Adsorption energy surfaces in faujasite type zeolites

doi:10.1016/S0009-2614(02)00167-7

Chemical Physics Letters

Volume 354, Issues 5–6, 18 March 2002, Pages 474-482

https://doi.org/10.1016/S0009-2614(02)00167-7 Get rights and content

Abstract

A perturbational approach is used to study the adsorption behavior of small molecules in faujasite type zeolites. From the evaluated interaction energies, iso-interaction energy plots for N₂, O₂, CO₂ and C₂H₄ in LSX faujasite exchanged with Na⁺ are calculated. From these plots the preferential adsorption sites in the zeolite cavity were located. The interaction energies on the preferential sites were found to predict rough trends for the separation behavior of the considered zeolites. A study of the different cation sites, based on an ab initio calculation of the atomic charges at the HF/3-21G level showed a good agreement with existing experimental results and previous theoretical calculations.

Introduction

Theoretical modeling of the separation capability and the related electrostatic properties of zeolites is essentially performed on the basis of ionic models [1], [2], [3] and by simple electrostatic calculations or more realistic models using, however, a molecular mechanics or molecular dynamics approach [4], [5], [6], [7], [8]. In line with earlier contributions on non-empirical calculations on adsorption interactions of small molecules in zeolite cavities [9], [10], [11], [12], [13] and in line with the global approach of our group in recent years on exploiting DFT both from the conceptual and computational points of view as a chemists' tool [14], [15], [16], [17], a non-empirical first principles approach is presented in which the interaction energy on a zeolite surface is calculated. The interaction energies are calculated inside the super cage of faujasite type zeolites for different small systems. When the interaction energies are plotted on the surface of the zeolite cavity, favorable regions for adsorption will be distinguished. In this sense, this study can be considered as a sequel to Bezus's ansatz of 15 years ago [3], using present day, much more powerful parameter free computational techniques. Calculations of this type may lead to the development of `tailor-made' zeolites.

Section snippets

Theory and computational details

In order to investigate the adsorption behavior on the zeolite surface, one has to quantify the interaction between the sorbates and the zeolite. The expression used in these calculations is based on the expansion of the energy of the adsorbed molecule with respect to the external potential perturbation induced by adsorption on the zeolite [10], [18]. The interaction energies were calculated up to first-order as $ΔE_{el} =∑_{j} q_{j} V(R_{j})$ (i.e. the electrostatic energy), where q_j is the charge of the cage

CHelpG charges on the faujasite cluster

In order to calculate the interaction energies, the electrostatic potential based CHelpG and wavefunction based Mulliken charges of the faujasite type cluster were calculated on the Hartree–Fock level in combination with an STO-3G and the 3-21G basis set. In contrast with other point charge calculations, e.g. in molecular dynamics approaches, where the point charges are considered as parameters [5], [6], every single atom has its own charge contributing to the reproduction of the ab initio

Conclusion

In the first part of this study, the CHelpG charges of a cluster, representing the inner atoms of the large cavity of faujasite, were calculated. Four different faujasite type zeolites were investigated: NaY, LiY, NaLSX and LiLSX. From this calculation, the following conclusions can be drawn: Lithium cations are more stabilized than sodium cations and S_III is more charged than S_II in an LSX zeolite. The cations in the Y-type are less stabilized than in the LSX-type.

In the second part, the

Acknowledgements

F.T. thanks the Flemish Institute for the support of Scientific-Technologic Research in Industry (IWT) for a postdoctoral fellowship. P.G. is indebted to the Free University of Brussels for a generous computer grant and to the FWO Flanders for continuous support.

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¹: IWT-post-doc (Institute for the support of Scientific-Technologic Research in Industry (IWT)).

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Adsorption energy surfaces in faujasite type zeolites

Abstract

Introduction

Section snippets

Theory and computational details

CHelpG charges on the faujasite cluster

Conclusion

Acknowledgements

Zeolites

J. Mol. Cat. A

Gas. Sep. Purif.

J. Chem. Soc., Faraday Trans.