Laser Raman spectroscopy of MoO3 and NiO–MoO3 supported on gallia and gallium–aluminum mixed oxides

The laser Raman spectra of MoO₃ and NiO–MoO₃ supported on gallia and gallium–aluminum mixed oxides are presented and correlated to the type of support and the hydrodesulfurization (HDS) activity of the catalysts. The results show that the support affects the species (orientation of catalyst) that are formed on its surface. For example, MoO₃ forms mainly HDS inactive tetrahedral species on gallia; on alumina surface, it forms the HDS active polymeric octahedral species. Also, bulk oxide is formed more readily on gallia than on alumina. On addition of NiO, as a promoter, to the MoO₃ catalyst supported on gallia, a band that can be assigned to HDS inactive species was also formed. The intensity of this band is low in the spectra of NiO–MoO₃ catalysts supported on alumina. This revised version was published online in July 2006 with corrections to the Cover Date.     

States of carbon nanotube supported Mo-based HDS catalysts

As HDS catalysts, the supported catalysts including oxide state Mo, Co–Mo and sulfide state Mo on carbon nanotube (CNT) were prepared, while the corresponding supported catalysts on γ-Al₂O₃ were prepared as comparison. Firstly, the dispersion of the active phase and loading capacity of Mo species on CNT was studied by XRD and the reducibility properties of Co–Mo catalysts in oxide state over CNTs were investigated by TPR while the sulfide Co–Mo/CNT catalysts were characterized by XRD and LRS techniques. Secondly, the activity and selectivity of hydrodesulfurization (HDS) of dibenzothiophene with Co–Mo/CNT and Co–Mo/γ-Al₂O₃ were studied. It has been found that the main active molybdenum species in the oxide state MoO₃/CNT catalysts were MoO₂, rather than MoO₃ as generally expected. The maximum loading before formation of the bulk phase was lower than 6%m (calculated in MoO₃). The TPR studies revealed that that active species in oxide state Co–Mo/CNT catalysts were more easily reduced at relatively lower temperatures in comparison to those in Co–Mo/γ-Al₂O₃, indicating that the CNT support promoted the reduction of active species. Among 0–1.0 Co/Mo atomic ratio on Co–Mo/CNT, 0.7 has the highest reducibility. It shows that the Co/Mo atomic ratio has a great effect on the reducibility of active species on CNT and their HDS activities and that the incorporation of cobalt improved the dispersion of molybdenum species on CNT and mobilization. It was also found that re-dispersion could occur during the sulfiding process, resulting in low valence state Mo₃S₄ and Co–MoS_2.17 active phases. The HDS of DBT showed that Co–Mo/CNT catalysts were more active than Co–Mo/γ-Al₂O₃ and the hydrogenolysis/hydrogenation selectivity of Co–Mo/CNT catalyst was also much higher than Co–Mo/γ-Al₂O₃. For the Co–Mo/CNT catalysis system, the catalyst with Co/Mo atomic ratio of 0.7 showed the highest activity, whereas, the catalyst with Co/Mo atomic ratio of 0.35 was of the highest selectivity.     

Effect of the activation process on thiophene hydrodesulfurization activity of activated carbon-supported bimetallic carbides

The effect of passivation and presulfidation after carbiding of activated carbon-supported Fe–Mo, Co–Mo and Ni–Mo catalysts on their thiophene HDS activity was evaluated. Catalytic precursors were prepared by co-impregnation of the support with solutions of ammonium heptamolybdate and the promotor nitrates or sulfates. Carbiding was achieved by means of the carbothermal method, employing pure H₂ as reductant and the support as the carbon source. Carbided samples were submitted to one out of three types of procedures before HDS tests: (a) passivation at room temperature followed by presulfiding; (b) presulfiding (no passivation); and (c) neither passivation nor sulfiding before HDS. Samples of passivated catalysts prepared from the sulfates of Fe, Co or Ni contained variable amounts of sulfur, as shown by XPS and elemental analysis, while XRD showed only metals and mixed Fe₃Mo₃C or η-M₆Mo₆C₂ (MCo, or Ni) phases. The nitrate-derived catalysts only presented β-Mo₂C and metals (XRD). Sulfur containing catalysts showed high initial activities although deactivate strongly during the first 40 min on the reaction stream, while the unsulfided nitrate-derived samples showed a more stable behavior and lower activities during the 2–3 h of testing. In general, samples submitted to passivation followed by presulfiding showed the higher steady state activities and those neither passivated nor sulfided were the less active. The results show the benefits of a passivating treatment on these carbon-supported catalysts, and point out to the importance of sulfided surface phases in HDS on carbides of transition metal catalysts.     

