Effects of Aging Treatment on the Hydrotreating Performance of the

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Kinetics, Catalysis, and Reaction Engineering

Effects of aging treatment on the hydrotreating performance of the unsupported catalyst Changlong Yin, Chengwu Dong, Yan Kong, Kunpeng Li, Haonan Zhang, Dong Liu, and Chenguang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04849 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Industrial & Engineering Chemistry Research

Effects of aging treatment on the hydrotreating performance of the unsupported catalyst Changlong Yin, Chengwu Dong, Yan Kong, Kunpeng Li, Haonan Zhang, Dong Liu, Chenguang Liu State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum, Qingdao, Shandong 266580, China

AUTHOR INFORMATION Corresponding Author * Changlong Yin, Prof

e-mail: [email protected]

Tel: +86-532-86984629

1

ACS Paragon Plus Environment

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 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: An unsupported Ni-Mo-W catalyst was synthesized by hydrothermal method and treated by aging treatment to improve its specific surface area and pore size. The effect of different aging conditions was studied by BET, XRD, SEM, and HRTEM techniques. The catalytic activity of the unsupported catalyst was evaluated with a simulated diesel feed. BET results showed that the catalyst specific surface area and pore volume increased significantly after aging treatment, and the hydrotreating results revealed that aging treatment resulted in higher catalytic activity than untreated catalyst, which is beneficial to the production of Ultra-Low Sulfur Diesel (ULSD).

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1. INTRODUCTION The rapidly growing industry has brought people numerous conveniences. However, a series of environmental problems, such as industrial waste gas and automobile exhaust, are considered to be the main cause of air pollution, which is caused by the deteriorating crude oil quality. Low-quality crude oil contains more sulfur and nitrogen compounds, which can cause serious air pollution.1-5 To solve this problem, it is necessary to develop high activity diesel desulfurization and denitrogenation catalysts. Traditional hydrogenation catalysts were SiO2 or Al2O3 as support.6-8 Due to their low price and excellent hydrogenation activity, these catalysts are widely used in petroleum refining, but the low loading greatly limits the further improvement of catalytic activity, and the stricter diesel standards cannot be met.9,10 Because of the questions, Albemarle, ExxonMobil, and the Nippon Ketjen Corporation jointly developed new bulk catalysts (NEBULA) in 2001,11,12 and achieved industrialization. Which overcome the disadvantage of supported hydrotreating catalyst, greatly improve the activity of hydrogenation catalyst.13 The unsupported catalyst consists mainly of Ni-Mo, Co-Mo, and Ni-W type catalysts. Ni-Mo and Co-Mo catalysts have better hydrogenolysis,14 while Ni-W catalyst is stronger in hydrogenation.15 To combine the advantages of these two kind of catalysts, Soled et al.16 developed a Ni-Mo-W type three-component hydrogenation catalyst, and the results showed that three-component hydrogenation catalyst has higher activity in hydrodesulfurization (HDS) than two -component catalysts.

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The extrusion technology of the unsupported catalyst is different from traditional supported catalyst. Alumina or silica is often used as binder for the unsupported catalyst. In the wet-mixing-kneading and extruding processes, some of the original pores in the precursor will be inevitably destructed, and the binder will also cover some of the active components and the pores, which will thereby reduce the activity of the extruded unsupported catalyst. So it is very important to find a solution for improving the strength and specific surface area of unsupported catalyst. As is well-known, the aging is a key factor in dough making in the food process.17 During the aging process, water will permeate into the protein colloidal particles and make them filled bibulous expansion, adhesion to form a network of dough. Through it, the water between the protein and starch can be adjusted automatically to achieve homogenization, which is beneficial to the improvement of dough toughness. Based on above views, we treated the unsupported catalyst mixture by aging before extruding, and the effects of aging temperature and aging time on the performance of the unsupported catalyst were evaluated.

