Selective Dehydration of Glucose Into 5-Hydroxymethylfurfural by Ionic Liquid-Zrocl2 in Isopropanol

catalysts

Article Selective Dehydration of Glucose into 5-Hydroxymethylfurfural by Ionic Liquid-ZrOCl2 in Isopropanol

Yubo Ma 1,2, Lei Wang 1,3, Hongyi Li 4,*, Tianfu Wang 2,* and Ronghui Zhang 5 1 School of Chemical Engineering, Chongqing Chemical Industry Vocational College, Chongqing 401220, China; [email protected] (Y.M.); [email protected] (L.W.) 2 Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China 3 Department of chemistry and Chemical Engineering, Xinjiang University, Urumqi 830001, China 4 Xinjiang Quality of Products Supervision & Inspection Institute of Technology, Urumqi 830001, China 5 Guangdong Key Laboratory of Intelligent Transportation System, School of Engineering, Sun Yat-Sen University, Guangzhou 510275, China; [email protected] * Correspondence: [email protected] (H.L.); [email protected] (T.W.)

Received: 29 August 2018; Accepted: 25 September 2018; Published: 18 October 2018

Abstract: In this work, a heterogeneous catalytic system consisting of [HO2CMMIm]Cl and ZrOCl2 in isopropanol is demonstrated to be effective for 5-hydroxymethylfurfural (HMF) synthesis with glucose as the feedstock. Various reaction conditions for HMF synthesis by glucose dehydration were investigated systematically. Under optimized reaction conditions, as high as 43 mol% HMF yield could be achieved. Increasing the water content to a level below 3.17% led to the production of HMF with a higher yield, while a lower HMF yield was observed when the water content was increased above 3.17%. In addition, the data also showed that ZrOCl2 could not only effectively convert glucose into intermediate species (which were not fructose, in contrast to the literature) but also catalyze the intermediate species’ in situ dehydration into HMF. [HO2CMMIm]Cl was used to catalyze the intermediate species’ in situ conversion to HMF. The kinetics data showed that a temperature increase accelerated the intermediate species’ dehydration reaction rate. The reaction of glucose dehydration was a strong endothermal reaction.

Keywords: glucose; 5-hydroxymethylfurfural; dehydration; [HO2CMMIm]Cl; ZrOCl2; kinetics

1. Introduction Carbohydrates, as a renewable and abundant biomass feedstock, represent a promising carbon source. Therefore, an efﬁcient and economical process that can convert carbohydrates into useful chemicals is highly desirable. Production of 5-hydroxymethylfurfural (HMF) with carbohydrates as the feedstock is one of the hot points in the process of material resources’ comprehensive utilization. HMF has been proposed by the US Department of Energy to be one of the top 12 platform chemicals, having the potential to be a building block in the synthesis of furanic-based polyamides, polyesters, and polyurethanes analogous to petroleum-based terephthalic acid [1–6]. HMF can be produced from glucose and fructose through acid catalysts [7]. In the past decade, HMF synthesis from fructose has been studied widely, and as high as 90% HMF yields have been readily achieved [8–22]. Glucose is the most abundant and cheap C6 sugar, so the syntheses of HMF from glucose is more attractive. A variety of solvents have been investigated for converting glucose into HMF, such as water [23,24], organic solvents (e.g., DMSO) [2], organic/aqueous binary mixtures [25–29], and ionic liquids [17,30–34]. Many more catalyst systems have also been developed for glucose dehydration, such as Lewis acid catalysts [35], tantalum compounds [36], boric acid [37],

