Self-Assembly of Hierarchically Porous ZSM-5/SBA-16 with Different Morphologies and Its High Isomerization Performance for Hydrodesulfurization of Dibenzothiophene and 4,6-Dimethyldibenzothiophene

Article abstract of DOI:10.1021/acscatal.7b04147

ZSM-5/SBA-16 (ZS) composite materials with different morphologies were synthesized successfully. The series supports were utilized to prepare NiMo/ZS for dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization (HDS) reactions. Series ZS supports and NiMo/ZS were well characterized to investigate their structure-property relationship. The NiMo/ZS catalyst (NiMo/ZS-3) with uniform morphology and well-ordered pore channels showed the maximum k_HDS and TOF values of DBT and 4,6-DMDBT HDS. The k_HDS value (13.9 × 10^-4 mol g^-1 h^-1) of DBT over NiMo/ZS-3 was more than 2 times greater than that over the reference NiMo/ZS-M catalyst (5.5 × 10^-4 mol g^-1 h^-1), 3 times greater than that over the NiMo/SBA-16 catalyst (4.4 × 10^-4 mol g^-1 h^-1), and almost 4 times greater than that over the NiMo/ZSM-5 catalyst (3.5 × 10^-4 mol g^-1 h^-1). Furthermore, the k_HDS value (8.4 × 10^-4 mol g^-1 h^-1) of 4,6-DMDBT over NiMo/ZS-3 was more than 3 times greater than that over the reference NiMo/ZS-M catalyst (2.8 × 10^-4 mol g^-1 h^-1), more than 4 times greater than that over the NiMo/SBA-16 catalyst (1.7 × 10^-4 mol g^-1 h^-1), and almost 5 times greater than that over the NiMo/ZSM-5 catalyst (1.6 × 10^-4 mol g^-1 h^-1). The superior DBT and 4,6-DMDBT HDS performances were assigned to the uniform morphology, well-ordered pore channels, and high B/L ratio of the NiMo/ZS-3 catalyst and the suitable dispersion of the MoS₂ active phases. HYD was the preferential route for DBT HDS, while ISO was the preferential route for 4,6-DMDBT HDS because of the high B/L ratio of NiMo/ZS-3. Moreover, the DBT and 4,6-DMDBT HDS reaction networks of the series NiMo/ZS are presented.

Full text of DOI:10.1021/acscatal.7b04147

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Article
Self-assembly of hierarchically porous ZSM-5/SBA-16 with
different morphologies and its high isomerization performance for
hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene
Xilong Wang, Jinlin Mei, Zhen Zhao, Peng Zheng, Zhentao Chen,
Daowei Gao, Jianye Fu, Jiyuan Fan, Aijun Duan, and Chunming Xu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04147 • Publication Date (Web): 19 Jan 2018
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Selfꢀassembly of hierarchically porous ZSMꢀ5/SBAꢀ16 with
different morphologies and its high isomerization
performance for hydrodesulfurization of dibenzothiophene
and 4,6ꢀdimethyldibenzothiophene
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Xilong Wang †, Jinlin Mei †, Zhen Zhao †, Peng Zheng †, Zhentao Chen †, Daowei Gao
‡, Jianye Fu †, Jiyuan Fan †, Aijun Duan *, †, Chunming Xu *, †
† State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Beijing, 18 Fuxue Road, Beijing, P.R. China, 102249
‡ School of Chemistry and Chemical Engineering, University of Jinan, Jinan, PR
China, 250022
Corresponding Author: *Eꢀmail: duanaijun@cup.edu.cn. Tel: +86 10 89732290.
*Eꢀmail: xcm@cup.edu.cn. Tel: +86 10 89733392.
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NiMo/ZSMꢀ5
NiMo/ZSꢀ2
NiMo/ZSꢀ3
NiMo/SBAꢀ16
ZSꢀ2 and ZSꢀ3 represent ZSMꢀ5/SBAꢀ16 with the SiO2/Al2O3 ratios of 80 and 100, respectively.
Abstract
ZSMꢀ5/SBAꢀ16 (ZS) composite materials with different morphologies were
synthesized successfully. The series supports were utilized to prepare the NiMo/ZS
for the dibenzothiophene (DBT) and 4,6ꢀdimethyldibenzothiophene (4,6ꢀDMDBT)
hydrodesulfurization (HDS) reactions. Series ZS supports and NiMo/ZS were well
characterized to investigate their structureꢀproperty relationship. The NiMo/ZS
catalyst (NiMo/ZSꢀ3) with uniform morphology and wellꢀordered pore channels
showed the maximum kHDS and TOF values of DBT and 4,6ꢀDMDBT HDS. The kHDS
(13.9×10ꢀ4 mol gꢀ1 hꢀ1) of DBT over NiMo/ZSꢀ3 was more than two times as that over
the reference NiMo/ZSꢀM catalyst (5.5×10ꢀ4 mol gꢀ1 hꢀ1), three times as that over the
NiMo/SBAꢀ16 catalyst (4.4×10ꢀ4 mol gꢀ1 hꢀ1), and almost four times as that over the
NiMo/ZSMꢀ5 catalyst (3.5×10ꢀ4 mol gꢀ1 hꢀ1). Furthermore, the kHDS (8.4×10ꢀ4 mol gꢀ1
hꢀ1) of 4,6ꢀDMDBT over NiMo/ZSꢀ3 was more than three times as that over the
reference NiMo/ZSꢀM catalyst (2.8×10ꢀ4 mol gꢀ1 hꢀ1), more than four times as that
over the NiMo/SBAꢀ16 catalyst (1.7×10ꢀ4 mol gꢀ1 hꢀ1) and almost five times as that
over the NiMo/ZSMꢀ5 catalyst (1.6×10ꢀ4 mol gꢀ1 hꢀ1). The superior DBT and
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4,6ꢀDMDBT HDS performance were assigned to the uniform morphology,
wellꢀordered pore channels, high B/L ratio of the NiMo/ZSꢀ3 catalyst and the suitable
dispersion of the MoS2 active phases. The HYD was the preferential route for DBT
HDS, while the ISO was the preferential route for 4,6ꢀDMDBT HDS because of the
high B/L ratio of NiMo/ZSꢀ3. Moreover, the DBT and 4,6ꢀDMDBT HDS reaction
networks of the series NiMo/ZS were presented.
