Mesoporous TS-1 Nanocrystals as Low Cost and High Performance Catalysts for Epoxidation of Styrene

Article abstract of DOI:10.1002/cjoc.201600701

Mesoporous TS-1 nanocrystals were facilely synthesized without involving additional templates. The usage of inorganic silicon source, less structure-directing agent and high yields of nearly 100% made this template-free strategy suitable for large-scale s

Full text of DOI:10.1002/cjoc.201600701

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DOI: 10.1002/cjoc.201600701
Mesoporous TS-1 Nanocrystals as Low Cost and High Perfor-
mance Catalysts for Epoxidation of Styrene
Tian-Lu Cui, Xin-Hao Li,* and Jie-Sheng Chen*
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 China
Mesoporous TS-1 nanocrystals were facilely synthesized without involving additional templates. The usage of
inorganic silicon source, less structure-directing agent and high yields of nearly 100% made this template-free
strategy suitable for large-scale synthesis with low cost. Furthermore, the as synthesized TS-1 exhibited comparable
epoxidation performance than those obtained by other methods.
Keywords TS-1, mesoporous, nanocrystal, low cost, epoxidation
[
7]
Introduction
zeolites with high yield. Meanwhile, many researches
were carried out to improve the catalytic activity espe-
cially in the diffusion aspect by employing soft or hard
template and fabricating other Ti-containing mesopo-
As an important class of crystalline metallosilicates,
titanium silicate-1 (TS-1) with uniform and ordered
networks of micropores has been studied extensively
with regards to various catalysts. In particular, epoxi-
dation of propylene, hydroxylation of phenol and am-
moximation of cyclohexanone catalytic oxidation reac-
tions using TS-1 and H O have been widely carried out
[8]
rous zeolites. However, seeds methods are either mul-
[
1]
tistep or of low activity; mesoporous and monolithic
materials often need various templates thus being high
cost; other zeolites such as Ti-MCM-41 show lower
activity due to the different titanium coordination envi-
ronment. More importantly, none of them can solve all
problems in one-pot.
2
2
[
9]
in industry for the production of epoxypropane, dihy-
droxybenzene and cyclohexanone oxime.
[1a,1b,2]
TS-1
catalysts with low cost and high activity are highly de-
sirable. It has been demonstrated that high catalytic ac-
tivity can be obtained with TS-1 particle size smaller
than 300 nm. Unfortunately, synthesis of small particle
size TS-1 often needs organic silicon source such as
tetraethylorthosilicate (TEOS) and a large amount of
tetrapropylammonium hydroxide (TPAOH, structure-
directing agent, SDA), both of which are extremely ex-
pensive. Besides, the cost of separation was highly
increased due to the small particle size. The high cost
and unsatisfied catalytic performance of TS-1 catalysts
drive materials scientists to redesign the mesoscale
structure by using cheaper precursors.
Herein, modified Kirkendall method which had been
reported in our previous work was utilized to synthesize
mesoporous TS-1 nanocrystals with the assistance of a
little water by using inorganic silicon source (silica
[10]
gel). High concentration of TPAOH can be achieved
by using less TPAOH (SiO /TPAOH＝8) due to the
2
quasi-solid-state system. The separating procedure can
also be omitted. Moreover, the as synthesized mesopo-
rous TS-1 nanocrystals exhibited comparable epoxida-
tion performance than those obtained by other methods.
[3]
Experimental
Synthesis of mesoporous TS-1 nanocrystals
Inexpensive silicon sources such as amorphous silica
and low cost SDA like tetrapropylammonium bromide
Ti(OCH CH CH CH ) (TBOT) was mixed with
2
2
2
3 4
(
TPABr) were utilized to partly or totally replace the
2 2
silica gel (40 wt% in H O) and TPAOH (25 wt% in H O)
[
3c,3d,4]
expensive raw materials of TEOS and TPAOH.
with the molar ratio of the composition to be 100SiO ∶
2
Seeds-assistance method was also employed to synthe-
size small particle sized TS-1 with less amount of ex-
12.5TPAOH∶1TiO . After the removal of water, the
as-obtained dry gel (2 g) was mixed with 0.2 mL of wa-
ter and then transferred to a Teflon-lined autoclave for
further crystallization at 120 ℃ for 8 h. The as synthe-
sized sample was dried and then heated at 550 ℃ in air
for 6 h to remove organic components.
2
[
5]
pensive organic ligands and silicon sources. Besides,
monolithic TS-1 materials were synthesized to reduce
[6]
the cost of separation. The dry-gel conversion and
solvent-free method were also employed to synthesize
*
E-mail: xinhaoli@sjtu.edu.cn, chemcj@sjtu.edu.cn
Received October 24, 2016; accepted December 3, 2016; published online XXXX, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600701 or from the author.
In Memory of Professor Enze Min.
Chin. J. Chem. 2017, XX, 1—4
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1

