Alkoxysilylation of Ti-MWW lamellar precursors into interlayer pore-expanded titanosilicates

Article abstract of DOI:10.1039/b910886f

Silylation of Ti-MWW lamellar precursor and subsequent calcination constructed an interlayer expanded structure, leading to novel titanosilicates with large pores. The silylating agents suitable for pore expansion were diethoxydimethylsilane, trimethylethoxysilane and triethoxymethylsilane containing methyl groups, which inhibited the intermolecular condensation of silanes effectively. In contrast to well-known 3D Ti-MWW with only medium pores of 10-membered rings, the silylation led to new crystalline structures with more open pores by ca. 2.5 A, as evidenced by the shift of layer-related diffractions to the lower-angle region in XRD patterns and the enlarged interlayer pores in HRTEM images. The interlayer expanded Ti-MWW was prepared readily from the corresponding hydrothermally synthesized precursors with a wide range of Ti contents (Si/Ti = 20-100). In addition, the pore expansion by silylation was realized under mild acid conditions with 0.1 M HNO₃. The interlayer expanded Ti-MWW exhibited 3-7 times higher turnover number than 3D Ti-MWW in the oxidation of cyclohexene with H₂O₂. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b910886f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Alkoxysilylation of Ti-MWW lamellar precursors into interlayer
pore-expanded titanosilicates†
a
a
a
a
a
a
Lingling Wang,ab Yong Wang, Yueming Liu, Haihong Wu, Xiaohong Li, Mingyuan He and Peng Wu
*
*
Received 3rd June 2009, Accepted 3rd September 2009
First published as an Advance Article on the web 24th September 2009
DOI: 10.1039/b910886f
Silylation of Ti-MWW lamellar precursor and subsequent calcination constructed an interlayer
expanded structure, leading to novel titanosilicates with large pores. The silylating agents suitable for
pore expansion were diethoxydimethylsilane, trimethylethoxysilane and triethoxymethylsilane
containing methyl groups, which inhibited the intermolecular condensation of silanes effectively. In
contrast to well-known 3D Ti-MWW with only medium pores of 10-membered rings, the silylation led
˚
to new crystalline structures with more open pores by ca. 2.5 A, as evidenced by the shift of layer-related
diffractions to the lower-angle region in XRD patterns and the enlarged interlayer pores in HRTEM
images. The interlayer expanded Ti-MWW was prepared readily from the corresponding
hydrothermally synthesized precursors with a wide range of Ti contents (Si/Ti ¼ 20–100). In addition,
the pore expansion by silylation was realized under mild acid conditions with 0.1 M HNO3. The
interlayer expanded Ti-MWW exhibited 3–7 times higher turnover number than 3D Ti-MWW in the
oxidation of cyclohexene with H2O2.
synthesis or by postsynthesis; for example, Ti-Beta,10,11
Ti-ZSM-12,12 Ti-MOR,13 Ti-ITQ-7,14 Ti-MCM-6815 and
Ti-MWW,16–18 all of which possess 12-MR channels or cages.
Among the above titanosilicates, Ti-MWW has attracted
particular interest because not only is Ti-MWW itself a prom-
ising catalyst for epoxidation17 and ammoximation,19,20 but also
the structural diversity of the MWW topology may convert
Ti-MWW into titanosilicates with large porosity, and expand its
abilities in catalytic applications. Structurally the same as the
well-known MCM-22 aluminosilicate, the pore system of
Ti-MWW is composed of 12-MR side cups and two independent
interlayer and intralayer 10-MR channels, one of which contains
the 12-MR supercages.21,22 Although it is characteristic of
reversible structural interchange between 3-dimensional (3D)
crystalline structure and the lamellar precursor,23 once the 3D
MWW structure is formed from the lamellar precursor through
interlayer dehydration/condensation, the accessibility of the
bulky molecules to the supercages is seriously restricted by the
10-MR pore window of the entrance. This limits the catalytic
potential of the Ti active sites inside the supercages in the case of
the oxidation of large molecules. Thus, the techniques of
delamination, pillaring and interlayer expansion have been used
to convert Ti-MWW into titanosilicates with open reaction
spaces, such as the fully delaminated material Del-Ti-MWW,24
the partially delaminated material Ti-MCM-56,25 the interlayer
pillared microporous-mesoporous hybrid material Ti-MCM-3626
as well as the interlayer expanded material Ti-YNU-1.27,28 In
comparison with 3D Ti-MWW, these titanosilicates generally
show improved turnover number for the epoxidation of cyclic
alkenes, as they not only preserve the basic structure unit of
MWW zeolite but also possess more open pores or more acces-
sible surfaces.
Introduction
The high reactivity of the oxirane ring makes epoxides industri-
ally important organic intermediates for producing poly-
urethanes, surfactants, epoxy adhesives, corrosion-protecting
reagents, and additives, etc.1 Conventional methods for epoxide
synthesis are facing drawbacks such as the use of hazardous
halogen reagents, co-production of salts with low value,
discharge of a large quantity of wastewater, and severe corrosion
of apparatus. Environmentally benign methods are thus required
urgently. It is well known that crystalline titanosilicates are
highly active and selective catalysts for the production of
oxygenated derivatives through the oxidation of hydrocarbons
and alcohols, and the ammoximation of ketones with H2O2 as an
oxidant, in which water is the sole byproduct.2–8 The represen-
tative titanosilicate is TS-1 with the MFI structure first reported
two decades ago.2 Extensive efforts made since then have led to
the successful applications of the TS-1/H2O2 catalytic system in
innovative commercial processes for clean production of cyclo-
hexanone oxime and propylene oxide.9
However, TS-1 suffers a major problem that its medium pores
of 10-membered rings (MR) impose serious mass transfer limi-
tations to substrates with relatively large molecular sizes. With
the purpose to overcome the problems that TS-1 encounters in
processing bulky substrates, researchers have developed many
other titanosilicates with larger pores, either by hydrothermal
aShanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, North
Zhongshan Rd. 3663, Shanghai, 200062, China. E-mail: pwu@chem.
ecnu.edu.cn; ymliu@chem.ecnu.edu.cn; Fax: +86-21 62232292
bSchool of Urban Development and Environmental Engineering, Shanghai
Second Polytechnic University, Jinhai Rd. 2360, Pudong, Shanghai,
201209, China
Nevertheless, the preparation procedures of Del-Ti-MWW
and Ti-MCM-36 involve a troublesome sonication step difficult
† This paper is part of a Journal of Materials Chemistry theme issue on
Green Materials. Guest editors: James Clark and Duncan Macquarrie.
