Assembly of heteropoly acid nanoparticles in SBA-15 and its performance as an acid catalyst

Article abstract of DOI:10.1002/chem.200401047

Keggin-type 12-tungstophosphoric acid (TPA) nanocrystals have been assembled inside the pores of mesoporous silica through a vacuum impregnation method by using largepore SBA-15 as a nanoreactor. The product was characterized by Brunauer-Emmet-Teller particle size distri bution (BET-PSD), NMR and FT-IR spectroscopy, X-ray diffraction (XRD), tranmsission electron microscopy (TEM), differential thermal analysis (DTA) and FT-IR of adsorbed pyridine. The experimental results illustrale that the TPA nanocrystals are excellent Bronsted acid catalytic materials at room temperature.

Full text of DOI:10.1002/chem.200401047

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Assembly of Heteropoly Acid Nanoparticles in SBA-15 and Its Performance
as an Acid Catalyst
Shu-Yuan Yu, Li-Ping Wang, Bo Chen, Ying-Ying Gu, Jing Li, Han-Ming Ding, and
Yong-Kui Shan*[a]
Abstract: Keggin-type 12-tungstophos-
phoric acid (TPA) nanocrystals have
been assembled inside the pores of
mesoporous silica through a vacuum
impregnation method by using large-
pore SBA-15 as a nanoreactor. The
product was characterized by Bruna-
uer–Emmet–Teller particle size distri-
bution (BET-PSD), NMR and FT-IR
spectroscopy, X-ray diffraction (XRD),
(TEM), differential thermal analysis
(DTA) and FT-IR of adsorbed pyri-
dine. The experimental results illustrate
that the TPA nanocrystals are excellent
Brønsted acid catalytic materials at
room temperature.
tranmsission
electron
microscopy
Keywords: dimerization
·
hetero-
poly acids · mesoporous silica ·
nanostructures · vacuum
Introduction
pounds. Pure heteropoly acids, generally containing at least
four different types of elements, are obtained by the con-
densation of two or more different types of related oxoan-
ions in an acidic solution followed by extraction under
acidic conditions. Heteropoly acids exist as an equilibrium
system of many species in the synthetic procedures, which
makes the aforementioned methods unsuitable for the prep-
aration of the pure heteropoly acid nanocrystals inside chan-
nels of porous materials. Although the conventional wet im-
pregnation method may incorporate some heteropoly acid
molecules into the channels of mesoporous materials, the
amount of heteropoly acid is too small to form nanocrys-
tals.[11–13]
There are three important prerequisites for the confine-
ment of the heteropoly acid nanoparticles inside the pores
of porous materials. Firstly, the size of the pores must be
large enough to accommodate the heteropoly acid nanopar-
ticles. Secondly, the amount of heteropoly acid incorporated
into the pores should be enough. Thirdly, conditions of crys-
tallization have to be mild to prevent the loss of water of
crystallization. Therefore, our synthetic strategy was to use
the mesoporous silica SBA-15 with large pore diameter as a
host for limiting the growth of crystals inside the pores; to
apply high-vacuum (10À7 Torr) pretreatment for clearing
water molecules, nitrogen, and other contaminations from
the pores of the SBA-15 in order to provide more capacity;
and to finish crystallization of heteropoly acid at relatively
low temperature. In this way, we have successfully synthe-
sized heteropoly acid nanoparticles within the channels of
mesoporous silica SBA-15.
