Organo-Laponites as Novel Mesoporous Supports for Manganese(III

Langmuir 2005, 21, 10825-10834

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Organo-Laponites as Novel Mesoporous Supports for Manganese(III) salen Catalysts Iwona Kuzńiarska-Biernacka,†,‡ Ana R. Silva,† Ana P. Carvalho,§ Joaõ Pires,*,§ and Cristina Freire*,† REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆ ncias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal, Department of Chemistry, University of Podlasie, 08-110 Siedlce, Poland, and Departamento de Quı´mica e Bioquı´mica and CQB, Faculdade de Cieˆ ncias, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal Received June 16, 2005. In Final Form: August 1, 2005 A Mn(III) salen complex was immobilized onto the Laponite surface using three different methodologies: method A, direct immobilization of the complex on the parent Laponite; method B, covalent anchoring through cyanuric chloride (CC); and method C, covalent anchoring through CC into a 3-aminopropyl)triethoxysilane (APTES) modified Laponite. All of the materials were characterized by FTIR, XPS, thermogravimetry, XRD, and nitrogen isotherms at 77 K, to gather information on the modifications introduced by the organo spacers within the Laponite surface, as well as on the anchored complex integrity; the Mn based materials were screened in the heterogeneous epoxidation of styrene. The results have shown that the immobilization of the manganese(III) salen complex by methods B and C have occurred at the edges of the clay particles through the spacers (APTES and CC) that have been anchored onto the Si-OH groups, whereas in method A, the complex is distributed throughout the clay surface, including the interlayer region. Therefore, the manganese loadings on the Laponites were as follows: materials prepared by method A . method B > method C. All of the heterogeneous catalysts showed high styrene epoxide selectivity, with that prepared by method A showing comparable styrene epoxide selectivity as the homogeneous phase reaction. The styrene epoxide yields decrease in the following order: materials prepared by method A > method B > method C (1st cycles), which parallel the respective support catalytic activity and decreasing of manganese content. The heterogeneous catalysts prepared using methods B and C could be reused at least for four times, with the former exhibiting the most stable catalytic activity, but that prepared by method A showed a significant decrease after two catalytic cycles.

1. Introduction The development of materials with adequate characteristics to be used in the immobilization of homogeneous catalysts is a subject that has been studied in recent years and reviewed by various authors.1-6 The most desired aim in this field is the heterogenisation of homogeneous catalysts capable of promoting enantioselective reactions. However, a number of other relevant chemical reactions also exist that can benefit from the major advantage of immobilization, namely, the easiest separation and the re-usability of the catalyst. Inorganic porous solids have been chosen to act as supports for numerous catalytic centers as they show high chemical inertness toward several reactions, for instance, oxidation reactions. When the homogeneous catalysts are transition metal complexes, several procedures of catalyst immobilization have been developed,1-4 such as, direct covalent bonding of the complex onto the support or by using spacers, physical adsorption, or encapsulation/ entrapment. * To whom correspondence should be addressed. † Universidade do Porto. ‡ University of Podlasie. § Universidade de Lisboa. (1) Fan, Q.-H.; Li, Y.-M.; Chan, A. S. C. Chem. Rev. 2002, 102, 3385. (2) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615. (3) Song, C. E.; Lee, S. Chem. Rev. 2002, 102, 3495. (4) Li, C. Catal. Rev.: Sci. Eng. 2004, 46, 419. (5) Corma, A. Catal. Rev.: Sci. Eng. 2004, 46, 369. (6) Bhattacharjee, S.; Dines, T. J.; Anderson, J. A. J. Catal. 2004, 225, 398.

Clays, either unmodified7,8 or after being pillared,9 have been used for the encapsulation/entrapment of homogeneous inorganic catalysts. Montmorillonite type clays have been the usual choice, mostly due to their availability and adequate characteristics of expandability. More recently, other clays were the object of study, in a limited number of works, as, for instance, the synthetic Laponite clay10,11 which belongs to the family of so-called swelling 2:1 clays. This synthetic material is similar to the smectite type clays, but the octahedral aluminium was substituted by magnesium.12 Additionally, Laponite has regular crystallites of small size, in the range of several nm. These characteristics make Laponite a potential candidate to act as a matrix for the heterogenisation of inorganic catalysts, with the cation exchange process the most used immobilization method.7,10,11,13 On the other hand, Laponite clay also possesses silanol groups, which are located on the edges of the clay sheets,14 (7) Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Ahmad, I.; Singh, S.; Jasra, R. V. J. Catal. 2004, 221, 234. (8) Fernańdez, A. I.; Fraile, J. M.; Garcıá, J. I.; Herrerıás, C. I.; Mayoral, J. A.; Salvatella, L. Catal. Comm. 2001, 2, 165. (9) Pires, J.; Francisco, J.; Carvalho, A.; Carvalho, M. B.; Silva, A. R.; Freire, C.; de Castro, B. Langmuir 2004, 20, 2861. (10) Cornejo, A.; Fraile, J. M.; Garcıá, J. I.; Gil, M. J.; Herrerıás, C. I.; Legarreta, G.; Merino, V. M.; Mayoral, J. A. J. Mol. Catal. A: Chem. 2003, 196, 101. (11) Fraile, J. M.; Garcıá, J. I.; Harmer, M. A.; Herrerıás, C. I.; Mayoral, J. A.; Reiser, O.; Werner, H. J. Mater. Chem. 2002, 12, 3290. (12) Velde, B. Introduction to Clay Minerals; Chapman & Hall: London, 1992. (13) Fraile, J. M.; Garcyá, J. I.; Massam, J.; Mayoral, J. A. J. Mol. Catal. A: Chem. 1998, 136, 47. (14) Wheeler, P. A.; Wang, J.; Baker, J.; Mathias, L. J. Chem. Mater. 2005, 17, 3012.