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Keywords
ZSMꢀ5/SBAꢀ16; Morphology, Dibenzothiophene; 4,6ꢀdimethyldibenzothiophene;
Hydrodesulfurization; Isomerization route
1. INTRODUCTION
Environmental pollution in many countries arised from undesirable toxic
emission from diesel engines has aroused public concern because of the high sulfur
content in diesel fuel.1ꢀ4 Increasing strict environmental legislation in many countries
limits the sulfur content of diesel lower than 10 ppm.5ꢀ7 The great challenge in
producing the ultraꢀlowꢀsulfur diesel is to eliminate the sulfur compounds with large
molecular weight, for instance, DBT and 4,6ꢀDMDBT.8ꢀ12 Nevertheless, complete
removal of 4,6ꢀDMDBT over the conventional industrial catalysts (NiꢀMoS2/γꢀAl2O3
and CoMoS2/γꢀAl2O3) is hard to achieve.13ꢀ16 In order to satisfy the increasing demand
for ultraꢀlowꢀsulfur diesel, many methods have been studied, among which the
exploition of more active support material is an economic way because the structure
of the sulfide active phase (orientation, morphology, size, sulfidation degree, etc) is
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strongly influenced by the nature of the support itself.17, 18
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Zeolites, for instance, Y, ZSMꢀ5, beta, mordenite and ferrierite, are important
candidates for petroleum industrial and environmental catalyst supports due to their
different shapes, sizes and connectivity of channels from 8 to 30 rings.19ꢀ21 Among
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different zeolites, ZSMꢀ5 is a versatile industrial catalyst due to its excellent
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hydrogenation and isomerization performance.22ꢀ23 Sugioka et al.
reported that
Pt/HZSMꢀ5 exhibited high and stable HDS catalytic activity of thiophene than that of
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commercial CoMo/Al2O3 catalyst at 400 ºC under 1 atm. Wu et al.
found that
nanoꢀsized HZSMꢀ5 synthesized hydrothermal crystallization method presented high
4,6ꢀDMDBT isomerization performance, which could remove the steric hindrance of
alkylꢀsubstitutes in the molecules of 4,6ꢀDMDBT. However, the application of ZSMꢀ5
in HDS process is limited because bulky sulfur compound molecules cannot diffuse
into the pore channels of ZSMꢀ5.26ꢀ28
Mesoporous silica materials are of great interest for hydrodesulfurization (HDS)
catalyst supports because of its high specific surface areas and tunable pore
diameters.29ꢀ33 The superior structure properties facilitate mass transfer and high
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dispersion of active phases. Valencia et al.
synthesized NiMo/SBAꢀ15 modified
with citric acid and found that modified NiMo/SBAꢀ15 displayed better HDS
performance compared with NiMo/SBAꢀ15 without citric acid. Gao et al. 35 found that
NiMo/AlꢀSBAꢀ15 with spherical morphology synthesized via hydrothermal synthesis
method exhibited high HDS activity with DBT conversion of 98.2% under 340 ºC and
4.0 MPa. However, these twoꢀdimensional hexagonal materials cannot provide more
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favorable mass transfer for reactant and product molecules.36 SBAꢀ16 material
exhibits threeꢀdimensional cubic arrangement (Im3m) of mesopores and each
nanocage pore in SBAꢀ16 is interconnected to eight neighboring cages through pore
entrance, which are in favor of the effective transport of reactant and product
molecules.37, 38 However, pure mesoporous silica materials have no B and L sites,
which are detrimental to the hydrogenation and isomerization performance for HDS
reaction.39 Moreover, the amorphous structure of pure mesoporous silica materials
indicates that the thermal and hydrothermal stabilities of mesoporous silica are
inherently lower than those of zeolites.
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It is well known that hydrogen spillover can vastly facilitate the hydrogenation
ability, which could induce to the great improvement of the HDS activity.40, 41 Tang et
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al.
synthesized the bunchy mordenite nanofibers and found that the spillover
hydrogen generated in micropores could transfer to the nearby active phases in the
mesopores. To overcome the limitations of individual zeolites and mesoporous
materials in the HDS reactions, the design of hierarchically porous composites
combining the superiority of zeolites and mesoporous materials is very important.
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Lots of microꢀmesoporous composites, for instance, BetaꢀMCMꢀ41
and
BetaꢀSBAꢀ15 44, were prepared and used in the catalysis. Certainly, our research
group has a good research foundation for the synthesis of microꢀmesoporous
composite material. There were previous work in our research group on the
hierarchically porous material and the application of the composite material in HDS
reaction. Zhang et al. 39 found that NiMo/BetaꢀKITꢀ6 showed much better DBT HDS
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activities than the commercial NiMo/γꢀAl2O3 catalyst. Wu et al.
found that
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NiMo/ZSMꢀ5ꢀKITꢀ6 catalyst showed the superior isomerization selectivity of
4,6ꢀDMDBT HDS. Through these previous researches we found that the synergistic
effect between microporous and mesoporous phases could vastly facilitate the HDS
activity. However, the phases of microporous phase and mesoporous phase in the
previous researches were separated. And the synergistic effect between microporous
and mesoporous phases was not utilized well. It was difficult to synthesize
homogeneous composites with uniform morphologies and it was also difficult to
prove the successful synthesis of homogeneous composites. Based on the
imperfection and the difficulties of the synthesis of the microꢀmesoporous composite
material, the exploration of the synthesis methods of homogeneous composite
materials with uniform morphologies was necessary. The phases and the
morphologies of homogeneous composite materials were controlled by the
precipitation rate of the silicon sources and the hydrolysis rate of the organic
templates.
