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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Hydrodesulfurization Properties of Dibenzothiophene over NiMo Catalysts Supported on Cubic Fm3m Mesoporous Structure and High-Framework Aluminum-Modified AlKIT‑5 Jiyuan Fan, Aijun Duan,* Chunming Xu,* Zhen Zhao, Peng Zheng, Xilong Wang, Haidong Li, Chengkun Xiao, Jinlin Mei, and Guiyuan Jiang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, P. R. China Energy Fuels Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/28/18. For personal use only.

S Supporting Information *

ABSTRACT: The AlKIT-5 sample with Fm3m symmetry order structure and different Si/Al ratios was first synthesized by chemical grafting method. The 27Al NMR result of the AlKIT-5 sample proves that most aluminum species have been incorporated into the silicon framework. The oxidized supports and catalysts were characterized by XRD, N2 adsorption, 27Al MAS NMR, SEM, TEM, Py-FTIR, and Raman spectroscopy, and the series of sulfided catalysts were investigated by XPS and HRTEM. The corresponding NiMo catalysts were obtained by impregnation method, and the hydrodesulfurization activity for dibenzothiophene (DBT) was tested in a high-pressure microreactor under different weight hourly space velocity (WHSV, 10− 100 h−1) values. All of the catalysts based on KIT-5 mesoporous material support showed better HDS efficiencies than that of the NiMo/AlMCM-41 catalyst, benefiting from the interconnected large pore structure. The NiMo/AK-10 catalyst exhibited the best HDS activity (99.2%) when the WHSV was 10 h−1, which resulted from the moderate acidity and good dispersion of Ni and Mo active species.

1. INTRODUCTION With increasingly prominent environment pollution, environmental regulations and the fuel specifications have become more and more strict. For controlling exhaust emission from engine combustion, the sulfur concentration in transportation fuels is strictly limited.1,2 A variety of technologies have been adopted to remove sulfur compounds from oil, such as oxidative desulfurization (ODS),3−6 extraction desulfurization (EDS),7,8 and hydrodesulfurization (HDS). Yang et al.3 prepared MoO3/4A catalyst and studied the catalytic ODS performance of DBT and BT by using cyclohexanone peroxide (CYHPO) as an oxidant. The results showed that the conversion of DBT was 99.0% and that the sulfur content reached 5 ppmw because the active components were highly dispersed on the catalyst surface. Wei et al.5 synthesized a novel heterogeneous Co/KIT-6 catalyst and evaluated the ODS performance for DBT under ambient temperature. The catalytic results confirmed that the Co/KIT-6 catalyst exhibited excellent ODS performance (98.68%), which could be attributed to the Ia3d mesoporous structures committing better dispersion for the active components, which favored the faster diffusion of reactants and resultants. Li et al.7 designed a series of deep eutectic solvents (DESs) and tested their extraction desulfurization activity for benzothiophene (BT). The tetrabutyl ammonium chloride-based DESs exhibited the highest extraction efficiency (99.48%) after five cycles, which was ascribed to the hydrogen bond formed between DESs and BT. The quality standard of vehicle fuels is constantly upgraded and gradually tends to be sulfur-free, which puts forward the high requirement of the hydrodesulfurization (HDS) technology. One of the most effective means to achieve ultra clean © XXXX American Chemical Society

fuels is to prepare a highly active hydrodesulfurization catalyst. The support plays an important role in the HDS catalyst, of which, the structure will affect the dispersion or sulfidation of the deposited active metals, and the morphology of sulfided active species would further affect the HDS activity of the catalyst. Alumina has been widely used in the catalytic process due to its high thermal stability, low price, and good regeneration properties. However, some typical shortcomings such as low surface area and strong metal support interaction were found in the traditional commercial alumina, which is not conductive to the reaction and removal of macromolecular sulfide reactant.9,10 In recent years, the ordered mesoporous materials possessing high surface area and large pore volume have attracted significant attention and exhibited good activity in deep HDS.5,11,12 MCM-41 belongs to the two-dimensional p6mm mesoporous structure, which is unfavorable to the transportation of the reactant and is easily blocked by the active components of the HDS catalyst. In addition, its poor hydrothermal stability and weak surface acidity limit the catalytic performance. Jaroszewska et al.13 studied the hydrodesulfurization performance of 4,6-DMDBT over the NiMo/TiMCM-41 and NiMo/ AlMCM-41 catalysts. The catalytic results showed that the NiMo/TiMCM-41 catalyst exhibits the highest HYD/DDS activity due to the higher stability of NiMoO4 phase over the TiMCM-41 surface. Sampieri et al.14 prepared MoS2/MCM-41 and MoS2/SBA-15 catalysts by means of thermal spreading of MoO3, and their activities of DBT HDS were investigated. The Received: June 25, 2018 Revised: August 10, 2018 Published: August 15, 2018 A

