Insight into the Formation of Anhydrosugars in Glucose Pyrolysis: A

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Insight into the formation of anhydrosugars in glucose pyrolysis: a joint computational and experimental investigation Bin Hu, Qiang Lu, Xiao-yan Jiang, Xiao-chen Dong, Min-Shu Cui, Changqing Dong, and Yongping Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01250 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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Energy & Fuels

Insight into the formation of anhydrosugars in glucose pyrolysis: a joint computational and experimental investigation Bin Hu, Qiang Lu*, Xiao-yan Jiang, Xiao-chen Dong, Min-shu Cui, Chang-qing Dong, Yong-ping Yang

National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China

KEYWORDS Anhydrosugars, Mechanisms, Pyrolysis, Glucose, Anomeric effects ABSTRACT

Fast

pyrolysis

of

glucose/cellulose

will

produce

abundant

1,6-anhydro-β-D-glucopyranose (known as levoglucosan, LG) as the predominant anhydrosugar product, together with certain amounts of other anhydrosugars, mainly including

1,6-anhydro-β-D-glucofuranose

(AGF),

1,4:3,6-dianhydro-α-D-glucopyranose

(DGP),

1,5-anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose

(APP),

1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC). The formation mechanisms of the latter four anhydrosugar products are not well known at present. Substantiated by analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments, quantum chemistry calculations are employed to give a deep insight into their formation mechanisms in this study. The anomeric effect of glucopyranose has a distinct effect to their formation. DGP mainly derives from α-D-glucopyranose which plays a vital role in the formation of 1,4-acetal ring. On the contrary, APP and LAC 1

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can merely derive from β-D-glucopyranose due to the dehydration at 1-OH + 2-H site. APP is important but not necessary to form LAC, and their formation pathways also compete with each other. Moreover, AGF is not influenced by the anomeric effect of the two anomers, because its formation relies on the acyclic D-glucose.

INTRODUCTION Fast pyrolysis is a clean and efficient utilization technique of biomass, which has attracted much attention in recent years. Cellulose is usually the most abundant component of lignocellulose biomass. Its fast pyrolysis will undergo depolymerization, fragmentation, dehydration and many other reactions to form various pyrolytic products including anhydrosugars, furans, linear carbonyls, and so on1-3. During the non-catalytic fast pyrolysis process, anhydrosugar products are usually the most abundant under medium pyrolysis temperatures, dominated by LG, together with certain

amounts

of

AGF,

DGP,

APP,

LAC,

1,6-anhydro-3,4-dideoxy-β-D-pyranosen-2-one (known as levoglucosenone, LGO), etc.4-6. These anhydrosugars are useful compounds that can be widely used in energy, chemical and medical industries7. Previous studies have confirmed that pyrolytic and catalytic conditions would significantly affect the distribution of anhydrosugars6-8. Therefore, thorough insight into the formation mechanisms of these anhydrosugars is of great significance to develop or optimize their selective preparation techniques. In the past decades, various investigations on the formation of LG and LGO have been reported9-13, but limited studies are available on the formation mechanisms of AGF, DGP, APP and LAC, especially based on quantum chemistry methods.

2


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AGF is an isomer of LG in furanose form. The key step for AGF formation from glucose/cellulose is the transformation of pyranose skeleton into furanose structure. Byrne et al.14 and Gardiner15 proposed that cellulose firstly broke hemiacetal bond to form a 1,2-anhydroglucose intermediate, which would then generate the glucofuranose unit to produce AGF. However, there were no direct evidences for the formation of 1,2-anhydrohexose. Differently, Fukutome et al.16 regarded that reducing sugar (acyclic D-glucose) would be formed during cellulose pyrolysis, and the chain structure made the transformation from pyranose to furanose possible. Seshadri et al.17 and Mayes et al.18 also favored that acyclic D-glucose was the bridge between pyranose and furanose in glucose pyrolysis. Moreover, both of them studied the AGF formation pathway in which glucopyranose transformed into glucofuranose involving acyclic

D-glucose

as the intermediate at quantum chemistry level. In addition,

Gardiner15 and Shafizadeh et al.19, 20 pointed that the glucose unit could directly form the furanose structure with the formation of 1,4-anhydroglucose, which might be a vital intermediate for the formation of both AGF and DGP in cellulose pyrolysis. DGP is a polycyclic anhydrosugar with 1,4-acetal, 1,5-acetal and 3,6-acetal rings. Shafizadeh et al.21 firstly reported the formation of DGP from acid-catalyzed pyrolysis of cellulose. They proposed a possible formation pathway for DGP in which 1,4-acetal bond was formed directly from the glucose unit, as stated above20. Differently, Ohnishi et al.22 put forward that the formation of 3,6-acetal bond before 1,4-acetal bond would be more feasible for the formation of DGP, because 3,6-anhydroglucose would provide a rigid B1,4 configuration which was benefit for the formation of 1,4-acetal ring.

