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Methods for the synthesis of supported mixed MoWS hydrotreating catalysts
Thomazeau et al. [81] conducted a study of catalytic systems prepared by incipient wetness coimpregnation of the support with solutions of ammonium heptamolybdate (AHM) ((NH4)6Mo7O24·4H2O), ammonium metatungstate (AMT) ((NH4)6H2W12O40·xH2O) and Co(Ni)(NO3)2‧6H2O with the expected optimal ratio Ni(Co)/(Ni(Co) + Mo + W) = 0.3. Before impregnation, the pH of the solutions was adjusted to 9.5 with diluted ammonia for monomeric species (WO42- or MoO42-) to be predominant. After co-impregnation, catalysts were oven-dried overnight at 393 K and calcined in air at 773 K during 2 h. It was found that the promoted Mo-W-S solid exhibits a new synergy effect in thiophene HDS when Ni was used as a promoter: NiMo0.5W0.5S2 catalyst reveals higher intrinsic HDS activity than bimetallic NiMoS and NiWS due to optimum metal-sulfur bond energy (Fig. 1.12). But at the same time, the CoMo0.5W0.5S2 catalyst led to catalytic results without synergetic effect corresponding to a linear combination of the HDS activities of the two CoWS and CoMoS reference systems.
Vázquez-Salas et al. [82] used the same method of synthesis to obtain NiMoW catalysts based on HMS modified with Ti (HMS-Ti) and conducted a comparative analysis of activities with Ni(Сo)Mo/Al2O3 catalysts. All synthetized Ni-promoted MoW catalysts exhibited improved specific activities in the HDS of DBT compared to that of two commercial NiMo/γ-Al2O3 and NiW/γ-Al2O3 catalysts. Moreover, NiMoW/HMS-Ti catalysts showed a high enhancement of dibenzothiophene conversion via hydrogenation route with respect to their Ti-free counterpart. The study of the genesis of NiMoW catalysts based on γ-Al2O3 was also developed by Hensen and co-workers [83]. All catalysts were synthetized by pore volume impregnation with aqueous solution of AHM ((NH4)6Mo7O24·4H2O), AMT ((NH4)6H2W12O40·xH2O) and nickel nitrate. It was found that the presence of Mo atoms does not affect the rate of sulfidation of tungsten under gas-phase conditions (H2/H2S (10%), 400⁰C). However, it was found that increasing the pressure during sulfidation to 15 bar helps to reduce the temperature of tungsten sulfidation and in this case mixed slabs with a random distribution of Mo and W atoms are formed, while at atmospheric pressure slabs with a core-shell structure are formed, where Mo is mainly located in the core, and W in the shell (Fig. 1.13). with a random distribution of atoms, which was explained by the sintering of particles together with the formation of slabs of greater length. Each structure was evaluated in only one reaction: the core-shell structure in the thiophene HDS reaction, the random structure – in the DBT HDS reaction, but in both cases, no pronounced synergistic effect between Mo and W was observed. More interesting data were obtained in in HDS of gas oil, according to the results of which it was found that mixed NiMo0.75W0.25 was the most active, which shows the potential of mixed systems [83].
Sigurdson et al. [84] reported that the catalytic activity in HDN and HDS reactions increases when phosphorus is added to NiMoW/Al2O3 as a promoter thanks to the P doping improving the dispersion and number of surface-active sites. A series of P-doped catalysts with different∼ amount of P (0–2.5 wt.%) were prepared by impregnation of alumina with aqueous solution (pH 4) containing ammonium heptamolybdate, ammonium tungstate, nickel nitrate. and phosphoric acid. The catalysts were tested under conditions of industrial hydrotreatment of light coking gas oil in a trickle bed reactor. The phosphorous content of 1.6 wt.% is optimal and the trimetallic catalyst exhibits better hydrotreating activity than bimetallic and commercial catalysts.
Various types of studies of mixed MoW systems can be found in the literature, and most of them are related to synthesis using traditional precursors of the active phase, such as AHM, AHT, AMT and ammonium tungstate [94–96].
