Thiophene Hydrodesulfurization Research Articles

Calculation of MoS2 slabs morphology descriptors from transmission electron microscopy data revisited. Case study of the influence of citric acid and treatment conditions on the properties of MoS2/Al2O3

Molybdenum sulfide MoS2 slabs length L and stacking S obtained from the transmission microscopy (TEM) images are the major characteristics of hydrotreating sulfide catalysts, widely used as morphology descriptors. In this work we demonstrate that the formulae routinely used to estimate mean L and S should be revisited. To obtain physically meaningful values L and S, both should be calculated as surface weighted averages. Moreover, correlation between L and S must be taken into account. Using surface weighted values of mean L and S, MoS2 phase morphology was studied for a series of MoS2/Al2O3 catalysts. The effects of Mo loading, addition of citric acid and sulfidation by H2S/N2 or H2S/H2 mixtures at different temperatures from 350 to 550°C were studied. The catalysts were evaluated in hydrodesulfurization of thiophene (HDS) and studied by temperature programmed reduction (TPR). Thorough statistical treatment of TEM, HDS and TPR data suggests that the intrinsic properties of MoS2 active phase depend primarily on the treatment atmosphere and temperature rather than on the addition of citric acid. If the impregnated catalyst is dried at temperature above 120°C, citric acid allows moderate improvement of MoS2 morphology. However the edge-specific activity of supported sulfide remains unaffected.

Theoretical Survey of the Thiophene Hydrodesulfurization Mechanism on Clean and Single-Sulfur-Atom-Modified MoP(001)

Molybdenum phosphide (MoP) has been extensively experimentally shown to possess high and surprisingly increasing hydrodesulfurization (HDS) activities during the HDS process. In order to understand the HDS mechanism, we investigate the HDS of thiophene on clean and single-sulfur-atom-modified MoP(001) using self-consistent periodic density functional theory (DFT). Thiophene strongly prefers flat adsorption, which is slightly weakened in the presence of a surface S atom. Thermodynamic and kinetic analyses of the elementary steps show that the HDS of thiophene takes place along the direct desulfurization (DDS) pathway on both clean and S-modified MoP(001), because of the very low C–S bond activation barriers as well as very high exothermicities involved. More importantly, the surface S atom does not elevate the C–S bond activation barriers but opens a new concerted pathway for the simultaneous rupture of both C–S bonds in thiophene. These results indicate that the presence of a surface S atom could be helpful for thiophene desulfurization. For comparison, we also investigate the influence of a surface S atom on the HDS of thiophene on Pt(111). The results show clearly a negative effect of the surface S atom, in accordance with the lower sulfur resistance of noble metals.

On the photocatalytic and electrocatalytic hydrogen evolution performance of molybdenum sulfide supported on TiO2

This study aimed to define key preparation and treatment parameters determining the hydrogen evolution reaction (HER) performance of molybdenum sulfide-based photocatalysts. Amorphous and nanocrystalline molybdenum sulfide catalysts supported on titania have been studied in photocatalytic and electrochemical HER. The results of HER measurements were compated to the thiophene hydrodesulfurization HDS activity. Titania polymorph plays a crucial role for HER: high-surface area anatase PC500 and anatase-rutile Degussa P25 show high HER performance, whereas rutile nanorods demonstrate poor activity. Two preparation methods applying impregnation and solution deposition techniques have been compared. Solution deposition affords superior activity, with a maximum HER rate for 0.5%wt. Mo loading, even if the dispersion of MoS2 phase and HDS activity are better for impregnated systems. Therefore, for the photocatalytic performance not the dispersion of MoS2 and abundance of edge sites but efficient electron hopping from the semiconductor to the co-catalyst is most important. In order to study the influence of the MoS2 edges state, the solids reduced under H2 or treated in pure H2S were compared. The presence edge S22− groups have a positive influence on the photo- and electrocatalytic HER activity.

