Effects of preparation conditions in hydrothermal synthesis of highly active unsupported NiMo sulfide catalysts for simultaneous hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

Unsupported NiMo sulfide catalysts were prepared from ammonium tetrathiomolybdate (ATTM) and nickel nitrate by using a hydrothermal synthesis method involving water, organic solvent and hydrogen. The activity of these catalysts in the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was much higher than that of the commercial NiMo/Al₂O₃ sulfide catalysts. Interestingly, the unsupported NiMo sulfide catalysts showed higher activity for hydrogenation (HYD) pathway than the direct desulfurization (DDS) pathway in the HDS of DBT. The same trends were observed for the HDS of 4,6-DMDBT. Morphology, surface area, pore volume and the HDS activity of unsupported NiMo sulfide catalyst depended on the catalyst preparation conditions. Higher temperature and higher H₂ pressure and addition of an organic solvent were found to increase the HDS activity of unsupported NiMo sulfide catalysts for both DBT and 4,6-DMDBT HDS. Higher preparation temperature increased HYD selectivity but decreased DDS selectivity. High-resolution TEM images revealed that unsupported NiMo sulfide prepared at 375 °C shows lower number of layers in the stacks of catalyst with more curvature and shorter length of slabs compared to that prepared at 300 °C. On the other hand, higher preparation pressure increased DDS selectivity but decreased HYD selectivity for HDS of 4,6-DMDBT. HRTEM images showed higher number of layers in the stack for the NiMo sulfide prepared under an initial H₂ pressure of 3.4 MPa compared to that under 2.1 MPa. The optimal Ni/(Mo + Ni) ratio for the NiMo sulfide catalyst was 0.5, higher than that for the conventional Al₂O₃-supported NiMo sulfide catalysts. This was attributed to the high dispersion of the active species and more active NiMoS generated. The present study also provides new insight for controlling the catalyst selectivity as well as activity by tailoring the hydrothermal preparation conditions.     

Activities of unsupported second transition series metal sulfides for hydrodesulfurization of sterically hindered 4,6‐dimethyldibenzothiophene and of unsubstituted dibenzothiophene

The applicability of transition metal sulfides (TMS) from the second transition series in deep hydrodesulfurization (HDS) was examined and compared to that of a traditional, supported CoMo/Al₂O₃ catalyst. Sulfides of Nb, Mo, Ru, Rh and Pd were studied for HDS of dibenzothiophene (DBT) and 4,6‐dimethyldibenzothiophene (4,6‐Me₂DBT). Measurements were carried out with unsupported TMS samples at different temperatures and H₂S partial pressures. The trend in DBT HDS activities agreed quite well with those found by previous authors. It was furthermore found that the activities of the metal sulfides towards the sterically hindered molecule 4,6‐Me₂DBT closely followed those for DBT. This is somewhat surprising since the direct sulfur abstraction route was of major importance for DBT while the prehydrogenation route, in which ring‐hydrogenation in the DBT skeleton precedes desulfurization, was prevalent for 4,6‐Me₂DBT. This suggests that common steps are involved in the two routes. For the unsupported metal sulfides, ring‐hydrogenated but not desulfurized DBT and 4,6‐Me₂DBT products were found in much larger amounts than for supported and promoted MoS₂‐based catalysts. This can be rationalized as being due to a relatively higher hydrogenation/desulfurization selectivity ratio for the different transition metal sulfides. Inhibition by H₂S was found to be most pronounced near the center of the transition series. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodesulfurization of Dibenzothiophene and its Hydrogenated Intermediates Over Bulk Ni2P

A bulk Ni₂P catalyst was prepared by co-precipitation of nickel phosphate followed by in situ temperature-programmed reduction (TPR) with H₂. The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and its hydrogenated intermediates 1,2,3,4-tetrahydro-dibenzothiophene (TH-DBT) and 1,2,3,4,4a,9b-hexahydro-dibenzothiophene (HH-DBT) was studied at 340 °C and 4 MPa both in the presence and absence of piperidine (Pi). Bulk Ni₂P exhibited a relatively low hydrogenation/dehydrogenation activity but high desulfurization activity. Pi retarded the hydrogenation of DBT to a greater extent than the desulfurization. The desulfurization of HH-DBT to 2-cyclohexen-1-yl-benzene (CHEB-2) occurred mainly by ??-elimination of the hydrogen atom attached to carbon atom C(4), whereas TH-DBT desulfurized mainly by hydrogenolysis to 1-cyclohexen-1-yl-benzene (CHEB-1). A minor amount of biphenyl (BP) observed in the HDS of TH-DBT and HH-DBT is due to the disproportionation of cyclohexenyl-benzenes. A reaction network of the HDS of DBT over Ni₂P is postulated in which both ??-elimination and hydrogenolysis play a role in the breaking of the C?CS bonds.     

