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Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43

Contents lists available at ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h om epage: ww w.elsev ier .com/ locate / jaap

he mechanism for the formation of levoglucosenone during pyrolysisf �-d-glucopyranose and cellobiose: A density functional theorytudy

iang Lua,∗, Yang Zhanga, Chang-qing Donga, Yong-ping Yanga, Hai-zhu Yub,∗

National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing, 102206, ChinaDepartment of Polymer Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China

r t i c l e i n f o

rticle history:eceived 5 May 2014ccepted 3 August 2014vailable online 14 August 2014

a b s t r a c t

Levoglucosenone (LGO) is a valuable anhydrosugar product from fast pyrolysis of cellulose/biomass,but its formation mechanism is still unclear at present. In this study, density functional theory (DFT)method was employed to examine the different LGO formation pathways from pyrolysis of both �-d-glucopyranose and cellobiose, and the most favorable pathways were clarified. The results corroborate

eywords:FTevoglucosenone-d-glucopyranoseellobioseyrolysis mechanism

the previous studies that levoglucosan (LG) is unlikely the essential intermediate to produce LGO. Themost feasible LGO formation pathways from both �-d-glucopyranose and cellobiose include sequen-tial 1,2-dehydration, six-membered hydrogen transfer and enol-keto tautomerization steps. Enol-ketotautomerization is the rate-determining steps for both reactions to form LGO.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The utilization of lignocellulosic biomass has attracted muchnterest in the past decades [1,2]. Among the various biomassonversion technologies, fast pyrolysis is a promising one to con-ert solid biomass into liquid bio-oil [2,3]. Bio-oil is a complexixture of water and hundreds of organic compounds. Many of

hese organic compounds are valuable chemicals, whereas theecovery of specific chemicals is difficult due to their low con-entrations in conventional bio-oils [4,5]. To solve this problem,elective pyrolysis has been proposed in recent years to producehe target chemicals predominantly via the selectively controllinghe biomass pyrolysis process [6,7]. Deep understandings on theormation mechanism of specific chemicals will help us clarify theate/selectivity-determining factors, and also benefit the develop-

ent and optimization of the selective pyrolysis techniques.Levoglucosenone (LGO, 1,6-anhydro-3,4-dideoxy-�-d-

yranosen-2-one) [8] is one of the most valuable chemicals

∗ Corresponding authors at: North China Electric Power University, Nationalngineering Laboratory for Biomass Power Generation Equipment, No.2 Beinongoad,Changping District, Beijing 102206, China.el.: +86 10 61772063/+86 10 62334748;ax: +86 10 61772032x801/+86 10 62334516.

E-mail addresses: qianglu@mail.ustc.edu.cn (Q. Lu), yuhz@ustb.edu.cn (H.-z. Yu).

ttp://dx.doi.org/10.1016/j.jaap.2014.08.002165-2370/© 2014 Elsevier B.V. All rights reserved.

in bio-oil, and is mainly derived from depolymerization anddehydration of cellulose. It is an anhydrosugar product witheasily modifiable functional groups (enone, ketone, acetal) anda bicyclic structure [9], and thus can be used for various organicsynthesis [10,11]. During the fast pyrolysis of cellulose or biomass,LGO is a minor product with very low yield. By contrast, LGOcan be selectively produced in several acid catalyzed pyrolysisprocesses, using acid catalysts such as H3PO4 [12,13], H2SO4[14], solid superacids (SO4

2−/TiO2 or SO42−/ZrO2) [15,16] or

1-butyl-2,3-dimethylimidazolium triflate ionic liquid [17].Unfortunately, the mechanism for LGO formation remains

unknown at present, which limits the optimization of the LGOselective pyrolysis techniques. In early studies, it was proposed thatLGO was formed through the dehydration of LG (1,6-anhydro-�-d-glucopyranose) [5,18,19], since LG was the most abundant productduring the pyrolysis of glucose-based carbohydrates (such as glu-cose, cellobiose and cellulose) [20,21]. This proposal was supportedby the observation that LGO could be generated from LG in someacid-catalyzed pyrolysis conditions [15,19,22,23]. Nonetheless, inour recent experimental studies on the fast pyrolysis of cellulose,we found that the formation characteristics of the LGO and LG weresignificantly different [5]. What’s more important, we confirmed

that LG was thermodynamically highly stable [24,25], and little sec-ondary cracking products (such as LGO) were detected even thetemperature was elevated to 600 ◦C [5,26]. Similarly, the yield ofLGO was also found to be independent of LG in the recent studies

