Magnetic deep eutectic solvents as efficient media for extraction of furfural and 5-hydroxymethylfurfural from aqueous samples

The extraction of furfural (FF) and 5-hydroxymethylfurfural (HMF) from hydrolysates is currently one of the main challenges in bio-refinery. In this work, the separation of FF and HMF from the aqueous phase was carried out using a new type of green solvents - Magnetic Deep Eutectic Solvents (MDES). A conductor-like screening model for realistic solvents (COSMO-RS) was used for the preselection of 400 MDES. MDES which exhibit the highest FF and HMF solubility were prepared. The detailed physicochemical (i.e. density, viscosity, melting point) and structural characterization of new solvents using spectroscopic analysis and density functional theory (DFT) were performed. The most important liquid-liquid extraction parameters, i.e. the type of MDES, pH, temperature, extraction time, and the ratio of MDES to aqueous phase were carefully optimized using Central Composite Design model (CCD). In addition, multistage extraction, MDES regeneration, and reusability were also examined. The obtained results indicate that MDES consisting of menthol, thymol, and FeCl3 in a 1:1:0.5 M ratio could easily extract both FF and HMF from model and real hydrolysates samples with 98.5 and 78.8 % efficiency, respectively. The MDES regeneration studies demonstrated that the extraction efficiency did not change after 15 regeneration cycles. The mechanism of FF and HMF extraction indicates that van der Waals interactions were the main driving force for the extraction process. A great advantage of the proposed method was the possibility to eliminate the tedious centrifugation step for phase separation by using solvents with magnetic properties.