Regulating the electronic state of Pd nanoclusters and Lewis acid density by the carbon doping for synergistically enhancing phenol hydrogenation in green solvent

Pd-based catalysts have been widely applied in selective hydrogenation of phenol. However, avoiding excessive hydrogenation of C=O functional groups is still challenging. Here, we synthesized Pd nanoclusters supported by N-doped C-Al 2 O 3 (Pd/CN-Al 2 O 3 ) catalyst for phenol hydrogenation. Remarkably, the cyclohexanone yield shows a volcano trend with the doping of carbon content, where Pd/CN-Al 2 O 3 with carbon content of 6.76 % (Pd/CN-Al 2 O 3 -2) exhibits the highest cyclohexanone yield. Under the optimized conditions, the phenol conversion and cyclohexanone selectivity over Pd/CN-Al 2 O 3 -2 are 99.5 % and 96.7 %, respectively. The corresponding TOF value is 1731.7 h −1 , which is 5.8 times higher than that of Pd/Al 2 O 3 (299.7 h −1 ). Mechanism studies reveal the excellent catalytic performance of Pd/CN-Al 2 O 3 -2 is mainly attributed to the synergistic effect of electron-deficient Pd clusters and Lewis acid sites. Pd nanoclusters with moderate electron-deficient state and moderate Lewis acid density are beneficial to the adsorption of phenol and the desorption of cyclohexanone, thereby exhibiting the best phenol conversion and cyclohexanone selectivity. This work provides a carbon modification strategy to regulate the structure of Pd-based catalysts, which is a helpful guide for the hydrogenation reactions.