Chemical Transformations of Poplar Lignin During Dilute Acid Pretreatment

Pretreatment prior to enzymatic hydrolysis to reduce biomass recalcitrance is considered a key step for a viable biomass to bioethanol process. Understanding the basic chemical mechanisms of lignin involved in pretreatment can be employed to help tailor the pretreatment more efficient and selective for the production of second generation bioethanol, as well as to fully utilize lignin to make a range of fuels, chemicals, or other co-products. In this study, lignin was isolated from poplar samples pretreated with dilute acid at varying severities. GPC and NMR techniques (13C, 13P, HSQC and DEPT-135 NMR) were utilized to elucidate structural transformations of poplar lignin during pretreatment. As revealed by NMR, the aryl-O-ether linkage (β-O-4) of lignin was extensively cleaved via homolysis and lignin condensation occurred during pretreatment. The lignin was also observed to have a decreased carboxylic group and methoxyl group content and these changes were accompanied with increases in condensed lignin. In summary, lignin changes significantly in structure and this influences plant recalcitrance and its use as a co-product in the biorefinery process.