Kinetics of sulfur model molecules competing with H2S as a tool for evaluating the HDS activities of commercial CoMo/Al2O3 catalysts

The relative-volume activities (RVAs) for real feedstocks HDS of four commercial CoMo/Al₂O₃ catalysts have been compared to the rates for thiophene and dibenzothiophene conversion. The reaction of thiophene competing with H₂S was studied in flow microreactors under a wide range of conditions: 300–400°C, overall pressure 0.1 or 3 MPa, thiophene pressure 8–125 kPa, H₂S content 0–15 mol%. The reaction of dibenzothiophene (DBT, 2 wt% in decaline) was carried out in a batch reactor at 335°C and 4 MPa.

The conversion of the two model molecules proceeds through the same mechanism with a preliminary dearomatization step followed by parallel hydrogenolysis and hydrogenation. From kinetic modeling, the global rates and the contribution of the hydrogenation and hydrogenolysis routes to HDS were determined. Under pressure, hydrogenolysis was predominant. In that case, thiophene and DBT behaved similarly and their initial relative rates did not correlate the RVA. Industrial HDS is controlled by hydrogenation as evidenced by the good correlation between RVA and the rates of dearomatization of thiophene at atmospheric pressure and hydrogenation of the product biphenyl from DBT under pressure. It is concluded that the reaction of model molecules under selected conditions can appraise rapidly industrial HDS.     

Hydrodesulfurization of thiophene on carbon nanofiber supported Co/Ni/Mo catalysts

Co, Mo, NiMo and CoMo catalysts supported on alumina, fishbone and platelet carbon nanofibers (CNFs) have been prepared. The dispersion of the oxide phases was qualitatively studied and compared using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The reducibility of the catalysts was studied by temperature programmed reduction (TPR). Hydrodesulfurization (HDS) of thiophene was used as a model reaction to compare the activity of different catalysts. The activity tests showed that the alumina supported catalysts exhibited higher activity compared to the corresponding CNF supported catalysts, and the NiMo catalysts were more active than the corresponding CoMo catalysts. The thiophene HDS activity was correlated with the dispersion of the molybdenum species and the reducibility of different catalysts. Interestingly, the CNF supported Co catalysts have higher thiophene HDS activity than the CNF supported Co(Ni)Mo catalysts.     

Hydrodeoxygenation of stearic acid using Mo modified Ni and Co/alumina catalysts: Effect of calcination temperature

Abstract

The calcination temperature (Cal-Temp) plays a vital role in the performance of supported metal catalysts. In this work, the alumina supported Ni, NiMo, Co, and CoMo catalysts were prepared at different Cal-Temp. The catalysts were characterized by various techniques to identify the catalytically active different surface species to correlate their role in the hydrodeoxygenation of stearic acid. With increasing Cal-Temp, the metal dispersion was increased for Ni, NiMo, and CoMo catalyst (up to 973 K) and decreased for Co catalyst. With increasing Cal-Temp, the catalytic activity was thus increased for Ni and NiMo catalyst and decreased for Co catalyst. The activity of CoMo catalyst was, however, enhanced with rising Cal-Temp up to 973 K and declined slightly after that. The optimum Cal-Temp for Ni, NiMo, Co, and CoMo catalyst was found to be 1023 K, 973 K, 773 K, and 973 K. The reaction followed the decarbonylation route over active metallic centers (Ni and Co) and the HDO route over oxophilic M²⁺?MoO₂ (M = Ni/Co) and reducible cobalt oxide species. The C₁₇ alkane was thus the principal product over Ni catalyst, whereas C₁₈ alkane was the primary product over CoMo and NiMo catalyst. In contrast, both C₁₇ and C₁₈ alkanes were significant over Co catalyst.     

A comparative study on the effect of promoter content of hydrodesulfurization catalysts at different evaluation scales

CoMo and NiMo supported Al₂O₃ catalysts have been investigated for hydrotreating of model molecule as well as industrial feedstock. Activity studies were carried out for thiophene and SRGO hydrodesulfurization (HDS) in an atmospheric pressure and batch reactor respectively. These activities on sulfided catalysts were evaluated as a function of promoter content [M/(M + Mo) = 0.30, 0.34, 0.39; M = Co or Ni] using fixed (ca. 8 wt.%) molybdenum content. The promoted catalysts were characterized by textural properties, XRD, and temperature programmed reduction (TPR). TPR spectra of the Co and Ni promoter catalysts showed that Ni promotes the easy reduction of Mo species compared with Co. With the variation of promoter content NiMo catalyst was found to be superior to CoMo catalyst for gas oil HDS, while at low-promoter content the opposite trend was observed for HDS of thiophene. The behavior was attributed to the several reaction mechanisms involved for gas oil HDS. A nice relationship was obtained for hydrodesulfurized gas oil refractive index (RI) and aromatic content, which corresponds to the Ni hydrogenation property.     

