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Hydroupgrading performance of Fluid Catalytic Cracking diesel over different crystal forms of alumina supported CoMo catalysts Xilong Wang, Jiyuan Fan, Zhen Zhao, Zhentao Chen, Peng Zheng, Jianmei Li, Yuyang Li, Longnian Han, Aijun Duan, and Chunming Xu Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Hydroupgrading performance of Fluid Catalytic Cracking diesel over different crystal forms of alumina supported CoMo catalysts Xilong Wang a †, Jiyuan Fan a †, Zhen Zhao a, Zhentao Chen a, Peng Zheng a, Jianmei Li a, Yuyang Li a, Longnian Han b, Aijun Duan a *, Chunming Xu a * a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

Beijing, 102249, P. R. China b

CNOOC Research Institute of Oil and Petrochemicals, Beijing, 102209, P. R. China.

ABSTRACT: A series of CoMo/Al2O3 catalysts was prepared using different crystal forms of alumina (including γ-Al2O3, δ-Al2O3, θ-Al2O3 and α-Al2O3) as supports for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of FCC diesel. The physicochemical properties of the supports and the corresponding catalyst were analyzed by various characterization methods. The results showed that δ-Al2O3 possessed moderate surface area, concentrated pore size distribution and less surface hydroxyl groups. The CoMo/δ-Al2O3 catalyst exhibited the highest HDS and HDN efficiencies, 98.4% and 96.1% respectively. This could be attributed to its reduced metal support interaction, moderate stacking morphology and highest sulfidation degree of active phases. The HDS and HDN efficiencies of the catalysts followed the order: CoMo/α-Al2O3 (87.3%, 72.2%) < CoMo/γ-Al2O3 (94.8%, 87.9%) < CoMo/θ-Al2O3 (96.2%, 90.1%) < CoMo/δ-Al2O3 (98.4%, 96.1%). KEYWORDS:

alumina;

crystal

forms;

FCC

diesel;

1

ACS Paragon Plus Environment

hydrodesulfurization;

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hydrodenitrogeneration 1. INTRODUCTION In recent years, with the increasing attention to environmental protection in the world, the maximum sulfur content of vehicle fuel has been limited to 10 ppm since 2009 according to the Europe V specification.1-3 This tendency makes the world petroleum refining industry facing a great challenge of quality upgrading, thus large-scale industrialization of clean diesel is imminent to the refinery. At present, HDS is the most important technology for large-scale production of clean oil. While developing effective hydrotreating catalyst is one of the most economic and effective methods to improve the HDS efficiency. The traditional γ-Al2O3 support has been widely used as the industrial hydrodesulfurization catalyst support because of its excellent mechanical properties, regeneration performance and because it is cheap. However, the increasingly stringent fuel specification limits the further application of γ-Al2O3 due to the mass transfer diffusion resistance and the strong metal support interaction. Many efforts have been devoted to improve the performance of alumina supported catalyst.4-7 Van Haandel et al.8 compared the gas-phase activation and liquid activation of CoMo/Al2O3 catalysts modified by phosphoric and citric acid. The results showed that the catalyst prepared by citric acid had the highest degree of promotion and was the most active one in thiophene and dibenzothiophene (DBT) HDS, whereas the catalyst with phosphoric acid performed the best in the gas-oil HDS process due to its good stability. In order to obtain novel material with high 2


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surface area and larger pore volume, many templating strategies were proposed to synthesize mesoporous alumina.9-10 Liu et al.11 synthesized hexagonally ordered mesoporous NiMo-Al2O3 catalysts by a one-pot method using P123 as surfactant. The catalytic results showed that the DBT conversion of a 10.0NiMoAl catalyst were 22.5% and 32.5% at 493K and 513K respectively, which were higher than the reference catalysts of NiMo/meso-Al2O3 and NiMo/γ-Al2O3. Bejenaru et al.12 evaluated the HDS catalytic performance for DBT of a CoMo catalyst supported by mesoporous alumina with different morphologies. Hierarchically ordered alumina was also studied as the catalyst support in the HDS reaction. Han et al.13 successfully fabricated three-dimensionally ordered macroporous γ-alumina by using dual templates of PMMA and F127. The excellent catalytic activity in the HDS of DBT could be contributed to the co-existence of the interconnected macroporous and mesoporous structure. Semeykina et al.14 studied the effect of the texture of a CoMoNi catalyst supported by macro/mesoporous alumina on the hydrodesulfuriztion activity and hydrodemetallization

conversion

of

heavy

tatar

oil.

