High-Performance Bimetal NiMo Catalysts Prepared over Novel Cubic

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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High-Performance Bimetal NiMo Catalysts Prepared over Novel Cubic Mesoporous Silica with a Cost-Efficient Method for the Removal of Dibenzothiophene Cong Liu, Yanjun Gong,* Aijun Duan,* Xin Wang, Xilong Wang, Jiyuan Fan, Qian Meng, Di Hu, Jinlin Mei, and Huiping Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China

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S Supporting Information *

ABSTRACT: Highly ordered mesoporous silica with Pm3n cubic mesostructure and cube-like morphology was facilely fabricated in a cost-efficient aqueous-phase synthesis system. The obtained cubic mesoporous silica (CMS) modified with various aluminum contents were taken as supports to prepare NiMo/Al-CMS catalysts, and the corresponding hydrodesulfurization performance of dibenzothiophene was subsequently assessed. The results showed that as-prepared CMS exhibited a large surface area (865 m2/g) and pore volume (0.92 cm3/g). The incorporated aluminum brought many advantages such as enhanced acidity, modulated metal−support interaction, and improved active metals dispersity, all of which contributed to the high sulfidation degree of catalysts. As a result, the NiMo/Al-CMS-10 catalyst presented superior HDS activity and a higher selectivity for the direct desulfurization route, deriving from a synergistic effect of its admirable textural characteristics, increased acidic property, highly sulfided active phases, and well-dispersed MoS2 slabs with moderate stacking layer and slab length.

1. INTRODUCTION With the growing serious environmental problems induced by exhaust emission, increasingly rigorous environmental regulations have been implemented to restrict the contents of contaminants,1 especially the sulfur compounds in transportation fuels.2 Many purification technologies for liquid fuel have been developed.3−5 As a highly effective method to remove sulfur compounds in fuel, hydrodesulfurization (HDS) technology confronts great challenges to achieve ultra clean fuels. The development of efficiently novel HDS catalysts is the most crucial point. As the primary component of HDS catalysts, the support attracts wide attention of the researchers. It has been found that the support can affect not only the structure and dispersity of the supported metal oxides, but the morphology of the active phases, thus further influencing the catalytic performance.6 Therefore, to develop a new support with both good physical and chemical properties is highly desired. The traditional γ-Al2O3 support has been extensively used in hydrotreating processes due to the low cost and good stability of alumina. Yet it possesses a low surface area, irregular pore © XXXX American Chemical Society

structure, and nonuniform pore size, which are unfavorable for the active metal dispersion and macromolecule reaction.7 Moreover, its interaction with active metals is relatively strong that will result in the formation of monolayered active phases. This kind of type-I active phase is not conducive to being reduced and sulfided, and thereby exhibits a low catalytic activity in HDS reactions.8 By comparison, mesoporous materials are advantageous for HDS processes to realize the ultra deep desulfurization due to the high surface area, large tunable pore size, and weak metal−support interaction with active species.9−12 In recent years, various mesoporous silicas with different pore structures, morphologies, and pore sizes were widely applied in HDS reactions. SBA-15 is a typical two-dimensional (2D) mesoporous material with tunable properties such as morpholReceived: February 26, 2019 Revised: May 16, 2019 Accepted: May 20, 2019

A

DOI: 10.1021/acs.iecr.9b01094 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

contents (molar ratio of Si/Al = 5−40) were further synthesized by applying postsynthesis approach. The NiMo/Al-CMS-x catalysts were prepared according to the impregnation method. The catalytic activities of the obtained catalysts were evaluated for HDS of DBT. The effects of Al incorporation into CMS on the physical-chemical characteristic and HDS performance of NiMo/Al-CMS have been systematically investigated. The properties of obtained mesoporous sieves and supported catalysts were performed by many characterization techniques involving XRD, TEM, 27Al NMR, Raman spectra, XPS, and so on. More importantly, the reaction network of DBT molecule over catalysts NiMo/Al-CMS has been proposed.

ogy, pore size, and doped metals; it then could be used as support for different catalytic processes. Kumaran et al.13,14 adopted the SBA-15-based catalysts for HDS reactions of different thiophenic compounds, and the catalytic activity of the SBA-15 supported catalysts was found to be higher than that of the γ-Al2O3 supported counterparts. The authors then proposed that the shorter length and higher stacks of MoS2 crystallites over the SBA-15 supported catalysts contributed more active sites to HDS than those of the corresponding Al2O3 catalyst. The study investigated by Gao et al.11 revealed that the different morphologies of SBA-15 had a significant influence on the active metals dispersion, and thus affect the HDS performance of dibenzothiophene (DBT) reactants over supported NiMo catalysts, especially the diffusion of reactant molecules. Furthermore, it has been accepted that the mesoporous materials with three-dimensional (3D) ordered structures are remarkably superior to the 2D materials due to the better active metals dispersion over the supports and the faster diffusion of reactants inside their 3D connected channels during the reaction. Soni et al.15 found that KIT-6 with a well-ordered 3D meso-structure was very effective as a support for thiophene HDS, and the corresponding CoMo and NiMo supported catalysts displayed higher activity than did the SBA-15-based catalysts. The detailed characterization evidence of BET and TEM confirmed that the combination of high surface area, 3D interconnected pore channel, and large pore volume was answerable for the predominant activities of KIT-6-based catalysts. For a support material to meet the requirement of industrial applications, it should not only possess suitable physicochemical and mechanical properties, but also have low cost and simple preparation process. The synthesis strategies of typical 3D mesoporous materials such as KIT-6, KIT-5, FDU-12, and so forth16−19 are usually a complicated hydrothermal treatment method that uses expensive amphiphilic block copolymers as the template; the expense would then be so high as to limit commercial utilizations. In recent years, a facile modified Stöber method has been explored to fabricate mesoporous silica with different morphologies and ordered structures by using cheap cetyltrimethylammonium bromide (CTAB) as the structure directing agent in a simple aqueous-phase system,20 which would open an avenue for developing a novel support candidate for HDS reactions. There has been no report on the usage of ordered mesoporous silica via this convenient synthesis strategy for HDS until now. Additionally, it is well-known that the moderate acidity is good for the hydrogenation and isomerization abilities of catalysts, which finally enhance the HDS activity.21,22 However, the neutral Si frameworks of pure silica materials result in its weaker acidity and lower hydrothermal stability, which are required to be improved by incorporation of various metal heteroatoms such as Al, Ti, and Zr.23−25 Among that, the substitution by Al atoms via postsynthesis method is a preferential choice.26 Moreover, it was reported that the incorporated heteroatoms could facilitate the dispersion degree of active species by altering the interaction between metals and supports from weak to medium.27 Herein, a novel mesoporous silica material with highly ordered Pm3n mesostructure and cube-like morphology was first applied in the HDS reaction system. The cubic mesoporous silica materials (CMS) were facilely and efficiently fabricated with high yield in a modified Stöber synthesis system. Moreover, series Al-modified CMS materials with various aluminum

