Green Chemistry

PERSPECTIVE

Cite this: Green Chem., 2014, 16,2015

Received 27th September 2013,Accepted 28th November 2013

DOI: 10.1039/c3gc42018c

www.rsc.org/greenchem

Hydroxymethylfurfural production frombioresources: past, present and future

Siew Ping Teong, Guangshun Yi and Yugen Zhang*

5-Hydroxymethylfurfural (HMF) has been known as a product from hexose dehydration for over 100 years

and is considered to be one of the most promising platform molecules that can be converted into a

variety of interesting chemicals. HMF, together with furfural and 2,5-furandicarboxylic acid (FDCA) are

derivatives of furan compounds, which were listed as the top 10 value-added bio-based chemicals by the

US Department of Energy. The great and increasing interest in the production of furan derivatives from

biomass resources is due to the great potential of furan derivatives as feedstock for bulk chemicals and

fuels. HMF can be synthesized by dehydration of all types of C6 carbohydrates, including monomeric and

polymeric carbohydrates, such as fructose, glucose, sucrose, starch, inulin, cellulose, and raw biomass.

Numerous improvements and milestones have been made in the dehydration process during the past

130 years. The big challenge for the process of HMF production is its suitability for industrial scale yet

being cost efficient. This perspective article will review the HMF development timeline, focusing on the

important events, landmark contributions, engineering and practical challenges of HMF production.

1. Introduction

Interest in making the best use of renewable natural resources,to reduce society’s reliance on synthetic petroleum-basedchemicals, is increasing due to the general awareness of theneed for a low-carbon economy. Currently, there is an

enormous amount of work being done by both academic andindustrial institutes to develop both new bio-based chemicalsand innovative bio-based routes to existing chemicals.1

Biomass is the most attractive sustainable feedstock, as it iswidely available and renewable. Biomass consists of carbo-hydrates, lignin and others. Among those, carbohydrates arethe major component of biomass and the largest naturalsource of carbon. With that, a major concern for the extensiveusage of biomass is that it would squeeze the production of

Siew Ping Teong

Miss Teong Siew Ping receivedher First Class Hons in Chem-istry and Biological Chemistryfrom Nanyang TechnologicalUniversity in 2012. She thenworked as a laboratory officerunder the guidance of Dr Zhangin Institute of Bioengineeringand Nanotechnology. Herresearch interests involve greenand environmentally benign cat-alytic reactions particularly theconversion of biomass-derivedsugars to biofuel and usefulchemicals.

Guangshun Yi

Dr Guangshun Yi is a seniorresearch scientist at the Instituteof Bioengineering and Nano-technology (IBN), A-Star, Singa-pore. He graduated fromTsinghua University (China),where he received his PhD inAnalytical Chemistry in 2003.After his PhD, he joined Shan-dong University of Technology asan Associate Professor. Prior tojoining IBN in 2009, he was aresearch fellow at NationalUniversity of Singapore

(2004–2008), and Chief Technology officer at NanoBright Techno-logies Pte Ltd (2008–2009). His main research areas are Nano-material, fluorescent technology, green & sustainable technology.

Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos,

Singapore 138669, Singapore. E-mail: ygzhang@ibn.a-star.edu.sg

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food crops.2 One solution to this concern is to use non-foodbiomass crops for the production of biochemicals. In addition,the scheme below also clearly demonstrates that the size ofchemical feedstock from fossil carbon and bio-based carbon ismuch smaller as compared to biomass resources. Comparingwith fuel and energy consumption, the market of biochemicalsis much smaller and has much less impact to food resources.However, biomass-based chemicals are highly value-added andcould make great impact to our low-carbon economy. Thedevelopment of biochemicals will reduce the consumption offossil resources, as well as producing biorenewable and bio-degradable products for our green society (Scheme 1).

5-Hydroxymethylfurfural (HMF), together with furfural and2,5-furandicarboxylic acid (FDCA) are derivatives of furan com-pounds, which were listed as the top 10 value-added bio-basedchemicals by the US Department of Energy.3 HMF can be natu-rally produced by thermal decomposition of carbohydrates.

HMF and its derivatives are widely available in a variety of foodsources, especially in processed foods.4 The great and increas-ing interest in production of furan derivatives from biomassresources is due to the great potential of furan derivatives asfeedstocks for bulk chemicals and fuels.5 As shown inScheme 2 above, HMF is the key intermediate to bridge thegap between biomass resources and biochemicals. HMF canbe synthesized by dehydration of all types of C6 carbohydrates,including monomeric and polymeric carbohydrates, such asfructose, glucose, sucrose, starch, inulin, cellulose, and rawbiomass. As the output from HMF, a wide variety of importantchemicals have been studied (Scheme 2). Although the directmarket for HMF is not remarkable, there is a huge market forHMF derivatives, such as FDCA (replacement of terephthalicacid (TPA) in the PET industry with ∼40 MT per year market)and adipic acid (∼3.2 MT per year market for the nylon indus-try).6,7 The HMF derivative-based plastic materials are gener-ally biodegradable, which will have great impact on our greenand low-carbon economy.

