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Adsorption mediated tandem acid catalyzed cellulose hydrolysis by ortho-substituted benzoic acids
Molecular Catalysis 475 (2019) 110459
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Adsorption mediated tandem acid catalyzed cellulose hydrolysis by orthosubstituted benzoic acids⋆ Danjo P. De Chaveza, Min Gaob, Hirokazu Kobayashib, Atsushi Fukuokab, Jun-ya Hasegawab, a b
T
⁎
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose DFT calculations Catalytic hydrolysis Mechanistic study
Carboxylic acid and phenol moiety containing carbon materials are proven to catalyze cellulose hydrolysis. Experiments utilizing benzoic acid and substituted derivatives as model catalysts revealed that vicinal oxygenated group containing catalysts showed significantly higher activities regardless of acidity. However, the increase in catalytic activity was not well understood. Using density functional theory, the mechanism of catalysis was elucidated. Calculated binding energy followed the experimental turnover number suggesting that the formation and the stability of the adduct complex is imperative to catalysis. In addition to this, protonated system effectively modeled the experimental activation energy implying that a hydronium participates in the reaction. These calculations indicated that during the life time of the adduct complex, a dissolved proton assists in the overall catalytic mechanism.
1. Introduction Cellulose has attracted great interest due to its high potential as a renewable carbon resource to reduce emission of greenhouse gases [1]. Cellulose is a polymer of glucose connected by β-1,4-glycosidic linkage and its depolymerization produces intermediates for synthesizing useful chemicals such as fuel additives, plastics and bioactive compounds [1–4]. One of the main products, 5-hydroxymethyfurfural (HMF), can then be converted to dimethylfuran, which is a potential fuel additive [2]. HMF also serves as a basic building block for synthetic applications [5]. Levulinic acid poses numerous applications such as chiral reagents, batteries and biopolymers [6,7]. γ-Valerolactone can be used as gasoline blender, green solvent and be post processed as valeric esters which can be used as biodiesel [8]. Hydrolysis is a practical method to depolymerize cellulose, and various catalysts have been employed to hydrolyze cellulose which include solid catalysts, [9,10] homogenous acids [11], and enzymes [12]. Many experimental and theoretical works have been done to reveal the reaction mechanism. Solid catalysts, particularly, carbon nanoparticles [9,13], and zeolites with ionic liquid as solvent [14] show high catalytic activity for hydrolysis of cellulose. An activated carbon with trace HCl shows high activity to give 88% yield of glucose [15] and active sites of the carbons locate at acidic oxygenated functional
groups. The interaction between cellulose and carbon-based materials also plays an important role in the catalytic performance. Homogenous catalysis was also considered for hydrolysis of cellulose. It is demonstrated that the maleic acid and oxalic acid possess high activity to hydrolyze cellulose [16]. Previously, our combined experimental and theoretical study showed that CH-πinteractions are the driving force for the initial adsorption of cellulosic molecules on the carbon catalyst [17]. As model active sites of the carbon materials, we also demonstrated high activity of o-hydroxybenzoic and phthalic acid among substituted benzoic acids despite the weak acidity [18]. Spectroscopic studies and density functional theory (DFT) calculations show that their activity higher than expected from pKa value [19]. A hydrogen bond formed between one oxygenated group and a hydroxyl group of cellulosic molecule increases the probability of attack of the adjacent carboxylic acid on the glyosidic bond. As the hydrolysis reaction is performed under slightly acidic conditions (pH ca. 3) in real experiments, the role of proton (hydronium) have not been discussed. In addition, the correlation between strength of the interactions and reaction results has not been quantitatively analyzed. Therefore, in the present work, we aim to clarify the proton effect on the reaction mechanism of cellulose hydrolysis with using of ortho-substituted benzoic acids catalyst by DFT calculations.
