Robust Nickel Aspartate Framework for Shape Recognition of Hexane Isomers

Adsorption separation of linear and branched alkane isomers provides an energy-saving alternative way to improve the octane number of gasoline compared to distillation, while zeolite 5A is nearly the only commercialized adsorbent for this process. Although metal-organic frameworks are widely considered to be more promising than traditional zeolites owing to their designability and tunability, the industrial application is hindered by their low stability, high cost, or other issues. Herein, using readily available biobased L-aspartic acid as a ligand, a sustainable and low-cost microporous material, termed as Ni-Asp, is constructed and proved to be effective for recognition and adsorption separation of n-hexane from its branched isomers. The appropriate rectangular pore with uniformly distributed O-sites from monodentate carboxyl groups along the channel endows Ni-Asp with strong affinity toward n-hexane, while excluding branched isomers, which is discussed in detail by DFT calculations and verified by column breakthrough experiments. In addition, intraframework hydrogen bonds between the ammino and carboxyl groups of L-Asp further stabilize the coordination network, making Ni-Asp an ultrastable MOF. Considering the environment-friendly aqueous growing conditions, Ni-Asp shows a great potential in the adsorption application for hexane isomer separation.