Origin of the Regioselectivity in the Aldol Condensation between

Feb 3, 2017 - Our previous work demonstrated that hydroxide ion (OH–) was able to catalyze aldol condensation reaction at room temperature between 5...
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Origin of the Regioselectivity in the Aldol Condensation between Hydroxymethylfurfural and Levulinic Acid: A DFT Investigation Liwei Zhao,† Nnenna Elechi,‡ Richard Qian,‡ Timila B. Singh,‡ Ananda S. Amarasekara,‡ and Hua-Jun Fan*,‡ †

Department of Material and Chemical Engineering, Ningbo University of Technology, Ningbo, Zhejiang 315016, China Department of Chemistry, Prairie View A&M University, Prairie View, Texas 77446, United States

‡

S Supporting Information *

ABSTRACT: Our previous work demonstrated that hydroxide ion (OH−) was able to catalyze aldol condensation reaction at room temperature between 5-hydroxymethylfurfural (HMF) and levulinic acid (LA). This work identified three primary reaction steps in this condensation reaction using density functional theory (DFT): (1) deprotonation of LA to generate LA ions, (2) LA ions addition at hydroxymethyl site of HMF, and (3) internal dehydration to form the condensation product. The reaction pathway through the C5 of LA forms a linear product that is favored with respect to both energy and configuration in all three elementary reaction steps. This is qualitatively consistent with the phenomenon observed in our previous experiment where the linear form is a main product. Further confirmation comes from the frontier orbital analysis of the transition states in the linear reaction route and explains the regioselectivity of product formation.

1. INTRODUCTION The depleting reserves of fossil energy resources and increasing demands of energy consumption are two pressing issues of the global energy crisis and provoke conflict among nations. Along with hydroelectric, wind, and solar energy, the efficient conversion of naturally abundant biomass into chemicals and liquid transportation fuels becomes an attractive new strategy to resolve this global energy crisis.1−3 Cellulose, hemicellulose, and lignin are three major sustainable sources of lignocellulosic biomass.4,5 Hydrolysis, particularly the hydrothermal process,6 is a common practice in order to transform the aforementioned biomass materials into potential fuels and fuel additives7 and further into a variety of useful chemicals such as acids, aldehydes, alcohols, and amines.8−12 Our previous work demonstrated that untreated cellulose can be converted into a mixture of ethyl levulinate and levulinic acid (LA) by heating with a Brönsted acidic ionic liquid (BAIL) catalyst in aqueous ethanol solution in a one-pot operation under mild conditions.13−16 Among the products converted from these feedstocks, furan-aldehydes such as HMF and furfural are the building blocks for further preparation of polymers as well as fuel precursors.17 The key step of all these reactions is through the aldol condensation reaction depicted in the following reaction Scheme 1. The aldol condensation between acetone and various furfurals such as furaldehyde, methyl furfural, and HMF can yield typical aldol condensation products in a 1:1 molar ratio reaction or double aldol condensation products with excess aldehyde.18,19 For example, Dumesic and co-workers first investigated the aldol con© XXXX American Chemical Society

Scheme 1. Aldol Condensation Reaction

densation with furan aldehydes and a series of ketones in a reactive aqueous phase containing NaOH as a catalyst, followed by an organic extraction to remove the aldol-adducts from the homogeneous catalyst, and the addition of salt (NaCl) to the aqueous phase to expand the miscibility gap between the aqueous and organic phases.19−21 Subbiah et al. have recently reported an efficient method for synthesis of 2,5-dihydroxymethylfuran (DHMF) and 5-hydroxymethyl-2-furoic acid (HMFA), in 80% yield from HMF in water using NaOH via a Cannizzaro reaction.17 Our previous work has shown that cellulose or dry corn stover powder can be directly converted to a mixture of HMF and furfural (an acetone aldol condensation products) in a single-reactor operation by heating in acetone at 120 °C in the presence of BAIL catalysts, where ionic liquid was used as a multipurpose catalyst for depolymerization, dehydration, and aldol condensation reactions.14 Interestingly, when we carried out a NaOH catalyzed condensation reaction between HMF and LA in water, the reaction gives a 2.5:1 mixture of isomeric aldol products: (E)-6-[5-(hydroxymethyl)furan-2-yl]-hex-4-oxo-5-enoic acid (a linear addition form) and Received: November 4, 2016 Revised: January 31, 2017 Published: February 3, 2017 A

