Carbon Dioxide Removal from Humid Atmosphere by a Porous Hierarchical Silicoaluminophosphate/Carbon Composite Adsorbent

A chabazite-type silicoaluminophosphate (SAPO-34) was grown within the meso- and macropores of activated carbon (AC) via a confined space synthesis and functionalized via the addition of strontium­(II) (i.e., Sr2+-CSAPO-34) for the selective adsorption of CO2 in the presence of humidity. The in situ growth of the SAPO phase was corroborated through SEM/EDAX, XRD, and pore size distribution profiles. About 80% of the meso- and macropores of AC were occupied by the SAPO. Sr2+-CSAPO-34 was further characterized via XRD, TGA, ICP-OES, and water contact angle measurements. A physical mixture of Sr2+-SAPO-34 and AC was also prepared to contrast against the hierarchical variant. The selectivity and capacity for trace CO2 removal were evaluated through single-component equilibrium and multicomponent fixed-bed adsorption. Bed tests (v = 200 mL min–1 and C i = 500, 1000, or 2500 ppm) showed that the CO2 capacity remains in the presence of 90% relative humidity, with no signs of roll-up. Specifically, the uptake capacity of the Sr2+-CSAPO-34 bed for a CO2 feed content of 1000 ppm was 0.11 mmol per cm3 of bed and with a breakthrough point greater than 2000 bed volumes; this is superior compared to other adsorbents for CO2 capture under humid conditions. The Sr2+-CSAPO-34 composite bed was also subjected to various cycles upon vacuum-assisted thermal regeneration, and no decrease in adsorption capacity was observed. The adsorbent hierarchical design approach showed that a synergistic combination of hydrophobicity and enhanced adsorbate–adsorbent interactions at the physisorption level is a promising strategy for removing trace CO2 under humid conditions.