The impact of electrostatics in bulk Linde Type A zeolites

Abstract

Extensive Molecular Dynamics (MD) simulations were performed to study the behavior of water and methane in Linde Type A (LTA) zeolites with sodium and potassium used as counter-ions (LTA-4A and LTA-3A zeolites). Diffusivity of water was calculated for both systems, using the Mean Square Displacement (MSD) approach to quantify the impact of different counter-ions. Water in LTA zeolite with potassium as counter-ions was found to have diffusivity an order of magnitude lower than that of water in LTA-4A zeolite. The impact of the counter-ions is further clarified by the use of heat maps detailing the spatial distribution of water. Simulations in interfacial systems containing zeolite in contact with methane showed an unusual behavior of the counter-ions, prompting a detailed study of the impact of Coulombic forces in zeolites. Thus we have calculated and visualized the electrostatic gradients and studied the sensitivity of water diffusion with respect to scaling of partial charges in the zeolite framework. It was found that the outcome was highly system dependent, with the diffusivity showing a pronounced dependence varying from being increased to reduced to only intra-cavity diffusion. We concluded that it is crucial to ensure correct termination with corresponding charge distribution in interface regions. Furthermore, if one aims to investigate the ingress of water and other components into zeolite, it will be crucially important to combine the correct termination of the zeolite framework with a scaling of partial charges in the interfacial region. In conclusion we emphasize the vital role of fitting dispersive and electrostatic force fields together against experimental results, especially in the context of MD simulations.