One critical issue in the development of immobilized amine sorbents for CO2 capture is sorbent degradation, which leads to a significant increase in the overall CO2 capture cost. Here we report a novel approach for enhancing the degradation resistance of a branched polyethylenimine (PEI) by hydroxyl groups (i.e., −OH) of a porous cross-linked poly­(vinyl alcohol) (PVA) support. The CO2 capture capacity of a PEI/PVA sorbent showed a 25% decrease and then leveled off. In contrast, the PEI/SiO2 sorbent exhibited more than a 3-fold decrease in CO2 capture capacity after exposure to a cyclic CO2 capture and oxidative degradation environment (15 vol % of CO2 in air at 130 °C). In situ infrared spectroscopic study revealed that the secondary amine is more liable to degrade than the primary amine on PEI/SiO2. On PEI/PVA, the PVA’s −OH groups interacted mainly with the secondary amine of branched PEI through a hydrogen bonding, which could contribute to enhancing the resistance of PEI against degradation. This new finding could provide new pathways in the development of low-cost and highly durable amine sorbents by using highly porous polymeric supports with −OH groups.

The structure and nature of adsorbed species on solid amine sorbent under various CO2 concentrations from ambient air (0.04 vol %) to an CO2/air stream (15 vol %) have been studied by using in situ IR spectroscopy. The in situ IR study (i) provided evidence to support the zwitterion pathway for CO2 adsorption/desorption and (ii) allowed the determination of the nature of the adsorbed species. The IR‐observable zwitterion was deprotonated to ammonium carbamate and carbamic acid reversibly. CO2 adsorbs on the primary amine site as a strongly adsorbed species in the form of ammonium carbamate and on the secondary amine site as a weakly adsorbed species in the form of carbamic acid. Another form of weakly adsorbed species could be ammonium carbamate on the secondary amine. An increase of the concentration of CO2 in the stream increased the CO2 capture capacity, the fraction of strongly adsorbed CO2, and the binding energy of adsorbed species. A concentration of 400 ppm CO2 in air competes over 0.5 % of water vapor for adsorption on Class I amine sorbents.

Water, a component in flue gas, plays a significant role in CO 2 capture through a complex interaction between water molecules and adsorbed CO 2 on amine sorbents. To determine how the H 2 O−CO 2 −amine interactions affect amine efficiency and the binding energy of adsorbed CO 2 , we used in situ infrared spectroscopy (IR) to determine the structure of adsorbed CO 2 and H 2 O as well as their relations to adsorption/desorption kinetics and CO 2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents. H 2 O enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO 2 and promoting the formation of hydronium carbamate and carbamic acid. CO 2 adsorbed on the surface of the TEPA film as a weakly adsorbed CO 2 in the form of hydronium and ammonium−carbamate with a low IR intensity of hydrogen bonding (−OH••• − OOC or −NH••• − OOC) between hydronium/ammonium ions and carbamate ions. CO 2 adsorbed on the middle layers (i.e., 0.2−0.4 μm below the surface) of TEPA films produced a strongly adsorbed species that exhibits an intensive hydrogen bonding band of ammonium−water−carbamate desorbing at temperatures above 120 °C. Comparison of IR spectra shows that the kinetic behaviors of adsorbed CO 2 on amine films are correlated well with those of adsorbed CO 2 on Class I amine sorbents. Thick amine films and high-amine-loading sorbents contain high-density amine sites that produce mainly strongly adsorbed CO 2 . Adsorbed H 2 O further increased amine efficiency and the binding energy of strongly adsorbed CO 2 through the formation of hydronium carbamate.

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