Optimization strategies for material

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 den- sity 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, hydroxyl ligands plus H₂O adsorbates. We show that the chemi- cal 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 towards partial methane oxidation (PMO) with N₂O. The models identify monomeric Fe²⁺ ex- changed near two proximal Al in six-membered ring as providing the optimal trade-off in reducing and oxidizing potential for PMO, consistent with experimental inference in other frameworks. The results illustrate a systematic approach for relating macroscopic zeolite composition to microscopic structure and a path towards rationale optimization of catalyst preparation and pretreatments to favor desired sites and promote desired reactivity.

• Related paper

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

S. Li, Y. Wang, T. Wu and W. F. Schneider*. ACS Catal., 2018, 8, 11, pp 10119-10130

Supporting information

• Related poster

2017 ACS annual meeting, Washington, D.C. (click the poster for high-resolution file)

• Related oral presentation

2017 25th North American Catalysis Society Meeting, Denver, CO