Direct synthesis, characterization and catalytic activity of titanium-substituted SBA-15 mesoporous molecular sieves

Abstract

Titanium-substituted SBA-15 mesoporous molecular sieves have been successfully synthesized under conventional hydrothermal conditions by using titanium trichloride and tetraethyl orthosilicate as titanium and silica source, respectively. The synthesized materials were characterized by powder X-ray diffraction patterns (XRD), diffuse reflectance UV–visible spectroscopy (UV–vis), X-ray fluorescence spectroscopy (XRF), N2 sorption isotherms and the reaction of catalytic oxidation of styrene. When the pH value of the gel solution was approximately 1.8 and titanium chloride was added after prehydrolysis of tetraethyl orthosilicate for a certain time, the titanium could be effectively incorporated into the framework of SBA-15. The Ti-SBA-15 so obtained was of high quality. With the present synthesis approach, the formation of anatase TiO2 in Ti-SBA-15 can be avoided. Moreover, the addition of H2O2 during the preparation process can improve the quality of Ti-SBA-15. The calcined Ti-SBA-15 materials showed high catalytic activity in the selective oxidation of styrene.

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Applied Catalysis A: General 273 (2004) 185–191
Direct synthesis, characterization and catalytic activity of
titanium-substituted SBA-15 mesoporous molecular sieves
Yangying Chena, Yanlei Huanga, Jinghai Xiub, Xiuwen Hana, Xinhe Baoa,∗
aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China
bState Key Laboratory of Fine Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, PR China
Received in revised form 14 June 2004; accepted 17 June 2004
Available online 22 July 2004
Abstract
Titanium-substitutedSBA-15mesoporousmolecularsieveshavebeensuccessfullysynthesizedunderconventionalhydrothermalconditions
byusingtitaniumtrichlorideandtetraethylorthosilicateastitaniumandsilicasource,respectively.Thesynthesizedmaterialswerecharacterized
by powder X-ray diffraction patterns (XRD), diffuse reflectance UV–visible spectroscopy (UV–vis), X-ray fluorescence spectroscopy (XRF),
N2sorption isotherms and the reaction of catalytic oxidation of styrene. When the pH value of the gel solution was approximately 1.8 and
titanium chloride was added after prehydrolysis of tetraethyl orthosilicate for a certain time, the titanium could be effectively incorporated
into the framework of SBA-15. The Ti-SBA-15 so obtained was of high quality. With the present synthesis approach, the formation of anatase
TiO2in Ti-SBA-15 can be avoided. Moreover, the addition of H2O2during the preparation process can improve the quality of Ti-SBA-15.
The calcined Ti-SBA-15 materials showed high catalytic activity in the selective oxidation of styrene.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Ti-SBA-15; TiCl3; Prehydrolysis; Styrene; Oxidation
1. Introduction
Since researchers at the Mobil Corporation reported a
series of ordered mesoporous silicates designated as M41S
[1] that were synthesized by using a surfactant templating
approach in the early 1990s, much effort has been devoted
to studies of the syntheses [2] and applications of ordered
mesoporous materials [3]. The most extensively studied
member of the ordered mesoporous materials is MCM-41,
which possesses a large specific surface area, a hexagonal
array and with uniform mesopore channels. Subsequently,
other mesoporous molecular sieves such as HMS [4] and
MSU [5] were reported. In 1998, a remarkable advance in
the synthesis of ordered mesoporous materials was made
by Zhao et al. [6], who used triblock copolymer surfactants
to template the formation of hydrothermally stable SBA-15
silica molecular sieves with uniform hexagonal channels
ranging from 50 to 300Å.
∗Corresponding author. Present address: Dalian Institute of Chemical
Physics, The Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, PR China. Tel.: +86 411 84379116; fax: +86 411 84694447.
E-mail address: xhbao@dicp.ac.cn (X. Bao).
