Identification of Mixed Valence Vanadium in ETS-10 Using Electron

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J. Phys. Chem. C 2009, 113, 10477–10484

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Identification of Mixed Valence Vanadium in ETS-10 Using Electron Paramagnetic Resonance, 51V Solid-State Nuclear Magnetic Resonance, and Density Functional Theory Studies Kristopher Ooms† and Tatyana Polenova Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716, USA

Anne-Marie Shough and Douglas J. Doren Center for Catalytic Science and Technology, Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716

Michael J. Nash‡ and Raul F. Lobo* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: March 13, 2009; ReVised Manuscript ReceiVed: April 21, 2009

Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron-nuclear dipolar interaction between the V(V) nuclei and the unpaired electron on V(IV). The NMR chemical shift and electric field gradient parameters estimated from the NMR spectra are used in conjunction with density functional theory calculations to propose a model where the V(V) species preferentially occupy sites at the ends of the octahedral chains. Introduction The development of new photocatalysts capable of decomposing a large variety of organic pollutants using solar energy is an area of intensive research.1,2 It has been shown that semiconductor photocatalysts, in particular TiO2, are well-suited for this purpose.3,4 Recently, microporous titanosilicate materials have also been shown to be effective photocatalysts for the decomposition of small organic molecules. One of the most promising microporous titanosilicates is ETS-10 (Engelhard titanosilicate).5-7 It is composed of octahedrally coordinated TiO6/2 units and tetrahedrally coordinated SiO4/2 units. The TiO6/2 octahedra connect through the apical oxygen atoms forming linear chains of corner-sharing units surrounded by SiO4/2 tetrahedra. The framework structure forms a three-dimensional 12-ring pore network, with an ideal unit cell composition of (Na,K)2TiSi5O13.8 The structure of ETS-10 is inherently disordered as the result of the intergrowth of two end polymorphs. The structure of polymorph B and an enlarged section of the * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: 302 831 1261. Fax: 302 831 2085. † Current address: Department of Chemistry, The King’s University College, 9125 50th Street, Edmonton, Alberta T6B 2H3, Canada. ‡ Current address: Eastman Chemical Company, P.O. Box 1972, Kingsport, Tennessee 37662.

Figure 1. Polymorph B of ETS-10.

chain of TiO6/2 octahedra are shown in Figures 1 and 2, respectively. In an effort to enhance the visible light photoreactivity of ETS-10, several groups have doped transition metals into the structure. Uma et al.9 have shown that the addition of Cr and

10.1021/jp902275f CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

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Figure 2. Quantum mechanics cluster region for 3M ONIOM model (perspective view) illustrating the structure of the titanium oxide chain in ETS-10. Some atoms are not shown as spheres to facilitate visualization of the structure and specific sites for V(V) substitution. The numbering scheme shown is used to identify different V(V) substituted structures. Purple atoms are counterions (Na+).

Co to the ETS-10 structure resulted in visible light activity for the decomposition of acetaldehyde. Nash et al.10 have also shown that the addition of V to the ETS-10 structure leads to visible light activity for ethylene oxidation. The location of the transition metals in the ETS-10 structure and their oxidation state have a large effect on the photoreactivity of the material. The band gap energy of ETS-10 (4.02 eV)11 is a property of the titanium octahedral chains and can therefore be modified when transition metals are substituted for the titanium atoms along the chain. Cr9 and V10,12 have been shown to substitute for titanium, resulting in lower band gap energy and visible light reactivity. In the case of Cr, however, this visible light reactivity was only observed from the assynthesized material after calcination at 500 °C.9 In the case of V, visible light reactivity was observed from the as-synthesized material.10,12 For the Cr-doped samples, changes in the oxidation states of the Cr atoms after calcination could be the reason for the increase in visible light reactivity. Using diffuse reflectance ultraviolet spectroscopy (DR-UV) and EPR, Cr was shown to substitute for the titanium atoms in an oxidation state of 3+ for the as-synthesized material.9,13 After calcination, Uma et al.9 observed changes in the DR-UV typical of the formation of Cr(VI) and attributed the visible light reactivity to the formation of this higher oxidation state of Cr. For the vanadium-doped materials, near edge X-ray absorption fine structure (NEXAFS) measurements have suggested the presence of V(IV) and V(V) sites.14 However, the resolution between the peaks and the fact that the results suggested a higher concentration of V(V) than V(IV), make it difficult to draw definitive conclusions about the incorporation of V(V) into the structure.10,14 It is clear that the oxidation state of the dopant metal has a large effect on the reactivity, and it is a parameter that needs to be determined to understand the relationship between composition and photoreactivity. Additionally, recent theoretical studies12 have suggested that different oxidation states may be required to form electron-hole traps that allow for visible light reactivity. To date, the experimental evidence for the presence of mixed oxidation states in these materials remains elusive, and the local electronic structure has been thus a subject of controversy. The presence of two different oxidation states for transition metals incorporated into titanosilicate materials is not uncommon, especially with V. In AM-2, another titanosilicate material, Ferdov and co-workers15 showed that V(IV) and V(V) were substituted into the structure using DR-UV and 51V NMR. They

