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Preparation and characterization of a titanium(IV) silsesquioxane epoxidation catalyst anchored into mesoporous MCM-41.
Inorganic Chemistry Communications 3 Ž2000. 557–562 www.elsevier.nlrlocaterinoche
Preparation and characterization of a titanium žIV/ silsesquioxane epoxidation catalyst anchored into mesoporous MCM-41. P. Smet a,) , J. Riondato b, T. Pauwels a , L. Moens b, L. Verdonck a a
Department of Inorganic and Physical Chemistry, Organometallics and Catalysis DiÕision, Ghent UniÕersity, Krijgslaan 281 (S3), 9000 Ghent, Belgium b Department of Analytical Chemistry, Ghent UniÕersity, Proeftuinstraat 86, 9000 Ghent, Belgium Received 28 April 2000; accepted 26 June 2000
Abstract This paper describes the synthesis and spectroscopic study of a titaniumŽIV. silsesquioxane complex, which is heterogenised in the pores of an MCM-41 host material. Its immobilization is performed via chemical bonding, not by means of physical adsorption. As a linking molecule between the MCM-41 carrier and the silsesquioxane, Ž3-glycidyloxypropyl.trimethoxysilane is used. Characterization is performed by using nitrogen adsorption techniques, TGA, diffuse reflectance infrared spectroscopy ŽDRIFT. and ICP-MS. Also its catalytic activity towards the epoxidation of alkenes shows interesting results. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Titanium; Silsesquioxane; MCM-41; Olefin epoxidation
1. Introduction
2. Experimental
Heterogenisation of homogeneous catalytic complexes is an important tool to obtain active heterogeneous catalysts which possess the same catalytic site as their homogeneous analogues, but which can be easily separated from the reaction mixture by simple filtration. TitaniumŽIV. silsesquioxanes have proven to be active and useful epoxidation catalysts w1–3x, while MCM-41 is used as a support because of its high specific surface area and its regular structure consisting of channels with a distinct diameter w4,5x. In this work, we report the heterogenisation of a titaniumŽIV. silsesquioxane complex to MCM-41. This is done by first modifying the support with Ž3-glycidyloxypropyl.trimethoxysilane, followed by the opening of the oxirane ring of the linker molecule using a deprotonated silsesquioxane. Finally, titaniumŽIV. butoxide is reacted with the modified MCM-41. The main characterization is performed by means of diffuse reflectance infrared spectroscopy ŽDRIFT. and supported by techniques such as nitrogen adsorption measurements, thermo-gravimetrical analysis and ICP-mass spectrometry.
2.1. Synthesis
)
Corresponding author. E-mail address: [email protected] ŽP. Smet..
MCM-41. The mesoporous, siliceous MCM-41 material was synthesized by a slightly modified literature procedure w4x. Its quality was established by XRD and by sorption ˚ measurements. The hexagonal unit cell length is ca. 40 A. (C 6 H 11 )7 Si 7 O 9 (OH)3 (1). The incompletely condensed silsesquioxane (1) was prepared according to the procedure reported by Feher et al. w6x. r GOPTMS. The procedure for the surface MCM-41r modification is based on a method reported previously w7x. 0.4 m L Ž 3-glycidyloxypropyl . trim ethoxysilane ŽGOPTMS. ŽFLUKA. are added to 3.0 g of freshly calcined MCM-41, suspended in 25 mL of dry toluene and refluxed for 24 h in an argon atmosphere. After reaction, the product is filtered off and washed with toluene, followed by a toluene Soxhlet extraction Ž24 h.. In a second and third experiment the amounts of GOPTMS used, are raised to respectively 1.0 mL and 2.0 mL per 3.0 g of MCM-41. r GOPTMSr r 1. 0.1562 g of 1 are dissolved MCM-41r in 30 mL of cyclohexane under argon. After complete dissolution, one equivalent Ž0.080 mL. cyclohexylmagnesium chloride ŽALDRICH. is added and the mixture refluxed for 1 h. In a separate flask, also under argon, 0.5174
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Table 1 Surface area of MCM-41rGOPTMS as a function of the amount of Ž3-glycidyloxypropyl.trimethoxysilane. Loading ŽmL GOPTMSr3 g MCM-41.
Surface area Žm2 rg.
