Directing zeolite structures into hierarchically nanoporous architectures

Article abstract of DOI:10.1126/science.1204452

Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

Full text of DOI:10.1126/science.1204452

Pores Within Pores−−How to Craft Ordered Hierarchical Zeolites
Karin Möller and Thomas Bein
Science 333, 297 (2011);
DOI: 10.1126/science.1208528
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PERSPECTIVES
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10.1126/science.1210358
CHEMISTRY
Thin walls of crystalline zeolites can be
assembled into hexagonal nanopore networks,
which expands the range of their catalytic
reactions to larger molecules.
Pores Within Pores—How to Craft
Ordered Hierarchical Zeolites
Karin Möller and Thomas Bein
eolites are aluminosilicate crys- rier was broken in the early 1990s with the blies such as the above surfactant micelles (5),
tals that have internal networks of synthesis of mesoporous oxides by using polymers (6), or objects such as carbon beads
angstrom-size pores, similar to the larger amphiphilic surfactant templates (4). or fibers (7) were used to create mesoporos-
Z
dimensions of small molecules. They are However, the local order in the resulting ity. Alternative approaches include chemical
among the most widely used materials in (alumino-)silicates was lost, and with it their treatments that partially dissolve the crystal-
heterogeneous catalysis (1, 2) because of strong acidity. Efforts to recrystallize these line zeolite lattice and create larger intrapar-
their defined structure and composition. amorphous walls into zeolitic structures ticle cavities (8). The need for a secondary
Although zeolites are very potent solid- were usually unsuccessful.
acid catalysts, their catalytic applications Dual-templatingapproachesarenowbeing assembly of nanosized zeolite particles, thus
have been limited to processing smaller explored that combine the advantages of zeo- creating mesoporous interparticle voids (9).
molecules; their internal pores are not read- lites and the mesoporous oxides. In addition to The other approach for allowing access
template can also be avoided by the direct
ily accessed by molecules exceeding 1 nm the molecular zeolite templates, larger assem- of larger molecules to zeolite pores is to pre-
in size. Major efforts have been directed to
overcoming this limitation. On page 328 of
A
this issue, Na et al. (3) present a new strat-
egy for creating thin zeolite walls, contain-
ing small pores, that grow into structures
forming larger pores that can catalyze reac-
tions with larger molecules.
To date, two major synthesis strategies
have been explored to create zeolites with
additional larger pores. One is to form a
secondary pore system of larger size than
the zeolitic micropores within the zeolite
crystal, thereby allowing faster diffusion of
larger molecules into the zeolite particles.
In many zeolite synthesis routes, molecular
500 nm
10 nm
“templates” are added to aid the growth of
the aluminosilicate crystals; when the syn-
thesis is completed and the template mol-
ecules are removed, zeolitic pore spaces
remain. Traditional methods in zeolite syn-
thesis use a single molecular template with
a size similar to or smaller than the micro-
pore dimensions. This micropore size bar-
B
Ordering the walls. Na et al. have used large molecular templates to grow thin walls of zeolites into mor-
phologies that create ordered mesopores. (A) Two electron microscopy images of the hexagonally grown
mesoporous MFI-type zeolite with extremely high surface area. The inset shows a schematic of the hierarchi-
cal structure and the MFI pore framework of the zeolite walls. (B) Examples of the bifunctional templating
molecules used in the synthesis that bear ammonium groups and long alkyl chains (white spheres, hydrogen;
gray spheres, carbon; red spheres, nitrogen).
Department of Chemistry, Ludwig-Maximilians University,
81377 Munich, Germany. E-mail: karin.moeller@cup.uni-
muenchen.de; bein@lmu.de
www.sciencemag.org SCIENCE VOL 333 15 JULY 2011
Published by AAAS
297

