Capture of carbon dioxide from air and flue gas in the alkylamine-appended metal-organic framework mmen-Mg ₂(dobpdc)

Two new metal-organic frameworks, M ₂(dobpdc) (M = Zn (1), Mg (2); dobpdc ^4- = 4,4′-dioxido-3,3′-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsaturated Mg ²⁺ cations lining the 18.4-Å-diameter channels of 2 were functionalized with N,N′-dimethylethylenediamine (mmen) to afford Mg ₂(dobpdc)(mmen) _1.6(H ₂O) _0.4 (mmen-Mg ₂(dobpdc)). This compound displays an exceptional capacity for CO ₂ adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO ₂ from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO ₂ capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg ₂(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixtures, with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO ₂ in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO ₂ in N ₂ at 40 °C. The purity of the CO ₂ removed from dry air and flue gas in these processes was estimated to be 96% and 98%, respectively. As a flue gas adsorbent, the regeneration energy was estimated through differential scanning calorimetry experiments to be 2.34 MJ/kg CO ₂ adsorbed. Overall, the performance characteristics of mmen-Mg ₂(dobpdc) indicate it to be an exceptional new adsorbent for CO ₂ capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions.