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Catalysis Today 200 (2013) 106– 116

Contents lists available at SciVerse ScienceDirect

Catalysis Today

j ourna l ho me p ag e: www.elsev ier .com/ lo cate /ca t tod

atalytic conversion of sugar into hydroxymethylfurfural in ionic liquids

ei Liu ∗, John Holladayacific Northwest National Laboratory, Richland, WA 99352, United States

r t i c l e i n f o

rticle history:eceived 30 March 2012eceived in revised form 21 July 2012ccepted 23 July 2012vailable online 27 September 2012

eywords:onic liquid

a b s t r a c t

Triisobutyl(methyl)phosphonium tosylate is discovered as a highly active and selective ionic liquidsolvent for conversion of fructose into HMF without any catalyst addition under moderate reaction condi-tions (80–110 ◦C). This ionic liquid provides high solubility for sugar and hydroxymethylfurfural (HMF),does not produce heavy or insoluble by-products, is available at a reasonable cost in large quantities,and thus, is promising for development of a practical HMF production process. This ionic liquid is alsoeffective for conversion of glucose into HMF in presence of CrCl2 catalyst. Formation of some unknowncompounds during the sugar conversion is reported first time in the field. In this work, a number of ionic

ugarydroxymethylfurfural (HMF)atalysiseaction

liquid + catalyst combinations are screened using a combinatorial experimental technique with fructose,high fructose corn syrup, and glucose as a feed. Reaction kinetics of sugars in the new ionic liquid istested in a batch reactor under various reaction conditions, in comparison to [EMIM]Cl – an ionic liquidextensively studied in the previous work. It is found that the catalytic activity of an ionic liquid is deter-mined by choice of both cation and anion, and can also be affected by its production source significantly.Reaction pathways are proposed based on the experimental results.

. Introduction

There has been an interest in hydroxymethylfurfural (HMF) as aeaction intermediate since the 1890s because of its potential appli-ation to a wide variety of desirable end products. A comprehensiveeview of synthesis chemistry of HMF and its derivatives wasrovided by Kunz [7] and Lewkowski [8]. Derivatives of HMF aretilized in agrochemistry as fungicides, and in galvanochemistry asorrosion inhibitors, and in cosmetic industry as flavor agents. HMFnd its furan derivatives are considered suitable starting materialsor production of some thermo-resistant polymers and complex

acrocycles. HMF can be converted into 2,5-furandicarbaldehydeFDC) and 2,5-furandicarboxylic acid (FDCA) by selective oxidation,hich can be used as a polymer monomer. Selective reduction ofMF can lead to products such as 2,5-hydroxymethylfuran and 2,5-is(hydroxymethyl)tetrahydrofuran, which can be used as alcoholomponents in the production of polyesters. These molecules canecome building blocks for numerous polyesters and polyamides1]. Thus, a new chemical and polymer industry may be developedompletely based on biomass-derived products [17]. Furthermore,iesel fuel additives can be prepared by reacting HMF with organic

cids or alcohols to form esters or ethers [5], and hydrocarbon fuelsay be made by hydrogenation and hydrotreating reactions ofMF [6]. Over 1000 papers have been published concerning HMF

∗ Corresponding author. Tel.: +1 509 375 2524.E-mail address: wei.liu@pnnl.gov (W. Liu).

920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2012.07.008

© 2012 Elsevier B.V. All rights reserved.

synthesis and applications. A variety of catalysts (including organicacids, inorganic acids, salts, Lewis acids, ion-exchange resins andzeolite) and solvents (aqueous, organic, mixed aqueous/organic)have been tried. A latest review by van Putten et al. [10] providescomprehensive information in various aspects of HMF fromconversion chemistry, process development, to applications.

The primary barrier to using HMF in high-volume chemical andfuel applications is its high cost and correspondingly low availabil-ity. To be commercially viable, HMF must be produced on a largescale at a cost comparable to petroleum-derived feedstocks suchas para-xylene (PX) and terephthalic acid (PTA). Fructose or glu-cose molecules can be converted into HMF by loss of three watermolecules, while sugars can be produced economically throughvarious processes. Production of HMF from sugar feedstock appearsto be a very attractive process route in terms of reaction stoi-chiometry and raw material supply. However, the actual conversionchemistry could be complex and a series of side-reactions mayoccur under reaction conditions, which strongly affect the pro-cess efficiency. As a result, HMF yield and/or selectivity reportedin earlier literature were often low.

More efficient and scalable catalytic conversion processes havebeen sought. A snapshot of various catalytic approaches canbe found in recent reviews [15]. An organic/aqueous two-phasereactor system was reported by Roman-Leshkov et al. [13] for

conversion of d-fructose to HMF. In the two-phase system, HMFproduct is extracted into the organic solvent phase while reactionsoccur in the aqueous phase. The authors reported 80% HMF selectiv-ity at 90% fructose conversion, i.e., 72% HMF yield, under optimum

ysis To

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owli1urbmbfomTuw

W. Liu, J. Holladay / Catal

eaction conditions. However, HMF solubility in the organic solvents low and the two-phase reaction involves a complex process.

