as a Lewis Acid in the Chemistry of Titanium Zeolites: Formation

nation to the Lewis acid site. However, the activated complex should be identified with the transient Ti-hydroperoxo + H3O. +. Figure 4. (Top left pan...
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J. Phys. Chem. B 2006, 110, 21651-21661

21651

On the Role of Ti(IV) as a Lewis Acid in the Chemistry of Titanium Zeolites: Formation, Structure, Reactivity, and Aging of Ti-Peroxo Oxidizing Intermediates. A First Principles Study Eleonora Spano´ , Gloria Tabacchi, Aldo Gamba, and Ettore Fois* Dipartimento di Scienze Chimiche ed Ambientali, UniVersity of Insubria at Como, and INSTM udr Como, Via Lucini 3, I-22100 Como, Italy ReceiVed: August 24, 2006; In Final Form: August 25, 2006

The ethylene epoxidation cycle in a H2O2/H2O-loaded Ti zeolite has been simulated by a Car-Parrinello approach. Results indicate a process where the zeolitic framework is the active oxygen mediator. The dissociative chemisorption of H2O2 leads, via a transient Ti-hydroperoxo species, to H2O and a Ti-peroxo zeolite intermediate. Transfer of active oxygen to ethylene follows, giving the epoxide and recovering the catalyst. A thorough theoretical characterization indicates that the active oxidizing species is an asymmetric η2-Ti-peroxo, absorbing in the visible range. The lability of the intermediate is found related to η2 T η1 interconversions of the Ti-peroxo structure. The interconversions, triggered by water molecules, could account for the experimentally found reduced catalytic activity in aged TS-1 catalysts. The results provide a microscopic picture of the reactivity and dehydration/aging processes of the catalyst fully consistent with experiments and highlight the fundamental role of the Lewis acid character of Ti in the formation, reactivity, and degradation of the active oxidizing species.

1. Introduction The relevance of Ti(IV) Lewis acidity in the chemistry of Ti-zeolites is here assessed by a comprehensive theoretical study in the context of hydrocarbons oxidative reactions. Such a property, due to low-lying empty d states, is shown to play a crucial role in the chemical processes characterizing this class of materials. Modulation of the Ti(IV) Lewis acid character by interactions with framework ions gives these materials their peculiar ability to act as environmentally benign oxidation catalysts. Aqueous hydrogen peroxide in the presence of Ti-zeolites oxidizes hydrocarbons in mild conditions.1 In particular, Tisilicalite (TS-1)2 is the choice catalyst for low olefin epoxidation.3-5 Moreover, TS-1 modified by Pd/Pt impregnation is found to directly oxidize olefins with an O2-H2 system.6 Despite the relevance of Ti-zeolites for environmentally friendly largescale plants and the wealth of available data, the role and the behavior of Ti in the oxidative cycle are still unknown in the microscopic details. In this context, knowledge of the reaction mechanism could be of great help in the development of a greener chemistry.7 A Ti-ligated hydroperoxo intermediate has been proposed8 as the oxygen-donating species in olefin epoxidations;1,4,9,10 however, indications of its presence have been reported only for conditions where the catalyst activity is drastically reduced,11-13 thus challenging the current opinion of a Tihydroperoxo-based mechanism. On the other hand, even if Tiperoxo moieties in zeolites have been experimentally detected,14 their role as oxidizing species has been ruled out because nonzeolitic Ti-peroxo complexes were found inactive in olefin epoxidation. TS-1 is one of the most-studied materials in the past decade,15-23 and excellent discussions on its properties can be * Corresponding author. E-mail: [email protected].

found elsewhere.1,24 Moreover, the properties and the aging behavior of a labile yellow species detected upon contacting dry TS-1 with H2O2(aq), identified as a Ti-peroxo moiety, have been the subject of a number of experiments.12-14,16,23,24 Also, computational studies have been performed to investigate Tizeolite-catalyzed olefin epoxidations.25-32 Most of the calculations, performed on molecular clusters or embedded cluster models, converge to a picture where the active oxygen-donating species, either H2O2 or HOO-, is coordinated to Ti as a ligand. On the other hand, a Ti-zeolite characterized by a Ti[OO]Si bridge has been proposed as the active epoxidizing agent.33 Moreover, whether the active sites are formed at nondefective Ti(IV) centers or associated with a hydrolyzed Ti-O-Si bridge has not been clarified yet.34,35 On the whole, there are still many open questions on the structure of the catalytically active species in TS-1 and on its aging properties. The aim of this work is to provide a coherent picture of the many facets of Ti-zeolite catalysts by presenting a thorough first-principles investigation on the microscopic origin of their challenging chemistry. 2. Methods of Calculation To study the room-temperature behavior and the reactivity of Ti(IV) catalysts by taking into account the full periodicity of the zeolitic framework, the first-principles molecular dynamics scheme due to Car and Parrinello36 (CP) has been adopted. The method, which is a molecular dynamics approach, allows one to explore the phase space of a classical set of nuclei by calculating forces “on the fly” from the ground-state electronic structure and is capable of describing hybridization changes as well as the formation and breaking of chemical bonds.36,37 Due to the complexity of the studied systems, the electronic problem is examined at the density functional theory (DFT) level.37 In the present study, the electronic structure calculations were performed via a gradient-corrected approximation to

