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

Abstract

Ni–Re/SiO2 bimetallic catalysts were prepared using a co-impregnation method and tested in vapor phase hydrodeoxygenation of m-cresol at 300°C and 1atm H2. In contrast to the use of unselective monometallic Ni/SiO2 for catalyzing deoxygenation, hydrogenation, and CC 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 CC 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 CO 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 CC 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 CC hydrogenolysis.