Ind. Eng. Chem. Res. 2007, 46, 4221-4225
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Ce-TUD-1: Synthesis, Characterization, and Testing of a Versatile Heterogeneous Oxidation Catalyst Leon G. A. van de Water,† Shaun Bulcock,‡ Anthony F. Masters,† and Thomas Maschmeyer*,† School of Chemistry and Electron Microscope Unit, UniVersity of Sydney, Sydney, NSW 2006, Australia
A cerium(IV)-containing mesoporous silica, Ce-TUD-1, has been synthesized using the sol-gel technique, where a Ce(IV) acetyl acetonate complex was used as the Ce(IV) precursor. The material proved to be an active catalyst for both the oxidation of p-tert-butyl toluene to p-tert-butyl benzaldehyde and the peroxidative halogenation of Phenol Red. Both reactions require the presence of bromide ions. The conversion of p-tertbutyl toluene over Ce-TUD-1, using 30 wt % aqueous hydrogen peroxide as the oxidant and potassium bromide as the co-catalyst, was 37%, somewhat higher than soluble Ce(III) and Ce(IV) salts. The aldehyde selectivity of 57% was comparable to that of both cerium salts. Regeneration experiments showed that the catalytic activity is slightly reduced in a subsequent run (90% compared to the first run). In the Phenol Red halogenation reaction, it is proposed that Ce-TUD-1 catalyzes the bromination via a pathway mechanistically distinct from well-known V, W, or Ti catalysts, which involve metal-peroxo intermediates. Ce(IV) is thought to act as a one-electron oxidant, reacting with bromide ions to yield bromine radicals, which then react with the organic substrate to give Phenol Red radicals. Reaction of hydrogen peroxide with bromide ions, in turn, yields bromine (Br2), which is proposed to react with the organic radical to give the brominated product. Introduction Mesoporous materials have been the subject of intense research since the earliest report on the first family of this kind, the M41S materials.1 In particular, their modification with catalytically active metals make these materials potential catalysts in conversions involving large substrate and/or product species.2 Many different types of mesoporous materials have since been reported and it has been shown frequently that these silicates can be modified with metal (oxide) species to render them catalytically active. One example is TUD-1,3 a 3-D mesoporous silicate material which can be easily modified with transition metals such as Ti,4 Co,5 Fe,6 and Cu.7 In the present paper, the introduction of Ce(IV) ions into the TUD-1 matrix is reported. Cerium is widely used in oxidation chemistry, where the facile oxidation/reduction of the Ce(III)/Ce(IV) redox system makes it particularly suitable in one-electron oxidation reactions. In this respect, Ce(IV) ammonium nitrate (CAN) has become a widely used, versatile reagent for a broad range of synthetic transformations,8 for example, in the oxidative bromination of aromatic substrates.9 In all these cases, stoichiometric quantities of CAN were used, which makes these processes less attractive from an economic and environmental point of view. Reports on the catalytic use of cerium salts include the CeCl3‚7H2O catalyzed hydroxylation of β-dicarbonyl compounds10 and the Ce(III)-acetate-catalyzed selective oxidation of toluenes to the corresponding aldehydes in the presence of hydrogen peroxide and bromide ions.11 Reports of heterogeneous oxidation catalysts featuring cerium include the Ce-MCM-41-catalyzed oxidation of cyclohexane12 and the catalytic oxidation of n-heptane.13 In this study, we report the preparation of a series of CeTUD-1 materials with different Si:Ce ratios and the testing of these materials in the partial oxidation of p-tert-butyl toluene to its aldehyde, applying the same conditions as used by Auty * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +61 2 9351 2581. Fax: +61 2 9351 3329. † School of Chemistry. ‡ Electron Microscope Unit.
