Characterisation of oxide-supported alkene conversion catalysts using X-ray absorption spectroscopy

Abstract

An X-ray absorption spectroscopy (XAS) cell has been developed to allow the loading of air-sensitive catalyst precursors in a glove box and their in situ activation and operation to be monitored. This has been used to investigate two alumina-supported propene metathesis catalysts derived from Mo₂(OAc)₄ and Li₄[Mo₂Me₈]. The EXAFS data so derived has been analysed by spherical wave methods using ab initio phase shifts and back-scattering factors. The detailed structural features obtained are sample-history dependent, but the following general observations can be drawn about the mean surface species present. In all cases the quadruple Mo—Mo bond is cleaved on the surface; some samples show evidence of one (or fewer) metal atoms at longer distances. For the latter sample activation in vacuo at 70°C yielded sites approximating to [MoO₂(Me)₂(O-)]ⁿ. The addition of propene caused a change in the XAS spectra. There was evidence of a shell with MoC distance of ca 1.8 Å that can be attributed to a terminal carbene ligand. For one of the Mo₂(OAc)₄-derived catalysts there was also evidence for a molybdacyclobutane unit. The Li₄[Mo₂Me₈]-based materials also showed evidence of residual MoO bonds after exposure to propene. The Zr K-edge EXAFS of Zr(allyl)₄/oxide (oxide = silica and alumina) were very similar for a range of sample conditions, showing evidence for a residual allyl group and Zr-O bonds.