FT-IR study of NO adsorption on MoS2/Al2O3 hydrodesulfurization catalysts: Effect of catalyst preparation

The effect of catalyst preparation on structure and performance of MoS2/Al2O3 hydrodesulfurization (HDS) catalysts was studied by NO adsorption, monitored by infrared spectroscopy (NO-IR) and complemented with EXAFS. The main bands at 1785cm−1 and 1690cm−1 were assigned to the ν(NO) stretching vibrations of coupled mono- or dinitrosyl surface complexes. NO adsorption on sulfided catalysts prepared with nitrilotriacetic acid (NTA) or citric acid (CA) gave rise to additional bands in the IR spectra at 1655cm−1, 1630cm−1 and 1615cm−1, assigned to mononitrosyl adsorption modes. Catalysts prepared with NTA and CA were more active in thiophene HDS than calcined or dried catalysts, despite comparable NO uptake (0.10-0.12 NO/Mo). This was attributed to a changed MoS2 edge structure as suggested by the NO-IR data, as well as to an improved degree of sulfidation as observed by EXAFS. Correlating NO uptake with MoS2 dispersion shows that not more than 30–40% of edge and corner sites were probed with NO. This implies that only a small portion of edge sites may act as active sites in HDS catalysis.

Selective hydrodesulfurization of gasoline on Co/MoS2 ± x catalyst: Effect of sulfur defects in MoS2 ± x

To improve the hydrodesulfurization activity while to minimize the hydrogen consumption from the hydrogenation of olefins is of importance for the deep desulfurization of gasoline. Herein, a series of Co/MoS2±x catalysts were prepared by impregnating Co-promoter to MoS2±x (x≤0.06) with controllably varied S/Mo atomic ratio. The S/Mo atomic ratio of MoS2±x in Co/MoS2±x catalysts influences the catalytic performance of Co/MoS2±x in simultaneously thiophene hydrodesulfurization (HDS) and cyclohexene hydrogenation (HYD) reactions. With decreasing the S/Mo atomic ratio in Co/MoS2±x, the HDS selectivity factor (kHDS/kHYD) increases to two times (from 37 to 89), although the catalytic activities of HDS and HYD decrease due to the low content of CoMoS phase. The higher selectivity of Co/MoS2±x with lower S/Mo atomic ratio could be well explained by larger size of sulfide crystallites exposed lower corner sites. Therefore, a high HDS selectivity catalyst could be developed by properly controlling the crystal size of sulfide crystallites.

Density Functional Theory Investigation on Thiophene Hydrodesulfurization Mechanism Catalyzed by ReS2 (001) Surface

We present density functional theory calculations on the reaction mechanism of thiophene hydrodesulfurization (HDS) over ReS2 (001) surface under typical HDS reaction conditions. It is found that thiophene adopts an “upright” adsorption configuration with the binding energy of 1.26 eV. Considering the factors such as Bader charge, two reaction mechanisms, named direct desulfurization (DDS) to the product of butadiene and hydrogenation (HYD) to 2-butene, 1-butene, and butane, are systematically investigated. Results show that H prefers to attack thiophenic C before the first C–S bond rupture but begins to hydrogenate ST (S atom of thiophene) after ring-opening. Prehydrogenation has different effect on the activity of C–S bond breaking. When the ring is intact, it has nominal effect; but when the ring is open, appropriate prehydrogenation can dramatically decrease the energy barrier while complete hydrogenation makes the barrier rise again due to stereohindrance effect. The DDS mechanism is proved to be kin...

Effect of high pressure sulfidation on the morphology and reactivity of MoS2 slabs on MoS2/Al2O3 catalyst prepared with citric acid

MoS2/Al2O3 catalysts were prepared by a simultaneous impregnation method with citric acid as chelating agent in order to investigate the effect of high pressure sulfidation on the morphology and reactivity of MoS2 slabs by infrared spectroscopy of CO adsorption (IR/CO), transmission electron microscopy, thiophene hydrodesulfurization (HDS) and 2,6-dimethylaniline hydrodenitrogenation (HDN) reactions. It is found that the MoS2 slabs are more truncated when sulfided at higher pressure, exposing larger S-edge/M-edge ratio. The thiophene HDS activity of MoS2/Al2O3 catalysts was significantly improved by high pressure sulfidation. The MoS2 edges formed after high pressure sulfidation are very stable toward H2-treatment, while the same H2-treatment can create sulfur vacancies on both M-edge and S-edge of the 0.1MPa sulfided MoS2 slabs. Meanwhile, parallel between the IR/CO results and HDN test suggests that the S-edge favors the C–N bond scission whereas the M-edge benefits the hydrogenation in HDN reaction.

