4232
J. Phys. Chem. B 1997, 101, 4232-4237
Computational and EXAFS Study of the Nature of the Ti(IV) Active Sites in Mesoporous Titanosilicate Catalysts Phillip E. Sinclair, Gopinathan Sankar, C. Richard A. Catlow,* John Meurig Thomas, and Thomas Maschmeyer The DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K. ReceiVed: August 7, 1996X
First principles, nonlocal density functional theory (DFT) calculations within the cluster approximation have been carried out in order to model the formation of Ti active sites in the pores of mesoporous silicas by the reaction of (η5-C5H5)2TiCl2 with terminal surface silanols. In line with recent experimental observations (Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature, 1995, 378, 159), the calculations predict that, after calcination, the dominant surface Ti species is the (≡SiO)3TiOH complex. However, our results imply that there could be an appreciable concentration of the (≡SiO)2Ti(OH)2 complex present. The (≡SiO)2TidO species is energetically unfavorable. Also in line with experiment, our calculations predict that the dominant surface Ti complex is formed Via a (≡SiO)3Ti(η5-C5H5) intermediate. Calculated structures of this Ti intermediate are not, however, in agreement with the experimental EXAFS structure, and it is suggested that the cause of this difference is the use of an oversimplified model during the EXAFS fit. Computational studies of the hydrolysis and hydration of Ti centers provide a possible rationale for the uncertainties in previous studies concerning the nature and coordination of such sites in materials synthesized by different methods and studied under a variety of conditions.
Introduction Titanium ions incorporated into the framework of siliceous microporous and mesoporous solids exhibit a range of interesting and useful catalytic properties.1-10 These materials combine the activity of titanium in selective oxidations using H2O2 and other peroxidic reagents on the one hand, with shape selectivity for catalytic activity commonly seen in microporous catalysts on the other.11 For example, TS-1 and TS-2,2 two medium pore zeolites, are active and selective catalysts for the oxidation of phenol to catechol and hydroquinone,12,13 of alkenes to epoxides,14,15 and of alkanes to alcohols and ketones.16-18 Debate still continues as to the precise nature of the Ti(IV) active sites both in the microporous, zeotype Ti-containing catalysts such as TS-1 and TS-2 and in the mesoporous silicas where Ti(IV) ions have been either grafted onto the inner walls or embedded within the walls of the structures. In the absence of adsorbates, however, framework-substituted Ti is almost universally accepted to be 4-coordinate.19-31 Recent theoretical studies of Ti substitution into the framework of zeotype materials also indicate that Ti is preferentially 4-coordinate.32,33 The catalytic activity of Ti(IV) framework-substituted microporous materials, e.g., TS-1 and TS-22 and Ti-Beta,6 is somewhat limited by the fact that only those reactants that are small enough to gain access to the Ti centers through the zeolitic channels can be converted. This prevents their use in the finechemical and pharmaceutical contexts where bulkier reactants dominate. However, the synthesis and use of Ti-MCM-41,34,35 a titanium framework-substituted mesoporous silicate with pore apertures in the range of 25-100 Å, have made these titanosilicate catalysts much more attractive and applicable. Maschmeyer et al.36 have recently described a method of introducing quite high concentrations, e.g., Ti:Si ) 0.04:1.0, of Ti(IV) ions onto the framework of MCM-41 silicas in a well-defined, siteseparated fashion using titanocene dichloride as a precursor. X
Abstract published in AdVance ACS Abstracts, April 15, 1997.
