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Exploring the catalytic activity of regular and ultralarge-pore Nb,Sn-SBA-15 mesoporous molecular sieves
1432
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Exploring the catalytic activity of regular and ultralarge-pore Nb,Sn-SBA-15 mesoporous molecular sieves Izabela Nowaka*, Agnieszka Feliczaka, Aleksandra Tomczaka, Iveta Nekoksováb and JiĜí ýejkab a
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland. Tel.: +48 618291207; Fax: +48 618658008; E-mail: [email protected] b J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3,182 23 Prague 8, Czech Republic
ABSTRACT A novel bifunctional mesoporous material Nb,Sn-SBA-15 has been synthesized in one pot synthesis by in-situ introduction of niobium and tin salts and by controlling the synthesis conditions. These molecular sieves possess a high dispersion of both metals resulting in both the strong and weak acidic sites being simultaneously formed on SBA-15. Optimization of acid concentration in synthetic gels resulted in tuning of pore geometries while maintaining high structural regularity and in pore size enlargement up to 14 nm. The nature and the coordination of T atoms in silica frameworks appropriate for catalytic applications were proved by UV-vis spectroscopy and catalytic reaction. Activity and selectivity of niobium/tincontaining mesoporous molecular sieves have been tested in liquid phase oxidation of cyclohexene. On the basis of this investigation, the new efficient NbSBA-15 catalyst prepared via conventional synthesis has been selected. 1. INTRODUCTION Recently discovered SBA-15 materials have a 2-dimensional hexagonal symmetry and expand the channel size to the mesoporous region up to 30 nm with pore wall thickness up to 6 nm [1]. They are synthesized in acidic media using a templating approach with triblock copolymers of a general formula: HO(EO)x(PO)y(EO)xH (associating two terminal poly(ethylene oxide) chains, PEO, containing x EO units with one central poly(propylene oxide) chain, PPO, containing y PO units) as structure directing agent. Initially, SBA-15 [1] was considered to be an extra-large-pore MCM-41 analogue with a honeycomb structure of disconnected channel-like pores. Later, much more evidence of microporosity [2,3] in SBA15 samples was reported. Such materials possessing a bimodal pore system, i.e., in which micropores exist within the walls of mesopores, are ideal porous materials for catalysis and adsorption, because molecules are first transported through the mesoporous channels and then strongly adsorbed in the micropores. Thus, the synthesis of SBA-15 materials has attracted much attention, especially when these materials can be prepared as metal-containing ones, as a pure SBA-15 shows very limited catalytic activity due to the absence of lattice defects,
1433 redox and acido-basic properties. Hence, SBA-15 has become a remarkable material introducing metals into mesoporous silicates to form the active sites and, thus, to improve the catalytic activity. The incorporation of various transition metals (T) into mesoporous molecular sieves has been extensively studied, however, there are not many reports on the simultaneous incorporation of two metals into SBA-15 structure. In most cases, the efficiency of T loading is not always high, especially in direct synthesis, which is expected to provide uniform distribution of heteroatoms in structures and to give an active oxidation catalyst. Tin, being tetrahedrally coordinated in those materials, acts as Lewis acid site, while pentacoordinated Nb introduces the oxidative properties. Tin has recently become a very interesting active metal when located in silicate matrices for Baeyer-Villiger oxidation of cyclic ketones to the respective lactones. Zeolite Beta as well as mesoporous MCM-41 and MCM-48 is highly active in this reaction after addition of Sn [4,5]. In a similar way, mesostructured NbMCM-41 has the advantage of being particularly efficient and long-lived catalysts for synthesis of one of the precursors of Nylon-6,6 – cyclohexene epoxide [6,7]. Multi-component incorporation can modify the surface of mesoporous silicate more effectively than monoheteroatom incorporation and could be widely used in the catalytic field. In this