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which may be used to covalently attach organic compounds (spacers) with derivatization options to further attach the catalytic active species.14-16 In fact, it has been found that local isolation of homogeneous complexes onto a matrix improves catalyst stability by hindering their deactivation processes.1-5 However, little work has been done on edgemodified clays, perhaps because the edge area is small relative to the surface for most clays. Nonetheless, Laponite is an ideal candidate for investigations on edge modification as it possesses a high ratio of edge-to-surface ratio (0.07), enabling its surface organic modification in sufficient amounts.14-16 In the present work, the immobilization of the [Mn(4OHsalophen)Cl] complex on the surface of a synthetic Laponite was studied. The solid material was used either as obtained or after surface modification. This modification was accomplished by reaction with cyanuric chloride and/ or an aminoalkylsilane derivative, 3-aminopropyl)triethoxysilane (APTES). Cyanuric chloride (CC) has been widely used as a linking agent, as it can react with a variety of groups, including hydroxyl and amino functionalized compounds, alkyl and aryl Grignard reagents, and organic hydrazine derivatives.17 The 3-aminopropyl)triethoxysilane (APTES) can also be used as a linking agent, as it has the appropriate functionalities to react with surface support groups (ethoxy groups) and the amino group which can coordinate directly to the metal centers18 or react with specific functionalities within the metal complex ligands.19 Three different methodologies were used in this work to immobilize the Mn(III) salen complex onto Laponite: method A, direct immobilization of the complex onto parent Laponite; method B, covalent anchoring through CC; and method C, covalent anchoring through CC into an APTES modified Laponite. After an extensive characterization of all Laponite-based materials by several techniques, those with the anchored Mn complex were screened in the heterogeneous epoxidation of styrene. With this work, we endeavor to explain how the nature of the linking agent between Laponite and the complex and their relative sizes influence the catalytic activity of the heterogenised metal complexes. To reach this goal, the changes in the textural and morphological properties that occurred upon surface modification of the Laponite were also assessed. Very few studies involving Laponite in the immobilization of catalysts7,10,11,13 have been published, and to the best of our knowledge, this is the first work where the surface of Laponite was modified with the goal of covalent anchoring of metal salen catalysts. 2. Experimental Section 2.1. Parent Material, Solvents, and Reagents. Laponite, from Laporte Industries Ltd., was used as received. Its structural formula, as indicated by the supplier, is Na0.7[(Si8Mg5.5Li0.3)O20(OH)4]. Cyanuric chloride (CC), 3-aminopropyl)triethoxysilane (APTES), styrene, and chlorobenzene were from Aldrich. Iodosylbenzene (PhIO) was synthesized according to procedures described in the literature.20 All solvents were from Merck, except acetonitrile used in catalytic experiments which was from Romil. (15) Herrera, N. N.; Letoffe, J.-M.; Reymond, J.-P.; Bourgeat-Lami, E. J. Mater. Chem. 2005, 15, 863. (16) Bourlinos, A. B.; Jiang, D. D.; Giannelis, E. P. Chem. Mater. 2004, 16, 2404. (17) Lin, A. W. C.; Yeh, P.; Yacynych, A. M.; Kunawa, T. J. Electroanal. Chem. 1977, 84, 411. (18) Baleizaõ, C.; Gigante, B.; Sabater, M. J.; Garcia, H.; Corma, A. Applied Catal. A: Gen. 2002, 228, 279. (19) Kim, G.-J.; Shin, J.-H. Tetrahedron Lett. 1999, 40, 6827. (20) Piaggio, P.; McMorn, P.; Murphy, D.; Bethell, D.; Bulman-Page, P. C.; Hancock, F. E.; Sly, C.; Kertoon, O. J.; Hutchings, G. J. J. Chem. Soc., Perkin. Trans. 2000, 2, 2008.

Kuz´ niarska-Biernacka et al. 2.2. Preparation of Materials. 2.2.1. Modification of Supporting Material. Reaction of Laponite with Cyanuric Chloride. Laponite (A1) (1 g) was added to a CC saturated solution in dry toluene (100 cm3) and the resulting mixture was refluxed for 48 h, under argon atmosphere. The solid, Lap_CC (A3) was separated by centrifugation, washed and dried at 120 °C in a vacuum. Reaction of Laponite with 3-(Aminopropyl)triethoxysilane. Laponite (A1) (3 g) was added to an APTES dry toluene solution (0.9 g in 30 cm3) and the suspension was refluxed for 48 h under argon atmosphere. The solid, Lap_APTES (A5) was separated by centrifugation, washed, and dried at 120 °C in a vacuum. Reaction of Lap_APTES with CC. A suspension of APTES functionalized Laponite (A5) (2 g) was refluxed with a saturated solution of CC in dry toluene (100 cm3) under argon atmosphere. The solid, Lap_APTES_CC (A6), was separated by centrifugation, washed, and dried at 120 °C in a vacuum. 2.2.2. Immobilization of [Mn(4-OHsalophen)Cl]. The synthesis and characterization of the [Mn(4-OHsalophen)Cl] complex has been reported elsewhere.21 [Mn(4-HOsalophen)Cl], chlorine-[N,N’-bis(4-hydroxysalicylaldehyde)phenylenediiminate] manganese(III): MnC20H14N2O4Cl. FAB-HRMS, m/z: calculated ([MnC20H14N2O4-Cl]+) 401.0334, experimental 401.0333. FTIR, υ j /cm-1: 1609 s, 1593 vs, 1576 s, 1546 s, 1497 m, 1435 m, 1374 s, 1327 vw, 1254 s, 1207 s, 1194 s, 1142 m, 1127 m, 986 w, 905 w, 848 m, 806 w, 754 m, 658 m, 609 w, 524 w, 496 m, 405 vw. UV-Vis, λmax/nm: 247, 297, ≈334(i), 420, ≈578(i), ≈690(i). The immobilization of the complex was performed by 3 different methods: Method A: Immobilization of [Mn(4-OHsalophen)Cl] complex onto parent Laponite. Laponite A1 (0.8 g) was added to 100 cm3 of a solution of [Mn(4-OHsalophen)Cl] (1 × 10-3 mol dm3) in dry tetrahydrofuran, and the mixture was refluxed for 48 h. The resulting material, A2, was separated by centrifugation, washed with ethanol, and dried at 120 °C in a vacuum. Method B: Immobilization of [Mn(4-OHsalophen)Cl] complex onto A3. The CC-functionalized Laponite A3 (0.8 g) was added to 100 cm3 of a solution of [Mn(4-OHsalophen)Cl] (1 × 10-3 mol dm3) in dry tetrahydrofuran, and the mixture was refluxed for 48 h. The resulting material, A4, was separated by centrifugation, washed with ethanol, and dried at 120 °C in a vacuum. Method C: Immobilization of [Mn(4-OHsalophen)Cl] complex onto A6. The functionalized Laponite A6 (1.5 g) was added to 100 cm3 of a solution of [Mn(4-OHsalophen)Cl] (1 × 10-3 mol dm3) in dry tetrahydrofuran, and the mixture was refluxed for 48 h. The resulting material, A7, was separated by centrifugation, washed with ethanol, and dried at 120 °C in a vacuum. 2.2.3. Characterization Methods. X-ray diffractograms were obtained with oriented mounts in a Philips PX 1820 instrument using Cu KR radiation. Nitrogen adsorption isotherms at -196 °C were measured in an automatic apparatus (Asap 2010; Micromeritics). Before the adsorption experiments, the samples were outgassed under vacuum during 2.5 h at 150 °C. Microporous volumes were estimated from the t-method and mesoporous volumes from the amounts adsorbed at high relative pressures (p/p0 ∼ 0.97), and specific surface areas were obtained by the BET method.22 Thermogravimetric curves were obtained in a TG-DSC model 111 (Setaram, France), which had sensitivity of 10 µg. The experiments were made under a flux of dry nitrogen with a ramp of 2.5 K/min between 298 and 773 K. The bulk Mn content was determined by atomic absorption spectroscopy in a Pye Unicam SP9 spectrometer. Typically one sample of 20 mg of solid, previously dried at 100 °C, was mixed with 2 cm3 of aqua regia and 3 cm3 of HF for 2 h at 120 °C, in a stainless steel autoclave equipped with a polyethylene-covered beaker (ILC B240). After reaching room temperature the solution was mixed with about 2 g of boric acid and finally adjusted to a known volume with deionized water. X-ray photoelectron spectroscopy was performed at “Centro de Materiais da Universidade do Porto” (Portugal), in a VG (21) Silva, A. R.; Figueiredo, J. L.; Freire, C.; de Castro, B. Micropor. Mesopor. Mater. 2004, 68, 83. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 209.

