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

Abstract

Hydrophobic metal-organic frameworks (MOFs) are typically overlooked for ammonia storage due to weak host-guest interactions. Here, we demonstrate that four structurally analogous aluminum-based MOFs exhibit a counterintuitive behavior whereby framework geometry, rather than ligand hydrophilicity, determines high-pressure NH3 adsorption performance. The hydrophobic CAU-23 achieved an exceptional capacity matching hydrophilic analogs despite its poor low-pressure uptake. This pressure-dependent enhancement stems from the unique 4-cis-4-trans geometry of CAU-23 compared to the purely cis arrangement of MIL-160 and KMF-1 and the alternating cis-trans configuration of MOF-303. Critically, CAU-23 retained 95% capacity over three high-pressure cycles, whereas hydrophilic MOFs suffered 39-46% irreversible losses due to strong NH3-framework interactions that compromise structural integrity. Grand canonical Monte Carlo simulations reveal that high pressure enables NH3 clustering through intermolecular hydrogen bonding, bypassing the need for strong host-guest interactions. High-pressure powder X-ray diffraction measurements confirm the exceptional mechanical resilience of CAU-23, showing complete structural recovery upon decompression despite exhibiting the highest pressure sensitivity among the studied MOFs. An extended analog, HE-CAU-23, validates this design principle with further enhanced capacity. These findings reveal a paradigm shift toward hydrophobic MOFs with optimized geometry for high-performance and regenerable gas storage applications.