Cui, Li & Chen
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Results and Discussion
Scheme 1 The proposed synthesis processes of mesoporous
TS-1 nanocrystals (a－d), the over-view transmission electron
microscopy (TEM) image (e) and photo of the obtained sample
The overall growth process of mesoporous TS-1
nanocrystals was depicted in Scheme 1. Commercial
silica nanoparticles were mixed with TPAOH with the
(
f)
2
molar ratio of 8SiO /TPAOH and then the mixture
(
Scheme 1a) was mixed with TBOT under stirring. The
homogeneous gel was dried at 60－70 ℃ to remove
water and dried intermediates were gotten (Scheme 1b).
During this process, intermediates were partially etched
by TPAOH for further crystallization. The obtained
mesoporous intermediates (dry gel) were transferred
into a polytetrafluoroethylene tube (Scheme 1c) for fur-
ther crystallization at 120 ℃ with assistance of limited
amount of water. With the crystallization proceeding,
the Kirkendall growth was triggered through chemical
etching of dry gel by TPAOH and water. With the grad-
ual diffusion of water vapor from outside of the precur-
sors to the inside, crystallization of TS-1 proceeded ac-
cordingly. Nanovoids simultaneously diffused into TS-1,
generating mesopores in situ as described in our previ-
[10]
ous work.
Besides, in this quasi-solid-state system
the concentration of TPAOH could be as high as 15
1
mol•L even though the molar ratio of TPAOH/SiO
2
was as small as 1/8 (much lower than 3/7 of conven-
tional method as shown in Table 1). In another word,
TPAOH molecules were used in a more efficient manner
here without a need of expensive and more complex
organic surfactants, reducing the production cost effec-
tively. On the other hand, the high concentration of
TPAOH could help to form more nuclei and thus small
size particles (Scheme 1e) even using inorganic silicon
source. Furthermore, the simplified process without
centrifuging (columnar sample could be obtained as
shown in Scheme 1f) or filtration procedure can give
high yields of mesoporous TS-1 nanocrystals more than
strategy. As shown in Figure 1a, the X-ray diffraction
patterns (XRD) of the typical sample demonstrated its
high crystallinity with characteristic peaks of MFI
structure. No other peaks assigned to titanium oxide
could be detected indicating that the pure phase of the
sample. Further diffuse reflectance UV-vis spectrum
was carried out to determine the coordination environ-
ment of titanium. As shown in Figure 1b, the as synthe-
sized sample showed a strong band centered at 205－
2
15 nm, characteristic of tetrahedrally coordinated Ti
species. Besides, it could be clearly observed that there
9
9%, much higher than that of samples obtained by the
was no obvious absorption band in the range of 300－
[11]
conventional method. It should be noted that the crystal-
lization time was shortened to 8 h even under a lower
reaction temperature of 120 ℃. All these advantages
made this strategy suitable for large scale production,
especially from the perspective of the environment and
economic issue. Detailed synthesis conditions of this
template-free method as well as the conventional meth-
od were listed in Table 1.
3
2
50 nm, excluding the formation of TiO particles.
Thus, it can be concluded that all the Ti atoms enter the
framework. The molar ratio of Ti/Si was estimated to be
1
/98 according to the X-ray fluorescence spectrometer
(
(
XRF) analysis, in agreement with that of the raw
1/100). The scanning electron microscope (SEM) im-
age and the corresponding elemental mapping image
Figure S1) further indicated the homogeneous
(
Further textural characterizations were conducted to
survey the property synthesized by this template-free
distribution of Ti species in the synthesized TS-1
sample.
The high resolution SEM image shown in Figure 2a
revealed the small size of the sample with a mean size
around 130 nm, matching well with the over-view
transmission electron microscopy (TEM) image of
Scheme 1e and demonstrated that nanosized TS-1 could
be obtained even without the involvement of organic
