Theoretical Studies on Thermochemistry for Conversion of 5

ARTICLE pubs.acs.org/JPCA

Theoretical Studies on Thermochemistry for Conversion of 5-Chloromethylfurfural into Valuable Chemicals Gang Liu,†,‡ Jianming Wu,§ Igor Ying Zhang,§ Zhe-Ning Chen,|| Yong-Wang Li,*,† and Xin Xu*,§ †

)

The State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China § Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China

bS Supporting Information ABSTRACT: Recently, 5-chloromethylfurfural (CMF) was proposed as a central intermediate in the conversion of carbohydrate-based material into useful organic commodities. In the present work, we have calculated the thermochemistry using the highly accurate G4 theory and several state-of-art density functional theory (DFT) methods (e.g., X1, M06-2X, B2PLYP-D, and XYG3) for the conversion from CMF to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) in water, and that to biofuels 5-ethoxymethylfurfural (EMF) and ethyllevulinate (EL) in alcohol. New reaction mechanisms have been proposed, which complement the well-recognized Horvat mechanisms. The assessment of DFT methods suggested that XYG3 be a viable method for biomass related thermochemistry calculations.

1. INTRODUCTION With increasing energy demands and environmental concerns, there is a growing interest in developing efficient biorefinery technologies for converting biomass into biofuels and valuable chemicals. Among other biomass-derived building-blocks, 5-hydroxymethylfurfural (HMF) is considered as an important intermediate due to its diverse possession approaches and rich applications.1 HMF can be produced from fructose, glucose, sucrose, and even cellulose, and it can be converted to biofuel 2,5-dimethylfuran (DMF) and other valuable chemicals such as 2,5-dimethyltetrahydrofuran, monomer 2,5 furandicarboxylic acid or 2,5 hydroxymethylfuran, as well as another value-added biomass platform levulinic acid (LA), and so on. Recently, Mascal et al.2
4 reported a new but possibly very valuable biomass-derived platform, 5-chloromethylfurfural (CMF). Not only glucose, sucrose, and cellulose, but also raw biomass (such as cotton, wood, corn stover, and straw) could be converted into CMF in high yields. It was predicted by these authors that CMF may emerge as the central intermediate in the conversion of carbohydrate-based material into useful organic commodities.4 Mascal and co-workers not only demonstrated the conversion from CMF to biofuels 5-ethoxymethylfurfural (EMF) or ethyllevulinate (EL) in alcohol, but also studied the conversion technologies from CMF into HMF or LA in water (cf., Figure 1). However, there is lack of theoretical investigation on the basic thermodynamic properties of CMF, such as its conformation distribution, heat of formation (HOF), etc. More importantly, there is no r 2011 American Chemical Society

accurate energetics being reported up to now for the hydrolysis of CMF to HMF and LA, or alcoholysis of CMF to EMF and EL. Such information is vitally important in addressing the challenge for thermal or catalytic conversion of biomass to fuels and useful chemicals. Predicting thermochemical data within chemical accuracy (∼1 kcal/mol) for medium size molecules (containing ∼15 nonhydrogen atoms) is still a nontrivial task. The Gn family methods5
8 and the CBS family methods9,10 would be the most successful methods in this context. The most recent G4 method8 shows a mean absolute deviation (MAD) from experiment of 0.83 kcal/mol on assessing the 454 energies in the G3/05 set, where the largest molecule contains 12 nonhydrogen atoms. Its widely used predecessor, the G3 method,7 shows an MAD of 1.13 kcal/mol. The CBS-QB3 method9 is also very popular and presents an MAD of 1.21 kcal/mol on assessing the 301 energies in the G2/97 set. However, these high-level composite methods are considerably time-consuming, making them unfeasible for routine use in study of medium size and larger molecules involved in biomass conversion. Noteworthily, some benchmark studies have just appeared.11
16 For example, Curtiss and coworkers have calculated the energetics for conversion of HMF to nonane12 and glucose to LA13 using the G4 method. Montoya Received: August 9, 2011 Revised: October 6, 2011 Published: October 17, 2011 13628

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Figure 1. Production of CMF and its conversion to EMF and EL, as well as its connection to other biomass platforms HMF and LA.

and co-workers have calculated the conformational and thermodynamic properties of LA14 using the G3//B3LYP and CBSQB3 methods. Bozzelli and co-workers have calculated the structures and thermochemical properties of methoxyfurans and corresponding radicals at the CBS-QB3 and G3MP2B3 levels of theory.15 Simmie and Curran have predicted HOFs and bond dissociation energies of alkylfurans using CBS-QB3, CBS-APNO, and G3 methods.16 Density functional theory (DFT) provides a valuable alternative. Because of its low computational cost in conjunction with good precision in calculating various physical and chemical properties of molecules, DFT has become the leading tool for molecular quantum chemical calculations in recent years. In particular, Shen et al.17 and Mu et al.18 have reported the DFT studies on the mechanisms for conversion of glucose to acrolein and HMF, respectively. Note that as the exact exchange-correlation functional is unknown, an approximated exchange-correlation functional has to be chosen in practical applications, whose quality critically influences the final results of the DFT calculations. High-quality Gn and CBS results, therefore, provide benchmarks in assessing and seeking the appropriate DFT method, which can be used for thermochemistry, as well as kinetics, in the study of biomass conversion. Indeed, Curtiss and co-workers13 have examined the performance of the most popular functional B3LYP,19
22 as well as some other functionals such as B3PW91,19,23,24 PW91,24 PBE,25 and RPBE26 against the G4 results for thermochemistry of conversion of glucose to LA. Generally, it was found that DFT results are in agreement with the G4 reaction energies within 2
5 kcal/mol.13 Nevertheless, the performance of a specific functional (e.g., B3LYP/6-31G*) could be good for a certain type of reaction (e.g., the dehydration steps), but poor for the other type of reaction (e.g., the rehydration steps), lacking a consistent description of the whole process. In the present work, we will focus on the thermochemistry for the conversion of CMF. In addition to B3LYP, and its neural network correction method X1,27 we will examine the performance of some recently developed DFT functionals M05-2X,28 M06-2X,29 B2PLYP,30 B2PLYP-D,31 and XYG3.32 Such information should be useful not only for understanding the new chemistry of CMF, but also for selecting the DFT functional suitable for biomass related thermochemistry calculations.

