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Levoglucosan formation mechanisms during cellulose pyrolysis
Journal of Analytical and Applied Pyrolysis 104 (2013) 19–27
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Levoglucosan formation mechanisms during cellulose pyrolysis Xiaolei Zhang a,∗ , Weihong Yang a , Changqing Dong b a b
Division of Energy and Furnace Technology, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 9 November 2012 Accepted 26 September 2013 Available online 8 October 2013 Keywords: Levoglucosan Cellulose pyrolysis Density functional theory
a b s t r a c t Levoglucosan is one important primary product during cellulose pyrolysis either as an intermediate or as a product. Three available mechanisms for levoglucosan formation have been studied theoretically in this paper, which are free-radical mechanism; glucose intermediate mechanism; and levoglucosan chain-end mechanism. All the elementary reactions included in the pathway of every mechanism were investigated; thermal properties including activation energy, Gibbs free energy, and enthalpy for every pathway were also calculated. It was concluded that free-radical mechanism has the highest energy barrier during the three levoglucosan formation mechanisms, glucose intermediate mechanism has lower energy barrier than free-radical mechanism, and levoglucosan chain-end mechanism is the most reasonable pathway because of the lowest energy barrier. By comparing with the activation energy obtained from the experimental results, it was also concluded that levoglucosan chain-end mechanism fits better with the experimental data for the formation of levoglucosan. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of biomass or bio-energy offers significant environmental advantages over fossil fuels; they produce fewer carbon dioxide emissions and less environmental pollution, and they can ease the global energy shortage. The technology of pyrolysis is an efficient way to convert biomass into usable energy, such as char from conventional slow pyrolysis and liquids or gases from fast pyrolysis. As the main component of biomass, cellulose pyrolysis plays an important role in the investigation of biomass pyrolysis. It was reported that several important primary products for cellulose pyrolysis, such as levoglucosan, glycolaldehyde, and 5hydroxymethylfurfural. Especially, levoglucosan is one important primary product during cellulose pyrolysis, either as a product [1–4] or as an important intermediate for the formation of other products [5–11]. Shafizadeh et al. [12] reported that cellulose is first decomposed into active cellulose without weight loss; the active cellulose can then be depolymerized into volatiles or polymerized to solid char. This reaction scheme is known as the Broido–Shafizadeh model, where volatiles, including tar (levoglucosan), represent a condensable fraction. The authors concluded that the activation energy for the formation of volatiles in vacumm is 198 kJ/mol. The activation energy for cellulose primary pyrolysis from Milosavljevic and Suuberg [13] and Antal et al. [14] are 218 kJ/mol and 228 kJ/mol,
∗ Corresponding author. Tel.: +46 8 790 85 31. E-mail address: [email protected] (X. Zhang). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.09.015
respectively. Capart et al. [15] studied cellulose pyrolysis in nitrogen and concluded that the activation energy is about 202 kJ/mol for the primary decomposition of cellulose. In the pyrolysis reaction scheme proposed by Banyasz et al. [10], it was concluded that cellulose can either be transferred into char, tar (levoglucosan), or gases (hydroxyacetaldehyde, formaldehyde, and CO) via depolymerizing cellulose. The activation energy for tar calculated in the Banyasz model is 151 kJ/mol. It was also reported that the activation energy of levoglucosan formation from cellulose fast pyrolysis is about 200 kJ/mol from Mamleev et al.’s work [16]. Lin et al. [17] also proposed that levoglucosan is the first resulting anhydromonosaccharide, then levoglucosan can undergo dehydration and isomerization reactions to form other anhydro-monoscaaharides such as dianhydro--d-glucopyranose, levoglucosenone, and 1,6anhydro--d-glucofuranose. Mayes and Broadbelt [18] summarized the formation mechanism of levoglucosan from cellulose, levoglucosan can be formed via glucose intermediate (Fig. 1a); via free-radical mechanism (Fig. 1b); via ionic mechanism (Fig. 1c). Free-radical intermediate mechanism which was proposed by Pakhomov [19], described the cellulose chain will break into dehydroglucose diradicals, then levoglucosan can be formed from these dehydroglucose diradicals. Another radical mechanism for formation of levoglucosan from cellulose was from Shen and Gu’s work [20]. Which proposed that levoglucosan radical with one unpaired electron will be transferred into levoglucosan by reacting with hydroxyl radical. This mechanism is not representative for levoglucosan formation because the limitation of hydroxyl groups existed in biomass pyrolysis process. Instead of radical intermediate, ionic mechanism
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Fig. 1. Main mechanisms for the formation of levoglucosan from cellulose pyrolysis.
[21] shows that the cellulose can produce levoglucosan by an ionic intermediate. Regarding radical cleavage and ionic cleavage, it is proved that under the gas atmosphere, which is the atmosphere for real biomass gasification, ionic cleavage need quite more energy than radical cleavage. Thus the ionic cleavage mechanism will not be considered in this paper. In Irvine and Oldham’s work [22], it was mentioned that levoglucosan can be formed from glucose which was produced by hydration of cellulose chain, this levoglucosan pathway is called glucose intermediate mechanism in this paper. There were also many studies [16,23–26] mentioned the levoglucosan formation via levoglucosan (LG) chain-end mechanism (Fig. 1d). Two transglycosylation steps are included in
the LG chain-end mechanism. During the first transglycosylation step, a cellulose chain is depolymerized into a levoglucosan-end intermediate and a short cellulose chain. During the second transglycosylation step, the levoglucosan-end intermediate which is formed from the first step will be unzipped into a levoglucosan molecule, with another levoglucosan-end intermediate formed by the remaining part. Even Mayes and Broadbelt [18] verified that LG chain-end mechanism has quite low energy barrier by density functional theory (DFT). Until now, there is still no systemic comparison for the cellulose decomposition via glucose, or free-radical intermediate mechanism, or via levoglucosan chain-end mechanism.