2. EXPERIMENTAL SECTION 2.1. Preparation of Unsupported Ni-Mo-W Catalyst Precursors The unsupported Ni-Mo-W catalyst precursors were synthesized by the hydrothermal method. Basic nickel carbonate, hexaammonium molybdate, and ammonium metatungstate (purchased from Sinopharm Chemical Reagent Company, PR China) with some deionized water were placed in a beaker in the molar ratio of 4


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Ni:Mo:W=2:1:1, the mixture was stirred with a magnetic stirrer. Then the mixed solution was put into an autoclave (S-2L, China), and reacted for 10 h at 150 °C to get a Ni-Mo-W catalyst precursor suspension. The suspension was filtered with a vacuum filter, washed several times with deionized water, and the filter cake was dried overnight at 120 °C to get the unsupported Ni-Mo-W catalyst precursor. 2.2. Preparation of Unsupported Ni-Mo-W Catalyst Alumina was used as binder of Ni-Mo-W composite oxide (precursor/alumina ratio = 80/20). Polyethylene glycol 2000 was used as the pore expanding agent. Acetic acid was used as glue solvent. The detailed synthesis steps are as follows: First, the right amount of alumina and the Polyethylene glycol 2000 were mixed in a beaker, the acetic acid and distilled water were slowly added to get a homogeneous mixture. Then Ni-Mo-W composite oxides were added, kneaded uniformly to get a green solid mixture. The solid mixture was putted in a sealed beaker, heated in a water bath for 1~4 h at 40~90 °C as aging. To obtain the fixed-bed catalysts, the aged solid mixtures were extruded into rods 1.6 mm in diameter with a single screw extruder, the products were dried at 120 °C and calcined at 400 °C to get the unsupported oxidic Ni-Mo-W catalyst. XRF result shows that the final composition of unsupported oxidic Ni-Mo-W catalyst is NiO (25.4%), MoO3 (23.7%), WO3 (31.2%). The unmodified catalyst was labeled as Cat-0. Catalysts aged at different temperature for 2 hours were labeled Cat-40-2, Cat-60-2, Cat-80-2, and Cat-90-2. The catalysts aged at 80 °C for various times were labeled Cat-80-1, Cat-80-2, Cat-80-3, and Cat-80-4.

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Theoretically, the aging temperature can be from room temperature to more than 100 °C. However, considering that treatment at high temperature may result in reduced strength of the catalyst, we choose a mild temperature condition, 40~90 °C in this manuscript. Also, too low temperature will make the aging effect very small. 2.3. Characterization The specific surface area, pore volume, and pore diameter of the catalysts were measured with a Micromeritics Tristar 3000 multi-function adsorption instrument (America). First, the samples were degassed in vacuum for 6 h at 300 °C, and then N2 was adsorbed at -196 °C, the specific surface area was calculated by the BET (Brunauer-Emmett -Teller) method, the pore volume and pore diameter distribution curve were calculated by the BJH (Barrett-Joyner-Halenda.) method. The catalyst crystal phase was analyzed with X-ray diffractometer with Cu-Kα radiation (Panalytical). The morphology of the catalysts was obtained by using a S-4800 cold field emission scanning electron microscope (Hitachi company) with a magnification of 30~800000. The dispersion of the sulfide active components was obtained by transmission electron microscopy (JEM-2100UHR) (Japanese Electronic Company). Then, the length (L) of the active component and the stacking layers number (N) were calculated by counting more than 600 layered crystalline grains in at least 15 representative micrographs. The average layer length (LA) and the average stacking layers number (NA) were calculated according to the formula (1) and (2).