8594 | J. Mater. Chem., 2009, 19, 8594–8602
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for scale-up synthesis, and also require the use of expensive
reactants (cetyl trimethyl ammonium salts and tetrapropyl
ammonium hydroxide). Ti-MCM-56 can be postsynthesized
simply by controlling the acid treatment of the MWW precursor,
but it lacks structural periodicity along the layer stacking direc-
tion, which leads to partial blocking of the intralayer chan-
nels.25,29 On the other hand, Ti-YNU-1 was obtained incidentally
when refluxing with acid solution the Ti-MWW precursors
postsynthesized through a reversible structural conversion
route.27 With an expanded interlayer spacing and well ordered
stacking of MWW layers, Ti-YUN-1 proves to be a highly active,
selective, and stable catalyst in the liquid-phase epoxidation of
bulky alkenes. However, the formation of Ti-YNU-1 depends
greatly on the Ti content of the precursors. It is obtained only
when the Si/Ti ratio is higher than 70. It is expected that new
methodology will be developed which can overcome this limita-
tion and is possible to synthesize interlayer expanded materials
independent of the Si/Ti ratio in Ti-MWW lamellar precursors.
Thereafter, a structure elucidation by using the combination of
HRTEM techniques and computational approach suggests that
the structure of Ti-YNU-1 is probably constructed through
interlayer pillaring with Si or Ti species.30
(Me3SiOEt) and tetraethyl orthosilicate (Si(OEt)4, TEOS).
Typically, 1 g of precursor was mixed with 20–50 g of an aqueous
solution of HNO3 (0–2 M) and the desired amount of silylating
agent. The mixture was then refluxed at 373 K for 20 h to induce
silylation. The amount of alkoxysilane was varied in the range
0–0.3 g per gram of precursor. The silylated sample was filtered
off and washed with deionized water repeatedly and dried at
393 K for 10 h. The samples were further calcined in air at 823 K
for 10 h to remove any organic species. The resulting interlayer
expanded zeolites are denoted as IEZ-Ti-MWW. As a control
experiment, the precursors were also refluxed in 2 M HNO3
solution and calcined at 823 K to obtain the usual titanosilicate
with the 3D MWW, Ti-MWW.
Characterization methods
X-Ray powder diffraction (XRD) patterns were measured on
a Bruker D8 ADVANCE diffractometer with Cu Ka radiation.
UV-visible diffuse reflectance spectra were recorded on a
Shimadzu UV-2400PC spectrophotometer with BaSO4 as
a reference. Scanning electron microscopy (SEM) was performed
on a Hitachi-4800 instrument after suspending the sample in
ethanol. High resolution transmission electron microscopy
(HRTEM) measurements were carried out with a 300 kV TEM
This stimulated us to propose a feasible strategy to expand the
interlayer space of layered zeolites in one step through alkoxy-
silylating the corresponding lamellar precursors.31 Such
a simple soft-chemical methodology expands efficiently the pore
structure and avoids stepwise modifications such as interlayer
expansion by ion-exchange with cationic surfactants, silylation,
alcoholysis, and subsequent condensation usually required by
layered silicas. With this unique interlayer silylation method,
a series of interlayer expanded zeolites (IEZ) have been obtained
successfully from the precursors of MWW, CDO, FER, MCM-
47.31–34 Hence this method should open new possibilities for
applying layered zeolites to petrochemical and fine-chemical
industries. Since titanosilicates particularly with a large porosity
are considered to weaken the adsorption restriction and diffusion
limitation caussed by the micropores of zeolites, and they are
expected to be promising oxidation catalysts for processing large
molecules, large-pore titanosilicates are urgently required in
heterogeneous oxidation. By using the feasible technique previ-
ously developed to post-construct new zeolite structures,31 we
have carried out the interlayer silylation of the lamellar precur-
sors of Ti-MWW to prepare IEZ-Ti-MWW in this study. Its
structure and porosity have been characterized, and its catalytic
properties in the oxidation of alkenes have been investigated to
show its potential application as a green heterogeneous catalyst.
(JEOL-3010, Cs 0.6 mm, resolution 1.7A). The 13C and 29Si MAS
˚
NMR spectra were recorded on a Bruker DSX 300 multinuclear
solid-state nuclear magnetic resonance spectrometer. FT-IR
spectra were collected on a Nicolet Fourier transform infrared
spectrometer (NEXUS 670) at room temperature using the KBr
technique. Inductively coupled plasma (ICP) analysis was
performed on a Thermo IRIS Intrepid II XSP atomic emission
spectrometer. The elemental analyses for carbon and nitrogen
contents were performed on an Elementar VarioEL III CHN
elemental analyzer.
