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Theoretical investigation on the carbon sources and orientations of the aldehyde group of furfural in the pyrolysis of glucose
Journal of Analytical and Applied Pyrolysis 120 (2016) 464–473
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Theoretical investigation on the carbon sources and orientations of the aldehyde group of furfural in the pyrolysis of glucose Meng Wang a , Chao Liu a,∗ , Xiaoxiao Xu a , Qibin Li b a Key laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400044, China b College of Aerospace Engineering, Chongqing University, Chongqing 400044, China
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 19 June 2016 Accepted 25 June 2016 Available online 27 June 2016 Keywords: Furfural Glucose Density functional theory Carbon sources Orientations of the aldehyde group
a b s t r a c t Furfural (FF) is a valuable and characteristic product of glucose pyrolysis, but little attention has been focused on its formation mechanism. In this study, density functional theory (DFT) methods were employed to investigate the possible pathways leading to the formation of FF from glucose. The results indicate that the pyrolysis of hydroxymethyl furfural (HMF) to FF is hardly to occur duo to the high activation energy (86.3 kcal/mol) required. The dominant FF formation pathway is Path-B2 which involves the 3-deoxy-glucose (3-DG) intermediate with overall energy barrier of 87.2 kcal/mol. The major proportions of FF are derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 is dominant over that at C5 . FF involving the C2 –C6 of glucose, which include two orientations (C2 and C6 ) of the aldehyde group only account for a minor proportion. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Thermochemical conversion of lignocellulosic biomass to biooil and high value chemicals is a hot issue in recent years. Many important chemicals such as glycolaldehyde, glyceraldehyde, acetone, 5-hydroxymethyl-furfural and furfural are indentified in the pyrolysis of cellulose [1]. Furfural is a highly versatile and key derivative widely used in oil refining, plastic, pharmaceutical and agrochemical industries [2,3]. Generally, furfural is considered as a characteristic product of the hemicellulose pyrolysis [4–7]. However, the high production of furfural was also reported in the pyrolysis of cellulose [1,8] and the production of HMF and FF was higher from glucose than from cellubiose and cellulose [9]. Therefore, it is suggested that low-polymerized sugars are more easily pyrolysis to furans, principally 5-hydroxymethyl furfural and furfural. Considering the abundance of cellulose in nature world [10], insight into the formation mechanism of FF will benefits the optimization of selective pyrolysis techniques and improvement of FF production. Early, it was suggested that FF was arisen from the further pyrolysis of levoglucosan (LG) which was the main product of the depolymerization of cellulose and dehydration of glucose unit
∗ Corresponding author. E-mail address: [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jaap.2016.06.019 0165-2370/© 2016 Elsevier B.V. All rights reserved.
[11]. However, in view of thermal stability of LG [12], the conversion of LG to FF could be a minor pathway. Lu et al. reported a very low yield of FF in the pyrolysis of LG at 600 ◦ C and suggested that FF and LG were formed in a competitive manner in the pyrolysis of cellulose [13]. More generally, it was suggested that FF was produced from HMF via the elimination of hydroxymethyl group of HMF [14]. Nonetheless, thermal decomposition of HMF conducted by Nikolov and Yaylayan revealed that main products were 5-methyfurfural, 2,5-furandicarboxaldehyde, and dimer [15], which indicates that the elimination of hydroxymethyl group is a minor route in the pyrolysis of 5-HMF. This is further confirmed by experimental and theoretical study by Shin et al., in which 5methyfurfural, 2,5-furandicarboxaldehyde were main products in the pyrolysis of 5-HMF and no FF was detected [16]. Theoretical study on pyrolysis mechanism of glucose indicated that concerted elimination of hydroxymethyl group of HMF resulted in FF and aldehyde required activation energy as high as 86.2 kcal/mol [17]. These results indicate that HMF and FF are formed through competitive pyrolysis reactions rather than sequential pyrolysis reactions in the pyrolysis of glucose. However, much less attention has been given to the formation mechanism of FF compared with HMF. It was suggested that the conversion of glucose to furanose structure involves an intermediate [8]. Fructose was generally considered as an intermediate in the formation of HMF [18], and the formation of FF could arise from decarbonylation that competes with the final dehydration
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to form HMF [19]. Another mechanism which involved the intermediate 3-deoxy-glucose (3-DG) was proposed in the formation of FF [20], which was supported by the identification of 3-DG in the pyrolysis of glucose [20] and heat sterilization of peritoneal dialysis (PD) fluids [21]. Recently, 13 C isotope labeling technique was employed to investigate the formation mechanism of furans during glucose pyrolysis and several formation routes of FF were proposed [22]. The experimental results indicated that FF could arise from different carbons of glucose and had different orientations of the aldehyde group. However, there is no consensus on the detailed formation mechanism of FF during glucose pyrolysis. Although enormous effects have been focused on improving the conversion of glucose to HMF [23,24], but the conversion of HMF to FF is difficult according to experimental and theoretical studies [15,16]. Therefore, density functional theory methods are employed to investigate the reaction mechanism for the conversion of glucose to FF in this study. The detailed energetics and reaction barriers are computed and these will provide fundamental information for the design of catalysts for the selective production of FF from glucose. 2. Computational methods All the calculations were conducted with Gaussian 09 suit of programs [25]. The M06-2X functional theory developed by Terhlar’ group [26] with 6–31 G +(d,p) basis set was used to optimize reactants and transition states, which has been shown to accurately estimate energy barriers. Stationary points on the potential energy surface of the reacting system were fully optimized using gradient minimization techniques, followed by evaluating harmonic vibration frequencies to characterize their nature as minima and to provide thermal corrections (vibrational contributions) to enthalpies and free energies which are used to analyze to the kinetics and thermodynamics of reactions in formation of FF [27]. All the reactants were verified to have zero imaginary frequencies, and transition states (TSs) have only one imaginary frequency. The transition states were located initially by QST3 method at low level M062 × 6–31(d,p) and then optimized by TS method with 6–31 + (d,p) basis set. Intrinsic reaction coordinate (IRC) was performed to identify the minimum energy path from a transition state to the two corresponding local minima at the same level. 3. Results and discussion 3.1. Initial elementary reactions of glucose decomposition to FF Based on the discussion above, the formation of FF competes with HMF in the pyrolysis of glucose. Therefore, the possible FF formation pathways analyzed in this study, which contain the mechanisms proposed by previous studies, such as “fructose intermediate” (Path-A), “3-DG intermediate” (Path-B), are shown in Fig. 1. Also, several mechanisms involving different decarbonylations, cyclizations and different carbon source (C1 –C5 and C2 –C6 ) are discussed in Path-C to Path-G. The enthalpies and free energies (in parentheses) of products and transition states are relative to that of glucose in Fig. 1 and following energy profiles. Path-A illustrates the formation pathways of FF via fructose intermediate, which competes with the formation of HMF. Firstly, -d-glucose decomposes into aldose P1 via ring-opening reaction with activation energy of 52.14 kcal/mol, which is in accordance with previous study [28]. Of course, that -d-glucose undergoes dehydrations to form anhydrosugars are also investigated as shown in supporting information Fig-S1. The results indicate that direct dehydrations are kinetically disfavored and ring-opening reaction could be the dominant reaction. The formation of fructose P-A from
