Morphological and Structural Properties of MoS2 and MoS2

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Catalysis and Kinetics

Morphological and structural properties of MoS2 and MoS2 - amorphous silica-alumina dispersed catalysts for slurry-phase hydroconversion Juliana Sanchez, Andres Moreno, Fanor Mondragon, and Kevin J. Smith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01081 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Morphological and structural properties of MoS2 and MoS2 - amorphous silica-alumina dispersed catalysts for slurry-phase hydroconversion Juliana Sáncheza, Andrés Morenoa, Fanor Mondragóna, Kevin J. Smithb* a

Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de

Ciencias Exactas y Naturales, Universidad de Antioquia, UdeA - Colombia, Calle 70 No. 52-21, Medellín, Antioquia, Colombia. b

Department of Chemical and Biological Engineering, University of British Columbia,

Vancouver, British Columbia V6T 1Z3, Canada. KEYWORDS: MoS2, slurry reactor, oil-dispersed catalyst, amorphous silica-alumina, hydroconversion, hydrogenation, hydrocracking, phenanthrene, decalin


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ABSTRACT

Unsupported MoS2 and bifunctional MoS2-amorphous silica-alumina (ASA) catalysts, active for hydrogenation and hydrocracking reactions, were obtained by in-situ thermal decomposition of molybdenum octoate in the presence of a sulfur source at temperatures between 300 °C and 400 °C. Unsupported MoS2 was formed at temperatures as low as 300 °C with yields higher than 76 %. Increasing temperature increased the degree of sulfidation but also increased particle dimensions and consequently, decreased the MoS2 dispersion. The presence of ASA reduced MoS2 particle dimensions compared to the unsupported MoS2, while the effect on sulfidation degree was dependent on temperature. High temperature (400 °C) was required to achieve high MoS2 yield due to interactions with the support at lower temperatures. The catalytic activity of the unsupported MoS2 showed that as a result of the opposing effects of temperature on the MoS2 dispersion and MoS2 sulfidation degree, the activity towards hydrogenation of phenanthrene, reported on a per edge site basis, was not affected by morphological and structural differences. The bifunctional MoS2-ASA catalyst at 400°C promoted cracking, isomerization and ring opening reactions due to the moderate Brønsted acidity of the ASA and changed the product distribution favorably when compared to the unsupported MoS2.


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1. INTRODUCTION The depletion of light crude oil reserves has increased interest in unconventional oils as a source of middle distillates. 1 Slurry phase hydroconversion is a recently developed heavy oil upgrading technology that aims to produce middle distillates from heavy oils that have high concentrations of heteroatoms (S, N), metals (V, Ni) and complex molecules such as asphaltenes, resins and aromatics. 1,2 Fine powders dispersed in the heavy oil, derived from iron-based mineral ores and mineral residues, can be used as an inexpensive catalyst for slurry phase hydroconversion. The hydrogenation activity of these materials is relatively low and the active phase (Fe1-xS) is mixed with other components present in the ore that affect reactant access to the catalyst hydrogenating function. For these reasons large quantities of catalyst are required (typical catalyst loadings are several wt %) and consequently, catalyst recovery and disposal after reaction is an issue.1,3 Conversely, nano-sized MoS2 exhibits highly accessible active sites because the MoS2 is unsupported and has high resistance to poisons (N, S). The MoS2 promotes low coke yield and high H2 uptake that improves the quality of the liquid products. 1,3,4 The MoS2 can be obtained in situ during the hydroconversion reaction in the presence of a sulfur source from complexes, salts or oxides.4 These catalysts are more expensive compared to those derived from mineral ores but their intrinsic activity towards hydrogenation reactions is higher. In practice, low concentrations of Mo catalyst are required (usually between 20-5000 ppm) and they can be prepared by the insitu decomposition of oil soluble molybdenum precursors. 1,4–7 The hydrogenating catalytic activity of MoS2 is due to the edge planes of the MoS2 crystallites, while the basal planes are inert, according to the rim-edge model proposed by Daage and Chianelli 8. The MoS2 crystallite corners and edges correspond to coordinatively unsaturated sites and sulfur ion vacancies (i.e. Lewis acid sites) which promote hydrogenation reactions. 2


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For this reason, MoS2 catalysts with a few layers and small particle size (high dispersion) are highly desired. Studies have established that MoS2 stacking number and particle length are factors that affect catalytic activity, depending on the hydrotreatment reaction being evaluated. 9– 12