Fig. 2. Cellulose homogeneous cleavage at two positions: C1 O position and O C4 position.
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Fig. 3. Energy illustration and geometries change of free-radical mechanism for LG formation.
In this work, we assessed the cellulose cleavage to form levoglucosan via glucose intermediate, free-radical mechanism, and LG chain-end mechanism by density functional theory. The detailed pathways including the geometries and thermal properties of all the involved structures for these mechanisms will be calculated and compared. Theoretical comparison under the same level can help to understand the formation mechanism of levoglucosan formation, and understand the biomass pyrolysis process further. 2. Computational details The optimizations of all the geometries (reactants, products, and transition states) were performed at the DFT/M062X level using the 6-31+G(d,p) basis set. Comparing with the method B3LYP which was used in our previous work [27–30], M062X was proved to be more accurate than B3LYP in many other studies [18,31–33] about carbohydrates systems. The unrestricted open-shell wave function was used in all open-shell cases. Calculations were carried out in the ground state. The transition states were calculated by the TS method and were confirmed by frequency analysis and intrinsic reaction coordinate (IRC) calculations, which were performed at the same basis set of that used for optimizations. The stabilities of all optimized structures were certified by frequency analysis. All the stable structures (reactants, intermediates, and products) have no imaginary frequency, and transition states have exactly one imaginary frequency. All the calculations were performed by the program Gaussian 09 [34]. And all of the calculations were carried out on the high performance computer in PDC at the Royal Institute of Technology (KTH) in Sweden. 3. Results and discussion 3.1. Free-radical mechanism (M1) For the free-radical mechanism (as called M1) for levoglucosan formation during cellulose pyrolysis, the cellulose chain firstly was decomposed into anhydroglucose radical units (as shown in Fig. 1b), then this anhydroglucose radical can be transferred into levoglucosan.
The free-radical can be formed by homogeneous cleavage of a cellulose chain, where bond breaking of -(1-4)-glycosidic linkage with two electrons separate equally. Two cleavage positions (C1 O bond and O C4 bond) in cellulose chain are possible, the relative cleavage pathways are called as C1 O cleavage and O C4 cleavage, respectively. Two types of free-radicals, free-radical A and free-radical B, can be produced, respectively, as shown in Fig. 2. The bond energy for these two bonds C1 O and O C4 are 422 kJ/mol and 426 kJ/mol, respectively, which was calculated by M062X/631+G(3df,2p) after basis set superposition error correction. It can be seen that breaking of these bonds and forming free-radical needs quite high external energy, indicating the difficulty for levoglucosan formation via free-radical mechanism. It can be seen that the C1 O bond is slightly weaker than O C4 bond, the probable reason is the C1 O5 bond linked with O is weaker than the C4 C3 bond on the other side of O atom. The two types of freeradical should be co-existed in the system because of the slightly difference of the bond energy, only 4 kJ/mol. The free-radical structure A formed from the C1 O bond cleavage will be considered in the following study, which is supported by two reasons, one is the slight weakness of C1 O bond than O C4 bond, and another reason is the geometry of free-radical A is easier for the formation of levoglucosan. Once the free radical is formed from cellulose chain cleavage, these radicals can be transferred into levoglucosan. The mechanism for levoglucosan formation from free-radical A was described detailed in our previous work [27]. As shown in Fig. 3, from freeradical A, firstly there is a hydrogen-donor reaction. A hydrogen atom H6c transfers from one oxygen atom O1 to another oxygen atom O. Then, there is a possible reaction because of a strong affinity between the active oxygen atom O1 and the active carbon atom C1. If there is a bond formed between them, product levolgucosan can be formed with a bridged structure. For this bridged structure formation step, the singlet state is more stable than the triplet state, which verifies that there is one bond formed by the two unpaired electrons from intermediate to form the product. The driving forces for this pathway are the two unpaired electrons from the oxygen atom O1 and the carbon atom C1. It can be seen that product levoglucosan is much more stable than the reactant free radical.
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Fig. 5. Two possible positions for the hydrolysis of cellobiosan.
Even the activation energy for the formation of levoglucosan from free-radical is not so high; however, free-radical mechanism needs huge external energy due to the homogeneous cleavage of cellulose chain, which is the determining step for the whole free-radical mechanism. The geometries of the two transition states in free-radical mechanism M1-TS1 and M1-TS2 were shown in Fig. 4. Both M1TS1 and M1-TS2 has exactly one imaginary frequency, which is −1717 icm−1 for M1-TS1, and −88 icm−1 for M1-TS2. The vibration of M1-TS1 is the stretching of hydrogen atom between two oxygen atoms. And for M1-TS2, there is a stretching between the atoms of O1 and C1. M1-TS1 has triplet ground state, and for M1-TS2, the ground state is singlet, indicated that there are two unpaired electrons during the first reaction step, the two unpaired electrons form one bond during the second step. 3.2. Glucose intermediate mechanism (M2)
Fig. 4. Structures of the two transition states during free-radical mechanism. The unit for bond length is Å. (a) Transition state M1-TS1, (b) Transition state M1-TS2.
