A simple two-step method to synthesize the well-ordered mesoporous composite Ti-FDU-12 and its application in the hydrodesulfurization of DBT and 4,6-DMDBT

Article abstract of DOI:10.1039/c4ta03691c

Mesoporous FDU-12 silicas with different morphologies have been synthesized under static and stirring conditions in a water bath. Furthermore, a series of mesoporous FDU-12 silicas modified by different amounts of titania have been synthesized, through a two-step method of prehydrolysis of TEOS followed by addition of titanium butoxide as a titanium precursor. Their respective NiMo supported catalysts were also prepared and characterized to elucidate the effects of titania, on the characteristics of the Mo surface species and their hydrodesulfurization (HDS) activities on dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The composites were characterized by various techniques (XRD, SEM, TEM, FT-IR, UV-vis, ²⁹Si NMR, Raman, XPS and N₂ adsorption-desorption); meanwhile, the catalysts were also tested in a higher pressure HDS reactor at T = 340 °C and P = 4 MPa with different liquid hourly space velocity (LHSV) values. The results show that the two-step synthetic route can prepare well-ordered mesoporous composites with high surface areas and large pore sizes and volumes. Obvious differences in the Mo species can be observed on the sulfided NiMo/Ti-FDU-12 catalysts. This showed that appropriate Ti loading could facilitate the dispersion of molybdena and the formation of the MoS₂ and NiMoS phases, which are the important factors for the HDS reaction. All of the catalysts presented high HDS activity, and when the Si:Ti molar ratio was increased to 5, the HDS activities on DBT and 4,6-DMDBT could reach as high as 97% and 65% respectively.

Full text of DOI:10.1039/c4ta03691c

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Materials Chemistry A
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A simple two-step method to synthesize the well-
ordered mesoporous composite Ti-FDU-12 and its
application in the hydrodesulfurization of DBT and
4,6-DMDBT†
Cite this: DOI: 10.1039/c4ta03691c
a
a
a
Zhengkai Cao, Aijun Duan, Zhen Zhao, Jianmei Li,b Yuechang Wei,a
*
*
Guiyuan Jianga and Jian Liua
Mesoporous FDU-12 silicas with different morphologies have been synthesized under static and stirring
conditions in a water bath. Furthermore, a series of mesoporous FDU-12 silicas modified by different
amounts of titania have been synthesized, through a two-step method of prehydrolysis of TEOS
followed by addition of titanium butoxide as a titanium precursor. Their respective NiMo supported
catalysts were also prepared and characterized to elucidate the effects of titania, on the characteristics
of the Mo surface species and their hydrodesulfurization (HDS) activities on dibenzothiophene (DBT) and
4,6-dimethyldibenzothiophene (4,6-DMDBT). The composites were characterized by various techniques
(XRD, SEM, TEM, FT-IR, UV-vis, 29Si NMR, Raman, XPS and N2 adsorption–desorption); meanwhile, the
catalysts were also tested in a higher pressure HDS reactor at T ¼ 340 ꢀC and P ¼ 4 MPa with different
liquid hourly space velocity (LHSV) values. The results show that the two-step synthetic route can
prepare well-ordered mesoporous composites with high surface areas and large pore sizes and volumes.
Obvious differences in the Mo species can be observed on the sulfided NiMo/Ti-FDU-12 catalysts. This
showed that appropriate Ti loading could facilitate the dispersion of molybdena and the formation of the
MoS2 and NiMoS phases, which are the important factors for the HDS reaction. All of the catalysts
presented high HDS activity, and when the Si : Ti molar ratio was increased to 5, the HDS activities on
DBT and 4,6-DMDBT could reach as high as 97% and 65% respectively.
Received 18th July 2014
Accepted 13th October 2014
DOI: 10.1039/c4ta03691c
www.rsc.org/MaterialsA
and pore volumes and uniform pore size distributions can
provide a favourable platform for TiO2 loading. Since the rst
ordered mesoporous silica MCM-41S was synthesized,6 different
kinds of titanium-containing mesoporous silica composites
have been reported, including Ti-MCM-41,7,8 Ti-MCM-48,9
Ti-SBA-15 (ref. 10 and 11) and Ti-SBA-16.12
1. Introduction
Recently, much attention has been paid to the environmental
problems caused by petroleum products with high sulfur
contents. Accordingly, the development of catalysts with higher
activities becomes important. Among different supports and
their combinations with catalysts, Mo catalysts supported on
TiO2 displayed a higher intrinsic hydrodesulfurization (HDS)
activity.1,2 This high HDS activity was attributed to the well
dispersed molybdena species and the support of titania made
the formation of MoS2 easier. However, the TiO2 supports have
an important disadvantage: they generally have low surface area
and small pore size, which is caused by the agglomeration of
TiO2 particles.3 TiO2-incorporated mesoporous silica with
appropriate physicochemical properties can solve this
problem.4,5 Mesoporous silica materials with large surface areas
There are two main methods for the synthesis of titanium-
containing mesoporous silica composites: the direct incorpo-
ration method13,14 and the post-synthetic method,15 both of
which have pros and cons. For the direct incorporation method,
the composites may maintain high surface areas and large pore
volumes and pore sizes, but the structure may collapse upon
addition of excess amounts of titanium.12 For the post-synthetic
method, the structures of the composites can maintain integ-
rity, but a decrease in surface areas and pore diameters occurs.16
Klimova and coworkers17 prepared Ti/MCM-41 samples by the
post-synthetic method using titanium butoxide as a titanium
source, and showed that the characteristic MCM-41 structure
remained almost intact aer Ti incorporation. It was found that
a better dispersion of Mo was obtained by this method, but that
it would also cause a decrease in pore diameter. Shen and
coworkers12 successfully synthesized Ti-SBA-16 mesoporous
aState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing,
102249, P. R. China. E-mail: duanaijun@cup.edu.cn; Fax: +86-10-69724721; Tel: +86-
10-89732290
bCollege of Science, China University of Petroleum, Beijing 102249, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta03691c
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silica through a simple two-step prehydrolysis method, using additives. The FDU-12 sample was synthesized in the following
TEOS and TiCl4 as silica and titanium sources, respectively. It steps: 2.0 g of triblock copolymer EO106PO70EO106 F127, 2.0 g of
showed that the approach of prehydrolysis of precursor TEOS TMB and 5.0 g of KCl were dissolved in 120 mL of 2 M HCl, then
was effective to synthesize the TiO2/silica mesoporous kept under stirring at 313 K for 24 h. 8.3 g of TEOS was added in
composites. The composites maintained large pore volumes, the mixture which was then kept under stirring in a water bath
pore sizes and surface areas.
