase conditions. When the 
GHSV was increased from 500 to 2000 h-1 under gas phase conditions, the CO conversion 
decreased from 29.2% to 18.9%, as shown in Table 4.1. It was also observed that the CO2 
selectivity decreased from 20.7% to 13.3% as the GHSV increased from 500 h-1to 2000 h-1. CH4 
selectivity was at the level of 60%, when the GHSV was within the range of 500 h-1to 2000 h-1 
under these gas phase conditions. The low space velocity values that were employed in this study 
(compared to higher values that have been used in the literature (Forzatti et al., 1991; Mahdavi et 
al., 2005; Mahdavi & Peyrovi, 2006; Majocchi et al., 1998; Subramani & Gangwal, 2008; 
Nguyen Tien-Thao et al., 2007) have resulted in very high methanation activity under these gas 
phase conditions, resulting from poor heat management within the catalyst. Table 4.1 also shows 
the C2+OH selectivity, which is the weight of C2+ alcohols over the total alcohol product. Under 
 95
these gas phase conditions, the low GHSV (500 h-1-2000 h-1) resulted in a liquid product that was 
composed of 90% methanol. Overall, the low space velocity under gas phase conditions favors 
the formation of methanol and CH4, and a GHSV value greater than 2000 h-1 would be suggested 
for the formation of higher alcohols over this Cu-Co catalyst in the gas phase.  
 
Figure 4.8 CO conversion, H2 conversion, CH4 selectivity, and CO2 selectivity as a function of 
GHSV in SC-HAS. The reaction conditions were as follows: catalyst = 0.5 wt% K 
promoted Cu/Co/ZnO/Al2O3, H2/CO ration = 2.0, temperature = 300 ?C, pressure = 18 
MPa, hexanes/syngas molar ratio = 3. GHSV was calculated based on the volumetric 
flow rate of syngas and the volume of catalyst bed. 
 96
 
Figure 4.9 Selectivity of methanol, ethanol, propanol and butanol isomers as a function of syngas 
flow rate in SC-HAS. Reaction conditions were as follows: catalyst = 0.5 wt% K 
promoted Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, temperature = 300 ?C, pressure = 18 
MPa, hexanes/syngas molar ratio = 3.  
 
Table 4.1 The effect of GHSV on the conversion and selectivities under gas phase conditions 
GHSV (h-1) CO Conversion (%) Activity (g/kgcat/h) Selectivity (%) C
2+OH CO2 CH4 
500 29.17 0.0659 Na 20.7 60.5 
1000 27.76 0.0603 10.1 17.8 57.7 
2000 18.92 0.1385 7.2 13.3 69.6 
Reaction conditions: T = 300 ?C, catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO = 2 
Alcohols: C1-C6 alcohols 
Sc2+OH: weight percentage of C2+ alcohols /(methanol+ C2+ alcohols) 
 
4.3.4 Using Argon as balance gas 
Investigations were also conducted to determine the impact of the presence of the supercritical 
hexanes on the activity and productivity by varying the partial pressure of the supercritical fluid 
and by maintaining constant space velocity by using argon as the balance gas. The addition of 
argon as an inert balance gas allowed gas phase HAS experiments to be performed with 
comparable total flow rate (i.e. contact time) and syngas concentration to those used in 
 97
supercritical phase higher alcohol synthesis (SC-HAS).  In the studies involving the effect of 
hexanes/syngas molar ratio (section 4.3.1 above), it was observed that CH4 selectivity decreased 
rapidly and higher alcohols productivity increased monotonically with an increase in the 
hexanes/syngas ratio under supercritical conditions. However, no definitive conclusion could be 
drawn on this effect due to the fact that both the total pressure and partial pressure of 
supercritical fluids were varied simultaneously. In this current experiment, a constant total 
pressure of 13.5 MPa and a partial pressure of syngas of 4.5 MPa were maintained, while argon 
was used to offset the change in the partial pressure of hexanes. Due to limitations in the pressure 
that could be supplied by the argon tank employed in this set of experiments (as limited by the 
manufacturer), the total system pressure was maintained at 13.5 MPa instead of 18 MPa. 
Therefore, a hexanes/syngas molar ratio of 2 was employed in this experiment instead of 3, as 
used in the sections above.  
Table 4.2 presents the effect of both argon and supercritical hexanes reaction media on the 
CO conversion as well as the selectivity towards CO2, alcohols and hydrocarbons. CO conversion 
was 55.5% under the argon gas phase conditions, which was higher than the average value under 
supercritical hexanes phase conditions (ca. 33.9%). CO2 selectivity was found to be quite stable 
as the reaction medium was changed from argon gas phase to supercritical hexanes phase. The 
selectivity towards alcohols was observed to be 16.0% under supercritical hexanes phase 
reaction conditions and 4.5% under the argon gas phase conditions, while the partial pressure and 
flow rate of syngas was kept constant. However, the light hydrocarbon selectivity (the SH shown 
in Table 4.2) was found to be much higher under argon gas phase conditions than under the 
supercritical hexanes phase conditions. The C8-C20 hydrocarbon selectivity under both reaction 
conditions was found to have a value of 6.0% (not included in Table 2.).  
 98
Table 4.2 The effect of reaction medium on the conversion and selectivity 
Reaction 
medium 
Total pressure 
(MPa) 
Psyngas 
(MPa) 
CO Conversion 
(%) 
Selectivity (%) 
CO2 SA SH Sc2+OH 
Argon 13.5 4.5 55.5 42.7 4.5 25.5 96.2 
SC Hexanes 13.5 4.5 33.9 43.5 16.0 18.9 73.7 
Reaction conditions: T = 300 ?C, catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO = 2, syngas flow rate 
= 50 sccm/gcat. 
SA: selectivity towards C1-C6 alcohols 
SH: selectivity towards C2-C5 hydrocarbons 
Sc2+OH: weight percentage of C2+ alcohols /(methanol+ C2+ alcohols) 
 
