Furfural production using ionic liquids: A review

Bioresource Technology 202 (2016) 181–191

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Review

Furfural production using ionic liquids: A review Susana Peleteiro, Sandra Rivas, José Luis Alonso, Valentín Santos, Juan Carlos Parajó ⇑ Chemical Engineering Department, University of Vigo (Campus Ourense), Faculty of Science, Polytechnical Building, As Lagoas, 32004 Ourense, Spain CITI (Centro de Investigación, Transferencia e Innovación), Universtity of Vigo, Tecnopole, San Cibrao das Viñas, 32900 Ourense, Spain

h i g h l i g h t s  Furfural is a renewable platform chemical with a bright future.  Technologies based on ionic liquids (ILs) are suitable for furfural production.  ILs can be used as additives, catalysts or reaction media for furfural manufacture.  Acidic ionic liquids may perform at once as reaction media and catalysts.

a r t i c l e

i n f o

Article history: Received 22 October 2015 Received in revised form 2 December 2015 Accepted 8 December 2015 Available online 15 December 2015 Keywords: Ionic liquids Lignocellulosic materials Biorefinery Hemicelluloses Furfural

a b s t r a c t Furfural, a platform chemical with a bright future, is commercially obtained by acidic processing of xylan-containing biomass in aqueous media. Ionic liquids (ILs) can be employed in processed for furfural manufacture as additives, as catalysts and/or as reaction media. Depending on the IL utilized, externally added catalysts (usually, Lewis acids, Brönsted acids and/or solid acid catalysts) can be necessary to achieve high reaction yields. Oppositely, acidic ionic liquids (AILs) can perform as both solvents and catalysts, enabling the direct conversion of suitable substrates (pentoses, pentosans or xylancontaining biomass) into furfural. Operating in IL-containing media, the furfural yields can be improved when the product is continuously removed along the reaction (for example, by stripping or extraction), to avoid unwanted side-reactions leading to furfural consumption. These topics are reviewed, as well as the major challenges involved in the large scale utilization of ILs for furfural production. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction 1.1. Importance of furfural Furfural (OC4H3CHO, also called 2-formylfuran, furan-2aldehyde, 2-furancarboxaldehyde, 2-furyl-methanal, pyromucic aldehyde, 2-furanaldehyde, 2-furancarbonal, carboxylic aldehyde, furan-2-carbaldehyde, furancarbonal, 2-furaldehyde, or 2-furfural), contains a heteroaromatic furan ring and an aldehyde functional group. Furfural was first isolated in 1832 by J.W. Döbereiner, and has been industrially produced since 1922. Today, furfural is used for multiple purposes, for example as a selective extraction agent (in the recovery of butadiene from oil steam cracking or in the refining of petroleum, diesel fuels, lubricants and vegetable oils), as a solvent (for anthracene or resins), as an agent for vulcanization, ⇑ Corresponding author at: Chemical Engineering Department, University of Vigo (Campus Ourense), Faculty of Science, Polytechnical Building, As Lagoas, 32004 Ourense, Spain. Tel.: +34 988387033; fax: +34 988387001. E-mail address: [email protected] (J.C. Parajó). http://dx.doi.org/10.1016/j.biortech.2015.12.017 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

as a nematicide and fungicide, as a flavoring agent in a variety of food products and alcoholic and non-alcoholic beverages, and as a component of commercial herbicides, insecticides, pesticides, antiseptics, disinfectors, and rust removers. Furfural is also involved in the manufacture of pharmaceuticals, cosmetics, fragrances, flavors and resins (in this latter case, by condensation with phenol, formaldehyde, acetone, or urea, to produce thermosetting resins with extreme physical strength); as well as in other products such as household cleaners and detergents. Furfural is a renewable platform chemical with a rich chemistry (Cai et al., 2014; Lange et al., 2012), suitable for yielding new families of bio-based, sustainable chemicals. Among these latter, furfuryl alcohol currently accounts for about 62% of the global furfural market or 75% of the USA market. Other important furfural-derived products are tetrahydrofurfuryl alcohol, furan, tetrahydrofuran, dihydropyran, acetylfuran, furfurylamine, and furoic acid (Zeitsch, 2000). Interestingly, furfural can also be converted into green fuels such as methylfuran, methyltetrahydrofuran, valerate esters, ethylfurfuryl and ethyltetrahydrofurfuryl

182

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

ethers, and C10–C15 coupling products. The aldol condensation with small ketones into larger compounds followed by selective deoxygenation (preserving C–C bonds while effectively breaking the C–O bonds) or hydrogenation followed by hydrodeoxygenation, acidcatalyzed rearrangement or etherification, acid-base-catalyzed coupling or metal-catalyzed decarbonylation yields products suitable as fuels. Levulinic acid can also be produced from furfural by furfuryl alcohol synthesis followed by acid hydrolysis. Based on factors such as manufacturing cost, market price and role as an intermediate for the production of other valuable chemicals, furfural was included among the top 30 added-value chemicals from biomass in a report commissioned by the US Department of Energy (Werpy and Petersen, 2004), which was further updated (Bozell and Petersen, 2010). These factors are boosting the demand, which is expected to double in the period 2014–2022 (DalinYebo, 2015). 1.2. Fundamentals of furfural production Furfural can be produced by dehydration of pentoses through a complex mechanism that involves a number of side reactions. Xylose has been the most studied substrate for furfural production, since it can be easily obtained by mild, selective, acidic processing of xylan-containing feedstocks (Peleteiro et al., 2015a). Xylan, characterized by a backbone made up of (1–4)-linked b-D-xylopyranosyl residues, is the most abundant hemicellulosic polymer. Native lignocellulosic materials contain complex xylans (in which the typical backbone presents a number of substituents, such as arabinosyl, glucopyranosyl uronic acid residues or its 4-O-methylated form, or esterified organic acids). The susceptibility of xylan to hydrolysis enables its selective separation from the rest of polymeric components of lignocellulose. For example, processes based on prehydrolysis or autohydrolysis–posthydrolysis (with hot, compressed water or steam) allow the production of hemicellulosic sugars from lignocellulosic feedstocks (Gullón et al., 2012), leaving a solid phase (mainly made up of cellulose and lignin) suitable for further utilization (for example, by cellulose hydrolysis or delignification). The hemicellulosic sugars obtained from typical xylan-containing lignocellulosics include xylose and minor amounts of arabinose, acetic acid and nonsaccharide components (for example, coming from extractives and acid-soluble lignin). Although arabinose is a potential substrate for furfural production, its potential contribution to commercial processes is usually neglected, owing to its low proportions in the usual lignocellulosic feedstocks and to its reaction rate (different than the one of xylose) (Cai et al., 2014). In aqueous media, xylose can either undergo retroaldol fragmentation into acids, aldehydes and ketones (Aida et al., 2010; Lange et al., 2012) or be converted into intermediates. According to a proposed mechanism, furfural formation would proceed through an anhydride (cyclic) intermediate (Antal et al., 1991): the C2 hydroxyl group of xylose is protonated and leaves the ring as water, generating a carbocation that forms a bond with the ring oxygen to give a reactive intermediate (2,5-anhydroxylose), which dehydrates into furfural (Enslow and Bell, 2012). Furfural generation by this mechanism was considered more favorable in mildly hot acidic solutions, based on quantum mechanical calculations supported by NMR data (Nimlos et al., 2006). A different pathway (acyclic intermediate) begins with xylose isomerization to its acyclic form and subsequent enolization (Zeitsch, 2000) or direct conversion to xylulose through hydride transfer (Enslow and Bell, 2012), with further furfural formation from the intermediate. Experimental evidence of furfural generation from xylulose (acting as an intermediate) has been confirmed for xylose dehydration in subcritical and supercritical water (Aida et al., 2010). Rasmussen et al. (2014) proposed that xylose could be converted into furfural

