Selective adsorption and separation of C6 hydrocarbons: the role of structural flexibility and functionalization in zeolitic imidazolate frameworks†

Abstract

This study investigates the selective adsorption and separation of C₆ hydrocarbons (benzene, cyclohexane, and n-hexane) by zeolitic imidazolate frameworks (ZIFs), focusing on their structural flexibility and functionalization. ZIF-8_CH₃ and ZIF-8_Br were synthesized and compared, indicating distinct adsorption behaviors. ZIF-8_CH₃ showed higher uptake for benzene (9.5 molecules per unit cell) and n-hexane (8.0 mlc uc⁻¹) compared to cyclohexane (1.0 mlc uc⁻¹). In contrast, ZIF-8_Br exhibited enhanced adsorption for cyclohexane (5.0 mlc uc⁻¹) and reduced n-hexane uptake (0.5 mlc uc⁻¹). Computational simulations supported these findings, identifying the involved host–guest interactions. Ideal adsorbed solution theory analysis confirmed that ZIF-8_CH₃ demonstrated virtually zero uptake of cyclohexane from binary mixtures containing either n-hexane or benzene, while ZIF-8_Br exhibited negligible adsorption of n-hexane from its mixtures with cyclohexane or benzene. It was concluded that bromine functionalization in ZIF-8_Br increased structural rigidity and selectivity for aromatic compounds. These results highlight the crucial role of functionalization and gate-opening phenomena in ZIFs to achieve efficient volatile organic compound capture and separation where traditional adsorbents may not be effective.