2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals were of analytical standard and used as obtained without subsequent treatment. CTAB, tetraethyl orthosilicate (TEOS, 98%), anhydrous aluminum chloride (AlCl3, 99%), (NH4)6Mo7O24·4H2O (99%), and Ni(NO3)2·6H2O (98%) were purchased from Sinopharm Reagent Co., Ltd. (China). Hexane, cyclohexane, and concentrated ammonia−water (28 wt %) were bought from Beijing Chemical Reagent. Dibenzothiophene (DBT) was bought from Sigma-Aldrich Co., Ltd. (U.S.). H2O used in all experiments was deionized water. 2.2. Synthesis of Supports. The CMS material was prepared on the basis of a simple Stöber process.20 First, 2 g of CTAB was mixed with 320 mL of H2O and stirred for awhile, and then 14 mL of ammonia−water was added and stirred to form a transparent solution at 35 °C. Afterward, 50 mL of TEOS (20 v/v % in hexane) was added into the above mixture dropwise under continuous stirring. The reaction was maintained at 35 °C for 20 h. After the reaction was completed, the sample was collected and washed by centrifugation several times. The obtained product was dried at 60 °C for 10 h. Finally, the white powder was calcinated to remove the template in a muffle furnace. The preparation of Al-modified CMS was done according to the postsynthesis method using AlCl3 as the aluminum source.28 The typical process was as followings: 1.5 g of CMS was dissolved in 120 mL of dry ethanol under stirring, and then a certain amount of AlCl3 was added and stirred for 12 h at 35 °C. Afterward, the solution was centrifuged and washed with absolute ethanol. The collected samples were dried overnight and calcinated at 550 °C under air atmosphere. The modified CMS with various aluminum contents were indicated as AlCMS-x, where x represented the Si/Al molar ratios of 5, 10, 20, 30, and 40. 2.3. Catalyst Preparation. The two-step incipient wetness impregnation method was adopted to prepare the series NiMobased catalysts, and (NH4)6Mo7O24·4H2O and (Ni(NO3)2· 6H2O were used as Mo and Ni precursors, respectively. The AlCMS supports were first mixed with a certain amount of Mo precursor, and then the samples were dried and calcinated. The followed impregnation of Ni precursor was the same as the above procedure. The prepared catalysts were expressed by NiMo/Al-CMS-x (x = 5, 10, 20, 30, 40), of which the nominal metal contents over the catalyst were 12 wt % of MoO3 and 3 wt % of NiO. 2.4. Characterization Methods. Small-angle X-ray diffraction (XRD) was conducted on a Bruker D8 Advance powder diffractometer with the scanning angle in the ranges of 0.5−5°. The wide-angle XRD was measured on the same diffractometer with the 2θ from 5.0° to 80°. B


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3. RESULTS 3.1. Small-Angle XRD of Supports. Figure 1 displayed the small-angle XRD of Al-CMS-x samples with various molar ratios

