Solvent-washable polymer templated synthesis of mesoporous materials and solid-acid nanocatalysts in one-pot

Article abstract of DOI:10.1039/b913035g

We report a new and simple one-pot synthetic method to produce mesoporous silica and nanoporous solid acid catalyst capable of catalyzing pinacole-pinacolone rearrangement and esterification reactions, by preparing a solvent washable phosphonated triblock copolymer template and self-assembling it in the presence of alkoxysilane.

Full text of DOI:10.1039/b913035g

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Solvent-washable polymer templated synthesis of mesoporous materials
and solid-acid nanocatalysts in one-potw
a
a
a
b
Richard E. Mishler II, Ankush V. Biradar, Cole T. Duncan, Eric A. Schiff* and
a
Tewodros Asefa*
Received (in Berkeley, CA, USA) 1st July 2009, Accepted 1st September 2009
First published as an Advance Article on the web 18th September 2009
DOI: 10.1039/b913035g
We report a new and simple one-pot synthetic method to produce
mesoporous silica and nanoporous solid acid catalyst capable of
catalyzing pinacole-pinacolone rearrangement and esterification
reactions, by preparing a solvent washable phosphonated
triblock copolymer template and self-assembling it in the
presence of alkoxysilane.
triblock copolymers that are hydrogen-bonded to and
5
occluded within micropores of the framework. Other
6
methods include Soxhlet extraction, stirring in supercritical
7
8
fluids and stirring under reflux conditions. Here we report
the synthesis of SBA15-type mesoporous material and
solid acid nanostructures by using phosphonated pluronics
P
P123 (designated P123 ), as a solvent-extractable template
In the early 1990’s, a family of mesoporous silicas (MCM41,
MCM48 and MCM50) were discovered that had a broad
range of uses including sensor applications, catalysis, drug
(Scheme 1).
P
Acid cleavable, phosphonated triblock copolymer (P123 )
template containing terminal phosphate ester groups was
synthesized by converting the terminal hydroxyl groups of
the PEG blocks of Pluronic P123 or poly(ethylene oxide)-
block-poly(propylene oxide)-block-poly(ethylene oxide) triblock
copolymer with dimethylphosphite (DMP) using a similar
method as reported by Gitsov and co-workers for synthesis
1
delivery, and use in solar cells. Depending upon the inorganic
material, surfactant template, and synthetic conditions
employed, mesoporous materials with widely varying
composition, structure and pore sizes can be synthesized.
The first report in 1998 on the synthesis of SBA15 mesoporous
silica with very large pore diameter by using triblock
copolymer templates was also a significant advance in the field
9
of block-copolymers via phosphorylation. The resulting
P
P123 polymer was then self-assembled with tetraethoxysilane
2
of mesoporous materials chemistry. The SBA15 material
(TEOS) in aqueous solution to produce mesostructured silica,
P
SBA15 , under conditions similar to those reported for
allows the immobilization via ‘‘host–guest’’ inclusion processes
of larger molecules that are unable to diffuse in the MCM41
3
mesoporous material.
making SBA15 with P123 (see ESI for details of the synthesis
and characterizationw).
The synthesis of well-ordered mesoporous structures
requires an extraction step to remove the templating agent.
The most common method is calcination, which involves
heating the material to a temperature sufficient to oxidize
the organic or polymer template. Calcination is widely used
The template extraction was performed under various
conditions with special attention to solvent extraction with
P
the expectation that the P123 template would be easily
removed through a low temperature solvent washing by
undergoing hydrolysis at its terminal phosphate ester groups.
1
,2
in SBA15-type materials. Although extraction of polymer
templates by calcination can completely remove the template,
the fairly high temperature is known to promote condensation
of surface hydroxyl (–OH) groups on the mesoporous
framework, reducing the number of hydroxyl groups utilized
for further organic functionalization of the material via
grafting and post-modification. Calcination also modifies the
framework, typically by shrinkage, and of course the template,
which could be potentially recycled, is lost.