The

hierarchically

macro/mesoporous catalyst showed lower coking rate of the hydrotreated products, as well as higher HDS and hydrodemetallization (HDM) conversions. Until now, few papers focused on the synthesis of the different crystal forms of alumina and on the structure-activity relationship of the corresponding catalysts. Laurenti et al.15 studied the catalytic performance of CoMoS catalyst supported by three kinds of alumina in hydrodesulfurization reactions of thiophene and 4,6-dimethyldibenzothiophene. The results showed that the δ-alumina supported 3


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CoMoS catalyst was the most intrinsically active catalyst for the HDS of refractory S-compounds. Wang et al.16 synthesized a series of NiMo catalysts supported by different crystal forms of alumina and tested their catalytic performance in the hydrodesulfurization of DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) compared with a commercial NiMo/Al2O3 catalyst. The DBT conversion of NiMo/δ-Al2O3 catalyst was about twice that of NiMo/Al2O3, while its 4,6-DMDBT conversion was more than four times that of NiMo/Al2O3. Zhang et al.17 prepared different crystal structure alumina using AlCl3·6H2O and Al(NO3)·9H2O as alumina source. The NiMo/δ-Al2O3 catalyst obtained from aluminum nitrate exhibited the highest HDS and HDN efficiencies in the HDS reaction for FCC diesel. The above reported material was obtained with addition of expensive template in the synthesis process. The industrial HDS process always requires the design of an inexpensive, environmental friendly and efficient catalyst. So, it is significant to optimize the catalyst support to reduce the metal support interaction and improve the dispersion active species by using a simple preparation process and low-cost raw material. In this research, alumina materials with four different crystal forms were successfully obtained by using low-cost pseudo boehmite as the raw material and calcining it at different temperatures. The corresponding CoMo catalysts were prepared by the impregnation method and the activity in the HDS reaction of FCC diesel was evaluated. Moreover, the textural properties of the supports and CoMo catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption isotherm, fourier transform infrared (FTIR) spectroscopy, hydrogen temperature 4


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programmed reduction (H2-TPR), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Raman characterization method and the effects of different supports of CoMo/Al2O3 on the catalytic activity of FCC diesel were discussed. The result showed that CoMo/δ-Al2O3 exhibited the highest HDS and HDN activities. 2. EXPERIMENTAL SECTION 2.1 Synthesis of Alumina with Different Crystal Forms The following chemicals were used as reagents for alumina material and the catalyst syntheses: pseudo boehmite (Al2O3·nH2O, Shandong aluminum Research Institute), ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, ≥99.0%, Tianjin Guangfu Fine Chemical Research Institute), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, ≥99.0%, Tianjin Guangfu Fine Chemical Research Institute) and deionized water. In a typical material synthesis, a certain amount of pseudo boehmite were put in an oven and dried at 353 K for 12 h, then the powder was calcined at 873 K for 6 h with a temperature ramping rate of 2 K/min. The obtained material was denoted as γ-Al2O3. The other three different crystal forms alumina material were calcined at 1173 K, 1323 K, and 1473 K with the same heating rate and are denoted as δ-Al2O3, θ-Al2O3 and α-Al2O3, respectively. 2.2 Preparation of Catalysts The CoMo supported catalysts were prepared by a two-step incipient wetness method using aqueous solutions of (NH4)6Mo7O24·4H2O and Co(NO3)2·6H2O. The solutions with Mo or Co were dropped the support in series, then the sample was 5


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dried at 383 K overnight and finally calcined at 823 K for 6 h. The catalysts obtained by different crystal forms of alumina were denoted as CoMo/γ-Al2O3, CoMo/δ-Al2O3, CoMo/θ-Al2O3 and CoMo/α-Al2O3, respectively. The catalysts had a chemical composition of 12% MoO3 and 3% CoO. In order to get a fix-bed catalyst, the CoMo supported catalysts were tabletted and pulverized into 40-60 mesh for reaction evaluation. 2.3 Characterization of Supports and Catalysts XRD pattern of the catalyst was analyzed with a Bruker D8 Advance diffractometer with Cu Kα radiation at 40 kV and 40 mA. The scanning speed and step were 2o/min and 0.02o, respectively. N2 adsorption-desorption experiments were measured with a Tristar II 3020 instrument at 77 K. All samples were pretreated at 623 K for 4 h prior to testing and the pore size distribution was determined by means of the Barret-Joyner-Halenda (BJH) method. FT-IR absorbance spectra were obtained with a FT-3000 spectrophotometer (Digilab company, USA). The test sample was dried at 373 K for 4 h and then mixed with potassium bromide in a mass ratio of 1: 100, with a scanning range of 3400 to 3900 cm-1 and a resolution of 2.5 cm-1. H2-TPR characterization was carried out on an Autosorb iQ type multifunction adsorption instrument (Quantachrome company, USA). HRTEM images of the sulfided catalysts were performed on a JEM 2100 instrument (FEI company, Netherlands). The average stacking lengths