The morphologies of supports were observed by scanning electron microscopy (SEM) on the SU8010 instrument (Hitachi company). The samples were dispersed on the aluminum sheet, andn then coated with gold before the SEM measurement. The microstructures of the samples and sulfided active phases were collected by transmission electron microscopy (TEM) on a Philips Tecnai F20 instrument (FEI Co.). The samples were first grounded and dispersed in ethanol under ultrasonic condition, and then deposited on carbon support films for measuring. The structure characteristics of samples were analyzed by N2 adsorption−desorption methods using a Micromeritics TriStar 3020 instrument at −196 °C. BET method and BJH model were applied to calculate the textural parameters.29 27 Al MAS nuclear magnetic resonance (NMR) spectra of materials were conducted on a Bruker AdvanceIII 600WB spectrometer using a MAS probe. The acidities of catalysts were assessed by Fourier transform infrared spectroscopy (FTIR) with pyridine molecule as the probe using MAGNAIR 560 FTIR instrument. Raman spectra were carried out on a Renishaw Micro-Raman spectrometer with the wavelength of 325 nm as th elaser source. X-ray photoelectron spectroscopy (XPS) measurements were progressed on a PerkinElmer PHI-1600 ESCA spectrometer. The relative contents of Mo4+, Mo5+, and Mo6+ were quantified by fitting XPS spectra using XPSPEAK software. 2.5. Catalytic Tests. The catalytic performances of series catalysts NiMo/Al-CMS-x were assessed in a tubular fixed-bed microreactor by using DBT as model reactant, and the filled catalysts are 0.5 g. Before the reaction, all catalysts were presulfided using 3.0 wt % CS2 in cyclohexane at the temperature of 340 °C and H2 pressure of 4 MPa. After presulfidation, the mixture (sulfur content: 500 ppm) containing DBT and cyclohexane was fed into the microreactor in the conditions of 340 °C, H2 pressure of 4.0 MPa, H2/oil volumetric ratio of 200, and various WHSV of 20−150 h−1. After the whole reaction period was completed, a RPP-2000SN analysis instrument was utilized to test the sulfur contents of reaction products and original liquid feed. The HDS conversions were calculated via eq 1: HDS (%) =

Cf − Cp Cf

× 100%

Figure 1. Small-angle XRD of Al-CMS-x: (a) Al-CMS-5; (b) Al-CMS10; (c) Al-CMS-20; (d) Al-CMS-30; (e) Al-CMS-40; and (f) pure CMS.

of Si/Al. It can be observed that pure CMS exhibits a wellresolved diffraction peak with high intensity at 2θ = 1.4° and three diffraction peaks with relatively weak intensities between 1° and 2.5° of 2θ, which are attributed to the (200), (210), (211), and (420) faces associated with the highly ordered mesostructure of three-dimensional cubic (Pm3n) symmetry.20 All of the Al-CMS-x materials present characteristic peaks similar to those of CMS but with lower intensity, suggesting the original 3D cubic pore structures of parent CMS material are well maintained after the incorporation of aluminum, but the order degree of pore channel is slightly decreased. Furthermore, the weakened peak intensities are probably derived from that the Al species are deposited onto the surface of CMS, consistent with the previous research.32 It is noteworthy that the peak positions of Al-CMS-x shift to larger 2θ values with the addition of aluminum, demonstrating that the pore sizes of Al-CMS-x decrease. 3.2. SEM and TEM of the Supports. The effect of Al modification on the morphologies of CMS was investigated by SEM as shown in Figure 2. The obtained samples all display monodispersed cube-like morphology with nonuniform size, revealing that the incorporation of aluminum does not result in significant changes in the morphology. Nevertheless, when doping excessive aluminum (Si/Al = 5) as displayed in Figure 2a, a fraction of oxide aluminum species emerged on the surface of CMS. In addition, the SEM-EDS elemental mapping (Figure 3) of the representative sample Al-CMS-20 reveals that the element Al is widely distributed in the Al-modified CMS materials, manifesting the Al species are embedded into the framework of CMS through the chemical grafting method. TEM was used to study the pore structures of Al-modified CMS series materials and displayed in Figure 4. The highly ordered Pm3n cubic mesostructures are clearly observed for all of the samples, demonstrating that the typical pore structure of pure CMS does not suffer damage after Al modification by posttreatment method. Yet from the TEM image of the Al-CMS-5

(1)

where Cf and Cp, respectively, indicate the sulfur contents of feedstock and product. The reaction product distributions were analyzed by a GC− MS instrument. The reaction rate constant (kHDS, mol g−1 h−1) and turnover frequency (TOF, h−1) for DBT HDS were obtained on the basis of the pseudo-first-order reaction,30,31 and can be expressed by the following equations: kHDS =

TOF =

F ij 1 yz zz lnjj m k1 − τ {

(2)

F ·τ F ·τ = Moe nMo × fMo

(3)

where F (mol h−1) represents the reactant feeding rate, m (g) is the mass of filled catalyst, τ expresses the total conversion rate of reactant, Moe (mol) is the number of edge Mo atoms of active sites, nMo (mol) is the total Mo atoms number, and f Mo is the proportion of edge Mo atoms in catalysts. C


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Figure 2. SEM images of Al-CMS-x: (a) Al-CMS-5; (b) Al-CMS-20; (c) Al-CMS-40; and (d) pure CMS.

Figure 3. SEM image and SEM-EDS elemental mapping of Al-CMS-20 material.

aluminum of Al-CMS-10 is preferable to others, which may generate more Brønsted (B) and Lewis (L) acid sites. 3.4. Wide-Angle XRD of Oxided NiMo Catalysts. Figure S2 exhibited the wide-angle XRD of the series oxided catalysts NiMo/Al-CMS-x. Moreover, the powder XRD patterns of the Al-CMS-10 support and pure CMS material are provided as comparison with NiMo/Al-CMS-10 catalyst in Figure 6. According to the previous reports,27,35 the characteristic peaks of bulk MoO3 and NiO crystallite phase are located at 2θ = 12.7°, 23.6°, 25.5°, 27.5°, and 2θ = 36.9°, 43.1°, 62.6°, respectively. Al-CMS-10 support and NiMo/Al-CMS-10 catalyst all display a broad peak centered at 2θ = 23° as well as CMS, confirming that the amorphous structure of CMS is retained after the introduction of Al species and NiMo active metals. There are no characteristic peaks corresponding to Al2O3 over Al-CMS-10 support, revealing the homogeneous distribution of extra-framework Al on the material Al-CMS-10 surface. Furthermore, no characteristic peaks belonging to Al2O3, bulk MoO3, and NiO were detected on the NiMo/AlCMS-10 catalyst, suggesting that no bulk NiMo species appear on the surface of the Al-CMS-10 support.

