Glucose Conversions Catalyzed by Zeolite Sn-BEA: Synergy among

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Glucose Conversions Catalyzed by Zeolite Sn-BEA: Synergy among Na+ Exchange, Solvent and Proximal Silanol Nest as Well as Critical Specifics for Catalytic Mechanisms Gang Yang, and Lijun Zhou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01157 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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Glucose Conversions Catalyzed by Zeolite Sn-BEA: Synergy among Na+ Exchange, Solvent and Proximal Silanol Nest as Well as Critical Specifics for Catalytic Mechanisms Gang Yanga,b,*, Lijun Zhoua a

College of Resources and Environment, Southwest University, Chongqing 400715,

China; b

Schuit Institute of Catalysis, Eindhoven University of Technology, Eindhoven,

5600MB, the Netherlands. * To whom correspondence should be addressed: Email: [email protected]; Phone: 086-023-68251504; Fax: 086-023-68250444.

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Abstract: Sn-BEA zeolite shows excellent catalytic performances for biomass transformation, and herein periodic density functional theory calculations accounting for the effect of zeolite framework are conducted to address the mechanistic aspects of glucose catalytic conversions. It is the synergistic effects of Na+ exchange, proximal silanol nest and solvent (water/methanol) that cause the epimerization path via the Bilik mechanism to occur facilely at ambient conditions, and each of them plays a definite while disparate role. Na+ exchange reverses the relative stabilities of critical intermediates for the epimerization vs. isomerization paths and drives the reaction towards the epimerization path with production of mannose. The proximal silanol nest participates in the Bilik reactions through the synchronous proton transfer to the sugar fragment, which is indispensable to promote the reaction thermodynamics and reduce the activation barriers. The activation barriers are generally lowered with increase of solvent (water/methanol) contents, and water (n = 4~6) achieves comparable catalytic results as methanol (n = 3). The difference between water and methanol solvents lies mainly in their divergent interactions with zeolite framework. Methanol rather than water constructs multiple methyl-H and lattice-O pairs and shows higher capability to retain Na+ ions, which account for its superior catalytic performances. At any solvent (water/methanol) content, the perfectly tetrahedral Sn(IV) site shows an apparently inferior catalytic effects than defect with the proximal silanol nest, owing the absence of the synchronous proton transfer. Keywords: biomass transformation; epimerization; silanol nest; Na+ exchange; solvent effects; synergetic effects; periodic density functional theory

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1. Introduction The search for alternative feedstocks to replace the exhausting fossil resources is among the global concerns and lignocellulosic biomass shows great potential in this regard.1-4 Isomerization of glucose to fructose, one of the key reactions for transformation of lignocellulosic biomass5, is efficiently catalyzed by Sn-BEA zeolite,6 while Na+ exchange drives the reaction towards the production of mannose.7,8 Although less abundant than glucose and fructose, mannose finds a wide spectrum of important applications such as antiviral drugs9, immune-modulating and anti-inflammatory agents10, UTI (urinary tract infection) prevention11 and chiral building blocks for natural products12. Therefore, the manufacture of mannose from glucose epimerization has also gained a plethora of attention. Mechanisms of glucose isomerization to fructose catalyzed by Sn-BEA zeolite have been addressed at a molecular level through the combined efforts of experiential techniques and quantum chemical calculations.5,13-17 NMR experiments with deuterium labeling at C2 position of glucose indicated the presence of substantial kinetic isotope effects (kH/kD = 1.98), and the intra-molecular hydride transfer from C2 to C1 was suggested to be the rate-decisive step13 (Scheme 1), as verified by subsequent density functional calculations that this step has the highest intrinsic activation energy.6,14-17 In addition, the isomerization reaction was demonstrated to be divided into five stages, as adsorption of glucose, opening of the pyranose ring, intra-molecular hydride transfer from C2 to C1, closure of the furanose ring and desorption of fructose.5,14-17 The silanol group adjacent to the framework Sn(IV) site significantly promotes the isomerization reaction due to the stabilization of related intermediates and transition states,14,16,17 while an adverse effect was predicted for the epimerization reaction occurring via the

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Bilik mechanism (Scheme 1); that is, the proximal silanol group raises the activation barriers of the Bilik reactions and hence was assumed to be the spectator.14 As indicated by Chethana and Mushrif18, the borate anion forms the stable complex with glucose that inhibits the isomerization reaction, and the framework Sn(IV) site corresponds to a lower activation barrier for the Bilik reactions than the framework Ti(IV) and Zr(IV) sites. It was proposed by Christianson et al.19 that the two sequential hydride transfer steps are an alternative for the Bilik mechanism (1,2-C shift), especially in absence of Na+ exchange. Isomerization and epimerization are two competitive paths for the catalytic transformation of glucose by Sn-BEA zeolite that produce fructose and mannose, respectively (Scheme 1).5-8 The effects of Na+ exchange, solvent (water/methanol) and proximal silanol nest are closely intertangled with each other and remain challenging to differentiate, which are, however, critical for the understanding and improvement of catalytic systems. To this end, periodic density functional theory (p-DFT) calculations accounting the effect of zeolite framework were conducted. The respective contributions of Na+ exchange, solvent and proximal silanol nest have been addressed, and we realized that each of them plays a definite while different role during the Bilik reactions, and their synergistic efforts drive the Bilik reactions to proceed facilely at ambient conditions. In addition, a number of critical specifics for the catalytic conversions have been offered; e.g., it was clearly shown that methanol and water differ mainly in their retaining capabilities of Na+ ions: At each content, methanol exhibits the superior inhibiting effects for Na+ leaching, which deciphers the catalytic difference for the two solvents. The perfectly tetrahedral Sn(IV) site was also considered as the active site, and the calculated results evidenced the indispensable role of the proximal silanol nest

