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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
7.12 Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts W Bensch, Christian-Albrechts-Universita¨t zu Kiel, Kiel, Germany ã 2013 Elsevier Ltd. All rights reserved.
7.12.1 7.12.2 7.12.3 7.12.4 7.12.5 7.12.6 7.12.7 7.12.7.1 7.12.7.2 7.12.8 7.12.9 7.12.10 7.12.11 7.12.12 7.12.12.1 7.12.12.2 7.12.13 7.12.14 7.12.14.1 7.12.14.2 7.12.15 7.12.16 7.12.17 7.12.18 7.12.19 References
7.12.1
General Introduction Sulfur Compounds in Diesel Feeds and Reactivities of These Compounds HDS Reaction Pathways and Mechanism Catalyst Components, Formation and Nature of Active Phases Analytical Tools for the Characterization of the Catalysts Formation of the Sulfide Phases Conversion of MoO3 into MoS2 Conversion of MoO3 into MoS2 Studied with XPS and IRES Temperature-Programmed Sulfidation of MoO3 Quick EXAFS Spectroscopy Investigation of the Local Environments in Ni-Promoted Molybdenum Sulfide Catalysts Sulfidation of Co-Promoted Molybdenum Oxide in the Presence of a Chelating Molecule Investigated with XPS Determination of the State of Co in CoMoS Phase Using In Situ Mo¨ssbauer Emission Spectroscopy The Local Structures of Co/Ni and Mo in CoMoS and NiMoS Phases Determined with EXAFS Sulfidation of NiW Catalysts Supported on Al2O3 x-Ray Absorption Spectroscopy Performed at the W LIII Absorption Edge Ni K Edge x-Ray Absorption Spectroscopy Probing the Active Sites: CO and NO Adsorption Structure of the Phases The Role of Carbon in HDS Catalysts Morphology and Microstructure of Catalysts Choice of Catalysts and Other Catalysts for HDS Poisoning and Deactivation Effects Synthesis Strategies of Catalysts Supported Catalysts and the Support Effect Conclusion
General Introduction
In 2010, about 3914 million tons of crude oil was produced worldwide. Crude oil is a very complex chemical mixture containing hydrocarbons (alkanes, alkenes, cycloalkanes, and aromatics), sulfur compounds (mercaptanes, sulfides, disulfides, thiophenes, and dibenzothiophenes), nitrogen compounds (anilines, aliphatic amines, quinoline, acridine, carbazole, and indole), oxygenates (phenols, ketones, and carboxylic acids), and metal atoms such as V and Ni that are present as porphyrin complexes of V4þ¼O and Ni2þ to mention the most important constituents. The composition of crude oil depends on the location where it is produced with the sulfur level varying between 1 and 6 wt%. Average values in weight% are for carbon 83–87, hydrogen 10–14, nitrogen 0.1–2, oxygen 0.1–1.5, sulfur 0.5–6, and metals <0.1. Different physical and chemical processes such as distillation, extraction, reforming, hydrogenation, cracking, and blending are used in a refinery converting crude oil to higher value products. The main products are liquid petroleum gas,
Comprehensive Inorganic Chemistry II
287 289 289 292 294 295 295 295 297 298 299 300 301 303 303 304 305 308 310 312 315 316 316 317 317 318
gasoline, jet fuel, diesel fuel, wax, lubricants, bitumen, and petrochemicals. The sulfur content increases with increasing boiling point of the different fractions from crude oil. Hydrotreating or hydroprocessing refers to a variety of catalytically promoted chemical reactions such as hydrogenation, which saturate unsaturated hydrocarbons and remove S by hydrodesulfurization (HDS), N by hydrodenitrogenation (HDN), O by hydrodeoxygenation (HDO), and metals by hydrodemetallization (HDM) from different petroleum streams in a refinery. Hydrotreating is mainly applied and is necessary to avoid air pollution emissions, to avoid poisoning of noble metals and acid catalysts used in catalytic reforming and cracking, and to improve fuel quality. The different streams are transformed into products through catalytically promoted chemical reactions such as hydrogenation, isomerization, aromatization, alkylation, cracking, and hydrotreating (Scheme 1). The important products of an oil refinery are diesel fuel and gasoline. About 1 ton of crude oil delivers 0.25 tons of diesel fuel. Due to stringent environmental regulations,1–3 the sulfur content of diesel fuel must be reduced to ultralow levels
http://dx.doi.org/10.1016/B978-0-08-097774-4.00723-3
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Gas concentration
Amine guard
H2 plant
Polybed PSA
From FCC From FCC Light straight run
Crude distillation
Hydrotreating
Light naphtha isomerization
Hydrotreating
Reforming
LPG Propylene
Gasoline
Naphtha Kerosene Kerosene jet fuel
Middle distillate Hydrotreating Desalted crude oil
Atmospheric gas oil
Diesel
Catalytic condensation C4 isomerization
Alkylation
SHP
FCC
Hydrotreating
Cycle oils Hydrotreating Hydrocracking Lube oils
Vacuum distillation
Asphalt Fuel oil Black oil
Solid fuel Refinery adsorbent locations for contaminant removal
Scheme 1 Process flow from crude oil to final products. http://www.uop.com/refining-flowscheme-2/. Abbreviations: FCC, fluid catalytic cracking; PSA, pressure swing adsorption; LPG, liquified petroleum gas.
(ultralow sulfur diesel, ULSD, 10–15 ppm) to decrease harmful exhaust emissions of diesel engines and to improve air quality (see also Chapter 7.19). In addition, there is also a need to strictly reduce the S content because even trace levels of S can poison other catalysts (e.g., Ni, Cu, Pd, or Pt) used in further catalytic processes (hydrocracking, reforming, automotive catalysis, fuel cells, etc.). Hydrotreating is one important catalytic step to convert crude oil streams into diesel fuel (Scheme 1). In the presence of a catalyst and molecular
hydrogen, double bonds of aromatics are hydrogenated and S, N, O, and metals are removed. The catalysts widely used are Mo and/or W-based sulfides promoted with Co and/or Ni (see also Chapter 5.09). The catalysts are supported on g-Al2O3 or are supplied as bulk material. Depending on the distinct process, typical reaction conditions for hydrotreating are 300–400 C and total pressure up to 15 MPa. The actual conditions depend on the boiling point of the feed. The required severe reduction of the sulfur level is a complicated chemical
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
and technical problem. Many different factors such as the catalysts,4–6 process parameters,7 feedstock source and quality,8,9 reactivities of sulfur compounds,10 inhibition effects of H2S,11 nitrogen compounds,12 and aromatics13,14 present in the feed influence the degree of desulfurization of diesel feeds. Thousands of papers and patents were published in the past and state of the art up to 2002 has been reviewed in Refs. [15–17]. The actuality of research in the field of hydrotreating was impressively demonstrated by F. Parlevliet and S. Eijsbouts in 2008: counting only sulfide-based catalysts, since 1986 more than 3700 patents and about 5300 nonpatents were published.18 In 2010, a review was published where the different scientific and technological aspects of ULSD production are thoroughly discussed and 702 references are cited.19 Further reviews and review-like papers present and discuss the state of the art in HDS catalysis at different times of development.15–17,20–36 A short discussion of the development of the knowledge about HDS catalysts is given in the last section of the chapter.
petroleum. The reactivity of BTs and DBTs decreases with increasing molecular geometry (Figure 2). Setting the relative rate for thiophene as 100, benzothiophene has a rate of 50, DBT of 30, 4-methyl-DBT of 5, and dior trisubstituted DBTs of 1 (see Scheme 2).37
7.12.3
HDS Reaction Pathways and Mechanism
During hydrotreating a complicated set of exothermic reactions occurs, and the main reactions are hydrogenation of aromatics, HDS, and HDN. Even for the simple substrates such as thiophene, the HDS networks are complex: a considerable number
S
7.12.2 Sulfur Compounds in Diesel Feeds and Reactivities of These Compounds
Compound
Relative rate
Thiophene
100
Benzothiophene
50
Dibenzothiophene
30
Methyldibenzothiophene
5
S
Diesel feeds from different sources were investigated with different analytic techniques to identify and quantify the Scontaining species. Many different sulfur compounds were identified with the most abundant classes being benzothiophenes (BTs) and dibenzothiophenes (DBTs). The BTs may contain alkyl substituents (1–7 C atoms) and the DBTs such substituents with 1–5 C atoms. Whereas sulfur can be relatively easily removed from BTs, b-DBTs (called refractory Scompounds) with alky groups in position 4 or 4 and 6 (Figure 1) require drastic catalytic conditions. The distribution of sulfur compounds mainly depends on the origin of the
S
S
CH3 Dimethyldibenzothiophene 1 S
CH3
CH3 Trimethyldibenzothiophene 1
CH3
S S
CH3
CH3
H3C
CH3
Scheme 2 Relative rates of different S-containing molecules relative to thiophene.
Figure 1 The molecular structure of 4,6-methyl-DBT.
Reactivity
R
Deep HDS R
R S S
R
R S R
R S R S
R R
R S
Molecular geometry Figure 2 Decrease of the reactivity of different S-containing compounds in diesel feed. Present state of the art and future challenges in the hydrodesulfurization (HDS) of polyaromatic sulfur compounds.32
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
One should note that the phenomena might not necessarily occur in the order shown here (see also Chapter 7.02). The reaction pathways of HDS of thiophene were intensively studied and it was proposed that the reaction proceeds via two parallel pathways (Scheme 3). In one reaction route (bottom path in Scheme 3), the sulfur from thiophene is removed directly (direct desulfurization, DDS) to form butadiene, which is then hydrogenated to give 1-butene and cisand trans-2-butene. In a further reaction, butenes are then hydrogenated to n-butane. In the second pathway (top path in Scheme 3), the first step is hydrogenation of thiophene to tetrahydrothiophene, which is further desulfurized to butadiene (hydrogenation pathway, HYD). Tetrahydrothiophene and butadiene were observed in trace amounts in the reaction products, and it was suggested that these intermediates are very reactive and they are transformed rapidly to butenes and butane.29 Due to this observation, the use of thiophene as the S-containing model compound is of less interest because the different reaction pathways cannot be distinguished. In a simplified picture, the general HDS of BTs and DBTs consists also of two routes: the DDS and the HYD pathway (Figure 3), both in the presence of molecular H2. One should note that both pathways occur in parallel employing different active sites of the catalyst surface. Which reaction pathway predominates depends on the nature of the sulfur compounds, the reaction conditions, and the catalyst used. The DDS pathway produces biphenyl (BP), while in the HYD route first hydrogenation of a benzene ring occurs followed by desulfurization, and in the case of DBT cyclohexylbenzene is obtained.38 In addition to hydrogenation of an aromatic ring before sulfur removal, hydrogenation of an aromatic ring may occur after sulfur was removed with a distinct
of elementary steps such as adsorption/desorption equilibria and hydrogenation and hydrogenolysis reactions, whose kinetics strongly depend on the reaction conditions and on the structure of the particular substrate, occur. The understanding of the fundamental steps is further complicated because results reported by different authors obtained on the same substrate such as thiophene applying similar catalysts and approximately identical reaction conditions are often very different, if not contradictory. Nevertheless, for the HDS reaction some primary phenomena involved can be summarized as follows:
• • • • • • •
adsorption (coordination) of the S-containing compound onto the active site, dissociative adsorption of hydrogen onto the surface of the catalyst, hydrogenation of unsaturated C¼C bonds, cleavage of two C–S bonds sequentially or simultaneously, addition of hydrogen to the broken bond of both S and C, release of the hydrocarbon product from the catalytic site, and release of H2S from the site.
S
S
Scheme 3 The two reaction pathways of HDS of thiophene with the different desulfurized and hydrogenated products.
Benzothiophene
S
S +
H2
+H2 -H2S
-H2S
S
+H2
Dibenzothiophene +H2
S +H2
S
S +H2
-H2S
-H2S
+H2
-H2S
S +H2
+H2
Figure 3 Simplified picture of the two principal pathways during HDS of S-containing molecules.15
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
rate constant for this reaction. This often leads to confusion in interpreting the results of experimental data as both routes can produce cyclohexylbenzene as the final product. The hydrogenation pathways are subject to thermodynamic equilibrium constraints and therefore partially hydrogenated intermediates such as tetrahydrodibenzothiophenes have lower equilibrium concentrations at higher temperatures. Consequently, a maximum of HDS via the hydrogenative route is observed as a function of temperature. Hence, HDS via the HYD pathway becomes limited at low pressures and high temperatures. In a different route, isomerization and transmethylation of methyl groups at the 4- or the 6- position may occur reducing the steric hindrance of the DBT molecule. It is assumed that the direct pathway DDS involves the insertion of a metal atom on the surface of the catalyst into a sulfur–carbon bond in the S-containing compound. After insertion of the metal atom, several reactions occur (Figure 4) leading to the removal of sulfur as H2S and the catalyst is restored to its original state. The formation of BP in the DDS pathway reflects the catalyst’s hydrogenolysis function for direct C–S bond cleavage, and the latter is due to the catalyst’s hydrogenation function to effect ring hydrogenation followed by C–S bond cleavage (see also Chapters 7.16 and 8.09 ).39 The two different desulfurization routes imply the presence of two different active sites in the catalysts. In Figure 5, the schemes of both reactions including details of the active sites are illustrated. The active site is a coordinatively unsaturated metal site where the S ligand is facile. The HL
S M
S I
S
S
S
S
S
S
S
Oxidative addition
H-
H
S
S
[H]
IV
M S
V H+
L +n
L M S
M S
S S
S-ring not opened
S
S
S
S
S
S-ring opened Reductive elimination (endo)
H+ Proton attack(exo) H
Hydride attack (endo or exo)
+n-2
H
L
M
S
S
L
-
S
S
+
M S
M S
M S
S
Proton attack (exo) H
H
H
H
S
M
III
M
L
+n
M
H
L
M
H
L
S
H H
S
S
+n-2
II H
L
S atom in aromatic rings can coordinate to the active center of HYD and DDS functions. There is evidence that in the case of DDS the initial adsorption of the S compound occurs through s-bonding, while for hydrogenation coordination is through p-bonding for both S removal and hydrogenation. The main reason for the low reactivity of b-DBTs seems to be a steric hindrance due to retarded adsorption onto the catalyst surface40,41 implying that adsorption is the rate-limiting step. Two types of chemisorption patterns of 4,6dimethyldibenzothiophene (4,6-DMDBT) on MoS2 are discussed: a flat adsorption and S-m3 type adsorption. The two molecules DBT and 4,6-DMDBT can interact well with the (10 1 0) edge of MoS2 by flat chemisorption. However, the chemisorption of 4,6-DMDBT is more difficult using the S-m3 type coordination due to the steric hindrance of the alkyl groups. This steric hindrance is expected to increase with increasing size of the alkyl groups (from methyl to ethyl to propyl). Consequently, an end-on adsorption mode is proposed involving interactions between the S atom and the active sites. A different explanation is that steric hindrance slows down the surface C–S bond scission.42 In this case, the rate-limiting step is not the Scentered adsorption and a side-on adsorption of the DBT molecule involving the p-electrons of the aromatic ring is proposed. Alkyl substituents in DBTs alter the overall HDS due to reduction of HDS reactivity and due to a change of the ratio between rates of the HYD and the DDS pathway. Alkyl substituents in the 4- and/ or 6-positions result in pronounced preference of the HYD route.7,23,38,40 In a recent study, the reaction sequence of the DDS route was determined as DBT > 4-MDBT > 4,6-DMDBT
L
Hydride attack (endo or exo)
291
S
S
S H+ proton attack
(a)
2,3-Dihydrothiophene
(b)
H2S, butadiene, thiobutene
Figure 4 Two different pathways to hydrogenation and ring-opening reactions leading to the products (a) and (b). Note that the mechanisms were determined with organometallic complexes. L ¼ ligands; M ¼ Co or Ni.
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
S
H
S
H
H
S Mo
S
S
Mo
Mo
S
H
S
S
Mo
S
Mo
S
Mo
Mo
S
S
S Mo
Mo
S
Mo
S
H
S
H
S
Mo
H
H
H
S
H
Mo
Mo
S
H
H
S
Hydrogenation
Simultaneous deS
Sequential deS
H
H
S Mo
S
S
H
H
S
S Mo
S
Mo
S
Mo
S
Mo
H H H S
S Mo
S
S
Mo
S
S
H
H
Mo
Mo
H
H S
S
S
S
Mo
S
Mo
Mo
S S
Mo
S
Mo
−H2S +H2 H S
S Mo
S
S
S
S
Mo
S
H
Mo
Mo
S
Mo
S
Mo
+H2 −H2S = Vacancy H
H
H
S
S Mo
H
S
Mo
S S
Mo
Figure 5 Vacancy model of the HDS mechanism as published in Ref. [32].
being explained by steric hindrance due to the methyl groups located adjacent to the S atom of DBT preventing s-bonding of S to the active site of the catalyst. However, no dependence of reaction rates of HYD pathways on the number of methyl groups was observed indicating that the alkyl groups do not hamper the p-adsorption of DBTs on the active sites.43 Deep desulfurization means to desulfurize the refractory S-containing molecules.
7.12.4 Catalyst Components, Formation and Nature of Active Phases Most industrially used catalysts consist of a support with a high specific surface (e.g., g-Al2O3) and active materials such as
MoS2/WS2 promoted with Co and/or Ni. In nature, MoS2 occurs as the mineral molybdenite in the form of black, lead-silvery gray crystallites or aggregates. In the structure of 2H-Mo(W)S2 (H¼ hexagonal, 2¼ two MS2 slabs in the unit cell, space group P63/mmc), layers consisting of S–Mo–S slabs are held together by weak van der Waals forces (Figure 6). The Mo/W atoms are in a trigonal-prismatic environment of six S atoms and the Mo(W)S6 polyhedra share edges within the slabs. Consecutive layers are stacked in an . . .ABAB. . . fashion along the crystallographic c-axis. The Mo–S bonds (Mo–S: 2.43 A˚) within the layers are covalent and in the trigonal-prismatic ligand field, a twoelectron band of mainly dz2 character is separated from the main part of the Mo 4d band and hybridizes strongly with the S 3p band. The dz2 band is completely filled and is separated by a
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
293
a b c
Figure 6 The crystal structure of 2H-MoS2.
gap of Eg 1.3 eV from the empty eg part of the Mo 4d band. Cleaving MoS2 crystallites between the layers generates a chemically inert (0 0 0 1) surface, while the (1 0 1 0) surface formed at the step edges of the basal planes exposes in alternating S–Mo–S sandwiches rows of coordinatively unsaturated S and Mo atoms, and is considered chemically reactive. In the following discussion, one should distinguish between the terms ‘active site’ and ‘active phase’: the active site is the transition metal ion where the adsorption of the reactant molecule such as a heterocyclic compound or hydrogen occurs through S vacancies or S ions. The active site and the ensemble of transition metal and sulfur ions are the active phase making this site active. The promoted catalysts are often abbreviated as CoMo(W) S/NiMo(W)S or CoMo(W)/NiMo(W), which are not chemical formula. The chemical compositions vary and the optimal promoter content depends on the HDS reaction conditions and the diesel feed. The Mo(W):S ratio is near 1:2, but nonstoichiometry has also been reported. In most applications and test reactions, the Co(Ni)/[Co(Ni) þ Mo(W)] ratio varies between 0.2 and 0.4. It is well documented that the structure and the dispersion of the catalytically active phase are dramatically altered during the life cycle of a typical supported Co–Mo/Ni–Mo catalyst, which is schematically shown in Figure 7. The preparation of the catalysts involves several individual steps: First, a Mo precursor such as (NH4)6Mo7O244H2O (AHM) (15–20 wt%) is impregnated on for example, g-Al2O3 (impregnation route) (see also Chapter 2.10). Then, different methods are used to introduce the promotors: the AHM-containing
support is first calcined at elevated temperatures yielding MoO3/g-Al2O3 followed by impregnation with the Co or Ni precursor (mainly Ni2þ/Co2þ salts such as nitrates) (1–5 wt%). The impregnated composite is calcined again to form (CoO/NiO) MoO3/g-Al2O3. There are reports that MoO3/g-Al2O3 is presulfided at elevated temperatures and the product is then impregnated with the promotor salt. A critical step is the transformation of the oxides into the sulfidic phases, and sulfide formation for one of the metals prior to the other should be avoided. It is well documented that Co is sulfided at lower temperatures than Mo. Hence, often additives such as chelating agents (citric acid, urea, glycol, nitriloacetic acid (NTA), and ethylene diamine tetraacetic acid (EDTA)) are added to increase sulfidization temperature of the promotors thus leading to a better dispersion of promotors. In addition, such additives lower the interaction strength between the catalysts and the support. Reactions between the Mo and Co/Ni precursors and the support should also be avoided and therefore the temperature of the sulfidization step should not be too high. Two strategies are followed to sulfidize the oxides: in the ex situ approach, the material is treated at elevated temperatures in, for example, 10% H2S/H2, 10% H2S/He, or 10% H2S/N2 flow; in the in situ method, the oxides are transferred to the HDS reaction chamber and sulfidization occurs in the presence of S-containing molecules. No general conclusion can be drawn whether the in situ route generates better catalysts than the ex situ method, and vice versa. In the as-prepared state, oxidic Co–Mo/Ni–Mo catalysts display a high Mo dispersion with well-dispersed MoO3 or polymolybdate phases and well-dispersed CoO/NiO is located near Mo particles.44–47 In some investigations, the presence of
294
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
a
b
MoO3
MoOxSy c MoS2
NiOxSy
h
NiSx
NiO
MoOxSy
g
i
j
MoS2 decorated with NiSx
d
MoS2 decorated with NiSx
NiSx MoS2 decorated with NiSx e f
Ni−Mo−NTA complex k Figure 7 Life cycle of a HDS catalyst: (a) Oxidic Mo and Ni/Co are bound to Al2O3; (b) partially sulfided catalysts, Ni/Co are sulfided, Mo oxysulfide is still bound to the support; (c) partially sulfided catalyst and part of Mo Mo is no longer bound to Al2O3; (d) partially sulfided catalyst consisting of small MoS2 slabs decorated with promoter Ni/Co; slabs are still anchored to the support; (e) fully sulfided catalyst with Mo being completely sulfided, MoS2 slabs decorated with sulfidic Ni/Co become mobile; (f) deactivation due to sintering of decorated small MoS2 slabs to form larger slabs, stacks, and crystals, edge surface area of the crystals is too low to accommodate all Ni/Co and separate Ni/Co sulfide crystals are formed; (g) regeneration – MoS2 crystals and some decorating Ni/Co sulfide get partly redispersed; (h) regeneration – large MoS2 crystals are only oxidized on the surface, Ni/Co sulfide crystals cannot redisperse on oxidation; (i) regeneration – small structures are fully oxidized, nearly the original state is reached; (j) high-temperature sulfidation with Mo being completely sulfided so fast that intermediate steps can be neglected; (k) NTA preparation route – Ni–Mo–NTA complex gets completely sulfided, and no linkages with Al2O3 are formed at any stage of the process.31
finely dispersed compounds such as CoMoO4/NiMoO4 was also observed.48,49 It is assumed that Mo is bound to the support via Mo–O–Al bridges (see also below).50–58 Several crystalline compounds such as MoO3, CoO, NiO, Al2(MoO4)3, CoAl2O4, NiAl2O4, CoMoO4, and NiMoO4 could be detected in x-ray powder patterns but mainly only in very small amounts if at all.
7.12.5 Analytical Tools for the Characterization of the Catalysts A large variety of analytic methods are used to characterize the catalysts in different states starting from the as-prepared precursors to catalysts recovered after intense use. In the following, some techniques are mentioned and some examples are discussed in detail demonstrating the kind of information that can be extracted by applying a distinct analytic method and combinations of methods. Chemisorption techniques are applied to receive information on the active phase dispersion and on the type of sites present on the catalyst surface. These methods include dynamic oxygen chemisorption (DOC),59,60 CO chemisorption (COC),61–69 and NO chemisorption (NOC).70–88
Temperature-programmed techniques such as temperatureprogrammed sulfidation (TPS),89–95 temperature-programmed reduction (TPR),96–103 and temperature-programmed oxidation (TPO)104–107 are suitable methods to understand the changes occurring during sulfidation of the catalyst and regeneration. Temperature-programmed desorption (TPD) or temperature-programmed surface reaction (TPSR) are sometimes applied.90,93 In a TPD experiment, a spent catalyst is heated and reaction intermediates and products desorbing from the surface are detected as a function of temperature. x-Ray photoelectron spectroscopy (XPS)110–115 was also often used for characterization of catalysts in different states. It yields information on the elemental composition and the oxidation state of elements. XPS is a surface-sensitive technique and, with this method, it is possible to determine how well particles are dispersed over a support.43,51,116–130 Extended x-ray absorption fine structure (EXAFS) spectroscopy and/or x-ray absorption near-edge spectroscopy (XANES) are powerful methods to probe the local environment of atoms and to estimate the oxidation state of the atoms. Hence, detailed information on the distance, number, and type of neighbors of the absorbing atom on a subnanometer length scale can be acquired.131–136
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
SIMS, Rutherford backscattering spectroscopy (RBS), infrared (IR) and transmission electron microscopy (TEM),64,137–140 Mo¨ssbauer emission spectroscopy,141–143 and in situ laser Raman spectroscopy (LRS)138,144,145 were also applied to characterize catalysts in different states.
7.12.6
Formation of the Sulfide Phases
The finely dispersed oxides MoO3/polyoxomolybdates and CoO/ NiO are transformed during the sulfidation,99,104,105,137,146–150 and a direct interaction between sulfidic MoS2 and Co/Ni is generated.151–156 If Mo oxide is fully sulfided, the catalyst consists of well-dispersed MoS2 nanoparticles, and sulfidic Co/Ni decorate the edges of the MoS2 slabs. The formation of large MoS2 and/or Co9S8 (Ni3S2, NiS) crystals is in most cases not observed.141,150,157–162 In cases where well crystalline compounds such as Al2(MoO4)3, CoAl2O4 (NiAl2O4), CoMoO4 (NiMoO4), CoO (NiO), and MoO3 are formed during calcinations, a full sulfidation of these compounds is often not achieved57,98,99,163 applying typical laboratory conditions such as gas phase, 673 K, 0.1 MPa, 10% H2S/H2, or commercial (e.g., liquid phase, þ/603 K, þ/3.0 MPa, light oil spiked with S compounds such as, dimethyl disulfide)164–167 sulfidation conditions. As outlined above, the different individual steps of the transformation of the oxidic precursors into the final sulfided catalysts were intensively studied with a variety of analytical tools. In the following, the results of fundamental studies on the sulfidation of oxidic precursors are discussed for selected examples.
7.12.7
Conversion of MoO3 into MoS2
Formally, the conversion of Mo oxide into the sulfide is a simple reaction according to eqn [1]:
Mo 3d
300400 C
MoO3 ! MoS2 H2 S=H2
7.12.7.1 Conversion of MoO3 into MoS2 Studied with XPS and IRES To reduce the complexity of the system, the sulfidation of crystalline MoO3 was investigated with XPS and infrared emission spectroscopy (IRES).168 Such an investigation allows comparison of elementary reaction steps with known structural features of Mo–O/Mo–S compounds. XPS is a surface-sensitive spectroscopic technique, being sensitive to changes of the oxidation state of the elements involved in the chemical reaction (see also Chapter 9.16) and IRES supplies information on the changes in Mo–O coordination during successive exchange of O atoms by S (see also Chapter 9.36). During conversion of MoO3 into MoS2, intermediate Mo oxysulfides are formed and therefore the thermal decomposition of (NH4)2MoO2S2 as a model compound was studied which decomposes according to eqn [2] and forms an {MoOS2} oxysulfide169 on the way to MoS2: ðNH4 Þ2 MoO2 S2 ! fMoOS2 g þ 2NH3 þ H2 O fMoOS2 g ! 0MoS2 0
[2]
A sequence of XPS spectra of MoO3 after sulfidation in H2S/ H2 at different temperatures is shown in Figure 8, left. The first spectrum of crystalline MoO3 exhibits a single Mo 3d doublet and the binding energy (Ebind) of 232.9 eV for the Mo 3d5/2
S 2s
S 2p
300 C 250 C 200 C 150 C 100 C 50 C 25 C
237 233 229 225 Binding energy (eV)
[1]
However, the structure of the nanocrystalline respectively amorphous oxidic catalyst precursor deposited on a support such as g-Al2O3 is not known and one can imagine a large variety of local Mo–O coordination environments generated in an unpredictable way during the preparation of the catalyst precursor. Hence, the investigation of real catalysts is a challenge requiring a combination of the results of different advanced analytical methods.