14

and the

number of layers (Nav) of the MoS2 slabs were obtained by statistical analysis method 6


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based on at least 300 crystallites and calculated according to the following equation.  n  n L av =  ∑ n i li  / ∑ n i  i =1  i =1 n   n N av =  ∑ n i N i  / ∑ n i  i =1  i =1

(1)

(2)

Where li is the length of MoS2 slab, ni is the number of MoS2 with a length of li, and Ni is the stacking layer of active phases. The dispersion level of MoS2 can be calculated by the following equation, where fMo represents the percentage of the active Mo atoms at the edge to the total Mo atoms.

Moedge ∑ ( 6ni − 6) fMo = = i i =1 2 Mototal ∑ 3ni − 3ni + 1 i =1 i

(3)

XPS of the sulfided catalysts was performed on a PerkinElmer PHI-1600 ESCA spectrometer using a Mg Ka X-ray source. The standard binding energy is 284.6 eV of C1s species. Raman characterization was recorded on an inVia Raman spectrometer (Renishaw company) with a laser light source at a wavelength of 325 nm and a power of 8 mW. 2.4 Evaluation of HDS Performance of Catalysts The catalyst HDS performance evaluation process was as follows: the loading amount of catalyst was 2 g, and all the catalysts were presulfided with a mixture of 2 wt.% CS2 in cyclohexane solution for 4 h under the conditions of 4 MPa and 593 K, and then switched to the diesel reaction. The HDS performance of Hohhot FCC diesel (with a sulfur content of 1013.8 µg ml-1 and a nitrogen content of 640.3 µg ml-1) was carried out in a fixed-bed reactor (Diameter = 9 mm, Length = 500 mm) at 623 K, 5 7


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MPa, a weight hourly space velocity (WHSV) of 1.0 h-1 and a H2/oil volumetric ratio of 600:1. The sulfur contents of the feedstock and products were analysized by RPP-2000SN sulfur and nitrogen testing instrument. The HDS and HDN reactivities of the various catalysts were denoted as: HDS (%) = (Sf - Sp)/Sf; HDN (%) = (Nf Np)/Nf, where Sf (Nf) and Sp

11

represent the sulfur (nitrogen) contents of FCC diesel

feedstock and its products, respectively. The typical properties of the feedstock are listed in Table S1. 3. RESULTS 3.1 XRD of the Supports and Oxidized Catalysts

*

*

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

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▼▼

*

*

*

*

* * *

e *

▼

▼ ▼ ▼ ▼

▼ ▼ ▼▼

◆ ◆ ◆ ●

d

▼▼▼▼

◆ ◆

◆

●

●

c b a

10

20

30

40 50 60 2θ, degree

70

80

Figure 1. XRD patterns of alumina materials with different crystal forms in the wide-angle domain. (a) pseudo boehmite, (b) γ-Al2O3, (c) δ-Al2O3, (d) θ-Al2O3, (e) α-Al2O3.

8


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●γ-Al2O3

★β-CoMoO4

▼δ-Al2O3

■MoO3

◆θ -Al2O3 □α-Al2O3

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

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□

□

□ ■ ★

□

□

◆ ◆◆ ★ ◆

★

◆ ◆

20

30

d

□

c

◆◆ ◆◆◆◆ ▼

b

▼ ●

●

●●

10

□□ ◆

▼ ▼

▼ ▼▼

□

a

40 50 2 θ, degree

60

70

80

Figure 2. XRD patterns of the oxidized catalysts supported by alumina with different crystal forms in the wide-angle domain. (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3.

