Mesoporous Materials

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2019, 58, 11831−11840

Effect of Inorganic Salts on Beta-FDU-12 Micro-/Mesoporous Materials with the Applications in Dibenzothiphene Hydrodesulfurization Huiping Li, Yuyang Li, Di Hu, Cong Liu, Jinlin Mei, Xiaowu Liu, and Aijun Duan* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18 Fuxue Road, Beijing 102249, PR China

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S Supporting Information *

ABSTRACT: Beta/FDU-12 (BF) series composite materials were successfully synthesized with different inorganic salts (KCl, MgCl2, MnCl2, and CuCl2). The supports and corresponding catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), pyridine-Fourier transform infrared spectroscopy (Py-FTIR), H2 temperatureprogrammed reduction (H2-TPR), Raman, X-ray photoelectron spectra (XPS), and high-resolution transmission electron microscopy (HRTEM). NiMo/BF catalysts were evaluated for dibenzothiophene (DBT) hydrodesulfurization (HDS). Consequently, NiMo/BF−Mg catalyst displayed the highest HDS performance (98.58%), as the weight hourly space velocity (WHSV) was 10 h−1. The synergistic attribution of its prominent support texture, the excellent acid and redox properties of NiMo catalyst, including more B and L acids (193.6 and 118.0 μmol·g−1 at 200 and 350 °C, respectively), strong metal−support interaction, smaller stacking number (3.75), shorter stacking length (3.11), higher dispersion degree (0.29), and higher sulfidation degree of Mo sulfide species (61.3%), result in high DBT HDS activity. Moreover, NiMo/BF−Mg catalyst exhibited a higher HYD/DDS ratio (0.29), due to the greater number of L acids and higher value of 0.23. tunable acidity, and high stability.13−15 Ding et al. reported that WNi/Beta catalyst showed reaction performance for light cycle oil much higher than that of WNi/Al2O3 catalyst especially in HDS, hydrodearomatation, and hydrodenitrogenation.16 Ding and co-workers prepared NiMo/Al2O3 and a series of NiMo/Al2O3−Beta catalysts blending different contents of Beta zeolite from 5 to 20 wt % with Al2O3, and they found that the catalysts containing Beta zeolite displayed HDS performance better than that of NiMo/Al2O3 catalyst, attributed to the improvement of hydrogenation rate by acidity from the Beta zeolite.17 Nevertheless, the limited pore size of less than 2 nm hindered the diffusion of macromolecule inside of zeolite. Hence, scientific researchers are positively exploring the applications of mesoporous zeolites in HDS. Since the first report that M41S mesoporous silica molecular sieve family are synthesized successfully,18 the mesoporous materials have comprehensive applications in the HDS process with uniform channels and high specific surface areas. The uniform mesoporous channels could reduce the impact of macromolecular diffusion restriction, such as DBT and 4,6DMDBT. In addition, the high specific surface areas could provide enough space for active metals to obtain a high

1. INTRODUCTION Environmental protection raises more and more concerns about public health and society’s economic development of the world. Exhaust emission as a source of atmospheric pollution, containing sulfur, nitrogen, and particulate matters, is seriously harmful to people’s health. To solve exhaust pollution, the sulfur contents of transportation fuels are limited to an ultra low level to meet the stringent gasoline and diesel quality standards; thus, some refractory macromolecule sulfocompounds, such as dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) need to be eliminated.1−3 At present, hydro-upgrading technology is one of the most effective methods in refinery to upgrade the quality of transportation fuels.4 However, the commercial (Co) Ni-(W) MoS2 alumina-supported catalysts face up with serious challenges on the strong metal support interaction (MSI) resulting in the low redox property of active metal.5−7 Meanwhile, the excellent structure property and surface acidity of supports are vital factors for ultradeep HDS. 8−10 Consequently, many novel materials, such as zeolites, graphene-analogous boron nitride, and mesoporous silicas molecular sieve, are receiving increasing attentions in HDS reaction for their suitable MSI, superior structural properties, and appropriate acidity.11,12 Microporous zeolites, such as Beta, ZSM-5, and Y zeolite, have extensive applications in many fields such as adsorption, separation, and biotechnology, due to their shape selectivity, © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11831

March 25, 2019 June 6, 2019 June 19, 2019 June 19, 2019 DOI: 10.1021/acs.iecr.9b01649 Ind. Eng. Chem. Res. 2019, 58, 11831−11840

