Preparation of meso-macro bimodal porous amorphous silica-alumina as an acidic catalyst

A bimodal amorphous silica-alumina with macropores and mesopores of sizes 2–4 µm and 10 nm, respectively, was prepared with an alumina content of up to 30 wt% using a sol–gel method. The requisite amount of water to generate the 2–4 µm macropores decreased with increasing alumina content. However, this decrease reached a limiting value, and the gelation temperature needed to be increased at alumina contents ≥20 wt%. Because the alkaline resistance increases with increasing alumina content, the aging temperature needed to be increased to obtain 10 nm mesopores. In the acidic isomerization of 1-octene over silica-alumina, homogeneous sol–gel samples were more active than those heterogeneously impregnated. The conversion of 1-octene increased with increasing alumina content up to 15 wt%, while the activity decreased above 30 wt% because of because of Al₂O₃ aggregation. The catalytic results, together with the temperature-programmed desorption of NH₃, indicate that the sol–gel sample generates Brønsted acid sites via the formation of Si-O-Al bonds, whereas the impregnated sample probably generates Lewis acid sites.