0.4 1.0 2.0
1191.09 959.82 734.79
added and refluxed during 24 h. After filtering and a 16 h diethyl ether Soxhlet extraction, the product is dried under vacuum at room temperature. Again three different samples are prepared by using the following amounts: 3.10 = 10y4 , 12.40 = 10y4 and 24.80 = 10y4 mol TiŽOBu.4r1 g support. 2.2. Sample characterization
g MCM-41rGOPTMS Žwith the highest loading of GOPTMS. are suspended in 20 mL of cyclohexane. To this suspension, the monomagnesium silsesquioxane is added by means of a syringe. After 24 h reaction at 323 K, 0.010 mL of water are added and the suspension is stirred for another 30 min. The product is filtered and subjected to a cyclohexane as well as a methanol Soxhlet extraction. The solvent is removed under reduced pressure at room temperature. The three different amounts of silsesquioxane used are: 3.10 = 10y4 , 6.20 = 10y4 and 12.40 = 10y4 mol 1r1 g MCM-41rGOPTMS. r GOPTMSr r 1 r Ti. To 1.095 g of MCMMCM-41r 41rGOPTMSr1 Žhighest loading. suspended in 30 mL of diethyl ether, 0.116 mL of TiŽOBu.4 ŽALDRICH. are
X-ray diffraction: XRD patterns were recorded using a Siemens D5000 diffractometer with monochromated CuŽK a . radiation. FT-infrared spectroscopy: DRIFT spectra were recorded on a Mattson Research spectrometer by filling a cup with grinded KBr onto which the product was deposited. If necessary, an inert DRIFT cell was used. Nitrogen adsorption measurements: N2 adsorption isotherms were measured at 77 K using a Gemini III 2375 analyser ŽMicromeritics.. Prior to the experiments, the sample of MCM-41 was dehydrated at 200 8C for 12 h. The other products were kept under vacuum at room temperature, also for 12 h. Surface areas were calculated using the BET ŽBrunauer–Emmett–Teller. w21x method
Fig. 1. DRIFT spectra of the OH region of MCM-41 and three successive loadings of GOPTMS.
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while pore-size distributions were collected via the BJH ŽBarrett–Joyner–Halenda. w22x method. ICP-MS: Measurements were done by means of a Finnigan MAT high resolution ICP-mass spectrometer, which was used in its standard configuration. NMR: Spectra are recorded using a Bruker WH-500 ¨ spectrometer.
3. Results and discussion As the amount of Ž3-glycidyloxypropyl.trimethoxysilane ŽGOPTMS. added to the MCM-41 is raised, the specific surface area undergoes a distinct decrease ŽTable 1.. This indicates that the hydroxyl groups within the channels are being modified. This is also clearly demonstrated by means of DRIFT analysis of the 2800–4000 cmy1 region. Since the band at 3700 cmy1 , which is attributed to hydroxyl groups on the inside of the pores w8,9x, completely disappears in the spectrum of the modified MCM-41 with the highest loading of GOPTMS ŽFig. 1.. Also note the decreasing intensity of the 3745 cmy1 signal which corresponds to isolated silanole groups located, for instance, at terminal sites w9x. As the MCM-41 material contains no organic functionalities, an intense signal between 2800 and 3000 cmy1 appears due to the presence of the GOPTMS. Further evidence is provided by the decreasing pore diameter and the fact that the total volume of adsorbed N2 diminishes as the amount of GOPTMS on the surface gets higher. Thermo-gravimetric analysis is useful, not only to demonstrate the presence but also to calculate approximately the amount of GOPTMS on the support Žvide supra.. Table 2 shows the experimental values of the TGA analysis. Sorption measurements of the samples containing the silsesquioxane (1) ŽMCM-41rGOPTMSr1. again show a further decrease of the specific surface area as well as a lowering in the total volume of adsorbed nitrogen gas as the amount of 1 increases. This indicates that during this step, the sites inside of the channels are also being modified. The presence of the silsesquioxane (1) can best be demonstrated by using DRIFT techniques. Especially the vibration band near 1448 cmy1 , which is only present in
Fig. 2. DRIFT spectra of MCM-41rGOPTMS and three successive loadings of silsesquioxane Ž1. Ž1450 cmy1 region..
the spectrum of structure 1, neatly illustrates the evolution of the amount of 1 bound into the support ŽFig. 2.. Also very convincing are the aliphatic infrared absorption bands. While the intensity of the peaks near 2947 and 2848 cmy1 Ž –CH 2 – asym. and sym. str. of GOPTMS. should stay constant, the ones near 2927 and 2854 cmy1 Ž –CH 2 – asym. and sym. str. of 1. should intensify as the amount of 1 is increased. The absorption band-also of GOPTMS-near 2877 cmy1 is used for normalization since
Table 2 Thermo-gravimetrical analysis of MCM-41rGOPTMS. Loading ŽmLr3 g.
