Deep HDS over NiMo/Zr-SBA-15 catalysts with varying MoO3 loading

In the present work, with the aim of searching for new, highly effective catalysts for deep HDS, a series of NiMo catalysts with different MoO₃ loadings (6–30 wt.%) was prepared using SBA-15 material covered with ZrO₂-monolayer as a support. Prepared catalysts were characterized by N₂ physisorption, small- and wide-angle XRD, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction, SEM-EDX and HRTEM, and their catalytic activity was evaluated in the 4,6-dimethyldibenzothiophene hydrodesulfurization (HDS). It was observed that ZrO₂ incorporation on the SBA-15 surface improves the dispersion of the Ni-promoted oxidic and sulfided Mo species, which were found to be highly dispersed, up to 18 wt.% of MoO₃ loading. Further increase in metal charge resulted in the formation of MoO₃ crystalline phase and an increase in the stacking degree of the MoS₂ particles. All NiMo catalysts supported on ZrO₂-modified SBA-15 material showed high activity in HDS of 4,6-DMDBT. The best catalyst having 18 wt.% MoO₃ and 4.5 wt.% NiO was almost twice more active than the reference NiMo/γ-Al₂O₃ catalyst. High activity of NiMo/Zr-SBA-15 catalysts and its evolution with metal loading was related to the morphological characteristics of the MoS₂ active phase determined by HRTEM.     

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.     

Hydrodesulfurization of hindered dibenzothiophenes on bifunctional NiMo catalysts supported on zeolite–alumina composites

A series of NiMo catalysts supported on HNaY(x)–Al₂O₃ composites with different amounts of HNaY zeolite (x = 0, 5, 10, 20 and 100 wt.% of HNaY) was prepared and tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyl-DBT (4,6-DMDBT). The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), FT-IR spectroscopy of pyridine and nitrogen oxide adsorption (Py and NO-FT-IR), temperature-programmed reduction (TPR), scanning electron microscopy (SEM-EDX) and high-resolution transmission electron microscopy (HRTEM). It was found that the increase in the zeolite content causes changes in the acidic properties of the catalyst (number of acid sites) as well as in the characteristics of the deposited metallic species (location and dispersion). Different activity trends with the amount of the zeolite were found for the DBT and 4,6-DMDBT hydrodesulfurization on NiMo/HNaY-Al₂O₃ catalysts. As for the HDS of DBT the alumina-supported catalyst presents the highest activity. The incorporation of the zeolite causes an initial drop and then the recovery of activity with zeolite content. In contrast, for the 4,6-DMDBT the HDS activity always increases with zeolite content. These two different catalytic behaviors seem to be due to two opposite effects, which affect the contribution of the reaction routes available for the HDS of each reactant, these effects are: (i) the decrease of MoS₂ dispersion caused by the incorporation of zeolite to the catalyst and (ii) the increase of the proportion of Brönsted acid sites with zeolite content. The reaction product distribution indicates that both types of sites, coordinatively unsaturated sites (CUS) of the MoS₂ and zeolite Brönsted acid sites, participate in the 4,6-DMDBT and DBT transformations.     

Hydroisomerization of model FCC naphtha over sulfided Co(Ni)–Mo(W)/MCM-41 catalysts

Deep hydrodesulfurization (HDS) of gasoline generally brings about the saturation of olefins and leads to the serious octane number losses. Conversion of linear olefins to branched ones followed by hydrogenation to isoalkanes would minimize such octane number losses. In this work, MCM-41-supported Co–Mo, Ni–Mo and Ni–W catalysts were prepared by the incipient wetness impregnation method, and compared with an industrial Co–Mo/γ-Al₂O₃ catalyst. The surface acidities were measured by the techniques of microcalorimetry and infrared spectroscopy for the adsorption of ammonia, and probed by the reaction of conversion of isopropanol. The isomerization and hydrogenation of 1-hexene as well as the HDS of thiophene were studied by using model FCC naphtha. It was found that the sulfidation enhanced significantly the surface Brønsted acidity that favored the skeletal isomerization of 1-hexene under the HDS conditions. Since the isomerization and hydrogenation of 1-hexene are the two competition reactions, the catalysts with relatively lower hydrogenation activity may have higher selectivity to the isomerization reactions. The Co–Mo/MCM-41 showed the high selectivity to the skeletal isomerization reactions due to its strong surface Brønsted acidity and the relatively low hydrogenation activity. On the other hand, the Ni–Mo/MCM-41 exhibited high hydrogenation activity and therefore low selectivity to the isomerization reactions although it possessed quite strong surface Brønsted acidity. The Ni–W/MCM-41 exhibited the low activity for the HDS of thiophene and isomerization of 1-hexene due to the poor dispersion of active metals.     

