Insight into the activation mechanism of methylene C–H bond over Ce–Al composite oxide

Abstract

Oxidation of methylene compounds remains a critical challenge due to the difficult activation of C–H bond. Herein, Ce–Al composite oxide was prepared and employed for oxidation of 2-alkyl-anthrone and anthracene to corresponding anthraquinone (AQ) which is used widely in the H₂O₂ production by the anthraquinone process and fine chemical industries, with air at mild conditions. Characterization results show that incorporation of Ce, existing partially in CeAlO₃ phase, results in the generation of oxygen vacancies and enhancement of reducibility. The optimal Ce–Al composite oxide (10%CeAl) exhibits superior performance in oxidation of 2-alkyl-anthrone and anthracene to AQ. The conversion of 100% could be achieved for 2-alkyl-anthrone to AQ at reaction time of 8 h and for 2-alkyl-anthracene to AQ at 15 h over 10%CeAl, with 100% selectivity. In-situ electron paramagnetic resonance measurements and DFT calculations reveal that the Ce–O–Al structure in CeAlO₃ phase promotes the formation of carbon-centered radical R^• through altering the C–H cleavage mechanism from hydrogen atom transfer to proton-coupled electron transfer (PCET), and superoxide anion radicals (O₂^•−) formed on oxygen vacancies promote oxidation of R^•.