Conversion of biomass derived carbohydrate to value added chemicals by liquid-phase thermochemical and biochemical processing

Abstract

Levulinic and lactic acid are considered as highly versatile chemicals with a potential to be building-block for the synthesis of various upgraded chemicals. The main objective of this study is to develop novel strategies for efficient production of these two chemicals by liquid-phase processing of biomass-derived carbohydrates. In that event, the kinetic aspect of homogeneous acid catalyzed levulinic acid (LA) production from glucose in aqueous media was investigated at first. The experimental data collected from batch reactor over the following range of conditions: 150-200 °C, sulfuric acid concentrations of 1-5 (wt. %) and initial glucose concentrations of 5-15 (wt. %) were fitted to a kinetic model. The model has shown a good agreement with experimental data. The kinetic model derived here was further used to model continuous reactor system. The model suggests that high temperature and shorter reaction times are required to maximize hydroxymethylfurfural (HMF), an intermediate in the process. It also predicts low temperature and longer reaction times are essential to maximize LA yield. We also developed an optimum temperature profile for the levulinic acid production from glucose. This study thus can serve as a useful tool for optimal design and operation of acid catalyzed LA production from glucose. Literature information suggests catalytic conversion of glucose to LA involves multi-step complex reaction sequence. The known reaction pathway includes glucose to fructose to HMF to LA and various side-reactions. Each of these reactions are independent, thus behave differently depending on the type of acid catalyst. Specifically, Lewis-acid has much higher selectivity for the first reaction (glucose to fructose) whereas Brønsted-acid favors the latter reactions. For this reason, we have prepared a series of solid Lewis acid catalysts by Solid State Ion Exchange method (SSIE) and tested them for glucose to fructose isomerization. We also tested these catalysts together with a commercial solid acid catalyst (Amberlyst-15) to produce LA from glucose as Amberlyst-15 was found to be very effective for the production of LA from fructose in aqueous media. Among the Lewis solid acid catalysts tested, a large-pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aqueous media with high activity and selectivity. The dual-catalyst system, Sn-beta and Amberlyst -15, has improved the yield of LA due to enhanced isomerization of glucose to fructose by Sn-Beta. Although, Sn-beta zeolite catalyst loses its activity during the course of the reaction, it can be fully regenerated by calcination. Ambelyst-15 also suffers from deactivation. The deactivation is primarily due to humin deposit on the surface of the catalyst. The 2nd part of the dissertation deals with the bioconversion of lignocellulosic biomass. The one of the main objective of this task is to investigate the technical feasibility of one-step bioconversion of cellulosic mixed feedstock into lactic acid through simultaneous saccharification and co-fermentation (SSCF). On that ground, hemp hurd, an industrial byproduct, was investigated as a complementary feedstock to paper mill sludge for lactic acid production via SSCF. The feedstock mixture was processed by Cellic C-Tec2 enzyme and Lactobacillus pentosus (ATCC-8041). The mixing ratio plays an important role in the production of lactic acid as the pH of the product titer depends on the sufficient supply of sludge to maintain at optimum level. Calcium carbonate, which retain as a fraction of ash in sludge react with lactic acid producing calcium lactate as final product. The final lactic acid concentration was obtained 66 g/L, which corresponds lactic acid yield 0.82 g/g-sugar, under 12 % solid loading at mixing ratio of sludge: hemp hurd =80:20. In the final section, the efficacy of chlorine dioxide (ClO2) as a secondary pretreatment reagent was investigated. Lignin is believed to be a major hindrance to bioconversion as it surrounds carbohydrates (cellulose and hemicellulose) making it highly recalcitrant to enzymes and microorganisms. ClO2, a selective bleaching agent, have been used in pulp and paper industry to remove lignin very effectively. Here, two different types of two-step pretreatment process, alkaline followed by ClO2 and dilute acid followed by ClO2 were evaluated as a measure of enzymatic deconstruction of biomass to monomeric sugars. The effectiveness of the secondary treatment (ClO2) is highly sensitive to the primary treatment reagent whether it is alkali or acidic. Thus, alkaline followed by ClO2 improve glucan digestibility significantly and in some cases, xylan digestibility as well. On the contrary, dilute acid followed by ClO2 has shown negative effect on enzymatic hydrolysis. Lignin quantity, lignin distribution, lignin structure of the treated feedstocks and lignin, which depend on the primary treatment reactions mechanism, were found to be the major parameters affecting the overall efficiency of the process. In summary, the work of this dissertation provides first-hand knowledge regarding the technical feasibility for the acid catalyzed production of LA from glucose and biochemical production of lactic acid from mixed feedstocks.