In the glucose intermediate mechanism (as called M2), cellulose firstly is decomposed into glucose units by hydrolysis, then levoglucosan can be formed from these glucose units by dehydration. For the hydrolysis step, similar with the two cleavage positions for the free-radical mechanism, water can react with cellulose at two possible positions, C1 O bond and O C4 bond as shown in Fig. 5. In this study, we assumed the process into a cellobiosan reacts
Fig. 6. Energy illustration and geometries change for hydrolysis of cellobiosan for formation of two glucose units.
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Fig. 9. Structure of the transition state for formation of levoglucosan during dehydration of glucose. The unit for bond length is Å.
Fig. 7. Structures of two transition states for the hydrolysis of cellobiosan. The unit for bond length is Å. (a) Transition state M2-TS1. (b) Transition state M2-TS1 .
with one water molecule to form two glucose units. The energy illustrations for the reaction at these two positions are shown in Fig. 6. One water molecule reacts with cellobiosan, and try to break the bond C1 O and O C4 respectively. Two glucose units can be produced by both pathways shown in Fig. 6. At position C1 O, one hydroxyl group from water bonds with C1, and the hydrogen atom from water bonds with oxygen atom O, simultaneously with the
breaking of C1 O. Similarly, at position O C4 , hydroxyl group and hydrogen atom from water can form bonds with C4 and oxygen O, respectively, with the breaking of O C4 bond and formation of two glucoses. It can be seen that the energy barrier for hydrolysis at C1 O position is 264 kJ/mol. However, for hydrolysis at O C4 position the energy barrier is 346 kJ/mol, indicating that C1 O is the possible position for the cleavage of C O bond in cellulose chain by hydrolysis. The possible reason is the weaker bond of C1 O than O C4 . However, the C1 O is only 4 kJ/mol slightly weaker than O C4 , so it can be concluded that the energy difference of C1 O and O C4 bond is enlarged when reacting with water rather than
Fig. 8. Energy illustration and geometries change for LG formation via glucose intermediate mechanism.
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Fig. 10. Energy illustration and geometries change for levoglucosan formation via LG chain-end intermediate mechanism.
the direct bond breaking, which indicates that the weakness of a bond can be enlarged during hydrolysis. The structures for the two transition states for the hydration of cellobiosan were shown in Fig. 7. Both of these two transition states have an imaginary frequency, which is −259 icm−1 for M2TS1, and −882 icm−1 for M2-TS1 . For M2-TS1, the vibrations under the imaginary frequency are stretching of the C1 O and O H bonds, with the tendency of new bonds formation between C1 OH and O H. Compared with M2-TS1 , M2-TS1 is a reasonable transition state for hydrolysis due to the lower activation energy. After hydrolysis of cellulose, the glucose unit formed from hydration of cellulose chain can produce levoglucosan by getting rid of water. The energy illustration and geometries changes for the whole glucose mechanism including of the hydrolysis and dehydration steps were shown in Fig. 8. For the hydration of cellulose chain, only the position C1 O was considered here because the lower energy barrier than the position of O C4 . For the dehydration step, it can be seen that a hydrogen atom from O1 and a hydroxyl group from C1 form a water molecule and released further from glucose, and the remained part is levoglucosan with a bond formation between O1 and C1. The activation energy for the dehydration step is 227 kJ/mol, compared with the activation energy of hydrolysis step, 264 kJ/mol, it can be concluded that the hydrolysis step is the rate-determining step for the whole levoglucosan formation process via glucose intermediate mechanism. The structure of the transition state for the dehydration from glucose to levoglucosan was shown in Fig. 9. The imaginary frequency for this transition state is −193 icm−1 , with vibration of stretching between O1 and hydrogen atom, C1 and hydroxyl group. 3.3. LG chain-end mechanism (M3) Levoglucosan formation via LG chain-end intermediate mechanism (as called M3) happens through two transglycosylation steps, the formation of the LG chain-end structure intermediate and the formation of levoglucosan from the LG chain-end structure
intermediate. The energy illustration and geometries change for LG chain-end mechanism were shown in Fig. 10. For the formation of LG chain-end intermediate, there is one transglycosylation elementary reaction. From the structure of M3TS1, we can see that three phenomena are happened during this elementary reaction: firstly the cleavage of the C1 O bond; secondly one hydrogen atom H6c transfers from O1 to the oxygen O; and at last a new bond formed by C1 and O1. Similar processes happen for second step: the formation of levoglucosan from LG chain-end intermediate. The activation energies for the two steps are 195 kJ/mol and 182 kJ/mol, respectively. Which showed the first step is the rate-determining step. The structures for the two transition states for LG chain-end mechanism were shown in Fig. 11. It can be seen that the two transition state has similar bond change. There is a ring-closing process with a hydrogen shifting. The imaginary frequency for these two transition states are −431 icm−1 and −413 icm−1 respectively. 3.4. Comparison of all the calculated mechanisms for levoglucosan formation The energy diagrams for the three mechanisms are shown in Fig. 12, it can be seen that levoglucosan formed from free-radical mechanism is the most difficult pathway due to the high energy barrier for the cleavage of the C O bonds in cellulose chain. The levoglucosan formed via glucose intermediate needs lower activation energy than the free-radical mechanism. However, LG chain-end mechanism gives the lowest energy barrier. The activation energy obtained from the experimental results [10,12,35] are also shown in Fig. 12, it can be seen that the general range for the activation energy obtained from experiments are 151–228 kJ/mol. The energy barrier for free-radical mechanism can be obtained from the energy barrier for direct breaking C1 O bond. The energy barrier for freeradical mechanism is quite high, about 422 kJ/mol. The activation energies for glucose intermediate mechanism and LG chain-end mechanism are 264 kJ/mol and 195 kJ/mol, respectively. It can be
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Fig. 11. Structure of two transition states involved in the LG chain-end intermediate mechanism. The unit for bond length is Å. (a) Transition state M3-TS1. (b) Transition state M3-TS2.
