Utilization of wood based hemicellulose pre-hydrolysate for the production of high value added platform chemicals

Abstract

The immense benefits of using renewable resources are because they can reduce global warming and other environmental issues which directly affect life on earth. Forest residues serve as potential renewable resources which can substitute the fossil based products with biobased products. In order to produce such renewable biobased products, biorefining concepts are needed to develop innovative and effective processes. However, biorefineries are at early stages of development and are directed towards making marketable products from forest residues. Development of efficient and economically viable methods is necessary. This thesis contributes to this endeavor by developing novel and efficient methods to produce valuable platform chemicals from a wood based resource, hemicellulose. In these studies, hemicellulose obtained from the poplar wood was used to produce xylitol, succinic acid and levulinic acid. They were listed among the top twelve chemicals which have the potential for high market demand, by the Department of Energy (DOE), US. The hemicellulose prehydrolysate used in this study was produced by a proprietary pretreatment process. As a pre-requirement to this study, I have analyzed its composition. Acid hydrolysis of the prehydrolysate resulted in a concentrated xylose rich hydrolysate. However, xylose undergoes several side reactions and results in undesired byproducts. In order to avoid the formation of these byproducts, the acid concentration was optimized to obtain a xylose rich hydrolysate with relatively low amounts of inhibitors. This hydrolysate was used in all the processes studied in this thesis. Microbial fermentation of the hydrolysates is inhibited by impurities in the media. In order to make the fermentation efficient, the detoxification (purification) of the hydrolysate has to be much effective. Therefore, a detoxification method was developed to remove the impurities such as furfural and acetic acid with minimal loss of xylose. This detoxification method includes the combination of vacuum evaporation and solvent extraction procedures. The fermentation of the hydrolysate detoxified with the developed process found to produce high yields of xylitol (0.59 g/g). Along with the hydrolysis and detoxification procedures, the conversion method was also improved further by using immobilized yeast strains. In these experiments, two yeast strains (Candida guilliermondii and Candida tropicalis) were used to produce xylitol. Candida tropicalis strain used in this study is a new strain isolated from a decaying wood biomass. The yield (0.92 g/g) and productivities (0.88 g/L/h) of xylitol obtained with the immobilized form of this yeast were significant and found to be higher than those reported in literature. It is evident that the hydrolysis, detoxification and fermentation methods developed are efficient in improving the yields of xylitol from hemicellulose prehydrolysates. Subsequently, the hydrolysed hemicellulose was used for the production of succinic acid (SA) by chemical routes. It was found that an important byproduct, furfural, formed during the acid hydrolysis of hemicellulose, can be converted to succinic acid. Therefore, a simple and facile method was developed to convert hemicellulose to furfural which was subsequently converted to succinic acid. Several challenges such as preventing humin formation, determination of an ideal solvent for furfural extraction etc. were addressed in this work. A heterogeneous acid catalyst, Amberlyst 15 used in this study provides an advantage of recycling. The ratio of catalyst to the substrate was also investigated to determine the optimum amount of catalyst loading. A biphasic system was developed for simultaneous production, separation and oxidation of furfural to succinic acid. These studies demonstrate the conversion of the hydrolysate into furfural and then to succinic acid with acceptable yields (49%) in the biphasic system. Another platform chemical, levulinic acid, was also studied for its production from hemicellulose. Levulinic acid can be produced by the acid catalysed reaction of furfuryl alcohol (FA), which can be produced from the hemicellulose derived furfural. In our study, ethanol was found to be a better solvent for the conversion of furfuryl alcohol to the ester of levulinic acid, ethyl levulinate (EL). A homogeneous acid (sulfuric acid) and a heterogeneous acid (Amberlyst 15) catalysts were compared in these studies and it was found that both are effective in ethanol than in water. In order to understand the conversion process, to improve and control the reaction, it necessary to know the actual reaction mechanism. The main focus of this study was to determine the reaction mechanism of this conversion process in the presence of sulfuric acid in ethanol. The intermediates were isolated and identified using nuclear magnetic resonance (NMR) spectroscopy and gas chromatography mass spectroscopy (GCMS). Though several intermediates were found, four intermediates could be identified based on GCMS and NMR data. Based on the interpreted intermediates, a reaction mechanism for the conversion of FA to EL was proposed. Besides, the molar yield of ethyl levulinate obtained from FA was found to be 85 – 90 % in ethanol. Based on the results obtained in this study, it is apparent that the high value products studied in this thesis can be produced from a cheap renewable resource, hemicellulose. The contribution of such processes, if bolted on the existing processes, can make the overall integrated biorefining process economically feasible.