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Synthesis of titanium silicalite-1 nanocrystals on silica nanofibers by steam-assisted dry gel conversion technique
Materials Letters 62 (2008) 3316–3318
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of titanium silicalite-1 nanocrystals on silica nanofibers by steam-assisted dry gel conversion technique Xuebin Ke a, Changfeng Zeng b, Jianfeng Yao a, Lixiong Zhang a,⁎, Nanping Xu a a b
State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China College of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 20 September 2007 Accepted 19 February 2008 Available online 4 March 2008 Keywords: Dry gel conversion Mesopore Nanocrystal Nanofiber Titanium silicalite-1
a b s t r a c t Titanium silicalite-1 (TS-1) nanocrystals were coated on silica nanofibers by steam-assisted dry gel conversion technique. The preparation was carried out by impregnating the silica nanofibers in the diluted TS-1 synthesis solution, followed by the steam treatment at 130 °C for 6 days. The products were characterized by XRD, UV– vis spectroscopy, N2 sorption, SEM, and TEM. The results showed that TS-1 nanocrystals were grown on the silica nanofibers with a particle size of 10–20 nm. The resulting composites, with a BET surface area of 428 m2/ g and a mesopore size of 4.4 nm, exhibited higher catalytic activity than conventionally hydrothermal synthesized TS-1 powders in the benzene hydroxylation with hydrogen peroxide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Titanium silicalite-1 (TS-1) is a remarkable catalyst for the selective oxidation of a variety of organic molecules using hydrogen peroxide as the oxidant [1,2]. The TS-1 powders prepared by conventional hydrothermal synthesis normally had a particle size of 100–300 nm [3,4]. The separation and recovery of these TS-1 powders were difficult in the extensive applications [5]. In addition, TS-1 with the above mentioned particle size suffers from intracrystalline diffusion limitations owing to its small pore size, which may result in the overoxidation in catalytic reactions. Thus, loading TS-1 nanocrystals on suitable supports could improve their catalytic performances. TS-1 crystals with a particle size of smaller than 50 nm were synthesized on the amorphous TiO2 powders [6] and the amorphous mesoporous titania–silica [7], respectively. On the other hand, one-dimensional nanoscale materials have shown many potential applications in heterogeneous catalysis [8] and molecular sensors [9], and it was reported that the fibrous TS-1 zeolites had enhanced the shapeselective adsorption of xylene isomers [10]. Therefore, combination of TS-1 nanocrystals with one-dimensional nanofibers could be assembled as composite materials for catalysis or fibrous membranes for separation with low pressure drop and high diffusion rate [11]. Herein, we report the in-situ growth of TS-1 nanocrystals on the silica nanofibers (SNFs) by the steam-assisted dry gel conversion technique. Various methods were applied to characterize the products. The catalytic performance of the product was examined by benzene hydroxylation with hydrogen peroxide.
2.1. Sample preparation
⁎ Corresponding author. Tel.: +86 25 83587186; fax: +86 25 83365813. E-mail address: [email protected] (L. Zhang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.069
Tetraethyl orthosilicate (TEOS, 98%), tetrapropylammonium hydroxide (TPAOH, 1 M solution in water), titanium tetrabutoxide (TBOT, 97%), and isopropanol (IPA, 99.5%) were used as received. SNFs were synthesized according to reference [12]. TS-1 synthesis solution was prepared following the procedure reported in reference [13] with a molar composition of 1SiO2: 0.037TBOT: 0.25TPAOH: 1.1IPA: 35H2O. TS-1 nanocrystals coated on SNFs (TS-SNF) were prepared as follows. First, the SNFs were dispersed in the diluted TS-1 synthesis solution with 10 folders of deionized water. The system was kept stirring at
Fig. 1. XRD patterns of SNFs (a), TS-SNFs (b), and TS-1 powders (c).
