Separation Techniques for Phenolic Compounds Using Ionic Liquids and Eutectic Solvents

Phenolic compounds, ubiquitous in industrial effluents and biomass‐derived oils, pose significant environmental and health concerns owing to their toxicity and persistence. Traditional separation methods often rely on volatile organic solvents, which suffer from flammability, toxicity and poor recyclability. In contrast, ionic liquids (ILs) and deep eutectic solvents (DESs) have emerged as versatile green alternatives. The tunable nature of ILs allows for precise control over cation–anion interactions, enabling selective solvation of phenolic moieties through hydrogen bonding, π–π stacking and electrostatic forces. DESs, formed by mixing two or more components that engage in strong hydrogen bonds, often exhibit lower cost, reduced viscosity and enhanced biodegradability compared to pure ILs. Advances in emulsion liquid membranes, aqueous two‐phase systems and salting‐out strategies have further broadened the toolkit for phenol recovery. Computational screening tools such as the conductor-like screening model for real solvents (COSMO-RS) streamline the identification of optimal solvent pairs, accelerating experimental validation. Key challenges remain in solvent regeneration, scale-up and managing high viscosities, yet ongoing efforts in solvent design and process integration are paving the way for sustainable, high-efficiency phenolic separations applicable to wastewater treatment, biorefinery streams and recycling of valuable aromatic chemicals.

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Recent studies have demonstrated the efficacy of amino acid-based natural deep eutectic solvents for extracting phenolic pollutants from aqueous streams. By combining L-proline with long-chain acids, researchers achieved extraction efficiencies exceeding 60 % under mild conditions, with solvents showing thermal stability and multiple reuse cycles. Computational–experimental work on deep eutectic solvents has focused on phenol extraction from model pyrolysis oil. Using COSMO-RS to screen ammonium- and phosphonium-based DESs, investigators confirmed that ammonium-based formulations provide superior distribution ratios, achieving high phenol enrichment in liquid–liquid equilibrium experiments at ambient temperature. In parallel, aqueous two-phase systems employing dicyanamide-based ionic liquids and inorganic salts have attained more than 90 % removal of phenol, o-cresol and chlorophenols from water. Optimisation of salt concentration and IL cation structure was shown to govern phase demarcation and partitioning behaviour, facilitating rapid separation and straightforward solvent regeneration.

Separation Techniques for Phenolic Compounds Using Ionic Liquids and Eutectic Solvents publication trend

The graph below shows the total number of articles in separation techniques for phenolic compounds using ionic liquids and eutectic solvents across all publications each year (not limited to Nature Index journals).

Technical terms

Ionic liquid (IL): A salt composed of organic cations and inorganic or organic anions that is liquid at temperatures below 100 °C, notable for negligible vapour pressure and tunable solvation properties.

Deep eutectic solvent (DES): A homogeneous mixture of hydrogen-bond donors and acceptors that exhibits a melting point significantly lower than that of its individual components, used as a green alternative to conventional solvents.

Natural deep eutectic solvent (NADES): A subclass of DESs prepared from naturally occurring metabolites (such as amino acids, sugars or organic acids) offering enhanced biodegradability and low toxicity.

Liquid–liquid extraction: A separation technique in which target compounds distribute between two immiscible liquid phases according to their relative solubilities and affinities, enabling selective recovery.

Salting-out effect: A phenomenon in aqueous systems where the addition of salts decreases the solubility of certain solutes (or ionic liquids), promoting phase separation and facilitating extraction of target molecules.

References

1. Amino Acid-Based Natural Deep Eutectic Solvents for Extraction of Phenolic Compounds from Aqueous Environments. Processes (2021).
2. Extraction of Phenolic Compound from Model Pyrolysis Oil Using Deep Eutectic Solvents: Computational Screening and Experimental Validation. Separations (2022).
3. Cleaning Phenolic Compounds Present in Water Using Salting-Out Effect with DCA-Based Ionic Liquids. Applied Sciences (2023).