Chemical and Biomolecular Engineering - Theses
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ItemDimensional design of the polymer/MOF composite structure for gas separation membranes( 2017)Global warming and climate change concerns have triggered global efforts to reduce the emission of carbon dioxide (CO2) to atmosphere. Post-combustion CO2 capture (PCC) is considered a crucial strategy for meeting this reduction targets. Membrane based separation technics are attractive in this field, representing the potential for a more energy efficient and eco-friendly separation process. Polymers are popular materials in gas separation membranes due to their high processability, mechanical strength and good selectivity. On the other hand, the metal-organic frameworks (MOFs) are also used in such membranes owing to their high porosity, which in turn leading to high gas permeability. In most of the current membrane studies, the MOFs merely serve as additives to polymer, i.e. the MOF particle is blended into polymer membrane to yield the mixed-matrix membrane (MMM). Therefore, the potential of MOF in gas separation applications are not fully developed. In addition, the effects of MOF morphology are not fully investigated yet. This thesis reports on several novel configurations of MOF/polymer composite structure and their applications in gas separations via membrane technology. The presentation is organized by the manipulation of MOF crystal morphologies, and the efficient use of different MOF topologies are demonstrated too. This study results in several novel membrane materials with excellent CO2/N2 separation performance. The relationship between performance and membrane architecture is investigated. In the 1st part, a novel polymer@metal-organic framework nanoparticle (P@MOF) was prepared via an in-situ ATRP on the surface of MOF nanoparticles. The P@MOF shows excellent pH dependent water dispersity, and used as the pH smart catalyst carrier. The investigation on the catalytic effect on 4-nitrophenol reduction clearly shows the efficient integration of the advantage from both heterogeneous and homogeneous catalysts. The 2nd part of this study directly applied the P@MOF particles in gas separation membranes. A novel approach to improve the selectivity of mixed matrix membrane (MMM) systems was developed. MOF nanoparticles (NPs) were chemical coated by a PEG based shell and then incorporated into a polymer matrix to yield a MMM. The unique design of the core-shell MOF NPs can enhance both the membrane permeability and selectivity simultaneously. This membrane material thus exhibits excellent CO2/N2 separation performance that surpasses the latest upper bound through the most direct way. This filler was also applied in the thin-film composite membrane system, showing promising performance located in the optimized zone for post-combustion CO2 capture proposed by Merkel et al. In the 3rd part, we have developed a bottom-up approach to fabricate an ultra-thin (~30 nm), continuous and defect-free polymeric membrane on a rough micro-scale MOF layer. This polymer-on-MOF architecture exhibits promising CO2/N2 separation performance with a CO2 permeance of > 3,000 GPU and a CO2/N2 selectivity of 34. To the best of our knowledge, this membrane has the best CO2/N2 separation performance compared to any membrane reported in the open literature. A novel concept of MMM is introduced in final part. This novel MMM has a unique configuration that the MOF fibres form a continuous interconnected sheet prior to the formation of polymer. Owing to this unique configuration, the permeability of the polymer is enhanced by 19 times without significant loss of selectivity, and surpasses the CO2/N2 separation upper bound.
