Spectroscopic Studies of Disorder in the Microporous Titanosilicate

Aug 31, 2002 - Mater. , 2002, 14 (10), pp 4209–4218. DOI: 10.1021/cm021199c ... The microporous titanosilicate ETS-10 is a potential photocatalytic ...
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Chem. Mater. 2002, 14, 4209-4218

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Spectroscopic Studies of Disorder in the Microporous Titanosilicate ETS-10 Peter D. Southon and Russell F. Howe* Department of Chemistry, University of Aberdeen, Aberdeen, United Kingdom AB24 3UE Received May 7, 2002. Revised Manuscript Received July 17, 2002

The microporous titanosilicate ETS-10 is a potential photocatalytic material with a disordered crystal structure. The disordered stacking of repeat units in the structure gives rise to defects that strongly influence the reactivity. This paper describes the spectroscopic study of a series of ETS-10 samples containing different levels of defects. All samples are well crystalline by X-ray powder diffraction, but they show significant variations in other spectroscopic properties that are sensitive to short-range order. Raman spectroscopy was particularly sensitive to disorder, and the 724 cm-1 stretching mode of the -Ti-O-Tichains broadened and shifted with increasing disorder through the series. Changes in the UV absorbance spectrum also indicated disruptions of the delocalized charge-transfer transitions along the chains. We propose that stacking defects interrupt the -Ti-O-Tichains and that differences in the relative nucleation and growth rates lead to different concentrations of these defects. Hydroxyl groups associated with defect sites were observed by FTIR and solid-state cross-polarization 1H-29Si NMR. Disorder is also correlated with the occurrence of faulting in the crystal morphology. No new cation exchange sites associated with the defects were observed by 3Q 23Na NMR.

Introduction ETS-10 is microporous titanosilicate with a large-pore (12-ring), three-dimensional channel structure, and an as-synthesized stoichiometry of (Na,K)2TiSi5O13. It has attracted attention as a heterogeneous catalyst1-4 and as an ion exchange material.5,6 Further interest in ETS10 arises from the presence in the structure of linear chains of corner-linked TiO6 octahedra. It has been proposed that these chains act as one-dimensional semiconductor “quantum wires”, and band structure calculations for such wires have been published.7 We have recently reported that the photoreactivity of ETS10 is significantly different from that of a conventional anatase photocatalyst.8 Differences between ETS-10 and anatase were also reported by Calza et al.,9 who describe shape selectivity effects in the oxidative photodegradation of organic molecules. These authors propose that small molecules such as phenol, which are able to penetrate the ETS-10 pores, are protected from photo* Corresponding author. E-mail: [email protected]. (1) Valente, A.; Lin, Z.; Brandao, P.; Portugal, P.; Anderson, M.; Rocha, J. J. Catal. 2001, 200, 99. (2) Waghmode, S. B.; Thakur, V. V.; Sudalai, A.; Sivasanker, S. Tetrahedron Lett. 2001, 42, 3145. (3) Zecchina, A.; Llabre´s i Xamena, F. X.; Paze´, Turnes Palomino, G.; Bordiga, S.; Otero Area´n, C. Phys. Chem. Chem. Phys. 2001, 3, 1228. (4) Bianchi, C. L.; Vitali, S.; Ragaini, V. Stud. Surf. Sci. Catal. 1998, 119, 167. (5) Al-Attar, L.; Dyer, A.; Blackburn, R. J. Radioanal. Nucl. Chem. 2000, 246, 451. (6) Kuznicki, S. M. US Patent 4,994,191, 1991. (7) Borello, E.; Lamberti, C.; Bordiga, S.; Zecchina, A. Appl. Phys. Lett. 1997, 71, 2319-2321. (8) Howe, R. F.; Krisnandi, Y. K. Chem. Commun. 2001, 15881589. (9) Calza, P.; Paze, C.; Pelizzetti E.; Zecchina, A. Chem. Commun. 2001, 2130-2131.

degradation reactions ocurring on the external surface. In our previous work, we found also that the photoreactivity of a highly defective sample of ETS-10 in the hydrogen-exchanged form was different from that of a well crystalline as-synthesized sample in the (Na,K) cation-exchanged form. This has prompted further investigation of the role of defects in ETS-10, particularly in influencing the photoreactivity. The detailed structure of ETS-10 was first established by Anderson et al. using a combination of electron microscopy, X-ray and electron diffraction, solid-state NMR, and molecular modeling.10 A subsequent singlecrystal diffraction study has confirmed the structure.11 The framework of ETS-10 is composed of corner-sharing SiO4 tetrahedra and TiO6 octahedra, as illustrated in Figure 1. The TiO6 octahedral chains are each linked to two folded chains of SiO4 tetrahedra, forming TiSi4O13 columns. These columns are packed into layers, with the columns in adjacent layers oriented perpendicular to each other. The double layers of orthogonal columns are stacked with a displacement of (1/4 unit cell in either the [100] or [010] direction. Two “ideal” polymorphs of ETS-10 have been identified, each formed by a particular stacking arrangement: polymorph A (tetragonal, P41 or P43) from zigzag stacking of layers, and polymorph B (monoclinic, C2/c) from a diagonal stacking arrangement. In reality, the stacking sequence is virtually random, resulting in an inherently disordered structure and the occurrence of stacking defects. These give rise to line defects, such as the double pores seen (10) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Malley, P. J. O.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Philos. Mag. B 1995, 71, 813. (11) Wang, X.; Jacobson, A. J. Chem. Commun. 1999, 973.