Low-temperature synthesis of rhodium phosphide on alumina and investigation of its catalytic activity toward the hydrodesulfurization of thiophene

In this study, the low-temperature synthesis of rhodium phosphide (Rh2P) on alumina (Al2O3) using triphenylphosphine (TPP) as a phosphorus (P) source and its catalytic activity toward hydrodesulfurization (HDS) were investigated to prepare a highly active HDS catalyst. TPP was more easily reduced than phosphate, and Rh2P was formed in the P(T)/Rh/Al2O3 catalyst prepared from TTP at lower temperature as compared with the temperature required by Rh-P(A)/Al2O3 catalyst prepared from a phosphate precursor. However, after reduction at a low temperature (450°C), excess P covered the surface of Rh2P. The optimal reduction temperature for HDS rate of the P(T)/Rh/Al2O3 catalyst (650°C) was lower than that of the Rh-P(A)/Al2O3 catalyst (800°C). Furthermore, this temperature was slightly higher than the optimal reduction temperature for CO uptake (600°C). These results are explained as follows: HDS rate is increased by both elimination of excess P on the active sites at higher reduction temperatures and enhancement of the crystallinity of Rh2P. Furthermore, because the particle size of the P(T)/Rh/Al2O3 catalyst (ca. 1.2nm) was substantially smaller than that of the Rh-P(A)/Al2O3 catalyst, the P(T)/Rh/Al2O3 catalyst exhibited greater HDS rate compared with the Rh-P(A)/Al2O3 catalyst.

Potential of templated mesoporous aluminas as supports for HDS CoMo catalysts

Activities in thiophene hydrodesulfurization were increased by a factor three when using aluminas, as support for CoMo catalysts, prepared using an optimized sol–gel method.

A controllable synthesis of nitrogen-doped mesoporous carbon supported MoS2catalysts for hydrodesulfurization of thiophene

A highly active and stable catalyst for hydrodesulfurization (HDS) was developed based on thein situformation of MoS2on nitrogen-doped mesoporous carbon (MoS2/NMC).

A NiMoS flower-like structure with self-assembled nanosheets as high-performance hydrodesulfurization catalysts.

Uniform 3D NiMoS nanoflowers with self-assembled nanosheets were successfully synthesized via a simple hydrothermal growth method using cheap and nontoxic elemental sulfur as sulfur sources. The structure and morphology of the nanomaterials were characterized by SEM, TEM, XRD, Raman and XPS analyses, revealing that the NiMoS nanoflowers were composed of ultrathin nanosheets with a thickness of approximately 6-12 nm. The HRTEM results indicate that the curve/short MoS2 slabs on the nanosheets possess the characteristics of dislocations, distortions and discontinuity, which suggests a defect-rich structure, resulting in the exposure of additional Ni-Mo-S edge sites. The obtained NiMoS nanoflowers exhibited an excellent activity for thiophene hydrodesulfurization (HDS) and 4,6-dimethyldibenzothiophene deep HDS due to their high density of active sites. The outstanding HDS performance suggests that these NiMoS composites with a unique flower-like nanostructure could be useful as promising catalysts for deep desulfurization of fuel oils.

Computational Chemistry Analysis of Hydrodesulfurization Reactions Catalyzed by Molybdenum Disulfide Nanoparticles

Molybdenum disulfide, MoS2, is a very versatile material used in several applications of science and engineering. In particular, when supported on alumina, it is a widely used catalyst for hydrotreating processes in petroleum refineries. Stringent environmental norms and uncertainty in import of low sulfur crude oil put pressure on refiners worldwide to develop more efficient catalysts to meet the demand of clean fuel. The hydrodesulfurization of thiophene and dibenzothiophene on both unpromoted and nickel-promoted MoS2 catalyst is analyzed using a multiscale approach including classical molecular dynamics (MD) and density functional theory (DFT). MD simulations are performed on a hydrocarbon mixture containing sulfur compounds to determine relative positions of thiophene and dibenzothiophene molecules with respect to the MoS2 catalytic surface previous to possible reactions that are then studied with DFT. The sample box contains a total of 57 515 atoms and is able to represent a realistic size of the nanocatalyst process. Under typical hydrodesulfurization conditions, vacancies are needed for adsorption of sulfur compounds and therefore activation of hydrogen becomes an important step. In addition, complexes of MoS2 with graphene and boron nitride, BN, surfaces are also analyzed. MoS2–graphene complexes show improvement in the efficiency of adsorption of thiophene on the catalyst edge. Both hydrogenation and direct desulfurization pathways are presented on pristine MoS2 clusters as well as under typical hydrodesulfurization conditions.

