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Catalysis and Kinetics
The influence of Inorganic salt on the ZSM-5/SBA-16 supported NiMo Catalysts for DBT and 4,6-DMDBT HDS Xilong Wang, Yuyang Li, Kebin Chi, Zhen Zhao, Peng Du, Jinlin Mei, Peng Zheng, Jiyuan Fan, Aijun Duan, and Chunming Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00708 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018
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The influence of Inorganic salt on the ZSM-5/SBA-16 supported NiMo Catalysts for DBT and 4,6-DMDBT HDS
Xilong Wang a†, Yuyang Li a†, Kebin Chi b, Zhen Zhao a, Peng Du a, Jinlin Mei a, Peng Zheng a, Jiyuan Fan a, Aijun Duan a*, Chunming Xu a*
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Beijing, 18 Fuxue Road, Beijing, P.R. China, 102249 b
Petrochemical Research Institute, PetroChina Company Limited, Beijing, P. R.
China, 100195
† Xilong Wang and Yuyang Li contributed equally to this work.
Corresponding Author: *E-mail:
[email protected]. Tel: +86 10 89732290. *E-mail:
[email protected]. Tel: +86 10 89733392.
Abstract Series ZSM-5/SBA-16 (ZS) composites were prepared by using various inorganic cations as additives and the associated catalysts were tested for the dibenzothiophene
(DBT)
and
4,6-dimethyldibenzothiophene 1
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(4,6-DMDBT)
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hydrodesulfurization (HDS). Structure-function relationship were investigated through various characterization methods. NiMo/ZS-Mg catalyst using MgCl2 as additive displayed higher HDS reaction rate constant (kHDS) and turn-over frequency (TOF)
values
which
could
be
attributed
to
its
major
acidity,
higher
metal-support-interaction (MSI), higher sulfidation degree, higher dispersion degree, moderate stacking number and shorter stacking length of sulfide Mo species, of which the higher acidity, higher MSI contributed more to the improvement of its HDS performance. Besides, the possible HDS pathways on NiMo/ZS-Mg catalyst were provided.
Keywords ZSM-5/SBA-16;
Inorganic
cation;
Hydrodesulfurization;
Dibenzothiophene;
4,6-dimethyldibenzothiophene
1. INTRODUCTION The toxic gases released from the vehicle engine give rise to serious problems on environment and human health.
1-3
Therefore, environmental regulations are being
frequently modified and become increasingly stringent, following by the current diesel specifications require the sulfur content at or below 10 ppm, meaning that the macromolecule with highly steric hindrance such as 4,6-DMDBT need to be removed. 4-8
The most effective way at present to eliminate S in diesel is HDS. However, the
traditional Ni (Co)-promoted (W) MoS2 catalysts supported on alumina are faced with an enormous challenge on account of their MSIs.
9-11
In order to satisfy the diesel
2
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desulfurization specifications, alternative supports with superior structural properties and acid intensities continue to be a focus area because the morphologies of active phases are closely related to the nature of the support.
12, 13
A variety of novel
materials, such as zeolites, and mesoporous silicas become the important issue of the design and synthesis of HDS catalysts because their distinct physico-chemical properties in structure and acidity are in favor of the improvement of HDS activities. ZSM-5, Beta, zeolite Y, ferrierite and mordenite are a significant group of heterogeneous catalysts because of their superior acid properties combining with thermal and hydrothermal stability.
14-16
Sugioka et al.
17
found that Pt/HZSM-5
catalyst showed higher thiophene HDS performance than that of the conventional CoMo/alumina catalyst. Wu et al.
18
found that HZSM-5 supported NiMo catalyst
displayed more isomerization (ISO) pathway of 4,6-DMDBT HDS than that of pure silica supported NiMo catalyst and the conventional NiMo/alumina catalyst, which vastly promoted the HDS performance. However, the accessibility of reactants to active phases is still limited by the smaller pore sizes less than 2 nm. 19-21 In order to promote the diffusion of the reactants and products, many researches took mesoporous silicas as the supports to estimate their HDS performance by decreasing the mass transfer resistance through the large pore channels. 22-24 Due to its ordered mesopores with three-dimensional cubic arrangement (Im3m) and relatively hydrothermal stability, SBA-16 material caught the research’s attention. 25 Guzmán et al. 26 found that P-loaded NiMoW/SBA-16 catalyst showed higher HDS performance than that over the P-free counterparts. Cao et al.
27
prepared NiMo/Al-SBA-16 and
3
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discovered that it displayed high DBT HDS performance. Nevertheless, pure silicas with no acidity and weak hydrothermal stability show less active for the HDS reaction.28 To resolve the imperfection of single microporous and mesoporous materials, the synthesis of composites with hierarchically pores gathering the preponderance of microporous and mesoporous materials is imperative. Zhang et al.
