Hydrothermally stable ordered mesoporous titanosilicates with highly active catalytic sites

Article abstract of DOI:10.1021/ja0170044

Ordered mesoporous titanosilicates with high stability and catalytic activity assembled from TS-1 nanoclusters(?). Mesoporous titanosilicates (MTS-9) are successfully prepared in strong acidic media by a two-step synthesis. MTS-9 has an ordered hexagonal structure and exhibits superior hydrothermal stability and high catalytic activity for the oxidation of the small molecules of phenol and styrene and also of the bulky molecule of trimethylphenol. Copyright

Full text of DOI:10.1021/ja0170044

Published on Web 01/15/2002
Hydrothermally Stable Ordered Mesoporous Titanosilicates with Highly Active
Catalytic Sites
Feng-Shou Xiao,* Yu Han, Yi Yu, Xiangju Meng, Miao Yang, and Shuo Wu
Department of Chemistry, Jilin UniVersity, Changchun 130023, China
Received September 4, 2001
There is currently great interest in titanium-containing zeolitic
catalysts for selective oxidation. Since the discovery of microporous
TS-1 and TS-2 by Enichem Company,1 a series of microporous
titanosilicates, Ti-ZSM-12,2 Ti-ZSM-48,3 and Ti-â,4 have been
reported which have remarkable catalytic properties.1-7 However,
one disadvantage of these titanosilicate catalysts is that their pores
are too small for access by bulky reactants in the fine chemical
and pharamaceutical industries. Recent progress in solving this has
been the incorporation of titanium ions into the framework sites of
mesoporous materials (MCM-41, HMS, SBA-15)8-16 and the
grafting of a titanocene complex onto mesoporous silica.17 These
mesoporous titanosilicate materials have pore diameters of 30-60
Å and exhibit catalytic properties for the oxidation of bulky reactants
under mild conditions, but unfortunately, when compared with those
of TS-1 and Ti-â, the oxidation ability and hydrothermal stability
are relatively low, which severely hinders their practical applica-
tions. The relatively low oxidiation ability and hydrothermal
stability, for example, of Ti-MCM-41, can be attributed to the
difference in the titanium coordination environment (amorphous
nature of the mesoporous wall).18 Recently, there had been great
progress in the preparation of mesostructured materials assembled
from nanoclusters, such as mesostructured metal germanium
sulfides19 and mesoporous aluminosilicate nanoclusters.20,21 In our
preliminary work,22 we have reported the synthesis of an ordered
mesoporous titanosilicate (MTS-9) by the assembly of preformed
titanosilicate precursors with triblock copolymers in a strong acidic
media. We demonstrate here that MTS-9 shows excellent hydro-
thermal stability and very high activity for the oxidation of the
smaller molecules of phenol and styrene and also of the bulky
molecule of trimethylphenol.
Figure 1. XRD patterns of (A) MTS-9, (B) Ti-MCM-41 and (C) SBA-
15 (left) before and (right) after hydrothermal treatment in boiling water
for 120 h. (Inset: magnified XRD of MTS-9.)
Table 1. Parameters over MTS-9, Ti-MCM-41, and SBA-15
Samples before and after Treatment in Boiling Water for 120 ha
2
pore size (nm)
wall thickness (nm)
surf. area (m /g)
samples
MTS-9
Ti-MCM-41
SBA-15
before
after
before
after
before
after
8.0
2.7
7.6
8.9
4.8
1.5
3.2
4.3
980
1080
870
720
55
187
a Ti-MCM-41 and SBA-15 were prepared from published procedures.12,14
Pore size distributions were determined by BJH method from N2 adsorption
isotherms at 77 K. The wall thicknesses were calculated as: a0-pore size
(a0 ) 2 × d(100)/31/2).
MTS-9 was hydrothermally synthesized from an assembly of
triblock polymers (P123) with preformed titanosilicate precursors
in a strong acidic media (pH < 1) by a two-step procedure. First,
precursors containing TS-1 nanoclusters were prepared. Second,
the preformed precursors were assembled with triblock copolymers
in a strong acidic media (pH < 1).14,22 The detailed synthesis
procedure for MTS-9 has been published previously.22
X-ray diffraction (Figure 1) and TEM images (Figure 1S) clearly
indicate that MTS-9 has ordered hexagonal arrays of mesopores
with uniform size. Notably, the results of XRD and N2 adsorption
isotherms (Figure 1, Table 1) clearly indicate that MTS-9 (with
surface area of 980 m2/g) retains ordered hexagonal structure (with
surface area of 720 m2/g) even after treatments in boiling water