DOI: 10.1021/acs.energyfuels.8b02172 Energy Fuels XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

results observed that the MoS2 particles were supported on MCM-41 material parallel to the direction of the mesoporous channels, which severely restricts the diffusion of DBT to the active sites and then restricted the HDS activity. Compared with two-dimensional mesoporous materials, the mesoporous molecular sieves with three-dimensional pore structure have more advantages in terms of diffusion, mass transfer, and other aspects of the reaction materials, and therefore, they show more application prospects.15 The KIT-5 material possesses 3D dimensional cubic Fm3m mesoporous structure, tunable pore diameter, and large specific surface area, which is attributed to better diffusion of reactants. Therefore, the KIT-5 material has received considerable attention and is expected to exhibit superior performance in many reaction processes. Varghese et al.16 synthesized a bimetallic nanoporous FeAlKIT5 catalyst and studied its application for the synthesis of 1,1-diarylalkanes and 1,1-diarylalkenes. The catalyst displayed much higher activity than other nanoporous catalysts such as AlSBA-15 and FeKIT-5 and could be reused several times without activity loss. The main reason was attributed to the large pore volume, high surface area, and combination of both Lewis and Brønsted acid sites. Srinivasu et al.17,18 prepared AlKIT-5 with different nSi/nAl ratios under highly acidic medium, and the corresponding catalyst was utilized for the acetylation of aromatics. The catalytic performance for the acetylation of veratrole showed that the AlKIT-5 (10) exhibited higher conversion and was superior to the catalysts supported on HY, Hβ, H-mordenite, and HZSM-5. Consequently, the authors believed that the three-dimensional cage-type porous network with high surface area and comparable acidity contributed more to good conversion. However, there has been no detailed report available for the synthesis of AlKIT-5 with highly alumina framework. Moreover, the effect of aluminum content on the hydrodesulfurization performance of supported AlKIT-5 catalysts deserves to be investigated in detail. The HDS catalyst should have an intrinsically large specific surface area, suitable acidity, and good hydrothermal stability, whereas the neutral framework of KIT-5 presents the shortcoming of poor stability and weak acidity. These problems can be solved by introducing aluminum atoms into the neutral Si skeleton.19−21 It is a challenge to obtain high content of the Al framework material by the direct incorporation method due to the highly acidic synthesis condition of KIT-5. Therefore, the postsynthesis method seems to be a better choice to prepare Al-KIT-5 with high aluminum framework content.22 In this research, a series of Al-substituted KIT-5 materials with high aluminum framework content up to the nSi/nAl ratio of 5 were synthesized by post-treatment method. The corresponding NiMo/AK-x catalyst was obtained by two-step incipient-wetness impregnation method. The effect of Al content on the physicochemical property of the material and catalyst were thoroughly characterized by XRD, N2 adsorption−desorption, SEM, TEM, 27Al NMR, Py-FTIR, Raman, HRTEM, and XPS. The catalytic activity of the series of catalysts were evaluated for the hydrodesulfurization of dibenzothiophene (DBT) under different weight hourly space velocity (WHSV) values, and the NiMo/Al-MCM-41 catalyst was taken as reference. Furthermore, the HDS reaction mechanism of dibenzothiophene (DBT) over the NiMo/AlKIT-5 catalyst with different Si/Al ratios was proposed.

