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Poly(maleic acid-co-styrene)-triruthenium cluster-catalysed oxidation of styrene with molecular oxygen
Reactive polymers ELSEVIER
Reactive Polymers 23 (1994) 33-38
Short c o m m u n i c a t i o n
Poly(maleic acid-co-styrene)-triruthenium cluster-catalysed oxidation of styrene with molecular oxygen C h e n g - G u o Jia *, F e n g - Y o u Jin, M e i - Y u H u a n g , Y i n g - Y a n Jiang Institute of Chemistry, Academia Smica, Beijing 100080, Peoples Republic of China (Received 5 October 1993; accepted in revised form 17 February 1994)
Abstract
Silica-supported poly(maleic acid-co-styrene)-triruthenium clusters have been prepared by carboxyl exchange of (CzHsCOO)6Ru3(/x3-O)(HzO)fC2HsCOO- with carboxyl groups of silica-supported poly(maleic acid-co-styrene). The supported cluster displays a high activity in the oxidation of styrene under very mild conditions. The main product of the oxidation is benzaldehyde which results from the oxidative cleavage of styrene by molecular oxygen. During the catalytic reaction, the cluster on the support degrades most likely to monoruthenium species such as ((~)-COO)3Ru(V)=O and ((~)-COO)3Ru(III), and the pentavalent ruthenium species may be the catalytically active species in the oxidation. Therefore, the supported cluster could be recycled more than eight times without a noticeable decrease of its activity although it is already fragmented in the first cycle. A possible mechanism for the styrene cleavage is proposed. Key words: Polymer-triruthenium cluster; Selective oxidation; Styrene; Molecular oxygen; Cluster degradation
I. Introduction
Metal cluster catalysts have been widely used in various reactions, such as hydroformylation and hydrogenation, but are rarely used in the catalytic oxidation of organic compounds [1-3]. On the other hand, it is difficult to find a selective catalyst to catalyse the oxidation of organic compounds to aldehydes and not further to acids [4]. Further,
* Corresponding author. Address for correspondence: Department of Organic Chemistry, University of Geneva, CH1211 Geneva 4, Switzerland.
only a few polymer-metal complex oxidation catalysts and even less polymer-supported cluster oxidation catalysts are available [5,6]. Recently, we found that the silica-supported poly(maleic acid-co-styrene)-triruthenium cluster (abbreviated as SiO 2P M S - R u 3) could catalyse the oxidation of styrene to benzaldehyde (yield 50-70%), phenyl acetaldehyde (15-35%) and a small amount of acetophenone ( < 15%) under an atmospheric pressure of oxygen. The oxidation stopped at the stage of aldehydes and did not proceed to form carboxylic acids. Even formaldehyde was released from the reaction mixture and polymerized as a white
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C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
34
film on the inner wall of the neck of the reaction flask:
2.2. Procedure for the oxidation of styrene
PhCH=CH 2 +
The oxidation was carried out under atmospheric pressure of oxygen. Olefin (10 mmol), Ru catalyst (total Ru content in the catalyst charged, 0.1 mmol) and toluene (5 ml) were added to a 25-ml round-bottom flask which was fitted with a reflux condenser (connected to an oxygen burette) and a magnetic stirrer. The reaction flask was placed in a constanttemperature bath (80 + 5°C), and the reaction was monitored by measuring the oxygen uptake. The reaction products were detected by GC using small aliquots of reaction liquid withdrawn with a syringe from the reaction mixture at regular intervals. The recycling of the catalyst was carried out as follows: the supported cluster catalyst was filtered out after a catalytic reaction and before the next cycle, washed with toluene for several times and again added to the reaction system together with styrene and toluene.
0 2 ---->P h C H O
+ -(CH20-)n
( + P h C H 2 C H O + PhCOCH3) The supported cluster catalyst could be recycled for more than eight times without a noticeable decrease of its activity. Fragmentation of the cluster was noticed during the oxidation.
2. Experimental
2.1. General procedures All solvents and olefins were purified prior to use. Infrared (IR) spectra were recorded on a Perkin-Elmer 983 spectrophotometer. X-Ray photoelectron spectroscopic (XPS) data were obtained on a PHI 5300 ESCA system (Perkin-Elmer) using an aluminum cathode (1486.6 eV) as the X-ray source; the system energy calibration was based on the binding energy of the adventitious carbon (284.6 eV). UV-visible spectra were taken on a Hitachi 340 spectrophotometer, and those of the undissolved samples were measured by reflectance techniques using MgO ( < 300 mesh) as standard. The oxidation products were analyzed on a Shanghai 103 gas chromatograph using 2-m steel column of SE-30 on silica (80-100 mesh) at 80°C and a 2.0 k g / c m 2 nitrogen atmosphere. They were identified by comparison with authentic samples which was further confirmed by gas chromatography-mass spectrometry ( G C MS) (GC, HP-5890; MS, HP-5988). The content of ruthenium in the catalyst was measured on a 197-70 Polarized Z e e m a n atomic absorption spectrophotometer. The catalysts were destroyed using a mixture of H N O 3 and HC1 before analyses.
