Partial oxidation of ethane over alumina-boria catalysts

Alumina-boria catalysts can be used in selective oxidation of ethane for ethylene production. The alumina-boria catalysts have been prepared from boric acid using a classical impregnation method on porous and non-porous alumina. To obtain more dispersed boron oxide on the surface of the support, some alumina-boria samples were prepared by chemical vapour deposition and a comparison with impregnated samples has been performed in the ethane selective oxidation. The contribution of gas-phase reactions to the formation of ethylene and the role of boron oxide at different percentages on alumina for ethane conversion are discussed. A maximum of activity (18.7% conversion) was observed with the sample with 30 wt.-% boron oxide on non-porous alumina. The main product was ethylene (16.7% yield) while CO and CH4 were the secondary products, coming from the decomposition of ethylene. A correlation has been demonstrated between catalytic activity and the acidity of the alumina-boria catalysts. The boron oxide (with its specific acidity) was responsible for the activation of ethane and for the stabilisation of the ethylhydroxyperoxide radical formed during the reaction and favouring the partial oxidation reaction leading to ethylene formation while alumina was shown to result into deep oxidation due to its too strong acidic sites. It is concluded that the role of alumina consists mainly in dispersing boria, resulting in a much more selective catalyst for ethane oxidation to ethylene than the support itself.