Effect of combination sequence of precursors on the structural and catalytic properties of Ti-SBA-15

Article abstract of DOI:10.1039/c3ra41304g

The combination sequence of Ti and Si precursors (TTIP and TEOS) in the synthesis solution of Ti-incorporated SBA-15 mesoporous silica (shortly termed Ti-SBA-15) was found for the first time to be critical for the distribution of tetrahedrally (T_d) coordinated Ti(iv) sites in the resultant Ti-SBA-15. It was found that TTIP pre-assembled with P123 in the synthesis solution for 3 h before the addition of TEOS gave T_d-coordinated Ti(iv) sites predominately incorporated near the superficial regions of the framework and were highly accessible to the reactants in catalytic reactions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41304g

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Effect of combination sequence of precursors on the
structural and catalytic properties of Ti–SBA-153
Cite this: RSC Advances, 2013, 3, 12604
a
ab
a
c
a
Feng-Ting Kuo, Shih-Yuan Chen, Tsung-Han Lin, Jyh-Fu Lee and Soofin Cheng*
Received 19th March 2013,
Accepted 29th May 2013
DOI: 10.1039/c3ra41304g
www.rsc.org/advances
1
5–17
The combination sequence of Ti and Si precursors (TTIP and TEOS)
in the synthesis solution of Ti-incorporated SBA-15 mesoporous
silica (shortly termed Ti–SBA-15) was found for the first time to be
of the active species.
Moreover, the complicated processing
steps and solvent waste are other drawbacks in comparison to the
co-condensation method. So far, the preparation of Ti–SBA-15 by
one-pot co-condensation is a difficult task due to the incongruent
hydrolysis and condensation rates of the silica and titania
precursors, which are mainly tetraethyl orthosilicate (TEOS) and
sodium silicate for silica, and titanium tetraisopropoxide (TTIP)
d
critical for the distribution of tetrahedrally (T ) coordinated Ti(IV)
sites in the resultant Ti–SBA-15. It was found that TTIP pre-
assembled with P123 in the synthesis solution for 3 h before the
d
addition of TEOS gave T -coordinated Ti(IV) sites predominately
6–8,12
incorporated near the superficial regions of the framework and
were highly accessible to the reactants in catalytic reactions.
and titanium(IV) chloride for titania.
indicated that mixed-phase products containing TiO crystallites
Previous reports have
2
and disordered porous SiO solids are frequently obtained by the
2
,8
7
12
co-condensation method. Chen et al. recently achieved evident
success in preparing well-ordered Ti-incorporated SBA-15 with a
1. Introduction
superficially T -coordinated Ti species in a self-generated acidic
d
Ti-incorporated zeolitic materials, such as TS-1 and Ti–MWW,
were reported to have remarkable catalytic activities in selective
oxidation reactions using hydrogen peroxide as the oxidant at
environment (pH = 1–2) combined with calcination treatment.
However, anatase TiO nanoparticles which gave a negative effect
2
on the selective oxidation reaction were found to co-exist in the
materials.
1–5
mild reaction temperatures. However, the pore sizes of zeolitic
materials are generally smaller than 1 nm and that limits their
applications in sorption and reactions of bulky molecules. In the
past two decades, many efforts have been devoted to the
preparation of Ti-incorporated mesoporous silica materials
because their relatively large pores facilitate the diffusion of bulky
In this study, the combination sequence of Ti and Si precursors
in preparing Ti–SBA-15 materials by one-pot co-condensation in
an acidic synthesis solution (1–2 M HCl) was examined. The
results showed that the combination sequence markedly affected
the Ti environment in SBA-15 and the catalytic properties of the
resultant materials in the selected oxidation of 2,3,6-trimethylphe-
nol (TMP) with H O .
6–12
molecules often encountered in fine chemicals synthesis.
Among the mesoporous materials, SBA-15 with pore diameters
larger than 5 nm and a high hydrothermal stability, has attracted
2
2
13,14
11
great attention.