10.1021/jp065494m CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006

21652 J. Phys. Chem. B, Vol. 110, No. 43, 2006 DFT.38-43 Specifically, the exchange and correlation functionals were those of Becke42 and Perdew,43 respectively (BP). The Kohn-Sham orbitals38 were expanded in plane waves up to a cutoff of 60 Ry, unless otherwise specified. Only valence electrons were explicitly accounted for, and the ionic coreelectron interactions were calculated by using norm-conserving nonlocal pseudopotentials.44-46 Nonlocality up to l ) 2 (d nonlocality) was considered for all atomic species, but for hydrogen atoms a norm-conserving local pseudopotential was adopted. Unless otherwise specified, perfect pairing of spin was imposed on the electronic states. The equations of motion36,47 were integrated by using a time step of 0.121 fs (5 au) and by adopting an inertia parameter of 500 au for the electronic coefficients. The combination of the CP approach with statistical sampling techniques can be adopted to simulate reactive events as it allows the sampling of the potential energy surface of very complex systems in the condensed phase.37 The free-energy profile of the reaction path was obtained through a series of molecular dynamics simulations using the CP approach combined with the blue moon ensemble48,49 sampling (CP-BM).50 The CPBM approach, used for the study of reactive processes in a wide range of systems (e.g., see refs 37, 50-52), needs the definition of an appropriate constraint Q,48,49 associated with the reaction coordinate. The reaction path consisted of 21 CP-BM constrained simulations of about 1.3 ps each, for a total of 26 ps. Geometry optimizations were carried out with a 60-Ry cutoff via simulated annealing procedures and/or using a quasi-Newton method.53 A convergence criterion of 5.0 × 10-4 au for the forces on the nuclei was adopted. Optimized geometries are available in the Supporting Information. For relevant geometries the electronic structure was analyzed by using the maximally localized Wannier orbitals,54 which provide a Lewis-like picture of the electronic distribution in condensed phases. Electronic excitation spectra were obtained by calculating the Franck-Condon optical conductivity σ(ω).55,56 The singleparticle Kohn-Sham states and eigenvalues for σ(ω) were calculated, with a cutoff of 90 Ry (BP/PW/90), on configurations evenly sampled along room-temperature trajectories. For selected geometries, the first excited singlet state was calculated via a low spin excitation (LSE) approach,57 already applied to the study of excited states in zeolites.58 Vibrational spectra were calculated from the Fourier transform of appropriate autocorrelation functions,59 obtained from the CP trajectories. The adopted combination of the DFT functional, pseudopotentials, and plane-wave basis set (BP/PW/60) is a balance between accuracy and computational cost in view of the long simulations needed for this study. This combination has been extensively tested, and in particular, it was shown to provide a very close representation of the structure and atomic motion correlation in framework minerals.60 Moreover, supramolecular arrangements of water molecules in zeolites,61,62 zeolitic Brønsted acid sites,63 and complex phenomena such as intracage proton transfers64 and dehydration processes65 in zeolites were correctly described by the CP method at the BP/PW/60 level of theory. In addition, the present scheme has been adopted for the study of Ti-zeolites,66-68 giving structural parameters of the Ti-site in line with results of both cluster34 and periodic calculations69 and with available experimental data as well.17 By considering the relevance and complexity of the problem, further tests on the accuracy of the BP/PW/60 scheme have been

Spano´ et al. carried out (see Supporting Information). In particular, on a benchmark system, TiCl4, for which experimental and theoretical data are available, the performance of the BP functional has been checked against that of the B3LYP functional with planewave basis sets70 and B3LYP and MP2 with localized bases.71 The degree of convergence of the adopted basis set is reported as well. Moreover, the BP/PW/60 energy difference between ethylene epoxide and acetaldehyde has been compared with the values obtained with larger plane wave basis sets (110 and 160 Ry) and with the B3LYP, MP2, and CCD approaches using localized basis sets. On the whole, the results of these test calculations further confirm the adequacy of the adopted computational scheme for the purposes of the present study. 3. Model Systems TS-1 is the most-studied Ti zeolite; however, its unit-cell content [TixSi98-xO196]72 makes extensive ab initio calculations unfeasible. Therefore, the present study has been performed on Ti offretite (Ti-off), a smaller but catalytically active Ti-zeolite.73 The chemical formula per unit cell of the Ti-off framework is [TiSi17O36]. All calculations were carried out with periodic boundary conditions using the hexagonal cell of offretite.74 Cell parameters and Ti-site location were as in refs 66 and 67. As a first step, to determine the starting structure of the Ti site, geometry optimizations were performed on two models, both with the stoichiometry [TiSi17O36]‚H2O. The first model, Ti-H, is characterized by a hydrolyzed Ti-O-Si bridge, i.e. containing one Ti-OH and one Si-OH group. In the second model, Ti-L, one H2O molecule is bonded to Ti as a ligand. Simulations of the ethylene epoxidation cycle were performed on two model systems, differing in concentrations of the H2O2/ H2O oxidizing mixture. Specifically, the dilution of H2O2 corresponded to 49 wt % in Model I and 39% in Model II. Model I was built by inserting two waters, one hydrogen peroxide, and one ethylene molecule in the channels of Ti-off;74 the stoichiometry per cell is [TiSi17O36]‚(H2O2)(C2H4)(H2O)2. Model II corresponds to a stoichiometry of [TiSi17O36]‚(H2O2)(C2H4)(H2O)3. The intermediates’ characterization has been performed on systems differing in degree of hydration. System W4, [TiSi17O37]‚ (H2O)4, was obtained by removing the ethylene molecule from the intermediate found in the simulations with Model II. Model W6 was built by adding two water molecules in the gmelinite cage of Model W4, thus simulating a higher degree of hydration. Finally, three model systems, characterized by a stoichiometry of [TiSi17O37]‚(H2O), were built. In the first model, W1, H2O was placed in the 12-ring cage, whereas in the second system, W0-1, H2O was positioned in the gmelinite cage. The third structure (HP) corresponds to a Ti-hydroperoxo system. 4. Results 4.1. Formation and Reactivity of the Intermediate. By considering the relevance of the mechanistic issues in low olefin epoxidation catalyzed by Ti-zeolites, the choice of the reaction coordinate was not based on a priori considerations, but it was determined by analyzing the structural and electronic properties of the model systems. Since the nonhydrolyzed Ti-L system, characterized by a H2O ligated to Ti, was found to be 21.2 kcal mol-1 more stable than the hydrolyzed zeolite Ti-H,75 the search of a proper reaction coordinate was performed on a nondefective Ti-zeolite model. An unconstrained CP simulation was carried out on Model I by positioning the hydrogen peroxide and the other guest

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Figure 1. (Left) Ball-and-stick representation of a snapshot taken from the unconstrained CP simulation on Model I. Ti is represented as a green sphere, Si atoms as red spheres, O as black spheres, H as gray spheres, and C as blue spheres. Hydrogen bonds are sketched as dashed lines. Arrow indicates the O(1)-O* separation. (Right) Densities of the maximally localized Wannier orbitals centered on O* (blue contour) and on O(1) (yellow contour).

Figure 2. Free-energy profiles vs reaction coordinates. (a) Free-energy profile for the first part of the reaction. The solid line refers to Model I, the dashed line refers to Model II. (b) Free-energy profile for the second part of the reaction.

molecules in the Ti-off 12-ring channel. A configuration taken from the trajectory is shown in Figure 1. In the simulation, 1.5 ps at room temperature, the two water molecules were hydrogen bonded to the Ti-ligated H2O2. The apolar C2H4 molecule diffused along the channel, at van der Waals distance from the framework and far from the H2O-rich Ti site. Aluminum-free zeolites are generally classified as hydrophobic and lyophilic molecular sieves.20,76 Actually, the Ti Lewis acid center is able to coordinate a protic ligand-like water or hydrogen peroxide67 and therefore to locally enhance the hydrophilic character of the zeolite. Remarkably, both H2O2 protons, involved in hydrogen bonds with water, interacted only transiently with the oxygen atoms of the Ti-O-Si bridges, whereas the water protons were hydrogen bonded to framework oxygens next nearest to Ti. This finding may indicate that Ti-O-Si bridges are difficult to hydrolyze. A clear indication of a reaction coordinate was found by calculating along the trajectory the maximally localized Wannier orbitals.54 Besides the ethylene π orbital, the most diffuse orbitals were those localized on the H2O2 oxygen ligated to Ti (labeled O*) and on the nearest framework oxygen bonded to Ti (labeled O(1)), as shown in Figure 1. Moreover, both Wannier orbitals became more diffuse whenever O* approached O(1). On this basis, the O(1)-O* distance has been chosen as the reaction coordinate Q for the CP-BM simulations.

