A New Functionalized Ionic Liquid for Efficient Glucose Conversion To5-Hydroxymethyl Furfural and Levulinic Acid

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Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121

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A new functionalized ionic liquid for efficient glucose conversion to5-hydroxymethyl furfural and levulinic acid

Nur Aainaa Syahirah Ramli, Nor Aishah Saidina Amin ∗

Chemical Reaction EngineeringGroup (CREG), EnergyResearch Alliance, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310UTM Johor,

Johor Bahru, Malaysia

a r t i c l e i n f o

Article history:

Received 7 January 2015Received in revised form 13 June 2015Accepted 22 June 2015Available online 29 June 2015

Keywords:

Glucose conversionFunctionalized ionic liquid5-hydroxymethyl furfuralLevulinic acid

a b s t r a c t

The conversion of glucose to 5-hydroxymethyl furfural (5-HMF) and levulinic acid (LA) using ionic liquidis a promising method for producing liquid fuels from renewable resources. In this study, three types of acidic functionalized ionic liquids (FILs) were prepared and used as catalysts in the conversion of glu-cose to 5-HMF and LA. The prepared FILs were characterized using CHNS elemental analysis and 1 H and13C NMR. The acidity of the FILs was examined using pyridine-FTIR, Hammett and acid-base titrationmethods. The FIL with high acidity and with both Brønsted and Lewis acid sites present seemed suitablefor 5-HMF and LA production. Among the tested FILs, 1-sulfonic acid-3-methyl imidazolium tetrachlo-roferrate ([SMIM][FeCl4]) demonstrated the highest catalytic performance. The yields of 5-HMF and LAreached as high as 18% and 68%, respectively after 4hat 150 ◦C.The catalyst was reusedfive times withoutsignificant loss of activity. Furthermore, for the kinetic analysis performed for glucose conversion, theactivation energy and pre-exponential factor for the reaction were 38 kJ mol−1 and 925 min−1, respec-tively. The experimental results demonstrated the potential of FIL as a catalyst for biomass transformationto platform chemicals under mild process condition.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Concerns about increasing petroleum oil prices compel thechemical industry to explore alternatives to fossil resources forbasic chemical productions [1]. Biomass resources are promisingalternatives for sustainable supply of fuels and valuable chem-icals [2–4]. Carbohydrates derived from biomass are abundant,relatively inexpensive, and are renewable energy sources. Car-bohydrate such as glucose is a compound from which variousbio-based chemicals can be derived [1]. Among those chemicals,5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA) havereceived significant attention as platform chemicals for synthesiz-ing a broad range of chemicals and bio-based fuels [5–7].

A versatile intermediate compound, 5-HMF, has been identi-fied as the petroleum-based building block and biomass-basedcarbohydrate chemical. 5-HMF is very useful not only for theproduction of furan derivatives, fine chemicals, promising liquidtransportation fuels, and polymers, but also for the productionof important molecules such as 5-hydroxy-4- keto-2-pentenoicacid and LA [8]. Besides, LA is also an important basic chemi-

∗ Corresponding author. Fax: +60 7 5588166.E-mail address: [email protected] (N.A.S. Amin).

cal compound with numerous potential uses such as textile dye,animal feed, coating material, antifreeze, solvent, food flavouringagent, pharmaceutical compound and resin [9]. In the last decade,an enormous amount of work in the development of sustainablemethods for the preparation of 5-HMF and LA from different feed-stocks including monosaccharides [10,11], disaccharides [12–14],polysaccharides [15–18], and raw lignocellulosic biomass [19–22]has been reported.

Theconversion of carbohydrates to 5-HMF andLA has been con-ducted in water and various organic solvents in the presence of various catalysts, including mineral acids [18,23–25], metal chlo-rides [5,26,27], and solid acid catalysts [17,28,29]. Basically, in thecatalytic conversion of glucose, glucose isomerizes to fructose anddehydrates to form 5-HMF, which will then catalytically rehydrateto form LA [27,30]. The isomerization of glucose to fructose is cat-alyzed by Lewis acid sites [31], whereas Brønsted acid sites arerequired for 5-HMF rehydration to LA [31]. Moreover, Lewis acidsites are also able to catalyze the decomposition of glucose to formhumins [31,32]. The glucose conversion reaction scheme to 5-HMFand LA is depicted in Fig. 1.

Ionic liquids are salts with melting point below 100◦C that areprepared from combinations of different cations and anions. Ionicliquids have received great attentionin various fields owing to theirpotential as eco-friendly solvents due to several unique proper-

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114 N.A.S.Ramli, N.A.S.Amin / Journal of Molecular CatalysisA: Chemical 407 (2015) 113–121

OHOH2C CHO

H3C COOH

O

H+

H+

LA

O

OH

OH

OH

OH

CH2OH

glucose

CH2OH

OH

OH OHHOH2C

fructose

isomerization

rehydration

dehydration5-HMF

Fig. 1. Reaction schemeof glucose conversion to 5-HMF and LA.

ties such as low vapour pressure, non-flammability, high thermalstability and easy to separate from reactions [33,34]. With thesepromising properties, ionic liquids are being used increasingly assolventsinthedissolutionofbiomassandinthedehydrationofcar-bohydrates to produce various useful chemicals including 5-HMFand LA [34–37]. In this regard, imidazolium based ionic liquids are

wellknownfortheirsolubilizingcompetency [36,37]. Assuch,ionicliquids containing [BMIM]+ are powerful solvents [38].