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APP was firstly identified as a pyrolytic product of cellulose by Shafizadeh and coworkers23. In the proposed APP formation pathway, glucose unit of cellulose eliminated glucosidic bonds substituent on C1, C4 firstly and then underwent enol-keto tautomerization of 3-OH. APP has been considered as the essential precursor of LAC which was firstly separated and identified from acid-catalyzed pyrolytic products of cellulose24. And an essential intermediate of LAC was regarded to be the diketone isomer of APP according to previous literatures24, 25. Recently, Marforio et al.26 deepened our knowledge of the LAC formation pathways from APP at quantum chemistry level. However, further studies are needed to certify that whether LAC can be generated from pathways without APP as the intermediate. So far, the formation mechanisms of the four anhydrosugars from pyrolysis of cellulose are still unclear. Studies at quantum chemistry level by density functional theory27 (DFT) are powerful to give a deep insight into the possible biomass pyrolytic pathways28-31. Selection of appropriate model compounds is necessary for the mechanism study of cellulose pyrolysis, because the real structure of cellulose is too enormous to be calculated efficiently. Typically, β-D-Glucopyranose is widely selected as the model compound of cellulose12, 28, 32, since it is the basic unit of cellulose33 and has similar pyrolytic product distribution to cellulose34. Additionally, it is worth noting that the anomeric effect which leads to co-existence of β-D-glucopyranose and α-D-glucopyranose during pyrolysis process, has a vital influence on the formation of anhydrosugars18,

35, 36

. Recently, Mayes et al.18 certified the existence of a

quasi-equilibrium process between β-D-glucopyranose and α-D-glucopyranose under pyrolytic conditions based on experimental and computational analyses, and further confirmed that the anomeric effect would result in different pyrolytic pathways of β-D-glucopyranose and α-D-glucopyranose. Hence, both β-D-glucopyranose and 4


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α-D-glucopyranose were selected to conduct quantum chemistry calculation in this work. Meanwhile, fast pyrolysis experiments were performed to substantiate the theoretical calculations. The joint computational and experimental investigation aims to clearly reveal the following three questions: a. How glucose forms these anhydrosugars in pyrolysis process; b. How the anomeric effect influences the formation of the four anhydrosugars; c. Whether APP is necessary or not for the formation of LAC. COMPUTATIONAL DETAILS All calculations were conducted by using Gaussian 09 rev D. 0137. The classical functional and basis set of B3LYP38 and 6-31G(d) were selected to initially optimize the structures of the reactants, intermediates, transition states and products. B3LYP is universal in different fields and can save resources and time. However, with the development of new calculation methods39,

40

, the precision of B3LYP is not

satisfactory in estimating energy barriers. Therefore, Thurlar’s functional of M06-2X41, which was reported to have an accurate estimation of energy barriers42, was used to re-optimize the above structures. And the 3-zeta basis set of 6-311+G(d,p) was selected accordingly, adding diffuse and polarization functions to heavy atoms but only polarization function to hydrogen. The above combined methods to gain the optimized structures of all compounds can save an enormous amount of time compared with direct optimization at the M06-2X/6-311+G(d,p) level. Next, frequency analyses43 were performed by using M06-2X/6-311+G(d,p). All the transition states were confirmed as the first order saddle point on the potential energy surface (PES) with exactly one imaginary frequency. And other compounds were the minimum points on the PES with no imaginary frequency. Furthermore, the 5


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transition states and corresponding minimum points were verified on the same PES by conducting intrinsic reaction coordinate (IRC) analyses44. Polarized continuum model45 (PCM) was used in all the calculations to stimulate the electrostatic environment in the pyrolysis process, because the primary pyrolytic reactions took place in the condensed phase during glucose/cellulose fast pyrolysis process46, 47. Mayes et al.11, 18 have excellently revealed the formation mechanisms of LG and 5-hydromethylfurfural (5-HMF) by using an implicit solvent model. Herein, we employed the same PCM implicit solvent model (ethanol) which was finely selected by Mayes’s group18. In addition, biomass fast pyrolysis is usually carried out at medium temperatures (500 °C typically). Hence, thermodynamic parameters were calculated at 500 °C which was the pyrolysis temperature employed in the present experiments. Based on the quantum chemistry calculation, 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 3D images and cartesian coordinates of the optimized structures for all compounds are shown in the supporting information (Figures S1-S5 and Section 3). EXPERIMENTAL SECTION The experimental materials used in the present study were three glucose-based carbohydrates, i.e.

