Application of Ionic Liquids in Separation and Fractionation Processes
- Aqueous biphasic systems
Liquid-liquid systems formed by two water-soluble components dissolved in water, which above given concentrations form two phases.
Facility with integrated processes to convert biomass into energy, fuels, materials, and commodity and value-added chemicals.
- Green chemistry
Concept addressing the design of chemical products and processes aiming at reduce or eliminate the use and generation of hazardous substances.
- Ionic liquids
Low melting temperature salts, composed of organic cations and organic/inorganic anions.
- Natural products
Compounds that can be obtained from biomass.
- Solid-liquid extraction
Process in which a solvent is added to solid biomass samples in order to extract target compounds.
Definition of the Subject
Research on the extraction and separation of value-added products from biomass has been a hot topic in the framework of biorefinery aiming a sustainable conversion of biomass into chemicals, materials, energy, and fuels. However, a complete use of the biomass potential is still limited by the lack of cost-effective extraction and separation processes. In the last years, a large number of studies on the application of ionic liquids (ILs) as alternative solvents for the extraction and separation of several bioactive compounds, for instance, alkaloids, antioxidants, phenolic/polyphenolic compounds, saponins, anthraquinones, and isoflavones from biomass, have been reported. Based on an extended compilation and analysis of the data hitherto reported, this entry provides an overview on the use of ILs in the extraction and separation of value-added compounds from natural sources. An overview on the use of solid-liquid and liquid-liquid extraction techniques for such purposes is outlined, highlighting and discussing the most relevant works in this area. New insights and directions to follow in IL-based processes within the biorefinery framework are suggested.
There are several large-scale (industrial) biorefinery schemes, which primarily differ in their biomass feedstock source. In particular, the lignocellulosic feedstock biorefinery has shown to be very promising, as it has the potential to accommodate a wide range of low-cost feedstocks (e.g., straw, reeds, grass, wood, paper-waste) that could yield competitive products over the petrochemical-dominated market , in addition to be able to generate chemicals with application in food, nutraceutical, cosmetic, and pharmaceutical industries.
The fractionation of biomass into its core constituents is an important step aiming the development of biorefining technologies. However, the development and application of biomass fractionation processes comprising cost-effective and sustainable technologies and solvents is still on-going, with bottlenecks that require fundamental and applied research to be overcome. For instance, most of the hemicellulose found in traditional kraft pulping is dissolved in the black liquor in the form of saccharified mono-sugars, along with lignin and inorganic pulping chemicals . This black liquor is traditionally burned for power generation and hemicellulose is not properly valued . If hemicellulose and lignin could be efficiently separated prior to combustion (fractionation of biomass), a more significant use for hemicellulose could be derived, such as for the production of fuels (e.g., ethanol) or high value chemicals (e.g., polyesters) .
The first stage of an integrated biorefinery scheme should be the extraction of valuable secondary metabolites, which is usually performed using volatile organic compounds (VOCs) [18, 19, 5]. Nevertheless, these solvents raise some concerns, such as the danger of handling large volumes of volatile and combustible solvents, with human risks and safety issues, and serious environmental impact [18, 19]. Furthermore, strict limitations on nonhazardous solvents exist when dealing with the processing of natural products, particularly when envisaging their use in food and nutraceutical industries. Therefore, there is a crucial need of using “greener” nonhazardous solvents and on the development of cost-effective and sustainable extraction and separation processes [20, 19].
Green chemistry focuses on the design of chemical products and processes by reducing or eliminating the use and generation of hazardous substances to human health and environment . The design of such environmentally benign products, solvents, and processes may be guided by the 12 Principles of Green Chemistry established by Anastas and Warner : (1) prevention, (2) atom economy, (3) less hazardous chemical syntheses, (4) design for safer chemicals, (5) safer solvents and auxiliaries, (6) design for energy efficiency, (7) use of renewable feedstocks, (8) reduction of derivatives, (9) catalysis, (10) design for degradation, (11) real-time analysis for pollution prevention, and (12) inherently safer chemistry for accident prevention. These principles are a categorization of the fundamental steps that should be considered to fulfill the green chemistry goals to obtain and develop sustainable products and processes. They have been used as guidelines and design criteria, changing the way by which academia and industry design chemical processes and select solvents . In this context, the minimization of the environmental and health impacts of volatile organic solvents, commonly used in biomass processing and biorefinery, became a priority in the past years. Novel solvents, such as solvents produced from renewable resources, water, supercritical CO2, deep eutectic solvents (DES), and ionic liquids (ILs), have been proposed in the past decades as “greener” alternatives .
