Recovery and recycling of deep eutectic solvents in biomass conversions: a review

Abstract

The assessment of the behavior of deep eutectic solvents (DESs) have been within the focus of numerous original research papers and review articles in the last two decades due to their unique properties such as low viscosity, low melting point, low volatility, high thermal stability, high conductivity, high surface tension, non-toxicity, and biocompatibility. DES have been used for the pretreatment of biomass, within conversion processes (e.g., as catalyst, co-solvent, extracting solvent) and within downstream processing. Related to an efficient use within integrated biorefineries, an important issue for the successful application of DES is their recovery and recycling. Thus, the aim of this review is to provide detailed information and assessments on current techniques (e.g., anti-solvent addition, crystallization, membrane filtration, solid–liquid extraction, liquid–liquid extraction, short path distillation, supercritical fluid extraction, separation due to density differences) to recover and recycle DES used for a specific purpose. The selection of the respective recovery option has to be based on the properties of the DES, the nature of the conversion/extraction process, the features of the target compound/product, the amount of energy required, and the equipment cost. Future aspects of DES recovery are also elaborated. However, further investigations are required to better understand the separation options while optimizing the product yields and minimizing the overall costs. Graphical abstract: [Figure not available: see fulltext.]