Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation

Article abstract of DOI:10.1039/c5ra00194c

Two titanium silicalite-1 samples with different crystal sizes were synthesized in the tetrapropylammonium bromide (TPABr) and tetrapropylammonium hydroxide (TPAOH) hydrothermal systems. The small-crystal TS-1 with a size of 600 nm was then treated with different organic bases. These TS-1 samples were evaluated in the epoxidation of propene, and characterized by ultraviolet-visible diffuse reflectance (UV-vis), X-ray absorption near edge structure (XANES) and Raman spectroscopies. The Ti L-edge absorption spectra show that a new Ti species, pentahedrally coordinated Ti, appears in some of the samples. This pentahedrally coordinated Ti species is correlated with the catalytic oxidation activity of TS-1, closely. Tetrahedrally coordinated Ti in TS-1 is the primary active center for selective oxidation reactions, but the existence of a small amount of pentahedrally coordinated Ti can further improve the catalytic activity. A high molar ratio of Si/Ti (n(Si/Ti)) in the synthesis process (n(Si/Ti) = 92.78) was beneficial for the generation of pentahedrally coordinated Ti. The improved catalytic activity of the TPAOH treated TS-1 is mainly due to the increasing amount of pentahedrally coordinated Ti, besides the elimination of diffusion limitation. Slowing down the crystallization rate can also increase the content of pentahedrally coordinated Ti. This journal is

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Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation
A new pentahedrally coordinated Ti was discovered in TS-1 and showed a higher catalytic activity
than the tetrahedrally coordinated one.

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Role of pentahedrally coordinated titanium in
titanium silicalite-1 in propene epoxidation
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
a
a,*
b
Yi Zuo, Min Liu, Ting Zhang, Luwei Hong, Xinwen Guo, Chunshan Song,
b,*
b
c
c
Yongsheng Chen, Pengyu Zhu, Cherno Jaye and Daniel Fischer ,
Received 00th January 2012,
Accepted 00th January 2012
Two titanium silicaliteꢀ1 samples with different crystal sizes were synthesized in the
tetrapropylammonium bromide (TPABr) and tetrapropylammonium hydroxide (TPAOH)
hydrothermal systems. The smallꢀcrystal TSꢀ1 with a size of 600 nm was then treated with
different organic bases. These TSꢀ1 samples were evaluated in the epoxidation of propene,
and characterized by ultravioletꢀvisible diffuse reflectance (UVꢀvis), Xꢀray absorption near
DOI: 10.1039/x0xx00000x
www.rsc.org/
edge structure (XANES) and Raman spectroscopies. The Ti Lꢀedge absorption spectra show
that a new Ti species, pentahedrally coordinated Ti, appears in some of the samples. This
pentahedrally coordinated Ti species is correlated with the catalytic oxidation activity of TSꢀ
1
, closely. Tetrahedrally coordinated Ti in TSꢀ1 is the primary active center for selective
oxidation reactions, but the existence of a small amount of pentahedrally coordinated Ti can
further improve the catalytic activity. A high molar ratio of Si/Ti ( (Si/Ti)) in the synthesis
process ( (Si/Ti)=92.78) was beneficial for the generation of pentahedrally coordinated Ti.
n
n
The improved catalytic activity of the TPAOH treated TSꢀ1 is mainly due to the increasing
amount of pentahedrally coordinated Ti, besides the elimination of diffusion limitation.
Slowing down the crystallization rate can also increase the content of pentahedrally
coordinated Ti.
mechanism well explains the effect of alcohol solvents in the
propene epoxidation. Thus, it is widely accepted. In this
1
. Introduction
mechanism, a pentahedrally coordinated Ti intermediate
species iv in Scheme 1) was provided and was considered as
an inert state for the oxidation reactions, due to the negative
charge of the species. The other two types of Ti species are
octahedrally coordinated Ti (usually called extraꢀframework Ti)
Since it was first hydrothermally synthesized by Taramasso et
al., titanium silicaliteꢀ1 (TSꢀ1) has attracted much attention
1
(
due to its excellent catalytic performances for selective
2
oxidation reactions, such as oxidation of alkanes, epoxidation
3
4
of alkenes, hydroxylation of aromatics and ammoxidation of
5
and anatase TiO . They are considered inert in oxidation
ketones. The hydroxylation of phenol over TSꢀ1 using
2
1
1
reaction.
Moreover, anatase TiO2 can even lead to
methanol and acetone as the solvents at 353 K to produce
6
decomposition of hydrogen peroxide. Therefore, efforts have
been to increase the content of framework Ti and decrease the
benzenediol was first commercialized by Enichem in 1986.