The publication of the synthesis of HMF by sugar de-hydration under aqueous acidic conditions goes back to 1875.8

Since then, there has been a continued growth of interest inHMF production from bioresources. As shown in Scheme 3,the history of HMF production could be summarized intoseveral periods, early stage before 1980, developing stage from1980 to 2000 and active stage after 2000. This perspectivearticle will review the HMF development timeline, focusing onthe important events and landmark contributions.

Yugen Zhang

Dr Yugen Zhang is a GroupLeader at the Institute of Bio-engineering and Nanotechnology(IBN), A-Star, Singapore. Hegraduated from the University ofScience and Technology of China(USTC), where he also receivedhis PhD in Chemistry in 1992.After his PhD, he joined USTC asa faculty member and was pro-moted to Professor in 1999. Hevisited Riken (Japan) (1996 to1997, 2000 to 2001), where heworked as visiting scholar in

Prof. Zhaomin Hou’ group. Before he joined IBN in 2004, he hadbeen working at Harvard University as a post-doctoral researchassociate in Prof. R. H. Holm’s group (2002–2004). His mainresearch areas are green catalysis, sustainable technology andbiomaterials.

Scheme 1 Global resources and markets. The same scale as the piecharts, for comparison.

Scheme 2 HMF represents a key intermediate for the conversion ofbiomass to biochemicals.

Scheme 3 The history and landmarks of HMF synthesis by carbo-hydrates dehydration.

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2. Hydroxymethylfurfural productionfrom bioresources: the past (lastcentury and before)2.1. Early stage before 1980

Although the production of HMF from carbohydrates has beenstudied since the 19th century, the research activities in thisfield were very inactive until the middle of the last century.The first review article for the production of furan from carbo-hydrates was published in 1951 by Newth.9 After that, theresearch interest in HMF production increased slowly.10 Overnearly a century time, until the 1980s, research in the develop-ment of efficient methods for HMF production from carbo-hydrates were almost exclusively focused on the aqueous-basedmineral acid-catalyzed system.10 Water, as a common solventfor traditional carbohydrate chemistry, has been widely usedin HMF production. In this early stage, fructose,11 glucose12

and polysaccharides (corn starch12 and wood chips13) weretested in the HMF production processes with mineral acid ororganic acid catalysts, as well as non-catalytic conditions. Thereaction temperature varied from below 100 °C to 300 °C, andthe reaction time was varied from seconds to hours. However,the aqueous system is limited by low selectivity toward HMFproduction. Typically, the HMF yield was less than 50% fromfructose. It was known later that HMF is not stable in waterunder acidic conditions.1 The low yield of HMF is sometimesalso accompanied by a high yield of levulinic acid and formicacid, which indicates the rehydration of HMF in such con-ditions. In addition, HMF may also self-polymerize or polymer-ize with sugars to solid by-products. These are the mainreasons for the low selectivity of HMF in acidic aqueoussystem. To improve the HMF selectivity, the first continuousextraction system in HMF synthesis from carbohydrates wasinvented by Peniston in 1956.14 The reaction was conducted in0.05 M aqueous sulfuric acid with 2 wt% of fructose and sameamount of n-butanol as organic phase. Improved HMFyield (68%) was achieved after 8 min at 170 °C. Theconcept here is to suppress undesired side reactions bycontinuously removing HMF from the aqueous phase. Thisimportant invention represents the earliest example of HMFproduction in a biphasic solvent system. Later in 1977, Kusterand Van der Steen studied a methyl isobutyl ketone (MIBK)–water biphasic system for the HMF synthesis from fructose.15

The reaction was carried out in a continuous stirred tankreactor (CSTR) and the best HMF yield of 69% was obtainedwith 1 M fructose and 0.1 M H3PO4 at 190 °C for 5 min andthe MIBK/water ratio was 7.5. It was reported that longer resi-dence time resulted in higher HMF yields without losses inselectivity. This observation indicated that continuouslyremoving HMF to organic phase improved the overall HMFselectivity.

In this early stage, HMF production had not yet gainedmuch attention. Carbohydrate dehydration to HMF technologywas slowly developed. The research interests were morefocused on increasing HMF selectivity. One of the most

important progresses during this period was the developmentof continuous/biphasic system for the HMF production.

2.2. Developing stage from 1980 to 2000

Kuster’s work15 indicated the positive effects of the addition oforganic solvents into aqueous system on the rate of HMF for-mation and the suppressing of HMF decomposition. Thisfinding actually led to major changes for HMF synthesis fromcarbohydrate dehydration after the 1980s. Kuster also pub-lished a review paper in 1990 for these developments.16 Thefructose dehydration to HMF in aqueous system has been con-tinuously developed by screening various homogeneous17 andheterogeneous18 catalysts. Meanwhile, many researchers havestarted to investigate the sugar dehydration in organic sol-vents. DMSO is the most popular organic solvent for HMF pro-duction as it has the highest solubility for sugars and highstability for HMF.

