Ind. Eng. Chem. Res. 1998, 37, 1769-1773
1769
Kinetic Study of Cycloolefin Oxidation with a Pd(0) Complex Paolo De Filippis,* Carlo Giavarini, and Rossella Silla Department of Chemical Engineering, University “La Sapienza”, via Eudossiana 18, 00184 Rome, Italy
A new chemical process is proposed for the oxidation of cyclic olefins, such as cyclooctene and cyclododecene, to cyclic ketones in the presence of hydrogen peroxide and a catalytic system based on tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4). The kinetic equation of the cyclooctene oxidation can be expressed by rolefin ) k[olefin]([catalyst] - k′[H+]1.7). Note that the reaction rate increases linearly with catalyst and olefin concentration and depends on the H+ concentration. The catalyst retains its activity when bound to a polymer. This allows catalyst recycling and better control of the reaction medium. Unlike most common processes for cyclic ketone production, the proposed novel method occurs in a single step and does not require any cocatalyst. Introduction Cyclic ketones have excellent solvent properties and their characteristic odor and reactivity of the carbonyl group makes them important in industrial applications (Kirk-Othmer Encyclopedia of Chemical Technology, 1994). Cyclohexanone is used for the manufacture of γ-caprolactam (nylon 6) and of adipic acid (nylon 6,6). Similarly, cyclododecanone is used to synthesize dodecanoic acid and laurolactam to make polyamides-12 (Ullman’s Encyclopedia of Industrial Chemistry, 1995). Higher cycloaliphatic ketones are easily obtained from the products of olefin oligomerization over Ni catalyst (Wilke, 1957, 1963). They are used as fragrances and as intermediates for the synthesis of pharmaceuticals, insecticides, and herbicides. Virtually all members of this ketone series have characteristic odors, corresponding to their ring size. Many cyclic ketones occur in natural oils, and the synthetic materials are highly valued perfume bases. These are generally produced by cyclization and decarboxylization of R,ω-difunctionalized precursors in the presence of various catalysts (Patai, 1966). Industrially, one of the most important reactions for the production of carbonyl compounds is the oxidation of olefins in the presence of palladium complex catalysts. An advantage of palladium chemistry, in the industrial Wacker process, is the easy reoxidation of Pd(0) to Pd(II) to regenerate in situ the active Pd(II). The reaction of R-olefins with palladium chloride and water is a classic reaction undergone by most olefins; in general, R-olefins produce methyl ketones, and cyclic olefins up to cycloheptene produce cycloalkanones (Smidt et al., 1959; Hu¨ttel et al., 1964). Side reactions of the Wacker catalytic system form chlorinated compounds, aldehydes, and internal ketones as byproducts. Other drawbacks are the precipitation of metallic palladium and corrosion (Tsuji et al., 1980; Roussel and Mimoun, 1980). Some improvements in the Wacker catalytic system have been achieved by using basic (Clement and Selvitz, 1964; Fahey and Zuech, 1974) or alcoholic (Grigore´v et al., 1972) solvents and phase-transfer catalysts (Mimoun et al., 1978), but some of the disadvantages of this system have not been eliminated. A variation of the Wacker reaction uses a Pd(0) complex which catalyzes the oxidation of linear C6-C18 R-olefins with hydrogen peroxide as the oxidizing agent
(Ioele et al., 1992). The complex converts C6-C18 R-olefins to methyl ketones with high conversions and high specificities. The reaction can be carried out either in water or in organic solvents. According to the authors, the catalytically active species is an anionic palladium(0) hydroperoxide complex soluble in water; the organic solvent improves the interaction between the water-insoluble olefinic substrate and the catalytic complex. This reaction appears promising as no cocatalyst is necessary and the reaction can be carried out under mild conditions. In addition, H2O2 has the distinct advantage of producing an environmentally friendly byproduct, water. H2O2 ranks first in terms of available oxygen in the list of oxygen donors (Shirmann and Delavarenne, 1980). The purpose of this research