large number of studies on the application of application...

Application of Ionic Liquids inSeparation and FractionationProcesses

Emanuel V. Capela, João A. P. Coutinho andMara G. FreireCICECO – Aveiro Institute of Materials,Chemistry Department, University of Aveiro,Aveiro, Portugal

Glossary

Aqueous biphasic systems Liquid-liquid sys-tems formed by two water-soluble componentsdissolved in water, which above given concen-trations form two phases.

Biorefinery Facility with integrated processes toconvert biomass into energy, fuels, materials,and commodity and value-added chemicals.

Green chemistry Concept addressing the designof chemical products and processes aiming atreduce or eliminate the use and generation ofhazardous substances.

Ionic liquids Low melting temperature salts,composed of organic cations and organic/inor-ganic anions.

Natural products Compounds that can beobtained from biomass.

Solid-liquid extraction Process in which a sol-vent is added to solid biomass samples in orderto extract target compounds.

Definition of the Subject

Research on the extraction and separation ofvalue-added products from biomass has been ahot topic in the framework of biorefinery aiminga sustainable conversion of biomass intochemicals, materials, energy, and fuels. However,a complete use of the biomass potential is stilllimited by the lack of cost-effective extractionand separation processes. In the last years, a

large number of studies on the application ofionic liquids (ILs) as alternative solvents for theextraction and separation of several bioactivecompounds, for instance, alkaloids, antioxidants,phenolic/polyphenolic compounds, saponins,anthraquinones, and isoflavones from biomass,have been reported. Based on an extended com-pilation and analysis of the data hitherto reported,this entry provides an overview on the use of ILsin the extraction and separation of value-addedcompounds from natural sources. An overviewon the use of solid-liquid and liquid-liquid extrac-tion techniques for such purposes is outlined,highlighting and discussing the most relevantworks in this area. New insights and directionsto follow in IL-based processes within thebiorefinery framework are suggested.

Introduction

The efficient use of low-cost high-volume agri-cultural and forest biomass, as well as of biomassresidues or by-products, for the production ofenergy, materials, and commodity and value-added chemicals plays an important role in theadvent of a sustainable society [1–3]. In this con-text, biorefinery plays a central role as an inte-grated plant that processes various biologicalnonfood feedstocks, converting them into arange of useful products, as represented inFig. 1 [4].

Despite being a recent topic of research,biorefinery is not a novel concept. The noveltyshown in the last decade is related to a moreefficient use of biomass (without waste genera-tion) and to the production of a wider range ofproducts using novel technologies and processes(physical, chemical, biochemical and thermal) inintegrated approaches [5]. The major challengethat biorefinery faces nowadays is the use of sus-tainable processes and “green” solvents to ensuremaximum extraction efficiencies and yields,along with minimal waste generation. In Fig. 2 it

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is provided a scheme on the biorefinery conceptnovelty/evolution through time.

There are several large-scale (industrial) bio-refinery schemes, which primarily differ in theirbiomass feedstock source. In particular, the ligno-cellulosic feedstock biorefinery has shown to bevery promising, as it has the potential to accommo-date a wide range of low-cost feedstocks (e.g.,straw, reeds, grass, wood, paper-waste) that couldyield competitive products over the petrochemical-dominated market [2], in addition to be able togenerate chemicals with application in food, nutra-ceutical, cosmetic, and pharmaceutical industries.

The fractionation of biomass into its core con-stituents is an important step aiming the develop-ment of biorefining technologies. However, the

development and application of biomass fraction-ation processes comprising cost-effective and sus-tainable technologies and solvents is stillon-going, with bottlenecks that require fundamen-tal and applied research to be overcome. Forinstance, most of the hemicellulose found in tra-ditional kraft pulping is dissolved in the blackliquor in the form of saccharified mono-sugars,along with lignin and inorganic pulping chemicals[2]. This black liquor is traditionally burned forpower generation and hemicellulose is not prop-erly valued [6]. If hemicellulose and lignin couldbe efficiently separated prior to combustion(fractionation of biomass), a more significant usefor hemicellulose could be derived, such as for the

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 1 Biorefinery concept, with thegeneration of chemicals, energy, fuels, and materials

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 2 Advances introduced in thebiorefinery concept up to date

2 Application of Ionic Liquids in Separation and Fractionation Processes

production of fuels (e.g., ethanol) or high valuechemicals (e.g., polyesters) [6].

The choice of the feedstock and final productsare quite important criteria in the design of abiorefinery scheme taking into account the large-scale production implications. Both the initialfeedstock availability and its potential use in mul-tiple production streams need to be considered[7]. Lignocellulosic matrices, such as hardwood,softwood, and agriculture residues, are the mainraw materials from renewable feedstocks,representing an abundant carbon source [1]. Ingeneral, biomass is composed of cellulose, hemi-celluloses, and lignin; yet, a large variety ofextractable compounds are also present in its com-position, such as proteins, lipids, vitamins, anti-oxidants, and other natural compounds[8–13]. Natural products, commonly designatedas “secondary metabolites”, can be highlighted asexamples of value-added compounds that can beobtained from biomass [14]. In fact, biomass is arich source of biologically active natural productsthat play an important role in the prevention andtreatment of several disorders. The complexchemical structures of such natural compoundsmake difficult their acquirement by other path-ways; thus, in most cases, their isolation from

natural sources is the most viable option. Of the1,135 new drugs approved from 1981 to 2010,50% were of natural origin (comprising also theirderivatives and analogues) [15, 16]. For instance,several medicinal compounds derived from bio-mass display analgesic (e.g., morphine), antitus-sive (e.g., codeine), antihypertensive (e.g.,reserpine), cardiotonic (e.g., digoxin), antineo-plastic (e.g., vinblastine and paclitaxel), or anti-malarial (e.g., artemisinin) properties [17], asdepicted in Fig. 3. Furthermore, some plant-derived compounds, such as Taxol/Paclitaxel(Taxus brevifolia), vinblastine and vincristine(Catharanthus roseus), topotecan and irinotecan(Camptotheca acuminata), and etoposide andteniposide (Podophyllum peltatum), were appro-ved by the Food and Drug Administration (FDA)for clinical use due to their anticancer potential.

The first stage of an integrated biorefineryscheme should be the extraction of valuable sec-ondary metabolites, which is usually performedusing volatile organic compounds (VOCs) [18,19, 5]. Nevertheless, these solvents raise someconcerns, such as the danger of handling largevolumes of volatile and combustible solvents,with human risks and safety issues, and seriousenvironmental impact [18, 19]. Furthermore,

Application of IonicLiquids in Separationand FractionationProcesses,Fig. 3 Summary of sometherapeutic propertiesdisplayed by naturalcompounds

Application of Ionic Liquids in Separation and Fractionation Processes 3

strict limitations on nonhazardous solvents existwhen dealing with the processing of natural prod-ucts, particularly when envisaging their use infood and nutraceutical industries. Therefore,there is a crucial need of using “greener” non-hazardous solvents and on the development ofcost-effective and sustainable extraction and sep-aration processes [20, 19].

Green chemistry focuses on the design ofchemical products and processes by reducing oreliminating the use and generation of hazardoussubstances to human health and environment[21]. The design of such environmentally benignproducts, solvents, and processes may be guidedby the 12 Principles of Green Chemistryestablished by Anastas and Warner [22]: (1) pre-vention, (2) atom economy, (3) less hazardouschemical syntheses, (4) design for saferchemicals, (5) safer solvents and auxiliaries,(6) design for energy efficiency, (7) use of renew-able feedstocks, (8) reduction of derivatives,(9) catalysis, (10) design for degradation,(11) real-time analysis for pollution prevention,and (12) inherently safer chemistry for accidentprevention. These principles are a categorizationof the fundamental steps that should be consideredto fulfill the green chemistry goals to obtain anddevelop sustainable products and processes. Theyhave been used as guidelines and design criteria,changing the way by which academia and indus-try design chemical processes and select solvents[23]. In this context, the minimization of the envi-ronmental and health impacts of volatile organicsolvents, commonly used in biomass processingand biorefinery, became a priority in the pastyears. Novel solvents, such as solvents producedfrom renewable resources, water, supercriticalCO2, deep eutectic solvents (DES), and ionic liq-uids (ILs), have been proposed in the past decadesas “greener” alternatives [24].