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In this research, ZS composite materials with different uniform morphologies
were synthesized from ZSMꢀ5 seeds by modulating silicon source, inorganic salts
(Figure S1 in Supporting Information), aging temperatures (Figure S2 in Supporting
Information), the concentrations of HCl (Figure S3 in Supporting Information), and
SiO2/Al2O3 ratios (60/80/100/120/140). It was also proved indirectly that partial Al
species attributed to ZSMꢀ5 were introduced into the SBAꢀ16 framework, indicating
the successful synthesis of ZSMꢀ5/SBAꢀ16 composites. Series NiMo/ZS catalysts
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were evaluated in the DBT and 4,6ꢀDMDBT HDS. Moreover, to study the influences
of pore channels and acidic sites on DBT and 4,6ꢀDMDBT HDS performance,
different supports such as ZSMꢀ5 zeolite, SBAꢀ16, ZSMꢀ5 seeds and SBAꢀ16 mixture
(ZSꢀM), were selected as the reference supports. The NiMo/ZS exhibited better DBT
and 4,6ꢀDMDBT HDS performance. HYD and ISO were the prior routes of the DBT
and 4,6ꢀDMDBT HDS, respectively, which were because of the higher of B/L ratio of
the NiMo/ZS catalyst. Moreover, the DBT and 4,6ꢀDMDBT HDS reaction network of
series NiMo/ZS were presented.
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2. EXPERIMENTAL SECTION
2.1 Synthesis of the supports and catalysts
2.1.1 Synthesis of ZSMꢀ5 seed
ZSMꢀ5 seed (SiO2/Al2O3 molar ratio = 30) was prepared from the mixture of
SiO2/Al2O3/TPABr/NaOH/H2O with a molar ratio of 30/1.0/5.0/21/450. 0.287 g
NaOH, 1.0 g NaAlO2 (45 wt% Al2O3) and 20 g H2O were stirred together for 1.0 h at
25 ºC (denoted as Solution A). 0.6 g NaOH, 5.3 g TPABr and 16 g H2O were stirred
together for 15 min (denoted as Solution B), then 20 g of colloidal silica (40 wt%
SiO2) was poured into solution B dropwise and stirred rapidly for 1.0 h at 25 ºC
(denoted as Solution C). Solution A was added into Solution C dropwise and stirred
rapidly for 3.0 h at 25 ºC. Afterwards, the homogenous gel was put into an autoclave
and crystallized at 170 ºC for 32 h to obtain zeolite ZSMꢀ5 seed solution. All details
of the reagents were listed in Table S1 (in Supporting Information).
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2.1.2 Synthesis of ZS materials
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ZS composites were synthesized using EO106PO70EO106 (F127) as the template
and tetraethylorthosilicate (TEOS) as the silica source. 4.0 g of F127, 10 g of KCl and
240 mL of HCl (1.5 mol/L) were stirred together for 2.0 h at 35 ºC. 12.0 g of
cosurfactant butanol was added into the mixed solutions and stirred rapidly for 2.0 h
at 35 ºC. Then 16.6 g of TEOS (28.4 wt.% SiO2) was added into the mixtures
dropwise and stirred for 30 min, after that, different amounts of zeolite ZSMꢀ5 seed
(37.14, 22.29, 15.92, 12.38 and 10.12 g, respectively) were added into these mixtures
and stirred for 1.0 h, subsequently, kept motionless for 24 h before putting into five
autoclaves and maintained at 100 ºC for 24 h. The solids were obtained by filtration,
dried at 90 ºC for 8.0 h, and calcined at 550 ºC for 6.0 h. The ZS materials with
different molar ratios of SiO2/Al2O3 (60, 80, 100, 120 and 140) synthesized with
different amounts of zeolite ZSMꢀ5 seed (37.14, 22.29, 15.92, 12.38 and 10.12 g)
were denoted as ZSꢀ1, ZSꢀ2, ZSꢀ3, ZSꢀ4 and ZSꢀ5, correspondingly. The preparation
scheme of ZSMꢀ5/SBAꢀ16 composites with various morphologies was displayed in
Scheme S1 (in Supporting Information).
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2.1.3 Synthesis of the reference materials
For comparison, reference materials (ZSMꢀ5, SBAꢀ16 and ZSꢀM) were
synthesized. ZSMꢀ5 with higher crystalline degree was obtained at the crystallization
time of 72 h. Mesoporous SBAꢀ16 material was prepared using the same method with
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ZS material without the addition of ZSMꢀ5 seed. ZSꢀM material was the mixture of
ZSMꢀ5 seed and SBAꢀ16 with a molar ratio of SiO2/Al2O3 = 100.
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2.1.4 Synthesis of the corresponding catalysts
Prior to the preparation of catalysts, ZS series materials, ZSMꢀ5 and ZSꢀM were
pretreated twice by ion exchange using ammonium chloride solution at 90 ºC for 2.0 h.
All of the corresponding NiMo catalysts contained 15.0 wt% MoO3 and 3.5 wt% NiO
were prepared as described in our previous work.45 The corresponding NiMo catalysts
were recorded as NiMo/ZSMꢀ5, NiMo/SBAꢀ16, NiMo/ZSꢀM, NiMo/ZSꢀ1,
NiMo/ZSꢀ2, NiMo/ZSꢀ3, NiMo/ZSꢀ4 and NiMo/ZSꢀ5, respectively.
2.2 Characterization of the supports and the catalysts
The detailed descriptions of different characterization methods, for instance,
Xꢀray diffraction (XRD), Nitrogen physisorptions, Scanning electron microscopy
(SEM), Transmission electron microscopy (TEM), PyridineꢀFTIR, Raman spectra,
Xꢀray photoelectron spectra (XPS), Highꢀresolution transmission electron microscopy
(HRTEM) and gas chromatographyꢀmass spectrometry (GCꢀMS), were provided in
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our previous work 45, 46. Al nuclear magnetic resonance (27Al NMR) measurements
of series supports were performed on a Bruker Avance III 500 MHz spectrometer.
