Regulated Threshold Pressure of Reversibly Sigmoidal NH 3 Absorption Isotherm with Ionic Liquids

In this work, flexible hydrogen-bonded frameworks composed of amino pyridinium-based protic ionic liquids (ILs) marked as [X-PyH][Tf 2 N] were designed as NH 3 absorption agents. Interestingly, NH 3 absorption isotherm of these ILs present sigmoidal absorption isotherm (S mode), and the threshold pressure (TP) decreases from 75 to 6 mbar with the tunable of hydrogen bonded frameworks by varying the substituted group X. Combined with XRD characterizations, Fourier transform infrared and 1 H NMR spectroscopies, thermometric analyses, and theory calculations, it indicates NH 3 fixed by these ILs via acid–base interaction as well as hydrogen bonding based on the activation of sites through the fracture of the hydrogen-bonded frameworks, which would reform along with heat release when NH 3 desorption. Therefore, S mode NH 3 absorption with [3NH 2 -PyH][Tf 2 N] presents low TP of 6 mbar (6000 ppm) at 30 °C, and its NH 3 capacity increases sharply and followed by a continuous increase to 3.85 mol/mol IL (175 mg/g IL) at 1.0 bar, which is higher than that of the reported ILs. The fixed NH 3 could be desorbed completely in vacuum at 80 °C and 1 mbar. [3NH 2 -PyH][Tf 2 N] maintained its high capacity within 20 time recycles. The sigmoidal NH 3 absorption would be considered as one of the potential approaches for its simultaneous efficient absorption and energy-saving separation at low concentrations.