Near-infrared spectroscopic study of [AlO4Al12(OH)23(H2O)12]7+-O-Si(OH)3 nitrate crystals formed by forced hydrolysis of Al3+ in the presence of TEOS

The polymer [AlO4Al12(OH)23(H2O)12]7+-O-Si(OH)3 was prepared by forced hydrolysis of Al3+ 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 Al13-sulfate and Al13-O-Si(OH)3 are very similar, there are differences related to the bonding of the -O-Si(OH)3 group to the Al13-unit. The strong complex of bands around 7000 cm(-1) associated with the overtones and combination bands of the OH-stretching modes for Al13-sulfate is much weaker for Al13-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 Al13-unit are involved in the formation of the new Al13-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 Al13-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 AlO4 in the Al13-unit around 2890-2935 cm(-1). The interaction between the -O-Si(OH)3 group and the Al13-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 Al13-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.