Halogen-C2 H2 Binding in Ultramicroporous Metal-Organic Frameworks (MOFs) for Benchmark C2 H2 /CO2 Separation Selectivity.

Acetylene (C2 H2 ) capture is a step in a number of industrial processes, but it comes with a high-energy footprint. Although physisorbents have the potential to reduce this energy footprint, they are handicapped by generally poor selectivity versus other relevant gases, such as CO2 and C2 H4 . In the case of CO2 , the respective physicochemical properties are so similar that traditional physisorbents, such as zeolites, silica, and activated carbons cannot differentiate well between CO2 and C2 H2 . Herein, we report that a family of three isostructural, ultramicroporous (<7 Å) diamondoid metal-organic frameworks, [Cu(TMBP)X] (TMBP=3,3',5,5'-tetramethyl-4,4'-bipyrazole), TCuX (X=Cl, Br, I), offer new benchmark C2 H2 /CO2 separation selectivity at ambient temperature and pressure. We attribute this performance to a new type of strong binding site for C2 H2 . Specifically, halogen⋅⋅⋅HC interactions coupled with other noncovalent in a tight binding site is C2 H2 specific versus CO2 . The binding site is distinct from those found in previous benchmark sorbents, which are based on open metal sites or electrostatic interactions enabled by inorganic fluoro or oxo anions.