Direct spectroscopic detection of framework-incorporated vanadium in mesoporous silica materials

Abstract

Framework-incorporated vanadium mesoporous silica materials with different contents in vanadium were obtained by a facile, direct synthesis at room temperature, using VOSO₄·5H₂O as the vanadium precursor. The porous characteristics of the samples and the coordination environment of the vanadia in the structure were studied by a combination of techniques: X-ray diffraction, N₂-adsorption/desorption, FT-Raman, FTIR-PAS and UV-Vis-DR, electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. A structural comparison is made using pulsed EPR and ENDOR spectroscopic techniques between vanadia deposited on the surface of MCM-41 by the Molecular Designed Dispersion method and as-synthesised samples of vanadia incorporated in the mesoporous silica framework using the above-mentioned synthesis method. The EPR study on the non-calcined samples proves the incorporation of a high amount of vanadium in the silica framework by the observation of a strong hyperfine coupling of the unpaired electron with ²⁹Si. It demonstrates the feasibility for EPR to reveal structural information on true incorporation of metal ions in framework positions leading to metal oxides.