Incorporation of transition-metal ions into the frame-
works of the molecular sieves is a general method for in-
troducing catalytic sites into mesoporous materials. So far,
numerous attempts have been made to prepare transition
metal-substituted mesoporous molecular sieves for making
them into effective catalysts which are capable of treating
large molecules [7]. Titanium-containing molecular sieves
have attracted much attention in past decades, since tita-
nium silicate-1 (denoted TS-1) molecular sieves have shown
excellent activities for the selective oxidation of a variety
of organic compounds by using hydrogen peroxide as ox-
idant [8]. However, the substrates which can be oxidized
in the presence of TS-1 are restricted to species having
kinetic diameters less than 0.6nm, owing to the pore size
limitation [9]. Titanium-substituted mesoporous materials
offer good alternatives to TS-1 for selective oxidation of
large molecules. Since Corma et al. [10] and Pinnavaia and
co-workers [11] reported the preparation and catalytic ac-
tivities of Ti-containing mesoporous materials using CTAB
or dodecylamine as the structure-directing agent under ba-
sic conditions, much effort has been directed to preparing
Ti-substituted mesoporous materials [12]. Although Pin-
navaia and co-workers [13] have reported the synthesis of
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2004.06.030

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Y. Chen et al./Applied Catalysis A: General 273 (2004) 185–191
Ti-MCM-41 under acidic conditions (pH ≈ 1.5), poor hy-
drothermal stability of the MCM-41-related materials has
limited their potential applications from the commercial
point of view. When highly hydrothermal stable SBA-15
materials were synthesized, then special attention has
been focused on the preparation of Ti-substituted SBA-15.
Titanium-substituted SBA-15, like Ti-MCM-41, could act
as a versatile selective oxidation catalyst for many large
molecules. Up to now, titanium-substituted SBA-15 molec-
ular sieves have been prepared by a postsynthesis method
[14], or directly obtained under microwave-hydrothermal
conditions [15]. Zhang et al. have reported an improved hy-
drothermal method to synthesize Ti-SBA-15. However, the
expensive titanium source (titanium isopropoxide) and the
noxious silica source (tetramethyl orthosilicate) have limited
its use [16]. In 2003, a process in which products obtained
by reacting titanium butoxide with acetylacetone were used
as the titanium precursor was reported [17]. In the rele-
vant reports, the content of framework titanium was either
very low or not determined, and much attention was paid
to avoid the formation of anatase TiO2. In addition, Xiao
and co-workers have used a complex method to synthesize
titanium-substituted SBA-15-like materials the framework
of which consists of nanocrystalline ZSM-5; this material
was quite different from the other mesoporous materials
that possess an amorphous framework [18]. According to
the catalytic testing results reported in the literature [16],
the higher the content of titanium Ti-SBA-15, the more
active the catalyst. Since Ti-MCM-41 is synthesized mostly
under basic conditions while Ti-SBA-15 is synthesized
under acidic conditions, the challenges in the synthesis of
Ti-SBA-15 lies on how to increase the content of titanium
in the framework while avoiding the formation of anatase
TiO2. Until now, reports on successful preparation of metal
doped mesoporous materials under acidic conditions are
very few, and this might be due to the easy dissocia-
tion of metal–O–Si bonds under acidic and hydrothermal
conditions.
In this paper, we report on a new approach to synthesize
Ti-SBA-15 by using titanium trichloride and tetraethyl or-
thosilicate as titanium and silica sources, respectively, un-
der conventional hydrothermal conditions. The Si/Ti ratio of
Ti-SBA-15 in final product can reach as high as 60, without
the formation of anatase TiO2. In addition, other potential
advantages of using titanium trichloride, including the rela-
tively slow hydrolysis rate and the hydrolysis product HCl
which can tune the acidity of synthetic solution, have been
demonstrated in the work.
2. Experimental
2.1. Synthesis of Ti-SBA-15 materials
The Ti-SBA-15 materials were synthesized using TEOS
and titanium trichloride as silicon and titanium sources,
respectively. Triblock copolymer P123 (EO20PO70EO20,
Mav = 5800, Aldrich) was used as the structure-directing
agent. Concentrated HCl aqueous solution was used as the
acid source. All reagents were used as received. The typi-
cal synthetic procedure of Ti-SBA-15 was as follows: 2g
of P123 was dissolved in 60ml HCl solution with varied
acidities. After the tetraethyl orthosilicate (TEOS, 4.25g)
was prehydrolyzed for a certain time at 40◦C, a calculated
amount of titanium trichloride was added to the above
acidic solution under vigorous stirring. In some experi-
ments, 2ml of H2O2pre-mixed with a certain amount of
TiCl3was needed, and the mixture was added into the gel
solution. The stirring was continued for about 24h. The
resultant mixture was then aged at 60◦C for another 24h
without stirring. The samples were recovered, washed, and
dried at 100◦C overnight. After calcination at 550◦C for
6h in air, the mesoporous Ti-SBA-15 samples were finally
obtained.
