Surface Modification of eta-Al2O3 by SiO2 Impregnation to Enhance Methanol Dehydration Activity

Abstract

Silica impregnated eta-Al2O3 catalysts ( Si( number) Al) were prepared for methanol dehydration, with the number indicating the Si wt % added to eta-Al2O3. The Si(0.25) Al and Si( 0.50) Al catalysts with the greatest number of acid sites had the highest methanol dehydration activities among the prepared catalysts. Si addition up to 0.5 wt % increased the number of the acid sites, which was slightly decreased by further Si addition. The number of weak acid sites correlated well with the catalytic activity for methanol dehydration, but was not enough to explain the higher activity of the catalysts containing silica compared with eta-Al2O3. The acid sites of the Si(1.0) Al catalyst were less than those of the eta-Al2O3 catalyst but the two had similar activity. Si addition primarily increased the Lewis acid sites, although it generated a few new Bronsted acid sites and chemical hydrogen bonded sites. Nonetheless, it was apparent that the increase of Lewis acid sites was the main reason for the enhancement of eta-Al2O3 activity by SiO2 addition. On the other hand, the new Bronsted and Lewis acid sites and the chemical hydrogen bonded sites created by Si addition should not be neglected.