Ionic liquids (IL) are a new solvent medium for catalytic con-ersion of HMF. Simple sugars, cellulose and even whole biomassre soluble in highly ionic media [4]. Conversion of fructose toMF in ionic liquids was reported by Moreau et al. [9]. Using an

onic liquid in combination with a soluble catalyst, Zhao et al.18] discovered that both glucose and fructose can be convertednto HMF with little formation of by-products. Therefore, HMFomprises 99% of the product in the water extract of the reactedixture, which is very beneficial to sugar utilization and HMF

roduct separation. HMF production in ionic liquids has becomen active catalysis research subject area since then. High HMFields (>70%) from fructose feedstock have been recently reportedith various new solvent and/or catalyst combinations, such as

olid heteropolyacid salt Ag3PW12O40 [3], tetraethyl ammoniumhloride (TEAC)–NaHSO4/H2O [2], phosphorous pentoxide (P2O5)12], FeCl3-tetraethyl ammonium bromide [16], and sulfated zirco-ia/ionic liquid [11]. Conversion of glucose and cellulose into HMF istill a difficult process without using chromium chloride catalysts.n those literature reports, however, delineation of by-products andarbon balances were often not conducted.

In this work, we aim to address some fundamental issues relatedo practical development of an ionic liquid catalytic process forMF production, such as determination of ionic liquids that areoth effective and low cost, impacts of major reaction conditionsn conversion and selectivity, and formation of by-products andarbon balances.

. Experimental

Three sugar feeds were evaluated in this work. Fructose99.9 wt%, Mallinckrodt), high fructose corn syrup (HFCS) (93.6 wt%,DMA Cornsweet), and d-glucose (99.5 wt%, Fluka) were used aseceived. Our analysis of these feeds using a HPLC method describedelow showed 75.5 wt% fructose in the HFCS and 96.2 wt% glu-ose in the glucose feedstock, indicating lower sugar content thanhe respective specification. The fructose feedstock was confirmedo be a pure compound. H2SO4, CrCl2, AlCl3, and CuCl2 cata-ysts were acquired from Aldrich as reagent grade. The catalyst

as added into the ionic liquid mixture typically at a loadingevel of 0.1 M. Tables 1 and 2 list ionic liquids tested in this

ork. 1-Ethyl-3-methylimidazolium chloride ([EMIM]Cl) was pur-hased from Fluka with two different purity levels (93%, 95%).EMIM]Cl was also acquired from BASF with kilogram quan-ities. Tetradecyl(trihexyl)phosphonium chloride (Cyphos 101)nd triisobutyl(methyl)phosphonium tosylate (Cyphos 106) werebtained from Cytec with large quantities.

Screening tests of catalysts and ionic liquids were conductedn the high throughput combinatorial testing apparatus at PNNL,hich was similar to what was used previously [18,14]. The ionic

iquid and catalyst were first added into a 3 cm3 glass vial at a load-ng level of 500 mg typically. The vial was heated under shaking to50 ◦C for 30 min to fully incorporate the catalyst into the ionic liq-id. Then, the vial was cooled and feed was added. Twenty-foureaction vials were hosted in one plate and multiple plates cane packed together in the reactor platform. Thus, tens of reactionixtures could be investigated under identical conditions in one

atch run. After the plate was heated at a targeted temperatureor a certain time, the plate was immediately opened, and 2 mLf water was added to each vial. The water/ionic liquid reaction

ixture was mixed well and centrifuged at 2000 rpm for 30 min.

he resulting liquid was analyzed on a high-performance liq-id chromatography (HPLC) instrument, Agilent 1100 series HPLCith Agilent G1362A refractive index (RI) Detector. The analytical

day 200 (2013) 106– 116 107

column used was BioRad Aminex HPX-87H (300 mm × 7.8 mm).The column temperature was maintained at 60 ◦C and a 0.005 MH2SO4 solution was used as the mobile phase at a flow rate of0.9 mL/min.

The reaction kinetics measurements were performed in a batchreactor. Typically, 50–200 g of a selected ionic liquid was loadedinto a glass reactor. The reactor was heated gradually in a heat-ing mantle to the targeted reaction temperature under continuousstirring and nitrogen gas purge. When the ionic liquid was fullymelted and the temperature was stabilized, sugar was graduallyadded into the reactor in less than 2 min. The reacting mixturewas sampled during the reaction. The reaction sample was dilutedwith de-ionized water and the mixture was filtered. The filtrate wasanalyzed using the HPLC method as described above.

The HPLC analysis gave weight content of the individual com-pound. The concentration of reaction products is normalized bythe initial concentration of the sugar feed on the C molar basis asfollows:

Ci

C0= ni(wi/MWi)

n0(w0/MW0)(1)

where Ci/C0 = ratio of product i to sugar feed on the basis of C num-ber. ni is the number of carbon atoms in compound i. wi is theweight content of compound i in a reaction mixture. MWi is themolecular weight of compound i. n0 is the number of carbon atomsin sugar molecule (6 for glucose and fructose). w0 is the weightcontent of sugar in initial reaction mixture. MW0 is the molecularweight of sugar molecule.

