Synergistic global and local flexibilities in Zr-based metal-organic frameworks enable sequential sieving of hexane isomers

Abstract

Separating hexane isomers based on the branching degree is crucial for their efficient utilization in the petrochemical industry, yet remains challenging due to their similar properties. Here we report a temperature-responsive Zr-based metal-organic framework, Zr-fum-FA, capable of sequentially sieving linear, mono-, and di-branched hexane isomers. Notably, the pore structure of Zr-fum-FA dynamically transforms from segmented triangular channels to an integrated rhombic configuration as the temperature increases, leading to distinct sieving effects. At low temperatures, the narrow triangular pores allow the exclusive adsorption of n-hexane while excluding branched isomers. In contrast, the expanded rhombic pores at high temperatures enable the sieving of mono- and di-branched isomers. Mechanistic studies reveal that this unique dual-sieving behavior originates from the synergistic effects of the global framework flexibility and the local dynamics of pendent hydroxyl groups. Furthermore, we demonstrate the decoupling of global and local flexibilities via two strategies: incorporating steric hindrance to dampen the global framework dynamics and enhancing the metal node rigidity to limit the local vibrations. These findings not only provide a promising adsorbent for the challenging separation of hexane isomers but also offer rational design principles for harnessing flexibility in MOFs.

Fig. 1. Schematic illustration of flexibility control strategies. (a) The tuned mass damper of skyscrapers for reducing the overall structural vibration and scaffold fastener for enhancing the local robustness. (b) The synergism of the global dynamics and the local flexibility in the original framework. (c) Damping block introduction, and (d) metal node enhancement strategies for regulating and decoupling the global and local flexibility. The aqua and orange balls represent hydroxyl and methyl groups respectively.

Fig. 2. Component and structure of the three materials. The metal clusters and organic ligands of (a) Zr-fum-FA, (b) Zr-mes-FA, and (c) Hf-fum-FA. (d–f) The double triangle 1D pores along a-axes and (g–i) the corresponding vertical view along b-axes of the three materials respectively. Each of the methyl groups on Zr-mes-FA and the –CH groups of formate have an occupancy of 0.5. Color scheme: black: C, red: O, blue: Zr, pink: Hf. H atoms are omitted for clarity.

Fig. 3. Single-component hexane isomer adsorption on the three materials. Hexane isomer isotherms on (a) Hf-fum-FA at 303 K, (b) Zr-fum-FA at 423 K, and (c) Zr-mes-FA at 423 K. (d) Separation performance comparison among Zr-fum-FA, Zr-mes-FA, Hf-fum-FA, and other materials at 303 K, 10 kPa. (e) Calculated and DSC-derived heats of adsorption of hexane isomers on the three materials, the error bars represent the standard error. (f) Adsorption kinetics of hexane isomers on Zr-fum-FA at 303 K, 15 kPa.

Fig. 4. Column breakthrough and cycling experiments. (a) Equimolar binary breakthrough curves of 3-MP/2,3-DMB at 393 K (10 cycles) on a column packed with Zr-fum-FA, and (b) the corresponding 3-MP uptake amounts during 10 cycles. Equimolar quinary breakthrough curves (c) on the same Zr-fum-FA column at 303 K, (d) on Zr-mes-FA (left) and Hf-fum-FA (right) columns at 393 K, and (e) on a sequence of two Zr-fum-FA columns at 303 and 393 K. (f) 20 Adsorption–desorption cycles of n-Hex on Zr-fum-FA among a temperature range of 363 to 403 K.

Fig. 5. Structural transformation and adsorption mechanism. (a) In situ PXRD patterns of Zr-fum-FA under a 3-MP saturated N₂ flow with a temperature range of 303 to 423 K. (b) DRIFTS spectra of Zr-fum-FA under N₂ flow with a temperature range of 303 to 423 K. (c) The representation of temperature- and guest-responsive structural transformation of Zr-fum-FA (the orange and pink balls represent hydroxyl groups and guest molecules, respectively), and (d) the corresponding interplanar spacing changes of (011) and (002) planes at varied temperatures. (e) In situ PXRD patterns of global flexibility-decoupled Zr-mes-FA under the same conditions. (f) DRIFTS spectra of local flexibility-decoupled Hf-fum-FA under the same conditions. (g) The diffusion pathways and corresponding energy barriers of n-Hex on the original Zr-fum-FA and the hypothetical one ignoring the pendent hydroxyl groups. The coordinate a denotes the cell parameter a of Zr-fum-FA (10.03 Å).