Journal of Catalysis 288 (2012) 8–15

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

Direct conversion of cellulose and lignocellulosic biomass into chemicalsand biofuel with metal chloride catalysts

Saikat Dutta a, Sudipta De a, Md. Imteyaz Alam a, Mahdi M. Abu-Omar b,⇑, Basudeb Saha a,⇑a Laboratory of Catalysis, Department of Chemistry, North Campus, University of Delhi, Delhi 110 007, Indiab Brown Laboratory of Chemistry, Department of Chemistry, Purdue University, West Lafayette, IN 47096, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 September 2011Revised 12 December 2011Accepted 21 December 2011Available online 2 February 2012

Keywords:5-Hydroxymethylfurfural5-Ethoxymethyl-2-furfuralBiomassMicrowavesSustainable process

0021-9517/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcat.2011.12.017

⇑ Corresponding authors. Fax: +1 765 494 0239.E-mail addresses: mabuomar@purdue.edu (M.M.

try.du.ac.in (B. Saha).

Direct transformation of cellulose and sugarcane bagasse into 5-hydroxymethylfurfural (HMF) was car-ried out using single or combined metal chloride catalysts in DMA–LiCl solvent under microwave-assisted heating. Among several metal chloride catalysts studied, Zr(O)Cl2/CrCl3 combined catalyst wasmost effective enabling 57% HMF from cellulose fiber. Zr(O)Cl2/CrCl3 catalyst was also effective for theconversion of sugarcane bagasse to HMF and 5-ethoxymethyl-2-furfural (EMF), a promising biofuel. Thisreport discloses one-pot synthesis of EMF from sugarcane bagasse for the first time and also reveals themaximum HMF yield known so far from sugarcane bagasse. Thus, the current catalytic system based oncheap and readily abundant zirconium and chromium salts presents an efficient method for EMF andHMF synthesis from inexpensive lignocellulosic biomass.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

The awareness of climate change and diminishing fossil fuelreserves necessitate the replacement of current hazardous andnonrenewable processes with sustainable, green and environmen-tally benign practices [1,2]. In the next two decades, the petroleumproduction is unlikely to keep pace with the growing demand forfuels and chemicals [3]. Thus, new synthetic routes and relatedtechnologies for generating fuels and chemicals from renewablefeedstock are needed. In this context, biomass-derived 5-hydrox-ymethylfurfural (HMF) [3,4–7] has emerged as an important plat-form chemical for the next generation plastics [8] and biofuel [9]production. Despite the versatile application of HMF, sustainablesynthetic routes for its production in scalable quantities are yetto be developed [10]. Facile HMF syntheses from sugar derivativesusing a wide range of catalysts including metal catalysts in ionic li-quid, acid catalysts in biphasic media have been developed [11].However, these existing processes of HMF production are primarilydependent on the edible monosaccharide substrates [12].

Conversely, direct transformation of the untreated lignocellu-losic biomass to HMF is not dependent on the edible crops-derivedcarbohydrates. Therefore, HMF synthesis from untreated biomassaddresses the sustainability issue and offers commercial scaleapplication opportunity. Several researchers have investigated

ll rights reserved.

Abu-Omar), bsaha@chemis-

catalytic routes for HMF synthesis from cellulose [13–20] and un-treated lignocellulosic biomass [6,13,18]. Binder and Raines [13]reported that CrCl2 catalyzed transformation of untreated biomass(corn stover) and purified cellulose can produce 48% and 54% HMF,respectively, in N,N-dimethyl acetamide (DMA) solvent containingLiCl and 1-ethyl-3-imidazolium chloride ([EMIM]Cl) additives at140 �C with reaction time 2–3 h.

In the same year, Zhang et al. [17] reported the formation of 58%HMF from cellulose using two metal chloride catalysts (CuCl2 andCrCl2) in ionic liquid at 120 �C with a reaction time of 8 h. Twoyears later, Wang et al. [21] reported the Brønsted–Lewis surfac-tant-combined heteropolyacid (HPA), Cr[(DS)H2PW12O40]3 (DS = O-SO3C12H25, dodecyl sulfate) micellar catalyzed transformation ofcellulose to HMF with a maximum of 53% HMF yield. The mostrecent publications [22,23] in this area of research showed the for-mation of 48% and 60% HMF from cellulose using CrCl2–NHC-car-bene/zeolite and combined CrCl2/RuCl3 catalytic systems in ionicliquids. Cellulose transformation with CrCl2–NHC-carbene/zeolitecatalyst is reported to occur via hydrolysis of cellulose followedby the dehydration of the resulting glucose [22]. The later reactionwith CrCl2/RuCl3 was carried out using 4:1 M ratio of CrCl2/RuCl3 at120 �C for 2 h [23]. Most of these reactions were carried out underthermal heating and hence required long reaction times. It isknown that the side reactions giving oligomeric species fromHMF and sugars become dominant at longer reaction times [24].In this context, microwave-assisted rapid synthesis of HMF fromcellulose [15] is significant, which affords 62% HMF yield in2 min using ionic liquids as reaction medium. Recent literature