Ooms et al. also suggested that these different V oxidation states could be located in different coordination environments within the AM-2 structure. It is therefore important to identify the vanadium oxidation states present in the doped ETS-10 materials as well as the location of these states within the structure. In this work, we report the combined use of solid-state EPR and 51V solid-state NMR (SSNMR) spectroscopy to study the local environments in vanadium-doped ETS-10 samples with varying vanadium concentrations. 51V SSNMR is especially useful because it is an effective method for determining the presence of diamagnetic vanadium and providing detailed information about its coordination environment in various inorganic materials and biological solids.16-21 On the basis of EPR and NMR results, we present clear evidence for V(IV) and V(V) in the V incorporated samples of ETS-10 (ETVS-10s) and for AM-6, the all-vanadium ETS-10 analogue. On the basis of the density functional theory calculations of the NMR parameters, we propose a model structure for the location and coordination environment of the V(V) species in vanadium-doped ETS-10 solids. The combination of experiment and theory thus presents a powerful approach yielding detailed information about the local structure of these photocatalytic materials. 51 V Solid-State NMR Spectroscopy and Electron-Nuclear Dipolar Interaction in Paramagnetic Solids. 51V spectra of diamagnetic V(V) solids are dominated by two interactions: quadrupolar coupling and chemical shift anisotropy. The former is the interaction of the nuclei possessing spins greater than 1/2 with the electric field gradient (EFG) around the nucleus. This interaction gives rise to splitting of the spin energy levels, resulting in multiple NMR transitions. In the case of a halfinteger quadrupolar nuclei such as 51V (I ) 7/2), the individual transitions are referred to as the central transition when mI ) +1/2 T -1/2 and as the satellite transitions when mI ) (7/2 T (5/2, (5/2 T (3/2, and (3/2 T (1/2. For many vanadium compounds, the electric field gradient can be determined best from the satellite transitions observed in the solid-state NMR, and the chemical shift anisotropy is determined from the central transition. The electric field gradient is a traceless second-rank tensor with three principal components, |VZZ| g |VYY| g |VXX|. In the Cartesian coordinate system, the electric field gradient tensor is defined by two independent parameters, the nuclear quadrupole coupling constant, CQ ) eQVZZ/h and the asymmetry parameter, ηQ ) (VXX - VYY)/VZZ, where e is the electronic charge and h is Planck’s constant.22 The 51V quadrupole moment Q is -0.048 × 10-28 V/m2.23 The chemical shift interaction (more precisely, its symmetric part that is detectable in the conventional solid-state NMR experiments) is expressed as a symmetric second-rank tensor with the principal components in the diagonal representation as |δzz - δiso| g |δxx - δiso| g |δyy - δiso|. Typically, the chemical shift is expressed in terms of the isotropic chemical shift, δiso ) (1/3)(δxx + δyy + δzz), chemical shift anisotropy, δσ ) δzz δiso, and asymmetry parameter, ησ ) (δyy - δxx)/(δzz - δiso).24 In general, chemical shift anisotropy provides information about the molecular orbitals at the vanadium, particularly the orbitals near the HOMO-LUMO gap that contain significant vanadium d character. When a 51V nucleus is near an unpaired electron, the electron-nuclear dipolar interaction influences the NMR spectrum. The presence of unpaired electrons typically makes V(IV) states undetectable by NMR because of the fast T2 relaxation that broadens the signal beyond detection. However, the NMR signal of V(V) can be detectable when the unpaired electrons are on

