Adsorption Separation Properties of Mono- and Di-Branched Isomers of Hexane on Dual-Linker Zif-8 Synthesized by Delayed Linker Addition (Dla)

Abstract

The separation of mono- and di-branched C6 alkanes 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 and kinetic properties of n-hexane (nHEX), 3-methylpentane (3MP), and 2,3-dimethylbutane (23DMB) on DLA-ZIF-8-IM were investigated. The dynamic separation performance of 3MP/ 23DMB binary mixture and nHEX/ 3MP/ 23DMB ternary mixture on DLA-ZIF-8-IM were carried out, and the dual-ligand ZIF-8 prepared by the solvent-assisted ligand exchange method (named as SALE-ZIF-8-IM) was used for comparison. The results show that compared with SALE-ZIF-8-IM, 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 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 23DMB on DLA-ZIF-8-IM (5.13) is 2.2 times than that on SALE-ZIF-8-IM. Moreover, at 30°C, 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 by 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 mixture (nHEX/3MP/23DMB) for obtaining single high-purity nHEX, 3MP and 23DMB product, respectively. This work provides a novel synthesis strategy for the development of C6 alkane (clear) adsorption separation materials by the modulation of microporous structures of dual-ligand MOFs.

Keywords: Clear adsorption separation, Preparation of dual-linker ZIF-8 by delayed linker addition, Modulation of micropore structure, Mono- and di-branched hexanes, Adsorption thermodynamic and kinetic