Elucidating the role of micropore-generating backbone motifs and amine functionality on H2S, CO2, CH4 and N2 sorption

Abstract

Microporous polymers have transformed the field of membrane-based gas separations over the past two decades. However, applying sorbent materials as membranes presents a unique challenge in quantifying emergent phenomena, such as how strongly sorbing penetrants influence transport of co-permeating species. These mechanistic features are of urgent concern for many current and emerging applications, yet they remain significantly understudied. In previous work, amine-functionalized PIM-1 (PIM-NH2) has proven to be an exemplar of these effects, exhibiting CO2/CH4 mixed-gas selectivities that are 2.6 times higher than selectivities calculated from pure-gas measurements. Here, we investigate the generalizability of this sorption-induced emergent phenomena through the synthesis of a novel amine-functional microporous poly(arylene ether) (PAE-NH2). The effects of amine functionalization on gas transport were analyzed through variable-temperature pure-gas sorption tests, ternary mixed-gas sorption modeling, and dual-mode sorption analysis for N2, CH4, CO2, and H2S. Compared to its nitrile-functional counterpart (PAE-CN), pure-gas sorption for PAE-NH2 was 69 % higher for CO2 and 26 % higher for H2S at 1 atm and 35 °C, suggesting increased affinity to both CO2 and H2S. However, the higher total sorption for H2S resulted in strong competitive sorption effects, decreasing permeability of both CO2 and CH4 for mixture experiments, which was reported in our complementary study on the mixed-gas separation performance of the same materials in this work. The strength of gas–polymer interactions were quantified by evaluating experimental isosteric heats of sorption for the acid gases in PIM-1, PIM-NH2, PAE-CN, and PAE-NH2. Amine-functionalized samples showed highly exothermic interactions, with minima in isosteric heats of approximately −44 kJ mol−1 for CO2 and −34 kJ mol−1 for H2S. Leveraging microporosity and amine-functionality in membranes are general approaches to access competitive sorption for many industrially relevant gas separations.