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. 2023 May 15;8(21):18400–18407. doi: 10.1021/acsomega.3c02029

Review of Molybdenum Disulfide Research in Slurry Bed Heavy Oil Hydrogenation

Xiaoning Zhang †, Buning Chen ‡, Jianwei Wang ‡, Yusheng Zhou ‡, Xueli Huang †,*, He Huang †, Xuefeng Wang †, Kaihong Li §
PMCID: PMC10233841  PMID: 37273628

Abstract

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With the growing demand for gasoline and diesel fuel and the shortage of conventional oil reserves, there has been extensive interest in upgrading technologies for unconventional feedstocks such as heavy oil. Slurry bed reactors with high tolerance to heavy oil have been extensively investigated. Among them, dispersive MoS2 is favored for its excellent hydrogenation ability for heavy oil even under harsh reaction conditions such as high pressure and high temperature, its ability to effectively prevent damage to equipment from deposited coke, and its ability to meet the requirement of high catalyst dispersion for slurry bed reactors. This paper reviews the relationship between the structure and hydrogenation effectiveness of dispersive molybdenum disulfide, the hydrogenation mechanism, and the improvement of its hydrogenation performance by adding defects and compares the application of molybdenum disulfide in heavy oil hydrogenation, desulfurization, deoxygenation, and denitrification. It is found that the current research on dispersive molybdenum disulfide catalysts focuses mostly on the reduction of stacking layers and catalytic performance, and there is a lack of research on the lateral dimensions, microdomain regions, and defect sites of MoS2 catalysts. The relationship between catalyst structure and hydrogenation effect also lags far behind the application of MoS2 in the precipitation of hydrogen, etc. Oil-soluble and water-soluble MoS2 catalysts eventually need to be converted to a solid sulfide state to have hydrogenation activity. The conversion history of soluble catalysts to solid-type catalysts and the key to their improved catalytic effectiveness remain unclear.

1. Introduction

In recent years, the growing demand for greener and lighter fuels, the shortage of crude oil reserves, and the shrinking conventional oil have created an urgent need to develop heavy oil refining technologies to ensure national defense security and national interests. In the field of petroleum catalyst hydrogenation, most of the research has focused on the advancement of refining technologies for low-quality industrial products (e.g., depressurized residual oil, coal tar, etc.) as well as unconventional oils (e.g., superheavy oil, oil sands, etc.), which are considered as alternative energy sources.1 With the rapid growth of global oil consumption, heavy or superheavy oil refineries will be substituted for refineries.2,3

Heavy oils usually contain high concentrations of heteroatoms (S, N, and O), transition metals (V, Ni, Fe, and Si), and complex molecules that are responsible for catalyst deactivation and coking in heavy oil refining.4 Depending on the polarity, heavy oils can be classified as saturates, aromatics, resins, and asphaltenes. The ultimate goals of hydroconversion of heavy crude oils are to reduce viscosity, remove heteroatoms, inhibit coking, increase the H/C ratio required for commercial products, and reduce hazards such as air pollution and acid rain during fuel consumption.

Catalytic hydroconversion is a more efficient way to upgrade heavy oil into liquid transportation fuels for modern refineries. Various hydrogenation reactor technologies using a fixed-bed, boiling-bed, moving-bed, and slurry-bed have been designed for the hydroconversion of heavy oils. Hydroprocessing using fixed-, boiling-, or moving-bed reactors often has the disadvantage of coking and heavy metal deposition leading to catalyst deactivation, especially for the hydroprocessing of heavier feedstocks. In addition, there are critical issues such as feed diffusion, pressure drop, and mass transfer in fixed-, boiling-, or moving-bed reactors. Slurry-bed reactors can overcome the limitations of these processes.5,6 Effective catalysts for the conversion of heavy oils into high-value products with lower boiling points by hydroconversion processes have been extensively studied for more than 50 years. Among these catalysts, MoS2 is favored as one of the main active catalysts due to its superior hydrogenation ability for heavy oils (including high sulfur content, impurities, and heavy components) even under harsh reaction conditions such as high pressure and high temperature and its ability to effectively stop the damage caused by deposited coke to the equipment. Although alumina-loaded Ni–Mo and Co–Mo nonhomogeneous catalysts have been used in fixed-bed and fluidized-bed reactors, unsupported Mo-based dispersion catalysts are used in slurry-bed reactors because of their strong resistance to deactivation, coking inhibition, and no diffusion limitation of reactants