Structure–Activity Correlations in Hydrodesulfurization Reactions over Ni-Promoted MoxW(1–x)S2/Al2O3 Catalysts

In this work, we have investigated the activation process and structure of Ni-promoted MoxW(1–x)S2/Al2O3 hydrodesulfurization (HDS) catalysts. Conversion of Mo and W oxides to the catalytically active MS2 (M = Mo, W) phase by sulfidation in gaseous H2S/H2 proceeded via different pathways, as found by XPS and EXAFS. The slower sulfidation kinetics of W on the alumina support formed NiMoxW(1–x) sulfides with a two-dimensional core–shell structure. Mo was mostly located in the core and W in the shell, as evidenced by EXAFS. Increasing the H2S/H2 pressure during sulfidation distributed Mo and W more homogeneously in the metal sulfide particles. This was attributed to the more favorable sulfidation of W under these conditions (i.e., below the temperature of MoS2 formation). Catalytic testing was consistent with these findings and demonstrated that a core–shell structure is the active phase in thiophene HDS (1 atm), whereas a homogeneously mixed MS2 phase catalyzes the HDS of dibenzothiophene at 40 bar. This is...

A Mechanistic Study of Thiophene Hydrodesulfurization by the Parahydrogen‐Induced Polarization Technique

AbstractThiophenic compounds are the least reactive organosulfur compounds in fossil fuels, and thiophene is widely used as a model substrate in studies of industrially important hydrodesulfurization (HDS) processes. It is generally presumed that the HDS process can proceed by two possible pathways, namely hydrogenation or direct desulfurization. In this work, the parahydrogen‐induced polarization technique was successfully utilized in a mechanistic study of a hydrodesulfurization reaction by example of a heterogeneous hydrodesulfurization of thiophene over supported catalysts in the presence of parahydrogen. It was demonstrated that the HDS of thiophene on a MoS2/γ‐Al2O3 catalyst proceeds preferentially by the hydrogenation pathway to form tetrahydrothiophene, followed by desulfurization. In contrast, if a Pt/TiO2 catalyst was used, direct desulfurization to 1,3‐butadiene and the hydrogenation pathway both contributed to the overall reaction mechanism.

Modeling the kinetics of sulfidation over NiMo/Al2O3 catalyst for thiophene hydrodesulfurization

A kinetic model has been developed for the sulfidation of NiMo/Al2O3 hydrodesulfurization (HDS) catalyst, which quantitatively establishes a relationship between the HDS activity and sulfidation conditions. The model parameters were estimated by fitting a series of experimental thiophene HDS conversion data over the catalysts sulfurized under different operations. The results show that the developed model satisfactorily predicts the HDS conversion with a total average relative deviation of less than 4 %. Moreover, parametric studies were made to validate the accuracy of the model. The comparison results show that the proposed model is fairly effective in simulating the sulfidation process. It is expected that the model will be used to guide the sulfidation research of supported Mo-based catalysts and to optimize laboratory/pilot-plant sulfiding procedures.

Synthesis and physicochemical characterization of ZrO2-doped NiMo/Al2O3 nanocatalyst via precipitation and sequential impregnation methods used in hydrodesulfurization of thiophene

A series of Al2O3-ZrO2 composite with various ZrO2 contents were prepared by precipitation and NiMo was dispersed over them via sequential impregnation. The samples were characterized by XRD, FESEM, EDX, BET, TPR-H2 and FTIR techniques. XRD patterns indicate the homogeneous dispersion of ZrO2 on Al2O3 in small amounts of ZrO2. Comparison of FESEM images implies that with the incorporation of small amounts of ZrO2 (5, 12 and 17 wt%) on Al2O3, the catalyst morphology changed from large ensembles to smaller particles with uniform distribution. According to TPR-H2 profiles, adding ZrO2 increases the reducibility of nanocatalysts. Catalytic activity results showed that the NiMo over Al2O3-ZrO2 composite showed higher hydrodesulfurization activity than that of Al2O3 supported catalyst. The maximum of hydrodesulfurization activity was observed for NiMo/Al2O3-ZrO2 nanocatalyst with 12 wt% of ZrO2 content related to enhanced physicochemical properties.

