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Advancing the characterization of open framework compounds by Positron Annihilation Spectroscopy (PAS)

Subject Area Physical Chemistry of Solids and Surfaces, Material Characterisation
Synthesis and Properties of Functional Materials
Term from 2021 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 464857745
 
Final Report Year 2025

Final Report Abstract

In a collaboration between TU Dresden and the Institute for Radiation Physics at HZDR, advancements in porosimetry for MOFs were explored. The study combined PALS with physical and chemical methods to improve the understanding of physisorption mechanisms of gases and vapors in environmentally relevant and chemically stable MOFs. In situ PALS was used to study phase transitions in MOFs, induced by temperature or guest molecules. This approach helped to decouple pore transitions and pore filling in switchable MOFs and identify residual closed porosity and defects. The interaction between positronium (Ps) and metal ions challenges PALS porosimetry, particularly in open-metal-site MOFs like CPO-27 (M). Diamagnetic Mg²⁺ promotes oxidation while maintaining longer lifetimes, whereas paramagnetic Co²⁺ and Ni²⁺ inhibit and quench o-Ps through spin conversion, and oxidation, leading to shorter lifetimes. In situ gas adsorption experiments by PALS showed that only O₂ reduced quenching, bringing lifetimes closer to expected values. These findings emphasize the strong influence of metal ions on the Ps lifetime and encourage the community to introduce corrections to existing models. PALS provided new insight into water adsorption in DUT-67 (Zr,Hf). In situ measurements revealed stepwise uptake, with DUT-67(Hf) exhibiting stronger water clustering and limiting pore filling compared to DUT-67(Zr). Similarly, PALS analysis of CALF-20 (Zn) showed selective CO₂ adsorption over H₂O at low humidity, making it promising for carbon capture. Combined PALS, PXRD, and gas adsorption data revealed a sequential CO₂ uptake mechanism, with molecules filling cage centers, forming chains, and adhering to pore walls. However, humidity disrupted CO₂ adsorption by forming hydrogen-bond networks. PALS identified residual porosity and defects in switchable MOFs (DUT-8(Ni), MIL-53(Al), and ZIF-7). In DUT-8(Ni), mesopores were detected, most likely from missing building blocks that hinder gas diffusion. These defects may aid structural relaxation during phase transitions. MIL-53(Al) exhibited less defects, while ZIF-7 (Zn) showed an interesting phenomenon, indicating the dependency of defect amount closely related to guest-induced phase transition and temperature. MIL-53(Al) showed an expected CO₂-induced np-lp transition at 195K, with reversible hysteresis. Ps intensity reached a minimum at the point of transition, suggesting defined changes in surface chemistry. Variable temperature experiments confirmed the switchability of the framework. In case of variable temperature CO2 physisorption in ZIF-7(Zn), the S-shaped o-Ps intensity with CO₂ pressure suggests framework flexibility. Unlike MIL-53(Al), ZIF-7 lacked a pronounced intensity drop, likely due to higher defect concentration. These findings indicate that surface alterations during transitions are prominent in defect-free MOFs but negligible in defect-containing ones. Further studies are needed to generalize these observations.

Link to the final report

https://zenodo.org/records/15582313

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