et al.11 This alkylbenzene oxidation reaction is of particular interest as the aldehyde product, p-tert-butyl benzaldehyde, is an important intermediate in the production of flavors and fragrances. Second, Ce-TUD-1 was tested in the catalytic peroxidative halogenation reaction of the dye Phenol Red. Peroxidative bromination reactions have attracted attention recently because they would offer a more benign alternative for bromination processes involving elemental bromine. The marine enzyme vanadium bromoperoxidase14 catalyzes this type of selective halogenation reaction at room temperature and at near-neutral pH for a variety of substrates, using hydrogen peroxide as the oxidant of bromide. Inspired by this enzyme, several catalysts for this reaction have been developed since, including a cis-dioxovanadium(V) complex,15 tungstateexchanged layered double hydroxides (WO42--LDH),16 and the mesoporous titanosilicate Ti-MCM-48.17 The Ce-TUD-1 catalyzed peroxidative bromination of Phenol Red presented here is the first example of a cerium catalyst for this reaction. Experimental Section Catalyst Preparation. Ce-TUD-1 was prepared following a modified procedure for the all-silica material TUD-1.3 R-Ce(acac)4 was used as the cerium precursor and was prepared according to Behrsing et al.18 The appropriate quantity of Ce(acac)4 was dissolved in 4.5 g of warm EtOH and mixed with 5.21 g (25.0 mmol) of tetraethyl orthosilicate (TEOS), with Si:Ce molar ratios of 20, 50, and 100. After homogenization of the mixture, 3.73 g (25.0 mmol) of triethanolamine was added and stirring was continued for 2 h. A mixture of 35 wt % tetraethyl ammonium hydroxide (TEAOH) solution (3.16 g, 7.50 mmol of TEAOH) and water (2.90 g, 161 mmol) was added and the resulting mixture was stirred for 24 h at room temperature. A yelloworange gel was obtained which was dried at 105 °C (24 h) and was then transferred into an autoclave and heated at 170 °C for 16 h. Most of the template was then removed from the dark brown products by Soxhlet extraction with EtOH (12 h). Finally, the products were calcined in static air at 600 °C (heating rate: 1 °C/min) for 10 h, yielding off-white (Ce-TUD-1(100)) to yellow (Ce-TUD-1(20)) products.
10.1021/ie061197s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007
Characterization. X-ray powder diffraction (XRD) patterns were recorded using a Siemens D5000 diffractometer using Cu KR radiation in the 1-60° 2θ range. Nitrogen sorption isotherms were recorded on a Micromeritics ASAP 2020 instrument at 77 K. Mesoporosity was calculated from the desorption branch using the BJH model. High-resolution TEM images were obtained on a JEOL 3000F microscope operating at 300 kV. EDS analysis over a broad area was performed by converging the beam to an estimated 100-150 nm diameter. Individual particles were then identified and analyzed using dark-field STEM with EDS spectra collected via a 1-nm stationary probe. The elemental composition of the samples was determined using a Varian Vista ICP-AES (axial plasma). Catalytic Testing. For the oxidation of p-tert-butyl toluene over Ce-TUD-1(20), p-tert-butyl toluene (801 mg, 5.40 mmol), KBr (119 mg, 1.00 mmol), Ce-TUD-1(20) (168 mg, 0.12 mmol of Ce), and approximately 100 mg of Cl-benzene (internal standard for GC analysis) were mixed with 10 mL of glacial acetic acid and the mixture was heated to 70 °C. A 30 wt % aqueous solution of hydrogen peroxide (1.25 g, 11.0 mmol) was added to this mixture in six aliquots over a period of 60 min. The mixture was stirred and heated for another hour, after which samples for GC-MS analysis were taken. The samples were neutralized with aqueous NaOH and extracted with ethyl acetate and toluene, and the combined organic fractions were subsequently analyzed on a Shimadzu GC-MS (QP 2010, using a fused silica Rtx-5Sil MS column (30 m, 0.25 mm i.d.)). Peroxidative halogenation of Phenol Red was performed using 6 mL of a solution containing 0.09 mM Phenol Red, 5 mM hydrogen peroxide, 0.05 M Hepes buffer, and 0.05 M KBr. The Ce-TUD-1 catalyst (corresponding to an amount of 0.031 mmol of Ce) was then added to this solution and the resulting slurry was