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.     

High loading of short WS2 slabs inside SBA-15: promotion with nickel and performance in hydrodesulfurization and hydrogenation

Layered nanoslabs of a WS₂ phase with a well-defined hexagonal crystalline structure, average slab length of 3.6 nm, and stacking number of 3.2 were inserted into the nanotubular channels of SBA-15, an ordered pure silica material (surface area of 800 m²/g, uniform mesopore diameter of 6.5 nm) at loadings up to 60 wt.%. Sonication of a slurry containing SBA-15 in a W(CO)₆–sulfur–diphenylmethane solution yielded an amorphous WS₂ phase inside the mesopores. By sulfidation with 1.5% dimethyldisulfide in toluene under a hydrogen flow at 593 K and 5.4 MPa, the amorphous phase was transformed into hexagonal crystalline WS₂ nanoslabs (as shown by XRD, HRTEM, and selected area electron diffraction (SAED)). The WS₂ nanoslabs were distributed exclusively inside the mesopores in a uniform manner (HRTEM, quantitative microanalysis), without blocking the pores (N₂-sorption), and were oriented with their edge planes toward the support surface. This study constitutes the first report of such a combination of high loading of a well-defined crystalline catalytic phase into the nanotubular channels of mesoporous silica without blocking them. The first well-resolved HRTEM images of the well-defined crystalline catalytic phase (WS₂) inside the SBA-15 nanotubes are presented. A Ni component was introduced into the WS₂/SBA-15 composite by impregnation from an aqueous solution of nickel acetate. It increased the catalytic activity up to a Ni/W ratio of 0.4. In the hydrodesulfurization (HDS) of dibenzothiophene and the hydrogenation (HYD) of toluene, the activity of the optimized NiWS/SBA-15 catalyst was 1.4 and 7.3 times higher, respectively, than that of a sulfided commercial CoMo/Al₂O₃. This finding illustrates the excellent potential of high loading NiWS/SBA-15 catalysts for deep hydrotreatment of petroleum feedstocks.     

Synthesis and characterization of W-SBA-15 and W-HMS as supports for HDS

W-modified HMS and SBA-15 mesoporous materials (Si/W molar ratio equal to 40) were synthesized using sodium tungstate as tungsten source. In order to prepare NiW catalysts these mesoporous materials were impregnated with an aqueous solution of nickel salt of 12-tungstophosphoric acid Ni_3/2PW₁₂O₄₀. The synthesized W-HMS, W-SBA-15 materials and NiW catalysts have been characterized by S_BET, XRD, UV–Vis DRS, FT-IR, TPD of NH₃, ²⁹Si MAS NMR, SEM and HRTEM. The influence of these particular supports on catalytic activity of NiW catalysts was studied in the reaction of hydrodesulfurization (HDS) of thiophene. The results from the FT-IR and UV–Vis DR spectroscopy confirm incorporation of W into the HMS and SBA-15 structures. Additionally ²⁹Si MAS NMR measurements revealed relatively stronger effect of W ion incorporation in HMS structure on degree of silica cross-linking as compared to the effect of W ion incorporation in SBA-15 structure. The catalytic study showed that both W-HMS and W-SBA-15 materials modified with W are good supports for NiW catalysts in the HDS reaction of thiophene. The catalysts show lower selectivity for butanes than a reference NiW/γ-Al₂O₃ catalyst leveling of about 10% for chosen experimental conditions.     

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.     

