Molecular dynamics studies of nanoconfined water in clinoptilolite and heulandite zeolites

Abstract

The complete periodic series of alkali and alkaline earth cation variants (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) of clinoptilolite (Si : Al = 5) and heulandite (Si : Al = 3.5) aluminosilicate zeolites are examined by large-scale molecular dynamics utilizing a flexible SPC water and aluminosilicate force field. Calculated hydration enthalpies, radial distribution functions, and ion coordination environments are used to describe the energetic and structural components of extra-framework species while power spectra are used to examine the intermolecular dynamics. These data are correlated to evaluate the impact of ion–zeolite, ion–water, and water–zeolite interactions on the behavior of nanoconfined water. Analysis of the correlated data clearly indicates that the charge density of extra-framework cations appears to have the greatest influence on librational motions, while the anionic charge of the framework (i.e.Si : Al ratios) has a lesser impact.