Organo-Laponites as Novel Mesoporous Supports Scientific ESCALAB 200A spectrometer using a nonmonochromatized Mg KR radiation (1253.6 eV). All of the materials were compressed into pellets prior to the XPS studies. To correct possible deviations caused by electric change of the samples, the C 1s line at 285.0 eV was taken as internal standard. FTIR spectra of the materials were obtained in KBr pellets (Merck, spectroscopic grade) in the range 400-4000 cm-1, with a Jasco FT/IR-460 Plus spectrophotometer; all spectra were collected at room temperature, with a resolution of 4 cm-1 and 32 scans. Diffuse reflectance UV-vis spectra were registered on a Shimadzu UV/3101PC spectrophotometer in the range 1500200 nm. GC-FID chromatograms were obtained with a Varian CP-3380 gas chromatograph using helium as carrier gas and a fused silica Varian Chrompack capillary column CP-Sil 8 CB Low Bleed/ MS (30 m × 0.25 mm i.d.; 0.25 µm film thickness). Conditions used: 60 °C (3 min), 5 °C/min, 170 °C (2 min), 20 °C/min, 200 °C (10 min); injector temperature, 200 °C; detector temperature, 300 °C. 2.2.4. Catalytic Experiments. The catalytic activity of the new materials in the epoxidation of styrene was assessed at room temperature using 0.500 mmol of styrene (substrate), 0.500 mmol of chlorobenzene (GC internal standard), 0.100 g of catalysts, and 0.250 mmol iodosylbenzene (PhIO) as oxidant in 5.00 cm3 of acetonitrile, under stirring. During the experiment, 0.1 cm3 aliquots were taken from the solution with a hypodermic syringe, filtered through 0.2 µm syringe filters, and directly analyzed by GC-FID. After the time needed for total consumption of PhIO (the ratio of areas of iodobenzene and chlorobenzene in chromatogram was constant), the catalyst was removed by centrifugation. To ensure that the epoxidation was only catalyzed heterogeneously, a new portion of PhIO (0.25 mmol) was added to the filtered solution and the composition of reaction media was re-monitored. After utilization, the catalyst was washed/ centrifugated with methanol (five times) and with acetonitrile (two times) to remove occluded reactants and products and then reused using the same experimental conditions described above. The cycle catalysis/washing was performed three times. The acetonitrile solution (after washing) has been checked for the existence of styrene. To assess the eventual catalytic activity of the support itself in the epoxidation of styrene, reactions using the same experimental conditions (vide supra) were also carried out in the presence of the support (0.1 g); only negligible catalytic activity was found using PhIO as oxidant. Identification and quantification of products were made by GC-FID analysis (internal standard method). The assignation was made by comparison with authentic samples; the retention times, under the experimental conditions used, are: chlorobenzene 6.4, styrene 7.5, benzaldehyde 9.7, iodobenzene 12.3, and styrene epoxide 13.1 min.

3. Results and Discussion 3.1. Modification of Laponite Surface. The FTIR spectra of the parent Laponite (A1) and its organo-modified materials (A3, A5, and A6) are shown in Figure 1. The spectrum of the Laponite exhibits two bands due to the presence of physisorbed water, namely the ν(O-H) stretching frequency at 3450 cm-1 and the δ(O-H) deformation band at 1640 cm-1. In the former band, there is a broad shoulder at around 3665 cm-1 which is usually assigned to surface hydroxyl groups. This poorly resolved shoulder is composed by the overlapping of two components corresponding to Si-OH (at ≈3628 cm-1) and Mg-OH stretching vibrations (at ≈ 3686 cm-1).15,16 The spectrum also shows, in the low energy region, one broad band with peak maximum at 1010 cm-1 assigned to Si-O and Si-O-Si stretching vibrations, one band around 660 cm-1 due to O-H bending vibration from adsorbed water, and one band at 550 cm-1 to Mg-O vibration. Upon individual grafting of CC and APTES onto Laponite (Figure 1: materials A3 and A5, respectively), several changes in the FTIR spectra can be observed in all the frequency range. In the high energy range, 3800-

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Figure 1. FTIR spectra of A1, A3, A5, and A6: (a) in the range 4000-500 and (b) in the range 2000-500 cm-1.

2600 cm-1, there is an overall intensity decrease of the broad band centered at 3450 cm-1 and the shoulder at 3686 cm-1 becomes sharper and well defined. This intensity decrease corresponds to a reduction in the adsorbed water content (confirmed by TG data, see below), and a decrease in the intensity of the peak due to Si-OH. The latter result is a consequence of the covalent bonding of APTES and CC to the Si-OH groups via the formation of new Si-O-Si-C and Si-O-C bonds, respectively. The APTES anchoring involves the hydrolysis of the methoxyl groups of the organosilane to silanol groups, by the adsorbed clay water, and then condensation with the accessible surface hydroxyls groups located at the edges of the clays layers, leading to a decrease in the water clay content. For CC, the adsorbed clay water may induce some hydrolysis of the highly reactive CC originating triazine based oligomers and/or cyanuric acid.23 The presence of cyanuric acid can be confirmed by the analysis of the FTIR spectrum of A3 in the region 1800-1650 cm-1 (Figure 1b), where sharp peaks at 1780 and 1726 cm-1 can be observed that are unambiguously assigned to cyanuric acid24 and by the XPS spectrum in the C 1s region (see below, XPS section).23 Further proof of the decrease in water content after grafting of APTES or CC can also be (23) Dautartas, M. F.; Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 51, 104. (24) Samaritani, S.; Peluso, P.; Malanga, C.; Menicagli, R. Eur. J. Org. Chem. 2002, 1551.

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checked by the decrease of the bands at 1640 cm-1 due to δ(O-H) and 660 cm-1 to bend (O-H). Supplementary evidence of the individual anchoring of both spacers can be gathered by the appearance of peaks in the region 2950-2850 cm-1, which are attributed to the ν(C-H) stretching vibrations and between 1500 and 1400 cm-1 due to aliphatic C-H bends of grafted APTES (A5),15,16 and among 1460-1350 cm-1 due to aromatic CdN ring stretching of anchored CC (A3).24 Further modification of material A5 with CC, material A6, leads to a FTIR spectrum quite different from that of A3 and A5, suggesting that CC has mainly reacted with the APTES grafted to Laponite as intended, and not with the remaining Si-OH clay groups; nevertheless some direct reaction of CC with the clay cannot be excluded. Relative to A5, material A6 shows an increase in the intensity and broadness of the typical intense clay bands (Figure 1a): the band centered at 3450 cm-1, the band at 1640 cm-1, and among 1000-1100 cm-1. In the latter region, new shoulders are clearly seen at 1252, 1215, and 1160 cm-1. These observations may suggest an increase in clay water content and a change in the Si-OH groups, but FTIR data do not allow further explanation on these changes; additional insights can be gathered from the techniques described below. Material A6 also shows the same peaks as A3 at 1781 and 1720 cm-1 (Figure 1b) which were assigned to cyanuric acid, the species resulting from the hydrolysis of CC with the clay adsorbed water, suggesting that some hydrolysis of CC has also occurred during the grafting to APTES modified clay. Among the region 1640-1300 cm-1 (Figure 1b), the spectrum of A6 is completely different from those of A3 and A5 as it shows a different band pattern. In this region, we expect to see the changes in the APTES aliphatic C-H bend vibrations due to CC grafting and also new bands due to CC species bound to the free NH2 groups of APTES. Although the band assignment in this region is not straightforward, some tentative assignments can be done considering the following experimental evidences gathered from the literature.24 Pure CC shows strong sharp bands at 1502 and 1275 cm-1 due to the ring stretching vibrations and at 854 and 794 cm-1 due to stretching C-Cl vibrations.24 The selective amination of CC gives triazine derivatives with one to three C-N-R bonds. Their IR spectra show several new bands from 1640 to 1300 cm-1 that can be assigned to ring stretching vibrations, one near 1130 cm-1 to the new C-N-R stretching vibrations, and those at 990, 800, and 700 cm-1 to the nonreacted C-Cl bonds. In this context, the new bands of A6 relative to A5, at 1570, 1519, 1482, 1451, 1414, and 1386 cm-1 (Figure 1b) may include the new aliphatic C-H bend vibrations of APTES bound to CC and the ring stretching vibrations of CC bound APTES. The vibration of the C-N-R bond, which is usually seen around 1130 cm-1, cannot be identified in A6 as it must be overlapped by the broad band due to Si-O and Si-O-Si stretching vibrations. Finally, the new band in A6 at 800 cm-1 (Figure 1b) can be assigned to C-Cl vibrations, suggesting that some of the bound CC have reactive Cl atoms needed for complex anchoring. Thermogravimetric results (not shown) can also give some information on the modifications of the surface of Laponite upon reaction with APTES and CC. The TG curve for the parent Laponite is similar to that usually observed for clays, showing a significant weight loss near 200 °C due to the adsorbed and bound water.12 In A5, the weight loss observed until 200 °C is lower than that observed for the starting Laponite, a confirmation that the silane

Kuz´ niarska-Biernacka et al. Table 1. XPS Results for Obtained Materials Based on Laponite atomic % sample

