Elucidating the Structure of Bimetallic NiW/SiO₂ Catalysts and Its Consequences on Selective Deoxygenation of <i>m</i>-Cresol to Toluene

Abstract

As a non-noble metal, Ni could offer significant economic advantages if used as a catalyst for hydrodeoxygenation (HDO) of lignin-derived phenolics to produce aromatics. However, on unmodified Ni catalysts, the desirable direct deoxygenation reaction must compete with high rates of phenyl-ring hydrogenation and C–C hydrogenolysis reactions, which lead to low aromatics yields. Here, we report on a bimetallic NiW/SiO2 (W/Ni = 1) prepared by coimpregnation that shows an HDO reaction rate of m-cresol almost an order of magnitude higher than that on Ni/SiO2 at 350 °C and 1 atm H2. More importantly, under these conditions, this catalyst exhibits a complete inhibition of CH4 formation, while at a temperature as low as 250 °C, the dominant product is still toluene, with minimal formation of ring-saturation products. To elucidate the structure of this catalyst, a detailed characterization was performed by combination of several techniques. It was found that the calcined NiW/SiO2 exhibits a large extent of Ni–W oxide interaction. After reduction at 500 °C, a thin NiW alloy shell with a small Ni core and WOx in close proximity are formed, with a strong interaction between Ni and adjacent W species. The electronic modifications of Ni and W species were monitored by X-ray photoelectron spectroscopy and it was found that these interactions alter the surface properties of the alloy, resulting in significantly weakened CO chemisorption. This unique structure provides a balanced hydrogenation, oxophilicity, and C–O cleavage activity, which result in a significantly improved rate and selectivity toward toluene with inhibition of CH4 and hydrogenation product formation.