(366g) Molecular Design of Selective Humid CO2 Adsorption Sites in Small-Pore Zeolites

While zeolites possess high volumetric CO₂ adsorption capacities under dry conditions, and weak interactions with CO₂, which are preferred for energetically facile adsorption/desorption, the inevitable presence of water in post-combustion gas streams results in strong competitive adsorption, which decreases CO₂ adsorption capacities and leads to creeping death in adsorption unit operations. Discovering selective CO₂ adsorption sites in zeolites that can function under humid conditions remains a grand challenge for enabling practical CO₂ capture from post-combustion gas mixtures with zeolitic adsorbents. We characterize humid CO₂ uptake in small pore zeolite RHO with various alkali cations and observe unusual behavior in zeolite Cs-RHO, in which H₂O adsorption actually increases CO₂ uptake at 5% relative humidity and 30 ^oC compared to dry CO₂ conditions, under equilibrium control. This is in stark contrast with other alkali cation-exchanged zeolites, which typically suffer significant deterioration in CO₂ adsorption capacity under humid versus dry conditions.¹ This result implies that the CO₂ adsorption site is not only water-resistant, but even water-promoted, going against the general phenomenon of water inhibition to CO₂ adsorption in zeolites. We investigate this behavior in Cs-RHO with a combined experimental and computational approach, which includes the first structural refinement of a humid CO₂ adsorption site in a zeolite. Our results draw a molecular picture of humid CO₂ adsorption in Cs-RHO, and demonstrate translocation of Cs⁺ cation from the center of the double eight-membered ring (D8R), as facilitated by cooperativity between H₂O and CO₂. This translocation leads to the creation of a unique CO₂ adsorption site within the D8R, which is not outcompeted by H₂O adsorption. Such a robust CO₂ adsorption site does not result from either H₂O or CO₂ adsorption alone, which elucidates the observed promotion of CO₂ adsorption by H₂O-promotion in Cs-RHO.

References

1. Xu, Le., et al., , 2021, 37, 13903−13908