Size Dependence of Vapor Phase Hydrodeoxygenation of <i>m</i>-Cresol on Ni/SiO₂ Catalysts

Abstract

Understanding the effect of metal particle size on the reactions during hydrodeoxygenation of phenolics is of great importance for rational design of a catalyst for selective control of a desirable reaction. To this end, vapor phase hydrodeoxygenation of m-cresol was studied over 5% Ni/SiO2 catalysts with varying Ni particle sizes (2–22 nm) at 300 °C and 1 atm H2. The Ni particle sizes were confirmed by several characterization techniques, and the varying surface concentration of terrace, step, and corner sites with Ni particle sizes was verified by H2 temperature-programmed desorption. Decreasing the Ni particle size from 22 to 2 nm improves the intrinsic reaction rate by 24 times and the turnover frequency (TOF) by 3 times. The TOFs for toluene and methylcyclohexanone/methylcyclohexanol formation increase by 6 and 4 times, respectively, while the TOF for CH4 formation decreases by 3/4, indicating that smaller particles with more defect sites (step and corner) favor deoxygenation and hydrogenation while larger particles with more terrace sites favor C–C hydrogenolysis. Density functional theory study shows that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211), and defected Ni(211) decreases from 175.6 to 145.6 and then to 120.5 kJ/mol. The results indicate that a highly coordinatively unsaturated surface Ni site is responsible for C–O cleavage through facile adsorption and stabilization of −OH in the transition state, thus facilitating deoxygenation toward toluene. Our results indicate that tuning the metal particle size is an effective approach to control reactions during hydrodeoxygenation.