Our results demonstrated that the contribution of vdW interactions is negligible and the contribution of electrostatic interactions plays a dominant role in the water adsorption in ZIFs.

No abstract

The objective of this work is to study the adsorptive separation of the biomass-derived furfural (Fur) and 5-hydroxymethylfurfural (5-HMF). Because Fur and 5-HMF have similar ellipsoidal shapes, the diffusion of the larger 5-HMF may be blocked by the elliptical window of adsorbents. Thus, two zeolitic imidazolate framework (ZIF) materials with the identical chemical composition, MAF-6 and MAF-5, were used as spherical and ellipsoidal pore adsorbents, respectively. In the dynamic column adsorption of a binary-component system (Fur/5-HMF, 5/5 wt %) in an aqueous solution, the adsorption amounts of Fur on MAF-6 display a 13% decrease compared with the single-component system, whereas the adsorption amounts of Fur on MAF-5 are identical to those obtained in the single-component system and the amounts of adsorbed 5-HMF are almost negligible. Thus, the Fur/5-HMF selectivity is only 6.4 for MAF-6, whereas the selectivity reached 115.8 for MAF-5. The results show that the narrow elliptical window/ellipsoidal tile of MAF-5 enhanced separation selectivity compared with the larger circular window/spherical cage of MAF-6. In addition, the general rate model and density functional theory calculations further verify that the elliptical window of MAF-5 exhibits a strong mass-transfer barrier that must be overcome for the diffusion of the larger 5-HMF molecules.

The flexibility and guest-responsive behavior of some metal-organic frameworks (MOFs) indicate their potential in the fields of sensors and molecular recognition. As a subfamily of MOFs, the flexible zeolitic imidazolate frameworks (ZIFs) typically feature a small displacive transition due to the rigid zeolite topology. Herein, an atypical reversible displacive transition (6.4 Å) is observed for the sodalite (SOD) cage in flexible ZIF-65(Zn), which represents an unusually large breathing effect compared to other ZIFs. ZIF-65(Zn) exhibits a stepwise II → III → I expansion between an unusual ellipsoidal SOD cage (8.6 Å × 15.9 Å for II) and a spherical SOD cage (15.0 Å for I). The breathing behavior of ZIF-65(Zn) varies depending on the nature of the guest molecules (polarity and shape). Computational simulations are employed to rationalize the differences in the breathing behavior depending on the structure of the ZIF-65(Zn) cage and the nature of the guest-associated host–guest and guest–guest interactions.