In situ fabrication of a perfect Pd/ZnO@ZIF-8 core-shell microsphere as an efficient catalyst by a ZnO support-induced ZIF-8 growth strategy

Controllable encapsulation of nanoparticles with metal organic frameworks (MOFs) has been an efficient way to impart the unique chemical and physical properties of the nanoparticles to metal organic frameworks and create new types of multifunctional MOF core-shell materials with enhanced properties. Here, a novel ZnO support-induced encapsulation strategy is reported to efficiently fabricate a Pd/ZnO@ZIF-8 core-shell catalyst, with Pd/ZnO as the core and ZIF-8 as the shell. The novel synthesis procedure involves first loading Pd nanoparticles onto the surface of the ZnO microsphere to form a Pd/ZnO core and then coating the core with a layer of defect-free ZIF-8 shell via ZnO-induced in situ ZIF-8 growth to obtain the Pd/ZnO@ZIF-8 core-shell catalyst. It was crucial that the ZIF-8 was in situ formed from the ZnO core in an ethanol solution only containing 2-methylimidazole under mild conditions. This strategy allowed for the growth of ZIF-8 right on the surface of Pd/ZnO via the reaction between ZnO and the 2-methylimidazole ligands, and thus avoided the random deposition of ZIF-8 crystals on the Pd/ZnO core as in the case of the conventional ZIF-8 synthesis solution. Furthermore, use of ethanol as the solvent also favored achievement of the well-defined Pd/ZnO@ZIF-8 structure, since the ethanol solution of 2-methylimidazole was able to keep the balance between ZnO dissolution and ZIF-8 formation. The as-prepared Pd/ZnO@ZIF-8 core-shell microsphere as an efficient catalyst displayed excellent performance in terms of size-selectivity, stability and anti-poisoning in the liquid hydrogenations of alkenes.