Ionic liquids (ILs) belong to the molten salts group, being defined as salts with melting temperatures below 100 °C. They are usually composed of a large organic cation and an organic or inorganic anion. The large dimensions of their ions lead to charge dispersion and do not allow their easy organization in a crystal structure; therefore, they are liquid at lower temperatures than conventional inorganic salts. Furthermore, due to their ionic character, ILs display a set of unique features, such as high ionic conductivity, negligible flammability and volatility, high thermal and chemical stability, and a strong solvation capability for a large variety of compounds [25, 26]. They are frequently designated as “green solvents” mainly due to their negligible flammability and volatility and also as “designer solvents” since there is a large number of cation/anion combinations allowing to tune their physic-chemical properties, their biodegradability, and toxicological impact, as well as their selective extractive potential for extractions/separations [27, 28, 29]. Unlike traditional molecular solvents, ILs can be regarded as nano-segregated media composed of a polar network permeated by nonpolar domains . These domains allow ILs to selectively interact with different types of solutes/solvents, and improved properties and applications are thus envisioned.
The first synthesized IL (ethylammonium nitrate – melting point 12 °C) dates from 1914, when Paul Walden was testing new explosives for the replacement of nitroglycerin . Later, several patents concerning the use of ILs were filled . Nevertheless, it was just in the beginning of the twenty-first century with the proposal of water and air stable ILs that the research on the synthesis and application of novel ILs significantly increased. Since then, the most studied ILs are composed of imidazolium-, piperidinium-, pyridinium-, pyrrolidinium-, phosphonium-, and ammonium-based cations, combined with anions such as chloride (Cl−), bromide (Br−), acetate ([CH3CO2]−), bis(trifluoromethylsulfonyl)imide ([NTf2]−), hexafluorophosphate ([PF6]−), and tetrafluoroborate ([BF4]−). Nonetheless, the research on ILs as alternative solvents is moving fast from these hydrophobic and fluorinated anions (such as [PF6]− and [BF4]−) with very poor water stability , towards less toxic and more biodegradable alternatives derived from natural sources, such as those based on carboxylic acids, amino acids, and mandelic acid derived anions [34, 35], often combined with the cholinium cation [36, 37]. The synthesis of novel biodegradable and low-toxicity ILs has considerably increased in the last few years, and several ILs are classified as biodegradable (at least 60% of the IL is biodegraded within 28 days) .
ILs have been investigated as alternative solvents in several applications, such as in synthesis and (bio)catalysis [39, 40], analytical applications , electrochemistry , among others. In addition to these applications, they have demonstrated a high performance in extraction and separations processes of value-added compounds from biomass , a trend that results from their designer solvents characteristic. Usually, ILs lead to higher extraction yields and to a higher selectivity, obtained at moderate conditions (time of extraction, temperature, and pressure), than those achieved with conventional VOCs .
Solid–Liquid Extraction of Value-Added Compounds from Biomass
In biorefinery solid-liquid extraction processes are employed when envisioning the recovery of value-added compounds from solid biomass samples. These consist on the addition of a solvent to the solid biomass under a set of operating conditions to obtain an extract rich in the target value-added compound . In this subchapter the most relevant studies concerning the use of ILs in this type of approach are reviewed. Unless the solvent is completely selective, which is seldom the case, these extracts are complex and need to be further fractionated, separated, or purified by other procedures, such as by the use of adsorbents  (out of the scope of this chapter), or by liquid-liquid extraction, namely, by IL-based aqueous biphasic systems (ABS) that are discussed in the next subchapter.
The pioneering work on the use of ILs as alternative solvents for solid-liquid extractions of value-added compounds from biomass was reported in 2007, by Du et al. , who demonstrated the successful application of IL aqueous solutions in the microwave-assisted extraction (MAE) of trans-resveratrol from a Chinese medicine herb. After this evidence on the potential of ILs as alternative solvents for extracting value-added compounds from biomass, the number of related works increased significantly . ILs, neat, or in aqueous or alcohol solutions, have been the most studied solvents. Although there is a large number of possible cation-anion combinations in ILs, imidazolium-based ILs remain the most widely investigated. Within this IL family, the most commonly applied anions are halogen- and [BF4]-based ones . Although it is well established that [BF4]-based ILs are not water stable , even at room temperature, this anion remains the selected choice of many authors – a tendency that in an era of natural-derived ILs urgently needs to be reversed. Biodegradable and biocompatible ILs can nowadays be synthesized at low cost, meaning that advances in the biorefinery field need to comprise these more attractive alternatives. Among the high-value compounds extracted from biomass using ILs and their solutions, alkaloids, flavonoids, and terpenoids have the largest representation (circa 50%) of all fine chemicals .
