Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal–Organic Framework mmen-Mg2(dobpdc)

Abstract

Two new metal–organic frameworks, M₂(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 Mg²⁺ cations lining the 18.4-Å-diameter channels of 2 were functionalized with N,N′-dimethylethylenediamine (mmen) to afford Mg₂(dobpdc)(mmen)_1.6(H₂O)_0.4 (mmen-Mg₂(dobpdc)). This compound displays an exceptional capacity for CO₂ 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 CO₂ from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO₂ capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg₂(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 CO₂ in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO₂ in N₂ at 40 °C. The purity of the CO₂ 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 CO₂ adsorbed. Overall, the performance characteristics of mmen-Mg₂(dobpdc) indicate it to be an exceptional new adsorbent for CO₂ capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions.