Cobalt-molybdenum Sulfide Catalysts Research Articles

Nanolayered Cobalt–Molybdenum Sulfides as Highly Chemo- and Regioselective Catalysts for the Hydrogenation of Quinoline Derivatives

Herein, a general protocol for the preparation of a broad range of valuable N-heterocyclic products by hydrogenation of quinolines and related N-heteroarenes is described. Interestingly, the catalytic hydrogenation of the N-heteroarene ring is chemoselectively performed when other facile reducible functional groups, including alkenes, ketones, cyanides, carboxylic acids, esters, and amides, are present. The key to successful catalysis relies on the use of a nanolayered cobalt–molybdenum sulfide catalyst hydrothermally synthesized from earth-abundant metal precursors. This heterogeneous system displays a tunable composition of phases that allows for catalyst regeneration. Its catalytic activity depends on the composition of the mixed phase of cobalt sulfides, being higher with the presence of Co3S4, and could also be associated with the presence of transient Co-Mo-S structures that mainly vanish after the first catalytic run.

Nickel–molybdenum and cobalt–molybdenum sulfide hydrogenation and hydrodesulphurization catalysts synthesized in situ from bimetallic precursors

Unsupported nickel–molybdenum and cobalt–molybdenum sulfide catalysts are synthesized via the in situ decomposition of water-soluble bimetallic precursors in a hydrocarbon feedstock using nickel–molybdenum and cobalt–molybdenum complexes with citric, oxalic, succinic, glutaric, and tartaric acids as precursors. The sulfide catalysts are characterized by means of transmission electron microscopy and X-ray photoelectron spectroscopy. The catalyst activity in the hydrogenation of bicyclic aromatic hydrocarbons and the hydrodesulfurization of dibenzothiophene is studied. The effect the composition of the precursor solution in the hydrocarbon feedstock emulsion has on the activity of the resulting catalyst is determined. It is shown that the activity reaches high values even after 1 h of reaction. The hydrogenation of mono-, di-, and trimethylnaphthalenes and ethylnaphthalene is studied. The optimum promoter-to-molybdenum ratio (0.25: 1) is found. It is shown that the catalyst activity does not fall during recycling, due to the elimination of the negative effect of water contained in the emulsion, which results in oxidation of the catalyst surface. After the second reaction cycle, the catalyst particles are longer and have a greater number of MoS2 layers than the respective parameters of the catalyst particles after the first cycle. XPS shows that the content of oxygen on the catalyst’s surface falls during recycling, while the fraction of metals in the sulfide environment and the sulfur in the sulfide state grows.

HDS of dibenzothiophene with CoMoS2 synthesized using elemental sulfur

An unsupported cobalt molybdenum sulfide catalyst was prepared using ammonium heptamolybdate, cobalt nitrate and elemental sulfur under reflux in ammonium hydroxide. The crystal structure of the precursor was identified using XRD and was determined to be a mix of CoMoO4 and elemental sulfur. The refluxed samples were decomposed in a tube furnace at 450°C under a constant flow of 10% H2:90% Ar gas mix. The freshly prepared catalyst was characterized using XRD and the LeBail fitting procedure, which confirmed the sample was a biphasic mixture of Co9S8MoS2. The synthesized sample was tested for the hydrodesulfurization of dibenzothiophene at 350°C. The removal of the dibenzothiophene was found to follow pseudo zeroth order reaction with two catalytic rates of 1.63×10−5molL−1·s−1 for the first 3.5h and 6.81×10−5molL−1·s−1 observed in the second 1.5h (or catalytic activities of 24.5 and 9.27molg−1s−1). After 5h of reaction, the catalyst showed approximately 93% conversion of the dibenzothiophene. The primary product of the hydrodesulfurization of the dibenzothiophene was biphenyl, which was approximately 80% of the products, and the minor product was cyclohexylbenzene at approximately 13%.

