Recent Advances in the Use of Ionic Liquids and Deep Eutectic Solvents for Lignocellulosic Biorefineries and Biobased Chemical and Material Production

Abstract

Biorefineries, which process biomass feedstocks into valuable (bio)products, aim to replace fossil fuel-based refineries to produce energy and chemicals, reducing environmental and health hazards, including climate change, and supporting a sustainable economy. In particular, lignocellulose-based biorefineries, utilizing the most abundant renewable feedstock on Earth, have significant potential to supply sustainable energy, chemicals and materials. Ionic liquids (ILs, organic salts with low melting temperatures) and deep eutectic solvents (DESs, mixtures with eutectic points lower than the ideal mixture) are capable of dissolving some of the key lignocellulose polymers, and even the whole biomass. Furthermore, they have intrinsic advantages over molecular solvents, including safer usage profiles and high tunability, which allow tailored physicochemical properties. Such properties provide unique opportunities for the development of new processes that could unlock the full potential of future biorefineries. Here, we review the current state of lignocellulosic biomass processing with ILs and DESs, with a specific focus on the pretreatment chemistry, process flow and products from each component; followed by discussions on sustainability assessments and technological challenges. We aim to inform the research community about the opportunities, challenges and perspectives in developing truly sustainable lignocellulose-based biorefineries.

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Common cations and anions used for synthesis of ILs.

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Representation of the chemical space, in terms of ionic content and polarity of their molecular component, occupied by the types of solvents under review in that document, DESs and ILs, compared to other related solvent systems as depicted by Abbot et al. Adapted with permission from ref . Copyright 2021 American Institute of Physics.

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Illustrations of possible feedstocks are depicted alongside the advantages and disadvantages associated with each generation of biofuel. Adapted with permission from ref . We also want to highlight that the 2G feedstock is an abundant feedstock in contrast to the note by authors on limited availability. , Adapted with permission from ref . Copyright 2023 the Public Library of Science under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Schematic representation of the plant cell wall of a lignocellulosic biomass consisting of cellulose, lignin, and hemicellulose. Adapted with permission from ref . Copyright 2023 Wiley-VCH.

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Representative structure of lignin to include various known units and interunit linkages in a range of grassy and woody lignocellulosic biomass. Adapted with permission from ref . Copyright 2024 Wiley-VCH.

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Some of the main types of LCC bonds found in lignocellulosic biomass. Adapted with permission from ref . Copyright 2023 Elsevier Ltd.

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Pretreatment and fractionation strategies for the utilization of lignocellulosic biomass. Adapted with permission from ref . Copyright 2023 Elsevier Ltd.

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Timeline of the main types of (a) cations and (b) anions introduced as pretreatment agents of lignocellulosic biomass. These studies performed pretreatment and enzymatic saccharification as a means of evaluation pretreatment performance. R and R′ denote either −H or −alkyl substituents. References from this figure are: Liu and Chen, 2006; Amarasekara and Owereh, 2009; Zhao et al., 2009; Liu et al., 2012; Anugwom et al., 2012; Achinivu et al., 2012; Tao et al., 2016; Li et al., 2009; Lee et al., 2009; Brandt et al., 2011; Ungurean et al., 2011.

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Method for the selection of a solvent for a defined application proposed by Jin et al. Adapted with permission from ref . Copyright 2017 Royal Society of Chemistry.

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Timeline of industrial processes based on the use of ILs. From Greer et al. 2020. Adapted with permission from ref . Copyright 2020 MDPI under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Precursors to bio-based ILs. Adapted with permission from ref . Copyright 2016 American Chemical Society.

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Schematic representation of the comparison of the SLE of a simple ideal eutectic mixture (red line) and a deep eutectic mixture (blue line). Adapted with permission from ref . Copyright 2018 Springer Nature.

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The most prevalent structures of hydrogen-bond donors and halide compounds employed in the synthesis of DESs. Adapted with permission from ref . 2015 American Chemical Society.