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ItemSoft polymeric nanoparticles as additives for CO2 separation membranes( 2014)The use of polymeric membranes for gas separation has experienced a major expansion in the past few decades with current applications which include the separation of CO2 from flue gas. Various approaches have been explored to fabricate membranes with superior separation performance that can exceed the current upper performance limit. These include the incorporation of hard inorganic nanoparticles into polymers to form mixed-matrix membranes (MMMs). The performance of MMMs can be further enhanced if they can be fabricated into asymmetric morphology. The fabrication of asymmetric membranes, in the form of a thin film composite (TFC) membrane, is more commercially viable due to the increased flux and reduced consumption of expensive nanoparticles. TFC membranes are typically composed of a porous support coated with a highly permeable gutter layer, which is in turn coated with a thin active layer. However, the development of effective asymmetric MMMs has been limited. This is due to the difficulty in fabricating nanoparticles in a size that does not exceed the thickness of the active layer and in avoiding defects in the resulting composite structure. The fabrication of next generation mixed-matrix gas separation membranes is also hampered by the need to ensure a defect-free polymer/inorganic particle interface. A similar approach can be applied to the addition of soft polymeric nanoparticles into a selective polymer matrix. In this case, the problem of defects occurring between the particle and the matrix can be avoided through the engineering of particles that are compatible with the polymer matrix. Hence, this thesis aims to synthesize novel soft polymeric nanoparticles with well-defined architectures and utilize these as additives to be incorporated into the thin active layer of TFC membranes. The requirements for these nanoparticles include (a) a soft and CO2 permeable core and (b) a corona which is compatible with the polymer matrix. The best candidate nanoparticles are then blended with a selective polymer matrix to form the active layer of TFC membranes, which are tested for their CO2 separation from N2. The size of the soft polymeric nanoparticles are significantly smaller than the thickness of the active layer and overcome the problem of blending larger inorganic nanoparticles to form asymmetric MMMs. The first soft polymeric nanoparticles studied were based on triblock copolymers containing polyimide (PI) and poly(dimethylsiloxane) (PDMS). Well-defined difunctional PI was initially prepared via step-growth polymerization. Subsequently, PI was functionalized and chain extended with different molar ratios of PDMS-monomethacrylate (PDMS-MA) via atom transfer radical polymerization (ATRP) to form a series of triblock copolymers. Self-assembly of triblock copolymers in a selective solvent for PI, followed by cross-linking via hydrogen abstraction, resulted in the formation of well-defined nanoparticles with a soft PDMS core. The second soft polymeric nanoparticles developed in this study was based on diblock copolymers containing poly(ethylene glycol) (PEG) and PDMS. Commercially available PEG was utilized as a substitute for the PI block due to the difficulty in synthesizing well-defined polymers via step-growth polymerization. Three different molecular weights of monomethyl ether PEG were initially functionalized to form macroinitiators suitable for ATRP. These macroinitiators were then chain extended with PDMS-MA and photoactive anthracene moeities in different molar ratios to afford a series of photoresponsive diblock copolymers. Self-assembly of diblock copolymers in a selective solvent for PEG, followed by photocross-linking via [4+4] photodimerization of anthracene moeities, resulted in the formation of another well-defined soft polymeric nanoparticles with various structures that range from spherical micelles to large compound micelles. The preparation of soft polymeric nanoparticles through the self-assembly of block copolymers is generally carried out in low concentration to avoid aggregation of nanoparticles. This hinders the preparation of nanoparticles on a larger scale. The third soft polymeric nanoparticles explored in this thesis were based on PEG and PEG-b-PDMS grafted star polymers that were synthesized via the ‘core-first’ approach. This method allows the preparation of nanoparticles in high yields as the crude reaction mixture only requires separation from unreacted monomers. Various grafted star polymers with different PEG and PDMS molar ratios were synthesized in high yields and high conversions utilizing a four-arm ATRP initiator. These grafted star polymers were then utilized as additives for existing gas separation membranes. TFC membranes were prepared from commercially available selective poly(amide-b-ether) (Pebax® 2533) that was blended with a series of PEG and PEG-b-PDMS grafted star polymers. These blends formed a thin film on microporous polyacrylonitrile substrates which have been pre-coated with a PDMS gutter layer. Their ability to selectively separate CO2 from N2 was studied at 35°C and an upstream pressure of 3.4 bar. The addition of soft polymeric nanoparticles into the thin active layer of TFC membranes resulted in greatly improved flux as these particles are able to form localized, high flux, soft domains within a selective polymer matrix. These results create an interesting route to further develop and utilize soft polymeric nanoparticles as additives in membranes for gas separation processes.