Figure 3. Snaphots taken from the CP-BM simulations representing the ethylene epoxide formation. (Top left panel) Contour plot of the density of the ethylene π orbital. Si atoms are represented as yellow spheres, oxygen atoms as red spheres, hydrogen atoms as white spheres, and carbon atoms as blue spheres. Ti is represented as a green sphere. The picture represents the incipient ethylene oxidation by partial delocalization of the ethylene π orbital toward the O(1)-O* antibonding orbital. The other panels represent configurations taken from the reactants’ side of the reaction (top right), at the transition state (bottom left), and on the products’ side (bottom right). In these panels, spheres representing the atoms are red for Si, black for O, green for Ti, blue for C, and gray for H. Water molecules are not shown.

The blue moon ensemble was sampled at room temperature for O(1)-O* distances ranging from 2.6 to 1.4 Å. The starting point of the first CP-BM run was constructed by positioning C2H4 in the gmelinite cage in order to avoid its diffusion along the 12-ring channel. The simulated free-energy profile, reported in Figure 2, shows a barrier of about 46 kcal mol-1, with a shallow free-energy minimum at Q ) 1.52 Å. Overall, the simulated path led to the insertion of one H2O2 oxygen (O*) into the framework and to

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Spano´ et al.

Figure 4. (Top left panel) Solvated H2O2 molecule ligated to Ti. (Top right panel) Transient [Ti-OOH-‚‚‚H3O+] hydrated hydroperoxo; Ti, green sphere; O, black spheres; Si, red spheres; H, gray spheres; C, blue spheres. (Bottom left panel) Density (gray contour) of the Wannier orbital localized between O* and O(1) in Ti-ligated hydrated H2O2. (Bottom right panel) Density of the Wannier orbital localized between O* and O(1) in the [Ti-OOH-‚‚‚H3O+] hydrated hydroperoxo species. In the bottom panels, oxygens are represented as red spheres. Snaphots are taken for Q ) 1.90 Å, corresponding to the Model II transition state.

the release of a H2O molecule in the zeolitic channel. The structure of the free-energy minimum, characterized by a Ti[O(1)-O*]Si intraframework peroxo moiety, corresponds to an oxidized zeolite (see Supporting Information, Figure S1). Since Ti is surrounded by five oxygens, its Lewis acid strength, as well as the Ti-site hydrophilic character, should be lower than in tetracoordinated Ti.77 This may facilitate the approach of lyophilic substances such as ethylene to the Ti center. On this basis, the final part of the catalytic cycle was started from the above-described free-energy minimum structure by repositioning ethylene in the 12-ring channel. A preliminary blue moon sampling was performed by using as reaction coordinate Q the separation between Ti and one of the C atoms of ethylene. The initial value of Q was 4.6 Å. For a value of Q ) 3.1 Å, the O* is transferred to ethylene with formation of ethylene epoxide. A configuration close to the transition state is shown in Figure 3. The ethylene π orbital is polarized toward O*, indicating the incipient alkene oxidation by partial occupation of an antibonding orbital on the O(1)-O* peroxo bond. Remarkably, such a result was obtained without imposing any constraint on O*. This indicates that O* is actually an active oxygen atom and that the oxidized zeolite is a reactiVe intermediate. Thereafter, a more accurate blue moon sampling of the second part of the cycle was accomplished by choosing as reaction coordinate the distance between O* and one of the ethylene C atoms. The calculated free-energy profile (Figure 2) and the behavior of the system along the path (Figure 3) indicate that the intermediate oxidizes C2H4 to ethylene epoxide with a barrier of 2.5 kcal mol-1. At the end of the reaction the Ti site is restored to its tetrahedral coordination.

The free-energy barrier for the first step is too high with respect to the experimental value of 16.7 kcal mol-1 reported for ethylene epoxidation in TS-1.78 As this result might be related to solvation, the first step of the catalytic cycle was carried out on Model II, characterized by a 3:1 H2O to H2O2 ratio. The form of the free-energy profile (Figure 2) is very similar to the one obtained for Model I, with a shallow minimum at Q ) 1.52 Å corresponding to the same zeolitic Ti-peroxo structure. However, the presence of a third water molecule has lowered the barrier to about 20 kcal mol-1.79 At the transition state, interconversions between a hydrated Ti-H2O2 and a Ti-hydroperoxo + H3O+ ion pair were found (Figure 4):

[Ti-HOOH‚‚‚H2O] h [Ti-OOH-‚‚‚H3O+] The [Ti-OOH-‚‚‚H3O+] ion pair is transient at room temperature, while the hydrated Ti-H2O2 species is predominant. Formation of the Ti-hydroperoxo is a crucial event in the reactive path, because it initiates the oxidation of the zeolitic framework, leading to the proposed intermediate:

[Si-O-Ti-O*OH-] + H3O+ f [ Si-OO*-Ti] + 2H2O The Wannier orbital localized between O* and O(1) in the two structures found at the transition state is represented in Figure 4. In both structures, a significant portion of the density is localized on the σ* antibonding orbital of hydrogen peroxide, indicating a weakening of the H2O2 peroxo bond upon coordination to the Lewis acid site. However, the activated complex should be identified with the transient Ti-hydroperoxo + H3O+

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Figure 6. Ball-and-stick representation of the structure of the Tiperoxo intermediate from the CP-BM simulation on Model II corresponding to Q ) 1.516 Å; distances in Å. Numbers in parentheses are the calculated standard deviations (in Å) at 300 K. Ti, green sphere; O, black spheres; Si, red spheres.

Figure 5. Ball-and-stick representation of the proton-transfer mechanism in the CP-BM simulations on Model II. Snapshots are taken from the simulation corresponding to Q ) 1.80 Å. Transfer is shown clockwise from panel A (bottom right). Ti, green sphere; O, black spheres; H, gray spheres. In the 1.9 > Q > 1.6 Å range, many solventmediated diffusive back-and-forth proton transfers occurred.

ion pair. Indeed, the formation of the intermediate occurs via multiple proton transfers from O* to the OH- group of the hydrogen peroxide, as shown in Figure 5. It is a Grotthus-like, diffusive proton-transfer mechanism, where a proton, donated to a solvent molecule, travels through the solvent hydrogenbonding network and reaches the OH- group, forming a H2O molecule. The transfer can be considered fully accomplished for Q = 1.52 Å, when H2O leaves the Ti center. 4.2. The Complex Chemistry of the Zeolitic Ti-Peroxo/ (H2O)n System: Structure, Excitation, and Degradation of the Intermediate. Characterization of the physico/chemical properties of the zeolitic Ti-peroxo moiety could help in establishing whether the elusive “active-oxidizing species” in TS-1 catalysts might be identified with the found intermediate. The free-energy minimum corresponds to an O(1)-O* distance of 1.516 Å. This separation was kept fixed in the CPBM simulation as it corresponded to the reaction coordinate, whereas the other interatomic separations fluctuate because of thermal motion. The Ti-O* and Ti-O(1) average distances are 1.879 and 2.071 Å, with SDs of 0.03 and 0.09 Å, respectively (Table 1). The intermediate, shown in Figure 6, could therefore be defined as an asymmetric η2-Ti-peroxo zeolite.

