Theoretical Elucidation of Glucose Dehydration to 5

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Theoretical Elucidation on the Glucose Dehydration to 5Hydroxymethylfurfural Catalyzed by a SOH-Functionalized Ionic Liquid 3

Jingjing Li, Jinghua Li, Dongju Zhang, and Chengbu Liu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b07773 • Publication Date (Web): 04 Oct 2015 Downloaded from http://pubs.acs.org on October 11, 2015

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The Journal of Physical Chemistry

Theoretical Elucidation on the Glucose Dehydration to 5-Hydroxymethylfurfural Catalyzed by a SO3H-functionalized Ionic Liquid †

‡

†

Jingjing Li, Jinghua Li, Dongju Zhang,*, and Chengbu Liu

†

†

Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, P. R. China ‡

Department of Chemistry, Duke University, Durham, NC 27708, Unite States

1

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ABSTRACT While catalytic conversion of glucose to 5-hydroxymethyl furfural (HMF) catalyzed by SO3H-functioned ionic liquids (ILs) has been achieved successfully, the relevant

molecular

mechanism

is

still

not

understood

well.

Choosing

1-butyl-3-methylimidazolium chloride [C4SO3HmimCl] as a representative of SO3H-functioned ILs, this work presents a density functional theory (DFT) study on the catalytic mechanism for conversion from glucose into HMF. It is found that the conversion may proceed via two potential pathways and that throughout most of elementary steps, the cation of IL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bond, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol, respectively, which are compatible with the observed catalytic performance of the IL under mild conditions (100 °C). The present results provide help for rationalizing the effective conversion from glucose to HMF catalyzed by SO3H-functionalized ILs and for designing IL catalysts used in biomass conversion chemistry.

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INTRODUCTION The shortage of fossil fuel combined with environmental pollution has been aggravating rapidly, making it imperative to search for sustainable and clean energy sources. Currently, considerable efforts1-7 have been put into the exploitation and utilization of biomass due to its renewability, abundance, and widespread distribution. Cellulose, (C6H10O5)n, is the most abundant and renewable biomass materials on earth,8,9 and its effective utilization has attracted great attention in academic and in industrial fields.10,11 Cellulose can be hydrolyzed into small polysaccharides and monosaccharides such as glucose, which can be further converted to various value-added chemicals, in particular 5-hydroxymethylfurfural (HMF) (Scheme 1),12-17 a versatile and multi-functional compound to generate liquid fuels and fine chemicals.

Scheme 1. Conversion of cellulose into glucose and further into HMF In the past decades, tremendous studies have been devoted to the production of HMF from glucose.18-22 Ionic liquids (ILs) are now recognized as the excellent solvents and catalysts for conversion of glucose to HMF.23-26 Using ILs as solvent, Zhao et al.27 have showed a cost effective conversion of glucose catalyzed by metal chlorides, giving a yield of HMF near 70%. The metal-free conversion of glucose to HMF in ILs is first reported by Riisager at al.28 who presented a HMF yield of up to 42% by using boric acid as a promoter. Recently, Jiang et al.29 provided a remarkable report that realized the direct conversion from glucose to HMF by solely using acidic ILs, such as 3