2. COMPUTATIONAL DETAILS Unless otherwise specified, the equilibrium geometry of each molecule was optimized at the level of B3LYP/6-311+G(d,p).33 Subsequently, analytical harmonic frequency calculation was performed at the same level of theory to present zero-point

energy (ZPE with scaling factor 0.987734) and thermo-corrections and to ensure that each geometry corresponds to a true local minimum. Final electronic energies were obtained by single point energy calculations using various flavors of DFT methods in conjunction with the 6-311+G(3df,2p) basis set.33 HOF (298 K, 1 atm) of each species of interest was calculated according to the standard atomization energy scheme as described in Gaussian.35 No spin
orbit corrections are included for all DFT methods. Having HOFs on hand, we calculated the reaction enthalpy changes (ΔrH) as the sum of the HOFs of the products subtracted by the sum of the HOFs of the reactants. Similar procedures were applied to obtain the reaction free energy changes (ΔrG). We also estimated the solvation effect by using the dielectric continuum solvent model. Solvation free energy calculations were actually carried out using the polarizable continuum model in conjunction with the united atom topological model with atomic radii optimized at the Hartree
Fock level (PCM-UAHF(HF)) at 298 K.36 The basis set used is 6-31+G(d). Recently, Coote and co-workers showed that this combination of solvation model leads to MAD of 1.4 kcal/mol for a test set of 50 neutral and ionic solutes, being most satisfactory.37 The free energy change of a reaction in solution phase was calculated according to eq 1. Δr Gsoln ¼ Δr Ggas þ þ RTð

∑ ðΔrGsolv Þ

∑ lnðRT=PÞproducts
∑ lnðRT=PÞreactants Þ

ð1Þ

where the first term refers to the reaction free energy change in the gas phase, the second term represents the sum of solvation free energy change, and the last two terms denote total contribution to free energy of products and reactants converted from the gas-phase standard state (1 atm) to the solution-phase standard state of 1 M. If a solvent molecule also serves as a reactant or a product, another item (ln[Csolvent]) should be included individually, where Csolvent stands for its concentration. PCM-UAHF (HF) calculations were performed using the Gaussian 03 suite of program38 as recommended by Coote and co-workers.37 All other calculations in the present study were carried out using the Gaussian 09 suite of program.39

3. RESULTS AND DISCUSSION The purpose of the present study is two-fold. One is to study the thermochemistry for the conversion of CMF to HMF and LA (routes 1 and 2) and EMF and EL (routes 3 and 4). The other is to use experimental data or high-quality G4 results to guide the selection of appropriate DFT functionals for routine application in biomass related thermochemistry. We first present a methodology 13629

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Table 1. Experimental HOFs (in kcal/mol) and Calculated Deviations for 18 Furan Derivatives at the Levels of G4 Theory and Seven DFT Methods expa

G4

B3LYP

X1

M05-2X

M06-2X

B2PLYP

B2PLYP-D

XYG3


8.3


0.1


4.7


1.7

5.0

3.2


5.3


4.2


0.6


126.3
17.3


0.7 0.6


7.6
5.6


2.9
0.4

4.5 5.0

4.7 2.4


6.1
8.2


3.9
6.0


1.8
0.9

tetrahydrofuran (C4H8O)


44.0


0.6


8.9


1.1

3.6

0.4


12.5


9.1


2.6

furfural (C5H4O2)


36.2

1.2


4.1

0.4

6.4

5.5


3.4


1.5

0.8

1.6

furancarboxylic acid (C5H4O3)


94.4

3.3


4.3

1.4

10.3

10.0


2.2

0.0

1.7

1.7

Furfural alcohol (C5H6O2)


50.8

1.9


7.2

0.0

8.1

6.1


7.6


4.9


0.8

1.1

furan (C4H4O)

1.2 1.3

dihydro-2,5-furandione (C4H4O3) 2,3-dihydrofuran (C4H6O)

1.4 1.5

1.8

2-methyl-4,5-dihydrofuran (C5H8O)


29.3

0.1


8.6


0.2

4.9

1.5


11.7


8.5


1.3

1.9

dihydro-5-methyl-2-furanone (C5H8O2)


97.2

1.2


9.6


0.4

6.2

3.4


10.9


6.6


0.2

1.10 1.11

tetrahydro-2-furanmethanol (C5H10O2) furylethylene (C6H6O)


88.2 6.6

2.2 1.3


12.0
6.0

0.9 1.6

7.8 7.1

4.2 3.9


14.8
7.2


9.3
4.6


2.3 0.5

1.12

5-hydroxymethylfurfural (C6H6O3, HMFb)


79.9

1.9


7.9

0.9

8.2

7.1


7.0


3.5


0.7

1.13

2-furancarboxylicacid (C6H6O3)


96.8


3.4


12.6


3.7

4.9

4.1


11.6


8.2

-4.7

1.14

2-furan acrolein (C7H6O2)


25.3

0.2


7.1

2.5

7.1

4.5


7.2


3.8

0.1

1.15

benzofurfuran (C8H6O)

3.3

0.5


11.4

0.4

10.2

4.2


9.2


5.7

1.9

1.16

2,3-dihydro-benzofurfuran (C8H8O)

1.17

dihydro-5-hexyl-2-furanone (C10H18O2)

1.18

dibenzofuran (C12H8O) MaxD1c MAD1d


11.1

1.5


12.2

2.1

11.1

4.3


11.6


6.9

2.1


118.6

5.6


19.5

7.3

12.8

5.2


23.5


12.2

3.1

11.3


0.4 5.6


19.9
19.9

0.7 7.3

14.0 14.0

4.0 10.0


14.5
23.5


8.3
12.2

3.2
4.7

9.4

1.6

7.6

4.4

9.7

6.0

1.6

MaxD2e

1.5


25.1

2.3

14.4

7.4


29.2


17.8


4.5

MAD2f

10.3

1.1

6.7

3.5

10.6

6.9

1.7

a

All experimental values are from ref 42. b From ref 11. c MaxD1 = maximum deviation (exp-theory). The occurrence of MaxD for each method is marked in boldface. d MAD1 = mean absolute deviation with respect to the experimental value. e MaxD2 = maximum deviation (G4-theory). f MAD2 = mean absolute deviation with respect to the G4 value.