2. 3. Preparation of silica-supported poly (maleic acid-co-styrene) (abbreviated as Si02-PMS) Under a nitrogen atmosphere, maleic anhydride (19.2 g), styrene (19.2 g) and divinyl benzene (4.6 g) were dissolved in benzene (300 ml); then silica (25 g, 370 mZ/g, < 300 mesh) was added along with 2,2-azo-isobutyronitrile (0.14 g). The reaction mixture was kept at 70-80°C for 6-8 h. After filtration, the white solid thus obtained was refluxed with a mixture of tetrahydrofuran (THF) and water (80 ml both) for 12 h. Finally, the support was filtered out, alternately washed with ethanol and THF, and dried at 80°C for 24 h under reduced pressure. The carboxyl content of the support was 4.69 m m o l / g which was measured by back neutralization titration with aqueous NaOH and HC1 solution.
C.-G. Jia et al. /Reactive Polymers 23 (1994) 33-38
2.4. Preparation of the supported cluster (Si0 2-PMS-Ru 3)
35
support. All these findings confirmed the structure of the supported cluster catalyst: ( ( ~ ) - C O O ) x ( C E H s C O O ) 7 x Ru3(/x3-O)(H 20),.
U n d e r a nitrogen atmosphere, (C2H 5C O O ) 6 R u 3 0 ( H 2 0 ) ~ C 2H 5 C O O (0.21 mmol) [7] and SiO2-PMS (1.0 g) were refluxed in ethanol (10 ml) for 15 h. The green solid obtained was filtered out, washed with ethanol for several times and dried at 60°C under reduced pressure for 8 h. The catalyst thus obtained contained 0.28 mmol R u / g of catalyst. (~)-COOH + (C2HsCOO)6Ru30(H
- -
20)+C 2 H s C O O -
((~-COO) (C2HsCOO)6Ru 30(H 20)3
(~)COOH ( ( ~ _ C O O ) 2(C2 HsCOO)5Ru 3 0 ( H 20) 3 ' ( ( ~ - C O O ) . ( C 2 H s C O O ) 7 _ x Ru 3 0 ( H 2 0 ) >, (SiO2-PMS-Ru3) (~)-COOH=SiO 2 - P M S
3. Results and discussion
S i O 2 - P M S - R u 3 was easily prepared by ligand exchange, as shown in the Experimental section, just as the carboxyl exchange of the unsupported cluster with small molecular acid ligands [8]. Its structure was confirmed by IR and UV-visible reflectance spectra based on its homogeneous counterpart. S i O 2 - P M S - R u 3 gave an absorption band with a maximum at 684 nm in the UV-visible reflectance spectrum, in good agreement with the absorption of the unsupported cluster (682 nm) in toluene (Table 1) [7]. The IR spectrum of the supported cluster displayed a band of m e d i u m intensity at ca. 1650 c m - 1 which was expected for the absorption of carboxyl anions and the carboxyl groups bound to ruthenium, and a strong band at 1725 cm-1 assigned to the absorption of the free carboxyl groups of maleic acid in the
The catalytic activity of both the supported and unsupported triruthenium cluster was investigated in the oxidation of olefins. They exhibited a high activity in the oxidation of styrene while they were almost inactive to 1-hexene, 1-heptene, cyclohexene and cyclopentene (only traces of 2-hexanone, 2heptanone, cyclohexanone and cyclopentanone respectively, were detected in the oxidation under the same conditions). This may result from the effect of the structure of olefins. During styrene oxidation, induction periods of ca. 0.2 and 1.0 h were observed for the unsupported and supported cluster catalysts, respectively (Table 2). The slightly longer induction period for the supported cluster may be due to the diffusion of the reactants, such as styrene and oxygen, to the inner sites of the catalytically active species in the framework of the polymer. This is a common p h e n o m e n o n in reactions catalyzed by polymer-supported catalysts [6]. After the induction period, the uptake of oxygen was very fast in both cases, ca. 50
Table 1 Binding energy (BE) of Ru 3p from XPS and maximum absorption band (Amax) from UV-visible spectra for both supported and unsupported triruthenium clusters Catalyst a
BE of Ru 3P3/2 (eV)
Amaxb (nm)
SiO 2 - P M S - R u 3 SiO 2 - P M S - R u 3 (after 2nd reuse) SiO 2 - P M S - R u 3 (after 8th reuse) Ru 3 (in toluene) Ru 3 (in toluene) (after catalytic reaction)
462.2 464.8 467.1 -
684 564
462,5 464.7 467.1
564
-
682 564
a Ru 3 = triruthenium cluster. b The undissolved samples like SiO 2 - P M S - R u 3 were milled with MgO and pressed to pellets for recording the UV-visible reflectance spectra.