Hydrodesulfurization of dibenzothiophene over alumina-supported nickel molybdenum phosphide catalysts

The activity of nickel molybdenum phosphide catalysts was studied for the hydrodesulfurization of dibenzothiophene at 573 K and total pressure of 2.0 MPa. The Al₂O₃-supported NiMo phosphide catalysts were prepared by successive and simultaneous methods. The effect of the reduction temperature on the catalyst activity was also studied. The simultaneous preparation was determined to be the best method for the preparation of the active supported catalyst for dibenzothiophene HDS. The 623 K-reduced catalyst had the highest HDS rate of the catalysts. Nickel migrated from the inside to the surface during the reaction and promoted the HDS activity. The active species in the dibenzothiophene HDS and the oxidation states of Mo, Ni and P in the catalyst before and after reaction and of S after the reaction were studied on the basis of an XPS analysis.     

Effect of Zr-SBA-15 support on catalytic functionalities of Mo, CoMo, NiMo hydrotreating catalysts

SBA-15 and ZrO₂ (10–50 wt.%) containing SBA-15 mesoporous materials were prepared by direct and post-synthesis methods. Characterization using low angle XRD, pore size distribution, CO₂ chemisorption indicate that hexagonal mesoporous structure is retained even after ZrO₂ addition (25 wt.%). Mo, CoMo and NiMo catalysts prepared using these supports were examined by XRD, oxygen chemisorption, temperature programmed reduction (TPR). The catalysts were tested for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene. HDS of thiophene for 8%Mo, 3%Co8%Mo, and 3%Ni8%Mo increases with increasing ZrO₂ loading in SBA-15 up to 25 wt.%. Oxygen chemisorption and TPR hydrogen consumption indicated that the molybdenum dispersion and anion vacancies, and catalytic activities are significantly influenced by ZrO₂ content in Zr-SBA-15. A comparison indicated that TiO₂-SBA-15, ZrO₂-SBA-15 supported CoMo catalysts show higher activities for hydrodesulfurization.     

Effect of Al and Ti content in HMS material on the catalytic activity of NiMo and CoMo hydrotreating catalysts in the HDS of DBT

Al- and Ti-containing HMS materials, with a Si/M (Me = Al(Ti)) molar ratio equal to 40, were used as supports for preparing NiMo and CoMo HDS catalysts. The supports and catalysts were characterized by N₂ adsorption–desorption (S_BET), X-ray diffraction (XRD), UV–vis diffuse reflectance (DRS UV–vis), temperature-programmed reduction (TPR) and Raman spectroscopy. The catalysts were tested in the hydrodesulphurization (HDS) reaction of dibenzothiophene (DBT). All supported NiMo and CoMo catalysts on Al-HMS and Ti-HMS substrates showed higher catalytic activity than their Me-free counterparts. We found two interesting correlations between the structure and chemical coordination of the supported oxide precursors and catalytic activity. The differences observed in catalyst performance are attributed to the structure and specific electronic properties of the supported active species. From our results, it appears possible to optimize the Al- and Ti-loading to maximize the HDS activity.     

The functionalities of Pt-Mo catalysts in hydrotreatment reactions

The catalytic functionalities of bimetallic Pt-Mo/γ-Al₂O₃ catalysts in hydrotreatment were studied by performing simultaneous and independent dibenzothiophene (DBT) hydrodesulfurization (HDS) and naphthalene hydrodearomatization (HDA) reactions as a function of the activating agent and the MoO₃ content. Pt-Mo/γ-Al₂O₃ catalysts always displayed a higher selectivity to both the direct route of desulfurization (DDS) of DBT and to HDS over HDA than the one exhibited by conventional CoMo and NiMo/γ-Al₂O₃. It was established that for the Pt-Mo catalytic system, the selectivity DDS to the hydrogenation route of desulfurization of DBT can be indirectly described by the selectivity HDS/HDA in simultaneous HDS-HDA catalytic tests. The model of an active phase composed of separated metallic Pt particles, PtS_x species, and sulfided Mo which can either act as independent or cooperative active centers seems to be suitable to explain both the observed kinetic trends and the synergy effect between Pt and Mo.     