Catalytic reactions
The liquid-phase oxidation of cyclohexene with H2O2 was
carried out to check the catalytic ability of titanosilicates for the
oxidation of bulky molecules. The reactions were performed in
a round-bottom flask (20 mL) equipped with a condenser. The
temperature was controlled with a water bath. The reaction
mixture containing 0.05 g of catalyst, 10 mL of acetonitrile as
solvent, 10 mmol of cyclohexene or 1-hexene and 10 mmol H2O2
(31 wt% aqueous solution) as oxidant was stirred vigorously at
333 K for 2 h. The products were analyzed on a gas chromato-
graph (Shimadzu 14B, FID detector) equipped with a 30 m DB-1
capillary column using cyclohexanone as an internal standard.
The amount of unconverted H2O2 was determined by the
standard titration method using a 0.1 M Ce(SO4)2 solution.
Experimental
Synthesis of materials
Results and discussion
Ti-MWW lamellar precursors with various Si/Ti ratios were
hydrothermally synthesized using hexamethyleneimine (HMI) as
a structure directing agent (SDA) from the gels with the molar
compositions of 1.0 SiO2 : (0–0.033) TiO2 : 0.67 B2O3 : 1.4 HMI :
19 H2O according to the previously reported procedures.35 The
precursors were alkoxysilylated with different types of silylating
agents such as diethoxydimethylsilane (Me2Si(OEt)2, DEDMS),
triethoxymethylsilane (MeSi(OEt)3), trimethylethoxysilane
Interlayer silylation of Ti-MWW lamellar precursors
Ti-MWW lamellar precursor synthesized with a Si/Ti molar ratio
of 20 was subjected to alkoxysilylation in acidic media with the
purpose to prepare highly crystalline materials with expanded
pores. Diethoxydimethylsilane (DEDMS) with two ethoxy
groups was first chosen to connect two up-and-down layers
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relatively broad ones, implying that it may possess ordered
linkages between the MWW sheets rather than the hydrogen-
bonded connection in the precursor. The structure of the sily-
lated sample was thermally stable, as the calcination at 823 K did
not cause obvious changes in its XRD pattern (Fig. 1d). The
˚
silylated sample had a layer spacing of 27.58 A (Table 1), which
was slightly larger than that of the lamellar precursor. As the unit
cell parameters along the a and b axes were almost same for the
precursor, 3D MWW and the silylated sample, the structural
change is considered to occur mainly along the direction of layer
stacking. In comparison with 3D MWW, the silylation expanded
˚
the pore windows at least by ca. 2.5 A between the layers, leading
Fig.
1 XRD patterns of (a) as-synthesized Ti-MWW precursor
to interlayer expanded zeolite of MWW sheet-containing tita-
nosilicate, IEZ-Ti-MWW. The N2 adsorption indicated that
IEZ-Ti-MWW with an open porosity had a higher specific
surface area than 3D Ti-MWW (Table 1).
(Si/Ti ¼ 20), (b) sample a directly calcined (3D MWW), (c) sample a sily-
lated with DEDMS, and (d) sample c further calcined (IEZ-Ti-MWW).
probably via condensation with the surface silanol groups.
Meanwhile, the presence of two methyl groups would avoid the
intermolecular condensation of the silane molecules themselves.
Fig. 1 shows the XRD patterns of the lamellar precursor as well
as the DEDMS-silylated samples before and after calcination.
The pattern of the precursor was fully consistent with that of
MWW-type lamellar precursor, which showed typical [001] and
[002] diffractions in the 2q region of 3–7ꢀ, characteristic of
a layered structure along the c-direction (Fig. 1a). The layer
structure is closely related to the interlayer occluded organic
SDA molecules which serve as the pillaring species. A direct
calcination of the lamellar precursor in air completely burned off
the organic species, leading to the conventional 3D MWW
structure.35 Thus, it displayed an XRD pattern greatly different
from that of the precursor (Fig. 1b). The [001] and [002]
diffractions almost disappeared, while the diffractions assigned
to the indexes of [h00] and [hk0], i.e. [100] and [310], remained
practically unchanged. This implies that the structural change
essentially takes place along the c-axis. The direct calcination
caused the dehydroxylation and condensation between the
Using the lamellar precursor synthesized at a Si/Ti ratio of 20
as a parent sample, we investigated in detail the effect of silyla-
tion conditions on the structural transformation to IEZ-Ti-
MWW. The formation of ordered structure of interlayer
expansion depended on the used amount of silylating agent. As
shown in Fig. 2, the 3D Ti-MWW sample was obtained when the
lamellar precursor was refluxed in 2 M HNO3 and further
calcined at 823 K (Fig. 2b). The diffraction due to the [002] plane
shifted to higher angle and was almost invisible as it was hidden
by the [100] diffraction. The silylation treatments of the
precursor with different amounts of DEDMS all led to the IEZ-
Ti-MWW structure showing [002] diffraction, the intensity of
which increased gradually with increasing amount of DEDMS.
When the amount of DEDMS used was 0.025 g per gram of the
˚
layers, and then made the layer spacing decrease from 26.93 A for
˚
the precursor to 25.09 A for the 3D Ti-MWW (Table 1). As
a result, the calcined sample had a narrower pore entrance
between the layers than that of the original lamellar precursor.
When the precursor was silylated with DEDMS, the resulting
sample still showed well resolved [001] and [002] diffractions in
the 2q region of 3–7ꢀ with the [100] and [310] diffractions almost
intact (Fig. 1c). Characteristic of a layered structure along the
c-direction, the silylated sample seemed to be structurally similar
to the MWW lamellar precursor obtained by direct hydro-
thermal synthesis. However, the silylated sample showed well
resolved diffractions in the 2q region of 10–25ꢀ instead of
Fig.
2 XRD patterns of (a) as-synthesized Ti-MWW precursor
(Si/Ti ¼ 20), (b) 3D Ti-MWW, and IEZ-Ti-MWW prepared by silylation
with (c) 0.025 g, (d) 0.05 g, (e) 0.07 g, (f) 0.1g, (g) 0.15 g, (h) 0.3 g of
DEDMS per gram of the precursor in 2 M HNO3 after further calcina-
tion in air at 823 K for 10 h.