S1089-5647(96)02396-6 CCC: $14.00
After calcination, the Cp2TiCl2 (Cp ) η5-C5H5-) treated MCM41 silica becomes an active catalyst for selective oxidation of even bulky substrates with a large number of accessible Ti sites offering the potential of shape selectivity through the structure of the mesoporous framework. All intermediate materials produced in the catalyst preparation were examined by in-situ Ti K-edge X-ray adsorption spectroscopy (EXAFS and XANES).19 Our aim in this work is to model the grafting of Cp2TiCl2 to surface silanols with the techniques of computational chemistry (density functional and force field methods). We show, in agreement with the earlier work from this laboratory,36 that (≡SiO)3TiCp rather than (≡SiO)2TiCp2 species are likely to be the first stable species formed after anchoring. Our calculations also show that, as a result of calcination, (≡SiO)3TiOH and (≡SiO)2Ti(OH)2 species are energetically more likely than (≡SiO)2TidO groups. The calculated structure of the (≡SiO)3TiOH active center is in excellent agreement with the EXAFS derived structure.36 However, the DFT and EXAFS structures for the (≡SiO)3TiCp intermediate are significantly different, and possible reasons for this are discussed. We also present the results of detailed calculations on the adsorption of H2O molecules at the tetrahedral Ti center of (≡SiO)3TiOH, (≡SiO)2Ti(OH)2, and (≡SiO)4Ti complexes (the latter being representative of Ti substituted into a zeotype framework). We show that, in accord with previous suggestions,22,30 the Ti-O-Si bridges are liable to hydrolyze to form (≡SiO)3TiOH, (≡SiO)2Ti(OH)2, and additional silanols. Results of calculations of the addition of water to the various possible TiO sites show, in agreement with experiment,22,27,30,31,37 that the coordination number of Ti in these systems can vary between 4 and 6 depending on the water partial pressure. The occurrence of an equilibrium between 4-coordinate (≡SiO)4Ti, (≡SiO)3TiOH, and (≡SiO)2Ti(OH)2 complexes and the variation in coordination of Ti with adsorbate concentration could explain the confusion concerning the nature and coordination number of © 1997 American Chemical Society
Ti Active Sites in Mesoporous Silicas
J. Phys. Chem. B, Vol. 101, No. 21, 1997 4233
Figure 1. DFT optimized geometries of the complexes relevant to the Ti active-site formation. Cp represents η5-C5H5-. The TiCp complex is modeled by (H3SiO)3TiCp, TiCp2 by (H3SiO)2TiCp2, Ti(OH)2 by (H3SiO)2Ti(OH)2, TiOH by (H3SiO)3TiOH, and TidO by (H3SiO)2Ti(dO)‚‚‚O(H)SiH3. Energy changes are in kJ mol-1.
Ti sites in materials prepared and studied by different methods and under different conditions.19-31 Method The nonlocal spin density formulation of approximate density functional theory (DFT)38 as implemented in the Cray Soft code DGauss39 has been used throughout. We employed the cluster approximation for all DFT calculations with dangling bonds being terminated by hydrogens. The Becke40 nonlocal exchange and the Perdew41 nonlocal correlation corrections to the local potential energy functional of Vosko, Wilk, and Nusair42 were used to calculate optimized geometries and energies of the species shown in Figure 1. These structures were chosen to represent possible intermediates formed during preparation of the surface titanium-oxygen catalysts studied by Maschmeyer et al.36 after anchoring (and subsequent calcination) of Cp2TiCl2 to silanols on an MCM-41 surface and correspond to the following reactions for initial anchoring
Cp2TiCl2 + 3(≡SiOH) f (≡SiO)3TiCp + 2HCl + CpH Cp2TiCl2 + 2(≡SiOH) f (≡SiO)2TiCp2 + 2HCl and to the following reactions for formation of the dehydrated TiO active site, i.e., calcination
(≡SiO)3TiCp + H2O f (≡SiO)3TiOH + CpH (≡SiO)2TiCp2 + 2H2O f (≡SiO)2Ti(OH)2 + 2CpH We also report the relative stability of TiOH and TidO surface species from calculation of the energetics involved in the following reaction
(≡SiO)3TiOH f (≡SiO)3(Od)Ti‚‚‚O(H)Si≡ where the TidO complex has a dative covalent bond between the Ti center and the O of an additional silanol. Hydrolysis
and hydration of the model TiO centers (see Figure 2) has been studied using the following reactions for hydrolysis
(≡SiO)4Ti + H2O f (≡SiO)3TiOH + ≡SiOH (≡SiO)3TiOH + H2O f (≡SiO)2Ti(OH)2 + ≡SiOH and the following reactions for hydration
(≡SiO)4Ti + H2O f (≡SiO)4Ti‚H2O + H2O f (≡SiO)4Ti‚2H2O (≡SiO)3TiOH + H2O f (≡SiO)3TiOH‚H2O + H2O f (≡SiO)3TiOH‚2H2O (≡SiO)2Ti(OH)2 + H2O f (≡SiO)2Ti(OH)2‚H2O + H2O f (≡SiO)2Ti(OH)2‚2H2O All calculations used a valence double-ζ basis set with polarization functions on heavy atoms only (DZVP). This basis set was specifically optimized for DFT calculations.43 In order to account for the likelihood that different intermediate and product species are favored at different surface sites, all three (in the notation of Figure 1) TiOH, Ti(OH)2, and TidO species were fully geometry optimized. The Ti(OSi)4 model (in the notation of Figure 2) used to represent Ti substituted into a zeolite framework was also fully optimized. The TiCp (in the notation of Figure 1) structure was then optimized with the Si fixed at the positions of the TiOH model and the TiCp2 species was optimized with the Si fixed at the positions of the Ti(OH)2 model. The Si centers of the TiOH, Ti(OH)2, and Ti(OSi)4 complexes were also kept fixed at their previously geometryoptimized positions (no water present) during the study of water adsorption at the Ti site. All other parameters were optimized. Thus, calculated energy differences are not reaction energies for equilibrium structures of the clusters shown in Figure 1,
4234 J. Phys. Chem. B, Vol. 101, No. 21, 1997
Sinclair et al.