for 24 h. The mixture was then transferred to an autoclave at 373
TiCl4 is generally used as a titanium source; however, its K for 24 h, and the solid was ltrated with deionized water and
property of easy hydrolysis make it difficult to add to the dried at 353 K for 6 h. Finally, the powder was calcined at 773 K
mixture. Because of this problem, titanium butoxide is used as a for 6 h, and the product was named as FDU-12 stirring. The
titanium precursor instead. Since the hydrolytic rate of the sample synthesized without stirring aer adding TEOS was
titanium precursor is faster than that of silicon, a method of denoted FDU-12 static. In this research, the sample of FDU-12
two-step prehydrolysis of TEOS followed by the addition of static served as a contrast to the other composites referred to as
titanium butoxide was used, which can facilitate the titanium FDU-12 stirring.
species to be effectively incorporated into the silica framework
The Ti-FDU-12 composite was prepared by a two-step
and protect the mesoporous structure from being destroyed, to method using different amounts of titanium in an acidic solu-
some extent.12 Furthermore, n-butanol is added to the solution tion. Typically, the synthetic procedure for ST-20 (Si : Ti initial
to decrease its hydrolytic rate, which can also serve as the molar ratio ¼ 20) was as follows: 2.0 g of F127, 2.0 g of TMB and
expansion agent to increase the pore diameter and reduce the 5.0 g of KCl were dissolved in 120 mL of 1.5 M HCl and stirred at
wall thickness of the pores.
313 K for 24 h. 6 g of n-butanol was added two hours earlier than
FDU-12 is a type of 3-dimensional material with a face-cen- TEOS (8.3 g). Aer three hours, 0.68 g of titanium butoxide was
tred cubic (Fm-3m) structure. The entrance size can be contin- dropped slowly into the resulting reaction mixture, which was
uously tailored (4–9 nm),18 indicating that FDU-12 is favourable kept understirring for a further 24 h at 313 K and then trans-
for the mass transfer and diffusions of guest molecules. ferred to an autoclave and heated at the desired temperature for
However, few works have been carried out on metal-incorpo- 24 h. The solid was isolated by ltration with deionized water
rated FDU-12, which is probably due to the difficulties and dried at 353 K for 6 hours. The resulting silica surfactant
encountered with the conditions of the composites synthesis, composite powder was calcined at 773 K for 6 h to obtain the
such as a strongly acidic environment. To the best of our composite, denoted ST-20.
knowledge, the synthesis and characterization of well-ordered
Ti-FDU-12 materials with high titanium loading and their 20, 10, 5, 2.5) were also prepared in a similar manner taking
application in the HDS reaction have not been reported in the appropriate amounts of the Si and Ti sources, and were denoted
Ti-FDU-12 with different Si : Ti initial molar ratios (x ¼ 40,
literature so far.
ST-x.
In this work, rstly, different morphologies of FDU-12 were
synthesized under static or stirring conditions in a water bath.
Secondly, face-centred cubic structured (Fm-3m) Ti-FDU-12
composites with different amounts of titanium were prepared
through a two-step method. The composites possessed well-
ordered mesoporous structures, and the molar ratio of Si : Ti
could be as high as 5. Finally, Ti-FDU-12 supported NiMo
catalysts were also prepared by a two-step incipient wetness
method. The sulded catalysts were also studied, and it was
found that the incorporation of titanium could enhance the
dispersion of molybdena and make the formation of MoS2 and
NiMoS phase easier. This was favourable for efficient hydro-
desulfurization. The NiMo/Ti-FDU-12 catalysts were tested in
the HDS of DBT and 4,6-DMDBT, and all the samples presented
2.2. Catalyst preparation
A series of NiMo/Ti-FDU-12 catalysts with different Si : Ti ratios,
denoted NiMo/ST-x (x was from 40 to 2.5) were prepared
by impregnation via a two-step incipient wetness method
with aqueous solutions of heptamolybdate tetrahydrate
((NH4)6Mo7O24$4H2O) and nickel nitrate hexahydrate
(Ni(NO3)2$6H2O), stepwise. Aer each impregnation, the cata-
lysts were dried at 80 ꢀC for 12 h and then calcined at 500 ꢀC for
6 h. The composition of the catalysts was 12 wt% of MoO3 and
3 wt% of NiO.
2.3. Catalytic activity
high HDS activities. The catalyst with the Si : Ti ratio of 5 The DBT HDS activity tests were performed in a continuous xed-
exhibited the highest activities for DBT and 4,6-DMDBT HDS, of bed reactor with 0.5 g catalyst. Before the catalytic activity testing,
97% and 65% respectively (the concentration of DBT was about the catalysts were sulded in situ with a mixture of 2 mol%
500 ppm).
CS2-cyclohexane at 340 ꢀC for 4 h under 4 MPa H2 pressure.
Aer sulidation, the DBT reactant, with a concentration of
500 ppm (cyclohexane was used as solvent), was fed to the
reactor under the conditions of the same volume ratio of H2 : oil
(200 : 1), the same temperature of 340 ꢀC and different LHSV
2. Experimental
2.1. Preparation of FDU-12 and Ti-FDU-12
Mesoporous structured pure FDU-12 silica was synthesized values from 20–150 hꢁ1. Finally, the products were analyzed by a
according to the literature18 using nonionic block copolymer sulfur and nitrogen analyzer (RPP-2000SN, Taizhou Central
EO106PO70EO106 (F127) as the template, and 1,3,5-trime- Analytical Instruments Company, China). The 4,6-DMDBT HDS
thylbenzene (TMB) together with inorganic salts (KCl) as activity tests were also performed with the above procedure.