C

O

N

Mg

Si

A1 A3 A5 A6 A2 A4 A7

11.40 10.42 16.95 16.06 12.58 12.00 12.45

50.88 46.74 43.51 30.41 49.43 49.83 42.09

0.42 1.30 1.65 2.39 0.13 0.71 4.19

14.82 13.33 10.52 4.19 13.68 13.9 9.37

20.21 19.53 20.84 15.47 20.48 20.67 20.82

Cl 1.94 1.20 8.32 1.35 1.43 0.98

Mn

Na

0.36 0.25 0.12

2.26 1.78 2.78 7.64 2.00 1.20 0.25

derivative molecule induces some hydrofobicity in the clay surface, resulting in lower water content for the modified clay. However, this material shows a much more evident weight loss after 300 °C, when compared with the starting Laponite, which is associated with the progressive decomposition of the anchored APTES molecules.25 The material A6 shows a TG curve with higher weight losses in all of the temperature range. Knowing that pure CC and cyanuric acid show weight losses at approximately 110 and 290 °C,26 in A6, the weight losses until 200 °C will have contributions from the physisorbed water within the clay structure, grafted CC and cyanuric acid. For temperatures above 300 °C, the weight losses are due to the simultaneous decomposition of anchored APTES, CC and CC derivatives. No results were obtained for A3 due to the low quantity available. All Laponite-based materials were characterized by XPS; the surface atomic contents are presented in Table 1 and the results from curve fitting for the different regions are summarized in Table 2. Low resolution XPS spectra of the materials show the presence of oxygen, silicon, magnesium, and sodium from the Laponite lattice. The high-resolution XPS spectra of parent Laponite show bands in the Si 2p region at about 102.9 eV, which corresponds to silicon from the tetrahedral sheets of the clay,27 a symmetrical band at 532.1 eV in the O 1s region, due to single bonded oxygen from silica lattice, a band centered at 1303.9 eV in the Mg 1s region, due to the Mg2+ cations present in the octahedral sheets, and a band centered at 1072.6 eV in the Na 1s region, due to the exchangeable Na+ cations within the interlayers. Low intense and broad bands at the C 1s region and N 1s are also observed which are due to the presence of some inorganic impurities resulting from the synthesis of the clay. Upon functionalization of the clay surface with CC (A3), an increase in nitrogen and chloride atomic contents is observed, confirming that the molecule was anchored onto the surface; the surface oxygen content shows a decrease of 8% due to the decrease of water bound clay, as explained before. The main product of CC hydrolysis is cyanuric acid that usually exhibits a peak in the C 1s region at 291.3 eV (Table 2), due to the existence of an amide like tautomer,23 which is clearly seen in the C 1s spectrum of A3. The presence of bound CC and possible triazine dimers can be evidenced by the low intense and broad peak in the C 1s region at 289.4 eV.23 No significant changes in the other elemental surface contents and binding energy values were observed (Table 2) when compared with parent Laponite, suggesting that no major transformations in the clay charge distribution/structure have occurred upon (25) Ahenach, J.; Cool, P.; Impens, R. E. N.; Vansant, E. F. J. Porous Mater. 2000, 7, 475. (26) Wieck, H. J.; Ianniello, R. M.; Osborn, J. O.; Yacynych, A. M. Anal. Chim. Acta 1982, 140, 19. (27) Zhao, D.; Yang, Y.; Guo, X. Inorg. Chem. 1992, 31, 4727.

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Table 2. Curve Fitting Data of the XPS Spectra in the Si 2p, O 1s, Mg 1s, Na 1s, C 1s, N 1s, Cl 2p, and Mn 2p3/2 Regions of the Laponite Materials binding energy (eV)a sample

a

Si 2p

O 1s

Mg 1s

Na 1s

C 1s

A1

102.9 (2.3)

532.1 (2.8)

1303.9 (2.6)

1072.6 (3.1)

A2

104.8 (2.3) 102.7 (1.7)

534.1 (2.5) 532.5 (2.8)

1303.6 (1.8) 1305.7 (2.4)

1074.4 (2.5) 1073.3 (3.3)

A3

102.9 (2.3)

532.1 (2.7)

1303.8 (2.5)

1072.3 (2.7)

A4

103.0 (2.2)

532.1 (2.6)

1305.1 (2.5)

1072.6 (2.7)

A5

103.0 (2.3)

532.3 (2.6)

1304.2 (2.4)

1072.4 (2.5)

A6

103.1 (2.4)

532.4 (2.7)

1304.3 (2.5) 1308.7 (3.9)

1074.5 (2.2) 1072.0 (1.8)

A7

102.9 (2.5)

532.2 (2.9)

1304.0 (2.6)

1072.5 (3.3)

284.6 (2.5) 286.3 (2.5) 288.7 (2.5) 284.6 (2.0) 286.7 (2.3) 288.8 (2.5) 284.7 (2.4) 286.6 (2.5) 289.4 (2.4) 291.6 (2.0) 284.9 (2.3) 286.8 (2.1) 289.2 (2.0) 291.6 (2.4) 285.0 (2.3) 286.5 (2.3) 288.5 (2.0) 284.8 (2.2) 285.9 (2.2) 287.1 (2.5) 284.7 (2.5) 286.2 (2.5) 288.6 (2.5) 292.1 (2.5)

N 1s

Cl 2p

Mn 2p3/2

399.8 (4.0) 401.6 (3.1)

b

400.5 (3.1)

198.6 (2.1) 200.2 (2.5)

400.4 (2.7)

198.9 (2.4) 200.1 (3.5)

399.7 (2.9)

198.5 (2.1) 200.2 (2.4) 199.3 (2.6) 201.1 (1.6) 202.6 (2.0)

399.8 (2.8) 399.9 (3.5)

199.3 (4.0)

643.7 (3.7)

642.4 (3.5)

642.3 (5.2)

Values between brackets refer to the fwhm of the bands. b Not fitted.

CC grafting, which has thus occurred within the edges of the Laponite crystallites. The grafting of APTES onto Laponite (A5) induces an increase in nitrogen and carbon surface contents and a decrease in oxygen content: these observations confirm the presence of APTES and validate that grafting reaction involves the clay adsorbed water. As in the case of A3, no significant changes were observed in the elemental surface contents and binding energies in the different regions (Table 2), suggesting that APTES grafting has also occurred within the edges of Laponite crystallites. Upon functionalization of A5 with CC (material A6), there is an increase in nitrogen as well as of chlorine content (much higher than expected), indicating the presence of bound CC; as in A3, cyanuric acid is present as a byproduct of its hydrolysis, confirmed by the band at 291.3 eV in the C 1s region. One evidence that CC has mainly reacted with the free amine groups of APTES (although direct reaction with clay free Si-OH groups cannot be excluded) can be gathered from the C 1s region: the new peak at 287.1 eV, positioned at lower binding energy than that observed in A3 assigned to Si-O-CC (BE ) 289.4 eV), Table 2, can be attributed to APTESN-CC, based on a similar result observed for the attachment of o-tolidine onto CC graphitic modified electrodes.23 A quite intriguing result is the observation of a significant decrease in the surface contents of Mg and Si, an increase in Na quantity and peak splitting with a shift to higher binding energies for the bands in Cl 2p, Mg 1s, and Na 1s regions (Table 2). These results suggest the presence of intrinsic structural changes within the clay. Taking into consideration that none of these observations were detected for A3 or A5, we propose that these results point toward the existence of chemical reactions between clay and the products of the grafting reaction between CC and anchored APTES. As the main product of this reaction is HCl, we propose that acid attack of the clay has occurred to some extent. In the presence of acid, the Mg-OH groups

Figure 2. X-ray diffraction patterns for the samples: A1, A2, A4, A5, A6, and A7; x axis in nm after conversion of the 2θ values (Bragg law).

can be easily attacked by the proton,28 resulting in the formation of water and the release of Mg2+ from the lattice, after which a second proton becomes attached to the highly nucleophilic silicon-oxygen system. As the di-hydroxilated silica behave as a weak acid, the hydrolyses reaction can still continue releasing silicon from the lattice, as low molecular weight silicates or silicon acid, leading to fresh exposed edges identical to those before acid attack, with Mg-OH and Si-OH groups.28 In this context, a decrease in Mg and Si surface contents and a change in the clay position of Na+ would be expected, and this is what XPS data indicate. Moreover, the high Cl content can be associated with the presence of surface adsorbed Cloriginated from the product reaction HCl. These post grafting structural changes can also be invoked to explain the changes that were observed in the FTIR spectrum of A6 when compared with A5. In Figure 2, the X-ray powder diffraction patterns for the parent Laponite, as well as the organo-modified materials A5 and A6, are shown; no results are given for A3 due to the low quantity available. As can be checked, (28) Grim, R. E. Clay Mineralogy, 2nd ed.; McGraw-Hill: New York, 1968.

