The influence of amine structure on the mechanism of CO2 facilitated transport across amine-functionalized polymer membranes: An operando spectroscopy study

Abstract

The influence of the amine structure (secondary, tertiary, pyridinic) in amine-functionalized polymeric membranes on the mechanism of CO2 transport across the membrane is investigated in this work using operando surface enhanced Raman spectroscopy (SERS) and in-situ transmission FTIR spectroscopy. Specifically, the mechanism of CO2 transport across poly-N-methyl-N-vinylamine (PMVAm), poly-N, N-dimethyl-N-vinylamine (PDVAm), and poly(4-vinylpyridine) (P4VP) membranes was investigated by measuring CO2 permeances/selectivities of the membranes and simultaneously detecting CO2 transport intermediates (e.g., carbamate, bicarbonate) formed in the membrane under operating conditions using SERS and FTIR spectroscopy. While permeation measurements suggest that CO2 moves across all membranes via a facilitated transport mechanism, operando SERS and in-situ FTIR results suggest that the molecular-level details of the facilitated transport process are highly sensitive to the structure of the amine functional group. For membranes with secondary (PMVAm) and tertiary (PDVAm) amines, CO2 moves across the membrane as a mixture of both carbamate and bicarbonate species. For P4VP, which has pyridinic amine groups, no CO2-derived intermediates were detected suggesting a new facilitated transport mechanism involving weak interactions between CO2 and the pyridinic nitrogen group without transformation of CO2 into carbamate, bicarbonate, or other intermediate species. Such a facilitated transport mechanism has not been reported in the literature to our knowledge.