ARTICLE pubs.acs.org/JPCB
Mechanistic Insights into the Decomposition of Fructose to Hydroxy Methyl Furfural in Neutral and Acidic Environments Using High-Level Quantum Chemical Methods Rajeev S. Assary,*,†,|| Paul C. Redfern,‡ Jeffrey Greeley,§ and Larry A. Curtiss*,†,§ Materials Science Division, ‡Chemical Sciences and Engineering Division, and §Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States Chemical & Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States
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†
bS Supporting Information ABSTRACT: Efficient catalytic chemical transformation of fructose to hydroxy methyl furfural (HMF) is one of the key steps for attaining industrial level conversion of biomass to useful chemicals. We report an investigation of the reaction mechanisms for the decomposition of fructose to HMF in both neutral and acidic environments at the Gaussian-4 level of theory including calculation of enthalpies, free energies, and effective solvation interactions. In neutral water solvent, the transformation of fructose to HMF involves a four step reaction sequence with four transition states. The effective activation energy relative to fructose in neutral water at 298 K is very large, about 74 kcal/mol, so that transformation in neutral media around this temperature is unlikely. In contrast, the computed potential energy surface is much more favorable for the transformation in acidic media at 498 K, as the effective activation barrier is about 39 kcal/mol. The transformation in acidic media is a much more complex mechanism involving dehydration and hydrogen transfer steps, which are more favorable when protonated intermediates are involved.
1. INTRODUCTION The development of fundamental understanding of the thermodynamics and kinetics of basic chemical transformations of sugar molecules is vital for the production of sustainable energy and useful chemicals from biomass. Chemical transformations such as hydrolysis of cellulose to carbohydrates and subsequent selective dehydration, rehydration, hydrogenation, condensation, and oxidation could lead to potential transportation fuels and industrial chemicals. Recent research18 on the catalytic conversion of carbohydrates to liquid alkanes (C3C15) and useful industrial chemicals such as 5-hydroxy methyl furfural (HMF), levulinic acid, lactic acid, and furfural highlight developments in this area. However, the design of more efficient catalysts and the development of new insights into the kinetics and thermodynamics of these basic chemical transformations still remain a challenge due to the complexity of chemical transformations and pathways. Obtaining accurate information on the energetics for these processes is an important aspect of addressing these challenges. Recent developments in advanced quantum chemical methods and faster computers, in turn, have now put such goals within reach for the large molecules involved in the biomass conversion processes. Most of the previous theoretical studies in this area have been aimed at determining the structures of carbohydrates, their conformational analysis, and the calculation of their interaction energies with other hydrocarbons.916 Quantum chemical investigations of the energies at 0 K of reaction intermediates and transition states of glycerol dehydration and xylose r 2011 American Chemical Society
decomposition under acidic conditions have been reported.17,18 In a recent study,19 we presented high-level quantum chemical predictions for thermochemical data of intermediates in a likely decomposition mechanism of glucose to levulinic acid and assessment of the performance of density functional methods. The computed gas phase reaction enthalpies indicated that the first two steps involving water elimination from fructose are highly endothermic (by up to 22 kcal/mol) and rehydration of hydroxy methyl furfural to levulinic acid is highly exothermic (32 kcal/mol);19 however, no kinetic barriers were reported in the indicated work. The investigation reported here focuses on elucidation of the thermodynamics and the activation barriers for fructose decomposition, through fructofuranosyl intermediates, to HMF, with an ultimate goal of providing an accurate, kinetically comprehensive analysis of this key pathway for biomass coversion.1,2,20 We have employed the Gaussian-4 series of theory,24,25 and free energies were also calculated to include entropic effects, which can be important. The calculations were done both for neutral media, for which we have previously calculated thermochemical data,19 and for acidic media. These results provide insights into kinetically significant steps and give useful benchmarks for the design of new and improved catalysts for the selective dehydration of fructose. Received: November 1, 2010 Revised: February 25, 2011 Published: March 28, 2011 4341
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Figure 1. Schematic representation of the reaction scheme predicted for the decomposition of fructose (A) to hydroxy methyl furfural (I) through various intermediates. All geometries are optimized at the B3LYP/6-31G(2df,p) level of theory. The transition states are shown in rectangular boxes.
