Synthesis and Characterization of Iron-Rich Highly Ordered

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MATERIALS AND INTERFACES Synthesis and Characterization of Iron-Rich Highly Ordered Mesoporous Fe-MCM-41 S. Samanta,† S. Giri,‡ P. U. Sastry,§ N. K. Mal,| A. Manna,⊥ and A. Bhaumik*,† Departments of Materials Science and Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India, Solid State Physics Division, Bhaba Atomic Research Centre, Trombey, Mumbai 400 085, India, Dynamic Materials Research Group, National Institute of Advance Science & Technology (AIST), Kansai Center, 1-8-31, Osaka 563-8577, Japan, and Materials Science Department, National University of Singapore, Kent Ridge Road, Singapore 117542

Iron-rich and highly ordered 2D-hexagonal mesoporous ferrisilicates containing predominantly tetrahedrally coordinated Fe3+ in the silica network have been prepared using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent (SDA) under mild alkaline hydrothermal conditions (initial pH ) 8-8.5 in the synthesis gel). The optimum limit of iron loading for the ordered mesophase in the present study was 8.2 wt %; beyond this limit, disordered iron oxide/silica phase was observed in the same pH range. These mesoporous ferrisilicate samples were characterized using XRD, N2 sorption, EPR, SEM-EDX, TEM, FT-IR and UV-visible spectroscopies, and Mo¨ssbauer measurements. The average pore diameters were 2.2-2.8 nm, and the mesopore volumes were 0.39-0.5 cm3g-1. Moderately high Brønsted acidity was observed in the temperature-programmed desorption (TPD) pattern of ammonia over these mesoporous ferrisilicate materials. Wet chemical analysis, EPR, and Mo¨ssbauer spectroscopic data suggested that most of the iron species are tetrahedrally coordinated Fe3+ attached to the silica framework. Introduction The substitution of various transition elements into open-framework microporous and mesoporous materials1-5 has received considerable attention because of the need to develop more efficient and stable materials for applications in catalysis, separations, coatings, and chemical sensing. The art of metal substitution received a big thrust since the discovery of microporous titanium silicate molecular sieve TS-16 and its versatile catalytic activity7 in liquid-phase partial oxidation reactions using dilute H2O2 as the oxidant. Iron-containing zeolites8 and related molecular sieves are of particular interest because of their unique catalytic activity in various selective gas-phase reactions, e.g., hydrocarbon oxidation,9 N2O decomposition,10 and selective catalytic NO and N2O reduction in the presence of hydrocarbon or ammonia.11,12 Mesoporous materials containing these heteroelements13-17 have still wider potential for use as catalysts, exchangers, and adsorbent because of their tunable nanoscale pore openings and exceptionally high internal surface areas accessible to bulky organic mol* To whom correspondence should be addressed. E-mail: [email protected]. † Department of Materials Science, Indian Association for the Cultivation of Science. ‡ Department of Solid State Physics, Indian Association for the Cultivation of Science. § Bhaba Atomic Research Centre. | National Institute of Advance Science & Technology (AIST). ⊥ National University of Singapore.

ecules. To date, a wide range of transition as well as nontransition elements have been incorporated in these silica-based mesostructures.4,5 Unlike tetravalent elements (Ti, Ge, Sn, Zr, etc.), whose incorporation does not introduce any charge in the neutral silica framework, each of the trivalent heteroelements incorporated into the silica matrixes (Al, B, Ga, Fe, In, etc.) introduces a negative charge that could be balanced by a cation. This cation can be a mono- or divalent metal ion, as well as ammonium or a proton. Thus, the incorporation of a trivalent heteroelement into the SiO2 framework can generate a cation exchange or Brønsted acid site. However, the adsorptive and catalytic properties of the other trivalent metal analogues of these micro- and mesoporous metallosilicates are usually different from those of their aluminum counterparts of zeolites. In addition to modifications of the acidity, the presence of multivalent metal cations in the framework can also create isolated redox centers, which can act as good heterogeneous oxidation catalysts in the presence of a mild oxidant.7 Moreover, their loading in the silica matrix can affect the overall acid strength, ion-exchange capacity,18 and catalytic performances19 extensively. Crystalline microporous ferrisilicates such as ZSM-5,20 -11,21 -beta,22 ferrite,23 etc., as well as amorphous mesoporous iron silicates,24,25 have been known for decades, and these materials have some unique characteristic features. Among the trivalent heteroelements, the incorporation of tetrahedral Fe3+ into a silica network is known for the generation of very strong acid sites.22 Usually, severe precautions are taken during the

