Effect of the fluorine-addition order on the hydrodesulfurization activity of fluorinated NiW/Al2O3 catalysts

Fluorinated NiW/Al2O3 catalysts with different orders of fluorine addition have been prepared, tested for hydrodesulfurization (HDS) of thiophene, and characterized using nitric oxide chemisorption and temperature-programmed sulfidation. The catalyst surface area has been affected by fluorine addition but not by the order of fluorination. The fluorine addition-order does not affect the amount of fluorine retained in the catalysts after the calcination and the reaction steps, either. On the other hand, the order of fluorine addition changes the dispersion of the nickel and the tungsten species, incorporation of nickel with the tungsten edge sites, and consequently the HDS activity of the catalysts. The catalyst fluorinated in the last step, i.e., after addition of both tungsten and nickel, shows the highest activity in thiophene HDS, which is supported by other experimental results indicating the most nitric oxide chemisorption and the largest incorporation of nickel with the tungsten species. Accordingly, enhancement of the catalyst activity by fluorination is due to the repartition of the metal species rather than to partial solubilization of alumina in the fluorine-addition step.