Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural

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Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural Bin Hu, Qiang Lu∗, Xiaoyan Jiang, Xiaochen Dong, Minshu Cui, Changqing Dong, Yongping Yang National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China

a r t i c l e

i n f o

Article history: Received 24 August 2017 Revised 2 November 2017 Accepted 16 November 2017 Available online xxx Keywords: Pyrolysis mechanism 5-HMF FF Density functional theory 13 C isotope labeling

a b s t r a c t Fast pyrolysis of biomass will produce various furan derivatives, among which 5-hydroxymethyl furfural (5-HMF) and furfural (FF) are usually the two most important compounds derived from holocellulose. In this study, density functional theory (DFT) calculations are utilized to reveal the formation mechanisms and pathways of 5-HMF and FF from two hexose units of holocellulose, i.e., glucose and mannose. In addition, fast pyrolysis experiments of glucose and mannose are conducted to substantiate the computational results, and the orientation of 5-HMF and FF is determined by 13 C-labeled glucoses. Experimental results indicate that C1 provides the aldehyde group in both 5-HMF and FF, and FF is mainly derived from C1 to C5 segment. According to the computational results, glucose and mannose have similar reaction pathways to form 5-HMF and FF with d-fructose (DF) and 3-deoxy-glucosone (3-DG) as the key intermediates. 5HMF and FF are formed via competing pathways. The formation of 5-HMF is more competitive than that of FF, leading to higher yield of 5-HMF than FF from both hexoses. In addition, compared with glucose, mannose can form 5-HMF and FF via extra pathways because of the epimerization at C2 position. Therefore, mannose pyrolysis results in higher yields of 5-HMF and FF than glucose pyrolysis. © 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction Fast pyrolysis is one of the promising ways for the utilization of lignocellulosic biomass materials [1,2]. During biomass fast pyrolysis process, a lot of furan derivatives will be produced from both cellulose and hemicellulose, including 5-HMF, FF, furan, and so on [3–6]. Among these furan compounds, 5-HMF and FF are usually the two most abundant products. Both of them are very useful in energy and chemical industries [7,8]. Deep understanding of their formation mechanism and pathways will be a great help for the industrial production of the two furan derivatives by selective pyrolysis techniques. During cellulose pyrolysis process, 5-HMF is usually the most abundant furan compound derived from the six-membered glucose unit [9,10]. However, 5-HMF has a five-membered furan ring. Hence, the formation of 5-HMF requires the conversion of pyranose into furanose. Acyclic d-glucose which is derived from the ring-opening reaction was considered necessary to build a bridge between pyranose and furanose [11–13]. 3-DG and DF were con-

∗

Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Lu).

sidered as the vital intermediates for the formation of the fivemembered furanose intermediates [13–19]. Early in 1968, inspired by acid-catalyzed sugar degradation mechanism, Kato and Komorita [16] identified that 3-DG was the intermediate to form 5-HMF in the thermal degradation of cellulose, known as 3-DG mechanism. A basic formation pathway of cellulose to 5-HMF was postulated involving d-glucose and 3-DG as the intermediates. Later, based on Curie-point pyrolysis-GLC experiments, Ohnishi et al. [17] also proposed that 5-HMF was one of the primary decomposition products of cellulose through 3-DG mechanism. Besides the 3-DG mechanism, DF was considered as another key intermediate to form 5-HMF from cellulose, known as DF mechanism. Antal et al. [18] reviewed the evidence for two hypotheses, i.e., DF mechanism and 3-DG mechanism. They also found the DF mechanism was more favorable for 5-HMF formation, which was consistent with their experimental results. Paine et al. [19] later proposed a detailed reaction pathway for 5-HMF formation based on DF mechanism, in which fructofuranose ring was formed and retained for 5-HMF generation through facile consecutive dehydration reactions. Also, the orientation (carbon sequence) of 5HMF from glucose was determined, with C1 in the aldehyde group and C6 in the hydroxymethyl substituent. Moreover, Yaylayan and coworkers [13] compared the conversion rates of several 5-HMF

https://doi.org/10.1016/j.jechem.2017.11.013 2095-4956/© 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013

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precursors to find that DF was more reactive than 3-DG, which indicated the major intermediate of 5-HMF formation should be DF. Additionally, other formation mechanisms of 5-HMF from cellulose have also been raised in previous literatures. Shafizadeh and Lai [20] suggested that the ring-opening of levoglucosan followed by dehydration and hemiacetal reaction could generate 5HMF. Shen and Gu [10] proposed a pathway to produce 5-HMF involving ring-opening of cellulose unit to form aldehyde structure, rearrangement to form double bond between C4 and C5, dehydration reaction to form double bond between C2 and C3, as well as acetal reaction. All the above 5-HMF formation mechanisms from cellulose were hypotheses based on experimental results. In order to evidence the inferences from experiments, density functional theory (DFT) [21,22] method has been applied as a powerful tool to investigate the pyrolysis mechanism of biomass and formation pathways of different pyrolytic products in recent decades [11,23,24]. Liu and coworkers [25] adopted DFT method to calculate four possible pathways of glucose pyrolysis from related experimental results. Calculations indicated that the pathway towards 5-HMF formation was a major reaction channel for glucose degradation, in which the formation of d-glucose through ring-opening reaction was necessary, followed by three consecutive dehydration reactions. Later, Wang et al. [26] also emphasized the significance of d-glucose for 5-HMF formation from glucose. Although 5-HMF formation through acyclic d-glucose prevailed in past decades, cyclic pathways were also proposed in theoretical studies. In the proposed cyclic pathways, the pyranose would transform into a five-membered intermediate in one step of ring contraction [27,28]. However, Mayes et al. [27] found the direct ring contraction had relative high energy barrier by quantum chemistry calculation. In addition to 5-HMF, FF was another important furan compound in cellulose pyrolysis process. Its formation requires not only the transformation of pyranose into furanose, but also the removal of a carbon containing group. FF was originally considered to be the secondary cracking product of 5-HMF via hydroxymethyl group elimination [29]. Consistent viewpoint has been found in many studies for years [10,20,26,30,31]. In 1968, considering that 5-HMF to FF was a minor role in the progress of cellulose pyrolysis, Kato and Komorita [16] pointed out that hexose should initially transform into pentose, then to form FF through intermediate 3-DG. Recent evidence from DFT calculations and experiments showed great support for the viewpoint that 5-HMF and FF were produced from cellulose concurrently. Evans and coworkers [32] calculated the bond energy of 5-HMF, and confirmed that C − O bond and C–H bond on hydroxymethyl group were relatively easy to cleave. Lu et al. [33] and Nikolov and Yaylayan [34] then verified this theoretical conclusion by conducting analytical pyrolysis of pure 5-HMF. Experimental results stated that the main products of 5-HMF decomposition were 5-methyl furfural and 2,5furandicarboxaldehyde, rather than FF. Paine et al. [19] discussed three classes of FF formation mechanisms in details based on isotopic labeling experiments of d-glucose. The most favorable mechanism showed that 5-HMF and FF shared the same precursor, which was able to lose the proton at C5 to form 5-HMF as well as lose the carbon at C6 to generate FF simultaneously. Additionally, experimental results suggested that about three quarters of FF was produced in this pathway with C1 in the aldehyde group. The remaining FF was generated with C2 or C5 in the aldehyde group. Wang et al. [11] obtained consistent result that FF was mainly derived from C1 to C5 of d-glucose with C1 in the aldehyde group, through theoretical investigation on carbon orientation of FF from d-glucose by DFT method. Hemicellulose is a group of complex amorphous polysaccharides, mainly consisting of hexose units (d-glucose, d-mannose,

