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Ordered Porous Nanoarchitectures with Specific Functions.

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Meeting Reviews
Ordered Porous
Nanoarchitectures with Specific
Functions**
Nicola Hsing
Porosity—or “the nothing”, as it was
recently termed—is an important and
integral part of the architectural design
of zeolites and related mesoscopically
organized materials, such as MCM-41
and SBA-15.[1, 2] Although these materials can be very different in terms of
pore size, wall structure (amorphous or
crystalline), chemical composition, and
synthesis, the introduction of the desired
(multi)functionality for possible applications, for example, through the incorporation of guest species in the pores, on
the walls, or as an integral part of the
pore walls, remains one of the key challenges. The growing interest in this field
can be seen in the fact that more and
more researchers—not only from classical solid-state science, which is typically
related to zeolite chemistry, but also
from areas of soft-materials chemistry,
such as sol-gel chemistry, organic and
polymer chemistry, supramolecular
chemistry, and even biochemistry—are
working on these materials.[3] This high
interdisciplinarity and the growing interest in this research field were taken into
account by Frank Marlow (MPI M1lheim, Germany) in the organization of
the EURESCO conference on guestfunctionalized molecular-sieve systems,
not only through the invitation of wellknown scientists from several different
[*] N. H0sing
Technische Universit3t Wien (Austria)
E-mail: nhuesing@mail.zserv.tuwien.ac.at
[**] EURESCO Conference on Zeolite Molecular Sieves: Euroconference on GuestFunctionalised Molecular Sieve Systems,
Hattingen (Germany), March 20–25,
2004.Organized by Dr. Frank Marlow.
3216
scientific areas, but
also
through
the
opportunity given to
young scientists from
Eastern
European
countries to participate at this meeting.
The main focus of
the conference was on
the functionalization
of molecular sieves by
the incorporation of
guest species. Molecular-sieve systems can
be composed of a
large variety of materials, from aluminosilicates and pure silicates, inorganic phosphates,
and
inorganic–organic hybrids
to purely carbonbased materials, to Figure 1. Inclusion of guest species in a molecular sieve, in this case
mention but a few. hexagonal MCM-41: A) inclusion by treatment of a preformed matrix
Many of these types with a gas or liquid; B) ship-in-a-bottle synthesis; C) grafting of funcof materials were re- tional entities through covalent bonding in a post-treatment step;
presented in the vari- D) co-condensation reactions of network-forming and functionalizing
molecules; E) bridged silanes in the synthesis of PMOs.
ous oral and poster
presentations.
Not
only because of this variety, many differ- is called grafting if carried out in two
ent options exist for the functionaliza- steps or true co-condensation if carried
tion
of
molecular-sieve
systems out in a single step. Some presentations
(Figure 1). Topics such as the inclusion covered the possibility of the post-treatof molecules or guest entities, such as ment of preformed matrices through the
dyes and biomolecules, by adsorption grafting of functional molecules. Eleinto a preformed porous matrix from gant approaches are based on true in
solution or the gas phase were discussed, situ co-condensation reactions well
as well as the in situ incorporation of known from sol-gel science. A highlight
noncovalently included functionalities in this area are the so-called periodic
during the synthesis of the host matrix. mesoporous organosilicas (PMOs), as
The latter approach is complex, as the was clear from the discussions after the
active entity should also act as a tem- talk of H. Garcia (Valencia, Spain) on
plate molecule so that the periodic co-condensation reactions of bifuncpore structure is not disturbed. An alter- tional, bridged organosilanes with tetnative approach presented by D. W;hrle raethoxysilanes to form a hybrid mate(Bremen, Germany) is stepwise synthe- rial with periodically arranged pores in
sis within a preformed porous host which the organic functionality is an
matrix, for example, of a dye molecule. integral part of the pore walls of the
This method, also termed “ship-in-a- porous material.[5] One focal point of
bottle synthesis”, is especially attractive, the discussion, which was led by M.
as it allows the incorporation of large Froeba (Giessen, Germany), was the
molecules, such as dyes, which can not difference between this type of material
be diffused into the matrix because of and “typical” hybrid sol-gel materials,
size restrictions. An additional advant- for example, in terms of the acidity of
age is that leaching of the active mole- the corresponding silanols. In conneccule is thus prevented.[4]
tion with this point, the question arose
Another approach is based on con- as to whether the term PMO truly classidensation reactions of functional enti- fies the uniqueness and significance of
ties with the host matrix. This method these systems.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460497
Angew. Chem. Int. Ed. 2004, 43, 3216 – 3217
Angewandte
Chemie
In addition to the scientific presentations, a lot of time was allocated during
the conference for general discussions,
many of which were particularly fruitful
because of the diverse research areas
represented by the scientists present.
Not only was the uniqueness of PMOs
analyzed, but also other issues, such as
the very important question as to
whether periodicity of the pores is
actually necessary for later applications.
