EFFECT OF FLUORIDE ON THE STRUCTURE AND ACTIVITY OF NIW AL2O3 CATALYSTS FOR HDS OF THIOPHENE AND HDN OF PYRIDINE/

The effect of adding different amounts of fluoride to a gamma-alumina used as support of NiW/Al2O3 catalysts was investigated using X-ray di ffraction, diffuse reflectance spectroscopy (DRS), infrared spectrosco py of adsorbed NO (IR-NO), X-ray photoelectron spectroscopy (XPS), sur face acidity, specific surface area and catalytic activity measurement s. The catalytic activity was evaluated by two model reactions, namely thiophene hydrodesulfurization (HDS) and pyridine hydrogenation (HDN) , in the temperature range 573-623 K using a flow system operated at 2 MPa (HDS) or 3 MPa (HDN) total pressure. DRS results of the oxidic pr ecursors suggested that the effect of fluoride incorporation to the al umina support is that it increases slightly the octahedrally coordinat ed fractions of Ni and W. The XPS results of the sulfided catalysts sh owed that the dispersion of the Ni and W phases decreased progressivel y with increasing fluoride content. This technique also showed that th ese phases were only partially sulfided under the present experimental sulfiding conditions, and that the extent of sulfidation of such phas es apparently tends to decrease slightly with increasing fluoride cont ent, in contrast with the results suggested by the IR-NO spectra. The sulfur content of sulfided samples, as measured by chemical analysis, revelated that the sulfidation degree increased only slightly with inc reasing fluoride content. The activity for thiophene HDS was almost in variant with fluoride addition, but the subsequent hydrogenation of th e intermediate products (butenes) was found to decrease linearly with the fluoride content. Possible interpretations for these changes in ac tivity and selectivity, which reflect changes in the relative distribu tion of the different active sites for the two reactions, are discusse d in relation with the structural variations of the active phases desc ribed above. In contrast, the activity for pyridine HDN increased cons iderably with the incorporation of small amounts of fluoride; this inc rease is mainly attributed to the increase in surface acidity which wa s observed with fluoride addition.