contribution we report on the incorporation of Nb and/or Sn into the mesoporous molecular sieves SBA-15 using different methods. The textural properties of these mesostructured materials were characterized and the investigation of their catalytic properties in oxidation of cyclohexene with hydrogen peroxide was carried out. 2. EXPERIMENTAL Initially, Nb,Sn-substituted mesostructured material was prepared following analogous synthesis conditions to those reported for pure silica SBA-15 (denoted as S) [1]. Due to problems with active phase leaching two different synthesis procedures were applied: NbSBA-15, Sn-SBA-15, and Nb,Sn-SBA-15 have been synthesized either using pH adjusting (A) [8] or dry gel conversion (D) [9] syntheses. P123 copolymer (EO20PO70EO20) was applied as a structure director, while tetraethyl orthosilicate (TEOS), SnCl4 and ammonium tris(oxalate) complex of niobium(V) were used as sources of Si, Sn and Nb, respectively. The synthesis of T-SBA-15-A using the pH adjusting method was carried out as follows: (1) 0.8 g of triblock copolymer was dissolved in 25 ml of HCl solution (2 M), followed by the addition of 1.7 g of TEOS. The mixture was stirred at 313 K for 4 h, and then requisite amounts of Nb or/and Sn salts were added to the mixture, followed by an additional stirring at 313 K for 20 h. Finally, the resulting mixture was transferred into polypropylene bottles for further condensation carried out at 373 K for 2 days. (2) After the procedure described above, the pH value of the synthesis system was adjusted up to 7.5 by adding ammonia dropwise at room temperature and the obtained mixture was hydrothermally treated again at 373 K for another 2 days. The final solid product was collected by filtration, washed out with water, and dried at room temperature. The surfactants were removed by calcination at 823 K for 6 h. The samples designated T-SBA-15S were prepared with a step (1) only followed by the filtration, drying and calcination at 823 K for 6 h. The synthesis of T-SBA-15-D: the resulting white solid prepared via step 1 was recovered by filtration and dried at 328 K for 3 h in an oven. The dried gel obtained in this way was ground into powder and transferred into a polypropylene beaker (60 ml) in a polypropylene vessel (250 ml), where 2 g of water was poured at the bottom of the vessel and physically separated from the dry-gel sample. After that, the vessel was sealed and heated at
1434 373 K for 24 h, the obtained material was washed out with deionized water, and then dried at 328 K for 12 h. To remove the template molecules, the material was heated from room temperature to 823 K at a heating rate of 2 K min-1 and followed by calcinations in air for 6 h. XRD, N2 physisorption, FTIR and Diffuse-Reflectance UV–VIS spectroscopies were used to find the location of niobium and/or tin ions in the SBA-15 matrix and to check the mesoporosity of the sample to verify how this incorporation may affect the properties and catalytic activity of the materials (oxidation of cyclohexene reaction using H2O2 as oxidant at 413 K). The DR-UV–vis spectra were recorded on a Varian 300 spectrometer with a diffusion reflection attachment in the 190–900 nm range with a resolution of 2 nm. Spectralon was used as a standard for measurements. The porous structure of the supports was determined from the adsorption/desorption isotherms of nitrogen (77 K) on a Micromeritics ASAP 2010 apparatus. The specific surface area was calculated by the BET method. Pore size distributions were calculated from the adsorption branch of isotherms in the range of capillary-condensation hysteresis by the KJSBJH method. Before measurement all samples were evacuated at 673 K for 2 h. The reaction of cyclohexene oxidation with H2O2 was carried out in a temperaturecontrolled glass vessel with stirrer and condenser. 2 mmol cyclohexene, 2 mmol H2O2 in 10 ml CH3CN and 40 mg catalyst were charged into the reactor and heated at 318 K for 40 h. GC was used to analyze the mixture of reaction products. 