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. 2023 Jan 9;8(2):2596–2606. doi: 10.1021/acsomega.2c07123

Novel Impregnation–Deposition Method to Synthesize a Presulfided MoS2/Al2O3 Catalyst and Its Application in Hydrodesulfurization

Xuehui Li 1,*, Xuefeng Wang 1, Jing Ning 1, Hongtao Wei 1, Long Hao 1,*
PMCID: PMC9850723  PMID: 36687028

Abstract

graphic file with name ao2c07123_0010.jpg

A novel impregnation–deposition method was applied to prepare presulfided MoS2/Al2O3 catalysts with large surface areas for the application of hydrodesulfurization (HDS). The synthesized catalysts were characterized systematically, and their catalytic performances were evaluated by the HDS of dibenzothiophene (DBT). It is found that the impregnation–deposition method improves the surface area of the synthesized catalysts by eliminating the micropores of the alumina support and adding mesostructured MoS2 particles within the support. Moreover, this method enhances the reducibility of the sulfided Mo species, as characterized by temperature-programed reduction (TPR) and X-ray photoelectron spectroscopy. Compared to the impregnation method, the impregnation–deposition method leads to the formation of more active sites as proved by TPR and CO-Fourier-transform infrared analyses. Hence, the reaction conversion rates and the hydrogenation/direct-desulfurization ratios of the DBT on the catalysts synthesized by the impregnation–deposition method are 1.3 times and 1.5 times as high as those of the catalysts made by the conventional impregnation method, respectively.

1. Introduction

Increasing attention is being paid to the fuel processing industries all over the world. Under the pressure of the lower-quality crude oil and the increasingly rigorous environmental problems, the sulfur content in fuels has been limited to an extremely low level (<10 ppm).1,2 Up until now, as one of the important techniques to produce ultra-clean fuels, the hydrodesulfurization (HDS) technology might be the most effective approach to remove the sulfur-containing compounds in refining industries.3−6

In order to meet the strict regulations,7,8 transition metal compounds, such as transition metal sulfides/phosphides, etc., have been widely used as catalysts for ultraclean fuel production.9−12 Generally, Co/Ni-promoted MoS2 catalysts supported on Al2O3 are the most commonly applied catalysts in HDS processes.13 Two types of active phases are usually contained in the catalysts: the lower active type I phase interacting strongly with the Al2O3 support via Mo–O–Al bonds and the higher active type II phase interacting weakly with Al2O3 by the van der Waals force.14 The conventional Mo-based HDS catalysts are usually prepared by the sulfidation of MoOx/Al2O3. However, a great number of Mo–O–Al linkages are formed during the preparation of MoOx/Al2O3, which will cause the subsequent insufficient sulfidation and the formation of majority type I active sites. All of these will finally lead to the decrease in the catalytic activity of MoS2/Al2O3. Consequently, improving the sulfidation degree and creating more type II active sites are very important.

Recently, ammonium tetrathiomolybdate [ATTM, (NH4)2MoS4] has attracted significant attention as an established active component for the preparation of catalysts in the sulfuric form (i.e., the presulfided catalysts).11,15,16 It was found that the molybdenum sulfide catalyst prepared by the decomposition of ATTM exhibited a good activity toward thiophene hydrogenolysis.17,18 Catalytic hydrogenolysis activity was directly correlated with the number of anion vacancies and the low valence state of the reduced MoS2/Al2O3 catalysts. Moreover, as ATTM has already tetrahedrally bonded to molybdenum atoms, which simplifies the sulfidation of oxide precursors,19 the decomposition of ATTM will lead to completely sulfided catalysts.

Up until now, the impregnation method has been widely used for the preparation of conventional HDS catalysts due to the simple and economical operations.20 Many studies have been conducted on supported catalysts prepared from the ATTM precursor.19,21,22 A series of highly loaded MoS2/Al2O3 HDS catalysts were prepared by impregnating and reducing ATTM with N2H4 in aqueous solution in the presence of Al2O3.23 The catalyst activity increased until the Mo content was up to 22 wt %, in contrast to those conventional oxide precursor routes, for which the catalytic activity stabilized after the Mo content was more than 15 wt %.24 Although ATTM is an excellent active component for HDS catalysts, the preparation methods are still unsatisfactory because these catalysts have highly decreased surface areas due to the blocked pore structures caused by aggregated active components. Therefore, taking advantage of the excellent properties of ATTM and optimizing the pore structures of the catalysts are a key point to improve the catalytic activity.