Compounds extracted and their yield, biomass, and IL-based solvent used and operating conditions applied in the solid-liquid extraction of value-added compounds from biomass
IL + H2O
[Hnmp]Cl and [Hnmp][CH3SO3]
Extraction time: 30 min
Temperature: 90 °C
85.94% and 56.02% of the lignin content
Miscanthus x giganteus
IL + H2O
Extraction time: 8 h
Temperature: 120 °C
80% of the lignin content
Cocos nucifera L. (coconut shell)
Extraction time: 6 h/2 h
Temperature: 110 °C / 150 °C
10% of regenerated lignin
Arachis Hypogea (peanut shells) and Castanea sativa (chestnut shells)
[C4mim]Cl and [C2mim][Ac]
Extraction time: 12 h
Temperature: 90 °C
75% and 95% of cellulosic material from peanut and chestnut shells, respectively
Lignin, cellulose and hemicellulose
Pure IL + microporous resins
Extraction time: 6 h
Temperature: 120 °C
Solid/liquid ratio: 1:20
42, 114 and 86 wt% of lignin, cellulose and hemicellulose, respectively
75.5% of phenolic compounds in the final of the process
Glaucium flavum Crantz (Papaveraceae)
IL + H2O
Camptotheca acuminata samara
Soak time: 2 h
Irradiation power: 385 W
Extraction time: 8 min
Solid/liquid ratio: 1:12
IL concentration: 0.80 M
674.5 μg∙g−1 (CPT)
104 μg∙g−1 (HCPT)
Camptotheca acuminata samara
Ultrasonic power: 239.42 W
Extraction time: 34.58 min
Solid/liquid ratio: 1:12
IL concentration: 0.50 M
100% (maximum extraction efficiency with 0.50 M of [C8mim]Br aqueous solutions defined as 100%)
Phellodendron amurense Rupr
Ultrasonic power: 100 W
Extraction time: 75 min
Solid/liquid ratio: 1:14
106.7% (extractions by heat reflux defined as 100%)
Larix gmelini bark
Soak time: 3 h
Irradiation power: 230 W
Extraction time: 10 min
Solid/liquid ratio: 1:20
IL concentration: 1.25 M
Flos Lonicera Japonicae
Temperature: 70 °C
Pectinase concentration: 1 mg∙mL−1
IL concentration: 0.75 M
Extraction time: 40 min
Eucommia ulmoides leaves
Temperature: 50 °C
Cellulase concentration: 2 mg∙mL−1
IL concentration: 0.50 M
Extraction time: 2 h
Cinnamomum cassia bark
Essential oil (enriched in coumarin)
Mixture of two ILs
Extraction time: 15 min
Temperature: 100 °C
Solid/liquid ratio: 1:10
Picea abies knots (Norway spruce knots)
IL + H2O
Extraction time: 280 min
Temperature: 25 °C
Solid/liquid ratio: 1:100
IL concentration: 1.50 M
IL + H2O
Extraction time: 60 min
Temperature: 80 °C
Solid/liquid ratio: 1:10
IL concentration: 0.50 M
Herba Artemisiae Scopariae
IL + methanol
Extraction time: 60 min
Temperature: 60 °C
Solid/liquid ratio: 1:120
IL concentration: 0.50 mg∙mL−1
10275.92 μg rutin∙g−1
899.73 μg quercetin∙g−1
554.32 μg scoparone∙g−1
ILs investigated, their names, acronyms, and chemical structures
Triethylammonium hydrogen sulfate
Analogues of glycine-betaine ILs
Table 1 clearly reinforces the potential of ILs as biomass extraction solvents to obtain lignin [53, 54, 55], cellulose , alkaloids [47, 49, 50, 58], proanthocyanidins , phenolic/polyphenolic compounds [57, 52, 51], essential oils , hydroxymatairesinol , and triterpenic acids  from natural sources.
ILs have been used in solid-liquid extraction processes for the recovery of lignin from biomass samples [53, 54, 55]. Zakaria et al.  investigated aprotic [C2mim]- and [C4mim]-based ILs, combined with acetate and chloride anions, for lignin extraction from coconut shell, showing that the dissolution of biomass can be achieved in 6 h at 110 °C, and in just 2 h at 150 °C, under a nitrogen atmosphere. Using this approach, 10% of regenerated lignin was obtained using [C2mim][Ac], where the main parameters affecting the extraction efficiency are temperature, time, particle size and IL type. In the same line, Ma et al.  extracted lignin from corn stalk using pyrrolidonium-based ILs. In this work, the authors  applied a pretreatment step using N-methyl-2-pyrrolidonium chloride ([Hnmp]Cl) and N-methyl-2-pyrrolidonium methanesulfonate ([Hnmp][CH3SO3]) during 30 min at 90 °C, leading to an yield of lignin of 85.94% and 56.02% of the original lignin content, respectively. Finally, through enzymatic hydrolysis, the authors  obtained a high yield of reducing sugars from the regenerated cellulosic feedstock (91.81%). The pyrrolidonium-based ILs combined with anions with strong hydrogen bond basicity used by the authors were considered to effectively disrupt the intricate cellulose network, thus leading to improved results in lignin extraction. In addition, the morphology of untreated corn stalk was assessed by the authors  by SEM, revealing a compact ordered and rigid fibril structure, which suffers pronounced changes when pretreated with adequate ILs. Lignin was regenerated from the IL using acetone/deionized water at room temperature.