D-(+)-glucose

(≥ 99.5%, GC, Aladdin),

D-(+)-cellobiose

(98%,

Aladdin) and microcrystalline cellulose (Avicel PH-101, Sigma). D-(+)-glucose was a mixture of α-D-glucopyranose and β-D-glucopyranose. Previous studies have confirmed that the product distributions of α-D-glucopyranose and β-D-glucopyranose pyrolysis were almost the same due to the quasi-equilibrium of the two anomers during pyrolysis18. Therefore, it is reasonable to omit the anomers ratio of the 6


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D-(+)-glucose

used here, although both anomers were studied by quantum chemistry

calculation. Analytical Py-GC/MS experiments were performed with the CDS Pyroprobe 5200HP pyrolyzer (Chemical Data Systems) connected with the Perkin Elmer GC/MS (Clarus 560). The sample weight in each experiment was strict to be 0.20 mg. Fast pyrolysis was conducted at 500 °C for 20 s with a heating rate of 20 °C/ms. Pyrolysis vapors were online analyzed by GC/MS. A capillary column (Elite-35MS, 30 m × 0.25 mm i.d., 0.25 mm film thickness) was used in the GC to separate the pyrolytic products. The GC oven program was set as follow: heating from 40 °C to 280 °C with a heating rate 15 °C/min, holding at 280 °C for 2 min. MS was in the electron ionization (EI) mode (70 eV) with a scan range of 20-400 AMU. The organic volatile compounds were identified by using the NIST MS database and literature data6. For both glucose and cellulose, the experiments were repeated several times. For each identified compound, its peak area and peak area percentage (peak area%) values were recorded, and its average and standard deviation values for the peak area and peak area% were also calculated. RESULTS AGF formation mechanism The formation of AGF requires the scission of 1,5-acetal bond as well as the linkages of 1,4-acetal and 1,6-acetal bonds. Among them, the formation of 1,4-acetal bond for a furanose structure is the key step to produce AGF. Hence, based on the different furanose intermediates, the formation mechanism of AGF can be classified into two categories, i.e., “via glucofuranose” mechanism (Paths GA1, GA2 and GA3) and “via 1,4-anhydroglucose” mechanism (Path GA4), with the possible pathways 7


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shown in Figure 1. The energy diagram of these pathways is illustrated in the supporting information (Figure S6). As shown in Figure 1, Path GA1 (in red) is one of the possible “via glucofuranose” pathways with β-D-glucofuranose (GA1-i1) as the intermediate17, 18. In this pathway, both β-D-glucopyranose and α-D-glucopyranose undergo ring-opening reactions via TSβG and TSαG respectively to form acyclic

D-glucose.

The two reactions have

similar activation free energies (△G†, 189.4 vs 190.4 kJ/mol), which indicates that the quasi-equilibrium of the two anomers via acyclic pyrolysis conditions18. Afterwards, acyclic

D-glucose

D-glucose

is reasonable under

forms the 1,4-acetal ring to

generate β-D-glucofuranose (GA1-ts1, △G† =183.1 kJ/mol). Both formations of acyclic D-glucose and β-D-glucofuranose are thermodynamically uphill (△Grxn =11.0 and 5.4 kJ/mol, respectively). Then β-D-glucofuranose directly forms AGF via the formation of 1,6-acetal ring. This reaction requires a relatively low activation free energy of 202.8 kJ/mol and is thermodynamically downhill by -58.1 kJ/mol, accompanied by the removal of 1-OH. Notably, the formation of 1,6-acetal ring can also be realized by the removal of 6-OH. However, due to the high activation free energy (△G† = 313.4 kJ/mol), removal of 6-OH is not favorable in this pathway. Corresponding structure of the transition state is shown in the supporting information (Figure S1). Besides the formation of β-D-glucofuranose in Path GA1, acyclic D-glucose can also form intermediate α-D-glucofuranose (GA2-i1) through the acetalization simultaneously, as shown in Path GA2 (in blue). Different from β-D-glucofuranose in Path GA1, α-D-glucofuranose firstly forms a carbene structure for AGF formation in Path GA218. In this reaction, the OH and H at C1 position of α-D-glucofuranose attract

8


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each other to dehydrate, resulting in the formation of the carbene structure GA2-i2. The free energy of the reactions to generate GA2-i2 is astonishing (△Grxn = 110.4 kJ/mol), which indicates the formation of carbene (GA2-i2) is extremely thermodynamically unfavorable. However, the carbene, if it can be formed, will easily generate AGF with a low activation free energy (△G† =129.8 kJ/mol). And the reaction is thermodynamically downhill by -165.3 kJ/mol. Similarly, the calculation results suggest that formation of the unstable carbene structure (GA2-i2) from β-D-glucofuranose is also thermodynamically unfavorable (Path GA3, in pink). Hence both Paths GA2 and GA3 can hardly occur due to the high reaction free energies as well as the unstable characteristic of carbene intermediate GA2-i2. In addition, 1,4-anhydroglucose was considered as an important intermediate for the formation of AGF by Gardiner15 and Shafizadeh et al.19,

20

. Based on their

conjecture, a typical pathway, Path GA4 (in green), involving 1,4-anhydroglucose is schemed in Figure 1. 1,4-anhydroglucose (GA4-i1) can be formed in different ways, which will be discussed in the formation mechanisms of DGP in detail. Path GA4 only

shows

the

α-D-glucopyranose

direct via

formation the

reaction

transition

state

of

1,4-anhydroglucose

GA4-ts1.