The deposition method is an optimal procedure to synthesize unsupported MoS2 catalysts with high specific areas and excellent activities due to the ease of preparation and homogeneity of the as-prepared catalysts.25−29 Moreover, several approaches have been proposed in the supported catalyst preparation based on this method, such as the chemical deposition method,30 the in situ deposition method,31 the chemical vapor deposition method,32 and the pulsed laser deposition method.27 Shi et al. reported a simple ethanol-assisted chemical deposition method to synthesize a MoS2/Al2O3 catalyst with a large specific surface area and high sulfidation degree of Mo species.33 A similar method was also employed by Gao et al. in the preparation of HDS catalysts using MoS3 nanoparticles as the precursor.34 They found that the as-synthesized MoS3 precursor not only promoted the sulfidation degree of the resulting catalyst but also realized a superior decoration of Ni atoms at the edges of MoS2 nanoflakes. The MoS2 active phases in the reported catalysts, however, are still generated by an ex situ sulfidation and thermal treatment process with the oxidic Mo species, which will lead to an insufficient utilization of Mo species.

The combination of impregnation and deposition is an effective way to synthesize catalysts with ideal porous structures and excellent catalytic performances. Nevertheless, to the best of our knowledge, this method has still not been applied in the presulfided HDS catalyst preparation. This work aims to develop a simple and convenient method to synthesize a presulfided HDS catalyst with superior catalytic behavior. For this target, ATTM was selected as the precursor due to its high sulfidation degree and excellent HDS activity. In the meantime, the impregnation–deposition method was also selected to optimize the structural and catalytic properties of the catalysts. The textural properties of the catalysts and the dispersity of the active components were characterized. The catalytic activity of MoS2/Al2O3 catalysts synthesized by the impregnation–deposition method was investigated.

2. Experimental Section

2.1. Catalyst Preparation

All of the chemicals were purchased from Sinopharm Chemical Reagent Company (PR China) without further purification. The cylindrical extrudates of the Al2O3 support (with a diameter of 1.2 mm and an average length of 3 mm) were prepared using a twin-screw extruder at a revolving speed of 300 rpm. Before extrusion, nitric acid (1.0 g), deionized water (70 mL), and sesbania powder (1.0 g) were added to pseudo-boehmite (100 g) and stirred well. Then, the extrudates were dried at 120 °C for 10 h and calcined at 550 °C for 4 h.

ATTM was prepared according to the method described by Alonso et al.35 Generally, ammonium heptamolybdate [(NH4)6Mo7O24·4H2O, 4.8 g] was first dissolved in ammonium hydroxide (50 mL) and purged with hydrogen sulfide (H2S) for sulfidation at room temperature for 1 h. Then, the mixture was heated to 60 °C with the continuous H2S purging until a deep-red color appeared. After this, the mixture was cooled in an ice bath for 12 h for the crystallization of ATTM. Finally, the ATTM crystals could be obtained after the vacuum filtration, washed five times with cold ethyl alcohol, and dried under vacuum.

The synthesized ATTM was dispersed on the Al2O3 support by the incipient wetness impregnation method. The impregnation solution was prepared by adding 3.0 g of ATTM and 1.5 g of ethanolamine in 9.0 mL of deionized water. Afterward, the solution was mixed with 10 mL of Al2O3 extrudates and placed at room temperature for 12 h. For the conventional impregnation method, the impregnated materials were dried in N2 at 120 °C for 12 h and calcined at 400 °C for 4 h with a 4 °C/min heating ramp. Active phase molybdenum (Mo) was prepared in a ratio of 20 % wt. The corresponding presulfided MoS2/Al2O3 catalyst was labeled MoS2-I/Al2O3. For the impregnation–deposition method, the same impregnating process was utilized, and the impregnated materials were added to a formic acid solution [0.92 g of formic acid (99%) added in 20 mL of deionized water] at room temperature for 1 h to guarantee a complete deposition. Then, the materials were filtered and washed several times with deionized water. After this, the as-synthesized materials were calcined in N2 at 400 °C for 4 h with a 4 °C/min heating ramp. Active phase molybdenum (Mo) was also prepared in a ratio of 20 % wt, and the corresponding catalyst was labeled MoS2-ID/Al2O3.

2.2. Characterization

A description in detail about all techniques has been supplied elsewhere,36 and only a brief summary is provided here. The X-ray diffraction (XRD) patterns of the support and catalysts were recorded using a Panalytical X’Pert Pro MPD diffractometer (Netherlands, radiation Cu Kα, accelerating voltage 40 kV, applied current 30 mA, scan rate 0.05° s–1).