Cumene hydrocracking and thiophene HDS on niobia-supported Ni, Mo and Ni–Mo catalysts

Results are reported on the XPS characterization and catalytic activity in cumene hydrocracking (2.8 MPa, 623 K) and thiophene HDS (2.8 MPa, 523–573 K) of sulfided Ni, Mo and Ni–Mo catalysts supported on alumina and on pure and phosphated niobia. From the XPS results, evidence was obtained for the formation of a surface niobium sulfide with stoichiometry close to NbS₂ during catalyst sulfidation. Sintering of supported nickel during sulfidation occurred to a much smaller extent with the niobia-supported catalysts than with the alumina-supported ones. The dispersion of alumina-supported molybdenum was little influenced by sulfidation, whereas, with the niobia supports, the molybdenum surface concentration increased with sulfidation. With the alumina support, the Ni–Mo combination caused the dispersion of the sulfided nickel to be improved, possibly due to formation of a NiMoS phase. This was not observed with the niobia-supported catalysts.

Reasonable linear correlations were also found between the intrinsic activity for cumene hydrocracking and the amount of sulfided niobium in the catalysts, but the catalysts supported on phosphated niobia had a higher intrinsic activity than the ones supported on pure niobia. In thiophene HDS, the activity of the niobia-supported nickel catalysts was much larger than the activity of the alumina-supported ones. The activity of the niobia-supported molybdenum catalysts was smaller than that of the alumina-supported catalyst. With the bimetallic catalysts, little or no synergy was observed with the niobia-supported catalysts, in sharp contrast with the alumina case.     

Performance of spent sulfide catalysts in hydrodesulfurization of straight run and nitrogen-removed gas oils

After the test run of several months two kinds of commercial catalysts (NiMo/Al₂O₃ and CoMo/Al₂O₃) were examined in hydrodesulfurization (HDS) of straight run (SRGO) and nitrogen-removed gas oils, at 340 °C under 50 kg/cm² H₂. Hydrogen renewal between stages was attempted to show additional inhibition effects of the by-products such as H₂S and NH₃. Spent NiMo/Al₂O₃ and CoMo/Al₂O₃ catalysts showed contrasting activities in HDS and susceptibility to nitrogen species, according to their catalytic natures, compared to those of their virgin ones. HDS over spent NiMo/Al₂O₃ was significantly improved by removal of nitrogen species, while that over spent CoMo/Al₂O₃ was much improved by H₂ refreshment. The activity for refractory sulfur species such as 4,6-dimethyldibenzothiophene was reduced more severely than that for the reactive sulfur species such as benzothiophenes over spent catalysts. The effects of both two-stage hydrodesulfurization and nitrogen-removal were markedly reduced over the spent NiMo when compared with those over virgin NiMo one. The acidity of the catalysts was correlated with the inhibition susceptibility by nitrogen species as well as H₂S and NH₃. Spent catalysts apparently lost their activity due to the carbon deposition, which covered the active sites more preferentially. The spent NiMo catalyst carried more deposited carbon with larger C/H ratio and nitrogen content. Higher acidity was found to be present on the NiMo catalyst, but this was greatly decreased by the carbon deposition. Additionally, the reactivity of nitrogen species in HDS was briefly discussed in relation to the acidity of the catalyst and its deactivation by carbon deposition.     

The effects of fluorine, phosphate and chelating agents on hydrotreating catalysts and catalysis

The effects of fluorine, phosphate and chelating agents on hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are reviewed. All three additives enhance the activity of NiMo/Al₂O₃ catalysts in HDN but have only a slightly positive or even a negative effect on the HDS activity of CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts. The positive effect on HDN is due to the enhancement of the hydrogenation of aromatic rings. On the other hand, these three additives diminish the rates of C–N bond breaking and alkene hydrogenation reactions.

All three additives are hard basic ligands that may interact strongly with hard acids such as coordinatively unsaturated Al³⁺ cations on the alumina surface. A strong interaction with the alumina support has several effects. First, molybdate and tungstate anions are no longer strongly bonded to the support and are predominantly present as polyanions, which can be easily sulfided to MoS₂ and WS₂ crystallites. The weaker interaction with the smaller support surface also leads to larger MoS₂ and WS₂ crystallites with a lower dispersion. Second, the Ni²⁺ and Co²⁺ cations will also interact more weakly with the alumina, and this makes the formation of Ni and Co promoter atoms in the catalytically active Ni–Mo–S and Co–Mo–S phases more efficient. Third, the weaker interaction of Mo and W with the support leads to a higher stacking of the MoS₂ and WS₂ crystallites and, thus, to the more active type II Ni–Mo–S and Co–Mo–S phases. The increased stacking is beneficial for geometrically demanding reactions such as the hydrogenation of aromatics. For less demanding reactions, such as alkene hydrogenation, aliphatic C–N bond breaking and thiophene HDS, the loss in dispersion is important.     