4.2.1. Molecular Dynamics Simulations of the Fully and Partially Hydrated Ti-Peroxo Zeolite Systems. The roomtemperature behavior of the intermediate has been studied on models W6 and W4 by CP simulations (i.e., without imposing the O(1)-O* constraint). The fully hydrated W6 system was found to be stable, and no significant change in the structure of the η2-Ti-peroxo site was detected in a 8-ps-long simulation. Remarkably, the average distance obtained for the O*-O(1) peroxo bond, 1.516 Å, is equal to the constrained value corresponding to the minimum in the free-energy profile, as shown in Table 1. On the other hand, the partially hydrated W4 system showed a particularly interesting modification, and its trajectory was therefore carried out for a longer simulation time, 16 ps. In particular the average O*-O(1) peroxo bond length decreases to 1.499 Å. The time behavior of the distances of Ti from O*, O(1), and Ow, a close water oxygen, are reported in Figure 7, along with the Ti-Si separation. In the first part of the trajectory, up to about 5 ps (W4-Part I), the W4 system essentially behaves like W6. After 5 ps (W4-Part II), a water molecule approaches Ti and enters its coordination shell as a ligand, while the TiO(1) distance increases. Such an event, in which a water molecule displaces O(1), is reminiscent of a SN2 mechanism (see top inset in Figure 7). Moreover, in the Ti-[OO]-Si bridge, the Ti-Si separation increases, while the O(1)-O* distance decreases slightly. The Ti-peroxo structural modification, which may be described as an η2 to η1 conversion, causes a change in Ti coordination. The Ti-O radial distribution function g(r) and running coordination number N(r) calculated for the W4 and W6 trajectories are reported in Figure 8. The same Ti-site structure is found in both W6 and W4-Part I, where 5 oxygen atoms are coordinated to Ti. On the other hand, in W4-Part II,

TABLE 1: Room-Temperature Averaged Relevant Interatomic Separationsa of the Ti Site (in Å) Obtained from the Constrained Simulation Corresponding to the Free-Energy Minimum on Model II (CP-BM), the Unconstrained Simulation on the W4 System (CP W4), and the Unconstrained Simulation on the W6 System (CP W6) CP-BM CP W4 CP W6

Ti-O*

Ti-O(1)

Si-O*

Si-O(1)

O*-O(1)

Ti-O′

Ti-O′′

Ti-O′′′

1.879 (0.03) 1.870 (0.03) 1.886 (0.05)

2.071 (0.09) 2.268 (0.26) 2.073 (0.06)

2.662 (0.15) 2.626 (0.11) 2.687 (0.06)

1.710 (0.04) 1.691 (0.04) 1.726 (0.06)

1.516

1.820 (0.03) 1.833 (0.03) 1.828 (0.04)

1.847 (0.02) 1.858 (0.03) 1.846 (0.03)

1.808 (0.03) 1.813 (0.03) 1.803 (0.03)

1.499 (0.05) 1.516 (0.06)

a Standard deviations (in Å) have been reported as well, as they provide indications on the width of the oscillations of a given geometrical parameter around its average value.

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Figure 7. Behavior of selected interatomic distances of the Ti-peroxo center as a function of time in the W4 simulation. In the top inset, graphical representations of the η2- and η1-Ti-peroxo structures found respectively in Part I (0-5 ps) and Part II (5-16 ps) of the W4 trajectory are shown.

the coordination number becomes 5 only at Ti-O separations beyond 2.25 Å. By the analysis of EXAFS data12 on TS-1/ (H2O2)(aq), Ti was found to be surrounded by 4.8 oxygens (2.8 atoms at 1.83 Å and 2 atoms at 2.01 Å), while the Ti coordination shell was found to be modified after 24 h aging (4 oxygen atoms at 1.83 Å).12 Such results compare favorably with the N(r) values at the same positions; indeed, the experimentally observed modifications are in line with the N(r) changes calculated from the W6 and W4-Part II simulations. Vibrational spectra were calculated from partial contributions due to the bending, symmetric, and asymmetric stretching modes of the T-O-T bridges, where T stands for all the Si and/or Ti

Spano´ et al. in the framework. The contributions due to the O*-Ti-O(1) system were calculated from the autocorrelation functions of the O*-Ti-O(1) cycle symmetric and asymmetric breathing modes and from the O*-O(1) stretching mode. Also the contributions of the three Ti-O-Si not involved in the Tiperoxo were calculated. The simulated spectra are shown in Figure 9. The O*-Ti-O(1) contribution shows two intense peaks located at 585 and 633 cm-1 in W6 and at 580 and 635 cm-1 in W4. While the lower wavenumbers’ components are partially overlapped with framework modes, the higher wavenumbers’ peak falls in a semitransparent window. In the 650800 cm-1 region, the absorptions due to the peroxo moiety may be obscured by framework modes. However, in the W4 system, two peaks at 856 and 879 cm-1 do not overlap with framework absorptions, while the corresponding ones in the W6 spectra (at 827 and 857 cm-1) are partially hidden. From the calculated spectra, fingerprints of the Ti-peroxo intermediate are therefore found at 633-635 cm-1 and 856-879 cm-1. Such wavenumbers are in fair agreement with the values of 625 and 875 cm-1 reported for the “labile yellow intermediate” in resonant Raman experiments on TS-1 samples.14 4.2.2. Electronic Excitation Spectra. Simulated electronic excitation spectra are reported in Figure 10, where the optical conductivities calculated for anhydrous Ti-off and a model allsilica offretite Si-off [Si18O36] have been reported as a reference. The comparison of the two reference spectra indicates that, for Ti-zeolites, DFT (BP/PW/90) calculated spectra give absorptions in the experimentally detected UV-vis region. Moreover, such an absorption range is also in line with that calculated by the more accurate TD-DFT method,71,80 as reported in ref 68. On the basis of this result, the electronic excitations of the model systems were calculated by DFT, and the optical conductivities were compared with experimental diffuse reflectance ultravioletvis (DRUV-vis) data.12 Differing from that of dry Ti-off, both W4 and W6 spectra show electronic excitation bands with tails extending into the visible region, in line with the DRUV-vis profiles obtained for TS-1/(H2O2)(aq).12 The two systems, both containing a Tiperoxo species but characterized by different water contents, show different excitation spectra. In general, intensity in the visible region decreases with decreasing water content as found in ref 12. The W6 spectrum is characterized by a strong peak

Figure 8. (Left panel) Calculated Ti-O radial distribution function g(r) and running coordination number N(r) in W4 (solid lines) and W6 (dashed lines). (Right panel) Contributions to the Ti-O g(r)’s and N(r)’s calculated for Part I (solid lines) and Part II (dashed lines) in W4. (Inset) Region between 1.5 and 2.5 Å is shown in detail . Lines corresponding to N ) 4 and 5 are reported as a guide for the eyes. Peak positions of g(r)’s are at 1.83, 2.05, and 3.35 Å in W6 and at 1.83, 2.25, and 3.10 Å in W4 (full trajectory).

