Operando observation of CO2 transport intermediates in polyvinylamine facilitated transport membranes, and the role of water in the formation of intermediates, using transmission FTIR spectroscopy

Abstract

Amine functional groups in polymeric facilitated transport membranes (FTMs) selectively facilitate CO₂ transport across the membrane over other gases to give rapid and selective CO₂ separation from mixed gas streams. However, the mechanisms by which the amine functional group facilitates CO₂ transport are not well understood. Additionally, water vapor is known to enhance CO₂ permeation across amine-based FTMs, but how exactly water influences the amine-facilitated CO₂ transport mechanism is unknown. To address these fundamental aspects of CO₂ facilitated transport, we have developed a novel operando transmission FTIR spectroscopy tool to directly observe the formation of CO₂ transport intermediates (e.g. carbamate, bicarbonate) formed in amine-based FTMs under realistic operating conditions, including in humid CO₂ gas mixtures, and correlate formation of specific CO₂ transport intermediates to CO₂ permeation rates measured simultaneously for the same sample. This work describes the design of this operando transmission FTIR spectroscopy tool and demonstrates its utility by investigating the mechanism of water-enhanced CO₂ transport across polyvinylamine (PVAm), a prototypical FTM for CO₂ separations. We show that CO₂ moves across PVAm most likely as carbamate (R-NHCOO⁻) species, based on infrared band positions and literature assignments, and formation of carbamate species is facilitated by water in the CO₂/N₂ feed gas mixture. During exposure of PVAm to dry CO₂/N₂ feeds, carbamate species or other intermediates were not observed spectroscopically, and the CO₂ flux across the membrane was below the detection limit. We speculate that water may facilitate the formation of carbamate, and thus enhance CO₂ transport across the membrane, by catalyzing deprotonation of a zwitterion (R-NH₂⁺COO⁻) intermediate to give carbamate species. This work clarifies the unknown mechanism of CO₂ facilitated transport across PVAm, and the role of water in the transport mechanism, and establishes a new operando spectroscopy tool that can be used with a wide range of membrane systems to probe transport mechanisms, fouling mechanisms, and changes in membrane structure/performance over time.