For instance, Tye and Smith 11 found that exfoliated MoS2 was more active for

hydrodesulfurization (HDS) and hydrodeoxygenation (HDO) reactions when compared to in-situ produced MoS2 from molybdenum naphthenate (Mo-Naph), due to a higher ratio of edge/rim sites on exfoliated MoS2 that results from a higher stack number. MoS2 derived from Mo-Naph was more active for hydrogenation reactions because of a higher dispersion.9–12 The specific conditions chosen for the transformation of the precursor into MoS2 impact catalyst morphology.13–15 It has been found that the type of activation (i.e., liquid-phase or gas-phase), the sulfiding precursor (i.e., CS2, DMDS, H2S), the S/Mo ratio and sulfidation temperature impact the sulfidation degree, the catalyst particle size, stacking number, the shape of the particles and consequently the activity. 12,15,16 Among the preparation conditions described above, several studies have identified temperature as a key parameter that determines the properties of the MoS2 catalyst. 13–15,17 Although an increase in sulfidation temperature favors particle agglomeration, 14 there is also a minimum temperature required to obtain the MoS2 active phase, depending on the preparation method. For instance, Yoosuk et al. 18 found that unsupported Ni-promoted MoS2 catalyst showed higher activity for HDS reactions when the hydrothermal preparation temperature increased from 300 °C to 375 °C using ammonium tetrathiomolybdate as the MoS2 precursor. Yin et al. 14 reported that the activity for HDS and hydrodenitrogenation (HDN) of Ni-promoted MoS2, prepared by a hydrothermal method of a mixed oxide precursor that was subsequently sulfided, increased as the gas phase sulfidation temperature increased up to 320 °C, while the activity for aromatic


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hydrogenation increased up to 340 °C and thereafter decreased. Zhang et al. 19 also used the hydrothermal method to prepare nano-MoS2 particles from MoO3 and found that a temperature of 320 °C was required to obtain crystalline MoS2 with a high number of defects, which impacted the catalytic activity for light cycle oil HDS. On the other hand, Afanasiev 15 reported that the decomposition of ammonium tetrathiomolybdate to produce unsupported MoS2 was complete at 400°C, and above this temperature the particle length and stacking increased along with the removal of over-stoichiometric sulfur, decreasing HDS activity. Okamoto et al. 17 have studied higher sulfidation temperatures (400 °C - 900 °C) for supported MoS2 obtained from inorganic salts by chemical vapor deposition (CVD) and reported that the turnover frequency (TOF) for HDS increased for sulfidation temperatures higher than 600 °C. Most studies of slurry phase hydroconversion assume that under the reaction conditions and in the presence of a sulfur source, the catalyst metal-organic precursor decomposes to MoS2, although the yield of MoS2 has not been quantified because of the difficulty in separating and recovering the catalyst from the solid residue. 1,2,4,7 Few reports have quantified the effect of reaction temperature on the morphology and activity of the MoS2 prepared in-situ from oil soluble precursors.20 Furthermore, most studies of the effect of sulfidation conditions on the catalyst activity have focused on HDS reactions, 10–12,14,15 with only few studies focusing on the hydrogenation of aromatic compounds with more than 2 rings. 11,16 Nano-sized MoS2 catalysts promote hydrogenation of aromatic compounds and heteroatom removal but the residue oil conversion is due to thermal cracking reactions, which usually require the use of high temperatures (> 420 °C). 1,5,21–23 Unsupported MoS2 catalysts dispersed in the residue oil can be recovered from the produced coke fraction and recycled. 6,7 Alternatively, the MoS2 can be dispersed on carbonaceous or inorganic support materials. 24,25 There are few


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reports regarding the incorporation of support materials with acidic properties for the hydroconversion of residue oils. 4,26,27 Eni Company has implemented a residue hydroconversion process using dispersed MoS2 incorporating acidic catalytic materials 27 and reported that with the addition of a cracking catalyst (FCC cracking catalyst), 4 residue conversion increased due to acid catalyzed isomerization, ring opening and cracking reactions. The hydrogenation function of the MoS2 also protected the cracking catalyst from poisoning by coke. 4 Other acidic solids such as talc, mica, alumina, silica, and silica-alumina have also been evaluated. 27 Although these reports provide evidence that the cracking catalyst improved conversion and the quality of hydroconversion products, several outstanding questions regarding dual-function slurry phase catalysts remain unanswered. For instance, the effect of the presence of the cracking catalyst on the properties of the in situ prepared MoS2 has not been established. Furthermore, although the required Mo concentration for hydroconversion is well known, the corresponding amount of cracking catalyst needed to increase conversion and limit deactivation has not been established. Due to the complexity of residue oil feedstocks and the associated difficulties of dispersed catalyst recovery and characterization, model reactants have been used as one approach to advance fundamental understanding of the chemical transformations that occur during hydroonversion.28 There is a particular interest in the evaluation of hydrogenation and cracking reactions of polyaromatic compounds with 2 - 4 aromatic rings

