Ammonia Adsorption at High Temperature and Pressure Using Sodium Containing Aluminosilicates: Selectivity over Nitrogen and Multicycle Working Capacities

Separating NH₃ from N₂ and H₂ using adsorption onto porous materials at elevated pressures and temperatures has been proposed as a promising alternative to optimize the Haber-Bosch (HB) synthesis process. However, experimental data on NH₃ adsorption at high pressure are scarce and necessary to properly evaluate NH₃ adsorption as a possible and viable separation option. Using commercial Na-LTA, Na-X, and Na-Y zeolites (Si/Al 1.00, 1.26, 2.80, respectively), we evaluated the NH₃ equilibrium adsorption up to 15 bar and temperatures of 323, 373, 423, and 473 K. N₂ adsorption isotherms were measured at 323 and 473 K. All the zeolite samples were activated at 673 K to ensure complete dehydration. At 36 bar total pressure and 473 K, ideal adsorbed solution theory (IAST) predicts selectivity for NH₃ over N₂ of 265, 234, and 211 for Na-LTA, Na-X, and Na-Y, respectively. Separation factors based on IAST and kinetic selectivity are estimated as 128, 39, and 46, respectively. The enhanced selectivity of Na-LTA is attributed to diffusion limitations for N₂ stemming from its narrower pore size. NH₃ working capacities through 5-cycle PSA tests at 473 K directly correlate with the Si/Al ratio, with Na-Y zeolite achieving the highest working capacity at 2.50 mmol cm^-3. However, VSA tests with desorption via dynamic vacuum for 10 min yielded a working capacity of up to 8.83 mmol cm^-3 for Na-X. Gibbs ensemble Monte Carlo simulations are carried out to investigate the adsorption of NH₃, N₂, H₂, and their mixture in Na-LTA. Analysis of the simulation trajectories indicates that the NH₃ molecules bind strongly to the Na⁺ cations and displace nitrogen in mixtures.