Molecular Modeling of Univalent Cation Exchange in Zeolite N

Abstract

Molecular dynamics simulations are used to investigate the hydration energy and ion-exchange properties of a synthetic zeolite, zeolite N with composition |K10(H2O)8Cl2|[Al12Si12O40]. The exchange of K+ ions with univalent ions such as NH4 +, Na+, Rb+, and Cs+ is investigated under a range of simulation conditions using a three-dimensional membrane in an electrolyte box containing explicit water molecules. Hydration energy calculations indicate that zeolite N prefers eight water molecules per cage, which is consistent with X-ray and neutron diffraction determination of the structure. Ion density profiles and calculated self-diffusion coefficients show that univalent ion exchange by zeolite N is selective toward NH4 + in preference to other ions. The methodology used here to simulate the uptake of ions from an electrolyte within the zeolite N membrane produces results that are consistent with experimental data and implements a low computational overhead.

The Exchange Mechanism of Alkaline and Alkaline-Earth Ions in Zeolite N

“…Moreover, Zwingmann et al [7] showed that ammonium exchanged zeolite N is an ideal slow release fertiliser for sandy soils due to the high retention capacity of zeolite N for NH 4 + ions. Our computational results from previous [10] and this study for NH 4 + retention are consistent with these experimental results. This study shows that NH 4 + has the highest retention between all exchanging systems in zeolite N membranes along both crystallographic directions.…”

“…Our previous simulation outcomes [10] show similar conditions regarding the total number of cations retained in the membrane. These results suggest that the zeolite N structure prefers to hold no more K or other cations than an approximately equilibrium value.…”

“…Zeolite N contains eight water molecules per cage [10]. Counting the average number of water molecules in each cage of ZM-110 (Figure S1e,f) demonstrates that the membrane adsorbs water molecules in K + /Li + , K + /Na + , K + /K + , K + /Rb + , K + /Mg 2+ and K + /Ca 2+ systems up to one extra water molecule per cage but releases water molecules in K + /NH 4 + and K + /Cs 2+ systems (Figure S1f).…”

“…These experimental studies demonstrate the high ion exchange capacity and exchange isotherms of zeolite N. However, these experiments are not able to explain the exchange and diffusion mechanism of cations within the complex porous structure of zeolite N. In our recent study [10] we simulated the exchange of univalent cations, NH 4 + , Na + , K + , Rb + and Cs + , in a zeolite N membrane using molecular dynamics calculations. We studied the structural and dynamic behaviour of ions inside a zeolite N membrane.…”

“…We explore ion retention within a zeolite N membrane along different crystallographic directions, [001] and [110], as well as the site preference of exchanged cations in the zeolite N structure. For our earlier study [10], we utilised partial charges for framework atoms (i.e., Si and Al) calculated by Salmas et al [11] for zeolite LTA as a guide. However, our recent study [12] shows that the partial charges of framework Si and Al atoms can be substantially different depending on the functional used in DFT calculations and on the zeolite type.…”