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.     

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.     

Preparation of Co–Mo/B2O3/Al2O3 catalysts for hydrodesulfurization: Effect of citric acid addition

The effect of citric acid (CA) addition was studied on the HDS of thiophene over Co–Mo/(B)/Al₂O₃ catalysts. The catalysts were characterized by means of LRS, Mo K-edge EXAFS, NO adsorption capacity measurements, and UV–vis spectra. The catalysts were subjected to a chemical vapor deposition (CVD) technique using Co(CO)₃NO as a precursor of Co in order to get deeper insights into the effect of citric acid addition. It was shown that the HDS activity was enhanced by the citric acid addition up to the CA/Mo mole ratio of around 1 and leveled off with further addition. The amount of Co anchored by the CVD was increased by the addition of citric acid, suggesting an increase in the dispersion of MoS₂ particles on the catalyst by the simultaneous presence of Co, Mo and citric acid, in conformity with the increase in the NO adsorption capacity. In contrast to Co–Mo catalysts, the edge dispersion of MoS₂ particles in Mo/B/Al₂O₃ was not affected by the addition of citric acid. The LRS, UV–vis spectra and Mo K-edge EXAFS showed that Co–CA and Mo–CA surface complexes are formed by the addition of citric acid. The Co–CA surface complex is more preferentially formed on CoMo/Al than on CoMo/B/Al, in agreement with a greater promoting effect of citric acid at a lower CA/Mo mole ratio for CoMo/Al than for CoMo/B/Al.     

Effect of EDTA on hydrotreating activity of CoMo/γ-Al2O3 catalyst

γ-Al₂O₃ supported Co (0–4.5 wt%) Mo (9.0 wt%) sulfide catalysts were prepared in the presence and the absence of ethylenediaminetetraacetic acid (EDTA). The hydrodenitrogenation (HDN) activity of these catalysts was studied in the model reaction of 2,6-dimethylaniline (DMA) at 300 °C under 4 MPa. The CoMo/Al₂O₃ catalysts prepared with the EDTA showed higher HDN of DMA than those prepared without EDTA. The maximum of 36% increase in rate constant of HDN of DMA was observed over the catalyst with 3% Co prepared using EDTA. The FT-IR spectroscopy of adsorbed CO on CoMo catalysts showed that EDTA addition promoted the formation of catalytically active “CoMoS” phase as evidenced from increases in intensity of band at 2070 cm⁻¹, which is maximum for 3% Co loaded catalysts. The HDN and hydrodesulfurization (HDS) activity of 3% Co loaded catalyst prepared using EDTA was tested and compared with those catalyst prepared without EDTA in a trickle bed reactor using heavy gas oil derived from Athabasca bitumen in the temperature range 370–400 °C and 8.8 MPa. Improved HDN and HDS conversion of heavy gas oil was obtained for the catalyst prepared with EDTA.     

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.     

Mechanisms of hydrodenitrogenation of alkylamines and hydrodesulfurization of alkanethiols on NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

The simultaneous hydrodenitrogenation (HDN) of alkylamines and hydrodesulfurization (HDS) of alkanethiols, with the NH₂ and SH groups attached to primary, secondary, and tertiary carbon atoms, were studied at 270–320 °C and 3 MPa over sulfided NiMo/Al₂O₃, CoMo/Al₂O₃, and Mo/Al₂O₃ catalysts. Pentylamine and 2-hexylamine reacted by substitution with H₂S to form alkanethiols and with another amine molecule to form dialkylamines. Alkenes and alkanes were not formed directly from pentylamine and 2-hexylamine, but indirectly by elimination and hydrogenolysis of the alkanethiol intermediates, as confirmed by their secondary behavior and the similar alkene/alkane ratios in the simultaneous reactions of amines and thiols. Only 2-methyl-2-butylamine, with the NH₂ group attached to a tertiary carbon atom, produced alkenes as primary products by E1 elimination. NiMo/Al₂O₃ and CoMo/Al₂O₃ have a higher activity for the HDS of alkanethiols than does Mo/Al₂O₃; H₂S has a negative influence. This shows that the thiols react on vacancies on the catalyst surface (Lewis acid sites). Mo/Al₂O₃ is the best HDN catalyst; H₂S has a positive influence on the HDN of amines with the NH₂ group attached to a secondary and a tertiary carbon atom. This indicates that the HDN of alkylamines occurs on Brønsted acid sites.     

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.     

Magnesia-Supported Mo, CoMo and NiMo Sulfide Catalysts Prepared by Nonaqueous Impregnation: Parallel HDS/HDN of Thiophene and Pyridine and TEM Microstructure

MgO-supported Mo, CoMo and NiMo sulfide catalysts were prepared by impregnation using slurry MoO₃/methanol and solutions of Ni and Co nitrates in methanol. The catalysts exhibited very high hydrodesulfurization activity and low hydrodenitrogenation activity in competitive reactions of thiophene and pyridine. The promotion effect for HDS of Ni and Co was higher for our MgO-supported MoS₂ catalysts than for conventional Al₂O₃-supported catalysts. The specific features in the TEM images of MgO-supported catalysts as compared to conventional Al₂O₃-supported catalysts were fairly broad MoS₂ slab length distribution and the presence of unusually long MoS₂ slabs.     

