Periodic DFT Study of the Opening of Fructose and Glucose Rings and

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Periodic DFT Study of the Opening of Fructose and Glucose Rings and the Further Conversion of Fructose to Trioses Catalyzed by M-BEA (M = Sn, Ti, Zr, or Hf) Brian D. Montejo-Valencia, and Maria C. Curet-Arana J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10211 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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The Journal of Physical Chemistry

Periodic DFT Study of the Opening of Fructose and Glucose Rings and the Further Conversion of Fructose to Trioses Catalyzed by M-BEA (M = Sn, Ti, Zr, or Hf)

Brian D. Montejo-Valencia, and María C. Curet-Arana* Department of Chemical Engineering, University of Puerto Rico - Mayaguez Campus, Road 108 km 1.1, Mayaguez, PR 00681-9000

*Corresponding Author: Department of Chemical Engineering, University of Puerto Rico Mayaguez Campus, Call Box 9000, Mayaguez, PR 00681-9000, Fax:787 265-3818, Tel.: 787 832-4040 ext. 2569, E-mail: [email protected]

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ABSTRACT Periodic density functional theory (DFT) calculations with long range corrections were used to analyze the opening of the fructose and glucose rings catalyzed by metal-substituted beta zeolites (M-BEA). The reaction mechanisms were systematically analyzed on BEA substituted with tin (Sn), titanium (Ti), zirconium (Zr), and hafnium (Hf). Here, we proposed a mechanism for the conversion of fructose to dihydroxyacetone (DHA) and glyceraldehyde (GLA) and a novel mechanism for the glucose ring opening. The preferential site of substitution of the metals in BEA were reported. The adsorption energies of fructose and glucose through their different oxygen atoms on M-BEA were also reported. The transition state energies were calculated using the nudge elastic band and dimmer methods. Among the zeolites studied, Sn-BEA displays the lowest energies barriers for the conversion of the fructose to its trioses and for the glucose ring opening.


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1. INTRODUCTION Lignocellulosic biomass is an abundant and inexpensive raw material from which high value added chemicals can be produced.1 It is primarily composed of cellulose and hemicellulose, which can be decomposed into sugars through acid hydrolysis.2 Glucose is the sugar obtained from the hydrolysis of cellulose and is in equilibrium with the fructose isomer. Previous studies have demonstrated that Lewis acid catalysts, such as tin substituted zeolite beta (Sn-BEA), can accelerate the isomerization of these sugars and promote the further reaction of fructose to produce lactic acid.

3–8

The main reactions involved in the conversion of glucose to lactic acid

with a Lewis catalyst are shown in Figure 1.

Figure 1. Catalytic path for the conversion of glucose with a Lewis catalyst. Despite the outstanding performance of Sn-BEA, the selective conversion of these sugars, however, is still a challenge. Previous studies have focused in optimizing the above paths by varying the solvents and catalysts.3–8 However, yields to obtain lactic acid from these sugars with Sn-BEA catalysts are still below 30%. These yields are mainly due to the Brønsted acid nature of lactic acid, which can catalyze the reaction through other paths that produce hydroxymethylfurfural (HMF) and levulinic acid.6 Furthermore, the vast number of functional groups in biomass-derived molecules imposes a major challenge on these reaction systems. Various theoretical studies have focused on the reaction mechanism for the conversion of glucose to fructose.

9–12

These previous studies agree that the hydride shift from two adjacent

carbon atoms within the molecule is the rate limiting step.9–12 We note, however, that the reaction



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mechanism for the conversion of fructose to its trioses, which is the rate determining step for the production of lactic acid6,7 has not been reported in the literature. Here we proposed two different mechanisms for the opening of the fructose ring with BEA zeolite catalysts, and the further conversion of the opened fructose to the trioses. Furthermore, despite the significant progress on the fundamentals of the glucose to fructose catalyzed reaction further studies are still necessary to clarify how the glucose ring opens. For instance, it has been proposed that the opening of the glucose ring can occur through two different paths as shown in Figure 2.10–12 These reaction mechanisms consist of two steps for the intramolecular hydrogen shift on glucose from O1 to O5. The main difference between the mechanisms illustrated in Figure 2 is on the adsorption of glucose, which has been proposed to occur through O110,12 or O511. We note that Yang and coworkers also performed periodic DFT calculations on the reaction mechanisms in Figure 2, but they analyzed closed sites instead of hydrolyzed sites.12

