Calculating the energy barriers to sodium cation motion through the six-rings of zeolite Y

The Mott-Littleton scheme has been used to calculate the energetic profile of sodium cation migration from the SI′ to SII sites in zeolite Y, including traversing the plane of zeolite 6-rings containing different configurations of aluminium atoms. Two barriers are encountered by a cation travelling from the SI′ site to the SII site. The first occurs between the SI′ and SII′ sites, and the second between SII′ and SII. The number of aluminium atoms and their relative positions are believed to influence the flexibility of the zeolite, and thereby affect the energetic barriers. The results show that the energy barriers for the path between the SI′ and SII′ sites vary between about 30 kJ mol−1 and 58 kJ mol−1. The barrier between SII′ and SII decreases with increasing numbers of aluminium atoms in the zeolite 6-ring, from around 19 kJ mol−1 to 4 kJ mol−1. In the event of a 6-ring containing two aluminium atoms, the scenario where the aluminium atoms are arranged in a para position has a lower energy barrier than the meta case by 8 kJ mol−1. Rationalisation for this observation in terms of elongation of oxygen–oxygen distances, and breathing of the 6-ring as the cation traverses is also presented.