Probing adsorption interactions in metal-organic frameworks using X-ray spectroscopy.

Area of Science:

• Materials Science
• Chemistry
• Physics

Background:

• Metal-organic frameworks (MOFs) possess unique porous structures with open metal sites.
• Understanding guest molecule adsorption in MOFs is crucial for applications like gas storage and separation.

Purpose of the Study:

• To investigate the local electronic signatures of molecular adsorption at coordinatively unsaturated Mg sites in Mg-MOF-74.
• To correlate X-ray spectroscopy data with theoretical calculations to understand adsorption mechanisms.

Main Methods:

• In situ Mg K-edge X-ray spectroscopy.
• First-principles density functional theory (DFT) calculations.
• Adsorption studies with CO2 and N,N'-dimethylformamide (DMF).

Main Results:

• Distinct pre-edge absorption features at the Mg K-edge were observed, linked to open Mg coordination sites.
• These features were suppressed upon adsorption of CO2 and DMF, indicating changes in local symmetry.
• DFT calculations confirmed that spectral changes are due to modified local symmetry and depend on metal-adsorbate binding strength.

Conclusions:

• X-ray spectroscopy is a powerful tool for probing local symmetry changes upon gas adsorption in MOFs with open metal sites.
• The findings provide a foundation for using spectroscopy to study adsorption dynamics in MOFs.
• Observed phenomena were consistent in both Mg-MOF-74 and the expanded Mg2(dobpdc) framework.