2.2. Characterization
XRD patterns were recorded on a Rigaku D/Max 3400
powder diffraction system using Cu K? radiation (40kV and
36mA) over the range of 0.5 ≤ 2θ ≤ 8. XRF results were
obtained on a Philips MagiX Spectrometer for determin-
ing the Si/Ti ratio. The nitrogen sorption experiments were
performed at 77K on an ASAP 2000 system in the static
measurement mode. The samples were outgassed at 250◦C
for 10h before the measurement. The pore-size distribu-
tion curves were obtained from analysis of the adsorption
branchoftheisothermbytheBarrett–Joyner–Halenda(BJH)
method. Diffuse-reflectance/UV–vis spectra were collected
on a JASCO V-550 scanning spectrophotometer equipped
with an integrating sphere of 200–800nm.
2.3. Catalytic reaction
The selective oxidation of styrene was used as the test re-
action. Typically, 5mmol of styrene, 5mmol of 30% H2O2,
10ml of CH3CN and 0.3g of catalyst were mixed in a 50ml
round-bottom flask and heated to 70◦C under stirring for
3h. The products were analysed with a GC equipped with
a capillary column and an FID detector.
3. Results and discussion
3.1. Influence of H2O2
Initially, we tried to synthesize Ti-SBA-15 under reported
synthetic conditions (2M HCl solution) for SBA-15 by
adding TEOS and TiCl3 simultaneously, but no evidence
proving the incorporation of titanium had been yielded.
When the concentration of hydrogen chloride of the gel so-
lution was reduced to 1M, then the UV–vis spectra showed
that titanium could be incorporated.

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187
Fig. 1. XRD patterns (A) and UV–vis spectra (B) of Ti-SBA-15 with different Si/Ti ratios in 1M HCl gel solution: (a) SBA-15; (b) Si/Ti = 40; (c) Si/Ti
= 30; and (d) Si/Ti = 20.
Fig. 1A presents the XRD patterns of Ti-SBA-15 synthe-
sized at 1M HCl with different titanium contents of the gel.
All samples show a very strong peak attributed to (100) in
low-angle range, indicating that these samples possess peri-
odic structures. However, the peaks of (110) and (200) are
absent, which means that the incorporation of titanium has
destroyed partly the ordered structure of SBA-15. Fig. 1B
displaystheUV–visspectraoftheTi-SBA-15samples.They
all show a very strong absorption band centering at 213nm,
which has a broader width than that of TS-1 [19]. This band
is generally believed to be related directly to the titanium
species of zeolite, and is arising from the electronic transfer
of the p?–d? between titanium and oxygen in the frame-
work. The band is usually used as a direct proof of the incor-
poration titanium atoms into the framework of the molecular
sieves [20]. The broader width may be due to the fact that a
part of or most of the titanium sites are in a distorted tetra-
hedral environment, which is a direct consequence of the
amorphous nature of the walls of the ordered mesoporous
materials [13,16]. The absence of a band at 330nm indicates
that no anatase TiO2is produced. Furthermore, the intensi-
ties of the band at 213nm are found to increase monotoni-
cally with a decrease of Si/Ti ratio from 40 to 20.
Fig. 2. XRD patterns (A) and UV–vis spectra (B) of Ti-SBA-15 with different Si/Ti ratios in 1M HCl gel solution in the presence of H2O2: (a) SBA-15;
(b) Si/Ti = 40; (c) Si/Ti = 30; and (d) Si/Ti = 20.
Some papers have reported that TS-1 and Ti-MCM-41
were synthesized in the presence of H2O2; however, the
role of H2O2was not very clear yet [21,22]. The influence
of H2O2 on the structure and composition of Ti-SBA-15
materials was also investigated in the present work. Fig. 2A
shows the XRD patterns of the Ti-SBA-15 samples prepared
in the presence of H2O2. It is shown that the (110) and
(200) peaks are clearly resolved, and they, together with
the (100) peak, all belong to SBA-15-type materials. The
result indicates that the hydrogen peroxide can improve the
ordered structures of the Ti-SBA-15 materials.
Fig. 2B displays the UV–vis spectra of Ti-SBA-15 sam-
ples prepared in the presence of H2O2. The intensity of the
peak at 213nm decreases with the increasing amount of ti-
tanium in gel solution, which is a trend opposite to that of
the samples prepared without H2O2.
