Controllable synthesis of spherical Al-SBA-16 mesoporous materials

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Controllable synthesis of spherical Al-SBA-16 mesoporous materials with different crystal sizes and its high isomerization performance for hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene Xilong Wang, Jinlin Mei, Zhen Zhao, Zhentao Chen, Peng Zheng, Jianye Fu, Haidong Li, Jiyuan Fan, Aijun Duan, and Chunming Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00109 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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Industrial & Engineering Chemistry Research

Controllable synthesis of spherical Al-SBA-16 mesoporous materials

with

different

crystal

sizes

and

its

high

isomerization performance for hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

Xilong Wang, Jinlin Mei, Zhen Zhao, Zhentao Chen, Peng Zheng, Jianye Fu, Haidong Li, Jiyuan Fan, Aijun Duan *, Chunming Xu *

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 18 Fuxue Road, Beijing, P.R. China, 102249

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

1

ACS Paragon Plus Environment


For Table of Contents Only

Decreased crystal size 6

4

TOF, h-1

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2 9

8 7 6 5 Crystal size of Al-SBA-16, µm

4

Abstract Series Al-SBA-16 supports with different crystal sizes were fabricated by adding different inorganic salts to the synthesis system and used as the catalyst support for dibenzothiophene

(DBT)

and

4,6-dimethyldibenzothiophene

(4,6-DMDBT)

hydrodesulfurization (HDS). The supports and the corresponding catalysts were well analyzed in depth to investigate their structure-property relationship. NiMo/AS-Mn catalyst with the smallest crystal sizes, the largest BET surface areas and the highest total acid & B acid contents displayed the highest kHDS of 4,6-DMDBT and DBT. HYD was the preferential route for the DBT HDS over NiMo/AS-Mn, while ISO was the preferential pathway for 4,6-DMDBT HDS. Moreover, the DBT and 4,6-DMDBT 2


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HDS reaction networks over NiMo/AS-Mn were proposed.

Keywords:

Al-SBA-16;

Crystal

size;

Inorganic

salt;

Hydrodesulfurization;

Dibenzothiophene; 4,6-dimethyldibenzothiophene.

1. INTRODUCTION For the past few years, oil and its fractions, diesel and gasoline are the main automotive fuel.

1

Air pollution caused by fuel combustion from gasoline or diesel

powered motor vehicles has attracted more and more attention around the globe because of high sulfur content in most fuels.

2-6

The stringent environmental

regulations are made to limit the sulfur content in fuels as low as 10 ppm.

7-9

Hydrodesulfurization (HDS) reaction is considered to be one of the most promising ways to eliminate sulfur from fuels. 10 Nevertheless, the ultra-low sulfur lower than 10 ppm is hard to be achieved using conventional catalysts due to the existence of highly refractory organosulfur compounds such as DBT and 4,6-DMDBT.

11-16

The steric

hindrance of alkyl groups at 4 and 6 positions of 4,6-DMDBT molecule inhibits the adsorption and diffusion on the HDS catalysts.

17

To realize the ultra-deep HDS,

many researches have been done in recent years. One of the most important topics is the design of HDS catalytic materials.

18

Alumina is considered as the typical support material in the industrial processes due to its low-cost, good properties and ease of synthesis.

19, 20

However, there are also

some drawbacks, mainly related to the presence of undesired metal-support

3


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interactions, which severely restricts the formation of NiMoS-II active phases.

21, 22

To overcome the aforementioned disadvantages of commercial support and realize the ultra-deep HDS, different supports like MCM-41 SBA-15

23, 24

, HMS

25, 26

, KIT-6

27, 28

and

29, 30

, have been used in HDS reactions. However, these one- or

two-dimensional supports cannot provide more space for reactant and product molecules to diffuse in and out.

31

Therefore, the application of three-dimensional

spherical supports will be required because the reactants tend to diffuse better in the HDS reaction. 32 SBA-16

is

a

mesoporous

cage-like

material

with

three-dimensional

body-centered-cubic structure corresponding to Im3m space group.

33

In this

three-dimensional cubic pore structure, each nanocage is interconnected to its eight neighboring cages to form a multidirectional system of mesoporous network.

34

Several researches have been studied to understand the HDS performance of SBA-16 supported catalysts. Klimova et al.

35

synthesized a novel NiMo/Al-SBA-16 catalyst,

and the catalytic performance evaluation results indicated that NiMo/Al-SBA-16 using AlCl3 as aluminum source showed a high 4,6-DMDBT HDS activity. Guzmán et al.

36

prepared a ternary NiMoW HDS catalyst supported on P-loaded SBA-16

material, and the HDS evaluation results displayed that NiMoW/SBA-16 loaded with an optimized phosphorous amount showed better DBT and 4,6-DMDBT HDS activities than P-free counterpart. Cao et al.

34

synthesized series NiMo/Al-SBA-16

catalysts with different morphologies and the evaluation results suggested that NiMo/Al-SBA-16 aging at 50 ºC exhibited highest DBT HDS performance. From the 4


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above results, it can be seen that the pure SBA-16 have no Lewis (L) and Brönsted (B) acid, which are not in favor of the improvement of the HDS performance. 37 Therefore, the introduction of heteroatom into the framework of pure SBA-16 material extensively improves the HDS catalytic activity. 38 Synthesis conditions such as acidity, aging temperature, surfactant type and inorganic salt have great influence on the morphology of SBA-16.