stirred at room temperature at a rate of 300 rpm. Samples were taken at regular intervals, subsequently centrifuged, and analyzed by UV-vis spectroscopy (Varian Cary 1E UV-vis spectrometer). The intensity of the absorption band of the tetrabrominated product, Bromophenol Blue, at 590 nm, was used to monitor the progress of the reaction. The relative intensity of this band, compared to its intensity after completion of the reaction, was plotted versus time to obtain semiquantitative data. An experiment involving excess substrate was performed by using 10 mL of a solution containing 1.5 mM Phenol Red, 75 mM hydrogen peroxide, 0.5 M Hepes buffer, and 0.75 M KBr and using 31 mg of Ce-TUD-1(100), corresponding to 5.0 µmol of Ce. Results and Discussion Catalyst Preparation and Characterization. The incorporation of heteroatoms into the mesoporous silica material TUD-1 is well-established. Silica-framework formation starts when water is added to a mixture of TEOS and triethanol amine, where simultaneous hydrolysis-condensation reactions yield a regular 3-D pore structure. To successfully incorporate heteroatoms into the framework in a highly dispersed fashion, it is essential that no precipitation of the metal-precursor oxide or hydroxide occurs prior to these hydrolysis-condensation reactions. To prevent premature metal-oxide formation, water was not added to the synthesis gel until a homogeneous mixture of the ethanolic Ce(acac)4 solution and TEOS and triethanolamine was obtained. In this way, the mixture remained clear and homogeneous at all times during the gel stage. The acetylacetonate complex of Ce(IV) was used because of its solubility in ethanol, thus making it miscible with TEOS and triethanol amine, and because its decomposition after the addition of water is slow. Some of the
Table 1. Porous and Compositional Properties of Ce-TUD-1 Samples
sample
BET surface area (m2/g)
total pore volume (cm3/g)
average pore diameter (Å)a
Si:Ce ratiob
Ce-TUD-1(20) Ce-TUD-1(50) Ce-TUD-1(100)
556 410 422
1.79 1.77 1.83
144 211 220
19.97 47.16 93.07
a Calculated from the desorption branch, using the BJH model. b Calculated from the Si and Ce concentrations measured by ICP.
Figure 1. HR-TEM images of Ce-TUD-1(20), showing the amorphous cerium-silicate matrix with some crystalline CeO2 particles of 2-5 nm.
properties of three samples with different Si:Ce ratios, Ce-TUD1(20), Ce-TUD-1(50), and Ce-TUD-1(100), are presented in Table 1. The BET surface areas are around 500 m2/g, the pore size distribution exhibits a maximum at around 200 Å, and the total pore volumes are around 1.80 cm3/g, which is in the same range as TUD-1 samples reported previously.7 XRD analysis revealed no peaks at higher angle, which is in accordance with the amorphous nature of the cerium silicate framework and the absence of large CeO2 crystallites, which would be detected by XRD. The peak associated with pores with a diameter of around 200 Å lies outside the detection range of the instrument (2θ > 1°) and could therefore not be detected. High-resolution TEM (HR-TEM) images of Ce-TUD-1(20) show the wormlike structure of the amorphous silica framework, along with 2-5 nm sized crystalline CeO2 particles; see Figure 1. This suggests that, despite attempts to prevent metal-oxide particle formation during synthesis, complete framework incorporation was not achieved at a Si:Ce ratio of 20. No crystalline CeO2 particles were observed in the Ce-TUD-1(100) sample, consistent with complete framework incorporation of all Ce atoms. The
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4223 Table 2. Composition of Ce-TUD-1(20) and Ce-TUD-1(100), Analyzed by EDSa
sample
Region 1
Region 2
Si
Si
Ce
Ce
Bright Spot Si
Ce
Dark Spot Si
Ce
Ce-TUD-1(20) 95.04 4.96 95.54 4.46 65.99 34.01 96.09 3.91 Ce-TUD-1(100) 98.92 1.08 99.02 0.98 97.19 2.81 99.41 0.59 a Broad regions were analyzed by converging the beam to an estimated diameter of 100-150 nm. Small spots (1 nm) were analyzed using darkfield STEM with EDS spectra collected via a 1 nm stationary probe, where bright spots indicate the presence of heavier elements and dark spots indicate the presence of lighter elements.