⋆
Peer review under responsibility of Holy Spirit University of Kaslik. 2214–4234/$ – see front matter © 2013 Holy Spirit University of Kaslik. Hosting by Elsevier B.V. All rights reserved. https://doi.org/10.1016/j.rgo.2013.10.012. ⁎ Corresponding author. E-mail address: [email protected] (J.-y. Hasegawa). https://doi.org/10.1016/j.mcat.2019.110459 Received 8 April 2019; Received in revised form 3 June 2019; Accepted 3 June 2019 Available online 08 June 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Molecular Catalysis 475 (2019) 110459
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2. Computational details
between cellobiose and o-chlorobenzoic acid (d) and benzoic acid (e) with the distance of ˜1.86 Å. Therefore, the binding energies for the corresponding geometries are much smaller, 39.0 kJ/mol and 37.0 kJ/ mol, respectively. Calculated Ebind shows a good correlation with the experimentally determined turnover frequency (TOF) of benzoic acid derivatives in hydrolysis of cellobiose [18]. This result indicates that the formation of a stable complex with cellulosic molecules is key to achieve high catalytic activity. Since a larger Ebind represents higher stability and in turn longer lifetime of the complex, a catalyst with a larger Ebind has a higher probability to proceed to the hydrolysis step during their lifetime. To calculate potential energy profile of the hydrolysis, a total of four water molecules was introduced to the cellobiose-catalyst complex. Three water molecules were added around the previous model; two were necessary for stabilizing charged species during the hydrolysis and the other was as a reactant to complete the reaction. As shown in Fig. 2, water molecules, W1 and W2, at the other side of cellobiose and were included to account for the nucleophilic attack to the transient carbocation. The third and fourth water molecules, W3 and W4, a part of hydrogen-bonding network with cellobiose and the catalyst. As the hydrolysis reaction occurs at acid condition with pH = 3.0, it is possible that the water molecule is pronated. Therefore, in present study, comparison of the reaction pathways with protonated and neutral W4 was done using the catalyst of o-hydroxybenzoic acid. The optimized structures and relative potential energies of the reactant (R), transition state (TS), and product (P) states in the hydrolysis of o-hydroxybenzoic acid are shown in Fig. 2. Calculated activation energy (Ea) of the neutral model was 167.8 kJ/mol which is relatively high, and the potential energy of the P state is larger than that of the R state (ΔE = 57.0 kJ/mol) which connotes the instability of P state. In contrast, the assistance of proton significantly reduced the activation barrier of the hydrolysis. The protonated model gave a much smaller activation energy, 109.8 kJ/mol, which reasonably agrees with the experimental data (˜124 kJ/mol) [18]. In addition, the P state was highly stabilized with the introduction of proton having a relative energy of -29.8 kJ/mol with respect to the R state. Structure variation along the reaction pathway of the proton assisted hydrolysis was also investigated. In the R state, the catalyst interacts both directly and indirectly via W3 and W4. The catalyst is in a near-attacking conformation in which the proton at the carboxyl group of the catalyst is hydrogen-bonding to the O atom of the glycosidic bond. To reach TS, the glycosidic OeC bond stretches to 1.87 Å, which is associated with proton transfers from the catalyst to the glycosidic O atom and from a hydrogen bond between W4 and the catalyst. According to the intrinsic reaction coordinate (IRC) calculation [28], the OeC bond stretching is connected to TS. In the calculated path, at OeC distance of 1.46 Å, the OeH distance already becomes 1.05 Å, and the proton at W4 is already transferred to the catalyst; see Figure S1 in supporting information (SI). Therefore, in a static view of the molecular dynamics based on IRC, the hydrolysis starts with proton transfer from the catalyst to the glycoside O atom. From TS to the P state, W1 attacks to the positively charged C atom at the carbocation in the dissociating glucose, and proton transfer also occurs from W1 to W2, which completes the hydrolysis. Without proton, significant changes were found in the geometries along the hydrolysis reaction path. The length of hydrogen bond between W4 and carboxyl group of the catalyst in the R state becomes much longer, indicating a weaker interaction between them. To reach the TS, the glycosidic OeC bond stretches from 1.40 Å of the R state to 2.10 Å which requires more energy to deform the complex. From the TS to the P state, OH group of W1 attacks to the positively charged C atom, and the proton transfers from W1 to the W2 follows. In the P state, different from the reaction with proton, the o-hydroxybenzoic acid anion is formed. The calculated Mulliken charge shows that in the presence of proton, the total charge at the catalyst (+0.02) was completely compensated at TS, and the positive charge migrates from W4 to W2 in the