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atoms in close proximity, and the interaction with these unpaired electrons is manifested in the broadening of the spectral lines via electron-nuclear dipolar coupling. Solid-state NMR spectroscopy of various nuclei dipolar coupled to paramagnetic centers has been previously used to probe local environments in a number of materials and bioinorganic systems25-31 Whether the NMR signal can be observed depends on the magnitude of the electron-nuclear dipolar coupling, which is determined by the magnitude of the electronic magnetic moment and the distance to the paramagnetic center. Paramagnetic interactions include the Fermi contact shift and the electron-nuclear dipolar interaction. In this work, we focus on the direct electron-nuclear interaction that because of its direct dependence on the electron-nuclear distance provides important information about the geometry of the paramagnetic centers in proximity to the V(V) sites. This interaction is expressed by the following Hamiltonian29,31,32 paramagn ˆ DIP H )

µ0 µ¯ 4π e

(∑ i

)

1 (δRβ - 3eReβ) µn ri3

(1)

where µ0 is the magnetic permeability, µ j e is the thermally averaged electronic magnetic moment, r is the electron-nuclear distance, δβR is the Kronecker delta, eR and eβ are the (x,y,z) components of the electron-nuclear dipolar vector, and µn is the nuclear magnetic moment. In the absence of spin-orbit coupling, which is a valid assumption for V(IV), the thermally averaged electronic magnetic moment can be expressed as

µ¯ e )

µBS(S + 1) g˜g˜H0 3kBT

(2)

where µB is the Bohr magneton, S is the electron spin quantum number, kB is the Botzmann’s constant, g˜ is the electron g tensor, and H0 is the magnetic field strength. Because of the fast thermal averaging of the electronic magnetic moment, the symmetry of the electron-nuclear dipolar interaction cannot be separated from the chemical shift anisotropy, δσ, in the magic-angle spinning (MAS) experiments. However, µ j je is temperature dependent. The temperature dependence of the electron-nuclear term will be much greater than that of the chemical shift, which typically has a very small temperature dependence that is rarely observed in solid compounds unless the nucleus of interest has an extremely large chemical shift range such as 59Co.22 Therefore, these two interactions can usually be disentangled using variable-temperature experiments.30,31 Experimental Section Sample Preparation and EPR Spectroscopy. The synthesis of the ETVS-10 samples with V/(V + Ti) ratios of 0.13, 0.33, 0.43, and 1.0 (AM-6) has been described previously.10 The characterization of the ETVS-10 and AM-6 samples was also reported previously, and samples were shown to be of high purity with small quartz and TiO2(anatase) impurities (less than 5%) in the mixed Ti and V materials.10 Samples were investigated in the as-synthesized form and were not heated prior to characterization. The EPR spectra of the V samples were obtained at room temperature using a Bruker EMX spectrometer equipped with a Bruker ER 4102ST cavity with X-band radiation. The spectra were simulated using the Bruker Simfonia software. Solid-State NMR Spectroscopy. Solid-state NMR spectra were collected at 157.61 MHz (B0 ) 14.1 T) on a Varian InfinityPlus spectrometer using a Varian 3.2 mm double