2.3 Evaluation of HDS performance of catalysts
The HDS catalytic activities were carried out by using DBT or 4,6ꢀDMDBT as
reactants in a fixꢀbed reactor. Before reaction, all the fresh catalysts were presulfided
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using 2.5 wt% CS2 in cyclohexane solution at 340 ºC, 4.0 MPa, 600 mL mLꢀ1, and 8.0
hꢀ1 for 4 h. Liquid feeds, DBT (500 ppm sulfur contents) dissolved in cyclohexane or
4,6ꢀDMDBT (500 ppm sulfur contents) dissolved in toluene, were fed into the reactor
by a 5963 Optos 1LM 3/32 Head Eldex pump. The catalysts were tested at 340 ºC, 4.0
MPa, 200 mL mLꢀ1, and different WHSVs of 10ꢀ100 hꢀ1.
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The sulfur contents of feedstock and resulting products were measured using a
RPPꢀ2000SN analyzer. The DBT or 4,6ꢀDMDBT HDS conversion (%) was calculated
from Equation (3):
HDS (%) = (Sf -Sp)/Sf ×100%
where Sf is the sulfur content (ppm) of the feeds and Sp the sulfur content (ppm)
of the products.
(3)
Figures S4 (in Supporting Information) displayed the DBT and 4,6ꢀDMDBT
HDS conversions of three catalysts (NiMo/ZSꢀ3, NiMo/ZSMꢀ5 and NiMo/SBAꢀ16)
with different particle sizes (40ꢀ60 mesh). In this DBT and 4,6ꢀDMDBT HDS
reaction systems, at the DBT and 4,6ꢀDMDBT HDS conversions of about 50%, the
influence of mass transfer could be ruled out. The calculation method of kHDS
,
Turnꢀover frequency (TOF, hꢀ1) and fMo were described in the previous published
researches. 47, 48, 49
3. RESULTS
3.1 Characterization of the supports
3.1.1 XRD of the supports
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(A)
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5 10 15 20 25 30 35 40 45 50
2θ, degree
0
10 20 30 40 50 60 70 80
Crystallization time, h
Figure 1. (A) Wide angle XRD and (B) relative crystallinity of ZSMꢀ5 obtained by different
crystallization times.
To assess the degree of crystallinity and control the particle size of ZSMꢀ5 seeds,
the ZSMꢀ5 seeds were obtained by different crystallization times (between 8 h and 72
h) at 170 ºC. The wideꢀangle XRD patterns and relative crystallinity curve of ZSMꢀ5
seeds obtained under different crystallization times are shown in Figure 1. The
asꢀsynthesized ZSMꢀ5 seeds show intense XRD peaks at 2θ = 7.9°, 8.8°, 20.3°, 23.1°,
23.9°and 29.9° corresponding well to the (101), (200), (103), (501), (303) and (503)
planes of ZSMꢀ5.50 As the crystallization time increases to 24 h, the diffraction peaks
assigned to ZSMꢀ5 crystal are detected, suggesting that the nanocrystalline units of
the ZSMꢀ5 crystals are beginning to form. ZSMꢀ5 seed obtained by the crystallization
time of 32 h possesses more nanocrystalline units and appropriate particle sizes,
which is in favor of synthesizing ZS microꢀmesoporous composite materials. As the
crystallization time increases to 72 h, the diffraction peak intensities attributed to
ZSMꢀ5 crystal are relatively strong, suggesting the crystallinity degree of the ZSMꢀ5
crystals is relatively high, which can be used as the reference Zeolite ZSMꢀ5 material.
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2θ, degree
Figure 2. Wide angle XRD of ZS series composites and ZSMꢀ5. (a) ZSꢀ1, (b) ZSꢀ2, (c) ZSꢀ3,
(d) ZSꢀ4, (e) ZSꢀ5, (f) ZSMꢀ5.
As shown in Figure 2, all the series ZS materials and ZSMꢀ5 obtained at the
crystallization time of 72 h exhibit six peaks corresponding to the planes of ZSMꢀ5.
The peak intensities change in the sequence of ZSMꢀ5 > ZSꢀ1 > ZSꢀ2 > ZSꢀ3 > ZSꢀ4
> ZSꢀ5, indicating that the amounts of the nanocrystalline units follow the same order.
a
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c
d
e
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2θ, degree
Figure 3. Small angle XRD of ZS series composites. (a) ZSꢀ1, (b) ZSꢀ2, (c) ZSꢀ3, (d) ZSꢀ4,
(e) ZSꢀ5.
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The small angle XRD patterns of ZS series materials are displayed in Figure 3.
All the samples show characteristic peaks at the region of 2θ = 0.8ꢀ1.5°,
corresponding to SBAꢀ16 with 3D cubic (Im3m) mesostructure.51 The characteristic
peaks also have these trends to become narrower and higher with the decrease of the
ZSMꢀ5 seed contents, manifesting that the degrees of order increase.
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3.1.2 N2 physisorption of the supports
The N2 physisorption isotherms and the pore size distribution of series ZS are
exhibited on Figures 4 (A) and (B). As shown in Figure 4 (A), the physisorption
isotherms of the ZS series supports exhibit typeꢀIV curves, demonstrating that all the
ZS supports possess mesoporous structure. The pore size distributions of the series ZS
supports show relatively narrow distributions of mesopores and the concentration
degree of the series ZS supports change in sequence of ZSꢀ1 < ZSꢀ2 < ZSꢀ3 < ZSꢀ4 <
ZSꢀ5, which are well consistent with the small angle XRD results.
(A)
a
(B)
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c
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d
d
e
e
0.0 0.2 0.4 0.6 0.8 1.0
Relative Pressure, P/P0
0
10
20
30
40
50
Pore diameter, nm
Figure 4. (A) physisorption isotherms and (B) pore size distribution patterns of series ZS.