Inhibition of CoMo/HMS catalyst deactivation in the HDS of 4,6-DMDBT by support modification with phosphate

A series of CoMoS catalysts supported on hexagonal mesoporous silica (HMS) modified with different amounts of phosphate (0.5, 1.0, 1.5 and 2.0 wt.%) were prepared in order to study the influence of phosphate on catalyst deactivation. The catalysts were characterized by a variety of techniques (X-ray fluorescence, N₂ adsorption-desorption at 77 K, FT-IR study of the framework vibration and NO adsorption, NH₃-TPD, H₂-TPR, XPS, ³¹P NMR and TPO/TGA). The sulfided catalysts were tested in the deep hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) performed in a fixed-bed flow reactor at 598 K, P = 5.0 MPa and WHSV = 46.4 h⁻¹. The catalyst with the largest phosphate content (2.0 wt.%) showed the best catalytic response linked with its low deactivation during on-stream reaction and a larger sulfidation degree of Co species. It was found that coking behavior is closely related with the location of the active sites in the support structure being a lower coke formation on the catalysts having active phases located within support structure. The catalysts modified with a large amount of phosphorous (1.5 and 2.0 wt.% of P₂O₅) were more susceptible to coking and produced a more polymerized coke than P-free sample, as confirmed by TPO/TGA experiments. The presence of P₂O₅ favours the sulfidation degree of Co species and the creation of medium strength acid sites leading to the enhancement of the 4,6-DMDBT HDS reaction toward the isomerization route.     

Discrepancy between TPD- and FTIR-based measurements of Brønsted and Lewis acidity for sulfated zirconia

Temperature-programmed desorptions (TPD) of isopropylamine (IPA), NH₃, and pyridine were compared with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine to determine the effect of H₂O on the Brønsted and Lewis acidities of two sulfated zirconia (SZ) catalysts. Although the traditional interpretation of pyridine infrared spectra showed an apparent increase in Brønsted acidity upon treating SZ with H₂O, TPD spectra showed that H₂O displaced IPA from approximately one-fifth of the Lewis sites with no corresponding increase in Bronsted acidity. Water treatment prior to TPD displaced similar amounts of both NH₃ and pyridine. The primary effect of H₂O is displacement of weakly adsorbed basic probe molecules from Lewis sites, rather than the conversion of Lewis sites to Brønsted sites. Finally, different types of analyses (e.g. infrared or TPD) of catalyst acidity yield dramatically different conclusions regarding Brønsted and Lewis acidity.     

Zeolite beta synthesized with acid-treated metakaolin and its application in diesel hydrodesulfurization

A new type of zeolite beta (denoted as MB) with multi-pore system was synthesized by using in situ synthesized method from kaolin mineral in this study. NiW/Al₂O₃–MB and NiW/TiO₂–Al₂O₃–MB catalysts were prepared and the hydrodesulfurization (HDS) activities of these catalysts were evaluated with FCC diesel feed. The samples were characterized by N₂ physisorption, XRD, SEM, TPR, FT-IR spectroscopy of pyridine adsorption, HRTEM and XPS techniques. The HDS results showed that the MB-containing catalyst exhibited much higher HDS conversion (98.7%) than that of NiW/γ-Al₂O₃ (97.5%). The incorporation of TiO₂ into the composite supports further increased the HDS conversion (99.3%) of NiW/TiO₂–Al₂O₃–MB. The higher HDS activity was mainly associated with the appropriate ratio of B/L (Brönsted acid/Lewis acid) and the enhanced hydrogenation activity.     

Study of the Keggin structure and catalytic properties of Pt-promoted heteropoly compound/Al-MCM-41 hybrid catalysts

Aluminum-containing mesoporous molecular sieves (referred as WSAn, where n = Si/Al molar ratio = 50, 30 and 10) were synthesized via a surfactant templated approach by using fumed silica and aluminum sulfate as Si and Al precursors, respectively. When the 12-tungstophosphoric acid was grafted onto the surface of WSAn, a high dispersion of the heteropolyacid was achieved on the heteropoly compound/WSAn hybrid catalysts. The Keggin structure of the dispersed 12-tungstophosphoric acid was primarily preserved without destruction, but it was distorted in some degree, as confirmed by FTIR, ³¹P NMR-MAS and UV–vis spectroscopic characterizations. The surface Brönsted acidity of the catalysts was greatly enhanced by several times in comparison to that of the bare WSAn support. In the hydroisomerization reaction of n-heptane, the Pt/H₃PW₁₂O₄₀/WSA30 catalyst exhibited the highest catalytic activity and the best isomerization selectivity among the catalysts tested, which can be generally correlated with its large number of Brönsted acid sites and high structural regularity. After the n-heptane hydroisomerization reaction, a high molar ratio of multibranched to monobranched isohexanes was obtained, indicating that Pt-promoted heteropoly compound/WSAn hybrid catalysts have a great potential for the hydroisomerization of long carbon chain hydrocarbons.     