resonance HX MAS probe or at 105.23 MHz (B0 ) 9.4 T) on a Tecmag Discovery spectrometer using a Doty 4 mm triple resonance XC4 MAS probe. Approximately 12 mg of sample was used for experiments performed at 14.1 T, and 64 mg of sample was used for experiments performed at 9.4 T. All spectra were acquired using one-pulse excitation with the pulse width of 0.82 µs at a B1 field of 58 kHz on the 14.1 T instrument, and 1.1 µs at a B1 field of 58 kHz on the 9.4 T instrument; sweep width was typically 2 MHz. It was found that T1 relaxation was fast enough that experimental recycle times could be as low as 0.1 s without compromising the sensitivity. The high frequency peak of an aqueous 0.5 M Na3VO4 solution was used to determine the B1 field strength and as a secondary reference (-534 ppm) and was calibrated to the primary reference of neat VOCl3 at 0 ppm. Spectra were processed by back-predicting the first rotor echo and performing a Fourier transformation; Gaussian line broadening of 1000 Hz was used. Computational Methods. All V-substituted ETS-10 models have been derived from the 3M ONIOM cluster models previously developed to investigate the effects of transition metal substitution in ETS-10.12,14,33 These systems are described using a hybrid quantum mechanics/molecular mechanics (QM/MM) method known as the two-layer ONIOM method34,35 as implemented in Gaussian 03.36 In this manner, the cluster is divided into two regions: (1) a region of interest described using QM that includes a single O-M-O chain incorporating three metal (M) sites surrounded by the immediate SiO2 framework and (2) an extended region described using MM that includes the SiO2 pore structure and regions at the front and ends of the O-Ti-O chain. Models have been considered where V(V) substitutes for Ti(IV) octahedral and Si(IV) tetrahedral sites. The numbering system used to identify the atoms of the cluster model is indicated in Figure 2. Using the nomenclature “coord-[n]-M”, where coord is the metal ion coordination (o is octahedral and t is tetrahedral), [n] is the position number or numbers of the substituted Ti site(s) (1, 2, and/or 3) used to distinguish models with octahedral substitution only, and M is the metal being substituted for Ti/Si, we have investigated the following cluster models: o-2-VV, o-1,3-VV, o-1,3-VV/o-2-VIV, and t-VV (Figure 2). The three models containing V(V) octahedral substitution (o-2-VV, o-1,3-VV, and o-1,3-VV/o-2-VIV) have been previously investigated.12,14 Details on the optimization of these systems can be found elsewhere.14 To create the t-VV system, a VdO group was substituted into a Si1 position of the 3Ti ONIOM model (see Figure 2 for atom assignments), initially forming an [OdVVO4]- structure. To form an [OdVVO3]0 species and correct charge balancing, one of the oxygen atoms must be capped with a proton. There are four V-O-Si/Ti bonds that may be capped to create [OdVVO3 HO-Si/Ti]. To ensure that we have investigated the lowest energy configuration, we have considered all four models. To distinguish among the four t-VV models, we will use the nomenclature t-VV-O1, t-VV-O2, t-VV-O3, and t-VV-O4, where the oxygen number denotes the atom that is capped to create the HO-Si/Ti unit (Figure 2). In all four t-VV models, the QM and MM regions of the 3M ONIOM systems were extended to allow for realistic geometric relaxation. This included 15 additional Si/O atoms to allow for two full coordination spheres of QM Si/Ti atoms surrounding the Si1 site as well as an extension of the MM region to fully encompass the new QM region. The same computational methods used to optimize the 3M ONIOM models, were employed in all t-VV model optimizations.14

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Figure 3. EPR spectra of ETVS-10 samples. The fine structure observed in the 0.13 ETVS-10 sample arises from the orientation dependence of the hyperfine and g tensors. In the samples with greater V loading, the fine structure is obscured by dipolar couplings to other paramagnetic centers.

For all models, the NMR parameters were calculated for the QM region of the optimized cluster models. The NMR properties were derived using Becke’s three parameter hybrid functional,37 B3LYP, with a 6-311++G basis set. Results and Discussion Identification of V(IV) Using EPR. The vanadium precursor used in the preparation of the ETVS-10 samples is VOSO4 · 5H2O, where V is in a 4+ oxidation state, and it is thus expected that V(IV) is present in the as-synthesized samples in significant amounts. Indeed, a clear signal from paramagnetic V(IV) was observed using electron paramagnetic resonance spectroscopy as illustrated in Figure 3. It can be seen that the EPR signal from the 0.13 ETVS-10 sample has a well-resolved hyperfine structure. The hyperfine structure arises because the electron g tensor and the hyperfine interaction tensor are anisotropic; the g and A tensor values determined experimentally (gxx ) 1.945, gyy ) gzz ) 1.975, Axx ) 187 gauss, Ayy ) Azz ) 62 gauss) are typical of vanadium in an octahedral coordination in an axial symmetric crystal field.38,39 As the V content is increased, the spectrum loses its hyperfine character and becomes one broad signal. This results from electron-electron coupling due to the presence of multiple V(IV) atoms in close proximity to each other. This spectral change suggests that the V(IV) atoms substitute for the titanium atoms in the chain and are isolated from one another at a low concentration. At higher vanadium concentrations, the distance between vanadium atoms decreases, causing the loss of the hyperfine structure. Identification of V(V) Using NMR.40,41 Figure 4 shows the 51 V solid-state NMR spectra of ETVS-10 samples prepared with V/(V+Ti) ratios of 0.13, 0.33, and 0.43 and a sample of AM-6 (containing no titanium in the structure). The first and trivial observation made is that a significant signal is obtained from the vanadium nuclei in these samples. This immediately indicates the presence of V(V) in the solids. We attribute the signal to V(V) sites, rather than V(IV), because direct detection of a signal from paramagnetic metal centers is not feasible under normal conditions using NMR because of the extremely fast T2 relaxation of the metal nuclei, which results in extreme line broadening. Additionally, the chemical shifts observed are