28
prepared
Beta-KIT-6 micro-mesoporous composites and found that NiMo/Beta-KIT-6 displayed preferable DBT HDS performance than that over the reference NiMo/Al2O3. Wu et al.
18
synthesized NiMo/ZSM-5-KIT-6 by using wrapped method and
confirmed that it exhibited higher 4,6-DMDBT HDS performance and ISO selectivity. Some researchers found that the additions of inorganic salts in the process of parent solution for mesoporous materials had distinct influence on the structural properties of mesoporous silicas.
29-31
However, the influence of inorganic cation on the structural
properties of ZS composites has not yet been reported and needs to be investigated systemically. This work was to investigate the influence of inorganic cations (K+, Mg2+, Mn2+, Zn2+ and Cu2+) on the structural properties and surface acidities of ZS composites. The corresponding catalysts were tested by employing DBT and 4,6-DMDBT as reactants. NiMo/ZS-Mg synthesized by using Mg2+ as the additive exhibited higher HDS activity than that over other reference catalysts.
2. EXPERIMENTAL SECTION 4
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2.1 Preparation of the supports 2.1.1 Synthesis of ZSM-5 seed ZSM-5 (molar ratio of SiO2/Al2O3 = 30) is synthesized as the following procedure. 0.143 g of NaOH, 0.5 g of NaAlO2 and 10 g of deionized water were mixed and kept stirring at 25 ºC for 1.0 h (marked as Solution X). 0.3 g of NaOH, 2.65 g of tetrapropylammonium bromide and 8.0 g of deionized water were mixed and kept stirring at 25 ºC for 15 min. Then 10 g of colloidal silica was put into the mixture. The mixture was kept stirring at 25 ºC for 1.0 h (marked as Solution Y). Afterwards, Solution X was put dropwise into Solution Y and kept stirring at 25 ºC for 3.0 h. The obtained solution was transferred to a reactor and kept static at 170 °C for 32 h. 2.1.2 Synthesis of ZS supports ZS supports prepared by using different inorganic cations (K+, Mg2+, Mn2+, Zn2+ and Cu2+) as additives were fabricated by the following procedures. Different amounts (5.0, 13.6, 13.3, 9.1 and 11.4 g) of (KCl, MgCl2·6H2O, MnCl2·4H2O, ZnCl2 and CuCl2·2H2O), 2.0 g of F127 and 120 mL of hydrochloric acid (2.0 mol/L) were mixed and stirred at 30 °C for 2.0 h. Subsequently, 6.0 g of n-butanol was put into the mixture and stirred at 30 °C for 2.0 h. 8.3 g of TEOS was put into the above mixture and stirred at 30 °C for 1.0 h. Afterwards, 9.28 g of ZSM-5 seed was put dropwise into the above mixture and agitated at 30 °C for 1.0 h. Then the obtained mixture was kept motionless at 30 °C for 24 h. The above mixture was moved to a reactor and kept static at 100 °C for 24 h. ZS series composites were acquired through calcination at 5
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550 ºC for 6 h. ZS by using different inorganic cations (K+, Mg2+, Mn2+, Zn2+ and Cu2+) as additives were labelled as ZS-K, ZS-Mg, ZS-Mn, ZS-Zn and ZS-Cu, correspondingly.
2.1.3 Synthesis of series catalysts Before preparing series NiMo catalysts, H-type ZS series supports were obtained through two-step ion exchanges in NH4Cl solution with a support/NH4Cl/H2O mass ratio of 1g/3g/30g at 90 °C for 3 h. After each ion exchange, the obtained supports were calcined at 550 °C for 3 h. NiMo supported catalysts were synthesized according to the published paper. 3 The metal contents of series catalysts were kept at 15 wt.% of MoO3 and 3.5 wt.% of NiO. Series NiMo catalysts were marked as NiMo/ZS-K, NiMo/ZS-Mg, NiMo/ZS-Mn, NiMo/ZS-Zn and NiMo/ZS-Cu, correspondingly.
