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Thermal Expansion of the Superhydrated Small-Pore Zeolite Natrolite

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Yongjae Lee*†, Chi-Chang Kao‡, and Thomas Vogt§

^† Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea

^‡ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025-7015, United States

^§ NanoCenter & Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

J. Phys. Chem. C, 2012, 116 (5), pp 3286–3291

DOI: 10.1021/jp209514q

Publication Date (Web): January 9, 2012

*E-mail: yongjaelee@yonsei.ac.kr. Tel.: +82-2-2123-5667. Fax: +82-2-2123-8169.

Section:

Mineralogical and Geological Chemistry

Abstract

Natrolite (Na₁₆Al₁₆Si₂₄O₈₀·16H₂O) at ambient conditions is the paradigmatic example of an auxetic material whose behavior under pressure can be rationalized using a “rotating squares” model with the squares being made up of T₅O₁₀ subunits (T = Al, Si). This model also rationalizes reversible superhydration where water is inserted under pressure (“pressure-induced hydration”). Using combined pressure and temperature in situ synchrotron powder diffraction techniques, we have investigated the structural changes occurring in “superhydrated” natrolite (Na₁₆Al₁₆Si₂₄O₈₀·32H₂O) between 20 and 300 K at 1.7(3) GPa. Rietveld refinements allowed us to identify significant changes within the sodium–water substructure located in the pores of the superhydrated natrolite. Despite the higher water content of the superhydrated phase, its thermal volumetric expansion coefficient is 15 times larger than that of natrolite at ambient pressure. We put forward a structural descriptor relating thermal expansion and the rotations of T₅O₁₀ units that measures the impact of nonframework cations and water contained in the pores on the “rotational squares” mechanism.