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Catalysts 2018, 8, x FOR PEER REVIEW 2 of 12 ionic liquids (ILs) [38], ion exchange resins [39], zeolites [40,41], chromium(0) nanoparticles [42], 2− Sn–MontionicCatalysts liquids catalyst 2018 ,(ILs) 8, x[ FOR4 [38],], bifunctionalPEER ion REVIEW exchange SO resins4 /ZrO [39],2 catalystszeolites [40,41], [43], mesoporous chromium(0) tantalum nanoparticles phosphate [42],2 of 12 Sn– [44], andMont so on. catalyst [4], bifunctional SO42−/ZrO2 catalysts [43], mesoporous tantalum phosphate [44], and so Recently,ionicon. liquids HMF (ILs) synthesis [38], ion exchange by sugar resins dehydration [39], zeolites with [40,41], ionic liquidschromium(0) (ILs) asnanoparticles solvents or [42], using Sn– acidic ionicMont liquidsRecently, catalyst [45– 47HMF [4],] as bifunctional catalystssynthesis hasby SO grownsugar42−/ZrO dehydration2 rapidly catalysts because [43], wi mesoporousth of ionic its unique liquids tantalum and (ILs) tunable phosphate as solvents properties. [44], or andusing In fact, a 70%acidicso HMF on. ionic yield liquids with [45–47] 1-ethyl-3-methylimidazolium as catalysts has grown chloride rapidly ([EMIM]Cl)because of asits theunique reaction and solvent tunable can beproperties. obtainedRecently, over In fact, aHMF chromium a 70% synthesis HMF (II) yieldby chloride sugar with dehydration 1-ethyl-3-methylimidazolium catalyst [17 wi];th a 83.4%ionic liquids HMF yield(ILs)chloride as can solvents ([EMIM]Cl) be obtained or using as over the a acidic ionic liquids [45–47] as catalysts has grown rapidly because of its unique and tunable chromiumreaction solvent (II) chloride can be catalyst obtained in 1,8-diazabicycloover a chromium [5.4.0]undec-7-ene-based (II) chloride catalyst [17]; ionica 83.4% liquids HMF [ 32yield]; a can 48.4% be properties.obtained over In fact, a chromium a 70% HMF (II) yield chloride with catalyst 1-ethyl-3-methylimidazolium in 1,8-diazabicyclo [5.4.0]undec-7-ene-based chloride ([EMIM]Cl) as the ionic HMF yield was observed over a germanium (IV) chloride catalyst in 1-butyl-3-methylimidazolium liquidsreaction [32]; solvent a 48.4% can beHMF obtained yield over was a chromium observed (II) over chloride a germanium catalyst [17]; (IV) a 83.4% chloride HMF yieldcatalyst can in chloride ([BMIM]Cl) with glucose as the feedstock [34]; a 58.3% HMF yield can be obtained over 1-butyl-3-methylimidazoliumbe obtained over a chromium chloride (II) chloride ([BMIM]Cl) catalyst in with 1,8-diazabicyclo glucose as the [5.4.0]undec-7-ene-based feedstock [34]; a 58.3% ionic HMF a tinliquids phosphate [32]; a catalyst 48.4% withHMF theyield ionic was liquidobserved EMIMBr over a asgermanium the solvent (IV) [48 chloride]; a 67.3% catalyst HMF in yield yield can be obtained over a tin phosphate catalyst with the ionic liquid EMIMBr as the solvent [48]; was reported1-butyl-3-methylimidazolium over a 1-hydroxyethyl-3-methylimidazolium chloride ([BMIM]Cl) with glucose tetraﬂuoroborate as the feedstock [34]; ionic a 58.3% liquid HMF catalyst a 67.3% HMF yield was reported over a 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate in DMSOyield can [38 ].be Only obtained a 50.6% over HMFa tin phosphate yield with catalyst glucose with as the the ionic feedstock liquid EMIMBr was achieved as the solvent in DMSO [48]; over ionic liquid catalyst in DMSO [38]. Only a 50.6% HMF yield with glucose as the feedstock was a 67.3% HMF yield was reported over a 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate theachieved 1-carboxypropyl-3-methyl in DMSO over the imidazoliumchloride 1-carboxypropyl-3-methyl ([CMIm]Cl) imidazoliumchloride and ZrOCl2 catalyst ([CMIm]Cl) system byand Hu et al.,ionic although liquid acatalyst 95.7% HMFin DMSO yield [38]. can Only be obtained a 50.6% HMF with fructoseyield with as glucose the feedstock as the feedstock over a [CMIm]Cl was ZrOCl2 catalyst system by Hu et al., although a 95.7% HMF yield can be obtained with fructose as achieved in DMSO over the 1-carboxypropyl-3-methyl imidazoliumchloride ([CMIm]Cl) and catalystthe feedstock [16]. over a [CMIm]Cl catalyst [16]. ZrOCl2 catalyst system by Hu et al., although a 95.7% HMF yield can be obtained with fructose as It is well known that the isomerization of glucose into fructose is a key step in the synthesis of theIt feedstock is well known over a that[CMIm]Cl the isomerization catalyst [16]. of glucose into fructose is a key step in the synthesis of HMF from glucose (Scheme1). Recently, it was reported that the isomerization of glucopyranose HMF fromIt is wellglucose known (Scheme that the 1). isomerization Recently, it was of glucose reported into that fructose the isomerization is a key step inof theglucopyranose synthesis of to tofructofuranose fructofuranoseHMF from glucose could could (Scheme be be effectively effectively 1). Recently, catalyzed catalyzed it was repoby by ZrOClrted ZrOCl that2 2[49]. the[49 isomerization ].Hu Hu et etal. al. also of also glucopyranosereported reported that that to the the isomerizationisomerizationfructofuranose of of glucose glucosecould intobe into effectively fructose fructose could couldcatalyzed bebe obtainedobtained by ZrOCl with 2 [49]. ZrOCl ZrOCl Hu2 2 etasas theal. the alsocatalyst catalyst reported [16]. [16 ].that the RecoveryisomerizationRecovery of of ionicof ionic glucose liquid liquid into is is fructose usually usually could donedone be byby obtained extraction with with with ZrOCl diethyl diethyl2 as the etherether catalyst (Et (Et2 O)[16].2O) [50]. [50 However,]. However, diethyldiethyl ether Recoveryether is is easily easily of ionic volatized, volatized, liquid is so sousually developingdeveloping done by a newextraction method with of of diethylrecovery recovery ether of of the(Et the 2ionicO) ionic [50]. liquid liquidHowever, without without anan organic diethylorganic solvent ether solvent is iseasily highlyis highly volatized, desirable. desirable. so developing In In our our previous previousa new method work, work, [HOof [HO recovery2CMMIm]Cl2CMMIm]Cl of the ionic couldcould liquid bebe recoveredwithoutrecovered by an organic solvent is highly desirable. In our previous work, [HO2CMMIm]Cl could be recovered centrifugation.by centrifugation. A 91.2% A HMF 91.2% yield HMF for fructoseyield for dehydration fructose coulddehydration be obtained could with be [HO obtained2CMMIm]Cl with as the[HO catalystby2 CMMIm]Clcentrifugation. in isopropanol as the A catalyst [91.2%51], but inHMF isopropanol only yield a 6.7% for [51], HMF fructose but yield only coulddehydration a 6.7% be HMF obtained could yield for couldbe glucoseobtained be obtained dehydration. with for glucose[HO2CMMIm]Cl dehydration. as theThus, catalyst a catalytic in isopropanol system [51],with but ZrOCl only2 aand 6.7% [HO HMF2CMMIm]Cl yield could was be obtained developed for in Thus, a catalytic system with ZrOCl2 and [HO2CMMIm]Cl was developed in this study to catalyze thisglucose study dehydration.to catalyze glucose Thus, aconversion catalytic system to HMF. with ZrOCl2 and [HO2CMMIm]Cl was developed in glucosethisconversion study to catalyze to HMF. glucose conversion to HMF. HO OHHO O OHHO OH HO OHHO O OHHO OH HO O O HO O O HO OH O HO OH HO OH O HO OH fructose glucose fructose 5-Hydroxymethylfurfural glucose 5-Hydroxymethylfurfural Scheme 1. Dehydration of glucose into 5-hydroxymethylfurfural (HMF). SchemeScheme 1. Dehydration1. Dehydration of of glucose glucose into 5-hydroxymethylfurfural 5-hydroxymethylfurfural (HMF). (HMF).