Ionic liquids (ILs) belong to the molten saltsgroup, being defined as salts with melting temper-atures below 100 �C. They are usually composedof a large organic cation and an organic or inor-ganic anion. The large dimensions of their ionslead to charge dispersion and do not allow theireasy organization in a crystal structure; therefore,they are liquid at lower temperatures than

conventional inorganic salts. Furthermore, due totheir ionic character, ILs display a set of uniquefeatures, such as high ionic conductivity, negligi-ble flammability and volatility, high thermal andchemical stability, and a strong solvation capabil-ity for a large variety of compounds [25, 26]. Theyare frequently designated as “green solvents”mainly due to their negligible flammability andvolatility and also as “designer solvents” sincethere is a large number of cation/anion combina-tions allowing to tune their physic-chemical prop-erties, their biodegradability, and toxicologicalimpact, as well as their selective extractive poten-tial for extractions/separations [27–29]. Unliketraditional molecular solvents, ILs can beregarded as nano-segregated media composed ofa polar network permeated by nonpolar domains[30]. These domains allow ILs to selectively inter-act with different types of solutes/solvents, andimproved properties and applications are thusenvisioned.

The first synthesized IL (ethylammoniumnitrate – melting point 12 �C) dates from 1914,when Paul Walden was testing new explosives forthe replacement of nitroglycerin [31]. Later, sev-eral patents concerning the use of ILs were filled[32]. Nevertheless, it was just in the beginning ofthe twenty-first century with the proposal of waterand air stable ILs that the research on the synthesisand application of novel ILs significantlyincreased. Since then, the most studied ILs arecomposed of imidazolium-, piperidinium-,pyridinium-, pyrrolidinium-, phosphonium-, andammonium-based cations, combined with anionssuch 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 movingfast from these hydrophobic and fluorinatedanions (such as [PF6]

� and [BF4]�) with very

poor water stability [33], towards less toxic andmore biodegradable alternatives derived from nat-ural sources, such as those based on carboxylicacids, amino acids, and mandelic acid derivedanions [34, 35], often combined with thecholinium cation [36, 37]. The synthesis ofnovel biodegradable and low-toxicity ILs has


considerably increased in the last few years, andseveral ILs are classified as biodegradable (at least60% of the IL is biodegraded within 28 days) [38].

ILs have been investigated as alternative sol-vents in several applications, such as in synthesisand (bio)catalysis [39, 40], analytical applications[41], electrochemistry [42], among others. Inaddition to these applications, they have demon-strated a high performance in extraction and sep-arations processes of value-added compoundsfrom biomass [43], a trend that results from theirdesigner solvents characteristic. Usually, ILs leadto higher extraction yields and to a higher selec-tivity, obtained at moderate conditions (time ofextraction, temperature, and pressure), than thoseachieved with conventional VOCs [23].

In this chapter, potential strategies proposed inthe past few years for the extraction and separa-tion of value-added compounds from biomass/natural sources using ILs are reviewed anddiscussed. Both solid-liquid and liquid-liquidapproaches are considered, including their inte-gration. Simultaneously, this chapter focuses onthe change and evolution of the type/family of ILs

used in these processes, as well as on the chal-lenges that remain to be accomplished and futuredevelopments. In Fig. 4, an outline of the presentchapter is presented.

Solid–Liquid Extraction of Value-AddedCompounds from Biomass

In biorefinery solid-liquid extraction processes areemployed when envisioning the recovery ofvalue-added compounds from solid biomass sam-ples. These consist on the addition of a solvent tothe solid biomass under a set of operating condi-tions to obtain an extract rich in the target value-added compound [44]. In this subchapter the mostrelevant studies concerning the use of ILs in thistype of approach are reviewed. Unless the solventis completely selective, which is seldom the case,these extracts are complex and need to be furtherfractionated, separated, or purified by other pro-cedures, such as by the use of adsorbents [45] (outof the scope of this chapter), or by liquid-liquidextraction, namely, by IL-based aqueous biphasic

Application of Ionic Liquids in Separation and Frac-tionation Processes, Fig. 4 Outline of the informationpresented in the present chapter, focused on the use of ionic

liquids in: (i) solid-liquid extraction processes and(ii) liquid-liquid separation processes of value-added com-pounds from biomass


systems (ABS) that are discussed in the nextsubchapter.

The pioneering work on the use of ILs asalternative solvents for solid-liquid extractions ofvalue-added compounds from biomass wasreported in 2007, by Du et al. [46], who demon-strated the successful application of IL aqueoussolutions in the microwave-assisted extraction(MAE) of trans-resveratrol from a Chinese medi-cine herb. After this evidence on the potential ofILs as alternative solvents for extracting value-added compounds from biomass, the number ofrelated works increased significantly [23]. ILs,neat, or in aqueous or alcohol solutions, havebeen the most studied solvents. Although there isa large number of possible cation-anion combina-tions in ILs, imidazolium-based ILs remain themost widely investigated. Within this IL family,the most commonly applied anions are halogen-and [BF4]-based ones [23]. Although it is wellestablished that [BF4]-based ILs are not waterstable [33], even at room temperature, this anionremains the selected choice of many authors – atendency that in an era of natural-derived ILsurgently needs to be reversed. Biodegradableand biocompatible ILs can nowadays be synthe-sized at low cost, meaning that advances in thebiorefinery field need to comprise these moreattractive alternatives. Among the high-valuecompounds extracted from biomass using ILsand their solutions, alkaloids, flavonoids, and ter-penoids have the largest representation (circa50%) of all fine chemicals [23].

In the past years it has been demonstrated thepotential of IL-assisted extractions of bioactivecompounds from several natural sources, over-coming some major drawbacks associated to con-ventional techniques that are energy-intensive andtime-consuming and often use high amounts ofVOCs, such as heat-reflux extraction (EHRE),Soxhlet extraction (ESE), and maceration extrac-tion (EME). Furthermore, it has been reported thatthe selectivity, yield of extraction, and puritylevels can be improved by applying ILs. Theselevels are further improved if combined withmicrowave- [47, 48], ultrasonic- [49, 50], orenzyme-assisted [51, 52] extractions (IL-MAE,IL-UAE, or IL-EAE, respectively). Table 1

summarizes the works that are discussed in thissubchapter, comprising the compounds extractedand their yield, biomass and IL-based solventused, and operating conditions applied. Table 2lists the ILs that have been investigated and arediscussed in this subchapter, including theirnames, acronyms, and chemical structures.

Table 1 clearly reinforces the potential of ILs asbiomass extraction solvents to obtain lignin[53–55], cellulose [56], alkaloids [47, 49, 50,58], proanthocyanidins [48], phenolic/polypheno-lic compounds [57, 52, 51], essential oils [59],hydroxymatairesinol [60], and triterpenic acids[61] from natural sources.

Figure 5 depicts the chemical structure of someof these value-added compounds, extracted bymeans of solid-liquid processes herein reviewed.Figure 6 summarizes the distribution of the num-ber of works dealing with solid-liquid extractionsusing IL-based solvents.

