Structure and Dynamics of Natural Zeolite Phases through Molecular Simulation

The molecular-scale mechanisms associated with natural zeolites used in gas separations, ion exchange, waste water treatment, contaminant transport, agriculture, and many other applications are often poorly understood. Our ability to understand the molecular control of these processes is provided by a few experimental and analytical methods such as X-ray absorption, vibrational, and NMR spectroscopies. However, due to the complex nature of zeolites and related minerals, and the inherent uncertainties of the experimental methods, it is essential to use computational tools for a fundamental understanding and interpretation of these phenomena. In this effort, we have developed a general force field suitable for the molecular simulation of zeolite mineral systems with particular emphasis on the cations and water molecules associated with the pore structure. The force field includes nonbonded terms to describe atomic interactions within the zeolite framework and for the exchangeable cations, plus a flexible SPC (simple point charge) model for the pore water molecules. Bulk structures, hydration behavior, and ion exchange processes are evaluated and compared to experimental and spectroscopic findings. Large-scale molecular dynamics simulations are used to derive equilibrium structures, enthalpies, and to describe the dynamical behavior of various hydration states of these phases. These theoretical methods provide a molecular perspective of the structure, dynamics, and reactivity of important natural zeolites including sodalite, clinoptilolite, and heulandite zeolites.