Localization of Water Molecules and Sodium Ions in Na-Mordenite, by Thermally Stimulated Current Measurement

Abstract

Dielectric relaxation properties of dehydrated and hydrated Na-mordenite are measured by means of thermally stimulated depolarization current (TSDC) spectroscopy. TSDC spectroscopy allows us to determine two characteristic terms which depend on the hydration state: on one hand, the site selectivity of sodium ions and water molecules and on the other hand, the sodium/network interaction energy. Upon hydration, half of the Na+ ions remain embedded in site A, whereas the other Na+ ions are extracted from the main channel sites (sites D and E). Besides, according to the water content, we show that water molecules are adsorbed in different sites, first in sites H or F, then in sites B, C, or G, and finally in sites G. Furthermore, it is pointed out that the interaction energy (Na+/zeolite) value depends on the sodium localization following the sequence: E site A > E site D > E site E. Finally, it is shown that the interaction energy decreases as the water content increases (typically from 0.79 eV to 0.44 eV). This evolution is successfully described by using a qualitative model referring to electronegativity and charge-transfer concepts.

TSDC spectroscopy of relaxational and interfacial phenomena

“…They showed that dipolar reorientation is due to Na + , and the TSDC spectrum analysis leads to an assessment of the interaction energies (70-90 kJ/ mol) between the hopped Na + ions and the zeolitic lattice. According to the Na + exchange degree and the nature of the occupied Na + sites, a quantitative and qualitative characterization of each site was given for dehydrated and hydrated Namordenite.The authors [153] determined characteristic terms dependent on (i) the hydration state, (ii) the site selectivity of sodium ions and water molecules, and (iii) the sodium/network interaction energy [154][155][156]. Upon hydration, half of the Na + ions remain embedded in site A, whereas the other Na + ions are extracted from the main channel sites (sites D and E).…”

“…Finally, it was shown that the interaction energy decreases as the water content increases (from 76 to 42 kJ/mol). This evolution is successfully described using a qualitative model referring to electronegativity and charge-transfer concepts [154][155][156].…”

A molecular simulation study of the distribution of cation in zeolites

“….). Many experimental and theoretical studies have shown that the location of these cations plays a crucial role on the thermodynamics, dynamics, and catalysis of adsorbed molecules (Meunier 2001;Devautour et al 2001;Moïse et al 2001;Maurin et al 2004Maurin et al , 2005Plant et al 2006).…”

Hydration of a Na+-montmorillonite studied by thermally stimulated depolarization current

“…So, the presence of adsorbed molecules, interacting directly or indirectly with the cations, can be measured. In previous papers, TSDC was successfully applied to dry and hydrated zeolites [7,8] and clays [9,10] though no quantitative detail about the thermodynamics of the first adsorption stages was extracted from the experimental data.…”