Ramsahye, Naseem Ahmed; (2003) Computer modelling of halocarbon sorption in zeolite Y. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The question of cation mobility upon the adsorption of polar molecules is crucial in terms of understanding the fundamental chemistry of basic zeolites, containing exchangeable alkali cations and electron-rich framework oxygen atoms. Experimental techniques have not as yet provided unambiguous answers. Computational methods, such as molecular dynamics (MD), can help to find answers to some of these questions. The MD study conducted in this thesis (chapters 3 and 4) has shown that sodium cations in NaY are mobile upon sorption of various halocarbon molecules and that cations located in different sites exhibit different types of mobility. The main migration activity takes place when an SII cation migrates from its site on a six-ring by interacting with the Cl atoms on the chloroform molecule. SI' cations are then able to traverse a six-ring window to take a position in a vacant SII site. The defect energy minimization technique has been used to study the energetic barriers to the cation motion throught zeolite six-rings, and the effect of the aluminium content on these energetic barriers. It has been found that the greater the A1 content, the lower the energy barriers for sodium cations traversing the six-ring from the SII' to the SII site. The six-ring has been found to exhibit a 'breathing' motion as the cation traverses through it. The full results are described in chapter 5. Electronic structure methods using Density Functional Theory and semi-empirical methods have been used to study the effect of the cation motion on the zeolite six-ring where a polar molecule is adsorbed. These calculations were performed using large clusters, embedded clusters, and using a periodic zeolite primitive cell. The results have shown the importance of including the long range electrostatic effects when calculating adsorption energies. The full results of this study are reported in chapter 6.

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