Porous Compounds Prefer Paraffins

Metal-organic framework (MOF) compounds can selectively adsorb paraffins from mixtures with olefins, according to researchers at Delft University of Technology, in the Netherlands, who discovered one MOF’s unusual preference for alkanes over alkenes (J. Am. Chem. Soc., DOI: 10.1021/ja1089765). To prepare pure feedstocks for polymerization and other industrial processes, paraffin-olefin mixtures are separated via distillation, which is energy intensive and costly. Separations based on sorbents would be a technologically simple and low-cost alternative if paraffins were retained on a sorbent column and a pure olefin, which is usually the desired product, passed through freely. As a rule, however, microporous crystalline compounds, which include MOFs, do just the opposite—they selectively adsorb olefins. Delft’s Jorge Gascon, Freek Kapteijn, and coworkers report that a zinc imidazolate MOF known as ZIF-7 selectively adsorbs ethane from mixtures with ethylene, allowing pure ethylene to pass through a packed column. The researchers attribute the unprecedented selectivity to interactions between ethane and benzene rings in the imidazole linker units, which discriminate between ethane and ethylene on the basis of subtle differences in the molecules’ shapes.