Nitrogen adsorption–desorption isotherms were measured on a Micromeritics TRISTAR 3020 adsorption analyzer (USA). N2 was used as the adsorption agent at −196 °C. Before the adsorption measurements, the samples should be degassed at 140 °C under a vacuum of 10–5 Torr for 5 h. The surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method. The pore size distributions were obtained from the branch of the isotherms in the Barrett–Joyner–Halenda (BJH) method.

The properties of surface sites were determined by Fourier-transform infrared (FT-IR) measurements after the adsorption of CO at low temperature (100 K). A Bruker VERTEX 70 digital FT-IR spectrometer equipped with a CO adsorption system has been used. For the CO-FTIR study, samples were carefully ground and placed into the IR cell. The samples were first dried in situ at 300 °C under normal pressure in N2 for 2 h. Then, they were cooled to 30 °C and treated with H2 at a heating rate of 10 °C/min to 300 °C for 2 h. Afterward, the sample was cooled to 100 K with liquid nitrogen under high vacuum (1 × 10–4 Pa), and the background was obtained under this condition. Small doses of CO were introduced by a calibrated volume at an equilibrium pressure of 133 Pa to obtain site saturation. The IR spectra of adsorbed CO were recorded using a mercury–cadmium–telluride detector with 4 cm–1 resolution and 256 scans.

The temperature-programed reduction (TPR) measurements were carried out with a Quantachrome CHEMBET-3000 instrument. Before the measurements, the samples (0.1 g) were first charged in a loop and heated to 180 °C at a rate of 10 °C/min and maintained for 1 h under Ar flow to remove the adsorbed materials. Then, the samples were cooled to room temperature and treated in H2/Ar (10 vol %) with a flow rate of 100 mL min–1 at room temperature for 1 h. After this, the samples were heated from 50 to 800 °C at a rate of 10 °C min–1. In the process, a thermal conductivity detector was used to measure the consumption of hydrogen.

X-ray photoelectron spectroscopy (XPS) analyses were performed on a VG ESCALAMBK II spectrometer equipped with a multi-channel detector and non-monochromatic Al K alpha radiation (1486.6 eV) with an excitation power of 15 kV and 10 mA. The analytic chamber vacuum prior to the test was 5.0 × 10–9 mbar. Before analyses, the sulfided catalysts were placed in a box under Ar to avoid the partial reoxidation. The samples were pressed onto the specimen holder, which was directly moved into the introduction chamber. All the bonding energies were determined by using C 1s (284.6 eV) as the reference. The Mo 3d, S 2p, and Al 2p spectra were analyzed with a XPSPEAK V4.1 software package. The Shirley background was subtracted from the data, and a Gaussian–Lorentzian decomposition with a 20/80 proportion was applied as the parameter. The binding energy values were estimated to be accurate within ±0.2 eV.

Transmission electron microscopy (TEM) was performed with a JEM-2100UHR electron microscope (JEOL, Japan), and the accelerating voltage was 200 kV. The catalysts were ground and suspended in ethanol using an ultrasonic bath and then placed in a Cu cellulose-coated grille. About 300 slabs from nearly 15 representative micrographs were taken to quantitatively compare the average length and stacking number of the MoS2 slabs on different catalysts, and the statistically calculated formulas are listed as eqs 1–4.11,37,38

2.2. 1
2.2. 2

where ni is the number of MoS2 units, Li is the length of MoS2 units, and Ni represents the layer number of stacked MoS2 units.

2.2. 3
2.2. 4

where ni′ is the number of Mo atoms along one side of the MoS2 slabs and fMo is the proportion of Mo at the edge site.

2.3. Catalytic Activity Assessment

The catalytic activity measurements were carried out in a continuous-flow fixed-bed high-pressure micro-reactor (15 mm inner diameter and 650 mm long). Prior to the reaction, 4.2 g of catalysts (5 mL) was placed at the center of the reactor. Then, the catalysts were activated in hydrogen under a pressure of 2.0 MPa at 400 °C for 1 h (the heating rate of the reactor is 10 °C/min). After this, the temperature was reduced to 300 °C, and 2.0 wt % dibenzothiophene (DBT) in toluene was introduced into the reactor as the model reactant at a liquid hour space velocity of 2 h–1 and a H2/oil volumetric ratio of 300/1. The measurement was kept at 300 °C for 4 h, and the products were cooled and separated into gas and liquid products in a high-pressure separator behind the reactor. The liquid products were then collected using chromatographic sample bottles and analyzed on a Bruker 450 GC gas chromatograph equipped with a flame ionization detector and HP-5 capillary column.

The reaction rates of the presulfided MoS2/Al2O3 catalysts toward DBT were calculated according to the equation39

2.3. 5

where rDBT is the reaction rate of DBT (mol/g·s), F is the molar flow rate of the reactant (mol/s), m is the weight of the presulfided MoS2/Al2O3 catalyst (g), and ConvDBT is the conversion of DBT.