ILs have been used in solid-liquid extractionprocesses for the recovery of lignin from biomasssamples [53–55]. Zakaria et al. [55] investigatedaprotic [C2mim]- and [C4mim]-based ILs, com-bined with acetate and chloride anions, for ligninextraction from coconut shell, showing that thedissolution of biomass can be achieved in 6 h at110 �C, and in just 2 h at 150 �C, under a nitrogenatmosphere. Using this approach, 10% ofregenerated lignin was obtained using [C2mim][Ac], where the main parameters affecting theextraction efficiency are temperature, time, parti-cle size and IL type. In the same line,Ma et al. [53]extracted lignin from corn stalk usingpyrrolidonium-based ILs. In this work, theauthors [53] applied a pretreatment step usingN-methyl-2-pyrrolidonium chloride ([Hnmp]Cl)and N-methyl-2-pyrrolidonium methanesulfonate([Hnmp][CH3SO3]) during 30 min at 90 �C, lead-ing to an yield of lignin of 85.94% and 56.02% ofthe original lignin content, respectively. Finally,through enzymatic hydrolysis, the authors [53]obtained a high yield of reducing sugars fromthe regenerated cellulosic feedstock (91.81%).The pyrrolidonium-based ILs combined withanions with strong hydrogen bond basicity usedby the authors were considered to effectively dis-rupt the intricate cellulose network, thus leading


Applicationof

IonicLiquidsin

Separationan

dFraction

ationProc

esses,Ta

ble

1Com

poun

dsextractedandtheiryield,biom

ass,andIL-based

solventusedandop

erating

cond

ition

sappliedin

thesolid

-liquidextractio

nof

value-addedcompo

unds

from

biom

ass

Biomass

Targetcom

poun

dSolvent

IlOperatin

gcond

ition

sYield

Ref.

Cornstalk

Lignin

IL+H2O

[Hnm

p]Cl

and[H

nmp]

[CH3SO3]

Extractiontim

e:30

min

Temperature:9

0� C

85.94%

and56

.02%

ofthelig

nincontent

[53]

Miscanthu

sxgiga

nteus

Lignin

IL+H2O

[N0222]

[HSO4]

Extractiontim

e:8h

Tem

perature:

120

� C

80%

ofthelig

nincontent

[54]

Cocos

nucifera

L.(coconu

tshell)

Lignin

PureIL

[C2mim

][Ac]

Extractiontim

e:6h/

2h

Tem

perature:

110

� C/1

50� C

Nitrog

enatmosph

ere

10%

ofregeneratedlig

nin

[55]

ArachisHypog

ea(peanu

tshells)andCastaneasativa

(chestnu

tshells)

Cellulosicmaterial

PureIL

[C4mim

]Cl

and[C

2mim

][A

c]

Extractiontim

e:12

hTem

perature:9

0� C

75%

and95

%of

cellu

losicmaterialfrom

peanut

andchestnut

shells,respectively

[56]

Wheatstraw

Lignin,

cellu

lose

andhemicellulose

Pheno

liccompo

unds

Pure

IL+micropo

rous

resins

[C2mim

][Ac]

Extractiontim

e:6h

Tem

perature:

120

� CSolid/liqu

idratio

:1:20

42,114

and86

wt%

oflig

nin,

cellu

lose

and

hemicellulose,respectively

75.5%

ofph

enoliccompo

unds

inthefinalo

ftheprocess

[57]

Glauciumflavum

Crantz

(Papaveraceae)

S-(+)-glaucine

IL+H2O

[C4mim

][Ac]

––

[58]

Cam

ptotheca

acum

inata

samara

Alkaloids

IL-M

AE

[C8mim

]Br

Soaktim

e:2h

Irradiationpo

wer:

385W

Extractiontim

e:8min

Solid/liqu

idratio

:1:12

ILconcentration:

0.80

M2cycles

674.5mg

∙g�1(CPT)

104mg

∙g�1(H

CPT)

[47]

(con

tinued)


Applicationof

IonicLiquidsin

Separationan

dFraction

ationProc

esses,Ta

ble

1(con

tinue

d)

Biomass

Targetcom

poun

dSolvent

IlOperatin

gcond

ition

sYield

Ref.

Cam

ptotheca

acum

inata

samara

Alkaloids

IL-U

AE

[C8mim

]Br

Ultrason

icpo

wer:2

39.42W

Extractiontim

e:34

.58min

Solid/liqu

idratio

:1:12

ILconcentration:

0.50

M

100%

(maxim

umextractio

nefficiency

with

0.50

Mof

[C8mim

]Braqueou

ssolutio

nsdefinedas

100%

)

[49]

Phello

dend

ronam

urense

Rup

rAlkaloids

IL-U

AE

[C4mim

]Br

Ultrason

icpo

wer:

100W

Extractiontim

e:75

min

Solid/liqu

idratio

:1:14

106.7%

(extractions

byheatreflux

definedas

100%

)[50]

Larixgm

elinib

ark

Proanthocyanidins

IL-M

AE

[C4mim

]Br

Soaktim

e:3h

Irradiationpo

wer:

230W

Extractiontim

e:10

min

Solid/liqu

idratio

:1:20

ILconcentration:

1.25

M2cycles

114.86

mg∙g�

1[48]

FlosLon

iceraJapo

nicae

Chlorog

enicacid

IL-EAE

[C6mim

]Br

Tem

perature:7

0� C

pH:4

.0Pectin

ase

concentration:

1mg∙mL�1

ILconcentration:

0.75

MExtractiontim

e:40

min

6.06

wt%

[51]


Eucom

mia

ulmoidesleaves

Chlorog

enicacid

IL-EAE

[C6mim

]Br

Tem

perature:5

0� C

pH:3

.0Cellulase

concentration:

2mg∙mL�1

ILconcentration:

0.50

MExtractiontim

e:2h

8.32

mg∙g�

1[52]

Cinna

mom

umcassia

bark

Essentialo

il(enrichedin

coum

arin)

Mixture

oftwo

ILs

[N0002][NO3]

[C4mim

]Cl

Extractiontim

e:15

min

Tem

perature:

100

� CSolid/liqu

idratio

:1:10

4.6wt%

[59]

Picea

abieskn

ots(N

orway

spruce

knots)

7- hydrox

ymatairesino

lIL

+H2O

[(C2) 3NC2]Br

Extractiontim

e:28

0min

Tem

perature:2

5� C

Solid/liqu

idratio

:1:10

0IL

concentration:

1.50

M

9.45

wt%

[60]

App

lepeels

Triterpenicacids

IL+H2O

[C14mim

]Cl

Extractiontim

e:60

min

Tem

perature:8

0� C

Solid/liqu

idratio

:1:10

ILconcentration:

0.50

M

2.62

wt%

[61]

Herba

Artem

isiaeScop

ariae

Rutin

Quercetin

Scoparone

IL+methano

l[C

4mim

]Br

Extractiontim

e:60

min

Tem

perature:6

0� C

Solid/liqu

idratio

:1:12

0IL

concentration:

0.50

mg∙mL�1

1027

5.92

mgrutin

∙g�1

899.73

mgqu

ercetin

∙g�1

554.32

mgscop

aron

e∙g�

1

[62]


Application of Ionic Liquids in Separation and Fractionation Processes, Table 2 ILs investigated, their names,acronyms, and chemical structures

IL Acronym Chemical structure

Imidazolium-based ILs

1-butyl-3-methylimidazolium bromide [C4mim]Br

1-hexyl-3-methylimidazolium bromide [C6mim]Br

1-octyl-3-methylimidazolium bromide [C8mim]Br

1-butyl-3-methylimidazolium chloride [C4mim]Cl

1-tetradecyl-3-methylimidazolium chloride [C14mim]Cl

1-ethyl-3-methylimidazolium acetate [C2mim][Ac]

1-butyl-3-methylimidazolium acetate [C4mim][Ac]

Ammonium-based ILs

Ethylammonium nitrate [N0002][NO3]

Triethylammonium hydrogen sulfate [N0222][HSO4]

Pyrrolidonium-based ILs

N-methyl-2-pyrrolidonium chloride [Hnmp]Cl

(continued)


to improved results in lignin extraction. In addi-tion, the morphology of untreated corn stalk wasassessed by the authors [53] by SEM, revealing acompact ordered and rigid fibril structure, whichsuffers pronounced changes when pretreated withadequate ILs. Lignin was regenerated from the ILusing acetone/deionized water at roomtemperature.