2.2 Characterization of series samples X-ray diffraction (XRD), Nitrogen physisorption isotherms, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), 27Al nuclear magnetic resonance (27Al NMR), Pyridine-FTIR, Raman, X-ray photoelectron spectra (XPS), High-resolution transmission electron microscopy (HRTEM) were characterized according to the published paper.3, 32
2.3 HDS evaluation DBT (sulfur contents = 500 ppm) or 4,6-DMDBT (sulfur contents = 500 ppm)
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HDS reaction were tested in a fix-bed reactor by using 1.0 g catalyst (0.25-0.37 mm). Prior to HDS evaluation, series NiMo/ZS were presulfided. Then the HDS efficiencies were evaluated at 340 °C, 4.0 MPa, 200 mL/mL and 10-100 h-1. Desulfurization degree (%) was obtained by Equations (c): HDS (%) = (Sf -Sp)/Sf ×100%
(c)
where Sp denotes the sulfur contents (ppm) of the feeds and Sf denotes the sulfur contents (ppm) of the products. Regarding DBT or 4,6-DMDBT HDS as a pseudo-first-order reaction, kHDS (mol·g-1·h-1) in this reaction system were determined by Equation (d): 33
k HDS =
F 1 ln m 1− τ
(d)
where F the flow rate of feed (mol·h-1), m the catalyst mass (g), and τ the desulfurization degree. HDS activities were reflected by a parameter of TOF (h-1), which was calculated according to Equation (e). 34 TOF = (F × x)/(nMo × fMo)
(e)
where F represents molar flow rate of reactants (mol h-1), x the desulfurization degree (%), nMo the moles of Mo atoms of series NiMo/ZS catalysts (mol), the dispersion degree (fMo) represents the percentage of Mo atoms located on the edges of MoS2 active phases, which is calculated through Equation (f). 35 Moedge ∑ (6ni − 6) fMo = = t i =1 2 Mototal ∑ (3ni − 3ni + 1) i =1 t
7
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(f)
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where Moedge represents effective Mo number located on the edges of sulfide Mo species, Mototal the total Mo number, ni the Mo number located on one side of MoS2 stacking, which is calculated according to the length of the MoS2 phases (L=3.2(2ni-1) Å) and t the total number of no less than 350 MoS2 slabs obtained through HRTEM images. The HDS products were detected through a gas chromatograph combined with a mass spectrometer (GC-MS).
3. RESULTS 3.1 Characterization of series materials 3.1.1 XRD of series supports
(A)
(B)
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
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a b c d e 1
2 3 4 2θ, degree
f a b c d e 10
5
20
30 40 2θ, degree
50
Figure 1. (A) Small and (B) wide-angle XRD of series materials. (a) ZS-K, (b) ZS-Mg, (c) ZS-Mn, (d) ZS-Zn, (e) ZS-Cu, (f) ZSM-5.
Small-angle XRD of series materials prepared are displayed in Figure 1(A). All series supports display the typical characteristic peaks in the 2θ range of 0.2°-1° which are ascribed to SBA-16.
36
In addition, ZS-Mg support displays a stronger
diffraction peak intensity than other supports, manifesting that it possesses higher crystalline degrees. 37 8
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Wide-angle XRD of series supports are presented in Figure 1(B). Series ZS supports prepared by using various inorganic cations as additives exhibit five typical characteristic peaks in the 2θ range of 8°-10° and 22.5°-25° which are attributed to ZSM-5 zeolites.
38
Compared to pure ZSM-5, the peak intensities of series ZS are
relatively weak, indicating that ZSM-5 embraces the primary and secondary unit structures of microporous ZSM-5 crystals in the series ZS composites, which confirms the incorporation of ZSM-5 seeds into the ZS supports.
3.1.2 N2 physisorption of series ZS
(A) a b c d e
0.0
(B)
dV/dD, cm3·g-1·nm-1
Volume, m3·g-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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0.2 0.4 0.6 0.8 Relative Pressure, P/P 0
1.0
0
a b c d e 10 20 30 Pore diameter, nm
40
Figure 2. (A) N2 physisorption isotherms and (B) pore size distributions of series ZS. (a) ZS-K, (b) ZS-Mg, (c) ZS-Mn, (d) ZS-Zn, (e) ZS-Cu.
N2 physisorption isotherms of series ZS prepared by using various inorganic cations as additives are exhibited in Figure 2(A). All the supports present type IV isotherms with H2 hysteresis loop which are assigned to mesoporous structures. 39 The pore size distributions of series ZS are presented in Figure 2(B). All series ZS present narrower mesopore distributions in the range of 3-5 nm. The physicochemical properties of series ZS are summarized in Table S1 (in Supporting
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Information). The specific surface areas, pore volumes and pore sizes of series ZS vary slightly with different organic cations as additives.
3.1.3 27Al MAS NMR of the supports 27
Al NMR of series ZS prepared by using various inorganic cations as additives
are presented in Figure 3. Series ZS supports display two typical peaks at δ = 0 and 54 ppm which are attributed to octahedral coordinated extra-framework and tetrahedral coordinated framework Al species, correspondingly. 40 ZS-Mg support exhibits higher peak intensities at δ = 54 ppm, manifesting that more Al species are incorporated into ZS-Mg composite.
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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a b c d e
-100
Figure 3.
27
0
100 ppm
200
Al NMR of series ZS materials. (a) ZS-K, (b) ZS-Mg, (c) ZS-Mn, (d) ZS-Zn, (e)
ZS-Cu.
3.1.4 TEM images of series ZS
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The pore channel structure of series ZS supports can be observed through TEM images (Figure S2, in Supporting Information). Series ZS supports all display ordered pore channels with cubic Im3m space group.