assessment using experimental HOFs for selected sets of 18 furan derivatives and 20 chloro compounds. We then validate DFT methods for prediction of the thermochemistry for convension of HMF to nonane using G4 results as reference.12 Finally, energetics involved in routes 1
4 in both gas phase and solution phase are predicted. 3.1. HOFs Calculations for Selected Sets of Furan Derivatives and Chloro Compounds. G4 is a highly accurate coupledcluster-based composite method. Specifically, it has an MAD of 0.80 kcal/mol from an assessment on the 270 HOFs in the G3/ 05 set.12 DFT methods are less accurate, albeit less expensive and potentially useful for larger molecules. B3LYP leads to an MAD = 5.58 kcal/mol for the 223 HOFs in the G3/99 set, which can be reduced, using a neural network-based correction X1 method, to 1.43 kcal/mol.27 M06-2X, being one of the best hybrid functionals available, gives an MAD of 2.93 kcal/mol32 for the G3/99 set, while MADs associated with doubly hybrid functionals for this G3/99 set are 1.81, 4.63, and 3.22 kcal/mol for XYG3, B2PLYP, and B2PLYP-D, respectively.32,40,41 Note that the later two functionals generally require larger basis sets for improved accuracy,31 while XYG3 was optimized using the present 6-311 +G(3df,2p) basis set.32 Thus, the state-of-art DFT functionals can achieve an MAD of 2
5 kcal/mol for HOFs via the standard atomization procedure. Tables 1 and 2 list selected sets of HOFs for furan derivatives and chloro compounds mostly from NIST chemistry webbook.42 In some cases, there are several experimental HOFs being reported for the same molecule. We chose the one that is most close to the G4 value. Table 1 shows that G4, XYG3, and X1 methods give the best performance for HOF calculations of the

furan derivatives. As compared to the experimental values, they all present an MAD around 1.6 kcal/mol. B3LYP is certainly not suitable for such purpose, whose MAD is as high as 9.4 kcal/mol. Table 1 shows that B3LYP consistently underestimates the stability of the species. This is also true for the B2PLYP method with the present basis set. Enlargement of the basis set shall reduce the errors by stabilizing the molecule more than its constituent atoms due to its MP2 component in B2PLYP. Nevertheless, this will increase dramatically the computational costs for larger molecules. The results are improved from B2PLYP to B2PLYP-D. MAD is reduced for this furan derivative set from 9.7 for the former to 6.0 kcal/mol for the latter. On the other hand, there is a clear tendency for M05-2X and M06-2X to overestimate the stability of each species. M06-2X is shown here to be superior to M05-2X. MAD is reduced from 7.6 for M05-2X to 4.4 kcal/mol for M06-2X. The maximum deviations (MaxDs) are also summarized in Table 1. MaxDs associated with X1, XYG3, and G4 are 7.3,
4.7, and 5.6 kcal/mol, respectively. M05-2X, M06-2X, and B2PLYP-D follow in turn by giving MaxDs of 14.0, 10.0, and
12.2 kcal/mol, respectively. The magnitudes for MaxDs of the former three are about one-half of those of the latter three. MaxDs for B2PLYP and B3LYP are
23.5 and
19.9 kcal/mol, respectively. Similar results can be seen from Table 2 in predicting HOFs of chloro compounds. G4 has the smallest MAD of 1.9 kcal/mol, while XYG3 and X1 results are about 0.6 and 0.9 kcal/mol inferior, giving MADs = 2.5 and 2.8 kcal/mol, respectively. These three methods also have the lowest magnitude of MaxDs with
6.9,
8.2, and 6.5 kcal/mol, respectively. M06-2X, M05-2X, and B2PLYP-D are grouped in the second class, having MADs of 3.7, 6.1, and 13630

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Table 2. Experimental HOFs (in kcal/mol) and Calculated Deviations for 20 Chloro Compounds at the Levels of G4 Theory and Seven DFT Methods

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20

chloro-ethane (C2H5Cl) 3-chloro-1-propene (C3H5Cl) chloroacetic acid methyl ester (C3H5O2Cl) chloromethyl-oxirane (C3H5OCl) propyl chloride (C3H7Cl) propyl chloroacetate (C5H9O2Cl) phenyl chloride (C6H5Cl) benzoyl chloride (C7H5OCl) 5-chloromethylfurfural (C6H5O2Cl, CMFb) benzyl chloride (C7H7Cl) n-butyl dichloro acetate (C6H10O2Cl2) 4-chlorobenzoic acid chloride (C7H4OCl2) 2-chlorodibenzo-p-dioxin (C12H7O2Cl) 1-chloro-dodecane (C12H25Cl) 3-chloro-1-butene (C4H7Cl) (Z)-1-chloro-1-butene (C4H7Cl) (E)-1-chloro-1-butene (C4H7Cl) 1-chloro-2-ethoxyethane (C4H9OCl) 1-chloro-3-methyl-butane (C5H11Cl) 3-ethyl-3-(chloromethyl)-oxetane (C6H11OCl) MaxD1c MAD1d MaxD2e MAD2f

expa

G4

B3LYP

X1

M05-2X

M06-2X

B2PLYP

B2PLYP-D

XYG3


26.8
1.3
106.1
25.8
31.7
111.7 13.01
26.1
48.9 4.5
119.0
30.8
17.7
77.0
10.9
4.6
4.0
71.0
42.9 g
46.2


0.4
1.1
6.9 1.8
0.3 0.2 1.0
0.8 2.3
2.2 0.0 2.0 2.6 0.1
3.0 2.8 3.1
5.1 0.5 1.3
6.9 1.9


3.7
5.6
13.7
5.7
6.2
11.4
8.4
11.5
8.1
14.2
18.7
12.3
19.4
30.7
11.2
4.4
3.8
14.1
12.7
18.8
30.7 11.7
30.8 11.6

0.7
0.1
5.2 1.9 1.2 4.4 1.9 1.8 5.0
0.2 5.8 4.9 6.5 2.5
2.6 4.1 4.6
0.9 1.4 0.5 6.5 2.8 5.8 2.1

1.9 1.5
1.7 6.8 2.7 6.5 8.8 7.4 9.1 6.0 7.1 11.3 21.1 7.3 0.4 5.8 6.2 0.0 4.3 6.3 21.1 6.1 18.5 6.0

0.1
0.9
1.6 6.6 0.0 4.7 3.3 2.7 7.8
0.8 4.9 7.5 13.8
4.0
3.0 3.0 3.3
2.5
0.2 3.6 13.8 3.7 11.1 3.0


6.6
8.3
14.0
6.6
9.7
13.1
7.0
8.3
5.7
13.4
17.9
6.1
12.0
39.6
13.8
7.6
7.1
18.0
16.5
19.6
39.6 12.5
39.6 12.4


5.2
6.5
11.6
4.3
6.9
8.4
4.2
4.4
2.4
9.1
11.5
1.9
5.2
24.5
10.4
4.5
4.2
14.0
10.0
11.2
24.5 8.0
24.6 7.9


0.3
1.7
8.2
0.4
0.2
1.1 3.3 1.9 1.8
0.2
2.1 4.8 4.9
1.7
3.6 2.2 2.6
6.8
0.1
2.4
8.2 2.5
3.7 1.5

a

Experimental values are from ref 42. b Data evaluated via isodesmic reactions based on averaged G4, G3, and CBS-QB3 reaction enthalpy and experimental data of the auxiliary set. c MaxD1 = maximum deviation (exp-theory). The occurrence of MaxD for each method is marked in boldface. d MAD1 = mean absolute deviation with respect to the experimental value. e MaxD2 = maximum deviation (G4-theory). f MAD2 = mean absolute deviation with respect to the G4 value. g From ref 45.

