On the diffusion of water in silicalite-1: MD simulations using ab initio fitted potential and PFG NMR measurements

Abstract

Molecular dynamics simulations of water diffusion in silicalite-1 are reported. The simulations are carried out using an ab initio fitted silicalite-1–water potential based on quantum chemical calculations. In addition, preliminary results of pulsed field gradient (PFG) NMR diffusion measurements of water and small alkane molecules in silicalite-1 samples are presented. Pre-adsorption of water in silicalite-1 samples was found to change the intra-crystalline diffusivities of small alkane molecules in silicalite-1. This is interpreted as an indirect evidence that under our experimental conditions water molecules occupy a significant part of the silicalite-1 channel system. The preliminary results of the PFG NMR diffusion measurements of water in silicalite-1 samples are discussed in terms of the contributions of extra- and intra-crystalline water to the measured signals. An-order-of magnitude agreement between the measured and the simulated intra-crystalline diffusivities of water in silicalite-1 is obtained.