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conditions used in the reaction and intractable mixtures in the product. In this study, we investigate the reaction mechanism of the aldol condensation reaction between HMF and LA using NaOH as the catalyst,22 which yields a 2.5:1 mixture of aldol products: (E)-6-[5-(hydroxymethyl)furan-2-yl]-hex-4-oxo-5enoic acid (linear form, PL‑3) and (E)-3-[5-(hydroxymethyl)furan-2-yl]methylene-4-oxo-pentanoic acid (branched form, PB‑3) in a combined yield of 82% (as shown in Scheme 2).

(E)-3-[5-(hydroxymethyl)furan-2-yl]methylene-4-oxo-pentanoic acid (a branched addition form) in 82% combined yield.22 Recently, Tao Zhang and co-workers reported this aldol condensation with heterogeneous catalysts, and two isomeric condensation products were also produced after acidification.23 In this paper, we will investigate the reaction mechanism of this aldol condensation reaction between HMF and LA through density functional theory. Particularly we want to identify the factors that control the reaction’s regioselectivity properties. Thermodynamic parameters of possible intermediates and transition states will not only provide valuable insight into the geometric and structural impact of reaction pathway but also supply strategy of building functional blocks that can be used to design and preparation of useful industrial chemicals and liquid fuel products.

Scheme 2. Adol Condensation between HMF and LA under Mild Reaction

2. THEORETICAL CALCULATIONS All molecule and transition states are constructed using GaussView optimized with the Gaussian 09 package.24 The popular B3LYP25 with hybrid exchange functional were used for all geometry optimization and frequency calculations. Additional calculations were performed using the M06-2X,26 TPSS,27 and MP228 in order to compare the results from various functionals. The M06-2X functional has a very good response under dispersion forces, improving one of the biggest deficiencies in DFT methods.29 The s6 scaling factor on Grimme’s long-range dispersion correction is 0.06 for M06-2X, rather small compared with the values of B3LYP or PBE0 (1.05 and 0.70, respectively).30 Compared to other density functionals, M06-2X was able to identify long-range correlation expression to make proper account of the delicate balance between the exchange and correlation functions. TPSS gives generally excellent results for a wide range of systems and properties, correcting overestimated PKZB (Perdew−Kurth− Zupan−Blaha) bond lengths in molecules, hydrogen-bonded complexes, and ionic solids. All structures were fully optimized, and frequency analyses were performed to ensure either a minimum was achieved, which had zero imaginary vibrational frequency as derived from a vibrational frequency analysis, or a transition state was identified, which had one imaginary vibrational frequency. QST2 was used to scan the transition states between the optimized reactants and products. The vibrational mode was visualized to confirm the expected motion of transition state. A separate IRC calculation was then performed on the transitionstate geometry to follow either the forward or reverse vibrational mode into the local minimum. The thermodynamic parameters of the reaction such as zero-point corrected energy (ΔEZPE), enthalpies (ΔH°), and Gibbs Free energies (ΔG°), were calculated at 298.15 K and 1 atm. In addition, the solvation correction (in H2O) has also been applied with the Polarizable Continuum Model (PCM)31,32 through a separate single-point energy calculation on the aforementioned optimized geometries. The 6-31G(d,p) basis set was used for the initial geometry optimization, and the 6-311+G(d,p) basis set was used for PCM and single-point calculations using various DFT functionals.

