A Novel Møller−Plesset Perturbation Based Potential for Determining the Structural and Dynamical Properties of Methane in Silicalite-1: A Molecular Dynamics Study

Abstract

A novel silicalite-1/methane potential function model has been developed using quantum chemical calculations at the second-order Møller−Plesset perturbation (MP2) level with the 6-31G* basis sets. Ab initio calculations have been performed at ∼150 methane configurations generated inside the three silicalite-1 segments, namely, O10Si10H20, O30Si22H44, and O35Si29H58. The interaction energies are subsequently fitted to an analytical form. We illustrate characteristics variant between the ab initio fitted potential and the available force-field models. The molecular dynamics simulations, consisting of two units of silicalite-1 cells and eight methane molecules, are performed at various temperatures. The calculated diffusion coefficient 5.53 × 10-9 m2·s-1 and the heat of adsorption −5.0 kcal·mol-1 at room temperature reasonably agree with the previous studies as well as an Arrhenius activation energy of 1.73 kcal·mol-1. The percentages of methane molecules residing in zigzag and straight channels and in the intersection are in good agreement with those reported previously. The methane/methane radial distribution function exhibits the first peak at 6.25 Å. This is in contrast to the previous one observed at ∼4.0 Å. It is, then, demonstrated that the appearance of the peak at 4.0 Å is caused primarily by an imbalance of the methane/methane and silicalite-1/methane pair potentials.