Efficient Splitting of Alkane Isomers by a Bismuth-Based Metal-Organic Framework with Auxetic Reentrant Pore Structures.

The isolation of di-branched alkanes from their isomers is vital in gasoline upgrading to achieve high octane numbers but is significantly challenging and energy-intensive. Here, we report the highly efficient separation of hexane isomers by combing molecular recognition and size-sieving in a bismuth-based MOF, UU-200. The unique auxetic structure with reentrant honeycomb-like pore cavities connected by narrow pore windows endows UU-200 with a complete rejection of di-branched alkanes and high capacities for linear and mono-branched isomers. The molecular sieving effect, unprecedented separation selectivities, and excellent efficiencies are proved via adsorption isotherms and breakthrough experiments with high research octane numbers obtained (>96), indicating a benchmark for alkane separation under ambient conditions. The molecular recognition mechanism was unveiled by theoretical simulation and in situ Fourier-transform infrared spectroscopy.