A Periodic DFT Study of Glucose to Fructose Isomerization on

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A periodic DFT study of glucose to fructose isomerization on tungstite (WO3·H2O): influence of group IVVI dopants and cooperativity with hydroxyl groups Guanna Li, Evgeny A Pidko, and Emiel J. M. Hensen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00869 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 28, 2016

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A periodic DFT study of glucose to fructose isomerization on tungstite (WO3·H2O): influence of group IV-VI dopants and cooperativity with hydroxyl groups Guanna Li,† Evgeny A. Pidko,†,‡* and Emiel J.M. Hensen†,* †

Inorganic Materials Chemistry group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. ‡ Institute for Complex Molecular Systems, Eindhoven University of Technology, PO Box 513, Eindhoven 5600 MB, The Netherlands. ABSTRACT: Periodic density functional theory (DFT) calculations were carried out to investigate the mechanism of glucose to fructose isomerization over tungstite (WO3·H2O). The isomerization reaction is catalyzed by under-coordinated W6+ sites. The reaction mechanism proceeds through an H-shift from C2 to C1 and involves a cooperative action of Lewis acidic tungsten sites with neighboring proton donors, which form a hydrogen-bonding surface network. Dopants of group IV-VI transition metals stabilize the pre-activated complex, which is the deprotonated open form of glucose adsorbed to the surface. In particular, calculation reveal that doping the tungstite structure with Nb5+ and Ti4+ ions is effective in lowering the overall barrier for glucose isomerization. KEYWORDS: biomass conversion; lignocellulose; aldose-ketose isomerization; tungstite; hydrogen bonding; surface doping; heterogeneous catalysis; cooperative catalysis 1. Introduction The efficient conversion of glucose to platform chemicals such as 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) is crucial to establish new green chemistry protocols for the valorization of cellulosic biomass (Scheme 1a).1-4 These biomass-derived platform chemicals open routes to a wide range of useful products ranging from transportation fuels to chemical intermediates and monomers, often compatible with the existing chemical industry infrastructure.5-7 However, the direct transformation of glucose - the dominant monosaccharide in cellulosic biomass - to these target platform chemicals suffers in general from low selectivity. In particular, Brønsted acid catalysis alone does not provide favorable and efficient paths from glucose to HMF.8 The successful reaction schemes for the conversion of glucose to HMF involve the intermediate isomerization step to its more reactive isomer fructose (Scheme 1a).9-12 Accordingly, the promise of bifunctional catalysts combining sites for this aldose-ketose isomerization step and subsequent dehydration steps has been recognized. In most cases, combinations of Lewis acid catalysts (such as Sn-Beta zeolite) to promote glucose isomerization and Brønsted acid catalysts (such as mineral acids) for the dehydration of fructose to HMF have been explored.13-16 The general mechanism of Lewis acid-catalyzed glucose to fructose isomerization is illustrated in Scheme 1b. It is comprised of three main reactions starting with the opening of the glucopyranose ring followed by an intramolecular C2→C1 H-shift reaction and fructofuranose ring closure. The H-shift is usually considered the most difficult step in this mechanism. The preferred heterogeneous catalyst for this reaction is SnBeta zeolite.17,18 Detailed mechanistic studies have shown that this H-shift step is promoted by a synergistic action of the Lewis acid lattice Sn sites and a neighboring proton donor,

which may be an internal silanol defect or a water molecule adsorbed to the Sn site (Scheme 1c).19-23 Despite many efforts have been put into optimization of this catalyst system toward its industrial implementation, only moderate improvements with regards its overall performance and stability could be achieved so far.24-29 Experimental and theoretical research suggested that although microporous Sncontaining zeolites (such as Sn/MFI, Sn/MOR, and Sn/MWW) could be active, their applicability as catalysts for sugar isomerization suffers from the same bottleneck of diffusion limitation.22,30-32 Therefore, Lewis acidic catalysts with open architectures and tunable properties can offer substantial advantages over the microporous zeolite catalysts in particular regarding to the improved transport of substrates and products from and to the active sites. An interesting class of potential isomerization catalysts are early transition metal oxides. Such materials offer more accessible surface sites and greater tunability of their acid-base properties compared to zeolites. TiO2, Nb2O5·nH2O, Ta2O5·nH2O, and mixed metal oxides of TiO2-ZrO2, WO3-ZrO2, ZrO2-AlO3 composition as well as their phosphates and sulphates have been applied as catalysts for sugar conversion reactions.5,33-39 These catalysts contain Lewis acid and basic sites on the surfaces, which have been linked to their isomerization activity. Recently, we reported on the promise of tungstite (WO3·H2O) as a catalyst for glucose dehydration to HMF.40 We demonstrated that the introduction of lower-valent Nb ions into tungstite resulted in an increased catalytic performance. This promotion effect has been tentatively attributed to increased Lewis acidity of the Nb/WO3 surface, facilitating the glucose to fructose isomerization step.