Figure 2. DFT optimized geometries of the surface TiO species before and after hydration with one and two H2O molecules. The Ti(OSi)4 complex is modeled by (H3SiO)4Ti, TiOH by (H3SiO)2TiOH, and Ti(OH)2 by (H3SiO)2Ti(OH)2. Energy changes are in kJ mol-1.
but they do reflect more accurately the differences in energy of the species at energetically likely surface sites. In order to probe the effect of the electrostatic field of the extended surface on the TiCp complex, a 50 × 50 × 10 Å slab of an amorphous silica glass surface (generated from libraries within Molecular Simulations Inc.’s graphical interface, InsightII44) with the complex situated in the middle was studied with the extensible systematic forcefield (ESFF) and MSI’s DISCOVER-3.0 code.45 This force field uses atomic parameters coupled with explicit rules to generate the actual force field for a given molecular species. While this method of force field generation enables the ESFF method to be widely transferable, it involves little explicit fitting to specific systems and is therefore of limited accuracy for the prediction of absolute observables of a given system. We use the method to compare Ti-C distances of the TiCp complex modeled on a silica slab and as the fragment shown in Figure 1 only after both models have been geometry optimized using the ESFF method. For this reason, absolute structural parameters arising from ESFF calculations are not reported. Results and Discussion Energetics. The results summarized in Figure 1 show immediately that if only local binding effects are important in the anchoring of Cp2TiCl2 to surface silanols then there is an 89 kJ mol-1 preference for formation of the TiCp complex over formation of the TiCp2 species. In the case of the latter structure, the second Cp ligand will interact with a silanol and be expelled as CpH. We note that the anchoring process shown in Figure 1 represents the reaction of Cp2TiCl2 with isolated silanol species (modeled by H3SiOH). In the real system, these silanols would probably be stabilized by hydrogen bonds which must be broken before anchoring can occur. Assuming that in the real system each surface silanol is involved in only one hydrogen bond and using the estimate of the strength of this hydrogen bond reported by Bleiber and Sauer of -27 kJ mol-1 (for two interacting H3SiOH species in the gas phase),46 then
in order to form the TiCp complex by the process in Figure 1, an initial energy input of 81 kJ mol-1 is required while in order to form the TiCp2 complex, an initial input of only 54 kJ mol-1 is required. This reduces the energy difference between the two pathways to about 60 kJ mol-1, formation of the TiCp complex still being the most favorable. The comparison of the energies of the two anchoring pathways is further complicated by the fact that the TiCp and TiCp2 complexes contain different numbers of Ti-O-Si bridges. The contribution to the energetics from the energy of condensation of hydroxyl species to form Ti-O-Si bridges will therefore be different in the two cases, and it is possible that this effect leads to the different energy changes for the anchoring pathways. However, it has been shown47 that this condensation energy is of the order of only 0-20 kJ mol-1, and it is thus likely that the reason for the differences in energies of formation of the two surface Cp complexes is primarily a steric one. The calcination step of the reaction has been modeled by the formation of TiOH and Ti(OH)2 species from the TiCp and TiCp2 complexes respectively (see Figure 1). Formation of the Ti(OH)2 species from its precursor (TiCp2) is much more favorable than formation of TiOH species from TiCp, presumably due to the relief of steric strain. Due to the large preference for the initial formation of the TiCp complex, however, we predict that the TiOH species is the dominant one (it