First-Principles Analysis of Site- and Condition-Dependent Fe Speciation in SSZ-13 and Implications for Catalyst Optimization

Metal-exchanged zeolites are common catalysts and adsorbents, but the relationship between their macroscopic composition (Si:Al and M:Al ratios) and microscopic details of exchange site composition and reactivity are difficult to infer. Here we address this general problem for Fe exchange in an SSZ-13 zeolite. We report periodic supercell density functional theory (DFT) calculations for the structures and energies of candidate Fe-exchange sites, including monomeric and dimeric Fe species with formal oxidation states ranging from 2+ to 5+ and charge-compensated by arbitrary combinations of framework Al, oxo, and hydroxyl ligands plus H₂O adsorbates. We show that the chemical identity of an Fe-exchange site depends strongly on the number and proximity of framework Al and, through first-principles thermodynamics models, that these sites evolve in distinct ways as a function of external treatment conditions. By placing the results on a common energy reference and combining with simulated Al distributions, we generate a composition phase diagram, relating macroscopic composition variables to the identity and relative number of monomeric and dimeric exchange sites. We use these models to predict the relative activities of the distinct exchange sites toward partial methane oxidation (PMO) with N₂O. The models identify monomeric Fe²⁺ exchanged near two proximal Al’s in a six-membered ring as providing the optimal trade-off in reducing and oxidizing potentials for PMO, consistent with experimental inference in other frameworks. The results illustrate a systematic approach for relating the macroscopic zeolite composition to the microscopic structure and a path toward rationale optimization of catalyst preparation and pretreatments to favor desired sites and promote desired reactivity.