Vanadium site in VAPO-5: Characterization and catalytic properties in liquid-phase alkene epoxidation and benzylic oxidation

Abstract

Solid state NMR ( 51V, 27Al, 31P) and ESR were used to characterize the vanadium site in VAPO-5, as obtained by a fluoride-assisted synthesis. It was found that the main species at low vanadium content is a monomeric vanadyl unit in square pyramidal or distorted octahedral coordination, both in oxidation state (IV) for the as-synthesized material and in oxidation state (V) for the calcined material. Chemical compositions of the samples and site geometries strongly suggest a phosphate environment, and vanadyl ions are thought to occupy aluminium sites. At higher vanadium contents (1.3% V/(V+Al+P) atomic ratio) tetrahedral vanadium (V) is also formed upon calcination. At 3.0% V/(V+Al+P), the material is not entirely stable towards calcination. The vanadium species readily interacts with water, tert-butanol and tert-butyl hydroperoxide (TBHP). VAPO-5 was found to be an active and very selective catalyst for the epoxidation of allylic alcohols with TBHP. It also catalyses the epoxidation of simple alkenes and the side chain oxygenation of aromatics using the same oxidant. The epoxidation reactions involve a heterolytic oxygen transfer from what is probably a vanadium alkylperoxo surface complex.