Q. Lu et al. / Journal of Analytical and Ap

oLdTmuN[c

2

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3

3

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Fig. 1. �-d-Glucopyranose.

f Patwardhan et al. [24]. In this context, it is able to suggest thatGO might be formed via the transformation of other intermediatesuring fast pyrolysis of cellulose, rather than the dehydration of LG.herefore, this study tends to clarify the mechanism for LGO for-ation from cellulose with the aid of DFT (B3LYP) calculations, by

sing �-d-glucopyranose and cellobiose as the model compounds.ote that B3LYP has been frequently used in mechanistic studies

27–43], especially the biomass pyrolytic mechanism for specificompounds [27,44–49].

. Computational methods

DFT calculations [28] were conducted using B3LYP/6-31 + G(d,p)ethod which has been widely used in the mechanistic studies of

rganic systems [45,50–55], especially the pyrolysis mechanisms ofellulose based model compounds [44–49]. Geometry optimizationf all the reactants, intermediates, transition states and productsas performed. During the optimization, four items were employed

s the convergence criteria. They are maximum force, root meanquare (RMS) force, maximum displacement and RMS displace-ent. The related thresholds for each item are 0.00045, 0.0003,

.0018 and 0.0012, respectively. Berny algorithm [38] was used toalculate the transition state. Frequency analysis was conductedith the same method as optimization to verify the stationaryoints to be real minima or saddle points and to obtain the ther-al corrections (vibrational contributions) to enthalpies and free

nergies. For each transition state, the intrinsic reaction coordinateIRC) analysis was carried out to confirm that it connected the cor-ect reactant and product. All the calculations were performed withaussian 03 package [56]. Considering that the entropic contrib-tions have been frequently proposed to be overestimated by DTFethods [57,58], we use the enthalpies for discussions on energet-

cs, under the condition of 298 K and 1 atm. For clarity reasons, theelative Gibbs free energies are also given in Part VII of the Sup-lementary data. The calculation results were further verified with06 method [59], and the details are provided in Part VIII of the

upplementary data.

. Results and discussion

.1. Mechanism for formation of LGO from ˇ-d-glucopyranose

Mechanistic analysis was firstly performed on LGO formationrom �-d-glucopyranose (monomer of cellulose). The conversionf �-d-glucopyranose to LGO involves several elementary stepsf 1,2-dehydration [60], enol-keto tautomerization [61], hydrogenransfer [62], and dehydration reaction to form the ether bond [45].

According to the structure of �-d-glucopyranose shown in Fig. 1,,2-dehydration can occur via seven pathways (i.e. 1-OH + 2-H;-OH + 3-H/1-H; 3-OH + 4-H/2-H; 4-OH + 5-H/3-H). Among these,hree 1,2-dehydration pathways (i.e. 3-OH + 2-H; 4-OH + 3-H;

plied Pyrolysis 110 (2014) 34–43 35

1-OH + 2-H) can result in the LGO formation, as shown in Fig. 2 (PathI–III). The detailed analysis in excluding the other possible 1,2-dehydration pathways is provided in Part I of the Supplementarydata. In addition to the 1,2-dehydration, �-d-glucopyranose canalso firstly undergo a dehydration process between the hydroxylgroups at C1 and C6 (Path IV in Fig. 2). The aforementioned pro-cesses produce the intermediates I-inter1, II-inter1, III-inter1 andIV-inter1 (Fig. 2), after which different mechanisms might occur toresult in the formation of LGO (vide supra).

In Path I, from the product of the 1,2-dehydration between 3-OH and 2-H (i.e. I-inter1), three pathways might occur to generateLGO. Their detailed transformations and related energy profiles areillustrated in Fig. 3 (Path I-1, Path I-2 and Path I-3). From I-inter1,the three pathways share the same elementary steps (i.e. 1,2-dehydration, enol-keto tautomerization and dehydration between6-OH and 1-OH), and are mainly differentiated by the sequence ofthese elementary reactions. In Path I-1, I-inter1 firstly undergoesthe enol-keto tautomerization reaction to form the intermediateI-1-inter2, which has the keto group at C2 position. The inter-mediate I-1-inter2 then undergoes the 1,2-dehydration between3-H and 4-OH and the dehydration between 6-OH and 1-OH toproduce LGO. Path I-2 differs from Path I-1 in that I-inter1 firstundergoes the dehydration between 6-OH and 1-OH to form thebridged intermediate I-2-inter2. The subsequent enol-keto tau-tomerization step enables the formation of I-2-inter3, from whicha final 1,2-dehydration between 3-H and 4-OH occurs to produceLGO. By contrast, Path I-3 take places with dehydration between 6-OH and 1-OH from I-1-inter2, following with the 1,2-dehydrationbetween 3-H and 4-OH to produce LGO.