3.1.5 SEM images of series ZS
(a)
(b)
20 µm
20 µm
(c)
(d)
20 µm
20 µm
(e)
20 µm
Figure 4. SEM images of series ZS. (a) ZS-K, (b) ZS-Mg, (c) ZS-Mn, (d) ZS-Zn, (e) ZS-Cu.
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The morphologies of series ZS prepared by using various inorganic cations as additives can be observed through SEM images (Figure 4). When using Mg2+ as additive, the ZS-Mg support displays relatively good morphology. While using Zn2+ and Cu2+ as additives, the ZS-Zn and ZS-Cu supports exist obvious aggregation. The hydrolysis rate of the template (F127) and the precipitation rate of the silicon source (TEOS) are closely related to the additives (inorganic cation), resulting in the generation of series ZS supports with various morphologies. 30
3.2 Characterization of series catalysts
(A)
B
L
L L B L+B
a b c d e
1700
1600 1500 Wavenumber, cm-1
Absorbance, a.u.
3.2.1 Pyridine-FTIR of series 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
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(B)
14001700
B L L B L+B
L a b c d e
1600 1500 Wavenumber, cm-1
1400
Figure 5. Pyridine-FTIR of series oxide catalysts degas at (A) 200 ºC and (B) 350 ºC. (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn and (e) NiMo/ZS-Cu.
Pyridine-FTIR spectra are usually used to measure the acidity of catalysts. The measuring results of series NiMo/ZS oxide catalysts are displayed in Figure 5. FTIR spectra obtained after desorption of pyridine molecules at 200 ºC and 350 ºC are attributed to the total acidic amounts, the medium and strong acidic amounts, respectively. Four peaks (1446, 1492, 1575 and 1622 cm-1) are ascribed to L acid, 12
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while the other two peaks (1546 and 1639 cm-1) are ascribed to B acid. 41, 42 B and L acid contents of series oxide catalysts are listed in Table S2 (in Supporting Information). The total acidic contents, the medium and strong acid contents decrease as the trend of NiMo/ZS-Mg > NiMo/ZS-K > NiMo/AS-Mn > NiMo/AS-Zn > NiMo/AS-Cu. The B acid amounts of the total, the medium and strong acid decrease as the same tendency.
3.2.2 Raman of the oxide and sulfide catalysts
a b c d e
200
(B)
Intensity, a.u.
(A)
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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400 600 800 1000 1200 Raman shift, cm-1
a b c d e 400
600 800 Raman shift, cm-1
1000
Figure 6. Raman of (A) oxide catalysts and (B) sulfide catalysts. (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn and (e) NiMo/ZS-Cu.
Figures 6(A) and (B) exhibit Raman spectra of series oxide and sulfide catalysts, correspondingly. The oxide catalysts display four typical peaks at 955, 908, 826 and 331 cm-1, of which the former three peaks at 955, 908 and 826 cm-1 are ascribed to NiMoO4 species, 43 and the last one at 331 cm-1 is assigned to the bending vibration of M = OT (T represents terminal bonding). In addition, there exists one wide peak at 955 cm-1, which is ascribed to the symmetric stretching vibrations of M = O. 44 13
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Series NiMo/ZS sulfide catalysts also show four typical peaks at 380, 405, 454 and 634 cm-1, of which the peaks at 380 and 405 cm-1 are ascribed to the E12g and A1g modes of sulfide Mo species, and the peaks at 454 and 634 cm-1 are assigned to the resonance Raman scatting.
45, 46
The peak intensities of series NiMo/ZS sulfide
catalysts change as the sequence of NiMo/ZS-Mg > NiMo/ZS-K > NiMo/ZS-Mn > NiMo/ZS-Zn > NiMo/ZS-Cu, manifesting that the sulfidation degrees of Mo active phases are subject to the same tendency. Most of the oxide Mo species are turned into the sulfide Mo species through presulfurization process.
3.2.3 XPS of series sulfide catalysts XPS characterization of series NiMo/ZS sulfide catalysts are performed to determine the chemical state and surface composition of active metals. The XPS peak-differentation-imitating analysis criterions are provided in the published paper. 47
The sulfidation degrees are calculated according to the published paper.
5
The
Mo3d XPS peak-differentation-imitating results of series catalysts are exhibited in Figure 7 and the statistical results are listed in Table S3 (in Supporting Information).