TiO2-Supported Mo Model Catalysts: Ti as Promoter for Thiophene HDS?

The combination of thiophene hydrodesulfurization (HDS) activity measurements and X-ray photoelectron spectroscopy on flat model systems of sulfided HDS Mo catalysts showed that sulfided Ti-species can act as a promoter in the same way as Co and Ni, although less effectively. This explains the higher thiophene HDS activity and hydrogenation selectivity of Mo/TiO₂ compared with Mo/Al₂O₃, while for Ni-promoted Mo catalysts the difference between the two supports is negligible.     

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.     

HDS of thiophene over CoMo/AlMCM-41 with different Si/Al ratios

A series of AlMCM-41 molecular sieves with different Si/Al ratios were synthesized followed by the deposition of cobalt and molybdenum oxides on these mesoporous supports by co-impregnation. Such materials were further calcined and catalysts with 15 wt.% of cobalt and molybdenum and a Co/(Co + Mo) atomic ratio of 0.30 were obtained. These materials were characterized by X-ray diffraction (XRD), transmission electron microscopy and selected area electron diffraction (TEM/SAED), X-ray fluorescence (XRF), and nitrogen adsorption. Hydrodesulphurisation (HDS) of thiophene was carried out at 350 °C in a fixed bed continuous flow micro reactor coupled on line to a gas chromatograph. The main XRD peaks of MCM-41 phase were observed in all samples and peaks due to MoO₃ and CoMoO₄ phases were also identified from XRD results. It was found that the as-synthesized catalysts presented reasonable conversion results for HDS of thiophene, when compared to other supported catalysts. The main products of HDS of thiophene were H₂S, isobutene, 1-butene, n-butane, 2-butene-trans, and 2-butene-cis. It was observed that the reactivity of the as-synthesized catalysts is a direct function of the Si/Al ratio, nature and concentration of the active species on the mesoporous supports.     

Redispersion effects of citric acid on CoMo/γ-Al2O3 hydrodesulfurization catalysts

Calcined CoMo/γ-Al₂O₃ catalysts were modified by citric acid (CA) with different CA/Co ratios and the corresponding structure evolutions were systematically characterized. Then combined with HDS activity results, potential redispersion effects of CA were suggested: (i) weaken the MoO₃-Al₂O₃ interaction via competitive interacting with the OH groups of alumina surface to realize the redispersion of Mo oxides; (ii) transform tetrahedral MoO₄^2 − or β-CoMoO₄ into octahedral polymolybdate species and promote bulk MoO₃ to form well-dispersed MoO₃; (iii) remove the CoAl₂O₄-like species. These effects probably together promote the resulting sulfided catalysts with more type II CoMoS active sites, thus enhancing the HDS activity.     

An attempt to predict the mechanism of Mo-Ni-O powders electrodeposition from the results of their TEM analysis

The Mo-Ni-O powders were electrodeposited from ammonium sulfate containing electrolytes with different Mo/Ni ion concentration ratios. The phase composition of these powders was investigated using TEM, EDS and XRD analysis. The TEM analysis showed that two types of particles were present in the powders: amorphous and crystalline. For crystalline particles it was found that two phases, MoO₃ and MoNi₄ prevail in all powders, while the NiO phase was detected in the powder electrodeposited at Mo/Ni = 0.3/1. These findings are in good agreement with our previous results where MoO₃, MoNi₄ and NiMoO₄ phases were detected in the recrystallized Mo-Ni-O powders. In this work we discovered that the NiMoO₄ phase was formed by solid state reaction between NiO and MoO₃ at elevated temperature.     