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during the catalytic conversions.

2. Computational Section 2.1. Models. The atomic coordinates of zeolite BEA were taken from the International Zeolite Association (IZA) website20, and based on the previous studies21-23, the Sn(IV) ion was substituted at one of framework T2 sites, see Figure 1A. Sn2-BEA zeolite is characteristic of the tetragonal unit cell with its lattice parameters being relaxed at a = b = 12.657 Å and c = 26.396 Å. In line with literature reports14-19, defect with the proximal silanol nest (SnOH, see Figure 1B) was constructed by removal of a framework Si atom and then the dangling Si-O- groups were saturated with H atoms. The perfectly tetrahedral Sn site (SnP, see Figure 1B) was also considered as the active site, and the calculated results help to understand the role of the proximal silanol nest during the catalytic reactions. As indicated in Scheme 1, 3 and 4 represent the starting intermediates respectively for the aldose-ketose isomerization (3→[TS1H]→4H)5,14-19 and Bilik (4→[TS1]→5)14,19 steps. Structures corresponding to defect with the proximal silanol nest (SnOH) and perfectly tetrahedral Sn site (SnP) were suffixed with the superscripts “OH” and “P”; e.g., TS1OH and TS1P stand for the transition states of Bilik reactions over SnOH and SnP, respectively. In agreement with the experimental procedures7,8,24, sodium was added to the catalytic systems in the salt (e.g., CH3ONa) or basic (e.g., NaOH) forms. Solvent (water/methanol) molecules construct direct bonds with Na+ ions and as many H-bonds as possible with zeolites (including the proximal silanol nest), and when the first shell of Na+ ions has been saturated, solvent molecules were subsequently added in vicinity of the silanol nest that may greatly stabilize the intermediates and transition states.

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2.2. Methods. Periodic density functional theory (p-DFT) calculations, as implemented in the Vienna Ab Initio Simulation Package (VASP) were conducted using the Perdew, Burke and Ernzrhof (PBE) functional25,26 was used to handle exchange-correlation interactions. All elements were described by the standard projector augmented-wave (PAW) potentials except Sn that employs the Sn_d potential to deal with its semi-core d-electrons15,21,27. The non-covalent interactions were accounted for by means of the damped C6 term28 (referred to as PBE+D2, which has been testified to produce accurate adsorption and reaction energies29-31). The default energy cutoff was set to 400.0 eV, which, as discussed in Section 3.3, achieves the fairly consistent structural and energetic results with the energy cutoff of 500.0 eV. The Brillouin zone sampling was restricted to Γ-point. Structural optimizations were converged when the forces on all atoms fall below 0.05 eV Å-1. Transition states were determined by means of the nudged elastic band method (NEB) with improved tangent estimates32,33, following by verifications that the only imaginary frequency coincides with the motion along the corresponding reaction coordinates. All atoms of zeolite framework were fully relaxed during structural optimizations and transition state searching.

3. Results and Discussion 3.1. Role of Na+ Exchange. The local structure of defect with the proximal silanol nest (SnOH) is shown in Figures 1B and S1. The silanol group of SiF1OF1H′ can get involved in the catalytic reactions as reported before14,16-19 and is stabilized by H-bonds with the other two silanol groups that are also produced from the removal of a framework Si atom. 3OH (Figure 2) is the starting structure for the aldose-ketose isomerization reaction5,14-19 and can be conformationally converted to 4OH (Figure 2), the starting

