Highly efficient and reversible absorption of NH3 by dual functionalised ionic liquids with protic and Lewis acidic sites

Abstract

Ammonia (NH3) pollution poses a serious environmental threat. Ionic liquids (ILs) are regarded as potential solvents to treat NH3-containing gases because of their unique properties, including tunable structures and high affinity for NH3 molecules. In order to achieve an efficient and reversible absorption of NH3, a series of protic ionic liquids (PILs) with multiple active sites were designed by introducing acidic protons, as well as the Li+ ion into ILs, resulting in PILs with two acidic protons on cations ([2-Mim][NTf2] and [Im][NTf2]), as well as dual functionalised PILs with a Lewis acidic Li+-based anion and acidic protons on cations, namely, [2-Mim][Li(NTf2)2] and [Eim][Li(NTf2)2]. The NH3 absorption capacity of [Im][NTf2] was found to be 3.46 mol NH3/mol IL, which is the highest among the NH3 absorption capacities of all known nonmetallic ILs, and is ascribed to the hydrogen bonding interactions between the NH3 and PIL molecules. Furthermore, [2-Mim][Li(NTf2)2] exhibited the highest NH3 absorption capacity, with a value of 7.01 mol NH3/mol IL, which is about 30 times higher than that of conventional ILs. The superior NH3 absorption performance of the dual functionalised PILs is attributed to the combination of hydrogen bonding interactions between the protons and the NH3 molecules, and the formation of coordination complexes between the Li+-based anion and the NH3 molecules. Moreover, the ILs studied here exhibited excellent recyclability, indicating a great potential for their use in applications involving efficient and reversible NH3 absorption from NH3-containing industrial gases.