Comparative Study of UMCM-9 Polymorphs: Structural, Dynamic, and Hydrogen Storage Properties via Atomistic...

Area of Science:

• Materials Science
• Computational Chemistry
• Chemical Engineering

Background:

• Metal-organic frameworks (MOFs) are porous materials with tunable properties for gas storage.
• UMCM-9 is a MOF with known polymorphs, but their structural and dynamic differences are not fully understood.
• Understanding polymorph behavior is crucial for optimizing MOF performance in applications like hydrogen storage.

Purpose of the Study:

• To investigate the structural and dynamic properties of UMCM-9-α and UMCM-9-β polymorphs.
• To propose a novel, more stable polymorph (UMCM-9-β) with reduced strain and increased flexibility.
• To correlate structural and dynamic properties with gas storage performance, specifically for molecular hydrogen.

Main Methods:

• Molecular dynamics (MD) simulations were employed to study the polymorphs.
• Density functional tight binding (DFTB) calculations were used for electronic structure analysis.
• The MACE-MP neural network potential (NNP) was utilized for efficient and accurate simulations.

Main Results:

• A new UMCM-9-β polymorph was proposed, exhibiting lower linker strain and higher flexibility than UMCM-9-α.
• UMCM-9-β demonstrated enhanced molecular hydrogen diffusion due to weaker host-guest interactions.
• UMCM-9-α showed stronger host-guest interactions, leading to improved hydrogen adsorption capacity.

Conclusions:

• Synthesis conditions can likely control the formation of UMCM-9 polymorphs, with UMCM-9-β as the thermodynamic product and UMCM-9-α as the kinetic product.
• The study highlights the potential for tailoring MOF structures and properties for specific gas storage applications.
• Optimizing MOFs like UMCM-9 can lead to enhanced performance in areas such as hydrogen storage and separation.