Berube et al. prepared well-ordered Ti–SBA-
2. Experimental
1
5 materials with Ti in T coordination up to a Ti/Si ratio of 5.6%
d
by a controlled post-grafting method using a special Ti precursor
2.1 One-pot synthesized Ti–SBA-15
of acetylacetone-chelated tetrapropylorthotitanate. However, the
grafting method has shown to give poor control in the distribution
The Ti–SBA-15 materials were prepared by a co-condensation
method based on the gel compositions of 0.017 P123 : 1
a
2
TEOS : 0.010–0.125 TTIP : 3.8–7.6 HCl : 210 H O. In the typical
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
E-mail: chem1031@ntu.edu.tw; Fax: +886-2-33668671; Tel: +886-2-33661662
synthesis, 4.0 g of Pluronic P123 triblock copolymer (Aldrich, M =
5800) was thoroughly dissolved in 160 mL of 2 M HCl solution at
n
b
Research Center for New Fuels and Vehicle Technology, National Institute of
Advanced Industrial Science and Technology, Tsukuba Central-5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
3
5 uC. To this solution, TTIP (Acros, 0.010–0.125 mol% of Si) was
added and hydrolyzed for 3 h before the addition of 8.72 g of TEOS
Acros). The reaction mixture was sealed in a polypropylene bottle
c
Research Division, National Synchrotron Radiation Research Center, Hsinchu 300,
(
Taiwan
and stirred at 35 uC for 24 h, followed by hydrothermal treatment
at 90 uC for another 24 h in a static condition. The solid
precipitates were collected by filtration, washed with deionized
3
Electronic supplementary information (ESI) available: Fig. S1 and S2 small-angle
XRD patterns; Fig. S3 wide-angle XRD patterns; Fig. S4 HRSEM of Ti–SBA-15. See
DOI: 10.1039/c3ra41304g
1
2604 | RSC Adv., 2013, 3, 12604–12610
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Communication
water and dried, then were calcined at 560 uC in air for 6 h with a 1
3
. Results and discussion
21
uC min ramping rate. The resultant materials were designated
as xTi–Si-y, where x is the Ti/Si molar percentage in the gels and y
is the molarity of the HCl solution. For comparison, another series
of Ti-incorporated SBA-15 materials, shortly termed xSi–Ti-y, were
3.1 Material characterization
The small-angle XRD patterns of the Ti-incorporated SBA-15
materials prepared by different combination sequences in 2 M
HCl medium, namely xTi–Si-2 and xSi–Ti-2, respectively, show
three distinct diffraction peaks, corresponding to the (100), (110)
and (200) planes of 2D hexagonal p6mm structure. They also show
steep H hysteresis loops at P/P = 0.6–0.8, suggesting that both
7,8
prepared based on the procedures of literature reports, which
were similar to those of xTi–Si-y samples except that TEOS was pre-
assembled with P123 micelles in the synthesis solution for 3 h
before the addition of TTIP.
1
0
series of Ti-incorporated SBA-15 materials possess well-ordered
2
.2 Characterizations
13
and narrowly-distributed mesopores (Fig. S1 and S2, ESI
Moreover, no diffraction peaks of crystalline TiO are observed in
the wide-angle region (Fig. S3, ESI ), implying that Ti should be
homogeneously incorporated in SBA-15.
Table 1, as well as Table S1 (ESI ), show that both series of Ti–
SBA-15 materials have high surface areas (796–896 m g ), large
3
).
Powder X-ray diffraction (XRD) patterns were measured by a
Philips X’pert Pro diffractometer using Cu-Ka radiation operated
at 40 mA and 45 kV. The divergent slits of 1/32 and 1/2 were set for
collecting small- and wide-angle XRD patterns, which showed 2h
regions of 0.5–5u and 15–80u, respectively. Nitrogen physisorption
isotherms were measured by a Micromeritics Tristar 3000
instrument at liquid nitrogen temperature (77 K). Materials were
degassed in vacuum (10 torr) for more than 6 h before the
measurement. The specific surface area was calculated by the
Brunauer–Emmett–Teller (BET) method in the P/P region of 0.05–
2
3
3
2
21
3
21
pore volumes (0.97–1.1 cm g ) and ordered nanopores (ca. 6
nm), akin to those of conventional SBA-15 (entry 1). The Ti
loadings in the solids were analyzed by ICP-MS technique, and
only ca. 5–12.8 mol% of Ti in the synthesis solutions was
incorporated in SBA-15. These results are consistent with previous
reports that Ti is difficult to incorporate in mesoporous silica
23
0
0.25. The pore size distributions (PSDs) were calculated by the
8
prepared in a strong acidic environment. However, the xTi–Si-2
Barrett–Joyner–Halenda (BJH) method using the desorption
branch of the isotherm. The materials were photographed with
a Hitachi S-800 Field Emission Scanning Electron Microscope
samples (entries 2–6), prepared by assembling a Ti precursor with
P123 micelles first, have slightly lower Ti loadings than xSi–Ti-2
(
entries 7–11), which was prepared by adding a Si precursor first.
