EPR detection of key radicals during coking process of lignin monomer pyrolysis

Lignin pyrolysis is a promising method for the sustainable production of phenolic compounds from biomass. However, detailed knowledge about the radicals involved in this process and their influence during the coking is missing. Herein, we report on the pyrolysis of guaiacol, a lignin monomer at 600 °C under inert and hydrogen gas conditions. During the pyrolysis process, the formed radicals were detected by two improved electron paramagnetic resonance spectroscopy methods. Successfully observed o-phenyl semiquinone radical anion, cyclopentadienyl radical, methyl radical, hydroxyl radical, methoxy radical, phenyl radical, phenoxy radical, etc. during the pyrolysis of guaiacol, as well as the changes of related radicals in different pyrolysis stages. It was found that o-semiquinone radical anions and cyclopentadienyl radicals were the key free radicals in the generation of coke and tar during the pyrolysis process. And through hydrogenation pyrolysis experiments, it was verified that the relative content of o-semiquinone radical and the solid coke were reduced with a hydrogen supply source. Our results showed that by controlling the key free radicals in the coking process, the formation of coke can be suppressed and the conversion efficiency can be improved.