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