Deep Eutectic Solvents for Biomass-Based Waste Valorization: Comparison
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Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Julia L. Shamshina.

Biomass waste streams are potential feedstocks for a variety of products such as fuels, polymers, and building blocks. The deep eutectic solvents (DESs), eutectic mixtures of Lewis (or Brønsted) acids and bases with incomplete proton transfer, were intentionally designed by choosing two or more distinct components that could interact via hydrogen bonding. The DESs, often with non-stoichiometric ratios between components, present melting points significantly lower than the starting materials and produce mixtures of charged and neutral species. 

  • biomass recovery
  • deep eutectic solvents
  • ionic liquids
  • circular economy

1. Deep Eutectic Solvents for the Recovery of Chemicals from Agri-Food Industrial Waste

In the extraction of phenolic compounds, DESs are employed mainly in the separation of carotenoids, flavonoids, and pigments with a wide range of polarity from lignocellulose. In the area of recovery of agri-food industrial by-products, a variety of [Cho]Cl-based DESs were used to extract phenolic compounds from virgin olive oils. Two of the DESs, [Cho]Cl/Xylitol and [Cho]Cl/1,2-Propanediol, showed an increase of extraction yield up to 20–33% and 67.9–68.3% compared to a conventional system, 80 vol% methanol/water [64][1]. In 2017, a study about the ultrasound-assisted [Cho]Cl/Malic acid extraction of wine lees anthocyanins was reported [65][2]. In tThis study, the optimum time conditions were 30.6 min extraction time, 341.5 W ultrasound power, and 35.4 wt% water content in the DES. Grudniewska et al., used [Cho]Cl/Glycerol for enhanced extraction of proteins from oilseed cakes [66][3]. They extracted the proteins into the DES, then the extract was precipitated upon the addition of water.
Fernandez et al., employed glucose/lactic acid, glucose/citric acid, and fructose/citric acid in 5:1, 1:1, and 1:1 mol/mol ratios, respectively, for ultrasound-based extraction of 14 phenolic compounds from onion, olive, tomato, and pear by-products at 40 °C [67][4]. The aqueous glucose/lactic DES resulted as the optimal solvent with the highest capacity of extraction, comparable to those of methanol and water. It was concluded that the glucose/lactic acid DES yielded higher extractability.
Deng et al., synthesized a series of water-soluble DESs composed of Hexafluoroisopropanol (HFIP) as HBD and L-Carnitine or Bet as HBAs to extract pyrethroid residues from tea beverages and fruit juices [68][5]. The extraction method based on L-Carnitine/HFIP (1:2 mol/mol) solvent showed several advantages, such as a short extraction time and a high enrichment factor. Eutectic mixtures can perform as reaction media or extraction solvents for the bioconversion of several components. The applicability of DESs for the removal of cadmium from rice flour was examined by Huang et al. [29][6]. Among the [Cho]Cl-based and glycerol-based DESs, the former demonstrated good removal of Cd (51–96%). The interesting point was that the DESs did not affect the structure or chemical components of rice flour.
Di Gioia et al., explored the possibility of a selective conversion of furfural, produced by biomass, to biofunctionalized cyclopentenone derivatives in [Cho]Cl/Urea [69][7]. In another study, cellulose derived from sunflower stalks was converted into value-added components [70][8]. Three DESs, namely, [Cho]Cl/Oxalic acid, [Cho]Cl/Citric acid, and [Cho]Cl/Tartaric acid, were used as solvents and catalysts. With [Cho]Cl/Oxalic acid-based DES under microwave irradiation, ~99% carbon efficiency was obtained at 180 °C in 1 min. Under such conditions, 4.1% of 5-hydroxymethyl furfural (5-HMF), 76.2% of levulinic acid, 5.6% of furfural, and 15.2% of formic acid were isolated.

