Low temperature oxidation of ethanol to acetic acid using gold-based catalysts

Abstract

Over the past decades it has become clear that gold supported nanoparticles are surprisingly active and selective catalysts for several green oxidation reactions of oxygen-containing hydrocarbons, in particular alcohols using molecular oxygen as the stoichiometric oxidant. In this research, ethanol was oxidized to acetic acid using molecular oxygen as oxidant in a presence of gold supported catalysts. Catalysts containing 1 wt.% Au supported on TiO2, ZnO, Al2O3, C and MgAl2O4 were prepared using different methods which includes inverse deposition precipitation, incipient wetness impregnation and immobilization of metal sol. It was found that TiO2 gave the most superior activity as support for gold in ethanol oxidation, followed by ZnO and Al2O3. High ethanol conversion of 93% and selectivity of 97% to acetic acid was achieved when using TiO2 as support for gold after 20 h reaction. The effect of using excess oxygen was also investigated in the study. It was noticed that at higher oxygen pressures it is possible to convert ethanol to 100% conversion. When excess oxygen was introduced into the autoclave, an ethanol conversion of 99.4%, selectivity of 99.8% and yield of 99.2% was achieved without altering any reaction conditions. The catalyst recycling studies were also conducted on Au/TiO2, The activity studies showed that the catalyst can only be used twice and still maintain the ethanol conversion of greater then 90%. When the catalyst was re-used for the third time it was noticed that the catalytic activity had dropped to 29% and this was due to leaching of gold on the catalyst which was observed in a third cycle. The concentration of ethanol was varied from 5 – 40 wt.% and it was noticed that at high ethanol concentration, the formation of ethyl acetate was most favoured compared to acetic acid. Acetic acid was only favoured at low ethanol concentration.