(a) ZSꢀ1, (b) ZSꢀ2, (c) ZSꢀ3, (d) ZSꢀ4, (e) ZSꢀ5.
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The corresponding properties of the series ZS supports are shown in Table S2 (in
Supporting Information). The BET surface area , pore size and volume of series ZS
change in sequence of ZSꢀ1 (647.42 m2 gꢀ1, 4.16 nm, 0.41 cm3 gꢀ1) < ZSꢀ2 (747.61 m2
gꢀ1, 4.19 nm, 0.51 cm3 gꢀ1) < ZSꢀ3 (751.96 m2 gꢀ1, 4.23 nm, 0.52 cm3 gꢀ1) < ZSꢀ4
(768.21 m2 gꢀ1, 4.30 nm, 0.54 cm3 gꢀ1) < ZSꢀ5 (792.62 m2 gꢀ1, 4.31 nm, 0.56 cm3 gꢀ1) <
SBAꢀ16 (842.16 m2 gꢀ1, 4.05 nm, 0.66 cm3 gꢀ1).
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3.1.3 27Al MAS NMR of the supports
a
b
c
d
e
f
200 150 100 50 0 ꢀ50 ꢀ100ꢀ150
ppm
Figure 5. 27Al NMR spectra of the series ZS supports. (a) ZSꢀ1, (b) ZSꢀ2, (c) ZSꢀ3, (d) ZSꢀ4, (e)
ZSꢀ5, (f) ZSMꢀ5.
The coordination information of Al atoms in the ZS series supports can be
obtained from 27Al MAS NMR. Figure 5 displays the 27Al MAS NMR spectra of
series ZS and ZSMꢀ5 zeolite. Two signals at 0 and 54 ppm are assigned to the
octahedral (extraꢀframework Al species) and tetrahedral coordinated aluminum atoms,
respectively.52 The framework/extraꢀframework Al ratios in the series ZS and ZSMꢀ5
27
supports follow the order of ZSMꢀ5 > ZSꢀ3 > ZSꢀ2 > ZSꢀ1 > ZSꢀ4 > ZSꢀ5. The Al
MAS NMR spectra (Figure S7, in Supporting Information) indicate that partial Al
atoms were incorporated into the framework of the silica SBAꢀ16 materials.
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The TEM images of the series ZS and pure SBAꢀ16 materials are exhibited in Figure
S5 (in Supporting Information). The series ZS and pure SBAꢀ16 supports display
wellꢀordered arrays of mesopores, revealing that they all have cubic bodyꢀcentred
Im3m symmetry structure. The mesoporous structures are partially destroyed with the
increase of the ZSMꢀ5 seed contents. It is hard to distinguish directly the MFI
topology structure of ZSMꢀ5, while we can get some information indirectly from the
HAADFꢀSTEM & EDX elemental mapping images (Figure S9). The Al mapping
image of ZSꢀ3 material shows that the Al atoms is uniformly dispersed in
ZSMꢀ5/SBAꢀ16 composite material, demonstrating that the primary and secondary
structures of ZSMꢀ5 construct ZSMꢀ5/SBAꢀ16.
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3.1.5 SEM images of the supports
The morphologies of the ZS series, ZSMꢀ5, pure SBAꢀ16 and ZSꢀM are studied
by using SEM and the images of these supports are displayed in Figure 6. As shown
in Figure 6 (a), the ZSMꢀ5 phase is present in the form of partial hexagonal prism
monoꢀcrystals. The pure SBAꢀ16 phase (Figure 6 (b)) is present in the form of
uniform spherical aggregates with the sizes ranging from 7 to 9 µm. For the
mechanical mixture of the pure SBAꢀ16 and ZSMꢀ5 seed, Figure 6 (h) clearly shows
two separated phases, of which the uniform spherical aggregates are ascribed to
SBAꢀ16 and the small particles are ZSMꢀ5 seeds. Figure 6 (c) and (g) show that the
tenacity of each aggregate decreases with the decreasing content of the ZSMꢀ5 seed in
the composite, while the regularity of the spherical aggregates increases with the
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decreasing content of the ZSMꢀ5 seed. ZSꢀ3 composite material is present in the form
of uniform spherical aggregates with the sizes ranging from 14 to 15 µm consisting of
uniform hexagonal prism monoꢀcrystals, which presents an absolutely different
morphology from that of ZSꢀM. The pH value of the media varies during the addition
of different amounts of ZSMꢀ5 seeds. The precipitation rate of the silicate species and
the hydrolysis rate of the surfactant are influenced greatly by the acid concentration.
Therefore, using different acidic concentrations and butanol additives modulate the
condensation rate of the inorganic chemicals, yielding different morphologies of the
ZS series supports.
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Combined with the 27Al MAS NMR spectra (Figure S7, in Supporting
Information), Al 2p XPS results (Figure S8, in Supporting Information) and
HAADFꢀSTEM & EDX elemental mapping images (Figure S9, in Supporting
Information), it could be deduced that parts of ZSMꢀ5 seeds were embedded in the
external surface and most of ZSMꢀ5 seeds with small particle sizes were incorporated
into the wall of ZSꢀ3 composite material via the AlꢀOꢀSi covalent bonding.
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20 ꢁm
20 ꢁm
(c)
(d)
20 ꢁm
20 ꢁm
(f)
(e)
20 ꢁm
20 ꢁm
(h)
(g)
20 ꢁm
20 ꢁm
Figure 6. SEM images of the series ZS and pure SBAꢀ16 supports. (a) ZSMꢀ5, (b) SBAꢀ16,
(c) ZSꢀ1, (d) ZSꢀ2, (e) ZSꢀ3, (f) ZSꢀ4, (g) ZSꢀ5, (h) ZSꢀM.
