(501e) First-Principles Development of Al Proximity Titration Strategy for SSZ-13 Zeolite through Comparison of Divalent Metal Cation Exchange Energy Landscapes

Zeolites are microporous, aluminosilicate materials that have wide applications as industrial and environmental catalysts. Most efforts to optimize zeolitic catalysts so far have focused on modulating the nature of catalytic active sites in zeolites, which are typically extra-lattice cations. Molecular-level density functional theory (DFT) calculations have been successful in characterizing such extra-lattice cations (e.g., Cu) in zeolites. [1]Recent literature, however, suggests Al distribution in zeolites is another critical parameter influencing the catalytic reactivity for a variety of reactions. [2] A practical obstacle to exploiting Al distribution differences in zeolite catalysis is the dearth of techniques capable of precisely characterizing Al proximity. Nonetheless, some progress has been made, as reported by using Co²⁺ exchange isotherms to titrate certain 6-member-ring (6MR) Al pairs in SSZ-13. [3]

In this work, we perform DFT calculations on SSZ-13, a model single tetrahedral-site zeolite framework, to determine the energy landscapes of lattices substituted with various Al-Al arrangements, and charge-compensated by a selection of alkaline earth (Mg, Ca, Sr,and Ba) and 3d transition metal (Mn, Fe, Co, Ni, Cu, and Zn) divalent cations. This work aims to explore the effect of counter-ion identity on energetic stability as a function of Al-Al distance, in order to identify titrants other than Co²⁺ that can selectively titrate Al-Al sites of different lattice proximity. We observe qualitatively similar energy landscapes among Mg and all the 3d transition metals studied, in which metal cations prefer siting at Al pairs in the 6MR. Al-Al energy landscapes for alkaline earth metals below Mg deviate more extensively with increasing row number. Al pairs in the 8MR, when charge-compensated by Ca²⁺, Sr²⁺ or Ba²⁺, become closer and even equal in energy to Al pairs in 6MR. Analyzing the energy landscapes through descriptor model, we rationalize the observed differences by the ionic radii differences among these cations, resulting in variation of optimal coordination environment. Coupling with the proton-form Al-Al energy landscape, we identify metal cations that exhibit distinct titration sequences for certain Al-Al pairs. These findings provide guidance for experimental titration approaches to probe various Al pair configurations, by using multiple divalent metal cation titrants with different Al-Al titration preference, allowing a more refined description of framework Al distribution and proximity in a given zeolite.
References:

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

[2] B. C. Knott, C. T. Nimlos, D. J. Robichaud, M. R. Nimlos, S. Kim and R. Gounder, ACS Catal. 2018, 8, 770

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