Rare earth oxides doped NiO/γ-Al<inf>2</inf>O<inf>3</inf> catalyst for oxidative dehydrogenation of cyclohexane

Abstract

The effect of rare earth oxides (RE=Ce, La, Gd, and Dy) doping of alumina support in NiO/γ-Al2O3 system was investigated on its catalytic performance in oxidative dehydrogenation (ODH) of cyclohexane. The physicochemical properties of various samples were followed up through N2 physisorption, temperature programmed reduction (H2-TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and potentiometric acid-base titration techniques. In the parent NiO/γ-Al2O3 catalyst, Ni species were found to be strongly interacted with alumina surface. Addition of rare earth dopants to γ-Al2O3 in the catalyst system affected the nickel-alumina interaction and resulted in significant modifications in the catalytic performances in the ODH reaction. The results revealed the beneficial role of both La2O3 and Gd2O3 doping in enhancing the ODH catalytic activity and selectivity to cyclohexene. H2-TPR and XPS results indicated that majority of Ni species in NiO/La2O3 modified γ-Al2O3 were more weakly interacted with La2O3 and alumina whereas both NiO like species and nickel aluminate were present on the surface. Doping with cerium or dysprosium increased the nickel-support interaction and led to a decrease in surface nickel concentration. In case of doping with Ce, surface concentration of cerium oxide was higher than those of the other RE oxides; the doped catalyst reached its steady state activity faster than the other catalysts. The acid-base results suggested that RE metals were interacted most likely with acidic surface hydroxyl groups. The degree of nickel-alumina interaction decreased in the following order: LaAl>GdAl>CeAl>DyAl.