Do titanyl groups exist in titanium silicates? An experimental study

Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 351–368

Do titanyl groups exist in titanium silicates? An experimental study Mark Crocker *, Ruud H.M. Herold, Bert G. Roosenbrand, Kees A. Emeis, Antonio E. Wilson Shell Research and Technology Centre, Amsterdam, Postbus 38000, 1030 BN Amsterdam, The Netherlands Received 13 December 1997; accepted 25 March 1998

Abstract Two synthetic routes have been investigated, aimed at the preparation of silica-supported titanyl (>TiNO) and titanol (>Ti(OH ) ) groups, the latter corresponding to the hydrated form of the titanyl group. In the first synthetic 2 route, the titanyl complex TiOCl (NMe ) was reacted with an aerosil, and the resulting material thermally treated 2 32 to remove residual Cl and NMe ligands. In an alternative route, silica (aerosil and silica-gel ) was reacted with 3 Ti(CH Ph) to afford mainly anchored >Ti(CH Ph) moieties, which were subsequently hydrolysed. Characterization 2 4 2 2 of the resulting materials using a combination of surface analytical techniques revealed that in all cases at least two titania phases were obtained, corresponding to isolated tetrahedral Ti sites, and an amorphous form of TiO containing 2 six-coordinate titanium. For the syntheses based on Ti(CH Ph) , UV-vis and XPS data indicated that the relative 2 4 proportion of the two phases formed was dependent on the support employed, aerosil affording predominantly (¬SiO) Ti(OH ) sites. No evidence was found for the presence of three-coordinate titanyl species, >TiNO, even 2 2 when the aerosil-supported >Ti(OH ) sites were calcined at 500°C. It is, therefore, concluded that titanyl groups are 2 unlikely to be present in significant concentrations in titanium silicates. When tested in the epoxidation of 1-octene with tert-butyl hydroperoxide, the model systems were found to display epoxidation activity comparable with that of a wide-pore Ti–zeolite, Ti-MCM-41. The observed turnover frequency was found to increase with increasing dispersion of the titania, consistent with the notion that isolated, Lewis acidic Ti(IV ) centres are the most active sites for epoxidation catalysis. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Catalyst; Epoxidation; Titania; Titanium silicates; Titanyl

1. Introduction Identification of the active site in heterogeneous catalysts is a task which is typically complicated by the presence of a broad spectrum of chemically inequivalent sites. For this reason, when studying heterogeneous catalysts it is frequently more instructive to prepare model catalysts, containing * Corresponding author. Tel: 0031 206303749; Fax: 0031 206303110. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 37 1 - 9

a chemically homogeneous dispersion of one type of site. Direct correlations can then be drawn between the activity of the particular site and its structure. In this context, we are interested in the preparation of well-defined chemical models for the active site in titanosilicates such as the Enichem Ti–silicalite ( TS-1) oxidation catalyst [1]. By comparing the physico-chemical (and especially catalytic) properties of model sites with those of Ti–zeolite catalysts, identification of the active site(s) present in the latter may be facilitated.

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The catalytic properties of TS-1, in which Si is isomorphously substituted by Ti in the MFI structure, are of considerable scientific and commercial interest [2]: using H O as oxidant, TS-1 is active 2 2 for a range of oxidation reactions including the selective epoxidation of olefins to epoxides, the oxidation of primary alcohols to aldehydes and secondary alcohols to ketones, the hydroxylation of aromatics, the oxidation of paraffins to alcohol/ketone mixtures and the ammoxidation of ketones. The remarkable properties of TS-1 have in turn led to renewed interest in the catalytic properties of amorphous titania–silica mixed oxides [3–6 ] and amorphous forms of titania supported on silica [7–10], as well as to research directed at the synthesis of large [11,12] and ultralarge pore Ti–zeolites [13,14]. In its IR spectrum, TS-1 shows a characteristic absorption band at about 960 cm−1 which is not present in the pure silicalite spectrum, and is absent in titanium oxides (rutile, anatase) and alkaline titanates. Significantly, the position of this band is similar to that found for amorphous titania deposited on silica, for which it has been proposed that the active site for epoxidation catalysis comprises surface titanyl ( TiNO) groups [15]. Support for this type of titanium site is provided by published IR spectral data on molecular titanyl complexes which characteristically show an absorption band in the region 930–980 cm−1 [16 ]. On this basis it was originally suggested that the active site in crystalline Ti/Si mixed oxides such as TS-1 may correspond to an isolated titanyl group [17,18] (structure I in Scheme 1) which undergoes reaction with added hydrogen peroxide; in the case of amorphous mixed oxide catalysts a peroxo-metal species would similarly be formed by reaction of the titanyl group with organic hydroperoxides. Note that in the presence of water, the titanium–oxygen double bond might be expected to undergo an addition reaction, affording a Ti(OH ) species (II ); this type of site can therefore 2 be regarded as the hydrated form of the titanyl group. In contrast, Zecchina and co-workers [19] have argued that the 960 cm−1 absorption of TS-1 corresponds to a vibration associated with framework [ TiO ] and [ TiO OH ] units or [SiO ] and 4 3 4

Scheme 1. Possible transformations of the titanyl group.

[SiO OH ] units perturbed by the presence of Ti 3 (or possibly a mixture of the two). As such, the 960 cm−1 band is representative of tetrahedrally coordinated Ti lattice sites, i.e. Ti(OSi¬) , a con4 clusion supported by the results of numerous other recent studies [2,20]. We were nevertheless intrigued by the possible existence of surface titanyl groups in titanium silicates and the chemistry that might be expected for them. Herein we report on studies aimed at the preparation of model systems containing sites I and II, and an investigation of the physicochemical properties of the resulting materials.