concluded that the activation energy calculated from LG chain-end mechanism is the closest with the experimental result. By comparing the three mechanism and their elementary steps, the most important thermal properties including activation energy, energy change, Gibbs free energy change, Gibbs free energy barrier, and enthalpy change were shown in Table 1. The first column gives the detailed activation energy for every pathway and elementary step, which is same with what can be obtained from Fig. 12. The Ea are listed here just for comparison. The second and fifth columns give the thermal energy changes and enthalpy changes for every pathway and elementary reactions, respectively. It can be seen that no matter which pathway, there is no big difference for the global energy change from the reactant cellulose and product levoglucosan. Take enthalpy for example, the enthalpy change is
106 kJ/mol for free-radical mechanism, 73 kJ/mol for glucose mechanism, and 102 kJ/mol for LG chain-end mechanism. All the thermal energy and enthalpy change shows that it is an endothermic reaction for the formation of levoglucosan from cellulose. The fourth column gives the Gibbs energy barrier for all the pathways, it also shows that LG chain-end mechanism gives lowest energy barrier, which is same trend which was reflected by the activation energy Ea . From the calculation for the three mechanisms for levoglucosan formation, it can be seen that the rate-determining step for free-radical mechanism is the direct breaking of C1 O bond. The rate-determining step for glucose intermediate mechanism is the formation of glucose from cellulose chain, which simultaneously with the breaking of C1 O bond. And for LG chain-end mechanism
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Fig. 12. Comparison of the energy barrier of experiment and different calculated levoglucosan formation mechanisms.
Table 1 Comparison of activation energy (Ea ), energy change (E), Gibbs free energy change (G), Gibbs free energy change between transition state and reactant (G‡ ), and enthalpy change (H) for the three mechanisms for LG formation from cellulose. The unit is kJ/mol. Ea Free-radical mechaniam (M1) Step 1: from cellulose to free-radical Step 2: from free-radical to LG Glucose mechanism (M2) Step 1: from cellulose to glucose Step 2: from glucose to LG LG chain-end mechanism (M3) Step 1: from cellulose to LG chain-end intermediate Step 2: from LG chain-end intermediate to LG
– – 48 264 264 227 195 195 182
for levoglucosan formation, the rate-determining step is the first transglycosylation step with the formation of a LG chain-end intermediate and a glucose, which happens with breaking of C1 O at the same time. So it can be concluded that breaking of C1 O bond is the rate-determining step for levoglucosan step no matter via which mechanism. Free-radical mechanism needs more energy because of the difficulty of breaking C1 O bond directly. Glucose intermediate mechanism needs lower energy than free-radical mechanism because it is easier to break C1 O bond due to the existing of water molecule in the process. And LG chain-end mechanism needs lowest energy barrier because of it is easiest to break the C1 O bond by transglycosylation.
E
G
G‡
H
81 406 −325 71 24 47 97 58 39
28 343 −315 18 14 4 12 6 6
– – 60 307 307 226 193 193 183
84 409 −325 73 24 49 102 60 42
enlarged during the hydrolysis process, the water molecule reacts at C1 O position gives much lower activation energy than that at O C4 position, the activation energy difference is about 78 kJ/mol. The rate-determining step for all these three mechanisms is the one with breaking of C1 O bond. Free-radical mechanism has high energy barrier because of the difficulty of breaking C1 O bond directly. C1 O bond can be easier broken when there exist water molecule to form glucose. During the three mechanisms for levoglucosan, LG chain-end mechanism which happens via two transglycosylation steps is the most reasonable pathway. Acknowledgments
4. Conclusions Three mechanisms for levoglucosan formation during cellulose pyrolysis are studied and compared in this work, which are free-radical mechanism, glucose intermediate mechanism, and LG chain-end mechanism. Firstly, it was concluded that the formation of levoglucosan from cellulose is endothermic no matter via which pathway. In cellulose chain, C1 O bond is slightly weaker than the bond O C4 , with an energy difference of 4 kJ/mol. But this trend can be
Financial support from Vetenskapsrådet (Swedish Research Council) is highly appreciated. One of the authors, Xiaolei Zhang, would also like to acknowledge financial support from the Chinese Scholarship Council (CSC). References [1] D.F. Arseneau, Canadian Journal of Chemistry 49 (1971) 632–638. [2] I. Milosavljevic, V. Oja, E.M. Suuberg, Industrial & Engineering Chemistry Research 35 (1996) 653–662.