The crystal phases of the materials calcined at various temperatures and the corresponding catalyst were confirmed by X-ray diffraction, and the results are presented in Figure 1 and 2. As can be seen from Figure 1, the wide-angle X-ray diffraction of different materials calcined at 873K shows three peaks at 37.50o, 46.10o, and 67.00o, corresponding to the (311), (400), and (440) planes (ICDD file no.: 10-425) of γ-Al2O3, respectively. The material b calcined at 1173K displays five major peaks at 32.76o, 37.6o, 46.48o, 45.62o, and 66.95o, assigned to the (220), (311), (400), (0012), and (440) of δ-Al2O3 (ICDD file no.: 56-1186), correspondingly. The characteristic peaks of 2θ at 31.05o, 33.05o, 37.15o, 39.52o, 45.10o, 48.00o, 60.20o, and 67.58o are attributed to θ-Al2O3 from the material c calcined at 1323K in Figure 1 (ICDD file no.: 47-1771). The peaks appearing at 2θ = 25.4o, 34.92o, 37.60o,43.16o, 9


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52.36o, 57.30o, 66.44o, and 68.00o can be well assigned to α-Al2O3 (ICDD file no.: 82-1467), with the calcined temperature increasing to 1473 K. The X-ray diffraction patterns of the catalysts in Figure 2 except for CoMo/γ-Al2O3 catalyst, show a peak at 2θ = 26.50o, which was ascribed to β-CoMoO4,18 indicating that the metals loaded on the surface of alumina exceeded than the monolayer dispersion state.19 As shown in the results, the intensities of the characteristic peak of β-CoMoO4 species gradually increase with the supports changing from γ-Al2O3 to α-Al2O3. It is generally considered that there is a strong interaction existed between the metals and the support. The state of Mo species on alumina surface is determined by the interaction of Mo and hydroxyl groups on the support surface. Therefore, the formation of β-CoMoO4 indicates that the interaction between the metals and supports is weak.20 This result was further confirmed by FT-IR analysis. In addition, CoMo/α-Al2O3 exhibits two peaks at 23.30o and 27.30o belonging to MoO3, which is due to its smallest surface area of α-Al2O3 support. 3.2 Specific Surface Area of the Supports

10


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b

dV/dD, cm 3·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

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Volume, m3·g-1

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c

a

d 0

20

40 60 80 100 120 140 Pore diameter, nm

a b c d

0.0

0.2 0.4 0.6 0.8 Prelative Pressure, P/P0

1.0

Figure 3. N2 adsorption-desorption isotherms and pore size distribution of alumina materials with different crystal forms. (a) γ-Al2O3, (b) δ-Al2O3, (c) θ-Al2O3, (d) α-Al2O3.

Brunauer-Emmett-Teller (BET) gas sorptometry measurements were conducted to examine the porous nature of the series supports. As shown in Figure 3, the N2 adsorption-desorption isotherms of the series supports exhibited type IV isotherm with a hysteresis loop, which is characteristic of mesoporous materials. The values of the BET surface area, mesoporous pore volume, microporous pore volume and pore diameter are listed in Table 1. Compared with γ-Al2O3, the α-Al2O3 material shows the greatest decrease in the surface area falling from 256 to 24 m2 g-1, and the pore diameter diminishes to 7.2 nm, which is due to the widespread collapse of the mesoporous channels under the high temperature calcination. It can be seen that the alumina surface area and pore volume decrease gradually with increased calcination temperature. The pore diameters of γ-Al2O3, δ-Al2O3, and θ-Al2O3 are 11.3, 17.6 and 20.5 nm respectively. Table S2 (in the Supporting Information) shows that the surface area, pore volumes and the pore size of the catalysts become smaller compared with 11


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the corresponding materials.

Table 1. Textural Properties of the Al2O3 supports with Different Crystal Forms. SBET

Vmes

Vmic

dBJH

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(nm)

γ-Al2O3

256

1.13

0.015

11.3

δ-Al2O3

158

0.91

0.012

17.6

θ-Al2O3

121

0.73

0.009

20.5

α-Al2O3

24

0.08

0.002

7.2

Samples

3.3 FTIR Results of the Supports

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

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3680 3730 3780 a b c d 3800

3700 3600 3500 Wavenumber, cm-1

3400

Figure 4. FT-IR spectra of alumina material with different crystal forms. (a) γ-Al2O3, (b) δ-Al2O3, (c) θ-Al2O3, (d) α-Al2O3.

FT-IR analyses were carried out in order to investigate the changes of the surface properties of alumina with different crystal forms. Figure 4 presents the infrared spectra of the surface hydroxyl groups of these four types of alumina materials after 12


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calcined at different temperatures. The peaks at 3780 cm-1 and 3730 cm-1 are ascribed to basic and neutral hydroxyl groups respectively, while the characteristic peak at 3680 cm-1 is assigned to acidic hydroxyl groups.21-22 The intensities of the basic hydroxyl and acidic hydroxyl characteristic peak decreased significantly, the characteristic peaks of the neutral hydroxyl groups also become broadened along with the crystal forms of alumina transforming from γ- to α- phases. Since the alumina crystal type transformation is a constant process of dehydration, it leads to a decrease of the surface hydroxyl groups. Due to a higher number of hydroxyl groups of γ-Al2O3, the formation of Mo-O-Al bond between the active species and the surface hydroxyls will make the active metals riveting on the surface of alumina, a strong metal support interaction (MSI) is obtained, which cannot be conductive to the reduction and sulfidation of the catalyst. In contrast, the decreasing amount of the surface hydroxyl groups of other crystal forms could modulate the interaction between metal and support. 3.4 H2-TPR Results of the Oxidized Catalysts

