Role of Cu–Mg–Al Mixed Oxide Catalysts in Lignin Depolymerization in Supercritical Ethanol

We investigated the role of Cu–Mg–Al mixed oxides in depolymerization of soda lignin in supercritical ethanol. A series of mixed oxides with varying Cu content and (Cu+Mg)/Al ratio were prepared. The optimum catalyst containing 20 wt % Cu and having a (Cu+Mg)/Al ratio of 4 yielded 36 wt % monomers without formation of char after reaction at 340 °C for 4 h. Comparison with Cu/MgO and Cu/γ-Al2O3 catalysts emphasized the excellent performance of Cu–Mg–Al oxides. These mixed oxides catalyze the reaction between formaldehyde and ethanol, which limits polymerization reactions between phenolic products and formaldehyde. The combination of Cu and basic sites catalyzes the associated Guerbet and esterification reactions. These reactions also protect lignin side-chains (e.g., aldehyde groups). Lewis acid sites of the catalyst, mainly Cu and Al cations, catalyze C- and O-alkylation reactions that protect phenolic products and phenolic moieties in lignin oligomers. Hydrogen produced by dehydrogenation reactions is involved in hydrogenolysis reactions of the chemical bonds in lignin and also to deoxygenate the monomeric and oligomeric products. Careful investigation of the influence of the acid and base functionalities allows concluding that Guerbet and esterification reactions are more important than alkylation reactions in avoiding formation of heavy products such as char. These insights point out directions for rational design of catalysts for lignin conversion.