Michèle Breysse

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.

Hydrodesulphurization activity and characterization of sulphided molybdenum and cobalt—molybdenum catalysts : Comparison of Alumina-, Silica-Alumina- and Titania-Supported Catalysts

The nature of the support effect in unpromoted (Mo) and promoted (CoMo) catalysts was examined by comparing the thiophene hydrodesulphurization activities of the catalysts supported on Al2O3, TiO2 and SiO2Al2O3. Catalyst samples were prepared by the incipient wetness impregnation method and characterized by photoelectron spectroscopy (XPS) and high-resolution electron microscopy (HREM). The activity per atom of molybdenum was higher for the TiO2-unpromoted and -promoted catalysts, but the synergistic effect was higher for the Al2O3, -supported catalysts. The absence of a significant shift in the titanium and molybdenum binding energies for catalyst supported on TiO2 and Al2O3 with respect to unsupported MoS2 catalyst does not support an electronic effect as the main cause of the great differences in activity between TiO2- and Al2O3- supported catalysts. The HREM results show clear evidence of the existence of smaller MoS2 particles on the TiO2-supported catalysts. Also, the addition of the promoter leads to the formation of smaller particles on the surface of the TiO2-supported catalysts. The differences in activity and synergistic effect for the different catalysts could be explained in terms of different activities for the smaller and larger particles. It is proposed that the difference in activity between the smaller and larger particles is related to the MoS2 crystallite orientation on the surface of the support.

Titania-alumina mixed oxides as supports for molybdenum hydrotreating catalysts

Abstract A series of coprecipitated TiO 2 -Al 2 O 3 supports containing between 0 and 100% TiO 2 were prepared and characterized by various methods. These solids were impregnated with molybdenum and tested in their sulphide form as catalysts in the hydrodesulphurization (HDS) of thiophene. The results showed that in the prepared mixed oxides supports, an interaction between TiO 2 and Al 2 O 3 exists and that the coprecipitated solid is different, in its surface properties, from a mechanical mixture of the two oxides. Acidity measurements show that Lewis and not Bronsted sites exist in the mixed oxides. The number of acid sites increases sharply at high TiO 2 content. High-resolution electron microscopy shows that a better dispersion of MoS 2 on the supports is obtained when the TiO 2 content is increased. Activity measurements in the thiophene hydrodesulphurization reaction show that the intrinsic activity, expressed on a molybdenum atom basis, increases sharply at high TiO 2 loadings in the mixed oxides supports. Comparison of the activity and acidity measurements point to a correlation between HDS activity and the number of acid sites on the support that affects the dispersion during the impregnation step and the reactivity of the MoS 2 sites.

HYDROGEN ACTIVATION BY TRANSITION METAL SULFIDES

The activity of the conventional catalysts during hydroprocessing results from the ability of Mo(W)S2 to adsorb and activate hydrogen. The optimum of hydrogen adsorption occurs at the S/Mo(W) ratio of about 1.95. This is consistent with removal of the corner and edge sulfur atoms leading to the formation of the sulfur anion vacancies. The heterolytic dissociation, involving both metal and sulfur ions, is the predominant mode of hydrogen activation. It leads to the formation of metal–H and S–H moieties. The homolytic dissociation, occurring on the sulfur ion pairs, is much less evident. For the supported catalysts, the amount of the adsorbed hydrogen per unit of Mo(W) is significantly greater. This results from the spillover of the active hydrogen from the active phase on the support. The surface hydrogen can migrate back from the support to the active phase. Promoters, such as Ni and Co, increase the rate of adsorption, whereas their effect on the total amount of the adsorbed hydrogen is much less pronounced. Similar effect may be observed by increasing hydrogen pressure. Among the noble metal sulfides, most of the attention has been paid to RuS2. The results obtained by spectroscopic techniques confirm the presence of the Ru–H and S–H species. The maximum of hydrogen activation is approached at about 20% reduction of RuS2. The homolytic and heterolytic hydrogen dissociation occur simultaneously. The extent of the former increases with increase in temperature. The activity of the RuS2 based catalysts increases with the decrease in particle size of RuS2, i.e., increasing the ratio of {111}/{100} surfaces. Suitable zeolite supports can significantly enhance the stability and activity of the RuS2 based catalysts.