加氢脱硫催化剂载体和活性组分的研究进展

总结了近年来加氢脱硫催化剂载体材料与活性组分的研究成果及其进展。指出介孔材料不仅具有高比表面积,而且孔径较大,与其他载体相比,对深度加氢脱硫中难以脱除的芳香大分子硫化物的脱除具有一定的优势。过渡金属元素,如Mo、Co、Ni、Pt和Pd由于电子特性和几何特性,其化合物是加氢脱硫催化剂活性组分的首选,对今后的研究工作做了展望。     

Support effect on the properties of iron–molybdenum hydrodesulfurization catalysts

Two series of Mo and Fe containing catalysts have been prepared over alumina and titania supports using H₃PMo₁₂O₄₀ heteropolyacid (HPMo) and Fe salt of HPMo. Catalysts have been characterized by BET, SEM, IR, TPR, XPS methods and by their HDS activity in the reaction of thiophene conversion. The TiO₂-supported catalysts with low Mo concentration (6 wt%) show higher HDS activity than the catalyst with 12 wt% Mo. Iron promoting effect (Fe/Mo ~ 0.1) is observed with both, the alumina- and titania-supported catalysts. Iron supported over alumina increases Mo reducibility and decreases it on TiO₂-supported catalysts. Compared to alumina-supported catalysts, the TiO₂-supported catalysts show higher surface concentration of Mo⁶⁺ and Mo⁵⁺ in octahedral coordination – Mo(Oh). Iron increases the Mo(Oh) concentration even more. After sulfidation the Fe-containing catalysts show formation of different Mo valence states (Mo⁴⁺, Mo⁵⁺, Mo⁶⁺), Fe–P, Mo–P and/or Fe–Mo–P bonds, which affect the HDS catalytic activity.     

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.     

Mo/MCM-41-Type mesoporous materials doubly promoted with Fe and Ni for hydrotreating reactions

A Si-MCM-41-type material was synthesized and impregnated with a Mo solution in order to get a 15% MoO₃ by weight. Then, it was doubly promoted with Ni and Fe in different proportions. Information on the structure of the precursors was revealed by XRD, TPR and surface area techniques. XRD results showed that the mesoporous materials were not affected for the successive impregnations carried out; while, their specific surface areas was partially blocked. These solids were tested for vanadyl octaethyl porphyrin hydrodeporphirinization (HDP), and individual and competitive reactions of dibenzothiophene (DBT) hydrodesulfurization (HDS) and 2-methyl naphthalene (2MN) hydrogenation (HYD).The DBT HDS activity was higher than 2MN HYD, and the presence of Fe in the catalysts did not produce a synergetic effect for this reactions. However, HDP activities of doubly promoted catalysts were superior to the monopromoted ones.     

Combustion of a high-calorific thermite mixture on the surface of a titanium substrate

The combustion and chemical transformation of highly exothermic MoO₃/NiO/Al/C andMoO₃/NiO/Al/B mixtures on the surface of a titanium substrate under centrifugal acceleration are studied. It is shown that chemical reactions occur not only in the combustion wave, but also on the surface of the titanium substrate, between the combustion products and the substrate material (titanium). The dynamics and mechanism of formation of layered (gradient) titanium–ceramics materials (Mo–Ni–C–Ti or Mo–Ni–B–Ti) is investigated.     

Effect of ethyleneglycol addition on the properties of P-doped NiMo/Al2O3 HDS catalysts: Part I. Materials preparation and characterization

Phosphorous-doped NiMo/Al₂O₃ hydrodesulfurization (HDS) catalysts (nominal Mo, Ni and P loadings of 12, 3, and 1.6 wt%, respectively) were prepared using ethyleneglycol (EG) as additive. The organic agent was diluted in aqueous impregnating solutions obtained by MoO₃ digestion in presence of H₃PO₄, followed by 2NiCO₃·3Ni(OH)₂·4H₂O addition. EG/Ni molar ratio was varied (1, 2.5 and 7) to determine the influence of this parameter on the surface and structural properties of synthesized materials. As determined by temperature-programmed reduction, ethyleneglycol addition during impregnation resulted in decreased interaction between deposited phases (Mo and Ni) and the alumina carrier. Dispersion and sulfidability (as observed by X-ray photoelectron microscopy) of molybdenum and nickel showed opposite trends when incremental amounts of the organic were added during catalysts preparation. Meanwhile Mo sulfidation was progressively decreased by augmenting EG concentration in the impregnating solution, more dispersed sulfidic nickel was evidenced in materials synthesized at higher EG/Ni ratios. Also, enhanced formation of the so-called “NiMoS phase” was registered by increasing the amount of added ethyleneglycol during simultaneous Ni–Mo–P–EG deposition over the alumina carrier. That fact was reflected in enhanced activity in liquid-phase dibenzothiophene HDS (batch reactor, T = 320 °C, P = 70 kg/cm²) and straight-run gas oil desulfurization (steady-state flow reactor), the latter test carried out at conditions similar to those used in industrial hydrotreaters for the production of ultra-low sulfur diesel (T = 350 °C, P = 70 kg/cm², LHSV = 1.5 h⁻¹ and H₂/oil = 2500 ft³/bbl).     

