The role of precursors in the structure of SiO₂-Al₂O₃ sols and gels by the sol-gel process

Abstract

Binary sols and gels of SiO₂-Al₂O₃ were prepared using tetraethyl orthosilicate and each of four aluminum compounds; aluminum di (sec-butoxide) ethylacetoacetic ester chelate (AC), aluminum nitrate nonahydrate (AN), aluminum formoacetate (AF), and boehmite sol (BS) made from aluminum i-propoxide. The structure and the evolution of the Si-O-Al bonds in SiO₂-Al₂O₃ sols and gels were investigated by ²⁷Al nuclear magnetic resonance (NMR), Infrared absorption spectra, DTA, and X-ray diffraction. The formation of Si-O-Al bonds differs depending on the aluminum compounds used as raw materials. The ratio of Al(IV) to {Al(IV) + Al(VI)} is related to the microstructural homogeneity of the gels. When AC is used as a raw material, the Si-O-Al bonds are formed in the sol state and resultant gel shows good microstructural homogeneity. In case of AN, the Si-O-Al bonds are not formed either in the sol or the wet gel state. The bonds are formed by drying the gel before heat-treatment temperature reaches 300 ∼ 400°C, resulting in good microstructural homogeneous gel. When AF is used, the Si-O-Al bonds are formed in the sol state but the ratio of Al(IV) to {Al(IV) + Al(VI)} is lower than when using AC. Microstructural homogeneity of the gel is ranked between AC or AN and BS. Using BS, the Si-O-Al bonds are not formed in the sol solution, and the change in the coordination number of the gel is similar to that of boehmite gel. The Microstructural homogeneity of the gel is the worst among the BS gels, which were prepared by using the four aluminum raw materials.