Deep Eutectic Solvents Composed of Urea and New Salts of a Choline Family for Efficient Ammonia Absorption
Olga V. Kazarina, Vira Agieienko, Anton N. Petukhov, Andrey V. Vorotyntsev, Maria E. Atlaskina, Artem A. Atlaskin, Sergey S. Kryuchkov, Atryom N. Markov, Alexander V. Nyuchev, И. В. Воротынцев
Abstract
Two new deep eutectic solvents (DESs) composed of urea (U) and salts of a choline family such as dimethyl-di(2-hydroxyethyl)-ammonium chloride [Me2COH2N]Cl and methyl-tri(2-hydroxyethyl)-ammonium chloride [MeCOH3N]Cl mixed in a 1:1 molar ratio were prepared. Their densities, viscosities, refractive indices, and properties related to the ammonia absorption were thoroughly investigated. We found that the obtained DESs show an absorption capacity of 2.078 and 2.632 molNH3·kg–1DES for [Me2COH2N]Cl/U and [MeCOH3N]Cl/U, respectively, at 313.2 K and 101.3 kPa, which is approximately two times higher than for the choline chloride/urea (2:3) DES at the same conditions. Also, the obtained Henry’s law constants are of the same sequence; that is, they increase from [MeCOH3N]Cl/U to [Me2COH2N]Cl/U and finally to choline chloride/urea. The mentioned tendencies show good correlation with DES’s molar volume and its free volume, and also with the number of the CH2CH2OH substituents in a quaternary ammonium salt cation. This suggests that the gas capture occurs via the interactions (most likely H-bonding) between an NH3 molecule(s) and the hydroxyalkyl fragments. Indeed, the results of 1H NMR and FTIR spectroscopy confirm strong coupling between ammonia and hydroxyls leading to a distortion of the inherent DES structure. This finding agrees well with the fact that an NH3 dissolution process in the studied DESs is favored by an entropic contribution.