3.1. Mechanism Analysis. Three major reaction steps were identified in the reaction shown in Scheme 2. Step 1, the catalyst Hydroxide ion (OH−) extracts a proton from levulinic acid at either β-carbon (C3, route to form a branched product PB‑3) or δ-carbon (C5, route to form a linear product PL‑3); for the sake of consistency and ease of discussion, route to form a branched product will be denoted as route-B, and route to form a linear product will be route-L; therefore, the transition states identified in step 1 for each route will be denoted as TSB‑1 and TSL‑1, respectively; step 2, the newly formed LA ion then attacks hydroxyl methyl site of HMF to form an addition intermediate through the transition states TSB‑2 for the branched route or TSL‑2 for the linear route; and step 3, the reaction undergoes an internal dehydration via TSB‑3 and TSL‑3 to form the final products PB‑3 and PL‑3. The route to form branched product PB‑3 will be colored black in the following figures, whereas the route to form linear product PL‑3 will be colored red. Step 1: Deprotonation of LA. In this step, the catalyst, hydroxyl group (OH−) approaches to levulinic acid to extract a proton from LA. An experiment found there are two locations where the deprotonation can occur: one is at the β-carbon (C3, route to form a branched product PB‑3) or δ-carbon (C5, route to form a linear product PL‑3). The atomic polar tensor (APT) atomic charges33 of C3 and C5 are −0.08 and −0.15, respectively. The calculated pKa values C2 (α-carbon) is 0.039, much larger than those of C3 and C5 (0.014 and 0.012, respectively). This is consistent with observation that only C3 and C5 can be deprotonated. This step of the reaction is endothermic in both routes, as shown in Figure 1. However, the route to form a linear product through C5 is slightly lower in energy overall than the route to form a branched product through C3. For example, the transition-state structure TSB‑1 (through C3 to form a branched product) has an energy barrier of 34.28 kcal/mol, where the transition structure TSL‑1 (through C5 to form a linear product) shows an energy barrier of 32.25 kcal/mol (all relative to the reactants). TSL‑1 of linear route is 2.03 kcal/mol more stable than TSB‑1 of the branched route. The product, the LA− shown to be 17.42 kcal/mol (branched route) and 15.10 kcal/mol (linear route), respectively, in energy relative to the reactants. The thermodynamic parameters for the reaction of step 1 such as the total energy ΔEtot, total energy with zero-point correction ΔEZPE, enthalpy ΔH, and Gibbs free energy ΔG

3. RESULTS AND DISCUSSION Liquefaction of biomass is one of the major methods in biomass conversion and can be carried out in water or in solvents. The main drawbacks in biomass liquefaction process are the harsh B

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Figure 1. Relative Gibbs free energy of step 1 (kcal/mol), route 1 (black, forms branched product), and route 2 (red, forms linear product) and the primary bonding distances for the transition states (the distances are reported in Å).

Table 1. Thermodynamic Parameters Total Energy ΔEtot, ΔEZPE, ΔH, ΔG, and ΔG(solv) for the Reaction in Step 1 (Reported in kcal/mol) −

branched route B

LA+OH TSB‑1 LA−···H2O LA+OH− TSL‑1 LA−···H2O

linear route L

ΔEtot

ΔEZPE

ΔH

ΔG

ΔG(solv)

