Aerobic oxidation of benzyl alcohol in methanol solutions over Au nanoparticles: Mg(OH)2 vs MgO as the support

Abstract

Magnesium oxide and magnesium hydroxide materials containing supported gold nanoparticles (NPs), Au/Mg(OH)2 and Au/MgO, were prepared from the commercial MgO through the deposition–precipitation (DP) method and characterized by XRD, XPS, HRTEM, FTIR spectroscopy and N2 adsorption techniques. It was found that the starting MgO support was fully transformed into the Mg(OH)2 phase during the DP procedure. A nearly complete dehydration of the magnesium hydroxide and formation of Au/MgO was achieved through the reductive treatment at 500°C, whereas the treatment at 350°C still resulted in the Au/Mg(OH)2 material. The FTIR analysis showed a much higher ability of the Au/MgO surface to adsorb both benzyl alcohol and benzaldehyde (ca. 10 and 3 times, respectively), as compared to Au/Mg(OH)2. Probably for this reason, the Au/MgO catalyst exhibited ca. 50% higher catalytic activity in the aerobic oxidation/oxidative methoxylation of benzyl alcohol in the methanol solutions with respect to the amount of surface gold atoms as compared to the Au/Mg(OH)2 catalyst, in spite of a larger size of the Au NPs. In addition, the thermal treatment of the catalyst at 500°C to dehydrate the support allowed to suppress the undesired side reaction between benzyl alcohol and primarily formed benzaldehyde to give benzyl benzoate.