Well-Defined Microenvironment in Metal–Organic Frameworks Enable Green, Benign, and Isolation-Free Catalytic Oxidation Reaction

Abstract

The catalytic oxidation reaction plays a pivotal role in scientific research and industrial applications. However, the development of green catalysts with high conversion and selectivity under mild conditions remains a challenging yet rewarding endeavor. Herein, a novel, stable four-connected Hf-based cationic metal–organic framework (MOF) PFC-30-Hf, is reported as a green catalyst for the oxidation of aniline to afford azoxybenzene. The catalytic reaction can proceed at room temperature using H2O2 as the green oxidant. More strikingly, the catalyst yields oxidation products with almost 100% conversion and selectivity, endowing the entire reaction with an isolation-free feature. The enhanced catalytic activity is attributed to the well-defined catalytic microenvironment created by the Lewis acidic Hf4+ metal clusters, the Brønsted acidic hydroxyl groups on metal nodes, and the cationic ligand with enhanced affinity for substrates. This synergistic effect was then evidenced by parallel studies on neutral isoreticular MOF PFC-30-Hf-N, HfO2, and Hf6 clusters. Detailed mechanistic investigations into the catalytic process revealed the occurrence of stepwise oxygen atom transfer and intermediate condensation during the reaction. Furthermore, PFC-30-Hf demonstrated equally satisfactory activity and isolation-free merit for sulfide oxidation to afford sulfone, demonstrating the great potential of MOFs to engineer the catalytic microenvironment for the pursuit of green and sustainable chemistry.