Characterization of hydrodesulfurization catalyst prepared by impregnating cobalt nitrate solution onto the sulfided MoO3/Al2O3 catalyst

CoMo/Al₂O₃ catalysts were prepared by impregnating Cobalt nitrate solution into oxidic or sulfided Mo/Al₂O₃. The properties of CoMo/Al₂O₃ catalysts were characterized by XRD, TPS, oxygen chemisorption and ESR. Catalytic activity of CoMo/Al₂O₃ catalyst was evaluated by thiophene HDS as a probe reaction. When CoMo/Al₂O₃ catalyst was prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al₂O₃, the interaction between Mo and alumina became weaker and the formation of synergic phase was facilitated. These structural changes may explain higher HDS activity of CoMo/Al₂O₃ catalyst prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al₂O₃.     

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.     

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.     

Nanosized HY zeolite-alumina composite support for hydrodesulfurization of FCC diesel

A series of hydrodesulfurization (HDS) NiMo catalysts supported on Al₂O₃-NY (denoted as ANY) composites with various amounts of nanosized H-type Y zeolite (denoted as NY) were prepared. The samples were characterized by XRD, BET, TPD, TPR, HRTEM, and FT-IR spectroscopy of pyridine adsorption. The characterization results showed that, compared with NiMo/Al₂O₃, the addition of NY reduced the Metal-support interaction and made the MoS₂ stacking degree higher, slabs shorter and dispersion of edge and corner Mo atoms bigger. The addition of NY also enhanced the overall acidity and the ratio of Brönsted acid to Lewis acid of these catalysts. The fluidized catalytic cracking (FCC) diesel HDS activity was increased with the addition of NY in these catalysts compared with NiMo/Al₂O₃ catalysts. The optimal NY content was found to be 20 wt% in ANY composite support. The highest HDS activity of NiMo/ANY20 was attributed to the synergy of hydrogenation activity, acid amount and textural properties.     

SBA-15 supports modified by Ti and Zr grafting for NiMo hydrodesulfurization catalysts

A series of Ti- and Zr-containing mesoporous SBA-15 supports and their respective NiMo catalysts were prepared to study the effect of the Ti and Zr loading into the support on the characteristics of Ni and Mo surface species and their catalytic activity in the 4,6-dimethyldibenzothiophene hydrodesulfurization (HDS). Ti and Zr-containing SBA-15 solids with different metal loading (up to 19 wt.% of TiO₂ or 22 wt.% of ZrO₂) were prepared by chemical grafting. The solids prepared were characterized by N₂ physisorption, X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR), chemical analysis and HRTEM. The results show that Ti- and Zr-SBA-15 supports with highly dispersed Ti and Zr species can be obtained without substantial loss of SBA-15 characteristics. Zr grafted species showed somewhat better dispersion on the SBA-15 surface than the corresponding Ti counterparts. It was found that TiO₂ and, especially, ZrO₂ incorporation in the SBA-15 support leads to stronger interaction of Mo and Ni species with the support providing better dispersion to the oxidic and sulfided metal species (XRD, TPR, HRTEM). NiMo catalysts supported on Ti- and Zr-containing SBA-15 showed high activity in 4,6-dimethyldibenzothiophene HDS. It can be concluded therefore that SBA-15 materials grafted with Ti or Zr species show promising features as supports for Mo-based hydrotreating catalysts.     

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.     

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.     

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.     

Bimetallic catalysts for the catalytic combustion of methane using microreactor technology

Pt–W and Pt–Mo based catalysts were evaluated for methane combustion using a sandwich-type microreactor. Alumina washcoated microchannels were impregnated with platinum in combination with and promoted with tungsten and molybdenum and compared with commercially available Pt/Al₂O₃ catalysts. Catalysts were tested in the range of 300–700 °C with flow rates adjusted to GHSV of 74,000 h⁻¹ and WHSV of 316 L h⁻¹ g⁻¹. Catalysts containing tungsten were found to be the most active and the most stable possibly due to a metal interaction effect. A Pt–W/γ-Al₂O₃ containing 4.6 wt% Pt and 9 wt% W displayed the highest activity with full conversion at 600 °C and a selectivity to CO₂ of 99%.     

Characterization of hydrodenitrogenation catalysts by O2 chemisorption

O₂ chemisorption on sulfided NiMo/Al₂O₃ catalysts with Ni/Ni+Mo = 0 to 1 was measured at 273 K. The quantities of chemisorbed O₂ were compared to the activities of the catalysts in biphenyl hydrogenation and in 1,2,3,4-tetrahydroquinoline hydrodenitrogenation (HDN) at 623 K, 3.5 MPa.     

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.     

Supported Molybdenum Carbides Lie Between Metallic and Sulfided Catalysts for Deep HDS

Hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene on alumina-supported Mo₂C has been studied. These catalysts are stable and active under deep HDS conditions (0-250 wt ppm S). However, although they are well known to have hydrogenation properties, they lead preferentially to a non-hydrogenated product of the HDS reaction: dimethylbiphenyl. For the same reaction, supported platinum and sulfided molybdenum oxide lead to the hydrogenated products dimethyldicyclohexyl and methylcyclohexyltoluene, respectively. The ranking of HDS activity is as follows: MoS₂/Al₂O₃ < Mo₂C/Al₂O₃ < Pt/SiO₂.