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P1 can be achieved through two alternative pathways. In one-step path, P-A is formed via a six-membered ring transition state TS-A in which the hydroxyl hydrogen atom transfers to O1 and meanwhile the hydrogen atom at C2 transfers to C1 with activation energy of 39.3 kcal/mol. In two-step path, P1 tautomerizes to 1,2 enediol intermediate through TS-A1 with a large activation energy of 69.9 kcal/mol and this reaction is thermodynamically uphill by 11.1 kcal/mol. This indicates that the tautomerization is both kinetically and thermodynamically disfavored. Then 1,2 enediol converts to P-A. The two-step path has higher activation energy than that of one-step path, which could be ascribed to the more sterically hindered four-membered ring TSs. Thus, the formation of fructose is probably via one-step path which involves 1,2 H transfer under vacuum pyrolysis condition. However, it is arguable whether the tautomerization of glucose to fructose involves 1,2 H transfer. Stahlberg et al. [29] proposed that the tautomerization proceeded through 1,2 enediol intermediate in imidazolium-based ionic liquids with boric acid as a promoter, which did not involved 1,2 H transfer and was evident by deuterium-labeling study. However, it was recently reported that [30] AlCl3 -catalyzed conversion of glucose to fructose involved 1,2 H transfer and confirmed by deuterium labeling study. Furthermore, glucose isomerase catalyzed tautomerization was reported reacted via 1,2 H transfer mechanism [31]. These indicate that the mechanism of conversion of glucose to fructose is notably catalyst-dependent. The subsequent reactions leading to the formation of FF are discussed in the next part. Path-B illustrates the formation of FF via 3-DG which is proposed as the intermediate for formation of FF in the pyrolysis of glucose [20]. 3-DG can be formed via IM1 which arisen from dehydration between H of 1-OH and 3-OH of 1,2 enediol with low activation enthalpy of 30.9 kcal/mol duo to the less distorted six-memebered TS-A3. In addition, P1 can convert to IM1 through directly 1,2 dehydration with activation enthalpy of 55.8 kcal/mol. Thus, P-B is possibly formed from IM1 which arisen from 1,2- dehydration of P1. Path-C to G contain several pathways that lead to the formation of FF, which compete with the formation of fructose and 3-DG in the pyrolysis of glucose. Path-C is initiated by grob fragmentation which was proposed to explain the pyrolysis of glycerol and resulted in the formation of formaldehyde and H2 O [32]. The six-membered ring grob fragmentation has a relatively lower activation energy (64.8 kcal/mol) for the cleavage of strong C C bond compared with the four-membered transition state in previous study [17], which could be a possible pathway for the elimination of hydroxymethyl in FF formation. Path-D is initiated by 1,2 dehydration between 4-H and 3-OH with activation energy of 73.8 kcal/mol, which illustrates the formation of FF with the aldehyde group derived from C5 of glucose. Path-E, Path-F and Path-G illustrate the formation of FF derived from the C2 –C6 of glucose by the elimination of C1 . Path-E is initiated by 1,2 dehydration between 5-H and 4-OH with activation energy of 71.7 kcal/mol to form P-E. Then P-E decomposes to FF and the aldehyde group is formed at C6, which are discussed in the next part. Both Path-F and Path-G illustrate the formation of FF with the aldehyde group formed at C2 . Path-F is initiated by the formation of P-F via multi-hydrogen transfer (TSF) with activation energy of 50.3 kcal/mol. The higher activation energy of TS-F than similar TS-A may be attributed to more strained geometry of TS-F (Fig. 2). Path-G is initiated by 1,2 dehydration between 3-H and 4-OH with activation energy of 76.9 kcal/mol, and this pathway illustrates a different mechanism for the elimination of C1 to Path-F. 3.2. Detailed formation pathways of FF from glucose The detailed reactions that lead to the formation of FF via fructose are illustrated in Fig. 3 and the energy profiles are illus-
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Fig. 1. Initial elementary reactions of FF formation from glucose, enthalpies and free energies (in parentheses) are relative to that of glucose (unit: kcal/mol).
Fig. 2. Transition states for initial reactions of glucose decomposition to FF.
trated in Fig. 4. Firstly, P-A converts to fructofuranose P-A1 though hydrogenation-cyclization and then P-A1 decomposes into FF that proposed for HMF formation in reference [33]. Then HMF decom-
poses to FF by the elimination of hydroxymethyl via distorted four-membered transition state (TS-A12) with activation energy as high as 86.3 kcal/mol. Thus, that the conversion of HMF to FF is
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Fig. 3. Elementary reactions for the conversion of glucose to FF via fructose intermediate.
Fig. 4. Energy profiles of FF formation pathways via fructose intermediate (unit: kcal/mol).
hardly to occur, which is consistent with the pyrolytic results of HMF[15,16]. In Path-A2, P-A6 directly decomposes into FF through less distorted six-membered grob-fragmentation (TS-A10) which leads to the production of H2 O, CH2 O and FF with much lower activation energy (63.6 kcal/mol) compared with TS-A12. In Path-A3, P-A1 decomposes into P-A3 through TS-A5 with energy barrier of 60.3 kcal/mol. P-A can also decompose into P-A3 through grobfragmentation (TS-A13) with relatively higher activation energy (69.4 kcal/mol) and subsequent hydrogenation-cyclization in PathA4. Thus, Path-A3 is more kinetically favored than Path-A4. Then P-A3 decomposes to FF through stepwise dehydrations and the rate-determining step is 1,2 dehydration between 1-H and 2-OH (TS-A6). Through analyzing the energy profiles of Path-A, Path-A2 is the most feasible pathway in FF formation with overall energy barrier of 100.3 kcal/mol, in which FF derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 . The formation of FF from glucose is highly endothermic by 47.9 kcal/mol because of the cleavage of strong C C bond and C OH bond. Fig. 5 illustrates the possible pathways that lead to the formation of FF via 3-DG intermediate which was identified in the pyrolysis of glucose [18]. In Path-B1 and Path-B2, 3-DG firstly undergoes
hydrogenation-cyclization (TS-B2) to form P-B1. In Path-B1, P-B1 undergoes 1,2 dehydration between 3-H and 2-OH (TS-B3) and subsequent grob-fragmentation between 4-OH and 6-hydroxymethyl (TS-A10, same as in Path-A) to produce FF. However, In Path-B2, P-B1 firstly undergoes 1,2 dehydration between 2-H and 3-OH (TS-B4) which has relatively lower activation energy than TS-B3, 56.6 and 62.8 kcal/mol respectively (Fig. 6). The subsequent grobfragmentation (TS-B7) of P-B4 produces FF with activation energy of 44.0 kcal/mol. Thus, Path-B2 is more kinetically favored than Path-B1. Path-B3 is initiated by 1,2 dehydration between 3-H and 4-OH (TS-B5) to produce P-B2 and subsequent hydrogenationcyclization (TS-B6) of P-B2 produces P-B4, which is kinetically disfavored compared with Path-B2 due to the high activation energy of TS-B5, 62.2 kcal/mol. In Path-B4, 3-DG decomposes to P-B3 through grob fragmentation (TS-B8) with activation energy of 63.8 kcal/mol. This step is highly endothermic by 37.2 kcal/mol and thermodynamically uphill by 12.6 kcal/mol, which indicates the reaction is both kinetically and thermodynamically disfavored. The overall energy barrier of Path-B4 is 108.5 kcal/mol. Therefore, Path-B2 should be the most kinetically favored in Path-B with over-
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Fig. 5. Elementary reactions for the conversion of glucose to FF via 3-DG intermediate.
