Now, Joachim Sauer, Omar Yaghi and co-workers report the direct capture of CO2 from open air using a covalent organic framework (COF). The authors strategy involves constructing a porous, crystalline COF containing pores decorated with polyamines. The COF material was synthesized through Knoevenagel condensation between 3,3′-bis[(6-azidohexyl)oxy]-4,4′-biphenyldicarbaldehyde and 1,3,5-tris(4-cyanomethylphenyl)benzene, followed by a Staudinger reaction to reduce the azide groups, and subsequent aziridine treatment to produce polyamines within the pores. This multiple-step process was conducted at various temperatures and pressures. Several designed strategies were employed to enhance structural stability, adsorption capacity and achieve a relatively low regeneration temperature: (1) the COF was constructed with olefin linkages to form a hydrophobic matrix, minimizing water absorption and resulting in a lower CO2 regeneration temperature; (2) the synthetic route was designed to allow covalent attachment of polyamines to the framework, preventing their loss during the recycling process; (3) the intentionally designed large pores ensured a high loading of polyamines and facilitated CO2 diffusion, leading to high CO2 adsorption capacity and rapid cycling; and (4) the olefin linkage provided the COF with a more thermally and chemically stable backbone.
As proof of concept, the as-prepared COF product demonstrated CO2 uptake of 0.91 mmol g–1 via gas sorption isotherm measurements at 0.4 mbar and 25 oC, with negligible uptake of N2, O2 and Ar. This condition is close to the CO2 partial pressure in air at this temperature. Multi-component breakthrough experiments under simulated air demonstrated rapid mass transfer of CO2 in the COF material. It is shown that a small amount of water within the pores promotes CO2 adsorption due to the formation of carbamates and bicarbonates. More impressively, this COF adsorbent was tested under practical conditions in outdoor air at Berkeley, California, USA. As outdoor air passed through the sorbent, 100 adsorption–desorption cycles were conducted continuously for more than 20 days. During this period, the outdoor CO2 concentration varied from 410 ppm to 517 ppm, and the relative humidity from 28% to 51%. This long cycle process delivered relatively stable CO2 productivity, averaging 1.28 mmol g–1 per cycle.
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