composition of different regions in both Ce-TUD-1(20) and CeTUD-1(100) samples was analyzed by EDS; see Table 2. The composition of the amorphous regions (analyzed area: 150 nm) in both samples corresponds to the ICP results of both materials; i.e., Si:Ce ratios very close to the expected values of 20 and 100 were found, respectively. The composition of the crystalline particles in Ce-TUD-1(20) was analyzed with dark-field STEM EDS (spot size: 1 nm). The high Ce content in these particles
suggests that some of the Ce precursor species crystallized to give CeO2 during the synthesis of Ce-TUD-1(20). HR-TEM analysis of different sections of the sample showed the number of CeO2 particles to be very small, which is in agreement with the fact that the amorphous regions in the same sample have a Ce concentration corresponding to the composition of the bulk material. Thus, the incorporation of Ce into the amorphous silica framework of the TUD-1 material was achieved completely in Ce-TUD-1(100), whereas in Ce-TUD-1(20) a small portion of Ce was found to be present as nanometer-sized CeO2 particles. Catalytic Testing. (a) Oxidation of p-tert-Butyl Toluene. The oxidation of p-tert-butyl toluene was carried using the same conditions as employed by Auty et al.,11 i.e., using a molar ratio of Ce(IV):substrate:hydrogen peroxide:KBr of 1:45:92:8. The p-tert-butyl toluene conversion over the Ce-TUD-1(20) catalyst was 37%, with a selectivity toward the desired p-tert-butyl benzaldehyde product of 57%. These values are very similar to those reported for cerium(III) acetate, i.e., 37% conversion and
Figure 2. Peroxidative halogenation of Phenol Red. The progress of the reaction was monitored by plotting the increasing relative intensity of the absorption band due to Bromophenol Blue at 590 nm against time.
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 Scheme 1. Catalytic Cycle for the Peroxidative Bromination with M ) V, W, Ti (after ref 16)a
Figure 3. Relative conversion of Phenol Red to Bromophenol Blue over different Ce catalysts.
51% aldehyde selectivity.11 In our hands, using Ce(III) acetate as the catalyst gave a conversion of 30% and an aldehyde selectivity of 56%. When Ce-TUD-1(50) or Ce-TUD-1(100) was used, conversion and aldehyde selectivity values similar to those found for Ce-TUD-1(20) were obtained. In all cases, the main byproduct was p-tert-butyl benzylbromide, while also small quantities of p-tert-butylbenzyl alcohol and p-tert-butyl benzylacetate were observed. These results suggest that the reaction mechanism over the heterogeneous Ce-TUD-1 and the homogeneous Ce(III) acetate catalysts is the same. A regeneration experiment was conducted, where the Ce-TUD-1(20) catalyst was removed by filtration after the reaction and re-used in a second cycle. The filtrate was analyzed for Ce by ICP, revealing that 7% ((2%) of the Ce had leached from the catalyst. The substrate conversion of the recovered catalyst in the second cycle was 90% of that in the first run, with an aldehyde selectivity of 55%. The high activity of Ce-TUD-1, compared to Ce(III) acetate, and the high activity of the heterogeneous catalyst in a consecutive run strongly suggest that the observed catalytic activity is not due to Ce ions leached into the reaction mixture. Research into the heterogeneously catalyzed toluene oxidation, involving Ce catalysts, is ongoing, and results of a more detailed study will be the subject of a subsequent publication. (b) Peroxidative Halogenation of Phenol Red. The peroxidative bromination of phenolsulfonephthalein (Phenol Red) to tetrabromophenolsulfonephthalein (Bromophenol Blue) and