For xSi–Ti-2 samples where TEOS is pre-hydrolyzed, the soft SBA-
5 mesostructure is formed prior to the addition of the Ti
(SEM), a JEOL JSM-7600F Field Emission Scanning Electron
Microscope (FE-SEM) and a Hitachi H-7100 transmission electron
microscope (TEM). Ti K-edge X-ray absorption spectra were
obtained at the Beamline 17C of National Synchrotron Radiation
Research Center (NSRRC) facility in Hsinchu, Taiwan. The
standard operating conditions were 1.5 GeV and 350 mA. The
photon energy was calibrated with a metallic Ti foil (K-edge, 4966
eV). The diffuse-reflectance (DR) UV-Vis spectra were obtained
with a Hitachi U-3310 spectrometer equipped with an integrating
sphere detector. Barium sulfate (Acros, 99 + %) was used as the
reference. The elemental contents in the bulk were obtained by
inductive-coupled plasma-mass spectrometry (ICP-MS) using a
Perkin Elmer SCIEX ELAN 5000 ICP-MS instrument. Prior to
measurement, the dried samples (ca. 20 mg) were dissolved in a
1
precursor, TTIP. When TTIP is added, the condensation between
Ti–OPr groups and silanol groups on the SBA-15 mesostructure
would form Ti–O–Si bonds and Ti is incorporated into the silica
matrix. In contrast, in xTi–Si-2 samples where TTIP is pre-
2
hydrolyzed first, the hydrolysis product TiOCl is soluble in acid.
The TiOCl species, which interacts with P123 micelles before the
2
addition of TEOS, is probably less effective when condensed with
Si–OEt groups on TEOS. As a result, a lesser amount of Ti is
incorporated into the silica framework of the xTi–Si-2 samples.
The morphology and pore structure of these materials were
photographed by high-resolution scanning electron microscopy
3
(HRSEM) using a retarding technique (Fig. S4, ESI ). The HRSEM
mixed HF–HNO solution and diluted to ppb level.
3
photos show that both series of Ti–SBA-15 materials are rod-like
aggregates in the micrometer level. Indeed, all of the xTi–Si-2 and
xSi–Ti-2 samples with different Ti loadings have rod-like
morphology 1–2 micrometers in size, similar to that of conven-
tional SBA-15. The channeling pores are well-aligned along the
long axis of the particles, and the pore mouths and concave
channels are seen on the top of particles, which are the superficial
2
.3 Catalytic reaction
Liquid phase oxidation of TMP (C
out in a 3-necked round bottom flask (50 mL) connected to a water
cooling condenser and a thermometer. The Ti-incorporated SBA-
9
H12O, Acros, 99+ %) was carried
1
5 catalyst was pre-dried at 200 uC overnight. Typically, a solution
of 0.16 g of TMP in 12 mL CH CN (Acros, 99+ %) was refluxed at
2 uC before adding 0.16 g of the catalyst and 0.408 g of H
Acros, 35%) as the oxidant. The TMP to catalyst weight ratio was
kept at 1, and the H O /TMP molar ratio was kept ca. 3.5. The
17–20
3
features of mesoporous silica templated by block copolymers.
8
(
2
O
2
Fig. 1 compares the DR UV-vis spectra of xTi–Si-2 and xSi–Ti-2
materials. All the spectra possess an intense band centered at 205–
209 nm, which is assigned to the ligand to metal charge transfer
2
2
4,12
products were separated by HP6890 Gas Chromatography (GC)
and qualitatively identified with a HP5973 mass selective detector.
The quantitative analyses of the products were carried out by a
Chrompack 9000 GC instrument equipped with a RTX-5 capillary
column (60 m in length, 0.53 mm in diameter) and a flame
ionization detector (FID), and toluene was used as an internal
standard.
(LMCT) from O to isolated Ti in tetrahedral coordination.