% Weight loss Ž%.
Including background corr. Ž%.
0.4 1.0 2.0
8.11 12.88 22.81
4.36 9.13 19.06
Fig. 3. DRIFT spectra of MCM-41rGOPTMS and three successive loadings of silsesquioxane Ž1. in the aliphatic region ŽLegend: see Fig. 2..
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Table 3 Overview of the absorption bands in the aliphatic region for each of the products Žvalues are in cmy1 .. GOPTMS
Ž1.
MCM-41rGOPTMSrŽ1.
TiŽOBu.4
2848 2877 q 2927 Žsh. 2947
2854 – 2927 –
2854 2877 2927 2947 Žsh.
– 2873 2930 2956
it shows no interference with the other vibration bands. This is clearly demonstrated in Fig. 3. In a third reaction step, a titaniumŽIV. butoxide is added to the MCM-41rGOPTMSr1 support. Due to the fact that the titanium centre still possesses one butoxide ligand Žsee below., infrared can be used to follow up the relative amount of wyTiŽOBu.x on the MCM-41r GOPTMSr1 support. In addition to the above, the vibration bands of the butoxide ligand appear at wavenumbers which are slightly different from those of the support. This is resumed in Table 3. These absorption bands correspond to stretching vibrations of the methylene Ž –CH 2 – . groups
of the anchored GOPTMS and silsesquioxane (1). No interference of the yOMe groups is expected, since these functionalities are all substituted by surface hydroxyl groups w7x. Fig. 4 clearly shows the appearance of the bands near 2956, 2930 and 2873 cmy1 demonstrating the increasing amount of wyTiŽOBu.x being bound by the support. The same observation is done for an absorption band near 1463 cmy1 , which is attributed to a –CH 2 – asymmetric bending. Very convincing is the presence of a new band near 957 cmy1 in the spectrum of MCM-41r GOPTMSr1rTi, which can be associated with Si-O-Ti bonds and attributed to Si-O-Ti stretching vibrations. Roesky et al. w10x have similarly reported the observation of an intense absorption band at 960–970 cmy1 for a number of cubic titanasiloxane compounds. For titanium silicates such as TS-1 and Ti-MCM-41 an absorption band is found at 960–970 cmy1 w11,12x while for amorphous titania-silica mixed oxides a similar band is observed in the region 940–960 cmy1 w13–15x. Also Crocker et al. w16x have come to the same conclusion after preparation of homogeneous titanium silsesquioxane complexes. So it is
Fig. 4. DRIFT spectra of MCM-41rGOPTMSr1 and three successive loadings of TiŽOBu.4 Žaliphatic region..
