The tunability of metal–organic frameworks offers the possibility of designing powerful new adsorbents that selectively adsorb and release gas molecules in a cooperative manner. An initial example of such a material was realized in mmen-Mg2(dobpdc), which exhibits step-shaped CO2 adsorption isotherms arising from a cooperative insertion mechanism that leads to ammonium carbamate chains running along the pore surface. This mechanism has now been widely elaborated, leading to diamine-appended adsorbents that cycle at high capacity in the presence of water, and are capable of efficiently separating CO2 from flue gas emissions, air, natural gas, and biogas. Stabilization of the materials through functionalization with appropriately structured tetraamines further enables cooperative CO2 capture at high temperatures, as well as regeneration via steam stripping. This type of cooperative mechanism has been extended to the selective adsorption of CS2, and a related mechanism has been shown to be operational in alcoholamine-appended frameworks. In addition, efforts to expand the scope of cooperative adsorption have led to new metal-organic frameworks containing: (i) chains of high-spin iron(II) sites that can cooperatively adsorb CO via a spin transition mechanism, (ii) chains of redox-active cobalt(II) sites that exhibit negative cooperativity upon uptake of O2, and (iii) binuclear metal complexes that enable the cooperative adsorption of NH3 through a ligand insertion mechanism.

Cooperative adsorption and gas separations in metal-organic frameworks

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