Heteropolyanions as effective precursors of hydrotreating catalysts
Works concerning synthesis of hydrotreating catalysts via heteropolyanions (HPAs) have been known since the middle of 1980s. Instead of the use of traditional Mo and W precursors as AHM, AHT, AMT and others inorganic heteropolyanions are more and more common in the literature as precursor of active phase [90, 97, 98]. For hydrotreating catalysts preparation, HPAs with the Keggin [67, 99] and Anderson [100–103] structures and their derivatives [104, 105] (Fig. 1.14) are the most frequently used.
Heteropolyanions are complex inorganic compounds, which composition can be described as an assembly of oxygen polyhedra (octahedra and tetrahedra) of limited extent obtained by sharing one or more oxo (or hydroxo) ligands, the polyhedra being joined at their corners, edges or faces [97]. The stability of the HPA is influenced primarily by the composition of the ligand sphere: HPA with W is much more stable than HPA with Mo [98].
Heteropolyanions as precursor for hydrotreating catalysts
Hetoropolyanions can be derived from the acids (heteropoly acids) and corresponding salts (Co, Ni, etc.) (heteropoly compounds). Heteropolyanions are favorable for the preparation of catalysts because of the uniform adsorption of HPA on the surface of the support [100, 103], the possibility to
avoid the calcination step due to the absence of NH4+, NO3- counterions (if metallic salts of heteropolyanions are used for example) and a higher sulfidation degree of metals, which contributes to an increase in catalytic activity.
The main advantage of Anderson HPA is the presence of the 3d-metal as heteroatom in the composition of the HPA, including the traditional promoters of Ni (Co) or Fe [98]. The literature data [67, 101, 104] shows that the activity of catalysts synthesized from HPA in the thiophene HDS depends on the molar ratio of promoter/molybdenum [67, 101]. Using HPA with Anderson structure, this ratio is 1/6, which is suboptimal. For this reason, the derived dimeric form [Co2Mo10O38H4]6-from the Anderson structure is more preferable as a precursor of the active phase, since it has a Co/Mo molar ratio in this structure equal to 1/5, which is much closer to the optimum value of 1/2. In addition, the lack of a promoter is compensated by using Co(Ni) salts of HPA type (Ni)2[Ni(OH)6Mo6O18] × nH2O or (Co)3[Co2Mo10O38H4] × nH2O.
In turn, the Keggin type HPA and its derivatives (Fig. 14 I) are more stable than Anderson HPC, accessible and important for catalysis [108, 109]. In the Keggin-type heteropolyanions, one non-metallic atom, such as P, Si, As, B, etc., exists in a regular tetrahedron, combined with 12 MO6 octahedra (where M is MoVI, WVI, VV), which are connected by shared edges to form trimetallic M3O13 groups joined together by their vertices [101]. Depending on the synthesis conditions, various isomers (α, β, etc) can be isolated.
It has been shown that supported phosphomolybdic heteropolyacid (H3PMo12O40) and its Co and Ni salts are efficient oxide precursors for thiophene HDS [102]. Moreover, Ni promoted molybdenum and tungsten HPA-based catalysts showed better performance in deep HDS of 4,6-DMDBT [103] and diesel hydrotreating [104] than the counterparts prepared from traditionally used Mo(W) ammonium salts. Blanchard et al. have shown that catalysts obtained from the PMo-HPA with Keggin structure are more active in the thiophene HDS than those prepared from AHM and H3PO4 due to a better metal dispersion and the absence of ammonium counterions [91]. Griboval et al. [92] showed by using of SiMo12 and PMo12HPAs that the nature of the heteroatom (Si or P) of these HPAs does not affect the thiophene HDS catalytic activity of the obtained catalysts.
