Hierarchical mesoporous TS-1 zeolite: A highly active and extraordinarily stable catalyst for the selective oxidation of 2,3,6-trimethylphenol

Article abstract of DOI:10.1039/c0cc00499e

We report the synthesis of a new hierarchical mesoporous TS-1 type zeolite by a simple steam-assisted crystallization method. This novel product exhibits high catalytic activity and a strongly prolonged lifetime in the selective oxidation of 2,3,6-trimethylphenol to trimethyl-p-benzoquinone.

Full text of DOI:10.1039/c0cc00499e

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Hierarchical mesoporous TS-1 zeolite: a highly active and
extraordinarily stable catalyst for the selective oxidation
of 2,3,6-trimethylphenolw
Jian Zhou, Zile Hua, Xiangzhi Cui, Zhengqing Ye, Fangming Cui and Jianlin Shi*
Received 20th March 2010, Accepted 30th April 2010
First published as an Advance Article on the web 1st June 2010
DOI: 10.1039/c0cc00499e
We report the synthesis of a new hierarchical mesoporous TS-1
type zeolite by a simple steam-assisted crystallization method.
This novel product exhibits high catalytic activity and a strongly
prolonged lifetime in the selective oxidation of 2,3,6-trimethyl-
phenol to trimethyl-p-benzoquinone.
stable mesostructure catalysts.7,8 Xiao et al. used zeolite seeds
as framework building units, which could self-organize into
mesoporous matrices with triblock polymers, to successfully
prepare strongly acidic micro/mesoporous titanosilicates.9
Unfortunately, however, the composites assembled from
zeolite seeds are inherently not hydrothermally stable in a
manner similar to that of nanocrystalline zeolites.7 Very
recently, a new TS-1/mesoporous hierarchical structure has
been synthesized by Tuel et al. using specially designed and
synthesized amphiphilic organosilane, however the material is
not suitable for reactions involving hydrophobic reactants
because of its hydrophilic character.10 Moreover, although
the hydrothermal stability of the catalysts was tested, few
reports have concerned the cyclical catalytic stability, which
is very important in practical applications.11
The selective oxidation of 2,3,6-trimethylphenol (TMP) is of
great importance for the preparation of 2,3,5-trimethyl-p-
benzoquinone (TMBQ), a key intermediate in the production
of Vitamin E.1 Various oxidant/catalyst, such as O2 or
H2O2/CuCl2 (especially with various co-catalysts) have been
employed in this oxidation process on the industrial scale.1,2
However, the main drawback of the above-mentioned systems
is the use of homogeneous catalysts which leads to the
separation and recycling problems of catalysts and purifica-
tion of products. Therefore, the development of more efficient
and ‘‘green’’ catalysts for the selective oxidation is still a great
challenge.
Herein, we report the synthesis of a new kind of hierarchical
mesoporous TS-1 zeolite by a simple steam-assisted crystalli-
zation method (SAC), which was developed recently in our
group for the synthesis of mesoporous ZSM-5 zeolite.12 This
novel material, which was denoted as c-Ti-TUD-1, possesses
high surface area, large pore volume, well-crystallized
framework of TS-1 zeolite, and high hydrothermal stability.
Moreover, it demonstrates significantly higher catalytic
activity and a strongly improved lifetime compared with
amorphous titanosilicate in the selective oxidation of TMP
to TMBQ.
Until now, Ti-doped microporous zeolites, especially the
TS-1, have been widely used as catalysts for selective oxidation
in industry due to their high activity and stability.3 However,
these zeolites are inefficient for the selective oxidation of bulky
reactants due to the slow diffusion of reactants and/or
products in their sub-2 nm sized channel systems and/or
deactivation caused by substance blockage in micropores.
