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Comparative spectroscopic study of TS-1 and zeolite-hosted extraframework titanium oxide dispersions
J. Weilkamp, H.G. Karge. H. Pfeikr and W. Hblderich (Eds.) Zeolites and Relaied Microporous Maicrials: Siare of lhe Ari 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All righls rcscrvcd.
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Comparative spectroscopic study of TS-1 and zeolite-hosted extraframework titanium oxide dispersions Joachim Klaas, Kornelia Kulawik, Giinter Schulz-Ekloff and Nils I. Jaeger *Institut fur Angewandte und Physikalische Chemie, F B 2, Universitat Bremen, PF 330440 D-28334 Bremen Diffuse UV-VIS reflectance spectroscopy (DRS) is shown to be an appropriate tool for the reliable distinction between isomorphously substituted Ti, molecular TiO, species attached to the zeolite framework, Q-size particles and Ti02 exhibiting bulk properties. These TiO, species can also be identified in TS-1 and other microporous materials. The high dispersion of titaniumoxide in TiOz zeolites accelerates the reactivity towards reducing gases. 1. INTRODUCTION The synthesis of the titanium silicalite TS-1 has led to intense investigations with respect to the structural assignment of the titanium and its relation to the unusual catalytic properties of the material [l]. In many cases following the synthesis of TS-1 extra-framework Ti-oxide species are present in the material. The assignment of the centers responsible for the observed catalytic properties therefore requires the unambiguous identification of the synthesized materials with respect to the location and the nature of the Ti-species within the zeolite structure. The preparation of suitable model systems for comparison purposes was in addition stimulated by the growing interest in size-quantization (Q-size) effects observed for highly dispersed semiconductors [2,3]. Faujasites can be used as a suitable matrix to host well defined highly dispersed titanium oxide [4].
2. EXPERIMENTAL 2.1. Sample preparation TS-1 samples were synthesized following procedures reported in the literature [5,6], i. e. tetraethyl orthosilicate (TEOS, Merck) and tetraethyl orthotitanate or tetrabutyl orthotitanate (TEOT, TBOT, Merck) were used as silicon and titanium source and tetrapropyl ammonium hydroxide (TPAOH, Alfa) as an alkali-free (Na+, K+ < 200 ppm) template. Different crystal sizes and titanium contents were obtained by the variation of the amount of educts. Low water content and a high [SiO,]/[OH-] ratio were needed for small crystals. All samples synthesized with i-propanol [5] contain small amounts or traces of Ti02 (anatase), which can be identified by a Raman vibration band at 144 cm-'.
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TiOr loaded faujasites were prepared in a flow reactor with a diameter of .55 mm providing shallow bed conditions by the following procedure: the zeolite powder (1.5 g Nay, N a X ) was dehydrated in a stream of dry nitrogen overnight at temperatures between 100°C and 400°C. Dry nitrogen (2.5 l/h), loaded with TiCI4 at room temperature (60% saturation), was led through the zeolite powder resulting in the chemisorption of the TiCll by the zeolite at temperatures between 100 "C and 400°C. Different temperatures of dehydration or for TiCI4 sorption and varied times were applied. To remove excess TiCI4 the reactor was flushed for at least 1 hour with dry nitrogen at the chemisorption temperature. The chemisorbed TiCI4 was then hydrolysed by a water saturated flow of nitrogen at the same temperature. The sample loaded at 100°C was calcined at 250°C in a stream of dry nitrogen. The titanium content was determined by analysing the amount of Tic14 not adsorbed by the zeolite and hydrolyzed in a gas-wash bottle. The adsorbed amount was determined by direct analysis of the loaded zeolite by X-Ray fluorescence and by chemical analysis. Both methods agreed within *20% . Zeolite-hosted TiO, prepared as described will be called TiO, zeolite further on. The dcxsignation of the samples contain the type of zeolite used, the temperature of dehydration Tdchydrand TiCI4 adsorption Tads.and the titanium loading in wt.%. In Table 1 the time of TiCI4 treatment tads. is given also. Table 1 Preparation conditions and titanium contents of the samples Sample TS-1 ( 2 p m ) 1's-1 (0.F) p m ) Ti0,NaX (400/ 100/0.1) Ti0,NaX (400/ 100/2.1) Ti0,NaX (100/100/1.9) li0,NaY (400/400/2.1) Ti0,NaY (400/400/3.9)
Tdehydr.