S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15 9

reports have disclosed efficient hydrolysis of cellulose in ionic li-quid–water mixed solvent [25,26]. Although ionic liquids are thepreferred solvents for HMF synthesis [27] due to their inherentcharacteristic properties such as low vapor pressure, good thermalstability, and a range of tunable hydrophobicity/hydrophilicity[4,28], their high cost limits their industrial application.

To minimize the use of expensive ionic liquids, DMA contain-ing LiCl has been utilized, which also has the added advantage ofdissolving purified cellulose. This report discloses microwave-as-sisted direct transformation of cellulose and lignocellulosic bio-mass into HMF using metal chloride catalysts in DMA�LiClsolvent with and without 1-butyl-3-methylimidazolium chloride([BMIM]Cl]) additive. Experimental results demonstrate thatZr(O)Cl2/CrCl3 is an effective catalyst for the direct conversionof biomass to HMF and 5-ethoxymethyl-2-furfural (EMF), apromising biofuel. The use of this catalyst combination for thesynthesis of HMF and EMF has not been reported previously. Thisreport also discloses one-pot synthesis of EMF from sugarcanebagasse for the first time.

2. Experimental

2.1. Chemicals

Metal salts, CrCl3�6H2O, AlCl3 (anhydrous), and Zr(O)Cl2�8H2O of�99% purity were purchased from Thomas Baker, India, and usedwithout further purification. Linter cotton cellulose fiber, a-cellu-lose powder, sigmacell cellulose powder, and 1-butyl-3-methyl-imidazolium chloride were purchased from Sigma–Aldrich andused as received. DMA and lithium chloride were purchased fromSpectrochem, India. Cellulose samples were oven-dried to constantweight at 120 �C prior to use. Sugarcane bagasse was collectedfrom local sugarcane juice shop and dried. The dried samples werecut into small pieces and grind to powder before use.

2.2. Instrumentation

Cellulose transformation reactions were performed in a CEMMatthews WC Discover microwave reactor, model no. 908010DV9068 equipped with programmable pressure and temperaturecontroller. 1H NMR spectral analysis was performed on a JEOLJNM ECX-400 P 400 MHz instrument, and data were processedusing JEOL DELTA program version 4.3.6. HMF yields were mea-sured by HPLC and UV–Visible spectrophotometric (UV-SPECORD250 analytikjena spectrometer) techniques. A Shimadzu HPLCinstrument (model: 20 AD) equipped with UV detector and pres-sure gradient pumps was used for determining HMF yields fromthe reaction mixture.

2.3. Catalytic conversion of cellulose and sugarcane bagasse into HMF

A microwave reactor tube was charged with the desired wt.%of cellulose substrates or sugarcane baggase and 0.5 g DMA–LiCl.The mixture was stirred at 80 �C for 5 min to dissolve the cellu-lose substrate. The tube was sealed with cap after loading withthe desired amount of metal chloride catalyst and ionic liquidadditive and was placed in the microwave reactor. The powerof the microwave reactor was set to 300 W. Upon completionof reaction for the set reaction time, the reaction mass wascooled to room temperature. The pale-yellow oily HMF productwas extracted with diethyl ether and analyzed by NMR tech-nique. 1H NMR (CDCl3): d 9.58 (s, 1H), 7.20 (d, J = 2.8 Hz, 1H),6.51 (d, J = 2.8 Hz, 1H), 4.70 (s, 2H). 13C NMR (CDCl3): d 177.76,160.97, 152.04, 123.47, 109.94, 57.25.

2.4. Catalytic conversion of sugarcane bagasse into EMF

One-pot synthesis of EMF from sugarcane bagasse was carriedout in ethanol by oil-bath heating. A round-bottom flask wascharged with 1 g sugarcane bagasse powder, 20 mL ethanol,20 mol% of Zr(O)Cl2/CrCl3 catalyst in 3:1 M ratio of Zr(O)Cl2/CrCl3