Nickel- and Cobalt-Molybdenum Sulfide Catalysts for Hydrogenation and Hydrodesulfurization Prepared in Situ from Bimetal Precursors

Non-supported niockel-molybdenum and cobalt-molybdenum sulfide catalysts were prepared through in situ decomposition of water-soluble bimetal precursors in hydrocarbon feedstock. The precursors were nickel- and cobalt-molybdenum complexes with citric, oxalic, amber, glutaric and tartaric acids. The prepared sulfide catalysts were characterized using transmission electron microscopy and X -ray photoelectron spectroscopy. The catalyst activities to hydrogenation of bicyclic aromatic hydrocarbons and to hydrodesulfurization of dibenzothiophene were studied. The composition of the ‘precursor solution in hydrocarbon feedstock’ emulsion was established to influence the catalyst activity. The high activity was reached as soon as in an hour after the reaction started. The reactions under consideration were hydrogenation of mono-, di- and trimethylnaphthalenes, as well as hydrogenation of ethylnaphthalene. The optimal promoter:molybdenum ratio equal to 0.25 : 1 was determined. The catalyst activity was demonstrated not to decrease upon recirculation because of the absence of the negative effect of water (constituent of the emulsion) that oxidizes the catalyst surface. After the second cycle of the reaction, the catalyst particles are longer and bear more MoS 2 layers than the particles after the first cycle. XPS studies revealed that recirculation results in a decrease in the total oxygen content on the catalyst surface, in an increase in the metal proportion in the sulfide surrounding and sulfur in the sulfide state.

Silicon decorated with amorphous cobalt molybdenum sulfide catalyst as an efficient photocathode for solar hydrogen generation.

The construction of viable photoelectrochemical (PEC) devices for solar-driven water splitting can be achieved by first identifying an efficient independent photoanode for water oxidation and a photocathode for hydrogen generation. These two photoelectrodes then must be assembled with a proton exchange membrane within a complete coupled system. Here we report the preparation of a Si/a-CoMoSx hybrid photocathode which shows impressive performance (onset potential of 0.25 V vs RHE and photocurrent jsc of 17.5 mA cm(-2) at 0 V vs RHE) in pH 4.25 phosphate solution and under simulated AM 1.5 solar illumination. This performance is among the best reported for Si photocathodes decorated with noble-metal-free catalysts. The electrode preparation is scalable because it relies on a photoassisted electrodeposition process employing an available p-type Si electrode and [Co(MoS4)2](2-) precursor. Investigation of the mechanism of the Si/a-CoMoSx electrode revealed that under conditions of H2 photogeneration this bimetallic sulfide catalyst is highly efficient in extracting electrons from illuminated Si and subsequently in reducing protons into H2. The Si/a-CoMoSx photocathode is functional over a wide range of pH values, thus making it a promising candidate for the construction of a complete solar-driven water splitting PEC device.

Unraveling the Redox Behavior of a CoMoS Hydrodesulfurization Catalyst: A Scanning Transmission X-ray Microscopy Study in the Tender X-ray Range

We visualize the elemental zoning in an alumina-supported cobalt molybdenum sulfide (CoMoS) catalyst with scanning transmission X-ray microscopy (STXM). We use the Canadian Light Source beamline 10-ID’s (SM) unique combination of soft X-ray and tender X-ray STXM to determine the spatial variation of Co, Al, Mo, and S species. The results clearly show the correlation between cobalt and molybdenum and the anticorrelation between cobalt and alumina, confirming that Co is closely associated with the MoS2 phase. The tender X-ray STXM images show that the fresh catalyst particle contains a 50 nm ring of molybdenum oxide encapsulating the supported MoS2 phase. After the reduction at 200 °C with H2, this oxide rim disappears and a uniform MoS2 distribution is found. Upon oxidation at 400 °C, the sulfur disappears from the catalyst sample and molybdenum is oxidized from a MoIV sulfide to a mainly tetrahedral MoVI oxide, while cobalt keeps its divalent nature and changes from a CoII sulfide to a CoII oxide.