"CI
Tads. 1"Cl
tads. [min]
400 400 100 400 400
100 100 100 400 400
10 30 30 30 60
Ti-content [wt %] 1.9 1.9 0.1 2.1 2.1 1.9 3.9
2.2. Sample characterization Diffuse reflectance UV-VIS spectra of the zeolite powders were recorded under ambient conditions on a Varian Cary 4 photospectrometer, equipped with a Praying Mantis. The light spot was about 1 to 2 mm. The powders were filled in a hole (10 mm in diameter and 3 mm deep) of a sample holder, and t h e surface was smoothed.The layer can be regarded as infinitely thick as required by the Kubelka-Munk theory. The recording parameters were spectral band width 4 n m , integration time 0.5s and data point distance 1 nm. Samples showing an absorbance F ( R )> 1.5 were diluted with untreated zeolite, if not noted otherwise. The diluent, or in t h e case of undiluted samples the untreated zeolite was used as a reference.
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The crystallinity of the samples was checked by X-Ray diffraction of the zeolite powders recorded on a Seifert diffractometer with a Bragg-Brentano setup using CuKa radiation and by physisorption of Nz. Backscattering Raman spectra were measured on a Bruker IFS FT-Raman-Spectrometer equipped with a Nd-YAG-Laser operating at a wavelength of 1064nm. 2.3. Catalysis and reactivity The hydroxylation of phenol in the presence of hydrogen peroxide into dihydroxybenzenes was used as the basic test reaction. The reaction was performed in a 100 ml batch reactor under reflux conditions at 343 K. A suspension of 10 g phenol, 0.5 g catalyst and 40 ml methanol was uniformly stirred. 4.65 g HzOz (34 wt.-%) was added over a time of 10 minutes. The products were analyzed by high-performance liquid chromatography (Merck) equipped with a LiChrosorb RP 18 column and an UV detector L-4000 at 280 nm. The hydrogen peroxide conversion was determined by standard iodometric titration. Gas-sensing properties were checked via UV-VIS DRS. The spectra were recorded as described above, but instead of the simple sample holder an evacuable and heatable cell (reaction cell manufactured by Harrick) was mounted under the Praying-Mantis. The zeolite powders were pressed to wafers of 5 mm diameter and 0.5 to 1mm thickness and placed into the cell. The sample was heated up to 600 "C in vacuum and subsequently in different gases ( lo5 Pa) in the following order: oxygen, hydrogen, oxygen and hydrogen. Spectra were recorded after the heating and gas treatment of the sample at ambient temperature with a n integration time of 0.1 s only.
3. RESULTS 3.1. Crystallinity With the exception of faujasite samples which were incompletely dehydrated prior to TiCI4 loading no loss of crystallinity could be detected by XRD. This result could be confirmed by N2-physisorption. The TS-1 structure could be unambiguously determined and no bulk Ti02 peaks could be observed in the X-ray diffractogram. 3.2. Spectroscopy Raman and IR spectra of TS-1 samples showed the 960cm-' signal typical for TS-1 with tetrahedral titanium in T-atom positions. This band is not visible in TiO, loaded zeolites even in the case of samples which show signals in their UV-VIS spectra typical for TS-1. Raman spectra of TS-1 usually show small peaks in the position around 144cm-' typical for anatase. This signal is not visible in spectra of TiO, zeolites except for sample Ti0,NaX (100/100/1.9) incompletely dehydrated at 100 "C or a highly loaded sample Ti0,NaX (400/400/2.1) treated with Tic14 at 400 "C (Figure 1). The measured diffuse reflectance Rs of the samples was related to the reflectance of the reference RR as Rb, = R ~ / R RThe . Kubelka-Munk values were calculated according to the Kubelka-Munk function (1 - R',)2 - _ It'
F ( R L )=
(2R',)
-
S NC h ' and S are the effective absorption and scattering coefficients, respectively. The effective absorption coefficient It' is proportional but not equal to the absorption coefficient
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I I I
Int a.u.
