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

Abstract

<p> Stable and easily synthesized metal&ndash;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 &ldquo;teabag&rdquo; approach. Under continuous flow, the catalyst was capable of withstanding 7 days of continuous operation at 110 &deg;C without deactivation. During this time, no leaching of metallic species was observed, and the material maintained its structural integrity.</p> <p> &nbsp;</p>