Protic, aprotic and choline-derived ionic liquids: towards enhancing

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Protic, aprotic and choline-derived ionic liquids: towards enhancing the accessibility of hardwood and softwood Victoria Rigual, Antonio Ovejero-Pérez, Sandra Rivas, Juan Carlos Domínguez, María Virginia Alonso, Mercedes Oliet, and Francisco Rodríguez ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b04443 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Sustainable Chemistry & Engineering

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PROTIC, APROTIC AND CHOLINE-

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DERIVED IONIC LIQUIDS: TOWARDS

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ENHANCING THE ACCESSIBILITY OF

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HARDWOOD AND SOFTWOOD

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Victoria Rigual*, Antonio Ovejero-Pérez, Sandra Rivas, Juan C. Domínguez, M. Virginia

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Alonso, Mercedes Oliet and Francisco Rodriguez

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Department of Chemical Engineering, Faculty of Chemistry, Complutense University of

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Madrid, Avda. Complutense s/n, 28040 Madrid, Spain.

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*E-mail: [email protected]; Tel: +34913948505

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Keywords: Choline ionic liquids, Ionic liquids, Protic Ionic liquids, 2D-NMR,

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Saccharification

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ABSTRACT

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A side-by-side comparison of softwood (pine) vs. hardwood (eucalyptus) pretreatment

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using 3 protic, 3 aprotic and 3 choline-derived ionic liquids (ILs) is proposed. While the

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protic ionic liquid 2-hydroxyethylammonium formate lead to alkali lignin dissolution

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at 30 °C after 1 h, the lack of interactions with the whole cell wall limits the biomass

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disruption. On the contrary, the protic ionic liquid 1-methylimidazolium chloride

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produces a catalytic effect that extracts almost all the hemicelluloses, and partially lignin.

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Remarkable digestibilities are obtained with choline acetate ([Ch][OAc]) in eucalyptus

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(69 %), while in pine, nor protic, neither choline-derive ILs tested look like to be a real

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“greener” alternative to conventional ILs such as 1-ethyl-3-methylimidazolium acetate

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(the highest digestibility, 84 %). Solid morphology revealed a smoother surface in pine

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pretreated with [Mim][Cl], and confocal fluorescence microscopy enhanced to

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distinguish surface holocellulose and lignin, highlighting differences in the accessibility

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of hardwood vs. softwood in the presence of surface lignin. 2D-NMR of saccharified

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samples pretreated with [Ch][OAc] showed the presence of groups derived from acetate.

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Finally, TGA and spectroscopy techniques evince the difficulties to recover the ionic

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liquid and conclude a work that offers an outlook of strengths and weaknesses in the ILs

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and biomasses studied.

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INTRODUCTION

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Lignocellulosic biomass is considered one of the main alternatives to face the gradual

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depletion of fossil fuels. 1In particular, woody biomass is potentially one of the main

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raw material in biorefineries. 2Softwood and hardwood are promising raw materials, due

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to their availability (pine is the most abundant wood in the whole Northern Hemisphere

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and eucalyptus is cultivated in template regions and covered 1.3 million of hectares in

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Europe in 2016), fast growth, and adaptability. 3-6 To exploit the maximum value of

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softwood and hardwood, the fractionation of its constituents through green and

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sustainable processes is essential. 1 Pretreatment comprises up to 40 % of the total cost

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of processing. 7 8 Among all the alternatives (steam explosion, autohydrolysis, alkali

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pretreatment, etc.), certain ionic liquids (ILs) offers the deconstruction of lignocellulosic

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biomass. ILs research boomed during the last decade due to their potential (of some of

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them) to replace conventional solvents, jumping from the basic research to the industrial


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scale. 9, 10 ILs potential advantages in this field of application are their task-specific

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properties, a wide range of polarities, and their capacity to dissolve a variety of

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chemicals, including polymers. 8

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Since Swalotski et al (2002) 11discovered the capacity of certain imidazolium ILs to

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dissolve cellulose, the employment of this group of ILs for biomass pretreatment has

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been boosted over the years. Certain imidazolium-based ILs such as 1-ethyl-3-

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methylimidazolium acetate ([Emim][OAc]) 1-ethyl-3-methylimidazolium

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dimethylphosphate ([Emim][DMP]) or 1-allyl-3-methylimidazolium chloride

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([Amim][Cl]) are conventional ILs employed for biomass pretreatment that has been

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shown to effectively decrease the recalcitrance of softwood and hardwood, enhancing

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saccharification.7, 12, 13 This approach included lignocellulose deconstruction through

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partial cellulose dissolution, and cellulose precipitation with the aid of an antisolvent 14.