the observed changes in the UV-vis spectra are shown in Figure 2. The reaction was monitored by measuring the UV-vis spectrum of the reaction mixtures at regular intervals. The bromination activity of different Ce-TUD-1 samples is shown in Figure 3. The reaction proceeds slowly, especially in the first hour, and complete conversion was achieved only after about 5 h, regardless of the density of the Ce sites in the catalyst. Similar activity was observed using a sample of commercially available CeO2, although initial low reactivity in the first hour was not observed in this case. The bromination mechanism over Ce-based catalysts is different from that over V,15 W,16 and Ti17 catalysts reported in literature. For all these metals, isolated metal sites are required, which form metal-peroxo species upon reaction with hydrogen peroxide. Subsequent reaction with an incoming bromide ion gives an electrophilic “Br+” species,19 which is involved in the electrophilic substitution reaction; see Scheme 1. In the case of Ce(IV), the formation of a metal-peroxo species is highly unlikely; we could find no examples of such complexes in the literature. To check whether Ce-TUD-1 was truly acting as a catalyst and did not serve as a stoichiometric reagent, a series of control experiments were performed. With use of an all-silica TUD-1 sample as the “catalyst”, no reaction was observed. An
a Hydrogen peroxide binds to the metal center in a η2 fashion, giving the metal-peroxo intermediate. Subsequent reaction of an incoming bromide ion results in decomposition of the peroxide bond whereby the bromide ion is oxidized to give a “Br+” species. This electrophile reacts with organic substrate species to give the brominated product.
experiment with a higher substrate-to-catalyst ratio was performed, where Ce-TUD-1(100) (5.0 µmol of Ce) was used with 10 mL of a 1.5 mM Phenol Red solution, where the Ce:substrate ratio was 1:3. It should be noted that as each substrate molecule is brominated four times, the turnover number per Ce atom was in fact 12 in this experiment. The reaction proceeded very slowly, only after 70 h was complete conversion to Bromophenol Blue achieved. The catalyst was removed by filtration and the filtrate was tested for Ce by ICP, which revealed that Ce leaching did not exceed 2%. The catalyst was used in a second cycle, employing the solution containing 0.09 mM Phenol Red, used in the original set of experiments. Complete conversion was achieved after 5 h, just as in the experiment with fresh Ce-TUD-1(100) (see Figure 3), showing that the catalytic activity of Ce-TUD-1 is retained in a consecutive cycle. These control experiments show that the peroxidative bromination reaction is catalytic in Ce-TUD-1. When hydrogen peroxide was omitted from a reaction mixture, also containing Ce-TUD-1, no reaction was observed, showing that both the Ce catalyst and hydrogen peroxide are required for the reaction. The catalytic activity over Ce-TUD-1 is not a function of the Si:Ce ratio, as the results in Figure 3 show. This is in sharp contrast with a corresponding series of Ti-TUD-1 catalysts, where it was found that the reaction rate decreased upon increasing Ti content. This finding is in agreement with the results of Walker et al.17 who found that a 18% Ti/MCM-48 sample had a lower catalytic activity per titanium than the corresponding 10% Ti/MCM-48 material. The difference is ascribed to the presence of titania clusters in the 18% material. Titanium atoms located in these clusters are not available to form titanium-peroxo species and hence are not involved in the catalytic cycle. In the case of