3. Results and Discussion

3.1. XRD Pattern Analyses

The XRD pattens of MoS2 and the corresponding catalysts synthesized by different methods are listed in Figure 1. For unsupported MoS2 particles MoS2-ID, ATTM was deposited by formic acid and calcined at 400 °C in a N2 atmosphere. As to MoS2-I, ATTM was only calcined at 400 °C in N2. Both samples exhibit a good MoS2 crystalline phase [14.1° (002), 33.2° (100), 39.3° (103), and 58.8° (110), JCPDS file no. 37-1492], and no significant difference exists in the peak intensity between them. When the MoS2 active phase was supported on alumina by the impregnation and the impregnation–deposition methods, characteristic peaks ascribed to γ-Al2O3 [45.9° (440) and 66.8° (400)] and MoS2 were observed in both samples. The crystallinities of the supported MoS2 samples (MoS2-ID/Al2O3 and MoS2-I/Al2O3) were obviously lower than those of the unsupported MoS2 samples (MoS2-ID and MoS2-I), indicating that the MoS2 crystals were well dispersed on the Al2O3 supports by both methods.9 The XRD patterns for the catalysts MoS2-ID/Al2O3 and MoS2-I/Al2O3 were also the same except for a broad diffraction peak at 14.1° in MoS2-ID/Al2O3. This indicates that the crystallinity of MoS2 on the supported catalysts could be affected by the preparation methods. It is known that the crystal structure of MoS2 consists of a vertical arrangement of MoS2 layers attached by van der Waals forces, and the (002) reflection peak comes from a certain stacking degree along the c direction.40 Hence, the stacking degree of MoS2 slabs prepared by the impregnation–deposition method is higher than that of MoS2 slabs prepared by the impregnation method. The impregnation–deposition method is more favorable to generate MoS2 crystals with a higher stacking degree.

Figure 1.

Figure 1

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XRD patterns of the presulfided catalysts prepared by different methods MoS2-ID/Al2O3, MoS2-I/Al2O3, and the support Al2O3 and MoS2 prepared by different methods MoS2-ID and MoS2-I and the JCPDS card no.37-1492.

3.2. Textural Properties

In this work, the precursor ATTM in both catalysts decomposed in the same form of (NH4)2MoS4 → MoS3 → MoS2. For the catalyst MoS2-ID/Al2O3 prepared by the impregnation–deposition method, ATTM was first impregnated in the Al2O3 support, and the as-synthesized wet sample was added to formic acid solution subsequently. The formic acid reacted with ATTM based on the following equation41

3.2. 6

Generated MoS3 deposited in the support channel immediately. As to the MoS2-I/Al2O3 catalyst, impregnated ATTM directly decomposed into MoS3 at 280 °C in the nitrogen atmosphere according to the following equation42

3.2. 7

Both of the above-mentioned samples were calcined at 400 °C in N2 for 4 h, and the MoS3 precursors turned into MoS2 active phases according to43

3.2. 8

In order to deeply investigate the formation of the pore structures, samples in different preparation stages were characterized by the nitrogen adsorption/desorption measurements.

The textural properties of the support and the catalysts were measured by nitrogen adsorption–desorption isotherms, and the results are listed in Figures 2 and S1. The isotherms are listed in Figure 2a: all samples exhibit typical IV isotherms with an H3 hysteresis loop in the relative pressure region higher than 0.5, revealing the presence of mesoporous structures according to IUPAC classification.44Figure 2b shows the BJH pore-size distributions for the support and catalysts. There are two types of pores that exist in the Al2O3 support, the serial narrow strong peaks in 4–10 nm and the broad weak peak that appears in 15–40 nm. For all the MoSx/Al2O3 catalysts, only the narrow strong peaks are reserved, indicating that the active phase of ATTM can be scattered inside the pores of the Al2O3 support, no matter which method is used.

Figure 2.

Figure 2

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(a) N2 adsorption isotherms and (b) pore size distributions of the presulfided catalysts MoS2-ID/Al2O3 and MoS2-I/Al2O3 and the precursor catalysts MoS3-ID/Al2O3 and MoS3-I/Al2O3 synthesized by different methods and the support Al2O3.