2. MoS2 Catalyst

MoS2 has numerous physicochemical properties and has several potential applications such as tribology, hydrogen precipitation reactions (HERs),7−11 lithium-ion batteries, and microelectronics12,13 and as a major catalyst in heavy oil refining processes.14 MoS2 crystals belong to the hexagonal crystal system and have three different forms of configuration such as 1T (a = 5.60 Å and c = 5.99 Å), 2H (a = 3.15 Å and c = 12.30 Å), and 3R (a = 3.17 Å and c = 18.38 Å).15,16 The 3R phase exhibited better catalytic activity than the 2H and 1T phases in hydrogen precipitation reactions,17 and the 2H phase was mainly applied in heavy oil hydrogenation. The thermodynamically stable and naturally occurring MoS2 is 2H (3% 3R content).18 Two of the substable states, 1T-MoS2 and 3R-MoS2, can be converted to the stable 2H-MoS2 form at high temperatures. In the UV–vis spectrum, 2H-MoS2 shows two significant peaks at (λ ∼ 580 nm) 2.1 eV and (λ ∼ 640 nm) 1.9 eV.19

The MoS2-catalyzed hydrogenation model, the Rim-edge model, was first proposed by Daage. The Rim-edge (Figure 1) model states that the basal atom is not reactive; the Rim atom is the active center of the hydrogenation reaction and the hydrodesulfurization reaction; and the Edge atom is the active center of the hydrogenation reaction. As shown in Figure 1a, when MoS2 catalysts have the same longitudinal dimensions, MoS2 with smaller lateral dimensions can expose more edges and exhibit higher catalytic hydrogenation activity. Figure 1b shows that MoS2 catalysts with the same lateral dimensions and different numbers of stacked layers show the same number of exposed edges, but MoS2 catalysts with fewer stacked layers show more catalytic activity (similar to mechanical stripping). That is, not all exposed edges of the S–Mo–S atomic layer are used as active sites for catalytic hydrogenation, while only the Rim atom is used as catalytically active sites for PAH hydrogenation. Therefore, it can be speculated that only the Rim atom has stronger catalytic hydrogenation activity. The MoS2 catalysts with theoretical minimum lateral size and stacked layers of 2 have the highest hydrogenation activity.5,20

Figure 1.

Figure 1

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MoS2 model: (a) the same stacked layers and different lateral dimensions, (b) the same lateral dimensions and different stacked layers, and (c) Rim-edge model.

Hydrogen molecules can be adsorbed at the edge positions of MoS2 and dissociate into hydrogen atoms bound to S2– or Mo atoms, forming acidic SH groups and metal hydrides.21−23 Metal hydride species (e.g., Mo–H) are not thermodynamically favored. Therefore, after the initial H2 dissociation, hydride species can react with adjacent S2– to form additional SH groups.24,25 Molecular hydrogen dissociation and atomic hydrogen surface migration can occur in the edge planes of MoS2 catalysts.26,27

The catalytic hydrogenation activity of MoS2 catalysts is not directly related to the physical properties of the BET surface area. The BET surface area does not determine the catalytic hydrogenation activity of MoS2 catalysts.28 However, the MoS2 substrate surface has good lubricity and photocatalytic activity.29

According to DFT calculations and STM studies, planar molecules with π-bond–polycyclic aromatic hydrocarbons can be adsorbed in parallel on the basal plane of MoS2 through strong van der Waals interactions.30,31 Hydrogenation may occur in the region located at the edge of the basal plane, where the SH group provides hydrogen. Only the activated H species at the Rim site can react with the polycyclic aromatic hydrocarbons adsorbed on the basal plane.32 Due to the spatial site resistance effect, the activated H species at the Edge position is difficult to react with PAHs. Accordingly, the Rim site can activate H2 and, at the same time, facilitate the reaction between activated H species and PAH molecules due to its accessibility. As the hydrogenation of PAHs proceeds, the deep hydrogenation products are nonplanar. Compared to Edge sites, macromolecules have a much smaller spatial site-blocking effect on Rim sites.33,34 In addition, PAHs can be adsorbed on the whole substrate; however, it is possible that only PAH molecules on the edge of the substrate near the edge sites can hydrogenate with SH at the edge sites, and the aromatic molecules adsorbed at positions far from the edge cannot react with SH. Therefore, the Rim position favors the adsorption of PAHs and the desorption of the corresponding products during catalytic hydrogenation.5 In addition, Iwata et al. found that the addition of Ni, for example, enhanced the number of inflection points in the substrate and greatly enhanced the MoS2 catalytic hydrogenation activity.35