DFT calculations were performed to evaluate binding energy and to elucidate the mechanism of the catalytic hydrolysis by ortho-substituted benzoic acids as models of active sites of carbon catalysts [18,20]. The transition state structures were verified by frequency calculations. B3LYP hybrid functional [21] was employed for the exchange-correlation functional. Self-consistent reaction field method with a polarizable continuum model [22–24] was adopted to account the solvent effect. The dielectric constant for water (εr = 78.3553) was used. The 6-31G** basis sets [25,26] were used for all of the atoms. Mulliken population analysis was used for charge distribution analysis. All the computations were done with Gaussian 09 [27]. The binding energy, Ebind, was calculated using a supermolecule scheme. An energy difference ΔE was calculated as the potential energy difference of cellobiose―catalyst complex and that of isolated cellobiose and catalyst as shown in Eq. (1). ΔE = Ecellobiose-catalyst − [Ecellobiose + Ecatalyst]
(1)
Ebind was defined to be Ebind = −ΔE
(2)
Using the above convention, eq. (2), a larger Ebind value corresponds to a larger stabilization by the complex formation. 3. Results and discussion In this work, cellobiose was chosen as the minimum unit to model cellulose, and model catalysts were limited to phthalic acid (R]COOH), o-hydroxybezoic (R]OH), o-chlorobenzoic (R]Cl) and benzoic acid (R]H) as shown in Scheme 1. The carboxyl groups of the catalysts were protonated because of the acidic condition (pH 3.0) in the real experiments [18]. For phthalic acid, mono anionic form dominates under the said condition, and hence mono anionic state was chosen as the model. An additional water molecule was included to the model to explicitly account for the solvent around the reaction site. In the initial structure of the optimization, the carboxylic moiety of the catalyst had a hydrogen bond with glycosidic O atom. Geometries of cellobiose, catalysts and cellobiose-catalyst adduct complex were then fully relaxed. These optimized geometries serve as a precursor to hydrolysis so that the calculated binding energy represents the stability of conformation prior to hydrolysis. The isolated cellobiose and cellobiose-catalyst complex interacts with the additional water molecule via hydrogen bonding as shown in Fig. 1. It should be noted that no significant substituent effect was seen as revealed by similar catalyst-water hydrogen bond lengths. O-hydroxybenzoic acid (b) and phthalic acid anion (c) interact with cellobiose with binding energies of 48.4 kJ/mol and 84.0 kJ/mol, respectively. Both oxygenated groups of the catalysts served as interaction sites for the cellobiose where one oxygenated group makes a hydrogen bond with primary hydroxyl group of cellobiose and the vicinal carboxylic groups retains the interaction with the glyosidic O atom. The anionic carboxylate group of phthalic acid anion increases the energy contribution due to the short hydrogen bond with distance of 1.69 Å. In contrast, only a single hydrogen bond exists
Scheme 1. Cellulose Hydrolysis by ortho-substituted benzoic acids. 2
Molecular Catalysis 475 (2019) 110459
D.P. De Chavez, et al.
Fig. 1. Optimized structures, binding energy (kJ/mol), and experimental turn over frequency (h−1) [18] for the cellobiose and cellobiose-catalyst complexes: (a) cellobiose, (b) o-hydroxybenzoic acid, (c) o-phthalic acid anion, (d) o-chlorobenzoic acid, and (e) benzoic acid. Bond length is indicated with unit of Å.
Fig. 2. Structures of R, TS, and P in the hydrolysis of cellobiose by o-hydroxybenzoic acid (a) with and (b) without H+. Bond length is indicated with unit of Å. Relative potential energy from the R state is also given. For TS, imaginary frequency obtained with normal mode analysis is included. For each segment, calculated charge is given inside parenthesis.
energetically unfavorable, the additional proton at the W4 site compensated the negative charge and alleviated the stabilization of potential energies of the hydrolysis reaction. Two-dimensional potential energy surfaces were plotted to overview the difference of the potential energy landscape between the
course of the hydrolysis. No significant change was found in the total charge distribution during the reaction pathways of hydrolysis. In the absence of proton, the total charge for each species is closed to zero in the R state, while the positively charged glucose and negatively charged catalyst appear in both TS and P states. Since a charge-separated state is 3
Molecular Catalysis 475 (2019) 110459
D.P. De Chavez, et al.
Fig. 3. Two-dimensional potential energy surfaces for the hydrolysis of cellobiose by o-hydroxybenzoic acid. Relative potential energies from the R state are plotted at each grid point defined by fixed O-C and O-H distances. The other structural variables were optimized. See Fig. 2 for the position of the O, C, and H atoms. The red and green surfaces denote hydrolysis with and without the H+ association, respectively. A red arrow is a possible reaction pathway indicated by the IRC calculation (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
hydrolysis, an adsorption mediated acid tandem mechanism is highly plausible, in which during the lifetime of the complex a proton in the form of hydronium approaches the active site and aids in the hydrolysis reaction. The results of this study can be generalized to other carbon-based materials. Katz et al. arrived with a similar conclusion that the weak acid sites in a mesoporous carbon nanoparticles (MCN) [13] and in zeolite templated carbon [29,30] are important to catalyze the hydrolysis reaction. Although the two different carbon materials differ in the particle size, the chemical environment of the active sites, which was characterized by CH-π interaction as well as weak acid sites (i.e. phenol and carboxylic acid), is expected to be similar to each other. Thus, MCN can be expected to exhibit adsorption mediated tandem acid catalysed hydrolysis in the context of cellulosic materials.
