Ba 2+ Complements Co 2+ Exchange as a Reporter of Al Proximity in CHA Zeolites

Co 2+ exchange is commonly used as a reporter of Al pair ensembles in zeolites. We combine density functional theory (DFT) calculations, statistical models, experimental titrations, and solid-state nuclear magnetic resonance (NMR) analyses to explore the utility of other 2+ ions as alternative reporters of proximal Al ensembles in the CHA zeolite. DFT calculations suggest that Ba 2+ will exchange into both eight- (8MR) and six-membered (6MR) CHA rings equally effectively, distinct from Co 2+, which exchanges solely into 6MR. Simulated titration curves highlight the potential for Co 2+ and Ba 2+ titrations to provide complementary information about specific proximal Al site ensembles as a function of Si/Al ratio. Experiments on CHA zeolites synthesized to express different Al distributions confirm that Ba 2+ uptake exceeds that of Co 2+ and that this uptake can be rationalized by Ba 2+ ions that titrate both 8MR and 6MR Al pair sites. Comparisons of absolute ion uptakes and two-dimensional NMR analyses of Al proximity with predictions reveal previously unrecognized differences in Al siting rules under different syntheses. These findings demonstrate that complementary titrations using divalent cations of differing ionic radii provide additional resolution on Al–Al pair ensembles and the underlying rules that govern Al distributions on zeolite frameworks.