7468
J. Phys. Chem. C 2008, 112, 7468-7476
Synthesis, Characterization and Catalytic Properties of the Novel Manganese-Containing Amorphous Mesoporous Material MnTUD-1 Anand Ramanathan,†,‡ Tanja Archipov,§ Rajamanickam Maheswari,| Ulf Hanefeld,⊥ Emil Roduner,§ and Roger Gla1 ser*,†,# Institute of Chemical Technology, UniVersity of Stuttgart, 70550 Stuttgart, Germany, Institute of Physical Chemistry, UniVersity of Stuttgart, 70550 Stuttgart, Germany, Department of Chemistry, Anna UniVersity, Chennai-600025, India, and Gebouw Voor Scheikunde, Technische UniVersiteit Delft, Julianalaan 136, 2638 BL Delft, The Netherlands ReceiVed: May 21, 2007; In Final Form: January 24, 2008
The incorporation of manganese into the amorphous mesoporous material TUD-1 (nSi/nMn ) 89-8; Mn content: 1-11 wt %) was achieved by the direct hydrothermal method in the presence of triethanol amine. Nitrogen sorption revealed the mesoporous nature (dP ) 4-9 nm) of MnTUD-1 and a decreasing specific surface area with increasing Mn content. FT-IR spectroscopy indicates the incorporation of Mn into the silica framework of TUD-1. As evident from DR-UV-vis and EPR spectroscopy, the Mn atoms in MnTUD-1 exist in the oxidation states +2 or +3. The fraction of Mn2+ is 98%), trans-stilbene (Alfa Aesar, 97%) and tert-butyl hydroperoxide (TBHP, 70 wt % aqueous solution, Aldrich) were used as reactants without further purification. Chlorobenzene (Acros Organics, >99%) served as an internal analytical standard.
J. Phys. Chem. C, Vol. 112, No. 19, 2008 7469 TABLE 1: nSi/nMn Ratios in the Synthesis Gel and after Calcination as Well as Results from Nitrogen Sorption at 77 K (Specific Surface Area SBET, Pore Diameter dP,BJH and Pore Volume VP,BJH) for MnTUD-1 Samples nSi/nMn sample MnTUD-1 (100) MnTUD-1 (50) MnTUD-1 (25) MnTUD-1 (10)
synthesis after SBET/ dP,BJH/ VP,BJH/ nm cm3 g-1 gel calcination m2 g-1 100 50 25 10
89 45 22 8
818 778 630 626
5.8 6.8 8.8 4.6
0.95 0.99 0.95 0.65
Toluene (Fisher Chemicals, >99%), cyclohexane (Acros Organics, >99%), acetone (Fisher Chemicals, >99.5%), or acetonitrile (Acros Organics, >99.5%) were applied as solvents. The catalytic experiments were carried out in the batch mode using a three-neck round-bottom flask (V ) 50 cm3) equipped with a condenser. After placing the solvent and the analytical standard in the reactor, equal molar amounts of styrene or trans-stilbene and TBHP were introduced. Then, the catalyst (dried overnight in air at 423 K) was added. Subsequently, the reaction flask was immersed into an oil bath preheated to the required reaction temperature (323-353 K), and the reaction mixture was stirred vigorously. Samples of the liquid reaction mixture were withdrawn periodically and analyzed by capillary gas chromatography using a flame ionization detector (gas chromatograph: Agilent 5890 Series II, column: INNOWAX (J&W), 60 m (length) × 0.32 mm (inner diameter) × 0.5 µm (film thickness)). The reactants and products were identified by comparison with authentic samples and additionally by GCMS (Agilent Technologies, 6890N Network GC system and 5975B inert XL MSD using DB-1 capillary column: 60 m (length) × 0.25 mm (inner diameter) × 1.0 µm (film thickness)). Both conversion and selectivities were calculated from the respective reactant or product peak areas in the gas chromatogram relative to that of the internal analytical standard (chlorobenzene). Similar to Zhang et al.,20 the TBHP efficiency was obtained as the conversion ratio of styrene and TBHP. 3. Results and Discussion 3.1. Preparation and Characterization. As obvious from the elemental analysis results (Table 1), the nSi/nMn ratio of the MnTUD-1 samples after calcination is close to, but slightly lower than that of the initial synthesis gels. This finding is different from those for most other transition metal-containing TUD-1 materials such as Fe-, Co-, or Cu-TUD-1, where typically