Tuning Ether Motifs in Polymer Membranes for CO2 Separation

Polymer membranes are an attractive, energy efficient alternative to traditional unit operations for gas separation. Polyethers have been leading membrane materials for CO2 separation due to their unique ether oxygen moiety that exhibits affinity towards CO2. We systematically study the effect of an ether-oxygen moiety on solubility using perturbed-chain statistical associating fluid theory equation of state calculations and on diffusivity using molecular dynamics simulations for CO2 separation. We investigate five different polymer materials with varying oxygen content, including commonly used polymers such as poly(ethylene oxide) as well as polymers with higher ether-oxygen content. Our results show that increasing the ether-oxygen moiety in the polymer membrane significantly increases the CO2/N2 solubility selectivity. Of the studied materials, polyoxymethylene has the highest oxygen to carbon ratio, and it has the highest CO2/N2 solubility selectivity. Molecular dynamics simulations indicates CO2/N2 diffusivity selectivity increases with increasing ether-oxygen content in the polymer, although the individual gas diffusion slows down. Moreover, we find that increasing the temperature increases the gas diffusion; however, the polymers lose their selective interactions with CO2, thus resulting in lower selectivity. We demonstrate that the ether-oxygen is a key functional group controlling the CO2/N2 solubility selectivity of polymer membranes.