Designing Low-Viscosity Deep Eutectic Solvents with Multiple Weak-Acidic Groups for Ammonia Separation

Developing NH3 separation technology with high efficiency is very meaningful for the advancement of the NH3 synthesis process. In this work, a new class of deep eutectic solvents (DESs) were designed by pairing N,N,N′,N′-tetramethyl-1,3-propanediamine dihydrochloride ([TMPDA]Cl2) with phenol (PhOH) at the molar ratios of 1:3∼7. [TMPDA]Cl2 + PhOH DESs have multiple weak-acidic groups and viscosities of as low as 48.1 cP at 298.2 K. They also exhibit excellent performance for the separation of NH3, showing not only efficient and selective but also reversible absorption of NH3. Especially, the solubilities of NH3 in [TMPDA]Cl2 + PhOH DESs at low pressures can reach 4.49 mol/kg of NH3 at 298.2 K and 13.3 kPa, being superior to those of most of the DESs and ILs reported in the literature. The mechanism of NH3 absorption was further elucidated by quantum chemistry calculations and 1H NMR spectra. It is validated that the efficient absorption of NH3 in DESs origins from the strong interaction of multiple weak-acidic groups of DESs with NH3.