Fig. 6. Energy profiles of FF formation pathways via 3-DG intermediate (unit: kcal/mol).
all energy barrier of 87.2 kcal/mol (TS-B1). In this process FF derives from C1 –C5 of glucose and the aldehyde group in FF forms at C1 . Path-C describes the formation of FF via intermediate that competes with the formation of fructose and 3-DG and involves different cyclic mechanism. The detailed elemental reactions in Path-C and energy profiles are shown in Figs. 7 and 8, respectively. In Path-C1, P-C undergoes dehydration between 2-H and 3-OH (TS-C1) and subsequent cyclodehydration (TS-C2) to form FF with overall energy barrier of 119.9 kcal/mol. Path-C2 contains the same elementary reactions with Path-C1, but with different order and higher overall energy barrier, 122.8 kcal/mol. The rate-determining steps are cyclodehydrations for Path-C1 and Path-C2. In Path-C3, P-C undergoes dehydration between H atom of 5-OH and 3-OH (TSC4) to form P-C3 with activation energy as low as 30.8 kcal/mol. This is attributed to less distorted six-membered transition state. Then P-C3 converts to P-C4 through hydrogenation-cyclization which has much lower activation energy than cyclodehydrations in PathC1 and Path-C2. Thus, hydrogenation-cyclization should be the possible cyclic mechanism in FF formation. Then P-C4 decomposes to FF through dehydration between 2-H and 5-OH. According to the energy profiles in Fig. 8, Path-C3 should be the most feasible path-
way with overall energy barrier of 107.7 kcal/mol in Path-C and FF derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 . However, Path-C3 is less kinetically favored than Path-A2 and Path-B2. Through analysis of energy profiles of several formation pathways of FF derived from the C1 –C5 of glucose and with the aldehyde formed at C1 . It is found that Path-B2 is the most kinetically favored pathway with overall energy barrier of 87.2 kcal/mol. What’s more, each step of Path-B2 is energetically feasible. This indicates that the 3-DG could be the intermediate that leads to the formation of FF, which is in accordance with experimental results that 3-DP was an intermediate in the pyrolysis of glucose [20,21]. The conversion of glucose to fructose is kinetically favored. Nonetheless, the subsequent conversion of fructose to FF is less kinetically favored than that of 3-DG. Path-D (Fig. 9) illustrates the possible pathways that lead to the formation of FF with the aldehyde group formed at C5 . Firstly, P-D undergoes enol-keto tautomerization to form P-D1 with activation energy of 48.2 kcal/mol. This reaction is thermodynamically downhill by 9.6 kcal/mol, which indicates the thermodynamically stability of keto form [34]. Then P-D2 is formed via retro-aldol con-
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Fig. 7. Elementary reactions for the conversion of glucose to FF in Path-C.
Fig. 8. Energy profiles of pathways for the conversion of glucose to FF in Path-C (unit: kcal/mol).
Fig. 9. Elementary reactions for the formation of FF from C1 –C5 of glucose and with the C5 aldehyde group.
densation which is proposed as the major reaction for the formation of small aldehydes [35,36]. This reaction requires activation energy of 33.4 kcal/mol and is thermodynamically uphill by 20.7 kcal/mol. Then P-D2 goes through hydrogenation-cyclization (TS-D3) to form P-D3 in Path-D1, or decomposes to P-D4 through 1,2 dehydration (TS-D5) in Path-D2. Both P-D3 and P-D4 can decompose to P-
D5 though 1,2 dehydration (TS-D4) and hydrogenation-cyclization (TS-D6), respectively. Then P-D5 decomposes to FF through dehydration between H atom of OH and 1-OH (TS-D7) with relatively lower activation energy of 41.6 kcal/mol duo to the less distorted six-membered transition state, and meanwhile the aldehyde group is formed at C5 of FF. According to the energy profiles in Fig. 10,
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Fig. 10. Energy profiles of pathways for the formation of FF from C1 –C5 of glucose and with the C5 aldehyde group (unit: kcal/mol).