3. MoS2 Defects

Typically, 0D point defects of MoS2 mainly include vacancies and heteroatom replacement defects. The presence of vacancies distorts the lattice and thus effectively modulates the electrical structure and chemical properties of the material. The formation energies of several vacancies, including Mo vacancies (VMo), S vacancies (VS), and mixed Mo and S vacancies (VMoS3), were calculated based on density functional theory by Barik et al. The results show that VS has the lowest formation energy, which is consistent with the frequently observed experimental results. Heteroatom doping by introducing substituted atoms is a common strategy for altering the microscopic morphology, structure, and electrical properties of MoS2 materials. Heteroatoms include both metallic and nonmetallic elements.36

The extended layer spacing of layered MoS2 nanosheets is defined as two-dimensional planar defect engineering. The reduction of stacked layers can greatly enhance their hydrogenation properties and improve structural stability during cycling. Typically, the introduction of small molecules, ions, particles, etc. can significantly improve the interlayer distance. About 60% of the interlayer expansion is caused by the introduction of oxygen from Mo precursors or by the insertion of NH4+ ions released by thiourea decomposition compared to pristine 2H-MoS2.36

4. MoS2 in Heavy Oil Hydrogenation

Heavy oils are mainly composed of irregularly structured macromolecular compounds with structures including, for example, paraffin, naphthenes, and aromatic hydrocarbons with different physicochemical properties (Table 1). Asphaltenes are the heaviest aromatic component of heavy oils but are the least reactive in destructive hydrogenation reactions, with a percentage of 8–15%. The rapid hydrogenation of asphaltenes is a key step in the conversion of heavy oils into clean fuels.28 According to the Yen Mullins model,45,46 asphaltene molecules form asphaltene nanoaggregates through intermolecular forces with aggregates of 6. These nanoaggregates can further aggregate to form aggregates of 8. The asphaltene molecular structure is usually considered a “continental” or “archipelagic” model.28

Table 1. Summary of Literature on Heavy Oil Hydrogenation.

Type Oil Catalytic conditions Catalysts Effects Source
Solids Iranian heavy oil 200 °C, 6.0 MPa, 24 h SiO2-C/MoS2 81% reduction in viscosity and a 7.7° increase in API (37)
Solids Light-circulating oil 375 °C, 10.3 MPa Dispersed MoS2 - (38)
Solids Pineapple oil and palm oil 330 °C, 5.0 MPa, 1.5 h–1 NiMo/γ-Al2O3, Deoxygenation rate: 94.6 (39)
Solids Dibenzothiophene 320 °C, 3.0 MPa, 1 h MoS2/SiO2/Fe3O4 Conversion rate: 80% (40)
Water-soluble Light cycle oil + bio-oil 360 °C, 7.9 MPa, 0.72 h–1 Ammonium secamolybdate tetrahydrate Oxygen removal rate: 90% (41)
Solids Straight-run gasoline 350 °C, 5.5 MPa, 750 rpm, 3 h Dispersed MoS2 HDS: 56%. Commercial CoMo/Al2O3: 52–53% (42)
Solids Nitrobenzene 3.0 MPa, 100 °C, 6 h Dispersed MoS2 100% conversion (commercial catalyst 52%) (43)
Solids Anthracene/phenanthrene 350 °C, 6–8.0 MPa, 4 h Dispersed MoS2 Conversion rate: 56.8, 68.9 (5)
Solids Thiophene/pyridine 300 °C, 1 h MoCoS/MoNiS Conversion rate: 80% and 50% (44)
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Asphaltenes cause the following problems during the thermal treatment of heavy oils: (1) Asphaltenes not only adversely affect the overall rate of the HDT reaction but also limit the maximum degradation during the thermal treatment of heavy oils. (2) During catalytic hydrogenation, asphaltene stability decreases, which in turn leads to a shift to coke resulting in catalyst deactivation and, in severe cases, clogging of the reactor.47