39

The addition of

inorganic salts like K2SO4, KCl, Na2SO4 and NaCl can accelerate the formation rate in acidic nonionic surfactant systems.

40-42

Yu et al.

41, 42

suggested that mesoporous

SBA-15 material can be obtained even at low concentration of triblock copolymer P123 and low temperature. However, the effects of inorganic salt on crystal size evolution of SBA-16 material, especially on HDS catalytic reaction, have not yet been found. In this research, the effect of inorganic salt (KCl, MgCl2, MnCl2, ZnCl2 and CuCl2) on the modulation of the crystal sizes of SBA-16 spheres was studied. And the synthetic methods of Al-containing SBA-16 were optimized using three different aluminum salts as aluminum source. The NiMo/Al-SBA-16 catalysts were synthesized for the DBT and 4,6-DMDBT HDS evaluation. The influence of different inorganic salts addition on DBT and 4,6-DMDBT HDS performances was investigated systematically. The NiMo/Al-SBA-16 (NiMo/AS-Mn) prepared with MnCl2 addition showed the higher desulfurization degree of DBT and 4,6-DMDBT HDS. HYD was the preferential route of DBT HDS reaction due to the higher B and L acid strength of NiMo/AS-Mn catalyst. While ISO was the preferential route of 5



4,6-DMDBT HDS because of the higher B acid strength of NiMo/AS-Mn catalyst. Moreover, the DBT and 4,6-DMDBT HDS reaction networks involving the intermediate products over NiMo/AS catalysts were proposed.

2. EXPERIMENTAL SECTION 2.1 Synthesis of the supports Silicas SBA-16 were synthesized using tetraethylorthosilicate (TEOS) as the silicon source and nonionic block copolymer EO106PO70EO106 (F127) as the surfactant. The synthetic process for mesoporous SBA-16 materials was carried out as follows: F127 (2.0 g) and different inorganic salts of KCl, MgCl2·6H2O, MnCl2·4H2O, ZnCl2 and CuCl2·2H2O (5.0, 13.62, 13.26, 9.13 and 11.42 g) were dissolved in a 2.0 mol/L HCl (120 mL), and the mixtures were stirred continuously at 35 ºC for 2.0 h. The cosurfactant n-butanol (6.0 g) was added dropwise into the above mixed solutions and stirred continuously at 35 ºC for 2.0 h. Then the silicon source TEOS (28.4 wt.% SiO2, 16.6 g) was added dropwise into the mixtures. The above mixtures were stirred at 35 ºC for 10 min and placed statically at 35 ºC for 24 h. Then the above mixtures were moved to autoclaves and maintained at 100 ºC for 24 h to obtain the series pure SBA-16 precursors. Subsequently, the series SBA-16 precursors with different inorganic salts were obtained through filtration, dried at 90 ºC for 12 h. Finally, all precursors were calcined at 550 °C for 6 h. Series SBA-16 supports prepared by different salts (KCl, MgCl2, MnCl2, ZnCl2 and CuCl2) addition were labeled as S-x, in

6


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which x represent inorganic cations K, Mg, Mn, Zn and Cu, respectively. Meanwhile, the commercial Al2O3 material was taken as the reference material. Al-containing SBA-16 (Si/Al molar ratio = 20) were synthesized by post alumination syntheticism using series pure mesoporous silica SBA-16 as precursor materials. S-K was used to optimize the superior aluminum source to inset aluminum atom into the framework of silica SBA-16. Three different kinds of aluminum sources, aluminum isopropoxide, AlCl3 and NaAlO2 were applied to the different ways to incorporate aluminum. The post-synthetic aluminum process for AS was obtained as listed below: the required amount of aluminum sources, aluminum isopropoxide (0.26 g), AlCl3 (0.17 g) and NaAlO2 (0.10 g), were dissolved in different solvents, isopropanol (75 mL), dry ethanol (75 mL) and water (75 mL), respectively. After the aluminum sources dissolved, SBA-16 (1.5 g) was dispersed into the solutions. The mixtures were stirred vigorously at 35 ºC for 12 h. The solid materials were obtained through filtration, dried at 90 ºC for 12 h. Finally, all the materials were calcined at 550 °C for 6 h to obtain the AS supports. The AS supports synthesized with aluminum isopropoxide, AlCl3 and NaAlO2 were denoted as AS-IP, AS-CL and AS-NA, respectively. Besides, S-K, S-Mg, S-Mn, S-Zn and S-Cu materials were all modified with AlCl3 and denoted as AS-K, AS-Mg, AS-Mn, AS-Zn and AS-Cu, respectively.

2.2 Preparation of the corresponding catalysts The preparation method of NiMo/AS series catalysts were described in the previous research. 13 Finally, the obtained NiMo series catalysts were all crushed into 7



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40-60 mesh. The loadings of NiO and MoO3 were 3.5 wt.% and 15 wt.%, respectively, and kept the same on all catalysts. The as-synthesized series catalysts were denoted as NiMo/AS-K,

NiMo/AS-Mg,

NiMo/AS-Mn,

NiMo/AS-Zn,

NiMo/AS-Cu and

NiMo/Al2O3, respectively.

2.3 Characterization of the supports and the catalysts The

characterization

methods

of

X-ray

diffraction

(XRD),

Nitrogen

adsorption-desorption isotherms, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Pyridine-FTIR, Raman spectra, X-ray photoelectron spectra (XPS), High-resolution transmission electron microscopy (HRTEM) were described in the previous research.