Path-D2 is more kinetically favored than Path-D1 and with overall energy barrier of 113.7 kcal/mol. However, the formation of FF with C1 aldehyde group (Path-B2) should be more preferential than that FF with C5 aldehyde group due to the relatively lower overall energy barrier. Furthermore, 13 C isotope labeling pyrolysis of glucose indicated that the formation of FF derived from the C2 –C6 of glucose, which was reflected by the incorporation of C6 in FF [22]. Of course, two orientational possibilities that the aldehyde group formed at C2 or C6 of glucose can occur. The formation of aldehyde group at C6 position should arise from dehydration between 6-H and 5-OH to form enol and subsequent enol-keto tautomerization. The detailed elementary reactions are shown in Fig. 11. Firstly, P-E converts to P-E1 through enol-keto tautomerization and this reaction is thermodynamically downhill by 6.9 kcal/mol. This further confirms that the keto form is more stable than the enolic form. Then P-E1 can decompose to FF by the two alternatives pathways. In Path-E1, P-E1 decomposes to P-E2 through 1,2 dehydration between 4-H and 3OH (TS-E2), then P-E4 is formed through hydrogenation-cyclization (TS-E4) of P-E2, and subsequent 1,2 dehydration and tautomerization leading to the formation of P-E8. Then P-E8 decomposes to FF through TS-E10 in which 5-H shifts to C1 and meanwhile C1 –C2 bond cleavages to form FF and CH2 O. Path-E2 is initiated by conversion of P-E1 to P-E3 through hydrogenation-cyclization (TS-E3), then P-E3 decomposes to FF through elementary reactions similar to that in Path-E1. According to the energy profiles in Fig. 12, it can be concluded that Path-E2 is more kinetically favored and with overall energy barrier of 97.5 kcal/mol. Path-F (Fig. 13) illustrates the formation of FF derived from C2 –C6 of glucose and the aldehyde group of FF formed at C2 . In PathF1, P-F converts to P-F1 through hydrogenation-cyclization (TS-F1). Then, the C1–C2 bond cleavages through grob-fragmentation (TSF2) to produce P-F2 with activation energy of 38.8 kcal/mol, P-F2 decomposes to P-F4 through dehydration between H atom of 2-OH and 4-OH, and meanwhile the aldehyde is formed at C2 . This reaction requires activation energy of 35.2 kcal/mol and is thermodynamically downhill by 9.7 kcal/mol. Then P-F4 decomposes to FF through 1,2 dehydration with activation energy of 56.6 kcal/mol. In Path-F2, firstly, the C1–C2 bonds cleavages through retro-aldol condensation (TS-F3) to form P-F3 with activation energy of 33.1 kcal/mol and this reaction is thermodynamically uphill by 20.9 kcal/mol. Then P-F3 decomposes to P-F2 through
cyclodehydration (TS-F4) with activation energy of 75.6 kcal/mol in Path-F2. Therefore, cyclodehydration is less kinetically favored than hydrogenation-cyclization (TS-F1) in ring-closing mechanism. In Path-F3, P-F3 firstly tautomerizes to P-F5, then P-F5 decomposes to FF through cyclodehydration and subsequent 1,2 dehydration. According to the energy profiles in Fig. 14, several steps of Path-F3 is highly energy demanded and overall energy barrier of Path-F3 is as high as 120.3 kcal/mol, therefore, Path-F1 is more kinetically favored with overall energy barrier of 112.5 kcal/mol. Path-G also induces the formation of FF containing C2 –C6 of glucose and with the aldehyde group formed at C2 as shown in Fig. 15. Firstly, P-G undergoes enol-keto tautomerization to form P-G1, then P-G1 decomposes into P-G5 through two pathways Path-G1 and Path-G2. According to the energy profiles in Fig. 16, Path-G2 is more energetically favored than Path-G1with energy barriers of 108.1 and 119.3 kcal/mol, respectively. Then P-G5 decompose into FF through Path-G4 and Path-G2 with different sequential of same elemental reactions, and with energy barriers of 112.5 and 108.5 kcal/mol, respectively. Thus, Path-G2 is more feasible than Path-G4. In Path-G3, P-G2 undergoes 1,2 dehydration (TS-G3) and subsequent four-membered TS-G4 to form P-G7 and CH2 O with energy barrier of 121.3 kcal/mol, which is less kinetically favored than Path-G2 that with energy barrier of 104.1 kcal/mol. The ratedetermining steps for three pathways in Path-G are the elimination of C1 , which involves four-membered transition states with high energy barriers. Path-G2 is the most feasible pathway for the formation of FF in Path-G with overall energy barrier of 108.5 kcal/mol. However, both Path-G2 and Path-F1 have high energy barriers, especially for Path-G2, each step has a higher activation energy. Through analysis of the possible pathways of the formation of FF, it is seen that the formation of FF involves the different carbon sources and orientations of the aldehyde group. The results suggest that Path-B2 is the most kinetically favored pathway with overall energy barrier of 87.2 kcal/mol and 3-DG could be the important intermediate in the formation of FF, in which FF drives from C1 –C5 of glucose and with the C1 aldehyde group. The isolation of 3-DG were reported in the pyrolysis of glucose [20,21]. Path-D2 is the possible pathway that leads to the formation of FF derived from C1 –C5 of glucose and with the C5 aldehyde group, and the overall energy barrier is 113.7 kcal/mol. Therefore, the formation of FF with the C1 aldehyde group is much more kinetically favored than FF with the C5 aldehyde group, which is consistent with experimen-
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Fig. 11. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the C6 aldehyde group.
Fig. 12. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the C6 aldehyde group (unit: kcal/mol).
Fig. 13. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-F.
tal result that the production of FF with the C1 aldehyde group is dominant over that of FF with the C5 aldehyde group [22]. The formation of FF deriving from C2 –C6 of glucose and with the C6 and C2 aldehyde group are discussed in Path-E, Path-F and Path-G, respectively. The results indicate that the formation of FF derived from C2 –C6 of glucose is less kinetically favored than that derived from C1 –C5 , which is consistent with experimental result that the incorporation of C6 into FF was a minor proportion [22]. Furthermore, it is found that HMF decomposes to FF is energetically disfavored, which is attributed to the distorted four-membered transition state
and consistent with experimental results that the production of FF is neglectable in the pyrolysis of HMF [15,16]. This indicates that the formation of HMF and FF is in a competitive manner in the pyrolysis of glucose. Previous studies [27,37] suggested that homolysis and heterolysis are highly energetically disfavored compared with concerted reactions. Therefore, the six-membered concerted grob fragmentation could be an important pathway for the cleavage of C C bond in the formation of FF duo to the relatively lower activation energy.
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Fig. 14. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 (unit: kcal/mol).
Fig. 15. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-G.
Fig. 16. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-G (unit: kcal/mol).
Additional, the 13 C isotope labeling pyrolysis of glucose indicated the possibility that FF arisen from bimolecular reactions [22], such as radical coupling reactions. Given the complex of radical
coupling reactions and minor proportion of FF arisen from bimolecular reactions, only the unimolecular decomposition pathways are considered in this study.