This

from

reaction

is

thermodynamically downhill by -29.4 kJ/mol with an activation free energy of 227.1 kJ/mol. It is obvious that the formation of AGF from 1,4-anhydroglucose will firstly break the 1,5-acetal ring. In this reaction, H at C1 shifts to O5, resulting in the formation of the carbene GA2-i2. This reaction is extremely thermodynamically uphill (△Grxn =156.6 kJ/mol) and needs to overcome a high activation free energy (△G† = 309.2 kJ/mol), suggesting this pathway is quite unfavorable for AGF formation.

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Comparing the above four pathways, three of them involve the carbene GA2-i2, which makes them all unfavorable for the formation of AGF. Hence, AGF should mainly derive from Path GA1 in which the generation of AGF relies on the successive formation of acyclic D-glucose and β-D-glucofuranose.

Figure 1. Possible formation pathways of AGF from glucose pyrolysis. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol. DGP formation mechanism The formation of DGP from glucose needs to form the 1,4-acetal bond involving 1-OH and 4-OH. Generally, the 1,4-acetal bond can be directly formed from glucopyranose or 3,6-anhydroglucose in the ring lock pathways, or generated from acyclic

D-glucose

involving ring-opening and ring-forming reactions. The direct

formation of 1,4-acetal bond from glucopyranose is vitally influenced by the anomeric effect. Hence, the DGP formation mechanism is classified into three categories based on the possible intermediates which result in different formation patterns of 1,4-acetal

10


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bond, i.e., “via α-D-glucopyranose” mechanism (Paths GB1 and GB2), “via β-D-glucopyranose” mechanism (Paths GB3 and GB4) and “via β-D-glucofuranose” mechanism (Path GB5). Notably, the formation of DGP is a ring lock process in both “via α-D-glucopyranose” and “via β-D-glucopyranose” mechanisms, while it involves the ring-opening reaction in the “via β-D-glucofuranose” mechanism. The possible pathways in the three mechanisms and the energy diagram are shown in Figure 2 and supporting information (Figure S7), respectively. The formation of β-D-glucofuranose from both materials (α-D-glucopyranose and β-D-glucopyranose) can be found in Figure 1. As shown in Figure 2, DGP can be produced by two possible “via α-D-glucopyranose” pathways (Paths GB1 and GB2) with different sequences of the formation of 1,4-acetal ring and 3,6-acetal ring. In Path GB1 (in red), 1,4-acetal ring is formed accompanied with dehydration between 1-OH and 4-OH. 1-OH is easier to be removed than 4-OH in this reaction by comparing the activation free energies (△G†, 227.1 vs 298.6 kJ/mol). Afterwards, DGP is formed from 1,4-anhydroglucose (GA4-i1) with the formation of 3,6-acetal ring. Similarly, both 3-OH and 6-OH can be removed in this reaction. The calculation results imply that the removal of 6-OH is easier than that of 3-OH (△G†, 294.2 vs 326.3 kJ/mol). In Path GB2 (in blue), 3,6-acetal ring is formed earlier than 1,4-acetal ring. Notably, all the reactions in the two pathways are thermodynamically downhill, implying that the two pathways are all thermodynamically favored. However, the activation free energies to form 3,6-acetal ring are quite high, especially when 3,6-acetal ring is formed firstly. Hence, Path GB1 is more favorable than Path GB2 in this mechanism.

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Possible pathways in “via β-D-glucopyranose” mechanism are affected by anomeric effect, as schemed in Paths GB3 (in pink) and GB4 (in green). In Path GB3, 1,4-acetal ring is formed before 3,6-acetal ring. Different from α-D-glucopyranose, β-D-glucopyranose must firstly form a carbene structure GB3-i1 with the removal of OH and H at C1 position for the generation of 1,4-anhydroglucose, due to the anomeric effect. This reaction is thermodynamically uphill (△Grxn = 123.9 kJ/mol) and requires a relatively high activation free energy (△G† = 278.8 kJ/mol). Then GB3-i1 cyclizes into 1,4-anhydroglucose, which finally forms DGP via the formation of 3,6-acetal bond. The carbene GB3-i1 is very unstable, and its BO,3 configuration is more favorable for the formation of LG (1,6-acetel ring)18. In Path GB4, the 3,6-acetal ring is formed firstly. Similarly, this reaction also favors the transition state removing 6-OH instead of 3-OH (△G†, 340.6 vs 372.9 kJ/mol). Both GB4-i1 and GB2-i1 are 3,6-anhydroglucose, but GB4-i1 must firstly form the carbene intermediate GB4-i2 to produce DGP due to the anomeric effect. The formation of carbene makes both pathways unfavorable for the generation of DGP. In the above two mechanisms, DGP is formed via the ring lock processes. In addition, the “via β-D-glucofuranose” pathway which involves a ring-opening reaction is studied in the present work, as schemed in Path GB5 (in purple) in Figure 2. In this pathway, β-D-glucofuranose (GA1-i1) which derives from the acetalization of acyclic D-glucose