X. Ke et al. / Materials Letters 62 (2008) 3316–3318
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Fig. 2. UV–vis spectra of SNFs (a), TS-SNFs (b), and TS-1 powders (c). Fig. 4. TEM image (a) and SAED pattern (b) of TS-SNFs.
room temperature for 24 h. Then, the SNFs were separated by centrifugation and transferred into a Teflon holder which was installed in the middle of an autoclave (125 ml), where 10 ml of water was previously poured into the bottom. The autoclave was heated at 130 °C for 6 days. Finally, TS-SNFs were obtained after washing, drying and calcining at 500 °C for 4 h. In addition, TS-1 powders were also prepared after hydrothermal crystallization of the TS-1 synthesis solution at 180 °C for 6 days. 2.2. Characterization X-ray powder diffraction (XRD) patterns were recorded on an X'Pert Pro X-ray diffractometer. UV–vis spectroscopic measurements were carried out on a Cary5000 UV–Vis–NIR Spectrophotometer. Scanning electron microscopy (SEM) images were collected by a Quanta 200 microscope. The morphology of the nanofibers and the selected area electron diffraction (SAED) patterns were recorded on a CM200 transmission electron microscope employing an accelerating voltage of 200 kV. The content of element was analysed using Inductively Coupled Plasma emission spectroscopy (ICP) on Optima 2000 instrument (Perkin-Elmer). The N2 sorption experiment was performed on a Tristar 3000 Analyzer at −196 °C to determine the Brunauer–Emmett–Teller (BET) surface area and the mesopore size. The mesopore size was calculated from the desorption-branch of the isotherm by using Barrett–Joyner–Halenda (BJH) method. The hydroxylation of benzene to phenol was carried out following the procedure reported in reference [14], which was conducted at 100 °C for 120 min, using benzene to H2O2 molar ratio of 3 and sulfolane as the solvent. The contents of components were determined by gas chromatography (capillary column, PEG-20M, 30 m × 0.25 mm, FID detector).The initial turn over frequency of the
titanium (TOF) was expressed as moles of reacted substrate/(moles of Ti × hour) [15]. 3. Results and discussion Fig. 1 shows the XRD patterns of SNFs, TS-SNFs, and TS-1 powders. The strong diffraction peaks indicating a highly ordered 2D hexagonal mesostructure of MCM-41 at 2.2°, 3.7°, 5.0° for SNFs (Fig. 1a) were observed [12]. The XRD pattern of TS-SNFs (Fig. 1b) exhibited peaks at 2.2°, 7.95°, 8.94°, 23.2°, 23.7°, 24.1°. The diffraction peaks at 2.2° attributed to the presence of 2D hexagonal mesostructure in SNFs, while those at 7.95°–24.1° suggested the presence of TS-1 crystals. The XRD pattern of TS-1 powders (Fig. 1c) was the same as that reported in literature [1], indicating the formation of TS-1 with high crystallinity. Fig. 2 presents the UV–vis spectra of SNFs, TS-SNFs, and TS-1 powders. Both TSSNFs and TS-1 powders exhibited an absorption band at about 210 nm, indicating the presence of tetrahedral Ti (IV) coordination species in the framework [16]. In addition, no absorption band at 330 nm could be observed, suggesting the non-existence of extra-framework Ti [3,7]. These data indicated that the Ti incorporated into the skeleton of silicalite-1 for both TS-SNFs and TS-1 powers. Fig. 3 displays the SEM images of SNFs, TS-SNFs, and TS-1 powders. It could be seen that SNFs were 2–3 μm in length and ca. 130 nm in diameter (Fig. 3a). The morphology of TS-SNFs (Fig. 3b), however, was different from that of SNFs since the surface of SNFs was covered by high dispersed nanosized particles. The particle size of TS-1 powders was about 300 nm (Fig. 3c). It was apparent that the particles covered on the surface of SNFs (Fig. 3b) were much smaller in size than TS-1 powders (Fig. 3c). Fig. 4a shows the TEM image of the TS-SNFs. The stripes along with the axis of SNFs were not continuous due to the spoiling or mantling of the loaded TS-1. The TS-1 crystals with particle sizes of 10–20 nm were dispersed on the SNFs. Some of the TS-1 nanoparticles were marked with arrows. SAED pattern (Fig. 4b) indicated the distinct crystal feature and the high crystallinity of TS-1 on TS-SNFs. N2 adsorption–desorption was employed to examine the pore structure of SNFs, TSSNFs, and TS-1 powders. Table 1 lists the resulting physical properties of three materials calculated from their N2 adsorption–desorption isotherms. The Ti/Si molar ratios analysed by ICP were also listed in Table 1. It can be seen that TS-SNFs exhibited larger surface area than TS-1 powders, which would result from the much smaller particle size of TS-1 on TS-SNFs and large surface area of SNFs. On the other hand, the mesopore size of SNFs was enlarged from 2.68 nm to 4.39 nm after the growth of TS-1 nanocrystals,
Fig. 3. SEM images of SNFs (a), TS-SNFs (b), and TS-1 powders (c).