43 27

Al nuclear magnetic resonance (27Al NMR)

analysis of Al-SBA-16 series materials was determined on a Bruker Avance III 500 MHz instrument.

2.4 HDS evaluation of catalysts The HDS feedstocks used in this research were liquid solutions of DBT dissolved in cyclohexane (sulfur contents = 500 ppm) or 4,6-DMDBT dissolved in toluene (sulfur contents = 500 ppm). The catalytic activities of the series NiMo supported catalysts were estimated in a fixed bed reactor. Before the HDS reactions, all catalysts should be presulfided using 2.5 wt.% CS2-cyclohexane and H2 mixture at 340 °C, 4.0 MPa, 8 h-1, 600 mL/mL for 4 h. After the presulfidation, the HDS evaluation was carried out at 340 °C, 4.0 MPa, 200 mL/mL and WHSVs in the range 8


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of 10 to 100 h-1. The desulfurization degree (%) of DBT or 4,6-DMDBT HDS are expressed by: HDS (%) = (Sf -Sp)/Sf ×100%

(a)

where Sp and Sf in the equation denote the sulfur content (ppm) of the feeds and the products. The reaction rate constant (kHDS) of DBT or 4,6-DMDBT over series catalysts were calculated according to Equation (b): 44

k HDS =

F  1  ln   m 1− τ 

(b)

where kHDS in the equation denotes DBT or 4,6-DMDBT HDS rate constant (mol·g-1·h-1), m mass of the catalysts (g), F flow rate of the feeds (mol·h-1), and τ total desulfurization degree of DBT or 4,6-DMDBT HDS. The HDS product compositions were analyzed by a Thermo-Finnigan Trace DSQ gas chromatograph-mass spectrometer (GC-MS).

3. RESULTS 3.1 Characterization of Al-SBA-16 materials with various aluminum sources 3.1.1 XRD of Al-SBA-16 series materials The small-angle XRD of Al-SBA-16 modified by using various aluminum sources are displayed in Figure S1 (in Supporting Information). All the as-synthesized materials exhibit well-resolved characteristic peaks at the region of 2θ = 0.8-1.5°, revealing that all the materials possess the typical three dimensional cubic (3D, Im3m) symmetries and well-ordered mesoporous structures of pure SBA-16 material. 9



45

Compared with pure SBA-16 material, the small-angle XRD diffraction peaks of

AS materials exhibit a shift to the wider angle which can be ascribed to the shrinking length of the unit cell 46, demonstrating that the length of the unit cell decreases with the incorporation of the aluminum atoms into the framework of SBA-16.

3.1.2 N2 physisorption of the Al-SBA-16 series materials The physico-sorption isotherms, pore size distribution of pure S-K and AS materials are exhibited in Figure S2A and S2B (in Supporting Information), respectively. The materials of AS-IP, AS-CL, AS-NA, and pure S-K possess type-IV isotherms, which are the typical characteristics of the ordered SBA-16 mesoporous structure. Compared with the pure S-K material, the isotherms of AS-IP, AS-CL and AS-NA materials show a slight decrease in the N2 adsorption capacity. The isotherms of AS-NA material exhibit a vertical shift in the P/P0 range from 0.70 to 0.80, which is attributed to the partially plugged mesopores. 47 The physico-chemical properties of the as-synthesized pure SBA-16 material and the series AS are summarized in Table S1 (in Supporting Information). The BET specific surface areas of all the series materials change in sequence of AS-NA (355 m2·g-1) < AS-IP (685 m2·g-1) < AS-CL (724 m2·g-1) < S-AK (842 m2·g-1).

3.1.3 27Al MAS NMR of Al-SBA-16 series materials 27

Al MAS NMR was recorded to study the coordination information of Al atoms

of the AS modified by different aluminum sources. The

27

Al MAS NMR spectra of

AS are shown in Figure S3 (in Supporting Information). Three signals centered at 54, 10


Page 10 of 43

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30 and 0 ppm are ascribed to the tetrahedral (framework aluminum species), pentahedral and octahedral coordinated aluminum species, respectively.

48

The

proportions of framework and extra-framework aluminum species in the AS materials modified using different aluminum sources follow the order of AS-NA > AS-CL > AS-IP. Among these three Al-SBA-16 materials modified by different aluminum sources, AS-CL material modified with AlCl3 can preserve the pore structure of SBA-16. Moreover, AS-CL material possesses more framework aluminum species. Therefore, chemical modification with AlCl3 in the framework of SBA-16 is the optimal post-synthetic route of the synthesis of Al-SBA-16.

3.2 Characterization of Al-SBA-16 supports obtained by the addition of different inorganic salts 3.2.1 XRD of the AS series supports The small-angle XRD patterns of AS series supports synthesized by using different inorganic salts are shown in Figure 1. All the as-synthesized series AS supports exhibit similar characteristic peaks at the region of 2θ = 0.8-1.5°, suggesting that the series AS supports all possess the typical three dimensional cubic (3D, Im3m) symmetries and well-ordered mesoporous structure of SBA-16. 47

11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity, a.u.


a b c d e 1

2

3 4 2θ, degree

5

Figure 1. Small angle XRD of AS series supports. (a) AS-K, (b) AS-Mg, (c) AS-Mn, (d) AS-Zn and (e) AS-Cu.