The conversion of phenol reached 25% in the Enichem route,
which was much higher than those in the Rhoneꢀpolenc route
1
2
extraꢀframework Ti proportion. However, the Ti content in
the TSꢀ1 framework was reported to be limited and the
(
5%) and Brichima route (10%). Epoxidation of propene is
1
3
maximum was 2.5 wt%, due to the expansion of lattice
constant. If the amount of titanium added to the hydrothermal
synthesis system exceeds 2.5 wt%, it will exist as extraꢀ
another important application of TSꢀ1. In recent years, the
hydrogen peroxide to propene oxide route (HPPO) has become
a mature technology and been commercialized by BASF/Dow
Chemical and Evonik/Uhde in Belgium and Korea,
framework Ti. Thus, the
synthesis gel.
n
(Si/Ti) is usually more than 33 in the
7
,8
respectively. The industrial applications of TSꢀ1 are more and
more attractive. However, the true active center for the
oxidation reactions in TSꢀ1 is still controversial.
Most researchers believe that there are three types of Ti
species in TSꢀ1. Tetrahedrally coordinated titanium, which is
called framework Ti, is usually considered as the active center
9
for the oxidation reactions. The tetrahedral Ti can form a five
Scheme 1
member ring (5MR) with hydrogen peroxide and alcohol (see
1
0
Scheme 1). This 5MR, which was reported by Clerici et al.,
can oxidize propene to prepare propene oxide. The 5MR
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12
In order to increase the Ti content in the framework, ratio of 0.3. The other synthesis conditions were the same as
researchers have explored another preparation method called that of TSꢀ1ꢀNA.
gasꢀsolid substitution method. Many silanol nests are created in
the dealuminum of ZSMꢀ5 or deboron of BꢀZSMꢀ5 by this
method, and Ti atoms are then planted in these nests to obtain
TiꢀZSMꢀ5. However, higher framework Ti content does not
bring a higher catalytic activity, probably due to the difference
of Ti locations in TSꢀ1 and TiꢀZSMꢀ5. In addition, it was also
reported that octahedrally coordinated extraꢀframework Ti with
a TiꢀOꢀTi bond was the true active center for the oxidation
reactions, according to the characterization of ultravioletꢀvisible
diffused reflectance and ultraviolet resonance Raman
2
.2. Characterization of TS-1
Ti L2,3ꢀedge (453.8 and 460.2 eV) Xꢀray absorption near edge
structure (XANES) spectroscopy was performed at the
National Institute of Standards and Technology U7A beamline
at National Synchrotron Light Source (NSLS) at Brookhaven
National Laboratory. The storage ring was operated with an
electron beam energy of 800 MeV and an average current of
6
00 mA. The XANES spectra were collected in a partial
1
4,15
electron yield mode with low energy electron compensation to
minimize charging effect for poor conductors. Additionally a
clean gold coated mesh was used to monitor I0, the incident
beam intensity. The data were processed using Athena. The
Raman spectra were recorded on a Bruker laser Raman
spectrometer, using a 532 nm laser as the excitation source. The
UVꢀvis spectra were collected on a Jasco UVꢀ550 spectrometer
from 190 to 500 nm, and pure BaSO was used as a reference.
A
Spectrometer provided the elemental composition (ICP).
spectroscopy.
Therefore, it is very important to recognize
the true active center for further improving the activity of TSꢀ1.
In our previous work, smallꢀcrystal TSꢀ1 (crystal size of
2
0
6
00 nm × 400 nm × 250 nm) was synthesized in a
tetrapropylammonium bromide (TPABr)ꢀethylamine (EA)
hydrothermal system by using the mother liquor of nanoꢀsized
1
6
TSꢀ1 (200 nm) as the seed. The treatment of smallꢀcrystal TSꢀ
with organic bases further improved its catalytic activity in
1
4
1
7
Perkin Elmer OPTIMA 2000DV Optical Emission
phenol hydroxylation. In the present contribution, smallꢀ
crystal TSꢀ1 and nanoꢀsized TSꢀ1 with different catalytic
oxidation activities were characterized by ultravioletꢀvisible
diffuse reflectance (UVꢀvis) spectroscopy, Xꢀray absorption 2.3. Epoxidation of propene
near edge structure (XANES) spectroscopy and Raman
The epoxidation of propene was carried out in a 400 mL
resonance spectroscopy. Via investigating the relationship
between Ti coordination structure and the catalytic activities,
we find a new Ti coordination structure in TSꢀ1, which is
pentahedrally coordinated Ti. This pentahedral Ti is more
active than tetrahedral one in oxidation reactions.