29

because they are an important

fraction of vacuum gas oils (VGO) and they can be found in fragments of larger molecules such as asphaltenes. In addition, the selective ring opening of naphthenic compounds is an important reaction for oil upgrading to produce alkyl-cycloalkanes, alkyl benzenes and aliphatic alkanes that increase cetane number of diesel fuels.29–32 Consequently, in the present study we have selected a phenanthrene-decalin mixture as a model of the polyaromatic and naphthenic


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compounds present in VGO and the fragments of larger molecular structures present in asphaltenes and resins.

16,29,33,34

Decalin was selected as a solvent due to its high boiling point

that ensures liquid phase reaction, compared to other solvents such us cyclohexane that would be in the vapor phase.35 The decalin also undergoes isomerization and cracking reactions that provide insight as to the effect of the acidic function on hydrogenated molecules. We also note that the model reactants used in the present study are free of organic S and N compounds that could have a significant impact on the acid function of the catalyst. Furthermore, the model reactants are far less prone to coke formation than the larger more complex molecules present in a residue oil.

Nonetheless, the reactants provide an opportunity to assess the impact of

bifunctional catalyst preparation conditions on the catalyst activity and selectivity, and to demonstrate that the acid function provides an opportunity for low temperature cracking and isomerization reactions to occur. Subsequent studies will be needed to demonstrate that the same reactions can be catalyzed in the presence of a residue oil. In the present study, we first evaluate the morphological, structural and catalytic properties of unsupported MoS2 obtained from molybdenum octoate at temperatures between 300 °C and 400 °C. Using this information, we then examine the effect of incorporating an acidic support amorphous silica-alumina (ASA) - on the MoS2 properties as a function of the reaction temperature. ASA’s are of interest for hydrocracking of real feedstocks 30,32,35–37 due to their mesoporosity, moderate acidity and thermal resistance, which reduce diffusional problems and poisoning by coke. 38,39 Hence, we report a systematic study on the incorporation of ASA and its effects on the in-situ preparation of MoS2 at different reaction temperatures, and the impact of the ASA on the activity of the obtained dispersed bifunctional catalysts.


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2. EXPERIMENTAL Materials Molybdenum-2-ethylhexanoate (Strem Chemicals, 15 wt % Mo) was used as the Mo precursor, carbon disulfide (Aldrich, ≥99.9 %) was used as the sulfur source and decalin (Aldrich, 99%) and phenanthrene (Aldrich, 98 %) were used as model reactants. Micro-crystalline MoS2 powder (dp < 2µm, Aldrich) was chosen as a commercially available, unsupported MoS2 catalyst reference for hydroconversion. As shown below, the micro-crystalline MoS2 powder had significantly larger particle dimensions than the MoS2 synthesized as part of this study and was chosen to emphasize the impact of particle dimensions on catalyst hydrogenation activity. Amorphous silica-alumina (ASA) supplied by the Sasol Company (Siral40) was used as the acidic function. Siral40 was received in hydrated form and prior to use was activated at 550 °C in air for 3 h. Catalyst preparation and assessment of activity MoS2 catalysts used in slurry phase hydroconversion are usually prepared in-situ i.e. the catalyst precursor is added to the reactant oil in the presence of a S source. During reactor heat-up and reaction at the desired temperature, the precursor is assumed to be converted to MoS2. However, to determine the effect of preparation temperature independent of the hydroconversion reaction temperature, unsupported MoS2 was first prepared ex-situ in a 300 mL stirred batch reactor (Autoclave Engineers) at temperatures of 300 °C, 350 °C and 400 °C for 2 h, at a cold pressure of 2.8 MPa H2 and a mixing speed of 1000 rpm. Molybdenum-2-ethylhexanoate was dissolved in 100 mL of decalin, to obtain a solution with a concentration of 5000 ppm of Mo, and CS2 was added to yield a S/Mo atomic ratio of 3 in the reactor. After the synthesis, the solids were