3. RESULTS AND DISCUSSION Sn-containing mesoporous SBA-15 type materials have been successfully prepared only by the hydrothermal method using pH adjusting or dry-gel conversion methods. Thus, only the full characteristic will be presented for samples with the proper hexagonally arranged SBA-15 structures. For Nb-SBA-15 the hexagonal structure was confirmed by XRD, but the amount of niobium introduced was lower than in the case of pH adjusting or dry-gel conversion methods (see Table 1). The main textural data such as specific surface area, average pore widths and specific pore meso- and micropore volume are presented in Table 1. The amount of micropore volume was determined using Į-plot method. The XRD showed the diffraction peaks corresponding to p6mm hexagonal lattice symmetry, characteristic of SBA-15 structure. Table 1. The structural/textural data for T-SBA-15 materials Surface Si/Nb and/or Catalysts area, a Si/Sn m2 g-1 Sn-SBA-15-A 34 (32) 390 Sn-SBA-15-D 32 (32) Nb-SBA-15-A 35 (32) 380 Nb-SBA-15-D 33 (32) 700 Nb-SBA-15-S 120 (32) 550 NbSn-SBA-15-A 64, 65 (64, 64) 420 NbSn-SBA-15-D 66, 67 (64, 64) a
in the synthesis gel in brackets
Pore vol., cm3 g-1 Total
Meso
Micro
1.13 1.16 1.02 0.59 1.26 -
1.10 1.12 0.90 0.36 1.15 -
0.03 0.03 0.03 0.18 0.04 -
Pore width, nm 13.2 14.1 11.5 7.4 15.2 -
1435 800
600
3
Amount adsorbed, cm (STP) g
-1
NbSBA-15-S NbSBA-15-A NbSBA-15-D
400
200
0.2
0 0
0
0
Relative pressure, p/p0
Fig. 1. N2 adsorption/desorption isotherms for Nb-SBA-15-S, –A and –D materials
All synthesized T-SBA-15-D and -A samples maintained the structure and good textural properties typical for SBA-15 material (e.g., Nb-SBA-15-D and –A in Fig. 1). The XRD patterns (not presented here) revealed that the materials exhibited a hexagonal arrangement of mesoporous structure, which remains intact after the heteroatom introduction. Also adsorption/desorption isotherms, showing the typical type IV shape, confirmed the preservation of mesoporous structure after template removal. Fig. 1 provides the N2 adsorption–desorption isotherms of Nb-SBA-15 obtained by three different methods. T-SBA15-D and -A samples exhibit typical IV isotherms comprising H1-type hysteresis with parallel adsorption and desorption branches, reflecting the regular array of cylindrical pore structure of SBA-15 materials. For the samples prepared with a standard method (e.g., Nb-SBA-15-S in Fig. 1), the isotherms show some pore blockage because the capillary evaporation was significantly delayed in relation to the capillary condensation. 3.0 NbSn-SBA-15-D
Nb-SBA-15-A
2.5
8
2.0
F(R)
6 1.5 Nb-SBA-15-D
NbSn-SBA-15-A
4
1.0 2
0.5 Sn-SBA-15-D
Sn-SBA-15-A
0.0 200
400
600 200
400
600
Wavelength, nm
Fig. 2. DR-UV-Vis spectra of T-SBA-15 samples
All synthesized samples showed sharp primary mesopore size distributions. The pore diameter gradually increased in the following order: T-SBA-15-S < T-SBA-15-D < T-SBA15-A (4.5, 11.5, 14.1 nm, respectively). The surface area and micro- and mesopore volume of T-SBA-15-D are all less than those of siliceous SBA-15-D (950 m2 g-1, 0.07 cm3g-1, and 1.2 cm3 g-1). These results indicate that the micropore and mesopore volumes are a little bit
1436 reduced after dry-gel conversion. This decrease in the textural parameters is probably due to the rearrangement of the micropores during the temperature treatment procedure. Diffuse reflectance UV–Visible spectroscopy is a very sensitive probe for the presence of framework and extraframework transition metal species in different heteroatomic mesostructures. Fig. 2 shows the diffuse reflectance UV–Vis spectra of Nb- and/or Sn-SBA15-A and –D materials. The DR-UV-vis spectra confirmed that heteroatoms were 80
50
Nb-SBA-15-D Nb,Sn-SBA-15-D Sn-SBA-15-D
30
20 40 10
0 0
800
1600
Time, min
20 2400
Cyclohexene conversion, %
60
80
40 60 30
20 40 10
0 0
800
1600
Epoxide selectivity, %
Nb-SBA-15-A Nb,Sn-SBA-15-A Sn-SBA-15-A
40
Epoxide selectivity, %
Cyclohexene conversion, %
50
20 2400
Time, min
Fig. 3. The catalytic activity for samples prepared via A- (left) and D-methods (right)