Ce, isolated metal atoms are not required for the reaction to occur, as the experiment with pure CeO2 shows. However, we cannot exclude the possibility that different mechanistic pathways occur over bulk CeO2 and CeTUD-1, in which most Ce atoms are incorporated in the silica framework. We propose that the Ce-catalyzed peroxidative bromination occurs via a different mechanism where free radicals play a key role. Ce(IV) is known8 to be an excellent radical generator. The combination of Ce(IV), hydrogen peroxide, and bromide ions is likely to generate bromine radicals, according to reactions (1) and (3):
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4225
Ce4+ + Br- f Ce3+ + Br•
(1)
Ce3+ + H2O2 f Ce4+ + •OH + OH-
(2)
•OH + Br- f OH- + Br•
(3)
Bromination results from initial reaction of Phenol Red with a bromine radical, giving an organic radical and HBr. The brominated product is formed after reaction of the organic radical with Br2, which is thought to be formed by reaction of bromide ions with hydrogen peroxide:
2Br- + H2O2 + 2H+ f Br2 + 2H2O
(4)
Thus, the role of Ce is twofold: Ce(IV) acts as a one-electron oxidant of Br- ions, while the Ce(III) formed upon this reaction generates hydroxyl radicals by reaction with hydrogen peroxide. This proposed radical mechanism differs markedly from the mechanism for V, W, and Ti catalysts depicted in Scheme 1. Concluding Remarks A series of mesoporous cerium-silicate Ce-TUD-1 materials has been prepared using the sol-gel technique. Nitrogen physisorption measurements revealed the mesoporous nature of the materials, with average pore sizes of 144-220 Å. XRD and HR-TEM analysis showed no evidence of large CeO2 crystallites in the materials, although in the case of Ce-TUD-1(20) a small number of 2-5 nm sized crystalline CeO2 regions was observed with HR-TEM. However, EDS analysis of large sections of the amorphous domains of this material, and of Ce-TUD-1(100) with a Si:Ce ratio of 100, revealed Si:Ce ratios very close to those of the corresponding synthesis gels, indicating that only a small amount of the Ce atoms was not present inside the amorphous silica framework. Combined with their low number, the small size of the CeO2 particles in Ce-TUD-1(20) is not expected to have a dramatic negative effect on the catalytic activity, as most Ce atoms will still be exposed to the surface and thus be available for catalysis. The catalytic results for both test reactions studied illustrate this point, as no differences in activity between the samples were observed. The catalytic activity of Ce-TUD-1 in the oxidation reaction of p-tert-butyl toluene was found to be slightly higher than that of homogeneous Ce(III) acetate, with the clear advantage of the ease of catalyst regeneration of the former. Surprisingly, Ce-TUD-1 also appeared to catalyze the peroxidative bromination of Phenol Red. The mechanism according to which bromination occurs is different from that for other catalytic systems reported in the literature, which involve either V, W, or Ti active sites. Instead of metal-peroxo intermediates in those cases, a radical mechanism is proposed to take place over Ce-TUD-1. Bromine radicals are formed in the presence of hydrogen peroxide and Ce(IV), generating organic radical species which react in a subsequent step with elemental bromine to give the brominated product. Literature Cited (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family Of
