Covalent anchoring of N-hydroxyphthalimide on silica via robust imide bonds as a reusable catalyst for the selective aerobic oxidation of ethylbenzene to acetophenone

Abstract

Currently, the aerobic oxidation of ethylbenzene to acetophenone has been found to be efficient and environmentally friendly, but homogeneous N-hydroxyphthalimide (NHPI), being active in the catalytic transformation suffers from separation and recovery. Here, NHPI was covalently anchored on commercial SiO₂ using symmetric aromatic dianhydrides and (3-aminopropyl) triethoxysilane as precursors and a linking molecule, respectively, and the grafting bonds formed were convincing based on their IR absorption, chemical state(s) of N atoms, and the decomposition temperatures. Also, the catalyst synthesized using pyromellitic dianhydride (PMDA) as the precursor presented a grafting density of 0.70 mmol_N–OH g_silica⁻¹. The immobilized NHPI catalyzed the selective aerobic oxidation of ethylbenzene to acetophenone, and an ethylbenzene conversion of 63.8% and a selectivity to acetophenone of 79.0% were observed under optimal reaction conditions. An excellent catalytic reusability of the grafted NHPI catalyst was observed in repeated evaluations and this could be attributed to the confirmed stability of the structure and composition arising from the robust imide bonds anchoring. The anchoring method and the resulting NHPI catalysts could open access to converting organics with benzylic C–H bond(s) into their oxygenated value-added products, suggesting a promising application prospect.