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The HDS activities (h) are calculated by the following (hv ¼ 1253.6 eV) X-ray source. The binding energies were cali-
formula:
brated using the C 1s peak of contaminant carbon (BE ¼ 284.6
eV) as an internal standard.
The XPS data of the promotion rate (PR) and the promoter
ratio (Ni : Mo)slabs, which are presented in Table 3, are calcu-
lated as follows:19
Sf ꢁ Sp
h ¼
ꢂ 100%
(1)
Sf
where Sf refers to the content of sulfur in the feed, and Sp refers
to the content of sulfur in the products.
½NiMoSꢃ
PR ¼
ꢂ 100
(2)
½Niꢃtotal
2.4. Characterization
½NiMoSꢃ
½MoS2ꢃ
ðNi : MoÞ
¼
(3)
The supports and catalysts were characterized by X-ray powder
diffraction (XRD), scanning electron microscopy (SEM), FT-IR,
UV-vis, 29Si NMR, Raman spectroscopy, N2 adsorption–desorp-
tion and X-ray photoelectron spectroscopy (XPS).
slabs
Low angle and wide angle XRD patterns were taken between
0.5–5ꢀ and 10–80ꢀ with an instrument (Shimadzu X-6000) at 40
kV using Cu Ka (k ¼ 0.154 nm, step size 0.02ꢀ, 0.5 s) radiation.
SEM images were obtained on a Cambridge S-360 apparatus
operating at 20 kV. The samples were coated with gold before
the SEM measurement.
TEM was carried out using a JEOL JEM 2100 electron
microscope equipped with a eld emission source at an accel-
erating voltage of 200 kV.
Fourier transform infrared (FT-IR) absorbance spectra were
recorded at wave numbers ranging from 4000 to 400 cmꢁ1 using
a FTS-3000 spectrophotometer. The measured wafer was
prepared with a weight ratio of sample to KBr of 1 : 100. The
resolution was set at 2 cmꢁ1 during the measurements.
The UV-visible diffuse reectance spectra of the supports
and catalysts were recorded in a 200–800 nm range at room
temperature using a UV-visible spectrophotometer (Hitachi
U-4100) with the integration sphere diffuse reectance
attachment.
3. Results and discussion
3.1. X-ray diffraction analysis
The low angle powder X-ray diffraction (XRD) pattern of the
pure FDU-12 silica is shown in Fig. 1A. It exhibits ve charac-
teristic peaks, which should be assigned to the (111), (220),
(311), (331), (442) reections of a face-centred cubic structure,
indicating that the mesoporous FDU-12 silica was successfully
synthesized. Low angle XRD patterns of different samples with
various Si : Ti molar ratios are shown in Fig. 1B. It is clear that
the typical peak (111) weakens and broadens and its intensity
reduces, which suggests that the order degree of mesoporous
structure decreases with the increasing Si : Ti molar ratio. The
mesostructure collapses with the Si : Ti molar ratio up to 2.5
(Fig. 1B curve f), which is identied by the weak characteristic
peak. The Si : Ti molar ratio can reach 5, at which a well-ordered
face-centred cubic structure is obtained. In addition, XRF
analysis of the ST-5 sample shows that the ratio of Si : Ti in the
nal product is 7.9 : 1, so the two-step method is effective for
the synthesis of the well-ordered mesoporous composite
Ti-FDU-12.
To conrm the type of TiO2 present in the samples, the
powder wide angle XRD patterns are shown in Fig. 1C. A
diffraction peak at 23ꢀ in all composites indicates the presence
of amorphous silica.20 There are no diffraction peaks of TiO2 in
the materials with no or less titanium content (FDU-12, ST-40,
and ST-20), suggesting that the titania crystal phase is absent.
The peaks at 36.2ꢀ, 48.1ꢀ and 54.0ꢀ appearing in the samples of
ST-5 and ST-2.5 correspond to anatase (JCPDS 21-1272). A weak
peak belonging to anatase at 25.2ꢀ appears in the sample of
29Si MAS NMR was carried out on a Bruker AVANCE III 600
spectrometer at a resonance frequency of 119.2 MHz using a 4
mm HX double-resonance MAS probe at a sample spinning rate
of 12 kHz. Single-pulse 29Si MAS NMR experiments with 1H
decoupling were performed with a p/4 pulse width of 2.6 ms, an
1
80 s recycle delay, a H decoupling strength of 60 kHz and for
400 scans. The chemical shis of 29Si were referenced to TMS.
Raman spectra were recorded with a Renishaw Micro-Raman
System 2000 spectrometer with a spectral resolution of 2 cmꢁ1
.
The laser line at 532 nm of a He/Cd laser was used as the
excitation source with an output of 20 mW. The spectra were
recorded at room temperature in conditions of 50 s integral
time at a 1 cmꢁ1 resolution.
The pore sizes and surface areas of different samples were
measured at ꢁ196 ꢀC using N2 adsorption–desorption with a
Micromeritics TriStar II 2020 porosimetry analyzer. The
Table 1 Physicochemical properties of pure FDU-12 silica and ST-x
(x ¼ 40, 20, 10, 5, 2.5)
Si : Ti
Pore volume
BJH (cm3 gꢁ1
)
Pore diameter
BJH (nm)
ꢀ
samples were degassed at 300 C for 8 h prior to the measure-
Samples molar ratio SBET (m2 gꢁ1
)
ments. The specic surface area was calculated via the Bru-
nauer–Emmett–Teller (BET) method. The total volumes of
micropores and mesopores were calculated from the amounts
of nitrogen adsorbed at a relative pressure of approximately
P/P0 ¼ 0.99.