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Scheme 1. a) Catalytic paths for the conversion of cellulosic biomass to HMF and LA platform chemicals and b) the mechanism of the key Lewis acid catalyzed step of the glucose isomerization to fructose, in which the rate-determining H-shift step can be facilitated through c) cooperative action of the Lewis acid and proton donor at the catalytic site.

In this work, we theoretically investigated in detail the mechanism of glucose isomerization to fructose over WO3·H2O catalyst by means of periodic DFT calculations. We explored the isomerization pathway and studied how the presence of water molecules near the Lewis acidic active surface sites influences the mechanism and energetics of the reaction. We also investigated the influence of the presence of guest Nb5+ and other transition metal ions across groups IVBVIB at the catalyst surface on the energetic of the ratedetermining H-shift step in an attempt to understand the promoting effect of surface doping. 2. Computational details All DFT calculations were performed with VASP version 5.3.341 using the PBE exchange-correlation functional42 based on generalized gradient approximation (GGA) and a planewave basis set with a cutoff energy of 500 eV. The electronion interactions were accounted for by the projected augmented wave (PAW) method.43 A Gaussian smearing of the population of partial occupancies with a width of 0.05 eV was used during iterative diagonalization of the Kohn-Sham Hamiltonian. The threshold for energy convergence was set to 10-5 eV. Geometry optimization was assumed to be converged when forces on each atom were less than 0.05 eV/Å. Spinpolarized calculations were performed throughout this study. The on-site Coulomb correction U for the W 5d states was used (DFT+U approach) with a value of U = 3.0 eV.44 Fig. 1a shows the crystal structure of bulk WO3·H2O. The structure is built up from octahedrally coordinated WO3·H2O units. Each tungsten atom is surrounded by one water molecule and five oxygen atoms are horizontally connected at four corners to form a layer. The layers are stacked through hydrogen bonding interactions. Optimized cell parameters for bulk orthorhombic WO3·H2O were a = 5.39 Å, b = 11.03 Å and c = 5.19 Å, which are in a reasonable agreement with the experimental values of a = 5.25 Å, b = 10.71 Å and c = 5.13 Å.45 Monkhorst-Pack mesh of 6x3x6 and 3x1x3 k-points were used to sample the Brillouin zone for the bulk and the surface models, respectively. The WO3·H2O (010) surface model was constructed to investigate glucose (α-D-glucopyranose in this study) adsorption and subsequent isomerization. The slab model was represented by a 2x1x2 supercell containing two

layers of WO3·H2O and a vacuum layer of 15 Å along the (010) direction (Fig. 1b). Each layer consists of eight in-plane WO2 units as well as four top and four bottom out-of-plane oxygen atoms and H2O molecules. Only the surface layer of WO3·H2O and the adsorbates were fully relaxed, while the other atoms and lattice parameters were fixed during the calculations.

Figure 1. a) Crystal structure of layered WO3·H2O; b) Slab model of WO3·H2O (010) surface.

The minimum-energy reaction pathways and their corresponding transition states were identified by using the nudged-elastic band method (NEB) with improved tangent estimate.46,47 The maximum energy geometry along the reaction path obtained with the NEB method was further optimized using a quasi-Newton algorithm. In this step only the adsorbates and the active center of metal site were relaxed. Frequency analysis of the stationary points was performed by means of the finite difference method as implemented in VASP. Small displacements (0.02 Å) were used to estimate the numerical Hessian matrix. The transition state was confirmed by the presence of a single imaginary frequency corresponding to the reaction coordinate. To account for the van der Waals (vdW) interactions between reactive surface and carbohydrates, all DFT computed energies were corrected for dispersion interactions by carrying out single point energy calculations with the DFT-D3(BJ) method on the DFToptimized structures.48 3. Results and Discussion 3.1 Glucose isomerization over (010) surface of WO3·H2O. The (010) surface of WO3·H2O was considered as