The first HMF synthesis in DMSO was reported in 1980with 90% of HMF yield catalyzed by solid resin DiaionPK-216.19 After which, DMSO, as well as some other polaraprotic solvents (sulfolane,20 DMF,21 ethers22), have beenwidely studied for HMF synthesis under various conditions.New types of catalysts, such as BF3·OEt2

21 and NH4Cl(NH4HSO4),

20 were tested in fructose dehydration in organicsolvents. In many cases high HMF yields (>90%) wereclaimed.20 However, due to the limitation of the analysismethod, some of the results are inconsistent. During thisperiod, there were also several reports working on glucosedehydration to HMF with catalysts, such as MgCl2

23 orHY-zeolite.24 However, only low HMF yields (<10%) were obtained.

It is notable that Fayet and Gelas first reported the de-hydration of fructose, glucose and polysaccharides in immo-nium and ammonium salts (also called ionic liquids).25 Forfructose, up to 70% HMF yield was achieved in the ionic liquidsystems at 120 °C for 30 min. HMF yields from glucose andsaccharose were about 5% and 30% under similar conditions.

In alcohols, HMF can be converted to ether in the presenceof an acid catalyst. Brown et al. reported the fructose de-hydration in various alcohol solvents in 1982.20 In this system,100 wt% of Amberlyst-15 was applied in various alcohols, suchas methanol, ethanol, iso-propanol and n-butanol. 19–55%HMF ether yields were obtained at 100 °C for 20 h. There aresome obvious advantages to using alcohol solvents, such asbeing greener and stable. The lower boiling point of alcoholsas compared to DMSO makes the HMF isolation process mucheasier. However, the HMF yield and product formation is notsatisfied at this stage (Scheme 4).

In addition, the biphasic solvent systems received moreattention after Kuster’s work in 1977. The system was mainlyfocused on water–MIBK during this period. Various solid acidcatalysts were tested for carbohydrate dehydration in biphasicsystems. For fructose dehydration, Lewatitt SPC 108 (61 mol%)gave 56% of HMF at 88 °C for 15 h.26 H-form zeolite (29 wt%)gave 69% of HMF at 165 °C for 1 h.24 Interestingly, a fixed bedreaction system with acidic IE resin and a DMSO–MIBK solventmixture under continuous flow conditions gave 97% of HMF

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yield at 76 °C.27 As the polar solvent changed to NMP or DMF,the HMF yields dropped to 88% and 84% respectively. Anotherfixed bed system has also been designed with SPC-108 as solidcatalyst and a water–MIBK biphasic solvent as eluent.26 Thissystem was tested for dehydration of various carbohydrates. Itgave an HMF yield of 72% from fructose and less than 10%from glucose at 78 °C. This system also produced HMF in 67%yield from inulin, 73% yield from Jerusalem artichoke, 41%yield from sucrose and 27% yield from raffinose. This fixedbed continuous flow reaction system represented an importanttrial toward the large scale dehydration process.

Carbohydrate dehydration to HMF has attracted more atten-tion during this period. Research interests were focused onscreening various solvents and catalysts (homogeneous andheterogeneous) to increase reaction selectivity. High boilingpoint polar aprotic solvents, such as DMSO, sulfolane, DMF,as well as ammonium ionic liquids were used in the carbo-hydrate dehydration and remarkable high selectivity wasachieved in these systems. However, the challenge of HMF iso-lation from such high polarity and high boiling point systemsremained untouched. Biphasic systems have been well de-veloped during this time period, while the development offixed bed reaction systems indicating the engineering concernof the HMF production have also received attention.

3. Hydroxymethylfurfural productionfrom bio-resources: the present(21st century)

As the time turns to the 21st century, HMF production fromcarbohydrate dehydration has attracted much more attention.Due to the variation of oil price, the growth in demand forresources and diminishing reserves, the requirements for thedevelopment of new and sustainable technology for the secur-ity of fuel and bulk chemical supplies is increasing. Researchactivities for HMF synthesis reached a high point in which agreat increase in the number of publications has beenobserved during the past decade.1 Many new systems and pro-tocols have been developed for carbohydrate dehydration inthe past ten years. Interestingly, many old methods andsystems have also been re-investigated with more powerful andaccurate analysis tools, exposing more reliable data, experi-mental details and insights that gave new life to many olddevelopments. There are also many review articles about HMFproduction,1,28 furan chemistry29 as well as general biomasstransformations30 published since 2000.

3.1. Single phase system

3.1.1. Simple aqueous phase system. Simple aqueoussystems have been continuously evaluated for carbohydratedehydration in the new century. Aqueous systems without acatalyst for fructose,31 glucose32 and poly-saccharide33 de-hydration were investigated at different temperatures, concen-trations and reaction times. The best result of 51% HMF yieldfor fructose dehydration was achieved at 200 °C for 30 minwith 11 wt% of fructose,34 which was even lower than theresult reported earlier.11 Glucose32 was typically dehydrated at200–350 °C in minutes and HMF yields were low (most of thecases less than 10%). Similar conditions were applied to poly-saccharides and again low HMF yields were obtained.33 Basi-cally, this process could be an autocatalytic system involvingformic acid as a by-product and catalyst.34 However, the reac-tion is limited by low HMF selectivity.

