(89f) Unsupported Sulphide and Nitride Catalysts for Hydrotreating: New Preparation Approaches

P. Afanasiev.

Institut de Recherches sur la Catalyse, 2, Avenue Albert Einstein, 69626 Villeurbanne Cedex, France.

pavel.afanasiev@catalyse.cnrs.fr

To minimize the negative environmental effects of exhaust emissions, the authorized sulfur level in motor fuels has been decreasing during the last years. Many different approaches were proposed to eliminate sulfur-containing molecules from the petroleum feedstocks. Because of relatively low investment costs, improvement of the catalysts is the most attractive solution for petroleum refiners. Such a demand for more active HDT catalysts incited a strong increase of research activity. One of the ways to improve the catalysts could be the use of unsupported or very highly loaded catalysts sulfides. Recently, new highly loaded sulfide catalysts emerged on the market. Beside sulfides, other bulk phases are extensively studied in hydrotreating, including nitrides, carbides and phosphides (1). Soft chemistry routes using solution reactions of provide an approach for preparation of new solids with interesting properties, such as for example new binary amorphous molybdenum sulphides or nonstoichiometric molybdenum oxysulfide materials with vesicle - like and tubular precipitates in organic solvents. Among the methods reported, solution preparations of thermally stable macro ? and mesoporous transition metal sulfides are particularly promising for the hydrorefining applications. Through control of precursors, solvents and reaction conditions, Ni(Co)-Mo-S catalytic materials can be prepared, with structural and morphological tuning over micro- and mesoscopic length scales (2,3). The preparation methods employed include soft hydrothermal syntheses, surfactant ? aided, or ultrasound-induced precipitations in the mixed non aqueous solutions, as well as solventothermal (polyol) reactions. The results of different preparation techniques are compared as concerns morphological properties of the materials obtained and their catalytic activity in model hydrodesulphurization (HDS) reactions. It has been shown that reactions of thiomolybdate MoS42-, thiodimolybdate Mo2S122- and oxothiomolybdate MoOS22- cores with nickel or cobalt salts in mixed solutions produce amorphous solids (pre-catalysts) with Ni(Co)/Mo ratio depending on the conditions and particularly on the nature of molybdenum thioanion applied. The morphology of Co(Ni)-Mo-S solids obtained after sulfidation of the pre-catalysts depends mostly on the preparation technique used. Homogeneity of the sulfided catalysts strongly depends on the reaction conditions in the solution and on the nature of precursors used. Residual carbon in these solids plays important role to stabilize their morphology and catalytic activity. Its content varies between 3 and 4 % wt from a catalyst to another; the absence of this residual carbon corresponds to the collapse of prous system and low catalytic activity. Numerous characterizations including XRD, TPR, scanning and transmission electron microscopy were used to understand the reactions occurring under the hydrothermal or in mixed solvents. EXAFS at the Co, Ni and Mo edges reveals evolution of transition metals coordination from the amorphous pre- catalysts to the Ni(Co)MoS sulfided phases. Optimization of the promoted NiMo and CoMo unsupported systems allows to obtain catalysts with very high activities in the thiophene and 4,6-DMDBT hydrodesulfurization, several times greater than those of commercial references available

Another class of extensively studied bulk catalytic systems is binary or ternary nitrides. To obtain them, decomposition of molecular precursors was applied. New molecular complexes of hexamethylenetetramine (HMTA) with transition metals including Co, Ni, Mo, V and their combinations have been prepared by simple aqueous reactions. Decomposition of the HMTA complexes produced dispersed nitride materials, with high specific surface areas, appropriate for catalytic use (4). Decomposition of Mo, V, Co-Mo and Ni-Mo compounds, containing binary nitrides such as Mo2N, Mo3N2 or VN, as well as ternary nitrides. It appears that slight modification of the gas flow composition allows qualitative control of the nature of the products formed. The reasons of such versatile behaviour are discussed. Catalytic activities of as synthesised binary and ternary nitrides have been evaluated in model hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) reactions. The impact of surface sulphidation on the catalytic activity was discussed in the case of HDS tests, assisted by the data of transmission electronic microscopy study. It has been shown that ternary nitrides are irreversibly deactivated by hydrogen sulphide or by sulphur containing organics. By contrast, our preliminary data suggest that the systems synthesised are highly active HDN catalysts. Next challenges in the field of bulk HDT catalysts are outlined. General advantages and drawbacks of sulphides and nitrides bulk systems as well as the perspectives of their industrial applications are critically reviewed.

References

1. D. J. Sajkowski , S. T. Oyama, Appl. Catal, 1996, 134, 339. 2. P. Afanasiev, I. Bezverkhy, J. Phys. Chem. B. 2003, 107(12), 2678. 3. D. Genuit, P. Afanasiev, M. Vrinat J. Catal. 2005, 235, 302. 4. P. Afanasiev Inorg. Chem. 2002, 41(21), 5317.