Mixed-Gas CO2/CH4 and CO2/N2 Separation With Fluorinated and Sulfonated Aromatic PEEK Membranes

The removal of CO₂ from mixtures with CH₄, N₂ and H₂ is of interest for a wide variety of applications such as natural gas separations, flue gas cleaning, and syngas processing^[1–3]. Polymeric membranes have received tremendous attention due to several advantages, such as low cost, high energy efficiency and low ecological footprint. This paper describes the performance of fluorinated and sulfonated aromatic poly(ether ether ketone) (FS-PEEK) membranes, directly prepared from the sulfonated monomer, for CO₂ separation from gas mixtures containing N₂ or CH₄. Dense membranes with different degrees of sulfonation were prepared via solvent evaporation. Increasing degree of sulfonation simultaneously improves the permeability and selectivity of both gas pairs. The selectivity decreased with increasing CO₂ concentration in the feed, and with increasing temperature. This was attributed to the increase in polymer chain mobility, leading to considerable increase in permeability of methane and nitrogen. Mixed gas selectivities were slightly lower than ideal selectivities due to the effect of the presence of highly condensable CO₂ in the gas mixture and lowered CO₂ solubility in the membranes in the presence of N₂ and CH₄. The effect of counterions was investigated by converting the FS-PEEK membranes from the Na⁺ -form in which they are prepared, to the H⁺ and multivalent cationic forms. It was observed that the gas permeation and selectivities increased by the exchange of monovalent ions by divalent and trivalent ions. This was attributed to both electrostatic cross-linking and steric hinderance effect. The results were compared with existing literature In order to evaluate the gas transport data of the FS-PEEK membranes in comparison with literature data of other polymers, a Robeson-type plot is constructed. The results obtained in this work showing encouraging performance in comparison to the exisiting literature. Moreover, the demonstrated anti-plasticization properties of FS-PEEK already proved the interesting opportunity of industrial applications of this polymer. In order to study the stability and potential industrial application of these membranes, they were tested at different conditions of feed pressure, temperature and CO₂ feed concentration.