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The three main categories of ILs used in the pretreatment of lignocellulosic biomass.

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Scheme of the ionoSolv process for biomass treatment. Adapted with permission from ref . Copyright 2022 Elsevier Ltd.

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Some relevant works in the field of biomass fractionation with the ionoSolv process. References from this figure are: Brandt et al. (2011), Brandt et al. (2013), Cox and Ekerdt (2013), Verdía et al. (2014), Achinivu et al. (2014), George et al. (2015), Brandt et al. (2015), Rocha et al. (2017), Brandt-Talbot et al. (2017), Weigand et al. (2017), Gschwend et al. (2018), Miranda et al. (2019), Gschwend et al. (2019), Nakasu et al. (2020), Chen et al. (2020), Gschwend et al. (2020), Das et al. (2021), Hennequin et al. (2022), Achinivu et al. (2022), and Ovejero-Pérez et al. (2023).

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SEM micrographs of: (a) untreated pine; (b) pine pretreated with [C₁im]Cl (c) pine pretreated with [C₂C₁im]­[C₁CO₂]; (d) pine pretreated with [Ch]­[C₁CO₂]; (e) untreated eucalyptus; (f) Eucalyptus pretreated with [C₁im]­Cl; (g) Eucalyptus pretreated with [C₂C₁im]­[C₁CO₂]; (h) Eucalyptus pretreated with [Ch]­[C₁CO₂]. Adapted with permission from ref . Copyright 2019 American Chemical Society.

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One-pot cellulosic ethanol production and delignification with biocompatible DES. Adapted with permission from ref . Copyright 2018 American Chemical Society.

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One-pot conversion of biomass into bioproducts using engineered poplar wood and biocompatible DESs. Adapted with permission from ref . Copyright 2022 Royal Society of Chemistry.

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Proposed structural evolution of lignin during the DES pretreatment. Adapted with permission from ref . Copyright 2022 Elsevier Ltd.

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Response surface for the delignification of sugarcane bagasse in time–temperature experiments with the PIL mono-ethanol ammonium acetate, [(OH)²C₂N]­[C₁CO₂], for a temperature range of 120–150 °C at residence times ranging from 30 to 150 min. Adapted with permission from ref . Copyright 2022 Elsevier Ltd.

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Predictive models for lignin removal from Eucalyptus red grandis with [C₂C₂C₂N]­[HSO₄]_80%. (A) Lignin removal as a function of R ₀. (B) Lignin removal as a function of H-factor. Adapted with permission from ref . Copyright 2020 Royal Society of Chemistry under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Fit of the classical severity factor log R ₀ (in orange, right) and the modified severity factor log R ₀* (in blue, left) with delignification, expressed as quadratic fits considering the results obtained from a BBD-RSM analysis where delignification showed a steep curvature. Adapted with permission from ref . Copyright 2023 American Chemical Society under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Correlation between glucose yield and delignification enzymatic hydrolysis was conducted for 72 h and pretreatment conditions (160–180 °C, 20–40 min, 10–30 wt% of water). Adapted with permission from ref . Copyright 2023 American Chemical Society under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Correlation of enzymatic glucose release and delignification of Pinus sylvestris after pretreatment with [C₄im]­[HSO₄]_80% at different temperatures and times with a solid loading of 10 wt%. Adapted with permission from ref . Copyright 2019 Royal Society of Chemistry.

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Four-stage model for lignin extraction during ionoSolv-type pretreatments. Adapted with permission from ref . Copyright 2018 Royal Society of Chemistry under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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(a) BBD-RSM response surface graphs and the (b) corresponding counter plots at the center point (left, IL concentration = 80 wt%; right, time = 30 min) for the pretreatment of pine with [C₄C₁C₁N]­[HSO₄]. Adapted with permission from ref . Copyright 2023 American Chemical Society under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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H₀ values for mixtures of [C₄C₁im]­[HSO₄] with water at different water concentrations. Adapted with permission from ref . Copyright 2023 American Chemical Society under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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CrI of pulps recovered after pretreatment of switchgrass with [C₂C₁im]­[C₁CO₂] and increasing water contents. Adapted with permission from ref . Copyright 2014 Royal Society of Chemistry.