Therefore, in recent years, mesostructured titanosilicates, for
example, TiO2–SiO2 aerogels, Ti-MCM-41, Ti-SBA-15 and
Ti-MMM-series et al.,2,4–6 have attracted increasing research
interest as heterogeneous catalysts for the selective oxidation
reactions involving bulky molecules due to their much
larger pore size than conventional zeolites. However, their
amorphous framework and consequently, low hydrothermal
stability have seriously limited their practical applications. In a
word, the successful synthesis of new materials combining
the advantages of high hydrothermal stability and catalytic
activity of zeolites and large pore size of mesoporous materials
is highly desired.
In our approach, tetrabutyl titanate, triethanolamine (TEA)
and tetraethylorthosilicate (TEOS) were dissolved in turn into
distilled water under continuous stirring. After that, tetra-
propylammonium hydroxide (TPAOH, 25% aqueous solution)
was added dropwise into the resultant sol. Therein, the molar
ratio of the precursors was TiO2:SiO2:TPAOH:TEA: H2O =
1 : 50 : 5 : 50 : 500. The resultant wet gel was dried and then
crystallized by the SAC method. Afterwards, the products
were washed repeatedly, dried in air and then calcined. Finally
white powders were obtained and labelled as c-Ti-TUD-1.
Part of the above mentioned wet gel was dried and calcined
directly, and the products were named as Ti-TUD-1.
Up to now, various synthetic procedures have been
attempted in the preparation of highly efficient and relatively
Fig. 1 shows the XRD patterns and N2 sorption isotherms
of Ti-TUD-1, c-Ti-TUD-1 and conventional TS-1 zeolite. In
the XRD pattern of TUD-1, there is no pronounced peak
except a broad one around 231 which suggests an amorphous
structure of the framework. However, for sample c-Ti-TUD-1,
its XRD pattern clearly shows the characteristic peaks of TS-1
in spite of weak intensity, indicating clear crystallization of the
material. For the N2 sorption isotherms, Ti-TUD-1 and
c-Ti-TUD-1 exhibit typical type-IV sorption isotherms with
State Key Lab of High Performance Ceramics and Superfine
Microstructures, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, No. 1295 Ding-xi Road, Shanghai, 200050, P. R. China.
E-mail: jlshi@mail.sic.ac.cn; Fax: +86 21 52413122;
Tel: +86 21 52412712
w Electronic supplementary information (ESI) available: Experimental
details, FTIR, SEM, N2 sorption, hydrothermal test and selectivity
data. See DOI: 10.1039/c0cc00499e
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H1 hysteresis loops. Their corresponding capillary condensa-
tions occur at the relative pressures of ca. 0.7–0.85 and 0.8–0.9,
respectively, indicating a relatively large mesopore size.
Comparatively, the negligible uptake in the isotherm of TS-1
zeolite at P/P0 4 0.1 suggests the absence of mesoporosity.
Alternatively, the sharp uptake at P/P0o0.02 reveals the
presence of micropores in both of c-Ti-TUD-1 and TS-1.
The textural properties of the catalysts are listed in Table 1.
Both c-Ti-TUD-1 and Ti-TUD-1 possess similar pore surface
area (SBET); however, it could be found that the former has an
increased mesopore size and pore volume after crystallization.
This could be ascribed to the framework contraction and
supporting effect of the ploy-TEA template during the
crystallization of amorphous mesopore walls.12–14 The micro-
pore surface area (Smicro), and more accurately, micropore
volume (Vmicro) of c-Ti-TUD-1 is greatly higher than that of
Ti-TUD-1, owing to the framework crystallization into a
zeolite phase. Hydrothermal treatments were applied to both
Ti-TUD-1 and c-Ti-TUD-1 to test their hydrothermal
stabilities (ESIw). Amorphous Ti-TUD-1 suffered from serious
structure collapse and only about 79% surface area remained
after 48 h hydrothermal treatment. Comparatively, only about
7% of the total surface area of c-Ti-TUD-1 was lost. The
much improved hydrothermal stability should be due to its
crystallized framework.
