Enhanced Catalytic Oxidation by Hierarchically Structured TS-1

Mar 17, 2010 - Yaquan Wang , Xinpeng Han , Haoyang Li , Xiao Wang. Transactions of ..... Li Chen , Yi Meng Wang , Ming-Yuan He. Materials Research ...
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J. Phys. Chem. C 2010, 114, 6553–6559

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Enhanced Catalytic Oxidation by Hierarchically Structured TS-1 Zeolite Hongchuan Xin,† Jiao Zhao,† Shutao Xu,† Junping Li,‡ Weiping Zhang,† Xinwen Guo,‡ Emiel J. M. Hensen,§ Qihua Yang,*,† and Can Li*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023 China, State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012 China, and Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed: December 23, 2009; ReVised Manuscript ReceiVed: March 5, 2010

A TS-1 zeolite with a disordered network of mesopores penetrating the microporous crystalline zeolite framework was successfully synthesized by a one-pot carbon hard-templating synthesis approach. Besides conventional methods to characterize the mesoporosity, the use of variable-temperature 129Xe NMR spectroscopy was explored. At low temperature, a new resonance of 129Xe adsorbed in the mesopores could be distinguished from the signal of Xe in the micropores. The similarity of UV-vis and UV resonance Raman spectra of this mesoporous TS-1 zeolite with a conventional microporous TS-1 zeolite shows that the local coordination environment of Ti in these samples is identical. Further characterization (TEM, XRD) indicates that phase separation of titanium oxide is absent. The mesoporous TS-1 zeolite exhibits improved catalytic activity in the hydroxylation of phenol and ammoxidation of methyl ethyl ketone. The catalytic activity is substantially improved by introducing mesoporosity in TS-1, whereas the selectivity to the desired products is very similar. The improved catalytic activity of the TS-1 with the hierarchical structure is mainly attributed to the improved mass transfer of reactants and products into and out of the zeolite micropores. The generation of the hierarchical pore structure by the one-pot carbon-templating route becomes a general strategy for the synthesis of hierarchical zeolite with different compositions. 1. Introduction TS-1 zeolite is one of the most efficient solid catalysts for selective oxidation reactions, such as the hydroxylation of phenols, epoxidation of alkenes, and ammoxidation of ketones.1–5 However, the small pores of TS-1 zeolite with the MFI topology limits its application to relative small molecules and imposes mass transport limitations.6,7 Considerable effort has been devoted to the synthesis of large pore Ti-containing zeolites, such as Ti-β zeolite and Ti-MCM-22, and Ti-substituted mesoporous silicas.8–14 The creation of mesoporosity in TS-1 zeolite crystals may provide another approach to improve the accessibility of the active sites in TS-1 catalysts. Different approaches15–25 have been developed for the synthesis of mesoporous zeolites with hierarchical structures, such as the partial crystallization of amorphous mesoporous walls,26,27 the use of zeolite “seeds” as framework building units,28,29 the deposition of zeolite precursors on the walls of mesoporous supports,30,31 and one-pot hydrothermal procedures using special templates.32–35 Most of these methods are efficient for the generation of mesopores in zeolite materials, but they usually do not produce fully crystalline pore walls. Mesopores can also be introduced into individual microporous zeolite crystals by post-treatment methods, such as controlled dealumination17,21 * To whom correspondence should be addressed. E-mail: [email protected] (Q.H.Y.), [email protected] (C.L.). Phone: 86-41184379552 (Q.H.Y.), 86-411-84379070 (C.L.). Fax: 86-411-84694447 (Q.H.Y.). † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Dalian University of Technology. § Eindhoven University of Technology.