sample, the order degree is found to be decreased. These findings are in line with the above small-angle XRD results. 3.3. 27Al NMR Analysis of Supports. 27Al NMR spectra were used to analyze the Al coordination state in the silica framework of Al-CMS materials. As exhibited in Figure 5, three types of peaks centered at 55, 0, and 31 ppm can be observed for all of the Al-modified samples, attributed to the tetrahedral, octahedral, and pentahedral coordinated aluminum, respectively,33 in which tetrahedral species are considered to be the framework Al species, but the other two coordination forms are supposed to be the extraframework alumina. The deconvolutions of the spectra show that the proportions of tetrahedral, pentahedral, and octahedral aluminum in Al-CMS-x with Si/Al ratios from 5 to 40 are 6.1:1:5.4, 7.6:1:6.1, 7.0:1:6.3, 7.5:1:6.6, and 6.7:1:6.2, indicating that about 48.8%, 51.7%, 49.0%, 49.7%, and 48.2% of Al atoms are successfully introduced into the silica framework of the CMS materials, respectively. Furthermore, the Al 2p XPS spectra in Figure S1 display the characteristic peaks located at the binding energy of about 74.9 eV for the series NiMo/Al-CMS-x catalysts, further confirming the successful introduction of partial Al species into the framework of the pure silica material CMS.34 Notably, the proportion of framework D


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Figure 4. TEM images of Al-CMS-x: (a) Al-CMS-5; (b) Al-CMS-10; (c) Al-CMS-20; (d) Al-CMS-30; (e) Al-CMS-40; and (f) pure CMS.

Figure 5. 27Al NMR spectra of Al-CMS-x composites: (a) Al-CMS-5; (b) Al-CMS-10; (c) Al-CMS-20; (d) Al-CMS-30; and (e) Al-CMS-40.

Figure 6. Powder XRD patterns of (a) NiMo/Al-CMS-10; (b) AlCMS-10; and (c) pure CMS.

Figure S2 reveals that the XRD spectra for all catalysts display a broad peak located at around 23° of 2θ, ascribed to the amorphous silica of CMS materials. There is no apparent signal of bulk MoO3 crystalline phase located at 2θ = 12−35° for all NiMo/Al-CMS-x catalysts, demonstrating no significant aggregation of Mo species emerged on the supports. Additionally, the element distributions of Al, Ni, and Mo are observed from the SEM-EDS elemental mapping of the representative catalyst NiMo/Al-CMS-10 as exhibited in Figure S3. It can be intuitively observed that the metals are well dispersed on the surface of Al-modified materials without apparent aggregations. Furthermore, the TEM and STEM-EDS mapping of NiMo/ Al-CMS-10 were performed. As presented in Figure 7, the ordered pore structures of support are still retained after loading active metals, and it can be noticed from the elemental mapping

(Figure 7b) that the loaded MoO3 (blue) and NiO (purple) species are distributed on the surface of Al-CMS-10 support with good dispersity, which is consistent with the above XRD and SEM-EDS analysis. 3.5. N2 Adsorption−Desorption of the Series Supports and Oxided Catalysts. The pore structure characters of the AlCMS-x supports and NiMo/Al-CMS-x catalysts were analyzed by the N2 adsorption−desorption as presented in Figure 8 and Figure S4. The corresponding textural parameters (SBET, Vt, and Dp) are given in Table 1. All of the Al-CMS materials and NiMo/ Al-CMS-x catalysts exhibit typical IV isotherms with H2 hysteresis loop, which present capillary condensation steps at the P/P0 ranging from 0.35 to 0.6, manifesting that all of the materials possess ordered pore structures with uniform mesopores. Moreover, the ordered pore structures are E


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Figure 7. TEM image (a) and STEM-EDS mapping (b) of NiMo/Al-CMS-10 catalyst.

Figure 8. (A) N2 adsorption−desorption and (B) pore size distribution patterns of Al-CMS-x supports: (a) Al-CMS-5; (b) Al-CMS-10; (c) Al-CMS20; (d) Al-CMS-30; (e) Al-CMS-40; and (f) pure CMS.

ratios reducing. That is because the incorporated framework Al species into CMS increase as confirmed by 27Al NMR results, and the pore channels of the supports may be plugged to some degree with the further increase of Al additions, thus causing the reduction of pore structure parameters. It is notable that the surface area and pore volume of Al-CMS-5 are significantly decreased on account of the presence of more extra-framework Al species. More importantly, the structural parameters of the series catalysts are all declined after the introduction of NiMo species, in which NiMo/CMS displays a sharp decline in surface area from 865 to 333 m2/g as well as the pore volume decreases from 0.92 to 0.36 cm3/g, revealing the partial pore channels of CMS are blocked to some degree by the active metals on account of the relatively weak metal−support interaction in NiMo/CMS. It can also be illustrated that the modification of aluminum for supports can improve the active metal dispersion due to the enhanced interaction between support and metal. 3.6. Py-FTIR of the Oxided Catalysts. Py-FTIR characterizations were performed at 200 and 350 °C for the purpose of evaluating the types and distributions of acid sites of the series catalysts as displayed in Figure 9, and the calculated results are given in Table 2. The spectra at 200 °C for all of the catalysts display three sharp adsorption peaks at 1450, 1491, and 1611 cm−1, and two weak peaks at 1540 and 1640 cm−1. It has been reported in the literature36 that the adsorption peaks at around

