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RESEARCH NOTES Preparation, Characterization, and Oxidation Catalysis of Polymer-Supported Ruthenium and Cobalt Complexes W. Trakarnpruk* Green Chemisty Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn UniVersity, Bangkok 10330, Thailand
W. Kanjina Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn UniVersity, Bangkok 10330, Thailand
The polymer-anchored Schiff base ligand was prepared by reacting chloromethylated poly(styrene-divinylbenzene) with 5% crosslinking with 4-hydroxybenzaldehyde, followed by condensing with 2-aminopyridine, then loading of ruthenium(III) and cobalt(II) ions in methanol. The loading of metal is 0.7%-0.8%. The polymer-supported Ru and Co complexes were characterized by different techniques, such as Fourier transform infrared (FTIR) and reflectance ultraviolet-visible light (UV-vis) spectroscopy. Thermogravimetric analysis (TGA) revealed the thermal stability of the catalysts. The catalytic activity of the polymer-supported metal catalysts was tested for oxidation of alcohols (1-phenylethanol, cyclohexanol, and benzyl alcohol) and ethylbenzene. The use of tert-butyl hydroperoxide (t-BuOOH) as an oxidant resulted in higher product yield, in comparison to using H2O2 and iodosylbenzene under the same reaction conditions. The solvent, reaction time, and amount of catalyst influenced the catalyst activity and selectivity. The oxidations performed without solvent at 70 °C show high activity. The catalytic activity of the Ru catalyst is greater than that of the Co catalyst. Introduction The oxidation of organic compounds is one of the most important transformations in industrial chemistry. The selective catalytic oxidation of alcohols and ethylbenzene to aldehydes and ketones, which are intermediates for chemicals and specialties, involves the use of transition-metal catalysts.1 Homogeneous ruthenium catalysts are known to oxidize alcohols and other substrates; the ligands in the complexes include salen,2 amine,3 phosphine,4 and other Schiff base ligands.5 The immobilization of homogeneous catalysts onto solid supports provides potential for extending the benefits of heterogeneous catalysts to homogeneous systems. Besides inorganic supports,6 polymeric supports have gained attention because they are inert, nontoxic, nonvolatile, insoluble, and often recyclable. Chloromethylated polystyrene crosslinked with divinylbenzene is one of the most widely used supports. Disadvantages of its use include the possible leaching of the active metal and a decrease in the activity upon recycling. The leaching problem can be prevented through the use of chelating ligands.7 Ruthenium- and cobalt-immobilized complexes have the advantage of better separation from the product.8 Polymeranchored Schiff bases catalysts have been reported.9 For the oxidants, many types have been used, such as molecular oxygen,10 iodosylbenzene,11 and oxone.12 The use of alkyl hydroperoxide is particularly interesting, given its ability to cause the selective oxo-functionalization of aliphatic C-H bonds in the presence of ruthenium catalyst complexes, as well as * To whom correspondence should be addressed. Fax: (662)022187671. E-mail address:
[email protected].