P. Smet et al.r Inorganic Chemistry Communications 3 (2000) 557–562 Table 4 ICP-MS analysis of MCM-41rGOPTMSr1rTi in order to determine the amount of Ti for the three different loadings. Ti Žmgrg. 3.23 20.73 23.59
obvious that the titanium complex is covalently bound to the support by means of siloxy-bridges. To support these results, the homogeneous reaction between glycidyl isopropyl ether and the monomagnesium silsesquioxane derivative is examined. The resulting product is further reacted with titaniumŽIV. butoxide. After filtration, extraction into tetrahydrofuran and evaporation of the solvent, the product is analysed by means of 1 H en 13 C NMR spectroscopy. It is obvious that the oxirane ring is opened due to the attack of the monomagnesium silsesquioxane, since the resonances of the three epoxide protons are no longer observed. Further evidence is provided by the absence of any hydroxyl proton resonance as well as a total integration of 21 protons relative to the silsesquioxane 1 H resonances as expected for ŽŽC 6 H 11 . 7 Si 7 O 9 ŽO. 2 OCH 2CHŽO.CH 2 OCHŽCH 3 . 2 .-TiŽOŽCH 2 . 3 CH 3 . (2). Assignment of individual peaks is difficult due to overlap of certain signals. Very interesting is the observation of 5 signals in a 1:2:2:1:1 ratio in the methine region of the carbon-13 NMR spectrum. This is in full agreement with structure 2 in which the silsesquioxane ligand is anchored to the titanium centre by means of two siloxy-bridges, as the third one is used to open the oxirane ring. The exact amount of titanium present on the support is determined by using ICP-MS analysis. Table 4 shows the amounts of titanium, expressed in mgrg of product analysed for the three different loadings. Plotting these data results in a curve reaching a maximum amount at about 24 mg of titaniumrg of sample. By using the TGA data listed in Table 2, it is possible to calculate Žapproximately. the amount of GOPTMS per gramme of MCM-41 present, being 2.18 mmolrg MCM41, indicating that the first reaction step has a yield of about 70% Žthese calculations are valid for the highest loadings.. Conversion into number of moles GOPTMS per gramme of sample Žs MCM-41rGOPTMS. gives a value of 1.65 mmolrg. So it is obvious that all of the added silsesquioxane Ž1.24 mmolrg. can be bound in case of 100% reaction yield in the second step. Data from the literature for the homogeneous reaction between the silsesquioxane (1) and titanium precursors, mention yields of 90% or higher w1,2,17–20x. Now assume that the reaction in which 1 is bound to MCM-41rGOPTMS goes to completion. This means that there are 0.562 mmol 1r1 g MCM-41rGOPTMSr1, corresponding to 0.562 mmolrg
561
support of possible binding sites for the titaniumŽIV. butoxide complex. Conversion of this value leads to 24.3 mg of titanium per gramme of sample, being the maximum amount found by means of ICP-MS. Although these calculations are approximate, they provide a good indication of the yields of the different steps. Initial testing of the catalytic epoxidation activity reveals that the system is highly active as well as selective. Using hydrogen peroxide as an oxidant and cyclooctene as substrate in a 250r75r1 ratio Žsubstr.rox.rcat.. at 60 8C a conversion of 50% and 100% selectivity is reached in one hour. When using tert-butyl hydroperoxide as oxidant, even 86% conversion is established, all other parameters being the same as for H 2 O 2 . Especially encouraging are the results with hydrogen peroxide, making this catalytic system very promising. Abbenhuis et al. w3x and Mojet et al. w19x reported the development of a quite similar compound. They immobilized a titaniumŽIV. silsesquioxane into an MCM-41 by physical adsorption of the cyclohexyl ligands onto the walls of the mesoporous material. No leaching occurred with an all silica MCM-41. They suggested a preserved accessibility of the titanium site when adsorbed in the MCM-41 host, as a result of the uncharged performance upon immobilization towards the epoxidation of olefins. These results, together with the above mentioned catalytic data, suggest that the titanium centre in the system developed here, is fully accessible, notwithstanding the use of a spacer molecule. Since in the work of Abbenhuis et al. w3x leaching occured when using an aluminium containing MCM-41, this approach could offer a solution to this problem.
References w1x H.C.L. Abbenhuis, S. Krijnen, R.A. van Santen, Chem. Commun. Ž1997. 331. w2x T. Maschmeyer, M.C. Klunduk, C.M. Martin, D.S. Shephard, J.M. Thomas, B.F.G. Johnson, Chem. Commun. Ž1997. 1847. w3x S. Krijnen, H.C.L. Abbenhuis, R.W.J.M. Hanssen, J.H.C. van Hooff, R.A. van Santen, Angew. Chem. Int. Ed. 37 Ž1998. 356. w4x J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 Ž1992. 10834. w5x C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 Ž1992. 710. w6x F.J. Feher, D.A. Newman, J.F. Walzer, J. Am. Chem. Soc. 111 Ž1989. 1741. w7x S. Rao, D. De Vos, T. Bein, P. Jacobs, Chem. Commun. Ž1997. 355. w8x P. Jacobs, R. van Ballmoos, J. Phys. Chem. 86 Ž1982. 3050. w9x A. Jentys, N.H. Pham, H. Vinck, J. Chem. Soc. Faraday Trans. 92 Ž1996. 3287. w10x A. Voigt, R. Murugavel, V. Chandrasekhar, N. Winkhofer, H.W. Roesky, H.-G. Schmidt, I. Uson, Organometallics 15 Ž1996. 1610. w11x B. Notari, Catal. Today 18 Ž1993. 163. w12x G. Bellussi, M.S. Rigutto, Stud. Surf. Sci. Catal. 85 Ž1994. 177. w13x S. Klein, S. Thorimbert, W.F. Maier, J. Catal. 163 Ž1996. 476.