13


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1068

740 TCD signal, 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

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a

734

b

722

c

1053 1043

711 1005

d 473

673 873 Temperature, K

1073

Figure 5. H2-TPR profiles of the oxidized catalysts. (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3.

The H2-TPR technique is used to characterize the ease of reduction of CoMo species on the catalyst surface in H2 atmosphere, which provides information of the interaction between CoMo species and the support surface. The TPR results for the CoMo/Al2O3 catalysts are shown in Figure 5. Two strong reduction peaks reduction peaks show upin the range of 673-773 K and 973-1173 K. The low temperature reduction peak can be attributed to the primary reduction of octahedrally coordinated Mo species (from Mo6+ to Mo4+), which can form type II Co-Mo-S active centers after sulfidation. The high temperature reduction peak can be ascribed to the complete reduction of all Mo species (from Mo4+ to Mo0).23,24 In addition, no obvious reduction peak of MoO3 is observed in the range of 873-9003 K, which is in agreement with the XRD results. As can be seen from the result, when the support is different, the TPR profiles of the CoMo catalysts change. The CoMo/γ-Al2O3 catalyst shows the highest 14


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reduction temperature, which indicates the strongest strength of Mo-support interaction on γ-Al2O3. The CoMo/α-Al2O3 catalyst shows the lowest reduction temperature, which can be attributed to less hydroxyl groups on the α-Al2O3 surface. From the above results it can be concluded that the strength of Mo-support interaction of different catalysts decrease in the order of CoMo/γ-Al2O3 > CoMo/δ-Al2O3 > CoMo/θ-Al2O3 > CoMo/α-Al2O3. It is shown that the interaction between the metal and support is weakened with the transformation of the crystal forms of alumina, and the active metal is more easily reduced to a MoS2 active phase in the presulfurization stage.25 3.5 Raman Results of the Oxidized Catalyst

839 818

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

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250 331

564

945 990

d c b a

200 400 600 800 1000 1200 1400 Raman shift, cm-1 Figure 6. Raman spectra of the oxidized catalysts (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3.

Figure 6 shows the Raman spectra of the oxide CoMo catalyst supported by different crystal forms of alumina. As can be seen from the spectra, the bands at 331 15


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cm-1 and 839 cm-1 belong to the Mo=O bending vibration of the tetrahedrally coordinated MoO42- species. The peak at 564 cm-1 is attributed to the Al-O stretching vibration with the effects of the active metals.26 This peak becomes broadened during the transformation from γ-Al2O3 to α-Al2O3, indicating that the surface hydroxyl groups decreases, resulting in a weakening of the interaction between the active metals and support, which is in consistent with the results of Py-IR and H2-TPR. The peaks at 818 cm-1, 877 cm-1, 940 cm-1 and 950 cm-1 are ascribed to the Mo-O bending vibration of asymmetric MoO4 tetrahedron in β-CoMoO4.27 The peak around 945 cm-1 is assigned to the Mo=O stretching vibration of two-dimensional polymer Mo7O246species. There is a good linear relationship between the HDS efficiency of the catalyst and the percentage of the peak area of the two-dimensional molybdenum polymer at about 945 cm-1, and the percentage can be used as an experimental basis for the preparation and improvement of the selective HDS catalyst.28 From the Raman spectra of the catalyst supported on α-Al2O3, a sharp peak of β-CoMoO4 appears at 818 cm-1, and a characteristic peak attributed to MoO3 at 990 cm-1, all of which indicate that the active metals on the surface of CoMo/α-Al2O3 catalyst tend to accumulation. Identical conclusions were obtained by XRD analysis. 3.6 XPS Results of the Sulfided Catalyst

16


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Ⅵ) Mo(Ⅵ (3/2),(5/2)

S2-

240

235 230 Binding energy, eV Ⅳ) Ⅴ) Mo(Ⅳ Mo(Ⅴ (3/2),(5/2) (3/2),(5/2)

S2-

235 230 Binding energy, eV

S 2-

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

(C)

224 (D)

Ⅵ) Mo(Ⅵ (3/2),(5/2)

S2-

240

225

(B)

Ⅵ) Mo(Ⅵ (3/2),(5/2)

240

225

Ⅵ) Mo(Ⅵ (3/2),(5/2)

240

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

(A) Intensity, a.u.

