In this study, microwave-assisted depolymerization of alkaline lignin in methanol/formic acid (FA) media was proposed. Different FA-to-lignin mass ratio, reaction temperature, and reaction time were applied to understand their effects on the product yield and distribution.The starting lignin and its depolymerized products were carefully characterized to propose the possible mechanism of lignin depolymerization in methanol/formic acid media under microwave heating.
The addition of formic acid to methanol as a solvent benefited the depolymerization of alkaline lignin under microwave heating. Reaction temperature and reaction time also had an effect on lignin depolymerization. 72.0 wt% yield of bio-oil including 6.7wt% monomers was obtained under the optimum condition . Formic acid acted mainly through acid-catalyzed cleavage of the linkages in lignin. MALDI-TOF MS result indicated that oligomers in bio-oil were mainly dimers (molecular weight: 253–378) and trimers (molecular weight: 379–510). This research provided a possibility to depolymerize lignin efficiently under microwave heating.