Preparation, characterization, and catalytic properties of the Mo2C/SBA-15 catalysts

A series of Mo₂C/SBA-15 catalysts with different Mo contents were prepared by temperature-programmed carburization (TPC). The materials obtained and their oxide precursors (MoO₃/SBA-15) were characterized by Nitrogen adsorption-desorption isotherms, X-ray diffraction (XRD), and Fourier transform-infrared (FT-IR) spectroscopy. The activities of the catalysts for deep hydrodesulfurization (HDS) of thiophene were evaluated. The results of N₂ adsorption-desorption isotherms indicated that the surface area and pore diameter of the oxide precursors increase after carburization. The XRD patterns show that Mo₂C particles are highly dispersed in the SBA-15 ordered mesoporous. The test results show that Mo₂C/SBA-15 catalysts have an excellent performance for the deep HDS under the lower temperature region.     

Rationalizing the catalytic performance of γ-alumina-supported Co(Ni)–Mo(W) HDS catalysts prepared by urea-matrix combustion synthesis

A comparative study of the influence of Co (or Ni) promoter loadings and the effect of different sulfurizing agents and sulfurizing temperatures on the structure, morphology and catalytic performance of Mo- or W-based hydrodesulfurization (HDS) catalysts was carried out. Catalyst performance using a tubular fixed-bed reactor and the HDS of thiophene as a model reaction was evaluated. The oxidic and sulfurized states of the HDS catalysts were characterized by laser Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and high resolution transmission electron microscopy (HRTEM). It has been found that the urea-matrix combustion (UMxC) synthesis is a simple tool for preparing supported catalysts in a short period of thermal treatment. Several consecutive stages such as urea melting, metal precursor dissolution and chemical reactions take place before and upon combustion process. The C₄H₄S/H₂-activated Co- (or Ni-) promoted MoS₂ (or WS₂) catalysts present a strong synergistic effect (SE) when the Co (or Ni)/Mo (or W) molar ratio is near to 0.5, whereas the C₄/C ₄ ⁼ molar ratios display a weak antagonistic effect. Alumina-supported Ni–W catalyst showed an optimal SE 2.5 times higher than those for Co (or Ni)-promoted Mo HDS catalysts. The kinetic parameters for thiophene-HDS reaction were also determined, suggesting that the C–S bond cleavage reaction for alumina-supported Co(Ni)–Mo HDS catalysts and H₂ activation reaction for Ni-promoted WS₂catalysts play an important role in the rate-limiting step.     

Hydrocracking of n-heptane. Study of NiO—MoO3 catalysts supported on a HY ultrastable zeolite

The hydrocracking of n-heptane in the temperature range of 573 to 623 K and at 2.45 × 10⁶ Pa pressure has been employed as a test reaction for the study of Ni—Mo bifunctional catalysts supported on a HY ultrastable zeolite. Two groups of catalysts containing 8 and 12 wt% of MoO₃ and different amounts of NiO have been studied. In both series a maximum in the activity has been obtained for catalysts with a Ni/Mo atomic ratio of 0.8-1.0. The order of the impregnation of the oxides can have little influence on the activity. The most active catalyst has been obtained when the zeolite is exchanged with NH⁺₄ ions until the Na⁺ level is less than 2% of the original and calcined at 823 K to obtain a HY ultrastable zeolite. Using this catalyst the rate controlling step could be the transformation of the carbonium ion on the acid sites.     

