H3PW12O40/mpg-C3N4 as an efficient and reusable bifunctional catalyst in one-pot oxidation–Knoevenagel condensation tandem reaction

Abstract

A single-site bifunctional catalyst for the oxidation–Knoevenagel condensation tandem reaction was prepared by the immobilization of phosphotungstic acid (HPW) on mesoporous graphitic carbon nitride (mpg-C₃N₄) via electrostatic interaction (HPW/mpg-C₃N₄). The results of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), solid-state ³¹P nuclear magnetic resonance (solid-state ³¹P NMR), zeta potentials, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) demonstrated that HPW was successfully immobilized on the protonated mpg-C₃N₄ by electrostatic interaction. The acid amounts of the catalysts were determined by NH₃ temperature programmed desorption (NH₃-TPD). The textural properties and morphology of HPW/mpg-C₃N₄ were characterized by N₂ adsorption–desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 30% HPW/mpg-C₃N₄ shows the best catalytic performance in the tandem reaction with 98.4% benzyl alcohol conversion and 96.2% selectivity to benzylidene malononitrile. The excellent catalytic performance of 30% HPW/mpg-C₃N₄ in the tandem reaction is due to the good catalytic performance of HPW in the oxidation and Knoevenagel condensation, respectively. Furthermore, protonated mpg-C₃N₄ not only acts as a support to facilitate good dispersion of HPW but also promotes the Knoevenagel condensation reaction effectively. Moreover, the HPW/mpg-C₃N₄ catalyst could be recycled easily without significant loss of catalytic activity.