and d-galactose) and pentose units (d-xylose and l-arabinose). Currently, published studies have confirmed that both 5-HMF and FF could be produced from the hexose units (mainly d-glucose and d-mannose) of hemicellulose [35–39]. However, few studies have been conducted to elucidate the formation mechanism of 5-HMF and FF from mannose unit of hemicellulose. Wang et al. [37] stated that mannose could undergo ring-opening reaction followed by ring-formation between C2 and C5 to generate 5-HMF. They also proposed that FF could be derived from 5-HMF in the presence of zeolite catalysts. Till now, although great efforts have been made on the formation mechanisms of 5-HMF and FF, there is still lacking systematic investigation of their formation pathways and mechanisms. Particularly, few studies have reported on 5-HMF and FF formation mechanisms from the mannose unit of hemicellulose. Hence, further study using combined computational and experimental methods is necessary to obtain acknowledged conclusions. In the present work, both of DFT calculations and fast pyrolysis experiments are performed to investigate the detailed formation pathways of 5-HMF and FF from glucose and mannose which are the main hexose units of holocellulose. β -d-Glucopyranose and β -dmannopyranose are chosen as the initial configurations for DFT calculations, because glucose and mannose units are linked by β −1,4-glucosidic bond in holocellulose [40,41]. In addition, β -dglucopyranose has been widely used as the model compound of cellulose in pyrolysis mechanism study [25,26,30,31]. Especially, 13 C labeled glucoses are used in fast pyrolysis experiments to determine the origins of 5-HMF and FF as well as the different pyrolytic characteristics between glucose and mannose. Combined with the computational and experimental methods, the present study is aimed at uncovering the detail formation mechanisms of 5-HMF and FF from glucose and mannose pyrolysis. In addition, attentions are paid to the relationship of the formation of 5-HMF and FF, as well as the effect of epimerization at C2 position between glucose and mannose on 5-HMF and FF formation. Furthermore, cellobiose is employed as another model compound to analyze the effects of neighbor unit on the formation of 5-HMF and FF, giving implications for holocellulose pyrolysis to produce the two furan derivatives. 2. Experimental 2.1. Experiments and characterization The materials utilized in the fast pyrolysis experiments were commercial unlabeled d-( + )-glucose ( ≥ 99.5%, GC, Aladdin), d( + )-mannose ( ≥ 99.5%, GC, J & K), 13 C1-labeled d-glucose ( ≥ 98% CIL), 13 C2-labeled d-glucose ( ≥ 99% CIL), 13 C5-labeled d-glucose ( ≥ 98% CIL) and 13 C6-labeled d-glucose (99%, GC, Sigma). All the materials were used as purchased without extra purification. Analytical Py-GC/MS experiments were performed using the CDS Pyroprobe 5200HP pyrolyzer (Chemical Data Systems) and the Perkin Elmer GC/MS (Clarus 560) which were connected together by a transfer line. The experimental procedure was described in our previous work [42] with a rough description as follows. Each experimental sample was prepared with strict 0.20 mg feedstock held by some quartz wool in a quartz tube. Fast pyrolysis experiments were conducted at temperatures of 400 °C, 500 °C and 800 °C for 20 s with the heating rate of 20 °C/ms. Pyrolysis vapors were online analyzed by GC/MS with an Elite-35MS capillary column (30 m × 0.25 mm i.d.; 0.25 mm film thickness) for GC separation. MS was in the electron ionization (EI) mode (70 eV) with a scan range of 20–400 AMU. Pyrolytic products were identified by NIST and Wiley library. The experiments for each sample were repeated at least three times. For every identified product, its peak area and peak area percentage (peak area%) values were

Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013

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Table 1. Selected structural parameters for glucose, mannose and the right glucose unit of cellobiose in the optimized structures.

Fig. 1. Optimized structures of glucose, mannose and cellobiose (grey ball: C; red ball: O; white ball: H).

recorded, including both average and standard deviation values. Peak area and peak area% values of a compound are considered to vary linearly with its quantity and relative concentration, respectively. Hence, for each product, the changes of its yield and concentration are evaluated by its average peak area and peak area% values, respectively.

2.2. Computational details All calculations were conducted using Gaussian 09 rev D. 01 [43]. The hybrid meta exchange-correlation approximation M062X functional [44] and extended basis set 6-311 + G(d,p) were selected for structures optimization of the reactants, intermediates, transition states and products to obtain their structural parameters and electronic energies (0 K). The selected method has a good accuracy in evaluating energy barriers [45] and has been widely used in mechanism study of biomass conversion [46–49]. Fig. 1 shows the optimized structures of the model compounds, with main structural parameters shown in Table 1. In addition, 3D images of the optimized structures for all compounds are shown in the Supplementary material (Figs. S11–S21 in Section S5), including the atomic coordinates (Section S6). Base on optimized structures, frequency analyses [50] were performed at the same level. All the transition states were confirmed as the first order saddle point on the potential energy surface (PES) with exactly one imaginary frequency. The other compounds were the minimum points on the PES with no imaginary frequency. Furthermore, intrinsic reaction coordinate (IRC) analyses [51] were necessary to verify the corresponding minimum points and first order saddle points on the same PES. In order to simulate the real pyrolysis conditions, it is necessary to obtain thermodynamic parameters at typical pyrolysis temperatures. For example, the Gibbs energies at different temperatures can be calculated by the following equation.

EG (T ) = Eele + EG_correct (T )

(1)

β -d-glucopyranose

β -d-mannopyranose

Cellobiose

˚ Bond length (A) C1 − O1 C1 − O5 C1 − C2 C2 − C3 C2 − O2 C4 − O4

1.389 1.410 1.521 1.515 1.411 1.411

1.392 1.407 1.526 1.522 1.411 1.412

1.391 1.410 1.528 1.519 1.406 1.420

Bond angles (°) O5 − C1 − C2 C1 − C2 − C3 C1 − C2 − O2 C1 − O5 − C5

110.2 109.0 111.7 113.7

112.3 109.8 111.2 114.0

110.7 110.8 109.7 112.4

Dihedral angles (°) O5 − C1 − C2 − O2 O5 − C5 − C6 − O6 O2 − C2 − C3 − O3

176.7 −57.1 63.7

−64.6 −57.8 −53.8

175.9 154.9 67.8

where, EG (T) is the Gibbs energy at a specific temperature, while Eele and EG_correct (T) are electronic energy as well as Gibbs energy correction at the specific temperature. The typical temperature of fast pyrolysis is 500 °C, which is also the fast pyrolysis temperature of our experiments (Section 3.1). Therefore the activation free energy (G† ) and free energies of the reactions (Grxn ) at 500 °C were reported to evaluate the competitiveness of different pathways. The calculation results at other temperatures are similar to the results at 500 °C. 3. Results and discussion 3.1. Fast pyrolysis results Fast pyrolysis of glucose and mannose produces various furan compounds, among which 5-HMF and FF are the two most important products. The total ion chromatograms of the two hexoses are similar, as shown in the Supplementary material (Fig. S1). Fig. 2 illustrates the peak area and peak area% values of 5-HMF and FF to evaluate the changes of their yields and concentrations under different pyrolysis temperatures. Both the peak area and peak area% values of 5-HMF are higher than those of FF for the pyrolysis of glucose and mannose, which implies that the formation of 5-HMF is more competitive than that of FF for both hexoses. Similar results were also reported in previous literatures [27,38]. In addition, mannose pyrolysis results in higher peak area and peak area% values of the two furan derivatives than glucose pyrolysis, which should be due to the epimerization at C2 position between glucose and mannose. Pyrolysis temperature has remarkable effects on the two furan derivatives. Maximal peak area and peak area% of 5-HMF from glucose and mannose are obtained at 400 °C and 500 °C, respectively. In regard to FF, its maximal peak area values are obtained at the same temperature of 500 °C for both glucose and mannose. The maximal peak area% value of FF from mannose appears at 800 °C, while the peak area% value of FF from glucose does not show significant changes under different temperatures. Based on the fast pyrolysis of 13 C-labeled glucoses, the origins of 5-HMF and FF can be determined. First of all, 5-HMF and FF are assumed to be generated from sequential carbon atoms (5-HMF from C1 to C6, FF from C1 to C5 or C2 to C6) [19,20]. For 5-HMF, it is only necessary to determine which carbon atom (C1 or C6) providing the aldehyde group. For FF, it is essential to determine which five carbon forming FF (C1–C5 or C2–C6) and which carbon providing the aldehyde group (C1, C5, C2 or C6). Typical mass spectra of unlabeled and 13 C-labeled 5-HMF are shown in the Supplementary material (Fig. S2a). For unlabeled

Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013

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Fig. 2. The peak area and peak area% of 5-HMF and FF from fast pyrolysis of glucose and mannose.

5-HMF, its molecular ion (M) peak is at m/z = 126, with the ion at m/z = 97 having a distinct intense. Perez Locas and Yaylayan [13] pointed that the m/z = 97 peak results from the removal of aldehyde group of M-1 peak. In the mass spectra of both unlabeled and 13 C1-labeled 5-HMF, m/z = 97 peak has a greater intensity over m/z = 98 peak, while opposite results are detected in the mass spectra of 13 C2, 13 C5 and 13 C6-labeled 5-HMF. Such results indicate that C1 predominantly provides the aldehyde group of 5HMF. For unlabeled FF, the M peak (m/z = 96) and M-1 peak (m/z = 95) are conspicuous in the mass spectrum (Fig. S2b). When FF is labeled by 13 C, the M peak and M-1 peak will become m/z = 97 and m/z = 96, respectively. In the present experiments, FF derives from the successive five carbon atoms (C1–C5 or C2–C6) of the six-carbon glucose (C1–C6), so the mass spectra of the unlabeled FF and 13 C-labeled FF are mixed together for 13 C1-labeled or 13 C6-labeled glucose. The isotopic enrichments (E) of 13 C1 and 13 C6 can be used respectively to evaluate the ratios of the FF from C1 to C5 and C2 to C6. Theoretically, the sum of the peak areas of m/z = 95, m/z = 96 and m/z = 97 should keep constant for all the samples. The increase of the peak area of m/z = 97 for 13 C-labeled glucose compared to unlabeled glucose can be used to evaluate the isotopic enrichment. To eliminate the errors resulting from the ineluctable mass differences of the samples, the relative values (rather than the absolute values) are used here, as shown in the following equations.



RC1−C5FF =

Pm/z=97 Pm/z=97 + Pm/z=96 + Pm/z=95





RC2−C6FF =

Pm/z=97 Pm/z=97 + Pm/z=96 + Pm/z=95



400 °C 500 °C 800 °C

RC1 – C5FF (%)

RC2 – C6FF (%)

94.7 92.7 93.1

5.3 7.3 6.9

brackets 0, C1 and C6 mean that the data are based on unlabeled, 13 C1-labeled and 13 C6-labeled samples. Normalization method is used to give the proportions of the FF from C1 to C5 and C2 to C6 which are shown in Table 2. Obviously, FF is mainly derived from C1 to C5, which is consistent with previous results [19,20]. Further analysis is conducted to determine which carbon providing the aldehyde group (C1, C5, C2 or C6) for FF. For 13 C-labeled FF, if 13 C is labeled in the aldehyde group, an m/z = 67 peak will emerge as 13 C-labeled aldehyde group departs, while an m/z = 68 peak will appear if 13 C is labeled at other positions. According to Fig. S2(b), the intensity of m/z = 67 peak is greater than that of m/z = 68 peak in the mass spectrum of 13 C1-labeled FF, while the mass spectrum of 13 C5-labeled FF has a more conspicuous m/z = 68 peak. The above results clearly indicate that the C1 mainly provides the aldehyde group of FF from C1 to C5, similar as 5-HMF. Moreover, C2 mainly provides the aldehyde group of FF from C2 to C6. 3.2. Formation mechanism of 5-HMF from glucose and mannose



C1

Pm/z=97 − Pm/z=97 + Pm/z=96 + Pm/z=95



Table 2. The proportions of FF from C1 to C5 and C2 to C6 (%).

(2)



0



C6

Pm/z=97 − Pm/z=97 + Pm/z=96 + Pm/z=95

(3) 0

where, RC1 – C5FF and RC2 – C6FF are the ratios of the FF from C1 to C5 and C2 to C6. Pm/z =95 , P m/z =96 and P m/z =97 are the peak areas of m/z = 95, m/z = 96 and m/z = 97 peaks, and subscripts outside the

During fast pyrolysis of β -d-glucopyranose and β -dmannopyranose, the key step to generate 5-HMF is the formation of five-membered intermediates which can be achieved in two different ways. In the first way, β -d-glucopyranose and β -dmannopyranose convert into the five-membered intermediates through ring-opening reaction and following ring-closure reaction. In the second way, they generate the five-membered intermediates through direct ring contraction without the occurrence of the acyclic structures. Hence, the mechanism for 5-HMF formation is classified into two categories in the present study, i.e., acyclic mechanism and cyclic mechanism. In acyclic mechanism, the fivemembered ring intermediate is formed involving acyclic d-glucose

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Scheme 1. Possible DF-involved pathways for the formation of 5-HMF (unit: kJ/mol).

or d-mannose. On the contrary, the five-membered intermediate is formed through direct ring contraction in cyclic mechanism.

3.2.1. Acyclic mechanism for 5-HMF formation In acyclic mechanism, β -d-glucopyranose and β -dmannopyranose firstly undergo ring-opening reaction to generate acyclic d-glucose and d-mannose, followed with the formation of five-membered intermediates to produce 5-HMF. Based on the analysis of all possible intermediates as well as previous studies, DF and 3-DG are the two important possible intermediates from both hexoses. In addition, due to the epimerization at C2 position between d-glucose and d-mannose, d-mannose itself can form other five-membered intermediates to produce 5-HMF. Therefore, pathways in acyclic mechanism are classified into three categories, i.e. DF-involved pathways, 3-DG-involved pathways, and d-mannose special pathways. Possible DF-involved pathways: As mentioned above, DF is one of the important possible intermediates for the formation of 5-HMF [11,15,19,27]. Scheme 1 shows the possible DF-involved pathways for 5-HMF formation, with structures of the key transition states shown in Fig. 3. The energy diagram of these pathways is shown in the Supplementary material (Fig. S3). As shown in Scheme 1, acyclic d-glucose firstly isomerizes into DF with an activation free energy of 176.9 kJ/mol. This reaction involves a concerted transition state, A1-ts1 (Fig. 3), in which 2– hydroxyl H shifts to 1-carbonyl O accompanied with another H at C2 position shifting to C1 position. Afterwards, DF forms 2,5acetal ring through transition state A1-ts2 (Fig. 3) to form β -dfructofuranose. In addition, DF can also form α -d-fructofuranose through similar transition state. The formation of the two epimers has close activation free energies (G† = 178.7 and 184.7 kJ/mol respectively). Both of them can form 5-HMF in similar reactions [27]. Scheme 1 only shows the five possible pathways (Paths A1– A5) involving β -d-fructofuranose. The pathways involving α -d-

