Structural characterization of vanadium environments in MCM-41 molecular sieve catalysts by solid state 51V NMR

Abstract

The structure of vanadium oxide (VO_x) species in vanadium containing MCM-41 catalysts prepared by co-condensation or grafting, respectively, was investigated by a combination of Raman scattering, UV-vis diffuse reflectance, ATR-IR, and magic angle spinning (MAS) ⁵¹V as well as ²⁹Si NMR spectroscopy techniques. Simulations of the ⁵¹V MAS NMR spectra allowed the determination of chemical shift and quadrupole tensor parameters, which give valuable information about the nature of the VO_x units. Structural transformations of the supported vanadium oxide species for the catalyst in the dehydrated state and hydrated state were investigated to examine the effect of water molecules on the VO_x structures. The results reveal the presence of different VO_x structures for the hydrated samples, including dimeric species, oligomeric chains and isolated trigonal pyramid units. Upon dehydration, the predominance of oligomeric and/or dimeric units for the sample prepared by grafting was observed, while a considerable amount of isolated units was additionally detected for the sample prepared by co-condensation.