Materials Characterization 50 (2003) 101 – 108
Sol–gel entrapped cobalt complex Omar J. de Lima a, Andre´a T. Papacı´dero a, Lucas A. Rocha a, He´rica C. Sacco a, Eduardo J. Nassar a, Katia J. Ciuffi a,*, Luciano A. Bueno b, Youne`s Messaddeq b, Sidney J.L. Ribeiro b a
Universidade de Franca, Avenida Armando Salles de Oliveira, 201, CEP 144040-600, Franca, SP Brazil b Instituto de Quı´mica-UNESP CP 355, CEP 14801-970, Araraquara, SP Brazil Received 15 January 2003; accepted 30 March 2003
Abstract This work describes optimized conditions for preparation of a cobalt complex entrapped in alumina amorphous materials in the form of powder. The hybrid materials, CoNHG, were obtained by a nonhydrolytic sol – gel route through condensation of aluminum chloride with diisopropylether in the presence of cobalt chloride. The materials were calcined at various temperatures. The presence of cobalt entrapped in the alumina matrix is confirmed by ultraviolet visible spectroscopy. The materials have been characterized by X-ray diffraction (XRD), surface area analysis, thermogravimetric analysis (TGA), differential thermal analyses (DTA) and transmission electron microscopy (TEM). The prepared alumina matrix materials are amorphous, even after heat treatment up to 750 jC. The XRD, TGA/DTA and TEM data support the increase of sample crystallization with increasing temperature. The specific surface area, pore size and pore diameter changed as a function of the heat treatment temperature employed. Different heat treatment temperatures result in materials with different compositions and structures, and influence their catalytic activity. The entrapped cobalt materials calcined at 750 jC efficiently catalyzed the epoxidation of (Z)-cyclooctene using iodozylbenzene as the oxygen donor. D 2003 Elsevier Inc. All rights reserved. Keywords: Alumina matrix; Heat treatment; Sol – gel
1. Introduction Sol – gel chemistry has been widely developed during the past decade for the preparation of mixed oxides for glasses, ceramics and catalytic applications [1]. Conventional sol – gel routes involve the hydrolysis and condensation of metal alkoxides in controlled pH * Corresponding author. Tel.: +55-16-3711-8871; fax: +55-163711-8878. E-mail addresses:
[email protected] (H.C. Sacco),
[email protected] (K.J. Ciuffi). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00074-3
conditions. Homogeneity at the molecular level is difficult to achieve in the preparation of heterometallic oxide and organic – inorganic hybrids due to the different reactivity of the various precursors toward hydrolysis and condensation [2]. Nonhydrolytic processes [3] are very attractive alternative methods for synthesis of multicomponent oxides where metal halides can react with metal alkoxides through in situ formation of alkyl halide. The arsenal of useful, heterogeneous, sol – gel entrapped metal complexes is growing steadily, covering many of the major families of catalytic reactions
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[2]. Many interesting developments are emerging in the design of recyclable oxidation catalysts containing the active center in constrained environments [4]. Cobalt-based catalysts are suited to produce high yields of long-chain alkenes in Fisher – Tropsch synthesis. The intermediate hydrogenation potential between nickel (active in methanation) and iron (selective in alkenes) appears to be useful for high selectivity in higher-molecular-weight alkane formation, especially when cobalt is supported (on SiO2 or Al2O3) [5,6]. This work describes optimized conditions for preparation of a cobalt complex entrapped in alumina amorphous materials in the form of powder. The hybrid materials, CoNHG, were obtained by a nonhydrolytic sol – gel route, through condensation of aluminum chloride with diisopropylether in the presence of cobalt chloride. The materials were heattreated at various temperatures (25, 750, 1000 and 1100 jC) and characterized by X-ray diffraction (XRD), surface area analysis, thermogravimetric analysis (TGA), differential thermal analyses (DTA) and transmission electron microscopy (TEM).