0.00 26.18 6.41 0.00 23.59 5.60

0.00 25.57 9.05 0.00 23.60 7.61

0.00 24.63 8.34 0.00 22.58 7.09

0.00 34.28 17.42 0.00 32.25 15.10

0.00 18.40 4.26 0.00 14.22 5.99

the deprotonated site. The calculation showed that in order to remove the water from this agnostic/solvated form, the branched route 1 would need 17.4 kcal/mol to do so while the linear form demands 13.03 kcal/mol. Step 2: LA− Addition. Once the levulinic acid deprotonated, the resulting LA− ion can then attack the HMF via the hydroxymethyl site. Similarly, the transition structure TSB‑2 is identified for the branched product route and TSL‑2 for the linear product route as shown in Figure 2. It is worthy pointing out that, by replacing the H2O from LA−···H2O site in step 1, linear route LA−···HMF agostic complex in step 2 is 37.76 and 40.07 kcal/mol more stable in energy for the branched and linear route, respectively (see Table 3). This is probably due to the new hydrogen bonds formed between HMF and LA−. Such H-bond is only feasible geometrically with HMF (see Figure 2). This newly formed H-bonds length HMF and LA− in the agostic complex is 1.57 and 1.55 Å for branched and linear route, respectively. One can see from Figure 2 that the linear route of step 2 is not only more favored but also has a much smaller energy barrier (4.49 kcal/mol) than that of branch route (13.33 kcal/ mol). Interestingly, step 2 suggests a lightly exothermic reaction for both routes if not an isothermic process (Figure 2). This larger energy barrier difference between two routes is probably due to the steric hindrance that occurred in the C3 site of LA− ion in the branched route. Upon the addition of LA− onto HMF, two notable bond lengths changes occurred: On the LA− side, the adjacent C−C bond at the addition site elongated. For example, C3−C4 of branch route increased from 1.38 to 1.49 Å, and C4−C5 of linear route increased from 1.38 to 1.50 Å (see Table 4). This is opposite to the observation of deprotonation of LA in step 1, and these bond length changes suggested this bond is migrating from a CC double bond in LA− species to a single C−C bond in the addition product. On the HMF side, the adjacent C6−C10 bond length also increases from 1.47 to 1.52 Å (branch route) and from 1.47 to 1.53 Å (linear route). The transition-state structure clearly revealed a newly formed C−C bond distance between LA− and HMF. For example, the C3−C10 bond length in TSB‑2 is 2.07 Å, while C5−C10 in TSL‑2 is 2.22 Å. The addition between LA− and HMF further weakened the H-bond between two fragment, as

are reported in Table 1. A single-point solvation energy calculation were used in solvation energy correction of Gibbs free energy ΔG(solv) are also reported in the same table. While the solvation corrected Gibbs free energy ΔG(solv) still follows the same barrier pattern for these two routes, the values is much stabilized because of anions in the complex. For example, ΔG(solv) for TSB‑1 is only 18.40 and 14.22 kcal/mol for TSL‑1. Not only does the animation of the negative frequency reveal such proton extraction from either C3 for branched route and C5 for linear route to form a water molecule, the transitionstate structures also support the protons’ extraction by OH− ion. For example, both transition-state geometries show strengthening the C−C bond at the extraction site, weakening the C−H bond of departing H atom, and forming a new O−H bond between the departing H and incoming OH− ligand. Table 2 lists selected bond lengths of reactant, TS, and product Table 2. Selected Bonding Lengths for the Reactant LA, Transition States, and Products of Step 1 (Reported in Å) branched route B

linear route L

bond

LA

TSB‑1

LA−···H2O

TSL‑1

LA−···H2O

C1−C2 C2−C3 C3−C4 C4−C5 C3−H C5−H O4−HC3 O4−HC5

1.58 1.52 1.51 1.53 1.10 1.10 -

1.58 1.53 1.45 1.54 1.31 1.10 1.41 -

1.56 1.52 1.39 1.55 2.34 1.10 0.98 -

1.57 1.53 1.53 1.48 1.10 1.24 1.51

1.57 1.53 1.55 1.42 1.10 1.96 1.01

of both routes. The C3−C4 bond length of TSB‑1 decreased from 1.51 Å (reactant LA form) to 1.45 Å, compared to 1.48 Å of the C4−C5 in TSL‑1. At the same time, the C−H bond of departing H atom at the extraction site is elongated to 1.31 Å of C3−H in TSB‑1 and to 1.24 Å of C5−H in TSL‑1. The newly formed O−H bond between the departing H atom of LA and the incoming OH− is 1.41 Å in TSB‑1 and 1.51 Å in TSL‑1, respectively. Interestingly, these deprotonated LA− forms are much more stable when the newly formed water molecule solvated around C

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Figure 2. Relative Gibbs free energy of step 2 (kcal/mol), route 1 (black, forms branched product), and route 2 (red, forms linear product), the primary bonding distances (the distances are reported in Å) and the Burgi−Dunitz angles for the transition states.