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w14x R.J. Davis, Z. Liu, Chem. Mater. 9 Ž1997. 2311. w15x R. Neumann, M. Levin-Elad, J. Catal. 166 Ž1997. 206. w16x M. Crocker, R.H.M. Herold, A.G. Orpen, M.T.A. Overgaag, J.Chem.Soc. Dalton Trans. Ž1999. 3791. w17x I.E. Buys, T.W. Hambley, D.J. Houlton, T. Maschmeyer, A.F. Masters, A.K. Smith, J. Mol. Catal. 86 Ž1994. 309. w18x M. Crocker, R.H.M. Herold, A.G. Orpen, Chem. Commun. Ž1997. 2411.
w19x S. Krijnen, B.L. Mojet, H.C.L. Abbenhuis, J.H.C. van Hooff, R.A. van Santen, Phys. Chem. Chem. Phys. 1 Ž1999. 361. w20x M.C. Klunduk, T. Maschmeyer, J.M. Thomas, B.F.G. Johnson, Chem. Eur. J. 5 Ž1999. 1481. w21x Thomas, Thomas, APrinciples and Practice of Heterogeneous CatalysisB, 1997, VCH. w22x E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 Ž1951. 373.
Preparation and characterization of a titanium žIV/ silsesquioxane epoxidation catalyst anchored into mesoporous MCM-41. P. Smet a,) , J. Riondato b, T. Pauwels a , L. Moens b, L. Verdonck a a
Department of Inorganic and Physical Chemistry, Organometallics and Catalysis DiÕision, Ghent UniÕersity, Krijgslaan 281 (S3), 9000 Ghent, Belgium b Department of Analytical Chemistry, Ghent UniÕersity, Proeftuinstraat 86, 9000 Ghent, Belgium Received 28 April 2000; accepted 26 June 2000
Abstract This paper describes the synthesis and spectroscopic study of a titaniumŽIV. silsesquioxane complex, which is heterogenised in the pores of an MCM-41 host material. Its immobilization is performed via chemical bonding, not by means of physical adsorption. As a linking molecule between the MCM-41 carrier and the silsesquioxane, Ž3-glycidyloxypropyl.trimethoxysilane is used. Characterization is performed by using nitrogen adsorption techniques, TGA, diffuse reflectance infrared spectroscopy ŽDRIFT. and ICP-MS. Also its catalytic activity towards the epoxidation of alkenes shows interesting results. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Titanium; Silsesquioxane; MCM-41; Olefin epoxidation
1. Introduction
2. Experimental
Heterogenisation of homogeneous catalytic complexes is an important tool to obtain active heterogeneous catalysts which possess the same catalytic site as their homogeneous analogues, but which can be easily separated from the reaction mixture by simple filtration. TitaniumŽIV. silsesquioxanes have proven to be active and useful epoxidation catalysts w1–3x, while MCM-41 is used as a support because of its high specific surface area and its regular structure consisting of channels with a distinct diameter w4,5x. In this work, we report the heterogenisation of a titaniumŽIV. silsesquioxane complex to MCM-41. This is done by first modifying the support with Ž3-glycidyloxypropyl.trimethoxysilane, followed by the opening of the oxirane ring of the linker molecule using a deprotonated silsesquioxane. Finally, titaniumŽIV. butoxide is reacted with the modified MCM-41. The main characterization is performed by means of diffuse reflectance infrared spectroscopy ŽDRIFT. and supported by techniques such as nitrogen adsorption measurements, thermo-gravimetrical analysis and ICP-mass spectrometry.
2.1. Synthesis
)
Corresponding author. E-mail address: [email protected] ŽP. Smet..