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recovered and washed with hexane several times using a high-speed centrifuge (1200 rpm for 15 min). The recovered solids were then placed in a U-tube and dried in a furnace at 150°C under N2 flow for 6 h. Samples were named as MoS2-X, X being the preparation temperature. The phenanthrene hydrogenation activity for the ex-situ prepared MoS2 catalysts (MoS2-X) and the micro-crystalline MoS2 were assessed in a 100 mL stirred batch reactor (Parr 4590 series) loaded with 40 g of the feed (10 wt% phenanthrene in decalin), using the required quantity of MoS2 catalyst to obtain 3000 ppm of Mo and CS2 in a S/Mo atomic ratio of 1. A simplified scheme of the reactor is shown in Figure S1. The reactor was operated under a cold pressure of 5 MPa of H2, a mixing speed of 1000 rpm, and a temperature of 300 °C, the lowest temperature used for the ex-situ catalyst preparation. Liquid samples (~2 mL) were withdrawn when the reactor reached the desired reaction temperature (reported as time zero) and every hour thereafter for a total reaction time of 4 h. The identification and quantitative analysis of the liquid products was performed using a GC/MS Shimadzu (2010-Plus) equipped with a SHRXI – 5MS (5 % phenyl 95 % methylpolysiloxane) capillary column (30 m × 0.25 mm x 0.25µm), using ntetradecane as internal standard. Phenanthrene (Xp) and decalin (XD) conversions were calculated using equations (1) and (2): = ⁄ − ⁄ ⁄ ⁄ ∗ 100

(1)

= ⁄ − ⁄ ⁄ ⁄ ∗ 100

(2)

where , , and correspond to the areas determined by GC-MS analysis for the decalin, the internal standard and the phenanthrene in the sample mixture. The ratios ⁄

and ⁄ refer to the area ratio for component j at the beginning of the reaction and after a

reaction period of t hours. Hydrogen consumption was not measured directly but estimates based


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on the phenanthrene conversion and product selectivity showed H2 consumption to be < 15 % of the initial moles of H2 at the maximum phenanthrene conversion. The bifunctional MoS2-ASA catalysts were prepared in-situ during the hydrocracking of the phenanthrene-decalin mixture. The ex-situ methodology described above could not be used because of potential ASA deactivation and decalin hydrocracking during the preparation. Hence, the MoS2-ASA catalyst preparation and catalytic assessment were carried out in a 100 mL stirred batch reactor (Parr, 4590) using 40 g of the feed (10 wt% phenanthrene in decalin) and the appropriate amounts of molybdenum-2-ethylhexanoate and ASA to obtain a concentration of 600 ppm Mo and 4 wt% of ASA with respect to the 40 g of 10 wt% phenanthrene in decalin feed (1.5 wt% Mo nominal content in the composite MoS2/ASA catalyst). CS2 was used as the S source with a S/Mo molar ratio of 3. The reactor was pressurized to 5 MPa of H2 at room temperature and heated to the reaction temperature (300 °C, 350 °C or 400 °C) and operated for a reaction time of 4 h. The catalysts characterized after reaction were identified as MoS2-ASA-Xi, where Xi is the in-situ synthesis/reaction temperature. These bifunctional catalysts were compared to bulk MoS2 prepared in-situ at the same conditions and denoted as MoS2-Xi. The identification and quantitative analysis of the liquid products was performed as explained above. Note that the in-situ prepared catalysts (MoS2-ASA-Xi and MoS2-Xi) were assessed similarly as described above for reaction periods of up to 4 h at the specified reaction/synthesis temperature; whereas, the ex-situ prepared MoS2-X catalysts were synthesized at the specified temperature for a period of 2 h and then reacted for a further 4h period at 300 °C.