and desilication.15,22,36 These treatments sometimes lead to changes of the zeolite composition and, in some cases, structural degradation. Hard-template methods are another appealing approach for the generation of mesoporosity in zeolites. Carbon black is a suitable template due to its low cost and wide availability.20,37 In a typical approach,38 carbon black is impregnated with the various components of a typical zeolite synthesis gel. The resulting composite is then autoclaved in steam to achieve crystallization of the aluminosilicate. Calcination of the final material removes structure-directing organic reagents as well as carbon templates. The resulting materials consist of randomly distributed mesopore-sized cavities imprinted in the crystalline zeolite material by the carbon black particle.15,20,21,39 However, when the aim is to introduce transition metals into the zeolite framework, the use of the carbon-templating method brings problems, such as inhomogeneity and phase separation of the catalytically active species.38,40 The number of studies dealing with the synthesis of mesoporous TS-1 zeolites is limited. Reports include soft-templating28 with a mesopore-directing surfactant and hard-templating38,40 approaches with sequential impregnation. Herein, we present a novel method for the one-pot synthesis of mesoporous TS-1 with improved catalytic properties compared with conventional TS-1 zeolite. The method is an improved version of the carbon template method and avoids phase separation during the crystallization step. 2. Experimental Section 2.1. Chemicals and Reagents. All materials were of analytical grade and used as received without any further purification. Carbon black (BP-2000) was provided by Cabot Corporation,

10.1021/jp912112h  2010 American Chemical Society Published on Web 03/17/2010

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and tetraethoxysilane (TEOS), tetrabutyl orthotitanate (TBOT), tetrapropylammonium hydroxide (TPAOH, 25 wt % aqueous solution), and other reagents were obtained from Shanghai Chemical Reagent, Inc. of Chinese Medicine Group. 2.2. Synthesis of Meso-TS-1. In a typical synthesis, 6.50 g of TEOS was added to 13.28 g of TPAOH (12.5 wt % in water) with vigorous stirring for 40 min, followed by the addition of a solution of TBOT (0.17 g) in 1-butanol (C4H10O, 1.65 g). After stirring the mixture for 1 h, 6.72 g of TPAOH (12.5 wt % in water) was added and the mixture was stirred overnight. The final molar composition of the mixture was 5 TBOT:312 TEOS:123 TPAOH:9722 H2O:223 C4H10O. Carbon black (5.00 g) dried overnight at 110 °C was slowly impregnated with the TS-1 precursor solution in about 0.5 h. The resulting material was aged for 5 h at room temperature. This impregnation/drying process was repeated two times. Finally, the impregnated carbon black was introduced into a small Teflon cup that was placed in a Teflon-lined autoclave containing 100 mL of water. Crystallization was carried out at 175 °C for 24 h. After cooling to room temperature, the product was suspended in water and filtered. This resuspension and filtration procedure was repeated four times. The filter cake was dried at 110 °C for 10 h, and the resulting material was crushed and calcined at 550 °C for 10 h at a ramp rate of 1 °C min-1 under air to give meso-TS-1. 2.3. Synthesis of TS-1 Zeolite. For comparison, TS-1 zeolite was synthesized in a similar manner as that of meso-TS-1 without the carbon black template. To this end, the TS-1 precursor solution was introduced into a Teflon-lined autoclave and crystallization was performed at 175 °C for 24 h. After cooling to room temperature, the product was centrifuged and washed with water three times. The filter cake was dried at 110 °C for 10 h, and the resulting material was crushed and calcined at 550 °C for 10 h with a ramp rate of 1 °C min-1 under air to give TS-1. 2.4. Characterization. The Ti content was determined by inductively coupled plasma optical emission spectrometry (ICPOES) with a Spectro CIROSCCD spectrometer equipped with a free-running 27.12 MHz generator at 1400 W. UV-vis spectra were recorded on a Shimadzu UV-2401 PC spectrometer in a diffuse-reflectance mode with a 60 mm integrating sphere with BaSO4 as a reference. FT-IR spectra were recorded on a Nicolet Avatar 360 spectrometer using the KBr method. XRD patterns were recorded on a Rigaku RINT D/Max-2500 powder diffraction system using Cu KR radiation (40 kV and 50 mA). Nitrogen sorption isotherms were determined at -196 °C on a Micromeritics ASAP2020 system in static measurement mode. Prior to N2 sorption, the sample was kept under vacuum at 400 °C for 8 h. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area (SBET) based on the adsorption data in the range of P/P0 ) 0.05-0.25. The mesopore volume (Vmeso) and BJH pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method using the adsorption branch. The external surface area (Sext) and micropore volume (Vmicro) were calculated from the t-plot curve at a thickness between 2 and 3.5 Å.41 Transmission electron microscopy (TEM) was performed using a JEOL JEM-2000EX and a FEI Tecnai G2 Spirit at an acceleration voltage of 120 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on a FEI Tecnai G2 F30 S-Twin. Scanning electron microscopy (SEM) was undertaken on a FEI Quanta 200F scanning electron microscope operating at an accelerating voltage of 1-30 kV. UV Raman spectra were recorded on a homemade UV Raman spectrometer using a Jobin-Yvon T64000 triple-stage spectrograph with a spectral

Xin et al.

