Abstract
This paper reports a study, performed by in-situ synchrotron X-ray Powder Diffraction, of the high pressure behavior of the natural zeolite amicite [K4Na4(Al8Si8O32)·10H2O], the GIS-type phase with ordered (Si, Al) and (Na, K) distribution. The experiments were carried out up to 8.13(5) GPa in methanol:ethanol:water = 16:3:1 (m.e.w.) and 8.68(5) GPa in silicone oil (s.o.). The crystal structure refinements of the patterns collected in m.e.w. were performed up to 4.71(5) GPa, while for the patterns collected in s.o. only the unit cell parameters were determined as a function of pressure. The observed framework deformation mechanism—similar to that reported for the other studied phases with GIS topology—is essentially driven by the distortion of the “double crankshaft” chains and the consequent changed shape of the 8-ring channels. The pressure-induced over-hydration observed in the experiment performed in aqueous medium occurs without unit cell volume expansion, and is substantially reversible. A comparison is made with the high pressure behavior of the other GIS-type phases, and the strong influence on compressibility of the chemical composition of both framework and extraframework species is discussed.
Highlights
The high pressure (HP) behavior of zeolites when compressed in non-penetrating fluids has recently been reviewed by Gatta and Lee (2014) [19] and summarized in the following way: (i) microporosity does not necessarily imply high compressibility, the range of compressibility is wide, with bulk modulus K0 ranging from ~15 to ~70 GPa; (ii) the flexibility observed in zeolites is based mainly on tetrahedra tilting; (iii) the deformation mechanisms are dictated by the framework topology; (v) the extraframework content governs the compressibility level in isotypic structures
This paper presents a study, performed by in-situ synchrotron X-ray Powder Diffraction (XRPD), of the HP stability and behavior of the natural zeolite amicite [K4 Na4 (Al8 Si8 O32 )·10H2 O], the GIS
HP XRPD data demonstrate that amicite does not undergo complete amorphization up to the highest investigated pressure, and the features characteristic of the pattern collected at ambient conditions are almost completely recovered upon decompression in both experiments
Summary
In the last 15–20 years, studies on the behavior of both natural and synthetic microporous materials under high pressure (HP) have multiplied noticeably, providing important information on their elastic behavior and stability, and opening new perspectives for technological applications.For instance, among the physical properties of microporous materials investigated under compression, worthy of mention are: the so called P-induced amorphization processes (PIA) (e.g., [1,2,3,4,5,6]), the effect of pressure on the ionic conductivity (e.g., [7,8]), the P-induced over-hydration (PIH) (e.g., [9,10,11,12,13,14]) and the penetration of gas, like Ar, Xe, and CO2 [15,16,17]. The HP behavior of zeolites when compressed in non-penetrating fluids has recently been reviewed by Gatta and Lee (2014) [19] and summarized in the following way: (i) microporosity does not necessarily imply high compressibility, the range of compressibility is wide, with bulk modulus K0 ranging from ~15 to ~70 GPa; (ii) the flexibility observed in zeolites is based mainly on tetrahedra tilting; (iii) the deformation mechanisms are dictated by the framework topology;. Zeolites with GIS topology [20] and GIS-like materials have been studied under both high temperature and high pressure, revealing widely variable degrees and mechanisms of deformation as a function of the non-ambient experimental conditions and the chemical composition of both the framework and extraframework. The study of gismondine dehydration [21] showed that this framework is flexible
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