A homogeneously dispersed silica dopant for control of the textural and structural evolution of an alumina aerogel

Abstract

We have prepared pure alumina and silica-doped alumina (9:1 Al:Si atom ratio) aerogels by supercritical drying of monolithic alcogels formed by slow, sub-stoichiometric (H 2O:Al=2.2) hydrolysis of solutions of alkoxide precursors (aluminum s-butoxide and tetraethyl orthosilicate) in s-butanol. Both aerogels were X-ray indifferent over a large temperature range, first ordering as the η transition form upon calcination at 1173 K. The silica dopant retarded sintering of the aerogel, thereby enhancing its surface area and delaying crystallization of α-alumina from 1373–1473 K. Silica did not, however, affect the surface-chemical properties of the X-ray amorphous alumina; both aerogels were inert to pyridine adsorption and butene isomerization probes at 523 K. Stabilization of structure and texture without alteration of surface-chemical properties is a result of the high dispersion of the silica dopant throughout the aerogel's bulk. Incorporation of silicon atoms into the growing pre-alumina network facilitates their insertion into vacancies in the transition alumina structure, thereby restricting lattice diffusion as a route for crystallization of the α phase.