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Delignification degree for four different feedstocks (Miscanthus, Pinus sylvestris, treated timber and waste wood) pretreated with [C₄C₁C₁N]­[HSO₄] containing different water concentrations at 170 °C for 30 min with a solid loading of 1:5 g·g⁻¹. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Lignin mass balances across six pretreatment cycles. (a) Using 1 water equiv as an antisolvent and (b) using 3 water equiv as an antisolvent. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Structures of guaiacylacetone and guaiacol, products from the breakage of lignin β-O-4′ linkages.

1. Proposed Lignin Reactivity Pathways under Acidic and Alkaline Conditions

2. Acid Catalyzed Mechanism for Hydrolysis of Lignin Leading to the Formation of the Hibbert Ketone

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Effect of cation–anion interactions within [C₂C₂C₂N]­[HSO₄] ILs on the solvation of the oxonium intermediate, before the water dissociation to form the activated complex. Adapted with permission from ref . Copyright 2016 Royal Society of Chemistry.

3. Mechanism of β-O-4′ Aryl Ether Bond Cleavage in ILs and DESs with Coordinating Anions

4. Mono- and Diketones Observed from Lignins Cleaved with Acidic DESs

5. Reaction Pathway Proposed for Acidic Pretreatment in ILs and DESs without Coordinating Anions

6. Possible Pathway for the Cleavage of β-O-4′ Linkages during Pretreatment with Lactic Acid as Proposed by Hong et al.

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Acid attack on β-5′ and β–β′ bonds.

7. Proposed Reaction Pathways for Lignin during Alkaline Pretreatment for Nonphenolic Subunits (above) and for Free Phenolic Subunits (below)

8. Proposed DES with [Ch]­Cl:LA (1:10), Pretreatment Mechanism of Lignin

9. Proposed Reaction Pathway for Repolymerization of Lignin to Form Diphenylmethane and Biphenyl Compounds

10. Possible Routes for Modifications at Side Chain Α Carbon of Monolignols during OrganoSolv–IonoSolv Pretreatment

11. Condensation of Aldehydes with Lignin Free-Phenolic Units

12. Proposed Reaction Pathway for Acylation of Lignin

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Cyclic ester formed between boric acid and lignin at the positions α and γ.

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SEM micrographs of (a) untreated Eucalyptus, (b) Eucalyptus after auto hydrolysis at 175 °C, (c) Eucalyptus after auto hydrolysis at 200 °C, and (d) Eucalyptus after pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C. Adapted with permission from ref . Copyright 2018 Elsevier Ltd.

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Confocal fluorescence microscopy images of (a) untreated Eucalyptus, (b) Eucalyptus after auto hydrolysis at 175 °C, (c) Eucalyptus after auto hydrolysis at 200 °C, and (d) Eucalyptus after pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C. Hemicelluloses (in blue) dyed with calcofluor are observed (Figures a and a) on the surface as well as lignin (in green). Adapted with permission from ref . Copyright 2018 Elsevier Ltd.

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SEM micrographs of (a) untreated pine, (b) pine after auto hydrolysis at 175 °C, (c) pine after auto hydrolysis at 200 °C, and (d) pine after pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C. Adapted with permission from ref . Copyright 2018 Elsevier Ltd.

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Confocal fluorescence microscopy images of (a) untreated pine, (b) pine after auto hydrolysis at 175 °C, (c) pine after auto hydrolysis at 200 °C, and (d) pine after pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C. Adapted with permission from ref . Copyright 2018 Elsevier Ltd.

