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

Abstract

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.