ARTICLE pubs.acs.org/EF
Formation Mechanism of Levoglucosan and Formaldehyde during Cellulose Pyrolysis Xiaolei Zhang,* Jun Li, Weihong Yang, and Wlodzimierz Blasiak Division of Energy and Furnace Technology, Department of Materials Science and Engineering, School of Industrial Engineering and Management, Royal Institute of Technology, SE 100 44 Stockholm, Sweden
bS Supporting Information ABSTRACT: Biomass pyrolysis is an efficient way to transform raw biomass or organic waste materials into useable energy, including liquid, solid, and gaseous materials. Levoglucosan (1,6-anhydro-β-D-glucopyranose) and formaldehyde are two important products in biomass pyrolysis. The formation mechanism of these two products was investigated using the density functional theory (DFT) method based on quantum mechanics. It was found that active anhydroglucose can be obtained from a cellulose homolytic reaction during high-temperature steam gasification of the biomass process. Anhydroglucose undergoes a hydrogen-donor reaction and forms an intermediate, which can transform into the products via three pathways, one (path 1) for the formation of levoglucosan and two (paths 2 and 3) for formaldehyde. A total of six elementary reactions are involved. At a pressure of 1 atm, levoglucosan can be formed at all of the temperatures (450750 K) considered in this simulation, whereas formaldehyde can be formed only when the temperature is higher than 475 K. Moreover, the energy barrier of levoglucosan formation is lower than that of formaldehyde, which is in agreement with the mechanism proposed in the experiments.
1. INTRODUCTION The use of biomass or bioenergy offers significant environmental advantages over fossil fuels; they produce fewer carbon dioxide emissions and less environmental pollution and can ease the global energy shortage. The technology of pyrolysis is an efficient way to convert biomass into usable energy, such as char from the conventional slow pyrolysis process and liquids or gases from fast pyrolysis. For the high-temperature steam gasification (HTSG) process, which was talked about extensively by our group,1 the biomass pyrolysis is the primary step for the biomass gasification. Regardless of the variability of biomass, its components generally include cellulose (4050%), hemicellulose (1525%), lignin (1530%), and a small amount of ash content. The study of the pyrolysis mechanism of these components is important for understanding the biomass pyrolysis process as a whole. Cellulose is a polymer of glucose.2 It decomposes at 598648 K,3 and an increasing heating rate is given by a narrower degradation temperature. Hemicelluloses2,4 are heterogeneous polymers made of pentoses, hexoses, and sugar acids. Xylose and arabinose are the main constituents of the hemicelluloses in agricultural residues.4 Hemicellulose decomposes at 498598 K3 and demonstrates a lower thermal stability than cellulose. The mass loss of lignin occurs at 523773 K.3 It is structurally variable with the plant species and is composed mainly of guaiacyl and syringyl in softwoods and hardwoods, respectively. The lignin present in straw and grass has a different structure than that in woods.5 Because cellulose is the main component of biomass, cellulose pyrolysis plays an important role in the investigation of biomass pyrolysis; it has been studied extensively using experimental methods. There are three product fractions from cellulose pyrolysis: volatiles, liquid products, and char residue.69 Kawamoto et al.1012 studied the cellulose pyrolysis mechanism in earlier r 2011 American Chemical Society
studies and concluded that levoglucosan is the primary direct product from cellulose decomposition, which is subsequently degraded to lower molecular-weight (LMW) products or polymerized into polysaccharide, which will be carbonized to form char; there is competition between the formation of LMW products or solid char. Shafizadeh et al.13 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, as shown in Figure 1. This reaction scheme is known as the BroidoShafizadeh model, and there is competition between the formation of volatile, char, and gas products from active cellulose, where volatiles, including tar (levoglucosan), represent a condensable fraction. The authors concluded that the activation energies for the formation of volatiles and char/gases are 198 and 153 kJ/mol, respectively. In the pyrolysis reaction scheme proposed by Banyasz et al.,14,15 however, the concept of active cellulose is excluded, as shown in Figure 2, and it was concluded that cellulose can either be transferred into char, tar (levoglucosan), or gases (hydroxyacetaldehyde, formaldehyde, and