Gas separations in metal-organic frameworks

Owing to their high surface areas, tunable pore dimensions, and adjustable surface functionality, metal-org. frameworks (MOFs) can offer advantages for a variety of gas storage and gas sepn. applications. In an effort to help curb greenhouse gas emissions from power plants, we are developing new MOFs for use as solid adsorbents in post- and pre-combustion CO2 capture, and for the sepn. of O2 from air, as required for oxy-fuel combustion. In particular, MOFs with open metal cation sites or alkylamine-functionalized surfaces are demonstrated to provide high selectivities and working capacities for the adsorption of CO2 over N2 under dry flue gas conditions. Breakthrough measurements further show compds. of the latter type to be effective in the presence of water, while calorimetry data reveal a low regeneration energy compared to aq. amine solns. MOFs with open metal cation sites, such as Mg2(dobdc) (dobdc4- = 2, 5-dioxido-1,4-benzenedicarboxylate), are highly effective in the removal of CO2 under conditions relevant to H2 prodn., including in the presence of CH4 impurities. Redox-active Fe2+ sites in the isostructural compd. Fe2(dobdc) allow the selective adsorption of O2 over N2 via an electron transfer mechanism. The same material is demonstrated to be effective at 45 °C for the fractionation of mixts. of C1 and C2 hydrocarbons, and for the high-purity sepn. of ethylene/ethane, propylene/propane, and CO/H2 mixts. Finally, it will be shown that certain structural features possible within MOFs, but not in zeolites, can enable the fractionation of hexane isomers according to the degree of branching or octane no.