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the active facet for glucose isomerization. Two distinct types of Lewis acid centers can potentially be distinguished on the tungstate surface, namely, the W ions of the terminal W=O and W-H2O surface moieties. However, only the latter is accessible by the sugar substrate via the ligand exchange mechanism and can therefore be considered as the catalytic site for the isomerization reaction. The terminal W=O sites on the surface are stable in the presence of water. The dissociation of surface-coordinated water molecule (W-H2OW=O) to form W-OH-W-OH hydroxyl groups destabilizes the system by 103 kJ/mol. This is consistent with the results of previous experimental and theoretical studies on surface chemistry of tungsten oxides49-55 and related chemical systems.56-58 The optimized structures of key intermediates and the corresponding energies of the glucopyranose ring-opening step are given in Fig. S1-S2 in the Supporting Information. Fig. 2 shows the acyclic intermediates and the energetics involved in the rate-determining H-shift reaction. The catalytic cycle starts with the exothermic (∆E = -116 kJ/mol) replacement of one of the surface-bound water molecules by cyclic glucose (Glu) substrate. The coordination of the O1H hydroxyl group of the sugar to the Lewis acidic W6+ site on the WO3·H2O (010) surface facilitates the ring-opening reaction, during which a proton from the terminal O1H group is transferred to the pyranose O5 site. This reaction is assisted by an adjacent surface W=O species acting as a proton mediator (Fig. S1-S2, in the supporting information). Glucopyranose ring-opening is a facile almost thermoneutral process (∆E = 1 kJ/mol) with an activation barrier of only 34 kJ/mol. Next, the coordination mode of the open form of glucose with the surface W6+ Lewis acid site changes from O1-W to O2-W coordination. Such a binding mode facilities the O2H-deprotonation by a neighboring basic surface W=O moiety resulting in a W-OH surface hydroxyl and W-bound anionic glucose intermediate (o-Glu−) (Fig. 2a). This intermediate is further stabilized by a hydrogen-bond network formed between the W-OH group and surface-bound H2O molecules. The activated O2-deprotonated o-Glu− intermediate isomerizes then to the open-chain form of fructose (o-Fru) via a C2 → C1 H-shift reaction accompanied by the protonation of the terminal aldehyde O1 site by a surface hydroxyl group (o-Glu– → [TS] → o-Fru, Fig. 2a). The barrier for this concerted reaction step is 98 kJ/mol and the reaction energy is -87 kJ/mol (Fig. 2b). The hydrogen-bond network between the surface OH and H2O species plays an important role in this process. It effectively mediates the proton transfer from the W-OH group to the aldehyde O1 site. This reaction yields the adsorbed fructose product (o-Fru) and it regenerates the initial W=O surface species. Although the cooperative action of Lewis acidic W sites and proton donors on the WO3·H2O surface allows establishing a favorable reaction pathway for the H-shift step, the endothermic nature of the initial O2Hdeprotonation (61 kJ/mol) required to pre-activate adsorbed glucose strongly increases the overall reaction barrier, which for the isomerization of o-Glu to o-Fru over WO3·H2O surface amounts to 159 kJ/mol (Fig. 2b). No change in the oxidation state of surface W sites is observed in the course of the reaction. The catalytic process is governed solely by acid-base mechanisms. 3.2 Role of surface and solvent water molecules. DFT calculations reveal a crucial role of water molecules adsorbed on the catalyst surface in facilitating proton-transfer processes

involved in sugar conversion reactions. Two different water states can be distinguished on the WO3·H2O surface, namely, the water that is part of the tungstite structure required to complete the octahedral coordination environment of W (Fig. 2a) and physisorbed water molecules, which are part of the solvent and which are forming the hydrogen-bond networks together with the proton-donating surface groups. Below we will discuss the important mechanistic contribution of these water molecules (Fig. 3). The formation of pre-activated anionic o-Glu– intermediate via O2H deprotonation by basic W=O surface sites (o-Glu → o-Glu–, Fig. 2a) is necessary for the catalytic H-shift reaction to proceed. Without proton mediators, the barrier for this step is higher than 300 kJ/mol, which is due to the need to transfer a proton over a more than 3 Å distance (r(O2H···O=W) = 3.128 Å). The presence of physisorbed water on the surface effectively decreases this distance and as the proton transfer reaction (Fig. 3) now involves the nearly barrierless rearrangement of the hydrogen-bond network. A similar strong promoting effect of solvent water molecules on sugar preactivation step will be discussed in the next section for doped tungstite catalysts.