Figure 4.10 presents a comparison of the alcohols productivity obtained from argon gas phase 
conditions and supercritical hexanes phase conditions. This data clearly demonstrated that the 
presence of the supercritical hexanes medium enhanced the productivity of alcohols significantly. 
An interesting observation is that the productivity of C1-C6 linear alcohols decreased 
monotonically with the increase in carbon number under the supercritical hexanes phase 
conditions; however, a maximum productivity was found at 1-butanol under argon gas phase 
reaction conditions. Figure 4.11 shows a comparison of the C8-C18 paraffin productivity obtained 
under both gas phase and supercritical phase HAS conditions. A nearly doubled C8-C11 paraffin 
productivity was observed in argon gas phase HAS compared with supercritical phase HAS. The 
higher productivity of C8-C11 paraffin observed in the liquid product analysis corresponds well 
with the higher light hydrocarbon selectivity obtained from the vapor phase product analysis as 
shown in Table 4.2. 
There are several proposed mechanisms for the formation of alcohols and hydrocarbons over 
Cu-Co catalyst systems (Chaumette et al., 1995; Forzatti et al., 1991; Kiennemann et al., 1989; 
M. Xu & Iglesia, 1999). Although many intermediates have been proposed regarding the carbon 
chain growth in higher alcohol synthesis, such as alkyl (Pan, Cao, & Griffin, 1988) or acyl 
entities (Chaumette et al., 1995), it is has been suggested that alcohols and hydrocarbons are 
formed from the same intermediates (Fang et al., 2009; Spivey, Egbebi, & Kumar, n.d.). 
 99
Moreover, the linear alcohol products have been reported to follow the Schulz-Flory distribution 
(REF), as described by: 
uni006Cuni006Funi0067g4666g1875g3041 g1866uni2044 g4667 g3404 g1866uni0009uni006Cuni006Funi0067g2009uni0009 g3397uni0009uni006Cuni006Funi0067g4666g4666uni0031 g3398 g2009g4667g2870 g2009uni2044 g4667 
where wn is the weight percentage of Cn alcohols and n is the number of carbon atoms. The 
probability of chain growth ? can be determined from the slope of the log(wn/n) versus n plot. 
Figure 4.12 shows the variation of log(wn/n) versus n for the alcohol synthesis and hydrocarbon 
synthesis, respectively. It can be observed that under supercritical hexanes phase conditions the 
formation of the alcohols obeys the Schulz-Flory distribution with a carbon chain growth number, 
?, of 0.52. As expected, the hydrocarbons also follow the ASF distribution and the carbon chain 
growth number was found to be ? = 0.76. As shown in Figure 4.10, the productivity of C1-C3 
linear alcohols is significantly lower in the argon phase than under the supercritical hexanes 
phase conditions. Combined with the observation that the light hydrocarbon selectivity was 
much higher under the argon gas phase conditions, as shown in Table 4.2, it is reasonable to 
speculate that the termination step of converting lower carbon number surface intermediates to 
paraffin was promoted under the argon phase conditions. Therefore, a deviation from the linear 
ASF type distribution was observed for the alcohol products in Argon as illustrated in Figure 
4.13.  
The productivity results in Figures 4.11 and 4.12 show that the termination step towards 
alcohols is preferred over the termination step towards hydrocarbons under the supercritical 
conditions. As a result, the productivity of alcohols is enhanced significantly. Because of the 
unique properties of supercritical fluids, the presence of the supercritical hexanes provided 
enhanced heat capacity and solubility compared to gas phase operation. As such, the heat 
removal and the mass transport were enhanced in the presence of supercritical hexanes. As a 
 100
result of the enhanced heat removal, localized hotspots in the reactor were avoided and the 
selectivity towards C1-C4 n-paraffin was significantly reduced. Moreover, the improved 
extraction capacity of the supercritical fluid solvent can allow for more efficient removal of FTS 
products from the catalyst resulting in higher C13-C20 hydrocarbons productivity under the SC-
HAS conditions.  
 
Figure 4.10 Productivity of C1-C8 alcohols in gas-phase and supercritical-phase HAS. The reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO 
ratio = 2.0, temperature = 300 ?C, pressure = 13.5 MPa, hexanes/syngas molar ratio = 2 
for the supercritical hexanes phase operation, argon/syngas molar ratio = 2 for the argon 
phase operation. 
 
Figure 4.11 Productivity of C8-C18 n-paraffin in gas phase and supercritical phase HAS. Reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO 
ratio = 2.0, temperature = 300 ?C, pressure = 13.5 MPa, hexanes/syngas molar ratio = 2 
for the supercritical hexanes phase operation, argon/syngas molar ratio = 2 for the argon 
phase operation. 
0
4
8
12
16
Methanol Ethanol 1-propanol 2-propanol 1-butanol 2-butanol isobutanol 1-pentanol 1-hexanol 1-heptanol 1-octanolPro
du
cti
vit
y (
g/k
g C
at
/h)
Alcohol  productivity
Supercritical hexanes phase Gas phase
0
4
8
12
C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18Pro
du
cti
vit
y (
g/k
g C
at
/h)
n-paraffin productivity 
Supercritical hexanes phase Gas Phase
 101
  
Figure 4.12 The variation of log(wn/n) with respect to n (the number of carbon atoms) of alcohols 
(left) and hydrocarbons (right) obtained in SC-HAS. Reaction conditions were as follows: 
catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, temperature = 
300 ?C, pressure = 13.5 MPa, hexanes/syngas molar ratio = 2. 
  
Figure 4.13 The variation of log(wn/n) with respect to n (the number of carbon atoms) of alcohols 
(left) and hydrocarbons (right) obtained in gas phase HAS. Reaction conditions were as 
follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, H2/CO ratio = 2.0, 
temperature = 300 ?C, pressure = 13.5 MPa, argon/syngas molar ratio = 2. 
  