The majority of recent investigations for simple aqueoussystems focused on testing various catalysts and different reac-tion conditions. A group of catalysts, including organicacids,35 metal halides,36 metal oxide32,37 and phosphates38

were newly tested in fructose dehydration. However, HMFyields suffered at a low level due to the low selectivity or con-version. The best performing system with FeVOP as catalyst39

(1.8 wt%) at 80 °C for 1 h gave 60% of HMF yield. Recently,systems with solid catalysts, such as niobic acid or silicotungs-tic acid, were also tested in aqueous media under differentconditions and more than 50% of HMF yield was achieved.40

For glucose feedstock, metal salts or oxides were applied asisomerization catalysts for their dehydration in water, however,the HMF yields were generally low. One best reported resultused 50 mol% of AlCl3 as catalyst at 120 °C (microwave) for20 min to give 40% HMF yield.36 Similarly, various catalystshave also been tested in poly-saccharide dehydration in simplewater systems.38,41 Generally, low HMF yields were obtained inthese systems. The HMF yield from inulin was typically lessthan 50%,38 while less than 20% was obtained for cellulose.32e

One exemption for cellulose dehydration in water is usingCr[(DS)H2PW12O40]3 catalyst at 150 °C for 2 h and a 53% HMFyield was reported.41a This high HMF yield may be due to ahighly efficient chromium-catalyzed isomerization process.42

After extensive screening of conditions and catalysts, theHMF yield in a single aqueous system is still relatively low.Water is an ideal green solvent for chemical transformations.However, the poor stability and high solubility of HMF inwater leads to low HMF selectivity and difficulty in HMF iso-lation, which would eventually limit the real application of asimple aqueous phase system.

3.1.2. Polar aprotic organic solvent systems. DMSO is stillthe most popular organic solvent used in carbohydrate de-hydration in the new century. Several groups re-investigatedthe fructose dehydration in DMSO without catalyst, and againinconsistent results were reported. Sidhpuria43 heated fructosein DMSO at 130 °C for 30 min with 0% conversion, whileYan et al.44 reported 72% HMF yield at 130 °C for 4 h; Deet al.36 obtained 22% HMF yield at 140 °C for 5 min. Such

Scheme 4 Equilibrium between HMF and its alcohol derivatives underacidic condition in alcohol systems.

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inconsistent results could be due to the different experimentalsetups and some minor contamination presented in thesystem.45

Various homogeneous and heterogeneous catalysts werecontinuously screened in DMSO for carbohydrate dehydration.Impressively high HMF yields from fructose were achieved inmany different systems. Seri et al.46 reported 95% HMF yieldfrom a system with 2.5 mol% LaCl3 catalyst in DMSO at 100 °Cfor 4 h. It was found that similar high yield was also achievedwhen DMA and DMF were used as solvent. In contrast, sulfo-lane (50%), dioxane (25%) and 1-butanol (25%) systems led tolower HMF yields. In another report, lanthanide triflates47

were used as catalysts for fructose dehydration and an 83%yield of HMF was obtained in DMSO with 10 wt% of Sc(OTf)3at 120 °C for 2 h. A variety of catalysts, such as ZrO2,

44,45

TiO2,48 modified acidic imidazolium salts,49 immobilized

ionic liquid,43 all kinds of solid acids,43,45,50 polytungstic acid(PTA) and MOF-supported PTA,51 were tested for fructose de-hydration in polar aprotic solvents, and relatively good resultswere achieved.

A remarkable system was reported by Shimizu et al.45 forfructose dehydration in DMSO with a variety of catalysts.Under a continuous water removal model, very high yields(>95%) were reported for several catalysts, such as Amberlyst-15, heteropolyacid FePW12O40 and zeolite H-BEA, at 120 °C for2 h. A quantitative yield could be achieved with Amberlyst-15catalyst even without continuous water removal and the cata-lyst can be recycled without any deactivation.

Qi et al.52 investigated an acetone–DMSO mixed solventsystem for fructose dehydration. 88% HMF yield was obtainedat 140 °C for 20 min in acetone–DMSO (70/30 w/w) withDOWEX 50WX8-100 resin as catalyst. The catalyst is stable andwas used for five recycles with very minor deactivation. Zirco-nia (ZrO2) and sulfated zirconia were also tested in the samesystem with an acetone–DMSO mixture and a moderate HMFyield (60–66%) was obtained.