either by an acyclic direct mechanism or by a cyclic direct mechanism; but furfural could also be formed bypassing the xylulose formation step. Once furfural has been formed, consumption reactions take place, including self-coupling or resinification reactions with itself, with xylose (Cai et al., 2014; Weingarten et al., 2010) or with the intermediate, leading to the formation of dark, resinous, insoluble substances (humins) or soluble polymers. In addition, furfural can undergo fragmentation to form several smaller molecules, such as formic acid, formaldehyde, acetaldehyde, pyruvaldehyde, glyceraldehyde, glycolaldehyde and lactic acid. These secondary reactions limit the furfural yields, which also decline in media containing increased xylose concentrations owing to the enhanced participation of xylose-consuming, side reactions. In some cases (for example, when using biomass-derived substrates) furfural production is carried out in media containing not only pentoses, but also hexoses. In these situations, the dehydration of pentoses is accompanied by hexose dehydration (which results in the formation of 5-hydroxymethylfurfural, here denoted HMF, see Section 3.6), to yield both furfural and HMF. 1.3. Technological aspects regarding furfural production The current commercial technology for furfural production is based on the acidic processing of xylan-containing raw materials in aqueous media. Oat hulls were the raw material employed in the first commercial process (Quaker Oats, USA); whereas corncobs and bagasse are currently used as feedstocks. Corncobs, oat hulls, cottonseed hull bran, almond husks, and bagasse are typical substrates with favorable composition for furfural production (Zeitsch, 2000). In the reaction media, xylan is first hydrolyzed to xylose, which is dehydrated to furfural, yielding the multiple byproducts cited above. This approach shows a number of drawbacks, including:  limited furfural yields (usually, within the range 45–55% of the stoichiometric one), owing to undesired side reactions. The participation of multiple biomass fractions in the side-reactions involving furfural production explains (at least, in part) the decreased furfural yields obtained when native biomass or complex saccharide mixtures are employed as substrates instead of pure xylose,  high energy consumption, owing to the huge amount of steam needed for both heating and furfural stripping,  equipment corrosion, caused by the mineral acid used as a catalyst,  impractical catalyst recovery, owing to the cost and inefficiency of downstream processing, which usually involve neutralization and disposal of sludges,  environmental hazards (for example, the management of the solid residues from processing is an important problem),  lack of valuable co-products. On the basis of these ideas, the need for novel, ecofriendly catalytic processes for furfural production has been pointed out, and a number of alternatives have been proposed: for example, furfural decomposition can be reduced by removing it from the reaction medium along the reaction by conventional distillation, stripping or flashing (Brownlee, 1938; Cai et al., 2014; Fitzpatrick, 2006; Hayes et al., 2008; Mandalika and Runge, 2012; Marcotullio and de Jong, 2011; Zeitsch, 2000); or by using biphasic reaction media (made up of a conversion phase where furfural is generated, and an insoluble organic phase to which furfural is selectively transferred, avoiding further decomposition) (Amiri et al., 2010; Campos Molina et al., 2012; Chheda et al., 2007; Ma et al., 2014; Rivas et al., 2013; Weingarten et al., 2010; Wettstein et al., 2012; Xing

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

et al., 2011; Yang et al., 2012). Other ways explored for improving the furfural yields include the utilization of new catalysts or catalytic mixtures, (Aellig et al., 2015; Antunes et al., 2012; Bhaumik and Dhepe, 2014; Choudhary et al., 2011; Dias et al., 2006, 2007) or combinations of some of the above operational strategies (for example, utilization of homogeneous or solid acid catalysts coupled with product removal or stripping) (Lessard et al., 2010; Li et al., 2014; vom Stein et al., 2011). On the other hand, the whole process of furfural production would be improved by implementing the ‘‘biomass refinery” concept: native lignocellulosic substrates can be fractionated into their major components (hemicellulose, cellulose and lignin), and the various fractions could then be used separately for specific purposes (including furfural production from the hemicellulosederived fraction) (Gullón et al., 2012). The interest in obtaining multiple commercial products from a given lignocellulosic feedstock in biorefineries operating under the principles of the green chemistry (for example, concerning the integral utilization of the considered raw material and the limitation of waste generation) has been highlighted in literature (Peleteiro et al., 2015b).

2. Ionic liquids: general aspects 2.1. Chemical nature and properties of ILs Ionic liquids (ILs) are salts composed of large organic cations and inorganic or organic anions, which differ from molecular solvents by their unique chemical nature, structure, organization, and properties. As a result, ILs offer a unique environment for chemistry, biocatalysts, separation, material synthesis, and electrochemistry (Niknam and Damya, 2009). The most important properties of typical ILs employed for furfural production include:  very low volatility, enabling operation at high temperature in conventional equipment with limited emissions, and facilitating the recovery of volatile reagents and products,  good dissolving capacity (Long et al., 2011),  green character,  chemical and thermal stability,  non-flammability, non-toxic nature and recyclability,  ‘‘tunable” or ‘‘designable” character, because their physical properties (for example, melting point, viscosity, density, solubility, and acidity/coordination properties), can be tuned according to different reactions or processes by selecting the appropriate combination of cation and anion,  improved conversion and selectivity, leading to improvements with respect to energy savings when compared to conventional solvents (Stark, 2011). The decreased energy consumption results from the improved catalytic ability, and/or by the implementation of efficient processing technologies (such as microwave- or ultrasound- assisted processing). Moreover, the ionic character of the ILs can play a positive role in enhancing the catalytic activity and the reaction selectivity (OlivierBourbigou et al., 2010), as well as in stabilizing the reactive catalytic species and/or the reaction intermediates (Zhang et al., 2011). ILs with strong Brönsted acidity (acidic ionic liquids, AILs) present catalytic activity, and may perform as multifunctional compounds, playing different roles at the same time (e.g. as solvents, ligands, catalysts, or stabilizing agents for catalysts or intermediates). The above properties confer to ILs favorable properties as agents for accomplishing a number of biorefinery goals (Peleteiro et al., 2015b).

183

2.2. ILs employed for furfural production ILs can be classified according to their cations. The most common ILs contain quaternary ammonium, N-alkylpyridinium or methylimidazolium cations. Imidazolium ILs are the most suited for furfural production, and most literature is focused on them. Fig. 1 shows the general formula of the imidazolium-type ILs, the nomenclature employed in this study and the specific ILs of this type employed for furfural production. Non-imidazolium ILs employed for furfural production cited in this article include pyridinium-type ILs (1-(4-sulfonylbutyl)pyridinium tetrafluoroborate and 1-(4-sulfonylbutyl)pyridinium methanesulfonate, denoted [SbPy]BF4 and [SbPy]MeSO3, respectively), and triethylammoniumtype ILs (1-(4-sulfonylbutyl)triethylammonium methanesulfonate and 1-(4-sulfonylbutyl)triethylammonium tetrafluoroborate, denoted [Sbe3N]MeSO3 and [Sbe3N]BF4, respectively).

3. Applications of ILs for furfural manufacture ILs can play a number of roles in processes dealing with furfural manufacture, including:  acidic catalysts for pentose dehydration in aqueous media, eventually in the presence of organic co-solvents,  additives for improving the furfural yields in reaction media made up of xylose or xylan, an organic solvent and externallyadded acidic catalyst(s), eventually in the presence of co-catalyst(s),  reaction media for furfural manufacture from pentoses, higher saccharides made up of pentoses, or pentosans (pure or present in native lignocellulosic substrates), eventually in the presence of co-solvents, either using externally-added catalysts or (in the case of AILs) catalyzed by the solvent. It can be noted that AILs enable the design of simple and efficient processes owing to the simplified recovery of solvent and product. Fig. 2 shows some of the above possibilities, whereas the next sections summarize representative data reported on these topics.

3.1. Utilization of ILs as catalysts for pentose dehydration in aqueous media As explained before, furfural production from suitable pentoses in aqueous, acid-catalyzed media involves both dehydration (loss of 3 mol of water/mol sugar) and a number of side reactions, which limit the target product yield. ILs with Brönsted acidity (such as [Sbmim]HSO4, [bmim]H2PO4, [SbPy]BF4, [SbPy]MeSO3, [Sbe3N] MeSO3, or [Sbe3N]BF4) have been employed as catalysts for furfural production in aqueous media (Tao et al., 2011; Serrano-Ruiz et al., 2012). In order to improve the target product yield, operation was carried out in biphasic systems (using an organic co-solvent such as tetrahydrofuran THF or methyl isobutyl ketone MIBK), in a way that the furfural generated in the aqueous phase was continuously transferred to the co-solvent, limiting the effects of furfural-consuming reactions taking place in the aqueous phase. The studies reported on this topic (see data in Table 1) cover a broad range of experimental conditions (particularly regarding the loadings of substrate and co-solvent). Interestingly, operating at comparatively high substrate charges, high furfural yields (up to 85% and 91.4% of the stoichiometric value) were achieved in media containing pyridinium and imidazolium-based ILs, respectively. As an important experimental trend, it can be highlighted that increased water proportions resulted in decreased furfural yields (Tao et al., 2011).

184

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

GENERAL NOMENCLATURE GENERAL FORMULA

R

N

N+

CH3

[Rmim]A A Anion

Substituent Methyl imidazolium cation (mim)

Type of R and nomenclature Butyl, b Ethyl, e 1-(4-sulfonic acid)butyl, Sb

IMIDAZOLIUM-TYPE ILs CITED IN THIS STUDY

Type of A− and nomenclature Chloride, ClBromide, BrDihydrogen phosphate, H2PO4Hexafluorophosphate, PF6 − Hydrogen sulfate, HSO4 − Methanesulfonate, MeSO3 −

[bmim]Cl [emim]Cl [Sbmim]HSO4 [bmim]Br [emim]HSO4 [bmim]PF6 [bmim]H2PO4 [bmim]MeSO3 [bmim]HSO4 Fig. 1. General formula and nomenclature of the imidazolium-based ILs cited in this study.

Fig. 2. Possible applications of ILs in biorefinery schemes based on furfural production from hemicelluloses.

3.2. Utilization of ILs as additives The potential stabilization of catalysts, products and reaction intermediates caused by ILs makes them potentially useful as additives for reaction media. Binder et al. (2010) studied the production of furfural from xylan or xylose in N,N-dimethylacetamide (DMA) catalyzed with chromium halides (CrCl2 or CrCl3) or HCl, and assessed the benefits derived from supplementing the media with LiCl and ILs ([emim]Cl] or [bmim]Br). Limited xylan conversions into furfural (up to 8%) were achieved operating for 2 h at 140 °C and 5% substrate charge in media containing LiCl and [emim]Cl, whereas significantly higher yields (up to 45% or 55%) were obtained when xylose was treated at 100 °C for the optimal time at 10% substrate charge in media containing [emim]Cl or [bmim] Br as additives.

