Polymer supported cobalt(II) catalysts for alkene epoxidation

Applied Catalysis A: General 247 (2003) 295–302

Polymer supported cobalt(II) catalysts for alkene epoxidation Grzegorz Kowalski a,∗ , Jan Pielichowski a , Marek Jasieniak b a

Department of Polymer Science and Technology, Cracow University of Technology, ul. Warszawska 24, Kraków 31-155, Poland b Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia Received 2 June 2002; received in revised form 29 October 2002; accepted 30 January 2003

Abstract A study of polyaniline (PANI), poly-o-toluidine (POT) and poly-o-anisidine (POA) cobalt supported catalysts has been presented. The catalysts were prepared by depositing cobalt species onto the polymers surfaces. Cobalt(II) acetate and N,N -ethylene-bis(salicylideneimine) cobalt(II) (Co(II)Salen) were used in the syntheses. The catalysts performance was examined in epoxidation of trans-stilbene by molecular oxygen and they turned out to be very efficient under mild conditions. The relationship between the surface exposure of cobalt species and the activity of these polymeric catalysts has been established. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer-supported catalysts; Epoxidation; Alkene; Cobalt complexes; Time-of-flight secondary ion mass spectrometry (ToF-SIMS); Scanning electron microscopy (SEM)

1. Introduction A totally new family of materials is in development to contribute important aspects to solving many different technical and ecological problems—the conducting polymers known as the organic metals. Due to their unique electronic properties, electrically conductive polymers are used in heterogeneous catalysis. Polymer supported metal complexes are gaining importance as efficient heterogeneous catalysts in a variety of organic transformations including oxidation. For example, polyaniline was employed as a catalyst support used in oxidation of a spectrum of organic compounds [1]. The polyaniline modifications with palladium salts were also used in dehydrogenative oxidation of 1-decene [2]. Pro´n et al. have used heteropolyanions, which are incorporated into conjugated polymer matrices via chemical doping [3]. These new ∗ Corresponding author. Fax: +48-12-628-2038. E-mail address: [email protected] (G. Kowalski).

catalysts exhibited enhanced activity and selectivity in a test reaction of catalytic ethanol conversion as compared to unsupported heteropolyacids studied under the same conditions [4]. Pielichowski and Iqbal [5], who used conductive polymer supported catalytic systems based on cobalt(II) salts and its complexes, have achieved remarkable results. They work yielded some efficient catalysts in oxidation of aromatic hydrocarbons, alcohols and alkenes with high efficiency at relatively low temperatures. Further research on application of catalysts based on polyaniline and cobalt(II) salts in oxidation reactions was continued by Das and Iqbal [6] as there is much interest in the use of these heterogeneous systems in the chemical industry as an environmentally more acceptable alternative compared with the classical stoichiometric oxidants. In this paper, we report the results on a series of novel conductive polymer supported cobalt catalysts. These catalysts, based on polyaniline (PANI), poly-o-toluidine (POT) and poly-o-anisidine (POA), have been designed and tested in trans-stilbene

0926-860X/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00099-1

296

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

epoxidation. The catalytic activity data were correlated with the cobalt exposures, which were determined by using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and electron dispersive X-ray spectroscopy (EDXS). The effect of the cobalt precursor (cobalt acetate and Co(II)Salen) on the final properties of the catalysts has also been investigated. 2. Experimental 2.1. Supports and cobalt salts PANI, POT and POA were obtained via oxidative polymerization, using the method described elsewhere [7]. Prior to use, the reagent grade aniline, o-toluidine and o-anisidine (POCh Gliwice) were vacuum distilled. N,N -ethylene-bis(salicylideneimine) cobalt(II) (Co(II)Salen) was prepared following the procedure given in [8]. Cobalt chloride and cobalt acetate were obtained from POCh Gliwice, whilst all the other chemicals were purchased from Aldrich and they were used without any further purification. 2.2. Polymer-supported cobalt(II) acetate A mixture of polymer (500 mg) and cobalt acetate (500 mg) was stirred in acetonitrile (25 ml) and acetic acid (25 ml) at room temperature for 72 h. After that, the reaction mixture was filtered and the solid catalyst was washed with acetonitrile (5 × 5 ml). The catalyst was dried at 110 ◦ C for 24 h. 2.3. Polymer-supported Co(II)Salen A mixture of polymer (500 mg) and Co(II)Salen [8] (500 mg) was stirred in 50 ml of acetonitrile for 48 h at room temperature. The reaction mixture was filtered and then the solid product was stirred again with 50 ml of acetic acid at room temperature for 1 h. The catalyst was separated by filtration and washed with acetic acid (3 × 10 ml) followed by acetonitrile (5 × 5 ml). The catalyst was dried at 110 ◦ C for 24 h. 3. Test reaction (trans-stilbene epoxidation) A mixture of 2-methylpropanal (1.08 g, 15 mmol) and polymer-supported catalyst (∼30 mg) was

