Reaction Analysis of Diaryl Ether Decomposition under Hydrothermal Conditions

The reactivity and decomposition pathway of models for α-O-4 and β-O-4 linkages, found within lignin, have been examined using methoxy-substituted (-OCH₃) and -unsubstituted (-H) aryl groups under hydrothermal conditions. α-O-4 model compounds readily underwent conversion at comparatively mild temperatures (140-300 °C) and short reaction times (5-80 min), in contrast with the β-O-4 containing model compounds which required temperatures up to 340 °C and longer reaction times up to 240 min. Pseudo-first-order rate constants and apparent activation energies were calculated for hydrothermal conversion of the model compounds based on experimental data. The cleavage of these linkages proceeded via hydrolysis and direct elimination pathways, with the resulting products prone to undergoing further reactions including condensation, and dehydration. The presence of methoxy functionalities on the aromatic rings was found to destabilize both the α-O-4 and β-O-4 ether linkages, decreasing the temperature and reaction times required to decompose them under hydrothermal conditions. In addition, the methoxy substituents were partially hydrolyzed under hydrothermal conditions at temperatures exceeding 280 °C, resulting in a number of substituted guaiacol products.