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Although at the beginning ILs were designated as “green solvents”, the risk assessment

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of these conventional ILs through effluents or soil, and the complexity for their

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synthesis arises concerns about their “green” character. 8Another aspect that must be

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overcome is the cost of ILs. [Emim][OAc] bulk production price has been estimated in

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2017 in 20-101 $·kg-1 . 15 In this context, the substitution of conventional ILs by

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emerging alternative ILs such as certain protic ionic liquids (PILs) and choline derived

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ILs are being studied in a wide range of biomasses.

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In the biorefinery context, some PILs have been proved to fractionate biomass and

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extract lignin due to the acidic behavior of these solvents.16 They have been considered

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“greener” than conventional ILs due to the easiness of synthesis, the availability of

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starting materials and the biodegradability.17 The PIL 2-hydroxyethylammonium

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formate ([OHEtAm][HCO2]) is an example of Brönsted acidic IL.17 [OHEtAm][HCO2]

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has been effectively proved in cotton stalk pretreatment, and is easy to synthesize by the



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combination of an amine and formic acid present in nature, with acidic hydrogens in

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cation and anion.18 Another kind of PIL is 1-methylimidazolium chloride

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([Mim][Cl]), a Brönsted acidic ionic liquid with acidic hydrogen in the cation, formed

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by the proton transfer from a Brönsted acid to a Brönsted base.17 The acidic behavior of

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this solvent enhanced the pretreatment of yellow pine for further saccharification. 19

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The strongest advantage of certain choline derived ILs is their friendliness towards

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cellulases and yeasts. Choline lysinate ([Ch][Lys]) was the prime example of

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biocompatible IL, firstly proved as effective pretreatment solvent in herbaceous biomass

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and more recently in hardwood and softwood. 12, 20, 21 Another checked (in herbaceous

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and softwood) example of effective choline IL is choline acetate ([Ch][OAc]) that

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benefits from the effectivity of acetate anion for biomass pretreatment combined with a

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“greener” cation. 22

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Given the powerful alternative and advantages of promising but not yet realized ILs,

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this work aims to study mechanisms that govern pretreatment of biomass with PILs and

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choline derived, in comparison to conventional ILs.

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Carboxylate (acetate) anion is employed in 1 protic, 1 aprotic and 1 choline derived

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IL. Chloride anions are employed when possible and imidazolium cations are employed

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in 3 aprotic ILs and 1PIL. Furthermore, in each group of ILs (protic, aprotic and choline

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ILs) variations are made systematically: in aprotic ILs, three of the most employed

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anions’ functional groups were employed: acetate, chloride and phosphate; in PILs,

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imidazolium chloride was included to compare the effect of [Amim][Cl] vs its acidic

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version [Mim][Cl] and ammonium ILs with two carboxylate anions were included to

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observe differences between formate and acetate anion; and in choline-derived ILs the

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acetate anion is employed to compare three groups of ILs. Furthermore, two amino

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acids ([Ch][Lys] and [Ch][Ser]) are used.


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ILs pretreatment is carried out over a model of softwood (pine) and hardwood

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(eucalyptus) to offer a side-by-side comparison of the different kinds of ILs in different

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wood biomasses. Enzymatic digestibility is the main, but other outstanding aspects,

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such as IL thermal degradation and IL recovery are also discussed. A deep study of

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biomass morpholology is performed to elucidate aspects that govern the pretreatment in

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every kind of IL. Additionally, the most promising saccharified by-stream samples are

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further characterized using 2D-NMR.