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Levoglucosan formation mechanisms during cellulose pyrolysis Xiaolei Zhang a,∗ , Weihong Yang a , Changqing Dong b a b
Division of Energy and Furnace Technology, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 9 November 2012 Accepted 26 September 2013 Available online 8 October 2013 Keywords: Levoglucosan Cellulose pyrolysis Density functional theory
a b s t r a c t Levoglucosan is one important primary product during cellulose pyrolysis either as an intermediate or as a product. Three available mechanisms for levoglucosan formation have been studied theoretically in this paper, which are free-radical mechanism; glucose intermediate mechanism; and levoglucosan chain-end mechanism. All the elementary reactions included in the pathway of every mechanism were investigated; thermal properties including activation energy, Gibbs free energy, and enthalpy for every pathway were also calculated. It was concluded that free-radical mechanism has the highest energy barrier during the three levoglucosan formation mechanisms, glucose intermediate mechanism has lower energy barrier than free-radical mechanism, and levoglucosan chain-end mechanism is the most reasonable pathway because of the lowest energy barrier. By comparing with the activation energy obtained from the experimental results, it was also concluded that levoglucosan chain-end mechanism fits better with the experimental data for the formation of levoglucosan. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of biomass or bio-energy offers significant environmental advantages over fossil fuels; they produce fewer carbon dioxide emissions and less environmental pollution, and they can ease the global energy shortage. The technology of pyrolysis is an efficient way to convert biomass into usable energy, such as char from conventional slow pyrolysis and liquids or gases from fast pyrolysis. As the main component of biomass, cellulose pyrolysis plays an important role in the investigation of biomass pyrolysis. It was reported that several important primary products for cellulose pyrolysis, such as levoglucosan, glycolaldehyde, and 5hydroxymethylfurfural. Especially, levoglucosan is one important primary product during cellulose pyrolysis, either as a product [1–4] or as an important intermediate for the formation of other products [5–11]. Shafizadeh et al. [12] reported that cellulose is first decomposed into active cellulose without weight loss; the active cellulose can then be depolymerized into volatiles or polymerized to solid char. This reaction scheme is known as the Broido–Shafizadeh model, where volatiles, including tar (levoglucosan), represent a condensable fraction. The authors concluded that the activation energy for the formation of volatiles in vacumm is 198 kJ/mol. The activation energy for cellulose primary pyrolysis from Milosavljevic and Suuberg [13] and Antal et al. [14] are 218 kJ/mol and 228 kJ/mol,
∗ Corresponding author. Tel.: +46 8 790 85 31. E-mail address: [email protected] (X. Zhang). 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.09.015
respectively. Capart et al. [15] studied cellulose pyrolysis in nitrogen and concluded that the activation energy is about 202 kJ/mol for the primary decomposition of cellulose. In the pyrolysis reaction scheme proposed by Banyasz et al. [10], it was concluded that cellulose can either be transferred into char, tar (levoglucosan), or gases (hydroxyacetaldehyde, formaldehyde, and CO) via depolymerizing cellulose. The activation energy for tar calculated in the Banyasz model is 151 kJ/mol. It was also reported that the activation energy of levoglucosan formation from cellulose fast pyrolysis is about 200 kJ/mol from Mamleev et al.’s work [16]. Lin et al. [17] also proposed that levoglucosan is the first resulting anhydromonosaccharide, then levoglucosan can undergo dehydration and isomerization reactions to form other anhydro-monoscaaharides such as dianhydro--d-glucopyranose, levoglucosenone, and 1,6anhydro--d-glucofuranose. Mayes and Broadbelt [18] summarized the formation mechanism of levoglucosan from cellulose, levoglucosan can be formed via glucose intermediate (Fig. 1a); via free-radical mechanism (Fig. 1b); via ionic mechanism (Fig. 1c). Free-radical intermediate mechanism which was proposed by Pakhomov [19], described the cellulose chain will break into dehydroglucose diradicals, then levoglucosan can be formed from these dehydroglucose diradicals. Another radical mechanism for formation of levoglucosan from cellulose was from Shen and Gu’s work [20]. Which proposed that levoglucosan radical with one unpaired electron will be transferred into levoglucosan by reacting with hydroxyl radical. This mechanism is not representative for levoglucosan formation because the limitation of hydroxyl groups existed in biomass pyrolysis process. Instead of radical intermediate, ionic mechanism
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Fig. 1. Main mechanisms for the formation of levoglucosan from cellulose pyrolysis.