3.3. Utilization of ILs as reaction media for converting pentoses into furfural in the presence of externally-added catalysts ILs show potential as solvents for furfural manufacture. Furfural can be produced from a number of monomeric substrates in ILs free from water at increased selectivity, since the decomposition reactions involving furfural rehydration are prevented. Xylose has been the most studied substrate for furfural production, even if other pentoses (such as arabinose, ribose or lyxose) are also suitable furfural precursors (Heguaburu et al., 2012; Peleteiro et al., 2014a; Zhang et al., 2014). Furfural can be produced from pentoses in IL media under mild conditions (particularly, at moderate temperatures and short reaction times) in the presence of one or several catalysts. Studies have been reported on the utilization of [emim]Cl, [bmim]Cl or [bmim]PF6

185

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191 Table 1 Results reported for furfural production from xylose in biphasic media containing xylose, water, an organic co-solvent and an acidic IL acting as a catalyst. Medium/co-solvent

IL (catalyst)

Conditions

% Conv. into furfural

Reference

Water/MIBK Water/MIBK Water/THF Water/THF Water/THF Water/THF

[Sbmim]HSO4 [bmim]H2PO4 [SbPy]BF4 [SbPy]MeSO3 [Sbe3N]BF4 [Sbe3N]MeSO3

60–150 °C, 5–60 min, 1 g xylose/1.5 mL water/8 mL co-solvent 150 °C, 25 min, 1 g xylose/1.5 mL water/8 mL co-solvent 150–180 °C, 30–60 min, 0.1 g xylose/1 g water/2 g co-solvent 150 °C, 60 min, 0.1 g xylose/1 g water/2 g co-solvent 150 °C, 60 min, 0.1 g xylose/1 g water/2 g co-solvent 150 °C, 60 min, 0.1 g xylose/1 g water/2 g co-solvent

13.9–91.4 68 55–85 36 40 28

Tao et al. (2011) Tao et al. (2011) Serrano-Ruiz et al. Serrano-Ruiz et al. Serrano-Ruiz et al. Serrano-Ruiz et al.

(2012) (2012) (2012) (2012)

Table 2 Results reported for furfural production from pentoses in ILs. SubstraTE

IL/(co-solvent)

Externally added catalyst(s)

Conditions

Conversion into furfural,%

Reference

Lyxose

[bmim]Cl

Solid acid catalyst (resin)

100 °C, 180 min, 10 g subs./100 g medium

75

Ribose

[bmim]Cl

Solid acid catalyst (resin)

100 °C, 180 min, 10 g subs./100 g medium

90

Ribose

[bmim]PF6

H2SO4 + MnCl2 or PEG-OSO3Ha + MnCl2

120 °C, 18 min, 0.075 g subs./2 mL IL

59–67

Arabinose

[bmim]Cl

CrCl3

160 °C, 5–600 min, 10 g subs./100 g IL

0.2–15.5

Arabinose

[bmim]Cl

100 °C, 180–360 min, 10 g subs./100 g medium

6–92

Arabinose

[bmim]PF6

CrCl3 or solid acid catalyst or solid acid catalyst + metal halide H2SO4 + MnCl2 or PEG-OSO3H + MnCl2

120 °C, 18 min, 0.075 g subs./2 mL IL

63–72

Xylose

[bmim]Cl

H2SO4

120 °C, 0–240 min, 1 g subs./20 g IL

3–13

Xylose

[bmim]Cl

CrCl3

100–160 °C, 5–600 min, 10 g subs./100 g IL

1–50

Xylose

[bmim]Cl

AlCl3

160 °C, 0.5–2 min, 38.3 mg subs/2 g IL

66–82

Xylose

[bmim]PF6

PEG-OSO3Ha

120 °C, 18 min, 0.075 g subs./2 mL IL

65

Xylose Xylose

[bmim]Cl [bmim]Cl

80–140 °C, 30–150 min, 0.2 g subs./2 g IL 100 °C, 360 min, 10 g subs./100 g medium

8.2–21 17–59

Xylose

[bmim]Cl

Solid acid catalyst (lignosulfonic acid) CrCl3 or solid acid catalyst (resin) with or without CrCl3 Solid acid catalyst (resin/other)

130–180 °C, 1.5–10 min, 38.3–130 mg/2 g IL

48–84.2

Xylose

[bmim]PF6

120 °C, 18 min, 0.075 g subs./2 mL IL

18–75

Xylose

Not usedd

100 °C, 240 min, 0.03 g subs./0.3 mL IL/0.7 mL co-solvent 100 °C, 30 min, 100 g subs./ L IL

44

Xylose

[bmim]Cl/ (Toluene) [emim]HSO4

H2SO4 + MnCl2 or PEG-OSO3H + metal halidesb H2SO4

Xylose

[bmim]HSO4

Not usedd

100–140 °C, 10–500 min, 10 g subs./100 g IL

0.6–36.7

100–140 °C, 15–480 min, 10 g subst./100 g IL/ 2.2–4.4 g co-solvent 140 °C, 30–360 min, 10 g subst./100 g IL/440 g co-solvent 140 °C, 30–360 min, 10 g subst./100 g IL/440 g co-solvent 100–120 °C, 30–360 min, 33–167 g subs./L IL/ 2.33 L co-solvent/L IL

3–73.8

Heguaburu et al. (2012) Heguaburu et al. (2012) Zhang et al. (2014) Peleteiro et al. (2014a) Heguaburu et al. (2012) Zhang et al. (2014) Sievers et al. (2009a) Peleteiro et al. (2014a) Zhang et al. (2013b) Zhang et al. (2014) Wu et al. (2014) Heguaburu et al. (2012) Zhang et al. (2013a) Zhang et al. (2014) Lima et al. (2009) Lima et al. (2009) Peleteiro et al. (2015c) Peleteiro et al. (2015a) Peleteiro et al. (2015c) Peleteiro et al. (2015c) Lima et al. (2009)

Xylose Xylose Xylose Xylose a b c d e

[bmim]HSO4/ (Toluene) [bmim]HSO4/ (MIBK) [bmim]HSO4/ (Dioxane) [emim]HSO4/ (Toluene)

Not used

d

Not used

d

Not usedd Not usedd

c

62e

26.5–80.3 32.5–82.2 33–84

PEG-OSO3H: polyethylene glycol-bound sulfonic acid. Metal halides: NaF, KF, LiCL, NaCl, KCl, ZnCl2, CuCl2, CuCl, FeCl3, AlCl3, SnCl2, SnCl4, MnCl2, BiCl3, NiCl2, BaCl2, CoCl2, CdCl2, CrCl3. Depending on the catalyst charge. The acidic ionic liquid performs as a solvent and as a catalyst. Furfural was continuously stripped from the reaction medium.

as solvents, supplemented with metal halides, solid acid catalysts, Brönsted acids (such as HCl, H2SO4 or polyethylene-bound sulfonic acid) or mixtures of them (Heguaburu et al., 2012; Peleteiro et al., 2014a; Sievers et al., 2009a; Wu et al., 2014; Zhang et al., 2013a,b; Zhang et al., 2014). Table 2 shows the operational conditions assayed for this approach, as well as the furfural yields obtained. Although the results cannot be assessed on a strictly comparative basis (owing to the diversity of operational conditions, and particularly, to the very different loadings of substrates and catalyst, which are strongly influential on the operational results), it can be highlighted that high yields (in the range 82–92%) have been reported

for operation with metal halides and solid acid catalysts alone or in combination (Heguaburu et al., 2012; Zhang et al., 2013a, 2013b). The mechanism of pentose dehydration in IL media follows the same general principles already described for aqueous media: furfural generation occurs through a complex set or reactions, involving (at least) productive generation of intermediates from the substrate, substrate losses by unwanted reactions, conversion of productive intermediates into furfural, and side-reactions involving furfural consumption that lead to a number of unwanted reaction byproducts (including organic acids and humins). Furfural