2. Deep Eutectic Solvents for Biofuels Production

The awareness of the significance of biomass contribution to energy consumption is increasing as follows: For instance, the EU has indicated that ~10% of the entire gross final-energy consumption in the EU must come from biomass, while the United States of America has set itself an ambitious goal aiming to achieve a total biofuel output of 136.3 billion liters by 2022 [1][9]. However, introducing new processing strategies within the biorefinery context can be challenging because of the typical discrepancy between the conditions used for pretreatment and those used downstream for saccharification and fermentation. For example, pretreatments under acidic or basic conditions are usually not compatible with downstream processing (e.g., enzymatic saccharification and microbial fermentation) and require neutralization and/or separation stages before proceeding with the next steps.
Delignification is the initial pretreatment step for biofuel production. It is a chemically intensive and environmentally problematic process [71][10]. Two of the most widely used methods are Kraft and OrganoSolv delignification. The Kraft process employs a hot mixture of water, sodium hydroxide, and sodium sulfide [72][11], and requires lots of energy to reduce sulfide-containing black liquor waste emissions. Handling this waste poses hazards to the environment [73][12]. The second type of pulping, OrganoSolv, uses catalysts in organic solvents, and the OrganoSolv family includes ASAM (alkali-sulfite-anthraquinone-methanol) [74][13], Organocell (sodium hydroxide-methanol-anthraquinone) [75][14], Formacell (acetic acid-formic acid) [76,77][15][16], Milox (multistage peroxyacid treatment) [78][17], etc. High capital and processing costs are associated with solvent recycling [79][18], and none of these processes has been permanently applied on an industrial scale [80][19]. Since the discovery of ILs’ biomass processing ability, the delignification of biomass using ILs has been reported [81,82,83[20][21][22][23][24],84,85], and there are multiple ILs that have been proposed as good solvents for lignin but not cellulose. Variables such as type of biomass, its load, time and temperature, particle size, etc. affect delignification and delignification rates [86][25]. The ILs are often used with a co-solvent and/or a catalyst (e.g., polyoxymetallates POMs [86][25]). Recently, a [C4mim][HSO4]/butanediol/water system was demonstrated to delignify wood to as low as <1% lignin content with an efficiency of 98% [87][26].
The DESs have also become widely acknowledged as eco-friendly delignification systems. Efficient delignification was reported using [Cho]Cl/lactic acid [88[27][28][29],89,90], propionic acid/Urea [89][28], [Cho]Cl/p-Toluenesulfonic acid [89][28], [Cho]Cl/formic acid [91][30], [Cho]Cl/acetic acid [91][30], [Cho]Cl/oxalic acid [92][31], [Cho]Cl/malic acid [91][30], etc. [93,94,95,96][32][33][34][35]. Due to their ability to remove lignin and xylan, DES such as [Cho]Cl/formic acid or triethylbenzylammonium chloride/lactic acid were proposed for the pretreatment of corn stover to produce biobutanol via acetone-butanol-ethanol (ABE) fermentation by Clostridium [97,98,99][36][37][38]. A similar approach, i.e., using [Cho]Cl-based DES for biomass, Bambara groundnut haulm pretreatment, was recently proposed for bioethanol production. Thus, [Cho]Cl/Lactic acid pretreatment at 100 °C for 1 h was observed to be the best condition for hemicellulose (54.5%) and lignin (60.7%) removal, along with optimum sugar recovery of 94.8% [100][39]. The resulting hydrolysate was concentrated, washed, and fermented for 72 h with Saccharomyces cerevisiae BY4743, and a maximum ethanol concentration of 11.57 g/L was achieved with an ethanol yield of 0.38 g/g sugar and productivity of 0.19 g/L/h. However, in these works, the rationale for the selection of the DES for this specific application was not discussed. Considering that their application was proposed in batches, with separation, filtration, and neutralization steps between the pre-treatment, saccharification, and fermentation steps, and that the number of available DESs is high [35][40], a comprehensive DES screening might be of value to identify the best type of DESs for in-batch biofuel production.
The recent development of biocompatible ILs, such as choline lysinate or ethanolamine acetate, allows for a consolidated one-pot biomass-to-biofuel conversion process that combines pretreatment, saccharification, and fermentation in one vessel [101,102,103][41][42][43]. Similarly, biocompatible DESs were evaluated for the conversion of biomass into biofuels and bioproducts using a one-pot process, an approach that can reduce the operating costs because it simplifies process design and reduces the energy input (avoids the mass transfer between reactors) [104][44]. A multistep ethanol conversion from corn stover was demonstrated in a single vessel as follows: First, the biomass was treated with [Cho]Cl/Glycerol (1:2 mol/mol) at 50 °C for 24 h, followed by a simultaneous saccharification and yeast fermentation at 37 °C for 48 h. Compared to conventional configurations, the one-pot process eliminated all solid/liquid separation steps and did not require any pH adjustment. The process generated 134 g of ethanol from 1 kg of corn stover, which is equal to a conversion yield of 77.5% based on the glucose present [104][44].
The anaerobic digestion of biological and food wastes produces biogas, which is considered a renewable energy supply. Biogas’ main impurity is CO2, which should be removed in the upgrading process. In a very recent study, the DES [Cho]Cl/Urea, in an aqueous form, was employed as a liquid absorbent in a conceptual process to upgrade biogas [105][45]. In comparison with a pure water process, the DES addition decreased energy use by 16%. Moreover, to study how the environment could be influenced by the process, they employed the Green Degree (GD) assessing method [106][46]. The DES loss was negligible due to its very low vapor pressure and thermal stability. They found that the calculated difference of GDs was higher than zero for aqueous [Cho]Cl/Urea-based processes, demonstrating that this process is environmentally benign.

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