Figure 4. 51V MAS NMR spectra of solid vanadium ETS-10 samples with V/(V + Ti) ratios of 0.13, 0.33, 0.43, and 1.0 (AM-6) acquired at 14.1 T. νr for spectra a-c is 20 kHz and for spectrum d is 15 kHz. The spectra were all acquired at room temperature.

characteristic of those for the V(V) species.16-21 The observed signal from vanadium nuclei confirms that V(V) must be present in the sample. Anisotropic 51V NMR Parameters. At the lowest loading of vanadium (0.13 ETVS-10), the 51V NMR spectrum acquired at the MAS frequency of 15 kHz contains a single sharp peak at -562 ppm (Figure 4d). No spinning sidebands are observed in the spectrum; these sidebands would be expected if there were significant chemical shift, paramagnetic anisotropy, or quadrupolar coupling present. This result is surprising and suggests that the diamagnetic vanadium (V) atoms are in sites of high local symmetry. This peak does not correspond to any of the possible starting or decomposition products of the ETVS10 samples, e.g., δiso(V2O5) ) -610 ppm, δiso(Na3VO4) ) -545 ppm (spectra not shown), and there are no obvious candidates for assignment of this signal in the framework sites of the endmember polymorphs of ETS-10. As more vanadium is added to the structure, a different pattern emerges in the51V NMR spectra (Figure 4a-c). The spectra obtained for the 0.33 ETVS-10, 0.43 ETVS-10, and AM-6 samples show 51V NMR signals that are effectively the same.

Identification of Mixed Valence Vanadium in ETS-10

Figure 5. 51V MAS NMR spectra of AM-6 acquired at magnetic fields of (a) and (c) 14.1 T and (b) and (d) 9.4 T. The expanded spectra, (c) and (d), illustrate that the line width of each ssb in ppm does not change, with field-dependence indicating the broadening comes from a chemical shift distribution and not second-order quadrupolar broadening. The peak at -2700 ppm in spectrum a corresponds to a 63Cu signal from the radio frequency coil of the probe.

The patterns show multiple spinning sidebands that arise from the central and satellite 51V NMR transitions (see above). This spinning sideband envelope suggests broadening by electronnuclear dipolar interaction at higher vanadium-doping levels. We also note that each spinning sideband is broad, ∼10 kHz at 14.1 T. This broadening can arise from two sources, secondorder quadrupolar coupling or variations in chemical shifts between multiple magnetically nonequivalent sites. The secondorder quadrupolar broadening (in Hz) scales inversely with the applied magnetic field, although broadening caused by chemical shift variations (in Hz) scales with the field. Spectra a and b of Figure 5 show two spectra of the AM-6 sample acquired at 9.6 and 14.1 T. Spectra c and d of Figure 5 illustrate that the spinning sidebands (ssbs) are very similar in width (in ppm) indicating that the breadth is dominated by the distribution of chemical shifts and not second-order quadrupolar broadening. If the second-order quadrupolar coupling were significant, the width of the spectra at 9.4 T would be a factor of 3/2 larger than that at 14.1 T, which is not the case. To separate the contributions from the chemical shift anisotropy and the electron-nuclear dipole interaction and confirm that the latter is present, we performed a series of variable temperature experiments. Figure 6 shows four 51V NMR spectra of AM-6 obtained at different temperatures. The relative intensities of the individual spinning sidebands correspond to the central transition change as a function of temperature, confirming the presence of paramagnetic broadening. The overall spinning sideband envelope was fit in SIMPSON, and an anisotropy parameter was extracted at each temperature. Because of the limited signal-to-noise ratio in these measurements arising from the low V(V) concentrations in the sample, there are substantial errors in the parameters extracted from the fits shown in Figure 6. However, because a monotonic linear change in the combined anisotropy was observed as a function of inverse temperature (Figure 7), these fits were further used to estimate the quadrupolar coupling and chemical shift and paramagnetic anisotropy parameters (Table 1). The fits were performed using three sites, which is the minimum number required to fit the major features of the spectra; additional sites could not be justified given the low signal-to-noise ratio. Although the three sites differed in chemical shifts, the anisotropy and asymmetry parameters were within the error of each fit.