Nevertheless, the concern towards the exorbitant price of ionicliquid is prevalent. The high price limits the use of ionic liquids inlarge scale reactions. Thus, the quantity of ionic liquid for reactionpurposes should be reduced. As such, the practicability of aque-ous ionic liquid [39] and combination of ionic liquid with othercompounds such as ammonia and oxygen [40,41] could be imple-mentedtoreducethequantityofionicliquid.Besides,thecapabilityto reuse ionic liquidis a good step towards reducing theoverall costfor any processes involving ionic liquids.

Functionalized ionic liquids (FILs) are defined as ionic liquidssynthesized and designed through the inclusion of different typesof functional groups on the cations and anions. To date, FILs have

exhibited enormous potential to be applied to various processesincluding catalytic conversion of renewable feedstocks to 5-HMFand LA[42–49]. The potential of sulfonic acid (SO3H) FIL for replac-ing conventional acid was first reported in 2002 [50]. SO3H FILs,which have strong Brønsted acidity, are flexible and recyclable,and could be used as both solvents and catalysts. Besides, SO3HFILs have been used in the conversion of cellulose and glucose to5-HMF and LA [43–45,47].

Many studies have demonstrated that metal salts (CrCl2, CrCl3,SnCl4, GeCl4) are effective catalysts for hydrolysis of carbohydrates[51–53]. Tao et al., found that the addition of MnCl2 in SO3H FIL was effective for the production of 5- HMF from cellulose, whereasLA was obtained as a side product [43]. Thus, the incorporation of metal metals with the SO3H FIL system can also be used for the

production of 5-HMF and LA, as the incorporated metals will actas the Lewis acid sites to catalyze the glucose conversion reaction.Meanwhile, in the research conducted by Zhang et al. [47], a com-plex FIL was used as a catalyst for the conversion of carbohydratesto 5-HMF.The complex FILwas prepared by grafting CrCl3·6H2O onthe polymeric heteropolyacid- SO3H FIL.

CrCl2 and CrCl3 are the mostly used metal salts for the conver-sion of carbohydrates; either the salts are used alone as a catalystor combined with ionic liquidin thereactionsystem[27,47,53–55].However, high price, toxicity and environmental pollution derivedfrom CrCl2 and CrCl3 have necessitated the search for non-toxicand low cost metal salts. Due to the low cost and non-toxic prop-erties of FeCl3 [56], FeCl3·6H2O was introduced into the groupof imidazolium based SO3H FIL. In the current study, several FILs([BMIM][FeCl4]), [SMIM][Cl]), [SMIM][FeCl4])withBrønstedand/or

Lewis acidic groups were synthesized by altering the anions of cations of the FIL. The acidic properties of the FILs were examinedaccordingly. Next, the performance of the FILs was evaluated forglucose conversion to main dehydration products (5-HMF and LA).The factors influencing the glucose conversion involving reactiontemperature, reaction time, glucose loading, and FIL loading were

examined. The reusability of FIL was also addressed. In addition,a simple kinetic analysis was performed for glucose conversion todetermine the reaction rate constant and activation energy of glu-cose conversion using the selected FIL. The findings from this studyare expected to improve the knowledge of catalytictransformationof renewable feedstocks to platform chemicals such as 5-HMF andLA.

2. Materials and methods

2.1. Materials

All chemicals were obtained from Merck (Germany) andused as received without any further purification. The chemi-

cals used for the preparation of FILs were 1-methylimidazole,chlorosulfonic acid, dichloromethane, iron (III) chloride hexahy-drate (FeCl3·6H2O), and 1-butyl-3-methyl imidazolium chloride([BMIM][Cl]). Glucose, 5-HMF, LA, sodium hydroxide (NaOH),3,5-dinitrosalicylic acid, potassium sodium tartrate tetrahydrate,sodium sulfite, ethyl acetate, and sulfuric acid (H2SO4) were usedfor the catalytic tests and product analysis. The chemicals involvedin the characterizations of FIL’s acidity were deuterated dimethylsulfoxide (DMSO-d6), chloroform (CDCl3), pyridine, 2,6-dichloro-4-nitroaniline, 2-propanol, phenolphthalein, and NaOH.

