Different activities and selectivities of silica-alumina catalysts synthesized in aqueous and alcohol solvents

Silica-aluminas having 15 wt% Al₂O₃ and subjected to identical heat treatment were synthesized starting from (i) sodium silicate and aluminum nitrate in water (ASA) and (ii) tetraethyl orthosilicate and aluminum isopropoxide in ethanol solvent (SA-SG). Both catalysts possess strong Brønsted acid sites which are capable of oligomerizing propene or ethene. With methanol as a feed, only ASA had a high hydrocarbon yield, while SA-SG produced mostly the dehydration product, dimethyl ether. While SA-SG possesses strong acidic sites, it is suggested that these sites become ineffective because of the strong chemisorption of the water resulting from dehydration. The weaker acidic sites also present on the SA-SG catalyst are probably responsible for the formation of the dehydration product.