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Figure 9. Vibrational spectra for the W4 and W6 systems. (Panel A) Contributions to the vibrational spectrum of the W6 system. (Panel B) Contributions to the vibrational spectrum of the W4 system. Dotted line + gray-shadowed region, framework T-O-T modes; thin solid line, Ti-O-Si modes not involved in the peroxo moiety; thick solid line, O*-Ti-O(1) modes.

Figure 10. (Left panel) Optical conductivities calculated from CP simulations of the W4 and W6 systems. The optical conductivities calculated separately for W4-Part I and W4-Part II are also shown. Optical conductivities of the dry Ti-off and Si-off systems are reported as a reference. (Right panel) Inset showing optical conductivities calculated for W6, full-W4, W4-Part I, and W4-Part II in the region of 300-600 nm.

at 365 nm and a less intense and very broad band peaking at 435 nm with a shoulder at 515 nm, tailing into the red region. A broad absorption centered at 380 nm with a shoulder at about 450 nm is observed in W4. By comparing the contributions of Parts I and II to the W4 spectrum, an interesting feature emerges: the 380 nm band is present only in Part I, while Part II does not show significant absorptions beyond 350 nm. This indicates that the η1-Ti-peroxo species, predominant in the second part of the W4 trajectory, should be uncolored, whereas the η2-Ti-peroxo could indeed be responsible for the yellow color of the H2O2/H2O-loaded Ti zeolite. Differences in the two calculated spectra are therefore due to the different behaviors of the Ti site in the W4 and W6 systems, and in particular to the η2-(colored) to η1-(uncolored) structural change in the partially hydrated system. 4.2.3. Properties and RelatiVe Stabilities of the η2- and η1Ti-Peroxo Species. Geometry optimizations were carried out starting from two configurations sampled from W4-Part I and W4-Part II. The geometry of the Ti-peroxo-center in the two optimized systems (Table 2) corresponds to a η2 and a η1 structure, respectively. In line with the room-temperature results, the Ti-O(1) separation increases significantly, and both Ti-

TABLE 2: Energy Gaps, in eV and nm, and Relevant Distances Obtained for the Optimized Structures Egap W6 η2 W4 η1 W4 W0-1 W1 η2 HP

(eV)

(nm)

O*-O(1) (Å)

Ti-O*

Ti-O(1)

3.15 3.20 4.17 3.66 4.28 4.44

394 387 297 339 290 279

1.497 1.496 1.462 1.497 1.487 1.473

1.876 1.881 1.858 1.869 1.860 1.890

2.068 2.081 2.518 2.072 2.232 1.967

Ti-Ow

2.226 2.293

a All distances in Å. Energy gaps are calculated with a basis set of 90 Ry, by using geometries optimized with the 60-Ry basis set.

O* and O*-O(1) distances shorten on passing from η2 to η1. The calculated HOMO-LUMO electronic energy gap, which is considered a reactivity index,34 is reported in Table 2. The gap is significantly higher and well outside the visible range (297 nm) for the η1 complex, while in the η2 case it is found at 387 nm, thus confirming that the η2 to η1 conversion detected in the W4 trajectory is accompanied by a blue-shift in the absorption edge. Quite remarkably, the energy difference between the two minima amounts to 5.6 kcal mol-1 in favor of

21658 J. Phys. Chem. B, Vol. 110, No. 43, 2006

Spano´ et al.

Figure 11. Optimized geometry of the Ti-hydroperoxo center in the HP system. Ti is represented as a green sphere, O as black spheres, Si as red spheres, and H as gray spheres. Hydrogen bonds are drawn as dashed lines.

η1. On the whole these results suggest that the uncolored η1 structure should be less reactive than the η2 one. 4.2.4. H2O Effects on the Ti-Peroxo Center. A geometry optimization has been performed starting from a configuration sampled from the W6 trajectory. The optimized geometry of the Ti site is very similar to that of the W4-η2 one, while the energy gap, 394 nm, is slightly lower. In the optimized W1 model, the water molecule is coordinated to Ti, with a Ti-Ow distance (2.293 Å) slightly longer than that in W4-η1 (2.226 Å). In W1 the peroxo-site geometry is between the W4-η1 and η2 structures, while the energy gap (290 nm) is close to that of W4-η1 (297 nm). In the optimized W0-1 system, where Ti is not reachable by H2O, the Ti-peroxo center has an η2 structure, and the gap (339 nm) is lower than in W1. The W1 system is found to be 2.6 kcal mol-1 more stable than W0-1, thus indicating that the η2 Ti-peroxo structure is also unfavored at low water contents. The geometry of the η1-peroxo site in W1 is very close to the one reported by Munakata et al.33 where a water molecule was ligated to Ti with a Ti-Ow distance of 2.26 Å. 4.2.5. The Optimized Ti-Hydroperoxo Center. Many catalytic paths have been modelled on hydrolyzed Ti-O-Si bridges with the formation, as an active oxygen-donating moiety,25-32 of a Ti-hydroperoxo center.81 Relevant data of the optimized Tihydroperoxo site (HP) are reported in Table 2, while a graphical representation is shown in Figure 11. The structure is characterized by hydrogen bonds between the silanol and the O* (1.792 Å) and between the silanol and O(1) (2.113 Å). The hydroperoxo proton is also involved in a very strong hydrogen bond with an oxygen of a neighboring Si-O-Si bridge (1.577 Å). The O*O(1) distance is 1.473 Å, and the Ti-O* and Ti-O(1) distances are 1.890 and 1.967 Å, respectively. Such a system can be considered an η2-Ti-hydroperoxo and has an energy gap of 279 nm. This structure, which has the same stoichiometry of the W1 and W0-1 systems, is found to be 16.6 kcal mol-1 less stable than W1.75 4.2.6. Electronic Structure of the Ti[OO]Si Center and its Photoexcitation. The electronic structure of the Ti-peroxo centers has been analyzed in terms of Wannier orbitals and

Figure 12. Position of the Wannier functions centers in the W4 η2Ti-peroxo intermediate. Ti, green sphere; Si, red spheres; O, black spheres. WFC are represented as yellow spheres.