400 C
241
295
MoO3 221 168
164 162 160 166 Binding energy (eV)
158
Figure 8 Evolution of Mo 3d (left) and S 2p (right) x-ray photoelectron spectra of crystalline MoO3 with increasing sulfidation temperature.168
296
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
S O VI Mo
+ H2S
S H
H
O H
H
H
H
O
O
S
VI Mo
VI Mo
VI Mo
Scheme 4 Attack on the terminal O atom by H2S to form intermediate species and release of H2O.
line is in accordance with Mo in formal oxidation state 6þ. Sulfidation at low temperatures leads to pronounced changes in the spectra and successively further signals appear in the spectra indicating the formation of different intermediate species. Sulfidation seems to be finished between 250 and 400 C and the spectrum of the sample treated at the highest temperature shows essentially one doublet and the binding energy of the Mo 3d5/2 peak of 229.0 eV corresponds to Mo4þ in MoS2.170 In the corresponding S 2p spectrum (Figure 8, right), there is only one doublet with Ebind ¼ 161.8 eV for the S 2p3/2 core level in agreement with the sulfide S2 anion present in MoS2. For different temperatures between 25 and 300 C, all the Mo 3d spectra can be explained on the basis of mixtures of Mo6þ and Mo4þ doublets and an additional doublet with a Mo 3d5/2 binding energy of 231.1 eV, which is already observed after sulfidation at room temperature (RT). A tentative assignment was done to Mo5þ present in oxysulfide intermediate phases. All spectra contain Mo 3d signals in the partially sulfided state from these three states in varying percentages giving evidence for the stepwise transition of Mo6þ in MoO3 to Mo4þ in MoS2. The S 2p peak in Figure 8, right, can be explained as the superposition of two doublets with Ebind ¼ 161.7 and 163.1 eV for the S 2p3/2 peaks. The signal located at 163.1 eV can be assigned to a bridging S22 disulfide anion,138,139 while the peak at 161.8 eV points to terminal disulfide and/or sulfide anions. According to the XPS results, one of the first elementary steps during sulfidation involves a metal–ligand redox reaction leading to the formation of Mo5þ and oxidized sulfur ligands (S22). Secondary ion mass spectrometry (SIMS) measurements gave evidence that the sulfidation of MoO3 leads to the formation of oxysulfides at the surface and Mo(IV) oxides in the interior of the crystallites at sulfiding temperatures between RT and 100 C, while MoS2 is formed above 125 C. For sulfur, two different chemical states are found and the most likely species are S2 and S22 or SH, but the presence of elemental S can be excluded. The S22 or SH species disappear when the temperature is raised to 150–200 C. Rutherford backscattering data yield S:Mo atomic ratios of 1–1.5 for T < 100 C and of 2–2.5 above 100 C.137 Combining the results obtained on sulfidation of MoO3 and the model compound (NH4)2MoO2S2 yields the following pictures of individual reaction steps for the two different temperature regimes RT to 200 C (generation of Mo oxysulfide) and 200–400 C (transformation of Mo oxysulfide into MoS2). The terminal Mo¼Ot groups are relatively unstable and play an important role in the uptake of sulfur. This instability is well known from solution chemistry where dissolved molybdate species react with H2S in NH4OH to form tetrathiomolybdate: MoO42 þ 4H2S ! MoS42 þ 4H2O. In solution, the reaction may proceed via H2S or SH formed in situ by deprotonation of H2S, which depends on NH4OH concentration. For the
O
S
O VI
VI
Mo
O
+ H2S - H2O
Mo
Mo
VI
S
V
VI
O
Mo
S
S
Mo
VI
O O
O S
S VI
Mo
Mo
O
O
V
Mo
O
Scheme 5 Replacement of two terminal O atoms by two S atoms followed by oxidation of two S2 to S2 2 and reduction of Mo6þ to Mo5þ.
O
O VI
Mo
+ 2H2S − 2H2O
S
S
S
S
VI
IV
Mo
Mo
Scheme 6 Exchange and subsequent redox reaction of edge metal centers involving two terminal O2 atoms.
MoVI
Ot + H2
MoIV + H2O
Scheme 7 Direct reaction between terminal O atom and H2 and reduction of Mo6þ to Mo4þ.
sulfidation of solid MoO3, a deprotonation of H2S is not possible and H2S is the reactive component. Hence, the terminal O atom is attacked by H2S to form intermediate species (Scheme 4) and H2O is released, which is the driving force of the reaction. Note that in this step the oxidation number of Mo is not changed. In the next reaction step (Scheme 5), Mo6þ is reduced to Mo5þ by a metal–ligand redox reaction in which two S2 anions bonded to adjacent Mo6þ centers are oxidized to S22: 2S22 ! S22 þ 2e and the Mo centers are reduced, 2Mo6þ þ 2e ! 2Mo5þ. The proposed reaction scheme is clearly supported by the XPS data and is in agreement with the IRES showing characteristic emission from disulfide species. In the low-temperature regime, the occurrence of Mo4þ in the XPS spectra may be explained by the reaction in Scheme 6, indicating a subsequent redox reaction with two terminal O22 anions. In principle, a direct reduction may occur according to Scheme 7 but as was demonstrated by TPS measurements,97 this type of reaction is more relevant at higher temperatures. Despite the S/Mo ratio in the XPS spectra measured after sulfidation at 50 and 100 C exhibiting very similar values, the relative amount of the different sulfur ligands changed. The fraction of S2 and/or terminal S22 anions increased because of bridging S22 ligands. In parallel, the proportion of Mo5þ increased, as opposed to the contribution of bridging S2 anions. A possible explanation is the release of S from S22 bridges according to Scheme 8.
297
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
H S S
V Mo
O
H
S V Mo
+ H2
S
V Mo
V Mo
O
− H2S
O O S
V Mo
V Mo
O O
–
entity with H2 forming two transient SH fragments, stabilization by H transfer (H2S complex), and subsequent emission
First, S22 reacts with H2 yielding two transient SH units. Such SH units must be kinetically stabilized by, for example, bulky ligands and sufficient electron density on Mo, which is highly unlikely. However, stabilization occurs easily by proton transfer to form H2S, which then immediately decomposes. After the emission of S from S22 units, the sterical hindrance on the surface of the crystal is reduced so that new S22 groups can be formed, leading to a higher S/Mo ratio in the XPS spectrum of the sample sulfided at 150 C. In the IRES, the low-intensity Mo¼Ot vibration indicates that most of the available Mo¼Ot moieties have reacted at 150 C. At 200 C sulfidation temperature, the analysis of the surface composition using the XPS spectrum demonstrates an increased degree of sulfidation. In the temperature range 200–400 C, a major change of composition is observed compared to that of the oxysulfides detected at lower temperatures. In this region, Mo6þ and Mo5þ species are mainly reduced to Mo4þ and the signal of bridging S22 in the XPS spectrum is absent. The formation of Mo4þ is mainly due to the reaction of Mo¼Ot with H2 (Scheme 8) followed by S uptake from the sulfiding atmosphere. The finally formed MoS2 contains imperfections in the sulfur planes and the IRE spectrum gives evidence that m2-S atoms are present on Mo–S–Mo bridges. Such defects are accompanied with a S deficiency and leads to a bending of the MoS2 layers (Scheme 9). Indeed, bent MoS2 nanocrystallites are often seen in TEM images (see below). It is well known that the formation of perfect MoS2 requires temperatures of about 800 C. An important conclusion of the study performed on MoO3 is that sulfidization of MoO3 at about 400 C, typically for catalyst pretreatments, generates MoS2 exhibiting an appreciable amount of defects.
7.12.7.2
Temperature-Programmed Sulfidation of MoO3
The sulfidation of the MoO3 model compound in a H2S/H2/Ar atmosphere as a function of temperature was studied with TPS,97,98 supporting the findings presented above. In the TPS pattern of MoO3 supported on Al2O3, negative peaks indicate that the corresponding gas is consumed, and positive peaks mean that the gas is released. As can be seen in Figure 9, at 400 K H2S is consumed and H2O is produced and no uptake of hydrogen can be detected. The observations suggest that at low temperatures an exchange of O by S atoms is the main reaction:
S
S
S Mo
Mo
Mo
Mo
Mo
S
S
S
S
S Mo
S
S
S Mo
S
Mo S
Scheme 9 A layer of MoS2 containing a m-S atom as defect.
TPS of MoO3/Al2O3
H2
Partial pressure (a.u.)
Scheme 8 Reaction of a S2 of H2S.
2
H2S
H 2O
400
600 800 1000 Temperature (K)
1200
Figure 9 TPS pattern of MoO3 supported on Al2O3 pretreated in Ar at 298 K.98
MoO3 þ H2 S ! MoO2 S þ H2 O Increasing the temperature to T 500 K, a sharp hydrogen consumption peak is seen while at the same temperature H2S and small amounts of H2O are produced. These observations may be interpreted as hydrogenation of excess sulfur formed by the decomposition of MoO2S according to the equations: MoO2 S ! MoO2 þ S S þ H2 ! H2 S
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Up to the final temperature, further oxygen is replaced by sulfur until MoS2 is formed: MoO2 þ 2H2 S ! MoS2 þ 2H2 O One should keep in mind that TP experiments detect only reactions accompanied by a net production or consumption of gases. If the following two reactions would occur instantaneously, no hydrogen signal would be detected: MoO2 þ 2H2 ! Mo þ 2H2 O Mo þ 2H2 S ! MoS2 þ 2H2 In any case, TPS is a powerful method to study the sulfidation of the catalysts, giving useful information about the stepwise conversion of the oxidic precursors into the final sulfide.
7.12.8 Quick EXAFS Spectroscopy Investigation of the Local Environments in Ni-Promoted Molybdenum Sulfide Catalysts
QEXAFS Mo K-edge of NiMo/SiO2
Fourier transform of k3 c(k)
Tsulf (C) 400 400 400 400 380 365 345 330 315 295 280 260 245 225 210 195 170 155 140 125 110 90 70 55 40 Fresh
0
1
2 r (Å)
3
5 a.u.
400 390 370 350 330 315 295 280 260 245 225 210 195 175 160 140 125 105 90 70 50 35 Fresh
|FT[c(k)k3]| (a.u.)
The changes of the local environment around Mo of a NiMo catalyst deposited on high surface area silica during sulfidation were investigated with a temperature-resolved Quick EXAFS (QEXAFS) study (Figure 10).171 The sample was heated with a heating rate of 6 C min1 in 10%H2S in H2 atmosphere. The resulting Fourier transform of the Mo K-edge EXAFS spectra is displayed in Figure 10.171 The sulfidation process can be divided into four regions. Up to about 110 C, Mo is present in the oxidic form; that is, Mo is mainly surrounded by O atoms as seen by Mo–O shells between 0.5 and 1.9 A˚ and a Mo–Mo shell at 3 A˚ (values not phase-corrected). Between about 110 and 225 C, the longest Mo–O shell and the Mo–Mo shell disappeared, whereas the shorter Mo–O can still be identified. A new signal appears at about 2 A˚ (not phase-corrected) caused by the presence of S
indicating the co-existence of Mo–O and Mo–S shells, with the Mo–O shell corresponding to Mo¼Ot groups. Near the 225 C region, a new signal at 2.5 A˚ (not phase-corrected) develops belonging to an intermediate product. The signal at about 2.5 A˚ dominates the temperature range between 225 and 280 C and in this region only contributions of the Mo–S and the Mo–Mo shells are seen attributed to sulfur-rich phases such as MoS3, which was also reported in other EXAFS studies.172 Above about 310 C, the spectrum of the final product MoS2 starts to form (region IV) and no significant changes are seen up to the final temperature of 400 C. The shoulder on the left side of the Mo–Mo shell at about 3 A˚ is caused by the interference of the Mo–S and the Mo–Mo shells. The results are in accordance with those obtained from XPS, RBS, and SIMS137,173 and slight differences of temperatures at which the changes occur are due to the different experimental setups of the methods. The intermediates formed during sulfidation of the Mo precursor are those discussed above. Due to the low loading of Ni of only about 1.3 wt%, EXAFS data were collected with a lower heating rate of 3 C min1. Fourier transform spectra (Figure 11) of the sulfidation of the fresh catalyst show Ni–O and Ni–Si shells at 1.65 and 2.9 A˚ (not phase-corrected), respectively, while the Ni–Si shell disappeared at 105 C. The transformation of the Ni oxide to the
Region IV MoS2 Region III MoS3-like Region II oxysulfides Region I oxidic
Surfidation temperature (C)
298
4
Figure 10 Fourier transforms of the Mo K-edge Quick EXAFS functions obtained during sulfidation of NiMo/SiO2. The four sulfidation regions are shown.171
0
1
2
3
4
r (Å)
Figure 11 Fourier transforms of the Ni K-edge Quick EXAFS functions measured during the sulfidation of NiMo/SiO2.171
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Ni sulfide is seen in the spectra by a shift of the signal by about 0.1 A˚ in the distance of the first shell. Definitive sulfidation occurs between 125 and 140 C and no further alteration can be detected in the spectra up to 400 C. That means full sulfidation is achieved for Ni at a significantly lower temperature than for the Mo oxide.
7.12.9 Sulfidation of Co-Promoted Molybdenum Oxide in the Presence of a Chelating Molecule Investigated with XPS The situation is similar for CoMoS catalysts: Co is sulfided at low temperatures and stable bulk sulfides such as Co9S8 are formed as large crystals while sulfidation of Mo only starts at moderate temperatures. Hence, it is surprising that a phase is formed in which Co atoms decorate the edges of MoS2 slabs and not a mixture composed of separated Co9S8 and MoS2. To retard sulfidation of Co or Ni, chelating agents such as NTA, EDTA, ethylenediamine (EN), or 1,2-cyclohexanediamineN,N,N,N-tetraacetic acid (CyDTA) are added during catalyst preparation,171,174–178 and the as-prepared precursor mixture is not calcined in order to leave the complexes formed with the ligands intact. The effect of NTA addition on the formation of the sulfides was studied with XPS on a CoMoS model catalyst deposited on SiO2. In the absence of NTA, Co is transformed into Co9S8 at very low temperatures (Figure 12, left). Even at RT, sulfidation of Co in 10% H2S/H2 atmosphere starts as can be seen by the appearance of a second doublet at 779.3 eV corresponding to the binding energy of bulk Co9S8. Increasing the temperature leads to disappearance of the doublet at 782.3 eV and only the signal at 779.3 eV remains, which means that cobalt has completely transformed into the sulfidic state. The addition of NTA retards sulfidation of Co and the
transformation to the sulfidic state starts above about 150 C and is completed at 225 C. Below 150 C, the typical signal of the oxide is seen and at higher temperatures the doublet of sulfidic cobalt appears (Figure 12, right). Recording the evolution of the N 1s signal as a function of sulfidation temperature demonstrates that the intensity starts to decrease and the signal is absent when sulfidation of cobalt is complete. These observations suggest that NTA is decomposed and Co ions are available for sulfidation. Sulfidation of Mo oxide calcined at 500 C in the absence of NTA (Figure 13) starts at a higher temperature than the formation of Co sulfide. In the as-prepared state, the binding energy of the Mo 3d5/2 core level matches well with Mo6þ. On increasing the temperature, MoO3 is successively transformed into MoS2 as discussed above and the typical spectrum of MoS2 is seen with a binding energy of 228.8 eV for the Mo 3d5/2 signal. The Co 2p3/2 and Mo 3d XPS spectra of a CoMo catalyst with an atomic ratio of 0.3 calcined at 500 C are shown in Figure 14, right. In the unsulfided state, the binding energy of the Mo 3d5/2 signal at 232.7 eV is typical for oxidic Mo in oxidation state 6þ. After sulfidation at T > 175 C, a Mo 3d doublet appears in the spectra, typical for MoS2. The results clearly show that sulfidation of Mo oxide occurs between T 50 and 175 C and several intermediates are formed containing Mo6þ, Mo5þ, and Mo4þ species. At about 150 C, the doublet of Mo4þ appears in the spectrum and at T > 175 C the signal of Mo6þ disappears. The sulfidation behavior of Co is shown in Figure 14. In the as-prepared state, a Co 2p doublet is seen at 781.8 eV with shake-up satellites at higher binding energy typical for Co2þ oxide. On increasing the temperature above 175 C, the spectrum shows a Co 2p doublet at a binding energy of 778.7 eV typical for Co2þ in a sulfidic environment. Shake-up satellites occurring at higher binding energy are less pronounced Sulfidation temperature
Sulfidation temperature
Co 2p
299
Co 2p
400 C 300 C
400 C
225 C
300 C 200 C
200 C
150 C
150 C
100 C
100 C
50 C Unsulfided
25 C Unsulfided Shake up Co 2p1/2
Shake up Co 2p 3/2
814 809 804 799 794 789 784 779 774 810 805 800 795 790 785 780 775 770 Binding energy (eV) Binding energy (eV) Figure 12 Co 2p XPS spectra of CoOx/SiO2 model catalysts (left) and Co 2p XPS spectra of Co(NTA)/SiO2 model catalysts as a function of sulfidation temperature.173
300
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Sulfidation temperature
compared to oxidic Co. According to the XPS spectra recorded over temperature, the sulfidation of Co starts at T > 50 C and is completed around T ¼ 150 C. Summarizing the results, in a mixed CoMo/SiO2 catalyst Co sulfide is formed between 50 and 150 C and Mo is sulfided between 100 and 200 C. Both regions overlap to an appreciable extent, but sulfidation of Co proceeds faster than that of molybdenum.
Mo 3d
400 C 300 C 250 C
7.12.10 Determination of the State of Co in CoMoS Phase Using In Situ Mo¨ssbauer Emission Spectroscopy
200 C 175 C
150 C 100 C 50 C 25 C Mo 3d3/2
Unsulfided 240
238
236
Mo 3d5/2
234 232 230 228 Binding energy (eV)
226
224
222
Figure 13 Evolution of the Mo 3d XPS spectra of MoO3/SiO2 model catalysts with increasing sulfidation temperature.173
It is well documented that Co-promoted MoS2 catalysts are in general much more active for HDS than unpromoted MoS2. Hence, the key question is to determine the chemical state of Co and the structural environment in sulfided CoMoS catalysts supported on Al2O3. Mo¨ssbauer spectroscopy probes the local environment of the absorber as well as the chemical state simultaneously. Because Co is not a suitable Mo¨ssbauer nucleus the experiments were performed in the emission mode, that is, the measured spectrum is not that of Co but of the decay product Fe. Radioactive 57Co was used as a promoter during catalyst preparation and the 57CoMoS catalyst was investigated under in situ conditions so that the Co spectrum and the changes could be directly correlated to the activity of the catalyst. The Mo¨ssbauer spectra of the CoMoS catalyst are compared with that of Co model compounds for which the state and the environment of Co are known.141,179 The analysis of the Mo¨ssbauer spectra of the sulfided CoMoS/Al2O3
Mo 3d
Sulfidation temperature
Sulfidation temperature
Co 2p
400 C 400 C
300 C
300 C 250 C
250 C
200 C
200 C
175 C
175 C 150 C
150 C
100 C 100 C
50 C
50 C
25 C
25 C Unsulfided Unsulfided
240 (a)
238
236
234
232
230
228
Binding energy (eV)
226
224
222
814 (b)
809
804
799
794
789
784
779
774
Binding energy (eV)
Figure 14 The Mo 3d (a) and Co 2p (b) XPS spectra of a CoOx/MoO3/SiO2 catalyst calcined at 500 C during sulfidation at different temperatures.173
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Co:Al2O3
Co Mo
Co9S8
40
Co-Mo-S Co9S8 (a)
0.27
(b)
30
mg Co/g Al2O3
0.09
20
Co–Mo–s
10
Absorption
301
(c)
0.53
Co:Al2O3
(a)
140 120
kT (cm3 min-1 g-1)
(d)
0.75
(e)
1.19
100 80 60 40 20
(b)
-6
-4
2 0 -2 Velocity (mm s-1)
4
6
Figure 15 In situ Mo¨ssbauer emission spectra of 57Co in sulfided CoMo/Al2O3 catalysts with different Co:Mo ratios.141
catalysts (Figure 15) essentially revealed contributions from Co ions located in the Al2O3 support, bulk Co9S8 dominating in catalysts with a high Co content, and a new formerly unknown state, which is labeled in Figure 15. This new state is best seen in sulfided CoMoS with a low Co content. The presence of this new phase correlates with the catalytic activity for the HDS reaction (Figure 16). Hence, one may conclude that this CoMoS phase is itself catalytically active or is closely related to the active site of the catalyst. This is because the catalytic activity of MoS2 is associated with edges and it is highly likely that the CoMoS phase is located at or near the edges. This assumption was verified with Auger spectroscopy and transmission electron microscopy clearly demonstrating that the basal planes of MoS2 do not contain measurable/detectable amounts of Co while the edges of the crystallites contain Co besides S and small amounts of O.180,181 In further MES experiments, it was demonstrated that Mo is not required to obtain the ‘Co–Mo–S’ MES spectrum. For Co/ Al2O3, Co–Mo/AI2O3, Co/C, and Co–Mo/C, Co sulfide species exhibiting the ‘Co–Mo–S’ type features such as quadrupole splitting (QS) were observed (Figure 17). Furthermore, the temperature dependence of the MES spectra of the CoMoS phase is identical with that of the Co sulfide phase present in Co–Mo/AI2O3 and Co/C catalysts.183 The important conclusion of these observations is that the
0.2
0.4
0.6
0.8 Co Mo
1.0
1.2
1.4
Figure 16 Correlation between the activity of CoMo/Al2O3 catalysts for the HDS reaction, expressed in the reaction rate constant kT, and the cobalt phases observed in Mo¨ssbauer spectra.141
Co sulfide species are essentially the same, and only the particle sizes and/or ordering may differ.157,183–185 Other important findings of the MES experiments are that the value of QS of the Co phase depends on the Co content in the catalysts and on the sulfidation temperature with large QS values indicating the presence of very small Co sulfide units and small values are related to larger well-ordered Co sulfide particles.
7.12.11 The Local Structures of Co/Ni and Mo in CoMoS and NiMoS Phases Determined with EXAFS The local environment of Mo and the promoter atoms were investigated with EXAFS/XANES in different states of the catalysts under in situ conditions allowing in situ sample presulfiding151,153,154,156,186–189 in a mixture of e.g., H2S/H2 at atmospheric pressure and at 673 K. Because industrial CoMoS catalysts are supported on Al2O3, such catalysts contain at least two Co species such as Co in the Al2O3 lattice and in the CoMoS phase. Hence, the CoMoS catalysts are deposited on carbon for the EXAFS investigations. The radial distance distribution (Fourier transform) of MoS2, sulfided Mo, and a CoMoS catalyst supported on carbon is shown in Figure 18.153 Evaluation of the data of MoS2 yields
302
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Co(2.25)Mo(6.84)/C
Co(2.45)/C 5.18
(S, 373 K)
4.86 3.06
5.11 2.83
(S, 473 K)
2.77 3.68
2.99 3.27
(S, 573 K)
3.58
3.20 3.27
3.64
(S, 673 K)
3.20
-4
-2
3.54
T = 300 K
T = 300 K -6
0
2
-6
4
Intensity (106 counts)
Intensity (106 counts)
4.94
Doppler velocity (mm. s–1)
-4
-2
0
2
4
Doppler velocity (mm. s–1)
Figure 17 MES spectra of carbon supported Co and CoMo catalysts measured at 300 K after various sulfidation treatments.182
30
Mo-Mo (3.18 Å)
10
MoS2 Mo/C Co–Mo/C
Co-S + Co-Co (1)
Co9S8 Co/C Co-Mo/C
Abs (FT)
Abs (FT)
Mo-s (2.41 Å)
15 Mo-Mo (6.32, 6.41 Å)
Co-Co (2)
Mo-S Mo-Mo (3.98 Å) (5.47 Å)
0 0
2
4
6
8
r (Å) Figure 18 Magnitude of the Fourier transforms of the MoS2 reference compound (solid line), the Mo/C catalyst (dotted line), and the Co–Mo/C catalyst (dashed line).153
a coordination number (CN) of 6 for Mo with the S atoms at a distance of 2.41 A˚. The next-nearest neighbors are six Mo atoms at a distance of 3.16 A˚. The reduced intensity of the first and the second peak in the spectrum of the two catalysts is an indication for a reduced number of neighbors; especially for the Mo–Mo coordination the number is lower than 6. To explain these observations, a structural model was developed taking into account the reduced CNs in the catalysts, leading to the average crystallite size for MoS2 of 1.5–3 nm. Such small particle crystallites will expose a relatively large number of catalytically active edge sites. The absolute part of the Fourier transform (FT) spectrum of Co9S8, of Co/C and Co–Mo/C samples, is displayed in Figure 19. In the crystal structure of Co9S8, three different Co–S coordinations are present, which are denoted in the spectrum as one overall Co–S coordination. The two Co–Co distances at about 3.5 A˚ are given as one overall Co–Co(2) coordination (indexed (2) to distinguish these from the first Co–Co coordination at 2.505 A˚). The first peak in the spectrum of
0 0
2
4 r (Å)
6
8
Figure 19 Fourier transforms of Co9S8 and the sulfided Co/C, Co–Mo/ C, and Co–Mo–S/C catalysts.131
Co9S8 is caused by a combined Co–S and Co–Co(1) coordination and the second peak is attributed only to the Co– Co(2) distance. In the spectra of Co/C and Co–Mo/C samples, the first peak is slightly shifted to lower r values. Overall, the two sulfided catalysts show identical features as Co9S8 but with reduced amplitudes. Despite the overlap of the Co–Co/Co–S nearest neighbors in the magnitude of the Fourier transform, a clear differentiation between these two contributions is seen in the imaginary part of the spectrum (Figure 20). In addition, a scattering contribution from Mo neighbors can also be identified. The detailed analysis of the spectra yields a Co–S distance of 2.2 A˚ with a high sulfur CN of 6.2(1.3). The Co atom is further surrounded by 1.7(0.35) Mo neighbors at a distance of 2.8 A˚. Using these geometric parameters, a model was developed fitting with a structure where Co is located on the edge of a MoS2 crystallite in the same plane as the Mo atoms. It was proposed that Co is coordinated to four S atoms on the edge, which are slightly displaced from their bulk positions. According to this model, Co is not exactly on the position of a
303
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
10
673 K
II
Co-Co (2)
Im (FT)
W−S
15
Co9S8 Co/C Co–Mo/C Co–Mo–S/C
|FT [c(k)k3]| (a.u.)
Co-S Co-Co (1)
0
573 K 10
473 K 5 W-O I 373 K 300 K 0 0
-10
b 0
1
2 r (Å)
3
4
Figure 20 Imaginary part of the Fourier transforms of Co9S8, sulfided Co/C, Co–Mo/C, and Co–Mo–S/C catalysts.131
Mo atom of a bulk crystal, which would give a Co–Mo distance of 3.16 A˚, but is shifted toward the MoS2 particle yielding the experimentally determined Co–Mo separation of 2.8 A˚. The S CN of 6 is achieved by placing two S atoms on Co to form a distorted trigonal prism together with the four S atoms of the MoS2 edge. A slightly different model was proposed on the basis of density functional theory (DFT) calculations indicating that some reconstruction may play a role but the model is also in agreement with the EXAFS data.