In this work, a [HO2CMMIm]Cl–ZrOCl2–isopropanol system was reported to catalyze glucose In thisIn work,this work, a [HO a [HO2CMMIm]Cl–ZrOCl2CMMIm]Cl–ZrOCl22––isopropanolisopropanol system system was was reported reported to catalyze to catalyze glucose glucose conversionconversionconversion into into into HMF, HMF, HMF, as as shownas shown shown in in in Scheme Scheme Scheme2 2..2. Interestingly, Interestingly,stingly, no no fructose fructose was was was found found found in inthe in the thereactions, reactions, reactions, andand moreand more more than than than 90% 90% 90% glucose glucose glucose conversion conversion conversion with withwith lessless than 3%3% 3% HMFHMF HMF yield yield yield was was was observed observed observed in in a infewa few a minutes. few minutes. minutes. ZrOClZrOCl2 was2 was used used not not only only for for glucose glucose conversionconversion but alsoalso forfor production production of of HMF; HMF; [HO [HO2CMMIm]Cl2CMMIm]Cl ZrOCl2 was used not only for glucose conversion but also for production of HMF; [HO2CMMIm]Cl waswas used used for for production production ofof HMF.HMF. TheseThese data suggest thatthat glucoseglucose was was first first converted converted into into was used for production of HMF. These data suggest that glucose was ﬁrst converted into intermediate intermediateintermediate species, species, but but the the intermediate intermediate speciesspecies were notnot fructose;fructose; therefore, therefore, the the possible possible reaction reaction species, but the intermediate species were not fructose; therefore, the possible reaction route was not in routeroute was was not not in in agreement agreement with with thethe routeroute reportedreported inin thethe literatureliterature [16,52]. [16,52]. Increasing Increasing the the water water agreement with the route reported in the literature [16,52]. Increasing the water content (below 3.17%) contentcontent (below (below 3.17%) 3.17%) led led to to a a higher higher HMF HMF yield,yield, but onlyonly aa 4.9% 4.9% HMF HMF yield yield was was obtained obtained with with 60% 60% ledwater towater a content. higher content. HMFFurthermore, Furthermore, yield, butthe the only catalystcatalyst a 4.9% systemsystem HMF could yield bebe recoveredrecovered was obtained by by centrifugation centrifugation with 60% without water without content.an an Furthermore,organicorganic solvent. solvent. the catalyst system could be recovered by centrifugation without an organic solvent.

HO OH HO HO OH HO OH OH ZrOCl2 ZrOCl2 O O [HO CMMIm]Cl O O 2 O [HO2CMMIm]Cl O HO OH glucoseHO OH 5-Hydroxymethylfurfural glucose 5-Hydroxymethylfurfural43% 43% Scheme 2. Glucose dehydration by [HO2CMMIm]Cl ionic liquid (IL)–ZrOCl2 in isopropanol. SchemeScheme 2. 2.Glucose Glucose dehydration dehydration by by[HO [HO22CMMIm]Cl ionic ionic liquid liquid (IL) (IL)–ZrOCl–ZrOCl2 in2 in isopropanol. isopropanol.

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2. Results & Discussion 2.1. Effect of IL [HO2CMMIm]Cl Loading on the Catalytic Performance