Wannier functions centers54 (WFC) for both the ground state and the first excited singlet state. While the ground-state analysis may provide insight on catalysis in dark conditions, excitations are relevant for photoreactivity. Indeed, it has been reported that ethylene epoxidation in aged TS-1 catalysts occurs only by photoirradiation.11 Relevant ground-state WFCs in the case of the W4-η2 system are represented in Figure 12, while some ground-state WFC atom distances are reported in Table 3. The WFC-based representations of the model systems show some common features. Each oxygen not involved in the peroxo bond is surrounded by 4 WFCs arranged in a quasi-tetrahedral geometry. They correspond to the eight valence electrons of sp3 O2-.82 Also, the peroxo oxygens are surrounded by 4 WFCs each; however, one WFC is shared (wp in Figure 12). The corresponding electronic density is localized along the O*-O(1) axis and has a σ symmetry, indicating a typical single O-O bond (Figure 13). However, the distribution of such a bonding electron pair is polarized; indeed, wp is always closer to O(1) than to O* (Table 3). The polarization is more pronounced in the η2 structures.83 Besides the asymmetry in the peroxo bond, a further difference between the η2 and η1 structures is pictured in Figure 13. In η2, the orbitals localized along the O*-Ti and O(1)-Ti axes are both combinations of oxygen p and titanium d functions. On the other hand, in η1 no titanium-d component is present in the orbital localized between O(1) and Ti. The orbital that becomes occupied upon photoexcitation was obtained from LSE calculations57 on all the optimized structures. The Wannier representation of the one calculated for the W6η2 structure is shown in Figure 14. Differing from the ground state, where all WFCs were close to oxygen atoms, the excited WFC is close to Ti, thus indicating a ligand-to-metal charge transfer (LMCT) process upon excitation. This singly occupied

TABLE 3: O*-O(1) Bond Distances and Distances of the Peroxo Oxygens from Relevant WFCs Calculated for the Optimized Structuresa W6 η2 W4 η1 W4 W0-1 W1 η2 HP a

O*-O(1)

O*-wp

O(1)-wp

O*-wa

O*-wb

O*-wc

O(1)-wa′

O(1)-wb′

O(1)-wc′

1.497 1.496 1.462 1.497 1.487 1.473

0.799 0.799 0.756 0.807 0.791 0.782

0.699 0.698 0.706 0.692 0.697 0.692

0.467 0.464 0.480 0.474 0.480 0.463

0.314 0.315 0.309 0.302 0.306 0.304

0.306 0.307 0.310 0.304 0.304 0.332

0.375 0.372 0.321 0.373 0.349 0.404

0.289 0.289 0.293 0.289 0.293 0.315

0.450 0.451 0.463 0.452 0.459 0.478

All distances in Å. Labeling of WFCs as in Figure 12.

Role of Ti(IV) as a Lewis Acid in Ti Zeolites

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21659 The oxidized zeolite intermediate, less hydrophilic in the proximity of the Ti center, reacts with the substrate Sub to give the oxidized substrate, recovering the catalyst:

[Ti-Zeo]Ox + SubRed f [Ti-Zeo]Red + SubOx In the case of ethylene, the overall reaction can be written as:

C2H4 + H2O2 f C2H4O + H2O

Figure 13. Contour plots of the density (gray-shadowed regions) of relevant Wannier orbitals calculated for the W4 η2-Ti-peroxo structure localized on O*-O(1) (top left), O*-Ti (top right), and O(1)-Ti (bottom left). The contour plot of the density of the Wannier orbital localized on O(1) and directed toward Ti for the W4 η1-Ti-peroxo complex is shown in the bottom right panel. Green spheres, Ti atom; yellow spheres, Si atoms; red spheres, oxygen atoms; white spheres, protons.

Figure 14. Contour plot of the density of the Wannier orbital corresponding to the first excited singlet state of the optimized W6 structure (grey-shadowed regions). Green sphere, Ti atom; yellow sphere, Si atom; red spheres, oxygen atoms.

orbital is partially delocalized on the Ti[OO] structure and consists of a Ti d function overlapped with the σ* antibonding orbital on the peroxo O*-O(1) bond. Excitation most probably occurs from the ground-state orbital localized along the TiO* bond which is characterized by a different symmetry of the titanium d component. Remarkably, both the shape and the WFC position (close to Ti) of the orbital that becomes occupied upon excitation are common to all the model systems here investigated. 5. Discussion 5.1. Role of the Ti(IV) Chemistry in the Epoxidation Cycle. The model catalytic cycle has been performed in two steps. The disproportionation reaction of (H2O2)(aq) at the Ti center leads to the formation of an oxidized zeolite and a water molecule via a Ti-hydroperoxo as transition state:

[Ti-Zeo]Red + H2O2 f [Ti-Zeo]Ox + H2O

The formation of the intermediate occurs via a synergic oxidative attack of H2O2 to the zeolitic Ti site ruled by the Lewis acid character of Ti. Indeed, Ti is attracting electronic density from the closest ligand (H2O2). The polarization of electronic density from H2O2 toward Ti is compensated by a lone pair of the closest anion, namely the framework oxygen bonded to Ti and located at about 2-3 Å from H2O2. Altogether, the effect of the Ti(IV) Lewis acidity is to increase the oxidizing properties of the hydrogen peroxide by enhancing its capability to attract electronic density from the closest framework oxygen. In the second part of the reaction, Ti acts as a Lewis acid as well by attracting electronic density from ethylene. The electronic structure at both transition states, where partial occupation of a Ti-d orbital occurs, indicates that reactivity is essentially due to the low-lying empty d states available in zeolitic Ti(IV). 5.2. Kinetic and Thermodynamic Aspects of the Cycle. The free-energy profile found for the model path fairly fits the experimental evidence of formation of a reactive and labile moiety. The rate-determining step of the oxidative cycle should be the formation of the active oxygen-donating species. The free-energy barrier, strongly dependent on the water content, amounts to about 20 kcal mol-1 with the H2O2 concentration of 39 wt %, in line with the experimental value of 16.7 kcal mol-1 reported for ethylene epoxidation in TS-1.78 The second reaction step involves a Ti-mediated transfer of electronic density from the ethylene π orbital to the σ* O-O antibonding orbital (see Figure 3), leading to the breaking of the O-O bond and the formation of the epoxide, with a barrier of 2.5 kcal mol-1. Therefore, transfer of active oxygen should be much faster than formation of the active oxygen-donating species. Actually, the epoxide formation could be diffusion limited, because the Ti center, due to its hydrophilic character, is hardly accessible to the apolar olefin molecule before the formation of the intermediate. The model cycle is found to be moderately endothermic, but the energy contributions of absorbing molecules inside the zeolitic channel are not taken into account in this approach. The different affinities of reactants and products to zeolites may play a role in the energetic cycle of the process.84 In line with thermodynamic considerations, both free-energy minima found in the simulated reaction path do correspond to situations where a stable compound is formed, i.e. water in the first minimum and ethylene oxide in the second. The formation of these products should be considered the driving force of the cycle. The different stabilities of the two minima are related to the zeolitic products formed in the two cases: a reactive intermediate for the first part of the reaction and the stable form of the Ti zeolite catalyst at the end of the cycle. 5.3. Solvent Effects on the Intermediate Formation. The degree of solvation affects the barrier height to a large extent. Responsibility for such an effect lies with the Grotthus-like proton diffusion mechanism, which facilitates the formation of the Ti-hydroperoxo activated complex. With a H2O to H2O2 ratio of 2:1, no H3O+ formation was observed, indicating that acidity can be developed only when

