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

Shang, Dawei, Zhang, Xiangping, Zeng, Shaojuan, Jiang, Kun, Gao, Hongshuai, Dong, Haifeng, Yang, Qingyuan, Zhang, Suojiang
Green chemistry 2017 v.19 no.4 pp. 937-945
absorption, ammonia, cost effectiveness, dissociation, green chemistry, hydrogen bonding, imidazoles, industry, ionic liquids, recycling, thermodynamics
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 (pKₐ). The results showed that protic ionic liquid (PIL) [Bim][NTf₂] with a moderate pKₐ 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.