Compositional Analysis and Advanced Distillation Curve for Mixed Alcohols Produced via Syngas on a K-CoMoSxCatalyst

The distillation behavior of mixed alcohols was studied by use of the Advanced Distillation Curve (ADC) methodology. Crude mixed alcohols (oxygenates) were generated from syngas over a potassium-promoted cobalt–molybdenum-sulfide catalyst and assayed for major and minor products. Distillation (boiling) curves were generated for the crude mixed oxygenate products and composition channel data were collected. The crude mixed alcohols consisted primarily of methanol with significant quantities of ethanol, 1-propanol, 1-butanol, methyl acetate, and ethyl acetate. These six species constitute 93.7%–95.8% (mass/mass) of the total product. Ester, ether, and aldehyde impurities were identified, as well as thiols and organic sulfides. Considering just the alcohol products without impurities, these can be blended into gasoline at 8.5% (v/v) and meet the requirements of the Octamix waiver if an appropriate corrosion inhibitor were also included (the blend would contain 3.0%–3.4% methanol, >2.5% higher alcohols (v/v), and a total oxygen content of 3.7% (mass/mass)). Distillation targeted at 50% methanol removal increased the volume of product that could be blended to over 9% (v/v). Methanol, aldehydes, and dimethyl sulfide were the first to vaporize from the mixture, and all C4+ alcohols remained within the last 20% of the distilled volume. Other products, including ethanol, propanols, esters, and organic sulfur species distilled over a range of boiling temperatures. ADCs suggest the presence of one or more azeotropes in the distillate, consistent with a large number of known binary azeotropes between components found in the mixed oxygenate product. Enthalpies of combustion were calculated for multiple distilled fractions and ranged from 890 kJ mol–1 in the first drop of distillate to 1150 kJ mol–1 in the first drop collected after distilling 80% of the original liquid volume. This energy density is low, compared to 91-octane gasoline at 3700 and 4940 kJ mol–1 in the first drop and at 80%, respectively. Comparisons of fractional distillation of the mixed oxygenate products showed directional agreement between experiment and simulation with Aspen Plus. This study provides useful insights into mixed oxygenate products derived from a sulfided catalyst, including considerations for process recycle, product constituents and their blending, and the applicability of distillation information from process simulators.

Conversion of synthesis gas into alcohols on supported cobalt-molybdenum sulfide catalysts promoted with potassium

The use of transition metal sulfides as catalysts for the synthesis of alcohols can solve the problem of catalyst resistance to sulfur. Catalysts based on molybdenum sulfide of different compositions (promoted with Co and K) were synthesized with the use of various supports (aluminum oxide, aluminum oxide modified with silicon oxide, Sibunit, and titanium silicate) and tested in the reactions of alcohol synthesis and the hydrofining of a mixture of thiophene with n-1-hexene. The dependence of catalyst activity in the synthesis of alcohols on support pore size was demonstrated. It was found that an increase in the potassium content of the active phase of a catalyst increased its activity in the synthesis of alcohols and decreased it in hydrodesulfurization and hydrogenation reactions. Transmission electron microscopy data made it possible to quantitatively evaluate the effect of a potassium additive on the morphology of the active phase; the hypothesis that potassium was intercalated between the layers of molybdenum sulfide was proposed.

Preparation and Characterization of SBA-15 Supported Cobalt–Molybdenum Sulfide Catalysts for HDS Reaction: An All Sulfide Route to Hydrodesulfurization Catalysts