2.1. Synthesis of Support. Pure KIT-5 was prepared following the procedure described by Kleitz et al.23 Typically, 2.5 g of Pluronic F127 (EO106PO70EO106, Sigma-Aldrich), 120 g of deionized water, and 5.25 g of concentrated hydrochloric acid (35 wt %, Beijing Chemical Work) were mixed and stirred at 45 °C for 6 h. To this mixture was added 12 g of tetraethyl orthosilicate (TEOS, 98%, Sinopharm Chemical Reagent Co., Ltd.), which was kept stirring for 24 h at 45 °C. The reaction mixture was transferred into a polytetrafluoroethylene bottle and heated at 100 °C under static conditions for 24 h. The product obtained by hydrothermal treatment was filtered, washed, and then dried for 12 h at 100 °C. Then, the solid sample was calcined at 550 °C to remove the organic template. The Al-KIT-5 substrates with Si/Al molar ratios of 5, 10, 20, and 30 were prepared by postsynthesized method using anhydrous alumina chloride (AlCl3, 99%, Tianjin Guangfu Fine Chemical Research Institute) as an alumina source.24,25 Then, 1 g of KIT-5 was dissolved in 100 mL of dry ethanol, and AlCl3 was added to the solution under stirring for 12 h. Subsequently, the solution was filtered and washed three times by dry ethanol. The obtained solid sample was dried at 100 °C and then calcined at 550 °C for 6 h. The Al-KIT-5 with different Si/Al molar ratios was denoted as AK-x (x = 5, 10, 20, and 30). The mesoporous MCM-41 material was synthesized according to the literature.26 The Al-MCM-41 with a molar ratio of Si/Al = 10 was obtained by the same procedure. 2.2. Synthesis of the NiMo-Supported Catalyst. The series of NiMo-supported catalysts were prepared by two-step incipient wetness method. First, the Al-KIT-5 supports were impregnated using an aqueous solution of hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99%, Sinopharm Chemical Reagent Co., Ltd.). Second, the support was stirred under ultrasonic conditions for 15 min and then dried at 80 °C. Finally, the support was calcined at 550 °C. The obtained material was impregnated with the nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 98%, Sinopharm Chemical Reagent Co., Ltd.) aqueous solution and then dried and calcined. The obtained catalyst was denoted as NiMo/AK-x (x = 5, 10, 20, and 30), where x represents the Si/Al molar ratio. The same approach was performed to prepare the NiMo/KIT-5 and NiMo/ AlMCM-41 catalysts. The nominal metal loadings are 12 wt % of MoO3 and 3 wt % of NiO for all catalysts. 2.3. Characterization Techniques. Wide-angle X-ray diffraction (XRD) patterns of the samples were recorded on Bruker D8 Advance diffractometer using a Cu Kα radiation resource according to the step scanning procedure in the 2θ ranges 0.5−1° and 5−50° operating at a power source of 40 kV and 40 mA. The morphology of the sample was analyzed by SU8010 (Hitachi Co.) scanning electron microscopy (SEM). The samples were dried and well distributed on the copper surface. Then, the copper surface was sprayed with gold and analyzed by electron microscope. The pore structure of the sample and the MoS2 phase of the sulfided catalysts were tested by F20 (FEI Co.) transmission electron microscopy (TEM). The sample was dispersed ultrasonically in ethanol, and the suspension was deposited on lacey support films. The Al species coordination environment information was tested on a Bruker Advance 600III MAS nuclear magnetic resonance (NMR) spectrometer. The texture properties of the series of oxided catalysts were obtained from the nitrogen adsorption−desorption isotherm, which was carried on the Micromeritics TriStar 3020 apparatus at −196 °C. The specific surface areas were calculated by the standard Brunauer− Emmett−Teller (BET) method. The acidity and acid type of the oxided catalyst were determined by the Digilab Fourier-transform infrared spectroscopy (FTIR) of adsorbed pyridine. The catalyst was vacuumed at 1.0 × 10−3 Pa for 2 h in an FTIR cell. Then, adsorption of pyridine proceeded at room temperature for 20 min followed by desorption for 30 min after the system was heated at 200 or 350 °C and the desorption signal was recorded. B


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Energy & Fuels Raman analysis of the catalyst was performed by inVia Raman spectrometer (Renishaw Co.) using 325 nm for laser source and with a power of 8 mW. The Ni and Mo contents of the series of fresh catalysts were analyzed by inductive coupled plasma emission spectrometer (ICP), and the results are listed in Table S1. The Mo species of the sulfided catalyst were analyzed with a PerkinElmer PHI-1600 ESCA spectrometer, and the corresponding X-ray photoelectron spectra (XPS) were obtained. XPS measurements were taken with Mg Kα (hv = 1253.6 eV) source, and the binding energies were corrected by using the C 1s peak (284.8 eV). 2.4. Catalytic Activity Measurements. The evaluation of hydrodesulfurization activity of dibenzothiophene (DBT) over the series of catalysts was performed in a high-pressure fixed bed microreactor. The loading amount of the catalyst was 1 g (40−60 mesh), and a certain amount of quartz sand was added to make the catalyst uniformly distributed in the constant temperature section. Before the test, the catalyst was presulfided using 2.5 wt % CS2 in a cyclohexane solution under a pressure of 4 MPa, a temperature of 340 °C, and a H2/oil ratio of 300 mL mL−1 for 4 h. After the presulfidation is complete, the DBT (500 ppm sulfur content) dissolved in cyclohexane was fed into the reactor by a pump. The catalytic performance was tested at 340 °C, reaction pressure of 4 MPa, H2/oil volume ratio of 200 mL ml−1, and different WHSVs of 10−100 h−1. Three samples were taken for each space velocity condition after the reaction system was stable, and then the samples were analyzed offline. The sulfur contents of feedstock and product were analyzed by RPP-2000SN sulfur nitrogen analyzer (Taizhou Zhonghuan Analysis Instruments Co., Ltd.). The HDS efficiency of catalyst was calculated using the formula

Figure 1. Small-angle XRD patterns of series of supports: (a) KIT-5, (b) AK-5, (c) AK-10, (d) AK-20, and (e) AK-30.

material structure is maintained.24 Figure S1 shows the wideangle XRD patterns of the series of supports. There is a broad peak at 2θ = ∼23° attributed to the amorphous silicon species.29 There is no characteristic peak appearing to belong to aluminum oxide crystal phases, indicating that no bulky Al2O3 aggregates formed on Al-KIT-5. 3.2. TEM Images of Supports. Transmission electron microscopy was performed to better understand the 3D cubic mesoscopic order structure after the addition of Al, and the results are shown in Figure 2. Compared with the pure KIT-5

