Density Functional Theory (DFT) Study on the Dehydration of

ARTICLE pubs.acs.org/EF

Density Functional Theory (DFT) Study on the Dehydration of Cellulose Minhua Zhang, Zhongfeng Geng, and Yingzhe Yu* Key Laboratory for Green Chemical Technology of the Ministry of Education, Tianjin University Research and Development Center for Petrochemical Technology, Tianjin 300072, People’s Republic of China ABSTRACT: Pyrolysis is a very important platform for the use of cellulose; however, the mechanism for the pyrolysis of cellulose is still not clear. A density functional theory (DFT) study was thus conducted to investigate the dehydration of cellulose using cellotriose as a model compound. The DFT study confirms that the location of hydroxyl groups has a significant influence on the dehydration of cellulose. The most active hydroxyl group is
O2H, followed by
O3H and
O6H. The pinacol rearrangement is a more likely mechanism for hydroxyl groups to be dehydrated. However, the barriers for dehydration are so high that dehydration reactions are difficult to occur. To obtain chemical products directly from the pyrolysis of cellulose, species with hydroxyl groups remaining might be targeted. Future studies are needed to explore the mechanism of cellulose pyrolysis, especially on the breaking of the glycosidic linkage and the opening of pyran rings.

1. INTRODUCTION With the inevitable depletion of fossil fuels, it is imperative to develop economical and energy-efficient processes for the sustainable production of fuels and chemicals. In this respect, cellulose is an important renewable source of organic carbon. Thus, it is essential to understand the chemistry about cellulose for its efficient use.1 To this end, pyrolysis of cellulose has been extensively studied and critically reviewed.2
6 According to Kilzer and Broido,2 dehydration is the first reaction in the degradation of cellulose, leading to anhydrocellulose. The existence of anhydrocellulose was later confirmed by Bradbury et al.3 Bradbury et al. also pointed out that pyrolysis of cellulose resulted in a tar containing various dehydration products.3 Scheris et al.7 reported that the total amount of water obtained from cellulose paper was measured to be 14.3% (w/w). Obviously, dehydration is a very important reaction during the pyrolysis of cellulose. There are three free hydroxyl groups in each glucose residue of a cellulose molecule. The dehydration reaction must have something to do with these free hydroxyl groups. Hence, the behavior of the three hydroxyl groups during pyrolysis of cellulose should be explored in detail. However, the mechanism of reactions involving the hydroxyl groups is still not clear. As the starting point of exploring the mechanism of cellulose pyrolysis via quantum chemical calulations, it is usually preferred to begin with a simple model reaction (rather than a comprehensive study). Nimlos et al.8 theoretically investigated the 1, 2-dehydration of alcohols as a model for water loss during the pyrolysis of carbohydrates. They found that the barriers to 1, 2-dehydration of neat alcohols were approximately 67.0
69.0 kcal/mol.8 They proposed a hypothesis that this barrier was similar to that of the carbohydrate based on of the fact that this barrier was independent of the size of the alcohol. This hypothesis was proven reasonable by the fact that the calculated barrier for the dehydration of levoglucosan was 68.0 kcal/mol. However, it is not immediately obvious how the hydrogen bonding formed r 2011 American Chemical Society

between adjacent glucose residues influences this chemistry. A more rigorous model compound for cellulose is needed to study the behaviors of hydroxyl groups in detail. Smith9 investigated two mechanisms to account for the rearrangement of glycol to acetaldehyde via a loss of one water molecule based on density functional theory (DFT) studies. The results showed that the pinacol rearrangement of the 1,2-diols prefers a concerted migration of a hydride: first, with the loss of water, next, the formation of enol, and finally, the formation of acetaldehyde. Pinacol rearrangement was an acid-catalyzed rearrangement mechanism and can be written as follows:10

This information might be very important in analyzing the dehydration of cellotriose. More recently, Nimlos et al.11 theoretically investigated the dehydration of neutral and protonated glycerol, which was a molecule more structurally similar to a carbohydrate. It was found that there was a high energy barrier of 70.9 kcal/mol for simple 1,2-dehydration, while the energy barrier for pinacol rearrangement of protonated glycerol was only 24.9 kcal/mol. When Nimlos et al. studied the dehydration mechanism of protonated glycerol, they considered the carbohydrates in aqueous acid solution.11 Aqueous acid solution could provide an ionconductive medium (electrolyte) and donors of mobile solvated protons, which were both necessary to launch acid-catalyzed Received: November 13, 2010 Revised: April 29, 2011 Published: May 02, 2011 2664

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Energy & Fuels

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Scheme 1

Figure 1. Optimized structure of cellotriose with selected bond lengths indicated in angstroms (gray ball, C; red ball, O; white ball, H).

