Computational Simulations of Zeolite Surfaces and Morphologies

  • Dom Turski

Student thesis: Doctoral ThesisPhD

Abstract

Zeolites are microporous aluminosilicate materials with a variety of industrial
uses, ranging from ion-exchange to catalysis. The potential uses of each
zeolite are determined by their physical properties, one of which is morphology. Certain shapes of zeolite crystals may be required for or better suited to a particular function. This thesis aims to answer the question, ’Can we use theoretical models to explain the morphology of zeolite crystals?’.

An up to date literature review outlining zeolite uses, synthesis and how
morphology is relevant to this field of research is initially presented. We
then describe the theory behind the computational methods used to carry out modelling on the chosen frameworks and the steps followed to generate data.

A range of potential libraries were screened for use in this project. The
molecular geometry of a siliceous D4R cluster minimised at constant pressure using each of the candidate force fields was compared to that of a DFT generated cluster. The CLAYFF and ERICAFF libraries deviated the least, therefore were chosen. Monte Carlo techniques were then used to investigate the behaviour of cations around aluminium atoms in D4R and D6R clusters. The cations were observed to move towards the aluminium species in each cluster.

We investigate the effects that altering the Si/Al ratio has on the morphology
of crystals with the LTA framework. Surface energies of LTA with Si/Al ratios
of 1 and ∞ were calculated using the CLAYFF and ERICAFF libraries and were
used to generate equilibrium morphologies. The maximum Miller index was varied between 1 and 4, to demonstrate the importance of including higher
indices in models. The CLAYFF library predicted a more spherical shape for
siliceous LTA and a more cubic shape for the Si/Al ratio of 1. The ERICAFF was only able to generate a morphology for siliceous LTA which was less spherical in shape than the CLAYFF equivalent. However, the surfaces expressed on the faces of the siliceous ERICAFF model had better agreement with experimental data than the CLAYFF equivalent. The low surface energies of the expressed surfaces were rationalised based on the number of water molecules required to hydroxylate the surface.

The effect of morphology on dye uptake was studied using different morphologies of zeolite L which has the LTL framework. The predicted morphology of siliceous LTL was generated using surface energies. The shape and surfaces expressed showed good agreement with synthesised crystals and other theoretical studies. The ability of rod and disc shaped crystals to uptake dye were tested to determine if one was more effective than the other. The rod morphology was found to absorb more dye than the disc shaped crystals.

To summarise, theoretical models of siliceous zeolite frameworks do not
always provide accurate predictions of the shape of crystals. However, the surfaces expressed on the faces of these models do agree with those experimentally observed. Zeolite crystals with an LTA framework become more spherical at higher Si/Al ratios. Morphology influences zeolite L crystals’ ability to absorb dye molecules from water. Small, rod-shaped crystals are more effective than larger disc-shaped ones.
Date of Award18 Jul 2024
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAsel Sartbaeva (Supervisor), Alexander O'Malley (Supervisor) & Steve Parker (Supervisor)

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