protonated and neutral models. Fig. 3 shows relative potential energies from the R state plotted at each grid point defined by fixed OeC and OeH distances. Starting from the R state, with decreasing of the OeH bond from ˜ 1.9 Å to 1.0 Å, no significant energy changes occurs in the potential surfaces for the two cases. The energy difference in the absence and presence of proton is only ˜ 25 kJ/mol when OeH bond is 1.0 Å, indicating that the energy difference has low dependence to the OeH bond length. The two potential surfaces showed high dependence on the OeC bond length with a shorter OeC bond corresponding to a lower energy on the calculated surfaces. Furthermore, it could be seen that an additional energy is required to stretch the OeC bond in the absence of proton. Remarkably, in the protonated system, the TS is characterized with a shorter OeC bond resulting in a lower activation energy for hydrolysis. Therefore, activity of a proposed catalyst can be approximated by the OeC bond length at the TS. As the reaction mechanisms were determined, only the reaction pathway with protonated W4 was adopted for the other catalysts. The activation energies and reaction energies for o-hydroxybenzoic acid, ochlorobenzoic acid, benzoic acid, and o-phthalic acid anion are summarized and compared with experimental value in Table 1. The reaction energy ΔE is defined as ΔE = Eproduct ̶ Ereactant. The Ea values are similar for o-hydroxybenzoic acid, o-chlorobenzoic acid and benzoic acid, and slightly higher for phthalic acid. It is notable that the order of calculated Ea and ΔG° are the same as that of experimental results (phthalic acid > o-hydroxybenzoic acid > benzoic acid > o-chlorobenzoic acid), thus showing good reliability of the calculation models employed. Furthermore, the P state is stable compared to the R state in all cases. In retrospect to the analyses done, results rationalize that the catalytic mechanism of benzoic acid and derivatives is aided by hydronium. As the hydronium is dissolved and wanders in the solvent system, the importance of the stability and lifetime of the substrate―catalyst complex is again emphasized. Forming the prerequisite to the
4. Conclusions The general reaction mechanism of the cellulose hydrolysis with respect to different catalyst was studied and proven to undergo similar pathways. The present calculations showed classical acid catalyzed hydrolysis mechanism where a proton is transferred from the catalyst to glycosidic oxygen. The origin of the increased catalytic activity of the vicinal oxygenated group containing carbon catalysts can be attributed to the lifetime of the adduct complex formed by the cellobiose and catalyst. The lifetime and stability of the adduct is proportional to the binding energy of the catalyst to the cellobiose molecule. Calculated binding energies increased with the experimental turnover number of the catalyst which proved that the complex formation is important to the catalysis. Based on the presented calculations, in addition to the benzoic acid-based catalyst, a dissolved proton aids the hydrolysis reaction. The calculations using a neutral system resulted in a high energy barrier, and the hydrolysis becomes endothermic which was not energetically favorable. The high energy barrier and the instability of hydrolysis can be explained due to a charge separated state. Calculated activation energies of the protonated systems are comparable with experimentally observed ones. This supported that a dissolved proton facilitates the hydrolysis. In congruent to these, an adsorption mediated tandem acid catalytic mechanism is suggested. During the lifetime of the adduct complex, which scales according to the calculated binding energies, the organic acid catalyst and an additional dissolved proton works in synergy to catalyze the hydrolysis of cellulose. It is also important to note that this mechanism is expected to hold in systems with similar chemical environment where a weak acid site is important to catalyze cellulose hydrolysis.
Table 1 Activation energy (Ea), reaction energy ΔE for the hydrolysis by the model carbon catalysts. Corresponding Gibbs free energies at 298 K are given in parenthesis. Calculations were performed with the B3LYP/6–311 G(d,p) level. Units are in kJ/mol. Optimized structures of the transition states are shown in Figure S2 in SI. Catalyst
o-Hydroxybenzoic acid o-Chlorobenzoic acid Benzoic acid Phthalic acid
ΔE (ΔG°)
Ea (ΔG°) Calc.
Exptl.
Calc.
105.6 (106.2) 99.6 (94.2) 102.8 (94.4) 125.1(111.7)
118 108 111 120
−11.7 −34.7 −30.2 −23.5
(-14.2) (-34.7) (-31.2) (-19.4)
4
Molecular Catalysis 475 (2019) 110459
D.P. De Chavez, et al.
Acknowledgements This work was financially supported by the Japan Science and Technology Agency (JST) ALCA, JSPS KAKENHI (Grant Number JP15H05805) and MEXT “Priority Issue on Post-K computer” (Development of new fundamental technologies for high-efficiency energy creation, conversion/storage and use). Part of the computations were carried out at RCCS (Okazaki, Japan), ACCMS (Kyoto University), and RIIT (Kyushu University).
