The stability and mobility of a set of organic structure-directing agents (OSDAs) with different molecular geometries and chargedistribution confined within the pear-like cavities of neutral and Al-containingmodels of AEI zeolites have been investigated by using static densityfunctional theory calculations and ab initio molecular dynamics simulations.The objective is to identify the role of electrostatic interactions between theOSDAs’ positive charge at N+ atoms and the anionic framework AlO4−centers on the preferential stabilization of Al at specific crystallographicpositions, opening the possibility to modulate the Al distribution in AEIzeolites. We find that several classical piperidinium-based OSDAs withdiverse methyl-substituent patterns in the N-containing ring but asymmetrical charge distribution, as well as bulkier nonclassical azoniabicycle-heptane-based OSDAs with the positive chargeasymmetrically located at one side of the molecule, behave similarly. All of them remain almost immobile at the center of the aeicavity along the simulations and always stabilize Al preferentially at the T1 crystallographic position. In contrast, an azabicyclo-octane-based OSDA with the positive charge located outside a cyclo-octane ring lacking substituents exhibits an enhanced mobilitythat includes full rotation within the aei cage and the ability to reach the regions of the cavity not accessible to the other OSDAsinvestigated. As a result, this highly mobile OSDA preferentially stabilizes Al in the T3 site, which might lead to differences incatalyst activity and stability for zeolite samples synthesized using this OSDA.
Abstract