HDS (%) = (Sf − Sp)/ Sf × 100% where Sf and Sp express the sulfur contents in feedstock and product, respectively. The distribution of hydrodesulfurization products was analyzed by Thermo Finnigan DSQ gas chromatograph mass spectrometer. The column model was HP-5MS elastic quartz thin column (60 m × 0.25 mm × 0.25 mm). The rate constant of HDS (kHDS mol g−1 h−1) reaction and the turnover frequency (TOF) of the series of catalysts for DBT HDS were calculated by the following equations base on pseuo-first-order kinetics.27,28 kHDS = TOF =

F ij 1 yz zz lnjj m k1 − x { F×x nMo × fMo

where F stands for DBT flow rate in mol h−1, m represents catalyst quality loading in a fix-bed reactor in grams, x is the total conversion of DBT, TOF represents the number of reacted DBT molecules per hour and per Mo atom at the edge sites, nMo is the total molar number of Mo atoms in the catalyst, and f Mo is the percentage of Mo atoms on the edge surface of MoS2 particles.

3. RESULTS 3.1. XRD Results of Supports. The small-angle X-ray diffraction (XRD) patterns of the pure KIT-5 and Al-KIT-5 materials are displayed in Figure 1. As can be seen from the spectra, highly resolved low-angle diffractions are observed at 2θ = 0.5−1° for all samples. The reflection peaks corresponding to (111) and (200) are well resolved and indexed to the face-centered-cubic Fm3m symmetry group of KIT-5, which is consistent with the result described by Kleitz et al.23 The incorporation of alumina did not result in a change in the intensity and position of the characteristic peaks, indicating that the original pore structure and order of the

Figure 2. TEM images of the series of supports: (a) KIT-5, (b) AK-5, (c) AK-10, (d) AK-20, and (e) AK-30. C


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Energy & Fuels sample, the series of materials modified by Al show ordered mesoporous structure. The results are consistent with the observation from XRD. Furthermore, the order degree of the series of materials becomes relatively weak with the increase of Al content. It is indicated that the AlKIT-5 materials obtained by post-treatment method remain in a highly ordered degree. The SEM images of AlKIT-5 material are presented in Figure S2. The series of materials present irregular morphology and show no significant difference after the Al incorporation. 3.3. 27Al NMR Results of Supports. The coordination of aluminum atoms in the as-synthesized samples was studied by 27 Al MAS NMR. Figure 3 shows the 27Al MAS NMR spectra of

Figure 4. N2 adsorption−desorption isotherms of the catalyst series: (a) NiMo/KIT-5, (b) NiMo/AK-5, (c) NiMo/AK-10, (d) NiMo/ AK-20, and (e) NiMo/AK-30.

Table 1. Texture Properties of the Series of Supports and Catalysts

Figure 3. 27Al NMR spectra of KIT-5 modified with different alumina contents: (a) AK-5, (b) AK-10, and (c) AK-20.

AlKIT-5 samples with different aluminum contents. Two types of peaks can be observed for all synthesized materials. The small signal at 0 ppm was assigned to the octahedral coordination extra-framework aluminum species.30 The signal at 53 ppm was attributed to the tetrahedral aluminum atoms.11 It is noted that the peak intensity of extra-framework Al species is significantly lower than those of framework alumina, indicating that most of the aluminum species have been introduced into the siliceous framework of KIT-5.25 3.4. BET Results of the Series of Oxidized Catalysts. Adsorption−desorption isotherms of nitrogen for the series of oxidized catalysts and the materials were studied at −196 °C, and the results are shown in Figure 4 and Figure S3, respectively. The N2 isotherms of the series of catalysts exhibit a type-IV hysteresis loop31 similar to those of Al-KIT-5 materials, indicating that the mesoporous structures of the supports are not damaged after metal loading followed by calcination. These results are consistent with the small-angle XRD results and TEM images. Table 1 summarizes the textural properties of the series of supports and catalysts. It is noted that the series of catalysts suffer a decrease in surface area and pore volume compared with pure material, which is due to the incorporation of Ni and Mo. The pore diameters are also slightly decreased. 3.5. Py-FTIR Results of the Series of Oxidized Catalysts. For the type and strength of acidity of the series of catalysts to be examined, pyridine adsorption measurements were performed, and the results are displayed in Figure 5. The results show that all of the catalysts exhibit strong adsorption

samples

SBET (m2 g−1)

V (cm3 g−1)

dBJH (nm)

KIT-5 AK-5 AK-10 AK-20 AK-30 NiMo/KIT-5 NiMo/AK-5 NiMo/AK-10 NiMo/AK-20 NiMo/AK-30

