The effective adsorption separation of n-hexane/3-methylpentane by vapor-phase linker exchange of Zeolitic-imidazolate framework ZIF-8

Abstract

Zeolitic-Imidazolate Framework 8 (ZIF-8), a porous crystalline absorbent, could be synthesized from the coordination reaction of zinc ions and 2-methylimidazole (2-mIM). ZIF-8 has a large specific surface area and a high adsorption capacity for n-hexane (nHEX). However, ZIF-8 suffers from low selectivity for both linear and mono-chain alkanes. Therefore, ZIF-8 was modified through 2-mIM ligand being partly replaced by 2-nitroimidazole (2-nIM) through Vapor-Phase post-synthesis modification. The modified materials were denoted as ZIF-8-2-nIM-X (X = 24h, 48h, 72h). ZIF-8-2-nIM-X has a higher specific surface area and better thermal stability than ZIF-8. Further thermodynamic and kinetic calculations show that the affinity of hexane isomers on ZIF-8-2-nIM-X is spontaneous physical adsorption combined with an exothermic process. In contrast, kinetic studies show that the adsorption of hexane isomers on ZIF-8-2-nIM-X is a pseudo-second-order kinetic model. The batch and dynamic adsorption and separation properties of ZIF-8-2-nIM-X for hexane isomers were studied. The results of the one-component vapor adsorption equilibrium experiment show that ZIF-8-2-nIM-72h has the best adsorption separation coefficient at 293 K. The adsorption capacity of nHEX and 3-methylpentane (3 MP) were 211.63 mg/g and 33.91 mg/g, respectively. Compared with ZIF-8 (selectivity coefficient: 1.08), the adsorption selectivity coefficient of the modified material is greatly improved. The adsorption selectivity coefficient of nHEX/3 MP is 6.24 (ZIF-8-2-nIM-72h).