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structure for the Bilik reaction (1,2-C shift) as suggested by Caratzoulas et al.14,19. Figures 2 and S2 indicate that in both 3OH and 4OH, the sugar fragments construct a number of H-bonds with framework-O atoms. In absence of Na+ exchange, the relative energy of 4OH vs. 3OH (∆E) is calculated to be 73.0 kJ/mol, and 4OH has an obviously lower stability than 3OH, probably due to the destruction of C1=O1 conjugation. In consequence, the Bilik reactions are not likely to occur in absence of Na+ exchange. Figures 2 and S2 also depict the 3OH and 4OH structures in presence of Na+ exchange. Water and methanol are used as solvents8,24, and the corresponding structures in these two solvents are found to resemble each other. Whether in water or in methanol, one hydroxyl-H atom of the sugar fragment transfers spontaneously to the O atom bonded to the Na+ ion and produces a water or methanol molecule. Owing to Na+ exchange, the C1=O1 conjugation is restored in 4OH; in addition, the ∠O1NaO3 strain in 4OH is less than the ∠O2NaO3 strain in 3OH, as inferred from the optimized angles (80.48o vs. 70.88o for water and 78.91o vs. 70.26o for methanol). As a result, Na+ exchange reverses the relative stabilities of 4OH vs. 3OH (∆E), which are calculated to be -79.5 and -71.4 kJ/mol for water and methanol, respectively. In consequence, Na+ exchange causes 4OH rather than 3OH to be significantly more favorable and further drives the reaction towards the epimerization path (via the Bilik mechanism) with production of mannose. It is consistent with the experimental observations that Na+ exchange enhances the product selectivity of mannose vs. fructose, even added halfway.8,24 Over the perfectly tetrahedral Sn(IV) site (SnP), the relative stabilities (∆E) of these two structures (4P and 3P) are also calculated and amount to 115.9, -59.2 and -43.4 kJ/mol respectively in absence of Na+ exchange, with Na+ exchange in water and with

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Na+ exchange in methanol, see Figure 2. The trend remains consistent with that of defect containing the proximal silanol nest (SnOH), whereas differences can still be detected: SnP rather than SnOH always corresponds to the more positive ∆E values implying that the Bilik reactions over SnP are more difficult to take place in all conditions (in absence of Na+ exchange, with Na+ exchange in water and with Na+ exchange in methanol). 3.2. Differences for two Solvents. Glucose and solvent molecules are competitive when binding to the framework Sn(IV) site of Sn-BEA zeolite, and owing to the higher binding affinity, water rather than methanol exhibits a larger inhibiting effect to the catalytic reactions.34,35 Nonetheless, the competitive binding of glucose and solvent molecules seems at a loss when interpreting the selectivity of fructose vs. mannose in different solvents, because the timely supplement of Na+ ions in water solvent can significantly enhance the selectivity and yield of mannose.8 It is assumed that the interactions with zeolite framework result in the divergent catalytic activities for these two solvents8,24, while direct evidence in this regard is lacking. Herein, with use of explicit solvent models (Figures 3 and 4), the interaction configurations and interaction energies of solvent molecules with zeolite framework are presented and the differences between two solvents are thus rationalized. With respect to the 4OH structures, the interaction energies of solvent molecules (Sol) with the rest fragments (Rest) and with zeolite framework (Zeo) are respectively expressed as, ∆ERest = E(4

OH

) – E(Sol) – E(Rest)

∆EZeo = E(Zeo-Sol) – E(Sol) – E(Zeo)

(1) (2)

where Zeo-Sol stands for the complex consisting of zeolite framework (Zeo) and solvent molecules (Sol). Note that all species employed to compute the interaction

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energies remain at their respective geometries in the 4OH structures. For both solvents (methanol and water), the interaction energies with the rest fragments of the 4OH structures (∆ERest) increase in a direct proportion with the number of solvent molecules (n), see Table 1. At a specific content, methanol rather than water exhibits an improved stabilization effect to the 4OH structures, mainly due to the formation of multiple methyl-H and lattice-O pairs that, for n = 3 (Figure 4), are counted to be 5 (19) below the threshold of 3.0 (3.5) Å. The interaction energies of zeolite framework with water/methanol (∆EZeo) are calculated to be -10.8/-17.3, -22.9/59.5 and -36.6/-75.3 kJ/mol for n = 1, 2 and 3, respectively (Table 1). It indicates that the ∆EZeo values of both solvents are enhanced with the larger contents, while the ascending rate of methanol is more pronounced and hence its interactions with zeolite framework are preferred at all contents, as also reflected from the calculated ∆EZeo/∆ERest ratios (Ω, see Table 1).

Tables S1 and S2 show that the presence of zeolite framework affects significantly the charge distributions of both water and methanol molecules. The non-covalent interactions between solvent molecules and zeolite framework in the 4OH structures (ENC) have been estimated using the following expression,27,36,37 ENC = ∆EZeo – ∆eZeo

(3)

where ∆eZeo is calculated similarly as ∆EZeo (Eq. 2) but in absence of the damped C6 term that accounts for non-covalent interactions28. The ENC values for water/methanol are calculated to be -10.3/-18.0, -19.4/-36.7 and -35.6/-67.7 kJ/mol for n = 1, 2 and 3, respectively. At any solvent content, the noncovalent interactions of zeolite framework with methanol are larger than those with water; In addition, with increase of solvent contents, the ENC differences between