Heavy oil macromolecules are easily activated at a higher reaction temperature, which is conducive to asphaltene hydrocracking and asphaltene composition changes with the increase in reaction temperature. In this process, the side chains of asphaltene molecules are easily removed, and the remaining thickened aromatic rings form larger molecules through dehydrogenation and condensation, thus generating coke.4 It is easy to activate at higher reaction temperatures, which is conducive to hydrolysis and cracking reactions. When the reaction temperature reached 410 °C, nearly 60% of the asphaltene in the feedstock was converted. This not only indicates that high reaction temperature is beneficial to asphaltene hydrocracking but also indicates that asphaltene composition changes with the increase in reaction temperature. At high temperatures, part of the alkyl side chains of asphaltene macromolecules are removed, and the remaining thickened aromatic rings react with each other through dehydrogenation and condensation to form larger molecules, thus increasing coke and gas generation.4 Meanwhile, the conversion of sulfur and oxygen follows an almost linear trend as the reaction temperature increases. The sulfur content decreases due to the destruction of the sulfur bridges on the alkyl chains. However, the nitrogen content in asphaltenes first decreases and then does not change significantly. Nitrogen compounds in asphaltene structure are one of the most difficult heteroatoms to deal with. The HDT of these compounds is not favorable to thermodynamics and therefore has high stability.4 Due to the small decomposition energy of the C–S bond, HDS is easier than HDN,2,48 and the decomposition energy of the C–S bond is smaller than that of other bonds in crude oil molecules. Therefore, these are the first bonds to break down, resulting in a decrease in crude oil viscosity and an increase in API gravity.37 XRD, 1H NMR, and XPS analysis of asphaltene showed that with the increase of temperature the molecular weight of asphaltene decreased; the aromaticity increased; and the space between the layers and chains of asphaltene decreased (asphaltene structure became denser). With the increase in reaction temperature, the content of S and molecular weight of asphaltene decreased first, while the content of N decreased first and then changed a little.4 A comparison of different crude oil samples revealed a relatively uniform distribution of different sulfur molecular groups in heavy oils, and compared to monocyclic and bicyclic aromatics, tricyclic aromatics have a higher tendency to adsorb in hydrotreating catalysts and therefore will strongly compete with sulfur compounds for catalyst sites.49 Partial hydrogenation of polycyclic aromatic hydrocarbons in heavy oil hydrogenation produces aromatic compounds attached to saturated rings, which are converted sequentially to monocyclic aromatic hydrocarbons by cracking. In the case of residual oil conversion, hydrocracking helps to obtain low boiling point distillates as well as hydrodesulfurization and hydrodenitrogenation.48

There are two main forms of MoS2 catalyst in the slurry bed: solid powder catalyst and dispersion catalyst. The dispersion of solid powder catalysts in crude oil is poor, and it is difficult to provide efficient catalytic hydrogenation. This is overcome by dispersion catalysts. However, from an economic point of view, oil-soluble precursors in dispersed catalysts, including molybdenum naphthenate and molybdenum diethyl caproate (molybdenum caprylate), are suitable for the recovery process rather than the primary method. The main function of the catalyst required by the slurry process is not hydrodesulfurization (HDS) but hydrogenation (HYD). At present, the performance of oil-soluble catalysts is close to the maximum. Under the same conditions, hydrogenation of heavy oil without a catalyst can increase the content of asphaltene by 15–20%.35

