Peptization and Al-Keggin Species in Alumina Sol

Vojmir Francetič and Peter Bukovec

University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerčeva 5, 1000 Ljubljana, Slovenija,
* Corresponding author: E-mail: vojmir.francetic@kabelnet.net

Abstract
A novel sol-gel proces for preparing alumina sol is reported in this article. Using this new method alumina sols can be prepared from inexpensive materials such as inorganic aluminium salts. Influence of the type and amount of acid (HCl(aq) and HNO₃(aq)), used for aluminium hydroxide peptization, were investigated.²⁷Al NMR, N₂ absorption (single- point BET), X-ray diffraction (XRD), infrared spectroscopy (IR) and dynamic thermoanalytical meauserments (TG) were used to detect the properties of the alumina sol and xerogel. The results of ²⁷Al NMR shows there are Keggin- Al₁₃ ⁷⁺ species in the sol. Besides Al₁₃ ⁷⁺ species some other reactive polymers with hexameric ring structure also exist in the sol. The results of this work show that the higher ratio n(acid) / n( Al(OH)₃) (peptization ratio) leads to an increase in crystallinity and amount of water in the aluminium xerogel, and to a decrease in specific surface area. The results also show that the role of acid is not only as catalyst for peptizing, but also as reactant to react with aluminium hydroxide to produce the Al₁₃ ⁷⁺ species, and other reactive aluminium polymers.

Keywords: Alumina, sol-gel, peptization