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4. Conclusions In order to understand the formation mechanism of FF and indentify the important intermediate in the pyrolysis of glucose, the possible formation pathways involving different intermediates, carbon sources of FF, cyclizations, and dehydroxymethylations are investigated by DFT method at M062X level. The following conclusions can be obtained. (1) The formation of HMF and FF is in a competitive manner in the pyrolysis of glucose. (2) The 3-DG could be the intermediate in the conversion of glucose to FF, which undergoes hydrogenation-cyclization, 1,2 dehydration between 3-H and 4-OH and grob fragmentation to form FF. (3) The six-membered grob-fragmentation plays a significant role in the formation of FF because of its relative low activation energy for the elimination of hydroxymethyl group. (4) FF is mostly derived from C1 C5 of glucose and with the aldehyde group formed at C1 . Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51576019 and 51506013) and Chongqing university postgraduates’ innovation project (CYB15016). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2016.06.019. References [1] D. Shen, S. Gu, Bioresour. Technol. 100 (2009) 6496. [2] J.N. Chheda, Y. Román-Leshkov, J.A. Dumesic, Green Chem. 9 (2007) 342. [3] C. Rong, X. Ding, Y. Zhu, Y. Li, L. Wang, Y. Qu, X. Ma, Z. Wang, Carbohydr. Res. 350 (2012) 77. [4] S. Wang, B. Ru, H. Lin, Z. Luo, Bioresour. Technol. 143 (2013) 378. [5] K. Werner, L. Pommer, M. Broström, J. Anal. Appl. Pyrolysis 110 (2014) 130. [6] S. Wang, B. Ru, H. Lin, W. Sun, Fuel 150 (2015) 243. [7] S. Wang, B. Ru, G. Dai, W. Sun, K. Qiu, J. Zhou, Bioresour. Technol. 190 (2015) 211.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Theoretical investigation on the carbon sources and orientations of the aldehyde group of furfural in the pyrolysis of glucose Meng Wang a , Chao Liu a,∗ , Xiaoxiao Xu a , Qibin Li b a Key laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400044, China b College of Aerospace Engineering, Chongqing University, Chongqing 400044, China
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 19 June 2016 Accepted 25 June 2016 Available online 27 June 2016 Keywords: Furfural Glucose Density functional theory Carbon sources Orientations of the aldehyde group
a b s t r a c t Furfural (FF) is a valuable and characteristic product of glucose pyrolysis, but little attention has been focused on its formation mechanism. In this study, density functional theory (DFT) methods were employed to investigate the possible pathways leading to the formation of FF from glucose. The results indicate that the pyrolysis of hydroxymethyl furfural (HMF) to FF is hardly to occur duo to the high activation energy (86.3 kcal/mol) required. The dominant FF formation pathway is Path-B2 which involves the 3-deoxy-glucose (3-DG) intermediate with overall energy barrier of 87.2 kcal/mol. The major proportions of FF are derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 is dominant over that at C5 . FF involving the C2 –C6 of glucose, which include two orientations (C2 and C6 ) of the aldehyde group only account for a minor proportion. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Thermochemical conversion of lignocellulosic biomass to biooil and high value chemicals is a hot issue in recent years. Many important chemicals such as glycolaldehyde, glyceraldehyde, acetone, 5-hydroxymethyl-furfural and furfural are indentified in the pyrolysis of cellulose [1]. Furfural is a highly versatile and key derivative widely used in oil refining, plastic, pharmaceutical and agrochemical industries [2,3]. Generally, furfural is considered as a characteristic product of the hemicellulose pyrolysis [4–7]. However, the high production of furfural was also reported in the pyrolysis of cellulose [1,8] and the production of HMF and FF was higher from glucose than from cellubiose and cellulose [9]. Therefore, it is suggested that low-polymerized sugars are more easily pyrolysis to furans, principally 5-hydroxymethyl furfural and furfural. Considering the abundance of cellulose in nature world [10], insight into the formation mechanism of FF will benefits the optimization of selective pyrolysis techniques and improvement of FF production. Early, it was suggested that FF was arisen from the further pyrolysis of levoglucosan (LG) which was the main product of the depolymerization of cellulose and dehydration of glucose unit
∗ Corresponding author. E-mail address: [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jaap.2016.06.019 0165-2370/© 2016 Elsevier B.V. All rights reserved.
[11]. However, in view of thermal stability of LG [12], the conversion of LG to FF could be a minor pathway. Lu et al. reported a very low yield of FF in the pyrolysis of LG at 600 ◦ C and suggested that FF and LG were formed in a competitive manner in the pyrolysis of cellulose [13]. More generally, it was suggested that FF was produced from HMF via the elimination of hydroxymethyl group of HMF [14]. Nonetheless, thermal decomposition of HMF conducted by Nikolov and Yaylayan revealed that main products were 5-methyfurfural, 2,5-furandicarboxaldehyde, and dimer [15], which indicates that the elimination of hydroxymethyl group is a minor route in the pyrolysis of 5-HMF. This is further confirmed by experimental and theoretical study by Shin et al., in which 5methyfurfural, 2,5-furandicarboxaldehyde were main products in the pyrolysis of 5-HMF and no FF was detected [16]. Theoretical study on pyrolysis mechanism of glucose indicated that concerted elimination of hydroxymethyl group of HMF resulted in FF and aldehyde required activation energy as high as 86.2 kcal/mol [17]. These results indicate that HMF and FF are formed through competitive pyrolysis reactions rather than sequential pyrolysis reactions in the pyrolysis of glucose. However, much less attention has been given to the formation mechanism of FF compared with HMF. It was suggested that the conversion of glucose to furanose structure involves an intermediate [8]. Fructose was generally considered as an intermediate in the formation of HMF [18], and the formation of FF could arise from decarbonylation that competes with the final dehydration
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to form HMF [19]. Another mechanism which involved the intermediate 3-deoxy-glucose (3-DG) was proposed in the formation of FF [20], which was supported by the identification of 3-DG in the pyrolysis of glucose [20] and heat sterilization of peritoneal dialysis (PD) fluids [21]. Recently, 13 C isotope labeling technique was employed to investigate the formation mechanism of furans during glucose pyrolysis and several formation routes of FF were proposed [22]. The experimental results indicated that FF could arise from different carbons of glucose and had different orientations of the aldehyde group. However, there is no consensus on the detailed formation mechanism of FF during glucose pyrolysis. Although enormous effects have been focused on improving the conversion of glucose to HMF [23,24], but the conversion of HMF to FF is difficult according to experimental and theoretical studies [15,16]. Therefore, density functional theory methods are employed to investigate the reaction mechanism for the conversion of glucose to FF in this study. The detailed energetics and reaction barriers are computed and these will provide fundamental information for the design of catalysts for the selective production of FF from glucose. 2. Computational methods All the calculations were conducted with Gaussian 09 suit of programs [25]. The M06-2X functional theory developed by Terhlar’ group [26] with 6–31 G +(d,p) basis set was used to optimize reactants and transition states, which has been shown to accurately estimate energy barriers. Stationary points on the potential energy surface of the reacting system were fully optimized using gradient minimization techniques, followed by evaluating harmonic vibration frequencies to characterize their nature as minima and to provide thermal corrections (vibrational contributions) to enthalpies and free energies which are used to analyze to the kinetics and thermodynamics of reactions in formation of FF [27]. All the reactants were verified to have zero imaginary frequencies, and transition states (TSs) have only one imaginary frequency. The transition states were located initially by QST3 method at low level M062 × 6–31(d,p) and then optimized by TS method with 6–31 + (d,p) basis set. Intrinsic reaction coordinate (IRC) was performed to identify the minimum energy path from a transition state to the two corresponding local minima at the same level. 3. Results and discussion 3.1. Initial elementary reactions of glucose decomposition to FF Based on the discussion above, the formation of FF competes with HMF in the pyrolysis of glucose. Therefore, the possible FF formation pathways analyzed in this study, which contain the mechanisms proposed by previous studies, such as “fructose intermediate” (Path-A), “3-DG intermediate” (Path-B), are shown in Fig. 1. Also, several mechanisms involving different decarbonylations, cyclizations and different carbon source (C1 –C5 and C2 –C6 ) are discussed in Path-C to Path-G. The enthalpies and free energies (in parentheses) of products and transition states are relative to that of glucose in Fig. 1 and following energy profiles. Path-A illustrates the formation pathways of FF via fructose intermediate, which competes with the formation of HMF. Firstly, -d-glucose decomposes into aldose P1 via ring-opening reaction with activation energy of 52.14 kcal/mol, which is in accordance with previous study [28]. Of course, that -d-glucose undergoes dehydrations to form anhydrosugars are also investigated as shown in supporting information Fig-S1. The results indicate that direct dehydrations are kinetically disfavored and ring-opening reaction could be the dominant reaction. The formation of fructose P-A from