2. Experimental methods 2.1. Materials Aerosil TT600 was obtained from Degussa (specific surface area, S , of 149 m2/g after BET calcination at 200°C ), silica-gel from GraceDavison (S =319 m2/g after calcination) and BET anatase from Rhone-Poulenc (G5; S = BET 122 m2/g). Oxygen-17 enriched water (26 at.%) was obtained from Campro and was degassed before use by repeated freeze–pump–thaw cycles using a high vacuum line. Oxygen-18 enriched water (95 at.%) was purchased from Aldrich and was similarly degassed. Toluene (99.5%), isooctane

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(99.5%), hydrogen peroxide (35% w/v) and 1-octene (98%) were purchased from Merck. ˚ molecular sieves and 1-Octene was dried over 4 A was checked for purity before each experiment using GLC. Anhydrous tert-butylhydroperoxide ( TBHP, ~3 M in isooctane) was purchased from ˚ molecular sieves. Fluka and was stored over 4 A Diethyl ether and THF were distilled from purple solutions of sodium benzophenone ketyl. Toluene was distilled from potassium metal and pentane from purple solutions of potassium benzophenone ketyl containing tetraglyme. Dichloromethane was dried over CaH . All solvents were distilled 2 under nitrogen immediately prior to use. TiOCl (NMe ) [21], Ti(CH Ph) [22] and 2 32 2 4 Ti-MCM-41 [13] were synthesized according to literature methods.

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high-speed Bruker 4 mm (double-bearing) MAS probe and zirconia rotors. Samples were spun at a spinning speed of 11 kHz, using dry nitrogen as the drive and bearing gas. Chemical shifts were measured relative to tetramethylsilane as an external standard. Typically, 800 transients were acquired using a small pulse angle (1.2 ms pulse width), a recycle delay of at least 4 s and a 20 kHz spectral width. In all cases a background signal, run under identical conditions with an empty rotor, was subtracted from the spectra. 17O MAS NMR spectra were collected on a Bruker MSL-300 spectrometer using a pulse length of 1.5 ms, a MAS spin rate of 5 kHz and a spectral window of 83 kHz. Samples were dried at 160°C under vacuum (10−5 mmHg) for a minimum of 20 h and were then loaded in a glove-box into 4 mm zirconia rotors for the MAS NMR measurements.

2.2. Physical measurements 2.2.1. General Elemental analysis of samples for Ti and Cl was carried out using wavelength dispersive X-ray fluorescence ( WDX ) spectroscopy on a Philips PW 1480 spectrometer. Pyrolysis combustion mass spectrometric element analyses (PCME) were performed employing a Balzers quadrapole mass spectrometer Type 511. X-ray powder diffraction analyses ( XRD) were performed on a Philips PW 1800 diffractometer using Cu Ka radiation. UV-vis spectra were collected on a Perkin Elmer 320 spectrometer operating in the diffuse reflectance mode. X-ray photo-electron spectroscopy ( XPS) spectra were recorded on a Kratos XSAM 800 spectrometer employing the C 1s line (284.6 eV ) as a standard. Infra-red spectra were recorded on thin selfsupporting wafers, diameter 18 mm, prepared in a nitrogen-filled glove-box from 25 mg of sample. Samples were first outgassed (10−5 mbar) in the spectrometer for 2 h (with heating as appropriate). All measurements were performed using a Mattson Cygnus 100 spectrometer at a resolution of 2 cm−1. Solid-state NMR spectra were acquired under magic angle spinning (MAS) on a Bruker AMX-500 spectrometer, operating at 500.13 MHz. The NMR experiments were performed using a

2.2.2. ISS measurements Low-energy ion scattering (LEIS) measurements were carried out at the Technical University Eindhoven ( TUE) using the facility called NODUS [23]. Samples were converted to pellets by pressing the powder in a Ta sample holder (1 cm diameter). In the LEIS experiments He+ ions were used with a primary energy of 3 keV and a beam current of ~120 nA. The beam current was regularly checked by using a Cu sample. The complicating effects of surface charging were prevented by using a ring-shaped electron shower [24], which flooded the samples with ~10 eV electrons. 2.2.3. Calculation of surface Ti concentrations from XPS and WDX data Surface Ti concentrations were calculated from XPS data using the expression: Ti atoms/ nm2=70×mol fraction Ti, where mol fraction TiNTi atomic concentration/100, and the figure of 70 represents the approximate number of atoms in a 1 nm2 surface ‘‘slab’’, four to five atom layers thick (the approximate mean escape depth of the photoelectrons analysed ). Surface Ti concentrations were also calculated on the basis of elemental analysis ( WDX ) data, combined with the surface area (S.A., in m2/g) of

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the support: Ti atoms/nm2=(wt.% Ti×6.02×103)/ (S.A.×47.88). The WDX results have a relative accuracy of ±10%, while the relative accuracy of atomic surface concentrations determined using XPS is in the order of ±5%. 2.3. Synthetic procedures Unless otherwise indicated, all manipulations were conducted in an inert atmosphere of argon using standard Schlenk and high vacuum line techniques, or in a nitrogen-filled Braun MB 200 glove-box. 2.3.1. Calcination of starting materials Silica supports were calcined for 4 h in an oven maintained at 200°C under an atmosphere of 50% N and 50% O . Afterwards the material was 2 2 flushed with N for 1 h at 200°C and then transfer2 red to the glove-box while still hot. 2.3.2. Preparation of model catalyst (A1/C) from TiOCl (NMe ) 2 3 2 Dichloromethane (50 ml ) was added to a mixture of aerosil (4.00 g) and TiOCl (NMe ) 2 32 (0.303 g, 1.20 mmol ) and the mixture refluxed vigorously for 24 h. The hot mixture was then filtered and washed with dichloromethane (5×50 ml ) and pentane (3×50 ml ) and dried under vacuum overnight. The material was then calcined for 3 h at 350°C in 100% O . After cooling 2 the product was flushed with nitrogen and transferred to the glove-box. 2.3.3. Preparation of model catalysts (A2, SG2) from Ti(CH Ph) 2 4 Pentane (150 ml ) was added to a cooled flask (0°C ) containing a mixture of aerosil or silicagel (10.0 g) and an appropriate quantity of Ti(CH Ph) (based on an approximate stoichio2 4 metric Ti:SiOH ratio of 1:2 and an assumed SiOH concentration of approximately 2.5 and 4.5 nm−2 for the aerosil and silica-gel supports, respectively). After stirring for 6 h at 0°C the silica was isolated