Many efforts have been dedicated to the utilization of meso-
porous materials as nanoreactors to prepare monodispersive
nanoparcticles or nanowires of uniform size, because of the
“particle-sieving effect” of mesoporous materials.[1–4] Many
nanoparticles or nanowires have been encapsulated into the
channels of mesoporous materials, such as Ge,[5] Ag,[6] Pd,[7]
Fe2O3,[8] GaAs,[9] Ru–Ag,[10] and ruthenium–carbonyl com-
plexes,[11] by incorporation of inorganic precursors into the
channels by sorption, phase transition, ion exchange, com-
plex or covalent grafting, impregnation, and gas-phase
chemical-vapor infiltration or deposition with further treat-
ment. The mesopores or channels can be regarded as hosts
and the nanoparticles or nanowires as guests like in the
host–guest chemistry. In addition, all of these nanoparticles
or nanowires are composed of mono- or bimetal elements
or binary compounds or complexes; nanoparticles contain-
ing more than two components have not been found in liter-
ature. The reason may be that incorporation of multiple
component compounds into the pore canals of porous mate-
rials is more difficult than that of mono- or binary com-
[a] S.-Y. Yu, L.-P. Wang, B. Chen, Y.-Y. Gu, J. Li, H.-M. Ding,
Prof. Y.-K. Shan
Department of Chemistry, East China Normal University
North Zhongshan Road 3663, Shanghai 200062 (China)
Fax : (+86)216-223-3503
E-mail: ykshan@chem.ecnu.edu.cn
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DOI: 10.1002/chem.200401047
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Results and Discussion
A well-defined step associated with the filling of the meso-
pores due to capillary condensation occurs approximately at
P/P0 =0.45–1.00 in TPA60/SBA-15v and at P/P0 =0.70–1.00
in SBA-15. These phenomena can be attributed to the en-
capsulation of TPA inside the channels of SBA-15, and fur-
thermore, the high loading of about 60 wt% (determined by
inductively coupled plasma (ICP) measurements) may lead
to a partial blocking of the pores.
12-Tungstophosphotic acid (TPA, H3[PW12O40]) was selected
as the heteropoly acid. The diameter of the Keggin-type
molecule is about 1 nm. Six loadings of TPA on SBA-15
ranging from 23–78 wt% were prepared.
The nitrogen adsorption–desorption isotherms and pore
size distribution at 77 K as shown in Figure 1 are of type IV
A
31P chemical shift of d=À15.4 ppm is observed by
NMR spectroscopy (Figure 2), which is just the same as that
of the bulk TPA.[14] It indicates that the crystals of TPA
exist in the pores of SBA-15 matrix and the structure of the
TPA did not experience modifications during the assembly
Figure 2. 31P NMR MAS spectrum of TPA60/SBA-15v.
Figure 1. Nitrogen adsorption–desorption isotherm plots and pore size
distribution curve for SBA-15 (top) and TPA60/SBA-15v (bottom).
classification for SBA-15 and TPA60/SBA-15v, which shows
that both SBA-15 and TPA60/SBA-15v possess the character
of a mesoporous framework. The SBA-15 matrix has regular
mesoporous channels and a narrow gaussian pore size distri-
bution; its Brunauer–Emmet–Teller (BET) surface area and
pore volume are 760 m2 gÀ1 and 2.1 cm3 gÀ1, respectively, and
the pore diameters calculated by the Barrett–Joyner–Halen-
da (BJH) method are in the range of 23–55 nm. The average
pore size is approximately 36 nm. Figure 1b demonstrates
that the pore structure was preserved during the TPA as-
sembly process in the channels of SBA-15, but the BET sur-
face area, pore volume, and pore diameters decreased
sharply to 143 m2 gÀ1, 0.14 cm3 gÀ1, and 2–7 nm, respectively.
Figure 3. FT-IR spectrum of a) bulk TPA, b) SBA-15, and c) TPA60/SBA-
15v.
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Y.-K. Shan et al.
angle range for the large-pore SBA-15 matrix. The crystal
diffraction signals for the crystalline phase of TPA are per-
fectly offered by TPA/SBA-15v samples with a TPA loading
of 60 wt% and higher in the 2q region from 5 to 608 and the
broadening of these signals can be clearly seen in the pat-
terns. This broadening reveals that the TPA crystals are on
the nanoscale size, and the average crystal size calculated by
the Scherrer formula is about 20 nm, which is consistent
with the pore diameter of the SBA-15 matrix. The XRD
patterns show that by using the large-pore SBA-15 as a
nanoreactor, the Keggin structure TPA nanoparticles can be
prepared easily in the channels when the loading is approxi-
mately 60 wt% by the new vacuum impregnation method.