Dimethyl acetamide (DMA)46,47 and N-methyl pyrrolidinone(NMP)50,53 were used for HMF production in several reports.As an example, Binder and Raines investigated dehydration offructose, glucose and cellulose in DMA with metal bromides oriodides and a yield of 90% was achieved from fructose at100 °C for 2 h.54

From the mechanistic point of view, glucose needs to beisomerized to fructose, followed by dehydration to HMF.55

Hence, one of the important points of progress for glucosedehydration was the development of an isomerization catalystfor the transformation of glucose to fructose. It was reportedthat Sn-beta zeolite facilitates the isomerization of glucose tofructose in aqueous media even at low pH.56 In order to obtainHMF in high yields from glucose, recent studies have aimed touse one-pot isomerization reactions to produce fructose byusing a Lewis acid or Lewis base, followed by Brønsted acid-catalyzed dehydration of fructose to HMF (Scheme 5). Tin57

and chromium54 were found to be very efficient for glucose iso-merization. AlCl3,

35 GeCl4,58 ZrO2

44 as well as hydrotalcite59

and zeolites60 were also reported to promote glucose

isomerization. It is very obvious that good to high HMF yields(up to 80% for Binder and Raines’s DMA/LiBr/Cr system54)could be achieved for glucose or cellulose dehydration inaprotic solvents with isomerization catalysts, while much lowerHMF yields (<20%) were obtained for those cases without iso-merization catalysts. Raines et al. also reported a phenylboronic-acid based catalyst with an excess of MgCl2 for theconversion of glucose to HMF in about 50% yield.61

Carbohydrate dehydration in high boiling point aprotic sol-vents generally gives higher HMF yields. Product isolationfrom such reaction systems has been studied and most of thecases involve using extraction or distillation. The isolation pro-cesses either consume a large amount of organic solvent or areenergy intensive, which are unlikely to be used in industries.Very recently, a new method for the isolation of HMF fromwater–DMSO solution by zeolite adsorption was reported.62

That process may promote new HMF isolation technologies tobe developed.

3.1.3. Economic and green solvent system. High boilingpoint polar solvents, such as DMSO and DMF, often give highHMF yields for carbohydrate dehydration with various cata-lysts. However, the separation and purification of HMF fromthese solvents are complicated. To find a low boiling point,readily available and cost-efficient solvent for HMF productionis one of the current interests.59 1,4-Dioxane, as a low boilingpoint solvent, has been used in carbohydrate dehydration toproduce HMF.46,47 A recent report by Jeong developed aprocess for dehydration of high fructose corn syrup in dioxanewith Amberlyst-15 as solid catalyst and 80% HMF yield wasachieved at 100 °C in 2 h.63 Both solvent and catalyst can berecycled. Dumesic’s group made efforts in developing bio-based solvents for dehydration process.64 Use of HMF-derived2,5-(dihydroxymethyl)-tetrahydrofuran (DHMTHF) or low-boiling tetrahydrofuran (THF) as co-solvent results inincreased selectivity (>70%) of HMF at fructose conversions ofca. 80%. In another report from the same group, high HMFyields were achieved from glucose by using a combination ofAmberlyst-70 and Sn-β as solid acid catalysts in various bio-based solvents: γ-valerolactone (GVL) 59%, γ-hexalactone(GHL) 55% and THF 63%, respectively.64b

Alcohols are another type of attractive solvent.21 They canbe synthesized from biomass, possess better dissolvingcapacity for sugars and have a variety of boiling points. Alco-hols are also environmental friendly, cost efficient and easily

Scheme 5 “One-pot” synthesis of 5-(hydroxymethyl)furfural fromcarbohydrates using tin-beta zeolite.57

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operated reaction media. HMF could react with alcohols underacidic condition to form HMF-ether that would possiblyprevent HMF from further decomposition or oligomerization.However, it may also be a problem for the conversion of HMFto other downstream chemicals. In 2005, Bicker65 reported asupercritical methanol system for fructose dehydration withH2SO4 (10 mM) catalyst at 180–240 °C, 15–35 MPa for 2 to 30 s.A 5-methoxymethyl-2-furfural (MMF) yield of 78% wasachieved. Tarabanko et al.66 worked on fructose dehydration inalcohols at lower temperature. HMF-ether yields of 60–66%were obtained with H2SO4 catalyst (1.8 M) at 80–90 °C inethanol or n-butanol. Zhu et al.67 also reported a methanol–THF mixture solvent system for fructose dehydration withAmberlyst-15 catalyst and yield of 65% for the mixture of HMFand MMF was obtained.

