Evidence of Three Kinds of Tetrahedral Vanadium (V) Species in VSiβ

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J. Phys. Chem. B 2000, 104, 6012-6020

Evidence of Three Kinds of Tetrahedral Vanadium (V) Species in VSiβ Zeolite by Diffuse Reflectance UV-Visible and Photoluminescence Spectroscopies† Stanislaw Dzwigaj,*,‡ Masaya Matsuoka,‡ Masakazu Anpo,§ and Michel Che‡,⊥ Laboratoire de Re´ actiVite´ de Surface, UMR 7609, CNRS, UniVersite´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Department of Applied Chemistry, College of Engineering, Osaka Prefecture UniVersity, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan ReceiVed: January 5, 2000; In Final Form: April 18, 2000

VSiβ zeolites prepared by contacting dealuminated Siβ zeolite with aqueous solutions of ammonium metavanadate at room temperature have been studied by diffuse reflectance UV-visible and photoluminescence spectroscopy. UV-visible spectroscopy shows that whatever the vanadium content, vanadium is present in as-prepared VSiβ in only one kind of tetrahedral coordination. After calcination and then rehydration of VSiβ with low V content, still only one kind of tetrahedral VV species is observed. However, in calcined/ rehydrated VSiβ with high V content, two kinds of tetrahedral and one kind of octahedral V species are revealed. In contrast, photoluminescence spectroscopy in static or dynamic mode can distinguish in as-prepared VSiβ three kinds of tetrahedral vanadium species. Their relative amounts depend strongly on the vanadium content and on calcination/rehydration treatments. They are present at two different types of framework sites. For samples with low V loading, the tetrahedral VV species are in a site poorly accessible to water, probably in the five-membered rings of the β structure. These VV species are little sensitive to calcination/rehydration treatments. In contrast, for higher V loading, the tetrahedral VV species become highly sensitive to such treatments suggesting that VV ions are in a site more accessible to water, probably in the 12-membered rings, where they can easily change their coordination upon dehydration/rehydration processes. Possible models of tetrahedral and octahedral V species and their probable location in the β structure are proposed.

Introduction The crystalline microporous metallosilicates are a new class of catalysts with remarkable catalytic properties in the selective oxidation of various organic molecules.1-15 In the recent years, particular attention has been paid to V-containing silicate molecular sieves with MFI,8-20 MEL,21 NCL-1,22,23 and ZSM4811 structures. These V-molecular sieves can be prepared by hydrothermal or by postsynthesis methods.8-23 The hydrothermal synthesis of V-loaded zeolite is confronted with the problem of the introduction of V ions into framework positions and of the stability of the resulting material. Centi et al.16 have reported on that V5+ ions could be incorporated in silicalite by hydrothermal synthesis only in small amounts and were related to defect sites. Recently, we have shown24,25 that catalytically active vanadium ions can be dispersed in a dealuminated β zeolite even at room temperature using ammonium metavanadate in aqueous solution as a precursor. Many techniques can be used to characterize the structure of VV centers in VSiβ zeolite.24-26 From our studies, the most common view is that VV ions are incorporated as isolated tetrahedral V species.24-26 Several XRD, IR, UV-visible, and NMR results are in favor of this hypothesis. In particular (i) Our XRD measurements show that the most significant diffraction line at 2θ ) 22.5° shifts to lower 2θ values as the * Corresponding author. E-mail: [email protected]. Fax: 33 1 44 27 60 33. Phone: 33 1 44 27 55 33. † The paper is dedicated to Professors M. Misono, Y. Moro-oka, and Y. Ono on the occasion of their retirement from the University of Tokyo (M.M.) and the Tokyo Institute of Technology (Y.M. and Y.O.). ‡ Universite ´ Pierre et Marie Curie. § Osaka Prefecture University. ⊥ Also at the Institut Universitaire de France.

V loading increases.26 These results are consistent with an expansion of the VSiβ lattice due to incorporation of V atoms in framework sites. This incorporation was confirmed by the shift of the asymmetric stretching of the T-O bond (T ) Si, V) from 1087 cm-1 (Siβ) to 1100 cm-1 (VSiβ) in IR spectra. (ii) The introduction of V ions within Siβ zeolite strongly modifies the IR peak of silanol groups: the single peak observed at 960 cm-1 in Siβ, which is a fingerprint of the Si-OH vibrators, is replaced by two peaks at 980 and 950 cm-1 in VSiβ probably due to the presence of Si-O-V and of silanol groups, polarized by V incorporated in the β framework, respectively.26 (iii) The UV-visible spectra of VSiβ confirm the tetrahedral coordination of the V ions, since the two bands at 270 and 340 nm can only be explained in terms of oxygen to vanadium charge-transfer transitions with isolated tetrahedral V species.16,24-30 (iv) The same conclusion was reached by 51V magic-angle spinning (MAS) and wide-line NMR studies with peaks at -633 ppm (MAS spectra) and -580 ppm (wide-line spectra) assigned to tetrahedral V species.26 However, it is difficult to conclude with IR, UV-visible, and NMR techniques that in V-loaded zeolite there is a unique type or a mixture of different types of tetrahedral V species. It is likely that only an averaged structure of the tetrahedral V species can be detected. We have shown recently25 that it is possible to distinguish in VSiβ zeolite different types of tetracoordinated V species by photoluminescence spectroscopy. Using its high sensitivity to the local environment of a luminescent center, we give in the present paper more information about the symmetry of these species and propose in which way they are formed in VSiβ. Moreover, using diffuse reflectance UV-visible, we show that calcination and rehydration of a VSiβ with high V content