According to the energy profiles in Fig. 3, the three pathwaysfirstly undergo the same 1,2-dehydration between 2-H and 3-OH with the energy barrier of 294.4 kJ/mol. Thereafter, Path I-1has to overcome the overall energy barrier of 230.0 kJ/mol (theenergy difference between I-inter1 and I-1-ts2). By contrast, theoverall energy barrier of Path I-2 is 248.4 kJ/mol (the energy dif-ference between I-inter1 and I-2-ts3), higher than that of PathI-1 by 18.4 kJ/mol, and thus Path I-2 can be excluded. For Path I-3, although its overall energy barrier is the same as that of Path I-1(i.e. 230.0 kJ/mol), it is less favorable than Path I-1 due to the follow-ing reasons. From the intermediate I-1-inter2, the overall energydemands for Path I-1 and Path I-3 are 226.7 kJ/mol (the energydifference between I-1-inter2 and I-1-ts3) and 246.5 kJ/mol (theenergy difference I-1-inter2 and I-2-ts4), respectively. Therefore,Path I-1 should be the most favorable pathway for Path I, the rate-determining step is the first step (1,2-dehydration between 2-Hand 3-OH), and the overall energy barrier is 294.4 kJ/mol.

Similar as Path I, Path II includes four different pathways (PathII-1, Path II-2, Path II-3 and Path II-4) from II-inter1 (the productof the 1,2-dehydration between 4-OH and 3-H) to form LGO, asshown in Fig. 4. These pathways consist of the same elementaryreactions (i.e. 1,2-dehydration, enol-keto tautomerization, dehy-dration between 6-OH and 1-OH, and hydrogen transfer). Due tothe skeleton structure of the six-membered ring, the enol-keto tau-tomerization tends to occur before the hydrogen transfer and the1,2-dehydration steps, and thus the different sequence of the dehy-dration between 6-OH and 1-OH results in four possible pathways.In Path II-1, Path II-2 and Path II-3, the dehydration between 6-OHand 1-OH occurs before the enol-keto tautomerization (Path II-1),the hydrogen transfer (Path II-2) and the 1,2-dehydration (Path II-3)steps, respectively. In Path II-4, the dehydration is the final step.

According to the energy profiles in Fig. 4, the four pathwaysfirstly undergo the same 1,2-dehydration between 4-OH and 3-H

with the energy barrier of 296.0 kJ/mol. After that, the energy bar-rier for the first step of Path II-1 (dehydration between 6-OH and1-OH) is much lower than that of the enol-keto tautomerizationstep in Path II-2, Path II-3, and Path II-4. The rate-determining step

36 Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43

durin

ooisP

P

Fig. 2. Initial dehydration processes

f Path II-1 is the final 1,2-dehydration step, and the relative energyf the related transition state (II-1-ts5) is 280.0 kJ/mol. This energys 3.3 kJ/mol lower than the relative energy of II-2-ts2 (transitiontate of the first step in Path II-2, Path II-3, and Path II-4). Hence,

ath II-1 is relatively more feasible than the other pathways in Fig. 4.

Fig. 5 shows the reaction pathways and energy profiles forath III and IV. Path III includes two possible pathways after the

Fig. 3. Reaction pathways and

g the �-d-glucopyranose pyrolysis.

formation of III-inter1 (product of the 1,2-dehydration between1-OH and 2-H) to generate LGO. In Path III-1, III-inter1 under-goes a concerted hydrogenation–cyclization step (proposed byAssary et al. [63]) to produce LG (IV-inter1), and the subsequent

steps from IV-inter1 are illustrated in Path IV. In Path III-2, inter-mediate I-2-inter2 is directly formed from III-inter1 through asix-membered transition state, during which one H2O molecule is

energy profiles for Path I.

Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43 37

and e

rILo3a(

pfHeoPwsaes

Fig. 4. Reaction pathways

emoved between 3-OH and the hydrogen atom of 6-OH. In PathV, the dehydration between 1-OH and 6-OH occurs to produceG (IV-inter1). Two subsequent 1,2-dehydration processes mightccur, between 3-OH and 2-H in Path IV-1, and between 4-OH and-H in Path IV-2. The produced intermediates, namely I-2-inter2nd II-1-inter2, can undergo the same transformations in Path IFig. 3) and Path II (Fig. 4) to produce LGO.

