Understanding, controlling, and utilizing the flexibility of adsorbents are of great importance and difficulty. Analogous with conventional solid materials, downsizing to the nanoscale is emerging as a possible strategy for controlling the flexibility of porous coordination polymers (or metal-organic frameworks). We report a unique flexibility controllable by crystal size at the micrometer to submillimeter scale. Template removal transforms [Cu 2 (pypz) 2 ]·0.5 p -xylene (MAF-36, Hpypz = 4-(1 H -pyrazol-4-yl)pyridine) with one-dimensional channels to α -[Cu 2 (pypz) 2 ] with discrete small cavities, and further heating gives a nonporous isomer β -[Cu 2 (pypz) 2 ]. Both isomers can adsorb p -xylene to give [Cu 2 (pypz) 2 ]·0.5 p -xylene, meaning the coexistence of guest-driven flexibility and shape-memory behavior. The phase transition temperature from α -[Cu 2 (pypz) 2 ] to β -[Cu 2 (pypz) 2 ] decreased from ~270°C to ~150°C by increasing the crystal size from the micrometer to the submillimeter scale, ca. 2-3 orders larger than those of other size-dependent behaviors. Single-crystal X-ray diffraction showed coordination bond reconstitution and chirality inversion mechanisms for the phase transition, which provides a sufficiently high energy barrier to stabilize the metastable phase without the need of downsizing to the nanoscale. By virtue of the crystalline molecular imprinting and gate-opening effects, α -[Cu 2 (pypz) 2 ] and β -[Cu 2 (pypz) 2 ] show unprecedentedly high p -xylene selectivities of 16 and 51, respectively, as well as ultrafast adsorption kinetics (<2 minutes), for xylene isomers.
CITATION STYLE
Yang, X., Zhou, H.-L., He, C.-T., Mo, Z.-W., Ye, J.-W., Chen, X.-M., & Zhang, J.-P. (2019). Flexibility of Metal-Organic Framework Tunable by Crystal Size at the Micrometer to Submillimeter Scale for Efficient Xylene Isomer Separation. Research, 2019. https://doi.org/10.34133/2019/9463719
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