(465h) Understanding the Ammonia/Nitrogen/Hydrogen Separation in Zeolite Nanosheet Membranes through Molecular Simulations

Ammonia synthesis is known as the chemical reaction that feeds the world.¹ However, NH₃ production accounts for about 2% of global energy consumption and more than 1% of global CO₂ emissions.² In the Haber-Bosch process, nitrogen reacts with hydrogen at very high temperatures (> 700 K to break the N₂ triple bond) and pressures (~ 200 bar to shift the equilibrium to the product side) to form ammonia, followed by an energy-intensive separation (involving condensation of ammonia) and recycle of the unreacted N₂ and H₂. The extreme process conditions make it difficult to operate on a small scale. A solution here is to use a membrane to separate ammonia from nitrogen and hydrogen at reaction conditions.

Our previous work has shown substantial ammonia selectivity at ambient³ and process–relevant conditions⁴ for MFI nanosheets (NS) with surface silanols. Although, experimentally synthesized membranes are composed of layered nanosheets with non-uniform inter-sheet regions. Here, we performed NpT-Gibbs ensemble Monte Carlo and NVT molecular dynamics simulations on membrane models formed by stacking MFI NS to understand the influence of inter-sheet regions, sheet-sheet alignment, and defects on gas adsorption and diffusion. Our simulations show a higher NH₃ selectivity for SNS as compared to both bulk MFI and NS. This work provides design criteria to guide the synthesis of membranes with high ammonia selectivity at industrial operating condition.

References

1. Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W.How a century of ammonia synthesis changed the world (2008). Geosci., 1, 636– 639
2. IEA (2021), Ammonia Technology Roadmap, IEA, Paris
3. Duan, X.; Kim, D.; Narasimharao, K.; Al-Thabaiti, S.; Tsapatsis, M. (2021). High-Performance Ammonia-Selective MFI Nanosheet Membranes. Commun., 57, 580–582.
4. Patel, R.; Castro, J.; Tsapatsis, M.; Siepmann, J. I. (2022). Molecular Simulations Probing the Adsorption and Diffusion of Ammonia, Nitrogen, Hydrogen, and Their Mixtures in Bulk MFI Zeolite and MFI Nanosheets at High Temperature and Pressure. Chem. Eng. Data., 67, 1779–1791.