2.2. Preparation of FILs

Three types of FILs, namely 1-butyl-3-methylimidazolium

tetrachloroferrate ([BMIM][FeCl4]), 1-sulfonic acid-3-methylimidazolium chloride ([SMIM][Cl]), and 1-sulfonicacid-3-methylimidazolium tetrachloroferrate ([SMIM][FeCl4])were prepared for catalytic conversion of glucose to 5-HMF andLA. Firstly, [BMIM][FeCl4] catalyst was prepared according to aprevious method [57]. Equimolar [BMIM][Cl] and FeCl3·6H2O weremixed under vigorous stirring, and the mixture was stirred for 24hat room temperature. Two distinct layers were formed, in whichthe darker layer contained [BMIM][FeCl4], and the upper layermostly consisted of water. The aqueous upper layer was separatedby centrifugation, and the [BMIM][FeCl4] layer was then dried at80 ◦C in order to remove the remaining water.

Meanwhile, [SMIM][Cl] was prepared by mixing 1-methylimidazole in dry CH2Cl2 (50mL) before SO3HCl was

added dropwise at room temperature. The molar ratio of 1-



N.A.S.Ramli, N.A.S.Amin / Journal of Molecular CatalysisA: Chemical 407 (2015) 113–121 115

N N

Cl

FeCl3.6H2O

N N + 6H2O

+

FeCl4

+

+

N N

+

SO3HClN N

SO3H

Cl

Cl

+

FeCl3.6H2O

N N

SO3H

+

+

FeCl4

CH2Cl2

(a)

(b)

(c)

N N

SO3H+

+

6H2O+

Fig. 2. The addition of ions involved in the preparation of (a) [BMIM][FeCl4] , (b)[SMIM][Cl], and (c) [SMIM][FeCl4].

methylimidazole and SO3HCl was 5:5.2 mol. The reaction mixturewas stirred for 20min, allowed to stand for 5 min, and then CH2Cl2was decanted. The precipitate was washed three times with dryCH2Cl2 and dried at 80◦C to give [SMIM][Cl]. This procedure is inaccordancewith the method described in the literature[58]. Mean-while, the preparation of [SMIM][FeCl4] involved the mixing of the

prepared [SMIM][Cl] and FeCl3·6H2O. Equimolar [SMIM][Cl] andFeCl3·6H2O were mixed under vigorous stirring, and the mixturewas stirred for 24h at room temperature. [SMIM][FeCl4] was driedat 80 ◦C to remove water. The addition of ions involved during thepreparation of [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4] isillustrated in Fig. 2.

2.3. Characterization of FILs

The elementalanalysis of the FILs wasconducted using Elemen-talAnalyzer Vario MICRO Cube. Theresults of 1HNMR and 13CNMR (Fig. S1–S6) were recorded on NMR Bruker Avance II 400 MHz inDMSO-d6 and CDCl3 solvents.

The acidictype (Brønstedand Lewis) of theFILs was determinedby FTIR using pyridine as a probe. The FILs were heated and mixedwith pyridine by a volume ratio of 5:2. The FTIR spectra of the FIL were recorded on a PerkinElmer infrared spectrometer using KBrpellets and pressed KBr discs (KBr pellets/film) in a resolution of 4 cm−1, and in a frequency range of 1350–1600cm−1. The ratio of Brønsted to Lewis acidity was determined based on the method inthe literature [59] shown in Eq. (1).

Ratio Brønsted to Lewis acidity = AB × C L

AL × C B(1)

Where AB and AL are the areas of Brønsted and Lewis acidityfrom FTIR (cm−1), respectively, C B is the coefficient of Brønstedacidity (188 cm/mmol), and C L is the coefficient of Lewis acidity

(142cm/mmol).

The acid strength of the FIL was determined by the Hammettmethod using UV–visible spectroscopy. A solution of 2,6-dichloro-4-nitroaniline, acted as an indicator, was added to the FIL, and themixture was stirred for 15min. The UV spectra were recorded onan Agilent 8453 UV–vis spectrophotometer (370 nm)at room tem-perature. The acidity of FIL was evaluated from the determinationof Hammett acidity function (Eq. (2)):

H o = pK(I)aq + log [I]

IH+

(2)

Where pK(I)aq isthepKa value of the indicator, and [I] and [IH+]arethe molar concentrations of unprotonatedand protonated forms of the indicator in the solvent, respectively.

The acid values of the FILs were determined by the acid-basetitration method. Approximately, 0.1g of FIL sample was dissolvedin10mL of 2-propanol as a solvent andphenolphthalein as an indi-cator. The mixture was then titrated with 0.1 M NaOH and shakenvigorously. The titration was stopped when the end point has beenreached once the clear mixture turns pink. The amount of NaOHused for the titration was recorded. In addition, a blank titrationwas also performed. The acid value, expressed as the amount of NaOH required to neutralize 1 g of FIL, was calculated using Eq. (3).

Acid value (g/g) =( A− B) ×M × 40

W (3)

Where W is the sample mass (g), A is the volume of NaOH requiredfor titration of the sample (mL), B is the volume of NaOH requiredfor titration of the blank (mL), and M is the molarity of the NaOHsolution.