Gschwend et al.  went further and developed a fractionation process for deconstructing the lignocellulosic matrix into a cellulose-rich pulp, a lignin fraction, and an organic distillate from the grass Miscanthus x giganteus, using low cost ILs. The authors  synthesized a low-cost IL, triethylammonium hydrogen sulfate ([N0222][HSO4]), which can be produced at bulk scale for $1.24/kg, a cost similar to that of common organic solvents, such as acetone or toluene. Using this IL-based approach, the authors showed the recovery of the solid pulp from biomass and the recovery of lignin from the IL liquor by precipitation with water. In addition, the authors  also investigated the IL complete recovery and reuse, showing that there are no losses on the solvent extraction efficiency. Through this approach, and by applying economic analysis, the authors  classified their process as efficient and economically viable. In the same line, Carneiro et al.  showed the potential of imidazolium-based ILs for biorefining purposes, by using [C4mim]Cl and [C2mim][Ac] to dissolve and fractionate peanut shells (Arachis Hypogea) and chestnut shells (Castanea sativa). Under the optimized operating conditions, up to 7 wt% of raw biomass can be dissolved, allowing an extraction of 87% of the cellulosic material. After a precipitation with an acetone/water mixture, the regeneration of the cellulosic material and the IL recovery was achieved, with a recovery of 75 and 95% for peanut and chestnut shells, respectively.
Mehta et al.  developed an efficient processing scheme for the extraction of essential oils enriched in coumarin from Cinnamomum cassia bark using a protic IL – [N0002][NO3] – through dissolution and further creation of a biphasic system with diethyl ether. The process was boosted in terms of biomass dissolution and essential oil yield by the addition of aprotic [C4mim]-based ILs, combined with chloride or acetate anions. The authors  demonstrated that the addition of these ILs allow to tailor the viscosity, solvation ability, and extraction efficiency of ILs, described as a synergic effect of IL ions (protic + aprotic). With the mixture of [N0002][NO3] and [C4mim][Ac], an yield of 4.7 wt% of essential oil was obtained, although some degradation of the IL was observed. However, when [C4mim]Cl was used, a similar yield was obtained (4.6 wt%) without any degradation. After the extraction of the essential oil, the cellulosic material and free lignin were regenerated from the biomass-IL solution by the addition of a mixture of acetone and water.
In addition to more complex structures and matrices such as lignin, cellulose, hemicellulose, and essential oils, there has also been a large interest on the study of ILs as alternative solvents to extract value-added chemicals from biomass. Bogdanov et al.  reported the successful substitution of methanol (a widely used solvent in industry to extract value-added molecules from natural sources) by a [C4mim][Ac] aqueous solution for the extraction of the biologically active alkaloid S-(+)-glaucine from the aerial parts of Glaucium flavum Crantz (Papaveraceae), proving that a higher extraction efficiency can be obtained using the IL-based solvent. The authors  performed consecutive extractions of biomass fresh samples using the same solvent up to saturation, avoiding the need of the IL recycling, while also contributing to a reduction of the process cost. The authors  finally showed the recovery of glaucine from the IL by a back-extraction step with chloroform that unfortunately decreases the “green” character of the proposed process. Ma et al.  optimized the extraction of bioactive compounds (rutin, quercetin, and scoparone) from Herba artemisiae scopariae, using IL-methanol solutions. Using 0.5 mg∙mL−1 of [C4mim]Br, at a reflux temperature of 60 °C and during 60 min of extraction, 10275.92 μg rutin/g, 899.73 μg quercetin/g, and 554.32 μg scoparone/g could be extracted from the natural source, corresponding to significant improvements when compared to the values obtained with pure methanol. The use of [C4mim]Br at low concentrations has the advantage of tuning the polarity of the extraction solvent while keeping a low viscosity. Lopes et al.  published an interesting work concerning the development of a sustainable process for the valorization of biomass, by exploring the fractionation of wheat straw and extraction and separation of high value phenolic compounds. Although the main goal of their work was the separation and purification of phenolic compounds through adsorption with specific polymeric resins, they firstly developed a scalable [C2mim][Ac]-based pretreatment and fractionation process of biomass. Taking only in consideration the step of interest to this subchapter, the authors found optimal conditions using a solid/liquid ratio of 1:20, a temperature of 120 °C, and continuous stirring during 6 h, yielding 75.5% of total phenolic compounds.
Faria et al.  investigated the potential of aqueous solutions of hydrotropes or surface-active ILs to improve the solubility and extraction of triterpenic acids from apple peel, a major residue of food industries. The authors  combined solubility and extraction studies to better understand the molecular-level mechanisms and ILs chemical structures which improve the extraction efficiency. The best conditions of the process were found to be an extraction time of 60 min, a temperature of 80 °C, a solid/liquid ratio of 1:10, and a concentration of [C14mim]Cl of 0.50 M. In such conditions, a total extraction yield of triterpenic acids of 2.62 wt% was attained, larger than the values obtained with volatile organic solvents, such as chloroform or acetone in similar conditions. The authors found an increase in the solubility of ursolic acid of 8 orders of magnitude in IL aqueous solutions when compared to pure water and thus proposed the use of water as an antisolvent to recover the target biomolecules from the IL aqueous solution.