CO) via another intermediate, namely, depolymerizing cellulose. The activation energies calculated in the Banyasz model are 151 and 196 kJ/mol for tar and formaldehyde, respectively. Despite the debate between these mechanisms, the existence of levoglucosan and formaldehyde in pyrolysis, as either an intermediate or a product, is a recognized fact. Furthermore, it has been reported that levoglucosan is the main component of tar and bio-oil,1620 which shows that the formation of levoglucosan from cellulose (cellulose f active cellulose f levoglucosan) is also important for biomass gasification and liquid-product formation. Received: January 13, 2011 Revised: June 27, 2011 Published: June 27, 2011 3739
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Energy & Fuels The previous reaction schemes can provide general descriptions for the reactants and products of cellulose pyrolysis. In fact, a cellulose cluster may pass through many different steps; different intermediates may exist; and a detailed mechanism for the product formation is needed. As the two main products of cellulose, the detailed formation mechanisms of levoglucosan and formaldehyde during the cellulose pyrolysis system were investigated in this paper. Several pathways for producing levoglucosan and formaldehyde were predicted and verified. All of the reactants, intermediates, transition states, and products included in these pathways were calculated. The Gibbs free-energy change was estimated to analyze the feasibility of each elementary step, and the activation energy was also obtained to reveal the energy barrier for each of these steps.
2. COMPUTATIONAL METHODS 2.1. Computational Models. Native cellulose is composed of two phases, IR and Iβ. The cellulose produced by bacteria and algae is enriched in IR. The cellulose from biomass contains IR and Iβ crystalline allomorphs together with surface and disordered chains,21,22 while the cellulose of higher plants consists mainly of Iβ. Cellulose IR was found to have a triclinic structure with one cellobiose chain in each unit cell, whereas cellulose Iβ has a monoclinic structure and two chains, as shown in Figure 3. The two chains are packaged in an up-and-down, parallel structure. One is in the corner, and the other is in the center of the unit cell.23,24 The repeating unit is D-glucose (C6H12O6) with β-1,4 linkages.25 2.2. Computational Details. The density functional theory (DFT)/B3LYP method based on quantum mechanics is used in this paper. DFT is a computational method that derives properties of a system based on the electron density of the system, in comparison to ab initio methods, which solve the wave function of a structure. The determination of the electron density is independent of the number of electrons, which makes solving the energy of the system easier. B3LYP, run with 6-31G* or a better basis set, is the best choice of a model chemistry for most systems. B3LYP/6-31G* is particularly good for organic molecules. In our previous study,26 B3LYP/6-31+G* was used to produce a better understanding of the high-temperature air/steam gasification (HTAG) process. In this work, structural optimizations of all of the reactants, intermediates, transition states, and products involved in the elementary reactions were performed at the DFT/B3LYP level27,28 using the 6-31+G(d) basis set. 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
Figure 1. BroidoShafizadeh model of the cellulose pyrolysis mechanism.
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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. The enthalpies and Gibbs free energies of all stable points (optimized reactants, products, and intermediates) were calculated at a series of temperatures (450750 K) and at the standard pressure (1 atm). The Gaussian09 package29 was used in this study, and all of the calculations were carried out on the highperformance computer in PDC at the Royal Institute of Technology (KTH) in Sweden.
3. RESULTS AND DISCUSSION 3.1. Depolymerization of Cellulose. There are two different methods for the depolymerization of a cellulose chain, the homolytic (radical) reaction and the heterolytic (ionic) reaction, proposed by Kislitzyn et al.30,31 and Shafizadeh,32 respectively. The method for cleaving of cellulose is dependent upon the conditions. The chemistry of the pyrolytic products testifies in favor of heterolytic mechanisms when accelerated by catalysis,31,33 either base catalysis or acid catalysis, in a high-temperature
Figure 3. Crystal structure for cellulose Iβ. The red spheres represent oxygen atoms; the gray spheres represent carbon atoms; and the white spheres represent hydrogen atoms.