FDU-12
ST-40
ST-20
ST-10
ST-5
N
40
20
10
5
539.4
631.7
854.5
646.2
611.7
530.4
0.46
0.63
0.75
0.61
0.36
0.38
5.13
7.97
7.50
8.54
4.16
4.03
X-ray photoelectron spectroscopy (XPS) was performed on a
PerkinElmer PHI-1600 ESCA spectrometer using a Mg Ka
ST-2.5
2.5
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Table 2 The distribution of Mo species over various Ti-modified
NiMo/FDU-12 catalysts, as determined by XPS analysisa
Catalysts
Mo4+/Mototal Mo5+/Mototal Mo6+/Mototal S : Mob
NiMo/FDU-12 0.49
0.19
0.18
0.15
0.21
0.32
0.32
0.27
0.41
1.28
1.30
1.78
1.32
NiMo/ST-10
NiMo/ST-5
NiMo/ST-2.5
0.50
0.58
0.38
a
b
Mototal (total molybdenum) ¼ Mo4+ + Mo5+ + Mo6+
.
Atomic ratios of
S : Mo determined by XPS analysis.
Table 3 The distribution of Ni species over different catalysts and their
sulphidity, as determined by XPS analysisa
Fig. 2 N2 adsorption–desorption isotherms of different samples: (a)
FDU-12, (b) ST-40, (c) ST-20, (d) ST-10, (e) ST-5, and (f) ST-2.5. All
isotherms are set vertically in order to observe them clearly.
Catalysts
NiS/NiT NiO/NiT Nisulphidity PR (Ni : Mo)slabs
NiMo/FDU-12 0.28
0.36
0.30
0.16
0.30
64
70
84
70
32
42
47
39
0.22
0.23
0.30
0.28
NiMo/ST-10
NiMo/ST-5
NiMo/ST-2.5
0.32
0.37
0.31
a
NiT ¼ NiS + NiMoS + NiO. Nisulphidity ¼ (NiS + NiMoS)/NiT ꢂ 100. PR
(promotion rate) ¼ (NiMoS/NiT) ꢂ 100. (Ni : Mo)slabs ¼ promoter ratio
¼ NiMoS : Mo4+
.
ST-10. Furthermore, there are some peaks appearing at 27.3ꢀ
and 41.2ꢀ in the samples of ST-5 and ST-2.5, which are assigned
to rutile (JCPDS 29-1360); meanwhile, the characteristic peak of
brookite at 30.8ꢀ is presented in the ST-2.5 sample, but the
intensity is very weak.
3.2. The results of BET
Fig. 3 FTIR spectra of different samples: (a) FDU-12 and (b) ST-20.
Normal N2 adsorption–desorption isotherms of different
samples are shown in Fig. 2. Mesoporous pure FDU-12 silica
shows a larger H2 hysteresis loop.18 The pore sizes and BET of
FDU-12 and the series of ST-x materials are listed in Table 1.
It is noteworthy that the pore sizes of Ti-modied composites
become larger than the pure FDU-12 silica while the ratios of
Si : Ti are low (40, 20 and 10), which is caused by adding the
expansion agent n-butanol. The sample ST-10 possesses the
largest pore diameter, of 8.54 nm, and the ST-20 material has
the largest surface area (854.5 m2 gꢁ1) and pore volume
(0.75 cm3 gꢁ1). The pore size of ST-5 decreases to some extent
but the surface area is well maintained. Meanwhile the same
physical parameters of ST-2.5 drastically decrease, which may
be due to the collapse of the mesoporous structure. This
will result in ST-2.5 having poor mass transfer and
diffusion abilities, and more so in it having fewer catalytic
active sites.
Fig. 1 (A) Low angle XRD pattern of FDU-12 (B) and (C) low angle and wide angle XRD patterns of different samples: (a) FDU-12, (b) ST-40, (c) ST-
20, (d) S T-10, (e) ST-5, and (f) ST-2.5.
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Fig. 4 Raman spectra of supports with different Si : Ti ratios (A–C); (D) Raman spectra of different catalysts: (a) NiMo/FDU-12, (b) NiMo/ST-40,
(c) NiMo/ST-20, (d) NiMo/ST-10, (e) NiMo/ST-5, and (f) NiMo/ST-2.5.
involving tetrahedrally coordinated Ti4+ ions.22 Finally, these
results conrm that titanium ions successfully enter the silicon
skeleton.
3.4. Raman spectroscopy
The Raman spectra of FDU-12 and ST-x (x ¼ 40, 20, 10) are
shown in Fig. 4A. The bands at 430, 491, 604, 800 and 977
cmꢁ1 should be ascribed to pure silica.23 Two bands at 491
and 604 cmꢁ1 are assigned to the tri- and tetracyclosiloxane
rings produced by condensation of surface hydroxyls. The
peak appearing at 977 cmꢁ1 is caused by the surface Si–OH
stretching mode. The band at 430 cmꢁ1 is attributed to
Si–O–Si bending mode, and the band at 800 cmꢁ1 is ascribed
Fig. 5 29Si NMR spectra for FDU-12 (upper) and ST-20 (lower).
to the symmetrical Si–O–Si stretching mode.24,25 When the
amount of titanium species is relatively less (ST-40, ST-20),
the characteristic peaks of titanium are not present; this
indicates that there is no bulk crystalline titania present in
the cubic Ti-FDU-12 mesoporous silica composites and that
the titanium ions are successfully incorporated into the
framework of the FDU-12. As for ST-10, the band at 144 cmꢁ1
belonging to anatase is very weak, which is consistent with
the results of wide angle XRD and UV-vis DRS. Raman spectra
of ST 5 and ST 2.5 are shown in Fig. 4B and C. For the samples
of ST-5 and ST-2.5 with comparatively high titanium
contents, the intense absorbance of anatase at 144 cmꢁ1 can
be observed. Furthermore, as the atomic ratios of Si and Ti
reach 5, two peaks assigned to anatase at 395 and 515 cmꢁ1
can be observed. Meanwhile the two broad bands attributed
to rutile at 445 and 607 cmꢁ1 become prominent, since the
3.3. FTIR spectroscopy
The FTIR spectra of FDU-12 and the ST-20 composites are
shown in Fig. 3. For the two samples, the clear adsorption peak
at 1068 cmꢁ1 is attributed to the asymmetric stretching vibra-
tions of the Si–O–Si band, while the bands at 457 and 808 cmꢁ1
are caused by the symmetric stretching vibration of the Si–O–Si
band. The peak at 1640 cmꢁ1 is observed in two of the samples,
which indicates the existence of a surface silanol group.21
Importantly, a peak at 948 cmꢁ1 can be observed from the
spectra aer modication with titanium butoxide; this is
attributed to the combination of the stretching modes of the Si–
Oꢁ species in the Si–OH groups and in the Si–O–Ti4+ sequences
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Fig. 6 (A) UV-vis DRS spectra of different samples: (a) ST-40, (b) ST-20, (c) ST-10, (d) ST-5, and (e) ST-2.5; (B) and (C) UV-vis DRS spectra and
plots of a1/2 versus photon energy (hn) of different catalysts: (a) NiMo/FDU-12, (b) NiMo/ST-40, (c) NiMo/ST-20, (d) NiMo/ST-10, (e) NiMo/ST-5,
and (f) NiMo/ST-2.5.