100 200 300 400 500 600 700 800 900 1000 raman shift [cm-'1 Figure 1. Raman spectra of TS-1 and Ti0,NaX samples determined by transmission experiments. The Kubelka-Munk theory [7] predicts that F ( R ) is proportional to the concentration of the absorbing species as long as the assumptions of the theory are fulfilled, namely that F ( R ) < 1.5. Samples showing an absorbance with F ( R ) higher than about 1.5 were diluted with untreated zeolite or BaS04. At a wavelength below 300nm the profiles of the spectra depend on the support and the diluent, since these materials show some absorption relative to Bas04 which itself has some absorption. In that case the assumption of the Kubelka-Munk theory that the reflectance of the references is close t o 1 is not fulfilled and care has to be taken in the interpretation of the spectra. The absorption edge was determined as the crossing point of the tangent through the point of inflexion with the x-axis, as shown in Figure 3. This point will also be referred to as absorption onset and labeled zo. Diffuse reflectance UV-VIS spectra for TS-1 and TiO, zeolites are displayed in Figure 2 and Figure 3. TS-1 and the sample Ti0,NaY (400/400/2.1) show an absorption onset at about 260 nm, while all other samples have a broad but structured absorption. It has to be mentioned that the sample Ti0,NaY (400/400/2.1) within a period of several months showed a marked shift of the absorption edge with time towards values observed in other Ti0,faujasites (Figure 2). The DRS UV-VIS spectra can be deconvoluted yielding up to four bands. However, not all observable bands are present in all samples. The bands are: (I) the absorption of anatas with a maximum at about 328 nm [8] and an absorption onset at about 370 nm; (11) a band centered at about 290nm with an absorption onset at 330-340nm; (111) a band centered around 240nm with an absorption onset between 300-310nm, and (IV) an absorption onset at 250-260 nm typical for TS-1 and attributed to tetrahedrally coordinated titanium. The spectrum of Ti0,NaX (400/100/0.1) is dominated by the band at 240 nm, while for
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Fig. 3
wavelength [nm]
wavelength [nm]
Figure 2. Normalised diffuse reflectance UV-VIS spectra of samples with tetrahedral titanium: (-) Ti0,NaY (400/400/%.1) left: fresh, right: aged sample; (- - ) TS-1 (0.5pm); ( - - - - ) TS-1 ( a p m ) , ) Ti0,NaX Figure 3. Normalised diffuse reflectance UV-VIS spectra of Ti0,zeolites: ((400/100/0.1); (- - ) Ti0,NaX (400/100/2.1); ( - - - - ) Ti0,NaX (100/100/1.9) (.-- ) anatas in zeolite NaY with the absorption onset 10 marked sample Ti0,NaX (400/100/2.1) with a higher titanium content the band at 290nm is stronger than the band at 240 nm. 3.3. Catalysis and reducibility The best results in testing the catalytic properties are observed for samples of TS-1 yielding high conversions of phenol (of about 15%) and hydrogen peroxide (100%))a high product selectivity and a low amount of by-products. The TiC14 treated samples with the faujasite structure show also a high phenol conversion (of about 15%), but a low hydrogen peroxide conversion (25%) and no selectivity towards dihydroxybenzenes. The reversible reduction of a titanium loaded faujasites Ti0,NaY (400/100/3.9) can be followed in situ by recording the reflectivity of the sample. Figure 4 depicts difference spectra obtained at room temperature after exposing the vacuum heated sample (600 "C) to oxygen (600°C) and subsequently to hydrogen (600°C)) oxygen (600°C) and again hydrogen (600°C). The difference spectra were obtained as A R = R,/& - 1 where & is the reflectivity of the vacuum treated and oxidized sample. The reflectance shows a broad decrease on the red side of the absorption edge under reducing conditions (heating in hydrogen) which is reversible under oxidizing conditions. The rate of change in reflectivity is much faster for the zeolite samples as compared to bulk TiOz. Quantitative evaluations are in progress.
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1
-0.3
300
200
500
400
600
700
800
wavelength [nm] Figure 4. Change in reflectivity of TiO,NaY(400/400/3.9) after heating to 600°C in different atmospheres relative to the sample after calcination at 600°C in oxygen: (- - ) HZ; () 0 2 ; ( - - - ) Hz; 4. DISCUSSION