2.4. Performance testing of FILs

The FILs performance was tested by conducting glucose con-version to 5-HMF and LA. The catalytic conversion of glucose wascarried out in a closed 100mL Schott bottle equipped with a ther-mocouple.In a typical reaction,the reactor was loadedwithglucose

(0.1 g) dissolvedin distilledwater(10 mL)andanFILcatalyst(10g).The solution was heated to the desired temperature at a constantstirring speed of 200rpm. After the reaction was completed, themixture was cooled down to room temperature. All samples werefiltered before further analysis usinghigh performance liquid chro-matography (HPLC). Randomly selected experimental runs wererepeated to test the reproducibility of the data.

The reusability of the FIL was tested for glucose conversionaccording to the following steps. After a completed run, 5g of distilled water was added to the reaction mixture to reduce theviscosity of the solution, as well as to facilitate the extraction of 5-HMF and LA.The liquid product wasthen separated from the FILbyextracting theproduct with ethyl acetate (5mL ×10) [60]. Aftertheextraction process, the remaining compounds in the solution were

FIL and water. The solution was then vaporized at 105◦

C until thewater in this system was completely removed. The dried FIL wasused directly in the next run by adding fresh feedstock under thesame reaction conditions.

2.5. Product analysis

For glucose conversion analysis, dinitrosalicylic acid (DNS)method was used throughout this study. DNS reagent was pre-pared by mixing 2g of NaOH in a 100 mL solution. About 80mL of NaOH solution was mixed with 1g of 3,5-dinitrosalicylic acid,30g of potassium sodium tartrate tetrahydrate, and 0.83g of sodium sulfite to give the DNS reagent. 1mL of supernatant fromthe product sample was mixed with 4mL of DNS reagent. The

resulting solution was heated in boiling water for 5min. Next,




1560 1540 1520 1500 1480 1460 1440 1420 1400

W avenumber (cm-1)

A b s o r b a n c e

[SMIM][FeCl4]

[SMIM][Cl]

[BMIM][FeCl4]

LB

Fig. 3. Pyridine-FTIR spectra of theFILs.

the absorbance of the mixture was measured at 540nm usingUV–vis spectrophotometer. Meanwhile, the concentrations of 5-HMF and LA in the liquid product were determined using HPLC(Waters 2690) under the following conditions: column= Hi Plex H;flow rate= 0.6 mL/min, mobile phase = 5 mM H2SO4, detector= UV250nm; retention time= 45min; and column temperature = 60◦C.Glucose conversion andproduct (5-HMF,LA)yields werecalculatedaccording to the following Eqs. (4)–(5).

Glucose conversion(%)

=Initial glucose amount(g)− final glucose amount(g)

Initial glucose amount(g) × 100%

(4)

Product yield(%) =Product amount(g)

Initial glucose amount(g) × 100% (5)

3. Results and discussions

3.1. Properties of FILs

The prepared [BMIM][FeCl4] and [SMIM][Cl] are viscous darkand yellowish liquids, respectively, while [SMIM][FeCl4] is a stiff solidat roomtemperature that melts at highertemperature around70 ◦C. The elemental CHNS analysis results show similar calculatedandfound values, which validatedthe prepared FILs. CHNSO analy-sis (%): [BMIM][FeCl4]: Calculated C: 28.52, H: 4.46, N: 8.32; FoundC: 28.61, H: 4.45, N: 8.35. [SMIM][Cl]: Calculated C: 24.19, H: 3.53,N: 14.11, S: 16.12; Found C: 24.26, H: 3.52, N: 14.15, S: 16.17.[SMIM][FeCl4]: Calculated C: 13.31, H: 1.94, N: 7.76, S: 8.87; FoundC: 13.28, H: 1.94, N: 7.74, S: 8.91.

For the determination of the FILs’ acidity types, pyridine canreact separately with Brønsted and Lewis acids in the FILs [61].Brønsted acid sites are observed at the absorption peak near1540cm−1 in the FTIR spectra, whereas the absorption peak of Lewis acid sites are detected close to 1450cm−1. By observingthese two peaks, the acidic type of the FILs can be determined. Thepyridine-FTIR spectra of theFILsare obtained as shown inFig.3. Theabsorption peaks near 1540 and 1450 cm−1 appear in the pyridine-FTIR spectra of [SMIM][FeCl4], suggesting that [SMIM][FeCl4]comprised of both Brønsted and Lewis acid sites. Meanwhile,[SMIM][Cl]and[BMIM][FeCl4]mostlycontainedBrønstedacidsitesand Lewis acid sites, respectively. The Brønsted acid sites in theFIL could come from the sulfonic acid group in the imidazoliumcation, whilst the Lewis acid sites could be contributed by FeCl4−

in the anion. The Brønsted to Lewis acid ratio of the FILs are 0.47,

Table 1

Acidic properties of the FILs.

