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

Abstract

We investigate 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/¿-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 catalyze 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. Production of hydrogen by dehydrogenation is important for catalyzing hydrogenolysis reactions to cleave the constituent bonds in lignin and also to deoxygenate the monomeric and oligomeric products. Careful investigation of the influence of the nature of the acid and base functionalities indicates that the Guerbet and esterification reactions are more important in avoiding heavy product formation than alkylation reactions. These insights point out directions for rationally designing catalyst for lignin conversion.