3.5. 29Si NMR spectroscopy
29Si NMR spectroscopy is a well-established method for char-
acterizing the heteroatoms incorporated in the framework of
crystalline silicate minerals, which should also be suitable for
mesoporous silicas.31 The 29Si NMR spectra of FDU-12 and ST-
20 are shown in Fig. 5. The peak appearing at about ꢁ110 ppm
(Q4) is attributed to Si(–O–)4 units but does not include silanol
(SiOH).32 The shoulder at ꢁ102 ppm (Q3) is ascribed to the
silicon nuclei with a single silanol group, Si(OH)(–O–)3. A signal
peak at ꢁ91 ppm (Q2) assigned to Si(OH)2(–O–)2 is very weak in
the FDU-12 and ST-20 samples. In comparison with FDU-12, a
peak region narrowing and characteristic intensity weakening
are observed for the Ti-containing sample ST-20. In particular,
the shoulders at ꢁ102 ppm and ꢁ91 ppm that are assigned to
silicon–oxygen bond containing silanol groups become weak,
which is probably caused by the titanium ions replacing the
hydrogen in silanol groups, meaning that the Ti4+ ions are
incorporated into the FDU-12 silica framework. Meanwhile, the
intensity of Q4 (Si(–O–)4) decreases more than Q2 and Q3, which
suggests that Ti4+ ions replace the silicon and form Si–O–Ti
bonds.
Scheme 1 Proposed schematic representation of a two-step method
to synthesise Ti-FDU-12 (OH ¼ surface silanol group, Si ¼ silyl group,
Ti ¼ titanium complex).
rutile phase is easily formed in strongly acid environments.26
The highest titanium loading composite, ST-2.5, exhibits a
complex spectrum. Four typical anatase bands at 144, 399,
515 and 634 cmꢁ1 are observed. It should also be noted that
the Raman spectrum of ST-2.5 sample shows one typical
rutile band at 445 cmꢁ1 and brookite bands at 249, 323, 366
cmꢁ1
.
These results indicate that the rutile and brookite
27
3.6. UV-vis DRS spectra of the supports and catalysts
phases with low specic surface areas and pore diameters
will reduce the physicochemical properties of the sample ST-
2.5, which is consistent with the low angle XRD analysis.
Fig. 4D shows the Raman spectra of the catalysts containing
different amounts of titanium in terms of the atomic ratio of
Si and Ti, from N to 2.5. The peaks appearing from 750–1000
cmꢁ1 are assigned to Mo species.28 A band at 958 cmꢁ1 is
observed in all samples; the peak centred at 958 cmꢁ1 should
Fig. 6A shows the UV-vis DRS spectra of Ti-FDU-12 with different
Si : Ti ratios. All the absorption ranges are associated with the
ligand-to-metal charge transfer from the O2ꢁ to Ti4+ which
forms its charge-transfer excited state, (Ti3+–Oꢁ).33 Meanwhile,
the absorption ranges become wider with an increasing amount
of Ti incorporation. The band appearing at about 200–210 cmꢁ1
in all samples can be assigned to the charge-transfer transitions
of oxygen to tetrahedrally coordinated Ti4+ ions within the
framework site of Ti(OSi)4,34,35 conrming that Ti4+ ions are
successfully incorporated into the framework of the silica. A
band at 220–230 nm can be observed when the Si : Ti ratio
reaches 20, which can be assigned to the framework site of
Ti(OH)–(OSi)3.36,37 Furthermore, when the Si : Ti ratios are low
(ST-40, ST-20), the bands of TiO2 cannot be observed. However,
a weak absorption of the composite ST-10 appearing at 390 nm
should be assigned to anatase. The ST-5 and ST-2.5 samples
present an absorption edge in the visible region around 405 nm,
be ascrꢁib4ed to large clusters of MoOx species, such as
Mo8O26
.
However, the peak at 943 cmꢁ1 belonging to
29
Mo7O24ꢁ6 species is not clearly observed, which suggests that
the amount of the Mo8O26 species is larger than that of
ꢁ4
Mo7O24ꢁ6. Meanwhile, there are no peaks appearing in the
990–1000 cmꢁ1 region, demonstrating that the large poly-
meric MoOx species must not be present.30 The characteristic
peaks of TiO2 can also be observed in the samples of
NiMo/ST-5 and NiMo/ST-2.5.