stainlessꢀsteel batch reactor. In a typical run, 0.2 g TSꢀ1, 32 mL
methanol and 30 wt% of H O were added to the reactor;
2
2
propene was then charged to reach 0.4 MPa. The initial
concentration of H O was 3.5 mol/L. After heating the mixture
2
2
with stirring at 313 K for 1 h, the residual H O was checked by
2
2
iodometric titration. The products were analyzed on a Tianmei
890F gas chromatograph with a flame ionization detector
FID) and a PEGꢀ20M capillary column (30 m × 0.25 mm × 0.4
ꢁm). Propene oxide (PO) was the main product, and propene
glycol (PG) and its monomethyl ethers (MME) were the byꢀ
2
. Experimental
7
(
2
.1. Preparation of TS-1
Smallꢀcrystal TSꢀ1 with different molar ratios of Si/Ti
(n(Si/Ti)) was hydrothermally synthesized according to
reference,
tetrachloride as silicon and titanium sources, respectively, and
TPABr as the template. The molar composition of the synthesis
gel was: SiO : TiO : TPABr : H O = 1 : (0.125~0.50) : 0.4 :
products. The conversion of H O (X(H O )), selectivity of PO
2
2
2 2
1
8
(
(
S
(PO)), utilization of H
O (U(H O )) and turnover frequency
2 2 2 2
using 30 wt% colloidal silica and titanium
TOF) were calculated using equations (1), (2), (3) and (4),
respectively:
X
(H O ) = (n0(H O ) ꢀ
n
(H O )) / n0(H O )
(1)
(2)
2
2
2
2
2
2
2
2
2
2
2
S
(PO) =
n
(PO) / (
n
(PO) +
n
(MME) +
n(PG))
1
2. The seed/SiO2 weight ratio was 0.06. The gel was
U
(H O ) = (
n
(PO) +
n
(MME) +
n
(PG)) / ( 0
n
(H O ) ·
X
(H O ))
2
2
2
2
2
2
crystallized for 48 h, and then the obtained solid was separated
from the mother liquor, washed, dried, and calcined at 813 K
for 6 h. The smallꢀcrystal TSꢀ1 with the feeding
(
3)
(4)
(H O ) are the initial and final molar
TOF = n0(H O ) ·
X
(H O ) / (
t
·
n
(Ti))
2
2
2
2
n
(Si/Ti) of 20,
where n0(H O ) and
n
2
2
2
2
5
1
0 and 80 were denoted as samples TSꢀ1ꢀ20, TSꢀ1ꢀ50 and TSꢀ
ꢀ80, respectively.
concentrations of H O ,
n
(PO),
the molar contents of PO, MME and PG,
which is 1 h in this paper, and (Ti) is the molar number of Ti,
n
(MME) and
n(PG) represent
2
2
t
is the reaction time,
Treatment of TSꢀ1 with the organic bases, including EA,
n
diethylamine (DEA), tetramethylammonium hydroxide
TMAOH), and tetrapropylammonium hydroxide (TPAOH),
was carried out in a Teflon lined autoclave with a solid/liquid
determined by ICP in the 0.2 g TSꢀ1.
(
1
7
3. Results and discussion
ratio of 1 g : 10 mL at 443 K for 48 h. The concentration of
the base solution was 0.06 mol/L. The solid was then washed
and calcined at 813 K for 6 h. The samples treated with EA, 3.1. Different molar ratio of Si/Ti
DEA, TMAOH and TPAOH were denoted as TSꢀ1ꢀEA, TSꢀ1ꢀ
DEA, TSꢀ1ꢀTMAOH and TSꢀ1ꢀTPAOH, respectively.
Smallꢀcrystal TSꢀ1 with different
n
(Si/Ti) was synthesized by
adding different amount of titanium tetrachloride in the
synthesis gel. Three TSꢀ1 samples were characterized and
evaluated in the epoxidation of propene to study the
relationship between the Ti coordination structure and catalytic
activity. All the smallꢀcrystal TSꢀ1 show a similar crystal size
The synthesis of nanoꢀsized TSꢀ1 was carried out
1
9
according to reference. Tetraethyl orthosilicate and tetrabutyl
orthotitanium were used as silicon and titanium sources, and
TPAOH, the template. The molar composition of the mixture
was: SiO : TiO : TPAOH : H O = 1 : 0.25 : 0.4 : 10. The postꢀ
2
2
2
(
600 nm × 400 nm × 250 nm), regardless of their n(Si/Ti) and
treatment of nanoꢀsized TSꢀ1 was the same as that of smallꢀ
crystal TSꢀ1. The asꢀsynthesized TSꢀ1 was denoted as TSꢀ1ꢀ
NA. The TSꢀ1ꢀAC was prepared by adding ammonium
carbonate in the synthesis gel with an ammonium/silicon molar
treatment with different organic bases (cf. Figs. S.9 and S.10).