2. Experimental All solvents and reagents were of commercial grades (Merck and Aldrich) unless otherwise stated. Dichloromethane (DCM) was suspended on anhydrous CaCl2 for 2.5 h, then filtered and distilled over P2O5 and kept over 0.4-nm molecular sieves. The (Z)cyclooctene substrate was purified using a basic alumina column just prior to use. 2.1. Preparation of entrapped cobalt chloride in alumina by nonhydrolytic sol – gel process (CoNHG) The preparation of gels was performed in ovendried glassware. The material was synthesized via modification of the method described by Acosta et al. [3] and Bourget et al. [7]. Aluminum chloride (AlCl3, 1.0 mol l 1) and diisopropylether (iPr2O, 1.5 mol l 1) were reacted in the presence of 8.7 10 6 mol (3.5 mg) of cobalt chloride(II) (CoCl2) under reflux at 110 jC in 50 ml of dry DCM (previously distilled) under argon atmosphere. The gel was formed after 4 h of reaction, and after 0.5 h, a solid
material started to precipitate. After reflux, the mixture was cooled and aged overnight in the mother liquor at room temperature. The solvent was then removed under vacuum. The solid was washed with several solvents (in the following order: DCM, acetonitrile and methanol) and heat-treated at 25, 750, 1000 and 1100 jC to form the samples CoNHG-25, CoNHG-750, CoNHG-1000 and CoNHG-1100, respectively. 2.2. Preparation of CoCl2 entrapped on hydrous alumina Hydrous alumina gel was prepared by reaction of 5.75 g (2.38 10 5 mmol) AlCl36H2O with 10 ml aqueous NH 3 (6 mol l 1) in 10 ml H 2O. The precipitated hydroxide was aged for 20 h and afterwards filtered and washed with pure water. It was then made into a clear sol by peptising with acetic acid under reflux at 90 jC. The sol was cooled and then mixed with 3.5 mg (8.7 10 6 mol) of CoCl2. Gelation was performed by the dehydration of aqueous sol under room temperature. The resultant xerogel was ground and washed with several solvents in the following order: acetone, methanol, water, methanol, acetone and DCM. The amount of leached CoCl2 from hydrous alumina was quantified by measuring the amount of CoCl2 in the combined washings through UV – vis spectroscopy. 2.3. Preparation of supported CoCl2 on commercial alumina CoCl2 supported on commercial neutral alumina was achieved by stirring a DCM solution of CoCl2 with a suspension of alumina for f 45 min. The resulting supported catalyst was washed with several solvents in the following order: acetone, methanol, water, methanol, acetonitrile, acetone and DCM. The amount of leached CoCl2 from support was quantified by measuring the amount of CoCl2 in the combined washings through UV –Vis spectroscopy. The electronic spectra of CoNHG materials were recorded on a UV – Vis spectrophotometer (HewlettPackard 8453, diode array). The spectra of the solid in DCM were recorded in a 2.0-mm path-length cell. Improved UV – Vis spectra were obtained using DCM as solvent when the suspension was prepared.
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TGA were carried out (Thermal Analyst 2100-TA instruments SDT 2960) in air, with a heating rate of 10 jC/min, from 25 to 850 jC. Specific surface areas were determined by analyzing the nitrogen adsorption isotherms according to the BET method [8] using a physical adsorption analyzer (Micrometrics AccSorb 2100E). XRD patterns were collected on a Siemens model D 5005 diffractometer using Cu Ka radiation. Specimens for TEM analysis were prepared by grinding the material into finer particles, which were subsequently deposited on carbon-coated palladium films supported on 300-mesh capped grids. TEM characterization was performed using a Phillips CM 200 transmission electron microscope. Iodosylbenzene (PhIO) was synthesized through the hydrolysis of iodosylbenzenediacetate [9]. The purity was measured by iodometric assay [9]. Controls for cyclooctene oxidation reactions were carried out in the absence of catalyst (CoNHG or homogeneous catalyst CoCl2) and in the presence of NHG alumina without CoCl2. Iodosylbenzene ( f 5.00 mg) was added to a 4-ml vial sealed with a Teflon-coated silicone septum containing 10 mg of CoAl-NHG, 1000 Al 1,2-dichloroethane, 150 Al (Z)-cyclooctene and 5 Al cyclohexanone. The products were analyzed by gas chromatography. These analyses were carried out using a chromatograph (HP 6890) equipped with a hydrogen flame ionization detector and capillary column (length 30 m, internal diameter 0.25 Am). Yields were determined by comparison with authentic samples using calibration curves with cyclohexanone as an internal standard.