Table 1 The unit cell parameters and specific surface areas of Ti-MWW lamellar precursor, 3D Ti-MWW and IEZ-Ti-MWW
a
˚
Unit cell parameters (A)
b
Sample
a
b
c
SSA (m2 gꢁ1
)
Ti-MWW lamellar precursor
3D Ti-MWW
IEZ-Ti-MWW
14.15
14.19
14.10
14.15
14.19
14.10
26.93
25.09
27.58
NDc
530
560
a
c
Given by XRD. b SSA, specific surface area measured by N2 adsorption at 77 K. Not determined.
8596 | J. Mater. Chem., 2009, 19, 8594–8602
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precursor, the intensity of the [002] diffraction was very weak,
indicating that the amount of DEDMS was not enough to
connect the up-and-down layers. The more DEDMS used, the
more intense [002] diffraction was observed as a result of the
gradually expanded layer structure along the c-direction
(Fig. 2c–h). This indicates that it is possible to control the phase
purity and crystallinity of IEZ-Ti-MWW by adjusting the
extent of silylation. When the amount of DEDMS was more than
0.15 g per gram of the precursor, the intensity of the [002]
diffraction displayed no obvious distinction, suggesting that
a saturated incorporation of silane groups could be realized at an
2850 cmꢁ1 bands were still observed after the silylation although
they decreased slightly in intensity. The silylation, however,
resulted in two bands at 2970 and 850 cmꢁ1 (Fig. 3b–g). These
bands were absent for the sample obtained by the acid treatment
without DEDMS, and are assigned to the CH3 asymmetric
stretching vibration and the rocking of CH3 groups attached to
Si, respectively.37,38 This confirmed the incorporation of the
(CH3)2Si moiety into the material. The intensity of the 2970 and
850 cmꢁ1 bands increased with increasing amount of DEDMS,
and was almost leveled off when the weight ratio of DEDMS to
precursor was over 0.15 g gꢁ1
.
optimal weight ratio of silane to precursor of 0.15 g gꢁ1
.
The amount of DEDMS actually incorporated was analyzed
by elemental analysis (Table 2). The precursor had a C/N molar
ratio of 5.88, which was very close to that of HMI, suggesting the
organic species existed inside the zeolite channels in the form of
HMI molecules. In comparison with the precursor, the silylation
in acid solution made the contents of both carbon and nitrogen
decrease as a result of removal of HMI species located mainly in
the interlayer space (Table 2, Nos. 2–5). Nevertheless, the carbon
content of the silylated samples increased slightly with increasing
amount of DEDMS, resulting in gradually increased C/N ratio
(Table 2, fifth column). This can be taken as evidence for the
incorporation of the silylating agent. The amount of carbon
actually gained by silylation was obtained by extracting from the
total carbon content sample with the remaining HMI species
which was determined on the basis of the nitrogen content (Table
2, sixth column). The number of DEDMS molecules incorpo-
rated per unit cell was then calculated according to the incor-
porated carbon and the chemical composition of samples
(Table 2, seventh column). According to the structure of the
MWW topology,21 every unit cell has two pillaring sites between
the layers. When the weight ratio of DEDMS to precursor was
over 0.15 g gꢁ1, the silane molecules incorporated per unit cell
were enough to consume the interlayer silanols to induce struc-
tural expansion, and even to silylate other sites such as surface
silanols and/or internal silanols on defect sites.
IR spectroscopy was employed to monitor the silylation
processes. Fig. 3 shows the IR spectra of the samples prepared by
silylating the lamellar precursor with different amounts of
DEDMS in 2 M HNO3 solution but without calcination. The
acid treated Ti-MWW precursor in the absence of DEDMS
showed the bands at 2926 and 2850 cmꢁ1 (Fig. 3a), which are due
to the stretching vibration of CH2 groups originating from the
HMI molecules incorporated in the pores during precursor
crystallization. The HMI molecules are reported to occupy both
the interlayer space and the intralayer 10-MR channels.36 The
cyclic HMI molecules with a relatively large dimension were not
extracted completely by the acid treatment, as the organic
molecules were confined firmly in the intralayer sinusoidal
channels in addition to the interlayer pores. Thus, the 2926 and
Fig. 4 shows the XRD patterns of IEZ-Ti-MWW prepared by
the silylation with 0.15 g DEDMS per gram of the precursor in
different concentrations of HNO3 to investigate the influence of
acid concentration on the formation of IEZ-Ti-MWW. When the
amount of DEDMS was fixed at 0.15 g per gram of Ti-MWW
lamellar precursor, IEZ-Ti-MWW was obtained in a wide range
of acid concentration. The acid concentration did not show
obvious influence on the intensity of the [002] diffraction,
Fig. 3 FT-IR spectra of (a) acid treated Ti-MWW with 2M HNO3, and
IEZ-Ti-MWW prepared by silylation with (b) 0.025 g, (c) 0.05 g, (d)
0.07 g, (e) 0.1 g, (f) 0.15 g, (g) 0.3 g of DEDMS per gram of the precursor
in 2 M HNO3 without calcination. The silylation resulted in bands at
2970 and 850 cmꢁ1 assigned to the asymmetric vibration CH3 groups and
the rocking vibration of Si–CH3, respectively.
Table 2 The results of chemical analyses after the silylation of Ti-MWW lamellar precursor with different amounts of DEDMS
Amount (wt%)
DEDMS incorporatedd (molecule
per unit cell)
No.