2.1.In Effect our ofpreviously IL [HO2CMMIm]Cl works, a 91.2 Loading% HMF on the yield Catalytic for fructose Performance dehydration could be obtained with [HO2CMMIm]Cl as the catalyst in isopropanol [51]. Therefore, a reaction of glucose dehydration was In our previously works, a 91.2% HMF yield for fructose dehydration could be obtained with carried out, but only a 6.7% HMF yield with 92% glucose conversion was observed. Based on the [HO2CMMIm]Cl as the catalyst in isopropanol [51]. Therefore, a reaction of glucose dehydration literature [16], ZrOCl2 was introduced into the catalytic system. A 25% HMF yield with 95.8% was carried out, but only a 6.7% HMF yield with 92% glucose conversion was observed. Based glucose conversion was achieved in a 3 h reaction at 150 °C, with 0.15 g ZrOCl2 and 0.1 g on the literature [16], ZrOCl2 was introduced into the catalytic system. A 25% HMF yield with [HO2CMMIm]Cl as the catalyst. Therefore, we studied the effect of◦ IL [HO2CMMIm]Cl loading on 95.8% glucose conversion was achieved in a 3 h reaction at 150 C, with 0.15 g ZrOCl2 and 0.1 g the catalytic performance, and the results are listed in Figure 1. It can be seen that the selectivity of [HO2CMMIm]Cl as the catalyst. Therefore, we studied the effect of IL [HO2CMMIm]Cl loading on the HMF increased gradually with the increase of the amounts of [HO2CMMIm]Cl; HMF yield first catalytic performance, and the results are listed in Figure1. It can be seen that the selectivity of HMF increased with [HO2CMMIm]Cl loading increase, reached a maximum of 29.3% at 50 mg increased gradually with the increase of the amounts of [HO2CMMIm]Cl; HMF yield ﬁrst increased [HO2CMMIm]Cl, then decreased with further increases of [HO2CMMIm]Cl loading. This suggested with [HO2CMMIm]Cl loading increase, reached a maximum of 29.3% at 50 mg [HO2CMMIm]Cl, then that [HO2CMMIm]Cl could increase HMF selectivity, and that ZrOCl2 not only was favorable to decreased with further increases of [HO2CMMIm]Cl loading. This suggested that [HO2CMMIm]Cl the conversion of glucose, but also could increase the yield of HMF. could increase HMF selectivity, and that ZrOCl2 not only was favorable to the conversion of glucose, but also could increase the yield of HMF.

32 105 100 30 95 HMF selectivity % 28 90 26 85 HMF yield % 80 24 HMF selectivity % 75 glucose conversion % 22 70

HMF yield % HMF yield 65 20 60 18 55

0 20406080100

IL loading /mg

FigureFigure 1. 1.EffectEffect ofof IL IL [HO [HO2CMMIm]Cl2CMMIm]Cl loading loading on on glucose glucose dehydration.Experimental dehydration.Experimental conditions: conditions: 0.15 g ◦ g ZrOCl2,, 0.1 0.1 g g glucose, glucose, 3.0 3.0 g g isopropanol, isopropanol, 3 3 h h reac reactiontion time, time, 150 °CC reaction reaction temperature.

2.2.2.2. Effect Effect of ofZrOCl ZrOCl2 Loading2 Loading onon the the Catalytic Catalytic Performance Performance

InIn the the above above section,section, ZrOClZrOCl2 2was was not not only only favoring favoring the conversionthe conversion of glucose, of glucose, but also but increasing also increasingHMF yield. HMF So, yield. the effects So, the of effects ZrOCl 2ofloading ZrOCl2 on loading the catalytic on the performancecatalytic performance were examined were examined with 50 mg with[HO 502CMMIm]Cl mg [HO2CMMIm]Cl as the catalyst, as the and catalyst, the results and the are results summarized are summarized in Figure2 .in it canFigure be seen2. it can that be the seenyield that of the HMF yield increased of HMF dramatically increased dramatically with ZrOCl with2 loading ZrOCl at2 loading the beginning, at the beginning, with up to with 30% up HMF to ◦ 30%yield HMF achieved yield atachieved 200 mg ZrOClat 200 2mgloading ZrOCl when2 loading the reaction when the was reaction conducted was at conducted 150 C. While at 150 the yield°C. Whileof HMF the decreasedyield of HMF as ZrOCl decreased2 loading as ZrOCl increased2 loading to 250 increased mg, the to selectivity 250 mg, the of HMFselectivity ﬁrst increased,of HMF firstthen increased, decreased. then decreased.

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100 30 95

35 105 HMFselectivity % 10090 25 30 9585 HMF yield % HMFselectivity % 9080 HMFselectivity % 20 75 25 glucose conversion % 85 HMF yield % 8070

15 HMFselectivity % HMF yield % HMF yield 20 7565 glucose conversion % 7060 10 15 55

HMF yield % HMF yield 65 50 100 150 200 250 60 10 55 ZrOCl2 loading /mg 50 100 150 200 250 Figure 2. Influence of ZrOCl2 loading on glucose conversion.Reaction conditions: 0.05 g

[HO2CMMIm]Cl, 0.1 g glucose, 3.0 g isopropanol, 3 h reaction time, 150 °C reaction temperature. ZrOCl2 loading /mg

