Geometric and electronic effects of bimetallic Ni-Re catalysts for selective deoxygenation of m-cresol to toluene

Ni-Re/SiO2 bimetallic catalysts were prepared using a co-impregnation method and tested in vapor phase hydrodeoxygenation of m-cresol at 300 degrees C and 1 atm H-2. In contrast to the use of unselective monometallic Ni/SiO2 for catalyzing deoxygenation, hydrogenation, and C-C hydrogenolysis reactions, bimetallic 5% Ni-2.5%Re/SiO2 improved the intrinsic reaction rate of the hydrodeoxygenation reaction by a factor of 6, with the turnover frequency for selective deoxygenation to toluene increased by four times, while that for C-C hydrogenolysis to methane was reduced by one-half. Characterization results from X-ray diffraction, Raman, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy of CO adsorption, H2 temperature-programmed reduction, and CO chemisorption indicate that adding Re increased Ni dispersion and resulted in Ni-Re surface alloy formation after reduction. The presence of Re in the surface alloy breaks the continuous Ni surface into smaller ensembles (geometric effect) and reduces the d-band electron density of Ni (electronic effect). Results from density functional theory calculations indicate that the Ni-Re neighboring site is the active site for breaking the C-O bond by adsorbing the O atom on Re and the phenyl ring on the neighboring Ni atoms, which facilitates deoxygenation to toluene. The reduced Ni ensemble size inhibits the hydrogenolysis of the C-C bond by destabilizing the transition state, whereas the reduction of the electronic density in d states of Ni weakens the adsorption of the phenyl ring, and both contribute to the greatly reduced methane production from successive C-C hydrogenolysis. (C) 2017 Elsevier Inc. All rights reserved.