Degradation of p-Toluene Sulfonic Acid–Choline Chloride Deep Eutectic Solvent

Abstract

The deep eutectic solvent (DES) formed between choline chloride (ChCl) and para-toluenesulfonic acid monohydrate (pTSA) is considered to be a promising solvent for green solvometallurgical processes. However, concerns regarding the green credentials of this DES and potential degradation pathways releasing toxic hydrochloric acid (HCl) remain. Here we report on the degradation of the ChCl-pTSA DES via the liberation of HCl gas. Dynamic thermogravimetric analysis – quadrupole mass spectrometry (TGA-QMS)(RT – 200 °C) confirmed both the loss of moisture, through dehydration, and the release of HCl, which was confirmed due to the presence of two peaks at 36 (HCl³⁵) and 38 (HCl³⁷). Isothermal TGA-QMS confirmed the release of HCl occurred at as low as 40 °C, the typical synthesis temperature of this DES, implying that HCl will be released no matter the operational temperature. Degradation led to the ChCl:pTSA solidifying, rendering it unusable as a solvent. Both TGA-QMS and subsequent ¹H NMR indicated no further degradation products were produced with the solid degradation product concluded to be choline tosylate. This degradation renders the classification of this mixture as a ‘green and sustainable solvent’ questionable, and further questions the use of acidic hydrogen bond donors as DES precursors.