Encapsulation of transition metal sulfides in faujasite zeolite for hydroprocessing applications

The dispersion of the sulfides of Co, Ni, Mo and W—and their combinations—in the micropores of acidic faujasite zeolite is a challenging task. While a close proximity of the hydrogenation function to the acidic cracking sites is thought to be beneficial for industrial hydrocracking applications, the genesis of well-defined metal sulfides in a tunable environment allows to address two important fundamental issues: (i) the possible existence of an optimum cluster size of metal sulfides with respect to hydrodesulfurization activity and (ii) an extension of the well-known support effect in hydrotreating catalysis. This review will concentrate on the various preparation methods—ranging from the brute-force chemistry involved in dispersing anion precursors to the occlusion of well-defined organometallic complexes in the zeolite micropore space, characterization to determine the successful genesis of suitable models and activity tests. Successful dispersion of these sulfides in the micropores of faujasite is only possible by careful preparative methods, either using well-dried cation-exchanged precursors or organometallic complexes. A synergistic effect of acidic protons on the metal sulfides is noted. However, the relatively low activities of ultra-dispersed Mo- or Co-sulfide particles compared to those of particles on more traditional supports force us to conclude that there must be an optimal size for metal sulfide clusters larger than those stabilized in the micropores. Although there are some indications that a sulfided CoMo cluster can be formed in the micropores using organometallic precursors, the synergistic effect between the two metals is quite small. From an application point of view, the detrimental effect of bases such as water or nitrogen compounds to the final dispersion of the metal sulfides is problematic. Nevertheless, model studies indicate that a close proximity of acidic and hydrogenation components decreases coking deactivation.