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Synthesis and Catalytic Activity of Doped Nanoporous Oxides (zeolites, AlPOs)
Modelling the incorporation of dopant ions in zeolite and aluminophosphate (AlPO) frameworks is a long-standing area of expertise of the group. At present, we consider the incorporation of vanadium dopants in AlPOs (Mr. M. Deifallah). The doped frameworks are described using periodic quantum mechanical calculations, to understand the interplay between the microporous framework structure and the atomic, chemical and structural properties of the dopants in determining adsorption and catalytic activity of the solid. The quantum mechanical calculations have also been used to parameterise a set of interatomic potentials suitable to describe the doped frameworks. These forcefields are now employed in studies of organic/inorganic structures, as required to model diffusion of organic molecules, but also the templated synthesis of the doped frameworks (Dr. L. Hortiguela, Prof. J. Pariente). These microporous oxides are in fact synthesized through hydrothermal methods and in the presence of water-soluble organic molecules, called structure directing agents (SDA). One of our goals is to understand the mutual interaction of dopants and SDAs during the synthesis, and ultimately to identify suitable SDAs able to yield ordered dopant incorporation as a way to taylor selectivity. Among the SDA molecules investigated, one interesting class is represented by amines with one or two benzyl rings, which have been demonstrated experimentally to be efficient templates for AlPO-5 and SAPO-5 materials with AFI framework topology. These molecules display self-assembly, driven by the overlap of the benzyl rings of adjacent molecules. Mono-benzyl SDAs self-assemble into stable dimers. The units that define the main channel of the AFI structure are rotated by 30° upon translation of half unit vector along the channel. This feature causes a preferential orientation of adjacent SDA dimers; if properly mastered to yield long-range order, it could give rise to chiral arrangements of the templates. To achieve this goal, we have designed and successfully synthesized a modified SDA, which contains a substituent in the region where adjacent SDA dimers interact. The steric and electrostatic effects align adjacent SDAs along one preferential direction; modelling work has demonstrated a spontaneous self-assemble of the modified SDA molecule into chiral helices. The stabilisation of chiral components during the synthesis of a heterogeneous catalyst is a feature of obvious practical interest. Although AFI is a non-chiral framework, we are now investigating whether it is possible to transfer the chirality from the SDA to the inorganic structure via the inclusion of suitable dopant atoms.
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