y = -0.2824x - 0.3708-3
-2
-1
0
0 2 4 6 8 10
log
(W
n/
n)
Carbon number
Alcohols
y = -0.1317x - 0.4397-3
-2
-1
0
0 4 8 12 16 20
log
(W
n/
n)
Carbon number
Hydrocarbons
y = -0.3099x - 0.464-4-3
-2
-1
0
0 2 4 6 8 10
log
(W
n/
n)
Carbon number
Alcohols
y = -0.4292x + 2.3858
-6
-4
-2
0
0 4 8 12 16 20
log
(W
n/
n)
Carbon number
Hydrocarbons
 102
4.4 Conclusion 
In summary, we have described the preparation of a K2O promoted Cu/Co/ZnO/Al2O3 catalyst 
and investigated the effect of supercritical hexanes on its catalytic performance as well as alcohol 
productivities. The presence of supercritical hexanes has been shown to enhance the heat transfer 
rate and significantly reduce the selectivity towards CH4. Additionally, the effect of 
hexanes/syngas molar ratio on the productivity of methanol and higher alcohols has been 
investigated. With increases in the hexanes/syngas molar ratio, the productivity towards higher 
alcohols increased monotonically among the conditions studied and reached its highest value at a 
hexanes/syngas molar ratio of 3.  
When the hexanes/syngas molar ratio was set at 3 and the reaction pressure was set at 18 MPa, 
it was found that decreasing the GHSV from 2000 h-1 to 500 h-1 had a positive effect on the 
formation of methanol and CH4. The use of a GHSV higher than 1000 h-1 would be suggested for 
subsequent studies of SC-HAS in order to further improve the formation of higher alcohols.  
The benefits of using supercritical hexanes as a reaction medium was confirmed when SC-
HAS was compared with argon gas phase HAS under the same operating conditions. The type of 
alcohol products obtained in SC-HAS was the same as that obtained in the gas phase studies. In 
addition, the formation of alcohols under the supercritical hexanes conditions was found to obey 
the Schulz-Flory distribution. The introduction of the supercritical fluid medium improved the 
heat removal from the active sites, resulting in a remarkably lower selectivity towards C1-C4 
paraffins. The significantly enhanced alcohols productivity, especially that of C1-C5 linear 
alcohols, was likely due to improved extraction of these alcohols from catalyst pores by the 
supercritical hexanes.  
 
  
 103
Chapter 5 Effect of H2/CO ratio on higher alcohol synthesis over a Cu-Co 
catalyst under supercritical hexanes conditions 
 
Abstract 
The effect of H2/CO ratio on higher alcohol synthesis was studied over a K 
promoted Cu/Co/ZnO/Al2O3 catalyst using supercritical hexanes as a reaction 
medium. A series of syngas mixtures with H2/CO ratios of 2, 1.75, 1.35 and 1 were 
employed in order to simulate the syngas compositions which can be derived from 
various carbonaceous resources. The catalytic performance of this Cu-Co based 
catalyst was investigated under both gas phase and supercritical hexanes phase 
conditions. As the H2/CO ratio decreased from 2 to 1, the CO conversion decreased 
considerably under gas phase conditions, while the CO conversion remained 
relatively stable under supercritical hexanes phase conditions. Methane selectivity 
was found to decrease as the H2/CO ratio decreased from 2 to 1 under supercritical 
hexanes phase conditions. The alcohol and hydrocarbon products formed over this 
Cu-Co catalyst follow the Schulz-Flory distribution. Productivity of higher alcohols 
was found to increase as a function with the H2/CO ratio under the gas phase 
conditions applied in this study. However, an opposite trend in higher alcohol 
productivity with H2/CO was observed under the supercritical hexanes phase 
conditions. Among all the conditions studied, the presence of the supercritical 
hexanes reaction medium combined with a syngas H2/CO ratio of 1 resulted in 
maximum higher alcohol productivity.  
 