PERSPECTIVES
pare extremely thin slices of zeolite crystals two unit cells of the important MFI zeolite
Changing the ammonium cation spacer
that expose all or most of the pores to sur- structure were obtained from gels contain- from an alkyl group to a phenyl group in
face adsorption. This approach would elim- ing the C22H45-N+(CH3)2-C6H12-N+(CH3)2- these bifunctional templates allowed Na et al.
inate the need for diffusion to the internal C6H13 template—alkyl chains joined by two to synthesize extremely small (10 nm) crys-
pores and can also produce very large sur- alkyl-bridged ammonium head groups (11). tallites of a different zeolite called beta.These
face areas. Indeed, there are a few examples A disordered assembly of these sheets was beta crystallites were intergrown in a disor-
of such architectures where two-dimensional obtained upon removal of the template, leav- dered assembly to produce very large accessi-
(2D) sheets of zeolites form with the thick- ing behind large open surfaces enclosing ble zeolite crystal surfaces. Further, Na et al.
ness of just a few unit cells, the repeat unit of mesopores of about 15 nm. systematically increased the zeolite domain
the crystal. These are formed during a zeolite Building on their groundbreaking dual- size by adding multiples of the molecular
synthesis relying solely on a small molecular templating concept, Na et al. have now zeolite-directing units into the large template
template (10). However, it is not easy to keep grown thin zeolite walls into beautifully molecule. We might surmise that the “zeolite
these 2D sheets exposed; during synthesis, ordered 3D hierarchical zeolite architec- template modules” can also be chemically
the sheets will tend to crystallize and stack tures (see the figure, panel A). They use altered in a rational way to direct the syn-
like a deck of cards, rather than stay open like specially designed surfactants featuring thesis toward other zeolite structures. These
a house of cards. Maintaining an open, hier- two long alkyl chains linked by differently rationally designed multifunctional templates
archical arrangement often calls for com- bridged ammonium groups to direct zeolite thus establish a new toolbox for the direct
plex multistep synthetic strategies, and more synthesis (see the figure, panel B). Strik- synthesis of complex hierarchical zeolite
direct routes for nudging zeolite structures ingly, these new gemini-type surfactants structures tuned and optimized toward spe-
into accessible architectures with nanoscale such as C18H37-N+(CH3)2-C6H12 -N+(CH3)2- cific applications.
thin walls are needed.
C6H12-N+(CH3)2-C18H37 stimulated the
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10.1126/science.1208528
COMPUTER SCIENCE
A rigorous mathematical proof shows that the
phase transition in an evolving random net-
work is continuous.
Networking—Smoothly Does It
Svante Janson
network, or graph, is a collection of components, but increasing the number of puters with physical connections, or as a col-
nodes together with a collection of links beyond a critical density leads to a phase lection of Web pages with logical links (5).
links joining some pairs of nodes. transition and the sudden emergence of one Graphs are also used to describe social net-
A
Networks are often used to describe and “giant” component. For disease modeling, the works, interactions between proteins in a
model many different types of structure in the latter case indicates the possibility of a large cell, collaborations between scientists, tele-
real world—the spread of infectious diseases, epidemic.There was substantial interest when phone calls in a given day, and so on.
for example, where nodes are people and links Achlioptas et al. (2) announced, based on
The mathematical study of random
represent contacts that may spread the disease numerical simulations, that a specific random graphs was begun 50 years ago by Erdös
(1). It is often useful to model such structures network model seemed to exhibit a behavior and Rényi (6). Their model is simple: Start
by random networks, constructed by add- at the phase transition that was different from with a large number of nodes and add links
ing links to the nodes by some well-defined all others known, with a much more abrupt one by one, each link joining two randomly
random procedure. Many different models or “explosive” behavior. On page 322 of this chosen nodes. One of their main results
of random networks have been studied. An issue, Riordan and Warnke (3) show that the is that a phase transition occurs when the
important feature of many such models is that numerical simulations in (2) were misleading, number of links is half the number of nodes,
if there are only a few links, then the network and that the phase transition for this model is and that the size of the largest component
consists of a large number of small, isolated continuous, thus confirming the results of da grows linearly immediately after the phase
Costa et al. (4). transition.
Another example of a network is the Many other random constructions have
Internet, seen either as a collection of com- been studied since. Some are motivated by
Department of Mathematics, Uppsala University, Uppsala
751 06, Sweden. E-mail: svante.janson@math.uu.se
15 JULY 2011 VOL 333 SCIENCE www.sciencemag.org
Published by AAAS
298