Efficient Separation of n-Butene and iso-Butene by Flexible Ultramicroporous Metal-Organic Frameworks with Pocket-like Cavities

Abstract

Efficient separation of n-butene (n-C₄ H₈ ) and iso-butene (iso-C₄ H₈ ) is of significance for the upgrading of C4 olefins to high-value end products but remains one of the major challenges in hydrocarbon purifications owing to their similar structures. Herein, we report a flexible metal-organic framework, MnINA (INA=isonicotinate), featuring one-dimensional pore channels with periodically large pocket-like cavities connected by narrow bottlenecks, for the first time for efficient n-/iso-C₄ H₈ separation. MnINA with smaller pore size (4.62 Å) compared with CuINA (4.84 Å), exhibits steep adsorption isotherms and high capacity of 1.79 mmol g^-1 for n-C₄ H₈ (4.46 Å) through strong host-guest interactions via C-H⋅⋅⋅π bonding. The narrow bottlenecks exert barriers for the large molecules of iso-C₄ H₈ (4.84 Å) within the gate-opening pressure range of 0-0.1 bar. This gives rise to MnINA with excellent separation selectivity of 327.7 for n-/iso-C₄ H₈ mixture. The adsorption mechanism for n-C₄ H₈ and the gate-opening effect were investigated by dispersion-corrected density functional (DFT-D) theory, verifying the strong interactions between n-C₄ H₈ and the frameworks as well as the gate-opening effect derived from the rotation of organic linkers. The breakthrough tests confirmed MnINA and CuINA can be promising candidates for n-/iso-C₄ H₈ separation.

Keywords: adsorption and separation; flexible MOFs; iso-butene; metal-organic frameworks; n-butene; ultramicroporous materials.