Intrinsic Flexibility of the Zeolitic Imidazolate Framework ZIF-7 Unveiled by CO₂ Adsorption and Hg Intrusion

Graphical Abstract

Opening the gate: By a convenient thermodynamic treatment of the CO₂ adsorption/desorption and Hg intrusion curves, a major entropic contribution to the elastic energy stored by ZIF-7 during the np→lp transition is unveiled.

Abstract

ZIF-7, built as an assembly of Zn^II centers and benzimidazolate ligands, shows prominent S-shaped isotherms upon CO₂ adsorption that can be attributed to sorbate-induced gate-opening phenomena involving a narrow-to-large pore phase transition. This peculiar sorption pattern can be captured via the formulation of thermodynamic isotherms, providing a direct enthalpic and entropic view of the gate-opening process. Relying on such an approach, an energy barrier with preferential enthalpic nature for CO₂ adsorption/desorption in the gate-opening region could be unveiled. Moreover, the elastic energy involved during the gate-opening process was revisited to 1.4–2.8 kJ mol⁻¹ of solid in the temperature range 273–323 K, matching the value measured by isostatic compression of a ZIF-7_lp sample filled with DMF and showing a dominant entropic contribution.