Article

Industrial & Engineering Chemistry Research dispersion degree.9 FDU-12 is a kind of three-dimensional (3D) mesoporous material with face-centered cubic structure possessed a high surface area (800−1000 m2·g−1) and a wide range of pores (4−27 nm).19,20 However, pure mesoporous silica materials lack acidity and display low hydrothermal stability which are disadvantageous for HDS activity. In order to cover the drawbacks of the FDU-12 material, Zr, Al, Ti, B, and other elements are used as additives to modify the acidity. Meng et al.21 found that Zr-FDU-12 synthesized by the postsynthesis exhibited a DBT HDS performance higher than that of direct synthesis, due to its higher surface areas, larger pore volume, and larger pore size. Cao et al. successfully synthesized Ti-FDU-12 by a simple two-step method and discovered that when the Si/Ti molar ratio was 5 the corresponding catalyst exhibited the best HDS performance attributed to its ordered channels, high specific surface areas, and incorporation of Ti.20 In recent years, novel composite supports with hierarchical pores containing micro- and mesopores structures attract more attentions of the scientific researchers. The hierarchical porous materials not only make up for the imperfect of pure mesoporous silica materials lacking of the acidity but also reduce the diffusion restriction of macromolecular sulfur compounds. Zhang et al. synthesized ZSM-5/FDU-12 micro-/mesoporous composites material with various molar ratios of Si/Al and found that all the catalysts of NiMo/ZSM5/FDU-12 exhibit higher DBT HDS activities than that of NiMo/FDU-12 catalyst.22 Du and co-workers synthesized CoMo/Beta-FDU-12 catalyst by a nanoassembly method, and the result confirmed that CoMo/Beta-FDU-12 displayed DBT HDS performance better than that of CoMo/γ-Al2O3 catalyst derived from the acidity of Beta combined with the mesopore of FDU-12.23 Some researchers discovered that inorganic salts distinctly influenced the properties of mesoporous silica material containing the morphology, structure property, and so on.24−26 Newalkar et al. prepared ordered SBA-15 silica sieve using varied amounts of sodium chloride, and they discovered that the contents of sodium chloride had distinct influences on the pore sizes, pore volumes, and surface areas.27 Zhong et al. reported that the kinds of inorganic salts used could affect the ordered degrees of meso-/macroporous monolithic silica materials following the sequence of NaCl > NaNO3 > Na2SO4, and the addition of inorganic salts could improve the condensation of silica species.28 Tang et al. systematically investigated the effect of inorganic salts on the morphology of FDU-12, and it was discovered that different contents of MgSO4 could influence the formation of uniform Fm3m pore structure, which was attributed to the coordination of Mg2+ with PEO (hydrophilic part) chains of Pluronic F127 surfactant.29 Therefore, the influence of inorganic salts (KCl, MgCl2, MnCl2, and CuCl2) in this work were investigated systematically on the structure properties, acidity, and MSI of NiMo/BF series catalysts. Moreover, the NiMo/BF series catalysts with different inorganic salts were evaluated for DBT HDS.

following specific procedures. First, 0.38 g of NaOH, 0.46 g of NaAlO2, and 42.86 g of tetraethylorthosilicate (TEOS) were mixed together with 51.90 g of an aqueous tetraethylammonium hydroxide (TEAOH) solution (25 wt %), and the mixture was kept under stirring at 298 K for 2−3 h. Then, the mixture was transferred to a Teflon-lined autoclave and heated at 413 K for 24 h. The Beta microcrystal solution was taken as the precursor. Second, BF supports prepared with different inorganic salts were obtained by the following steps. BF support with KCl as additive is used as an example, where 4.00 g of triblock copolymers F127 was used as a textural template agent, 4.00 g of 1,3,5-trimethylbenzene (TMB), and 10.00 g of KCl were added into 240 mL of 2.00 M HCl aqueous solution with continuous stirring for 24 h at 288 K. Next, 16.60 g of TEOS was added dropwise into the above solution and stirred for 2 h. Subsquently, the presynthesized Beta microcrystal precursor was joined with the above mixture with vigorous agitation for 24 h at 298 K, and the mixture is then heated in the Teflon-lined autoclave for 48 h at 373 K. Finally, the above mixture was filtered and washed several times using distilled water to obtain a white gel. The white gel was dried in the oven for 24 h at 353 K and ultimately calcined for 6 h at 823 K in the muffle furnace to get the final product. In addition, the synthetic process of other BF materials with MgCl2, MnCl2, and CuCl2 additives followed the same procedure as the above BF synthetic process with KCl, in which the amount of additives were 27.2 g of MgCl2, 26.6 g of MnCl2, and 22.8 g of CuCl2. Besides, the BF materials with a variety of inorganic salts (KCl, MgCl2, MnCl2, and CuCl2) were denoted as BF−K, BF−Mg, BF−Mn, and BF−Cu, respectively. 2.2. Preparation of the Catalysts. H-type BF series materials were obtained by a two-step ion exchange with 1.00 M NH4Cl solution. Each ion exchange proceeded for 3 h at 353 K, and the obtained product was calcined at 823 K in air for 6 h after filtering. Then, NiMo/BF series catalysts were prepared by a two-step incipient-wetness impregnation with ammonium molybdate solution and nickel nitrate solution. The metal contents were 15 wt % MoO3 and 3.5 wt % NiO. NiMo/BF series catalysts were labeled as NiMo/BF−K, NiMo/BF−Mg, NiMo/BF−Mn, and NiMo/BF−Cu, respectively. 2.3. Catalytic Performance Evaluation. The catalytic HDS activities were evaluated in a fixed-bed reactor using 1.0 g of catalyst with the particle sizes of 40−60 mesh. Before the catalytic HDS tests, the NiMo/BF series catalysts were presulfided for 4 h using 2 wt % CS2-cyclohexane mixture to obtain MoS2 active phases under 4 MPa H2 and 340 °C. After presulfurization, the catalytic HDS reactions using DBT (sulfur content is 500 ppm) as the model reactant proceeded under the conditions of 340 °C, 4.0 MPa, H2/oil 200 mL/mL, and weight hourly space velocities (WHSVs) in the range of 10− 100 h−1. The liquid yield for each product was greater than 99%. The HDS efficiency was calculated according to HDS (100%) = (Sf − Sp)/Sf × 100%