2.3. InfluenceFigureFigure 2. 2.of InfluencetheInfluence Reaction of ZrOClof Time ZrOCl2 loadingon 2the loading Catalytic on glucose on Performance conversion.Reactionglucose conversion.Reaction conditions: 0.05conditions: g [HO2CMMIm]Cl, 0.05 g ◦ [HO0.12CMMIm]Cl, g glucose, 3.0 0.1 g isopropanol,g glucose, 3.0 3 hg reactionisopropanol, time, 3 150 h reactionC reaction time, temperature. 150 °C reaction temperature. The influence of the reaction time on glucose conversion, HMF yield, and selectivity was also 2.3.investigated,2.3. Influence Influence ofand ofthe the theReaction Reaction data areTime Time summarized on on the the Catalytic Catalytic in Figure Performance Performance 3. It can be seen that the conversion of glucose increased sharply to 93% in the first 5 min, then increased gradually from 93% to 98% in the TheThe influence influence of of the the reaction reaction time time on on glucose glucose conversion, conversion, HMF HMF yield, yield, and and selectivity selectivity was was also also following 3 h, while the yield of HMF was only 1% in the first 5 min and then gradually increased to investigated,investigated, and and the the data data are are summarized summarized in inFigure Figure 3.3 It. Itcan can be be seen seen that that the the conversion conversion of of glucose glucose 29.3% in the following 3 h; HMF selectivity gradually decreased from 90% to 66% in 3 h. These data increasedincreased sharply sharply to to 93%93% inin the the first first 5 min,5 min, then then increased increased gradually gradually from 93%from to 93% 98% to in the98% following in the suggested that glucose could be converted into intermediate species within a short time. The following3 h, while 3 theh, while yield the of HMF yield was of HMF only was 1% in only the 1% first in 5 the min first and 5 then mingradually and then increasedgradually toincreased 29.3% in to the intermediate species were not fructose according to the results from HPLC, and a longer reaction 29.3%following in the 3 following h; HMF selectivity 3 h; HMF gradually selectivity decreased gradually from decreased 90% to from 66% in90% 3 h. to These 66% in data 3 h. suggested These data that time was necessary for HMF production from the intermediate species by the [HO2CMMIm]Cl– suggestedglucose couldthat beglucose converted could into be intermediateconverted into species intermediate within a shortspecies time. within The intermediatea short time. species The ZrOCl2–isopropanol system. intermediatewere not fructose species according were not to fructose the results according from HPLC, to the andresults a longer from reactionHPLC, and time a waslonger necessary reaction for HMF production from the intermediate species by the [HO CMMIm]Cl–ZrOCl –isopropanol system. time was necessary for HMF production from the intermediate2 species by the2 [HO2CMMIm]Cl– ZrOCl2–isopropanol system. 35 100 30

35 HMFselectivity % 25 30 100 20 HMFselectivity % 25 15 HMF yield % 80 20 HMFselectivity % 10 glucose conversion % 15 HMF yield %

HMF yield % 80 5 HMFselectivity % 10 glucose conversion % 0 HMF yield % 5 60 0 20 40 60 80 100 120 140 160 180 200 0 Reaction time /h 60 0 20 40 60 80 100 120 140 160 180 200 FigureFigure 3. 3. EffectEffect of of reaction reaction time time on on glucose glucose de dehydrationhydration Reaction Reaction conditions: conditions: 0.15 0.15 ZrOCl ZrOCl2, 20.15, 0.15 g g ◦ [HO[HO2CMMIm]Cl,CMMIm]Cl, 0.1 0.1 g glucose, g glucose, 3.0 3.0 g isopropanol, g isopropanol, 150 150 °C Creaction reaction temperature. temperature. 2 Reaction time /h

Figure 3. Effect of reaction time on glucose dehydration Reaction conditions: 0.15 ZrOCl2, 0.15 g [HO2CMMIm]Cl, 0.1 g glucose, 3.0 g isopropanol, 150 °C reaction temperature.

Catalysts 2018, 8, x FOR PEER REVIEW 5 of 12 Catalysts 2018, 8, 467 5 of 12 2.4. Effect of the Reaction Temperature on the Catalytic Performance 2.4.The Effect effect of the of Reaction the reaction Temperature temperature on the Catalytic on glucose Performance dehydration in isopropanol over the [HO2CMMIm]Cl–ZrOCl2 catalyst system (reaction time of 3 h) was also examined, and the data are The effect of the reaction temperature on glucose dehydration in isopropanol over the listed in Figure 4. The conversion of glucose gradually increased from 80% to 95.8% when [HO2CMMIm]Cl–ZrOCl2 catalyst system (reaction time of 3 h) was also examined, and the data increasingare listed the in Figuretemperature4. The from conversion 120 °C to of 160 glucose°C, while gradually the yield increased of HMF from increased 80% toto 95.8%29.3% whenfrom

6.7%increasing with a thereaction temperature temperature from 120increase◦C to from 160 ◦120C, while°C to the150 yield°C and of HMFremained increased unchanged to 29.3% as fromthe reaction6.7% with temperature a reaction further temperature increased increase to 160 from°C. 120The ◦changeC to 150 in◦ Cthe and selectivity remained of unchangedHMF was asvery the limited,reaction ranging temperature from 68% further to 66% increased under the to studied 160 ◦C. reaction The change temperature. in the selectivity of HMF was very limited, ranging from 68% to 66% under the studied reaction temperature.

35 105 100 30

95 HMFselectivity % 25 90

20 85 80 15 HMF yield % HMFselectivity % 75 glucose conversion % HMF yield % 10 70 65 5 60 120 130 140 150 160 Reaction temperature oC