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3.2 Characterization of the catalysts
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3.2.1 PyridineꢀFTIR of the oxide catalysts
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(B)
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d
e
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e
1700
1600
1500
1400 1700
1600
1500
1400
Wavenumber, cmꢀ1
Wavenumber, cmꢀ1
Figure 7. FTIRꢀpyridine spectra of different catalysts (a) NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2, (c)
NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4 and (e) NiMo/ZSꢀ5 after degassing at (A) 200 ºC and (B) 350 ºC.
PyridineꢀFTIR spectroscopies of the series NiMo/ZS were recorded to
investigate the acidic strength and types. The IR adsorption spectra of pyridine
molecules (degas at 200 ºC) are ascribed to the total acid amounts of the series
catalysts, and the IR adsorption spectra of pyridine molecules (degas at 350 ºC) are
ascribed to the medium and strong acid amounts. Figure 7 shows the pyridineꢀFTIR
adsorption spectra of series NiMo/ZS. The peaks at 1446, 1492, 1575 and 1622 cmꢀ1
are assigned to L acid and the peaks at 1546 and 1639 cmꢀ1 are assigned to B acid.39
The peak intensities increase in the following order: NiMo/ZSꢀ1 > NiMo/ZSꢀ2 >
NiMo/ZSꢀ3 > NiMo/ZSꢀ4 > NiMo/ZSꢀ5.
The acid amounts of the NiMo/ZS are summarized in Table S3 (in Supporting
Information). The total acid, the medium and strong acid amounts all increase with
the following order: NiMo/ZSꢀ1 (208 ꢁmol gꢀ1, 133 ꢁmol gꢀ1) > NiMo/ZSꢀ2 (192
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ꢁmol gꢀ1, 91 ꢁmol gꢀ1) > NiMo/ZSꢀ3 (165 ꢁmol gꢀ1, 65 ꢁmol gꢀ1) > NiMo/ZSꢀ4 (145
ꢁmol gꢀ1, 50 ꢁmol gꢀ1) > NiMo/ZSꢀ5 (131 ꢁmol gꢀ1, 26 ꢁmol gꢀ1). The B/L ratios of the
total acid, and the medium and strong acid follow the order of NiMo/ZSꢀ3 (0.23,
0.25) > NiMo/ZSꢀ2 (0.21, 0.21) > NiMo/ZSꢀ1 (0.20, 0.20) > NiMo/ZSꢀ4 (0.17,
0.19) > NiMo/ZSꢀ5 (0.14, 0.18), which is in accordance with the 27Al MAS NMR
results, indicating that the incorporation of the Al atoms into SBAꢀ16 framework
favors the improvement of B acid.
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3.2.2 H2ꢀTPR of the oxide catalysts
H2ꢀTPR of series NiMo/ZS was recorded to investigate the metalꢀsupport
interaction (MSI) and the reducibility of the oxide phases, the results are displayed in
Figure 8. The series NiMo/ZS show the lowꢀtemperature characteristic peaks due to
the reduction of polymeric octahedral coordination Mo species
(Mo6+→HxMoO3→Mo4+) at the range of 400ꢀ600 oC and highꢀtemperature
characteristic peaks due to the deep reduction (Mo4+ + 2e–→Mo0) of tetrahedrally
coordinated monomeric Mo species at the range of 700ꢀ900 oC.53 The first reduction
temperatures and the deep reduction temperatures all follow the order of NiMo/ZSꢀ1
o
o
o
o
o
o
(534 C, 832 C) > NiMo/ZSꢀ2 (520 C, 824 C) > NiMo/ZSꢀ3 (512 C, 817 C) >
o
o
o
o
NiMo/ZSꢀ4 (505 C, 812 C) > NiMo/ZSꢀ5 (499 C, 792 C), demonstrating that the
MSIs of series NiMo/ZS change in the identical rule. The NiMo/ZSꢀ3 catalyst not
only has much weaker MSI than NiMo/ZSꢀ1 and NiMo/ZSꢀ2 catalysts, but also
obtains suitable dispersion of active Mo species than NiMo/ZSꢀ4 and NiMo/ZSꢀ5
catalysts.
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Figure 8. H2ꢀTPR of various oxide catalysts. (a) NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2, (c) NiMo/ZSꢀ3, (d)
NiMo/ZSꢀ4 and (e) NiMo/ZSꢀ5.
3.2.3 Raman of the oxide and sulfide catalysts
Figure 9 displays the Raman spectra of the NiMo/ZS series oxide and sulfide
catalysts. Figure 9 (A) shows that the Raman spectra of the series oxide catalysts have
three peaks at 826, 908 and 955 cmꢀ1 assigned to the NiMoO4 phase 54, a peak at 331
cmꢀ1 attributed to the terminal Mo=O bending mode of octahedral MoO4 species,
2ꢀ
and a peak at 564 cmꢀ1 assigned to AlꢀO stretching mode.55, 56 Especially, the broad
peak at 890ꢀ1000cmꢀ1 are characteristic of M=O stretching vibrations, indicating a
weak MSI.57 The peak intensities at 890ꢀ1000 cmꢀ1 follow the order of NiMo/ZSꢀ5 >
NiMo/ZSꢀ4 > NiMo/ZSꢀ3 > NiMo/ZSꢀ2 > NiMo/ZSꢀ1, which demonstrates that the
MSIs decrease in the inverse order. The result agrees well with the H2ꢀTPR
characterization. Figure 9 (B) displays the Raman spectra of the series sulfide
catalysts have two peaks at 380 and 405 cmꢀ1 attributed to E1 and A1g of MoS2
2g
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crystalline structures and two peaks at 454 and 634 cmꢀ1 are ascribed to the resonance
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scatting.58,
The intensities of the bands follow the order of NiMo/ZSꢀ3 >
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NiMo/ZSꢀ2 > NiMo/ZSꢀ1 > NiMo/ZSꢀ4 > NiMo/ZSꢀ5, demonstrating that the
sulfidation degree of the active phases changes in the same rule. The Raman spectra
of the NiMo/ZS series oxide and sulfide catalysts show that the majority of oxide Mo
species are sulfided to MoS2 phases after the presulfurization treatment.
