Isomerization of n-Butane over Sulfated Al- and Ga-Promoted Zirconium Oxide Catalysts. Influence of Promoter and Preparation Method

Isomerization of n-butane has been investigated at 250°C over sulfated zirconia, and Al- and Ga-promoted zirconias calcined at 650°C and activated at 450°C. The promoter (3 mol% as Al2O3 and Ga2O3) was introduced by both impregnation and coprecipitation, in order to examine the influence of the preparation method and the nature of the promoter on the catalytic properties. The catalysts were characterized by X-ray diffraction, Raman, and photoelectron spectroscopy (XPS). The total acidity was determined from ammonia-TPD measurements. The sulfate and OH stretching regions and the spectral modifications occurring upon ammonia adsorption were investigated by DRIFT spectroscopy. The nature of the acid sites prevailing at the activation and reaction temperature was clearly of Brønsted type, as observed by the IR characteristic NH4+ deformation band. With respect to sulfated zirconia, the promoted catalysts exhibited higher fractions of tetragonal structure, sulfate densities, and Brønsted acidity, all intervening in the catalyst efficiency and stability. Incorporation of the promoter via coprecipitation provided more efficient catalysts than via impregnation. Platinum addition improved the performances (conversion, long-term stability) of all catalysts, with this beneficial effect being relatively more spectacular for nonpromoted than for promoted sulfated zirconias.