Intensity, a.u.

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

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

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Intensity, a.u.

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236 232 228 Binding energy, eV

224

Figure 7. Mo3d XPS spectra of the sulfided catalysts. (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3.

XPS analysis can provide information on the types, chemical states and chemical interactions of the surface active species of the catalyst. Figure 7 shows the Mo 3d XPS fitting spectra of the sulfided catalysts and the calculation results are listed in Table 2. As can be seen from Figure 7, the Mo 3d spectra are decomposed into three types of peaks.29 The strongest intensity peaks at the binding energies of about 228.9 ±0.1 eV and 231.7±0.1 eV are ascribed to Mo4+ 3d5/2 and 3d3/2 species of MoS2 respectively. The weakest intensity peaks at the binding energy of 230.2±0.1 eV and 233.5±0.1 eV belong to Mo5+ 3d5/2 and 3d3/2 species of MoSxOy. The characteristic peaks of Mo6+ 3d5/2 and 3d3/2 of MoO3 species appeared at 232.5±0.1 eV and 235.8 ±0.1 eV. In addition, the peak at 226.3±0.1 eV binding energy is assigned to S2s 17


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species. The sulfidation degree of Mo species is obtained by SMo=Mosulfidation= Mo4+/(Mo4++Mo5++Mo6+). From the results of Table 2, we can see that the SMo of the CoMo/Al2O3 catalysts increase in the order CoMo/α-Al2O3 < CoMo/γ-Al2O3 < CoMo/θ-Al2O3 < CoMo/δ-Al2O3. The CoMo/α-Al2O3 catalyst shows the lowest sulfidation degree (44%) of Mo species, whereas the CoMo/δ-Al2O3 catalyst exhibits the highest sulfidation degree (64%). Compared with the CoMo/γ-Al2O3 (51%) catalyst, the CoMo/θ-Al2O3 displays higher sulfidation degree (58%). Table 2. XPS Characterization Results of the CoMo Sulfided Catalysts Mo4+ ar.% a

Mo5+ ar.%

ar.%

ar.%

Mo6+ ar.%

ar.% SMo b

Catalysts (228.9

(232.0

(230.5

(233.6

(232.5

(235.6

eV)

eV)

eV)

eV)

eV)

eV)

CoMo/γ-Al2O3

31

20

9

7

19

14

51

CoMo/δ-Al2O3

38

26

6

4

15

11

64

CoMo/θ-Al2O3

35

23

7

6

17

12

58

CoMo/α-Al2O3

28

16

10

9

21

16

44

Note: aar.% means the area percent of XPS peak. b

SMo= Mosulfidation = Mo4+/(Mo4++Mo5++Mo6+).

3.7 HRTEM Results of the Sulfided Catalyst

The interaction between the support and the active species will affect the morphology and dispersion of the MoS2 species, especially the stacking degree of the crystalklites and the length of the lamellae will have an influence on the activity of the 18


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HDS catalyst. In order to obtain information of the MoS2 microcrystals on different supported catalysts, the sulfided catalyst were characterized by HRTEM. The corresponding HRTEM image of the catalyst and the statistical results are presented in Figure 8 and Table 3. As can be seen from the results, the length of the active phase and the number of stacking layers vary with the change of the crystal forms of alumina. The sulfided CoMo/γ-Al2O3 catalyst possesses a lower average stacking number (Nav) and a short slab length

14

of MoS2 crystalline, 2.5 and 3.4 nm

respectively. This indicates that a strong interaction between Mo species and γ-Al2O3, resulting in the mono-layer dispersion of active species, which is easy to form CoMoS-I type species. Compared with CoMo/γ-Al2O3, the average stacking number and slab length of CoMo/δ-Al2O3 and CoMo/θ-Al2O3 catalysts increase, the Nav values are 3.1 and 3.6 while the Lav values are 3.6 nm and 4.2 nm respectively. It is due to the reduction of hydroxyl groups on the surface of alumina, resulting in a weakening MSI. This will be conductive to the formation of the CoMoS-II type active species, which is beneficial for the HDS activity of aromatic sulfur containing compounds.30-31 The fMo values of the series catalyst decrease in the order: CoMo/γ-Al2O3 (0.32) > CoMo/δ-Al2O3 (0.30) > CoMo/θ-Al2O3 (0.28) > CoMo/α-Al2O3 (0.16). Therefore, CoMo/δ-Al2O3 shows relatively higher stacking degree of MoS2 slabs and moderate dispersion of MoS2 species, which will increase the activity of HDS reaction.

