Relationship between the formation of oligomers and monophenols and lignin structure during pyrolysis process

Abstract

This study investigated the relationship between the formation of oligomers and monophenols and lignin structure during pyrolysis regarding two technical lignins from alkali lignin (AL) and corn cob hydrolysis residue lignin (CHRL). The results showed that the molecular weight distribution and inter-unit linkages varied substantially between AL and CHRL, resulting in profound differences in the content of monophenols and oligomers and their distributions in pyrolysis oil (PO). Thereinto, CHRL-PO containing higher monomers content (20.48%) and lower oligomers content (14.31%) relative to AL-PO (5.02%, 22.4%). This was attributed to the higher proportion of guaiacyl (G) units existed in AL, which tended to form oligomers and char. Additionally, the formation pathways of main monophenols from two POs were also different. In light of these findings, the thermal cracking behavior of lignin was closely bound up with its structural features. On one hand, the formation pathways of lignin-derived dimers and tetramers in PO originated from the breaking of a large amount of β-O-4, α-O-4 and Cβ-Cγ bonds during pyrolysis process. Meanwhile, the interconversion relationship between oligomers and monophenols was also existed. Polycyclic aromatic hydrocarbon (PAH) derivatives, on the other hand, were formed by the continuous growth of smaller free radicals because of the evolutionary components (such as phenols, syringols) could form oxygen-related free radicals during the secondary reaction process. Overall, this work provided the research basis and theoretical guidance for balancing the production of oligomers and monophenols through the selection of specific lignin feedstock.