fructofuranose are shown in the Supplementary material (Scheme S1). Path A1 (in red) is one of the typical DF-involved pathways [14,27]. In this pathway, β -d-fructofuranose dehydrates at 2-OH + 1-H (hydroxyl group at C2 position and hydrogen at C1 position) site to form intermediate A1-i3 via the four-membered transition state A1-ts3 (G† = 285.5 kJ/mol). The structure of A1ts3 is shown in Fig. 3, and structures of similar transition states involved in other dehydration reactions are shown in the Supplementary material (Figs. S12–S21). Then, A1-i3 tautomerizes into its keto isomer A1-i4 (2,5-anhydro-d-mannose) with an activation free energy of 253.2 kJ/mol. Afterwards, A1-i4 undergoes dehydration at 3-OH + 2-H (hydroxyl group at C3 position and hydrogen at C2 position) site with the formation of A1-i5 which finally produces 5HMF through following dehydration at 4-OH + 5-H (hydroxyl group at C4 position and hydrogen at C5 position) site. Alternatively, A1i4 can also produce 5-HMF when dehydration at 4-OH + 5-H site occurs before dehydration at 3-OH + 2-H site, as schemed in Path A2 (in blue). However, Path A2 is not as favorable as Path A1 due to the high activation free energy of dehydration at 4-OH + 5-H site (G† = 292.8 kJ/mol). As shown in Scheme 1, Path GA3 (in green) shows an alternative electrocyclic dehydration reaction of A1-i3. In this pathway, A1-i3 undergoes electrocyclic dehydration instead of tautomerization (in Path A1) to form A1-i5 directly in one step [13,19,27,42]. This reaction involves six-membered transition state A3-ts4 (Fig. 3) in which 3-OH and 1-H (OH) dehydrates, accompanied with the formation of C2=C3 bond and C1=O bond. It is notable that this reaction is not only thermodynamically downhill (Grxn = − 111.8 kJ/mol), but also requires relatively low activation free energy (G† = 164.8 kJ/mol). Eventually, A1-i5 results in the formation of 5-HMF through dehydration at 4-OH + 5-H site. In addition, Path A4 (in pink) schemes another formation way of 5-HMF from A1-i3. Different from Path A3, intermediate A1-i3 undergoes

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Fig. 3. Optimized transition states structures of possible 5-HMF formation pathways in acyclic mechanism.

dehydration at 4-OH + 5-H site before electrocyclic dehydration in this pathway. However, the activation free energy of the dehydration at 4-OH + 5-H is as high as 301.0 kJ/mol, which is not favorable for the formation of 5-HMF. In the above four pathways (Paths A1 − A4), β -d-fructofuranose firstly undergoes dehydration at 2-OH + 1-H site. Whereas in Path A5 (in purple), β -d-fructofuranose dehydrates at 4-OH + 5-H site firstly to form intermediate A5-i3. Although this reaction is thermodynamically downhill (Grxn = − 81.0 kJ/mol), it requires much high activation free energy (G† = 311.0 kJ/mol). Then, A5-i3 forms A4-i4 by dehydrating at 2-OH + 1-H site, with a high activation free energy of 316.8 kJ/mol. Finally, A4-i4 undergoes electrocyclic dehydration to produce 5-HMF. Among the five pathways, Paths A1–A4 involve the initial dehydration of β -d-fructofuranose at 2-OH + 1-H site (G† = 285.5 kJ/mol), whereas Path A5 involves the dehydration at 4-OH + 5-H site (G† = 311.0 kJ/mol). It is obvious that β -d-fructofuranose trends to dehydrate at 2-OH + 1-H site to form A1-i3. And A1-i3 prefers to undergo electrocyclic dehydration (Path A3) due to the low activation free energy. Therefore, Path A3 is the most favorable DF-involved pathway to form 5-HMF. Notably, acyclic d-mannose can also form d-fructose via isomerization with a relatively low activation free energy of 148.6 kJ/mol. As a result, d-mannose shares similar pathways to produce 5-HMF with DF as the intermediate. Possible 3-DG-involved pathways: Besides DF, 3-DG (or its enol isomer) is another possible intermediate for the formation of 5HMF [14,16,27]. The possible 3-DG-involved pathways are shown in Scheme 2, with the structures of the key transition states shown in Fig. 3. The energy diagram of these pathways is illustrated in the Supplementary material (Fig. S4). As shown in Scheme 2, acyclic d-glucose firstly dehydrates at 3-OH + 2-H site with the formation of 3-DG’s enol isomer B1-i1 (B1-ts1, G† = 223.9 kJ/mol). Then B1-i1 tautomerizes into 3-DG through the four-membered transition state B1-ts2 (G† = 247.4 kJ/mol). Both reactions are thermodynamically downhill (Grxn = − 67.4 and − 31.6 kJ/mol). Similar to DF, 3-DG forms 2,5-acetal ring through hemiacetal reaction to form five-membered

intermediate B1-i3. The formation of B1-i3 involves a fourmembered transition state, B1-ts3, with its structure shown in Fig. 3. Meanwhile, 3-DG can also form another five-membered intermediate, B1-i3’, through similar transition state. The activation free energies for the formation of the two epimers are close to each other (G† = 180.3 and 172.6 kJ/mol, respectively). Both of them can lead to the formation of 5-HMF in similar reactions. Only three possible pathways (Paths B1 − B3) involving B1-i3 are illustrated in Scheme 2. The pathways involving B1-i3’ are shown in the Supplementary material (Scheme S2). For B1-i3, it will produce 5-HMF in Path B1 (in red) or Path B2 (in blue) through successive dehydration reactions. In path B1, B1-i3 dehydrates at 2-OH + 3H (hydroxyl group at C2 position and hydrogen at C3 position) site (B1-ts4, G† = 257.6 kJ/mol), with the formation of B1-i4 (A1i5), which finally forms 5-HMF through dehydration at 4-OH + 5H site. In Path B2, dehydration reactions occur at the same sites with the opposite sequence. B1-i3 firstly undergoes dehydration at 4-OH + 5-H site and then at 2-OH + 3-H site in this pathway. The dehydration at 4-OH + 5-H site has a high activation free energy (G† = 301.7 kJ/mol), which makes Path B2 less favorable than Path B1 to generate 5-HMF. In addition, B1-i1 can lose another molecular of water through dehydration at 4-OH + 5-H site, as shown in Path B3 (in green). This dehydration leads to the formation of a diene intermediate B3-i2, which can directly form 5-HMF through acetal reaction of 2-OH and 5-OH accompanied with the generation of water [27]. Because of the limitation of the diene structure resulted from the successive dehydration, the formation of the five-membered ring requires a high activation free energy of 296.0 kJ/mol. However, the formation of the five-membered ring in the other two pathways (Paths B1 and B2) only requires an activation free energy of 180.3 kJ/mol. Hence, 5-HMF can hardly be generated once the diene intermediate is formed. Among the above three pathways, Path B1 is the most feasible one due to the low activation free energy of the rate-determining step (B1-i3 → B1-i4). It is notable that although Path B3 involves less steps than the other two pathways, the formation of 5-HMF in this pathway is not easy due to the high activation energy to from

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Scheme 2. Possible 3-DG-involved pathways for the formation of 5-HMF (unit: kJ/mol).