In addition to the works previously described carried out only with inputs of temperature and agitation, microwave- [47, 48], ultrasonic- [49, 50], and enzyme-assisted [51, 52], processes combined with ILs as solvents were also proposed. In fact, there are several roles that ILs play in these kind of processes, for instance, leading to more efficient extraction by reducing the mass transfer barrier, or by enhancing the activity of enzymes. Up to date, only imidazolium-based ILs were studied for the extraction of alkaloids, proanthocyanidins, and chlorogenic acid from biomass. All of these studies were performed using halogen-based ILs, most of the times with the bromide anion. The use of more benign and sustainable ILs was still not investigated in this type of strategies. Although some improvements could be achieved in terms of extraction yield, it should be remarked that these processes require an extra step, the use of more sophisticated equipment, and/or higher energetic inputs.
In summary, ILs and their mixtures with alcohols or water are remarkable extraction solvents for value-added compounds from biomass. Neat ILs may act as solvents and as pretreatment tools of biomass that usually presents a compact ordered and rigid structure, inducing pronounced changes in the morphological structure of biomass (usually visualized by SEM) and a better access of the solvent to the target compounds . When using a coupled extraction technology, for example, an enzymatic one, morphological changes in biomass are also observed, thus leading to higher extraction efficiencies. In fact, the chemical structures of ILs have an important role to deconstruct biomass when pure ILs are employed in the process, where ILs comprising anions with high hydrogen bond basicity are the most efficient. On the other hand, aqueous or alcoholic solutions of ILs are not expected to destroy the lignocellulosic part of biomass. These solutions are thus more selective to extract target compounds, while also leading to a reduced IL consumption and to a viscosity decrease, ultimately resulting in improvements in the mass transfer phenomenon. Large increments in the solubility of the target natural value-added compounds in this type of mixtures involving ILs have also been reported, which can occur by a micelle-mediated or by hydrotropic phenomena [61, 63]. Both phenomena are highly dependent on the IL chemical structure (surfactant-like or hydrotropic ILs, respectively) and on the target compounds chemical structure and hydrophobicity. In the cases where the micelle-mediated phenomenon prevails, it is frequently noticed a higher performance of ILs composed of longer alkyl side chains to solubilize and extract bioactive compounds, following their critical micellar concentration (CMC) trend. On the other hand, the formation of solute–hydrotrope complexes has been to occur with ILs, leading to significant enhancements in the solubility of poorly soluble compounds in aqueous media [61, 63].
Based on the exposed, the high efficiency of ILs and ILs solutions in the extraction of bioactive compounds from biomass is not only related with their ability to disrupt of the biomass structure, but also with the enhanced solubility of the target compounds in these solvents. This possibility also opens doors for the development of simpler recovery procedures, e.g., by using water as antisolvent to induce the precipitation of the extracted molecules and to recover the IL solvents. Although the recovery of value-added compounds and ILs regeneration are crucial factors in the process development, both are scarcely explored in the literature. The extracts obtained by IL-based solid-liquid extraction approaches are usually complex mixtures enriched in the desired products, thus requiring further fractionation and purification steps. These can be achieved, for instance, by means of liquid-liquid approaches (that are discussed in the next subchapter). The process proposed by Ferreira et al.  should be however highlighted in this field, since the authors designed ILs with high performance to extract 7-hydroxymatairesinol (HMR) from Norway spruce knots and capable of being directly used in the final extract for the envisioned applications.
Separation of Value-Added Compounds Using Liquid–Liquid Extraction (LLE) Techniques
Most of the works discussed in the previous subchapter refer to the use of aqueous solutions of ILs to extract value-added compounds from biomass. However, no significant attempts have been carried out to separate and purify the valuable compounds extracted, and as such no information on the purity degree of the target compounds is usually given. The use of aqueous solutions presents several advantages, such as the increase in the solubility of the target compounds by a hydrotropic or micelle-mediated solubilization phenomena and the decrease of the high viscosity of most ILs, enhancing the mass transfer and reducing energetic inputs. Furthermore, the most green and low cost solvent (water) is added to the process, also contributing to increase the selectivity for target chemicals.
All the extracts obtained by solid-liquid extractions using IL aqueous solutions are enriched in the desired chemicals, but they are usually complex mixtures unless completely selective solvents have been used, which is seldom the case. Therefore, depending on the target application, further fractionation and purification steps are required. Since almost all ILs used in solid-liquid extractions from biomass are completely miscible with water, they can be used to form aqueous biphasic systems (ABS). ABS are two-phase systems that can be used for liquid-liquid extraction. They are formed by water and two water-soluble components, such as two polymers, a polymer and a salt, or two salts. Recently, IL-based ABS have been proposed  by the combination of ILs with a wide variety of compounds, such as salts, polymers, carbohydrates, and amino acids in aqueous solution [26, 65, 66]. Above given concentrations, these systems form two aqueous phases, allowing therefore the partition of different compounds between them . Due to the wide range of IL cation-anion combinations, plus the mixture with different salts, polymers, carbohydrates, and amino acids, IL-based ABS allow the tailoring of the phases’ polarities and affinities, and thus selective and effective extractions of target natural compounds . In this subchapter, the most recent and relevant studies concerning the application of IL-based ABS for the separation and fractionation of value-added compounds are reviewed and discussed.
