Reductive depolymerization of lignin by bifunctional Ru-based catalysts supported on tungstated-zirconia

Lignin depolymerization is crucial for producing fuels and chemicals. Catalytic hydrodeoxygenation offers a distinct method for lignin depolymerization, leading to improved bio-oil conversion yields. This approach involves breaking C-O bonds, hydrogenating rings, and opening rings. In this study, Ru-based bifunctional catalysts with a low Ru metal loading of 0.2 to 0.5 wt.% on tungstated zirconia (Ru/W-Zr-O) were investigated for the ring hydrogenation and hydrogenolysis of lignin model compounds (β-hydroxypropiovanillone and guaiacol) as well as corn stover lignin. The Ru/W-Zr-O catalyst exhibited similar selectivity for ring hydrogenation as the Ru/Al2O3 catalyst. However, Ru/W-Zr-O consistently demonstrated high selectivity for ring-opening hydrogenation products (e.g., linear alkanes) from the model compounds and lignin, particularly in ethanol compared to water. "The highest selectivity towards long-chain alkanes in bio-oil was 79.4%, with an overall lignin conversion rate of 90.6%. By comparing and analyzing experimental data with the activation energies of bonds in lignin model compounds using density functional theory, the reaction mechanism was determined to follow this route: 'aromatics → cyclic hydrocarbon → linear hydrocarbon.' By optimizing the characteristics of bifunctional catalysts and solvents, this work suggests the potential for designing effective catalysts supported on tungstated zirconia for the selective hydrogenolysis of lignin into long-chain alkanes.