7.12.12 Sulfidation of NiW Catalysts Supported on Al2O3 The extent of sulfidation of W is crucial for catalytic performance.190–192 It is well documented that the sulfidation of W in NiW/Al2O3 occurs at a lower rate than that of Mo of Al2O3supported CoMo or NiMo catalysts. It was pointed out that the relatively strong W–O–Al bond94 or the stronger W–O bond compared to the Mo–O bond are responsible for this behavior and hence higher temperatures are required to fully transform W oxide to W sulfide.193–195 It is well documented that the final degree of W sulfidation strongly depends on the calcination and the sulfidation temperatures.191,192,196 At the working temperature of 673 K, Al2O3-supported NiW catalysts are only partially converted to WS2 and other species such as WS3 or oxysulfides are still present.191,192 In contrast to Mo-based HDS catalysts, the addition of Co as promoter seems to have no positive effect,193 while Ni addition enhances the catalytic performance of the W-based catalysts. For NiW-based materials, it was observed that Ni can easily be sulfided and sulfidation is completed before W sulfidation has started.193,197 At low sulfidation temperatures, Ni sulfide phases are present as a separated compound or as small Ni sulfide particles interacting with WOxSy phases. At higher sulfidation temperatures, WS2 slabs are formed accompanied by a redispersion of Ni sulfide species and Ni–W–S-type structures are formed,198–201 which was shown by a 57Co Mo¨ssbauer emission study on 57Co-doped NiW/Al2O3.202 As for Co(Ni)MoS-based catalysts, two types of
2
4
6
8
r (Å)
Figure 21 W EXAFS Fourier transforms of NiW/Al2O3 after sulfidation at indicated temperatures. Intervals I and II: W–W contributions of WO3 and WS3, respectively (R not corrected for phase-shift).203
NiW catalysts are proposed: a type I Ni–W–S phase obtained at low-temperature sulfidation exhibiting residual oxygen linkages showing a high hydrogenation activity and a type II phase formed at higher sulfidation temperatures exhibiting a high HDS activity.191 The sulfidation behavior of NiW materials supported on different materials (g-Al2O3, carbon, and amorphous silica-alumina (ASA)) was studied applying different analytic tools and some selected results are discussed in more detail. The supports were impregnated using aqueous solutions of ammonium metatungstate ((NH4)6W12O39xH2O) and nickel nitrate. The resulting material was dried at 383 K and subsequently calcined at elevated temperature in air. The sulfidation was done stepwise in a mixture of 10 vol.% H2S in H2 at 300, 373, 473, 573, 673, and 773 K for 1 h, atmospheric pressure. For catalytic measurements, the catalysts were sulfided at 673 K for 2 h.203
7.12.12.1 x-Ray Absorption Spectroscopy Performed at the W LIII Absorption Edge The Fourier transforms of a catalyst that has been calcined at 673 K prior to sulfidation are shown in Figure 21. At low sulfidation temperature, the spectra are dominated by two W–O contributions at 1.76 and 1.91 A˚ corresponding to the W–O bonds in the first coordination shell of WO3. The peaks in the region labeled I (Figure 21) are caused by second shell W and third shell O backscatters. These findings indicate that at low sulfidation temperatures an oxidic tungsten phase is present with a WO3-like structure. The sulfidation of W starts at 573 K and a W–S contribution at a distance of 2.42 A˚ with a CN of 0.9 can be detected in the spectrum. In this spectrum, a second W–W contribution (2.71 A˚, CN 1) is seen indicating the presence of a WS3-type structure (W–S ¼ 2.40 A˚, CN W–S ¼ 5; W–W ¼ 2.75 A˚, CN W–W ¼ 2). While at 673 K the W–O contributions are clearly reduced, contributions of the oxidic W phase are still present. The characteristic W–W distance in WO3 at 3.73 A˚ cannot be detected in the spectrum (T ¼ 673 K) suggesting that the original W oxide phase has largely disappeared. In addition, an increase of the W–S coordination from 0.9 to 1.6 is observed
304
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
while the W–W CN associated with the WS3-type structure decreased from 1.0 to 0.2. The lower than expected W–S CN for a-WS3 indicates that sulfidation of the WS3-type phase is not complete at 673 K. A small W–W contribution at 3.16 A˚ points to the onset of WS2 formation, and it can be assumed that a partial transition from WOxSy and WS3-like phases to WS2 already occurred before complete sulfidation to WS3 was achieved. In accordance with results reported earlier,95,193 the W species formed during the sulfidation procedure may be regarded as a WOxSy phase. If a NiW catalyst supported on Al2O3 is sulfided in a single step at T ¼ 673 K for 2 h, much more WS2 is present compared to the stepwise sulfidation. In addition, the amount of oxygen is also substantially lower. A higher calcination temperature of 823 K and a single-step sulfidation leads to a lower degree of sulfidation and the catalyst contains more oxygen and less sulfur. Several NiW catalysts with a varying Ni:W ratio were calcined at 823 K and then sulfided at T ¼ 673 K at a total pressure of 15 bar (H2S/H2 ratio: 0.1). The analysis of the EXAFS data indicate that in all catalysts WS2 slabs are present pointing to a higher degree of sulfidation at higher pressure. With increasing Ni:W ratio, sulfidation seems to be more complete and at low Ni:W ratio the low W–S CNs suggest the presence of some oxygen. A larger number of W–W CNs at higher Ni:W ratios may be interpreted as a higher order of the crystallites and a more complete sulfidation.
7.12.12.2
occurrence of Ni–Ni contributions indicates that NiO and Ni– (oxy)sulfide are not well dispersed over the support surface. In the XANES spectra, a continuous decrease of the area of the white line is accompanied by a decrease of the Ni–O CN suggesting a progressive Ni sulfidation. In the spectrum of the sample sulfided at 573 K, the Ni–O contribution is absent and most of Ni is sulfided. In the XANES spectra (Figure 22, right), a pre-edge peak near 8333 eV characteristic for the 1s to 3d transition shows an increase of the intensity related to a change of the symmetry around Ni. For the Ni sulfide obtained at 573 and 673 K, the bond length for Ni–S amounts to 2.21 A˚ and that for Ni–Ni is 2.93 A˚. Especially the Ni–Ni separation does not match with any well-known bulk Ni sulfide such as Ni3S2 (Ni–Ni¼ 2.50 A˚, 3.80 A˚), NiS (Ni–Ni ¼ 2.68 A˚), NiS with millerite structure (Ni–Ni ¼ 3.16 A˚), NiS2 (Ni–Ni ¼ 3.92–4.12 A˚), or Ni3S4 (Ni–Ni¼ 2.50 A˚), and together with the Ni–S bond length of 2.21 A˚ the formation of the ‘Ni–W–S’ structure may be assumed. The results of the studies can be schematically summarized as shown in Figure 23. Tungsten oxide is more difficult to be sulfided due to the stronger metal–oxygen bond than that of Mo oxide in Co(Ni) Mo catalysts. Hence, over a broad range of sulfiding conditions incomplete sulfided tungsten phases are observed and these may be viewed as oxysulfides WOxSy. For these materials, indications are found for the presence of WO3, WS3, and WS2 with differing amounts of oxygen and sulfur atoms. Interaction between WO3 and the support g-Al2O3 makes it more difficult to sulfide W and calcination even increases the interaction with the support. Investigations with carbon as support demonstrate a real support effect and because metal–support interactions are negligible or even absent, WS2 formation is more or less complete at 673 K for NiW/C, while alumina or amorphous silica–alumina supports require sulfidation at higher temperatures being accompanied by a loss of WS2 dispersion. Because sulfidation of W is very slow, Ni sulfide particles tend to aggregate as larger crystallites, being not segregated from the WOxSy phase. Under typical sulfiding conditions (673 K, 1 bar), not all NiS species are redispersed to the WS2 slabs. This is caused by the sulfidation sequence (NiO is sulfided earlier than WO3) and because not all tungsten has converted to WS2. Compared to sulfided Co(Ni)Mo catalysts with
Ni K Edge x-Ray Absorption Spectroscopy
In Figure 22, the Fourier transform functions at the Ni K edge of stepwise sulfided Al2O3-supported NiW catalysts (left) and the near-edge spectra (right) are displayed. After treatment at T ¼ 300 K, the spectrum contains peaks of four different contributions. At a distance of 1.98 A˚ a Ni–O contribution (CN 4.2), at 2.17 A˚ a Ni–S contribution (CN 2.3), and two Ni–Ni contributions at larger distances of 2.60 A˚ (CN 0.8) and 3.02 A˚ (CN 1.5) are seen. Both the Ni–O distance of 1.98 A˚ and the Ni–Ni separation of 3.02 A˚ match well with the geometric parameters of NiO (Ni–O ¼ 2.08 A˚, Ni–Ni ¼ 2.96 A˚). The other observed peaks in the spectrum point to a NiOxSy species formed during sulfidation. Compared to bulk NiS (Ni– S ¼ 2.39 A), the Ni–S distance of 2.17 A˚ is much smaller. The
0.6 Ni−S
300 K 373 K 473 K
Ni−Ni
0.4
673 K
mx (a.u.)
|FT [c(k)k3]| (a.u.)
15
10 573 K 473 K
5
Ni−O
573 K
Whiteline
673 K
0.2 Pre edge peak
373 K Ni−Ni 300 K
0 0
2
4 r (Å)
6
8
0.0 8300
8320
8340 E (eV)
8360
Figure 22 (Left) Ni EXAFS Fourier transforms of NiW/Al after sulfidation at indicated temperatures and (right) the XANES region at the Ni K edge during sulfidation (distance R not corrected for phase-shift).203
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
305
NiO WO3
Ni
Ni
Al2O3
Fresh
NiOxSy Ni
15 bar sulfidation
WO3
Ni
373 K
1 bar sulfidation
NiS WO3
Ni
Ni
473 K
‘Ni−W−S’
NiS−WOxSy
“WS2”
NiS WOxSy
S S W Ni S S
573 K
NiS
‘Ni−W−S’
‘Ni−W−S’
NiS−WOxSy Ni
WOxSy
WS2
S S W Ni S S
Ni
WS2
Ni
Ni
WS2
Ni
673 K
‘Ni−W−S’
NiS Ni
WS2
Ni
923 K Figure 23 The sulfidation process of g-Al2O3-supported NiW catalyst. Left: sulfidation at atmospheric pressure; right: sulfidation at 15 bar. Fresh material contains Ni and W oxide; between 373 and 573 K small NiS crystallites are formed being attached to WOxSy (NiS–WOxSy), which may be a precursor to the Ni–W–S phase. At 673 K and 1 bar, NiS–WOxSy is partially converted to ‘Ni–W–S.’ NiS particles are redispersed to smaller crystallites over the WS2 edges during this transformation process. At 973 K and 1 bar, full sulfidation is achieved and a loss of WS2 dispersion as well as an increased crystallinity of the WS2 slabs occurs. Sulfidation at 15 bar and T ¼ 673 K is more complete, WS2 is reasonably good dispersed, and more Ni–W–S phase is formed.203
promoter ions being present in the Co(Ni)–Mo–S phase, NiW catalysts may be regarded as atomically dispersed Ni species in Ni–W–S phases together with less dispersed NiS–WOxSy phases.
7.12.13 Probing the Active Sites: CO and NO Adsorption Different spectroscopic techniques are used to obtain information regarding the surface binding sites of HDS catalysts.30 A powerful approach is to monitor the changes of adsorption of small probe molecules such as NO70–87 or CO61–69 with IR spectroscopy. Applying IR studies of NO adsorption allowed identifying different adsorption complexes for unpromoted and promoted catalysts.69,71–73 It was also shown by IR NO adsorption experiments and EXAFS investigations that NO molecules are adsorbed onto MoS2 edges and not onto the basal planes in sulfided Mo/Al2O3 catalysts.142 In the following, the results of a CO IR study are discussed in detail.69 Different g-Al2O3-supported Mo catalysts calcined at 773 K exhibiting different Mo loadings (4.2, 9.8, and 11.9 wt%,
abbreviated as Mo4/Al, Mo9/Al, and Mo12/Al), Co-promoted catalysts (impregnation with Co nitrate, calcinations at 773 K, 1.6, 3.1, and 4.7 wt% on the Mo9/Al catalyst), and a Mo catalyst supported on silica (Mo7Si, 7.2 wt% Mo, calcination temperature 623 K) were pressed into discs and placed in an IR cell. Then, sulfidation was done (1) under H2S/H2 flow or (2) under H2S/H2 flow followed by a post-treatment under pure H2 flow. IR CO adsorption experiments were done at 100 K. Three absorptions are seen in the IR spectrum of CO on the sulfided Mo7/Si catalyst (2158, 2120, and 2075 cm1) (Figure 24) with the band at 2158 cm1 being caused by the interaction of CO with silanol groups, while the remaining absorptions are due to CO adsorption onto unpromoted Mo sites of the sulfide phase. The spectra of CO adsorbed onto catalysts with different Mo loading are shown in Figure 24.69 The peaks located at 2189 and 2156 cm1 are caused by CO adsorption onto Al3þ vacancies and hydroxyl groups of alumina, while the band at 2110 cm1 is characteristic of CO interacting with Mo sites. This absorption increases with Mo loading and in the same direction the two former peaks decrease because of a higher coverage of the support by MoS catalyst. A band should appear at 2075 cm1 in the IR
306
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
2110
2120
A
0.1 A 2156
0.01 A
2075
2158 2189
Mo12/Al 2075 Mo9/Al
Mo7/Si Mo5/Al 2250 (a)
2200
2150
2100
2050
2000
1950
Wavenumbers (cm−1)
2250
2200
(b)
2150 2100 2050 Wavenumbers (cm−1)
2000
1950
Figure 24 (a) IR spectra of CO adsorbed onto sulfided Mo catalysts supported on silica and (b) on g-Al2O3.69
spectrum for the catalyst with the highest Mo loading (Mo12/ Al). Compared to Mo7/Si, the band related to the Mo sites on Mo/Al catalysts is shifted toward lower wavenumber, whereas the absorption at 2075 cm1 is not significantly affected. MoS2-based catalysts often display a hexagonal shape and two types of crystallographic edge planes (1 0 1 0) and (1 0 1 0) can be distinguished called the molybdenum and the sulfur edges. The nature of these two MoS2 edges may explain the presence of the two CO absorptions: the absorption at 2075 cm1 is caused by CO in interaction with Mo atoms on the sulfur edge and the peak at 2110–2120 cm1 is related with CO adsorbed onto Mo atoms on the molybdenum edge. These assignments are in agreement with DFT calculations where a difference of the two absorption bands of 40 cm1 was proposed67,68 in good agreement with 35 cm1 for the Mo/Al catalyst and 40 cm1 for the Mo/Si catalyst. IR CO spectra of the Co-promoted CoMo9/Al catalysts (Figure 25) show a new peak at 2070 cm1, which increases in intensity with increasing Co content up to 3.1 wt% Co, while the band characteristic of unpromoted Mo sites (2110 cm1) decreases. Increasing the Co content to 5 wt% does not lead to a further intensity gain of the absorption at 2070 cm1. For a sulfided Co/Al2O3 catalyst (3 wt% Co), an absorption at 2094 cm1 and a shoulder with low intensity at 2056 cm1 is observed. Therefore, the band in the IR spectra of the Copromoted catalysts can be assigned to Co-promoted Mo sites, in agreement with Ref. [63]. For the highest Co content, that is, for an atomic ratio for Co/(Co þ Mo) of 0.48, the peak at 2110 cm1 is still observable suggesting that promotion of the MoS2 slabs by Co is not complete in the CoMo/Al2O3 catalysts. The presence of Co also results in a decrease of the absorptions caused by CO-support interactions and this decrease is more pronounced at higher Co loading. A possible explanation is that an increasing amount of cobalt spinel or cobalt sulfide phase is formed at the higher Co contents.
A 0.05 A 2156 2190 Co5Mo9/Al
Co3Mo9/Al
2070
Co1Mo9/Al
2110 Mo9/Al 2250
2200
2150
2100
2050
2000
1950
Wavenumbers (cm−1) Figure 25 IR spectra of CO adsorbed onto CoMo9/Al catalysts.69
The results demonstrate that for CoMo/Al catalysts the concentration of unpromoted Mo sites is reduced with increasing Co content while in the same direction new Co-promoted sites are generated. However, the sum of the amount of unpromoted and promoted sites does not vary significantly as function of Co content. This observation suggests that the creation of one promoted site leads to the loss of about one unpromoted Mo site. Interestingly, by increasing the atomic ratio of Co/(Co þ Mo) above 0.36 the number of Co-promoted sites is not increased, and the amount of unpromoted Mo sites almost does not decrease. These observations indicate that above a
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
2070 A A
2110
2070 2055
0.05
2156
0.1
2098 2110
2055
307
2190
Co5Mo9/Al
Co3Mo9/Al
Co1Mo9/Al
0.75
Mo9/Al
2300 (a)
2200 2100 Wavenumbers (cm−1)
2000
1900
2300 (b)
2200 2100 2000 Wavenumbers (cm−1)
1900
Figure 26 The influence of the H2 post-treatment at 573 K on the CO adsorption behavior on sulfided Mo/Al and CoMo/Al catalysts. (a) IR spectra of CO adsorbed onto the Mo9/Al catalyst after sulfidation (dotted line) and after sulfidation and H2 treatment (full line). (b) Difference spectra derived from IR spectra of CO adsorbed after sulfidation and H2 post-treatment minus spectra measured after sulfidation.69
certain Co/(Co þ Mo) ratio, additional Co atoms are not significantly deposited on MoS2 edges. In Figure 26, the IR CO spectra of the catalysts after a post-treatment in H2 are shown and the difference spectra are displayed in Figure 26, right. For Mo9/Al, the treatment leads to an increased intensity of the band at 2110 cm1 (Mo sulfide sites) and to the appearance of a new absorption at 2098 cm1. It seems also that the H2 post-treatment generates additional Al3þ vacancies and hydroxyl groups (bands at 2156 and 2190 cm1), which may be explained by breaking of Al–O–Al bridges during H2 treatment creating new Al3þ vacancies and OH groups. A detailed analysis of the spectra without and with H2 treatment yields an increase of Mo sites by a factor 2.3 after H2 treatment (Mo12/Al show a slightly smaller increase). Reversibility of the effects of the H2 treatment was checked by heating the catalysts in a H2S/H2 atmosphere at 573 K for 2 h. The IR spectra are very similar to those of catalysts not treated in H2, and especially the peak at 2098 cm1 totally disappeared. The H2 post-treatment at 573 K removes S atoms from the MoS2 edge slabs significantly, increasing the amount of accessible Mo sites. The observation that Mo12/Al exhibits only an increase of such sites by a factor of 1.8 may be due to larger size of the MoS2 slabs that are less easily reduced by H2. The H2 post-treatment also generates additional sites on the CoMo/Al catalysts (Figure 26). Independent of the Co content, the amount of unpromoted Mo sites increases by a factor of about 1.8 being slightly lower than for Mo9/Al (factor of 2.3). This increase can be explained by the generation of Mo sites characterized by the band at 2098 cm1. For the concentration of promoted sites, a factor of 1.3 is estimated as being smaller than for unpromoted sites, and for the catalyst with the highest Co content the factor is only 1.1. Because the increase of
Co-promoted sites by H2 treatment is smaller than that of unpromoted Mo sites, it is most likely that most of the Copromoted sites are already generated during the sulfidation procedure, whereas unpromoted Mo sites are more difficult to be created. In the IR spectra, no additional absorptions could be detected after the H2 treatment excluding the creation of new adsorption sites, that is, the environment of Co sites is not modified by the reductive treatment. Like for unpromoted catalysts, the number of Al3þ vacancies and OH groups increased after the H2 treatment. The adsorption properties of sulfided CoMo/Al2O3 catalysts were also investigated applying in situ Fourier transform infrared spectroscopy (FTIR) spectroscopy combined with NO adsorption (Figure 27).73,204 The IR absorptions associated with NO adsorbed onto Co sites at 1850 cm1 start to evolve by the addition of small amounts of Co to a Mo/Al2O3 catalyst and the intensity of the band associated with NO adsorbed onto Mo sites at 1690 cm1 starts to decrease. The catalytic activity in the HDS of thiophene correlates with the absorption of NO onto Co sites and not onto Mo sites suggesting the formation of the CoMoS phase upon Co addition to the Mo/ Al2O3 catalyst (Figure 27). In another in situ IR study, sulfided CoMo/Al2O3 catalysts were investigated after sulfidation at different temperatures. On a comparison of the changes of the surface properties by increasing the sulfiding temperature from 673 to 873 K, different types of CoMoS structures were detected. The type II CoMoS structure being obtained at the higher sulfiding temperature is characterized by none or very few bonds to Al2O3. At the lower temperature, type I CoMoS is formed having Mo– O–Al linkages, which is much less active in the HDS reaction than the type II catalyst.205
308
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Absorbance (a.u.)
8
Co 4 Moedge
2
Absorbance
2 CoMo
4 6 8 wt% Co added
(b)
10
1900
1800 1700 1600 1500 Wavenumber (cm−1)
kHDS 102 (mol h−1 g−1)
Mo
Co
(a)
6
1.5
1.0
0.5
1 (c)
2
3
4
Absorbance of NO on Co
Figure 27 (a) IR spectra of NO adsorbed onto sulfided 2% Co/Al2O3, 8% Mo/Al2O3, and 2%Co 8%Mo/Al2O3. (b) Absorbance of IR bands of NO adsorbed onto Co (1850 cm1) and Mo (1690 cm1) plotted as function of Co/Mo ratio at constant Mo loading. (c) Thiophene HDS activities versus absorbance of IR band of NO adsorbed onto Co.73
Atomic-scale insights into the nature of NO adsorption onto MoS2 and CoMoS are obtained combining IR spectroscopy investigations with scanning tunneling microscopy (STM) experiments and DFT calculations.88 The NO molecule does not adsorb at fully sulfided MoS2 edges when these edges do not contain hydrogen. However, it can be expected that sulfided catalysts contain hydrogen at the edge in the form of SH groups. The results suggest a ‘push–pull’ type mechanism where simultaneously vacancies are created, and NO adsorption and H2S release occur. Dosing the catalyst with atomic hydrogen, STM images reveal formation of vacancies and at these vacancies NO dimers are adsorbed. In the IR spectra measured during TPD, several NO adsorption complexes are observed. On the basis of DFT calculations, it was shown that mononitrosyl species dominate at the Mo edges and stable dinitrosyl moieties are found at both the unpromoted and the Co-promoted S-edges.
7.12.14 Structure of the Phases On the S-terminated (0 0 0 1) plane of MoS2, only very weak physisorption occurs, and hence the basal plane of the crystallites is more or less inactive in catalytic reactions. Coordinatively unsaturated sites (CUS) and/or sulfur ion vacancies at the corners and edges of the Mo(W)S2 slabs are the predominantly catalytically active sites (Lewis acid character).23 The vacancies or coordinatively unsaturated sites are created in a reaction with hydrogen. Due to the Lewis acid character of
Diameter
Sites Rim Edge
n Layers or stacking height
Basal
Figure 28 Scheme of the rim-edge model.
CUS, molecules with the unpaired electrons can be adsorbed. Using probe molecules such as CO, NO, or O2, the number of CUS could be determined and correlated with catalytic activity.88 Under real reaction conditions (presence of H2, H2S, and excess substrates and reaction products), it is highly questionable whether vacancies/CUS remain empty for any length of time. For unpromoted MoS2 catalysts, a two-site model was developed for MoS2 to explain the HDY and DDS pathways (Figure 28). These sites are located on the edge of MoS2 nanocrystals at different positions. The so-called rim-site terminating layers of the crystal stack on which HDY and DDS occur are located at the edge of exterior layers with adjacent basal planes. The second sites called edge-sites are located on interior layers
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
that have no exposed basal plane surfaces. On these sites only the desulfurization reaction occurs.206 Promotion by adding Co decreases the strength of the Mo–S bond and increases the electron density on the sulfur atoms.207,208 More electron density at the S centers increases the basicity of surface S species, which may facilitate C–S bond cleavage. Indeed, supported CoMoS catalysts exhibit a pronounced selectivity toward DDS.209 Because of the electronic density reorganization, the S atom bonded to Co and Mo exhibits an intermediate bond strength optimal for HDS activity. The bond weakening effect due to Co(Ni) promotion is supported by theoretical calculations as well as experimental results.210,211 For promoted catalysts, the Co–Mo–S (or Ni–Mo–S) phase model proposed by Topsøe et al.141,179,212,213 is now widely accepted: the catalysts consist of MoS2 nanocrystals with Co (or Ni) promoter atoms substituting Mo atoms at the edges of the MoS2 layers in the same plane as the Mo atoms, but their local coordination is different. The CN of the promotor atoms is still under debate and evidence was presented either for CN ¼ 4 or for CN ¼ 5. Practical Co–Mo HDS catalysts are generally composed of several phases: Co9S8 nanoparticles, Co2þ species
2
1
3
3
1
2 3
3
Figure 29 Scheme of a CoMoS catalyst composed of Codecorated single slab MoS2 (1), Co9S8 nanoparticles (2) and Co in the support (3).212
309
strongly interacting with the support, Mo (oxy)sulfides, as well as Co–Mo–S, schematically shown in Figure 29. Two structural types of Co(Ni)MoS catalysts were identified155: type-I with low activity and type-II with high activity. Type-I structures are incompletely sulfided and contain remaining Mo–O–Al linkages to the support. The formation of such Mo–O–Al bonds occurs during catalyst preparation (see below). Type-II Co(Ni)MoS phases display a weaker interaction to the support and they are fully sulfided. Often, type-II catalysts consist of multiple MoS2 slabs that are not linked to the support. The degree of stacking can be controlled by appropriate selection of support properties and synthesis conditions leading to stable single slabs of the catalytic materials on g-Al2O3 supports.214,215 A type-III catalyst was identified on SiO2 support and should consist of Co sulfide clusters with S2 dimers between the Co atoms located at the edges of the MoS2 slabs.135,216 A new model concerning hydrogen activation by MoS2 was recently presented. Unsupported overstoichiometric MoS2þx catalysts containing S22 species on the edges react with H2 to form –SH groups. These –SH groups participate in the catalytic reaction of thiophene conversion directly to butane (HYD pathway). Removal of the extra sulfur leads to a suppression or loss of the HYD function.217 Evidences for the presence of H groups on MoS2 were recently presented.218,219 Progress in the identification of active sites in MoS2 and CoMoS/NiMoS model catalysts was achieved by the combination of theoretical calculations (DFT; see also Chapters 7.15 and 9.02) and advanced analytical tools such as STM, aberrationcorrected high-resolution electron microscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).220–225 The local structure of pure MoS2 (grown on Au(11 1)) has a special electronic state identified as metallic brim sites (Figure 30). In the brim edge states, Mo is fully covered by S atoms forming S2 dimers on the edge. The edges can accept or donate electrons and thus act as a catalyst. Hence, an S-containing molecule may be bonded at the brim site and when hydrogen is available at the neighboring edge sites as a SH group, hydrogen transfer and hydrogenation reaction can occur. The results strongly indicate that the brim sites are catalytically active for the HYD pathway. The proposed formation of sulfur vacancies in MoS2 under H2 atmosphere has also been observed directly by STM.
Mo78S204 cluster
-S
Mo edge (100%) (a)
(b)
-S -S 2 S1
-S S2 1
-S -S 2
-S 1
2
-S 1
2
1
Figure 30 STM image of a MoS2 nanocrystal on Au(1 1 1) (left) showing the bright brim sites, the ball model of the structure with top view (middle, top) and side view (middle, bottom), and a model cluster with the S2 groups.226
310
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts profile (Figure 32, right) are 2.34 A˚ (0.1 A˚) for Mo–Co, 2.7 A˚ (0.1 A˚) for Co–Mo), and 2.7 A˚ (0.1 A˚) for Mo–Mo for the atoms labeled A–B–C–D. This result may be explained by the partial substitution of Mo by Co in the lattice of MoS2. Similar results were obtained for Co-promoted WS2 and the experimental finding is summarized in Figure 33. According to DFT calculations, the 50% Co partial occupation of the Mo-edge covered by 25% S is stable and the simulations also indicated the existence of two different configurations with such S-saturations that are close in energy (Figure 33(a) and 33(b)). In these configurations, either –Mo–Co–Mo–Co– alternates or a pairing of the atoms in the order –Mo–Mo–Co–Co is present, and in both cases the Mo-edge structure contains mixed Co–Mo sites. The S atoms are located at the top of the Mo atoms. However, DFT calculations also hinted that S atoms might be located in bridging positions suggesting that the mobility of S atoms on the Mo edge is high.228,229 In Figure 33(c), the model obtained experimentally is schematically shown. While DFT calculations yield an optimized local Co–Mo distance between 2.74 and 2.84 A˚, a pairing of Mo and Co edge atoms (2.34 A˚) is observed. The model for Co-promoted WS2 (Figure 33(d)) exhibits a sequence –W–Co–W–Co–W– supporting the idea that Co atoms are incorporated into the WS2 structure by substituting W atoms at the edge in an alternating fashion.