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n

n

i 1

i 1

m

m

i 1

i 1

LA   ni Li /  ni

(1)

N A   mi N i /  mi

(2)

The length of the active components was labeled as Li, the stacking layers number were labeled as Ni. The number of crystallites with specific length i was presented with ni, and the number of crystallites with the specific stacking layers number i was presented with mi. The MoS2/WS2 dispersion, fMo/W, was calculated by dividing the total number of Mo/W atoms at the edge surface by the total number of Mo/W atoms. By assuming that MoS2/WS2 slabs are present as perfect hexagons, the following equation was derived. 18 n

f Mo /W 

 (6 x  6) i

i 1

n

 (3x i 1

2 i

 3x i  1)

(3)

Where xi is the number of Mo/W atoms along one edge of a MoS2/WS2 slab determined from its length (L=3.2(2xi-1) (Å)), and n is the total number of slabs shown in the TEM micrographs. 2.4. Catalytic Activity Evaluation The catalyst evaluation was carried out on a 10 mL high pressure micro-reactor. A volume of 5 mL catalyst was placed in the middle of the reaction tube, and the upper and bottom parts of the tube were filled with quartz sand. The size of the catalysts and 7



quartz sand particles were 20-40 meshes. The catalysts were pre-sulfided at 320 °C for 10 h with a cyclohexane solution containing 3 wt.% CS2 at a pressure of 5 MPa and a H2/oil volume ratio of 300:1. A simulated diesel (1.5 wt.% DBT, 5 wt.% naphthalene, and 2 wt.% quinoline) in petroleum ether was used as the feed. The reaction conditions were: a total H2 pressure of 4 MPa, a liquid hourly space velocity (LHSV) of 2.0 h-1, a H2/oil volume ratio of 300:1, and a reaction temperature of 280 °C. The samples were collected after 12 h, and analyzed with a GC-214 type chromatograph (Bruker Corporation). And the space time yield of the catalyst was calculated by the following formula: N

Y (A) = VConv (A)

(4)

Where Y (A) is the space time yield of A (mol/(mL·h)), N is the moles of feed per hour (mol/h), Conv (A) represents the conversion of A, and V is the volume of catalyst (mL).

3. RESULTS AND DISCUSSION 3.1. Effects of Different Aging Conditions 3.1.1. N2 Adsorption-Desorption

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Table 1. Physical Properties of Unsupported Ni-Mo-W Oxide Catalyst Treated at Different Aging Temperature Catalysts

SBET

VP

DP

Bulk density

Strength

/ (m2·g-1)

/ (cm3·g-1)

/ nm

/ (g·mL-1)

/ (N·cm-1)

Cat-0

193

0.24

4.4

1.3

90

Cat-40-2

202

0.30

5.0

1.2

104

Cat-60-2

238

0.33

5.6

1.1

107

Cat-80-2

241

0.32

5.3

1.1

113

Cat-90-2

234

0.32

5.2

1.1

113

Table 2. Physical Properties of Unsupported Ni-Mo-W Oxide Catalyst Treated after Different Aging Time Catalysts

SBET

VP

DP

Bulk density

Strength

/ ( m2·g-1)

/ (cm3·g-1)

/ nm

/ (g·mL-1)

/ (N·cm-1)

Cat-0

193

0.24

4.4

1.3

90

Cat-80-1.0

238

0.31

5.6

1.1

111

Cat-80-2.0

241

0.32

5.3

1.1

113

Cat-80-3.0

237

0.30

5.4

1.1

111

Cat-80-4.0

232

0.31

5.0

1.1

110

9



Table 1 shows the N2 adsorption-desorption results at different aging temperature. The specific surface area, pore volume, pore diameter, and mechanical strength of the catalysts are improved after aging. On the whole, specific surface area first increases and then decreases with increasing aging temperature. When the aging temperature is 80 °C, the specific surface area reaches a maximum value of 241 m2·g-1, but at a different aging temperature, the pore volume, pore diameter, and strength hardly change. On the whole, after aging, the specific surface area, pore volume, and pore diameter of the catalysts increased by about 19%, 32%, and 20% respectively. Table 2 presents the N2 adsorption-desorption results after different aging time. From Table 2, with the extension of aging time, the catalyst has the highest specific surface area of 241 m2·g-1 with the aging time of 2 h. On the whole, after aging, the specific surface area, pore volume, and pore diameter of the catalysts increased by about 23%, 29%, and 21% respectively. It is suggested that the catalyst needs an optimal aging time, too long or too short is not conducive to the improvement of specific surface area and pore structure 3.2. Effects of Aging on the Physicochemical Properties of Unsupported Ni-Mo-W Catalysts 3.2.1. XRD

