Thermal stability of choline chloride deep eutectic solvents by TGA/FTIR-ATR analysis

doi:10.1016/j.molliq.2018.03.076

Journal of Molecular Liquids

Volume 260, 15 June 2018, Pages 37-43

https://doi.org/10.1016/j.molliq.2018.03.076 Get rights and content

Highlights

•
Eight choline chloride-based DESs thermal stability has been studied.
•
Isothermal and dynamic FTIR-ATR spectra have shown the behavior of their constituent groups.
•
Dynamic TGA has provided the onset temperature for the DESs.
•
Isothermal TGA has been employed to determinate the thermal limit of the DESs application.
•
The less stable DESs are ChCl:Mal (1:1) and ChCl:EG (1:2) and the most stable is ChCl:Gluc (2:1).

Abstract

Deep eutectic solvents (DESs) based on the cation choline have been proposed to date for a variety of applications due to their remarkable physicochemical properties. The thermal stability is one of the first properties of DESs that needs to be known since it limits the maximum operating temperature for which these solvents are useful in many applications. In this work, the thermal stability of eight different choline chloride-based DESs formed using levulinic acid, malonic acid, glycerol, ethylene glycol, phenylacetic acid, phenylpropionic acid, urea, and glucose as hydrogen bond donors (HBDs) has been studied using isothermal and dynamic thermogravimetric analysis/Fourier transform infrared-attenuated total reflectance spectroscopy (TGA/FTIR-ATR) techniques. Isothermal and dynamic FTIR-ATR were carried out to confirm the formation and to show the structural changes with temperature of the DESs, respectively. The onset decomposition temperatures of the DESs were obtained from dynamic TGA. However, the maximum operating temperatures determined by isothermal TGA in long-term scenarios have demonstrated to be significantly lower than the onset decomposition temperatures for every DES studied. The thermal stability and the boiling point of HBDs have a crucial impact on the maximum operating temperature of DESs.

Graphical abstract

Introduction

The research into a novel generation of environmentally sustainable solvents derived from renewable resources began in 2003 when Abbott et al. reported a mixture of choline chloride and urea in a 1:2 mol ratio with a low melting point eutectic, which was liquid at ambient temperature due to their ability to form hydrogen bonds [1]. Deep eutectic solvents (DESs) are eutectic mixtures of Lewis or Brønsted acids and bases that are commonly composed of a quaternary ammonium salt with a hydrogen bond donor (HBD). The charge delocalization occurs through the hydrogen bonds and is responsible for the lower melting point of the eutectic mixture [2]. DESs can be binary or ternary mixtures of Lewis or Brønsted acids and bases. Ternary deep eutectic solvents have been recently investigated to increase the number and applications of DESs. Kadhom et al. have reported the thermal properties for this kind of eutectic mixtures based on choline chloride [3] and different authors have studied their potential use in CO₂ capture [4,5]. As ionic liquids, the physicochemical properties of DESs depend on the composition of the eutectic mixture [6]. Besides their non-flammability nature, DESs have demonstrated extra advantages such as low cost, easiness of handling and synthetizing, no purification is required [7], and they are formed from non-toxic, biodegradable, and biocompatible substances [8].

In this work, we have used choline chloride as the quaternary ammonium salt because it is a non-toxic and biodegradable substance that is used nowadays as additive in animal feed and human nutrition. In addition, choline chloride is inexpensive and ton-scale available [8]. As HBD, we have used several organic compounds namely levulinic acid, malonic acid, glycerol, ethylene glycol, phenylacetic acid, phenylpropionic acid, urea, and glucose since DESs derived from these compounds are liquids at room temperature and can be used as solvents in different separation processes [9,10]. In order to confirm the potential use of DESs as alternative solvents, one of the most relevant properties to determine is their thermal stability. The aim of this work has been to study the thermal stability of the above-mentioned eight choline-based DESs. First, FTIR-ATR measurements for the DESs and their pure constituents at a constant temperature of 298 K have been made to guarantee the eutectic mixture was formed. Next, dynamic thermogravimetric analyses (TGA) have been made at a temperature range from (323.2 to 673.2) K with heating rates of 5 and 10 K·min⁻¹ to determinate the onset decomposition temperatures. Then, dynamic FTIR-ATR measurements at a temperature range from (313.2 to 473.2) K with a heating rate of 10 K·min⁻¹ have been made to analyze the behavior of the different functional groups of the DESs with increasing temperature scenarios. Finally, isothermal TGA have been performed at temperatures from (323.2 to 403.2) K for 20 h to evaluate the thermal stability of the DESs in long-term scenarios. Isothermal TGA have been demonstrated in our previous publications to be essential to characterize the thermal stability of ionic liquids as onset temperatures overestimate their maximum operational temperatures [11].