was to investigate the catalytic activity and kinetics of a tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4)-hydrogen peroxide system for the oxidation of cyclic olefins to cycloalkanones; no systematic study on this subject has been previously reported. Experimental Section Cyclooctene and cyclododecene (Fluka) were used without further purification. Pd(PPh3)4 (97% Pd, supplied by Fluka) and Pd(PPh3)4 polymer bound (0.075 mmol of Pd/g, Fluka) were used as catalysts. Reagentgrade THF (tetrahydrofuran; Fluka), MEK (methyl ethyl ketone; Merck), 2-octanone (Aldrich), and NMP (N-methylpyrrolidone; Aldrich) were used as solvents. H2O2 (40 wt %, Carlo Erba) was the oxidizing agent. The reaction was carried out in a 100-mL two-necked flask, equipped with magnetic stirrer and reflux condenser (T ) -10 °C). The reactants were added in the following order: olefin, solvent (10 mL), catalyst, and H2O2 (total volume ∼ 50 mL). The experiments were performed at temperatures between 50 and 80 °C. Parameters varied included olefin and H2O2 concentrations and the solvent to catalyst ratio. The mixture was kept at specific reaction temperatures for 24 h. During this period the reaction was monitored by GLC analysis of aliquot samples to measure olefin consumption. After separation of the organic layer and the water phase, separated water was
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1770 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 1. Conditions and Results of the Preliminary Reactions olefin run no. 1 2 3 4 5 6 7 8 9 10 11 12 13
PdL4 (mol/L)
type
C0 (mol/L)
∆C/C0 (at 8 h)
H2O2 (mol/L)
T (°C)
initial rate (mol L-1 min-1)
solvent type
10-4
C8 C8 C8 C8 C8 C8 C8 C8 C8 C8 C8 C12 C12
0.86 0.86 0.86 0.86 0.79 1.87 1.87 1.87 1.87 1.87 1.87 2.23 0.53
0.10 0.19 0.25 0.30 0.31 0.28 0.28 0.27 0.52 0.24 0.38 0.18 0.15
7.80 7.80 7.80 7.81 7.94 1.84 3.16 5.98 5.67 5.68 5.68 4.48 7.80
50 60 65 70 80 70 70 70 70 60 70 70 70
0.000 20 0.000 35 0.000 39 0.000 49 0.000 76 0.002 10 0.001 40 0.001 30 0.004 90 0.000 91 0.002 70 0.000 50 0.000 10
H2O H2O H2O H2O H2O H2O H2O H2O MEK THF 2-octanone 2-octanone THF
4.37 × 4.37 × 10-4 4.24 × 10-4 4.37 × 10-4 4.80 × 10-4 1.37 × 10-3 9.64 × 10-4 9.64 × 10-4 9.64 × 10-4 9.64 × 10-4 9.64 × 10-4 1.14 × 10-3 4.09 × 10-4
extracted with CH2Cl2; the organic phases were passed through a column of activated alumina to remove the catalyst and to decompose any residual hydrogen peroxide. The oxygenated products were identified by GLC-MS (Fisons MD 800 GLC-MS), and comparison of the mass spectra were made with those of authentic samples. GLC analyses were performed with an HP 5890 GLC gas chromatograph equipped with a flame ionization detector, using a 25 m i.d., 0.32 mm SE 52 column. The main parameters were as follows: carrier gas, helium with a flow of 2 mL/min; injector temperature, 250 °C; detector temperature, 250 °C; split ratio, 1/10; temperature program, 60 °C for the first minute, 60-100 °C at 5 °C/min, 100 °C for 1 min, 100-220 °C at 15 °C/ min, 220 °C for 5 min. Results and Discussion The main reactions involved in Pd(0)-catalyzed olefins oxidation are (Ioele et al., 1992)
L4Pd a L3Pd + L
(1)
L3Pd + H2O2 a [L3PdOOH]-H+
(2)
[L3PdOOH]-H+ + olefin a [L2Pd(olefin)OOH]-H+ + L (3) [L2Pd(R-CHdCH2)OOH]-H+ a RCOCH3 + [L2PdOH]-H+ (4) where L ) PPh3. The catalytically active species [L3PdOOH]- in eq 2 is generated by ligand exchange between PPh3 and H2O2; the anionic complex could coordinate the olefin, by further ligand exchange (eq 3) and then undergo an oxygen transfer, yielding the methyl ketone and the anionic Pd(0) complex (eq 4), from which the catalytic species is regenerated. This catalytic system was tested using cyclooctene as the substrate. Figure 1 shows a typical plot of the olefin conversion vs time. The plot shows an induction period of about 30 min; this period is presumed to be the formation of the catalytic active complex. Since the catalyst activity decayed gradually with time, only the initial rate in the exponential zone was determined. Exploratory Experiments. To determine the more favorable conditions for the kinetic study, a number of
Figure 1. Typical plot of the olefin (cyclooctene) conversion vs time.