Figure 2. Banyasz model of the cellulose pyrolysis mechanism. 3740
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Figure 4. Homolytic cleavage and heterolytic cleavage of cellulose.
Figure 6. Anhydroglucose radical from the homolytic cleavage of cellulose. The red spheres represent oxygen atoms; the gray spheres represent carbon atoms; and the white spheres represent hydrogen atoms.
Figure 5. Mulliken atomic charge distribution for the two-glucose cellulose model.
atmosphere without catalysis; however, the homolytic mechanism is preferred. The bond dissociation energies (BDEs) of both the homolysis and heterolysis of cellulose are evaluated, as shown in Figure 4. The BDEs were calculated as the standard enthalpy change before and after cleavage at 0 K (absolute zero). Two glucose radicals with one unpaired electron each are obtained from a homolytic reaction, while two ionic glucoses are obtained from a heterolytic reaction. The CO BDE for homolytic cleavage is 331 kJ/mol, while the energies for the heterolysis reactions are 659 and 925 kJ/mol for heterolysis 1 and heterolysis 2, respectively. Thus, it was concluded that heterolytic bond cleavage is more difficult than homolysis. The reason is due to the weak polarity of the CO covalent bond. In the two glucose model shown in Figure 5, the charges are 0.207 for C3, 0.314 for O30, and 0.047 for C25; thus, the charge difference between C3 and O30 is 0.521 and between C25 and O30 is 0.351. The active radical can be obtained from homolysis because of conjugation, which distributes the unpaired electron to the whole
Figure 7. Energy illustrations for the three pathways of the formation of levoglucosan and formaldehyde from anhydroglucose. The superscript before every structure represents the spin multiplicity of this structure.
six-member ring with 97.2 kJ/mol of energy decreased, and then, the radical becomes more stable. For the product of this 3741
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Figure 8. Geometry changes for the six elementary reactions. The red spheres represent oxygen atoms; the gray spheres represent carbon atoms; and the white spheres represent hydrogen atoms.
homolysis cleavage, the most stable state is triplet; however, for the reactant, the most stable state is singlet. This indicates that a CC bond is broken during the depolymerization. Because the simulation in this work is under pure atmosphere without catalysts, the cellulose pyrolysis that we studied here is the first step of the HTSG process. It is well-known that the
cellulose pyrolysis prefers homolytic cleavage during steam treatment34 and under high temperature.35 Thus, the homolysis product, an anhydroglucose molecule (as shown in Figure 6), will be the focus of the following work. 3.2. Reaction Pathways from Anhydroglucose to Levoglucosan and Formaldehyde. According to the DFT simulation 3742
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Table 1. Relative Energies, Frequency, Spin Multiplicity, Total Spin S2, and Spin Contamination for Every Reactant, Intermediate, Transition State, and Producta species
ΔER
3
0
3
39.1 4.4
IIA (anhydroglucose)
TS1 3 IIC
frequency
spin multiplicity
S2
spin contamination (%)b
73.5
triplet
2.0083
0.42
1584.4 65.7
triplet triplet
2.0097 2.0071
0.49 0.36
3
2.2
35.4
triplet
2.0070
0.35
3
13.3
145.4
triplet
2.0068
0.34
2.0075
0.38
2.0057
0.29
IID TS2
3
IIF
9.1
32.2
triplet
1
IIF0
27.0
61.2
singlet
3
TS3
262.8
1857.5
triplet
1
TS30
2.3
101.4
singlet
1
298.9 71.7
89.9 256.3
singlet triplet
2.0171
0.86
3
57.8
20.7
triplet
2.0076
0.38
3
6.3
41.4
triplet
2.0074
0.37
3
214.3
960.7
triplet
2.0268
1.34
3
0.8
45.1
triplet
2.0069
0.34
3
91.9
375.8
triplet
2.0211
1.05
3
41.5
26.1
triplet
2.0072
0.36
IIH (levoglucosan) TS4
3
IIJ (C5O4H8 + formaldehyde) IIK TS5 IIL TS6 IIN (C5O4H8 + formaldehyde)
a
The superscript before every species represents the spin multiplicity for the related structure. The units for relative energies are kJ/mol, and the units for frequencies are cm1. b The allowable spin contamination is 10%.