727 675 663 664 680 230 357 348 346 334

0.48 0.46 0.45 0.45 0.46 0.25 0.27 0.27 0.26 0.27

5.84 5.78 5.88 5.79 5.78 3.33 4.09 4.06 4.06 4.26

bands at 1450 cm−1 assigned to Lewis (L) acid sites after degassing at 200 °C. The characteristic adsorption band at 1490 cm−1 was attributed to the combination of Brönsted (B) and Lewis acid sites, whereas the weak infrared adsorption at 1540 cm−1 is ascribed to the pyridine adsorption on the Brönsted acid sites.32 After degassing at 350 °C, similar spectra still exist, and the intensities of the characteristic adsorption band became weakened. This indicates that the catalysts possess medium/strong acidity. Table 2 lists the acid distribution and the acid type of the catalysts. It can be seen that the B acid sites present in the series of catalysts after the incorporation of Al species, which can thus assumed that the bridging hydroxyl groups (Si-OH-Al) have been generated on Al-KIT-5.11,33 This is consistent with the observation from the framework Al species of 27Al NMR. 3.6. Raman Results of the Series of Oxidized Catalysts. For better understanding the oxide phases on the surface of the catalyst, Raman spectroscopic investigation was also conducted. The Raman spectra of the series of oxidized catalysts are displayed in Figure 6. For all samples, four characteristic peaks are observed at 220, 564, 826, and 955 cm−1. The peaks at 220 cm−1 can be assigned to the Mo−O− Mo deformation peak in the polymolybdate species.25 Peaks at 564 cm−1 are also observed, which could be attributed to the Al−O stretching vibration of Mo−O−Al. The bands at 826 and 955 cm−1 belong to the Mo=O stretching vibration of D


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Figure 5. FTIR spectra of pyridine adsorbed on the catalyst series: (a) NiMo/AK-5, (b) NiMo/AK-10, (c) NiMo/AK-20, and (d) NiMo/AK-30.

7 show representative images of the series of sulfide catalysts. The average length (Lav) and average number layer (Nav) of the MoS2 slab are obtained by counting 300 slabs and calculated by the formula

Table 2. Surface Acid Amounts and Acid Types of the Series of Catalysts 200 °C/μmol g−1 catalyst

B

L

B+L

NiMo/AK-5 NiMo/AK-10 NiMo/AK-20 NiMo/AK-30

2.58 2.34 1.91 1.59

79.41 62.39 54.60 47.40

81.99 64.77 56.51 48.99

350 °C/μmol g−1 B

L

B+L

32.45 25.80 18.60 16.20

32.45 25.80 18.60 16.20

n n ji zy Lav = jjjj∑ xili zzzz/∑ xi j i=1 z i=1 k {

ij n yz n Nav = jjjj∑ xiNi zzzz/∑ xi j i=1 z i=1 k {

where li is the length of slab, xi is the number of slab in li or stacking number (Ni) in li. The dispersion degree of MoS2 (f Mo) is determined by the percentage of Moedge accounting for the total amount of Mo atoms (Mototal); ni is the number of molybdenum atoms along the edge of a MoS2 slab, and the value is calculated by the slab length (L = 3.2(2ni − 1) Å).36,37 fMo =

Moedge Mototal

i

=

∑i = 1 (6ni − 6) i

∑i = 1 3ni2 − 3ni + 1

The length distribution of the MoS2 slab is depicted in Figure 7(f), and the statistical results of Lav, Nav, and f Mo are summarized in Table 3. The results demonstrate that the length and stacking number of NiMoS change with the incorporation of Al species. From Figure 7(f) and Table 3, it is clearly found that the NiMo/KIT-5 catalyst exhibits a higher stacking number and lowest dispersion degree (0.24), whereas the highest dispersion degree of Mo species (f Mo = 0.30) was obtained over the NiMo/AK-10 catalyst with an average length of 3.6 nm and Nav of 3.1. Furthermore, for the NiMo/AK-30 catalyst with Lav of 3.9 nm, a slab length greater than 8 nm occupies the highest ratio (33.3%), which is not conducive to the HDS reaction. The f Mo values over the series of catalysts increase gradually following the order NiMo/KIT-5 < NiMo/ AK-30 < NiMo/AK-5 = NiMo/AK-20 < NiMo/AK-10. This is consistent with the Raman results. The MoS2 slabs with shorter lengths and moderate stacking numbers would favor to the formation of type-II active phase.38 The NiMo/AK-10 catalyst possessing suitable slab length and stacking layer number will result in more active sites, which facilitates the HDS reaction. 3.8. XPS Results of Series of Sulfide Catalysts. X-ray photoelectronspectroscopy (XPS) was used to characterize the chemical states of Mo species and sulfidation degree. The decomposition of the Mo 3d and S 2s spectra of the series of

Figure 6. Raman spectra of the oxidized catalysts: (a) NiMo/AK-5, (b) NiMo/AK-10, (c) NiMo/AK-20, and (d) NiMo/AK-30.

