Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

Abstract

To cite this version:Gerardo Rodriguez-Fuentees, Sabine Devautour-Vinot, Sekou Diaby, François Henn. Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy. Phys. Chem. Minerals, 2011, 38 (xx) ABSTRACT.Purified natural clinoptilolite from the Tasajeras deposit, Cuba, and some of its metal exchanged forms are studied, at the dehydrated state, by means of Dielectric Relaxation Spectroscopy (DRS) using two different modus operandi: Complex Impedance Spectroscopy and Dielectric Dynamic Thermal Analysis. Data analysis yields the determination of the extra-framework cation (EFC) population into the various possible crystallographic sites of the zeolitic framework as well as of the activation energy characterizing the localized hopping mechanism of EFC. First, it is shown that the DRS responses obtained here match well with the previous reported data which were previously localized EFCs in positions close to M1 and M2 sites when the clinoptilolite is modified to almost homoionic form. From this outcome, it can be concluded that all EFCs are in the same crystallographic situation regarding solvation or, in other terms, that no steric effect can be taken into account to explain cationic selectivity. Second, based on the assumption that the activation energy for EFC hopping is directly connected to the EFC/framework interaction and on simple thermodynamics consideration, we show this interaction does not govern the EFC 2 exchange reaction. So, it is emphasized that EFC/H 2 O interaction is the key factor for cation exchange selectivity.

Behavioural Change according to the Si/Al Ratio of Successive Na-Mordenites Observed through Their Dielectric Relaxation during Water Vapour Adsorption Process

“…In this work, we have shown how the use of the complex impedance spectroscopy allows characterizing the evolution of the surface properties of microporous solids according to the water molecules adsorption process. The studies that we previously performed on the same materials in a dehydrated state [9][10][11][12] led us to discuss the nature of the energy driving the exchangeable cation hopping between two adjoining sites onto the surface of the zeolite. The obtained results by dielectric relaxation method make it possible to determine the change in the energy according to the number of adsorbed water molecules.…”

“…The parameter in which we are interested in this type of experiment is the activation energy (Δ ex ) that characterizes the cation hopping responsible for the dielectric losses. The relations between the cation hopping and the dielectric loss measurements have been described in detail, in a previous series of works [9][10][11][12]. To determine the activation energy, first we had to draw the change curve of the imaginary permittivity ( ) of the sample in relation to the frequency [13][14][15][16][17][18], at a given measuring temperature ( ).…”

“…An example of straight line thus obtained is shown with the diagram in Figure 2. Proceeding as described, first we determined the energy (Δ ex ) values, by assessing them with a margin of error of 0.01 eV [9][10][11][12] according to the equilibrium temperatures (TT) to which the Na-mordenite samples previously hydrated were successively submitted. Then, we were able to determine the degree of hydration of the Na-mordenite at each temperature (TT), thanks to the results obtained with the use of a thermobalance.…”

“…In actual fact, the method consists in measuring, as accurately as possible, the variations of a parameter related to the surface structure and depending on the vapour pressure imposed on the system, at constant temperature. The work we did previously on the zeolites [9][10][11][12] moved us to examine a Na-mordenite…”