A General Synthetic Approach for Integrated Nanocatalysts of Metal-Silica@ZIFs

Integration of different nanocomponents into a greater assemblage or object for applications poses a significant challenge to materials chemists. At present, it still remains extremely difficult to achieve high monodispersivity for such assembled products. To gain better synthetic controllability, ideally, an integration of this type should be done in a stepwise manner. Herein, we report a versatile stepwise approach for preparation of integrated nanocatalysts of metal-mSiO₂@ZIFs (metal = Pt, Pd, Ru, Ag, and Pt₅₃Ru₄₇; mSiO₂ = mesoporous silica; and ZIFs = ZIF-8 and ZIF-67). Starting with uniform solid Stöber silica spheres in submicrometer scale, mesoporous channels with desired length and diameter can be created for silica which serves as a support. With measurements of amino-modification of mesopores and selection of metal precursors applied, subsequently, ultrafine metal nanoparticles (2–5 nm) can be deposited evenly onto the inner walls of silica channels. Resultant metal-mSiO₂ spheres are then modified by a layer of anionic polymer which imparts negative charges around and facilitates coating of ZIF-8 shell and thus formation of metal-mSiO₂@ZIF-8. Through coordination interaction between polyvinylpyrrolidone (PVP; as surfactant molecules) and unsaturated Zn²⁺ ions exposed on the ZIF-8 shell, uniform metal-mSiO₂@ZIF-8 spheres with desired shape and size can be obtained and simultaneously well-dispersed. Fundamental study and optimization are also carried out, aiming at a greater generality of this synthetic approach. The workability of these catalysts is demonstrated with hydrogenation of different alkenes using as-produced Pd-mSiO₂@ZIF-8 catalyst. Indeed, reactant-selective hydrogenation is achieved based on different interactions of the alkene molecules with the shell structure of ZIF-8, possibly influencing the flexible gate opening of ZIF-8.