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.