The Type III Deep Eutectic Solvents: Physicochemical Properties and Applications for the Extraction of Value-Added Compounds from Spent Coffee Grounds

This work describes the preparation of two-component DES based on proline and choline chloride. DESs for their eco-friendly nature are recognized, and their involvement in various chemical processes has prompted detailed investigations into properties pertinent to chemical engineering. The formation of new hydrogen bonds between the two components by FTIR spectra and ¹H and ¹³C NMR spectroscopy was demonstrated. The properties such as density (1.20–1.37 g/cm³), electrical conductivity (0.129–00.003 S/m), DPPH^· radical scavenging activity (IC₅₀ radical scavenging activity at 45–50 mg/L), and thermal stability (T_oncet is equal to 85, 120, and 150 °C, respectively, for DES-1 (proline-lactic acid), DES-2 (proline-xylitol), and DES-3 Choline Chloride-xylitol) have been thoroughly examined in relation to their implications in chemical technology. The variation HBD in the DESs enables deliberate manipulation of the density, electrical conductivity, and pH (it changes from 3.27 to 7.18) of the DESs was observed. The studied DESs have moderate thermal stability ranging between 100 °C ≤ T_onset < 300 °C and can be used for the extraction of natural organic compounds from plant materials. The practical application of the DESs in the extraction of phenolic compounds from coffee waste was demonstrated. The proline-based DES that favors the selective extraction of phenolic compounds, namely the extraction yield of 3-feruloylquinic acid, is 4.8 µg g⁻¹. According to the results, the combination of proline and xylitol extracted phenolic compounds with maximal concentration, color intensity, and antioxidant activity more efficiently than any other combination of the three produced DESs.