Thermostable Deep Eutectic Solvents for Thermochemical Reactions: Novel Stability Indicators

This study re-examines the widely touted thermostability of deep eutectic solvents (DESs) using novel stability indicators. Thermostability is critical for the application of thermochemical reactions. Type II and Type III DESs receive the most attention due to their versatility, with Type II DES demonstrating superior thermostability, often remaining stable at temperatures up to 200 °C. Of the Type II DES, ChCl:CoCl2·6H2O was the most thermostable at 250 °C, followed by ChCl:MgCl2·6H2O and ChCl:ZnCl2·2H2O that showed moderate stability at 200 °C. Type III DESs typically exhibited thermolability at 150 °C, although ChCl:Lac, ChCl:Gly, and ChCl:Ur were capable of retaining their liquid state even after heating. It was through simple visual observation that out of the 10 DESs studied, we were able to exclude DESs that were no longer liquid after heating. Visual observation is an important complement to TGA and DSC, as most of the DESs we examined were previously reported to be stable within the 150 to 200 °C range. The other part of our study is to quantitatively measure changes to the molar ratio post-heat using GC-FID and to identify possible decomposition products using GC-MS. Determining the molar ratio is crucial, as it facilitates the molar adjustment of DES for reuse, an area that presently receives less attention.