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Mild Methods for Removing Organic Templates from Inorganic Host Materials.

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Highlights
Porous Materials
Mild Methods for Removing Organic Templates from
Inorganic Host Materials
Jol Patarin*
Keywords:
host–guest systems · solid-state structures ·
template removal · zeolites
Inorganic porous materials with controlled pore-size distribution are very
attractive for a wide range of applications such as catalysis, adsorption, and
separation.[1–3] Two main families have
been reported: crystalline microporous
solids and organized mesoporous materials.[4] In the former, zeolites and related materials (e.g., aluminophosphates and gallophosphates) are the
most representative solids. The second
family comprises the so-called M41S
materials[5] and related solids (e.g.,
SBA type[6, 7] and MSU type[8]). Most of
these solids are prepared by hydrothermal or solvothermal treatment of an
inorganic mixture in the presence of an
organic template around which the inorganic framework forms. After synthesis, the resulting material is in fact a
nonporous organic–inorganic hybrid
since the organic agent is usually trapped in the cavities of the solid (Figure 1). Depending on the type of porous
solid, different templating agents have
been introduced in the starting mixtures: molecules such as amines, ammonium ions, or crown ethers for the
preparation of crystalline microporous
solids, and surfactants or more generally
amphiphilic molecules for the synthesis
of organized mesoporous solids. To
obtain a completely porous solid, it is
necessary to remove the organic species
from the as-made material. The most
[*] Dr. J. Patarin
Laboratoire de Mat&riaux Min&raux
ENSCMu
Universit& de Haute-Alsace
UMR CNRS 7016
3 rue Alfred Werner
68093 Mulhouse Cedex (France)
Fax: (+ 33) 3-8933-6885
E-mail: j.patarin@uha.fr
3878
Figure 1. Removal of the organic template (black and gray spheres) from an as-made material
to obtain a porous solid.
common method consists of calcining
the materials in a flow of oxygen or air.
Under these conditions the template,
which is the high-cost reactant, is destroyed. Such a procedure may also
damage the porous material. Indeed
during the calcination high local temperatures and water formation may occur,
and therefore extra-framework species
may be formed. This is the case for some
aluminosilicates, for instance, where extra-framework aluminum species can be
detected after removal of the template
by calcination. The procedure can be
optimized by first heating the sample
under an inert atmosphere with a slow
heating rate, and then introducing air or
oxygen to remove the carbon residues.
However, pore shrinkage is generally
observed. One way to decrease the cost
of synthesis of the porous inorganic
materials is to develop methods for the
nondestructive removal of the organic
template, which would allow the recovery and reuse of these expensive organic
species.
Such alternative methods have been
developed in particular for mesoporous
materials. In these solids, the frame-
work–surfactant interactions are weak
(electrostatic, van der Waals or hydrogen-bonding interactions), and the pore
size is larger than that observed for
microporous solids. Therefore, the extraction of the template can be carried
out by mild treatments such as liquid
extraction using acidic solutions, neutral
salt solutions, alcohols, or mixture of
these.[9, 10] For instance, about 70 % of
the hexadecyltrimethylammonium ions
used as templates in the synthesis of AlMCM-41 could be removed by extraction with acidic ethanol (EtOH/H2SO4,
1 h, 78 8C, procedure repeated twice).[9]
Under these acidic conditions, exchange
of the sodium ions for protons is achieved simultaneously. For HMS-type
materials prepared with nonionic surfactants, the removal of the template by
solvent extraction is more effective,
which is probably due to the fact that
the framework–surfactant interactions
are weaker.[10] Similar behavior was
observed for the SBA-15 silica material
prepared in the presence of a nonionic
triblock copolymer.
In zeolites the removal of the template, which is also referred to as a
DOI: 10.1002/anie.200301740
Angew. Chem. Int. Ed. 2004, 43, 3878 –3880
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
Chemie
“structure-directing agent” (SDA), by
solvent extraction is more difficult. Attempts were reported on MFI and BEA
structure types by Jones et al.[11] For
both molecular-sieve topologies, the
amount of SDA that could be removed
by extraction was found to be dependent
on the size of the SDA and the strength
of its interaction with the molecularsieve framework. These authors showed
that for different BEA-type materials
(zincosilicate, aluminosilicate, and borosilicate) the ease of liberation of the
organic template (tetraethylammonium
ions (TEA)) is in the order Zn > B > Al
and follows the reverse trend of the
known Brønsted acidity of the various
types of sites. However, in these materials, the SDA is only partly extracted.
Moreover, for the boron-containing
samples, in particular, solvent extraction
(aqueous acetic acid, 80 8C) results in
the removal of 60 % of the boron from
the framework.[11] It is worth noting that
under these experimental conditions
complete removal of the TEA ions from
a pure silica b-zeolite is possible.
Other mild procedures have been
tested for the removal of organic species
from mesoporous inorganic frameworks. Microwave-assisted template removal was performed on organized
SBA-15 silica materials in the presence
of nitric acid and H2O2 as solvents (0.1–
0.2 g SBA-15, 1.5 mL of 15 m HNO3,
1 mL of 9.3 m H2O2).[12] In this procedure
the maximum temperature was close to
200 8C and the template (triblock copolymer, Pluronic P123, EO20PO70EO20)
can be removed completely within minutes. Compared to the ordinary calcined samples, these materials have
more highly ordered frameworks with
higher surface areas, larger pore volumes, lower structural shrinkage, and a
higher silanol-group density. Such a
procedure was also successfully applied
to mesoporous films and macromesoporous membranes.
