Effect of preparation methods on properties of amorphous alumina/silicas

Abstract

Alumina/silicas have been prepared by three methods: complexing-agent-assisted sol–gel, coprecipitation and hydrogel kneading. The properties of these alumina/silicas were investigated by solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR), ammonia temperature-programmed desorption (TPD) and nitrogen adsorption. Al—O—Si bond formation is more effectively performed in the sol–gel alumina/silicas than in the conventional ones. This was deduced from the shift profile in the ²⁹Si NMR spectra of the sol–gel alumina/silicas with the change in the Al/Si ratio, which is comparable to that of zeolites. Because of the effective Al—O—Si bond formation by the sol–gel method, the sol–gel alumina/silicas show much higher acid amounts than the corresponding conventional ones. As certain complexing agents can level the hydrolysis rates of silicon and aluminium alkoxides and bind both metal atoms, the use of a complexing agent is effective for the formation of Al—O—Si bonds and the preparation of alumina/silicas with high miscibility and controlled properties.