Table 1. Textural Characters of Series Al-CMS-x Supports and NiMo/Al-CMS-x Catalysts samples

SBET (m2/g)

Vt (cm3/g)

Dp (nm)

Al-CMS-5 Al-CMS-10 Al-CMS-20 Al-CMS-30 Al-CMS-40 CMS NiMo/Al-CMS-5 NiMo/Al-CMS-10 NiMo/Al-CMS-20 NiMo/Al-CMS-30 NiMo/Al-CMS-40 NiMo/CMS

755 811 824 849 858 865 492 494 496 554 549 333

0.76 0.84 0.85 0.89 0.91 0.92 0.52 0.52 0.52 0.57 0.57 0.36

4.6 5.0 5.1 5.1 5.1 5.3 4.3 4.5 4.5 4.5 4.6 3.7

maintained well after loading active metals. The pore size distributions of Al-CMS-x also reflect the uniformity of mesopores and show slight decreases in pore sizes with the increasing incorporation of aluminum as displayed in Figure 8B, which are in accordance with the aforementioned TEM and small-angle XRD results. Furthermore, the results in Table 1 showed pure CMS possessed a high surface area that can reach up to 865 m2/g, and the SBET, Vt, and Dp suffer decreased tendencies with the Si/Al F


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Figure 9. Pyridine IR spectra: (a) NiMo/Al-CMS-5; (b) NiMo/Al-CMS-10; (c) NiMo/Al-CMS-20; (d) NiMo/Al-CMS-30; (e) NiMo/Al-CMS-40; and (f) NiMo/CMS.

Table 2. Acid Site Amounts Calculated from Py-FTIR 200 °C/μmol g−1

350 °C/μmol g−1

catalysts

B

L

B+L

B

L

B+L

NiMo/Al-CMS-5 NiMo/Al-CMS-10 NiMo/Al-CMS-20 NiMo/Al-CMS-30 NiMo/Al-CMS-40 NiMo/CMS

15.16 24.24 11.94 10.34 10.39 2.10

60.90 75.31 67.02 61.11 44.37 30.44

76.06 99.55 78.96 71.45 54.76 32.54

2.89 3.10 3.16 1.66 1.71 0

25.67 26.83 25.56 23.64 8.34 6.13

28.56 29.93 28.72 25.30 10.05 6.13

Figure 10. Raman spectra of oxided catalysts (A) and sulfided catalysts (B): (a) NiMo/Al-CMS-5; (b) NiMo/Al-CMS-10; (c) NiMo/Al-CMS-20; (d) NiMo/Al-CMS-30; (e) NiMo/Al-CMS-40; and (f) NiMo/CMS.

1610 and 1450 cm−1 belong to L acid sites, and the peaks at 1640 and 1540 are ascribed to the adsorption of pyridine molecules on protonated B acid sites; however, the band at around 1490 is attributed to the integration of B and L acid sites. After degassing at 350 °C, the spectra still exhibit similar adsorption bands with weaker intensities, indicating the catalysts have medium-strong acid sites. Additionally, the data in Table 2 reveal that the total acid amounts increase significantly after the Al incorporation and present rising tendency along with the increase of Al doping contents, and the total acid amounts increase from 32.54 μmol g−1 for NiMo/CMS to 99.55 μmol g−1 for NiMo/Al-CMS-10. Among all of the catalysts, NiMo/Al-CMS-10 possesses the highest total acid and middle acid amounts of 99.55 μmol g−1 (200 °C) and 29.13 μmol g−1 (350 °C), respectively. When the Si/Al ratio attains 5, the acid amounts decrease instead, because the excessive Al species aggregate on the surface, and thus Si4+ ions cannot be substituted by Al3+ sufficiently.37 In general, the preferable B and L acidities of NiMo/Al-CMS-x are expected to

improve the hydrogenation and isomerization ability for reactant DBT. 3.7. Raman Analysis of Catalysts. To study the state of active species on the series oxided and sulfided catalysts, Raman spectra were adopted and shown in Figure 10. The spectra in Figure 10A appear as three peaks at about 240, 710, and 960 cm−1 for the oxided catalysts. According to the literature,38 the broad band at 240 cm−1 is supposed to have arisen from the vibration deformation of Mo−O−Mo bands, and the weak peak at 710 cm−1 could be originated from the symmetrical stretching vibration of the α-NiMoO4 crystalline phase. Moreover, the peaks at 948 and 960 cm−1 belong to the MoO stretch vibrations in the Mo7O246− species, which indicate good distributions of Mo oxides on account of the weak metal− support interaction; in turn, the type-II active phases with high activity are easy to be formed during the sulfidation process. Additionally, no peaks appeared within the scope of 990−1000 cm−1, manifesting the absence of bulk Mo oxide species.39 G


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Figure 11. HRTEM images: (a) NiMo/Al-CMS-5; (b) NiMo/Al-CMS-10; (c) NiMo/Al-CMS-20; (d) NiMo/Al-CMS-30; (e) NiMo/Al-CMS-40; and (f) NiMo/CMS.

coordinated by sulfur atoms and Ni locates on the edge of the MoS2 layers. Therefore, HRTEM was performed to explore the morphology and dispersion of MoS2 slabs, which are significant for the study on catalytic activity. Figure 11 exhibits the typical images of the corresponding sulfided catalysts. It is found that MoS2 particles have a good dispersion on the Al modified supports, while the agglomerated and large MoS2 particles emerge in the sulfided NiMo/CMS. The average length and stacking layers are calculated statistically by the following equation on the basis of about 300 MoS2 slabs:41