Table 3. Thermodynamic Parameters Total Energy ΔEtot, ΔEZPE, ΔH, ΔG, and ΔG(solv) for the Reaction in Step 2 (Reported in kcal/mol) branched route B

linear route L

LA−···HMF TSB‑2 C−C addition product LA−···HMF TSL‑2 C−C addition product

ΔEtot

ΔEZPE

ΔH

ΔG

ΔG(solv)

0 9.36 −6.02 −4.84 −1.77 −8.25

0 10.65 −3.97 −3.95 −0.18 −5.36

0 9.74 −4.57 −4.22 −1.08 −6.29

0 13.33 −2.26 −2.94 1.55 −3.46

0 8.39 −1.27 0.48 9.39 0.21

Table 4. Selected Bonding Lengths for the Reactant LA, Transition States, and Products of Step 2 (Reported in Å) branched route B

linear route L

bond

LA−···HMF

TSB‑2

product

LA−···HMF

TSL‑2

product

C1−C2 C2−C3 C3−C4 C4−C5 C6−C10 C3−C10 C5−C10 O2−H O7−H

1.55 1.50 1.38 1.55 1.47 1.42 1.07

1.56 1.52 1.44 1.55 1.50 2.07 1.57 1.03

1.56 1.54 1.49 1.52 1.52 1.69 1.64 1.01

1.54 1.53 1.55 1.38 1.47 1.44 1.06

1.55 1.53 1.55 1.44 1.49 2.22 1.55 1.03

1.55 1.53 1.53 1.50 1.53 1.68 1.60 1.01

Figure 3. Relative Gibbs free energy of step 3 (kcal/mol), route 1 (black, forms branched product) and route 2 (red, forms linear product) and the primary bonding distances for the transition states (the distances are reported in Å).

products. Like step 1, step 3 also shows an endothermic reaction on both routes. The transition-state TSB‑3 in the branch route has a barrier of 38.66 kcal/mol, and the transitionstate TSL‑3 in the linear route has a barrier of 33.96 kcal/mol, 4.70 kcal/mol lower in energy barrier than that of the branched route (see Figure 3). Because of the double bond, the linear product PL‑3 can have two conformations: cis and trans. The trans product is 5.24 kcal/mol more stable than the cis form. Interestingly, the relative energy of the final linear product is

indicated by the O2−H from 1.42 to 1.64 Å (branch route) and 1.44 to 1.60 Å (linear route). The Burgi−Dunitz angles for the transition states were also investigated. The ∠Nu-CO in TSB‑2 is 112.26°, somewhat larger than the Burgi-Dunitz angle in TSL‑2 which is calculated as 109.54°. These angles are both in reasonable agreement with the generally preferred angle for a nucleophilic attack on a carbonyl group (∼110°).34,35 Step 3: Forming CC Double Bond. The final step 3 is to form a CC double bond at the addition site to yield the final D

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route where the branched form still maintained at 1.54 Å (see Figure 3). 3.2. HOMO, LUMO Analysis. In a quest to better understand the difference between the linear route and the branch route, the frontier orbitals are examined here. For example, the electronic density map of the highest occupied molecular orbital (HOMO) of TSB‑1 and TSL‑1 are shown in Figure 4. This reaction is quite accessible because not only does

9.30 kcal/mol higher in energy than that of the branched products. However, by adding the solvation correction, ΔG(solv) shows that the linear route is still more stable product than the branch route (Table 5). Table 5. Thermodynamic Parameters Total Energy ΔEtot, ΔEZPE, ΔH, ΔG, and ΔG(solv) for the Reaction in Step 3 (Reported in kcal/mol) branched route B

linear route L

C−C addition product TSB‑3 product C−C addition product TSL‑3 product

ΔEtot

ΔEZPE

ΔH

ΔG

ΔG(solv)

0.00

0.00

0.00

0.00

0.00

38.97 18.08 −2.23

35.57 14.88 −1.39

35.86 16.39 −1.72

38.66 5.27 −1.20

34.86 −1.97 −1.48

35.45 27.62

33.49 25.00

33.21 26.24

32.76 14.57

31.17 −5.01

Figure 4. Electronic density map of the highest occupied molecular orbital (HOMO) of TSB‑1 and TSL‑1.