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1-butyl-3-methylimidazolium chloride [C4SO3Hmim]Cl, in a single-pot reaction. They found that the yield of total reducing sugar (mainly including glucose and HMF) can be as high as 90%, indicating a promising method for the direct transformation of cellulose into HMF by replacing metal catalysts and inorganic acid catalysts with environmentally-benign ILs. Such a direct catalytic conversion from glucose to HMF by solely using ILs is a ground-breaking technology, which is greener than ones10,20 involving metal chlorides and therefore deserves more attention. To understand the fundamental reaction chemistry of the IL-promoted conversion, we have recently presented a theoretical explanation for how acid ILs promote the conversion of cellulose to glucose (the first step in Scheme 1),30 and proposed an enzyme-like mechanism of ILs breaking the glycosidic bond through a retaining mechanism or/and an inverting mechanism. However, the molecular mechanism for the conversion from glucose to HMF (the second step in Scheme 1) catalyzed by acid ILs has not yet been illustrated. This work, as one of our serial researches on the dissolution and conversion of cellulose in ILs,31-33 reports the density functional theory calculations of the transformation concerned. In the experiment of Jiang et al.,29 [C4SO3Hmim]Cl is found to be one of the ILs with relatively high efficiency for the conversion from glucose to 5-HMF. In this case, we chose [C4SO3Hmim]Cl for the present study. It should be emphasized that Hensen et al.34 has reported the mechanism of CrClx-promoted conversion of glucose to HMF, where the metal chloride, acting as a Lewis acid, was proposed to promote the conversion through its coordination to O atom of glucose to stabilize intermediates and hence reduces the free-energy barrier. In the present work, our calculations show that the sulfonic group in the cation of [C4SO3Hmim]Cl plays a substantial role, which acts as a proton shuttle to promote the 4


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conversion. Thus the catalytic mechanism of SO3H-functionalized ILs is substantially different from that of the metal chloride. The calculated results are expected to provide help for understanding the catalytic role of acid ILs in the transformation from glucose to HMF.

COMPUTATIONAL METHOD Scheme 2 shows the model system studied in this work, which describes the conversion of glucose to HMF in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) catalyzed by 1-butyl sulfonic acid-3-methylimidazolium chloride([C4SO3Hmim]Cl), the most active catalyst found by Jiang et al.29

Scheme 2. Catalytic conversion of glucose into HMF by [C4SO3Hmim]Cl in [Bmim]Cl. The calculations were carried out by using the meta GGA BB95 functional,35 which is proposed to be the best functional to accurately calculate barrier heights,36 with the standard 6-31G (d, p) basis set, as implemented in Gaussian 09 software package.37 All geometries for the reactants, intermediates, transition states and products were first fully optimized without any constraints in the gas phase, and further refined using solvation model based on density (SMD).38,39 Previous studies40,41 demonstrated the SMD model is accurate enough for describing the solvation effect of ILs. To mimic the solvent effect of a imidazolium chloride

IL

using

SMD

model,

the

necessary

descriptors

5


include

the


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dielectric constant ( ε ), the index of refraction (n), the macroscopic surface tension ( γ ), the fraction of aromatic carbon atoms ( φ ), the fraction of electronegative halogen atoms (ψ ), and Abraham’s hydrogen bond acidity and basicity parameters (

∑ αH 2

and

∑ βH 2

).

For

[Bmim]Cl,

n=1.515, γ =69.41, φ =0.273, ψ =0.091,

these

∑ αH 2

parameters

=0.203, and

are ε =15.0,

∑ βH 2

=0.666.42-45

Vibrational frequency calculations were also carried out to verify the optimized structures as minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency) and to provide free energies at 298.15 K, which include entropic contributions by taking into account the vibrations, rotations, and translations of the structures. The intrinsic reaction coordinate (IRC)46,47 pathways of transition states have been traced to confirm that each of them actually connects the desired reactant and product. RESULTS AND DISCUSSION Based on previous experimental27,28 and computational34,48,49 studies on the catalytic conversion of glucose to HMF by Lewis acidic metal salts34,48 or Brønsted acids,49 it is generally recognized that the conversion occurs through an enediol intermediate which then either evolves to fructose and further to HMF (pathway I in Scheme 3) or directly dehydrates to HMF (pathway II in Scheme 3). For the system under study (Scheme 2) with [C4SO3Hmim]Cl acting as catalyst, we conjecture that the reaction may follow the general mechanism shown in Scheme 3. In the following sections, we will show the calculated mechanism details of pathways I and II to understand the IL-catalyzed conversion of glucose to HMF.