Sample Ti0,NaX (100/100/2.1) is the only one containing anatase detectable in the XRD pattern. This can be explained by hydrolysis of TiC14 and agglomeration during loading due to incomplete dehydration. In the sample of Ti0,NaY (400/400/2.1) with a comparable high loading and a reaction temperature of 400 "C an agglomeration to bulk Ti02 can also be observed. The same was observed by Kooyman et al. [9] for highly loaded samples. This subject is discussed in detail in ref. [lo]. A strong Raman band at 144 cm-' can be seen in Figure 1 even though the UV-VIS spectrum is dominated by bands at 240 nm and 290 nm. This demonstrates the sensitivity of Raman spectroscopy towards TiOz. The weak band at 144cm-' occasionally detected for TS-1 samples (Figure 1) indicates that only traces of anatase are present giving rise to a very weak tailing around 3.50 nm in the UV-VIS spectrum (Figure 2). Figure 3 demonstrates how the spectrum of Ti0,NaX is affected by successive loading of fully dehydrated samples. Low loading (0.1 wt. %) yields a spectrum dominated by a maximum at 240nm (Figure 3) while a tenfold increase of the Ti content leads to a broadening of the spectrum and a pronounced maximum at 290nm (Figure 3). The two maxima are tentatively assigned to two distinguishable Ti-species attached to the zeolite lattice via reaction of the Tic14 with OH-groups present in the zeolite. A mono- and a bifunctional reaction as an initial step is suggested according to the following scheme [Ill: \
-Si-0,
/
\ -Si-O, /
%-OH /
,.H
+ TiC14
-+
+ Tic14
+
Si-0, Si-O/
TiClz
+ 2 HCl
bifunctional
H \
-Si-O-TiC13 /
+ HCl
monofunctional
Following Ritala et al. [12] and Haukka et al. [13] TiC14 'reacts preferentially with
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neighhouring Si-OH groups bridged by hydrogen. These sites are apparently available and low loading at relatively low temperature results in the formation of a double-bonded species first. Subsequent loading at higher temperature leads to monofunctional reaction with Si-OH groups. Complete hydrolysis then leaves two distinguishable species double (Ti0,NaX (400/100/0.1)) and single bonded (Ti0,NaX (400/400/2.1)) to the zeolite lattice which can be clearly distinguished in the absorption spectrum (Figure 3). Comparison of the spectra obtained for TS-1 and Ti0,NaY (400/400/2.1) reveals an initial similarity which indicates, that species of similar tetrahedral coordination can be obtained in faujasites (Figure 2). The species, however, is not stable and is transformed under ambient conditions within several weeks into a species normally observed in faujasites (Figure 3). The initial species can be attributed to the reaction of TiC14 with defect sites present in NaY as was reported by Kraushaar and van Hooff for dealuminated ZSM-5 [14]. The shoulder and the tailing observed in the spectra of many but not all TS-1 samples can in turn be assigned to double- and single-bonded Ti-species formed also in this structure. The infrared spectra of the samples containing mono- or bifunctionally bonded titanium species do not show the band at 960 cm-' typical for TS-1 and mostly attributed to a stretching mode of [Si04]units in the neighbourhood of Ti4+[15,16] or to the vibration of a titanyl group [17]. The absence of this band in the samples with tetrahedral Ti-species in the faujasite structure suggests a structure dependence of the 960 cm-' band and/or an instability of tetrahedrally coordinated titanium in molecular sieves of the faujasite type. The spectra of sample Ti0,NaY (400/400/2.1) (Figure 2) document the temporal change from predominantly tetrahedrally coordinated Ti-species in the as-synthesized sample to predominantly mono- and bifunctionally bonded TiO, in the aged sample. This observation can be correlated with the catalytic properties. In spite of a considerable activity none of the TiO, loaded faujasites show any significant selectivity towards dihydroxybenzenes. This emphasizes again the role of stable isomorphously substituted titanium in combination with shape selectivity and the absence of acidic catalytic centers in the TS-1. The exposure of the Ti-oxide loaded faujasite to a reducing atmosphere demonstrates, how the sensitivity of a rather unreactive bulk material can be enhanced by suitable dispersion. The broad absorption below 450 nm following a reduction cycle corresponds to a reduction of bulk TiOz to TiO, with 5 close to 2 [18]. Compared to the slow reduction of bulk rutile [19] the response time can be improved by one order of magnitude for TiO, zeolites. 5. Conclusions
Diffuse reflectance spectroscopy in the UV-VIS region has proved to be an appropriate tool for the reliable distinction between isomorphously substituted Ti, TiO, species double- and single-bonded to the zeolite framework, Q-size and bulk TiOz. The spectra of TS-1 could be assigned to the coexistence of tetrahedral and different molecular Ti0,-species. It could be shown that the response towards a reducing atmosphere can be significantly accelerated by the dispersion of the oxides.
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Acknowledgement Financial support by the University of Bremen (N/FSP3/90) and by the Bundesminisfer fur Forschung und Technologie (NT2063-9) is gratefully acknowledged. We thank Mr. Ch. Kulinna (GHS Essen) for providing the Rarnan spectra.