The
intensity of the O A Ti(T ) LMCT increases with the Ti loadings,
d
suggesting that more isolated Ti species are incorporated. When
the Ti/Si ratio in gels is increased to ca. 8–12.5 mol%, two weak
bands appear at ca. 240 and 300 nm. They are assigned to the
LMCT from O to Ti in octahedral coordination and the band gap
transition of TiO
2
nanocrystallites, respectively. These two bands
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 12604–12610 | 12605

View Article Online
Communication
RSC Advances
Table 1 Physicochemical and catalytic properties of 560 uC calcined xTi–Si-2 and xSi–Ti-2 samples
Ti/Si (mol%)
Product Select. (%)
2
21
a
b
21
Entry
Materials
Gel
Solid
S
BET (m g
)
TMP conv. (%)
TMBQ
HTMBQ
TMCHT
BP
TOF (h
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
SBA-15
0
0
801
858
874
813
824
810
—
5.3
12.4
23.2
47.7
65.8
92.0
93.2
91.5
36.1
45.7
53.7
57.5
60.1
57.3
56.8
0
0
0
0
0
0
0
100
75.5
0
2Ti–Si-2
4Ti–Si-2
5Ti–Si-2
8Ti–Si-2
12.5Ti–Si-2
12.5Ti–Si-2
Recycl. 12.5Ti–Si-2
2Si–Ti-2
4Si–Ti-2
5Si–Ti-2
8Si–Ti-2
12.5Si–Ti-2
12.5Si–Ti-2
2.0
4.0
5.0
8.0
12.5
—
0.10
0.40
0.70
1.0
1.6
—
24.5
51.1
48.3
51.9
68.2
69.1
67.3
0
39.1
41.5
50.3
60.4
61.8
61.3
3.3
3.3
3.6
3.8
4.3
—
—
0
2.8
2.7
2.3
1.4
—
48.9
39.8
34.9
24.3
25.0
26.0
9.9
9.6
4.5
3.5
3.5
0
1.2
12.6
10.5
7.6
5.6
6.2
2.0
3.6
3.0
2.4
3.2
0
1.2
2.8
8.3
14.8
13.2
14.5
c
d
—
—
—
2.0
4.0
5.0
8.0
12.5
—
0.40
0.70
0.90
1.4
2.8
—
796
837
799
870
896
—
100
0
1
2
3
4
5
58.5
42.5
30.4
17.2
19.4
18.0
c
d
Recycl.12.5Si–Ti-2
—
—
—
—
a
b
Reaction conditions: 82 uC, 4 h, 0.16 g catalyst, 0.16 g TMP, 0.408 g H
2
O
2
(35 wt%), 12 mL CH
3
CN. Molar yield of TMBQ per mole of Ti in
c
d
the first hour. New batches of samples. The catalyst of the new batch of sample was regenerated by calcination at 500 uC for 3 h with a
ramping rate of 1 uC min
2
1
.
on the xSi–Ti-2 series are more intense than those of the xTi–Si-2
series, when comparing materials of the same Ti/Si ratios in the
synthesis gels. Elemental analyses show that the Ti loadings in the
solids giving 240 and 300 nm bands are 1–2.8 mol%. These results
imply that the maximum Ti/Si ratio in Ti–SBA-15 materials to
this region when the Ti/Si ratio in the synthesis gels is lower than
5% using 2 M HCl, while that decreases to 4% with 1.5 M HCl and
to 2% with 1 M HCl. Under these Ti/Si ratios, the Ti loadings in
the solid products do not exceed 1 mol% based on ICP-MS
analysis (Tables 1 and 2). These results suggest that the maximum
amount of Ti which can substitute the framework Si in SBA-15
prepared by one-pot co-condensation should be ca. 1 mol%, and
that value is not significantly influenced by the acidity of the
synthesis solutions. Above this ratio, Ti has the tendency to form
retain Ti in a T
mol%.
The Ti loadings in Ti–SBA-15 products can be increased up to
d
coordination geometry should be lower than 1
4
.4–5.1 mol% by using 1–1.5 M HCl instead of 2 M HCl to reduce
the acidity of the synthesis solution. Table 2 shows that the
materials thus prepared by assembling the Ti precursor with P123
micelles first still have well ordered mesostructures, high surface
2
TiO nanocrystallites.
The chemical environments of Ti(IV) species in both series of Ti-
incorporated SBA-15 materials prepared using 2 M HCl were
examined by Ti K-edge X-ray absorption near edge structure
(XANES) spectroscopy at beamline 17C of NSRRC, Hsinchu,
Taiwan. Samples 12.5Ti–Si-2 and 12.5Si–Ti-2 were selected because
they have relatively high loadings of Ti meaning that their Ti
K-edge X-ray absorption spectra are more clearly resolved.