10.1021/cm021199c CCC: $22.00 © 2002 American Chemical Society Published on Web 08/31/2002

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Figure 1. Structure of ETS-10. TiO6 octahedra are shaded light gray, and SiO4 tetrahedra are shaded dark gray. Table 1. Synthesis conditions and elemental composition of as-synthesized ETS-10 samples A

B

C

D

E

F

G

Synthesis conditions temperature 200°C 200°C 200°C 200°C 200°C 200°C 170°C time (days) 2 6 4 1 2 2.6 11 Na:Ti K:Ti Si:Ti

1.55 0.40 4.97

Elemental analysis 1.50 1.39 1.41 1.41 0.40 0.39 0.43 0.35 5.02 4.94 4.98 4.97

1.35 0.35 4.87

1.34 0.39 4.71

in high-resolution electron micrographs.10,12 The 12-ring pore structure of ETS-10 is three-dimensional; the pores are straight in the [100] and [010] directions and crooked in the direction of disorder. In this paper, we have examined a series of different preparations of ETS-10 in which the concentrations of stacking defects differ. By examining the effects of these differences on the spectroscopic properties of ETS-10 we hoped to better understand how and why the photoreactivity of this fascinating material varies with defect levels. Experimental Section Seven batches of ETS-10 were synthesized hydrothermally according to the following method. NaOH pellets (1.45 g) were dissolved in 8.0 g of water and then mixed with 10.0 g of sodium silicate solution (27 wt % SiO2, 14 wt % NaOH). Into this solution was dissolved 0.33 g of NaCl and 0.91 g of KCl, followed by the slow addition of 8.40 g of 15 wt % TiCl3 solution with stirring. Approximately 80 mg of ETS-10 (Engelhard Corp.) was then added as crystallization seeds. The pH was adjusted with ∼0.1 g of NaOH to between 10.2 and 10.6. The molar ratios were TiO2:SiO2:Na2O:K2O:H2O 1.0:5.5:4.8:0.75: 140. The mixtures were then sealed in a Teflon-lined autoclave and heated for the times and temperatures listed in Table 1. The resulting fine white powders were washed, filtered, and dried at 110 °C. The elemental composition of sample “A” (that giving the least defective crystals in the SEM) was determined by X-ray fluorescence (Siemens SRS300 instrument). This sample was then used as a standard for analysis of the remaining samples by atomic absorption and emission spectrophotometry. A 100 mg portion of each sample was fused with LiBO2, dissolved in 4 wt % nitric acid, and analyzed for Ti and Si by atomic absorption spectroscopy and for K and Na by flame emission spectroscopy. The size and morphology of the crystals were determined by scanning electron microscopy (Hitachi S4500 SEM). X-ray powder diffraction patterns were measured with a Philips (12) Anderson, M. W.; Agger, J. R.; Hanif, N.; Terasaki, O. Microporous Mesoporous Mater. 2001, 48, 1-9.

XPERT diffractometer, using Cu KR radiation. Raman spectra were measured with a Renishaw Raman 2000 microprobe, using 633 nm laser excitation. Ultraviolet absorption spectra were measured by diffuse reflectance (Cary 5 spectrophotometer), with scattering intensity measured with respect to a BaSO4 standard. A standard Kubelka-Munk conversion was then applied to the UV spectra. Samples were pressed into wafers and outgassed in a high vacuum cell, and infrared absorption spectra were measured in transmission mode with a Nicolet Nexus FTIR spectrometer. The X-ray absorption spectra of several samples were recorded at the Australian National Beamline Facility, located at the Photon Factory synchrotron, Tsukuba Science City, Japan. Powders were pressed into a flat disk and held in a cryostat at 10 K, and spectra were recorded in transmission mode across the Ti K-edge. The near-edge region was recorded with a resolution of 0.2 eV. Analysis and modeling of the EXAFS region were carried out using the X-FIT software package13 and FEFF theory.14 29 Si MAS NMR spectra were measured with a Varian Innova 300 MHz spectrometer at the University of New South Wales, using 40° pulses of 1.7 µs, a recycle delay of 60 s, and a sample spinning of rate 3 kHz. Cross-polarization 29Si MAS NMR spectra were obtained with a Varian UNITYplus 300 MHz spectrometer at the EPSRC National Solid State NMR Facility, University of Durham, using a recycle delay of 0.2 s, a spinning rate of 5 kHz, and variable 1H-29Si contact times. All 29Si chemical shifts are reported with respect to tetramethylsilane. Triple-quantum 23Na MAS NMR spectra were also measured with the Varian UNITY 300 MHz spectrometer, using a standard two-pulse sequence with optimized pulses of 4.6 and 1.6 µs (197° and 69°, respectively). A total of 576 data points were collected in the first dimension with an acquisition time of 10 ms and 80 increments in the second dimension. The sweep width was 30 kHz in both dimensions, the recycle delay 0.2 s, and the sample spinning rate 16 kHz. The 23Na chemical shifts are given with respect to aqueous NaCl. Processing and shearing of the data were carried out with commercial VNMR software.