safety and environment concerns that are involved with the use of alkyl hydroperoxide.13 In this work, we report the synthesis and characterization of the polymer-supported ruthenium and cobalt catalysts (hereafter referenced as Ru and Co catalysts). These catalysts can be used for the oxidation of alcohols and ethylbenzene using tert-butyl hydroperoxide (t-BuOOH). The catalysts can be recycled and exhibit high thermal stability. Experimental Section Materials. A chloromethylated styrene-divinylbenzene copolymer with 5% cross-link (17% chlorine content, 16-50 mesh; from Fluka) was washed with methanol and dried under vacuum. RuCl3‚xH2O, CoCl2‚6H2O, t-BuOOH (70% aqueous solution), 1-phenylethanol, benzyl alcohol, cyclohexanol, and ethylbenzene were purchased from Fluka. All other chemicals and solvents were of analytical grade. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet FT-IR Impact 410 spectrophotometer. Elemental analysis was performed using a carbon-hydrogen-nitrogen (CHN) elemental analyzer. The metal content was determined using atomic absorption spectrometry (AAS) (Varian, Model Spectra-AA300). Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TA Instruments, Model SDT 2960) at a heating rate of 10 °C/min under an air/nitrogen (30/20) atmosphere. Ultraviolet-visible light (UV-vis) reflectance spectra of the solid samples were recorded on a spectrophotometer (Shimadzu, Model UV-2550), with reference to nonabsorbing BaSO4 as a standard. The swelling
10.1021/ie070710e CCC: $40.75 © 2008 American Chemical Society Published on Web 01/09/2008
Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 965 Scheme 1. Synthesis of Metal-Supported Poly(styrene-divinylbenzene) Schiff Base Complexes
behavior of the catalysts in polar solvents, as well as nonpolar solvents, was determined as previously described.14 The analysis of products of catalytic oxidation was performed using a gas chromatograph (Varian, Model CP-3800) in conjunction with a flame ionization detector (CP-Wax (30 m × 0.32 mm). Syntheses of Polymer Containing Schiff Base Ligand. To functionalize the polymer support with a Schiff base, chloromethylated poly(styrene-divinylbenzene) was reacted with 4-hydroxybenzaldehyde (4 equiv, with respect to the chlorine content), potassium carbonate, and 18-crown-6 (molar ratio of 20:10:1). The mixture was refluxed under N2 for 24 h. The polymer beads were filtered, washed with dioxane and warm distilled water, and then dried in vacuo. After that, the beads was allowed to swell in methanol, and a solution of 2-aminopyridine and a drop of concentrated HCl was added. The contents were refluxed for 48 h. The color of the polymer beads changed from white to pale yellow, indicating attachment of the ligand. The beads were filtered, washed with methanol and deionized water, and then dried in vacuo. The formation of a Schiff base of the polymer resin was confirmed using analytical and infrared (IR) spectral data. Loading of Metal. The polymer-anchoring ligand (P-L) (10 g) was kept in contact with methanol and a methanol solution (50 mL) of metal chloride (5% w/v) (RuCl3 or CoCl2) was added slowly. The mixture was refluxed for 3 days (8 days in the literature7). The color of the beads changed from pale yellow to dark green in the presence of ruthenium and light green in the presence of cobalt. This indicated the formation of the metal complexes on the polymer matrix. The beads were filtered, washed with methanol (to remove unreacted metal chloride), and dried in vacuo for 24 h at 70 °C. Catalytic Oxidation Reactions. To evaluate the catalytic activity of the polymer-supported metal catalysts, the oxidation of 1-phenylethanol was conducted in the presence of an oxidant. The oxidation was studied by varying the reaction conditions, which included the type of solvent (CH2Cl2, CH3CN, toluene), the type of oxidant (t-BuOOH, H2O2, iodosylbenzene (PhIO)), the temperature (30-90 °C), the reaction time, and the catalyst amount (0.01-0.1 g). The catalyst was allowed to swell in the solvent or substrate (2 mL) for 30 min in a round-bottom flask.