The weight contents of silicon and titanium in the samples
obtained from ICP was settled in Table 1 with their catalytic
2
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ART LE
IC
performances. The actual
n(Si/Ti) slightly differed from the 200ꢀ210 nm is assigned to tetrahedrally coordinated Ti, while
2
7
feeding one. Fig. 1 shows the Raman spectra of the three that at 310ꢀ330 nm corresponds to anatase TiO2. The
samples. The vibration bands at 144, 390, 516 and 637 cm are contribution of the band at 240ꢀ290 nm is controversial. Blasco
ꢀ
1
2
1
assigned to the characteristic bands of anatase TiO , and the et al. claimed that the band belonged to an octahedrally
2
ꢀ
1
28
peak at 380 cm corresponds to fiveꢀmemberꢀring structure of coordinated Ti containing TiꢀOꢀTi bond, while Geobaldo et
2
2
MFI topology. The most anatase TiO appears in TSꢀ1ꢀ20. al. attributed it to isolated Ti species that coordinated with two
2
ꢀ
1
29
The characteristic band of MFI topology (380 cm ) is almost water molecules. In our opinion, more than one kind of Ti
covered by that of anatase TiO (390 cm ). This suggests that species appear between the band of 230 and 290 nm in the UVꢀ
the amount of the framework titanium is limited. When the vis spectra of TSꢀ1. The band at about 250ꢀ290 nm is assigned
amount of titanium added to the synthesis gel past the to the octahedrally coordinated Ti species, which is inactive for
ꢀ
1
2
1
3
maximum, extra titanium will exist as anatase TiO , which is the oxidation reactions, and that at 220ꢀ250 nm is isolated Ti
2
octahedrally coordinated extraꢀframework Ti. In recent years, species with less coordination number of oxygen (such as
many researchers tried to increase the amount of framework Ti. pentahedral Ti). The underꢀcoordinated Ti has a higher energy
Fan et al. found that the addition of ammonium salts in the than the octahedral one, so its band shifts to shorter wavelength
hydrothermal system could not only decrease the basicity and (hypsochromic shift). The contents of tetrahedral Ti, octahedral
the rate of crystallization, but also change the crystallization Ti and anatase TiO increase with the decrease of
n
(Si/Ti), and
2
mechanism. They obtained TSꢀ1 with a
almost no anatase TiO was present after being washed by HCl others, indicating that the introduction of Ti in the framework is
n(Si/Ti) of 33, and the increase of anatase TiO is much stronger than those of the
2
2
2
3
ꢀ1
solution. The Raman bands at 390, 516 and 637 cm nearly limited. There is no absorption at 300ꢀ310 nm in TSꢀ1ꢀ80,
disappear and that at 144 cm becomes weaker for TSꢀ1ꢀ50, which demonstrates that anatase TiO2 is absent. This is
ꢀ
1
indicating that the content of anatase TiO decreases sharply consistent with its Raman spectrum. A new band appears at
2
when the
inferred that titanium exists mainly as tetrahedrally coordinated coordinated Ti (proved by XANES). This band does not appear
framework Ti. Further increase of the (Si/Ti) to 92.78 results in other samples.
n(Si/Ti) increases from 18.82 to 52.00. It can be ~235 nm in TSꢀ1ꢀ80, which is considered to be pentahedrally
n
ꢀ
1
in the presence of only Raman band at 380 cm , which is the
characteristic band of MFI topology, demonstrating that anatase
TiO2 is not present in TSꢀ1ꢀ80. Evidently, the content of
anatase TiO in smallꢀcrystal TSꢀ1 decreases with the decrease
2
of the amount of titanium added to the synthesis process. The
5000
4000
3000
2000
1000
0
existence of anatase TiO may lead to the decomposition of
2
2
4
H O and the coverage of the active framework Ti centers.
2
2
Therefore, the mitigation of anatase TiO2 generation is
beneficial for the oxidation reactions. Based on the Raman
spectra, we believe that a high n(Si/Ti) may have a positive
TS-1-20
effect on the catalytic reactions because it results in a low
anatase TiO content.
2
TS-1-50
Raman spectroscopy provided us the information about
TS-1-80
1000
anatase TiO , but it did not tell us how the content of
2
framework Ti changed with the variation of n(Si/Ti). UVꢀvis
spectroscopy, one of the first spectral techniques used for the
detection of Ti coordination states in titanium silicalites,
2
00
400
600
800
1200
-
1
2
5
Raman shift ( cm )
provides important information. Fig. 2 shows the UVꢀvis
spectra of smallꢀcrystal TSꢀ1 with different (Si/Ti). Peak
deconvolutions were performed using the PeakFit program with
n
Fig. 1 Raman spectra of smallꢀcrystal TSꢀ1 with different
n(Si/Ti).
2
6
the Gaussian fitting method. In the spectra of smallꢀcrystal
TSꢀ1, there are usually three major absorption bands, which are
centered in 200ꢀ210, 240ꢀ290 and 310ꢀ330 nm. The band at
a
Table 1 Catalytic performances of propene epoxidation and IH/L of TSꢀ1 samples
b
Cat.
