Materials Sciences and Applicatio ns, 2011, 2, 661-668
doi:10.4236/msa.2011.26091 Published Online June 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
661
Preparation and Catalytic Properties of Ti-SBA-15
Mesoporous Materials
Wenhao Ye, Zheng Lin, Beibei Dong, Jinfeng Kang, Xiucheng Zheng*, Xiangyu Wang
Department of Chemistry, Zhengzhou University, Zhengzhou, China.
Email: *zhxch@zzu.edu.cn
Received November 15th, 2010; revised December 29th, 2010; accepted May 17th, 2011.
ABSTRACT
Ti-SBA-15 mesoporous materials were directly synthesized via a hydrothermal process and characterized by using ni-
trogen adsorption-desorption measurements, X-ray diffraction, scanning electron microscopy, transmission electron
microscopy, diffuse reflectance ultraviolet-visible spectroscopy, and infrared ray spectroscopy. The effect of synthesis
temperatures on the structure and catalytic efficiency in epoxidation of cyclohexene was discussed in details. The re-
sults showed that the optimal temperature was 100˚C under the reaction conditions.
Keywords: Ti-SBA-15; Hydrothermal S ynthesis , Synthesis Temperatures, Cyclohexe ne, Epoxidation
1. Introduction
It is known that epoxidation reactions of alkenes gener-
ally require the presence of a catalyst. However, several
side reactions can take place, such as oxidation in the
allylic positions, ring-opening of the epoxides by hy-
drolysis or solvolysis, epoxide rearrangement, or even
total break-down of the C=C double bonds [1]. Cyclo-
hexene epoxidation to yield cyclohexene oxide is one of
the most difficult cases, in which the first two problems,
namely allylic oxidation and epoxide ring-opening, occur
considerably. Cyclohexene oxide is an important organic
intermediate consumed in the production of pharmaceu-
ticals, plant-protection agents, pesticides, and stabilizers
for chlorinated hydrocarbons. Therefore, it is very essen-
tial and vital to develop new active and selective cyclo-
hexene epoxidation catalysts that circumvent the side
reactions and the subsequent formation of large amounts
of by-products [2].
On the other hand, mesoporous molecular sieves have
attracted great interest in many fields, such as catalysis
and materials science, since their disclosure in 1992 [3].
The most important features for their use are the ex-
tremely high surface area and well defined pore size
which can be adjusted over a large range (2 - 20 nm).
However, pure mesoporous molecular sieves have no
intrinsic catalytic activity. It is necessary to replace part
of the silicon of the structure or graft onto the internal
surface of the pores heteroatoms such as Al, Ti, W or Mo
which are able to make the solids catalytically active [4].
In particular, titania modified silica takes advantages
of titania and mesoporous silica, making it more active,
stable and easier to recover [5]. To date, much effort has
been made to the study of the catalytic efficacy of
mesoporous molecular sieves containing titanium. For
example, Galacho et al. [6] prepared Ti-MCM-41 at am-
bient temperature and pressure and evaluated their cata-
lytic activities in the reaction of oxidation of cyclohexene.
They found that the sample with Si/Ti = 10 exhibited the
highest conversion and excellent selectivity. Eimer et al.
[7] studied the influence of Ti-loading on the acid be-
havior and the catalytic efficiency in the oxidation of
cyclohexene of Ti-MCM-41. The results showed that the
reaction presented a high velocity during the 1st hours
and cyclohexene epoxide was the main product.
Siliceous mesoporous materials SBA-15 possess a
regular two-dimensional array of tubular channels [8,9].
In comparison with the other regular mesoporous materi-
als, SBA-15 can be prepared with larger pores, resulting
in a more stable structure because of thicker pore walls.
Hence, SBA-15 have recently attracted considerable at-
tention because of the promising applications in shape-
selective catalysis, separations involving large molecular,
etc [10]. As we all know, the catalytic properties of the
catalysts are affected greatly by their composition, struc-
ture, preparation method, and process treatment. Scholars
have done many studies in the case of functionalization
of SBA-15 materials with isolated titanium sites, such as
Reference [11,12]. However, to the best of our knowledge,
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials
662
only a few researches have been done on the direct syn-
thesis and catalytic properties of Ti-SBA-15 in the reac-
tion of cyclohexene oxidation to yield cyclohexene ep-
oxide.