the orbital symmetry match between the lowest unoccupied molecular orbital (LUMO) of LA and the highest occupied molecular orbital (HOMO) of OH− group but also the energy difference is relatively small (0.04 hartree). However, there is a subtle difference between the linear route and the branched route. One can see that the HOMO of TSB‑1 has more spread electron density distribution throughout the molecule, while the HOMO of TSL‑1 has more localized electron density around the C5. This is probably because the location of C3 makes it more susceptible to the induction effect of carboxylate functional group on C1. As such, it requires more energy to remove the hydrogen atom from C3 than C5. In the C−C bond addition step 2 between the LA− and HMF, the HOMO energy of reactant, TS, and product of linear route is more stable than those in branch route, and additionally, the energy difference between the frontier orbitals of HMF and LA− of branch route is much larger than those of linear route. This is consistent with the fact that TSB‑2 is higher in energy than TSL‑2. Similar electron density profile is observed in step 3 of intramolecular deprotonation of C3 (branch route) or C5 (linear route) to form C−C double bond. Overall, the linear route represents more energy viable route to yield the final product than the branch route. The HOMO/LUMO and electron density analysis further confirm the observed experimental regioselectivity. 3.3. Solvation Effect. In order to calculate the solvation effect, a single-point energy calculation under polarizable continuum model (PCM) was first carried out using B3LYP functional and 6-311+G(d,p) basis set. The energy difference between this PCM-single-point calculation and previous calculation under vacuum was then added onto the Gibbs free energy in vacuum without reoptimizing the geometries. This new Gibbs free energy with the correction is traditionally treated as the Gibbs free energy with solvation correction. Water was used as the PCM solvent according to the experiment. The PCM corrected energy profile (dash line) is shown in Figure 5. Overall, the energies of species in linear route are still lower in energy than those in branch route, except TSL‑2. Even so, the energy difference between the linear and branch routes in TS2 is only by 1 kcal/mol, which is probably due to the stabilization from the charge in the PCM model. Further investigation on this abnormality will be discussed in the following true solvated Gibbs free energy calculation. One can see from the figure that the rest of activation energies were

The transition-state structure of TSB‑3 in branch route shows the motion of forming CC double bond at the addition site of LA− and HMF when extracting H from C3 by OH group of HMF. For example, this H atom being extracted has distances to the OOH of HMF fragment and to C3 of LA fragment of 1.35 and 1.38 Å, respectively. As a result, C3−C10 bond distance decreased from 1.61 to 1.46 Å, a clear motion to form a CC double bond. The adjacent bond C6−C10 of HMF moiety was enhanced slight from 1.53 to 1.50 Å but C10−O6 was weakened from 1.32 to1.41 Å (Table 6). The internal hydrogen Table 6. Selected Bonding Lengths for the Reactant LA, Transition States, and Products of Step 3 (Reported in Å) branched route B

linear route L

bond

reactant

TSB‑3

product

reactant

TSL‑3

product

C1−C2 C2−C3 C3−C4 C4−C5 C6−C10 C3−C10 C5−C10 O2−HOH O7−HOH C10−O6 O6−H C3−H C5−H

1.56 1.54 1.53 1.52 1.53 1.61 1.61 1.02 1.32 1.09 1.10

1.56 1.53 1.56 1.54 1.50 1.46 1.54 1.04 1.41 1.35 1.38 1.09

1.61 1.49 1.52 1.51 1.44 1.36 0.97 1.09

1.55 1.53 1.53 1.50 1.53 1.68 1.60 1.01 1.31 1.09 1.09

1.58 1.54 1.53 1.56 1.50 1.46 0.98 1.41 1.36 1.09 1.34

1.58 1.52 1.50 1.51 1.44 1.35 0.97 1.10 1.09

bond between the hydroxyl moiety of HMF and carboxyl group of LA− still exists to stabilize the TSB‑3 and O2−H bond was enhanced from1.61 Å to 1.54 Å. The transition-state structure of linear route TSL‑3 also has similar geometric changes. For example, H atom distances to the OOH of HMF fragment and to C5 of LA fragment are 1.36 and 1.34 Å, respectively. Subsequently, C5−C10 bond distance decreases from 1.68 to1.46 Å in preparation to form CC double bond. The adjacent bond C6−C10 of HMF moiety was enhanced slightly from 1.53 to 1.50 Å, and C10−O6 was weakened from 1.31 to 1.41 Å (Table 6). Due to the geometric constraint, the internal H-bond between the hydroxyl moiety of HMF and carboxyl group of LA− was further weakened to 1.92 Å in the linear E

