Computational Modeling of the Mobility, Stability, and Al Positioning Ability of Cyclic Cationic Organic Structure-Directing Agents in AEI Zeolite

Abstract

The stability and mobility of a set of organic structure-directing agents (OSDAs) with different molecular geometries and charge distribution confined within the pear-like cavities of neutral and Al-containing models of AEI zeolites have been investigated by using static density functional theory calculations and ab initio molecular dynamics simulations. The objective is to identify the role of electrostatic interactions between the OSDAs’ positive charge at N⁺ atoms and the anionic framework AlO₄^– centers on the preferential stabilization of Al at specific crystallographic positions, opening the possibility to modulate the Al distribution in AEI zeolites. We find that several classical piperidinium-based OSDAs with diverse methyl-substituent patterns in the N-containing ring but a symmetrical charge distribution, as well as bulkier nonclassical azoniabicycle-heptane-based OSDAs with the positive charge asymmetrically located at one side of the molecule, behave similarly. All of them remain almost immobile at the center of the aei cavity 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 mobility that includes full rotation within the aei cage and the ability to reach the regions of the cavity not accessible to the other OSDAs investigated. As a result, this highly mobile OSDA preferentially stabilizes Al in the T3 site, which might lead to differences in catalyst activity and stability for zeolite samples synthesized using this OSDA.