186

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

formation from xylose in IL media has been reported to proceed through the isomerization into xylulose (Binder et al., 2010; Enslow and Bell, 2012; Zhang et al., 2014). Operating in [bmim]Cl catalyzed with an acidic ion exchange resin, substrates such as arabinose, ribose, or lyxose were more reactive than xylose, a fact ascribed to the different reaction intermediate involved in furfural generation (ribose and arabinose dehydration proceed through the formation of ribulose, which enables higher furfural yields than xylulose, the intermediate involved in the dehydration of xylose and lyxose) (Heguaburu et al., 2012). However, operating in [bmim]Cl catalyzed with CrCl3, the furfural yield from arabinose was significantly lower than the one from xylose operating at the same catalyst loading (Peleteiro et al., 2014a). Substrate concentrations above a given threshold resulted in decreased product yields, resulting from the increased participation of furfural-consuming reactions (including xylose coupling with xylose-to-furfural intermediates, self-coupling furfural/resinification reactions, and reaction between furfural and either xylose or xylose-to-furfural intermediates). Xylose fragmentation has also been considered in the kinetic modeling of xylose dehydration/degradation in catalyzed [bmim]PF6 (Zhang et al., 2014). Increased formation of humins in IL media under harsh conditions has been reported frequently, but the results are strongly dependent on the operational conditions. Since furfural shows a surprising stability in [bmim]Cl/sulfuric acid media (Sievers et al., 2009a), it has been concluded that other compounds must participate in humin formation. Decreased furfural yields have been ascribed to coupling loss reactions (Binder et al., 2010; Enslow and Bell, 2012). Oppositely, Wu et al. (2014) found that humin generation was suppressed when xylose was reacted in [bmim]Cl using a solid acid catalyst (lignosulfonic acid) derived from biomass. Furfural generation from xylan entails its prior hydrolysis to xylose, whereas when starting from native lignocellulosic materials, reactive species derived from fractions different from xylan can be present in the medium and contribute to furfural consumption. Fig. 3 summarizes some of the above ideas in a simplified mechanism. The dielectric properties of ILs permit an efficient bulk heating by microwave absorption (Lima et al., 2011). The ‘‘microwave effects” are responsible for an efficient internal heating or in-core volumetric effect heating by direct coupling of microwave energy with a number of types of molecules present in the reaction mixture (solvent, reagent or catalyst); together with possible decrease of the activation energy and/or increase of the pre-exponential factor in the Arrhenius law due to an orientation effect of polar species in an electromagnetic field (Zhang and Zhao, 2010). As a result, high furfural yields can be achieved at very short reaction times (seconds or minutes, in comparison with several hours for typical experiments performed with conventional heating), enabling energy savings.

The utilization of co-catalysts has been explored as a way for optimizing the production of furfural in ILs. A typical approach is the combination of a metal halide (behaving as a Lewis acid, which facilitates the isomerization of xylose into xylulose) and a Brönsted acid (which facilitates the dehydration of xylulose into furfural (Zhang et al., 2014). The results in Table 2 show that this approach can result in high furfural yields from ribose, arabinose and xylose (Heguaburu et al., 2012; Zhang et al., 2014). The utilization of toluene as a co-solvent for increasing the furfural yields in [bmim]Cl catalyzed with H2SO4 has been considered as a potential way to achieve two different goals: limitation of the possible unwanted, furfural-consuming side-reactions that take place in the IL phase, and improved product recovery (Lima et al., 2009). However, the furfural conversion achieved (44%, see Table 2) is clearly below the ones reported for alternative strategies. 3.4. Conversion of xylose into furfural using acidic ionic liquids AILs, characterized by their strong Brönsted acidity, can play the roles of a reaction media and a catalyst at once. AILs have been reported to combine the advantages of both homogenous and heterogeneous catalysts, such as high acid density, uniform catalytic active centers, and easy separation from the media (Long et al., 2011). Since no externally-added catalyst are necessary, AILs offer the advantage of avoiding typical problems of catalyzed systems, for example the ones coming from pollution, catalyst separation and recycling. Both [bmim]HSO4 and [emim]HSO4 have been employed for furfural production from xylose in the presence or absence of cosolvents (see Table 2). In media containing xylose and [bmim] HSO4 or [emim]HSO4 as sole components, maximum furfural yields of 36.7–62% have been achieved (Lima et al., 2009; Peleteiro et al., 2015c), even if this latter result was obtained with continuous product stripping. The furfural yields were significantly better (optimal results in the range 73.8–84%) when a co-solvent (toluene, MIBK or dioxane) was added to the reaction medium (Lima et al., 2009; Peleteiro et al., 2015a,c). Using this approach, most furfural generated in the AIL phase was transferred to the co-solvent, limiting the product losses, and enabling multiple effects (generation, separation and purification) in one-pot operation. Details on the operational conditions are provided in Table 2. 3.5. Utilization of ILs as reaction media for furfural production from pure xylan The kinetic mechanism of furfural production from purified xylan is closely related to the one discussed for xylose, beyond

Fig. 3. Simplified reaction mechanism for furfural production in ILs.

187

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

the need of hydrolyzing first xylan into xylose. This additional reaction requires the participation of some water (1 mol water/ mol anhydroxylose unit in xylan), whereas the overall mechanism also involves water generation upon xylose dehydration (which is likely to provide the main water source for additional hydrolysis) (Sen et al., 2012). Improvements can be obtained when the initial amount of water is kept to a minimum, and/or additional water is added gradually along the reaction period, in order to limit the formation of unwanted furfural rehydration products. Zhang et al. (2013b) proposed a mechanism for furfural production from xylan in [bmim]Cl–AlCl3 media based on the formation of complexes that weaken the glycosidic bonds, facilitating the hydrolysis of xylan to xylose, which is further dehydrated into furfural. Furfural production from xylan has been assayed in [emim]Cl or [bmim]Cl in absence or presence of homogeneous catalysts (Brönsted acids, Lewis acids or mixtures of both) or solid acid catalysts (Binder et al., 2010; Zhang and Zhao, 2010; Zhang et al., 2013a, 2013b). Table 3 summarizes the experimental conditions employed in these studies, as well as the corresponding product yields. Utilization of uncatalyzed ILs resulted in limited conversions into furfural (3.6–18%), whereas high optimal conversions (in the range 79–93.7%) were achieved in microwave-heated, catalyzed [bmim]Cl. Experiments were performed with AlCl3, combined catalysts or solid acid catalysts (Zhang et al., 2013a,b), all of them using limited substrate charges. In general, the furfural yields from xylan in [bmim]Cl media were lower than those obtained with xylose, a fact ascribed to the difference in solubility of xylose and xylan (Zhang et al., 2013b). The near quantitative yield obtained using the [bmim]Cl-H3PW12O40 system (higher than the one determined for xylose under the same conditions) was justified on the basis that the decreased xylose concentrations in the medium, which limited the influence of cross-polymerization reactions between furfural and xylose and/or the reactions between furfural and the intermediates (Zhang et al., 2013b). Regarding the general patterns of the effects caused by influential operational variables, the optimal furfural yields from xylan were achieved at defined catalyst loadings, and the furfural yields decreased rapidly when the initial xylan concentration increased. As observed for xylose, harsh operational conditions resulted in increased humin formation. Finally, the AIL-co-solvent approach ([emim]HSO4 – toluene) was assayed using xylan as a substrate (Lima et al., 2009). Under mild conditions (100 °C), 29 wt% yield (corresponding to near 40% xylan conversion into furfural) was achieved after 4 h. 3.6. Furan production from lignocellulosic materials: general aspects ILs have been used for a number of biomass-related applications, for example as pretreatment agents for the enzymatic hydrolysis of cellulose, as fractionation agents (to recover one or several of the structural components of native lignocellulosic

materials), as solvents for biomass dissolution or delignification, or as reaction media for cellulose derivatization. These research subjects are out of the scope of this article. ILs have been considered as a very attractive media to perform reactions involving solid biomass, due to their ability to overcome the physical and biochemical barriers responsible for the recalcitrant lignocellulose structure (Li et al., 2008; Serrano-Ruiz et al., 2012). In particular, ILs containing anions with strong hydrogen bond basicity can effectively weaken the hydrogen bond network of the biomass polymers, allowing the alteration of the physicochemical properties of the macromolecular components (including reduction of the cellulose crystallinity, and disruption of the hydrogen bond network) (Carvalho et al., 2015). The improved accessibility of the glycosidic bonds caused by ILs allows the hydrolysis reaction to take place at considerably lower temperatures than in aqueous media under comparable conditions (Sievers et al., 2009a). Additionally, the presence of ILs may facilitate biomass depolymerization, enhancing the yields of dehydration products by stabilizing the reactive intermediates and/or the active catalytic species (Zhang et al., 2011). The chemical saccharification of polysaccharides (hemicelluloses and/or cellulose) in ILs has been reported. Both native lignocellulosics and model compounds (xylan or cellulose) have been used as substrates to produce monosaccharides (reducing sugars from hemicelluloses, glucose from cellulose or both). This topic is closely related to the subject of the present study, but it cannot be reviewed in deep here owing to space limitation. However, just as complementary information, it can be highlighted that representative saccharification studies in catalyzed ILs or in AILs, in the presence or absence of co-solvents, have been reported by Binder and Raines (2010), Brandt et al. (2011), Carvalho et al. (2015), da Costa Lopes et al. (2013a), Enslow and Bell (2012), Li et al. (2008), Sen et al. (2012), Sievers et al. (2009b) and Stark (2011). Even if furfural has been occasionally cited as a reaction byproduct in these studies, its production has not been optimized, and the reported information is not comprehensively considered in the following discussion. One-pot processing of suitable native lignocellulosic materials in ILs enabling the simultaneous hydrolysis and dehydration of hemicellulose and cellulose would result in the co-generation of furfural and HMF from xylan and cellulose, respectively. This idea would allow the integration of dissolution, fractionation, hydrolysis and/or conversion of biomass in a single step, depicting an attractive operation scheme from both economic and technological points of view, and enabling the synthesis of processes that could be greener and more sustainable and than the traditional ones (da Costa Lopes and Bogel-Lukasik, 2015). However, this approach is hindered by the comparatively slower reaction of cellulose, resulting mainly from its crystalline nature and higher polymerization degree. As a consequence, furfural can be substantially degraded under the optimal conditions for HMF production from cellulose.