Here we propose a third path for the

opening of glucose that involves only one step with a lower energy barrier than the reaction mechanisms that have been previously proposed in the literature. In this study, we analyzed the conversion of fructose to its trioses, dihydroxyacetone (DHA) and glyceraldehyde (GLA), and the glucose ring opening using BEA zeolite catalysts substituted with tin (Sn), hafnium (Hf), titanium (Ti), and zirconium (Zr).

We analyzed two

mechanisms for the fructose ring opening, one composed by two catalytic steps, and the other one comprised by only one step. The lowest transition state (TS) energy was obtained with SnBEA (73.1 kJ/mol). Furthermore, the TS energy for the glucose opening in Sn-BEA (96.9 kJ/mol) presented here is lower than those previously reported (129 and 194 kJ/mol).12


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Figure 2. Previously reported reaction mechanisms for the glucose ring opening on Sn-BEA. (a) Reaction mechanism proposed by Li and coworkers10 (b) Reaction mechanism proposed by Bermejo-Deval and coworkers.11 2. METHODOLOGY In this work, the reaction mechanism for the conversion of fructose to DHA and GLA and the reaction mechanism for the ring opening of glucose on M-BEA were analyzed using periodic DFT calculations. As shown in Figure S1 in the Supporting Information, α-D-Glucopyranose (GP) and β-D-fructofuranose (FF) were selected as the conformers of glucose and fructose, respectively. We analyzed the polymorphism A of zeolite beta and used the coordinates as reported by the International Zeolite Association (IZA).13 BEA has a tetragonal unit cell with lattice parameters: a = b = 12.6 Å and c = 26.2 Å, and has nine crystallographic sites, as shown in Figure S2 in the Supporting Information. Tetravalent metal substituted BEA zeolites (M-BEA) were analyzed with Sn, Hf, Zr, and Ti. The calculations were performed using the Vienna ab initio simulation package (VASP 5.4)14,15 with the generalized gradient approximation Perdew–Burke– Ernzrhof (PBE) exchange-correlation.16,17 Standard PAW potentials15,18 were used for all the elements, except for the metal atoms, for which higher electronic PAW pseudopotentials were 5 ACS Paragon Plus Environment


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used instead of the default ones. Thus, for the substituted metals, we employed Sn (Sn_d), Hf (Hf_pv), Zr (Zr_sv), and Ti (Ti_sv) to treat their semicores s, p, or d as valence states.19 The dispersion interactions were taken into account with the DFT-D3 method.20 The Brillouin zone sampling was restricted to the G-point. The energy cutoff was set to 500 eV. Full geometry optimizations were performed with fixed cell parameters. For all the optimizations and TS, the converge was assumed to be reached when the forces on all atoms were less than 0.05 eV/A. Similar methodologies have been previously used to describe BEA zeolites substituted with metals.12,21–24 The nudged elastic band method (NEB) was used to determine the minimum energy path and to locate the transition-state (TS) structures.25,26 Then, the dimer method was used to obtain the saddle point.27 The TS were confirmed by frequency calculations, which showed only one imaginary frequency. The frequency calculations involved only the motions of the atoms of the adsorbed molecules, as well as the metal substituted with its four bonded oxygens.

3. RESULTS AND DISCUSSION The following sections discuss the results obtained for the conversion of fructose to its trioses, GLA and DHA, the opening of glucose ring on BEA zeolite substituted with Sn, Ti, Zr, and Hf. First, the preferential locations of the metals in BEA are presented. Then, the adsorption energies of fructose through the different oxygen atoms in M-BEA are discussed, followed by the analysis of the reaction mechanisms for the opening of the fructose ring and its further conversion to the trioses. For the opening of the fructose ring, two reaction mechanisms were considered, and they are illustrated in Figure 3 (a) and Figure 3 (b). The mechanism for the further conversion of the acyclic fructose to DHA and GLA is represented in Figure 4. Lastly, a novel mechanism for the opening of the glucose ring on M-BEA (shown in Figure 5) is discussed.


