Metal-organic framework (MOF) membranes for gas separations : a thesis presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Chemistry at Massey University, Manawatu, New Zealand
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2024-04-29
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Massey University
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The study presented in this thesis involves the design, preparation and evaluation of metal-organic frameworks (MOFs) for gas separation applications, with a focus on the development of MOF membranes with both high gas permeability and good selectivity. Here, two types of membranes, including crystal glass composite membranes (CGCMs) and mixed matrix membranes (MMMs), were prepared and characterized, followed by a comprehensive evaluation of their performance for CO₂ separation.
Separating gases using membranes is appealing due to their efficiency and low energy requirements. Recently, glasses have been discovered that are produced by the melt-quenching of ZIFs. In chapter 2, we propose that useful membranes can be prepared by combining ZIF glasses with MOFs. We prepared these crystal-glass composite membranes by ball milling ZIF-62 with various crystalline MOFs followed by pressing into a tablet, heating to melt the ZIF-62 into a glass, and subsequent cooling. This fabrication process delivers membranes with homogenous dispersion of crystalline MOF particles in a ZIF glass matrix. As an alternative, we show how pre-forming ZIF-62 glass allows membranes to be formed with MOFs with relatively low stability. The resulting crystal-glass composite membranes show ultrahigh CO₂ permeance.
MMMs offer great potential for gas separation through the integration of nanofillers into polymers with excellent processability. Among these, MOF-based MMMs have emerged as promising candidates for advanced gas separation applications. However, a major obstacle hindering their performance is the inherent interfacial incompatibility between MOFs particles and the polymer matrix. In this thesis, we present a viable solution utilizing a scalable ball milling approach to address this challenge, specifically targeting the interfacial incompatibility between MUF-16 and the polymer matrix. Significantly, our approach involves the production of MUF-16 with varying crystal sizes through ball milling prior to MMM fabrication. In chapter 3 and 4, we illustrate that the nanosizing of MUF-16 (nsMUF-16) enhances its dispersion within three different polymer matrices (Pebax, 6FDA-DAM and 6FDA-Durene), effectively minimizing non-selective voids around the particles. Importantly, MMMs incorporating nsMUF-16 consistently exhibit superior CO₂ separation performance compared to those utilizing micro-sized MUF-16. Overall, this study highlights a versatile methodology employing engineered approaches to develop high-performance membranes with enhanced interfacial compatibility for gas separation. Furthermore, this approach holds promise for extending the processability of other MOF adsorbents into matrix materials, thereby broadening the scope of potential applications in gas separation technologies.
Due to the distinctive structural arrangement of MUF-15, where the phenyl rings of the ipa ligands protrude into the void space, there exists a potential for substituting the ipa ligand with various functional groups. This opens avenues for obtaining analogues of MUF-15. These functionalized variants, denoted as MUF-15-X (with X representing substituents such as F, Br, CH₃, and NO₂), offer the prospect of imbuing MUF-15 with diverse gas sorption properties. Consequently, our interest is piqued to explore the structural characteristics and gas separation capabilities of these analogues across different functional groups. In chapter 5, we present a comprehensive investigation into 6FDA-DAM-based MMMs incorporating MUF-15 and its analogues for CO₂ separation from CH₄. Notably, this study constitutes the pioneering utilization of MUF-15 analogues as fillers in MMMs specifically designed for CO₂/CH₄ separation. It introduces a spectrum of emerging MUF-15 derivatives as potential filler materials for MMM development and underscores the efficacy of pore structure functionalization in augmenting the separation performance of MMMs.
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Gas separation membranes, Materials, Metal-organic frameworks, Evaluation, Carbon dioxide, MOFs, membranes, gas separation