Keywords: Copper-Cobalt catalyst; Higher alcohol synthesis; H2/CO ratio; Supercritical fluid; 
Supercritical reaction media 
5.1 Introduction 
Higher alcohol synthesis (HAS) has long been developed as one of the primary pathways for 
converting syngas (a mixture of CO and H2) in to valuable fuels and fuel additives. As the 
demand for alternative fuels increases, the higher alcohol synthesis (HAS) process is becoming 
increasingly more important and is receiving attention from both academic and industrial sectors. 
Syngas can be derived from a variety of carbonaceous resources, such as coal, natural gas and 
biomass, via gasification or partial oxidation. The H2/CO ratio of a certain syngas is determined 
by the type of the carbonaceous resource used and the processing technology and conditions 
 104
employed. For instance, depending on the gasification method employed, syngas produced from 
biomass can have a broad range of H2/CO ratios that vary from 0.45 to 2 (Faaij, Hamelinck, & 
Hardeveld, 2002).  As another example, the H2/CO ratio of syngas produced from certain coal 
species is correlated to the H/C ratio of the coal, with the H2/CO ratio regularly close to 1 (Cao, 
Gao, Jin, Zhou, & Cohron, 2008). In addition, syngas can also be derived from natural gas via 
steam reforming or partial oxidation. The H2/CO ratio of the syngas produced from the steam 
reforming process is typically rich in hydrogen (e.g. it can be as high as H2/CO = 3); however, 
the H2/CO ratio of syngas formed in the partial oxidation process commonly varies from 1.5 to 2 
(Fong & Wilson, 1996). 
The H2/CO ratio is one of the operating variables that can significantly affect the catalytic 
behavior and the selectivity towards higher alcohols in the HAS process. As such, it is expected 
that the use of syngas that is derived directly from various carbonaceous resources would lead to 
different alcohol and other product distributions and yields in the HAS process. Boz et al. (1994) 
investigated the effect of varying the H2/CO ratio over a 0.5 wt% K promoted Cu/ZnO/Al2O3 
catalyst under the reaction conditions of 4 MPa, 290 ?C and 3000 h-1. The CO conversion was 
found to increase with an increase in the H2/CO ratio from 0.5 to 3.3.  Lower CO content was 
found to favor the formation of higher alcohols while also favoring the formation of 
hydrocarbons. Higher H2/CO ratios produced more methanol instead. Additionally, at a H2/CO 
ratio as low as 0.5, the overall activity was quite low, such that the productivity of alcohols was 
actually lower than the value obtained at an intermediate H2/CO ratio. Two possible reason were 
given a) low H2/CO ratio results in a limiting H2 availability, b) Cu is more likely to be reduced 
to Cu0 in the presence of excess CO. Forzatti et al. (1991) studied the effect of H2/CO feed ratio 
on the product distribution using a ZnCrCsO catalyst and found that a H2/CO ratio of 2 was the 
 105
optimal condition for the production of methanol. For the selectivity towards higher alcohols, 
however, the maximum was reported when the H2/CO ratio was ca. 1. Moreover, due to different 
side reactions involved in the HAS processes, especially the parallel water-gas shift reaction, the 
H2/CO ratio within the reactor changed as the reaction process goes on. Mahdavi et al. (2005) 
examined three H2/CO ratios over a Cu-Co catalyst at 285 ?C, 4 MPa and GHSV = 3410 h-1. 
They reported that the activity and selectivity towards alcohols were stable when the H2/CO ratio 
was changed from 2 to 1. A further decrease in H2/CO ratio to 0.5 decreased the activity rapidly. 
Methane selectivity increased significantly as H2/CO ratio decreased from 1 to 0.5.  
It has been shown that higher alcohol synthesis from syngas can benefit from the utilization of 
a supercritical fluid solvent as the reaction medium given that the HAS process is a highly 
exothermic polymerization reaction (Jiang et al., 2001; Qin, Liu, & Wang, 2004; Zhang et al., 
2006). Supercritical fluids have unique characteristics in terms of their thermodynamic and 
transport properties which are intermediate to those of a liquid and those of a gas (Baiker, 1999; 
Fan, Yokota, & Fujimoto, 1995; McHugh & Krukonis, 1994; Subramaniam & McHugh, 1986). 
For example, the density, which is typically less than that of a normal liquid, is remarkably 
higher than that of a gas.  As such, density dependent properties can be tuned in a supercritical 
fluid reaction medium in order to affect desired changes on catalytic reaction performance 
through improved heat transfer, mass transfer, as well as product and reactant solubilities, just to 
name a few. For instance, the presence of a supercritical fluid can significantly enhance the heat 
transfer rate in an exothermic heterogeneous catalytic process compared to gas phase operation. 
Accordingly, it has been shown that the use of supercritical hexanes as reaction media in HAS 
has significantly reduced the formation of CH4, which is thermodynamically favored (as shown 
in Chapter 3 and 4). As another example, the diffusivity of a supercritical fluid, while lower than 
 106
that of a gas, is significantly higher than that of a liquid and can provide improved transport 
properties in heterogeneous catalysis applications (Baiker, 1999; Elbashir et al., 2010; 
Subramaniam & McHugh, 1986). A supercritical fluid reaction medium can also offer enhanced 
solubility of certain reactants and products. Compounds that are largely insoluble in a fluid at 
ambient conditions can be made more soluble in the solvent medium at supercritical conditions 
and vice versa (Savage, Gopalan, Mizan, Martino, & Brock, 1995). These differences in the 
solubilities and diffusivities of H2 and CO can give rise to a consequence that the actual H2/CO 
ratios within the reactor differ substantially from that of the gas that is fed to the reactor. 
Since it is well known that the H2/CO ratio at the catalyst surface affects liquid product 
distribution, as well as the formation of free carbon (Stern, T. Bell, & Heinemann, 1983), it 
becomes quite important to establish the extent to which the presence of a supercritical fluid 
solvent influences the H2/CO ratio in the reaction media. In this work, higher alcohol synthesis 
was examined using syngas of different H2/CO ratios over a K promoted Cu-Co based catalyst. 
These studies were performed both with and without the presence of supercritical solvent. In 
particular, the effect of H2/CO ratio on the distribution of higher alcohol products and the 
conversion of CO has been studied.  
5.2 Experimental Section 
5.2.1 Catalyst preparation 
The preparation of the Cu-Co based catalysts employed in this study involves the continuous 
coprecipitation of nitrate solutions of respective metals under controlled conditions. A 
Cu/Co/Zn/Al mixed nitrate salt solution was first prepared at a mass ratio of 43.5:14.5:24:18. 
The precursor was prepared by co-precipitating this mixed nitrate salt solution with a 1.0 M 
K2CO3 solution in a 1L flask, which was initially filled with 200 ml of deionized water at 80 ?C. 
 107
The pH value was strictly kept at 7 in order to achieve a homogeneous dispersion. After aging 
for an hour, the precipitate was decanted and then washed with sufficient deionized water at 
80 ?C in order to eliminate the excess K introduced with the carbonate. The moisturized 
precursor (filter cake) was dried at 80 ?C and ambient pressure for 24 hours. The dried 
precipitate was then ground and calcined at 350 ?C under continuous air flow to give the 
corresponding mixed oxides. The promotion of K was carried out through the incipient wetness 
method using an aqueous potassium carbonate solution. The slurry paste was then dried at 80 ?C 
overnight and calcined in air at 350 ?C for 4 hours. 
5.2.2 HAS Catalytic Testing 
H2/CO ratio studies of higher alcohol synthesis from syngas over the Cu-Co catalyst were 
carried out using a stainless steel fixed-bed reactor. Four syngas mixtures of varying 
compositions were prepared by Airgas, with the H2/CO ratios ranging from 1 to 2, as shown in 
Table 5.1. N2, with a consistent concentration of 4.0%, was used as an internal standard in order 
to calculate the molar flow rate of the effluent. 
Table 5.1 Syngas inlet compositions with respect to different H2/CO ratio 
H2/CO ratio H2 (mol%) CO (mol%) N2  (mol%) 
1.00 48.0 48.0 4.0 
1.35 55.2 40.8 4.0 
1.75 61.1 34.9 4.0 
2.00 64.0 32.0 4.0 
 
