Selective adsorption of ethylene over ethane and propylene over propane in the metal-organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn)

A significant redn. in the energy costs assocd. with the cryogenic sepn. of ethylene-ethane and propylene-propane mixts. could potentially be realized through the use of selective solid adsorbents that operate at higher temps. The metal-org. frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) are of particular interest for this application, owing to their high d. of coordinatively unsatd. M2+ cation sites that can selectively interact with unsatd. hydrocarbons. Here, we present gas adsorption data for ethylene, ethane, propylene, and propane at 45, 60, and 80 °C for the entire series. The means of sample prepn. and activation is found to be important for achieving high sepn. selectivities and capacities. While all of the compds. investigated show good performance characteristics, Fe2(dobdc) and Mn2(dobdc) exhibit the highest selectivities for the sepn. of ethylene-ethane and propylene-propane mixts., resp. Crystal structures detd. from neutron powder diffraction data elucidate the binding of ethane, ethylene, and propylene in Mn2(dobdc) and Co2(dobdc).