Frontier Orbital Interpretation of Gas Release Pathways in Lignin Thermolysis

Abstract

An attempt was made to interpret the experimentally observed gas release pathways in lignin thermolysis by the frontier molecular orbital (FMO) theory. A set of 20 monoaromatic molecules that mimic the moieties in Freudenberg and Harkin’s model of spruce lignin were thermolyzed at 250–550 °C and fractional conversions from 0.0005 to near unity. Guaiacol substrate, a model of the prevalent coniferyl alcohol monomer residues in lignin, thermolyzed by two parallel paths, namely, (R1) demethanation to CH4 and catechol and (R2) decarbonylation to CO and phenol, with their relative rates in the ratio of ∼5:1. Thermolyses of three substituted guaiacols, namely, 2,6-dimethoxyphenol, isoeugenol, and vanillin, also resulted in demethanation and decarbonylation, analogous to R1 and R2 for guaiacol, but vanillin decarbonylation, to guaiacol and CO, was more akin to and roughly 1000 times faster than the single decarbonylation path (R3) of benzaldehyde to benzene and CO. Pathway R1 was mechanistically interpreted as a...