Atomistic Simulations of Zeolite Surfaces and the Zeolite -Water Interfaces: Towards an Understanding of Zeolite Growth

Abstract

The aim of the work in this thesis is to use atomistic simulation to model the interface between zeolite-structured alumina-silicates and water to gain a better understanding of factors controlling the surface structure, stability and morphologies. To improve the understanding of growth processes, the adsorption and desorption of monomeric growth units and template molecules is also investigated.

Chapter 1 describes recent experimental and computational work on the structure and properties of zeolites, illustrating how the microporous framework of pores and channels leads to applications such sieves, ion exchangers and catalysts. This is followed by a description of the computational techniques used in this work, as well as the approaches used for modelling the interatomic forces using potential models, in Chapter 2.

Chapter 3 discusses the results from static simulations on the surfaces of four zeolites: sodalite, zeolite L (LTL), silicalite and zeolite A (LTA) in siliceous and aluminosilicate forms. The most stable surfaces are found to be stabilised by dissociatively adsorbed water and the resulting predicted equilibrium crystal morphologies compare favourably with experiment. Molecular dynamics simulations of zeolites immersed in water are described in Chapter 4. The results show the ordering of water molecules around the siliceous and alumino-silicate LTA slabs, which caused the diffusion coefficients to become anisotropic. A notable feature of the alumino-silicate slabs is that the charge compensating cations leach into solution. Chapter 5 uses thermodynamic integration to begin to calculate growth and dissolution mechanisms. The free energy profiles of Si(OH)4 and Si(OH)3O− monomers and an OH− group adsorbing, desorbing and diffusing through {100} LTA slabs were calculated. In Chapter 6, the influence of growth directing templates is considered. The work focuses on the adsorption of (DEAH)+ and TPA+ molecular ions on LTA and silicalite surfaces, respectively. Adsorption, segregation and surface energies for both templates were evaluated. The results show that the molecular ions have different free energies of adsorption for different surfaces, as is shown for TPA+ on {100} and {010} silicalite surfaces.

Finally, Chapter 7 concludes with a brief account of the major results and an outline of several areas where this work could usefully be continued.

Date of Award	1 Jun 2010
Original language	English
Awarding Institution	University of Bath
Supervisor	Steve Parker (Supervisor)