C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
36
Table 2 Oxidation of styrene catalyzed by the supported and unsupported triruthenium clusters Catalyst
a
IP b
Time
Conversion
Yield c (%)
(h)
(h)
(%)
PhCHO
PhCH2CHO
PhCOCH 3
SiO2-PMS-Ru 3 1st reuse 2nd reuse 3rd reuse 4th reuse 5th reuse 6th reuse 7th reuse 8th reuse Ru 3
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.2
Ru 3 d
0.2
10 10 10 9 10 11 10 10 12 3 6 12 16
92.2 91.3 87.7 89.6 93.2 91.9 94.1 96.6 100 64.2 74.7 76.2 87.9
65.4 64.1 65.8 66.8 56.8 54.9 66.3 61.3 69.0 39.5 47.2 48.3 56.4
15.3 18.1 19.4 19.8 28.3 34.6 25.1 31.8 18.7 18.7 20.2 20.5 21.8
11.5 9.1 2.5 3.0 8.1 2.7 2.5 3.5 12.3 6.0 7.3 7.4 9.7
a Ru 3 = triruthenium cluster. b IP = induction period; this is the interval between the beginning of the reaction and the m a x i m u m rate of the oxygen uptake. Before IP, the uptake of oxygen is very slow. c The yield was based on styrene. d Re-addition of styrene to the reaction mixture of the u n s u p p o r t e d cluster after the reaction above and connection to the reaction system for an additional 16 h.
m l / h . In the recycles, the supported cluster catalyst kept almost the same activity as the fresh catalyst (Table 2). In the UV-visible spectrum, the supported cluster after the 2nd and 8th reuse gave a broad absorption band with a maximum at 564 nm, while the typical absorption band of the triruthenium cluster at 684 nm disappeared (Table 1). This blue shift of the absorption maximum indicated that the cluster on the support degraded during the oxidation [1]. According to the work of Upadhyay et al. [9], the absorption band at c a . 560 nm belongs to the Ru(V)=O species. Therefore, it is reasonable that one of ruthenium species fragmented from the triruthenium cluster on the support is ( ( ~ ) - C O O ) 3Ru(V)=O, which is believed to be the catalytically active species in various oxidations [12]. In the XPS spectra, two shoulder peaks of R u 3P3/2 in the high-energy area (binding energy = 464.8 and 467.1 eV) (Fig. 1) for the fresh supported cluster became sharp and high in intensity after its 8th reuse, indicating the formation of a high-valent ruthenium
species, which agrees with the results of the UV-visible spectra. On the other hand, the highest peak at 462.2 eV for the fresh catalyst slightly changed to a higher energy area after the 8th recycle (Fig. 1) (possibly this change was in the range of experimental error). This implies that some trivalent m o n o r u t h e n i u m species such as ( ( ~ ) 10
2
0
I
474.4
!
r
470.0 466.0 462.0 Binding energy £n eV
458. O
Fig. 1. XPS spectra of Ru 3p in the supported triruthenium cluster (dashed line) and the same supported cluster after the 8th recycle in the oxidation of styrene (solid line) (zaC = 1.4
eV).
37
C.-G. Jia et al. /Reactive Polymers 23 (1994) 33-38
COO)3Ru(III) were still present on the support after the degradation of the cluster and might be partially converted to the pentavalent catalytically active species by molecular oxygen during the catalytic reactions. Thus the cluster catalyst could be recycled in the oxidation without noticeable decrease of its activity.