19


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Figure 8. HRTEM images of the sulfided catalysts (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3; (E) Length distribution of MoS2 layer dispersed on the supported catalysts.

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Table 3. Average Length and Average Layer Number (Nav) of MoS2 Crystallites. Catalyst

Lav (nm)

Nav

fMo

CoMo/γ-Al2O3

3.4

2.5

0.32

CoMo/δ-Al2O3

3.6

3.1

0.30

CoMo/θ-Al2O3

4.2

3.6

0.28

CoMo/α-Al2O3

5.9

4.6

0.16

3.8 Catalytic Activity for FCC Diesel 100

HDS and HDN efficiency (%)

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Energy & Fuels

HDS HDN

95 90 85 80 75 70 CoMo/γ-Al2O3 CoMo/δ-Al2O3 CoMo/θ-Al2O3

CoMo/α-Al2O3

Figure 9. HDS and HDN activities of CoMo/Al2O3 series catalysts (623 K, 5 MPa, 1 h-1 and H2/Oil of 600).

The catalytic activity of the prepared catalysts was evaluated on the JQ-III type hydrogenated high pressure microreactor using FCC diesel as feedstock. Figure 8 shows the HDS and HDN activities of the CoMo/Al2O3 catalysts. From the results, the CoMo/δ-Al2O3 and CoMo/θ-Al2O3 catalyst exhibited higher catalytic activities 21


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than the commercial CoMo/γ-Al2O3 catalyst. The CoMo/δ-Al2O3 catalyst showed the highest HDS and HDN activities, 98.4% and 96.1% respectively. CoMo/θ-Al2O3 gives a better HDS activity (96.2%) and HDN efficiency (90.1%). Comparably, CoMo/γ-Al2O3 exhibited relatively low catalytic activity, which can be ascribed to the strong MSI of γ-Al2O3 material and the mass transfer diffusion resistance caused by its small pore size. The CoMo/α-Al2O3 catalyst displayed the lowest HDS (87.3%) and HDN (72.2%) activities. The specific surface area decreased and the pore volume greatly reduced during the higher temperature calcination, so that the active metals can not be dispersed on the surface of the support resulting in the decrease of hydrotreating activity. The HDS and HDN efficiencies of the series catalysts follow the order: CoMo/δ-Al2O3 > CoMo/θ-Al2O3 > CoMo/γ-Al2O3 > CoMo/α-Al2O3. 4. DISCUSSION

The HDS activity of FCC diesel over the series catalyst supported on different crystal forms of alumina could be related to the physicochemical properties of the supports and the dispersion of active phases.4 Four kinds of crystal form of alumina with different textural properties were obtained by using pseudo boehmite as the raw material at different calcination temperatures. The specific surface areas and pore volumes of the supports gradually decreased with increasing temperature (Table 1), in the order γ-Al2O3 (256 m2 g-1, 1.13 cm3 g-1) > δ-Al2O3 (158 m2 g-1, 0.91 cm3 g-1) > θ-Al2O3 (121 m2 g-1, 0.73 cm3 g-1) > α-Al2O3 (24 m2 g-1, 0.08 cm3 g-1). Moreover, the θ-Al2O3 showed the maximal pore size and wider pore size distribution, which could be due to the adjacent pore structures of alumina partly collapsed with the increasing calcination temperature, so that the micropores and the smaller mesopores might

22


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connect to form larger mesopores and even macropores. The δ-Al2O3 exhibited a moderate surface area, suitable pore size and a more concentrated pore size distribution, which would contribute to balance the weak diffusion hindrance and metal dispersion, then improve the activity of catalyst. In addition, the number of Al-OH groups decreased with increasing calcination temperatures (Figure 4), caused by the dehydration of pseudo boehmite during the increasing calcination temperature, resulting in the decrease of the number of hydroxyl groups on the supports surface. Therefore, the MSI of the series catalyst decreased as the order: CoMo/γ-Al2O3 > CoMo/δ-Al2O3 > CoMo/θ-Al2O3 > CoMo/α-Al2O3. The strong MSI made the Mo species on the CoMo/γ-Al2O3 surface difficult to be reduced. The corresponding inferior catalytic activity of CoMo/γ-Al2O3 catalyst could be attributed to the relatively lower stacking degree and lower sulfidation degree (51%) of active species. The XRD results of different crystal forms of alumina supported catalysts showed that the characteristic peak of β-CoMoO4 appeared on the CoMo/δ-Al2O3 catalyst, and the peak intensity also enhanced on the CoMo/θ-Al2O3 catalyst. These phenomena could be elucidated by two factors, the first was the weakened MSI, and the second was appropriate accumulation of the active metals with the reduction of specific surface area. As can be seen from the H2-TPR results, CoMo/θ-Al2O3 and CoMo/δ-Al2O3 catalyst were more easily reduced than CoMo/γ-Al2O3 due to their weakened MSI. The XRD pattern of CoMo/α-Al2O3 catalyst showed the characteristic peak of MoO3 due to the smallest surface area. This was consistent with XRD and 23