According to Fig. 5, the energy barriers of the 1,2-dehydrationrocesses of LG (IV-inter1) are considerably higher (324.4 kJ/molor Path IV-1 and 306.3 kJ/mol for Path IV-2) than other pathways.ence, the pathways involving the transformations of LG to LGO arenergetically disfavored. This proposal agrees with the previouslybserved high thermodynamic stability of LG [5,26]. In other words,ath III-2 is the most plausible pathway among all possible path-ays in Fig. 5. The overall energy barrier of Path III-2 is 290.8 kJ/mol,

lightly lower than those of Path I-1 and Path II-1 (i.e. 294.4 kJ/molnd 296.0 kJ/mol). What’s more important, based on the detailednergy profiles of different pathways, it is noted that only onetep in Path III-2 (i.e. enol-keto tautomerization, 290.8 kJ/mol) is

Fig. 5. Reaction pathways and ener

nergy profiles for Path II.

energy-demanding, while the other steps are much more facile(<260 kJ/mol). Especially for the dehydration step from III-inter1 toI-2-inter2, the energy barrier in Path III-2 is as low as 214.4 kJ/mol,much lower than the related energy barriers in Path I-1, II-1, andIV. The reason might be mainly attributed to that the dehydrationfrom III-inter1 proceeds through the less distorted six-memberedtransition state, rather than the highly distorted four-memberedtransition states in the 1,2-dehydration processes in Path I, II and IV.Accordingly, Path III-2 is the most favorable pathway for LGO forma-tion from ˇ-d-glucopyranose. For clarity reasons, an overall profileof Path III-2 is provided in Fig. 6.

3.2. Mechanism for formation of LGO from cellobiose

Based on the above LGO formation analysis from �-d-

glucopyranose, further mechanistic analysis was performed onthe LGO formation from cellobiose (the minimal repeating unitin cellulose) [27,64]. Cellobiose is usually employed as the modelcompound for cellulose pyrolysis mechanism study [45,48,64].

gy profiles for Path III and IV.

38 Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43

rmat

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3

fimdbd7ddntsde2atapiaiLtiISae(L

3

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Fig. 6. Most favorable pathway for fo

he transformation of cellobiose to LGO can be classified intowo categories via two different mechanisms: “decomposition-first

echanism” (the glycosidic bond cleavage firstly occurs) and “1,2-ehydration-first mechanism” (the 1,2-dehydration occurs beforehe glycosidic bond cleavage).

.2.1. Decomposition-first mechanismIn this mechanism, the cleavage of the glycosidic bond is the

rst step. According to the previous studies [44,45,48], this stepight occur via homolytic [21,65], heterolytic [66,67] or concerted

ecomposition processes [63,64]. Our calculations indicate that theond dissociation energies (BDEs) for homolytic and heterolyticissociation of the �(1–4) glycosidic bond are as high as 365.7 and08.4 kJ/mol, respectively (please see Part II in the Supplementaryata for more details). On the other hand, the concerted cellobioseecomposition step might occur via three possible mechanisms,amely, Path CRI, CRII and CRIII (CR is short for “concerted reac-ion”), and the related intermediates and transition states werehown in Fig. 7 (Some other mechanisms were excluded, and theetails were given in Part III in the Supplementary data). Thenergy barriers for Path CRI, CRII and CRIII are 230.1, 191.6 and44.9 kJ/mol, respectively, much lower than those of the homolyticnd heterolytic decomposition processes. The results indicate thathe concerted transformations are more favorable for the cleav-ge of the glycosidic bond. This conclusion agrees well with therevious studies [45,64]. In this mechanism, four products (i.e. CRI-

nter1, CRII-inter1, CRIII-inter1 and Glucose) can be generated,nd their detailed transformations to LGO have all been providedn Section 3.1 (Figs. 4–6). Among the four products, CRII-inter1 isG. Its transformation to LGO is energetically unfavorable. Glucoseransforms to LGO via III-inter1, which is the same as CRI-inter1n Fig. 7. Hence, only CRI-inter1 and CRIII-inter1 (the same as theI-inter1) need to be considered for the formation of LGO. Based onection 3.1, the most plausible pathways via the two intermediatesre shown in Figs. 8 and 9, respectively. The overall energy barri-rs for Path CRI and Path CRIII are 274.5 (Fig. 8) and 271.1 kJ/molFig. 9), respectively. Therefore, Path CRIII should be the most feasibleGO formation pathway via the Decomposition-First Mechanism.