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Scheme 3. General mechanism for the production of HMF from glucose

Formation of 1,2-enediol. As shown in Scheme 3, 1,2-enediol is an open-chain structure, which is formed via two elementary steps: the opening of the pyranose ring of glucose, followed by a ketone-enol isomerization process. Figure 1 displays the calculated energy profile with schematic representation of calculated structures, where the energy of the initial complex between a glucose molecule and an ionic pair of [C4SO3Hmim]Cl, denoted as R, is taken as the zero-energy reference point. In R, the chloride anion interacts with the hydroxyl group at C7 through H-bond. The ring opening transition state located is TSR-1, a typical eight-membered-ring transition state, where the sulfonic group acts as a proton shuttle, accepting a proton from the hydroxyl group at C1 atom and transferring another one to O6 atom, to promote the break of C1-O6 bond and formation of C1=O bond, leading to intermediate IM1. This process involves a free energy barrier of 8.3 kcal/mol. Prior to the subsequent ketone-enol isomerization, IM1 evolves into a more stable structure 7



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IM2 via reposition between the substrate and catalyst. IM2 is converted to IM3, realizing the transformation of the substrate from ketone to enol through TS2-3. It is noted that TS2-3 is also an eight-membered-ring transition state where the sulfonic group plays again a role of a proton shuttle to carry out the ketone-enol isomerization. However, the formation of 1,2-enediol involves a barrier of 26.3 kcal/mol, which is considerably higher than that of the ring opening process, 8.3 kcal/mol.

Figure 1. Calculated Gibbs free energy profile with schematic geometries of intermediates and transition states for the formation of 1,2-enediol intermediate. Isomerization of 1,2-enediol to fructose. Along pathway I, 1,2-enediol first evolves into fructose. Figure 2 shows the calculated energy profile for this isomerization reaction with schematic representation of calculated structures. This process consists of two elementary steps: the enol-ketone isomerization and ring closure. As shown in Figure 2, along the reaction coordinate, IM3 evolves to it’s an 8


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energetically less stable conformational isomer, IM4, which is geometrically ready to carrying out the enol-ketone isomerization through transition state TS4-5 with the sulfonic group acting as the proton shuttle. The forward minimum from TS4-5 is intermediate IM5, where the substrate has been converted to the open-fructose that can be easily converted into fructose via a ring-closure transition state TS5-6, as indicated by the low barrier 7.2 kcal/mol from IM5 to IM6. From Figure 2, it is found that the overall barrier for the formation of fructose is 28.2 kcal/mol, and the whole process is slightly exothermic by 2.7 kcal/mol. Vlachos et al50 reported the conversion from glucose to fructose in aqueous using CrCl3 as a catalyst with an apparent activation energy (15.3 kcal/mol), implying that the metal chloride is more efficient for the catalytic conversion. However, the catalytic conversion solely using the IL is expected to have the great potential due to the environment-friendly characteristics of ILs.

9



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Figure 2. Calculated Gibbs free energy profile with schematic geometries for the isomerization of 1,2-enediol intermediate to fructose.

Dehydration of Fructose to HMF. Formally, the transformation of fructose to HMF involves three dehydration processes. Calculated mechanism details are shown in Figures 3-5. Considering the possibility that both the cation and anion of [C4SO3Hmim]Cl may take part in the dehydration reaction, two ion pairs are necessary to mimic the catalysis of the IL. In the originally designed mechanism, the cation of one ionic pair acts as a Brønsted acid and the anion of another ionic pair functions as a nucleophile, and the presences of both the cation and anion are accompanied by another anion and another cation, respectively, resulting in two ion pairs occurred in the system. Figure 3 presents the first dehydration process. Along the reaction coordinate, the 10


