Immobilization of Large, Aliovalent Cations in the Small‐Pore Zeolite K‐Natrolite by Means of Pressure

Abstract

Selective ion exchange is one of the fundamental properties of zeolites enabling various environmental and industrial applications. [1] The ion-exchange selectivity of a zeolite is controlled in part by the geometry of the pores and channels through which exchanging cations diffuse and bind via framework oxygen atoms. [2] Most studies on the ion-exchange properties of zeolites have been carried under near-ambient conditions to establish the selectivity series between monoand divalent cations [1b] and, to a lesser extent, for trivalent cations such as lanthanides and actinides. Due to steric hindrance, however, small-pore zeolites such as natrolite and related fibrous zeolites have not received much attention with regard to ion-exchange applications. [3] The composition of natural aluminosilicate natrolites lie exclusively near the pure sodium form (Na 16 Al 16 Si 24 O 80 ·16 H 2 O), and nonframework cation substitutions have been known to be feasible for smaller aliovalent cations such as Li + and NH 4 + . [4] Li and NH 4 exchange increases the ellipticity of the 8-ring channel by reducing the unit-cell volume by about 2 and 4 %, respectively, making these materials less favorable for further ionexchange applications.We recently established that natrolite can also incorporate larger cations under ambient conditions by successive replacement of the sodium cations in the channel. Sodiumto-potassium exchange expands the unit-cell volume by about 10 %, and even enables further exchange to give rubidium and cesium forms with about 15.7 and 18.5 % larger unit-cell volumes, respectively. [5] During this process, the elliptical 8ring channel transforms progressively into circular shape by rotating the T 5 O 10 subunits perpendicular to the channel axis. Exchange of monovalent by divalent cations such as Ca 2+ and Sr 2+ has also been achieved for the K form of natrolite. [6] We have further developed the chemistry of natrolites, since this framework can show auxetic behavior leading to expansion of the channels under pressure, which can be rationalized in terms of a "rotating-squares" model in which the squares are made up of the T 5 O 10 subunits (T = Al, Si). [7] An auxetic material has a negative Poissons ratio; when it is compressed or stretched, it becomes thinner or thicker perpendicular to the applied force. [8] Recently Grima et al. [9] showed experimentally that Na-natrolite is an auxetic zeolite with negative Poissons ratio. This provides an explanation for the reversible superhydration observed in Na-natrolite, whereby water is inserted under pressure (pressure-induced hydration), [10] and the pressure-induced adsorption of small gas molecules such as Ar and CO 2 into Na-natrolite. [7,11] We utilize the auxetic properties of the natrolite framework to exchange under pressure and trap under ambient conditions trivalent cations such as lanthanides, formerly thought to be non-exchangeable, in K-natrolite.Pressure-induced cation exchange has recently been shown by ex situ high-pressure quenching experiments t...

Zeolites at high pressure: A review

“…60ºC for 10 h for Ar, 110ºC for 1 h for CO 2 ). In a recent study by Lee et al (2012), pressure-induced volume expansion was used to exchange and immobilize large aliovalent cations in natrolite channels. Yellow and red balls depict Na + and H 2 O oxygen atoms, respectively.…”