Elsevier

Bioresource Technology

Volume 202, February 2016, Pages 181-191
Bioresource Technology

Review
Furfural production using ionic liquids: A review

https://doi.org/10.1016/j.biortech.2015.12.017Get rights and content

Highlights

  • 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.

Abstract

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 xylan-containing 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.

Introduction

Furfural (OC4H3CHO, also called 2-formylfuran, furan-2-aldehyde, 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, 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 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, acid-catalyzed 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).

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 β-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 non-saccharide 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.

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 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, Dias et al., 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 hemicellulose-derived 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).

Section snippets

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

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 externally-added 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

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

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.

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