Molecular Mechanism of Swing Effect in Zeolitic Imidazolate Framework ZIF‐8: Continuous Deformation upon Adsorption

Abstract

Zeolitic imidazolate framework ZIF‐8 displays flexibility of its structure by rotation of its imidazolate linker. This “swing effect” has been widely described in the literature, both experimentally and theoretically, as a bistable system where the linker oscillates between two structures: “open window” and “closed window”. By using quantum chemistry calculations and first‐principles molecular dynamics simulations, it is shown that the deformation upon adsorption is in fact continuous upon pore loading, with thermodynamics of packing effects being the reason behind stepped adsorption isotherms experimentally observed. Finally, we study a variant of ZIF‐8 with a different linker, highlighting the influence of the linker and the balance of microscopic interactions on the framework's flexibility.

Supramolecular Organization in Confined Nanospaces

“…c) Ball‐and‐stick representation of a 2×2 supercell of ZIF‐8. The SOD topology is highlighted by brown lines connecting Zn 2+ ions (green balls) (adapted with permission from Wiley‐VCH) . d) Ball‐and‐stick representation of the six‐membered and four‐membered rings in ZIF‐8.…”

“…Figure 22 shows a popular member -ZIF-8, an archetypal example of MOF's resistance to pressure. [423] ZIF-8 is also a choice system for modeling studies, aimed at the molecular-level description of the geometry [424] and flexibility [425] of the pore windows, useful in sorption applications, or the modification of its band structure for electronic or photocatalytic uses. [426,427] Zeolite-like frameworks can be built with other linkers besides imidazolates, and are usually termed ZMOF (Zeolitic MOF).…”

“…The SOD topology is highlighted by brown lines connecting Zn 2 + ions (green balls) (adapted with permission from Wiley-VCH). [425] d) Ball-and-stick representation of the six-membered and four-membered rings in ZIF-8. Colors: Zn = green, N = blue, C = black (H not shown).…”

“…However, molecules are flexible, and can often manage to pass through the pore orifices even if they have nominally a larger size. Framework flexibility, which is a distinctive feature of MOFs such, e. g., ZIF-8, [423][424][425]610,611] may also play a key role in facilitating the penetration of oversized guests via a window-swing effect. [425] Contrary to common expectations, also zeolites manifest a surprising structural flexibility, which allows for the vapor-phase incorporation of relatively bulky molecules (e. g., perylene-diimide) in nanosized channels like those of zeolite L (Figure 40).…”

“…Framework flexibility, which is a distinctive feature of MOFs such, e. g., ZIF-8, [423][424][425]610,611] may also play a key role in facilitating the penetration of oversized guests via a window-swing effect. [425] Contrary to common expectations, also zeolites manifest a surprising structural flexibility, which allows for the vapor-phase incorporation of relatively bulky molecules (e. g., perylene-diimide) in nanosized channels like those of zeolite L (Figure 40). [607] Despite all progress in the fabrication and characterization of channel-based materials, the incorporation mechanisms and the host-guest interactions determining the composites' properties are still little understood, often because of the complexity of both encapsulated species and confining matrix.…”