Role of the Cu-ZrO2 interface in the hydrogenation of levulinic acid to γ-valerolactone

Abstract

Here we exquisitely fabricated Cu/ZrO2-dp catalysts with plentiful Cu-ZrO2 interfaces by depositing amorphous ZrO2 onto Cu nanoparticles for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). With the created plentiful Cu-ZrO2 interfaces, the optimal catalyst 3Cu/ZrO2-dp exhibited exceptional catalytic performance under mild reaction conditions, and achieved the highest GVL mass productivity of 266.0 mmol GVL·h−1·g−1 Cu, which was 12.5 and 2.3 times of Cu/ZrO2 catalysts with equivalent Cu loadings prepared by traditional impregnation (3Cu/ZrO2-im) or co-precipitation (3Cu/ZrO2-cp). As far as we know, this GVL mass productivity stood at the highest level compared with those obtained using non-noble metal catalysts under similar reaction conditions. By systematic investigation with multiple characterizations, density functional theory (DFT) calculations, and kinetic studies, it was found that interfacial active centers were created at Cu-ZrO2 interfaces, which contained oxygen vacancies (Ov), negatively charged Cuδ− and partially reduced Zr3+. The Ov favored the adsorption and activation of LA via its ketone group, while negatively charged Cuδ− was able to enhance heterolysis of H2, which resulted in the formation of H+-Cuδ− and Zr3+-H− active species via hydrogen spillover. Also, plentiful acid sites, which derived from coordinatively unsaturated and defective Zr species, generated at Cu-ZrO2 interfaces. With the cooperation of interfacial active centers (Cuδ−-Ov-Zr3+) and acid sites, the fabricated 3Cu/ZrO2-dp with plentiful Cu-ZrO2 interfaces achieved excellent catalytic performance for the hydrogenation of LA to GVL. Hence, the synergistic catalysis of Cu-ZrO2 interfaces provided an effective strategy for designing catalysts with a satisfactory performance for the hydrogenation of LA, which also can be expanded to other hydrodeoxygenation reactions.