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Figure 6. 51V MAS NMR spectra of AM-6 (black) acquired at four different temperatures. The change in the relative intensities of the spinning sidebands indicates temperature dependence in the anisotropy, revealing that the anisotropy has a large paramagnetic component. Calculated spectra (brown) were obtained from SIMPSON using three sites and the NMR parameters listed in Table 1.

Figure 7. Plot of measured anisotropy as a function of reciprocal temperature. At each temperature, the parameters and error bars from the three-site fit are plotted. Inset bar graphs represent the relative signal intensity of the spinning sidebands for the 51V spectra at the indicated temperatures. It is apparent there are systematic changes in the intensities that correspond to the monotonic linear changes in the anisotropy.

A number of observations can be made from the fits in Figure 6. The quadrupolar coupling is estimated to be small. A CQ value of 1.5 MHz was used and resulted in a reasonable fit of the spinning sideband envelope for the satellite transitions. This is consistent with the field dependence of the spinning sideband linewidths discussed above. Furthermore, the quadrupolar asymmetry parameter, ηQ, is close to 1, resulting in a pattern of decreasing intensity as one moves away from the isotropic chemical shift. In contrast, an axially symmetric EFG, ηQ ) 0, would result in “horns” near the edges of the ssb envelope. The anisotropy parameter (reflecting the combined chemical shift, δCS, and electron-nuclear interactions, δp) is significant but relatively small compared to that of the other vanadium compounds. Similar to the EFG, the combined anisotropy is not axially symmetric. As the temperature is lowered, the magnitude of the anisotropy increases from -180 ppm at 323 K to -260 ppm at 243 K. Figure 7 shows a plot of the anisotropy as a function of the reciprocal temperature that reveals a clear linear trend. By extrapolating the linear trend to

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TABLE 1:

Ooms et al.

51

V NMR Parameters for Vanadium ETS-10 and AM-6 δ (ppm)

VETS-0.13 VETS-0.5 VETS-0.75 AM-6 (323 AM-6 (293 AM-6 (263 AM-6 (243

η

δCS (ppm)a

δiso (ppm)b

CQ (MHz)b

ηQc

1.5 ( 0.5

0.8 ( 0.2

-562 ( 5 -205 ( 15 K) K) K) K)

-185 ( 15 -205 ( 15 -230 ( 10 -260 ( 15

0.7 ( 0.2

65 ( 15

-610 ( 5, -600 ( 8 -621 ( 8

a Values are based on the extrapolation of the temperature-dependent δ value to infinite temperature, i.e., where there is no electron-nuclear dipolar contribution to the anisotropy. b The three δiso values in the samples with V(V+Ti) ratios >0.13 are for the three sites as simulated in Figure 6. The other chemical shift and quadrupolar parameters did not show differences greater than experimental error of the fits. c There was not enough effect to justify changing the quadrupolar interaction parameters between simulations. Errors are (0.5 MHz and (0.2 for CQ and ηQ, respectively.