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a
300 400 500 600 700 800 9001000
Raman shift, cmꢀ1
200 400 600 800 1000 1200
Raman shift, cmꢀ1
Figure 9. Raman spectra of (A) oxide catalysts and (B) sulfided catalysts. (a) NiMo/ZSꢀ1,
(b) NiMo/ZSꢀ2, (c) NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4 and (e) NiMo/ZSꢀ5.
3.2.4 XPS of the sulfide catalysts
The surface composition and chemical state of the series NiMo/ZS and
NiMo/ZSꢀM sulfide catalysts were recorded by XPS characterizations and the Mo3d
XPS deconvolution results of the NiMo/ZS and NiMo/ZSꢀM series sulfide catalysts
are shown in Figure 10. The standards for the fitting are described in our previous
work.45 The related Mo3d XPS results of series sulfide catalysts are listed in Table S4
(in Supporting Information). The sulfidation degrees change in the sequence of
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NiMo/ZSꢀ3 (69 %) > NiMo/ZSꢀ2 (63 %) > NiMo/ZSꢀ1 (56 %) > NiMo/ZSꢀ4 (54 %)
> NiMo/ZSꢀ5 (52 %) > NiMo/ZSꢀM (49 %), which is in well accordance with the
Raman results of the corresponding sulfide catalysts.
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Mo(Ⅳ)
(3/2),(5/2)
(b)
Mo(Ⅳ)
(3/2),(5/2)
(a)
Mo(Ⅴ)
(3/2),(5/2)
Mo(Ⅴ)
(3/2),(5/2)
Mo(Ⅵ)
(3/2),(5/2)
Mo(Ⅵ)
(3/2),(5/2)
S2ꢀ
S2ꢀ
240 236 232 228 224
Binding energy, eV
240 236 232 228 224
Binding energy, eV
Mo(Ⅳ)
(3/2),(5/2)
Mo(Ⅳ)
(3/2),(5/2)
(c)
(d)
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Mo(Ⅵ)
(3/2),(5/2)
Mo(Ⅵ)
S2ꢀ
(3/2),(5/2)
S2ꢀ
240 236 232 228 224
Binding energy, eV
240 236 232 228 224
Binding energy, eV
(e) Mo(Ⅳ)
(f)
Mo(Ⅳ)
(3/2),(5/2)
Mo(Ⅴ)
(3/2),(5/2)
(3/2),(5/2)
Mo(Ⅴ)
(3/2),(5/2)
Mo(Ⅵ)
(3/2),(5/2)
Mo(Ⅵ)
S2ꢀ
(3/2),(5/2)
S2ꢀ
240 236 232 228 224
Binding energy, eV
240 236 232 228 224
Binding energy, eV
Figure 10. Mo3d XPS spectra of various sulfided catalysts. (a) NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2,
(c) NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4, (e) NiMo/ZSꢀ5 and (f) NiMo/ZSꢀM.
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864 862 860 858 856 854 852 850
Binding energy, eV
Figure 11. Ni2p XPS spectra of the sulfided catalysts. (a) NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2,
(c) NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4, (e) NiMo/ZSꢀ5 and (f) NiMo/ZSꢀM.
The Ni2p XPS spectra of series NiMo/ZS and NiMo/ZSꢀM sulfide catalysts are
displayed in Figure 11 and the Ni2p XPS deconvolution results of the NiMo/ZS and
NiMo/ZSꢀM series sulfide catalysts are listed in Table S5 (in Supporting Information).
The NiMoS ratios of the NiMo/ZS and NiMo/ZSꢀM series sulfide catalysts follow the
order of NiMo/ZSꢀ3 (91.3 %) > NiMo/ZSꢀ2 (87.6 %) > NiMo/ZSꢀ1 (81.6 %) >
NiMo/ZSꢀ4 (76.1 %) > NiMo/ZSꢀ5 (71.5 %) > NiMo/ZSꢀM (66.2 %), indicating that
the NiMoS active phases increase in the same order, which is well in accordance with
the Mo3d XPS results of the corresponding sulfide catalysts. For the NiMo/ZSꢀ3
catalyst, the moderate MSI, ordered pore structure and uniform morphology are
expected to be responsible for the promotion of sulfidation degree, which is beneficial
to the improvement of the HDS performance.
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More information on the morphology of MoS2 active phases formed can be
investigated by the statistical results of around 400 slabs from various HRTEM
images. Figures 12 (a)ꢀ(f) display the representative HRTEM images of series sulfide
NiMo/ZS and NiMo/ZSꢀM catalysts and the corresponding stacking number
distributions of MoS2 crystallites. Figure 13 exhibits the length distribution of the
MoS2 crystallites dispersed on the series NiMo/ZS and NiMo/ZSꢀM sulfide catalysts.