and 9 wt.% [BMIM]Cl. The mixture was refluxed with continuousstirring at 120 �C for 15 h. After 15 h reaction, the reaction mixturewas cooled to room temperature, evaporated ethanol under vac-uum, and the oily residue was run through a column chromato-graph made with silica gel of 200–400 mesh as a stationaryphase and a mixed dichloromethane/diethyl ether solvent (2:1 vol-ume ratio) as a mobile phase. After separating [BMIM]Cl compo-nent by column chromatograph, the oily product layer wascollected and analyzed by NMR spectroscopy. Both 1H and 13CNMR spectra of the oily layer revealed the formation of EMF andethyl levulinate (EL) as the sole products, and their observed char-acteristic signals in the NMR spectra are as follows; EMF: 1H NMR(CDCl3): d 9.59 (s, 1H), 7.19 (d, J = 2.8 Hz, 1H), 6.50 (d, J = 2.8 Hz,1H), 4.53 (s, 2H), 3.69 (q, 2H), 1.20 ppm (t, 3H); 13C NMR (CDCl3):d 14.98, 64.67, 66.58, 111.01, 122.3, 152.22, 160.88, 177.79 ppm.EL: 1H NMR (CDCl3): d 4.08 (q, 2H), 2.71 (t, 3H), 2.54 (t, 2H), 2.15(s, 3H), 1.20 ppm (t, 3H); 13C NMR (CDCl3): d 14.2, 28.1, 30.5,38.1, 61.4, 173.0, 207.4 ppm. Based on the peak integration of 1HNMR spectra, the ratio of EMF to EL in the product mixture was9:1. Total isolated yield of the product mixture containing EMFand EL in 9:1 ratio was 0.24 g.

2.5. Catalytic conversion of HMF into EMF

The synthesis of EMF from HMF was carried out in a round-bot-tom flask by refluxing a reaction mixture containing 0.25 g (5 wt.%)HMF, 6 mL ethanol, 20 mol% dual Zr(O)Cl2/CrCl3 catalyst in 3:1 Mratio of Zr(O)Cl2/CrCl3 and 9 wt.% [BMIM]Cl at 120 �C for 8 h. Theproduct, containing a mixture of EMF and EL, was isolated andcharacterized by NMR technique as described in Section 2.4. Totalisolated yield was 0.29 g. Based on the peak integration of 1H NMRspectra, the ratio of EMF to EL in the product mixture was 9:1.

2.6. Determination of HMF yield

The yield of HMF was determined by both HPLC and UV–Visiblespectrophotometric techniques.

2.6.1. HPLC methodHPLC measurements were obtained using a LC 20 AD Shimadzu

instrument equipped with a UV detector, low pressure gradientpump, and C18 reverse-phase column of dimension 250 mm �4.6 mm � 5.0 micron. The product solution containing HMF wasrun using a mobile phase of acidic water (0.05% H2SO4) at 35 �C.About 20 lL injection loop was used with a 1.0 mL/min flow rate.LC solution software was used for the analysis of the data and cal-culation of HMF yield. The HMF peak was identified by its retentiontime in comparison with authentic sample and integrated. The ac-tual concentration of HMF was determined from the pre-calibratedplot of peak area against concentrations.

2.6.2. UV–Visible methodThe UV–Visible spectrum of pure HMF solution has a distinct

peak at 284 nm with corresponding molar extinction coefficient(e) value of 1.66 � 104 M�1cm�1. The percentage of HMF in eachof the reaction product was calculated from the measured absor-bance values at 284 nm and the extinction coefficient value. First,a standard HMF solution of 99% purity was analyzed for correlatingthe percentage of actual and calculated amount of HMF. Once agood correction was established, the extracted HMF product

10 S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15

samples were run and the percentage of HMF yield was calculated.Repeated measurement of the same solution shows the percentageof error associated with this measurement was ±3%. HMF yieldsobtained from UV–Vis measurement compares well with that ofthe HPLC measurement.

2.7. Recyclability study of catalyst for HMF synthesis

The reusability of the catalyst and solvent was tested by perform-ing an experiment in microwave irradiation under the followingreaction conditions: 4 wt.% cellulose fiber, 20 mol% Zr(O)Cl2/CrCl3

catalyst in 3:1 M ratio of Zr(O)Cl2/CrCl3, 2.0 g DMA–LiCl solvent,9 wt.% [BMIM]Cl, 120 �C, and 5 min reaction time. After completingthe reaction for 5 min, the reaction mass was cooled to room tem-perature and HMF was extracted with diethyl ether for three timesto ensure complete extraction. About 4 mL of diethyl ether was usedin each time. The spent reaction mixture containing the catalyst, anyunreacted substrate and solvent, was reused for the next cycle with-out isolating and characterizing the catalytic species. The next cyclewas started by adding fresh cellulose fiber. The process was repeatedfor 4 cycles.