The XRF results (Table 1) show that the real amounts
of titanium incorporated into the framework of SBA-15 is
much smaller than the amount added to the gel solution. For
example, although the Si/Ti ratio was 20 in the gel solution,
the real Si/Ti ratio in the final product was only 300. The
result confirms that the biggest difficulty for incorporating
titanium into the SBA-15 framework is the strong acidity of

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Table 1
Texture properties of different Ti-SBA-15 samples prepared in the presence or absence of H2O2
Samplea
Si/Ti (initial gel, mol%)Si/Ti (final product)BET suface area (m2/g) Pore diameterb(nm) Pore volume (mL/g)
SBA-15
a1
b1
c1
a2
b2
c2
599
841
701
873
652
603
736
5.70
5.53
5.26
5.64
4.29
4.61
4.37
0.82
0.88
0.72
0.77
0.48
0.52
0.65
40
30
20
40
30
20
950
540
530
550
300
300
aThe subscript 1 represents the material prepared in the presence of H2O2; subscript 2 represents the material prepared in the absence of H2O2.
bBJH adsorption average pore diameter.
the gel solution, which increases the solubility of titanium.
By comparing the XRF results of the Ti-SBA-15 prepared
in the presence and absence of H2O2, it is found that the
real titanium content of Ti-SBA-15 in the presence of H2O2
is about half of that of the Ti-SBA-15 without H2O2, al-
though the Si/Ti ratios in the gel solution are almost the
same. Hence, the added H2O2 can enhance the solubility
of titanium in acidic solution. The maximum Si/Ti ratios in
these Ti-SBA-15 samples prepared with or without H2O2
are 530 and 300, respectively. In combination with the XRD
patterns, the ordered mesostructure of SBA-15 is maintained
only when there is H2O2present in the synthetic process,
despite the fact that the final Si/Ti ratios are around 550.
In other words, the H2O2is beneficial for maintaining the
mesostructure of Ti-SBA-15.
The porosity of the Ti-SBA-15 samples was measured by
N2sorption method. Fig. 3 gives the adsorption and des-
orption isotherms of Ti-SBA-15 samples having similar ti-
tanium contents in final products, but either prepared in the
presence or in the absence of H2O2. The siliceous SBA-15
and Ti-SBA-15 (Si/Tifinal= 540) prepared in the presence
of H2O2yielded Type IV isotherms, and showed steep hys-
teresis of Type H1 at higher relative pressures, which is a
Fig. 3. N2isotherms of: (a) SBA-15; (b) Ti-SBA-15 (Si/Ti = 540 in final
product, prepared in the presence of H2O2); (c) Ti-SBA-15 (Si/Ti = 550
in final product, prepared in the absence of H2O2).
characteristic of mesoporous materials with larger pore sizes
and narrow size distributions [23]. In addition, an obvious
hysteresis of the H3 type of at P/P0 = 0.8 was observed
which was an indication of N2condensation and evapora-
tion within interparticles [23], or was caused by some im-
purity phase generated by liquid-crystal-templated materials
[24]. However, the Ti-SBA-15 (Si/Tifinal= 550) prepared in
the absence of H2O2showed a placid hysteresis loop with a
small closed area, indicating that the sample had relatively
small mesopores and wider size distribution.
The texture properties of other Ti-SBA-15 samples are
summarized in Table 1. It is clear that the incorporation of
titanium into the framework of SBA-15 makes the pore di-
ameter and pore volume to decrease, due to the fact that the
diameter of a titanium atom is larger than that of a silicon
atom. Surprisingly, the surface areas of Ti-SBA-15 are larger
than that of pure SBA-15, and the presence of H2O2in the
preparation procedure made the surface area and pore diam-
eter larger than those of samples in the absence of H2O2.
3.2. Influence of pH value
On the basis of the above discussion, it was found that
the titanium contents of Ti-SBA-15 samples were very low
whentheHClconcentrationwas1M.Sincetheacidityofthe
gel solution was the key factor in limiting the titanium con-
tent, the influence of pH value on the titanium content was
investigated. Fig. 3 shows the UV–vis spectra of Ti-SBA-15
synthesized at different pH values. Although the UV–vis
spectra cannot be used to evaluate the real content of tita-
nium, the variation observed on the peaks may reflect the
changes in amount and species of titanium in Ti-SBA-15.