In the hydrogenation of heavy oil, the hydrogenation of polycyclic aromatic hydrocarbons (PAHs) is carried out in successive steps. The first ring hydrogenation kinetics of PAHs is favorable. This result indicates that the hydrogenation rate of the subsequent rings is slower, and the last ring hydrogenation is more difficult compared to the first ring hydrogenation. In the case of anthracene, for example, AN can be hydrogenated first to the preliminary AH2, and the reaction is kinetically very easy even without a catalyst. However, deep hydrogenation will become increasingly difficult. The MoS2 catalyst with the smallest lateral size and the smallest stacking layer exhibited the highest catalytic activity for the hydrogenation of phenanthrene and naphthalene. According to the GC-MS results, the hydrogenation of polycyclic aromatic hydrocarbons (PAHs) proceeds sequentially: PAHs are first hydrogenated to preliminary products and then further hydrogenated to full saturation. Only trace amounts of hydrocracking products could be detected.5 The hydrogenation reaction occurs mainly for bicyclic aromatic hydrocarbons, which are reduced to monocyclic compounds; monocyclic and polycyclic aromatic hydrocarbons are almost not hydrogenated. This phenomenon confirms the weak hydrogenation ability of the carrier-free catalysts.38 Kadiev et al. found through DFT calculation that the catalytic hydrogenation conversion catalyst containing molybdenum and nickel was conducive to reducing the content of asphaltene and resin in the reconversion products. In contrast to the case of molybdenum, benzene molecules are chemically adsorbed to nickel by double bonds.47 The addition of nickel atoms can improve the stability of the hydrogenated intermediate and promote the hydrogenation of heavy oil.50

The mechanism of residue slurry bed hydrocracking is similar to viscosity-reducing cracking in the presence of free radical H2, and the main reactions in these processes are free radical reactions, including cracking and condensation. Although the addition of molybdenum catalysts is effective in suppressing coking,51 increasing the catalyst dosage does not always improve the catalytic effect. Li et al. derived the highest Mo4+ concentration and colloidal stability of the system after sulfidation for Mo-based catalyst concentrations up to 300 ppm, resulting in the lowest coke yield and highest catalytic hydrogenation performance in the reduced viscous residue passion.52

Persi et al. obtained water-soluble Ni–Mo catalysts by preparing mixed solutions at room temperature, resulting in the reduction of crude oil S content from 5.5 to 3.1% and N content from 750 to 392 ppm. The increase of aromatic and saturated hydrocarbon compounds at the expense of asphaltenes and resins increased the volume of gasoline and diesel to 8 and 14%, respectively.48 Liu et al. dissolved the catalyst precursor cetyltrimethylammonium heptamolybdate The resin fraction decreased from 25.21 wt % to 3.54 wt %, and the C7-asphalt content decreased from 6.82 wt % to almost zero. The yield of liquid products increased from 75.03 wt % to 96.43 wt %.53 Abdullahi et al. performed catalytic cracking of Oman thick oil with an oil-soluble NiMo viscosity reduction catalyst and glycerol as a hydrogen donor, and the maximum viscosity was reduced by 69% in a 30 h reaction at 277 °C.54 Since carrier-free MoS2 is a hydrocracking catalyst without cracking capacity, heavy oil decomposes by thermal cracking during hydroconversion using a dispersion catalyst.14

5. MoS2 in Heavy Oil Decontamination

Due to the burning of light fuels, sulfur dioxide (SO2) and nitrogen oxides (NOx) are prone to environmental problems such as acid rain. Due to the requirement for environmental protection, most countries have increasingly strict control over the heteroatom content in fuel oil.55,56

Carrier-free Mo catalysts were evaluated for hydrodeoxygenation using model compounds such as phenol, methyl phenol, and methoxyphenyl.57,58 These carrier-free Mo catalysts were found to favor the direct deoxygenation pathway.59 Hydrotreating of hydrothermal liquefied bio-oil using carrier-free catalysts showed hydroxyl removal rates comparable to those of commercial NiMo/γ-Al2O3. Hydrodeoxygenation leads to a decrease in oxygen content and a decrease in acidity. After hydrotreating, the atomic O/C ratio decreased significantly, while the effective H/C of the feed mixture increased slightly from 1.07 to 1.26 to 1.23–1.31. The significant decrease in O/C can be attributed to hydrodeoxygenation, and the slight increase in the H/C ratio implies limited ring saturation of the oil product.41 Carrier-free MoS2 catalysts with low stacking are more selective for the hydrogenation of C–OH bonds in phenol molecules, while MoS2 with high stacking is more favorable for the hydrogenation of aromatic rings.59 Co-MoS2 with nanoflower morphology exhibited higher p-cresol conversion performance (rate constant of 0.37 h–1) compared to Co-MoS2 with nanotube morphology (rate constant of 0.23 h–1). The reason for the improved catalytic performance of Co-MoS2 with nanoflower morphology can be attributed to the presence of various exposed edges and the synergistic effect between Co- and MoS2-based nanomaterials.60,61 These studies were analyzed using the atom-vacancy interface model and found that isolated cobalt atoms promote hydrogen activation and that the underlying sulfur atoms promote carbon–oxygen fracture and vacancy regeneration.62 Zhang et al. hydro-deoxidized fatty acids and triglycerides to produce diesel fuel. Based on the equilibrium constant of the reverse water-vapor transfer reaction, it is indicated that CO will be the main gaseous product of the hydrodecarbonization reaction.39 Comparing the variation of CO and CO2 concentrations, it was found that the decarboxylation effect of molybdenum disulfide is slightly higher (1–5 times) than the decarbonylation effect.41