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P1 can be achieved through two alternative pathways. In one-step path, P-A is formed via a six-membered ring transition state TS-A in which the hydroxyl hydrogen atom transfers to O1 and meanwhile the hydrogen atom at C2 transfers to C1 with activation energy of 39.3 kcal/mol. In two-step path, P1 tautomerizes to 1,2 enediol intermediate through TS-A1 with a large activation energy of 69.9 kcal/mol and this reaction is thermodynamically uphill by 11.1 kcal/mol. This indicates that the tautomerization is both kinetically and thermodynamically disfavored. Then 1,2 enediol converts to P-A. The two-step path has higher activation energy than that of one-step path, which could be ascribed to the more sterically hindered four-membered ring TSs. Thus, the formation of fructose is probably via one-step path which involves 1,2 H transfer under vacuum pyrolysis condition. However, it is arguable whether the tautomerization of glucose to fructose involves 1,2 H transfer. Stahlberg et al. [29] proposed that the tautomerization proceeded through 1,2 enediol intermediate in imidazolium-based ionic liquids with boric acid as a promoter, which did not involved 1,2 H transfer and was evident by deuterium-labeling study. However, it was recently reported that [30] AlCl3 -catalyzed conversion of glucose to fructose involved 1,2 H transfer and confirmed by deuterium labeling study. Furthermore, glucose isomerase catalyzed tautomerization was reported reacted via 1,2 H transfer mechanism [31]. These indicate that the mechanism of conversion of glucose to fructose is notably catalyst-dependent. The subsequent reactions leading to the formation of FF are discussed in the next part. Path-B illustrates the formation of FF via 3-DG which is proposed as the intermediate for formation of FF in the pyrolysis of glucose [20]. 3-DG can be formed via IM1 which arisen from dehydration between H of 1-OH and 3-OH of 1,2 enediol with low activation enthalpy of 30.9 kcal/mol duo to the less distorted six-memebered TS-A3. In addition, P1 can convert to IM1 through directly 1,2 dehydration with activation enthalpy of 55.8 kcal/mol. Thus, P-B is possibly formed from IM1 which arisen from 1,2- dehydration of P1. Path-C to G contain several pathways that lead to the formation of FF, which compete with the formation of fructose and 3-DG in the pyrolysis of glucose. Path-C is initiated by grob fragmentation which was proposed to explain the pyrolysis of glycerol and resulted in the formation of formaldehyde and H2 O [32]. The six-membered ring grob fragmentation has a relatively lower activation energy (64.8 kcal/mol) for the cleavage of strong C C bond compared with the four-membered transition state in previous study [17], which could be a possible pathway for the elimination of hydroxymethyl in FF formation. Path-D is initiated by 1,2 dehydration between 4-H and 3-OH with activation energy of 73.8 kcal/mol, which illustrates the formation of FF with the aldehyde group derived from C5 of glucose. Path-E, Path-F and Path-G illustrate the formation of FF derived from the C2 –C6 of glucose by the elimination of C1 . Path-E is initiated by 1,2 dehydration between 5-H and 4-OH with activation energy of 71.7 kcal/mol to form P-E. Then P-E decomposes to FF and the aldehyde group is formed at C6, which are discussed in the next part. Both Path-F and Path-G illustrate the formation of FF with the aldehyde group formed at C2 . Path-F is initiated by the formation of P-F via multi-hydrogen transfer (TSF) with activation energy of 50.3 kcal/mol. The higher activation energy of TS-F than similar TS-A may be attributed to more strained geometry of TS-F (Fig. 2). Path-G is initiated by 1,2 dehydration between 3-H and 4-OH with activation energy of 76.9 kcal/mol, and this pathway illustrates a different mechanism for the elimination of C1 to Path-F. 3.2. Detailed formation pathways of FF from glucose The detailed reactions that lead to the formation of FF via fructose are illustrated in Fig. 3 and the energy profiles are illus-
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Fig. 1. Initial elementary reactions of FF formation from glucose, enthalpies and free energies (in parentheses) are relative to that of glucose (unit: kcal/mol).
Fig. 2. Transition states for initial reactions of glucose decomposition to FF.
trated in Fig. 4. Firstly, P-A converts to fructofuranose P-A1 though hydrogenation-cyclization and then P-A1 decomposes into FF that proposed for HMF formation in reference [33]. Then HMF decom-
poses to FF by the elimination of hydroxymethyl via distorted four-membered transition state (TS-A12) with activation energy as high as 86.3 kcal/mol. Thus, that the conversion of HMF to FF is
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Fig. 3. Elementary reactions for the conversion of glucose to FF via fructose intermediate.
Fig. 4. Energy profiles of FF formation pathways via fructose intermediate (unit: kcal/mol).
hardly to occur, which is consistent with the pyrolytic results of HMF[15,16]. In Path-A2, P-A6 directly decomposes into FF through less distorted six-membered grob-fragmentation (TS-A10) which leads to the production of H2 O, CH2 O and FF with much lower activation energy (63.6 kcal/mol) compared with TS-A12. In Path-A3, P-A1 decomposes into P-A3 through TS-A5 with energy barrier of 60.3 kcal/mol. P-A can also decompose into P-A3 through grobfragmentation (TS-A13) with relatively higher activation energy (69.4 kcal/mol) and subsequent hydrogenation-cyclization in PathA4. Thus, Path-A3 is more kinetically favored than Path-A4. Then P-A3 decomposes to FF through stepwise dehydrations and the rate-determining step is 1,2 dehydration between 1-H and 2-OH (TS-A6). Through analyzing the energy profiles of Path-A, Path-A2 is the most feasible pathway in FF formation with overall energy barrier of 100.3 kcal/mol, in which FF derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 . The formation of FF from glucose is highly endothermic by 47.9 kcal/mol because of the cleavage of strong C C bond and C OH bond. Fig. 5 illustrates the possible pathways that lead to the formation of FF via 3-DG intermediate which was identified in the pyrolysis of glucose [18]. In Path-B1 and Path-B2, 3-DG firstly undergoes
hydrogenation-cyclization (TS-B2) to form P-B1. In Path-B1, P-B1 undergoes 1,2 dehydration between 3-H and 2-OH (TS-B3) and subsequent grob-fragmentation between 4-OH and 6-hydroxymethyl (TS-A10, same as in Path-A) to produce FF. However, In Path-B2, P-B1 firstly undergoes 1,2 dehydration between 2-H and 3-OH (TS-B4) which has relatively lower activation energy than TS-B3, 56.6 and 62.8 kcal/mol respectively (Fig. 6). The subsequent grobfragmentation (TS-B7) of P-B4 produces FF with activation energy of 44.0 kcal/mol. Thus, Path-B2 is more kinetically favored than Path-B1. Path-B3 is initiated by 1,2 dehydration between 3-H and 4-OH (TS-B5) to produce P-B2 and subsequent hydrogenationcyclization (TS-B6) of P-B2 produces P-B4, which is kinetically disfavored compared with Path-B2 due to the high activation energy of TS-B5, 62.2 kcal/mol. In Path-B4, 3-DG decomposes to P-B3 through grob fragmentation (TS-B8) with activation energy of 63.8 kcal/mol. This step is highly endothermic by 37.2 kcal/mol and thermodynamically uphill by 12.6 kcal/mol, which indicates the reaction is both kinetically and thermodynamically disfavored. The overall energy barrier of Path-B4 is 108.5 kcal/mol. Therefore, Path-B2 should be the most kinetically favored in Path-B with over-
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Fig. 5. Elementary reactions for the conversion of glucose to FF via 3-DG intermediate.