Figure 2. Reaction mechanism for the conversion of HMF to nonane.12

8.0 kcal/mol, respectively. The three methods possess their MaxDs of 13.8, 21.1, and
24.5 kcal/mol, respectively. B2PLYP and

B3LYP methods have MADs of 12.5 and 11.7 kcal/mol, while the corresponding MaxDs are
39.6 and
30.7 kcal/mol, respectively. 13631

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Table 3. Reaction Enthalpies and Reaction Free Energy Changes (ΔrH/ΔrG) for the Conversion of HMF into Nonane: G4 Results and the DFT Deviations (in kcal/mol) G4

X1

M05-2X

M06-2X


0.8/0.7

R1


3.4/
1.5


3.9/
2.4

2.3/3.9

R2 R3


10.8/
2.2
25.1/
17.6


5.2/
5.5
5.7/
6.3


2.3/
2.5
3.1/
3.6

R4


26.9/
18.2


4.5/
4.6

R5


36.9/
18.8


7.2/
7.3

B2PLYP

B2PLYP-D

XYG3


2.4/
0.8


3.1/
1.5


2.4/
0.8


3.0/
1.4

2.1/1.9 2.1/1.6

0.6/0.4 1.2/0.8


3.7/
3.9
3.4/
3.9


2.6/
2.8
2.0/
2.5


1.6/
1.9
0.1/
0.5


1.5/
1.6

2.5/2.4

1.5/1.4


2.4/
2.5


0.5/
0.6

0.7/0.6


1.5/
1.5

4.1/4.1

0.7/0.7


4.5/
4.6


1.0/
1.0

0.5/0.4

R6


37.9/
19.7


7.0/
6.6


1.7/
1.2

3.5/4.0

0.2/0.7


4.5/
4.1


1.5/
1.0

0.3/0.7

R7


13.6/
3.7


3.8/
3.2


0.8/
0.1

1.9/2.6

0.3/0.9


2.7/
2.0


1.7/
1.0


1.0/
0.4

R8


36.1/
41.4

2.8/2.4

2.6/2.3

0/
0.4


2.8/
3.1

1.6/1.3

0.3/0.0

1.2/0.8

R9


33.5/
29.3

2.6/2.7

1.6/1.7

1.3/1.4


0.7/
0.6

1.4/1.5

1.2/1.4

1.3/1.4


3.1/3.9 1.9/2.1

4.1/4.1 2.1/2.1


2.8/
3.1 1.2/1.0


4.5/
4.6 3.0/2.8


2.6/
2.8 1.5/1.3


3.0/
1.9 1.1/0.9

MaxDa MADb a

B3LYP


7.2/
7.3 4.8/4.5

MaxD = maximum deviation (G4-DFT). The occurrence of MaxD for each method is marked in boldface. b MAD = mean absolute deviation.

Figure 3. Reaction mechanisms for the conversion of CMF into LA. Route 1 follows Horvat’s original proposal.43

It should be pointed out that the experimental data may also suffer from large errors. MaxD for the furan derivatives occurs at

dihydro-5-hexyl-2-furanone (no. 1.17 in Table 1) for the G4 method (5.6 kcal/mol). Such a large error is suspicious. Possibly, 13632

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Table 4. Reaction Enthalpies and Reaction Free Energy Changes (ΔrH/ΔrG) for the Conversion of CMF into LA: Gas-Phase G4 Results and the DFT Deviations (in kcal/mol)a G4

