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Valorization of lignocellulosic biomass and food residues to obtain valuable chemicals is essential to the establishment of a sustainable and biobased economy in the modern world. The latest and greenest generation of ionic liquids (ILs) are deep eutectic solvents (DESs) and natural deep eutectic solvents (NADESs); these have shown great promise for various applications and have attracted considerable attention from researchers who seek versatile solvents with pretreatment, extraction, and catalysis capabilities in biomass- and biowaste-to-bioenergy conversion processes. The present work aimed to review the use of DESs and NADESs in the valorization of biomass and biowaste as pretreatment or extraction solvents or catalysis agents.
Keywords:
deep eutectic solvent, natural deep eutectic solvent, biomass, food residue, pretreatment, extraction
DESs are eutectic mixtures with their eutectic points lower than that of the ideal liquid mixture [99]. DESs are liquid when they have a eutectic or near-eutectic composition, formed of an appropriately mixed molar ratio of Lewis or Brønsted acids and bases [5,6]. DESs with ionic components are regarded as a new generation of IL analogues, since they have some similarities with ILs. They usually consist of large nonsymmetrical ions, most commonly a quaternary ammonium cation coupled with a halide anion, which is complexed with a metal salt or a hydrogen bond donor (HBD). shows a number of common salts as hydrogen bond acceptors (HBAs) and HBDs used to make DESs.
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Open in a separate windowDESs are classified in based on the nature of their HBDs. Type I DESs are made up of nonhydrated metal halide, MClx, and quaternary ammonium salt, Cat+X&#;, in the general form of Cat+X&#;zMClx where X&#; is a Lewis base (x and z refer to the number of Cl&#; and MClx, respectively). However, the number of nonhydrated metal halides appropriate for a low melting point mixture is limited. Type II DESs are made of hydrated metal halides, MClx.yH2O, combined with salts (y refers to the number of H2O molecules). Type III DESs typically contain a combination of choline chloride (ChCl) and HBDs such as alcohols, amides, and carboxylic acids. Appropriate HBDs can be mixed with suitable metal halides to form Type IV DESs. For example, ZnCl2 suitably mixed with several HBDs, including ethylene glycol, urea, acetamide, and 1,6-hexandiol has been reported by Abbott et al. to form eutectic mixtures [100]. Finally, non-ionic compounds can be used to make mixtures with decreased freezing points to establish a new class, type V, of DESs [101].
The term &#;natural deep eutectic solvent&#;, NADES, was proposed to represent mixtures formed by cellular metabolites such as alcohols, amino acids, organic acids, and sugars [9], as shown in . They are ubiquitous in nature and are highly applicable because of their superiority over ILs and DESs as being more nontoxic, sustainable, and environmentally benign [102]. In the same way as for a DES, a NADES is obtained by combining HBDs and HBAs in appropriate molar ratios to develop interspecies H-bonds, causing a significant melting point drop. NADESs play major roles in cellular metabolism; many biological phenomena can be explained when considering their formation and existence. For example, many water-insoluble metabolites are transferred into plants because of the presence of such natural solvents. Plants can also survive extremely cold temperatures since the membranes, enzymes, and metabolites are stabilized in plant cells rich in NADESs [103]. In the following section, the physicochemical properties of DESs are discussed. In most cases, the discussion also holds true for NADESs.
Open in a separate windowDifferent components of the biomass and food residue are chemically influenced by solvents during any specific stage of bioconversion. Depending on the type of the DES and other conditions such as temperature, pressure, and pH of the mixture, the structures of these components may change. For extraction purposes, structural modifications of the isolated species are highly avoided. There are varieties of experimental techniques with which the structural changes of target constituents are revealed upon solvent addition and through the process. Among the experimental methods, XRD and SEM analysis, and FTIR, UV-vis, and NMR spectroscopies are of high importance for structural exploration. Additionally, the use of such techniques may help to identify and prove the existence of the desired components and to study the extraction mechanism.