3. Results and discussions

3.1. Cellulose to HMF conversion

The direct transformation of cellulose into HMF involves threesteps: (1) hydrolysis of polymeric cellulose into monosaccharides,(2) isomerization of glucopyranose to fructofuranose, and finally(3) dehydration of fructofuranose to HMF [13,23] (Scheme 1). Thus,a multifunctional catalytic system is required to perform these threesteps simultaneously for direct transformation of cellulose intoHMF. In an attempt to investigate the direct transformation ofcellulose and untreated biomass into HMF, single metal chloridesincluding Zr(O)Cl2, CrCl3, AlCl3, and combination of two metal chlo-rides including Zr(O)Cl3/CrCl3, CrCl3/AlCl3, Zr(O)Cl2/AlCl3 were usedas catalyst. DMA–LiCl solvent was able to dissolve cellulose by form-ing DMA�Li+ macrocations, resulting high concentration of weaklyion-paired Cl� [13]. The Cl� can form H-bonds with –OH groups ofcellulose, disrupting its extensive network of intra- and interchainhydrogen bonds. In this process, the Cl� concentration can be furtherincreased by the addition of [BMIM]Cl as an additive, which wouldaccelerate the cellulose hydrolysis step.

Single metal chloride catalyzed transformations of both fiberand powder cellulose samples to HMF were carried out at 120 �Cfor 5 min under microwave-assisted heating. The results as shown

OHO

OHO

OH

OOH

OOH

HO

n

Cellulose

Hydrolysis

HO

H OHHO H

H OHH OH

CH2OH

isomerization

Glucopyranose

Scheme 1. Chemical pathways for c

in Table 1 reveal that Zr(O)Cl2 catalyst is the most active and pro-duced 30% HMF from cellulose fiber in DMA–LiCl without[BMIM]Cl additive (Table 1, entry 4). Under comparable reactionconditions, CrCl3 and AlCl3 catalyzed reaction afforded 24% and21% HMF from cellulose fiber, respectively (Table 1, entry 1 and7). When reactions 1–9 of Table 1 were repeated in the presenceof 9 wt.% [BMIM]Cl additive (Table 1, entries 10–18), Zr(O)Cl2, CrCl3

and AlCl3 catalyzed the conversion of cellulose fiber produced 37%,29%, and 27% HMF, respectively (Table 1, entries 10, 13, 16). Thecorresponding conversion of cellulose fiber with Zr(O)Cl2 and CrCl3

catalysts was 75% and 70%, respectively. In the presence of 9 wt.%[BMIM]Cl additive, the yield of HMF consistently increased byabout 5–7% for all cellulose substrates. [BMIM]Cl is believed to in-crease the Cl�ion concentration in the reaction medium and henceaccelerate the cellulose hydrolysis pathway in the three-step reac-tion sequence as shown in Scheme 1. A significant variation in HMFyields from three different cellulose substrates was noted, whichcan be interpreted by their different microstructures and strengthof H-bonding network [29].

To further test the catalytic effectiveness of the single metalchloride catalysts in the presence of another metal chlorides, a ser-ies of experiments were designed for cellulose substrates transfor-mation under microwave-assisted heating at 120 �C in DMA–LiClsolvent with and without [BMIM]Cl. Combined metal chloride cat-alysts that were tested in the present work include Zr(O)Cl2/CrCl3,Zr(O)Cl2/AlCl3, and CrCl3/AlCl3. A total of 20 mol% combined metalchlorides at 3:1 M ratio of Zr(O)Cl2 to other metal chlorides wereused. Among these three different combinations, Zr(O)Cl2/CrCl3

catalyst exhibited the best performance resulting 43% HMF fromcellulose fiber in the presence of 9 wt.% [BMIM]Cl (Table 1, entry19). The conversion of cellulose fiber in this reaction was 82%.The cellulose conversion was determined based on the observedHMF yields, as determined by HPLC analysis, and unconverted re-duced sugars in the product solution, as analyzed by the reportedphenol–sulfuric acid method [30,31]. Previous reports have shownthe formation of 5-chloromethylfurfural (CMF), levulinic acid (LA),formic acid (FA), and humin as side products along with the de-sired HMF in aqueous phase reaction [24,32]. To test the hypothe-sis of the formation of similar side products in the present reaction,the reaction product obtained from cellulose fiber conversion withZr(O)Cl2–CrCl3 catalyst was extracted with diethyl ether and ana-lyzed by 1H NMR spectroscopic technique. 1H NMR spectra of thereaction product showed small peaks for formic acid (d 8.1 ppm)and LA (d 2.5–2.7 ppm), and broad peaks for glucose units (d 3–4.4 ppm). The integration of proton signals in 1H NMR spectra sug-gested the formation of formic acid occurred in 1:33 M ratio of FA/HMF and that of LA was formed in 1:77 M ratio of LA/HMF. The

CH2OHO

HO HH OHH OH

CH2OH

O

HO

HOOH

OHHO

Fructofuranose

Dehydration

OOHO

HMF

ellulose transformation to HMF.