by filtration, washed with pentane (5×30 ml ) and dried under vacuum. Diethyl ether (150 ml ) and degassed water (three-fold molar excess based on titanium content) were added and the mixture stirred for 6 h. The mixture was filtered and the solid washed with diethyl ether (3×50 ml ) and dried under vacuum. 2.4. Epoxidation procedure Epoxidation tests were performed batch-wise in a magnetically stirred 250 ml glass reactor, equipped with a condenser, thermometer probe and septum for withdrawing samples. All runs were performed under an atmosphere of dry nitrogen. Typically, toluene (3 g, 0.03 mol as ISTD), 1-octene (73 g, 0.6 mol ), a quantity of catalyst (equivalent to 0.2 mmol of Ti) and a stirrer bar were placed in the reactor. The mixture was warmed to 80°C and 10 ml of TBHP solution added via syringe. Immediately a sample was taken for analysis (GLC and iodometric titration), further samples for analysis being taken at regular intervals. GLC analyses were performed on a Hewlett-Packard HP 5890 instrument, with flame ionization detection (FID), a 25 m×0.32 mm (0.52 mm film thickness) HP-1 (cross-linked methyl silicone gum) fused silica capillary column, and helium as carrier gas. An injection temperature of 140°C was employed, which was found to be sufficiently low to avoid the occurrence of secondary reactions in the injection port. TBHP was determined by iodometric titration with sodium thiosulphate.

3. Results 3.1. Preparation of model catalysts employing TiOCl (NMe ) as a precursor 2 3 2 3.1.1. Adsorption of TiOCl (NMe ) onto aerosil 2 3 2 Two synthetic routes aimed at the preparation of anchored titanyl complexes were studied in this work, the first of these relying on grafting of the titanyl complex TiOCl (NMe ) onto silica. 2 32 TiOCl (NMe ) , first synthesized by Wallon and 2 32 co-workers [21], possesses a strong band at

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976 cm−1 in its IR spectrum, consistent with the presence of a terminal TiNO group. The results of a single crystal X-ray analysis [26 ] are reported to indicate a trigonalbipyramidal structure for the complex, the titanium–oxygen bond length of ˚ falling at the lower end of the range 1.613(11) A of values reported for other molecular titanyl ˚ ) [27]. complexes (typically 1.61–1.66 A Treatment of aerosil (hereafter denoted as A) with approximately one molar equivalent of TiOCl (NMe ) (assuming a reaction stoichiome2 32 try of 1 Ti:2 SiOH and a surface silanol concentration of approximately 2.5 per nm2 [28]) in refluxing dichloromethane afforded a material (A1) containing 1.44 wt.% Ti (by WDX ). This loading, corresponding to a surface titanium concentration of 1.2 Ti atoms/nm2, is consistent with complete uptake of the titanium complex. Elemental analysis and XPS data also revealed the presence of chlorine in the product, the atomic Cl/Ti ratio determined by elemental analysis being 2.0, while a ratio of 1.8 was measured using XPS. On this basis, it appears that the TiOCl (NMe ) 2 32 starting material is adsorbed intact onto the support. The IR spectrum of A1 (Fig. 1) reveals: (a) the presence of residual silanol groups (note that the main silanol band has shifted to a lower frequency, approximately 3250 cm−1, suggesting the involvement of hydrogen bonding, and that no isolated silanol groups can be observed at 3740 cm−1); (b) the absence of a band at 976 cm−1 due to the TiNO group; and (c) the continued presence of at least one NMe group, as indicated by the presence 3 of a band at 1480 cm−1 (CH deformation vibra3 tion). Subtraction of the IR spectrum of the aerosil support from that of A1 confirms the presence of H-bonded silanol groups absorbing in the region 3000–3500 cm−1, and a simultaneous decrease in the intensity of the absorptions in the region 3600–3740 cm−1. Two species which would appear to be consistent with the above data are depicted in Scheme 2. Interaction of the highly nucleophilic titanyl oxygen with the hydrogen of a surface silanol group would result in a lowering of the TiNO stretching frequency to a position where it could easily be masked by other absorption bands.

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Simultaneously, the OH stretch of those silanol groups interacting with the titanyl group would be expected to show a large shift to lower wavenumber, as observed. 3.1.2. Calcination of adsorbed TiOCl (NMe ) 2 3 2 Thermal treatment (350°C under O ) of 2 adsorbed TiOCl (NMe ) resulted in the formation 2 32 of new surface species. Thermogravimetry and PCME data revealed that two processes occur, corresponding to elimination of HCl (30–100°C ), followed by loss of NMe (100–350°C; see 3 Table 1). The complete loss of NMe from the 3 surface Ti groups was confirmed by the results of in situ IR measurements ( Fig. 2), the band at 1480 cm−1 in A1 disappearing upon heating at 350°C in air. Calcination also resulted in the loss of the most strongly hydrogen bonded silanol groups (~3500–3000 cm−1); the simultaneous increase in the intensity of the band corresponding to isolated silanol groups (3740 cm−1) suggests that some of the H-bonded silanol groups are converted to isolated groups, presumably as a consequence of the reaction of their H-bonded neighbours with the adsorbed titanium complex. No significant changes were observed in the region 800–1000 cm−1, where an absorption corresponding to a titanyl group might be expected. Similarly, Raman spectra obtained on samples calcined in situ were found to be featureless in the region 800–1100 cm−1. Elemental analysis and XPS data for the calcined material (A1/C ) are summarized in Table 2. These show the presence of residual chlorine (Cl/Ti#0.2), while a comparison of the measured surface Ti concentration (0.6 Ti atoms/nm2, by XPS) with that calculated from WDX data (1.3 Ti atoms/nm2) indicates that the Ti is rather poorly dispersed. This conclusion is supported by the measured Ti 2p binding energy (see Fig. 3), 3/2 which at 459.2 eV, is somewhat below that expected for atomically dispersed Ti(IV ) (approximately 459.8 eV, as reported for TS-1 [29]; note that the Ti 2p binding energy of anatase corres3/2 ponds to a value of 458.3 eV ). Additionally, the X-ray powder diffractogram of A1/C, while showing it to be largely amorphous, also reveals that anatase is present in low concentration (Fig. 4).

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Fig. 1. IR spectrum of: (a) A1; (b) aerosil support; (c) difference spectrum (a–b).

Fig. 2. IR spectra pertaining to the in situ calcination of A1: (a) at 20°C in vacuum; (b) after calcination at 350°C (measured under vacuum); (c) difference spectrum (b–a).

