Synthesis and comparative catalytic study of zinc oxide (ZnOx) nanoparticles promoted MnCO3, MnO2and Mn2O3for selective oxidation of benzylic alcohols using molecular oxygen

Abstract

Selective oxidation catalysts play an important role in several industrial processes. Efforts towards finding better oxidation catalysts have always been a focus of studies. In this study we report the synthesis of ZnOxdoped MnCO3[ZnOx(1%)–MnCO3] via a facile co-precipitation method, which upon calcination at different temperatures yields different manganese oxides i.e., [ZnOx(1%)–MnO2] and [ZnOx(1%)–Mn2O3]. A comparative catalytic study was carried out to evaluate the catalytic performance of carbonate and oxides for the selective oxidation of benzyl alcohol. During this study various catalysts were prepared by varying the w/w percentage of ZnOxand calcination temperature. The catalytic performance in the liquid-phase oxidation of benzyl alcohol with molecular oxygen as the oxidant was examined and the influence of various parameters such as reaction temperature, reaction time and catalyst concentration has been thoroughly investigated. The ZnOx(%1)–MnCO3obtained after calcination at 300ºC showed the best catalytic performance and possessed highest surface area which suggests that the calcination temperature and surface area play a crucial role. Typically, an extremely high specific activity of 60 mmol·g-1·h-1with complete benzyl alcohol conversion as well as product selectivity of >99% was achieved within very short reaction time (4 min). It was found that ZnOxnanoparticles also play an important role in enhancing the catalytic activity for the aerobic oxidation of alcohols. A selective conversion of substituted benzyl alcohols and high benzaldehyde selectivity was observed at different reaction times under mild conditions. Additionally, the catalyst was recycled six times without considerable loss of the catalytic performance and the selectivity remained almost unchanged.