10.1021/jp0000331 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/06/2000

Three Kinds of Tetrahedral V Species in Beta Zeolite induce not only changes in the symmetry of the tetrahedral V species but also lead to octahedral ones. Experimental Section Materials. The TEAβ sample with a framework Si/Al ratio of 11 was provided by RIPP (China). It was dealuminated by treatment in a 13 mol L-1 HNO3 solution for 4 h at 353 K according to the procedure described by Bourgeat-Lami et al.31 The dealuminated Siβ zeolite thus obtained (Si/Al > 1300) was recovered by centrifugation, washed with distilled water and dried overnight at 353 K. The Siβ samples were contacted with an aqueous solution of ammonium metavanadate in a great excess (2.3 g of zeolite in 10 mL of NH4VO3 solution).24 Because of the low concentration (10-2-10-3 mol L-1) and pH ) 6, the initial aqueous solution of NH4VO3 is expected to contain mainly monomeric HVO3 species.32-34 The concentration of the NH4VO3 solutions was varied (from 0.25 × 10-2 to 7 × 10-2 mol L-1) in order to obtain samples with different V contents. The suspension was left for 3 days at room temperature without any stirring. The solids with vanadium were recovered by centrifugation and dried at 353 K overnight. VSiβ samples with low (0.05 V wt %) and high (0.5 and 1.5 V wt %) V content were prepared and labeled V0.05Siβ, V0.5Siβ, and V1.5Siβ, respectively. The V0.05Siβ, V0.5Siβ, and V1.5Siβ samples were further calcined (50 K/h) at 773 K for 2 h in flowing oxygen (120 mL/min) and denoted C-V0.05Siβ, C-V0.5Siβ and C-V1.5Siβ, respectively. These samples were then cooling to room temperature and rehydrated at 298 K in moist air for 30 h and labeled C-Hyd-V0.05Siβ, C-Hyd-V0.5Siβ, and C-Hyd-V1.5Siβ, respectively, where C stands for calcined and Hyd for rehydrated. Methods. Diffuse reflectance UV-visible spectra were recorded at 298 K in the range 200-800 nm on a Cary 5E spectrometer equipped with a diffuse reflectance accessory. The parent, V-free, materials were used as references. Static and dynamic photoluminescence measurements were performed at 77 K with a Spex Fluorolog II spectrofluorimeter and a Spex 1934D phosphorimeter, respectively. Prior to the photoluminescence and lifetime measurements, the powdered VSiβ, C-VSiβ, and C-Hyd-VSiβ samples were placed in a quartz cell with window and furnace sections that were permanently connected to a vacuum line and outgassing at 353-473 K for 2 h at pressure of 10-3 Pa. Emission spectra were recorded using 250 nm excitation light and excitation spectra were recorded by monitoring the 520 nm emission light. The decay curves (logarithmic scale) were monitored at 77 K at the emission of 520 nm. The lifetimes were calculated from the averaged slope between 2 and 60 ms. Results To favor the incorporation of vanadium, vacant T atom sites, i.e., silanol nests, are first created in the zeolite framework by dealumination of β zeolite with nitric acid. Then, the resulting Siβ zeolite is contacted at room temperature with an aqueous solution of NH4VO3. This leads to a strong decrease of the pH from 6 to 2.5 while the color of the solution changes from light yellow (color of the initial solution of NH4VO3) to pale yellow (VO2+ ions). Then, the color of the solution changes from pale yellow to colorless within several hours. Simultaneously, the absorbance of the aqueous solution of NH4VO3 measured at 380 nm, in the region of O2- f VV charge transfer transitions, decreases rapidly and reaches zero within a few hours. It indicates that the mononuclear cationic VO2+ species present in the highly acidic (pH ) 2.5) and diluted (10-2 mol L-1) solution of NH4VO3 react with the Siβ zeolite.