of styrene in the range 1.94-2.17 M and the relationship between the initial reaction rate and the partial pressure of oxygen in the range 0.5-1.0 atm, it was found that the oxidation catalyzed by a fixed amount of the unsupported triruthenium cluster (0.33 mmol) was first order in the first case and zero order in the second case:
Lx(/-~3-O)Ru 3 02
r = kCP ° = kC
) L3Ru(V)=O + L3Ru(III)
1lI°
L3Ru(V)=O L = ( ~ ) - C O O , C2H5COO Therefore, the ruthenium species for the supported cluster after the 8th recycle were of mixed valences (see above). The absorption bands of the low-valent ruthenium species, such as Ru(III) in UV-visible spectra, were in the high-energy area and usually overlapped with those of the aromatic compounds [9]. They sometimes also overlapped with silica. Extremely similarly, the reaction mixture of the oxidation catalyzed by unsupported cluster showed an absorption maximum at 564 nm with the disappearance of the band at 682 nm. This reaction mixture was also active during the readdition of styrene (Table 2). Degradation of the triruthenium cluster was not reported when the cluster was bound to the soluble polyethylene carboxylate and used as alcohol oxidation catalyst [10]. We also observed that the triruthenium cluster dissolved in toluene was degraded when heated with oxygen at 80°C, even without styrene. Thus our results were reasonable. In all cases, the main oxidation product was benzaldehyde, resulting from the oxidative cleavage of styrene by molecular oxygen. From detailed investigations of the relationship between the initial reaction rate after the induction period and the concentration
(1)
where r, k, C and P denote the reaction rate, rate constant, concentration of styrene and partial pressure of oxygen, respectively. According to Eq. 1 and the mechanism in the oxidative cleavage of olefins to acids suggested by other authors [11,12], the mechanism in this oxidation can be proposed as follows: Ru + 0 2
fast Ru(O2)
(2) /
Ru(02)
+ PhCH=CH
2
slow
//
R x\u
"1 i i I I
'O "'~" Ph
(3) /'
I I
Ru
1 O ""
'
%'Ph
fast
, Ru + PhCHO + C H 2 0
PhCH=CH 2 + 0 2
(4)
) PhCHO + C H 2 0
where Ru = active ruthenium species, r = k'[PhCH=CH2][Ru(O2)], [Ru] = [Ru(O2)] and k = K'[Ru], hence, r = k C . Reaction 3 may be the slow step that involved a five-center intermediate. The oxidation mechanism catalyzed by the supported cluster catalyst may be similar to the mechanism above because the fundamental chemical steps are essentially the same, whether the oxidation occurs in the coordination sphere of a soluble metal complex or on the supported metal species [12].
38
C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
It is very interesting that a white film was formed on the inner wall of the neck of the reaction flask when the oxidation proceeded: the faster the uptake of oxygen, the faster the film formed. Elemental analysis of the film gave the formula of CH20, in accordance with the composition of polyformaldehyde, indicating that formaldehyde formed during the reaction was released from the reaction mixture. It is well known that the cleavage of olefins can be carried out in the presence of stoichiometric R u O 4 [4] and by RuO2-aldehyde catalyst systems [11]. However, the oxidation products are carboxylic acids and ketones, not aldehydes. In this respect, the two cluster catalysts reported in this paper are much more selective. Besides benzaldehyde, phenyl acetaldehyde and a small amount of acetophenone were also obtained, they arised from the Wacker-type oxidation [12]. The yield of all three products increased with reaction time, indicating the competition between the cleavage and Wacker-type oxidation. The main advantage of the supported cluster catalyst over its homogeneous counterpart was its easy recovery by filtration and stable activity in the recycles. Also, we noticed that the supported cluster was more active during the oxidation (high conversion of styrene) (Table 2) though a slightly longer
induction period was needed. The high activity of the supported cluster may result from the high concentration of ruthenium on the support; the concentration of ruthenium on the support may be higher than that in the solution of the unsupported cluster [6]. In summary, the supported triruthenium cluster catalysts displayed a high activity and stability in the oxidation of styrene. During the oxidation, the cluster degraded to monoruthenium species. 4. References [1] B.C. Gates, L. Guzi and H. Knozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986. [2] G. Schmid, Chem. Rev., 92 (1992) 1709. [3] C.G. Jia, Y.P. Wang, H.Y. Feng, Reactive Polym., 18 (1992) 203. [4] A.H. Haines, Methods for the Oxidation of Organic Compounds, Academic Press, London, 1985. [5] D.C. Sherrington, Pure Appl. Chem., 60 (1988) 401. [6] F.R. Hartly, Supported Metal Complexes, D. Reidel, Dordrecht, 1988. [7] A. Spencer and G. Wikinson, J. Chem. Soc., Dalton, (1972) 1570. [8] A. Spencer and G. Wikinson, J. Chem. Soc., Chem. Commun., (1990) 1467. [9] M.J. Upadhyay, P.K. Bhattacharya, P.A. Ganeshpure and S. Satish, J. Mol. Catal., 80 (1993) 1. [10] D.E. Bergbreiter and D.R. Treadwell, Reactive Polym., 12 (1990) 290. [11] K. Kaneda, S. Haruna, T. Imanaka and K. Kawamoto, J. Chem. Soc., Chem. Commun., (1990) 1467. [12] R.A. Sheldon and J.K. Kochi, Metal Catalyzed Oxidation of Organic Compounds, Academic Press, New York, NY, 1981.