the five-membered ring. Similar to d-glucose, acyclic d-mannose can also form B1-i1 (3-DG’s enol isomer) with close activation free energy. Hence, d-mannose shares similar pathways to produce 5HMF through the above 3-DG-involved pathways. Possible D -mannose special pathways: According to Schemes 1 and 2, both d-mannose and d-glucose can form 5HMF involving DF and 3-DG as the intermediates. Due to the epimerization at C2 position, there are extra possible pathways for d-mannose to form 5-HMF compared to d-glucose. These pathways may explain the fact that d-mannose produces more 5-HMF than d-glucose in the pyrolysis process (Fig. 1). Based on the structural analysis, special 5-HMF formation pathways from dmannose are discussed specially in Scheme 3. The energy diagram for these pathways is shown in the Supplementary material (Fig. S5). As shown Scheme 3, Path MC1 (in red) involves the direct formation of five-membered intermediate MC1-i1 through acetal reaction. Intermediate MC1-i1 is exactly A1-i4 (2,5-anhydro-dmannose) which will form 5-HMF following the reactions in Paths A1 and A2. In this reaction, the formation of 2,5-acetal results in the removal of 2-OH or 5-OH in the form of water. Both the removal of 2-OH and 5-OH are examined, and the activation free energy for removing 2-OH is lower than removing 5-OH (G† , 301.0 vs 317.2 kJ/mol). In addition, d-mannose can firstly undergo successive dehydration at 4-OH + 5-H site (MC2-ts1, G† = 286.7 kJ/mol) and tautomerization (MC2-ts2, G† = 239.9 kJ/mol) to form intermediate MC2-i2 in Paths MC2 (in blue) and MC3 (in green). Afterwards, MC2-i2 forms 2,5-acetal bond through hemiacetal reaction, resulting the formation of five-membered intermediates MC2-i3. This reaction involves four-membered transition state MC2-ts3, of which the structure is shown in Fig. 3. MC2-i3 can also form another fivemembered intermediate, MC2-i3’, which is the C5-epimer of MC2i3. Both epimers can produce 5-HMF through similar reactions, with the pathways involving MC2-i3’ shown in the Supplementary

material (Scheme S3). For MC2-i3, it will undergoes dehydration at 5-OH + 4-H (hydroxyl group at C5 position and hydrogen at C4 position) and 3-OH + 2-H sites in different orders to form 5-HMF, as shown in Paths MC2 (in blue) and MC3 (in green), respectively. In fact, similar as d-mannose, d-glucose is also able to form similar five-membered intermediates C1-i1 and C2-i3 through reactions as shown in Paths C1 and Path C2. Unfortunately, both C1i1 and C2-i3 can hardly dehydrate at 3-OH + 2-H site, because 3OH and 2-H are in opposite sides of the ring for the two intermediates, thus, not feasible to generate 5-HMF. Hence, the epimerization at C2 position between d-mannose and d-glucose makes d-mannose has more pathways (Paths MC1-MC3) to form 5-HMF than d-glucose. As a result, mannose pyrolysis produces more 5HMF than glucose pyrolysis (Fig. 2). 3.2.2. Cyclic mechanism for 5-HMF formation In cyclic mechanism, direct ring contraction of pyranose takes place to form five-membered ring intermediate [27]. Both β d-glucopyranose and β -d-mannopyranose can undergo the ring contraction reactions to form intermediate A1-i4, as shown in Scheme 4. As discussed above, A1-i4 can form 5-HMF through two successive dehydration reactions in Paths A1 and A2. However, these ring contraction reactions require much more energies than the ring-opening reactions, although the ring contraction can result in the transformation from pyranose to furanose in one-step. Hence, it is reasonable to conclude that the acyclic mechanism, rather than the cyclic mechanism, is predominant for the formation of 5-HMF during the fast pyrolysis process. 3.3. Formation mechanism of FF from glucose and mannose The key steps for the formation of FF from the two hexoses include the transformation from pyranose to furanose and the removal of a carbon containing group. According to the experimental results, FF mainly derives from C1 to C5 segment and slightly

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Scheme 3. Possible d-mannose special pathways to form 5-HMF (unit: kJ/mol).

Scheme 4. Cyclic mechanism for the formation of 5-HMF (unit: kJ/mol).

from C2 to C6 segment, and their formation mechanisms will be discussed, respectively. As clearly indicated in the above section, acyclic d-glucose and d-mannose are necessary for the formation of five-membered intermediates. Hence, only the pathways involving acyclic d-glucose and d-mannose are investigated here. Similar to 5-HMF, d-glucose and d-mannose share the same/similar pathways in the formation of FF, and moreover, d-mannose has some special pathways to form FF. The different pathways from dglucose and d-mannose are compared to explain the epimerization effect on the formation characteristics of FF. 3.3.1. Formation mechanism for the FF from C1 to C5 Based on structural analysis, the key steps to generate the FF from C1 to C5 include the formation of five-membered intermediates and removal of C6 group. Hence, the formation mechanism can be classified into two categories based on the sequence of five-

membered ring formation and C–C bond cleavage, i.e., cyclization first mechanism and decarbonization first mechanism. In cyclization first mechanism, five-membered intermediates generate before the removal of C6 group. On the contrary, C6 group will be removed firstly in decarbonization first mechanism. Similar as 5HMF formation mechanism, DF and 3-DG are vital intermediates for the generation of FF from both d-glucose and d-mannose. In addition, d-mannose also involves other special intermediates to produce FF, which leads to more FF production from mannose pyrolysis than glucose pyrolysis. Hence, pathways in the two mechanisms will be discussed in three categories, i.e., DF-involved pathways, 3-DG-involved pathways and d-mannose special pathways, the same as the 5-HMF formation pathways (acyclic mechanism). Cyclization first mechanism: Scheme 5 shows the possible FF formation pathways in cyclization first mechanism, with the structures of the key transition states shown in Fig. 4. These pathways involve DF (Paths D1 and MD1) and 3-DG (Paths D2 and MD2) as the intermediates for both hexoses, and A1-i4 (Path MD3) as the special intermediate for d-mannose, respectively. The energy diagram of these pathways is shown in the Supplementary material (Fig. S6). The formation of these intermediates has been discussed in details in the formation mechanism of 5-HMF as illustrated in Schemes 1–3, respectively. Path D1 (in red) is the typical DF-involved pathway from, in which DF firstly forms the five-membered intermediates β -dfructofuranose or α -d-fructofuranose [27]. Similarly, Path D1 only gives the pathway involving β -d-fructofuranose, and the detailed information involving α -d-fructofuranose is shown in the Supplementary material (Scheme S4). β -d-Fructofuranose undergoes dehydration between 4-OH and 6-H(OH) through six-membered transition state D1-ts3 (Fig. 4). This reaction results in the scission of C5–C6 bond as well as the formation of unsaturated C4=C5

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Scheme 5. Possible pathways for the formation of FF from C1 to C5 in cyclization first mechanism (unit: kJ/mol).