Table 1Results of microwave-assisted direct transformation of cellulose into HMF with metal chloride catalysts in DMA–LiCl and [BMIM]Cl.

Entry Cellulose (4 wt.%)a Catalyst (20 mol%)b Additive, wt.%c HMF yield (%)d

1 Fiber CrCl3 242 a-Cellulose CrCl3 233 Sigmacell CrCl3 284 Fiber Zr(O)Cl2 305 a-Cellulose Zr(O)Cl2 196 Sigmacell Zr(O)Cl2 247 Fiber AlCl3 218 a-Cellulose AlCl3 249 Sigmacell AlCl3 2110e Fiber CrCl3 [BMIM]Cl, 9 2911 a-Cellulose CrCl3 [BMIM]Cl, 9 2712 Sigmacell CrCl3 [BMIM]Cl, 9 3113f Fiber Zr(O)Cl2 [BMIM]Cl, 9 3714 a-Cellulose Zr(O)Cl2 [BMIM]Cl, 9 2515 Sigmacell Zr(O)Cl2 [BMIM]Cl, 9 3116 Fiber AlCl3 [BMIM]Cl, 9 2717 a-Cellulose AlCl3 [BMIM]Cl, 9 3318 Sigmacell AlCl3 [BMIM]Cl, 9 2819g Fiber Zr(O)Cl2/CrCl3 [BMIM]Cl, 9 4320 a-Cellulose Zr(O)Cl2/CrCl3 [BMIM]Cl, 9 3521 Sigmacell Zr(O)Cl2/CrCl3 [BMIM]Cl, 9 3822 Fiber CrCl3/AlCl3 [BMIM]Cl, 9 3423 a-Cellulose CrCl3/AlCl3 [BMIM]Cl, 9 3224 Sigmacell CrCl3/AlCl3 [BMIM]Cl, 9 3925 Fiber Zr(O)Cl2/AlCl3 [BMIM]Cl, 9 3826 a-Cellulose Zr(O)Cl2/AlCl3 [BMIM]Cl, 9 2827 Sigmacell Zr(O)Cl2/AlCl3 [BMIM]Cl, 9 3428 Fiber Zr(O)Cl2/CrCl3 [BMIM]Cl, 17 4729 Fiber Zr(O)Cl2/CrCl3 [BMIM]Cl, 29 5330 Fiber Zr(O)Cl2/CrCl3 [BMIM]Cl, 38 5631 Fiber Zr(O)Cl2/CrCl3 [BMIM]Cl, 44 57

Reaction conditions: solvent: DMA–LiCl (LiCl = 10 wt.%), T = 120 �C, t = 5 min.a Cellulose wt.% is related to the total mass of the reaction mixture.b Catalyst loading is related to starting cellulose concentration.c Additive wt.% is related to the total mass of reaction mixture.d HMF yield obtained from HPLC data.e Cellulose fiber conversion = 70%.f Cellulose fiber conversion = 75%.g Cellulose fiber conversion = 82%.

10 20 30 40 50

40

44

48

52

56

60

HM

F Yi

eld

(%)

[BMIM]Cl (wt%)

Fig. 1. Effect of [BMIM]Cl on HMF yields from cellulose fiber (conditions: cellulose4 wt.%, Zr(O)Cl2/CrCl3 20 mol% with respect to cellulose (Zr(O)Cl2/CrCl3 = 1:1 Mratio), DMA–LiCl 10 wt.%, [BMIM]Cl 9 to 44 wt.%, T = 120 �C, t = 5 min).

S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15 11

formation of a small amount of humin via cross-polymerization ofHMF with fructofuranose is also possible as dark brown colordeveloped in the reaction mixture [32]. When reaction 19 of Table 1was carried out in the presence of 10 wt.% water, the yield of HMFdecreased to 34% with almost similar cellulose fiber conversion(80%). However, significant rehydration of HMF with added waterwas evidenced from the formation of higher concentration of for-mic acid (FA/HMF = 1:4.6).

Under similar reaction conditions, a-cellulose and sigmacelltransformation reaction with Zr(O)Cl2/CrCl3 catalyst produced35% and 38% HMF (Table 1, entries 20 and 21), respectively. A closelook of the data listed in entries 19–27 of Table 1 revealed that theeffectiveness of CrCl3/AlCl3 catalyst was comparable with that ofZr(O)Cl2/CrCl3 catalyst in some cases; for example, the yield ofHMF from Sigma cell substrate with CrCl3/AlCl3 and Zr(O)Cl2/CrCl3

catalysts were 39% and 38%, respectively (Table 1, entries 24 and21). The effectiveness of the Zr(O)Cl2/CrCl3 catalyst improved inthe presence of higher concentrations of [BMIM]Cl (Table 1, entries19 and 28–31). The yield of HMF increased from 43% to 57% fromcellulose fiber upon increasing the [BMIM]Cl concentrations from9 to 44 wt.% (Fig. 1). Controlled experiment showed thatZr(O)Cl2/CrCl3 catalyzed reaction produced 6–7% less HMF without[BMIM]Cl than that obtained with 9 wt.% [BMIM]Cl (Table S1, entry34, 35, and 36).

