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

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