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MATERIALS AND METHODS

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Materials and reagents

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The ILs [OHEtAm][HCO2], (97 %, Iolitec GmbH), 2-hydroxyethylammonium

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acetate ([OHEtAm][OAc], 97 %, Iolitec, GmbH), (98 %, Iolitec GmbH),

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[Emim][OAc], (95 %, Iolitec GmbH), 1-ethyl-3-methylimidazolium

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dimethylphosphate ([Emim][DMP], 98 %, Iolitec, GmbH), [Amim][Cl], >(98 %,

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Iolitec GmbH), [Ch][OAc], (98 %, Iolitec GmbH), [Ch][Lys], (95 %, Iolitec

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GmbH) and choline serinate ([Ch][Ser], 95 %, Iolitec, GmbH) were employed for

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wood dissolution. Pinus radiata (P) and Eucalyptus globulus (E) sawdust were

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provided by the Instituto Nacional de Investigación y Tecnología Agraria

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(CIFOR - INIA). Accellerase 1500® enzyme cocktail (protein content: 70 mg of

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protein/mL cocktail) was kindly provided by DuPont Industrial BioSciences.

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Biomass pretreatment

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Due to the high viscosity of ILs, pine and eucalyptus were milled and sieved at a

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particle size [Ch][OAc]_E > Org_E. The higher

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S/G ratio obtained with [Emim][OAc]_E evinces the G units removal during the

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pretreatment with this IL, which also favors the material digestibility. This selective G

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lignin extraction can be one of the reasons why [Emim][OAc]_P digestibility is higher

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than [Emim][OAc]_E, as pine lignin is formed by G units. Side chain regions of pine

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and eucalyptus saccharified cell wall samples are observed in Figure S6. The non-

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oxygenated aliphatic region (δC ~ 50 – 0 ppm) shows the α/β-CH2- groups, γ-CH3 in n-

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propyl side chains of lignin, and acetyl hemicelluloses.47 In this case, higher amounts of

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non-oxygenated compounds were observed in [Ch][OAc]_P and [Ch][OAc]_E samples.

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Organosolv samples had lower amounts of non-oxygenated aliphatic compounds, and

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the non-oxygenated aliphatic compounds present in saccharified cell wall samples may

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be attributed mainly to lignin, but also acylated hemicelluloses. The presence of β-O-4

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interunit linkages (Cβ-Hβ at δC/ δH = 83.4/4.27 ppm; Cγ−Hγ in γ-hydroxylated β-O-4′

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substructures at δC/ δH = 59.4/3.40 and 3.72 ppm; Cγ−Hγ in γ-acylated β-O-4′


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substructures at δC/ δH = 63.5/3.83 and 4.30 ppm; Cα−Hα in β-O-4′ substructures (A)

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linked to a G-unit at δC/ δH = 70.9/4.71 ppm; and Cα−Hα in β-O-4′ substructures (A)

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linked to an S-unit at δC/ δH = 71.8/4.83 ppm) are observed in all the samples.46

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However, phenylcoumaran substructures (Cγ−Hγ at δC/ δH = 63.4/3.7 ppm) were only

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observed in the standard Org_P. Resinol substructures (Cβ−Hβ in β−β′ resinol

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substructures at δC/ δH = 53.5/3.05 ppm and; Cγ−Hγ in β−β′ resinol substructures at

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δC/ δH = 71.0/3.81 and 4.17 ppm) were observed in Org_P, Org_E, [Ch][OAc]_P and

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[Ch][OAc]_E. 46, 48More resinol structures were observed in [Ch][OAc]_P than in

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[Ch][OAc]_E, which may support the previous statement about modification of

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softwood lignin, as one of the causes that limit enzyme digestibility.15 The decreased

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intensities in β-β’ linkages indicates that these linkages were modified, as it is unlikely

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that these C-C bonds have been truly broken and more likely they have been chemically

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altered.15 A substantial amount of carbohydrates were also appreciated, suggesting that

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they may be still bound to the lignin.15

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Recovery capacity of ILs

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To evaluate the recyclability of ILs, the FTIR-ATR spectra of fresh and recovered ILs

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are shown in Figure S7. It must be highlighted that in all the cases recovered ILs still

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contain relative amounts of water (up to 15 %). All the spectra obtained of the recovered

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IL used with pine and eucalyptus are analogous, so the recovery of the IL is independent

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of the hardwood/softwood kind of biomass. [OHEtAm][HCO2] spectra decreases the

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intensity of 1340 cm-1 band, assigned to C-N stretching, as well as 1589 cm-1 band

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(assigned to C-O-O asymmetric stretching), keeping all the bands.49 [OHEtAm][OAc]

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spectra keep all the bands, increasing the intensity of the band at 1658 cm-1, assigned to

