Regulated Threshold Pressure of Reversibly Sigmoidal NH3 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][Tf2N] were designed as NH3 absorption agents. Interestingly, NH3 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 H-1 NMR spectroscopies, thermometric analyses, and theory calculations, it indicates NH3 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 NH3 desorption. Therefore, S mode NH3 absorption with [3NH(2)-PyH] [Tf2N] presents low TP of 6 mbar (6000 ppm) at 30 degrees C, and its NH3 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 NH3 could be desorbed completely in vacuum at 80 degrees C and 1 mbar. [3NH(2)-PyH] [Tf2N] maintained its high capacity within 20 time recycles. The sigmoidal NH3 absorption would be considered as one of the potential approaches for its simultaneous efficient absorption and energy-saving separation at low concentrations.