An efficient H2O2-based propylene to propylene oxide (HPPO) reaction catalyzed by ZnO/ZnO2 materials

Abstract

Propylene oxide (PO) is an essential feedstock in the plastic industry. Herein, unprecedented, inexpensive and robust zinc oxide (ZnO)-based catalysts were prepared. A ZnO nanorod (ZnO-NR) catalyst was synthesized using a solvothermal method. Another type of ZnO catalyst supported and immobilized on a mesoporous MCM-41 material (ZnO/MCM-41(x)) was also prepared with varied proportions (x = 0.82–9.41 wt%) of Zn content. The catalytic reactions of propylene epoxidation over ZnO-NR and ZnO/MCM-41(x) with H₂O₂ as an oxidant and acetonitrile as a solvent were studied at 30–70 °C and 5–20 bar. The ZnO catalysts were found capable of catalyzing the reaction with high H₂O₂ utilization and PO selectivity. Among them, ZnO/MCM-41(7.99) achieved nearly 100% PO selectivity and the highest turnover number of 124.4 (moles of PO per moles of Zn). The fresh catalysts, including ZnO-NR and ZnO/MCM-41(7.99), the spent catalysts, and the ZnO₂ nanoparticles were characterized by synchrotron powder X-ray diffraction, transmission electron microscopy, and X-ray absorption spectroscopy. The fractions of ZnO₂ in the spent catalysts were quantified to be higher than 50%. Electron paramagnetic resonance and X-ray photoelectron spectroscopy revealed that the ZnO₂ phase formed by reacting ZnO with H₂O₂ in acetonitrile, stored as green redox oxide materials, contained hydroperoxide, peroxide and superoxide species, which can be essential for catalytic production of PO. The observed ¹⁸O enrichment in PO using H₂¹⁸O₂ suggested that the reactive oxygenated species are generated from H₂O₂ and anchored on/in ZnO₂ for electrophilic epoxidation, with the assistance of acetonitrile.