In a study, delignification of corncob was performed with three ChCl-based DESs as pretreatment solvents [22]. XRD, FTIR, and SEM were employed to explore the structure of the sample during the process. The XRD experiment revealed that the crystallinity index of corncob residues had a minor increase upon DES pretreatment. Because the crystallinity index of cellulose considerably decreased after the same pretreatment process, it was concluded that the relative amount of cellulose in corncob residues increased due to hemicellulose and lignin removal. The SEM images also indicated that the surface of the corncob was roughened and disordered after being pretreated. This was attributed to destructuration of the corncob by DESs via lignin and cellulose removal. In the FTIR analysis, the decrease in the amplitude of the wavenumber bands assigned to H-bonded hydroxyls in cellulose after DES addition indicated the formation of stronger H-bonds between DESs and corncob. The intensity decrease and disappearance of the band at cm&#;1 after pretreatment by {ChCl:Carboxylic acid} and {ChCl:Polyalcohol} was ascribed to the rupture of the ether bonds between hemicellulose and lignin. Furthermore, the decrease of the band at 834 cm&#;1 after pretreatment was indicative of delignification. In another study, FTIR and NMR spectroscopies were used to explore the molecular structure of lignin, isolated from wheat straw biomass, before and after pretreatment with {ChCl:ZnCl2} at a 1:2 molar ratio [245]. The FTIR results indicated that the backbone structure of lignin did not change much after DES pretreatment. However, the phenolic hydroxyl in the precipitates increased as the carbonyl groups decreased. The 13C-NMR analysis also suggested that the DES used had little impact on the amount of aromatic ring substitution.
In another study, a 2D NMR experiment on the lignin extracted from switchgrass via {ChCl:Ethylene glycol} pretreatment revealed the cleavage of β-O-4 linkages in lignin, which facilitated the solubilization of lignin. This clearly showed the importance of the acidic protons in the DES [26]. Huang et al. [182] employed several techniques, namely FTIR, XRD, and TGA, to explore the chemical composition changes of the extracted chitin from shrimp shells using {ChCl:Malic acid} NADES and acidic/alkaline solvents. Regarding FTIR spectroscopy, they found that the spectra of the shrimp shells was considerably different from those obtained from NADES-/acid-/alkali-extracted chitin. The XRD of NADES-extracted samples showed a crystal lattice type of α&#;chitin. The increase in crystallinity index indicated that mineral and proteins were extracted from shells by NADES. For the TGA experiment, in the range of 200 to 250 °C (the range typically observed for proteins in shrimp shells) the mass loss was absent in the NADES-extracted chitin, indicating that the proteins were removed by NADES.
A series of strongly basic DESs was used to pretreat wheat straw for delignification [233]. XRD analysis of the sample was carried out before and after DES pretreatment. The results of the untreated sample showed that its crystalline structure was the native cellulose I crystal type. As the pretreated sample did not reveal any alteration in crystal type, it was concluded that the DESs that were used could not disrupt the crystalline structure of the wheat straw. However, the crystallinity index suggested higher crystallinity of the sample after DES pretreatment. The IR analysis indicated decreases in the characteristic bands of DES-pretreated wheat straw compared to untreated samples. This implies the depolymerization of lignin and hemicellulose via pretreatment. Very recently, FTIR spectroscopy was used to study the structural modifications of the used lignocellulosic biomass, beech wood polymers upon DES ({ChCl:KOH} and {ChCl:Oxalic acid}) pretreatment. As illustrated in , the size of the peaks at 990, , and cm&#;1 assigned to C&#;O, C=C, C&#;C&#;O, and C&#;O&#;C groups of cellulose, were reduced when pretreated by {ChCl:KOH}, confirming the cellulose removal from the biomass sample. Furthermore, the disappearance of the two peaks at and cm&#;1, attributed to C&#;C and C&#;O vibrations of the lignin aromatic ring, indicated lignin removal from biomass upon {ChCl:Oxalic acid} pretreatment. Considering the hydroxyl stretching region, the peak of lignin is usually wider and that of cellulose is deeper. In , the peak at around cm&#;1 is broader for {ChCl:KOH} and deeper for {ChCl:Oxalic acid} DESs, indicating that the hydroxyls were from phenols (lignin) and cellulose molecules, respectively.
Open in a separate windowThe FTIR and UV-vis spectroscopies were examined for collagen peptides extracted with DESs from cod skins [31]. The strong absorbance at 218 nm of the collagen peptides in {ChCl:Ethylene glycol} DES was assigned to n&#;π* transitions of carbonyl in the peptide bonds, indicating that neither of the DESs affected the peptide structure, or any chemical bond formed between peptide and the DES. However, the other DES, {ChCl:Oxalic acid}, behaved differently. The IR spectra showed that the bands assigned to collagen peptides disappeared when peptides were dissolved in {ChCl:Oxalic acid} DES, meaning that the functional groups of peptides were broken under the effect of this acidic DES. Grudniewska et al. [27] obtained the solid state 13C-NMR, FTIR, and TGA of oil cakes (RC and EC), biomass residues (RCBR and ECBR), and precipitates (RCP and ECP) after biomass pretreatment with {ChCl:Glycerol} to investigate any structural change of the biomass and characterize the proteins in the precipitates.