In 2011, Zhang et al.68 developed an iso-propanol mediatedreaction system for production of HMF from fructose with HCl(5 mol%) as catalyst. Up to 87% of HMF yield was achieved at120 °C in 3 h. Interestingly, only HMF was produced in iso-pro-panol solvent, while a mixture of HMF and HMF-ether wasobtained when other alcohols, such as methanol, ethanol, 1-propanol and 1-butanol, were used as solvent (Scheme 6).Solid acid (Amberlyst 15) was also used as a recyclable catalystin this iso-propanol system, but lower yield and HMF selectivitywas obtained. The solvent and catalyst can be easily recycled byevaporation, giving the HMF product. Following that, Liu et al.69

screened various metal halides and ammonium salts as cata-lysts for the fructose dehydration in alcohols. It was found thatNH4Cl is an efficient catalyst for fructose dehydration inethanol to give a 71% HMF (containing a small amount ofEMF) yield at 100 °C for 12 h. Recently, alcohol-mediated carbo-hydrate dehydration systems have attracted increasingattention.70

To replace a batch reaction system with a continuous flowreactor is one important step toward large scale production.Dumesic et al.71 developed a tubular reactor designed for theproduction of HMF continuously from fructose in a single-phase solution of THF and H2O (4 : 1 w/w). The reactor waspacked with solid catalysts and operated at 403 K for extendedperiods, up to 190 h. The HMF selectivity using propylsulfonicacid-functionalized silica with ordered pore structures as

catalyst ranged from 60 to 75% (Scheme 7). Hermans et al.72

reported a continuous flow reaction system by using Amber-lyst-15 as catalyst and THF–DMSO solvent mixture for fructosedehydration. The HMF yield reached 92% in a continuous flowreactor at 110 °C, though at a residence time of only 3 min.Amberlyst-15 was shown to be stable over 96 h without loss ofactivity. In another report,73 a working setup for producingHMF continuously out of fructose in a coupled two-step reac-tion, or out of glucose, in THF–water solvent system was intro-duced. These continuous flow systems further demonstratedthe potential for large scale HMF production.

Low boiling point solvents, especially those low or non-toxic and bio-renewable solvents, represent a new direction forthe green carbohydrate dehydration process. These systemshave a lower environmental burden and require less energy forproduct isolation. They could have better potential in largescale production.

3.2. Biphasic system

In the new century, biphasic systems have been extensivelystudied in water–solvent system as the in situ HMF extractioncould prevent the decomposition of HMF in aqueous solution.Biphasic systems were also investigated in ionic liquid–solventsystems and this will be discussed later.

The major contributions in the new century for the carbo-hydrate dehydration in biphasic system came from Dumesic’sgroup. This group extensively studied various conditions andapplications of the biphasic systems.74–76 In 2006, 55% ofHMF yield from fructose in a MIBK–water system with HCl(0.25 M) catalyst was obtained.74 Soon after that, the yield wasimproved to 87% by modifying two phases. To avoid the cor-rosive acid catalyst, Dumesic’s group also tested solid catalysts,such as acidic ion-exchange resin and acid polymer modifiedsilica, in the biphasic system and good HMF yield was alsoachieved. A variety of two phases including water–MIBK, water(DMSO)–MIBK(2-butanol), water(DMSO)–CH2Cl2, water(NMP)–MIBK and water–alcohol were investigated.75 Several mineralsalts were tested for the saturation of the aqueous phase inwhich chlorides showed a beneficial effect. Dumesic’s groupalso tested their biphasic system for glucose dehydration,especially combined with other Brønsted acid catalyst, such asAlCl3, and 62% of HMF yield was reported.76

One important progress for HMF production is to demon-strate the large scale process for HMF production and theprocess from sugars to downstream chemicals via HMF. In2000, an interesting reaction system was developed for direct

Scheme 6 Yield of fructose to HMF in alcohols with different sterichindrance.67

Scheme 7 Dumesic’s continuous flow reactor.71

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conversion of fructose to furandicarboxylic acid (FDCA) in onepot.77 The reactor was separated into two parts by a membranewhich only allows HMF to pass through. In this design, theacidic dehydration solution will not affect the oxidization reac-tion. However, the efficiency of this system is very low. In 2007,it was reported that a combined process linking biphasic fruc-tose dehydration to HMF and a Ru/Cu catalyzed reductionprocess could convert HMF to dimethylfuran, a biofuel(Scheme 8).78 This process directly converts fructose to biofuelvia HMF, and the HMF isolation step was avoided. This rep-resented a good example of solving the HMF isolation chal-lenge. Since then, there have been other reports for directconversion of sugars to downstream chemicals via HMF as anin situ intermediate.79

Biphasic systems have also been widely studied by manygroups for the dehydration of sugars and polysaccharides.Various catalysts, such as H3BO3,

80 Ag3PW12O40,81a Al-TUD-1,82

acidic resins,27,76 Diaion PK216,76 zeolites,83

Cs2.5H0.5PW12O40,81 NA-p,82 SBA-15,83 SPC-108,82 TiO2 and

Taa-A380,84 have been used in dehydration reactions. For fruc-

tose dehydration, several systems gave over 85% HMF yields.For glucose dehydration, 76% HMF yield was reported in 2011in a water–MIBK system with Ag3PW12O40 catalyst at 130 °C for4 h.81a However, no high yield was reported for the de-hydration of glucose-based polysaccharides.76,81–84

Biphasic systems have been well developed in the newcentury. Solvent/aqueous biphasic systems can potentially beapplied on a large scale, especially when these systems arecombined with downstream conversion reactions of HMF.However, to ensure high extraction efficiency and high solventrecyclability is the key issue in the biphasic system.

