Symmetry and Location of Titanium Within Titanium Silicalite Framework of MFI Structure

Symmetry and Location of Titanium Within Titanium Silicalite Framework of M[FI Structure

D. Trong On, I. Denis, C. Lortie, C. Cartier and L. Bonneviot Departement de Chimiet, CERPIC, Universith Laval, G1K 7P4, Ste Foy, Qc, Canada. ABSTRACT A series of titanium silicalites of MFI structure, active in n-hexane oxyfunctionalization, were investigated by FT-IR, XPS, XANES and EXAFS spectroscopies to characterize the titanium sites. Most of the titanium ions are sited in a non-substitutional framework sites of C4v symmetry rather than Td. The framework IR bands reveal that the [SiO4] units, linked to titanium via double Ti-0-Si bridges, have a symmetry lowered from Td to at least CzV. The decrease of the 960 cm-1 IR band upon the effect of adsorption of water or H202 is attributed to the partial hydrolysis of the double bridges leading to a linkage by single bridges. A molecular simulation investigation shows that such sites can be accommodated in the structure by disruption of the four (Si) membered rings. INTRODUflION Titanium silicalites (TS)were recently found selective for various reactions. Among them, oxyfunctionalization of alkanes with H202 is probably one of the most interesting [l-31. The titanium ions responsible for the catalytic activity are believed, on the basis of unit cell expansion with increasing Ti content, to occupy substitutional T sites. Such lattice expansion has been confirmed to occur up to 4% molar fraction of titanium in TS-1 [4,5]. Titanium can also be incorporated in the framework of other silicalites of MEL (TS2),p, or ZSM-48 (TS-48) structure 16-10]. Despite this success, the incorporation of transition metal ions into a zeolite framework and its characterization are still a challenge since these ions preferentially occupy octahedral sites that a zeolite framework can not provide. The location of the titanium site is a puzzling case not yet fully understood. Though the lattice expansion is a good criteria for framework incorporation, it does not necessitate the occupancy of substitutional site to take place. Our recent EXAFS investigation indeed proved that the framework TiOx species are non-substitutional species in TS2 [7,81. They are linked to framework SiO4 tetrahedra via an edge-sharing type of binding. This was later confirmed for TS-1 whose dehydration effect was 101

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investigated by EXAFS [12]. It was found that the double Ti-O-Si bridge that connects a [Ti041 unit to a [SiO4] unit is partially hydrolyzed leaving those two units linked via a corner, i.e., through a single Ti-0-Si bridge. This study deals with the problem of rationalization concerning the 960 cm-I IR band evolution in connection with the titanium symmetry upon adsorption of water or hydrogen peroxide and, with the titanium location in the framework. EXPERIMENTALS Titanium silicalites (TS-I) were prepared from the addition of water to a mixture of tetra(ethoxy)silicon(IV) and tetra(iso-propoxy)titanium(IV)compounds in presence of a solution of tetrapropylammonium hydroxide in propanol. The hydrothermal treatment was carried out at 175OC for 4 days in a Teflon coated stainless steel autoclave. The solid materials was filtered, washed and calcined at 500 "C. Four samples were synthesized with a Ti/Ti+Si ratio of 1.2, 2.1, 2.6 and 3.4% obtained from chemical analysis. Dehydrated samples were obtained by evacuation at 45OOC in N2 a n d transferred under dry N2 in the appropriate cell for measurements. The Ba2Ti04 and hexadecaphenyloctaeiloxyspiro(9,9)titanate(WtHDPOSST, were prepared as indicated in the literature [ll]. The XRD were recorded on a Rigaku D-Max IIIVC X-ray spectrometer. IR spectra were recorded on self supporting pellets of samples diluted in KBr using a Bomem 102 FTIR spectrometer. The XPS data were performed on a V.G. Scientific Escalab Mark I1 [5]. The X-ray Absorption spectra at the titanium edge were collected at the radiation synchrotron facility of the LURE (France) and treated as previously [7,8,12]. The white radiation was monochromatized by a Si (311) two-crystal monochromator. The Fourier transform were obtained on filtered and k3 weighted EXAFS signals (Kaiser window [z =3.7], kl = 2.50 A-1 to k2 = 12.30 A-1) and the simulation were performed as previously [7,8,12,131. The reaction of n-hexane with hydrogen peroxide in methanol was performed at 55OC in a pyrex flask with a reflux condenser. The catalysts/hexane, hexane/H202, hexane/MeOH ratios were kept constant at 43.5 g/mol, 1.15 mol/mol and 34.3 mol/mol while, in comparison, they were held at 42.9,1.17 and 34.9 (same units), respectively, in the work by Clerici et al. [31. The product analysis was performed on a GC equipped with a capillary column. H202 was titrated at the end of the reaction. RESULTS AND DISCUSSION The XRD confirmed that the synthesized materials had the MFI structure with a crystallinity of 90-95% within this series. A linear dependence of the 960 cm-I IR band and lattice expansion with the Ti loading was taken as a fingerprint of the titanium incorporation in the framework 141.