.2.2. 1,2-Dehydration-first mechanismIn this mechanism, 1,2-dehydration of cellobiose occurs firstly.

imilar as the discussions in Section 3.1, three possible 1,2-ehydration mechanisms (i.e. 3-OH + 2-H; 4-OH + 3-H; 1-OH + 2-H)

ion of LGO from �-d-glucopyranose.

might result in the formation of LGO. Considering the absence ofhydroxyl groups at C1 and C4′ positions in cellobiose, four possible1,2-dehydration pathways were examined (i.e. Path DI: 3-OH + 2-H;Path DII: 3′-OH + 2′-H; Path DIII: 4-OH + 3-H; Path DIV: 1′-OH + 2′-H) for LGO formation, as shown in Fig. 10.

The energy barriers for the initial 1,2-dehydrations in Path DI,DII, DIII and DIV are 260.0, 293.6, 259.5 and 245.1 kJ/mol, respec-tively. Because the energy barrier for Path DII is higher than theoverall energy barrier of the decomposition-first mechanism (PathCIII in Fig. 9), and thus the subsequent transformations in Path DIIomitted.

For Path DI, different types of concerted decomposition mech-anisms have been examined, and the details are given in theSupplementary data (Part IV). The most favorable pathway wasfound to occur via the 1,2-dehydration between 2-H and 3-OH,enol-keto tautomerization, concerted decomposition and 1,2-dehydration between 3-H and 4-OH steps to form LGO, as shownin Fig. 11 (Path DI-2). According to the energy profile of Path DI-2,the rate-determining step is the enol-keto tautomerization, and theoverall energy barrier is 272.9 kJ/mol.

For Path DIII, the most favorable reaction pathway to form LGO(i.e. Path DIII-3) was determined in similar ways (see Supplemen-tary data, Part V), as shown in Fig. 12. In Path DIII-3, the productof the initial 1,2-dehydration between 4-OH and 3-H (DIII-inter1)undergoes enol-keto tautomerization to obtain intermediate DIII-1-inter2, which then goes through a concerted decompositionreaction to form the bridged intermediate DIII-2-inter3 and glu-cose. From DIII-2-inter3, the hydrogen transfer from hydroxylgroup to ketone group then occurs to produce DIII-2-inter4. Finally,the 1,2-dehydration between 3-OH and 4-H occurs and results inthe formation of LGO. The rate-determining step of this pathwayis the enol-keto tautomerization, and the overall energy barrier is273.1 kJ/mol.

For Path DIV, Path DIV-1 was concluded as the most favor-able reaction pathway to form LGO (see Supplementary data formore details, Part VI), as shown in Fig. 13. In Path DIV-1, after theinitial 1,2-dehydration process between 1′-OH and 2′-H, the gener-ated intermediate DIV-inter1 transforms to DIV-1-inter2 througha six-membered transition state. Thereafter, DIV-1-inter2 under-

goes enol-keto tautomerization and produces the intermediateDIV-1-inter3, in which the keto group lies at C2′ position. The gly-cosidic bond cleavage finally takes place through the concerteddecomposition. In this pathway, the enol-keto tautomerization

Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43 39

Fig. 7. Concerted decomposition mechanisms of cellobiose.

Fig. 8. Reaction pathway and energy profile for Path CRI.

Fig. 9. Reaction pathway and energy profile for Path CRIII.

40 Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43

Fig. 10. 1,2-Dehydration mechanisms of cellobiose.

Fig. 11. Reaction pathway and energy profile for the most favorable pathway (Path DI-2) in Path DI.

Fig. 12. Reaction pathway and energy profile for the most favorable pathway (Path DIII-3) in Path DIII.

Q. Lu et al. / Journal of Analytical and Applied Pyrolysis 110 (2014) 34–43 41

the m

i2

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Fig. 13. Reaction pathway and energy profile for

s the rate-determining step, and the overall energy barrier is62.4 kJ/mol.

Comparing the different 1,2-dehydration-first mechanisms inigs. 11–13, the overall energy barrier of Path DIV-1 is relativelyower than those of Path DI-2 and Path DIII-3. Therefore, PathIV-1 should be the most feasible LGO formation pathway via 1,2-ehydration-First Mechanism.