(1)

where Sf represents the sulfur concentration of the feedstock, and Sp represents the sulfur concentration of the product. In addition, the sulfur contents of feedstock and products were tested by RPP-2000 SN sulfur analyzer, of which the deviation was within 2 μg·mL−1. After the reactions, the HDS products were analyzed by a gas chromatograph combined with a mass spectrometer (GC-MS) to obtain product distributions.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Support Material. BF series supports with different inorganic salts (KCl, MgCl2, MnCl2, and CuCl2) were synthesized by an in situ nanoassembly method by adding the preformed Beta microcrystal solution. Nanosized Beta microcrystal solution was prepared by the 11832

DOI: 10.1021/acs.iecr.9b01649 Ind. Eng. Chem. Res. 2019, 58, 11831−11840

Article

Industrial & Engineering Chemistry Research

Figure 1. Small-angle (A) and wide-angle (B) XRD of BF supports.

ratio was 1 cm−1. XPS analysis was measured by a PerkinElmer PHI-1600 ESCA spectrometer with Mg Kα X-ray source. The binding energy (BE = 284.6 eV) of C 1s spectrum was used as standard to calibrate the obtained data. Lastly, HRTEM image was obtained from a JEM 2100 device. The dispersion degree of Mo atoms (f Mo) was calculated according to30

Because the DBT HDS is a pseudo-first-order reaction, kHDS (mol g−1 h−1) was calculated by26 kHDS =

F ji 1 zy zz lnjj m k1 − x {

(2) −1

where F is the flow rate of the feed (mol h ), m is the catalyst mass (g), and x is the desulfurization degree. Turnover frequency (TOF, h−1) could reflected the natural differences of catalytic activities, which was calculated by eq 326 TOF = (Fx)/(nMofMo )

t

fMo =

∑ (6ni − 6) Moc + Moe = t i=1 2 MoT ∑i = 1 (3ni − 3ni + 1)

(4)

where Moc and Moe represent the number of Mo atoms at corner and edge sites of MoS2, which exhibit high catalytic activities for HDS, MoT denotes the total Mo number, ni represents the number of Mo atoms located on one of six sides of Mo stacking, which is determined by its length, t is the number of more than 300 MoS2 slabs sampled from HRTEM images. The ratio of the dispersion degree of Mo at the corner and edge sites ((fc/fe)Mo) was expressed by31

(3)

where F represents the flow rate of the reactor (mol h−1), τ is the desulfurization degree (%), and m is the catalyst mass (g). 2.4. Catalytic Characterization. In this research, the series supports were characterized by X-ray powder diffraction (XRD), nitrogen (N2) adsorption−desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 27Al nuclear magnetic resonance (NMR). Small- and wide-angle XRD patterns were recorded on an apparatus (Shimadzu X-6000 using Cu Kα radiation at 40 kV) in the 2θ ranges of 0−5° and 5−50°, respectively. The N2 adsorption−desorption isotherms were tested by Micromeritics TriStar II 2020 porosimetry analyzer. Before the test, the series supports were degassed at 350 °C for 4 h and tested at −196 °C. The Brunauer−Emmett−Teller method and Barrett−Joyner−Halenda model were used to calculate the specific surface areas and the pore size distributions, respectively. Besides, the total micropore and mesopore volumes could be obtained from the amounts of adsorbing nitrogen at a relative pressure about P/P0 = 0.99. The SEM and TEM images were taken via a Cambridge S-360 apparatus and a JEOL JEM 2100 device. 27Al NMR analysis was carried out with an instrument of Bruker Avance III at 500 MHz. The series catalysts prepared by using different inorganic salts were analyzed by pyridine-Fourier transform infrared spectroscopy (Py-FTIR), H2 temperature-programmed reduction (H2-TPR), Raman, X-ray photoelectron spectra (XPS), and high-resolution transmission electron microscopy (HRTEM). Py-FTIR was analyzed on a MAGNAIR 560 FTIR spectrophotometer with a resolution ratio of 1 cm−1. H2TPR analysis was recorded with a Quantachrome apparatus (Autosorb-iQ USA). The sample was heated from 50 to 850 °C at a rate 10 °C/min. The Raman spectra was scanned with a Renishaw Micro-Raman 2000 spectrometer using a He/Cd laser at 532 nm under room temperature, while the resolution

