CO2 uptake-Relative humidity-Pore size relationship: A new tool for the design of amine-containing adsorbents

Optimization of amine-containing CO2 adsorbents, including CO2 uptake, adsorption and desorption kinetics, energy requirements, and material stability, is a recurrent theme since the inception of this field. The objective of this work is (1) to establish a relationship between CO2 uptake in amine-containing adsorbents as a function of relative humidity (RH) of feed gas and material pore size, and (2) to use this relationship to design adsorbents that maximize CO2 uptake depending on RH. To this end, periodic mesoporous silicas with similar morphology and different pore sizes, ranging from 3 to 9.2 nm, are synthesized and grafted with comparable amounts of triaminosilane. All materials exhibit S-shaped water adsorption isotherms, with a steep rise as the water vapor reaches the capillary condensation pressure. All humid CO2 adsorption isotherms versus RH, show prominent maxima at vapor pressures corresponding to the water capillary condensation. The enhanced CO2 uptake at specific RHs is attributed to the occurrence of liquid-like water at capillary condensation pressure, which facilitates the formation of ammonium bicarbonate. Hence, maximum CO2 uptake may be achieved using an adsorbent whose average pore size corresponds to the water capillary condensation at the feed gas RH.