DEDMSa (g gꢁ1
)
C
N
C/Nb
C incorporatedc (wt%)
1
2
3
4
5
0
10.23
8.70
8.95
9.13
9.26
2.03
1.48
1.46
1.45
1.46
5.88
6.86
7.15
7.38
7.40
0
0
0.05
0.10
0.15
0.30
1.09
1.44
1.67
1.75
1.9
2.6
3.0
3.1
a
The amount of DEDMS used per gram of lamellar precursor in the silylation. b Molar ratio. The precursor had a C/N molar ratio of 5.88, similar to
that of SDA of HMI. c Calculated by deducting the C amount of HMI from the total amount of C according to the equation: C ꢁ (N/14) ꢂ 6 ꢂ 12.
d
Every two incorporated C atoms correspond to one DEDMS molecule.
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Fig. 4 XRD patterns of IEZ-Ti-MWW prepared by the silylation with
0.15g DEDMS per gram of the precursor in different concentration of
HNO3 (a) 0.1 M, (b) 0.3 M, (c) 0.5 M, (d) 1 M, (e) 2 M and further
calcined in air at 823 K for 10 h.
Fig. 6 XRD patterns of (a) lamellar precursor, (b) 3D Ti-MWW, and
IEZ-Ti-MWW prepared by the silylation with (c) Si(OEt)4, (d)
MeSi(OEt)3, (e) DEDMS, (f) Me3SiOEt and further calcined in air at
823 K for 10 h.
agents (Fig. 6), indicating these silanes are all applicable to
induce the interlayer expansion. In the process of silylation, the
molecules of monomeric silanes react with the silanol groups on
the layer surface to realize incorporation of silane groups and
formation of Si–O–Si linkages. The subsequent calcination burns
off the methyl groups coordinated to the Si atoms as well as the
SDA species occluded in the pores, leading to a new pore
structure which is composed of the Si(OH)2 pillar together with
the original framework T atoms.31 According to this possible
mechanism of interlayer silylation and pore expansion, it is
anticipated that the above four monomeric silylating agents can
serve as pore expansion pillars. Nevertheless, TEOS with four
ethoxy groups was inclined to impose a serious intermolecular
condensation after hydrolysis in acid solution, which led to the
deposition of amorphous silica on the zeolite crystals. This may
influence the catalytic activity of the corresponding IEZ-Ti-
MWW, to be shown later. Although trimethylethoxysilane has
only one ethoxy group available for the silylation with silanol, it
is presumed that once the Me3Si– group was attached to a MWW
sheet through Si–O–Si bonding, the connection to the other sheet
also via Si–O–Si bonding could be achieved during the process of
CH3 group removal by calcination.
indicating the interlayer expansion can be realized effectively
under mild conditions in 0.1 M HNO3 solution.
Fig. 5 shows the XRD patterns of the Ti-MWW samples with
various Si/Ti molar ratios prepared without or with the silylation
treatment. The 3D Ti-MWW samples were obtained readily from
the corresponding lamellar precursors by calcination at 823 K in
air, which made the layer-related [001] and [002] diffractions
disappear completely (Fig. 5A). However, when the precursors
were silylated with DEDMS at a silane to precursor weight ratio
of 0.15 g gꢁ1 in 1 M HNO3 solution, IEZ-Ti-MWW materials
with different Si/Ti molar ratios were obtained. They were all
featured by developing the [001] and [002] diffractions (Fig. 5B),
and then by expanded pore windows between the layers. It is
worth mentioning that the formation of the IEZ structure by the
present silylation method was independent of the Si/Ti molar
ratio in the precursors which could be hydrothermally synthe-
sized. This would overcome the limitation encountered in the
preparation of Ti-YNU-1, in which the expanded pore structure
can only be constructed for the postsynthesized lamellar
precursors with Si/Ti > 70.27,28
In addition to DEDMS, other monomeric silylating agents
such as tetraethoxysilane (TEOS), triethoxymethylsilane and
trimethyethoxylsilane were also employed to induce the structure
expansion. The molar amount of the different types of silylating
agent used was comparable to 0.15 g of DEDMS per gram of
precursor. The intensity of the [002] diffraction had no obvious
distinction among the samples prepared using different silylating
Characterization of IEZ-Ti-MWW
The UV-visible spectra of the as-synthesized Ti-MWW
precursor, 3D Ti-MWW and IEZ-Ti-MWW are shown in Fig. 7.
Fig. 5 XRD patterns of (A) 3D Ti-MWW and (B) IEZ-Ti-MWW
produced from the same lamellar precursors hydrothermally synthesized
at Si/Ti molar ratios of (a) 30, (b) 50, (c) 70, (d) 100.
Fig. 7 UV-visible spectra of (a) as-synthesized Ti-MWW (Si/Ti ¼ 20),
(b) 3D Ti-MWW, and (c) IEZ-Ti-MWW.
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The precursor synthesized at a Si/Ti molar ratio of 20 exhibited
a main band at 260 nm together with a weak shoulder around
220 nm (Fig. 7a). The 220 nm band, resulting from the charge
transfer from O2ꢁ to Ti4+, has been widely found for
Ti-substituted zeolites and is characteristic of tetrahedrally
coordinated Ti species highly dispersed in the framework.4,5 The
260 nm band has been attributed to octahedral Ti species. The
occurrence of the 260 nm band for the present samples is
presumed to be closely related to the layered structure of the
MWW precursor irrespective of the Si/Ti ratio in synthetic gels.35
The layer surface is favorable for the incorporation of octahedral
Ti therein. These octahedral Ti species are usually removed
selectively by refluxing the precursor in acid solution before
calcination, resulting in 3D Ti-MWW which showed only the
220 nm band assigned to tetrahedral Ti (Fig. 7b). The acid
treatment at 373 K for 20 h in the presence of silylating agent also
extracted the extraframework species effectively (Fig. 7c), sug-
gesting the presence of DEDMS had no negative effect on the
removal of the octahedral Ti species. This phenomenon was
clearly observed for the IEZ-Ti-MWW samples prepared with
different amounts of silylating agent, as they showed only the
narrow 220 nm band in the UV-visible spectra (Fig. 8).