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Fig. 7 SEM images and particle size distribution of different samples: (a) FDU-12 static (b) FDU-12 stirring and (c) ST-20. The image in the same
column represents the same sample.
demonstrating that the rutile phase appears in these two Ti(OSi)4. Secondly, as the Si : Ti ratio reaches 20, the framework
composites. Other lower-energy absorption regions at about 250 sites of Ti(OH)–(OSi)3 in the surface coexist with the former site
and 300–330 nm are apparent in the samples with higher tita- of Ti(OSi)4. Finally, with the increase in Ti content (ST-10, ST-5
nium contents (ST-5 and ST-2.5), which are assigned to Ti–O–Ti and ST-2.5), the TiO2 phases (Ti–O–Ti bonds) are formed out of
bonds and the presence of high coordination number titanium the FDU-12 framework. In addition, the above two framework
sites. The band at 250 nm is generally attributed to isolated sites also exist in these three samples as well. Accordingly, there
penta-coordinated titanium sites or TiO2 oligomers.38,39 In are three main forms of Ti atoms: Ti(OSi)4, Ti(OH)–(OSi)3 and
addition, it should be noted that the adsorption of ST-2.5 shows titania phases (anatase, rutile and brookite), which change with
a red-shied displacement compared with the ST-5 sample. The the various amounts of titanium incorporated.
ST-2.5 sample also presents a broader visible-light region,
which may caused by the appearance of brookite.40
In conclusion, the Ti-FDU-12 synthesis route is shown in
Scheme 1. Firstly, the silicate species interact with randomly
From the above results, the form of titanium in the arranged micellar units through Coulomb force, then SiO2 is
composites can be deduced. Firstly, when the amount of tita- formed on the surface of the micellar units. Secondly, the units
nium incorporation is low (ST-20), the titanium atoms substi- pile up together and spontaneously form a well-ordered cage-
tute the silicon atoms and form the internal framework sites of like structure. Finally, aer the addition of a titanium source
Fig. 8 (A) XPS spectra (O 1s) of different samples: (a) Pure silica FDU-12 and (b) ST-5; (B) Ti 2p XPS scan of ST-5.
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Table 4 Product distribution of the HDS of DBT over NiMo/FDU-12,
NiMo/ST-10, NiMo/ST-5 and NiMo/ST-2.5
HYD route
DDS route
BP (%)
THDBT
CHB (%) (%)
Catalysts
HYD : DDSa ratio
NiMo/FDU-12 96.4
2.4
15.6
22.5
7.3
1.2
1.5
3.5
2.0
0.04
0.21
0.35
0.10
NiMo/ST-10
NiMo/ST-5
NiMo/ST-2.5
82.9
74.0
90.7
a
The HYD : DDS ratio of the DBT reaction is determined by (CHB +
THDBT)/BP under the conditions that the DBT conversion was xed at
about 50%.
Fig. 9 XPS envelope of the Mo 3d region of the sulfided Ti-modified
NiMo/FDU-12 catalysts loaded with different amounts of titanium.
Scheme
2 Reaction scheme for the DBT HDS routes over the
NiMo/FDU-12 and NiMo/Ti-FDU-12 catalysts: hydrogenation (HYD),
direct desulfurization (DDS), tetrahydrodibenzothiophene (THDBT),
biphenyl (BP), and cyclohexylbenzene (CHB).
silicon. Furthermore, based on the FTIR and 29Si NMR results, a
peak attributed to Si–OH bonds is observed, indicating that
Fig. 10 Ni 2p scan of sulfided Ti-modified NiMo/FDU-12 catalysts titanium doesn't replace the hydrogen in silanol groups
prepared with different amounts of titanium.
completely. Moreover, the groups of Si(–O–)4, Si(OH)(–O–)3 and
Si(OH)2(–O–)2 coexist in the Ti-FDU-12 structure. In addition,
there are three main forms of titanium as deduced from the
and calcinations, the template F127 is removed and well-
ordered mesoporous Ti-FDU-12 composites are successfully
synthesized. Si–O–Ti bonds are also formed through titania
atoms replacing the hydrogen atoms in the silanol groups and
UV-vis spectra analysis, including Ti(OSi)4, Ti(OH)–(OSi)3 and
titania phases, which change with an increase in the titanium
contents. More importantly, in this two-step process, the
Fig. 11 DBT and 4,6-DMDBT HDS activities of the catalysts modified with different amounts of titanium at various LHSV values.
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characteristic cage-like structure units are protected from observation suggests that the FDU-12 product has a uniform,
destruction due to titanium incorporation of some degree. well ordered face-centred cubic (Fm-3m) structure without
UV-vis DRS spectroscopy was also used to study the species intergrowth, indicating that FDU-12 was successfully synthe-
of molybdenum oxide in the NiMo supported catalysts modied sized. Moreover, in the images of ST-40, ST-20 and ST-10, the
by different amounts of titanium. In Fig. 6B, the absorption Ti-FDU-12 composites also have well-ordered mesoporous
bands in the 200–400 nm region are attributed to the ligand-to- structures, but the order degree of the composites decreases
metal charge transfer from (LMCT)O2ꢁ to Mo6+. The charac- slightly with an increase in titanium content. The TiO2 phase is
teristic band at about 250 nm is assigned to the isolated presented in the ST-5 and ST-2.5 samples. Meanwhile, (111)
molybdate in tetrahedral coordination, Mo(Td). The poly- reection still exists in ST-5, indicating that the composite has a
molybdate species in octahedral coordination Mo(Oh) are relatively ordered mesoporous structure. The image of ST-2.5
observed in a wider region between 260 and 330 nm.41 The exhibits a disordered structure, which suggests that the struc-
spectra clearly show that the bands of Mo oxide species change ture has already been destroyed by the addition of excess
with different amounts of titanium in the catalysts. The char- titanium.
