Pore structure modification of ZIF-8 by ligand exchange for separation mono- and di-branched isomers of hexane via thermodynamic and kinetic mechanism

Isomeric C6 alkanes are high-quality raw materials for producing high octane gasoline. Among them, the octane number of di-branched C6 alkanes is significantly higher than that of mono-branched C6 alkanes. Therefore, efficient and economical separation of mono- and di-branched C6 alkanes has received widespread attention. Herein, ZIF-8 with mixed ligands (named as ZIF-8-IM) was successfully synthesized by exchanging some 2-methylimidazole (MIM) ligands in the framework of ZIF-8 with imidazole (IM), and the microstructure of ZIF-8-IM were analyzed from N2 adsorption. Adsorption thermodynamic and kinetic properties of n-hexane (nHEX), 3-methylpentane (3MP) and 2,3-dimethylbutane (23DMB) on ZIF-8-IM were determined by IGA-100 intelligent weight analyzer. Column breakthrough experiments were carried out to confirm the separation performance of of 3MP/23DMB binary mixture and nHEX/3MP/23DMB ternary mixture on ZIF-8-IM and ZIF-8. Results show that the microporous structure of ZIF-8 is changed by ligand exchange, which means the most probable pore diameters at 1.08 nm and 1.34 nm of ZIF-8 are converted to 1.06 nm and 1.19 nm of ZIF-8-IM. The adsorption equilibrium and kinetic characteristics of mono-branched 3MP are significantly enhanced due to the microstructural changes in ZIF-8-IM. The equilibrium adsorption capacity of 3MP on ZIF-8-IM is 1.6 times that of 23DMB, and the diffusional time constant is 4–13 times that of 23DMB. By relying on the synergistic effect of thermodynamic and kinetic characteristics, efficient adsorption and separation of 3MP/23DMB mixtures can be achieved with a separation factor of 2.24 on ZIF-8-IM. In addition, ZIF-8-IM can also achieve clear separation of nHEX/3MP/23DMB ternary mixtures, obtaining individual nHEX, 3MP, and 23DMB products. The results of this study show excellent application prospects in the production of the raw materials for high octane gasoline and molecular oil refining processes.