Tuning electron density of metal nickel by support defects in Ni/ZrO2 for selective hydrogenation of fatty acids to alkanes and alcohols

Abstract

Catalysts with tunable activity and selectivity to targeted products are highly demanded when using complex biomass-based resources for the production of fuels and chemicals. Here we synthesize a series of Ni/ZrO2 catalysts, denoted as iwi-Ni/m-ZrO2, pc-Ni/t-ZrO2 and pfc-Ni/t-ZrO2, prepared by incipient-wetness impregnation (iwi), positive co-precipitation (pc) and parallel flow co-precipitation (pfc) methods, respectively, with the purpose to tune the electron density of nickel in the Ni/ZrO2 catalysts for the selective hydrogenation of fatty acids to fuel-like alkanes or fatty alcohols. The structural and electronic properties of the catalysts are systematically investigated by N2 adsorption, XRD, H2-TPR, H2-TPD, HRTEM, XPS, in situ FTIR, and DFT calculations. The catalytic performance shows that monoclinic ZrO2 supported nickel catalysts (iwi-Ni/m-ZrO2) exhibit the highest selectivity to 1-octadecanol but the lowest selectivity to n-heptadecane, while tetragonal ZrO2 supported nickel catalysts (pc-Ni/t-ZrO2 and pfc-Ni/t-ZrO2), especially the latter possess the highest selectivity to n-heptadecane with the lowest selectivity to 1-octadecanol. This variation in the selectivity is found correlated with the oxygen deficiency of ZrO2 support which affects the electron density of supported Ni metals. Charge transfer from the support to metal occurs on all the three catalysts, which stems from the electrons trapped in the oxygen vacancies after removing the interfacial O atom in the process of reduction of the ZrO2 support. The negatively charged metal Ni promotes the heterolysis of hydrogen and the subsequent hydrogenation of adsorbed fatty acids to aldehyde intermediate. Ni metals of much higher excess electron density on m-ZrO2 (due to its higher reducibility) further catalyze the hydrogenation of the C=O bond of aldehyde to 1-octadecanol while Ni metals with approximate excess electron density on t-ZrO2 preferentially catalyze the cleavage of the C−CHO bond to produce n-heptadecane.