Surface Modification of η–Al₂O₃ by SiO₂ Impregnation to Enhance Methanol Dehydration Activity

Silica impregnated η-Al₂O₃ catalysts (Si(number)Al) were prepared for methanol dehydration, with the number indicating the Si wt % added to η-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 η-Al₂O₃. The acid sites of the Si(1.0)Al catalyst were less than those of the η-Al₂O₃ catalyst but the two had similar activity. Si addition primarily increased the Lewis acid sites, although it generated a few new Brønsted 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 η-Al₂O₃ activity by SiO₂ addition. On the other hand, the new Brønsted and Lewis acid sites and the chemical hydrogen bonded sites created by Si addition should not be neglected.