Section snippets

Chemicals

The compounds used to prepare the DESs were supplied by Sigma-Aldrich (choline chloride, levulinic acid, malonic acid, glycerol, ethylene glycol, phenylacetic acid, 3-phenylpropionic acid, and d-(+)-glucose), and by Merck KGaA (urea). Table 1 shows the purities along with other important information for these chemicals.

Preparation of DESs

Choline chloride was dried under vacuum (10 kPa) prior to use in a Büchi Glass Oven B-585 connected to a Büchi Vacuum Pump V-700 at a temperature of (323 ± 1) K for 12 h; it was also

Isothermal FTIR-ATR study on the formation of DESs

Fig. S1a–h in the Supplementary Material represents normalized FTIR-ATR spectra at 298 K for the eight DESs (red dashed line) along with those for their two pure constituents (blue straight line for ChCl and green dotted line for HBD). Pure choline chloride shows several functional groups but few of them co-exist after the DESs formation [15]; vibrational bands at 3200 cm⁻¹ and 1200–880 cm⁻¹ refer to a hydroxyl or amino group (N single bond H stretching first ones and CN⁺ symmetric stretching second ones),

Conclusions

In this work, the stability of eight choline chloride-based deep eutectic solvents (DESs) has been studied. As hydrogen bond donors (HBDs) have been used levulinic acid, malonic acid, glycerol, ethylene glycol, phenylacetic acid, phenylpropionic acid, urea, and glucose. Isothermal and dynamic FTIR-ATR analyses have been made to confirm the formation of the DESs and to study the effect of temperature on the structural changes of the DESs, respectively. Dynamic and isothermal TGA were carried out

Acknowledgments

The authors are grateful to Ministerio de Economía y Competitividad (MINECO) of Spain and Comunidad Autónoma de Madrid for financial support of Projects CTQ2014–53655-R and S2013/MAE-2800, respectively. Noemí Delgado-Mellado and Victoria Rigual thank MINECO for awarding them an FPI grant (Reference BES–2015–072855 and BES-2014-067788, respectively), Pablo Navarro also thanks Fundação para a Ciência e a Tecnologia for awarding him a postdoctoral grant (Reference SFRH/BPD/117084/2016).

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Thermal stability of choline chloride deep eutectic solvents by TGA/FTIR-ATR analysis

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals

Preparation of DESs

Isothermal FTIR-ATR study on the formation of DESs

Conclusions

Acknowledgments

Chin. Chem. Lett.

J. Mol. Liq.

J. Mol. Liq.

Thermochim. Acta

Spectrochim. Acta A

J. Mol. Liq.

J. Chem. Thermodyn.

Thermochim. Acta

Novel solvent properties of choline chloride/urea mixtures

Chem. Commun.

Deep eutectic solvents (DESs) and their applications

Chem. Rev.

Studying two series of ternary deep eutectic solvents (choline chloride-urea-glycerol) and (choline chloride-malic acid-glycerol), synthesis and characterizations

Arab. J. Sci. Eng.

Ternary deep eutectic solvents tasked for carbon dioxide capture

ACS Sustain. Chem. Eng.

Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids

J. Am. Chem. Soc.

Deep eutectic solvents: syntheses, properties and applications

Chem. Soc. Rev.

Low melting mixtures in organic synthesis – an alternative to ionic liquids?

Green Chem.

Ionic liquids and deep eutectic mixtures: sustainable solvents for extraction processes

Chem. Sus. Chem.

Choline chloride-based deep eutectic solvents in the dearomatization of gasolines

ACS Sustain. Chem. Eng.

Thermal properties of cyano-based ionic liquids

J. Chem. Eng. Data

Functionalization of graphene using deep eutectic solvents

Nanoscale Res. Lett.