Table 1 Results of the polymer-supported cobalt catalyst epoxidation of trans-stilbene with molecular oxygena Catalyst

Yield (%)

Reaction time (h)

Temperature (◦ C)

PANI + Co(CH3 COO)2 POT + Co(CH3 COO)2 POA + Co(CH3 COO)2 PANI + Co(II)Salen POT + Co(II)Salen POA + Co(II)Salen

90 93 92 85 85 91

1 1 1 3 3 3

20 20 20 20 20 20

a

Products were characterized by analytical and spectroscopic

data.

dissolved in 30 ml of acetonitrile. The mixture was bubbled through with oxygen for 15 min at room temperature. Subsequently trans-stilbene (0.90 g, 5 mmol) was added. The reaction was carried out under oxygen atmosphere for time period indicated in Table 1. After completion of the reaction, the catalyst was filtered off and the solvent evaporated to yield a residue, which was dissolved in ethyl acetate and washed with the sodium bicarbonate solution and water. The organic phase was dried over MgSO4 and the evaporation of solvent yielded the desired product, which was purified by Kugelrohr distillation.

4. Instrumentation 4.1. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) ToF-SIMS measurements were performed using a PHI TRIFT 2100 time-of-flight secondary ion mass spectrometer (ToF-SIMS) equipped with gallium liquid metal ion gun (LMIG). This instrument allows spectroscopy, for characterization of surface chemical composition, and microscopy (imaging) for characterization of surface chemical heterogeneities. The system uses a pulsed primary ion beam to desorb and ionise species from a sample surface. To retain the molecular information it is essential that primary ion strikes the same surface region once only. This mode of operation is referred to as static SIMS. Static SIMS is uniquely suited for surface

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

characterization of surface-modified polymers. Damage to the uppermost monolayer was minimised in the present work by applying extremely low primary ion fluxes. In order to maintain static conditions, typical analysis time ranged from 4 to 5 min with primary ion doses of around 3 × 1012 ions/ cm2 . Each sample was deposited onto a piece of indium and introduced into the spectrometer preparation

297

chamber. Prior to analysis the samples were outgassed under vacuum for around 2 h. 4.2. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) The specimen analyses were carried out using a CamScan CS44FE field emission scanning electron

Fig. 1. SEM images of (a)PANI + Co(II)Salen; (b) PANI + Co(CH3 COO)2 .

298

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

microscope (SEM) fitted with a CamScan energy dispersive x-ray spectrometer (EDXS). A focussed electron beam is rastered across the sample surface. The secondary or backscattered electrons produced are detected which are used to map the surface topography and compositional contrast that is based on density differences. The X-rays emitted when the electron beam strikes a sample yield information as to the chemical composition of the sample surface (EDXS). The analysis depth of EDXS varies between 0.3 and 4.5 ␮m depending on the material analysed and the primary beam energy. Two polyaniline supported cobalt catalysts [PANI+ Co(CH3 COO)2 and PANI + Co(II)Salen] were examined by SEM/EDXS. Prior to analysis the catalyst samples were coated with conductive carbon films to a thickness of 20–30 nm.

4.3. Gas chromatography (GC) Gas chromatographic analyses were carried out on a Hewlett-Packard HP-5890 Series II gas chromatograph, fitted with an Ultra 1 column type (30 m × 0.25 mm × 0.25 ␮m). 5. Results and discussion The epoxidation reaction results given in Table 1 indicate that PANI, POT and POA are effective as supports for cobalt-based catalysts. 1,2-Diphenyloxirane was the main product of trans-stilbene oxidation with molecular oxygen. The yield of 1,2-diphenyloxirane was catalyst dependant and varied between 85 and 93%. There was very little effect of the support on

Fig. 2. EDXS spectra for: (a) PANI + Co(II)Salen; (b) PANI + Co(CH3 COO)2 (please refer to Fig. 1).

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

the catalyst activity. The results indicate that the catalysts based on cobalt acetate are significantly more active compared to those based on Co(II)Salen. The activity ratio between the cobalt acetate and Co(II)Salen based catalysts, which was estimated using the time required to achieve a similar yield of the main product was 3:1, respectively. As shown in our earlier research polyaniline cobalt supported catalysts turned out to be very efficient in epoxidation of other

299

alkenes [9].The secondary electron images for PANI + Co(CH3 COO)2 and PANI + Co(II)Salen are given in Fig. 1. The micrographs show that these heterogeneous catalysts form the 10–20 ␮m agglomerates consisting of 2–4 ␮m “spheres”. The relevant EDXS spectra in Fig. 2 yield information on the cobalt exposures at the catalysts surfaces. These values for PANI + Co(II)Salen and PANI + Co(CH3 COO)2 were 0.5 and 2.0 at.%, respectively. The results clearly

Fig. 3. Secondary (SEI) and backscattered (BEI) electron images of PANI + Co(CH3 COO)2 . BEI shows no bright spots indicating that the Co species is evenly distributed over the PANI surface.

