(329d) Mixed Metal Oxide Catalysts for Microwave-Assisted Kraft Lignin Depolymerization to Produce Phenolic Monomers

For the economic viability of biorefineries, the efficient valorization of biomass-derived wastes, lignin, is important. Lignin, which accounts for 15-30 % of the available renewable carbon can be processed to produce value-added phenolic chemicals. In this study, microwave-assisted depolymerization of a Kraft lignin in methanol / formic acid (FA) solvent to produce high-value added phenolic monomers is investigated, in which methanol is a solvent, while FA acts as an acid catalyst and in-situ hydrogen donor. The highest lignin conversion of 78 % was achieved at 140°C and a FA-to-lignin mass ratio of 4 after 20 mins reaction time. Acid-catalyzed cleavage of the linkages in lignin occurs in the presence of FA, which results in the formation of a liquid product. Further, the catalytic depolymerization of Kraft lignin with added methanol / FA solvent system was conducted in the presence of mixed metal oxides to maximize liquid product yield and minimize the solid residue formation. Three different mixed metal oxide catalysts were prepared using hydrotalcite-structured materials i.e., Mg₃AlCO₃, Mg₃AlSO₄, and Mg₃AlCl. Mixed metal oxide-Mg₃AlCl catalyst having a particle size of 50 nm was found to be effective for Kraft lignin depolymerization. The effects of the catalyst composition, catalyst amount, and reaction conditions on the lignin conversion and product yields were explored. The highest lignin conversion of 89 % (with 81% bio-oil yield) was obtained at an optimized reaction temperature of 100°C with a catalyst-to-lignin ratio of 0.25, FA-to-lignin mass ratio of 4, and at a 20 min reaction time. The depolymerized liquid product contains mainly G-type phenolic monomers (~60 %) in the presence of mixed metal oxide-Mg₃AlCl. This work provides an efficient approach for the production of value-added phenolic compounds from lignin by microwave heating in the presence of mixed metal oxides and without external hydrogen.