Gschwend et al. [54] went further and devel-oped a fractionation process for deconstructingthe lignocellulosic matrix into a cellulose-richpulp, a lignin fraction, and an organic distillatefrom the grass Miscanthus x giganteus, using lowcost ILs. The authors [54] synthesized a low-costIL, triethylammonium hydrogen sulfate ([N0222][HSO4]), which can be produced at bulk scale for

$1.24/kg, a cost similar to that of common organicsolvents, such as acetone or toluene. Using thisIL-based approach, the authors showed the recov-ery of the solid pulp from biomass and the recov-ery of lignin from the IL liquor by precipitationwith water. In addition, the authors [54] alsoinvestigated the IL complete recovery and reuse,showing that there are no losses on the solventextraction efficiency. Through this approach, andby applying economic analysis, the authors [54]classified their process as efficient and economi-cally viable. In the same line, Carneiro et al. [56]showed the potential of imidazolium-based ILsfor biorefining purposes, by using [C4mim]Cland [C2mim][Ac] to dissolve and fractionate pea-nut shells (Arachis Hypogea) and chestnut shells

Application of Ionic Liquids in Separation and Fractionation Processes, Table 2 (continued)


N-methyl-2-pyrrolidonium methanesulfonate [Hnmp][CH3SO3]

Analogues of glycine-betaine ILs

Triethyl[2-ethoxy-2-oxoethyl]ammonium bromide [(C2)3NC2]Br

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 5 Schematic representation of thesolid-liquid extraction processes reviewed, and the chemical structures of some compounds extracted


(Castanea sativa). Under the optimized operatingconditions, up to 7 wt% of raw biomass can bedissolved, allowing an extraction of 87% of thecellulosic material. After a precipitation with anacetone/water mixture, the regeneration of thecellulosic material and the IL recovery wasachieved, with a recovery of 75 and 95% forpeanut and chestnut shells, respectively.

Mehta et al. [59] developed an efficient pro-cessing scheme for the extraction of essential oilsenriched in coumarin from Cinnamomum cassiabark using a protic IL – [N0002][NO3] – throughdissolution and further creation of a biphasic sys-tem with diethyl ether. The process was boosted interms of biomass dissolution and essential oilyield by the addition of aprotic [C4mim]-basedILs, combined with chloride or acetate anions.The authors [59] demonstrated that the additionof these ILs allow to tailor the viscosity, solvationability, and extraction efficiency of ILs, describedas 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 wasobtained, although some degradation of the ILwas observed. However, when [C4mim]Cl wasused, a similar yield was obtained (4.6 wt%) with-out any degradation. After the extraction of the

essential oil, the cellulosic material and free ligninwere regenerated from the biomass-IL solution bythe addition of a mixture of acetone and water.

In addition to more complex structures andmatrices such as lignin, cellulose, hemicellulose,and essential oils, there has also been a largeinterest on the study of ILs as alternative solventsto extract value-added chemicals from biomass.Bogdanov et al. [58] reported the successful sub-stitution of methanol (a widely used solvent inindustry to extract value-added molecules fromnatural sources) by a [C4mim][Ac] aqueous solu-tion for the extraction of the biologically activealkaloid S-(+)-glaucine from the aerial parts ofGlaucium flavum Crantz (Papaveraceae), provingthat a higher extraction efficiency can be obtainedusing the IL-based solvent. The authors [58]performed consecutive extractions of biomassfresh samples using the same solvent up to satu-ration, avoiding the need of the IL recycling,while also contributing to a reduction of the pro-cess cost. The authors [58] finally showed therecovery of glaucine from the IL by a back-extraction step with chloroform that unfortunatelydecreases the “green” character of the proposedprocess. Ma et al. [62] optimized the extraction ofbioactive compounds (rutin, quercetin, and

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 6 Number of articles discussed inthis subchapter using ILs for the extraction of natural compounds from biomass


scoparone) from Herba artemisiae scopariae,using IL-methanol solutions. Using0.5 mg∙mL�1 of [C4mim]Br, at a reflux tempera-ture of 60 �C and during 60 min of extraction,10275.92 mg rutin/g, 899.73 mg quercetin/g, and554.32 mg scoparone/g could be extracted fromthe natural source, corresponding to significantimprovements when compared to the valuesobtained with pure methanol. The use of[C4mim]Br at low concentrations has the advan-tage of tuning the polarity of the extraction solventwhile keeping a low viscosity. Lopes et al. [57]published an interesting work concerning thedevelopment of a sustainable process for the val-orization of biomass, by exploring the fraction-ation of wheat straw and extraction and separationof high value phenolic compounds. Although themain goal of their work was the separation andpurification of phenolic compounds throughadsorption with specific polymeric resins, theyfirstly developed a scalable [C2mim][Ac]-basedpretreatment and fractionation process of bio-mass. Taking only in consideration the step ofinterest to this subchapter, the authors found opti-mal conditions using a solid/liquid ratio of 1:20, atemperature of 120 �C, and continuous stirringduring 6 h, yielding 75.5% of total phenoliccompounds.

Faria et al. [61] investigated the potential ofaqueous solutions of hydrotropes or surface-active ILs to improve the solubility and extractionof triterpenic acids from apple peel, a major resi-due of food industries. The authors [61] combinedsolubility and extraction studies to better under-stand the molecular-level mechanisms and ILschemical structures which improve the extractionefficiency. The best conditions of the process werefound to be an extraction time of 60 min, a tem-perature of 80 �C, a solid/liquid ratio of 1:10, anda concentration of [C14mim]Cl of 0.50 M. In suchconditions, a total extraction yield of triterpenicacids of 2.62 wt% was attained, larger than thevalues obtained with volatile organic solvents,such as chloroform or acetone in similar condi-tions. The authors found an increase in the solu-bility of ursolic acid of 8 orders of magnitude in ILaqueous solutions when compared to pure waterand thus proposed the use of water as an

antisolvent to recover the target biomoleculesfrom the IL aqueous solution.

Recently, Ferreira et al. [60] reported on a moresustainable and greener extraction-recovery pro-cess, both by using biocompatible ILs and byreducing the number of steps required to recoverthe target compound/extract. The authors usedanalogues of glycine-betaine (naturally occurringand low cost amino acid, AGB) ILs for the extrac-tion of 7-hydroxymatairesinol (HMR) from Nor-way spruce knots (Picea abies). Several operatingconditions were optimized, namely, the IL con-centration, extraction time, and solid–liquid ratio,for which a response surface methodology wasapplied. The best conditions were obtained withan aqueous solution of 1.5 M of [(C2)3NC2]Br,with a solid-liquid ratio of 1:100, and during280 min at 25 �C, in which extraction yields of9.46 wt% of HMR were obtained. The authorsthen proposed the use of the IL-water-extract forcosmetic, food, and nutraceutical applications,without requiring an additional step for the targetproduct recovery [60]. The IL aqueous solutionenriched in hydroxymatairesinol demonstrated nocytotoxicity towards a macrophage cell line aswell as a higher anti-inflammatory potential thanthe recovered extracts (no IL present). If suitableILs are chosen, it is thus possible to carry out theextraction and the direct application of the naturalextracts, without requiring a step for productrecovery. Figure 7 summarizes the integrated pro-cess proposed by the authors [60], starting withthe solid-liquid extraction step, followed by afiltration step, and finishing it with the direct useof the IL-extract in industry applications. Thiswork represents a huge advance in biorefineryapproaches using ILs, since it comprises biocom-patible ILs, a moderate temperature, and the effec-tiveness and cost of the process were improved.