B3LYP

X1

M05-2X

M06-2X

B2PLYP

B2PLYP-D

XYG3

PCM-UAHF
4.3

R10

5.2/5.6

1.2/1.2


1.7/
1.7

1.1/1.1

1.4/1.4

0.6/0.6

0.7/0.7

0.4/0.4

R11 R12


2.7/9.1 0.8/12.3


6.9/
6.9
7.2/
7.2


4.2/
4.2
4.8/
4.7

1.4/1.4 1.3/1.4

1.2/1.2 1.2/1.3


4.7/
4.7
5.0/
4.9


2.3/
2.4
2.9/
3.0


1.1/
1.1
1.2/
1.1

R13 R14

5.0/
5.5
10.6/0.1

2.3 2.7

3.4/3.5

0.3/0.3


2.4/
2.4


2.9/
2.9

1.6/1.6

0.0/0.0


0.8/
0.8


3.7/
3.9


0.9/
1.0

2.3/2.2

2.7/2.6


1.6/
1.7

0.2/0.1

1.0/0.9


2.0 0.5

2.8

R15


0.2/
3.1

4.3/4.4

3.0/3.1


2.5/
2.4


0.6/
0.5

3.4/3.5

3.7/3.8

2.6/2.7

R16

9.7/
1.4

4.9/5.2

2.1/2.3


3.2/
3.0


3.7/
3.5

2.4/2.6

0.3/0.5


0.6/
0.4

R17


8.0/
8.8

3.1/3.0

1.9/1.8

2.4/2.3

2.3/2.3

2.1/2.0

1.5/1.4

1.6/1.5


2.9

3.8/3.6


2.6/
2.8


1.4/
1.7


0.6/
0.8


5.0

0.8/0.4 2.7/2.5

1.3/0.9 1.6/1.4

2.1/1.7 1.8/1.6

3.7
5.9
1.7


4.0/
4.3


0.6/
0.8

4.3/4.1

R19 R20


30.0/
30.5
4.6/
6.1


0.3/
0.7 4.3/4.2


0.5/
0.9 3.7/3.6


0.4/
0.8 2.8/2.6

R18


7.1/4.1

0.2/
0.2 3.5/3.3

0.4

R21


13.1/
3.5


1.1/
1.0

0.7/0.8

1.0/1.2

1.7/1.9

0.1/0.3

2.7/2.7

1.9/2.0

R22

12.1/0.5

5.1/5.2

2.0/2.2


2.8/
2.8


3.3/
3.2

2.6/2.6

0.0/0.1


0.6/
0.5

0.6

R23


17.9/
30.0

4.9/4.5

1.6/1.2


3.2/
3.5


3.0/
3.4

3.4/3.0

1.3/1.0

1.5/1.2

4.3

MaxDb


7.2/
7.2


4.8/
4.7

4.3/4.1

3.8/3.6


5.0/
4.9

3.7/3.8

2.6/2.7

MADc

3.9/3.9

2.0/2.1

2.2/2.2

2.3/2.2

2.4/2.4

1.5/1.4

1.3/1.2

a

Solvation free energy changes are listed in the last column. b MaxD = maximum deviation (G4-DFT). The occurrence of MaxD for each method is marked in boldface. c MAD = mean absolute deviation.

it is due to the error in the experimental data (HOF =
118.6 kcal/ mol), rather than the G4 value (
124.2 kcal/mol). Note that the no. 1.17 molecule in Table 1 differs only from the no. 1.9 molecule by the alkyl chain, while it is known that G4 leads to MAD less than 0.2 kcal/mol for the C7 or C6 alkanes;8 therefore, we suggest that the G4 value be more reliable. The corresponding X1, M06-2X, and XYG3 values are
125.9,
123.8, and
121.7 kcal/mol, respectively, lending support to the G4 value. Similarly, we suspect the reliability of the experimental data (
106.1 kcal/ mol) for the no. 2.3 molecule (cholroacetic-acid-methyl-ester ClCH2COOCH3) in Table 2. G4, X1, and XYG3 methods predict the HOF of
99.2,
100.9, and
97.9 kcal/mol, respectively, which differ from the experimental data by more than 6 kcal/mol. MADs and MaxDs using the G4 data as reference are also summarized in Tables 1 and 2. The functionals follow the same order in their abilities of predicting HOFs as that when the experimental data are used as reference. Experimental HOFs for furan derivatives are scarce. Some of the existing ones should be subjected to larger errors as discussed above. HOF for HMF (no. 1.12 molecule) was measured to be
79.9 kcal/mol.11 The corresponding G4, X1, and XYG3 values are
81.8,
80.8, and
79.2 kcal/mol, respectively, confirming the experimental data. The calculated data with other functionals can also be found in Table 1, which differ more from the experimental data. There is no experimental HOF being reported for CMF (no. 2.9 molecule). The “experimental” data (
48.9 kcal/mol) listed in Table 2 are estimated via isodesmic reactions with reaction enthalpy averaged from the calculated values of CBS-QB3, G3, and G4, in conjunction with experimental HOFs of the auxiliary species. Table 2 also lists HOF of CMF with the methods tested in the present work. The G4 and XYG3 values, being most satisfactory, are
51.2 and
50.7 kcal/mol. The X1 and M06-2X values are too low by 5.0 and 7.8 kcal/mol, respectively. Currently, we are embarking on a project to evaluate HOFs of many furan derivatives via isodesmic reactions. The results will be published in due time. 3.2. Thermodynamic Calculations on Conversion of HMF to Nonane. Recently, Curtiss and co-workers12 have investigated

reaction thermochemistry from HMF to nonane at the level of G4 theory. The reaction mechanisms are repeated here in Figure 2, where nine reactions are involved through seven furan derivative intermediates. The enthalpy changes ΔrH and free energy changes ΔrG at the G4 level provide a platform where evaluation of the DFT performance can be carried out. Some of the intermediates may possess complicate conformational space, which has not been exploited systematically in the present work. Yet we reperform the G4 calculations to keep the configurations consistent between those with G4 and with the DFT methods. The results are summarized in Table 3. The optimized structures of intermediates 1
7, as well as those of HMF and nonane, can be found in the Supporting Information. Generally, our G4 results reproduce well the original data of Curtiss and co-worker’s.12 For ΔrH, the deviation is generally less than 0.2 kcal/mol, while for ΔrG, the agreement is also good, with one exception for R8 in Table 3, where the deviation is around 0.8 kcal/mol. The general chemistry from HMF to nonane is qualitatively well reproduced by all methods examined here. All methods agree that hydrogenation of the acyclic double bond (R3) is thermodynamically more favorable than the reduction of keto group (R2). Thus, the reaction proceeds in the sequence of R1fR3fR5fR7, instead of R1fR2fR4fR6. If we compare ΔrH and ΔrG data in Table 3, one immediately finds that their error distributions are similar. This indicates that B3LYP/6-311+G(d,p) method used for DFT calculations leads to entropy contributions similar to those of the G4 method where B3LYP/6-31G(2df,p) method is used for geometry optimizations and frequency calculations. Quantitatively, B3LYP results are the least satisfactory with MADs = 4.8 and 4.5 kcal/mol for ΔrH and ΔrG, respectively, as compared to the G4 results. The largest error (
7.2 kcal/mol) occurs at R5, which involves the hydrogenation of the furan ring. The same is true for M05-2X (MaxD = 4.1 kcal/mol) and B2PLYP (MaxD =
4.5 kcal/mol). R6 shares the same reaction character with R5 and suffers from similar calculation error. Generally, XYG3, M062X, and B2PLYP-D methods give the lowest MADs for either 13633

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Figure 4. Reaction enthalpies (A) and free energies (B) for the conversion of CMF into LA at the G4 level. The net reaction is CMF + 3H2O f LA + HCOOH + HCl.