is more stable by 22 kJ mol-1). However, 22 kJ mol-1 is of a similar magnitude to the strength of silanol-silanol hydrogen bonds. Thus, if we account for the rupture of such a hydrogen bond before the Ti(OH) to Ti(OH)2 conversion could take place, the energy difference is small, and we predict that the two species probably coexist. The prediction of TiOH as a primary titanium-oxygen complex is in agreement with the EXAFS and XANES analysis.36 We note however that EXAFS cannot easily distinguish between the TiOH and Ti(OH)2 complexes. Additionally, Figure 1 shows that within the cluster approximation (which omits longer range interactions) there is a clear energetic preference for the TiOH and Ti(OH)2 species over the TidO
Ti Active Sites in Mesoporous Silicas complex, a species that was originally thought to be present in titanium-substituted silicates.49-52 However, the absence of a suitable absorption band in the UV-vis spectrum31 strongly suggests that the species is not present in appreciable concentration, unlike the situation in the mineral fresnoite52 and in the synthetic titanosilicate catalyst JDF-L1,53 in which TidO species have been unambigously characterized. Figure 2 shows the DFT optimized structures of the Ti(OSi)4, TiOH, and Ti(OH)2 complexes before and after hydration with one and two water molecules. At first it appears that the Ti(OSi)4 complex is the most stable 4-coordinate form of Ti, probably as a result of the larger number of T-O-T bridges.47 However, conversion between each 4-coordinate form involves a silanol, which, as discussed above, would probably be hydrogen bonded to other silanols or surface oxygens in a real system. Using, as before, the estimate of 27 kJ mol-1 for the strength of these hydrogen bonds,46 which must be broken prior to reaction of the silanol, we see that there is little energy difference between the three 4-coordinate Ti complexes. Thus, we predict that Ti subtituted into a zeotype framework, as in TS-1, would readily hydrolyze to form TiOH and Ti(OH)2 species with neighboring silanol groups as suggested in refs 22 and 30 as a means to relieve the strain imposed on the framework when Ti is substituted for Si. We note that there is unlikely to be a low-energy pathway to form Ti(OSi)4 from either the TiOH or Ti(OH)2 species when Ti is grafted onto the surface since suitably positioned silanols would probably be unavailable. Figure 2 also shows that in the presence of water the coordination shell around Ti expands. Little can be inferred from the relative binding energies of water in the different configurations as they are too similar, given the errors implicit in the model, e.g., the use of the cluster approximation. However, it is clear that only in the complete absence of water will pure 4-coordinate Ti be observed and only in the limit of a saturated system will 6-coordinate Ti be observed. Intermediate regimes of water partial pressures will lead to an equilibrium of Ti species with coordination numbers varying between 4 and 6. Conversion between the 4-coordinate Ti species and their subsequent hydrated structures could explain the previous uncertainty concerning the nature and coordination numbers of Ti centers in framework19-31 and grafted materials (as discussed in this work) prepared in different ways and studied with different techniques and under different conditions. Geometries. Table 1 shows selected geometrical parameters for the optimized structures shown in Figure 1 (and Ti(OSi)4 from Figure 2). Also listed are the geometries found by the EXAFS study.36 The observed geometries of the Cp2TiCl2 precursor and the TiOH species are in good agreement with the current calculated values. Previous theoretical studies of related Ti-O species also agree well with the current work with reported Ti-OSi distances of about 1.79-1.83 Å.32,33,59,60 The current