(582bv) Nature and Consequences of Al - Al Interactions in SSZ-13 Zeolite

Cu-exchanged SSZ-13 zeolite is now widely used in diesel vehicles for on-board selective catalytic reduction (SCR) of NO_x to nitrogen. It is now generally accepted that catalytic activity is associated with isolated, exchanged Cu ions within the zeolite pores, and that those ions can be present either charge-compensated by two framework Al (Z₂Cu) or by a single Al (ZCuOH) [1]. Under some synthesis conditions those Al appear to avoid first-nearest-neighbor locations but to be otherwise randomly distributed [2], and synthesis conditions can be adjusted to bias towards one type of site or the other [3]. The origins and consequences of these biases remain to be fully elucidated, both for the catalytic activity and durability of the catalyst. Dealumination is a common mode of zeolite catalyst failure, and the number and proximity of Al may have an influence on the tendency towards or resistance against dealumination in addition to its influence on Cu speciation. Thus, it is important to understand how the Al environment influences the stability of Al within the framework. In this work, we use density functional theory (DFT) methods to compute the energetics of Al siting in SSZ-13 as a function of Al proximity.

We calculated the total energies of two protons each associated with an O adjacent to Al, and reported these energies relative to the isolated Brønsted site energy. At a given Al separation there is a range of energies corresponding to the proton associated with each of the four symmetry-distinct O atoms. At third-nearest-neighbor separation, energies begin to decrease relative to the infinite limit. Energies are minimized at first-nearest-neighbor separation, suggesting that Al sites experience an attractive potential in the acid-form, and in this form the Al will tend to aggregate. As laboratory-prepared zeolites generally follow Löwenstein’s rule, the DFT results show that this rule is not a consequence of the underlying energy landscape experienced by the Al sites. Further, at conditions at which Al become mobile on the SSZ-13 lattice, e.g. high temperature steaming, the Al experience an attractive interaction that may lead to Al aggregation and ultimately provide a route to dealumination. The results provide a baseline for considering the influence of other extra-lattice ions.

[1] Paolucci, C., Di Iorio, J. R., Ribeiro, F. H., Gounder, R, and Schneider, W. F. Adv. Catal. 57 (2016).

[2] Paolucci, C., Parekh, A. A., Khurana, I., Di Iorio, J. R., Li, H., Albarracin Caballero, J. D., Shih, A. J., Anggara, T., Delgass, W. N., Miller, J. T., Ribeiro, F. H., Gounder, R., and Schneider, W. F. J Am. Chem. Soc. 138, 6028 (2016).

[3] Di Iorio, J. R., and Gounder, R. Chem. Mater. 28 2236 (2016).