The CO2 and H2 Permeability of Membranes Composed of Polyfluoroacrylate, Perfluoropolyether (PFPE), PFPE-Plasticized Amorphous Teflon, and Polyfluoroalkoxyphosphazene

Membranes could be an integral part of the coal gasification/water-gas-shift reaction process to transform coal, oxygen and water feed streams into hydrogen for fuel and CO₂ for sequestration. The objective of this work is to design polymeric membranes that have very high CO₂ permeability and high selectivity toward CO₂ (i.e. very low H₂ permeability). It is our hypothesis that the favorable thermodynamic interactions that enable certain polymers to dissolve in dense CO₂ at extremely high pressure will also allow membranes composed of these polymers to exhibit high CO2 permeability at low pressure. Candidates include perfluoro polyether (PFPE), polyfluoroalkoxy phosphazene (PFAP), poly(fluoroacrylate) (PFA) and PFPE-plasticized Teflon AF2400. PFPE is a liquid oligomer that was transformed into a diacrylate and then cross-linked to form flexible membranes. PFAP and PFA were processed into films via dip coating or spin coating. Although PFA is known to be one of the most CO₂-philic amorphous polymers ever identified, and the solubility of PFPE is comparable to PDMS at lower molecular weights, the CO₂-permeability of these membranes has not (to the best of our knowledge) been previously established.

The CO₂ and H₂ permeability values of these fluorinated membranes were determined using a constant volume apparatus or constant pressure apparatus. These constant volume experiments were conducted with a single gas at 295K and CO₂ (or H₂) with partial pressure gradients no greater than 450 kPa. Ideal selectivity values were determined as the ratio of CO₂ permeability to H₂ permeability. The mixed gas constant pressure experiments were conducted within a temperature range of 310 to 423 K and CO₂ (or H₂) partial pressure gradients near atmospheric conditions. Permselectivity values were determined as the ratio of CO₂ permeability to H₂ permeability.

PFPE membranes exhibited relatively high permeability values (comparable to PDMS), but CO₂/H₂ selectivity values did not exceed 5. Despite its incredible CO₂-philicity, the PFA membrane exhibited disappointingly low permeance and selectivity values. Results for the PFPE-plasticized amorphous Teflon and the PFAP membranes will also be presented.