Platinum–zeolite interactions in alkaline L zeolites. Correlations between catalytic activity and platinum state

Alkaline platinum L zeolites have been studied with regard to the state of the platinum and the catalytic activity of the platinum sites in benzene hydrogenation and n-hexane dehydrocyclisation. X-ray diffraction, electron micrography and infrared studies of CO adsorption led to the conclusion that there are four types of Pt particles: large 100–600 A particles outside the channels, crystals 10–25 A in diameter and small metallic cylinders inside and outside the channels and very small particles in cavities, the latter giving reversible Pt carbonyl clusters upon CO adsorption. The Pt active sites in benzene hydrogenation and n-hexane dehydrocyclization (10–25 A crystals and small cylinders) are strongly dependent upon their environment. In the absence of any acidity their activity increases with the zeolite basicity, while no electron deficiency is observed. The infrared band at 2060–2065 cm–1 of adsorbed CO suggests that Pt particles have an excess of electrons and/or typical faces, corners or edges. It is suggested that the L zeolite structure and field induced these unusual Pt state and catalytic properties.

Catalysis of carbon monoxide oxidation by cerium dioxide: I. Correlations between catalytic activity and electrical conductivity

A mechanism involving the oxidation and reduction of the solid is presented for carbon monoxide oxidation on cerium dioxide. Expressions for the reaction rate and the electrical conductivity of the catalyst are derived from this mechanism. Experiments have been performed in which these two quantities have been simultaneously recorded. The results agree with the proposed mechanism.

Influence of preparation method on the acidity of MoO3(WO3)/ZrO2 catalysts

Zirconia-supported Mo and W oxides have been prepared by three methods: a conventional impregnation technique, a molten salt method and calcination of zirconium hydroxide impregnated with Mo(W) salt. The state of the Mo(W) and the acidity of the solids were studied, versus Mo(W) loading and calcination temperature. It has been shown that superficial acidity depends strongly on the preparation method used. Among the systems studied, solids prepared by calcination of impregnated zirconium hydroxide manifested the strongest Lewis and Bronsted acidity. The latter was, however, lower than the acidity of sulfated zirconias, as revealed by catalytic tests. A model of the structure of the acidic centres was proposed.

Niobium sulfides as catalysts for hydrotreating reactions

Abstract Hydrotreating properties of unsupported and supported niobium sulfides are compared to those of molybdenum sulfide catalysts. Niobium catalysts demonstrate higher activities than molybdenum sulfide ones and remarkable selectivity in cracking and isomerisation reactions.

Promoting effect of fluorine on cobalt—molybdenum/ titania hydrodesulfurization catalysts

Abstract The effect of fluoride incorporation on the titania support on the surface structure of promoted cobalt-molybdenum catalysts and their catalytic activity for thiophene hydrodesulfurization (HDS) has been studied. The relative activity between fluoride-modified and unmodified catalysts was found to be maximum and close to 5 for a fluorine content of 0.8 wt.-%. From high-resolution electron microscopy (HREM) examination of sulphided samples, the increase in HDS could be partly explained by smaller molybdenum sulphide crystallites (increase in the dispersion of the active phase). Diffuse reflectance spectroscopy (DRS) of the oxidic precursors shows that the effect of fluoride incorporation is also to increase the amount of the Mo VI in tetrahedral surroundings and to enhance the part of cobalt involved in the generation of the “CoMoS” active phase (probably cobalt in octahedral symmetry).

Catalytic properties of niobium sulphides in the conversion of nitrogen containing molecules

Abstract The catalytic properties of niobium sulphides, i.e. NbS 3 and Nb 1.12 S 2 have been examined in the conversion of nitrogen-containing molecules (n-pentylamine and pyridine), and in the hydrogenation of biphenyl. Compared to MoS 2 , NbS 3 and Nb 1.12 S 2 show higher activities for all these reactions and a peculiar ability to perform CN and CC bond cleavage. The activities for these last reactions have been related to the acid-base properties of niobium compounds. Moreover, the trisulphide is more active than Nb 1.12 S 2 . These differences in activities are discussed in relation to the structural features of both sulphides and the existence in NbS 3 of S 2 2− groups and metal-metal pairing.