Near-infrared spectroscopic study of [AlO 4Al 12(OH) 23(H 2O) 12] 7+-O -Si(OH) 3 nitrate crystals formed by forced hydrolysis of Al 3+ in the presence of TEOS

Abstract

The polymer [AlO 4Al 12(OH) 23(H 2O) 12] 7+-O-Si(OH) 3 was prepared by forced hydrolysis of Al 3+ up to an OH/Al molar ratio of 2.0 in the presence of monomeric orthosilicic acid. Crystalline material was obtained by slow evaporation. Although the near-infrared spectra of the Al 13-sulfate and Al 13-O-Si(OH) 3 are very similar, there are differences related to the bonding of the -O-Si(OH) 3 group to the Al 13-unit. The strong complex of bands around 7000 cm −1 associated with the overtones and combination bands of the OH-stretching modes for Al 13-sulfate is much weaker for Al 13-O-Si(OH) 3 and the opposite is true for the complex of bands around 5000 cm −1 associated with the water overtone and combination modes, suggesting that the outer OH-groups of the Al 13-unit are involved in the formation of the new Al 13-O-Si(OH) 3 units. A weak band around 7370–7631 cm −1 is interpreted as the overtone of the SiOH stretching vibration around 3740 cm −1. A low intensity band, absent for Al 13-sulfate and -nitrate is observed around 5550–5570 cm −1 and is interpreted as the overtone of the OH-stretching mode of the OH-groups in the vicinity of the central AlO 4 in the Al 13-unit around 2890–2935 cm −1. The interaction between the -O-Si(OH) 3 group and the Al 13-unit has a small influence on other bands like the combination modes of water in the 4400–4800 cm −1 region, which show a small shift towards higher wavenumbers. The internal OH-groups in the Al 13-complex are relatively shielded by the water molecules and therefore do not reflect the influence of the -O-Si(OH) 3 in their band positions.