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

• Synthesized periodic mesoporous silicas with 3 to 9 nm pore sizes, with similar morphology and amine loading. • Investigated the effect of pore size and relative humidity (RH) on CO2 uptake. • Impressive increase in CO2 uptake was achieved at vapor pressures corresponding to the water capillary condensation. • Maximum CO 2 uptake vs RH was pore-size dependent, allowing precise tailoring of adsorbents for feed gases with different RHs. Optimization of amine-containing CO 2 adsorbents, including CO 2 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 CO 2 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 CO 2 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 CO 2 adsorption isotherms versus RH, show prominent maxima at vapor pressures corresponding to the water capillary condensation. The enhanced CO 2 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 CO 2 uptake may be achieved using an adsorbent whose average pore size corresponds to the water capillary condensation at the feed gas RH.