Development of Mixed Matrix Membranes for Carbon Dioxide Capture
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Global warming is one of the world’s major environmental issues. Reduction of greenhouse gas emissions by the capture of carbon dioxide from flue gas is one out of different kinds of methods to try to address this problem. Due to the economic and environmental advantages of membrane separation over other separation technologies, a lot of research activities are being carried out for development of sophisticated membrane materials such as facilitated transport membranes, polymeric, inorganic and mixed matrix materials for CO2 capture. When used in industrial applications the membrane will need to be chemically resistant to the harsh environment in flue gas. Polyimides combine excellent thermal and chemical stability with a very wide range of CO2 permeability. Some polyimides, particularly those incorporating the group 6FDA, possess high CO2 permeability. Thus some aromatic polyimides with a favorable balance of properties such as solubility, crystallinity, glass transition temperature, density or free volume have achieved importance for application in gas separation technology. Polyimides are generally synthesized by the reaction of a diamine and dianhydride in an aprotic solvent to form a polyamic acid. This polymer then undergoes a polycondensation reaction to form desired polyimide. Polyether block amide (PEBA) resin is best known under the trademark PEBAX, and is a thermoplastic elastomer combining linear chains of rigid polyamide segments interspaced with flexible polyether segments. The structure of the PEBAX repeating unit includes an aliphatic polyamide “hard segment” and an amorphous polyether “soft segment.” The soft segment is either poly(ethylene oxide) or poly(tetramethylene oxide). This crystalline/amorphous structure creates a blend of properties of thermoplastics and rubbers. In application to permeation, it was believed that the hard amide block provides the mechanical strength, whereas gas transport occurs primarily in the soft ether segments. In applications involving the removal of CO2 from gas mixtures, PEBAX was found to have high selectivity for polar or quadrupolar/nonpolar systems like CO2/N2. Composite materials could combine the basic properties of organic and inorganic materials and offer specific advantages for the preparation of mixed matrix membranes (MMMs) with excellent separation performances. Composite materials such as polymers with embedded nano-inorganic particles have been investigated in the recent years and possess the potential to provide a solution to the trade-off between permeability and selectivity for polymer membranes published by Robeson. Metal organic frameworks (MOFs) are a relatively new class of microporous materials comprised of transition metals and transition metal oxides connected by organic linkages to create microporous structures. It is important to choose a polymer with the quality which matches the nano fillers as much as possible. In Article 1, a new kind of self-supported dual layer mixed matrix membrane was developed in this work using ZIF-8 as inorganic filler in PEBAX-2533 polymer matrix. The developed dual layer flat sheet mixed matrix membrane was characterized to investigate the morphology of organic-based and inorganic-based layer. The gas separation properties of the mixed matrix membranes were tested using single gases CO2, CH4, N2, and O2 and a mixture of CO2 and N2 in dry and humidified conditions. The permeability of all examined gases increased as the inorganic filler content increased in the matrix membranes, and specifically it increased dramatically for CO2 in all cases, single feed gas and dry and humidified mixed gas. The CO2/N2 selectivity decreased slightly from 33.8 for the pure PEBAX membrane to 32.3 for the mixed matrix membrane with 35% ZIF-8 loading, while a more significant drop in CO2/N2 selectivity was observed in experiments using mixed gases. In article 2, mixed matrix membranes of ZIF-8 as inorganic filler in polymer matrix of synthesized 6FDA-Durene Diamine was developed. Pure gas permeation for the gases CO2, CH4, N2, and O2 and mixed gas of CO2/N2 was carried out at 2 and 6 bar for single gas and 2.6 bar for mixed gas using the mixed matrix membrane of ZIF-8/6FDA-Durene Diamine in different inorganic filler loading. The homogenous dispersion of ZIF-8 particles in polymer confirmed by SEM resulted in dramatic increase in gas permeability for all tested gases and significant increase in CO2 permeability from 1468 Barrer to 2185 Barrer corresponding to pure polymer membrane and 30 wt.% loaded ZIF-8 mixed matrix membrane at 2 bar feed gas pressure, respectively. Incorporation of inorganic fillers in membrane results in increment of decomposition temperature of mixed matrix membranes while no significant change in Tg was observed. In article 3, nanocomposite membranes of modified Si nanoparticles as inorganic filler in two different polymer matrixes were developed. Two polymer matrixes from two different categories were chosen to investigate the effect of the inorganic nanofiller in polymer matrix on different properties of the polymeric membrane and specifically the gas separation properties. Synthesized 6FDA-Durene Diamine as a glassy polymer and PEBAX-2533 as a block copolymer were used as the polymer matrix to develop the nano composite membranes of modified Si nanoparticles in polymer matrix. The STEM result showed the nano size dispersion of inorganic nanofillers in the polymer matrix in both cases. Pure gas permeation for the gases CO2, CH4, N2, and O2 and mixed gas of CO2/N2 was carried out at 2 and 6 bar for single gas and 2.6 bar for mixed gas using the developed nanocomposite membranes. The increment of loading inorganic fillers in PEBAX-2533 polymer matrix resulted in dramatic increase in gas permeability for all tested gases and significant increase was observed in CO2 permeability at 2 bar feed gas pressure from 301 Barrer to 400 Barrer corresponding to pure polymer membrane and nanocomposite membrane, respectively. A decrease was observed in CO2/N2 and CO2/CH4 selectivity with loading small amount of inorganic filler while adding more inorganic filler does not change the aforementioned selectivities which were probably due to the formation of nanogap around the nanoparticles in polymer matrix. Incorporation of inorganic fillers in PEBAX-2533 resulted in increment of decomposition temperature of nanocomposite membranes, while no significant change in melting point was observed. The dispersion of the nanoparticle inorganic fillers in 6FDA-Durene polymer matrix caused an increase in the fraction free volume of the polymer matrix confirmed by the density results. Therefore, this disruption resulted on the increment of gas permeability confirmed by both single and mixed gas experiment while the CO2/N2 and CO2/CH4 selectivity also increased. The permeability of 6FDA-Durene of CO2 at 2 bar is 1468 Barrer while with loading inorganic filler it increased the permeability up to 3785 Barrer, while the ideal CO2/N2 selectivity changed from 26 to 31 and ideal selectivity of CO2/CH4 changed from 23 to 46. No significant change on Tg of the polymer matrix was observed after loading inorganic nanoparticle filler and the thermal decomposition increased up to 495ºC.