For comparison purposes, a material templated with
as-received P123 was also synthesized and subjected to similar
extraction conditions. The samples were then characterized
with transmission electron microscopy (TEM), gas adsorption,
small angle X-ray scattering (SAXS), powder X-ray diffraction
(XRD), elemental analysis, thermogravimetric analysis and
solid-state NMR and FT-IR spectroscopy (FTIR).
4
The second method is solvent-extraction, which is often
able to remove ionic template molecules, but has been shown
to be less successful in removing non-ionic templates such as
a
Department of Chemistry, 111 College Place, Syracuse University,
Syracuse NY 13244, USA. E-mail: tasefa@syr.edu;
Fax: +1-315-443-4070; Tel: +1-315-443-3360
Department of Physics, Syracuse University, Syracuse, NY 13244,
b
USA. E-mail: easchiff@syr.edu; Fax: +1-315-443-9103;
Tel: +1-315-443-2566
w Electronic supplementary information (ESI) available: Synthesis,
experimental details, catalysis results, and solution NMR and FT-IR
spectra. See DOI: 10.1039/b913035g
Scheme 1 Synthesis of hexagonal mesoporous material and solid acid
catalyst with solvent extractable phosphonated pluronics template,
P
P123 .
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Fig. 1 Small angle X-ray scattering (SAXS) of mesoporous silica as
P
synthesized from phosphonated template, P123 (SBA15 ) and
P
P
following solvent washing (SBA15 -sw) and calcination (SBA15 -cal).
P
P
Fig. 2 Nitrogen gas adsorption/desorption isotherms of SBA15
P
(from P123 ) and SBA15 (from P123) after template extraction with
P
Inset: transmission electron micrograph of SBA15 -sw.
solvent washing and calcination.
P
Fig. 1 shows SAXS for material made with P123 in its
P
P
as-synthesized state (SBA15 ), after solvent washing (SBA15 -sw),
to graphs in Fig. 3B (P123 template), a significant difference
particularly between the two different solvent washes
P
and after calcination (SBA15 -cal). All three samples yield a
ꢁ1
P
was observed. For the samples synthesized from P123 , the
peak near 0.6 nm ; higher order peaks are also discernible.
P
The scattering for sample SBA15 -sw is similar to that of
calculated difference between the weight losses in the range of
B200 1C–600 1C for unextracted material and solvent washed
for 24 h was 14%; when extracted for 48 h, an additional
weight loss of 6% was observed (Fig. 3A). For the SBA15
sample synthesized from P123, the solvent washed sample lost
only about 3% mass compared to the untreated sample. This
clearly indicates that a simple solvent washing that is unable to
P
SBA15 , which supports the idea that the gentler approach
of solvent extraction will not change the mesostructure.
P
SBA15 -cal showed slight unit cell shrinkage indicating that
high temperature calcination reduced the unit cell of the
material. The XRD patterns also exhibited that the materials
have ordered mesostructures with unit cell dimensions of 11.5,
P
P
P
P
1
1.5, and 10.3 nm for SBA15 , SBA15 -sw, and SBA15 -cal,
remove P123 template does remove most of the mass of P123
respectively. The TEM images are consistent with an
hexagonally ordered mesostructure (Fig. 1).
template. We attribute the latter to the hydrolytic cleavage of
P
terminal phosphate ester groups of P123 , facilitating bulk
Fig. 2 illustrates nitrogen gas adsorption isotherms for
several template-extracted samples; they all have type IV
isotherms, which is indicative of the presence of mesoporous
structures in the materials. Compared with SBA-15, the
removal of the template leaving residual phosphate or
phosphonic acid groups intact on the walls of the material.
