Hydrophobic Metal-Organic Frameworks Enable Superior High-Pressure Ammonia Storage through Geometric Design

Area of Science:

• Materials Science
• Chemical Engineering
• Nanotechnology

Background:

• Hydrophobic metal-organic frameworks (MOFs) are generally not considered for ammonia storage due to weak host-guest interactions.
• Existing research overlooks the significant impact of framework geometry on gas adsorption performance.

Purpose of the Study:

• To investigate the ammonia adsorption performance of hydrophobic aluminum-based MOFs.
• To determine the role of framework geometry versus ligand hydrophilicity in high-pressure ammonia storage.
• To explore the potential of hydrophobic MOFs for regenerable gas storage applications.

Main Methods:

• Synthesis and characterization of four structurally analogous aluminum-based MOFs.
• High-pressure ammonia adsorption measurements.
• Grand canonical Monte Carlo (GCMC) simulations.
• High-pressure powder X-ray diffraction (PXRD) for structural integrity analysis.

Main Results:

• The hydrophobic CAU-23 MOF demonstrated exceptional high-pressure ammonia adsorption capacity, comparable to hydrophilic analogs.
• Framework geometry (4-cis-4-trans in CAU-23) was identified as the key factor for high-pressure performance, overriding ligand hydrophilicity.
• CAU-23 retained 95% of its capacity over three cycles, unlike hydrophilic MOFs which suffered significant irreversible losses.
• GCMC simulations indicated NH3 clustering via intermolecular hydrogen bonding at high pressures.
• PXRD confirmed CAU-23's structural resilience and recovery upon decompression.

Conclusions:

• Framework geometry is a critical design parameter for high-pressure ammonia storage in MOFs.
• Hydrophobic MOFs with optimized geometry offer a promising avenue for high-performance and regenerable gas storage.
• This study presents a paradigm shift, highlighting the potential of hydrophobic MOFs for challenging gas storage applications.