10830


Kuz´ niarska-Biernacka et al. Table 3. Specific Surface Areas (ABET) and Microporous (Vmicro) and Mesoporous Volumes (Vmeso) Estimated from the Nitrogen Adsorption Isotherms at -196 °Ca sample

ABET (m2 g-1)

Vmicro (cm3 g-1)

Vmeso (cm3 g-1)

A1 A5 A6 A2 A4 A7

378 122 58 280 314 80

0.03 0 0 0.01 0.04 0

0.25 0.13 0.08 0.22 0.23 0.10

Mn (µmol g-1) beforeb afterb XPSc

123.6 37.9 26.9

118.3 32.6 21.1

183 127 63

a Mn contents, determined by atomic absorption spectroscopy, before and after the catalytic tests, are also given. b Determined by AAS. c Mn amount per weight of sample (before catalytic tests) calculated from XPS data in Table 1: mmol Mn/weight of sample ) at. % Mn/[at. % C × Ar(C) + at. % N × Ar(N) + at. % O × Ar(O) + at. % Mg × Ar(Mg) + at. % Si × Ar(Si) + at. % Cl × Ar(Cl) + at. % Mn × Ar(Mn) + at. % Na × Ar(Na)].

Figure 3. Nitrogen adsorption-desorption isotherms at -196 °C. The labels are the same as in Figure 2.

the organization of the Laponite crystallites was modified after reaction with APTES (A5) and CC (A6). No clear d001 value could be obtained for parent Laponite due to the broadness of the (001) peak, but an approximate value for the interlayer distance of approximately 1.28 nm can be estimated; it should be emphasized that other authors also reported this situation, which was attributed to the very low dimensions of the Laponite crystals29,30 and to the fact that Laponite tends to form partially disordered aggregates through edge-to-edge and edge-toface interactions. The XRD results for A5 and A6 show that, upon grafting of APTES and then CC to Laponite, a pronounced broadening that prevents any clear evaluation of the (001) peak occurs. These results suggest that clay surface modification by the organic molecules proceeds on the outer surfaces of the crystallites and not within their interlayer space and have induced a higher degree in the disorganization of the crystallites, caused by the modification of the edge-to-edge and face-to-edge interactions of the Laponite crystallites. Further information on the textural properties of these materials can be gathered by the nitrogen adsorptiondesorption isotherms at -196 °C in Figure 3. All isotherms present a hysteresis loop, which is related to the presence of mesopores:31 the specific surface areas and pore volumes are given in Table 3. The parent Laponite shows the highest values for ABET, meso and micropore volumes, decreasing upon its functionalization with APTES (A5) and APTES plus CC (A6) as expected. The evolution of the shape of the nitrogen adsorption isotherms, on going from A1 to A7, is in line with the conclusions gathered from XRD, as they also suggest a high degree of disorganization of the organo-modified Laponites. The lowest adsorbed amounts in the sample A6 can be a direct consequence of some disruption of the structure due to the acid attack mentioned above. 3.2. Complex Immmobilization onto Laponites. The FTIR spectra of the complex based Laponite materials (A2, A4, and A7) show, besides the main features of the clay supports, some changes in the preexisting bands due to the spacers (A4 and A7) and new weak bands in the region 1600-1100 cm-1, Figure 4, (where no strong clay (29) Occelli, M. L.; Landau, S. D.; Pinnavaia, T. J. J. Catal. 1984, 90, 256. (30) Cool, P.; Vansant, E. F. Micropor. Mater. 1996, 6, 27. (31) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

vibrations are observed). These latter bands can be assigned to the anchored complex vibrations, as they are quite similar to those of the free complex (see in the Experimental Section):21 1576, 1546, 1496, 1437, 1376, 1252 and 1204 cm-1 for A2 and 1579, 1543, 1494, 1437, 1374 and 1254 cm-1 for A4. In A7, the Mn complex bands are very badly defined, but when combined with the observed big changes in the CC characteristic vibrations in this region indirectly prove the presence of the complex. The frequency of the typical clay bands remain unchanged upon complex immobilization, indicating that no significant changes in the structure of the different supports took place and the changes observed in the characteristic bands of the spacers (A4 and A7) indicates that Mn complex has been mainly anchored through them onto the Laponite surface. The surface atomic contents and Mn bulk analysis of the materials A2, A4, and A7 are presented in Tables 1 and 3; XPS curve fitting values are summarized in Table 2. The XPS spectra of all of the [Mn(4-OHsalophen)Cl] modified materials (A2, A4, and A7) show, besides the typical elements for the clay Laponite, the presence of manganese(III): in the Mn 2p region, a broad band at about 642 eV is observed, which is in agreement with the values reported for manganese(III) complexes with salen32 and porphyrinic33 ligands. The AAS results clearly show that the manganese loadings decrease in the order: A2 > A4 > A7 and allows the estimation of complex anchoring efficiency in the corresponding parent materials which are 86, 26, and 18%, respectively. These results combined with the information on the supports, suggest that in A2 the complex must be immobilized throughout the clay surface, as no specific anchoring sites are present, whereas in the modified Laponites, complex anchoring has been limited to the linkers that have been grafted preferentially at the edges of the clay crystallites. Further insights into the complex localization within the clay can be provided by the comparison of Mn bulk (AAS) and surface (XPS) contents, Table 3. Actually, manganese contents obtained by AAS are always lower than those obtained by XPS, especially for A4 and A7 (70.3 and 57.4% respectively) suggesting that in the two latter materials the manganese(III) salen complex is mainly anchored onto the most external surface of the Laponite, i.e., at the edges of the clay crystallites. (32) Domeńech, A.; Formentin, P.; Garcia, H.; Sabater, M. J. Eur. J. Inorg. Chem. 2000, 1339. (33) Li, Z.; Xia, C.-G.; Zhang, X.-M. J. Mol. Catal. A: Chem. 2002, 185, 47.