Compounds fractionated/separated and their extraction efficiency/recovery yield, IL-based solvent used, and ABS phase-forming components applied in the separation of value-added compounds from biomass
Abs phase-forming components
Extraction efficiency/recovery yield
[Cnmim]Cl, [C4mpip]Cl, [C4mpyr]Cl and [N4444]Cl
IL + C6H5K3O7/C6H8O7, pH 7
PEG + K2HPO4/KH2PO4 + IL as adjuvant, pH 7
47.1% – 100%
[C4mim]Cl, [C4mpyr]Cl, [C4mpip]Cl, [N4444]Cl and [P4444]Cl
PEG + C6H5K3O7/C6H8O7 + IL as adjuvant
[Cnmim][CF3SO3], [C4mim]Br, [C4mim][CH3SO4], [C4mim][C2H5SO4], [C4mim][OctylSO4], [Cnmim]Cl and [C4mim][N(CN)2]
IL + K3PO4
IL + K2HPO4/KH2PO4
IL + Na2SO4
Up to 98.80%
[C4mim][CF3SO3] and [C4mim][N(CN)2]
IL + Na2CO3
IL + Na2SO4
[C12mim]Cl and [C14mim]Cl
PEG + NaPA + IL as electrolyte
[Ch]Cl, [Ch][Bit] and [Ch][DHC]
IL + acetonitrile
IL + tween 20
97% of total phenols
89% of ferulic acid 93% of p-coumaric acid
IL + K2CO3
[C2-5Tr]Br and [C2-6Qn]Br
IL + K3PO4, K2HPO4, K2CO3, Na3C6H5O7, or K3C6H5O7
IL + K3PO4
IL + NaH2PO4
IL + Na2SO4
IL + K2HPO4
ILs investigated, their names, acronyms, and chemical structures
Cholinium dihydrogen citrate
n-alkyl(ethyl to pentyl)-tropinium bromide
(Example for [C4Tr]Br that lead to the best results in the work)
n-alkyl(ethyl to hexyl)-quinolinium bromide
(Example for [C4Qn]Br)
Santos et al.  studied the potential of IL-based ABS to separate antioxidants from aqueous solutions. In this work, ILs were studied in two ways: as main phase-forming components of IL-based ABS, combined with potassium citrate (C6H5K3O7/C6H8O7) at pH 7, and as adjuvants in more conventional systems composed of polyethylene glycol (PEG) and potassium phosphate (K2HPO4/KH2PO4) at pH 7. In the first approach, the authors  studied imidazolium-, piperidinium-, pyrrolidinium-, and ammonium-based ILs, and in the second case, only imidazolium-based ILs were studied as adjuvants. Using both types of ABS, the authors were able to attain a 100% extractive performance of these systems for the natural antioxidants eugenol and propyl gallate. In general, higher extraction efficiencies were achieved using the IL + salt ABS (from 75.9 to 100%) than with PEG + salt + IL ABS (between 47.1% and 100%). The poorer efficiency of polymer-based ABS was suggested to be a result of less tuned and nonspecific interactions, contrarily to what is observed in systems where the ILs are present. π-π stacking interactions (between ILs aromatic cations and antioxidants aromatic rings) and other antioxidant–IL interactions may be decisive for the success of the separation process. Almeida et al.  also demonstrated the relevance of IL-solute interactions in the separation of antioxidants carried out in polymer + salt ABS with 5 wt% of IL (as adjuvant). These results were further interpreted in more detail by Sousa et al. , who studied a larger range of ABS, ILs, and natural compounds. In general, the addition of ILs as adjuvants to polymer-based ABS changes the coexisting phases’ characteristics and thereby modifies the partition of biomolecules. An increase in the extraction of more hydrophobic biomolecules is observed when using ILs as adjuvants in PEG-salt systems, whereas IL + salt ABS perform better in the extraction of more hydrophilic biomolecules. In summary, the favorable partition of more hydrophilic biomolecules in IL + salt ABS seems to be ruled by specific solute-IL interactions, while the favorable partition of more hydrophobic biomolecules in PEG + salt and PEG + salt + IL seems to be governed by the phases hydrophobicity/polarity.