experiments were carried out, both in aqueous media and in the presence of an organic solvent. Cyclooctene was the reactant of choice experiments for most runs. Cyclododecene was also tested to confirm the performance of the catalytic system on higher cyclic olefins. Since the mixture was a heterogeneous system, efficient mixing was necessary to ensure a homogeneous composition of the bulk liquids. Reaction conditions and conversion results are summarized in Table 1. The conversion is expressed as ∆C/C0 after 8 h. Cyclooctene oxidation produced cyclooctanone and cyclooctene oxide (6-oxabicyclo[6.1.0]nonane). Variation of reaction temperatures in the range of 50-80 °C shortened the time required for reaching equilibrium (from 24 to 5 h). Initial rate values seem to be unaffected by hydrogen peroxide concentration (runs 7 and 8), whereas the catalyst concentration affects the reaction rates (runs 6 and 7). The presence of an organic solvent instead of water has different effects on the reaction rate and on the olefin conversion (runs 9-11). Palladium catalyst is active in donor solvents, like ketones or tetrahydrofuran, but not in N-methylpyrrolidone. Tetrahydrofuran provides a homogeneous mixture and therefore improves the substrate/oxidant interactions. However, the use of this solvent is not suggested because of its low boiling point and its tendency to form peroxides. Methyl ethyl ketone was excluded because it produces a significant decrease in the specificity of the catalyst. The main reaction product is cyclooctene oxide.
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1771
Figure 2. Reaction order with respect to olefin, from the van’t Hoff plot.
Figure 3. Influence of various catalyst amounts on the oxidation reactions: b, 3.64 × 10-4 mol/L; 9, 1.86 × 10-3 mol/L; 2, 2.08 × 10-3 mol/L.
Table 2. Cyclooctene Oxidation in the Presence of 2-Octanone run no.
PdL4 (mol/L)
13 14 15 16 17 18 19 20 21
1.33 × 10-3 1.33 × 10-3 1.33 × 10-3 1.33 × 10-3 1.33 × 10-3 1.33 × 10-3 3.64 × 10-4 1.86 × 10-3 2.08 × 10-3
olefin initial rate C0 ∆C/C0 H2O2 (mol L-1 solvent (mol/L) (at 8 h) (mol/L) min-1) (mol/L) 2.70 2.70 2.70 4.19 5.32 5.78 2.63 2.63 2.63
0.37 0.37 0.36 0.40 0.41 0.38 0.36 0.39 0.41
0.9 1.8 2.7 2.7 2.7 2.7 2.8 2.8 2.8
0.0044 0.0046 0.0041 0.0065 0.0087 0.0090 0.0016 0.0055 0.0068
2.70 2.70 2.70 1.40 0.54 0.13 1.40 1.40 1.40
Best results in the cyclooctanone formation were obtained by employing 2-octanone as a solvent; the initial rate was twice the rate obtained without any organic solvent. Experiments carried out with cyclododecene show that the catalyst is active for the oxidation of higher cyclic olefins, giving the corresponding cycloalkanones (Table 1). Kinetic Study. For the kinetic study of cyclooctene oxidation, 2-octanone was selected as the organic solvent and all runs were conducted at 70 °C. Table 2 lists the results of experiments designed to evaluate the influence of olefin, H2O2, and catalyst concentrations. When the kinetics of the heterogeneous reaction is expressed in terms of pseudohomogeneous rate, the general form of the kinetic expression is
r ) -d[olefin]/dt ) k[olefin]n[H2O2]m[catalyst]p (5) Runs 13-15 in Table 2 show that the reaction rate is unaffected by the hydrogen peroxide concentration. This agrees with the previous data reported in Table 1 for the runs in aqueous media. The Pd(0) catalyst used in this work has the same behavior of the Pd(II) Wacker type catalyst, where H2O2 can be used at any dilution (Mimoun et al., 1978). The value of index m for the kinetic expression (5) is equal to zero. Runs 15-18 show an increase in the reaction rate with the olefin concentration; therefore, the kinetic expression is dependent on the olefin concentration and the reaction order n, calculated from the slope of the van’t Hoff plot, is equal to 1 (Figure 2). In these runs the amount of solvent has negligible influence on the reaction rate.