Figure 10B displays that all of the sulfided catalysts show two bands located at 380 and 405 cm−1, which should be assigned to the 1E2g and A1g crystalline structures of molybdenum sulfide.40 It is notable that the catalyst NiMo/Al-CMS-10 exhibits the stronger peak intensity, suggesting the active phases possess the highest sulfidation degree after the presulfurization. 3.8. HRTEM of the Sulfided Catalysts. The oxided catalysts need to be presulfided to transform into the sulfide phases of metal species MoS2 with high HDS activity by using CS2 in cyclohexane at 340 °C in a fix-bed reactor. After the presulfidation procedure, the sulfided catalysts could be obtained, which were perceived to be active for HDS. In generally, the active sites for HDS reaction are usually considered to be the NiMoS active phases, in which Mo is

n ij n zy Lav (Nav) = jjjj∑ xiMi zzzz/∑ xi j i=1 z i=1 k {

H


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3.10. HDS Catalytic Activity over Sulfided Catalysts. HDS catalytic behaviors of the series catalysts were assessed with DBT as the model compound under various WHSVs of 20−150 h−1 as displayed in Figure 13. It is obvious that the HDS activities for all catalysts increase along with a reduction in WHSV values. Moreover, the overall trend of the catalytic performance for the series catalysts reveals that the catalysts NiMo/Al-CMS-x are much more active than NiMo/CMS, and the activity of NiMo/Al-CMS-x presents an increased tendency with NiMo/Al-CMS-40 < NiMo/Al-CMS-5 < NiMo/Al-CMS30 < NiMo/Al-CMS-20 < NiMo/Al-CMS-10. The catalyst NiMo/Al-CMS-10 presents the highest HDS activity of 93.5% at WHSV = 20 h−1 and 21.2% at WHSV = 150 h−1, which is about 3 times more active than NiMo/CMS at high WHSV. These results indicate that the Al modification for the materials significantly enhances the HDS activity of catalysts. To further investigate the reaction process of DBT HDS, GC−MS methods were utilized to analyze the products at the DBT conversion of about 50%. The GC−MS chromatograph of products of HDS over the representative catalyst NiMo/AlCMS-10 is presented in Figure S5. It has been well accepted that there are two main pathways for the reaction of DBT HDS to proceed.26 One is the direct desulfurization (DDS) route resulting in the generation of biphenyl (BP) as the primary product, in which a minute part of BP is further converted to cyclohexylbenzene (CHB). Another is the hydrogenation (HYD) route, in which DBT is prehydrogenated to generate product tetrahydrodibenzothiophene (THDBT), then followed by eliminating the sulfur atom to cyclohexenylbenzene (CHEB) with CHB as the principal product.44 Furthermore, the hydrogenation reaction occurs at CHB and makes it convert to dicyclohexyl (DCH). Besides, it is noted that such products involving cyclopentylmethylcyclohexane (CPMCH), isophenyl hexadiene (PHDi), cyclopentylmethylbenzene (CPMB) and (3methylcyclopentyl)-benzene (MECB) were also observed, which may be obtained from the hydrogenation−isomerization process promoted by the acidity of the catalyst. According to the foregoing analysis of the GC−MS results, a probable DBT HDS reaction network over NiMo/Al-CMS-x is proposed and exhibited in Figure 14. The corresponding results of the product distributions are given in Table 5. The HDS/DDS ratio is used to represent the proportion of the two reaction routes, which is counted on the basis of the selectivity of product CHB to BP at the condition of about 50% DBT HDS conversion. By analyzing the results in Table 5, noteworthy is that the HDS/DDS ratio is far below 1 for all of the catalysts, which shows a decreased tendency with the addition of more aluminum (up to Si/Al = 10), revealing the priority to generate BP through the DDS pathway. These results are closely related to the acid properties obtained from Py-IR. By comparing all of the catalysts, NiMo/Al-CMS-10 presents the highest HDS rate constant kHDS (9.80 × 10−4 mol g−1 h−1) and TOF (2.48 h−1), which are 2.8 times (kHDS) and 1.5 times (TOF) higher than those of the NiMo/CMS catalyst, respectively. In addition, the orders of the kHDS and TOF for different catalysts are the same as those of the HDS activity in Figure 13, illustrating that the modification of Al to CMS improves the DDS ability of the catalysts, and thereby results in the high HDS rate.

where Mi represents the slab length or layer stacking number, and xi indicates the number of slab lengths or stacking number in a certain value. The proportion of edge Mo atoms in MoS2 slabs is expressed as f Mo, which is determined by the formula: fMo =

Moedge Mototal

t

=

ni =

∑i = 1 (6ni − 6) t

∑i = 1 (3ni 2 − 3ni + 1) L + 0.5 0.64

in which ni represents the Mo atoms number at the edge of MoS2 crystallite, and t indicates the total number of MoS2 slab. The obtained Lav, Nav, and f Mo values are listed in Table 3. The Table 3. Average Slab Length (Lav) and Stacking Number (Nav) of MoS2 Crystallites catalysts

Lav (nm)