To this was added 10 mmol of 1-phenylethanol, followed by 20 mmol of oxidant. The reaction mixture was stirred at the desired temperature. At the end of the specified time, the contents were analyzed by gas chromatography (GC). The peak positions of the products were matched with the retention times of authentic samples. A control experiment in the absence of a catalyst was also conducted. The oxidation of other substrates such as benzyl alcohol, cyclohexanol, and ethylbenzene using the polymer-supported Ru and Co catalysts was also performed under the same reaction conditions. Test of Catalyst Leaching from Polymer Support. To study the metal leaching in the polymer-supported catalyst, it was refluxed in methanol for 6 h. It then was filtered and the filtrate was treated with substrate and oxidant under the aforementioned reaction conditions. Results and Discussion The polymer-supported Ru and Co catalysts were prepared by subsequent steps: (I) anchor the ligand on the polymer bead, and (II) load the metal on the polymeric ligands. The preparation steps and the proposed structures of metal-supported poly(styrene-divinylbenzene) Schiff base complexes are shown in Scheme 1. They were characterized by FTIR, AAS, elemental analysis, and TGA techniques. Their activity in the oxidation of alcohols and ethylbenzene was evaluated. Characterization of the Catalyst. Elemental analysis of the polymer-supported catalysts confirms the functionalization and metal loading of the polymer (Table 1). The nitrogen content in the catalysts was ∼6.6%-7.2%. The metal loading of cobalt (0.8%) was greater than that of ruthenium (0.7%). The choice of suitable solvent is an important factor in regard to studying the catalytic behavior of polymer-supported catalysts. The extent of swelling is dependent on the solvent-polymer interaction, which is determined not only by the nature of the solvent and polymer matrix but also by the active groups introduced into the polymer matrix. Swelling studies have been conducted for many types of solvents. A decrease in swelling was observed as the nature of solvent was changed from polar
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Table 1. Elemental Analysis of Polymer, Polymer Anchoring Ligand (P-L), and Supported Metal Catalysts Composition (%) compound
C
H
N
Cl
Ma
polymer polymer anchoring ligand Ru catalyst Co catalyst
78.67 75.44 64.02 62.32
6.03 6.11 7.09 5.80
7.26 6.95 6.61
17.30 3.29 NDb NDb
0.8 0.9
a
M ) metal. b ND ) not determined.
Table 2. Swelling Property of the Catalysts Swelling (mol %) solvent
Ru catalyst
Co catalyst
water methanol acetonitrile toluene dichloromethane hexane
2.65 1.50 1.00 0.55 0.38 0.08
2.63 1.44 0.96 0.54 0.36 0.10
to nonpolar (Table 2). This is in good agreement with those reported.5,7 Water was determined to be the solvent that results in the highest swellability of the catalysts. The IR spectra of the P-L and the polymer-supported Ru and Co catalysts are shown in Figure 1. The spectrum of P-L shows an absorption band at 1260 cm-1, which is attributed to the presence of the C-Cl bond of residual chloromethyl in the polymer beads. A strong band at 1640 cm-1 is ascribed to the CdN stretching of the Schiff base ligand. This confirms the attachment of the ligand on the polymer beads. The CdN band undergoes a downshift in the polymer-supported metal catalysts (1634 cm-1).15 Pyridyl ring breathing vibrations are observed at 1510 cm-1. The UV-vis reflectance spectra of the Ru and Co catalysts in the BaSO4 matrix are shown in Figure 2. A band appears at 338 and 328 nm, respectively. This is assigned to a ligand-tometal charge transfer (CT) band.16 The thermal analysis of polymer-supported catalysts is useful to evaluate their applications in high-temperature reactions and to provide proof for the complexation of metal ions with the Schiff base ligand. TGA data are summarized in Table 3. TGA examination of the polymer indicated a single-step degradation in the temperature range of 410-440 °C with a weight loss of 35%. On the other hand, the Ru and Co catalysts degrade at lower temperatures. Figure 3 shows a representative TGA plot of the Co catalyst. Both catalysts show weight losses of
Figure 1. Fourier transform infrared (FTIR) spectra of the polymer anchoring ligand (P-L) and the polymer-supported Ru and Co catalysts.