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preparation of the synthesis gel of such materials to avoid the precipitation of iron oxides and hydroxides, as the synthesis is usually carried out under moderate to strong alkaline conditions.26 The mode of addition of the reagents and, in some cases, the addition of the iron source in the form of iron complexes27 have been recommended for such preparations. Successful incorporation of Fe3+ into the tetrahedral lattice site of silica can be achieved through careful preparation of the synthesis gel and its subsequent hydrothermal treatment. However, the maximum loading of Fe in crystalline microporous silica networks having defined interatomic distances and bond angles is restricted to 3.2 mol %,27 and amorphous mesoporous HMS materials with broad distributions of bond lengths and bond angles can accommodate a maximum of only 5.7 wt % of iron25 in their frameworks. To date, reports on the synthesis28 and catalytic properties29,30 of Fe-MCM-41 have been restricted to 1.1-2 wt % loadings of Fe only using direct hydrothermal synthesis, template ion exchange, incipient wetness impregnation, solid-state impregnation, and other postsynthesis methods. Because the acidities of these ferrisilicates are related to the loading of iron, more isolated tetrahedral Fe3+ could be conducive to the generation of more catalytically active materials. Thus, iron-rich ferrisilicates having predominantly tetrahedral Fe3+ and highly ordered mesoporous frameworks, but devoid of iron oxide impurities, are most desirable. Here, we show that as high as 8.2 wt % Fe can successfully be incorporated preserving the highly ordered mesoporous MCM-41 framework using a simple hydrothermal method, avoiding coprecipitation of iron oxides during the synthesis. Detailed characterizations of these ironrich 2D-hexagonal mesoporous Fe-MCM-41 materials were carried out using powder XRD, TEM, SEM, N2 sorption, EPR, FT-IR, Mo¨ssbauer, and magnetic measurements and surface, as well as wet chemical analysis. Experimental Section Tetraethyl orthosilicate (TEOS, E-Merck, Germany) was used as the silica source in all syntheses. The cationic surfactant cetyltrimethylammonium bromide (CTAB, Loba Chemie, India) was used as the structuredirecting agent, and ferric chloride (Loba Chemie, India) was used as the iron source. NaOH (Loba Chemie, India) was used to maintain the pH of the medium. For the syntheses of the mesoporous Fe-MCM-41materials, TEOS was first added to an aqueous solution of CTAB, which was then stirred for 15 min. The aqueous solution of CTAB was prepared by dissolving CTAB in deionized water and then stirring the mixture for 15 min. Then, the desired amount of FeCl3 was taken in water and added to this aged silica sol in different mole ratios corresponding to the desired loadings of iron. TEOS was then allowed to hydrolyze in acidic pH slowly. After 1 h, aqueous NaOH solution was added to the solution until the pH rose to 8-8.5. The final mixture was vigorously stirred for 1 h and then autoclaved at 353 K for 2-3 days. The molar ratio of various constituents of the hydrothermal gels was

SiO2/CTAB/Fe(III)/H2O ) 1/0.25/(0.1-0.0125)/90 After the hydrothermal treatment, the solid products were filtered, washed with water, and dried in air. The mesophases of both the as-synthesized and templatefree samples were identified by small-angle powder XRD