(refer to Figure 1), undergoes acetalization reaction to form 1,5-acetal ring

to produce 1,4-anhydroglucose (GA4-i1). This reaction requires a relatively low activation free energy (△G† = 216.0 kJ/mol) and is thermodynamically downhill (△Grxn = -49.4 kJ/mol). Finally, 1,4-anhydroglucose forms DGP following the reaction in Path GB1.

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Based on the above calculation results, Paths GB1 and GB5 have obvious advantages over other pathways. However, the two pathways are competing with the pathways to form AGF (Path GB1 vs Path GA4, Path GB5 vs Path GA1). Comparing these pathways, it can be concluded that 1,4-anhydroglucose (GA4-i1) is more favorable for the formation of DGP (Path GB1 vs Path GA4), and β-D-glucofuranose (GA1-i1) is more feasible to produce AGF (Path GB5 vs Path GA1). Therefore, Path GB1 is the main channel for the formation of DGP, which exactly implies the vital role that anomeric effect plays in the formation of DGP.

Figure 2. Possible formation pathways of DGP from glucose pyrolysis. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol. APP formation mechanism APP has a simple structure with C1=C2 and C3=O bonds which result from two dehydration reactions (1-OH + 2-H and 4-OH + 3-H) and one enol-keto 13


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tautomerization. Based on the different sequences of the three reactions, possible formation pathways of APP are schemed in Figure 3. Notably, α-D-glucopyranose cannot dehydrate at 1-OH + 2-H site, because 1-OH and 2-H are in the opposite side of the ring for α-anomer. Hence, Figure 3 only shows the pathways in which APP derives from the decomposition of β-D-glucopyranose. The energy diagram of these pathways is illustrated in the supporting information (Figure S8). In Path GC1 (in red), β-D-glucopyranose successively undergoes dehydration at 1-OH + 2-H site (GC1-ts1, △G† = 238.2 kJ/mol), 4-OH + 3-H site (GC1-ts2, △G† = 291.7 kJ/mol) to form diene intermediate GC1-i2. Then GC1-i2 tautomerizes into APP via transition site GC1-ts3 with an activation free energy of 220.4 kJ/mol. In Paths GC2 (in blue) and GC3 (in pink), β-D-glucopyranose firstly dehydrate at 4-OH + 3-H site (GC2-ts1, △G† = 305.9 kJ/mol). Then GC2-i1 undergoes tautomerization (GC2-ts2, △G† = 243.3 kJ/mol) in Path GC2, or undergoes dehydration at 4-OH + 3-H site (GC3-ts2, △G† = 229.6 kJ/mol) in Path GC3. All these reactions in the three pathways are thermodynamically downhill, suggesting the formation of APP is thermodynamically favored at 500 °C. In addition, APP is more stable than its diene isomer (GC1-i2) and its diketone isomer (GD1-i1, vide infra), which means APP is a thermodynamically stable compound. The stability attributes to the conjugated enone structure and the hydrogen bond between 2-hydroxyl group and 3-carbonyl group23, 24.

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Figure 3. Possible formation pathways of APP from glucose pyrolysis. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol. It can be seen that dehydration at 4-OH + 3-H site in all the three pathways requires a high activation free energy, and is the rate-determining step for the formation of APP. Among the three pathways, Path GC1, in which β-D-glucopyranose initially dehydrates at 1-OH + 2-H site, is more competitive than the other two pathways, due to the relatively low activation free energy of the rate-determining step. LAC formation mechanism LAC has a complex structure as compared to the other three anhydrosugars. It was considered to be derived from APP via benzylic rearrangement in previous studies24-26. But no convincing proofs have been found to support that APP is the only intermediate for LAC during pyrolysis of cellulose. Hence, the formation mechanisms of LAC are divided into two categories as shown in Figures 4 and 5, based on whether its formation involves APP as the intermediate. The energy diagrams of these pathways are illustrated in the supporting information (Figures S9 and S10).

