Since its first discovery in nature, natrolite has been largely known as a sodium aluminosilicate zeolite, showing very limited preference toward cation exchange. Here we show that fully K-exchanged natrolite can be prepared from natural Na-natrolite under mild aqueous conditions and used to subsequently produce Rb-and Cs-exchanged natrolites. These cation-exchanged natrolites exhibit successive volume expansions by ca. 10, 15.7, and 18.5% for K-, Rb-, and Cs-forms, respectively, compared to the original Na-natrolite. This constitutes the largest, ever-reported volume expansion via cation substitution observed in zeolites and occurs by converting the elliptical channels into progressively circular ones. The observed cation-dependent changes in the channel volume and shape thus show the flexibility limits of the natrolite framework and suggest the possible existence of compositionally altered analogues in suitable environments as well as a novel means to tailor the cation selectivity of this class of small pore zeolites toward various industrial and environmental applications.

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Synchrotron infrared (IR) and micro-Raman spectroscopic studies have been performed on zeolite natrolites as a function of the non-framework composition at ambient conditions. This establishes the spectroscopic characterization of the ion-exchanged natrolites in the alkali-metal series both in the as-prepared hydrated (M-NAT-hyd, M = Li, Na, K, Rb, and Cs) and some stable dehydrated forms (M-NAT-deh, M = Rb and Cs). The former series exhibits non-framework cation-size dependent opening of the helical channels to span ca. 21° range in terms of the chain rotation angle, ψ (or ca. 45° range in terms of the chain bridging angle, T-O2-T). For these hydrated phases, both IR and Raman spectra reveal that the degree of the red-shifts in the frequencies of the helical 8-ring channel as well as the 4-ring unit is proportional to the ionic radius of the non-framework cations. Linear fits to the data show negative slopes of -55.7 from Raman and -18.3 from IR in the 8-ring frequencies and ionic radius relationship. The spectroscopic data are also used to identify the modes of the dehydration-induced "collapse" of the helical 8-ring channels as observed in the stable anhydrous Rb-NAT-deh and Cs-NAT-deh. In addition, we demonstrate that the spectroscopic data in the hydrated series can be used to distinguish different water arrangements along the helical channels based on the frequency shifts in the H-O-H bending band and the changes in the O-H stretching vibration modes.

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