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

Area of Science:

• Materials Science
• Thermodynamics
• Chemistry

Background:

• Metal-organic frameworks (MOFs) are porous materials with applications in gas sequestration.
• Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, are particularly promising for carbon dioxide (CO2) capture.
• ZIF-8, a specific ZIF, has shown potential for CO2 sequestration.

Purpose of the Study:

• To investigate the low-temperature heat capacity of ZIF-8 with varying amounts of sorbed CO2.
• To identify thermodynamic evidence of structural changes induced by CO2 sorption in ZIF-8.
• To understand the interactions between CO2 and the ZIF-8 framework at a molecular level.

Main Methods:

• Low-temperature heat capacity measurements were performed on ZIF-8 samples containing different quantities of CO2 (up to 0.99 mol CO2 per mol ZIF-8).
• Measurements were conducted across a temperature range of 1.8 K to 300 K.
• Analysis focused on identifying anomalies in heat capacity to infer structural transitions and interactions.

Main Results:

• Samples with significant CO2 loading (≥0.56 mol CO2/mol ZIF-8) exhibited a broad heat capacity anomaly between 70-220 K, indicative of sorption-induced structural transitions.
• This anomaly was partially attributed to a gate-opening transition and CO2 rearrangement/lattice expansion.
• A subtle anomaly below 70 K was observed in all CO2-loaded samples, suggesting new vibrational modes due to CO2/ZIF-8 interactions.

Conclusions:

• The study provides the first thermodynamic evidence of structural transitions occurring in ZIF-8 upon CO2 sorption.
• CO2 sorption induces significant changes in the ZIF-8 framework, including gate-opening phenomena and lattice modifications.
• The findings enhance the understanding of gas-framework interactions in MOFs, crucial for designing advanced CO2 capture materials.