Fig. 6. Energy profiles of FF formation pathways via 3-DG intermediate (unit: kcal/mol).
all energy barrier of 87.2 kcal/mol (TS-B1). In this process FF derives from C1 –C5 of glucose and the aldehyde group in FF forms at C1 . Path-C describes the formation of FF via intermediate that competes with the formation of fructose and 3-DG and involves different cyclic mechanism. The detailed elemental reactions in Path-C and energy profiles are shown in Figs. 7 and 8, respectively. In Path-C1, P-C undergoes dehydration between 2-H and 3-OH (TS-C1) and subsequent cyclodehydration (TS-C2) to form FF with overall energy barrier of 119.9 kcal/mol. Path-C2 contains the same elementary reactions with Path-C1, but with different order and higher overall energy barrier, 122.8 kcal/mol. The rate-determining steps are cyclodehydrations for Path-C1 and Path-C2. In Path-C3, P-C undergoes dehydration between H atom of 5-OH and 3-OH (TSC4) to form P-C3 with activation energy as low as 30.8 kcal/mol. This is attributed to less distorted six-membered transition state. Then P-C3 converts to P-C4 through hydrogenation-cyclization which has much lower activation energy than cyclodehydrations in PathC1 and Path-C2. Thus, hydrogenation-cyclization should be the possible cyclic mechanism in FF formation. Then P-C4 decomposes to FF through dehydration between 2-H and 5-OH. According to the energy profiles in Fig. 8, Path-C3 should be the most feasible path-
way with overall energy barrier of 107.7 kcal/mol in Path-C and FF derived from C1 –C5 of glucose and the aldehyde group in FF formed at C1 . However, Path-C3 is less kinetically favored than Path-A2 and Path-B2. Through analysis of energy profiles of several formation pathways of FF derived from the C1 –C5 of glucose and with the aldehyde formed at C1 . It is found that Path-B2 is the most kinetically favored pathway with overall energy barrier of 87.2 kcal/mol. What’s more, each step of Path-B2 is energetically feasible. This indicates that the 3-DG could be the intermediate that leads to the formation of FF, which is in accordance with experimental results that 3-DP was an intermediate in the pyrolysis of glucose [20,21]. The conversion of glucose to fructose is kinetically favored. Nonetheless, the subsequent conversion of fructose to FF is less kinetically favored than that of 3-DG. Path-D (Fig. 9) illustrates the possible pathways that lead to the formation of FF with the aldehyde group formed at C5 . Firstly, P-D undergoes enol-keto tautomerization to form P-D1 with activation energy of 48.2 kcal/mol. This reaction is thermodynamically downhill by 9.6 kcal/mol, which indicates the thermodynamically stability of keto form [34]. Then P-D2 is formed via retro-aldol con-
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Fig. 7. Elementary reactions for the conversion of glucose to FF in Path-C.
Fig. 8. Energy profiles of pathways for the conversion of glucose to FF in Path-C (unit: kcal/mol).
Fig. 9. Elementary reactions for the formation of FF from C1 –C5 of glucose and with the C5 aldehyde group.
densation which is proposed as the major reaction for the formation of small aldehydes [35,36]. This reaction requires activation energy of 33.4 kcal/mol and is thermodynamically uphill by 20.7 kcal/mol. Then P-D2 goes through hydrogenation-cyclization (TS-D3) to form P-D3 in Path-D1, or decomposes to P-D4 through 1,2 dehydration (TS-D5) in Path-D2. Both P-D3 and P-D4 can decompose to P-
D5 though 1,2 dehydration (TS-D4) and hydrogenation-cyclization (TS-D6), respectively. Then P-D5 decomposes to FF through dehydration between H atom of OH and 1-OH (TS-D7) with relatively lower activation energy of 41.6 kcal/mol duo to the less distorted six-membered transition state, and meanwhile the aldehyde group is formed at C5 of FF. According to the energy profiles in Fig. 10,
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Fig. 10. Energy profiles of pathways for the formation of FF from C1 –C5 of glucose and with the C5 aldehyde group (unit: kcal/mol).
Path-D2 is more kinetically favored than Path-D1 and with overall energy barrier of 113.7 kcal/mol. However, the formation of FF with C1 aldehyde group (Path-B2) should be more preferential than that FF with C5 aldehyde group due to the relatively lower overall energy barrier. Furthermore, 13 C isotope labeling pyrolysis of glucose indicated that the formation of FF derived from the C2 –C6 of glucose, which was reflected by the incorporation of C6 in FF [22]. Of course, two orientational possibilities that the aldehyde group formed at C2 or C6 of glucose can occur. The formation of aldehyde group at C6 position should arise from dehydration between 6-H and 5-OH to form enol and subsequent enol-keto tautomerization. The detailed elementary reactions are shown in Fig. 11. Firstly, P-E converts to P-E1 through enol-keto tautomerization and this reaction is thermodynamically downhill by 6.9 kcal/mol. This further confirms that the keto form is more stable than the enolic form. Then P-E1 can decompose to FF by the two alternatives pathways. In Path-E1, P-E1 decomposes to P-E2 through 1,2 dehydration between 4-H and 3OH (TS-E2), then P-E4 is formed through hydrogenation-cyclization (TS-E4) of P-E2, and subsequent 1,2 dehydration and tautomerization leading to the formation of P-E8. Then P-E8 decomposes to FF through TS-E10 in which 5-H shifts to C1 and meanwhile C1 –C2 bond cleavages to form FF and CH2 O. Path-E2 is initiated by conversion of P-E1 to P-E3 through hydrogenation-cyclization (TS-E3), then P-E3 decomposes to FF through elementary reactions similar to that in Path-E1. According to the energy profiles in Fig. 12, it can be concluded that Path-E2 is more kinetically favored and with overall energy barrier of 97.5 kcal/mol. Path-F (Fig. 13) illustrates the formation of FF derived from C2 –C6 of glucose and the aldehyde group of FF formed at C2 . In PathF1, P-F converts to P-F1 through hydrogenation-cyclization (TS-F1). Then, the C1–C2 bond cleavages through grob-fragmentation (TSF2) to produce P-F2 with activation energy of 38.8 kcal/mol, P-F2 decomposes to P-F4 through dehydration between H atom of 2-OH and 4-OH, and meanwhile the aldehyde is formed at C2 . This reaction requires activation energy of 35.2 kcal/mol and is thermodynamically downhill by 9.7 kcal/mol. Then P-F4 decomposes to FF through 1,2 dehydration with activation energy of 56.6 kcal/mol. In Path-F2, firstly, the C1–C2 bonds cleavages through retro-aldol condensation (TS-F3) to form P-F3 with activation energy of 33.1 kcal/mol and this reaction is thermodynamically uphill by 20.9 kcal/mol. Then P-F3 decomposes to P-F2 through