incorporated into the framework of SBA-15 (band at 214 nm - Sn4+ in tetrahedral coordination, band at 221 nm – isolated framework Nb species). The lack of an absorption in the region >300 nm indicated the absence of separated oxide phases. Mesoporous tinsilicate and/or niobiumsilicate catalysts were investigated in the liquid phase oxidation of cyclohexene using diluted H2O2 (35% aqueous solution) as an oxidation agent under mild reaction conditions. The catalytic properties of T-SBA-15 materials in terms of cyclohexene conversion and epoxide selectivity with time-on-stream are presented in Fig. 3. The catalytic activity increases in the following order: T-SBA-15-S < T-SBA-15-A < T-SBA15-D. It seems that the structural/textural properties, especially the external surface area and the average mesopore diameter, strongly influenced the catalytic activity. The Nb-based catalysts were found to be the best catalysts for the epoxidation of cyclohexene. Cyclohexene conversion reached almost 50% in 40 h over Nb-SBA-15-D. Although the catalyst was as active as NbMCM-41, the selectivity to epoxide on NbSBA-15 was lower than in the case of NbMCM-41 [6]. It is evident that the NbMCM-41 samples have much superior performance than NbSBA-15. This effect can be due to different location of niobium and the micropores present in the latter samples. The addition of niobium into the Sn-containing mesoporous molecular sieves improved both the conversion and selectivity to epoxide compared with pure Sn-SBA-15. By changing the amount of Sn and Nb incorporated, two main catalyst properties were modified: textural properties, and strength and number of acid sites. All these parameters of the catalysts play an important role in this process and control the selectivity towards epoxide. Non-framework
1437 incorporation of Sn and Nb by impregnation provides a catalyst with a lower performance than the framework substituted counterparts. The DR-UV-vis spectra of leached catalysts (not shown here) indicated that the exposure to the oxidation conditions did not cause any change (especially the Nb coordination) in the case of Nb-SBA-15-D sample in contrast to other catalysts. It is worthy to add that the NbSBA-15-D catalyst showed the highest activity in oxidation reaction and the highest epoxide selectivity in the product distribution. It is important to note that unlike many vanadium-based molecular sieve catalysts, which exhibit a significant loss in activity owing to a continuous leaching of active vanadium ions from the matrix, the Nb-SBA-15-D sample suffered from this behavior only slightly, i.e. there was a negligible decrease in the activity of the catalyst run for the second time in comparison to the first catalytic reaction. 4. CONCLUSIONS Ultra-large pore SBA-15 with Nb and Sn in the framework was synthesized at the presence of triblock copolymers. The size of the pores from 7 up to 14 nm was controlled by proper adjustment of pH of the synthesis mixture. UV-vis spectroscopy provided clear evidence on the incorporation of Nb and Sn into the siliceous matrix without the formation of separated oxide phases. Particularly Nb-SBA-15 was highly active in cyclohexene oxidation with hydrogen peroxide. No leaching of the active phase to the liquid reaction mixture confirmed the true heterogeneous character of this catalyst. ACKNOWLEDGEMENT BASF and Companhia Brasilia de Metalurgia e Mineraçáo are acknowledged for donating the block copolymer surfactant (Pluronics) and the sources of niobium (ammonium trisoxalate complex) used in this study, respectively. Izabela Nowak thanks the Polish Ministry of Science and Higher Education for the financial support (N204 084 31/1965; 2006-09). Iveta Nekoksová and JiĜí ýejka thank the Ministry of Industry and Trade of the Czech Republic (grant FT-TA/040) and the Academy of Sciences of the Czech Republic (project 1ET400400413). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548; I. Nowak, Stud. Surf. Sci. Catal., 154 (2005) 2936. M. Kruk, M. Jaroniec, C. H. Ko and R. Ryoo, Chem. Mater., 12 (2000), 1961. W. W. Lukens Jr., P. Schmidt-Winkel, D. Zhao, J. Feng and G. D. Stucky, Langmuir, 15 (1999), 5403. A. Corma, S. Iborra, M. Mifsud, M. Renz and M. Susarte, Adv. Synth. Catal., 346 (2004) 57. I. Nekoksová, N. Žilková and J. ýejka, Stud. Surf. Sci. Catal., 158 (2005) 1589. I. Nowak, B. Kilos, M. Ziolek and A. Lewandowska, Catal. Today, 78 (2003) 487. I. Nowak and M. Ziolek, Microporous Mesoporous Mater., 78 (2005) 281. S. Wu, Y. Han, Y. C. Zou, J. W. Song, L. Zhao, Y. Di, S. Liu and F. S. Xiao, Chem. Mater., 16 (2004) 486. S. Shen , F. Chen, P. S. Chow, P. Phanapavudhikul, K. Zhu and R. B. H. Tan, Microporous Mesoporous Mater., 92 (2006) 300. I. Nowak, Stud. Surf. Sci. Catal., 154 (2004) 2936.
