We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(OiPr)3) to thermally pretreated silica under solvent-free conditions. VO(OiPr)3 is found to react with both site-isolated silanol (Si-OH) groups and strained siloxane (≡Si-O-Si≡) bridges at the silica surface. Solid-state 51V and 13C MAS NMR confirms the formation of two slightly different vanadium species associated with the two anchoring mechanisms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ Raman spectroscopy, and thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) were used to study the subsequent calcination, revealing the formation of a transient V - OH intermediate upon the release of propene, followed by the formation of isolated VO4 surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and 51V MAS NMR of the calcined material confirm the conversion of the two original vanadium sites to a species with a single isotropic shift, confirming the formation of isolated, tetrahedral VO4 sites.
Bibliographical noteFunding Information:
The authors acknowledge financial support from the University of Wisconsin-Madison. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We specifically acknowledge support from Sam Burt and beamline engineer Dale Brewe, who assisted in the collection of XANES data reported in this publication.
© 2016 American Chemical Society.