One gram of catalyst (45-70 mesh) diluted with 2.5 ml of the same sized glass beads was 
loaded into the high pressure reactor. A mixture of hexane isomers (refer to as hexanes, 
purchased from VWR) was selected as the inert supercritical reaction medium. The standard 
conditions were as follows: total pressure = 4.5 MPa in gas phase operation, total pressure = 13.5 
 108
MPa in SCF phase operation, partial pressure of syngas = 4.5 MPa in both gas phase and SCF 
phase operation, T = 290 ?C, syngas flow rate = 50 sccm, hexanes pumping rate = 0.51 ml/min 
(only in SCF phase operation), H2/CO ratio = 2, hexanes/syngas molar ratio = 2 (only in SCF 
phase operation). 
The effluent from the reactor was passed through a hot trap, a heat exchanger, and a cold trap, 
in which it was separated into gas phase and liquid phase product streams. The gas phase 
products were analyzed using a Varian CP-3380 Gas Chromatograph equipped with a Haysep-
DB column and a TCD detector. The liquid phase products were analyzed using a Bruker 430 
Gas Chromatograph equipped with a capillary DB-wax column and a FID detector.  
In the gas phase HAS experiments, 200 ml hexanes were injected into the cold trap in order to 
facilitate collection of the volatile alcohol products. This process was repeated after each liquid 
sample collection. When switching from one H2/CO ratio to another in the gas phase operation, 
the residual products inside the catalyst and/or reactor were extracted with hexanes at reaction 
temperature and pressure for 1 hour. These recovered products were also analyzed by GC-FID in 
order to determine whether heavy products were retained in the catalyst bed during gas phase 
operation.   
5.3 Results and Discussion 
The effect of H2/CO ratio on higher alcohol synthesis was investigated under gas phase 
conditions where the H2/CO ratio was maintained at the different values of 2.0, 1.75, 1.35 and 
1.0, respectively. Figure 5.1 shows the CO conversion, H2 conversion, CO2 selectivity and CH4 
selectivity in this gas phase higher alcohol synthesis (GP-HAS) experiment under these four 
H2/CO molar reaction conditions. It must be noted that the conversion and selectivity are the 
average values of the data obtained once the reaction had reached steady state (as evidenced by 
 109
stable CO conversion). As shown in Figure 5.1, the CO conversion increased considerably from 
22.4% at H2/CO ratio = 1 to 47.6% at H2/CO ratio = 2. H2 conversion shows a similar trend but 
changed less dramatically from 10.8% to 17.1% when the H2/CO ratio was changed from the 
value of 1 to 2. CO2 selectivity was stable with the change in H2/CO ratio. It is interesting to note 
that under gas phase conditions the CH4 selectivity exhibited a maximum at an intermediate 
H2/CO ratio, approximately between the values of 1.35 and 1.75.  
 
Figure 5.1 Effect of H2/CO ratio on conversion and gas product selectivity in GP-HAS. The reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas 
flow rate = 50 sccm, temperature = 290 ?C, pressure = 4.5 MPa 
The effect of H2/CO ratio on the reaction performance of higher alcohol synthesis under 
supercritical hexanes phase conditions (SC-HAS) is shown in Figure 5.2. It can be seen that the 
CO conversion remained stable in the range of 30.0% to 35.0% under supercritical conditions 
when changing the H2/CO ratio from 1 to 2, while the CO conversion under gas phase conditions 
increased monotonically from a value that was lower than supercritical phase at H2/CO = 1 to a 
value considerably higher than supercritical phase at H2/CO = 2. Fan and coworkers (Fan et al., 
0%
10%
20%
30%
40%
0%
10%
20%
30%
40%
50%
0.75 1 1.25 1.5 1.75 2 2.25
Se
lec
tiv
ity
  (%
)
Co
nv
er
sio
n (
%)
H2/CO molar ratio
CO Conversion CO? Selectivity
H? Conversion CH? Selectivity
 110
1995) observed a lower CO conversion in the supercritical-phase FTS reaction compared to the 
gas-phase FTS reaction and attributed this to the fact that the diffusion of synthesis gas in the 
supercritical phase was slower than in the gas phase. A similar trend was observed in this HAS 
experiment when the H2/CO ratio was maintained at 1.75 or 2.0. However, at the H2/CO ratio of 
1.0 or 1.35, the opposite trend was observed where the CO conversion in SC-HAS was actually 
higher than that obtained in GP-HAS, illustrating that this cannot be the effect of differences in 
diffusion alone. The H2 conversion also follows a similar trend when the H2/CO ratio was 
changed from 1 to 2 in SC-HAS, where the H2 conversion was relatively steady across all of the 
H2/CO ratios employed. In addition, CO2 selectivity decreased slightly as the H2/CO ratio 
increased from 1 to 2 in the SC-HAS experiment. However, the CH4 selectivity was found to 
increase remarkably in SC-HAS from 5.3% at H2/CO ratio = 1 to a value of 13.4% at H2/CO 
ratio = 2.  
 
Figure 5.2 Effect of H2/CO ratio on conversion and gas product selectivity in SC-HAS. The reaction 
conditions were as follows: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, syngas 
flow rate = 50 sccm, temperature = 290 ?C, pressure = 13.5 MPa, syngas partial pressure 
= 4.5 MPa, Hexanes/syngas molar ratio = 2 
0%
10%
20%
30%
40%
0%
10%
20%
30%
40%
50%
0.75 1 1.25 1.5 1.75 2 2.25
Se
lec
tiv
ity
  (%
)
Co
nv
er
sio
n (
%)
H2/CO molar ratio
CO Conversion CO? Selectivity
H? Conversion CH? Selectivity
 111
Table 5.2 demonstrates the selectivity towards alcohol products and hydrocarbon products as 
a function of the H2/CO ratio in GP-HAS. Under gas phase operation, alcohol products were 
comprised of C1-C8 linear mixed alcohols, and hydrocarbons were collected up to C20 n-paraffin. 
It can be seen that selectivity towards alcohol products increased from 2.98% to 5.71% when the 
H2/CO ratio increased from 1 to 2 in gas phase HAS. Selectivity towards C8-C20 n-paraffin did 
not demonstrate a clear trend as the H2/CO ratio was increased from 1 to 2, and was in the range 
of 2.5% to 6.5%. In addition, Table 5.2 also presents the alcohol product distribution in terms of 
alcohol weight percentage (ROH wt %), which is defined as the weight of alcohols having a 
certain carbon number over the weight of total alcohol products. It has been found that this Cu-
Co based catalyst has a good selectivity towards higher alcohols, with methanol selectivity lower 
than 6% for the conditions studied in the gas phase.  
Table 5.2 Catalytic performance as a function of H2/CO ratio in gas phase HAS 
H2/CO 
Molar Ratio CO Conversion (%) 
Selectivity (%) ROH (wt.%) ? 
ROH HC C1 C2 C3 C4 C5+ ROH HC 
1.00 22.5 2.98 2.59 3.93 25.52 32.38 27.28 10.90 0.35 0.62 
1.35 28.4 3.51 6.37 3.12 25.65 30.50 25.68 15.05 0.42 0.60 
1.75 24.7 3.91 2.55 5.63 32.21 33.78 19.38 8.99 0.36 0.61 
2.00 47.6 5.71 5.69 3.06 23.72 30.63 23.68 18.91 0.49 0.59 
Reaction conditions: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, T = 290 ?C, P = 4.5 MPa, syngas flow 
rate = 50 sccm/gcat. 
ROH: C1-C8 alcohols 
HC: C8-C20 hydrocarbons 
 