[21] shows that the cellulose can produce levoglucosan by an ionic intermediate. Regarding radical cleavage and ionic cleavage, it is proved that under the gas atmosphere, which is the atmosphere for real biomass gasification, ionic cleavage need quite more energy than radical cleavage. Thus the ionic cleavage mechanism will not be considered in this paper. In Irvine and Oldham’s work [22], it was mentioned that levoglucosan can be formed from glucose which was produced by hydration of cellulose chain, this levoglucosan pathway is called glucose intermediate mechanism in this paper. There were also many studies [16,23–26] mentioned the levoglucosan formation via levoglucosan (LG) chain-end mechanism (Fig. 1d). Two transglycosylation steps are included in
the LG chain-end mechanism. During the first transglycosylation step, a cellulose chain is depolymerized into a levoglucosan-end intermediate and a short cellulose chain. During the second transglycosylation step, the levoglucosan-end intermediate which is formed from the first step will be unzipped into a levoglucosan molecule, with another levoglucosan-end intermediate formed by the remaining part. Even Mayes and Broadbelt [18] verified that LG chain-end mechanism has quite low energy barrier by density functional theory (DFT). Until now, there is still no systemic comparison for the cellulose decomposition via glucose, or free-radical intermediate mechanism, or via levoglucosan chain-end mechanism.
Fig. 2. Cellulose homogeneous cleavage at two positions: C1 O position and O C4 position.
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Fig. 3. Energy illustration and geometries change of free-radical mechanism for LG formation.
In this work, we assessed the cellulose cleavage to form levoglucosan via glucose intermediate, free-radical mechanism, and LG chain-end mechanism by density functional theory. The detailed pathways including the geometries and thermal properties of all the involved structures for these mechanisms will be calculated and compared. Theoretical comparison under the same level can help to understand the formation mechanism of levoglucosan formation, and understand the biomass pyrolysis process further. 2. Computational details The optimizations of all the geometries (reactants, products, and transition states) were performed at the DFT/M062X level using the 6-31+G(d,p) basis set. Comparing with the method B3LYP which was used in our previous work [27–30], M062X was proved to be more accurate than B3LYP in many other studies [18,31–33] about carbohydrates systems. The unrestricted open-shell wave function was used in all open-shell cases. Calculations were carried out in the ground state. The transition states were calculated by the TS method and were confirmed by frequency analysis and intrinsic reaction coordinate (IRC) calculations, which were performed at the same basis set of that used for optimizations. The stabilities of all optimized structures were certified by frequency analysis. All the stable structures (reactants, intermediates, and products) have no imaginary frequency, and transition states have exactly one imaginary frequency. All the calculations were performed by the program Gaussian 09 [34]. And all of the calculations were carried out on the high performance computer in PDC at the Royal Institute of Technology (KTH) in Sweden. 3. Results and discussion 3.1. Free-radical mechanism (M1) For the free-radical mechanism (as called M1) for levoglucosan formation during cellulose pyrolysis, the cellulose chain firstly was decomposed into anhydroglucose radical units (as shown in Fig. 1b), then this anhydroglucose radical can be transferred into levoglucosan.
The free-radical can be formed by homogeneous cleavage of a cellulose chain, where bond breaking of -(1-4)-glycosidic linkage with two electrons separate equally. Two cleavage positions (C1 O bond and O C4 bond) in cellulose chain are possible, the relative cleavage pathways are called as C1 O cleavage and O C4 cleavage, respectively. Two types of free-radicals, free-radical A and free-radical B, can be produced, respectively, as shown in Fig. 2. The bond energy for these two bonds C1 O and O C4 are 422 kJ/mol and 426 kJ/mol, respectively, which was calculated by M062X/631+G(3df,2p) after basis set superposition error correction. It can be seen that breaking of these bonds and forming free-radical needs quite high external energy, indicating the difficulty for levoglucosan formation via free-radical mechanism. It can be seen that the C1 O bond is slightly weaker than O C4 bond, the probable reason is the C1 O5 bond linked with O is weaker than the C4 C3 bond on the other side of O atom. The two types of freeradical should be co-existed in the system because of the slightly difference of the bond energy, only 4 kJ/mol. The free-radical structure A formed from the C1 O bond cleavage will be considered in the following study, which is supported by two reasons, one is the slight weakness of C1 O bond than O C4 bond, and another reason is the geometry of free-radical A is easier for the formation of levoglucosan. Once the free radical is formed from cellulose chain cleavage, these radicals can be transferred into levoglucosan. The mechanism for levoglucosan formation from free-radical A was described detailed in our previous work [27]. As shown in Fig. 3, from freeradical A, firstly there is a hydrogen-donor reaction. A hydrogen atom H6c transfers from one oxygen atom O1 to another oxygen atom O. Then, there is a possible reaction because of a strong affinity between the active oxygen atom O1 and the active carbon atom C1. If there is a bond formed between them, product levolgucosan can be formed with a bridged structure. For this bridged structure formation step, the singlet state is more stable than the triplet state, which verifies that there is one bond formed by the two unpaired electrons from intermediate to form the product. The driving forces for this pathway are the two unpaired electrons from the oxygen atom O1 and the carbon atom C1. It can be seen that product levoglucosan is much more stable than the reactant free radical.
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Fig. 5. Two possible positions for the hydrolysis of cellobiosan.