Inhibition and deactivation in staged hydrodenitrogenation and hydrodesulfurization of medium cycle oil over NiMoS/Al2O3 catalyst

Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of a commercial medium cycle oil (MCO) were performed over a commercial NiMoS/Al₂O₃ catalyst through both single- and two-stage hydrotreatments at 340°C. The reaction atmosphere was replaced with fresh hydrogen, with or without additional dose of catalyst, for the second-stage treatment to determine the mechanism of reduced activity. Sulfur and nitrogen molecular species in MCO were identified by gas chromatography with an atomic emission detector (GC-AED) to quantify their respective reactivities and susceptibilities to inhibition. Under single-stage (30 min) conditions, the reactivity orders in HDS and HDN were BT>DBT>4-MDBT>4,6-DMDBT and In>alkylIn>Cz>1-Cz>1,8-Cz, respectively. Additional reaction time beyond the initial 30 min, without atmosphere or catalyst replacement, gave little additional conversion. Replacement of the first-stage gas with fresh hydrogen strongly improved second-stage conversions, particularly those of the more refractory species. An additional dose of catalyst for the second stage with hydrogen renewal facilitated additional HDS of dibenzothiophene (DBT), 4-monomethylated DBT (4-MDBT), and 4,6-dimethylated DBT (4,6-DMDBT) which was independent of their initial reactivity, while HDN of carbazole (Cz), 1-Cz, and 1,8-Cz was improved, the least reactive species being most denitrogenated. Such results suggest the strong inhibition of the gaseous products H₂S and NH₃. The catalyst deactivation was most marked with HDN of 1,8-Cz, suggesting that acidity is essential to the reaction. H₂S is suspected to inhibit both S elimination and hydrogenation of S and N species at the level of concentration obtained during desulfurization. The inhibition by remaining substrates may still influence the HDS and HDN of refractory species in the second stage, even if their contents were reduced by the first stage. It appears very important to clarify the inhibition factor of all species on the refractory sulfur species, and to determine the inhibition susceptibility of these species at their lowered concentration to enable the effective achievement of 50 ppm sulfur level in distillate products. The conversions of inhibitors must be accounted for during reactions. Catalyst and reaction configuration to reduce the inhibition by the gaseous products are the keys for deep refining.     

Promotion Effect of 2-Methylpiperidine on the Direct Desulfurization of Dibenzothiophene over NiMo/γ-Al2O3

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) was studied at 300 °C, 4.8 MPa H₂ and 35 kPa H₂S in the presence of 0 to 6 kPa 2-methylpyridine and 2-methylpiperidine. 2-Methylpyridine suppressed the hydrogenation pathway by a factor of 6 to 30, depending on its partial pressure, and moderately inhibited the direct desulfurization pathway of the HDS of DBT (by a factor of 2 or less). 2-Methylpiperidine suppressed the hydrogenation by a factor of 10 to 50 but promoted the direct desulfurization of DBT at low partial pressures of 2-methylpiperidine. Both pathways were inhibited at high concentrations of 2-methylpiperidine. Structural and electronic factors may account for the promoting effect of 2-methylpiperidine.     

Competitive adsorption of nitrogen species in HDS: Kinetic characterization of hydrogenation and hydrogenolysis sites

Through modeling of the transient response of dibenzothiophene (DBT) hydrodesulfurization (HDS) to inhibition by 3-ethylcarbazole (3ECBZ), the hydrogenation (HYA) and hydrogenolysis (HYL) functions of a sulfided CoMo/Al₂O₃–SiO₂ catalyst are characterized. The HYL sites, accounting for about one third of the total active sites, have a lower adsorption affinity for 3ECBZ than for DBT. The opposite is true for the HYA sites. The adsorbed nitrogen species are denitrogenated more rapidly on the HYL sites than on the HYA sites. As a result, the HYL sites are less inhibited by 3ECBZ and drive the HDS of DBT almost single-handedly in the presence of 3ECBZ. The catalyst surface is sparsely occupied by sulfur species because the HDS rate is much faster than the hydrodenitrogenation rate on both HYA and HYL sites. These results give a quantitative understanding of why 3ECBZ is a far weaker inhibitor to the HDS of DBT than it is to the HDS of 4,6-diethyldibenzothiophene. There appears a tradeoff between the HYA and HYL functions, which can be exploited for HDS catalyst and process optimization.     