In contrast, Table 5.3 shows the catalytic performance in SC-HAS as a function of H2/CO 
ratio. It can be seen that selectivity towards alcohol products slightly decreased from 15.09% 
when H2/CO ratio =1 to a value of 12.61% when H2/CO ratio = 2. Table 5.3 also lists the alcohol 
products distribution as a function of H2/CO ratio. Increasing the H2/CO ratio under supercritical 
hexanes phase conditions favors the formation of methanol and ethanol. Little to no change was 
observed in the formation of C3 and C4 alcohols when H2/CO ratio increased from 1 to 2. 
 112
However, C5+ alcohol decreased significantly with the increase in H2/CO ratio. Table 5.3 also 
presents the selectivity towards hydrocarbon products in SC-HAS. Under supercritical hexanes 
phase conditions, up to C24 linear paraffin was observed in the FID chromatogram, suggesting 
that the presence of supercritical hexanes facilitates the extraction of longer carbon chain 
hydrocarbons (C20 to C24) from the catalytic sites.  
Table 5.3 Catalytic performance as a function of H2/CO ratio in SC-HAS 
H2/CO 
Molar Ratio CO Conversion (%) 
Selectivity (%) ROH (wt.%) ? 
ROH HC C1 C2 C3 C4 C5+ ROH HC 
1.00 33.6 15.09 16.20 2.11 12.97 22.53 22.37 40.01 0.70 0.81 
1.35 29.1 13.44 7.11 4.23 17.21 25.53 23.03 30.00 0.62 0.78 
1.75 35.8 12.65 7.70 8.30 21.12 23.13 20.91 26.55 0.59 0.80 
2.00 36.1 12.61 7.87 9.39 21.86 21.89 21.46 25.40 0.58 0.78 
Reaction conditions: catalyst = 0.5 wt% K promoted Cu/Co/ZnO/Al2O3, T = 290 ?C, P = 13.5 MPa, Psyngas = 4.5 
MPa, syngas flow rate = 50 sccm/gcat, hexanes/syngas molar ratio = 2. 
ROH: C1-C8 alcohols 
HC: C8-C24 hydrocarbons 
 
The observation that the production of higher alcohols can be enhanced by the presence of 
supercritical solvent in the HAS process has been reported by our group and others previously 
(Fang et al., 2009; Qin et al., 2004). A comparison of the data in Table 2 and Table 3 illustrates 
that the ROH selectivity in SC-HAS was significantly higher (average of 13.5%) than the ROH 
selectivity observed in GP-HAS (average of 4.0%), confirming these prior observations. It is also 
worth pointing out that under supercritical hexanes phase conditions the lower H2/CO ratios 
actually promoted the formation of higher alcohols significantly. The difference in the effect that 
changes in the H2/CO ratio can have on the formation of higher alcohols in GP-HAS compared 
to SC-HAS can also be seen in the different trends that are observed for the higher alcohol 
productivity, as shown in Figures 3 and 4. Figure 3 demonstrates that with the decrease in the 
H2/CO ratio from 2 to 1, the productivity of higher alcohols decreased almost monotonically in 
GP-HAS. The stoichiometry of alcohol synthesis from syngas reveals that two moles of H2 and 
 113
one mole of CO are required for the synthesis of alcohols heavier than methanol. However, due 
to a multitude of side reactions that occur in parallel to the HAS process, such as the water-gas-
shift reaction, the optimum H2/CO ratio in actual practice may be different from the theoretical 
value of 2. It has been reported by several investigators that the optimum H2/CO ratio for HAS is 
below the stoichiometric value of 2 (Forzatti et al., 1991; Mahdavi et al., 2005). In general, 
decreasing the H2/CO ratio while maintaining the same reaction pressure will result in an 
increase in the CO partial pressure. Higher CO partial pressure would favor the CO insertion and 
promote the C-C chain growth in the HAS process, thereby resulting in an increased production 
of higher alcohols and/or hydrocarbons (X. Xu et al., 1987). However, higher H2 partial pressure 
(higher H2/CO ratio) could diminish coke formation, thereby sustaining catalytic activity. From 
Figure 5.3, it can be observed that of the four reaction conditions applied in this study, the H2/CO 
ratio of 2 seems to be the optimum value in GP-HAS for the production of each of the C2 to C8 
higher alcohols . This can primarily be attributed to the higher CO conversion (ca. 47.6%) at the 
H2/CO ratio of 2, compared to the much lower CO conversion (ca. 22.5%) at the H2/CO ratio of 
1. An opposite trend in the productivity of each higher alcohol is observed in SC-HAS as a 
function of the H2/CO ratio, as shown in Figure 5.4. Among the H2/CO ratios studied, the 
productivity of higher alcohols was much higher in SC-HAS than in GP-HAS. When the H2/CO 
ratio decreased from 2 to 1, a monotonic increase in the formation of higher alcohols was 
observed, as demonstrated in Figure 5.4. Combined with the fact that the CO conversion was 
relatively stable with respect to the change in H2/CO ratios in SC-HAS, it can be inferred that the 
presence of the supercritical solvent as a reaction medium further enhanced the carbon chain 
growth process at low H2/CO ratios. Further evidence of this can be found in the fact that the C5+ 
alcohol weight percentage under SC-HAS conditions was significantly higher than that observed 
 114
in GP-HAS conditions (e.g. 40.01% in SC-HAS versus 10.90% in GP-HAS when the H2/CO 
ratio = 1). Increased higher alcohol formation under supercritical conditions and at low H2/CO 
ratios is also demonstrated by the differences between the carbon chain growth probability 
factors for both GP-HAS and SC-HAS, as discussed below.  
 