3.2.2 SEM images of the AS series supports with different crystal sizes Figure 2a-2e show the changes in crystal diameters and morphologies of the series AS supports due to the addition of different inorganic salts. The series supports show the uniform spherical aggregates with the sizes ranging from 3.0 to 10.0 µm. And the crystal diameters of the uniform spherical aggregates decrease in the following order: AS-Mn (3.0-4.8 µm) < AS-K (4.6-5.7 µm) < AS-Mg (5.7-6.8 µm) < AS-Zn (6.8-7.9 µm) < AS-Cu (8.6-9.2 µm).

12


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(a)

(b)

10 µm

10 µm (d)

(c)

10 µm

10 µm

(e)

10 µm Figure 2. SEM images of the series AS supports. (a) AS-K, (b) AS-Mg, (c) AS-Mn, (d) AS-Zn and (e) AS-Cu.

3.2.3 N2 physisorption of the AS series supports

13


(A) a b c d e 0.0

Page 14 of 43

dV/dD, cm3·g-1·nm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume, m3·g-1


0.2 0.4 0.6 0.8 1.0 Prelative Pressure, P/P0

(B) a b c d e 0

10 20 30 40 Pore diameter, nm

50

Figure 3. (A) N2 physisorption isotherms and (B) pore size distribution patterns of AS series supports. (a) AS-K, (b) AS-Mg, (c) AS-Mn, (d) AS-Zn and (e) AS-Cu.

The physisorption isotherms and pore size distribution of AS series supports are displayed in Figure 3A and 3B, respectively. The series AS supports all possess type-IV isotherms which can be ascribed to the characteristics of the ordered SBA-16 mesoporous structure. Compared with other four supports, the isotherms of AS-Mn support show the higher N2 adsorption capacity. Table 1. Physico-chemical properties of AS series supports. Samples

SBET 2

Vmes

-1

3

dBJH -1

(m ·g )

(cm ·g )

(nm)

AS-K

724

0.55

4.1

AS-Mg

699

0.53

4.1

AS-Mn

817

0.67

4.2

AS-Zn

652

0.49

4.1

AS-Cu

594

0.43

4.0

The physico-chemical properties of AS series supports are summarized in Table 1. The specific surface area and pore volume of all the series AS supports decrease in the order of AS-Mn > AS-K > AS-Mg > AS-Zn > AS-Cu.

14


Page 15 of 43

3.2.4 TEM images of the AS series supports The TEM images of the AS series supports are revealed in Figure S4a-S4e (in Supporting Information). It is obvious that these AS series supports possess well-ordered arrays of mesoporous structure, demonstrating their 3D Im3m cubic structures of SBA-16, which are consistent with the above XRD results.

3.3 Characterization of the oxide and sulfide catalysts

(A)

L L B

1700

L

B

L+B

1600 1500 Wavenumber, cm-1

a b c d e

Absorbance, a.u.

3.3.1 Pyridine-FTIR of the oxide catalysts

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

1400 1700

L B L L

B

L+B

1600 1500 Wavenumber, cm-1

a b c d e 1400

Figure 4. FTIR spectra of pyridine adsorbed on different catalysts. (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn and (e) NiMo/AS-Cu after degassing at (A) 200 ºC and (B) 350 ºC.

The acid strength and types over series NiMo/AS are determined through Py-FTIR spectroscopy and the results are presented in Figure 4. It can be found from Figure 4 that the bands located at 1446, 1492, 1575 and 1622 cm-1 can be ascribed to L acid, while the bands at 1546 and 1639 cm-1 can be attributed to the B acid.

49, 50

The IR spectra adsorbed by pyridine (degas at 200 °C) are attributed to the total acid strength, while the IR spectra adsorbed by pyridine (degas at 350 °C) are ascribed to the medium and strong acid strength. The B & L acid strength decrease in the 15



sequence of NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu. The B acid strength follow the same order.

3.3.2 Raman of the oxide and sulfide catalysts The UV Raman spectra (325 nm) of the series oxide and sulfide catalysts are listed in Figure 5A and 5B, respectively. As observed in Figure 5A, three peaks centered at 826, 908 and 955 cm-1 are assigned to NiMoO4 phase.

51

The peak

centered at 331 cm-1 is ascribed to terminal Mo=O of octahedral MoO42- species, while the peak centered at 564 cm-1 is assigned to Al-O stretching mode.

52, 53

The

broad peak in the range of 890-1000cm-1 is ascribed to M=O stretching vibrations, indicating an easier performance of reducibility and sulfurization. 54 The intensities of the peaks at 890-1000 cm-1 decrease in the following order: NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu, suggesting that the performance of reducibility and sulfurization increase in the same order.

(A)

(B) a b c d e

200

a

Intensity, a.u.

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 43

400 600 800 1000 1200 Raman shift, cm-1

b c d e 400

600 800 Raman shift, cm-1

1000

Figure 5. Raman spectra of (A) the oxide catalysts and (B) the sulfide catalysts. (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn and (e) NiMo/AS-Cu.

16


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The UV Raman spectra of the sulfide NiMo/AS are listed in Figure 5B. The peaks centered at 380 and 405 cm-1 are assigned to the E12g and A1g of the MoS2 crystalline structures, while the peaks at 454 and 634 cm-1 are attributed to the resonance Raman scatting.