ΔrH or ΔrG, all being below 1.5 kcal/mol. Also, all three methods have MaxDs below
3.1 kcal/mol. X1 indeed significantly reduces the B3LYP error, but is surpassed by M06-2X and B2PLYP-D for both ΔrH and ΔrG predictions. As compared to the accuracy in predicting HOFs shown in Tables 1 and 2, calculation results are much more satisfactory for all functionals for reaction enthalpy and free energy changes. Curtiss et al. found12 that the computationally less demanding G3MP2B3 method predicted the energetics within 1
2 kcal/ mol of the G4 method, except for hydrogenolysis reaction R8, where a deviation of 5.7 kcal/mol was encountered. We note in passing that G3MP2B3 is still more expensive than the DFT methods examined here, while XYG3, M06-2X, and B2PLYP-D methods all perform better than G3MP2B3 for this set of reaction energies. 3.3. Thermodynamic Calculations on CMF Conversion Reactions in the Gas Phase. 3.3.1. Thermodynamics for

Conversion of CMF into HMF and LA (Routes 1 and 2). The

process for conversion of CMF into LA is shown in Figure 3. Besides the first step, where chlorine in CMF is substituted by hydroxyl in water to produce HMF and hydrogen chloride, the subsequent process is the hydrolysis of HMF into LA. On the basis of the intermediates revealed by 13C NMR spectroscopy, the mechanism of the HMF to LA has been proposed by Horvat et al.43 (cf., route 1 in Figure 3). Curtiss and co-workers have recently predicted the net reaction energy changes from HMF to LA (eq 2). To our knowledge, no detailed thermodynamic information for the detailed mechanism involved in the HMF to LA conversion is available up to now. HMF þ 2H2 O f LA þ HCOOH

ð2Þ

Table 4 and Figure 4 provide such information. At the G4 level, the replacement of Cl by OH in converting CMF into HMF 13634

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Figure 5. Proposed reaction mechanism for the conversion of CMF to EL.

(R10 in Figure 3) is uphill by 5.2 kcal/mol. Such a process should be much enhanced in aqueous solution due to the much stronger acidity of HCl than H2O (see discussion in section 3.4). It was proposed that there are two possibilities to hydrate HMF. The 2,3-addition of water leads to a compound that undergoes polymerization rather than hydrolysis to LA (R11 in Figure 3). The desired reaction was supposed to be 4,5addition (R12 in Figure 3) to intermediate 9 (Int 9). This step is essentially thermoneutral (0.8 kcal/mol), possibly because the breakdown of the π-electron conjugation is paid back by forming additional hydrogen bonding. Nevertheless, R13 imposes a substantial free energy barrier due to the penalty of entropy loss. After overcoming this barrier, the reaction proceeds more smoothly and eventually goes downhill. Dehydration (R13) from Int 9 leads to Int 10, where the π-electron conjugation reappears with the formation of the butadiene moiety. Rehydration of Int 10 (R14) occurs via a 1,4-like

addition of butadiene by water, which is accompanied by 10.6 kcal/mol exothermicity to form Int 11. The reaction comes to a fork at Int 11. We will first follow route 1 as proposed by Horvat.43 Int 11 undergoes tautomerization to form Int 12, where furan ring is opened (R15). R15 is thermoneutral (
0.2 kcal/mol), but is favored in terms of free energy by 3.1 kcal/mol. Dehydration of Int 12 to produce Int 13 is uphill by 9.7 kcal/mol (R16), possibly due to the disruption of the hydrogen-bond net work, but it is favored in terms of free energy as water is released to increase the entropy contribution. Originally, Horvat et al.43 proposed that Int 13 reacts with two water molecules to form Int 15 and formic acid, as indicated by the dashed arrow in Figure 3. We decompose it stepwise, where Int 13 first reacts with one water molecule to form formic acid plus Int 14, which then reacts with the second water to form Int 15. At the G4 level, the first step (R17) is exothermic by
8.0 kcal/mol, while the second step (R18) is exothermic by another 13635

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Table 5. Reaction Enthalpies and Reaction Free Energy Changes (ΔrH/ΔrG) for the Conversion of CMF into EL: Gas-Phase XYG3 Results and the Differences of Other DFT Methods (in kcal/mol)a XYG3

X1

M05-2X

M06-2X

B2PLYP

B2PLYP-D

PCM-UAHF


1.3

1.3

0.0

0.4


0.7

0.4

2.5

R25 R26

1.8/13.7 9.8/
1.8


8.3 6.2


3.8 0.6

1.2
0.5

1.4
1.1


4.7 3.2


1.3 0.6

5.4
1.0

R27


16.0/
3.9


7.5


2.5

0.2

0.8


3.8


1.2

1.1

3.3


0.9


4.5


3.6

1.5

0.9


2.6
2.8

R24

R28


0.4/1.3

B3LYP


7.4/
11.8

R29

14.6/2.1

8.1

1.9


1.7


2.1

4.2

0.7

R30


14.4/
14.3

0.1

0.5

0.5

0.2


0.1


0.3

0.3

R31


10.1/2.6


5.8

2.6

4.3

3.9


3.0


0.8


1.4

R32


33.2/
33.5


1.9


2.2


2.5


2.2


1.1


1.0

4.0

R33 R34


15.7/
15.8
13.9/
3.7

2.5
7.0

5.8
4.8

0.4
1.4

0.3
0.3

1.0
3.4


0.2 1.4


2.6 7.9

R35

13.4/0.6

R36


34.4/
36.1

R37


1.2/
2.6

9.0

5.2


0.9


1.7

4.4


0.1


6.3


0.7


4.4


2.2


2.2


0.6


1.7


1.6
5.6

1.2


2.2

0.3

0.0

0.5


0.7

MaxDb

9.0

5.8


4.5

3.9


4.7


1.7

MADc

4.5

2.8

1.5

1.5

2.3

0.8

a

Solvation free energy changes are listed in the last column. b MaxD = maximum deviation (G4-DFT). The occurrence of MaxD for each method is marked in boldface. c MAD = mean absolute deviation.


7.1 kcal/mol, making a total of
15.1 kcal/mol from Int 13 to Int 15. Indeed, Int 15 is just a transient, which undergoes tautomerization, leading to the final product LA. The large exothermicity (∼30 kcal/mol in terms of both enthalpy and free energy) provides the thermodynamic driving force to form LA. We then check route 2, which is proposed in the present work as an alternative to the Horvat mechanism43 shown in route 1. In this route, hydration to release formic acid occurs first (R20), which is then followed by furan ring-opening (R21). In contrast to R15 and R16, these two steps are all the way downhill in terms of both reaction enthalpy and reaction free energy changes (see Table 4 and Figure 4). It would not be necessary to undergo dehydration to form Int 15 via R22. Instead, dehydration via R23 from Int 15 leads to direct formation of LA, which is once again exothermic. The Horvat mechanism (route 1) is a widely recognized mechanism for HMF conversion into LA. The present calculations at the G4 theory suggest an alternative pathway (route 2), which is more thermodynamically favored (see Figure 4). More detailed study is required to characterize the kinetics of routes 1 and 2, which is under way in our laboratory. In addition to those of CMF, HMF, and LA, the optimized structures of intermediates 8
17 can be found in the Supporting Information. The accurate G4 values for routes 1 and 2 offer a benchmark set where we can evaluate the performance of the DFT methods. As shown by the data in Table 4, B3LYP significantly overestimates the endothermicity of R12 (i.e., 4,5 water addition to HMF, see Figure 3) by 7.2 kcak/mol. X1 reduces this error to 4.8 kcal/mol, but is still not satisfactory. The smallest error for R12 occurs at M05-2X, M06-2X, and XYG3, with the magnitude of ∼1.2 kcal/mol. As compared to the G4 data, it is observed that XYG3 and B2PLYP-D methods perform generally better than the other functional examined here. MADs associated with XYG3 and B2PLYP-D are around 1.3 kcal/mol. X1, M05-2X, M06-2X, and B2PLYP show their MADs for routes 1 and 2 higher slightly above 2.0 kcal/mol. B3LYP has the largest MAD

