Delayed linker addition (DLA) synthesis of dual linker ZIF-8 for separation mono- and di-branched isomers of hexane

Abstract

The separation of mono- and di-branched C6 alkane isomers is one of the most challenging industrial processes for the production of high-octane gasoline components. In this work, ZIF-8 with dual ligands of imidazole (IM) and 2-methylimidazole (2-MIM), named as DLA-ZIF-8-IM, was synthesized by the method of delayed linker addition, and the microstructure of DLA-ZIF-8-IM was analyzed. Adsorption thermodynamic, kinetic and separation properties of C6 alkane isomers were investigated. The results show that compared with SALE-ZIF-8-IM prepared by the solvent-assisted ligand exchange method, the synthesis method of DLA-ZIF-8-IM is simpler and the BET specific surface area and pore volume (2136 m2·g−1 and 0.72 cm3·g−1) of DLA-ZIF-8-IM are increased by 21 % and 14 %, which is more conducive to adsorption and diffusion of 3-methylpentane (3MP) on DLA-ZIF-8-IM. The equilibrium adsorption capacity of 3MP on DLA-ZIF-8-IM reaches up to 2.89 mmol·g−1, which is 1.1 times than that on SALE-ZIF-8-IM, and the ratio of diffusion time constants between 3MP and 2,3-dimethylbutane (23DMB) on DLA-ZIF-8-IM (5.13) is 2.2 times than that on SALE-ZIF-8-IM. Moreover, at 30℃, the breakthrough adsorption capacity of 3MP on DLA-ZIF-8-IM is 1.46 mmol·g−1, which is 1.6 times more than that of SALE-ZIF-8-IM (0.90 mmol·g−1), while the breakthrough adsorption capacity of 23DMB is basically the same. The separation factor between 3MP and 23DMB on DLA-ZIF-8-IM is 2.96 via the synergy of thermodynamic and kinetic mechanism and the dynamic adsorption capacity of 3MP reaches 1.46 mmol·g−1. DLA-ZIF-8-IM achieves complete separation of C6 alkane mixtures for obtaining single high-purity product. This work provides a novel synthesis strategy for developing C6 alkane (clear) adsorption separation materials by modulating microporous structures of dual-ligand MOFs.