acteristic region of Mo(Oh) (between 260 and 330 nm) widens
with an increasing titanium contents, which indicates that the
incorporation of titania is conducive to the formation of
3.8. XPS results
Mo(Oh). Band gap energy can be estimated by a plot of (a)1/2 The O 1s XPS analysis performed on the FDU-12 and ST-5
versus photon energy (hv); the position of the absorption edge samples is presented in Fig. 8A. The peak at 532 eV should be
can then be determined by extrapolating the linear part of the assigned to the Si–O–Si unit. The spectrum of ST-5 (Fig. 8A,
rising curve to zero. The average domain size of the oxide curve b) shows an extra peak at 530 eV that may be attributed to
nanoparticles increases along with a decrease in the energy the Ti–O–Ti or Si–O–Ti units that do not appear in the FDU-12
gap.42,43 In Fig. 6C, the energy band gap of molybdenum oxides sample (Fig. 8A, curve a). A weak shoulder appearing at about
in the catalysts becomes larger with an increase in the amount 531 eV is assigned to the Ti–OH unit,44–46 which is consistent
of titanium loading, indicating a decrease in the average with the UV-vis DRS spectra. In addition, the Ti 2p XPS scan of
domain size of molybdenum, which will result in a decrease in ST-5 is shown in Fig. 8B. The Ti 2p peaks at 458.2 eV and 464.1
aggregation and an improvement in the dispersion of molyb- eV can be assigned to Ti4+ 2p3/2 and Ti4+ 2p1/2. This conrms
dena. These results clearly propose that the addition of titanium that TiO2 is present at the surface of the ST-5 composite. The
species has a great inuence on the oxidic molybdenum species analysis results demonstrate the appearance of TiO2 in the ST-5
and domain size.
sample, which is consistent with the Raman spectroscopy
observations.
Fig. 9 shows the XPS proles of the Mo 3d region of various
sulded Ti-modied NiMo/FDU-12 catalysts. The data of peak
3.7. SEM and TEM observations
The SEM image of the pure mesoporous FDU-12 silica under deconvolution for various Mo species are summarized in Table
stirring conditions is shown in Fig. 7a. It can clearly be seen that 2. Beccat et al. proposed that Mo 3d signals included Mo4+
these particles have an inerratic polyhedral morphology and a (corresponding to MoS2), Mo5+ (oxysulde species) and Mo6+
uniform particle size distribution. The SEM image of FDU-12 (oxidic molybdenum).47 Different types of Mo species have
under static conditions is given in Fig. 7b. It was found that doublet binding energies (BEs) (Mo 3d5/2 and 3d3/2). Peak
these particles have a spherical morphology, but that the deconvolution is in accordance with the BE values (Mo 3d5/2)
particle size distribution is non-uniform. The image in Fig. 7c for Mo4+, Mo5+ and Mo6+, which are about 228.6 eV, 230.7 eV,
shows that ST-20 exhibits polyhedral but uniform particle 232.3 eV respectively. The BE values of Mo 3d3/2 are xed at 3.1
morphology, and aggregation is also observed. These results eV more than those of Mo 3d5/2, and the intensity ratio of Mo
suggest that the conditions, either stirring or static, in the water 3d5/2 and Mo 3d3/2 is approximately xed at 3 : 2. Furthermore,
bath have a great inuence on the morphology and particle size a peak at about 226.0 eV is assigned to the S2s line and is shown
distribution of pure FDU-12 silica. Furthermore, the incorpo- in the spectrum of Mo 3d. Sulphidity can be observed by the
ration of titanium also affects the morphology of the composite. ratio of Mo4+ : Mototal (Mototal ¼ Mo4+ + Mo5+ + Mo6+) in Table 2.
The particle-size distributions are also shown in Fig. 7. The It is apparent that TiO2 affects the Mo species distribution. As
average particle size of sample 1 (FDU-12 static) is about 2.5 mm, the amount of titanium addition is relatively low (Si : Ti ¼ 10),
that of sample 2 (FDU-12 stirring) is about 5 mm and that of the change in Mo species distribution is small. Meanwhile, the
sample 3 (ST-20) is about 7 mm. It can be observed that the Mo4+ : Mototal and S : Mo ratios increase much more when the
composite synthesized under stirring conditions has a narrow titanium loading is high (Si : Ti ¼ 5), whereas the ratios
particle size distribution and a small average particle size. decrease when the Si : Ti ratio is as high as 2.5. From the Raman
Moreover, the average particle size of the composite ST-20 is spectrum (Fig. 4C) and BET analysis (Table 1) of the NiMo/ST-
larger than the pure FDU-12 silica, which is probably caused by 2.5 catalyst, the brookite phase appears and the surface area
titanium incorporation.
decreases in comparison with the NiMo/ST-5 catalyst. There-
TEM images of different samples are presented in Fig. S1.† fore, the decreases int he Mo4+ : Mototal and S : Mo ratios should
The image of FDU-12 is consistent with the literature.18 The be due to the appearance of the brookite phase and the decrease
highly ordered lattice array over large domains seen under TEM of the surface area is adverse to the formation of Mo4+. These
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results indicate that an appropriate Ti loading content can the excess addition of titanium (sample ST-2.5) causes a
facilitate the formation of MoS2. collapse of the well-ordered mesoporous structure. Secondly, as
The Ni 2p XPS spectra of the sulded Ti-modied NiMo/FDU- can be seen using the BET measurement, the pore volume and
12 catalysts are presented in Fig. 10. The Ni 2p envelope is surface area of ST-2.5 have drastically decreased. The above two
composed of three contributing compounds (NiS, NiO and results indicate a high diffusion resistance of the reactants and
NiMoS).48 The binding energies of NiS (2p3/2 and 2p1/2) for fewer catalytic active sites, which are not favourable for the HDS
each of the catalysts are 852.6 and 873 eV. The peak appearing at reaction. Finally, as the amount of titanium incorporation is
862 eV is assigned to NiO.49 The binding energy of the NiMoS excessive, the appearance of the brookite phase and the
phase appeared between 854.6 and 855.0 eV.19 Two parameters decrease in the surface area will decrease the percentages of
of promotion rate (PR) and promoter ratio (see the corre- Mo4+ : Mototal
, Nisulphidity, S : Mo NiMoS (PR value) and
sponding formulas in the footnote in Table 3) are closely related (Ni : Mo)slabs, which are the important factors for a high HDS
to the formation of NiMoS active phases over the sulded NiMo efficiency.