To confirm this, we characterized the sample with elemental
analysis, FT-IR, which showed residual phosphorus after
solvent washing (Fig. S10 and Table S2, ESIw). Because
the content of phosphorus in the final samples is small
P
samples prepared from P123 showed two desorption steps
indicating that they have bottle-neck shaped pores or pore
1
0
31
clogging. This may be due to the presence of residual
phosphate ester groups on the walls of the materials after
solvent washing or possible residues of phosphorus oxides
from oxidation of phosphate esters inside the mesopores upon
(Table S2, ESIw), P solid-state NMR and X-ray photo-
emission spectroscopy (XPS) did not show peaks corresponding
to phosphorus. The low amount of residual phosphorus may
1
1
calcination. The pore size distribution in Fig. S7 (ESIw) that
was calculated from the isotherms revealed that sample
P
P
P
SBA15 -sw, SBA15 -cal and SBA15 have major pore
diameters of 8.5, 8.5, and 8.0 nm, respectively. It is worth
noting that SBA-15 has insignificant complementary or
interconnecting small pores with only 0.09 cc/g pore volume,
P
while samples synthesized with P123 have significant
amounts of such pores with an average size of 2.3 nm and
with 40.2 cc/g pore volume (Table S1 and Fig. S7, ESIw).
The thermogravimetric analysis (TGA) traces (Fig. 3) for
the mesoporous samples show two regions of major weight
loss. In the first B25–100 1C, a distinct drop in weight is
observed which is attributed to evaporation of physisorbed
water. In the second drop between B200 1C–600 1C, another
distinct weight loss is observed due to the oxidation of the
organic template. Upon comparing the graphs in Fig. 3A
P
mesoporous samples made using the new P123 template)
Fig. 3 Thermogravimetric traces before and after solvent washing of
P
mesoporous materials made with (a) P123 and (b) as-received P123.
(
6
202 | Chem. Commun., 2009, 6201–6203
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demonstrated its extraction by a simple solvent washing; the
solvent washing procedure had little effect on material
prepared using the P123 template. The residual phosphate
ester groups could be converted into phosphonic acids,
making the material a solid acid nanocatalyst; its catalytic
activity was demonstrated by acid-catalyzed pinacole to
pinacolone transformation.
Scheme 2 Catalytic transformation of benzopinacol to benzopinaco-
P
lone using P123 as a catalyst.
We gratefully acknowledge the financial support by the
Empire State Development Corporation through the Syracuse
Center of Excellence in Environmental and Energy Systems.
We thank Prof. I. Gitsov of SUNY-ESF, Prof. Mietek
Jaroniec at Kent State University and Dr. Maura Weathers
at Cornell University for valuable discussion.
be a result of hydrolysis of phosphate esters over several
9c
minutes, upon self-assembly at low pH.
Besides its ease of extraction by hydrolytic cleavage, the use
P
of the P123 template allowed the placement of residual
phosphate ester, which was converted to phosphonic acid
groups on the mesoporous framework in one-pot, creating a
solid acid nanocatalyst (see ESI for detailsw).z We tested the
solid acid catalytic properties of the material by first acidifying
its phosphate esters with dilute HCl solution and then
using the resulting material as a solid acid catalyst in the
Notes and references
z A solid acid catalyzed pinacole–pinacolone reaction was conducted
P
by using the mesoporous material, SBA15 -sw, as
a catalyst.
Typically, 0.1 g of benzopinacol was mixed and stirred with 0.1 g
1
2,13
P
SBA15 -sw catalyst and 10.0 mL of toluene at 80 1C. Samples were
benzopinacole–benzopinacolone rearrangement
(Scheme 2)
1
4
P
collected at 0, 6, 12 and 24 h after starting the reactions for three
separate runs using the same catalyst to show recyclability. Three
different temperatures (80, 100, and 120 1C) were also tried close the
boiling point of solvent used (toluene B 110 1C) to determine the most
efficient temperature for the transformation of benzopinacol into
benzopinacolone. Control experiment using SBA15 as a catalyst was
also conducted in the same way. The reaction mixture was taken with a
as well as esterification reactions. The acidified SBA15
material showed nearly a 100% conversion of benzopinacole
with 100% selectivity towards benzopinacolane in 12 h at
1
20 1C (Table 1). Furthermore, the catalyst was recycled three
times without loss of its catalytic activity. The product was
1
confirmed by GC, GC-MS, FT-IR, and H NMR spectro-
1
filter syringe and characterized by H NMR spectroscopy.
scopy. To perform the control reaction, we used an acidified
SBA15 (see ESIw). This sample resulted in no reaction under
the same conditions. It is also worth noting that this catalytic
reaction is dependent on the reaction temperature, where
higher temperatures favored this catalytic transformation
1
(a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710; (b) S. Inagaki, Y. Fukushima
and K. Kuroda, J. Chem. Soc., Chem. Commun., 1993, 680.