Even the activation energy for the formation of levoglucosan from free-radical is not so high; however, free-radical mechanism needs huge external energy due to the homogeneous cleavage of cellulose chain, which is the determining step for the whole free-radical mechanism. The geometries of the two transition states in free-radical mechanism M1-TS1 and M1-TS2 were shown in Fig. 4. Both M1TS1 and M1-TS2 has exactly one imaginary frequency, which is −1717 icm−1 for M1-TS1, and −88 icm−1 for M1-TS2. The vibration of M1-TS1 is the stretching of hydrogen atom between two oxygen atoms. And for M1-TS2, there is a stretching between the atoms of O1 and C1. M1-TS1 has triplet ground state, and for M1-TS2, the ground state is singlet, indicated that there are two unpaired electrons during the first reaction step, the two unpaired electrons form one bond during the second step. 3.2. Glucose intermediate mechanism (M2)
Fig. 4. Structures of the two transition states during free-radical mechanism. The unit for bond length is Å. (a) Transition state M1-TS1, (b) Transition state M1-TS2.
In the glucose intermediate mechanism (as called M2), cellulose firstly is decomposed into glucose units by hydrolysis, then levoglucosan can be formed from these glucose units by dehydration. For the hydrolysis step, similar with the two cleavage positions for the free-radical mechanism, water can react with cellulose at two possible positions, C1 O bond and O C4 bond as shown in Fig. 5. In this study, we assumed the process into a cellobiosan reacts
Fig. 6. Energy illustration and geometries change for hydrolysis of cellobiosan for formation of two glucose units.
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Fig. 9. Structure of the transition state for formation of levoglucosan during dehydration of glucose. The unit for bond length is Å.
Fig. 7. Structures of two transition states for the hydrolysis of cellobiosan. The unit for bond length is Å. (a) Transition state M2-TS1. (b) Transition state M2-TS1 .
with one water molecule to form two glucose units. The energy illustrations for the reaction at these two positions are shown in Fig. 6. One water molecule reacts with cellobiosan, and try to break the bond C1 O and O C4 respectively. Two glucose units can be produced by both pathways shown in Fig. 6. At position C1 O, one hydroxyl group from water bonds with C1, and the hydrogen atom from water bonds with oxygen atom O, simultaneously with the
breaking of C1 O. Similarly, at position O C4 , hydroxyl group and hydrogen atom from water can form bonds with C4 and oxygen O, respectively, with the breaking of O C4 bond and formation of two glucoses. It can be seen that the energy barrier for hydrolysis at C1 O position is 264 kJ/mol. However, for hydrolysis at O C4 position the energy barrier is 346 kJ/mol, indicating that C1 O is the possible position for the cleavage of C O bond in cellulose chain by hydrolysis. The possible reason is the weaker bond of C1 O than O C4 . However, the C1 O is only 4 kJ/mol slightly weaker than O C4 , so it can be concluded that the energy difference of C1 O and O C4 bond is enlarged when reacting with water rather than
Fig. 8. Energy illustration and geometries change for LG formation via glucose intermediate mechanism.
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Fig. 10. Energy illustration and geometries change for levoglucosan formation via LG chain-end intermediate mechanism.
the direct bond breaking, which indicates that the weakness of a bond can be enlarged during hydrolysis. The structures for the two transition states for the hydration of cellobiosan were shown in Fig. 7. Both of these two transition states have an imaginary frequency, which is −259 icm−1 for M2TS1, and −882 icm−1 for M2-TS1 . For M2-TS1, the vibrations under the imaginary frequency are stretching of the C1 O and O H bonds, with the tendency of new bonds formation between C1 OH and O H. Compared with M2-TS1 , M2-TS1 is a reasonable transition state for hydrolysis due to the lower activation energy. After hydrolysis of cellulose, the glucose unit formed from hydration of cellulose chain can produce levoglucosan by getting rid of water. The energy illustration and geometries changes for the whole glucose mechanism including of the hydrolysis and dehydration steps were shown in Fig. 8. For the hydration of cellulose chain, only the position C1 O was considered here because the lower energy barrier than the position of O C4 . For the dehydration step, it can be seen that a hydrogen atom from O1 and a hydroxyl group from C1 form a water molecule and released further from glucose, and the remained part is levoglucosan with a bond formation between O1 and C1. The activation energy for the dehydration step is 227 kJ/mol, compared with the activation energy of hydrolysis step, 264 kJ/mol, it can be concluded that the hydrolysis step is the rate-determining step for the whole levoglucosan formation process via glucose intermediate mechanism. The structure of the transition state for the dehydration from glucose to levoglucosan was shown in Fig. 9. The imaginary frequency for this transition state is −193 icm−1 , with vibration of stretching between O1 and hydrogen atom, C1 and hydroxyl group. 3.3. LG chain-end mechanism (M3) Levoglucosan formation via LG chain-end intermediate mechanism (as called M3) happens through two transglycosylation steps, the formation of the LG chain-end structure intermediate and the formation of levoglucosan from the LG chain-end structure
intermediate. The energy illustration and geometries change for LG chain-end mechanism were shown in Fig. 10. For the formation of LG chain-end intermediate, there is one transglycosylation elementary reaction. From the structure of M3TS1, we can see that three phenomena are happened during this elementary reaction: firstly the cleavage of the C1 O bond; secondly one hydrogen atom H6c transfers from O1 to the oxygen O; and at last a new bond formed by C1 and O1. Similar processes happen for second step: the formation of levoglucosan from LG chain-end intermediate. The activation energies for the two steps are 195 kJ/mol and 182 kJ/mol, respectively. Which showed the first step is the rate-determining step. The structures for the two transition states for LG chain-end mechanism were shown in Fig. 11. It can be seen that the two transition state has similar bond change. There is a ring-closing process with a hydrogen shifting. The imaginary frequency for these two transition states are −431 icm−1 and −413 icm−1 respectively. 3.4. Comparison of all the calculated mechanisms for levoglucosan formation The energy diagrams for the three mechanisms are shown in Fig. 12, it can be seen that levoglucosan formed from free-radical mechanism is the most difficult pathway due to the high energy barrier for the cleavage of the C O bonds in cellulose chain. The levoglucosan formed via glucose intermediate needs lower activation energy than the free-radical mechanism. However, LG chain-end mechanism gives the lowest energy barrier. The activation energy obtained from the experimental results [10,12,35] are also shown in Fig. 12, it can be seen that the general range for the activation energy obtained from experiments are 151–228 kJ/mol. The energy barrier for free-radical mechanism can be obtained from the energy barrier for direct breaking C1 O bond. The energy barrier for freeradical mechanism is quite high, about 422 kJ/mol. The activation energies for glucose intermediate mechanism and LG chain-end mechanism are 264 kJ/mol and 195 kJ/mol, respectively. It can be
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Fig. 11. Structure of two transition states involved in the LG chain-end intermediate mechanism. The unit for bond length is Å. (a) Transition state M3-TS1. (b) Transition state M3-TS2.