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Figure 3. Reaction mechanisms for fructose ring opening through (a) one and (b) two elementary steps.

Figure 4. Reaction mechanism for the conversion of open fructose to DHA and GLA.

Figure 5. Proposed reaction mechanism for the opening of the glucose ring.



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The preferential substitution sites for Sn, Hf, Zr, and Ti in BEA were calculated by replacing the metal in each T-Site of the zeolite and optimizing the BEA unit cell structure. The substitution energies for the different sites within BEA zeolite are reported relative to the energy of the most stable substitution. The nine T-sites of BEA and the calculated relative energies are illustrated in the Figures S3-S6 in the Supporting Information. As shown in the Supporting Information, our calculations demonstrate that the most stable substitution site for all the metal atoms in BEA is in the T6 site, in agreement with experimental and other computational results.21,28–30 For instance, Bare and co-workers, using EXAFS spectroscopy, found that the substitution of Sn in BEA is predominantly in the T6 site.28

Furthermore, previous works with DFT calculations have

demonstrated that the preferential substitution site of Zr and Ti in BEA is also in T6.21,29,30 As far as we know, studies on the substitution of Hf in BEA have not been reported, but based on our results, the preferred substitution site for Hf in BEA is also in T6. In this work, all the calculations involving the adsorption of fructose and glucose were performed with the metal atoms substituted in the T6 site of BEA.

3.1 Fructose Adsorption, Ring Opening and Conversion to Trioses. The adsorption energies of fructose through the different oxygen atoms of the molecule on M-BEA are shown in Figure 6. The optimized structures for the adsorption of fructose on MBEA were obtained in monodentate configurations. The only exception is when the adsorption of fructose is through O5, in which the fructose is also coordinated to the metal atom through O6, due to the steric hindrances cause by its hydroxides (O6H, O1H). The distances between the adsorbed oxygen of the fructose and the metal of M-BEA vary between 2.17 to 2.74 Å, as shown on Table S1 and Figure S7 in the Supporting Information. In general, the O5-M distances obtained by the bidentate adsorption through the O5 and O6 are larger (2.55 – 2.74 Å) than the ones obtained through the other oxygen atoms.


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The adsorption energies of fructose on M-BEA are also strongly affected by the substituted metal atom and by the fructose oxygen that is coordinated to the metal. Comparing the average values of the adsorption energies on each M-BEA catalyst, the strongest adsorption of fructose was obtained on Zr-BEA (-163.3 kJ/mol), followed by Hf-BEA (-153.9 kJ/mol), Sn-BEA (-131.8 kJ/mol) and Ti-BEA (-122.70 kJ/mol). Furthermore, on average, the strongest interaction of fructose on M-BEA are through the oxygen O2, closely followed by O1, O6, O3, O4, and O5. We note that this result differs to that reported by Yang and co-workers for the adsorption of fructose on Sn-BEA, in which fructose adsorbs more strongly through O4 when compared to the other oxygens.33

Figure 6. Adsorption energies of fructose on M-BEA through the different oxygen atoms of the molecule. The oxygen atoms are identified in Figure S1. Av stands for the average adsorption energy. Figure 3 (a) and Figure 3 (b) illustrate two plausible reaction mechanisms for the opening of the fructose ring. In the mechanism of Figure 3 (a), the intramolecular hydrogen shift occurs in one elementary step (B  C). The reaction mechanism of Figure 3 (b) is analogous to the reaction mechanisms that have been suggested for glucose, shown in Figure 2, in which the intramolecular hydrogen shift occurs through the zeolite and involves two elementary steps (B’  I1  C’). The reactions energies for the fructose ring opening by the two mechanisms proposed in Figures 3 are shown in Figures 7 (a) and 7 (b). The optimized geometries of the species involved in the



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reaction mechanisms on Hf-BEA are shown in Figure S8 and Figure S9 in the Supporting Information, and the TS optimized geometries are shown in greater detail in Figure 8.