Fig. 4. Optimized transition states structures of possible formation pathways of the FF from C1 to C5 in cyclization first mechanism.

bond to form intermediate D1-i3. It requires an activation free energy as high as 361.8 kJ/mol, which is kinetically unfavorable. Afterwards, D1-i3 undergoes dehydration at 2-OH + 1-H site to form D1-i4 which finally produces FF via electrocyclic dehydration. As mentioned above, 3-DG can form two five-membered epimers, B1-i3 and B1-i3’. Scheme 5 only shows the FF formation pathway involving B1-i3 (Path D2), and the pathway involving B1-i3’ is shown in the Supplementary material (Scheme S4). In Path D2 (in blue), B1-i3 undergoes dehydration between 4-OH and 6-H (OH), leading to the formation of intermediate D2-i4 with a high activation free energy of 339.5 kJ/mol. This reaction involves six-membered transition state D2-ts4 whose structure is similar to that of D1-ts3, as shown in Fig. 4. Eventually, D2-i4 forms FF through dehydration at 2-OH + 3-H site. As shown in Scheme 5, d-mannose can also produce FF with DF and 3-DG as the intermediates through Paths MD1 (in purple) and MD2 (in green), respectively. In addition, d-mannose will undergo special reactions in Path MD3 (in pink) to generate FF. In this pathway, intermediate A1-i4 dehydrates at 4-OH + 6-H (OH) site to break C5–C6 bond, with the formation of MD3-i2. This reaction also involves six-membered transition state MD3-ts2 (Fig. 4)

which has similar structure to D1-ts3 and D2-ts4. Similarly, this dehydration reaction also requires a high activation free energy (G† = 352.5 kJ/mol). Intermediate MD3-i2 finally undergoes dehydration at 3-OH + 2-H site to form FF. As mentioned above, dglucose can also form similar five-membered intermediate C1-i1 (Scheme 3). However, this intermediate can barely dehydrate at 3OH + 2-H site due to 3-OH and 2-H in opposite side of the ring, and thus, cannot form FF through similar reactions as d-mannose does. Based on the above results, the dehydration between 4-OH and 6-H (OH) for each of the five-membered intermediate to break C5–C6 bond has a high activation free energy in all the above pathways, implying all these pathways are not predominant for the formation of FF. Conversely, these five-membered intermediates trend to dehydrate successively to form 5-HMF, as discussed above (Schemes 1–3). Decarbonization first mechanism: Different from cyclization first mechanism, d-glucose and d-mannose remove the C6 group before cyclization in decarbonization first mechanism for the formation of FF from C1 to C5. The possible pathways are shown in Schemes 6–8, involving DF and 3-DG as the intermediates for both

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Scheme 6. Possible DF-involved pathways in decarbonization first mechanism for the formation of FF from C1 to C5 (unit: kJ/mol).

Scheme 7. Possible 3-DG-involved pathways in decarbonization first mechanism for the formation of FF from C1 to C5 (unit: kJ/mol).

hexoses (Schemes 6 and 7) and special pathways for d-mannose (Scheme 8). Scheme 6 shows the possible pathways involving DF as the intermediate, with the structures of two main transition states shown in Fig. 5. The energy diagram of these pathways is illustrated in the Supplementary material (Fig. S7). In Path E1 (in red), DF breaks C5–C6 bond via a four-membered transition state E1-ts2, with the formation of intermediate E1-i2. This reaction requires an activation free energy as high as 413.4 kJ/mol, which is not kinetically favorable for producing FF. Afterwards, E1-i2 forms the fivemembered intermediate E1-i3 by forming 2,5-acetal bond. E1-i3 has two C2-epimers E1-i3 and E1-i3’. Both epimers can form FF through similar reactions. Only the pathways involving E1-i3 are illustrated in Scheme 6. Pathways involving E1-i3’ are shown in the Supplementary material (Scheme S5). Then E1-i3 dehydrates at 2-OH + 1-H site to form E1-i4 with an activation free energy of 285.3 kJ/mol. E1-i4 undergoes electrocyclic dehydration to generate E1-i5 which finally produces FF through dehydration at 4-OH + 5-H site. Different from Path E1, intermediate E1-i3 can produce FF through three dehydration reactions in other orders, as shown in

Paths E2 (in blue) and E3 (in pink), respectively. In Path E2, E1-i3 undergoes dehydration at 4-OH + 5-H site firstly. This reaction results in the formation of E2-i4 (D1-i3) which will form FF through successive dehydration at 2-OH + 1-H site and electrocyclic dehydration. In Path E3, intermediate E1-i4 will undergo dehydration at 4-OH + 5-H site and electrocyclic dehydration in sequence to form FF. It is notable that all the reactions in the above three pathways are thermodynamically favorable (Grxn < 0), and the high activation free energy of breaking C5–C6 limits the whole reaction rate. DF can also produce FF via Path E4 (in green), in which DF breaks C5–C6 bond via dehydration between 4-OH and 6-H (OH). This dehydration involves a six-membered transition state E4-ts2 in a stable structure, so that only requires a relatively low activation free energy, compared to the scission of C5–C6 bond in Path E1 (G† , 276.7 vs 413.4 kJ/mol). The structure of E4-ts2 is similar to those of D1-ts3, D2-ts4 and MD3-ts2 in Fig. 4. Then the dehydrated intermediate E4-i2 forms the five-membered intermediate E2-i4 through four-membered transition state E4-ts3 which has similar structure to those of A1-ts2, B1-ts3 and MC2-ts3 in Fig. 3. Afterwards, E2-i4 will from FF through the reactions in blue

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Scheme 8. Possible d-mannose special pathways to form FF from C1 to C5 in decarbonization first mechanism (unit: kJ/mol).

arrows. Additionally, E4-i2 can also form another five-membered intermediate, E2-i4’, which is the C2-epimer of E2-i4. E2-i4’ can produce FF via similar reactions, which is shown in the Supplementary material (Scheme S5). Due to the relatively low activation free energy for breaking C5–C6 bond, Path E4 is much more favorable to generate FF than the other three pathways in these DFinvolved pathways. The possible pathways involving 3-DG as intermediate are illustrated in Scheme 7, with the structures of two main transition states shown in Fig. 5. Their energy diagram is shown in the Supplementary material (Fig. S8). Based on the analysis of the above DF-involved pathways, the dehydration between 4-OH and 6-H (OH) of 3-DG to break the C5–C6 is firstly taken into consideration in these pathways, as shown in Path E5 (in red). In this pathway, 3-DG removes the C6 group via a six-membered transition state E5-ts3 to form intermediate E5-i3, with an activation free energy of 264.4 kJ/mol. Transition sate E5-ts3 has a similar structure to E4ts2, as shown in Fig. 5. Then E5-i3 cyclizes into two C2-epimers E5-i4 and E5-i4’ with the same activation free energy coincidentally (G† = 179.5 kJ/mol). Both epimers can form FF through sim-

ilar reactions, whereas, Scheme 7 only shows the pathway involving one of the epimers (E5-i4). The pathway involving another one (E5-i4’) is shown in the Supplementary material (Scheme S6). As shown in Path E5, FF is formed from E5-i4 via one step of dehydration at 2-OH + 3-H site. Paths E6 (in blue) and E7 (in pink), in which 3-DG breaks C5–C6 bond via a four-membered transition state, are also investigated in the present work, although the scission of C5–C6 bond in the two pathways is more difficult than that in Path E5. As shown in Scheme 7, the removal of C6 group in the two pathways results in the formation of E6-i3, with a high activation free energy of 388.7 kJ/mol. Then E6-i3 undergoes hemiacetal reaction to form five-membered C2-epimers E6-i4 and E6-i4’. The formation of the two five-membered C2-epimers is thermodynamically downhill by − 4.7 and − 8.0 kJ/mol. Detailed reactions involving the two epimers to form FF are shown in Scheme 7 and Supplementary material (Scheme S6), respectively. For the five-membered intermediate E6-i4, it will produce FF through dehydration at 4-OH + 5H and 2-OH + 3-H sites in different orders in Paths E6 and E7, respectively. Obviously, Path E5 is more feasible to generate FF than

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Fig. 5. Optimized transition states structures of possible formation pathways of the FF from C1 to C5 in decarbonization first mechanism.

