New insights into the production of platform chemicals from biomass using deep eutectic solvents

Platform chemicals are versatile compounds produced from sustainable biomass resources using chemical and/or biological processes. These platform chemicals include C6-sugars, 5-hydroxymethylfurfural / 2-furfural, 2,5-furandicarboxylic acid, levulinic acid, xylitol, and others, that could be used for direct applications, as polymer building blocks or as derivatives for synthesizing commodity chemicals. Production of these platform chemicals through chemical approaches includes hydrothermal but also solvothermal methods or catalyzed reaction paths in organic (monophasic or biphasic) solvents systems. Driven by the quest of sustainability, ionic liquids have thus progressively emerged as strategic and tunable media for the production of these biobased platform molecules in high yields and selectivities. In particular also, deep eutectic solvents (DES) and natural deep eutectic solvents (NADES) were recently proposed as other competitive sustainable alternatives. These stable systems are defined as a particular mixture composition of two or more entities (including a hydrogen-bond acceptor and a hydrogen-bond donor) that becomes liquid at room temperature. Offering the possibility to perform reactions at low temperatures, DES (and NADES) were found as efficient "green" solvents for the production of an array of platform chemicals, minimizing the formation of side-products. The tunable acid character of these deep eutectic systems allows performing some specific reactions without the introduction of Lewis or Bronsted acids. That is particularly the case for the benchmark dehydration of C6 or C5-sugars into 5-hydroxymethylfurfural or 2-furfural (and their rehydration into levulinic acid). This contribution (that includes original results) aims thus to offer an exhaustive overview of the effectiveness of (natural) deep eutectic solvents as viable liquid systems for the production of top valuable platform compounds from renewable biomass.