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methanol and water are considerably magnified, which corroborate the critical role of methyl-H and lattice-O pairs in methanol played during the stabilization of the 4OH structures. Accordingly, methanol molecules within Sn-BEA zeolite are less mobile than water molecules and exhibit a more profound inhibiting effect for Na+ leaching, which accords with the superior catalytic performances observed experimentally.8,24 3.3. The Bilik Reactions with Na+ Exchange. Structures of the Bilik reactions over defect with the proximal silanol nest (SnOH) are shown in Figures 3, 4, S2 and S3. In addition to solvent molecules, the Na+ ions in all structures (4OH, 5OH and TS1OH) form direct bonds with the O1 and O3 atoms of the sugar fragments, and as in the condition with no Na+ exchange, a number of H-bonds have been detected between the sugar fragments and framework-O atoms. During the catalytic processes (4OH→[TS1OH]→ 5OH), the C2-C3 bonds are gradually elongated until ruptured whereas the interactions between C1 and C3 atoms are gradually reinforced until with construction of C1-C3 direct bonds (Tables 2 and 3); e.g., at n = 3, the C1-C3 and C2-C3 distances in water/methanol are optimized at 2.480/2.505 and 1.554/1.546 Å in 4OH, 2.152/1.707 and 2.208/2.124 Å in TS1OH and 1.565/1.563 and 2.469/2.371 Å in 5OH, respectively. Meanwhile, the silanol nest adjacent to the Sn(IV) site is closely involved in the Bilik reactions by transferring one proton (H′) to the O1 atom of the sugar fragment forming the hydroxymethyl group (-CH2OH′), which is similar to the participatory effects of the proximal silanol group or adsorbed water molecules observed during the isomerization reactions.14,16,17 As indicated in Figure 5, the activation barriers of the Bilik reactions (∆Ea) generally decline with increase of solvent contents and at n = 3 amount to 100.5 and 70.7 kJ/mol for water and methanol, respectively. When employing the energy cutoff of 500.0 eV, the activation barrier (∆Ea) for three water molecules changes to

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100.8 kJ/mol and causes a difference of merely 0.3 kJ/mol from the value obtained using the default energy cutoff (400.0 eV). It can also be found from Table 2 that the two energy cutoffs correspond to the rather consistent distances and angles. Accordingly, the default energy cutoff (400.0 eV) is reliable with respect to structural optimizations and energetic calculations.15,17,21,35 It indicates that methanol has a more pronouncedly promoting effect on the Bilik reactions. However, according to the results of 9-T cluster models14, the proximal silanol nest is not beneficial to reduce the activation barriers of the Bilik reactions, implying that the reductionist of models may be not sufficient to understand the catalytic activities of zeolites.38 Structures and potential energy diagrams over the perfectly tetrahedral Sn(IV) site (SnP) are given in Figures 6 and 7. Along the Bilik reactions, the C2-C3 distances are gradually enlarged whereas the C1-C3 distances are gradually shortened (Tables S3 and S4), which are similar to the condition of SnOH. The major difference lies in that over SnP, no proton (H′) transfers to the O1 atom of the sugar fragment. At a specific solvent content, the activation barriers (∆Ea) of SnP are always higher than those of SnOH, and at n = 3 amount to 120.3 and 128.9 kJ/mol for water and methanol, respectively. Accordingly, the Bilik reactions are significantly promoted by defect with the proximal silanol nest (SnOH), probably due to the more unfavorable reaction thermodynamics over SnP than over SnOH (Figures 5 and 7) as corroborated subsequently; e.g., at n = 3, the reaction energies (∆Er) for water/methanol are 61.4/49.5 kJ/mol over SnP while 18.0/-9.2 kJ/mol over SnOH. 3.4. Increase of Solvent Contents. Mechanisms for the Bilik reactions in presence of Na+ exchange and solvent molecules (n ≤ 3) have been discussed so far. Over SnOH, one proton is transferred synchronously from the silanol nest to the O1 atom of the sugar

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fragment, while no such proton transfer occurs over SnP. In fact, the similar mechanism as in the condition of SnP is also tenable over SnOH. For water (n = 3), structures of the Bilik reactions over SnOH corresponding to no synchronous proton transfer (4OH#→[TS1OH#]→5OH#) are given in Figure 8, and the activation barrier (∆Ea) is calculated to be 130.3 kJ/mol (Figure 9). The substantially higher reaction energy (∆Er = 72.6 kJ/mol) accounts for the much higher ∆Ea value than that with the synchronous proton transfer (∆Ea = 100.5 kJ/mol, see Figure 5), in line with the results of SnP where the unfavourable reaction thermodynamics results in the very high activation barriers. In consequence, defect with the proximal silanol nest allows the synchronous proton transfer to the sugar fragments, which facilitates the thermodynamics of the Bilik reactions greatly and further lowers the activation barriers substantially. More solvent molecules are then added to inspect whether the activation barriers continue to reduce and to determine the optimal solvent contents for the Bilik processes. Structures of the Bilik reactions in presence of four solvent molecules are shown in Figure 10, with the characteristic distances being given in Tables 2 and 3. It has been testified that the fourth solvent (water/methanol) molecule will not remain in the first hydration shell of Na+ ions as the others previously added. The fourth water constructs H-bonds with the previously added water molecules and silanol groups (Figures 10 and S4), which may facilitate the synchronous proton transfer from the silanol nest to the sugar fragment. It shows that when the number of water molecules ascends up to 4, the reaction mechanism corresponding to no synchronous proton transfer (4OH#→[TS1OH#] →5

