Bioresource Technology 102 (2011) 4179–4183

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Bioresource Technology

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Catalytic hydrolysis of lignocellulosic biomass into 5-hydroxymethylfurfuralin ionic liquid

Pan Wang, Hongbing Yu ⇑, Sihui Zhan ⇑, Shengqiang WangCollege of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China

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

Article history:Received 31 August 2010Received in revised form 16 December 2010Accepted 17 December 2010Available online 23 December 2010

Keywords:Lignocellulosic biomassCelluloseAcid catalysisIonic liquid5-Hydroxymethylfurfural

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.12.073

⇑ Corresponding authors. Tel./fax: +86 22 23502756E-mail addresses: hongbingyu1130@sina.com (H. Y

(S. Zhan).

Production of 5-hydroxymethylfurfural (HMF) from cellulose catalyzed by solid acids and metal chlorideswas studied in the 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) under microwave irradiation.Among the applied catalysts, the use of CrCl3/LiCl resulted in the highest yield of HMF. The effects of cat-alyst dosage (mole ratio of catalyst to glucose units in the feedstock) and reaction temperature on HMFyields were investigated to obtain optimal process conditions. With the 1:1 mol ratio of catalyst to glu-cose unit, the HMF yield reached 62.3% at 160 �C for 10 min. Untreated wheat straw was also investigatedas feedstock to produce HMF for the practical use of raw biomass, in which the HMF yield was compa-rable to that from pure cellulose. After the extraction of HMF, [BMIM]Cl and CrCl3/LiCl could be reusedand exhibited no activity loss after three successive runs.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Cellulose is abundantly available and has a potential use as analternative to fossil resource for production of chemicals and fuelswhich decrease the generation of CO2 from the utilization of fossilfuels (Huber et al., 2006; Klemm et al., 2005). Cellulose composedof D-glucoses monomers joined together by b-1,4-glycosidic bondscould be primarily converted into glucose (Chheda et al., 2007;Tian et al., 2010). Then glucose can be efficiently converted intochemicals, such as 5-hydroymethylfurfural (HMF). Preparation ofHMF from cellulose is one of the most important approaches totransform biomass into useful chemicals, as HMF and its deriva-tives are known as promising surrogates for petroleum-basedchemicals (Román-Leshkov et al., 2007). Many biomass materialscontaining higher cellulose, such as agricultural residues, forestwaste, are currently used as resources to produce chemicals andbiofuel (Chang et al., 2007; Lin and Tanaka, 2006). Among the agri-cultural residues, wheat straw is the second largest biomass feed-stock after rice straw in the world (Kim and Dale, 2004). At present,most of wheat straw is burnt directly in China, which results inenvironment pollution and waste of biomass resource. Due to hav-ing a rather high content of cellulose, wheat straw is a good rawmaterial to produce HMF. If wheat straw can be directly used forHMF production, it would be a promising approach to gain useful

ll rights reserved.

(H. Yu).u), sihuizhan@nankai.edu.cn

organic compound and make the cost of HMF production lowergreatly.

So far, there are many methods, including acidic hydrolysis(Torget et al., 2000), enzymatic hydrolysis (Zhang and Lynd,2004) and hydrolysis in supercritical water (Sasaki et al., 2000),which have been devoted to depolymerization of cellulose. How-ever, the acidic hydrolysis results in serious corrosion hazard.Furthermore, the use of acid is wasteful and energy-inefficient,which requires separation, recycling and treatment of the wastesulfuric acid. Although the enzymatic process took place undermild conditions, the rate of the hydrolysis was slow and enzymeswere expensive and easy to lose activity. Supercritical water hasrecently been investigated as a medium for hydrolysis of cellulosewith showing some particular advantages, such as high reactionrate, no catalyst requirement and no product inhibition, yet it re-quired high temperature (i.e. 380 �C) and high pressure (i.e.22 MPa). Considering these disadvantages of above methods, it isurgently necessary to develop a green, economical, reusable, andeasy separable process. Because solid acids can be easily separatedfrom the liquid reaction mixture and do not require energy for sep-aration and recovery, it has received much attention in the hydro-lysis of cellulose. Onda et al. (2009) found that sulfonatedactivated-carbon (AC-SO3H) showed excellent catalytic propertiesfor the hydrolysis of cellulose under hydrothermal conditions.

