Valorization of Coffee Waste for the Preparation of Caffeine-Containing ZnCl2:Ethylene Glycol Eutectic Systems: Rheological and Thermal Properties

Abstract

Valorizing coffee agro-industrial waste provides a sustainable route for producing value-added chemical precursors. Here, caffeine hydrochloride (CafCl) was prepared from commercial caffeine and from a caffeine-rich fraction recovered from coffee pulp. Protonation with in situ-generated HCl yielded CafCl and promoted the removal of part of the nonvolatile impurities present in the recovered fraction, as supported by FTIR, ¹H NMR, and TGA analyses. Subsequently, caffeine-containing ZnCl₂:ethylene glycol eutectic systems were prepared by incorporating different molar amounts of CafCl into the ZnCl₂:ETG liquid matrix. Rheological measurements showed that increasing CafCl content increased viscosity, reaching values up to approximately 50,000 cP at 20 °C, while slightly decreasing density. Arrhenius and Vogel–Fulcher–Tammann modeling indicated higher apparent activation energies for viscous flow after CafCl incorporation. Thermal analysis showed that, excluding initial moisture-related mass loss, the prepared systems did not exhibit major degradation events below approximately 60 °C and displayed a glass transition near –45 °C. Overall, caffeine recovered from coffee pulp produced eutectic systems with thermal and rheological properties comparable to those prepared from commercial caffeine, supporting the use of coffee-processing residues as low-cost feedstocks for sustainable liquid materials.