Selective aerobic oxidation of benzyl alcohol on inexpensive and reusable ZnO/MnCO3 catalyst

The selective synthesis of benzaldehyde (B) by oxidation of benzyl alcohol (BA) was studied at 373 K using ZnO-MnCO3 catalysts, O2 as oxidant and mild reaction conditions. An inexpensive commercial MnCO3 sample was impregnated with aqueous solutions containing different concentration of Zn2+ cations to obtain a series of ZnO-MnCO3 catalysts with 1.0–22.6 wt% Zn. Catalysts were characterized by N2 physisorption, atomic absorption spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy techniques. Benzaldehyde was selectively formed during liquid phase oxidation of benzyl alcohol but the catalytic performance of the ZnO-MnCO3 materials depended on Zn loading. The maximum initial reaction rate value was measured on the most promising catalyst (15.4 wt% Zn) and a maximum of 88% benzaldehyde yield was achieved on this catalyst after 6 h of reaction using toluene as solvent. The catalytic activity of the ZnO-MnCO3 materials could be assigned to a synergistic contribution of Zn and Mn species. While the main role of ZnO would be related to its capacity to chemisorb the alcohol, the role of highly oxidized Mn surface species (mainly Mn4+) generated during catalyst preparation, is to participate in de redox mechanism involved in benzyl alcohol oxidation toward benzaldehyde. The effect of chemical nature of polar and apolar aprotic solvents was investigated during the aerobic oxidation over 15.4ZnMn. Apolar solvents, in which the oxygen solubility is higher than in the benzyl alcohol, are more suitable for the reaction. An optimum in solvent basicity requirements (hydrogen-bond-acceptance) in order to optimize the catalyst activity was found. The regeneration and reuse of the most promising catalyst were also investigated. A mild thermal treatment at 493 K in flowing air was enough to fully recover the activity of the 15.4ZnO-MnCO3 catalyst.