[15]
[16]
[17]
Appendix A. Supplementary data [18]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110459.
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Adsorption mediated tandem acid catalyzed cellulose hydrolysis by orthosubstituted benzoic acids⋆ Danjo P. De Chaveza, Min Gaob, Hirokazu Kobayashib, Atsushi Fukuokab, Jun-ya Hasegawab, a b
T
⁎
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Cellulose DFT calculations Catalytic hydrolysis Mechanistic study
Carboxylic acid and phenol moiety containing carbon materials are proven to catalyze cellulose hydrolysis. Experiments utilizing benzoic acid and substituted derivatives as model catalysts revealed that vicinal oxygenated group containing catalysts showed significantly higher activities regardless of acidity. However, the increase in catalytic activity was not well understood. Using density functional theory, the mechanism of catalysis was elucidated. Calculated binding energy followed the experimental turnover number suggesting that the formation and the stability of the adduct complex is imperative to catalysis. In addition to this, protonated system effectively modeled the experimental activation energy implying that a hydronium participates in the reaction. These calculations indicated that during the life time of the adduct complex, a dissolved proton assists in the overall catalytic mechanism.
1. Introduction Cellulose has attracted great interest due to its high potential as a renewable carbon resource to reduce emission of greenhouse gases [1]. Cellulose is a polymer of glucose connected by β-1,4-glycosidic linkage and its depolymerization produces intermediates for synthesizing useful chemicals such as fuel additives, plastics and bioactive compounds [1–4]. One of the main products, 5-hydroxymethyfurfural (HMF), can then be converted to dimethylfuran, which is a potential fuel additive [2]. HMF also serves as a basic building block for synthetic applications [5]. Levulinic acid poses numerous applications such as chiral reagents, batteries and biopolymers [6,7]. γ-Valerolactone can be used as gasoline blender, green solvent and be post processed as valeric esters which can be used as biodiesel [8]. Hydrolysis is a practical method to depolymerize cellulose, and various catalysts have been employed to hydrolyze cellulose which include solid catalysts, [9,10] homogenous acids [11], and enzymes [12]. Many experimental and theoretical works have been done to reveal the reaction mechanism. Solid catalysts, particularly, carbon nanoparticles [9,13], and zeolites with ionic liquid as solvent [14] show high catalytic activity for hydrolysis of cellulose. An activated carbon with trace HCl shows high activity to give 88% yield of glucose [15] and active sites of the carbons locate at acidic oxygenated functional
groups. The interaction between cellulose and carbon-based materials also plays an important role in the catalytic performance. Homogenous catalysis was also considered for hydrolysis of cellulose. It is demonstrated that the maleic acid and oxalic acid possess high activity to hydrolyze cellulose [16]. Previously, our combined experimental and theoretical study showed that CH-πinteractions are the driving force for the initial adsorption of cellulosic molecules on the carbon catalyst [17]. As model active sites of the carbon materials, we also demonstrated high activity of o-hydroxybenzoic and phthalic acid among substituted benzoic acids despite the weak acidity [18]. Spectroscopic studies and density functional theory (DFT) calculations show that their activity higher than expected from pKa value [19]. A hydrogen bond formed between one oxygenated group and a hydroxyl group of cellulosic molecule increases the probability of attack of the adjacent carboxylic acid on the glyosidic bond. As the hydrolysis reaction is performed under slightly acidic conditions (pH ca. 3) in real experiments, the role of proton (hydronium) have not been discussed. In addition, the correlation between strength of the interactions and reaction results has not been quantitatively analyzed. Therefore, in the present work, we aim to clarify the proton effect on the reaction mechanism of cellulose hydrolysis with using of ortho-substituted benzoic acids catalyst by DFT calculations.
