Fractionation and characterization of lignin streams from pretreatment of engineered switchgrass

Development of biomass feedstocks with desirable traits for cost effective conversion is one of the main focus areas in biofuels research. Pretreatment is a crucial step for making biomass feedstocks more amenable to biological conversion by unlocking sugars for fermentation. Nevertheless, as suggested by techno-economic analyses, the success of a lignocellulose-based biorefinery largely relies on the utilization of lignin to generate value-added products i.e. fuels and chemicals. The fate of lignin and its structural/compositional changes during pretreatment have received increasing attentions recently; however, the effect of genetic modification on the fractionation, depolymerization and catalytic upgrading of lignin from engineered plants is not well understood. This study aims to fractionate and characterize the lignin streams from wild-type and engineered switchgrass species (with low/high lignin content and high S or G lignin content) using three different pretreatment methods, i.e. dilute acid, ammonia hydroxide, and ionic liquid (cholinium lysinate). The molecular weight of the lignin fractions recovered from the liquid and solids streams after pretreatment and enzymatic hydrolysis was determined by gel permeation chromatography (GPC); while the cleavage of inter-unit lignin linkages was identified by H¹C¹³ HSQC and compared with results from lignin in untreated switchgrass. Fast catalytic pyrolysis of lignin streams was carried out in a customized pyro-GC/MS under inert or hydrogen atmosphere to analyze the distribution of lignin degradation/upgrading products. Results from this study provide a better understanding on how lignin engineering of switchgrass influences lignin fractionation and upgrading during conversion processes based on different pretreatment technologies.