using a Rigaku 12-kW rotating-anode X-ray generator on which small- and wide-angle gonio- meters were mounted. The X-ray source was Cu KR radiation (R ) 0.154 06 nm) with a voltage and current of 40 kV and 30 mA, respectively. For transmission electron microscopy measurements, a Hitachi H-600 instrument was used. N2 adsorption measurements were carried out using a BELSORP 28SA apparatus at 77 K. Prior to N2 adsorption, samples were degassed for 2 h at 323 K. For the Fourier transform infrared (FT-IR) measurements, a Nicolet MAGNA IR 750 instrument was used. EPR spectra were recorded in the X-band mode on a Varian E9 spectrometer at 298 K. Before the EPR measurements, the samples were dried under vacuum for 4 h at 373 K. Magnetic measurements were carried out using an EG&G Princeton Applied Research vibrating sample magnetometer 155 at 295 K, with an applied magnetic field of up to 10 kG. 57Fe Mo¨ssbauer spectra were recorded at room temperature in a transmission geometry using a 25 mCi 57Co source in a Rh matrix with a Wissel velocity drive unit in constant acceleration mode. All Mo¨ssbauer parameters were estimated with respect to metallic R-Fe. Temperature-programmed desorption (TPD) of ammonia was carried out with a homemade apparatus equipped with a mass spectrometer. Before the TPD measurements, samples were purged at 373 K in nitrogen atmosphere for 2 h. This was followed by ammonia gas adsorption at 373 K. Decomposition of ammonium ions and desorption of NH3 were monitored (at a heating rate of 10 K/min) through the mass spectrometer. For the determination of the chemical composition and the oxidation state of Fe in various samples, a Dionex 500 ion chromatograph was employed. An atomic absorption spectrophotometer (Perkin-Elmer AAS 3310) with a flow-injection hydride generation attachment was used for chemical weight analysis, with a detection limit of F1-Hg-AAS each 3 ppb and 95% accuracy. Prior to chemical analysis, solid samples were dissolved in concentrated HF and HCl solution, followed by evaporation, three times before the volume was made up with distilled water. The surfactant was removed from the as-synthesized mesoporous ferrisilicate materials by acid extraction using ethanolic hydrochloride acid for 4 h at room temperature under vigorous stirring. For a typical acid extraction, 1.0 g of the as-synthesized sample was added to a mixture of 75 mL of dry ethanol and 1.0 mL of 1 M HCl. Then, the resulting mixture was stirred at room temperature for 6 h. After this extraction, the solid was filtered, washed with dry ethanol, and then air-dried at room temperature. Surfactant was also removed in some cases through calcination under a flow of air at 773 K for 8 h. Results and Discussion We have prepared Fe-rich Fe-MCM-41, optimized its iron-loading limit, and identified the nature of Fe species present in this mesoporous material. Physicochemical properties of various ferrisilicate samples synthesized in the present study are reported in Table 1. Sample 1, pure silica MCM-41, was synthesized under otherwise identical synthesis conditions except for the addition of the Fe3+ precursor in the gel. Chemical analysis of these ferrisilicate samples after the acidsolvent extraction revealed the absence of any organic SDA. Si/Fe mole ratios for various Fe-MCM-41 samples as determined by AAS increased with the increasing input gel ratio (Table 1). An optimum of this ratio for

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Table 1. Physicochemical Properties of Different Fe-MCM-41 Samples

a

Si/Fe mole ratio

sample no.

gel

product

nature of the sample

BET surface area (m2 g-1)

pore diameter (Å)

mesopore volume (cm3 g-1)

1 2 3 4 5 6

∞ 5.0 10.0 20.0 40.0 80.0

-a 10.1 23.2 41.5 77.6

ordered amorphous ordered ordered ordered ordered

1025 176 464 614 636 666

27.5 22.0 22.4 25.7 28.0

0.63 0.39 0.42 0.46 0.50

Si/Fe mole ratio in this iron-silica mixed oxide was 6.7.

Figure 2. TEM image of Fe-MCM-41 (sample 4).

Figure 1. XRD patterns of different Fe-MCM-41 samples: (a) sample 2 (Si/Fe ) 6.7), (b) sample 3 (Si/Fe ) 10.1), and (c) sample 4 (Si/Fe ) 23.2). The d spacings of different planes for sample 4 are shown in the inset.

the ordered ferrisilicate mesophase was observed for sample 3 at 10.1 (8.2 wt %). The materials obtained with Si/Fe molar ratios of 10 or more in the synthesis gel were two-dimensional hexagonally ordered Fe-MCM41 materials, whereas increasing the iron loading further in the gel resulted in disordered porous structures (sample 2, Si/Fe ) 5). One of the great advantage of these iron-containing mesoporous Fe-MCM-41 is that, unlike Fe-incorporated zeolites,23 which turn pale yellow to brown coloration during calcination, the cationic surfactants used in the present study can easily be removed by simple acid-ethanol extraction without changing the color of the sample and, thus, preserving the coordination of the iron in the lattice. A typical XRD pattern of the as-synthesized mesoporous ferrisilicate Fe-MCM-41 (sample 4) is shown in Figure 1c. The other two patterns in Figure 1, a and b, correspond to samples 2 and 3, respectively. As seen from Figure 1, all four low-angle peaks of the 100, 110, 200, and 210 phases corresponding to the 2D-hexagonal mesophase of MCM-4113 were observed in samples 3 and 4, and no distinctive higher-order peaks were observed. Among the samples whose characterization data are presented in Table 1, except for sample 2, all other samples (samples 3-6) were highly ordered and exhibited similar X-ray diffraction patterns.13-17 The d spac-