TSꢀ1ꢀ20
TSꢀ1ꢀ50
TSꢀ1ꢀ80
TSꢀ1ꢀEA
Si/wt%
43.58
43.50
46.01
44.97
44.94
44.96
44.86
45.43
45.20
Ti/wt%
3.97
1.50
0.85
2.04
2.09
2.04
2.21
1.59
1.89
n(Si/Ti)
18.82
52.00
92.78
37.79
36.86
37.78
34.80
48.98
40.99
X(H O )/%
S(PO)/%
82.0
78.7
83.7
84.4
83.6
85.8
90.2
87.7
U(H O )/%
TOF/(mol H O /(h·mol Ti))
IH/L
2
2
2
2
2
2
21.8
26.2
26.9
52.0
52.8
52.1
58.3
29.1
52.0
87.1
88.2
89.6
86.2
85.1
84.3
88.5
87.4
85.6
156.8
498.8
903.8
728.0
721.5
729.4
753.4
522.7
785.8
0.92
1.59
2.43
1.34
1.31
1.46
1.86
2.15
3.32
TSꢀ1ꢀDEA
TSꢀ1ꢀTMAOH
TSꢀ1ꢀTPAOH
TSꢀ1ꢀNA
TSꢀ1ꢀAC
91.1
a
b
Reaction conditions: catalyst 0.2 g, methanol 32 mL, H O 3.5 mol/L, propene pressure 0.4 MPa, 313 K, 1 h.
IH/L stands for the relative intensity of the higher energy peak of L3 edge to the lower one in XANES.
2
2
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Xꢀray absorption spectroscopy (XAS) has been used by
many researchers to examine the local structure of active
3
0,31
centers.
coordination structure in titanium silicalites and oxides.
Several in situ Ti ꢀedge XAS studies revealed that active Ti
center was present in tetrahedral coordination.
XANES is also used for studying the coordination of Ti in Tiꢀ
Ti
K
ꢀedge XANES is often used to determine Ti
1.0
.8
3
2ꢀ35
TS-1-20
TS-1-50
TS-1-80
0
258 nm
K
3
6,37
Ti
Lꢀedge
0.6
0.4
0.2
3
8
silicate glasses. The Ti
two sets of doublets (see Fig. 3), which correspond to the 2p1/2
and 2p3/2 transitions of the 3
at higher energy (462ꢀ470 eV) and the L3 edge is at lower
energy (455ꢀ462 eV). The splitting of each edge is attributed to
the teg and eg symmetry of the
of L3 edge (459ꢀ462 eV) consists of two peaks in some samples,
a main peak and a shoulder one, which is on the lower energy
side (~460 eV, b) for rutile TiO and the higher energy side
461 eV, c) for anatase TiO2. The intensities of the two peaks
are reversed for the substances that they represent. For
example, the intensity of peak b is higher than that of peak c in
Lꢀedge XANES spectrum consists of
0
5
1
39
d to 2p 3d states. The L2 edge is
0.0
0.8
0.6
0.4
d
orbital. The higher energy peak
258 nm
2
3
8
(
0.2
.0
0
the spectrum of anatase TiO . The tetrahedrally coordinated Ti
0.8
2
is characterized by the absence of splitting of the peak at 459ꢀ
235 nm
4
62 eV and a relatively weaker intensity of the peaks at lower
0.6
energy of both L2 and L3 edges (peaks a and d) than those at
higher energy (peaks b and e). Pentahedrally coordinated Ti
was first discovered in fresnoite, whose coordination structure
is that a Ti atom centered in a tetragonal pyramid, formed by
five oxygen atoms. The pentahedral Ti is characterized by a
slight shift to higher energy and a drastic decrease of peak a, a
shift of peak e to lower energy and the appearance of a shoulder
peak on peak d.
0.4
0.2
0.0
200
250
300
350
400
Wavelength ( nm )
Fig. 2 UVꢀvis spectra of smallꢀcrystal TSꢀ1 with different
(Si/Ti).
n
TSꢀ1ꢀ50 contains primarily tetrahedrally coordinated Ti,
because the relative intensity of peak a is obviously lower than
that of peak b. Anatase TiO is observed in TSꢀ1ꢀ20 and TSꢀ1ꢀ
2
5
0, which is proved by the slight splitting of peaks b and c. The
content of anatase TiO in TSꢀ1ꢀ20 is more than that in TSꢀ1ꢀ
2
5
0, illustrated by the relatively weaker intensity of peak b to
peak a (denoted as IH/L). This result confirms that the content of
Ti in the framework is limited, and the excessive Ti will exist
as anatase TiO . The peak intensity of TSꢀ1ꢀ80 is much weaker
2
than those of the others, which is due to the least Ti content.
Moreover, the peak a is very weak and shifts to higher energy
slightly (from 458.1 eV to 458.5 eV), the peak e shifts to lower
energy (from 466.0 eV to 465.8 eV), and a weak shoulder peak
appears in peak d, confirming the existence of pentahedrally
coordinated Ti in TSꢀ1ꢀ80. We can provide a possible reason
16ꢀ18
for this phenomenon, according to our previous works.