Extraction with supercritical CO2
was used by Kawi et al.[13] and subsequently by van Grieken et al.[14] for removing organic templates from MCM41 and SBA-15 silica materials, respectively. It appears that under similar
experimental conditions no surfactant
removal is observed for MCM-41 materials in the absence of alcohol as cosolvent, whereas the solvating strength of
Angew. Chem. Int. Ed. 2004, 43, 3878 –3880
pure CO2 is sufficient to extract the
nonionic surfactant from the SBA-15
samples. However, the highest extraction efficiency was achieved in the
presence of ethanol. According to Kawi
et al.,[13] for a pure siliceous MCM-41
material, 93.2 % of the template (hexadecyltrimethylammonium ions) could
be removed from the as-made solid after
3 h under the following experimental
conditions: extraction temperature
85 8C, extraction pressure 350 bar, CO2
flow rate of 1 mL min 1, and methanol
flow rate of 0.2 mL min 1. This method
allowed the recovery and reuse of organic templates.
Ozone treatments[15, 16] have been
used to eliminate the organic template
from as-synthesized MCM-41 silica materials. In an initial study, ozone was
generated by using a UV lamp (estimated temperature close to 250 8C)
operating at a wavelength that created
ozone from atmospheric oxygen, and in
a second study, ozone was produced
directly by an ozone generator. In the
former study, after 24 h of treatment,
complete removal of the organic species
was observed (the ozone-treated sample
showed 1 % weight loss by thermogravimetric analysis at 1000 8C). In the second study, the treatment was performed
on dry samples and on as-made MCM41 samples resuspended in water or in
their mother liquors. The best results
were obtained for the resuspended samples. Thus, after 14 h, 82 % of the
organic species was removed from the
ozone-treated sample in water. However, when the treatment time was too
long (up to 26 h) the pore structure
collapsed. Compared to the calcined
samples, ozone-treated samples displayed higher surface areas, larger pore
sizes, narrower pore-size distributions,
and a higher silanol-group density.
Recently SchIth and co-workers[17]
used ether cleavage by an acid to partly
remove the triblock copolymer template
(Pluronic P123) from an as-synthesized
SBA-15 silica material. The SBA-15
material prepared at low temperature
displays micropores and mesopores, and
it was proposed that the poly(ethylene
oxide) chains (EO) of the surfactant are
occluded in the silica walls, whereas the
more hydrophobic poly(propylene oxide) chains (PO) are located in the
mesopores of the SBA-15 material.[18–20]
The authors clearly showed that the
ether cleavage by sulfuric acid at quite a
low temperature (95 8C under reflux)
generated mesopores by the removal of
the PO groups. The EO chains, which
are less accessible to the acid, could be
decomposed by a subsequent thermal
treatment in air at 200 8C to create
micropores. Such acid and low-temperature treatments led to a SBA-15 material with better characteristics than
those of the classical calcined SBA-15
material (larger mesopore size, larger
micropore volume).
A similar approach was used by Lee
et al. for removing the structure-directing agents in as-synthesized zeolites.[21]
The strategy developed by these authors
is summarized in Figure 2. It is based on
an SDA that can be cleaved easily into
fragments (A and B in Figure 2) without
Figure 2. Strategy proposed by Lee et al. for synthesizing zeolites (figure adapted from ref. [21]).
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3879
Highlights
affecting the zeolite structure. The
fragements are removed from the zeolite structure, and then recombined to
regenerate the SDA.
This strategy was successfully applied to the synthesis of the zeolite
ZSM-5. The SDA, a derivative of 1,4dioxa-8-azaspiro[4,5]decane which contains a cyclic ketal, was found occluded
intact in the as-synthesized zeolite. It
was then cleaved into ethylene glycol
and a ketone by treatment of the zeolite
in acidic solution (HCl 1m, 80 8C, 20 h).
After this treatment the ketone was still
present in the structure, and it was
removed by a subsequent treatment
with a NaOH/NaCl solution (100 8C,
72 h). Under these conditions the structure of the empty zeolite was not
affected. The two components of the
SDA can then be recombined for a
subsequent synthesis. This strategy was
also applied to the synthesis of the
zeolites ZSM-11 and ZSM-12 and might
be extended to SDAs having acetals and
ortho-ester functionalities.
In conclusion, numerous studies
have been performed to remove the
organic species from inorganic frameworks. For mesoporous organized materials, most of the mild routes developed allow the reuse of the organic
templates for a subsequent synthesis and
3880
lead to an inorganic solid with better
characteristics (higher surface area,
larger pore volume, less structural
shrinkage, and higher silanol-group density). For zeolites and related materials,
the strategy proposed by Lee and coworkers[21] opens new perspectives in
this field.
Published Online: July 7, 2004
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[4] For recent advances on these solids see:
Handbook of Porous Solids, Vol. 1–5
(Eds.: F. SchIth, K. S. W. Sing, J. Weitkamp), Wiley-VCH, Weinheim, 2002.
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[12] B. Tian, X. Liu, C. Yu, F. Gao, Q. Luo, S.
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[14] R.van Grieken, G. Calleja, G. D. Stucky,
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