University of Bath PHD Gas Separation Exploiting Molecular Trapdoors in Small Pore Zeolites Alexander, Thomas Award date: 2019 Awarding institution: University of Bath Link to publication Alternative formats If you require this document in an alternative format, please contact: [email protected] General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 11. Oct. 2021 Gas Separation Exploiting Molecular Trapdoors in Small Pore Zeolites Thomas Alexander A thesis submitted for the degree of Doctor of Philosophy University of Bath Department of Chemical Engineering May 2019 COPYRIGHT Attention is drawn to the fact that copyright of this thesis/portfolio rests with the author and copyright of any previously published materials included may rest with third parties. A copy of this thesis/portfolio has been supplied on condition that anyone who consults it understands that they must not copy it or use material from it except as licenced, permitted by law or with the consent of the author or other copyright owners, as applicable. This thesis may be made available for consultation within the University Library and may be photocopied or lent to other libraries for the purposes of consultation. Declaration of authorship and previous submission of work The material presented here, for examination for the award of a higher degree by research, has not been incorporated into a submission for another degree. Wherever contributions of others are involved in the work, every effort has been made to indicate this clearly, with due reference to literature and acknowledgement of collaborative research and discussions. Candidate’s signature: Thomas Alexander Abstract Zeolite RHO has recently been identified as a promising candidate for the separation of CO2 and CH4. This is due to the presence of extraframework cations in the eight membered rings (8MR) which occupy the spaces between cages and act as gatekeepers, selectively allowing the uptake of CO2 but restricting the uptake of CH4. The mechanism by which the cations move to allow the passage of CO2 molecules is not fully understood and computationally has only been studied at the quantum level. This does not allow the gating phenomenon to be observed directly and so the aim of this work is to use faster classical simulations to gain insight into the separation mechanism. Zeolite RHO is a particularly flexible zeolite and on gas loading undergoes both a phase transition and cell expansion. This makes these simulations particularly challenging to model correctly. One of the first stages in this work is therefore to ensure that the behaviour framework is reproduced adequately. Using an optimised set of force field parameters, two mechanisms are found for CO2 diffusion. The first occurs when a gate-keeping 8MR cation is pushed through a double eight ring (D8R) by a CO2 molecule and the second, less common mechanism, occurs when the D8R is completely unoccupied by a cation. The work focuses mainly on the diffusion of CO2 but other gases are also examined. Studies of the diffusion of noble gases through Na-RHO show that Xe and Kr are blocked by Na⁺ cations, whilst Ar shows low diffusivity. He shows very high diffusivity due to its small size. The gas diffusion rates through RHO can be tuned by adjusting the Si/Al ratio as well as the choice of cation. For mixed Li/Na-RHO systems, increasing the Na⁺ content increases CO2 equilibrium uptake but leads to a drop in diffusivity. Higher silicon content frameworks have larger limiting pore diameters, giving faster diffusion rates. i Acknowledgements First of all, I would like to thank my supervisors, Professor Tina Düren, Dr Valeska Ting and Dr Carmelo Herdes at the University of Bath and Dr Carole Morrison at the University of Edinburgh for their valuable inputs through the course of the project. I would also like to thank the three post-docs who I have worked with during my project: Dr Claire Hobday for her assistance in carrying out DFT simulations and discussions about crystallography; Dr Gaël Donval for his insights into Monte Carlo and Molecular Dynamics simulations and running them efficiently on HPC clusters; and Dr Stephen Wells for useful discussions about zeolites. I would also like to say thank you to the other members of the EPSRC cation- gating consortium: Professor Stefano Brandani, Dr Enzo Mangano and Dr Maarten Verbraeken at the University of Edinburgh for carrying out practical adsorption work; Professor Paul Wright, Dr Veselina Georgieva, Dr Magdalena Lozinska and Elliott Bruce at the University of St. Andrews for synthesizing the samples and carrying out XRD measurements; Dr John Casci & Dr Alessandro Turrina at Johnson Matthey and Dr Bill Casteel at Air Products for providing useful industrial feedback. I would like to say a particular thank you to Professor Julian Gale for providing invaluable assistance with GULP during the early days of the project. I would also like to thank Dr James Grant, Dr Tom Underwood and Dr Andrey Brukhno for help resolving problems with early versions of DL-MONTE and Professor Ilian Todorov and Dr Alin Marin Elena for help in sorting early problems encountered with DL-POLY. I am also grateful to Dr Marco Sant, Dr Carlos Nieto-Draghi and Dr Salvador Rodríguez-Gómez Balestra for assistance with various zeolite forcefields tried through the course of this project. ii I would also like to thank everyone who helped me get started with molecular simulations as an undergraduate: Professor Tina Düren, Professor Lev Sarkisov, Dr Matthew Lennox, Dr Ana Maria Banu, Dr Linjiang Chen and Dr Peyman Moghadam. I would also like to thank everyone in the IT departments at the University of Bath and University of Edinburgh who has provided support with both my office PC and their respective clusters (Balena and Eddie). Finally, I would like to thank the University of Bath and EPSRC (grant number EP/N032918/1) for funding this project. iii Table of Contents 1 Introduction ................................................................................................................ 1 2 Simulation techniques and theoretical background ........................................... 6 2.1 Preface .................................................................................................................... 6 2.2 The structure of zeolite RHO .............................................................................. 6 2.3 Aluminium distribution in zeolite RHO ......................................................... 11 2.4 Computer simulation techniques .................................................................... 12 2.4.1 Monte Carlo (MC) ....................................................................................... 12 2.4.2 Pressure, Fugacity, Fugacity Coefficients and the Chemical Potential in the Grand Canonical Ensemble .......................................................................... 20 2.4.3 Molecular dynamics (MD) ......................................................................... 23 2.4.4 Calculation of self-diffusion coefficient ................................................... 27 2.4.5 Periodic boundary conditions ................................................................... 28 2.5 Previous work on RHO zeolite in literature................................................... 30 3 Forcefield selection and optimisation ................................................................. 33 3.1 Preface .................................................................................................................. 33 3.2 Background ......................................................................................................... 33 3.3 Overview of CO2 model used for testing ........................................................ 35 3.4 Overview of forcefield used to describe the zeolite ...................................... 36 3.4.1 Functional form of forcefield .................................................................... 36 3.4.2 Development of Gabrieli et al. [46] forcefield ........................................... 37 3.5 Validation of model ........................................................................................... 39 3.5.1 Charges used to model RHO (Si/Al = 3.92) ............................................. 39 3.5.2 Choice of exclusion policy and Lennard-Jones parameters ................. 40 iv 3.5.3 Effect of adsorbed CO2 ............................................................................... 43 3.6 Reproduction of experimental sitings ............................................................. 49 3.6.1 Metadynamics characterisation of energy wells .................................... 53 3.6.2 Comparison with DFT calculations .........................................................