Fig. 3 (left) UV-Vis spectra of TS-1, c-Ti-TUD-1, Ti-TUD-1 and
nano-TiO2; (right) Conversions of TMP in the selective oxidation
reaction using c-Ti-TUD-1, Ti-TUD-1 and TS-1 as catalysts. The
catalysts were recycled and reused for five times.
It has been proved that catalytic activity of Ti-doped
molecule sieves strongly depends on the nature and coordina-
tion of Ti species in the silica networks3,6,10 and these
parameters can be characterized by UV-Vis spectroscopy
(Fig. 3 (left)). The band of Ti-TUD-1 shows a broad peak
which is characteristic of Ti-doped amorphous mesoporous
silicates or nano-TiO2 particles (Fig. 3 (left) for comparison)
and such a peak could be attributed to the simultaneous
presence of tetra-, penta- and hexa-coordinated Ti species.10,16
Conventional TS-1 zeolite shows a maximum at 205 nm, with
the line width at half maximum (LWHM) no larger than
45 nm. This band is the characteristic of O(2p) - Ti(3d)
charge transfer transition, and is generally identified as a
‘‘fingerprint’’ for the isolated, tetrahedrally coordinated Ti
species in hydrophobic zeolite frameworks.10 The band profile
of c-Ti-TUD-1 is very similar to that of TS-1 which clearly
suggested that most of the Ti atoms in c-Ti-TUD-1 should be
tetrahedrally coordinated. In addition, the very weak signals
above 300 nm in the band of c-Ti-TUD-1 indicates the absence
of extra framework TiO2 particles which is one of the
challenging problems in the synthesis of high Ti-content
amorphous mesoporous sieves.
Representative TEM images of c-Ti-TUD-1 are given in
Fig. 2 to reveal the micro/meso-structure. Among them, Fig. 2
(left) shows a part of a typical particle of c-Ti-TUD-1. There is
a dark domain in the center of the particle, which should be
much thicker than the edge, and SEM image (ESIw) reveals
that two particles were actually 901 crosswise intergrown into
one. This distinctive feature can be attributed to the inter-
growth behavior which is one of the typical growth patterns
for MFI type zeolites.12,15 HR-TEM image shown in Fig. 2
(right) demonstrates a clear vision of lattice fringe co-existing
with the meso-structure. The d-placing of 0.97 nm measured
from the HR-TEM image is in agreement with XRD results
(at 2y = 8.91 corresponding to (020) or (200) lattice plane).
The co-existence of meso-structure and zeolite lattice in one
particle could be mostly attributed to the TEA template used
in this synthesis: As TEA molecules polymerized into
Poly-TEA during the steam treatment at increased
temperature, the Poly-TEA could support the mesoporous
structure and prevent the framework from collapse during
the phase transformation from the amorphous to the
crystalline.12–14
Fig. 3 (right) shows the conversions of TMP using
Ti-TUD-1, c-Ti-TUD-1 and TS-1 as catalysts and the catalytic
performances during the lifetime tests for up to 5 cycles. The
TMP conversions using TS-1 as the catalyst in all of the five
cycles are typically lower than 1% in the given time, suggesting
negligible catalytic activity in the selective oxidation. As the
sizes of both TMP and TMBQ molecules are larger than the
diameter of micropores in TS-1 zeolite (ca. 0.55 nm), TS-1 is
ineffective for the selective oxidation of such bulky reactants
due to the diffusion difficulty of reactants and products in its
channel systems. In contrast, Ti-TUD-1 and c-Ti-TUD-1 have
extensive mesoporosity (Table 1), and consequently, both
reactant and product molecules can diffuse into/out of the
mesoporosity without difficulty. This deduction is confirmed
by the greatly higher selective oxidation activities of TMP
when using Ti-TUD-1 and c-Ti-TUD-1 as catalysts than TS-1.
