Effects of preparation methods on the property and hydrodesulfurization activity of NiAlZrW catalysts derived from tungstate intercalated NiAlZr layered double hydroxides

Abstract

A series of tungstate intercalated NiAlZr layered double hydroxides (LDHs) with various Zr contents were synthesized by an ion exchange method and a coprecipitation method, respectively. They were further calcined and used as hydrodesulfurization (HDS) catalysts. The effects of preparation methods on the property and HDS activity of catalysts were investigated. It is found that the introduction of Zr by the ion exchange method facilitates the formation of amorphous and well dispersed NiWO4 species, because the highly ordered layered structure can be well retained after calcination, acting as support and separator to disperse NiWO4 species and prevent them from aggregation. Moreover, the reducibility and sulfidability of catalysts are improved, because the substitution of Al by Zr in the brucite-like layers could decrease the strong WOAl linkages and hence weaken the metal-support interaction. Therefore, the HDS activity and hydrogenation selectivity for dibenzothiophene are significantly improved with the increase in Zr content up to Zr/Al ratio of 0.3/0.7 whose HDS rate constant is 55.7% higher than that of the Zr-free catalyst. However, the coprecipitation method leads to the formation of crystalline NiWO4 species which grow into bigger crystallites with the addition of Zr. This may be attributed to the lower crystallinity of LDHs and the formation of NiWO4 salt during the coprecipitation process. As the Zr content increases, the reducibility and sulfidability of catalysts are declined remarkably, due to the crystal growth and aggregation of NiWO4. Therefore, the introduction of Zr by the coprecipitation method decreases the HDS activity of catalysts.