Figure 2. Reflectance ultraviolet-visible light (UV-vis) analysis of the polymer-supported Ru and Co catalysts. Table 3. Thermogravimetric Analysis (TGA) Data of the Polymer Support and the Catalysts compound
temperature (°C)
weight loss (%)
polymer
410-440
35
Ru catalyst Ru catalyst
420-450 260-310
36 14
Co catalyst Co catalyst
425-450 260-308
36 14
14% and 36% in the temperature ranges of 260-310 °C and 420-450 °C, respectively. The former step is due to the dissociation of the covalently bound ligand or the chloride from the catalyst, as well as partial scission of the polymeric chain, whereas the latter step is resulted from the degradation of the polymer.17 Therefore, the catalysts are thermally stable and could be used in the high-temperature range, up to 100 °C. Catalytic Activity for Oxidation. To determine a suitable oxidant, three types of oxidant were tested: t-BuOOH, H2O2, and PhIO. The oxidation reaction of 1-phenylethanol was conducted using a polymer-supported Ru catalyst. The results are shown in Table 4. The catalytic activity is dependent on the type of oxidant. The oxidation with t-BuOOH gave a higher yield (expressed as a percentage) than H2O2, because t-BuOOH is a more-efficient oxidant, due to weaker O-O bonds, with respect to H2O2. This trend is similar to that which was reported previously.18 PhIO gave the lowest yield. According to the literature, when alcohol was oxidized by PhIO, the suitable amount of oxidant used is 2.5 equiv and acetonitrile is a suitable solvent.19 Therefore, t-BuOOH was selected as the oxidant for further study. To understand the effect of various reaction parameters on catalytic oxidation, a systematic study was performed on the oxidation of 1-phenylethanol as the substrate, using a Ru catalyst and t-BuOOH as an oxidant. The results are shown in Table 5. The results clearly show that the temperature, amount of catalyst, and type of solvent affect the product yield (1-phenylethanol). The blank (control) experiment revealed that no reaction occurred in the absence of the oxidant. When the oxidation of 1-phenylethanol was performed in different solvents, the results show that the catalytic activity is the highest in the absence of a solvent. This is explained by the fact that, in the neat reaction, the polymer-supported catalyst was highly swelled in water (from the aqueous t-BuOOH), as shown by the swelling study in Table 2, so that the substrate and oxidant can easily approach the active sites of the catalyst. For the reactions performed in organic solvents, the results show that, using toluene and dichloromethane, which have less ability to swell the catalyst, moderately high product yield can still be obtained. This is due to the water that is present in the oxidant, which itself swells the catalyst. In contrast, an acetonitrile solvent, which better swells the catalyst, will retard the access
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Figure 3. Thermogravimetric analysis (TGA) curve of the polymer-supported Co catalyst. Table 4. Oxidation Using Different Oxidantsa
Table 6. Oxidation of Various Substratesa
oxidant
yield (%)
H2O2 PhIO t-BuOOH
15 17 100
a
Reaction conditions: 0.025 g Ru catalyst, 10 mmol 1-phenylethanol, 20 mmol oxidant; 70 °C, 6 h. Table 5. Oxidation under Different Reaction Conditionsa catalyst weight (g)
temperature (°C)
time (h)
solvent
yield (%)
0.025 0.025 0.025 0.025
Effect of Solvent 70 6 70 6 70 6 70 6
0.025 0.025 0.025 0.025
Effect of Temperature 30 6 50 6 70 6 90 6
40 91 100 60
0.01 0.025 0.05 0.10
Effect of Catalyst Weight 70 6 70 6 70 6 70 6 70 6
trace 71 100 92 85
CH2Cl2 CH3CN toluene
87 29 95 100
a Reaction conditions: 10 mmol 1-phenyl ethanol, 20 mmol t-BuOOH. Yield is based on the substrate that is taken.
of the substrate and oxidant to the catalyst’s active site, and, therefore, the lowest yield resulted. This trend is in good agreement with that from a previous report.20 In regard to the effect of temperature, the experiments showed that the product yield (expressed as a percentage) increased with temperature. At 70 °C, the product yield is 100%. Further increasing the temperature to 90 °C, the oxidation of 1-phenylethanol was decreased, which was due to the decomposition of t-BuOOH. The catalytic activity of the Ru catalyst was compared using different amounts of catalyst (0.01-0.10 g) at 70 °C and 6 h,
time (h)
substrate
6 6 3
1-phenylethanol cyclohexanol benzyl alcohol
6
ethylbenzene
24 6 6 24
1-phenylethanol cyclohexanol benzyl alcohol ethylbenzene
product
yield (%)
selectivity (%)
Ru Catalyst 1-phenylethanol cyclohexanone benzaldehyde benzoic acid 1-phenylethanol acetophenone
100 65 21 63 50 8
100 100 25 75 86 14
Co Catalyst 1-phenylethanol cyclohexanone benzaldehyde 1-phenylethanol acetophenone
78 50 50 34 8
100 100 100 80 19
a Reaction conditions: 0.025 g catalyst, 10 mmol substrate, 20 mmol t-BuOOH; 70 °C.