Figure 2. a) Local geometries of optimized structures for reaction intermediates and the transition states and b) the respective reaction energy diagram for o-Glu isomerization to o-Fru over WO3·H2O (color code: W: green; orange: surface O atoms; light grey: H; dark grey: C; dark red: O atoms of the adsorbed carbohydrate). Relative energies (∆E) for elementary transformation are given in kJ/mol.


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Figure 3. a) Schematic representation of the mechanism for o-Glu isomerization to o-Fru facilitated by one solvent molecule in the vicinity of the carbohydrate O2H group and b) local geometries for the respective optimized structures and transition state involved in the waterpromoted o-Glu to o-Fru isomerization.

While physisorbed water assists the pre-activation of oGlu, structural water molecules are essential to the low-barrier concerted H-shift/O1-protonation mechanism. The preactivation step that is the O2H deprotonation yielding a surface W-OH moiety is followed by reorganization of the surface water layer involving both structural and physisorbed water molecules. Because of the periodic nature of the slab model, this hydrogen-bond network allows connecting the surface W-OH group with the sugar C1=O1 carbonyl via structural water and provides in this way a pathway for the proton transfer during the H-shift reaction (Fig. 3). The C2 → C1 H-shift step involves redistribution of the negative charge from O2 to O1 site. The simultaneous protonation of O1 via this channel allows for efficient compensation of the negative charge developing at the non-coordinated O1 atom and provides an additional driving force for the overall reaction. To additionally support this mechanistic proposal we calculated the barrier for the direct H-shift reaction over a WO3·H2O model constructed in such way that the surface hydrogen-bond network is disrupted (Fig. S3). In this hydrogen-bond free system, the activation barrier for the C2 → C1 H-shift step increased to 142 kJ/mol and the elementary step became thermodynamically unfavorable. The formation

of o-Fru− proceeds now with the reaction energy of 100 kJ/mol. These findings clearly demonstrate the need for the cooperative action of Lewis acidic surface W6+ sites and surface water molecules to establish a low-energy path for glucose to fructose isomerization over WO3·H2O. In summary, WO3·H2O can catalyze sugar isomerization because of the presence of water-tolerant Lewis acid W6+ centers, the presence of basic oxo species in the form of W=O groups and the formation of a hydrogen-bond network within the surface water layer. This mechanism occurring in metal oxide-catalyzed glucose to fructose isomerization resembles that proposed for glucose conversion catalyzed by zeolite and enzyme catalysts.20-23,59-60 A similar reaction path is established in these very different systems by involving the proton donor of “catalytic water” or defective silanol group to efficiently stabilize the redistributing negative charge in the course of the intramolecular H-shift step of glucose isomerization. 3.3 Transition metal doping. The main factors causing the relatively high overall barrier for glucose isomerization reaction on WO3·H2O (Fig. 2) are the low oxophilicity of W6+ cations and the low basicity of the terminal oxo ligands in the tungstite structure, as they determine the energy change upon


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the O2H deprotonation step which pre-activates glucose. We explored doping of the surface by early transition metals as a means to modify the acid-base properties of the surface. Such an approach has been successfully applied previously to modify tungsten oxide surfaces and achieve materials with improved properties for such applications as solar energy conversion, electrochromism, and gas sensing.61-64