(fc /fe )Mo =

2 10 × L̅ /3.2 − 3

(5)

where fc and fe are the dispersion degrees of Mo at corner and edge sites, respectively. The average layer number (N̅ ) and average length (L̅ ) were calculated by the following:32 ij n yz n N̅ = jjjj∑ niNi zzzz/∑ ni j i=1 z i=1 k {

ij n yz n L̅ = jjjj∑ nili zzzz/∑ ni j i=1 z i=1 k {

(6)

(7)

where ni is the same as in eq 4, Ni denotes as the layer number, and li represents the length of MoS2 slab.

3. RESULTS 3.1. Characterizations of BF Series Supports. 3.1.1. XRD Characterization. The small-angle XRD patterns of BF series supports are displayed in Figure 1A. All supports show the characteristic peaks in the 2θ range of 0.5−1°, which correspond to the (111) plane of a face-centered cubic structure,27 manifesting that all the supports possess a highly ordered mesoporous structure of FDU-12. It is worth mentioning that the diffraction peak intensities of BF−K and BF−Mg supports are stronger than that of BF−Mn and BF− 11833


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Figure 2. N2 physisorption isotherms (A) and pore size distribution (B) of BF supports.

Cu supports, indicating that they possess higher crystalline degrees.33 The wide-angle XRD patterns of BF series supports are exhibited in Figure 1B. All the supports display two peaks at around 2θ = 7.8 and 22.4° which are attributed to Beta zeolite.34 Compared to the pure Beta zeolite, their peak intensities are relatively weaker, due to the Beta zeolite seeds being incorporated into FDU-12 mesoporous materials by in situ nanoassembly method. 3.1.2. N2 Adsorption−Desorption Isotherms Characterization. The surface areas and pore structures of BF series composite materials with different inorganic salts were detected by N2 adsorption−desorption characterization. All the supports show the characteristic type IV isotherm and H2 hysteresis loop at the relative pressure ratio P/P0 between 0.45 and 0.9 in Figure 2A,35 proving that all the supports possess a mesoporous structure.36 Figure 2B shows that a series of supports display narrow pore size distributions in a range of 8− 20 nm which are beneficial to diffusion of the reactants and products. Besides, the structure properties including the surface area, pore volume, and pore size are summarized in Table S1, which are slightly varied with different inorganic salts. From Table S1, the differences in the surface areas of BF−Mn (954 m2·g−1), BF−K (924 m2·g−1) and BF−Mg (902 m2·g−1) are not significant; however, the surface area of BF− Cu (849 m2·g−1) is relative lower. Besides, the pore sizes of BF−Cu (12.22 nm) and BF−Mn (11.82 nm) are larger than those of BF−Mg (11.06 nm) and BF−K (11.02 nm), and the pore volume of BF−Mn (0.80 cm3·g−1) is larger than those of the other supports which are around 0.7 cm3·g−1. The hydrophilic EO group in F127 (EO106PO70EO106) can interact with a silicon precursor through electrostatic interaction and hydrogen bonding. Metal cations have a strong interaction with the long-chain EO group, which affects the state of the EO group in the F127 surfactant core, and the net effect of this interrelated interaction is that the core and pore sizes are different after the assembly of silicate surfactant to form FDU12.29,37 3.1.3. TEM and SEM Characterizaton. TEM images display the pore channels of BF series supports prepared with different inorganic salts. Figure 3 confirms that BF−K, BF−Mg, and BF−Mn supports have ordered pore channels with facecentered cubic mesostructures; however, BF−Cu has less ordered mesostructures with a wormlike channel on the edge. Besides, the mesopores (∼10 nm) are clearly shown in TEM

Figure 3. TEM images of BF series supports: (A) BF−K, (B) BF− Mg, (C) BF−Mn, and (D) BF−Cu.

images, which is consistent with N2 adsorption−desorption isotherms characterizations. The morphology of BF materials can be investigated by SEM images, and the corresponding SEM images of BF series supports are displayed in Figure S1. Compared to the SEM image of Beta, it is observed that Beta seeds are incorporated into FDU-12 mesoporous zeolite incompletely, and parts of Beta seeds are exposed on the surface of FDU-12 particles. In addition, hexagonal disk crystals of BF−Mg material are easily to be found in Figure S1C, while irregular and aggregated particles of BF−K, BF−Mn, and BF−Cu materials can be observed clearly in Figure S1B,D,E. 3.1.4. 27Al NMR Characterization. Figure 4 is performed to present 27Al NMR of BF materials adding different inorganic salts. The images of 27Al NMR exhibit two characteristic peaks at δ = 0 and 54 ppm attributed to the octahedral-coordinated extra-framework Al and tetrahedral-coordinated framework Al, respectively.38 These two kinds of Al species have significant influence on the acidity of BF materials, of which the former contribution to the formation of B acid and the latter is related to L acid. From Figure 4, the intensity of tetrahedralcoordinated framework Al is stronger than that of octahe11834


