Confinement effects facilitate low-concentration carbon dioxide capture with zeolites

Engineered systems designed to remove CO₂ from the atmosphere need better adsorbents. Here, we report on zeolite-based adsorbents for the capture of low-concentration CO₂. Synthetic zeolites with the mordenite (MOR)-type framework topology physisorb CO₂ from low concentrations with fast kinetics, low heat of adsorption, and high capacity. The MOR-type zeolites can have a CO₂ capacity of up to 1.15 and 1.05 mmol/g for adsorption from 400 ppm CO₂ at 30 °C, measured by volumetric and gravimetric methods, respectively. A structure-performance study demonstrates that Na⁺ cations in the O33 site located in the side-pocket of the MOR-type framework, that is accessed through a ring of eight tetrahedral atoms (either Si⁴⁺ or Al³⁺: eight-membered ring [8MR]), is the primary site for the CO₂ uptake at low concentrations. The presence of N₂ and O₂ shows negligible impact on CO₂ adsorption in MOR-type zeolites, and the capacity increases to ∼2.0 mmol/g at subambient temperatures. By using a series of zeolites with variable topologies, we found the size of the confining pore space to be important for the adsorption of trace CO₂. The results obtained here show that the MOR-type zeolites have a number of desirable features for the capture of CO₂ at low concentrations.