Cellulose is hardly soluble in conventional solvents, such aswater, because of its intermolecular hydrogen bonds. Many studieshave shown that cellulose can be dissolved in some hydrophilic io-nic liquids (ILs). Ionic liquids are attracting increasing attention asa new ‘‘green’’ solvent. ILs have some specific properties, such as

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negligible vapor pressure, nonflammability, high thermal andchemical stability, and adjustable solvent power for organic andinorganic substances (Hu et al., 2009). It was reported that espe-cially ILs containing chloride have more solubility for cellulose,for example, 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)and 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) (Swatloskiet al., 2002; Zhang et al., 2005). Su et al. (2009) reported that metalchlorides could effectively depolymerize cellulose in 1-ethyl-3-methylimidazolium chloride. In this system, a high HMF yield of55.4 ± 4.0% was obtained with mixtures of two metal chlorides(CuCl2 and CrCl2) at a temperature of 120 �C but for a long reactiontime of 8 h.

Since the last decade or so, the use of microwave heating hasgained more attention in organic reaction because of its high heat-ing efficiency and easy operation (Krzan and Kunaver, 2006).Microwave heating produces efficient internal heating or in-corevolumetric heating by direct coupling of microwave energy withthe molecules that may be the solvents, reagents, or catalystswhich are present in the reaction mixture. Because of containinganions and cations, ILs have excellent microwave conductivityand can be heated up more rapidly than other heating methods,such as oil bath, salt bath and electrical furnaces.

In this work, we investigated various solid acids, metal halidesand double metal halides as catalysts to produce HMF from cellu-lose in [BMIM]Cl under microwave irradiation (MI). Among thosecatalysts, CrCl3/LiCl was found to be effective for the reaction. Thenprocess conditions (catalyst dosage and reaction temperature)were optimized to promote the yield of HMF. Untreated wheatstraw was also examined to produce HMF to verify the feasibilityof raw biomass utilization.

Fig.1. Conversion. of cellulose into HMF catalyzed with various catalyst in[BMIM]Cl under MI. Reaction conditions: [BMIM]Cl 2 g, cellulose 50 mg (0.31 mmolof glucose units), solid catalysts and H2SO4 were at the maximal dosage(H3PW12O40, 0.25 g; Nb2O5, 0.62 mmol; Zr3(PO4)2, 0.93 mmol; Cs2.5H0.5PMo12O40,0.25 g; SO2�

4 /TiO2, 0.2 g; SO2�4 /ZrO2/SBA-15, 0.25 g; NKC-9, 0.3 g; H2SO4, 0.31 mmol)

for producing HMF, metal chlorides 0.31 mmol, 140 �C, 40 min under MI.

2. Methods

Microcrystalline cellulose ‘‘Avicel’’ was purchased from Sigma–Aldrich. Wheat straw investigated in this study was obtained froma local farm in Tianjin, north part of China. After being dried, thewheat straw was ground using a high-speed rotary cutting milland then screened to limit the particle size smaller than0.18 mm. The main compositions of wheat straw were cellulose38.45%, hemicellulose 24.59% and lignin 19.31%. In this study, thesample was dried at 105 �C to a constant weight before the exper-iment. [BMIM]Cl (99%) was supplied by Shanghai Cheng Jie Chem-ical Co. Ltd. (Shanghai, China). All other chemicals were purchasedfrom local chemical company and used without further purifica-tion. MCR-41 microwave reactor with a K-type thermocouple, anelectromagnetic stirring system and a control system was pur-chased from Yuhua Instrument Co. Ltd. (Gongyi, China).