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Figure 4 shows the possible pathways of LAC through intermediate APP. Both Paths GD1 (in red) and GD2 (in blue) involve the diketone isomer of APP (GD1-i1), as proposed in previous studies24-26. The formation of GD1-i1 requires a high activation free energy (△G† = 307.3 kJ/mol) and is thermodynamically uphill (△Grxn = 13.3 kJ/mol). GD1-i1 then forms hemiketal GD1-i2 (Path GD1) or hemiketal GD2-i2 (Path GD2) via cyclization. The two reactions are thermodynamically uphill (△Grxn = 6.2 and 17.2 kJ/mol) and have close activation free energies (△G† = 206.6 and 199.7 kJ/mol). GD1-i2 can directly generate LAC with a relatively low activation free energy (△G† = 177.9 kJ/mol). This reaction is thermodynamically downhill by -48.9 kJ/mol. Differently, GD2-i2 forms LAC via an essential isomerization to GD1-i2. This reaction is thermodynamically downhill (△Grxn = -10.9 kJ/mol) and requires a quite low activation free energy (△G† = 137.4 kJ/mol). Notably, both Paths GD1 and GD2 involve the diketone isomer of APP (GD1-i1). However, the formation of GD1-i1 from APP is unfavorable both in thermodynamics (△Grxn > 0) or kinetics (△G† = 307.3 kJ/mol), which is not beneficial for the formation of LAC. Hence, another possible pathway (Path GD3, in pink) involving an initial feasible cyclization is studied, as shown in Figure 4. In this pathway, APP firstly undergoes cyclization to form hemiketal GD3-i1. This reaction has an advantage over the formation of GD1-i1 in kinetics with an activation free energy of 241.4 kJ/mol. Then GD3-i1 tautomerizes into GD2-i2 thermodynamically downhill by -31.3 kJ/mol and with an activation free energy of 263.3 kJ/mol. Finally, GD2-i2 forms LAC through successive isomerization and benzylic rearrangement. Compared with the other three pathways, Path GD3 is more favorable for the formation of LAC involving APP as the intermediate.

16


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Figure 5 shows the reaction network for the formation of LAC without involving APP. Because of the anomeric effect, LAC can only derive from β-D-glucopyranose. For all the pathways in Figure 5, β-D-glucopyranose initially involves the dehydration at 1-OH + 2-H site or 4-OH + 3-H site, with the formation of intermediate GC1-i1 and GC2-i1 respectively. For GC1-i1, it can form LAC following the reactions in Path GE1 (in red) or Path GE3 (in pink). For GC2-i1 it can produce LAC through the reactions in Path GE2 (in blue) or Path GE6 (in purple). In Path GE1, intermediate GE1-i2 may alternatively undergo the reaction in Path GE4 (in gray) to form intermediate GE2-i3 which then form LAC following the reactions in Path GE2 (in blue) or Path GE5 (in green). For GE2-i4 (GD1-i1), it can also undergo reaction in Path GE7 (in black) to form intermediate GD2-i2 which produces LAC by successive isomerization and benzylic rearrangement. Among these pathways in Figure 5, Paths GE2, GE3, GE4, GE6 and GE7 involve the formation of intermediates GD1-i1, GD2-i2 and (or) GD3-i1, which lead to the formation of LAC following the pathways in Figure 4. In terms of Paths GE1 and GE5, both of them result in the formation of GD1-i2 which is the precursor of LAC. Overall, all the seven pathways will form crucial intermediates for the formation of LAC without involving APP. Comparing the seven pathways, Path GE5 is more feasible to generate LAC with a relatively low activation free energy of the rate determining step. In summary, paths GD3 and GE5 are feasible for the formation of LAC with or without APP as the intermediate respectively, which implies that the formation of LAC does not completely rely on the formation of APP.

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Figure 4. Possible formation pathways of LAC involving APP as the intermediate in glucose pyrolysis. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol.

Figure 5. Possible formation pathways of LAC without involving APP as the intermediate in glucose pyrolysis. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol.

18


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Fast pyrolysis experimental results In order to further confirm the calculation results, non-catalytic analytical fast pyrolysis experiments of glucose were conducted. Meanwhile, to analyze the influence of glycosidic bonds on the formation of the four anhydrosugars, fast pyrolysis experiments of cellobiose and cellulose were also carried out. Typical total ion chromatograms are shown in Figure 6. Results suggest that fast pyrolysis of glucose, cellobiose and cellulose shared similar pyrolytic product distribution but different product concentrations. Table 1 shows the peak area% values of the six major anhydrosugar products, and results of other pyrolytic products are given in the supporting information (Table S1). LG was the predominant anhydrosugar (also the most abundant pyrolytic product) from the three glucose-based carbohydrates. With the increase of the degree of polymerization, the peak area% values of LGO, APP and LAC increase, while for AGF and DGP, the trends are opposite. Similar results were also observed by Patwardhan et al.5 and Zhou et al.34 Combining the experimental results and the structural characteristics of the three materials, it can be deduced that the formation mechanisms of the four anhydrosugars are similar in the pyrolysis of the three glucose-based carbohydrates. The different peak area% values should be attributed to the influence of glycosidic bonds.

