Computational Screening of Metal-Organic Frameworks for Ammonia Capture from H₂/N₂/NH₃ Mixtures

The separation of ammonia from H₂/N₂/NH₃ mixtures is an important step in ammonia decomposition for hydrogen production and ammonia synthesis from H₂ and N₂ based nonaqueous technologies. Metal-organic frameworks (MOFs) are considered as potential materials for capturing ammonia. In the present work, high-throughput screening of 2932 Computation-Ready Experimental MOFs (CoRE MOFs) was carried out for ammonia capture from H₂/N₂/NH₃ mixtures by Grand Canonical Monte Carlo (GCMC) simulations. It was found that the high-performing MOFs are characterized by tube-like channels, moderate LCD (largest cavity diameter) (4-7.5 Å), and high Q ⁰(NH₃) (the isosteric heat of NH₃ adsorption) (>45 kJ/mol). MOFs with high NH₃ adsorption capacity often feature moderate surface area, while the surface area of MOFs with high NH₃ selectivity is relatively lower, which limits the NH₃ adsorption capacity. Q ⁰ and the Henry's constant (K ) are two energy descriptors describing the interactions between adsorbents and adsorbates. The former has a stronger correlation with the adsorption selectivity, while the latter has a stronger correlation with the adsorption capacity. By analyzing the molecular density distribution of adsorbates in high-performing MOFs, it was found that unsaturated coordinated metal sites provide the main functional binding sites for NH₃. Most MOFs with high NH₃ selectivity have multiple different metal nodes or other atoms except C, O, and H, such as N and P. Multiple metal nodes and nonmetallic atoms provide more functional binding sites. Finally, the adsorption behavior with various concentrations of gas mixtures was examined to verify the universality of the screening calculations, and the effect of framework flexibility on adsorption performance was explored.