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Peer review under responsibility of Holy Spirit University of Kaslik. 2214–4234/$ – see front matter © 2013 Holy Spirit University of Kaslik. Hosting by Elsevier B.V. All rights reserved. https://doi.org/10.1016/j.rgo.2013.10.012. ⁎ Corresponding author. E-mail address: [email protected] (J.-y. Hasegawa). https://doi.org/10.1016/j.mcat.2019.110459 Received 8 April 2019; Received in revised form 3 June 2019; Accepted 3 June 2019 Available online 08 June 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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2. Computational details
between cellobiose and o-chlorobenzoic acid (d) and benzoic acid (e) with the distance of ˜1.86 Å. Therefore, the binding energies for the corresponding geometries are much smaller, 39.0 kJ/mol and 37.0 kJ/ mol, respectively. Calculated Ebind shows a good correlation with the experimentally determined turnover frequency (TOF) of benzoic acid derivatives in hydrolysis of cellobiose [18]. This result indicates that the formation of a stable complex with cellulosic molecules is key to achieve high catalytic activity. Since a larger Ebind represents higher stability and in turn longer lifetime of the complex, a catalyst with a larger Ebind has a higher probability to proceed to the hydrolysis step during their lifetime. To calculate potential energy profile of the hydrolysis, a total of four water molecules was introduced to the cellobiose-catalyst complex. Three water molecules were added around the previous model; two were necessary for stabilizing charged species during the hydrolysis and the other was as a reactant to complete the reaction. As shown in Fig. 2, water molecules, W1 and W2, at the other side of cellobiose and were included to account for the nucleophilic attack to the transient carbocation. The third and fourth water molecules, W3 and W4, a part of hydrogen-bonding network with cellobiose and the catalyst. As the hydrolysis reaction occurs at acid condition with pH = 3.0, it is possible that the water molecule is pronated. Therefore, in present study, comparison of the reaction pathways with protonated and neutral W4 was done using the catalyst of o-hydroxybenzoic acid. The optimized structures and relative potential energies of the reactant (R), transition state (TS), and product (P) states in the hydrolysis of o-hydroxybenzoic acid are shown in Fig. 2. Calculated activation energy (Ea) of the neutral model was 167.8 kJ/mol which is relatively high, and the potential energy of the P state is larger than that of the R state (ΔE = 57.0 kJ/mol) which connotes the instability of P state. In contrast, the assistance of proton significantly reduced the activation barrier of the hydrolysis. The protonated model gave a much smaller activation energy, 109.8 kJ/mol, which reasonably agrees with the experimental data (˜124 kJ/mol) [18]. In addition, the P state was highly stabilized with the introduction of proton having a relative energy of -29.8 kJ/mol with respect to the R state. Structure variation along the reaction pathway of the proton assisted hydrolysis was also investigated. In the R state, the catalyst interacts both directly and indirectly via W3 and W4. The catalyst is in a near-attacking conformation in which the proton at the carboxyl group of the catalyst is hydrogen-bonding to the O atom of the glycosidic bond. To reach TS, the glycosidic OeC bond stretches to 1.87 Å, which is associated with proton transfers from the catalyst to the glycosidic O atom and from a hydrogen bond between W4 and the catalyst. According to the intrinsic reaction coordinate (IRC) calculation [28], the OeC bond stretching is connected to TS. In the calculated path, at OeC distance of 1.46 Å, the OeH distance already becomes 1.05 Å, and the proton at W4 is already transferred to the catalyst; see Figure S1 in supporting information (SI). Therefore, in a static view of the molecular dynamics based on IRC, the hydrolysis starts with proton transfer from the catalyst to the glycoside O atom. From TS to the P state, W1 attacks to the positively charged C atom at the carbocation in the dissociating glucose, and proton transfer also occurs from W1 to W2, which completes the hydrolysis. Without proton, significant changes were found in the geometries along the hydrolysis reaction path. The length of hydrogen bond between W4 and carboxyl group of the catalyst in the R state becomes much longer, indicating a weaker interaction between them. To reach the TS, the glycosidic OeC bond stretches from 1.40 Å of the R state to 2.10 Å which requires more energy to deform the complex. From the TS to the P state, OH group of W1 attacks to the positively charged C atom, and the proton transfers from W1 to the W2 follows. In the P state, different from the reaction with proton, the o-hydroxybenzoic acid anion is formed. The calculated Mulliken charge shows that in the presence of proton, the total charge at the catalyst (+0.02) was completely compensated at TS, and the positive charge migrates from W4 to W2 in the