Figure 7. Reaction energies of the opening of the fructose ring on M-BEA through (a) the one-step mechanism shown in Figure 3(a) and (b) through the two-steps mechanism shown in Figure 3(b).

Figure 8. TS configurations for the opening of fructose ring. (a) TS configuration obtained for the one step mechanism shown in Figure 3(a). (b) and (c) illustrate the configurations of TS1’ and TS2’ that were obtained for the two-steps mechanism of Figure 3(b). On the basis of Figure 3 (a), upon fructose adsorption the hydrogen atom is shifted from O2 to O5 to form the acyclic fructose. The TS configuration on Figure 8 (a) shows the synergistic interactions between the closed site of the zeolite and fructose, which are reflected in the distances dO2-M, and dH-Oz (see Table 1). These distances range between 2.23 - 2.34 Å for dO2-M 10 ACS Paragon Plus Environment

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and 2.29 - 2.31 Å for dH-Oz. The energy barriers obtained for Sn-, Hf-, Zr-, and Ti-BEA are similar, ranging from 130 to 141 kJ/mol. A more pronounce difference, however, is obtained in the apparent activation energies. Based on Figure 7 (a), the lowest apparent activation energy for the opening of fructose is on Hf-BEA, followed by Zr-BEA, Sn-BEA, and Ti-BEA. The reaction energies for the conversion of the adsorbed cyclic fructose to adsorbed acyclic fructose (B C) increase as follows: Hf-BEA (14.1 kJ/mol) < Zr-BEA (14.8 kJ/mol) < Sn-BEA (18.2 kJ/mol) < TiBEA (20.4 kJ/mol). Table 1. Optimized distances for the fructose ring opening TS on M-BEA.

Sn-BEA Hf-BEA Zr-BEA Ti-BEA

dO2-H (Å) dO5-H (Å) dO2-M (Å) dH-Ozeo (Å) 1.31 1.21 2.27 2.31 1.31 1.21 2.23 2.29 1.31 1.21 2.27 2.31 1.31 1.21 2.34 2.30

In the second proposed mechanism, shown in Figure 3 (b), the hydrogen is shifted from O2 to O5 through two consecutive steps. We note that the optimized geometry of the adsorbed fructose is slightly different on the two reaction mechanisms of Figure 3 (a) and 3 (b). Although in both cases fructose is adsorbed through the oxygen O2, the optimized structure of adsorbed fructose that yields the two-steps mechanism shown in Figure 3 (b) exhibits adsorption interactions that are weaker than the ones obtained for the one-step mechanism in Figure 3 (a). For the intermediate B’ in Figure 3 (b), the adsorption energies decrease as follows: Ti-BEA (117.1 kJ/mol) < Sn-BEA (-129.6 kJ/mol) < Zr-BEA (-134.3 kJ/mol) < Hf-BEA (-156.0 kJ/mol). Upon fructose adsorption, one of the hydrogen atoms of fructose ends up coordinated to an oxygen atom of the zeolite (B’ I1). This step is endothermic, and the reaction energies range from 27.3 kJ/mol on Sn-BEA to 53.4 kJ/mol on Ti-BEA. The TS configurations involved in this step, (TS1’) are described in Figure 8 (b) and Table 2. The configurations show how the hydrogen is transferred from O2 to Oz, which is an oxygen atom in the zeolite, and simultaneously O2 interacts with the metal of the zeolite (M). These interactions are reflected in the distances dO2-H, 11 ACS Paragon Plus Environment


