The influence of carbon supports and their surface modification on aqueous phase highly selective hydrogenation of phenol to cyclohexanol over different Ni/carbon catalysts

Cyclohexanol is an important feedstock in the chemical industry. Highly selective hydrogenation of phenols to cyclohexanol over non-noble metal catalysts remains a challenge. Herein, four distinctive carbon materials, carbon black (CB), activated carbon (AC), activated carbon fiber (ACF) and carbon nanotube (CNT) were selected as catalyst supports to load Ni nanoparticles for effective phenol hydrogenation. The results show that the phenol conversion and cyclohexanol selectivity in aqueous phase hydrogenation reaction at 180 °C for 2 h over 10 wt% Ni/AC catalyst could reach 99.7% and 94.8%, respectively. These values are better than those obtained over 10 wt% Ni/ACF, Ni/CNT and Ni/Al₂O₃. After surface modification of carbon supports, the catalytic activity and reuse effect of modified catalysts are significantly enhanced and the optimal temperature decreases to 150 °C. This is because that modified carbon supports have abundant surface functional groups so as to provide a high dispersion and stable anchor of ultrafine active Ni nanoparticles (∼12 nm). Phenol hydrogenation catalyzed by different Ni/carbon catalysts conforms to a first-order reaction, and their apparent activation energies are in a range of 49∼68 kJ mol⁻¹ (49 corresponds to the modified catalyst Ni/MAC), which are markedly lower than that of Ni/Al₂O₃ (∼96 kJ mol⁻¹). The potential mechanism of highly selective hydrogenation of phenol to cyclohexanol in this work is mainly controlled by the reaction kinetics, which is closely related to the different desorption abilities of cyclohexanone and cyclohexanol from the catalysts.