Confining Water Nanotubes in a CuO-Based Metal-Organic Framework for Propylene/Propane Separation with Record-High Selectivity.

Energy-efficient separation of propylene (CH)/propane (CH) is in high demand for the chemical industry. However, this process is challenging due to the imperceptible difference in molecular sizes of these gases. Here, we report a continuous water nanotube dedicatedly confined in a CuO-based metal-organic framework (MOF) that can exclusively adsorb CH over CH with a record-high selectivity of 1570 (at 1 bar and 298 K) among all the porous materials. Such a high selectivity originates from a new mechanism of initial expansion and subsequent contraction of confined water nanotubes (∼4.5 Å) caused by CH adsorption rather than CH. Such unique response was further confirmed by breakthrough measurements, in which one adsorption/desorption cycle yields each component of the binary mixture high purity (CH: 98.8%; CH: >99.5%) and good CH productivity (1.6 mL mL). Additionally, benefiting from the high robustness of the framework, the water nanotubes can be facilely recovered by soaking the MOF in water, ensuring long-term use. The molecular insight here demonstrates that the confining strategy opens a new route for expanding the function of MOFs, particularly for the sole recognition from challenging mixtures.