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, 2.79, 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, 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.