NEWSDASFORALPOS · Use of newly designed organic molecules as large and efficient structure directing agents for the synthesis of microporous aluminophosphates

Use of newly designed organic molecules as large and efficient structure directing agents for the synthesis of microporous aluminophosphates

Nowadays, most of the processes in the chemical industry exploit the benefits of performing the reactions with catalysts, preferably heterogeneous catalysts due to its improved sustainability; in this context, microporous zeolite-type materials had a tremendous impact on the chemical industry, especially in petrochemistry. Chemical processing of the large molecular substrates typical of fine and pharmaceutical chemistry strongly relies on the discovery of extra-large-pore microporous materials, which represented the main objective of the present research project. Microporous materials usually require the addition of organic molecules to the synthesis mixture that direct the crystallization towards a particular framework type, and hence they are referred to as structure-directing agents (SDAs). Indeed, such SDAs provided the most direct strategy to yield large-pore microporous materials by using large organic molecules. However, the increase of the molecular size of SDAs is limited by the chemical requirements to be efficient, and hence alternative structure-direction strategies are required. Supramolecular chemistry, which drives the formation of multi-molecular aggregates, for instance by using aromatic amines that self-assemble through π-π type intermolecular interactions between the rings, represents an exceptional tool to increase the size of the organic molecules to be used as SDAs while fulfilling their chemical requirements. In this project, the fundamental basis of knowledge of the different chemical factors governing the supramolecular behaviour of these organic molecules has been settled down, which will be extremely helpful for rationally designing new efficient self-assembling SDA molecules to potentially produce new extra-large-pore microporous materials. Several aromatic amines have been successfully employed as SDAs for the synthesis of aluminophosphate-based microporous materials, including molecules with one aromatic ring ((1R,2S-ephedrine, benzylpyrrolidine and (S)-N-benzyl-2-pyrrolidine-methanol and its fluorinated derivatives in the aromatic ring) as well as with two aromatic rings (diphenylguanidine, 1,3-Di-o-tolylguanidine, phenylimidazol). All these molecules yielded different types of host-guest materials, most of which with unknown frameworks. Some of these materials are low-dimensional layered-like frameworks, while others are actual microporous material ready to be used as catalysts. Indeed, we have been able to solve, for the first time, the framework structure of one of these materials, where a very strong supramolecular self-assembly has been observed. All this work has allowed gaining fundamental insights on the structure-directing action of these self-assembling molecules which is governed by their associated supramolecular chemistry: the supramolecular behaviour depends i) on the π-electronic density of the aromatic ring, which is the main driving force for the self-assembly, which in turn depends on the nature of the aromatic ring and the presence of substituents, ii) on the molecular structure (in terms of size and shape) of the N-substituents, and specially on the presence of hydroxyl (or hydrophilic in general) moieties that will dictate the interaction with polar species such as water molecules or the aluminophosphate network, and iii) on the steric repulsion generated in the aromatic rings through the presence of bulky substituents, which will determine the nature and size of the supramolecular aggregates (in terms of number of molecules composing the aggregate), which in turn control the type (layered or 3D-microporous) framework that is formed. All these factors have to be taken into account in order to rationally design new and efficient self-assembling SDAs. A very interesting case has been observed in particular when using (1R,2S)-ephedrine as SDA. This molecule has led to a complete supramolecular aggregation when occluded within the one-dimensional large pores of the AFI structure; only dimers are observed within the channels. Interestingly, molecular simulations indicate that these dimers are arranged in a helicoidal fashion within the pores, with consecutive dimers rotated always an angle of 30 degrees. Moreover, such supramolecular helicoidal organic arrangement can be potentially transferred to the spatial distribution of the dopants within the AFI framework, resulting in a helicoidal enantiomorphically pure double-helix of dopants, thus imparting chirality in the microporous material, which could potentially induce enantioselectivity in chemical processes. The results obtained in this research project have contributed to the achievement of a Ramón y Cajal position to the fellow, which is a five(plus two)-years fellowship position that usually precedes the achievement of a permanent position.

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