In earlier research, it was shown that no TPA crystal phase
exits in the TPA/SBA-15 materials prepared by traditional
wet impregnation below TPA loadings of 80%.[18–20]
Figure 4. XRD patterns for a) TPA23/SBA-15v, b) TPA46/SBA-15v,
c) TPA60/SBA-15v, d) TPA67/SBA-15v, e) TPA73/SBA-15v, f) TPA78/SBA-
15v, and g) bulk TPA.
The TEM images for SBA-15 and TPA/SBA-15v and
TPA/SBA-15i samples are presented in Figure 5a-f. SBA-15
(Figure 5a) has channels with very large diameters, into
which TPA molecules can enter easily. TPA nanoparticles
can be found both in the pores and on the surface of the
matrix in TPA60/SBA-15v samples (Figure 5b,c); however,
most of the nanoparticles are distributed in the channels. As
the loading is increased, more TPA nanoparticles are
formed on the surface of the matrix, as shown in Figure 5d.
The crystal sizes of the TPA nanoparticles, both inside and
outside the pores, estimated from TEM image (Figure 5b–d)
are in agreement with those calculated from XRD. In addi-
tion no TPA nanoparticles are observed in the pores or on
the surface of the matrix in the TPA74/SBA-15i sample (Fig-
ure 5e). The results suggest that TPA nanoparticles can just
process; this result is different from those obtained by
means of common silica impregnation.[13,15,16]
The FT-IR spectra for TPA60/SBA-15v and bulk TPA are
shown in Figure 3. For the bulk TPA, characteristic bands
À
are observed at 1081 (P O), 985 (W=O), 890 (W-O-Wcorner),
and 802 cmÀ1 (W-O-Wedge), which coincide with those report-
ed in the literature for the [PW12O40]3À Keggin unit.[17] The
bands of TPA60/SBA-15v appear at 1081, 982, 890, and
801 cmÀ1 without overlapping, which are also a good proof
of the existence of the Keggin structure [PW12O40]3À anions
without depolymerization or degradation.
The XRD patterns for bulk TPA and TPA/SBA-15v are
displayed in Figure 4. No signal is detected in the small-
Figure 5. TEM images for large-pore SBA-15 (a), TPA60/SBA-15v (b,c), TPA73/SBA-15v (d), and TPA74/SBA-15i (e).
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Mesoporous Materials
FULL PAPER
Table 2. Cyclodimerization of a-methylstyrene catalyzed by TPAx/SBA-
15v and TPA74/SBA-15i.
be assembled in the channels of the large-pore SBA-15
through the vacuum impregnation method.
Thermal stability of TPA/SBA-15v has also been investi-
gated by XRD. The results show that the TPA nanoparticles
are quite stable up to 4108C and degrade completely at
4608C due to the destruction of the TPA Keggin structure
with formation of phosphate and tungsten trioxide species,
similar to an earlier report.[21] The initial decomposition
temperature of nanoparticles is a bit lower than that of bulk
TPA (4658C);[22] this could be attributed to the interaction
between TPA and silica, which can weaken the stability of
TPA.[13]
The acidity of TPA73/SBA-15v and TPA74/SBA-15i with
the same loading of TPA was determined by FT-IR spec-
troscopy of adsorbed pyridine. The characteristic bands at
1540 and 1635 cmÀ1 are assigned to pyridine protonated by
Brønsted acid sites, while the bands from pyridine coordi-
nated to Lewis acid sites appear at 1450 and 1608 cmÀ1.[23,24]
The intensities of the 1450 and 1540 cmÀ1 bands are listed in
Table 1. The Brønsted acid sites are estimated to be much
Selectivity [%]
Conversion
[%]
indan
linear
byproducts
dimers
TPA23/SBA-15v
TPA46/SBA-15v
TPA60/SBA-15v
TPA67/SBA-15v
TPA73/SBA-15v
TPA78/SBA-15v
TPA74/SBA-15i
99.8
99.0
99.0
99.8
100
20.3
37.0
53.4
89.7
97.2
66.7
46.1
68.1
51.5
35.0
3.1
11.6
11.5
11.6
7.2
2.8
0
6.3
100
83.9
27.0
20.3
33.6
Conclusion
Table 1. The intensity of B (1540 cmÀ1) and L (1450 cmÀ1) bands for
TPA73/SBA-15v and TPA74/SBA-15i.