3. Results and discussions

3.1. Batch reactor tests of [EMIM]Cl-based reactions

[EMIM]Cl was an ionic liquid (IL) system that was extensivelyused in the previous combinatorial chemistry studies at PNNL andidentified as an effective IL for HMF production [18,14]. Thus, thisionic liquid was chosen initially for reaction kinetics study in abatch glass reactor. The experimental results are represented byvariations of sugar and HMF concentration inside the reactor withreaction time. It was found that purity of this ionic liquid had a dra-matic impact on catalytic reactivity. The 93% pure [EMIM]Cl wasactive by itself for conversion of fructose into HMF. Fig. 1 showsthat fructose content in the reacting mixture decreases with timeand HMF content increases concomitantly. 90% of the sugar feedwas converted within 60 min. By contrast, the 95% pure [EMIM]Clshowed no catalytic activity for fructose conversion, as evidencedby no change in the fructose content in the reacting mixture andno formation of HMF product. The catalytic activity is significantlyenhanced with addition of 0.1 M catalyst into the 93% pure ionicliquid. In the presence of a catalyst, nearly complete conversionof fructose occurred in 10 min, and correspondingly, formation ofHMF rapidly reached a plateau. The results indicate that conversionof fructose into HMF is catalyzed by acidic catalysts, which is con-sistent with earlier knowledge in the field. HMF yield obtained withthe H2SO4 catalyst appears higher than with the AlCl3 catalyst.

dCFru

dt= −k1 · CFru (2)

dCHMF

dt= k2 · CFru (3)

where k1 is the rate constant for conversion of fructose. k2 is the

rate constant for formation of HMF.

The above 1st order kinetics equations (2) and (3) are used tofit fructose conversion and HMF yield profiles, respectively. Exper-imental data points at two reaction temperatures are plotted in

108 W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116

Table 1Materials used in first group of combinatorial tests.

Feed stock Catalyst Ionic liquid Abbreviation

Fructose None 1-Ethyl-3-methylimidazolium chloride (95%, BASF) [EMIM]Cl

High fructose corn syrup (HFCS) H2SO4 1-Ethyl-3-methylimidazolium ethyl sulfate (95%, BASF) [EMIM] ethyl sulfate

Glucose CrCl2 1-Ethyl-3-methylimidazolium acetate (90%, BASF) [EMIM] acetateAlCl3 1-Ethyl-3-methylimidazolium diethyl phosphate (98%, Aldrich) [EMIM] diethyl phosphateCuCl2 Tetradecyl(trihexyl)phosphonium chloride (Cytec) Cyphos 101

Triisobutyl(methyl)phosphonium tosylate (Cytec) Cyphos 106

Table 2Ionic liquids used in second group of combinatorial tests.

Sample # Name Source

1 Triisobutyl(methyl)phosphonium tosylate – Cyphos 106 Cytec2 1-Butyl-3-methylimidazolium tosylate 97%, Fluka3 1-Ethyl-3-methylimidazolium tosylate 98%, Sigma4 1-Ethyl-3-methylimidazolium tosylate 99%, Solvent Innovation5 Tetrabutylphosphonium toluene-4-sulfonate6 Tetrabutylphosphonium methanesulfonate

7 Tributylphosphonium methyl sulfate

(a) Variation of fructose with ti me

(b) Variation of HMF yield with ti me

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80 10 0 12 0

C/C 0

Time, min

93% IL

93% IL+

93% IL+

95% IL

93% IL+ AlCl393% IL+ H2SO4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 10 0 12 0

HMF/

C 0

Time, min

93% IL

93% IL +AlCl3

93% IL +H2SO4

95% IL

93% IL+ AlCl393% IL+ H2SO4

Fig. 1. Conversion of fructose in [EMIM]Cl ionic liquids (Fluke) of different purityand catalyst addition in batch reactor testing (10 wt% fructose and 0.1 M catalystloading; reaction temperature of 84–87 ◦C). (a) Variation of fructose with time and(b) variation of HMF yield with time.

98%, Fluka98%, Fluka95%, Aldrich

Fig. 2 in comparison to the kinetics curves. The resulting appar-ent activation energy for fructose conversion in the 93% pure ionicliquid alone is 78 kJ/mol. It is noted that formation of levulinic andformic acids was not detected in these reaction runs. However, Fig. 2shows that HMF yield is less than the fraction of fructose converted.Thus, there are likely parallel reactions that convert fructose intosomething else, which will be discussed later.