Figure 4. Apparent activation energy calculated from the Arrhenius plot.
Figure 3 shows the influence of the catalyst concentration (Table 2, runs 19-21) on the oxidation reaction. The catalyst concentration does not affect the final conversion but has a positive effect on the initial reaction rate. Because the reaction rate increases linearly with the catalyst concentration, the value of index p in the kinetic expression (5) is equal to 1; therefore, the kinetic expression (5) can be rewritten:
r ) k[olefin][catalyst]
(5a)
Figure 4 shows the Arrhenius plot for runs 1-5 in Table 1. The apparent activation energy calculated from the slope of the best fit of the experimental data was Ea ) 41 kJ/mol. Influence of pH. Additional experiments were conducted to explain the decay of catalyst activity with time. Although several modifications of the reaction medium were made to improve olefin conversion these did not affect the amount of olefin converted. This cannot be attributed to lack of oxidant. The addition of fresh H2O2 to the reaction mixture did not reinitiate the reaction. The observed final pH value of the aqueous phase ranged from 2.2 to 2.4. To study the pH influence on the reaction, pH values were routinely measured during the reaction. Figure 5 shows a typical plot of [H+] increase vs time, compared to olefin conversion. This suggested that [H+] probably interferred with the formation of the catalytic active species. As indicated in the figure, an increase in the hydrogen ion concentration shifts the equilibrium to the left (eq 2).
1772 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 3. Reactions with Polymer-Bound Catalyst run no.
Pd (equiv/L)
C0 (mol/L)
23 24 25 26
1.62 × 10-4 5.68 × 10-4 5.68 × 10-4 5.68 × 10-4
0.8 1.2 1.2 1.1
olefin ∆C/C0 (at 8 h) 0.08 0.14 0.13 0.26
H2O2 (mol/L)
initial rate (mol L-1 min-1)
solvent (mL)
solvent type
7.9 3.6 3.6 3.6
0.000 10 0.000 47 0.000 30 0.000 38
10 5.5 5.5 5.5
H2O 2-octanone 2-octanone 2-octanone
Conclusions
Figure 5. Influence of H+ concentration on olefin conversion: b, olefin concentration; 0, H+ concentration.
This implies that the kinetic expression should contain a term involving proton inhibition. From the experimental data (Figure 5) the kinetic equation for the studied reaction becomes
r ) k[olefin]([catalyst] - k′[H+]1.7)
(6)
where k ) 2.4 (L mol-1 min-1) and k′ ) 3.76 (L0.7 mol-0.7). Attempts to carry out runs in the presence of buffer solutions failed, and decomposition of the catalyst leads to precipitation of metallic Pd. Reactions with Pd(PPh3)4 Polymer Bound. To investigate the inhibition role of H+ and to restore the initial pH value, the aqueous phase from the reaction mixture had to be replaced with fresh H2O2. This was easily accomplished using a catalyst complex anchored to a styrene-divinylbenzene (ST-DVB) resin. The amount of catalyst complex used was calculated to have Pd concentrations similar to those used in the previous runs. This heterogeneous catalyst had an activity similar to that of the “free” complex and gave the same products (Table 3). Such behavior is interesting if compared with similar Ni-heterogenized systems (Braca et al., 1995). The presence of a polymeric skeleton normally produces a change in the activity and selectivity of the catalyst due to steric hindrance. Note that catalytic activity in the presence of solvent 2-octanone (runs 23 and 24) improves olefin conversion brought about by a more favorable environment, one that “swells” the resin and makes more accessible the catalytic sites. As expected, substitution of the aqueous phase with fresh H2O2 made the reaction reinitiate and provided reaction rates and olefin conversions comparable with those of the first step (Table 3, runs 24 and 25). The same catalyst, removed by filtration, was used in another batch reaction (run 26). This demonstrates that the catalytic species remains active and can be recycled.
