Details of Research Outputs

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.