At the same reaction temperature, catalytic reaction activity forglucose is much less than for fructose. The 93% pure ionic liquidalone did not have any catalytic activity for glucose conversion.Glucose conversion occurred only when a catalyst was addedinto the ionic liquid. Fig. 3 shows that the glucose content in thereacting mixture decreases monotonically with reaction time.At the same loading level (0.1 M), relative catalytic activity forglucose conversion looks to be AlCl3 > H2SO4 > CrCl2. This confirmsthat glucose conversion is catalyzed by an acidic catalyst as well.However, the glucose conversion activity does not correlate withformation of HMF. HMF yield with the CrCl2 catalyst increasesmonotonically with reaction time, while HMF yield tends to reacha plateau with the other two catalysts. HMF yield is much less

than the glucose converted, due to formation of by-products. CrCl2appears to be the most effective catalyst for HMF production from

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 10 0 12 0

C/C 0

Time, min

Fructose (87oC) HMF (87oC)

Fructose (120oC) HM F(120oC)

87oC 87oC

120oC 120oC

Fig. 2. Reaction kinetics of fructose in [EMIM]Cl (93% purity, Fluke).

W. Liu, J. Holladay / Catalysis To

(a). Variation of glucose with time

(b). Variation of HMF yield with time

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 10 0 12 0

C/C

0

Time, min

93% IL

+H2SO4 +CrCl2+ H2SO4

+ AlCl3+ CrCl2

0.0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 10 0 12 0

HM

F/C

0

Time, min

93% IL +AlCl 3

+H2SO4 +CrCl2+ H2SO4 + CrCl2

+ AlCl3

Fcg

gm

Hciofo

cl

3

svrwwpia

ig. 3. Conversion of glucose in [EMIM]Cl (93%, Fluke) with addition of differentatalysts (10 wt% glucose loading, batch reactor testing at 85 ◦C). (a). Variation oflucose with time. (b) Variation of HMF yield with time.

lucose feedstock, which is consistent with the earlier discoveryade in this research laboratory [18].Current research results show that dehydration of fructose into

MF is a fast reaction process in the ionic liquid, which may beatalyzed by an acid catalyst. Glucose may need to be convertednto fructose to form HMF. CrCl2 is likely to catalyze isomerizationf glucose into fructose. Hydrolysis of HMF into levulinic acid andormic acid does not appear to be a problem in current batch reactorperation.

Given the large variability, high melting point (∼80 ◦C) and highosts of [EMIM]Cl, search of more effective and less expensive ioniciquids was pursued.

.2. Combinatorial screening tests of new ionic liquid + catalyst

Six ionic liquids and four catalysts as listed in Table 1 werecreened on the high-throughput combinatorial apparatus for con-ersion of different sugars into HMF. Compared to the above batcheactor test, only a small amount of the ionic liquid (about 500 mg)as used in the combinatorial tests. Thus, the material balance

as poor, and the results mostly serve for relative comparison pur-oses. Figs. 4–6 show the compositions of final reacted mixtures

n various ionic liquid + catalyst combinations for fructose, HFCS,nd glucose feeds, respectively. In these plots, concentrations of

day 200 (2013) 106– 116 109

individual compounds in a reacted vial are normalized to the initialfeed on the basis of carbon number. Theoretically, those individualcompounds in each reaction vial should add up to 100% after reac-tion. The discrepancy reflects that some reaction products werenot quantified by the current HPLC analytical method and/or thatthe reaction products in the ionic liquid were not fully extractedinto water during the HPLC sample preparation.

As shown in Fig. 4, HMF, fructose, and glucose were the mainconstituents in the reacted mixture with the fructose feed. Smallamounts of cellobiose were found in some reaction vials, while lev-ulinic acid and mannose were not detected. Formic acid was foundonly in one reaction vial that was loaded with [EMIM] acetate andH2SO4. Without any catalyst additive, Cyphos 106 produced a sig-nificant amount of HMF, while no or little HMF yield was obtainedusing the other five ionic liquids. Addition of H2SO4, AlCl3, or CuCl2catalyst into this ionic liquid had little impact on the HMF yield.By contrast, HMF yield in [EMIM]Cl (95%, BASF) was dramaticallyenhanced with addition of H2SO4, AlCl3, or CuCl2 catalyst. The com-binatorial testing results of the [EMIM]Cl are consistent with thebatch reactor testing results with the same ionic liquid but obtainedfrom a different vendor (95%, Fluke). HMF yields in the other ionicliquids were still small even with the catalyst addition. The poorcarbon balances observed with Cyphos 101 were largely caused byin-efficient water extraction of the reactant and products from thisionic liquid. The present screening results indicate that molecularstructures of cation and anion in an ionic liquid play a dominantrole in conversion activity and selectivity.

By comparing Fig. 5 to Fig. 4, one can see that the productdistribution with HFCS feed is similar to that with fructose feed,because fructose is the main constituent of HFCS. Significant HMFyield was obtained in the Cyphos 106 ionic liquid without any cat-alyst addition, while no HMF was produced in the [EMIM]Cl ionicliquid without catalyst. Addition of an acidic catalyst dramaticallyenhanced HMF yield in [EMIM]Cl but showed little impact on HMFyield in Cyphos 106.