The procedure of cellulose hydrolysis was as follows: 2 g of[BMIM]Cl and a certain amount of catalyst were loaded into thetube of 10 mm � 100 mm. The tube was then heated at 100 �C un-der MI and stirred at the speed of 500 rpm for 5 min to make the[BMIM]Cl and catalysts well-mixed. After the mixture was cooledto room temperature, 50 mg of Avicel was added to each tube. Inorder to make sure the cellulose was dissolved absolutely in the io-nic liquid, the tube was put into the microwave reactor and heatedat 105 �C for 10 min with stirring speed of 500 rpm. This reactionsolution was cooled to room temperature again, and 5 ll of H2Owas added into each tube. Then the tube was put into the reactoragain at different temperatures for a certain time as representedin this paper and stirred at 500 rpm. After desired reaction timepassed, the microwave reactor was turned off and the reactionwas cooled down by adding 6 ml of ultra pure water. The aqueoussolution was filtered and subjected to total reducing sugars (TRS)and HMF analysis. Each experiment was performed three times,and the averaged value was used for analysis.

The amount of TRS was measured using DNS method (Li andZhao, 2007). The HMF and furfural concentrations were deter-mined using HPLC (Agilent 1200) with an Ultraviolet Detector at265 nm and an Agilent Eclipse XDB-C18 column. The column oventemperature was 30 �C, and mobile phase was methanol/water(20/80 v/v) at a flow rate of 0.6 ml/min. Other products were deter-mined using HPLC (Agilent 1200) with refractive index detectorand a Bio-Rad Organic Acid column Aminex HXP-87H. The columnoven temperature was 60 �C, and mobile phase was 5 mmol/LH2SO4 at a flow rate of 0.55 ml/min. Each sample was diluted withultra pure water before measured by HPLC to prevent the overload-ing of the column with organic compounds.

3. Results and discussion

3.1. Influence of catalysts on the formation of HMF from cellulose

We explored the effects of numerous catalysts on the conver-sion of cellulose into HMF in [BMIM]Cl under MI at 140 �C for40 min (Fig. 1). Among the catalysts including H3PW12O40, Nb2O5,Zr3(PO4)2, Cs2.5H0.5PMo12O40, SO2�

4 =TiO2, SO2�4 =ZrO2=SBA� 15,

NKC-9 (macroporous styrene-based sulfonic acid resin), H2SO4,CrCl3, CrCl3/LiCl, metal chlorides displayed higher activity for pro-ducing HMF from cellulose. As can be seen in Fig. 1, TRS yields withsolid acids and H2SO4 were higher than those with chromium. Incontrast, HMF yields were lower than those catalyzed by chro-mium. The process of producing HMF from cellulose in [BMIM]Clwas proposed via saccharification of cellulose followed by isomer-ization of the glucose monomers into fructose and dehydration offructose to form HMF (Binder and Raines, 2009). Many efforts havebeen devoted to the dehydration of fructose with solid acids.According to Shimizu et al. (2009), solid acids, such asH3PW12O40, Cs2.5H0.5PMo12O40, SO4

2�/ZrO2, gave more than 90%of HMF yields, even 100% with Amberlyst-15 from fructose.H2SO4 also afforded a high HMF yield of 80% from fructose in ionicliquid solvent (Zhao et al., 2007). A reason for the low yields ofHMF from cellulose with solid acids and H2SO4 was that these cat-alysts could facilitate the hydrolysis of cellulose into saccharidesbut lack enough catalytic activity for the isomerization from glu-cose to fructose.

