Double-Supported Silica-Metal–Organic Framework Palladium Nanocatalyst for the Aerobic Oxidation of Alcohols under Batch and Continuous Flow Regimes

Stable and easily synthesized metal–organic framework MIL-88B-NH2 represents an attractive support for catalysts employed in oxidation reactions, which are typically performed under relatively harsh conditions. However, MIL-88B-NH2, the thermodynamic polymorph of the more popular MIL-101-NH2, has been rarely employed in catalytic applications because of a difficult impregnation process caused by the flexible nature of the framework. We report herein a new catalyst denoted Pd@MIL-88B-NH2 (8 wt % Pd), the first example of metallic nanoparticles successfully impregnated in the pores of MIL-88B-NH2. Furthermore, by enclosing the MOF crystals in a tailored protective coating of SiO2 nanoparticles, an even more enduring material was developed and applied to the aerobic oxidation of benzylic alcohols. This doubly supported catalyst Pd@MIL-88B-NH2@nano-SiO2 displayed high activity and excellent performance in terms of endurance and leaching control. Under batch conditions, a very convenient and efficient recycling protocol is illustrated, using a “teabag” approach. Under continuous flow, the catalyst was capable of withstanding 7 days of continuous operation at 110 °C without deactivation. During this time, no leaching of metallic species was observed, and the material maintained its structural integrity.