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(Ⅳ Ⅳ)
Mo (3/2),(5/2)
2-
235 230 225 Binding energy, eV
(Ⅵ Ⅵ)
Mo Ⅳ (3/2),(5/2)
Mo Ⅵ (3/2),(5/2)
Mo (3/2),(5/2) 2-
235 230 225 Binding energy, eV
2-
235 230 225 Binding energy, eV (Ⅳ Ⅳ)
(Ⅵ Ⅵ)
Mo Ⅳ (3/2),(5/2)
Mo Ⅵ (3/2),(5/2)
240
(Ⅴ Ⅴ)
Mo Ⅴ (3/2),(5/2) 2-
S
235 230 225 Binding energy, eV
(Ⅳ Ⅳ)
Mo Ⅳ (3/2),(5/2)
(e) (Ⅵ Ⅵ)
Mo Ⅵ (3/2),(5/2)
240
(Ⅴ Ⅴ)
Mo Ⅴ (3/2),(5/2) S
(d)
(Ⅴ Ⅴ)
S
240
(Ⅵ Ⅵ)
Mo Ⅳ (3/2),(5/2)
Mo Ⅵ (3/2),(5/2)
240
(Ⅳ Ⅳ)
(c) Intensity, a.u.
Mo (3/2),(5/2) S
240
(Ⅳ Ⅳ)
(b)
(Ⅴ Ⅴ)
Intensity, a.u.
(Ⅵ Ⅵ)
Mo Ⅳ (3/2),(5/2)
Intensity, a.u.
Intensity, a.u.
(a)
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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(Ⅴ Ⅴ)
Mo Ⅴ (3/2),(5/2) 2-
S
235 230 225 Binding energy, eV
Figure 7. Mo3d XPS spectra of series catalysts. (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn and (e) NiMo/ZS-Cu.
Table S3 (in Supporting Information) lists sulfidation degrees of series catalysts, which decrease as the trend of NiMo/ZS-Mg (73 %) > NiMo/ZS-K (67 %) > 15
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NiMo/ZS-Mn (62 %) > NiMo/ZS-Zn (57 %) > NiMo/ZS-Cu (50 %), and these are in well consistent with Raman results of various sulfide catalysts.
3.2.4 HRTEM of the sulfide catalysts
The morphologies of MoS2 layers of series sulfide catalysts can be seen intuitively through HRTEM images. The statistical results of stacking number and stacking length of series sulfide catalysts are presented in Figure 8. NiMo/ZS-Mg sulfide catalyst exhibits lower stacking number and shorter stacking length than those over other sulfide catalysts. The average stacking length (Lav), stacking number (Nav) and the dispersion degree (fMo) of sulfide Mo layers on various catalysts are listed in Table 1. Lav and Nav of sulfide Mo layers on series sulfide catalysts decrease as the following sequence: NiMo/ZS-Cu > NiMo/ZS-Zn > NiMo/ZS-Mn > NiMo/ZS-K > NiMo/ZS-Mg. Nevertheless, fMo values of sulfide Mo layers of various catalysts decrease with the reverse trend. NiMo/ZS-Mg catalyst possesses relatively short stacking length and relatively high dispersion degree of Mo active species which can improve the HDS performance.
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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
50 40 30 20 10 0
(D)
%
%
(C)
1 2 3 4 5 6 7 8 Number of layers
10 nm
(E)
50 40 30 20 10 0 1 2 3 4 5 6 7 8 Number of layers
10 nm
10 nm
1 2 3 4 5 6 7 8 Number of layers
10 nm
50 40 30 20 10 0
(F)
%
80 NiMo/ZS-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
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1 2 3 4 5 6 7 8 Number of layers
NiMo/ZS-Mn NiMo/ZS-Zn
40
NiMo/ZS-Cu
20 0
10 nm
NiMo/ZS-Mg
60
0-20 20-40 40-60 60-80 >80 Slab length, 10-1 nm
Figure 8. HRTEM images of series catalysts. (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn, (e) NiMo/ZS-Cu, and (f) length distribution of sulfide Mo layers. 17
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Table 1. Lav and Nav of sulfide Mo species.
Catalyst
Lav (nm)
Nav
fMo
NiMo/ZS-K
3.5
2.4
0.36
NiMo/ZS-Mg
3.3
2.3
0.38
NiMo/ZS-Mn
3.7
2.6
0.33
NiMo/ZS-Zn NiMo/ZS-Cu
4.2
3.1
0.27
4.4
3.4
0.24
3.3 Results of the DBT HDS 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
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100 80 60 ba c 40 d 20 fe 100
80
60 40 WHSV, h-1
20
0
Figure 9. DBT HDS results at various WHSVs (340 ºC, 4.0 MPa, 200 mL/mL). (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn, (e) NiMo/ZS-Cu and (f) NiMo/Al2O3.