Exploring the activity of a novel Au/TiC(001) model catalyst towards CO and CO2 hydrogenation

Small metallic nanoparticles supported on transition metal carbides exhibit an unexpected high activity towards a series of chemical reactions. In particular, the Au/TiC system has proven to be an excellent catalyst for SO2 decomposition, thiophene hydrodesulfurization, O2 and H2 dissociation and the water gas shift reaction. Recent studies have shown that Au/TiC is a very good catalyst for the reverse water–gas shift (CO2+H2→CO+H2O) and CO2 hydrogenation to methanol. The present work further expands the range of applicability of this novel type of systems by exploring the catalytic activity of Au/TiC towards the hydrogenation of CO or CO2 with periodic density functional theory (DFT) calculations on model systems. Hydrogen dissociates easily on Au/TiC but direct hydrogenation of CO to methanol is hindered by very high activation barriers implying that, on this model catalyst, methanol production from CO2 involves the hydrogenation of a HOCO-like intermediate. When dealing with mixtures of syngas (CO/CO2/H2/H2O), CO could be transformed into CO2 through the water gas shift reaction with subsequent hydrogenation of CO2 to methanol.

Hydrodesulfurization of thiophene over Mo/AC catalyst presulfided by ammonium thiosulfate

Through the impregnation of Mo/AC catalyst with an aqueous solution of ammonium thiosulfate (ATS), the ex-situ presulfided Mo/AC-ATS catalyst was prepared; the influence of activation temperature and time on the catalytic performance in the hydrodesulfurization (HDS) of thiophene as a probe reaction was investigated. The result indicated that under the optimized activation conditions, viz., 300°C and 0.5 h, the conversions of thiophene over the presulfided Mo/AC-ATS catalyst are 34% and 42% higher than those obtained over the Mo/AC catalysts sulfided with traditional DMDS and CS2 sulfiding agents, respectively. The XPS, TPR-MS and TEM results demonstrated that the presulfided Mo/AC-ATS has a high content of Mo4+, which may contribute to its high activity in HDS.

Characterization and catalytic performance of {Mo2O2S2}-based oxothiomolybdenum cyclic clusters supported on mesoporous SBA-15

Cyclic polyoxothiomolybdate clusters containing {Mo2O2S2} building unit supported on SBA-15 have been prepared by immersion SBA-15 in the corresponding cluster's solution. The measurements of low angle X-ray diffraction (LXRD), nitrogen adsorption, transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy indicated that the cyclic clusters were successfully loaded on SBA-15. The supported catalysts were tested in thiophene hydrodesulfurization at different temperatures, respectively. Results show that the supported catalysts have an excellent desulfurization catalytic performance for thiophene.

Applications of CoMo/calcined quaternary hydrotalcites for hydrotreatment reactions

Co2+, Ni2+, Zn2+ and Al3+ quaternary hydrotalcite-type materials were synthesized at: 0.33; 0.25 and 0.20 (M3+)/(M3++M2+) ratios. Then, these as-synthesized hydrotalcites were calcined and impregnated with Mo (15wt.% MoO3) and Co (3Mo:Co) to obtain the catalytic precursors. As-synthesized and impregnated calcined hydrotalcites were characterized by different physico-chemical techniques such as: X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), BET surface area measurements and NH3-temperature programmed desorption (NH3-TPD). Finally, CoMo/calcined hydrotalcites were tested in hydrodesulfurization (HDS) and hydrogenation (HYS) reactions of thiophene and cyclohexene. Results showed that the 0.25 (M3+)/(M3++M2+) ratio was the most active for the HDS reaction of thiophene, while the 0.33 (M3+)/(M3++M2+) ratio showed the best performance for the HYD reaction of cyclohexene.