Regulating Extra-Framework Cations in Faujasite Zeolites for Capture of Trace Carbon Dioxide

The development of cost-effective sorbents for direct capture of trace CO₂ (<1 %) from the atmosphere is an important and challenging task. Natural or commercial zeolites are promising sorbents, but their performance in adsorption of trace CO₂ has been poorly explored to date. A systematic study on capture of trace CO₂ by commercial faujasite zeolites reveals that the extra-framework cations play a key role on their performance. Under dry conditions, Ba-X displays high dynamic uptake of 1.79 and 0.69 mmol g^-1 at CO₂ concentrations of 10000 and 1000 ppm, respectively, and shows excellent recyclability in the temperature-swing adsorption processes. K-X exhibits perfect moisture resistance, and >95 % dry CO₂ uptake can be preserved under relative humidity of 74 %. In situ solid-state NMR spectroscopy, synchrotron X-ray diffraction and neutron diffraction reveal two binding sites for CO₂ in these zeolites, namely the basic framework oxygen atoms and the divalent alkaline earth metal ions. This study unlocks the potential of low-cost natural zeolites for applications in direct air capture.