Reactive Polymers 23 (1994) 33-38
Short c o m m u n i c a t i o n
Poly(maleic acid-co-styrene)-triruthenium cluster-catalysed oxidation of styrene with molecular oxygen C h e n g - G u o Jia *, F e n g - Y o u Jin, M e i - Y u H u a n g , Y i n g - Y a n Jiang Institute of Chemistry, Academia Smica, Beijing 100080, Peoples Republic of China (Received 5 October 1993; accepted in revised form 17 February 1994)
Abstract
Silica-supported poly(maleic acid-co-styrene)-triruthenium clusters have been prepared by carboxyl exchange of (CzHsCOO)6Ru3(/x3-O)(HzO)fC2HsCOO- with carboxyl groups of silica-supported poly(maleic acid-co-styrene). The supported cluster displays a high activity in the oxidation of styrene under very mild conditions. The main product of the oxidation is benzaldehyde which results from the oxidative cleavage of styrene by molecular oxygen. During the catalytic reaction, the cluster on the support degrades most likely to monoruthenium species such as ((~)-COO)3Ru(V)=O and ((~)-COO)3Ru(III), and the pentavalent ruthenium species may be the catalytically active species in the oxidation. Therefore, the supported cluster could be recycled more than eight times without a noticeable decrease of its activity although it is already fragmented in the first cycle. A possible mechanism for the styrene cleavage is proposed. Key words: Polymer-triruthenium cluster; Selective oxidation; Styrene; Molecular oxygen; Cluster degradation
I. Introduction
Metal cluster catalysts have been widely used in various reactions, such as hydroformylation and hydrogenation, but are rarely used in the catalytic oxidation of organic compounds [1-3]. On the other hand, it is difficult to find a selective catalyst to catalyse the oxidation of organic compounds to aldehydes and not further to acids [4]. Further,
* Corresponding author. Address for correspondence: Department of Organic Chemistry, University of Geneva, CH1211 Geneva 4, Switzerland.
only a few polymer-metal complex oxidation catalysts and even less polymer-supported cluster oxidation catalysts are available [5,6]. Recently, we found that the silica-supported poly(maleic acid-co-styrene)-triruthenium cluster (abbreviated as SiO 2P M S - R u 3) could catalyse the oxidation of styrene to benzaldehyde (yield 50-70%), phenyl acetaldehyde (15-35%) and a small amount of acetophenone ( < 15%) under an atmospheric pressure of oxygen. The oxidation stopped at the stage of aldehydes and did not proceed to form carboxylic acids. Even formaldehyde was released from the reaction mixture and polymerized as a white
0923-1137/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0923-1 137(94)00028-4
C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
34
film on the inner wall of the neck of the reaction flask:
2.2. Procedure for the oxidation of styrene
PhCH=CH 2 +
The oxidation was carried out under atmospheric pressure of oxygen. Olefin (10 mmol), Ru catalyst (total Ru content in the catalyst charged, 0.1 mmol) and toluene (5 ml) were added to a 25-ml round-bottom flask which was fitted with a reflux condenser (connected to an oxygen burette) and a magnetic stirrer. The reaction flask was placed in a constanttemperature bath (80 + 5°C), and the reaction was monitored by measuring the oxygen uptake. The reaction products were detected by GC using small aliquots of reaction liquid withdrawn with a syringe from the reaction mixture at regular intervals. The recycling of the catalyst was carried out as follows: the supported cluster catalyst was filtered out after a catalytic reaction and before the next cycle, washed with toluene for several times and again added to the reaction system together with styrene and toluene.
0 2 ---->P h C H O
+ -(CH20-)n
( + P h C H 2 C H O + PhCOCH3) The supported cluster catalyst could be recycled for more than eight times without a noticeable decrease of its activity. Fragmentation of the cluster was noticed during the oxidation.
2. Experimental
2.1. General procedures All solvents and olefins were purified prior to use. Infrared (IR) spectra were recorded on a Perkin-Elmer 983 spectrophotometer. X-Ray photoelectron spectroscopic (XPS) data were obtained on a PHI 5300 ESCA system (Perkin-Elmer) using an aluminum cathode (1486.6 eV) as the X-ray source; the system energy calibration was based on the binding energy of the adventitious carbon (284.6 eV). UV-visible spectra were taken on a Hitachi 340 spectrophotometer, and those of the undissolved samples were measured by reflectance techniques using MgO ( < 300 mesh) as standard. The oxidation products were analyzed on a Shanghai 103 gas chromatograph using 2-m steel column of SE-30 on silica (80-100 mesh) at 80°C and a 2.0 k g / c m 2 nitrogen atmosphere. They were identified by comparison with authentic samples which was further confirmed by gas chromatography-mass spectrometry ( G C MS) (GC, HP-5890; MS, HP-5988). The content of ruthenium in the catalyst was measured on a 197-70 Polarized Z e e m a n atomic absorption spectrophotometer. The catalysts were destroyed using a mixture of H N O 3 and HC1 before analyses.
