Thermodynamic Evidence of Structural Transformations in CO₂-Loaded Metal–Organic Framework Zn(MeIm)₂ from Heat Capacity Measurements

Abstract

Metal−organic frameworks are a class of porous compounds with potential applications in molecular sieving, gas sequestration, and catalysis. One family of MOFs, zeolitic imidizolate frameworks (ZIFs), is of particular interest for carbon dioxide sequestration. We have previously reported the heat capacity of the sodalite topology of the zinc 2-methylimidazolate framework (ZIF-8), and in this Article we present the first low-temperature heat capacity measurements of ZIF-8 with various amounts of sorbed CO 2 . Molar heat capacities from 1.8 to 300 K are presented for samples containing up to 0.99 mol of CO 2 per mol of ZIF-8. Samples with at least 0.56 mol of CO 2 per mol of ZIF-8 display a large, broad anomaly from 70 to 220 K with a shoulder on the low-temperature side, suggesting sorption-induced structural transitions. We attribute the broad anomaly partially to a gate-opening transition, with the remainder resulting from CO 2 rearrangement and/or lattice expansion. The measurements also reveal a subtle anomaly from 0 to 70 K in all samples that does not exist in the sorbate-free material, which likely reflects new vibrational modes resulting from sorbate/ ZIF-8 interactions. These results provide the first thermodynamic evidence of structural transitions induced by CO 2 sorption in the ZIF-8 framework.

A Review on Removal of Carbon Dioxide (CO2) using Zeolitic Imidazolate Frameworks: Adsorption and Conversion via Catalysis

“…The low-temperature heat capacity measurements of ZIF-8 confirm the structural change of the framework upon adsorption [223] . Data analysis revealed CO2 rearrangement and lattice expansion [223] . The adsorption mechanism depends on several parameters, including temperature [225] .…”

“…Several mechanisms have been reported for gas adsorption, including the gate-opening mechanism [223,224] , the flip motion of the linkers [88] , and the linker swing motion [191] . The low-temperature heat capacity measurements of ZIF-8 confirm the structural change of the framework upon adsorption [223] . Data analysis revealed CO2 rearrangement and lattice expansion [223] .…”

Heat capacity and thermodynamic functions of partially dehydrated sodium and zinc zeolite A (LTA)

“…The sensitivity of heat capacity data to changes in a material's density of states offers a unique perspective into sorption-induced flexibility transitions (Dickson et al, 2019;Rosen et al, 2020). Thus, heat capacity is a promising way to investigate the flexibility transition uncovered by Guo and coworkers (Guo and Navrotsky, 2018; in ion-exchanged zeolite A.…”

“…Because the features of the Na-A apparent water heat capacity qualitatively match those in the analcime and mordenite data, it seems probable that they have a common source. A more promising possible explanation is framework flexibility, either due to structural differences between hydrated and dehydrated samples or possibly shifts in lattice parameters that become favorable at low temperatures and appear as a peaked contribution, like those observed in a metal-organic framework by Rosen et al, 2020.…”

Low-Temperature Heat Capacity and Thermodynamic Functions of Tetrameric Cobalt(II) Acetylacetonate

“…Low-temperature heat capacity data are very informative. They enable calculating integral thermodynamic functions, such as entropy, enthalpy, and reduced Gibbs energy, as well as gaining information on phase transitions, , phonon spectrum parameters, , etc. In addition, accumulation of reliable thermodynamic data provides opportunities in searching for correlations and appropriate theoretical generalizations. , …”