OH#

) is no longer existent, and only the preferred reaction mechanism with the

synchronous proton transfer (4OH→[TS1OH]→5OH) survives, which is beneficial to the manufacturing of mannose. The fourth water causes a considerable reduction of reaction

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energy (∆Er = -35.3 kJ/mol), due to the profound stabilization effects of water molecules to the deprotonated silanol nest in 5OH; Meanwhile, the activation barrier (∆Ea) descends substantially to 75.4 kJ/mol, see Figure 9. Different from the fourth water forming Hbonds with the previously added water molecules, the fourth methanol constructs only one H-bond with the silanol groups and disturbs the H-bonds among the silanol groups (Figures 10 and S4), which produces the adverse stabilization effects for transition states. The activation barrier of four methanol molecules (n = 4) is calculated to be 78.9 kJ/mol and increases somewhat as compared to the value of n = 3. Structures of the Bilik reactions in presence of six water molecules (n = 6) resemble the corresponding ones of n = 4, see Figure S5 and Table 2. At n = 6, the activation barrier (∆Ea) and reaction energy (∆Er) respectively amount to 71.4 and -41.1 kJ/mol, which are very close to those of n = 4. In consequence, at sufficient contents (n = 4~6), water achieves the comparable catalytic results as methanol (n = 3) and the major difference between two solvents should be attributed to their divergent interactions with zeolite framework as discussed earlier. It is the synergistic effects of Na+ exchange, proximal silanol nest and solvents that cause the Bilik reactions to occur facilely at ambient conditions, and each of them plays a definite while disparate role: 1) Na+ exchange stabilizes the catalytic intermediates for the epimerization path (4OH) that drives the reaction towards the production of mannose via the Bilik mechanism; 2) Proximal silanol nest promotes the reaction thermodynamics of the Bilik reactions significantly due to the synchronous proton transfer from the proximal silanol nest to the sugar fragment that further lowers the activation barriers pronouncedly; 3) Solvents cause a further reduction of activation barriers. With the sufficient solvent (water/methanol) contents, the activation barriers are moderate so that the Bilik

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reactions occur facilely at ambient conditions.

4. Conclusions The catalytic conversions of glucose by Sn-BEA zeolite recently have triggered wide interest, and p-DFT calculations that account for the effect of zeolite framework have been conducted to address the mechanistic aspects of the Bilik reactions (1,2-C shift). It is the synergistic effects of Na+ exchange, proximal silanol nest and solvents that cause the Bilik reactions to occur facilely at ambient conditions, and each of them plays a definite while different role. Relative stabilities of the critical intermediates for the epimerization vs. isomerization paths (4OH vs. 3OH) are reversed due to Na+ exchange, regardless of the choice for solvents (water/methanol). Na+ exchange in water/methanol solvent stabilizes the 4OH structures pronouncedly and drives the reaction towards the epimerization path with production of mannose, consistent with the experimental observations. The trends of relative stabilities remain consistent over SnP, whereas the epimerization paths are always less preferred as compared to SnOH. Using explicit solvent models, the interaction configurations and interaction energies of solvent molecules with the catalytic intermediates (4OH) have been presented. At each content, methanol rather than water shows the superior capability to retain Na+ ions due to the construction of multiple methyl-H and lattice-O pairs. Accordingly, Na+ ions in methanol are more difficult to leach than in water, which account for the superior catalytic performances. The proximal silanol nest of SnOH is closely involved in the Bilik reactions through the synchronous proton transfer to the sugar fragment forming the hydroxymethyl group,

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which greatly promotes the reaction thermodynamics and substantially reduces the activation barriers. The activation barriers generally decline with increase of solvent contents and at n = 3 amount to 100.5 and 70.7 kJ/mol for water and methanol, respectively. Accordingly, methanol exhibits a more promoting effect on the Bilik reactions. The promoting effects of the proximal silanol nest have not been detected by means of 9-T cluster models, however. Different from the condition of SnOH, no synchronous proton transfer occurs over SnP, which is responsible for the more unfavorable reaction thermodynamics and larger activation barriers. The similar mechanism over SnOH corroborates that the synchronous proton transfer is indispensable to lower the activation barriers. When water ascends up to 4, only the more thermodynamically favorable mechanism corresponding to the synchronous proton transfer survives for SnOH. At relatively high contents (n = 4~6), water achieves the comparable catalytic results as methanol (n = 3), and accordingly it is verified that the major difference between two solvents lies in their divergent interactions with zeolite framework.