Figure 4. FTIR spectra in the range 1700-1000 cm-1 of (a) A1 and A2, (b) A3 and A4, and (c) A6 and A7; in all panels, C corresponds to the IR spectrum of the complex.

XPS spectra can also give some insights into the nature of complex immobilization onto the Laponite supports by analysis of the surface atomic contents and binding energies of the different elements. Direct immobilization of the manganese(III) salen complex onto the Laponite (method A) leads to no changes in the elemental surface contents (only minor decreases in the Na+ and Mg2+ contents are detected, Table 1). However, the bands at Mg 1s, Na 1s, O 1s, and Si 2p regions show peak splitting and peak shifts to higher binding energies (Table 2), suggesting that the complex has diffused into the clay interlayer region and become irreversibly immobilized by strong interactions within the clay sheets at no specific sites. Apparently, no ion exchange

Langmuir, Vol. 21, No. 23, 2005 10831

mechanism can be proposed, since no considerable decrease in the exchangeable cation (Na+) was detected. After complex anchoring onto the A3, material A4 (method B), there is a decrease in the chloride content, suggesting the occurrence of a reaction between the hydroxyl groups of the complex and the reactive chlorines from bound CC; the complex has been anchored through an ether bond, as described in the literature.23,34 The non existence of changes in the Si 2p, Mg 1s, and O 1s regions suggest no major modifications within the clay sheets (no complex intercalation has occurred) and that the complex has been anchored mainly at the edges of the clay crystallites through bound CC. Finally, the complex immobilization onto the A6 modified Laponite, material A7 (method C), leads to a significant decrease in chloride surface content, suggesting the reaction of the complex with APTES-CC, through the formation of an ether bond as described elsewhere.23,34 Relative to A6, there are several changes in elemental surface contents, such as increases in oxygen, magnesium, and silicon contents and a decrease in sodium, but in terms of corresponding band maxima, there are no changes and no peak splitting, except for that of Cl 2p, as expected. The latter observation points toward the reaction of the complex with the spacer grafted at the edges of the clay crystallites. On the other hand, the changes in the elemental contents may indicate some structural rearrangements of the clay crystallites. The proposed structures of the complexes immobilized by the different methodologies are summarized in Scheme 1. The X-ray diffraction patterns for the complex based Laponites A2, A4, and A7 are shown in Figure 2. Direct immobilization of the complex onto the Laponite (A2) leads to a small peak broadening and an increase in the d001 value (from 1.28 to 1.40 nm) when compared to the parent support A1. The former result suggests that no significant changes occurred in the preexistent crystallite face-toface and face-to-edge interactions, but the increase in the d001 value confirms the location of complex within the interlayer region of the clay, in the case of the complex immobilization by method A. As no results could be obtain for CC modified Laponite (A3), no information on the changes induced by the complex on this support could be obtained. For A7, when compared to A6, there is a further broadening of the peak, suggesting some changes in the crystallite edge-to-edge and face-toface interactions upon complex anchoring onto the APTES_CC spacer. Nitrogen adsorption-desorption isotherms at -196 °C (Figure 3) can give further information on the textural properties of these materials. Among A2, A4, and A7, the latter one shows the lowest mesopore volume as is the material with the two linkers, but the trend is less clear for A2 and A4 materials which have similar mesopore volumes (Table 3). This apparent contradiction can be overcome by noting that A2 has a complex content that is 3.6 times higher than A4, but comparison is difficult for samples from different supports. The comparison of ABET and Vmeso values of the complex based Laponites with the corresponding supports also leads to some intriguing results: an increase of these values on going from A6 to A7, contrary to the expected decrease which is observed from A1 to A2. The increase in the mesopore volumes from A6 to A7 is most probably related with the reorganization of the Laponite crystals (34) Silva, A. R.; Freire, C.; de Castro, B.; Freitas, M. M. A.; Figueiredo, J. L. Micropor. Mesopor. Mater. 2001, 46, 211.

10832


Kuz´ niarska-Biernacka et al.

Scheme 1. Anchoring Methods to Heterogenise [Mn(4-OHsalophen)Cl] Complex

that have been submitted to acid attack, upon complex immobilization. 3.3. Catalysis Experiments. The results in the epoxidation of styrene, at room temperature using the manganese(III) salen heterogeneous catalysts and PhIO as oxygen source in acetonitrile, are summarized in Table 4 and Figure 5; data from the homogeneous phase and the blank experiments run under comparable conditions are also included. The results from Table 4 and Figure 5 show that these new materials are active and chemoselective in the epoxidation of styrene. The styrene epoxide selectivity of the A2 heterogeneous catalyst is comparable to the reaction run in homogeneous phase with the free complex, under similar experimental conditions, but A4 and A7 show lower styrene epoxide selectivity values. For the three materials, the styrene epoxide selectivities follow

the order A2 > A4 > A7, and thus there is a decrease in styrene epoxide selectivity upon clay surface functionalization. These results are a consequence of the respective support activity toward styrene conversion, as it also increases upon support functionalization, A1 < A3 < A6, with the parent Laponite exhibiting no significant styrene conversion (1%) and A6 showing about 40%. The increase of the support activity in styrene conversion upon surface functionalization is due to different catalytic pathways involving the modified Laponite matrix, as lead preferentially to the formation of benzaldehyde and other byproducts (Table 4). When it is compared with the homogeneous counterpart, the heterogeneous catalyst prepared by method A (A2) shows slightly higher styrene epoxide yield in the 1st reaction cycle, whereas the other heterogeneous catalysts (A4 and A7) show lower styrene epoxide yield. This result


Langmuir, Vol. 21, No. 23, 2005 10833

Figure 5. Epoxidation of styrene catalyzed by [Mn(4-OHsalophen)Cl] immobilized onto the Laponite based materials with PhIO as oxygen source in acetonitrile: (a) styrene epoxide selectivity % and (b) styrene epoxide yield %.

