Mechanism Study on Depolymerization of the α-O-4 Linkage Lignin Model Compound in Supercritical Ethanol System

The aim of this study was to explore the conversion mechanism of alpha-O-4 linkage lignin dimer model compound (monobenzone) in supercritical ethanol. The decomposition processes of monobenzone were investigated at different reaction time (0-12 h), conversion temperature (250-310 degrees C) and initial concentrations (0.01-0.03 g/mL). The reaction mechanism and pathways were proposed based on the distribution of products, the bond dissociation energies and the free radical theory. The results showed almost complete degradation was achieved in supercritical ethanol system. A higher temperature, a longer reaction time and a higher initial concentration significantly promoted the formation of solid residue due to the condensation reactions of the degradation intermediates/products. Based on the formation mechanism of products, the conversion products were classified into three types: (1) its own fragmentation compounds, (2) condensation compounds of its intermediates and (3) the second fragmentation compounds of its intermediates. A depolymerization reaction mechanism of monobenzone that primarily involved homolytic cleavage of the C-alpha-O linkage was proposed. In addition, the bibenzyl were an important and unstable intermediate product, it can continue to produce many different radical species that can participate in a variety of reaction mechanisms, resulting in complex reaction pathways and products distribution, even forming higher-molecular weight solid residue.