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COO- asymmetric stretching. 50 [Mim][Cl] FTIR-ATR is also modified, decreasing the

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intensity of 1116 cm-1 band (assigned to N1-C5-H rock vibration) and broadening the



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band observed at 1450 cm-1 (CH3(C2) scissor vibration).51 The spectra observed for

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[Emim][OAc] remained unaltered and all the bands observed in the fresh IL were

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present in the recovered IL. The band at 1394 cm-1 (H-C-H rock vibration) was shifted

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due to the presence of water.52, 53 Although in different experiments the alteration of

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acetate anion is demonstrated, in this case, bands that confirm the presence of the

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acetate anion are kept (1005 cm-1 correspond to C-O stretching in the acetate anion and

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1174cm-1 corresponds to C=O stretching of the acetate anion).54 [Emim][DMP] spectra

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of fresh and recovered IL keep all the bands, highlighting some of them such as

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1180 cm-1 (N1-C2-H13 twist), or 784 and 742 cm-1 (P-H bend).52, 55 The spectra

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comparison of fresh and recovered [Amim][Cl] evinces that all the bands are still

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present and no apparent differences in the chemical structure of this IL are observed, as

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was previously observed in other pretreatments.56 In the case of [Ch][OAc], some bands

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decrease the intensity (at 1089 cm-1 CHn rocking and twisting, and at 1011 cm-1

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asymmetric stretching C3(CH3)N-CH2).57, 58 Other bands of the recovered [Ch][OAc]

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spectra increases the intensity at 1487 cm-1 (CH3 scissor vibration of choline) and at

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1475 cm-1 C-H stretching vibration of CH3. [Ch][Lys] spectra of raw and recovered IL

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keep the representative bands (1569 cm-1 due to the asymmetric stretching of the

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carboxylate C=O; 1475 cm-1 corresponding to C-H stretching vibration of CH3;

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1399 cm-1 corresponding to CO2 symmetric stretching; 1091 cm-1 corresponding to CHn

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rocking/twisting, and 955 cm-1 corresponding to C-N stretching).58 In the case of

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[Ch][Ser], all the bands are also kept (1573 cm-1 due to the asymmetric stretch of

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carboxylate function (C=O); 1475 cm-1 due to C-H stretching vibration of CH3; 955 cm-

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1

due to C-N stretching; and 1399 cm-1 due to CO symmetric stretching). 58-60


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The thermal decomposition temperature of every fresh and recovered IL is shown in

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Figure 3 and in the Supplementary material (Figure S8). The decomposition

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temperature of fresh ILs increase following the order:

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[OHEtAm][OAc] (174 ℃) < [Ch][Lys] (199 ℃) < [Ch][Ser] (218 ℃ ) < [Ch][OAc]

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(219 ℃) < [OHEtAm][HCO2] (245 ℃) < [Emim][OAc] (248 ℃) < [Mim][Cl] (265 ℃)

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< [Amim][Cl] (275 ℃) < [Emim][DMP] (308 ℃).

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Certain amounts of water are present in fresh and recovered IL (evinced by the loss of

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weight below 100 ℃ in all the cases). Recovered PILs [OHEtAm][HCO2] and

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[Mim][Cl] decreased their decomposition temperatures around 20 ℃, indicating the

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structure has changed or some compounds are being accumulated in the IL.

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Hemicelluloses degradation occurs in the range of temperatures of 150 - 260 ℃, so the

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accumulation of these compounds may be one of the causes of the decrease of

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decomposition temperatures.61 Furthermore, the global mass balance evinced that 32

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and 43 % of the initial hemicelluloses were removed from [OHEtAm][HCO2]_P and

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[OHEtAm][HCO2]_E, and 97 and 98 % of the initial hemicelluloses present in

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[Mim][Cl]_P and [Mim][Cl]_E were also extracted. Note that Tpeak of recovered IL used

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with eucalyptus was lower than the one used with pine, in the same way that

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hemicelluloses extracted with eucalyptus are higher than with pine. However, taking

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into account the FTIR-ATR results of PILs, IL degradation or partial evaporation has

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also occurred. Choline based ILs show Tpeak lower with the recovered IL than with the

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fresh IL. In the case of [OHEtAm][OAc] Tpeak is similar between fresh and recovered IL.