Table 1. Selected Structural Parameters of the Second Glucose Residue in the Optimized Structure parameters

values

bond length (Å)

parameters

values

C3
H

1.108

C1
C2

1.537

C4
H

1.106

C2
C3

1.538

C5
H

1.107

C3
C4

1.541

C6
H

C4
C5

1.558

1.102

Scheme 2

bond angles (deg)

C5
C6

1.551

τ

118.0

C1
O1 C2
O2

1.426 1.436

τ0 C1
O5
C5

117.1 112.4

C3
O3

1.441

C1
C2
O2

112. 9

C4
O10

1.456

C2
C3
O3

109.9

C5
O5

1.441

C5
C6
O6

115.2

C6
O6

1.429

dihedral angles (deg)

O2
H

0.989

O6
C6
C5
O5

O3
H

0.982

j


108.3

162.0

O6
H C1
H

0.984 1.104

ψ j0


157.3
120.0

C2
H

1.108

ψ0


158.0

reactions.12 The cellulose microfibrils in biomass have both crystalline and amorphous regions. The major part of cellulose (around 2/3 of the total cellulose) is in the crystalline form.13 The solid matrix of the cellulose crystalline region might hamper the access of acidic groups. Besides, cellulose is an infusible polymer, and therefore, its matrix cannot be likened to an electrolyte.12 Therefore, “neutal” cellulose was selected as the focus of this paper. Cellobiose has been employed as a good model compound for exploring the structure of cellulose in the literature,14
18 because of the relative complexity of cellulose. Cellotriose is employed here as the model compound of cellulose to study the mechanism of the intramolecular dehydration of cellulose. For the reason of computational capability, the interaction of different cellulose chains was ignored.

2. COMPUTATIONAL DETAILS All of the DFT calculations were carried out with the Materials Studio DMol3 program from Accelrys.19
21 A double numerical plus polarization (DNP) basis set was employed to describe the valence orbits of O, C, and H atoms. The revised Perdew
Burke
Ernzerhof (RPBE) functional of generalized gradient approximation (GGA) was used to calculate the nonlocal exchange and the correlation energies of reactants, products, and transition states.22,23 The convergence criteria included threshold values of 1  10
5

Hartree (Ha), 0.002 Ha Å
1, and 0.005 Å for energy, force, and displacement convergence, respectively, while the self-consistent-field (SCF) density convergence threshold value of 1  10
5 Ha was specified. A Fermi smearing of 0.005 Ha was used to improve the calculation performance. The geometries of stationary points were fully optimized at this level. The vibrational frequency analysis at the same level was performed to determine the nature of stationary points. The minimum structures were characterized by no imaginary frequency, while transition states were characterized by only one imaginary frequency. The linear synchronous transit (LST) and quadratic synchronous transit (QST) methods were used to study the transition states.24,25 The transition states were confirmed by the nudged elastic band (NEB) method. The accuracy of the DNP basis set has been analyzed by Delley in detail.19 Some previous works26
30 that employed such a method achieved good accuracy.

3. RESULTS AND DISCUSSION 3.1. Structure of Cellotriose. In a computational study of dehydration of cellotriose, it is important to establish a starting structure for cellotriose. The backbone of the cellulose molecule 2665

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Energy & Fuels is determined by the glycosidic dihedral angles, which might be affected by the orientation of the exosyclic hydroxyl groups and hydroxymethyl groups.17,31
33 In this paper, the glycosidic dihedral angles j (j0 ) and ψ (ψ0 ) are defined as the dihedral angles of O50
C10
O10
C4 (O5
C1
O1
C400 ) and C10
O10
C4
C5 (C1
O1
C400
C500 ), respectively. The hydroxymethyl side chains are described by the orientation of the C5
C6 (C50
C60 , C500
C600 ) bond and written conventionally as gg (gg0 , gg00 ), gt (gt0 , gt00 ), or tg (tg0 , tg00 ). The orientations of the exocylcic hydroxyl groups on the sugar Scheme 3