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According to the previous report, CrCl3 showed an effective cata-lytic activity for depolymerization of cellulose (Li et al., 2009). Inaddition, it was found that double metal chlorides present a highactivity for producing HMF from cellulose (Su et al., 2009). In thiswork, LaCl3, LiCl, LiBr (with different size of cation radius and anionradius) were chosen to be mixed with CrCl3 to catalyze the hydroly-sis of cellulose. It was found that adding metal halides to [BMIM]Cl /CrCl3 could promote the yield of HMF compared with single CrCl3 ascatalyst when the total amount of metal chloride and glucose units(based on the cellulose feedstock) was maintained at a mol ratio of1:1. Furthermore, the mol ratio of CrCl3 to other metal halide playedan important role on the HMF yields. As can be seen in Table 1, theyields of HMF and TRS were obtained as 52.6% and 26.3%, respec-tively, with CrCl3 in the microwave reactor at 140 �C for 40 min (en-try 2). Among the paired catalyst, CrCl3/LiCl exhibited a bettercatalytic activity than CrCl3/LaCl3 and CrCl3/LiBr. The yields of HMFand TRS increased with the increasing amounts of LiCl from 10 to50 mol%, and then they decreased with more usage of LiCl catalyst.Furthermore, adding 100 mol% LiCl showed little catalytic activityfor producing HMF, which was comparable with the experiment ofwithout catalyst. Maximal yields of HMF and TRS were 61.9% and30.3%, respectively, when loading 50% mol of CrCl3 and 50% mol ofLiCl (entry 11). It has been reported that chromium catalysts enabledsynthesis of HMF from glucose in alkylimidazolium ionic liquids(Binder and Raines, 2009), but single LiCl presented low activities(Zhao et al., 2007). Previous literatures reported that cellulose couldbe efficiently hydrolyzed to glucose in the ionic liquid catalyzed byBrønsted acid and Lewis acid (Li et al., 2009; Onda et al., 2008). Themechanism of cellulose transformation to HMF can be proposed thatCrCl3 in [C4mim]Cl forms [CrCl3+n]n complexes which would pro-mote conversion of a-glucopyranose anomers to b-glucopyranoseanomers through hydrogen bonding of chloride ions with the carbo-hydrate hydroxyl groups. Then [BMIM]CrClx leads to the isomeriza-tion of glucopyranose to fructofuranose, followed by rapiddehydration of fructofuranose to HMF (Zhang and Zhao, 2010; Zhaoet al., 2007; Remsing et al., 2006). For only LiCl, it did not work forconversion of cellulose and glucose to HMF. However, N,N-Dimeth-ylacetamide (DMAc) containing lithium chloride is a very commonsolvent system in cellulose chemistry. We surmise that the mecha-nism of cellulose degradation in [BMIM]Cl/LiCl is similar to that in

Table 1Results of hydrolysis of Avicel cellulose catalyzed by metal halides.a

Entry Catalyst, mol% Yield (%)

HMF TRS

1 Non catalyst 0.9 24.32 CrCl3, 100 52.6 26.33 CrCl3, 90; LaCl3, 10 56.9 27.54 CrCl3, 70; LaCl3, 30 57.9 30.05 CrCl3, 50; LaCl3, 50 55.4 29.66 CrCl3, 30; LaCl3, 70 52.9 28.67 CrCl3, 10; LaCl3, 90 30.2 25.48 LaCl3, 100 2.5 33.19 CrCl3, 90; LiCl, 10 56.4 27.1

10 CrCl3, 70; LiCl, 30 57.6 28.711 CrCl3, 50; LiCl, 50 61.9 30.312 CrCl3, 30; LiCl, 70 59.2 28.413 CrCl3, 10; LiCl, 90 33.5 27.514 LiCl, 100 1.3 39.515 CrCl3, 90; LiBr, 10 58.6 30.716 CrCl3, 70; LiBr, 30 60.8 34.117 CrCl3, 50; LiBr, 50 59.3 32.718 CrCl3, 30; LiBr, 70 57.4 29.619 CrCl3, 10; LiBr, 90 31.8 27.220 LiBr, 100 2.4 37.6

a Reaction conditions: cellulose (50 mg), [BMIM]Cl (2.0 g), H2O (5 mg), 140 �C,40 min under MI, and molar ratio of catalyst to glucose units calculated based onthe amount of cellulose 1:1.

DMAc/LiCl (Potthast et al., 2002). In our work, CrCl3 and LiCl werefirst dissolved in ionic liquid at 100 �C for 5 min to obtain a homoge-neous solution, and then cellulose was dissolved in this solution at105 �C for a period of time. During this procedure, we suppose thatadding LiCl in the ionic liquid result in further degradation of cellu-lose, which facilitated the hydrolysis reaction. Other products wereidentified by HPLC with refractive index detector. The major com-pounds were cellotriose (Retention Time (RT) = 7.36 min), cellobiose(RT = 7.66 min), glucose (RT = 9.27 min), fructose (RT = 9.98 min),anhydroglucose (RT = 12.2 min), formic acid (RT = 14.19 min), levu-linic acid (RT = 15.48 min) and HMF (RT = 30.86 min).

