Identification and characterisation of stable phases of silicotungstic acid, H4SiW12O40·nH2O

Abstract

Thermal analysis data recorded from silicotungstic acid show the existence of stable phases of composition H4SiW12O40·nH2O where n=24, 14, 6 and 0. FT-IR data confirms the presence of the Keggin anion, SiW12O404−, in each of these phases. In addition, dehydration of the higher hydrates (n>6) is shown to occur via the loss of zeolitic type water followed by (n<6) the removal of water associated with the acidic protons. Each of the hydrated phases, as well as the anhydrous phase, has been characterised by X-ray powder diffraction and the unit cell parameters are reported. 29Si MAS NMR spectroscopy is consistent with a single silicon site in each of the four single phases and deshielding of the silicon nucleus in the range n=6 to n=0 is associated with the transfer of acidic protons from the H5O2+ ions to random locations on oxygen atoms in the Keggin unit. The observed increase in magnitude of T1 (29Si) for the hexahydrate is interpreted in terms of a reduction in motional freedom of the water molecules in this hydrate compared to those in the higher hydrates. Nominal hydrated states between n=6 and 14, as well as those between n=14 and 24, are shown by X-ray powder diffraction to be composed of the stable hydrated phases in the given ranges.