MCM-41. The mesoporous, siliceous MCM-41 material was synthesized by a slightly modified literature procedure w4x. Its quality was established by XRD and by sorption ˚ measurements. The hexagonal unit cell length is ca. 40 A. (C 6 H 11 )7 Si 7 O 9 (OH)3 (1). The incompletely condensed silsesquioxane (1) was prepared according to the procedure reported by Feher et al. w6x. r GOPTMS. The procedure for the surface MCM-41r modification is based on a method reported previously w7x. 0.4 m L Ž 3-glycidyloxypropyl . trim ethoxysilane ŽGOPTMS. ŽFLUKA. are added to 3.0 g of freshly calcined MCM-41, suspended in 25 mL of dry toluene and refluxed for 24 h in an argon atmosphere. After reaction, the product is filtered off and washed with toluene, followed by a toluene Soxhlet extraction Ž24 h.. In a second and third experiment the amounts of GOPTMS used, are raised to respectively 1.0 mL and 2.0 mL per 3.0 g of MCM-41. r GOPTMSr r 1. 0.1562 g of 1 are dissolved MCM-41r in 30 mL of cyclohexane under argon. After complete dissolution, one equivalent Ž0.080 mL. cyclohexylmagnesium chloride ŽALDRICH. is added and the mixture refluxed for 1 h. In a separate flask, also under argon, 0.5174
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Table 1 Surface area of MCM-41rGOPTMS as a function of the amount of Ž3-glycidyloxypropyl.trimethoxysilane. Loading ŽmL GOPTMSr3 g MCM-41.
Surface area Žm2 rg.
0.4 1.0 2.0
1191.09 959.82 734.79
added and refluxed during 24 h. After filtering and a 16 h diethyl ether Soxhlet extraction, the product is dried under vacuum at room temperature. Again three different samples are prepared by using the following amounts: 3.10 = 10y4 , 12.40 = 10y4 and 24.80 = 10y4 mol TiŽOBu.4r1 g support. 2.2. Sample characterization
g MCM-41rGOPTMS Žwith the highest loading of GOPTMS. are suspended in 20 mL of cyclohexane. To this suspension, the monomagnesium silsesquioxane is added by means of a syringe. After 24 h reaction at 323 K, 0.010 mL of water are added and the suspension is stirred for another 30 min. The product is filtered and subjected to a cyclohexane as well as a methanol Soxhlet extraction. The solvent is removed under reduced pressure at room temperature. The three different amounts of silsesquioxane used are: 3.10 = 10y4 , 6.20 = 10y4 and 12.40 = 10y4 mol 1r1 g MCM-41rGOPTMS. r GOPTMSr r 1 r Ti. To 1.095 g of MCMMCM-41r 41rGOPTMSr1 Žhighest loading. suspended in 30 mL of diethyl ether, 0.116 mL of TiŽOBu.4 ŽALDRICH. are
X-ray diffraction: XRD patterns were recorded using a Siemens D5000 diffractometer with monochromated CuŽK a . radiation. FT-infrared spectroscopy: DRIFT spectra were recorded on a Mattson Research spectrometer by filling a cup with grinded KBr onto which the product was deposited. If necessary, an inert DRIFT cell was used. Nitrogen adsorption measurements: N2 adsorption isotherms were measured at 77 K using a Gemini III 2375 analyser ŽMicromeritics.. Prior to the experiments, the sample of MCM-41 was dehydrated at 200 8C for 12 h. The other products were kept under vacuum at room temperature, also for 12 h. Surface areas were calculated using the BET ŽBrunauer–Emmett–Teller. w21x method
Fig. 1. DRIFT spectra of the OH region of MCM-41 and three successive loadings of GOPTMS.
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while pore-size distributions were collected via the BJH ŽBarrett–Joyner–Halenda. w22x method. ICP-MS: Measurements were done by means of a Finnigan MAT high resolution ICP-mass spectrometer, which was used in its standard configuration. NMR: Spectra are recorded using a Bruker WH-500 ¨ spectrometer.
