Solubility selective membrane materials for carbon dioxide removal from mixtures with light gases
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Membrane technology has attracted interest for the selective removal of carbon dioxide from mixtures with light gases such as H2, CH4 and N2. While conventional structure-property correlations have focused mainly on improving the separation performance by increasing polymer size sieving ability (i.e., diffusivity selectivity), this project explores the possibility of harnessing favorable interactions between CO2 and polymers containing polar groups to improve permeability/selectivity properties. Ether oxide groups are discovered to be among the best moieties known to interact with CO2, leading to high CO2 solubility and CO2/light gas solubility selectivity, while still providing polymer chain flexibility, leading to high CO2 diffusivity and favorable CO2/H2 diffusivity selectivity. Poly(ethylene oxide) (PEO) has a high concentration of ether oxygen groups and exhibits high CO2/light gas selectivities. However, gas permeability is low due to the high crystallinity in PEO. Crosslinking and introduction of short chain branching are efficient methods to inhibit crystallization. Three series of crosslinked poly(ethylene oxide) rubbers have been prepared using prepolymer solutions containing: (1) poly(ethylene glycol) diacrylate (PEGDA) and H2O, (2) PEGDA and poly(ethylene glycol) methyl ether acrylate (PEGMEA), and (3) PEGDA and poly(ethylene glycol) acrylate (PEGA). Independent of the prepolymer composition, all of these polymers have similar ethylene oxide (EO) content (approximately 82 wt.%). Crosslink density decreases with decreasing PEGDA content in the prepolymer solution, which is estimated from water swelling experiments and/or dynamic mechanical testing and has essentially no effect on gas transport properties. Increasing PEGMEA content increases the average size of free volume elements, resulting in a decreased glass transition temperature, and increased CO2 permeability and CO2/H2 selectivity. In contrast, the presence of PEGA or water has a negligible impact on these properties. Due to the affinity between EO and CO2, decreasing temperature increases CO2/light gas solubility selectivity. PEGDA/PEGMEA-30 (a copolymer containing 30 wt.% PEGDA and the balance PEGMEA) demonstrates very favorable mixed gas CO2/H2 and CO2/CH4 separation performance. The separation properties improve with decreasing temperature and have excellent resistance to impurities in the feed streams such as high pressure CO2, higher hydrocarbons and water. Interestingly, the highly sorbing impurities can even improve CO2/H2 separation. Pure and mixed gas diffusivity and permeability in these crosslinked PEO rubbers were successfully modeled using a new free volume model, which was based on the Cohen-Turnbull and Fujita free volume models coupled with Chow’s model for estimating glass transition temperatures in polymer/gas mixtures as a function of temperature and penetrant concentration.