Energy & Fuels

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FTIR results. HRTEM characterization displayed that the MoS2 slab length and the stacking number of CoMo/θ-Al2O3 (4.2 nm, 3.6) and CoMo/δ-Al2O3 (3.6 nm, 3.1) increased than those of CoMo/γ-Al2O3 (3.4 nm, 2.5). Compared with the CoMo/δ-Al2O3 catalyst, the CoMo/θ-Al2O3 catalyst possessed a relatively smaller surface area and a less accumulation of the active metals on the support surface, which led to a lower sulfidation degree (58%) than CoMo/δ-Al2O3 (64%). The synergy of MSI and the textural properties contributed more to the excellent catalytic activity of CoMo/δ-Al2O3 catalyst. 5. CONCLUSION

Four crystal forms of alumina were obtained by using pseudo boehmite as raw material under different roasting conditions. The characterization results showed that the CoMo/δ-Al2O3 possessed relatively concentrated pore size distribution, moderate specific surface area and appropriate MSI. The catalytic evaluation results showed that the HDS and HDN activities of the corresponding CoMo catalysts increased in the order: CoMo/α-Al2O3 < CoMo/γ-Al2O3 < CoMo/θ-Al2O3 < CoMo/δ-Al2O3. The excellent HDS and HDN activities over CoMo/δ-Al2O3 catalyst could be attributed to the synergy effects of the appropriate texture property, moderate MSI, high sulfidation degree and suitable stacking degree of the MoS2 phases. The hydrotreating catalyst supported on δ-Al2O3 exhibited an outstanding HDS and HDN activity and might be a promise candidate in future industrial application. AUTHOR INFORMATION Corresponding Author

† These authors contributed equally to this work.

24


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

Notes

The authors declare no competing financial interest. 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), Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2015K003), CNPC Key Research Project and KLGCP (GCP201401).

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(22) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L., In situ FTIR spectra of pyridine adsorbed on SiO2-Al2O3, TiO2, ZrO2 and CeO2 : general considerations for the identification of acid sites on surfaces of finely divided metal oxides. Coll.Surf. A 2001, 190, (3), 261-274. 28


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FIGURES Figure 1. XRD patterns of alumina materials with different crystal forms in the

wide-angle domain. (a) pseudo boehmite, (b) γ-Al2O3, (c) δ-Al2O3,(d) θ-Al2O3, (e) α-Al2O3. Figure 2. XRD patterns of the series oxidized catalysts supported by alumina with

different crystal forms in the wide-angle domain. (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3. Figure 3. N2 adsorption-desorption isotherms and pore size distribution of alumina

materials with different crystal forms. (a) γ-Al2O3, (b) δ-Al2O3, (c) θ-Al2O3, (d) α-Al2O3. Figure 4. FT-IR spectra of alumina material with different crystal forms.(a) γ-Al2O3,

(b) δ-Al2O3, (c) θ-Al2O3, (d) α-Al2O3. Figure 5. H2-TPR profiles of the oxidized catalysts. (a) CoMo/γ-Al2O3, (b)

CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3. 30


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Figure 6. Raman spectra of the oxidized catalysts(a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3. Figure 7. Mo3d XPS spectra of the sulfided catalysts.(a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3. Figure 8. HRTEM images of the sulfided catalysts (a) CoMo/γ-Al2O3, (b) CoMo/δ-Al2O3, (c) CoMo/θ-Al2O3, (d) CoMo/α-Al2O3; (E) Length distribution of MoS2 layer dispersed on the supported catalysts. Figure 9. HDS and HDN activities of CoMo/Al2O3 series catalysts TABLES Table 1. Textural Properties of the Series Al2O3 with Different Crystal Forms. Table 2. XPS Characterization Results of the Series of CoMo SulfidedCatalysts. Table 3. Average Length and Average Layer Number (Nav) of MoS2Crystallites.

Graphic Abstract

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