The CO₂ permeability and mixed gas CO₂/H₂ selectivity of membranes composed of CO₂-philic polymers

The objective of this work was to design polymeric membranes that have very high CO₂ permeability and high mixed gas selectivity toward CO₂ rather than hydrogen. Therefore the membranes were based on “CO₂-philic” polymers that exhibit thermodynamically favorable Lewis acid:Lewis base and hydrogen bonding interactions with CO₂. CO₂-philic polymers that are solid at ambient temperature include polyfluoroacrylate (PFA); polyvinyl acetate (PVAc); and amorphous polylactic acid (PLA). Literature CO₂ permeability values for PVAc and PLA are disappointingly low. The cast PFA membranes from this study had low permeabilities (45barrers at 25̊C) and very low CO₂/H₂ selectivity of 1.4. CO₂-philic polymers that are liquid at ambient conditions include polyethylene glycol (PEG), polypropylene glycol (PPG), polybutylene glycol with a linear -((CH₂)₄O)-repeat unit (i.e., polytetramethylene ether glycol (PTMEG)), polybutylene glycol (PBG) with a branched repeat unit, perfluoropolyether (PFPE), poly(dimethyl siloxane) (PDMS), and polyacetoxy oxetane (PAO). A small compound, glycerol triacetate (GTA) was also considered because it is similar in chemical structure to a trimer of PVAc. These liquids were tested as supported liquid membranes (SLM) and also (with the exception of PAO and GTA) as rubbery, crosslinked materials. Mixed gas permeability was measured using equimolar mixtures of CO₂ and H₂ feed streams at one atmosphere total pressure in steady-state flux experiments over the 298–423K temperature range. The most promising SLMs were those composed of PEG, PTMEG, GTA, and PDMS. For example, at 37̊C the PEG-, PTMEG-, GTA- and PDMS-based SLMs exhibited CO₂/H₂ selectivity values of ∼11, 9, 9, and 3.5, respectively, and CO₂ permeability values of ∼800, 900, 1900, and 2000barrers, respectively. Crosslinked versions of the PEG, PTMEG and PDMS membranes at 37̊C exhibited selectivity values of ∼5, 6, and 3.5, respectively, and CO₂ permeability values of ∼50, 300, and 3000barrers, respectively.