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dOz-H, and dO2-M shown in Table 2, which range between 1.37 - 1.39 Å, 1.10 - 1.12 Å, and 2.06 2.17 Å, respectively. The energy barriers obtained for this step (B’  I1) increase as follows: SnBEA (73.1 kJ/mol) > Zr-BEA (79.7 kJ/mol) > Hf-BEA (98.2 kJ/mol) > Ti-BEA (107.0 kJ/mol). Table 2. Optimized distances for the fructose ring opening TS on M-BEA. dO2-H (Å) dOz-H (Å) dO2-M (Å) 1.39 1.11 2.15 Sn-BEA 1.39 1.12 2.13 Hf-BEA 1.37 1.12 2.17 Zr-BEA 1.39 1.10 2.06 Ti-BEA The second elementary step on the mechanism of Figure 3 (b) (I1  C’) involves the formation of adsorbed acyclic fructose. This step consists in the shift of the hydrogen atom from the zeolite to O5, which is an oxygen atom in the adsorbed molecule. The reaction is exothermic for all the zeolites, and the reaction energies range from -28.4 kJ/mol on Ti-BEA to -10.7 kJ/mol on Sn-BEA. The TS configurations in this step (TS2’) are described in Figure 8 (c) and Table 3. The TS2’ configurations show that the hydrogen shift is mediated by the interaction of O2 with the metal of the zeolite. The reaction has a low energy barrier for all the zeolites ranging from 0.84 kJ/mol for Zr-BEA to 19.2 kJ/mol for Ti-BEA. Table 3. Optimized distances for the fructose ring opening TS on M-BEA. dO5-H (Å) dOz-H (Å) dO2-M (Å) 1.25 1.19 2.08 Sn-BEA 1.26 1.20 2.07 Hf-BEA 1.23 1.23 2.09 Zr-BEA 1.19 1.27 1.98 Ti-BEA When comparing the energies obtained in Figure 7, we note that for all the M-BEA catalysts analyzed here, the energy barriers obtained for the one-step mechanism shown in Figure 3 (a) are higher than the ones obtained for the two-steps mechanism shown in Figure 3 (b). These results suggest that the opening of the cyclic fructose ring to form the acyclic fructose is more favorable through the two-steps mechanism. The lower energy barriers obtained in the 12 ACS Paragon Plus Environment

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two steps mechanism could be explained, due to the stronger interactions of the fructose with the zeolite, which are reflected in the O2-M distances obtained in the transition states. The O2-M distances for the two steps mechanism (1.98 – 2.09 Å) are lower than the ones obtained for the one step mechanism (2.06 – 2.17 Å). Furthermore, among all the metal substituted zeolites analyzed in this work, Sn-BEA yields the lowest energy barrier for the opening of the fructose ring. After the opening of the fructose ring, the adsorbed acyclic fructose is close to its conversion to the trioses, DHA and glyceraldehyde GLA. The subsequent steps include the intramolecular hydrogen shift between the atoms O4 to C3. We analyzed a reaction mechanism in which the hydrogen atom is directly shifted from O4 to C3 on Sn-BEA. The reaction mechanism is shown in Figure 4. The optimized geometries and the reaction energies for that reaction mechanism are shown in Figure S10 in the Supporting Information and in Figure 9, respectively. The first step (C’  D’) is a rearrangement of the acyclic fructose, in which the fructose leaves its coordination through O2 with the zeolite and passes to be coordinated through O4. This rearrangement is endothermic with a reaction energy of 37.3 kJ/mol. The second step (D’  E’) is the hydrogen shift from O4 to C3, and the transition state (TS3’) is shown in Figure 10. The reaction energy of this step is energetically favorable (-8.5 kJ/mol), but the energy barrier is high (266.2 kJ/mol). The high TS energy is due to the long distance between O4 to C3, which is reflected in the distances dO4-H (1.32 Å) and dH-C3(1.48 Å) shown in Figure 10.



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Figure 9. Reaction energies for the conversion of acyclic fructose to DHA and GLA.

Figure 10. TS3’ configuration for the conversion of acyclic fructose to DHA and GLA.

3.2 Glucose Adsorption and Ring Opening The adsorption energies of glucose on M-BEA were calculated through each of the oxygen atoms of the adsorbate, which are identified in Figure S1 in the Supporting Information. The different adsorption energies for glucose on M-BEA are shown in Figure 11. The optimized adsorbed structures were obtained in monodentate configurations, which have been reported to be energetically more favorable than the bidentate modes.31 For instance, Yang and coworkers reported adsorption energies of cyclic glucose on Ti-BEA, and the lowest value reported is -123