Fig. 10 13C MAS NMR spectra of (a) Ti-MWW lamellar precursors,
and (b) after silylation with DEDMS in 2 M HNO3 without calcination.
The silylation resulted in a signal at ꢁ2.0 ppm assigned to carbon species
in Si(CH3)2 groups.
assumed to take place at a sub-nanometer level but not
a micrometer scale of the crystals.
The incorporation of (CH3)2Si groups into the lamellar
precursor by silylation has been further investigated by 13C and
29Si MAS NMR techniques. The 13C MAS NMR spectra of
Ti-MWW lamellar precursor and the silylated sample without
calcination both showed resonances at 48.5, 57 and 26.5 ppm
(Fig. 10). The former two resonances, assigned to the carbon
species at C1 position of HMI, are presumably associated with
the residence of HMI in two distinct void spaces, that is, inter-
layer channels and intralayer channels.36 The latter one is
attributed to the overlapped signals of the carbon species at the
C2 and C3 positions of HMI. What is more attractive is that the
silylation developed a new resonance at ꢁ2.0 ppm in the 13C
MAS NMR spectrum (Fig. 10b), which is due to the configura-
tion of (CH3)2Si groups.37
The crystals of Ti-MWW precursor showed the morphology of
platelets (ca. 1 ꢂ 1 ꢂ 0.1 mm) which generally formed aggregates
of 10–15 mm size (not shown). 3D Ti-MWW obtained by the acid
treatment of the precursor showed the same morphology
(Fig. 9a). Upon acid refluxing in the presence of silylating agent,
that is, after forming the interlayer expanded crystalline struc-
ture, no change in morphology was observed (Fig. 9b). These
results confirmed that the morphologies were preserved after the
chemical modification. The structural expansion is therefore
The 29Si MAS NMR spectra provided structural information
complementary to that given by the above diffraction techniques.
The Ti-MWW lamellar precursor and the silylated sample
without calcination showed the main resonances at ꢁ110
and ꢁ120 ppm (Fig. 11), which are attributed to the silicon atoms
coordinated with four silicon atoms (Q4). In addition, the
precursor showed an obvious resonance at ꢁ103 ppm due to the
Fig. 8 UV-Visible spectra of (a) 3D Ti-MWW, and IEZ-Ti-MWW
prepared by silylation with (b) 0.025 g, (c) 0.05 g, (d) 0.07 g, (e) 0.1 g, (f)
0.15 g, (g) 0.3 g of DEDMS per gram of the precursor (Si/Ti ¼ 20) in 2 M
HNO3 and further calcined in air at 823 K for 10 h.
Fig. 11 29Si MAS NMR spectra of (a) Ti-MWW lamellar precursor, (b)
after silylation with DEDMS in 2 M HNO3 without calcination, and (c)
after further calcination at 823 K. The silylation consumed a portion of
the silanol groups (Q3) and developed at ꢁ12 ppm a signal assigned to
]Si–(CH3)2 groups (D2) instead, which were converted to ]Si(OH)2
groups (Q2) around ꢁ90 ppm after calcination.
Fig. 9 SEM images of (a) Ti-MWW, (b) IEZ-Ti-MWW.
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Q3 site, Si(OH)(OSi)3 or Si(OH)(OTi)(OSi)2. Compared with
the precursor, the silylated sample showed a much less intense
ꢁ103 ppm resonance, while the samples exhibited a new reso-
nance at ꢁ12 ppm (Fig. 11b). This signal is attributed to the
silicon atom from Si(CH3)2(OSi)2 groups (D2).39 The results
further confirmed that the silylation occurred through the reac-
tion of silane groups with the silanols on the layer surface to form
pillaring silicon species between the layers. When the silylated
sample was further calcined at 823 K, the organic species
including the methyl groups and remaining HMI were burned off
completely. The D2 signal disappeared, whereas the signal
corresponding to the Q2 groups, Si(OH)2(OSi)2, was observed at
ca. ꢁ90 ppm. This suggests that the incorporated silane groups
lost the methyl moieties, but the Si species cross-linked to the
MWW sheets remained in the interlayer spaces to pillar
the expanded pore structure.
The changes in pore structure as a result of silylation were
investigated by HRTEM measurement. The TEM image taken
of the platelet crystal surface, that is along the [001] incidence, is
shown in Fig. 12a together with the corresponding Fourier dif-
fractogram (FD). IEZ-Ti-MWW had ordered arrays of 12-MR
side cups or side pockets in this projection, which is almost the
same as that of the normal 3D MWW structure reported previ-
ously, and reveals the characteristic hexagonal framework
topology of MWW type materials.27,30 The image did not show
any aperiodically or periodically repeating defects. It is thus
apparent that the silylation did not alter the structure within the
MWW sheets. The TEM image and FD taken perpendicular to
the side of the platelet, that is along the [100] incidence, indicated
that IEZ-Ti-MWW also showed a well ordered pore structure
along the stacking direction of MWW sheets (Fig. 12b). It is
concluded that analogous to the 3D MWW topology, IEZ-Ti-
MWW also has a 3D-connected framework structure composed
of MWW sheets. This should be a result of uniform silylation
achieved throughout the crystals. The average layer spacing of
˚
˚
IEZ-Ti-MWW was 27.4 A, while that of 3D Ti-MWW was 25 A
as reported previously.30 In agreement with the data given by
XRD (Table 1), the silylation caused an expansion in the c
˚
direction of ca. 2.4 A. This would contribute mainly to the
enlargement of pore windows between the layers in IEZ-Ti-
MWW. Although the real structure of IEZ-MWW is still waiting
for structural resolution by synchrotron X-ray diffraction,
a 12-MR pore window is suggested to be constructed by inserting
two pillaring Si species between the layers, instead of 10-MR
pores of 3D MWW.