Conversion of glucose into HMF is much more difficult thanfructose. Reaction tests with glucose feed were conducted at ahigher temperature than what was used for fructose and HFCSfeeds (100 ◦C versus 80 ◦C). Fig. 6 shows that no or little HMF wasformed in any ionic liquid without catalyst addition. Glucose wasvery reactive in [EMIM] acetate and [EMIM] diethyl phosphate, butthe reaction was not selective toward HMF. Instead, glucose wasconverted into water-insoluble matters in those ionic liquids. Asa result, the carbon balances were very poor in those runs. Smallamounts of HMF yield were obtained by adding H2SO4 or AlCl3 cat-alyst into [EMIM]Cl and Cyphos 106. However, the highest HMFyield was obtained using CrCl2 catalyst. No significant differencewas shown between [EMIM]Cl and Cyphos 106 for glucose conver-sion into HMF with addition of CrCl2 catalyst. The results suggestthat Lewis or Brønsted acidic catalysis is less important to glucoseconversion into HMF.

Impacts of anion and cation in an ionic liquid on catalytic reac-tion activity were studied. Seven ionic liquids selected for this groupof experiments are listed in Table 2. Samples #3 and 4 are the sameionic liquid but obtained from two different vendors. The molec-ular structures of these ionic liquids are drawn in Fig. 7. Samples#1–5 have the same anion but different cations. Samples #5–6 havethe same cation but different anions. Samples #6–7 have the sameanion but slightly different cations. The seven ionic liquids weretested for fructose and glucose conversion without any catalystaddition, and glucose conversion with addition of the CrCl2 cat-alyst. Fig. 8 shows the distribution of fructose, glucose and HMF in

each reaction vial after the reacting mixture was heated at 100 ◦Cfor 3 h.

For fructose conversion, Cyphos 106 still appears superior tothe other ionic liquids tested. The catalytic function of Cyphos 106

110 W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116

id + ca

dccys

Fig. 4. Product distribution of fructose reaction in various ionic liqu

oes not result from tosylate anion alone, and is attributed to con-erted effects of tosylate anion and tributyl(methyl)phosphonium

ation. In general, phosphonium ionic liquids produced higher HMField than the imidazolium ionic liquids. None of the ionic liquidshowed an activity for appreciable HMF yield from glucose without

Fig. 5. Product distribution of HFCS reaction in various ionic liquid + cata

talyst mixtures (10 wt% loading, 3-h heating at 80 ◦C in a glass vial).

catalysts. With addition of CrCl2 catalyst, appreciable amounts ofHMF yield were produced in the ionic liquid samples #1, 3, 5, and

6. It is interesting to note that the samples #3 and 6 also exhib-ited a significant activity for isomerization of glucose to fructose.Comparison of samples #6 to #7 suggests that a slight difference

lyst mixtures (10 wt% loading, 3-h heating at 80 ◦C in a glass vial).

W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116 111

d + cat

iiouc

syaatactts

Fig. 6. Product distribution of glucose reaction in various ionic liqui

n cation between those two ionic liquids could have a significantmpact on glucose-to-fructose isomerization activity. This groupf experiments confirms that the catalytic activity of an ionic liq-id for sugar conversion is affected by choice of both anions andations.

The testing results with [EMIM]Cl are consistent with previoustudies [18], and catalyst addition is necessary to obtain high HMFields. However, this ionic liquid was too expensive to be used at

larger quantity and its melting temperature (about 80 ◦C) is rel-tively high. Discovery of Cyphos 106 in this work is importanto the process research and development activities that require

much larger quantity of the ionic liquid (a few gallons in theurrent laboratory scale) than that used in the combinatorial vial

esting (a few milliliters). Based on above screening tests, fruc-ose and Cyphos 106 was selected for systematic reaction processtudies.

Fig. 7. Molecular structures of io

alyst mixtures (10 wt% loading, 3-h heating at 100 ◦C in a glass vial).

3.3. Reaction process studies of fructose conversion in Cyphos 106

TGA/MS characterization of Cyphos 106 +fructose reactivity. Stabil-ity of Cyphos 106 and its reactivity with fructose were characterizedon a Thermogravimetric Analyzer (TGA)/Differential ScanningCalorimeter (DSC) apparatus equipped with an on-line MassSpectrometer (MS). Sugar powder was mixed with the ionic liquidsolid by grinding at room temperature to generate a homogenouspaste. Weight change and heat flux profiles for the ionic liquid, purefructose, and three fructose/ionic liquid mixtures are compared inFig. 9a and b, respectively. The ionic liquid was stable up to 190 ◦Cand no decomposition products were detected by MS. There wasabout ∼0.6 wt% weight loss during initial heating. The slight weight

loss was due to vaporization of adsorbed moisture. The ionic liquidlooked the same before and after the TGA test. By contrast, purefructose was completely turned into char after the TGA test. The

nic liquids listed in Table 2.