Results Characterization of the seven batches of ETS-10 strongly indicated that some aspects of the local structure varied considerably. Each of the batches were given the label “A” through to “G”, in approximate order of the variations, with sample A being the most similar to previous reports of highly crystalline ETS-10. It was immediately obvious that there was no correlation between this series and the known synthesis conditionss either the pH of the gel before synthesis, which varied slightly between batches, or the hydrothermal treatment time, which varied considerably. A probable exception to this observation is the sample G, which was synthesized at a lower temperature and which the results presented below show to be by far the most disordered. Elemental Analysis. Table 1 summarizes the elemental compositions of the as-synthesized ETS-10 samples, expressed relative to the titanium content. In comparison with the theoretical stoichiometry of (Na,K)2TiSi5O13, it can be seen that all samples have the expected Si:Ti ratio, with the possible exception of sample G. Across the series from A to G there is, however, a decrease in the exchangeable cation content, (13) Ellis, P. A.; Freeman, H. C. J. Synchrotron Rad. 1995, 2, 190. (14) Ab initio Multiple-Scattering X-ray Absorption Fine Structure and X-ray Absorption Near Edge Structure Code; FEFF Project: Department of Physics, University of Washington.

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Figure 2. Scanning electron microscope images of ETS-10 samples A-G.

from 1.95 cations per titanium for sample A to 1.70 for sample F and 1.73 for sample G. SEM. Representative micrographs of each of the batches are shown in Figure 2. It can be seen that there are very significant variations in the size and shape of the crystals in each sample, with increasing disorder through the series. The crystals in sample A are fairly uniform in size, typically ∼4-5 µm, but are frequently intergrown. They possess a well-defined habit (truncated tetragonal bipyrimid), and their surfaces are quite flat, with occasional “steps” of 50-100 nm. The other samples have smaller crystals, generally between 1 and 2 µm, but the crystal size does not change uniformly through the series. The crystals in sample B have a similar morphology to those in sample A, but the crystals in the rest of the samples have various irregularities and faulting on the surfaces. Some show “blocky” morphology, such as in sample D, while others possess a “layered” morphology, such as in sample C. Diffraction. The diffraction patterns of each of the samples are shown in Figure 3. All peaks observed correspond to those reported previously for ETS-10,10 except for two minor peaks that are assigned to a very small AM-1 impurity.15 The patterns are very similar in peak positions, intensities, and peak widths. There is no indication of significant variations in crystallinity as measured by powder diffraction across the series. UV Absorption Spectra. The diffuse-reflectance UV absorption spectra, converted using the Kubelka-Munk equation, are shown in Figure 4. Considerable differ(15) Lin, Z.; Rocha, J.; Branda˜o, P.; Ferreira, A.; Esculcas, A. P.; Pedrosa de Jesus, J. J. Phys. Chem. B 1997, 101, 7114.

Figure 3. X-ray diffraction patterns for ETS-10 samples A-G. Peaks assigned to the AM-1 phase are indicated by asterisks.

ences can be seen through the series. The broad, asymmetric absorption band centered at ∼280 nm was readily fitted by two Gaussian peaks, also shown in Figure 4, which are located at approximately 295 nm (4.2 eV) and 270 nm (4.6 eV). These two component bands vary somewhat in shape, intensity, and position, with the ∼270 nm band becoming much broader and

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Figure 5. Raman spectra of ETS-10 samples, over the range 200-1200 cm-1, and expanded over the range 100-600 cm-1. In each set, spectra are normalized to the most intense band.

