Ca-Na cation exchange in zeolite A: A microscopic approach using molecular dynamics simulations

(ricevuto il 15 Dicembre 2008; pubblicato online il 13 Febbraio 2009)Summary. — Molecular dynamics computer simulation technique was applied tothe study of Ca-Na cation exchange in hydrated zeolite A, one of the most widelyexploited cation exchange processes in practical applications. The exchange canoccur only by breaking and reconstructing the coordination shell of the cations,so that some steps of the mechanism show a high activation energy, even if theoverall energy difference between the starting and the final states of the processis relatively small. Therefore, special procedures such as umbrella sampling mustbe used to force the system to overcome the energy barriers. The cation exchangeappeared to follow a highly coordinated mechanism, and a complete exploration ofthe free-energy hypersurface is required to obtain quantitative results. In this papersome interesting qualitative features of the cation exchange process are anticipated.PACS 82.75.Jn – Measurements and modeling of molecule migration in zeolites.