Nav

f Mo


4.0 3.2 3.7 3.9 4.6 5.0

3.0 3.1 3.3 3.4 3.7 3.9

0.28 0.33 0.29 0.29 0.27 0.20

sulfided catalyst NiMo/Al-CMS-10 is found to display a short average slab length of 3.2 nm and a low stacking number of 3.1 as compared to other catalysts, while the catalyst NiMo/CMS shows higher stacking number (3.9) and overlong MoS2 slabs (5.0 nm). Furthermore, in terms of the MoS2 dispersion, f Mo values present the sequence of NiMo/CMS (0.20) < NiMo/AlCMS-40 (0.27) < NiMo/Al-CMS-5 (0.28) < NiMo/Al-CMS30 (0.29) ≈ NiMo/Al-CMS-20 (0.29) < NiMo/Al-CMS-10 (0.33), indicating that the incorporated aluminum species definitely improve the dispersion state of active phases. It has been accepted that the shorter slab length and suitable stacking number would be beneficial to expose more active edge sites and form type-II active phases for the Mo active species;42 in turn, the high stacking number and long slab length would lead to exposing less active sites. Thus, the short slab length, appropriate stacking layer, and better MoS2 dispersion of NiMo/Al-CMS-10 would result in its superior activity. 3.9. XPS of the Sulfided Catalysts. Figure 12 presented the Mo 3d XPS spectra so as to analyze the Mo species states. The binding energies located at about 229.0 and 232.0 eV are attributed to Mo 3d5/2 and Mo 3d3/2 standards of Mo4+ species (MoS2), respectively. Those for Mo5+ species (MoOxSy) are located at around 230.5 and 233.8 eV, and those for Mo6+ species (MoO3) are about at 232.8 and 236.0 eV.43 Besides, the peak attributed to the S 2s level is located at around 226.0 eV. SMo is introduced to represent the sulfidation degree and expressed by SMo = Mo4+/(Mo4+ + Mo5+ + Mo6+). The calculated results are displayed in Table 4 and show that the SMo of the catalysts changes with the sequence of NiMo/Al-CMS-10 (74.6%) > NiMo/Al-CMS-20 (69.9%) > NiMo/Al-CMS-30 (67.9%) > NiMo/Al-CMS-5 (65.5%) > NiMo/Al-CMS-40 (64.8%) > NiMo/CMS (51.8%), in accordance with the results of Raman. Notable is that the sulfidation degrees of NiMo/AlCMS catalysts are enhanced after Al modification, indicating the incorporated Al species improve the metal−support interaction and active metals’ dispersion, which is in keeping with the above HRTEM results.

4. DISCUSSION Generally, the differences in catalytic performance of the series catalysts NiMo/Al-CMS-x for HDS of DBT derive from I


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Figure 12. Mo 3d XPS spectra: (a) NiMo/Al-CMS-5; (b) NiMo/Al-CMS-10; (c) NiMo/Al-CMS-20; (d) NiMo/Al-CMS-30; (e) NiMo/Al-CMS40; and (f) NiMo/CMS.

Table 4. Calculated Results Obtained from Mo 3d Spectra Mo4+

Mo5+

Mo6+

catalysts

ar. % (229.0 eV)

ar. % (232.0 eV)

ar. % (230.5 eV)

ar. % (233.8 eV)

ar. % (232.8 eV)

ar. % (236.0 eV)

SMo


38.9 46.0 44.7 42.8 38.0 30.2

26.6 28.6 25.2 25.1 26.8 21.6

9.9 6.6 9.7 1.7 1.5 1.2

12.9 12.1 8.5 20.3 15.4 16.8

4.7 4.8 9.6 5.7 9.1 10.7

7.0 1.9 2.3 4.4 9.2 19.5

65.5 74.6 69.9 67.9 64.8 51.8

the HDS performance of DBT. High specific surface area can accommodate more active metals with good dispersion, and thus could improve the accessibility of the DBT molecules to active sites. Also, the large pore size can efficiently reduce the resistance of mass transfer and diffusion of large reactant molecules. In

comprehensive influence factors such as the textural properties, the acidity, the sulfidation of active phase, and MoS 2 morphology. First, the porous structure characteristics of the support, involving surface area, pore volume, and pore size, are crucial for J


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Second, the acidic natures of the catalysts also act as a crucial part for enhancing HDS activity. The increased amounts of total acid sites and stronger B and L acid sites of catalysts NiMo/AlCMS improved hydrogenolysis/isomerization and hydrogenation ability. It is found that there is a good association between the HDS performance and the acidity of catalysts as proved in Figure 13 and Table 5. The results in Table 5 reveal that the HYD/DDS ratios decrease from 0.38 to 0.22 with the addition of Al species, confirming that the increased amounts of acid sites can facilitate both the activities of DDS and HYD, particularly for the DDS route.45 Moreover, the reaction products of the catalysts contain the component of CPMCH, PHDi, and CPMB, which derive from the products of the isomerization pathway. Also, the catalyst NiMo/Al-CMS-10 with the highest amounts of total acid (99.55 μmol g−1) and B acid (24.24 μmol g−1) sites present the preferential DDS and isomerization products, suggesting that the enhanced B acidity benefits the isomerization pathway. Third, the high sulfidation degree would promote the formation of active phases with high catalytic activity. From the results in Raman and XPS, the sulfidation degrees of the series catalysts increase with increasing aluminum content and reach the highest value of 74.6% at Si/Al = 10. This is due to the enhanced metal−support interaction after grating Al species into CMS support. In consequence, the higher sulfidation degree of active metal oxides may contribute to exposing more active sites (Ni−Mo−S) after presulfurization treatment. Finally, the MoS2 slabs, as the main part of the active phase, have a significant effect on the reaction process, and its stacking number and slab length can affect the adsorption and reaction pathway for DBT reactants, thereby influencing the HDS activity. The commercial supports Al2O3 generally make the active species form a type-I active phase with a low stacking degree due to the strong interaction between metal species and