55, 56

The peak intensities decrease in the following order:

NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu, demonstrating that the sulfidation degree of the Mo active species changes in the same sequence.

3.3.3 XPS of the sulfide catalysts In order to investigate the changes in the sulfidation degrees, the chemical states of the Mo active species and the influence of the inorganic salts on the MoS2 active phases, the NiMo/AS series sulfide catalysts were recorded by XPS characterizations. The deconvolution results of the Mo3d XPS spectra are revealed in Figure 6 and Table S2 (in Supporting Information). The fitting standard for each Mo active phases is according to our previous work.

59

The sulfidation degree of series sulfide

NiMo/AS decrease in the following order: NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu, which is well in accordance with the Raman results of series sulfide NiMo/AS.

17


(Ⅵ Ⅵ)

Mo (3/2),(5/2)

Ⅴ) Mo(Ⅴ (3/2),(5/2) 2-

S

236 232 228 224 Binding energy, eV Ⅳ) Mo(Ⅳ (3/2),(5/2)

240

(Ⅵ Ⅵ)

Mo (3/2),(5/2)

240

S2-

Page 18 of 43

(b) Ⅴ) Mo(Ⅴ (3/2),(5/2) 2-

S

236 232 228 224 Binding energy, eV Ⅳ) Mo(Ⅳ (3/2),(5/2) (Ⅵ Ⅵ)

Mo (3/2),(5/2)

240

236 232 228 224 Binding energy, eV Ⅳ) Mo(Ⅳ (3/2),(5/2)

Mo (3/2),(5/2)

(c)

(Ⅵ Ⅵ)

Mo (3/2),(5/2)

(Ⅵ Ⅵ)

240

Intensity, a.u.

Intensity, a.u.

(Ⅳ Ⅳ)

Mo Ⅳ (3/2),(5/2)

(a) Intensity, a.u.

Ⅳ) Mo(Ⅳ (3/2),(5/2)

240

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity, a.u.


(d) Ⅴ) Mo(Ⅴ (3/2),(5/2)

S2-

236 232 228 224 Binding energy, eV

(e) Ⅴ) Mo(Ⅴ (3/2),(5/2)

S2-

236 232 228 224 Binding energy, eV

Figure 6. Mo3d XPS spectra of various sulfided catalysts. (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn and (e) NiMo/AS-Cu.

18


Page 19 of 43

Ni(II) Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiMoS NiSx

a b c d e

864 862 860 858 856 854 852 850 Binding energy, eV Figure 7. Ni2p XPS spectra of various sulfided catalysts. (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn and (e) NiMo/AS-Cu.

The Ni2p XPS results of series sulfide NiMo/AS are displayed in Figure 7 and Table S3 (in Supporting Information). The NiMoS ratios of the NiMo/AS series sulfide catalysts increase in the following order: NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu, which is well in accordance with the Mo3d XPS results of series sulfide NiMo/AS.

3.3.4 HRTEM of the sulfide catalysts

19



50 40 30 20 10 0

50 40 30 20 10 0

(B)

%

%

(A)

1 2 3 4 5 6 7 8 Number of layers

50 40 30 20 10 0

(D)

%

50 40 30 20 10 0

%

(C)


10 nm

(E)


10 nm

10 nm


10 nm 50 40 30 20 10 0

(F)

%

80 NiMo/AS-K

Fraction, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 43


NiMo/AS-Mg

60

NiMo/AS-Mn NiMo/AS-Zn

40

NiMo/AS-Cu

20 0 10 nm

0-20 20-40 40-60 60-80 >80 Slab length, 10-1 nm

Figure 8. HRTEM images of various sulfide catalysts. (A) NiMo/ZS-K, (B) NiMo/ZS-Mg, (C) NiMo/ZS-Mn, (D) NiMo/ZS-Zn, (E) NiMo/ZS-Cu; and (F) the distribution of lengths of the MoS2 particles dispersed on the sulfide catalysts. 20


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HRTEM of sulfide catalysts is considered to be very important for the characterization of morphologies of MoS2 active phases on the sulfide catalysts. The representative HRTEM images of series sulfide NiMo/AS and the layer number distribution MoS2 active phases are shown in Figure 8A-E, and the corresponding length distribution of MoS2 slabs are shown in Figure 8F. It can be observed that sulfide NiMo/AS-Mn shows appropriate stacking number (2-3 layers) and relatively short slab length (2-4 nm) of MoS2 phases. The average stacking number (Nav) and length (Lav) of MoS2 active phases are summarized in Table S4 (in Supporting Information). As observed in Table S4, the Nav and Lav values of MoS2 phases over series sulfide NiMo/AS decrease in the following sequence: NiMo/AS-Mn < NiMo/AS-K < NiMo/AS-Mg < NiMo/AS-Zn < NiMo/AS-Cu. It can be found that the NiMo/AS-Mn sulfide catalyst possesses appropriate stacking layers and short stacking lengths, in favor to create more available edge and corner sites, which promote the perpendicular adsorption through the sulfur atoms and then improve the intrinsic HDS reactivity. 58, 59

3.4 Results of the DBT HDS

21



100

Desulfurization degree, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 43

80 60 c a 40 b d f 20 e 100

80

60 40 WHSV, h-1

20

0

Figure 9. HDS results of DBT at different WHSVs (340 oC, 4 MPa, 200 mL/mL). (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn, (e) NiMo/AS-Cu and (f) NiMo/Al2O3.

