Monte Carlo Simulations of N2and O2Adsorption in Silicalite and CaLSX Zeolites

Abstract

Grand Canonical Monte Carlo simulations are performed with two different zeotype structures, silicalite and CaLSX, chosen for their different adsorption and separation properties toward N₂ and O₂ at ambiant temperature and pressure. The purpose of this work is to obtain from GCMC simulations a consistent approach to N₂ and O₂ adsorption in both structures, especially when compared to experimental data. This study investigates how far a simple model including a Lennard-Jones term and a coulombic term for electrostatic interactions is able to provide a significant description of the zeolite-sorbate system, using a minimum of adjustable parameters. Silicalite adsorption properties are well captured by the simulations as no N₂/O₂ selectivity is obtained. Energy distribution density grids show that the silicalite structure provides an homogeneous surface toward N₂ and O₂ molecules and induces no energetic discrimination between both sorbates. The CaLSX zeolite provides a highly heterogeneous surface toward N₂ molecules owing to the interaction of the quadrupolar N₂ molecules with the electric field gradient generated by the extraframework cations. The simulations, while failing in capturing the experimental N₂ isotherm in the whole [0–1] bar pressure range, clearly show that the high N₂/O₂ selectivity is correlated to the distribution of the Ca²⁺ cations over the sites S^II of the supercages.