112 W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116

Fig. 8. Reaction conversion of fructose and glucose in ionic liquids of different

(a). Variation of weight with ti me and te mperature

(b). Variation of heat flux with ti me and te mperature

0

50

100

150

200

250

80

85

90

95

100

105

0 30 60 90 120 150 180 210 24 0

Te

mp

era

ture

, oC

We

igh

t c

ha

ng

e, %

Time, min

Ionic liqu id

5% fructose10%

20%

fructose

0

50

100

150

200

250

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0 30 60 90 120 150 180 210 240

Te

mp

era

ture

, oC

He

at fl

ux

, m

W/m

g

Time, min

Ionic liqu id

Fructose

5 % Fructose10 % Fructose

20 %

Fig. 9. TGA/DSC characterization of fructose reactivity in Cyphos 106. (a) Varia-tion of weight with time and temperature. (b) Variation of heat flux with time andtemperature.

anions and cations (10 wt% loading, 3-h heating at 100 ◦C in a glass vial).

initial weight loss for fructose was due to desorption of adsorbedmoisture. Pure fructose started losing its weight at temperaturesabove 110 ◦C due to decomposition and charring. Weight loss of thefructose/ionic liquid mixture started at lower temperatures around60 ◦C. This weight loss was accompanied with evolution of H2Omolecule in the TGA effluent gas as detected by MS. Taking intoaccount the batch reaction testing results at different temperatures,we think that this weight loss was likely due to dehydration of fruc-tose into HMF. Two heat flux peaks are shown in the early stage ofheating process. The first one is believed to be due to evaporationof adsorbed moisture, while the second one is due to melting of theionic liquid. Interestingly, no large heat flux peak is shown duringthe reaction stage, which suggests that the reaction heat of fructoseto HMF in the ionic liquid would not be large. A large heat flux peakis shown for charring of fructose in absence of the ionic liquid.

Reaction kinetics of fructose in Cyphos 106 in a batch reactor. Reac-tion kinetics of fructose was measured in a batch reactor underdifferent conditions. Fig. 10 shows variations of fructose and HMFcontent in the ionic liquid with reaction time at different temper-atures. As expected, the fructose content decreases as HMF yieldincreases with time. This ionic liquid system is very active for fruc-tose conversion. Appreciable HMF conversion occurred at 60 ◦C in100 min. The reaction became very fast when the temperature wasraised to 80 and 110 ◦C. At 110 ◦C, mixing of fructose with the ionicliquid and reaction conversion proceeded almost simultaneously.Nearly complete conversion of fructose happened at 110 ◦C in afew minutes. Two duplicate runs were conducted at 80 and 110 ◦C,which are denoted as respective (I) and (II) in the figures. Theexperimental data were regressed with first order reaction kineticsequations. The kinetics simulation results are represented by con-tinuous lines in Fig. 10. An apparent activation energy for fructoseconversion of 81 kJ/mol is obtained, which is close to the number forfructose conversion in [EMIM]Cl. However, reaction rate constantfor Cyphos 106 is three to four times higher than that for [EMIM]Clat the same reaction temperature. The HMF formation kinetics is

similar to the fructose conversion except an upper threshold seemsto exist for HMF yield.

Since the ionic liquid is considered an expensive solvent,impacts of fructose loading on HMF formation are examined. Fig. 11

W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116 113

(a). Variation of fructose with ti me

(b). Variati on of HMF yield with time

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 10 100

C/C

0

Time, min

60oC

80oC (I)

80oC (II)

110oC (I)110oC (II)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 10 10 0

HM

F/C

0

Time, min

60oC

80oC (I)80oC (II)

110oC (I)

110oC (II)

Fig. 10. Reaction kinetics of fructose conversion in Cyphos 106 ionic liquid (10 wt%ff

sffpqTufu

eiciat1veto

(a). Variation of fructose with ti me

(b). Variation of HM F yield with time

0.0

0.2

0.4

0.6

0.8

1.0

0 1 10 10 0

C/C

0

Time, min

5 % fructose

10% fructose

25 % fructose

0.0

0.2

0.4

0.6

0.8

1.0

0 1 10 10 0

HM

F/C

0

Time, min

5 % fructose

10% fructose

25 % fructose

shows the carbon balance of reaction batches after 0.5 and 1-h reac-tion. HMF product and un-converted fructose were the only twocomponents by the current HPLC method, as content of formic acidand levulinic acid was zero. However, the carbon balance obtained

Table 3Carbon balance of a reacted batch with identified compounds only (reaction condi-tions: 10 wt% fructose in Cyphos 106, 80 ◦C).

Reaction time, h Composition of reacted mixture, wt% C balance, mol/mol

ructose loading, batch reactor testing at different temperatures). (a) Variation ofructose with time. (b) Variation of HMF yield with time.

hows rapid conversion of fructose at 110 ◦C with 5, 10, and 25 wt%ructose loading. However, HMF yield decreases with increasingructose loading. It takes a longer time for HMF yield to reach alateau at higher fructose loading. This is partially explained byuicker dispersion of sugar into the ionic liquid at lower loading.he higher fructose loading means a less amount of the ionic liq-id needed for a practical process. The present result suggests thatructose loading in Cyphos 106 may be significantly increased byse of efficient, rapid mixing methods.