As the acidity of the gel solution decreases from 1M HCl to
pH 1, the intensity of peak at 213nm increases considerably
(Fig. 4). So, we have to consider a decreasing of the acid-
ity of the gel solution for synthesizing high titanium content
Ti-SBA-15. Moreover, it is also found that Ti-SBA-15 ma-
terials obtained in a solution of weaker acidic show broader
UV–vis spectra, and with a tail approaching 350nm, while
the spectra of the Ti-SBA-15 obtained in a strong acidic so-
lution are much more narrow, and without any absorption
band higher than 300nm. Two assignments of the 300nm
absorption are possible: either ascribing to six-coordinate Ti

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Fig. 4. UV–vis spectra of Ti-SBA-15 samples with Si/Ti = 40 in gel
solution synthesized at different acidic solutions: (a) 0.5M of HCl con-
centration; (b) pH 0.5; and (c) pH 1.
where at least two of the ligands are not framework oxy-
gens, or to partially condensed Ti–O–Ti species [25]. The
lack of the band at 330nm for the Ti-SBA-15 suggests that
no bulk titania is formed in Ti-SBA-15 because bulk titania
usually gives a broad absorption band at 330nm.
3.3. Influence of TCl3addition style
High quality SBA-15 materials have been reported to be
synthesized at pH values lower than 2 [8], so we further
increased the pH value to 1.8 for synthesizing Ti-SBA-15.
Interestingly, when the Si/Ti ratio was 40 in the gel solution
and the pH value was 1.8 and with TEOS and TiCl3added
simultaneously into the acidic solution, most of titanium
species were anatase TiO2, as shown in Fig. 5A(b). Under
Fig. 5. UV–vis spectra (A) and XRD patterns (B) of Ti-SBA-15 samples synthesized at pH 1.8: (a) Si/Ti = 40 in gel solution and TiCl3 was added
after TEOS prehydrolysis for 6h; (b) Si/Ti = 40 in gel solution and TiCl3and TEOS were added simultaneously; and (c) Si/Ti = 30 in gel solution and
TiCl3was added after TEOS prehydrolysis for 6h.
the present conditions, the hydrolysis rate of TiCl3does not
match with that of TEOS, namely, the hydrolysis rate of
TiCl3is faster than that of TEOS at pH 1.8. When TEOS
was prehydrolyzed either for 2h or 4h, the UV–vis spectra
show that anatase TiO2was still the main titanium species.
When the prehydrolysis time of TEOS was extended to 6h
and TiCl3 was then added, the UV–vis spectrum showed
that most titanium species were tetrahedral titanium species
and no anatase TiO2appeared (Fig. 5A(a)). However, when
further decreasing the Si/Ti ratio from 40 to 30 under the
same synthetic conditions, a large amount of anatase TiO2
appeared again (Fig. 5A(c)). In contrast, if the Si/Ti ratios
was increased either to 100 or to 150, no anatase TiO2was
produced, as shown in Fig. 6A.
The XRD patterns of the Ti-SBA-15 prepared with vary-
ingprehydrolysistimesareshowninFig.5B.Thesamplesof
the Ti-SBA-15 prepared by TEOS and TiCl3added simulta-
neously and Ti-SBA-15 with a Si/Ti ratio of 30 showed only
a broad peak in lower angle region, whereas the Ti-SBA-15
with a Si/Ti ratio of 40 displayed three peaks (100) (110)
(200) which were typical of highly ordered SBA-15 ma-
terial. Thus, it is proposed that the anatase TiO2produced
could have great influence on the formation of high quality
Ti-SBA-15.
Fig. 6A shows the UV–vis spectra of the Ti-SBA-15,
prepared at pH 1.8 and with different Si/Ti ratios. As the
Si/Ti ratio in the gel solution decreased from 150 to 40, the
intensity of the peak at 213nm increased relatively, and no
peaks attributed to anatase TiO2were observed. The XRF
results (Table 2) show that, when the Si/Ti ratios were, re-
spectively, 150 and 100 in the gel solution, the final titanium
content of the Ti-SBA-15 was higher than that in the gel so-
lution in both cases, which was similar to the phenomena of
Ti-MCM-41 prepared under basic conditions [26]. However,
forthesamplewithSi/Tiratioof40,therealtitaniumcontent
was less than that in the gel solution. Since the hydrolysis