Natrolite may not be a "soda-stone" anymore: Structural study of fully K-, Rb-, and Cs-exchanged natrolite

Abstract

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.

Synthesis and structural study of the Linde Type-A zeolite prepared from kaolinite

“…This may result from a little structural changes. Similar changes was described in the case of natrolite exhanged with Rb and Cs [15]. …”

Spectroscopic and Computational Characterizations of Alkaline‐Earth‐ and Heavy‐Metal‐Exchanged Natrolites

“…[9] It has recently been shown that various alkali, alkaline-earth, and heavy-metal cations can be exchanged into the natrolite channels, and that the unit cell volume and channel ellipticity of ion-exchanged natrolites have a nearly linear dependence on the cation radius. [4,5] The larger the cation, the less elliptical the channel becomes, and hence, the unit cell volume expands by up to 25 % in the alkali metal series. For example, from Lito Cs-natrolite, the chain rotation angle and unit cell volume vary from 25.58 and 2118.5 3 to 2.98 and 2658.8 3 , respectively.…”

“…Therefore, EFCs and water molecules in Li + -, Na + -, Ca 2 + -, Ag + -, and Sr 2 + -exchanged natrolites (r < 1.3 ) are ordered and distributed over well-separated sites with full site occupancies, [5] whereas those in K + -, Rb + -, and Cs + -exchanged natrolites (r > 1.3 ) reveal statistically disordered arrangements over closely separated sites. [4] The overall symmetry of the ordered cation forms is then dictated by the type of cation. Thus, all the alkaline-earth forms studied so far (Ca 2 + -, Sr 2 + -, and Ba 2 + -exchanged natrolites) adopt a scolecite-like structure in space group Cc, whereas the alkali and heavymetal forms (Li + -, Na + -, Cd 2 + -, Ag + -, and Pb 2 + -exchanged natrolites) crystallize in the original natrolite-like structure in space group Fdd2 (Figure 1).…”

“…[3] However, we have shown that complete cation exchange can be achieved in natrolites by converting the natural sodium form into the potassium form, which can subsequently be exchanged by other isovalent alkali metals (Li + , Rb + , and Cs + ), aliovalent alkaline earths (Ca 2 + , Sr 2 + , and Ba 2 + ), and selected metal cations (Cd 2 + , Pb 2 + , and Ag + ). [4,5] The comparative crystal chemistry of these natrolites has been established and related to the framework expansion or contraction under pressure or during hydration/dehydration, and has been found to be proportional to the size of the EFC. This size dependence also determines the topology of the EFCs and water molecules along the natrolite channels.…”

“…The TÀO2ÀT angles are from previous crystallographic studies. [4,5,16] The lines are fits to the data from the alkali-metal forms.…”