FIL H o Acid value (g NaOH/g FIL)

[BMIM][FeCl4] 3.58 0.28[SMIM][Cl] 3.40 0.45[SMIM][FeCl4] 3.36 0.94

3.36, and 1.16 for [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4],respectively.The acidic properties of the FILs, based on the Hammett and

acid-base titration methods, are summarized in Table 1. The H ovalues of the [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4] are3.58, 3.40, and 3.36, respectively. Since lower value of H o corre-sponds to greater acid strength [62], [SMIM][FeCl4] has higher acidstrength than the other two FILs. It is important to note that theacidity amount of the FILs determined by acid- titration is con-sistent with the Hammett test according to the following order:[SMIM][FeCl4]>[SMIM][Cl]>[BMIM][FeCl 4]. Based on the tests, itis suggested that the acidity of [SMIM][FeCl4] is predominantlycontributed by the presence of sulfonic acid group.

3.2. Glucose conversionwith different FIL catalysts

FILs with different cations and anions were employed as cata-lysts for glucose dehydration reaction. As indicated in Fig. 4, theconversions of glucose in the reactions carried out for 1 to 5h at110 to 170 ◦C depended on the FIL catalysts. The FILs were effi-cient for glucose conversion, and the acidity of the FIL influencedthe reaction to a great extent. Large number of acid sites is essen-tial in all the reaction pathways involved including isomerizationand dehydration/rehydration reactions. The isomerization of glu-cose into fructose has been reported in favour of Lewis acid sites[10,32,63,64].

A complete conversion of glucose was achieved using[BMIM][FeCl4] (Fig. 4a). Meanwhile, [SMIM][Cl] exhibited poor

activity, with the highest glucose conversion registered was 80%at 170 ◦C after 5h reaction time (Fig. 4b). The poor performanceof [SMIM][Cl] for the conversion of glucose is probably due tothe absence of Lewis acid sites. Thus, glucose conversion by using[SMIM][Cl] as a catalyst was attributed by the Brønsted acid sites.It is verified that Lewis acid site has better activity in catalyzing theisomerization of glucose than Brønsted acid site [65,66].

The trend of glucose conversion using [SMIM][FeCl4] as a cat-alyst was similar to [BMIM][FeCl4], suggesting that the Lewis acidsites generated from FeCl4− enhanced the conversion of glucosethrough isomerization reaction (Fig. 4c). It was observed that glu-cose conversion increased considerably at elongated reaction timeand then remained relatively stable after 3h, except at 110◦C,for both [BMIM][FeCl4] and [SMIM][FeCl4]. For all FILs, low glu-

cose conversion rates were observed at low reaction temperature(110 ◦C) compared to theconversion at highertemperatures,whichconfirmed that elevated temperature contributes to the accelera-tion of reaction rate and conversion efficiency.

The effect of reaction temperature and time on 5-HMF and LAyieldsusing three differentFILs is illustrated inFig.5. Reaction timeand temperature played important roles in both glucose conver-sion and product yields. The presence of LA was substantial at allreaction conditions. This implies that 5-HMF from the dehydrationof glucose was converted to rehydration product, mainly LA. It wasalsoobservedthatthecolourofthesolutionturneddarkercolourasthe reaction proceeded, which might be the evidence of decompo-sition of the formed5-HMF to LA. The dark coloured solution mightalso be due to the presence of insoluble byproduct. The change in

the solution’s colour was clearly observed when using [SMIM][Cl]




1 2 3 4 50

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G l u c o s e

c o n v e r s i o n ( % )

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(a)

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G l u c o s e c o n v e r s i o n ( % )

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Fig. 4. Effect of reaction temperature and time on glucose conversion using[BMIM][FeCl4] (a), [SMIM][Cl] (b), and [SMIM][FeCl4] (c) as catalysts. 170 ◦C,150 ◦C, 130 ◦C, ×110 ◦C. (0.1g glucose, 10g FIL catalyst, 10mL water).

and [SMIM][FeCl4] as the catalysts since the two FILs solutions areyellowish.

The presence of water in the reaction system has facilitatedthe 5-HMFrehydration. As reported previously, 5-HMF, unstable inwater, could be further converted to LA and formic acid [27]. Theo-retically, formic acid produced from 5-HMF rehydration is in equalmolarratiowith LA[28,67]. However, theproduction of formicacid

is not reported since low formic acid yield was observed through-

out this study, which probably due to the tendency of formic acidto decompose to CO2, H2, C O a n d H2O in heat and acidic mediumsas reported previously [68,69].

The performance of [BMIM][FeCl4] as a catalyst for glucose con-version (Fig. 5a) demonstrated that LA yield was enhanced withincreasing temperature from 110 to 170 ◦C and at prolonged reac-tion time. Meanwhile, the 5-HMF yield decreased after 3 h reactiontime at 170 ◦C, and after 4h at150 ◦C. Generally, the LA yield fromglucose using [BMIM][FeCl

4] as a catalyst was lower than 5-HMF

yieldthroughout the reactions.This condition wasduetothescarcepresence of Brønsted acid sites in [BMIM][FeCl4] which is essentialfor the rehydration of 5-HMF to LA. Thus, 5-HMF was not produc-tively rehydrated to LA. Fig. 5b shows the distributions of 5-HMFand LA yields when [SMIM][Cl], a Brønsted FIL, was used for glu-cose conversion reactions. The highest 5-HMF yield was recordedat 110 ◦C after 5h reaction time, while the highest LA yield wasobserved at 170◦C after 5 h reaction time. The remaining 5-HMF inthe product was low since the produced 5-HMF rehydrated to LAin the presence of Brønsted acid sites. Since glucose was not fullyconverted in the reaction system with [SMIM][Cl] as a catalyst, lowLA yields were also generated. Meanwhile, when [SMIM][FeCl4]was used as a catalyst, 5-HMF and LA yields surged up to 31%and 67%, respectively (Fig. 5c). The high activity demonstrated by[SMIM][FeCl4] is most likely because of its high acidity comprisedof both Lewis and Brønsted acid sites.