The pore structures of the Al2O3 support and catalysts are shown in Table 1. The specific areas and pore volumes of unsupported MoS3 prepared by different methods are smaller than those of as-synthesized MoS2, indicating that the calcination of MoS3 under N2 at 400 °C can change the pore structures to a certain extent. The surface areas of unsupported MoS2 prepared by different methods are almost the same, but the pore volume of MoS2-ID is 5 times as big as that of MoS2-I. This indicates that the depositing–calcinating process can endow MoS2 particles with better pore structures. The average pore size, surface area, and pore volume of Al2O3 supports decreased after the loading of Mo. However, compared to the impregnation method, the impregnation–deposition method creates more surface areas and pore volumes for the MoS3/Al2O3 precursors. A narrow peak appearing at 6 nm is observed in MoS3-ID/Al2O3 (Figure 2b), indicating that new mesopores are created during the depositing process. For both catalysts, more mesopores are created after calcination at higher temperatures due to the release of S and the sintering of amorphous MoS3. It is noteworthy that the surface area of MoS2-ID/Al2O3 is almost the same as that of Al2O3 because the porous structure of MoS2 is reserved after deposition.

Table 1. Characteristics of the Alumina Support and Presulfided Catalysts.

sample average pore size (nm) pore volume (cm3/g) BET surface area (m2/g)
Al2O3 10.6 0.85 320.5
MoS2-ID 3.0 0.10 37.1
MoS3-ID 3.8 0.06 18.6
MoS2-I   0.02 31.0
MoS3-I 4.2 0.01 4.3
MoS3-ID/Al2O3 9.1 0.68 298.5
MoS2-ID/Al2O3 9.2 0.73 319.9
MoS3-I/Al2O3 8.7 0.55 252.5
MoS2-I/Al2O3 8.6 0.59 269.4
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Compared to the conventional impregnation method, the impregnation–deposition method leads to about a 50 m2/g increasement in the specific surface area (Table 1). This may be induced by the addition of the acid and, what is more, the formation of MoS3 particles. Contreras and co-workers45 put forward that the acidification of ATTM could endow the material with a large surface area in the presence of polyvinylpyrrolidone. In this experiment, although no template was added, the aggregation of MoS3 particles was inhibited by the pore structures of Al2O3. Consequently, smaller MoS3 particles were generated inside the support (as shown in Figure S2) and provided certain surface areas. After calcination in the N2 atmosphere at 400 °C, MoS3 decomposed into MoS2, and more pores were created due to the loss of S. Hence, the surface area of the catalyst became even larger.

3.3. Morphological Characterizations

It is widely accepted that the morphology of MoS2 particles has a significant influence on the catalytic performance of Mo-based HDS catalysts.46,47 Therefore, representative high-resolution TEM (HRTEM) micrographs of MoS2-ID/Al2O3 and MoS2-I/Al2O3 catalysts were characterized, and the results are shown in Figure 3. The black thread-like fringes (with an interlayer spacing of 0.62 nm) corresponding to MoS2 slabs are well dispersed on the surface of both catalysts. In order to evaluate the influence of preparation methods on the morphology and dispersion of the MoS2 slabs, statistical processing was performed on TEM images. The slab length and stacking number distributions for both catalysts are shown in Figure 4. The average slab length (L̅A) and average stacking number (N̅A) of MoS2 slabs are summarized in Table 2. For both catalysts, the slab length of MoS2 particles is mainly distributed in 2–5 nm, and the average length is concentrated at 4 nm. The stacking number of MoS2 slabs is mainly distributed in 1–3 in MoS2-I/Al2O3 and 2–5 in MoS2-ID/Al2O3. It can be seen that the catalyst obtained by the impregnation–deposition method possesses more layers in slabs, and its average stacking number NA is higher than that of the catalyst obtained by the impregnation method, which is in accordance with the results of XRD. Hence, the deposition of ATTM could change the morphology of MoS2 slabs.

Figure 3.

Figure 3

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HRTEM images of presulfided catalysts synthesized by the impregnation method (a) MoS2-I/Al2O3 and the impregnation–deposition method (b) MoS2-ID/Al2O3.

Figure 4.

Figure 4

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(a) Slab length and (b) stacking number distribution of presulfided catalysts synthesized by the impregnation method MoS2-I/Al2O3 and the impregnation–deposition method MoS2-ID/Al2O3.

Table 2. Characteristics of the Morphology of the Presulfided Catalysts Synthesized by Different Methods.

sample average stacking number NA average slab length LA (nm) fMo
MoS2-I/Al2O3 2.02 4.06 0.28
MoS2-ID/Al2O3 3.13 4.07 0.26
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The proportion of Mo atoms at the edge sites (fMo) is listed in Table 2. It can be seen that fMo of the MoS2-I/Al2O3 catalyst is slightly higher than that of the MoS2-ID/Al2O3 catalyst, indicating that the dispersion of Mo on MoS2-I/Al2O3 is better but not overbearing. However, the activity of the catalyst MoS2-ID/Al2O3 is higher than that of the MoS2-I/Al2O3 catalyst. It is well accepted that the compromise between the dispersion and the stacking number of MoS2 slabs is crucial for achieving optimal HDS activity and selectivity.48 To form the sufficient and accessible MoS2 active phases, a higher and suitable stacking number is more favorable.