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350 °C, which are 193.6 and 118 μmol·g−1, respectively. However, the NiMo/BF−K catalyst has the lowest B+L acid amount (85.4 μmol·g−1) at 200 °C, and the NiMo/BF−Cu owns the lowest B+L acid amount (29.0 μmol·g−1) at 350 °C. 3.2.2. H2-TPR Characterization. H2-TPR characterization is used to investigate the reducibility of the active metal oxides and the interaction between active metals and supports. From Figure 6, all the TCD signals of NiMo/BF oxided catalysts

Figure 4. 27Al NMR images of series BF supports.

dral-coordinated extra-framework Al, implying that most Al is incorporated into BF composite materials. It is remarkable that the intensities at δ = 54 ppm of BF−K and BF−Mg are stronger than BF−Mn and BF−Cu. 3.2. Characterizations of NiMo/BF Series Catalysts. 3.2.1. Py-FTIR Characterization. Py-FTIR characterization is used to analyze the acid strength and types of NiMo/BF series catalysts which play important roles in the selective hydrogenolysis route of DBT HDS reaction. The Py-FTIR spectra of the series catalysts can be observed in the region of 1300− 1700 cm−1 in Figure 5. The absorbed pyridines are desorbed at 200 °C, assigned to the total amount acid site, and at 350 °C, ascribed to the medium and strong acid site. From the images, the bands at 1450 and 1610 cm−1 are assigned to Lewis acid sites, while the band at 1540 cm−1 corresponds to Brønsted acid sites. The band at 1490 cm−1 is ascribed to both Lewis acid and Brønsted acid sites.23,39 It is noted that the peak intensity of NiMo/BF−Mg is stronger than that of other catalysts which shows that the acid of NiMo/BF−Mg is higher than that of other catalysts. The acid amounts of NiMo/BF catalysts prepared with various inorganic salts are obtained after the desorption of pyridine molecules at 200 and 350 °C which are ascribed to the total acidity and the medium and strong acidity, respectively.40 The data of B, L, and B+L acid amounts of catalysts are listed in Table S2. From Table S2, the NiMo/BF− Mg catalyst possesses the highest B+L acid amounts at 200 and

Figure 6. H2-TPR of the oxided catalysts.

show two reductions in the regions of 300−550 °C and 650− 750 °C. The reduction peak at the lower temperature represents the reduction of amorphous multilayered octahedral Mo species (Mo6+ →Mo4+), and the lower reduction temperatures of NiMo/BF oxided catalysts decrease in the order of NiMo/BF−Mg > NiMo/BF−K > NiMo/BF−Mn > NiMo/BF−Cu, which indicates that the same descending trend of MSI over the NiMo/BF series catalysts. Comparably, the higher reduction temperature in the range 650−750 °C corresponds to the reduction of tetrahedral coordinated monomeric Mo specie. MSI is one of the important properties for NiMo/BF catalyst to realize a high dispersion degree and suitable stacking number and slab length of MoS2 active phases, which can facilitate the formation of Type-II NiMoS active phases. 3.2.3. Raman Characterization. The Raman spectrum is usually used to detect the crystalline symmetry and reflect the nature of active metals in the oxided and sulfided states. The

Figure 5. Pyridine-FTIR of the oxided catalysts degassed at 200 °C (A) and 350 °C (B). 11835


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Figure 7. Raman of NiMo/BF oxided catalysts (A) and sulfided catalysts (B).