Moreover, the UV-vis spectra of these samples illustrate that the
2
21
3
21
areas (>835 m g ) and pore volumes (>1.00 cm g ). However,
the DR UV-vis spectra (Fig. S5, ESI ) show that the bands
corresponding to TiO nanocrystallites at ca. 240 and 300 nm
3
2
are more easily detected, especially in the synthesis condition of
higher pH values. The UV-vis spectra can retain low intensity in
majority of the Ti species are in T
spectra are compared with that of anatase TiO
Japan Reference Catalysts), as shown in Fig. 2. For anatase TiO
d
-coordination. These XANES
2
(JRC-TIO-1 of
2
,
three main-features of B (4980 eV), C (4987 eV) and D (5004 eV)
1
are attributed to the electron transition from an initial state of the
Ti 1s energy level to the excited states of the predominately Ti 4p
energy level, and three pre-edge features of A
eV) and A (4974 eV) are due to the electron transition from the Ti
s energy level to bound 3d (or O 2p) molecular orbitals.
Since Ti in anatase TiO is in octahedral (O ) coordination and has
1 2
(4969 eV), A (4971
3
12,20–25
1
2
h
the center of symmetry, the 1s A 3d transition is Laporte
forbidden. That accounts for the weak intensity of the pre-edge
features.
The Ti K-edge XANES spectrum of calcined 12.5Ti–Si-2 material
resembles that of anatase TiO , indicating that most of the Ti
2
h
species in this sample are in O coordination. However, when the
Fig. 1 DR UV-vis spectra of 560 uC calcined samples: (a) xTi–Si-2 and (b) xSi–Ti-2.
calcined 12.5Ti–Si-2 sample was dehydrated at 200 uC in vacuum
This journal is ß The Royal Society of Chemistry 2013
1
2606 | RSC Adv., 2013, 3, 12604–12610

View Article Online
RSC Advances
Communication
Table 2 Physicochemical and catalytic properties of 560 uC calcined xTi–Si-1.5 and xSi–Ti-1.5 samples
Ti/Si (mol%)
Product Select. (%)
2
21
a
b
21
Materials
Gel
Solid
S
BET (m g
)
TMP conv. (%)
TMBQ
HTMBQ
TMCHT
BP
78.5
28.9
20.4
18.6
64
100
100
90.2
8.7
3.7
TOF (h
)
1
2
4
5
8
1
1
2
4
5
8
1
Ti–Si-1
Ti–Si-1
Ti–Si-1
Ti–Si-1
1
2
4
5
8
10
1
2
4
5
0.2
0.7
1.0
1.8
3.4
5.1
0.08
0.2
0.9
1.1
2.6
4.4
927
986
923
932
867
864
900
911
915
930
835
884
20.1
54.0
59.0
75.2
25.0
17.1
7.0
21.5
46.3
54.9
57.9
32.5
0
0
9.8
62.3
65.9
48.9
32.1
0
0
2.4
3.9
3.6
2.7
0.3
0
18.9
15.1
11.9
0
0
0
5.9
9.6
11.6
3.5
0
0
0
16.1
11.4
8.2
4.3
Ti–Si-1
0Ti–Si-1
Ti–Si-1.5
Ti–Si-1.5
Ti–Si-1.5
Ti–Si-1.5
Ti–Si-1.5
0Ti–Si-1.5
0
9.2
0
0.5
4.6
4.4
1.3
0.3
60.3
67.3
64.7
32.7
12.9
19.0
11.0
14.7
8
10
31.9
48.9
a
b
Reaction conditions: 82 uC, 4 h, 0.16 g catalyst, 0.16 g TMP, 0.408 g H
2
O
2
3
(35 wt%), 12 mL CH CN. Molar yield of TMBQ per mole of Ti in
the first hour.
for 8 h, the three pre-edge A features are replaced by a relatively
strong peak centered at ca. 4970.2 eV. The pre-edge peak of the 1s
A 3d transition becomes intense when the selection rule is
released, which means that the Ti atom no longer has the center
of symmetry. In other words, the pre-edge feature suggests that the
Ti species in the dehydrated 12.5Ti–Si-2 material are most likely to
be at the tetrahedral center, and that can occur only when Ti is
substituting the Si sites on the silica framework. Moreover, these
d
material are in a T coordination. Since these Ti sites are
insensitive to moisture and dehydration, they are likely in the
inner regions of the SBA-15 framework and not easily accessed by
water molecules.