3. Results and discussion As the amount of Ž3-glycidyloxypropyl.trimethoxysilane ŽGOPTMS. added to the MCM-41 is raised, the specific surface area undergoes a distinct decrease ŽTable 1.. This indicates that the hydroxyl groups within the channels are being modified. This is also clearly demonstrated by means of DRIFT analysis of the 2800–4000 cmy1 region. Since the band at 3700 cmy1 , which is attributed to hydroxyl groups on the inside of the pores w8,9x, completely disappears in the spectrum of the modified MCM-41 with the highest loading of GOPTMS ŽFig. 1.. Also note the decreasing intensity of the 3745 cmy1 signal which corresponds to isolated silanole groups located, for instance, at terminal sites w9x. As the MCM-41 material contains no organic functionalities, an intense signal between 2800 and 3000 cmy1 appears due to the presence of the GOPTMS. Further evidence is provided by the decreasing pore diameter and the fact that the total volume of adsorbed N2 diminishes as the amount of GOPTMS on the surface gets higher. Thermo-gravimetric analysis is useful, not only to demonstrate the presence but also to calculate approximately the amount of GOPTMS on the support Žvide supra.. Table 2 shows the experimental values of the TGA analysis. Sorption measurements of the samples containing the silsesquioxane (1) ŽMCM-41rGOPTMSr1. again show a further decrease of the specific surface area as well as a lowering in the total volume of adsorbed nitrogen gas as the amount of 1 increases. This indicates that during this step, the sites inside of the channels are also being modified. The presence of the silsesquioxane (1) can best be demonstrated by using DRIFT techniques. Especially the vibration band near 1448 cmy1 , which is only present in
Fig. 2. DRIFT spectra of MCM-41rGOPTMS and three successive loadings of silsesquioxane Ž1. Ž1450 cmy1 region..
the spectrum of structure 1, neatly illustrates the evolution of the amount of 1 bound into the support ŽFig. 2.. Also very convincing are the aliphatic infrared absorption bands. While the intensity of the peaks near 2947 and 2848 cmy1 Ž –CH 2 – asym. and sym. str. of GOPTMS. should stay constant, the ones near 2927 and 2854 cmy1 Ž –CH 2 – asym. and sym. str. of 1. should intensify as the amount of 1 is increased. The absorption band-also of GOPTMS-near 2877 cmy1 is used for normalization since
Table 2 Thermo-gravimetrical analysis of MCM-41rGOPTMS. Loading ŽmLr3 g.
% Weight loss Ž%.
Including background corr. Ž%.
0.4 1.0 2.0
8.11 12.88 22.81
4.36 9.13 19.06
Fig. 3. DRIFT spectra of MCM-41rGOPTMS and three successive loadings of silsesquioxane Ž1. in the aliphatic region ŽLegend: see Fig. 2..
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Table 3 Overview of the absorption bands in the aliphatic region for each of the products Žvalues are in cmy1 .. GOPTMS
Ž1.
MCM-41rGOPTMSrŽ1.
TiŽOBu.4
2848 2877 q 2927 Žsh. 2947
2854 – 2927 –
2854 2877 2927 2947 Žsh.
– 2873 2930 2956
it shows no interference with the other vibration bands. This is clearly demonstrated in Fig. 3. In a third reaction step, a titaniumŽIV. butoxide is added to the MCM-41rGOPTMSr1 support. Due to the fact that the titanium centre still possesses one butoxide ligand Žsee below., infrared can be used to follow up the relative amount of wyTiŽOBu.x on the MCM-41r GOPTMSr1 support. In addition to the above, the vibration bands of the butoxide ligand appear at wavenumbers which are slightly different from those of the support. This is resumed in Table 3. These absorption bands correspond to stretching vibrations of the methylene Ž –CH 2 – . groups
of the anchored GOPTMS and silsesquioxane (1). No interference of the yOMe groups is expected, since these functionalities are all substituted by surface hydroxyl groups w7x. Fig. 4 clearly shows the appearance of the bands near 2956, 2930 and 2873 cmy1 demonstrating the increasing amount of wyTiŽOBu.x being bound by the support. The same observation is done for an absorption band near 1463 cmy1 , which is attributed to a –CH 2 – asymmetric bending. Very convincing is the presence of a new band near 957 cmy1 in the spectrum of MCM-41r GOPTMSr1rTi, which can be associated with Si-O-Ti bonds and attributed to Si-O-Ti stretching vibrations. Roesky et al. w10x have similarly reported the observation of an intense absorption band at 960–970 cmy1 for a number of cubic titanasiloxane compounds. For titanium silicates such as TS-1 and Ti-MCM-41 an absorption band is found at 960–970 cmy1 w11,12x while for amorphous titania-silica mixed oxides a similar band is observed in the region 940–960 cmy1 w13–15x. Also Crocker et al. w16x have come to the same conclusion after preparation of homogeneous titanium silsesquioxane complexes. So it is
Fig. 4. DRIFT spectra of MCM-41rGOPTMSr1 and three successive loadings of TiŽOBu.4 Žaliphatic region..