Fig. 12 TEM images of IEZ-Ti-MWW taken along (a) [001] incidence
and (b) [100] incidence. FDs are also inset in the images.
On the basis of the aforementioned XRD, IR, NMR and
HRTEM investigations, the procedures for interlayer silylation,
pore expansion and formation of regular large pores between the
layers are graphically described in Scheme 1 using DEDMS as
a representative silylating agent. In acid solution, the SDA
molecules were extracted from the interlayer space, making the
silane molecules accessible to the silanol groups on the layer
surface to induce silylation. The formation of Si–O–Si bonds
with up-and-down layers resulted in the incorporation of the
silane groups, while the interlayer structure expanded. Finally,
a large-pore titanosilicate, IEZ-Ti-MWW, was obtained.
XRD and HRTEM investigations verified consistently that the
structure of IEZ-Ti-MWW was very similar to that of previously
reported Ti-YNU-1.30 However, the present silylation method is
Scheme 1
independent of the Ti content of precursors, and proves to be
more feasible and useful for the preparation of large-pore tita-
nosilicates.
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Catalytic properties of IEZ-Ti-MWW
Ti-MWW has been reported to be a promising catalyst for the
epoxidation of various linear alkenes with H2O2 as an
oxidant.17,40,41 The IEZ materials, having larger pores than the
3D MWW structure, are expected to show higher catalytic
activities especially for bulky molecules. We thus investigated
the catalytic properties of IEZ-Ti-MWW in the epoxidation of
1-hexene and cyclohexene.
Table 3 compares the results of liquid-phase epoxidation of
1-hexene and cyclohexene with H2O2 between 3D Ti-MWW and
IEZ-Ti-MWW samples prepared with different amounts of
silylating agent. The molecules of linear 1-hexene are small
enough to enter into both the interlayer and intralayer 10-MR
channels of the 3D MWW structure, and reach readily the active
Ti sites without mass transfer limitation. Thus, 3D Ti-MWW and
IEZ-Ti-MWW show no obvious differences in the oxidation of
1-hexene. In contrast, the cyclohexene molecules with a larger
kinetic diameter hardly penetrate the distorted intralayer 10-MR
channels. Actually, the Ti sites in the supercages of 3D MWW
are potential active sites for the oxidation of bulky molecules, but
the entrance of bulky molecules into the supercages is seriously
restricted by the 10-MR windows. Thus, for 3D Ti-MWW, only
the Ti sites on the external surface and within the side pockets are
considered to act as active sites for the oxidation of cyclohexene.
Since the 2D intralayer sinusoidal 10-MR channels are not
connected to the supercages, the interlayer space expansion is the
most effective way to make the Ti sites in the supercages available
for oxidizing bulky substrate molecules. As described above,
a saturated incorporation of silane groups was realized at a silane
to precursor weight ratio of 0.15 g gꢁ1. Consequently, the
accessibility of the Ti sites to cyclohexene molecules was the
highest for the IEZ-Ti-MWW prepared under that condition.
This sample exhibits a turnover number (TON) about 7 times
that of 3D Ti-MWW. But once the addition amount of DEDMS
exceeded 0.15 g gꢁ1, the activity declined probably due to pore
blocking by the condensation of the excess silanes. Therefore, it is
necessary to use an optimal amount of DEDMS to achieve active
catalysts for bulky molecules.
Fig. 13 Comparison of the catalytic activity between 3D Ti-MWW and
IEZ-Ti-MWW with different Si/Ti mole ratios in the epoxidation of
cyclohexene. Reaction conditions: cat., 0.05 g; cyclohexene, 10 mmol;
H2O2, 10 mmol; MeCN, 10 mL; temp., 333 K; time, 2 h.
expected the cyclohexene conversion increased with increasing
amount of Ti active sites for both 3D Ti-MWW and IEZ-Ti-
MWW. However, the increase was very limited in the case of 3 D
Ti-MWW, as only the Ti species are usable for this reaction. IEZ-
Ti-MWW, with expanded interlayer pores and the Ti species
inside the supercages accessible to the cyclohexene molecules,
showed much higher conversion than 3D Ti-MWW. The
distinction became more obvious at a higher Ti content.
Table 4 lists the catalytic properties obtained on the IEZ-Ti-
MWW catalysts which were prepared using different types of
silylating agents. TEOS was inclined to induce serious intermo-
lecular condensation, which led to the deposition of amorphous
silica on the zeolite crystals. The IEZ-Ti-MWW prepared with
TEOS thus showed a lower activity in comparison with the
others. The methyl groups contained in triethoxymethylsilane,
trimethylethoxysilane and DEDMS effectively restrained the
condensation of silane molecules, leading to IEZ-Ti-MWW
showing much higher activity than that prepared with TEOS.
The IEZ-Ti-MWW catalysts also had an advantage in product
selectivity. They showed higher epoxide selectivities (43.3–57.1%)
than conventional 3D Ti-MWW (32.5%).
As a result, the expanded structure enhanced greatly the
catalytic activity of Ti-MWW in the epoxidation of alkenes with
bulky molecular dimensions. This implies that the post-treatment
through alkoxysilylating the Ti-MWW lamellar precursor is an
To further ensure the activity improvement by alkoxysilylation,
3D Ti-MWW and IEZ-Ti-MWW with different Ti contents were
checked in the epoxidation of cyclohexene with H2O2 (Fig. 13). As
Table 3 The catalytic activity obtained on 3D Ti-MWW and IEZ-Ti-MWW synthesized with different amounts of DEDMS in the epoxidation of 1-
hexene and cyclohexenea
Catalyst
Cyclohexene oxidation
1-Hexene oxidation
Product sel.
(mol%)
Product sel.
