image: (a) Comparison of binding of benzene molecules in quadrangular, triangular and circular channels. (b) Side view of the one-dimensional coordination chain. (c) Coordination mode of ligand. (d) View of the framework structure of NUK-300. Art by Zhonghang Chen and Jiangnan Li.
Credit: Art by Zhonghang Chen and Jiangnan Li.
Single-crystal X-ray diffraction analysis shows that NKU-300 has a three-dimensional open framework with triangular channels decorated with crown ether sites. Surprisingly, desolvated NKU-300 shows a notable adsorption of benzene vapour but negligible uptake of cyclohexane, thus demonstrating a great potential for their separation.
Liquid-phase purification is considered more energy-efficient than gas-phase separation and has been performed in this study. Remarkably, NKU-300 can discriminate trace benzene from cyclohexane, exhibiting an unprecedented selectivity of 8615(10) for their separation with ultrafast sorption kinetics and excellent stability. "This material shows an excellent separation performance with fully retained selectivity over 20 consecutive adsorption-desorption cycles." says Prof. Shi.
To figure out the mechanism behind the superior selectivity, the team employed in situ single-crystal X-ray diffraction analysis, which shows that there are four different adsorption sites (a, b, c, and d) in the channel of NKU-300. There are multiple host-guest and guest-guest interactions, facilitated by the abundant aliphatic bonds on the crown ether, that collectively confine the benzene molecules to the triangular channels. Interestingly, the adsorbed benzene molecules are stacked in a manner similar to solid benzene.
To determine the strength of each adsorption site (d < c < a < b), in situ synchrotron single-crystal X-ray diffraction data were collected during the desorption of benzene from NKU-300 at UK Diamond Light Source.
"Synchrotron X-ray diffraction analysis has revealed the underlying molecular mechanism that drives the superb separation performance" Prof. Yang says.
The practical potential of NKU-300 is confirmed by its remarkable stability over repeated cycles and fast sorption kinetics. Structural analysis coupled with modelling provides key insights for optimal host-guest and guest-guest interactions, which underpin the superior performance in this challenging separation.
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See the article:
Rapid extraction of trace benzene by a crown-ether-based metal-organic framework
https://doi.org/10.1093/nsr/nwae342
Journal
National Science Review