Synthesis of Methanol and Higher Alcohols from Syngas over K Promoted Cu Based Catalysts in Supercritical Solvent
Type of Degreedissertation
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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 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.
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