DFT calculations were performed to evaluate binding energy and to elucidate the mechanism of the catalytic hydrolysis by ortho-substituted benzoic acids as models of active sites of carbon catalysts [18,20]. The transition state structures were verified by frequency calculations. B3LYP hybrid functional [21] was employed for the exchange-correlation functional. Self-consistent reaction field method with a polarizable continuum model [22–24] was adopted to account the solvent effect. The dielectric constant for water (εr = 78.3553) was used. The 6-31G** basis sets [25,26] were used for all of the atoms. Mulliken population analysis was used for charge distribution analysis. All the computations were done with Gaussian 09 [27]. The binding energy, Ebind, was calculated using a supermolecule scheme. An energy difference ΔE was calculated as the potential energy difference of cellobiose―catalyst complex and that of isolated cellobiose and catalyst as shown in Eq. (1). ΔE = Ecellobiose-catalyst − [Ecellobiose + Ecatalyst]
(1)
Ebind was defined to be Ebind = −ΔE
(2)
Using the above convention, eq. (2), a larger Ebind value corresponds to a larger stabilization by the complex formation. 3. Results and discussion In this work, cellobiose was chosen as the minimum unit to model cellulose, and model catalysts were limited to phthalic acid (R]COOH), o-hydroxybezoic (R]OH), o-chlorobenzoic (R]Cl) and benzoic acid (R]H) as shown in Scheme 1. The carboxyl groups of the catalysts were protonated because of the acidic condition (pH 3.0) in the real experiments [18]. For phthalic acid, mono anionic form dominates under the said condition, and hence mono anionic state was chosen as the model. An additional water molecule was included to the model to explicitly account for the solvent around the reaction site. In the initial structure of the optimization, the carboxylic moiety of the catalyst had a hydrogen bond with glycosidic O atom. Geometries of cellobiose, catalysts and cellobiose-catalyst adduct complex were then fully relaxed. These optimized geometries serve as a precursor to hydrolysis so that the calculated binding energy represents the stability of conformation prior to hydrolysis. The isolated cellobiose and cellobiose-catalyst complex interacts with the additional water molecule via hydrogen bonding as shown in Fig. 1. It should be noted that no significant substituent effect was seen as revealed by similar catalyst-water hydrogen bond lengths. O-hydroxybenzoic acid (b) and phthalic acid anion (c) interact with cellobiose with binding energies of 48.4 kJ/mol and 84.0 kJ/mol, respectively. Both oxygenated groups of the catalysts served as interaction sites for the cellobiose where one oxygenated group makes a hydrogen bond with primary hydroxyl group of cellobiose and the vicinal carboxylic groups retains the interaction with the glyosidic O atom. The anionic carboxylate group of phthalic acid anion increases the energy contribution due to the short hydrogen bond with distance of 1.69 Å. In contrast, only a single hydrogen bond exists
Scheme 1. Cellulose Hydrolysis by ortho-substituted benzoic acids. 2
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Fig. 1. Optimized structures, binding energy (kJ/mol), and experimental turn over frequency (h−1) [18] for the cellobiose and cellobiose-catalyst complexes: (a) cellobiose, (b) o-hydroxybenzoic acid, (c) o-phthalic acid anion, (d) o-chlorobenzoic acid, and (e) benzoic acid. Bond length is indicated with unit of Å.
Fig. 2. Structures of R, TS, and P in the hydrolysis of cellobiose by o-hydroxybenzoic acid (a) with and (b) without H+. Bond length is indicated with unit of Å. Relative potential energy from the R state is also given. For TS, imaginary frequency obtained with normal mode analysis is included. For each segment, calculated charge is given inside parenthesis.
energetically unfavorable, the additional proton at the W4 site compensated the negative charge and alleviated the stabilization of potential energies of the hydrolysis reaction. Two-dimensional potential energy surfaces were plotted to overview the difference of the potential energy landscape between the
course of the hydrolysis. No significant change was found in the total charge distribution during the reaction pathways of hydrolysis. In the absence of proton, the total charge for each species is closed to zero in the R state, while the positively charged glucose and negatively charged catalyst appear in both TS and P states. Since a charge-separated state is 3
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Fig. 3. Two-dimensional potential energy surfaces for the hydrolysis of cellobiose by o-hydroxybenzoic acid. Relative potential energies from the R state are plotted at each grid point defined by fixed O-C and O-H distances. The other structural variables were optimized. See Fig. 2 for the position of the O, C, and H atoms. The red and green surfaces denote hydrolysis with and without the H+ association, respectively. A red arrow is a possible reaction pathway indicated by the IRC calculation (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
hydrolysis, an adsorption mediated acid tandem mechanism is highly plausible, in which during the lifetime of the complex a proton in the form of hydronium approaches the active site and aids in the hydrolysis reaction. The results of this study can be generalized to other carbon-based materials. Katz et al. arrived with a similar conclusion that the weak acid sites in a mesoporous carbon nanoparticles (MCN) [13] and in zeolite templated carbon [29,30] are important to catalyze the hydrolysis reaction. Although the two different carbon materials differ in the particle size, the chemical environment of the active sites, which was characterized by CH-π interaction as well as weak acid sites (i.e. phenol and carboxylic acid), is expected to be similar to each other. Thus, MCN can be expected to exhibit adsorption mediated tandem acid catalysed hydrolysis in the context of cellulosic materials.
