ZnO-template synthesis of rattle-type catalysts with supported Pd nanoparticles encapsulated in hollow ZIF-8 for liquid hydrogenation

- A rattle-type Pd/ZnO@hollow ZIF-8 yolk-shell catalyst was obtained by a ZnO-template approach.
- High-concentration ZIF-8 growth solution helps to obtain a perfect yolk-shell catalyst.
- Pd/ZnO@hollow ZIF-8 yolk-shell catalyst exhibits enhanced catalytic performance.

MOFs-based yolk-shell composites are newly-presented but promising catalysts in the area of catalysis. In this work, a kind of unique rattle-type Pd/ZnO@hollow ZIF-8 yolk-shell catalyst is developed via a ZnO-template approach, in which there exist two processes of both ZnO-induced ZIF-8 formation and ZnO dissolution. Of prime importance is that ZnO was selected as the sacrificial template since it can not only be etched in the solution of ligands but also assist the formation of compact ZIF-8 shell by providing nucleation sites via linker activation. It is also proposed that sufficient amount of Zn(NO3)2·6H2O and 2-methylimidazole added in the growth solution of ZIF-8 should accelerate the outward growth of ZIF-8 layer as well as dissolution of ZnO. This way, the final construction of Pd/ZnO@hollow ZIF-8 yolk-shell materials is achieved. Compared to the Pd/ZnO counterparts, Pd/ZnO@hollow ZIF-8 rattle-type catalysts demonstrate a promoted behavior in terms of size-selectivity for the liquid hydrogenations of alkenes due to the outer ZIF-8 shell acting as a molecular sieve. It can also protect the catalytically active Pd NPs from poisoning and leaching, giving rise to an excellent stability. Moreover, the yolk-shell catalysts manifest a better activity beyond their core-shell counterparts since the presented interior hollow space between core and shell is likely to expose more active surfaces to reactants.

In this work, a unique rattle-type Pd/ZnO@hollow ZIF-8 yolk-shell catalyst was fabricated via a ZnO-template strategy. ZnO was selected as sacrificial template since it can not only be etched in the solution of ligands but also assist the formation of a compact ZIF-8 shell. Concentration of ZIF-8 growth solution also facilitates the formation of a well-defined yolk-shell structure. The as-prepared Pd/ZnO@hollow ZIF-8 rattle-type catalyst demonstrated a promoted behavior in terms of size-selectivity and stability.274