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Exploring the catalytic activity of regular and ultralarge-pore Nb,Sn-SBA-15 mesoporous molecular sieves Izabela Nowaka*, Agnieszka Feliczaka, Aleksandra Tomczaka, Iveta Nekoksováb and JiĜí ýejkab a
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland. Tel.: +48 618291207; Fax: +48 618658008; E-mail: [email protected] b J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3,182 23 Prague 8, Czech Republic
ABSTRACT A novel bifunctional mesoporous material Nb,Sn-SBA-15 has been synthesized in one pot synthesis by in-situ introduction of niobium and tin salts and by controlling the synthesis conditions. These molecular sieves possess a high dispersion of both metals resulting in both the strong and weak acidic sites being simultaneously formed on SBA-15. Optimization of acid concentration in synthetic gels resulted in tuning of pore geometries while maintaining high structural regularity and in pore size enlargement up to 14 nm. The nature and the coordination of T atoms in silica frameworks appropriate for catalytic applications were proved by UV-vis spectroscopy and catalytic reaction. Activity and selectivity of niobium/tincontaining mesoporous molecular sieves have been tested in liquid phase oxidation of cyclohexene. On the basis of this investigation, the new efficient NbSBA-15 catalyst prepared via conventional synthesis has been selected. 1. INTRODUCTION Recently discovered SBA-15 materials have a 2-dimensional hexagonal symmetry and expand the channel size to the mesoporous region up to 30 nm with pore wall thickness up to 6 nm [1]. They are synthesized in acidic media using a templating approach with triblock copolymers of a general formula: HO(EO)x(PO)y(EO)xH (associating two terminal poly(ethylene oxide) chains, PEO, containing x EO units with one central poly(propylene oxide) chain, PPO, containing y PO units) as structure directing agent. Initially, SBA-15 [1] was considered to be an extra-large-pore MCM-41 analogue with a honeycomb structure of disconnected channel-like pores. Later, much more evidence of microporosity [2,3] in SBA15 samples was reported. Such materials possessing a bimodal pore system, i.e., in which micropores exist within the walls of mesopores, are ideal porous materials for catalysis and adsorption, because molecules are first transported through the mesoporous channels and then strongly adsorbed in the micropores. Thus, the synthesis of SBA-15 materials has attracted much attention, especially when these materials can be prepared as metal-containing ones, as a pure SBA-15 shows very limited catalytic activity due to the absence of lattice defects,
1433 redox and acido-basic properties. Hence, SBA-15 has become a remarkable material introducing metals into mesoporous silicates to form the active sites and, thus, to improve the catalytic activity. The incorporation of various transition metals (T) into mesoporous molecular sieves has been extensively studied, however, there are not many reports on the simultaneous incorporation of two metals into SBA-15 structure. In most cases, the efficiency of T loading is not always high, especially in direct synthesis, which is expected to provide uniform distribution of heteroatoms in structures and to give an active oxidation catalyst. Tin, being tetrahedrally coordinated in those materials, acts as Lewis acid site, while pentacoordinated Nb introduces the oxidative properties. Tin has recently become a very interesting active metal when located in silicate matrices for Baeyer-Villiger oxidation of cyclic ketones to the respective lactones. Zeolite Beta as well as mesoporous MCM-41 and MCM-48 is highly active in this reaction after addition of Sn [4,5]. In a similar way, mesostructured NbMCM-41 has the advantage of being particularly efficient and long-lived catalysts for synthesis of one of the precursors of Nylon-6,6 – cyclohexene epoxide [6,7]. Multi-component incorporation can modify the surface of mesoporous silicate more effectively than monoheteroatom