results, there are three pathways for the formation of levoglucosan and formaldehyde from anhydroglucose: one for the formation of levoglucosan and two for formaldehyde. The total energy changes of the three pathways under singlet and triplet states are illustrated in Figure 7. A total of six elementary reactions are included in these pathways. For each elementary reaction, a transition state was found and the activation energy was calculated. The geometries of the six elementary reactions and the activation energies are illustrated in Figure 8. The atom numbers in the structures shown in Figure 8 are the same as those atom numbers shown in Figure 6. Table 1 gives the relative energy, frequency, and spin multiplicity of every reactant, intermediate, transition state, and product and also gives the S2 value and spin contamination for all open-shell systems. The stabilities of the calculated points were verified by frequency analysis, where reactants, intermediates, and products have no imaginary frequencies and transition states have exactly one imaginary frequency. To an open-shell system with a multiplicity other than one, instead of a spin-restricted calculation as used in singlet systems, an unrestricted calculation is performed. The disadvantage of unrestricted calculation is that the wave function is no longer an eigenfunction of the total spin; thus, some error called spin contamination may be introduced into the calculation. To check the spin contamination, the value of the total spin, S2 in Table1, should differ from s(s + 1) by less than 10%, where s equals 1/2 times the number of unpaired electrons. The s(s + 1) value for a triplet system is 2. From Table 1, it can be seen that the spin contaminations for all of the open-shell systems are far less than 10%, which are allowable. From the potential energy surfaces of the three pathways under both singlet and triplet states, as shown in Figure 7, it is resulted that, for the two pathways of the formation of formaldehyde, from IIA to IIJ or from IIA to IIN, the singlet potential energies are always higher than those of the triplet, which indicates that the whole pathway for the formation of formaldehyde is under the triplet state and there are always two unpaired electrons
existing in the system. For the formation of levoglucosan (from IIA to IIH), however, there is one crossing seam between the singlet and triplet potential energy surfaces; the most stable state for the structures IIA, TS1, IIC, IID, TS2, and IIF is triplet, but for the structures IIF0 , TS3, and IIH, the most stable is the singlet structure. Therefore, as shown in Figure 7, the pathway for the formation of levoglucosan is 3IIA3TS13IIC3IID3TS2 3 IIF1IIF0 1TS30 1IIH and the two pathways for the formation of formaldehyde are 3IIA3TS13IIC3IID3TS2 3 IIF3TS43IIJ and 3IIA3TS13IIC3IIK3TS53IIL 3 TS63IIN. One important elementary step in the formation of both levoglucosan and formaldehyde is the hydrogen-donor reaction, ER1 (3IIA3TS13IIC) shown in Figure 8. A hydrogen atom H21 transfers from one oxygen atom O13 to another oxygen atom O11. The atom number can be seen clearly from Figure 6. Thus, the active position is transferred from O11 to O13 because of the energy difference of 4.4 kJ/mol, as shown in Table 1. The product of ER1 (configuration 3IIC) is slightly more stable than the reactant (3IIA), and a transition state 3TS1 between 3IIA and 3 IIC was found. This elementary reaction also indicates that one of the unpaired electrons existing in the anhydroglucose radical is located at the CH2O+ moiety, which becomes one of the active centers and tends to bond with other active positions. From 3IIC, there is a possible reaction because of a strong affinity between the active oxygen atom O13 and the active carbon atom C3. If there is a bond formed between them, structure 1IIH can be formed, which is exactly levoglucosan; this pathway is called path 1. There are three elementary steps in path 1: ER1, ER2 (3IID3TS23IIF), and ER3 (1IIF0 1TS30 1 IIH), as shown in Figure 8. First, in the elementary reaction ER2, there is a consecutive rotation of 90° for the CH2O+ moiety, while the rest of the structure remains fixed. To form an intermediate 3IIF, during this rotation, only atoms O13, H19, and H20 changed their positions, while other atoms remain fixed. The bond C8O13 is at last vertical to the six-member ring plane 3743