catalysts. The PR value represents the total percentage of nickel
in the NiMoS phase. The promoter ratio signies the active
species.19,50 It is clear that the PR, (Ni : Mo)slabs and Nisulphidity
(shown in Table 3) of NiMo/ST-5 are higher than NiMo/FDU-12
3.10. The selectivity of DBT HDS
and NiMo/ST-10, which is probably caused by the appearances The selectivity of the DBT HDS reaction is also studied. The
of anatase and rutile. However the above three values of the product distributions of the HDS of DBT over the catalysts
NiMo/ST-2.5 catalyst decrease in comparison with the NiMo/ST- detected by GC-MS are shown in Table 4. There are two major
5 catalyst, which could be also attributed to the appearance of routes in the HDS of DBT, as shown in Scheme 2. These are: the
brookite and the decrease in the surface area, which makes the direct desulfurization (DDS) route, which involves the hydro-
formation of the active phase more difficult. In conclusion, an genolysis of C–S bonds without hydrogenation of the aromatic
appropriate amount of Ti loading can facilitate the formation of ring; and the hydrogenation (HYD) route, which requires ring
more activity sites and lead to
desulfurization.
a
high efficiency of saturation prior to the desulfurization step.51 In this research,
the main products of the DBT HDS are biphenyl (BP), cyclo-
hexylbeneze (CHB) and traces of tetrahydrodibenzothiophene
(THDBT). It has been reported that selectivity is related to the
3.9. HDS performance on DBT and 4,6-DMDBT
mode of DBT adsorption. BP is the main product when using all
The pore properties of the support play an important role in the of the catalysts, which indicates that DBT adsorbs on the active
HDS reaction. The composites with large surface areas and pore phase perpendicularly according to a vertical geometry.52 For all
sizes can facilitate the diffusion of reactant molecules and form the catalysts, the amount that the HYD route produced is less
abundant active sites. In addition, an appropriate amount of Ti than the DDS route, This is because the adsorption of p-elec-
incorporation is conducive to the formation of MoS2 and NiMoS trons of the DBT molecule, which facilitates the HYD route,
(XPS test), which can also improve the HDS activity of the appears more difficult than direct C–S bond cleavage.53 The
catalysts. The hydrodesulfurization performances on DBT and HYD : DDS ratios in the supported NiMo catalysts follow the
4,6-DMDBT at different LHSV values (150 hꢁ1, 120 hꢁ1, 80 hꢁ1
,
order: NiMo/FDU-12 < NiMo/ST-2.5 < NiMo/ST-10 < NiMo/ST-5.
50 hꢁ1 and 20 hꢁ1) of NiMo/ST-x are shown in Fig. 11. It is The HYD : DDS ratios of the Ti-incorporated catalysts are higher
obvious that the DBT and 4,6-DMDBT HDS activities become than that of the NiMo/FDU-12 catalyst, indicating that the
better with a decreasing LHSV value. The DBT and 4,6-DMDBT desulfurization of DBT over the Ti-modied supported catalysts
HDS activities of NiMo/FDU-12 are lower than the other Ti- promotes the HYD route more than the pure silica supported
incorporated samples, conrming that the incorporation of catalyst; this is consistent with the literature.54–57 The effect of Ti
titanium can increase the HDS efficiency. Furthermore, when incorporation can be attributed to the structure and specic
the amount of titanium added is relatively low (NiMo/ST-40, 20, electronic properties of the active phases formed on the Ti-
10 and 5), the catalysts show higher activity with an increasing loaded support.54,56 It has also been reported that the accelera-
titanium content at the same LHSV value. It should also be tion of the HYD pathway must be associated with a good
noted that the 4,6-DMDBT HDS conversion rates over all the dispersion of the active phase.58 From the above analysis results
catalysts are lower than those for DBT, since the steric (Fig. 6C), it appears that the titania incorporation can lead to a
hindrance of the methyl groups in the 4,6-DMDBT molecule decrease in the aggregation and an improvement in the
resulted in a lower HDS.2 The NiMo/ST-5 catalyst exhibited the dispersion of molybdena. Hence, the Ti-incorporated catalysts
highest HDS activity; the DBT and 4,6-DMDBT HDS conversions present relatively higher HYD selectivities than the NiMo/FDU-
could reach 97% and 65%, respectively, at the low LHSV value of 12 sample. The NiMo/ST-5 sample presents the highest
20 hꢁ1. This is probably because the ST-5 support keeps a HYD : DDS ratio, while the ratio decreases in the ST-2.5 sample.
relatively ordered structure and an appropriate amount of tita- This may be explained by the fact that the excess titania coated
nium incorporation helps to increase the HDS activity. Mean- on the surface inhibits the adsorption of DBT molecules on the
while the NiMo/ST-2.5 catalyst presented a poorer HDS activity. active sites.53,58 Indeed, the excess titania loading causes a
There are three main factors to consider when explaining the decrease in surface area and the appearance of brookite, which
reasons for this. Firstly, according to low angle XRD analysis, will lead to a decrease in amount of active phases (MoS2 and
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4. Conclusion
In summary, pure FDU-12 silicas with spherical and inerratic
polyhedral morphologies were synthesized under static and
stirring conditions. Highly ordered mesoporous Ti-modied
FDU-12 composites with an Fm-3m structure were synthesized
through a simple two-step method of prehydrolysis of TEOS
followed by the addition of titanium butoxide as a titanium
source. Low angle XRD spectra and TEM results showed that
even when the Si : Ti ratio of the composites reaches 5, a rela-
tively ordered structure can still be obtained. N2-adsorption and
desorption measurement indicated their high surface areas,
and large pore volumes and sizes. XPS analysis of the sulded
catalysts showed that titanium incorporation facilitates the
dispersion of molybdena, and an appropriate Ti incorporation
make the formation of MoS2 and NiMoS phases easier. These
are very important for the HDS reaction. HDS tests of DBT and
4,6-DMDBT showed that all catalysts present high activities,
with the NiMo/ST-5 catalyst exhibiting the highest HDS effi-
ciency due to its relatively ordered structure, high surface area
and the incorporation of TiO2.
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Acknowledgements
The authors acknowledge nancial support from the National
NSFC (no. 21276277), the Opening Project of the Guangxi Key
Laboratory of Petrochemical Source Processing and Process
Intensication Technology (2012K01), the CNPC-Petrochemical
Research Key project, and the PetroChina Innovation Founda-
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