Figure 4. Ultraviolet absorption spectra for ETS-10 samples A-G. Spectra have been converted using the Kubelka-Munk equation and fitted with two Gaussian peaks and a mixed Gaussian-Lorentzian peak. The sum of these peaks is plotted as a solid line passing through the data points.

more intense. A further absorption band at 216 nm (5.7 eV) is also observed, and this band becomes much less intense through the series. Raman Spectra. The Raman spectra of each of the samples are shown in Figure 5. The spectra of most samples are very similar to those reported elsewhere for ETS-10.16-19 The intense, asymmetric band at 724 cm-1 dominates the spectrum of sample A and has previously been assigned to the Ti-O stretch of the -Ti-O-Ti- chains.16 This band broadens somewhat through the series, shifts slightly to higher frequency and becomes less intense with respect to the other bands below 700 cm-1. The spectra of the last few members of the series show a band that is greatly broadened and shifted by up to 50 cm-1. A band at about 724 cm-1 can also be detected in the infrared spectra of samples A-E (not shown), but it is extremely weak and strongly overlapped with other stronger bands in this region. Other changes take place in the low-frequency region of the Raman spectrum. The bands at 305, 480, and 535 cm-1, which are relatively weak in the spectrum of (16) Su, Y.; Balmer, M. L. J. Phys. Chem. B 2000, 104, 8160. (17) Ashtekar, S.; Prakash, A. M.; Kevan, L.; Gladden, L. F. Chem. Commun. 1998, 91. (18) Hong, S. B.; Kim, S. J.; Uh, Y. S. Korean J. Chem. Eng. 1996, 13, 419. (19) Rocha, J.; Branda˜o, P.; Pedrosa de Jesus, J.; Philippou, A.; Anderson, M. W. Chem. Commun. 1999, 471.

sample A, grow strongly in relative intensity through the series, while bands at 320 and 425 cm-1 decrease in intensity. These bands have been broadly assigned to various M-O bending modes,17,18 but a more detailed assignment of this complex region has not been made. Furthermore, some bands change in intensity mainly in the first part of the series, between A and D, while other changes occur mainly between D and G. This spectral region is particularly sensitive to changes in the local structure. Ti K-Edge X-ray Absorption Spectra. Ti K-edge EXAFS for four of the samples, A, D, F, and G, is shown in Figure 6a, and the Fourier transformed data are given in Figure 6b. A k range up to 13 Å-1 was used for modeling, except for sample A, where experimental problems prevented data beyond 10 Å-1 being used. Sankar et al. have presented an optimized model for the Ti EXAFS spectrum of ETS-10, using a single length for the four Ti-O(Si) bonds and different lengths for each of the two Ti-O(Ti) bonds.20 This implies that the -Ti-O-Ti- chains have alternating long and short Ti-O bonds. We have applied models using one, two, and three oxygen shells to our data and found that the model with three oxygen shells (that used by Sankar et al.) is by far the most suitable. The optimized parameters from these fits are given in Table 2. Most values for bond lengths are consistent with those determined by Sankar et al. The Debye-Waller factors, σ2, are somewhat smaller than those reported previously, but this is expected for data measured at 10 K. There was no significant variation between the samples, within the limits of uncertainty. The parameters for sample A are less reliable, due to the smaller k range available for modeling. Modeling of further Ti and O shells was difficult. Each of the six oxygen atoms around the titanium is located almost diametrically opposite another oxygen atom, leading to a high “multiple scattering” contribution to (20) Sankar, G.; Bell, R. G.; Thomas, J. M.; Anderson, M. W.; Wright, P. A.; Rocha, J. J. Phys. Chem. 1996, 100, 449.

Disorder in the Microporous Titanosilicate ETS-10

Figure 6. EXAFS spectra of ETS-10 samples A, D, F and G, plotted as (a) background-subtracted data, and (b) Fouriertransformed data with the refined model as a broken line. Note that the data for sample A was Fourier-transformed over a smaller k-range than the other samples.

the EXAFS. The intensity of the multiple scattering contribution is strongly dependent on the deviation of the O-Ti-O and Ti-O-Ti angles from 180°. Although Sankar et al. reported a successful multiple scattering model, we found that a realistic model provided too many sensitive parameters to reliably fit to the current data set. The Ti K-edge XANES spectra for samples A, D, F, and G are shown in Figure 7. Each spectrum has a single peak at 4971.8 eV, just below the absorption edge. Such pre-edge peaks are usual in titanates, and the location and relatively low intensity (with respect to the

