Tuning Ether Motifs in Polymer Membranes for CO₂ Separation

Abstract

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.

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