Construction of aliphatic channels in covalent organic framework for efficient separation of benzene and cyclohexane

Abstract

Separation of benzene from cyclohexane is a significant challenge due to their similar boiling points and molecular sizes, rendering conventional methods like distillation and adsorption separation via molecular sieving ineffective. Selective separation by adsorbents having distinct molecular interaction with benzene and cyclohexane could be a potential solution. Therefore, a novel covalent organic framework (COF-IPR) with aliphatic channels enriched by isopropyl groups was designed in this study to separate benzene from cyclohexane via C-H⋯π interactions. COF-IPR achieves an impressive IAST selectivity of 30.1 at 50 %:50 % of benzene/cyclohexane (298 K and 12.5 kPa), comparable to the best available materials like HFPB-TAB-COF (36.0) and Zn-TCNQ-bpy (29.9). COF-IPR also exhibits a breakthrough separation factor of 2.8, a 47.4 % enhancement over the leading COFs, i.e., JUC-641 (1.9) and HFPB-TAB-COF (1.9). The C–H⋯π interactions of COF-IPR with benzene, but not cyclohexane, were elucidated through a combination of Fourier transform infrared (FT-IR), Raman spectra, Hirshfeld surface analysis and DFT calculations. Furthermore, COF-IPR is a hydrophobic material with water contact angle up to 135°, ensuring excellent benzene/cyclohexane separation performance even at 99 % relative humidity. Additionally, COF-IPR exhibits robust thermal stability up to 380 °C and water/chemical stability in solutions with pH ranging from 1 to 13.