Electron spin resonance study of rearrangements in the co-ordination of Cr5+ and V4+ complexes due to chemisorption

Abstract

The surface chemistry of Cr⁵⁺ complexes in chromia/silica catalysts for the polymerization of ethene is investigated. To this end not only the chromia/silica system is studied, but also the related combinations chromia/alumina, vanadia/silica and vanadia/alumina. Electron spin resonance spectra are used to follow the rearrangements in the co-ordination of the Cr⁵⁺ and V⁴⁺ ions, caused by contact with a number of different gases of vapours: H₂O, air, CO, C₂H₄, HCI and NH₃. It is found that the atomic arrangement at the surface of some of the systems studied is very flexible and easily influenced by, for instance, contact at room temperature with H₂O, HCl or NH₃. It is suggested that such a flexibility—encountered especially in chromia/silica—is a prerequisite for the catalytic activity. The active species in the fresh catalyst is Cr⁵⁺ in tetrahedral co-ordination. A mechanism is proposed according to which in the initiation step of the polymerization the co-ordination number is increased from four to five. An empty sixth co-ordination site may then be available to accommodate ethene monomers.