The
crystallization of smallꢀcrystal TSꢀ1 starts from the
combination of one [TiO ] and several [SiO ], forming Ti[ꢀOꢀ
4
4
SiO ] species. This species then combines with each other to
4
x
form the TSꢀ1 framework under the structureꢀdirect of
template. Finally, the left [SiO ] grows along the crystal
4
surface. In other words, the combination of [SiO ] with [TiO ]
4
4
makes silicon easier to be inserted into framework. Thus,
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ARTICLE
[
SiO ] trends to combine with [TiO ] to crystallize in a H O /(h·mol Ti). This results confirms that pentahedral Ti is
4
4
2
2
supersaturating condition. If the n(Si/Ti) was very high (may be the most active species among the three coordination structures.
more than 80), [SiO ] would show a competitive effect with It was also reported that pentahedrally coordinated Ti in a
4
each other. Therefore, it can be inferred that one titanium atom homogeneous catalyst showed a high catalytic activity in the
4
0,41
needs to combine with more than four [SiO ]. At this time, epoxidation of cyclohexene.
Moreover, a pentahedrally
4
pentahedrally coordinated Ti is easy to be formed. Furthermore, coordinated Ti formed by tetrahedrally coordinated Ti in Tiꢀ
it was reported that octahedrally coordinated Ti was generated MWW, H O , methanol and piperidine was reported and
2
2
by
a tetrahedral one with two molecular water. The considered to show a sharp increasing activity in the
4
2
combination between them was the oxygen atom from water. epoxidation of propene, 1ꢀhexene and cyclopentene.
[
[
SiO ] can also provide the oxygen atom to combine with However, the controllable synthesis of TSꢀ1 with a large
4
TiO ]. Therefore, [TiO ] has the ability to bond with more than amount of pentahedral Ti is still impossible. When more Ti was
4
4
four [SiO ], although the bonding may lead to a lattice added to the synthesis gel, the pentahedral Ti would transform
4
distortion.
to tetrahedral and octahedral ones, due to the losing of
pentahedral Ti generation conditions mentioned previously.
Table 2 Contents of different coordinated Ti in smallꢀcrystal
0
0
0
0
0
.12
.10
.08
.06
TSꢀ1
4
66.0 eV
4
58.1 eV
e
Percentage of different coordinated Ti/%
Cat.
b
Tetrahedral Ti
Pentahedral Ti
Octahedral Ti
d
a
c
TS-1-20
TSꢀ1ꢀ20
TSꢀ1ꢀ50
TSꢀ1ꢀ80
42
76
50
0
21
50
58
3
0
TS-1-50
TS-1-80
3
.2. Treatment with organic bases
4
65.8 eV
Shoulder peak
465 470
.04
4
Although zeolites are widely used in industry, one problem is
the diffusion limitation by zeolite channels. Compared with
those molecular sieves with mesopores or macropores, the
hydrothermal stability of zeolite is excellent, but the diffusion
resistance in channels is quite significant. Therefore, improving
diffusion within the channels is one way to improve
performance of zeolites. Many studies were focused on the
treatment of zeolites with organic bases, because the treatment
4
58.4 eV
50
455
460
475
Photon energy ( eV )
Fig. 3 Ti
Lꢀedge XANES spectra of smallꢀcrystal TSꢀ1 with
different (Si/Ti). (Peaks aꢀc are the characteristic peaks of L3
edge, while peaks d and e are those of L2 edge.)
n
4
3ꢀ46
could generate some mesopores in the zeolite crystals,
which may decrease the diffusion resistance. In a previous
work, we treated smallꢀcrystal TSꢀ1 with different organic
The catalytic performances of the smallꢀcrystal TSꢀ1 with
different
n(Si/Ti) were evaluated in the propene epoxidation
1
7
bases forming hierarchical structures.
The catalytic
(
Table 1). Although the Ti content in TSꢀ1ꢀ20 is the largest, the
performances of phenol hydroxylation over treated samples
were improved to different extents, partly due to the unblocking
of channels by the dissolution of silicon in the crystals. In this
conversion of H O2 over it is the lowest among the three
2
samples, indicating that the Ti coordination state is more
important than its content for the catalytic activity. The content
of Ti in TSꢀ1ꢀ80 is the least, but its activity is higher than those
of the other two. The highest TOF was obtained over TSꢀ1ꢀ80,
indicating that the pentahedrally coordinated Ti was the most
active species of all the Ti coordination. The lowest TOF were
obtained over TSꢀ1ꢀ20, indicating that octahedrally coordinated
Ti showed only negative effects on the oxidation reaction.
