Acid site properties of thermally stable, silica-doped alumina as a function of silica/alumina ratio and calcination temperature

Abstract

Acid site properties of silica-doped aluminas prepared by a simple solvent deficient hydrolysis of the alkoxides was investigated. The total acid concentration (Brønsted and Lewis sites) of silica-doped aluminas (SDAs) calcined in the range of 700–1200°C with Si/Al ratios of 5, 15, 27wt% was determined using temperature-programmed desorption of ammonia (ammonia-TPD). 27Al solid state MAS NMR (Al SS MAS NMR) was used to measure the intrinsic Lewis acid site concentration, and FTIR was also used as a separate measure of the Brønsted and Lewis acid site concentration. Results indicate that removing hydroxyl groups in the form of water molecules through calcination result in a lower concentration of Brønsted acid sites. Calcination at higher temperature also results in the transformation of unsaturated 5-coordinated aluminum (a strong Lewis acid) to higher concentrations of 6 and 4-coordinated aluminum in 5, 15 and 27% silica-doped alumina samples. Therefore, the total acid site concentration (Brønsted and Lewis sites) decreases by increasing the calcination temperature. In addition, the data show that increasing the silica/alumina ratio increases both the Brønsted and Lewis acid site concentrations. Based on these results, the acid site concentrations can be controlled by altering the Si/Al ratio and calcination temperature while maintaining high surface areas, large pore volumes, and large pore diameters.