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

Abstract

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 Qst0(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. Qst0 and the Henry’s constant (KH) 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.