Spectroscopic study of cation exchange in small-pore zeolite sodium natrolite

Small-pore zeolite natrolite exhibits a distinct cation-exchange behavior that profoundly influences both framework and non-framework configurations. Herein, we investigate the structural evolution of Na⁺-natrolite upon K⁺ exchange using a suite of spectroscopic approaches, including solid-state ²⁹Si, ²⁷Al, and ²³Na magic angle-spinning NMR, FT-IR, and Raman spectroscopy. An order-disorder transition is observed in the Na⁺-natrolite framework during progressive K⁺ substitution, accompanied by ∼10 % unit cell volume expansion and ∼46 % decrease in the chain rotation angle, reflecting a gradual transformation toward a more circular 8-ring channel geometry. The ²⁹Si NMR spectra show a significant line broadening up to 73.7 % K⁺-exchange, followed by two distinct resonances at −89.1 and −92.3 ppm for fully exchanged K⁺-substituted natrolite, confirming the structural reordering. The ²⁷Al NMR results reveal the progressive development of asymmetric shoulder resonances during partial substitution, which disappeared upon full exchange, indicating the reorganization of tetrahedral Al ordering. Meanwhile, ²³Na NMR shows a monotonic decrease in signal intensity without line-shape alteration, suggesting either fast Na⁺ release or preservation of ordered Na⁺ environments. Raman and FT-IR spectroscopy further reveal vibrational shifts and line broadening in both the framework and O−H stretching modes, indicating the transition from ordered “zeolitic ice” to disordered “zeolitic water” upon full K⁺ exchange. These results provide molecular-level insights into the ion-water interaction and structural flexibility of natrolite, with implications for the cation-exchange behavior in nanoporous zeolitic materials.