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However, DTG curves show two peaks corresponding to both ions of the IL.62 The

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second peak is sharper in recovered IL that in fresh [OHEtAm][OAc], which may

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indicate that ratios between cation and anion have changed after pretreatment.



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In the case of [Ch][OAc], Tpeak is lower, but the curve keeps the same sigmoid shape.

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In the case of [Ch][Lys], the DTG curve aspect change considerably. In this case, the IL

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degrade at different steps due to the decarboxylation of amino acid. While Tpeaks in the

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fresh IL are 199, 227, 253, 351 and 422 ℃, the degradation produced at 227 ℃ does

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not appear in the recovered IL, indicating that degradation step does not occur probably

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due to the degradation of this compound during the pretreatment. In the case of

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[Ch][Ser], Tpeak between fresh and recovered IL are similar and DTG curves keep the

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same aspect.

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AILs from the other side showed decomposition temperatures similar or in

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[Emim][OAc], higher in [Amim][Cl] and lower in [Emim][DMP]. The variation of

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degradation temperature produced in the recovered [Amim][Cl] and [Emim][DMP] is

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attributed to the presence of lignin impurities (lignin degradation occurs in the range

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260 - 400 ℃) accumulated in the ionic liquid, and hemicellulose.61, 63, 64 The results

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obtained are in accordance with the bands observed in the FTIR-ATR spectra, which

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keep all the representative bands. The higher biocompatibility of PILs and choline

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derived ILs contrast with low thermal stabilities.63 Biomass accumulation occurred in all

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the cases and may be solved by the precipitation of its constituents with the aid of an

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antisolvent, but the degradation observed will alter the pretreatment if the IL pretends to

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be reused. 47


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Figure 3. Tdecomp of raw and recovered ILs

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CONCLUSIONS

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In this work, 3 PILs, 3 AILs, and 3 choline derived ILs have been employed in

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softwood and hardwood biomass pretreatment. Results have demonstrated different

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reactivity and tendencies in softwood versus hardwood. While choline derived ILs may

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be considered a potential “greener” alternative in comparison with ILs in hardwood

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([Ch][OAc] yielded a digestibility of ̴70 %). Softwood digestibilities obtained with both

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PILs and choline derived ILs are < 55 %, considerably lower compared to the obtained

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with [Emim][OAc] (84 %). The catalytic effect was observed with the IL [Mim][Cl],

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while [OHEtAm][HCO2] barely pretreated the material despite its capacity to dissolve

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alkali lignin at room temperature. Confocal fluorescence microscopy points out that

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lignin reactivity with PILs and choline derive ILs is one of the main factors that difficult

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its utilization in softwood. Thermal degradation analysis shows Tpeak of recovered PILs

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and choline derived ILs is lower than in the fresh IL.

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ACKNOWLEDGMENTS



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This work was performed thanks to the financial support of the “Ministerio de

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Economía y Competitividad“, under the funded project CTQ2017-88623-R, the contract

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BES-2014-067788, and the “Juan de la Cierva” postdoctoral grant FJCI-2015-23765.

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Authors thank DuPont Industrial Biosciences for the donation of enzymatic cocktails.

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Graphical artwork was done thanks to the work of Oscar Notario. 2D-NMR experiments

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were developed in the technical facilities of Nuclear Magnetic Resonance and Electron

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Spin of the Complutense University of Madrid.

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ASSOCIATED CONTENT

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Supporting information

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Detailed materials and methods; summary of the ILs screening visual observations;

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micrograph and images of [OHEtAm][HCO2] + lignin mixtures; representative SEM and

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CFM micrographs, and 2D-NMR spectra; FTIR-ATR and DTg curves of recovered ILs.

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Figure 1. Enzymatic hydrolysis of a) Pine untreated and pretreated samples and b) Eucalyptus untreated and pretreated samples. Results are expressed according to Eq. S1 177x66mm (300 x 300 DPI)



Figure 2. a) Ratio surface lignin and enzymatic hydrolysis and; b) ratio surface lignin vs total lignin in pine (blue) and eucalyptus (red) 84x124mm (300 x 300 DPI)


Page 34 of 35

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Figure 3. Tdecomp of raw and recovered ILs 84x64mm (600 x 600 DPI)


Protic, aprotic and choline-derived ionic liquids: towards enhancing

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