and (iPr)2O is heated a monolithic alumina gel is formed, followed by precipitation of an amorphous alumina powder. AlX3 þ 3=2ROR ! AlO3=2 þ 3RX
ð1Þ
where RX = residual alkyl halide groups. The CoNHG materials were prepared by adding the cobalt complex (CoCl2) to a starting solution of AlCl3 and (iPr)2O. Heating the solution in the presence of the cobalt complex leads to the formation of an alumina gel containing the cobalt complex confined in its skeleton (Fig. 1). When the alumina powder precipitates, the cobalt complex remains entrapped inside the alumina network. At this point, the network is supple and further condensation and bond formation occur during aging in the mother liquor and during slow removal of solvent. This stage is of great importance to consolidate the alumina network and the new bonds formed help to restrain the cobalt complex entrapped into the alumina network. After aging, the UV – Vis spectra of the mother liquor did not have the bands characteristic of cobalt complex, indicating that 100% cobalt complex was entrapped in the CoNHG material. We have used the same strategy for the entrapment of metalloporphyrins [10], rare earth ions [11] and Ni complex [12] into alumina nonhydrolytic gels. The CoNHG materials were in the form of a blue powder. Since the pure nonhydrolytic alumina gel is white, the blue color observed in the CoNHG materi-
3. Results and discussion 3.1. Synthesis of sol –gel powder materials The preparation of oxide materials using nonhydrolytic condensation reactions is well documented. Alumina nonhydrolytic gels were first prepared by Acosta et al. [3] and Bourget et al. [7] through the reaction of equimolar amounts of aluminum halide and aluminum alkoxide (Eq. (1)), where the alkoxide can be added to the reaction medium or produced in situ from AlCl3 and (iPr)2O. When a solution of AlCl3
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Fig. 1. Scheme of Co+ 2 entrapped in an alumina matrix.
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als is conferred by the presence of the cobalt complex. The blue color is characteristic of anhydrous cobalt species. Further heat treatment was performed at different temperatures: 25, 750, 1000 and 1100 jC. The resulting blue CoNHG powder was washed with several solvents to remove any CoCl2 not entrapped into the CoNHG. After the final washing, the CoNHG materials remained blue and exhibited UV – Vis spectra characteristic of CoCl2 at 656 nm, which corresponds to the anhydrous species, indicating that the structure of the complex was preserved in the matrix. To confirm that the cobalt complex was entrapped in CoNHG materials and not only adsorbed on the surface of the alumina matrix, the same CoCl2 was adsorbed on the surface of commercial neutral alumina by stirring a suspension of commercial neutral alumina in a solution of CoCl2 in DCM. After anchoring, the cobalt complex was totally leached when the material was washed with DCM. The absence of leaching in the CoNHG materials confirms that in these materials the CoCl2 is entrapped and not only adsorbed on the surface of alumina matrix. The cobalt complex was also entrapped on an alumina matrix using the conventional hydrolytic sol – gel process through the reaction of AlCl36H2O with aqueous NH3 in the presence of the water, but we observed the complete leaching of cobalt from the matrix using this methodology. These data emphasize the importance of using the nonhydrolytic methodology for obtaining the entrapped cobalt complex. 3.2. XRD The X-ray diffractograms (Fig. 2) suggest the initiation of CoNHG crystallization when the sample is heated to 750 jC. Nonhydrolytic alumina materials are reported to be amorphous up to about 750 jC, and crystallization begins at about 850 jC [3,7,10,11]. The DTA of CoNHG samples (Fig. 3) indicates one exothermic transformation at 900 jC, in regions where no weight loss was observed by TGA. This exothermic transformation is known in literature [3,7] to be due to the formation of a more ordered alumina transition phase (g-alumina). The XRD peaks of the CoNHG materials heated at 1100 jC correspond to a structure of cobalt and alumina oxides. The peak definition in the XRD pattern is indicative of material crystallization (Fig.
Fig. 2. XRD patterns obtained from CoNHG heat-treated at (a) 25, (b) 750, (c) 1000 and (d) 1100 jC. (*) Al2O3, (z) CoO.