The aim of the present work was to study the influence
of synthesis temperatures on the structure and catalytic
properties of Ti-SBA-15 composite prepared by a di-
rectly hydrothermal process. The physiochemical proper-
ties were investigated by using nitrogen adsorption-de-
sorption measurements (N2 sorption), X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmis-
sion electron microscopy (TEM), diffuse reflectance ul-
traviolet-visible spectroscopy (DR UV-vis), and infrared
ray spectroscopy (IR). The catalytic properties of Ti-
SBA-15 were tested for the cyclohexene epoxidation
reaction with tert-butyl hydroperoxide (TBHP). The re-
sults will be helpful for the development of the study of
mesoporous materials and epoxidation reactions.
2. Experimental
2.1. Catalysts Preparation
Ti-SBA-15 composite catalysts were prepared by directly
hydrothermal synthesis using Pluronic P123 triblock
polymer (EO20PO70EO20, Mav = 5800, Aldrich) as struc-
ture-directing reagent. Tetraethoxysilane (TEOS) and
titanium isopropoxide (TIP) were used as the Si and Ti
sources, respectively. 1.00 g of P123 was added to 35.00
mL of 1.72 mol/L HCl solution to yield a transparent
solution at 40˚C. Then 2.30 mL of tetraethylorthosilicate
(TEOS) and 0.34 mL mixed solution of acetylacetone
and TIP (molar ratio 1:1) were added dropwise and the
solution was stirred for another 24 h. The prepared gel
was transferred into Teflon-lined stainless-steel autoclave
and crystallized at different temperatures (80, 100, 120,
140, and 160˚C, respectively) for 24 h. The final solid
was filtered, washed with H2O and dried at 110˚C over-
night. Finally, the samples were calcined at 500˚C for
6 h.
Pure SBA-15 synthesized at 100˚C, herein, was also
prepared with the same method for the comparison pur-
pose.
2.2. Catalysts Characterization
N2 adsorption-desorption experiments were performed
with a Quantachrome NOVA 1000e surface area & pore
size analyzer. Before measurement, the samples were
degassed at 150˚C under vacuum (1.33 Pa) for 1.5 h. The
specific surface area of samples was determined using
the Brunauer-Emmett-Teller (BET) method. The total
pore volume was evaluated at a relative pressure about
0.99. The pore size distributions were derived from the
desorption profiles of the isotherms using the Barrett-
Joyner-Halenda (BJH) method, based on the Kelvin
equation [13].
Low-angle X-ray powder diffraction (XRD) patterns
were collected on a PW3040/60 diffractometer using Cu
Kα radiation of wavelength 0.15418 nm. Diffraction data
were recorded at an interval of 0.01671 in the 2θ range of
0.5˚ - 5˚. The interplanar distance (d100) was obtained by
the Bragg’s law using the position of the first X-ray dif-
fraction line. Large-angle XRD patterns of the samples
were recorded with the same diffractometer in the 2θ
range of 10˚ - 80˚.
Scanning electron microscopy (SEM) images were
taken using a HITACHI S4800 scanning electron micro-
scope. For the SEM observations, the samples were de-
posited on a sample holder and coated with Au, using an
accelerating voltage of 10.0 kV.
The transmission electron microscopy (TEM) meas-
urements were performed on Tecnai G2F20 (USA) to
observe the inner pore structure of samples.
IR spectra were recorded on a Thermoscientific
Nicolet 380 FT-IR spectrometer at 2 cm–1 resolution us-
ing a KBr pellet technique. Before measurement, all
samples and KBr were dried in loft drier at 373 K over-
night. The sample diluted in KBr (2.0 wt%) was pressed
into a wafer (40.5 mg·cm–2 thickness). The spectra were
collected in transmittance mode.
Diffuse reflectance UV-vis spectra were collected on a
Hitachi U-3010 UV-vis spectrophotometer in the 200 -
800 nm range with a resolution of 1.0 nm. BaSO4 was
used as reference for measurements.
2.3. Catalytic Experiments
The catalytic efficiency studies were carried out using
cyclohexene as substrate and TBHP as the primary oxi-
dant. In a typically reaction, 50 mL round-bottomed flask
was fitted with a reflux condenser and placed in a con-
stant temperature oil bath. 5.0 mL of cyclohexene, 20.0
mL of solvent (1, 2 Dichloroethane), 4.0 mL of TBHP,
and 0.25 g of Ti-SBA-15 composite catalyst was added
into the flask, respectively. Products were analyzed by a
gas chromatography equipped with a capillary column
(KF1701, 30 m × 0.2 mm × 0.5 m) and a flame ioniza-
tion detector (FID).
