Hydrogen bonding effects assisted organic ammonium polyoxometalate salts catalysis for the selective oxidation of methacrolein and mechanism study

The selective oxidation of methacrolein (MAL) to methacrylic acid (MAA) is a key step in C4 oxidation for polymethyl methacrylate production, but the limited catalytic activity of polyoxometalates (POMs) hinders their industrial use. Here, we synthesized POM-IL hybrid catalysts via ion exchange and calcination. Ionic liquids disrupted the POM structure through hydrogen bonding, creating oxygen vacancies that optimized adsorption energy and prevented MAL over-oxidation to COx. The hydrogen bonding between ionic liquids and polyoxometalate anions, coupled with ammonium ions derived from ionic liquids, generated a synergistic effect that markedly improves the acidity and redox properties of the catalyst. The catalyst achieved 82.9 % MAL conversion, 88.9 % MAA selectivity, and exceptional stability over 80 h. In-situ infrared spectroscopy and partial density of states (PDOS) analysis identified Mo=Od as the active site, with ester intermediate formation as the rate-determining step. This study uncovers the influence of hydrogen bonding in ionic liquids on the structure-activity relationship of polyoxometalate anions, offering crucial insights into the mechanism of methacrolein selective oxidation.