ings of various planes for sample 4 and the corresponding hkl values are shown in the inset of Figure 1. The unit cell parameter, a0, calculated from these values was 4.32 nm. Figure 2 displays the TEM image of a representative mesoporous ferrisilicate sample. The existence of a hexagonal arrangement of uniform mesopores is very clear. These results suggest that this ferrisilicate material has a highly ordered two-dimensional hexagonal mesophase. The peak positions remained almost unchanged after removal of the surfactant, although a slight change in the intensity was observed. Because there were no noticeable high-angle reflections in the XRD patterns of these mesoporous ferrisilicate materials, the pore walls were concluded to be amorphous in nature. Sample 2, with an input Si/ Fe mole ratio of 5.0, did not exhibit any well-resolved diffraction peaks (Figure 1a), indicating the absence of any long-range ordering. No diffraction line corresponding to iron oxide crystallites was observed in either of the samples synthesized using this method. For sample 2, also, no high-angle reflection was observed. This indicates that the Fe2O3 crystallites, if formed, were very small (less than 3 nm size) or comprised disordered aggregates.31,32 In Figure 3, N2 adsorption/desorption isotherms for Fe3+-containing mesoporous silica (sample 4, taken as representative) are shown. The pore size distribution for this sample is shown in the inset. The isotherms, as shown in Figure 3, were of type IV.13,33 The pore size distribution was very narrow and relatively lower in range vis-a`-vis mesoporous materials synthesized using CTAB surfactant.13-18 The average pore diameter for this mesoporous ferrisilicate sample, determined according to the BJH model,33 was 2.24 nm. The adsorption/desorption isotherms and corresponding pore size distributions for samples 3, 5, and 6 followed an identical pattern. The BET surface areas of these mesoporous ferrisilicates were medium to high, considering the moderately high concentrations of iron. The surface area decreased with increasing loading of Fe, as expected from the increase in the unit cell mass of the composites.

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Figure 4. UV-visible diffuse reflectance spectra of (a) surfactantfree Fe-MCM-41 (sample 4) and (b) R-Fe2O3.

Figure 3. N2 adsorption/desorption isotherms of sample 4. Adsorption points are marked with filled circles, whereas desorption points are marked with empty circles. The BJH pore size distribution is shown in the inset.

The pore walls were relatively thicker (1.6-2.2 nm) than those of siliceous mesoporous materials (1-1.5 nm).13-18 The pore size also decreased from 2.8 to 2.2 nm as the Fe3+ loading increased from samples 6 to 3. A similar lowering in the pore diameter was also observed for Eudoped silica34 vis-a`-vis a pure-silica MCM-41 material synthesized under the same conditions. The pore diameter and d spacings determined by TEM image analysis agreed well with the experimental data obtained from XRD and N2 sorption. In Figure 4, a UV-visible diffuse reflectance spectrum of iron-rich mesoporous Fe-MCM-41 (a, sample 4, representative) is shown. The corresponding spectrum for bulk Fe2O3 is given in the Figure 4b for comparison. The UV-visible spectra of the ferrisilicate materials showed a very strong absorption band in the 200-330 nm wavelength range with maxima at ca. 219 and 255 nm. Other surfactant-free mesoporous materials showed identical peak patterns. A similar high-energy absorption band associated with ligand-to-metal charge transfer (LMCT) that is characteristic of isolated tetrahedral coordination of Fe3+ has been observed for mesoporous FeS-124 and Fe-HMS.25 These two bands have been assigned to electronic transitions of the anion (e.g., O2-) to the t2g and eg orbitals of Fe3+ within [FeO4]- tetrahedra.35 The high-energy absorption band of this mesoporous ferrisilicate indicated that tetrahedral coordination of Fe is dominant in these materials. Bulk R-Fe2O3 exhibited a broad adsorption band at 320-670 nm with an absorption maximum at 560 nm (Figure 4b). The absence of any peak above 320 nm in the UVvisible spectra of these ferrisilicate materials indicated the absence of clustered Fe2O3 or Fe3O4 (having octahedral Fe) in these Fe-MCM-41 samples.

Figure 5. EPR spectrum of Fe-MCM-41 (sample 4).