3.2 Catalytic studies
In order to examine the catalytic performance of Ti-incorporated
SBA-15 materials prepared by a different combination sequence of
Ti and Si precursors, selective oxidation of TMP with H O was
d
T -coordinated Ti sites readily coordinate with water molecules
2
2
from a humid atmosphere to form O -coordinated Ti species, and
h
studied as the model reaction. The targeting product is 2,3,6-
trimethylbenzoquinone (TMBQ), an important intermediate for
the synthesis of Vitamin E. The by-products are 2,29-3,39-5,59-hex-
amethyl-4,4-biphenol (BP), 2-hydroxyl-3,5,6,-trimethyl-benzoqui-
none (HTMBQ), and 3,5,6-trimethyl-cyclohex-5-ene-1,2,4-trione
the process is reversible upon dehydration. These results are the
evidence that most of the Ti incorporated in the 12.5Ti–Si-2
material, prepared by adding Ti first, is located near the superficial
regions of the pore walls.
The Ti K-edge XANES spectra of the calcined 12.5Si–Ti-2
material, prepared by adding Si first, and that after dehydration
at 200 uC for 8 h have only one intense pre-edge peak (Fig. 2(d) and
(TMCHT), as shown in Scheme 1. Fig. 3 shows the selective
oxidation of TMP with H in CH CN at 82 uC as a function of
2
O
2
3
time over the 12.5Ti–Si-2 material. The TMP conversion rapidly
reaches 92% at 4 h and retains the value thereafter, while the
TMBQ yield increases continuously up to 82% in 7 h. These results
imply that there are side products other than TMBQ formed. By
tracing the distribution of by-products as a function of time, it is
found that BP appears rapidly at the early stage, grows to an
appreciable concentration during 1–4 h, and then gradually
decreases with time and almost disappears after 7 h. In contrast,
the amount of HTMBQ and TMCHT is negligible at the beginning
of the reaction, and their yields increase with reaction time.
HTMBQ and TMCHT are considered to be produced by further
oxidation of TMBQ. Since BP is formed by dimerization of two
TMP molecules, BP should be a parallel product of TMBQ. The
observation of the maximum yield of BP during the reaction
period is elucidated by that dimerization of TMP to form BP is a
reversible reaction. BP is consumed as the TMP is used up in
(e)), implying that a large portion of the Ti species in 12.5Si–Ti-2
26
oxidation to form TMBQ. For the purpose of comparing the
catalytic activities of the Ti-incorporated SBA-15 materials pre-
pared by varying the combination sequences of Ti and Si
precursors, the results of the catalytic reaction after 4 h are
compared hereafter.
Fig. 2 Ti K-edge XANES spectra of (a) anatase TiO
2
, (b) calc. 12.5Ti–Si-2, (c) dehyd.
The catalytic results over the two series of Ti–SBA-15 materials
prepared in 2 M HCl are tabulated in Table 1, and the graphic
12.5Ti–Si-2, (d) calc. 12.5Si–Ti-2, and (e) dehyd. 12.5Si–Ti-2.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 12604–12610 | 12607

View Article Online
Communication
RSC Advances
2 2
Scheme 1 Liquid phase oxidation of TMP with H O catalyzed by Ti-incorporated SBA-15 materials.
only product obtained. Both xTi–Si-2 and xSi–Ti-2 series materials
can efficiently catalyze the oxidation of TMP with H to form
2 2
O
TMBQ, except catalyst 2Si–Ti-2. The TMP conversion and TMBQ
selectivity increase with Ti loading, while the BP selectivity varies
in the reverse trend. Since UV-vis spectra show the amount of T
coordinated Ti species increases with the Ti loading in these Ti–
SBA-15 materials, the T -coordinated Ti should be the main active
site for selective oxidation of TMP to form TMBQ. A possible
d
-
d
26
mechanism has been proposed by Kholdeeva et al. to explain
that high surface Ti(IV) concentration in T coordination is
d
favourable for TMBQ formation, while low T -coordinated Ti(IV)
d
concentration will lead to high BP selectivity. The xTi–Si-2 series
always gives higher TMBQ selectivities than the xSi–Ti-2 series
when comparing samples with similar Ti content in the solids. It
Fig. 3 Selective oxidation of TMP with H
2 2
O as a function of time over the calcined
12.5Ti–Si-2 material.
is an indication that the xTi–Si-2 catalysts should have more T
d
-
coordinated Ti(IV) species on the superficial region.
The TOF is defined as the initial TMBQ yield per Ti site in the
first hour, and a higher TOF value infers the better accessibility of
correlations of the catalytic performances as a function of Ti
loadings in the solids are shown in Fig. 4. Siliceous SBA-15 gives a
the T -coordinated Ti sites in the catalysts. The TOFs of xTi–Si-2
d
21
very low activity in the oxidation of TMP with H
2
2
O , and BP is the
catalysts are around 3.3–4.3 h , and the values slightly increase
Fig. 4 Graphic correlation of catalytic performances (A) TMP conversion, (B) TMBQ selectivity and (C) BP selectivity after 4 h, and (D) TOF in the first hour of TMP oxidation with
at 82 uC, as a function of Ti loadings in Ti–SBA-15 materials prepared in 2 M HCl: (a) xTi–Si-2 prepared by adding the Ti precursor first, and (b) xSi–Ti-2 prepared by
adding the Si precursor first.