In addition to the works previously describedcarried out only with inputs of temperature andagitation, microwave- [47, 48], ultrasonic-[49, 50], and enzyme-assisted [51, 52], processescombined with ILs as solvents were also pro-posed. In fact, there are several roles that ILsplay in these kind of processes, for instance, lead-ing to more efficient extraction by reducing themass transfer barrier, or by enhancing the activity


of enzymes. Up to date, only imidazolium-basedILs were studied for the extraction of alkaloids,proanthocyanidins, and chlorogenic acid frombiomass. All of these studies were performedusing halogen-based ILs, most of the times withthe bromide anion. The use of more benign andsustainable ILs was still not investigated in thistype of strategies. Although some improvementscould be achieved in terms of extraction yield, itshould be remarked that these processes requirean extra step, the use of more sophisticated equip-ment, and/or higher energetic inputs.

In summary, ILs and their mixtures with alco-hols or water are remarkable extraction solventsfor value-added compounds from biomass. NeatILs may act as solvents and as pretreatment toolsof biomass that usually presents a compactordered and rigid structure, inducing pronouncedchanges in the morphological structure of biomass(usually visualized by SEM) and a better access ofthe solvent to the target compounds [56]. Whenusing a coupled extraction technology, for exam-ple, an enzymatic one, morphological changes inbiomass are also observed, thus leading to higher

extraction efficiencies. In fact, the chemical struc-tures of ILs have an important role to deconstructbiomass when pure ILs are employed in the pro-cess, where ILs comprising anions with highhydrogen bond basicity are the most efficient.On the other hand, aqueous or alcoholic solutionsof ILs are not expected to destroy the lignocellu-losic part of biomass. These solutions are thusmore selective to extract target compounds,while also leading to a reduced IL consumptionand to a viscosity decrease, ultimately resulting inimprovements in the mass transfer phenomenon.Large increments in the solubility of the targetnatural value-added compounds in this type ofmixtures involving ILs have also been reported,which can occur by a micelle-mediated or byhydrotropic phenomena [61, 63]. Both phenom-ena are highly dependent on the IL chemicalstructure (surfactant-like or hydrotropic ILs,respectively) and on the target compounds chem-ical structure and hydrophobicity. In the caseswhere the micelle-mediated phenomenon pre-vails, it is frequently noticed a higher performanceof ILs composed of longer alkyl side chains to

Application of Ionic Liquids in Separation and Frac-tionation Processes, Fig. 7 Process proposed byFerreira et al. [60] for the extraction of

7-hydroxymatairesinol (HMR) from Norway spruceknots using aqueous solutions of glycine-betaine ana-logues (AGB) ILs


solubilize and extract bioactive compounds, fol-lowing their critical micellar concentration(CMC) trend. On the other hand, the formationof solute–hydrotrope complexes has been to occurwith ILs, leading to significant enhancements inthe solubility of poorly soluble compounds inaqueous media [61, 63].

Based on the exposed, the high efficiency ofILs and ILs solutions in the extraction of bioactivecompounds from biomass is not only related withtheir ability to disrupt of the biomass structure, butalso with the enhanced solubility of the targetcompounds in these solvents. This possibilityalso opens doors for the development of simplerrecovery procedures, e.g., by using water as anti-solvent to induce the precipitation of the extractedmolecules and to recover the IL solvents.Although the recovery of value-added compoundsand ILs regeneration are crucial factors in theprocess development, both are scarcely exploredin the literature. The extracts obtained by IL-basedsolid-liquid extraction approaches are usuallycomplex mixtures enriched in the desired prod-ucts, thus requiring further fractionation and puri-fication steps. These can be achieved, for instance,by means of liquid-liquid approaches (that arediscussed in the next subchapter). The processproposed by Ferreira et al. [60] should be howeverhighlighted in this field, since the authorsdesigned ILs with high performance to extract7-hydroxymatairesinol (HMR) from Norwayspruce knots and capable of being directly usedin the final extract for the envisioned applications.

Separation of Value-Added CompoundsUsing Liquid–Liquid Extraction (LLE)Techniques

Most of the works discussed in the previous sub-chapter refer to the use of aqueous solutions of ILsto extract value-added compounds from biomass.However, no significant attempts have been car-ried out to separate and purify the valuable com-pounds extracted, and as such no information onthe purity degree of the target compounds is usu-ally given. The use of aqueous solutions presentsseveral advantages, such as the increase in the

solubility of the target compounds by a hydrotro-pic or micelle-mediated solubilization phenomenaand the decrease of the high viscosity of most ILs,enhancing the mass transfer and reducing ener-getic inputs. Furthermore, the most green and lowcost solvent (water) is added to the process, alsocontributing to increase the selectivity for targetchemicals.

All the extracts obtained by solid-liquid extrac-tions using IL aqueous solutions are enriched inthe desired chemicals, but they are usually com-plex mixtures unless completely selective sol-vents have been used, which is seldom the case.Therefore, depending on the target application,further fractionation and purification steps arerequired. Since almost all ILs used in solid-liquidextractions from biomass are completely misciblewith water, they can be used to form aqueousbiphasic systems (ABS). ABS are two-phase sys-tems that can be used for liquid-liquid extraction.They are formed by water and two water-solublecomponents, such as two polymers, a polymer anda salt, or two salts. Recently, IL-based ABS havebeen proposed [64] by the combination of ILswith a wide variety of compounds, such as salts,polymers, carbohydrates, and amino acids inaqueous solution [26, 65, 66]. Above given con-centrations, these systems form two aqueousphases, allowing therefore the partition of differ-ent compounds between them [67]. Due to thewide range of IL cation-anion combinations,plus the mixture with different salts, polymers,carbohydrates, and amino acids, IL-based ABSallow the tailoring of the phases’ polarities andaffinities, and thus selective and effective extrac-tions of target natural compounds [68]. In thissubchapter, the most recent and relevant studiesconcerning the application of IL-based ABS forthe separation and fractionation of value-addedcompounds are reviewed and discussed.

IL-based ABS have been investigated for theseparation of antioxidants [69–71], phenolic/polyphenolic compounds [72–77], saponins [78,79], polysaccharides [80], anthraquinones [81],and isoflavones [82], which are reviewed herein.Figure 8 provides a schematic representation ofthe ABS-based strategies herein reviewed, as wellas the chemical structures of some value-added


compounds investigated. Figure 9 depicts the dis-tribution of the articles using this approach for theseparation and purification of natural compounds.

Table 3 summarizes the works that arediscussed below, including the value-added com-pounds studied, ILs and additional phase-formingcomponents investigated, as well as the extractionperformance of the process. It should be remarkedthat although many of these studies were carriedout with model aqueous solutions comprising the

compounds of interest [69, 73, 76, 72, 71], eightworks have been found on the use of ABS to frac-tionate and purify value-added compounds fromcrude biomass extracts [75, 74, 77–79, 81, 80].A list of the ILs employed in these works isprovided in Table 4, including their names, acro-nyms, and chemical structures.

Santos et al. [69] studied the potential ofIL-based ABS to separate antioxidants from aque-ous solutions. In this work, ILs were studied in

Application of Ionic Liquids in Separation and Frac-tionation Processes, Fig. 8 Schematic representation ofthe liquid-liquid extraction approach herein reviewed

based on ABS, and chemical structures of the compoundsfractionated/separated

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 9 Number articles discussed in thischapter regarding the use of IL-based ABS for the separation of natural compounds


two ways: as main phase-forming components ofIL-based ABS, combined with potassium citrate(C6H5K3O7/C6H8O7) at pH 7, and as adjuvants inmore conventional systems composed of polyeth-ylene glycol (PEG) and potassium phosphate

(K2HPO4/KH2PO4) at pH 7. In the first approach,the authors [69] studied imidazolium-,piperidinium-, pyrrolidinium-, and ammonium-based ILs, and in the second case, onlyimidazolium-based ILs were studied as adjuvants.