Figure 9 displays DBT HDS efficiencies of various catalysts at 10-100 h-1. In addition, the reference NiMo/Al2O3 catalyst is used as comparison. The liquid yield for each product is greater than 99%. The DBT desulfurization degrees of series NiMo/ZS catalysts increase with the reducing WHSVs because of the enhancing contact time between the reactants and the active phases. DBT HDS activities of 18
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series NiMo/ZS at 100 h-1 decrease as the trend of NiMo/ZS-Mg > NiMo/ZS-K > NiMo/ZS-Mn > NiMo/ZS-Zn > NiMo/Al2O3 > NiMo/ZS-Cu. NiMo/ZS-Mg catalyst exhibits the highest DBT HDS performance than those over other catalysts. Figure S7 (in Supporting Information) displays DBT HDS product distributions on different catalysts at various WHSVs, and the GC spectrogram of NiMo/ZS-Mg catalyst at various WHSVs are displayed in Figure S6 (in Supporting Information). Biphenyl (BP) selectivity increases with the decreasing WHSV, manifesting that the improvement of the proportion of DDS route. DDS is the prior DBT HDS route at 10-100 h-1, which can be obtained through that the BP selectivities exceed 50 % during 10-100 h-1. The selectivities of cyclopentylmethylbenzene (CPMB), isophenyl hexadiene (PHDi) and cyclopentylmethylcyclohexane (CPMCH) increase slightly, while tetrahydrodibenzothiophene (THDBT) selectivity decreases with the decrease of WHSVs. The possible DBT HDS reaction pathway (Figure S8, in Supporting Information) on series NiMo/ZS catalysts is deduced through the product distributions by means of GC-MS analysis method under the same DBT desulfurization degree of ~50 % (Figures S5, in Supporting Information). The possible DBT HDS reaction pathway includes two primary routes: (1) DDS route, S atoms are eliminated directly and BP is the final product; (2) HYD route, the aromatic rings are hydrogenated firstly before the elimination of S atoms, while CHB, CHEB, THDBT, PHDi, CPMB and CPMCH are the intermediate or final products. HYD is the prior DBT HDS pathway on NiMo/ZS-Mg. 19
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Table 2 lists DBT HDS product distributions of different catalysts. The HYD/DDS ratios of different catalysts decrease as the trends of NiMo/ZS-Mg (1.27) > NiMo/ZS-K (1.08) > NiMo/ZS-Mn (0.89) > NiMo/ZS-Zn (0.67) > NiMo/ZS-Cu (0.61) > NiMo/Al2O3 (0.56). NiMo/ZS-Mg catalyst exhibits higher HYD/DDS ratio of DBT HDS, which can improve DBT HDS activities. kHDS and TOF (Table 2) of DBT HDS on various catalysts change as the trends of NiMo/ZS-Mg > NiMo/ZS-K > NiMo/ZS-Mn > NiMo/ZS-Zn > NiMo/Al2O3 > NiMo/ZS-Cu. NiMo/ZS-Mg catalyst displays higher kHDS and TOF of DBT HDS than those over other catalysts.
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Table 2. DBT HDS product distributions on different catalysts a.
Catalyst
NiMo/ZS-K NiMo/ZS-Mg NiMo/ZS-Mn NiMo/ZS-Zn NiMo/ZS-Cu NiMo/Al2O3
kHDS (10-4 mol g-1 h-1)
TOFb
11.9 14.5 10.8 7.9 6.7 7.3
4.8 5.2 4.5 4.0 3.7 3.9
Product selectivity (%) HYD
DDS
THDBT
CHEB
PHDi
CHB
CPMB
CPMCH
BP
HYD/DDS ratio
2 2 2 1 1 1
1 1 1 1 1 2
2 3 2 1 1 0
40 42 36 32 31 33
4 5 4 3 3 0
3 3 2 2 1 0
48 44 53 60 62 64
1.08 1.27 0.89 0.67 0.61 0.56
(h-1)
a
Obtained at about 50% of total desulfurization degree by adjusting WHSVs (340 oC, 4 MPa, 200 mL/mL).
b
Number of reacted DBT molecules per hour and per Mo atom at the edge sites.
HYD: THDBT + CHEB + PHDi + CHB + CPMB + CPMCH; DDS: BP.
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3.4 Results of 4,6-DMDBT HDS 100 Desulfurization degree, %
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80 60 40 b a c 20 f d e 0 100
80
60 40 WHSV, h-1
20
0
Figure 10. 4,6-DMDBT HDS performance at different WHSVs (340 oC, 4 MPa, 200 mL/mL). (a) NiMo/ZS-K, (b) NiMo/ZS-Mg, (c) NiMo/ZS-Mn, (d) NiMo/ZS-Zn, (e) NiMo/ZS-Cu, and (f) NiMo/Al2O3.