MoS2 hydrodesulfurization (HDS) catalysts promoted with Co supported on SBA-15 were synthesized from sulfur-containing Mo sources [ammonium thiomolybdate (ATM), and tetramethylammonium thiomolybdate (TMATM)] and Co complexes cobalt dimethylthiocarbamate by using different synthesis strategies in order to achieve active catalysts. The (Co)-MoS2/SBA-15 catalysts were characterized with X-ray diffraction, N2-physisorption and High-Resolution Transmission Electron Microscopy. The catalytic performance in the HDS reaction of dibenzothiophene was examined at T = 623 K and P H2 = 3.4 MPa. The results of the experiments suggest that the sequence of impregnation steps has no significant influence on the HDS activity. On the other hand, the use of different thiomolybdate precursors significantly affects the catalytic activity. The catalysts derived from TMATM show lower HDS activities compared to the catalysts synthesized from ATM which is probably due to the presence of pronounced pore blocking as well as the generation of big needle-like aggregates of the Co–MoS2 phase. It seems that the formation of intermediate MoS3 is not a prerequisite for the generation of catalytic active CoMoS phases. The high activity and high selectivity for the direct desulfurization pathway of catalysts prepared with ATM despite the large MoS2 stacking could be due to the generation of a large number of coordinately unsaturated sites. Cobalt promoted molybdenum sulfide catalysts supported on mesoporous SBA-15 were prepared by an all sulfide route using sulfur-containing Mo and Co complexes. High catalytic activity at high Mo loading is observed for Co–MoS2/SBA-15 catalysts derived from ammonium thiomolybdate despite a pronounced stacking of MoS2 slabs. The catalysts prepared with tetramethylammonium thiomolybdate show lower HDS activities probably due to a pronounced pore blocking as well as the generation of big needle-like aggregates of the Co–MoS2 phase. The sequence of impregnation steps has no significant influence on the HDS activity. It seems that the occurrence of intermediate MoS3 is not necessary for the generation of catalytic active Co–MoS phases.

SBA-15 as Support for MoS2 and Co-MoS2 Catalysts Derived from Thiomolybdate Complexes in the Reaction of HDS of DBT

Molybdenum sulfide and cobalt-molybdenum sulfide catalysts supported on mesoporous SBA-15 were prepared by thermal decomposition of ammonium thiomolybdate (ATM). SBA-15 was synthesized at 353 K and 413 K to obtain pore diameters of about 6 and 9 nm, respectively. The (Co)-MoS2/SBA-15 catalysts were characterized with X-ray diffraction (XRD), N2-physisorption and high-resolution transmission electron microscopy (HRTEM). HRTEM images give evidence for the presence of a poorly dispersed MoS2 phase with long MoS2 slabs and a pronounced MoS2 stacking. The catalytic performance in the hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT) was examined at T = 623 K and P = 3.4 MPa. The Co-MoS2/SBA-15 materials show a relatively high catalytic activity with a strong preference for the direct desulfurization (DDS) pathway. This is an interesting result in view of the significant stacking of MoS2 particles and the size of the slabs. The generation of the catalytically active CoMoS phase and a large number of coordinately unsaturated sites (CUS) may explain the high performance of Co promoted MoS2/SBA-15 catalysts in the HDS reaction. A confinement effect of the mesoporous channels of SBA-15 is observed for the unpromoted MoS2/SBA-15 catalysts. SBA-15 with 9 nm channel diameter with 11 wt.% Mo loading shows a higher selectivity for the hydrogenation pathway than SBA-15 with 6 nm channel and 16 wt.% Mo loading.

Effect of Boron Addition on the Intrinsic Activity of Al2O3-Supported Cobalt–Tungsten and Cobalt–Molybdenum Sulfide Catalysts for the Hydrodesulfurization of Thiophene