J. Phys. Chem. B, Vol. 104, No. 25, 2000 6013 It seems that the VO2+ species react with the silanol groups created in vacant T atom sites of Siβ. In our previous paper,26 we have reported on that intensities of the FT-IR bands at 3706 and 3530 cm-1 characteristic of SiO-H vibrators decrease with increasing V loading and simultaneously new FT-IR bands at 3650 and 3620 cm-1 appear, characteristic of VO-H and SiO-H vibrators, respectively. The formation of these two bands has been related to the incorporation of V ions into vacant T atom sites.26 The resulting V0.05Siβ, V0.5Siβ, and V1.5Siβ are white, and almost all V ions incorporated in the β structure are retained in the samples after treatment in an aqueous solution of ammonium acetate (NH4OAc), in agreement with earlier data.24 The C-V0.05Siβ, C-V0.5Siβ, C-V1.5Siβ, and C-Hyd-V0.05Siβ also are white and the C-Hyd-V0.5Siβ and C-Hyd-V1.5Siβ ones are yellow. Their treatments with NH4OAc (200 mg of zeolite stirred with 20 mL of 1 mol L-1 NH4OAc for 12 h at room temperature) lead to white samples with lower amount of V. The percentage of V ions removed increases with V loading, in agreement with earlier data.35 The white and yellow color of the above samples are likely to be due to the presence of isolated tetrahedral and octahedral V species, respectively. Nature and Environment of Tetrahedral and Octahedral V Species: UV-Visible Results. We have studied the nature and environment of the V species in the VSiβ, C-Siβ, and C-Hyd-VSiβ by diffuse reflectance UV-visible spectroscopy. This technique allows to distinguish both tetrahedral and octahedral V species. The selected UV-visible spectra of these samples are presented in Figure 1. The diffuse reflectance UV-visible spectra of V0.05Siβ, V0.5Siβ, and V1.5Siβ exhibit two bands at around 270 and 340 nm (Figure 1a-c). Because of the absence of (d-d) transition in the range 600-800 nm and of any VIV ESR signal, these bands can only involve VV ions and are attributed to π(t2) f d(e) and π(t1) f d(e) oxygen-to-tetrahedral VV charge transfer transitions, involving oxygen in bridging (V-O-Si) and terminal (VdO) positions, respectively, in line with earlier results.16,36,37 These UV-visible spectra indicate the presence in V0.05Siβ, V0.5Siβ, and V1.5Siβ of only one kind of tetrahedral VV. The intensities of both bands increase with vanadium loading (Figure 1a-c) with a larger increase for the band at 270 nm than for that at 340 nm. The diffuse reflectance UV-visible spectra of the C-V0.5Siβ and C-V1.5Siβ are composed of a large band in the range 230340 nm, with a maximum at 235 nm and shoulder at about 270 nm. This is illustrated in Figure 1d for the C-V1.5Siβ sample. A very weak signal of VIV (not shown) registered by the more sensitive ESR technique clearly indicates that the amount of VIV ions in these samples is negligible and the large band in the range of 230-340 nm can be only related to the presence of tetrahedral VV. For C-V0.05Siβ (spectrum not shown) and C-Hyd-V0.05Siβ (Figure 1e), only two bands at 270 and 340 nm are observed, similar to those found for V0.05Siβ (Figure 1a). In contrast, for C-Hyd-V0.5Siβ and C-Hyd-V1.5Siβ, two broad bands at around 265-270 and 370-375 nm with a shoulder at 235 nm appear (Figure 1f-g). As reported earlier,16,25 the band around 235270 nm can be attributed to π(t2) f d(e) oxygen-to-tetrahedral VV charge transfer transition and the band at around 370-375 nm to an oxygen-to-octahedral VV charge transfer transition with oxygen in terminal position (VdO). It is important to mention here that the bands attributed to π(t1) f d(e) oxygen-totetrahedral VV charge transfer transition (340 nm) and to an oxygen-to-octahedral VV charge-transfer transition with oxygen

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Figure 2. Photoluminescence (a-c) and excitation spectra (a′-c′) of V1.5Siβ outgassing at 353 K (10-3 Pa) (a, a′), of C-V1.5Siβ outgassing at 473 K for 2 h (10-3 Pa) (b, b′), and of C-Hyd-V1.5Siβ outgassing at 473 K for 2 h (10-3 Pa) (c, c′). Photoluminescence spectra obtained under irradiation of 250 nm light. Excitation spectra were monitored at 520 nm. The spectra were measured at 77 K.

Figure 1. Diffuse reflectance UV-visible spectra of (a) V0.05Siβ, (b) V0.5Siβ, (c) V1.5Siβ, (d) C-V1.5Siβ, (e) C-Hyd-V0.05Siβ, (f) C-Hyd-V0.5Siβ, and (g) C-Hyd-V1.5Siβ samples.

in bridging position (V-O-Si) are not seen, since they are probably masked by the large intense bands at around 370375 and 265-270 nm respectively (Figure 1f-g). The shoulder at 235 nm and the band at 265 nm suggest the presence of two different kinds of tetrahedral VV, strongly and less distorted, respectively, in agreement with the results reported for KVOF4 compound and the VO43- complex.38,39 The intensities of the bands at around 265-270 and 365-370 nm increase with the amount of vanadium (Figure 1f-g) and time of rehydration, during the first hour of exposure to moist air. Then, an equilibrium is reached and the intensities of the bands remain constant. When the C-Hyd-V1.5Siβ and C-Hyd-V0.5Siβ are calcined at 773 K for 2 h in flowing oxygen or air, the samples change the color from yellow to white and the spectra characteristic of C-V0.5Siβ (not shown) and C-V1.5Siβ (Figure 1d) are restored. The subsequent exposure of the latter samples to moist air resulted in a UV-visible spectrum similar to that observed for C-Hyd-V0.5Siβ and C-Hyd-V1.5Siβ, respectively (Figure 1fg), indicating that the dehydration (during calcination) and rehydration (upon exposure to moist air) processes are reversible. Nature and Environment of Tetrahedral V Species: Photoluminescence Results. Photoluminescence spectroscopy in static or dynamic mode only detects tetrahedral V species.40-46