2 2
H O
1
2608 | RSC Adv., 2013, 3, 12604–12610
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Communication
results, 2 M HCl is favourable to 1–1.5 M HCl in preparing
catalytically active Ti–SBA-15 materials.
4. Conclusions
The present studies demonstrate that pre-assembly of P123
micelles and the Ti precursor, TTIP, for 3 h before the addition
of the Si precursor TEOS in synthesizing Ti–SBA-15 is important in
d
preparing Ti–SBA-15 materials with more T -coordinated Ti(IV)
dispersed on the superficial regions of the pore walls. These Td-
coordinated Ti(IV) centers are highly accessible by the reactants,
and they contribute to the high catalytic activities of the Ti–SBA-15
2 2
materials in liquid phase oxidation of TMP with H O to form
TMBQ. In contrast, more Ti(IV) ions are hidden in the inner
regions of the SBA-15 framework and lose their accessibility to the
reactants in conventional Ti–SBA-15 prepared by pre-assembling
P123 and the Si precursor before the addition of the Ti precursor.
These results demonstrate the importance of a combination
sequence in preparing catalytically active metal-substituted
mesoporous materials. The used Ti–SBA-15 catalysts can be easily
regenerated by calcination at 500 uC for 3 h, and the recycling
catalysts were found to retain their catalytic activities well.
Fig. 5 Graphic correlation of catalytic performances (A) TMP conversion and (B)
2 2
TMBQ selectivity in TMP oxidation with H O at 82 uC, 4 h as a function of Ti
loadings in Ti–SBA-15 materials prepared by pre-assembly of P123 micelles and TTIP
for 3 h before the addition of TEOS in (a) 2 M, (b) 1.5 M, and (c) 1 M HCl.
with the increase in Ti loading up to the maximal Ti/Si ratio of 1.6
Acknowledgements
mol%. It is noticeable that the xSi–Ti-2 catalysts, which have
21
This work was supported by the National Science Council and
Ministry of Education, Taiwan. Acknowledgements were also
extended to C. Y. Tang, C. Y. Lin and S. J. Ji of Instrumentation
Center, National Taiwan University for EM measurement.
relatively higher Ti content, give lower TOFs of 1.4–2.8 h . The
maximal TOF is achieved over 4Si–Ti-2, and the TOF value
decreases almost linearly with further increase of Ti loading in
xSi–Ti-2 catalysts. These results should be due to the fact that a
large portion of the Ti sites in the xSi–Ti-2 series materials are not
accessible by the reactants. That is consistent with the observation
on Ti K-edge XANES spectra. In contrast, the higher TOFs of xTi–
Si-2 catalysts prepared by adding the Ti precursor first are due to
References
1
M. A. Uguina, D. P. Serrano, G. Ovejero, V. Grieken and
M. Camacho, Appl. Catal., A, 1995, 124, 391.
the higher accessibility of T -coordinated Ti(IV) sites on the
d
2 G. Tozzola, M. A. Mantegazza, G. Ranghino, G. Petrini,
S. Bordigay, G. Ricchiardiy, C. Lambertiy, R. Zuliany and
A. Zecchina, J. Catal., 1998, 179, 64.
superficial region of the SBA-15 framework.
Entries 6 and 7 of 12.5Ti–Si-2, as well as entries 13 and 14 of
1
2.5Si–Ti-2 in Table 1 are different batches of materials prepared
3 S. C. Laha and R. Kumar, J. Catal., 2002, 208, 339.
4 W. Fan, R. G. Duan, T. Yokoi, P. Wu, Y. Kubota and T. Tatsumi,
J. Am. Chem. Soc., 2008, 130, 10150.
5 F. Song, Y. Liu, H. Wu, M. He, P. We and T. Tatsumi, J. Catal.,
2006, 237, 359.
by different students. The similarity in catalytic performances of
the materials from different batches demonstrates that the
materials are easily reproduced. The used catalysts are regenerated
by calcination at 500 uC for 3 h. The catalytic results of the
recycling catalysts shown in entries 8 and 15 illustrate that the
catalytic activities of these Ti–SBA-15 materials can be well
retained.