Figure 1. SEM images of (a) TS-1 and (b) meso-TS-1.

resolution of 2 cm-1.42 The 290 nm line from a Coherent Innova 300 Fred laser was used as an excitation source. The power of the 290 nm line at samples was below 1.0 mW. Laserhyperpolarized (HP) 129Xe NMR experiments were carried out at 110.6 MHz on the Varian Infinity-plus 400 spectrometer using a 7.5 mm probe head. Prior to each experiment, samples were pressed, crushed, and sieved into 20-40 mesh particles, then dehydrated at 673 K under vacuum ( 0.9) results from the presence of interparticle voids. The N2 sorption isotherms of meso-TS-1 combine type I and IV shapes. The sharp uptake at relative pressures P/P0 < 0.05 confirms the presence of micropores. In addition, an H1 hysteresis loop at relative pressures P/P0 > 0.8 shows the presence of mesopores in mesoTS-1. The BJH pore size distribution shows that the mesopore diameter is about 25 nm. This particle size is in good agreement with the particle size of the carbon black hard template of about 20-30 nm. The textural parameters show the coexistence of micropores and mesopores in both zeolite samples. The Vmeso/ Vmicro ratio of meso-TS-1 (2.36) is much higher than that of TS-1 (1.91). The external surface area, which includes the mesopore surface area, is higher for meso-TS-1 (207 m2 g-1) than for TS-1 (162 m2 g-1). 129 Xe NMR spectroscopy is a useful tool to characterize porous materials.43–46 Mesopores in ZSM-5 zeolites with hierarchical porous structures can be directly observed by laserhyperpolarized 129Xe NMR.47 Figures 4 and 5 show the variabletemperature laser-hyperpolarized 129Xe NMR spectra of TS-1 and meso-TS-1, respectively. The peaks at 0 ppm are due to gas-phase 129Xe. In the temperature range of 293-163 K, the spectra for TS-1 show only one signal downfield of the gasphase Xe signal. This signal originates from Xe adsorbed in the microporous channels of TS-1. The chemical shift of this signal increases as the temperature decreases. This shift is due to the increasing Xe-surface and Xe-Xe interactions at

decreasing temperatures. Similarly, the intense downfield signal in the spectra of meso-TS-1 is due to Xe adsorbed in the micropores of meso-TS-1. At 173 K, a new upfield line emerges at 158 ppm, which shifts downfield (161 ppm) with decreasing temperature (163 K). The signals we observed at low temperatures could not be assigned to the liquid or solid Xe because its chemical shift of liquid Xe and solid Xe should be at about 240 and 300 ppm, respectively.48 This implies the presence of another environment of adsorbed Xe. The absence of this signal above 173 K points to the fast exchange of Xe among the mesopores and micropores. Combining with the above TEM and N2 sorption results, the new upfield peak is suggested to be Xe adsorbed in the mesopores of meso-TS-1, which is also consistent with that observed in mesoporous ZSM-5 zeolites.47 Moreover, there is no big chemical-shift difference between meso-TS-1 and TS-1 at higher temperatures, which may suggest good connectivity between micropores and mesopores in mesoTS-1. The 129Xe NMR characterization clearly shows the existence of mesopores penetrating through the microporous framework of meso-TS-1, and the mesopores in meso-TS-1 and TS-1 from N2 sorption isotherms indeed have different natures. 3.3. Crystallinity. The high-angle XRD patterns of mesoTS-1 and TS-1 in Figure 6 correspond to the powder pattern of crystalline MFI zeolite. No typical diffraction peaks due to large TiO2 were observed, further underpinning that this one-pot synthesis method avoids phase separation that has been observed during the synthesis of transition-metal substituted zeolites using the hard-templating method.40 The relative crystallinity of MFI zeolites was also evaluated from FTIR spectra.49 These spectra are shown in Figure 6B, and the corresponding crystallinity data are listed in Table 1. The relative crystallinity was estimated from (I550/I450) × 100%/ 0.72, with I550 and I450 being the intensities of the infrared bands near 550 cm-1, which is the characteristic vibration of the double-five ring in MFI zeolite, and 450 cm-1, which is the Si-O vibration,49,50 respectively. The crystallinity of TS-1 and meso-TS-1 is 96 and 103%, respectively. These results confirm that meso-TS-1 has a very similar crystallinity to TS-1, in line with the XRD results. 3.4. UV-vis and UV-Raman Spectroscopy. The Ti content of TS-1 and meso-TS-1 is 1.06 and 1.10 wt % by ICP, respectively, indicating that both samples have almost the same Ti content (Table 1). Figure 7A shows the UV-vis spectra of TS-1 and meso-TS-1 zeolites. The spectra are dominated by a characteristic band at 215 nm that originates from the pπ-dπ charge-transfer transition between Ti and O of Ti-O-Si species in the framework. The tail of the band centered at 215 nm extends to 350 nm due to the presence of extraframework titanium species.51 The UV-vis spectra of meso-TS-1 and TS-1 are virtually indistinguishable, indicating that the Ti speciation is very similar. UV resonance Raman spectroscopy is a useful technique to identify the transition metal in the framework of zeolites and mesoporous silica.42,52 With UV excitation, low fluorescence interference and strong resonance Raman enhancement make detection of framework Ti in TS-1 at low concentrations possible.53 The UV-Raman spectra of meso-TS-1 and TS-1 excited with a 290 nm laser line are very similar (Figure 7B).