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Catalyst Characterization For the unsupported MoS2 catalysts (MoS2-X) the Mo content was determined by atomic absorption spectroscopy using an Agilent MP-AES 4100, while C and S contents were determined using a 2400 Perkin-Elmer CHNS/O analyzer. Structural characterization was determined by X-ray diffraction (XRD) on a Bruker D8 Focus (0−20, LynxEye detector) diffractometer with Co Kα (λ= 1.789 Å) radiation operating at 35 kV and 40 mA, with a scan range of 2θ = 5 ° to 80 ° and a step size of 0.033 °. X-ray photoelectron spectroscopy (XPS) was performed on all catalysts using a Leybold Max200 spectrometer with a Mg Kα photon source generated at 1253.6 eV. The C 1s peak at 284.5 eV was taken as a reference to calculate binding energies. XPSPEAK41 software was used for deconvolution of the experimental spectra. Shirley background subtraction was applied and the peaks were deconvoluted through mixed Gaussian−Lorentzian functions (80 %−20 %). Morphology, stacking and length distribution of MoS2 particles in both the unsupported MoS2 and the MoS2-ASA catalysts were determined from transmission electron microscopy (TEM) using a (JEOL) JEM 200 scanning transmission electron microscope operated at 200 kV. Samples were prepared by dispersing the ground solids in acetone and placing one drop on a holey carbon grid. The length and stacking number of at least 200 slabs were used to generate histograms for both properties of each sample. As a measure of the dispersion, the average fraction of Mo atoms at the MoS2 edge surface (fMo) was calculated using equation (3) based on the assumption that the MoS2 slab is a perfect hexagon, as presented by Kasztelan et al. 40

=

∑

… 6 − 6 ∑ … 3# − 3 +

1

3


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where ni is the number of Mo atoms along one side and t the total number of slabs. ni is calculated according to the equation % = 3.22 − 1 Å, using the lengths (% ) reported in the histograms. Additionally, the Mo atoms located at the rim sites were calculated using the histograms for stacking number and equation (4) 2 ()* +),-+ % = + / 0 1 4 ) #

where ) is the stacking number and and are the percentage of particles with a stacking number of 1 or ), respectively. Surface areas of unsupported MoS2 were determined from N2 adsorption − desorption isotherms measured at −196 °C using a Micromeritics ASAP 2020 analyzer. Samples were degassed at 150 °C under vacuum for 8 h before analysis. The surface area was calculated using the Brunauer−Emmett−Teller (BET) equation in the relative pressure range (P/Po) of 0.05−0.20. Finally, Raman spectra were obtained with a LabRam HR Raman spectrometer (Horiba-Jobin Yvon) equipped with a confocal microscope and using a He-Ne laser at 632.8 nm. 3. RESULTS AND DISCUSSION Catalyst Characterization According to the catalyst compositional data reported in Table 1 for the ex-situ prepared MoS2-X catalysts, the Mo and S content increased with temperature; whereas, carbon content determined by elemental analysis decreased from 4.7 wt% to 2.0 wt% (Table 1), indicating that most of the carbon from the molybdenum octoate precursor decomposed at temperatures as low as 300 °C.


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Table 1. Compositional and textural properties of MoS2-X prepared at 300 °C, 350 °C and 400 °C from molybdenum octoate Properties

MoS2-300

MoS2-350

MoS2-400

Mo content (wt %)

37.7

40.8

49.3

C content (wt %)

4.7

2.5

2.0

S content (wt %)

30.08

35.30

36.46

S/Mo atomic ratio

2.3

2.5

2.2

BET surface area (m2/g)

99

119

182

0.23

0.27

0.51

Pore volume (cm3/g)

Figure 1 shows the XRD patterns of the MoS2-350 and MoS2-400 catalysts with diffraction peaks at 2θ =15.9 º, 38.7 º, 46.1 º and 69.6 º corresponding to (002), (100), (103) and (110) planes of the MoS2 crystalline phase, respectively. 16 The MoS2-300 only showed low-intensity signals for (100) and (110) planes, which could be due to incomplete formation of the MoS2 or small crystallite size. The shape and intensity of the (002) peak indicates the MoS2 layer stacking and was used to calculate an approximate value of the MoS2 stacking number. Table 2 shows that for the samples prepared at 350 °C and 400 °C, stacking numbers of 8 and 9 were obtained, respectively.


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Figure 1. XRD patterns of MoS2-X catalysts prepared at 300 ºC, 350 ºC and 400 ºC from molybdenum octoate The TEM micrographs of the MoS2-X catalysts (Figure 2) show a laminar morphology, evidence of MoS2 formation. 41 Figure 3 shows the length and stacking number histograms and the fit to log-normal or normal distributions which were used to estimate the mean values reported in Table 2. Table 2 shows that the MoS2 particle length increased from 8.0 nm to 13.6 nm and stacking number increased from 1.4 to 4.1 as the preparation temperature increased from 300 °C to 400 °C. Note that increasing synthesis temperature from 300 to 350 ºC (MoS2-300 versus MoS2-350) resulted in a larger change in stacking than in length; whereas, from 350 to 400 ºC (MoS2-350 and the MoS2-400) the MoS2 particles increased in length but the stacking number did not change significantly. Stacking values obtained by TEM were lower than the values calculated by XRD data, the difference likely because XRD does not detect the smallest particles (< ~5 nm).