results of the series oxided and sulfided metal catalysts are represented in Figure 7A,B. It can be observed from Figure 7A that all oxided catalysts display peaks at 960 cm−1 assigned to Mo8O264−.41 Meanwhile, the bands at 948 cm−1 are assigned to the terminal MoO stretching vibration of Mo7O246− which is an octahedrally coordinated polymolybdate.42 Mo7O246− is easily sulfided to form active phases in the presulfiding process. Furthermore, the bands at 901 and 849 cm−1 are involved to MoO bending vibration of MoO42− and MoO stretching vibrations of orthorhombic MoO3, respectively.43 In Figure 7B, there are two obvious signals of NiMo/BF series sulfided catalysts at 385 and 408 cm−1 which correspond to the E12g and A1g modes of MoS2.44 MoS2 is the active phase for DBT HDS reaction which influences directly on the reaction performance. The peak intensities of NiMo/BF−Mg sulfided catalyst are stronger than that of other sulfided catalysts, indicating that NiMo/BF−Mg sulfided catalyst possesses the highest sulfidation degree and Mo utilization percentage. 3.2.4. XPS Characterization. XPS analysis can be performed to investigate the sulfidation degree of Mo species on the surface of NiMo/BF series sulfided catalysts. Figure 8 displays the Mo 3d XPS spectra which are decomposed into three contributions including Mo4+, Mo5+, and Mo6+; the data of peak deconvolution to three Mo species are listed in Table S3. All the catalysts showed the strong doublet peaks of Mo4+ 3d (3/2) and 3d (5/2) at around 228.9 ± 0.1 eV and 232.0 ± 0.1 eV which are attributed to MoS2 active phases.45 In addition, the relatively weaker peak intensities at around 233.5 ± 0.1 eV and 235.9 ± 0.1 eV are assigned to Mo6+ 3d (3/2) and 3d (5/ 2), respectively, which demonstrate the existence of MoO3 species in oxided state.20 The bimodal peaks of Mo5+ 3d (3/2) and 3d (5/2) at around 233.5 ± 0.1 eV and 235.9 ± 0.1 eV are assigned to MoOxSy.40 From Table S3, the sulfidation degree (SMo = Mo4+/(Mo4+ + Mo5+ + Mo6+)) of NiMo/BF series catalysts decreases in the order of NiMo/BF−Mg (61.3%) > NiMo/BF−Cu (58.0%) > NiMo/BF−K (50.6%) > NiMo/ BF−Mn (46.3%). 3.2.5. HRTEM Characterization. HRTEM characterization is carried out to obtain f Mo and (fc/fe)Moof Mo sulfided species on different catalysts and to observe intuitively the morphologies of MoS2 slabs. The stacking number and stacking length distributions of the NiMo/BF series sulfided catalysts are exhibited in Figure 9 and Figure 10, in which the data were achieved through a statistical analysis to 350 slabs

Figure 8. Mo 3d XPS spectra of NiMo/BF series catalysts: (A) NiMo/BF−K, (B) NiMo/BF−Mg, (C) NiMo/BF−Mn, and (D) NiMo/BF−Cu.

from different parts of each catalyst. As shown in Figure 9 that the stacking numbers of MoS2 on all sulfided catalysts are concentrated on 2−4 layers, meaning that more type II active sites are exposed for HDS. In addition, the NiMo/BF−Mg sulfided catalyst hardly exists one layer of active phase. The length of MoS2 slabs are concentrated on 2−8 nm, due to the suitable MSI. The average layer number (N̅ ), average length (L̅ ), and the dispersion degree (f Mo) and (fc/fe)Mo are obtained from eqs 4−7, respectively, and the corresponding values are listed in Table 1. N̅ of Mo sulfided species reduces in the order of NiMo/BF−Cu > NiMo/BF−K > NiMo/BF−Mg = NiMo/BF−Mn, and L̅ of MoS2 changes in the sequence of NiMo/BF−Cu > NiMo/BF−K > NiMo/BF−Mn > NiMo/ BF−Mg. However, f Mo and (fc/fe)Mo of Mo sulfided species are in a descending order of NiMo/BF−Mg > NiMo/BF−Mn > NiMo/BF−K = NiMo/BF−Cu. The stacking number, stacking length, and dispersion degree of Mo sulfided species are closely related to the MSI. From all of the data, the NiMo/BF−Mg sulfided catalyst possesses a smaller stacking number (3.11), a relatively shorter length (3.75), and a high dispersion degree (0.28) of the MoS2 active phase, which are necessary to form a suitable amount of accessible active sites to improve HDS 11836


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Figure 11. DBT HDS results at various WHSV (340 °C, 4.0 MPa, and 200 mL/mL).

Figure 9. HRTEM images of NiMo/BF series catalysts: (A) NiMo/ BF−K, (B) NiMo/BF−Mg, (C) NiMo/BF−Mn, and (D) NiMo/ BF−Cu. .

