Selective adsorption of ethylene over ethane and propylene over propane in the metal-organic frameworks M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn)

A significant reduction in the energy costs associated with the cryogenic separation of ethylene-ethane and propylene-propane mixtures could potentially be realized through the use of selective solid adsorbents that operate at higher temperatures. The metal-organic frameworks M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) are of particular interest for this application, owing to their high density of coordinatively unsaturated M2+ cation sites that can selectively interact with unsaturated hydrocarbons. Here, we present gas adsorption data for ethylene, ethane, propylene, and propane at 45, 60, and 80 degrees C for the entire series. The means of sample preparation and activation is found to be important for achieving high separation selectivities and capacities. While all of the compounds investigated show good performance characteristics, Fe-2(dobdc) and Mn-2(dobdc) exhibit the highest selectivities for the separation of ethylene-ethane and propylene-propane mixtures, respectively. Crystal structures determined from neutron powder diffraction data elucidate the binding of ethane, ethylene, and propylene in Mn2(dobdc) and Co-2(dobdc).