Defect-less, layered organo-titanosilicate with superhydrophobicity and its catalytic activity in room-temperature olefin epoxidation

Article abstract of DOI:10.1039/c2cc30737e

A new type of defect-less, layered organo-titanosilicate is synthesized using a simple, template-free, evaporation-induced self-assembly process. The obtained layered material has superhydrophobicity and exhibits promising catalytic activity in the epoxidation of olefins using 30% H₂O ₂ aqueous solution as oxidant at room temperature.

Full text of DOI:10.1039/c2cc30737e

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Defect-less, layered organo-titanosilicate with superhydrophobicity and
its catalytic activity in room-temperature olefin epoxidationw
Hui Zhou,a Liping Xiao,*a Xiaonao Liu,a Sha Li,a Hisayoshi Kobayashi,b Xiaoming Zhenga
and Jie Fan*a
Received 2nd February 2012, Accepted 22nd May 2012
DOI: 10.1039/c2cc30737e
A new type of defect-less, layered organo-titanosilicate is
synthesized using a simple, template-free, evaporation-induced
self-assembly process. The obtained layered material has super-
hydrophobicity and exhibits promising catalytic activity in the
epoxidation of olefins using 30% H2O2 aqueous solution as
oxidant at room temperature.
a superhydrophobic TS material has not been successfully
prepared.
Here, we report a defect-less, layered organo-titanosilicate
(denoted as LOTS) with covalently linked phenyl groups
through a simple and template-free synthetic route. Comparing
with other TS catalysts, LOTS displays superhydrophobicity,
and exhibits superior catalytic activity in epoxidation of olefins
using 30% H2O2 as oxidant at room temperature.
Layered organic–inorganic hybrid materials have attracted
much attention because of their emerging applications in ion
exchange, adsorption and catalysis.1,2 These novel materials
provide an ordered array of organic moieties with a stable and
robust inorganic matrix. Tailoring of the organic components
and inorganic matrix is very important to optimize their
performance in a particular application. Many research efforts
have focussed on the modification of these layered nanocomposites
by altering the organic groups or inserting another sheet of
inorganic cations (e.g. Ca, Mg, Sn, Al)2,3 into the organosilicate
layers. However, the partial substitution of silicon atoms with other
functional metal cations such as Ti4+ has been rarely reported.
The unique reduction/oxidation properties of titanium-
containing silica-based materials (TS) make them particularly
attractive for heterogeneous catalysis in selective oxidation.4–8
One of the most important findings in this field is that the
catalytic activity is not only related to well-dispersed isolated
tetrahedral Ti(IV) but also to the location of Ti in hydrophobic/
hydrophilic channels or cavities in the structure.9,10 It has been
proven that hydrophobicity can prevent poisoning of the active
site by water as well as unproductive decomposition of H2O2
and then leads to a high catalytic performance.11 Two main
strategies have been developed to synthesize hydrophobic TS
materials via: (1) grafting hydrophobic organic groups onto the
surface;12,13 (2) directly incorporating them into the frameworks.9,14
However, the restricted incorporation of organic content and large
portion of uncondensed silanol groups resulted in a limited
enhancement of hydrophobicity.15–17 To the best of our knowledge,
LOTS is synthesized via a facile one-step method as illustrated
in Fig. 1a. In a typical synthesis, 10 mmol phenyltrimethoxysilane
(PTMS) and 0.7 mmol titanium n-butoxide (TBOT) were mixed in
a acidic ethanol solution with a PTMS : TBOT : HCl : HAc :
EtOH : H2O molar ratio of 1.0 : 0.07 : 1.2 : 4.0 : 53 : 4.1. The
mixture was stirred vigorously for 2 h at room temperature.
A transparent viscous liquid was formed after the ethanol solvent
was completely evaporated. The liquid sample was further aged at
elevated temperature to ensure full condensation of titanosilicate
frameworks. The final LOTS sample is obtained in the form of a
solid powder after grinding.