Hydrodesulfurization and hydrogenation of model compounds on silica-alumina supported bimetallic systems

Silica-alumina supported Pd catalysts with a second metal such as Cu, Ru or Pt have been prepared (nominal content: 1 wt%) and tested in the naphthalene hydrogenation (HYD) and dibenzothiophene hydrodesulfurization (HDS) model reactions. These catalysts were characterized by means of temperature-programmed reduction, NH₃ temperature-programmed desorption and photoelectron spectroscopy techniques. The combined use of all these techniques revealed that only in the case of PtPd system a rather uniform distribution of the metals across the pore network is achieved. Another important observation was that the PtPd system exhibited strong Brönsted acid sites. The highest HYD and HDS activities of the PtPd bimetallic catalyst can be related not only to a high dispersion of the metals and their uniform distribution but also to its strong Brönsted acidity. The PtPd system presented the highest DBT conversion (83.1%) and the highest C-S hydrogenolysis activity as illustrated by the lowest S-containing products (21.9 wt%) as compared with the much higher S-containing products (54.8 wt%) obtained with the monometallic Pd/ASA catalyst under the same experimental conditions.     

Synergism Between Ni and W in the Niw/ γ-Al2O3 Hydrotreating Catalysts

A series of NiW/ γ-Al₂O₃ catalysts (20 and 30 wt% W and 1–5 wt% Ni) have been prepared and studied by TPR and XPS. HDS activity has been tested in the thiophene conversion. The effect of Ni and W loadings on the formation of different structures is presented. In the calcined catalysts several phases coexist, concentrations of which depend on the Ni/(Ni+W) atomic ratio. The Ni synergistic effect in the HDS reaction is confirmed by the increase in the HDS activity (~10–15 times). This effect is ascribed to the formation of an active NiWS phase of high dispersion from the mixed NiW oxide precursors. At higher Ni/Ni+W ratio a redistribution of active components in additional amount ofNiWS phase during sulfidation is suggested.     

MoO3 supported on montmorillonite type pillared clays: Characterization, surface acidity and catalytic properties towards the oxidation of methanol

Two different MoO₃ catalysts (30 wt%) supported on montmorillonite type pillared clays have been synthesized by (i) impregnation with an ammonium molybdate solution (sample Mo-APC-i) and (ii) mechanical mixing with MoO₃ (sample Mo-APC-m). After preparation both catalysts were calcined at 400°C. It has been established by X-ray phase analysis, temperature programmed reduction and IR spectroscopy that impregnation leads to a better dispersion of molybdenum oxide on the support, which ensures (i) a higher concentration of Brønsted acid sites (measured by adsorption of pyridine) and (ii) a higher catalytic activity in methanol oxidation of the Mo-APC-i catalyst.     

Modulating the active phase structure of NiMo/Al2O3 by La modification for ultra-deep hydrodesulfurization of diesel

Herein, a series of mesoporous NiMo/LaAlO_x with various La contents were constructed through a solvent evaporation-induced self-assembly protocol, and their catalytic activities were investigated for hydrodesulfurization (HDS) of 4,6-DMDBT. It has been confirmed that the incorporation of La influences the electronic structure and morphology of NiMoS active phase. The lower amount of La (x ≤ 1.0 wt.%) could facilitate the formation of “Type II” NiMoS phase by weakening the interaction of Mo-O-Al leakage and promoting the sulfidation of both Mo and Ni species as well as increasing the ratio of “Type II” NiMoS phase, thereafter boosting the HDS performances. Further increasing La incorporation, however, leads to the generation of inactive NiS_x phase and decrease of NiMoS active phase proportion, the HDS activities of corresponding catalysts were suppressed. NiMo/LaAlO_x-1.0 exhibits the highest activity for 4,6-DMDBT HDS because of its moderate metal-support interaction, optimal morphology and the largest proportion of “Type II” NiMoS active phase.     