3. Results and Discussion
Low-angle XRD analysis is an effective probe for the
meso-structure materials. Figure 1 shows the low-angle
XRD patterns of pure SBA-15 and Ti-SBA-15 samples.
It can be seen that all the samples exhibited XRD pat-
terns with one very intense diffraction peak and two
weak peaks, which are characteristic of 2-D hexagonal
(P6mm) structure with excellent textural uniformity. The
results indicated that Ti-SA-15 composite catalysts B
Copyright © 2011 SciRes. MSA
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials
Copyright © 2011 SciRes. MSA
663
Figure 1. Low-angle XRD patterns of pure SBA-15 (a) and Ti-SBA-15 synthesized at
different temperatures: (b) 80˚C, (c) 100˚C, (d) 120˚C, (e) 140˚C, (f) 160˚C.
retained the ordered mesoporous structure of SBA-15.
Moreover, compared with pure SBA-15, there was a shift
towards lower angles for the characteristic peaks of the
Ti-SBA-15 composite except for the one synthesized at
80˚C. This may indicate that the pore diameter increased
when titanium oxide is introduced into silica due to the
fact that the Ti-O bond is longer than that of Si-O, simi-
lar to that of W-SBA-15 [14]. However, as seen in Figure
1f (insert), when the temperature rose up to 160˚C, the
relative intensity of the (110) and (200) peaks decreased.
The results clearly indicated that the synthesis tempera-
tures remarkably affected the structure. In other words,
although the ordered mesoporous structure of SBA-15
are retained at low crystallization temperature, higher
temperature leads to some disruption of the structure.
Figure 2 shows the large-angle XRD pattern of
Ti-SBA-15 composite synthesized at 100˚C. Only a sin-
gle broad peak appeared at about 22.7˚ characteristic of
amorphous silica [12]. The characteristic peaks of an
anatase structure associated with TiO2 are not present in
the pattern. This suggests that TiO2 was in the fine dis-
persion state and no obvious crystallites were formed.
Figure 2. Large-angle XRD pattern of Ti-SBA-15 synthe-
sized at 100˚C.
well-defined wheat-like macro-structure of pure SBA-15
[14]. And the relatively uniform particles sizes were
about of 4 - 6 m. However, although no strong charac-
teristic peaks of crystalline TiO2 appeared in XRD pat-
terns as shown in Figure 2 and the rope-like domains
were still largely maintained, a significant degradation in
macroscopic structure can be observed, suggesting that
Figure 3 shows the SEM micrographs of pure SBA-15
and Ti-SBA-15 composite synthesized at 100˚C. The
micrographs reveal that the Ti-SBA-15 retained the
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials
664
(a)
(b)
Figure 3. SEM micrographs of pure SBA-15 (a) and
Ti-SBA-15 synthesized at 100˚C (b).
titanium oxide begins to congregate.
TEM was furthermore applied to investigate the inte-
rior of the prepared Ti-SBA-15 composite materials.
Figure 4 shows the micrographs viewed normal to or
along the axis of the hexagonal pores of Ti-SBA-15
composite synthesized at 80˚C and 140˚C, respectively.
The images clearly indicated that both the samples re-
tained the characteristic p6mm structure of pure SBA-15.
Furthermore, as indicated in Figure 4, the pore size of
Ti-SBA-15 synthesized at 140˚C was remarkably larger
than that of the sample synthesized at 80˚C.
Figure 5 shows the forms of the nitrogen adsorption-
desorption isotherms and pore size distributions of
Ti-SBA-15 composite materials. All the curves are type
IV in the classification of Brunauer with H1-type hys-
teresis loops. This also suggested that the doping of tita-
nium species retained the typical mesoporous structure of
SBA-15. Comparing with the others, the hysteresis loop
of Ti-SBA-15 synthesized at 80˚C appeared at low rela-
tive pressure (between 0.50 and 0.85), indicating the
smallest pore size. Furthermore, the hysteresis loops
(a)
(b)
Figure 4. TEM micrographs of Ti-SBA-15 synthesized at
80˚C (a) and 140˚C (b).
moved slightly to higher relative pressure with the in-
crease of synthesis temperatures under the present condi-
tions. This revealed that the pore size increased with the
increase of synthesis temperatures as mentioned in Table
1. It is noteworthy that the curves of pore size distribu-
tion for the sample synthesized at low temperatures
(80˚C and 100˚C) exhibited relatively narrower than the
others (Figure 5(B)).