In addition to the previously works focused on the separation of phenolic compounds, yet only carried out with model aqueous solutions comprising the target solutes, Santos et al.  went further and reported the extraction and recovery of capsaicin, a bioactive phenolic compound with different therapeutic properties (anticancer, antioxidant, antiobesity) directly from pepper Capsicum frutescens, followed by the use of ABS for its separation. ABS consisting of acetonitrile and cholinium-based ILs ([Ch]Cl, [Ch][Bit] and [Ch][DHC]) were investigated. First, the solid-liquid extraction of the target bioactive compound from the natural biomass was conducted using mixtures of water and acetonitrile. By using a mixture of 40% of water and 60% of acetonitrile, the maximum extraction yield of capsaicin was 0.146 mg∙g−1. The water-acetonitrile-extract mixture was then used to create ABS with cholinium-based ILs. An ABS composed of 30% of acetonitrile + 35% of [Ch]Cl allowed the successful recovery and purification of capsaicin to the acetonitrile-rich phase, with an extraction efficiency of 90.57% and a purification factor of 3.26. In the end, an integrated process for the extraction and purification of capsaicin from biomass was proposed, combining the extraction step from biomass, the purification stage with ABS, and the recycling of the phase-forming components. More recently, Xavier et al.  reported a more environmentally friendly and competitive strategy aiming at the extraction of ferulic and p-coumaric acids from wheat straw biomass. In this approach, the biomass was pretreated by an acid hydrolysis and a subsequent alkaline delignification. Afterwards, the authors used an ABS composed of polyoxyethylene (20) sorbitan monolaurate (Tween 20) and a biocompatible IL – choline dihydrogencitrate ([Ch][DHC]) – to optimize the extraction of phenolic compounds from the alkaline hydrolysate samples. The surfactant content revealed to be a key parameter in ruling the extraction performance, where higher surfactant concentrations lead to higher extraction levels of phenolic acids: 97% total phenolic acids, 89% of ferulic acid, and 93% of p-coumaric acid. Moreover, by using this biocompatible approach, no bioactivity impairment was reported by the authors, as addressed by the free radical scavenging capacity and trolox equivalents antioxidant capacity.
Other type of (bio)molecules that can be found in biomass are anthocyanins, water-soluble bioflavonoids (polyphenolic compounds) presenting a vast range of applications in human health, mainly as inhibitors of lipid peroxidase, as anticancer and anti-inflammatory compounds, and as neuro- and cardioprotector agents. Based on these features, Lima et al.  developed a process for the extraction and purification of anthocyanins from biomass using IL-water mixtures. The strategy reported by the authors includes a solid-liquid extraction step of anthocyanins from the agricultural waste product grape pomace using aqueous solutions of [C2mim][Ac] and a sequential purification step using ABS formed by the addition of a salt. Under optimized conditions for the solid-liquid extraction, the authors reported an yield of 3.58 mg of anthocyanins per g of biomass. Then, using the ABS, the undesired flavonoids are separated from the anthocyanins, with the best purification factor (16.19 fold) achieved with an ABS composed of 29.38% of [C2mim][Ac] + 29.40% of K2CO3 at 35 °C.
Tan et al. [80, 81] published two works regarding the extraction and separation of value-added compounds from Aloe vera L. (Liliaceae) using IL-based approaches. The Aloe vera contains 75 potentially active substances [84, 85], but two major bioactive compounds were targeted, namely, aloe polysaccharides present in the aloe fillet and aloe anthraquinones in leaves. Aloe polysaccharides are responsible for the pharmacological activities of wound healing, anti-inflammation, and immunomodulating properties , while aloe anthraquinones have antibacterial, anti-inflammation, hemostatic, and antispasmodic features . In both studies, the authors started by the pretreatment of the plant material, soaking aloe peel powder in an aqueous solution containing 60% of ethanol. For the aloe polysaccharides work, the colloid was dried and used . For the anthraquinones , sulfuric acid and chloroform were added into the extract, refluxed, and the chloroform finally removed. After evaporation, a yellowish-brown colloid was used as the crude extract and dissolved in methanol as the stock solution . In a strategy similar to that adopted in the previous works, the stock solutions containing the compounds of interest were used in the formation of IL-based ABS, composed of [C4mim][BF4] and sodium-based salts, and the fractionation and purification of the value-added components investigated. Based on the gathered results, the authors shown that an ABS composed of 18.52% [C4mim][BF4] and 25.93% NaH2PO4 is able to retain the aloe polysaccharides into the salt-rich phase (93.12%), while the other proteins and impurities are extracted to the IL-rich phase (extraction efficiency of 95.85%). The purity of the final product was demonstrated by thermogravimetric analysis (TGA) . Concerning the aloe anthraquinones, a maximal extraction efficiency of 92.1% of anthraquinones into the IL-rich phase was obtained under optimum conditions, using an ABS composed of [C4mim][BF4] and Na2SO4 at 25 °C and pH 4.0 . In the end, a last step for the IL recovery was proposed using reverse extraction experiments, carried out by taking the IL-rich phase containing anthraquinones and forming a new ABS by adding a salt with alkaline characteristics. The alkaline medium led to the speciation of the molecules of interest and consequent migration into the salt-rich phase, allowing the IL-rich phase to be recovered and reused in the extraction procedure, being in agreement with the back-extraction procedure proposed by Cláudio et al.  for phenolic acids. Although these works demonstrate the potential of IL-based ABS to separate and purify specific value-added compounds from natural sources, the authors focused on ABS formed by [C4mim][BF4], which has some drawbacks in terms of IL toxicity, stability in water and cost, strengthening the notion that more benign ILs should be explored in the development of sustainable bioprocesses. Moreover, the authors [80, 81] performed the extraction of the target compounds from biomass using a common organic solvent, and the IL-based strategy was only applied for the fractionation and purification. Thus, based on the high performance of aqueous solutions of IL for the extraction of value-added compounds from biomass, as discussed in the previous subchapter, an integrated process could be designed, in which the extraction of the target compounds from the biomass can be carried out with an IL aqueous solution that could be directly used in the formation of ABS for the fractionation and purification steps, in a similar strategy as that proposed by Lima et al.  and He et al.  for anthocyanins and saponins, respectively.
