Alkali-enhanced oxygen vacancies driven high catalytic performance of Pd/WO3 for selective hydrogenation of phenol to cyclohexanone

Abstract

Production of cyclohexanone through selective hydrogenation of phenol under mild conditions has attracted much attention. Herein, a NaOH-modified Pd/WO₃ rich in oxygen vacancies for selective hydrogenation of phenol was prepared through impregnation method. The highest catalytic activity was exhibited on NaOH-Pd/WO₃ with 25 wt % addition of NaOH, and phenol achieved 100 % conversion at 90 °C and 0.7 MPa H₂ for 2 h, with a cyclohexanone selectivity of 95.3 %. The turnover frequency (TOF) of NaOH-Pd/WO₃ is approximately 3.5 times higher than that of the Pd/WO₃. Detailed catalyst characterization and theoretical calculations reveal that the high activity and selectivity of NaOH-Pd/WO₃ for phenol hydrogenation to cyclohexanone are highly correlated with the concentration of oxygen vacancies in the catalyst. In-situ infrared and Raman analyses reveal that the OH^- introduced by NaOH reacted with the lattice oxygen in WO₃ to abstract oxygen from the lattice, generating a significant number of oxygen vacancies. The possible reaction mechanisms for phenol hydrogenation were elucidated through combined density functional theory calculations and isotopic labeling experiments. This strategy of improving catalytic activity and selectivity by regulating the concentration of oxygen vacancies provides a theoretical basis for the design and optimization of industrial catalysts.