D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,
B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.
(a) A. Katiyar and N. G. Pinto, Small, 2006, 2, 644; (b) J. Lu,
M. Liong, S. Sherman, T. Xia, M. Kovochichm, A. E. Nel,
J. I. Zink and F. Tamanoi, NanoBiotechnology, 2007, 3, 89;
2
3
(
Table 1).
We also used this catalyst successfully for esterification
(
c) M. Vallet-Regı, Chem.–Eur. J., 2006, 12, 5934.
´
reaction of acetic acid with ethanol. The catalyst produced
4
5
(a) T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin,
Nature, 1999, 402, 867; (b) S. Huh, J. W. Wiench, J.-C. Yoo,
M. Pruski and V. S.-Y. Lin, Chem. Mater., 2003, 15, 4247.
(a) M. Kruk, M. Jaroniec, C. H. Ko and R. Ryoo, Chem. Mater.,
40% conversion with 100% selectivity to ethyl acetate at
75 1C in 24 h whereas in a control run with SBA15 only 25%
conversion in the same time was observed. Also this catalyst was
successfully recycled 5 times with only marginal loss in activity.
This slight loss in activity was mainly due to handling losses of
catalyst (see ESIw). These experiments suggest that we have
generated an important catalyst with a systematic design and
synthetic approach using easily extractable polymer template.
In conclusion, we have synthesized mesoporous silica by
2
000, 12, 1961; (b) C.-M. Yang, H.-A. Lin, B. Zibrowius,
B. Spliethoff, F. Schuth, S.-C. Liou, M.-W. Chu and
C.-H. Chen, Chem. Mater., 2007, 19, 3205.
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¨
2
P
preparing and using a phosphonated template, P123 , and
8
X. Wang, K. S. K. Lin, J. C. C. Chan and S. Cheng, J. Phys. Chem.
B, 2005, 109, 1763.
Table 1 % Conversion data for benzopinacol to benzopinacolone
transformation
a
9 (a) K. Troev, I. Tsatcheva, R. Georgieva and I. Gitsov, J. Polym.
Sci., Part A: Polym. Chem., 2007, 45, 1349; (b) R. Georgieva,
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Entry
Sample
t/h
T/1C
% Conversion
p
1
2
3
4
5
6
7
8
9
SBA15
SBA15
SBA15
6
12
24
6
12
24
6
80
80
100
80
33.0
39.6
66.3
33.0
5797; (c) I. Gitsov and F. E. Johnson, J. Polym. Sci., Part A:
Polym. Chem., 2008, 46, 4130.
p
p
p
1
1
1
0 E. B. Celer, M. Kruk, Y. Zuzek and M. Jaroniec, J. Mater. Chem.,
2
b
b
b
c
c
c
SBA15 -1
006, 16, 2824.
1 D. Carta, D. M. Pickup, J. C. Knowles, M. E. Smith and
R. J. Newport, J. Mater. Chem., 2005, 15, 2134.
p
SBA15 -1
80
47.6
p
SBA15 -1
120
120
120
120
B100
94.8
98.3
p
SBA15 -2
2 (a) M. Jurado-Gonzalez, D. L. Ou, B. Ormsby, A. C. Sullivan and
J. R. H. Wilson, Chem. Commun., 2001, 67; (b) B.-Y. Hsu and
S. Cheng, Microporous Mesoporous Mater., 1998, 21, 505.
3 D. J. Upadhyaya and S. D. Samant, Appl. Catal., A, 2008, 340, 42.
p
SBA15 -2
12
24
p
SBA15 -2
B100
a
1
b
H NMR. The catalyst after
1
%
recycling once. The catalyst after recycling twice.
Conversion determined by
c
14 Y.-M. Liu, J. Xu, L.-C. Wang, Y. Cao, H.-Y. He and K.-N. Fan,
Catal. Lett., 2008, 125, 62.
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6201–6203 | 6203