Supporting Information Bader charge analyses (Tables S1 and S2), Structural parameters of intermediates and transition states for SnP (Tables S3 and S4), Local structure of SnOH (Figure S1), Enlarged structures for intermediates and transition states (Figures S2∼S4), Structures of the Bilik reactions in presence of six water molecules (Figures S5) and Atomic coordinates for all structures. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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Acknowledgments We thank the financial supports from the National Natural Science Foundation of China (21473137) and Natural Science Foundation Project of CQ CSTC, China (cstc2017jcyjAX0145). This manuscript was previously submitted to ACS Catal. in 2017, and the authors appreciate the helpful comments from two anonymous reviewers.

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References (1) Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411–2502. (2) Arvela, P. M.; Salmi, T.; Holmbom, B.; Willfför, S.; Murzin, D. Y. Synthesis of Sugars by Hydrolysis of Hemicelluloses - A Review. Chem. Rev. 2011, 111, 5638–5666. (3) Delidovich, I.; Leonhard, K.; Palkovits, R. Cellulose and Hemicellulose Valorisation: An Integrated Challenge of Catalysis and Reaction Engineering. Energy Environ. Sci. 2014, 7, 2803–2830. (4) Ennaert, T.; van Aelst, J.; Dijkmans, J.; de Clercq, R.; Schutyser, W.; Dusselier, M.; Verboekend, D.; Sels, B. F. Potential and Challenges of Zeolite Chemistry in the Catalytic Conversion of Biomass. Chem. Soc. Rev. 2016, 45, 584–611. (5) Bermejo-Deval, R.; Assary, R. S.; Nikolla, E.; Moliner, M.; Román-Leshkov, Y.; Hwang, S. J.; Palsdottir, A.; Silverman, D.; Lobo, R. F.; Curtiss, L. A.; Davis, M. E. Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites. Proc. Natl. Acd. Sci. USA 2012, 109, 9727–9732. (6) Moliner, M.; Román-Leshkov, Y.; Davis, M. E. Tin-containing Zeolites Are Highly Active Catalysts for the Isomerization of Glucose in Water. Proc. Natl. Acd. Sci. USA 2010, 107, 6164–6168. (7) Gunther W. R., Wang Y. R., Ji Y. W., Michaelis V. K., Hunt S. T., Griffin R. G., Román-Leshkov Y. Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift. Nature Commun. 2012, 3, 1109. (8) Bermejo-Deval R., Orazov M., Gounder R., Hwang S. J., Davis M. E. Active Sites in Sn-Beta for Glucose Isomerization to Fructose and Epimerization to Mannose. ACS Catal. 2014, 4, 2288–2297.

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(16) Li, Y. P.; Head-Gordon, M.; Bell, A. T. Analysis of the Reaction Mechanism and Catalytic Activity of Metal-substituted Beta Zeolite for the Isomerization of Glucose to Fructose. ACS Catal. 2014, 4, 1537–1545. (17) Li, G. N.; Pidko, E. A.; Hensen, E. J. M. Synergy between Lewis Acid Sites and Hydroxyl Groups for the Isomerization of Glucose to Fructose over Sn-containing Zeolites: A Theoretical Perspective. Catal. Sci. Technol. 2014, 4, 2241–2250. (18) Chethana, B. K.; Mushrif, S. H. Brønsted and Lewis Acid Sites of Sn-beta Zeolite, in Combination with the Borate Salt, Catalyze the Epimerization of Glucose: A Density Functional Theory Study. J. Catal. 2015, 323, 158–164. (19) Christianson, J. R.; Caratzoulas, S.; Vlachos, D. G. Computational Insight into the Effect of Sn-beta Na Exchange and Solvent on Glucose Isomerization and Epimerization. ACS Catal. 2015, 5, 5256–5263. (20) Baerlocher, C.; McCusker, L. B. Database of Zeolite Structures. http://www.izastructure.org/databases/ (Accessed on July 8th, 2015). (21) Yang, G.; Pidko, E.A.; Hensen, E.J.M. Structure, Stability, and Lewis Acidity of Mono and Double Ti, Zr, and Sn Framework Substitutions in BEA Zeolites: A Periodic Density Functional Theory Study. J. Phys. Chem. C 2013, 117, 3976–3986. (22) Shetty, S.; Kulkarni, B. S.; Kanhere, D. G.; Goursot, A.; Pal, S. A Comparative Study of Structural, Acidic and Hydrophilic Properties of Sn-BEA with Ti-BEA Using Periodic Density Functional Theory. J. Phys. Chem. B 2008, 112, 2573–2579. (23) Montejo-Valencia, B. D.; Curet-Arana, M. C. DFT Study of the Lewis Acidities and Relative Hydrothermal Stabilities of BEC and BEA Zeolites Substituted with Ti, Sn, and Ge. J. Phys. Chem. C 2015, 119, 4148–4157. (24) Bermejo-Deval, R.; Gounder, R.; Davis, M. E. Framework and Extraframework

