Elucidating the mechanism of densifying lignocellulosic biomass with acidic chemicals (DLC) for lignocellulosic biorefinery

Abstract

DLC pretreatment is a novel and efficient pretreatment method for lignocellulosic biomass, but its mechanism of action has not been illustrated yet. In this study, a systematic investigation was conducted to find that the synergy of acidic chemical and mechanical actions during DLC pretreatment played an important role to ensure excellent performance. Mechanical actions likely exposed the internal polysaccharides of biomass by disrupting the cell wall structure, and the exposed polysaccharides reacted with concentrated acid until they were partially deconstructed, thereby significantly reducing the severity of subsequent dilute acid treatment. This synergism increased the accessible breakpoints of crystalline cellulose to cellulase by reducing the cellulose crystallinity and polymerization degree, as well as by digesting amorphous cellulose. Additionally, the large removal (about 60%) of hemicellulose increased biomass porosity. The native lignin structure was well preserved without the generation of pseudo-lignin and large amounts of toxic degradation products. These properties contribute to the high sugar recovery (97.2%), great digestibility (83.4% total yield of monosaccharides), and outstanding fermentability of DLC biomass; these results are not achievable for traditional dilute acid pretreatment. Simultaneous saccharification and co-fermentation of DLC-pretreated rice straw resulted in an ethanol titer as high as 82.9 g L⁻¹ without detoxification. Detailed structural analysis revealed that the well-preserved inter-unit linkages (>80%) and non-condensed subunits remained in the DLC lignin. This study revealed the mechanism of DLC technology in promoting biomass conversion, and thus, provides guidance to potentially realize efficient and economical biorefinery.