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Figure 5. Relative Gibbs free energy profile (solid line) and the PCM corrected energy profile (dash line) (reported in kcal/mol).

between the linear route and the branched route increased to 2.21 kcal/mol. Although there are no quantitative kinetic and thermodynamic data available for direct comparison, our computational results agree qualitatively with the previous experimental observations (1:2.5 ratio for the branched:linear products). Geometry-wise, there are subtle changes upon reoptimization under PCM model. For example, the hydrogen bond between the hydroxyl moiety of HMF and carboxyl group of LA is 1.74 Å in the branch route and the C3−C10 bond length is 2.15 Å, a little weakened than the bond formed under nonsolvated condition (2.07 Å). Similarly, the aforementioned hydrogen bond is 1.68 Å, and the C5−C10 bond length is 2.16 Å, slightly enhanced than the bond formed under nonsolvated condition (2.22 Å) and shorter than those in the TSB‑2 under the solvation mode. To further discuss the role of the solvent water molecule in the reaction system, one water molecule has been added in the model of transition state of step 3, as an example to investigate the role of water in the formation of the transition states of steps 3. As shown in Figure 6, even though this explicit water

getting smaller in the PCM model overall. For example, the TS1 of branch route and linear route is 18.40 and 14.22 kcal/mol, respectively. Compared to those in vacuum, these energy barriers dropped by 16−18 kcal/mol. In step 2, such energy barriers dropped from 13.33 to 8.40 kcal/mol for branch route, but the linear route increased slightly from 4.48 to 8.92 kcal/ mol. Similarly in step 3, the transition-state energy decreases from 38.66 to 34.86 kcal/mol for branch route and from 33.96 to 32.65 kcal/mol for linear route. The single-point energy calculation was also carried out under the PCM model using MP2 functional and 6-311+G(d,p) basis set. This PCM corrected energy profile is shown in Figure S1. The linear route is favored in energy over the branch route through the overall observation. The energy differences of transition states between linear and branched route in step 1 and step 3 were found minor increased in all three step (from 4.18 to 5.24 kcal/mol in step 1, from 0.52 to 0.90 kcal/mol in step 2 and from 2.21 to 4.24 kcal/mol in step 3). To further investigate the geometric relaxation upon the PCM solvation model was applied, the geometries in step 2 were further optimized under PCM model using B3LYP functional and 6-311+G(d,p) basis set. The calculated thermodynamic parameters are summarized in Table 7. Now, Table 7. Relative Gibbs Free Energy (G), PCM Corrected Relative Gibbs Free Energy (G(solv)) and Relative Gibbs Free Energy Optimized under PCM (G(solv)-Opt) for the Agostic Complexes, Transition States and Products of Step 2 (Reported in kcal/mol) branched route B

linear route L

LA−···HMF TSB‑2 C−C addition product LA−···HMF TSL‑2 C−C addition product

G

G(solv)

G(solv)-Opt

0 13.33 −2.26 −2.94 1.55 −3.46

0 8.39 −1.27 0.48 9.39 0.21

0 10.48 6.47 −0.50 9.07 2.04

Figure 6. Primary bonding distances for the transition states of step 3 with one water molecular added (the distances are reported in Å).