3.3. Ionic liquid system

Although Fayet and Gelas first used pyridinium chloride basedionic liquid in fructose dehydration,25 the strong growth ofinterest towards the use of ionic liquids in biomass conversionstarted from Moreau’s work with imidazolium salts. In 2003,

Moreau et al.85 carried out fructose dehydration in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) with andwithout DMSO as co-solvent and Amberlyst 15 as catalyst. 87%HMF yield was achieved with DMSO at 80 °C in 32 h and 52%HMF yield was obtained without DMSO in 3 h. In 2006, Moreauet al.86 reported dehydration of fructose and sucrose in methyl-imidazolium chloride [HMIM]Cl, a Brønsted acid type of IL.More than 90% of the HMF yield was achieved from fructosewithout additional catalyst (Scheme 9). Since then, a variety ofimidazolium-based ionic liquids has been used in the dehy-dration of fructose, glucose and polysaccharides.23,87

Fructose dehydration in acidic ionic liquids without othercatalyst was further investigated. Various ionic liquids, such asimidazolium salts,28 choline chloride (ChoCl),88 ammoniumsalts,89 were used as reaction media for fructose dehydrationand up to 92% of HMF yield was achieved. A biphasic [EMIM]-(HSO4)/solvent system was also tested for fructose dehydration,yielding 88% and 79% of HMF from MIBK and the tolueneorganic phase, respectively.90

Using ionic liquids as solvents combined with other cata-lysts have attracted more attention, especially for dehydrationof glucose and polysaccharides. One of the important break-throughs in sugar dehydration to HMF was reported by Zhaoet al. in 2007.91 In ionic liquid [EMIM]Cl and CrCl2 catalyst,similar HMF yields were obtained from fructose (65%) andglucose (68%) at 100 °C for 3 h (Scheme 10). This is the firsttime that such high HMF yield was achieved from glucosefeedstock. Zhang et al. used chromium chlorides in fructoseand glucose dehydration in imidazolium ionic liquid in combi-nation with N-heterocyclic carbene (NHC) ligands, andimproved HMF yields from fructose (96%) and from glucose(81%) were achieved.92 Since then, chromium salts, chromiumnanoparticles and other solid chromium catalysts have beenwidely used in carbohydrate dehydration.93 Starting from2009, chromium salts combined with other conditions havebeen used in cellulose dehydration in ionic liquids. Impress-ively good yields (>50%) were achieved.94 Good yields were alsoachieved in similar systems with other polysaccharide feed-stocks, such as pine wood, cellobiose, starch and sucrose.95

Other catalysts have also been combined with ionic liquidsfor the dehydration process. Amberlyst-15, Dowex 50WX8, PTA,AuCl3, CuCl2, FeCl3 and ZrO2 were used in fructose

Scheme 8 Direct conversion of sugars to biofuel via a combinedprocess.78

Scheme 9 Moreau et al. reported dehydration of sucrose to HMF inmethylimidazolium chloride [HMIM]Cl.86

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dehydration.28,96 In 2009, Qi et al.97 performed dehydration offructose in an ionic liquid with an Amberlyst catalyst at roomtemperature, while Zhang et al.98 also reported a room temp-erature fructose dehydration system with WCl6 as catalyst in animidazolium ionic liquid. In THF/IL biphasic system, WCl6catalyzed fructose dehydration gave more than 80% HMFyield. At the same time, metal catalysts, such as GeCl4, SnCl4,CoCl2, CuCl2/PdCl2, CuCl2/CrCl2, MnCl2, H3BO3, Yb(OTf)3 andZrO2, were performed in fructose, glucose and polysaccharidedehydration in IL.44,57,58,99 Other solid acids, such as lignin-derived solid acid and carbon-based solid acid have also beenused in sugar dehydration.100 Microwave irradiation was alsoapplied for HMF production in ionic liquid.101 Generally, HMFyields from glucose in ionic liquid with various catalysts werein the range of 60 to 80%. There were two reported higherHMF yields (91%) in the [BMIM]Cl/CrCl3 system with MIBK asthe second phase or with microwave irradiation.90,94 For cellu-lose dehydration, Zhang reported a CuCl2–CrCl2 catalystsystem that gave 55% HMF yield in [EMIM]Cl;102 Qi alsoreported 55% HMF yield from cellulose in CrCl3/[BMIM]Clsystem.95a Wang reported 60% HMF yield from cellulose withCrCl3/LaCl3(LiCl) catalyst in [BMIM]Cl.103 The highest HMFyield (89%) from cellulose was reported by Zhang et al. in aCrCl2/[EMIM]Cl system at 120 °C for 6 h.104

Imidazolium salts were also modified as Brønsted acid cata-lysts in the dehydration reaction. For example, Jiang et al.105

reported the hydrolysis of cellulose in [BMIM]Cl with[C4H8SO3HMIM]Cl catalyst and 15% HMF yield was obtained.In addition, acidic ionic liquids were also widely used as cata-lysts in other solvent systems.106

