Ultrahigh selectivity of benzene/cyclohexane separation by ZIF-8 framework: Insights from spin-probe EPR spectroscopy

Liquid-phase separation of cyclohexane and benzene is both demanded and challenging, and the use of selective sorbents, such as metal-organic frameworks (MOFs), for extraction of one component from the mixture is the most promising solution. We report new mechanistic insights into diffusion of benzene and cyclohexane in well-known MOF ZIF-8 using recently developed methodology of encapsulated spin probes detected by Electron Paramagnetic Resonance (EPR). We demonstrate unambiguously that only one component of the hydrocarbons mixture, benzene, can penetrate into ZIF-8 particles, and these EPR-based conclusions are confirmed by high values of sorption selectivity measured by gas chromatography. However, systematic analysis of the mixtures containing small fractions of benzene (e.g. similar to 1%) shows noticeable decrease in separation efficiency and only partial removal of benzene from diluted benzene/cyclohexane mixtures. This occurs due to the reverse diffusion of benzene from MOF particles into solution in equilibrium. At the same time, our diffusion study clearly points out that the membrane separation of benzene/cyclohexane using ZIF-8 should be highly perspective and pursued in the future.

Automated summary

The liquid-phase separation of cyclohexane and benzene is both in demand and challenging. Metal-organic frameworks (MOFs) are promising selective sorbents for extracting components from the mixture. The study provides mechanistic insights into the diffusion of benzene and cyclohexane in MOF ZIF-8 and confirms that only benzene can penetrate the particles. However, there is a decrease in separation efficiency and only partial removal of benzene in mixtures with small fractions of benzene due to reverse diffusion.