Fan et al.  investigated ILs as phase-forming components of ABS for the extraction and isolation of puerarin from Radix Puerariae Lobatae extracts. Puerarin is an important isoflavone with beneficial effects concerning the treatment of hypertension, arteriosclerosis, and diabetes mellitus. In this work, the separation/purification step was optimized using pure puerarin, and some important key factors affecting the extraction process were appraised, namely, the IL nature, the presence of short-chain alcohols, the salting-out ability of the salt, and the pH of the medium. When applying the optimized system (ABS composed of [C4mim]Br and K2HPO4) to Radix Puerariae Lobatae extracts, the authors  achieved an extraction efficiency higher than 99% to the IL-rich phase. However, no information was provided by the authors regarding the purity levels afforded by using this IL-based ABS approach.
Based on the aforementioned information and discussion, and on the summary given in Tables 1 and 3, most of the studies carried out hitherto on the use of IL-based strategies for extraction, separation, and purification purposes within a biorefinery framework applied imidazolium-based ILs combined with a limited number of anions (e.g., chloride, bromide, acetate, dicyanimide, and tetrafluoroborate). Nevertheless, in the past few years, some promising works appeared reporting successful results in terms of extraction, fractionation and purification processes using ammonium-, phosphonium-, cholinium-, and betaine-glycine-based ILs. However, there is still a wide range of IL ions that can be explored for the solid-liquid extraction and further isolation of value-added compounds from natural sources. Also, most works addressing the application of IL-based ABS were carried out using model solutions of target value-added compounds; therefore, more studies are needed by employing more complex and real biomass extracts in the separation and purification steps, as well as on the development of combined and integrated extraction-separation processes to fulfill the requirements of a sustainable biorefinery.
Conclusions and Future Directions
In the past decade, ILs have demonstrated their potential for the extraction, fractionation, and purification of value-added compounds from natural sources. The most relevant property of ILs behind such successful results is their “designer solvent” ability, allowing to tailor the extraction and purification performance of target compounds. In this chapter, it was overviewed the potential and suitability of IL-based solvents for solid-liquid extractions from biomass, followed by IL-based separation processes by the application of ABS. IL-based processes have been used to extract, separate, and purify a wide range of value-added bioactive compounds, such as antioxidants, phenolic/polyphenolic compounds, saponins, polysaccharides, anthraquinones, and isoflavones, in addition to more complex structures such as lignin, cellulose and hemicellulose.
Imidazolium-based ILs are still the preferred choice for most researchers working on solid-liquid extraction and liquid-liquid separation approaches. This may be due to the well-established knowledge on their properties and phase behavior. However, recent works are moving into a different direction by using quaternary ammonium-based ILs, such as cholinium- and glycine-betaine-based. In the coming years, it is expected that more biocompatible and benign ILs will start to be comprehensively investigated, without forgetting the compromise between the IL chemical structure and the performance of the extraction and separation processes. It is interesting to note that the solid-liquid and liquid-liquid strategies discussed in this chapter can complement each other since the development of an integrated process can be foreseen, e.g., by extracting value-added compounds from biomass using IL aqueous solutions, which can be further used to form IL-based ABS for separation and purification purposes. In fact, some pioneering studies on the development of this type of integrated processes appeared in the recent years and were discussed in this chapter, demonstrating that sustainable and efficient IL-based processes can be developed and applied in biorefinery.
Based on the promising evidences reported and discussed herein concerning the use of ILs in biorefinery processes, some challenges still needed to be addressed. In summary, the ILs community working in this area is already aware of the potential of IL-based extraction and separation processes, but more progress is required, mainly on the replacement of the widely used imidazolium-based by more biocompatible ILs, on the development of integrated processes or reduction of the number of steps involved, and on decreasing the cost of the processes. Furthermore, it is of high importance to explore and develop processes for both the target compounds recovery and IL recycling, as well as to demonstrate the potential of scaling-up such processes. Finally, both economic and life cycle analyses are of high relevance to evaluate and guide the design of IL-based processes, so that they can become an industrial reality in biorefinery plants.
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), and project MultiBiorefinery (POCI-01-0145-FEDER-016403), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. E.V. Capela also acknowledges FCT for his PhD grant (SFRH/BD/126202/2016). M.G. Freire acknowledges the European Research Council (ERC) for the Starting Grant ERC-2013-StG-3377.
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