EPR spectroscopy is an important tool for identifying the oxidation states of the transition elements in these ferrisilicate materials.23 It has been observed that the loading of iron has very little effect on the oxidation states of such materials. Figure 5 displays an EPR spectrum of mesoporous Fe-MCM-41 (sample 4). In this EPR spectrum, two different signals at geff values of 4.28 and 2.00 corresponding to the presence of trivalent iron in two distinct environments were observed. On the basis of earlier assignments,25 the signal at 4.28 can be ascribed to the presence of Fe3+ in an isolated distorted tetrahedral environment. The contribution of this signal

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Figure 6. Mo¨ssbauer spectra of Fe-MCM-41: (a) sample 4 and (b) sample 3.

was much lower than that with g ) 2.00. The very strong signal at 2.00 can support the presence of both framework and extraframework trivalent iron in the mesoporous silica.25 The absence of a weak signal at 2.18 can be attributed to the absence of nonframework (superparamagnetic) iron oxide/oxi-hydroxide nanoparticles dispersed in the mesoprous silica channels. Room-temperature Mo¨ssbauer spectra of samples 3 and 4 after surfactant removal are shown in Figure 6. Unfortunately, we could not observe Mo¨ssbauer patterns for samples 5 and 6 (Si/Fe ) 41.5 and 77.6, respectively), possibly because of the low concentrations of Fe3+ in these samples. On the other hand, a weak doublet spectrum was observed for samples 3 and 4. The values of the average isomer shift (IS) and quadrupolar splitting (QS) for samples 3 and 4 were estimated as 0.12 and 0.17 mm/s and 0.82 and 0.68 mm/s, respectively. It has been observed that the values of IS and

Figure 7. SEM image of sample 4.

QS are considered as important parameters in understanding the nature of the framework and oxidation state of iron. At room temperature, IS < 0.3 mm/s is indicative of a tetrahedral iron framework, whereas IS > 0.3 mm/s characterizes octahedral Fe3+.20,21,25 The values of IS and QS for these Fe-MCM-41 samples indicate a tetrahedral framework around the iron present and an iron oxidation state of Fe3+.20-25 In agreement with other experimental results on Fe zeolites23 and Fe-HMS,25 the UV-visible diffuse reflectance spectra (Figure 4) and Mo¨ssbauer results for ironrich Fe-MCM-41 support a tetrahedral environment and Fe3+ oxidation state in the system. It was observed that the value of the magnetic moment did not change as the applied magnetic field was increased to 10 kG for the Fe-MCM-41 samples. In the present system, Fe3+ existed randomly as islands in the silica network because of low iron concentration. The absence of any field dependence of the magnetization indicates that there is no linkage of exchange path between nearestneighbor iron ions. The SEM image of mesoporous FeMCM-41 (sample 4) shown in Figure 7 exhibits uniform spherical crystallites ∼0.3 to 0.4 µm in size. The UV-visible diffuse reflectance, EPR, and Mo¨ssbauer spectral patterns of these materials indicate that the iron species are essentially present as tetrahedrally coordinated Fe3+ moieties in the Fe-MCM-41 samples. These trivalent iron atoms substitute for some of the silicon atoms in the pore wall and thus generate Brønsted acidity. To understand the nature of the interactions36 and the acid sites present in the surface, TPD of ammonia over calcined Fe-MCM-41was carried out. Ammonia was found to diffuse very slowly in the temperature range of 373-673 K, having maximum desorption in the range of 523-573 K for all samples. This indicates medium to strong interaction of ammonia with the Brønsted acid sites and the framework iron species are located at the surface of the mesoporous pore wall similar to that observed for Fe-HMS25 materials. With increasing Fe3+ loading, the concentration of Brønsted acid sites increases (signal intensity for the desorption maximum continues to increase from sample 6 to 3), indicating that the incorporation of Fe3+ into

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and Fe3+ moieties. However, there was a limit for the uptake of Fe3+ in the silica mesophase. The sample with the highest Fe3+ content in the gel preserving the mesostructure was obtained with a lowest Si/Fe mole ratio of 10 (sample 3, Si/Fe ) 10.1, 8.2 wt % Fe in the solid). The mechanism for the formation of this mesostructure involved the formation of rodlike micelles of surfactant molecules39 encapsulated by ferrisilicate nuclei. This was followed by the polymerization of the ferrisilicate species to form the regular array of mesopores separated by the thick (1.6-2.2 nm thickness, Table 1) ferrisilicate pore walls. The self-assembly of the surfactant-silicate mesostructure is largely dependent on the strong ionic interactions between surfactant micelles and inorganic metallo-silicate species. As observed for other transitionmetal- (viz., Ti-, V-, Cr-, etc.)40 containing mesoporous silica, in the case of Fe3+, there was also an optimum limit to its incorporation. This limitation seems to be guided by the fact that, beyond an 8.2 wt % loading of Fe3+ in the metallosilicate precursor nuclei, more local strain in the mesostructure is created, leading to a disordered mesophase. Conclusions Figure 8. FTIR spectra of (a) surfactant-free and (b) assynthesized Fe-MCM-41 (sample 4) and ammonia adsorbed on samples (c) 3 and (d) 4. Impurity peaks due to CO2 are marked by an asterisk.