However, one should keep in mind that STM was performed under high vacuum conditions in the absence of substrate molecules, H2/H2S, and at ambient temperature. STM provided also evidences for significant changes of the morphology when the Co/Ni promoter is added. Pure MoS2 particles adopt a triangular shape and when Co is added, the particles become truncated hexagons (Figure 31, left). Two different edges can then be distinguished: one edge is covered by 100% and the second one by 50% S atoms. The situation is more complex for NiMoS because two different shapes of the nanoparticles are observed. The first one is identical with that found for CoMoS but the second one exhibits higher indexed edges exhibiting a lower degree of S coordination (Figure 31, right). The results suggest that Co/Ni plays two different roles: promotion of MoS2 with Co/Ni alters the electronic structure as seen as modified brim sites in STM; lower S coordination on one type of NiMoS edges is an attractive structural situation for adsorption of S-containing molecules. The occurrence of the second edge type in NiMoS clusters may be a first step to explain the different selectivities of the two catalytic systems in the hydrotreating processes. Concerning the activation of H2 on MoS2, it is proposed that a dissociative chemisorption occurs forming S–H groups at the edges. The most stable position of hydrogen seems to be on top of the S atom adjacent to the brim states. The promoter atoms Co in an unsupported CoMoS catalyst could be located using probe aberration-corrected STEM (Figure 32).225 Due to the lower atomic number, the Co atoms have a lower intensity in the HAADF-STEM image compared to Mo. The distances between the Mo–Co–Mo–Mo atoms along the edge of the nanosized particle determined by the line
The Role of Carbon in HDS Catalysts
It was also suggested54,55 that carbon plays an important role in stabilizing the active phase. It is assumed that excess of sulfur located on the surface of MoS2 is replaced by carbon, yielding MoSxCy phases with a carbidic species (see also Chapter 7.14).
NiM
CoM
oS
oS(1
0)
7.12.14.1
010)
(10
)
0 01
01
10)
S2
(1 2 oS
Mo
) N 10 NiMoS(1120) iMoS
10
( oS
(1
(10
10
iM
)
N
NiMoS(1010)
M
MoS2(1010) (100% S)
MoS2(1010) edge (100% S)
NiMoS(1010) (50% S) NiMoS(1010) (50% S)
CoMoS2(1010) edge (50% S)
Ni
Mo
S(
10
)
10
(10
10
) NiMoS(1120) iMoS N
NiMoS(1120) high index edge
Figure 31 Structure of promoted nanocrystals derived from STM investigations and DFT calculations.227
Mo Co A
Mo
B
Mo
C D
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140 130 120 110 100 90 80 70 60 50 40 30
Mo Mo Mo Co
A
C
B 0.234
0.0
0.1
0.2
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0.3
0.4
0.5
D 0.27
0.6
0.7
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0.9
nm
Figure 32 HAADF-STEM image of a Co-doped unsupported MoS2 nanowire catalyst (left) and the corresponding line profile (right).225
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Mo-edge (1010)
Mo-edge (1010) Mo
Co
Mo
S
Co
Mo
Co
Mo
Co
S
S-edge (1010)
S-edge (1010)
Mo–Mo–Co–Co
Mo–Co–Mo–Co
(a)
(b)
Mo-edge Mo-edge (1010)
2.34 Å Mo1
311
Mo-edge W-edge (1010)
2.70 Å
2.70 Å Mo2
Co
2.24 Å Mo3
2.72 Å
2.72 Å
2.24 Å
S W1
Co1
W2
Co2
W3
S
S-edge
S-edge
(1010)
(c)
(1010)
(d)
Figure 33 Two different views (perspective and top) of (a) Mo-edge, 50% Co in an alternate position and 25%S; (b) Mo-edge with 50% Co, pairing configuration, 25% S. (c) and (d) perspective view of CoMoS (CoWS) structures for the experimentally observed Mo–Co–Mo–Mo (W–Co–W–Co–W) sequence at the Mo/W-edge. Mo (W) atoms: blue (orange), S atoms yellow.225
800
Intensity (counts s−1)
MS833 before N2/DMS treatment MS833 after N2/DMS treatment
600
structure (NEXAFS)232 and the most important findings are discussed in detail. Catalysts were prepared by thermal decomposition of (NH4)2MoS4 (ATTM) in inert atmosphere according to the equation: ðNH4 Þ2 MoS4 ! MoS3 þ 2NH3 þ H2 S
400
200
0 0
10
20
30
40
50 60 2q
70
80
90 100
Figure 34 Comparison of the synchrotron XRD patterns of a sample before and after DMS treatment (T ¼ 793 K).232
The incorporation of carbon and the nature of the carbon species in MoS2-based catalysts were investigated with x-ray powder diffractometry, IR spectroscopy, electron energy loss spectroscopy (EELS), and near-edge x-ray absorption fine
Above 573 K, MoS2þx phases are obtained by autoreduction of MoS3 with the value x decreasing with increasing temperature. The MoS2þx samples were treated between 573 and 833 K with a gas flow of CH3–S–CH3 (DMS) diluted in N2 or H2. A second set of samples was synthesized by heating ATTM with a DBT solution in decalin. The sample was treated at 623 K in a bomb for 3 days. The x-ray powder pattern of the catalyst treated with DMS at 793 K (sample composition: MoS2.05C1.12) shows no new peaks (Figure 34) but a reduction of the absolute intensity compared to the sample before the treatment (note: identical sample volumes were used) indicating a lower density of the material after DMS treatment. The (1 1 0) plane at 2y ¼ 47 exhibits a stronger intensity relative to the (1 0 0) plane after the DMS treatment. The change of the intensity can be explained with a more pronounced distortion/more folding along the c axis after heating the precursor in DMS. The TEM image of a material obtained
312
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20 nm
20 nm
Figure 35 TEM micrographs of a MoS2 sample prepared by thermal decomposition of ATTM (left) and of a sample treated with DMS at 793 K (right).232
Intensity (a.u.)
s*
CK edge p*
B A 292 294 296 298 300 302 304
a b 280
290
300
310 320 330 Energy loss (eV)
340
350
Figure 36 EELS C K-edge spectra of Mo2C (a) and a sample treated at 793 K with N2/DMS. The inset shows the fine structure at the s* transition.232
by thermal decomposition of ATTM at 833 K in N2 (Figure 35) shows that MoS2 particles are present in closely packed groups of large slabs exhibiting highly folded shapes. The average slab length was estimated to about 94 A˚ and the average stacking number as 9.3, which is in reasonable agreement with the value of 7.8 obtained from the x-ray powder pattern. In the TEM image of a sample treated with DMS, heterogeneous regions with some highly stacked layers and regions with less stacked and less curved layers are observed. The average particle size was reduced by a factor of 2 to about 47 A˚ and the stacking number decreased from 9.3 to 4.0. In the EELS (Figure 36), an intense peak to the p* antibonding state is observed and by a comparison with a spectrum of graphite a graphitic structure can be excluded. In addition, the peak shape structure near the ionization edge is also different from that for amorphous, graphite, or carbide structures. Two features A and B are observed for graphite and carbide for the s* transition. Feature A is more intense for graphite than feature B, while the opposite is observed for carbides and diamond. The intensity difference is caused by the different density of states for
graphitic sp2 carbon bonds and tetrahedral sp3 bonds in carbides. The feature B is more intense than A in the MoS2 sample treated with DMS suggesting that carbon is not present as a graphitic-like material and that Mo–C bonds of a carbide-like structure are present. Because the p* peak is less intense than for Mo2C, one may conclude that the sample may contain some amorphous carbon. The replacement of S by C on edges of MoS2 can be crystallographically accommodated. Hence, it cannot be ruled out that both the Co(Ni)Mo(W)SC and Co(Ni)Mo(W)S phases constitute the active catalyst. As mentioned recently, the role of carbon in understanding the function of the catalysts may be of great importance and was more or less neglected.233
7.12.14.2
Morphology and Microstructure of Catalysts
In real catalysts, phase segregation was found and Co9S8 and mainly Ni3S2, besides other Ni sulfides, are formed under reaction conditions. In the ‘remote control’ concept, Co9S8 and Ni3S2 may play a distinct catalytic role, providing spillover hydrogen for the MoS2. Real catalysts display different MoS2 morphologies such as straight and curved MoS2 single slabs, and stacks of slabs.215 The MoS2 nanoparticles are either parallelly57,234,235 or perpendicularly234,235 oriented to the support. Spent NiMoS/Al2O3 and CoMoS/Al2O3 catalysts prepared by gas phase or liquid phase sulfidation (Table 1) of the oxidic precursors exhibit a large variety of microstructures.215 The x-ray powder patterns of the two catalysts NiMo1-T2-LF and NiMo2-T2-L-F show no clear reflection at 2y ¼ 14 indicating the absence of significant MoS2 stacking (Figure 37). The shape and the broadness of the reflections are typical for MoS2 with a low crystallinity. In addition, no indications are found for the presence of crystalline Ni sulfides. However, in about 30% of analyzed TEM regions of NiMo1T2-L-F Ni3S2 crystals are visible, which was verified with energydispersive x-ray spectroscopy (EDX), electron diffraction, and lattice imaging (Figure 38(a)). The TEM image in Figure 38(b) shows that Ni3S2 crystals are decorated with MoS2 layers. Ni sulfide particles with diameters between 5 and 50 nm can be imaged using STEM-EDX. An example is shown in Figure 39 for NiMo3-T2-L-U. From an analysis of the STEM-EDX data,
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Table 1 Type I/II (T1/2), type II (T2), liquid-phase sulfidation (L), gas-phase sulfidation (G), FCC-PT (F), ULSD (U), FCC-PT canister (F/C), HC-PT (H), diesel HDS (D) Catalyst properties Catalyst annotationa
Description
Metal loadings
Sulfidation
Originating fromb
NiMo1-T2-L-F NiMo2-T2-L-F NiMo3-T2-L-U NiMo3-T2-L-F/C NiMo3-T2-G-H NiMo3-T2-G NiMo4-T1/2-L-F CoMo1-T2-L-F CoMo2-T2-L-F CoMo3-T1/2-L-D
Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 1/2 Ni–Mo/Al2O3 Type 2 Co–Mo/Al2O3 Type 2 Co–Mo/Al2O3 Type 1/2 Co–Mo/Al2O3
3.2 at Ni and 5.8 at Mo per nm2 1.8 at Ni and 5.6 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 1.5 at Ni and 3.1 at Mo per nm2 1.7 at Co and 4.8 at Mo per nm2 2.4 at Co and 6.7 at Mo per nm2 1.5 at Co and 3.1 at Mo per nm2
Liquid phase Liquid phase Liquid phase Liquid phase Gas phase Gas phase Liquid phase Liquid phase Liquid phase Liquid phase
FCC-PT FCC-PT ULSD FCC-PT canister HC-PT – FCC-PT FCC-PT ULSD Diesel HDS
a
Type 1/2 (T1/2), Type 2 (T2), liquid phase sulfidation (L), gas phase sulfidation (G), FCC-PT (F), ULSD (U), FCC-PT canister (F/C), HC-PT (H), diesel HDS (D). Fluid catalytic cracker pretreat (FCC-PT), ultralow sulfur diesel (ULSD), hydrocracker pretreat (HC-PT), hydrodesulfurization (HDS).215
b
* = MoS2 + = gamma-Al2O3
* Intensity
*
+
+
*
+
*
+
*
NiMo1-T2-L-F
NiMo2-T2-L-F
Reference
5
10
20
30
40
50
60
70
80
2q Figure 37 x-Ray powder patterns of two liquid phase sulfided type II Ni–Mo/Al2O3 catalysts NiMo1-T2-L-F (top) and NiMo2-T2-L-F (middle) together with the pattern of a reference sample containing 1.5% Ni3S2 (bottom). Vertical bars: reflection positions of crystalline Ni3S2.215
it can be estimated that 32–47% of Ni has segregated in these catalysts corresponding to 1.3–3.0 wt% Ni3S2. Combining these data with the MoS2 dispersion estimated from the TEM micrographs and about 80% of Mo atoms located on edge and corner (e þ c) sites, the amount of not segregated Ni is sufficient to reach Ni/Mo (e þ c) atom ratios of 0.24–0.29. The Ni sulfide segregation phenomenon is also detected in type I/II Ni–Mo catalysts. A detailed analysis also demonstrates that the sulfidation procedure significantly influences the morphology of type II catalysts. The MoS2 stacking is more pronounced when sulfidation is done in H2S/H2 atmosphere, while liquid-phase sulfidation seems to prevent it. In a H2S/H2 atmosphere, only few but much larger Ni3S2 crystals are formed than after
liquid-phase sulfidation. This may be related to the exothermicity of the H2S/H2 sulfidation, an effect that is suppressed if liquid is present during the sulfidation. In contrast to Ni-promoted catalysts, no Co-rich particles are visible in Co–Mo type II catalysts displaying the same composition and preparation, when they are used under the same conditions. It is highly likely that Co9S8 aggregates less rapidly than Ni3S2. For liquid-phase sulfided type II commercial NiMo catalysts, MoS2 stacking is not pronounced and is mainly absent in some catalysts (Figure 40). The MoS2 dispersion is high after liquid-phase sulfidation and using short reaction times, while longer reaction times and/or more severe reaction conditions produce longer MoS2 slabs (i.e., the dispersion is lower) (NiMo3-T2-L-F/C, used for 1 year in a unit
314
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5 nm
10 nm
(a)
(b)
Figure 38 TEM micrographs of liquid-phase sulfided type II Ni–Mo/Al2O3 catalysts: (a) shows a lattice image of a Ni sulfide crystallite in NiMo1-T2-L-F; (b) decoration of a Ni sulfide particle with MoS2 slabs in NiMo2-T2-L-F.215
0.2 mm (a)
0.1 mm (b)
Figure 39 STEM-EDX images of Ni (red) and Mo (blue) of liquid phase sulfided type II Ni–Mo/Al2O3 catalyst NiMo3-T2-L-U.215
5 nm
10 nm (a)
(b)
Figure 40 TEM micrographs of liquid-phase sulfided type II NiMo/Al2O3 catalyst NiMo3-T2-L-F/C showing long curved MoS2 slabs.215
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20 nm
20 nm (a)
315
(b)
Figure 41 HAADF-STEM images of MoS2/C (a) and WS2/C (b) catalysts. All clusters in the two images are single-layer structures.220
20 nm
(1 0 1 0) (so-called Mo(W) edges) and (1 0 1 0) (so-called S edges) being exposed. The addition of promoter atoms to MoS2 modifies the morphology to more heavily truncated triangles (Figure 42), that is, a change toward a more hexagonal shape is induced. This observation is in line with STM images of Co-promoted MoS2 deposited on Au(1 1 1).236 The Co atoms in the CoMoS structures are located at the (1 0 1 0) S edges and Co addition causes exposure of more promoted (1 0 1 0) S-type edges while the amount of Mo-type edges (1 0 1 0) is reduced. The situation is similar for NiMoS and NiWS catalysts. Promoter atoms may have other effects than changing the degree of truncation. There are particles that are more round and in some cases additional new higher index edges are present. For some of the edges, angles of about 150 to the dominant edges are observed indicating that the promoted structures also expose (1 1 2 0) edges. This finding is in agreement with atomically resolved STM images of NiMoS structures for small clusters.221 The present results give evidence that extended (1 1 2 0) edges may indeed be stable features of the promoted structures.
Figure 42 Promotion of MoS2 with Co changes the morphology toward heavily truncated triangles.220
7.12.15 Choice of Catalysts and Other Catalysts for HDS operating at high temperature). Nevertheless, the degree of stacking is rather limited and most MoS2 slabs consist of only one or two layers. It seems that MoS2 stacking is not required for desulfurization of molecules such as DBT. An interesting feature occurs for this catalyst, namely the curvature of the MoS2 layers (Figure 40). Type I and II catalysts often show MoS2 morphologies such as Al2O3 needles or plates coated with MoS2. Carbon-supported MoS2, WS2, CoMoS, NiMoS, and NiWS catalysts were prepared by impregnation with an aqueous solution containing (NH4)2[MS4] (M ¼ Mo or W) and, for promoted catalysts, also with Me(acetate)2 (Me ¼ Co, Ni). After impregnation, the samples were dried in air at 110 C and then treated with 10% H2S in H2 for 6 h at 1073 K. The morphology of the catalysts was investigated with HAADFSTEM and an example is shown in Figure 41.220 Most of the single-layered clusters exhibit a regular shape in the form of a truncated triangle. The angles at the vertices of the nanostructures amount to about 120 and are consistent with both the
Whether CoMoS or NiMoS is used depends on several factors because these catalysts behave differently. Generally, CoMoS/ g-Al2O3 catalysts are very efficient in the HDS process but they are less efficient for HDN or hydrogenation of aromatic molecules. NiMoS/g-Al2O3 phases exhibit better HDN activity and are better hydrogenation catalysts. Hence, they are preferably applied for feedstock with a high concentration of unsaturated compounds because more aromatic molecules are saturated leading to a higher cetane number and density. However, this is accompanied by higher hydrogen consumption, requiring appropriate hydrogen availability, hydrogen recycle rate, and compression costs. For aromatic hydrogenation reactions, NiWS-based catalysts are highly effective but the higher costs compared to NiMoS catalysts have limited their uses. At low pressures and high temperatures, a CoMoS catalyst can be a better choice than a NiMoS catalyst. Other approaches to obtain active catalysts were also reported and some are mentioned in brief. Using bimetallic complexes containing, for example, Mo3S4M cores (M ¼ e.g.,
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Ru, Ir, Rh) deposited on g-Al2O3 yields active catalysts. The results suggest that the activity is at least partially caused by a better distribution of MoS2 slabs on the support. No clear evidences were found whether the noble metals are located in direct contact with the MoS2 phase.237 Are there compounds exhibiting better HDS performance? Yes, as can be seen in Figure 43; the HDS activity scales with the M–S bond strength (M ¼ transition metals) and the best catalysts are found for noble metal sulfides. Keeping in mind that more than 120 000 tons of catalysts are used in refineries and that roughly the same amount must be replaced every year due to catalyst deactivation, the high prices and restricted availability of the noble metals prevent the use of such sulfides. During the last few years, unsupported metal phosphides have been studied due to their high activity for HDS and HDN.239. Both MoP and WP exhibit only moderate activity but Ni2P shows an excellent activity in hydroprocessing. According to results of several studies performed for simultaneous HDS and HDN reactions, the overall activity was found to be in the order Fe2P < CoP < MoP < WP < Ni2P.
7.12.16 Poisoning and Deactivation Effects The poisoning effects of H2S, nitrogen compounds, and aromatics are different for the HYD and DDS routes.240–242 According to experimental and theoretical DFT-based investigations, the general trend for poisoning is: nitrogen compounds> organic sulfur compounds > polyaromatics oxygen compounds H2S > monoaromatics. The H2S produced during HDS may react with the CUS to form saturated sites containing –SH groups.243 Hence, the number of CUS is reduced inhibiting removal of S via DDS, while the HYD pathway is only slightly affected.244–248 Different catalysts exhibit different sensitivities against H2S poisoning. NiMoS/g-Al2O3 is more sensitive than Co-promoted catalysts.39 The inhibition effect is more pronounced for HDS of DBT than for alkyl-substituted DBTs.244 The
1.00E + 03
Log (HDS activity per mole of metal)
RuS2 IrS2
OsS2
Rh2S3
1.00E + 02
ReS2 PdS
1.00E + 01
PtS
MoS2
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NbS2 TaS2
Co9S8 VS
1.00E + 00 MnS
1.00E · 01 30
40
50
60
70
80
M−S strength bond (kcal mol−1)
Figure 43 HDS activity of transition metal sulfides plotted against M–S bond strength.238
identification of the Mo edge brim site (see above) as the HYD site explains the low inhibiting effect of H2S on hydrogenation. H2S partial pressure also plays an important role in poisoning.244 However, at low partial pressure H2S may exert a promotional effect.249 Model compound studies have shown that some basic nitrogen-containing molecules (e.g., acridine) are orders of magnitude more inhibiting than other Ncontaining molecules (e.g., aniline or indole). The differences in the degree of inhibition between different catalysts, different types of S-compounds, and between DDS and HYD pathways have been explained on the basis of different catalytic sites involved in DDS and HYD reactions, as well as based on the mode of adsorption of the reactants. The HDS reaction is also significantly inhibited by organic nitrogen compounds. Experimental and theoretical data suggest that competitive adsorption between N-containing and S-containing compounds occurs on active sites. The HDS reaction is then inhibited due to the strong adsorptive strength of the N-containing molecules. The extent of inhibition, however, depends on the type and the concentration of organic nitrogen compounds. Catalysts applied in the hydrotreating of light feeds (socalled middle distillates) mainly deactivate due to coke deposition and sintering. Metal deposition as a deactivation source is found in hydroprocessing of heavy feeds.250 Coverage of the active sites by N-compounds and coke formation are the most important factors in catalyst deactivation. Basic N-containing compounds may adsorb irreversibly onto catalyst acidic sites, and may be converted to coke. HDS reaction conditions such as T, partial H2 pressure, and H2/oil ratio strongly influence coke formation. Coke is formed at the beginning of the reaction, but the catalyst may retain a substantial portion of its original activity.251
7.12.17 Synthesis Strategies of Catalysts In the literature, numerous different preparation approaches are reported. Hence, only some of these methods are presented in brief. A variety of unsupported NiMoWS catalysts are commercially used with the commercial trade name NEBULA. The synthesis procedure is patented and therefore the exact preparation method is not known. There are studies where the chemical vapor deposition (CVD) route was applied to deposit Co on supported MoS2. The MoS2 was treated with Co(CO)3NO vapor at RT for a distinct time followed by sulfidization. HDS activity was substantially higher than that of a conventionally prepared catalyst (impregnation route). More interestingly, adding Co by the CVD technique to a conventionally prepared CoMoS/Al2O3 catalyst, the HDS activity increased significantly.252 Besides the use of oxidic starting compounds, the suitability of S-containing precursors was investigated. Such precursors are either ammonium thiomolybdate (ATM)/thiotungstate (ATT), tetraethylammonium salts of the type [M0 (MS4)2]n (n ¼ 2 or 3) (M0 ¼ Co or Ni, M ¼ Mo or W)253, or tetraalkylammonium thiometalates with formula (R4N)2MS4 (R ¼ methyl, ethyl, propyl, pentyl, hexyl, etc.; M ¼ Mo, W). These precursors are thermally decomposed at relatively low temperatures often
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts yielding finely dispersed unsupported catalysts.230,254–256 Using alkyl-containing precursors, the decomposed MoS2þx contains carbon and small amounts of nitrogen. The carbon content increases with the size of R. The catalyst is then impregnated with promotor salts and the Co(Ni)Mo(W)S catalyst is sulfided either ex situ or in situ.257–259 Unpromoted MoS2 catalysts with large carbon contents and a good performance are obtained by thermal decomposition and different activation modes.260 To avoid the use of any oxidic materials in the catalyst preparation, a combination of ATTM/(Prop4N)2MoS4 and nickel diethyldithiocarbamate or cobalt dimethylthiocarbamate was used.261,262 These catalysts are characterized by a good dispersion, variable carbon content, and high activity. Another approach is the use of single source precursors such as (ML)(MoO4) (L ¼ amine such as EN, M ¼ transition metal), for example, Co(en)3MoO4. The precursor is first thermally decomposed followed by sulfidization at elevated temperature in a H2S/H2 atmosphere.10 There are also reports where the oxidic catalyst precursors are sulfided in solution at elevated temperature and high H2 pressure using, for example, dimethyldisulfide (DMDS) dissolved in an inert organic solvent.
7.12.18 Supported Catalysts and the Support Effect For most supported Co(Ni)MoS catalysts g-Al2O3 is used: this material has a high specific surface area (200–350 m2 g1), pore size can be controlled, it has a good affinity to sulfide yielding a high dispersion, it contains acidic and basic sites, can be easily formed into desired shapes, mechanical strength is suitable, and cost is low. To improve the performance of the catalysts several support materials such as carbon, SiO2, TiO2, ZrO2, MgO, zeolites, and mesoporous silicas are used.263–266 Further strategies are modifying g-Al2O3 by mixing with different materials such as zeolites or other metal oxides and adding additives such as P, F, B, Si, Mg, Zn, La, V, and Ga as modifiers, to alter the acidic and basic properties of g-Al2O3. The consequences of the application of such mixtures are altered interaction between the impregnation solution and the support, a change of the dispersion of the active phase, and reducibility of the oxidic precursor catalyst. In recent years, phosphorus is mainly used as a modifier and in several reports an improvement of the performance was reported influencing especially the HDN activity of NiMoS/Al2O3. The addition of boron to gAl2O3 seems to have a positive effect for CoMoS because the formation of catalytically inactive bulk Co9S8 is minimized.267 Some oxides have acid–base properties promoting desulfurization of alkyl DBTs. Acidic supports such as zeolites facilitate isomerization and trans-alkylation of alkylated DBT resulting in an enhanced HDS activity of these refractory sulfur species. However, the disadvantage of such supports is an extensive coking shortening the lifetime of the catalyst. To increase the dispersion of the catalytically active phase, mesoporous silica materials such as SBA-15 or MCM-41 with very large surface areas of about 1000 m2 g–1 were tested (see also Chapters 5.06 and 7.11).268–274 Under laboratory conditions, all these materials showed comparable or even better HDS performance than industrial catalysts. Most of the studies focus on a high dispersion of the catalysts enabling a good decoration of Mo(W)S2 edges with promoter
317
atoms. In addition, a weak catalyst-support interaction favors formation of type II Co(Ni)Mo(W)S structures with a low degree of stacking. Several disadvantages of promising supports should be shortly mentioned. An upscale of mesoporous materials such as SBA-15 and MCM-41 yielding kilograms or even hundred of kilograms was not possible until now. Zeolites have very small pore diameters (less than 2 nm) and dispersion of the catalysts is relatively poor. Moreover, the pores are so small that large DBT molecules cannot enter the pores.
7.12.19 Conclusion Co(Ni)MoS/Al2O3 and related catalysts have been commercially used for more than 80 years. During this time period, different models and theories were developed and the evolution of the models/theories is summarized in several reviews.23,25–27 Most models developed are directly related to the layered crystal structure of MoS2. The first structural model of CoMo/ Al2O3 was the monolayer model developed by Schuit and Lipsch (Figure 44).275 After calcination, Mo species are on the surface of the support forming a monolayer. The interaction between Mo and Al2O3 occurs via Mo–O–Al bridges. Incorporation of Mo6þ ions was proposed to be charge compensated by a capping O2 layer located on top of the monolayer, and Co2þ was assumed to substitute Al3þ ions in the surface and being in tetrahedral environment. The promotional effect of Co2þ was suggested to increase the stability of the monolayer. During sulfidation, the capping oxygen atoms are replaced by S2 anions. The presence of H2 under HDS conditions causes a removal of S2 leading to a reduction of Mo6þ to Mo3þ, which were assumed to be the catalytically active sites. Another model was the intercalation model (Figure 45)276,277 assuming that the sulfided catalysts contain MoS2/WS2 on the g-Al2O3 and the Co/Ni atoms occupy octahedral sites in the van der Waals gaps between the MoS2/WS2 layers. Later it was shown that intercalation of Co/Ni is energetically not possible and it was proposed that the promoter cations are located at the edges of the slabs (pseudo-intercalation).278 The model implicitly assumes a three-dimensional multilayer structure of MoS2/WS2 so that intercalation can take place. S
S
S
O
O
S Mo
Mo O
O Co
Co Co
Figure 44 Schematic illustration of the monolayer model.
MoS S Mo S S
Co
Bulk intercalation ‘Surface’ intercalation Co
Figure 45 Schematic representation of the intercalation pseudointercalation model.
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
MoS2
Co9S8
Figure 46 The contact synergy model.
The contact synergy model also assumes the presence of layered MoS2 (Figure 46).279,280 In this model, Co9S8 and MoS2 are in tight contact and results in spill-over of hydrogen from Co9S8 to MoS2 enhancing the intrinsic activity.281 During the last decades, a detailed picture of the CoMoS phase on the atomic scale was developed based on the findings and observations made with a variety of analytical tools and theoretical calculations. The catalyst contains nanosized MoS2 particles decorated with Co on the edges to form the catalytically highly active CoMoS phase. After impregnation of the alumina support with molybdenum and cobalt compounds, followed by drying and/or calcination, well-dispersed oxides of these elements are obtained. The oxides are then converted into the sulfides achieving the catalytically active phase. A more detailed picture of the Co/NiMoS catalysts will be obtained applying more in situ studies to probe the structure– activity relationship at the individual sites on nanosized catalysts in their working state. Such studies are challenging especially if industrial-type conditions are envisaged. The results of the experiments will help to rationally design catalysts for the different applications in the HDS reaction. HDS research has moved to the most refractory sulfur compounds, that is, DBTs with alkyl substituents in the 4- and 6-positions. In addition, improvements of conventional metal sulfide catalysts and the development of new materials are required to meet the increasing demand for energy.