2. COMPUTATIONAL METHODS The Gaussian-4 (G4) theory21 was developed with the goal of calculating molecular energies within chemical accuracy. G4 theory has an average absolute deviation from experiment of 0.83 kcal/mol from an assessment on the 454 energies in the G3/05 test set composed of enthalpies of formation, ionization energies, electron affinities, proton affinities, and hydrogen bond energies.22 Also, several recent studies23,24 have shown that 1 kcal/mol accuracies can be obtained by using G4 methods for prediction of transition state barriers compared to a very accurate large basis set coupled cluster calculations. We have employed the G4 level of theory to calculate the thermochemical data and reaction barriers for the decomposition of fructose to HMF. In addition to the G4 method, the computationally less demanding G4MP225 method has also been used in this study. The geometries and zero-point energies (scaled by 0.9854) for the G4 methods were calculated at the B3LYP/6-31G(2df,p) level of theory. Frequency calculations were performed to verify the nature of all the stationary points as either minima or transition states and to provide zero point energy corrections. We have not carried out an accurate conformational analysis of the molecular species involved in the reaction mechanisms. This would require a detailed study due to the large number of conformations possible for species such as glucose and fructose. Instead, we have selected conformers based on previous investigations.13,14,19 We have used the lowest energy conformer for glucose and for the subsequent structures resulting from the dehydration steps and tautomerization steps. The single point MP4/6-31G(2df,p) energies required for the G4 method were calculated using 10 f functions.22,25 In order to account for the effects of aqueous environment, calculations were also performed in water dielectric using the recently developed SMD26 solvation model at the B3LYP/6-31G(2df,p) level of theory.
The calculations for this investigation were done using Gaussian 03,27 Gaussian 09,28 and NWCHEM.29
3. RESULTS AND DISCUSSION 3.1. Neutral Fructo-furanose to HMF. Enthalpic Profile. The mechanism proposed from previous experimental studies1,2,30,31 for dehydration of neutral fructo-furanose to HMF in neutral media is shown in Figure 1. This is the mechanism that we have investigated here. The chemical transformations involve initial dehydration of fructose (A) to produce an enol (C) and one water molecule through transition state B†, tautomerization from enol form C to the corresponding aldehyde (E) through a transition state D†, and two subsequent dehydration processes to produce HMF (I) (E to G, through F†, and G to I, through H†). Equilibrium geometries for the five reaction intermediates (A, C, E, G, H) and four transition state geometries (B†, D†, F†, H†) were located at the B3LYP/6-31G(2df,p) level of theory, and the optimized geometries, with selected bond distances, are shown in Figure 1. The computed energetics ΔEe (change in absolute energy), ΔE0 (change in energy including zero-point vibration energy), ΔH298K (enthalpy change at 298 K), and ΔG298K (Gibbs free energy change at 298 K) of all intermediates and transition states (A to I) at the B3LYP/6-31G(2df,p), G4MP2, and G4 levels of theory are shown in Table 1. Also given is ΔGsolv, the solvation energy computed from the difference in energy between the SCRF energy at water dielectric and gas phase with nonelectrostatic free energy terms at the B3LYP/6-31G (2df,p) level of theory using the SMD solvation model. The G4 energies of the intermediates in Table 1 were also previously reported in our G4 study of the decomposition of glucose to levulinic acid.19 The predicted gas phase enthalpy profile at 298 K for the formation of HMF from fructose at the G4 level of theory is 4342
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Table 1. Relative Gas Phase Energetics of Various Intermediates during the Chemical Transformation of Fructose to HMF at DFT (B3LYP/6-31G (2df,p)), G4MP2, and G4 Levels of Theorya
species
B3LYP 6-31G(2df,p)
B3LYP 6-31G(2df,p)
G4MP2
ΔEe
ΔGsolv
ΔEe
0.0
G4 ΔEe
ΔE0
ΔH298K
ΔG298K
A
0
0
0
0
0
0
B†
59.3
4.8
66.6
66.4
61.4
61.5
61.5
C þ H2O D† þ H2O
24.7 87.6
3.8 8.1
23.4 87.9
24.3 88.8
19.8 80.4
21.5 81.9
9.2 69.9
Eþ H2O
17.5
11.2
15.8
16.5
11.6
13.3
0.6
F† þ H2O
76.8
2.0
80.3
80.7
72.5
73.6
62.4
G þ 2H2O
29.1
3.9
25.4
26.8
17.7
20.9
3.3
H† þ 2H2O
85.5
4.3
84.2
85.2
71.2
74.7
50.0
I þ 3H2O
22.6
8.4
18.5
20.4
7.8
12.3
22.9
a ΔGsolv is calculated at the B3LYP/6-31G(2df,p) level of theory using the SMD-PCM model using Gaussian 09 software. All energies are given in kcal/ mol and with respect to that of fructose (A). For details of the labels see Figure 1.