Due to its high sensitivity to the local environment of a luminescent center, different kinds of tetrahedral V species can be distinguished. Static Photoluminescence Measurements. Figure 2 shows both excitation and photoluminescence spectra of V1.5Siβ, C-V1.5Siβ, and C-Hyd-V1.5Siβ. The excitation spectra of all these samples exhibit two bands at around 240 and 290 nm whose shapes and positions as well as relative intensities strongly depend on the pretreatment conditions (Figure 2a′-c′). The former is always more intense than the latter one, in agreement with the results reported for VOF3 and VO43- complex.29 Indeed, the ratio of the intensity of the band at around 290 nm to the intensity of the band at around 240 nm increases from 0.5 for V1.5Siβ to 0.77 for C-V1.5Siβ and decreases to 0.32 for C-Hyd-V1.5Siβ. The V1.5Siβ, C-V1.5Siβ, and C-Hyd-V1.5Siβ samples exhibit photoluminescence spectra with maxima at around 500 nm together with complex vibrational fine structure (Figure 2ac). Similar emission spectra of the surface vanadyl groups have been previously reported for V2O5 supported on porous Vycor glass40,41 and on SiO2, MgO, and γ- and R-Al2O3.43 The emission spectra correspond to transitions from the lowest vibrational level of the excited triplet state T1(V4+-O-)* to the various vibrational levels of the ground singlet state S0(V5+dO2-).40-46 Since the positions of the peak maxima of the vibrational fine structure do not change upon varying the excitation UV wavelength from 250 to 320 nm, the two maxima observed in the excitation spectra at around 240 and 290 nm are likely to be related to the same kinds of V species. The positions, as well as intensities of the components of the fine structure strongly depend on the pretreatment conditions (Figure 2a-c). For V1.5Siβ, two vibrational fine structures can be distinguished (Figure 2) but they are not very well resolved. A broad band at around 425 nm suggests the presence in V1.5Siβ of an important amount of hydroxyl groups, in agreement with our previous FT-IR results.24 For C-V1.5Siβ, the bands of vibrational fine structure are more intense and better resolved and photoluminescence maxima are shifted to higher wavelength. In

Three Kinds of Tetrahedral V Species in Beta Zeolite

Figure 3. Photoluminescence spectra at 77 K of V0.5Siβ sample outgassing at 353 K for 2 h (10-3 Pa), of C-V0.5Siβ outgassing at 473 K for 2 h (10-3 Pa), and of C-Hyd-V0.5Siβ outgassing at 473 K for 2 h (10-3 Pa).

Figure 4. Photoluminescence spectra at 77 K of C-V0.05Siβ outgassing at 473 K for 2 h (10-3 Pa) and of C-Hyd-V0.05Siβ outgassing at 473 K for 2 h (10-3 Pa).

contrast, for C-Hyd-V1.5Siβ, the bands of the vibrational fine structure are less intense and photoluminescence maxima are shifted to lower wavelength by comparison with C-V1.5Siβ. Similar photoluminescence spectra with vibrational fine structure are observed for VSiβ samples with smaller V content (Figures 3 and 4). Different pretreatments lead to changes of intensities and positions of the bands of the vibrational fine structure. For V0.05Siβ, the photoluminescence spectrum is not presented because of the interference of the strong luminescence of OH

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Figure 5. Second derivative of the photoluminescence spectra at 77 K of V1.5Siβ outgassing at 353 K for 2 h (10-3 Pa), of C-V1.5Siβ outgassing at 473 K for 2 h (10-3 Pa), and of C-Hyd-V1.5Siβ outgassing at 473 K for 2 h (10-3 Pa).