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Figure 1. XRD patterns of the unsupported Ni-Mo-W catalyst before (A) and after (C) aging treatment and unsupported Ni-Mo-W catalyst precursor (B)

XRD patterns of the unsupported Ni-Mo-W catalyst precursors before and after aging treatment are illustrated in Figure 1. All catalysts have the same diffraction peaks closed to amorphous structure, which can be attributed to the unsupported Ni-Mo-W catalyst precursors, indicating that aging treatment does not affect the crystallization of unsupported Ni-Mo-W catalysts, but changes its pore structure. 3.2.2. SEM

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Figure 2. SEM micrographs of oxide and sulfided catalysts before and after aging treatment: Oxide catalyst before (A) and after (B) aging; Sulfided catalysts before (C) and after (D) aging treatment

The SEM images of oxidized and sulfided catalysts are shown in Figure 2. Figure 2 (A) and (B) shows that the catalyst before aging has a lamellar structure, which is not conducive to the formation of loose and porous structure, resulting in a smaller specific surface area. However, after aging, a more uniform pore structures appears, which is favorable for hydrotreating catalysts. The sulfided images (C) and (D) also show that the catalyst treated with aging is composed of smaller and more dispersed particles, and their overall structure is more loose and porous, which favors the 12


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dispersion of active components. 3.2.3. HRTEM

Figure 3. HRTEM photographs of sulfided unsupported Ni-Mo-W catalysts before (A) and after (B) aging treatment

 



-Ni3S2 







-MoS2/WS2 

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


Cat-80-2

Cat-0

10

20

30

40

50

60

70

2θ（ Cu Kα（ / degree

Figure 4. XRD pattern of sulfided unsupported Ni-Mo-W catalysts before and after aging treatment 13



It is generally believed that the active component of sulfided Ni-Mo-W catalyst has a MoS2/WS2 structure, and these structures are usually observed by HRTEM photographs. Figure 3 presents the HRTEM results of the sulfided Ni-Mo-W catalyst before and after aging. The photograph shows many striped black lines. These threadlike fringes exist in a parallel cluster structure with about 0.6 nm interplanar spacing, which is the typical MoS2/WS2 interplanar spacing,13,19,20 implying the existence of MoS2/WS2 structures. In addition, another lamellar structure with about 0.3 nm interplanar spacing can be seen in Figure 3 (B), which can be attributed to the Ni3S2 lamellar structure.21,22 As a whole, MoS2/WS2 is evenly distributed on the edge of Ni3S2, so that the two structures can be fully combined, which improves their synergistic effect.22 Figure 4. shows the XRD pattern of sulfided unsupported Ni-Mo-W catalysts before and after aging treatment, XRD results also show the existence of Ni3S2 and MoS2/WS2 structures, but the diffraction peaks of MoS2/WS2 are overlapping, we can't tell them apart. In order to further study the change of MoS2/WS2 structure after aging, the stacking layer number and laminar length of MoS2/WS2 structures were calculated, and the statistical results are summarized in Figures 5 and 6, while, the average slab length and layers of MoS2/WS2 are summarized in Table 3.

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25

Cat-80-2

20 15 10

Frequency (%)

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


5 0 25

Cat-0

20 15 10 5 0 0

2

4

6

8

10

12

14

16

Length (nm)

Figure 5. The relationship between MoS2/WS2 slab lengths and frequency of occurrence

15



30

Cat-80-2

25 20 15 10

Frequency (%)

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

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5 0 30

Cat-0

25 20 15 10 5 0 0

2

4

6

8

10

12

Stacking layer number

Figure 6. The relationship between MoS2/WS2 stacking layer number and frequency of occurrence

Table 3. The Average Slab Length (LA) and Layers (NA) of MoS2/WS2 Catalysts

LA/ nm

LA/ nm (L10)

