Cooperative Adsorption by Porous Frameworks: Diffraction Experiment and Phenomenological Theory

Area of Science:

• Materials Science
• Chemistry

Background:

• Metal-organic frameworks (MOFs) are advanced materials with applications in gas storage, sensors, and medicine.
• Understanding guest adsorption in MOFs is crucial for optimizing their performance.

Purpose of the Study:

• To investigate the adsorption-desorption behavior of guest molecules in two nanoporous MOFs with varying structural flexibility: γ-Mg(BH4)2 and MIL-91(Ti).
• To elucidate the mechanisms underlying cooperative guest adsorption and lattice deformation in these MOFs.

Main Methods:

• In situ X-ray diffraction experiments during guest adsorption-desorption cycles.
• Utilizing a mean-field Gorsky-Bragg-Williams (GBW) approach for the lattice-gas Ising model to rationalize experimental observations.

Main Results:

• Both γ-Mg(BH4)2 and MIL-91(Ti) frameworks exhibited cooperative guest adsorption.
• This cooperativity was linked to lattice deformation and long-range interactions between guest molecules, mediated by the MOF's elastic response.
• The GBW model successfully rationalized the experimental adsorption thermodynamics.

Conclusions:

• Cooperative guest adsorption in MOFs is a phenomenon driven by guest-guest interactions and the elastic properties of the framework.
• The combination of in situ synchrotron powder diffraction and the adjusted GBW model provides an effective method for characterizing MOF adsorption thermodynamics.
• This approach allows for the characterization of both total guest uptake and specific guest site interactions within MOFs.