Ammonia storage performance of thiocyanate-based pseudo ionic liquids: experimental study and computational chemistry analysis

Abstract

Ammonia (NH₃) is one of the most important industrial chemicals and shows potential application as a hydrogen energy vector. The separation and storage of NH₃ plays a key role in its safe transportation and effective utilization. In the present study, five thiocyanate-based pseudo ionic liquids (PILs) with ultra-high NH₃ absorption capacity were proposed as innovative NH₃ absorbents via phase change. The effects of the cation species, temperature, and pressure on their NH₃ absorption performance were experimentally evaluated. PILs involved progressive liquefaction from a solid to a liquid state upon integration with NH₃. NH₄SCN demonstrated a robust and outstanding reversible absorption capacity of 0.402 g NH₃ per g PIL at 303.15 K and 0.1 MPa, markedly higher than that of the majority of ionic liquids (ILs) and deep eutectic solvents (DESs) reported to date. Furthermore, the NH₃ absorption mechanism and liquefaction process were comprehensively elucidated using NMR and FTIR spectroscopy, quantum chemical calculations, and molecular dynamics simulations. The results showed that absorption process involved six consecutive steps of NH₃ adsorption, NH₄⁺ formation via proton transfer, NH₄⁺ diffusion, solid surface area expansion, ion migration, and final liquefaction. The binding enthalpy of NH₃ was correlated with the molar capacities for PILs and exhibited moderate −58.13 kJ mol⁻¹ for NH₄SCN.