The functionalities of Pt/γ-Al2O3 catalysts in simultaneous HDS and HDA reactions

A Pt/γ-Al₂O₃ catalyst was tested in simultaneous hydrodesulfurization (HDS) of dibenzothiophene and hydrodearomatization (HDA) of naphthalene reactions. Samples of it were subjected to different pretreatments: reduction, reduction–sulfidation, sulfidation with pure H₂S and non-activation. The reduced catalyst presented the best performance, even comparable to that of Co(Ni)Mo catalysts. All catalyst samples were selective to the HDS reaction over HDA, and to the direct desulfurization pathway of dibenzothiophene HDS over the hydrogenation reaction pathway of HDS. The effect of H₂S partial pressure on the functionalities of the reduced Pt/γ-Al₂O₃ catalyst was studied. The results showed that an increase in H₂S partial pressure does not cause poisoning, but an inhibition effect, without changing the catalyst selectivity. Accordingly, the activity trends were ascribed to adsorption differences between the different reactive molecules over the same catalytic active site. TPR characterization along with a thermodynamics analysis showed that the active phase of reduced Pt/γ-Al₂O₃ is constituted by Pt⁰ particles. However, presulfidation of the catalyst leads to a mixture of PtS and Pt⁰ which has a negative effect on the catalytic performance without changing catalyst functionalities.     

V-Mo based catalysts for oxidative desulfurization of diesel fuel

Catalytic oxidative desulfurization (ODS) of a Mexican diesel fuel on a spent HDS catalyst, deactivated by metal deposits, was carried out during several reactive-batch cycles in order to study the catalytic performance to obtain low sulfur diesel. To explain catalytic activity results, Mo and/or V oxides supported on alumina pellets were prepared and evaluated in the ODS of a model diesel using tert-butyl hydroperoxide (TBHP) or H₂O₂ as oxidant. The catalytic results show that V-Mo based catalysts are more active during several ODS cycles using TBHP. The performance of the catalysts was discussed in terms of reduced species of vanadium oxide, prevailing on the catalysts, which increase the sulfone yield of refractory HDS compounds (DBT, 4-MDBT and 4,6-DMDBT).     

Unsupported NiMo Sulfide Catalysts Obtained from Nickel/Ammonium and Nickel/Tetraalkylammonium Thiomolybdates: Synthesis and Application in the Hydrodesulfurization of Dibenzothiophene

Ammonium and tetraalkylammonium tetrathiomolybdates impregnated with nickel nitrate were used as precursors of unsupported NiMo sulfide catalysts. The precursors were decomposed either in situ during the course of a dibenzothiophene (DBT) hydrodesulfurization (HDS) test or ex situ through sulfidation by H₂S/H₂ (15% v/v H₂S). The catalysts were characterized by thermogravimetric analysis, N₂ adsorption, scanning electron microscopy (SEM), and X-ray diffraction. Textural and catalytic properties of these NiMo catalysts were strongly influenced both by the nature of the precursor and the activation procedure. For ex-situ activated NiMo catalysts, the use of carbon-containing tetraalkylammonium thiosalts as precursors did not lead to a significant improvement in HDS activity. For in situ activated NiMo catalysts, the role of carbon is more complex. The use of tetramethyl- or tetrapropylammonium tetrathiomolybdate salts led to a poor final HDS activity while using tetrabutylammonium tetrathiomolybdate, a net increase in HDS activity was observed compared to the use of the non-carbon containing ammonium tetrathiomolybdate. This was related to the development of a mesoporous structure and to a high increase in surface area. This result is in agreement with those found previously for CoMo catalysts and confirms that tetraalkylammonium tetrathiomolybdate salts with long alkyl chains lead to Co- or Ni-promoted MoS₂-based catalysts with enhanced HDS activity if in situ activated.     