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edge jump) is characteristic of 6-fold coordination.21 There is no significant difference in the position or intensity of this peak between spectra. 29Si NMR Spectra. The MAS 29Si NMR spectra for all samples are given in Figure 8. The spectrum for sample A is closely similar to those reported previously for ETS-10.10,19,22 On the 300 MHz instrument used here, the peaks are not as well resolved as those measured at higher field. We have however confirmed by measuring the spectrum of sample A on a 400 MHz instrument that this sample is identical to those previously described in the literature. The group of peaks between -94 and -97 ppm has been assigned to silicon sites bonded through oxygen to three silicons and one titanium, while the -103.4 ppm peak is due to silicon bonded through oxygen to four silicons.10,19,22 The Si(OSi)3OTi sites further fall into two crystallographically inequivalent groups, each of which gives two distinct peaks.10 The peak at -94.5 ppm contains a higher field shoulder, which is not observed at 300 MHz. The second doublet, at about -96.0 and -96.6 ppm, is only partially resolved at 300 MHz. The spectra of the other samples retain the same features as those seen in sample A, but with some significant variations. All of the peaks become somewhat broader through the series of samples from A to G, although the ratio of the areas under the three groups of peaks remains constant at 2:2:1. The peak at -96 ppm shifts upfield slightly and changes in shape, probably due to a decrease in the spacing between the two components in the peak. The -103.4 ppm peak also shifts slightly to -103.9 ppm. In sample G an additional very weak signal is observed at -89.3 ppm, representing approximately 1% of the Si sites. Cross-polarization MAS 29Si NMR was used to investigate the nature of any defect sites in the framework. The cross-polarization experiment selectively enhances signals from those silicon sites that are dipolar coupled to protons. Spectra of samples C, F, and G were measured for a range of 1H-29Si cross-polarization contact times between 1 and 20 ms, as shown in Figure 9. Sample A (not shown) gave no detectable signals in the cross-polarization experiment. The three main peaks in the MAS 29Si spectra are also present in the CP spectra of samples C, F, and G, and in Figure 9 they are labeled 1, 2, and 3. The intensities of these peaks reach a maximum at a contact time of approximately 5 ms, but the peaks are still observed at 20 ms. One additional peak is clearly observed in each set of CP spectra, located at -89.2 ppm and labeled with an asterisk. It is particularly prominent in sample G and coincides with the very weak signal detected at this chemical shift in the MAS spectrum of the same sample. This signal cross-polarizes more efficiently than the others, reaching a maximum intensity within 3 ms, and also decays more rapidly with increasing contact time. The CP spectra also show a broad underlying signal between about -80 and -110 ppm not seen in the MAS spectra. This broad signal also cross-polarizes efficiently (21) Farges, F.; Brown, G.; Rehy, J. J. Phys. Rev. B 1997, 56, 1809. (22) Rocha, J.; Ferreira, A.; Lin, Z.; Anderson, M. W. Microporous Mesoporous Mater. 1998, 23, 253.

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Table 2. Refined values for models fitting the EXAFS spectra of ETS-10 samples A, D, F and G, compared with refined values published by Sankar et al.20 A

D

atom pair

N

r (Å)

σ2

(Å2)

Ti-O(Ti) Ti-O(Si) Ti-O(Ti)

1 4 1

1.69 2.03 2.01

0.000* 0.006 0.000*

F

Sankar et al.20

G

r (Å)

σ2

(Å2)

r (Å)

σ2

(Å2)

1.70 1.98 2.09

0.000* 0.005 0.000*

1.72 1.99 2.11

0.001 0.003 0.001

r (Å)

σ2

(Å2)

1.70 1.99 2.09

0.002 0.005 0.002

r (Å)

σ2 (Å2)

1.71 2.02 2.11

0.006 0.007 0.006

* Refined value less than uncertainty limits.

Figure 7. Pre-edge section of the X-ray absorbance spectra for ETS-10 samples A, D, F and G.

for Na+ cations.23,24 Triple quantum spectra were measured for most samples in the series, and those for samples A and G are shown in Figure 10. All spectra showed a single composite peak. For sample A this comprises two sites, as shown by the presence of a shoulder on the isotropic F1 projection. These two sites become less well resolved through the series A-G. For sample G, the line width in the isotropic projection is significantly larger, and the two sites are no longer resolved. Infrared Spectra. Figure 11 shows FTIR spectra over the range 3000-4000 cm-1 for each of the samples, measured following outgassing under vacuum at the temperatures indicated in the figure caption. The spectra of the unheated samples (not shown) are dominated by broad bands between 3400 and 3600 cm-1, assigned to adsorbed water. On outgassing under vacuum these bands are gradually diminished, revealing a weak but clearly distinguishable pattern of narrower bands due to hydroxyl groups in ETS-10. The low intensities of the hydroxyl bands precluded measurements of absolute or relative concentrations of hydroxyl groups. For convenience, the spectra in Figure 11 have been normalized to give the same intensity for the highest frequency band at 3735 cm-1, which was present in all samples. Some samples, most prominently sample G, show a second sharp band at 3705 cm-1. At lower frequencies, samples E, F, and G show additional bands at 3488 cm-1 and ∼3200 cm-1. The 3488 cm-1 band is particularly prominent in sample G. Note that the wafer for sample G was considerably thinner than for the other samples, leading to an improved signal:noise ratio. Discussion

Figure 8. MAS G.