2. 3. Preparation of silica-supported poly (maleic acid-co-styrene) (abbreviated as Si02-PMS) Under a nitrogen atmosphere, maleic anhydride (19.2 g), styrene (19.2 g) and divinyl benzene (4.6 g) were dissolved in benzene (300 ml); then silica (25 g, 370 mZ/g, < 300 mesh) was added along with 2,2-azo-isobutyronitrile (0.14 g). The reaction mixture was kept at 70-80°C for 6-8 h. After filtration, the white solid thus obtained was refluxed with a mixture of tetrahydrofuran (THF) and water (80 ml both) for 12 h. Finally, the support was filtered out, alternately washed with ethanol and THF, and dried at 80°C for 24 h under reduced pressure. The carboxyl content of the support was 4.69 m m o l / g which was measured by back neutralization titration with aqueous NaOH and HC1 solution.
C.-G. Jia et al. /Reactive Polymers 23 (1994) 33-38
2.4. Preparation of the supported cluster (Si0 2-PMS-Ru 3)
35
support. All these findings confirmed the structure of the supported cluster catalyst: ( ( ~ ) - C O O ) x ( C E H s C O O ) 7 x Ru3(/x3-O)(H 20),.
U n d e r a nitrogen atmosphere, (C2H 5C O O ) 6 R u 3 0 ( H 2 0 ) ~ C 2H 5 C O O (0.21 mmol) [7] and SiO2-PMS (1.0 g) were refluxed in ethanol (10 ml) for 15 h. The green solid obtained was filtered out, washed with ethanol for several times and dried at 60°C under reduced pressure for 8 h. The catalyst thus obtained contained 0.28 mmol R u / g of catalyst. (~)-COOH + (C2HsCOO)6Ru30(H
- -
20)+C 2 H s C O O -
((~-COO) (C2HsCOO)6Ru 30(H 20)3
(~)COOH ( ( ~ _ C O O ) 2(C2 HsCOO)5Ru 3 0 ( H 20) 3 ' ( ( ~ - C O O ) . ( C 2 H s C O O ) 7 _ x Ru 3 0 ( H 2 0 ) >, (SiO2-PMS-Ru3) (~)-COOH=SiO 2 - P M S
3. Results and discussion
S i O 2 - P M S - R u 3 was easily prepared by ligand exchange, as shown in the Experimental section, just as the carboxyl exchange of the unsupported cluster with small molecular acid ligands [8]. Its structure was confirmed by IR and UV-visible reflectance spectra based on its homogeneous counterpart. S i O 2 - P M S - R u 3 gave an absorption band with a maximum at 684 nm in the UV-visible reflectance spectrum, in good agreement with the absorption of the unsupported cluster (682 nm) in toluene (Table 1) [7]. The IR spectrum of the supported cluster displayed a band of m e d i u m intensity at ca. 1650 c m - 1 which was expected for the absorption of carboxyl anions and the carboxyl groups bound to ruthenium, and a strong band at 1725 cm-1 assigned to the absorption of the free carboxyl groups of maleic acid in the
The catalytic activity of both the supported and unsupported triruthenium cluster was investigated in the oxidation of olefins. They exhibited a high activity in the oxidation of styrene while they were almost inactive to 1-hexene, 1-heptene, cyclohexene and cyclopentene (only traces of 2-hexanone, 2heptanone, cyclohexanone and cyclopentanone respectively, were detected in the oxidation under the same conditions). This may result from the effect of the structure of olefins. During styrene oxidation, induction periods of ca. 0.2 and 1.0 h were observed for the unsupported and supported cluster catalysts, respectively (Table 2). The slightly longer induction period for the supported cluster may be due to the diffusion of the reactants, such as styrene and oxygen, to the inner sites of the catalytically active species in the framework of the polymer. This is a common p h e n o m e n o n in reactions catalyzed by polymer-supported catalysts [6]. After the induction period, the uptake of oxygen was very fast in both cases, ca. 50
Table 1 Binding energy (BE) of Ru 3p from XPS and maximum absorption band (Amax) from UV-visible spectra for both supported and unsupported triruthenium clusters Catalyst a
BE of Ru 3P3/2 (eV)
Amaxb (nm)
SiO 2 - P M S - R u 3 SiO 2 - P M S - R u 3 (after 2nd reuse) SiO 2 - P M S - R u 3 (after 8th reuse) Ru 3 (in toluene) Ru 3 (in toluene) (after catalytic reaction)
462.2 464.8 467.1 -
684 564
462,5 464.7 467.1
564
-
682 564
a Ru 3 = triruthenium cluster. b The undissolved samples like SiO 2 - P M S - R u 3 were milled with MgO and pressed to pellets for recording the UV-visible reflectance spectra.
