Synthesis and characterization of tantalum silicalite molecular sieves with MFI structure

Tantalum silicalite-1 (TaS-1) with MFI structure was prepared using the hydrothermal synthesis method. X-ray diffraction, scanning and transmission electron microscopies, Fourier transform infrared spectroscopy, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, Si-29 magic-angle spinning nuclear magnetic resonance spectroscopy, temperature-programmed desorption of ammonia (NH3-TPD), physical adsorption of nitrogen and elemental analysis were then performed to evaluate its physico-chemical properties and provided evidence for the incorporation of Ta5+ into the zeolite framework. The unit cell volume of tantalum silicalite-1 increased linearly with increasing tantalum content, suggesting isomorphous substitution of Si4+ by Ta5+ in the lattice framework. The characteristic framework infrared spectra shifted to lower frequencies as Ta5+ was incorporated into the silicalite lattice and showed a characteristic absorption band at 965 cm(-1), probably due to Si-O-Ta linkages. In, the UV-vis spectrum, the absorption assigned to the presence of Ta5+ in the zeolite framework was observed at around 200 nm. In addition, NH3-TPD studies revealed weak acidity in tantalum silicalite-1 due possibly to [Ta ...(OH)-Si] sites in the framework. (C) 1999 Elsevier Science B.V. All rights reserved.