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Figure 2. TEM images of MoS2 -X catalysts prepared at different temperatures from molybdenum octoate. a and b MoS2-300; c and d MoS2-350; e and f MoS2-400


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Figure 3. Length and stacking number histograms for MoS2 prepared at different temperatures from molybdenum octoate The increase in particle length with preparation temperature also impacted fMo, as reported in Table 2. As the preparation temperature increased from 300 °C to 400 °C, fMo decreased from 13.3 % to 8.1 %, indicative of a lower number of Mo atoms at the crystallite edges as temperature increases. The values obtained are in agreement with the fMo values close to 10 % reported for in-situ prepared MoS2 from molybdenum naphthenate and ammonium heptamolybdate precursors. 9


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Table 2. MoS2 particle dimensions of MoS2-X fresh and after use in phenanthrene hydrogenation and for MoS2-ASA-Xi catalysts obtained in-situ in the hydrocracking of phenanthrene-decalin

Catalyst

Length

Dispersion fMo

Rim sites

(nm)

(%)

(%)

Stacking number

Fresh

1.4 ± 0.2

8.0 ± 2.9

13.3

92

Used

2.8 ± 1.3

8.0 ± 1.5

12.1

75

Fresh

3.7 ± 1.3 (8*)

9.5 ± 2.9

10.2

64

Fresh

4.1 ± 1.4 (9*)

13.6 ± 4.6

8.1

62

Used

5.1 ± 2.0

12.8 ± 5.9

8.1

45

MoS2-ASA-300i

1.6± 1.6

6.20± 1.6

16.0

MoS2-ASA-350i

1.7± 1.7

6.4± 1.6

14.3

MoS2-ASA-400i

2.0± 1.7

7.7 ± 1.5

12.6

MoS2-300

MoS2-350

MoS2-400

* Stacking number as determined from XRD

BET surface areas of the three MoS2-X catalysts, reported in Table 1, increased from 99 m2/g to 182 m2/g as temperature increased from 300 °C to 400 °C. The isotherms for the three materials (Figure S2, Supporting Information) are Type IV, characteristic of mesoporous materials with low microporosity with an H3 type hysteresis loop, usually associated with aggregates of platelike particles giving rise to slit-shaped pores. 42 With an increase in temperature, the TEM images (Figure 2) indicate that the morphology of the aggregated particles transformed to a more open and curved structure, induced by the presence of highly disordered MoS2 lamellae, which generated voids between MoS2 stacks, probably inducing a higher surface area and porosity, as observed from the N2 adsorption isotherms. 43


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Figure S3 (Supporting Information) compares the Raman spectra of the MoS2-X catalysts and the commercial micro-crystalline MoS2. Blanco et al. 44 reported a correlation between the intensity ratio of the 228 cm-1 to 405 cm-1 bands, associated with disorder degree, and the MoS2 crystallite size. For the catalysts of the present study it was found that the intensity ratio of these bands decreased from 0.97 to 0.63 as the preparation temperature increased from 300 °C to 400 °C, indicating that the crystallite size increased with temperature, which is consistent with the calculated particle length estimated by TEM (Table 2). Note that this technique is useful for the characterization of small crystallite sizes like the ones obtained for the MoS2-300 catalyst, that could not be observed by XRD. 44 Figure 4 shows high-resolution TEM micrographs of the bifunctional MoS2-ASA-Xi catalysts. Histograms for length and stacking number are reported in Figure 5 and mean values are presented in Table 2. Note that the MoS2-ASA-Xi catalyst had lower MoS2 particle dimensions compared to the bulk MoS2-X catalyst at the same preparation temperature, indicating that the presence of ASA limits interactions among MoS2 nanoparticles. Differences in particle dimensions between MoS2-X and MoS2-ASA-Xi catalysts increased with temperature. At 300 °C, the presence of ASA decreased particle length while stacking number remained close to 1.6. At temperatures of 350 °C and 400 °C both stacking number and length decreased significantly, compared to the unsupported MoS2-X samples. Mo dispersion (fMo) increased in the presence of ASA because of the decrease in particle length (refer to Table 2). Moreover, stacking number histograms for the three MoS2-ASA-Xi catalysts did not differ significantly with the mean values increasing marginally from 1.6 to 2.0.