CoMo bimetallic nitride catalysts for thiophene HDS

CoMo bimetallic nitride catalysts were prepared by temperature-programmed reduction of CoMo oxides with flowing ammonia, and the effects of cobalt addition on the thiophene hydrodesulfurization (HDS) reaction were investigated. MoO₃ transformed into Mo₂N, while cobalt oxide was just reduced to Co metal rather than Co nitride. When Co was added to Mo, the surface area decreased significantly but the new bimetallic nitride phase of Co₃Mo₃N was produced. The addition of Co could improve the HDS conversion, and the high specific activity of the bimetallic nitride catalyst was believed to be related with the new Co₃Mo₃N phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physico-chemical characterization and catalysis on SBA-15 supported molybdenum hydrotreating catalysts

SBA-15 supported Mo, CoMo, NiMo catalysts were prepared. The supports were characterized by surface area, pore size distribution, and X-ray diffraction. The finished catalysts in oxide state were characterized by surface area analysis and X-ray diffraction in the region where the molybdenum oxide lines are seen. The sulfided catalysts were examined by oxygen chemisorption at low temperatures. The catalytic functionalities of these catalysts viz hydrodesulfurization (HDS) and hydrogenation were evaluated on sulfided catalysts. The catalytic activities of these catalysts are compared with γ-Al₂O₃- and SiO₂-supported catalysts. An attempt is made to understand the relationship between oxygen chemisorption and catalytic activities with the help of other characterization results.     

On the formation of cobalt–molybdenum sulfides in silica‐supported hydrotreating model catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO₂ layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a Co–Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower activity in thiophene HDS. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrotreating of gas oil on SBA-15 supported NiMo catalysts

The siliceous and the metal substituted (B or Al)-SBA-15 molecular sieves were used as a support for NiMo hydrotreating catalysts (12 wt.% Mo and 2.4 wt.% Ni). The supports were characterized by X-ray diffraction (XRD), scanning electron microscopy and N₂ adsorption–desorption isotherms. The SBA-15 supported NiMo catalysts in oxide state were characterized by BET surface area analysis and XRD. The sulfided NiMo/SBA-15 catalysts were examined by DRIFT of CO adsorption and TPD of NH₃. The HDN and HDS activities with bitumen derived light gas oil at industrial conditions showed that Al substituted SBA-15 (Al-SBA-15) is the best among the supports studied for NiMo catalyst. A series of NiMo catalysts containing 7–22 wt.% Mo with Ni/Mo weight ratio of 0.2 was prepared using Al-SBA-15 support and characterized by BET surface area analysis, XRD and temperature programmed reduction and DRIFT spectroscopy of adsorbed CO. The DRIFT spectra of adsorbed CO showed the presence of both unpromoted and Ni promoted MoS₂ sites in all the catalysts, and maximum “NiMoS” sites concentration with 17 wt.% of Mo loading. The HDN and HDS activities of NiMo/Al-SBA-15 catalysts were studied using light gas oil at temperature, pressure and WHSV of 370 °C, 1300 psig and 4.5 h⁻¹, respectively. The NiMo/Al-SBA-15 catalyst with 17 wt.% Mo and 3.4 wt.% of Ni is found to be the best catalyst. The HDN and HDS activities of this catalyst are comparable with the conventional Al₂O₃ supported NiMo catalyst in real feed at industrial conditions.     

Investigation of CoNiMo/Al2O3 catalysts: Relationship between H2S adsorption and HDS activity

Sulfidation of trimetallic CoNiMo/Al₂O₃ catalysts was studied by thermogravimetry at 400 °C under flow and pressure conditions. Results were compared with those obtained on prepared and industrial CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts. The amount of sorbed H₂S on the sulfided solids was measured at 300 °C in the H₂S pressure range 0–3.5 MPa at constant H₂ pressure (3.8 MPa). The adsorption isotherms were simulated using a model featuring dissociated adsorption of H₂S on supported metal sulfides and bare alumina. The amount of sulfur-vacancy sites could thus be determined under conditions close to industrial practice. A relationship with activity results for thiophene HDS and benzene hydrogenation was sought for.     