29

Si NMR spectra for ETS-10 samples A to

but decays more slowly than the sharp -89.2 ppm signal. 23Na NMR Spectra. Conventional single-quantum MAS 23Na spectra of ETS-10 show a broad poorly defined peak at about -9 ppm, but triple-quantum 2-D MAS spectra have been used to identify several sites

The spectroscopic and other characterization data presented above fall into two categories. X-ray diffraction, 29Si MAS NMR, 23Na NMR, and Ti K-edge XANES, and EXAFS show little or no differences between the different samples of ETS-10. The SEM images, on the other hand, show substantial differences in crystal morphology, and the Raman, UV absorption, infrared, and 1H-29Si CPMAS NMR spectra show wide variation across the series. There is no evidence to suggest that any major secondary phases, amorphous or crystalline, are formed in any of these samples. Furthermore, the diffraction patterns show that the long-range order of the ETS-10 crystals is maintained in all samples. Therefore, the variations must lie in the short-range structure of the framework. The SEM images show that, with the exception of sample A, all samples have similar crystal sizes of (23) Ganapathy, G.; Das, T. K.; Vetrivel, R.; Ray, S. S.; Sen, T.; Sivasanker, S.; Delevoye, L.; Fernandez, C.; Amoureux, J. P. J. Am. Chem. Soc. 1998, 120, 4752. (24) Anderson, M. W.; Agger, J. R.; Luigi, D.-P.; Baggaley, A. K.; Rocha, J. Phys. Chem. Chem. Phys. 1999, 1, 2287-2292.

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Figure 9. Cross-polarization 29Si NMR spectra for ETS-10 samples C, F and G, with contact times varied between 1 and 20 ms.

around 2 µm, so that there is no large variation in external surface area across the series. However, the micrographs do show that the crystals are increasingly faulted across the series from A to G; we therefore discuss the characterization data in terms of stacking defects in the ETS-10 structure. As noted above, each layer in the ETS-10 structure is able to be stacked on top of the previous layer in one of four ways. During crystal growth, each layer is formed on the top of the previous one by a process of nucleation and growth. Each layer may be nucleated at several places, and these nuclei grow out to form two-dimensional domains. Anderson has suggested that stacking defects are formed where these domains meet.12 The most prominent of these are the “double channels”, readily observed end-on by high-resolution electron microscopy.10,12 Another defect at the interface must be the termination of -Ti-O-Ti- chains. Figure 12 schematically illustrates a single ETS-10 layer containing several domains, and the defects that will occur at the boundaries. Termination of -Ti-O-Ti- chains will involve replacement of Ti-O-Ti and Ti-O-Si-O-Si linkages with Ti-OH and Ti-O-Si-OH sites. The introduction of hydroxyl groups into the structure in this way would certainly account for the observed cross-polarization of the three framework silicon signals in the 29Si NMR spectra. The signals observed in the CP spectra at -94, -97, and -103 ppm are attributed to Si(OSi)3(OTi) and Si(OSi)4 sites, respectively, that are adjacent to protonated sites. The broadening and slight shifting of these signals in the MAS spectra with increasing disorder

(from A to G) is due to the increased number of stacking defects causing local structural distortions. The new signal at -89.3 ppm appearing in the CP spectra (and also visible in the MAS spectrum of sample G) is assigned to either Si(OSi)3OH or Si(OSi)2(OTi)OH sites. The difference between Si(OSi)3OH and Si(OSi)4 sites in silicate frameworks is generally 8-14 ppm; since the new signal is shifted 5.2 and 7 ppm from the Si(3Si,Ti) peaks and 14 ppm from the Si(4Si) peak, the precise environment of this signal cannot be readily defined. The intensity of the very broad signal appearing in the CP spectra at ca. -100 ppm is not correlated with any of the sharp peaks observed. It is assigned to a range of Si(OSi)3OH sites, present either in a small quantity of amorphous hydroxy-rich silica or on the external crystal surfaces. The presence of hydroxyl groups can also be probed by infrared spectroscopy. The 3735 cm-1 band found in all samples is frequently observed in siliceous zeolites and is assigned to isolated Si-OH groups on the external surface. The 3705 cm-1 band, apparent in all samples except A and C, has been tentatively assigned by others to isolated Ti-OH groups.3,25 Although this band is most evident in the most defective sample G, its presence also in sample B suggests that it is not due to TiOH groups terminating disrupted -Ti-O-Tichains. If it is accepted from the SEM and Raman results that samples E, F, and G are the most defective, (25) Robert, R.; Rajamohanan, P. R.; Hegde, S. G.; Chandwadkar, A. J.; Ratnasamy, P. J. Catal. 1995, 155, 345.

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Figure 11. FTIR spectra for ETS-10 samples A to G, outgassed under vacuum at 250 °C (samples A, C, E, F, and G) or 160°C (samples B and D). Spectra have been normalized to the intensity of the 3735 cm-1 band.

Figure 10. and G.