Table 3 Results reported for furfural production from pure xylan.

a b

Medium

External catalyst(s)/(co-solvent)

Conditions

% Conversion into furfural

Reference

[bmim]Cl [bmim]Cl [emim]Cl [emim]Cl [bmim]Cl [bmim]Cl [bmim]Cl [emim]HSO4

Not used/not used Not used/not used HCl/not used CrCl2 + HCl/not used CrCl3/not used AlCl3/not used Solid acid catalystb/not used Not used/(toluene)

Temperature n.a.a, 2 min, 100 mg subs./2.0 g IL 150 °C, 2.5 min, 38 mg subs./2 g IL 140 °C, 120 min, 5 g subs./100 g medium 140 °C, 120 min, 5 g subs./100 g medium Temperature n.a.a, 2 min, 100 mg subs./2.0 g IL 140–170 °C, 0–10 min, 38 mg subs./2 g IL 140–180 °C, 1–10 min, 38–130 mg subs./2.0 g IL 100 °C, 240 min, 100 g subs./L IL

18 3.6 2 11–25 63 53 8–84.8 37–93.7 39.7

Zhang and Zhao (2010) Zhang et al. (2013b) Binder et al. (2010) Binder et al. (2010) Zhang and Zhao (2010) Zhang et al. (2013b) Zhang et al. (2013a) Lima et al. (2009)

Temperature not available, heating conditions: microwave irradiation at 400 W for the considered reaction time. Solid acid catalyst: resin (Amberlyst or NKC9) or H3PW12O40.

188

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

Alternatively, HMF and furfural can be co-produced in one-pot reaction when softwoods are employed as raw materials, since their hemicellulose fractions contain both xylan and polymers made up of hexoses (glucomannan, galactoglucomannan or related components), which can yield the two types of furans upon hydrolysis–dehydration. Even if the susceptibility of hemicellulosic polymers made up of pentoses or hexoses to this type of reactions is different, furfural and HMF can be obtained at significant yields operating under compromise conditions (see below). 3.7. Production of furfural from xylan-rich native substrates The furfural yields from biomass reported for experiments using externally catalyzed ILs or AILs were remarkably dependent on both the type of raw material and the operational conditions. No furfural was obtained when wheat straw was treated for 5 h at 125 °C in uncatalyzed [emim]Cl (van Spronsen et al., 2011), but the addition of acetic acid as a catalyst increased the furfural yield up to 14% (see Table 4). Lower furfural production was achieved in the same medium and conditions starting from pine wood, a more lignified substrate. ILs such as [emim]Cl and [bmim]PF6 catalyzed with combinations of Brönsted and Lewis acids allowed furfural yields in the range 22–36% starting from susceptible or lignified raw materials (corncob, switchgrass, straw, corn stover or woods) (Binder et al., 2010; Zhang et al., 2014). Details on the operational conditions are provided in Table 4. Alternatively, the catalytic activity of selected AILs makes them suitable for the direct hydrolysis–dehydration of biomass polysaccharides. The AILs [bmim]HSO4 and [bmim]MeSO3 have been employed to produce furfural from Miscanthus at 33% and 13% furfural yields, respectively (Brandt et al., 2011). As a general trend, the furfural yields obtained when processing native feedstocks were lower than the ones achieved when starting from pure xylose or xylan under comparable reaction conditions, a fact ascribed to the negative influence of the lignin–cellulose–xylan network on the whole furfural production process and to the complex composition of the reaction media (which contain biomass-derived compounds different from xylan responsible for negative effects on yield, catalysts and selectivity). 3.8. Combined production of furfural and hydroxymethylfurfural from native lignocellulosic materials As explained before, the coproduction of furfural and HMF from hemicelluloses may take place when softwoods are used as raw materials, since they possess hemicellulosic polymers made up of pentoses and hexoses. On the other hand, some studies have reported on the co-production of furfural and HMF from feedstocks having xylan-type hemicelluloses, such as corn stalks, corncobs, grass, straw or bagasse (see Table 5); evidencing that some

cellulose has been hydrolyzed into glucose and further dehydrated in the reaction media. As xylan is more susceptible to hydrolysis– dehydration than polymers made up of hexoses, the optimal furfural yields were achieved under milder conditions than those leading to the highest HMF production (i.e., furfural was already consumed in part when the optimal conditions for HMF production were achieved). The results in Table 5 show the yields of furfural and HMF achieved in [bmim]Cl operating in the presence of externally added catalysts (solid acid catalysts, Brönsted or Lewis acids, or mixtures of them). Limited yields of furans from pine wood were obtained when CF3COOH was employed as a catalyst (Sievers et al., 2009b), whereas higher yields of both furfural and HMF (up to 33.6% and 24.2%, respectively) were obtained when the same raw material was processed in [bmim]Cl – AlCl3 (Zhang et al., 2013b). These results are better than the ones reported for media containing solid acid catalysts alone or in combination with AlCl3 (Zhang et al., 2013a). In comparative terms, high HMF yields (up to 52%) were obtained using [bmim]Cl catalyzed with CrCl3 (Zhang and Zhao, 2010), revealing that cellulose was significantly involved in the reactions. Intermediate furan yields were obtained when less lignified materials were processed in [bmim] catalyzed with a strong mineral acid (Zhang et al., 2013b). Alternatively, the AIL [bmim]HSO4 enabled a selective hydrolysis–dehydration of xylan, achieving up to 36.2% furfural yield with scarce HMF generation. The major drawback of the approaches listed in Table 5 is the low substrate charges (it can be noted that 10% substrate loading was employed just in one study), particularly when considering that the true substrates for reaction (polysaccharides) account just for a fraction of the feedstock weight. 3.9. Production of furans from crude solutions obtained by fractionation of native lignocellulosic feedstocks The partial dissolution of solid biomass in ILs requires extensive washing to recover the IL retained by the treated solids. IL recycling is facilitated when the reaction media contain substantial amounts of water, but this approach is not feasible in many cases owing to the negative effect of water on furfural stability. To improve furan production in ILs, an alternative approach has been envisaged: the raw material can be first fractionated (in catalyzed or uncatalyzed aqueous media) to yield a solution rich in soluble hemicellulose-derived saccharides (including low molecular polymers, oligomers and/or sugars), which are then converted into furfural in a separate stage involving the utilization of ILs. For this purpose, hydrothermal processing (or autohydrolysis) is especially interesting, since this technology enables the selective removal of hemicelluloses and has lower operating and capital costs than conventional prehydrolysis (Archambault-Léger et al., 2015; Kazi et al., 2010; Kumar and Murthy, 2011). On the other hand, the selective removal of hemicelluloses before processing with ILs is interesting

Table 4 Selective production of furfural from native lignocellulosic material using ILs.

a

Substrate

Medium

External catalyst(s)

Conditions

% Conv. into furfural

Reference

Wheat straw Pine wood Corncob Switchgrass Straw Corn stover Corn stover Pine wood Poplar wood Miscanthus giganteus Miscanthus giganteus

[emim]Cl [emim]Cl [bmim]PF6 [bmim]PF6 [bmim]PF6 [emim]Cl [bmim]PF6 [bmim]PF6 [bmim]PF6 [bmim]HSO4 [bmim]MeSO3

CH3COOH CH3COOH PEG-OSO3H + MnCl2 PEG-OSO3H + MnCl2 PEG-OSO3H + MnCl2 HCl + CrCl2 PEG-OSO3H + MnCl2 PEG-OSO3H + MnCl2 PEG-OSO3H + MnCl2 Nonea Nonea

100–125 °C, 300 min, 1 g subs./20 g IL 100–125 °C, 300 min, 1 g subs./20 g IL 120 °C, 18 min, 0.3 g subs./2 mL IL 120 °C, 18 min, 0.3 g subs./2 mL IL 120 °C, 18 min, 0.3 g subs./2 mL IL 140 °C, 120 min, 5 g subs./100 g medium 120 °C, 18 min, 0.3 g subs./2 mL IL 120 °C, 18 min, 0.3 g subs./2 mL IL 120 °C, 18 min, 0.3 g subs./2 mL IL 120 °C, 22 h, 0.5 g subs./5 ml IL 120 °C, 22 h, 0.5 g subs./5 ml IL

1.9–14 0.1–4.5 35 22 25 22 36 30 24 33 13

van Spronsen et al. (2011) van Spronsen et al. (2011) Zhang et al. (2014) Zhang et al. (2014) Zhang et al. (2014) Binder et al. (2010) Zhang et al. (2014) Zhang et al. (2014) Zhang et al. (2014) Brandt et al. (2011) Brandt et al. (2011)

The acidic ionic liquid performs as a solvent and as a catalyst.

