Polyethylene Glycol–Bridged Diimidazolium Polyoxometalates: Thermoregulated Reusable Catalysts for Selective Oxidation of Toluene to Benzaldehyde

The oxidation of toluene to benzaldehyde holds great industrial significance, and it typically involves toxic reagents and/or affords difficult-to-separate product mixtures. Herein, polyethylene glycol (PEG)–bridged diimidazolium polyoxometalates (POMs) with a Keggin-type tungstovanadophosphoric heteropolyacid anion and various PEG-functionalized diimidazolium cations were prepared and evaluated as catalysts for the eco-friendly oxidation of toluene using aqueous tert -butyl hydroperoxide in acetonitrile. The bridging of diimidazolium cations by PEG enabled the fine-tuning of heteropolyanion redox properties and increased catalyst reactivity and solubility in the reaction system. The best-performing POM exhibited thermoregulated phase-separation behavior, that is, it was fully soluble in the reaction medium at elevated temperatures and hardly soluble at room temperature. Further, it showed almost no performance deterioration after sevenfold reuse and achieved an acceptable toluene conversion (82%) and benzaldehyde selectivity (68%). Thus, our work enables the development of eco-friendly processes for the catalytic oxidation of industrially relevant hydrocarbon and thus facilitates the establishment of a more sustainable society.