(461d) Update On the Redistribution of Lignin Caused by Dilute Acid Pretreatment of Biomass and Its Effect On the Enzymatic Digestion of Cellulose
The purpose of this research was to advance our understanding of the effect of pretreatment on the lignin component in biomass so that engineering research can be directed more efficiently. Research conducted at NREL over the past few years has shown that lignin undergoes a phase transition during thermochemical pretreatments conducted above its glass transition temperature. The lignin coalesces within the plant cell wall and appears as microscopic droplets on the surfaces of cells. In recent years, we have studied the redistribution of lignin that occurs during dilute acid pretreatment using small lab scale reactors operated under conditions that were not strictly process relevant. Typically, samples were pretreated in flow reactors with high liquid to solid ratios, with no acid, and for relatively long periods of time. However, microscopic analysis showed that changes were occurring both within the cell wall and on cell surfaces. It was clear that pretreatment could cause significant changes in the distribution of lignin. More recently we have studied the changes in lignin distribution that could occur under more process relevant conditions. Dilute acid pretreatments have been performed in a steam gun reactor at high solids loadings (43%), although at somewhat lower temperatures (150 and 170oC) and longer times (5-60 min) than normally used in our pilot scale reactors. Our studies have shown that lignin still undergoes a significant change in distribution and that coalescence and droplet formation still occur under these conditions. A method for selectively isolating these lignin droplets from the surface of pretreated cell walls using extraction with aqueous dioxane has been successfully employed. Characterization of the extracts by 2D NMR (HSQC) showed they contained carbohydrates and that the carbohydrates were covalently bound to the lignin in at least 2 types of lignin-carbohydrate complexes. Chemical analysis of the extracts showed the extracts contained only low levels of carbohydrates (2-6%) with the highest levels observed in the extracts from pretreatments performed at the lowest severity conditions. There was also evidence that the carbohydrates were at least oligomeric in size (i.e., not monomers or dimers). Based on this evidence, it appears likely that the frequency of the lignin-carbohydrate linkages found in the lignin extracts were low at about one cross-linkage per 80 to 100 lignin units. The effect of lignin redistribution on the digestibility of the pretreated solids was also tested, using the solids remaining after extracting the lignin droplets from the cell wall surfaces It was clear that removal of the droplets produced an increase in the digestibility of the pretreated corn stover. Whereas the digestibility of the cellulose was strongly correlated with the level of xylan removal, droplet removal increased cellulose conversion by about 15%. The improved digestibility could be due to a decrease in non-specific binding of the enzymes to the lignin in the droplets, or it could be because the droplets no longer blocked access to cellulose in the pretreated solids.