8-05: Investigation of lignin deposition on cellulose during hydrothermal pretreatment, its effect on cellulose hydrolysis, and underlying mechanisms

During thermochemical pretreatments above the lignin glass transition temperature, lignin is believed to coalesce and form droplets, move out of the cell wall, and deposit back on the cellulose surface, where it can retard cellulose hydrolysis. Previous studies have shown that to improve the cellulose digestibility in cellulosic biomass, the decompartmentalization and/or relocalization of lignin during dilute acid and hydrothermal pretreatment is as important as lignin removal. However, studies of this nature are limited for hydrothermal pretreatment, with the result that the mechanism for hydrolysis slowdown is poorly understood. Therefore, in this study, batch hydrothermal pretreatments of Avicel PH-101 mixed with poplar wood as a lignin source were performed at two different temperatures to determine the mobility of lignin in forming “lignin droplets” that could impede cellulose enzymatic hydrolysis.  Following each pretreatment, Avicel cellulose solids were physically and chemically characterized and hydrolyzed with cellulase. SEM imaging and wet chemistry analysis confirmed that lignin from poplar wood relocated onto the surface of Avicel, and this lignin deposition had a strong negative impact on cellulose enzymatic hydrolysis. The structural composition of deposited lignin was also characterized using NMR. Based on these observations, a series of experiments were performed to test mechanisms that could explain how lignin deposited on cellulose surface retards enzymatic hydrolysis. The results provided new insights into the fate of lignin during biomass deconstruction, its effect on cellulose hydrolysis, and mechanisms that could explain hydrolysis slow down.