Mesoporous Molecular-Sieves Prepared With Liquid-Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834. (2) Thomas, J. M. Molecular-Sieves - The Chemistry Of Crystalline Sponges. Nature 1994, 368, 289. (3) Jansen, J. C.; Shan, Z.; Marchese, L.; Zhou, W.; van der Puil, N.; Maschmeyer, T. A new templating method for three-dimensional mesopore networks. Chem. Commun. 2001, 713. (4) Shan, Z.; Gianotti, E.; Jansen, J. C.; Peters, J. A.; Marchese, L.; Maschmeyer, T. One-step synthesis of a highly active, mesoporous, titaniumcontaining silica by using bifunctional templating. Chem.sEur. J. 2001, 7, 1437. (5) Anand, R.; Hamdy, M. S.; Hanefeld, U.; Maschmeyer, T. Liquidphase oxidation of cyclohexane over Co-TUD-1. Catal. Lett. 2004, 95, 113. (6) Hamdy, M. S.; Mul, G.; Jansen, J. C.; Ebaid, A.; Shan, Z.; Overweg, A. R.; Maschmeyer, T. Synthesis, characterization, and unique catalytic performance of the mesoporous material Fe-TUD-1 in Friedel-Crafts benzylation of benzene. Catal. Today 2005, 100, 255. (7) Hamdy, M. S.; Mul, G.; Wei, W.; Anand, R.; Hanefeld, U.; Jansen, J. C.; Moulijn, J. A. Fe, Co and Cu-incorporated TUD-1: Synthesis, characterization and catalytic performance in N2O decomposition and cyclohexane oxidation. Catal. Today 2005, 110, 264. (8) Nair, V.; Balagopal, L.; Rajan, R.; Mathew, J. Recent advances in synthetic transformations mediated by cerium(IV) ammonium nitrate. Acc. Chem. Res. 2004, 37, 21. (9) Roy, S. C.; Guin, C.; Rana, K. K.; Maiti, G. An efficient chemo and regioselective oxidative nuclear bromination of activated aromatic compounds using lithium bromide and ceric ammonium nitrate. Tetrahedron Lett. 2001, 42, 6941. (10) Christoffers, J.; Werner, T.; Unger, S.; Frey, W. Preparation of acyloins by cerium-catalyzed, direct hydroxylation of beta-dicarbonyl compounds with molecular oxygen. Eur. J. Org. Chem. 2003, 425. (11) Auty, K.; Gilbert, B. C.; Thomas, C. B.; Brown, S. W.; Jones, C. W.; Sanderson, W. R. The selective oxidation of toluenes to benzaldehydes by cerium(III), hydrogen peroxide and bromide ion. J. Mol. Catal. A: Chem. 1997, 117, 279. (12) Yao, W. H.; Chen, Y. J.; Min, L.; Fang, H.; Yan, Z. Y.; Wang, H. L.; Wang, J. Q. Liquid oxidation of cyclohexane to cyclohexanol over cerium-doped MCM-41. J. Mol. Catal. A: Chem. 2006, 246, 162. (13) Araujo, A. S.; Aquino, J.; Souza, M. J. B.; Silva, A. O. S. Synthesis, characterization and catalytic application of cerium-modified MCM-41. J. Solid State Chem. 2003, 171, 371. (14) Vilter, H. Peroxidases from Phaeophyceae - a Vanadium(V)Dependent Peroxidase from Ascophyllum-Nodosum. 5. Phytochemistry 1984, 23, 1387. (15) Clague, M. J.; Butler, A. On The Mechanism Of Cis-Dioxovanadium(V)-Catalyzed Oxidation Of Bromide By Hydrogen-Peroxide Evidence For A Reactive, Binuclear Vanadium(V) Peroxo Complex. J. Am. Chem. Soc. 1995, 117, 3475. (16) Sels, B.; De Vos, D.; Buntinx, M.; Pierard, F.; Kirsch-De Mesmaeker, A.; Jacobs, P. Layered double hydroxides exchanged with tungstate as biomimetic catalysts for mild oxidative bromination. Nature 1999, 400, 855. (17) Walker, J. V.; Morey, M.; Carlsson, H.; Davidson, A.; Stucky, G. D.; Butler, A. Peroxidative halogenation catalyzed by transition-metal-iongrafted mesoporous silicate materials. J. Am. Chem. Soc. 1997, 119, 6921. (18) Behrsing, T.; Bond, A. M.; Deacon, G. B.; Forsyth, C. M.; Forsyth, M.; Kamble, K. J.; Skelton, B. W.; White, A. H. Cerium acetylacetonates - new aspects, including the lamellar clathrate [Ce(acac)(4)]‚10H(2)O. Inorg. Chim. Acta 2003, 352, 229. (19) Sels, B. F.; De Vos, D. E.; Buntinx, M.; Jacobs, P. A. Transition metal anion exchanged layered double hydroxides as a bioinspired model of vanadium bromoperoxidase. J. Catal. 2003, 216, 288.
ReceiVed for reView September 13, 2006 ReVised manuscript receiVed November 20, 2006 Accepted November 27, 2006 IE061197S