Paths E6 and E7, because of the relatively easy scission of C5–C6 bond in Path E5. In addition to DF and 3-DG, d-mannose can involve other special intermediates to produce FF via extra possible pathways (Paths ME8 and ME9) as shown in Scheme 8. The structures of two main transition states are shown in Fig. 5. The energy diagram of these pathways is illustrated in the Supplementary material (Fig. S9). In Path ME8 (in red), d-mannose firstly undergoes dehydration at 4OH + 6-H (OH) site to break C5–C6 bond, with the formation of ME8-i1 (ME8-ts1, G† = 246.8 kJ/mol). Similarly, this reaction involves six-membered transition state ME8-ts3 which has similar structure to E4-ts2 and E5-ts3, as shown in Fig. 5. Afterwards, ME8-i1 tautomerizes into its keto isomer ME8-i2 which will form 2,5-acetal ring to generate the five-membered intermediate ME8i3. This reaction also results in another C5-epimer ME8-i3’ which can form FF in similar reactions. The formation pathways involving ME8-i3 and ME8-i3’ are schemed in Scheme 8 and Supplementary material (Scheme S7), respectively. Finally, ME8-i3 undergoes dehydration at 5-OH + 4-H and 3-OH + 2-H sites in sequence to form FF. In Path ME9, FF is produced from ME8-i3 by the above two dehydration reactions in an opposite order. All the reactions in the two pathways are thermodynamically favored (Grxn < 0). As shown in Scheme 8, a pentose-involved pathway (Path ME10, in pink) is also investigated based on the conjecture of Kato and Komorita [16]. However, the formation of the pentose ME10i1 involves a four-membered transition state to break the C5–C6 bond with a high activation free energy of 380.2 kJ/mol. It is obvious that d-mannose is easier to crack C5–C6 bond through dehydration between 4-OH and 6-H (OH). Hence, pentose is not a potential intermediate to generate FF in the pyrolysis of hexose. Following reactions for ME10-i1 forming FF are omitted. Among the three d-mannose special pathways, Paths ME 8 and ME 9 are feasible for the formation of FF. In fact, d-glucose can also form the similar intermediates E8-i3 and E9-i1. However, for E8-i3, it can barely dehydrate at 3-OH + 2-H site because of their opposite orientations. For E9-i1, its formation requires a high activation free energy (390.0 kJ/mol). Hence, both E8-i3 and E9-i1 are not favorable for the formation of FF. As a result, the d-mannose

special pathways (Paths ME8 and ME9) should be mainly responsible for the experimental result that mannose produces more FF than glucose in the pyrolysis process (Fig. 2). 3.3.2. Formation mechanism of FF from C2 to C6 According to the experimental results in glucose pyrolysis, only about 7% FF derives from C2 to C6 segment (Table 2), with C6 mainly providing the aldehyde group. The formation mechanisms of FF from C2 to C6 in both glucose and mannose pyrolysis are investigated in similar ways, with the possible formation pathways shown in Scheme 9. The energy diagram of these pathways is illustrated in the Supplementary material (Fig. S10). As schemed in Scheme 9, Paths F1 and F2 are the possible formation pathways of FF from d-glucose [11]. In Path F1 (in red), d-glucose firstly undergoes dehydration at 4-OH + 5-H site, enol-keto tautomerization and cyclization in sequence to form the five-membered intermediate F1-i3 (C2-i3). Similarly, F1-i3 has a C5-epimer F1-i3’, whereas, only F1-i3 can lead to the formation of FF. F1-i3 dehydrates at 5-OH + 6-H site and then tautomerizes into F1-i5. Afterwards, F1-i6 with an axisymmetric structure is formed through the dehydration at 3-OH + 4-H site of F1-i5 (F1-ts6, G† = 263.0 kJ/mol). Finally, F1-i6 removes the aldehyde group to form FF through a four-membered transition state (F1ts7, G† = 232.1 kJ/mol). The axisymmetric structure of F1-i6, which relies on the tautomerization of F1-i4, is necessary for the removal of the aldehyde group. F1-i4 can also result in another C5-epimer with the two aldehyde groups in the same side of the ring, which is not favorable to generate FF. In addition, F1-i6 can remove either C1 group or C6 group due to the axisymmetric structure. Hence, this pathway forms the FF from C2 to C6 only with a low selectivity. In Path F2 (in blue), intermediate F1-i2 undergoes dehydration at 3-OH + 4-H site to form F2-i3 (F2-ts3, G† = 240.9 kJ/mol). Then F2-i3 cyclizes into the five-membered intermediates F2-i4 or F2i4’, with FF formation pathways involving F2-i4’ shown in the Supplementary material (Scheme S8). Afterwards, F2-i4 undergoes dehydration at 5-OH + 6-H site and enol-keto tautomerization in sequence to form the axisymmetric F1-i6 which finally produces FF

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Scheme 9. Possible pathways for the formation of FF from C2 to C6 (unit: kJ/mol).

by removing one of the aldehyde groups. Due to the similar reasons as mentioned in Path F1, Path F2 also forms the FF from C2 to C6 with a low selectivity. In regard to d-mannose, it can form the FF from C2 to C6 via Paths MF1 (in purple), MF2 (pink) and MF3 (in green). Paths MF1 and MF2 are similar to Paths F1 and F2, while Path MF3 is the special pathway of d-mannose. In Path MF3, d-mannose firstly forms 3,6-acetal bond to generate five-membered intermediate MF3-i1. 6-OH is easier to be removed than 3-OH (G† , 281.9 vs 307.1 kJ/mol). Afterwards, MF3-i1 breaks C1–C2 bond through a four-membered transition sate, with the activation free energy as high as 397.2 kJ/mol. Finally, FF is formed via successive dehydration at 4-OH + 3-H and 5-OH + 6-H sites. Different from Paths MF1 and MF2, Path MF3 leads C2 providing the aldehyde group of the FF from C2 to C6. In fact, d-glucose can also form the similar five-

membered intermediate F1-i3 as shown in Path F3. However, F1i3 can barely form FF, because 4-OH and 3-H are at the opposite sides of the ring, making dehydration at 4-OH + 3-H site impossible. It is notable that Path MF3 is not favorable for the formation of FF due to the high activation free energy to break the C1–C2 bond. Therefore, this pathways should not be responsible for the fact that mannose pyrolysis produces more FF than glucose pyrolysis. 3.4. Discussion Experimental results clearly indicate that 5-HMF is a typical and abundant furan derivative in the pyrolysis of glucose and mannose. Based on the quantum chemistry investigation (Schemes 1–4), acyclic d-glucose is necessary for the transformation of six-membered pyranose to five-membered furanose intermediate.

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Scheme 10. Effects of the neighbor unit for the major formation pathways of 5-HMF and FF (unit: kJ/mol).