estimate of TidO of 1.65 Å is also close to other theoretical60 and experimental53 values. The calculated values of the Ti-C distance in the TiCp complex of 2.43-2.44 Å, however, are in marked disagreement with the reported value of 1.99 Å. The Ti-Si distances for this species also show substantial differences due to the large ∠TiOSi angle in the optimized structure. Full relaxation of the TiCp model (removing restraints on the Si atoms) did little to remove the disagreement. Given the good accord between the EXAFS derived and the DFT calculated Ti-Si distances for the TiOH complex, the disagreement between experiment and theory for the Ti-Si distances of the TiCp complex is possibly due to the fact that the EXAFS measurement is for a species
J. Phys. Chem. B, Vol. 101, No. 21, 1997 4235 TABLE 1: DFT Optimized Geometries for the Models Used To Describe the Ti Active-Site Formation: Distances Are in Å; Angles Are in deg model Cp2TiCl2 (H3SiO)3TiCp
(H3SiO)2TiCp2
(H3SiO)3TiO′H
(H3SiO)2Ti(O′H)2
(H3SiO)2TidO′‚‚‚O′′(H)SiH3
(H3SiO)4Ti
parameter
DFT
EXAFS36
Ti-C Ti-Cl C-C Ti-C Ti-O Si-O Ti-Si C-C ∠Ti-O-Si Ti-C Ti-O Si-O C-C ∠Ti-O-Si Ti-O′ O′-H Ti-O Si-O Ti-Si ∠Ti-O-Si ∠Ti-O′-H Ti-O′ Ti-O Si-O ∠Ti-O-Si ∠Ti-O′-H Ti-O′ Ti-O Ti-O′′ Si-O Ti-O Si-O ∠Ti-O-Si
2.42-2.48 2.35 1.41-1.43 2.43-2.44 1.83-1.84 1.66-1.67 3.42-3.45 1.42-1.43 154-162 2.43-2.50 1.89-1.90 1.65 1.41-1.44 160-162 1.82 0.98 1.81-1.82 1.67-1.68 3.35-3.42 148-158 123 1.82 1.82 1.67-1.68 137-158 123-125 1.65 1.85 2.13 1.66 1.81 1.68 144-153
2.39 2.33 1.99 1.82 3.14
1.81 1.81 3.30
formed under kinetic control and is therefore strained (small TiOSi angle). Calcination at high temperatures to produce the TiOH from the TiCp complex will allow some structural relaxation to occur around the Ti center. We might therefore expect that, as found, the TiOH structure, being more thermodynamical stable, should be comparable to the DFT energyminimized structure. Experimentally, a reduction in the metal-Cp distance has been observed a number of times when the species is chemically anchored on surface silanols.54,55 However, examination of X-ray structures in the Cambridge Structural Data Base56 did not reveal any Ti-Cp complexes with such extremely short TiCp distances. A number of factors could be responsible for these short distances including hydrogen bonding of the Cp ligand to nearby silanols and the high electronegativity of surface SiO4 groups leading to a perturbation in the Ti-Cp bonding on anchoring.54,55 In order to probe the effect of increasing the electronegativity of the SiO- groups to which the TiCp complex is anchored, one of the H atoms on each of the H3SiO- groups in the (H3SiO)3TiCp model was replaced by F. With the Si centers kept fixed as before, this cluster was reoptimized. Little effect on the Ti-Cp distance was observed. This result is not surprising since the highest occupied molecular orbital (HOMO) of the (H3SiO)3TiCp species is a Ti-Cp bonding orbital. All other factors being equal, polarization of the HOMO away from the metal-Cp region (i.e., by increasing the electronegativity of the surface) is expected to increase the bonding distance rather than to decrease it. The possibility of hydrogen bonding of the cyclopentadienyl ligand to nearby silanols was probed using the model shown in Figure 3. The originally optimized (H3SiO)3TiCp complex was
4236 J. Phys. Chem. B, Vol. 101, No. 21, 1997
Sinclair et al.
a
Figure 3. DFT optimized model used to probe the effect of hydrogen bonding of the cyclopentadienyl ring to surface silanol groups on the Ti-Cp distance. The positions of Si1 and Si2 were kept fixed at the positions of the “TiOH” model. The dashed lines highlight the [C-H‚‚‚O(H)Si] hydrogen bond.
b
Figure 4. ESFF force field optimized amorphous silicate slab with a (-OSi)3TiCp complex substituted in the center (represented as a CPK model).