groups which completely masks the photoluminescence of V species. The C-V0.05Siβ and C-Hyd-V0.05Siβ give very good photoluminescence spectra (Figure 4), composed of different vibrational fine structures. For the C-V0.05Siβ, two vibrational fine structures can be identified (Figure 4) but they are not very well resolved. In contrast, for C-Hyd-V0.05Siβ, three vibrational fine structures can be clearly distinguished. In order to ease the analysis of the complex photoluminescence spectra due to the superposition of different vibrational fine structures, the second-derivative presentation of the photoluminescence is used. For V1.5Siβ (Figure 5), three vibrational fine structures are clearly seen which can be related to the presence of three (R, β, γ) different kinds of tetrahedral VV species, R and β in larger concentration than γ. The second-derivative photoluminescence spectra of the above samples are not strongly affected by the photoluminescence due to silanol groups and the energy separation between the (0 f 0) and (0 f 1) vibrational transitions can thus be determined for each V species, in good agreement with the vibrational energy of the surface VdO bond obtained by IR or Raman measurements for various vanadia supported catalysts.29,42,43,46-50 The vibrational energy calculated from the second-derivative spectrum of β species corresponds with 1054 cm-1. On the basis of the linear correlation between the wavenumber of the VdO bond and its bond length in several V compounds,43 a bond length of 1.54 Å is derived for the Vd O bond length of the β species. For C-V1.5Siβ, from the three species, γ and β appear as the dominating ones (Figure 5). The vibrational energy of the γ species corresponds with 1036 cm-1 with a longer VdO bond length (1.56 Å) and a higher symmetry than for the β species. With the increase of VdO bond length, the symmetry moves toward the perfect tetrahedral one which would have identical single V-O bonds. The second-derivative spectrum shows that for C-Hyd-V1.5Siβ, the R species is the main one (Figure 5). The vibrational energy of the R species corresponds with 1018 cm-1 with yet a longer VdO bond length (1.58 Å) and a higher symmetry than for γ. When the C-Hyd-V1.5Siβ sample is calcined at 773

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Figure 6. Second derivative of the photoluminescence spectra at 77 K of V0.5Siβ outgassing at 353 K for 2 h (10-3 Pa), of C-V0.5Siβ outgassing at 473 K for 2 h (10-3 Pa), and of C-Hyd-V0.5Siβ outgassing at 473 K for 2 h (10-3 Pa).

K for 2 h in flowing oxygen, the second-derivative photoluminescence spectrum, characteristic of C-V1.5Siβ, is restored. The subsequent exposure of the latter sample to room atmosphere resulted in the second-derivative photoluminescence spectrum of C-Hyd-V1.5Siβ again, indicating that the dehydration and rehydration processes are reversible. The vibrational energy found for the R, β, and γ species is in good agreement with that of surface vanadyl groups.41 Both, the VdO bond length and the symmetry of V species decrease in the following order: R > γ > β along with a concomitant increase of the OdV-O(Si,H) bond angle, on the basis of Valence shell electron pair repulsion (VSEPR) arguments which apply for d0 systems where there is no stabilization effect due to crystal field.51 As seen in Figures 6 and 7, the second derivative photoluminescence spectra are better resolved for VSiβ samples with lower V loading. For V0.5Siβ, C-V0.5Siβ, and C-Hyd-V0.5Siβ three vibrational fine structures (Figure 6) can be distinguished related to R, β, and γ species. It should be noted here that after rehydration R is the main species as in the case of C-Hyd-V1.5Siβ. For V0.05Siβ, the second-derivative spectrum is not presented, because of its very poor resolution related to the interference of the luminescence of OH groups. In contrast, for C-V0.05Siβ and C-Hyd-V0.05Siβ, three different vibrational fine structures can be clearly distinguished. For C-V0.05Siβ, β is the main species. After rehydration of this sample, the relative intensities of the vibrational fine structures change a little and C-HydV0.05Siβ exhibits also three kinds of species in contrast to the high V loading samples where the main species is R. The values of intensities of the transitions in the range 400-450 nm for C-Hyd-V0.05Siβ are uncertain because of interference of luminescence of OH groups. Because of this problem, the vibrational transitions for R, β, and γ species in this range are determined from the second derivative of C-V0.05Siβ. Time-ResolVed Photoluminescence Measurements. Timeresolved photoluminescence spectroscopy is a powerful technique to distinguish species with different lifetimes. Figure 8

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Figure 7. Second derivative of the photoluminescence spectra at 77 K of C-V0.05Siβ outgassing at 473 K for 2 h (10-3 Pa) and of C-HydV0.05Siβ outgassing at 473 K for 2 h (10-3 Pa).

Figure 8. Photoluminescence (a, b, c) and their second derivative (a′, b′, c′)* spectra of C-Hyd-V0.05Siβ sample outgassing at 473 K for 2 h (10-3 Pa) measured under continuous irradiation at 250 nm (a, a′), measured between 0.05 and 3 ms after pulse irradiation at 250 nm (b, b′), and measured between 50 and 100 ms after pulse irradiation at 250 nm (c, c′). (*Here the second derivatives are multiplied by -1.)

shows the photoluminescence spectra of C-Hyd-V0.05Siβ and their second derivatives measured under continuous irradiation

Three Kinds of Tetrahedral V Species in Beta Zeolite

Figure 9. Decay curves (logarithmic scale) obtained from dynamic photoluminescence of C-Hyd-V1.5Siβ outgassing at 353 K for 2 h (10-3 Pa) (a), of C-V0.05Siβ outgassing at 473 K for 2 h (10-3 Pa) (b), and of C-V1.5Siβ outgassing at 473 K for 2 h (10-3 Pa) (c) monitored at 77 K.