for 120 h. In comparison, SBA-15 and Ti-MCM-41 lose most of
their mesostructure (with surface area less than 200 m2/g) (Figure
1, Table 1) by the same treatments. These results indicate that
MTS-9 is extremely hydrothermally stable compared to Ti-MCM-
41 and SBA-15.
TS-1 are summarized in Table 2. In phenol hydroxylation, Ti-MCM-
41 and Ti-HMS shows very low catalytic activity (2.5 and 0.5%,
respectively), but MTS-9 exhibits very high catalytic activity, with
a phenol conversion of 26% which is comparable with that of
TS-1.1,6 In styrene epoxidation, MTS-9 shows activity and selectiv-
ity similar to those of TS-1, which are much different from those
of Ti-MCM-41. In 2,3,6-trimethylphenol hydroxylation, Ti-MCM-
41 is inactive due to the relatively low oxidation ability of Ti species
in the amorphous wall of Ti-MCM-41, and TS-1 is also inactive
due to the inaccessibility of the small micropores of TS-1 to the
large diameter of a bulky molecule like 2,3,6-trimethylphenol.
However, MTS-9 is very active for this reaction with a conversion
of 18.8%, indicating that MTS-9 is an effective catalyst for the
oxidation of bulky molecules. Additionally, the leaching of Ti
during the aforementioned reactions is characterized by element
analysis, and the results show no detectable leaching of Ti in our
experiments, suggesting the high stability of Ti species in MTS-9.
The Ti species in MTS-9 have been characterized by UV-visible
and UV-Raman techniques. The UV-vis spectrum (Figure 2S)
for MTS-9 has an adsorption band near 215 nm, indicating that
Catalytic activities for the oxidation of aromatics by H2O2 over
various catalysts, including MTS-9, Ti-MCM-41, Ti-HMS, and
9
888 VOL. 124, NO. 6, 2002 J. AM. CHEM. SOC.
10.1021/ja0170044 CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Catalytic Activities in Oxidation Reactions by H2O2 over
MTS-9, Ti-MCM-41, and TS-1 Samples
Because of the strong acidic condition in the second step, the
nanoclusters prepared in the first step would not grow continuously
into large crystals, and thus the appearance of TiO2 as a separate
phase is avoided. Generally, a TiO2 phase, which is easily formed
under the basic conditions used in the preparation of porous
titanosilicates such as TS-1 and Ti-MCM-41, often acts as a catalyst
poison in oxidation reactions.25
product selectivity
(%)
conv.
samples
MTS-9
reactions
TOF (%)
P1
P2
P3
0.7
1.9
0.5
1.0
phenol hydroxylationc
6.8 26.3 59.5 39.8
0.5 2.5 60.1 38.0
0.1 0.5 58.5 41.5
5.5 28.0 50.4 48.6
Ti-MCM-41 phenol hydroxylationc
Ti-HMSa
phenol hydroxylationc
phenol hydroxylationc
styrene epoxidationd
TS-1∧
Acknowledgment. ThisworkwassupportedbyNSFC(29825108,
29733070, and 20173022), the State Basic Research Project
(G2000077507). We thank Prof. Dezeng Wang (Department of
Chemical Engineering, Tsinghua University, China) for helpful
suggestions and discussions.
MTS-9
9.4 56.4 28.0 29.3 42.7
6.1 48.3 100
5.2 54.6 13.3 58.3 29.0
trimethylphenol hydroxylatione 7.4 18.8 66.7 21.1 12.2
Ti-MCM-41 styrene epoxidationd
TS-1b
styrene epoxidationd
MTS-9
Ti-MCM-41 trimethylphenol hydroxylationd 1.4 4.1 25.5 69.8
4.6
5.0
Ti-HMSa
TS-1b
trimethylphenol hydroxylatione 0.5 2.0 25.0 70.0
trimethylphenol hydroxylatione 0.3 1.2 71.1 17.6 11.3
Supporting Information Available: Figures S1-S5 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org
a Ti-HMS with Si/Ti ratio of 30 was synthesized according to published
procedure.9 b TS-1 with Si/Ti ratio of 30 was synthesized according to
published procedure.1 c Reaction conditions: water as a solvent, reaction
temperature at 80 °C, phenol/H2O2 ) 3/1 (molar ratio), reaction time for 4
h, catalyst/phenol ) 5% (weight ratio). The products are catechol (P1),
hydroquinone (P2), and benzoquinone (P3). The product of tar is not
included. d Reaction conditions: acetone as a solvent, reaction temperature
at 45 °C, styrene/ H2O2 ) 3/1 (molar ratio), reaction time for 5 h, catalyst/
phenol ) 5% (weight ratio). The products are styrene epoxide (P1),
phenylacetaldehyde (P2), and benzaldehyde (P3). e Reaction conditions:
acetonitrile as a solvent, reaction temperature at 80 °C, trimethylphenol/
H2O2 ) 3/1 (molar ratio), reaction time for 4 h, catalyst/phenol ) 5%
(weight ratio). The product are trimethylhydroquinone (P1), trimethylben-
zoquinone (P2), others (P3).
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