Application of Ionic Liquids in Separation and Frac-tionation Processes, Table 3 Compounds fractionated/separated and their extraction efficiency/recovery yield,

IL-based solvent used, and ABS phase-forming compo-nents applied in the separation of value-added compoundsfrom biomass

Value-addedcompound IL

Abs phase-formingcomponents

Extractionefficiency/recovery yield Ref.

EugenolPropyl gallate

[Cnmim]Cl, [C4mpip]Cl, [C4mpyr]Cl and[N4444]Cl

IL + C6H5K3O7/C6H8O7, pH 7PEG + K2HPO4/KH2PO4 + IL asadjuvant, pH 7

47.1% – 100% [69]

Gallic acidVanillic acidEugenolNicotineCaffeine

[C4mim]Cl, [C4mpyr]Cl, [C4mpip]Cl,[N4444]Cl and [P4444]Cl

PEG + C6H5K3O7/C6H8O7 + IL asadjuvant

– [76]

Gallic acid [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% [72]

Gallic acidVanillic acidSyringic acid

[C4mim][CF3SO3] and [C4mim][N(CN)2] IL + Na2CO3

IL + Na2SO4

73–99% [71]

Caffeic acidVanillic acidGallic acidVanillinSyringaldehyde

[C12mim]Cl and [C14mim]Cl PEG + NaPA + IL aselectrolyte

40.73–82.52% [73]

Capsaicin [Ch]Cl, [Ch][Bit] and [Ch][DHC] IL + acetonitrile 90.57% [75]

Phenolic acidsFerulic acidp-coumaricacid

[Ch][DHC] IL + tween 20 97% of totalphenols89% of ferulicacid 93% ofp-coumaric acid

[74]

Anthocyanins [C2mim][Ac] IL + K2CO3 31.90% [77]

Triterpenesaponins

[C2-5Tr]Br and [C2-6Qn]Br IL + K3PO4,K2HPO4, K2CO3,Na3C6H5O7, orK3C6H5O7

> 99% [78]

PolyphenolsSaponins

[Ch]Cl IL + K3PO4 35–70% [79]

Aloepolysaccharides

[C4mim][BF4] IL + NaH2PO4 93.12% [80]

Aloeanthraquinones

[C4mim][BF4] IL + Na2SO4 92.1% [81]

Puerarin [C4mim]Br IL + K2HPO4 > 99% [82]


Application of Ionic Liquids in Separation and Fractionation Processes, Table 4 ILs investigated, their names,acronyms, and chemical structures


Imidazolium-based ILs

1-butyl-3-methylimidazolium bromide [C4mim]Br

1-butyl-3-methylimidazolium chloride [C4mim]Cl

1-dodecyl-3-methylimidazolium chloride [C12mim]Cl

1-tetradecyl-3-methylimidazolium chloride [C14mim]Cl

1-ethyl-3-methylimidazolium acetate [C2mim][Ac]

1-butyl-3-methylimidazoliumtetrafluoroborate

[C4mim][BF4]

1-butyl-3-methylimidazoliumtrifluoromethanesulfonate

[C4mim][CF3SO3]

1-butyl-3-methylimidazolium dicyanamide [C4mim][N(CN)2]

1-butyl-3-methylimidazolium methylsulfate [C4mim][CH3SO4]

1-butyl-3-methylimidazolium ethylsulfate [C4mim][C2H5SO4]

(continued)




1-butyl-3-methylimidazolium octylsulfate [C4mim][OctylSO4]

Piperidinium-based ILs

1-butyl-1-methylpiperidinium chloride [C4mpip]Cl

Pyrrolidinium-based ILs

1-butyl-1-methylpyrrolidinium chloride [C4mpyr]Cl

Ammonium-based ILs

Tetrabutylammonium chloride [N4444]Cl

Cholinium chloride [Ch]Cl

Cholinium bitartrate [Ch][Bit]

(continued)


Using both types of ABS, the authors were able toattain a 100% extractive performance of thesesystems for the natural antioxidants eugenol andpropyl gallate. In general, higher extraction effi-ciencies were achieved using the IL + salt ABS

(from 75.9 to 100%) than with PEG + salt + ILABS (between 47.1% and 100%). The poorerefficiency of polymer-based ABS was suggestedto be a result of less tuned and nonspecific inter-actions, contrarily to what is observed in systems



Cholinium dihydrogen citrate [Ch][DHC]

Phosphonium-based ILs

Tetrabutylphosphonium chloride [P4444]Cl

Tetrabutylphosphonium bromide [P4444]Br

Tropinium-based ILs

n-alkyl(ethyl to pentyl)-tropinium bromide [C2-5Tr]Br (Example for [C4Tr]Br that lead to the best resultsin the work)

Quinolinium-based ILs

n-alkyl(ethyl to hexyl)-quinolinium bromide [C2-6Qn]Br (Example for [C4Qn]Br)


where the ILs are present. p-p stacking interac-tions (between ILs aromatic cations and antioxi-dants aromatic rings) and other antioxidant–ILinteractions may be decisive for the success ofthe separation process. Almeida et al. [70] alsodemonstrated the relevance of IL-solute interac-tions in the separation of antioxidants carried outin polymer + salt ABS with 5 wt% of IL(as adjuvant). These results were furtherinterpreted in more detail by Sousa et al. [76],who studied a larger range of ABS, ILs, andnatural compounds. In general, the addition ofILs as adjuvants to polymer-based ABS changesthe coexisting phases’ characteristics and therebymodifies the partition of biomolecules. Anincrease in the extraction of more hydrophobicbiomolecules is observed when using ILs as adju-vants in PEG-salt systems, whereas IL + salt ABSperform better in the extraction of more hydro-philic biomolecules. In summary, the favorablepartition of more hydrophilic biomolecules inIL + salt ABS seems to be ruled by specificsolute-IL interactions, while the favorable parti-tion of more hydrophobic biomolecules in PEG +salt and PEG + salt + IL seems to be governed bythe phases hydrophobicity/polarity.

Cláudio et al. [72] focused their studies ingallic acid, a phenolic compound present in rela-tively high concentrations in a large number ofbiomass sources presenting antioxidant, anti-inflammatory, antifungal, and antitumoral proper-ties [83]. The authors [72] aimed at the develop-ment of an efficient purification process based onABS formed by a wide variety of imidazolium-based ILs and Na2SO4, K3PO4 or K2HPO4/KH2PO4. In general, the partition of the phenoliccompound to the IL-rich phase decreases in theorder: Na2SO4 (pH ca. 3–8) > > K2HPO4/KH2PO4 (pH ca. 7) > K3PO4 (pH ca. 13), dem-onstrating a strong dependence of the partition ofthe target compound between the phases with themedium pH. For pH values below the pKa ofgallic acid (pKa = 4.4), the uncharged moleculepreferentially partitions to the IL-rich phase,whereas for higher pH values the preferential par-tition to the salt-rich phase is observed. Theseresults led the authors to propose a subsequentwork on the back-extraction of antioxidants

present in biomass, as well as on the recoveryand reuse of the IL-rich phases without loss ofextraction efficiency [71]. Two types of IL-basedABS were studied for the extraction of phenolicacids (gallic, syringic, and vanillic acids), namely,IL + Na2CO3 and IL + Na2SO4, in order to tailorthe pH values of the coexisting phases. From theseveral [C4mim]-based ILs investigated, the mostpromising were [C4mim][CF3SO3] and [C4mim][N(CN)2], which were used in sequential two-stepcycles (comprising both the product and IL recov-eries). In four sequential partitioning experimentsinvolving phenolic acids, extraction efficienciesranging between 73% and 99% were attained,while allowing the regeneration of the IL to befurther reused. In Fig. 10, it is depicted a schemewith the IL-based approach proposed by theauthors for the extraction and recovery of gallicacid, and subsequent recovery of the IL-richphase. This work supports the development ofgreener cost-effective IL-based ABS with a sub-stantial reduction in the environmental footprintand economic issues.

