Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts

Abstract

We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(O<sup>i</sup>Pr)<sub>3</sub>) to thermally pretreated silica under solvent-free conditions. VO(O<sup>i</sup>Pr)<sub>3</sub> 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 <sup>51</sup>V and <sup>13</sup>C 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 VO<sub>4</sub> surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and <sup>51</sup>V 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 VO<sub>4</sub> sites.