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Scheme 2. Preparation of model catalysts from TiOCl L (L=NMe ). 2 2 3 Table 1 PCME data for A1 Element

Weight loss calculated (%)

Weight loss observed (%)

C H N Cl

2.17 0.57 0.42 2.19

2.07 0.60 0.46 2.43

Recent publications have shown diffuse reflectance spectroscopy to be a useful technique for probing the coordination sphere of Ti ions in Ti–silicates [30–32]. As shown in Fig. 5, calcination of TiOCl (NMe ) adsorbed on aerosil affords 2 32

a material having absorption maxima at 211 and 258 nm. On the basis of the literature, the band at 211 nm may be assigned to a CT transition involving tetrahedrally coordinated titanium, a similar band being observed at approximately 208 nm in the spectrum of TS-1 [30–32]. The band at 258 nm is indicative of the presence of an amorphous form of TiO containing octahedrally coordinated tita2 nium, although some contribution to this band may arise from the presence of octahedrally coordinated titanium sites formed by the interaction of water with tetrahedral titanium sites (due to the fact that the sample was measured in air). Additionally, the presence of an absorption edge at approximately 330 nm is indicative of the pres-

Table 2 XPS and elemental analysis ( WDX ) data for model catalysts prepared from TiOCl (NMe ) 2 32 Code

A1 A1/C

Sample

Aerosil/TiOCl (NMe ) 2 32 Al, calcined

aPeak width at half-height.

XPS data

WDX data

Ti 2p 3/2 B.E. (eV )

WHHa (eV )

[ Ti ] (wt%)

Ti/nm2

[Cl ] (wt%)

Cl/nm2

[ Ti ] (wt%)

Ti/nm2

[Cl ] (wt%)

Cl/nm2

458.9 459.2

2.6 3.3

3.10 2.20

0.7 0.6

3.7 0.3

1.1 0.1

1.44 1.52

1.2 1.3

2.190 0.2

2.4 0.2

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Fig. 4. X-ray diffractograms of model catalysts: (a) A1/C; (b) A2/H; (c) SG2/H.

Fig. 3. XPS spectra of model catalysts: (a) anatase; (b) A1; (c) A1/C; (d) A2; (e) A2/H; (f ) SG2/H.

ence of crystalline anatase in the sample. Addition of hydrogen peroxide results in the appearance of a broad absorption in the region 350–450 nm, which, as for TS-1, is assigned to a Ti–peroxo complex [30]. The 1H MAS NMR spectrum of A1/C shown in Fig. 6, is particularly informative. In addition to an intense signal at d=1.9 ppm which is assigned to isolated silanol groups, a signal is observed at d=3.2 ppm. This signal can be most readily assigned to the hydroxyl protons of titanol groups, Ti(OH ) and/or Ti(OH ). In this context, 2 it should be noted that it is not possible to distinguish between hydroxyl groups of the type SiOH and Si(OH ) using 1H MAS NMR spectroscopy, 2 since the difference between the corresponding values of d is less than 0.1 ppm; this also appears H to be the case for titanol groups [33]. Although we defer a detailed discussion concerning the inter-

pretation of 1H MAS NMR spectra of amorphous and crystalline forms of titania to a separate paper [33], we note that a chemical shift of d=3.2 ppm has been observed previously for titanol groups in SiO -containing anatase [34]. Further, recent 2 work [35] suggests that in such mixed oxide systems, where silicon is present in low concentration, the silicon tends to form a nanolayer of titanium silicalite on the titania particles, i.e. the surface titanium is present in tetrahedral coordination. In contrast, anatase has been shown to give rise to signals at approximately d=2.3 and 6.5 ppm, corresponding to terminal and bridging hydroxyl groups, respectively [33,34]. On this basis, the signal observed at d=3.2 ppm can be assigned to hydroxyl groups bound to tetrahedrally coordinated titanium species, which we propose are formed via the pathways depicted in Scheme 2. Note that although detected by XRD, the presence of anatase in A1/C cannot be inferred from the 1H MAS NMR spectrum, due to the fact that the expected peak at d=2.3 ppm is obscured by the SiOH signal. That no signal is observed corre-

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Fig. 6. 1H MAS NMR spectra of: (a) A1/C; and (b) SG2/H.

Fig. 5. Diffuse reflectance spectra of model catalysts: (a) aerosil; (b) TiOCl (NMe ) ; (c) A1; (d ) A1/C; (e) A2/H; (f ) 2 32 SG2/H+H O ; (g) SG2/H. 2 2

sponding to bridging hydroxyl groups (d= 6.5 ppm) is presumably a consequence of partial (thermal ) dehydroxylation of the anatase crystallites. 3.2. Preparation of model catalysts employing Ti(CH Ph) as a precursor 2 4 The reaction between Ti(CH Ph) and various 2 4 hydroxylic supports has been reported by Ballard [36 ] and Yermakov and co-workers [37,38], the resulting supported Ti species being active in olefin polymerization. In the present procedure, a slight excess of aerosil (A) or silica-gel (SG) was reacted with Ti(CH Ph) in pentane to form anchored 2 4

Ti(CH Ph) units (n=1–3). Subsequently, the 2 n material was hydrolysed by treatment with an excess of water (in ether). Quantitative analysis (GLC ) of the toluene formed in these reactions (see Scheme 3) was used to determine the reaction stoichiometries. Defining x as the number of mols of toluene released per mol titanium, it was found that for the first step (i.e. anchoring of the Ti complex), x=2.2 when aerosil was employed as the silica source, and x=2.3 when silica-gel was used. The residual benzyl content of the reaction products (1.7–1.8 meq/mmol Ti) shows good agreement with the value of 1.6 meq/mmol Ti reported by Yermakov [37] for the reaction product of Ti(CH Ph) with silica vacuum dried at 2 4 25°C. Note that these reaction stoichiometries, assuming that only bi- and tripodal sites are formed (i.e. (¬SiO) Ti(CH Ph) and (¬SiO) 2 2 2 3 Ti(CH Ph)), correspond to a distribution of 80% 2