REFERENCES
G.Bellussi and V. Fattore, in: P. Jacobs, N. Jaeger, L. Kubelkovaand B. Wichterlova, editors. Zeolite Chemistry and Catalysis, Stud. Surf. Sci. Catal., Vol 69, Elsevier, Amsterdam, 1991, p. 79. 2. A. Henglein, Chem. Rev., 89, (1989), 1861. 3. M. Wark, G. Schulz-Ekloff and N. I. Jaeger, Catalysis Today, 8 , (1991), 469. 4. V. Kornarov, L. Shirinskaya and N. Bohan, Russian Journal of Physical Chemistry, 50, (1976), 1478. 5. A. Thangaraj, M. J. Eapen, S. Sivasanker and P. Ratnasarny, Zeolites, 12, (1992), 945. 6. G. Bellussi, A. Carati, M. G. Clerici, G. Maddinelli and R. Millini, J . Catal., 133, (1992), 220. 7 . G. Kortiirn, RefEe2ionsspek.troskopie, Springer, Berlin, 1969. 8. D. R. Huybrechts, P. Buskens and P. Jacobs, J . Mol. Catal., 71, (1992), 129. 9. P. .J. Kooyman, P. van der Waal, P. A. Verdaaskonk, K. C. Jansen and H. van Bekkum, Catal. Lett., 13, (1992), 229. 10. J. Klaas, G. Schulz-Ekloff and N. I. Jaeger, to be submitted, 1994. 11. R. Morrow and A. Hardin, J . Phys. Chem., 83, (1979), 3135. 12. M. Ritala, M. Leskela, E. Nykanen, P. Soininen and L. Niinisto, Thin Solid Films 225, (1993), 288. 13. S . Haukka, E.-L. Lakornaa and A. Root, J . Phys. Chem., 97,(1993), 5085. 14. B. Kraushaar and J . H. C. van Hooff. Catal. Lett., 1, (1988), 81. 15. M. Roccuti, K. Rao, A. Zecchina, G. Leofanti and G. Petrini, in: C. Morterra, A. Zecchina and G. Costa, editors, Structure and Reactivity of Surfaces, Stud. Surf. Sci. Catal., Vol 48, Elsevier, Amsterdam, 1989, p. 1. 16. J. S. Reddy and R. Kumar, J. Catal., 130. (1991), 440. 17. H. Notari, in: P. J. Grobet, W. J. Mortier, E. F. Vansant and G . Schulz-Ekloff, editors, Iiinovation in Zeolite Materials Science, Stud. Surf. Sci. Catal., VoZ. 37, Elsevier Amsterdam, 1988, p. 413. 18. R. Clark, The Chemistry of Titanium and Vanadium Elsevier, Amsterdam, 1968. 19. M. Khader, F.-N. Kheiri, B. El-Anadoouli and B. Ateya, J . Phys. Chem., 97,(1993), 6074. 1.
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Comparative spectroscopic study of TS-1 and zeolite-hosted extraframework titanium oxide dispersions Joachim Klaas, Kornelia Kulawik, Giinter Schulz-Ekloff and Nils I. Jaeger *Institut fur Angewandte und Physikalische Chemie, F B 2, Universitat Bremen, PF 330440 D-28334 Bremen Diffuse UV-VIS reflectance spectroscopy (DRS) is shown to be an appropriate tool for the reliable distinction between isomorphously substituted Ti, molecular TiO, species attached to the zeolite framework, Q-size particles and Ti02 exhibiting bulk properties. These TiO, species can also be identified in TS-1 and other microporous materials. The high dispersion of titaniumoxide in TiOz zeolites accelerates the reactivity towards reducing gases. 1. INTRODUCTION The synthesis of the titanium silicalite TS-1 has led to intense investigations with respect to the structural assignment of the titanium and its relation to the unusual catalytic properties of the material [l]. In many cases following the synthesis of TS-1 extra-framework Ti-oxide species are present in the material. The assignment of the centers responsible for the observed catalytic properties therefore requires the unambiguous identification of the synthesized materials with respect to the location and the nature of the Ti-species within the zeolite structure. The preparation of suitable model systems for comparison purposes was in addition stimulated by the growing interest in size-quantization (Q-size) effects observed for highly dispersed semiconductors [2,3]. Faujasites can be used as a suitable matrix to host well defined highly dispersed titanium oxide [4].
2. EXPERIMENTAL 2.1. Sample preparation TS-1 samples were synthesized following procedures reported in the literature [5,6], i. e. tetraethyl orthosilicate (TEOS, Merck) and tetraethyl orthotitanate or tetrabutyl orthotitanate (TEOT, TBOT, Merck) were used as silicon and titanium source and tetrapropyl ammonium hydroxide (TPAOH, Alfa) as an alkali-free (Na+, K+ < 200 ppm) template. Different crystal sizes and titanium contents were obtained by the variation of the amount of educts. Low water content and a high [SiO,]/[OH-] ratio were needed for small crystals. All samples synthesized with i-propanol [5] contain small amounts or traces of Ti02 (anatase), which can be identified by a Raman vibration band at 144 cm-'.
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TiOr loaded faujasites were prepared in a flow reactor with a diameter of .55 mm providing shallow bed conditions by the following procedure: the zeolite powder (1.5 g Nay, N a X ) was dehydrated in a stream of dry nitrogen overnight at temperatures between 100°C and 400°C. Dry nitrogen (2.5 l/h), loaded with TiCI4 at room temperature (60% saturation), was led through the zeolite powder resulting in the chemisorption of the TiCll by the zeolite at temperatures between 100 "C and 400°C. Different temperatures of dehydration or for TiCI4 sorption and varied times were applied. To remove excess TiCI4 the reactor was flushed for at least 1 hour with dry nitrogen at the chemisorption temperature. The chemisorbed TiCI4 was then hydrolysed by a water saturated flow of nitrogen at the same temperature. The sample loaded at 100°C was calcined at 250°C in a stream of dry nitrogen. The titanium content was determined by analysing the amount of Tic14 not adsorbed by the zeolite and hydrolyzed in a gas-wash bottle. The adsorbed amount was determined by direct analysis of the loaded zeolite by X-Ray fluorescence and by chemical analysis. Both methods agreed within *20% . Zeolite-hosted TiO, prepared as described will be called TiO, zeolite further on. The dcxsignation of the samples contain the type of zeolite used, the temperature of dehydration Tdchydrand TiCI4 adsorption Tads.and the titanium loading in wt.%. In Table 1 the time of TiCI4 treatment tads. is given also. Table 1 Preparation conditions and titanium contents of the samples Sample TS-1 ( 2 p m ) 1's-1 (0.F) p m ) Ti0,NaX (400/ 100/0.1) Ti0,NaX (400/ 100/2.1) Ti0,NaX (100/100/1.9) li0,NaY (400/400/2.1) Ti0,NaY (400/400/3.9)
Tdehydr.
