Extraction of caffeine from coffee husk employing choline-based ionic liquids: Optimization of the process and theoretical study on solute-salts interactions

Abstract

The effectiveness of a series of five aqueous solutions of choline (Ch)-based ionic liquids (IL), where the corresponding counterions were propanoate [Prop], hexanoate [Hex], nicotinate [Nic], citrate [Cit], and tartrate [Tar], was compared to extracting caffeine (Caf) from a sample of coffee husk (CH), an abundant agro-industrial by-product. The [Ch][Hex] was the most efficient of the series, whereas no significant differences were found among the others. A response surface methodology (RSM) using a three-factor central composite design 23 was used to optimize the process, considering temperature (T), solid/liquid ratio (R) and [Ch][Hex] concentration (C) as the more influential variables. The estimated optimal conditions for the extraction were 95 °C, 0.021 g/mL, and 2.34 M, respectively, which yield 4347.50 ± 115 mgCaf/kgCH; in addition, the [Ch][Hex] solution was reusable, at least twice, without drastically reducing its efficiency. On the other hand, the non-covalent interactions between ILs and caffeine within the complexes IL-Caf were obtained at a DFT level of calculation, including temperature and the solvation effect of water, through AIM and NCI analyses. A net of hydrogen bonds can be observed among the components of the complexes, the strongest ones taking place between hydrogen atoms and hydroxyl or carboxylic groups of the anions, mainly in the citrate and tartrate anions. In contrast, weak hydrogen bonds and van der Waals interactions are predominant between hexanoate and caffeine. Thus, a hydrotropic effect seems to be the driving force in the extraction of the alkaloid with [Ch][Hex]. Thermochemistry of the complexation process was also calculated, and findings were in good agreement with experimental results for the ILs under study.