Synthesis of fluorinated ether free aromatic backbone copolymers containing trifluoromethyl and dimethylamino groups for CO2 separation: Investigation of pure and mixed gas performance under dry and humid conditions

Abstract

A novel series of linear, high molecular weight aromatic copolymers containing trifluoromethyl and dimethylamino groups was synthesized via facile super-acid catalyzed polyhydroxyalkylation and investigated as CO₂ gas separation membrane materials. The molar ratio of the two ketone comonomers used in copolymerization, trifluoroacetophenone (TFAp) and N,N-dimethylamino trifluoroacetophenone (dMATFAp), was systematically varied to produce five copolymers with different dMATFAp contents ranging from 12 to 73 %. The influence of dMATFAp content on physicochemical and mechanical properties as well as pure gas and water vapor separation performance was studied. All synthesized copolymers displayed excellent film forming ability, high thermal stability and high T_g values (T_gs > 380 °C). Pure gas permeability measurements revealed that the increase of dMATFAp molar content from 12 % to 34 % resulted in improved gas permeability, while, on the contrary, further increase of dMATFAp content from 34 to 73 % led to substantial gas permeability decrease ascribed to FFV decrease. In particular, among copolymers, the one with dMATFAp content of 34 % presented the highest CO₂ permeability value of 290 Barrer due to its highest CO₂ diffusion and CO₂ solubility coefficients associated with its higher FFV value. The CO₂/CH₄ performance of synthesized copolymers fell very close to 2008 upper bound limit and exceeded most of the super-acid catalyzed copolymer membranes reported in the literature and commercial membranes. CO₂/CH₄ and CO₂/N₂ selectivity values for the different synthesized copolymers were in the range of 29.9–40 and 22.7–44, respectively. The study of the effect of dMΑTFA on water permeability unveiled that the membrane with the highest dMΑTFAp molar content (73 %) showed the highest water permeability value (23,100 Barrer) as well. This was attributed to the increased concentration of N-methyl substituent of dMΑTFAp which can interact with water molecules thereby increasing solubility. Under process gas stream conditions using a mixture of 3 % H₂O-48.5 % CO₂-48.5 % CH₄, both CH₄ and CO₂ permeability were significantly reduced (by 35 % and 47 %, respectively) due to the preferential sorption of water vapor over other gases present in the mixture, highlighting the negative effect of water vapor on gas permeability. Accordingly, the CO₂/CH₄ separation factor was slightly increased from 23.8 to 29. Finally, stability test of the copolymer (BP-TFAp)₆₆-(BP-dMATFAp)₃₄ at 40 °C under humid mixed gas conditions showed that CO₂ and CH₄ permeability remained unchanged for one month after an initial condition stabilization that is required implying that these copolymer structures are promising materials for CO₂ separation.