Preparation of Highly Active Alumina-Pillared Clay Montmorillonite-Supported Platinum Catalyst for Hydrodesulfurization

Effect of Pt precursor and pretreatment on hydrodesulfurization (HDS) activity of Pt/Al-PILM catalyst was examined to prepare highly active Pt-supported HDS catalyst. The order of HDS activities of Pt/alumina-pillared clay montmorillonite (Al-PILM) catalysts prepared by various Pt precursors was Pt(C₅H₇O₂)₂ > H₂PtCl₆ · 6H₂O > [Pt(NH₃)₄](NO₃)₂ > [Pt(NH₃)₄]Cl₂ · H₂O > H₂Pt(OH)₆. This order was in accordance with that of Pt dispersion. Thus, high Pt dispersion is essential factor to prepare highly active Pt/Al-PILM catalyst for HDS reaction. On the other hand, the effect of pretreatment on the HDS activities of Pt/Al-PILM catalysts prepared by various Pt precursors was also evaluated. The UC-TPS Pt/Al-PILM catalyst showed the highest HDS activity among various pretreated Pt/Al-PILM catalysts, in which uncalcined catalyst was sulfided by temperature programmed sulfidation (TPS). We assumed that high HDS activity of UC-TPS Pt/Al-PILM catalyst is caused by partly sulfided Pt particle with high dispersion. It is concluded that the highly active Pt/Al-PILM catalyst for the HDS reaction could be prepared by using Pt(C₅H₇O₂)₂ as a precursor and UC-TPS treatment.     

Catalyst’s Sulfur Displacement by Thiophene, as Monitored by Exchange of 35S Radiotracer

The isotopic exchange has been studied between catalyst radiosulfur and H₂S, formed in thiophene hydrodesulfurization (HDS) (named S-displace) on alumina supported molybdena, on CoMoO_x, PdMoO_x, PtMoO_x and on silica–alumina supported NiWO_x. S-displace was compared with radiosulfur exchange data between catalyst radiosulfur and gas phase H₂S (S^exc) determined previously. The extent of S^exc was higher than that of the S-displace for Mo, CoMo in and NiW, whereas the extent of S-displace from PdMoO and PtMoO was significantly higher, than that of S^exc. Thiophene HDS product distribution data are discussed in terms of increased C=C hydrogenation and C–C hydrogenolysis activity, explained by increasing H₂S production with longer circulation time of the thiophene/H₂ mixture, The C₁/C₃<1 ratios among C₄-hydrogenolysis products indicate some coke formation. The decrease of thiophene HDS activity is presumably a consequence of increasing site-blocking with the formation of more H₂S and coke with longer duration of thiophene treatment.     

Silica Gel- and MCM-41-Supported MoS2 Catalysts for HDS Reactions

MCM-41- and silica gel-supported MoS₂ catalysts were prepared. MCM-41 was synthesized and impregnated with precursor, then activated to obtain the active phase. The sol–gel method was used for providing the SiO₂ support as well as for including the catalyst precursors in one single step of preparation. Such catalysts have applications particularly in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes. A comparison of the activities of the catalysts was made. The catalytic activity results showed the method of preparation used in this study was successful in producing very efficient catalysts for the HDS of dibenzothiophene (DBT).A higher selectivity for direct C–S bond cleavage was observed for the MoS₂ catalyst supported on SiO₂ by the sol–gel method. X-ray diffraction studies showed that the catalysts were poorly crystallized with a very weak intensity of the (002) line of 2H-MoS₂.     

A role of molybdenum and shape selectivity of catalysts in simultaneous reactions of hydrocracking and hydrodesulfurization

The hydrocracking and hydrodesulfurization (HDS) of n-heptane containing 0.2 mole% dibenzothiophene (DBT) were performed simultaneously using NiPtMo catalysts supported on HZSM-5, LaY and γ-Al₂O₃ in a high pressure fixed bed reactor. Molybdenum played an important role in both hydrocracking and hydrodesulfurization (HDS). We found that the sulfur compound, dibenzothiophene (DBT). in the reactant was adsorbed on a molybdenum site and converted to hydrogen sulfide so that the active sites of the catalysts for hydrocracking were less poisoned by DBT and the conversion of n-heptane over molybdenum impregnated catalyst was higher than that over molybdenum-free catalyst. The crystal structures of the molybdenum supported on the zeolite and γ-Al₂O₃ were mainly MoO_2.5 (OH)_0.5[021] and MoO₃[210] respectively as shown by XRD analysis. The structure of MoO_2.5(OH)_0.5 was easily reduced to MoS₂[003] during the reaction. After the reaction of 100 hours over the catalyst supported on γ-Al₂O₃ the crystal structure of MoO₃[210] partially changed to MoO₃[300] and the structure of MoS₂[003] was not observed. Because of the reactant shape selectivity of zeolite, the acid and the metal sites in the intracrystalline of the catalysts supported on zeolites were less poisoned by DBT. Therefore, both hydrocracking and HDS using n-heptane containing 0.2 mole% of DBT were successfully demonstrated over the prepared catalysts.     