Water content in the reaction mixture is an important param-ter for a practical process. Water content may become significantn a large commercial reactor. The recycled ionic liquid in a practi-al process may contain some water. Thus, impact of water contentn the reaction mixture on reaction performances was assessed bydding water and sugar together into the ionic liquid at the reactionemperature. At reaction temperature of 80 ◦C (Fig. 12), addition of

or 5 wt% water into the reacting mixture reduced fructose con-ersion rate and HMF yield as well. Thus, water has some inhibition

ffect on the reaction. At a higher temperature of 110 ◦C (Fig. 13),he conversion profiles with 5 wt% water and without any waterverlap each other. Addition of the water did not make a difference.

Fig. 11. Impacts of fructose loading on conversion and HMF yield (batch reactortesting at 110 ◦C, Cyphos 106). (a) Variation of fructose with time. (b) Variation ofHMF yield with time.

This can be explained by evaporation of water at this temperature.Thus, this reaction process is preferably conducted at temperaturesabove the boiling point of water so that liquid water is removedfrom the ionic liquid.

Carbon balance and unknown by-products. Carbon balance wasconducted for the batch reaction runs. In these runs, the batch reac-tor was quenched by adding deionized water at a weight ratio ofabout 1:1 to the initial charge after a certain time to avoid potentialside reactions. The resulting batch mixture was analyzed. Table 3

Fructose Levulinic acid HMF Reacted/initial

0.5 0.90 0.00 2.30 93.0%1.0 0.14 0.00 2.87 93.4%

114 W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116

(a). Variation of fructose with time

(b). Variation of HM F yield with time

0.00.10.20.30.40.50.60.70.80.91.0

0 10 20 30 40

C/C 0

Time, min

no water

1 % water5 wt% water

0.00.10.20.30.40.50.60.70.80.91.0

0 10 20 30 40

HMF/

C 0

Time, min

no water1 % water

5 wt% water

Fig. 12. Impacts of water addition on fructose conversion and HMF yield (10 wt%lt

iascsrrgHls

rtld1Wrw

(a). Variation of fructose with time

(b). Variation of HMF yield with time

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60

C/C

0

Time, min

10% Fru

10% Fru +5% water

10% Fru +12% water

20% Fru + 25% water

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60

HM

F/C

o

Time, min

10% Fru

10% Fru +5% water

10% Fru +12% water

20% Fru + 25% water

Fig. 13. Impacts of fructose loading and water addition on fructose conversion and

oading in Cyphos 106, batch reactor testing at 80 ◦C). (a) Variation of fructose withime. (b) Variation of HMF yield with time.

n this way was only around 93%. Some carbon atoms were notccounted for. Examination of HPLC chromatograms found thatome peaks in reacted mixtures could not be identified with thealibration compounds we have known so far. As shown in Fig. 14,everal significant peaks exist between the ionic liquid and fructoseetention time, which are denoted as unk (i.e., unknown). Then, theeaction mixture was characterized with LC/MS. The analysis sug-ests the presence of a compound with molecular weight of one2O molecule less than fructose. Due to interference of the ionic

iquid, we could not clearly elucidate the structures of the unknownpecies.

By lumping all the unknown peaks together and using the HPLCesponse factor of fructose, we try to get a rough assessment ofhe carbon balance by taking into account the unknown. Table 4ists the carbon balances of a few batch runs under different con-itions. Although there is variation in carbon balance from 97.7 to

03.3%, majority of the carbon can be accounted for in this way.e understand that those unknown species may have different

esponse factors from fructose. The unknown is quantified in thisay for approximation only. Impacts of reaction conditions on the

HMF yield (Cyphos 106, batch reactor testing at 110 ◦C). (a) Variation of fructosewith time. (b) Variation of HMF yield with time.

unknown yield were studied to further delineate formation mech-anisms of the unknown.

Fig. 15a and b shows variations of the unknown content withreaction time at 80 ◦C and 110 ◦C, respectively. Significant amountsof the unknown were formed at the beginning of the reaction run.The unknown tends to decrease with reaction time. The yield ofthe unknown tends to be higher at the higher sugar loading. Thecontent of the unknown tends to decrease with increasing reaction

temperature. Impacts of fructose loading and water addition onformation of the unknown are shown in Fig. 16. The unknown tendsto increase at the beginning and then, decline. Clearly, formation of

W. Liu, J. Holladay / Catalysis Today 200 (2013) 106– 116 115

Table 4Carbon balances of reacted batches by taking into account unknown by-products.