undoped surface. The corresponding reaction energy diagrams for this reaction are shown in Fig. 5, while the geometries of the intermediates and transition states are given in Fig. S4. The substitution of W6+ with Cr6+ and Mo6+ does not lead to significant changes in the reaction energetics. The reaction energy for the O2H deprotonation increases by only 11 and 3 kJ/mol upon Cr6+- and Mo6+-doping, respectively. The barrier for the H-shift reaction remains unchanged for Cr6+-doped WO3·H2O, while it is lowered to 77 kJ/mol for Mo6+-doped catalyst. The overall barrier decreases upon substitution of surface W6+ sites with Cr6+ and Mo6+ by 11 and 18 kJ/mol, respectively. Doping with pentavalent transition metal ions, that is V5+, Nb , and Ta5+ does not alter the square-pyramidal environments of the surface site, in which the axial coordination site is occupied by an OH ligand instead of an oxygen. The other four equatorial sites are bridging oxygen ligands (Fig. 4). The computed energetics for the o-Glu to oFru isomerization by these M5+-doped WO3·H2O and optimized structures of intermediates and transition states are shown in Fig. 6 and Fig. S5, respectively. Doping in this case results in a strong decrease of the reaction energy for the O2H deprotonation step in the order of 25-56 kJ/mol. The intrinsic barrier for the H-shift, on the other hand, remains nearly the same as for WO3·H2O (Fig. 5). The overall barrier for the isomerization reaction decreases for these doped systems to 138, 106, and 96 kJ/mol for V5+, Nb5+, and Ta5+-doped catalysts, respectively. 5+

Figure 4. Local structures of early transition metal doped Mn+/WO3·H2O.

Figure 5. Reaction energy diagram for o-Glu isomerization to oFru over M6+-doped tungstite (M6+/WO3·H2O).

The models for doped tungstite catalysts were systematically constructed by replacing W6+ cations on the tungstite surfaces for other transition metal cations such as Ti4+, Zr4+, V5+, Nb5+, Ta5+, Cr6+, and Mo6+ (Fig. 4). Figs. 5-7 show the influence of these dopants on the computed reaction energy diagrams for glucose isomerization. Because the initial glucopyranose ring-opening step is a facile reaction, in this section we focus only on the most difficult transformation of o-Glu → o-Fru. This transformation over the transition metaldoped surfaces can be promoted by structural and physisorbed water molecules in a manner similar to that for the parent WO3·H2O systems. The respective computational results are given in the supporting information section (Figs. S7-S9). The activity trends predicted for the water-assisted paths are similar to those observed for the water-free models. Below we focus on the analysis of the influence of surface doping on the intrinsic reactivity of WO3·H2O catalysts. The isovalent substitution of the surface W6+ with Cr6+ or 6+ Mo creates 5-fold coordinated square pyramidal Lewis acid centers (Fig. 4), which will catalyze the o-Glu to o-Fru isomerization via a mechanism similar to that described for the




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Figure 8. a) DFT computed o-Glu adsorption energies (∆ ) on Mn+/WO3·H2O surfaces and the respective b) overall o-Glu isomerization barrier (∆# ). c) Deformation energy (∆ ), and d) electronic interaction energy (∆ ) components.

In contrast to the situations discussed above, doping with tetravalent Ti4+ and Zr4+ ions substantially alters the coordination environment of the surface sites. The dopant in this case forms a strongly distorted 4-fold coordinated surface Lewis acid sites (Fig. 4), which are highly reactive towards oGlu activation. The high exothermicity of the initial o-Glu deprotonation over Ti4+- and Zr4+-doped catalysts (∆E = –56 –59 kJ/mol, Fig. 7) is due to formation of a favorable square pyramidal coordination of the M4+ site upon O2H dissociation (Fig. S6). The increased stability of the anionic o-Glu– intermediate is also reflected in the increased barrier for the Hshift reaction (111 and 102 kJ/mol, for Ti4+/WO3·H2O and Zr4+/WO3·H2O, respectively, Fig. 7). Nevertheless, the Ti4+and Zr4+-doped catalysts provide the reaction paths with the lowest overall barrier for glucose to fructose isomerization among the systems considered here. 3.4 Correlation between Lewis acidity and catalytic activity. To gain a deeper insight in factors that control the reactivity of the doped tungstites, we analyzed the different contributions to interaction energy of the sugar substrate with the modified surface. We employed an energy decomposition analysis schemes, in which the o-Glu adsorption energies (∆ ) were represented as a sum of two terms, namely the electronic energy gain due to formation of the bond between the adsorbate and the Lewis acid site ( ∆ ), and the deformation energy (∆ ) that accounts for the energy losses due to structural alternations of the stable non-bonded states upon the formation of the adsorption complex. The respective contributions are then calculated according to: ∆ ∆ ∆