TABLE 1: Physicochemical Properties of TS-1 and Meso-TS-1 catalyst

Ti content (wt %)

SBET (m2 g-1)

Sext (m2 g-1)

Vmicro (cm3 g-1)

Vmeso (cm3 g-1)

crystallinity (%)

TS-1 meso-TS-1

1.06 1.10

458 423

162 207

0.12 0.11

0.23 0.26

96 103

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Figure 3. Nitrogen sorption isotherms (left) and BJH pore size distributions (right) of (a) TS-1 and (b) meso-TS-1 (+100 in the left panel means that the nitrogen isotherms of meso-TS-1 are shifted manually upward along the y axis by 100 cm3 g-1 STP).

129

Figure 5. Laser-hyperpolarized 129Xe NMR spectra of Xe adsorbed in meso-TS-1. The temperature is varied from 293 to 163 K.

The band at 380 cm-1 is assigned to the νs(Si-O-Si) modes of the five-membered rings of the MFI structure.42 In addition to the characteristic bands of the MFI structure at 290, 380, and 800 cm-1,51 UV resonance Raman bands at 510 and 1125 cm-1 are attributed to the framework titanium species in tetrahedral coordination. The bands are, respectively, assigned to the symmetric and asymmetric stretching vibrations of the framework Ti-O-Si species.28,51,53,54 The band at 960 cm-1 is not a resonance-related Raman band53 and is, therefore, not directly associated with the framework titanium ions of TS-1. These results strongly suggest that the Ti species are located within the crystalline framework environment of hierarchical meso-TS-1 zeolite and that these species are comparable to those of TS-1.

3.5. Catalytic Performance. Hydroquinone and catechol are widely used in chemical, pharmaceutical, and food industries,36,55,56 and they are prepared industrially via the Hamilton system using Fenton chemistry in homogeneous media. Methyl ethyl ketoxime (MEKO) is a very important fine chemical with a gradually increasing market share. Conventional manufacturing processes of MEKO include the noncatalyzed oximation of methyl ethyl ketone (MEK) with a hydroxylamine derivative, such as (NH2OH)2 · H2SO4, and the separation of MEKO by neutralization with ammonia.57 These processes have serious disadvantages because they involve poisonous agents, the formation of large quantities of low-value byproducts, and significant production of pollutant gases, such as nitrogen oxides. Compared with traditional synthesis processes, the phenol hydroxylation58 and synthesis of MEKO59 catalyzed by heterogeneous catalysts are highly desirable in view of separation,

Figure 4. Laser-hyperpolarized Xe NMR spectra of Xe adsorbed in TS-1. The temperature is varied from 293 to 163 K.