"CI
Tads. 1"Cl
tads. [min]
400 400 100 400 400
100 100 100 400 400
10 30 30 30 60
Ti-content [wt %] 1.9 1.9 0.1 2.1 2.1 1.9 3.9
2.2. Sample characterization Diffuse reflectance UV-VIS spectra of the zeolite powders were recorded under ambient conditions on a Varian Cary 4 photospectrometer, equipped with a Praying Mantis. The light spot was about 1 to 2 mm. The powders were filled in a hole (10 mm in diameter and 3 mm deep) of a sample holder, and t h e surface was smoothed.The layer can be regarded as infinitely thick as required by the Kubelka-Munk theory. The recording parameters were spectral band width 4 n m , integration time 0.5s and data point distance 1 nm. Samples showing an absorbance F ( R )> 1.5 were diluted with untreated zeolite, if not noted otherwise. The diluent, or in t h e case of undiluted samples the untreated zeolite was used as a reference.
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The crystallinity of the samples was checked by X-Ray diffraction of the zeolite powders recorded on a Seifert diffractometer with a Bragg-Brentano setup using CuKa radiation and by physisorption of Nz. Backscattering Raman spectra were measured on a Bruker IFS FT-Raman-Spectrometer equipped with a Nd-YAG-Laser operating at a wavelength of 1064nm. 2.3. Catalysis and reactivity The hydroxylation of phenol in the presence of hydrogen peroxide into dihydroxybenzenes was used as the basic test reaction. The reaction was performed in a 100 ml batch reactor under reflux conditions at 343 K. A suspension of 10 g phenol, 0.5 g catalyst and 40 ml methanol was uniformly stirred. 4.65 g HzOz (34 wt.-%) was added over a time of 10 minutes. The products were analyzed by high-performance liquid chromatography (Merck) equipped with a LiChrosorb RP 18 column and an UV detector L-4000 at 280 nm. The hydrogen peroxide conversion was determined by standard iodometric titration. Gas-sensing properties were checked via UV-VIS DRS. The spectra were recorded as described above, but instead of the simple sample holder an evacuable and heatable cell (reaction cell manufactured by Harrick) was mounted under the Praying-Mantis. The zeolite powders were pressed to wafers of 5 mm diameter and 0.5 to 1mm thickness and placed into the cell. The sample was heated up to 600 "C in vacuum and subsequently in different gases ( lo5 Pa) in the following order: oxygen, hydrogen, oxygen and hydrogen. Spectra were recorded after the heating and gas treatment of the sample at ambient temperature with a n integration time of 0.1 s only.
3. RESULTS 3.1. Crystallinity With the exception of faujasite samples which were incompletely dehydrated prior to TiCI4 loading no loss of crystallinity could be detected by XRD. This result could be confirmed by N2-physisorption. The TS-1 structure could be unambiguously determined and no bulk Ti02 peaks could be observed in the X-ray diffractogram. 3.2. Spectroscopy Raman and IR spectra of TS-1 samples showed the 960cm-' signal typical for TS-1 with tetrahedral titanium in T-atom positions. This band is not visible in TiO, loaded zeolites even in the case of samples which show signals in their UV-VIS spectra typical for TS-1. Raman spectra of TS-1 usually show small peaks in the position around 144cm-' typical for anatase. This signal is not visible in spectra of TiO, zeolites except for sample Ti0,NaX (100/100/1.9) incompletely dehydrated at 100 "C or a highly loaded sample Ti0,NaX (400/400/2.1) treated with Tic14 at 400 "C (Figure 1). The measured diffuse reflectance Rs of the samples was related to the reflectance of the reference RR as Rb, = R ~ / R RThe . Kubelka-Munk values were calculated according to the Kubelka-Munk function (1 - R',)2 - _ It'
F ( R L )=
(2R',)
-
S NC h ' and S are the effective absorption and scattering coefficients, respectively. The effective absorption coefficient It' is proportional but not equal to the absorption coefficient
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I I I
Int a.u.