The highest LA yield using [SMIM][Cl] and [SMIM][FeCl4] wasobtained for glucose conversion reaction at 150◦C. Meanwhile, thehighest LA yield was achieved at 170◦C when [BMIM][FeCl4] wasemployed. It is a known fact that elevated temperature contributesto the acceleration of reaction rate and conversion efficiency. Athigher temperature, atoms donate or receive electrons more eas-ily, increasing chemical reaction rate [70,71]. Nevertheless, in thisstudy, lower 5-HMF and LA yields were recorded after 3h reac-tion time at higher reaction temperatures (150–170◦C), possiblydue to the formation of other soluble and insoluble byproducts[63,72]. The insoluble black solid residues observed in this studywere regarded as humins based on previous reports [44,73]. It

was observed that the formation of humins were obvious when[BMIM][FeCl4] and [SMIM][FeCl4] were used as catalysts. Thismight be due to the presence of Lewis acid sites in the FILs, sinceLewis acid sites are known to catalyze the formation of huminsfrom glucose decomposition reaction [32].

Collectively, the results indicated that strong acidity of the FILisan important aspect in glucose conversion for the production of 5-HMF and LA. Among the three FILs, [SMIM][FeCl4] was identifiedasthemosteffectiveFIL,wherethemediumstrongacidsitesofSO3H+

groupstogetherwithFeCl4− inthebulkof[SMIM][FeCl4]musthave

fully played the roles as active sites for the dehydration reaction.From the experimental results of glucose conversion at differentreaction temperatures and time, reaction conditions of 150◦C and4 h were chosen for the subsequent experiments.Table 2 compares

the performances of various ionic liquids used [44,74,75] f or glu-cose conversion reaction in order to evaluate thecompetency of thecatalysts used in this study. It is evident that the FILs in this studyhave improved or are comparable to the dehydration reaction withrespect to reaction time,reaction temperature, glucose conversion,5-HMF yield, and LA yield. The relatively high glucose conversion,as well as LA yield using [SMIM][FeCl4], could be well explained bythe use ofwater as the reaction mediumand due tothe high acidityof the catalyst.

3.3Effect of glucose and FIL loading on glucose conversion

catalyzedby [SMIM][FeCl4]

As [SMIM][FeCl4] was the most effective FIL in this study,[SMIM][FeCl4] was chosen for further investigation on the influ-ence of other reaction conditions. Thus, theeffect of glucose loading

was studied using [SMIM][FeCl4] as a catalyst. The effect of this




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y i e l d ( % )

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(a)

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y i e l d ( % )

Reaction time (h)

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2 3 4 50

15

30

45

60

75

L A y i e l d ( % )

Reaction time (h)

(c)

Fig. 5. Effectof reaction temperatureand time on 5-HMF andLA yieldsusing[BMIM][FeCl4] (a), [SMIM][Cl] (b),and [SMIM][FeCl4] (c)as catalysts.170 ◦C, 150 ◦C,130 ◦C,×110 ◦C (0.1g glucose, 10g FIL catalyst, 10mL water).

parameter was investigated because process economics and envi-ronmentalfriendlinesscan be improved if higherglucose loading isusedwithaconstantamountofFIL. Fig.6 exhibits theconversion of glucose and yields of 5-HMF and LA at various glucose loadings inthe range of 0.05–0.3g. When the glucose loading increased from0.05 to 0.2 g, the product yields did not decrease significantly. Thisinfers that excess reactive sites are available for extra conversionof glucose under the applied reaction condition. Further increase

in the glucose weight (0.2–0.3g) decreased the glucose conversionfrom97%to91%,whereas5-HMFandLAyieldsdecreasedby5%and14%, respectively. Additional increase in theglucose loading ledto adecrease in 5-HMF and LA yields, which probably due to the gener-ation of the insoluble byproducts, humins, in the reaction system.5-HMF can combine with fructose and cross-polymerize to formhumins, especially in an aqueous mixture system [44]. Besides,the decrease in the glucose conversion at higher glucose loading




Table 2

Comparison of different ionic liquids for glucose conversion reaction.

