H5PV2Mo10O40 encapsulated into Cu3(BTC)2 as an efficient heterogeneous nanocrystalline catalyst for styrene epoxidation

Abstract

The Keggin-type polyoxometalate H₅PV₂Mo₁₀O₄₀ (HPMoV) was encapsulated into the Cu₃(BTC)₂ framework (H₃BTC = 1,3,5-benzenetricarboxylic acid) via a liquid-assisted grinding (LAG) method to obtain nanocrystalline POM@MOF, i.e. NENU-9N, formulated as [Cu₁₂(BTC)₈(H₂O)₁₂][H₅PV₂Mo₁₀O₄₀]. The NENU-9N was characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and nitrogen adsorption–desorption analysis, indicating that NENU-9N has good crystallinity, excellent thermal stability, permanent porosity, and uniform particle size (about 70 nm). The catalytic performance of NENU-9N as a heterogeneous catalyst for the epoxidation of styrene using oxygen as the oxidant was studied. As a result, NENU-9N showed excellent catalytic performance (97.2% styrene conversion with 93.5% selectivity to styrene oxide) which can be attributed to the synergistic catalysis between Cu₃(BTC)₂ and HPMoV. The nanocrystalline catalyst had good stability and recyclability which can be recycled at least 5 times with no significant loss of catalytic activity. Furthermore, the catalytic mechanism of NENU-9N on the aerobic epoxidation of styrene was investigated by radical quenching experiments.