Abstract The effect of boron addition was studied on the intrinsic activity of Al2O3-supported cobalt–molybdenum and cobalt–tungsten sulfide catalysts prepared by a CVD technique using [Co(CO)3NO] as a precursor of Co. The catalysts were characterized by means of NO adsorption and Raman spectroscopy. The intrinsic activity of the catalysts are discussed in terms of the turnover frequency (TOF) of the hydrodesulfurization of thiophene, calculated on the basis of the amount of the Co–Mo(W)–S phase. The intrinsic activity on the Co–Mo–S phase and the Co–W–S phase was increased by the addition of boron in a different way depending on the presulfidation temperature. The addition of a proper amount of boron weakened the interactions between molybdenum (tungsten) oxides and the Al2O3 surface, leading to an increase in the TOF on the Co–Mo–S phase and the Co–W–S phase by a factor of 1.6 and 1.3, respectively, when Mo(W)S2/Al2O3 was presulfided at 673 K. As for the catalyst presulfided at 773 K, the addition of boron increased the TOF on both CVD-Co/MoS2/B/Al2O3 and CVD-Co/WS2/B/Al2O3 by a factor of 1.9. A reverse correlation was observed between the strength of the molybdenum (tungsten) oxides–Al2O3 interactions and the intrinsic activity on the Co–Mo(W)–S phase.

Effective supports to moderate H 2S inhibition on cobalt and nickel molybdenum sulfide catalysts in deep desulfurization of gas oil

Deep desulfurization of gas oil was attempted in the presence of H 2S over commercially available catalysts. Hydrotreated gas oil containing 340 ppm of sulfur (ppmS) was successfully desulfurized to less than 10 ppmS under the presence of 1.67% H 2S over CoMoS and NiMoS supported on acidic supports. Such supports are believed to enhance the sulfur release from active sites and the hydrogenation of refractory sulfur species to accelerate their deep desulfurization.

Selectively Hydrodesulfurization of FCC Naphtha over Monolayer Dispersion Catalysts

Metal oxides and salts can disperse spontaneously to the surface and pores of zeolite or alumina as atomically monolayer dispersion. In this paper a series of supported solid base catalysts were prepared by the method of monolayer dispersion, and the dispersion thresholds were determined by means of X-ray diffraction quantitative phase analysis. The fundamental functions of cobalt-molybdenum sulfide catalysts on monolayer dispersion catalysts were evaluated by hydrodesulfurization of FCC naphtha in a 100 mL high-pressure reactor unit. Experimental results indicated that the catalyst prepared by monolayer dispersion method in this article gives higher selectivity than the traditional hydrogenation catalyst and the oxide catalyst from hydrotalcite-like compounds. And the catalyst may show greater hydrodesulfurization activity and less olefin saturation under the moderate reaction conditions.

Cobalt-molybdenum sulfide catalysts prepared by in situ activation of bimetallic (Co-Mo) alkylthiomolybdates

Unsupported cobalt-molybdenum sulfide catalysts were prepared from bimetallic CoMo alkyl precursors by in situ activation during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The bimetallic CoMo precursors were prepared by reaction of tetraalkylammonium thiomolybdate salts, (R4N)2MoS4 (where R = H, methyl, butyl, pentyl or hexyl), with CoCl2 in water at a Co/Mo molar ratio of 0.3. These catalysts exhibit a “Swiss-cheese”-like morphology, high surface areas (from 52 up to 320 m2/g), high content of carbon (C/Mo = 2.2-3.3) and type IV adsorption-desorption isotherms of nitrogen. The in situ activation of these tetraalkylammonium thiobimetalate precursors leads to a mesoporous structure with pore size ranging from 2 to 4.5 nm. X-ray diffraction showed that the structure of unsupported cobalt-molybdenum sulfide catalysts corresponds to a poorly crystalline structure characteristic of 2H-MoS2 with low-stacked layers. The nature of the alkyl group strongly affects both the surface area and the HDS activity. The catalytic activity is strongly enhanced when using carbon-containing precursors; the CoMo catalysts prepared by in situ activation of Co/[N(C4H9)4]2MoS4 presents the highest HDS activity. The highest surface area of the catalysts was observed for the CoMo catalyst formed from Co/[N(C6H13)4N]2MoS4.

