Na⁺-gated nanochannel membrane for highly selective ammonia (NH₃) separation in the Haber-Bosch process

Currently, cryogenic condensation is the predominant process for recovering ammonia (NH₃) in the Haber-Bosch (HB) process, which is highly energy intensive. To be more compatible with the reaction conditions in the HB process and thus minimize the pressure and temperature swing during reactant recycling, energy-efficient technologies for NH₃ extraction at elevated temperature and pressure are greatly needed. In this work, the Na⁺-gated nanochannel membrane, shown exclusively for water conduction in our previous work, also exhibited highly NH₃-selective performance, with NH₃/H₂ selectivity as high as 4,280 and NH₃/N₂ selectivity > 10,000 at temperature up to 250 °C and pressure up to 35 bar. Excellent stability of the Na⁺-gated nanochannel membrane was demonstrated during a 100-h run in ternary NH₃/H₂/N₂ gas mixture at 200 °C and 35 bar, consistent with structural characterization by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transfer infrared (FTIR) spectroscopy. A techno-economic analysis (TEA) was conducted for the HB process using the Na⁺-gated nanochannel membrane and the traditional HB process with condenser. Under the optimized separation conditions, > 80 % energy savings and approximately 20 % reduction of the net NH₃ production cost can be achieved, demonstrating the great potential of the Na⁺-gated nanochannel membrane for NH₃ separation in the HB process.