Investigation of the Fate of Switchgrass Lignin Chemical Structures During Ammonia Fiber Expansion, Soaking in Aqueous Ammonia, Dilute Acid, and Ionic Liquid Pretreatment

With anticipated future biorefineries processing biomass, a large amount of hydrolysis lignin will become available. Exploiting the chemical value of lignin is economically preferable to its use as process heat. The effective utilization of lignin is largely predicated on pretreatment technologies, since the chemical characteristics of lignin are affected by the choice of pretreatment. In the present study, we present our results of characterizing  the chemical structures of switchgrass lignin before and after pretreatment from the three DOE funded Bioenergy Research Centers; dilute sulfuric acid (DA), ammonia fiber expansion (AFEX), soaking in aqueous ammonia (SAA), and ionic liquid with [C₂mim][OAc]. The impact of pretreatments on substrate reactivity was evaluated by enzymatic hydrolysis yield, mass balance, and cross polarization/magic angle spinning (CP/MAS) ¹³C nuclear magnetic resonance (NMR). The enzymatic glucan digestibility was enhanced due to pretreatment in the following order: IL > DA > SAA > AFEX. After pretreatment, ¹³C-¹H Heteronuclear Single-Quantum Correlation (HSQC) NMR analysis showed that β-O-4 aryl linkages were dominant linkages in lignin regardless of pretreatment type. β-O-4 aryl linkages were 50.44, 44.85, 31.22, and 29.83 per 100 aromatic units for IL for IL, AFEX, SAA, and DA, respectively. Major lignin structural changes were observed from DA including severely cleaved β-O-4 aryl linkages along with a decrease in β-5 and β-β substructures. Lignin condensation as well as repolymerization occurred during DA and AFEX pretreatment, as shown in an increase in molecular mass of lignin by size exclusion chromatography.