Zr(O)Cl2 is believed to play an important role in isomerization ofglucopyranose to fructofuranose [33,34] (Scheme 3) The transfor-mation of fructofuranose to HMF via the formation of oxoniumion and cyclic intermediates has been reported in the literature

[13,32]. The observed improvement in catalytic performance ofthe combined Zr(O)Cl2–CrCl3 catalyst can be attributed to the pres-ence of Cr metal, which is believed to promote the isomerization ofglucopyranose to fructofuranose [35]. A recent literature reportalso showed the similar beneficial effect of the combined metalchloride catalysts, RuCl3–CrCl2, for the direct conversion of cellu-lose to HMF in [EMIM]Cl solvent [23].

The possibility of metal chloride hydrolysis with resulting waterfrom cellulose conversion process, and hence the possibility of HCl

Table 2Effect of reaction time on HMF yield for the conversion of 4 wt.% cellulose fiber a with20 mol% Zr(O)Cl2/CrCl3 (3:1 mol ratio) catalystb in DMA–LiCl solvent.

Entry Cellulose (wt.%)c t (min) HMF yield (%)c

1 Fiber, 4 2 492 Fiber, 4 5 573 Fiber, 4 20 62

Reaction conditions: solvent: DMA–LiCl (LiCl = 10 wt.%), 44 wt.% [BMIM]Cld,T = 120 �C.dAdditive wt.% is related to the total mass of reaction mixture.

a Cellulose wt.% is related to the total mass of the reaction mixture.b Catalyst loading is related to starting cellulose concentration.c HMF yield obtained from HPLC data.

0

20

40

60

432Run number

1

HM

F Yi

eld

(%)

Fig. 2. Reuse of the Zr(O)Cl2/CrCl3/DMA–Li/[BMIM]Cl catalytic system for thetransformation of cellulose fiber into HMF (Conditions: 4 wt.% cellulose fiber,20 mol% Zr(O)Cl2/CrCl3 (Zr(O)Cl2/CrCl3 = 3:1) with respect to cellulose concentra-tion, solvent DMA–LiCl (LiCl 10 wt.%), [BMIM]Cl 44 wt.%, T = 120 �C, t = 5 min).

OO

O

OO OH

OO O

HMF

EMF

sugarcane bagasse

Zr(O)Cl2-CrCl3ethanol

[BMIM]Cl

Zr(O)Cl2-CrCl3ethanol

[BMIM]Cl8h15h

EL

+(9 : 1)

Scheme 2. Synthesis of EMF from sugarcane bagasse and HMF with Zr(O)Cl2/CrCl3

catalyst in ethanol in the presence of [BMIM]Cl.

12 S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15

formation that could catalyze cellulose conversion, was consid-ered. To test this hypothesis, the pH of two reaction mixtures, pre-pared in water and DMA–LiCl under identical conditions, wasmeasured for the Zr(O)Cl2/CrCl3 catalytic system. The pH of thereaction mixture in DMA–LiCl was significantly higher (pH = 5.8)than that measured in aqueous medium (pH = 3.0). The pH of thereaction mixture in DMA–LiCl containing 10 wt.% water was alsohigher (5.1) than that of purely aqueous medium. This excludesthe possibility of considerable hydrolysis of the metal chloride cat-alyst in DMA–LiCl and DMA–LiCl-10% water media.

3.2. Effect of reaction time on HMF yield

The reaction time of cellulose conversion was varied to studythe rate of HMF formation as a function of time. As shown in Ta-ble 2, the yield of HMF improved from 49% to 62% upon increasingthe reaction time from 2 min to 20 min for Zr(O)Cl2/CrCl3 catalyzedconversion of 4 wt.% cellulose fiber at 120 �C. Thus, the kinetic ofHMF formation is rapid in first 2 min followed by a slow reaction.This behavior of HMF formation with an increase in reaction timeagreed well with the trend observed for fructose, glucose, and su-crose conversion to HMF with anhydrous AlCl3 catalyst [32]. Tostudy the effect of starting cellulose concentration on HMF yield,a reaction was carried out between 8 wt.% cellulose fiber and20 mol% Zr(O)Cl2/CrCl3 catalyst (Zr(O)Cl2/CrCl3 = 3:1) at 120 �C inDMA–LiCl solvent using 44 wt.% [BMIM]Cl. The reaction startingwith higher concentration of cellulose fiber (8 wt.%) produced sim-ilar HMF yield as that obtained from 4 wt.% starting cellulose fiber.This result suggests that either (i) maximum solubility of cellulosefiber under the present reaction conditions attained when 4 wt.%cellulose fiber was used or (ii) the yield of HMF is not dependenton the starting substrate concentrations [33].