.2.3. Most favorable mechanism for formation of LGO fromellobiose

Based on the above results, Path CRIII and Path DIV-1 are con-luded as the most favorable pathways for LGO formation viaecomposition-first mechanism and 1,2-Dehydration-First Mech-nism, respectively. It’s noteworthy that the overall energy barrierf Path DIV-1 is relatively lower than that of Path CRIII, andhe dehydration steps occur through either a relatively facileour-membered transition state or a less distorted six-memberedransition state. Therefore, Path DIV-1 should be the most favorableathway for LGO formation from cellobiose. The rate determining stepf this mechanism is the enol-keto tautomerization step (Fig. 13).

.3. Discussion

According to the calculation results in Sections 3.1 and 3.2,t is interesting to note that the energy barrier of the enol-keto

Fig. 14. Proposed formation mechanism

ost favorable pathway (Path DIV-1) in Path DIV.

tautomerization step is always high, and in most cases, it is therate-determining step. The high energy barrier is mainly attributedto the distorted four-membered transition state [63,68]. Theenergy difference between the reactant (enol form) and product(keto form) is around 50 kJ/mol (ranging from 46.8 kJ/mol to55.6 kJ/mol), which is similar with experimental energy differencebetween compounds with enol form and corresponding keto form(50–58 kJ/mol) [69,70]. Due to the large energy gap, the enol-ketoequilibrium would shift to the keto side. The keto/enol ratio isreported to reach 1011 in some previous studies [71,72]. In spite ofthe high energy barrier, the enol-keto tautomerization is generallyirreversible, and thus provides thermodynamic facility for theformation of LGO.

Meanwhile, the formation of double bonds is essential for thegeneration of LGO. The double bond can be produced througheither dehydration or concerted decomposition step on cellobiose.For both �-d-glucopyranose and cellobiose, the 1,2-dehydrationbetween 1-OH and 2-H is relatively more feasible than the other1,2-dehydration steps. The reason might be attributed to thatthe C1 hydroxyl group results in a less distorted transition state.For the concerted decomposition of cellobiose, this step involv-

ing 2-H exhibits lower energy barrier than the counterparts with3′-H. In general, concerted decomposition reactions consume lessenergy to produce intermediates with double bonds than the 1,2-dehydration steps, suggesting that the glycosidic bond is important

of LGO during cellulose pyrolysis.

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or the formation of LGO. This observation helps to explain the LGOield difference between glucose and cellobiose (about tripled inellobiose than in glucose) [24]. Cellulose is a natural polymer of-d-glucopyranose, which are connected by �(1,4) glycosidic links.or the monomer unit in cellulose, double bond and ether bond cane formed through the concerted cleavage of the glycosidic links onoth sides. Finally, based on the most favorable pathway for forma-ion of LGO from cellobiose, we propose a possible mechanism forormation of LGO from cellulose, as shown in Fig. 14.

. Conclusion

In the present study, systematic analysis on the LGO forma-ion mechanism from both �-d-glucopyranose and cellobiose wasarried out with DFT calculations. The following conclusions arebtained.

1) The most favorable pyrolytic pathways for the LGO forma-tion from both �-d-glucopyranose (Path III-2) and cellobiose(Path DIV-1) are determined. The two reactants firstly undergosimilar 1,2-dehydration, six-membered hydrogen transfer andenol-keto tautomerization steps, while the last step in forma-tion of the double bond between C3 and C4 of LGO are distinct.In Path III-2, the last step takes place through 1,2-dehydration,while in Path DIV-1, concerted decomposition of the glyco-sidic bond occurs as the final step. The overall energy barriersfor LGO formation from �-d-glucopyranose and cellobiose are290.8 kJ/mol and 262.4 kJ/mol, respectively.

2) The results agree well with previous studies that LGO is unlikelyto be formed from LG.

3) In the most favorable pyrolytic pathways from both reactants,enol-keto tautomerization is the rate-determining step.

This study is the first theoretical study on the mechanism forhe formation of LGO from �-d-glucopyranose and cellobiose. Itrovides the detailed energetics of each reaction step, the spatialtructural diagrams for all the concerned intermediates, transitiontates and the relative facility of different pathways. All these infor-ation might be helpful for deep understanding of the mechanism

nvolved in pyrolysis of biomass.

cknowledgements

The authors thank the National Natural Science Foundation ofhina (51106052, 21202006), National High Technology R&D Pro-ram (2012AA051803), National Torch Plan (2013GH561645), 111roject (B12034), and Fundamental Research Funds for the Centralniversities (2014ZD17) for financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.jaap.2014.08.002.

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