Ionic liquid / co-solvent Reaction condition Glucose conversion (%) Product yield (%) Reference

min ◦C 5-HMF LA

[SO3HMIM][CF3SO3]/MIBK 360 120 97.4 75.1 2.8 [44][EMIM][HSO4] 240 100 95.0 9 NA [74][C2OHMIM][BF4]/DMSO 60 180 NA 67.3 NA [75][BMIM][FeCl4]/H2O 240 150 98.9 48.3 22.4 This study[SMIM][Cl]/H2O 240 150 60.0 6.3 25.8 This study[SMIM][FeCl4]/H2O 240 150 99.9 17.7 67.8 This study

80

85

90

95

100

0.05 0.10 0.15 0.20 0.25 0.300

20

40

60

80

100

Gl u c o s e c onv er s i on ( % )

P r o d u c t y i e l d ( % )

Glucose loading (g)

LA

5-HMF Glucose

Fig. 6. Effect of glucose loading on glucose conversion and 5-HMF and LA yieldsusing [SMIM][FeCl4] as catalyst (10g FIL catalyst, 10mL water,150 ◦C,4 h).

implied that there were insufficient catalytic sites available for thesubstrate glucose in the reaction system at the experimental con-ditions [27]. From the analyses, 0.2g of glucose was used for thenext testing.

FIL dosage is also an important parameter that determines glu-cose conversion, as well as 5-HMF and LA yields. Different amountsof [SMIM][FeCl4] (3–15g/0.2g of glucose) were employed in theglucose dehydration reaction at 150◦C for 4 h. Fig. 7a shows thatvarying the amount of [SMIM][FeCl4] catalyst affected the glucoseconversion and product yields. The glucose conversion increased asthe amount of [SMIM][FeCl4] increased from3 to10g. A minimumamount of 10g [SMIM][FeCl4] was needed for a complete conver-sion of glucose. Upon increasing the amount of [SMIM][FeCl4], thecorresponding increase in the number of FeCl4− in the anions andthe SO3H+ groups in the cations seems to favour the overall reac-tions. As the amount of Lewis acid sites increased, the conversionof glucose conversion also increased. When [SMIM][FeCl4] dosagewas above 10g, side effects such as aldol condensation occurred,

which led to the decrease of product yields. Besides, the largedecrease in the glucose conversion and product yields at higher[SMIM][FeCl4]loadingmightbeattributedtomoreviscousreactionmixture. This condition will hamper the glucose conversion andconsequently inhibit the rehydration of 5-HMF to LA. The signifi-canceofhighviscosityonglucoseconversionandproductyieldshasbeen discussed previously [76,77]. Since the difference of LA yieldbetween5–10gand10–13gof[SMIM][FeCl4]dosagewasconsider-ably small, 5 g of [SMIM][FeCl4] was chosen as the optimum initialamount of FIL to reduce the cost of reaction.

In order to confirm the role of FIL as a catalyst in the conver-sion of glucose, an experiment using blank sample(sample without[SMIM][FeCl4]) was also performed. The glucose conversion, 5-HMF yield, and LA yield reached 47%, 5%, and 2%, respectively, at

150◦

C after 4h reaction. The huge difference of the glucose con-

60

70

80

90

100

3g 5g 10g 13g 15g0

20

40

60

80

100(a)


Catalyst loading (g)

LA

5-HMF

Gl u c o s e c onv er s i on

( % )

Glucose

80

85

90

95

100

2:1 1.5:1 1:1 1:1.5 1:20

20

40

60

80

100

(b)

Gl u c o

s e c onv er s i on ( % )

P r o d u c t y i e

l d ( % )

Water : catalyst loading

LA

5-HMF Glucose

Fig. 7. Effectof FILloading (a)and water:FIL loading (b)on glucose conversion and5-HMF and LA yields using [SMIM][FeCl4] as catalyst (0.2g glucose, 10mL water,

150◦

C,4 h (a),5 g FIL catalyst, 150◦

C,4 h (b)).

version and product yields between the sample with and without[SMIM][FeCl4] verified the role of FIL as a catalyst in the glucoseconversion reaction.

Since water is used as a solvent in the reaction, the effect of water loading was also examined in this study. Given the highcost of ionic liquid, the addition of water that reduces the purityof ionic liquid may offset the quantity of ionic liquid used. Waterlowers the viscosity of ionic liquid, increases the solubility of feed-stocks andcatalyst, andat thesame time encourages thehydrolysisreaction and mass transfer [55,78–80]. The effect of water to FIL ([SMIM][FeCl4]) ratio from 2:1 to 1:2 on the glucose conversionand dehydration product yields are shown in Fig. 7b. A complete

conversion of glucose was consistently reported regardless of the




60

70

80

90

100

Fresh 1st reuse 2nd reuse 3rd reuse 4th reuse0

20

40

60

80

100

Gl u c o s e c onv

er s i on ( % )


Reusability

LA

5-HMF

Glucose

Fig. 8. Reusability of [SMIM][FeCl4] for glucose conversion to 5-HMF and LA (0.2gglucose, 5g FIL catalyst, 5 mL water, 150 ◦C,4 h).

