Simultaneously enhanced benzene adsorption and benzene/cyclohexane separation via decorating the pore environment of metal–organic frameworks

Abstract

Benzene/cyclohexane separation remains a considerable challenge due to the subtle difference in boiling points between benzene and cyclohexane. A promising avenue is to capture benzene molecules and efficiently separate benzene and cyclohexane through the pore functionalization strategy of MOFs. Herein, we introduce F and azo functional groups into a hydrothermally stable Zr-MOF to produce compound 1, (Zr₆O₄(OH)₄(F₂abdc)₆, where F₂abdc = azobenzene-3,3′-difluoro-4,4′-dicarboxylic acid). This material exhibits a high benzene adsorption capacity of 12.15 mmol g⁻¹ under 12 kPa and an efficient separation selectivity of 76.73 for a benzene/cyclohexane mixture (v/v = 1/1) at 298 K. Compared with isostructural compounds 2 (lacking the –F group) and 3 (lacking –F and –NN– groups), the benzene adsorption capacity and the benzene/cyclohexane separation selectivity for 1 are significantly superior to those of the unmodified counterparts 2 and 3. Dynamic vapor sorption measurements reveal that compound 1 has the fastest benzene adsorption kinetics in the series. In situ FTIR spectroscopy reveals the binding dynamics of benzene in the pores of 1. Grand Canonical Monte Carlo (GCMC) simulations are employed to elucidate the binding mechanisms of benzene and cyclohexane. The results unveil that benzene is preferentially adsorbed on the pore channels of compounds 1–3 compared to cyclohexane, with strong π–π interactions between benzene and the functionalized framework. Especially for 1, fluorine incorporation enhances the electropositive character of the ligand, promoting interactions with electron-rich molecules. This work highlights that pore functionalization of MOFs is a powerful strategy for enhancing benzene adsorption and benzene/cyclohexane separation.