incorporation and could be widely used in the catalytic field. In this contribution we report on the incorporation of Nb and/or Sn into the mesoporous molecular sieves SBA-15 using different methods. The textural properties of these mesostructured materials were characterized and the investigation of their catalytic properties in oxidation of cyclohexene with hydrogen peroxide was carried out. 2. EXPERIMENTAL Initially, Nb,Sn-substituted mesostructured material was prepared following analogous synthesis conditions to those reported for pure silica SBA-15 (denoted as S) [1]. Due to problems with active phase leaching two different synthesis procedures were applied: NbSBA-15, Sn-SBA-15, and Nb,Sn-SBA-15 have been synthesized either using pH adjusting (A) [8] or dry gel conversion (D) [9] syntheses. P123 copolymer (EO20PO70EO20) was applied as a structure director, while tetraethyl orthosilicate (TEOS), SnCl4 and ammonium tris(oxalate) complex of niobium(V) were used as sources of Si, Sn and Nb, respectively. The synthesis of T-SBA-15-A using the pH adjusting method was carried out as follows: (1) 0.8 g of triblock copolymer was dissolved in 25 ml of HCl solution (2 M), followed by the addition of 1.7 g of TEOS. The mixture was stirred at 313 K for 4 h, and then requisite amounts of Nb or/and Sn salts were added to the mixture, followed by an additional stirring at 313 K for 20 h. Finally, the resulting mixture was transferred into polypropylene bottles for further condensation carried out at 373 K for 2 days. (2) After the procedure described above, the pH value of the synthesis system was adjusted up to 7.5 by adding ammonia dropwise at room temperature and the obtained mixture was hydrothermally treated again at 373 K for another 2 days. The final solid product was collected by filtration, washed out with water, and dried at room temperature. The surfactants were removed by calcination at 823 K for 6 h. The samples designated T-SBA-15S were prepared with a step (1) only followed by the filtration, drying and calcination at 823 K for 6 h. The synthesis of T-SBA-15-D: the resulting white solid prepared via step 1 was recovered by filtration and dried at 328 K for 3 h in an oven. The dried gel obtained in this way was ground into powder and transferred into a polypropylene beaker (60 ml) in a polypropylene vessel (250 ml), where 2 g of water was poured at the bottom of the vessel and physically separated from the dry-gel sample. After that, the vessel was sealed and heated at
1434 373 K for 24 h, the obtained material was washed out with deionized water, and then dried at 328 K for 12 h. To remove the template molecules, the material was heated from room temperature to 823 K at a heating rate of 2 K min-1 and followed by calcinations in air for 6 h. XRD, N2 physisorption, FTIR and Diffuse-Reflectance UV–VIS spectroscopies were used to find the location of niobium and/or tin ions in the SBA-15 matrix and to check the mesoporosity of the sample to verify how this incorporation may affect the properties and catalytic activity of the materials (oxidation of cyclohexene reaction using H2O2 as oxidant at 413 K). The DR-UV–vis spectra were recorded on a Varian 300 spectrometer with a diffusion reflection attachment in the 190–900 nm range with a resolution of 2 nm. Spectralon was used as a standard for measurements. The porous structure of the supports was determined from the adsorption/desorption isotherms of nitrogen (77 K) on a Micromeritics ASAP 2010 apparatus. The specific surface area was calculated by the BET method. Pore size distributions were calculated from the adsorption branch of isotherms in the range of capillary-condensation hysteresis by the KJSBJH method. Before measurement all samples were evacuated at 673 K for 2 h. The reaction of cyclohexene oxidation with H2O2 was carried out in a temperaturecontrolled glass vessel with stirrer and condenser. 2 mmol cyclohexene, 2 mmol H2O2 in 10 ml CH3CN and 40 mg catalyst were charged into the reactor and heated at 318 K for 40 h. GC was used to analyze the mixture of reaction products. 