Figure 13. HDS ratios of DBT: (a) NiMo/Al-CMS-5; (b) NiMo/AlCMS-10; (c) NiMo/Al-CMS-20; (d) NiMo/Al-CMS-30; (e) NiMo/ Al-CMS-40; and (f) NiMo/CMS.

addition, the 3D cubic cage-like interconnected mesostructure of the material also promotes the mass transfer and diffusion of reagents and products during the reaction process.24 In this work, the CMS material with 3D Pm3n mesostructure possesses high SBET (865 m2/g), and large Vt (0.92 cm3/g) and Dp (5.3 nm); in particular, the favorable pore structure parameters are still retained after incorporating Al species and loading NiMo active metals on the basis of the textural properties as displayed in Table 1, which provide suitable surface and space for the good dispersion of active metals as confirmed by Figure S2. Furthermore, the element mappings of NiMo/Al-CMS-10 in Figure S3 and Figure 7b intuitively indicate the homogeneous distributions of Ni and Mo oxides over the Al-CMS support. Finally, the HDS efficiencies show a direct relationship with the structural characteristics, which confirm the contribution of good Ni and Mo dispersions to the high catalytic activity.

Figure 14. Proposed reaction network of HDS over catalysts NiMo/Al-CMS-x. K


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Industrial & Engineering Chemistry Research Table 5. Rate Constants (kHDS), TOF, HYD/DDS Ratio, and Product Selectivity over Different Catalysts product selectivity (%)

NiMo/Al-CMS-5

NiMo/Al-CMS-10

NiMo/Al-CMS-20

NiMo/Al-CMS-30

NiMo/Al-CMS-40

NiMo/CMS

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

62.5 14.2 8.7 4.2 0.7 6.0 0.4 2.8 0.5 6.66 2.21 0.23

65.2 14.1 5.7 4.5 0.2 6.2 0.5 3.0 0.6 9.80 2.48 0.22

66.0 16.4 6.7 3.9 0.4 4.1 0.2 1.7 0.6 7.54 2.34 0.25

64.1 16.9 9.1 4.1 0.1 5.2 0 0 0.5 6.45 2.09 0.26

67.4 19.9 4.7 3.1 0.1 3.5 0 0.9 0.4 5.44 1.96 0.30

64.7 24.8 3.5 3.0 0.7 1.3 0.9 0.3 0.8 3.39 1.65 0.38

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support, which helps with the planar adsorption of the sulfurcontained molecules, but is not easy to access the active sites for the large molecules through planar adsorption and thus decreases the catalytic activity.44 A suitable stacking number can increase the edge and corner sites and enhance the utilization efficiency of the internal metals, which are indispensable to eliminating sulfur via the perpendicular adsorption of reactants through S atoms, thus facilitating the DDS process.42 Consequently, the silica-based supported catalysts with Al modification display moderate stacking degree by modulating metal−support interaction due to Al incorporation, which benefits to form type-II active phases in high activity. As compared to NiMo/CMS, the NiMo/Al-CMS catalysts modified by aluminum possess less stacked MoS2 slabs with around 3−4 layers as demonstrated by HRTEM results in Table 3. The NiMo/Al-CMS-10 catalyst displays the best dispersion degree (f Mo = 0.33) of the active species with the appropriate MoS2 slab length of 3.2 nm and stacking number of 3.1, which therefore displays the highest intrinsic activity for HDS (93.5%).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01094. Al 2p XPS spectra (Figure S1); wide-angle XRD (Figure S2); SEM image and EDS mapping (Figure S3); N2 adsorption−desorption (Figure S4); and GC−MS chromatograph (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yanjun Gong: 0000-0002-8801-3337 Aijun Duan: 0000-0001-5964-7544 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (nos. 21878330, 21676298, 21503152, and U1463207), the CNPC Key Research Project, and KLGCP (GCP201401).

5. CONCLUSION A facile and simple modified Stöber method was introduced to fabricate highly ordered cubic mesoporous silica. The series cubic mesoporous silico-aluminate materials with various aluminum contents were obtained through the Al grafting method. The differences in physicochemical properties and catalytic activities of catalysts caused by various Si/Al ratio were systematically studied. The obtained Al-CMS materials presented highly ordered 3D cubic mesostructure and were rich in B/L acid sites. Additionally, the suitable metal−support interaction for NiMo/Al-CMS catalysts facilitated the good metal dispersion and thus led to a high sulfidation degree (74.6%). The presented HDS efficiencies over the series NiMo catalysts showed good correlations with the textural characters, acid nature, and the morphologies of active phases, among which NiMo/Al-CMS-10 catalyst achieved the highest HDS activities (93.5%) at WHSV = 20 h−1, and possessed the maximum TOF (2.48 h−1) and kHDS (9.80 × 10−4 mol g−1 h−1). The good performance of NiMo/Al-CMS-10 catalyst could be derived from the superior structure nature (SBET = 865 m2/g, Vt = 0.92 cm3/g), well-dispersed active species, moderate acidity, and the highly sulfided MoS2 slabs with the appropriate stacking number (3.1) and slab length (3.2 nm). The simple and efficient synthesis method of ordered mesoporous silica might be one of the potential candidates for designing excellent HDS catalysts.

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