A series of NiMo/AS with various crystal sizes were used to evaluate the catalytic activities for the DBT HDS reaction. The evaluation results are displayed in Figure 9. As observed, the desulfurization degree at all the WHSVs over the series NiMo/AS changes in the sequence of NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/Al2O3 > NiMo/AS-Cu. NiMo/AS-Mn shows the highest desulfurization degree of DBT HDS at all the WHSVs, of which at 100 h-1 its desulfurization degree of DBT HDS (49.5 %) is almost 1.5 times as that over the NiMo/Al2O3 (33.6 %), and more than two times as that over NiMo/AS-Cu (20.5 %). The highest DBT HDS performance of NiMo/AS-Mn is assigned to the synergistic effects of its small crtstal sizes of the AS-Mn support, relatively strong acidity, moderate stacking degree and high sulfidation degree of MoS2 active phases on the NiMo/AS-Mn sulfide catalyst. To study the influence of crystal sizes of the series AS supports on the reaction routes of DBT HDS, the products were analyzed using a GC-MS chromatograph at 22


Page 23 of 43

the similar desulfurization degree (50 %) of DBT HDS (Figure S7, in Supporting Information). The DBT HDS reaction network of series NiMo/AS with different crystal sizes is proposed in Figure S8 (in Supporting Information). For the DBT HDS reaction, direct desulfurization (DDS) and hydrogenation (HYD) are two parallel routes. In the DDS pathway, the S atom can be directly removed to form biphenyl (BP) as the final DDS product and no other products or intermediates will be formed. In the HYD route, DBT firstly hydrogenated to tetrahydrodibenzothiophene (THDBT), then either the S atom can be removed to produce cyclohexenylbenzene (CHEB) and cyclohexylbenzene (CHB) intermediate or further hydrogenated to dicyclohexyl (DCH).

60

Moreover,

some

HYD

products

were

isomerized

to

cyclopentylmethylcyclohexane (CPMCH), cyclopentylmethylbenzene (CPMB) and isophenyl hexadiene (PHDi) over the NiMo/AS series catalysts.

Selectivity, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 BP CHB

40 20

THDBT CPMCH CPMB PHDi

0 100

80

60 40 WHSV, h-1

20

0

Figure 10. DBT HDS product distributions over NiMo/AS-Mn at various WHSVs.

The DBT HDS product distributions (Figure S9, in Supporting Information) at different WHSVs over the NiMo/AS-Mn catalyst are presented in Figure 10. It can be found from Figure 10 that the BP selectivity increases while the CHB selectivity

23



decreases with the decrease of the WHSVs, so it can be concluded that the DDS reaction rate of the DBT HDS is promoted more than that of the HYD rate at low WHSV. For NiMo/AS-Mn, DDS is the preferential DBT HDS reaction route, reflected by the BP selectivity is more than 50 % when the WHSVs is less than 50 h-1. The selectivities of CPMCH, CPMB and PHDi increase with the decreasing WHSVs, while the selectivity of THDBT decrease with the decreasing WHSVs.

24


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Table 2. Catalytic performances for HDS of DBT over various catalysts.

Catalyst

Conversiona (%)

Product selectivity (%)b

kHDS （10-4 mol g-1 h-1）

THDBT

CHEB

PHDi

CHB

CPMB

CPMCH

BP

HYD/DDS ratio

HYD

DDS

NiMo/AS-K

49.8

14.7

4

1

2

38

8

4

43

1.33

NiMo/AS-Mg

49.9

12.1

2

1

2

35

6

3

51

0.96

NiMo/AS-Mn

50.2

16.4

1

1

2

41

13

6

36

1.77

NiMo/AS-Zn

50.3

8.6

1

1

2

33

5

2

56

0.79

NiMo/AS-Cu

49.8

6.9

1

1

2

31

4

2

59

0.69

NiMo/Al2O3

50.1

7.3

1

2

0

34

0

0

63

0.59

a

The DBT HDS conversion was obtained by changing WHSV (340 ºC, 4 MPa, 200 mL/mL).

b

Determined at about 50% of total DBT conversion by changing WHSV.

HYD: THDBT + CHEB + PHDi + CHB + CPMB + CPMCH; DDS: BP.

25



The DBT HDS product distributions over series NiMo/AS are summarized in Table 2. The HYD/DDS ratios for the series catalysts change in the sequence of NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu > NiMo/Al2O3. NiMo/AS-Mn exhibits the maximum HYD/DDS ratio of DBT HDS, which is ascribed to the relatively high acidity. The kHDS of the DBT of series NiMo/AS are also listed in Table 2. The kHDS changes in the following sequence: NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/Al2O3 > NiMo/AS-Cu. The kHDS of DBT over NiMo/AS-Mn catalyst is greater than 2.2 times as that over reference NiMo/Al2O3, and greater than 2.3 times as that over NiMo/AS-Cu.

26


Page 26 of 43

Page 27 of 43

3.5 Results of the 4,6-DMDBT HDS 100

Desulfurization degree, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 c a b 20 d e f 100

80

60 40 WHSV, h-1

20

0

Figure 11. 4,6-DMDBT HDS results at various WHSVs (340 oC, 4 MPa, 200 mL/mL). (a) NiMo/AS-K, (b) NiMo/AS-Mg, (c) NiMo/AS-Mn, (d) NiMo/AS-Zn, (e) NiMo/AS-Cu and (f) NiMo/Al2O3.