3. RESULTS AND DISCUSSION Sn-containing mesoporous SBA-15 type materials have been successfully prepared only by the hydrothermal method using pH adjusting or dry-gel conversion methods. Thus, only the full characteristic will be presented for samples with the proper hexagonally arranged SBA-15 structures. For Nb-SBA-15 the hexagonal structure was confirmed by XRD, but the amount of niobium introduced was lower than in the case of pH adjusting or dry-gel conversion methods (see Table 1). The main textural data such as specific surface area, average pore widths and specific pore meso- and micropore volume are presented in Table 1. The amount of micropore volume was determined using Į-plot method. The XRD showed the diffraction peaks corresponding to p6mm hexagonal lattice symmetry, characteristic of SBA-15 structure. Table 1. The structural/textural data for T-SBA-15 materials Surface Si/Nb and/or Catalysts area, a Si/Sn m2 g-1 Sn-SBA-15-A 34 (32) 390 Sn-SBA-15-D 32 (32) Nb-SBA-15-A 35 (32) 380 Nb-SBA-15-D 33 (32) 700 Nb-SBA-15-S 120 (32) 550 NbSn-SBA-15-A 64, 65 (64, 64) 420 NbSn-SBA-15-D 66, 67 (64, 64) a
in the synthesis gel in brackets
Pore vol., cm3 g-1 Total
Meso
Micro
1.13 1.16 1.02 0.59 1.26 -
1.10 1.12 0.90 0.36 1.15 -
0.03 0.03 0.03 0.18 0.04 -
Pore width, nm 13.2 14.1 11.5 7.4 15.2 -
1435 800
600
3
Amount adsorbed, cm (STP) g
-1
NbSBA-15-S NbSBA-15-A NbSBA-15-D
400
200
0.2
0 0
0
0
Relative pressure, p/p0
Fig. 1. N2 adsorption/desorption isotherms for Nb-SBA-15-S, –A and –D materials
All synthesized T-SBA-15-D and -A samples maintained the structure and good textural properties typical for SBA-15 material (e.g., Nb-SBA-15-D and –A in Fig. 1). The XRD patterns (not presented here) revealed that the materials exhibited a hexagonal arrangement of mesoporous structure, which remains intact after the heteroatom introduction. Also adsorption/desorption isotherms, showing the typical type IV shape, confirmed the preservation of mesoporous structure after template removal. Fig. 1 provides the N2 adsorption–desorption isotherms of Nb-SBA-15 obtained by three different methods. T-SBA15-D and -A samples exhibit typical IV isotherms comprising H1-type hysteresis with parallel adsorption and desorption branches, reflecting the regular array of cylindrical pore structure of SBA-15 materials. For the samples prepared with a standard method (e.g., Nb-SBA-15-S in Fig. 1), the isotherms show some pore blockage because the capillary evaporation was significantly delayed in relation to the capillary condensation. 3.0 NbSn-SBA-15-D
Nb-SBA-15-A
2.5
8
2.0
F(R)
6 1.5 Nb-SBA-15-D
NbSn-SBA-15-A
4
1.0 2
0.5 Sn-SBA-15-D
Sn-SBA-15-A
0.0 200
400
600 200
400
600
Wavelength, nm
Fig. 2. DR-UV-Vis spectra of T-SBA-15 samples
All synthesized samples showed sharp primary mesopore size distributions. The pore diameter gradually increased in the following order: T-SBA-15-S < T-SBA-15-D < T-SBA15-A (4.5, 11.5, 14.1 nm, respectively). The surface area and micro- and mesopore volume of T-SBA-15-D are all less than those of siliceous SBA-15-D (950 m2 g-1, 0.07 cm3g-1, and 1.2 cm3 g-1). These results indicate that the micropore and mesopore volumes are a little bit
1436 reduced after dry-gel conversion. This decrease in the textural parameters is probably due to the rearrangement of the micropores during the temperature treatment procedure. Diffuse reflectance UV–Visible spectroscopy is a very sensitive probe for the presence of framework and extraframework transition metal species in different heteroatomic mesostructures. Fig. 2 shows the diffuse reflectance UV–Vis spectra of Nb- and/or Sn-SBA15-A and –D materials. The DR-UV-vis spectra confirmed that heteroatoms were 80
50
Nb-SBA-15-D Nb,Sn-SBA-15-D Sn-SBA-15-D
30
20 40 10
0 0
800
1600
Time, min
20 2400
Cyclohexene conversion, %
60
80
40 60 30
20 40 10
0 0
800
1600
Epoxide selectivity, %
Nb-SBA-15-A Nb,Sn-SBA-15-A Sn-SBA-15-A
40
Epoxide selectivity, %
Cyclohexene conversion, %
50
20 2400
Time, min