Table 1 summarizes the structure properties of the
Copyright © 2011 SciRes. MSA
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials665
(A)
(B)
Figure 5. N2 adsorption-desorption isotherms (A) and pore
size distributions (B) of Ti-SBA-15 synthesized at different
temperatures: (a) 80˚C, (b) 100˚C, (c) 120˚C, (d) 140˚C, (e)
160˚C.
prepared Ti-SBA-15 composite materials. As it can been
seen from Table 1, the value of surface area, pore vol-
ume, pore size, cell parameter and wall thickness for the
sample synthesized at 100˚C were higher than those for
the one synthesized at 80˚C. When the synthesis tem-
peratures increased further, the value of surface area, and
wall thickness, especially Ti-SBA-15 synthesized at
160˚C, decreased obviously. Meanwhile, the mean pore
size increased greatly with only a little change of pore
volume and cell parameter. The results clearly indicated
that the crystallization temperature affected the pore
structure greatly. As we all know, wall thickness of
mesoporous materials is a key factory to ensure its
hydrothermal stability. Therefore, 100˚C was the optimal
crystallization temperature under the reaction conditions.
It is known that diffuse reflectance UV-vis (DR UV-vis)
spectroscopy is a very sensitive probe for the presence of
extra-framework transition metal oxides in different het-
eroatomic mesostructures [14,15]. Hence, we used
UV-vis spectroscopy to characterize the coordination and
the chemical nature of the titanium species in the pre-
pared Ti-SBA-15 samples. According to the Reference
[16-18], the existence of titanium (IV) isolated in the
framework was characterized by a band at about 210 -
230 nm for a tetrahedral environment and 240 - 250 nm
for an octachedral environment. Chiker et al. [12] re-
ported that the formation of titanium oxide clusters in the
anatase form leads to a displacement of the band towards
higher wavelengths. In other words, the absorption
threshold for anatase is at about 350 nm. Other scholars
[14,15,19,20] reported that the band centered at about
220 nm implies the presence of a ligand-to-metal charge
transfer involving isolated transition metal sites, and is
generally considered direct proof that transition metal
atoms have been incorporated into the framework of a
molecular sieve.
The DR UV-vis spectra of Ti-SBA-15 composite in
the region of 200 - 800 nm are depicted in Figure 6. The
spectrum of pure SBA-15 was also included for com-
parison purposes. As can be seen from Figure 6, pure
SBA-15 had no characteristic absorbance band. While
the curves of the Ti-SBA-15 composite exhibited sig-
nificant bands. It is noteworthy that when the synthesis
temperature was low, the relative intensity of the band at
ca. 220 nm was high, indicating titanium species suc-
cessfully incorporated into the framework SBA-15 with
the presence in tetrahedral coordination. However, the
relative intensity of the band in the region of 280 - 380
nm related to anatase increased with the increase of syn-
thesis temperatures (e.g. 160˚C). Meanwhile, the band at
ca. 240 nm related to octachedral coordination environ-
ment became clear. The results indicated that the effect
of synthesis temperatures on the nature and the disper-
sion of the titanium species were obvious. Oki et al. [21]
reported that Ti-O-Si was the active center for catalysis.
Therefore, lower synthesis temperatures (e.g. 80 and
100˚C) promoted the form of the active center Ti-O-Si.
The fourier transform infrared (FT-IR) spectra of pure
SBA-15 and Ti-SBA-15 composite materials are shown
in Figure 7. The bands around 3440 cm–1 can be as-
signed to the O-H stretching vibrations mode of the si-
lanols involved in hydrogen interactions with the adsorbed
water molecules [22]. The band at ca. 1080 cm–1 and 810
cm–1 corresponded to characteristic of anti-symmetric
vibration nonbridging oxygen atoms (Si-Oδ) of Si-O-H
bonds and symmetric stretching vibration (Si-O-Si)sym of
tetrahedral SiO4 structure units [23]. Meanwhile, the
band at ca. 459 cm–1 corresponded to characteristic of
Copyright © 2011 SciRes. MSA
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials
Copyright © 2011 SciRes. MSA
666
Table 1. Pore structure parameters of the prepared Ti-SBA-15 samples.
Synthesis
temperatures (˚C) BET area (m2·g–1) Pore volume (cm3·g–1)Pore size D (nm)d100 (nm) a0a (nm) Wall thickness
tb (nm)
80 557.3 0.67 4.8 8.41 9.71 4.91
100 754.5 1.01 5.4 9.52 10.99 5.99
120 610.0 0.98 6.4 9.28 10.72 4.32
140 454.1 1.08 9.5 9.83 11.35 1.85
160 362.1 1.00 11.1 9.78 11.29 0.19
a:100
0
2
3
d
a. b: t = a0 – D.