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Figure 6. High-angle XRD patterns (A) and FT-IR spectra (B) of (a) TS-1 and (b) meso-TS-1.

Figure 7. UV-vis spectra (A) and UV resonance Raman spectra (B) of (a) TS-1 and (b) meso-TS-1.

recovering, and environment. Conventional TS-1 zeolite has been considered as a promising and environmentally friendly candidate to replace the traditional route for both the hydroxylation of phenol and the ammoxidation of methyl ethyl ketone with H2O2 as an oxidant.4 Therefore, these two reactions were employed as model reactions to test the role of mesopores in catalytic performance.

The yields of diphenol (catechol and hydroquinone) in the hydroxylation of phenol as a function of reaction time are shown in Figure 8A. The catalytic results are summarized in Table 2. The diphenol yield increases sharply with increasing reaction time by meso-TS-1 and TS-1. The diphenol yield of hierarchically structured meso-TS-1 is much higher than that of TS-1. The phenol conversion for meso-TS-1 is 33%, which is close

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Figure 8. Yields of diphenol (catechol and hydroquinone) (A) and the yields of methyl ethyl ketoxime (MEKO) (B) as a function of reaction time for (a) TS-1 and (b) meso-TS-1.

TABLE 2: Catalytic Performance of TS-1 and Meso-TS-1 in Phenol Hydroxylationa

TABLE 3: Catalytic Performance of TS-1 and Meso-TS-1 in the Ammoxidation of Methyl Ethyl Ketonea

methyl ethyl ketone phenol b

-1

c

d

H 2 O2

catalyst

TOF (h )

X (%)

S (%)

X (%)

S (%)

TS-1 meso-TS-1

15 25

19.3 33.0

74.8 73.1

49.5 90.8

86.8 79.1

Reactions were performed at 80 °C for 4 h with 0.470 g of phenol, 0.047 g of catalyst, 15 mL of water, and phenol/H2O2 ) 3:1 (molar ratio). b Calculated when the reaction time is 30 min. c Conversion. d Selectivity. a

to the maximum based on a phenol-to-H2O2 ratio of 3. The conversion for TS-1 is only 19.3% under identical reaction conditions. The turnover frequency (TOF) of meso-TS-1 (25 h-1) is considerably higher than that of TS-1 (15 h-1). Note that the TOF for meso-TS-1 is a lower limit as equilibrium has been attained. The diphenol selectivity is directly related to the microenvironment of titanium species in TS-1 zeolite.28 The diphenol selectivity is very similar for TS-1 and meso-TS-1, which is in good agreement with the large body of characterization presented above. Figure 8B shows the yields of MEKO as a function of the reaction time in the ammoxidation of methyl ethyl ketone. The MEKO yield of meso-TS-1 is considerably higher than that of TS-1. The corresponding reaction data are listed in Table 3. The conversion of methyl ethyl ketone on meso-TS-1 is 80.2%, significantly higher than that on TS-1 (64.9%). The selectivity to MEKO is close to 100% for both meso-TS-1 and TS-1.

H 2O 2

catalyst

TOFb (h-1)

Xc (%)

Sd (%)

X (%)

S (%)

TS-1 meso-TS-1

964 1328

64.9 80.2

99.2 99.5

87.1 85.1

38.4 48.7

a Reactions were performed at 75 °C for 3 h with 1.50 mL of methyl ethyl ketone, 5 mL of ammonia solution (25 wt %), 2.50 mL of hydrogen peroxide (H2O2, 30 wt % aq), 0.030 g of catalyst, 4.50 mL of H2O, and 3.75 mL of tert-butanol. b Calculated when the reaction time is 60 min. c Conversion. d Selectivity.