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residues are defined in this paper as c (clockwise) or r (anticlockwise). Figure 1 shows the structure of cellotriose optimized with DFT, with key structural parameters of the second glucose residue presented in Table 1. Crystallography indicates that molecules in crystalline cellulose either have 2-fold screw-axis (21) symmetry or closely approximate it.34 Approximately 97% of the conformations in crystal structures of molecules related to cellobiose are in a region with j = from
180° to
20° and ψ = from
220° to
60°.29 The parameters of conformer X, which was reported by Strati et al.14 as the lowest conformer in energy among the regular conformers, were selected as the starting conformer of our DFT calculation. The calculated values of j (j0 ) and ψ (ψ0 ) were
108.3° (
120.0°) and
157.3° (
158.0°), respectively. These values just fell in the region French and Johnson mentioned.31 Therefore, the values of j (j0 ) and ψ (ψ0 ) calculated in this work were rational. The orientation of hydroxymethyl groups in the optimized conformer were tg0 , tg, and tg00 for the three glucose residues, respectively. This was the same as the conformer X obtained by Strati et al.14 The hydroxyl group orientation of this conformation was all counterclockwise. Therefore, it was rational to employ cellotriose to simulate the dehydration of cellulose. 3.2. Dehydration with
O2H Involved. The dehydration with
O2H involved is shown in Schemes 1
3. Schemes 1 and 2 follow the mechanism of 1,2-dehydration, where the hydroxyl group
O2H is lost, with the hydrogen atom connected to either C1 or C3. As shown, the transition states for Schemes 1 and 2 have four atom centers. Nimlos et al. have used this mechanism to explain the dehydration of alcohols and glycerol.11,13

Figure 2. Structures of transition states and products of dehydration involving
O2H, with the critical bond length indicated in angstroms. 2666

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Energy & Fuels Paine III et al.35
38 proposed a mechanism of hydrogen-bondingassisted Grob fragmentation for carbohydrate pyrolysis based on solid experiental works. A similar mechanism, hydrogen-bondingassisted pinacol rearrangement, would be another possible pathway for
O2H on cellotriose to dehydrate. Hydrogen-bonding-assisted pinacol rearrangement can be illustated using 2,3-butanediol as an example in the following scheme. A five-membered ring would be generated via hydrogen-bonding involving adjacent hydroxyl groups. When electron migration is forced around this ring system, 2, 3-butanediol can be made to dehydrate in concerted electrocyclic fashion with the methyl group migrating.

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Figure 3. Potential energy profiles of cellotriose dehydration pathways involving
O2H. The dehydration pathways are R1, R2, and R3 described in Schemes 1
3, respectively.

Scheme 4

Scheme 3 follows such a mechanism assisted by intramolecular hydrogen bonding, O2 3 3 3 H
O3. The loss of a water molecule occurs with a concerted migration of a hydride, H
C3. This mechanism was employed by Nimlos et al. to explain the dehydration of glycerol.11 The structures of transition states and products of Schemes 1
3 are shown in Figure 2, with the distances between critical atoms indicated. Consistent with the convention of 1,2-dehydration of alcohols, the β hydrogen atom is H
C1 for Scheme 1 and H
C3 for Scheme 2, whereas the R and β carbon atoms are C2 and C1 for Scheme 1 and C2 and C3 for Scheme 2, respectively. For TS 1 and TS 2, the Cβ
Hβ bond lengths were 1.615 and 1.509 Å, the CR
Cβ bond lengths were 1.451 and 1.425 Å, and the CR
O bond lengths were 1.847 and 1.937 Å. The values for TS 1 were similar to those calculated for the transition state of 1,2-dehydration of glycerol.11 However, the values for TS 2 were larger than those calculated for glycerol. The difference of the CR circumstance might cause the difference of the bond lengths in the transition states. The effect of dehydration on the glycosidic dihedral angle can also be seen from Figure 2. The vibrational analysis of TS 1 and TS 2 suggested that the leaving of β hydrogen atoms from β carbon atoms was during the kinetic controlling step of the 1,2-dehydration. As shown, the transition states for Scheme 3 had five atom centers. The bond lengths were 1.990, 1.315, 1.467, and 3.016 Å for O3
H30 , O3
C3, C3
C2, and C2
O2, respectively. The vibrational analysis of the TS 3 implied that the migration of H3 from C3 to C2 was during the kinetic controlling step of hydrogen-bonding-assisted pinacol rearrangement. The potential energies and energy barriers of the species involved in Schemes 1
3 are summarized in Figure 3. The energy of cellotriose is set to 0 point reference. All energy values of the other types are shown as they relate to this 0 point reference. The energy barrier for Scheme 3 is 63.2 kcal/mol, which compares well to the value calculated11 using the B3LYP/ 6-311G**(d, p) method, 65.2 kcal/mol. There was only about 2 kcal/mol difference. This confirms the accuracy of the calculation at the GGA/RPBE/DNP level.