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between hydrolysis of the Ti species and either a strong adsorption of the added water on the residual silanol groups of the surface, or reaction of the water with strained siloxane bridges to form new silanol groups. Elemental analysis and XPS data for the hydrolysed materials (denoted as A2/H and SG2/H ) are collected in Table 3. WDX data show that, employing aerosil as support, this synthesis route results in lower titanium loadings on the surface of the support in comparison with the route based on TiOCl (NMe ) . The higher surface 2 32 area of the silica-gel results in higher Ti loadings than when aerosil is used as support. According to Ti 2p binding energies determined from the 3/2 XPS spectra ( Fig. 3), non-octahedrally coordinated titanium is present in the hydrolysed materials. Also of note is the observation that the Ti 2p binding energy of A2/H is 0.7 eV higher than 3/2 that of SG2/H (459.7 versus 459.0 eV ), suggesting that a higher Ti dispersion is obtained on the nonporous aerosil support. Further, the Ti 2p bind3/2 ing energy of A2/H is 0.5 eV higher than that observed for A1/C, indicative of a lower Ti dispersion in the latter. This conclusion is supported by the results of powder X-ray diffraction, which reveal the materials prepared by the Ti(CH Ph) 2 4 route to be completely amorphous, in contrast to materials prepared from TiOCl (NMe ) . 2 32 IR spectra of A2 and SG2, containing anchored Ti(CH Ph) groups, revealed the presence of resid2 n ual silanol groups (both isolated and H-bonded), in addition to benzyl groups (C–H stretching bands at 2880 and 2960 cm−1). Upon hydrolysis the latter absorptions disappeared, while the remainder of the spectrum was little changed. Significantly, the

Scheme 3. Preparation of model catalysts from Ti(CH Ph) : 2 4 (i) pentane, 0°C; (ii) ether/H O r.t. 2

bipods and 20% tripods for x=2.2, and 70% bipods and 30% tripods for x=2.3. For the hydrolysis reaction, the measured value of x was typically 1.7 for the aerosil material (A2) and 1.6 for its silica-gel analogue (SG2), consistent with essentially complete protolysis of the benzyl ligands. The hydrolysis step was found to be fairly selective with respect to toluene formation, the only other organic products detected comprising trace amounts of benzyl alcohol and bibenzyl (detected by GC-MS). Interestingly, in order to achieve complete hydrolysis of the anchored Ti(CH Ph) species on the time-scale of a typical 2 n experiment (6 h), it proved necessary to employ an excess of water (at least three-fold). We interpret this observation in terms of a competition

Table 3 XPS and elemental analysis ( WDX ) data for model catalysts prepared from Ti(CH Ph) 2 4 Code

Sample

XPS data Ti 2p

A2 A2/H SG2/H —

Aerosil/Ti(CH Ph) 2 4 Aerosil/Ti(CH Ph) /hydrol. 2 4 Silica-gel/Ti(CH Ph) /hydrol. 2 4 Ti-MCM-41

aPeak width at half-height.

459.5 459.7 459.0 459.8

3/2

B.E.

WDX data WHHa (eV )

[ Ti ] (wt%)

Ti/nm2

[ Ti ] (wt%)

Ti/nm2

3.5 3.4 2.9 3.2

3.28 2.94 4.86 0.48

0.9 0.8 1.3 0.14

— 0.80 2.98 1.43

— 0.7 1.0 0.3

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Fig. 7. IR spectra of: (a) A2/H; (b) aerosil support; (c) difference spectrum (a–b).

expected TiNO stretching band at approximately 970 cm−1 was found to be at best observable only as an extremely weak absorption for both A2/H and SG2/H (Fig. 7). Further, this band could not be unambiguously assigned to a titanyl group, since absorptions of a similar intensity were also observed at 965 cm−1 in spectra of the supports, being attributable to the Si–O stretching vibration of silanol groups [39]. Spectra of materials prepared using H 18O failed to show any new bands 2 in the region anticipated for a TiN18O group (930–940 cm−1). Raman spectra of the hydrolysed materials were featureless in the region 800–1100 cm−1 even after in situ drying under vacuum at 200°C or calcination in dry air at 500°C. Diffuse reflectance spectra of the hydrolysed aerosil and silica-gel materials are shown in Fig. 5. As for the material prepared from TiOCl (NMe ) , two absorptions are observed, 2 32

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possessing maxima at 212 and 251 nm for A2/H, and at 218 and 270 nm for SG2/H. Comparison of the spectra obtained for A1/C and A2/H reveals some similarity, the main difference being the reduced intensity of the 251 nm band (relative to the band at 212 nm) for the material prepared from Ti(CH Ph) . Comparing the spectra of A2/H 2 4 and SG2/H reveals that the 270 nm absorption maximum of SG2/H is relatively more intense than the 251 nm absorption of A2/H (with respect to the 212 nm band), while being also significantly broader and shifted to longer wavelength. These features are most readily interpreted in terms of the formation of significant amounts of octahedrally coordinated Ti in the silica-gel material, i.e. non-crystalline TiO , in addition to tetrahe2 drally coordinated Ti. In support of this assignment, it should be noted that extra-framework TiO in titanium silicalites is reported to give rise 2 to an absorption in the region 240–330 nm [31]. Further, the presence of non-crystalline TiO is 2 also supported by XPS data, which, as noted above, indicate that titanium in SG2/H is less highly dispersed than in its aerosil equivalent. As for A1/C, upon addition of H O , both A2/H and 2 2 SG2/H show a new absorption band in their UV-vis spectra in the region 350–450 nm (Fig. 5), indicative of the formation of Ti–peroxo species. 1H MAS NMR spectra of A2/H and SG2/H contain a number of features of interest (Fig. 6). A signal at d=7.2 ppm is assigned to adsorbed aromatic residues resulting from hydrolysis of the benzyl ligands, while other signals are observed at d=2.2 ppm (-CH - and -CH protons of the organic 2 3 residues, possibly combined with a signal arising from terminal silanol groups of the support) and d=1.1 ppm (silanol groups). Also present is a broad signal at d=3.4 ppm which can be assigned a priori to titanol groups. Washing the material with dry THF leads to the almost complete disappearance of the signal at 7.2 ppm, while the other signals are unchanged. As for the model catalyst prepared from TiOCl (NMe ) , the signal at d=3.4 ppm is 2 32 assigned to a hydroxyl group bound to titanium which is in tetrahedral coordination. It is, therefore, apparent that during hydrolysis of the surface Ti(CH Ph) moieties, Ti(OH ) groups are formed 2 2 2 (Scheme 3), a process which may or may not involve the intermediacy of titanyl groups.