of 3.9 kcal/mol. Besides, XYG3 and B2PLYP-D also give the lowest MaxDs of 2.6 and 3.7 kcal/mol, respectively, while X1, M05-2X, M06-2X, and B2PLYP display their MaxDs of
4.8, 4.3, 3.8, and
5.0 kcal/mol, respectively. Generally speaking, according to the calculated reaction enthalpy and free energy changes from HMF to nonane (Table 3) and CMF to LA (Table 4), the XYG3 method is the best performer, and M06-2X and B2PLYP-D are similarly good. B2PLYP-D shows a clear improvement over B2PLYP, suggesting the importance of dispersion interaction in the systems examined. M06-2X is superior to M05-2X for the description of conversion from HMF to nonane, while they perform similarly for the conversion from CMF into LA. B3LYP, although being very popular, is the least satisfactory, while X1, its neural network correction method, can be of great help in reducing B3LYP errors. 3.3.2. Thermodynamics for Conversion of CMF into EMF and EL (Routes 3 and 4). We propose the reaction mechanisms for the conversion of CMF into EMF and EL as shown in Figure 5. These processes involve alcoholysis as compared to hydrolysis of CMF to produce HMF and LA in Figure 3. There are similarities between alcoholysis for the first four steps from R24 to R27 in Figure 5 with the corresponding steps in hydrolysis from R10, R12, to R14. Hence, Cl is first replaced by C2H5O in R24 to yield HCl plus EMF. There occurs a 4,5 addition of C2H5OH in R25 to make Int 18, which is followed by the release of C2H5OH through R26, leading to the formation of a butadiene moiety in Int 19. In analogy to R14 in hydrolysis, a 1,4-like addition of butadiene by alcohol is invoked in R27, giving rise to Int 20. The reaction divides into routes 3 and 4 at Int 20. Tautomerization that happens in Int 11 in hydrolysis (R15) is not applied to Int 20 in alcoholysis due to the lack of hydroxyl group in the latter. We propose that the furan ring is opened either by addition of hydrogen chloride to produce Int 21 plus ethyl chloride (R28 in route 3) or alcohols to produce Int 24 with release of HCOOC2H5 (R33 and R34 in Route 4). Dealcoholization from Int 21 (R29) leads to the formation of Int 13. In analogy to R17 13636

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Figure 6. Reaction enthalpies (A) and free energies (B) for the conversion of CMF into EL at the XYG3 level. The net reaction is CMF + 3C2H5OH f EL + HCOOC2H5 + C2H5Cl.

and R18 in hydrolysis, stepwise alcoholysis via R30 and R31 gives rise to Int 22, which then undergoes a similar tautomerization as R19, leading to the final product of EL via R32 in route 3. On the other hand, dealcoholization from Int 24 (R35) leads to the formation of Int 25. Unlike tautomerization from Int 15 to LA (R19 in Figure 3), we propose the conversion from Int 25 to EL (R36 in Figure 5) with the help of hydrogen chloride, which completes route 4. Routes 4 and 3 may be connected, for example, via R37 by converting Int 25 to Int 22. Along alcoholysis, there are two to six more carbon atoms involved as compared to hydrolysis. We therefore only perform DFT calculations and use XYG3 results as the reference to see the differences of the other DFT methods. The results are summarized in Table 5. The energetics are depicted in Figure 6.

Both enthalpy change and free energy change of R24 are mild. The calculated values are
0.4 and 1.3 kcal/mol, respectively. Hence, increasing alcohol concentration and temperature should favor the conversion from CMF to EMF. The as-formed acid (HCl) should catalyze the transformation from EMF to EL, but here we will focus only on the thermodynamics. The next two steps, alcoholization (R25) or dealcoholization (R26), are uphill either in free energy (Δr G = 13.7 kcal/mol for the former) or in reaction enthalpy (Δr H = 9.8 kcal/mol for the latter). Afterward, the reactions shall proceed more smoothly as indicated in Figure 6. Route 4 undergoes favorable decarboxylation (R33) and alcoholization (R34) first, followed by unfavorable dealcoholization (R35), whereas route 3 undergoes unfavorable dealcoholization (R29) first, which is then compensated by 13637

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Figure 7. Methanolysis of 2-hydroxymethylfuran as proposed by Horvat et al.43

Figure 8. Calculated free energy changes for the CMF conversions in solution phase. Free energy changes in gas phase are calculated (A) using the G4 method to LA and (B) using the XYG3 method to EL. The solvation free energy changes are calculated with the PCM-UAHF model.

favorable decarboxylation (R30) and alcoholization (R31). This difference in reaction sequence makes route 3 thermodynamically less favorable than route 4. Previously, Horvat et al. have studied

the methanolysis of 2-hydroxymethylfuran (1) by 13C NMR43 and indicate a reaction mechanism of 1f7f8f9f10 (see Figure 7). Here, 1, 7, 8, 9, and 10 may correspond to EMF, Int 18, 13638