in the absence of solvent. The product yield increased as the amount of catalyst increased and was maximum at a catalyst amount of 0.025 g. Further increases in the amount of catalyst resulted in lower yield. Similar results have also been reported.7b,7d One explanation might be the excess amount of catalyst causing a too-rapid decomposition of oxidant before oxidizing the substrate or the mass-transfer limitation; it was observed that stirring is difficult when a large amount of catalyst was used. The ability of the synthesized polymer-supported Ru and Co catalysts to catalyze the oxidation of various alcohols and ethylbenzene were examined in the presence of t-BuOOH as the oxidant. The results are shown in Table 6. The result obtained from oxidation of cyclohexanol to cyclohexanone using both Ru and Co catalysts in this work shows higher yields (65% and 50%, respectively) than that of the reported homogeneous Ru catalyst (27% in 4 h).20 The oxidation of benzyl alcohol with Ru catalyst and t-BuOOH resulted in the formation of benzaldehyde as the main reaction product at the early stages of the reaction. It was further oxidized to benzoic acid. The yield of benzoic acid increased progressively when the reaction was continued for prolonged
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periods of time (80% benzoic acid for 6 h). In contrast, the Co catalyst gave 100% selectivity to benzaldehyde. The oxidation of ethylbenzene was performed using t-BuOOH as an oxidant. The oxidation occurs on the R-carbon of the ethylbenzene, and the main products are R-alcohol (1-phenylethanol) and R-ketone (acetophenone). The product yield and selectivity obtained in this work are higher than that obtained from a polymer-supported Cu catalyst using oxygen as an oxidant (10.7% conversion, 68.2% selectivity to acetophenone; 10 h, 90 °C).7b Heterogeneity of the Reaction. To determine if the oxidations involve a heterogeneous or homogeneous catalysis, the reaction was performed by stirring the catalyst in hot solvent for 6 h, then the catalyst was removed by filtration. The filtrate was added to the substrate and t-BuOOH. The results show that no product occurred; this reveals no metal leaching from either the polymer-supported Ru or Co catalysts and the catalysts behave in a truly heterogeneous manner. Conclusions Polymer-supported Ru and Co catalysts have been successfully prepared and used to catalyze the oxidation reactions of alcohols and ethylbenzene, using t-BuOOH as the oxidant, under mild conditions. The activity and selectivity of the catalysts was dependent on the experimental conditions, solvent, reaction time, and reaction temperature. The Ru catalyst is more active than the Co catalyst. The catalysts show no leaching of metal, and they are heterogeneous in nature. Acknowledgment The authors would like to thank the Graduate School, Chulalongkorn University for partial financial support. Literature Cited (1) Choudhary, D.; Paul, S.; Gupta, R.; Clark, J. H. Green Chem. 2006, 8, 479. (2) Irie, R.; Katsuki, T. Chem. Rec. 2004, 4, 96. (3) Rodriguez, M.; Romero, I.; Llobet, A.; Deronzier, A.; Biner, M.; Parella, T.; Stoechkli-Evans,s H. Inorg. Chem. 2001, 40, 4150. (4) (a) Meijer, R. H.; Ligthart, G. B.W. L.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A.; Mills, A. M.; Kooijman, H.; Spek, A. L. Tetrahedron 2004, 60, 1065. (b) Elzinga, A. J. M.; Li, Y.-X.; Arends, I.