used as a basis for the model, and optimization proceeded with the two Si centers that are not involved in the hydrogen bond being fixed. Little effect on the coordination of the ring to the metal center was observed. Another possibility for the observed short distance could be long range electrostatic effects which were not included in the cluster calculations. In order to model this effect a 50 × 50 × 10 Å slab of amorphous silica glass was constructed (as summarized in the Methods section) with the TiCp complex substituted at the center, as shown in Figure 4. A 10 Å radius of the slab centered at the titanium atom was optimized using the ESFF force field.45 Since this force field was not paramterized specifically for this type of system and since we are only interested in the specific effect of the long range electrostatics, the fragment (H3SiO)3TiCp was also optimized with the ESFF force field for comparison. Both the slab and the fragment calculations led to similar Ti-Cp distances; i.e., no bond length reduction was evident. The slab calculations did, however, lead to the observation that due to steric interactions of the ring with nearby surface silanols, the cyclopentadienyl ring could not always be exactly η5 bonded. Indeed, with this simple method, variations of up to 0.1 Å were observed for the Ti-C distances in a single Ti-Cp complex. Finally, the Ti‚‚‚OH2 distances of the complexes shown in Figure 2 varied from 2.24 to 2.47 Å. The change in the Ti coordination number (from 4 to 6) was accompanied by an increase in the Ti-OSi distances of up to 0.14 Å and by an increase in the Ti-OH distances of up 0.1 Å as observed spectroscopically by Boccuti et al.31 and Zecchina et al.37 Comparison of DFT and EXAFS Data for the TiCp Complex. Although a reduction of the Ti-C distances of the TiCp complexes could not be achieved by any of the abovementioned computational approaches, it is evident that only one Cp ligand is attached to the titanium after anchoring, as observed by the EXAFS investigation.36 In order to highlight the difference between the DFT optimized cluster representing the TiCp complex and the experimental results, we have plotted
Figure 5. (a) Comparison of EXAFS data for the grafted TiCp complex; (b) comparison of the Fourier transforms of the EXAFS data. In both cases, solid lines show the experimental data36 and dotted lines show the simulated data based on the DFT structure.
experimental and simulated EXAFS based on the DFT calculated structure in Figure 5a. Figure 5b shows the Fourier transforms (FTs) of the EXAFS data. The EXAFS was simulated using the atom types, bond distances, and bond angles from the DFT model and the Debye-Waller factors from the experimental fit reported in ref 36. It is clear from Figure 5 that there are considerable differences between the two sets of data. Refitting of the experimental EXAFS data36 with a split carbon shell and using the EXAFS fitting code Xfit61 resulted in a fit with three Ti-C bonds of 1.96 Å and two at 2.52 Å, Debye-Waller factors of 0.014 and 0.08, respectively, and an R factor of 0.17. While a split shell was suggested by the force field calculations, the EXAFS derived structure based on a split C shell is still substantially different from the DFT structure. It is also possible that the experimental EXAFS data contains contributions from water coordinated to the TiCp complex or that one or more of the TiOSi bridges of some of the complexes are hydrolyzed. Initial studies suggest that such factors could indeed lead to artifacts in the Ti-Cp distance when using a model exclusively based on the dominant species. Clearly, more work is required to characterize fully all of the intermediate surface species. Nevertheless, our calculations have almost certainly established that the previously reported, unusually short Ti-C bond results from use of an oversimplified model. Conclusions Through the use of the techniques of computational chemistry we have rationalized and amplified the structural information in earlier experimental work from this laboratory.36 We have confirmed that (≡SiO)3TiCp is more likely to form than (≡SiO)2TiCp2 on initial anchoring of the titanocene dichloride
