Immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on metal-substituted hexagonal mesoporous silica

Hexagonal mesoporous silicate (HMS) materials containing Ti, Zr and Al ions were prepared using the neutral template route. The effect of the cation on the immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on mesoporous materials was studied. The texture of the samples was studied by the N-2 adsorption-desorption isotherms, and their structure by X-ray diffraction (XRD), UV-Vis diffuse reflectance (DRS), infrared (IR) and X-ray photoelectron spectroscopy (XPS), and H-1 and P-31 nuclear magnetic resonance (NMR). Incorporation of different metals into the framework of HMS led to change in the structural and surface properties of mesoporous silica. All metal-containing samples showed a lower surface area compared to that of pure silica. The introduction of Zr and Al cations led to the development of textural mesoporosity in addition to the framework mesoporosity, characteristic of HMS and Ti-containing mesoporous silica (TiHMS) materials. IR and H-1 and P-31 NMR spectroscopic data revealed that the species present on the surface of HMS and TiHNIS mesoporous materials after impregnation with solutions of 12-molybdophosphoric and 12-tungstophosphoric acid are non-degraded heteropoly anions with a preserved Keggin unit. Partial degradation of the anion was observed for zirconium-containing silicate samples (ZrHMS)-suppor-ted acids, the extent of which was stronger for supported molybdophosphoric acid in comparison with that of tungstophosphoric acid. (C) 2003 Elsevier Science B.V. All rights reserved.