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from parallel to the six-member ring plane. This rotation reaction is due to the σ single bond between two carbon atoms C7 and C8; thus, a series of different configurations around this bond are formed. The energy barrier for this rotation is only 14 kJ/mol. Then, in the next elementary reaction ER3, a bridge-shape transition state 1TS30 is formed on the basis of the singlet structure 1IIF0 . The singlet state is more stable than the triplet Table 2. Activation Energies for Three Pathways, Compared to Experimentsa EA in this calculation
a
BroidoShafizadeh
Banyasz
Capart
model13
model15
model36
path 1
93
EA,tar
153
151
202
path 2 path 3
63 219
EA,gas
198
196
255
The unit of energy is kJ/mol.
state in this step, which indicates that there is one bond formed by the two unpaired electrons from configuration 1IIF0 . The driving forces for this pathway are the two unpaired electrons from the oxygen atom and the carbon atom. Obviously, ER3 is the rate-determining step because of its higher activation energy, and the activation energy for the whole pathway is 93 kJ/mol, which is based on the rate-determining elementary step. For 1 IIH, levoglucosan, there is no unpaired electron in this structure, which indicates that path 1 is a termination of the radical reactions; thus, path 1 is a favored pathway. Another possibility is that a formaldehyde molecule can be formed by a fragmentation reaction from the intermediate 3IIF. The bond between two carbon atoms, C7C8, is broken, and a structure 3IIJ including a formaldehyde molecule and a new radical is formed. This process is path 2. Three elementary steps are included in path 2: ER1, ER2, and ER4 (3IIF3TS43IIJ). ER4 is based on intermediate 3IIF. The energy barrier for this pathway is not so high, 63 kJ/mol, which is determined by ER4.
Figure 9. Mechanism proposed in this work. 3744
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Figure 10. Relationship of the enthalpy changes with the temperature for the six elementary reactions and three pathways.
Figure 11. Relationship of the Gibbs free-energy changes with the temperature for the six elementary reactions.
There are still two unpaired electrons in the product, which reveals that this pathway is not a termination for the cellulose pyrolysis. The third possible pathway (path 3) is a rearrangement for 3 IIC, where a CC bond is broken and a new bond between the carbon atom and the oxygen atom is connected to form an intermediate 3IIL, which then will be transformed into formaldehyde through the breaking of this CO bond. Formaldehyde in the complex 3IIL is connected with the other radical by hydrogen bonds. For path 3, three elementary steps are included: ER1, ER5 (3IIK3TS53IIL), and ER6 (3IIL3TS63IIN). For ER5, a CC bond is broken and a new CO bond is formed. 3 IIL is a little more stable than 3IIK because of the 7.1 kJ/mol energy difference. The intermediate 3IIL can go through a transition state 3TS6 to obtain the product 3IIN, which includes a formaldehyde molecule and a new radical. The activation energies for ER5 and ER6 are 219 and 93 kJ/mol, respectively, which indicates that the first one is the rate-determining step. Structure 3IIN has a lower energy than 3IIJ (Figure 7); thus, 3IIN is the preferred method for the combination of formaldehyde and the new C5O4H8 radical. Table 2 gives the activation energy calculated in this simulation compared to the previous pyrolysis models. The activation energy of path 1, which gives the formation of levoglucosan, has a lower energy barrier than the formation of formaldehyde. This tendency is in agreement with the values from the experimental models.13,15,36 It can also be seen that the activation energy of levoglucosan is lower than the energy barrier of tar formation; thus, we predict that there is another main component from tar, which has a higher energy barrier than levoglucosan. From the aforementioned analysis, a detailed mechanism for the formation of levoglucosan and formaldehyde can be proposed, as shown in Figure 9. First, under higher temperature steam treatment and without catalysis, there is a homolytic reaction for cellulose fragmentation. Then, from anhydroglucose (3IIA), which is formed from the cellulose homolysis cleavage, there are three pathways for the formation of the products, with six elementary reactions. The hydrogen-donor reaction happens to form the active intermediate 3IIC, with transition state 3TS1, and the activation energy is 39 kJ/mol. From 3IIC, levoglucosan can be formed, with an activation energy of 93 kJ/mol.