Hydrodesulfurization can be considered the most efficient technique to remove sulfur from oil, resulting in clean hydrocarbons.63−68 In this context, MoS2 proved to be an effective catalytic material for the hydrodesulfurization reaction. Nickel nanoparticles deposited on the surface of molybdenum disulfide were used as an efficient catalytic material for the hydrodesulfurization of nickel and its derivatives.69−71 Ni-MoS2-based nanocomposites were synthesized by a solution chemistry method through a two-step approach. The particle size of the composites was determined based on MoS2/Ni ratio, concentration, and temperature. 50.2 wt % of the catalytic system exhibited the highest catalytic performance with 99%, 98%, and 94% conversion of thiophene, benzothiophene, and dibenzothiophene, respectively.72 Del Valle et al. concluded that exfoliated MoS2 is more HDS active than crystalline MoS2. The catalytic activity of crystalline MoS2 exfoliated MoS2, and MoS2 was prepared from molybdenum naphthenate and ammonium heptamolybdate for the same HDS reactions. When crystallized and stripped MoS2 was used, the yield of biphenyl was higher than that of tetrahydrodibenzomethine for all dibenzothiophene transformations. In addition, higher yields of trihydrodibenzothiophene were obtained when the other two catalysts were used. In the HDS of dibenzothiophene, MoS2 derived from ammonium heptamolybdate exhibited higher catalytic activity than MoS2 derived from molybdenum naphthenate.73 Seyyedmajid et al. prepared the magnetic catalyst MoS2/SiO2/Fe3O4 by mixed micellar polymerization and hydrothermal synthesis, and the addition of surfactants CTAC and SDS during the preparation resulted in MoS2 crystals of small size. Its high dispersion in the catalyst resulted in more than twice the desulfurization performance without the addition of surfactants,40 and the direct desulfurization activity (DDS) of molybdenum disulfide was found to be much higher than the hydrodesulfurization (HDS) activity.40 Quantum chemical techniques have shown that the chemisorption of hydrogen molecules on valence-unsaturated Mo atoms leads to H–H bond breakage and migration of H atoms to other valence-unsaturated Mo and S atoms. The study of the interaction of H2S molecules with Mo2S4 and Mo3S6 clusters shows that the chemisorption of H2S molecules occurs on valence-unsaturated Mo atoms, followed by the extraction and migration of H atoms on the cluster surface, with unsaturated S atoms playing the role of donors and hydrogen carriers. The results showed that sulfur-containing compounds (mercaptans, thiophenes, and dibenzothiophenes) are also chemisorbed via S atoms on valence-unsaturated Mo atoms located on the surface of MoS2 clusters.74 Hydrotreating of this complex feedstock was performed in an intermittent reactor using dispersed MoS2 catalysts. The reactivity of different sulfides in light cycle oil followed the trend of benzothiophene > 1-methyl benzothiophene≫ 2-methyl benzothiophene > 3-methyl benzothiophene > 4-methyl benzothiophene ≫ dibenzothiophene ≈ 1-methyldibenzothiophenol ≈ 2-methyldibenzothiophenol ≈ 3-methyldibenzothiophenol. The removal of nitrogen compounds from LCO followed the trend of aniline > indole > carbazole.38 Both theoretical calculations and experimental studies have shown that the ligand-unsaturated Mo sites on the surface of MoS2 catalysts are active species in hydrodesulfurization catalysis. For example, density flooding theory studies suggest that the unsaturated molybdenum atoms exposed on the MoS2 surface are responsible for the high activity of the HDS reaction.75 Experimental results confirmed that the coordinated unsaturated Mo species are the active centers of molybdenum sulfide catalysts loaded on Al2O3 and TiO2 for the hydrodesulfurization of thiophene and the hydrogenation of butadiene. Also, XPS results show that nano-MoS2 shows a redshift of 0.2 eV compared to commercial MoS2. This indicates that nano-MoS2 has a higher amount of low Mo valence state, which implies a more coordinated unsaturated Mo site on the MoS2 microfluidic surface.43 Guo et al. synthesized MoCoS and MoNiS monolayers as catalysts for HDS and HDN in one step. The monolayer structure of MoS2 ensures better activity for basic catalysts due to the high density of active centers. In HDS and HDN, the monolayer metal oxide semiconductor shows enhanced catalytic activity compared to the multilayer MoS. Rationally, cobalt-based MoS catalysts show higher conversions than nickel-based MoS in HDS, which is the counterpart of HDN.44