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kJ/mol in a monodentate configuration, whereas the adsorption energy is -98 kJ/mol in a bidentante configuration.31 The distances between the adsorbed oxygen of the glucose and the metal of M-BEA vary between 2.22 to 3.37 Å, as shown on Table S2 and Figure S11 in the Supporting Information. In general, the O-M distances obtained when the molecules adsorbs through O5 are larger (2.84 - 3.37 Å) than the ones obtained through the other oxygens (2.222.57 Å). This significant difference is due to the steric hindrance caused by the hydroxyl groups when the glucose is adsorbed through the O5 oxygen. However, despite this steric hindrance, the adsorption energies through O5 are strong. This is caused by the interaction between a hydroxyl group (O6H) of the glucose with the zeolite as shown in the Figure S11 and Table S2 in the Supporting Information. This interaction is reflected in the optimized distances dO6H-OzM, which range between 2.12 - 2.50 Å.

Figure 11. Adsorption energies of glucose on M-BEA through the different oxygen atoms of the molecule. The oxygen atoms are identified in Figure S1. Av stands for the average adsorption energy. On the basis of Figure 11, the adsorption energies of glucose on M-BEA is strongly affected by the nature of the metal as well as by the glucose oxygen coordinated to the metal. In average, the strongest adsorption of glucose is on Hf-BEA (-98.7 kJ/mol), followed by Zr-BEA (-



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93.5 kJ/mol), Sn-BEA (-86.7 kJ/mol), and Ti-BEA (-71.2 kJ/mol). Yang and co-workers reported the same trend for the adsorption of glucose in BEA zeolites substituted with Zr, Sn, and Ti.31 The strongest interactions of glucose on Hf-, Zr-, and Sn-BEA are through the oxygen O1 of glucose, which are below -120 kJ/mol. For Ti-BEA, the strongest interactions were obtained through O5 closely followed by O1. However, in average, the adsorption of fructose is stronger than the adsorption of glucose on M-BEA. Previous DFT studies have reported that the isomerization of glucose to fructose on Sn-BEA starts when glucose is adsorbed on O1 or O5.11,12 The low binding energies of glucose through O3 is mainly because of the weak interaction of the other glucose atoms with zeolite pore. The minimum energy path and the reaction mechanism for the glucose ring opening are shown in Figure 12 and Figure 5, respectively. Our calculations suggest that the glucose ring can be opened in one elementary step, in which the hydrogen atom is shifted from O1 to O5 as illustrated in the TS configuration in Figure 13. The confinement effect of the closed sites of M-BEA allows the simultaneous interaction between several atoms of the glucose molecule and the zeolite, which are manifested in the distances of the TS shown in Table 4. As the intramolecular hydrogen shift occurs between O1 and O5, O1 interacts simultaneously with the metal atom (M), while the hydrogen atom interacts with one of the oxygen atoms coordinated to the metal (Oz). These interactions are reflected in the distances dO1-M which range between 2.20 - 2.27 Å and the distances dH-Oz which range between 2.43 - 2.55 Å. This TS configuration can be obtained on the closed sites of M-BEA and is not possible to obtain with open sites due to the steric hindrance caused by the hydrolyzed groups (M-OH, Si-OH) in the open site. The energy barriers increase as follows: Sn-BEA (96.9 kJ/mol) < Hf-BEA (104.9 kJ/mol) < Zr-BEA (107.1 kJ/mol) < Ti-BEA (111.9 kJ/mol). Thus, Sn-BEA has the lowest energy barrier for the opening of the glucose ring, and it is well known that Sn-BEA exhibits high catalytic activity for the isomerization of glucose to fructose.4,8


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Frequency calculations for the stable states of adsorbed fructose and glucose on the zeolites were calculated in order to estimate the energy barriers with the zero point energy (ZPE) correction for the one-step mechanism. The results are shown in Table S3 and Table S4 in the Supporting Information. The tendencies obtained with the ZPE corrections are the same. For the fructose ring opening, the energy barriers increase as follows Hf-BEA > Zr-BEA > Sn-BEA > Ti-BEA. For the glucose ring opening, the energy barriers increase as follows: Sn-BEA > Hf-BEA > Zr-BEA > Ti-BEA.

