Poly(ethylene glycol) Diacid-Based Deep Eutectic Solvent with Excellent Denitrogenation Performance and Distinctive Extractive Behavior

Abstract

Poly­(ethylene glycol) diacid [dcPEG, HOOCCH2(OCH2CH2) n OCH2COOH] can form a deep eutectic solvent (DES) with tetraethylammonium chloride (TEACl). The dcPEG-based DES (dcPEG-250/TEACl) as extractant (Ex) showed high performance for both basic and nonbasic N-species. In n-octane model oil, N-distribution coefficient [D N, (μgN·gEx –1)/(μgN·goil –1)] can be more than 100 for both indole and pyridine. In real oils, the DES was still effective (D N > 2.0; oil yield > 96.0%). The excellent performance was due to contributions of TEACl: (1) weakening the intermolecular interaction of dcPEG; (2) releasing Lewis basic −(OCH2CH2) n –; (3) intensifying polarity of the extractant. Extractive behaviors for the both N-species can be reflected by Freundlich- or Langmuir-like relationships between equilibrium N-contents in the extractant and the oil phases, respectively. It was interesting that, from dcPEG-250 to dcPEG-250/TEACl, the behaviors significantly changed. For the nonbasic, the extraction changed from chemical behavior with H-bonding to pseudo-physical one caused by solvent polarization. For the basic, it moved away from reactive behavior and changed to the chemical one involving acid–base interaction.

A simple model for precisely describing liquid–liquid equilibrium and better understanding extraction mechanism

“…These systems had the plateau of concentration and the higher value of n than unity, for which the Freundlich might fail, the assumption of the Langmuir ( n = 1) could not apply, and the Redlich‐Peterson was not good enough either. In Table S2, moreover, the model can be still comparable with the linear model in term of the precision 29,36‐38,49 . It is worth noting that all the systems tested can be well correlated by the present model, including separation of organic substances (phenols, acids, nitrogens, and sulfurs) from oil or water and recovery of inorganic metal ions (Li + , Cu 2+ , and Pr 3+ ) from the water.…”

“…According to the Nernst partition law, the system with very weak interaction usually shows a linear distribution 23,42 . In Figure 2a, however, indole and dcPEG250/TEACl possess the strong H‐bonding as characterized by our previous work 29 . So, the linear distribution sometimes does not mean a weak interaction axiomatically.…”

“…Besides, the model (Equation )) becomes linear when n = 1 and K [ C A (α)] ≪ 1. Thereby, the model was used to correlate the concentration distribution data that vary linearly, as shown in Figure S2 29,36‐38 . As seen in Figure 2, the non‐linear model is precisely applicable for the linear systems as well.…”

“…Influence of extractant on parameters. Note : I. the data were from the literatures: Reference 29 for (a), References 41 and 40 for (b), Reference 39 for (c), and Reference 17 for (d); II. T = 303.15 K for (a), 298.15 K for (b), 301.15 K for (c), and 293.15 K for (d)…”

“…This resulted in the non‐ideality. In recent years, classical isotherm models, that is, the Langmuir and the Freundlich, had been used to express the relationship of C A (β)– C A (α) in some LLE cases 16,29 . The Langmuir model was applicable to the reactive systems, and the Freundlich model seemed suitable for the systems with weak interaction.…”