Figure 2. Computed enthalpy changes (kcal/mol) for the conversion of fructose (A) to hydroxy methyl furfural (I) in the gas phase and at 298 K at the G4 level of theory.
shown in Figure 2. All energies are relative to that of the fructose molecule. The first dehydration barrier to form the enol form (C) from fructose is calculated to be 61.5 kcal/mol, where the tertiary alcohol group and hydrogen atom from the hydroxy methyl side group combine to form the leaving water molecule. This is very similar to the previously reported 1,2-dehydration barrier for neutral glycerol17 (70.9 kcal/mol) and glyoxal32 (67.5 kcal/mol). This reaction is endothermic due to breaking of a strong COH bond and a HCH bond while the only energy stabilization occurs through the formation of the HOH bond and relative strengthening of the single CC bond to form a double bond. The positive charge produced at the carbon center after the cleavage of the COH bond is stabilized by the presence of an oxygen atom, and hence formation of a partial double bond (1.30 Å), between the carbon and the oxygen atom.
The barrier for the second reaction, hydrogen shift from the enol hydroxy group to the electron deficient carbon center, is found to be very large with a height of 61.4 kcal/mol relative to intermediate C and 81.9 kcal/mol relative to fructose. This is similar to the calculated range for ketoenol tautomerization reactions33 (7677 kcal/mol) for acetaldehyde and acetone in the gas phase. The tautomerization barrier may actually be lower than this, as theoretical studies have shown that explicit inclusion of either one water molecule or water clusters decreases the enolketo transition barriers significantly (by ∼30 kcal/mol) for acetone.37,38 We have done a calculation with a single water molecule added, and the barrier to ketoenol tautomerization (C f E) is reduced by about 30 kcal/mol at the B3LYP/ 6-31G(2df,p) level (see Supporting Information, Figure S2). The large barrier for this transition state is due to the weakening of the 4343
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Table 2. Comparison of Computed Reaction Barriers (ΔEe, kcal/mol) at Various Levels of Theory on Geometries Obtained from the B3LYP/6-31G(2df,p) Level of Theorya Reaction †
G4MP2
G4
CCSDT(T)/CC-pVTZ
66.4
66.6
66.7
C f D†
64.5
64.5
64.0
E f F†
64.5
64.2
63.4
G f H†
58.8
58.4
61.8 (60.40)
1 f 2† 4 f 5†
11.0 18.7
11.7 18.7
12.3 18.7
AfB
6 f 7†
0.9
0.8
0.9
8 f 9†
20.7
20.7
20.6
10 f 11†
20.6
20.8
21.5
12 f 13†
43.3
43.0
42.7
14 f 15†
10.3
10.8
11.9
a
For reactions labels (A to H) and (1 to 15), see Figures 1 and 4, respectively. The number given in the parentheses corresponds to the barrier computed at the CCSD(T)/G4Large level of theory.
Figure 3. Relative Gibbs free energy changes (kcal/mol) at the G4 level of theory for various reactions involved in the dehydration of fructose to HMF. (a) Gas phase at 298 K. (b) Gas phase at 498 K. Values in parentheses are changes in free energy of reactions in water dielectric at the respective temperatures. The solvation energy was calculated at the B3LYP/6-31G(2df,p) level of theory using the SMD solvation model. For details of labels see Figure 1.
CO bond in the ring and the breaking of the enolic OH bond, thus forming a sterically hindered four membered transition state. When at least one explicit water molecule is included in the calculation, the transition state becomes a six membered ring. Overall tautomerization to form the aldehyde from enol is exothermic by 8.2 kcal/mol. The activation barriers for the second and third dehydration processes are 60.3 and 53.9 kcal/ mol, respectively, from their precursors, and 73.6 and 74.7 kcal/ mol relative to that of fructose. These are similar to that of the first hydration step. The second dehydration is found to be less endothermic (þ7.6 kcal/mol) compared to that of the first