The reactivity of cycloolefins such as cyclooctene and cyclododecene toward H2O2 in the presence of Pd(PPh3)4 is similar to that observed with linear R-olefins. The lower conversion levels, as compared with that of linear R-olefins, are probably explained in terms of steric hindrance. The proposed kinetic expression is consistent with the catalytic scheme reported in the literature. The reaction rate increases linearly with catalyst and olefin concentrations; however, the kinetic expression should contain a term involving proton inhibition. When anchored to a solid resin such as ST-DVB, the catalyst retains most of the reactivity observed for the “free” complex. The catalyst can be recycled, and the reaction medium (mostly, H+ concentration) can be more easily controlled. The heterogeneous catalyst keeps its activity after recycling and does not require any cocatalyst. Unlike most common syntheses of cyclic ketones, the proposed reaction occurs in a single step and simplifies complex and more difficult routes applied for oxidation of olefins to ketones; this novel method has a potential utility in laboratory and industrial processes. Literature Cited Braca, G.; Raspolli Galletti, A. M.; Di Girolamo, M.; Sbrana, G.; Silla, R.; Ferrarini, P. Organometallic Nickel Catalysts Anchored on Polymeric Matrices in the Oligomerization and/or Polymerization of Olefins. Part II. Effect and Role of the Components of the Catalytic System. J. Mol. Catal. A 1995, 96, 203-213. Clement, W. H.; Selvitz, C. M. Converting Higher R-Olefins to Methyl Ketones with PdCl2. J. Org. Chem. 1964, 29, 241. Fahey, D. R.; Zuech, E. A. Aqueous Sulfolane as Solvent for Rapid Oxidation of Higher R-Olefins to Ketones Using Palladium Chloride. J. Org. Chem. 1974, 39, 3276. Grigore´v, A. A.; Moiseev, I. I.; Klimenko, M. J. A.; Lipina, U. N. Oxidation of Higher R-Olefins into Methyl Alkyl Ketones. Khim. Prom. 1972, 48, 14; Chem. Abstr. 1972, 76, 85317. Hu¨ttel, R.; Dietl, H.; Christ, H. π-Allyl-Palladiumchlorid-Komplexe von Cycloolefin und Cyclodienen. Chem. Ber. 1964, 97, 2037. Ioele, M.; Ortaggi, G.; Scarsella, M.; Sleiter, G. Oxidation of Terminal Olefins by Hydrogen Peroxide Catalysed by Tetrakis(triphenylphosphine)Palladium(0). Gazz. Chim. Ital. 1992, 122, 531. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley & Sons: New York, 1994; Vol. 14. Mimoun, H.; Perez Machirant, M. M.; Seree de Roch, I. Activation of Molecular Oxygen: Rodium-Catalyzed Oxidation of Olefins. J. Am. Chem. Soc. 1978, 100, 5437. Patai, S. The Chemistry of Functional Groups; John Wiley & Sons: New York, 1966; Vol. II. Roussel, H.; Mimoun, H. Palladium-Catalyzed Oxidation of Terminal Olefins to Methyl Ketones by Hydrogen Peroxide. J. Org. Chem. 1980, 45, 5387. Shirmann, J. P.; Delavarenne, S. Y. Hydrogen Peroxide in Organic Chemistry; Ed. Documentation Industrielle: Paris, 1980. Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, J.; Sieber, R.; Ru¨ttinger, R.; Kojer, H. Catalytic Reactions of Olefins on Compounds of the Pt Group. Angew. Chem. 1959, 71, 176.
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1773 Tsuji, J.; Nagashima, H.; Kori, K. A New Preparative Method for 1,3-Dicarbonyl Compounds by the Regioselective Oxidation of R,β-Unsaturated Carbonyl Compounds Catalyzed by Palladium(II) Chloride Using Hydroperoxides as the Reoxidant of Palladium (Pd0). Chem. Lett. 1980, 257. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Publishers: Weinheim, Germany, 1995; Vol. A8. Wilke, G. Oligomerization of 1,3-Diolefins with Ziegler-type Catalysts. Angew. Chem. 1957, 69, 397.
Wilke, G. Cyclooligomerization of Butadiene and Transition Metal π-Complex. Angew. Chem. 1963, 75, 10.
Received for review September 18, 1997 Revised manuscript received February 16, 1998 Accepted February 24, 1998 IE970671E
