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Porous solids get organized

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A powerful combination of analytical techniques is used to shed light on the complex crystallizations of porous solids. Molecular recognition creates the seeds of order from which complex lattices grow.

Although the era of empirical chemistry is seemingly long gone, many chemical processes are still poorly understood. The formation of crystalline porous solids with nanoscale cavities is a case in point. These materials are extremely useful catalysts for many industrial chemical reactions, but we have little understanding of how they crystallize from their disordered precursors. Such insight could enable better catalysts to be designed. A novel combination of spectroscopic tools has now been used to unravel the mechanistic details of the synthesis of complex nanoporous solids, as reported in two papers in the Journal of the American Chemical Society1,2. The results suggest that molecular complexes in the synthetic mixture 'recognize' template molecules, so forming clusters from which ordered structures may grow.

Zeolites are porous solids that contain channels up to 1 nanometre long. The building-blocks of these solids are tetrahedra of oxygen atoms, with a cation at the centre of each tetrahedron. Three-dimensional networks are formed through corner-sharing of the tetrahedra. From the infinite number of theoretical frameworks that can be constructed from these building-blocks, only about 150 different zeolites are found experimentally.

Archetypal zeolites have a lattice framework that is electrostatically neutral, such as is seen in silicalite, which is constructed from SiO4 tetrahedra. Similar structures, known as aluminophosphates, can be constructed from combinations of aluminium-based tetrahedra (AlO4) and phosphorus-based tetrahedra (PO4) (ref. 3). A good example of this is the zeolite known as AlPO4-5, which contains one-dimensional channels.

Zeolites become catalysts only when reactive complexes are incorporated into the structure. This can be done in two ways. First, one can replace cations in the lattice framework with cations that have a lower positive charge. For example, when cobalt ions (Co2+) substitute for some of the aluminium ions (Al3+) in aluminophosphates, a porous solid is generated that catalyses oxidations4. Such cation substitutions result in a framework that has an overall negative charge. This can be balanced by placing cations in the nanoporous channels. If reactive cations are used, this is a second way to activate catalytic behaviour.

A breakthrough in zeolite science came with the discovery that previously unknown structures could be made by adding organic bases to the chemical reactions used to prepare zeolites5. These organic bases seem to act as a template for the formation of micropores and are now commonly used in zeolite synthesis, although the molecular details of their role are poorly understood.

Now, Weckhuysen and colleagues1,2 have advanced our understanding of the molecular mechanism for the organic-base-mediated synthesis of zeolites. Zeolites usually form as gels, which then crystallize into the desired microporous solids. The authors used a combination of in situ techniques to examine this crystallization process. They applied X-ray scattering methods1 previously also used to study silicalite synthesis from solution6 to probe the dimensions of particle aggregates and the presence of crystals. They combined this with spectroscopy2 to investigate the local environment of the atoms.

Weckhuysen and colleagues compared the formation of the chargeless AlPO4-5 framework with the negatively charged framework (known as ZnAPO-34) that is formed by replacing aluminium ions in AlPO4-5 with zinc ions (Zn2+). The authors followed not only the structural changes in the aluminophosphate gel in real time, but also the conformational features of the organic base (tetraethylammonium hydroxide) used as a template for the crystallization. The tetraethylammonium cation adopts one of two conformations once it takes up its position in the aluminophosphate nanopores.

The authors prepared the ZnAPO-34 structure, which contains spherical cavities rather than channels, simply by adding zinc ions to the aluminophosphate gel. They also made similar structures by adding either cobalt ions (Co2+) or manganese ions (Mn2+) to the gel2, showing that the influence of dipositive cations on the lattice topology is general. Their studies revealed previously unknown details of the crystallization process — for example, particles of crystalline material, about 11.5 nanometres in size, initially form in the ZnAPO-34 gel before increasing in size1. Most intriguingly, not only do ZnO4 tetrahedra form in the recursor gel before crystallization begins, but the tetraethylammonium ion also adopts the conformation that it will ultimately assume in the crystal. In the case where no zinc is present and AlPO4-5 forms, the organic template takes on the alternative conformation.

These results1,2are highly relevant to the debate on the mechanism of zeolite formation and the role of organic base molecules6,7,8,9. The implication is that the tetraethylammonium ion forms a complex with developing zeolite subunits in the gel (Fig. 1), adopting a molecular structure close to that found in the final crystal2. This molecular recognition process determines which type of crystal lattice is formed. For ZnAPO-34, the distinctive spherical cavities of the crystal may actually form first in the gel, forcing the tetraethylammonium ion into the conformation seen in the solid. The near absence of this conformation in the alternative AlPO4-5 system corroborates this theory. Furthermore, Weckhuysen and colleagues' data are in line with proposals on the mechanism of silicalite formation from its synthesis solution6,9. It is thought that precursor nanoparticles of silicalite might appear before crystallization, and that these nanoparticles are aggregates of complexes formed between SiO4 units and the organic base used in the reaction.

Figure 1: Template-directed crystallization of microporous solids.
figure1

a, On synthesis, aluminophosphate crystallizes as a porous solid known as AlPO4-5, which contains regular arrays of microscopic channels. This process is directed by an organic base (tetraethylammonium hydroxide; TEAOH) in the reaction mixture. Weckhuysen and colleagues' data1,2 suggest that TEAOH forms a complex with the aluminophosphate molecules that acts as a template for constructing the AlPO4-5 lattice. b, When zinc is incorporated into some of the aluminophosphate molecules, TEAOH may adopt an alternative conformation. This yields a different intermediate complex that acts as a template for the formation of a structure known as ZnAPO-34. (Structures courtesy of Andrew M. Beale.)

Weckhuysen and colleagues1,2 have thus shed light on the mechanism of zeolite formation, providing experimental support for the idea that molecular organization occurs before crystallization. The insights obtained from this work will certainly benefit the search for the next generation of microporous materials. More generally, the authors' powerful combination of analytical techniques will enable the physical chemistry of many other synthetic processes to be monitored in situ, so that the structural changes involved can be observed as they happen.

References

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    Beale, A. M. et al. J. Am. Chem. Soc. 128, 12386–12387 (2006).

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    O'Brien, M. G., Beale, A. M., Catlow, C. R. A. & Weckhuysen, B. M. J. Am. Chem. Soc. 128, 11744–11745 (2006).

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    Flanigan, E. M., Lok, B. M., Patton, R. L. & Wilson, S. T. in New Developments in Zeolite Science and Technology Vol. 28 (eds Murakami, Y., Ijima, A. & Ward, J. W.) 103–112 (Elsevier, Amsterdam, 1986).

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    Raja, R., Sankar, G. & Thomas, J. M. J. Am. Chem. Soc. 123, 8153–8154 (2001).

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    Barrer, R. M. & Denny, P. J. J. Chem. Soc. 971–982 (1961).

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    de Moor, P.-P. E. A. et al. Chem. Eur. J. 5, 2083–2088 (1999).

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    Davis, T. M. et al. Nature Mater. 5, 400–408 (2006).

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    Nokdakis, V., Kokkoli, E., Tirell, M., Tsapatsis, M. & Vlachos, M. Chem. Mater. 12, 845–853 (2000).

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    Kirschhock, C. E. A., Ravishankar, R., van Looveren, L., Jacobs, P. A. & Martens, J. A. J. Phys. Chem. B 103, 4972–4978 (1999).

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