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Figure 6. Total ion chromatograms of glucose, cellobiose and cellulose pyrolysis at 500 °C. 1 hydroxyacetaldehyde (HAA); 2 hydroxyacetone (HA); 3 furfural (FF); 4 LGO; 5 LAC; 6 DGP; 7 5-HMF; 8 APP; 9 LG; 10 AGF. Table 1. Main anhydrosugars in the pyrolysis of glucose, cellobiose and cellulose (peak area%)

Compounds

Glucose

Cellobiose

Cellulose

LG

36.30±1.62

41.16±1.44

43.70±2.57

LGO

0.29±0.01

0.98±0.01

1.17±0.23

AGF

8.61±1.91

2.54±0.77

2.13±0.36

DGP

1.78±0.22

1.42±0.02

1.11±0.17

APP

1.14±0.24

3.24±0.16

5.23±0.45

LAC

1.00±0.05

1.87±0.11

3.25±0.76

DISCUSSION Influence of anomeric effect on the formation of the four anhydrosugars 20


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Among the four anhydrosugars, AGF is the only compound whose formation is not influenced by the anomeric effect. Basically, the formation of AGF requires the transformation from pyranose to furanose. Acyclic D-glucose was believed as the key for the transformation by Fukutome and co-workers16, which is exactly accordance with the calculation results in the present work. As illustrated in Figure 1, AGF mainly derives from Path GA1 which is the typical “via glucofuranose” pathway. In this pathway, acyclic D-glucose is easy to form β-D-glucofuranose which then produces AGF through acetalization. The formation of acyclic D-glucose is not affected by the anomeric effect. Therefore, the acyclic structure, instead of the anomeric effect has a vital influence on the formation of AGF. According to the experiment results (Table 1), fast pyrolysis of cellobiose and cellulose results in lower peak area% values of AGF than that of glucose pyrolysis, which should be attributed to the fact that glucose may generate acyclic

D-glucose

more easily than cellobiose and cellulose34,

48

. Since

cellulose contains numerous glycosidic bonds, its fast pyrolysis results in the lowest peak area% value of AGF. Thus both experimental and theoretical studies imply the importance of the acyclic structure for the formation of AGF. Based on the above analyses, a key point to increase the yield of AGF is to promote the depolymerization of cellulose, thus broaden the access to the formation of β-D-glucofuranose. Different from AGF, the formation of DGP is seriously influenced by the anomeric effect for the generation of 1,4-acetal ring. Path GB1, in which 1,4-acetal ring derives from α-D-glucopyranose, is the predominant channel for the formation of DGP. Cellulose and cellobiose consist of glucose units linked by β-1,4-glycosidic bonds. Hence, they cannot produce the α-anomers as many as glucose does in the fast pyrolysis process48. Therefore, cellulose produces less DGP than glucose (Table 1). It also confirms the importance of the α-anomers for the formation of DGP. In addition, 21


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1,4-acetal ring is much easier to be formed than 3,6-acetal ring in all the possible formation pathways of DGP in Figure 2. Moreover, the calculation results suggest the pathway forming 1,4-acetal ring first (Paths GB1 or GB3) is more favorable than the pathway forming 3,6-acetal ring first (Paths GB2 or GB4) in the same mechanism. However, Ohnishi and co-workers22 supported the formation of 3,6-acetal bond before 1,4-acetal bond. They proposed that the formation of 3,6-acetal ring turned glucose into a rigid B1,4 configuration, which favored the formation of 1,4-acetal ring. Besides, Gardiner14

detected

a

large

amount

of

DGP

during

the

pyrolysis

of

3,6-anhydroglucose. According to the calculation results, the formation of 1,4-acetal ring from 3,6-anhydro-α-glucose (GB2-ts2 in Path GB2, △G† = 174.1 kJ/mol) is really easier than that from α-D-glucopyranose (GB1-ts1 in Path GB1, △G† = 227.1 kJ/mol). Therefore, promoting the formation of 3,6-acetal ring, especially before the formation of 1,4-acetal ring deserves further exploitation to increase the yield of DGP. The anomeric effect also has a prominent influence on the formation of APP and LAC. Both of them can only derive from the β-anomer of glucopyranose, because the necessary dehydration at 1-OH + 2-H site can hardly occur for α-D-glucopyranose. Their formation also involves the dehydration at 4-OH + 3-H site which is the rate determining step for most of the pathways in Figures 3 and 5. In addition, the formation of APP and LAC require ring-locking of the pyranose. According to the study of Wong et al.48, the pyran ring structure is more favorable to be remained due to the presence of glycosidic bond linkage in pyrolysis process. Hence, cellobiose and cellulose pyrolysis produce more APP and LAC than glucose pyrolysis according to the experimental results (Table 1). Therefore, it is meaningful for increasing the yield of APP and LAC to lock the ring and promote the dehydration at 4-OH + 3-H site.