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SEM microscopy images of (a) untreated pine wood, (b) pine pulp after autohydrolysis at 150 °C followed by pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C, (c) pine pulp after autohydrolysis at 150 °C followed by pretreatment with [C₂C₁im]­[C₁CO₂] at 150 °C, (d) pine pulp after autohydrolysis at 175 °C followed by pretreatment with [C₂C₁im]­[C₁CO₂] at 120 °C, (e) pine pulp after autohydrolysis at 200 °C followed by pretreatment with [C₂C₁im]­[C₁CO₂] at 80 °C, and (f) pine pulp after autohydrolysis at 150 °C followed by pretreatment with [C₂C₁im]­[C₁CO₂] at 150 °C. Adapted with permission from ref . Copyright 2019 Elsevier Ltd.

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Lignin yield recovered at different water equivalents. Fractionation experiments were conducted on pine using [C₄C₁C₁N]­[HSO₄]_80% at 170 °C for 30 min using a 1:5 g·g^–1 solid loading. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Lignin fractionation process by which lignins were precipitated and recovered using sequential antisolvent addition. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Mass balance of lignins isolated by sequential precipitation after fractionations of Miscanthus, willow, and pine with [C₄C₁C₁N]­[HSO₄]_80t% at 150 °C for 120, 90, and 90 min, respectively. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Molecular weight distribution of full precipitation and sequential fractionated lignin as a function of water amount added for Miscanthus lignins with [C₄C₁C₁N]­[HSO₄]_80wt%. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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Chemical paths to substitute different fossil fuel-based chemicals with furan based. Adapted with permission from ref . Copyright 2023 Royal Society of Chemistry under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Coordination of the anion, cation, and Brønsted acid catalyst with the hydroxyl group of fructose.

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Relationship between the K-T β parameter of several ILs and their cellulose dissolving ability.

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The two-platform concept of utilization of lignocellulosic biomass (adapted from Santos et al. ).

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Cellulosic ethanol production process. SHF, separate hydrolysis and fermentation; SSF, simultaneous saccharification and fermentation; SScF, simultaneous saccharification and co-fermentation; HSF, hybrid saccharification and fermentation; CBP, consolidated bioprocessing. Adapted from ref . Copyright 2016 Springer Nature.

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Cellulose materials that can be formulated using CRM.

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Methods to produce cellulose fibers.

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Opportunities of ILs in the production of RCFs. Adapted with permission from ref . Copyright 2022 Springer Nature under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Example of cellulose dissolution and material formulation using ILs and DMSO. Adapted with permission from ref . Copyright 2023 Royal Society of Chemistry under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Nanocellulose and nanofibrillated cellulose obtained using ILs. Adapted with permission from ref . Copyright 2021 American Chemical Society.

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LCNF films production using DES. Adapted with permission from ref . Copyright 2023 Elsevier Ltd.

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Workflow for spinning and conversion of high lignin content fibers using the IL [C₄C₁C₁N]­[HSO₄]. Adapted with permission from ref . Copyright 2023 American Chemical Society under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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The most commonly characterized adhesive properties of lignin and IL modified lignin-based phenol-formaldehyde (PF), phenol-glyoxal (PG), lignin-glyoxal (LG), and urea-formaldehyde resins. − ,−

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Flavonoids purification process from Apocynum venetum leaves. Adapted with permission from ref . Copyright 2016 MDPI under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Essential oils recovery from Dryopteris fragrans using [C₂C₁im]­[C₁CO₂]. Adapted with permission from ref . Copyright 2013 Elsevier Ltd.

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Process to recover betulin from Birch bark using [C₂C₁im]­[C₁CO₂]. Adapted with permission from ref . Copyright 2012 Royal Society of Chemistry.

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Recovery of stilbene glycoside and anthraquinones from Polygonum multiflorum roots using ILs. Adapted with permission from ref . Copyright 2017 Elsevier Ltd.

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Cynaropicrin purification from Cynara cardunculus L. leaves using [C₁₄C₁im]­Cl. Adapted with permission from ref . Copyright 2018 Springer Nature.