189

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191 Table 5 Co-production of furfural and HMF from native lignocellulosic material using ILs. Substrate

Medium

External catalyst(s)

Conditions

% Conv. into furfural/ HMF

References

Corn stalk

[bmim]Cl

CrCl3

Temperature n.a.a, 3 min,100 mg subs./2 g IL

23/45

Corncob Corncob Corncob Bagasse Grass Grass Straw

[bmim]Cl [bmim]Cl [bmim]Cl [bmim]Cl [bmim]Cl [bmim]Cl [bmim]Cl

HCl or H2SO4 Individual or paired metal halidesb Solid acid catalystc + AlCl3 HCl AlCl3 Solid acid catalystd CrCl3

160 °C, 3 min, 50 mg subs./2 g IL 160 °C, 3 min, 50 mg subs./2 g IL 160–170 °C, 3–10 min, 50 mg subs./2 IL 130 °C, 30 min, 10 g subs./100 g IL 160 °C, 3–4 min, 50 mg subs./2 g IL 160 °C, 10 min, 50 mg subs./2 g IL Temperature n.a.a, 3 min, 100 mg subs./2 g IL

14.4–14.9/9.9–12.2 16.9–27.4/15.2–37.5 11.6–17.6/7.2–16.9 0.7–32/0–7.9 25.7–31.4/21.3–22.5 22.5–26/18.6–22.6 25 /47

Pine wood

[bmim]Cl

CF3COOH

120 °C, 120 min, 0.25 g subs./5 g IL

0–1/0–3d

Pine wood Pine wood

[bmim]Cl [bmim]Cl

160 °C, 3–4 min, 50 mg subs./2 g IL 160 °C, 10 min, 50 mg subs./2 IL

27.3–33.6/23.1–24.2 19.7–22.3/15.4–17.9

Pine wood

[bmim]Cl

AlCl3 Solid acid catalyst with or without AlCl3c CrCl3 or HCl

Zhang and Zhao (2010) Zhang et al. (2013b) Zhang et al. (2013b) Zhang et al. (2013a) Zhang et al. (2012) Zhang et al. (2013b) Zhang et al. (2013a) Zhang and Zhao (2010) Sievers et al. (2009b) Zhang et al. (2013b) Zhang et al. (2013a)

Temperature n.r.a or 100–200 °C, 3–60 min, 100 mg subs./2 g IL 85–175 °C, 26.7–163.3 min, 0.4 g subs./4 g IL

4.4–31/2.1–52

Wheat straw a b c d e

[bmim] HSO4

e

None

0.1–36.2/0.6–3.0

Zhang and Zhao (2010) Carvalho et al. (2015)

Temperature not reported in some experiments, in which the media were heated by: microwave irradiation (400 W for 3 min). Metal halides: AlCl3 or CrCl3; paired metal halides: CrCl3/AlCl3, FeCl3/AlCl3, SnCl2/AlCl3, CuCl2/AlCl3, CeCl3/AlCl3. Solid acid catalyst: Resin (Amberlyst or NKC9) or H3PW12O40. Hemicellulose-derived sugars were the major reaction products. Yields expressed as wt%. The acidic ionic liquid performs as a solvent and as a catalyst.

because many IL-based protocols for lignocellulose utilization pay little attention to the fate of hemicelluloses, which remains dissolved in the IL or are lost (Stark, 2011). Additionally, the treated solids (mainly composed of cellulose and lignin) can processed in separate stages according to the biorefinery philosophy (Gullón et al., 2012). Based on this general concept, Peleteiro et al. (2014b) reported on the co-production of furfural and HMF in [bmim]Cl – CrCl3 from a crude, complex saccharide mixture (containing monosaccharides, higher saccharides made up of pentoses and hexoses, and nonsaccharide compounds) obtained by hydrothermal processing of pine wood. Operating under optimal conditions (10 g substrate/100 g IL, 160–170 °C, 10 or 45 min of reaction), the maximum individual yields of furfural and HMF were 37.7% and 36.1%, respectively. However, the conversion into HMF achieved under the optimal conditions for furfural production was just about 30%, owing to the comparatively faster furfural generation (and further consumption). In comparison with the results achieved using pure hexoses or pentoses as substrates for furan production, the conversion of the crude fraction was slower and led to decreased yields. These facts were ascribed to the influence of additional reaction steps (hydrolysis of higher saccharides to monosaccharides), to the presence of non-saccharide products (acting as potential substrates for parasitic reactions), and to the participation of a number or reactive intermediates (from hexose, pentoses and non-saccharide compounds) in the side reactions involved in the dehydration of the various monosaccharides present in the medium. In a related study (Serrano-Ruiz et al., 2012) started from an aqueous biorefinery syrup obtained by acidic treatment of cereal straws (containing arabinose, xylose, oligomers made up of pentoses, minerals and other organic compounds), which was subjected to hydrolysis–dehydration in a biphasic medium formulated with the syrup, the AIL [SbPy]MeSO3 (acting as a catalyst), water and a co-solvent (THF). Within the operational range tested (150–180 °C for 2–4 h in media formulated with 0.1 g of syrup with 45 wt% sugar content, 0.1 g AIL, 1 g water and 2 g THF), furfural yields in the range 14–45% were obtained.

Alternatively, hemicellulose hydrolyzates obtained in treatments with ILs can be converted into furfural in a separate step after solid separation, facilitating the recovery of furfural (van Spronsen et al., 2011). Interestingly, the hydrolysis of hemicelluloses in IL media results in the disruption of the hemicellulose–cel lulose–lignin network in the biomass, making the treated solid highly susceptible to cellulose hydrolysis by enzymes (and so, suitable for multiple purposes in biorefineries). 4. Challenges in the utilization of ILs for furfural production Although ILs show important advantages for furfural production; their industrial utilization in large-scale processes is hindered by issues of economic, scientific, technological and environmental nature. The cost of ILs is an important economic drawback frequently highlighted in literature, (particularly when the IL is used as a solvent for the partial dissolution of biomass), and has been identified as the major process cost driver. For a feasible operation, the IL recovery has to be as efficient as possible: for example, 98% recovery of [emim]Cl has been cited as a threshold for economic profitability (Sen et al., 2012). However, it has to be considered that not all ILs are equally expensive, particularly when considered at industrial scale (Tadesse and Luque, 2011); and that the development of bulky low-price ILs capable of efficiently pretreating biomass was recently reported, suggesting that the IL costs could no longer be a major obstacle for the development of industrial processes (da Costa Lopes and Bogel-Lukasik, 2015). In this field, information on the costs of some ILs has been reported (Tadesse and Luque, 2011), as well a techno-economic estimates for a biorefinery based on IL fractionation (Klein-Marcuschamer et al., 2011; Sen et al., 2012). Increased knowledge on the properties of ILs (including physicochemical and equilibrium data) is needed for process development. Some ILs are highly viscous, presenting mass transfer limitations and hindering the separation operations. Water tolerance (regarding both the stability of the IL and the product distribution resulting from the chemical transformation stages) is

190

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191

desirable for biomass-based processes, as it would help in handling both the moisture of the feedstocks and the recovery of ILs. Furthermore, increased amounts of water would result in lower IL requirements and in decreased viscosity. The development of effective methods for the recovery and recycling of ILs is a major challenge (Liu et al., 2012), identified as a main hurdle to the industrialization of these processes (Zhu, 2008). When the considered IL is employed for biomass saccharification to obtain mixtures of saccharides (for example, for further transformation into furfural), its recovery is complicated (owing to the high affinity of sugars for the IL and the lack of volatility of both components) and expensive (for example, technologies such as chromatography have been proposed for this purpose) (Binder and Raines, 2010; Stark, 2011). When ILs are employed for furfural production, liquid–liquid extraction (for example with ethyl acetate or diethyl ether) has been proposed for product recovery, enabling the further separation and purification of the target product by distillation (Zhang et al., 2013b, 2014). This problem can be addressed by using biphasic reaction systems, but some furfural still remains in the IL phase after reaction and further processing would be needed for total separation. On the other hand, IL refining treatments would be necessary, for example to remove humins and contaminants. Alternative solvents, such as supercritical fluids, can be easily integrated for product extraction from ILs (da Costa Lopes and Bogel-Lukasik, 2015). Additionally, the chemostability and thermostability of ILs, as well as the catalyst performance and recovery, need to be assessed in the long run under practical processing conditions (i.e., in the presence of the contaminants derived from biomass processing), because the scarce information available is focused on a limited number of cycles. Further research is also necessary to develop efficient processes able to manage increased substrate loadings, in order to improve the volumetric concentrations of the target products. Finally, some concerns have been pointed out regarding the environmental implications and the ‘‘greenness” of processes based in ILs, including the ones for furfural production (Wu et al., 2014). Lack of toxicity, biodegradability and recyclability have been cited as characteristics of an ideal IL; but some chloridebased ILs are toxic, corrosive, and very hygroscopic (Xie et al., 2012). The lack of toxicological data for many ILs has been highlighted (Zhu, 2008), even if this topic has been extensively studied in the past years (da Costa Lopes et al., 2013a). Another basic principle of the green chemistry lies on the integral benefit of raw materials, which seems directly feasible from a technological point of view, but the profitable production of added-value compounds from all the structural components of the feedstocks needs complementary research efforts (particularly regarding the applications of lignin-rich fractions isolated from IL media, that have been scarcely studied). The energy efficiency seems a strong point furfural manufacturing processed based on ILs, owing to: (i) the current facilities in China are very demanding in terms of energy (25–35 tons of steam at 8–14 kg/cm2 are used per furfural ton); (ii) operation in ILs media can be run under comparatively mild conditions, (iii) energy-efficient microwave heating enables high furfural yields at very short reaction times in a number of ILs, with or without externally-added catalysts, saving energy respect classical thermal methods (da Costa Lopes et al., 2013b). Acknowledgements We are grateful to the Spanish ‘‘Ministry of Economy and Competitivity” for supporting this study in the framework of the research project ‘‘Advanced processing technologies for biorefineries” (reference CTQ2014-53461-R), partially funded by the FEDER program of the European Union.