zero, one can obtain an estimate of the chemical shift anisotropy from the y-intercept yielding δ (ppm) ) (-7.8 × 104)/[T (K)] + 65 ppm, i.e., δσ ) 65 ppm. In this analysis, we assume that the paramagnetic broadening can be extrapolated linearly to infinite temperature, which may be an oversimplification. Regardless, 65 ppm is a small chemical shift anisotropy for vanadium and, along with the small observed CQ, indicates that the vanadium is in highly symmetric sites. Vanadium has been incorporated into a number of siliceous zeolite frameworks (MFI, beta, MCM-22, and others).42-46 Physicochemical studies of the isolated vanadium species indicate that vanadium can be incorporated predominantly in tetrahedral coordination sites with three V-O-Si bonds and one VdO bond. A silanol group is present adjacent to the vanadium site.47,48 51V NMR studies of these samples show that significant chemical shift anisotropy is observed for these tetrahedral species,38 much more than the 65 ppm observed in our studies after accounting for the the paramagnetic broadening. This observation, along with the models described below, makes it unlikely that the majority of the 51V NMR signals observed in Figures 4 and 5 are from vanadium atoms substituting for tetrahedral Si atoms in the configuration observed in zeolites. Given the signal-to-noise ration of the spectra, we cannot rule out tetrahedral sites, but it is unlikely that they dominate the observed spectra. Structural Models for Interpreting NMR Results. On the basis of previous computational and experimental results, it has been suggested that the vanadium atoms replace the titanium atoms in the metal-oxygen chains. The axial symmetry of the hyperfine tensor and g tensor measured using EPR are consistent with V(IV) in the octahedral sites of the metal-oxygen chains. The presence of electron-nuclear dipolar broadening in the 51 V NMR spectra indicates that the V(V) must be close enough to the V(IV) species to result in observable electron-nuclear interaction. To estimate the magnitude of the electron-nuclear dipolar interaction, we calculated the paramagnetic anisotropy using eq 1 for several different models, based on the atomic coordinates in the geometry optimized 3Ti ONIOM model.33 The results (Table 2), indicate that to get close to the experimentally observed value of -200 ppm at room temperature, V(V) must be next to V(IV) (i.e., a single oxygen bridge away). When both vanadium atoms are in octahedral sites, this produces an anisotropy of -274 to -308 ppm, depending on the arrangement of metal atoms in the third position of our models (Table 2). Models were also checked where V(V) was substituted for a silicon in the silicate framework and V(IV) was placed in different adjacent octahedral sites. These models showed a greater range of values, a number of which could correspond to the 200 ppm observed experimentally (Table 2).

TABLE 2: Calculated Electron-Nuclear Dipolar Parameters, δpa octahedral

δp (ppm)

Ti-VV-VIV VIV-VV-VIV VV-VIV-VV VIV-Ti-VV-Ti-VIV

-274 -547 -308 -69

tetrahedral VV VV VV VV VV VV VV

VIV-Ti-Ti Ti-VIV-Ti Ti-Ti-VIV VIV-VIV-Ti VIV-Ti VIV Ti-VIV-VIV VIV-VIV-VIV

δp (ppm) -136 -637 -104 -454 120 -458 -463

a Parameters are for a series of model complexes where the octahedral titanium sites in a three-member chain were substituted for vanadium (V) and vanadium (IV) as indicated.

From these results, we can conclude that most of the V(V) nuclei must be close to the V(IV) sites. At the same time, a small number of V(V) may be next to Ti in each sample, as this is the likely source of the small single peak observed most clearly in the 0.13 ETVS-10 sample (δiso ) -562 ppm). There is also a chance that this peak could arise from vanadium substituting for Si in the framework. One Si site is surrounded by other Si tetrahedra and is not connected to the octahedral sites. This site would experience very little paramagnetic broadening due to the distance from the V(IV). The 51V chemical shift and quadrupolar coupling parameters do not conclusively indicate whether the V(V) species are in the octahedral chain sites or some other environment on the surface of the channels or are substituted into the silicate framework. The small CQ and δσ parameters do indicate that the V(V) is in a site of high symmetry, with respect to electronic charge distribution and molecular orbital structure. To identify the environment of the V(V), we have compared the experimental data to results predicted by the DFT calculations of the NMR parameters in the optimized structural models. The predicted NMR parameters for V(V) substituted into different octahedral and tetrahedral environments are presented in Table 3. Comparison of the results for the models with octahedral substitution (o-VV), the o-2-VV model, which considers V(V) substitution into a bulk octahedral position, yield predicted CQ and δσ parameters that are considerably larger than the experimental values. In contrast, the o-1,3-VV and o-1,3VV/o-2-VIV models, which contain V(V) in octahedral coordination at the end of the chain, are predicted to have smaller CQ and δσ parameters, which are in better agreement with the experimental results. For all o-VV models, the smallest CQ and δσ parameters are predicted for o-1,3-VV/o-2-VIV, where the V(V) is substituted next to a V(IV). In addition, the calculations indicate that the two tensors are not axially symmetric, which is in agreement with the experiment (see Results and Discussion).