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Figure 4. TEM images of MoS2-ASA-Xi catalysts obtained in-situ at 300°C, 350°C and 400°C in the hydrocracking of phenanthrene-decalin

Figure 5. Number of stacks and length histograms for MoS2 particles in MoS2-ASA-Xi catalysts obtained in-situ at 300°C, 350°C and 400°C in the hydrocracking of phenanthrene-decalin


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Figure 6 reports the Mo (3d) and S (2p) XPS spectra of the MoS2-X catalysts. The Mo (3d) spectra consist of two peaks (3d3/2 and 3d5/2) due to the spin-orbit (j-j) coupling separated by 3.1 eV with a peak area ratio of 1.5. 45 The raw data were deconvoluted into three doublets - the main Mo 3d5/2 peak, with binding energy (B.E) close to 229.6 eV for Mo4+ associated with MoS2, 45,46 a second doublet with Mo 3d5/2 signal at 230.8 eV is considered to be associated with Mo5+ in an intermediate oxysulfidic environment 45 and finally, the doublet with Mo 3d5/2 at 232.5 eV attributed to Mo6+ of residual oxide species. The intensities of the latter two signals decreased with increased temperature. The molar composition of the Mo states (sulfidation degree) reported in Table 3 indicate that the MoS2 surface concentration increased from 76.5 mol% to 88.9 mol% as the preparation temperature increased from 300 °C to 400 °C.


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Figure 6. Mo (3d) and S (2p) XPS spectra deconvolution of MoS2-X prepared at 300 °C, 350 °C and 400 °C from molybdenum octoate The S (2p) spectra are shown in Figure 6 together with the peak located at 226.8 eV, corresponding to S (2s) and present as a shoulder in the Mo (3d) spectra. The S (2p) signals are also comprised of a doublet corresponding to S 3p1/2 and S 3p3/2 separated by 1.16eV with a peak area ratio of 0.51. 45,46 The measured data could be fitted to two doublets. The main doublet for the three samples (S 3p3/2) located at 162.4 eV is related to S2- in MoS2 and the second doublet at 163.5 eV is assigned to S2- species in MoOxSy. 45 Table 3 summarizes the B.E values and the molar compositions determined from the XPS spectra. The data show that the sulfur in a MoS2


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environment is between 91.2 mol% for the MoS2-300 catalyst and 96.4 mol% for the MoS2-400 catalyst. Previous literature results 16,47,48 have shown similar values for the sulfidation degree at the temperatures studied here. Yiwen et al.16 reported a molar composition of 87.29 mol% of Mo4+ and 95.52 mol% of S2- in MoS2 environment, when MoS2 was prepared from molybdenum octoate in the presence of carbon black at a pressure of 7 MPa and 400 °C.

Table 3. XPS data for MoS2-X fresh and MoS2-ASA-Xi catalysts obtained in-situ in the hydrocracking of phenanthrene-decalin and S/Mo ratio for MoS2-X after used in the phenanthrene hydrogenation Mo 3d5/2

S 2p3/2

Composition (mol %)

Composition (mol %)

S/Mo atomic Mo/(Si+Al)

ratio

Catalyst

atomic Mo

4+

B.E(eV)

Mo

5+

B.E (eV)

Mo

6+

B.E (eV)

MoS2

MoOxSy

B.E (eV)