Scheme 5

The energy barriers for Schemes 1 and 2 are 99.1 and 88.8 kcal/mol, respectively. These values are much higher than those calculated for 1,2-dehydration of glycerol11 and simple alcohols.8 Calculated barriers for the dehydration of a number of alcohols8 are reported to be 66
70 kcal/mol, and experimental values are 2667

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Energy & Fuels 66.2 kcal/mol for tert-butyl alcohol39 and 64.7 kcal/mol for 2, 3-dimethybutan-2-ol.40 Such a high difference in energy barriers might be caused by the difference of model compounds selected. Specifically speaking, C2
OH is closed to the acetyl group at C1, which might make it difficult for
O2H to leave according to the 1,2-dehydration mechanism. In addition, the existence of hydrogen bonding O2
H 3 3 3 O600 makes the 1,2- dehydration for O2
H more difficult. These calculated results suggested that the Scheme 6

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pinacol rearrangement was a more energy-preferable mechanism for dehydration involving the
O2H group. 3.3. Dehydration with
O3H Involved. There is a similarity between the dehydration reactions of
O3H and
O2H. The dehydration reactions involving the hydroxyl group
O3H are shown in Schemes 4
6. Schemes 4 and 5 follow the mechanism of a 1,2-dehydration, while Scheme 6 follows a pinacol rearrangement mechanism assisted by intramolecular hydrogen bonding, O3 3 3 3 H
O2. The loss of a water molecule occurs with a concerted migration of a hydride, H
C2. The structures of transition states and products of Schemes 4
6 are shown in Figure 4, with the distances between critical atoms indicated. The β hydrogen atom was H2 for Scheme 4 and H4 for Scheme 5, whereas the R and β carbon atoms are C3 and C2 for Scheme 4 and C3 and C4 for Scheme 5. As shown, the transition states for Schemes 4 and 5 had four atom centers. For TS 4 and TS 4, the Cβ
Hβ bond lengths were 1.415 and 1.378 Å, the CR
Cβ bond lengths were 1.457 and 1.435 Å, and the CR
O bond lengths were 2.083 and 2.166 Å. The values for TS 4 and TS 5 were similar to those calculated for the transition state of 1,2-dehydration of glycerol.11 The vibrational analysis of TS 4 and TS 5 suggested that the leaving of β hydrogen atoms from β carbon atoms was during the kinetic controlling step of the direct 1,2-dehydration. This was similar to that of the dehydration involving
O2H. Scheme 6 followed a pinacol rearrangement mechanism assisted by hydrogen bonding, O3 3 3 3 H
O2, with a transition state of five atom centers. As shown in Figure 3, the bond lengths

Figure 4. Structures of transition states and products of dehydration involving
O3H, with the critical bond length indicated in angstroms. 2668

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Energy & Fuels were 1.811, 1.311, 1.482, and 2.819 Å for O2
H20 , O2
C2, C2
C3, and C3
O3, respectively. The vibrational analysis of TS 3 implied that the kinetic controlling step of hydrogenbonding-assisted pinacol rearrangement was the migration of H2 from C2 to C3, which was similar to the case of
O2H. The potential energies and energy barriers of the species involved in Schemes 4
6 are summarized in Figure 5. The energy of cellotriose is set to 0 point reference, and all energy values of the other groups given are relative to this 0 point reference. The energy barriers are 76.4 and 71.9 kcal/mol for Schemes 4 and 5, respectively. These values are closed to the reported 66
70 kcal/mol for the dehydration of a number of alcohols.8 This confirms the accuracy of the GGA/RPBE/DNP method for the second time. The energy barrier for Scheme 6 is 69.2 kcal/mol, which also compares well to the reported 65.2 kcal/mol.11 These calculated results also suggested that

Figure 5. Potential energy profiles of cellotriose dehydration pathways involving
O3H. The dehydration pathways are R4, R5, and R6 described in Schemes 4
6, respectively.