protonated and neutral models. Fig. 3 shows relative potential energies from the R state plotted at each grid point defined by fixed OeC and OeH distances. Starting from the R state, with decreasing of the OeH bond from ˜ 1.9 Å to 1.0 Å, no significant energy changes occurs in the potential surfaces for the two cases. The energy difference in the absence and presence of proton is only ˜ 25 kJ/mol when OeH bond is 1.0 Å, indicating that the energy difference has low dependence to the OeH bond length. The two potential surfaces showed high dependence on the OeC bond length with a shorter OeC bond corresponding to a lower energy on the calculated surfaces. Furthermore, it could be seen that an additional energy is required to stretch the OeC bond in the absence of proton. Remarkably, in the protonated system, the TS is characterized with a shorter OeC bond resulting in a lower activation energy for hydrolysis. Therefore, activity of a proposed catalyst can be approximated by the OeC bond length at the TS. As the reaction mechanisms were determined, only the reaction pathway with protonated W4 was adopted for the other catalysts. The activation energies and reaction energies for o-hydroxybenzoic acid, ochlorobenzoic acid, benzoic acid, and o-phthalic acid anion are summarized and compared with experimental value in Table 1. The reaction energy ΔE is defined as ΔE = Eproduct ̶ Ereactant. The Ea values are similar for o-hydroxybenzoic acid, o-chlorobenzoic acid and benzoic acid, and slightly higher for phthalic acid. It is notable that the order of calculated Ea and ΔG° are the same as that of experimental results (phthalic acid > o-hydroxybenzoic acid > benzoic acid > o-chlorobenzoic acid), thus showing good reliability of the calculation models employed. Furthermore, the P state is stable compared to the R state in all cases. In retrospect to the analyses done, results rationalize that the catalytic mechanism of benzoic acid and derivatives is aided by hydronium. As the hydronium is dissolved and wanders in the solvent system, the importance of the stability and lifetime of the substrate―catalyst complex is again emphasized. Forming the prerequisite to the
4. Conclusions The general reaction mechanism of the cellulose hydrolysis with respect to different catalyst was studied and proven to undergo similar pathways. The present calculations showed classical acid catalyzed hydrolysis mechanism where a proton is transferred from the catalyst to glycosidic oxygen. The origin of the increased catalytic activity of the vicinal oxygenated group containing carbon catalysts can be attributed to the lifetime of the adduct complex formed by the cellobiose and catalyst. The lifetime and stability of the adduct is proportional to the binding energy of the catalyst to the cellobiose molecule. Calculated binding energies increased with the experimental turnover number of the catalyst which proved that the complex formation is important to the catalysis. Based on the presented calculations, in addition to the benzoic acid-based catalyst, a dissolved proton aids the hydrolysis reaction. The calculations using a neutral system resulted in a high energy barrier, and the hydrolysis becomes endothermic which was not energetically favorable. The high energy barrier and the instability of hydrolysis can be explained due to a charge separated state. Calculated activation energies of the protonated systems are comparable with experimentally observed ones. This supported that a dissolved proton facilitates the hydrolysis. In congruent to these, an adsorption mediated tandem acid catalytic mechanism is suggested. During the lifetime of the adduct complex, which scales according to the calculated binding energies, the organic acid catalyst and an additional dissolved proton works in synergy to catalyze the hydrolysis of cellulose. It is also important to note that this mechanism is expected to hold in systems with similar chemical environment where a weak acid site is important to catalyze cellulose hydrolysis.
Table 1 Activation energy (Ea), reaction energy ΔE for the hydrolysis by the model carbon catalysts. Corresponding Gibbs free energies at 298 K are given in parenthesis. Calculations were performed with the B3LYP/6–311 G(d,p) level. Units are in kJ/mol. Optimized structures of the transition states are shown in Figure S2 in SI. Catalyst
o-Hydroxybenzoic acid o-Chlorobenzoic acid Benzoic acid Phthalic acid
ΔE (ΔG°)
Ea (ΔG°) Calc.
Exptl.
Calc.
105.6 (106.2) 99.6 (94.2) 102.8 (94.4) 125.1(111.7)
118 108 111 120
−11.7 −34.7 −30.2 −23.5
(-14.2) (-34.7) (-31.2) (-19.4)
4
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Acknowledgements This work was financially supported by the Japan Science and Technology Agency (JST) ALCA, JSPS KAKENHI (Grant Number JP15H05805) and MEXT “Priority Issue on Post-K computer” (Development of new fundamental technologies for high-efficiency energy creation, conversion/storage and use). Part of the computations were carried out at RCCS (Okazaki, Japan), ACCMS (Kyoto University), and RIIT (Kyushu University).
[15]
[16]
[17]
Appendix A. Supplementary data [18]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110459.
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