100 200 300 400 500 600 700 800 900 1000 raman shift [cm-'1 Figure 1. Raman spectra of TS-1 and Ti0,NaX samples determined by transmission experiments. The Kubelka-Munk theory [7] predicts that F ( R ) is proportional to the concentration of the absorbing species as long as the assumptions of the theory are fulfilled, namely that F ( R ) < 1.5. Samples showing an absorbance with F ( R ) higher than about 1.5 were diluted with untreated zeolite or BaS04. At a wavelength below 300nm the profiles of the spectra depend on the support and the diluent, since these materials show some absorption relative to Bas04 which itself has some absorption. In that case the assumption of the Kubelka-Munk theory that the reflectance of the references is close t o 1 is not fulfilled and care has to be taken in the interpretation of the spectra. The absorption edge was determined as the crossing point of the tangent through the point of inflexion with the x-axis, as shown in Figure 3. This point will also be referred to as absorption onset and labeled zo. Diffuse reflectance UV-VIS spectra for TS-1 and TiO, zeolites are displayed in Figure 2 and Figure 3. TS-1 and the sample Ti0,NaY (400/400/2.1) show an absorption onset at about 260 nm, while all other samples have a broad but structured absorption. It has to be mentioned that the sample Ti0,NaY (400/400/2.1) within a period of several months showed a marked shift of the absorption edge with time towards values observed in other Ti0,faujasites (Figure 2). The DRS UV-VIS spectra can be deconvoluted yielding up to four bands. However, not all observable bands are present in all samples. The bands are: (I) the absorption of anatas with a maximum at about 328 nm [8] and an absorption onset at about 370 nm; (11) a band centered at about 290nm with an absorption onset at 330-340nm; (111) a band centered around 240nm with an absorption onset between 300-310nm, and (IV) an absorption onset at 250-260 nm typical for TS-1 and attributed to tetrahedrally coordinated titanium. The spectrum of Ti0,NaX (400/100/0.1) is dominated by the band at 240 nm, while for
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Fig. 3
wavelength [nm]
wavelength [nm]
Figure 2. Normalised diffuse reflectance UV-VIS spectra of samples with tetrahedral titanium: (-) Ti0,NaY (400/400/%.1) left: fresh, right: aged sample; (- - ) TS-1 (0.5pm); ( - - - - ) TS-1 ( a p m ) , ) Ti0,NaX Figure 3. Normalised diffuse reflectance UV-VIS spectra of Ti0,zeolites: ((400/100/0.1); (- - ) Ti0,NaX (400/100/2.1); ( - - - - ) Ti0,NaX (100/100/1.9) (.-- ) anatas in zeolite NaY with the absorption onset 10 marked sample Ti0,NaX (400/100/2.1) with a higher titanium content the band at 290nm is stronger than the band at 240 nm. 3.3. Catalysis and reducibility The best results in testing the catalytic properties are observed for samples of TS-1 yielding high conversions of phenol (of about 15%) and hydrogen peroxide (100%))a high product selectivity and a low amount of by-products. The TiC14 treated samples with the faujasite structure show also a high phenol conversion (of about 15%), but a low hydrogen peroxide conversion (25%) and no selectivity towards dihydroxybenzenes. The reversible reduction of a titanium loaded faujasites Ti0,NaY (400/100/3.9) can be followed in situ by recording the reflectivity of the sample. Figure 4 depicts difference spectra obtained at room temperature after exposing the vacuum heated sample (600 "C) to oxygen (600°C) and subsequently to hydrogen (600°C)) oxygen (600°C) and again hydrogen (600°C). The difference spectra were obtained as A R = R,/& - 1 where & is the reflectivity of the vacuum treated and oxidized sample. The reflectance shows a broad decrease on the red side of the absorption edge under reducing conditions (heating in hydrogen) which is reversible under oxidizing conditions. The rate of change in reflectivity is much faster for the zeolite samples as compared to bulk TiOz. Quantitative evaluations are in progress.
