Influence of alumina fluoridation on the dispersion and hydrotreating activity of W/Al2O3 catalysis

The influence of alumina fluoridation on the activity of alumina-supported tungsten catalysts for simultaneous gas oil hydrodesulfurization (HDS) and pyridine hydrodenitrogenation (HDN) has been studied. Both HDS and HDN activities increased significantly with increasing fluoride content in the alumina; the promoting effect of fluoride being relatively stronger for HDN than for HDS. Transmission electron microscopy measurements of sulfided catalysts revealed that fluoridation of the alumina support increases the proportion of WS, crystallites which can be visualized by this technique, probably because it improves the sulfidation of the oxidic W species, and also increases the size and stacking of the WS, crystallites. These results are consistent with the increase in polytungstates shown recently by temperature-programmed reduction, Raman spectroscopy and diffuse reflectance spectroscopy results for the oxidic state of the same catalysts. Possible explanations for the different improvements in HDS and HDN activity based on the observed changes in morphology and sulfidation, and also the higher surface acidity induced by fluoride on alumina, are discussed.