Enhancing the matching of acid/metal balance by engineering an extra Si–Al framework outside the Pd/HBeta catalyst towards benzene hydroalkylation†
The one-step hydroalkylation of benzene to cyclohexylbenzene (CHB) needs to occur on bifunctional metal/zeolite catalysts, and introducing more acid sites to match the metal sites plays a key role in improving the matching of acid/metal sites toward the requirements of benzene hydroalkylation. Herein, we designed and constructed a Si–Al framework coating with extra acid sites outside the surface of the Pd/HBeta catalyst and obtained a zeolite-encaged Pd NP structure (Pd@HBeta). Notably, the Pd@HBeta catalysts achieved excellent catalytic performance with 44.0% CHB selectivity at about 85.0% benzene conversion, which is much higher than that of the Pd/HBeta catalyst with a CHB selectivity of 25.1% at the same benzene conversion. The remarkable increment of CHB selectivity of Pd@HBeta is mainly attributed to the extra acid sites provided by the coated Si–Al framework as well as the increase of the B/L ratio. Moreover, the adsorption capacity of benzene and cyclohexene is greatly improved after introducing extra acid sites by coating the Si–Al framework, which is beneficial for both hydrogenation and alkylation. Further DFT calculations reveal that C6H10 can easily desorb from Pd NPs and migrate to the acidic channel of HBeta in the Pd@HBeta catalyst than that in Pd/HBeta. Our work presents an elegant example of catalytic hydroalkylation selectivity regulation by coating an extra Si–Al framework to enhance the matching of acid/metal balance.