Based on the XANES and TOF data, we calculated the contents
of different coordinated Ti in the three samples (see Table 2)
and the TOF of each coordinated Ti. The process of calculation
was provided in Electronic Supplementary Information. The
activity of TSꢀ1 is not affected only by the Ti coordination, but
also by the crystal size and diffusion property. Therefore, the
results obtained may be just fit for the untreated smallꢀcrystal
TSꢀ1. The highest content of pentahedrally coordinated Ti was
generated in TSꢀ1ꢀ80 (~0.43 wt%), while that of tetrahedral one
in TSꢀ1ꢀ50 (~1.14 wt%). The content of pentahedrally
coordinated Ti decreases and octahedral one increases in TSꢀ1ꢀ
work, we characterize the treated samples with Ti
Lꢀedge
XANES, and find that the generation of pentahedrally
coordinated Ti can be another reason for the increasing activity
of TPAOH treated TSꢀ1.
The Ti Lꢀedge XANES of the treated samples are shown in
Fig. 4. The IH/L can be considered to be positive related with the
percentage of the pentahedrally coordinated Ti in all kinds of Ti
coordination states. The larger the IH/L is, the higher pentahedral
Ti contains. The IH/L decreased to different extents after the
treatment except that of TSꢀ1ꢀTPAOH (Table 1). The IH/L of
TSꢀ1ꢀTPAOH is higher than those of the other treated samples,
indicating that pentahedral Ti was generated after being treated
with TPAOH. A shoulder peak appears in the spectra of TSꢀ1ꢀ
DEA and TSꢀ1ꢀTMAOH on the higher side of the higher
energy peak of L3 edge, but it is very small in TSꢀ1ꢀEA and TSꢀ
1
ꢀTPAOH. The treatment with organic bases leads to the
dissolution of silicon and titanium. The silicon and titanium
4
7
will recrystallize in the present of
a
template.
The
5
0, compared with those in TSꢀ1ꢀ80, suggesting that the
recrystallization rate depends on the structureꢀdirecting effect
of the base. The strong structureꢀdirecting effects of EA and
TPAOH cause a faster rate than those of DEA and TMAOH.
Furthermore, TPAOH has a protection effect for framework Ti,
insertion of Ti into the framework is nearly the maximum in
this synthesis condition. Continuing adding of Ti leads to the
sharp increase of octahedrally coordinated Ti. The TOFs of
pure tetrahedrally, pentahedrally and octahedrally coordinated
Ti in smallꢀcrystal TSꢀ1 are 373.3, 1434.3 and 0 mol
1
7
which may prevent the dissolution of framework titanium.
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Therefore, the contents of anatase TiO in TSꢀ1ꢀEA and TSꢀ1ꢀ 3.3. Nano-sized TS-1
2
TPAOH are less than those in TSꢀ1ꢀDEA and TSꢀ1ꢀTMAOH.
We also investigated the effect of pentahedrally coordinated Ti
in nanoꢀsized TSꢀ1. Nanoꢀsized TSꢀ1 usually needs to be
synthesized with the presence of a very expensive structureꢀ
directing template, TPAOH. The substitution of TPAOH by
TPABr often makes the crystal size larger, leading to a poorer
b
e
4
8
activity. Therefore, further improving the activity of nanoꢀ
sized TSꢀ1 to reduce its unit synthesis cost is significant.
Increasing the content of framework Ti by adding ammonium
d
c
a
2
3
salts is one of the methods. It was considered that the slowing
down of crystallization rate was beneficial for the incorporation
of Ti into the TSꢀ1 framework. The framework Ti is accepted
by most researchers as the active center for oxidation reactions.
Therefore, the addition of ammonium salts can improve the
catalytic activity. In our opinion, its effect on crystallization
rate may be one of the reasons. The two samples with and
without adding of ammonium carbonate show a similar crystal
size (Fig. S.9), indicating that the size is not the main factor
TS-1-50
TS-1-EA
TS-1-DEA
TS-1-TMAOH
TS-1-TPAOH
450
455
460
465
470
475
affecting their catalytic performances. From the Ti
Lꢀedge
Photon energy ( eV )
XANES of nanoꢀsized TSꢀ1 (Fig. 6), it can be seen that the IH/L
of TSꢀ1ꢀAC is higher than that of TSꢀ1ꢀNA (3.32 versus 2.15),
indicating that the amount of pentahedrally coordinated Ti in
the former is more than the latter. The UVꢀvis spectra of these
two samples (Fig. 7) confirms that the coordination state of Ti
between 230 and 290 nm in TSꢀ1ꢀAC is mainly pentahedrally
coordinated Ti (239 nm), but it is octahedrally coordinated Ti
Fig. 4 Ti
Lꢀedge XANES spectra of organic bases treated
smallꢀcrystal TSꢀ1. (Peaks aꢀc are the characteristic peaks of L3
edge, while peaks d and e are those of L2 edge.)