higher nSi/nM ratios (M, metal) are found in the calcined products than in the synthesis mixtures.29,31,32 It can, however, be explained by small losses of the silicon source during gel preparation in the present study. The manganese content in the samples with nSi/nMn ratios of 89, 45, 22, and 10 after calcination amount to 1.0, 1.9, 3.8, and 10.6 wt %, respectively. The XRD patterns of the MnTUD-1 samples show a single broad reflection in the low-angle region of 0° e 2θ e 10° with a maximum at 2θ ) 0.6° (Figure 1, upper part), indicating that the materials are meso-structured. With increasing manganese content, the intensity of this reflection decreases without a noticeable change in the line position or shape. This contrasts with the behavior of TUD-1 materials with increasing amounts of incorporated copper.31 It suggests that the manganese species included during synthesis do not affect the meso-structural order and that these species are evenly distributed over the MnTUD-1 material, either within the walls or within the pores. For samples with higher contents of manganese, that is, nSi/nMn e 50, additional reflections at higher 2θ values are observed (Figure
7470 J. Phys. Chem. C, Vol. 112, No. 19, 2008
Figure 1. Powder X-ray diffractograms of MnTUD-1 with different nSi/nMn ratios (upper part, 0° e 2θ e 3°; lower part, 5° e 2θ e 80° compared with Mn2O3).
1, lower part). These can be assigned to manganese(III) oxide Mn2O3. Thus, apart from framework-incorporated manganese species, a significant amount of crystalline Mn2O3 is present as a separate phase in the synthesized products with higher manganese content. Evidently, the fraction of this Mn2O3 phase increases with the amount of manganese in the synthesized MnTUD-1 sample. The exclusive presence of manganese as isolated sites in the silica framework of TUD-1 can, thus, only be expected for low manganese contents, that is, with nSi/nMn ratios above 50. The assumption that the manganese sites are isolated is in analogy to other metal-containing TUD-1-type materials prepared by the same synthesis route, for example, Ti-TUD-127 or, more recently, Zr-TUD-1.35 This phenomenon is based on the formation of atranes from the silicon and the metal precursors, respectively, with the triethanolamine in the synthesis gel, resulting in the isolation of the metal sites.36,37 The mesoporous and amorphous character of MnTUD-1 was further confirmed by high-resolution transmission electron micrographs (HR-TEM, Figure 2). As typical for TUD-1-type materials,26-33 these micrographs show an irregular, spongelike, three-dimensional arrangement of mesopores. Moreover, extraframework particles with a size in the range of 2-3 nm can be observed in the HR-TEM micrographs for MnTUD-1 (10) (Figure 2, lower right part, also showing lattice fringes of these particles). These most likely consist of Mn2O3 in accordance with the results from XRD (vide supra). Note, however, that no separate particles could be detected by HRTEM for all other MnTUD-1 samples with nSi/nMn > 10. Together with the results from XRD, the incorporation of
Ramanathan et al. approximately 2 wt % (nSi/nMn > 50) manganese into the TUD-1 walls with only negligible occurrence of extraframework species appears to be feasible. The nitrogen sorption isotherms for all MnTUD-1 samples (Figure 3, upper part) are of the type IV with a broad H3-type hysteresis loop at relative pressures of p/p0 ) 0.4-0.9, also typical for TUD-1-type materials.26-33 The specific surface area SBET, pore diameter dP,BJH and pore volume VP,BJH determined from the sorption isotherms are listed in Table 1 and the pore size distribution is shown in Figure 3, lower part. The BET surface area decreases with an increase in the Mn content of the TUD-1 samples. On the other hand VP,BJH remained almost constant. This is consistent with the formation of extraframework Mn