The unloaded MoCo(Ni)S catalysts also performed better in HDS and HDN compared to the conventional loaded catalysts.76−78 Therefore, the one-step synthesis of carrier-free Mo(W)Co(Ni)S catalysts with well-defined structures has great potential to improve their catalytic performance in HDS and HDN reactions. Kun et al. studied the interaction effect between HDS and HDN under the catalysis of different Mo series catalysts,44 proving the superiority of nitrogen-containing organic matter in hydrogenation.79−81 In the presence of Ni, the adsorption effect of nitrogen-containing organic matter on the catalyst will be better.82,83 The opening rate of Ni-MoS2 under γ-Al2O3 loading is the best, while the opening rate of Ni/MoS2 under unloading is the lowest. The HDN reaction was temperature dependent: as the temperature increased, due to the presence of Ni, the reaction with γ-Al2O3-loaded Ni-MoS2 also showed a higher removal rate compared to Al2O3-loaded Ni-MoS2.84 For polymetallic catalysts, such as Ni–Mo–W, the simultaneous presence of Mo and W in the same slab hinders the growth of pro-Ni species and favors nucleation, which allows the largest fraction of Ni to be bound. In addition, the slow growth in the Z-direction leads to the maximization of active sites around the particles. Compared to bimetallic catalysts, trimetallic catalysts have the highest concentration of active centers and larger specific perimeters, leading to the highest rates of hydrodenitrogenation and hydrodesulfurization.85

The differences in catalytic properties are caused not only by the different properties of metals and auxiliary materials but also by the purity of the feedstock. For example, K in the feedstock causes an increase in the electronic charge on the MoS2 material, raises the surface alkalinity, and blocks the Mo and S edges, inhibiting the catalytic activity and limiting the access of hydrogen to the MoS2 surface. In particular, the decrease in HDS and HDO activity and the slight decrease in hydrogenation activity are related.39

6. Conclusion

Slurry bed reactors have recently gained momentum in the field of heavy oil hydrogenation because they overcome the challenges of pressure drop and mass transfer limitations present in conventional fixed-bed reactors and other reactors. As one of the important hydrogenation catalysts in slurry beds, the literature has focused on the reduction of the number of MoS2 stacking layers, but there are a lack of studies on the lateral dimensions, microdomain regions, and defect sites. The relationship between molybdenum disulfide structure and hydrogenation effect also lags far behind the application of MoS2 in hydrogen precipitation, etc. The current studies mostly focus on the variation of hydrogenation performance but lack detailed studies on active hydrogen transfer, etc. In slurry bed hydrogenation applications, oil-soluble molybdenum is a hot topic of research, but all need to be converted to a solid state to have hydrogenation activity. How does the conversion of soluble molybdenum-based catalysts to solid molybdenum disulfide during heavy oil hydrogenation positively affect the effect of heavy oil hydrogenation? This question is still unclear and poses a significant obstacle to reducing the cost of soluble Mo precursors and affecting their industrialization.

Acknowledgments

This work was financially supported by the Major Science and Technology Special Projects in the Autonomous Region (No. 2021A01002-2).

The authors declare no competing financial interest.

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