Charge distribution in NaY zeolite from charge-transfer molecular dynamics

Abstract

Charge-transfer molecular dynamics simulations of NaY zeolite in thermodynamical equilibrium in the microcanonical ensemble were carried out in order to obtain the charge distribution of all charge-transferring species. The interaction potentials used were parameterised ab initio calculations and take into account the Al–O and Si–O partially covalent bonds. The different kinds of O and Si atoms are characterised based on their charge distributions which are comparable with experimental ${}^{29}\text{Si}$ MAS NMR data.

Introduction

Faujasites are the most widely used catalysts in the oil industry. Because of their importance as catalysts and molecular sieves, many attempts have been made to model their chemical and structural characteristics. Their catalytic activity strongly depends on the existence and location of acid sites, and their interaction with the adsorbed molecules. From the physical point of view one of their most important qualities is their electronic structure. Quantum-chemical simulations of zeolites have been recently performed [1]. However, this methodology is still quite expensive in computational terms and size and simulation times are rather limited. Experimentally, it is necessary to employ a series of techniques to characterise their structure. Recently, the usage of ${}^{29}\text{Si}$ MAS NMR spectroscopy has shine some light on the existence of ${}^4\text{Q}$(nAl) of different kinds in zeolitic networks 2, 3. Even though in the last few years experimental studies have improved our understanding of these complex systems, there still are open questions to investigate, such as the combined effect of the existence and location of Brönsted and Lewis acid sites, and the role of water in their formation, which utlimately is related to proton transfer; the mechanisms of formation of extraframework alumina, and the catalytic role it plays. All these basic phenomena related to the catalytic properties of zeolites are amenable to numerical simulation.

The presence of aluminium in zeolites modifies the electronic environment and consequently produces electrostatic field gradients which are directly related to the catalytic activity [4]. Charge-transfer molecular dynamics (CTMD) [5]was developed to study systems in which bond making and breaking occurs, and in which electric charge is crucial and determines the way in which a number of phenomena such as diffusion and modifications of structure occur. In order to increase the realism of our simulations of silicates in general and of zeolites in particular we have fitted parameters for CTMD interaction potentials in order to represent the partially covalent nature of the Al–O bond, based on periodic ab initio calculations [6]. This potential along with the already existing one for the Si–O interaction allows us to simulate the zeolitic framework, improving the quality of the simulations relative to our own previous works 7, 8. In this Letter, we present and discuss one of the most interesting features of NaY zeolite, namely the charge distribution of Al, O, and Si atoms, as a step forward in the wider study of zeolite physico-chemical properties through CTMD simulations.