Raman and infrared spectroscopic investigation of the neutralisation of aluminium in the presence of monomeric orthosilicic acid

The tridecameric aluminium 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 with increasing Si/Al ratio up to 1.00. Crystalline material was obtained by slow evaporation. The Raman spectra are characterised by the similar bands as the Al-13 nitrate. The Fourier transform-IR spectrum of the Al-13 crystals without any Sio(OH)(3) added indicates that previous assignments of the bands between 450 and 800 cm(-1) to the sulphate in the crystal structure of basic aluminium sulphate are not correct for the Al-13 in this study. These bands are more likely to correspond with deformation bands within the Al octahedra of the Al-13 complex. Upon increasing the Si/Al ratio, a part of the nitrate is replaced as evidenced by the decrease in intensity of one of each set of double nitrate bands in both the Raman and Fourier transform-IR spectra. The incorporation of SiO(OH)(3) in the crystal structure results in the formation of new Fourier transform-IR bands at 3740 cm(-1) assigned to silanol groups, 1106 cm(-1) assigned to Si-O stretching mode and a band at 724 cm(-1) assigned to Si-O-Al deformation mode. The formation of the [AlO4Al12(OH)(23)(H2O)(12)](7+)-O-Si(OH)(3) therefore is thought to proceed via the replacement of incorporated nitrate groups in the Al,, structure instead of via the hydroxyl groups. (C) 1999 Elsevier Science B.V. All rights reserved.