at 250 nm (Figure 8a,a′), measured between 0.05 and 3 ms (Figure 8b,b′) and between 50 and 100 ms (Figure 8c,c′) after pulse irradiation at 250 nm. In Figure 8, all second derivatives were multiplied by -1 to emphasize the good correspondence between the vibrational bands shown in the normal and their second-derivative presentation, respectively. The photoluminescence spectrum of the C-Hyd-V0.05Siβ measured under continuous irradiation is not well resolved; however, its corresponding second derivative clearly shows the three different vibrational fine structures due to the presence of three kinds of tetrahedral V species, with R and β as main species. In the photoluminescence spectrum and its second derivative measured between 0.05 and 3 ms after pulse irradiation at 250 nm (Figure 8b,b′), R is the main species while in the photoluminescence spectrum and its second derivative measured between 50 and 100 ms after pulse irradiation (Figure 8c,c′) β is the main species, indicating that the lifetime of the excited β is much longer than that of R. Dynamic Photoluminescence Measurements. Since, the decay time of the triplet-to-singlet emission is known to depend on the degree of distortion of the symmetry of transition metal ions from the ideal tetrahedral one,38 measurements of such decay times have been performed. The lifetimes of the excited triplet states of R, β, and γ species have been determined to be 28, 88, and 49 ms, respectively from the decay curves of the dynamic photoluminescence of C-Hyd-V1.5Siβ (Figure 9a), C-V0.05Siβ (Figure 9b), and C-V1.5Siβ (Figure 9c), respectively. The increase of the lifetime from 28 to 88 ms in the sequence R < γ < β can be interpreted in terms of an increase of the distortion of the tetrahedral VV species and a decrease of the VdO bond length. The three kinds of tetrahedral VV species are likely to be due to three different framework species with different V-O-Si angles and VdO bond lengths. Moreover, the very long lifetimes of the excited triplet states of the β (88 ms) and γ (49 ms) species, much longer than those obtained with impregnated vanadium oxides52 or grafted vanadium oxide prepared by the photo-CVD method,40 suggest that vanadium ions are highly dispersed within the VSiβ zeolite. Discussion Since the active sites of selective oxidation of alkenes and alkanes on crystalline microporous metallosilicates have been shown to be transition metal cations in tetrahedral coordination,1,5,14,53,54 the study of their environment has become an

J. Phys. Chem. B, Vol. 104, No. 25, 2000 6017 important issue. While it is possible to discriminate an octahedral from a tetrahedral environment, using different techniques (UVvisible, NMR, ESR, Raman, ...), it seems more difficult to distinguish one tetrahedral environment from the others. Nature and Environment of VV Species Obtained from UV-Visible Spectroscopy. Tetrahedral V Species. For asprepared VSiβ samples, UV-visible spectroscopy shows the presence of one kind of tetrahedral VV with two charge transfer bands at around 265-270 and 340 nm (Figure 1). The UVvisible results agree with 51V NMR studies. Indeed, 51V NMR signals at -633 ppm (MAS) and at -580 ppm (wide line) (not shown) confirm the presence of only one tetrahedral VV species. This species does not change its coordination even when VSiβ samples are kept several months in the moist air. The large width of the UV-visible bands and the change of their relative intensities with V content can suggest the presence of different kinds of tetrahedral VV species. The shift of the UV-visible charge transfer bands to lower wavelengths upon calcination (Figure 1d) and the simultaneous decrease of their intensity suggest a greater distortion of tetrahedral VV species in C-VSiβ than in VSiβ, in agreement with the conclusions reported for KVOF4 and the VO43complex.38,39 Mixture of Tetrahedral and Octahedral V Species. The UVvisible spectra of C-Hyd-VSiβ with high V content show the presence of two kinds of tetrahedral VV. The first is identified by the shoulder at 235 nm and the second one by the band at about 270 nm, which is less distorted than that observed at 235 nm (Figure 1f-g). Moreover, the octahedral VV are identified by the band at 375 nm. It suggests that upon rehydration of C-VSiβ, some of the more distorted tetrahedral V species are still present in the sample but the major part is transformed into less distorted tetrahedral and into octahedral V species. The increase of the intensity and the constant position of the band at 375 nm as a function of time of exposure to moist air indicate that this band represents mononuclear VV in octahedral coordination. The high dispersion in the zeolite structure of these species and their interaction with water molecules are confirmed by the disappearance of the band at 375 nm after removal of adsorbed water upon calcination and the reappearance of a band at wavelength lower than 340 nm (not shown), which characterizes isolated tetrahedral VV ions.55 Furthermore, the large width of the band at 375 nm suggests that this band can be related to a mixture of different octahedral VV species. The UV-visible spectrum of C-Hyd-VSiβ with low V content shows only one kind of tetrahedral VV species identified by two charge transfer bands at around 270 and 340 nm (Figure 1e), very similar to those observed for VSiβ (Figure 1a). The white color of C-Hyd-VSiβ with low V content in moist air and the absence of the band at 375 nm suggest that tetrahedral VV ions are in a site poorly accessible to water, such as S1 site in the five-membered rings of the β structure (Figure 10) and thus do not change their coordination upon calcination/rehydration treatments. The UV-visible studies show that tetrahedral VV species in C-VSiβ with higher V content can change their coordination in contact with moist air to square-pyramidal or octahedral. This suggests that when the V content increases, VV ions are incorporated in a site more accessible to water, such as S2 site in the 12-membered rings (Figure 10) where they can easily change their coordination upon calcination/rehydration treatments. It can be noticed that the spectrum of unsupported vanadia, which can be considered as a model for distorted octahedral VV species, is characterized by an intense band at

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Figure 10. Possible locations of two different type of sites for incorporation of V in β structure.