Figure 6. DR UV-vis spectra of pure SBA-15 (a) and Ti-
SBA-15 synthesized at different temperatures: (b) 80˚C, (c)
100˚C, (d) 120˚C, (e) 140˚C, (f) 160˚C.
Figure 7. FT-IR spectra of pure SBA-15 (a) and Ti-SBA-15
synthesized at different temperatures: (b) 80˚C, (c) 100˚C,
(d) 120˚C, (e) 140˚C, (f) 160˚C.
tetrahedral bending of Si-O bonds [24]. No typical IR
band located at around 960 cm–1 can be observed for the
pure SBA-15. However, Ti-SBA-15 exhibits a strong
band, especially for the samples synthesized at low tem-
peratures (80˚C - 120˚C), at around 960 cm–1.
The band at around 960 cm–1 has been widely used to
characterize the incorporation of metal ions in the silica
framework as the stretching Si-O vibration mode per-
turbed by the neighboring metal ions [25,26]. According
to Gao et al. [27] and Kochkar et al. [28], the Ti-O-Si
infra-red vibration is generally observed between 910
and 960 cm–1, with the exact band position depending on
the chemical composition of the sample as well as cali-
bration and resolution of the instrument. Therefore, the
presence of the band at ca. 960 cm–1 is a piece of direct
evidence for the isomorphous substitution of Si by Ti
ions in Ti-SBA-15.
In addition, the appearance of a weak band at ca. 560
cm–1, characteristic band for Titania (TiO2) in the
Ti-SBA-15 synthesized at 160˚C may be an indication of
the appearance of the titania phase [29]. This was con-
sistent with that of UV-vis results mentioned above.
Experiments approved that the conversion of cyclo-
hexene can be as high as 88% - 95% over all the present
Ti-SBA-15 composite materials under the reaction con-
ditions. Here, the catalytic efficiencies of Ti-SBA-15
composite materials for cyclohexene epoxidation as a
function of reaction time in the region of 1 - 10 hours are
presented in Figure 8. It is generally acknowledged that
the selectivity of cyclohexene epoxide increased signifi-
cantly with the increase of reaction time under the reac-
tion conditions for all the present catalysts. In addition,
our experiment exhibited that the by-products of the
catalytic reaction were 2-cyclohexen-1-ol, 2-cyclohexe-
none, and 2-tert-butyl-cyclohexen.The GC-MS analytic
results showed that the percent of by-products decreased
with the increase of reaction time. Meanwhile, the per-
cent of cyclohexene epoxide increased.
Furthermore, it is clear from Figure 8 that the catalytic
efficiency varies with the synthesis temperatures. Ti-
SBA-15 synthesized at 100˚C exhibited slightly higher
Preparation and Catalytic Properties of Ti-SBA-15 Mesoporous Materials667
Figure 8. Selectivity of cyclohexene epoxide over the pre-
pared Ti-SBA-15 samples.
selectivity than that of the sample synthesized at 80˚C
and both the efficiencies of them obvious higher than
those of the others. Then, the catalytic efficiency de-
creasedremarkably with the increase of synthesis tem-
peratures. The highest selectivity of cyclohexene epoxide
was only about 44% for Ti-SBA-15 synthesized at 160˚C.
While the value was about 77% for Ti-SBA-15 synthe-
sized at 100˚C under the same reaction conditions. This
indicated that increasing the synthesis temperatures can
promote the catalytic efficiency of Ti-SBA-15 composite
catalysts to some extent, but it is not always so. From the
DR UV-vis spectra, we know that low synthesis tem-
peratures promoted the form of Ti-O-Si which was the
active center. Thus, 100˚C was the optimal synthesis
temperature under the reaction conditions.
4. Conclusions
In the present work, a series of Ti-containing mesoporous
catalysts, Ti-SBA-15 composite, has been synthesized
via direct hydrothermal route and characterized by using
various technologies. The influence of synthesis tem-
peratures on the structure and catalytic properties of
these materials has been studied. The results showed that
the prepared composite retained the ordered mesoporous
structure of SBA-15 and showed very good catalytic ef-
ficiency in the epoxidation of cyclohexene. The sample
synthesized at 100˚C exhibited the highest catalytic effi-
ciency. Thus, 100˚C was the optimal temperature for the
direct synthesis of Ti-SBA-15 composite catalysts under
the present conditions.
5. Acknowledgements
This work was partly financially supported by the Inno-
vation Experiment Fund of Zhengzhou University (2010
cxsy090).
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