Unsupported Ni/Mo(W)S2 Catalysts from Hexamethylenediammonium Thiometallates Precursors: In Situ Activation During the HDS of DBT

Unsupported Ni/MoS₂ and Ni/WS₂ HDS catalysts were prepared by in situ activation of hexamethylenediammonium thiometallates promoted with Ni. The method involved an aqueous solution reaction of ammonium thiomolybdate (ATM) or ammonium thiotungstate (ATT) with Ni(NO₃)₂·6H₂O and hexamethylenediamine. Ni-promoted precursors were then in situ activated during the hydrodesulfurization (HDS) of dibenzothiophene (DBT) producing Ni/MoS₂ and Ni/WS₂ catalysts. Solids were analyzed after the in situ activation by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and for textural properties by using the BET and BJH methods. Catalysts with relatively high surface area and type IV N₂ adsorption–desorption isotherms were obtained. The use of the hexamethylenediammonium precursor led to a significant nickel promotion of MoS₂ and WS₂ catalysts. For Ni/MoS₂, the use of this carbon-containing precursor was found to be more beneficial for the final HDS catalytic activity than using the classical ammonium tetrathiomolybdate (ATM) without carbon. For Ni/WS₂, compared to tetraalkylammonium thiosalts, the lower amount of carbon in excess formed during the decomposition of the hexamethylenediammonium precursor coupled with a lower crystallization rate of WS₂ favors a correct nickel accommodation on the WS₂ edges.     

Selectivity enhancement of Co-Mo/Al2O3 FCC gasoline hydrodesulfurization catalysts via incorporation of mesoporous Si-SBA-15

Mesoporous Si-SBA-15 was applied to enhance the FCC gasoline selective hydrodesulfurization (HDS) performance of conventional Co-Mo/Al₂O₃ catalysts and the physicochemical properties of the resulting catalyst were compared with those of Co-Mo/Al₂O₃ catalysts incorporated with macroporous kaolin, mesoporous Si-MCM-41 and microporous Si-ZSM-5. The selective HDS performances of all the catalysts were assessed with different FCC gasolines as feedstocks. The results showed that the HDS selectivity of the catalysts was closely related to the Mo sulfidation that depends on catalyst surface area and metal-support interaction. With the superior Mo sulfidation, the Co-Mo/Si-SBA-15-Al₂O₃ catalyst had the optimal HDS selectivity for not only the full-range FCC gasolines but also the heavy fractions thereof. The present article demonstrates the significance of enhancing Mo sulfidation in improving HDS selectivity and thus sheds a light on the development of highly selective HDS catalysts.     

Effect of Ni promoter in the oxide precursors of MoS2/MgO–Al2O3 catalysts tested in dibenzothiophene hydrodesulphurization

NiMoS catalysts supported on MgO–Al₂O₃ oxides, with 95 and 80 mol% of MgO, were synthesized by sol–gel method. In order to study the Ni promoter effect, MgO–Al₂O₃ supports were impregnated with a pH = 9 solution of Mo and Ni–Mo, respectively; the catalysts were dried (D) and calcinated (C). Catalytic tests showed a Ni promoter effect of 4.5 on the NiMoMg95Al5-D catalyst and 8.5 on the calcinated one. The latter catalyst is more active than a commercial NiMo/Al₂O₃ catalyst. On the other side, the catalyst supported on Mg80Al20 solid did not show any Ni promoter effect. Raman and UV–vis diffuse reflectance spectroscopy showed that during the impregnation step, a strong support interaction with the ion MoO₄^2? takes place on the Mo/MgO–Al₂O₃ solids. After calcination, MoO₄^2? ion remained on the catalyst surface, but increased its interaction with the support. The presence of Ni²⁺_Th, Ni²⁺_Oh and MoO₄^2? ions on dried NiMo/Mg95Al5 catalysts was confirmed, as well as the presence of Ni²⁺_Th, Ni²⁺_Oh, MoO₄^2? and Mo₇O₂₄^6? ions on the calcinated catalyst. This suggests that Ni²⁺ ion allows polymerization of MoO₄^2? to Mo₇O₂₄^6?, produced by Ni²⁺_Oh–MoO₄^2? and Ni²⁺_Oh–Mo₇O₂₄^6? close interactions. The NiMo/Mg80Al20 solids also showed MoO₃ species and a high Ni²⁺_Th concentration. Thus, the Ni promoter effect and therefore, catalytic activity decreased, due to the formation of Ni²⁺_Th–MgO and Ni²⁺_Th–Al₂O₃ spinels.