ratio

Fresh Used

B.E (eV) 163.8 229.6

230.8

232.8

162.5

MoS2-300

76.5

11.3

12.2

91.2

8.8

2.6

2.3

-

MoS2-350

79.3

9.7

11.0

93.0

7.0

2.5

2.8

-

MoS2-400

88.9

7.0

4.1

96.4

3.6

2.4

2.6

-

MoS2-ASA-300i

71.5

6.9

21.6

-

-

-

-

0.015

MoS2-ASA-350i

79.9

4.0

16.1

-

-

-

-

0.011

MoS2-ASA-400i

94.5

5.5

0.0

-

-

-

-

0.011


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The S/Mo ratios obtained for the three catalysts by both XPS (values close to 2.5) and bulk composition analysis (reported in Table 1) are higher than the stoichiometric value expected for MoS2. Previous studies have shown the advantageous role of over-stoichiometric sulfur present in the form of edge-located S22- species, which are believed to increase the catalytic activity owing to the activation of H2 molecules. 15 Figure 7 reports the Mo (3d) XPS spectra for the MoS2-ASA-Xi catalysts and Table 3 reports the molar composition of the Mo states obtained from the spectra. Sulfidation degree, calculated as the Mo4+ content for the MoS2-ASA-300i and MoS2-ASA-350i were analogous to the ones obtained for the MoS2-300 and MoS2-350 catalysts, respectively. At these temperatures, it was noticeable that Mo in an oxysulfide environment (Mo5+) decreased and the molybdenum oxide (Mo6+) increased compared to the percentages obtained for the MoS2-X catalyst, which is probably an effect of the interaction of Mo species with the alumina phase that avoids the partial sulfidation of molybdenum oxide species to yield Mo in an oxysulfide environment. Interactions between transition metal oxides and the support have been extensively studied in the literature with the purpose of decreasing these interactions such that a higher degree of sulfidation can be obtained at lower temperatures. 49–52 According to the results of this study, formation of MoS2 from molybdenum octoate requires temperatures higher than 350 °C to achieve a high degree of sulfidation and to avoid the interaction with the ASA that results in the production of a molybdenum oxide phase. By contrast, for catalysts prepared at 400 °C, sulfidation increased from 88.9 % for the MoS2-400 catalyst to 94.5 % for the MoS2-ASA-400i catalyst and the XPS peaks corresponding to molybdenum oxide species (i.e., Mo6+) were not present. The increase in sulfidation degree could be a result of both the presence of the ASA which decreased particle dimensions and


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Energy & Fuels

favored almost complete sulfidation and the higher temperature compared to 300 ºC and 350 ºC, that diminished the interactions between molybdenum species and the ASA support. Table 3 shows that the Mo/(Si+Al) atomic ratios for the three MoS2-ASA-Xi catalysts were similar and the Mo concentration on the surface was not affected by the reaction temperature.

Figure 7. Mo (3d) spectra deconvolution of MoS2-ASA-Xi catalysts obtained in-situ at 300°C, 350°C and 400°C in the hydrocracking of phenanthrene-decalin Catalyst activity Figure 8 reports the phenanthrene conversion at 300 °C for the three MoS2-X catalysts versus reaction time, showing that highly active catalysts for the hydrogenation of polyaromatic compounds were obtained, even at synthesis temperatures as low as 300 °C. Phenanthrene conversions after 4 h reaction time were close to 60 % in contrast to the conversion of 11 %


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obtained with the micro-crystalline commercial MoS2. The low activity of the micro-crystalline MoS2 has been reported previously for the HDS of dibenzothiophene 9 and can be attributed to the large crystallite size of the commercial MoS2 according to Raman data, and the high stacking number with a value of 191 deduced from the (002) peak in the XRD spectra (Figure S4, Supporting Information).

Figure 8. A. Conversions kinetics for the hydrogenation of phenanthrene at 300°C, 5.0 MPa H2 with MoS2-X catalysts prepared at 300°C, 350°C and 400°C and micro-crystalline commercial MoS2 B. Selectivities to phenanthrene hydrogenation products vs Time Figure 8 shows that there is no significant difference in the phenanthrene conversion vs reaction time for the MoS2-X catalysts prepared at temperatures between 300 °C and 400 °C. For a better understanding of the effect of the morphological and structural properties on the catalyst activity, the rate constants for the phenanthrene hydrogenation were calculated using the conversion data, assuming 1st-order kinetics in phenanthrene concentration and zero order in H2 because of the large H2 excess in the reactor. The estimated rate constants were normalized to the amount of MoS2 using the estimated Mo4+ concentration obtained by XPS to account for the Mo that was converted to MoS2. (k, Table 4, column 2). The rate constants were also calculated per Mo edge


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Energy & Fuels

site (Mo4+e), 8 using the dispersion values fMo (k*, Table 4, column 3). Equations (5) and (6) describe the calculation of both k and k*: 3 = 4. 5 ⁄*67 869:

(5)

3 ∗ = 4. 5 ⁄*67 86;9:

(6)

where 4 is the slope of the ln(1-Xp) vs , graph, XP is the conversion of phenantherene at time t, 5 is the volume of the liquid in the reactor (mL) and *67 86 9: = *

Morphological and Structural Properties of MoS2 and MoS2

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