Where is the electronic energy of the adsorption complex, and are the electronic energies of clean surface and isolated o-Glu in the gas phase before adsorption, and are the electronic energies of the two fragments of surface and adsorbate of the adsorption complex. Fig. 8 summarizes the results of this energy decomposition analysis for glucose adsorption on the doped surfaces. The comparison of Fig. 8a and 8b evidences a correlation between the overall isomerization barrier and o-Glu adsorption strength: the stronger the adsorption interaction is - the lower the overall isomerization barrier is. In all cases, the deformation in the sugar molecule upon adsorption is relatively small compared to the deformation of the surface (Fig. 8c). The largest deformation energy is computed for the Ti4+-doped surface upon the glucose adsorption. The results in Fig. 8c suggest that doping of penta- and tetravalent ions in the tungstite surface allow for a generally larger surface deformation following sugar adsorption. These deformation energy losses can in turn be compensated by forming stronger electronic interactions between dopants and the sugar adsorbate. The results in Fig. 8d show that the electronic interaction energy between the surface and the transition metal cations becomes more negative upon the decrease of the formal oxidation valence of the dopants. This gradual enhancement of the interaction strength between the Lewis site and the sugar is


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also reflected by the gradual decrease of the r(Mn+−O2) bond length in the same order (Fig. S4-S6). The decrease of the overall isomerization barriers for the penta- and tetravalent cations is due to the stabilization of both the reaction intermediate and transition state by the dopants (Fig. 5-7 and Fig. 8b). The electronic interaction strength, which directly correlates with the Lewis acidity of the metal center, is dominating the changes in the overall reaction barrier. This can be seen from the almost perfect correlation in the trends of the respective energy parameters shown in Fig. 8b and 8c. We further employed Bader charge analysis to assess basicity of the oxygen atoms coordinated to tungsten and transition metal dopants (Table S1). The insignificant variation in the computed atomic charges points to only a minor effect of the surface doping on the basicity of these oxygen centers. The coordinative unsaturation and the flexibility of the doped surfaces determines in turn the actual possibility of the formation of the adsorption complex with an efficient sugarsurface electronic interactions. 4. Conclusion In this work we employed periodic density functional theory calculations to investigate the reaction mechanism of catalytic isomerization of glucose to fructose over tungstitebased (WO3·H2O) catalysts. The promise of such materials for selective sugar activation stems from the simultaneous availability of Lewis acid sites (W6+), Lewis basic sites (terminal W-oxo groups), and proton mediators ( “structural” and physisorbed water) on the oxide surface. Calculations indicate that the key aspect of the catalytic mechanism is the synergistic action of the Lewis acid site promoting the C2→C1 H-shift rate-determining step and the hydrogen-bond network formed on the surface, which facilitates the accompanying proton-transfer processes. Such a cooperative reaction environment formed by the different functional groups on the surface provides a highly favorable reaction path for a concerted transformation of the activated glucose substrate to fructose product. These new mechanistic insights indicate that the activity of tungstites can be increased by doping the surface with more oxophylic transition metal cations. Periodic DFT calculations on model systems, in which single surface W centers were substituted by different cations across groups IV-VI confirm this hypothesis. The introduction of more oxophilic metal dopants of lower oxidation state (e.g., Nb5+, Ta5+, Ti4+, Zr4+) allows more efficient stabilization of activated sugar intermediates on the tungstite surface and, in this way, lowering the overall barrier for the isomerization reaction. These computational findings demonstrate the promise of tuning the surface chemical composition of early transition metal oxides to obtain heterogeneous catalysts for selective sugar conversion to platform molecules.

ASSOCIATED CONTENT Supporting Information. Reaction energy diagrams and local geometries for intermediates and TS involved in the glucopyranose ring-opening reaction; comparison of H-shift activation barriers with and without hydrogen-bond network on the catalyst surface; local geometries of intermediates and TS for o-Glu to o-Fru transformations for all catalyst models; reaction energy diagrams for o-Glu to o-Fru conversion for all catalyst models assisted by a water molecule at O2 site, Atomic Bader charge analysis of surface oxygen atoms. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E.A.P.: [email protected]; E.J.M.H.: [email protected].

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the European Union FP7 NMP project NOVACAM (Novel cheap and abundant materials for catalytic biomass conversion, FP7-NMP-2013-EU-Japan604319). NWO is acknowledged for providing access to the supercomputer facilities.

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