Binding and decomposition of oxovanadium(IV) phthalocyanine, tetraphenylporphyrin and etioporphyrin on hydrotreating catalysts studied by X-ray photoelectron and ultraviolet–visible spectroscopies. Relevance to catalytic demetallisation

Abstract

The interactions of oxovanadium(IV) phthalocyanine, VOPc, oxovanadium(IV) tetraphenylporphyrin, VOTPP, and oxovanadium(IV) etioporphyrin, VOETP, with alumina, silica–alumina, fluorided silica, Co₃O₄, MoO₃, MoS₂, MoO₃/Al₂O₃ and Co–Mo/Al₂O₃ catalysts in the oxide and sulphide forms have been studied by X-ray photoelectron spectroscopy and u.v.–visible diffuse reflectance spectroscopy. Partial demetallisation of VOTPP and VOPc, but not VOETP, occurs on acidic (H⁺) supports. The VO²⁺ thereby released is reduced in the X.p.s. spectrometer; porphyrin-bound vanadium is not reduced.

Changes of core-electron binding energies of vanadium and nitrogen caused by interaction of the porphyrins with the supports is small, and there is no evidence of a strong interaction with the surface through vanadium or nitrogen. It is therefore suggested that the porphyrins are bound to the surfaces by a donor–acceptor, or charge-transfer, interaction through the π system of the porphyrin ring and Brønsted- and/or Lewis-acid sites of the surfaces.