Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO₂ on Supported Amines

A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO₂ have been developed, yet the effect of amine-solid support interactions on the CO₂ adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, commercial γ-Al₂O₃ and MIL-101(Cr), they show different trends in CO₂ sorption when the temperature (-20 to 25 °C) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO₂ sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al₂O₃-supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement observed at -20 °C. However, while equilibrium H₂O sorption is high at cold temperatures (e.g., -20 °C), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H₂O uptake kinetics. This work suggests that the CO₂ capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H₂O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20 °C) or ambient temperature (e.g., 25 °C) operations.