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X. Ke et al. / Materials Letters 62 (2008) 3316–3318
Table 1 Composition and N2 sorption results of SNFs, TS-SNFs, and TS-1 powers Samples
Ti/Si (mol)
SNFs TS-SNFs TS-1 powders
0 0.010 ± 0.002 0.033 ± 0.004
SBET (m2/g) 1343 ± 28 428 ± 12 389 ± 15
Mesoporous size (nm) 2.7 ± 0.1 4.4 ± 0.3 –
Acknowledgements This work is supported by the National Natural Science Foundation of China (NNSFC; No. 20141003 and No. 20201007) and “Green–blue” Project of Jiangsu Province.
References which could be ascribed to the corroding of the silica wall in the basic synthesis solution during the ageing and steam treating processes [7,14]. Hydroxylation of benzene reaction was used to examine the catalytic performance of TS-SNFs. 9.6% of benzene conversion could be obtained after a reaction time of 120 min for TS-SNFs, which was higher than that for TS-1 powders (6.8%), although both catalysts exhibited a selectivity of phenol over 99%. On the other hand, the turn over frequency (TOF) [15] for TS-SNFs (TOF = 14.61) was 50% higher than that for TS-1 powders (TOF = 9.78), indicating that TS-SNFs had much higher catalytic activity than TS-1 powders, despite of the low Ti content of TS-SNFs (Table 1).
4. Conclusions In this paper, we reported a simple steam-assisted dry gel conversion technique to coat TS-1 nanocrystals on the silica nanofibers. XRD and UV–vis spectrum results verified the existence of TS-1 nanocrystals on the silica nanofibers. SEM and TEM results indicated that the crystal size of TS-1 nanocrystals on the silica nanofibers was less than 20 nm. The TSSNFs exhibited much higher benzene conversion and TOF than TS-1 powders in the hydroxylation of benzene reaction.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Taramasso, G. Perego, B. Notari. US Patent 4410501, 1983. P.T. Tanev, M. Chibwe, T.J. Pinnavala, Nature 368 (1994) 321–323. G.Y. Zhang, J. Sterte, B.J. Schoeman, Chem. Mater. 9 (1997) 210–217. A.J.H.P. van der Pol, J.H.C. Van Hooff, Appl. Catal. A 92 (1992) 93–111. J. Ozaki, K. Takahashi, M. Sato, A. Oya, Carbon 44 (2006) 1243–1249. H. Wang, X. Guo, X. Wang, W. Zhang, X. Bao, L. Lin, Chin. J. Catal. 20 (1999) 557–560. D. Trong-On, D. Lutic, S. Kaliaguine, Microporous Mesoporous Mater. 44–45 (2001) 435–444. V. Lordi, N. Yao, J. Wei, Chem. Mater. 13 (2001) 733–737. J. Kong, M.G. Chapline, H. Dai, Adv. Mater. 13 (2001) 1384–1386. Y.K. Hwang, J. Chang, S. Park, D.S. Kim, Y. Kwon, S.H. Jhung, et al., Angew. Chem. Int. Ed. 44 (2005) 556–560. X.B. Ke, H.Y. Zhu, X.P. Gao, J.W. Liu, Z.F. Zheng, Adv. Mater. 19 (2007) 785–790. B. Wang, C. Chi, W. Shan, Y.H. Zhang, N. Ren, W.L. Yang, et al., Angew. Chem. Int. Ed. 45 (2006) 20880–20890. A. Thangaraj, M.F. Eapen, S. Sivasanker, P. Ratnasamy, Zeolites 12 (1992) 943–950. X.B. Ke, L. Xu, C.F. Zeng, L.X. Zhang, N.P. Xu, Microporous Mesoporous Mater. 106 (2007) 68–75. L. Balducci, D. Bianchi, R. Bortolo, R. DAloisio, M. Ricci, R. Tassinari, et al., Angew. Chem. Int. Ed. 42 (2003) 4937–4940. X. Wang, X. Guo, Catal. Today 51 (1999) 177–186.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of titanium silicalite-1 nanocrystals on silica nanofibers by steam-assisted dry gel conversion technique Xuebin Ke a, Changfeng Zeng b, Jianfeng Yao a, Lixiong Zhang a,⁎, Nanping Xu a a b
