A novel bifunctional Pd–ZIF-8/rGO catalyst with spatially separated active sites for the tandem Knoevenagel condensation–reduction reaction

Abstract

A novel bifunctional catalyst with spatially separated active sites was prepared by the immobilization of Pd nanoparticles (NPs) via covalent interaction and coordination of a zeolitic imidazolate framework (ZIF-8) on the surface of graphene oxide (GO), respectively, which was used as an efficient catalyst for the Knoevenagel condensation–reduction tandem reaction. The results of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) demonstrated that Pd and ZIF-8 were successfully immobilized on the surface of GO, and the GO was reduced to reduced graphene oxide (rGO) using NaBH₄ as the reductant in the preparation of Pd–ZIF-8/rGO. The textural properties and morphology of Pd–ZIF-8/rGO were characterized by N₂ adsorption–desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Pd–ZIF-8/rGO shows excellent catalytic performance in the tandem reaction with 100% benzaldehyde conversion and 98.3% selectivity to benzylmalononitrile. The excellent catalytic performance of Pd–ZIF-8/rGO in the tandem reaction is due to the high catalytic activities of spatially separated Pd NPs and ZIF-8 active sites and concentrated reactants on the surface of Pd–ZIF-8/rGO due to the π–π interaction between rGO and the reactants. The anchoring and stabilization effects of oxygenated groups of GO inhibit the aggregation and leakage of active sites, leading to good catalytic recyclability with almost unchanged catalytic activity for more than eight cycles in the tandem reaction.