Batch # Reaction conditions Carbon balance, mol %

Temp, ◦C Time, min N2 purge, sccm Fructose in feed, wt% Reacted/initial Fructose conversion HMF yield Unknown yield

#1 110 20 50 9.5 97.7 99.1 83.3 13.1

Htp

F

F

F

so

Ft

#2 110 20 50 24.7

#3 85 20 50 10.6

#4 110 10 50 10.1

MF product follows a different kinetic pathway from formation ofhe unknown by-products. Thus, the following reaction paths areroposed to describe fructose conversion in the ionic liquid.

ructose → HMF + 3H2O (4)

ructose ↔ Reaction intermediate + H2O → HMF + H2O (5)

ructose + HMF ↔ Reaction intermediate (6)

These reaction intermediates are considered as unknownpecies. Their formation rate is faster than direct conversion ratef fructose into HMF. These intermediates are soluble compounds

ig. 14. Un-identified peaks in HPLC chromatogram (batch reactor testing condi-ions listed in Table 3). (a) At reaction time of 0.5 h. (b) At reaction time of 1.0 h.

103.3 98.5 74.0 26.0101.8 98.4 77.4 22.5100.5 97.9 78.7 19.4

in the ionic liquid and water. Production of the heavier such aspolymers and acids is avoided in this ionic liquid reaction system.Further research is needed to determine molecular identifies of thereaction intermediates and understand their conversion chemistry.

Reactivity and conversion of glucose are described with the fol-lowing reaction equations:

Glucose ↔ Fructose → HMF + 3H2O (7)

Glucose → Other products (8)

Glucose → Heavier (polymers, particulate, etc.) (9)

(a). Reaction temperature of 80oC

(b). Reaction te mperature of 110oC

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1501209060300

Unk

now

n/C 0

Time, min

5 % fructose

10% fructose

0.0

0.2

0.4

0.6

0.8

1.0

1.2

403020100

Unk

now

n/C 0

Time, min

5 % fructose10% fructose25 % fructose

Fig. 15. Impact of fructose loading and reaction temperature on formation ofunknown (batch reactor testing, Cyphos 106). (a) Reaction temperature of 80 ◦Cand (b) reaction temperature of 110 ◦C.

116 W. Liu, J. Holladay / Catalysis To

0.0

0.2

0.4

0.6

0.8

1.0

6050403020100

Un

kn

ow

n/C

0

Time, min

10% Fru

10% Fru +5% water

10% Fru +12% water

20% Fru + 25%water

F(

pshaTicge

4

tiiaHaatefbl

[

[

[[[

[[

ig. 16. Effect of fructose loading and water addition on formation of unknownbatch reactor testing at 110 ◦C, Cyphos 106).

Isomerization of glucose into fructose is proposed as a mainathway for production of HMF from glucose. This is a slow reactiontep relative to dehydration of fructose into HMF. Glucose can beighly reactive in an ionic liquid with addition of an acidic catalyst,nd converted into insoluble matters and/or other by-products.hus, an ionic liquid catalyst system of strong activity for glucosesomerization is preferred to obtain high HMF yields. So far, Crhloride is still the most active catalyst additive for conversion oflucose into HMF, although both [EMIM]Cl and Cyphos 106 are anffective solvent medium for this catalytic process.

. Conclusion

A number of ionic liquid/catalyst combinations are screenedo produce HMF from sugars. The catalytic activity of anonic liquid is determined by both cation and anion. Tri-sobutyl(methyl)phosphonium tosylate – Cyphos 106 is discovereds an effective, low-cost ionic liquid for conversion of fructose intoMF. Conversion of fructose in this ionic liquid without any catalystddition occurs under moderate conditions: 10–60 min, 80–110 ◦C,tmospheric pressure. No acids and solid maters are produced inhe ionic liquid at nearly complete conversion of fructose. How-

ver, some unknown by-products (or reaction intermediates) areound present in the reacted mixture and could not be determinedy the current HPLC method. Formation of the unknown, which

ikely occurs concomitantly with dissolution of sugar molecules,

[

[

day 200 (2013) 106– 116

could be not eliminated by adjusting the reaction conditions.Conversion of the unknown into HMF will be reported in thenear future. Identifying and understanding these unknown com-pounds could be an interesting subject for fundamental catalysisstudies.

Triisobutyl(methyl)phosphonium tosylate – Cyphos 106 is com-parable to [EMIM]Cl for conversion of glucose into HMF. However,addition of CrCl2 catalyst into the ionic liquid is necessary to achieveappreciable HMF yields from glucose.

Acknowledgements

We would like to thank our colleagues at PNNL, Alan Cooper andHeather Brown for conducting the experimental work, Drs. JamesWhite, Michael Lilga, and John Lee, for helpful discussions and con-sulting, Dr. Abhi Karkamkar for conducting TGA measurements, Dr.Al Robertson at Cytec for providing the ionic liquid samples, andstudent interns, Joanne Li and Joshua Croshaw, for their assistanceto some experimental work. We also would like to thank our indus-trial partner – UOP, Timothy Brandvold, Joseph Kocal, Sharry Lynch,for starting this project. This project was funded by USDA underGrant Agreement # 68-3A75-7-613.

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