Preparation and characterization of zeolite-supported molybdenum and cobalt-molybdenum sulfide catalysts

Preparation, characterization and catalysis of zeolite-supported Mo, Co(Ni) and Co(Ni)Mo sulfide catalysts have been reviewed in the present article. Incomplete sulfidation of Mo oxides and poor dispersion of Mo sulfides were observed for zeolite-supported Mo catalysts prepared by impregnation methods, accompanied by a considerable crystallinity loss at a high calcination temperature. A significant migration of Mo oxide species into zeolite pores was noted during the calcination, producing Mo oxo-species in tetrahedral coordinations. Incorporation of Mo into zeolite using ion-exchange techniques and solid-solid reactions is briefly described. Metal carbonyl techniques provided highly dispersed and fully sulfided intrazeolite Mo and Co sulfide clusters by use of Mo(CO) 6 and Co(CO) 3NO, respectively. The Mo sulfide catalysts encaged in NaY exhibited much higher HDS activities than the impregnation catalysts. CoMo binary sulfide clusters were successfully synthesized in the pores of NaY as evidenced by EXAFS and XPS. The Co sites of the clusters were found to play important roles in hydrogenation and HDS. Thermally stable and catalytically active binary sulfide clusters for HDS are proposed to be intrazeolite Co 2Mo 2S 6 molecular clusters. It was demonstrated that catalytic synergy between Co and Mo sulfides was generated by CoSMo chemical bond formations.

CATALYTIC PRODUCTION OF ELEMENTAL SULFUR FROM THE THERMAL DECOMPOSITION OF H2S IN THE PRESENCE OF CO2

Abstract The feasibility of using a cobalt-molybdenum (Co-Mo) sulfide catalyst that was prepared from a commercial Co-Mo oxide catalyst for the production of elemental sulfur from hydrogen sulfide (H2S) and carbon dioxide (CO2) in a packed bed catalytic reactor was studied. It was demonstrated that the desired sulfide catalyst could be prepared by first reducing, then sulphiding the corresponding oxide. The results showed that the prepared catalyst was capable of producing elemental sulfur from the thermal decomposition of H2S in the presence of CO2 over a temperature range of 465–700°C and at atmospheric pressure. A specific rate coefficient was calculated as well as the Arrhenius parameters for the non-equilibrated reaction. The H2S decomposition reaction was found to be a second order reaction and have an activation energy of 114.4kJ/mol(27.3kcal/mol).

Preparation and Catalytic Properties of Highly Dispersed Molybdenum and Cobalt-Molybdenum Sulfide Catalysts Supported on Alumina

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPreparation and Catalytic Properties of Highly Dispersed Molybdenum and Cobalt-Molybdenum Sulfide Catalysts Supported on AluminaYasuaki Okamoto, Mitsuhiro Odawara, Hiroyuki Onimatsu, and Toshinobu ImanakaCite this: Ind. Eng. Chem. Res. 1995, 34, 11, 3703–3712Publication Date (Print):November 1, 1995Publication History Published online1 May 2002Published inissue 1 November 1995https://doi.org/10.1021/ie00038a005RIGHTS & PERMISSIONSArticle Views550Altmetric-Citations24LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Structure of the molybdenum sulfide phase in carbon-supported molybdenum and cobalt-molybdenum sulfide catalysts as studied by EXAFS

The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided (at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage local ordered structure of5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C coordination distance of 2.1 * 0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8 f 0.1 A.

Unsupported cobalt molybdenum sulfide catalysts Part II: Characterization and evolution of physicochemical properties during catalytic reaction

Cobalt molybdenum sulfide catalysts were prepared by two different methods, comaceration (CM) and homogeneous sulfide precipitation (HSP). Both types o catalysts were characterized by several techniques, DTA, XPS, AEM, conductivity measurements and temperature programmed reduction. Physicochemical properties were determined for different states of the catalyst:precursor, fully sulfided and catalytically tested. It is confirmed t

Unsupported cobalt molybdenum sulfide catalysts Part I: Catalytic activities for hydrodesulfurization of dibenzothiophene

Unsupported sulfided CoMo, MoS 2 and CogS 8 catalysts have been studied in hydrodesulfurization of dibenzothiophene under medium/high pressure co