dehydration, while the third dehydration to form a stable aromatic furan ring is exothermic by 8.6 kcal/mol. The G4 methodology is a composite method that includes a CCSD(T)/6-31G* calculation. We have also calculated the barriers using the CCSD(T) method with a larger basis set (CCSD(T)/CC-pVTZ) to verify the G4 barriers. The results are given in Table 2 and indicate good agreement between G4 and the larger CCSD(T) calculations for the barrier heights (generally within about 1 kcal/mol). This is in accord with previous assessments on smaller systems.24 The G4 dehydration and tautomerization barriers are very high and require a large amount of thermal energy to overcome them. Since at high temperatures there is a clear possibility of ring-opening of fructose and rearrangements such as enolization and carboncarbon bond cleavage occurring also, the dehydration pathway is likely to be much more complex and beyond the scope of this present study as it requires detailed investigation of alternate decomposition pattern through acyclic intermediates. Importantly, at higher temperature water itself is capable of acting as an acid or base catalyst due to its ionization constant, and this would lead to other reaction mechanisms; thus, the pathways for an acidic media discussed in the next section are more appropriate.34 Free Energy Profile. The G4 gas phase free energy changes at 298 and 498 K for fructose decomposition are shown in a schematic representation in Figure 3. In addition to the gas phase values, the computed free energy changes associated with all reactions in water dielectric at 298 and 498 K are given in the figure. Inclusion of solvation effects with explicit water molecules for sugar molecules is essentially intractable due to the extensive hydrogen bond formation and intramolecular hydrogen bond networks within the molecules.15,16 Therefore, as discussed in Computational Methods, the solvation energy from the SMD model (Table 1, ΔGsolv) is used to approximate the solvation effects from the water medium. At 298 K in the gas phase, the apparent activation free energy barriers for the first, second, and third dehydration processes are 61.5, 62.4, and 50.0 kcal/mol, respectively, with respect to the neutral fructose molecule. The activation free energy of hydrogen shift for the tautomerization is higher (69.9 kcal/mol, D) than that of the dehydration barrier. Inclusion of solvent effects reduces the activation free energy barriers by 2.4, 4.3, and 8.4 kcal/mol, respectively, for the first, second, and third dehydration processes. The activation free energy barrier for the hydrogen shift was also decreased by 3.9 kcal/mol due to the stabilization by solvent. Overall, inclusion of solvent effects makes the fructose to HMF conversion process more downhill by 11.2 kcal/mol. Apart from the first dehydration process, all the other free energy barriers have a significant reduction at 498 K compared to those at 298 K (see Supporting Information, Table S1). The activation free energy barriers for tautomerization and the second and the third dehydration processes are computed to be lower by 8.0, 7.4, and 16.9 kcal/mol, respectively, at 498 K compared to at 298 K. The overall free energy of the reaction at 498 K is 46.8 kcal/mol, and this becomes more downhill by the inclusion of solvation effects, to 57.9 kcal/mol. It should be noted that at higher temperatures, such as 498 K, ionization of water increases, and the reaction mechanism is likely to become similar to that in acidic media. There is evidence for this from experimental studies of this reaction in water at higher temperatures where the rates are similar to those in solutions with added acids.35 The following analysis of the barriers for fructose decomposition is done at 298 K for this reason. 4344
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Table 3. Relative Gas Phase Energetics of Various Intermediates during the Chemical Transformation of Protonated Fructose (1) to HMF (17) at DFT (B3LYP/6-31G (2df,p)), G4MP2, and G4 Levels of Theorya B3LYP 6-31G(2df,p)
B3LYP 6-31G(2df,p)
G4MP2
ΔEe
ΔGsolv
ΔEe
1
0.0
61.9
2†
11.2
62.7
3 4
24.6 28.4
62.9 67.3
5†
45.9
6
35.6
7†
G4 ΔEe
ΔE0
ΔH298K
ΔG298K
0.0
0.0
11.0
11.7
0.0
0.0
0.0
9.2
10.2
21.9 26.5
7.7
23.1 27.7
18.0 22.7
20.3 24.8
6.3 11.2
64.0 66.2
45.2
46.4
40.3
41.7
29.6
33.1
34.2
29.9
31.4
36.5
19.3
65.5
34.0
35.0
30.3
31.6
8
19.9
35.6
66.2
33.1
34.2
29.9
31.4
19.3
9†
57.4
64.6
53.8
54.9
47.9
50.1
36.4
10
70.0
69.1
61.8
63.3
54.6
57.6
33.7
11† 12
85.3 33.9
69.8 64.4
82.4 29.1
84.1 30.9
72.4 21.3
75.8 24.7
50.8 0.1
13†
74.3
66.8
72.4
73.9
62.9
66.1
41.1
14
44.2
61.4
37.1
39.5
26.0
30.9
5.8 7.0
species
15
31.2
58.3
26.8
28.7
16.5
20.9
16†
40.9
61.5
41.2
42.7
29.4
33.0
7.7
17
69.9
7.2
63.0
65.0
53.5
58.0
21.4
ΔGsolv is calculated at the B3LYP/6-31G(2df,p) level of theory using the SMD-PCM model using Gaussian 09 software. All energies are given in kcal/mol and are with respect to that of fructose (1). For details of the labels see Figure 4.