23

Na 3Q MAS NMR spectra of ETS-10 samples A

then it is the lower frequency bands (3488 and 3200 cm-1) that correlate best with defect levels, suggesting that these bands may be associated with the stacking faults. The frequencies are unusually low for isolated hydroxyl groups, and the Ti-OH and Si-OH groups involved may therefore be strongly hydrogen bonded. We have no way at present to distinguish between these or to identify which is responsible for the new signal at -89.3 ppm in the cross-polarization 29Si NMR spectra. The Ti EXAFS and XANES data show no direct evidence for Ti-OH sites. If, however, these sites are terminating -Ti-O-Ti- chains, the coordination around titanium would remain octahedral, and no new features would be expected in the XANES spectra. Although the Ti-O bond length in such chain-terminating TiOH species might be expected to differ from those in an intact chain, the number of such sites is insufficient for any such effect to be observed in the Ti EXAFS. The

Figure 12. Schematic diagram of defects that may occur in a single layer of the ETS-10 structure. Each layer consists of parallel columns of -Ti-O-Ti- chains (black), surrounded by silicate framework (gray), separated by channels (white). If growth domains within the layer have grown on the underlying layer with different stacking sequences, then defects will occur at the domain boundaries. Termination of chains will produce Ti-OH (shown) and Si-OH groups (now shown).

EXAFS data for all samples measured fit well the alternating short-long Ti-O bond model for the -TiO-Ti- chains proposed by Sankar et al.20 Disruption of -Ti-O-Ti- chains is also expected to reduce the cation content of the ETS-10, as observed. 23Na NMR spectra were measured to investigate any changes in the cation sites. Previous 23Na NMR studies have identified up to five different cation sites in well crystalline ETS-10. The major signal in the 2-D spectra in Figure 10 corresponds to that assigned by Anderson et al. and by Ganapathy et al. to cations in sites I and

Disorder in the Microporous Titanosilicate ETS-10

II, sites clos to the -Ti-O-Ti- chains at the apexes of the 12-ring pores.23,24 In the most ordered sample (A) these two sites are partially resolved in the isotropic dimension. In sample G, this signal is broadened considerably, indicating increased disorder in the cation environment. A similar broadening effect is observed when Al is substituted into the Si sites in the ETS-10 framework,24 which will also decrease short-range order. However, there is no evidence of any new cation exchange sites in the more defective materials. Both previous 23Na NMR studies also report a second, weaker signal at about 2 ppm in the isotropic dimension, attributed to sodium cations in sites III and V within the 12-ring pore. No signal at this position is observed in the spectra in Figure 10. However, if sites I and II have different quadrupole coupling constants from sites III and V, the relative position of peaks in the isotropic dimension will change with magnetic field. Since these spectra are obtained at a lower magnetic field than in previous studies, a referee has suggested that the second site corresponds to the weak peak at -2.5 ppm, which is not present in those spectra measured at higher field. Further measurements at higher field, coupled with simulations of the spectra, are needed to confirm this suggestion. A striking feature of this work is the observation that the Raman spectrum of ETS-10 is sensitive to the presence of defects in the structure. The largest effect seen is the perturbation of the intense 724 cm-1 Raman band associated with Ti-O vibrations in the -Ti-OTi- chains. Introduction of stacking defects will shorten the average chain length. The broadening and shift to higher frequency of the Ti-O stretching band that occurs when this happens indicates strongly that this band arises from a coupling of Ti-O stretching modes. Similar effects are seen with the coupled C-C stretching vibrations in the Raman spectrum of graphite as the crystal size is reduced.26 The disappearance of the corresponding Ti-O infrared band in more disordered samples, although much less apparent, can be accounted for similarly. The more subtle changes occurring in the lower frequency region of the Raman spectrum are more difficult to interpret, but the introduction of lattice defects would certainly be expected to perturb also the Si-O-Si, Si-O-Ti, and Ti-O-Ti bending modes in the vicinity of the defects. Other Raman studies have observed that with the isomorphous substitution of B, Al, Co, Nb, and Ga into the ETS-10 framework, the intense 724 cm-1 band is considerably broadened and shifted by approximately 20 cm-1 to higher frequency.19,27-29 These changes were attributed to substitution, and it would be reasonable to associate them with heterogeneous framework strain, although they may also be related to the stacking disorder discussed here. Interruption of the -Ti-O-Ti- chains will also affect the UV absorption spectrum of ETS-10. The previously (26) Lespade, P.; Al-Jishi, R.; Dresselhaus, M. Carbon 1982, 20, 427. (27) Rocha, J.; Branda˜o, P.; Anderson, M. W.; Ohsuna, T.; Terasaki, O. Chem. Commun. 1998, 667. (28) Lin, Z.; Rocha, J.; Ferreira, A.; Anderson, M. W. Colloid Surf. A 2001, 179, 133. (29) Eldewik, A. N. Spectroscopic Studies of Heavy Metals in Microporous Materials, Ph.D. Thesis, University of New South Wales: Sydney, 1998.