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7.12.1 7.12.2 7.12.3 7.12.4 7.12.5 7.12.6 7.12.7 7.12.7.1 7.12.7.2 7.12.8 7.12.9 7.12.10 7.12.11 7.12.12 7.12.12.1 7.12.12.2 7.12.13 7.12.14 7.12.14.1 7.12.14.2 7.12.15 7.12.16 7.12.17 7.12.18 7.12.19 References
7.12.1
General Introduction Sulfur Compounds in Diesel Feeds and Reactivities of These Compounds HDS Reaction Pathways and Mechanism Catalyst Components, Formation and Nature of Active Phases Analytical Tools for the Characterization of the Catalysts Formation of the Sulfide Phases Conversion of MoO3 into MoS2 Conversion of MoO3 into MoS2 Studied with XPS and IRES Temperature-Programmed Sulfidation of MoO3 Quick EXAFS Spectroscopy Investigation of the Local Environments in Ni-Promoted Molybdenum Sulfide Catalysts Sulfidation of Co-Promoted Molybdenum Oxide in the Presence of a Chelating Molecule Investigated with XPS Determination of the State of Co in CoMoS Phase Using In Situ Mo¨ssbauer Emission Spectroscopy The Local Structures of Co/Ni and Mo in CoMoS and NiMoS Phases Determined with EXAFS Sulfidation of NiW Catalysts Supported on Al2O3 x-Ray Absorption Spectroscopy Performed at the W LIII Absorption Edge Ni K Edge x-Ray Absorption Spectroscopy Probing the Active Sites: CO and NO Adsorption Structure of the Phases The Role of Carbon in HDS Catalysts Morphology and Microstructure of Catalysts Choice of Catalysts and Other Catalysts for HDS Poisoning and Deactivation Effects Synthesis Strategies of Catalysts Supported Catalysts and the Support Effect Conclusion
General Introduction
In 2010, about 3914 million tons of crude oil was produced worldwide. Crude oil is a very complex chemical mixture containing hydrocarbons (alkanes, alkenes, cycloalkanes, and aromatics), sulfur compounds (mercaptanes, sulfides, disulfides, thiophenes, and dibenzothiophenes), nitrogen compounds (anilines, aliphatic amines, quinoline, acridine, carbazole, and indole), oxygenates (phenols, ketones, and carboxylic acids), and metal atoms such as V and Ni that are present as porphyrin complexes of V4þ¼O and Ni2þ to mention the most important constituents. The composition of crude oil depends on the location where it is produced with the sulfur level varying between 1 and 6 wt%. Average values in weight% are for carbon 83–87, hydrogen 10–14, nitrogen 0.1–2, oxygen 0.1–1.5, sulfur 0.5–6, and metals <0.1. Different physical and chemical processes such as distillation, extraction, reforming, hydrogenation, cracking, and blending are used in a refinery converting crude oil to higher value products. The main products are liquid petroleum gas,
Comprehensive Inorganic Chemistry II
287 289 289 292 294 295 295 295 297 298 299 300 301 303 303 304 305 308 310 312 315 316 316 317 317 318
gasoline, jet fuel, diesel fuel, wax, lubricants, bitumen, and petrochemicals. The sulfur content increases with increasing boiling point of the different fractions from crude oil. Hydrotreating or hydroprocessing refers to a variety of catalytically promoted chemical reactions such as hydrogenation, which saturate unsaturated hydrocarbons and remove S by hydrodesulfurization (HDS), N by hydrodenitrogenation (HDN), O by hydrodeoxygenation (HDO), and metals by hydrodemetallization (HDM) from different petroleum streams in a refinery. Hydrotreating is mainly applied and is necessary to avoid air pollution emissions, to avoid poisoning of noble metals and acid catalysts used in catalytic reforming and cracking, and to improve fuel quality. The different streams are transformed into products through catalytically promoted chemical reactions such as hydrogenation, isomerization, aromatization, alkylation, cracking, and hydrotreating (Scheme 1). The important products of an oil refinery are diesel fuel and gasoline. About 1 ton of crude oil delivers 0.25 tons of diesel fuel. Due to stringent environmental regulations,1–3 the sulfur content of diesel fuel must be reduced to ultralow levels
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Gas concentration
Amine guard
H2 plant
Polybed PSA
From FCC From FCC Light straight run
Crude distillation
Hydrotreating
Light naphtha isomerization
Hydrotreating
Reforming
LPG Propylene
Gasoline
Naphtha Kerosene Kerosene jet fuel
Middle distillate Hydrotreating Desalted crude oil
Atmospheric gas oil
Diesel
Catalytic condensation C4 isomerization
Alkylation
SHP
FCC
Hydrotreating
Cycle oils Hydrotreating Hydrocracking Lube oils
Vacuum distillation
Asphalt Fuel oil Black oil
Solid fuel Refinery adsorbent locations for contaminant removal
Scheme 1 Process flow from crude oil to final products. http://www.uop.com/refining-flowscheme-2/. Abbreviations: FCC, fluid catalytic cracking; PSA, pressure swing adsorption; LPG, liquified petroleum gas.
(ultralow sulfur diesel, ULSD, 10–15 ppm) to decrease harmful exhaust emissions of diesel engines and to improve air quality (see also Chapter 7.19). In addition, there is also a need to strictly reduce the S content because even trace levels of S can poison other catalysts (e.g., Ni, Cu, Pd, or Pt) used in further catalytic processes (hydrocracking, reforming, automotive catalysis, fuel cells, etc.). Hydrotreating is one important catalytic step to convert crude oil streams into diesel fuel (Scheme 1). In the presence of a catalyst and molecular
hydrogen, double bonds of aromatics are hydrogenated and S, N, O, and metals are removed. The catalysts widely used are Mo and/or W-based sulfides promoted with Co and/or Ni (see also Chapter 5.09). The catalysts are supported on g-Al2O3 or are supplied as bulk material. Depending on the distinct process, typical reaction conditions for hydrotreating are 300–400 C and total pressure up to 15 MPa. The actual conditions depend on the boiling point of the feed. The required severe reduction of the sulfur level is a complicated chemical
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and technical problem. Many different factors such as the catalysts,4–6 process parameters,7 feedstock source and quality,8,9 reactivities of sulfur compounds,10 inhibition effects of H2S,11 nitrogen compounds,12 and aromatics13,14 present in the feed influence the degree of desulfurization of diesel feeds. Thousands of papers and patents were published in the past and state of the art up to 2002 has been reviewed in Refs. [15–17]. The actuality of research in the field of hydrotreating was impressively demonstrated by F. Parlevliet and S. Eijsbouts in 2008: counting only sulfide-based catalysts, since 1986 more than 3700 patents and about 5300 nonpatents were published.18 In 2010, a review was published where the different scientific and technological aspects of ULSD production are thoroughly discussed and 702 references are cited.19 Further reviews and review-like papers present and discuss the state of the art in HDS catalysis at different times of development.15–17,20–36 A short discussion of the development of the knowledge about HDS catalysts is given in the last section of the chapter.
petroleum. The reactivity of BTs and DBTs decreases with increasing molecular geometry (Figure 2). Setting the relative rate for thiophene as 100, benzothiophene has a rate of 50, DBT of 30, 4-methyl-DBT of 5, and dior trisubstituted DBTs of 1 (see Scheme 2).37
7.12.3
HDS Reaction Pathways and Mechanism
During hydrotreating a complicated set of exothermic reactions occurs, and the main reactions are hydrogenation of aromatics, HDS, and HDN. Even for the simple substrates such as thiophene, the HDS networks are complex: a considerable number
S
7.12.2 Sulfur Compounds in Diesel Feeds and Reactivities of These Compounds
Compound
Relative rate
Thiophene
100
Benzothiophene
50
Dibenzothiophene
30
Methyldibenzothiophene
5
S
Diesel feeds from different sources were investigated with different analytic techniques to identify and quantify the Scontaining species. Many different sulfur compounds were identified with the most abundant classes being benzothiophenes (BTs) and dibenzothiophenes (DBTs). The BTs may contain alkyl substituents (1–7 C atoms) and the DBTs such substituents with 1–5 C atoms. Whereas sulfur can be relatively easily removed from BTs, b-DBTs (called refractory Scompounds) with alky groups in position 4 or 4 and 6 (Figure 1) require drastic catalytic conditions. The distribution of sulfur compounds mainly depends on the origin of the
S
S
CH3 Dimethyldibenzothiophene 1 S
CH3
CH3 Trimethyldibenzothiophene 1
CH3
S S
CH3
CH3
H3C
CH3
Scheme 2 Relative rates of different S-containing molecules relative to thiophene.
Figure 1 The molecular structure of 4,6-methyl-DBT.
Reactivity
R
Deep HDS R
R S S
R
R S R
R S R S
R R
R S
Molecular geometry Figure 2 Decrease of the reactivity of different S-containing compounds in diesel feed. Present state of the art and future challenges in the hydrodesulfurization (HDS) of polyaromatic sulfur compounds.32
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One should note that the phenomena might not necessarily occur in the order shown here (see also Chapter 7.02). The reaction pathways of HDS of thiophene were intensively studied and it was proposed that the reaction proceeds via two parallel pathways (Scheme 3). In one reaction route (bottom path in Scheme 3), the sulfur from thiophene is removed directly (direct desulfurization, DDS) to form butadiene, which is then hydrogenated to give 1-butene and cisand trans-2-butene. In a further reaction, butenes are then hydrogenated to n-butane. In the second pathway (top path in Scheme 3), the first step is hydrogenation of thiophene to tetrahydrothiophene, which is further desulfurized to butadiene (hydrogenation pathway, HYD). Tetrahydrothiophene and butadiene were observed in trace amounts in the reaction products, and it was suggested that these intermediates are very reactive and they are transformed rapidly to butenes and butane.29 Due to this observation, the use of thiophene as the S-containing model compound is of less interest because the different reaction pathways cannot be distinguished. In a simplified picture, the general HDS of BTs and DBTs consists also of two routes: the DDS and the HYD pathway (Figure 3), both in the presence of molecular H2. One should note that both pathways occur in parallel employing different active sites of the catalyst surface. Which reaction pathway predominates depends on the nature of the sulfur compounds, the reaction conditions, and the catalyst used. The DDS pathway produces biphenyl (BP), while in the HYD route first hydrogenation of a benzene ring occurs followed by desulfurization, and in the case of DBT cyclohexylbenzene is obtained.38 In addition to hydrogenation of an aromatic ring before sulfur removal, hydrogenation of an aromatic ring may occur after sulfur was removed with a distinct
of elementary steps such as adsorption/desorption equilibria and hydrogenation and hydrogenolysis reactions, whose kinetics strongly depend on the reaction conditions and on the structure of the particular substrate, occur. The understanding of the fundamental steps is further complicated because results reported by different authors obtained on the same substrate such as thiophene applying similar catalysts and approximately identical reaction conditions are often very different, if not contradictory. Nevertheless, for the HDS reaction some primary phenomena involved can be summarized as follows:
• • • • • • •
adsorption (coordination) of the S-containing compound onto the active site, dissociative adsorption of hydrogen onto the surface of the catalyst, hydrogenation of unsaturated C¼C bonds, cleavage of two C–S bonds sequentially or simultaneously, addition of hydrogen to the broken bond of both S and C, release of the hydrocarbon product from the catalytic site, and release of H2S from the site.
S
S
Scheme 3 The two reaction pathways of HDS of thiophene with the different desulfurized and hydrogenated products.
Benzothiophene
S
S +
H2
+H2 -H2S
-H2S
S
+H2
Dibenzothiophene +H2
S +H2
S
S +H2
-H2S
-H2S
+H2
-H2S
S +H2
+H2
Figure 3 Simplified picture of the two principal pathways during HDS of S-containing molecules.15
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
rate constant for this reaction. This often leads to confusion in interpreting the results of experimental data as both routes can produce cyclohexylbenzene as the final product. The hydrogenation pathways are subject to thermodynamic equilibrium constraints and therefore partially hydrogenated intermediates such as tetrahydrodibenzothiophenes have lower equilibrium concentrations at higher temperatures. Consequently, a maximum of HDS via the hydrogenative route is observed as a function of temperature. Hence, HDS via the HYD pathway becomes limited at low pressures and high temperatures. In a different route, isomerization and transmethylation of methyl groups at the 4- or the 6- position may occur reducing the steric hindrance of the DBT molecule. It is assumed that the direct pathway DDS involves the insertion of a metal atom on the surface of the catalyst into a sulfur–carbon bond in the S-containing compound. After insertion of the metal atom, several reactions occur (Figure 4) leading to the removal of sulfur as H2S and the catalyst is restored to its original state. The formation of BP in the DDS pathway reflects the catalyst’s hydrogenolysis function for direct C–S bond cleavage, and the latter is due to the catalyst’s hydrogenation function to effect ring hydrogenation followed by C–S bond cleavage (see also Chapters 7.16 and 8.09 ).39 The two different desulfurization routes imply the presence of two different active sites in the catalysts. In Figure 5, the schemes of both reactions including details of the active sites are illustrated. The active site is a coordinatively unsaturated metal site where the S ligand is facile. The HL
S M
S I
S
S
S
S
S
S
S
Oxidative addition
H-
H
S
S
[H]
IV
M S
V H+
L +n
L M S
M S
S S
S-ring not opened
S
S
S
S
S
S-ring opened Reductive elimination (endo)
H+ Proton attack(exo) H
Hydride attack (endo or exo)
+n-2
H
L
M
S
S
L
-
S
S
+
M S
M S
M S
S
Proton attack (exo) H
H
H
H
S
M
III
M
L
+n
M
H
L
M
H
L
S
H H
S
S
+n-2
II H
L
S atom in aromatic rings can coordinate to the active center of HYD and DDS functions. There is evidence that in the case of DDS the initial adsorption of the S compound occurs through s-bonding, while for hydrogenation coordination is through p-bonding for both S removal and hydrogenation. The main reason for the low reactivity of b-DBTs seems to be a steric hindrance due to retarded adsorption onto the catalyst surface40,41 implying that adsorption is the rate-limiting step. Two types of chemisorption patterns of 4,6dimethyldibenzothiophene (4,6-DMDBT) on MoS2 are discussed: a flat adsorption and S-m3 type adsorption. The two molecules DBT and 4,6-DMDBT can interact well with the (10 1 0) edge of MoS2 by flat chemisorption. However, the chemisorption of 4,6-DMDBT is more difficult using the S-m3 type coordination due to the steric hindrance of the alkyl groups. This steric hindrance is expected to increase with increasing size of the alkyl groups (from methyl to ethyl to propyl). Consequently, an end-on adsorption mode is proposed involving interactions between the S atom and the active sites. A different explanation is that steric hindrance slows down the surface C–S bond scission.42 In this case, the rate-limiting step is not the Scentered adsorption and a side-on adsorption of the DBT molecule involving the p-electrons of the aromatic ring is proposed. Alkyl substituents in DBTs alter the overall HDS due to reduction of HDS reactivity and due to a change of the ratio between rates of the HYD and the DDS pathway. Alkyl substituents in the 4- and/ or 6-positions result in pronounced preference of the HYD route.7,23,38,40 In a recent study, the reaction sequence of the DDS route was determined as DBT > 4-MDBT > 4,6-DMDBT
L
Hydride attack (endo or exo)
291
S
S
S H+ proton attack
(a)
2,3-Dihydrothiophene
(b)
H2S, butadiene, thiobutene
Figure 4 Two different pathways to hydrogenation and ring-opening reactions leading to the products (a) and (b). Note that the mechanisms were determined with organometallic complexes. L ¼ ligands; M ¼ Co or Ni.
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
S
H
S
H
H
S Mo
S
S
Mo
Mo
S
H
S
S
Mo
S
Mo
S
Mo
Mo
S
S
S Mo
Mo
S
Mo
S
H
S
H
S
Mo
H
H
H
S
H
Mo
Mo
S
H
H
S
Hydrogenation
Simultaneous deS
Sequential deS
H
H
S Mo
S
S
H
H
S
S Mo
S
Mo
S
Mo
S
Mo
H H H S
S Mo
S
S
Mo
S
S
H
H
Mo
Mo
H
H S
S
S
S
Mo
S
Mo
Mo
S S
Mo
S
Mo
−H2S +H2 H S
S Mo
S
S
S
S
Mo
S
H
Mo
Mo
S
Mo
S
Mo
+H2 −H2S = Vacancy H
H
H
S
S Mo
H
S
Mo
S S
Mo
Figure 5 Vacancy model of the HDS mechanism as published in Ref. [32].
being explained by steric hindrance due to the methyl groups located adjacent to the S atom of DBT preventing s-bonding of S to the active site of the catalyst. However, no dependence of reaction rates of HYD pathways on the number of methyl groups was observed indicating that the alkyl groups do not hamper the p-adsorption of DBTs on the active sites.43 Deep desulfurization means to desulfurize the refractory S-containing molecules.
7.12.4 Catalyst Components, Formation and Nature of Active Phases Most industrially used catalysts consist of a support with a high specific surface (e.g., g-Al2O3) and active materials such as
MoS2/WS2 promoted with Co and/or Ni. In nature, MoS2 occurs as the mineral molybdenite in the form of black, lead-silvery gray crystallites or aggregates. In the structure of 2H-Mo(W)S2 (H¼ hexagonal, 2¼ two MS2 slabs in the unit cell, space group P63/mmc), layers consisting of S–Mo–S slabs are held together by weak van der Waals forces (Figure 6). The Mo/W atoms are in a trigonal-prismatic environment of six S atoms and the Mo(W)S6 polyhedra share edges within the slabs. Consecutive layers are stacked in an . . .ABAB. . . fashion along the crystallographic c-axis. The Mo–S bonds (Mo–S: 2.43 A˚) within the layers are covalent and in the trigonal-prismatic ligand field, a twoelectron band of mainly dz2 character is separated from the main part of the Mo 4d band and hybridizes strongly with the S 3p band. The dz2 band is completely filled and is separated by a
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
293
a b c
Figure 6 The crystal structure of 2H-MoS2.
gap of Eg 1.3 eV from the empty eg part of the Mo 4d band. Cleaving MoS2 crystallites between the layers generates a chemically inert (0 0 0 1) surface, while the (1 0 1 0) surface formed at the step edges of the basal planes exposes in alternating S–Mo–S sandwiches rows of coordinatively unsaturated S and Mo atoms, and is considered chemically reactive. In the following discussion, one should distinguish between the terms ‘active site’ and ‘active phase’: the active site is the transition metal ion where the adsorption of the reactant molecule such as a heterocyclic compound or hydrogen occurs through S vacancies or S ions. The active site and the ensemble of transition metal and sulfur ions are the active phase making this site active. The promoted catalysts are often abbreviated as CoMo(W) S/NiMo(W)S or CoMo(W)/NiMo(W), which are not chemical formula. The chemical compositions vary and the optimal promoter content depends on the HDS reaction conditions and the diesel feed. The Mo(W):S ratio is near 1:2, but nonstoichiometry has also been reported. In most applications and test reactions, the Co(Ni)/[Co(Ni) þ Mo(W)] ratio varies between 0.2 and 0.4. It is well documented that the structure and the dispersion of the catalytically active phase are dramatically altered during the life cycle of a typical supported Co–Mo/Ni–Mo catalyst, which is schematically shown in Figure 7. The preparation of the catalysts involves several individual steps: First, a Mo precursor such as (NH4)6Mo7O244H2O (AHM) (15–20 wt%) is impregnated on for example, g-Al2O3 (impregnation route) (see also Chapter 2.10). Then, different methods are used to introduce the promotors: the AHM-containing
support is first calcined at elevated temperatures yielding MoO3/g-Al2O3 followed by impregnation with the Co or Ni precursor (mainly Ni2þ/Co2þ salts such as nitrates) (1–5 wt%). The impregnated composite is calcined again to form (CoO/NiO) MoO3/g-Al2O3. There are reports that MoO3/g-Al2O3 is presulfided at elevated temperatures and the product is then impregnated with the promotor salt. A critical step is the transformation of the oxides into the sulfidic phases, and sulfide formation for one of the metals prior to the other should be avoided. It is well documented that Co is sulfided at lower temperatures than Mo. Hence, often additives such as chelating agents (citric acid, urea, glycol, nitriloacetic acid (NTA), and ethylene diamine tetraacetic acid (EDTA)) are added to increase sulfidization temperature of the promotors thus leading to a better dispersion of promotors. In addition, such additives lower the interaction strength between the catalysts and the support. Reactions between the Mo and Co/Ni precursors and the support should also be avoided and therefore the temperature of the sulfidization step should not be too high. Two strategies are followed to sulfidize the oxides: in the ex situ approach, the material is treated at elevated temperatures in, for example, 10% H2S/H2, 10% H2S/He, or 10% H2S/N2 flow; in the in situ method, the oxides are transferred to the HDS reaction chamber and sulfidization occurs in the presence of S-containing molecules. No general conclusion can be drawn whether the in situ route generates better catalysts than the ex situ method, and vice versa. In the as-prepared state, oxidic Co–Mo/Ni–Mo catalysts display a high Mo dispersion with well-dispersed MoO3 or polymolybdate phases and well-dispersed CoO/NiO is located near Mo particles.44–47 In some investigations, the presence of
294
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
a
b
MoO3
MoOxSy c MoS2
NiOxSy
h
NiSx
NiO
MoOxSy
g
i
j
MoS2 decorated with NiSx
d
MoS2 decorated with NiSx
NiSx MoS2 decorated with NiSx e f
Ni−Mo−NTA complex k Figure 7 Life cycle of a HDS catalyst: (a) Oxidic Mo and Ni/Co are bound to Al2O3; (b) partially sulfided catalysts, Ni/Co are sulfided, Mo oxysulfide is still bound to the support; (c) partially sulfided catalyst and part of Mo Mo is no longer bound to Al2O3; (d) partially sulfided catalyst consisting of small MoS2 slabs decorated with promoter Ni/Co; slabs are still anchored to the support; (e) fully sulfided catalyst with Mo being completely sulfided, MoS2 slabs decorated with sulfidic Ni/Co become mobile; (f) deactivation due to sintering of decorated small MoS2 slabs to form larger slabs, stacks, and crystals, edge surface area of the crystals is too low to accommodate all Ni/Co and separate Ni/Co sulfide crystals are formed; (g) regeneration – MoS2 crystals and some decorating Ni/Co sulfide get partly redispersed; (h) regeneration – large MoS2 crystals are only oxidized on the surface, Ni/Co sulfide crystals cannot redisperse on oxidation; (i) regeneration – small structures are fully oxidized, nearly the original state is reached; (j) high-temperature sulfidation with Mo being completely sulfided so fast that intermediate steps can be neglected; (k) NTA preparation route – Ni–Mo–NTA complex gets completely sulfided, and no linkages with Al2O3 are formed at any stage of the process.31
finely dispersed compounds such as CoMoO4/NiMoO4 was also observed.48,49 It is assumed that Mo is bound to the support via Mo–O–Al bridges (see also below).50–58 Several crystalline compounds such as MoO3, CoO, NiO, Al2(MoO4)3, CoAl2O4, NiAl2O4, CoMoO4, and NiMoO4 could be detected in x-ray powder patterns but mainly only in very small amounts if at all.
7.12.5 Analytical Tools for the Characterization of the Catalysts A large variety of analytic methods are used to characterize the catalysts in different states starting from the as-prepared precursors to catalysts recovered after intense use. In the following, some techniques are mentioned and some examples are discussed in detail demonstrating the kind of information that can be extracted by applying a distinct analytic method and combinations of methods. Chemisorption techniques are applied to receive information on the active phase dispersion and on the type of sites present on the catalyst surface. These methods include dynamic oxygen chemisorption (DOC),59,60 CO chemisorption (COC),61–69 and NO chemisorption (NOC).70–88
Temperature-programmed techniques such as temperatureprogrammed sulfidation (TPS),89–95 temperature-programmed reduction (TPR),96–103 and temperature-programmed oxidation (TPO)104–107 are suitable methods to understand the changes occurring during sulfidation of the catalyst and regeneration. Temperature-programmed desorption (TPD) or temperature-programmed surface reaction (TPSR) are sometimes applied.90,93 In a TPD experiment, a spent catalyst is heated and reaction intermediates and products desorbing from the surface are detected as a function of temperature. x-Ray photoelectron spectroscopy (XPS)110–115 was also often used for characterization of catalysts in different states. It yields information on the elemental composition and the oxidation state of elements. XPS is a surface-sensitive technique and, with this method, it is possible to determine how well particles are dispersed over a support.43,51,116–130 Extended x-ray absorption fine structure (EXAFS) spectroscopy and/or x-ray absorption near-edge spectroscopy (XANES) are powerful methods to probe the local environment of atoms and to estimate the oxidation state of the atoms. Hence, detailed information on the distance, number, and type of neighbors of the absorbing atom on a subnanometer length scale can be acquired.131–136
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
SIMS, Rutherford backscattering spectroscopy (RBS), infrared (IR) and transmission electron microscopy (TEM),64,137–140 Mo¨ssbauer emission spectroscopy,141–143 and in situ laser Raman spectroscopy (LRS)138,144,145 were also applied to characterize catalysts in different states.
7.12.6
Formation of the Sulfide Phases
The finely dispersed oxides MoO3/polyoxomolybdates and CoO/ NiO are transformed during the sulfidation,99,104,105,137,146–150 and a direct interaction between sulfidic MoS2 and Co/Ni is generated.151–156 If Mo oxide is fully sulfided, the catalyst consists of well-dispersed MoS2 nanoparticles, and sulfidic Co/Ni decorate the edges of the MoS2 slabs. The formation of large MoS2 and/or Co9S8 (Ni3S2, NiS) crystals is in most cases not observed.141,150,157–162 In cases where well crystalline compounds such as Al2(MoO4)3, CoAl2O4 (NiAl2O4), CoMoO4 (NiMoO4), CoO (NiO), and MoO3 are formed during calcinations, a full sulfidation of these compounds is often not achieved57,98,99,163 applying typical laboratory conditions such as gas phase, 673 K, 0.1 MPa, 10% H2S/H2, or commercial (e.g., liquid phase, þ/603 K, þ/3.0 MPa, light oil spiked with S compounds such as, dimethyl disulfide)164–167 sulfidation conditions. As outlined above, the different individual steps of the transformation of the oxidic precursors into the final sulfided catalysts were intensively studied with a variety of analytical tools. In the following, the results of fundamental studies on the sulfidation of oxidic precursors are discussed for selected examples.
7.12.7
Conversion of MoO3 into MoS2
Formally, the conversion of Mo oxide into the sulfide is a simple reaction according to eqn [1]:
Mo 3d
300400 C
MoO3 ! MoS2 H2 S=H2
7.12.7.1 Conversion of MoO3 into MoS2 Studied with XPS and IRES To reduce the complexity of the system, the sulfidation of crystalline MoO3 was investigated with XPS and infrared emission spectroscopy (IRES).168 Such an investigation allows comparison of elementary reaction steps with known structural features of Mo–O/Mo–S compounds. XPS is a surface-sensitive spectroscopic technique, being sensitive to changes of the oxidation state of the elements involved in the chemical reaction (see also Chapter 9.16) and IRES supplies information on the changes in Mo–O coordination during successive exchange of O atoms by S (see also Chapter 9.36). During conversion of MoO3 into MoS2, intermediate Mo oxysulfides are formed and therefore the thermal decomposition of (NH4)2MoO2S2 as a model compound was studied which decomposes according to eqn [2] and forms an {MoOS2} oxysulfide169 on the way to MoS2: ðNH4 Þ2 MoO2 S2 ! fMoOS2 g þ 2NH3 þ H2 O fMoOS2 g ! 0MoS2 0
[2]
A sequence of XPS spectra of MoO3 after sulfidation in H2S/ H2 at different temperatures is shown in Figure 8, left. The first spectrum of crystalline MoO3 exhibits a single Mo 3d doublet and the binding energy (Ebind) of 232.9 eV for the Mo 3d5/2
S 2s
S 2p
300 C 250 C 200 C 150 C 100 C 50 C 25 C
237 233 229 225 Binding energy (eV)
[1]
However, the structure of the nanocrystalline respectively amorphous oxidic catalyst precursor deposited on a support such as g-Al2O3 is not known and one can imagine a large variety of local Mo–O coordination environments generated in an unpredictable way during the preparation of the catalyst precursor. Hence, the investigation of real catalysts is a challenge requiring a combination of the results of different advanced analytical methods.