C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
36
Table 2 Oxidation of styrene catalyzed by the supported and unsupported triruthenium clusters Catalyst
a
IP b
Time
Conversion
Yield c (%)
(h)
(h)
(%)
PhCHO
PhCH2CHO
PhCOCH 3
SiO2-PMS-Ru 3 1st reuse 2nd reuse 3rd reuse 4th reuse 5th reuse 6th reuse 7th reuse 8th reuse Ru 3
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.2
Ru 3 d
0.2
10 10 10 9 10 11 10 10 12 3 6 12 16
92.2 91.3 87.7 89.6 93.2 91.9 94.1 96.6 100 64.2 74.7 76.2 87.9
65.4 64.1 65.8 66.8 56.8 54.9 66.3 61.3 69.0 39.5 47.2 48.3 56.4
15.3 18.1 19.4 19.8 28.3 34.6 25.1 31.8 18.7 18.7 20.2 20.5 21.8
11.5 9.1 2.5 3.0 8.1 2.7 2.5 3.5 12.3 6.0 7.3 7.4 9.7
a Ru 3 = triruthenium cluster. b IP = induction period; this is the interval between the beginning of the reaction and the m a x i m u m rate of the oxygen uptake. Before IP, the uptake of oxygen is very slow. c The yield was based on styrene. d Re-addition of styrene to the reaction mixture of the u n s u p p o r t e d cluster after the reaction above and connection to the reaction system for an additional 16 h.
m l / h . In the recycles, the supported cluster catalyst kept almost the same activity as the fresh catalyst (Table 2). In the UV-visible spectrum, the supported cluster after the 2nd and 8th reuse gave a broad absorption band with a maximum at 564 nm, while the typical absorption band of the triruthenium cluster at 684 nm disappeared (Table 1). This blue shift of the absorption maximum indicated that the cluster on the support degraded during the oxidation [1]. According to the work of Upadhyay et al. [9], the absorption band at c a . 560 nm belongs to the Ru(V)=O species. Therefore, it is reasonable that one of ruthenium species fragmented from the triruthenium cluster on the support is ( ( ~ ) - C O O ) 3Ru(V)=O, which is believed to be the catalytically active species in various oxidations [12]. In the XPS spectra, two shoulder peaks of R u 3P3/2 in the high-energy area (binding energy = 464.8 and 467.1 eV) (Fig. 1) for the fresh supported cluster became sharp and high in intensity after its 8th reuse, indicating the formation of a high-valent ruthenium
species, which agrees with the results of the UV-visible spectra. On the other hand, the highest peak at 462.2 eV for the fresh catalyst slightly changed to a higher energy area after the 8th recycle (Fig. 1) (possibly this change was in the range of experimental error). This implies that some trivalent m o n o r u t h e n i u m species such as ( ( ~ ) 10
2
0
I
474.4
!
r
470.0 466.0 462.0 Binding energy £n eV
458. O
Fig. 1. XPS spectra of Ru 3p in the supported triruthenium cluster (dashed line) and the same supported cluster after the 8th recycle in the oxidation of styrene (solid line) (zaC = 1.4
eV).
37
C.-G. Jia et al. /Reactive Polymers 23 (1994) 33-38
COO)3Ru(III) were still present on the support after the degradation of the cluster and might be partially converted to the pentavalent catalytically active species by molecular oxygen during the catalytic reactions. Thus the cluster catalyst could be recycled in the oxidation without noticeable decrease of its activity.