For Ti-TUD-1, the TMP conversion was 86% in the first
cycle, then decreases gradually and only 42% TMP was
oxidized in the fifth cycle. Comparatively, the reactant
conversion in the first cycle using c-Ti-TUD-1 as catalyst
was as high as ca. 99%; then decreased to 86% in the next
cycle but remained unchanged at about 80% in the following
cycles. In the last cycle that we performed, the reactant
Fig. 1 XRD patterns (left) and N2 sorption isotherms of (K)
c-Ti-TUD-1, (m) Ti-TUD-1 and (’) TS-1. For comparative purpose,
the isotherms of c-Ti-TUD-1, amorphous Ti-TUD-1 are offset by 100
and 200 cmÀ3 À1, respectively.
g
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Table 1 Textural properties of Ti-TUD-1, c-Ti-TUD-1 and TS-1
Catalyst
Tia (wt%)
SBETb/m2 gÀ1
Smicroc/cm3 gÀ1
Dmesod/nm
VBJHd/cm3 gÀ1
Vmicroc/cm3 gÀ1
UV-Vis lmaxe/nm
Ti-TUD-1
c-Ti-TUD-1
TS-1
1.80
1.54
1.53
445
456
378
29
97
196
8.5
11.2
—
1.08
1.37
0.17
0.007
0.049
0.087
219
208
205
a
b
c
d
e
Derived from ICP-AES. Calculated by BET method, t-plot method and BJH method, respectively. The value of l with the maximum
intensity.
In summary, a hierarchical mesoporous TS-1 zeolite has
been synthesized by a simple steam-assisted crystallization
method, and the material shows greatly increased catalytic
activity and a strongly prolonged lifetime in the selective
oxidation of TMP, as compared to both amorphous
Ti-TUD-1 and conventional TS-1. This excellent catalytic
performance of mesoporous TS-1 is closely related to its
hierarchical micro/meso-structure, and high hydrothermal
stability against mesostructural collapse and Ti leaching
during reactions.
The authors thank the NSFC (Nos. 20633090, 20703055,
Fig. 2 Transmission electron microscopy (TEM) image of c-Ti-TUD-1.
50872140) for financial support.
Part of a representative particle (left) with an ED pattern in the inset;
HR-TEM image at the particle edge exhibiting the lattice stripes
(right) with an enlarged image in the inset.
Notes and references
1 W. Bonrath and T. Netscher, Appl. Catal., A, 2005, 280, 55;
W. Bonrath, M. Eggersdorfer and T. Netscher, Catal. Today,
2007, 121, 45.
conversion using c-Ti-TUD-1 as catalyst was almost twice of
that of Ti-TUD-1, which still kept a further downward
tendency in the following cycles.
2 N. N. Trukhan, A. Yu. Derevyankin, A. N. Shmakov,
E. A. Paukshtis, O. A. Kholdeeva and V. N. Romannikov,
Microporous Mesoporous Mater., 2001, 44–45, 603; N. N. Trukhan,
V. N. Romannikov, E. A. Paukshtis, A. N. Shmakov and
O. A. Kholdeeva, J. Catal., 2001, 202, 110; O. A. Kholdeeva,
N. N. Trukhan, M. P. Vanina, V. N. Romannikov, V. N. Parmon,
J. Mrowiec-Bialon and A. B. Jarzebski, Catal. Today, 2002, 75, 203.
3 B. Notari, Catal. Today, 1993, 18, 163; S. Gontier and A. Tuel,
Appl. Catal., A, 1994, 118, 173.
4 J. Mrowiec-Bialon, A. B. Jarzebski, O. A. Kholdeeva,
N. N. Trukhan, V. I. Zaikovski, V. V. Kriventsov and
Z. Olejniczak, Appl. Catal., A, 2004, 273, 47.
5 Y. Chen, Y. Huang, J. Xiu, X. Han and X. Bao, Appl. Catal., A,
2004, 273, 185.