Although ionic liquids have been widely used as reactionmedia for carbohydrate dehydration and good results wereachieved, there are also limitations for ionic liquids to be usedin large scale HMF production. ILs are generally expensive andtheir stability at high temperature is also a concern. Due to thehigh solubility of HMF in ILs, HMF isolation is another greatchallenge, especially for large scale production. Very recently,an interesting design involving an entrainer-intensifiedvacuum reactive distillation (EIVRD) process was developed forthe in situ separation of HMF from imidazolium ionic liquidreaction mixture.107 However, it is also an energy intensiveprocess due to the high vacuum with entrainer, especially forlarge scale production.

4. Hydroxymethylfurfural productionfrom bioresources: a perspective

HMF has been known as a product from hexose dehydrationfor over 100 years and is considered to be one of the mostpromising platform molecules that can be converted into avariety of interesting chemicals. However, the direct market forHMF itself is small. HMF production is essentially targeted atdownstream bulk chemicals, such as fuel and the plasticsindustry. The big challenge for the process of HMF productionis its suitability for industrial scale, yet being cost efficient.Although numerous improvements have been made in thedehydration process, most of the research is still focused onthe batch reaction system instead of efficient and economicallyviable large scale process. Some of the reaction systems gavevery high HMF yield from the dehydration of fructose, glucoseand even cellulose, for example in DMSO, however, such asystem may not be suitable on an industrial scale due to theHMF isolation issue. In fact, isolation and purification are themajor bottleneck for large scale HMF production.

Water and high boiling point aprotic solvents (includingionic liquids) are the most suitable solvents for carbohydratedehydration in terms of solubility and reaction efficiency.However, isolation of HMF from these solvents is a bigproblem and a large amount of organic solvent may be neededfor extraction. Improvements have been made in recent yearsby applying a biphasic system that may be suitable for a largescale process, although such a process still requires a relativelylarge amount of extraction solvent. In addition, solvent andcatalyst recycling have not yet been well-studied in biphasicsystems, especially for large scale production. Recent develop-ments in continuous reaction system represent a good trial forincreasing catalyst efficiency; however, there is still much to bedone for such a system, such as solvent recycling and HMF iso-lation (Table 1).

Low boiling point organic solvent systems represent apromising future for HMF production where HMF can beeasily isolated by a distillation process. Efforts by Dumesic’sgroup on using bio-based low boiling point solvents in de-hydration provide new opportunities, since such solvents willbe entirely green and have a lower environmental burden. Inalcohols, HMF was mostly obtained in its alkyl ether form and

Scheme 10 Chromium catalyzed glucose isomerization.91

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the yield was relatively low. However, recent work has shownthat the right combination of alcohol and catalyst could selec-tively produce HMF instead of the ether and the HMF yieldcould also be improved. These new findings on low boilingpoint solvent systems are certainly promising, but more workneeds to be done before they can be used for large scale syn-thesis. The dehydration of glucose and polysaccharides in alco-hols has not yet been well-demonstrated.

One challenge for carbohydrate dehydration to HMF is stillthe lack of an efficient and scalable system for glucose andglucose-based polysaccharides. Although high yields wereachieved in some systems, such as ionic liquid or high boilingpoint polar aprotic medium with chromium catalyst, these pro-tocols are unlikely to be suitable for large scale production.Therefore, more effort is needed for more environmentallyfriendly systems that are efficient and sustainable for glucose-based carbohydrate dehydration.

HMF is an important intermediate chemical, which servesas a platform to synthesize many essential bulk industrialchemicals, such as fuel, furandicarboxylic acid (FDCA) andadipic acid. Most of the research for biomass-based bulkchemicals is separated into two groups, one of which focuseson the biomass dehydration to HMF and another groupstudies the conversion of HMF to related chemicals. However,the gap between them is the quality or purity of HMF. In fact,isolation and purification of HMF is currently the bottleneckfor highly efficient HMF production. Therefore, it is importantto develop technology for direct conversion of carbohydratesinto bulk chemicals in one system. For example, FDCA ispotentially a replacement for terephthalic acid in PET industry.Oxidization of HMF to FDCA has been extensively studied andan almost quantitative yield could be achieved.108 However,all of these studies are based on pure HMF. In fact, rawHMF from dehydration could poison the catalyst and deacti-vate the system. Recently, Dumesic’s group demonstrated atwo-step system for conversion of fructose and glucose toFDCA.109 This system still requires distillation or multiplecycle extractions for HMF purification and the maximumFDCA overall yield is 50%. An efficient system that can inte-grate two steps together would potentially bring the process ofthe conversion of biomass to bulk chemicals closer for indus-trial applications.

Acknowledgements

This work was supported by the Institute of Bioengineeringand Nanotechnology (Biomedical Research Council, Agency forScience, Technology and Research, Singapore).

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