400 C
241
295
MoO3 221 168
164 162 160 166 Binding energy (eV)
158
Figure 8 Evolution of Mo 3d (left) and S 2p (right) x-ray photoelectron spectra of crystalline MoO3 with increasing sulfidation temperature.168
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
S O VI Mo
+ H2S
S H
H
O H
H
H
H
O
O
S
VI Mo
VI Mo
VI Mo
Scheme 4 Attack on the terminal O atom by H2S to form intermediate species and release of H2O.
line is in accordance with Mo in formal oxidation state 6þ. Sulfidation at low temperatures leads to pronounced changes in the spectra and successively further signals appear in the spectra indicating the formation of different intermediate species. Sulfidation seems to be finished between 250 and 400 C and the spectrum of the sample treated at the highest temperature shows essentially one doublet and the binding energy of the Mo 3d5/2 peak of 229.0 eV corresponds to Mo4þ in MoS2.170 In the corresponding S 2p spectrum (Figure 8, right), there is only one doublet with Ebind ¼ 161.8 eV for the S 2p3/2 core level in agreement with the sulfide S2 anion present in MoS2. For different temperatures between 25 and 300 C, all the Mo 3d spectra can be explained on the basis of mixtures of Mo6þ and Mo4þ doublets and an additional doublet with a Mo 3d5/2 binding energy of 231.1 eV, which is already observed after sulfidation at room temperature (RT). A tentative assignment was done to Mo5þ present in oxysulfide intermediate phases. All spectra contain Mo 3d signals in the partially sulfided state from these three states in varying percentages giving evidence for the stepwise transition of Mo6þ in MoO3 to Mo4þ in MoS2. The S 2p peak in Figure 8, right, can be explained as the superposition of two doublets with Ebind ¼ 161.7 and 163.1 eV for the S 2p3/2 peaks. The signal located at 163.1 eV can be assigned to a bridging S22 disulfide anion,138,139 while the peak at 161.8 eV points to terminal disulfide and/or sulfide anions. According to the XPS results, one of the first elementary steps during sulfidation involves a metal–ligand redox reaction leading to the formation of Mo5þ and oxidized sulfur ligands (S22). Secondary ion mass spectrometry (SIMS) measurements gave evidence that the sulfidation of MoO3 leads to the formation of oxysulfides at the surface and Mo(IV) oxides in the interior of the crystallites at sulfiding temperatures between RT and 100 C, while MoS2 is formed above 125 C. For sulfur, two different chemical states are found and the most likely species are S2 and S22 or SH, but the presence of elemental S can be excluded. The S22 or SH species disappear when the temperature is raised to 150–200 C. Rutherford backscattering data yield S:Mo atomic ratios of 1–1.5 for T < 100 C and of 2–2.5 above 100 C.137 Combining the results obtained on sulfidation of MoO3 and the model compound (NH4)2MoO2S2 yields the following pictures of individual reaction steps for the two different temperature regimes RT to 200 C (generation of Mo oxysulfide) and 200–400 C (transformation of Mo oxysulfide into MoS2). The terminal Mo¼Ot groups are relatively unstable and play an important role in the uptake of sulfur. This instability is well known from solution chemistry where dissolved molybdate species react with H2S in NH4OH to form tetrathiomolybdate: MoO42 þ 4H2S ! MoS42 þ 4H2O. In solution, the reaction may proceed via H2S or SH formed in situ by deprotonation of H2S, which depends on NH4OH concentration. For the
O
S
O VI
VI
Mo
O
+ H2S - H2O
Mo
Mo
VI
S
V
VI
O
Mo
S
S
Mo
VI
O O
O S
S VI
Mo
Mo
O
O
V
Mo
O
Scheme 5 Replacement of two terminal O atoms by two S atoms followed by oxidation of two S2 to S2 2 and reduction of Mo6þ to Mo5þ.
O
O VI
Mo
+ 2H2S − 2H2O
S
S
S
S
VI
IV
Mo
Mo
Scheme 6 Exchange and subsequent redox reaction of edge metal centers involving two terminal O2 atoms.
MoVI
Ot + H2
MoIV + H2O
Scheme 7 Direct reaction between terminal O atom and H2 and reduction of Mo6þ to Mo4þ.
sulfidation of solid MoO3, a deprotonation of H2S is not possible and H2S is the reactive component. Hence, the terminal O atom is attacked by H2S to form intermediate species (Scheme 4) and H2O is released, which is the driving force of the reaction. Note that in this step the oxidation number of Mo is not changed. In the next reaction step (Scheme 5), Mo6þ is reduced to Mo5þ by a metal–ligand redox reaction in which two S2 anions bonded to adjacent Mo6þ centers are oxidized to S22: 2S22 ! S22 þ 2e and the Mo centers are reduced, 2Mo6þ þ 2e ! 2Mo5þ. The proposed reaction scheme is clearly supported by the XPS data and is in agreement with the IRES showing characteristic emission from disulfide species. In the low-temperature regime, the occurrence of Mo4þ in the XPS spectra may be explained by the reaction in Scheme 6, indicating a subsequent redox reaction with two terminal O22 anions. In principle, a direct reduction may occur according to Scheme 7 but as was demonstrated by TPS measurements,97 this type of reaction is more relevant at higher temperatures. Despite the S/Mo ratio in the XPS spectra measured after sulfidation at 50 and 100 C exhibiting very similar values, the relative amount of the different sulfur ligands changed. The fraction of S2 and/or terminal S22 anions increased because of bridging S22 ligands. In parallel, the proportion of Mo5þ increased, as opposed to the contribution of bridging S2 anions. A possible explanation is the release of S from S22 bridges according to Scheme 8.
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Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
H S S
V Mo
O
H
S V Mo
+ H2
S
V Mo
V Mo
O
− H2S
O O S
V Mo
V Mo
O O
–
entity with H2 forming two transient SH fragments, stabilization by H transfer (H2S complex), and subsequent emission
First, S22 reacts with H2 yielding two transient SH units. Such SH units must be kinetically stabilized by, for example, bulky ligands and sufficient electron density on Mo, which is highly unlikely. However, stabilization occurs easily by proton transfer to form H2S, which then immediately decomposes. After the emission of S from S22 units, the sterical hindrance on the surface of the crystal is reduced so that new S22 groups can be formed, leading to a higher S/Mo ratio in the XPS spectrum of the sample sulfided at 150 C. In the IRES, the low-intensity Mo¼Ot vibration indicates that most of the available Mo¼Ot moieties have reacted at 150 C. At 200 C sulfidation temperature, the analysis of the surface composition using the XPS spectrum demonstrates an increased degree of sulfidation. In the temperature range 200–400 C, a major change of composition is observed compared to that of the oxysulfides detected at lower temperatures. In this region, Mo6þ and Mo5þ species are mainly reduced to Mo4þ and the signal of bridging S22 in the XPS spectrum is absent. The formation of Mo4þ is mainly due to the reaction of Mo¼Ot with H2 (Scheme 8) followed by S uptake from the sulfiding atmosphere. The finally formed MoS2 contains imperfections in the sulfur planes and the IRE spectrum gives evidence that m2-S atoms are present on Mo–S–Mo bridges. Such defects are accompanied with a S deficiency and leads to a bending of the MoS2 layers (Scheme 9). Indeed, bent MoS2 nanocrystallites are often seen in TEM images (see below). It is well known that the formation of perfect MoS2 requires temperatures of about 800 C. An important conclusion of the study performed on MoO3 is that sulfidization of MoO3 at about 400 C, typically for catalyst pretreatments, generates MoS2 exhibiting an appreciable amount of defects.
7.12.7.2
Temperature-Programmed Sulfidation of MoO3
The sulfidation of the MoO3 model compound in a H2S/H2/Ar atmosphere as a function of temperature was studied with TPS,97,98 supporting the findings presented above. In the TPS pattern of MoO3 supported on Al2O3, negative peaks indicate that the corresponding gas is consumed, and positive peaks mean that the gas is released. As can be seen in Figure 9, at 400 K H2S is consumed and H2O is produced and no uptake of hydrogen can be detected. The observations suggest that at low temperatures an exchange of O by S atoms is the main reaction:
S
S
S Mo
Mo
Mo
Mo
Mo
S
S
S
S
S Mo
S
S
S Mo
S
Mo S
Scheme 9 A layer of MoS2 containing a m-S atom as defect.
TPS of MoO3/Al2O3
H2
Partial pressure (a.u.)
Scheme 8 Reaction of a S2 of H2S.
2
H2S
H 2O
400
600 800 1000 Temperature (K)
1200
Figure 9 TPS pattern of MoO3 supported on Al2O3 pretreated in Ar at 298 K.98
MoO3 þ H2 S ! MoO2 S þ H2 O Increasing the temperature to T 500 K, a sharp hydrogen consumption peak is seen while at the same temperature H2S and small amounts of H2O are produced. These observations may be interpreted as hydrogenation of excess sulfur formed by the decomposition of MoO2S according to the equations: MoO2 S ! MoO2 þ S S þ H2 ! H2 S
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Up to the final temperature, further oxygen is replaced by sulfur until MoS2 is formed: MoO2 þ 2H2 S ! MoS2 þ 2H2 O One should keep in mind that TP experiments detect only reactions accompanied by a net production or consumption of gases. If the following two reactions would occur instantaneously, no hydrogen signal would be detected: MoO2 þ 2H2 ! Mo þ 2H2 O Mo þ 2H2 S ! MoS2 þ 2H2 In any case, TPS is a powerful method to study the sulfidation of the catalysts, giving useful information about the stepwise conversion of the oxidic precursors into the final sulfide.
7.12.8 Quick EXAFS Spectroscopy Investigation of the Local Environments in Ni-Promoted Molybdenum Sulfide Catalysts
QEXAFS Mo K-edge of NiMo/SiO2
Fourier transform of k3 c(k)
Tsulf (C) 400 400 400 400 380 365 345 330 315 295 280 260 245 225 210 195 170 155 140 125 110 90 70 55 40 Fresh
0
1
2 r (Å)
3
5 a.u.
400 390 370 350 330 315 295 280 260 245 225 210 195 175 160 140 125 105 90 70 50 35 Fresh
|FT[c(k)k3]| (a.u.)
The changes of the local environment around Mo of a NiMo catalyst deposited on high surface area silica during sulfidation were investigated with a temperature-resolved Quick EXAFS (QEXAFS) study (Figure 10).171 The sample was heated with a heating rate of 6 C min1 in 10%H2S in H2 atmosphere. The resulting Fourier transform of the Mo K-edge EXAFS spectra is displayed in Figure 10.171 The sulfidation process can be divided into four regions. Up to about 110 C, Mo is present in the oxidic form; that is, Mo is mainly surrounded by O atoms as seen by Mo–O shells between 0.5 and 1.9 A˚ and a Mo–Mo shell at 3 A˚ (values not phase-corrected). Between about 110 and 225 C, the longest Mo–O shell and the Mo–Mo shell disappeared, whereas the shorter Mo–O can still be identified. A new signal appears at about 2 A˚ (not phase-corrected) caused by the presence of S
indicating the co-existence of Mo–O and Mo–S shells, with the Mo–O shell corresponding to Mo¼Ot groups. Near the 225 C region, a new signal at 2.5 A˚ (not phase-corrected) develops belonging to an intermediate product. The signal at about 2.5 A˚ dominates the temperature range between 225 and 280 C and in this region only contributions of the Mo–S and the Mo–Mo shells are seen attributed to sulfur-rich phases such as MoS3, which was also reported in other EXAFS studies.172 Above about 310 C, the spectrum of the final product MoS2 starts to form (region IV) and no significant changes are seen up to the final temperature of 400 C. The shoulder on the left side of the Mo–Mo shell at about 3 A˚ is caused by the interference of the Mo–S and the Mo–Mo shells. The results are in accordance with those obtained from XPS, RBS, and SIMS137,173 and slight differences of temperatures at which the changes occur are due to the different experimental setups of the methods. The intermediates formed during sulfidation of the Mo precursor are those discussed above. Due to the low loading of Ni of only about 1.3 wt%, EXAFS data were collected with a lower heating rate of 3 C min1. Fourier transform spectra (Figure 11) of the sulfidation of the fresh catalyst show Ni–O and Ni–Si shells at 1.65 and 2.9 A˚ (not phase-corrected), respectively, while the Ni–Si shell disappeared at 105 C. The transformation of the Ni oxide to the
Region IV MoS2 Region III MoS3-like Region II oxysulfides Region I oxidic
Surfidation temperature (C)
298
4
Figure 10 Fourier transforms of the Mo K-edge Quick EXAFS functions obtained during sulfidation of NiMo/SiO2. The four sulfidation regions are shown.171
0
1
2
3
4
r (Å)
Figure 11 Fourier transforms of the Ni K-edge Quick EXAFS functions measured during the sulfidation of NiMo/SiO2.171
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Ni sulfide is seen in the spectra by a shift of the signal by about 0.1 A˚ in the distance of the first shell. Definitive sulfidation occurs between 125 and 140 C and no further alteration can be detected in the spectra up to 400 C. That means full sulfidation is achieved for Ni at a significantly lower temperature than for the Mo oxide.
7.12.9 Sulfidation of Co-Promoted Molybdenum Oxide in the Presence of a Chelating Molecule Investigated with XPS The situation is similar for CoMoS catalysts: Co is sulfided at low temperatures and stable bulk sulfides such as Co9S8 are formed as large crystals while sulfidation of Mo only starts at moderate temperatures. Hence, it is surprising that a phase is formed in which Co atoms decorate the edges of MoS2 slabs and not a mixture composed of separated Co9S8 and MoS2. To retard sulfidation of Co or Ni, chelating agents such as NTA, EDTA, ethylenediamine (EN), or 1,2-cyclohexanediamineN,N,N,N-tetraacetic acid (CyDTA) are added during catalyst preparation,171,174–178 and the as-prepared precursor mixture is not calcined in order to leave the complexes formed with the ligands intact. The effect of NTA addition on the formation of the sulfides was studied with XPS on a CoMoS model catalyst deposited on SiO2. In the absence of NTA, Co is transformed into Co9S8 at very low temperatures (Figure 12, left). Even at RT, sulfidation of Co in 10% H2S/H2 atmosphere starts as can be seen by the appearance of a second doublet at 779.3 eV corresponding to the binding energy of bulk Co9S8. Increasing the temperature leads to disappearance of the doublet at 782.3 eV and only the signal at 779.3 eV remains, which means that cobalt has completely transformed into the sulfidic state. The addition of NTA retards sulfidation of Co and the
transformation to the sulfidic state starts above about 150 C and is completed at 225 C. Below 150 C, the typical signal of the oxide is seen and at higher temperatures the doublet of sulfidic cobalt appears (Figure 12, right). Recording the evolution of the N 1s signal as a function of sulfidation temperature demonstrates that the intensity starts to decrease and the signal is absent when sulfidation of cobalt is complete. These observations suggest that NTA is decomposed and Co ions are available for sulfidation. Sulfidation of Mo oxide calcined at 500 C in the absence of NTA (Figure 13) starts at a higher temperature than the formation of Co sulfide. In the as-prepared state, the binding energy of the Mo 3d5/2 core level matches well with Mo6þ. On increasing the temperature, MoO3 is successively transformed into MoS2 as discussed above and the typical spectrum of MoS2 is seen with a binding energy of 228.8 eV for the Mo 3d5/2 signal. The Co 2p3/2 and Mo 3d XPS spectra of a CoMo catalyst with an atomic ratio of 0.3 calcined at 500 C are shown in Figure 14, right. In the unsulfided state, the binding energy of the Mo 3d5/2 signal at 232.7 eV is typical for oxidic Mo in oxidation state 6þ. After sulfidation at T > 175 C, a Mo 3d doublet appears in the spectra, typical for MoS2. The results clearly show that sulfidation of Mo oxide occurs between T 50 and 175 C and several intermediates are formed containing Mo6þ, Mo5þ, and Mo4þ species. At about 150 C, the doublet of Mo4þ appears in the spectrum and at T > 175 C the signal of Mo6þ disappears. The sulfidation behavior of Co is shown in Figure 14. In the as-prepared state, a Co 2p doublet is seen at 781.8 eV with shake-up satellites at higher binding energy typical for Co2þ oxide. On increasing the temperature above 175 C, the spectrum shows a Co 2p doublet at a binding energy of 778.7 eV typical for Co2þ in a sulfidic environment. Shake-up satellites occurring at higher binding energy are less pronounced Sulfidation temperature
Sulfidation temperature
Co 2p
299
Co 2p
400 C 300 C
400 C
225 C
300 C 200 C
200 C
150 C
150 C
100 C
100 C
50 C Unsulfided
25 C Unsulfided Shake up Co 2p1/2
Shake up Co 2p 3/2
814 809 804 799 794 789 784 779 774 810 805 800 795 790 785 780 775 770 Binding energy (eV) Binding energy (eV) Figure 12 Co 2p XPS spectra of CoOx/SiO2 model catalysts (left) and Co 2p XPS spectra of Co(NTA)/SiO2 model catalysts as a function of sulfidation temperature.173
300
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Sulfidation temperature
compared to oxidic Co. According to the XPS spectra recorded over temperature, the sulfidation of Co starts at T > 50 C and is completed around T ¼ 150 C. Summarizing the results, in a mixed CoMo/SiO2 catalyst Co sulfide is formed between 50 and 150 C and Mo is sulfided between 100 and 200 C. Both regions overlap to an appreciable extent, but sulfidation of Co proceeds faster than that of molybdenum.
Mo 3d
400 C 300 C 250 C
7.12.10 Determination of the State of Co in CoMoS Phase Using In Situ Mo¨ssbauer Emission Spectroscopy
200 C 175 C
150 C 100 C 50 C 25 C Mo 3d3/2
Unsulfided 240
238
236
Mo 3d5/2
234 232 230 228 Binding energy (eV)
226
224
222
Figure 13 Evolution of the Mo 3d XPS spectra of MoO3/SiO2 model catalysts with increasing sulfidation temperature.173
It is well documented that Co-promoted MoS2 catalysts are in general much more active for HDS than unpromoted MoS2. Hence, the key question is to determine the chemical state of Co and the structural environment in sulfided CoMoS catalysts supported on Al2O3. Mo¨ssbauer spectroscopy probes the local environment of the absorber as well as the chemical state simultaneously. Because Co is not a suitable Mo¨ssbauer nucleus the experiments were performed in the emission mode, that is, the measured spectrum is not that of Co but of the decay product Fe. Radioactive 57Co was used as a promoter during catalyst preparation and the 57CoMoS catalyst was investigated under in situ conditions so that the Co spectrum and the changes could be directly correlated to the activity of the catalyst. The Mo¨ssbauer spectra of the CoMoS catalyst are compared with that of Co model compounds for which the state and the environment of Co are known.141,179 The analysis of the Mo¨ssbauer spectra of the sulfided CoMoS/Al2O3
Mo 3d
Sulfidation temperature
Sulfidation temperature
Co 2p
400 C 400 C
300 C
300 C 250 C
250 C
200 C
200 C
175 C
175 C 150 C
150 C
100 C 100 C
50 C
50 C
25 C
25 C Unsulfided Unsulfided
240 (a)
238
236
234
232
230
228
Binding energy (eV)
226
224
222
814 (b)
809
804
799
794
789
784
779
774
Binding energy (eV)
Figure 14 The Mo 3d (a) and Co 2p (b) XPS spectra of a CoOx/MoO3/SiO2 catalyst calcined at 500 C during sulfidation at different temperatures.173
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Co:Al2O3
Co Mo
Co9S8
40
Co-Mo-S Co9S8 (a)
0.27
(b)
30
mg Co/g Al2O3
0.09
20
Co–Mo–s
10
Absorption
301
(c)
0.53
Co:Al2O3
(a)
140 120
kT (cm3 min-1 g-1)
(d)
0.75
(e)
1.19
100 80 60 40 20
(b)
-6
-4
2 0 -2 Velocity (mm s-1)
4
6
Figure 15 In situ Mo¨ssbauer emission spectra of 57Co in sulfided CoMo/Al2O3 catalysts with different Co:Mo ratios.141
catalysts (Figure 15) essentially revealed contributions from Co ions located in the Al2O3 support, bulk Co9S8 dominating in catalysts with a high Co content, and a new formerly unknown state, which is labeled in Figure 15. This new state is best seen in sulfided CoMoS with a low Co content. The presence of this new phase correlates with the catalytic activity for the HDS reaction (Figure 16). Hence, one may conclude that this CoMoS phase is itself catalytically active or is closely related to the active site of the catalyst. This is because the catalytic activity of MoS2 is associated with edges and it is highly likely that the CoMoS phase is located at or near the edges. This assumption was verified with Auger spectroscopy and transmission electron microscopy clearly demonstrating that the basal planes of MoS2 do not contain measurable/detectable amounts of Co while the edges of the crystallites contain Co besides S and small amounts of O.180,181 In further MES experiments, it was demonstrated that Mo is not required to obtain the ‘Co–Mo–S’ MES spectrum. For Co/ Al2O3, Co–Mo/AI2O3, Co/C, and Co–Mo/C, Co sulfide species exhibiting the ‘Co–Mo–S’ type features such as quadrupole splitting (QS) were observed (Figure 17). Furthermore, the temperature dependence of the MES spectra of the CoMoS phase is identical with that of the Co sulfide phase present in Co–Mo/AI2O3 and Co/C catalysts.183 The important conclusion of these observations is that the
0.2
0.4
0.6
0.8 Co Mo
1.0
1.2
1.4
Figure 16 Correlation between the activity of CoMo/Al2O3 catalysts for the HDS reaction, expressed in the reaction rate constant kT, and the cobalt phases observed in Mo¨ssbauer spectra.141
Co sulfide species are essentially the same, and only the particle sizes and/or ordering may differ.157,183–185 Other important findings of the MES experiments are that the value of QS of the Co phase depends on the Co content in the catalysts and on the sulfidation temperature with large QS values indicating the presence of very small Co sulfide units and small values are related to larger well-ordered Co sulfide particles.
7.12.11 The Local Structures of Co/Ni and Mo in CoMoS and NiMoS Phases Determined with EXAFS The local environment of Mo and the promoter atoms were investigated with EXAFS/XANES in different states of the catalysts under in situ conditions allowing in situ sample presulfiding151,153,154,156,186–189 in a mixture of e.g., H2S/H2 at atmospheric pressure and at 673 K. Because industrial CoMoS catalysts are supported on Al2O3, such catalysts contain at least two Co species such as Co in the Al2O3 lattice and in the CoMoS phase. Hence, the CoMoS catalysts are deposited on carbon for the EXAFS investigations. The radial distance distribution (Fourier transform) of MoS2, sulfided Mo, and a CoMoS catalyst supported on carbon is shown in Figure 18.153 Evaluation of the data of MoS2 yields
302
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Co(2.25)Mo(6.84)/C
Co(2.45)/C 5.18
(S, 373 K)
4.86 3.06
5.11 2.83
(S, 473 K)
2.77 3.68
2.99 3.27
(S, 573 K)
3.58
3.20 3.27
3.64
(S, 673 K)
3.20
-4
-2
3.54
T = 300 K
T = 300 K -6
0
2
-6
4
Intensity (106 counts)
Intensity (106 counts)
4.94
Doppler velocity (mm. s–1)
-4
-2
0
2
4
Doppler velocity (mm. s–1)
Figure 17 MES spectra of carbon supported Co and CoMo catalysts measured at 300 K after various sulfidation treatments.182
30
Mo-Mo (3.18 Å)
10
MoS2 Mo/C Co–Mo/C
Co-S + Co-Co (1)
Co9S8 Co/C Co-Mo/C
Abs (FT)
Abs (FT)
Mo-s (2.41 Å)
15 Mo-Mo (6.32, 6.41 Å)
Co-Co (2)
Mo-S Mo-Mo (3.98 Å) (5.47 Å)
0 0
2
4
6
8
r (Å) Figure 18 Magnitude of the Fourier transforms of the MoS2 reference compound (solid line), the Mo/C catalyst (dotted line), and the Co–Mo/C catalyst (dashed line).153
a coordination number (CN) of 6 for Mo with the S atoms at a distance of 2.41 A˚. The next-nearest neighbors are six Mo atoms at a distance of 3.16 A˚. The reduced intensity of the first and the second peak in the spectrum of the two catalysts is an indication for a reduced number of neighbors; especially for the Mo–Mo coordination the number is lower than 6. To explain these observations, a structural model was developed taking into account the reduced CNs in the catalysts, leading to the average crystallite size for MoS2 of 1.5–3 nm. Such small particle crystallites will expose a relatively large number of catalytically active edge sites. The absolute part of the Fourier transform (FT) spectrum of Co9S8, of Co/C and Co–Mo/C samples, is displayed in Figure 19. In the crystal structure of Co9S8, three different Co–S coordinations are present, which are denoted in the spectrum as one overall Co–S coordination. The two Co–Co distances at about 3.5 A˚ are given as one overall Co–Co(2) coordination (indexed (2) to distinguish these from the first Co–Co coordination at 2.505 A˚). The first peak in the spectrum of
0 0
2
4 r (Å)
6
8
Figure 19 Fourier transforms of Co9S8 and the sulfided Co/C, Co–Mo/ C, and Co–Mo–S/C catalysts.131
Co9S8 is caused by a combined Co–S and Co–Co(1) coordination and the second peak is attributed only to the Co– Co(2) distance. In the spectra of Co/C and Co–Mo/C samples, the first peak is slightly shifted to lower r values. Overall, the two sulfided catalysts show identical features as Co9S8 but with reduced amplitudes. Despite the overlap of the Co–Co/Co–S nearest neighbors in the magnitude of the Fourier transform, a clear differentiation between these two contributions is seen in the imaginary part of the spectrum (Figure 20). In addition, a scattering contribution from Mo neighbors can also be identified. The detailed analysis of the spectra yields a Co–S distance of 2.2 A˚ with a high sulfur CN of 6.2(1.3). The Co atom is further surrounded by 1.7(0.35) Mo neighbors at a distance of 2.8 A˚. Using these geometric parameters, a model was developed fitting with a structure where Co is located on the edge of a MoS2 crystallite in the same plane as the Mo atoms. It was proposed that Co is coordinated to four S atoms on the edge, which are slightly displaced from their bulk positions. According to this model, Co is not exactly on the position of a
303
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
10
673 K
II
Co-Co (2)
Im (FT)
W−S
15
Co9S8 Co/C Co–Mo/C Co–Mo–S/C
|FT [c(k)k3]| (a.u.)
Co-S Co-Co (1)
0
573 K 10
473 K 5 W-O I 373 K 300 K 0 0
-10
b 0
1
2 r (Å)
3
4
Figure 20 Imaginary part of the Fourier transforms of Co9S8, sulfided Co/C, Co–Mo/C, and Co–Mo–S/C catalysts.131
Mo atom of a bulk crystal, which would give a Co–Mo distance of 3.16 A˚, but is shifted toward the MoS2 particle yielding the experimentally determined Co–Mo separation of 2.8 A˚. The S CN of 6 is achieved by placing two S atoms on Co to form a distorted trigonal prism together with the four S atoms of the MoS2 edge. A slightly different model was proposed on the basis of density functional theory (DFT) calculations indicating that some reconstruction may play a role but the model is also in agreement with the EXAFS data.