The 4,6-DMDBT HDS catalytic performance of series NiMo/AS with various crystal sizes were evaluated and the results are shown in Figure 11. The desulfurization degrees of 4,6-DMDBT HDS at all the WHSVs over the investigated catalysts change in the following sequence: NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu > NiMo/Al2O3. NiMo/AS-Mn shows the maximum desulfurization degrees of 4,6-DMDBT HDS at all the WHSVs, of which at 100 h-1 its desulfurization degree of 4,6-DMDBT HDS (31.9 %) is greater than 2.3 times as that over reference NiMo/Al2O3 (13.8 %), and almost two times as that over NiMo/AS-Cu (16.2 %). The 4,6-DMDBT HDS products were studied using a GC-MS chromatograph at the similar desulfurization degree (50 %) of 4,6-DMDBT HDS (Figure S10, in Supporting Information). The 4,6-DMDBT HDS reaction network over series NiMo/AS

with various crystal sizes is proposed in Figure S11 (in Supporting 27



Information). For the 4,6-DMDBT HDS, DDS, HYD and isomerization (ISO) are three parallel pathways. In the DDS route, S atom can be directly removed to form 3,3’-dimethylbiphenyl (3,3’-DMBP) and no other products or intermediates will be formed.

In

the

HYD

route,

4,6-DMDBT

4,6-tetrahydro-dimethyldibenzothiophene

firstly

hydrogenated

(4,6-THDMDBT)

to and

4,6-hexahydro-dimethyldibenzothiophene (4,6-HHDMDBT) intermediate, then either the

S

atom

can

be

removed

3,3’-dimethylcyclohexylbenzene

to

produce

(3,3’-DMCHB)

(3,3’-DMCHEB)

intermediates

or

and further

hydrogenated to generate 3,3’-dimethylbicyclohexyl (3,3’-DMBCH). In the ISO pathway, 4,6-DMDBT was isomerized to 3,7-DMDBT and/or 3,6-DMDBT intermediates firstly. Then the S atom is eliminated via HYD and DDS pathways.

50

Selectivity, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 43

Iso-MIPT

40 30

3,3’-DMCHB 4,4’-DMBP THDMDBT 3,3’-DMBCH 3,3’-DMBP

20 10 0 100

80

60 40 20 WHSV, h-1

0

Figure 12. 4,6-DMDBT HDS product distributions over NiMo/AS-Mn at various WHSVs.

The 4,6-DMDBT HDS product distributions (Figure S12, in Supporting Information) over NiMo/AS-Mn catalyst at different WHSVs are shown in Figure 12. The low WHSV can improve the 4,6-DMDBT HDS reaction rate through ISO pathway rather than via the HYD pathway, reflected by the increases of 4,4’-DMBP 28


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and Iso-MIPT selectivity and the decrease of the THDMDBT selectivity. For the NiMo/AS-Mn catalyst, DDS is not the preferential reaction route of 4,6-DMDBT HDS, reflected by the 3,3’-DMBP selectivity always less than 10 % at different WHSVs. The selectivity to form 3,3’-DMCHB decreases, while 3,3’-DMBCH increases with the decreasing WHSV.

29


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 30 of 43

Table 3. Catalytic performances for HDS of 4,6-DMDBT over various catalysts.

Catalyst

a b

Conversiona (%)

kHDS （10-4 mol· g-1·h-1）

Product selectivity (%)b HYD

DDS

Total

ISO

ISO

4,6-THDMDBT + 4,6-HHDMDBT

3,3’-DMCHB

3,3’-DMBCH

3,3’-DMBP

4,4’-DMBP

Iso-MIPT

NiMo/AS-K

49.8

7.9

7

28

10

5

16

34

50

NiMo/AS-Mg

50.1

6.8

8

29

11

5

15

32

47

NiMo/AS-Mn

50.2

8.7

6

27

9

3

18

37

55

NiMo/AS-Zn

49.9

5.4

9

31

11

7

13

29

42

NiMo/AS-Cu

50.1

4.3

11

33

13

8

9

26

35

NiMo/Al2O3 49.8 4.1 2 38 5 The DBT HDS conversion was obtained by changing WHSV (340 ºC, 4 MPa, 200 mL/mL).

55

0

0

0

Determined at about 50% of total 4,6-DMDBT conversion by changing WHSV.

HYD: 4,6-THDMDBT + 4,6-THDMDBT + 3,3’-DMCHB + 3,3’-DMBCH; DDS: 3,3’-DMBP; Total ISO: 4,4’-DMBP + Iso-MIPT.

30


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The 4,6-DMDBT HDS product distributions of series NiMo/AS are listed in Table 3. The proportions of the primary products of ISO for the series catalysts change in the sequence of NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu > NiMo/Al2O3. NiMo/AS-Mn displays the highest ISO proportion of 4,6-DMDBT HDS, which is ascribed to the relatively high B acidity of NiMo/AS-Mn catalyst. The kHDS of the 4,6-DMDBT over series NiMo/AS are also listed in Table 3. The kHDS of the 4,6-DMDBT over series NiMo/AS change in the sequence of NiMo/AS-Mn > NiMo/AS-K > NiMo/AS-Mg > NiMo/AS-Zn > NiMo/AS-Cu > NiMo/Al2O3. The kHDS of 4,6-DMDBT over NiMo/AS-Mn is greater than 2.1 times as that over the reference NiMo/Al2O3, and greater than two times as that over the NiMo/AS-Cu.