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In accord with the above findings, the spectrum of A2 prepared using 17O-enriched water in the hydrolysis step showed no signals in the region expected for a titanyl group (approximately d= 1000 ppm [16 ]) although a signal was observed at approximately d=210 ppm, in addition to the signal at approximately d=50 ppm expected for the siloxy oxygens of the silica support ( Fig. 8). Assignment of the signal at d=210 ppm is problematic; of possible relevance, however, are the results of a study of amorphous silica employing 17O MAS NMR reported by Walter et al. [40], in which a resonance showing a more shielded isotropic chemical shift (~20 ppm) than the siloxy oxygen resonance was attributed to silanol species present both on the surface of the silica and at internal defect sites. Thus, if the silanol oxygen is shifted to higher field in comparison with the siloxy group (Si–O–Si), it seems likely that the signal for the oxygen of a Ti–O–Ti group is also found at the high field side of the Ti–O–Ti resonance (d= 557 ppm [41]). On this basis, we tentatively assign the signal at approximately d=210 ppm to a Ti–OH group. 3.3. ISS study of model catalysts The unique surface sensitivity of ion scattering spectrometry (ISS ) makes it a valuable tool for the determination of the elemental composition of the outermost atomic layer of solids [24]. In this context, ISS was employed with the aim of ascertaining whether the Ti sites in the model catalysts

were indeed situated above the surface of the silica support, as opposed to being incorporated into the surface (as for titania–silica mixed oxides). For the purposes of the study four samples were examined. The first sample, A1/C , consisted of in TiOCl (NMe ) adsorbed on aerosil and was 2 32 calcined in situ at 350°C (1 bar O ) for a period 2 of 2 h. The second model catalyst, A1/C , was ex similar to sample A1/C , but had already been in calcined ex situ (350°C ). As a surface cleaning treatment, A1/C was heated at 150°C under ex vacuum for 30 min. Also examined was catalyst A2/H, for which it would be expected that Ti is present in the outermost atomic layer only. No special cleaning treatment was applied other than outgassing at 150°C in the vacuum system of the spectrometer. Additionally, the surface composition of the clean aerosil carrier material was analysed. Fig. 9 summarizes the most important results, the various peak area ratios being listed in Table 4. Note that these ratios are not corrected for the different sensitivities of the elements in ISS. The curves in Fig. 9 are scaled in such a way that the inelastic background between the O-peak (1203 eV ) and the Si-peak (1797 eV ) is the same for all spectra. It is clear that the three model catalysts all have a pronounced Ti-peak at 2225 eV, that of A1/C being the most intense. In the in spectra corresponding to the Ti-containing materials, the Si-peak tends to be lower as compared to the Si-peak originating from the clean carrier. The most important observation with respect to

Fig. 8. 17O MAS NMR spectrum of SG2 hydrolysed with 17O-enriched water.

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Fig. 9. ISS spectra of model catalysts: (a) A1/C ; (b) A1/C ; (c) A2/H; (d ) aerosil. in ex

Fig. 9 is that all the Ti peaks in the ISS spectrum arise from elastic scattering. This indicates that Ti atoms are present in the outermost atomic layer of the model catalysts. As can be seen from Table 4, the Ti:Si and Ti:O ratios for A1/C , and A2/H in are comparable. The same ratios for A1/C are ex smaller. Therefore, assuming that a similar amount of Si atoms is present per unit area in the outermost layer of all the samples, the amount of Ti, as

found by ISS, is comparable for A1/C and A2/H, in and is the lowest in A1/C . This observation is in ex agreement with XPS results, calcination of adsorbed TiOCl (NMe ) lowering the amount of 2 32 surface Ti detectable by XPS as a consequence of anatase formation (see Table 5). The presence of anatase in A1/C is also reflected in the observaex tion that the Ti 2p level of catalyst A1/C is 3/2 ex slightly shifted towards lower binding energy with respect to that of model system A2/H ( Fig. 3). In

Table 4 ISS peak area ratios for model catalysts Table 5 XPS atomic ratios for model catalysts

Catalyst

Ti:Si Ti:O O:Si

A1/C in

A1/C ex

A2/H

Catalyst

0.74 0.78 0.95

0.38 0.40 0.95

0.69 0.61 1.14

A1

A1/C ex

A2/H

0.074

0.031

0.040

Ti:Si

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this respect, it is noteworthy that the in situ calcination treatment results in a higher Ti:Si peak area ratio for A1/C as compared with A1/C . in ex Apparently, less anatase is formed during the in situ calcination treatment. This can be rationalized on the basis that in the in situ calcination the reaction conditions are expected to be somewhat milder as compared with those for A1/C , due to ex radiative heat loss from the sample cell. Also of note is the finding that the two model systems A1/C and A1/C show a O:Si peak area in ex ratio of 0.95, which is lower than the corresponding ratio amounting to 1.09 for the clean carrier material, and 1.14 for catalyst A2/H. This is presumably a consequence of the fact that A2/H and the clean carrier were exposed to lower temperatures than A1/C and A1/C , and hence should have in ex retained a larger number of hydroxyl groups at their surfaces. 3.4. Epoxidation of 1-octene with TBHP Numerous studies have shown titanium silicates to be active and selective catalysts for liquid phase olefin epoxidation [2–14]. In order to investigate the catalytic properties of the model systems in selective oxidation reactions, the oxidation of 1-octene with tert-BuOOH (TBHP) was chosen as a test reaction. Experiments were performed batchwise using the olefin as substrate and solvent. For reference purposes a wide pore zeolite, Ti-MCM-41 (containing 1.43 mol% Ti), was similarly tested. Physical data for the samples are collected in Table 3. Use of Ti-MCM-41 as a reference, rather than TS-1, was necessitated by the incompatibility of TS-1 with organic hydroperoxides (due to pore size restrictions) [10]. Conversely, the hydrophilic nature of titania-onsilica prohibits the use of hydrogen peroxide as oxidant [10]. The generalized rate expression for metal-catalysed epoxidation reactions is given by [25]: rate=

d[epoxide] dt

=k [catalyst][ROOH ][olefin]. 3

Assuming that the catalyst remains unchanged during the reaction and that high olefin/

hydroperoxide ratios are employed, this reduces to the pseudo-first order expression: rate=k [ROOH ]. 1 Thus, at the olefin/TBHP molar ratio of approximately 20 employed, pseudo-first order reaction kinetics were generally observed, corresponding to the rate equation: d[epoxide] dt