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TABLE 3: Predicted NMR Parameters from DFT Calculations, B3LYP/6-311G(d,p)a o-1,3-V [V -Ti-V ] o-1,3-VV/2-VIV [VV-VIV-VV] o-2-VV [Ti-VV-Ti] t-VV-O1 t-VV-O2 V

V

V

relatively homogeneous in the samples with V(IV) in the bulk of the chains and V(V) at the ends.

δCS (ppm)

ηCS

CQ (MHz)

ηQ

-165 149 -438 -295 -348

0.12 0.62 0.52 0.75 0.25

-4.6 -1.7 -20.3 -4.5 -8.0

0.63 0.96 0.03 1.0 0.62

a

Parameters are for a series of model complexes where the octahedral titanium sites and tetrahedral Si sites were substituted for vanadium (V) and vanadium (IV) as indicated.

Of the four structural models having tetrahedral substitution (t-VV), the lowest energy configurations were found for t-VV-O1 and t-VV-O2. In both of these systems, the “capped oxygen” is bonded to a surface Si, allowing more structural flexibility to accommodate the neighboring OdVVO3 HO-SiO3 states. As reported in Table 3, the calculated chemical shift anisotropies for these t-VV systems are significantly larger than those for the o-1,3-VV and o-1,3-VV/o-2-VIV models and the experimental values. Consequently, VV substitution into the tetrahedral Si sites is unlikely. Here, we have considered V(V) substitution into a single tetrahedral Si site. In the ETS-10 structure, there are several unique Si sites, which may result in different [OdVVO3 HO-SiO3] environments than those modeled here and, therefore, would yield different 51V NMR properties. For example, more flexible surface Si states may be able to adopt more symmetric [OdVVO3 HO-SiO3] tetrahedral geometries, resulting in lower chemical shift anisotropies and quadrupolar couplings. However, although we cannot rule out tetrahedral V(V) substitution, the data presented above support the substitution of V(V) into the octahedral Ti(IV) sites. Previous theoretical studies14 have proposed that V(IV) will energetically favor bulk octahedral site substitution, while the octahedral end sites of the chain will preferentially favor V(V) substitution. It is highly probable that there are many octahedral chain ends in the as-synthesized samples due to small the crystallite size and presence of crystal defects within the framework. Therefore, the existence of a significant 51V NMR signal could arise from V(V) in either type of octahedral end site. On the basis of this experimental-theoretical approach, we conclude that the NMR data support the hypothesis that the V(V) sites are at the ends of the octahedral chains. Our model, however, is not consistent with the relative intensities of V(IV) and V(V) observed in NEXAFS spectra,10 showing a preponderance of V(V) in AM-6 and VETS-10 samples. However, the NEXAFS spectra were collected using electron yield detection, and under this detection mode, mostly the external surface of the crystal is sampled. Therefore, it is not surprising that more V(V) are observed by NEXAFS than suggested by 51NMR spectroscopy because surface sites are necessarily chain ends. Summary and Conclusions EPR spectra clearly indicate that there is V(IV) in these ETVS-10 samples, and that the vanadium occupies the octahedral sites in the structure. 51V NMR spectra have revealed the presence of V(V) species which, except for at the lowest vanadium loadings, occupy sites adjacent to V(IV). In addition, on the basis of the estimated 51V chemical shift, EFG parameters, and DFT calculations of the NMR parameters, we conclude that the hypothesis stating that V(V) occupyies sites at the ends of the octahedral chains is consistent with the NMR results. Clearly, in the ETVS-10 samples, there is a mixture of vanadium oxidation states, and the local structures of the two species are

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