Computational Screening of Metal-Organic Frameworks for Ammonia Capture from H/N/NH Mixtures.

The separation of ammonia from H/N/NH mixtures is an important step in ammonia decomposition for hydrogen production and ammonia synthesis from H and N 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 H/N/NH 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 (NH) (the isosteric heat of NH adsorption) (>45 kJ/mol). MOFs with high NH adsorption capacity often feature moderate surface area, while the surface area of MOFs with high NH selectivity is relatively lower, which limits the NH adsorption capacity. and the Henry's constant ( ) 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 NH. Most MOFs with high NH 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.