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1
-0.3
300
200
500
400
600
700
800
wavelength [nm] Figure 4. Change in reflectivity of TiO,NaY(400/400/3.9) after heating to 600°C in different atmospheres relative to the sample after calcination at 600°C in oxygen: (- - ) HZ; () 0 2 ; ( - - - ) Hz; 4. DISCUSSION
Sample Ti0,NaX (100/100/2.1) is the only one containing anatase detectable in the XRD pattern. This can be explained by hydrolysis of TiC14 and agglomeration during loading due to incomplete dehydration. In the sample of Ti0,NaY (400/400/2.1) with a comparable high loading and a reaction temperature of 400 "C an agglomeration to bulk Ti02 can also be observed. The same was observed by Kooyman et al. [9] for highly loaded samples. This subject is discussed in detail in ref. [lo]. A strong Raman band at 144 cm-' can be seen in Figure 1 even though the UV-VIS spectrum is dominated by bands at 240 nm and 290 nm. This demonstrates the sensitivity of Raman spectroscopy towards TiOz. The weak band at 144cm-' occasionally detected for TS-1 samples (Figure 1) indicates that only traces of anatase are present giving rise to a very weak tailing around 3.50 nm in the UV-VIS spectrum (Figure 2). Figure 3 demonstrates how the spectrum of Ti0,NaX is affected by successive loading of fully dehydrated samples. Low loading (0.1 wt. %) yields a spectrum dominated by a maximum at 240nm (Figure 3) while a tenfold increase of the Ti content leads to a broadening of the spectrum and a pronounced maximum at 290nm (Figure 3). The two maxima are tentatively assigned to two distinguishable Ti-species attached to the zeolite lattice via reaction of the Tic14 with OH-groups present in the zeolite. A mono- and a bifunctional reaction as an initial step is suggested according to the following scheme [Ill: \
-Si-0,
/
\ -Si-O, /
%-OH /
,.H
+ TiC14
-+
+ Tic14
+
Si-0, Si-O/
TiClz
+ 2 HCl
bifunctional
H \
-Si-O-TiC13 /
+ HCl
monofunctional
Following Ritala et al. [12] and Haukka et al. [13] TiC14 'reacts preferentially with
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neighhouring Si-OH groups bridged by hydrogen. These sites are apparently available and low loading at relatively low temperature results in the formation of a double-bonded species first. Subsequent loading at higher temperature leads to monofunctional reaction with Si-OH groups. Complete hydrolysis then leaves two distinguishable species double (Ti0,NaX (400/100/0.1)) and single bonded (Ti0,NaX (400/400/2.1)) to the zeolite lattice which can be clearly distinguished in the absorption spectrum (Figure 3). Comparison of the spectra obtained for TS-1 and Ti0,NaY (400/400/2.1) reveals an initial similarity which indicates, that species of similar tetrahedral coordination can be obtained in faujasites (Figure 2). The species, however, is not stable and is transformed under ambient conditions within several weeks into a species normally observed in faujasites (Figure 3). The initial species can be attributed to the reaction of TiC14 with defect sites present in NaY as was reported by Kraushaar and van Hooff for dealuminated ZSM-5 [14]. The shoulder and the tailing observed in the spectra of many but not all TS-1 samples can in turn be assigned to double- and single-bonded Ti-species formed also in this structure. The infrared spectra of the samples containing mono- or bifunctionally bonded titanium species do not show the band at 960 cm-' typical for TS-1 and mostly attributed to a stretching mode of [Si04]units in the neighbourhood of Ti4+[15,16] or to the vibration of a titanyl group [17]. The absence of this band in the samples with tetrahedral Ti-species in the faujasite structure suggests a structure dependence of the 960 cm-' band and/or an instability of tetrahedrally coordinated titanium in molecular sieves of the faujasite type. The spectra of sample Ti0,NaY (400/400/2.1) (Figure 2) document the temporal change from predominantly tetrahedrally coordinated Ti-species in the as-synthesized sample to predominantly mono- and bifunctionally bonded TiO, in the aged sample. This observation can be correlated with the catalytic properties. In spite of a considerable activity none of the TiO, loaded faujasites show any significant selectivity towards dihydroxybenzenes. This emphasizes again the role of stable isomorphously substituted titanium in combination with shape selectivity and the absence of acidic catalytic centers in the TS-1. The exposure of the Ti-oxide loaded faujasite to a reducing atmosphere demonstrates, how the sensitivity of a rather unreactive bulk material can be enhanced by suitable dispersion. The broad absorption below 450 nm following a reduction cycle corresponds to a reduction of bulk TiOz to TiO, with 5 close to 2 [18]. Compared to the slow reduction of bulk rutile [19] the response time can be improved by one order of magnitude for TiO, zeolites. 5. Conclusions
Diffuse reflectance spectroscopy in the UV-VIS region has proved to be an appropriate tool for the reliable distinction between isomorphously substituted Ti, TiO, species double- and single-bonded to the zeolite framework, Q-size and bulk TiOz. The spectra of TS-1 could be assigned to the coexistence of tetrahedral and different molecular Ti0,-species. It could be shown that the response towards a reducing atmosphere can be significantly accelerated by the dispersion of the oxides.
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Acknowledgement Financial support by the University of Bremen (N/FSP3/90) and by the Bundesminisfer fur Forschung und Technologie (NT2063-9) is gratefully acknowledged. We thank Mr. Ch. Kulinna (GHS Essen) for providing the Rarnan spectra.
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