Physicochemical and catalytic properties of NiO–TiO 2 modified with WO 3 for ethylene dimerization

Abstract

NiO–TiO 2/WO 3 catalysts was prepared by drying powdered Ni(OH) 2–Ti(OH) 4 with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. The presence of NiO and WO 3 prevented the phase transition of TiO 2 from anatase to rutile forms. From 600 °C of calcination temperature, nickel titanium oxide (NiTiO 3) was observed due to the reaction between TiO 2 and NiO; its amount increased with increasing calcination temperature. Upon the addition of tungsten oxide to titania up to 15 wt.%, the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content, due to the interaction between tungsten oxide and titania. Since the TiO 2 stabilizes the tungsten oxide species, for the samples equal to or less than 25 wt.%, tungsten oxide was well dispersed on the surface of titania, but for the samples containing 25 wt.% or above 25 wt.%, the triclinic phase of WO 3 was observed at calcination temperature above 400 °C. The high acid strength and acidity was responsible for the W O bond nature of the complex formed by the interaction between WO 3 and TiO 2. The catalytic activities for ethylene dimerization were correlated with the acidity of the catalysts measured by the ammonia chemisorption method. Catalysts without NiO or WO 3 were inactive as catalysts for ethylene dimerization, because the active sites of ethylene dimerization consist of low valent nickel ion and acidity.