Scheme 7

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the pinacol rearrangement was a more energy-preferable mechanism for the dehydration-involved
O3H group. 3.4. Dehydration with
O6H Involved. The dehydration reactions involving the hydroxyl group
O6H are shown in Scheme 7. Scheme 7 follows the mechanism of 1,2-dehydration, where the hydroxyl group
O6H is lost, with the hydrogen atom connected to C5. As shown, the transition states for Scheme 7 had four atom centers. The structures of transition states and products of Scheme 7 are shown in Figure 6, with the distances between critical atoms indicated. As shown, the transition states for Scheme 7 had four atom centers. For TS 7, the Cβ
Hβ bond length was 1.667 Å, the CR
Cβ bond length was 1.457 Å, and the CR
O bond length was 1.735 Å. The values for TS 7 were similar to those calculated for the transition state of 1,2-dehydration of glycerol.9 The vibrational analysis of TS 7 suggested that the leaving of β hydrogen atoms from β carbon atoms was the kinetic controlling step during the direct dehydration. The potential energies and energy barriers of the species involved in Scheme 7 are summarized in Figure 7. The energy of cellotriose is set to 0 point reference, and all energy values of the other species are stated as relative to this 0 point reference. The energy barrier is 73.1 kcal/mol for Scheme 7, which is a little higher than the 1,2-dehydration of alcohols.11 This may be caused by hydrogen bonding, O20
H 3 3 3 O6. Considering the single pyranose ring, which would be produced during the pyrolysis of cellulose, hydrogen-bondingassisted Grob fragmentation might be another possible mechanism of dehydration.36
38 This is a 1,3-diols dehydration mechanism and is shown in the following scheme. Ring flipping is needed prior to Grob fragmentation. The calculated barriers for ring flipping and Grob framentation were 27.3 and 96.3 kcal/mol, respectively. It is clear that the Grob fragmentation is the kinetic controlling step. This pathway is less likely to occur compared to pinacol rearrangement. This Grob fragmentation should also be available to the glucopyranose subunits of cellulose; for example,

Figure 7. Potential energy profiles of cellotriose dehydration pathways involving
O6H. The dehydration pathway is R7 described in Scheme 7.

Figure 6. Structures of transition states and products of dehydration involving
O6H, with the critical bond length indicated in angstroms. 2669

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Energy & Fuels the interaction between the 6-hydroxyl group and the 4-ether linkage can convert the 6-hydroymethyl substiture to formaldehyde, while cleaving the cellulose chain via Grob fragmentation.36 The method employed here should be applicable to study these reactions. This would be meaningful work for exploring a further pyrolysis mechanism of cellulose and should be carried out in more detail in future studies.

4. CONCLUSION The present DFT study confirms that the location of hydroxyl groups has a significant influence on the dehydration of cellulose. The pinacol rearrangement is a more likely mechanism for the dehydration of hydroxyl groups. The energy barrier for the 1, 2-dehydration of
O2H is much higher than the values found in the literature.8,11 It might be due to the fact that C2-OH is closed to the acetyl group at C1, which might make it difficult for
O2H to leave, according to the 1,2-dehydration mechanism. In addition, the existence of hydrogen bonding O2
H 3 3 3 O600 makes 1,2-dehydration for O2
H more difficult. The energy barriers for the 1,2-dehydration of
O3H in the central glucose residue are slightly higher than those in alcohols reported by Nimlos et al.8 The possible reason would be the hydrogen-bonding effect formed between hydroxyl groups from adjacent glucose residues. The energy barriers to the pinacol rearrangement were 63.2 and 69.2 kcal/mol for
O2H and
O3H, respectively. The direct dehydration was the only way for
O6H, with an energy barrier of 73.1 kcal/mol. Therefore, it can be concluded that the most active hydroxyl group was
O2H, followed by
O3H and
O6H. Although the barrier for the dehydration of
O2H is the lowest, it is still so high that the dehydration reaction needs to occur at a high temperature. This means that the hydroxyl group is hard to separate from the pyran ring. To gain chemical products directly from the pyrolysis of cellulose, various hydroxyl groups might be targeted. This paper just provides information about the reactions of hydroxyl groups. In the pyrolysis of cellulose, a complete study of the reaction network of the pyrolysis is critically important. In addition to the dehydration, there are many other reactions involved during the pyrolysis of cellulose to be investigated, such as the break of glycosidic linkage and the opening of the pyran ring. Therefore, more efforts to explore the mechanisms of cellulose pyrolysis are needed in the future. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ86-22-2740-6119. Fax: þ86-22-2740-6119. E-mail: [email protected].

’ REFERENCES

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