Pebax-based composite membranes with high gas transport properties enhanced by ionic liquids for CO2 separation

Abstract

Membrane-based separation technology has been reported as one of the possible methods to efficiently and economically separate carbon dioxide (CO₂). To provide synergistic enhancements in the gas separation performance, organic polymer (Pebax 1657), zeolite imidazolate framework-8 (ZIF-8) nanoparticles, and ionic liquid (IL) have been integrated to develop three-component composite membranes. To achieve high separation performance of three-component membranes, the effects of IL anions and ZIF-8 content on gas permeability and selectivity were investigated first. The ILs were 1-butyl-3-methyl imidazolium ([Bmim]) cation based on different anions of bis(trifluoromethylsulfonyl)imide ([NTf₂]), dicyanamide ([DCA]), and tetrafluoroborate ([BF₄]). Gas transport properties of all the prepared membranes were investigated at 23 °C and 1 bar. The results showed that the anion of IL is a key factor to determine the CO₂ permeability of the membranes, which is similar to the principle of CO₂ solubility in pure ILs. In addition, ZIF-8 could increase both CO₂ diffusivity and solubility coefficients of the Pebax/ZIF-8 membranes, resulting in a two-fold increase in the CO₂ permeability. For the Pebax/ZIF-8(15%)/[Bmim][NTf₂] membranes, it has been revealed that [Bmim][NTf₂] acts as a low molecular weight additive, leading to a more amorphous structure and a higher FFV (fractional free volume) of the membranes, which are beneficial for gas diffusion. The addition of IL can improve the compatibility between the inorganic particles and the polymer matrix; thus, the non-selective voids decrease, which leads to a higher CO₂/N₂ selectivity. The CO₂ permeability of the Pebax/ZIF-8(15%)/IL(80%) membrane was 4.3 times that of the pure Pebax membrane without sacrificing the CO₂/N₂ selectivity. Therefore, the high gas transport properties of the Pebax/ZIF-8/IL membranes make them promising candidates for CO₂-effective separation materials.