species such as Mn2O3 that may reside within the pores of the TUD-1 material. A similar decrease of the specific surface area upon incorporation of increasing amounts of copper into TUD-1 was also attributed to the formation of CuO as an oxide phase within the pores of Cu-TUD-1.31 This decrease of SBET is particularly pronounced for the sample with the highest manganese content, that is, nSi/nMn ) 10, the specific pore volume of which is considerably lower than that of all other MnTUD-1 samples. This is probably another consequence of the high fraction of extraframework Mn2O3 in this sample that leads to partial pore blocking. In agreement with this assumption, the sample MnTUD-1 (10) shows a rather broad pore size distribution with a maximum at a much lower value than for all other samples (Figure 3, lower part). However, it cannot be excluded that the low specific surface area and pore volume of MnTUD-1(10) are due to the high manganese content in the synthesis gel. This could disturb the formation of the TUD-1type pore structure and result in a lower degree of mesostructural order as evidenced by the low intensity of the reflection at 2θ ) 0.6° in the XRD pattern (Figure 1, upper part). Interestingly, the average pore size of MnTUD-1 with nSi/ nMn 25 slightly increases with increasing amounts of incorporated manganese (Table 1 and Figure 3, lower part). At variance, the average pores size of Fe- or Cu-containing TUD-1-type materials decreases with or is largely unaffected by the amount of incorporated transition metal, respectively.31 This effect of the manganese content on the pore size of TUD-1 is, however, rather small when compared with that of the synthesis conditions such as the time of the hydrothermal treatment, which can lead to changes of the pore size in the range of 6 to 20 nm.26 The DR-UV-vis spectra of the MnTUD-1 samples with different nSi/nMn ratios are dominated by two broad bands: one near 270 and another one around 500 nm (Figure 4). These two bands are also reported for manganese-containing MCM-41type materials.20,38-40 The first band, which occurs at 250 nm in the spectrum of Mn3O4, is associated with an O2- f Mn2+ charge-transfer transition, while a band due to the charge-transfer transition of O2- f Mn3+ occurs at 320 nm. The band at 270 nm is assigned to the charge-transfer transition of O2- f Mn3+ for manganese atoms in tetrahedral oxygen coordination.20,39 The band at 500 nm is due to the 6A1g f 4T2g crystal field transition of Mn2+ as observed, for example, for Mn3O4 or MnO.39 This latter band has also been assigned to Mn2+ species on the surface of MnMCM-41.20 The presence of the two bands around 500 and 270 nm in the DR-UV-vis spectra of MnTUD-1 with 10 e nSi/nMn e 100 (Figure 4), therefore, indicates that both Mn2+ and Mn3+ species are present in these materials. The DR-UV-vis spectra, however, do not allow an unambiguous, quantitative interpretation with respect to the fractions of Mn2+ and Mn3+ species, respectively.
Mn-Containing Amorphous Mesoporous Material
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Figure 2. HR-TEM images of MnTUD-1 with different nSi/nMn ratios. The left- and right-hand parts show images at different magnifications (lower right-hand side: extraframework particles, presumably Mn2O3, with size of 2-3 nm).
Figure 5 displays the FT-IR spectra in the region of the framework vibrations for the MnTUD-1 samples with different
nSi/nMn ratios in comparison with a Mn-free TUD-1 material. Similar to ordered mesoprous M41S-type materials containing
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Ramanathan et al.
Figure 5. FT-IR spectra of MnTUD-1 with different nSi/nMn ratios in comparison to Mn-free TUD-1.
Figure 3. Nitrogen sorption isotherms (upper part, successive curves offset by 150 cm3 g-1) and pore size distribution (lower part) of MnTUD-1 with different nSi/nMn ratios.