Symmetry a n d Location of Titanium i n MFI Structure

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The reaction products of n-hexane with hydrogen peroxide were very similar within this series of samples. After 2 hours the H202 consumption was almost completed (93-98%) with an hexane conversion of 42-45%. The yield of H202 toward oxyfunctionalization was in the range of 70-83%. The functionalization in position 2 which produced 2-hexanol and 2-hexanone was favored in comparison to functionalization in position 3. The C2/C3 ratio was varying in the range of 2.8-3.5. This results were quite similar to those reported by Clerici et al. (see table 1). Table 1. Reactivity of the catalysts in n-hexane functionalization sample Ti/Ti+Si Hexane conv./% H202 conv./% H202 yield/% 0.026 43.5 97.5 74.4 TSI a TS-1 b 0.028 90 0 4 0 Ti02 C SiO2 d 0 0 0

C2/W ol/one 3.1 0.47 2.3 0.67

a) this work, b) Clerici et al. c) anatase, Degusssa P 25 and fumed silica Cab-0-Sil M5. Frame work IR characterization The characteristic IR band at 965 cm-1 was found to decrease in intensity by about 30% and 60% after adsorption of H20 and H202, respectively, in comparison with its original intensity in the calcined material. In the same time, its position was shifted from 965 to 970 cm-1. A complete restoration of this band was obtained after H20 adsorption followed by a subsequent calcination. Only a partial recovery was found to occur after a first H202 adsorption-calcination cycle. After subsequent cycles no more loss of intensity was observed. To investigate the IR 1000-11200 cm-1 range, the pellets were prepared with a higher dilution of the sample in KBr to avoid the saturation of the transmitted signal. Figure 1 depicts the absorbance of a pure silicalite superimposed with the absorbance of a TS-1. This clearly gives evidence that the incorporation of titanium is not only associated with the new peak at 965 cm-1 but also with a shift toward lower energies of the main peak position as well as a modification of the peak shape on the high energy range. These changes were further examined on difference IR spectra of TS-1 of various loadings with pure silicalite as a reference. The results shown on figure 2, were found systematically reproducible. The signal to noise ratio was still very good to trust the signal shape obtained by difference. In the central part of the figure, a very sharp peak looks like a differential spectrum. This is not due to new oscillators, this is rather the result of the difference between two strong bands slightly shifted one with another by about few cm-1. Such an effect is probably due to a slight shift toward

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0.4

8

8

5

5

e

e

P

9

8

2

4

4

900

1000

1100

1200

1300

cm-' Fig. 1. FT-IR spectra of (- - -) pure silicalite and (---) 2,6% TSI.

-0.3

I0

1000

..1200

1400

cm-1

Fig. 2. Difference IR spectra of dehydrated a) 1.2, b) 2.1 and c) 2.6%TS-I.

lower energies of the overall framework vibrational frequencies in presence of titanium. There is also a negative peak whose linewidth is large enough to discard any artefact previously described. This negative peak at about 1130 cm-1 is the trace of the suppression of one type of oscillators when the titanium is incorporated in the framework. Finally, two other peaks are revealed by the difference IR spectra at about 1080 and 1200 cm-1. These peaks and the 965 cm-1 peak could account for the splitting of the asymmetric stretching mode of the [SiO4] units linked to titanium by three new vibrational modes. A splitting by three of such magnitude occurs for the sulfato, anion whose symmetry is lowered for Td to CzV in complexes where it binds two cobalt cations [14]. For the reasoning part, this interpretation applied to [SiO41 is in agreement with previous authors [15,161 with, nevertheless, the difference that instead of two bands at 967 and 1083 cm-1, there is three bands to take into account at 965, -1080 and -1200 cm-1. Along this line, the IR results and the EXAFS data will be consistent with the double bridge formation between Ti and Si.