3.3. Recyclability of catalyst

To test the recyclability of the catalyst, the reaction mixturecontaining spent Zr(O)Cl2/CrCl3 catalyst, solvent (DMA–LiCl), andadditive ([BMIM]Cl) was reused for the next run after separatingHMF by diethyl ether extraction. The catalytic activity of the spentreaction mixture was tested by adding fresh cellulose fiber into thereaction mixture, without isolating and characterizing the activecatalytic species. Fresh catalyst was not added to compensateany loss of the catalyst in the prior run. In a similar fashion, thespent reaction mixture was reused for three catalytic cycles. Theyields of HMF obtained from each cycle are shown in Fig. 2. The re-sult shows minimal loss of activity of the spent catalyst; in terms ofHMF yield, the loss is only 4% after four cycles. This suggests thatthe homogeneous metal chloride catalysts can be recycled forHMF synthesis from cellulose and untreated biomass.

3.4. HMF synthesis from sugarcane bagasse

The conversion of untreated lignocellulosic biomass, sugarcanebagasse, with Zr(O)Cl2/CrCl3 catalyst was carried out in DMA–LiClat 120 �C with and without [BMIM]Cl additive. Without [BMIM]Cladditive, a reaction between 4 wt.% sugarcane bagasse and20 mol% Zr(O)Cl2/CrCl3 catalyst under microwave-assisted heatingproduced 29% HMF in 5 min. The effectiveness of the catalyst, interms of HMF yield, improved when the above reaction was re-peated in the presence of [BMIM]Cl additive. As shown in Table 3,the yield of HMF increased from 29% to 37% upon increasing[BMIM]Cl concentration from 9 wt.% to 38 wt.%. A further improve-ment in HMF yield to 42% was noted when sugarcane bagasse con-version was carried out in [BMIM]Cl as a solvent (100% [BMIM]Cl).Previous report has shown the formation of only 5–7% HMF fromsugarcane bagasse with metal oxide catalysts (TiO2, ZrO2, and

TiO2–ZrO2) at 250 �C in hot compressed water [36]. When com-pared with the literature data, the catalytic performance ofZr(O)Cl2/CrCl3 catalyst is really effective even at lower temperature(120 �C). Controlled experiments to explore the effect of reactiontime on HMF yield showed a 4% increase in HMF yield uponincreasing the reaction time from 5 min to 15 min (Table 3, entries3, 7, 8).

3.5. One-pot synthesis of EMF from sugarcane bagasse and HMF

EMF, one of the promising next generation biofuels, has a com-parable energy density (8.7 kW h L�1) with those of standard gaso-line (8.8 kW h L�1) and diesel fuel (9.7 kW h L�1) [37,38]. Previousliterature report has shown the synthesis of EMF from corn stover

Table 3Results of microwave-assisted direct transformation of sugarcane bagasse into HMFwith Zr(O)Cl2/CrCl3 (3:1 mol ratio) catalyst in DMA–LiCl.

Entry Substrate(4 wt.%)a

Catalyst(20 mol%)b

Additive,wt.%c

HMF yield(%)d

1 Bagasse Zr(O)Cl2/CrCl3 – 292 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 9 313 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 17 344 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 29 355 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 38 376 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 100 427 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 17 37f

8 Bagasse Zr(O)Cl2/CrCl3 [BMIM]Cl, 17 38g

Reaction conditions: solvent: DMA–LiCl (0.5 g, LiCl, and 10 wt.%), T = 120 �C,t = 5 min.

a Sugarcane baggase wt.% is related to the total mass of the reaction mixture.b Catalyst loading is related to cellulose concentration.c Additive concentration is related to the total mass of the reaction mixture.d HMF yield obtained from HPLC data.f 10 min.g 15 min.