the tetrahedral lattice site with cation exchange capacity has occurred. FT-IR spectra of the representative as-synthesized and surfactant-free Fe-MCM-41 samples are shown in Figure 8 (a and b; sample 4). A broad band in the hydroxyl region between 3700 and 3000 cm-1 with a maximum in the range of 3400-3450 cm-1 was observed in both cases. This band can be assigned to the framework Si-O-H, as well as Si(OH)Fe groups in interaction with defect sites and adsorbed water molecules. Various C-H stretching vibrations due to the presence of the organic surfactant molecules, which appeared in the as-synthesized sample at 2925 and 2840 cm-1, disappeared after the removal of the surfactant. T-O-T framework vibrations, which appeared in the region 860-1400 cm-1, became much narrower in the region 900-1360 cm-1 after surfactant removal. The-ammonia TPD results were confirmed by the FT-IR spectroscopic data for the ammonia-adsorbed samples. The characteristic intensity of the band for ammonium ions at 1400-1500 cm-1 continues to grow with increasing Fe3+ loading (Figure 8c vis-a`-vis 8d, because of the increase in number of acid sites), whereas the frequency showed a slight downshift38 with increasing Fe3+ loading. A liquid-crystal templating pathway for cationic surfactant micelles in aqueous solution is commonly employed for the synthesis of mesoporous silica-based materials. The pH in such synthesis processes can be varied widely, from mild to strong alkaline (8.5-12.0) to strong acid (0.5-2.0) conditions. In the present case, silicon alkoxide was first hydrolyzed by acidic ferric chloride solution. This was followed by condensation of the silica and iron species as the pH was increased with aqueous NaOH solution. Unlike in previous synthesis methods,26-30 here, the final pH of the hydrothermal gels was kept at mild alkaline condition (between pH ) 8.0-8.5), which assists the condensation of ferrisilcate nuclei rather than that of individual silicate

Highly ordered mesoporous ferrisilicate Fe-MCM-41 materials have been synthesized using a cationic surfactant as the structure-directing agent under mild hydrothermal condition and at weakly alkaline pH. The loading of iron was found to have a direct relation to the ordering of these materials. An ordered Fe-MCM41 phase was observed up to the highest loading of iron (8.2 wt % with respect to the template-free solid product) in these mesoporous ferrisilicates. Beyond this loading limit, disordered ferrisilicate materials were obtained. N2 adsorption measurements indicated medium to high BET specific surface areas, uniform mesopore openings, and high pore volume in these Fe-MCM-41 samples synthesized using the present hydrothermal method. UV-visible, EPR, and Mo¨ssbauer measurements suggested tetrahedral coordination of Fe3+ in an isolated silica environment. FT-IR and acidity measurements indicated that surface Brønsted acidity is generated by the incorporation of each Fe3+ into the silica network of MCM-41. Acknowledgment A.B. thanks DST, Govt. of India, New Delhi, India, for providing financial assistance. S.S. is grateful to DST for providing a Junior Research Fellowship. Literature Cited (1) Szostak, R. Molecular Sieves: Principles of Synthesis and Identification; Van Nostrand Reinhold: New York, 1989. (2) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations. Chem. Rev. 2002, 102, 3615. (3) Corma, A.; Kan, Q.; Rey, F. Synthesis of Si and Ti-SiMCM-48 Mesoporous Materials with Controlled Pore Sizes in the Absence of Polar Organic Additives and Alkali Metal Ions. Chem. Commun. 1998, 579. (4) Jerrylaine, V.; Morey, M.; Carlsson, H.; Davidson, A.; Stucky, G. D.; Butler, A. Peroxidative Halogenation Catalyzed by Transition-Metal-Ion-Grafted Mesoporous Silicate Materials. J. Am. Chem. Soc. 1997, 119, 6921. (5) Zhang, W.; Fro¨ba, M.; Wang, J.; Tenev, P. T.; Wong, J.; Pinnavaia, T. J. Mesoporous Titanosilicate Molecular Sieves prepared at Ambient Temperature by Electrostatic (S+I-, S+X-I+)

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Received for review November 12, 2002 Revised manuscript received March 19, 2003 Accepted April 16, 2003 IE020905G