3.2. Effect of catalyst dosage on the yield of HMF produced fromcellulose

The influence of catalyst dosage with respect to cellulose con-version into HMF in [BMIM]Cl was investigated and the resultsare shown in Fig. 2. The molar ratios(R) of total paired metal chlo-ride (CrCl3, 50% mol; LiCl, 50% mol) to glucose units calculatedbased on the amount of cellulose were 0.5:1, 1:1 and 1.5:1, respec-tively. It can be seen from Fig. 2 that increasing the catalyst loadingled to an increase in yields of HMF. However, when catalyst wasused with R-values of 1:1 and 1.5:1, yields of HMF were almostthe same after reaction periods beyond 40 min. The reason mightbe that more catalyst (R = 1.5:1) accelerated the formation ofHMF from cellulose while it also favored the rehydration of HMFinto levulinic acid, which offset increase in HMF yield (Qi et al.,2008). Therefore, the molar ratio of catalyst to glucose was chosenas 1:1 in subsequent experiments if not otherwise indicated.

3.3. Effect of reaction temperature on the yield of HMF produced fromcellulose

Experiments were conducted at 140, 150, 160 and 170 �C tostudy the effect of reaction temperature on the hydrolysis of cellu-lose into HMF. Yields of HMF at different temperatures as a func-tion of reaction time are shown in Fig. 3, from which it can beseen that reaction temperature had a great effect on the formationof HMF. When reaction temperature was 140 �C, the HMF yieldwas obtained as 61.9% for 40 min of reaction time. However, it onlytook 7.5 min to reach HMF yield of 61.5% at 170 �C. With increasingtemperature, the reaction rate of cellulose hydrolysis increased and

Fig. 2. Effect of catalyst dosage on the cellulose conversion into HMF. Conditions:[BMIM]Cl 2 g, cellulose 50 mg, temperature 140 �C. Total mol ratios of CrCl3/LiCl(CrCl3, 50% mol; LiCl, 50% mol) to glucose unit calculated based on the amount ofcellulose were 0.5:1, 1:1 and 1.5:1, respectively.

Fig. 3. Effect of reaction temperature on the cellulose conversion into HMF.(Conditions: [BMIM]Cl 2 g, cellulose 50 mg, catalyst (CrCl3, 50% mol; LiCl, 50% mol),molar ratio of catalyst to glucose units calculated based on the amount of cellulose1:1).

Table 3Results of recycling of [BMIM]Cl and catalyst.a

Feedstockb Time (min) HMF yield (%)

C 1st run 10 62.5C 2nd run 10 63.5C 3rd run 10 62.9WS 1st run 15 61.1WS 2nd run 15 61.7WS 3rd run 15 62.2

a Reaction conditions: [BMIM]Cl 2 g, cellulose 50 mg, wheat straw 50 mg,160 �C,catalyst (CrCl3, 50% mol; LiCl, 50% mol), molar ratio of catalyst to glucose unitscalculated based on the amount of cellulose 1:1.

b Cellulose (C), wheat straw (WS).

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the time when yields of HMF reached their peak values was short-ened. For the range of temperature observed in this study, themaximal HMF yield was 62.3% obtained at 160 �C. When it was170 �C, the peak value of HMF yield was lower than that at160 �C, which was due to that higher temperature gave rise to sidereactions that formed undesired byproducts (Yong et al., 2008).Thus, a reaction temperature of 160 �C and a reaction time of10 min were chosen as the best conditions for conversion of cellu-lose into HMF.