2). The XRD pattern of the materials heat-treated at 1100 jC showed the presence of very broad peaks indicative of small crystalline particles. The average particle size has been calculated to be about 3 nm using the relation: t ¼ 0:9k=BcoshB where t is the average particle size in angstroms, B is the width of peak at half the peak height in radians, k is the wavelength in angstroms and hB is the Bragg angle in degrees. 3.3. TGA – DTA TGA of CoNHG in air (Fig. 3) revealed a mass loss corresponding to the loosening of water molecules weakly bound in the CoNHG materials [50% (wt.%), between 63 and 200 jC]. These water molecules were probably adsorbed in the material after preparation. We observed weight losses between 200 and 600 jC corresponding to the pyrolysis and the oxidation of residual alkyl halide groups [3,7,10] (see Eq. (1)). 3.4. Nitrogen adsorption isotherms and surface area The porosity and surface properties data for the CoNHG heat-treated at different temperatures determined using the BET method [5] are shown in Table 1. The isotherms of nitrogen adsorption have characteristics of the type II isotherm according to the BDDT classification [13], indicating a larger popula-
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Fig. 3. TGA and DTA in air of CoNHG.
tion of pore size diameter between 2.0 and 50.0 nm. The hysteresis loop of the isotherms of nitrogen absorption changed according to the sample heat treatment temperature. Samples heat-treated at 750 and 1000 jC have an H3-type hysteresis loop according to IUPAC classification [14], suggesting the presence of cones or parallel planes. The CoNHG material calcined at 1100 jC has an H1-type hysteresis loop, suggesting the presence of agglomerates with open cylindrical pores in a more organized structure. These results are in agreement with XRD patterns that predict the crystalline character of the materials heated at 1100 jC. The porosity and surface properties were determined using nitrogen adsorption according to the BET method. Table 1 shows that the hybrid materials have a low surface area, between 1 and 45 m2 g 1. The pore size distribution of the heattreated materials are between 20 and 31 nm (Table 1) depending on the heat treatment temperature used. The calcination temperature controls pore size distribution on the prepared CoNHG materials. The pore size is much larger in the CoNHG heat-treated at 1100 jC. It seems that as the temperature increases, the rearrangement of the network takes place with the
elimination of the organic residues and solvent to yield a solid crystalline structure. Summarizing, we observed that the higher calcination temperature resulted in the decrease in surface area and the increase of pore diameters [15]. In fact, with the higher temperatures, the capillary forces are responsible for shrinkages, damaging and breaking of the pore walls, resulting in larger pore sizes. When the drying is performed at low temperatures, the capillary forces are reduced so that a larger portion of mesopores is preserved [15]. This sensitivity of texture and pore diameters to calcination temperature observed with the CoNHG materials is in agreement with the results reported in the literature for the systems MoSe/ alumina [16]. 3.5. TEM TEM bright-field images and selected-area electron diffraction patterns of the CoNHG materials heattreated at different temperatures are shown in Figs. 4– 7. The micrograph of the sample obtained at 25 jC shows that this material is amorphous, whereas we observed the gradual crystallization of the materials heat-treated at 750 and 1000 jC, and the appearance of crystals in the sample heat-treated at 1100 jC, in agreement with the XRD pattern presented previously. All these data support the increase of sample crystallization with increasing temperature. In contrast to reports in the literature for other materials [17], high-
Table 1 Structural properties of CoNHG heat-treated at different temperatures Sample
BET specific surface area (m2 g 1)
Pore volume (cm3 g 1)
Pore diameter (nm)
CoNHG CoNHG-750 CoNHG-1000 CoNHG-1100
0.8 45 38 10
– 0.23 0.21 0.09
– 20 22 31
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Fig. 4. TEM image of CoNHG-25.
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Fig. 5. (A) Bright-field TEM image, (B) corresponding selected-area electron diffraction pattern and (C) high-resolution TEM image of CoNHG-750.
temperature heating did not cause complete material sintering and the elimination of porosity. At 1100 jC, the material is crystalline and not yet fully dense. 3.6. Oxidation reactions The catalytic activity of the CoNHG materials was tested using (Z)-cyclooctene as paradigmatic substrate and PhIO as oxygen donor. (Z)-Cyclooctene is a reactive alkene that has been used in previous studies both with homogeneous and heterogeneous catalysts;
we observed the formation of only cyclooctene oxide products [18]. Controls for all reactions were carried out in the absence of cobalt. Overall accountability of oxidant was achieved by measuring the iodobenzene (PhI) yield (100% in all reactions), showing that all the oxidant was converted to PhI, and the competitive reaction between the active intermediate and another molecule of PhIO to form PhIO2 did not occur [18]. The catalysis results are shown in Table 2. For comparison of the catalytic activity of CoNHG mate-
Fig. 6. (A) Bright-field TEM image, (B) corresponding selected-area electron diffraction pattern and (C) high-resolution TEM image of CoNHG-1000.