Mo7O246− species, which reflect the weak metal−support interaction.34,35 These species are easily sulfided in the process of hydrodesulfurization and consequently improve the catalytic activity. As can be seen from the results, the NiMo/AK-10 catalyst exhibits the strongest characteristic peak at 955 cm−1 among the series of catalysts, which will favor enhancing the corresponding catalytic performance. The same results can be further verified by HRTEM. It is noted that no peaks corresponding to the MoO3 phase are observed, which indicates that the molybdenum oxides are highly dispersed on the surface of the catalyst. 3.7. HRTEM Results of Series of Sulfide Catalysts. The high-resolution transmission electron microscopy (HRTEM) images of the sulfided catalysts were using an F20 microscope operating at 285 kV accelerating voltage. Panels a−e in Figure E


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Figure 7. HRTEM images of the series of sulfide catalysts: (a) NiMo/KIT-5, (b) NiMo/AK-5, (c) NiMo/AK-10, (d) NiMo/AK-20, (e) NiMo/ AK-30, and (f) the length distribution of the MoS2 slab.

are listed in Table 4. From the results, it can be seen that Ssulfidation of the series of catalysts decreases in the order of NiMo/AK-10 > NiMo/AK-20 > NiMo/AK-5 > NiMo/AK-30 > NiMo/KIT-5. Identical conclusions were obtained by HRTEM characterization. The NiMo/AK-10 catalyst exhibits the highest sulfidation degree (56.5%), which can be attributed to the appropriate texture property and moderate acidity; therefore, it will favor the HDS reaction. 3.9. Catalytic Activity of DBT Hydrodesulfurization. Figure 9 shows the catalytic activity of the series of catalysts for the HDS of DBT at different WHSV values from 10 to 100 h−1. It should be noted that the NiMo/KIT-5 and NiMo/AK-x catalysts exhibit superior HDS activities relative to those of the NiMo/Al-MCM-41 catalyst at every WHSV value. The HDS efficiencies of DBT over the series of catalysts decrease following the order NiMo/AK-10 > NiMo/AK-20 > NiMo/ AK-5 > NiMo/AK-30 > NiMo/KIT-5 > NiMo/AlMCM-41. The NiMo/AK-10 catalyst presents the highest DBT conversion at all WHSVs; furthermore, the HDS efficiency (69.9%) at 100 h−1 is ∼2.2-times that of the NiMo/AlMCM41 catalyst (31.7%) and 1.8-times that of the NiMo/KIT-5 catalyst (38.4%). Cao et al.41 synthesized a series of AlSBA-16 materials in different morphologies with Si/Al molar ratios of

Table 3. Average Length (Lav) and Average Layer Number (Nav) of MoS2 Slabs catalyst

Lav (nm)

Nav

f Mo

NiMo/KIT-5 NiMo/AK-5 NiMo/AK-10 NiMo/AK-20 NiMo/AK-30

4.0 3.8 3.6 4.1 3.9

3.4 3.5 3.1 3.2 3.1

0.24 0.27 0.30 0.27 0.26

sulfided catalysts were analyzed by XPSPEAK 4.1 version software, and the results are shown in Figure 8. As shown in Figure 8, the peaks around 229.0 ± 0.1, 229.6 ± 0.1, and 233.1 ± 0.1 eV are ascribed to Mo 3d5/2 of MoS2 species (Mo4+), oxysulfide MoOxSy compounds (Mo5+), and MoO3 species (Mo6+), respectively.39 The binding energy located at 232.2 ± 0.1, 232.8 ± 0.1, and 236.0 ± 0.1 eV belong to Mo 3d3/2 of MoS2 species (Mo4+), oxysulfide MoOxSy compounds (Mo5+), and MoO3 species (Mo6+), respectively.40 The peak at 226.0 ± 0.1 eV binding energy is attributed to the S 2s peak. The sulfidation degree of Mo species was calculated by Ssulfidation = SMo4+ = Mo4+/(Mo4+ + Mo5+ + Mo6+). The analysis concentration of different Mo species over the sulfided catalyst F


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Figure 8. XPS spectra of the series of sulfided catalysts: (a) NiMo/KIT-5, (b) NiMo/AK-5, (c) NiMo/AK-10, (d) NiMo/AK-20, and (e) NiMo/ AK-30.

Table 4. Distribution of Mo Species Over the Series of Sulfide Catalystsa Mo4+

Mo5+

Mo6+

catalyst

area %

area %

area %

area %

area %

area %

SMo

NiMo/KIT-5 NiMo/AK-5 NiMo/AK-10 NiMo/AK-20 NiMo/AK-30

11.0 20.7 33.9 33.1 19.2

7.4 13.8 22.6 22.1 12.8

20.6 13.0 0 0 15.5

13.7 8.7 0 0 10.3

28.4 26.2 26.1 26.9 25.3

18.9 17.6 17.4 17.9 16.9

18.4 34.5 56.5 55.2 32.0

a

Area % means the area percent of the XPS peak. SMo = Ssulfidation = Mo4+/(Mo4+ + Mo5+ + Mo6+).