P. Smet et al.r Inorganic Chemistry Communications 3 (2000) 557–562 Table 4 ICP-MS analysis of MCM-41rGOPTMSr1rTi in order to determine the amount of Ti for the three different loadings. Ti Žmgrg. 3.23 20.73 23.59
obvious that the titanium complex is covalently bound to the support by means of siloxy-bridges. To support these results, the homogeneous reaction between glycidyl isopropyl ether and the monomagnesium silsesquioxane derivative is examined. The resulting product is further reacted with titaniumŽIV. butoxide. After filtration, extraction into tetrahydrofuran and evaporation of the solvent, the product is analysed by means of 1 H en 13 C NMR spectroscopy. It is obvious that the oxirane ring is opened due to the attack of the monomagnesium silsesquioxane, since the resonances of the three epoxide protons are no longer observed. Further evidence is provided by the absence of any hydroxyl proton resonance as well as a total integration of 21 protons relative to the silsesquioxane 1 H resonances as expected for ŽŽC 6 H 11 . 7 Si 7 O 9 ŽO. 2 OCH 2CHŽO.CH 2 OCHŽCH 3 . 2 .-TiŽOŽCH 2 . 3 CH 3 . (2). Assignment of individual peaks is difficult due to overlap of certain signals. Very interesting is the observation of 5 signals in a 1:2:2:1:1 ratio in the methine region of the carbon-13 NMR spectrum. This is in full agreement with structure 2 in which the silsesquioxane ligand is anchored to the titanium centre by means of two siloxy-bridges, as the third one is used to open the oxirane ring. The exact amount of titanium present on the support is determined by using ICP-MS analysis. Table 4 shows the amounts of titanium, expressed in mgrg of product analysed for the three different loadings. Plotting these data results in a curve reaching a maximum amount at about 24 mg of titaniumrg of sample. By using the TGA data listed in Table 2, it is possible to calculate Žapproximately. the amount of GOPTMS per gramme of MCM-41 present, being 2.18 mmolrg MCM41, indicating that the first reaction step has a yield of about 70% Žthese calculations are valid for the highest loadings.. Conversion into number of moles GOPTMS per gramme of sample Žs MCM-41rGOPTMS. gives a value of 1.65 mmolrg. So it is obvious that all of the added silsesquioxane Ž1.24 mmolrg. can be bound in case of 100% reaction yield in the second step. Data from the literature for the homogeneous reaction between the silsesquioxane (1) and titanium precursors, mention yields of 90% or higher w1,2,17–20x. Now assume that the reaction in which 1 is bound to MCM-41rGOPTMS goes to completion. This means that there are 0.562 mmol 1r1 g MCM-41rGOPTMSr1, corresponding to 0.562 mmolrg
561
support of possible binding sites for the titaniumŽIV. butoxide complex. Conversion of this value leads to 24.3 mg of titanium per gramme of sample, being the maximum amount found by means of ICP-MS. Although these calculations are approximate, they provide a good indication of the yields of the different steps. Initial testing of the catalytic epoxidation activity reveals that the system is highly active as well as selective. Using hydrogen peroxide as an oxidant and cyclooctene as substrate in a 250r75r1 ratio Žsubstr.rox.rcat.. at 60 8C a conversion of 50% and 100% selectivity is reached in one hour. When using tert-butyl hydroperoxide as oxidant, even 86% conversion is established, all other parameters being the same as for H 2 O 2 . Especially encouraging are the results with hydrogen peroxide, making this catalytic system very promising. Abbenhuis et al. w3x and Mojet et al. w19x reported the development of a quite similar compound. They immobilized a titaniumŽIV. silsesquioxane into an MCM-41 by physical adsorption of the cyclohexyl ligands onto the walls of the mesoporous material. No leaching occurred with an all silica MCM-41. They suggested a preserved accessibility of the titanium site when adsorbed in the MCM-41 host, as a result of the uncharged performance upon immobilization towards the epoxidation of olefins. These results, together with the above mentioned catalytic data, suggest that the titanium centre in the system developed here, is fully accessible, notwithstanding the use of a spacer molecule. Since in the work of Abbenhuis et al. w3x leaching occured when using an aluminium containing MCM-41, this approach could offer a solution to this problem.
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