Characterization of tetrahedral vanadium-containing MCM-41 molecular sieves synthesized at room temperature

A series of mesoporous vanadosilicate V-MCM-41 molecular sieves with variable Si/V ratios have been synthesized at room temperature using a minimum amount of template. The chemical environment of the vanadium centers in V-MCM-41 was investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, diffuse reflectance UV-visible (UV-vis) spectroscopy, electron spin resonance (ESR), and Si-29 and V-51 nuclear magnetic resonance (NMR), and the optimal. synthesis condition have been established. The increase of the unit cell parameter and decrease of Q(3)/Q(4) ratio of Si-29 NMR spectra with vanadium content suggest the incorporation of vanadium in the framework of the MCM-41 structure. UV-vis and V-51 NMR studies confirm the presence of V5+ ion in the tetrahedral position. In the case of a freshly synthesized sample, VO2+ in a square pyramidal arrangement was detected by ESR. After calcination, V5+ species remain in the tetrahedral arrangement, confirming the stability of the material. Moreover, these solids were found to be catalytically active in the oxidation of toluene and hydroxylation of benzene with H2O2.