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Tin Sites in Zeolite Beta React Glucose Differently. ACS Catal. 2012, 2, 2705–2713. (25) Kresse, G.; Furthmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (27) Yang, G.; Li, X.; Zhou, L. J. Adsorption of Fructose in Sn-BEA Zeolite from Periodic Density Functional Calculations. RSC Adv. 2016, 6, 8838–8847. (28) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787–1799. (29) Svelle, S.; Tuma, C.; Rozanska, X.; Kerber, T.; Sauer, J. Quantum Chemical Modeling of Zeolite-Catalyzed Methylation Reactions: Toward Chemical Accuracy for Barriers. J. Am. Chem. Soc. 2009, 131, 816–825. (30) Hansen, N.; Kerber, T.; Sauer, J.; Bell, A. T.; Keil, F. J. Quantum Chemical Modeling of Benzene Ethylation over H-ZSM-5 Approaching Chemical Accuracy: A Hybrid MP2:DFT Study. J. Am. Chem. Soc. 2010, 132, 11525–11538. (31) Fang, H. J.; Kamakoti, P.; Zang, J.; Cundy, S.; Paur, C.; Ravikovitch, P. I.; Sholl, D. S. Prediction of CO2 Adsorption Properties in Zeolites Using Force Fields Derived from Periodic Dispersion-corrected DFT Calculations. J. Phys. Chem. C 2012, 116, 10692–10701. (32) Ulitsky, A.; Elber, R. A New Technique to Calculate Steepest Descent Paths in Flexible Polyatomic Systems. J. Chem. Phys. 1990, 92, 1510–1511. (33) Mills, G.; Josson, H.; Schenter, G. K. Reversible Work Transition State Theory: Application to Dissociative Adsorption of Hydrogen. Surf. Sci. 1995, 324, 305–337.

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(34) Gounder, R.; Davis, M. E. Monosaccharide and Disaccharide Isomerization over Lewis Acid Sites in Hydrophobic and Hydrophilic Molecular Sieves. J. Catal. 2013, 308, 176–188. (35) van der Graaff, W. N. P.; Tempelman, C. H. L.; Li, G. N.; Mezari, B.; Kosinov, N.; Pidko, E. A.; Hensen, E. J. M. Competitive Adsorption of Substrate and Solvent in SnBeta Zeolite During Sugar Isomerization. ChemSusChem 2016, 9, 3145–3149. (36) Burns, L. A.; Vázquez-Mayagoitia, Á.; Sumpter, B. G.; Sherrill, C. D. DensityFunctional Approaches to Noncovalent Interactions: A Comparison of Dispersion Corrections (DFT-D), Exchange-Hole Dipole Moment (XDM) Theory, and Specialized Functionals. J. Chem. Phys. 2011, 134, 084107. (37) Cheng, L.; Curtiss, L. A.; Assary, R. S.; Greeley, J.; Kerber , T.; Sauer, J. Adsorption and Diffusion of Fructose in Zeolite HZSM-5: Selection of Models and Methods for Computational Studies. J. Phys. Chem. C 2011, 115, 21785–21790. (38) Pidko, E. A. Toward the Balance between the Reductionist and Systems Approaches in Computational Catalysis: Model versus Method Accuracy for the Description of Catalytic Systems. ACS Catal. 2017, 7, 4230–4234.

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Table 1. Interaction energies of solvent molecules with the rest portion (∆ERest), and zeolite framework (∆EZeo) in the 4OH structures a

Solvent

n=1

b

n=2

∆ERest

∆EZeo (Ω)

-72.9

Methanol -75.1

Water

a

c

∆ERest

∆EZeo (Ω)

-10.8 (14.8%)

-129.9

-17.3 (23.1%)

n=3 c

c

∆ERest

∆EZeo

-22.9 (17.6%)

-243.8

-36.6 (15.0%)

-159.2 -59.5 (37.4%)

-241.3

-75.3 (31.2%)

Energy units are kJ/mol;

b

n stands for the number of solvent molecules;

c

Ω stand for the ratio of stands for ∆EZeo/∆ERest.

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ACS Catalysis

Table 2. Characteristic distances (r) and angles (θ) for the Bilik reactions in water solvent, catalyzed by defect with the proximal silanol nest (SnOH) a,b r(C2-C3)

r(C1-C3)

n=1 n=2 n=3

θ(C1C3C2)

4OH

TS1OH

5OH

4OH

TS1OH

5OH

4OH

TS1OH

5OH

2.539

2.204

1.548

1.545

1.906

2.448

33.83

40.30

36.75

2.481 2.114 1.564 2.480 2.152 1.565 (2.482) (2.143) (1.566)

1.555 2.214 2.476 1.554 2.208 2.469 (1.555) (2.212) (2.468)

35.29 38.18 35.65 35.37 37.95 35.73 (35.40) (38.00) (35.81)

n=4

2.489

2.069

1.562

1.544

2.232

2.481

34.79

38.13

35.11

n=6

2.493

2.013

1.562

1.543

2.227

2.477

34.64

38.53

35.28

a

Units of distances and angles are angstrom and degree, respectively;

a

Data in parentheses are reported with the energy cutoff of 500.0 eV.