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catalyst-fructose complex IM6 evolves into a less stable conformer IM7, from which the proton of –SO3H group is transferred to the tertiary alcohol group at C2 atom via TS7-8 with a barrier of 29.4 kcal/mol, resulting in C2-OH bond cleavage. After that, the substrate forms an oxocarbenium species, as indicated by the geometrical parameters shown in IM8. This step is calculated to be endothermic by 22.1 kcal/mol. Subsequently, IM8 is converted to the first dehydration product IM9 through TS8-9, where the abstraction of a hydrogen atom from C1 by water molecule co-occurs with proton transfer from water molecule to -SO3 anion group. The barrier of this process is calculated to be 32.9 kcal/mol. The transformation of IM9 to IM10 is calculated to be exothermic by 16.8 kcal/mol, which is mainly owing to the entropy contribution with the removal of a water molecule. From Figure 3, it is found that the overall barrier of the first dehydration process is 32.9 kcal/mol. During this process, the sulfonic group in the cation of [C4SO3Hmim]Cl plays a role of acid/base catalyst, and a water molecule participates the reaction as a proton transfer mediator. In contrast, the chloride anion acts as a spectator which interacts with the substrate via H-bond rather than a nucleophile.

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Figure 3. Calculated Gibbs free energy profile with schematic geometries of the first dehydration process.

Then, our attention turns to the second dehydration process. As shown in Figure 4, two potential pathways, denoted as the step-wise (black line) and synergistic (red line), respectively, have been calculated. Along the step-wise pathway, IM10 evolves to its ketone form IM10′, from which the alcohol group at C3 atom is protonated via TS10′-11 with a barrier of 40.0 kcal/mol. Note that in TS10′-11, the chloride anion, as a nucleophile is attacking C3 atom to assist the proton transfer event. Subsequently, H-shift from C2 atom to O atom of water molecule occurs via TS11-12 with a barrier of 37.0 kcal/mol, resulting in IM12 and further IM13. Due to the high barriers involved in the two elementary steps, the step-wise pathway seems to be difficult to be carried 12


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out under thermal conditions. Alternatively, the synergistic pathway is found to be energetically much more favorable. This pathway proceeds through TS10-12, a ten-membered-ring transition state structure, where the sulfonic group acts as a proton shuttle, protonating of the alcohol group at C3 atom and accepting a proton from the hydroxyl group at C1 atom, directly leading to intermediate IM12. The barrier of this pathway is calculated to be 16.5 kcal/mol, which is in contrast to 40.0 kcal/mol located in the step-wise pathway. Thus the second dehydration process prefers to occur along the synergistic pathway.

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Figure 4. Calculated Gibbs free energy profile with schematic geometries of the second dehydration process along the synergistic pathway (red line) and the step-wise pathway (black line). 14


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With elimination of a water molecule and reposition between the substrate and catalyst, IM12 evolves to a more stable intermediate IM13, which is geometrically ready to carrying out the third dehydration reaction. The calculated results are shown in Figure 5. The relevant mechanism is intrinsically similar to that of the first dehydration reaction: initial protonation of hydroxyl group at C4 followed by migration of the hydrogen from C5 to O atom of water molecule, leading to formation of HMF and release of IL catalyst, completing the catalytic cycle. The overall barrier is found to be 17.7 kcal/mol (the energy difference between TS14-P and IM13), which is much lower than that in the first dehydration step. Thus the first dehydration step with the barrier of 32.9 kcal/mol is the bottleneck of whole process. This value is comparable to the experimental value, 34.6 kcal/mol, reported by Wei et al.51 who realized the conversion from glucose to HMF by using Ir and Au chlorides as catalysts and ionic liquids as solvents.