Figure 12. Reaction energies of the opening of the glucose ring on M-BEA.

Figure 13. TS configuration for the opening of the cyclic glucose



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Table 4. Optimized distances for the glucose ring opening TS on M-BEA. dO1-H (Å) dO5-H (Å) dO1-M (Å) dH-Oz (Å) Sn-BEA 1.34 1.19 2.22 2.45 Hf-BEA 1.33 1.19 2.24 2.43 Zr-BEA 1.34 1.19 2.27 2.46 Ti-BEA 1.37 1.17 2.2 2.55 The TS energy obtained here for Sn-BEA is lower than the TS energies reported in previous works, in which the opening of the glucose ring was analyzed through two consecutive steps, as shown in Figure 2. TS energies reported in the literature for the opening of the glucose ring on Sn-BEA are 194 kJ/mol and 129 kJ/mol for O5 and O1 oxygen atoms of the glucose, respectively.12 We note that we have to be careful when comparing our results with the reported energies of the reaction mechanisms of Figure 2 (a)10 and Figure 2 (b).11 For instance, Li and coworkers reported the reaction energies using the solvated glucose as reference, instead of the glucose in gas phase that we are reporting.10 On the other hand, Bermejo-Deval and coworkers reported the energies as enthalpies at 298K with a polarizable continuum model using the water dielectric constant and small cluster models.11 However, we can compare our energies with the ones reported by Yang and coworkers.12 They studied the reaction mechanisms illustrated in Figure 2, but with closed sites, and they used a similar methodology to ours. The transition state energy for the glucose opening on Sn-BEA that we obtained (96.9 kJ/mol) is lower than those reported by Yang and coworkers (129 and 194 kJ/mol).12 On the basis of Figure 12, the apparent activation energies of Hf-BEA, Sn-BEA, and ZrBEA are very similar. On the other hand, the apparent activation energy obtained for Ti-BEA is much higher when compared to the other zeolites. It is well known that Ti-BEA has a lower catalytic activity than Sn-BEA for the isomerization of glucose to fructose.3,32 The reaction energies for the conversion of the adsorbed cyclic glucose to adsorbed acyclic glucose increase


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as follows: Sn-BEA (28.0 kJ/mol) < Zr-BEA (30.0 kJ/mol) < Hf-BEA (30.2 kJ/mol) < Ti-BEA (36.1 kJ/mol).

4. CONCLUSIONS Our results demonstrate that the preferential site of substitution of Sn, Hf, Zr, and Ti in BEA is in T6. The adsorption of fructose and glucose on M-BEA is strongly affected by the nature of the metal and by the oxygen through which the sugar is adsorbed on the zeolite. In average, the strongest adsorption of fructose and glucose were obtained on Zr-BEA and Hf-BEA, respectively. Furthermore, the strongest interactions of glucose on M-BEA were obtained through the oxygens O1 and O5, which are the ones that trigger the isomerization reaction. We found two different mechanisms for the opening of the fructose ring. In the first mechanism, the intramolecular hydrogen shift occurs in one elementary step. In the second mechanism, the intramolecular hydrogen shift occurs through the zeolite and involves two elementary steps analogous to the reaction mechanisms that have been suggested for the opening of the glucose ring. Our results demonstrate that the two steps mechanism has lower reaction barriers than the one-step mechanism. We also found a path for the opening of glucose that involves only one elementary step with a lower energy barrier than the reaction mechanisms that have been previously proposed in the literature. Among the metal substituted zeolites analyzed in this work, our results demonstrate that the lowest TS energy for the fructose and glucose rings opening are obtained with Sn-BEA.

SUPPORTING INFORMATION DESCRIPTION Supporting information includes relative substitution energies of the metals on BEA, geometries of stable intermediates and transition states on the proposed mechanisms, and xyz coordinates for all stable and transition states.



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ACKNOWLEDGMENTS This research was supported by the National Science Foundation through grant OIA-1632824. This research used computational resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and the HighPerformance Computing Facility of the Institute for Functional Nanomaterials, which is supported by NSF through grants EPS-1002410.

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Periodic DFT Study of the Opening of Fructose and Glucose Rings and

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