22


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Competitiveness of the formation pathways of the four anhydrosugars Based on the calculation results, the favorable formation pathways of the four anhydrosugars are illustrated in Figure 7, with the pathways for AGF, DGP, LAC and APP indicated by red, blue, purple and green arrows, respectively. As shown in Figure 7, these pathways compete with each other during the pyrolysis process. Firstly, acyclic D-glucose derived from ring-opening reaction is necessary for AGF, whereas, the formation of the other three anhydrosugars is a ring-locking process. Glucopyranose undergoes ring-opening reaction more readily than dehydration or acetalization. Hence, AGF has a higher peak area% value than other three anhydrosugars in the pyrolysis process. Secondly, both AGF and DGP can derive from β-D-glucofuranose via Paths GA1 and GB5 respectively. However, the formation of AGF is more competitive than that of DGP due to the lower activation free energy. Thirdly, the formation of LAC not only involves APP as an intermediate (Path GD3), but also compete with the formation of APP (Path GC1 vs Paths GE3 and GE5) in glucose pyrolysis. As shown in Figure 7, the dehydrated intermediate GC1-i1 can undergo enol-keto tautomerization or dehydration at 4-OH + 3-H site. The enol-keto tautomerization results in the formation of GE1-i2 which then forms LAC following the reactions in Path GE5. The dehydration at 4-OH + 3-H site leads to the formation of GC1-i2, which can form APP (Path GC3) or LAC (Path GE3) at the same time. Additionally, APP can continue to form LAC following the reactions in Path GD3. Overall, LAC is not only a product and but also a competitor of APP.

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Figure 7. Favorable formation pathways of the four anhydrosugars. The free energies of activation (△G†, kJ/mol) and free energies of the reaction (△Grxn, kJ/mol) at 500 °C are calculated with M06-2X/6-311+G(d,p) with implicit ethanol. It is notable that all the above calculations only concentrate on primary pyrolytic reactions of glucose to form anhydrosugars. In fact, pyrolysis is a very complex process, in which besides the primary pyrolytic reactions, secondary reactions are usually ineluctable. During the pyrolysis process, secondary reactions will take place via different ways, i.e., cracking of primary products, interaction reactions among the feedstock and primary products, etc. For example, a considerable amount of water will be produced from the primary pyrolysis process due to the dehydration reactions, which may have a distinct interaction influence on the pyrolytic reactions. Marforio et al.26 concluded that water could lower the energy barriers of the pyrolytic reactions by assisting the proton shift. But the rate-determining reactions remained the same in 24


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Energy & Fuels

both cases with or without water. In addition, Seshadri et al.17 regarded that the pyrolytic reactions of the condensed phase glucose could be catalyzed by nearby hydroxyl groups on adjacent glucoses. All these secondary effects will affect the formation of the four anhydrosugars. In the present study, although we only concentrate on constructing a closed system to study the favorable pathways for anhydrosugars through primary pyrolytic reactions, the quantum chemistry calculation and non-catalytic fast pyrolysis experiment results can well sustain each other. The fundamental results primarily confirm the reliability of our research methodology, and provide the foundation for further theoretical investigation into various secondary effects on the formation of anhydrosugars in our future works. CONCLUSIONS In the present work, fast pyrolysis experiments and quantum chemistry calculations are performed to provide a deep insight on the formation mechanism of four anhydrosugars (AGF, DGP, APP and LAC) in glucose pyrolysis. The two anomers of glucopyranose (α-D-glucopyranose and β-D-glucopyranose) have distinct effect to the formation of the four anhydrosugars. The main formation pathway of AGF demands the appearance of acyclic

D-glucose,

thus will not be influenced by the anomeric

effect. On the contrary, α-D-glucopyranose is necessary for the formation of the 1,4-acetal ring to form DGP. β-D-glucopyranose is essential for the formation of APP and LAC, due to the dehydration at 1-OH + 2-H site. APP is important but not necessary for the formation of LAC, and their formation pathways will compete with each other in the pyrolysis process.

ASSOCIATED CONTENT 25


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Supporting Information Complete ref. 37; 3D images and atomic coordinates for all structures; energy diagrams of all pathways; distribution of major organic products. AUTHOR INFORMATION

Corresponding Authors

*Qiang Lu, Email: [email protected], Tel.:+86 10 61772063;

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (51576064, 51676193), Beijing Nova Program (Z171100001117064), Beijing Natural Science Foundation (3172030), Foundation of Stake Key Laboratory of Coal Combustion (FSKLCCA1706) and Fundamental Research Funds for the Central Universities (2016YQ05, 2015ZZD02) for financial support.

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Table of Contents Graphic:

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Insight into the Formation of Anhydrosugars in Glucose Pyrolysis: A

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