amount of water loaded in the reaction system. It appears that thepresence of wateronlyaffects theproduct yields. When theamountof water was low compared to the dosage of [SMIM][FeCl4] (1:2),theproductyieldswerelowduetothehighviscosityofthereactionmixture, which resulted in low mass transfer rate and affected theoccurrence of the dehydration /rehydration reaction. Fig. 7b alsoindicates that the yield of 5-HMF decreased with the increase of waterto FILratioloading (1:1.5to 1.5:1). The reason was that whenthe dosage of water increased, the 5-HMF rehydration occurredprogressively andthe yield of LA correspondinglyincreased. The LAyield continued to increase with increasing water to FIL ratio until1.5:1, beyond which the LA yield decreased when more water wasloaded into the reaction system. This observation is probably dueto the opposition between H+ in [SMIM][FeCl4] and water to cat-alyze the rehydration reaction, which suppresses the intermediatereactions. Previous studies on glucose conversion in ionic liquids

reported that the presence of water in the reaction system givesa negative effect on the reaction [55,78,81]. However, this studyverified that the FIL used in the aqueous system registered a goodperformance for glucose conversion to 5-HMF and LA. Thus, at theoptimum water:FIL ratio, the reaction system applied in this studyshould have the potential to be economically viable.

3.4. FIL reusability

ToconfirmthereusabilityofthesynthesizedFILs,[SMIM][FeCl 4]was selected forthe dehydration reaction over fivecycles. Thecata-lystwas recoveredafterextracting the productsusingethyl acetate.The performance of [SMIM][FeCl4] reusability was conducted forglucose conversion at 150◦C for 4h using 5 g of [SMIM][FeCl4] and

water, and 0.2g of glucose. As shown in Fig. 8, the catalytic activ-ity of [SMIM][FeCl4] did not decrease significantly after five runs,which demonstrated that [SMIM][FeCl4] was stable in this system.The conversion of glucose maintained at around 90% until the fifthrun.Thedecreaseby6%and8%in5-HMFandLAyields,respectively,were observed.

3.5. Kinetic analysis of glucose conversion reaction

A kinetic analysis of the conversion of glucose catalyzedby [SMIM][FeCl4] was carried out in a temperature range of 110–170 ◦C. The values of -ln (1-X) (where X is the conversion of glucose) were plotted against reaction time (t ) at different reac-tion temperatures to determine the reaction rate constants (k)

(Fig. 9). The linearity of Fig. 9 supported the first order depen-

0

2

4

6

0 50 100 150 200 250 300

- l n ( 1 - X )

Reaction time (min)

Fig. 9. The kinetic profiles of glucose conversion catalyzed by [SMIM][FeCl4] atdifferent reaction temperatures.170 ◦C, 150 ◦C, 130 ◦C, ×110 ◦C.

Table 3

Reaction rate constant (k) of glucose conversion catalyzed by [SMIM][FeCl4] atdif-ferent reaction temperatures.

Reaction temperature(◦C)

Reaction rate constant,k (min−1)

Correlationcoefficient

110 0.0052 0.978130 0.0121 0.993150 0.0195 0.981170 0.0265 0.980

Theexperimentswere carried out at thespecified temperature for5 h with 10g FIL catalyst and 0.1g glucose in 20mL H2O.

Table 4

Kinetic parameters for glucose conversion catalyzed by [SMIM][FeCl4].

Parameter Value

Reaction order, n 1.0Activation energy, E a (kJmol−1) 38.1Pre- exponential factor, A (min−1) 925.0Correlation coefficient 0.968

dence of theglucose conversion reactions. Theresultsshow that thevalue of reaction rate constant increased with increasing temper-

ature, indicating that higher temperature accelerates the reactionrate of glucose conversion (Table 3). The activation energy, E a andpre-exponential factor, A, were obtained by applying the Arrheniusplot using the values of the rate constants and are listed inTable 4.A value of 38kJmol−1 was reported in this study for the activa-tion energy of glucose conversion, which is lower with than thosereported by using mineral acid [23,82,83] and [C2OHMIM][BF4]ionic liquid [75] as catalysts, and comparable to the activationenergy reported by using Cr-SO3H polymeric ionic liquid [47] asa catalyst. This study signifies that the introduction of functionalgroups (SO3H and FeCl3) in ionic liquid catalyst has accelerated thereaction, consequently lowering the activation energyand increas-ing the reaction rate.

4. Conclusions

Efficient glucose conversion to 5-HMF andLA has been achievedusing acidic FIL as a catalyst. [SMIM][FeCl4] displayed the high-est catalytic activity in the conversion of glucose, with 18% and68% yields of 5-HMF and LA, respectively, which was achievedafter 4h at 150 ◦C. The acidity of the solution registered a largeeffect on glucose conversion and product yields, where the cat-alyst with stronger acidity led to higher activity. The presenceof both Brønsted and Lewis acid sites seemed suitable for 5-HMF and LA production. The significantly high activity of aqueous[SMIM][FeCl4] reduced the quantity of catalyst needed for thereaction. [SMIM][FeCl4] catalyst was reused and exhibited favor-able catalytic activity over five successive cycles. In addition, the

kinetic analysis implied that the introduction of functional groups



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