Design of Novel Zeolites

  • Lisa Price

Student thesis: Doctoral ThesisPhD

Abstract

The Flexibility Window (FW) is a universal, geometric feature that can explain thefeasibility of all existing zeolites. Using template-based geometric simulations, it hasbeen shown that all known, IZA framework types can be made geometrically idealover a range of densities, yet less than 10% of hypothetical structures have thisproperty. This is an important distinction that can help narrow the search for newframeworks. The geometric method employed is highly efficient for routine screeningover large databases, where more exhaustive techniques involving energy minimisation and atomic potentials are too demanding. A FW search should, therefore, be the first point of call when evaluating the feasibility of a hypothetical structure.In this study, 255 existing and 5,824 hypothetical zeolite structures have beenevaluated and characterised using the FW criterion. High-similarity groups of existing and hypothetical frameworks have been identified based on (i) common FW descriptors (shape, length and area), and (ii) structural characteristics (d6r/d8r connectivity). As a result, 26 target, ABC-6 hypothetical frameworks have been selected that are of practical interest. Remarkably, five of these predicted structures, 194_3_1140 (SWY), 194_4_47060 (ANO), 164_2_612 (PTT), 166_4_600690 (AVE) and 229_3_4866 (PWN), have been approved as real materials during the course of this work (2018-2021).The FW is, however, more than just a yes/no parameter of feasibility. It providesvaluable insight into fundamental relationships between framework geometry, flexibility and physical properties. Using geometric simulations, it is possible to predict behaviour of known and hypothetical frameworks at the atomic level under non-ambient conditions. Within the FW, frameworks can respond to external stimuli, such as temperature, pressure and guest materials, by “ideal” deformations i.e. those that leave the tetrahedra undistorted. Furthermore, it is the orientation and connectivity of closed polyhedral building units, d6r, d8r and can, that determines the specific flexibility of each topology. This is confirmed experimentally for K-L zeolites which have been studied under high-pressures using synchrotron, ToF neutron diffraction. As a result, a new method of combining geometric modelling with Rietveld refinement has been developed to determine the crystal structures of zeolites at high-pressures. Furthermore, it is possible to predict framework compositions based on the observed trend in FW areas and ∠T-O-T distributions. Now, more than ever, there is a great demand for new zeolite materials with unique topologies that offer potential solutions to tackle many of the 21st century global sustainability challenges. Here, we have developed a robust protocol that should be used to narrow the search so that efforts are focussed on targeting only the most promising framework types. It is hoped that the strategies presented here will assist in the discovery of many new zeolites in the near future.
Date of Award14 Sep 2022
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAsel Sartbaeva (Supervisor), Adam Squires (Supervisor) & Stephen Wells (Supervisor)

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