3.4. XPS Analyses

The binding energies of Mo 3d and S 2p and the relative atomic concentration for both catalysts are listed in Table 3. The Mo/Al ratio of the catalyst MoS2-ID/Al2O3 is slightly smaller than that of the catalyst MoS2-I/Al2O3, indicating that the dispersion of Mo on MoS2-I/Al2O3 is better. Meanwhile, the S/Mo ratio of MoS2-ID/Al2O3 is 2.04, which is smaller than the S/Mo ratio of MoS2-I/Al2O3, approaching the stoichiometric ratio of MoS2. This means that the decomposition of ATTM is more complete in the impregnation–deposition method.

Table 3. Binding Energies and Atomic Ratios Determined by XPS Experiments.

sample MoS2-I/Al2O3 MoS2-ID/Al2O3
Mo 3d5/2 (eV) 228.4 228.6
S 2p (eV) 161.4 161.4
Al 2p (eV) 74.3 74.2
C 1s (eV) 284.6 284.7
Atomic ratios    
Mo/Al 0.07 0.06
S/Mo 2.26 2.04
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Furthermore, XPS experiments allow evaluating the relative contents of Mo and S involved in different chemical phases on the surface of the catalysts. The Mo 3d and S 2p spectra for MoS2-I/Al2O3 and MoS2-ID/Al2O3 are presented in Figure 5: all samples were decomposed according to the reported literature,49,50 which detailedly discussed the decomposition of MoS2 samples. Concerning the spectral region of Mo 3d (Figure 5a,b), the catalysts contain two different doublets and one S 2s peak, which is not taken into account in the quantification. The doublet with the higher binding energy (232.5 and 235.7 eV) is ascribed to incompletely decomposed ATTM(VI), and the doublet with the lower energy (228.4 and 231.5 eV) is attributed to MoS2(IV). In order to investigate the presence of sulfur in the catalysts, the S 2p envelopes (Figure 5c,d) are decomposed into two doublets, which contain a main peak (S 2p3/2) and an associated peak (S 2p1/2). The S 2p3/2 peak at higher energy (162.6 eV) is attributed to S22–, which often abundantly exists in MoS3 or slightly at the edge of MoS2 slabs.51,52 The one at lower binding energy (161.4 eV) is attributed to S2– (MoS2).

Figure 5.

Figure 5

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Mo 3d and S 2p photoelectron spectra of the catalysts synthesized by different methods: (a) Mo 3d-MoS2-ID/Al2O3, (b) Mo 3d-MoS2-I/Al2O3, (c) S 2p-MoS2-ID/Al2O3, and (d) S 2p-MoS2-I/Al2O3.

The relative contents of different Mo and S species are listed in Table 4. The ratio of Mo4+/(Mo4+ + Mo6+) for both samples is around 95%, showing that most Mo atoms in (NH4)2MoS4 have turned into MoS2, whether in the impregnation–deposition method or in the impregnation method. The atom percentage of S22– in MoS2-I/Al2O3 is twice that in MoS2-ID/Al2O3, indicating that more MoS2 active slabs are created by the impregnation–deposition method, which is in accordance with the S/Mo ratios given in Table 3. This could be attributed to the different structural compositions of MoS3 prepared by different methods. Known as an amorphous compound,53 the structural formula of MoS3 is MoVI (S2–)(S22–), which will turn into MoS2 after calcination at 400 °C in N2 (eq 8). The reductive removal of S22– ligands is based on the following equation

3.4. 9

Table 4. XPS Parameters of the Different Contributions of Mo 3d5/2 and S 2p3/2 Obtained from the Catalysts.

  Mo6+
 Mo4+
 S2–
 S22–
catalysts BE (eV) % at. BE (eV) % at. BE (eV) % at. BE (eV) % at.
MoS2-ID/Al2O3 232.6 4.5 228.4 95.5 161.4 96.5 162.6 3.5
MoS2-I/Al2O3 232.7 5.2 228.6 94.8 161.4 92.6 162.5 7.4
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After the loss of one S per MoVI center, reactive {Mo3S6} units may be preserved in each layer and aggregate to build up the MoS2 lattice.54 In this work, the easier formation and better pore structures of MoS3 prepared by the impregnation–deposition method would lead to the easier reductive removal of S22– ligands and finally create more MoS2 active slabs.

