Protic ionic liquid [Bim][NTf2] with strong hydrogen bond donating ability for highly efficient ammonia absorption

Abstract

Cost-efficient and environmentally benign treatment of NH₃-containing exhaust gas has been a challenge. Ionic liquids (ILs) due to their unique structures and properties are recognized as potential solvents to absorb NH₃. In this study, three types of ILs, including [Bmim][NTf₂], [Bim][NTf₂], [HOOC(CH₂)₃mim][NTf₂], were designed and synthesized with various hydrogen bond donating abilities and characterized based on their thermodynamic dissociation constants (pK_a). The results showed that protic ionic liquid (PIL) [Bim][NTf₂] with a moderate pK_a value had the highest NH₃ absorption capacity (up to 2.69 mol NH₃ per mol IL, 313 K, 100 kPa). Experimental characterization and theoretical calculations verified that such extremely high NH₃ absorption capacity for [Bim][NTf₂] resulted from the interactions between H-3 on the imidazole ring and the NH₃ molecule and NH₃ molecules sintering themselves. Furthermore, stable absorption performance was recorded for [Bim][NTf₂] over four cycles, implying potential applications in the industry for NH₃ recycling.