In situ observation of radicals and molecular products during lignin 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 on the molecular products is missing. Herein, we report on the pyrolysis of hard- and softwood Klason lignins under inert gas conditions in a temperature range between 350-550 °C. During the pyrolysis process, the formed radicals were detected by in situ high-temperature electron paramagnetic resonance spectroscopy. The overall formation of volatile products during lignin pyrolysis was determined using thermogravimetric analysis. The volatile molecular products were characterized and quantified using GC-MS analysis. Major differences were observed between hardwood and softwood lignins. Hardwood lignins form more radicals and volatile products than softwood lignins at temperatures from 350 to 450 °C. In the late stages of the pyrolysis process at 550 °C radical quenching reactions become dominant in hardwood lignins. We identified the disproportionation of two semiquinone radicals to quinone and hydroquinone species as the most likely quenching reaction. Our results show that both the pyrolysis temperature and the type of lignin source have a major influence on radical formation and the molecular products during the depolymerization of lignin.