Figure 4. DR-UV-vis spectra of MnTUD-1 with different nSi/nMn ratios.
titanium or manganese,38,40-43 the absorption bands at around 1030 and 1080 cm-1 are due to asymmetric stretching vibrations of Si-O-Si bridges. The absorption band at 960-970 cm-1 is associated with the stretching vibrations of the Si-OH groups, while the bands at 780-800 cm-1 and at 540-560 cm-1 (not shown) correspond to the symmetric stretching vibrations of Si-O-Si bridges.38,40-43 The increase in band intensity located at 960-970 cm-1 is often assigned to the stretching vibration of Si-O-M (M, metal) entities. This effect and a slight decrease
in the vibration frequencies indicate the incorporation of metal into the silica framework, as reported, for example, for MCM41 containing titanium or vanadium.44,45 The intensity of this band clearly increases for MnTUD-1 with the manganese content in the sample, that is, with a decrease in the nSi/nMn ratio. Thus, increasing amounts of manganese are incorporated in the framework of TUD-1 with increasing amounts of manganese in the synthesis gel. However, the band intensity decreased upon increasing the manganese content from 3.8 to 10.6 wt %, that is, decreasing the nSi/nMn ratio from 25 to 10 (Figure 5). This provides further evidence that in the case of MnTUD-1 (10) not all of the manganese atoms are incorporated within the framework but are rather present as part of a separate phase such as Mn2O3. Electron paramagnetic resonance (EPR) was applied to further clarify the oxidation state and the local environment of the manganese atoms in the MnTUD-1 samples. Mn3+ which was observed by DR-UV-vis spectroscopy is EPR silent due to its large zero-field splitting.46,47 On the other hand, both Mn2+ (S ) 5/2) and Mn4+ (S ) 3/2) can be observed by EPR. The reported g values of Mn2+ are about 2.010 with a hyperfine coupling constant (a) in the range of 80-100 G,20,38,40,48 while those of Mn4+ are less than 2.000 with a about 70 G.39,49 The calcined (and template-free) MnTUD-1 samples were first measured in air at room temperature. The obtained spectra exhibit the typical Mn2+ six-line (2I + 1) hyperfine structure with g ) 2.027 and a ) 92 G (Figure 6, upper part), which corresponds to the transition mS|-1/2〉 f |1/2〉 (∆mI ) 0). The identical shape of the isotropic signal for all samples independent of manganese content indicates that all Mn2+ species are located in a similar environment of octahedral coordination by oxygen20,38,40 The amplitude of the signals decreases with increasing nSi/nMn ratio (Figure 6, upper part). The existence of Mn4+ in the MnTUD-1 samples can be excluded, because no finestructure lines appear at higher field as it would have been expected for the lower g value of Mn4+. After dehydration of the MnTUD-1 samples by evacuation, the same signals were detected in the EPR spectra but with reduced intensity. Only in the EPR spectrum of MnTUD-1 (10), an additional small narrow line was detected at g ) 2.004. This signal is attributed to an organic radical, possibly from incomplete removal of the template (TEA). The absolute values of spin concentrations permit a quantitative determination of the Mn2+ content in the MnTUD-1 samples. The fraction of Mn2+ with respect to the overall manganese content in the
Mn-Containing Amorphous Mesoporous Material
J. Phys. Chem. C, Vol. 112, No. 19, 2008 7473
Figure 7. Curie-Weiss plots for MnTUD-1 (100) and MnTUD-1 (10).
Figure 6. EPR spectra of MnTUD-1 with different nSi/nMn ratios measured at room temperature (upper part) and at 20 K (lower part).
TABLE 2: Molar Fraction of Mn2+ (in %, Relative to the Total Amount of Manganese) in MnTUD-1 with Different nSi/nMn Ratios (Calcination at 673 K in Flowing Oxygen, Re-Hydration at Room Temperature) sample fresh samples after calcination after re-hydration a
MnTUD-1 MnTUD-1 MnTUD-1 MnTUD-1 (10) (25) (50) (100) 0.05a 0.00 0.23a
0.20a 0.00 0.47a
0.58a 0.00 0.54a
0.71a 0.00 0.72a
Corrected for the antiferromagnetic character of the samples.
samples as obtained from chemical analysis is summarized in Table 2. It is very low (