NA

fMo/W

Cat-0

5.42.5

11.9

3.61.5

0.18

Cat-80-2

4.92.2

10.8

4.01.9

0.20

Values are the means ± standard deviations

Figures 5 and 6 show the distribution of the lengths and stacking layers of MoS2/WS2 crystallites respectively. The slab lengths of MoS2/WS2 are within the range of 1~15 nm, and the stacking numbers are between 2 and 12. Specifically, aging results in 16


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lower slab lengths and higher stacking layers of MoS2/WS2 crystallites, which is mainly concentrated in 3~5 nm and 2~8 respectively. The average slab lengths and stacking numbers of MoS2/WS2 crystallites are presented in Table 3. From the data we can find that the average slab lengths and the average stacking numbers of the catalysts with and without aging treatment are almost consistent in considering of the standard deviations. However the average length of the larger slabs (≥10 nm) for the aged catalyst decreased from 11.9 nm to 10.8 nm, indicating that aging treatment can mainly disperse the larger catalyst particles into smaller particles. In addition, the fMo/W also was shown in Table 3, the fMo/W can reflect the dispersion of the active component, the higher the fMo/W is, the better the catalyst dispersion is. From Table 3, the fMo/W is slightly higher for Cat-80-2 (0.20) than Cat-0 (0.18), and this is chiefly because aging treatment can disperse large particles into small ones, then improve the dispersion of the catalyst, which is also beneficial to the improvement of the hydrogenation activity of the catalyst. 23 3.3. Catalytic Performance Evaluation

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Table 4. The Product Distribution of the HDS of DBT under Different Aging Conditions Catalysts

BPa

CHBb

BCHc

BCPd

HDS (%)

YeHDS

Cat-0

82.2

13.3

2.3

1.2

89.0

9.4

Cat-40-2

82.3

13.2

2.4

1.2

90.1

9.9

Cat-60-2

72.8

12.7

9.3

5.0

92.8

10.2

Cat-80-2

70.4

14.8

9.2

5.5

99.9

11.0

Cat-80-1

81.4

13.5

2.6

1.8

95.3

10.4

Cat-80-3

76.4

13.9

6.8

2.7

97.8

10.7

Cat-80-4

76.3

13.6

8.9

1.0

98.8

10.8

a: Biphenyl.

b: Cyclohexylbenzene.

c: Bicyclohexane.

d: Benzyl cyclopentane

e: The space time yield (mol/mL (cat.)·h/10-5)

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Table 5. The Product Distribution of Quinoline under Different Aging Conditions Catalysts

THQa

DHQb

Quinoline

HDN (%)

YcHDN

Cat-0

34.3

0.2

2.1

60.3

12.1

Cat-40-2

38.7

0.9

1.8

60.4

12.1

Cat-60-2

26.3

1.1

0.9

72.6

14.6

Cat-80-2

23.1

1.8

0.5

73.5

14.8

Cat-80-1

34.2

1.2

1.2

63.4

12.7

Cat-80-3

24.3

1.7

1.6

72.4

14.6

Cat-80-4

19.9

1.8

3.2

75.1

15.1

a: Tetrahydroquinoline.

b: Decahydroquinoline.

c: The space time yield ((mol/mL (cat.)·h/10-5)

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Table 6. The Product Distribution of Naphthalene under Different Aging Conditions Catalysts

Tetralin

Cis-Decalin

Trans-Decalin

HDAr (%)

YaHDAr

Cat-0

25.8

1.3

2.4

29.5

15.0

Cat-40-2

26.8

0.9

1.8

29.9

15.2

Cat-60-2

27.4

1.1

2.1

30.6

15.5

Cat-80-2

30.4

2.3

5.6

38.3

19.4

Cat-80-1

16.6

1.5

3.1

31.6

16.0

Cat-80-3

23.8

1.7

2.9

32.2

16.3

Cat-80-4

23.1

1.4

3.0

31.7

16.1

a: The space time yield ((mol/mL (cat.)·h/10-5)