- x

. .

The XPS spectra of the dehydrated and the hydrated materials were found identical and exhibits a single doublet characteristic of tetravalent titanium. The 2P3/2 line rise at 459.7 eV at the same position found for tetrahedral titanium species in the titanium glasses or TS-2 materials [71. After adsorption of H202, a second doublet appears at the expense of the other, the new 2P3/2 line rising at 458.3 eV like for octahedral titanium in Ti02 anatase.

Symmetry and Location of Titanium in MFI Structure

lal5.iA c/-I

r

anatase

I

u

0 0

2 2

0 4

0 4

RIA

6 6

Fig. 3. EXAFS FI' transforms of 2.6%TSand anatase.

8

4 2

* 1

2

R / i

3

Fig. 4. EXAFS FT transforms of (---) dehydrated 2.6%TS-1 and (- - -) BCTST compound

. .

ed X-rav A

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b b The full analysis and simulation of the EXAFS signal of TS-1 and TS-2 samples was previously performed [7,8, 121. For the sake of brevity, the focus will be restricted on a qualitative comparison of the Fourier Transformed (FT) of the samples in various T profiles arising in the 0.5-1 A range is mainly due to the states. The first peak of the l mathematical residue of the baseline extraction, no further comments will be made about it (see figure 3). The second and more intense peak is attributed to the first shell of oxygen neighbors. Since the lT transform is not phase corrected, this peak is at 0.5 8, lower than the real Ti-0 distance. This first peak at 1.3 A is simulated at 1.79 8, for tetrahedral siloxytitanium compound, HDOSST, consistently with its XRD structure (1,78-1,79&. By comparison, the dehydrated TS-1 has a much lower first shell. This decrease has been described previously as the effect of the presence of a very close Si neighbor at about 2.2-2.3 A that negatively interferes with the oxygen EXAFS oscillations [81. The strong second peak in the HDOSST is due to 4 Si atoms at about 3.5 A, i. e, exactly where should lie the silicon neighbors in a regular T site. This comparison with the model compound and TS-1 clearly supports the first EXAFS simulations made previously [8,12]. The comparison between anatase and TS-1 FT's demonstrates that after water or H202 adsorption, the Ti-0 distance never reached the values expected for an octahedral environment (dTi-0 1.95 A). A slight increase of the peak at about 2.9 A in agreement with previous results 1121. This effect is more pronounced for H202 than H20. This can be related to the framework IR data assuming that the opening of the double bridge via the hydrolysis of one of the Ti-0-Si bridge is more efficient with H202. Finally, the Ti-0

-

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D. T. On. 1. Denis, C. Lortie. C . Cartier and L. Bonneviot

0

15 30 45 Energy /eV

60

0

Fig. 5. XANES at Ti K-edge of dehydrated 2.6%TS-I, and reference compounds

15

30 45 Energy /eV

60

B

Figure 6 : XANES at Ti K-ed es of 2.6% TS-1 after various treatments an anatase

distances in the H202 adduct seems to be distributed in two groups of distances, short and long distances (-1.8 8, and 2.1 A), consistent with the formation of peroxotitanium species [171.

. .

X-rav AbsorDtion Near Edge characThough the titanium near edges of dehydrated TS-1 are similar to those of tetrahedral titanium in Ba2Ti04 and HDOSST compounds, there is some striking differences (see figure 5 and 6). Along the series, Ba2TiO4, BCTST and TS-I, the pre-edge position shifts from 2.6, 2.9 to 3.2 eV, a shoulder at about 13 eV increases progressively and the post-edge evolves toward longer distances. These evolution are consistent with a distortion leading to a stronger crystal field in the xy plane and a weaker field effect in the z direction. This would be drastically produced in a square planar symmetry 1181. A square planar titanium phtalocyanin indeed exhibits a preedge at 3.5 eV, close to the TS-1 pre-edge position [19]. Nevertheless, the shoulder at 13 eV is not strong enough for TS-1to account to for a pure D4h symmetry. Titanium is more likely to occupy a C4" site. The hydration might be understood as an addition of a water molecule that increases the coordination number from 4 to 5 accompanied with an equilibrated reaction of hydrolysis of one of the two Ti-0-Si bridges that links Ti to Si. By contrast, hydrogen peroxide leads to a substantial displacement toward an hexacoordinated state in a strongly distorted octahedral symmetry (intense pre-edge, mixture of short and long Ti-0 distances and XPS 2P3/2 shift). In the same time, most of the rings formed by

