Characterization of Amine-Functionalized Facilitated Transport Membranes for CO₂ Separation

Amine functional groups in polymeric facilitated transport membranes (FTMs) selectively and reversibly facilitate the transport of CO₂ over other gases, which results in rapid and selective CO₂ separation from mixed gas streams, such as CO₂/N₂. However, the mechanisms of CO₂ transport by amine functional groups across membranes are not well understood. Water vapor is known to enhance CO₂ permeation rates through amine-functionalized polymeric membranes; however, it is unclear how exactly water influences the CO₂ transport. To address the fundamental issues of CO₂ facilitated transport, we have developed a novel operando transmission FTIR spectroscopic permeation tool to directly observe the formation of CO₂ transport intermediates through amine-based FTMs under realistic operating conditions, including dry and humid CO₂ gas mixtures, and simultaneously correlate the formation of these intermediate species with the CO₂ permeation rates through the same sample. This work describes the development of this novel operando transmission FTIR spectroscopic tool, and first demonstrates its utility by probing the mechanism of water-enhanced CO₂ transport through polyvinylamine (PVAm), a model primary amine that it a typical FTM used for CO₂ separations. We show that water enhances CO₂ permeation across PVAm by facilitating the conversion of CO₂ to a reactive intermediate, which is most consistent with an ammonium carbamate species based on the infrared band positions. To further probe the fundamental issues of CO₂ permeation, we will study the effect of water-enhanced CO₂ permeation on poly(N-methyl-N-vinylamine) (PMVAm), poly(N,N-dimethyl-N-vinylamine) (PDVAm), polyvinylpyridine, and quaternized polyvinylpyridine materials to investigate the effect of amine structure on CO₂ transport through membranes. Next, we will further investigate how the presence or absence of water influences CO₂ transport through these same materials through exposure to dry CO₂/N₂ and humid N₂. These investigations allow us to clarify the effect of amine structure and the role of water for CO₂ transport through amine-functionalized membranes, which ultimately allows us to propose new CO₂ transport mechanisms based on the direct observation of intermediate species. This work establishes a novel operando spectroscopy tool that can be used to further study a wide range of membrane systems to probe transport mechanisms, fouling mechanisms, and correlate any changes in membrane structure to its performance over time.