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Title: A molecular dynamic study of molecular gas separation for clean energy applications
Author: Luo, H.
ISNI:       0000 0004 7660 1953
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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Gaseous molecular separation is crucial for carbon dioxide capture and storage, hydrogen purification, and natural gas processing. Graphene-based membranes are promising candidates for such purposes, where their performance can be enhanced by the tunable pathways consisting of the nanopore, interlayers and inter-edge spacing. However, there is a lack of understanding of the molecular behaviours within the versatile pathways, due to the practical complexity of the process and the limitation of experimental techniques. Molecular Dynamics (MD) simulations can offer significant insights into the mechanisms of molecular transport characteristics inside graphene-based nanostructures, and hence predict the membrane performance and optimisation for gas separations. A newly proposed monolayer porous graphene membrane was first evaluated as a primary step for the separation of H2 from CH4, N2, or CO2 impurities. The membrane showed high performance for the H2/CH4 separation under various pressure gradients; with the selectivity-permeance relationship far surpassing the upper bound for conventional polymer membranes. For H2/N2 separation, the selectivity-permeance relationship closely approached the upper bound. For H2/CO2 separation, CO2 molecules can be strongly adsorbed at the centre of the porous membrane, implying that the membrane can also function as a highly selective sorbent for CO2 removal. Furthermore, the characteristics of CO2 and N2 diffusion inside different interlayer spaces of graphene-based membranes were investigated under both dry and iii wet conditions. Based on the solution-diffusion mechanism, the predicted selectivity of CO2/N2 separation was improved 42 times by the presence of water, as a result of the single-file diffusion of CO2 through the interlayers of graphenebased structures; this could help explain the experimental observations in the literature. An in-depth investigation into the mechanism of the enhancement on the selectivity of CO2/N2 separation showed that water formed hydrogen bond networks with rich oxygen-containing groups of graphene-based membranes and restricted the diffusion of CO2 and N2, leading to the self-diffusivity of CO2 and N2 approximating to that of H2O. Under the confinement of graphene-based interlayer spaces, the solubility of both CO2 and N2 were improved, with the solubility of CO2 being larger than that of N2 due to the stronger binding between oxygen-containing groups and CO2 than N2. Finally, the diffusion of H2, CH4, CO2 and CO through the inter-edge spacing of graphene-based membranes was investigated. The results showed that high selectivity and permeance for CO2 removal from H2, CH4, CO impurities were achieved by modifying the chemistry of the inter-edge spacing of the graphene-based membrane. The highest enhancement is 136% for H2/CO2, 208% for CH4/CO2, and 180% for CO/CO2 separations when the edges were enriched with carboxyl groups. Much of the enhancement was due to the presence of carboxyl and amide groups which forced gases to diffuse in a larger distance from the edges of graphene-based structures, where H2, CH4, CO showed higher mobility, except for CO2 due to its strong binding with various functional groups. This study provides a fundamental understanding of gas transport characteristics through the complex pathways of graphene-based nanostructures and is of great significance to practical design and development of membranes for gas separations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available