Figure 10 shows 4,6-DMDBT HDS efficiencies of series catalysts at 10-100 h-1. The HDS performance of different catalysts decrease as the trends of NiMo/ZS-Mg > NiMo/ZS-K > NiMo/ZS-Mn > NiMo/Al2O3 > NiMo/ZS-Zn > NiMo/ZS-Cu. NiMo/ZS-Mg catalyst shows higher HDS desulfurization degree than those over other catalysts. Figure 11 exhibits 4,6-DMDBT HDS product distributions on NiMo/ZS-Mg catalystat at various WHSVs, and the GC spectrogram at various WHSVs are presented in Figure S10 (in Supporting Information). 4,4'-dimethylbiphenyl (4,4'-DMBP) and iso-methyl-isopropyltetralin (Iso-MIPT) selectivities increase with the decreasing WHSV, manifesting that the improvement of the proportion of 22
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isomerization (ISO) route. DDS route is not preferential 4,6-DMDBT HDS pathway in the WHSV range of 10-100 h-1, which can be obtained through that the 3,3'-dimethylbiphenyl (3,3'-DMBP) selectivity is less than 10 % at 10-100 h-1. The selectivities
of
3,3'-dimethylcyclohexylbenzene
(3,3'-DMCHB)
and
3,3'-dimethylbicyclohexyl (3,3'-DMBCH) increase slightly with the decreasing WHSVs. 3,3'-DMCHB
40
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
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4,4'-DMBP
30 20 10 0
Iso-MIPT THDMDBT 3,3'-DMBCH 3,3'-DMBP
100
80
60 40 WHSV, h-1
20
0
Figure 11. 4,6-DMDBT HDS product distributions over NiMo/ZS-Mg at various WHSVs.
The possible 4,6-DMDBT HDS reaction pathway (Figure S11, in Supporting Information) on series catalysts is deduced through the products in virtue of GC-MS analysis method under the same 4,6-DMDBT desulfurization degree of ~50 % (Figures S9, in Supporting Information). The possible 4,6-DMDBT HDS reaction pathway includes three routes: (1) DDS route, S atoms are eliminated directly; (2) HYD route, the aromatic rings are hydrogenated firstly before the elimination of S atoms, 4,6-THDMDBT, 4,6-HHDMDBT, 3,3'-DMCHB and 3,3'-DMBCH are the intermediate or final products; (3) ISO route, the methyl groups of 4-, 6-sites are 23
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transferred into 3-, 6-sites or 3-, 7-sites firstly before the elimination of S atoms through HYD or DDS route continuously, 4,4'-DMBP and Iso-MIPT are the intermediate or final products. ISO is the preferential route of 4,6-DMDBT HDS on NiMo/ZS-Mg. Table 3 presents 4,6-DMDBT HDS product distributions of different catalysts. The ISO proportions on different catalysts decrease as the following trends: NiMo/ZS-Mg (51 %) > NiMo/ZS-K (49 %) > NiMo/ZS-Mn (46 %) > NiMo/ZS-Zn (43 %) > NiMo/ZS-Cu (39 %) > NiMo/Al2O3 (0). NiMo/ZS-Mg catalyst displays higher proportion of ISO route for 4,6-DMDBT HDS, which can promote the improvement of HDS performance. kHDS and TOF (Table 3) of 4,6-DMDBT HDS on various catalysts decrease as the following trends: NiMo/ZS-Mg > NiMo/ZS-K > NiMo/ZS-Mn > NiMo/ZS-Zn > NiMo/Al2O3 > NiMo/ZS-Cu. NiMo/ZS-Mg catalyst exhibits higher kHDS and TOF of 4,6-DMDBT HDS than those on other catalysts.
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Table 3. 4,6-DMDBT HDS product distributions over various catalystsa.
Catalyst
kHDS (10-4 mol· g-1·h-1)
Product selectivity (%) b
TOF
HYD
DDS
(h )
4,6-THDMDBT + 3,3'-DMCHB 3,3'-DMBCH 3,3'-DMBP 4,6-HHDMDBT 6 30 9 6 NiMo/ZS-K 7.9 2.6 7 26 9 7 NiMo/ZS-Mg 8.3 2.7 8 30 10 8 NiMo/ZS-Mn 6.3 2.2 8 29 11 9 NiMo/ZS-Zn 4.1 1.5 9 31 12 9 NiMo/ZS-Cu 3.8 1.4 4.3 1.6 4 36 6 54 NiMo/Al2O3 a Obtained at about 50% of total desulfurization degree by adjusting WHSVs (340 oC, 4 MPa, 200 mL/mL). b
Total
ISO
-1
ISO 4,4'-DMBP
Iso- MIPT
33 36 31 30 28 0
16 15 15 13 11 0
Number of reacted 4,6-DMDBT molecules per hour and per Mo atom at the edge sites.
HYD: 4,6-THDMDBT + 4,6-THDMDBT + 3,3’-DMCHB + 3,3’-DMBCH; DDS: 3,3’-DMBP; Total ISO: 4,4’-DMBP + Iso-MIPT.