The layered structure of the LOTS materials is verified by a
complementary combination of X-ray diffraction (XRD) and
transmission electron microscopy (TEM) analysis. The XRD
pattern of LOTS displays two distinct peaks at 2y = 6.72, 19.391
corresponding to (001) and (020)/(110) reflections, which are
characteristic for layered inorganic–organic nanocomposites
a Key Lab of Applied Chemistry of Zhejiang Province, Department of
Chemistry, Zhejiang University, Hangzhou, Zhejiang Province
310027, China. E-mail: lpxiao@zju.edu.cn, jfan@zju.edu.cn;
Fax: 86-571-87952338; Tel: 86-571-87952338
b Department of Chemistry and Materials Technology, Kyoto Institute
of Technology, Matsugasaki, Sakyo-ku, Kyoto, Japan
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc30737e
Fig. 1 (a) Scheme of synthetic procedure for LOTS, (b) TEM image
(XRD pattern as inset) and (c) HRTEM image of LOTS aged at 250 1C.
c
6954 Chem. Commun., 2012, 48, 6954–6956
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(Fig. 1b, inset). The sharp diffraction peak at 2y = 6.721
(d001 = 1.3 nm) can be attributed to a bilayer structure with a
linear arrangement of two phenyl groups. The interlayer
spacing d of LOTS is about 1.3 nm from TEM measurement
(Fig. 1b), in good agreement with XRD characterization. This
value is much larger than that of a layered organosilicate
containing covalently linked phenylene groups (0.76 nm) and is
similar to that with biphenylene groups (1.19 nm),18 suggesting
the existence of a bilayer structural motif of the phenyl groups
in LOTS.
mapping and FTIR spectra. A detailed EDX element mapping
measurement of LOTS at a resolution of B10 nm shows the
uniform X-ray intensity of Ti, Si and C signals throughout the
particles (Fig. S1, ESIw), revealing that Ti atoms are homo-
geneously distributed within the organo-silicate frameworks. A
vibration band at B960–970 cmÀ1 in FTIR is the characteristic
peak of Ti–O–Si for Ti containing silica zeolites24 and related
materials.25 For LOTS samples, there is a new absorption band
at 926 cmÀ1 (Fig. 2c, Fig. S2, ESIw), which is much lower in
intensity than that at B960–970 cmÀ1 for Ti-containing zeolites
and related materials, due to the organic groups covalently
attached to silicon, and can be assigned to the Ti–O–Si hetero-
linkages.26,27 The XPS spectrum of LOTS is shown in Fig. S3
(ESIw). The binding energy for Ti2p3/2 of the sample is centered
at 459.5 eV, well above the typical value of 458.5 eV for anatase
TiO2,27,28 is also taken as a proof of Ti incorporation into the
organo-silica framework in a tetrahedral environment.
The intense and broad peak centred at 2y = 19.391 (d =
0.46 nm) is assigned to the uniform spacing between phenyl
groups (in-plane), which is consistent with the spacing of alkyl
chains (d = 0.47 nm) of the n-alkyltrialkoxysilane monolayer as
revealed by grazing incidence X-ray diffraction.3,19 Molecular
structure simulation also suggested that the spacing of covalently
linked phenylene in an ordered mesoporous benzene-silica hybrid
is about 0.44 nm.20 Here, we first observed the spacing between
phenyl groups of LOTS by HRTEM analysis. As shown in
Fig. 1c, many lattice fringes with a uniform spacing of 0.46 nm
were observed, which directly confirmed the existence of molecular-
scale periodicity within the titano-silicate sheets.
LOTS has been confirmed to be superhydrophobic owing to
the nature of large portion of incorporated organic moieties and
highly condensed organo-titanosilicate frameworks. Contact
angle (CA) measurement is usually used to characterize the
hydrophobic/hydrophilic properties of materials.29 The CA of
LOTS aged at 250 1C is 153 Æ 0.51 (Fig. 2d), suggesting that a
superhydrophobic material has been obtained. The materials
changed from superhydrophobic to hydrophilic after calcination
at 500 1C due to the loss of phenyl groups as confirmed by
thermogravimetric analysis (TGA, Fig. 2d). A B51% weight loss
occurs in the temperature range of 300–700 1C corresponding to
the phenyl functional groups of LOTS. This value is close to the
starting organic species concentration in as-synthesized LOTS
(51.3%), indicating that the phenyl groups can be fully retained
after aging at 250 1C.
Epoxidation of olefins is one of fundamental reactions for
organic chemistry and is of great importance for laboratory
and commercial processes.30,31 In this work, we investigated
the catalytic performance of LOTS in the epoxidation of
cyclohexene and compared it with microporous titanosilicate
TS-1 and mesoporous titanosilicate (m-TS). The detailed
characterizations of TS-1 and m-TS are shown in Fig. S4
and S5, respectively (ESIw). The catalytic reaction was performed
at room temperature by using 30% H2O2 aqueous solution as
oxidant and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as solvent.