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Figure 5. Calculated Gibbs free energy profile with schematic geometries of the third dehydration process. Transformation of 1, 2-enediol to HMF. As indicated in Scheme 3, pathway II involves the direct transformation of 1,2-enediol intermediate to HMF. Figure 6 show calculated mechanism details, which is characterized by four fundamental sub-processes: i) protonation of the hydroxyl group at C3 atom and migration of proton of the hydroxyl group at C1 atom to -SO3H group via a ten-member-ring transition state (TS3-A1) to give a conjugated aldehyde species (A1), ii) formation of a diketone species (A3) via transition state TSA2-A3, which is similar to TS3-A1, discussed above, iii) protonation of the tertiary hydroxyl group at C2 atom to give an oxocarbenium species A5, and iv) formation of the aim product HMF and release of IL catalyst through H-shift.

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Figure 6. Mechanism details for the transformation of 1, 2-enediol to HMF according to the pathway II shown in Scheme 3. Figure 7 shows the calculated free energy profile along the reaction coordinate. The barriers of four sub-processes are calculated to be 29.3, 31.0, 21.2, and 17.7 kcal/mol, respectively, indicating that the second dehydration process is the bottleneck of the transformation. The overall barrier (31.0 kcal/mol) of this pathway is comparable with 17



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that (32.9 kcal/mol) in pathway I, Therefore, both the pathways may contribute to formation of HMF. From the discussion above, we find that in most elementary steps, the –SO3H group in the cation plays a substantial catalytic role, functioning as a proton shuttle, while Cl- plays an auxiliary role which interacts with the substrate via H-bond to stabilize intermediates and transition states. In a previous study of the conversion from fructose to HMF promoted by 1-butyl-3-methylimidazolium bromide (BmimBr), He et al.52 proposed a different catalytic mechanism, where Br- of the IL interacts with hydroxyl group of substrate via H-bond to induce the dehydration processes. The different mechanism might originate from the different cation and anion components of ILs. In addition，it should be noted that the imidazolium ring also plays an important role for the catalytic conversion. As shown in Figures 1-6 the acidic proton of imidazolium ring forms effective H-bond with the chlorine anion to stabilize the intermediates and transition states. If the imidazolium ring is replaced by a pyridine ring, we expect that the reaction would be more difficult to occur due to the weaker acidity of the pyridine ring.

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Figure 7. Calculated Gibbs free energy profile with schematic geometries for the transformation of 1, 2-enediol to HMF according to the pathway II shown in Scheme 3. CONCLUSIONS DFT calculations have shown the mechanism details for the conversion from glucose to HMF promoted by SO3H-functionalized IL. There exist two possible pathways that contribute to formation of HMF, and 1, 2-enediol species is identified as a necessary intermediate along both pathways. Pathway I characterizes the formation of fructose intermediate, whereas pathway II involves the direct formation of HMF from 1, 2-enediol species. Calculated comparable overall barriers of two pathways (32.9 and 31.0 kcal/mol) indicate both of them contribute to formation of HMF. Throughout most of elementary steps, the –SO3H group of the cation of [C4SO3Hmim]Cl plays a substantial role, which acts as a proton donor as a Brønsted acid and on the other hand functions as a proton acceptor as the conjugate base, just like a proton shuttle. The chloride anion interacts with the substrate and acidic proton in the imidazolium ring 19



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via H-bond interaction, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. In addition, it is noted that water molecules produced during the conversion from glucose to HMF also generally take part in the reaction, assisting the proton transfer processes. The present theoretical results provide a clear elementary-step mechanism profile of the transformation from glucose to HMF catalyzed by SO3H-functionalized ILs.

ASSOCIATED CONTENT Supporting Information The Cartesian coordinates of all stationary points located in this work. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]; Tel: +86-531-88365833; Fax: +86-531-88564464. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was jointly supported by National Natural Science Foundation of China (Nos. 21433006 and 21273131) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130131110012). REFERENCES (1) Corma, A.; Iborra, S.; Velty, A. Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 2007, 107, 2411–2502. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044–4098. 20


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TOC

DFT calculations show the mechanism details of the conversion from glucose to HMF catalyzed by a SO3H-functionalized Ionic Liquid.

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Theoretical Elucidation of Glucose Dehydration to 5

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