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4. DISCUSSION Different inorganic cations (K+, Mg2+, Mn2+, Zn2+ and Cu2+) affect the structural properties, acidity and MSI of series NiMo/ZS oxide catalysts and then influence the morphologies of sulfide Mo species on series NiMo/ZS sulfide catalysts, consequently result in the changes of HDS activities. The structural properties of series ZS supports, involving pore sizes, surface areas, pore volumes, and the pore orderliness, and the surface morphologies are closely related to the diffusion of the reactants and products, which can improve the HDS reaction rates. 40 The addition of inorganic cations have slightly influence on the specific surface areas, pore sizes and pore volumes of series ZS. ZS-Mg catalyst possesses relatively good pore distributions and relatively ordered pore channels which can promote the HDS reaction. The acidities (B & L acid) of series catalysts play a significant role on the selectivity of HDS reaction. NiMo/ZS-Mg catalyst presents higher HYD/DDS ratios of DBT HDS, which can be assigned to its more B & L acidic amounts, resulting in the improvement of accessibility between reactants and active MoS2 species. Besides, NiMo/ZS-Mg catalyst presents higher ISO selectivity of 4,6-DMDBT HDS, which can be assigned to its more B acidic amounts, resulting in the reduction of steric hindrance for the 4,6-DMDBT molecules. 48, 49 MSI of series NiMo/ZS catalysts affects the dispersion degree and the morphologies of MoS2 phases.
32
NiMo/ZS-Mg sulfide catalyst possesses higher
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dispersion degrees and shorter stacking lengths of MoS2 phases which can be attributed to its higher MSI, resulting in the exposure of more active sites. The morphologies, involving stacking number and stacking length of sulfide Mo species affect HDS activity directly. 5 Moderate stacking number and relatively short stacking length of MoS2 slabs are in favour of the improvement of HDS activity. NiMo/ZS-Mg sulfide catalyst owns suitable stacking number and shorter stacking length of MoS2 slabs which are closely to its better structural properties and higher MSI, resulting in its higher HDS efficiencies. In addition, NiMo/ZS-Mg sulfide catalyst exhibits higher sulfidation degree which can promote directly HDS reactions. The better HDS activities of NiMo/ZS-Mg catalyst can be attributed to the synergistic effect of its relatively surface areas, relatively concentrated pore size distributions, ordered pore channels, higher acidity, higher MSI, higher sulfidation degree, suitable stacking number and shorter stacking length of MoS2 slabs, of which its MSI and acidity contributes more to the improvement of HDS efficiencies.
5. CONCLUSION A series of ZS with various structural properties and acidity were successfully synthesized by using different inorganic cations as additives. NiMo/ZS-Mg catalyst showed higher HYD selectivities of DBT and higher ISO selectivities of 4,6-DMDBT, which could be assigned to its more acidities (B & L acid), resulting in the improvement of the accessibility between reactants and MoS2 species. NiMo/ZS-Mg sulfide catalyst displayed higher kHDS and TOF of DBT and 27
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4,6-DMDBT HDS which could be assigned to its higher sulfidation degree, higher dispersion degree, moderate stacking number and shorter stacking length of sulfide Mo species. The higher MSI and acidity of NiMo/ZS-Mg catalyst contributed more to the higher HDS activity. For NiMo/ZS-Mg catalyst, the DBT HDS reaction pathway included HYD and DDS, and HYD was its preferential route; the 4,6-DMDBT HDS reaction pathway included HYD, DDS and ISO, and ISO was its prior pathway. HYD of DBT HDS and ISO of 4,6-DMDBT HDS enhanced the accessibility between reactants and Mo active species, leading to the enhancement of HDS activities over NiMo/ZS-Mg catalyst.
ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.21676298, U1463207 and 21503152), and CNPC Key Research Project.
ASSOCIATED CONTENT Supporting Information. Desulfurization degree and particle size (Figure S1), TEM images of series ZS (Figure S2), DBT HDS performance at various WHSVs (Figure S3), HDS performance with time on stream (Figure S4), The GC spectrogram of DBT HDS product on different catalysts (Figure S5), The GC spectrogram of DBT HDS product on NiMo/ZS-Mg at various WHSVs (Figure S6), Product distributions of DBT HDS on series catalyst at various WHSVs (Figure S7), DBT HDS reaction network (Figure S8), The GC spectrogram of 4,6-DMDBT HDS product on different 28
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catalysts (Figure S9), The GC spectrogram of 4,6-DMDBT HDS product on NiMo/ZS-Mg at different WHSVs (Figure S10), 4,6-DMDBT HDS reaction network (Figure S11), 4,6-DMDBT to 3,7-DMDBT on various NiMo/ZS (Figure S12), Sructural properties of various ZS (Table S1), Acid distribution and acid amounts of series NiMo/ZS (Table S2), and Mo3d XPS statistical results of different NiMo/ZS (Table S3).
For Table of Contents Only DBT HDS 4,6-DMDBT HDS
6
TOF, h-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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4
2
0 NiMo/ZS-K
NiMo/ZS-Mg NiMo/ZS-Mn NiMo/ZS-Zn NiMo/ZS-Cu
ZS-x represent ZSM-5/SBA-16 composite materials using different inorganic cations as additives, in which x represent K+, Mg2+, Mn2+, Zn2+ and Cu2+, respectively.
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