Figure 4. Effect of the reaction temperature on HMF yield, during glucose conversion.Experimental Figure 4. Effect of the reaction temperature on HMF yield, during glucose conversion.Experimental conditions: 0.15 g ZrOCl2, 0.05 g [HO2CMMIm]Cl, 0.1 g glucose, 3.0 g isopropanol, 3 h reaction time. conditions: 0.15 g ZrOCl2, 0.05 g [HO2CMMIm]Cl, 0.1 g glucose, 3.0 g isopropanol, 3 h reaction time. 2.5. Effect of Substrate Loading on HMF synthesis 2.5. Effect of Substrate Loading on HMF synthesis The effect of the glucose loading on glucose dehydration was also examined under the conditions The effect of the glucose loading on glucose◦ dehydration was also examined under the of 0.05 g [HO2CMMIm]Cl, 0.15 g ZrOCl2, 150 C reaction temperature, and 3 h reaction time, and conditionsthe data areof 0.05 listed g in[HO Figure2CMMIm]Cl,5. HMF yield0.15 g decreased ZrOCl2, 150 from °C 43% reaction to 27% temperature, with the increase and 3 h of reaction glucose time,loading and fromthe data 30 mg are to listed 120 mg. in Figure This could 5. HMF arise yield from decreased increased from glucose 43% concentration to 27% with the in isopropanol,increase of glucosewhich likelyloading favored from 30 side mg reactions to 120 mg. such This as polymerization could arise from and increased rehydration, glucose producing concentration humins in as isopropanol,the main by-product. which likely The favored selectivity side reactions of HMF such showed as polymerization a similar trend. and The rehydration, conversion producing of glucose huminsdecreased as the gradually. main by-product. The selectivity of HMF showed a similar trend. The conversion of glucose decreased gradually. 2.6. Effect of Water Content on the Catalytic Performance 2.6. Effect of Water Content on the Catalytic Performance Figure6 shows the inﬂuence of water on glucose dehydration in isopropanol at 150 ◦C. In isopropanol,Figure 6 shows glucose the influence was ﬁrstly of convertedwater on intoglucose the intermediatedehydration in species, isopropanol and the at intermediate 150 °C. In isopropanol,species were glucose subsequently was firstly converted converted to HMF. into A 29.3%the intermediate HMF yield wasspecies, achieved and afterthe intermediate a 3 h reaction. speciesThe conversion were subsequently of the intermediate converted to species HMF. to A HMF29.3% was HMF dramatically yield was achieved enhanced after by a the 3 h addition reaction. of The3.17 conversion wt % water. of the The intermediate yield of HMF species increased to HM to 35.2%F was withdramatically the addition enhanced of 3.17 by wt the % addition water to of the 3.17isopropanol. wt % water. The The intermediate yield of HMF species increased convertion to 35.2% to HMF with was the inhibited addition in of the 3.17 presence wt % water of more to the than isopropanol.10 wt % water. The Onlyintermediate a 4.9%HMF species yield convertion was observed to HMF in was the inhibited presence in of the 60 wtpresence % water. of more These than data 10suggested wt % water. that Only the catalytic a 4.9% HMF reaction yield could was be observed accelerated, in the and presence the concentration of 60 wt % of water. the catalytic These data active suggestedspecies could that bethe decreased catalytic byreaction increasing could the be water accelerated, content toand a certain the concentration range. of the catalytic active species could be decreased by increasing the water content to a certain range.

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46 105 46 105 44 100 44

42 100 HMFselectivity % 95

42 HMFselectivity % 40 95 40 38 90 90 38 36 HMF yield % 85 HMF HMFselectivity yield % % 36 34 85 HMFselectivity glucose conversion % % 80 34 32 80 glucose conversion % 75 32 30 75 30HMF yield % 70 HMF yield % 28 70 28 26 65 65 26 24 60 24 20 40 60 80 100 120 60 20 40 60 80 100 120 Glucose loading /mg Glucose loading /mg Figure 5. Effect of Substrate loading on glucose conversion.Reaction conditions: 0.15 g ZrOCl2, 0.05 g

FigureFigure[HO 5.2 5.EffectCMMIm]Cl,Effect of ofSubstrate Substrate 3.0 g isopropanol, loading loading on on glucose 3 glucose h reacti conversion.Reaction conversion.Reactionon time, 150 °C reaction conditions: conditions: temperature. 0.15 0.15 g gZrOCl ZrOCl2, 20.05, 0.05 g g ◦ [HO[HO2CMMIm]Cl,2CMMIm]Cl, 3.0 3.0 g isopropanol, g isopropanol, 3 h 3 reacti h reactionon time, time, 150 150 °C Creaction reaction temperature. temperature.

40 40 35 35 30 30 25 25

20 15 HMF yield % 15 HMF yield % HMF yield % 10 HMF yield % 10 5 5 0 102030405060 0 102030405060 water content % water content % FigureFigure 6. Effect6. Effect of water of water content% content% on glucose on glucose dehydration.Experimental dehydration.Experimental conditions: cond 0.1itions: g glucose, 0.1 g 0.15glucose, g ◦ FigureZrOCl0.15 6.2 , gEffect 0.05 ZrOCl g [HOof2 , water0.052CMMIm]Cl, gcontent% [HO2CMMIm]Cl, 3.0 on g glucose isopropanol, 3.0 dehydr g isopropanol, 3 hation.Experimental reaction time,3 h re andaction 150cond time,Citions: reaction and 0.1 150temperature. g glucose, °C reaction 0.15 temperature.g ZrOCl2, 0.05 g [HO2CMMIm]Cl, 3.0 g isopropanol, 3 h reaction time, and 150 °C reaction 2.7.temperature. Kinetics Studies 2.7.We Kinetics considered Studies that, in our work, glucose dehydration consisted of two processes, the first of 2.7. Kinetics Studies which wasWe considered glucose convertion that, in our into work, the intermediate glucose dehydrat species,ion and consisted the second of two was processes, the intermediate the first of specieswhichWe considered convertion was glucose that, into convertion HMF,in our whichwork, into couldglucose the intermediate be dehydrat assumedion species, to beconsisted the and rate-determining theof two second processes, was step. the the intermediate As first shown of whichelsewhere,species was glucoseconvertion in this case,convertion into the overallHMF, into which rate the canintermediate could be written be assumed inspecies, terms to and of be a productthethe rate-determiningsecond of equilibrium was the intermediate step. constants As shown for speciestheelsewhere, steps convertion that in are this notinto case, rate-determining, HMF, the whichoverall could rate multiplied can be beassumed wr byitten the to equilibriumin be terms the rate-determiningof a constant product for of equilibrium the step. formation As shown constants of the elsewhere,transitionfor the insteps state this forthat case, the are the rate-determining not overall rate-determining, rate can step be wr [ 53multipliitten,54]. in Accordingly,ed terms by the of aequilibrium product the overall of constant rateequilibrium can befor expressed theconstants formation in forterms theof thesteps of atransition single that are equilibrium notstate rate-determining, for constantthe rate-determining (k, which multipli is aed product st byep the[53,54]. of equilibrium the individualAccordingly, constant equilibrium the for overall the constants, formation rate can for be ofexample, theexpressed transition for HMF)in stateterms for for theof the formationa singlerate-determining equilibrium of the rate-determining st epconstant [53,54]. (k, Accordingly, transition which is state a theproduct from overall the of reactant ratethe canindividual R [be55 ]. expressedequilibriumTherefore, in terms constants, in our of work,a singlefor theexample, equilibrium kinetics for of glucoseHMF) constant for dehydration the (k, formationwhich was is of analyzeda theproduct rate-determining according of the individual to the transition HMF equilibriumyield.state The from firstconstants, the few reactant intermediate for Rexample, [55]. species for HMF) (IS) [IS] fort data the pointsformation were of used the torate-determining construct liner ln([IS] transitiont/[IS] 0) stateversus from t the plots reactant (Equation R [55]. (1)), and the first-order rate constants, k, were obtained from the slope

