Aerobic Baeyer-Villiger oxidation of ketones over mesoporous Mn-Ce and Mn-Co composite oxides in the presence of benzaldehyde: The effect of valence state
Aerobic Baeyer-Villiger oxidation of ketones into corresponding esters were performed over mesoporous Mn-Ce and Mn-Co composite oxides prepared by co-impregnation method and benzaldehyde was used as the sacrificing agent. It was found that the best conversion and selectivity were obtained when the Mn/Ce mole ratio was 1:2 (denoted as Mn 0.33 Ce 0.67 O s ) through investigating the effect of Mn/Ce mole ratio on the catalytic activity. Interestingly, Mn 0.33 Co 0.67 O s (Mn/Co mole ratio was 1:2) catalyst had no catalytic activity for this reaction. X-ray diffraction (XRD) results showed that MnO x was completely dissolved into the CeO 2 lattice, and Mn 0.33 Co 0.67 O s sample was existed in the form of spinel structure (Co, Mn) (Mn, Co) 2 O 4 and obvious peaks of Co 3 O 4 . Nitrogen adsorption-desorption measurements revealed that Mn 0.67 Ce 0.33 O s , Mn 0.5 Ce 0.5 O s , Mn 0.33 Ce 0.67 O s and Mn 0.33 Co 0.67 O s materials possessed mesoporous structure. X-ray photoelectron spectroscopy (XPS) analysis showed that the highest concentration of Ce 3+ on the surface of Mn 0.33 Ce 0.67 O s catalyst, which could create unsaturated chemical bonds, charge imbalance, and oxygen vacancies on the surface of samples, it is beneficial to the oxidation reaction. Additionally, XPS data revealed that Mn 0.33 Ce 0.67 O s catalyst has great potential in catalytic oxidation due to the large amount of active oxygen species on the surface. In addition, manganese and cobalt species on the surface of Mn 0.33 Co 0.67 O s are almost present in the form of Mn 2+ and Co 3+ , it is known that Mn 2+ can serve as a potent antioxidant and scavenge the activity of active oxygen species, while Co 3+ ions can prevent the formation of oxygen vacancies and inhibit the release of reactive oxygen species, thus the presence of Mn 2+ and Co 3+ inhibited the catalytic activity of Mn 0.33 Co 0.67 O s catalyst. The diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-vis) and hydrogen temperature-programmed reduction (H 2 -TPR) results are in accordance XPS data.
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