a
If one assumes that the highest point on the free energy surface determines the effective free energy of activation for the decomposition of fructose to HMF (this analysis corresponds to applying the quasi-steady state assumption to the reactive intermediates in the pathway), this will correspond to the tautomerization (second) step at 298 K. The activation energy (enthalpy) relative to fructose for this step is 82 kcal/mol. Since our results indicate that explicit inclusion of a water molecule in the calculation significantly reduces the tautomerization reaction barrier, however, the highest point is likely to be either the second or third dehydration step with barriers of about 74 kcal/ mol, when such explicit effects are taken into account. 3.2. Protonated Fructo-Furanose to HMF. Experimentally, glucose or fructose decomposition is performed in the presence of mineral acid or zeolite such as HZSM5 or large pore Sn/Tibeta.39,36 Both in mineral acids and in HZSM5, fructose decomposition is believed to follow a similar mechanism starting from protonated fructose and subsequent acid catalyzed dehydration and rearrangements. In this study, we have investigated the dehydrationdecomposition mechanism of protonated fructose to HMF, similar to a previously reported xylose decomposition study.18 Initially we have assessed the free energy of protonation of various hydroxyl groups of the fructose molecule (see Supporting Information, Figure S1) and identified that the protonation is thermodynamically more favorable at the tertiary hydroxyl group. It is, however, possible that there will be competition for the proton between the water solvent and the hydroxyl groups of the solvated fructose molecule. It is known that the standard free energy of solvation of a proton in water is in the range 252262 kcal/mol.37 This is comparable to the standard free energy of protonation of the various hydroxyl groups of solvated fructose, which range from 255 to 265 kcal/ mol from B3LYP/6-31G(2df,p) calculations (see Supporting Information, Figure S1). Considering the most favorable protonation
site, the tertiary hydroxyl group (265 kcal/mol), the free energy change for conversion of a solvated proton to a solvated protonated fructose molecule would be in the range 3 to 13 kcal/mol. This free energy change indicates that protonation of fructose is likely to occur in acidic media. Reaction Pathway. Assuming protonation of the tertiary hydroxyl group, we have investigated a possible mechanism of decomposition of fructose to HMF through various intermediates including dehydration and hydride shift transition states. Optimized structures of all the intermediates including seven transition states (117) are shown in Figure 4. Initially, protonated fructose (1) with the tertiary hydroxyl group as the protonation site acts as a good leaving group. Hence, the initial COH2 bond cleavage occurs through the dehydration transition state (2† TS). Complete detachment of water from this geometry results in formation of a double bond [R(CdO) = 1.26 Å] between the oxygen and the carbocation to form intermediate (3). Rotation of the OH hydrogen atom of the hydroxy-methyl group of 3 toward to the adjacent hydroxyl group to form an intramolecular hydrogen bond (2.25 Å) results in the formation of intermediate (4). This is followed by a 1,2-hydrogen shift through a transition state (5†) to form a precursor ion to formation of an aldehyde intermediate (6). This intermediate transfers a proton to an adjacent OH group through a proton transfer transition state (7†) to form an intermediate (8). Dehydration of (8) through a transition state (9†) leads the formation of intermediate (10) upon complete removal of the second water molecule, and the final hydroxyl group in the fructose ring forms a bridged complex. This is followed by a rearrangement through hydrogen transfer (1,2-hydride shift) to form a CH2 group adjacent to the hydroxyl group (12) through a transition state (11†). Similar to the transition state 9, the third dehydration proceeds through a transition state via 1,2-dehydration (13†) to form protonated HMF (14). Further, we have 4345
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Figure 4. Optimized equilibrium and transition state geometries of various intermediates involved in the chemical transformation of protonated fructose (1) to hydroxy methyl furfural (17) at B3LYP/6-31G(2df,p) level of theory.
