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

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 degrees C and 1 atm H-2. 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 H-2 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 CH, 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.