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rials with the cobalt complex in homogeneous solution, all the reaction conditions were maintained constant. The control reactions showed that in the absence of cobalt complex, alumina or CoNHG material, no detectable oxidation of substrate occurred. We observed that alumina without cobalt (NHG) can catalyse cyclooctene oxidation using PhIO as oxygen donor in yields between 4% and 22%, depending on the temperature of material heat treatment. The higher the heat treatment temperature, the lower the product yields. Acosta et al. [3] detected the presence of AlV sites in alumina prepared by nonhydrolytic process in the gels dried at 150 jC, suggesting a special catalyst activity related to them [3]. The AlV pentacoordinates aluminum sites are associated with disordered materials and may be regarded as structural defects, which hinder the transformation of the amorphous solid to an ordered structure [3]. In our samples, the decrease of catalytic yields (Table 2) coincides with the disappearance of AlV sites and crystallization (Fig. 2 and Figs. 4 – 7). Higher catalytic yields (98% cyclooctenoxide) were obtained using the amorphous CoNHG material heat-treated at 750 jC. The crystalline CoNHG material heat-treated at 1100 jC and the CoNHG material prepared at 25 jC have lower specific surface areas (10 and 0.8 m2 g 1, respectively, and 45 m2 g 1 for CoNHG material heat-treated at 750 jC). It is likely that the active catalytic sites in the materials with lower specific surface area are less accessible than in
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Table 2 Catalytic activity of CoNHG obtained at different temperatures (jC) in the oxidation of (Z)-cyclooctene using PhIO as the oxygen donor Catalyst
Oxygen donor
– NHG-25 NHG-750 NHG-1000 NHG-1100 CoCl2 CoNHG-25 CoNHG-750 CoNHG-1000 CoNHG-1100
PhIO PhIO PhIO PhIO PhIO PhIO PhIO PhIO PhIO PhIO
Cyclooctene epoxide yield (%) 1h
24 h
0 9 5 2 1 33 27 17 9 13
0 22 15 8 4 39 69 98 65 46
Reaction conditions: T = 25 jC, magnetic stirring, solvent: 1,2dichloroethane, (Z)-cyclooctene/oxygen donor/Co molar ratio = 1.2 104:100:1, [Co] in both homogeneous or heterogeneous reaction = 3.0 10 4 mol l 1, [oxygen donor] = 5.7 10 3 mol l 1.
the material heat-treated at 750 jC, making diffusion from the heterogeneous catalyst of the product cisepoxycyclooctane to bulk solution difficult [10].
4. Summary A hybrid material containing cobalt complex entrapped in alumina amorphous materials has been successfully prepared. The CoNHG materials were obtained by a nonhydrolytic sol –gel route, through the condensation of aluminum chloride with diisopropylether in the presence of cobalt chloride. The
Fig. 7. (A) Bright-field TEM image, (B) corresponding selected-area electron diffraction pattern and (C) bright-field TEM image of CoNHG1100.
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materials were further characterized after heat treatments at 25, 750, 1000 and 1100 jC. XRD, TGA/DTA and TEM data support the increase of sample crystallization with increasing heat treatment temperature. Specific surface areas, pore sizes and pore diameters changed according to the heat treatment temperature employed. The NHG and CoNHG materials were tested in cyclooctene oxidation reactions. Different heat treatment temperatures result in materials with different composition and structure, and influence their catalytic activity. Higher catalytic yields (98% cyclooctenoxide) were obtained using the amorphous CoNHG material heat-treated at 750 jC. It is likely that the high catalytic activity of this material is correlated with its high surface area (45 m2 g 1).
[5]
[6]
[7]
[8] [9] [10]
[11]
[12]
Acknowledgements Financial support from FAPESP is gratefully acknowledged.
[13]
[14]
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