Formaldehyde can be formed through one intermediate (3IIL) and two transition states (3TS5 and 3TS6), and the ratedetermining step is the formation of 3IIL. The activation energy is 219 kJ/mol, and there is another pathway to form formaldehyde (3IIJ), whose activation energy is only 63 kJ/mol. 3.3. Feasibilities of the Three Reaction Pathways. The enthalpy change and Gibbs free-energy change for every elementary reaction were calculated at a series of temperatures and under standard pressure (1 atm), as shown Figures 10 and 11 for the enthalpy change and the Gibbs free-energy change, respectively. The temperature range chosen is 450750 K, which includes nearly the entire decomposition temperature range for the three main compounds 3: cellulose (598648 K), hemicellulose (498598 K), and lignin (523773 K). From the curves shown in Figure 10, the enthalpy changes with the temperature are small. The values of the enthalpy changes for ER1ER6 indicate that ER2, ER4, ER5, and ER6 are endothermic, while ER1 and ER3 are exothermic, and the values for the three pathways indicate that the formation of levoglucosan is exothermic, while two pathways for the formation of formaldehyde are endothermic. Path 1 can release energy because of the highly exothermic elementary step ER3 as a result of the stability of the new bond formation in 1IIH. The Gibbs free-energy change varies significantly with the temperature. The values of ER1, ER2, ER3, and ER5 are negative at the whole simulated temperature range, but the value of ER4 is negative only when the temperature is higher than 625 K. For ER6, the value is negative when the temperature is higher than 475 K. It can therefore be concluded that the conversion from anhydroglucose to levoglucosan is feasible at the whole range of temperatures (450 750 K) and the formation of formaldehyde from anhydroglucose is feasible only at the temperatures higher than 475 K. When the temperature is higher than 625 K, there are two pathways to form formaldehyde. The formation of levoglucosan can occur at the complete temperature range of cellulose pyrolysis, while formaldehyde can only be formed when the temperature is higher than 475 K; this conclusion is in agreement with the experimental results that, at lower temperatures, more liquid products (levoglucosan) can be formed, while at higher temperatures, more volatiles can be formed. 3745
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4. CONCLUSION For a cellulose chain, active anhydroglucose can be formed from the homolytic reaction, and then, after a hydrogen-donor reaction, anhydroglucose can be transformed into levoglucosan and formaldehyde through three pathways, for a total of six elementary reactions. Levoglucosan can be formed at the full range of temperatures from 450 to 750 K, while formaldehyde can only be formed when the temperature is higher than 475 K. The formation of levoglucosan from anhydroglucose is exothermic, while the reaction for the formation of formaldehyde is endothermic. The energy barrier of levoglucosan formation is lower than that of formaldehyde, which is in agreement with the experiments. ’ ASSOCIATED CONTENT
bS
Supporting Information. Cartesian coordinates of structures 3IIA, 1IIH, 3IIJ, 3IIN, 3TS1, 3TS2, 1TS30 , 3TS4, 3TS5, and 3 TS6 and energy illustration for the homolysis cleavage pathway. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +46-8-790-85-31. E-mail:
[email protected].
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dx.doi.org/10.1021/ef2005139 |Energy Fuels 2011, 25, 3739–3746