References Aellig, C., Scholz, D., Dapsens, P.Y., Mondelli, C., Perez-Ramirez, J., 2015. When catalyst meets reactor: continuous biphasic processing of xylan to furfural over GaUSY/Amberlyst-36. Catal. Sci. Technol. 5, 142–149. Aida, T.M., Shiraishi, N., Kubo, M., Watanabe, M., Smith Jr., R.L., 2010. Reaction kinetics of D-xylose in sub- and supercritical water. J. Supercrit. Fluids 55, 208– 216. Amiri, H., Karimi, K., Roodpeyma, S., 2010. Production of furans from rice straw by single-phase and biphasic systems. Carbohydr. Res. 345, 2133–2138. Antal Jr., M.J., Leesomboon, T., Mok, W.S., Richards, G.N., 1991. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 217, 71–85. Antunes, M.M., Lima, S., Fernandes, A., Pillinger, M., Ribeiro, M.F., Valente, A.A., 2012. Aqueous-phase dehydration of xylose to furfural in the presence of MCM22 and ITQ-2 solid acid catalysts. Appl. Catal. A 417–418, 243–252. Archambault-Léger, V., Losordo, Z., Lynd, L.R., 2015. Energy, sugar dilution, and economic analysis of hot water flow-through pre-treatment for producing biofuel from sugarcane residues. Biofuels. Bioprod. Biorefin. 9, 95–108. Bhaumik, P., Dhepe, P.L., 2014. Exceptionally high yields of furfural from assorted raw biomass over solid acids. RSC Adv. 4, 26215–26221. Binder, J.B., Raines, R.T., 2010. Fermentable sugars by chemical hydrolysis of biomass. PNAS 107, 4516–4521. Binder, J.B., Blank, J.J., Cefali, A.V., Raines, R.T., 2010. Synthesis of furfural from xylose and xylan. ChemSusChem 3, 1268–1272. Bozell, J.J., Petersen, G.R., 2010. Technology development for the production of biobased products from biorefinery carbohydrates. The U. S. Department of Energy´s Top 10 revisited. Green Chem. 12, 539–554. Brandt, A., Ray, M.J., To, T.Q., Leak, D.J., Murphy, R.J., Welton, T., 2011. Ionic liquid pretreatment of lignocellulosic biomass with ionic liquid–water mixtures. Green Chem. 13, 2489–2499. Brownlee, H.J., 1938. Process for producing furfural. US Pat 2,140–572. Cai, C.M., Zhang, T., Kumar, R., Wyman, C.E., 2014. Integrated furfural production as a renewable fuel and chemical platform from lignocellulosic biomass. J. Chem. Technol. Biotechnol. 89, 2–10. Campos Molina, M.J., Mariscal, R., Ojeda, M., López, Granados M., 2012. Cyclopentyl methyl ether: a green co-solvent for the selective dehydration of lignocellulosic pentoses to furfural. Bioresour. Technol. 126, 321–327. Carvalho, A.V., da Costa Lopes, A.M., Bogel-Łukasik, R., 2015. Relevance of the acidic 1-butyl-3-methylimidazolium hydrogen sulphate ionic liquid in the selective catalysis of the biomass hemicellulose fraction. RSC Adv. 5, 47153–47164. Chheda, J.N., Roman-Leshkov, Y., Dumesic, J.A., 2007. Production of 5hydroxymethylfurfural and furfural by dehydration of biomass derived monoand poly-saccharides. Green Chem. 9, 342–350. Choudhary, V., Pinar, A.B., Sandler, S.I., Vlachos, D.G., Lobo, R.F., 2011. Xylose isomerization to xylulose and its dehydration to furfural in aqueous media. ACS Catal. 1, 1724–1728. da Costa Lopes, A.M., Bogel-Lukasik, R., 2015. Acidic ionic liquids as sustainable approach of cellulose and lignocellulosic biomass conversion without additional catalysts. ChemSusChem 8, 947–965. da Costa Lopes, A.M., João, K.G., Bogel-Lukasik, E., Roseiro, L.B., Bogel-Lukasik, R., 2013a. Pretreatment and fractionation of wheat straw using various ionic liquids. J. Agric. Food Chem. 61, 7874–7882. da Costa Lopes, A.M., João, K.G., Morais, A.R., Bogel-Łukasik, E., Bogel-Łukasik, R., 2013b. Ionic liquids as a tool for lignocellulosic biomass fractionation. Sustainable Chem. Processes 1, 1–31. DalinYebo Trading and Development, 2015. Information retrieved from: http:// www.dalinyebo.com/media/k2/items/cache/84c42b9986b8cecdea81ed6abb66c108_ XL.jpg. Last accessed October, 2015. Dias, A.S., Lima, S., Brandão, P., Pillinger, M., Rocha, J., Valente, A.A., 2006. Liquidphase dehydration of D-xylose over microporous and mesoporous niobium silicates. Catal. Lett. 108, 179–186. Dias, A.S., Lima, S., Pillinger, M., Valente, A.A., 2007. Modified versions of sulfated zirconia as catalysts for the conversion of xylose to furfural. Catal. Lett. 114, 151–160. Enslow, K.R., Bell, A.T., 2012. The kinetics of Brönsted acid-catalyzed hydrolysis of hemicellulose dissolved in 1-ethyl-3-methylimidazolium chloride. RSC Adv. 2, 10028–10036. Fitzpatrick, S.W., 2006. The biofine technology: a bio-refinery concept based on thermochemical conversion of cellulosic biomass. ACS Symp. Ser. 921, 271–287. Gullón, P., Romaní, A., Vila, C., Garrote, G., Parajó, J.C., 2012. Potential of hydrothermal treatments in lignocellulose biorefineries. Biofuels Bioprod. Biorefin. 6, 219–232. Hayes, D.J., Fitzpatrick, S., Hayes, M.H.B., Ross, J.R.H., 2008. The Biofine process– production of levulinic acid, furfural, and formic acid from lignocellulosic feedstocks, in biorefineries. In: Kamm, B. (Ed.), Industrial Processes and Products, vol. 2. Wiley-VCH VerlagGmbH, Weinheim, pp. 139–164. Heguaburu, V., Franco, J., Reina, L., Tabarez, C., Moyna, G., Moyna, P., 2012. Dehydration of carbohydrates to 2-furaldehydes in ionic liquids by catalysis with ion exchange resins. Catal. Commun. 5, 88–91. Kazi, F.K., Fortman, J., Anex, R., Kothandaraman, G., Hsu, D., Aden, A., Dutta, A., 2010. Techno-economic analysis of biochemical scenarios for production of cellulosic ethanol. National Renewable Energy Laboratory, Golden, Colorado. Report retrieved from: http://www.nrel.gov/docs/fy10osti/46588.pdf. Last accessed: October, 2015.

