The Effects of Dopants on the Cu-ZrO₂ Catalyzed Hydrogenation of Levulinic Acid

Catalytic hydrogenation of levulinic acid to form γ-valerolactone was studied over Cu-ZrO₂ catalysts doped with metal oxides from the first-row transition metals. The Cu-ZrO₂ material was prepared by oxalate gel coprecipitation, and dopants were added by an incipient wetness approach. The addition of 1% Mn into Cu-ZrO₂ significantly increases the yield of γ-valerolactone, and the catalytic activity of Mn/Cu-ZrO₂ was found to be 1.6 times higher than that of the undoped Cu-ZrO₂ catalyst. Catalyst characterization suggests that the Mn dopant improves the dispersion of Cu on the surface of ZrO₂. Kinetic studies show that the reaction order with respect to the substrate concentration is approximately zero. However, the order of reaction with respect to the partial pressure of H₂ is different for the Mn/Cu-ZrO₂ and Cu-ZrO₂ catalysts. Comparison of reaction products from reactions carried out in H₂O and D₂O solvents using ¹H NMR and ¹³C NMR show that there is a pre-equilibrium keto-enol isomerization step under our reaction conditions. DFT calculations show that the enol isomers have a higher affinity for the Cu surface, which may improve the availability of the substrate in the hydrogenation step of the reaction.