Value-added Bioproducts Development from Lignocellulosic Biomass
Abstract
Alternative fuels and chemicals from renewable biomass can be used to reduce U.S. dependence on foreign oil, increase national energy security, and address environmental challenges. While renewable biomass can be used for the production of biofuels and value-added bioproducts, the key impediment to commercialization is the lack of a cost-effective process for converting biomass into biofuels. Therefore, development of value-added bioproducts from lignocellulosic biomass is critically needed for biofuels development. The production of lactic acid and acrylic acid from biomass hemicellulose can generate extra revenue for the biorefinery process, which can grow the bio-based industries potentially and make the biofuels production economically viable. Pretreatment is required to disrupt the cellulose-lignin matrix in lignocellulose and enhance the accessibility to digestive enzymes, but residual lignin and xylan may decrease the efficiency of enzymatic hydrolysis. However, the interactions between lignin/xylan and cellulolytic enzymes are not very well understood. My initial work was therefore concerned with quantifying and correlating the residual lignin/xylan content with enzymatic hydrolysis rates and yields. It was found that a higher residual xylan (9.7%) in organosolv pretreated Sweetgum (OPSG) resulted in a slower initial hydrolysis rate (1.19 g/L/h), and a higher amount of residual lignin (18.6%) in organosolv pretreated Loblolly Pine (OPLP) resulted in a lower final hydrolysis yield of glucan (76.4%). A more accurate fundamental understanding of the roles of xylan and lignin in limiting the enzymatic hydrolysis has been developed showing that the initial hydrolysis rate is dominated by the xylan content whereas the final hydrolysis yield is controlled by lignin content. The catalytic conversion of hemicellulose sugars to lactic acid under alkaline conditions was investigated. It was observed that the kinetics of sugars degradation under aqueous alkaline condition followed a pseudo-first-order reaction based on the overall interconverted isomers. The yields of lactic acid from each sugar in 0.5 M NaOH at 60°C were 55% (fructose), 46% (glucose), 38% (mannose), and 35% (xylose). Meanwhile, we found that Ba(OH)2 significantly increased the yield of lactic acid from xylose from 33 to 46%. In addition, the production of gluconic acid and xylonic acid was studied by microbial fermentation with Gluconobacter oxydans ATCC621. The presence of xylonic acid resulted in strong inhibition of glucose and xylose fermentation, which can be alleviated by addition of CaCO3. Finally, cellulose nanocrystals (CNCs) were prepared from sulfuric acid (H2SO4) and hydrochloric acid (HCl) hydrolysis of Avicel and filter paper and characterized for the development of potential piezoelectric materials. The H2SO4 treated CNCs (H2SO4-CNCs) and HCl treated CNCs (HCl-CNCs) were characterized and compared based on their morphology, thermo-stability, and surface charge. The H2SO4-CNCs isolated from Avicel and filter paper resulted in a different lytropic phase behavior indicated by their birefringence patterns. Time domain NMR was used to analyze the interaction of water molecules with CNCs. Three locations of water molecules were identified in the CNCs. In addition, the CNCs with negatively charged groups strongly impeded the enzymatic hydrolysis. Finally, chemical modifications with acetic anhydride (Ac2O) and polyisobutylene succinic anhydride (PIBSA) were carried out to improve the dispersibility and compatibility of CNCs.