300

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

indicate that the catalyst activity correlates with the surface exposure of cobalt: the higher the cobalt exposure the higher the catalyst activity. Fig. 3 shows the secondary electron (SE) and backscattered electron (BE) images for PANI + Co(CH3 COO)2 . The low contrast between the catalyst particles and the background (double sided sticky tape) reflects a low surface coverage of the polymer support with cobalt. The BE image is free of any high contrast (high intensity) areas that indicates the cobalt species is evenly distributed on the surface. The positive SSIMS spectra for PANI+Co(II)Salen and PANI + Co(CH3 COO)2 are shown in Fig. 4. The surface exposure of cobalt for PANI + Co(CH3 COO)2 is significantly higher compared to that for PANI + Co(II)Salen. The ToF-SIMS and EDXS results are consistent. The ToF-SIMS images given in Fig. 5 indi-

cate the surface character of the cobalt species. Cobalt originated from acetate forms quite “thick” overlayers on the PANI support, whilst for the Co(II)Salen based system—the cobalt distribution is “patchy”. Since the emission of secondary ions from complex (matrix effect; ionization efficiency) the method of quantification in static SIMS is, however, purely comparative in nature. The results demonstrate that the higher activities of the acetate based catalysts compared to those obtained from Co(II)Salen systems are directly related to the surface exposure of cobalt species. Cobalt acetate seems to be more promising precursor of the conductive polymer supported oxidation catalysts compared to Co(II)Salen. This result likely reflects a low chemical affinity of Co(II)Salen to polyaniline and its derivatives.

Fig. 4. (+) SSIMS spectra for: (a) PANI + Co(II)Salen; (b) PANI + Co(CH3 COO)2 .

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

301

Fig. 5. ToF-SIMS images. Differences in the cobalt distribution on PANI modified with: (a) Co(II)Salen; (b) Co(CH3 COO)2 .

6. Conclusions A series of novel conductive polymer supported cobalt catalysts based on polyaniline and its derivatives (poly-o-toluidine and poly-o-anisidine) and cobalt(II) compounds have been developed. They turned out to be the efficient and selective catalysts for epoxidation of trans-stilbene. The cobalt acetate based systems showed higher activity compared to catalysts obtained from Co(II)Salen. The ToF-SIMS and EDXS results connect these performance characteristics with the catalyst structure and composition. The catalyst activity reflects the exposure of cobalt at the polymer surface. The higher the cobalt exposure the higher the catalyst activity. The distribution of cobalt species on the supports was found to be even.

In view of the results reported here, the conductive polymer supported cobalt catalysts promise to be excellent in promoting oxidation of trans-stilbene by molecular oxygen in high yields under mild conditions. Acknowledgements Authors thank the Polish State Committee for Scientific Research (KBN), grant No. 7. T09B 03721, for financial support. The assistance of Len Green and Nobuyuki Kawashima with the SEM/EDXS investigation is gratefully acknowledged. References [1] (a) T. Hirao, M. Higuchi, I. Ikeda, Y. Ohshiro, J. Chem. Soc., Chem. Commun. (1993) 194–195;

302

G. Kowalski et al. / Applied Catalysis A: General 247 (2003) 295–302

(b) M. Higuchi, D. Imoda, T. Hirao, Macromolecules 29 (1996) 8277–8279. [2] T. Hirao, M. Higuchi, B. Hatano, I. Ikeda, Tetrahedron Lett. 36 (1995) 5295–5298. [3] (a) M. Hasik, W. Turek, E. Stochmal, M. Łapkowski, A. Pro´n, J. Catal. 147 (1994) 544–551; (b) M. Hasik, J. Pó´zniczek, Z. Piwowarska, R. Dziembaj, A. Bielañski, A. Pro´n, J. Mol. Catal. 89 (1994) 329–344; (c) E. Stochmal-Pomarza´nska, M. Hasik, W. Turek, A. Pro´n, J. Mol. Catal. 114 (1996) 267–275. [4] M. Hasik, A. Pro´n, J. Pó´zniczek, A. Biela´nski, Z. Piwowarska, K. Kruczała, R. Dziembaj, J. Chem. Soc., Faraday Trans. 90 (1994) 2099–2106.

[5] J. Pielichowski, J. Iqbal, PL Appl. Patent P-323292 (1997). [6] (a) B.C. Das, J. Iqbal, Tetrahedron Lett. 38 (1997) 1235–1238; (b) B.C. Das, J. Iqbal, Tetrahedron Lett. 38 (1997) 2903–2906; (c) T. Punniyamurthy, J. Iqbal, Tetrahedron Lett. 38 (1997) 4463–4466; (d) E.N. Prabhakaran, J.P. Nandy, S. Shukla, J. Iqbal, Tetrahedron Lett. 42 (2000) 333–337. [7] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 2305–2311. [8] R.H. Bailes, M. Calvin, J. Am. Chem. Soc. 69 (1949) 1886– 1893. [9] G. Kowalski, J. Pielichowski, Synlett (2002) 2107–2109.