State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China College of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 20 September 2007 Accepted 19 February 2008 Available online 4 March 2008 Keywords: Dry gel conversion Mesopore Nanocrystal Nanofiber Titanium silicalite-1
a b s t r a c t Titanium silicalite-1 (TS-1) nanocrystals were coated on silica nanofibers by steam-assisted dry gel conversion technique. The preparation was carried out by impregnating the silica nanofibers in the diluted TS-1 synthesis solution, followed by the steam treatment at 130 °C for 6 days. The products were characterized by XRD, UV– vis spectroscopy, N2 sorption, SEM, and TEM. The results showed that TS-1 nanocrystals were grown on the silica nanofibers with a particle size of 10–20 nm. The resulting composites, with a BET surface area of 428 m2/ g and a mesopore size of 4.4 nm, exhibited higher catalytic activity than conventionally hydrothermal synthesized TS-1 powders in the benzene hydroxylation with hydrogen peroxide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Titanium silicalite-1 (TS-1) is a remarkable catalyst for the selective oxidation of a variety of organic molecules using hydrogen peroxide as the oxidant [1,2]. The TS-1 powders prepared by conventional hydrothermal synthesis normally had a particle size of 100–300 nm [3,4]. The separation and recovery of these TS-1 powders were difficult in the extensive applications [5]. In addition, TS-1 with the above mentioned particle size suffers from intracrystalline diffusion limitations owing to its small pore size, which may result in the overoxidation in catalytic reactions. Thus, loading TS-1 nanocrystals on suitable supports could improve their catalytic performances. TS-1 crystals with a particle size of smaller than 50 nm were synthesized on the amorphous TiO2 powders [6] and the amorphous mesoporous titania–silica [7], respectively. On the other hand, one-dimensional nanoscale materials have shown many potential applications in heterogeneous catalysis [8] and molecular sensors [9], and it was reported that the fibrous TS-1 zeolites had enhanced the shapeselective adsorption of xylene isomers [10]. Therefore, combination of TS-1 nanocrystals with one-dimensional nanofibers could be assembled as composite materials for catalysis or fibrous membranes for separation with low pressure drop and high diffusion rate [11]. Herein, we report the in-situ growth of TS-1 nanocrystals on the silica nanofibers (SNFs) by the steam-assisted dry gel conversion technique. Various methods were applied to characterize the products. The catalytic performance of the product was examined by benzene hydroxylation with hydrogen peroxide.
2.1. Sample preparation
⁎ Corresponding author. Tel.: +86 25 83587186; fax: +86 25 83365813. E-mail address: [email protected] (L. Zhang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.069
Tetraethyl orthosilicate (TEOS, 98%), tetrapropylammonium hydroxide (TPAOH, 1 M solution in water), titanium tetrabutoxide (TBOT, 97%), and isopropanol (IPA, 99.5%) were used as received. SNFs were synthesized according to reference [12]. TS-1 synthesis solution was prepared following the procedure reported in reference [13] with a molar composition of 1SiO2: 0.037TBOT: 0.25TPAOH: 1.1IPA: 35H2O. TS-1 nanocrystals coated on SNFs (TS-SNF) were prepared as follows. First, the SNFs were dispersed in the diluted TS-1 synthesis solution with 10 folders of deionized water. The system was kept stirring at
Fig. 1. XRD patterns of SNFs (a), TS-SNFs (b), and TS-1 powders (c).
