13C NMR study of amino acid salts in facilitated transport membranes for post-combustion carbon capture

Some of amino acid salts (AASs) have been demonstrated to be effective mobile carriers in facilitated transport membranes (FTMs) to achieve superior CO2 permeance and CO2/N2 selectivity for CO2 capture from flue gas. Understandings of how the structures of different AASs affect the chemistry of the amine–CO2 reaction are essential for the future development of more efficient AAS mobile carriers. In this study, the reaction chemistry of selected AASs with CO2 was investigated by 13C nuclear magnetic resonance (NMR) spectroscopy. The CO2 loading and the distribution of major reaction products, including carbamate and bicarbonate products of each studied AAS were quantitatively analyzed. The positive correlation between the CO2 loading of AAS (mol CO2/g AAS) and the CO2 permeance of FTMs suggested that AAS with a higher CO2 loading may improve the performance of FTMs. Our results also showed that increasing the steric hindrance of AAS could be a practical way to promote the bicarbonate reaction pathway and thus potentially increase the CO2 loading (mol CO2/mol AAS). Moreover, 2-(1-piperazinyl)ethylamine (PZEA) was found to be a more effective multi-amine than piperazine (PZ) for synthesizing AASs due to the presence of more effective nitrogen sites per molecule. The superior CO2 permeance of the PZEA-Sar membrane is also attributed to its more uniform membrane formation. The knowledge gained from this study will inform the rational design of more effective AAS carriers for CO2 capture.