Flexibility and distortion of the framework of natrolite: crystal structures of ion-exchanged natrolites

Abstract

The crystal structure of the Li-exchanged form of natrolite, Li 1.6Na04AJ,Si301O . 2H20, was refined based on 568 F obs' R = 0.071. For comparison the structure of a natrolite, NazAJ,Si30lO· 2H10, was also refined (1278 F obs' R = 0.040). The diffraction data collected for K-natrolite were of insufficient quality, therefore a computer simulation of the K-form was based on the experimentally determined cell edges and space group. The mean rotation angles ljI of the chains (composed of 4 = 1 secondary building units) relative to the a and b cell axes are observed to be 250 (Li-compound), 240 (Na-compound) and 180 (K-compound). A detailed comparison shows that only the transformation to K-natrolite conforms approximately to the currently widely accepted model of rotating chains. Surprisingly the Li-form instead achieves the observed shortened cell constants a and b by a distortion of the chain itself, the rotation is only a secondary effect. Both cation exchanged forms display also shortened c-axes, which is an indication that the chains of 4 = I units are tilted and twisted within themselves. It is most likely that the angle of the hinge at the oxygen atom 02 is in Na-natrolite already at the lower attainable limit beyond wh ich it would become energetically unfavorable, and thus it cannot be reduced any furt her when the Na-natrolite is transformed by ion-exchange into a Li-natrolite.

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

“…10% expanded unit cell compared to the original Na-form. Soon after, Baur et al (1990) used computer simulation on insufficient single-crystal data to propose that the K and H 2 O distribution in the fully K-exchanged natrolite would be similar to that found in the Na-form.…”

“…Nonframework cation substitutions, on the other hand, have only been successful via high-temperature melt exchange to fully replace Na with small univalent cations such as Li and NH 4 , which led to volume contractions by ca. 5% (Baur et al 1990). Larger cations such as Rb and Cs simply cannot access the narrow pores of natrolite at ambient conditions, and natrolite does not take up any significant amounts of smaller divalent or transition metal cations such as Ca, Co, Ni, Cd, and Zn, i.e., maximum exchange level of ca.…”

Thermodynamic study of alkali and alkaline-earth cation-exchanged natrolites

“…However, cation exchange of natrolite was found to be more difficult than in other zeolites due to its small pore size and tightly bound cations [1]. For many years, only isovalent cation substitution was reported in Na-NAT by high-temperature melt exchange methods to produce partially Li-exchanged and anhydrous NH 4 -forms [7,8]. Recently, natrolites containing large isovalent (Rb + , Cs + , Ag + ) and divalent cations (Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Pb 2+ ) as well as hydrous NH 4 -NAT were synthesized through ion exchange using K-NAT as a starting material by Lee's group [9][10][11][12].…”

Structural characterization of various alkali cation forms of synthetic aluminosilicate natrolites

“…3 also shows that the introduction of larger alkali cations into both zeolites is accompanied by the progressive opening of the elliptical natrolite channel to more circular one. This can be quantitatively described using the chain rotation angle, j, defined as the relative orientation of the quadrilateral edges around T 5 O 10 chain projected onto the ab plane to the a and b unit cell edges [11], and T-O2-T angle, q, which provides the soft hinge for the rotation of fibrous natrolite chains, postulated by Pauling [9]. As seen in Fig.…”