21660 J. Phys. Chem. B, Vol. 110, No. 43, 2006 the degree of hydration at the Ti site is sufficiently high. Indeed, the increase in the local concentration of water around the Ti center and therefore in the degree of solvation of H2O2 favors the Grotthus mechanism by a local increase of the dielectric constant. The described mechanism may also explain why moderate alkalinity increases reaction rates: the presence of other OHgroups in the zeolite should increase the number of protonic diffusive paths, thus enhancing the transformation of the transition state. Furthermore, the observed H3O+ formation could rationalize both the acid-catalyzed collateral reactions detected in TS-11 and the acidity developed upon contact with aqueous H2O2.12,13 5.4. The Yellow Active η2 Ti-peroxo Intermediate. The structural, vibrational, and electronic properties of the reactiVe intermediate found in the model catalytic cycle indicate that the “labile yellow complex” formed upon contact of Ti-zeolites with aqueous H2O212-14 could be an oxidized zeolite with a peroxo bond inserted between a Ti and a Si tetrahedral center. Such a finding might explain why complete disappearance of titrable oxidizing species in TS-1 occurs only upon heating at 673 K.12,13 This temperature could be considered too high in the case of “active oxygen” ligated to Ti as a molecular complex (peroxo or hydroperoxo), whereas its value could be better explained if the active oxygen were chemically inserted in the zeolitic framework, as found in the present study. An η1-(Ti-[OO]-Si) intermediate has been proposed by Munakata et al.33 on the basis of ab initio calculations on cluster models. It is worth noticing that an oxidized zeolite intermediate characterized by an Al-[OO]-Si bridge has been also found in the simulation of the oxidation of NO2- to NO3- by molecular oxygen in sodalitic cages.51,85 The theoretical characterization of the asymmetric η2-Tiperoxo intermediate gives results in fair agreement with a wide range of experiments. Even more relevant is the finding that such a species oxidizes ethylene in the model cycle, therefore indicating that the “yellow labile intermediate” detected in TS1/ (H2O2)(aq)12,14 and the active form of the catalyst should be actually the same species. A rationalization of the reactivity of the proposed intermediate toward olefins could be found in its electronic structure. Indeed, the charge distribution along the O*-O(1) bond, polarized toward O(1), makes O* more electrophilic than an oxygen atom of an unpolarized symmetric side-on peroxo.86 The presented results are also in line with the photochemistry of Ti zeolites contacted with H2O2: upon excitation an antibonding σ* orbital, localized on the O-O bond, becomes partially populated, thus further enhancing the reactivity of the active O* atom. 5.5. Peroxo vs Hydroperoxo Intermediates. The catalytic cycle was performed on a nondefective Ti zeolite by adopting periodic boundary conditions on the basis of its higher stability with respect to a hydrolyzed system. However, the presence of four Ti-O-Si bridges is not a key factor in the presented reaction path, which might occur at hydrolyzed Ti sites as well. In such a case, the outcome of the dissociative chemisorption of H2O2 should be a Ti-hydroperoxo site, such as Model HP. On the other hand, HP has an electronic gap larger than both η2- and η1-Ti-peroxo moieties. These results allow us to hypothesize a minor role of Ti-hydroperoxo zeolitic structures as active catalytic intermediates in TS-1. An interesting feature of the proposed asymmetric η2-Tiperoxo intermediate is that its geometrical parameters are

Spano´ et al. comparable with η2-Ti-hydroperoxo structures obtained from ab initio calculations on model clusters (e.g., see ref 32). 5.6. Aging Behavior of the Catalyst. The aging and dehydration processes were not directly simulated because only the picosecond scale is accessible to ab initio molecular dynamics. However, their effects were mimed by studying systems characterized by different degrees of hydration. The reported η2 to η1 conversion could explain the lability of the intermediate. In such a conversion, water molecules play a relevant role. As shown by the Ti-Ow distances (Table 2), water is more tightly bound to Ti when other water molecules are present. Such a cooperative effect66 that makes the H2O ligand a stronger Lewis base is due to the hydrogen-bonding network among the caged water molecules. The effect of water on the properties of the intermediate is very subtle. With decreasing water content, the long wavelength optical conductivity decreases and the energy gap increases, in line with DRUV-vis data.12 In addition, comparison of models characterized by the same water content but different structure also indicates that the η2 to η1 conversion contributes to the increase of the gap. By comparing the W1 and W0-1 systems an interesting feature emerges. In W1, where one H2O is ligated to Ti, an η1 structure is found with a gap of 290 nm. Such a large energy gap indicates a low reactivity.34 The Ti site in W01, not accessible to the water molecule, may be considered equivalent to a fully dehydrated site. This system, characterized by an η2 structure and a gap of 339 nm, should be more reactive than W1 (η1) but less reactive than both the η2-solvated systems W6 (394 nm) and W4 (387 nm). A picture of the reactivity-aging relationship emerges from the presented data. The most active system is the fresh, fully hydrated η2-Ti-peroxo zeolite. Upon dehydration, its activity is reduced due to the η2 to η1 conversion. When the dehydration is fully accomplished, an η1 to η2 reconversion could take place; however, the dehydrated η2-Ti-peroxo zeolite is less reactive than the fully hydrated intermediate. By recalling that both the binding and the release of a ligand at Ti sites are activated processes,67 it can be concluded that kinetics should play a role in the catalyst aging because the η2 T η1 interconversions are ruled by changes in the Ti coordination shell. Therefore, besides the formation of the η2-Ti-peroxo intermediate and its reactivity toward olefins, the capability of zeolitic Ti sites to act as Lewis acids also drives the η2 T η1 interconversions, thus rationalizing the catalyst aging at a microscopic level. 6. Conclusion A first-principles investigation on the catalytic epoxidation cycle of an olefin inside a Ti-zeolite in the presence of aqueous H2O2 is presented. At the end of the cycle, ethylene epoxide and water are obtained, and the catalyst is recovered. The reaction path, suggested by the analysis of the electronic distribution in equilibrium structures at 300 K, predicts as the active, oxidizing intermediate an asymmetric η2-Ti-peroxo species inserted in the zeolitic framework. The theoretical characterization of the intermediate, and its lability, validate the proposed path, as they are in fair agreement with experimental findings. Moreover, they may help to reconcile apparently contrasting facts: the known inactivity of well-characterized non-zeolitic Ti-peroxo species in olefin epoxidation,9 the signatures of Ti-peroxo in TS-1,14 and the known activity of TS-1 in olefin epoxidation.3 Finally, the results presented here provide a fully consistent picture of the complex behavior of such a relevant class of materials and can explain many facets of the chemistry of Ti sites. In a wider perspective, the deeper understanding of the general features of zeolitic Ti(IV) achieved