It can be observed that as the amount of ZSMꢀ5 seeds increase in the ZS series
supports, the length and stacking degree of MoS2 slabs decrease obviously. The
sulfide NiMo/ZSꢀ1 catalyst contains a lower degree of stacking layer numbers
ascribed to the strong MSI, which is restrictive to the generation of NiMoSꢀII active
phases. Compared to NiMo/ZSꢀ1 catalyst, NiMo/ZSꢀ5 and NiMo/ZSꢀM catalysts
show relatively high stacking layer numbers of MoS2 active phases, which are
restrictive to expose enough brim and edge sites for adsorption and reaction of the
reactants.60 The locations of the reactants are good for the elimination of sulfur
through the perpendicular adsorption of reactants.61 NiMo/ZSꢀ1 catalyst exhibits the
appropriate stacking degree of MoS2 phases, which can promote the generation of
more NiMoSꢀII phases.57 The statistical results of the average length (Lav), average
layer number (Nav) and dispersion degree (fMo) of MoS2 active phases are summarized
in Table 1. The Lav and Nav values of the MoS2 slabs over the NiMo/ZS and
NiMo/ZSꢀM series sulfide catalysts change in the sequence of NiMo/ZSꢀM (5.2 nm,
3.8) > NiMo/ZSꢀ5 (4.6 nm, 3.5) > NiMo/ZSꢀ4 (4.3 nm, 3.1) > NiMo/ZSꢀ3 (3.9 nm,
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2.8) > NiMo/ZSꢀ2 (3.4 nm, 2.5) > NiMo/ZSꢀ1 (3.2 nm, 2.3). The fMo values of MoS2
over the NiMo/ZS and NiMo/ZSꢀM series sulfide catalysts change in the sequence of
NiMo/ZSꢀ1 (0.37) > NiMo/ZSꢀ2 (0.35) > NiMo/ZSꢀ3 (0.33) > NiMo/ZSꢀ4 (0.29) >
NiMo/ZSꢀ5 (0.26) > NiMo/ZSꢀM (0.21), which is consistent with the Raman spectra
results. NiMo/ZSꢀ3 catalyst not only possesses the appropriate stacking degree of
MoS2 slabs than the other five catalysts, but also possesses the relatively high MoS2
dispersion degree, which are essential for the improvement of the HDS performance.
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10 nm
10 nm
10 nm
10 nm
(c)
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(e)
10 nm
Figure 12. HRTEM images of various sulfide catalysts. (a) NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2, (c)
NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4, (e) NiMo/ZSꢀ5 and (f) NiMo/ZSꢀM.
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0
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NiMo/ZSꢀ2
NiMo/ZSꢀ3
NiMo/ZSꢀ4
NiMo/ZSꢀ5
NiMo/ZSꢀM
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0ꢀ20 20ꢀ40 40ꢀ60 60ꢀ80 >80
Slab length, 10ꢀ1 nm
Figure 13. The length distribution of the MoS2 slabs dispersed on various sulfide catalysts.
Table 1. Lav and Nav values of MoS2 slabs.
Catalyst
Lav (nm)
Nav
fMo
NiMo/ZSꢀ1
NiMo/ZSꢀ2
NiMo/ZSꢀ3
NiMo/ZSꢀ4
NiMo/ZSꢀ5
NiMo/ZSꢀM
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3.3 Results of the DBT HDS
Figure 14 displayed the DBT HDS performance of series NiMo/ZS and reference
catalysts at different WHSVs in the range of 10 to 100 hꢀ1. The DBT HDS activities of
series catalysts increase with the decrease of WHSVs, which are derived from the
extension of the contact time between reactants and catalysts. Furthermore, the DBT
HDS conversions of different catalysts at all WHSVs follow the similar orders of
NiMo/ZSꢀ3 > NiMo/ZSꢀ2 > NiMo/ZSꢀ1 > NiMo/ZSꢀ4 > NiMo/ZSꢀ5 > NiMo/ZSꢀM >
NiMo/SBAꢀ16 > NiMo/ZSMꢀ5. The NiMo/ZSꢀ3 exhibits the highest DBT HDS
conversions, of which its DBT HDS conversion (58.2 %) is more than two times as
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that over the reference NiMo/ZSꢀM (25.2 %), and about three times as that over the
reference NiMo/SBAꢀ16 (20.8 %), while about four times as that over NiMo/ZSMꢀ5
(15.4 %) at 100 hꢀ1. The highest DBT HDS efficiency can be ascribed to the
synergistic effects of its ordered pore channel, uniform morphology, moderate MSI,
high sulfidation degree and moderate stacking degree, which can contribute more to
deep DBT HDS reaction.
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Figure 14. HDS results of DBT at different WHSVs (340 oC, 4 MPa, 200 mL/mL). (a)
NiMo/ZSꢀ1, (b) NiMo/ZSꢀ2, (c) NiMo/ZSꢀ3, (d) NiMo/ZSꢀ4, (e) NiMo/ZSꢀ5, (f) NiMo/ZSꢀM,
(g) NiMo/SBAꢀ16 and (h) NiMo/ZSMꢀ5.
The product distributions (Figures S15, in Supporting Information) in the DBT
HDS over NiMo/ZSꢀ3 catalyst as a function of WHSVs are shown in Figure 15. From
Figure 15, it can be found that the low WHSV can improve the DBT HDS reaction
rate through direct desulfurization (DDS) other than hydrogenation (HYD), reflected
from the increase of biphenyl (BP) selectivity and the decrease of cyclohexylbenzene
(CHB) selectivity. DDS is the prior DBT HDS reaction route over NiMo/ZSꢀ3,
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reflected by the BP selectivity more than 50 % at different WHSVs. The selectivities
of cyclopentylmethylcyclohexane (CPMCH), cyclopentylmethylbenzene (CPMB) and
isophenyl hexadiene (PHDi) increase with the decreasing WHSVs, while the
selectivity of tetrahydrodibenzothiophene (THDBT) decrease with the decreasing
WHSVs.
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PHDi
THDBT
0
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WHSV, hꢀ1
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Figure 15. DBT HDS product distributions over NiMo/ZSꢀ3 as a function of WHSVs.
In order to study the influence of morphologies of the series ZS materials on the
DBT HDS reaction pathways, the products were analyzed at the similar total DBT
conversion (50 %) using a GCꢀMS chromatograph. The DBT HDS reaction network
of the series NiMo/ZS is proposed in Figure 16. Two parallel routes are found for the
DBT HDS: DDS and HYD. In the DDS route, the S atom is directly removed via the
breakage of CꢀS bonds, the characteristic of the formation of BP to be the final DDS
product. In the HYD route, the hydrogenation of one aromatic ring in DBT occurs
firstly, yielding THDBT intermediate. Then the intermediates are further desulfurized
to generate CHEB and CHB intermediate.62 Furthermore, the CHB intermediate might
be fully hydrogenated to dicyclohexyl (DCH). Additionally, three isomerization
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