In a recent work, Santos et al. [73] proposed anABS approach to fractionate five phenolic com-pounds resulting from lignin depolymerization,namely, caffeic acid, vanillic acid, gallic acid,vanillin, and syringaldehyde. ABS formed bysodium polyacrylate (NaPA 8000) and polyethyl-ene glycol (PEG 8000) were used, in which cat-ionic and anionic commercial surfactants andionic liquids with surface-active nature([C12mim]Cl and [C14mim]Cl) were used as elec-trolytes at concentrations below 1 wt%. Therecovery, partition coefficients, and selectivity ofeach system were evaluated, and recoveries rang-ing between 40.73% for syringaldehyde and82.52% for caffeic acid were reported. The inves-tigated systems were finally used in the design ofan integrated process comprising the fractionationof phenolic compounds, their isolation, andrecycling of the phase-forming components. Fig-ure 11 represents a schematic overview of theintegrated process proposed by the authors [73]for the fractionation/separation, isolation, andrecovery of the phenolic compounds andrecycling of the phase-forming components.


In addition to the previously works focused onthe separation of phenolic compounds, yet onlycarried out with model aqueous solutions com-prising the target solutes, Santos et al. [75] wentfurther and reported the extraction and recovery ofcapsaicin, a bioactive phenolic compound withdifferent therapeutic properties (anticancer, anti-oxidant, antiobesity) directly from pepper Capsi-cum frutescens, followed by the use of ABS for itsseparation. ABS consisting of acetonitrile andcholinium-based ILs ([Ch]Cl, [Ch][Bit] and [Ch][DHC]) were investigated. First, the solid-liquidextraction of the target bioactive compound fromthe natural biomass was conducted using mixturesof water and acetonitrile. By using a mixtureof 40% of water and 60% of acetonitrile, themaximum extraction yield of capsaicin was

0.146 mg∙g�1. The water-acetonitrile-extractmixture was then used to create ABS withcholinium-based ILs. An ABS composed of 30%of acetonitrile + 35% of [Ch]Cl allowed the suc-cessful recovery and purification of capsaicin tothe acetonitrile-rich phase, with an extractionefficiency of 90.57% and a purification factor of3.26. In the end, an integrated process for theextraction and purification of capsaicin from bio-mass was proposed, combining the extraction stepfrom biomass, the purification stage with ABS,and the recycling of the phase-forming compo-nents. More recently, Xavier et al. [74] reported amore environmentally friendly and competitivestrategy aiming at the extraction of ferulic andp-coumaric acids from wheat straw biomass. Inthis approach, the biomass was pretreated by an

Application of Ionic Liquids in Separation and Frac-tionation Processes, Fig. 10 Approach proposed by

Cláudio et al. [71] for the extraction and recovery of gallicacid, comprising the reuse of the IL-rich phase

Application of Ionic Liquids in Separation and Frac-tionation Processes, Fig. 11 Overview of the integratedprocess proposed by Santos et al. [73] for the fractionation/

separation, isolation, and recovery of phenolic compoundsobtained from lignin depolymerization, and recycling ofthe phase-forming components


acid hydrolysis and a subsequent alkalinedelignification. Afterwards, the authors used anABS composed of polyoxyethylene (20) sorbitanmonolaurate (Tween 20) and a biocompatible IL –choline dihydrogencitrate ([Ch][DHC]) – to opti-mize the extraction of phenolic compounds fromthe alkaline hydrolysate samples. The surfactantcontent revealed to be a key parameter in rulingthe extraction performance, where higher surfac-tant concentrations lead to higher extraction levelsof 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 theauthors, as addressed by the free radical scaveng-ing capacity and trolox equivalents antioxidantcapacity.

Other type of (bio)molecules that can be foundin biomass are anthocyanins, water-solublebioflavonoids (polyphenolic compounds) pre-senting a vast range of applications in humanhealth, mainly as inhibitors of lipid peroxidase,as anticancer and anti-inflammatory compounds,and as neuro- and cardioprotector agents. Basedon these features, Lima et al. [77] developed aprocess for the extraction and purification ofanthocyanins from biomass using IL-water mix-tures. The strategy reported by the authorsincludes a solid-liquid extraction step of anthocy-anins from the agricultural waste product grapepomace using aqueous solutions of [C2mim][Ac] and a sequential purification step usingABS formed by the addition of a salt. Underoptimized conditions for the solid-liquid extrac-tion, the authors reported an yield of 3.58 mg ofanthocyanins per g of biomass. Then, using theABS, the undesired flavonoids are separated from

the anthocyanins, with the best purification factor(16.19 fold) achieved with an ABS composed of29.38% of [C2mim][Ac] + 29.40% of K2CO3 at35 �C.

Natural saponins have immunopotentiation,cardio-protective, antidiabetic, and anthypnoticeffects, motivating He et al. [78] to develop amethod for the extraction of ginseng saponinsfrom the root of Panax ginseng C. A. Mey. Thenovel approach proposed comprises the use oftwo IL + salt ABS, using n-alkyl-tropinium andn-alkylquinolinium bromide ILs ([C2-5Tr]Br/[C2-

6Qn]Br). The authors evaluated initially the abil-ity of these ILs to induce aqueous phase separa-tion in presence of several inorganic salts,followed by the exploration of the generatedABS to separate genosides (Rg1, Re, Rd, andRb1) from crude extracts of ginseng roots. At thebest conditions (35% [C4Tr]Br + 20%NaH2PO4 + 3% ginseng extracts + 42% water for60 min at room temperature), extraction efficien-cies higher than 99% were achieved, although nomajor discussions on the purification factors andselectivity of the systems were provided (Fig. 12).

Ribeiro et al. [79] studied the extraction ofsaponins and polyphenols from dried leaves andaerial parts of Tea (Camellia sinensis) and Mate(Ilex paraguariensis) using aqueous solutions ofILs. A wide range of ILs were studied by theauthors, ranging from imidazolium-based ILs dif-fering in their alkyl side chain length to moresustainable ones, such as cholinium based. Atthe solid-liquid extraction, most of the ILs studiedallowed higher extraction yields than thoseobtained using a 30% ethanol aqueous solution,commonly applied in such processes. ThenK3PO4 and Na2CO3 were added to the IL aqueous

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 12 Overview of the processproposed by He et al. [78] for the extraction of ginseng saponins


solution of [Ch]Cl aqueous solution enriched inthe extract in order to form ABS to further frac-tionate and purify the compounds of interest. Yet,only with K3PO4 an ABS is formed. In general,interesting results were obtained for both tea andmate matrices, since partition coefficients twoorders of magnitude higher were obtained forsaponins and phenolic compounds than withethanolic solutions, with extraction efficienciesaround 70% and 35% achieved for mate and teasaponins, respectively. Moreover, after the two-phase separation, a nonwater miscible IL –[C4mim][NTf2] – was added to the [Ch]Cl-richphase resulting in the formation of two newphases: one containing the two ILs and the otheran aqueous phase containing saponins and phe-nols, allowing the value-added compounds recov-ery from the IL-rich phase. Figure 13 depicts aschematic representation of the process developedby the authors.

