Mechanism study on the pyrolysis of a synthetic β-O-4 dimer as lignin model compound

In this study, a β-O-4 dimer with abundant oxygen substituents was successfully synthesized, and was used as the model compound for lignin to study the pyrolysis mechanism. Py-GC/MS (micro pyrolyzer coupled with gas chromatography/mass spectrometry) was employed to identify the distribution of pyrolytic products under temperatures of 150-850 °C. It was found that the yields of methoxylated monoaromatics underwent tendencies of first increase and then decrease at high temperature. Polyaromatics and benzofuran were only detected at high temperature. Based on experimental analysis, a detailed pyrolysis kinetic model was developed by combining the density functional theory (DFT) and the transition state theory (TST). The homolysis of Cβ-O was the most favorable route for the initial depolymerization of dimer rather than the concerted retro-ene fragmentation. For the evolution of intermediate products, the breakage of ether bond and the keto-enol tautomerization of enols were the most favorable routes, while intramolecular cyclization and group dissociation exhibited high energy barriers and low reaction rates. The methoxyl in guaiacol preferred to undergo demethylation leading to the formation of catechol.