Upgrading of gas oils: the HDS kinetics of dibenzothiophene and its derivatives in real gas oil

The hydrodesulphurization (HDS) of dibenzothiophene (DBT), 4-methyl dibenzothiophene (4 M-DBT), 4,6-dimethyl dibenzothiophene (4,6 DM-DBT) and 4,6-diethyl dibenzothiophene (4,6 DE-DBT) as real gas oil components on NiMo/Al₂O₃ catalyst was investigated. On the basis of the first order rate constants of HDS of the individual sulphur compounds reactivities of the investigated compounds decreased in the order DBT ≫ 4 M-DBT > 4,6 DE-DBT ≈ 4,6 DM-DBT. Apparent activation energies of HDS of above sulphur compounds increased from 80.0 to 120.5 kJ/mol.     

Mechanisms of hydrodesulfurization and hydrodenitrogenation

To study the problems inherent in deep hydrodesulfurization (HDS), the separate and simultaneous HDS of 4,6-dimethyldibenzothiophene and hydrodenitrogenation (HDN) of pyridine were investigated over a Ni-MoS₂/γ-Al₂O₃ and a Pd/γ-Al₂O₃ catalyst. The HDS of 4,6-dimethyldibenzothiophene and its three intermediates, 4,6-dimethyl-tetrahydro-dibenzothiophene, 4,6-dimethyl-hexahydro-dibenzothiophene and 4,6-dimethyl-dodecahydro-dibenzothiophene, demonstrated that, over the Pd catalyst, the (de)hydrogenation reactions were relatively fast compared to the C–S bond breaking reactions, whereas the reverse was true over the metal sulfide catalyst. The methyl groups of 4,6-dimethyldibenzothiophene strongly hinder the direct desulfurization HDS pathway over both catalysts. On the Pd catalyst the hydrogenation pathway is strongly promoted by the methyl groups, so that the total HDS rate does not decrease. Pyridine and piperidine were strong poisons for the hydrogenation pathway and H₂S was a strong poison for the direct desulfurization pathway.HDN of nitrogen-containing aromatic molecules occurs by hydrogenation of the aromatic heterocycle followed by C–N bond breaking. The C–N bond breaks by substitution of the alkylamine by H₂S to form an alkanethiol, followed by the loss of H₂S by elimination or hydrogenolysis. The NH₂-SH substitution does not occur by a classic organic substitution reaction but through a multi-step reaction pathway via an alkylimine. DFT calculations showed that the hydrogenolysis of ethanethiol to ethane and the elimination of ethane are relatively easy reactions.     

Synthesis, characterization and hydrotreating performance of supported tungsten phosphide catalysts

Supported tungsten phosphide catalysts were prepared by temperature-programmed reduction of their precursors (supported phospho-tungstate catalysts) in H₂ and characterized by X-ray diffraction (XRD), BET, temperature-programmed desorption of ammonia (NH₃-TPD) and X-ray photoelectron spectroscopy (XPS). The reduction-phosphiding processes of the precursors were investigated by thermogravimetry and differential thermal analysis (TG-DTA) and the suitable phosphiding temperatures were defined. The hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities of the catalysts were tested by using thiophene, pyridine, dibenzothiophene, carbazole and diesel oil as the feedstock. The TiO₂, γ-Al₂O₃ supports and the Ni, Co promoters could remarkably increase and stabilize active W species on the catalyst surface. A suitable amount of Ni (3%–5%), Co (5%–7%) and V (1%–3%) could increase dispersivity of the W species and the BET surface area of the WP/γ-Al₂O₃ catalyst. The WP/γ-Al₂O₃ catalyst possesses much higher thiophene HDS and carbazole HDN activities and the WP/TiO₂ catalyst has much higher dibenzothiophene (DBT) HDS and pyridine HDN activities. The Ni, Co and V can obviously promote the HDS activity and inhibit the HDN activity of the WP/γ-Al₂O₃ catalyst. The G-Ni5 catalyst possesses a much higher diesel oil HDS activity than the sulphided industrial NiW/γ-Al₂O₃ catalyst. In general, a support or promoter in the WP/γ-Al₂O₃ catalyst which can increase the amount and dispersivity of the active W species can promote its HDS and HDN activities.     