Conversely, it is difficult for β -d-glucopyranose to form fivemembered intermediate via the direct ring contraction. Hence, it is the acyclic mechanism, rather than the cyclic mechanism, that is predominant for the formation of 5-HMF. Among the pathways in acyclic mechanism, Paths A3 and B1 are the main channels for 5-HMF formation from d-glucose involving DF and 3-DG as the intermediates respectively. Moreover, C1 group will provide the aldehyde group of 5-HMF in the two pathways, which is in accordance with the 13 C-labeled glucose pyrolysis results. Similar to glucose pyrolysis, 5-HMF predominantly derives form acyclic mechanism for mannose pyrolysis. d-Mannose can produce 5-HMF through the similar pathways with DF and 3-DG as the intermediates. In addition, 5-HMF can also derive from d-mannose through some special pathways of Paths MC1, MC2 or MC3 as shown in Scheme 3, which makes mannose pyrolysis produces more 5-HMF than glucose pyrolysis. FF is another important furan derivative in the pyrolysis of glucose and mannose. The formation of FF requires not only the transformation of pyranose to furanose, but also the removal of a carbon containing group. The DFT calculation results also imply that FF mainly derives from acyclic d-glucose, similar to 5-HMF. The C1–C5 segment instead of C2–C6 segment is the main origin of FF, which is in accordance with the 13 C-labeled glucose pyrolysis results. For the formation of the FF from C1–C5 (Schemes 5–8), it is difficult for the five-membered intermediates to break C5–C6 bond. On the contrary, C5–C6 bond can be easily broken before the formation of the five-membered intermediates. Therefore, decarbonization first mechanism is predominant for the C1–C5 segment to produce FF. The favorable formation pathways are Paths E4 and E5, with DF and 3-DG as the intermediates, respectively. The aldehyde group of FF derives from C1 group of d-glucose in these favorable pathways, in accordance with the experimental results. Similar to d-glucose, d-mannose can form the FF from C1 to C5 through similar pathways with DF and 3-DG as the intermediates. In addition, there are also some special pathways for d-mannose to generate FF from C1 to C5 (Paths ME8 and ME9), which results in higher yield of FF in mannose pyrolysis than glucose pyrolysis. In regard to the FF from

C2 to C6, according to Scheme 9, both d-glucose and d-mannose produce it with a low selectivity. In summary, both experimental and computational methods gain the coincident results. FF mainly derives from the C1 to C5 segment, with C5–C6 bond broken before forming the five-membered intermediates. Based on the above analyses, the main formation pathways of 5-HMF and FF and their energy diagrams are illustrated in Fig. 6. Two 5-HMF formation pathways and four FF formation pathways are included. Attentions can be paid to the relationship of the formation of 5-HMF and FF. It is seen that the favorable formation pathways of both 5-HMF and FF involve DF and 3-DG as the intermediates. For both DF and 3-DG, they undergo cyclization reaction more easily than breakage of C5–C6 bond. When the fivemembered intermediates are generated from DF and 3-DG, they trend to undergo dehydration instead of removal of C6 group. In addition, 5-HMF is difficult to form FF directly with the activation free energy as high as 358.9 kJ/mol, as presented by the purple arrows. Based on the above reasons, the formation pathways of 5HMF and FF compete with each other, and 5-HMF can barely produce FF through secondary decomposition, supporting the experimental results that 5-HMF pyrolysis results in trace amount of FF [33,34]. The formation pathways of 5-HMF are more competitive than those of FF, because DF and 3-DG are more favorable to generate 5-HMF than FF. As a result, glucose pyrolysis results in higher yield of 5-HMF than FF (Fig. 2). Due to the similar structure, mannose pyrolysis has the similar mechanism for 5-HMF and FF formation. In addition, the epimerization at C2 position makes mannose produce more 5-HMF and FF through extra pathways. In addition to glucose and mannose, cellobiose is employed as another model compound to investigate the effects of neighbor unit on the formation of 5-HMF and FF, and thus, giving implications for holocellulose pyrolysis to produce the two furan derivatives. Scheme 10 shows the major pyrolytic pathways based on cellobiose, and the structures of the compounds in these pathways are shown in the Supplementary material (Fig. S21). Based on the comparison of the pathways in Fig. 6 and Scheme 10, it is able to obtain the following information. First, the neighbor unit linked

Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013

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Fig. 6. (a) Major pathways and (b) their energy diagrams for 5-HMF and FF formation.

by β −1,4-glycosidic bond almost does not influence the formation pathways of 5-HMF and FF. Especially, FF formation pathways from both glucose and cellobiose involve the same intermediates after removing the C6 group. Second, the structures of the transition states are almost the same regardless of the neighbor unit. Third, the rate-determining steps keep consistent for the corresponding pathways in the presence or absence of the neighbor unit. Therefore, it is reasonable to deduce the formation pathways of 5-HMF and FF during holocellulose pyrolysis based on the results from

glucose and mannose, because the neighbor unit has no crucial influence on 5-HMF and FF formation. Based on the above calculation analyses, acyclic structures are the premise for the formation of the two furan derivatives during pyrolysis of glucose, mannose and holocellulose. Hence, to increase the yield of 5-HMF and FF, it is essential to promote the formation of acyclic structures in the first place. For the production of 5-HMF following the acyclic structures, all the reactions are thermodynamically favored (Grxn < 0) and require relative low

Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013

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activation free energies, except one reaction step (the formation of the five-membered intermediates) which is not thermodynamically favored. Therefore, to increase the production of 5-HMF from acyclic structures, it is only necessary to promote the cyclization of DF and 3-DG (or polymers containing DF and 3-DG structures). In regard to FF, there are two reaction steps which require relative high activation free energies, i.e., the formation of fivemembered intermediates and the scission of C5–C6 bond. Hence, the increased production of FF from acyclic structures of glucose, mannose and holocellulose requires efforts to accelerate not only the cyclization reaction but also the scission of C5–C6 bond. 4. Conclusions In the present study, combined fast pyrolysis experiments as well as DFT calculations are conducted to provide a deep insight into the formation mechanisms of 5-HMF and FF from glucose and mannose. The conclusions can be drawn as follows. (1) During the fast pyrolysis process, both glucose and mannose produce more 5-HMF than FF. Mannose pyrolysis results in higher yields of the two furan derivatives than glucose pyrolysis. For 5-HMF, its aldehyde group mainly derives from C1 group and its hydroxymethyl group derives from C6 group. FF predominantly derives from the C1 to C5 segment, with C1 mainly providing the aldehyde group. (2) Acyclic d-glucose, generated by the ring-opening reaction from β -d-glucopyranose, is essential for the formation of 5-HMF and FF with DF and 3-DG as the important intermediates. DF and 3-DG can readily form 5-HMF through successive dehydration reactions after forming the fivemembered intermediates in Paths A3 and B1, respectively. For d-mannose, it can produce 5-HMF through similar pathways as d-glucose. Besides, due to the influence of epimerization at C2 position, d-mannose can also form 5-HMF via special pathways (Paths MC1–MC3), which results in the higher yield of 5-HMF in mannose pyrolysis than glucose pyrolysis. (3) DF and 3-DG are also the vital intermediates for the formation of FF in glucose pyrolysis. The two intermediates trend to break C5–C6 bond before forming the five-membered ring in the favorable pathways for FF formation (Paths E4 and E5). Similarly, d-mannose can form FF through the similar pathways as d-glucose. Moreover, due to the influence of epimerization at C2 position, d-mannose can also produce FF through extra Paths ME8 and ME9, which contributes to the higher yield of FF in the pyrolysis of mannose than glucose. (4) The favorable formation pathways of 5-HMF and FF compete with each other with trace amount of FF deriving from the secondary decomposition of 5-HMF. The pathways for 5HMF formation are more competitive than those for FF formation, thus resulting in higher yield of 5-HMF than FF. Acknowledgments The authors thank the National Natural Science Foundation of China (51576064, 51676193), Beijing Nova Program (Z17110 0 0 01117064), Beijing Natural Science Foundation (3172030), the Foundation of Stake Key Laboratory of Coal Combustion (FSKLCCA1706) and the Fundamental Research Funds for the Central Universities (2017MS071, 2016YQ05) for financial support.

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Please cite this article as: B. Hu et al., Pyrolysis mechanism of glucose and mannose: The formation of 5-hydroxymethyl furfural and furfural, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.11.013