Synthesis of Pd@ZIF-8 via an assembly method: Influence of the molar ratios of Pd/Zn²⁺ and 2-methylimidazole/Zn²⁺

Pd@ZIF-8 nanoparticles with uniform size and morphology were prepared via an assembly method that enables the crystallization of ZIF-8 in methanol in the presence of polyvinylpyrrolidone (PVP)-modified Pd nanoparticles. The molar ratios of Pd/Zn²⁺ and 2-methylimidazole (Hmim)/Zn²⁺ are important parameters affecting the physical and catalytic properties of the Pd@ZIF-8. The catalytic activities of the hybrid catalysts were evaluated by the hydrogenation of p-nitrophenol to p-aminophenol. The results indicate that the increase of Pd loading and p-nitrophenol conversion can be tuned by adding more amounts of PVP-Pd colloid, but a suitable amount of PVP-Pd (Pd/Zn²⁺ ≤ 0.012) is helpful to the control of particle size of Pd@ZIF-8 and encapsulation of the Pd nanoparticles within the ZIF-8 crystals. Pd@ZIF-8 catalysts prepared with excessive or equal molar of Hmim relative to Zn;bsupesup have similar morphology and uniform particle size, where the Pd nanoparticles are fully confined within the ZIF-8 crystals without aggregation. Furthermore, the Pd@ZIF-8 catalysts exhibit the better catalytic performance for the p-nitrophenol hydrogenation owing to the high loading of well-dispersed Pd nanoparticles with the decreased Hmim/Zn;bsupesup molar ratio. This work would aid the development of high-performance Pd@ZIF-8 catalysts.