Unsupported (bulk) sulfide hydrotreating catalysts
Recently, the number of studies aimed at the development of unsupported sulfide hydrotreating catalysts has significantly increased. Currently, new catalysts appeared in the market, which contains very large amounts of active components and can be considered as bulk catalysts. The metal composition of the active phase in bulk catalysts is the same than that in the supported one, but at the same time, their hydrotreating volume activity may exceed two or more times that of supported catalysts [112]. The concentration of the active component can vary from 80% (if various additives and modifiers are used or if the role of the support is played by inactive sulfide) to 100% (pure bulk catalyst).
Methods for the synthesis of unsupported sulfide hydrotreating catalysts
The literature has described various methods for preparing bulk catalysts. One of the first methods for growing nano-sized MoS2 particles inside inverse micellar cages in nonaqueous solvents was proposed more than 20 years ago and described by Wilcoxon and Samara [113]. The MoS2 clusters are formed by first dissolving a molybdenum (IV) halide salt in solution with micellar cages and then combining this solution with another inverse micelle solution containing a sulfiding agent (e.g., metal sulfide or H2S). The authors managed to get MoS2 nanoparticles ranging in size from 2 to 15 nm. The size of the particles depends on the size of the initial micelles containing salt of Mo(IV).
Methods of hydrothermal [122, 123] and solvothermal [124, 125] processes for the synthesis of bulk catalysts have also been described. Devers et al. [114] synthesized unsupported MoS2 by hydrothermal method using (NH4)2MoS4 (ATTM) as Mo source. The catalysts consisted in multi-layered slabs with an average length of 5 to 50 nm and a surface area (from 50 to 100 m2/g) depending on the synthesis conditions (temperature and acidity of the reaction mixture). It was found that samples with a large particle length exhibit a higher activity in hydrogenation (HYD), while samples with a low particle length are more active in hydrodesulfurization (HDS). Vrinat and co-workers [117] investigated bulk molybdenum disulfide obtained by the solvothermal method using molybdenum naphthenate as precursor and n-hexadecane or 1-methylnaphthalene as hydrocarbon solution. The samples had a high surface area (more than 200 m2/g) and showed higher catalytic activity in thiophene HDS than the sample obtained by the hydrothermal method. A variable amount of a carbonaceous phase was detected in catalysts (6-8 wt.% of the total solids). It has been suggested that the coke phase originates from the organic-molybdenum precursor, while the organic solvent is not involved in the formation of a carbonaceous phase. The authors did not establish a direct dependence of the catalytic activity on the surface area. However, changes in the texture of coke can affect the accessibility of reagents to the MoS2 surface and, therefore, the catalytic activity can depend on the interface between coke and molybdenum sulfide [117]. Polyakov et al. [118] reported the method of high temperature decomposition of ATTM. The catalysts were tested in the model reactions of hydrogenation and isomerization of alkenes, as well as deuterium exchange. It was found that MoS2 microcrystalline obtained at high-temperature was completely inactive in hydrocarbon transformations (isomerization of alkenes and ethylene hydrogenation) and H2/D2 scrambling, but after high-energy ball milling was transformed into an active catalyst. The activity of MoS2 correlates with the degree of structural defects such as split nanoslab projecting from an aggregation of bent nanoslabs and stepwise truncations of the layers (Fig 1.17.).