7.12.12 Sulfidation of NiW Catalysts Supported on Al2O3 The extent of sulfidation of W is crucial for catalytic performance.190–192 It is well documented that the sulfidation of W in NiW/Al2O3 occurs at a lower rate than that of Mo of Al2O3supported CoMo or NiMo catalysts. It was pointed out that the relatively strong W–O–Al bond94 or the stronger W–O bond compared to the Mo–O bond are responsible for this behavior and hence higher temperatures are required to fully transform W oxide to W sulfide.193–195 It is well documented that the final degree of W sulfidation strongly depends on the calcination and the sulfidation temperatures.191,192,196 At the working temperature of 673 K, Al2O3-supported NiW catalysts are only partially converted to WS2 and other species such as WS3 or oxysulfides are still present.191,192 In contrast to Mo-based HDS catalysts, the addition of Co as promoter seems to have no positive effect,193 while Ni addition enhances the catalytic performance of the W-based catalysts. For NiW-based materials, it was observed that Ni can easily be sulfided and sulfidation is completed before W sulfidation has started.193,197 At low sulfidation temperatures, Ni sulfide phases are present as a separated compound or as small Ni sulfide particles interacting with WOxSy phases. At higher sulfidation temperatures, WS2 slabs are formed accompanied by a redispersion of Ni sulfide species and Ni–W–S-type structures are formed,198–201 which was shown by a 57Co Mo¨ssbauer emission study on 57Co-doped NiW/Al2O3.202 As for Co(Ni)MoS-based catalysts, two types of
2
4
6
8
r (Å)
Figure 21 W EXAFS Fourier transforms of NiW/Al2O3 after sulfidation at indicated temperatures. Intervals I and II: W–W contributions of WO3 and WS3, respectively (R not corrected for phase-shift).203
NiW catalysts are proposed: a type I Ni–W–S phase obtained at low-temperature sulfidation exhibiting residual oxygen linkages showing a high hydrogenation activity and a type II phase formed at higher sulfidation temperatures exhibiting a high HDS activity.191 The sulfidation behavior of NiW materials supported on different materials (g-Al2O3, carbon, and amorphous silica-alumina (ASA)) was studied applying different analytic tools and some selected results are discussed in more detail. The supports were impregnated using aqueous solutions of ammonium metatungstate ((NH4)6W12O39xH2O) and nickel nitrate. The resulting material was dried at 383 K and subsequently calcined at elevated temperature in air. The sulfidation was done stepwise in a mixture of 10 vol.% H2S in H2 at 300, 373, 473, 573, 673, and 773 K for 1 h, atmospheric pressure. For catalytic measurements, the catalysts were sulfided at 673 K for 2 h.203
7.12.12.1 x-Ray Absorption Spectroscopy Performed at the W LIII Absorption Edge The Fourier transforms of a catalyst that has been calcined at 673 K prior to sulfidation are shown in Figure 21. At low sulfidation temperature, the spectra are dominated by two W–O contributions at 1.76 and 1.91 A˚ corresponding to the W–O bonds in the first coordination shell of WO3. The peaks in the region labeled I (Figure 21) are caused by second shell W and third shell O backscatters. These findings indicate that at low sulfidation temperatures an oxidic tungsten phase is present with a WO3-like structure. The sulfidation of W starts at 573 K and a W–S contribution at a distance of 2.42 A˚ with a CN of 0.9 can be detected in the spectrum. In this spectrum, a second W–W contribution (2.71 A˚, CN 1) is seen indicating the presence of a WS3-type structure (W–S ¼ 2.40 A˚, CN W–S ¼ 5; W–W ¼ 2.75 A˚, CN W–W ¼ 2). While at 673 K the W–O contributions are clearly reduced, contributions of the oxidic W phase are still present. The characteristic W–W distance in WO3 at 3.73 A˚ cannot be detected in the spectrum (T ¼ 673 K) suggesting that the original W oxide phase has largely disappeared. In addition, an increase of the W–S coordination from 0.9 to 1.6 is observed
304
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
while the W–W CN associated with the WS3-type structure decreased from 1.0 to 0.2. The lower than expected W–S CN for a-WS3 indicates that sulfidation of the WS3-type phase is not complete at 673 K. A small W–W contribution at 3.16 A˚ points to the onset of WS2 formation, and it can be assumed that a partial transition from WOxSy and WS3-like phases to WS2 already occurred before complete sulfidation to WS3 was achieved. In accordance with results reported earlier,95,193 the W species formed during the sulfidation procedure may be regarded as a WOxSy phase. If a NiW catalyst supported on Al2O3 is sulfided in a single step at T ¼ 673 K for 2 h, much more WS2 is present compared to the stepwise sulfidation. In addition, the amount of oxygen is also substantially lower. A higher calcination temperature of 823 K and a single-step sulfidation leads to a lower degree of sulfidation and the catalyst contains more oxygen and less sulfur. Several NiW catalysts with a varying Ni:W ratio were calcined at 823 K and then sulfided at T ¼ 673 K at a total pressure of 15 bar (H2S/H2 ratio: 0.1). The analysis of the EXAFS data indicate that in all catalysts WS2 slabs are present pointing to a higher degree of sulfidation at higher pressure. With increasing Ni:W ratio, sulfidation seems to be more complete and at low Ni:W ratio the low W–S CNs suggest the presence of some oxygen. A larger number of W–W CNs at higher Ni:W ratios may be interpreted as a higher order of the crystallites and a more complete sulfidation.
7.12.12.2
occurrence of Ni–Ni contributions indicates that NiO and Ni– (oxy)sulfide are not well dispersed over the support surface. In the XANES spectra, a continuous decrease of the area of the white line is accompanied by a decrease of the Ni–O CN suggesting a progressive Ni sulfidation. In the spectrum of the sample sulfided at 573 K, the Ni–O contribution is absent and most of Ni is sulfided. In the XANES spectra (Figure 22, right), a pre-edge peak near 8333 eV characteristic for the 1s to 3d transition shows an increase of the intensity related to a change of the symmetry around Ni. For the Ni sulfide obtained at 573 and 673 K, the bond length for Ni–S amounts to 2.21 A˚ and that for Ni–Ni is 2.93 A˚. Especially the Ni–Ni separation does not match with any well-known bulk Ni sulfide such as Ni3S2 (Ni–Ni¼ 2.50 A˚, 3.80 A˚), NiS (Ni–Ni ¼ 2.68 A˚), NiS with millerite structure (Ni–Ni ¼ 3.16 A˚), NiS2 (Ni–Ni ¼ 3.92–4.12 A˚), or Ni3S4 (Ni–Ni¼ 2.50 A˚), and together with the Ni–S bond length of 2.21 A˚ the formation of the ‘Ni–W–S’ structure may be assumed. The results of the studies can be schematically summarized as shown in Figure 23. Tungsten oxide is more difficult to be sulfided due to the stronger metal–oxygen bond than that of Mo oxide in Co(Ni) Mo catalysts. Hence, over a broad range of sulfiding conditions incomplete sulfided tungsten phases are observed and these may be viewed as oxysulfides WOxSy. For these materials, indications are found for the presence of WO3, WS3, and WS2 with differing amounts of oxygen and sulfur atoms. Interaction between WO3 and the support g-Al2O3 makes it more difficult to sulfide W and calcination even increases the interaction with the support. Investigations with carbon as support demonstrate a real support effect and because metal–support interactions are negligible or even absent, WS2 formation is more or less complete at 673 K for NiW/C, while alumina or amorphous silica–alumina supports require sulfidation at higher temperatures being accompanied by a loss of WS2 dispersion. Because sulfidation of W is very slow, Ni sulfide particles tend to aggregate as larger crystallites, being not segregated from the WOxSy phase. Under typical sulfiding conditions (673 K, 1 bar), not all NiS species are redispersed to the WS2 slabs. This is caused by the sulfidation sequence (NiO is sulfided earlier than WO3) and because not all tungsten has converted to WS2. Compared to sulfided Co(Ni)Mo catalysts with
Ni K Edge x-Ray Absorption Spectroscopy
In Figure 22, the Fourier transform functions at the Ni K edge of stepwise sulfided Al2O3-supported NiW catalysts (left) and the near-edge spectra (right) are displayed. After treatment at T ¼ 300 K, the spectrum contains peaks of four different contributions. At a distance of 1.98 A˚ a Ni–O contribution (CN 4.2), at 2.17 A˚ a Ni–S contribution (CN 2.3), and two Ni–Ni contributions at larger distances of 2.60 A˚ (CN 0.8) and 3.02 A˚ (CN 1.5) are seen. Both the Ni–O distance of 1.98 A˚ and the Ni–Ni separation of 3.02 A˚ match well with the geometric parameters of NiO (Ni–O ¼ 2.08 A˚, Ni–Ni ¼ 2.96 A˚). The other observed peaks in the spectrum point to a NiOxSy species formed during sulfidation. Compared to bulk NiS (Ni– S ¼ 2.39 A), the Ni–S distance of 2.17 A˚ is much smaller. The
0.6 Ni−S
300 K 373 K 473 K
Ni−Ni
0.4
673 K
mx (a.u.)
|FT [c(k)k3]| (a.u.)
15
10 573 K 473 K
5
Ni−O
573 K
Whiteline
673 K
0.2 Pre edge peak
373 K Ni−Ni 300 K
0 0
2
4 r (Å)
6
8
0.0 8300
8320
8340 E (eV)
8360
Figure 22 (Left) Ni EXAFS Fourier transforms of NiW/Al after sulfidation at indicated temperatures and (right) the XANES region at the Ni K edge during sulfidation (distance R not corrected for phase-shift).203
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
305
NiO WO3
Ni
Ni
Al2O3
Fresh
NiOxSy Ni
15 bar sulfidation
WO3
Ni
373 K
1 bar sulfidation
NiS WO3
Ni
Ni
473 K
‘Ni−W−S’
NiS−WOxSy
“WS2”
NiS WOxSy
S S W Ni S S
573 K
NiS
‘Ni−W−S’
‘Ni−W−S’
NiS−WOxSy Ni
WOxSy
WS2
S S W Ni S S
Ni
WS2
Ni
Ni
WS2
Ni
673 K
‘Ni−W−S’
NiS Ni
WS2
Ni
923 K Figure 23 The sulfidation process of g-Al2O3-supported NiW catalyst. Left: sulfidation at atmospheric pressure; right: sulfidation at 15 bar. Fresh material contains Ni and W oxide; between 373 and 573 K small NiS crystallites are formed being attached to WOxSy (NiS–WOxSy), which may be a precursor to the Ni–W–S phase. At 673 K and 1 bar, NiS–WOxSy is partially converted to ‘Ni–W–S.’ NiS particles are redispersed to smaller crystallites over the WS2 edges during this transformation process. At 973 K and 1 bar, full sulfidation is achieved and a loss of WS2 dispersion as well as an increased crystallinity of the WS2 slabs occurs. Sulfidation at 15 bar and T ¼ 673 K is more complete, WS2 is reasonably good dispersed, and more Ni–W–S phase is formed.203
promoter ions being present in the Co(Ni)–Mo–S phase, NiW catalysts may be regarded as atomically dispersed Ni species in Ni–W–S phases together with less dispersed NiS–WOxSy phases.
7.12.13 Probing the Active Sites: CO and NO Adsorption Different spectroscopic techniques are used to obtain information regarding the surface binding sites of HDS catalysts.30 A powerful approach is to monitor the changes of adsorption of small probe molecules such as NO70–87 or CO61–69 with IR spectroscopy. Applying IR studies of NO adsorption allowed identifying different adsorption complexes for unpromoted and promoted catalysts.69,71–73 It was also shown by IR NO adsorption experiments and EXAFS investigations that NO molecules are adsorbed onto MoS2 edges and not onto the basal planes in sulfided Mo/Al2O3 catalysts.142 In the following, the results of a CO IR study are discussed in detail.69 Different g-Al2O3-supported Mo catalysts calcined at 773 K exhibiting different Mo loadings (4.2, 9.8, and 11.9 wt%,
abbreviated as Mo4/Al, Mo9/Al, and Mo12/Al), Co-promoted catalysts (impregnation with Co nitrate, calcinations at 773 K, 1.6, 3.1, and 4.7 wt% on the Mo9/Al catalyst), and a Mo catalyst supported on silica (Mo7Si, 7.2 wt% Mo, calcination temperature 623 K) were pressed into discs and placed in an IR cell. Then, sulfidation was done (1) under H2S/H2 flow or (2) under H2S/H2 flow followed by a post-treatment under pure H2 flow. IR CO adsorption experiments were done at 100 K. Three absorptions are seen in the IR spectrum of CO on the sulfided Mo7/Si catalyst (2158, 2120, and 2075 cm1) (Figure 24) with the band at 2158 cm1 being caused by the interaction of CO with silanol groups, while the remaining absorptions are due to CO adsorption onto unpromoted Mo sites of the sulfide phase. The spectra of CO adsorbed onto catalysts with different Mo loading are shown in Figure 24.69 The peaks located at 2189 and 2156 cm1 are caused by CO adsorption onto Al3þ vacancies and hydroxyl groups of alumina, while the band at 2110 cm1 is characteristic of CO interacting with Mo sites. This absorption increases with Mo loading and in the same direction the two former peaks decrease because of a higher coverage of the support by MoS catalyst. A band should appear at 2075 cm1 in the IR
306
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
2110
2120
A
0.1 A 2156
0.01 A
2075
2158 2189
Mo12/Al 2075 Mo9/Al
Mo7/Si Mo5/Al 2250 (a)
2200
2150
2100
2050
2000
1950
Wavenumbers (cm−1)
2250
2200
(b)
2150 2100 2050 Wavenumbers (cm−1)
2000
1950
Figure 24 (a) IR spectra of CO adsorbed onto sulfided Mo catalysts supported on silica and (b) on g-Al2O3.69
spectrum for the catalyst with the highest Mo loading (Mo12/ Al). Compared to Mo7/Si, the band related to the Mo sites on Mo/Al catalysts is shifted toward lower wavenumber, whereas the absorption at 2075 cm1 is not significantly affected. MoS2-based catalysts often display a hexagonal shape and two types of crystallographic edge planes (1 0 1 0) and (1 0 1 0) can be distinguished called the molybdenum and the sulfur edges. The nature of these two MoS2 edges may explain the presence of the two CO absorptions: the absorption at 2075 cm1 is caused by CO in interaction with Mo atoms on the sulfur edge and the peak at 2110–2120 cm1 is related with CO adsorbed onto Mo atoms on the molybdenum edge. These assignments are in agreement with DFT calculations where a difference of the two absorption bands of 40 cm1 was proposed67,68 in good agreement with 35 cm1 for the Mo/Al catalyst and 40 cm1 for the Mo/Si catalyst. IR CO spectra of the Co-promoted CoMo9/Al catalysts (Figure 25) show a new peak at 2070 cm1, which increases in intensity with increasing Co content up to 3.1 wt% Co, while the band characteristic of unpromoted Mo sites (2110 cm1) decreases. Increasing the Co content to 5 wt% does not lead to a further intensity gain of the absorption at 2070 cm1. For a sulfided Co/Al2O3 catalyst (3 wt% Co), an absorption at 2094 cm1 and a shoulder with low intensity at 2056 cm1 is observed. Therefore, the band in the IR spectra of the Copromoted catalysts can be assigned to Co-promoted Mo sites, in agreement with Ref. [63]. For the highest Co content, that is, for an atomic ratio for Co/(Co þ Mo) of 0.48, the peak at 2110 cm1 is still observable suggesting that promotion of the MoS2 slabs by Co is not complete in the CoMo/Al2O3 catalysts. The presence of Co also results in a decrease of the absorptions caused by CO-support interactions and this decrease is more pronounced at higher Co loading. A possible explanation is that an increasing amount of cobalt spinel or cobalt sulfide phase is formed at the higher Co contents.
A 0.05 A 2156 2190 Co5Mo9/Al
Co3Mo9/Al
2070
Co1Mo9/Al
2110 Mo9/Al 2250
2200
2150
2100
2050
2000
1950
Wavenumbers (cm−1) Figure 25 IR spectra of CO adsorbed onto CoMo9/Al catalysts.69
The results demonstrate that for CoMo/Al catalysts the concentration of unpromoted Mo sites is reduced with increasing Co content while in the same direction new Co-promoted sites are generated. However, the sum of the amount of unpromoted and promoted sites does not vary significantly as function of Co content. This observation suggests that the creation of one promoted site leads to the loss of about one unpromoted Mo site. Interestingly, by increasing the atomic ratio of Co/(Co þ Mo) above 0.36 the number of Co-promoted sites is not increased, and the amount of unpromoted Mo sites almost does not decrease. These observations indicate that above a
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
2070 A A
2110
2070 2055
0.05
2156
0.1
2098 2110
2055
307
2190
Co5Mo9/Al
Co3Mo9/Al
Co1Mo9/Al
0.75
Mo9/Al
2300 (a)
2200 2100 Wavenumbers (cm−1)
2000
1900
2300 (b)
2200 2100 2000 Wavenumbers (cm−1)
1900
Figure 26 The influence of the H2 post-treatment at 573 K on the CO adsorption behavior on sulfided Mo/Al and CoMo/Al catalysts. (a) IR spectra of CO adsorbed onto the Mo9/Al catalyst after sulfidation (dotted line) and after sulfidation and H2 treatment (full line). (b) Difference spectra derived from IR spectra of CO adsorbed after sulfidation and H2 post-treatment minus spectra measured after sulfidation.69
certain Co/(Co þ Mo) ratio, additional Co atoms are not significantly deposited on MoS2 edges. In Figure 26, the IR CO spectra of the catalysts after a post-treatment in H2 are shown and the difference spectra are displayed in Figure 26, right. For Mo9/Al, the treatment leads to an increased intensity of the band at 2110 cm1 (Mo sulfide sites) and to the appearance of a new absorption at 2098 cm1. It seems also that the H2 post-treatment generates additional Al3þ vacancies and hydroxyl groups (bands at 2156 and 2190 cm1), which may be explained by breaking of Al–O–Al bridges during H2 treatment creating new Al3þ vacancies and OH groups. A detailed analysis of the spectra without and with H2 treatment yields an increase of Mo sites by a factor 2.3 after H2 treatment (Mo12/Al show a slightly smaller increase). Reversibility of the effects of the H2 treatment was checked by heating the catalysts in a H2S/H2 atmosphere at 573 K for 2 h. The IR spectra are very similar to those of catalysts not treated in H2, and especially the peak at 2098 cm1 totally disappeared. The H2 post-treatment at 573 K removes S atoms from the MoS2 edge slabs significantly, increasing the amount of accessible Mo sites. The observation that Mo12/Al exhibits only an increase of such sites by a factor of 1.8 may be due to larger size of the MoS2 slabs that are less easily reduced by H2. The H2 post-treatment also generates additional sites on the CoMo/Al catalysts (Figure 26). Independent of the Co content, the amount of unpromoted Mo sites increases by a factor of about 1.8 being slightly lower than for Mo9/Al (factor of 2.3). This increase can be explained by the generation of Mo sites characterized by the band at 2098 cm1. For the concentration of promoted sites, a factor of 1.3 is estimated as being smaller than for unpromoted sites, and for the catalyst with the highest Co content the factor is only 1.1. Because the increase of
Co-promoted sites by H2 treatment is smaller than that of unpromoted Mo sites, it is most likely that most of the Copromoted sites are already generated during the sulfidation procedure, whereas unpromoted Mo sites are more difficult to be created. In the IR spectra, no additional absorptions could be detected after the H2 treatment excluding the creation of new adsorption sites, that is, the environment of Co sites is not modified by the reductive treatment. Like for unpromoted catalysts, the number of Al3þ vacancies and OH groups increased after the H2 treatment. The adsorption properties of sulfided CoMo/Al2O3 catalysts were also investigated applying in situ Fourier transform infrared spectroscopy (FTIR) spectroscopy combined with NO adsorption (Figure 27).73,204 The IR absorptions associated with NO adsorbed onto Co sites at 1850 cm1 start to evolve by the addition of small amounts of Co to a Mo/Al2O3 catalyst and the intensity of the band associated with NO adsorbed onto Mo sites at 1690 cm1 starts to decrease. The catalytic activity in the HDS of thiophene correlates with the absorption of NO onto Co sites and not onto Mo sites suggesting the formation of the CoMoS phase upon Co addition to the Mo/ Al2O3 catalyst (Figure 27). In another in situ IR study, sulfided CoMo/Al2O3 catalysts were investigated after sulfidation at different temperatures. On a comparison of the changes of the surface properties by increasing the sulfiding temperature from 673 to 873 K, different types of CoMoS structures were detected. The type II CoMoS structure being obtained at the higher sulfiding temperature is characterized by none or very few bonds to Al2O3. At the lower temperature, type I CoMoS is formed having Mo– O–Al linkages, which is much less active in the HDS reaction than the type II catalyst.205
308
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Absorbance (a.u.)
8
Co 4 Moedge
2
Absorbance
2 CoMo
4 6 8 wt% Co added
(b)
10
1900
1800 1700 1600 1500 Wavenumber (cm−1)
kHDS 102 (mol h−1 g−1)
Mo
Co
(a)
6
1.5
1.0
0.5
1 (c)
2
3
4
Absorbance of NO on Co
Figure 27 (a) IR spectra of NO adsorbed onto sulfided 2% Co/Al2O3, 8% Mo/Al2O3, and 2%Co 8%Mo/Al2O3. (b) Absorbance of IR bands of NO adsorbed onto Co (1850 cm1) and Mo (1690 cm1) plotted as function of Co/Mo ratio at constant Mo loading. (c) Thiophene HDS activities versus absorbance of IR band of NO adsorbed onto Co.73
Atomic-scale insights into the nature of NO adsorption onto MoS2 and CoMoS are obtained combining IR spectroscopy investigations with scanning tunneling microscopy (STM) experiments and DFT calculations.88 The NO molecule does not adsorb at fully sulfided MoS2 edges when these edges do not contain hydrogen. However, it can be expected that sulfided catalysts contain hydrogen at the edge in the form of SH groups. The results suggest a ‘push–pull’ type mechanism where simultaneously vacancies are created, and NO adsorption and H2S release occur. Dosing the catalyst with atomic hydrogen, STM images reveal formation of vacancies and at these vacancies NO dimers are adsorbed. In the IR spectra measured during TPD, several NO adsorption complexes are observed. On the basis of DFT calculations, it was shown that mononitrosyl species dominate at the Mo edges and stable dinitrosyl moieties are found at both the unpromoted and the Co-promoted S-edges.
7.12.14 Structure of the Phases On the S-terminated (0 0 0 1) plane of MoS2, only very weak physisorption occurs, and hence the basal plane of the crystallites is more or less inactive in catalytic reactions. Coordinatively unsaturated sites (CUS) and/or sulfur ion vacancies at the corners and edges of the Mo(W)S2 slabs are the predominantly catalytically active sites (Lewis acid character).23 The vacancies or coordinatively unsaturated sites are created in a reaction with hydrogen. Due to the Lewis acid character of
Diameter
Sites Rim Edge
n Layers or stacking height
Basal
Figure 28 Scheme of the rim-edge model.
CUS, molecules with the unpaired electrons can be adsorbed. Using probe molecules such as CO, NO, or O2, the number of CUS could be determined and correlated with catalytic activity.88 Under real reaction conditions (presence of H2, H2S, and excess substrates and reaction products), it is highly questionable whether vacancies/CUS remain empty for any length of time. For unpromoted MoS2 catalysts, a two-site model was developed for MoS2 to explain the HDY and DDS pathways (Figure 28). These sites are located on the edge of MoS2 nanocrystals at different positions. The so-called rim-site terminating layers of the crystal stack on which HDY and DDS occur are located at the edge of exterior layers with adjacent basal planes. The second sites called edge-sites are located on interior layers
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
that have no exposed basal plane surfaces. On these sites only the desulfurization reaction occurs.206 Promotion by adding Co decreases the strength of the Mo–S bond and increases the electron density on the sulfur atoms.207,208 More electron density at the S centers increases the basicity of surface S species, which may facilitate C–S bond cleavage. Indeed, supported CoMoS catalysts exhibit a pronounced selectivity toward DDS.209 Because of the electronic density reorganization, the S atom bonded to Co and Mo exhibits an intermediate bond strength optimal for HDS activity. The bond weakening effect due to Co(Ni) promotion is supported by theoretical calculations as well as experimental results.210,211 For promoted catalysts, the Co–Mo–S (or Ni–Mo–S) phase model proposed by Topsøe et al.141,179,212,213 is now widely accepted: the catalysts consist of MoS2 nanocrystals with Co (or Ni) promoter atoms substituting Mo atoms at the edges of the MoS2 layers in the same plane as the Mo atoms, but their local coordination is different. The CN of the promotor atoms is still under debate and evidence was presented either for CN ¼ 4 or for CN ¼ 5. Practical Co–Mo HDS catalysts are generally composed of several phases: Co9S8 nanoparticles, Co2þ species
2
1
3
3
1
2 3
3
Figure 29 Scheme of a CoMoS catalyst composed of Codecorated single slab MoS2 (1), Co9S8 nanoparticles (2) and Co in the support (3).212
309
strongly interacting with the support, Mo (oxy)sulfides, as well as Co–Mo–S, schematically shown in Figure 29. Two structural types of Co(Ni)MoS catalysts were identified155: type-I with low activity and type-II with high activity. Type-I structures are incompletely sulfided and contain remaining Mo–O–Al linkages to the support. The formation of such Mo–O–Al bonds occurs during catalyst preparation (see below). Type-II Co(Ni)MoS phases display a weaker interaction to the support and they are fully sulfided. Often, type-II catalysts consist of multiple MoS2 slabs that are not linked to the support. The degree of stacking can be controlled by appropriate selection of support properties and synthesis conditions leading to stable single slabs of the catalytic materials on g-Al2O3 supports.214,215 A type-III catalyst was identified on SiO2 support and should consist of Co sulfide clusters with S2 dimers between the Co atoms located at the edges of the MoS2 slabs.135,216 A new model concerning hydrogen activation by MoS2 was recently presented. Unsupported overstoichiometric MoS2þx catalysts containing S22 species on the edges react with H2 to form –SH groups. These –SH groups participate in the catalytic reaction of thiophene conversion directly to butane (HYD pathway). Removal of the extra sulfur leads to a suppression or loss of the HYD function.217 Evidences for the presence of H groups on MoS2 were recently presented.218,219 Progress in the identification of active sites in MoS2 and CoMoS/NiMoS model catalysts was achieved by the combination of theoretical calculations (DFT; see also Chapters 7.15 and 9.02) and advanced analytical tools such as STM, aberrationcorrected high-resolution electron microscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).220–225 The local structure of pure MoS2 (grown on Au(11 1)) has a special electronic state identified as metallic brim sites (Figure 30). In the brim edge states, Mo is fully covered by S atoms forming S2 dimers on the edge. The edges can accept or donate electrons and thus act as a catalyst. Hence, an S-containing molecule may be bonded at the brim site and when hydrogen is available at the neighboring edge sites as a SH group, hydrogen transfer and hydrogenation reaction can occur. The results strongly indicate that the brim sites are catalytically active for the HYD pathway. The proposed formation of sulfur vacancies in MoS2 under H2 atmosphere has also been observed directly by STM.
Mo78S204 cluster
-S
Mo edge (100%) (a)
(b)
-S -S 2 S1
-S S2 1
-S -S 2
-S 1
2
-S 1
2
1
Figure 30 STM image of a MoS2 nanocrystal on Au(1 1 1) (left) showing the bright brim sites, the ball model of the structure with top view (middle, top) and side view (middle, bottom), and a model cluster with the S2 groups.226
310
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts profile (Figure 32, right) are 2.34 A˚ (0.1 A˚) for Mo–Co, 2.7 A˚ (0.1 A˚) for Co–Mo), and 2.7 A˚ (0.1 A˚) for Mo–Mo for the atoms labeled A–B–C–D. This result may be explained by the partial substitution of Mo by Co in the lattice of MoS2. Similar results were obtained for Co-promoted WS2 and the experimental finding is summarized in Figure 33. According to DFT calculations, the 50% Co partial occupation of the Mo-edge covered by 25% S is stable and the simulations also indicated the existence of two different configurations with such S-saturations that are close in energy (Figure 33(a) and 33(b)). In these configurations, either –Mo–Co–Mo–Co– alternates or a pairing of the atoms in the order –Mo–Mo–Co–Co is present, and in both cases the Mo-edge structure contains mixed Co–Mo sites. The S atoms are located at the top of the Mo atoms. However, DFT calculations also hinted that S atoms might be located in bridging positions suggesting that the mobility of S atoms on the Mo edge is high.228,229 In Figure 33(c), the model obtained experimentally is schematically shown. While DFT calculations yield an optimized local Co–Mo distance between 2.74 and 2.84 A˚, a pairing of Mo and Co edge atoms (2.34 A˚) is observed. The model for Co-promoted WS2 (Figure 33(d)) exhibits a sequence –W–Co–W–Co–W– supporting the idea that Co atoms are incorporated into the WS2 structure by substituting W atoms at the edge in an alternating fashion.