Fig. 3. The catalytic activity for samples prepared via A- (left) and D-methods (right)
incorporated into the framework of SBA-15 (band at 214 nm - Sn4+ in tetrahedral coordination, band at 221 nm – isolated framework Nb species). The lack of an absorption in the region >300 nm indicated the absence of separated oxide phases. Mesoporous tinsilicate and/or niobiumsilicate catalysts were investigated in the liquid phase oxidation of cyclohexene using diluted H2O2 (35% aqueous solution) as an oxidation agent under mild reaction conditions. The catalytic properties of T-SBA-15 materials in terms of cyclohexene conversion and epoxide selectivity with time-on-stream are presented in Fig. 3. The catalytic activity increases in the following order: T-SBA-15-S < T-SBA-15-A < T-SBA15-D. It seems that the structural/textural properties, especially the external surface area and the average mesopore diameter, strongly influenced the catalytic activity. The Nb-based catalysts were found to be the best catalysts for the epoxidation of cyclohexene. Cyclohexene conversion reached almost 50% in 40 h over Nb-SBA-15-D. Although the catalyst was as active as NbMCM-41, the selectivity to epoxide on NbSBA-15 was lower than in the case of NbMCM-41 [6]. It is evident that the NbMCM-41 samples have much superior performance than NbSBA-15. This effect can be due to different location of niobium and the micropores present in the latter samples. The addition of niobium into the Sn-containing mesoporous molecular sieves improved both the conversion and selectivity to epoxide compared with pure Sn-SBA-15. By changing the amount of Sn and Nb incorporated, two main catalyst properties were modified: textural properties, and strength and number of acid sites. All these parameters of the catalysts play an important role in this process and control the selectivity towards epoxide. Non-framework
1437 incorporation of Sn and Nb by impregnation provides a catalyst with a lower performance than the framework substituted counterparts. The DR-UV-vis spectra of leached catalysts (not shown here) indicated that the exposure to the oxidation conditions did not cause any change (especially the Nb coordination) in the case of Nb-SBA-15-D sample in contrast to other catalysts. It is worthy to add that the NbSBA-15-D catalyst showed the highest activity in oxidation reaction and the highest epoxide selectivity in the product distribution. It is important to note that unlike many vanadium-based molecular sieve catalysts, which exhibit a significant loss in activity owing to a continuous leaching of active vanadium ions from the matrix, the Nb-SBA-15-D sample suffered from this behavior only slightly, i.e. there was a negligible decrease in the activity of the catalyst run for the second time in comparison to the first catalytic reaction. 4. CONCLUSIONS Ultra-large pore SBA-15 with Nb and Sn in the framework was synthesized at the presence of triblock copolymers. The size of the pores from 7 up to 14 nm was controlled by proper adjustment of pH of the synthesis mixture. UV-vis spectroscopy provided clear evidence on the incorporation of Nb and Sn into the siliceous matrix without the formation of separated oxide phases. Particularly Nb-SBA-15 was highly active in cyclohexene oxidation with hydrogen peroxide. No leaching of the active phase to the liquid reaction mixture confirmed the true heterogeneous character of this catalyst. ACKNOWLEDGEMENT BASF and Companhia Brasilia de Metalurgia e Mineraçáo are acknowledged for donating the block copolymer surfactant (Pluronics) and the sources of niobium (ammonium trisoxalate complex) used in this study, respectively. Izabela Nowak thanks the Polish Ministry of Science and Higher Education for the financial support (N204 084 31/1965; 2006-09). Iveta Nekoksová and JiĜí ýejka thank the Ministry of Industry and Trade of the Czech Republic (grant FT-TA/040) and the Academy of Sciences of the Czech Republic (project 1ET400400413). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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