Catalysts 2018, 8, x FOR PEER REVIEW 7 of 12

Therefore, in our work, the kinetics of glucose dehydration was analyzed according to the HMF yield. The first few intermediate species (IS) [IS]t data points were used to construct liner ln([IS]t/[IS]0) versus t plots (Equation (1)), and the first-order rate constants, k, were obtained from the slope (the glucose dehydration reaction was established to occur as follows: glucose was first converted into fructose, then fructose was converted into HMF; the rate constant of fructose Catalysts 2018, 8, 467 7 of 12 conversion to HMF was described as first-order in the literature; therefore, a first-order rate constant was assumed for glucose dehydration in this work): (the glucose dehydration reaction was established to occur as follows: glucose was first converted into [IS] fructose, then fructose was converted into HMF;ln 〈 the rate〉 =−k constantt of fructose conversion to HMF was(1) [IS] described as first-order in the literature; therefore, a first-order rate constant was assumed for glucose dehydrationwhere [IS]t in= IS this concentration work): at time t, [IS]0 = initial IS concentration, k = observed rate constant, and t = time in minutes. [IS] lnh t i = −kt (1) As summarized in Table 1, the value [ofIS ]0k increased with the reaction temperature increase, indicating that the dehydration reaction of the intermediate species would be accelerated by a higher where [IS]t = IS concentration at time t, [IS]0 = initial IS concentration, k = observed rate constant, and reaction temperature: t = time in minutes. As summarized in Table1, the value of k increasedE with the reaction temperature increase, lnk =− +lnA (2) indicating that the dehydration reaction of the intermediateRT species would be accelerated by a higher reactionwhere temperature:k is the observed rate constant, E is the activation energy, T is the temperature in kelvin, R is E the ideal gas constant, A is the pre-exponentiallnk = − factor+ lnA (2) RT where k isTable the observed 1. Reaction rate rate constant, constants E (k) isthe of glucose activation dehydration energy, Tat is various the temperature reaction temperatures in kelvin, Ra. is the ideal gas constant, A is the pre-exponential factor. Entry Temperature °C k × 103 (min−1) Correlation Coefficient Table 1. Reaction1 rate constants140 (k) of glucose dehydration 89.1 at various reaction 0.986 temperatures a. 2 150 118.7 0.989 Entry Temperature ◦C k × 103 (min−1) Correlation Coefficient 3 160 152.4 0.977 1 140 89.1 0.986 a Experimental conditions: 3 h reaction time, 0.05 g [HO2CMMIm]Cl, 0.15 g ZrOCl2, 0.1 g glucose, and 2 150 118.7 0.989 3 g isopropanol. 3 160 152.4 0.977 a ExperimentalAn Arrhenius conditions: plot 3 hwas reaction obtained, time, 0.05 gas [HO shown2CMMIm]Cl, in Figure 0.15 g ZrOCl 7. 2Based, 0.1 g glucose, on these and 3 gdata, isopropanol. the kinetic parametersAn Arrhenius for [HO plot2 wasCMMIm]Cl–ZrOCl obtained, as shown2-catalyzed in Figure glucose7. Based dehydratio on these data,n could the kinetic be calculated. parameters An apparent activation energy of 48.01 KJ/mol was calculated. for [HO2CMMIm]Cl–ZrOCl2-catalyzed glucose dehydration could be calculated. An apparent activation energy of 48.01 KJ/mol was calculated.

5.1

5.0

4.9

4.8

4.7 lnk

4.6

4.5

4.4 0.00231 0.00234 0.00237 0.00240 0.00243 -1 1/T (K )

Figure 7. Figure Arrhenius7. Arrhenius plot plot for for glucose glucose dehydration dehydration in isopropanolin isopropanol over over [HO [HO2CMMIm]Cl–ZrOCl2CMMIm]Cl–ZrOCl2. 2.

