Niobium sulfide as a dopant for hydrotreating NiMo catalysts

Niobium sulfide has been recently found to be an interesting new active phase for hydrodesulfurization. In this work, niobium was used as a dopant for a conventional hydrotreating catalyst. A NiMo hydrotreating catalyst in the oxide form was doped with various contents of Nb precursor salt (02 sulfiding agent in a high pressure vessel. The use of this new dopant increased the catalytic activity in both HDS and HYD model reactions. Highest activities were obtained with an optimum Nb content of 5 wt.%. The selectivity of the products was also modified since more isomerized compounds where produced. Various techniques were used to determine structural and morphological characteristics of the materials. TEM pictures only showed the presence of lamellar particles similar to MoS₂. EDX analysis demonstrated the homogeneous distribution of the transition metal elements (Ni, Mo, Nb) even with small electron probes at high magnification. EXAFS was used to determine the local environment of Nb atoms and showed that Nb was present in the form of “NbS₂” entities similar to the bulk phase.     

Deactivation of MoS2 catalysts during the HDS of thiophene

Catalytic properties of unsupported MoS₂ catalysts in the thiophene hydrodesulfurization reaction were determined in the temperature range 623–653 K. The catalysts were prepared by ex situ decomposition of ammonium thiomolybdate (ATM) crystals in a mixture of 15% H₂S in H₂ at 673 K. Activity of catalysts decreased very rapidly before reaching a steady state after 15 h on‐stream. The thiophene conversion went down from 10–12 to 3–4% in that time. The surface area of the catalysts also decreased during the catalytic reaction from 40–50 to 8–10 m²/g. Selectivity for hydrodesulfurization, hydrogenation and isomerization reactions was affected distinctly by the deactivation process. By increasing the reaction time, double‐bond isomerization increased, hydrogenation of butenes decreased and hydrodesulfurization remained constant. Results indicate that the main cause of activity decay was surface area loss that was due to sintering of MoS₂ crystallites. Selectivity variation indicates that different active sites are involved for the three reactions. A deactivation model involving diminution of active sites located in edge and rim sites of small MoS₂ particles is proposed to explain the variation of product distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Use of CO as probe molecule for characterization of mixed Ru-Ni sulphide phases supported over alumina

Infrared spectroscopy was used to characterize mixed ruthenium-nickel sulphides prepared by step-wise impregnation over alumina and further sulphidation. The hydrogenation of biphenyl was compared over samples containing different amounts of nickel and ruthenium and a large increase was observed for a catalyst with an atomic Ni /(Ni + Ru) ratio near 0.4. Infrared spectra of adsorbed CO indicate that the increase in catalytic activity is ascribed to the presence of a very large number of Ru-Ni sites.     

Adsorption of tetrahydrothiophene on faujasite type zeolites: Breakthrough curves and FTIR spectroscopy study

Adsorption of tetrahydrothiophene (THT) on NaX, CaX, AgX, and H-USY was studied by dynamic adsorption method and FTIR spectroscopy. The stoichiometric adsorption capacities are not very different for all materials, which is indicative of a complete micropore filling. In contrast, the overall rate constant of adsorption, determined from fitting the breakthrough curves with Bohart–Adams equation, is two times greater for H-USY than for type X zeolites. For NaX, CaX and H-USY, heating under N₂ flow at 300 °C allows to restore completely their initial adsorption capacities. In the case of AgX the same treatment results in a twofold lose of capacity accompanied by a collapse of the zeolite structure as evidenced by XRD. The nature of interaction between THT molecules and zeolites was characterized by FTIR spectroscopy. Only a weak interaction through hydrogen bonding was observed for THT on H-USY leading to an almost complete desorption at 100 °C. For NaX and CaX adsorbed molecules are gradually eliminated when heated to 300 °C while completely different pattern was observed for THT adsorbed on AgX on which the amount of the adsorbate remains nearly constant between 25 and 200 °C, but rapidly decreases on further heating. Such a behavior is indicative of much stronger interaction between THT molecules and Ag⁺ cations in agreement with the data on the regeneration of the adsorbents.     

Deep desulfurization: reactions, catalysts and technological challenges

Very stringent regulation in the maximal S content of gas oil have led to an intense activity of research dealing with all the aspects of desulfurization. The design of future processes is based on the identification of the refractory sulfur compounds and the knowledge of their individual reactivity and in the presence of inhibitors, as illustrated in this paper. This knowledge have oriented the research towards new catalysts such as molybdenum sulfide supported on zeolites, combination of sulfide and noble metal catalysts, and molybdenum carbides. Non-catalytic approaches like charge transfer complex were also examined. This paper summarises these various aspects of desulfurization.     

Conjugaison froid mécanique/froid cryogénique

The combination, in parallel or in series, of systems using mechanical refrigeration and cryogenic cooling enables one to realize assemblies whose overall performance is interesting from various aspects. A review of the main definitions relating to freezing tunnels, as well as an analysis of phenomena linked to the enthalpy variation of a product being frozen, can assist in quantifying the performance of these combined assemblies, depending upon whether one is seeking to maintain the intrinsic qualities of fragile products or to temporarily increase the output of current products.     

Hydrothermal Syntheses and Catalytic Properties of Dispersed Molybdenum Sulfides

Reactions of ammonium thiomolybdate under mild hydrothermal conditions were studied, and the range of conditions leading to the dispersed MoS₂ product has been determined. The morphology and the catalytic properties of the hydrothermal MoS₂ preparations have been compared with those of reference MoS₂ produced from the thermal decomposition of thiosalts. It has been shown that the length of the slabs and their stacking number of the crystallites of hydrothermal MoS₂ preparations differ considerably from those in the reference sulfides. The morphological differences strongly influence the hydrogenation to hydrodesulfurization activity ratio in these systems.     

Preparation, characterization and catalytic properties of unsupported vanadium sulphides

This work concerns the preparation and the ability of unsupported vanadium sulphide to catalyze different reactions representative of hydrotreatment. Pure V₂S₃ was prepared by decomposition of ammonium thiovanadate at relatively low temperature. The reactivity of the decomposed solid towards oxygen confirms the high oxophilic character of this group V sulphide. The oxidation mechanism involves a superficial oxidation followed by an insertion of oxygen into the lacunar structure of the solid. The comparison of the catalytic properties of this chalcogenide with those of molybdenum or tungsten sulphides shows the superiority of vanadium sulphide for performing hydrogenation reaction of various cyclic molecules. In contrast, the hydrodesulphurization properties are of the same order.