Tan et al. [80, 81] published two works regard-ing the extraction and separation of value-addedcompounds from Aloe vera L. (Liliaceae) usingIL-based approaches. The Aloe vera contains75 potentially active substances [84, 85], but twomajor bioactive compounds were targeted,namely, aloe polysaccharides present in the aloefillet and aloe anthraquinones in leaves. Aloepolysaccharides are responsible for the pharmaco-logical activities of wound healing, anti-inflammation, and immunomodulating properties[86], while aloe anthraquinones have anti-bacterial, anti-inflammation, hemostatic, and anti-spasmodic features [87]. In both studies, theauthors started by the pretreatment of the plantmaterial, soaking aloe peel powder in an aqueoussolution containing 60% of ethanol. For the aloe

polysaccharides work, the colloid was dried andused [80]. For the anthraquinones [81], sulfuricacid and chloroform were added into the extract,refluxed, and the chloroform finally removed.After evaporation, a yellowish-brown colloidwas used as the crude extract and dissolved inmethanol as the stock solution [81]. In a strategysimilar to that adopted in the previous works, thestock solutions containing the compounds ofinterest were used in the formation of IL-basedABS, composed of [C4mim][BF4] and sodium-based salts, and the fractionation and purificationof the value-added components investigated.Based on the gathered results, the authors shownthat an ABS composed of 18.52% [C4mim][BF4]and 25.93% NaH2PO4 is able to retain the aloepolysaccharides into the salt-rich phase (93.12%),while the other proteins and impurities are extra-cted to the IL-rich phase (extraction efficiency of95.85%). The purity of the final product was dem-onstrated by thermogravimetric analysis (TGA)[80]. Concerning the aloe anthraquinones, a max-imal extraction efficiency of 92.1% of anthraqui-nones into the IL-rich phase was obtained underoptimum conditions, using an ABS composed of[C4mim][BF4] and Na2SO4 at 25 �C and pH 4.0[81]. In the end, a last step for the IL recovery wasproposed using reverse extraction experiments,carried out by taking the IL-rich phase containinganthraquinones and forming a new ABS byadding a salt with alkaline characteristics. Thealkaline medium led to the speciation of the mol-ecules of interest and consequent migration intothe salt-rich phase, allowing the IL-rich phase tobe recovered and reused in the extraction proce-dure, being in agreement with the back-extractionprocedure proposed by Cláudio et al. [71] for

Application of Ionic Liquids in Separation and Fractionation Processes, Fig. 13 Overview of the integratedprocess proposed by Ribeiro et al. [79] for the extraction and fractionation/purification of saponins and polyphenols


phenolic acids. Although these works demon-strate the potential of IL-based ABS to separateand purify specific value-added compounds fromnatural sources, the authors focused on ABSformed by [C4mim][BF4], which has some draw-backs in terms of IL toxicity, stability in water andcost, strengthening the notion that more benignILs should be explored in the development ofsustainable bioprocesses. Moreover, the authors[80, 81] performed the extraction of the targetcompounds from biomass using a commonorganic solvent, and the IL-based strategy wasonly applied for the fractionation and purification.Thus, based on the high performance of aqueoussolutions of IL for the extraction of value-addedcompounds from biomass, as discussed in theprevious subchapter, an integrated process couldbe designed, in which the extraction of the targetcompounds from the biomass can be carried outwith an IL aqueous solution that could be directlyused in the formation of ABS for the fractionationand purification steps, in a similar strategy as thatproposed by Lima et al. [77] and He et al. [78] foranthocyanins and saponins, respectively.

Fan et al. [82] investigated ILs as phase-forming components of ABS for the extractionand isolation of puerarin from Radix PuerariaeLobatae extracts. Puerarin is an important isofla-vone with beneficial effects concerning the treat-ment of hypertension, arteriosclerosis, anddiabetes mellitus. In this work, the separation/purification step was optimized using purepuerarin, and some important key factors affect-ing the extraction process were appraised, namely,the IL nature, the presence of short-chain alco-hols, the salting-out ability of the salt, and the pHof the medium. When applying the optimizedsystem (ABS composed of [C4mim]Br andK2HPO4) to Radix Puerariae Lobatae extracts,the authors [82] achieved an extraction efficiencyhigher than 99% to the IL-rich phase. However,no information was provided by the authorsregarding the purity levels afforded by using thisIL-based ABS approach.

Based on the aforementioned information anddiscussion, and on the summary given in Tables 1and 3, most of the studies carried out hitherto onthe use of IL-based strategies for extraction,

separation, and purification purposes within abiorefinery framework applied imidazolium-based ILs combined with a limited number ofanions (e.g., chloride, bromide, acetate,dicyanimide, and tetrafluoroborate). Neverthe-less, in the past few years, some promisingworks appeared reporting successful results interms of extraction, fractionation and purificationprocesses using ammonium-, phosphonium-,cholinium-, and betaine-glycine-based ILs. How-ever, there is still a wide range of IL ions that canbe explored for the solid-liquid extraction andfurther isolation of value-added compounds fromnatural sources. Also, most works addressing theapplication of IL-based ABS were carried outusing model solutions of target value-added com-pounds; therefore, more studies are needed byemploying more complex and real biomassextracts in the separation and purification steps,as well as on the development of combined andintegrated extraction-separation processes to ful-fill the requirements of a sustainable biorefinery.

Conclusions and Future Directions

In the past decade, ILs have demonstrated theirpotential for the extraction, fractionation, andpurification of value-added compounds from nat-ural sources. The most relevant property of ILsbehind such successful results is their “designersolvent” ability, allowing to tailor the extractionand purification performance of target com-pounds. In this chapter, it was overviewed thepotential and suitability of IL-based solvents forsolid-liquid extractions from biomass, followedby IL-based separation processes by the applica-tion of ABS. IL-based processes have been usedto extract, separate, and purify a wide range ofvalue-added bioactive compounds, such as anti-oxidants, phenolic/polyphenolic compounds,saponins, polysaccharides, anthraquinones, andisoflavones, in addition to more complex struc-tures such as lignin, cellulose and hemicellulose.

Imidazolium-based ILs are still the preferredchoice for most researchers working on solid-liquid extraction and liquid-liquid separationapproaches. This may be due to the well-


established knowledge on their properties andphase behavior. However, recent works are mov-ing into a different direction by using quaternaryammonium-based ILs, such as cholinium- andglycine-betaine-based. In the coming years, it isexpected that more biocompatible and benign ILswill start to be comprehensively investigated,without forgetting the compromise between theIL chemical structure and the performance of theextraction and separation processes. It is interest-ing to note that the solid-liquid and liquid-liquidstrategies discussed in this chapter can comple-ment each other since the development of anintegrated process can be foreseen, e.g., by extra-cting value-added compounds from biomassusing IL aqueous solutions, which can be furtherused to form IL-based ABS for separation andpurification purposes. In fact, some pioneeringstudies on the development of this type of inte-grated processes appeared in the recent years andwere discussed in this chapter, demonstrating thatsustainable and efficient IL-based processes canbe developed and applied in biorefinery.

Based on the promising evidences reported anddiscussed herein concerning the use of ILs inbiorefinery processes, some challenges stillneeded to be addressed. In summary, the ILs com-munity working in this area is already aware of thepotential of IL-based extraction and separationprocesses, but more progress is required, mainlyon the replacement of the widely usedimidazolium-based by more biocompatible ILs,on the development of integrated processes orreduction of the number of steps involved, andon decreasing the cost of the processes. Further-more, it is of high importance to explore anddevelop processes for both the target compoundsrecovery and IL recycling, as well as to demon-strate the potential of scaling-up such processes.Finally, both economic and life cycle analyses areof high relevance to evaluate and guide the designof IL-based processes, so that they can become anindustrial reality in biorefinery plants.

Acknowledgments This work was developed within thescope 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 fundsthrough the FCT/MEC and when appropriate co-financedby FEDER under the PT2020 Partnership Agreement.E.V. Capela also acknowledges FCT for his PhD grant(SFRH/BD/126202/2016). M.G. Freire acknowledges theEuropean Research Council (ERC) for the Starting GrantERC-2013-StG-3377.

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