investigated the conversion of protonated HMF to HMF catalyzed by a single water molecule. Initially protonated HMF (14) is complexed with one water molecule (15), followed by a proton transfer transition state (16†) that leads to the formation of HMF (17) and a hydronium ion. Enthalpic Profile. The computed energetics ΔEe (change in absolute energy), ΔE0 (change in energy including zero-point vibration energy), ΔH298K (enthalpy change at 298 K), and ΔG298K (Gibbs free energy change at 298 K) of all intermediates (117) at B3LYP/6-31G(2df,p), G4MP2, and G4 levels of theory are given in Table 3. Also given is ΔGsolv, the solvation energy computed from the difference in energy between the SCRF energy in a water dielectric and in the gas phase with nonelectrostatic free energy terms at the B3LYP/6-31G (2df,p) level of theory using SMD solvation model. The predicted G4 enthalpy profile at 298 K is shown at Figure 5. Due to significant electrostatic stabilization of protonated intermediates from solvent interactions, we have incorporated the electrostatic contribution from the water solvent in the enthalpies shown in Figure 5. Note that all enthalpies and free energies in the figures and text are relative to protonated fructose instead of the solvated proton because the free energy change favors protonation of
fructose over a solvated proton by about 313 kcal/mol as discussed above. The first dehydration reaction barrier (1 f 3) for protonated fructose is only 8.5 kcal/mol, compared to 61.5 kcal/mol for the first dehydration barrier in the neutral medium. This dramatic reduction of the barrier is due to the fact that the protonated hydroxyl group at the tertiary carbon atom is a good leaving group; however, removal of the initial water molecule from protonated fructose is still endothermic. The activation enthalpy required for the 1,2-hydride shift (4 f 6) is quite large, 31.2 kcal/ mol. The subsequent proton transfer and aldehyde formation (6 f 8), on the other hand, do not require significant expenditure of energy. From the aldehyde (8), the second dehydration activation barrier (9) is þ19.9 kcal/mol, and the second dehydration process (8 f 10) is endothermic by þ15.3 kcal/mol. The 1,2-hydride shift (10 f 12) through transition state 11† requires 17.1 kcal/mol activation enthalpy. The resulting intermediate 12 is surprisingly stable compared to the other intermediates, and this 1,2-hydride shift is calculated to be very exothermic from the previous intermediate 10 (28.5 kcal/ mol). The relative stability of this intermediate is predominantly due to the formation of the oxonium ion, and similar 4346
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Figure 5. Predicted enthalpic profile (298 K) of decomposition of protonated fructose to HMF at G4 level of theory at water dielectric medium. Contribution from the water dielectric medium is calculated at the B3LYP/6-31G (2df,p) level of theory using SMD ionic radii recommended in G09 software. See Supporting Information Table S4 for details of enthalpy changes at gas phase and at 498 K. Note: The water molecule is completely removed from the species 2† (TS), which is considered as the intermediate 3; therefore, the local minimum after the transition state 2† is not shown.
Figure 6. Relative Gibbs free energy profile for the conversion of protonated fructose to HMF at 298 and 498 K in water dielectric using G4 level of theory and SMD solvation model.
intermediates have been experimentally reported in tetrahydro furan derivatives.38 The third 1,2-dehydration step (12 f 14) through transition structure (13†) has a barrier of þ38.6 cal/mol relative to intermediate 12 and is slightly endothermic (þ0.8 kcal/mol). Complexation with a single water molecule and protonated fructose is a slightly exothermic process, and the proton transfer barrier (16†) from the complex (15) to the water molecule is þ9.7 kcal/mol, which leads to formation of HMF (17) and a hydronium ion. Overall, the formation of HMF from protonated fructose is slightly endothermic by þ1.7 kcal/mol on the basis of enthalpies at 298 K. Similar to our analysis in neutral media, we have calculated the barriers for the pathway in acidic media using the CCSD(T) method with the much larger basis set (CCSD(T)/CC-pVTZ). The results given in Table 2 indicate good agreement between G4 and the larger CCSD(T) calculations for the barrier heights (within about 1 kcal/mol, for all cases). As in the case of the barriers for the neutral media, this is in accord with previous assessments on smaller systems.24
Free Energy Profile. Since the feasibility of decomposition of protonated fructose to HMF is controlled by both thermodynamic and kinetic factors, we have analyzed the free energy profile at both 298 and 498 K in water dielectric, and the results are shown in Figure 6. The detailed relative G4 free energies of all intermediates at 298 and 498 K, both in gas phase and aqueous dielectrics, are provided in the Supporting Information (Figure S2 and Tables S2 and S3). The free energy for the overall process is down hill in aqueous solution by 32.7 and 57.4 kcal/mol at 298 K and 498 K, respectively. In addition, the free energy barriers in the reaction pathway are also reduced (1520 kcal/mol) when the temperature is increased. Contrary to the first dehydration pattern in neutral media, in acidic conditions the free energy for the initial dehydration process is exothermic. The free energies of two other intermediates (12 and 14) are also very exothermic with respect to protonated fructose. If one assumes that the highest point on the free energy surface determines the effective activation free energy barrier for the decomposition of fructose to HMF in acidic media (corresponding to a quasi-steady state assumption for the reactive intermediates), then the barrier corresponds to the second dehydration step (8 f 10) at 498 K, the temperature at which most such experiments are carried out.35The activation energy (enthalpy) relative to protonated fructose for this step is 38.8 kcal. At 298 K, on the other hand, the highest point on the free energy surface corresponds to a hydrogen shift (10 f 12), yielding a barrier (11) of 51 kcal/mol. Thus, our calculations indicate that at 298 K in aqueous acid solutions, hydrogen rearrangements (1,2-hydride shift) control the kinetic feasibility and overall outcome of the reaction, rather than the 1,2-dehydration process. This could be one of the reasons for improved catalytic reaction rate of production of HMF from fructose when using catalysts such as large pore Sn-beta, which is known to catalyze 1,2-hydride shifts effectively.39 4347
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The Journal of Physical Chemistry B Experimentally reported activation barriers for fructose decomposition to form HMF assuming first order reaction kinetics reaction in the presence of mineral or Lewis acid are in the range of 3134 kcal/mol.34,4042 Thus, the predicted barrier of about 39 kcal/mol for fructose decomposition at 498 K relative to protonated fructose is in reasonable agreement with experiment, considering the uncertainties that may exist in the inclusion of solvation effects. In addition, no rearrangements are required for the first acid catalyzed dehydration step which thus has a lower barrier than the subsequent dehydration processes that, in turn, require rearrangements to facilitate 1,2-dehydration of the cationic intermediates.
