In situ-created Mn(iii) complexes active for liquid-phase oxidation of alkylaromatics to aromatic ketones with molecular oxygen

Abstract

The liquid-phase oxidation of alkylaromatics with molecular O₂ over manganese hydroxide catalysts supported on various oxides was examined. The reaction kinetics in ethylbenzene oxidation with O₂ indicated some configurational/electronic alterations of Mn species during the induction period of the reaction due to their interaction with benzoic acid. Characterization by means of FT-IR and XAFS revealed that the Mn(OH)_x species was easily transformed into a monomeric Mn(III) species coordinated with the PhCOO⁻ ligand and anchored on the oxide support surface during the initial induction period of the reaction, which was the real active species. Among the catalysts examined, the in situ-created Mn species supported on γ-Al₂O₃ particularly exhibited superior catalytic activity with good ketone selectivity compared to the other supported Mn catalysts. Moreover, leaching and agglomeration of the active Mn(III) species were scarcely observed, and the recovered catalyst could be reused without significant loss of activity in the presence of the benzoic acid ligand. Such an in situ-created Mn(III) species offers a simple, reusable and efficient oxidation system for alkylaromatics, such as ethylbenzene and diphenylmethane, with activity greater than that of its unsupported counterpart.