Zeolite Dehydration and Extra-Framework Cations - Dr. Yongjae Lee
JULY 24, 2017
New work led by HPSTAR scientists, Dr. Yongmoon Lee and Prof. Yongjae Lee presents new results in Zeolite. They performed comprehensive high-temperature diffraction and spectroscopic experiments on natrolites, containing extra-framework cations (EFC) of various size and charge. They find systematic yet unusual structural and chemical behaviors. This work is published in the latest American Mineralogist, and is selected as “Editor's pick of notable papers” for this month in American Mineralogist: Journal of Earth and Planetary Materials.
The team presents a comprehensive structural and spectroscopic study of the thermal behavior of monovalent cation exchanged natrolites as a model system of a small-pore zeolite.
Natrolite (NAT), one of the representative members in mineral zeolites, contain tiny, regularly spaced elliptical pores. There two types of NAT depending on the distribution pattern of extra-framework cations (EFC) and H2O in the pores. In Li-, Na- and Ag-NAT, the EFC’s are located in the middle of the pores and water molecules are closer to the amuminosilicate framework, whereas in K-, NH4-, Rb-, and Cs-NAT, this distribution is reversed in a disordered fashion.
In this work, the team aimed to find out the influence of EFC on the temperature-induced structural and chemical changes. They found that the dehydration temperature is inversely proportional to the size of the EFC in such a way that Li-NAT shows the highest dehydration temperature while the dehydration temperature for Cs-NAT is the lowest in the NAT families. This can be understood given a possible Coulomb’s Law effect on bonding strength.
What is special in this study is that those isostructural K-, Rb- and Cs-NATs with disordered EFC-H2O distribution exhibit negative thermal expansion behaviors before dehydration. Their thermal expansion coefficients increase linearly from K-, Rb- to Cs-NATs, at which it becomes comparable to that of ‘zero thermal expansion’ alloys known to date. After dehydration, the EFC distribution of K-, Rb- and Cs-NATs becomes ordered, and their thermal expansion coefficients become positive.
In contrast, in Li-, Na- and Ag-NATs with ordered EFC-H2O distribution, thermal expansion coefficients are positive in Li- and Ag-NATs and negative in Na-NAT, which becomes reversed after dehydration.
To understand these phenomena, the team further probed the bonding interactions of EFC and H2O molecules via X-ray absorption spectroscopy.
“Moreover, our structural work on NATs suggest that EFC type has a greater influence over the stability of zeolite structures than degree of dehydration, with stability increasing with decreasing EFC radius ”, Dr. Yongmoon Lee explained who is the leading author in the series of publications on natrolites in American Mineralogist.
Caption: Dependency of the (a) one set of dehydration temperature of NAT phases(Td),(b) Chain rotation angle.
钠沸石Natrolite (NAT)是沸石族矿物的典型代表，具有小的，规则的椭圆形的空洞。根据钠沸石中骨架阳离子，及空洞中水分子的位置分布的不同，钠沸石被分为两种：Li-,Na- and Ag-NAT及K-,NH4-, Rb-, and Cs-NAT。通过对钠沸石两种体系进行系统的高温结构及光谱学研究，该研究团队发现钠沸石的脱水温度与骨架阳离子的大小成反比并提出骨架阳离子的类型在钠沸石的结构稳定性方面起着比脱水程度更重要的作用。