12-Tungstophosphoric acid nanoparticles with diameters of
around 20 nm were successfully assembled inside the pores
of large-pore SBA-15 by the novel vacuum impregnation
method. We believe that this facile method of preparing
tungstophosphoric acid nanoparticles could be applicable in
synthesizing other multicomponent nanocrystals inside the
pore system of SBA-15 and other mesoporous materials as
the host for nanomanufacturing. Moreover, the tungstophos-
phoric acid nanoparticle system is an excellent acid catalytic
material operating at low temperatures.
T [8C]
B (1540 cmÀ1
)
L (1450 cmÀ1
)
TPA73/SBA-15v
RT
100
150
200
250
300
1.928
2.595
2.904
3.109
3.106
2.550
0.912
0.602
0.582
0.564
0.548
0.539
TPA74/SBA-15i
RT
100
150
200
250
300
1.235
2.098
2.626
3.025
3.253
2.985
0.514
0.285
0.192
0.142
0.111
0.119
Experimental Section
The siliceous large-pore SBA-15 was synthesized according to the proce-
dure reported previously.[25,26] In
a typical synthesis, Pluronic P123
(EO20PO70EO20) template (4.0 g) was dissolved with stirring in a solution
of water (30 g) and HCl (120 g, 2m) at 408C, and 1,3,5-trimethylbenzene
(TMB; 6.0 g) was then added to this homogeneous solution. After 2 h,
tetraethoxysilane (TEOS; 8.5 g) was also added. The resulting mixture
was stirred at 408C for 20 h, and then aged at 1008C for 24 h under static
conditions. The as-prepared sample was recovered by filtration and air-
dried. After the template was removed by calcination in air at 5508C for
6 h, the large-pore SBA-15 (0.5 g) was pumped to a vacuum of 10À7 Torr
with a turbo pump. An excess aqueous solution of tungstophosphoric
acid (TPA) (10, 20, 30, 40, 50, 60 wt%) was introduced and kept in con-
tact for 24 h under vacuum. Then the solution was pumped out and the
solid was dried at 1008C in air. The materials are denoted as TPAx/SBA-
15v, in which x denotes the loading amount of TPA as wt%, which was
determined by ICP. TPA was also impregnated into the SBA-15 with the
similar procedure except for the vacuum treatment. This sample is denot-
ed as TPA74/SBA-15i.
more numerous than the Lewis acid sites in both samples,
and the acidity of TPA73/SBA-15v is higher than that of
TPA74/SBA-15i according to the intensity ratio of the bands.
The catalytic activities of TPAx/SBA-15v and TPA74/SBA-
15i for the cyclodimerization reaction of a-methylstyrene
(AMS) have been investigated. The results are shown in
Table 2. The TPAx/SBA-15v catalysts exhibit high activities.
The conversions are more than 99% and the selectivity of
the cyclic dimer (indan) increases with increasing the TPA
loading. The maximum selectivity of indan is observed over
TPA73/SBA-15v. The conversion of AMS catalyzed by
TPA73/SBA-15v and the selectivity of indan and linear
dimers are 100, 97.2, and 2.8% respectively, while those cat-
alyzed by TPA74/SBA-15i are 83.9, 46.1, and 20.3%, respec-
tively. The TPAx/SBA-15v catalyst has a higher activity
probably due to its higher acidity. The higher selectivity
might be due to its higher crystallinity.
The cyclodimerization reaction of a-methylstyrene (AMS) was per-
formed in a continuous flow stainless steel fixed bed microreactor system
with N2 (20 mLminÀ1) as carrier gas. The catalyst (0.3 g) was pretreated
at 1008C for 10 h. The reaction was carried out at room temperature
without extra heating.
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