Computational study of the structure-directing effect of benzylpyrrolidine and its fluorinated derivatives in the synthesis of the aluminophosphate AIPO-5

Using a combination of computer modeling techniques, we have investigated the ability of benzylpyrrolidine and its fluorinated derivatives in ortho, meta, and para positions of the phenyl ring to direct the synthesis of the aluminophosphate AIPO-5. The o- and p-fluoro derivatives are not good templates because of the poor packing of the template molecules inside the AIPO-5 pores, due to a repulsion provoked by the fluorine atoms. However, benzylpyrrolidine and the m-fluoro derivative do direct the synthesis of AIPO-5, the latter being a better template due to higher electrostatic interactions with the framework. We demonstrate that, at least when the synthesis is performed with an excess of template molecules, the ability of organic templating molecules to direct the synthesis of microporous materials depends not on the host-guest interaction energy per unit of template molecules, as usually calculated, but on the density of interaction energy, i.e., the energy per formula unit of the microporous network. The packing density of molecules inside the channel system must be taken properly into account. From the calculated location of the benzylpyrrolidine molecules and their m-fluoro derivative inside the inorganic network, we would expect the formation of stable dimers.