4. CONCLUSION In this paper, we report on a high level, CCSD-based (G4) quantum chemical investigation of the energies and reaction barriers for the dehydration of fructose through fructofuranosyl intermediates to produce HMF in neutral and acidic environments. This is a key process for biomass conversion to useful precursors for alternative fuels and industrial chemicals. In neutral water solvent, the transformation of fructose to HMF involves a four step reaction sequence with four transition states. The computed enthalpic barriers are very large, ranging from 62 to 82 kcal/mol. From a free energy perspective, the highest point on the free energy surface at 298 K corresponds to tautomerization. The effective activation barrier for this step is very large, about 74 kcal/mol, indicating that reactivity in neutral media is not favorable at lower temperatures The thermodynamics and kinetics of the acid-catalyzed fructose conversion mechanism are considerably more favorable. The enthalpy surface is more complex, and the barriers relative to protonated fructose range from 10 to 50 kcal/mol and include dehydration and hydrogen transfer steps. The free energy for the overall reaction to form HMF from fructose is very exothermic. At 498 K, the effective enthalpy of activation is about 39 kcal/mol, which is much smaller than that found in neutral media. This is because dehydration is much more favorable when protonated intermediates are involved. ’ ASSOCIATED CONTENT
bS
Supporting Information. Complete citations for refs 2729, free energy of protonation of various hydroxyl groups of solvated fructose (Figure S1), gas phase proton affinity of glucose and fructose alcohol sites (Scheme S1), schematic representation of alternative decomposition pathways (Scheme S2), effect of explicit water molecule on the activation barrier for the ketoenol tautomerization (Figure S2), relative free energies of the intermediates during decomposition of fructose in neutral aqueous solution at 298 and 498 K (Table S1), relative free energies of the intermediates during decomposition of protonated fructose in gas phase and water dielectric at 298 K (Figure S3), comparison of solvation energies computed with SMD and CPCM methods for all the intermediates (117) during the of decomposition of protonated fructose (Table S2), relative enthalpies and free energies of all intermediates in the gas phase and water dielectric for dehydration of protonated fructose to HMF at 298 and 498 K (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (R.S.A.),
[email protected] (L.A.C.).
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’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy under Contract DE-AC0206CH11357. This material is based upon work supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences. We gratefully acknowledge grants of computer time from EMSL, a national scientific user facility located at Pacific Northwest National Laboratory, the ANL Laboratory Computing Resource Center (LCRC), and the ANL Center for Nanoscale Materials. ’ REFERENCES (1) Huber, G. W.; Chheda, J. N.; Barrett, C. J.; Dumesic, J. A. Science 2005, 308, 1446–1450. (2) Chheda, J. N.; Roman-Leshkov, Y.; Dumesic, J. A. Green Chem. 2007, 9, 342–350. (3) Kunkes, E. L.; Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; Gartner, C. A.; Dumesic, J. A. Science 2008, 322, 417–421. (4) West, R. M.; Liu, Z. Y.; Peter, M.; Gartner, C. A.; Dumesic, J. A. J. Mol. Catal. A: Chem. 2008, 296, 18–27. (5) West, R. M.; Kunkes, E. L.; Simonetti, D. A.; Dumesic, J. A. Catal. Today 2009, 147, 115–125. (6) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. Ed. 2007, 46, 7164–7183. (7) Bond, J. Q.; Alonso, D. M.; Wang, D.; West, R. M.; Dumesic, J. A. Science 2010, 327, 1110–1114. (8) Holm, M. S.; Saravanamurugan, S.; Taarning, E. Science 2010, 328, 602–605. (9) Csonka, G. I.; French, A. D.; Johnson, G. P.; Stortz, C. A. J. Chem. Theory Comput. 