S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15 13

by treating with LiCl in the presence of aqueous HCl in chloro sol-vent, followed by further treatment with ethanol [39]. In the pres-ent study, Zr(O)Cl2/CrCl3 catalyst in 3:1 M ratio of Zr(O)Cl2/CrCl3

was effective for the conversion of HMF to EMF in ethanol in thepresence of 9 wt.% [BMIM]Cl additive. Under identical reactionconditions, the same catalyst combination was also effective forone-pot synthesis of EMF from sugarcane bagasse (Scheme 2). De-tails preparative method and isolation of EMF from HMF and sug-arcane bagasse are shown in the experimental section. 1H NMRspectral data showed the formation of EMF as a major productalong with EL, another potential biofuel component, as a minorcomponent (Fig. 3). In both cases, the ratio of EMF to EL was 9:1,as compared to 5:2 reported in the literature for CrCl2 catalyzedconversion of microalgae-derived agar to a mixture of EMF andEL in [EMIM]Cl [40]. Total isolated yield of EMF and EL from sugar-cane bagasse (1 g) and HMF (1 g) in 15 h and 8 h was 0.24 g and0.29 g, respectively, under oil-bath heating. Upon extending thereaction time from 15 h to 25 h for sugarcane bagasse substrate,the ratio of EMF to EL remained unchanged. Thus, Zr(O)Cl2/CrCl3

catalyst is highly selective for EMF preparation. Controlled experi-ments showed that [BMIM]Cl additive is essential for the

O

OHHO

HO

OHHO O

OHO

HO

OHHOZrOCl2

ZrCl Cl

O

Zr ClO

HO

H OHO H

H OHH OH

CH2OH

Z

O

OHHO

HO

OHHO

α-glucopyranose

ZrOCl2 O

OHO

HO

OHHO

HCl

Zr OCl

Scheme 3. Isomerization of gluco

conversion of sugarcane bagasse to EMF, whereas HMF can be con-verted to EMF in the absence of [BMIM]Cl additive without affect-ing EMF yield and selectivity. The essential role of [BMIM]Cladditive for sugarcane substrate can be explained by the presenceof Cl�ion in [BMIM]Cl, which accelerates the hydrolysis of biomass.The metal chloride catalyzed route for the formation EMF and EL ispromising when compared with the highly acidic –SO3H function-alized ionic liquid catalyzed method, which produced EL as majorproduct [41]. This one-pot method of conversion of biomass toEMF promises a sustainable route to utilize the untreated biomass.

4. Conclusions

In conclusion, the present report demonstrates the use of metalchloride catalysts for the synthesis of HMF form cellulose and sug-arcane bagasse substrates under microwave-assisted heating.Among several single and combined metal chloride catalyststested, Zr(O)Cl2/CrCl3 catalyst was found to be most effective forHMF from both cellulose and sugarcane bagasse in DMA–LiCl sol-vent with and without [BMIM]Cl additive. The maximum HMFyields achieved from cellulose and sugarcane bagasse were 57%and 42%, respectively. An enhanced HMF yield in the presence of[BMIM]Cl additive is due to the increase in Cl� ion concentration,which favors cellulose hydrogen bond disruption. Excellent reus-ability of the catalyst was demonstrated by recycling the spent cat-alyst for four catalytic consecutive cycles without significant loss inHMF yield. The combined Zr(O)Cl2/CrCl3 catalyst also effectivelyconverted sugarcane bagasse and HMF substrates into a mixtureof EMF and EL with about 90% selectivity in EMF. Further studieson the metal’s interaction with cellulosic material are underway.

Acknowledgments

The authors gratefully acknowledge financial support by theUniversity Grant Commission (UGC), India. SD thanks UGC, India,for a DS Kothari Postdoctoral Research Fellowship. SD thanksUGC, India, for a Junior Research Fellowship. MMA-O acknowl-edges support from the Center for Direct Catalytic Conversion ofBiomass to Biofuels (C3Bio), an Energy Frontier Research Centerfunded by the US Department of Energy, Office of Science, Officeof Basic Energy Sciences under Award Number DE-SC0000997,

r O

HOO

HO HH OHH OH

CH2OH

Zr OH

Cl

Cl CH2OHO

HO HH OHH OH

CH2OH

-ZrOCl2

OOH

OHHO

HO

HOO CHOHO

HMF

H shift

O

OHOHO

OHHO

H ClZr

Cl O

-ZrOCl2 OOH

HOHO

OHHO

β-glucopyranose

fructofuranose

-3H2O

pyranose to fructofuranose.

Fig. 3. (a) 1H NMR plot (d 0–10.5 ppm) of the mixture of EMF and EL (9:1) obtained from the direct conversion of 4 wt.% sugarcane bagasse with 20 mol% Zr(O)Cl2/CrCl3

catalyst in ethanol in the presence of 10 wt.% [BMIM]Cl. (b) Enlarge view of the CHO signals of EMF and HMF (c) Enlarge view of the signal corresponding to –CH2– of EL andEMF. Peak integration showed their ratio as 9:1.

14 S. Dutta et al. / Journal of Catalysis 288 (2012) 8–15

for providing laboratory space and equipment for Prof. Saha duringhis sabbatical interim at Purdue University.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcat.2011.12.017.

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