cyclodehydration (TS-F4) with activation energy of 75.6 kcal/mol in Path-F2. Therefore, cyclodehydration is less kinetically favored than hydrogenation-cyclization (TS-F1) in ring-closing mechanism. In Path-F3, P-F3 firstly tautomerizes to P-F5, then P-F5 decomposes to FF through cyclodehydration and subsequent 1,2 dehydration. According to the energy profiles in Fig. 14, several steps of Path-F3 is highly energy demanded and overall energy barrier of Path-F3 is as high as 120.3 kcal/mol, therefore, Path-F1 is more kinetically favored with overall energy barrier of 112.5 kcal/mol. Path-G also induces the formation of FF containing C2 –C6 of glucose and with the aldehyde group formed at C2 as shown in Fig. 15. Firstly, P-G undergoes enol-keto tautomerization to form P-G1, then P-G1 decomposes into P-G5 through two pathways Path-G1 and Path-G2. According to the energy profiles in Fig. 16, Path-G2 is more energetically favored than Path-G1with energy barriers of 108.1 and 119.3 kcal/mol, respectively. Then P-G5 decompose into FF through Path-G4 and Path-G2 with different sequential of same elemental reactions, and with energy barriers of 112.5 and 108.5 kcal/mol, respectively. Thus, Path-G2 is more feasible than Path-G4. In Path-G3, P-G2 undergoes 1,2 dehydration (TS-G3) and subsequent four-membered TS-G4 to form P-G7 and CH2 O with energy barrier of 121.3 kcal/mol, which is less kinetically favored than Path-G2 that with energy barrier of 104.1 kcal/mol. The ratedetermining steps for three pathways in Path-G are the elimination of C1 , which involves four-membered transition states with high energy barriers. Path-G2 is the most feasible pathway for the formation of FF in Path-G with overall energy barrier of 108.5 kcal/mol. However, both Path-G2 and Path-F1 have high energy barriers, especially for Path-G2, each step has a higher activation energy. Through analysis of the possible pathways of the formation of FF, it is seen that the formation of FF involves the different carbon sources and orientations of the aldehyde group. The results suggest that Path-B2 is the most kinetically favored pathway with overall energy barrier of 87.2 kcal/mol and 3-DG could be the important intermediate in the formation of FF, in which FF drives from C1 –C5 of glucose and with the C1 aldehyde group. The isolation of 3-DG were reported in the pyrolysis of glucose [20,21]. Path-D2 is the possible pathway that leads to the formation of FF derived from C1 –C5 of glucose and with the C5 aldehyde group, and the overall energy barrier is 113.7 kcal/mol. Therefore, the formation of FF with the C1 aldehyde group is much more kinetically favored than FF with the C5 aldehyde group, which is consistent with experimen-
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Fig. 11. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the C6 aldehyde group.
Fig. 12. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the C6 aldehyde group (unit: kcal/mol).
Fig. 13. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-F.
tal result that the production of FF with the C1 aldehyde group is dominant over that of FF with the C5 aldehyde group [22]. The formation of FF deriving from C2 –C6 of glucose and with the C6 and C2 aldehyde group are discussed in Path-E, Path-F and Path-G, respectively. The results indicate that the formation of FF derived from C2 –C6 of glucose is less kinetically favored than that derived from C1 –C5 , which is consistent with experimental result that the incorporation of C6 into FF was a minor proportion [22]. Furthermore, it is found that HMF decomposes to FF is energetically disfavored, which is attributed to the distorted four-membered transition state
and consistent with experimental results that the production of FF is neglectable in the pyrolysis of HMF [15,16]. This indicates that the formation of HMF and FF is in a competitive manner in the pyrolysis of glucose. Previous studies [27,37] suggested that homolysis and heterolysis are highly energetically disfavored compared with concerted reactions. Therefore, the six-membered concerted grob fragmentation could be an important pathway for the cleavage of C C bond in the formation of FF duo to the relatively lower activation energy.
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Fig. 14. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 (unit: kcal/mol).
Fig. 15. Elementary reactions for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-G.
Fig. 16. Energy profiles of pathways for the formation of FF from C2 –C6 of glucose and with the aldehyde group formed at C2 in Path-G (unit: kcal/mol).
Additional, the 13 C isotope labeling pyrolysis of glucose indicated the possibility that FF arisen from bimolecular reactions [22], such as radical coupling reactions. Given the complex of radical
coupling reactions and minor proportion of FF arisen from bimolecular reactions, only the unimolecular decomposition pathways are considered in this study.
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4. Conclusions In order to understand the formation mechanism of FF and indentify the important intermediate in the pyrolysis of glucose, the possible formation pathways involving different intermediates, carbon sources of FF, cyclizations, and dehydroxymethylations are investigated by DFT method at M062X level. The following conclusions can be obtained. (1) The formation of HMF and FF is in a competitive manner in the pyrolysis of glucose. (2) The 3-DG could be the intermediate in the conversion of glucose to FF, which undergoes hydrogenation-cyclization, 1,2 dehydration between 3-H and 4-OH and grob fragmentation to form FF. (3) The six-membered grob-fragmentation plays a significant role in the formation of FF because of its relative low activation energy for the elimination of hydroxymethyl group. (4) FF is mostly derived from C1 C5 of glucose and with the aldehyde group formed at C1 . Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51576019 and 51506013) and Chongqing university postgraduates’ innovation project (CYB15016). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2016.06.019. References [1] D. Shen, S. Gu, Bioresour. Technol. 100 (2009) 6496. [2] J.N. Chheda, Y. Román-Leshkov, J.A. Dumesic, Green Chem. 9 (2007) 342. [3] C. Rong, X. Ding, Y. Zhu, Y. Li, L. Wang, Y. Qu, X. Ma, Z. Wang, Carbohydr. Res. 350 (2012) 77. [4] S. Wang, B. Ru, H. Lin, Z. Luo, Bioresour. Technol. 143 (2013) 378. [5] K. Werner, L. Pommer, M. Broström, J. Anal. Appl. Pyrolysis 110 (2014) 130. [6] S. Wang, B. Ru, H. Lin, W. Sun, Fuel 150 (2015) 243. [7] S. Wang, B. Ru, G. Dai, W. Sun, K. Qiu, J. Zhou, Bioresour. Technol. 190 (2015) 211.
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