X. Ke et al. / Materials Letters 62 (2008) 3316–3318
3317
Fig. 2. UV–vis spectra of SNFs (a), TS-SNFs (b), and TS-1 powders (c). Fig. 4. TEM image (a) and SAED pattern (b) of TS-SNFs.
room temperature for 24 h. Then, the SNFs were separated by centrifugation and transferred into a Teflon holder which was installed in the middle of an autoclave (125 ml), where 10 ml of water was previously poured into the bottom. The autoclave was heated at 130 °C for 6 days. Finally, TS-SNFs were obtained after washing, drying and calcining at 500 °C for 4 h. In addition, TS-1 powders were also prepared after hydrothermal crystallization of the TS-1 synthesis solution at 180 °C for 6 days. 2.2. Characterization X-ray powder diffraction (XRD) patterns were recorded on an X'Pert Pro X-ray diffractometer. UV–vis spectroscopic measurements were carried out on a Cary5000 UV–Vis–NIR Spectrophotometer. Scanning electron microscopy (SEM) images were collected by a Quanta 200 microscope. The morphology of the nanofibers and the selected area electron diffraction (SAED) patterns were recorded on a CM200 transmission electron microscope employing an accelerating voltage of 200 kV. The content of element was analysed using Inductively Coupled Plasma emission spectroscopy (ICP) on Optima 2000 instrument (Perkin-Elmer). The N2 sorption experiment was performed on a Tristar 3000 Analyzer at −196 °C to determine the Brunauer–Emmett–Teller (BET) surface area and the mesopore size. The mesopore size was calculated from the desorption-branch of the isotherm by using Barrett–Joyner–Halenda (BJH) method. The hydroxylation of benzene to phenol was carried out following the procedure reported in reference [14], which was conducted at 100 °C for 120 min, using benzene to H2O2 molar ratio of 3 and sulfolane as the solvent. The contents of components were determined by gas chromatography (capillary column, PEG-20M, 30 m × 0.25 mm, FID detector).The initial turn over frequency of the
titanium (TOF) was expressed as moles of reacted substrate/(moles of Ti × hour) [15]. 3. Results and discussion Fig. 1 shows the XRD patterns of SNFs, TS-SNFs, and TS-1 powders. The strong diffraction peaks indicating a highly ordered 2D hexagonal mesostructure of MCM-41 at 2.2°, 3.7°, 5.0° for SNFs (Fig. 1a) were observed [12]. The XRD pattern of TS-SNFs (Fig. 1b) exhibited peaks at 2.2°, 7.95°, 8.94°, 23.2°, 23.7°, 24.1°. The diffraction peaks at 2.2° attributed to the presence of 2D hexagonal mesostructure in SNFs, while those at 7.95°–24.1° suggested the presence of TS-1 crystals. The XRD pattern of TS-1 powders (Fig. 1c) was the same as that reported in literature [1], indicating the formation of TS-1 with high crystallinity. Fig. 2 presents the UV–vis spectra of SNFs, TS-SNFs, and TS-1 powders. Both TSSNFs and TS-1 powders exhibited an absorption band at about 210 nm, indicating the presence of tetrahedral Ti (IV) coordination species in the framework [16]. In addition, no absorption band at 330 nm could be observed, suggesting the non-existence of extra-framework Ti [3,7]. These data indicated that the Ti incorporated into the skeleton of silicalite-1 for both TS-SNFs and TS-1 powers. Fig. 3 displays the SEM images of SNFs, TS-SNFs, and TS-1 powders. It could be seen that SNFs were 2–3 μm in length and ca. 130 nm in diameter (Fig. 3a). The morphology of TS-SNFs (Fig. 3b), however, was different from that of SNFs since the surface of SNFs was covered by high dispersed nanosized particles. The particle size of TS-1 powders was about 300 nm (Fig. 3c). It was apparent that the particles covered on the surface of SNFs (Fig. 3b) were much smaller in size than TS-1 powders (Fig. 3c). Fig. 4a shows the TEM image of the TS-SNFs. The stripes along with the axis of SNFs were not continuous due to the spoiling or mantling of the loaded TS-1. The TS-1 crystals with particle sizes of 10–20 nm were dispersed on the SNFs. Some of the TS-1 nanoparticles were marked with arrows. SAED pattern (Fig. 4b) indicated the distinct crystal feature and the high crystallinity of TS-1 on TS-SNFs. N2 adsorption–desorption was employed to examine the pore structure of SNFs, TSSNFs, and TS-1 powders. Table 1 lists the resulting physical properties of three materials calculated from their N2 adsorption–desorption isotherms. The Ti/Si molar ratios analysed by ICP were also listed in Table 1. It can be seen that TS-SNFs exhibited larger surface area than TS-1 powders, which would result from the much smaller particle size of TS-1 on TS-SNFs and large surface area of SNFs. On the other hand, the mesopore size of SNFs was enlarged from 2.68 nm to 4.39 nm after the growth of TS-1 nanocrystals,
Fig. 3. SEM images of SNFs (a), TS-SNFs (b), and TS-1 powders (c).