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Table 3. Characteristic distances (r) and angles (θ) for the Bilik reactions in methanol solvent, catalyzed by defect with the proximal silanol nest (SnOH) a r(C1-C3)

a

r(C2-C3)

θ(C1C3C2)

4OH

TS1OH

5OH

4OH

TS1OH

5OH

4OH

TS1OH

5OH

n=1

2.537

1.946

1.548

1.544

2.137

2.448

33.93

40.75

36.73

n=2

2.538

1.643

1.550

1.541

2.248

2.512

33.81

40.30

34.30

n=3

2.505

1.707

1.563

1.546

2.124

2.371

34.33

42.48

38.26

n=4

2.488

1.645

1.563

1.550

2.162

2.369

34.93

42.16

38.28

Units of distances and angles are angstrom and degree, respectively.

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ACS Catalysis

Scheme 1. Isomerization (H-shift from C2 to C1) and epimerization (1,2-C shift, also referred to as Bilik reaction) paths of glucose catalyzed by Sn-BEA zeolite (R = H for water solvent and R = OCH3 for methanol solvent; X = H for defect and X = Si for perfectly tetrahedral Sn(IV) site).

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Figure 1. A) Periodic model of Sn-BEA zeolite labeling the framework Sn2 site and B) local structures of perfectly tetrahedral Sn site (SnP) and defect with the proximal silanol nest (SnOH).

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ACS Catalysis

Figure 2. Structures of 3OH and 4OH and relative energies (∆E) of 4OH vs. 3OH: A) No Na+ exchange; B) Na+ exchange in water solvent; C) Na+ exchange in methanol solvent. Relative energies of perfectly tetrahedral Sn site (i.e., 4P vs. 3P) are given in parentheses.

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Figure 3. Optimized intermediate structures and transition states for the Bilik reaction catalyzed by defective Sn-BEA zeolite with the proximal silanol nest (SnOH), in presence of Na+ exchange and explicit water molecules (n = 1, 2, 3 as indicated in the legends).

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ACS Catalysis

Figure 4. Optimized intermediate structures and transition states for the Bilik reaction catalyzed by defective Sn-BEA zeolite with the proximal silanol nest (SnOH), in presence of Na+ exchange and explicit methanol molecules (n = 1, 2, 3 as indicated in the legends).

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ACS Catalysis

140 120

Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

140

water

120

100

100

80

80

60

60

40

40

20 0

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n=1 n=2 n=3

methanol

20 0

n=1 n=2 n=3

Figure 5. The Bilik reaction energies of defective Sn-BEA zeolite with the proximal silanol nest (SnOH), in presence of Na+ exchange and explicit solvent molecules (Left panel: water; Right panel: methanol). The first solvent molecule is formed as a result of the spontaneous deprival of one hydroxyl-H atom from sugar fragments.

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ACS Catalysis

Figure 6. Optimized intermediate structures and transition states for the Bilik reaction catalyzed by the perfectly tetrahedral Sn site in Sn-BEA zeolite (SnP), in presence of Na+ exchange and explicit solvent molecules (n = 3).

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ACS Catalysis

140

140

water

120

120

100

100

80

80

60

60

Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 20 0

n=1 n=2 n=3

Page 32 of 36

methanol

40 20 0

n=1 n=2 n=3

Figure 7. The Bilik reaction energies of the perfectly tetrahedral Sn site in Sn-BEA zeolite (SnP), in presence of Na+ exchange and explicit solvent molecules (Left panel: water; Right panel: methanol). The first solvent molecule is formed as a result of the spontaneous deprival of one hydroxyl-H atom from sugar fragments.

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ACS Catalysis

Figure 8. Optimized intermediate structures and transition states for the Bilik reaction catalyzed by defective Sn-BEA zeolite with the adjacent silanol nest (SnOH), in presence of Na+ exchange and explicit water molecules (n = 3). The superscript “#” added in the nomination of structures indicates no synchronous proton transfer from the proximal silanol nest to the sugar fragments.

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ACS Catalysis

130 110

Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

90 70 50 30 10 -10 -30 -50

n=3 (SnOH) n=4 (SnOH) n=6 (SnOH) n=3 (SnOH#)

Figure 9. The Bilik reaction energies of defective Sn-BEA zeolite with the proximal silanol nest (SnOH), in presence of Na+ exchange and explicit water molecules. The superscript “#” added in SnOH# indicates no synchronous proton transfer from the silanol nest to the sugar fragments.)

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ACS Catalysis

Figure 10. Optimized intermediate structures and transition states for the Bilik reaction catalyzed by defective Sn-BEA zeolite with the proximal silanol nest (SnOH), in presence of Na+ exchange and explicit solvent molecules (n = 4). OW4 and OM4 stand for the O atoms from the fourth water and methanol molecules, respectively.

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TOC Graphic

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Glucose Conversions Catalyzed by Zeolite Sn-BEA: Synergy among

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