3.5. TPR and CO-FTIR Analyses

The TPR measurement has been commonly used to characterize the active sulfur species of the MoS2-based catalysts.55Figure 6 illustrates the TPR profiles of the catalysts prepared by the impregnation and the impregnation–deposition method. Two peaks appear in the range of 200–300 and 400–600 °C for both catalysts. The low-temperature peak originates from the reduction of the nonstoichiometric sulfur atoms (Sx) weakly adsorbed on coordinated unsaturated sites (CUSs),56 which are believed to be the catalytic active sites of MoS2 slabs.57 The signal of the high-temperature peak is attributed to the partial reduction of the MoS2 crystal.58,59 A 65 °C downward shift in the reduction temperature for the first peak is observed in Figure 6. According to Afanasiev et al.,56 the position of the low-temperature peak is related to the strength of the metal–sulfur bond on the MoS2-based catalysts. In this work, when the Mo–S bonds became weaker, H2 interacted more easily with Sx in the reduction process and finally led to the easier formation of a CUS. Besides, the areas of the low-temperature peaks allow the determination of the total number of active sites.11 An enhancement of the low-temperature peak areas is also observed from the picture insert in Figure 6, indicating that more active sites will be created on MoS2-ID/Al2O3 after activation in H2 at 400 °C. Hence, the impregnation–deposition method is more beneficial for the formation of active sites compared to the impregnation method.

Figure 6.

Figure 6

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TPR profiles of the sulfided catalysts synthesized by the impregnation–deposition method MoS2-ID/Al2O3 and the impregnation method MoS2-I/Al2O3.

As a proven technique, IR spectroscopy obtained after CO adsorption (IR/CO) has been intensively used to probe the edges sites of MoS2 slabs on MoS2-based catalysts.60 The IR/CO spectra of the catalysts MoS2-ID/Al2O3 and MoS2-I/Al2O3 are presented in Figure 7. Three bands appear at 2183, 2126, and 2055 cm–1 for both catalysts. The band at 2183 cm–1 can be attributed to the coordination of CO on the CUS Al3+ sites of the Al2O3 support.61 The bands at 2126 and 2055 cm–1 are due to CO adsorbed on the different active sites of the MoS2 phase. The former adsorption takes place at the Mo-edge sites of MoS2 particles (six-coordinated Mo atoms with 50% S-coverage), while the latter takes place at the S-edge sites (100% S-coverage).62

Figure 7.

Figure 7

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IR spectra of CO adsorbed (T = 100 K) on sulfided catalysts synthesized by the impregnation–deposition method MoS2-ID/Al2O3 and by the impregnation method MoS2-I/Al2O3.

As the amount of CO adsorption can provide information on the number of active sites,63 the relative areas of the peaks at 2126 and 2055 cm–1 for both catalysts can be used to reveal the structure–activity relationships. The total band area of the peaks 2126 and 2055 cm–1 in the catalyst MoS2-ID/Al2O3 is larger than that of the catalyst MoS2-I/Al2O3, which corelates nicely with TPR results, indicating that more active sites exist in this catalyst. This could be related to the easier formation of CUSs, which is caused by the weaker Mo–S bonds in the catalyst MoS2-ID/Al2O3. Moreover, the appearance of 2055 cm–1 in MoS2-ID/Al2O3 indicates that the impregnating–depositing process is more favorable for the formation of S-edge sites in the MoS2 slabs. It is well accepted that the sulfur vacancies at different edges of MoS2 slabs have a great influence on the reaction routes of HDS. Zheng et al.64 considered that Mo edges were more beneficial for the direct desulfurization (DDS) pathway, while the hydrogenation (HYD) route mainly proceeded at the S-edges. In consequence, the catalyst prepared by the impregnation–deposition method might possess a higher activity and HYD selectivity than the catalyst synthesized by the impregnation method.

3.6. Catalytic Activity Assessments

DBT is a typical sulfur-containing compound in petroleum refining products and is always used to study the catalytic performance of the HDS catalyst.65 It is well known that the HDS of DBT occurs by two parallel pathways66−68 (Figure S3). The first route is DDS, and its main product is biphenyl (BP), which is obtained via the σ-adsorption of DBT and the following cleavage of the C–S bond. The second route is HYD to form cyclohexylbenzene (CHB) and bicyclohexane (BCH). In this route, DBT is first hydrogenated via the π-adsorption of the aromatic rings, producing the intermediates 1,2,3,4-tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT). The next step involves the cleavage of the C–S bond, which yields CHB and BCH.69 The catalytic selectivity, therefore, is expressed by the ratio between the HYD route and DDS route in the following equation

3.6.