402 nm.56 The blue shift of 27 nm observed here for octahedral VV species (375 nm) indicates that in C-Hyd-VSiβ octahedral V are well dispersed. Such a shift has already been reported for supported molybdenum oxides57 and for titanium dispersed in silicates,58 indicating the presence in these systems of mononuclear Mo and Ti species, respectively. The presence of well-dispersed mononuclear VV species in C-Hyd-VSiβ is also supported by the fact that the color change between yellow and white is fully reversible upon calcination/rehydration cycles. Such a reversible behavior would be difficult to imagine with oligomeric VV species. Nature and Environment of Tetrahedral V Species Obtained from Photoluminescence Spectroscopy. Because of the high sensitivity of luminescent sites to their local environment,42 photoluminescence spectroscopy can be useful in discriminating different tetrahedral V species. The excitation spectra of tetrahedral VV sites at about 230-300 nm are attributed to an oxygen to vanadium charge transfer O2- ) V5+ f (O--V4+)* transition and emission spectra at about 500 nm are attributed to a reverse radiative decay process, i.e., the charge transfer from the excited triplet state to the singlet ground state (O-V4+)* f O2- ) V5+.40-46 The changes of the positions and the relative intensities of the absorption bands at 240 and 290 nm in the excitation spectra after calcination and then rehydration (Figure 2a′-c′) as well as variations of the relative intensities of the different components in the photoluminescence spectra of different VSiβ, C-VSiβ, and C-Hyd-VSiβ samples (Figures 5, 6, and 7) indicate that the relative populations of vanadium framework sites change with pretreatment conditions and V loading. The advantage of measuring the excitation in addition to the emission spectra is the greater sensitivity than of standard absorption measurements. As described above, the contacting dealuminated Siβ with NH4VO3 at room temperature leads to incorporation of V ions

Figure 11. Proposed ways of formation of three kinds of tetrahedral V species in VSiβ zeolite.

in the β framework. Photoluminescence spectra show that these V ions are present in VSiβ samples in three different kinds of symmetry (R, β, and γ) as depicted in Figure 11. The formation of each kind of tetrahedral V species is related to the state of vacant T atom sites (silanol nests). These species are formed by reaction of VO2+ ions, present in the highly acidic and diluted solution of NH4VO3, with silanol groups. For R species, we propose hydroxylated (SiO)2(HO)VdO sites with structure A; for β species, non-hydroxylated (SiO)3VdO sites with stucture B; and for γ species, non-hydroxylated (SiO)3Vd O sites with structure C. A ball and stick representation, obtained using the MSI-Cerius 2 program and polymorph A structure taken from Newsan et al.’s paper,59 of the framework β structure with these three tetrahedral VV species is shown in Figure 12. The tetrahedral VV species in VSiβ with low V content are poorly sensitive to calcination/rehydration treatments (Figures 1 and 7), suggesting that for low V content vanadium ions are first incorporated inside the zeolite structure in particular sites (such as S1, Figure 10) where they are poorly accessible to water. The dramatic changes of relative concentrations of different tetrahedral VV species observed by photoluminescence spectroscopy upon calcination and then rehydration for the sample with high V content (Figures 5 and 6) suggest that for higher V contents vanadium ions are incorporated in a second type of sites, such as S2 probably in the 12-membered rings where they are more accessible to water and can easily change their coordination (Figure 10). The possible formation of the Si+ Lewis acid sites shown in Figure 11 upon dehydration is confirmed by the appearance of IR bands at 1596 and 1445 cm-1 after contact of pyridine with dehydrated VSiβ zeolite.26

Three Kinds of Tetrahedral V Species in Beta Zeolite

J. Phys. Chem. B, Vol. 104, No. 25, 2000 6019

Figure 12. Ball and stick representation of the framework structure of VSiβ with the three kinds of tetrahedral (structures A, B, C) and the two kinds of octahedral (structures D and E) V species.