Regarding only the 13C-NMR analysis, the spectra revealed signals of cellulose and other structural polysaccharides for the oil cakes and biomass residues. The spectra also signified a broad peak attributed to the carbonyl groups of proteins, hemicellulose, and lignin. The intensity of the carbonyl group of the biomass residues, compared to that of oil cakes, decreased and the signal features of cellulose increased. For the precipitants (RCP and ECP), the intensity of carbonyl group increased, while cellulose C1 signal disappeared (ECP) or diminished (RCP). This also happened to polysaccharide sugar units where C2, C4, C5, and C6 signals decreased compared to oil cakes. In both precipitates, the signals at 65&#;48 ppm and 56&#;54 ppm were attributed to α-carbons found in the proteins and CH3O in lignin.
In this article, we reviewed DESs and NADESs in state-of-the-art technologies for biomass/biowaste valorization, where DESs and NADESs were used as reaction media, pretreatment or extraction solvents, catalysts, or as multifunctional solvents. A variety of multipurpose eutectic mixtures can be prepared with properties superior to those reported for ILs; the eutectic mixtures can be made to be less toxic, more biodegradable, and quicker and easier to prepare. Their unfavorable properties can be surmounted by tailoring them, for example by changing the nature of the salt or its molar ratio to HBD, adding appropriate cosolvents, or simply by changing temperature or pressure. If the DES or NADES suffers from high viscosity, adding water in measured amounts works well. In biomass and food waste valorization, materials can be pretreated to enhance enzymatic hydrolysis and selectively solubilize the desired components or catalyze the thermochemical processes. They can also be used as reaction media for chemical and biochemical processes. In some cases, the efficiency of the all the above-mentioned functions of DESs or NADESs could be increased if combined with another technique. For example, the pretreatment power of the solvents improved when coupled with microwave or ultrasonic irradiation or hydrothermal methods. Eutectic solvents can, however, have serious impacts on the structures and functional groups of biomass components.
The existing routes for the bioconversion of biomass and food residue should be optimized, with the possibility of taking full advantage of the features and advantages of eutectic solvents. We looked into the future prospects of the use of DESs and NADESs for valorization of real food waste, and the feasibility of a successive two-step process for biofuel and bio-oil production through sugar fermentation and hydrothermal liquefaction, where DESs and NADESs have the potential to be employed as multifunctional agents. There are three aspects of future study that we think are important.
i. Using real food waste instead of only lignocellulosic biomass, single-component biowaste, or even food waste models for production of chemicals, biofuel, and bio-oil: Food waste can provide free biomass from many sources, including households, restaurants, and food processing industries. There are several methods able to transform biomass, single-component wastes, or multi-food waste into liquid, solid, or gaseous fuels [37,65,68]. However, food waste is seldom used and, to our knowledge, no single study has yet explored the use of DESs or NADESs for such purposes.
ii. A successive two-step process for biofuel and bio-oil production via sugar fermentation and hydrothermal liquefaction: Food waste can be first pretreated and enzymatically hydrolyzed to produce fermentable sugars, after which biofuel is obtained through microbial fermentation. The unhydrolyzed residue usually contains undigested recalcitrant carbohydrates, lipids, and proteins, and can be transferred to hydrothermal reactors for further processing. Hydrothermal processes involve thermochemical conversion of material using high-pressure and high-temperature water to decompose the polymeric material structures. Depending on the type of the hydrothermal analysis, bio-oil, biochar, or biogas is produced by hydrothermal liquefaction, carbonization, and gasification, respectively. The efficient conversion of unhydrolyzed residue into bio-oil, biochar, or biogas fuels enhances the overall efficiency of food waste conversion. Employing DESs or NADESs in (co)solvent-requiring or catalyst-requiring stages is believed to be a major step towards building a sustainable bioeconomy.
iii. For this type of successive two-step process, DESs or NADESs should be employed as (co)solvents or catalysts. This requires innovative design of highly efficient eutectic solvents.
The Natural Sciences and Engineering Research Council of Canada (NSERC) partially financially supported this research.
The authors state that they have no conflicts of interest.
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