catalysts decrease with increasing WHSVs from 10 to 100 h−1, because the contact times between reactants and catalysts are reduced. Furthermore, the NiMo/BF−Mg catalyst exhibits the best HDS efficiency among the NiMo/BF series of catalysts. When WHSV is 10 h−1, the maximal HDS efficiency of NiMo/ BF−Mg is 98.58%; nevertheless, for NiMo/BF−K it is down to 80.63% under the same WHSV value. The selectivities of the products over the NiMo/BF series of catalysts are listed in Table 2. It is observed that the HYD/ DDS ratios of the series catalysts are less than 1, indexing that DDS is the major pathway in DBT HDS reaction. The HYD/ DDS ratios of all catalysts decrease with the sequence of NiMo/BF−Mg (0.29) > NiMo/BF−Mn (0.24) > NiMo/BF− K (0.22) = NiMo/BF−Cu (0.22). The NiMo/BF−Mg catalyst displays the highest HYD/DDS ratio among the series of catalysts, derived from its greater number of L acids and higher (fc/fe)Mo of MoS2, which finally can promote DBT HDS reaction. In Table 2, kHDS and TOF values of DBT HDS over the NiMo/BF series of catalysts decrease in the order of NiMo/BF−Mg > NiMo/BF−Mn > NiMo/BF−K (0.22) > NiMo/BF−Cu, which is consistent with the DBT HDS performance of the NiMo/BF series of catalysts. As we know, the possible DBT HDS reaction includes the following two pathways. The first one is the DDS route, in which hydrogen reacts with an S atom directly to realize elimination of an S atom through the C−S bond cleavage. Then, BP becomes the final product. The second one is the HYD route, in which the aromatic ring is hydrogenated first and then the S atom is removed. The products are more complicated through the HYD route, involving many different kinds of intermediate and final products, including THDBT, CHEB, CHB, PHDi, CPMB, and MECB. Figure S2 shows the DDS route and the HYD route of DBT in the HDS reaction.

Figure 10. MoS2 slab length distribution of NiMo/BF series catalysts.

Table 1. L̅ , N̅ , f Mo, and (fc/fe)Mo of NiMo/BF Series Catalysts catalyst

L̅ (nm)

N̅

f Mo

(fc/fe)Mo

NiMo/BF−K NiMo/BF−Mg NiMo/BF−Mn NiMo/BF−Cu

4.33 3.75 4.18 4.34

3.13 3.11 3.11 3.25

0.24 0.28 0.25 0.24

0.19 0.23 0.20 0.19

activities.46 Besides, MoS2 active phase is considered to a perfect hexagonal model on which there are two active sites, including corner and edge sites. The HYD route is more likely to occur in the corner sites of active phases and that the edge sites of MoS2 are in charge for the DDS route. Compared to the other catalysts, the NiMo/BF−Mg catalyst has a higher ( fc/fe)Mo (0.23) value of MoS2, which can be directly correlated with the HYD/DDS ratio. 3.3. HDS Performance of DBT. In this research, HDS activities of NiMo/BF series catalysts prepared with different inorganic salts were evaluated by using DBT as the probe molecule. As shown in Figure 11, the DBT HDS activities of all

4. DISCUSSION Various inorganic salts (KCl, MgCl2, MnCl2, and CuCl2) have an intimate effect on the textural properties, acidity, and MSI of NiMo/BF series catalysts. The above properties further influence the reducibility of the active metal oxides, dispersion degree, stacking number, stacking length, and sulfidation degree of MoS2 species over different supports, ultimately contributing to the different DBT HDS performances of the NiMo/BF series of catalysts. 11837


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Industrial & Engineering Chemistry Research Table 2. DBT HDS Product Distributions of the NiMo/BF Series of Catalysts product selectivity (%)a HYD catalysts NiMo/BF−K NiMo/BF−Mg NiMo/BF−Mn NiMo/BF−Cu

−4

kHDS (10 ) (mol g 2.03 8.64 6.31 5.86

−1

−1

h )

DDS

TOF

THDBT

CHEB

PHDi

CHB

CPMB

MECB

BP

HYD/DDS ratiob

0.59 2.14 1.90 1.65

1.42 7.18 3.59 5.18

1.45 0.46 1.27 1.28

6.42 0.47 6.25 7.33

5.03 13.45 5.75 1.65

2.93 1.16 2.37 2.27

0.74 0 0.20 0.34

82.01 77.27 80.58 81.95

0.22 0.29 0.24 0.22

a

The selectivity data were calculated from the GC-MS analysis results. bThe HYD/DDS ratio of the DBT reaction is determined by (THDBT + MECB + PHDi + CHB + CPMB + MECB)/BP under the conditions that the DBT conversion at about 50%, where tetrahydrodibenzothiophene (THDBT), cyclohexenylbenzene (CHEB), isophenyl hexadiene (PHDi), cyclohexylbenzene (CHB), cyclopentylmethylbenzene (CPMB), 3methylcyclopentane-benzene (MECB), biphenyl (BP).

smaller stacking number (3.11) of MoS2 slabs. The above factors are conductive to make NiMo/BF−Mg catalyst exposing greatly active sites for the reactants. In summary, the NiMo/BF−Mg catalyst exhibits the best DBT HDS performance, due to the synergistic effects of its prominent support texture, the excellent acid and redox properties of NiMo catalyst, including more B and L acidities (193.6 and 300 μmol·g−1 at 200 and 300 °C, respectively), the stronger MSI, the higher dispersion degree (0.28), the shorter stacking length (3.75 nm), the smaller stacking number (3.11), and the higher sulfidation degree (61.3%) of MoS2 slabs. Among the above factors, the acidity and MSI contribute more to the DBT HDS improvement.