6 A. Sorokin and A. Tuel, Catal. Today, 2000, 57, 45; A. Tuel,
Microporous Mesoporous Mater., 1999, 27, 151; A. Tuel and
L. G. Hubert-Pfalzgraf, J. Catal., 2003, 217, 343.
7 X. Yang, Y. Li, A. Lemaire, J. Yu and B. Su, Pure Appl. Chem.,
2009, 81, 2265; J. Cejka and S. Mintova, Catal. Rev. Sci. Eng.,
2007, 49, 457; K. Egeblad, C. Christensen, M. Kustova and
C. Christensen, Chem. Mater., 2008, 20, 946.
8 X. Meng, F. Nawaz and F. Xiao, Nano Today, 2009, 4, 292.
9 F. Xiao, Y. Han, Y. Yu, X. Meng, M. Yang and S. Wu, J. Am.
Chem. Soc., 2002, 124, 888.
10 Y. Cheneviere, F. Chieux, V. Caps and A. Tuel, J. Catal., 2010,
269, 161.
As we have mentioned, compared with amorphous
Ti-TUD-1, crystallized c-Ti-TUD-1 possesses higher
hydrothermal stability owing to the formation of crystallized
zeolite frameworks. The oxidant used in the selective oxidation
of TMP was H2O2 aqueous solution (30%) and the reaction
temperature was as high as 80 1C. Generally, amorphous
mesoporous materials are not competent for long-term
employment in such hydrothermal environment.4–6 However,
c-Ti-TUD-1 shows the much improved recycling capability
which should be ascribed to the high hydrothermal stability of
its crystallized frameworks. Moreover, the speciality of the
present mesoporous TS-1 zeolite lies in its single crystal-like
zeolite structure throughout the particles with penetrating
mesopore channels in them,12,17 as can be found in Fig. 2 of
its coherent and continuous lattice stripes across the whole
particle framework and the single crystalline ED pattern on a
large part of the particle (inset), which is essentially different
from the mesostructure randomly self-assembled by zeolite
nanoseeds.9
In addition to the mesostructure collapse, the Ti species
leaching during the reaction may also account for the declined
catalytic activity.2 We examined the Ti contents of the recycled
catalysts (ESIw), amorphous Ti-TUD-1 actually experienced
significant Ti species leaching (by ca. 16% after being reused
five times), however, the Ti contents of c-Ti-TUD-1 did not
significantly change. The titanium atom loss from amorphous
frameworks of Ti-TUD-1 debases the catalytic active site
concentration owing to its hydrophilic surface, but in contrast
crystallized zeolite frameworks of c-Ti-TUD-1 possess a
hydrophobic surface of TS-1 type zeolite which can effectively
prevent the loss of heteroatoms such as Ti.
11 O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti, C. Pirovano,
N. Ravasio, M. V. Barmatova and Y. A. Chesalov, Adv. Synth.
Catal., 2009, 351, 1877.
12 J. Zhou, Z. Hua, J. Shi, Q. He, L. Guo and M. Ruan, Chem.–Eur.
J., 2009, 15, 12949.
13 J. C. Jansen, Z. Shan, L. Marchese, W. Zhou, N. von der Puil and
T. Maschmeyer, Chem. Commun., 2001, 713.
14 Z. Shan, J. C. Jansen, L. Marchese and T. Maschmeyer, Micro-
porous Mesoporous Mater., 2001, 48, 181.
15 C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and
A. Carlsson, J. Am. Chem. Soc., 2000, 122, 7116.
16 W. Zhang, M. Froba, J. Wang, P. T. Tanev, J. Wong and
T. J. Pinnavaia, J. Am. Chem. Soc., 1996, 118, 9164.
17 J. Zhao, J. Zhou, Y. Chen, Q. He, M. Ruan, L. Guo, J. Shi and
H. Chen, J. Mater. Chem., 2009, 19, 7614.
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This journal is The Royal Society of Chemistry 2010
4996 | Chem. Commun., 2010, 46, 4994–4996