Maximization of Kraft lignin depolymerization using synthetic mixed oxide catalysts under microwave exposure

The efficient utilization of biomass-derived waste, particularly lignin, is crucial for the economic success of biorefineries. Lignin, comprising 15–30 % of the available renewable carbon, can be transformed into valuable aromatic chemicals. This study explores microwave-assisted depolymerization of Kraft lignin using a methanol/formic acid solvent system, with formic acid acting as both a hydrogen donor and acid catalyst. A maximum 78 % of lignin conversion was achieved at 140°C with a formic acid-to-lignin mass ratio of 4 after 20 minutes. To further enhance product yield and reduce solid residues, mixed-metal oxide (MMO) catalysts i.e., Mg₃AlCO₃, Mg₃AlSO₄, and Mg₃AlCl were used. The optimized reaction conditions (100°C, 20 minutes, formic acid-to-lignin mass ratio of 4, and catalyst-to-lignin ratio of 0.25) resulted in a bio-oil yield of 81 % and a lignin conversion of 89 %. The primary liquid product was G-type phenolic monomers (∼60 %), highlighting the effective role of MMOs in selectively cleaving lignin linkages. This method provides an efficient, external hydrogen-free route for producing valuable phenolic compounds from lignin.