The results of catalytic tests are shown in Fig. 3a and b and
Table S1 (ESIw). LOTS aged at 250 1C exhibits an exceptionally
high activity as compared to other TS catalysts. The conversion
of cyclohexene reaches 98.8% in 2 h with a selectivity of 98% to
the epoxide at room temperature. The turnover number (TON)
for LOTS is about 5–10 times as high as that of m-TS (calcined
at 450 1C) or commercial TS-1, indicating that LOTS showed
the highest activity among these titanosilicates. The LOTS
material can be easily recycled and exhibits an almost constant
catalytic performance. After four repeated reaction runs with
regeneration, the cyclohexene conversion slightly decreased
from 98.8 to 93.0%, showing that the prepared organo-
titanosilicate is a relatively stable catalyst (Table S1 and
Fig. S6, ESIw). It is noteworthy that the surface area of LOTS
is only 6.8 m2 gÀ1, which is 50–60 fold less than that of m-TS
The covalently linked phenyl groups of LOTS were confirmed
by solid-state 29Si, 13C MAS NMR spectra and FTIR spectro-
scopy. 29Si MAS NMR for LOTS aged at 250 1C exhibits a strong
peak around À78.5 ppm and a weak peak around at À72.2 ppm,
which are assigned to T3 [RSi(OSi)3] and T2 [RSi(OH)(OSi)2],
respectively.21 It is noted that the proportion of T3 is 490%
(Fig. 2a), which is much higher than in previous reports (40–65%)
for ordered mesoporous organosilica and layered hybrid silica,21,22
suggesting nearly completely condensed organo-titanosilicate
frameworks have been formed. The 13C NMR spectrum also
confirms the presence of phenyl groups in LOTS (Fig. 2b).
FTIR spectroscopy not only indicates the characteristic peaks
of phenyl groups (1431, 1384, 738, 696 cmÀ1), but also confirms
that the Si–C bond (1134 cmÀ1 23
of phenyl organosilane
)
remained intact during the synthesis and aging process (Fig. 2c).
The homogeneous incorporation of titanium into the silica
sheets is revealed by energy dispersive X-ray (EDX) element
Fig. 2 (a) Solid-state 29Si, (b) 13C MAS NMR spectra and (c) FTIR
spectrum for LOTS aged at 250 1C; (d) TG analysis and CA (insets)
for LOTS aged at 250 and 500 1C.
(423 m2
g g
À1) or TS-1 (339 m2 À1). This implies that the
genuine activity of accessible Ti4+ sites of LOTS may be two
c
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Fig. 3 (a) Catalytic activity of LOTS (aged at 250 1C) in epoxidation
of cyclohexene; (b) yield of epoxycyclohexane vs. reaction time using
m-TS, TS-1 and LOTS as catalysts.
orders of magnitude higher than those of other TS catalysts.
LOTS also shows good catalytic performance in methanol or
acetonitrile and its catalytic activity can compete with those of
layered alkoxysilylated Ti-containing silicates32 under the
same reaction condition despite of its much lower surface area
value (Table S2, ESIw). Besides, the better catalytic activity for
LOTS in methanol than that in acetonitrile further confirmed
that LOTS is hydrophobic.33–35 The exceptional catalytic
activity is probably attributable to the superhydrophobicity
resulting from high phenyl group content (Si : phenyl = 1 : 1)
and the fully condensed titanosilicate frameworks.
In summary, we have described a green, simple and template-
free synthetic route for the synthesis of defect-less, organic-inserted
layered titanosilicates. The layered hybrid materials provide a
superhydrophobic surface and well-dispersed titanium species.
They have high thermal stability and display outstanding catalytic
activity in the epoxidation of olefins at room temperature. The
materials are expected to provide new opportunities for applying
superhydrophobic materials in industrial catalytic applications.
We are grateful for financial support from the National Science
Foundation of China (20873122, 21003106), the Innovative Team
Project of Zhejiang Province (2010R50014-06), Science Foundation
of Zhejiang Province (Y4090097), Fok Ying Tung Education
Foundation (131015) and the Fundamental Research Funds from
the Central Universities (2012QNA3014). We thank Dr Yan Xu
and Dr Ningdong Feng for help on NMR measurement.
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