20, and the DBT HDS activity was evaluated over the corresponding catalyst. The NiMo/AK-20 catalyst prepared in this research exhibited higher HDS activity (97.5%) compared with that of the NiMo/AlSBA-16 catalyst (95.2%) when WHSV = 20 h−1, which might be attributed to larger specific

area and pore volumes. The highest HDS efficiency (99.2%) of DBT was achieved at WHSV = 10 h−1 over the NiMo/AK-10 catalyst. The stability of the NiMo/AK-10 catalyst was evaluated, and the results are displayed in Figure S5. G


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metal species, thus influencing the catalytic performance of the catalyst. Large pore size and specific surface area can effectively reduce the diffusion resistance of large molecules and then increase the accessibility of reactants on the active sites. Compared with MCM-41 (with pore sizes of 2−4 nm), the KIT-5 material possesses three-dimensional cage-type pores (6−10 nm), which are more resistant to pore blocking and can provide more active sites. As can be seen from Table 1, the pure material and the Al-incorporated material (AK-10) exhibit large specific surface areas (727 and 663 m2/g, respectively) and pore volumes (0.48 and 0.45 cm3/g, respectively). As depicted in Figure S1 and Figure 2, the Al species incorporated well into the framework of the material and did not destroy the ordered structure. In addition to the effect of pore size, the pore structure of the material also has an important influence on the HDS catalytic reaction. In previous studies,32,42 it was found that the 3D mesoporous pores are more favorable to the diffusion and mass transfer of the reactants than in the cylindrical pore network (MCM-41 and SBA-15). This is consistent with the result obtained on the catalyst with KIT-5 material as the support, which exhibits better catalytic HDS performance than that of NiMo/AlMCM-41 catalyst in Figure 9. From the N2 adsorption results of the series of NiMo catalysts (Figure 5), the mesoporous structure was still maintained after the loadings of MoO3 and NiO, which will favor the HDS reaction. In addition, the HDS catalytic performance over Mo-supported and NiMo catalyst were compared, and the results are displayed in Figure S6. As can be seen from the result, the addition of Ni promoter could effectively increase the HDS activity. Seven types of products were detected43 over the NiMo/ AK-x catalysts, and the relative selectivities of each substance are summarized in Table 5. Biphenyl (BP) was affirmed to be the product of direct desulfurization (DDS), whereas tetrahydrodibenzothiophene (THDBT), cyclohexenylbenzene (CHEB), and cyclohexylbenzene (CHB) were considered to be the products of hydrogenation desulfurization (HYD).44 THDBT was obtained after the hydrogenation of the aromatic ring of DBT molecule, and it further transformed into CHEB via the cleavage of the C−S bond; then, CHB was formed through the following hydrogenation pathway. It is noted that cyclopentylmethylbenzene (CPMB) was observed in the product, which was derived from the hydroisomerization reaction of CHEB. Moreover, CHB would generate dicyclohexyl (DCH) and cyclopentylmethylcyclohexane (CPMCH) through the combination route of hydrogenation and isomerization processes.45 On the basis of the above

Figure 9. DBT hydrodesulfurization efficiency over the series of catalysts at different WHSVs.

Gas chromatography−mass spectrometer (GC-MS) characterization was adopted to analyze and identify the DBT HDS product distribution, which provides information for the study of HDS path reaction and mechanism over the sulfided catalysts. The product was obtained at the DBT conversion of ∼50% by adjusting the WHSV. The GC-MS chromatographs of DBT HDS products obtained over the series of catalysts are shown in Figure S4. The kHDS and TOF value were calculated, and the results are listed in Table 5. From the data in Table 5, the global HDS rate of the series of catalysts decreased in the order: NiMo/ AK-10 (10.93 × 10−4 mol g−1 h−1, 4.76 h−1) > NiMo/AK-20 (10.72 × 10−4 mol g−1 h−1, 4.36 h−1) > NiMo/AK-5 (9.15 × 10−4 mol g−1 h−1, 4.06 h−1) > NiMo/AK-30 (8.43 × 10−4 mol g−1 h−1, 3.82 h−1) > NiMo/KIT-5 (7.71 × 10−4 mol g−1 h−1, 2.88 h−1). It should be noted that, for all of the NiMo/AK-x catalysts, the DBT HDS reaction followed the pseudo-firstorder kinetics and the corresponding rate constants are higher than that of NiMo/KIT-5. In particular, the kHDS value of the NiMo/AK-10 catalyst is more than 1.4-times greater than that of the NiMo/KIT-5 catalyst, and the TOF value is more than 1.7-times greater than that of the NiMo/KIT-5 catalyst. This order is similar to that of the catalytic activity.