Consequently, the TOF of meso-TS-1 (1328 h-1) is higher than that of TS-1 (964 h-1). In both the hydroxylation of phenol and the ammoxidation of methyl ethyl ketone, meso-TS-1 zeolite shows much higher catalytic activity than TS-1. On the basis of the characterization results above, meso-TS-1 and TS-1 have similar crystallinity and Ti species. Therefore, the difference in catalytic performance can be directly related to the difference in the textural properties. The 129Xe NMR evidences that the nature of the mesopore volume of TS-1 and meso-TS-1 obtained from nitrogen sorption measurement is different. The mesoporous volume of mesoTS-1 is originated from the penetrating mesopores templated by carbon black, whereas that of TS-1 is from intercrystal voids. The fact that meso-TS-1 (Vmeso ) 0.26 cm3 g-1) with a slightly higher mesopore volume than TS-1 (Vmeso ) 0.23 cm3 g-1) shows much higher catalytic activity suggests that the meospore formed by carbon black is more penetrable than that formed from intercrystal voids. The enhanced activity of meso-TS-1

Enhanced Catalytic Oxidation by TS-1 Zeolite could be mostly attributed to the improved mass transport to and out of micropores through mesopores in meso-TS-1. For both reactions, the selectivity of meso-TS-1 and TS-1 are similar due to the similarity of active centers (Ti sites), as indicated by UV-vis and UV-Raman spectra. From the catalytic data, it can be concluded that the current one-pot carbon-templating method is highly efficient for the preparation of mesoporous zeolite with transition-metal substitution. 4. Conclusions In summary, the one-pot carbon-templating method is efficient for the synthesis of meso-TS-1 with mesopores penetrating through the crystalline framework of zeolite. Moreover, the Ti species in meso-TS-1 have almost the same coordination environment as those in TS-1, suggesting that this carbontemplating method will not change the coordination environment of Ti species. Meso-TS-1 can efficiently catalyze the hydroxylation of phenol and ammoxidation of methyl ethyl ketone using H2O2 as an oxidant. Compared with TS-1, meso-TS-1 shows similar product selectivity but significantly improved catalytic activity. The increased catalytic activity of meso-TS-1 is attributed to the rapid diffusion rate to and out of micropores conferred by mesopores. This carbon-templating method may open a new window for the synthesis of other hierarchically structured zeolites with specific active sites. Acknowledgment. This work was supported by the Programme Strategic Scientific Alliances between China and The Netherlands (2008DFB50130) and the National Basic Research Program of China (2009CB623503 and 2005CB221407). References and Notes (1) Bordiga, S.; Bonino, F.; Damin, A.; Lamberti, C. Phys. Chem. Chem. Phys. 2007, 9, 4854. (2) Vayssilov, G. N. Catal. ReV. 1997, 39, 209. (3) Saxton, R. J. Top. Catal. 1999, 9, 43. (4) Cavani, F.; Teles, J. H. ChemSusChem 2009, 2, 508. (5) Corma, A.; Garcia, H. Chem. ReV. 2002, 102, 3837. (6) Corma, A. Chem. ReV. 1997, 97, 2373. (7) Zhang, L.; Chmelik, C.; van Laak, A. N. C.; Karger, J.; de Jongh, P. E.; de Jong, K. P. Chem. Commun. 2009, 6424. (8) Corma, A.; Navarro, M. T.; Pariente, J. P. J. Chem. Soc., Chem. Commun. 1994, 147. (9) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (10) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (11) Zhang, W. H.; Lu, J. Q.; Han, B.; Li, M. J.; Xiu, J. H.; Ying, P. L.; Li, C. Chem. Mater. 2002, 14, 3413. (12) Tang, J. T.; Liu, J.; Yang, J.; Feng, Z. C.; Fan, F. T.; Yang, Q. H. J. Colloid Interface Sci. 2009, 335, 203. (13) Zhang, L.; Abbenhuis, H. C. L.; Gerritsen, G.; Ni Bhriain, N.; Magusin, P.; Mezari, B.; Han, W.; van Santen, R. A.; Yang, Q. H.; Li, C. Chem.sEur. J. 2007, 13, 1210. (14) Thomas, J. M.; Sankar, G. Acc. Chem. Res. 2001, 34, 571. (15) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Chem. Soc. ReV. 2008, 37, 2530. (16) Hartmann, M. Angew. Chem., Int. Ed. 2004, 43, 5880. (17) van Donk, S.; Janssen, A. H.; Bitter, J. H.; de Jong, K. P. Catal. ReV. 2003, 45, 297. (18) Cejka, J.; Mintova, S. Catal. ReV. 2007, 49, 457. (19) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494. (20) Egeblad, K.; Christensen, C. H.; Kustova, M.; Christensen, C. H. Chem. Mater. 2008, 20, 946.

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