W. C. E.; Sheldon, R. A. Tetrahedron: Asymmetry 2002, 13, 879. (c) Takahashi, M.; Oshima, K.; Matsubara, S. Tetrahedron Lett. 2003, 44, 9201. (d) Chen, W.; Roberts, S. M.; Whittal, J. Tetrahedron Lett. 2006, 47, 4263. (5) (a) Dalal, M. K.; Upadhyay, M. J.; Ram, R. N. J. Mol. Catal., A: Chem. 1999, 142, 325. (b) Ramesh, R. Inorg. Chem. Commun. 2004, 7 (2), 274. (6) (a) Das, B. K.; Clark, J. H. Chem. Commun. 2000, 605. (b) Kockritz, A.; Sebek, M.; Dittmar, A.; Radnik, J.; Bruckner, A.; Bentrup, U.; Pohl, M.-M.; Hugl, H.; Magerlein, W. J. Mol. Catal., A: Chem. 2006, 246, 85. (c) Ebitani, K.; Ji, H.-B.; Mizugaki, T.; Kaneda, K. J. Mol. Catal., A: Chem. 2004, 212, 261. (d) Sheldon, R. A.; Arends, I. W. C. E.; Dijkaman, A. Catal. Today 2000, 57, 157. (7) (a) Antony, R.; Tembe, G. L.; Ravindranathan, M.; Ram, R. N. Eur. Polym. J. 2000, 36, 1579. (b) Wang, Y.; Chang, Y.; Wang, R.; Zha, F. J. Mol. Catal., A: Chem. 2000, 159, 31. (c) Maurya, M. R.; Kumar, M.; Kumar, U. J. Mol. Catal., A: Chem. 2007, 273, 133. (d) Gupta, K.C.; Sutar, A. K. J. Mol. Catal., A: Chem. 2007, 272, 64. (8) (a) Wang, C.-c.; Li, W.-S.; Cheng, S.-K.; Chen, C.-Y.; Chen, C.-Y.; Kuo, J.-F. React. Funct. Polym. 2002, 51, 69. (b) Lei, Z. React. Funct. Polym. 2000, 43, 139. (9) (a) De, B. B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K. Macromolecules 1994, 27, 1291. (b) Antony, R.; Tembe, G. L.; Ravindranathan, M.; Ram, R. N. Polymer 1998, 18, 4327. (10) Gauli, K.; Ram, R. N.; Soni, H. P. J. Mol. Catal., A: Chem. 2005, 242, 165. (11) Che, C. M.; Leung, W. H.; Poon, C. K. J. Chem. Soc., Chem. Commun. 1987, 173. (12) Khan, M. M. T.; Chatterjee, D.; Merchant, R. R. J. Mol. Catal., A: Chem. 1991, 66, 1. (13) Shepherd, R. E. Inorg. Chim. Acta 1993, 209, 201. (14) Thornton, D. A. Coord. Chem. ReV. 1990, 104, 251. (15) Chandra, S.; Gupta, K. Trans. Met. Chem. 2002, 27, 329. (16) Aneetha, H.; Padmaja, J.; Zacharias, P. S. Polyhedron 1996, 15, 2445. (17) Valodkar, V. B.; Tembe, G. L.; Ravindranathan, M.; Rama, H. S. J. Mol. Catal., A: Chem. 2004, 223, 31. (18) (a) Peyrovi, M. H.; Mahdavi, V.; Salehi, M. A.; Mahmoodian, R. Catal. Commun. 2005, 6, 476. (b) Valodkar, V. B.; Tembe, G. L.; Ravindranathan, M.; Ram, R. N.; Rama, H. S. J. Mol. Catal., A: Chem. 2004, 208, 21. (19) Kang, Q.-X.; Luo, J.-J.; Bai, Y.-B.; Yang, Z.-W.; Lei, Z.-Q. J. Organomet. Chem. 2005, 690, 6309. (20) Patel, S. A.; Sinha, S.; Mishra, A. N.; Kamath, B. V.; Ram, R. N. J. Mol. Catal., A: Chem. 2003, 192, 53. (21) Chatterjeea, D.; Mitra, A.; Roy, B. C. J. Mol. Catal., A: Chem. 2000, 161, 17.
ReceiVed for reView May 17, 2007 ReVised manuscript receiVed November 21, 2007 Accepted December 24, 2007 IE070710E