Symmetry and Locarion of Titanium i n MFI Structure

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the double bridge are open with respect to the strong intensity decrease of the 965 cm-* band.

e work

The lattice expansion, the symmetry, the binding mode of titanium and the loss of crystallinity clearly characterize a disruptive framework site. The search for the site using Polygen Quanta from Molecular Simulations was dictated by the separation by about 4.3-4.6%i (about twice the Ti--Si EXAFS distance) between two framework silicon ions. The result shows that the four silicon membered rings fulfill the conditions.

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According to this ,one can envisage the monomeric site as shown on the scheme. Such sites are most probably those active in alkane oxyfunctionalization owing to its highly strained environment, its open coordination shell and its capacity to form the peroxotitanium species. CONCLUSION The titanium environment in TS-1 materials has been investigated with a combination of techniques designed to probe long and short range structure as well as local symmetry. A consistent picture of the monomeric site has been proposed where the titanium is tetracoordinated in a C4" symmetry. Its linkage to the framework occurs via two double bridges. The disrupted four silicon membered ring of the framework is the more reasonable location to accommodate such an odd site. REFERENCES 1 T. Tatsumi, M. Nakamura, S. Nagashi & H. Tominaga, J.Chem.Soc.,Chem. Comm., (1990)476 2 D.R.C. Huybrechts, L. De Bruycker, & P. A. Jacobs, Nature, 345 (1990) 240 3 M. G . Clerici, Appl.Catal., 68 (1991) 249. 4 M. Tamarasso, G. Perego, and B. Notari, U. S. Pat.4410501 (1983). 5 A.J.H.P. van der Pol and J.H.C. van Hoof, Appl. Catal., 92 (1992) 93. 6 J. R. Reddy and R. Kumar, J. Catal., 130 (1991) 440. 7 D. Trong On, L. Bonneviot, A. Bittar, A. Sayari and S. Kaliaguine, J. Mol. Catal., 74 (1992) 233; A. Bittar, D. Trong On, L. Bonneviot, S. Kaliaguine and A. Sayari, in R. von Ballmoos, J. B. Higgins and M. M. Treacy (Eds.), Proc. 9th Int. Zeolite Conf., Montreal, July 1992, Butterworth-Heinemann, Boston, 1993, p. 453 8 D. Trong On, A. Bittar, A. Sayari, S. Kaliaguine and L. Bonneviot, Catal.Letters, 16 (1992) 95. 9 M. A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. Soc., Chem. Comm., (1992) 589. 10 D. P. Serrano, H.-X. Li and M. E. Davis, J. Chem. SOC.,Chem. Comm., (1992) 745. 11 M. B. Hursthouse and Md. A. Hossain, Polyhedron, 3 (1984) 95. 12 L. Bonneviot, D. Trong On and A. Lopez, J. Chem. Soc., Chem. Comm., (1993) 685. 13 A. Michalowicz in Logiciel pour la Chirnie, SOC.Fr. Chimie, Paris, 1991, p. 102; A. Michalowicz and V. Voinville, ibid,p.l16. 14 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th Edition, Wiley, New York, 1986, p. 249. 15 M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal., 48 (1990) 133. 16 G. Deo, A. M. Turek, I. E. Wachs, D. R. C. Huybrechts and P. A. Jacobs, Zeolites, 13 (1993)365. 17 F. A. Cotton and G. Wilkinson, Advance in Inorganic Chemistry, 5th edition, Wiley, New York, 1988, p.659. 18 C. Cartier, M. Momenteau, E. Dartyge, A. Fontaine, G. Tourillon, A. Michalowicz and M. Verdaguer, J. Chem. SOC.Dalton Trans., 19 C. A. Yarker, P. A. Johnson, A. C. Wright,J. Wong, B. Greegor, F. W. Lytle and R. N. Sinclair, J. Non-Crystalline Solids, 791(986) 117.