All the catalysts possess excellent structural properties involving mesopore size, large specific surface areas, and pore volumes, which are slightly varied with different inorganic salts, due to the interaction of the metal cation and the EO group in the F127 surfactant. Mesoporous size and large pore volumes can decrease the diffusion resistance to improve the accessibility of the internal active sites for large reactant molecules. Moreover, the large specific surface areas can provide sufficient space for the active metals to obtain a suitable dispersion degree.40 The BF−Mg support possessed higher crystalline degree, ordered pore channel structure which could improve DBT HDS performance. The acidity is a crucial factor for the DBT HDS reaction. The presence of B acid can facilitate the cleavage of C−S bond, and the L acid is beneficial to the hydrogenation of the aromatic ring.40,47 The synergy of B and L acids is conducive to high DBT HDS performance. Furthermore, a greater L acid amount also results in a high HYD/DDS ratio which also can improve the DBT HDS activity. The NiMo/BF−K catalyst also shows the preferable structural properties as mentioned above; nonetheless, it fails to achieve a satisfactory DBT HDS result. The most important reason is probably that it lacks adequate B and L acidities. According to the calculation of density functional theory (DFT), the corner sites of MoS2 are more favorable for adsorption and dissociation of H2 molecules to produce active hydrogen atom.48 Hence, the HYD route is more likely to occur in the corner sites of active phases, and the edge sites of MoS2 are in charge for the DDS route.49 (fc/fe)Mo of MoS2 is linearly related to the HYD/DDS ratio in the DBT HDS reaction. With the higher (fc/fe)Mo of MoS2, the corresponding catalyst could result in the higher HYD/DDS ratio. On the basis of the eq 5, (fc/fe)Mo is linearly related to the stacking length of MoS2, meaning that the MSI has a great influence on (fc/fe)Mo. The NiMo/BF−Mg catalyst has a higher (fc/fe)Mo value (0.23) and a higher HYD/DDS ratio (0.29) than do the other catalysts. In light of the above analysis, the L acid and (fc/fe)Mo of MoS2 could affect the HYD/DDS ratio together, and a higher HYD/DDS ratio is ascribed to more L acid and a higher (fc/fe)Mo value. MSI has an intimate influence on dispersion degree, stacking number, stacking length, and the sulfidation degree of MoS2 slabs. Generally speaking, the strong MSI favors the dispersion of active Mo species; nevertheless, the weak MSI results in the aggregation of Mo species which is detrimental to the sulfurization of metal Mo species. According to H2-TPR characterization, the NiMo/BF−Mg catalyst has a stronger MSI which consequently contributes to the higher dispersion degree (0.28), the shorter stacking length (3.75 nm), and the

5. CONCLUSION In this research, NiMo/BF series supports were successfully synthesized using different inorganic salts (KCl, MgCl2, MnCl2, and CuCl2) to be the additives. On the basis of the characterization results of different supports by XRD, N2 adsorption−desorption, SEM, TEM, and 27Al NMR techniques, it could be found that all the catalysts possess favorable structure properties including high specific surface areas, large pore volumes, and mesopore sizes. Among these supports, the BF−Mg support had the higher crystalline degrees and relatively ordered pore channels. The Py-FTIR characterization displayed the NiMo/BF−Mg catalyst possesses more B and L acidity; moreover, more L acid and a higher (fc/fe)Mo value resulted in a higher HYD/DDS ratio. H2-TPR characterization showed that the NiMo/BF−Mg catalyst possesses a stronger MSI, which promoted the active Mo species dispersing well on the support, finally improving the MoS2 phases to get a smaller stacking number, a shorter stacking length, a higher dispersion degree, and a higher (fc/fe)Mo value. The XPS characterization confirmed that the NiMo/BF−Mg catalyst possesses the highest sulfidation degree. When WHSV was 10 h−1, the NiMo/BF−Mg catalyst displayed the highest DBT HDS (98.58%) performance among the NiMo/BF series of catalysts. The prominent textural properties, greater B and L acidities (193.6 and 118 μmol·g−1 at 200 and 350 °C, respectively), stronger MSI, smaller stacking number (3.11), shorter stacking length (3.75), higher dispersion degree (0.28), higher (fc/ fe)Movalue (0.23), and higher sulfidation degree (61.3%) of MoS2 slabs were considered to be responsible for the highest HDS efficiency. 11838


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01649.

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Textural characteristics of BF series supports, SEM images of BF series supports, amounts of B and L acid sites determined by FT-IR of pyridine adsorption over NiMo/BF catalysts, XPS characterization results of NiMo/BF series catalysts, possible reaction network of DBT HDS over NiMo/BF series catalysts (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 10 89732290. ORCID

Aijun Duan: 0000-0001-5964-7544 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely acknowledge the financial support by the National Natural Science Foundation of China (Nos. 21676298 and 21878330), the China National Petroleum Corporation (CNPC) Key Research Project, and KLGCP (GCP201401).

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REFERENCES

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