A solvent-free,selective liquid phase hydrogenation of nitroarenes to aniline in slurry bubble mode on porous NiMo bimetallic catalyst

Ni Mo bimetallic catalysts were prepared by a solid reaction method. On the Ni Mo catalyst, the selective liquid phase hydrogenation of nitrobenzene to aniline was achieved in slurry bubble mode. And the high yields(98.9%) were obtained under the conditions of 80 °C, solvent-free and atmospheric pressure. The effect of Mo on the catalytic behavior of Ni based catalyst was investigated. The characterization displayed that the inclusion of Mo could improve the specific surface area and pore volume, and the solid reaction method made metal Mo enrichment on the surface of catalyst. These two aspects should be responsible for excellent catalytic performance of Ni Mo catalyst. In sum, we described a simple and efficient Ni Mo catalyst and provided a facile and green procedure for liquid phase hydrogenation of nitrobenzene to aniline.     

A Review of Deep Hydrodesulfurization Catalysis

The increasing importance of hydrodesulfurization (HDS) in petroleum processing in order to produce clean-burning fuels has led to a surge of research on the chemistry and engineering of HDS. Most of the earlier works are focused on catalyst characterization by physical methods; on low-pressure reaction studies of compounds like thiophene having relatively high reactivities; on process development; or on CoMo, NiMo, or NiW catalysts supported on alumina, often doped by fluorine or phosphorus. Almost all the reviews have concentrated on alumina-supported CoMo, NiMo, and NiW sulfide catalysts for hydrotreating. Even reviews that are not limited to the above catalytic systems essentially deal with studies of simple compounds like thiophene.     

Alumina-, niobia-, and niobia/alumina-supported NiMoS catalysts: Surface properties and activities in the hydrodesulfurization of thiophene and hydrodenitrogenation of 2,6-dimethylaniline

The activity of NiMoS catalysts supported on niobia, alumina, and niobia/alumina was compared for the thiophene hydrodesulfurization (HDS) and 2,6-dimethylaniline (2,6-DMA) hydrodenitrogenation (HDN) reactions. To evaluate the acidity of the supports and identify the nature of the sulfide sites, adsorption of 2,6-dimethylpyridine, pyridine, and CO was performed and followed by IR spectroscopy. This study has shown that with niobia as a support, the activity of NiMoS catalysts in thiophene HDS and in HDN of 2,6-DMA was no longer promoted by the synergy between Ni and Mo. The absence of synergy between molybdenum and nickel on niobia can be explained by the strong interaction of each metal with niobia at the expense of interaction with each other. Moreover, it has been shown that on a niobia/alumina support, the formation of the NiMoS phase can be directly linked to the presence of alumina not covered by niobia. However, niobia is an interesting support for the HDN of 2,6-DMA, because it favors the formation of xylene through direct ammonia elimination involving low H₂ consumption. The activity for xylene formation on niobia is linked to the electron-deficient nature of the Mo sulfide site, as demonstrated by CO adsorption followed by IR.     

Sulfided Mo and CoMo supported on zeolite as hydrodesulfurization catalysts: transformation of dibenzothiophene and 4,6-dimethyldibenzothiophene

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out on sulfided Mo and CoMo on HY catalysts, and also on sulfided Mo and CoMo on alumina catalysts (fixed bed reactor, 330°C, 3 MPa hydrogen pressure). On all the catalysts, the two reactants transformed through the same parallel pathways: direct desulfurization (DDS) leading to biphenyl-type compounds, and desulfurization after hydrogenation (HYD) leading first to tetrahydrogenated intermediates, then to cyclohexylbenzene-type products. However, additional reactions were observed with the zeolite-supported catalysts, namely methylation of the reactants, cracking of the desulfurized products, and, in the case of 4,6-DMDBT, displacement of the methyl groups and transalkylation. The global activity of Mo/zeolite in DBT or 4,6-DMDBT transformation as well as its activity for the production of desulfurized products (HDS) were much higher than those of Mo/alumina. On the other hand, cobalt exerted a promoting effect on the activity in the transformation of DBT or 4,6-DMDBT of all the molybdenum catalysts. However, this effect was much less significant with the zeolite support than with the alumina support, which indicated that the promoter was not well associated to molybdenum on the zeolite support. Therefore, the activity of CoMo/zeolite in the HDS of DBT was much lower than that of CoMo/alumina. On the contrary, in the case of 4,6-DMDBT CoMo/zeolite was more active in HDS than CoMo/alumina. This increase in HDS activity was attributed to the transformation of 4,6-DMDBT into more reactive isomers through an acid-catalyzed methyl migration. The consequence was that on the zeolite-supported catalyst 4,6-DMDBT was more reactive than DBT.