Deterpenation of Citrus Essential Oils Using Glycerol-Based Deep Eutectic Solvents

Citrus essential oils are complex hydrocarbon mixtures mainly composed of terpenes and terpenoids, which are widely used as raw materials in food, pharmaceutical and fine chemical industries. However, essential oil deterpenation (i.e., separation of terpenes and terpenoids) is required to preserve the quality of the final product for practical applications. Currently, there is a need to find efficient and environmentally friendly solvents to replace the harmful volatile organic compounds that are conventionally used as extraction solvents. Therefore, alternative solvents with more benign and environmentally friendly characteristics to develop sustainable citrus essential oil deterpenation processes are crucial. In this work, bio-renewable deep eutectic solvents (DES) composed of glycerol (Gly) and choline chloride (ChCl) are evaluated as sustainable solvents for citrus essential oil deterpenation, using model mixtures and real citrus crude orange essential oils (COEO). The liquid-liquid equilibrium data of the deterpenation process were obtained at 298.15 K and 1 atm and compared against the predicted results from COnductor-like Screening Model for Real Solvents (COSMO-RS), and the solvent performance was evaluated in terms of the distribution coefficient and selectivity values. The effect of solvent composition (i.e. hydrogen bond acceptor/donor ratio) and the addition of water (pure DES vs. aq. DES solutions) were also explored. Further thermodynamic understanding of the results was completed by analysis of sigma (σ) profiles and excess enthalpy of mixing obtained with COSMO-RS. Overall results indicate the feasibility of using DES as extraction solvents for citrus essential oil deterpenation, with pure ChCl:Gly 1:2 providing the highest extraction yield, while addition of water decreased the distribution coefficient but increased the selectivity of the process.