Calcination of VSiβ changes the relative intensities of the different tetrahedral VV species (Figures 5 and 6) and γ and β become the main species. At the same time, the shift of the UV-visible charge transfer absorption bands to lower wavelengths than observed for as-prepared VSiβ, coupled with the simultaneous decrease in their intensities (Figure 1), suggest a greater distortion of the tetrahedral VV species in C-VSiβ than in VSiβ. These changes are related to transformation of part of hydroxylated (SiO)2(HO)VdO sites with structure A to nonhydroxylated (SiO)3VdO ones with structures B and/or C shown in Figure 13. The appearance of R as the main species after rehydration of C-VSiβ in moist air suggests that this kind of V species is formed as a result of hydrolysis of the V-O-Si bridge in structures B and/or C (reactions 1 and 1′ in Figure 13). The presence of hydroxylated (SiO)2(HO)VdO sites with structure A in C-VSiβ was confirmed by IR spectroscopy26 which shows creation of VO-H and SiO-H acidic groups whose appearance or disappearance was very sensitive to calcination/rehydration process. Moreover, UV-visible spectra of C-Hyd-VSiβ show that during rehydration of C-VSiβ not only part of tetrahedral VV species is transformed to less distorted tetrahedral VV species but also a part of the tetrahedral VV species is coordinated with two molecules of water with formation of octahedral VV species (such as δ and ) with structures D and/or E (reactions 2 and 2′, Figure 13), suggested by the broadness of the UV-visible band at 375 nm. A ball and stick representation of the polymorph A framework β structure with these two octahedral VV species is shown in Figure 12. The UV-visible and photoluminescence studies show that three different kinds of tetrahedral V species (R, β, γ) can occur at two different types of sites, such as S1 and S2, inside the β structure (Figure 10). The low sensitivity of tetrahedral VV species at low V loading and the high sensitivity at high V loading to calcination/rehydration treatments suggest that va-

Figure 13. Proposed ways of formation of tetrahedral and octahedral V species in C-VSiβ and C-Hyd-VSiβ zeolites.

nadium is first incorporated in a site poorly accessible to water, such as S1 site in the five-membered rings and then with increasing the V content in a site more accessible to water, such as S2 site in the 12-membered rings where its symmetry can

6020 J. Phys. Chem. B, Vol. 104, No. 25, 2000 change upon calcination/rehydration treatments. In VSiβ zeolite, the calcination/rehydration process is reversible. Conclusions Diffuse reflectance UV-visible and photoluminescence spectroscopies have been used to distinguish different kinds of tetrahedral and octahedral VV species in VSiβ zeolites with low (0.05 V wt %) or high (0.5 and 1.5 V wt %) V content. UV-visible spectroscopy shows in as-prepared VSiβ zeolite whatever the V content and in calcined/rehydrated VSiβ with low V content only one kind of tetrahedral VV species. However, this technique reveals the presence in calcined/rehydrated samples with high V content two kinds of tetrahedral and one of octahedral VV species. Photoluminescence spectroscopy in static or dynamic mode clearly allows to distinguish in as-prepared VSiβ three kinds of tetrahedral VV species. The relative amounts of these species strongly depend on vanadium content and calcination/rehydration treatments. They are present at the two different types of framework sites. For samples with low V loading these V species are in a site poorly accessible to water, such as S1 site in the five-membered rings where they are poorly sensitive to calcination/rehydration treatments. In contrast, for higher V loading, the tetrahedral VV species become highly sensitive to such treatments suggesting that VV ions are in a site more accessible to water, such as S2 site in the 12-membered rings, where they can easily change their coordination upon calcination/rehydration treatments. Photoluminescence spectroscopy is able not only to show the presence of these species but also to determine their relative distortion and give information about their VdO bond length. With this technique octahedral species cannot be observed but only tetrahedral ones. It is very sensitive to the tetrahedral species particularly at low V content and gives electronic and vibrational information; i.e., it observes vibrational fine structures and gives vibrational energies allowing to differentiate them. Possible models of the three kinds of tetrahedral and two kinds of octahedral VV species and their probable location in the β structure are proposed. Further studies are underway on VSiβ zeolite, in particular its catalytic activity is examined in alkane oxidative dehydrogenation. Acknowledgment. S.D. gratefully acknowledges the CNRS for financial support as “Chercheur Associe´”. Special thanks are due to Dr. D. Costa (Laboratoire de Re´activite´ de Surface) for her help in the preparation of ball and stick models of framework V species. References and Notes (1) Notari, B. Stud. Surf. Sci. Catal. 1988, 37, 413. (2) Camblor, M. A.; Corma, A.; Martinez, A.; Perez-Pariente, J. J. Chem. Soc., Chem. Commun. 1992, 8. (3) Thangaraj, A.; Sivasanker, S.; Ratnasamy, P. J. Catal. 1991, 131, 394. (4) Romano, U.; Esposito, A.; Maspero, F.; Neri, C.; Clerici, M. G. Stud. Surf. Sci. Catal. 1990, 55, 33. (5) Taramasso, M.; Perego, G.; Notari, B. U.S. Patent 1983, 4 410 501. (6) Roffia, P.; Padovan, M.; Moretti, E.; De Alberti, G. Eur. Patent 1987, 0208311. (7) Clerici, M.; Ingallina, P. J. Catal. 1993, 140, 71. (8) Whittington, B. I.; Anderson, J. R. J. Phys. Chem. 1993, 97, 1032. (9) Sen, T.; Rajamohanan, P. R.; Ganapathy, S.; Sivasanker, S. J. Catal. 1996, 354, 163.

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