Jack M. Carraher [56] reported that ∆H = Ea − RT. On the basis of this equation, ∆H at different temperatures could be obtained, and the results are listed in Table2. Obviously, all values of ∆H were higher than 47 kJ mol−1, suggesting that the reaction of glucose dehydration was a strong endothermal reaction, and thus a high reaction temperature was favorable to obtaining the intermediate species conversion. Catalysts 2018, 8, 467 8 of 12

Table 2. The values of ∆H at different reaction temperatures.

Entry Reaction Temperature ∆H/kJ mol−1 1 140 47.976 2 150 47.975 3 160 47.974

2.8. Recycling of the Catalysts Tests were carried out for the recycling of the catalyst, and the results are summarized in Table3. It can be seen that there was only a slight decrease of the catalytic performance of the catalyst, suggesting that the catalyst could be recycled and reused. A possible reason for the decreased yield of HMF may be that a certain amount of catalyst was not recycled by centrifugation.

Table 3. Recycling of the catalyst.

Run Glucose Conversion % HMF Yield % 1 99 43 2 99 41 3 99 40 4 99 39 5 99 38

Reaction conditions: 0.15 g ZrOCl2, 0.05 g [HO2CMMIm]Cl, glucose 0.03 g, 3.0 g isopropanol, 3 h reaction time, 150 ◦C reaction temperature.

3. Experimental Section

3.1. Materials

The IL [HO2CMMIm]Cl) was supplied by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. All the other materials, such as glucose, fructose, ZrOCl2, isopropanol, were analytic-grade, purchased from Alfa Aesar and used as received without further puriﬁcation.

3.2. Reaction Testing Glucose dehydration was carried out in a 10 mL thick-walled glass reactor made by Alltech. An oil bath with an Isotemp digital stirring hot plate was used to heat the reactor rapidly to the desired reaction temperature. In addition, a triangular stir bar was placed in the bottle of the reactor before adding the reactants, so as to give an adequate agitation. A typical experiment was as followed: 0.1 g glucose, 0.05 g [HO2CMMIm]Cl, 0.15 g ZrOCl2, and 3.0 g isopropanol were introduced into the reactor. Then the reactor was sealed and immersed in the oil bath at 150 ◦C and stirred at 400 rpm. After 120 min, the reactor was removed from the oil bath and then cooled rapidly to room temperature with water. Other reactions were carried out by changing the reaction temperature, reaction time, glucose loading, [HO2CMMIm]Cl and ZrOCl2 amounts. For all the reactions, some amounts of brownish humins formed. Humin formation precluded a closed carbon balance analysis, and thus the only product analyzed was HMF. Each reaction was repeated at least three times, and the average yield was reported.

3.3. Sample Analysis All the liquid samples were precipitated by centrifugation. The upper liquid was analyzed with a Shimadzu LC-20A liquid chromatography. HMF was measured with a SPD-20A UV detector at 280 nm and an InterSustain C18 column at 313 K, with a methanol/5 mM H2SO4 (8:2 v/v) binary solvent as the mobile phase at a ﬂow rate of 0.6 mL/min. The amount of glucose was analyzed by introducing 2 mL of water into the upper liquid and then measured with a refractive index (RI-20A) Catalysts 2018, 8, 467 9 of 12

−1 detector, under the conditions of 333 K, with 5 mM H2SO4 at a flow rate of 0.6 mL min as the mobile phase, with an Aminex HPX-87H Column. In this manuscript, conversion and selectivity are defined as follows: conversion of glucose is defined as the amount of reacted glucose divided by the moles of initial glucose added; selectivity of HMF is defined as the moles of HMF produced divided by the amount of reacted glucose. The yield of HMF is defined as converted glucose multiplied by HMF selectivity.

4. Conclusions

In summary, a combination of the [HO2CMMIm]ClZrOCl2 catalyst system and isopropanol for HMF synthesis with glucose as the feedstock was developed. A series of experiments was examined over a broad range of conditions (reaction temperature, reaction time, water content, and catalyst and substrate loadings) to explore the rate and selectivity of glucose dehydration. Up to 43% HMF yield could be obtained with 0.03 g glucose, 0.05 g [HO2CMMIm]Cl, 0.15 g ZrOCl2 in 3 mL isopropanol, at 150 ◦C reaction temperature and in 3 h reaction time. It was demonstrated that [HO2CMMIm]Cl could increase HMF selectivity and that ZrOCl2 not only was favorable to glucose conversion but also increased HMF yield under the examined reaction conditions. ZrOCl2 affected the reaction in two opposite ways, and these opposite factors resulted in the highest yield corresponding to twice the mass loading of ZrOCl2. Glucose could be converted to intermediate species within a short time (about 5 min), and the intermediate species were not fructose according to the HPLC results; a longer reaction time (3 h or longer) were needed for HMF production from the intermediate species over the [HO2CMMIm]Cl-ZrOCl2 catalyst. The conversion of the intermediate species to HMF was dramatically increased by the addition of 3.17 wt % water. The performance of the catalytically active species could be enhanced by increasing the water content, and the concentration of the catalytically active species could be decreased by increasing the water content to a certain range. The kinetics data showed that a higher temperature accelerated the intermediate species dehydration reaction rate [51]. The glucose dehydration reaction was a strong endothermal reaction, thus a high reaction temperature was favorable to glucose and intermediate species conversion.

Author Contributions: Y.M., L.W., and R.Z. performed the experiments; H.L. and T.W. conceived the idea. All the authors contributed to the writing of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (21506246, U1703128, 21808192), and Chinese Government “Thousand Talent” Program (Y42H291501). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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