=k [ TBHP] 1

At moderate TBHP conversions (>30%), however, some deviation from first order kinetics was observed, corresponding to a decrease in the reaction rate. This can be attributed to the fact that the reaction is auto-retarded by the tert-butanol co-product, a phenomenon observed previously for a variety of homogeneous and heterogeneous Ti and V catalysts [25]. The calculated second order rate constants (k =k /[ Ti]) for the various catalysts tested are 2 1 collected in Table 6. Values of k were in all cases 1 determined from the slope of the (initially) linear portion of the rate plot ( Fig. 10). In general, the model catalysts possess moderate activity in the epoxidation reaction, while showing good selectivity to 1,2-epoxyoctane based on the TBHP conTable 6 Performance of model catalysts in the epoxidation of 1-octene using TBHP Catalyst

[ Ti ] (wt.%)

k ×102 2 (M−1/s−1)

Selectivity to epoxidea

A1 A1/C A2/H SG2/H Ti-MCM-41 TiO (G5) 2

1.44 1.52 0.80 2.98 1.43 59.94

0 2.0 5.2 2.2 2.6 (7.7)b <0.1

—88% 95% 92% 94% 33%c

Conditions: T=80°C, Ti=0.2 mmol, TBHP=30 mmol, 1-octene (75 g) as solvent. aSelectivity=(mol 1,2-epoxyoctane formed/mol TBHP consumed )×100; measured at 50% TBHP consumption unless otherwise stated. bCalculated on basis of surface titanium concentration determined by XPS (Table 3). cSelectivity measured at 4% TBHP consumption.

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showed only very low activity for TBHP decomposition (k <0.1 M−1 s−1), with poor selectivity to 2 the epoxide (33%). This result confirms that the octahedrally coordinated titanium-containing phases in the model catalysts would not be expected to contribute substantially to the observed epoxidation activity.

4. Discussion 4.1. Characterization of model systems

Fig. 10. Pseudo-first order rate plots for the epoxidation of 1-octene with TBHP.

sumed. For all the catalysts tested the selectivity based on 1-octene was 100%, i.e. 1,2-epoxyoctane was the only product formed. The exception to this was A1, which showed no activity for olefin epoxidation or TBHP decomposition. That the calcined material was active is in agreement with the notion that a coordinatively unsaturated, Lewis acidic Ti centre is a pre-requisite for activation of the hydroperoxide [15]. Considering the results obtained with catalysts A1/C, A2/H and SG2/H, it is apparent that a clear correlation exists between Ti dispersion and catalyst activity. Thus, the value of k ×102 deter2 mined for A2/H was significantly higher than that found for its silica-gel analogue, SG2/H (5.2 versus 2.2 M−1 s−1). Likewise, A2/H was found to be both more active and more selective than A1/C. When similarly tested, the Ti-MCM-41 sample showed comparable activity (k ×102= 2 2.6 M−1 s−1) and selectivity to the model systems. In contrast, a sample of high surface area anatase

On the basis of the analytical data presented above, it is clear that both synthetic routes employed afford products containing a mixture of at least two titanium phases. Calcination of TiOCl (NMe ) adsorbed on aerosil yields a small 2 32 quantity of anatase, detected by XRD, together with amorphous material. For the latter, UV-vis and XPS data permit a further distinction to be made between phases containing tetrahedrally and octahedrally coordinated titanium, both of which are present. The tetrahedrally coordinated titanium observed can be assigned to isolated, anchored titanium sites of the type (¬SiO) Ti(OH ) , the x 4−x presence of titanol groups in A1/C being confirmed by 1H MAS NMR spectroscopy. The nature of the octahedrally coordinated titanium phase is unclear: possibly it may correspond to anatase particles which are too small to be detected by XRD (i.e. <4 nm in diameter), or to a highly dispersed TiO (B) phase [42]. Unfortunately, the 2 low titanium loadings employed in this work preclude the use of Raman spectroscopy to distinguish between these possibilities. Hydrolysis of anchored (¬SiO) Ti(CH Ph) x 2 4−x species leads to the formation of amorphous titania only, the presence of both tetrahedrally and octahedrally coordinated titanium again being indicated. XPS and UV-vis data indicate that the relative proportion of the two phases varies according to the support employed. For SG2/H, the observed Ti 2p binding energy of 459.0 eV lies 3/2 approximately midway between that expected for atomically dispersed (tetrahedrally coordinated) Ti(IV ) (459.8 eV ) and anatase (458.3 eV ). In contrast, the Ti 2p binding energy of 459.7 eV 3/2

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observed for A2/H suggests that predominantly isolated (¬SiO) Ti(OH ) species are present, x 4−x where x=2 (on the basis of the observed reaction stoichiometry). Although the parameters affecting the Ti dispersion are hard to delineate, it should be noted that a lower surface Ti concentration is obtained using aerosil as support in comparison to the silica-gel (0.7 versus 1.0 Ti atoms/nm2 on the basis of WDX data). Clearly, the probability of isolated species oligomerizing to form dispersed anatase or TiO (B) will be lower at low surface Ti 2 concentrations. Additionally, the porosity of the support may in part determine the occurrence of such reactions: the convoluted surface of a porous material such as silica-gel may favour the formation of pockets of H-bonded titanol groups (as for silanol groups), thereby facilitating the occurrence of condensation and associated oligomerization reactions. In no cases was evidence found for the formation of silica-anchored titanyl species. Instead, the combined IR, Raman and 1H and 17O MAS NMR data suggest that the tetrahedrally coordinated Ti(IV ) sites present bear hydroxyl groups as capping ligands, as opposed to terminal oxygens. Attempts to dehydrate the titanol groups by in situ treatments at 500°C were also unsuccessful according to IR and Raman data. In the case of A1/C, titanol group formation can be explained in terms of the reaction of initially formed titanyl species with water, the latter rising from the condensation of silanol groups during calcination of A1. An alternative pathway, involving addition of a silanol group across the titanyl group in adsorbed TiOCl (NMe ) , would similarly afford 2 32 an anchored titanol species (Scheme 2). The apparent elusiveness of surface titanyl groups, as depicted in Scheme 1, is in our opinion not surprising. The extreme Lewis acidity of such a three-coordinate Ti(IV ) centre would render it highly reactive, 1,2-addition reactions of the TiNO group (with, for example, water or SiOH groups) providing a facile pathway to four-coordinate species lower in energy. Similar reasoning can be used to explain the elusiveness of siloxide groups, SiNO, on the silica surface [43]. Indeed, preliminary calculations performed in our laboratory, employing a density functional theory (DFT ) quantum chemistry method (B.C.H. Krutzen et al.,