Catalytic activity results of sulfided unsupported Ni-Mo-W catalysts are summarized in Tables 4, 5, and 6. Table 4 show the hydrogenation product distribution of the HDS of DBT under different aging conditions for unsupported Ni-Mo-W catalysts. Aging treatment leads to higher DBT conversion than untreated Ni-Mo-W catalyst. Especially the catalyst matured at 80 °C for 2 h has the highest DBT conversion of 99.9 %. Table 5 gives the hydrogenation result of quinoline. Before aging, the hydrodenitrogenation rate of quinoline is 60.3 %, aging at 80 °C for 2 h results in a higher hydrodenitrogenation rate of 73.5 %, which is very considerable due to the low hydrodenitrogenation (HDN) rate of diesel.24 It illustrated that the catalyst matured at 80 °C for 2 h has the highest hydrogenation activity. Table 6 shows the product 20


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distribution of naphthalene. The hydrogenation products of naphthalene are mainly tetralin and decalin. On the whole, the conversion rate of naphthalene is relatively low. The reason is that there is a competition of different substances in the process of hydrogenation,25,26 but it can be enhanced to 38.3 % from 29.5 % by aging, which further illustrates that the hydrogenation activity of the catalyst can be improved with aging. 3.4. The Mechanism of Aging on Improving the Hydrogenation Activity of Catalyst

Figure 7. Sketch for aging of unsupported Ni-Mo-W catalyst

According to the SEM results of catalyst precursors, the mixture of binder, glue solvent, and unsupported Ni-Mo-W catalyst precursor powder were kneaded into catalyst mixture. At the beginning, the binder, glue solvent, and unsupported Ni-Mo-W catalyst precursor powder aggregate together without fully absorbing water, which is detrimental to the formation of the porous structure and the particles 21



dispersion. The catalyst particles aggregate together without being uniformly dispersed on the binder, causing low specific surface area, which restricts the fully exploitation of active components, reducing the hydrogenation activity of the unsupported Ni-Mo-W catalyst. After aging, the binder can fully extend to the surrounding by absorbing water, so as to form a larger space for the catalyst particles dispersion, which leads to a more loose and porous structure (Figure 7). On the other hand, aging can also promote the fully absorbing water of catalyst particles. Therefore, the initially aggregated catalyst particles are dispersed into smaller particles uniformly dispersed on the binder, which improves the specific surface area and pore structure of the catalysts, so that the active components are exposed more fully, and increase the probability of contacting with the impurities in the diesel, then the hydrogenation effect is improved. The HRTEM results of sulfided unsupported Ni-Mo-W catalyst revealed that aging is also conducive to the uniform dispersion of the active phase, promoting the synergistic effect of Ni3S2 and MoS2/WS2, which is the key factor to improve the hydrogenation activity of the catalyst.27

4. CONCLUSION The solid mixture kneaded with binder, glue solvent, and unsupported Ni-Mo-W catalyst precursor powder was treated by aging. The results show that aging can not

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only reduce the particle size of the catalyst and thus improve the specific surface area, but also significantly enhance the strength of the catalyst, which is conducive to industrial application. The HRTEM results reveal that aging also promotes the synergistic effect of Ni3S2 and MoS2/WS2, which favors the improvement of the catalyst hydrogenation activity. The aging condition of the catalyst was investigated, and we found that the catalyst aged at 80 °C for 2 h had the higher specific surface area. The hydrogenation performance of the catalyst before and after aging was evaluated with a simulated diesel feed. The results suggest that aging can give a higher hydrogenation activity.

Acknowledgements

This work was financially supported by the National key R &

D program of China (2017YFB0602500), the National Natural Science Fund of China (Grant No. 21676301), and Shandong Province Natural Science Foundation (Grant No. ZR2016BM19). Financial support from PetroChina Corporation Limited was also greatly appreciated.

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148-149, 221-230. (2) Song, C., An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, (1), 211-263. (3) Souza, M. J. B.; Pedrosa, A. M. G.; Cecilia, J. A.; Gil-Mora, A. M.; Rodríguez-Castellón,

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Effects of Aging Treatment on the Hydrotreating Performance of the

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