Table of contents :
Chapter 1
1.Literature review
1.1 Introduction
1.2 The development of hydrotreating processes
1.2.1 The role and purpose of hydrogenation processes in oil refining
1.2.2 Main compounds and target reactions of hydrotreating
1.3 Hydrotreating sulfide catalysts
1.3.1 The composition and structure of the active phase
1.3.2 On active phase and active sites of unpromoted hydrotreating catalysts
1.3.3 Relations between active phase composition and catalytic properties
1.3.4 The influence of the support on the catalytic activity
1.3.5 Mesostructured silica as a promising support for hydrotreating catalysts
1.4 Supported MoWS catalysts
1.4.1 Methods for the synthesis of supported mixed MoWS hydrotreating catalysts
1.4.2 Heteropolyanions as effective precursors of hydrotreating catalysts
1.4.2.1 Type, composition and structures of heteropolyanions
1.4.2.2 Heteropolyanions as precursor for hydrotreating catalysts
1.4.2.3 Mixed MoW-heteropolyanions as interesting precursors for hydrotreating catalysts
1.5 Unsupported (bulk) sulfide hydrotreating catalysts
1.5.1 Methods for the synthesis of unsupported sulfide hydrotreating catalysts
1.5.2 Promoted unsupported sulfide catalysts
1.5.3 Unsupported mixed (Ni)MoW catalysts
1.5.4 The bulk catalysts: from science to industry
1.6 Conclusion
References
Chapter 2
2.Unpromoted MoWS hydrotreating catalysts supported on alumina
2.1 Introduction
2.2 Synthesis and characterization of H4SiMonW12-nO40 Keggin-type heteropolyacids
2.2.1 Synthesis of H4SiMonW12-nO40 Keggin-type heteropolyacids
2.2.1.1 Synthesis of monometallic H4SiMo12O40 and H4SiW12O40 heteropolyacids
2.2.1.2 Synthesis of mixed H4SiMonW12-nO40 (n = 1 and 3) heteropolyacids using lacunar salts
2.2.1.3 One-step synthesis of mixed H4SiMonW12-nO40 (n = 6 and 9) heteropolyacids
2.2.2 Characterization of H4SiMonW12-nO40 Keggin-type heteropolyacids
2.2.2.1 IR and Raman analysis
2.2.2.2 Single-crystal XRD analysis
2.3 Preparation and characterization of the supported oxidic precursors
2.4 Characterization of supported Mo(W)/Al2O3 sulfide catalysts
2.4.1 Transmission electron microscopy (TEM)
2.4.2 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
2.4.3 X-ray photoelectron spectroscopy (XPS)
2.5 Influence of the Mo/(Mo+W) atomic ratio in the active phase on the catalytic properties
2.6 Conclusion
References
Chapter 3
3.Ni-promoted MoWS hydrotreating catalysts supported on alumina
3.1 Introduction
3.2 Synthesis of NiMo(W)/Al2O3 catalysts
3.3 Characterization of sulfided NiMo(W)/Al2O3 catalysts
3.3.1 Transmission electron microscopy (TEM)
3.3.2 X-ray photoelectron spectroscopy (XPS)
3.4 Determination of the composition and structure of the active phase
3.4.1 HAADF characterization of gas phase sulfided Ni(Mo)W/Al2O3 catalysts
3.4.2 EXAFS characterization of gas phase sulfided Ni(Mo)W/Al2O3 catalysts
3.5 Catalytic tests in hydrotreating of model and real feeds
3.5.1 NiMo(W)/Al2O3 catalysts in hydrotreating of DBT and naphthalene
3.5.2 NiMo(W)/Al2O3 catalysts in сo-hydrotreating of DBT, naphthalene and quinoline
3.5.3 Hydrotreating of SRGO
3.6 Conclusions
References
Chapter 4
4.Bulk mixed MoWS hydrotreating catalysts
Introduction
Full Published Article
Supporting Information for Chapter
Chapter 5
5.Mixed MoW catalysts supported on mesostructured silica
5.1 Introduction
5.2 Comparison of the catalytic properties of supported mixed MoW catalysts based on alumina and mesostructured silica (SBA-15 and COK-12).
5.2.1 Supports and catalysts preparation
5.2.2 Characterization of MoW/Sup catalysts
5.2.3 Transmission electron microscopy (TEM)
5.2.4 X-ray photoelectron spectroscopy (XPS) of MoW/Sup catalysts
5.2.5 MoW/Sup catalysts in hydrotreating of mixture of DBT and naphthalene.
5.3 Influence of the Mo/W ratio on the catalytic activity of mixed SBA-15 based catalysts
5.3.1 Preparation and characterization of the Mo(W)/SBA-15 catalysts
5.3.2 Transmission electron microscopy (TEM)
5.3.3 X-ray photoelectron spectroscopy (XPS)
5.3.4 Mo(W)/SBA-15 catalysts in hydrotreating of DBT and naphthalene
5.4 Conclusions
References
General Conclusion
