Control of the Acidity and Surface Area of Silica-Aluminas by a Chemical Mixing Procedure

Abstract

In order to control the properties of silica-alumina, silica-aluminas were synthesized from aluminum alkoxides and tetraalkoxysilanes in diols by chemical mixing procedure. The influence of diol structures on the specific surface areas and acidities of the silica-aluminas were investigated.
The bulkier diols generally gave the silica-aluminas with higher specific surface areas. Ethylene glycol and 1, 3-propanediol which can strongly coordinate to Si and Al, gave also the silica-aluminas with high specific surface areas. The less branched 1, 4-diols gave the silica-aluminas having extremely low specific surface areas.

According to Hammett indicators method, acidities of silica-aluminas were mainly Ho≥-8.2. Among 1, 2-diols, ethylene glycol and 1, 2-propanediol were found to give si licaaluminas with large acid amounts. Silica-aluminas prepared in 1, 3-diols had almost the same acid amounts irrespective of diol structure. It was found from the NH₃-TPD curves in the range of 200-600°C that the diol structure had big influence on the acidity of the silicaaluminas. The 1, 2-diols which tend to remain in the silica-aluminas before calcination gave silica-aluminas with high acid strength. This tendency was completely reverse when 1, 3and 1, 4-diols were used as solvents.
Accordingly, it seemed possible to control specific surface areas and acidities with this preparation method.