(44h) Interrogation of Alkaline Earth Ion Exchange and CO Vibrational Spectroscopy As Reporters of Al Proximity in CHA Zeolites

The proximity of framework aluminum on a zeolite framework is well known to influence the catalytic reactivity of associated Brønsted acids or of associated exchanged metal ion species and is suggested to influence hydrothermal stability. Methods to interrogate proximity are thus of value for characterizing zeolites. Here we use density functional theory (DFT) methods to compare the energetics of alkaline earth ion exchange energies across a family of candidate proximal Al pair sites on the CHA zeolite framework. We show that the tendency to exchange into larger rings increases with ion size, and that Ba²⁺ in particular is expected to occupy six- and eight-membered ring Al pair sites. Aqueous Ba²⁺ exchange experiments on CHA zeolites prepared to contain different Al distributions confirm that Ba²⁺ uptake exceeds that of smaller 2+ ions (e.g., Co²⁺), consistent with the expected occupancy of larger rings. Cryogenic CO adsorption and vibrational spectroscopy experiments reveal distinct vibrational features associated with Ba²⁺ exchange. DFT calculations are used to compute CO adsorption isotherms and vibrational spectra as a function of Al distribution, extent of Ba²⁺ exchange, and CO dosing. Results rationalize observed experimental features and support the interpretation of Ba²⁺ as a titrant of larger ring features in CHA and likely other frameworks.