Cooperative effect of multi-active sites in WOx-modified ZrO2 supported highly dispersed Pd catalysts for promoting benzene hydroalkylation

Abstract

Designing efficient metal–acid bifunctional catalysts is a challenging task to achieve the one-step benzene hydroalkylation to produce cyclohexylbenzene (CHB), which has significant importance due to the economic potential for the industrial production of phenol. In this study, a series of WOx-modified ZrO2 supported highly dispersed Pd catalysts were developed by modulating the surface acidity of ZrO2 through the introduction of WOx species. The as-constructed Pd/WOx-ZrO2 catalyst bearing a low Pd loading of 0.4 wt% and a W content of 21 wt% exhibited a superior catalytic performance, along with an unprecedentedly high productivity of CHB (144.6 mmolCHB·gcat−1·h−1), much higher than those obtained by using the state-of-the-art catalysts. It was demonstrated that highly dispersed Pd species could promote the formation of active hydrogen species and favor the hydrogen spillover on the catalyst surface that induced the generation of surface oxygen vacancies and Brønsted acidic sites originating from surface-loaded WOx. Comprehensive structural characterizations and catalytic experiments showed that the favorable metal–acid cooperation on Pd/WOx-ZrO2 catalysts and the cooperation between Lewis acid sites and Brønsted acid sites, as well as the presence of abundant defective oxygen vacancies, efficiently regulated the adsorption and activation of molecular hydrogen, benzene, and cyclohexene intermediate, leading to the improved catalytic hydroalkylation activity. These findings provide new high-performance metal–acid bifunctional catalysts with multi-active site cooperation structures for one-step hydroalkylation of benzene to CHB.