molecule was initially placed near the TS site, this water molecule was favorably migrated to the carboxylate site in the branched route TSB‑3-H2O where the H-bond was feasible. Interestingly, the same explicit water in the linear TS-state TSL‑3-H2O still stays at the TS site. According to the calculations, after one water molecule was added, the hydrogen bond between H atom and the OOH of HMF fragment was enhanced from 1.35 to 1.24 Å, while the distance of H···O−C3 was slightly weakened from 1.38 to 1.43 Å in route 1. Both H atoms in water molecule were found to form hydrogen bonds with the carbonyl and carboxyl group of LA. In route 2, the water molecule was found play a more important role in the formation of transition state. The hydrogen bond between H

one can see that with the optimization under PCM model, the “corrected” energy barrier was “restored” and consistent with linear route being a favored route. The linear route is favored in energy over the branch route through the overall observation. For example, the transition state of linear route TSL‑2 has an energy barrier of 9.57 kcal/mol, ∼1 kcal/mol smaller than that of the branched route (10.48 kcal/mol). In the following CC double bond forming step, the difference of energy gap F

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Article

The Journal of Physical Chemistry A

addition of LA−-HMF; and (3) intramolecular deprotonation to form CC double bond. According to this DFT investigation, the linear route is overall thermodynamically more favored than the branch route. The activation energy barriers of step 1 and 3 are the largest among three primary steps. Molecular orbital analysis also confirms the greater induction influence on C3 (branch route) that cause the activation energy being higher in branch route than the linear route. The results from the solvation model, M06-2X, TPSS, and MP2 functionals are consistent with B3LYP functional. This work highlights a promising strategy in building structural and functional blocks for the preparation of useful industrial chemicals and liquid transportation fuels.

atom and the OOH of HMF fragment was enhanced from 1.36 to 1.20 Å, while the distance of H···O−C3 was apparently weakened from 1.34 to 1.51 Å. One H atom in the water molecule was found to form a strong hydrogen bond with the carbonyl group of LA (0.99 Å), and the other H atom formed one weak hydrogen bond with the carbonyl group of HMF moiety (2.22 Å). Due the geometric constraint, the energy difference between TSB‑3 and TSL‑3 was increased from 7.1 kcal/mol without a water molecule to 11.27 kcal/mol. 3.4. A Comparative Study of the DFT Method and MP2 Method. First, to further investigate the effect of different functionals on the energy profile of the reaction in this study, single-point energies with B3LYP optimized geometries were calculated using M06-2X and TPSS functionals, and the results are summarized in Figure 7. The calculation results

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11100. HOMO, LUMO and their energies, computational results obtained using M06-2X and TPSS and PCM model, Cartesian coordinates and thermodynamic parameters, imaginary frequencies of transition states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liwei Zhao: 0000-0002-3719-8773 Notes

Figure 7. Relative energy plots for the transition states calculated by B3LYP, comparing with the single-point energies plots calculated by M06-2X and TPSS functionals.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.-J.F. gratefully acknowledges the Department of Chemistry at Prairie View A&M University for release time and funding support of this work by the U.S. Department of Energy, National Nuclear Security Administration grant (DE-NA 0001861 & DE-NA 0002630) and the Welch foundation Grant (No. L0002).

demonstrated that the predicted activation energies of steps 1 and 2 by the M06-2X and TPSS functionals are both lower in both routes than those calculated by B3LYP functional. This is presumably because the Minnesota functional M06-2X and TPSS functionals are doing better job in modeling the weak interaction than the B3LYP functional. Furthermore, the calculation with Minnesota functional M06-2X is very slow to converge in this reaction step. The slow convergence is probably due to the basis set limit for intermolecular interactions, a typical behavior of their exchange functional inhomogeneity correction factors (ICF). Then, in order to compare the results from density functionals, additional calculations on step 2 were performed using the MP2 functional and 6-31G(d,p) basis set (Table S4). The energy barrier increased slightly from 13.33 using the B3LYP method to 14.58 kcal/mol, whereas the linear route increased from 4.48 to 9.79 kcal/mol. The linear route still represents more energy viable route than the branch route under the MP2 method.

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REFERENCES

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DOI: 10.1021/acs.jpca.6b11100 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.6b11100 J. Phys. Chem. A XXXX, XXX, XXX−XXX