Michel Vrinat

Overview of support effects in hydrotreating catalysts

The commercial hydrotreating catalysts are usually composed of a sulfide active phase supported on alumina. It has been known for more than 15 years that other supports might lead to enhanced catalytic properties. The present article summarizes the progress which have been made in this area, often related to new methods of preparation of supports, active phases deposition or activation procedures. The properties of active phases supported on oxides, pure or mixed, basic and acidic supports, zeolites, mesoporous materials, carbon and clays are examined.

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.

Comparative inhibiting effect of polycondensed aromatics and nitrogen compounds on the hydrodesulfurization of alkyldibenzothiophenes

The influence of anthracene, phenanthrene, fluorene, acridine and carbazole on the transformation of 4,6-dimethyldibenzothiophene on a NiMo/Al2O3 catalyst was studied. Reactivities have been evaluated under increasing concentrations of inhibitors and negative effect of some polycondensed ring aromatic compounds has been shown to be in the same order of magnitude than that of N-containing compounds. Moreover, such inhibition was proposed to be due to some partially hydrogenated molecule and conclusions relative to the practice of deep hydrodesulfurization (HDS) were drawn.

New generation of titanium dioxide support for hydrodesulfurization

Mesoporous titanium oxide with a high specific surface area of 120 m 2 /g prepared by a novel method developed by Chiyoda was used for supporting molybdenum sulfide. In order to examine the influence of the surface area on the properties of the molybdenum sulfide phase, two different samples of titanium oxide were studied, a commercial one with a surface area of 72 m 2 /g and that prepared by Chiyoda. Molybdenum was deposited on the TiO2 supports by incipient wetness impregnation with ammonium heptamolybdate in one or two steps depending on the Mo loading. Some samples were also prepared by impregnation of ammonium heptamolybdate basified by ammonia. Raman spectroscopy and XPS were used to examine the nature of the molybdate phase and its dispersion in the oxidic state. HREM and XPS were used for studying the sulfided state. As expected, the maximum amount of well-dispersed molybdenum is higher on the Chiyoda support than on the reference support with a lower surface area. The catalytic properties of the catalysts were studied in dibenzothiophene conversion. For the Chiyoda support, the catalytic activity varied linearly with the Mo loading up to 6–7 Mo/nm 2 then became nearly constant for the higher loadings. Much higher activities (six times, expressed per gram of catalyst) were obtained compared to molybdenum sulfide supported on alumina.

Effect of experimental parameters on the relative reactivity of dibenzothiophene and 4-methyldibenzothiophene

Abstract Numerous studies concerning the hydrodesulfurization of crude oil have already been realized and have shown that substituted dibenzothiophenes, and especially the 4-methyldibenzothiophene and the 4,6-dimethyldibenzothiophene are refractory molecules of the process. The reaction mechanism of their transformation is still discussed. In this work, we have chosen to carry out the transformation of dibenzothiophene and 4-methyldibenzothiophene on various catalysts and under different experimental conditions. Attention has been particularly paid to the effect of H2S on the two different reaction pathways: the DDS (direct desulfurization) and the HDS (hydrogenation followed by hydrogenolysis) routes. The relative importance between the two routes is greatly modified by the presence of H2S: the DDS route appears to be more inhibited by this compound. An original approach to hydrodesulfurization, the competitive catalytic reaction kinetics, was also used in this study: it demonstrates that the dibenzothiophene and the 4-methyldibenzothiophene have the same adsorption equilibrium constant and suggests that the molecules are adsorbed flat on the catalyst surface. The lower reactivity of the 4-methyldibenzothiophene can then be due to a steric hindrance of the methyl group during the approach of the hydrogen molecule for the hydrogenolysis reaction.

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.

Ultra-deep desulfurization of transportation fuels via charge-transfer complexes under ambient conditions

Very stringent regulations on the maximum sulfur content of gas oil have led to intense research into all aspects of desulfurization. Deep desulfurization of diesel fuels is particularly challenging due to the difficulty of removing refractory sulfur compounds, particularly 4,6-dialkyldibenzothiophenes, using conventional hydrodesulfurization processes (HDS). A novel approach to the potential desulfurization of fuels such as diesel is proposed. It relies on the ability of 4,6-dialkyldibenzothiophene to form charge-transfer complexes (CTC) with π-acceptor molecules. We present the synthesis of a new π-acceptor molecule, 4,5-dicyano-2,7-dinitrofluorenone, solution redox behaviour and the crystal structure of the charge-transfer complex with the refractory 4,6-dimethyldibenzothiophene. This π-acceptor compound was then immobilized on a hydrophobic support (poly(styrene-co-divinylbenzene)). The selectivity of the CTC process was confirmed by the analysis of the sulfur compounds trapped from straight run Arabian Light (SR) containing 13600 ppm S. The functionalized polymer can be used in multiple cycles for the removal of refractory S-containing compounds from hydrotreated SR. It can also be regenerated with toluene. The high selectivity of this material permits diesel fuel to be desulfurized to a level that meets future regulatory requirements, i.e. less than 10 ppm S, at ambient temperature and without hydrogen consumption.

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).