S. Peleteiro et al. / Bioresource Technology 202 (2016) 181–191 Klein-Marcuschamer, D., Simmons, B.A., Blanch, H.W., 2011. Techno-economic analysis of a lignocellulosic ethanol biorefinery with ionic liquid pre-treatment. Biofuels Bioprod. Biorefin. 5, 562–569. Kumar, D., Murthy, G.S., 2011. Impact of pre-treatment and downstream processing technologies on economics and energy in cellulosic ethanol production. Biotechnol. Biofuels 4, 27–46. Lange, J.P., van der Heide, E., van Buijtenen, J., Price, R., 2012. Furfural-a promising platform for lignocellulosic biofuels. ChemSusChem 5, 150–166. Lessard, J., Morin, J.F., Wehrung, J.F., Magnin, D., Chornet, E., 2010. High yield conversion of residual pentoses into furfural via zeolite catalysis and catalytic hydrogenation of furfural to 2-methylfuran. Top. Catal. 53, 1231–1234. Li, C., Wang, Q., Zhao, Z.K., 2008. Acid in ionic liquid: an efficient system for hydrolysis of lignocellulose. Green Chem. 10, 177–182. Li, H., Deng, A., Ren, J., Liu, C., Wang, W., Peng, F., Sun, R., 2014. A modified biphasic system for the dehydration of D-xylose into furfural using SO24 /TiO2–ZrO2/ La3+as a solid catalyst. Catal. Today 234, 251–256. Lima, S., Neves, P., Antunes, M.M., Pillinger, M., Ignatyev, N., Valente, A.A., 2009. Conversion of mono/di/polysaccharides into furan compounds using 1-alkyl-3methylimidazolium ionic liquids. Appl. Catal. A 363, 93–99. Lima, S., Antunes, M.M., Pillinger, M., Valente, A.A., 2011. Ionic liquids as tools for the acid-catalyzed hydrolysis/dehydration of saccharides to furanic aldehydes. ChemCatChem 3, 1686–1706. Liu, C.Z., Wang, F., Stiles, A.R., Guo, C., 2012. Ionic liquids for biofuel production: opportunities and challenges. Appl. Energy 92, 406–414. Long, J., Guo, B., Teng, J., Yu, Y., Wang, L., Li, X., 2011. SO3H-functionalized ionic liquid: efficient catalyst for bagasse liquefaction. Bioresour. Technol. 102, 10114–10123. Ma, S., Li, P., Zhu, T., Chang, H., Lin, L., 2014. Reaction extraction of furfural from pentose solutions in a modified Scheibel column. Chem. Eng. Process. 83, 71–78. Mandalika, A., Runge, T., 2012. Enabling integrated biorefineries through high-yield conversion of fractionated pentosans into furfural. Green Chem. 14, 3175–3184. Marcotullio, G., De Jong, W., 2011. Furfural formation from D-xylose: the use of different halides in dilute aqueous acidic solutions allows for exceptionally high yields. Carbohydr. Res. 346, 1291–1293. Niknam, K., Damya, M., 2009. 1-Butyl-3-methylimidazolium hydrogen sulfate [bmim] HSO4: an efficient reusable acidic ionic liquid for the synthesis of 1,8dioxo-octahydroxanthenes. J. Chin. Chem. Soc. 56, 659–665. Nimlos, M.R., Qian, X., Davis, M., Himmel, M.E., Johnson, D.K., 2006. Energetics of xylose decomposition as determined using quantum mechanics modeling. J. Phys. Chem. A 110, 11824–11838. Olivier-Bourbigou, H., Magna, L., Morvan, D., 2010. Ionic liquids and catalysis: recent progress from knowledge to applications. Appl. Catal. A 373, 1–56. Peleteiro, S., Garrote, G., Santos, V., Parajó, J.C., 2014a. Conversion of hexoses and pentoses into furans in an ionic liquid. Afinidad 567, 202–206. Peleteiro, S., Garrote, G., Santos, V., Parajó, J.C., 2014b. Furan manufacture from softwood hemicelluloses by aqueous fractionation and further reaction in a catalyzed ionic liquid: a biorefinery approach. J. Cleaner Prod. 76, 200–203. Peleteiro, S., da Costa Lopes, A.M., Garrote, G., Bogel-Łukasik, R., Parajó, J.C., 2015a. Manufacture of furfural in biphasic media made up of an ionic liquid and a cosolvent. Ind. Crops Prod. 77, 163–166. Peleteiro, S., Rivas, S., Alonso, J.L., Santos, V., Parajó, J.C., 2015b. Utilization of ionic liquids in lignocellulose biorefineries as agents for separation, derivatization, fractionation, or pretreatment. J. Agric. Food. Chem. 63, 8093–8102. Peleteiro, S., da Costa Lopes, A.M., Garrote, G., Parajó, J.C., Bogel-Łukasik, R., 2015c. Simple and efficient furfural production from xylose in media containing 1-butyl3-methylimidazolium hydrogen sulfate. Ind. Eng. Chem. Res. 54, 8368–8373. Rasmussen, H., Sørensen, H.R., Meyer, A.S., 2014. Formation of degradation compounds from lignocellulosic biomass in the biorefinery: sugar reaction mechanisms. Carbohydr. Res. 385, 45–57. Rivas, S., González-Muñoz, M.J., Santos, V., Parajó, J.C., 2013. Production of furans from hemicellulosic saccharides in biphasic reaction systems. Holzforschung 67, 923–929. Sen, S.M., Binder, J.B., Ronald, T.R., Christos, T.M., 2012. Conversion of biomass to sugars via ionic liquid hydrolysis: process synthesis and economic evaluation. Biofuels Bioprod. Biorefin. 6, 444–452.

191

Serrano-Ruiz, J.C., Campelo, J.M., Francavilla, M., Romero, A.A., Luque, R., MenéndezVázquez, C., García, A.B., García-Suarez, E.J., 2012. Efficient microwave-assisted production of furfural from C5 sugars in aqueous media catalyzed by Brönsted acidic ionic liquids. Catal. Sci. Technol. 2, 1828–1832. Sievers, C., Musin, I., Marzialetti, T., Valenzuela-Olarte, M.B., Agrawal, P.K., Jones, C. W., 2009a. Acid-catalyzed conversion of sugars and furfurals in an ionic-liquid phase. ChemSusChem 2, 665–671. Sievers, C., Valenzuela-Olarte, M.B., Marzialetti, T., Musin, I., Agrawal, P.K., Jones, C. W., 2009b. Ionic-liquid-phase hydrolysis of pine wood. Ind. Eng. Chem. Res. 48, 1277–1286. Stark, A., 2011. Ionic liquids in the biorefinery: a critical assessment of their potential. Energy Environ. Sci. 4, 19–32. Tadesse, H., Luque, R., 2011. Advances on biomass pretreatment using ionic liquids: an overview. Energy Environ. Sci. 4, 3913–3929. Tao, F., Song, H., Chou, L., 2011. Efficient process for the conversion of xylose to furfural with acidic ionic liquid. Can. J. Chem. 89, 83–87. van Spronsen, J., Cardoso, M.A.T., Witkamp, G.J., de Jong, W., Kroon, M.C., 2011. Separation and recovery of the constituents from lignocellulosic biomass by using ionic liquids and acetic acid as co-solvents for mild hydrolysis. Chem. Eng. Process Intensif. 50, 196–199. vom Stein, T., Grand, P.M., Leitner, W., Domínguez de María, P., 2011. Iron-catalyzed furfural production in biobased biphasic systems: from pure sugars to direct use of crude xylose effluents as feedstock. ChemSusChem 4, 1592–1594. Weingarten, R., Cho, J., Conner Jr., W.C., Huber, G.W., 2010. Kinetics of furfural production by dehydration of xylose in a biphasic reactor with microwave heating. Green Chem 12, 1423–1429. Werpy, T., Petersen, G., 2004. Top value added chemicals from biomass. Report retrieved from: http://www.nrel.gov/docs/fy04osti/35523.pdf. Last accessed October, 2015. Wettstein, S.G., Alonso, D.M., Gürbüz, E.I., Dumesic, J.A., 2012. A roadmap for conversion of lignocellulosic biomass to chemicals and fuels. Curr. Opinion Chem. Eng. 1, 218–224. Wu, C., Chen, W., Zhong, L., Peng, X., Sun, R., Fang, J., Zheng, S., 2014. Conversion of xylose into furfural using lignosulfonic acid as catalyst in ionic liquid. J. Agric. Food Chem. 62, 7430–7435. Xie, R.Q., Li, X.Y., Zhang, Y.F., 2012. Cellulose pretreatment with 1-methyl-3ethylimidazolium dimethylphosphate for enzymatic hydrolysis. Cellul. Chem. Technol. 46, 349–356. Xing, R., Qi, W., Huber, G.W., 2011. Production of furfural and carboxylic acids from waste aqueous hemicellulose solutions from the pulp and paper and cellulosic ethanol industries. Energy Environ. Sci. 4, 2193–2205. Yang, Y., Hu, C.W., Abu-Omar, M.M., 2012. Synthesis of furfural from xylose, xylan, and biomass using AlCl3
6H2O in biphasic media via xylose isomerization to xylulose. ChemSusChem 5, 405–410. Zeitsch, K.J., 2000. The chemistry and technology of furfural and its many byproducts. In: Ads, A. (Ed.), Sugar Series, vol. 13. Elsevier Science, Amsterdam. Zhang, Z., Zhao, Z.K., 2010. Microwave-assisted conversion of lignocellulosic biomass into furans in ionic liquid. Bioresour. Technol. 101, 1111–1114. Zhang, Q., Zhang, S., Deng, Y., 2011. Recent advances in ionic liquid catalysis. Green Chem. 13, 2619–2637. Zhang, Z., O’Hara, I.M., Doherty, W.O.S., 2012. Pretreatment of sugarcane bagasse by acid-catalysed process in aqueous ionic liquid solutions. Bioresour. Technol. 120, 149–156. Zhang, L., Yu, H., Wang, P., Dong, H., Peng, X., 2013a. Conversion of xylan, D-xylose and lignocellulosic biomass into furfural using AlCl3 as catalyst in ionic liquid. Bioresour. Technol. 130, 110–116. Zhang, L., Yu, H., Wang, P., 2013b. Solid acids as catalysts for the conversion of Dxylose, xylan and lignocellulosics into furfural in ionic liquid. Bioresour. Technol. 136, 515–521. Zhang, Z., Du, B., Quan, Z.J., Da, Y.X., Wang, X.C., 2014. Dehydration of biomass to furfural catalyzed by reusable polymer bound sulfonic acid (PEG-OSO3H) in ionic liquid. Catal. Sci. Technol. 4, 633–638. Zhu, S., 2008. Use of ionic liquids for the efficient utilization of lignocellulosic materials. J. Chem. Technol. Biotechnol. 83, 777–779.