Anchoring Cu sites in a hierarchical single-crystalline ZSM-5 zeolite for enhanced diffusion and benzene oxidation†

Abstract

Phenol is an important intermediate for high-value chemicals. Current phenol production via the three-step cumene process leads to significant energy waste and environmental problems. The conversion of benzene to phenol under mild conditions can be achieved by oxidation with Cu-based zeolites. However, conventional microporous zeolites suffer from severe diffusion limitations, especially when bulky molecules, such as benzene, are involved. In this work, we used a hierarchically macro–meso–microporous ZSM-5 single crystal (Hier-ZSM-5) as a substrate for Cu species (Cu@Hier-ZSM-5). The irregular morphology of the opal-like Hier-ZSM-5 exhibited abundant surface Si–OH groups and provided a platform for stabilizing Cu²⁺ sites. Meanwhile, hierarchical porosity significantly improved the diffusion ability of bulky molecules. As a result, excellent selective oxidation performance of benzene was obtained with Cu@Hier-ZSM-5, achieving a conversion of 77% and a phenol selectivity of 73%, which were 1.5 times and 2 times higher than those obtained with a catalyst based on microporous ZSM-5, respectively. CuOOH species were identified as an important intermediate in benzene oxidation. This hierarchical zeolite system, with a synergistic effect of site anchoring and molecular diffusion, provides an excellent platform for catalyst design.