Ti Active Sites in Mesoporous Silicas precursor to the surface of MCM-41. We have also been able to show that as a result of calcination of the TiCp complex, the dominant surface TiO species is likely to be the TiOH complex (or an equilibrium of this and the Ti(OH)2 complex) and that TidO species are unstable in the absence of extra stabilizing species. On the basis of DFT cluster calculations, we have shown that the previously reported contraction of metal-Cp distances of metallocene-derived precursors on anchoring to silanolated surfaces is not due to the high electronegativity of the surface, nor due to hydrogen bonding of the cyclopentadienyl ring with nearby silanol groups, nor to geometry constraints imposed by the surface. ESFF force field calculations also showed that the bond length reduction could not be attributed to longer range electrostatic effects. The force field calculations did, however, suggest that the cyclopentadienyl ring is not exactly η5 bonded to Ti due to steric interactions of the ring with nearby silanols. An exact model to explain the discrepancy between the EXAFS and DFT structures for this intermediate remains to be fully defined. The calculations were able to verify that partial hydrolysis of Ti-O-Si bridges in framework-substituted silicas is energetically feasible. Thus, a mixture of Ti(OSi)4, TiOH, and Ti(OH)2 species, all of which can expand their coordination shell to incorporate more water molecules, could explain the previous uncertainties concerning the nature and coordination number of non-extra-framework Ti centers in microporous and mesoporous silicas. Acknowledgment. We are grateful to EPSRC for support of this work, and in particular for a studentship to one of the authors (P.E.S.). References and Notes (1) Notari, B. AdV. Catal. 1996, 41, 253. (2) Taramasso, M.; Perego, G.; Notari, B. U.S. Patent 4410501, 1983. (3) Tuel Z. Zeolites 1995, 15, 236-242. (4) Serrano, D. P.; Li, H. X.; Davis, M. E. J. Chem. Soc., Chem. Commun. 1992, 745-747. (5) Reddy, K. M.; Kaliaguine, S.; Sayari, A.; Ramaswamy, A. V.; Reddy, V. S.; Bonneviot, L. Catal. Lett. 1993, 23, 175-187. (6) Cambior, M. A.; Corma, A.; Perez-Pariente, J. Zeolites 1993, 13, 82-87. (7) Ulagappan, N.; Krishnasamy, U. J. Chem. Soc., Chem. Commun. 1995, 373-374. (8) Tuel, A.; Ben-Taarit, Y. J. Chem. Soc., Chem. Commun. 1994, 1667-1668. (9) Thomas, J. M. Nature 1994, 368, 289. (10) Thomas, J. M.; Greaves, G. N. Science 1994, 265, 1675. (11) Thomas, J. M. Philos. Trans. R. Soc. London A 1990, 333, 173207. (12) Reddy, J. S.; Kumar, R.; Ratnasamy, P. Appl. Catal. 1990, 58, L1. (13) Thangaraj, A.; Kumar, R.; Mirajkar, S.; Ratnasamy, P. J. Catal. 1990, 130, 1. (14) Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 1. (15) Khouw, C. B.; Dartt, C. B.; Li, X.; Davis, M. E. Symposium on New Catalytic Chemistry Utilizing Molecular SieVes; 206th National Meeting of the American Chemical Society, Chicago, IL, 1993; American Chemical Society: Washington, DC, 1993. (16) Huybrechts, D. R. C.; Buskens, Ph. L.; Jacobs, P. A. Stud. Surf. Sci. Catal. 72, 21. (17) Clerici, M. G.; Bartolomeo, A.; Bellussi, G. Eur. Pat. Appl. 412596, 1991. (18) Sudhakar Reddy, J.; Sivasanker, S. Catal. Lett. 1991, 11, 241. (19) Sankar, G.; Rey, F.; Thomas, J. M.; Greaves, G. M.; Corma, A.; Dobson, B. R.; Dent, A. J. J. Chem. Soc., Chem. Commun. 1994, 2279.
