Many zeolites contain a variety of topological regions, such as MFI which contains two distinct channel environments which form larger voids at their intersections. Confinement effects, often influential in zeolite performance, vary within these distinct void environments. Therefore, we can create families of single-framework zeolite catalysts with diverse performance, if we can control where Al resides within these materials.

Zeolite synthesis occurs with the assistance of structure-directing agents (SDAs), which are encapsulated by the framework during synthesis. The size and shape of these SDAs directs which framework is synthesized by their fit within zeolite voids to maximize dispersive interactions with the framework. These shape-driven factors have been studied for many SDA-framework interactions, using density functional theory (DFT) calculations to predict the SDA binding energies within the framework. These studies, however, have often neglected the presence of Al during zeolite synthesis, focusing on Si-only zeolite forms to expedite SDA-framework screening studies.

More recently, we have examined how SDAs can be engineered based on their cation (N) locations and H-bonding capability to alter Al locations. Here, we focus on two SDA sets: tetrapropylammonium (TPA) and a 1:1 combination of diazobicyclooctane (DABCO) and methylamine (MA).

TPA, being a quaternary amine, has a permanent cation (N⁺) centrally located among four propyl ligands, which resides near the center of the intersections of MFI. When TPA is present within those intersections, Al atoms are driven towards the intersection to minimize N–Al (cation–anion) distances, and these factors reinforce the intrinsic stability of Al at the T-12 sites, resulting in a large DFT-predicted preference for T-12: 10 kJ mol⁻¹ more stable than all other T-sites.

DABCO, in contrast to TPA, is a tertiary amine with N atoms on its periphery. Furthermore, also in contrast to TPA, DABCO can form strong H-bonded complexes with species present, such as MA and H₂O. These DABCO-MA or DABCO-H₂O complexes can readily protonate to form a cation, and this cation can form H-bonds to AlO_2/4⁻ sites. These interactions stabilize Al in locations far from MFI intersections, such as within the sinusoidal channels (e.g., T-4).

In this talk, we show these SDA–Al interactions in detail, compare other SDAs, and show how these synthesis changes improve p-xylene selectivity during toluene methylation.

Sopuruchukwu Ezenwa

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Appearances