(mol%)
H2O2
(mol%)
H2O2 (mol%)
DEDMS (g gꢁ1
)
Si/Ti
Conv. (mol%)
TONb
oxide
othersc
conv.
sel.
Conv. (mol%)
oxide
othersc
conv.
sel.
0
68
95
86
94
82
94
105
4.1
9.2
33
105
149
172
159
233
178
41
39
59
41
58
57
36
59
61
41
59
42
43
64
4.6
11.2
20.6
17.0
20.1
19.9
16.0
89
79
72
92
75
98
89
13.1
10.2
13.9
9.1
12.6
12.1
9.8
92.7
95.6
96.8
93.0
95.6
95.0
95.6
7.3
4.4
3.2
7.0
4.4
5.0
4.4
19.6
15.2
20.1
12.0
15.8
17.6
15.1
66.7
63.0
63.8
75.8
79.7
68.8
64.9
0.025
0.05
0.07
0.1
0.15
0.3
14.4
15.3
15.8
20.7
14.2
a
b
Reaction conditions: cat., 0.05 g; alkenes, 10 mmol; H2O2, 10 mmol; MeCN, 10 mL; temp., 333 K; time, 2 h. TON in mol (mol Ti)ꢁ1 which was
calculated by dividing the amount of alkene converted with the used Ti amount. 2-Cyclohexene-1-ol, 2-cyclohexene-1-one, glycols.
c
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Table 4 The catalytic properties shown by 3D Ti-MWW and IEZ-Ti-MWW prepared with different silylating agents in the oxidation of cyclohexenea
Cyclohexene oxidation
Product sel. (mol%)
oxide
H2O2 (mol%)
conv.
Silylating agent
Conv.(mol%)
othersb
sel.
None
TEOS
Triethoxymethylsilane
Trimethyethoxylsilane
DEDMS
4.1
8.8
19.2
19.8
20.7
32.5
43.3
53.5
54.6
57.1
67.5
56.7
46.5
45.4
42.9
5.6
9.4
20.4
21.0
19.9
69.6
92.9
90.3
94.3
98.3
a
Reaction conditions: cat., 0.05 g; cyclohexene, 10 mmol; H2O2, 10 mmol; MeCN, 10 mL; temp., 333 K; time, 2 h. b 2-Cyclohexene-1-ol, 2-cyclohexene-
1-one, glycols.
12 A. Tuel, Zeolites, 1995, 15, 236.
effective way to obtain crystalline zeolites with enlarged pores
useful for fine chemical synthesis.
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14 M. Diaz-Cabanas, L. A. Villaescusa and M. A. Camblor, Chem.
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Conclusions
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Chem. Commun., 2008, 6224.
16 P. Wu, T. Tatsumi, T. Komatsu and T. Yashima, Chem. Lett., 2000,
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17 P. Wu, T. Tatsumi, T. Komatsu and T. Yashima, J. Catal., 2001, 202,
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18 P. Wu and T. Tatsumi, Chem. Commun., 2001, 897.
19 F. Song, Y. Liu, H. Wu, M. He, P. Wu and T. Tatsumi, J. Catal.,
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20 F. Song, Y. Liu, L. Wang, H. Zhang, M. He and P. Wu, Appl. Catal.,
A, 2007, 327, 22.
21 M. E. Leonowicz, J. A. Lawton and S. L. Lawton, Science, 1994, 264,
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22 S. L. Lawton, M. E. Leonowicz and P. D. Partridge, Microporous
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23 L. Wang, Y. Liu, W. Xie, H. Wu, X. Li, M. He and P. Wu, J. Phys.
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24 P. Wu, D. Nuntasri, J. Ruan, Y. Liu, M. He, W. Fan, O. Terasaki and
T. Tatsumi, J. Phys. Chem. B, 2004, 108, 9126.
25 L. Wang, Y. Wang, Y. Liu, L. Chen, S. Cheng, G. Gao, M. He and
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29 G. Juttu and R. Lobo, Microporous Mesoporous Mater., 2000, 40, 9.
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31 P. Wu, J. Ruan, L. Wang, L. Wu, Y. Wang, Y. Liu, W. Fan, M. He,
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32 S. Inagaki and T. Tatsumi, Chem. Commun., 2009, 2583.
33 S. Inagaki, T. Yokoi, Y. Kubota and T. Tatsumi, Chem. Commun.,
2007, 5188.
The alkoxysilylating treatment of Ti-MWW lamellar precursor
leads to new crystalline titanosilicates having an interlayer
expanded structure in comparison with the conventional 3D
Ti-MWW. The expanded structure is formed as a result of the
incorporated Si species serving as pillars. The silylation process is
independent of Ti content in directly synthesized Ti-MWW
precursors, leading to IEZ-Ti-MWW catalysts with a wide range
of Ti content. Compared with 3D Ti-MWW, IEZ-Ti-MWW
contains much more open interlayer spaces, showing a signifi-
cantly improved catalytic activity in the liquid-phase epoxidation
of alkenes with large molecular dimensions. IEZ-Ti-MWW is
expected to open up green oxidation processes for synthesizing
large oxygenated fine chemicals. This study may offer a new
route for designing and preparing large-pore titanosilicate
catalysts.
Acknowledgements
We gratefully acknowledge the NSFC of China (20673038,
20873043), Science and Technology Commission of Shanghai
Municipality (08JC1408700, 09XD1401500, 07QA14017,
08QA1402700), 973 Program (2006CB202508), 863 Program
(2007AA03Z34), and Shanghai Leading Academic Discipline
Project (B409). Y.W. thanks the PhD Program Scholarship Fund
of ECNU 2008. We thank Dr Juanfang Ruan for measuring the
HRTEM images and helpful discussion.
34 J. Ruan, P. Wu, B. Slater, Z. Zhao, L. Wu and O. Terasaki, Chem.
Mater., 2009, 21, 2904.
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