Synthesis of an yttrium-modified bulk Ni2P catalyst with high hydrodesulfurization activity

Yttrium-modified bulk Ni₂P (Y_xNi₂P) catalysts showing high hydrodesulfurization (HDS) activity are described. The incorporation of Y into the bulk Ni₂P catalyst can suppress the formation of the Ni₅P₄ phase and therefore promote formation of the more active Ni₂P phase. Y can also greatly increase the surface area of the catalyst, leading to a smaller crystallite size and better dispersion of active Ni₂P particles. The obtained Y_xNi₂P catalysts show much higher Dibenzothiop (DBT) HDS activity than the bulk Ni₂P catalyst.     

HDS reactivities of dibenzothiophenic compounds in a LC-finer LGO and H2S/NH3 inhibition effect

The hydrodesulfurization (HDS) reactivities and the inhibition effects of H₂S and NH₃ were experimentally investigated for 11 difficult-to-remove sulfur compounds contained in LC-finer light gas oil using a commercial NiMo/Al₂O₃ hydrotreating catalyst. Among these sulfur compounds, 4-MDBT was the most reactive while 4,6-DMDBT was the least reactive. It was found that the presence of methyl groups away from the sulfur atom slightly enhanced the HDS reactivities of the methyl substituted DBTs. The observed low reactivity of 4,6-DMDBT was mainly caused by the steric hindrance of the two methyls at the 4 and 6 positions. Both H₂S and NH₃ significantly inhibited the HDS reaction rates for all 11 sulfur species of interest, while 4,6-DMDBT was one of the most inhibited species. At the same concentration (1.0 vol%), H₂S showed a stronger inhibition effect than NH₃. Measurable catalyst deactivation was observed in the course of the experiment that had a relatively even effect on the HDS of all reactants investigated in this study.     

Activity and thermal stability of sonochemically synthesized MoS2 and Ni-promoted MoS2 catalysts

Sonochemically synthesized MoS₂/Al₂O₃, which had a hydrodesulfurization (HDS) activity that was significantly greater than that of a catalyst prepared by impregnation, exhibited low thermal stability due to sintering of MoS₂ crystallites at high temperatures. The thermal stability was improved when the catalyst was promoted with Ni. In this study, we compared the activity and thermal stability of different Ni-promoted MoS₂ catalysts, which were prepared by addition of Ni to MoS₂ using either impregnation (IMP) or chemical vapor deposition (CVD). After use in the HDS of dibenzothiophene (DBT) at 673 K for 2 h, the initial activity of the un-promoted catalyst was partially lost, while that of the Ni-promoted catalysts was preserved. Ni added by CVD interacted more intimately with MoS₂ than Ni added by impregnation because CVD allowed selective deposition of Ni on the MoS₂ edge sites. Another advantage of the CVD method over the impregnation method is that Ni(CO)₄, which was used as the Ni precursor in the former method, could be decomposed at much lower temperatures than in the case of Ni(NO₃)₂, which was used in the impregnation method. As a result, Ni-promoted catalysts prepared using Ni-CVD showed superior HDS activity compared with those prepared using Ni-impregnation.     

Dibenzothiophene hydrodesulfurization over MoP/SiO<Subscript>2</Subscript> catalyst prepared with sol-gel method

Silica-supported molybdenum phosphide, MoP/SiO₂ catalysts with different Mo weight loadings were prepared by temperature programmed reduction of the oxidic catalyst precursors, which were prepared via sol-gel technique using ethyl silicate-40 as silica source. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area measurements, and their catalytic activity in hydrodesulfurization (HDS) was tested with dibenzothiophene (DBT) as model compound. XRD analysis revealed the amorphous nature of the catalyst up to 10 wt% Mo loading and the formation of crystalline MoP phase on amorphous silica support with higher Mo loading. BET surface area showed high surface area for catalysts prepared by sol-gel technique with lower Mo content, and the surface area decreased with increasing in Mo loading. The HDS results showed that prepared MoP/SiO₂ exhibited high HDS activity and stability toward the catalytic test. Among the series of catalysts prepared, MoP/SiO₂ containing 20 wt% Mo was found to be the most active catalyst. And the effects of reaction temperature and hydrogen pressure on conversion and product selectivity were investigated.