Role of Ti(IV) as a Lewis Acid in Ti Zeolites in this study may also be of relevance for stimulating the design, synthesis, and application of novel framework materials. Supporting Information Available: Cartesian coordinates of optimized geometries of the model systems, results of benchmark calculations on the TiCl4 species (Table S1), results of benchmark calculations on ethylene epoxide and acetaldehyde (Table S2), basis-set dependency of calculated ∆E between selected model systems (Table S3 and S4), and graphical representation of relevant steps of the Ti-peroxo species formation with Model I (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bellussi, G.; Carati, A.; Clerici, M. G.; Maddinelli, G.; Millini, R. J. Catal. 1992, 133, 220. (2) Taramasso, M.; Perego, G.; Notari, B. U.S. Patent 441051, 1983. (3) Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 159. (4) Clerici, M. G.; Ingallina, P. J. Catal. 1993, 140, 71. (5) Clerici, M. G. Stud. Surf. Sci. Catal. 1993, 78, 21. (6) Laufer, W.; Hoelderich, W. F. Appl. Catal., A 2001, 213, 163. (7) Hill, C. L. Nature 1999, 401, 436. (8) Clerici, M. G. Appl. Catal. 1991, 68, 249. (9) Clerici, M. G. Top. Catal. 2001, 15, 257. (10) Notari, B. Catal. Today 1993, 18, 163. (11) Lin, W.; Frei, H. J. Am. Chem. Soc. 2002, 124, 8292. (12) Bonino, F.; Damin, A.; Ricchiardi, G.; Ricci, M.; Spano´, G.; D’Alosio, R.; Zecchina, A.; Lamberti, C.; Prestipino, C.; Bordiga, S. J. Phys. Chem. B 2004, 108, 3573. (13) Prestipino, C.; Bonino, F.; Usseglio, S.; Damin, A.; Tasso, A.; Clerici, M. C.; Bordiga, S.; D’Acapito, F.; Zecchina, A.; Lamberti, C. ChemPhysChem 2004, 4, 1799. (14) Bordiga, S.; Damin, A.; Bonino, F.; Ricchiardi, G.; Lamberti, C.; Zecchina, A. Angew. Chem., Int. Ed. 2002, 41, 4734. (15) Boccuti, M. R.; Rao, K. M.; Zecchina, A.; Leofanti, A.; Petrini, G. Stud. Surf. Sci. Catal. 1988, 48, 133. (16) Tozzola, G.; Mantegazza, M. A.; Ranghino, G.; Petrini, G.; Bordiga, S.; Ricchiardi, G.; Lamberti, C.; Zulian, R.; Zecchina, A. J. Catal. 1998, 179, 64. (17) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125. (18) Noc, L. L.; On, D. T.; Solomykina, S.; Echchahed, B.; Beland, F.; Moulin, C. C. D.; Bonneviot, L. Stud. Surf. Sci. Catal. 1996, 101, 611. (19) Parker, W. O.; Millini, R. J. Am. Chem. Soc. 2006, 128, 1450. (20) Vayssilov, G. N. Catal. ReV.sSci. Eng. 1997, 39, 209. (21) Srinivas, D.; Manikandan, P.; Laha, S. C.; Kumar, R.; Ratnasamy, P. J. Catal. 2003, 217, 160. (22) Bonoldi, L.; Busetto, C.; Congiu, A.; Marra, G.; Ranghino, G.; Spano´, G.; Giamello, E. Spectrochim. Acta, Part A 2002, 58, 1143. (23) Bordiga, S.; Damin, A.; Bonino, F.; Ricchiardi, G.; Zecchina, A.; Tagliapietra, R.; Lamberti, C. Phys. Chem. Chem. Phys. 2003, 5, 4390. (24) Zecchina, A.; Bordiga, S.; Spoto, G.; Damin, A.; Berlier, G.; Bonino, F.; Prestipino, C.; Lamberti, C. Top. Catal. 2002, 21, 67. (25) Neurock, M.; Manzer, L. E. Chem. Commun. 1996, 10, 1133. (26) Karlsen, E.; Scho¨ffel, K. Catal. Today 1996, 32, 107. (27) Limtrakul, J.; Inntam, C.; Truong, T. N. J. Mol. Catal. A: Chem. 2004, 207, 137. (28) Sever, R. R.; Root, T. W. J. Phys. Chem. B 2003, 107, 4080. (29) Sever, R. R.; Root, T. W. J. Phys. Chem. B 2003, 107, 4090. (30) Wells, D. H. J.; Delgass, W. N.; Thomson, K. T. J. Am. Chem. Soc. 2004, 126, 2956. (31) Vayssilov, G. N.; vanSanten, R. A. J. Catal. 1998, 175, 170. (32) Barker, C. M.; Gleeson, D.; Katosyannis, N.; Catlow, R. A.; Sankar, G.; Thomas, J. M. Phys. Chem. Chem. Phys. 2002, 4, 1228. (33) Munakata, H.; Oumi, Y.; Miyamoto, A. J. Phys. Chem. B 2001, 105, 3493. (34) Zhanpeisov, N. U.; Anpo, M. J. Am. Chem. Soc. 2004, 126, 9439. (35) To, J.; Sokol, A. A.; French, S. A.; Catlow, C. R. A.; Sherwood, P.; van Dam, H. J. Angew. Chem., Int. Ed. 2006, 45, 1633. (36) Car, R.; Parrinello, M. Phys. ReV. Lett. 1985, 55, 2471. (37) Marx, D.; Hutter, J. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J., Ed.; Forschungzentrum Ju¨lich: Ju¨lich, Germany, 2000; Vol. 1. (38) Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133. (39) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, 1989. (40) Ceperly, D. M.; Alder, B. J. Phys. ReV. Lett. 1980, 45, 566. (41) Perdew, J. P.; Zunger, A. Phys. ReV. B: Condens. Matter Mater. Phys. 1981, 23, 5048. (42) Becke, A. D. Phys. ReV. A: At., Mol., Opt. Phys. 1988, 38, 3098.

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