3.4. Production of HMF from wheat straw

The hydrolysis of wheat straw to produce HMF is a good alter-native for practical utilization for this abundant biomass, since ithas a rather high content of cellulose. The milled wheat straw withparticle size smaller than 0.18 mm was not treated by any otherchemical treatment and used as raw materials in the experiment.It can be seen from Table 2 that 57.9% of HMF yield based on thecellulose content of the wheat straw was obtained at 160 �C for10 min of reaction time. As the time prolonged to be 15 min, theyield of HMF reached the maximal value of 61.4% which was com-parable to that from pure cellulose. This result indicated that othercomponents of wheat straw, such as lignin and protein, had littleinfluence on the hydrolysis of cellulose (Zhang and Zhao, 2010).However, the reaction time was littler longer, which was due tothe interactions of lignin-cellulose-xylan exerting a great influenceon degradation of lignocellulosic biomass (Talebnia et al., 2009). Itwould be highly efficient to utilize the pentose present in hemicel-lulose in the process for producing HMF from biomass. For thispurpose, yields of the industrial chemical furfural from pentosein the hemicellulose component were investigated (Table 2). The

Table 2HMF, TRS and furfural yields from hydrolysis of wheat straw.a

Entry Time (min) Yield (%)

HMF TRS Furfural

1 10 57.9 25.1 41.12 12.5 58.6 26.8 43.83 15 61.4 28.0 43.24 17.5 60.5 27.3 41.8

a Reaction conditions: [BMIM]Cl 2 g, wheat straw 50 mg, 160 �C, catalyst (CrCl3,50% mol; LiCl, 50% mol), molar ratio of catalyst to glucose units calculated based onthe amount of cellulose 1:1. Yields of HMF from wheat straw are based on celluloseanalysis of 38.5%, and yields for furfural are based on xylan analysis of 20.9%.

maximal furfural yield of 43.8% was obtained at 12.5 min. The rea-son was that hemicellulose is more susceptible to acid hydrolysisthan cellulose.

3.5. Recycling of [BMIM]Cl and catalyst

After the first reaction run for hydrolyzing pure cellulose, 1 g ofultra pure water was added into the reaction liquid to decrease theviscosity of ionic liquid and facilitate the extraction of HMF. Thenthe HMF product was separated from the [BMIM]Cl solvent byextracting 10 times with 5 ml of ethyl acetate (Qi et al., 2009).We examined the compounds extracted by ethyl acetate usingGC/MS, which demonstrated that HMF was the sole product inthe extraction solvent. The result was similar to that reported byQi et al. (2009) and Hu et al. (2009). The amount of extractedHMF was examined by HPLC, which indicated that ethyl acetateextraction could recover 95% of the HMF. After extraction, CrCl3/LiCl catalyst was still in the reaction solution. Then the solutionwas heated at 65 �C in a vacuum drier until the water in this sys-tem was removed absolutely. The dried [BMIM]Cl solvent withCrCl3/LiCl catalyst was used directly in the next run by adding freshfeedstock under the same reaction conditions. In order to examinethe changes of ionic liquid and catalyst, the recycling test was con-ducted for three times. As listed in Table 3, the catalytic activity ofthe catalysts for the hydrolysis of cellulose and wheat straw didnot decrease after the 3rd run, which demonstrated that the cata-lyst was stable in this reaction. The HMF yield in the recycling testwas even higher than that in the 1st run, which might be due to theretention of HMF and unreacted feedstock in the previous run.

4. Conclusion

In this study, numerous catalyst such as solid acid and metal ha-lides, were investigated for production of HMF from cellulose with[BMIM]Cl as solvent under MI. Among the catalysts, CrCl3/LiClshowed relatively high activity for the hydrolysis of cellulose intoHMF. HMF yield from cellulose was obtained as high as 62.3% at160 �C for 10 min with CrCl3/LiCl. When reaction time proceededa little longer, HMF yield from untreated wheat straw was compa-rable to that from cellulose. The ionic liquid and CrCl3/LiCl catalystcould be easily recycled with stable catalytic activity after the HMFwas extracted with ethyl acetate.

Acknowledgements

The authors gratefully acknowledge the National NaturalScience Foundation of China (NSFC, Grant No. 20907022,21003094, 20806041) and the National Science & Technology PillarProgram of China (2008BAC43B01) for the financial support.

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