Discerning molecular-level CO2 adsorption behavior in amine-modified sorbents within a controlled CO2/H2O environment towards direct air capture

Sorbents designed for direct air capture (DAC) play a crucial role in the pursuit of achieving net-zero carbon dioxide emissions. This study elucidates CO2 adsorption from dilute, humidified CO2 streams onto an amine-modified benchmark DAC adsorbent via solid-state NMR spectroscopy. Various NMR techniques, including 1D H-1 MAS, C-13 MAS, 2D H-1-C-13 HETCOR NMR, and H-1 R-2 and R-1 rho relaxometry reveal the impact of CO2 partial pressure and H2O on CO2 adsorption behavior. We find that CO2 concentration governs the stepwise formation of ammonium carbamate, carbamic acid, and physisorbed CO2, where relative humidity (RH) at a desired low (<400 ppm) CO2 loading affects total CO2 uptake. The relaxation studies reveal the cooperative or competitive nature of H2O-CO2 sorption in CO2-dilute humid gas, and in particular polymer swelling upon humidification. From those results, we demonstrate that the observed absorption capacity enhancement by humidity is caused by pore opening due to sorbent swelling, and not by bicarbonate formation. This NMR-discerned speciation provides insights into sorption behavior at different RHs in dilute CO2 gas streams, simulating real-world atmospheric conditions, and governs the design of efficient and adaptable material-process combinations for solid sorbent DAC.