However, one should keep in mind that STM was performed under high vacuum conditions in the absence of substrate molecules, H2/H2S, and at ambient temperature. STM provided also evidences for significant changes of the morphology when the Co/Ni promoter is added. Pure MoS2 particles adopt a triangular shape and when Co is added, the particles become truncated hexagons (Figure 31, left). Two different edges can then be distinguished: one edge is covered by 100% and the second one by 50% S atoms. The situation is more complex for NiMoS because two different shapes of the nanoparticles are observed. The first one is identical with that found for CoMoS but the second one exhibits higher indexed edges exhibiting a lower degree of S coordination (Figure 31, right). The results suggest that Co/Ni plays two different roles: promotion of MoS2 with Co/Ni alters the electronic structure as seen as modified brim sites in STM; lower S coordination on one type of NiMoS edges is an attractive structural situation for adsorption of S-containing molecules. The occurrence of the second edge type in NiMoS clusters may be a first step to explain the different selectivities of the two catalytic systems in the hydrotreating processes. Concerning the activation of H2 on MoS2, it is proposed that a dissociative chemisorption occurs forming S–H groups at the edges. The most stable position of hydrogen seems to be on top of the S atom adjacent to the brim states. The promoter atoms Co in an unsupported CoMoS catalyst could be located using probe aberration-corrected STEM (Figure 32).225 Due to the lower atomic number, the Co atoms have a lower intensity in the HAADF-STEM image compared to Mo. The distances between the Mo–Co–Mo–Mo atoms along the edge of the nanosized particle determined by the line
The Role of Carbon in HDS Catalysts
It was also suggested54,55 that carbon plays an important role in stabilizing the active phase. It is assumed that excess of sulfur located on the surface of MoS2 is replaced by carbon, yielding MoSxCy phases with a carbidic species (see also Chapter 7.14).
NiM
CoM
oS
oS(1
0)
7.12.14.1
010)
(10
)
0 01
01
10)
S2
(1 2 oS
Mo
) N 10 NiMoS(1120) iMoS
10
( oS
(1
(10
10
iM
)
N
NiMoS(1010)
M
MoS2(1010) (100% S)
MoS2(1010) edge (100% S)
NiMoS(1010) (50% S) NiMoS(1010) (50% S)
CoMoS2(1010) edge (50% S)
Ni
Mo
S(
10
)
10
(10
10
) NiMoS(1120) iMoS N
NiMoS(1120) high index edge
Figure 31 Structure of promoted nanocrystals derived from STM investigations and DFT calculations.227
Mo Co A
Mo
B
Mo
C D
0.5 nm
140 130 120 110 100 90 80 70 60 50 40 30
Mo Mo Mo Co
A
C
B 0.234
0.0
0.1
0.2
0.27
0.3
0.4
0.5
D 0.27
0.6
0.7
0.8
0.9
nm
Figure 32 HAADF-STEM image of a Co-doped unsupported MoS2 nanowire catalyst (left) and the corresponding line profile (right).225
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
Mo-edge (1010)
Mo-edge (1010) Mo
Co
Mo
S
Co
Mo
Co
Mo
Co
S
S-edge (1010)
S-edge (1010)
Mo–Mo–Co–Co
Mo–Co–Mo–Co
(a)
(b)
Mo-edge Mo-edge (1010)
2.34 Å Mo1
311
Mo-edge W-edge (1010)
2.70 Å
2.70 Å Mo2
Co
2.24 Å Mo3
2.72 Å
2.72 Å
2.24 Å
S W1
Co1
W2
Co2
W3
S
S-edge
S-edge
(1010)
(c)
(1010)
(d)
Figure 33 Two different views (perspective and top) of (a) Mo-edge, 50% Co in an alternate position and 25%S; (b) Mo-edge with 50% Co, pairing configuration, 25% S. (c) and (d) perspective view of CoMoS (CoWS) structures for the experimentally observed Mo–Co–Mo–Mo (W–Co–W–Co–W) sequence at the Mo/W-edge. Mo (W) atoms: blue (orange), S atoms yellow.225
800
Intensity (counts s−1)
MS833 before N2/DMS treatment MS833 after N2/DMS treatment
600
structure (NEXAFS)232 and the most important findings are discussed in detail. Catalysts were prepared by thermal decomposition of (NH4)2MoS4 (ATTM) in inert atmosphere according to the equation: ðNH4 Þ2 MoS4 ! MoS3 þ 2NH3 þ H2 S
400
200
0 0
10
20
30
40
50 60 2q
70
80
90 100
Figure 34 Comparison of the synchrotron XRD patterns of a sample before and after DMS treatment (T ¼ 793 K).232
The incorporation of carbon and the nature of the carbon species in MoS2-based catalysts were investigated with x-ray powder diffractometry, IR spectroscopy, electron energy loss spectroscopy (EELS), and near-edge x-ray absorption fine
Above 573 K, MoS2þx phases are obtained by autoreduction of MoS3 with the value x decreasing with increasing temperature. The MoS2þx samples were treated between 573 and 833 K with a gas flow of CH3–S–CH3 (DMS) diluted in N2 or H2. A second set of samples was synthesized by heating ATTM with a DBT solution in decalin. The sample was treated at 623 K in a bomb for 3 days. The x-ray powder pattern of the catalyst treated with DMS at 793 K (sample composition: MoS2.05C1.12) shows no new peaks (Figure 34) but a reduction of the absolute intensity compared to the sample before the treatment (note: identical sample volumes were used) indicating a lower density of the material after DMS treatment. The (1 1 0) plane at 2y ¼ 47 exhibits a stronger intensity relative to the (1 0 0) plane after the DMS treatment. The change of the intensity can be explained with a more pronounced distortion/more folding along the c axis after heating the precursor in DMS. The TEM image of a material obtained
312
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
20 nm
20 nm
Figure 35 TEM micrographs of a MoS2 sample prepared by thermal decomposition of ATTM (left) and of a sample treated with DMS at 793 K (right).232
Intensity (a.u.)
s*
CK edge p*
B A 292 294 296 298 300 302 304
a b 280
290
300
310 320 330 Energy loss (eV)
340
350
Figure 36 EELS C K-edge spectra of Mo2C (a) and a sample treated at 793 K with N2/DMS. The inset shows the fine structure at the s* transition.232
by thermal decomposition of ATTM at 833 K in N2 (Figure 35) shows that MoS2 particles are present in closely packed groups of large slabs exhibiting highly folded shapes. The average slab length was estimated to about 94 A˚ and the average stacking number as 9.3, which is in reasonable agreement with the value of 7.8 obtained from the x-ray powder pattern. In the TEM image of a sample treated with DMS, heterogeneous regions with some highly stacked layers and regions with less stacked and less curved layers are observed. The average particle size was reduced by a factor of 2 to about 47 A˚ and the stacking number decreased from 9.3 to 4.0. In the EELS (Figure 36), an intense peak to the p* antibonding state is observed and by a comparison with a spectrum of graphite a graphitic structure can be excluded. In addition, the peak shape structure near the ionization edge is also different from that for amorphous, graphite, or carbide structures. Two features A and B are observed for graphite and carbide for the s* transition. Feature A is more intense for graphite than feature B, while the opposite is observed for carbides and diamond. The intensity difference is caused by the different density of states for
graphitic sp2 carbon bonds and tetrahedral sp3 bonds in carbides. The feature B is more intense than A in the MoS2 sample treated with DMS suggesting that carbon is not present as a graphitic-like material and that Mo–C bonds of a carbide-like structure are present. Because the p* peak is less intense than for Mo2C, one may conclude that the sample may contain some amorphous carbon. The replacement of S by C on edges of MoS2 can be crystallographically accommodated. Hence, it cannot be ruled out that both the Co(Ni)Mo(W)SC and Co(Ni)Mo(W)S phases constitute the active catalyst. As mentioned recently, the role of carbon in understanding the function of the catalysts may be of great importance and was more or less neglected.233
7.12.14.2
Morphology and Microstructure of Catalysts
In real catalysts, phase segregation was found and Co9S8 and mainly Ni3S2, besides other Ni sulfides, are formed under reaction conditions. In the ‘remote control’ concept, Co9S8 and Ni3S2 may play a distinct catalytic role, providing spillover hydrogen for the MoS2. Real catalysts display different MoS2 morphologies such as straight and curved MoS2 single slabs, and stacks of slabs.215 The MoS2 nanoparticles are either parallelly57,234,235 or perpendicularly234,235 oriented to the support. Spent NiMoS/Al2O3 and CoMoS/Al2O3 catalysts prepared by gas phase or liquid phase sulfidation (Table 1) of the oxidic precursors exhibit a large variety of microstructures.215 The x-ray powder patterns of the two catalysts NiMo1-T2-LF and NiMo2-T2-L-F show no clear reflection at 2y ¼ 14 indicating the absence of significant MoS2 stacking (Figure 37). The shape and the broadness of the reflections are typical for MoS2 with a low crystallinity. In addition, no indications are found for the presence of crystalline Ni sulfides. However, in about 30% of analyzed TEM regions of NiMo1T2-L-F Ni3S2 crystals are visible, which was verified with energydispersive x-ray spectroscopy (EDX), electron diffraction, and lattice imaging (Figure 38(a)). The TEM image in Figure 38(b) shows that Ni3S2 crystals are decorated with MoS2 layers. Ni sulfide particles with diameters between 5 and 50 nm can be imaged using STEM-EDX. An example is shown in Figure 39 for NiMo3-T2-L-U. From an analysis of the STEM-EDX data,
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
313
Table 1 Type I/II (T1/2), type II (T2), liquid-phase sulfidation (L), gas-phase sulfidation (G), FCC-PT (F), ULSD (U), FCC-PT canister (F/C), HC-PT (H), diesel HDS (D) Catalyst properties Catalyst annotationa
Description
Metal loadings
Sulfidation
Originating fromb
NiMo1-T2-L-F NiMo2-T2-L-F NiMo3-T2-L-U NiMo3-T2-L-F/C NiMo3-T2-G-H NiMo3-T2-G NiMo4-T1/2-L-F CoMo1-T2-L-F CoMo2-T2-L-F CoMo3-T1/2-L-D
Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 2 Ni–Mo/Al2O3 Type 1/2 Ni–Mo/Al2O3 Type 2 Co–Mo/Al2O3 Type 2 Co–Mo/Al2O3 Type 1/2 Co–Mo/Al2O3
3.2 at Ni and 5.8 at Mo per nm2 1.8 at Ni and 5.6 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 2.5 at Ni and 8.1 at Mo per nm2 1.5 at Ni and 3.1 at Mo per nm2 1.7 at Co and 4.8 at Mo per nm2 2.4 at Co and 6.7 at Mo per nm2 1.5 at Co and 3.1 at Mo per nm2
Liquid phase Liquid phase Liquid phase Liquid phase Gas phase Gas phase Liquid phase Liquid phase Liquid phase Liquid phase
FCC-PT FCC-PT ULSD FCC-PT canister HC-PT – FCC-PT FCC-PT ULSD Diesel HDS
a
Type 1/2 (T1/2), Type 2 (T2), liquid phase sulfidation (L), gas phase sulfidation (G), FCC-PT (F), ULSD (U), FCC-PT canister (F/C), HC-PT (H), diesel HDS (D). Fluid catalytic cracker pretreat (FCC-PT), ultralow sulfur diesel (ULSD), hydrocracker pretreat (HC-PT), hydrodesulfurization (HDS).215
b
* = MoS2 + = gamma-Al2O3
* Intensity
*
+
+
*
+
*
+
*
NiMo1-T2-L-F
NiMo2-T2-L-F
Reference
5
10
20
30
40
50
60
70
80
2q Figure 37 x-Ray powder patterns of two liquid phase sulfided type II Ni–Mo/Al2O3 catalysts NiMo1-T2-L-F (top) and NiMo2-T2-L-F (middle) together with the pattern of a reference sample containing 1.5% Ni3S2 (bottom). Vertical bars: reflection positions of crystalline Ni3S2.215
it can be estimated that 32–47% of Ni has segregated in these catalysts corresponding to 1.3–3.0 wt% Ni3S2. Combining these data with the MoS2 dispersion estimated from the TEM micrographs and about 80% of Mo atoms located on edge and corner (e þ c) sites, the amount of not segregated Ni is sufficient to reach Ni/Mo (e þ c) atom ratios of 0.24–0.29. The Ni sulfide segregation phenomenon is also detected in type I/II Ni–Mo catalysts. A detailed analysis also demonstrates that the sulfidation procedure significantly influences the morphology of type II catalysts. The MoS2 stacking is more pronounced when sulfidation is done in H2S/H2 atmosphere, while liquid-phase sulfidation seems to prevent it. In a H2S/H2 atmosphere, only few but much larger Ni3S2 crystals are formed than after
liquid-phase sulfidation. This may be related to the exothermicity of the H2S/H2 sulfidation, an effect that is suppressed if liquid is present during the sulfidation. In contrast to Ni-promoted catalysts, no Co-rich particles are visible in Co–Mo type II catalysts displaying the same composition and preparation, when they are used under the same conditions. It is highly likely that Co9S8 aggregates less rapidly than Ni3S2. For liquid-phase sulfided type II commercial NiMo catalysts, MoS2 stacking is not pronounced and is mainly absent in some catalysts (Figure 40). The MoS2 dispersion is high after liquid-phase sulfidation and using short reaction times, while longer reaction times and/or more severe reaction conditions produce longer MoS2 slabs (i.e., the dispersion is lower) (NiMo3-T2-L-F/C, used for 1 year in a unit
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5 nm
10 nm
(a)
(b)
Figure 38 TEM micrographs of liquid-phase sulfided type II Ni–Mo/Al2O3 catalysts: (a) shows a lattice image of a Ni sulfide crystallite in NiMo1-T2-L-F; (b) decoration of a Ni sulfide particle with MoS2 slabs in NiMo2-T2-L-F.215
0.2 mm (a)
0.1 mm (b)
Figure 39 STEM-EDX images of Ni (red) and Mo (blue) of liquid phase sulfided type II Ni–Mo/Al2O3 catalyst NiMo3-T2-L-U.215
5 nm
10 nm (a)
(b)
Figure 40 TEM micrographs of liquid-phase sulfided type II NiMo/Al2O3 catalyst NiMo3-T2-L-F/C showing long curved MoS2 slabs.215
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts
20 nm
20 nm (a)
315
(b)
Figure 41 HAADF-STEM images of MoS2/C (a) and WS2/C (b) catalysts. All clusters in the two images are single-layer structures.220
20 nm
(1 0 1 0) (so-called Mo(W) edges) and (1 0 1 0) (so-called S edges) being exposed. The addition of promoter atoms to MoS2 modifies the morphology to more heavily truncated triangles (Figure 42), that is, a change toward a more hexagonal shape is induced. This observation is in line with STM images of Co-promoted MoS2 deposited on Au(1 1 1).236 The Co atoms in the CoMoS structures are located at the (1 0 1 0) S edges and Co addition causes exposure of more promoted (1 0 1 0) S-type edges while the amount of Mo-type edges (1 0 1 0) is reduced. The situation is similar for NiMoS and NiWS catalysts. Promoter atoms may have other effects than changing the degree of truncation. There are particles that are more round and in some cases additional new higher index edges are present. For some of the edges, angles of about 150 to the dominant edges are observed indicating that the promoted structures also expose (1 1 2 0) edges. This finding is in agreement with atomically resolved STM images of NiMoS structures for small clusters.221 The present results give evidence that extended (1 1 2 0) edges may indeed be stable features of the promoted structures.
Figure 42 Promotion of MoS2 with Co changes the morphology toward heavily truncated triangles.220
7.12.15 Choice of Catalysts and Other Catalysts for HDS operating at high temperature). Nevertheless, the degree of stacking is rather limited and most MoS2 slabs consist of only one or two layers. It seems that MoS2 stacking is not required for desulfurization of molecules such as DBT. An interesting feature occurs for this catalyst, namely the curvature of the MoS2 layers (Figure 40). Type I and II catalysts often show MoS2 morphologies such as Al2O3 needles or plates coated with MoS2. Carbon-supported MoS2, WS2, CoMoS, NiMoS, and NiWS catalysts were prepared by impregnation with an aqueous solution containing (NH4)2[MS4] (M ¼ Mo or W) and, for promoted catalysts, also with Me(acetate)2 (Me ¼ Co, Ni). After impregnation, the samples were dried in air at 110 C and then treated with 10% H2S in H2 for 6 h at 1073 K. The morphology of the catalysts was investigated with HAADFSTEM and an example is shown in Figure 41.220 Most of the single-layered clusters exhibit a regular shape in the form of a truncated triangle. The angles at the vertices of the nanostructures amount to about 120 and are consistent with both the
Whether CoMoS or NiMoS is used depends on several factors because these catalysts behave differently. Generally, CoMoS/ g-Al2O3 catalysts are very efficient in the HDS process but they are less efficient for HDN or hydrogenation of aromatic molecules. NiMoS/g-Al2O3 phases exhibit better HDN activity and are better hydrogenation catalysts. Hence, they are preferably applied for feedstock with a high concentration of unsaturated compounds because more aromatic molecules are saturated leading to a higher cetane number and density. However, this is accompanied by higher hydrogen consumption, requiring appropriate hydrogen availability, hydrogen recycle rate, and compression costs. For aromatic hydrogenation reactions, NiWS-based catalysts are highly effective but the higher costs compared to NiMoS catalysts have limited their uses. At low pressures and high temperatures, a CoMoS catalyst can be a better choice than a NiMoS catalyst. Other approaches to obtain active catalysts were also reported and some are mentioned in brief. Using bimetallic complexes containing, for example, Mo3S4M cores (M ¼ e.g.,
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Ru, Ir, Rh) deposited on g-Al2O3 yields active catalysts. The results suggest that the activity is at least partially caused by a better distribution of MoS2 slabs on the support. No clear evidences were found whether the noble metals are located in direct contact with the MoS2 phase.237 Are there compounds exhibiting better HDS performance? Yes, as can be seen in Figure 43; the HDS activity scales with the M–S bond strength (M ¼ transition metals) and the best catalysts are found for noble metal sulfides. Keeping in mind that more than 120 000 tons of catalysts are used in refineries and that roughly the same amount must be replaced every year due to catalyst deactivation, the high prices and restricted availability of the noble metals prevent the use of such sulfides. During the last few years, unsupported metal phosphides have been studied due to their high activity for HDS and HDN.239. Both MoP and WP exhibit only moderate activity but Ni2P shows an excellent activity in hydroprocessing. According to results of several studies performed for simultaneous HDS and HDN reactions, the overall activity was found to be in the order Fe2P < CoP < MoP < WP < Ni2P.
7.12.16 Poisoning and Deactivation Effects The poisoning effects of H2S, nitrogen compounds, and aromatics are different for the HYD and DDS routes.240–242 According to experimental and theoretical DFT-based investigations, the general trend for poisoning is: nitrogen compounds> organic sulfur compounds > polyaromatics oxygen compounds H2S > monoaromatics. The H2S produced during HDS may react with the CUS to form saturated sites containing –SH groups.243 Hence, the number of CUS is reduced inhibiting removal of S via DDS, while the HYD pathway is only slightly affected.244–248 Different catalysts exhibit different sensitivities against H2S poisoning. NiMoS/g-Al2O3 is more sensitive than Co-promoted catalysts.39 The inhibition effect is more pronounced for HDS of DBT than for alkyl-substituted DBTs.244 The
1.00E + 03
Log (HDS activity per mole of metal)
RuS2 IrS2
OsS2
Rh2S3
1.00E + 02
ReS2 PdS
1.00E + 01
PtS
MoS2
Cr2S3
WS2
Ni
FeS
NbS2 TaS2
Co9S8 VS
1.00E + 00 MnS
1.00E · 01 30
40
50
60
70
80
M−S strength bond (kcal mol−1)
Figure 43 HDS activity of transition metal sulfides plotted against M–S bond strength.238
identification of the Mo edge brim site (see above) as the HYD site explains the low inhibiting effect of H2S on hydrogenation. H2S partial pressure also plays an important role in poisoning.244 However, at low partial pressure H2S may exert a promotional effect.249 Model compound studies have shown that some basic nitrogen-containing molecules (e.g., acridine) are orders of magnitude more inhibiting than other Ncontaining molecules (e.g., aniline or indole). The differences in the degree of inhibition between different catalysts, different types of S-compounds, and between DDS and HYD pathways have been explained on the basis of different catalytic sites involved in DDS and HYD reactions, as well as based on the mode of adsorption of the reactants. The HDS reaction is also significantly inhibited by organic nitrogen compounds. Experimental and theoretical data suggest that competitive adsorption between N-containing and S-containing compounds occurs on active sites. The HDS reaction is then inhibited due to the strong adsorptive strength of the N-containing molecules. The extent of inhibition, however, depends on the type and the concentration of organic nitrogen compounds. Catalysts applied in the hydrotreating of light feeds (socalled middle distillates) mainly deactivate due to coke deposition and sintering. Metal deposition as a deactivation source is found in hydroprocessing of heavy feeds.250 Coverage of the active sites by N-compounds and coke formation are the most important factors in catalyst deactivation. Basic N-containing compounds may adsorb irreversibly onto catalyst acidic sites, and may be converted to coke. HDS reaction conditions such as T, partial H2 pressure, and H2/oil ratio strongly influence coke formation. Coke is formed at the beginning of the reaction, but the catalyst may retain a substantial portion of its original activity.251
7.12.17 Synthesis Strategies of Catalysts In the literature, numerous different preparation approaches are reported. Hence, only some of these methods are presented in brief. A variety of unsupported NiMoWS catalysts are commercially used with the commercial trade name NEBULA. The synthesis procedure is patented and therefore the exact preparation method is not known. There are studies where the chemical vapor deposition (CVD) route was applied to deposit Co on supported MoS2. The MoS2 was treated with Co(CO)3NO vapor at RT for a distinct time followed by sulfidization. HDS activity was substantially higher than that of a conventionally prepared catalyst (impregnation route). More interestingly, adding Co by the CVD technique to a conventionally prepared CoMoS/Al2O3 catalyst, the HDS activity increased significantly.252 Besides the use of oxidic starting compounds, the suitability of S-containing precursors was investigated. Such precursors are either ammonium thiomolybdate (ATM)/thiotungstate (ATT), tetraethylammonium salts of the type [M0 (MS4)2]n (n ¼ 2 or 3) (M0 ¼ Co or Ni, M ¼ Mo or W)253, or tetraalkylammonium thiometalates with formula (R4N)2MS4 (R ¼ methyl, ethyl, propyl, pentyl, hexyl, etc.; M ¼ Mo, W). These precursors are thermally decomposed at relatively low temperatures often
Hydrotreating: Removal of Sulfur from Crude Oil Fractions with Sulfide Catalysts yielding finely dispersed unsupported catalysts.230,254–256 Using alkyl-containing precursors, the decomposed MoS2þx contains carbon and small amounts of nitrogen. The carbon content increases with the size of R. The catalyst is then impregnated with promotor salts and the Co(Ni)Mo(W)S catalyst is sulfided either ex situ or in situ.257–259 Unpromoted MoS2 catalysts with large carbon contents and a good performance are obtained by thermal decomposition and different activation modes.260 To avoid the use of any oxidic materials in the catalyst preparation, a combination of ATTM/(Prop4N)2MoS4 and nickel diethyldithiocarbamate or cobalt dimethylthiocarbamate was used.261,262 These catalysts are characterized by a good dispersion, variable carbon content, and high activity. Another approach is the use of single source precursors such as (ML)(MoO4) (L ¼ amine such as EN, M ¼ transition metal), for example, Co(en)3MoO4. The precursor is first thermally decomposed followed by sulfidization at elevated temperature in a H2S/H2 atmosphere.10 There are also reports where the oxidic catalyst precursors are sulfided in solution at elevated temperature and high H2 pressure using, for example, dimethyldisulfide (DMDS) dissolved in an inert organic solvent.
7.12.18 Supported Catalysts and the Support Effect For most supported Co(Ni)MoS catalysts g-Al2O3 is used: this material has a high specific surface area (200–350 m2 g1), pore size can be controlled, it has a good affinity to sulfide yielding a high dispersion, it contains acidic and basic sites, can be easily formed into desired shapes, mechanical strength is suitable, and cost is low. To improve the performance of the catalysts several support materials such as carbon, SiO2, TiO2, ZrO2, MgO, zeolites, and mesoporous silicas are used.263–266 Further strategies are modifying g-Al2O3 by mixing with different materials such as zeolites or other metal oxides and adding additives such as P, F, B, Si, Mg, Zn, La, V, and Ga as modifiers, to alter the acidic and basic properties of g-Al2O3. The consequences of the application of such mixtures are altered interaction between the impregnation solution and the support, a change of the dispersion of the active phase, and reducibility of the oxidic precursor catalyst. In recent years, phosphorus is mainly used as a modifier and in several reports an improvement of the performance was reported influencing especially the HDN activity of NiMoS/Al2O3. The addition of boron to gAl2O3 seems to have a positive effect for CoMoS because the formation of catalytically inactive bulk Co9S8 is minimized.267 Some oxides have acid–base properties promoting desulfurization of alkyl DBTs. Acidic supports such as zeolites facilitate isomerization and trans-alkylation of alkylated DBT resulting in an enhanced HDS activity of these refractory sulfur species. However, the disadvantage of such supports is an extensive coking shortening the lifetime of the catalyst. To increase the dispersion of the catalytically active phase, mesoporous silica materials such as SBA-15 or MCM-41 with very large surface areas of about 1000 m2 g–1 were tested (see also Chapters 5.06 and 7.11).268–274 Under laboratory conditions, all these materials showed comparable or even better HDS performance than industrial catalysts. Most of the studies focus on a high dispersion of the catalysts enabling a good decoration of Mo(W)S2 edges with promoter
317
atoms. In addition, a weak catalyst-support interaction favors formation of type II Co(Ni)Mo(W)S structures with a low degree of stacking. Several disadvantages of promising supports should be shortly mentioned. An upscale of mesoporous materials such as SBA-15 and MCM-41 yielding kilograms or even hundred of kilograms was not possible until now. Zeolites have very small pore diameters (less than 2 nm) and dispersion of the catalysts is relatively poor. Moreover, the pores are so small that large DBT molecules cannot enter the pores.
7.12.19 Conclusion Co(Ni)MoS/Al2O3 and related catalysts have been commercially used for more than 80 years. During this time period, different models and theories were developed and the evolution of the models/theories is summarized in several reviews.23,25–27 Most models developed are directly related to the layered crystal structure of MoS2. The first structural model of CoMo/ Al2O3 was the monolayer model developed by Schuit and Lipsch (Figure 44).275 After calcination, Mo species are on the surface of the support forming a monolayer. The interaction between Mo and Al2O3 occurs via Mo–O–Al bridges. Incorporation of Mo6þ ions was proposed to be charge compensated by a capping O2 layer located on top of the monolayer, and Co2þ was assumed to substitute Al3þ ions in the surface and being in tetrahedral environment. The promotional effect of Co2þ was suggested to increase the stability of the monolayer. During sulfidation, the capping oxygen atoms are replaced by S2 anions. The presence of H2 under HDS conditions causes a removal of S2 leading to a reduction of Mo6þ to Mo3þ, which were assumed to be the catalytically active sites. Another model was the intercalation model (Figure 45)276,277 assuming that the sulfided catalysts contain MoS2/WS2 on the g-Al2O3 and the Co/Ni atoms occupy octahedral sites in the van der Waals gaps between the MoS2/WS2 layers. Later it was shown that intercalation of Co/Ni is energetically not possible and it was proposed that the promoter cations are located at the edges of the slabs (pseudo-intercalation).278 The model implicitly assumes a three-dimensional multilayer structure of MoS2/WS2 so that intercalation can take place. S
S
S
O
O
S Mo
Mo O
O Co
Co Co
Figure 44 Schematic illustration of the monolayer model.
MoS S Mo S S
Co
Bulk intercalation ‘Surface’ intercalation Co
Figure 45 Schematic representation of the intercalation pseudointercalation model.
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MoS2
Co9S8
Figure 46 The contact synergy model.
The contact synergy model also assumes the presence of layered MoS2 (Figure 46).279,280 In this model, Co9S8 and MoS2 are in tight contact and results in spill-over of hydrogen from Co9S8 to MoS2 enhancing the intrinsic activity.281 During the last decades, a detailed picture of the CoMoS phase on the atomic scale was developed based on the findings and observations made with a variety of analytical tools and theoretical calculations. The catalyst contains nanosized MoS2 particles decorated with Co on the edges to form the catalytically highly active CoMoS phase. After impregnation of the alumina support with molybdenum and cobalt compounds, followed by drying and/or calcination, well-dispersed oxides of these elements are obtained. The oxides are then converted into the sulfides achieving the catalytically active phase. A more detailed picture of the Co/NiMoS catalysts will be obtained applying more in situ studies to probe the structure– activity relationship at the individual sites on nanosized catalysts in their working state. Such studies are challenging especially if industrial-type conditions are envisaged. The results of the experiments will help to rationally design catalysts for the different applications in the HDS reaction. HDS research has moved to the most refractory sulfur compounds, that is, DBTs with alkyl substituents in the 4- and 6-positions. In addition, improvements of conventional metal sulfide catalysts and the development of new materials are required to meet the increasing demand for energy.
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