Many porous sol–gel materials can be
utilized in similar or the same applications as the corresponding microporous
zeolites or mesoporous molecular
sieves, for example, in separations, in
catalysis, or as a support for active species.[6] In both cases (that is, whether a
system is arranged periodically or not),
an activity enhancement can be found,
for example, for dyes or biomolecules,
or in certain catalytic applications (P.
Wright (St. Andrews, UK), D.
W;hrle). C. Sanchez showed an intelligent combination of sol–gel and surfactant solution chemistry, which yieled
mesoporous materials in a wide variety
of chemical compositions and nanostructures. The establishment of efficient communication pathways between
the different scientific disciplines is
indispensable for efficient research in
this area in the future. For some applications, such as those that require excellent reproducibility in the synthesis,
alignment, or preferential orientation
of guest molecules within the pores, an
ordered arrangement of the pore channels has some distinct advantages. For
dye molecules in particular, an ordered
arrangement can lead to specific properties, such as frequency doubling or optical switching. F. Laeri (Darmstadt, Germany) presented one potential application in zeolite-based lasing systems.
It was evident from all presentations
that the characterization techniques
necessary for a thorough investigation
of the structure and properties of these
molecular-sieve systems must be highly
advanced and again interdisciplinary.
Depending on the type of host–guest
system, optical, electrical, or magnetic
techniques may be required (F. Tihay
(Rueil-Malmaison, France), B. Weckhuysen, (Utrecht, The Netherlands)).
The art of generating and evaluating
aesthetic transmission and scanning
electron microscopy images in the charAngew. Chem. Int. Ed. 2004, 43, 3216 – 3217
acterization of periodically
arranged
materials was impressively demonstrated
by
O.
Terasaki
(Stockholm,
Sweden). An inspiring
example
of
the
characterization of
porous materials was
provided
by
C.
BrBuchle (M1nchen,
Germany),
who
showed that singlemolecule
spectroscopy is a useful tool
for the analysis not
only of the architectural features of the
channel systems of
porous
substances,
Figure 2. Energy transfer by different dye molecules aligned in the channel
but also of the oriensystem of an aluminophosphate (AlPO4-5; with the kind permission of D.
tation of dye moleW@hrle, Bremen).
cules within the channels (Figure 2).[7]
Technologically important questions direction, bringing together scientists
were also addressed, such as the fabrica- from different fields in a scientifically
tion of molecular-sieve systems in mor- stimulating environment. It was clearly
phologies other than powders, for exam- shown that zeolites and related molecuple, as zeolite membranes (J. Caro lar sieves are not only aesthetic materi(Hannover, Germany)). Films are quite als, but that emerging fields for ordered
easy to prepare from systems with host systems can be found in catalysis
larger pores and amorphous pore walls, and nanotechnology, for example, in
such as M41S-type molecular sieves. enantioselective catalysis, sensors, solid
However, in the case of a one-dimen- lasers, intelligent switches, membranes,
sional channel system the pores are waveguides, and solar cells, by combinalways aligned parallel to the substrate. ing the know-how and expertise from
The alignment of mesoporous channels the various disciplines. The next EURin thin films vertical to the substrate sur- ESCO conference of this series will be
face by applying external fields (T. Bein organized by M. LindEn and held in Hel(M1nchen, Germany)) and the fabrica- sinki (Finland).
tion of zeolitic architectures as shapeselective membranes are areas still in
their infancy. The first zeolite membranes for hydrophilic pervaporation [1] C. S. Cundy, P. A. Cox, Chem. Rev. 2003,
103, 663.
are ready to enter the market. However,
for shape-selective applications, the reli- [2] J. Y. Ying, C. P. Mehnert, M. S. Wong,
Angew. Chem. 1999, 111, 58; Angew.
ability and performance of the correChem. Int. Ed. 1999, 38, 56.
sponding membranes must still be
[3] M. E. Davis, Nature 2002, 417, 813.
improved.
[4] A. Corma, H. Garcia, Eur. J. Inorg.
Although zeolites and molecularChem. 2004, 6, 1143.
sieving systems of different types have [5] G. Kickelbick, Angew. Chem. 2004, 116,
3164; Angew. Chem. Int. Ed. 2004, 43,
been known for quite some time, true
3102.
large-scale commercial applications for
these materials are still rare. To broaden [6] F. Ribot, C. Sanchez, Comments Inorg.
Chem. 1999, 20, 327.
the prospects for these materials, new
[7] M. Ganschow, C. Hellriegel, E. Kneuper,
fields and interdisciplinary networking
M. Wark, C. Thiel, G. Schulz-Ekloff, C.
have to be established. The EURESCO
BrBuchle, D. W;hrle, Adv. Funct. Mater.
conference was a big step forward in this
2004, 14, 269.
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3217
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