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published UV spectra do not resolve two components to the band at around 280 nm. However, the experiments reported by Bordiga et al. in which ETS-10 is doped with adsorbed sodium vapor appear to show the presence of a second component to this band that is less affected by sodium doping than the first (although this difference is not commented on by the authors).30 The 280 nm band has been assigned by Borello et al. to a delocalized transition from the top of the valence band of the -Ti-O-Ti- chains (filled oxygen 2p orbitals from the bridging oxide ions) to the bottom of the conduction band (vacant Ti4+ d orbitals).7 Sodium doping was shown to reduce Ti4+ to Ti3+, thereby reducing the intensity of the O2- to Ti4+ charge-transfer transition.30 The exact wavelength of the band gap transition in the -Ti-O-Ti- chains depends on the diameter of the titania “wires”, the effective mass of the electron, and the length of the wire. Borello et al. estimated a band gap shift from bulk TiO2 of 0.84 eV, assuming an effective mass of about 2 me and a chain length of greater than 10 nm. This agrees well with the observed position of the 280 nm band. The introduction of stacking defects into the -Ti-O-Ti- chains will shorten the average chain length and should therefore shift the band gap to shorter wavelength. The other consequence of interrupting the chains will be to introduce defect states within the band gap associated with localized charge-transfer transitions. The changes in the profile of the 280 nm band across the series A-G (Figure 4) cannot easily be accounted for in terms of the above model. There is no obvious blue shift in the absorption edge across the series. The lower wavelength component does appear to broaden significantly, which would be consistent with the development of a wider distribution of chain lengths in the more disordered samples, and the higher wavelength component shifts to longer wavelength, perhaps due to the introduction of localized states in the band gap. Nevertheless, we have to conclude at this point that the simple model of Borello et al. is unable to convincingly explain these changes. A more straightforward trend in the spectra in Figure 4 is the progressive reduction in the relative intensity of the 216 nm band across the series. This band has been attributed to localized charge-transfer transitions from oxide ions, bridging between Si and Ti, to Ti4+.7 The loss in intensity of this band is too great, however, to be attributed directly to loss of Ti-O-Si bonds. We suggest that this band also may be associated with delocalized transitions, which are greatly reduced in intensity as the average chain length is shortened. We note that the relative intensities of the 280 and 216 nm bands do vary in UV spectra of ETS-10 reported by others,31 supporting the suggestion that average -TiO-Ti- chain lengths may vary in different preparations of ETS-10. The abundance of stacking defects will be dependent on the relative rates of nucleation and growth during the synthesis of ETS-10. Where nucleation of each layer is much slower than the growth, a relatively defect-free (30) Bordiga, S.; Turnes Palomino, G.; Zecchina, A.; Ranghino, G.; Giamello, E.; Lamberti, C. J. Chem. Phys. 2000, 112, 3859. (31) Yang, X.; Paillaud, J.-L.; van Breukelen, H. F. W. J.; Kessler, H.; Duprey, E. Microporous Mesoporous Mater. 2001, 46, 1.

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crystal will grow. Conversely, to increase the number of defects it is necessary to increase the rate of nucleation relative to the rate of layer growth. As discussed recently by Anderson et al., these relative rates should be influenced by variables such as synthesis temperature, heating rate, or addition of surface modifiers.12 These variables are not yet well enough understood to be able to produce samples with predicted defect concentrations. An alternative approach is to introduce defects by postsynthesis treatment of the ETS-10. For example, we have found recently that ammonium ion exchange followed by calcination to produce the proton exchanged form of ETS-10 produces UV and Raman spectra very similar to those shown above for defective samples.32 It is interesting to compare previously published ETS10 spectra with these results. Most UV absorption7,33 and Raman16-18 spectra resemble the spectra of samples D or E, with a medium degree of disorder. These two techniques have proved the most sensitive to variations in disorder. From the viewpoint of photoreactivity, the level of defects in ETS-10 has two important consequences. (32) Krisnandi, Y.; Southon, P. D.; Howe, R. F. To be published. (33) Lamberti, C. Microporous Mesoporous Mater. 1999, 30, 155163.

Southon and Howe

Interruption of the -Ti-O-Ti- chains will modify the semiconductor properties (both electronic and optical), which may influence photocatalytic performance. Second, the stacking defects interrupting the chains produce exposed titanium sites that would not be present in a well crystalline sample. We are currently investigating both of these factors. Furthermore, an increased concentration of double-pores will increase the adsorption capacity for larger molecules.12 Acknowledgment. Part of this work was undertaken in the School of Chemistry at the University of New South Wales. Support from the Australian Research Council for the initial work and from the University of Aberdeen for the later work is gratefully acknowledged. Access to the Photon Factory was provided by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. We also acknowledge the EPSRC National Solid State NMR Centre at the University of Durham for access and Dr David Apperley for his invaluable assistance and advice. CM021199C