unpublished results), indicate that the hydrated form of the titanyl group, as represented by the model complex Ti(OH ) , is more stable than its 4 non-hydrated congener, Ti(O)(OH ) , by approxi2 mately 43 kcal/mol. In view of the foregoing, it therefore appears unlikely that titanyl groups are present in significant concentrations in mixed Ti/Si oxides or amorphous titania-on-silica catalysts, and still less under epoxidizing conditions when protic species are present. In our view the 960 cm−1 absorption band found in IR spectra of these materials can be assigned in all cases to a Si–O–Ti stretching mode of titanium occupying lattice positions. For amorphous titania-on-silica this implies that during calcination of the adsorbed titanium species, some incorporation of titanium into the silica lattice occurs, presumably via a process of surface annealing. In the present work, the apparent absence of the 960 cm−1 band is most probably a consequence of the low titanium loadings and low calcination temperatures employed. As shown by the results of ISS measurements, significant incorporation of titanium into the silica surface does not occur under these conditions. 4.2. 1-Octene epoxidation The results obtained with the model catalysts in the epoxidation of 1-octene show that epoxidation activity increases with increasing dispersion of the titania. This is logical, since the Lewis acidity of the Ti(IV ) sites is responsible for the activation of the alkyl hydroperoxide, coordination of the peroxide enhancing the electrophilicity of the oxygen atoms and thereby rendering the peroxo group more susceptible to nucleophilic attack by the olefin. Isolated, coordinatively and electronically unsaturated Ti(IV ) centres would, therefore, be anticipated to be the most active sites for epoxidation catalysis. It is now generally accepted that a four-coordinate lattice titanium site is the catalytically active species in titanium silicates [2]. Further, recent studies point toward the involvement of titanium sites possessing tripodal geometry, i.e. (¬SiO) Ti(OH ), representing so-called open lat3 tice sites [30,44], although the presence of closed lattice sites ( Ti(OSi¬) ) and bipodal sites 4 ((¬SiO) Ti(OH ) ) has also been inferred 2 2

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[2,30,44]. Whilst bipodal sites may exist as defect sites within the zeolite structure, they may additionally be formed from the hydrolysis of lattice Ti(OSi¬) sites (for reactions involving aqueous 4 hydrogen peroxide) [30]: Ti(OSi¬) L +H O<(¬SiO) Ti(OH )L 4 n 2 3 n +¬SiOH,

(1)

(¬SiO) Ti(OH )L +H O<(¬SiO) Ti(OH ) L 3 n 2 2 2 n +¬SiOH (2) (L=H O, H O ; n=0, 1, 2). 2 2 2 Note that spectroscopic evidence suggests that the lattice titanium sites are additionally coordinated by weakly bound water molecules. Presumably, hydrolysis of one or more of the Ti–O–Si bonds provides a manner of releasing the strain around the lattice Ti sites. Indeed, it can be envisaged that the unique properties of TS-1 may in large part be attributable to the ability of the zeolite lattice to accommodate changes in the coordination sphere of the Ti sites, without the formation of oligomeric Ti species. In a recent publication we have shown that soluble titanium silsesquioxane complexes can be used to model the various titanium sites which may occur in titanium silicates, and that on the basis of the observed activities, the most active site corresponds to a tripodal species, (¬SiO) Ti(OH ) 3 [45]. In contrast, a titanium complex containing a bidentate silsesquioxane ligand, being a model for a bipodal site (¬SiO) Ti(OH ) , was found to be 2 2 about an order of magnitude less active in the epoxidation of 1-octene with TBHP than tripodal complexes (containing a terdentate ligand ). A complex modelling the closed lattice site was similarly found to possess low epoxidation activity. Accurate comparisons of the intrinsic activities of the titanium sites in the model catalysts prepared in this work with those in Ti-MCM-41 are hampered by the fact that as for the model systems, a certain fraction of the Ti sites in the Ti-MCM-41 sample will not be active, due to the fact that they correspond to octahedrally coordinated Ti (e.g. anatase crystallites) or are inaccessible to the reactants. However, the finding that the value of k 2 calculated for the Ti-MCM-41 sample on the basis

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of the surface Ti concentration (measured by XPS) is higher than that determined for the model catalysts is at least consistent with the notion that whilst bipodal titanium sites are active in epoxidation catalysis, they do not correspond to the most active site present in titanium silicates. In a subsequent publication we will present a detailed discussion of the relationship between titanium site geometry and epoxidation activity.

5. Conclusions All attempts to prepare silica-anchored titanyl groups in this work were unsuccessful. The apparent elusiveness of surface titanyl groups is most probably related to the extreme Lewis acidity of such a three-coordinate Ti(IV ) site: DFT calculations indicate that the hydrated form of the titanyl group, as represented by the model complex Ti(OH ) , is more stable than its non-hydrated 4 congener, Ti(O) (OH ) , by approximately 2 43 kcal/mol. It therefore appears unlikely that titanyl groups are present in significant concentrations in mixed Ti/Si oxides or amorphous titania-onsilica catalysts, and still less under epoxidizing conditions. In our view, the 960 cm−1 absorption band found in IR spectra of these materials can be assigned in all cases to a Si–O–Ti stretching mode of titanium occupying lattice positions. Note added in proof A recent computational study [46 ], also employing a DFT method, has confirmed that (OSiO) TiNO groups are energetically unfavour2 able with respect to (OSiO) Ti(OH ) and 2 2 (OSiO) TiOH species. 3

Acknowledgement The authors thank Dr B.C.H. Krutzen for performing the DFT calculations, J.B. van Mechelen for XRD measurements and G.M. Mulder for XPS measurements. Shell International Chemicals is thanked for permitting publication of this work.

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