4. DISCUSSION In general, the physicochemical properties of the support material can affect the dispersion and morphology of the active

Table 5. Selectivity of the Products Over the Series of Catalysts for DBT HDS selectivity (%)

NiMo/KIT-5

NiMo/AK-5

NiMo/AK-10

NiMo/AK-20

NiMo/AK-30

BP CHB THDBT CPMB CHEB DCH CPMCH kHDS (10−4 mol g−1 h−1) TOF (h−1) HYD/DDS

45.9 42.1 2.1 6.2 1.8 1.6 0.4 7.71 2.88 0.92

56.7 28.9 2.7 5.6 3.7 1.3 0.9 9.15 4.06 0.51

38.5 38.3 7.0 6.6 5.1 2.6 1.8 10.93 4.76 1.00

45.6 40.4 2.0 6.2 3.3 1.6 1.0 10.72 4.36 0.89

44.7 39.7 4.5 5.9 2.9 1.5 0.9 8.43 3.82 0.88

H


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Energy & Fuels

Figure 10. Reaction network of DBT HDS over the series of NiMo/AK-x catalysts.

5. CONCLUSIONS A series of highly ordered aluminosilicates (AlKIT-5) with different Si/Al molar ratios combining with high framework alumina content, which possessed three-dimensional cubic Fm3m mesoporous structure, were synthesized by facile method. The mesoporous structure of the parent KIT-5 sample was maintained after the Al incorporation. The effect of Al content on the physicochemical properties and catalytic activities of NiMo/AK-x catalysts were systematically investigated. The catalytic performance of the series of catalysts decreased in the order: NiMo/AK-10 > NiMo/AK-20 > NiMo/AK-5 > NiMo/AK-30 > NiMo/KIT-5 > NiMo/AlMCM-41. The NiMo/KIT-5 catalyst exhibited a higher HDS activity than that of the NiMo/AlMCM-41 catalyst, which was ascribed to the cubic Fm3m mesoporous structure having superior mass transfer properties than those of the two-dimensional p6mm pore network. The highest HDS efficiency (99.2%), kHDS (10.93, 10−4 mol g−1 h−1) and TOF values (4.76) were observed over NiMo/AK-10 catalyst, which could be attributed to the good dispersion of active species (0.30), moderate acidity, and especially the cubic mesoporous structure.

discussion and previous research results, the HDS reaction mechanism of DBT over the series of NiMo/AK-x catalysts are proposed and displayed in Figure 10. The acidity type and the total amount of acid catalyst are key factors for the DBT HDS reaction.46 The catalyst supported on the Al-KIT-5 exhibited much higher HDS activity than the catalyst supported on the pure KIT-5 material. Py-FTIR results in Table 2 showed that the total amount of acidity of the series of catalysts follows the order: NiMo/AK-30 (65.19 μmol/g) < NiMo/AK-20 (75.11 μmol/g) < NiMo/AK-10 (90.57 μmol/ g) < NiMo/AK-5 (114.44 μmol/g). Furthermore, the 27Al NMR results showed that peak intensity at 53 ppm belonged to tetrahedral alumina species gradually strengthened with the decreasing molar ratio of Si/Al, indicating the existence of the high framework content Al species and the formation of the Al−O−Si bond. The higher selectivity (56.7%) to BP and lower value of HYD/DDS ratio (0.51) over NiMo/AK-5 catalyst demonstrated that the DDS pathway was accelerated due to the increasing amount of strong Brönsted acid. Moreover, the isomerization and hydrogenation products of CPMB and CPMCH were obtained through route 5 and routes 8−10 based on the synergy interaction between Brönsted acid and Lewis acid. According to the GC-MS results, seven types of products (including BP, CHB, THDBT, CPMB, CHEB, DCH, and CPMCH) were obtained, and no lighter cracking products (C3−C6) were detected over the series of catalysts, demonstrating that no secondary cracking reactions occurred.11 The distribution and morphology of MoS2 species were also affected by the change of the acidity properties of support. The commercial Al2O3 always present lower stacking layers due to the strong metal−support interaction,27 whereas the NiMo/ KIT-5 catalyst possesses multilayer stacks (Nav = 3.4) of MoS2 as its neutral silicon species. The HRTEM results demonstrated that the Lav and Nav of the MoS2 slab decreased with the incorporation of aluminum species. The NiMo/AK-10 catalyst with moderate length (3.6 nm) and stacking number (3.1) of MoS2 slabs possesses the highest dispersion value (0.3) of Mo species, which facilitates the formation of more type-II NiMoS active phases. The XPS results of the series of sulfided catalysts further confirmed the good dispersion of active species (f Mo = 0.30) and the highest sulfidation degree (56.5%) obtained over those of the NiMo/AK-10 catalyst. In summary, the interconnected three-dimensional mesoporous pore structure, suitable acidity, good dispersion, and appropriate morphology of MoS2 active phases synergistically contribute to the excellent catalytic performance over that of the NiMo/AK-10 catalyst.

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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.energyfuels.8b02172. Wide-angle XRD patterns, SEM images, N2 adsorption− desorption isotherms, GC chromatographs, DBT HDS results, and NiMo contents of the series of catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 10 89732290. *E-mail: [email protected]. Tel: +86 10 89733392. ORCID

Aijun Duan: 0000-0001-5964-7544 Guiyuan Jiang: 0000-0003-1464-3368 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 21676298, U1463207, and 21503152), CNPC Key Research Project, and KLGCP (GCP201401). I


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