2009, 5, 679–692. (10) Appell, M.; Strati, G.; Willett, J. L.; Momany, F. A. Carbohydr. Res. 2004, 339, 537–551. (11) Momany, F. A.; Appell, M.; Strati, G.; Willett, J. L. Carbohydr. Res. 2004, 339, 553–567. (12) Appell, M.; Willett, J. L.; Momany, F. A. Carbohydr. Res. 2005, 340, 459–468. (13) Momany, F. A.; Appell, M.; Willett, J. L.; Bosma, W. B. Carbohydr. Res. 2005, 340, 1638–1655. (14) Momany, F. A.; Appell, M.; Willett, J. L.; Schnupf, U.; Bosma, W. B. Carbohydr. Res. 2006, 341, 525–537. (15) Ma, B. Y.; Schaefer, H. F.; Allinger, N. L. J. Am. Chem. Soc. 1998, 120, 3411–3422. (16) Cramer, C. J.; Truhlar, D. G. J. Am. Chem. Soc. 1993, 115, 5745–5753. (17) Nimlos, M. R.; Blanksby, S. J.; Qian, X. H.; Himmel, M. E.; Johnson, D. K. J. Phys. Chem. A 2006, 110, 6145–6156. (18) Nimlos, M. R.; Qian, X.; Davis, M.; Himmel, M. E.; Johnson, D. K. J. Phys. Chem. A 2006, 110, 11824–11838. (19) Assary, R. S.; Redfern, P. C.; Hammond, J. R.; Greeley, J.; Curtiss, L. A. J. Phys. Chem. B 2010, 114, 9002–9009. (20) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Nature 2007, 447, 982–985. (21) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2007, 126, 084108. (22) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2005, 123, 124107. (23) Wheeler, S. E.; Ess, D. H.; Houk, K. N. J. Phys. Chem. A 2008, 112, 1798–1807. (24) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Chem. Phys. Lett. 2010, 499, 168–172. (25) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 2007, 127, 124105. (26) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378–6396. 4348
dx.doi.org/10.1021/jp1104278 |J. Phys. Chem. B 2011, 115, 4341–4349
The Journal of Physical Chemistry B
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(27) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (28) Frisch, M. J.; et al. Gaussian 09; Gaussian, Inc.: Wallingford CT, 2009. (29) Valiev, M.; et al. NWChem: a comprehensive and scalable open-source solution for large scale molecular simulations. Comput. Phys. Commun. 2010, 181, 1477. (30) Antal, M. J.; Mok, W. S. L.; Richards, G. N. Carbohydr. Res. 1990, 199, 91–109. (31) Antal, M. J.; Mok, W. S. L.; Richards, G. N. Carbohydr. Res. 1990, 199, 111–115. (32) Ohshima, T.; Yamamoto, Y.; Takaki, U.; Inoue, Y.; Saeki, T.; Itou, K.; Maegawa, Y.; Iwasaki, T.; Mashima, K. Chem. Commun. 2009, 2688–2690. (33) Lee, D.; Kim, C. K.; Lee, B.-S.; Lee, I.; Lee, B. C. J. Comput. Chem. 1997, 18, 56–69. (34) Li, Y.; Lu, X.; Yuan, L.; Liu, X. Biomass Bioenergy 2009, 33, 1182–1187. (35) Yao, C.; Shin, Y.; Wang, L.-Q.; Windisch, C. F.; Samuels, W. D.; Arey, B. W.; Wang, C.; Risen, W. M.; Exarhos, G. J. J. Phys. Chem. C 2007, 111, 15141–15145. (36) Moliner, M.; Roman-Leshkov, Y.; Davis, M. E. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 6164–6168. (37) Tawa, G. J.; Topol, I. A.; Burt, S. K.; Caldwell, R. A.; Rashin, A. A. J. Chem. Phys. 1998, 109, 4852–4863. (38) Donohoe, T. J.; Williams, O.; Churchill, G. H. Angew. Chem. 2008, 120, 2911–2913. (39) Collyer, C. A.; Blow, D. M. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 1362–1366. (40) Yao, C.; Shin, Y.; Wang, L. Q.; Windisch, C. F.; Samuels, W. D.; Arey, B. W.; Wang, C.; Risen, W. M.; Exarhos, G. J. J. Phys. Chem. C 2007, 111, 15141–15145. (41) Moreau, C.; Finiels, A.; Vanoye, L. J. Mol. Catal A: Chem. 2006, 253, 165–169. (42) Moreau, C.; Durand, R.; Razigade, S.; Duhamet, J.; Faugeras, P.; Rivalier, P.; Ros, P.; Avignon, G. Appl. Catal., A 1996, 145, 211–224.
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