Mechanism insights into enol ether intermediate formation during β-O-4-type lignin pyrolysis

Enol ether (EE) is considered to be one of the important intermediates in lignin pyrolysis, but its formation process is still unclear. In this study, the generation of EE structure from the pyrolysis of beta-O-4 linkages under vairous structural environments was studied systematically with simplified lignin models, covering phenyl dimers, phenolic dimers, and phenolic polymers, and its formation mechanism and inducing factors were summarily proposed. Results showed that EE intermediates were difficult to generate in beta-O-4-type phenyl structures, but the activation of alpha/beta-substituted side chains promoted the EE formation, confirming that elimination reactions in pyrolysis was the main driving force for EE generation. Free phenolic hydroxyl groups and corresponding phenoxy radicals were very effective EE formation activators, as they can both induce the production of quinone intermediates, which were easily converted to EE structure. These conclusions provided a new connotation for the pyrolysis of lignin beta-aryl ether linkages.

Automated summary

This study systematically investigates the formation mechanism of enol ether (EE) from the pyrolysis of beta-O-4 linkages in simplified lignin models. Results show that EE intermediates are difficult to generate in beta-O-4 phenyl structures, but the activation of alpha/beta-substituted side chains promotes EE formation. Free phenolic hydroxyl groups and corresponding phenoxy radicals are effective activators for EE formation.