concluded that the activation energy calculated from LG chain-end mechanism is the closest with the experimental result. By comparing the three mechanism and their elementary steps, the most important thermal properties including activation energy, energy change, Gibbs free energy change, Gibbs free energy barrier, and enthalpy change were shown in Table 1. The first column gives the detailed activation energy for every pathway and elementary step, which is same with what can be obtained from Fig. 12. The Ea are listed here just for comparison. The second and fifth columns give the thermal energy changes and enthalpy changes for every pathway and elementary reactions, respectively. It can be seen that no matter which pathway, there is no big difference for the global energy change from the reactant cellulose and product levoglucosan. Take enthalpy for example, the enthalpy change is
106 kJ/mol for free-radical mechanism, 73 kJ/mol for glucose mechanism, and 102 kJ/mol for LG chain-end mechanism. All the thermal energy and enthalpy change shows that it is an endothermic reaction for the formation of levoglucosan from cellulose. The fourth column gives the Gibbs energy barrier for all the pathways, it also shows that LG chain-end mechanism gives lowest energy barrier, which is same trend which was reflected by the activation energy Ea . From the calculation for the three mechanisms for levoglucosan formation, it can be seen that the rate-determining step for free-radical mechanism is the direct breaking of C1 O bond. The rate-determining step for glucose intermediate mechanism is the formation of glucose from cellulose chain, which simultaneously with the breaking of C1 O bond. And for LG chain-end mechanism
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Fig. 12. Comparison of the energy barrier of experiment and different calculated levoglucosan formation mechanisms.
Table 1 Comparison of activation energy (Ea ), energy change (E), Gibbs free energy change (G), Gibbs free energy change between transition state and reactant (G‡ ), and enthalpy change (H) for the three mechanisms for LG formation from cellulose. The unit is kJ/mol. Ea Free-radical mechaniam (M1) Step 1: from cellulose to free-radical Step 2: from free-radical to LG Glucose mechanism (M2) Step 1: from cellulose to glucose Step 2: from glucose to LG LG chain-end mechanism (M3) Step 1: from cellulose to LG chain-end intermediate Step 2: from LG chain-end intermediate to LG
– – 48 264 264 227 195 195 182
for levoglucosan formation, the rate-determining step is the first transglycosylation step with the formation of a LG chain-end intermediate and a glucose, which happens with breaking of C1 O at the same time. So it can be concluded that breaking of C1 O bond is the rate-determining step for levoglucosan step no matter via which mechanism. Free-radical mechanism needs more energy because of the difficulty of breaking C1 O bond directly. Glucose intermediate mechanism needs lower energy than free-radical mechanism because it is easier to break C1 O bond due to the existing of water molecule in the process. And LG chain-end mechanism needs lowest energy barrier because of it is easiest to break the C1 O bond by transglycosylation.
E
G
G‡
H
81 406 −325 71 24 47 97 58 39
28 343 −315 18 14 4 12 6 6
– – 60 307 307 226 193 193 183
84 409 −325 73 24 49 102 60 42
enlarged during the hydrolysis process, the water molecule reacts at C1 O position gives much lower activation energy than that at O C4 position, the activation energy difference is about 78 kJ/mol. The rate-determining step for all these three mechanisms is the one with breaking of C1 O bond. Free-radical mechanism has high energy barrier because of the difficulty of breaking C1 O bond directly. C1 O bond can be easier broken when there exist water molecule to form glucose. During the three mechanisms for levoglucosan, LG chain-end mechanism which happens via two transglycosylation steps is the most reasonable pathway. Acknowledgments
4. Conclusions Three mechanisms for levoglucosan formation during cellulose pyrolysis are studied and compared in this work, which are free-radical mechanism, glucose intermediate mechanism, and LG chain-end mechanism. Firstly, it was concluded that the formation of levoglucosan from cellulose is endothermic no matter via which pathway. In cellulose chain, C1 O bond is slightly weaker than the bond O C4 , with an energy difference of 4 kJ/mol. But this trend can be
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