Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium-Olsalazine Frameworks

Diamine-appended Mg-2(dobpdc) (dobpdc(4-) = 4,4 & PRIME;-dioxidobiphenyl-3,3 & PRIME;-dicarboxylate) metal-organicframeworks have emerged as promising candidates for carbon captureowing to their exceptional CO2 selectivities, high separationcapacities, and step-shaped adsorption profiles, which arise froma unique cooperative adsorption mechanism resulting in the formationof ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents,in particular, exhibit excellent stabilities and CO2 adsorptionproperties. However, these frameworks display double-step adsorptionbehavior arising from steric repulsion between ammonium carbamates,which ultimately results in increased regeneration energies. Herein,we report frameworks of the type diamine-Mg-2(olz)(olz(4-) = (E)-5,5 & PRIME;-(diazene-1,2-diyl)bis(2-oxidobenzoate))that feature diverse diamines with bulky substituents and displaydesirable single-step CO2 adsorption across a wide rangeof pressures and temperatures. Analysis of CO2 adsorptiondata reveals that the basicity of the pore-dwelling amine inaddition to its steric bulk is an important factor influencingadsorption step pressure; furthermore, the amine steric bulk is foundto be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2-Mg-2(olz) (ee-2= N,N-diethylethylenediamine), adsorbs>90% of the CO2 from a simulated coal flue stream andexhibitsexceptional thermal and oxidative stability over the course of extensiveadsorption/desorption cycling, placing it among top-performing adsorbentsto date for CO2 capture from a coal flue gas. Spectroscopiccharacterization and van der Waals-corrected density functional theorycalculations indicate that diamine-Mg-2(olz) materialscapture CO2 via the formation of ammonium carbamate chains.These results point more broadly to the opportunity for fundamentallyadvancing materials in this class through judicious design.