Figure 5.3 Effect of H2/CO ratios on the productivity of higher alcohols under gas phase conditions. 
The reaction conditions were as follows: catalyst = 0.5 wt% K promoted 
Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 290 ?C, pressure = 4.5 
MPa 
0.0
4.0
8.0
12.0
2.00
1.75
1.35
1.00
Productivity
(g/kgcat/h)
H2/CO
molar ratio
 115
 
Figure 5.4 Effect of H2/CO ratios on the productivity of higher alcohols under supercritical hexanes 
phase conditions. The reaction conditions were as follows: catalyst = 0.5 wt% K 
promoted Cu/Co/ZnO/Al2O3, syngas flow rate = 50 sccm, temperature = 290 ?C, pressure 
= 13.5 MPa, syngas partial pressure = 4.5 MPa, Hexanes/syngas molar ratio = 2 
The alcohol products produced over Cu-Co catalysts have been reported to obey the Schulz-
Flory distribution (Forzatti et al., 1991; Herman, 2000; X. Xu et al., 1987). The carbon chain 
growth probability factor (? value) is commonly derived from the slope of a plot of ln(Wt/n) vs. 
n, where Wt is the weight percentage of a certain alcohol product and n is the carbon number of 
this product. Bailliard et al. (1989) reported a same ? value (ca. 0.45) for both the alcohol and 
hydrocarbon products, indicating that the formation of alcohols and hydrocarbons occurs in 
parallel. In the current study, as shown in Table 2, an ? value of alcohol formation was found to 
be 0.49 at the standard reaction conditions (H2/CO ratio of 2 in GP-HAS), while the ? value of 
hydrocarbon formation was slightly higher (ca. 0.59). As the H2/CO ratio decreased from 2 to 1, 
little to no difference was observed in the ? values for both the alcohol and hydrocarbon products 
in GP-HAS. However, very different values and trends for the carbon chain growth probability 
factor were observed in SC-HAS with respect to changes in the H2/CO ratio, as shown in Table 3. 
0.0
4.0
8.0
12.0
1.00
1.35
1.75
2.00
H2/CO
molar ratio
Productivity 
(g/kgcat/h)
 116
Compared to ? = 0.49 for higher alcohols in GP-HAS, an alpha value of 0.58 was found in SC-
HAS for the same H2/CO ratio of 2, illustrating a higher probability for carbon chain growth. 
This difference is even more obvious in the ? values for the formation of hydrocarbons, where an 
? = 0.78 was obtained in SC-HAS versus an ? = 0.59 in GP-HAS at the same H2/CO ratio of 2. 
Furthermore, the decrease in the H2/CO ratio from 2 to 1 under supercritical hexanes phase 
conditions resulted in a further increase in the ? value for the formation of alcohols (from 0.58 to 
0.70), as shown in Table 5.3. Elbashir and Roberts (Elbashir et al., 2005) found that availability 
of active sites increased in supercritical Fischer-Tropsch synthesis relative to gas phase operation, 
and that the supercritical media may promote both the adsorption of the reactant molecules (CO 
and H2) and possible incorporation of primary products (? olefins) into the chain growth process.  
Based on the results presented above, the presence of supercritical hexanes may similarly 
improve the accessibility of catalyst active sites in the SC-HAS process thereby resulting in 
improved production of higher alcohols. As a result, when the partial pressure of CO increased 
(H2/CO ratio decreased from 2 to 1), the CO insertion reaction pathway was promoted. As such, 
the ? value of alcohol products increased from 0.58 to 0.70. Hydrocarbon formation, however, 
was not affected by the change in H2/CO ratio under both gas phase and the supercritical hexanes 
phase conditions, though it should be noted that the value was significantly higher under 
supercritical phase conditions in all instances.  
 
 
 117
5.4 Conclusion 
In the present study we investigated the effect of H2/CO ratio on the synthesis of higher 
alcohols under both gas phase and supercritical hexanes phase conditions. From our study, four 
conclusions have been drawn as listed below: 
1. Under gas phase conditions, the CO conversion decreased with decreases in the H2/CO 
ratio from 2 to 1. However, under supercritical hexanes phase conditions, the CO 
conversion remained quite stable with changes in H2/CO ratio. 
2. Changes in H2/CO ratio had little effect on CO2 selectivity in GP-HAS and slightly 
influenced CO2 selectivity in SC-HAS, indicating that the water gas shift reaction 
approached equilibrium at the conditions studied. CH4 selectivity exhibited a 
maximum at moderate H2/CO ratios in GP-HAS, but decreased significantly with 
decreasing H2/CO in SC-HAS.  
3. Under gas phase conditions, higher H2/CO ratios favor the productivity and selectivity 
of higher alcohols by promoting CO hydrogenation. Under supercritical hexanes phase 
conditions, however, a lower H2/CO ratio resulted in much higher alcohol productivity 
and enhanced carbon chain growth probability factor.  
4. Linear alcohols and hydrocarbon products followed the Schulz-Flory distribution. 
Much higher carbon chain growth probability factors for both the alcohol and the 
hydrocarbon formation were observed in SC-HAS compared to the GP-HAS at the 
same reaction conditions. An increase in the carbon chain growth probability factor for 
alcohol formation was found as the H2/CO ratio decreased from 2 to 1 under 
supercritical hexanes phase conditions. This change can be attributed to improved CO 
insertion resulting from enhanced CO partial pressure in SC-HAS. 
 118
The benefits of using supercritical hexanes as a reaction medium was confirmed by 
comparing SC-HAS with GP-HAS under the same operating conditions and H2/CO ratios. The 
total alcohol selectivity in SC-HAS is remarkably higher than that obtained in GP-HAS, 
indicating that the supercritical hexanes facilitated the removal of alcohol products from the 
active sites of the catalyst and therefore promoted the conversion of syngas into alcohols.  
 
