Chemistry of Sol-Gel Synthesis of Aluminum Oxides with In Situ Water Formation: Control of the Morphology and Texture

Abstract

Sol-gel synthesis of aluminum oxide from mixtures containing Al(O-s-Bu)_3, acetic acid, and sec-butyl alcohol, whereby water was produced in situ, was characterized by infrared, ^1H NMR, ^(13)C NMR, and ^(13)C and ^(27)Al magic angle spinning NMR spectroscopies. The molar ratio (R) of acetic acid to Al(O-s-Bu)_3 strongly influenced the nature of the product. When R was < 2, the product was an opaque gelatinous precipitate, and when R was ≥ 2, transparent gels were formed. Acetic acid played crucial roles in both the hydrolysis and condensation reactions that constitute the essential chemistry of the sol-gel synthesis. Acetic acid influenced the rate of hydrolysis by (1) reacting rapidly with Al(O-s-Bu)_3 to give Al(O-s-Bu_2-OCOCH_3), which is much less reactive than Al(O-S-BU)_3 in hydrolysis, (2) catalyzing condensation reactions, and (3) catalyzing the dehydration of sec-butyl alcohol to give water. Acetic acid is involved in all the reactions of the sol-gel synthesis and thus provides a delicate control of the chemistry. The ratio R also influenced the morphology of the products formed as a result of drying at temperatures in the range 50-300 °C; these products were characterized by infrared spectroscopy, scanning electron microscopy, and surface area/pore volume measurements. Upon heating, the gelatinous precipitate was transformed into a dense material with a low surface area and low pore volume. Prior to drying, this material contained principally oligomeric species with few OH ligands and little unreacted acetic acid, thus favoring intermolecular condensation reactions and collapse of the structure during drying. In contrast, gels produced at values of R ≥ 2 gave materials with higher surface areas and higher pore volumes. Prior to drying, these materials contained principally polymeric species incorporating acetate and OH ligands located close to each other, thus favoring intramolecular condensation reactions during drying. Removal of unreacted acetic acid from the gel led to high-surface-area and high-pore-volume materials.