Elucidating the Impact of <i>Cis</i>–<i>Trans</i> Organic Structure Directing Agent Isomer Ratios on\nthe Aluminum Distribution Within SSZ-39

Despite\ntheir widespread use, the mechanisms governing the synthesis\nof zeolite catalysts are still poorly understood. A notable example\nof this problem is the uncertainty surrounding the influence of synthesis\nconditions on the placement of Al atoms in the zeolite framework,\nwhich determines the active sites available for catalytic species.\nIn this work, the role of the <i>cis</i> to <i>trans</i> isomer ratio of the OSDA <i>N</i>,<i>N</i>-dimethyl-3-5-dimethylpiperidinium\non the energetics of 26 distinct Al pair distributions in SSZ-39 is\nexamined both in the presence and absence of Na using density functional\ntheory calculations. The initial orientation of the OSDA was found\nto have a significant impact on the final energies present, necessitating\nthe screening of a large number of initial orientations with force\nfield calculations and single point DFT calculations. Ground state\nenergies were found to vary significantly with the ratio of <i>cis</i> to <i>trans</i> OSDAs with a Boltzmann distribution\nrevealing the most likely Al pair distributions shift from sharing\nthe same 8 membered rings to sharing the same double 6-membered rings\nto having no shared subunits as one increases the amount of <i>cis</i> OSDA present within the framework. The presence of Na\nwas found to favor Al pair distributions where both Als occupied the\nsame 6-membered ring. When an implicit solvent model was used to evaluate\nground state energies the ideal Na sites shifted from 6-membered rings\nto empty SSZ-39 cages while OSDA positions and orientations remained\nlargely the same. To provide insight on how kinetic factors may influence\nAl distributions, formation energies we calculated for connected double\n6-membered rings. These formation energies revealed a preference for\nAl pairs to occupy the same 4-membered ring, which indicates kinetic\nand thermodynamic control may lead to different Al distributions in\nSSZ-39.