Figure 5 shows the Raman spectra of the samples treated
with different organic bases. After the treatment, all the
samples show weaker bands than the untreated ones due to the
decrease of the amount of anatase TiO . Some part of the TiO
(
261 nm) in TSꢀ1ꢀNA. The inhibition of anatase TiO formation
2
with the addition of (NH ) CO3 is probed by UVꢀvis
4 2
2
2
spectroscopy. A very sharp decrease of the amount of anatase
is probably dissoluted in the treatment or converted to
pentahedrally coordinated Ti (such as TSꢀ1ꢀTPAOH). This
conversion to pentahedral Ti is beneficial for the oxidation
reactions. The TOFs of the treated samples are all higher than
the untreated TSꢀ1ꢀ50. However, the reasons for the increased
TOF may not be the same. TSꢀ1ꢀEA, TSꢀ1ꢀDEA and TSꢀ1ꢀ
TMAOH show a smaller IH/L than TSꢀ1ꢀ50. Thus, the
unblocking of channels is considered as the primary factor for
the improvement of activity. The lower activity over TSꢀ1ꢀDEA
than that over TSꢀ1ꢀEA is due to the weaker structureꢀdirecting
effect of the former, leading to a slower recrystallization rate.
The highest TOF, which can be inferred, is obtained over TSꢀ1ꢀ
TPAOH, accounted for the high IH/L demonstrating a relatively
high pentahedral Ti content.
TiO was observed in the spectrum of TSꢀ1ꢀAC. However, the
2
amount of framework tetrahedrally coordinated Ti was not
improved, but decreased slightly. This is due to the slowing
down of crystallization rate by the addition making the
insertion of Ti to the framework slower. The Ti in the liquid
phase, thus can be combined with more than four [SiO4],
forming pentahedral Ti. In other words, the tetrahedral Ti,
extraꢀframework octahedral Ti and anatase TiO2 were
transferred to the framework pentahedral Ti by the effect of
ammonium carbonate.
TS-1-EA
TS-1-AC
TS-1-NA
TS-1-DEA
TS-1-TMAOH
TS-1-TPAOH
450
455
460
465
470
475
Photon energy ( eV )
TS-1-50
Fig. 6 Ti Lꢀedge XANES spectra of nanoꢀsized TSꢀ1.
200
400
600
800
1000
1200
-
1
Raman shift ( cm
)
Fig. 5 Raman spectra of organic bases treated smallꢀcrystal
TSꢀ1.
In section 3.1, we found that a small amount of
pentahedrally coordinated Ti could show a very high activity.
Therefore, the increasing pentahedral Ti amount in TSꢀ1ꢀAC
could also lead to an obvious increase of activity. The catalytic
6
| J. Name., 2012, 00, 1-3
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ARTICLE
performances of the two nanoꢀsized TSꢀ1 samples exhibit the Certain commercial names are mentioned in this manuscript
same trend with what we supposed according to the XANES only for the purpose of illustration and do not represent an
and UVꢀvis spectra. TSꢀ1ꢀAC shows a much higher TOF than endorsement by the National Institute of Standards and
TSꢀ1ꢀNA. The increase of the content of pentahedrally Technology.
coordinated Ti, but not tetrahedrally coordinated Ti, is
considered as the main reason for the improvement of catalytic
activity.
Notes and references
a
State Key Laboratory of Fine Chemicals, PSUꢀDUT Joint Center for
Energy Research, School of Chemical Engineering, Dalian University of
Technology, Dalian 116024, PR China
b
EMS Energy Institute, PSUꢀDUT Joint Center for Energy Research and
0
0
0
0
.8
.6
.4
.2
Department of Energy & Mineral Engineering, Pennsylvania State
University, University Park, Pennsylvania 16802, USA
TS-1-NA
261 nm
c
National Institute of Standards and Technology, Gaithersburg, MD
2
*
0899, USA
Corresponding Authors
X. Guo. Tel.: +86 411 84986133. Fax: +86 411 84986134. Eꢀmail:
guoxw@dlut.edu.cn
Y. Chen. Tel.: +1 814 8659834. Eꢀmail: yzc2@psu.edu
0
0
.0
.6
TS-1-AC
Electronic Supplementary Information (ESI) available: Standard Ti Lꢀ
2
39 nm
edge XANES spectra of different coordinated Ti, and the characterization
of samples by Xꢀray powder diffraction, Fourierꢀtransform infrared
spectroscopy, nitrogen physisorption and transmission electron
microscopy were settled in the Electronic Supplementary Information,
accompanied with the calculation process of different coordinated Ti
0
0
0
.4
.2
.0
contents
and
their
TOFs
in
smallꢀcrystal
TSꢀ1.
See
DOI: 10.1039/b000000x/
200
250
300
350
400
450
Wavelength ( nm )
1
2
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. Conclusions
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in TSꢀ1, but pentahedrally coordinated Ti is considered to be
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Foundation (2014M551094), the program for the New Century
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