Effect of acid sites on catalytic destruction of trichloroethylene over solid acid catalysts

The catalytic destruction of trichloroethylene (TCE) over several solid acid catalysts (HZSM-5, γ-Al₂O₃ and SBA-15/P) was evaluated under dry conditions. The activity order was found to be: HZSM-5>SBA-15/P>γ-Al₂O₃. It was reported that Brønsted and Lewis acid sites of catalysts both played an important role on TCE catalytic destruction, while the Brønsted acid sites were more decisive. Additionally, the formation of the polychlorinated by-product (tetrachloroethylene, PCE) over HZSM-5 and γ-Al₂O₃ catalysts was observed and attributed to the presence of Lewis acid sites and basic O^2?, and PCE was not detected over SBA-15/P catalyst due to the presence of only Brønsted acid sites. The TCE/O₂-TPSR studies demonstrated that the main oxidation products during TCE catalytic destruction are CO, CO₂ and Cl₂, and the carbon in TCE was firstly converted to CO and then further oxidized into CO₂ by gas phase O₂.     

Mesoporous silica–alumina as support for Pt and Pt–Mo sulfide catalysts: Effect of Pt loading on activity and selectivity in HDS and HDN of model compounds

The potential of mesoporous silica–alumina (MSA) material as support for the preparation of sulfided Pt and Pt–Mo catalysts of varying Pt loadings was studied. The catalysts were characterized by their texture, hydrogen adsorption, transmission electron microscopy, temperature programmed reduction (TPR) and by activity in simultaneous hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine. Sulfided Pt/MSA catalysts with 1.3 and 2 wt.% Pt showed almost the same HDS and higher HDN activities per weight amounts as conventional CoMo and NiMo/Al₂O₃, respectively. The addition of Pt to sulfided Mo/MSA led to promotion in HDS and HDN with an optimal promoter content close to 0.5 wt.%. The results of TPR showed strong positive effect of Pt on reducibility of the MoS₂ phase which obviously reflects in higher activity of the promoted catalysts. The activity of the MSA-supported Pt–Mo catalyst containing 0.5 wt.% Pt was significantly higher than the activity of alumina-supported Pt–Mo catalyst. Generally, Pt–Mo/MSA catalysts promoted by 0.3–2.3 wt.% Pt showed lower HDS and much higher HDN activities as compared to weight amounts of CoMo and NiMo/Al₂O₃. It is proposed that thiophene HDS and pyridine hydrogenation proceed over Pt/MSA and the majority of Pt–Mo/MSA catalysts on the same type of catalytic sites, which are associated with sulfided Pt and MoS₂ phases. On the contrary, piperidine hydrogenolysis takes place on different sites, most likely on metallic Pt fraction or sites created by abstraction of sulfur from MoS₂ in the presence of Pt.     

Vanadium loaded carbon-based monoliths for the on-board No reduction: Influence of vanadia and tungsten loadings

Carbon-coated catalysts doped with tungsten and vanadia oxides with different V and W loadings have been prepared by the ionic exchange method and characterized. The surface, structure and composition have been investigated by XPS, Raman, N₂ sorption at 77 K, TPD-NH₃ and reactivity tests for the SCR of NO with NH₃ at low temperatures. Under reaction conditions, NO conversions were found to go through a maximum with vanadia surface coverage at approximately half a monolayer. The observed decrease in the SCR activity at higher vanadia loadings can be attributed to either a loss of dispersion or loss of textural properties. Maximum NO conversion is ascribed to the higher Brönsted proton acidity (V⁴⁺) of the centres that decreases with increasing vanadia loadings up to 3 wt% loading due to the increase of V⁴⁺/V⁵⁺ ratio.Large amounts of tungsten (5%, w/w) upon or before addition of vanadia do not provide an enhancement of activity. The results indicate that W addition increases surface acidity leading to stronger Brönsted or even Lewis acid centre creation.     

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

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