silicon source as well as large amounts of TPAOH. Fig-
ure 2b showed the high resolution TEM (HRTEM) im-
age of nanosized TS-1. The clearly distinguishable lat-
tice fringes revealed the high crystallinity of the whole
nanoparticle which was also reflected by the XRD pat-
Table 1 Synthesis conditions of samples obtained by different
methods
Conventional method
Our method
silica gel
1/8
Silicon source
TPAOH/SiO
TEOS
3/7
2
Temperature/℃
Time/h
170
120
72
8
Yield/%
30－80
＞99
2
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Mesoporous TS-1 Nanocrystals as Low Cost and High Performance Catalysts
Figure 1 (a) XRD pattern and (b) diffuse reflectance UV-visible
spectrum of the as synthesized sample.
terns as show in Figure 1a. It could also be observed
that there existed brighter areas in the TEM image and
the observation was more clearly in the large scale TEM
image of Scheme 1e, suggesting the formation of meso-
pores distributing homogeneously in each nanoparticles.
The mesoporous properties of the as synthesized TS-1
nanoparticles could also be reflected by the N
tion/desorption isotherms and the corresponding pore
size distribution as shown in Figures 1c and 1d. The N
2
adsorp-
Figure 2 (a) SEM image, (b) HRTEM image, (c) N adsorp-
tion-desorption isotherm and (d) BJH pore size distribution of the
as synthesized sample.
2
2
adsorption/desorption isotherms (Figure 1c) exhibit a
steep uptake at the low relative pressure which signifies
the presence of microporosity in the TS-1 nanoparticles,
being agree with the XRD and HRTEM results as
shown in Figures 1a and 2b. Furthermore, the isotherms
ticular interest from both scientific and commercial
point of view. As shown in Figure 3, both mesoporous
TS-1 nanocrystals (meso-TS-1) and the control sample
(bulk-TS-1) gave the same products of styrene oxide,
benzaldehyde and phenylacetaldehyde. The selectivity
of each product was similar while the conversions were
various. As expected, meso-TS-1 offered much higher
conversions as compared with the corresponding bulk
phases (bulk-TS-1). Meso-TS-1 sample offered a much
higher conversion of 51.7 %, almost two times of that of
bulk-TS-1 (26.7%). It should be noted that the Ti con-
tent of meso-TS-1 was lower than that of bulk-TS-1 as
shown in Table S1. The turn over frequency (TOF) val-
ues for oxidation of styrene over meso-TS-1 and
bulk-TS-1 were also estimated to better reflect the cata-
lytic performance of the two samples. As shown in Ta-
ble S1, meso-TS-1 offered a TOF value of 71.7 mol
exhibit two hysteresis loops at relative pressure p/p
0
of
0
.20－0.40 and 0.85－1.00, indicating the formation of
bimodal mesopores in the sample. These results were
also consistent with the TEM observation (Scheme 1e
and Figure 2b) and could be clearly reflected by the
Barret-Joyner-Halenda pore size distribution as shown
in Figure 2d. Moreover, the as synthesized mesoporous
TS-1 nanocrystals showed high total as well as meso-
pore volume of 0.46 cc/g and 0.34 cc/g, respectively
(Table S1).
In order to confirm the advantages of mesoporous
TS-1 nanocrystals, much effort has also been directed
[
12]
towards catalytic performance.
Epoxidation of sty-
1
1
rene was selected as a model reaction, which is of par-
(styrene) mol metal h , while that of bulk-TS-1 was
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3

Cui, Li & Chen
COMMUNICATION
only 28.2 mol styrene mol1 metal h . Obviously, it is
the unique mesoporous nanostructure of TS-1 with
shortened mass transfer path that promoted their cata-
lytic activity. Moreover, the TOF values of meso-TS-1
surpassed those of typical TS-1 and other MFI based
catalysts for the same reaction in the literature (Table
S2). All these results promise great potentials of meso-
porous TS-1 nanocrystals as efficient catalysts.
1
(21331004, 21301116) and Shanghai Eastern Scholar
Program.
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Figure 3 Catalytic performance of styrene epoxidation over
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4
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