The Ti contents in the solids can be further increased to 4–5
mol% by lowering the acid concentrations using 1–1.5 M HCl in
the synthesis solutions (Table 2), yet the highest TMP conversions
and TMBQ selectivities are obtained from the samples with Ti
loadings in the solids around 1.1–1.8 mol% (Fig. 5). Indeed, the
UV-vis spectra show that these materials contain the highest
6 B. L. Newalker, J. Clanrewaju and S. Komarneni, Chem. Mater.,
2001, 13, 552.
7
8
9
W. H. Zhang, J. Lu, B. Han, M. Li, J. Xiu, P. Ying and C. Li,
Chem. Mater., 2002, 14, 3413.
Y. Chen, Y. Huang, J. Xiu, X. Han and X. Bao, Appl. Catal., A,
2004, 273, 185.
K. Lin, P. P. Pescarmona, K. Houthoofd, D. Liang, G.
V. Tendeloo and P. A. Jacobs, J. Catal., 2009, 263, 75.
1
0 T. Yokoi, T. Karouji, S. Ohta, J. N. Kondo and T. Tatsumi, Chem.
Mater., 2010, 22, 3900.
1 F. Berube, B. Nohair, F. Kleitz and S. Kaliaguine, Chem. Mater.,
010, 22, 1988.
2 S. Y. Chen, C. Y. Tang, J. F. Lee, L. Y. Jang, T. Tatsumi and
d
amount of T -coordinated Ti species in these two series of
1
samples. Although both xTi–Si-1 and xTi–Si-1.5 materials were also
prepared by adding the Ti precursor first and the Ti(IV) ions are
supposed to locate at superficial regions of SBA-15 framework,
relatively large amounts of TiO crystallites are formed when the
synthesis solutions are of higher pH values. Based on these
2
1
S. Cheng, J. Mater. Chem., 2011, 21, 2255.
13 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B.
F. Chmelka and G. D. Stucky, Science, 1998, 179, 548.
2
14 S. Y. Chen and S. Cheng, Chem. Mater., 2007, 19, 3041.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 12604–12610 | 12609

View Article Online
Communication
RSC Advances
1
5 L. H. Hsiao, S. Y. Chen, S. J. Huang, S. B. Liu, P. H. Chen, J. C.
C. Chan and S. Cheng, Appl. Catal., A, 2009, 359, 96.
6 S. Y. Chen, J. F. Lee and S. Cheng, J. Catal., 2010, 270, 196.
21 Z. Y. Wu, G. Ouvrard, P. Gressier and C. R. Natoli, Phys. Rev. B:
Condens. Matter, 1997, 55, 10382.
22 D. Cabaret, Y. Joly, H. Renevier and C. R. Natoli, J. Synchrotron
Radiat., 1999, 6, 258.
1
17 S. Y. Chen, T. Yokoi, C. Y. Tang, L. Y. Jang, T. Tatsumi, J. C.
C. Chan and S. Cheng, Green Chem., 2011, 13, 2920.
8 S. Che, K. Lung, T. Tatsumi, S. Iijiima, H. Joo, R. Ryoo and
O. Terasaki, Angew. Chem., Int. Ed., 2003, 42, 2182.
9 A. Endo, M. Yamada, S. Kataoka, T. sano, Y. Jnagi and
A. Miyaki, Colloids Surf., A, 2010, 357, 11.
0 S. Y. Chen, C.-Y. Hung, T. Yokoi, C.-T. Tang, S.-J. Huang, J.
J. Jau, J. C. C. Chan, T. Tatsumi and S. Cheng, J. Mater. Chem.,
23 F. Farges, G. E. Brown and J. J. Rehr, Phys. Rev. B: Condens.
Matter, 1997, 56, 1809.
1
24 J. M. Fraile, J. Garcya, J. A. Mayoral, M. G. Proietti and M.
C. Sanchez, J. Phys. Chem., 1996, 100, 19484.
1
2
5 M. Bandyopadhyay, A. Birkner, M. W. E. van den Berg, K.
V. Klementiev, W. Schmidt, W. Grunert and H. Gies, Chem.
Mater., 2005, 17, 3820.
2
26 O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti, N. Ravasio,
2012, 22, 2233.
M. Sgobba and M. V. Barmatova, Catal. Today, 2009, 141, 330.
1
2610 | RSC Adv., 2013, 3, 12604–12610
This journal is ß The Royal Society of Chemistry 2013