A Review of Ammonia (NH3) Adsorption in Porous Solids.

Ammonia (NH3) plays a crucial role in the agricultural sector as a fertilizer and is a dense hydrogen carrier that offers a potential renewable and carbon-free energy source. These significant advantages necessitate the development of efficient NH3 synthesis and separation systems. Conventionally, the Haber-Bosch (HB) process involves synthesis at high temperature (>673 K, 100-200 bar) and separation of NH3 from the equilibrium-limited reaction mixture through condensation at low temperatures. Among various alternative NH3 separation techniques, adsorption approaches in porous solids have been investigated as a potentially more energy-efficient method. However, identifying the optimal porous materials for reversible NH3 separation with high NH3 capacity remains challenging. This review discusses the potential of porous solid adsorbents, such as zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), carbons, and their composite materials with NH3 absorbents like ionic liquids and metal halides, for NH3 separation and other potential uses like storage and sensing. The review also discusses active sites for adsorption, and adsorbent structural and mechanical properties. It also emphasizes NH3 selectivity over N2 and H2, highlighting the solid sorbent's reversibility, stability, and regeneration conditions. Future directions for NH3 adsorption research are highlighted.