4. DISCUSSION The DBT and 4,6-DMDBT HDS catalytic performances of series NiMo/AS are closely associated with the structural properties, of which crystal sizes of the supports, the acidic center type and acid strength of the catalysts contribute more to the good catalytic performance. From the above research results, NiMo/AS-Mn revealed the excellent DBT and 4,6-DMDBT HDS catalytic performance. The main reasons could be ascribed to the following aspects. Firstly, the physicochemical properties of the series AS supports contributed more to the diffusion process of DBT and 4,6-DMDBT molecules. The AS-Mn support possessed the open pore channels and well-ordered arrays of mesoporous 31



structure and the 3D Im3m cubic structure, which could provide enough space for the good diffusion properties the reactants and products. The physicochemical properties of the series AS supports also contributed more to the dispersion of the active metals. The AS-Mn support possessed the highest BET specific surface area (817 m2·g-1), and pore volume (0.67 cm3·g-1), which could promote the dispersion of the active metals on the NiMo/AS-Mn catalyst. Secondly, the crystal sizes of the series Al-SBA-16 supports had an important effect on the diffusion of the reactants and the accessibility of MoS2 active phases. The small crystal size (3.0-4.8 µm) of the uniform spherical AS-Mn support enhanced the diffusion and accessibility between the reactants and MoS2 active phases. Thirdly, the acid properties of the catalysts had great influence on HYD and ISO pathways during the HDS process, which could enhance the desulfurization degree of the DBT and 4,6-DMDBT. NiMo/AS-Mn with the maximum total acid content (B+L) exhibited the maximum HYD/DDS selectivity (1.77), the maximum kHDS (16.4×10-4 mol·g-1·h-1) of DBT HDS. NiMo/AS-Mn with the highest B acid content showed the highest ISO selectivity (55 %) and the highest kHDS (8.7×10-4 mol·g-1·h-1) of 4,6-DMDBT HDS reaction. The steric hindrances were reduced after the migration of the methyl groups from the 4 and 6 positions via ISO in 4,6-DMDBT HDS. 61 The stacking degree of the active metals played an important role in the reduction and sulfidation. NiMo/AS-Mn possessed higher BET surface area, which could promote the formation of appropriate stacking layer and short stacking length of MoS2 active phase that expose more brim and edge active sites, then improving the 32


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HDS performance. NiMo/AS-Mn exhibited the maximum NiMoS ratio (93.5 %), Mosulfidation (71 %) and the highest desulfurization degree of DBT and 4,6-DMDBT. The above discussions confirmed that the highest DBT and 4,6-DMDBT HDS catalytic performances of NiMo/AS-Mn were derived from the cooperative contribution of the highest BET specific surface area and smallest crystal sizes of AS-Mn, the highest B acid and total acid contents of the catalysts, appropriate stacking layer and shortest stacking length of MoS2 active phases.

5. CONCLUSION A series of AS supports with different crystal sizes, textural structure and acidity were synthesized via a hydrothermal crystallization method. The NiMo/AS-Mn catalyst with relatively high B acid and total acid content combining with relatively small crystal size is beneficial for the isomerization reactions and eliminating the diffusion resistance in the DBT and 4,6-DMDBT HDS. NiMo/AS-Mn showed the maximum kHDS of the DBT amd 4,6-DMDBT due to its synergistic effect of the highest BET specific surface area and smallest crystal sizes of AS-Mn, the highest B acid and total acid contents of the catalysts. The higher total acid content of NiMo/AS-Mn could favor the HYD selectivity of DBT HDS, and the higher B acid content could favor the ISO selectivity of 4,6-DMDBT HDS. For DBT HDS, HYD was the preferential route over NiMo/AS-Mn, while for 4,6-DMDBT HDS reaction, ISO was the preferential route. The ISO route greatly enhanced the 4,6-DMDBT HDS performance of NiMo/AS-Mn. 33



ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.21676298, U1463207 and 21503152), CNOOC project (CNOOC-KJ 135 FZDXM 00 LH 003 LH-2016) and CNPC Key Research Project and KLGCP (GCP201401).

ASSOCIATED CONTENT Supporting Information. Small angle XRD of Al-SBA-16 series materials (Figure S1), N2 physisorption isotherms and pore size distribution of series materials (Figure S2), 27

Al NMR spectra of the serie materials (Figure S3), TEM images of the series AS

supports (Figure S4), HDS results of DBT at different WHSVs (Figure S5), DBT and 4,6-DMDBT HDS results with time on stream (Figure S6), The DBT HDS product distributions (Figure S7), A DBT HDS possible reaction network (Figure S8), The DBT HDS product distributions at various WHSVs (Figure S9), The 4,6-DMDBT HDS product distributions (Figure S10), A possible 4,6-DMDBT HDS reaction network (Figure S11), The 4,6-DMDBT HDS product distributions at various WHSVs (Figure S12), Physico-chemical properties of the series materials (Table S1), Mo3d XPS results of series sulfide catalysts (Table S2), Ni2p XPS results of series sulfide catalysts (Table S3), Average length and layer number of MoS2 phases (Table S4).

34


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Controllable synthesis of spherical Al-SBA-16 mesoporous materials

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