Quantum Chemical Simulation Of Alkali And Alkali Earth Ions Hydration In The Vicinity Of Zeolite 8member Structural Rings
Quantum Chemical Simulation of Alkali and Alkali Earth Ions Hydration in the Vicinity of Zeolite 8‑member Structural Rings
E. V. Aksenenko1, Yu. I. Tarasevich2
Institute of Colloid Chemistry and Chemistry of Water
Ukrainian National Academy of Sciences
42 Vernadsky Avenue, Kyiv (Kijow) 03680, Ukraine;
1eugene@thomascat.kiev.ua, http://www.ln.com.ua/~thomasca/;
As an initial step in the approach to the quantum chemical description of ion exchange involving alkali and alkali earth cations in zeolite voids, the calculations of energetic and structural characteristics of clusters were performed to model certain states of the systems involved in such exchange processes. In particular, the cation localisation sites in the vicinity of zeolite 8-member structural rings were chosen for the simulation.
In this regard, also it was interesting to verify the reliability of the fact believed to be of a common knowledge (however non-referenced), namely that the semi-empirical quantum chemical methods [1] which rely essentially on the point charges approximation are incapable for the correct description of the systems involving alkali metal cations, for which the single electron constituting the external shell is, in fact, de-localised. The procedure chosen for the simulation was the MNDO approximation implemented in the HyperChem evaluation version [2]; for Na, Mg and Ca the parameterisation described in [3] was used.
The portion of the clinoptilolite structure considered as a cluster was the 8-member zeolite ring; the initial geometry for the optimisation was taken from [4]. The ring cluster was formed by 8 silicon/oxygen tetrahedra with the broken valences saturated by hydrogen atoms. To meet the electroneutrality requirement, one isomorphic Al®Si substitution was made when an alkali metal ion was introduced into the centre of the ring, while for the alkali earth ion two substitutions were made at opposite tetrahedra in the ring.
The optimisation procedure was global: no atoms were fixed. The hydration neighbourhood of a ion was formed by one or two water molecules; in the latter case these two molecules were located on the opposite sides of the ring plane. The calculated energetic yields of the hydration reaction per one water molecule are listed below; here n (1 or 2) is the number of hydrating molecules, DH is the hydration enthalpy in kJ/mol.
n
DH
n
DH
Alkali metal cations
Alkali earth metal cations
Li
Mg
1
−33.2
1
-1.7
2
−4.40
2
-7.0
Na
Ca
1
+23.9
1
+37.2
2
+10.9
2
+7.9
The result presented here are the preliminaries only; it is seen, however, that the cluster used for the simulation, being obviously small, is therefore quite deficient. In particular, positive values of the hydration enthalpy (corresponding to the unstable configuration) are due to the fact that no real zeolite void is present in the model. In our further studies the size of optimised cluster will be increased.
References
1. T.Clark, A Handbook of Computational Chemistry, Wiley, NewYork, 1985.
2. HyperChem6 evaluation version at http://www.hyper.com/.
3. A.A.Vojtiuk, Zhurn. Struct. Khim. (USSR) 28, No.5, p.156 (1987); 28, No.6, p.128 (1987); 29, No.1, p.138 (1988).
4. J.R.Smyth, A.T.Spaid, D.L.Bish Amer. Miner.,- 1990.- 75, No.5‑6, p.522(1990).
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