Chemisorbed O2-Driven Radical-Mediated Baeyer–Villiger Oxidation on Cu Surface

Abstract

Our study addresses the critical gap in understanding the aerobic Baeyer–Villiger oxidation (BVO) of cyclohexanone on transition-metal oxide and their heavily reduced metallic surfaces. BVO, which produces caprolactone, is rarely reported on metallic surfaces and lacks a complete mechanistic pathway comparison for these two surfaces. Moreover, identifying intermediates, reactive species, or nucleophilic oxidants in the BVO reaction and justifying the catalyst’s role in their formation and overall reaction remain challenging. The study attempts to reveal the feasibility of the formation of adsorbed O₂^•– on the metallic Cu(111) surface and that the O₂^•– adsorption on the Cu(111) surface could make it an active catalyst for BVO. In this regard, four different Cu-based catalysts (S1, S2, S3, and S4) were prepared using an aqueous method with increasing concentrations of the reducing agent of l-ascorbic acid. S1, S2, and S3 are Cu-oxide-rich, while S4 is predominantly metallic Cu. A multiscale experimental and theoretical approach proposed that the BVO reaction, in the presence of benzaldehyde as a sacrificial agent and molecular oxygen (O₂) as an oxidant, follows a radical-mediated pathway on the Cu metallic surface (S4). The S4 catalyst achieved 97% oxidation of cyclohexanone, attributed to an optimal density of chemisorbed O₂^•– and abundant Cu(111) surfaces compared with 45% observed with the S1 catalyst. Nudge elastic band (NEB) calculations, in situ Fourier transform infrared (FTIR) studies, and spectroscopic analysis revealed that the reaction proceeds via a benzoyl radical intermediate, formed through the abstraction of the α-H atom from benzaldehyde (by chemisorbed O₂^•–) and activation of the carbonyl group of cyclohexanone via Lewis’s acid–base interaction. Adsorption energy interpretations further corroborate the higher reactivity of O₂^•– radical over peroxide radical (O₂^2–), which are formed by O₂ adsorption on the Cu(111) surface in parallel and perpendicular configurations.