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X. Ke et al. / Materials Letters 62 (2008) 3316–3318
Table 1 Composition and N2 sorption results of SNFs, TS-SNFs, and TS-1 powers Samples
Ti/Si (mol)
SNFs TS-SNFs TS-1 powders
0 0.010 ± 0.002 0.033 ± 0.004
SBET (m2/g) 1343 ± 28 428 ± 12 389 ± 15
Mesoporous size (nm) 2.7 ± 0.1 4.4 ± 0.3 –
Acknowledgements This work is supported by the National Natural Science Foundation of China (NNSFC; No. 20141003 and No. 20201007) and “Green–blue” Project of Jiangsu Province.
References which could be ascribed to the corroding of the silica wall in the basic synthesis solution during the ageing and steam treating processes [7,14]. Hydroxylation of benzene reaction was used to examine the catalytic performance of TS-SNFs. 9.6% of benzene conversion could be obtained after a reaction time of 120 min for TS-SNFs, which was higher than that for TS-1 powders (6.8%), although both catalysts exhibited a selectivity of phenol over 99%. On the other hand, the turn over frequency (TOF) [15] for TS-SNFs (TOF = 14.61) was 50% higher than that for TS-1 powders (TOF = 9.78), indicating that TS-SNFs had much higher catalytic activity than TS-1 powders, despite of the low Ti content of TS-SNFs (Table 1).
4. Conclusions In this paper, we reported a simple steam-assisted dry gel conversion technique to coat TS-1 nanocrystals on the silica nanofibers. XRD and UV–vis spectrum results verified the existence of TS-1 nanocrystals on the silica nanofibers. SEM and TEM results indicated that the crystal size of TS-1 nanocrystals on the silica nanofibers was less than 20 nm. The TSSNFs exhibited much higher benzene conversion and TOF than TS-1 powders in the hydroxylation of benzene reaction.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Taramasso, G. Perego, B. Notari. US Patent 4410501, 1983. P.T. Tanev, M. Chibwe, T.J. Pinnavala, Nature 368 (1994) 321–323. G.Y. Zhang, J. Sterte, B.J. Schoeman, Chem. Mater. 9 (1997) 210–217. A.J.H.P. van der Pol, J.H.C. Van Hooff, Appl. Catal. A 92 (1992) 93–111. J. Ozaki, K. Takahashi, M. Sato, A. Oya, Carbon 44 (2006) 1243–1249. H. Wang, X. Guo, X. Wang, W. Zhang, X. Bao, L. Lin, Chin. J. Catal. 20 (1999) 557–560. D. Trong-On, D. Lutic, S. Kaliaguine, Microporous Mesoporous Mater. 44–45 (2001) 435–444. V. Lordi, N. Yao, J. Wei, Chem. Mater. 13 (2001) 733–737. J. Kong, M.G. Chapline, H. Dai, Adv. Mater. 13 (2001) 1384–1386. Y.K. Hwang, J. Chang, S. Park, D.S. Kim, Y. Kwon, S.H. Jhung, et al., Angew. Chem. Int. Ed. 44 (2005) 556–560. X.B. Ke, H.Y. Zhu, X.P. Gao, J.W. Liu, Z.F. Zheng, Adv. Mater. 19 (2007) 785–790. B. Wang, C. Chi, W. Shan, Y.H. Zhang, N. Ren, W.L. Yang, et al., Angew. Chem. Int. Ed. 45 (2006) 20880–20890. A. Thangaraj, M.F. Eapen, S. Sivasanker, P. Ratnasamy, Zeolites 12 (1992) 943–950. X.B. Ke, L. Xu, C.F. Zeng, L.X. Zhang, N.P. Xu, Microporous Mesoporous Mater. 106 (2007) 68–75. L. Balducci, D. Bianchi, R. Bortolo, R. DAloisio, M. Ricci, R. Tassinari, et al., Angew. Chem. Int. Ed. 42 (2003) 4937–4940. X. Wang, X. Guo, Catal. Today 51 (1999) 177–186.