indicates that the complex immobilized with no linkers, A2 (method A), has a catalytic behavior similar to the free complex, but those that have been anchored through spacers (methods B and C) show a change in their catalytic efficiency. Although, in this case the intrinsic catalytic activity of the respective supports have an important contribution for these results (for example, support A6 has a similar alkene conversion as A7), it must be remembered that spacers used to graft the complexes onto the supports can induce changes in the ligand electronic density, which in turn, can modify the catalytic properties of the metal center; consequently, the choice of the linker is of crucial importance in the heterogenisation of catalytic metal complexes. Reutilization of catalyst A2 led to a significant decrease in styrene epoxide yield, and thus it was not reused further. For the A4 and A7 catalysts, reutilization experiments also gave a progressive reduction of styrene epoxide yield, but not as significant as in the case of the catalyst A2; in addition, catalyst A4 shows an almost constant styrene

epoxide selectivity during the reuse cycles, whereas A7 shows a slight decrease. Hence, Mn(III) complex anchored onto Laponite matrix using the shorter and rigid spacer CC is preferable to the longer spacer APTES and to that just adsorbed at the as-received Laponite surface. The bulk manganese contents of the heterogeneous catalysts after reutilization are similar to those obtained before the catalytic reactions, suggesting that no significant leaching of the active phase has occurred (Table 1). Therefore, the progressive deactivation of the heterogeneous catalysts upon reutilization can be attributed to the formation of non catalytically active complex forms (µ-oxo-Mn(IV) dimer),1 when migration of the metal complexes are allowed as a consequence of the catalytic media, as for example in A2, or to ligand degradation, where apparently no migration of the anchored complexes can occur, A4 and A7. The comparison of the FTIR spectra (not presented) of three heterogeneous catalysts before and after the last reaction cycle showed some band broadening in the 1620-

10834


Kuz´ niarska-Biernacka et al.

Table 4. Epoxidation of Styrene Catalyzed by Homogeneous and Heterogenised [Mn(4-OHsalophen)Cl] Complexa % selectivityf catalyst

%

[Mn(4-OHsalophen)Cl]

Mnb

A2

2.5 1.0 0.5 2.5

A4

1.0

A7

0.5

cycle

1st 2nd 1st 2nd 3rd 4th 1st 2nd 3rd 4th

A1i A3i A6i

tc

(h)

5 3 42 48 48 48 48 48 48 48 48 48 48 48 48 48

Cd,e

SE

B

O

% yield SE

TONg

TOFh (h-1)

57 70 89 66 29 42 30 25 18 35 22 16 20 1 11 40

82 75 92 81 76 65 72 75 72 61 65 56 54 0 7 8

6 4 8 8 19 19 20 21 25 34 34 43 45 100 70 54

12 21 0 11 6 16 9 4 2 5 1 1 1 0 23 38

47 52 82 54 22 27 21 19 13 21 14 9 11 0 1 3

6 24 20 7 7 11 15 18 25 10 12 10 18

1 8 0.5 0.1 0.1 0.2 0.3 0.4 0.5 0.2 0.2 0.2 0.3

%

a In acetonitrile, at room temperature, molar ratio styrene/PhIO ) 2:1. b % of Mn relative to styrene. c Time need for complete consumption of oxidant. d Styrene conversion corrected for the limiting reagent (PhIO). e Determined by GC-FID against internal standard (chlorobenzene). f SE ) styrene epoxide, B ) benzaldehyde, and O ) other reaction products. g Total TON ) mmol epoxide/mmol Mn. h TOF ) TON/reaction time. i Carried out under comparable experimental conditions but with 0.1 g of supporting material.

1200 cm-1 region, which corresponds to the frequency range where vibration bands of the complex occur. On the contrary, the bands typical of the clay matrix do not show significant changes after the catalytic reaction. These observations suggest that no structural changes on the Laponite matrix took place during the consecutive catalytic cycles, but some metal complex decomposition, probably by partial oxidation,13 must occur under the catalytic experimental conditions. Thus, the styrene epoxide yield decrease with reuse of the heterogeneous catalysts might be correlated with some active phase deactivation. It is noteworthy that in the heterogeneous phase reactions time increases when compared with the homogeneous counterparts. This is a general effect that arises upon immobilization of metal complex catalyst in porous matrixes and has been attributed to diffusion constraints imposed on substrates and reactants by the porous network of the matrix.35,36 The diffusion of the reactants to the metal center can be particularly hindered for PhIO, solid with low solubility in acetonitrile, and that solubilization is controlled by its rate of consumption.35 4. Conclusions Three new heterogeneous catalysts have been prepared by immobilization of the [Mn(4-OHsalophen)Cl] complex onto Laponite clay matrix by three different methodologies: direct anchoring (method A) and using spacers, CC, and APTES (methods B and C). The immobilization of the manganese(III) salen complex by methods B and C occurs at the edges of the clay particles, where the spacers have been anchored (Si-OH groups), whereas in method A, the complex is distributed throughout the clay surface, including the interlayer region, Scheme 1.

The heterogeneous catalysts showed slightly higher (A2) and lower (A4 and A7) styrene epoxide yields than the respective reaction run in homogeneous phase with the free complex. The styrene epoxide yields (1st cycle) decrease in the order: A2 > A4 > A7 (1st cycle) which is in agreement with the intrinsic support activity (A1 < A3 < A6) and decreasing of manganese loading. Moreover, A4 and A7 catalysts coud be reused at least for four times with a small decrease in their catalytic activity, but for A2 a significant decrease is observed after the 2nd reuse cycle. Therefore, the heterogeneous catalyst A4 prepared by method B shows the most stable catalytic activity, suggesting that local isolation of the Mn(III) complex on the matrix using the short and rigid spacer CC was preferable relative to the others prepared by methods A and C. FTIR spectra of heterogeneous catalysts after the consecutive catalytic reactions suggest no structural changes of the Laponite matrix occur in the reaction media, but some decomposition of ligand should take place, which must responsible for the decrease in styrene conversion and styrene epoxide yield after the four reuse cycles. The comparison with results from literature, obtained with other matrixes as natural montmorillonite clays, silicas or polymeric supports13,21,35-37 is not straightforward. However, we believe that the methodology described in this work can be of interest for the immobilization of large complexes. Acknowledgment. This work was funded by FCT Fundaç aõ para a Cieˆncia e a Tecnologia (FCT) and FEDER, through the project ref. POCTI/QUI/42931/2001. A.R.S. and I.K.B. thank FCT for a Postdoctoral fellowship. LA051619N

(35) De, B. B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K. Macromolecules 1994, 27, 1291. (36) Silva, A. R.; Vital, J.; Figueiredo, J. L.; Freire, C.; Castro, B. New J. Chem. 2003, 27, 1511.

(37) Angelino, M. D.; Laibinis, P. E. J. Polym. Sci.: A: Polym. Chem. 1999, 37, 3888.

Organo-Laponites as Novel Mesoporous Supports for Manganese(III

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