dx.doi.org/10.1021/jp207641g |J. Phys. Chem. A 2011, 115, 13628–13641

The Journal of Physical Chemistry A Int 24, Int 25, and EL in Figure 5. Thus, Horvat’s methanolysis mechanism from 1 to 10 lends certain support to our proposed alcoholysis mechanism from EMF to EL, favoring the reaction path along route 4. Besides EL, the optimized structures of intermediates 18
25 can also be found in the Supporting Information. 3.4. Reaction Free Energy Changes from CMF to LA and EL in Solution Phase. Solvation effects on the reaction free energy changes are investigated using the polarizable continuum model in conjunction with the united atom topological model with atomic radii optimized at the Hartree
Fock level (PCMUAHF).36 We used the 6-31+G(d) basis set as recommended by Coote et al.37 Even though a complicated hydrogen-bond network is involved in our systems, it has been reported that the PCM model can qualitatively explain the relative solvation energy differences in a system such as the glucose decomposition reaction pathway.13 As a first test, we find that the free energy change for gas-phase HCl dissociation to become hydrochloric acid in water is
4.2 kcal/mol, while the experimental datum estimated from thermodynamic quantities is around
8.7 kcal/ mol.44 Hence, the present solvation model is able to give a qualitatively correct description of this process, although quantitatively it is not quite satisfactory. Solvation free energy change for each step in the CMF conversion into LA in water is summarized in Table 4, while the corresponding free energy changes in aqueous solution are depicted in Figure 8A, which may be compared to the gas-phase reaction free energy changes depicted in Figure 4B. Formation of hydrochloric acid favors the transformation from CMF to HMF. R10 should be downhill if the HCl dissociation constant is not underestimated by the present solvation model. Hydration can be an unfavorable process (e.g., R11, R12) due to the consumption of water molecule during the reaction. Indeed, the 4,5 water addition (R12, destabilized further by 2.7 kcal/mol in solution) is still the bottleneck of the whole conversion process. The strongest stabilization is gained in R20 (by
5.9 kcal/mol). This decarboxylation process is favored for the formation of formic acid, which enhances the preference of route 2 as compared to route 1. Solvation free energy change for each step in the CMF conversion into EL in alcohol is summarized in Table 5. The corresponding free energy changes in solution are depicted in Figure 8B, which may be compared to the gas-phase reaction free energy changes depicted in Figure 6B. Alcoholization in alcohol is found to be even more unfavorable than hydration in water. In fact, the 4,5 alcohol addition (R25, destabilized further by 5.4 kcal/mol in solution) is the highest uphill process in the free energy surface of the whole conversion process. The stabilization is gained in the dealcoholization processes such as R29 (by
2.8 kcal/mol) and R35 (by
6.3 kcal/mol). The decarboxylation process, mostly favored due to the formation of formic acid in water in CMF conversion to LA (by
5.9 kcal/mol), is thermo neutral as in R30, or less downhill (by
2.6 kcal/mol) as in R33 in alcohol in CMF version to EL. Because of ester formation, Int 24 is less favorable than Int 23 by 4.0 kcal/mol in alcohol, whereas the corresponding species Int 17 is more stable than Int 16 by 4.9 kcal/mol in water. As shown in Figure 8A, the preference of route 2 is enhanced as compared to route 1 due to solvation. Figure 8B suggests that solvation downplays route 4 as compared to its gas-phase counterpart due to the uphill in R34. As far as the CMF conversion to LA in water is concerned, solvation effects as predicted by PCM model do not change

ARTICLE

qualitatively the chemistry as predicted by the gas phase calculations. Generally, solvation favors the transformation of CMF to LA in water. Solvation may have a stronger influence for alcoholysis of CMF. It disfavors route 4, which is actually more favorable than route 3 in the gas phase. Nevertheless, a quantitative description of solvation effect is a challenge in theoretical calculations. Solvation will certainly play a significant role in characterizing accurately the inter- and intramolecular hydrogen network in the biomass related systems. Furthermore, detailed characterization of reaction barrier heights is necessary to give a better understanding of the conversion mechanism.

4. CONCLUDING REMARKS In this work, we have presented, for the first time, the detailed quantum chemical calculations for the conversion of CMF to HMF and LA in water, as well as that of CMF to EMF and EL in alcohol. New mechanisms have been proposed and evaluated at the highly accurate G4 level of theory. The G4 results have been used to examine the performance of some of the state-of-art density functional methods. The following conclusions can be drawn from this study: (1) Alcoholysis is thermodynamically more favorable than hydrolysis. The exothermicity (ΔrH) in the gas phase from CMF to LA is found to be 35.2 kcal/mol for hydrolysis, which is increased to 55.3 kcal/mol from CMF to EL for alcoholysis. The corresponding gas-phase free energy changes are predicted to be
27.2 and
45.6 kcal/mol, respectively, while those in solution phase are
31.3 and
40.1 kcal/mol, respectively. (2) The 4,5 addition into furan ring to form the dihydrofuran species (Int 9 or Int 18) imposes the free energy bottleneck either in hydrolysis (R12) or in alcoholysis (R25). Afterward, the reactions go generally downhill. In solution phase, the formation of hydrochloric acid favors the desired transformation. Further study on reaction kinetics is necessary to understand how the as-formed acid can catalyze the conversion. (3) The reaction divides at the followed dihydrofuran species (Int 11 or Int 20). Routes 2/4 undergo decarboxylation (R20/R33) and hydration/alcoholization (R21/R34) first, followed by dehydration/dealcoholization (R21/ R35), whereas routes 1/3 undergo dehydration/de-alcoholization (R16/R29) first, which is then complemented by decarboxylation (R17/R30) and hydration/alcoholization (R18/R31). This difference in reaction sequence makes routes 2/4 thermodynamically more favorable than routes 1/3 for LA/EL formation in the gas phase. Noteworthily, such preference may be enhanced or depressed in solution. The new mechanisms provide alternatives to the well-recognized Horvat mechanism. (4) Despite the emerging importance of biorefinery, experimental HOFs for furan derivatives are scarce. According to the calculated HOFs for furan derivatives and chloro compounds listed in Tables 1 and 2, the XYG3 method shows the best performance. HOF for HMF is calculated to be
81.8 (G4) and
79.2 (XYG3) kcal/mol, confirming the recent experimental data of
79.9 kcal/mol. There is no experimental HOF being reported for CMF. The predicted G4 and XYG3 values are
51.2 and
50.7 kcal/ mol, respectively. Currently, we are embarking a project to 13639

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The Journal of Physical Chemistry A evaluate HOFs of many furan derivatives, whose results will be published in due time. (5) Generally speaking, as compared to the G4 calculated reaction enthalpy and free energy changes from HMF to nonane (Table 3) and CMF to LA (Table 4), the XYG3 method is the best performer. B2PLYP-D shows a clear improvement over B2PLYP, suggesting the importance of dispersion interaction in the systems examined. M06-2X is superior to M05-2X for the description of conversion from HMF to nonane, while they perform similarly for the conversion from CMF into LA. B3LYP, although being very popular, is the least satisfactory, while X1, its neural network correction method, can be of great help in reducing B3LYP errors.

’ ASSOCIATED CONTENT

bS

Supporting Information. Optimized geometries. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (X.X.); [email protected] (Y.-W.L.).

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