of styrene in the range 1.94-2.17 M and the relationship between the initial reaction rate and the partial pressure of oxygen in the range 0.5-1.0 atm, it was found that the oxidation catalyzed by a fixed amount of the unsupported triruthenium cluster (0.33 mmol) was first order in the first case and zero order in the second case:
Lx(/-~3-O)Ru 3 02
r = kCP ° = kC
) L3Ru(V)=O + L3Ru(III)
1lI°
L3Ru(V)=O L = ( ~ ) - C O O , C2H5COO Therefore, the ruthenium species for the supported cluster after the 8th recycle were of mixed valences (see above). The absorption bands of the low-valent ruthenium species, such as Ru(III) in UV-visible spectra, were in the high-energy area and usually overlapped with those of the aromatic compounds [9]. They sometimes also overlapped with silica. Extremely similarly, the reaction mixture of the oxidation catalyzed by unsupported cluster showed an absorption maximum at 564 nm with the disappearance of the band at 682 nm. This reaction mixture was also active during the readdition of styrene (Table 2). Degradation of the triruthenium cluster was not reported when the cluster was bound to the soluble polyethylene carboxylate and used as alcohol oxidation catalyst [10]. We also observed that the triruthenium cluster dissolved in toluene was degraded when heated with oxygen at 80°C, even without styrene. Thus our results were reasonable. In all cases, the main oxidation product was benzaldehyde, resulting from the oxidative cleavage of styrene by molecular oxygen. From detailed investigations of the relationship between the initial reaction rate after the induction period and the concentration
(1)
where r, k, C and P denote the reaction rate, rate constant, concentration of styrene and partial pressure of oxygen, respectively. According to Eq. 1 and the mechanism in the oxidative cleavage of olefins to acids suggested by other authors [11,12], the mechanism in this oxidation can be proposed as follows: Ru + 0 2
fast Ru(O2)
(2) /
Ru(02)
+ PhCH=CH
2
slow
//
R x\u
"1 i i I I
'O "'~" Ph
(3) /'
I I
Ru
1 O ""
'
%'Ph
fast
, Ru + PhCHO + C H 2 0
PhCH=CH 2 + 0 2
(4)
) PhCHO + C H 2 0
where Ru = active ruthenium species, r = k'[PhCH=CH2][Ru(O2)], [Ru] = [Ru(O2)] and k = K'[Ru], hence, r = k C . Reaction 3 may be the slow step that involved a five-center intermediate. The oxidation mechanism catalyzed by the supported cluster catalyst may be similar to the mechanism above because the fundamental chemical steps are essentially the same, whether the oxidation occurs in the coordination sphere of a soluble metal complex or on the supported metal species [12].
38
C.-G. Jia et al. / Reactive Polymers 23 (1994) 33-38
It is very interesting that a white film was formed on the inner wall of the neck of the reaction flask when the oxidation proceeded: the faster the uptake of oxygen, the faster the film formed. Elemental analysis of the film gave the formula of CH20, in accordance with the composition of polyformaldehyde, indicating that formaldehyde formed during the reaction was released from the reaction mixture. It is well known that the cleavage of olefins can be carried out in the presence of stoichiometric R u O 4 [4] and by RuO2-aldehyde catalyst systems [11]. However, the oxidation products are carboxylic acids and ketones, not aldehydes. In this respect, the two cluster catalysts reported in this paper are much more selective. Besides benzaldehyde, phenyl acetaldehyde and a small amount of acetophenone were also obtained, they arised from the Wacker-type oxidation [12]. The yield of all three products increased with reaction time, indicating the competition between the cleavage and Wacker-type oxidation. The main advantage of the supported cluster catalyst over its homogeneous counterpart was its easy recovery by filtration and stable activity in the recycles. Also, we noticed that the supported cluster was more active during the oxidation (high conversion of styrene) (Table 2) though a slightly longer
induction period was needed. The high activity of the supported cluster may result from the high concentration of ruthenium on the support; the concentration of ruthenium on the support may be higher than that in the solution of the unsupported cluster [6]. In summary, the supported triruthenium cluster catalysts displayed a high activity and stability in the oxidation of styrene. During the oxidation, the cluster degraded to monoruthenium species. 4. References [1] B.C. Gates, L. Guzi and H. Knozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986. [2] G. Schmid, Chem. Rev., 92 (1992) 1709. [3] C.G. Jia, Y.P. Wang, H.Y. Feng, Reactive Polym., 18 (1992) 203. [4] A.H. Haines, Methods for the Oxidation of Organic Compounds, Academic Press, London, 1985. [5] D.C. Sherrington, Pure Appl. Chem., 60 (1988) 401. [6] F.R. Hartly, Supported Metal Complexes, D. Reidel, Dordrecht, 1988. [7] A. Spencer and G. Wikinson, J. Chem. Soc., Dalton, (1972) 1570. [8] A. Spencer and G. Wikinson, J. Chem. Soc., Chem. Commun., (1990) 1467. [9] M.J. Upadhyay, P.K. Bhattacharya, P.A. Ganeshpure and S. Satish, J. Mol. Catal., 80 (1993) 1. [10] D.E. Bergbreiter and D.R. Treadwell, Reactive Polym., 12 (1990) 290. [11] K. Kaneda, S. Haruna, T. Imanaka and K. Kawamoto, J. Chem. Soc., Chem. Commun., (1990) 1467. [12] R.A. Sheldon and J.K. Kochi, Metal Catalyzed Oxidation of Organic Compounds, Academic Press, New York, NY, 1981.