The HDS reaction results of DBT over the sulfided catalysts synthesized by the impregnation–deposition and impregnation method are listed in Table 5. The reaction rate of MoS2-ID/Al2O3 (4.5 × 10–8 mol/g·s) is 1.3 times as high as that of the catalyst MoS2-I/Al2O3 (3.4 × 10–8 mol/g·s). This indicates that more DBT reacted on MoS2-ID/Al2O3 under the same reaction conditions. The DBT conversion is 53.2% for the catalyst MoS2-I/Al2O3 while 70.5% for the catalyst MoS2-ID/Al2O3. The content of BP is almost the same for both catalysts due to the similar steric hindrance effect of DBT on the DDS pathway. The higher HDS activity of the catalyst MoS2-ID/Al2O3 can be explained by the higher amounts of active sites created during the preparation and reaction process. Compared to the impregnation method, MoS3 was directly deposited on the Al2O3 support by the precipitation process under mild conditions and could be reduced to MoS2 more completely, as evidenced by the S/Mo ratio of XPS results (Table 3). Moreover, it appears from the TPR results (Figure 6) that for the impregnation–precipitation method, the MoS2 particles were more easily reduced, and more active sites could be created accordingly. Furthermore, the higher amounts of active sites created by this method could also be proved by the results of CO-FTIR (Figure 7).

Table 5. Distribution of HDS Reaction Products.

  distribution (wt %)
    
samples BCH CHB BP HH-DBT TH-DBT DBT HDSa/% rDBT/×10–8 mol/g·s SHYD/SDDSb
MoS2-ID/Al2O3 3.6 35.6 22.8 0.8 7.7 29.5 70.5 4.5 2.1
MoS2-I/Al2O3 3.0 22.4 22.2 0.3 5.3 46.8 53.2 3.4 1.4
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a

HDS (%) = DBT conversion (%) = TH-DBT + HH-DBT + CHB + BCH + BP.

b

SHYD/SDDS: (CHB + BCH + HH-DBT + TH-DBT)/BP.

The ratio of HYD/DDS shows that the dominant route of HDS over the two catalysts is the hydrogenation route. Particularly, the SHYD/SDDS ratio in the catalyst MoS2-ID/Al2O3 is 1.5 times as large as that of the catalyst MoS2-I/Al2O3, indicating the superior activity of the former catalyst provided by the HYD route. This can be attributed to two factors: (1) the higher average stacking number and (2) the more S-edge sites in the MoS2 slabs of the catalyst MoS2-ID/Al2O3. Hensen et al.70 put forward that the HYD rate increased with an increasing stacking number of MoS2 slabs, which would favor the less hampered planar adsorption of DBT. Besides, the S-edge sites of the MoS2 slabs are mainly responsible for the HYD route as proved by the CO-FTIR results. Hence, more DBT turns into CHB, BCH, and TH-DBT in the catalyst prepared by the impregnation–deposition method.

4. Conclusions

In conclusion, a novel impregnation–deposition method was used to synthesize a presulfided MoS2/Al2O3 catalyst with large surface area and excellent HDS active performance. Compared with the conventional impregnation method, the impregnation–deposition method produces catalysts with larger surface areas mainly by generating mesostructured MoS2 particles inside the Al2O3 support, as proved by the N2 adsorption–desorption analyses. The average slab length of the MoS2 particles in both catalysts is almost the same (about 4.0 nm), but the stacking number of the MoS2 particles is higher for the impregnation–deposition method than that of the impregnation method, as proved by the HRTEM images and XRD patterns. The formation of MoS2 active phases is also more completed by the impregnation–deposition method based on the XPS analyses. TPR reveals that the Mo–S bonds in the catalysts become weaker when the impregnation–deposition method is used. Hence, more active sites, especially more active S-edges, are created in the as-synthesized catalyst as proved by the TPR and CO-FTIR data. The reaction rate and the HYD/DDS ratio of the DBT on the catalysts synthesized by the impregnation–deposition method are 1.3 times and 1.5 times as high as those of the impregnation method, respectively. All the results prove that the novel impregnation–deposition method not only increases the activity of the presulfided Mo catalyst but also changes the contribution of the HYD and DDS pathways.

Acknowledgments

This work was supported by the Doctoral Fund of Qingdao Agriculture University (665-1120048).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07123.

  • Textural properties of MoS3, morphologies of the presulfided MoS3/Al2O3 and MoS2/Al2O3 catalysts, and reaction networks for the HDS of DBT (PDF)

Author Contributions

† X.L. and X.W. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao2c07123_si_001.pdf (409.5KB, pdf)

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