 
 
 
 
  
 119
Chapter 6 Future work 
6.1 Catalyst characterization of the fresh and spent catalysts 
In chapter 4&5, we have shown that the presence of supercritical hexanes as a reaction 
medium can promote the carbon chain growth probability factors over this Cu-Co based catalyst, 
and significantly reduce the CH4 formation. However, it has been reported that the activity of 
Cu-Co based catalysts (usually referred to as IFP type catalysts) highly depends on the 
homogeneity of the dispersion of Cu and Co, which can be affected by various step in 
preparation and reduction process (Courty et al., 1988, 1982). The use of STEM-EDS would 
allow us to be able to determine the extent of interaction between Cu and Co. The comparison of 
the EDS results of catalysts used for SC-HAS and GP-HAS might provide more evidence from a 
characterization perspective that the supercritical solvent improves the catalyst maintenance. 
Moreover, TEM, XPS and ICP would also be interesting to utilize to garner further fundamental 
understanding of the catalyst characteristics under these operating conditions. 
6.2 Modified catalyst for supercritical higher alcohol synthesis from syngas 
As discussed in chapter 1, there are four types of catalysts that have been under development 
for the synthesis of higher alcohols. This works has focused on the modified low temperature 
methanol synthesis catalyst (Cu-based) and the Cu-base catalyst modified with Fischer-Tropsch 
synthsis element (Cu-Co based). The Rh modified catalyst has also been shown to provide a high 
selectivity towards both higher alcohols while also generating significantly more CH4 under gas 
phase conditions.  Since it has been proven that the presence of supercritical hexanes would 
result in a significant reduction in the CH4 formation over both the Cu based and Cu-Co based 
catalyst in this study, it would be valuable to investigate the application of a supercritical solvent 
when using a Rh modified catalyst system in HAS.  
 120
6.3 Comparison of supercritical phase and slurry phase higher alcohol synthesis 
Besides the exploration of the catalysts, the innovation of different reactor and process 
technology is also important in order to further improve yields and selectivity. In the current 
work, the catalytic performance in supercritical phase higher alcohol synthesis (SC-HAS) has 
been compared with the performance in gas phase higher alcohol synthesis (GP-HAS). Benefits 
of introducing supercritical hexanes in to HAS as a reaction medium has been partially attributed 
to the enhance heat transfer provided by the supercritical solvent. Similarly, because of the 
highly exothermic nature of the HAS process, slurry phase reactor systems, which also provide 
improved heat transfer capability, would be interesting to investigate and to further compare with 
SC-HAS over a same catalyst.  
By operating the same catalyst in each reaction media (gas-phase, supercritical, and slurry 
phase) at the same CO conversion levels, the function of the supercritical solvent would be more 
distinguishable and therefore better understand.  
6.4 Dual catalyst bed reactor design 
In chapter 3, it has been demonstrated that the formation of methanol was enhanced 
significantly by the presence of supercritical hexanes over the traditional low temperature 
methanol synthesis catalyst, namely 0.5 wt% promoted Cu/ZnO/Al2O3 catalyst. Additionally, in 
chapter 4, it has been shown that by adding cobalt into the Cu based methanol synthesis catalyst 
and applying the supercritical hexanes as a reaction medium, the formation of higher alcohols 
has been enhanced. Combining these two observations with previously reported dual reactor bed 
configuration concept (Herman, 2000), it would be interesting to develop a set of experiments on 
a supercritical hexanes phase higher alcohol synthesis using a dual catalyst bed reactor system. 
 121
The idea of this configuration is to maximize the formation of methanol from syngas in the 
first bed, and convert methanol into the higher alcohols as much as possible at the second bed. 
Beretta et al. (1995) investigated a dual bed system using Cs/Cu/ZnO/Cr2O3 catalyst in the first 
bed and Cs/ZnO/Cr2O3 catalyst in the second bed. They found the isobutanol productivity was 
more than doubled compared with only using the Cs/Cu/ZnO/Cr2O3 catalyst in a single bed 
operation. In their reaction scheme, methanol reacted with ethanol or propanol in the second 
catalyst bed via aldol condensation over the Cs/ZnO/Cr2O3 catalyst.  
By using Cu-Co based catalyst (the same catalyst in Chapter 4&5) in the second catalyst bed 
and introducing the supercritical hexanes as a reaction medium, methanol whose productivity has 
been maximized in the first bed, would be expected to readsorb onto the Cu-Co active sites and 
be converted into higher alcohols via CO insertion. If this assumption proved true, then the 
formation of a certain higher alcohol (i.e. certain carbon number) could be maximized by 
adjusting the bed sizes and other reaction conditions, such as GHSV, temperature, and H2/CO 
molar ratio, etc. 
6.5 Mechanistic study of higher alcohol synthesis over the Cu-Co catalysts 
In chapter 5, the formation of higher alcohols in SC-HAS is enhanced by the presence of 
supercritical hexanes. The carbon chain growth probability factors of alcohol was found distinct 
from the values of hydrocarbon in both SC-HAS and GP-HAS. A clear understanding of the 
carbon chain growth intermediate and fundamental mechanism would be extraordinarily 
meaningful for elucidating the beneficial effect of the supercritical solvent on formation of 
higher alcohols. 
 122
6.6 Phase behavior study of the reaction mixtures in HAS  
The composition of the reaction mixture varies along the length of the reactor due to 
conversion of the syngas to various products thereby causing the critical properties of the 
mixture to continually change as the reaction proceeds. As such these reaction studies need to be 
complemented with investigations of the phase-behavior of the reaction mixture at various 
conversions and conditions in order to gain an understanding of the fundamentals of the 
thermodynamics and kinetics of the HAS chemical reactions in solution. The SCF-HAS mixture 
consists of a solvent (e.g. hexane), reactant gases (CO, H2), light hydrocarbons (CH4, CO2), and 
light alcohols (C1-C4 alcohols). As a result, near-critical and supercritical phase HAS operations 
are governed not only by the properties of the supercritical fluid solvent, but also by the 
properties of the HAS mixture obtained when the process reaches steady state. Gao et al. (2001) 
reported critical points for hexane (> 94 mol %) and reactants (CO and H2) mixtures at different 
moles ratios. They observed that the critical temperature and critical density of the solvent-
reactant mixture decreased as the concentration of CO and H2 increased, whereby the critical 
pressure increased with CO and H2 concentration. In future studies, a series of mixtures that 
contain different compositions of CO, H2, CO2, CH4, hexanes(or other supercritical media) and 
C1-C4 alcohols should be investigated in order to better understand the phase behavior of the 
reaction mixtures along the length of the reactor. 
  
 123
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