J. Phys. Chem. B, Vol. 101, No. 21, 1997 4237 (20) Pei, S.; Zajac, G.; Kaduk, J.; Faber, J.; Boyanov, B.; Duck, D.; Fazzini, D.; Morrison, T.; Yang, D. Catal. Lett. 1993, 21, 333. (21) Trong On, D.; Bittar, A.; Sayari, A.; Kaliaguine, S.; Bonneviot, L. Catal. Lett. 1992, 16, 85. (22) 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, 4125-4132. (23) Lopez, A.; Tuilier, M.; Guth, J.; Delmotte, L.; Popa, J. J. Solid State Chem. 1993, 102, 480. (24) Behrens, P.; Felsche, J.; Vetter, S.; Schulz-Ekloff, G.; Jaeger, N.; Niemann, W. J. Chem. Soc., Chem. Commun. 1991, 678. (25) Trong On, D.; Bonneviot, L.; Bittar, A.; Sayari, A.; Kaliaguine, S. J. Mol. Catal. 1992, 74, 233. (26) Deo, G.; Turek, A.; Wachs, I.; Huybrechts, D.; Jacobs, P. Zeolites 1993, 13, 365. (27) Scarano, D.; Zecchina, A.; Bordiga, S.; Geobaldo, F.; Spoto, G.; Petrini, G.; Leofanti, G.; Padovan, M.; Tozzola, G. J. Chem. Soc., Faraday Trans. 1993, 89, 4123. (28) Dartt, C.; Khouw, C.; Li, H. X.; Davis, M. Microporous Mater. 1994, 2, 425. (29) Blasco, T.; Camblor, M.; Corma, A.; Perez-Pariente, J. J. Am. Chem. Soc. 1993, 115, 11806. (30) Geobaldo, F.; Bordiga, S.; Zecchina, A.; Giamello, E.; Leofanti, G.; Petrini, G. Catal. Lett. 1992, 16, 109. (31) Boccuti, M. R.; Rao, K. M.; Zeccina, A.; Leofanti, G.; Petrini, G. Structure and ReactiVity of Surfaces; Morterra, C., et al., Eds.; Elsevier; Amsterdam, 1989; p 133. (32) de Man, A. J. M.; Sauer, J. J. Phys. Chem. 1996, 100, 5025-5034. (33) Jentys, A.; Catlow, C. Catal. Lett. 1993, 22, 251. (34) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321323. (35) Corma, A.; Navarro, M. T.; Perez-Pariente, J. J. Chem. Soc., Chem. Commun. 1994, 147-148. (36) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159-162. (37) Zecchina, A.; Spoto, G.; Bordiga, S.; Geobaldo, F.; Leofanti, G.; Petrini, G.; Padovan, M.; Mantegazza, M.; Roffina, P. Proceedings of the 10th International Congress on Catalysis, Budapest, July 19-24, 1992; p 105. (38) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1991, 96, 1280. (39) DGauss 3.0, Cray Research Inc., 1995. (40) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (41) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (42) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Chem. 1980, 58, 1200. (43) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. (44) The molecular graphics package InsightII, Molecular Simulations Inc., San Diego, CA, 1995. (45) Discover 3 and the ESFF Forcefield, Molecular Simulations Inc., San Diego, CA, 1995. (46) Bleiber, A.; Sauer, J. Chem. Phys. Lett. 1995, 238, 243-252. (47) Catlow, C. R. A.; George, A.; Freeman, C., in press. (48) Jorgensen, K. A. Chem. ReV. 1989, 89, 431. (49) Sheldon, R. A.; van Doorn, J. A. J. Catal. 1973, 31, 427-437. (50) Sheldon, R. A.; van Doorn, J. A.; Schram, C. W. A.; de Jong, A. J. J. Catal. 1973, 31, 438-443. (51) Sheldon, R. A. J. Mol. Catal. 1980, 7, 107-126. (52) Markgraf, S. A.; Halliyal, A.; Bhalla, A. S.; Newnham, R. E.; Prewitt, C. T. Ferroelectrics 1985, 62, 17. (53) Roberts, M. A.; Sankar, G.; Thomas, J. M.; Jones, R. H.; Du, H.; Chen, J.; Pang, W.; Xu, R. Nature 1996, 381, 401. (54) Maschmeyer, T.; Masters, A. F.; Smith, A. K.; Joyner, R. J. Mol. Catal., in press. (55) Sankar, G.; Maschmeyer, T.; Oldroyd, R.; Troeger, L.; Thomas, J. M., in preparation. (56) Cambridge Structural Data Base, Daresbury, UK. (57) Sankar, G.; Bell, R. G.; Thomas, J. M.; Anderson, M. W.; Wright, P. A.; Rocha, J. J. Phys. Chem. 1996, 100, 449. (58) Lewis, D. W.; Catlow, C. R. A.; Sankar, G.; Carr, S. W. J. Phys. Chem. 1995, 99, 2377. (59) Millini, R.; Perego, G.; Seiti, K. Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier: Amsterdam, 1994, Vol. 84, pp 2123-2129. (60) Neurock, M.; Manzer, L. E. J. Chem. Soc., Chem. Commun. 1996, 1133-1134. (61) Ellis, P. Y.; Freeman, H. C. J. Synchrotron Radiat. Res. 1995, 2 (Pt 4), 190.
