Active Sites of a NiSO4/γ-Al2O3 Catalyst for the Oligomerization of Isobutene

Structural characterization, the mechanism of catalytic activity generation and the nature of active sites of a NiSO4/gamma-Al2O3 catalyst for isobutene oligomerization were studied by temperature programmed reduction (TPR), X-ray diffraction (XRD), diffuse reflectance infrared fourier transformed (DRIFTS) and X-ray photoelectron spectroscopy (XPS) techniques. The TPR measurements together with the XRD data indicated that calcination of the catalyst at 500 A degrees C did not form either nickel oxide or nickel aluminate. The presence of only one type of surface nickel species formed by the incorporation of nickel ions into the surface vacant sites of gamma-alumina lattice was indicated by XPS with Ar+ ions sputtering and TPR measurements. XPS analysis of the calcined catalyst suggested that the oxidation state of nickel ions in the calcined catalyst was (+2) and after calcination the nickel ions were coordinated to relatively more basic ligands. The surface acid centers of the catalyst were found to be only Lewis type. SO4 (2-) ions were found to be present as a chelating bidentate ligand and enhanced the acidity of metal ( and/or ) Lewis acid centers. The results suggested that the combined effects of the presence of the bidentate SO4 (2-) ligand and dehydroxylation generate coordinatively unsaturated that interact with isobutene during the oligomerization reaction. The formation of lower-valent nickel ions () was demonstrated by in situ DRIFTS using CO as a probe molecule and by XPS measurements. Formation of a binuclear bridging carbonyl complex, suggested that some lower-valent nickel species were formed via in situ reduction by isobutene. Analysis of Ni 2p photolines indicated the appearance of a new lower-valent nickel species () during the course of isobutene oligomerization. Hence it is plausible that lower-valent nickel species might act as the active center for the oligomerization reaction, while the SO4 (2-) ions enhance the acidity of the Lewis acid sites on the surface and assist in the adsorption of reactant molecules on the surface.