Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions

40 Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions YOSHIO KAMIYA

Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch040

Faculty of Engineering, University of Tokyo, Hongo, Tokyo, Japan The autoxidation of hydrocarbons catalyzed by cobalt salts of carboxylic acid and bromide ions was kinetically studied. The rate of hydrocarbon oxidation with secondary hydrogen is exactly first order with respect to both hydrocarbon and cobalt concentration. For toluene the rate is second order with respect to cobalt and first order with respect to hydrocarbon concentration, but it is independent of hydrocarbon concentration for a long time during the oxidation. The oxidation rate increases as the carbon number of fatty acid solvent as well as of cobalt anion salt are decreased. It was suggested that the cobalt salt not only initiates the oxidation by decomposing hydroperoxide but also is responsible for the propagation step in the presence of bromide ion.

*Tphe rate of metal salt-catalyzed autoxidation of hydrocarbons reaches a maximum at a certain catalyst concentration ( J , 7, 13), and any further increases i n this concentration do not accelerate the rate. H o w ever, when bromide ion is added to the solution of hydrocarbon and fatty acid with metal salts, the oxidation rate increases over the maximum value of & 3 ( R H ) / 2 f c as a function of metal concentration. Although cobalt and bromide ion catalysis have been studied b y Ravens ( I I ) and recently by H a y and Blanchard (4), there are many important aspects yet to be elucidated. In this paper we hope to clarify the features of cobalt bromide catalysis using various hydrocarbons and a neutral bromide as sodium bromide at low temperatures and at moderate concentrations of cobalt. 2

2

6

Experimental The experimental technique has been described ( 6 ) . Hydrocarbons were oxidized at temperatures from 35° to 95 °C. i n a 1:1 mixture by 193

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

194

OXIDATION

O F ORGANIC COMPOUNDS

II

volume of fatty acid and hydrocarbon unless otherwise stated. The oxidation products were analyzed by gas-liquid chromatography. Isotactic polypropylene (powdered, M W , 2.5 Χ 10 ), atactic polypropylene ( M W , 6 Χ 10 ), and polystyrene ( M W , 2 Χ 10 ) were the polymers used. T o analyze the oxidation products in a long run, a 50-ml. cylindrical flask with condenser, oxygen inlet, and sample outlet was used under vigorous agitation. 5

4

5


Results When the cobalt salt of carboxylic acid and bromide ion are dis solved in acetic acid, a cobalt bromide complex is formed instantaneously. For cobalt dibromide a pronounced induction period was observed, but adding sodium acetate eliminates entirely the induction period, suggesting that cobalt monobromide is responsible for the catalysis. Typical oxygen absorption curves ( Figure 1 ) show a short induction period and steady rate even after considerable absorption of oxygen at NaBr/cobalt ratio above 1/1.

Time, min.

Figure 1. Oxygen absorption curves in the autoxidation of 4.07M ethylbenzene catalyzed by 2 X 10~ M CoAc and NaBr in acetic acid 2

A: NaBr, 8 X 10~*M B: NaBr, 2 X 10~ M C: NaBr, 1 Χ 10~'Μ 2

Cobalt Concentration. The effect of bromide ion becomes appreci able at cobalt concentrations above 0.001M and quite remarkable above 0.01M. The ratio of /o (oxidation rate i n the presence of 0.1M N a B r ) to p (oxidation rate i n the absence of N a B r ) at cobalt concentration Br

0


40.

195

Autoxidation of Hydrocarbons

KAMiYA

0.05M is shown i n Table I. The effect of bromide ion on the cobaltcatalyzed oxidation of methylbenzenes is quite large. Table I. Effect of NaBr on Oxidation Rate of Hydrocarbons Catalyzed by 5 X 10" M Cobalt Acetate in Acetic Acid


2

Hydrocarbon

Temperature, °C.

pBr/po

Tetralin Cumene Ethylbenzene n-Dodecane p-Xylene

50 60 80 80 80

4 4 30 86 400

The activation energy of over-all oxidation catalyzed by 0.02M cobalt acetate and 0.04M N a B r is very small—8.3 kcal./mole for ethylbenzene, 8.7 for p-xylene, and 14.9 for n-dodecane. The broken line in Figure 2 shows that the steady rate of oxidation of 4.07M ethylbenzene in acetic acid, catalyzed by cobalt acetate, reaches a limiting rate of 2.2 X 10~ mole liter" sec." (after correcting for the dielectric effect (5, 9), 1.8 X 10' ), which is i n excellent agreement with the theoretical limiting rate of 1.85 X 10" mole liter" sec." as calculated by fc (RH) /2fc . 5

1

1

5

r>

2

3

2

1

1

6

However, the rate of oxidation in the presence of bromide ion (Figure 2) is exactly first order with respect to cobalt. The autoxidation of hydrocarbons catalyzed by cobalt and bromide ion is characterized by the fact that the rate increases with increasing cobalt concentration, while the rate at high cobalt concentrations reaches a limiting value in the absence of bromide ion. Similar first-order correlation between cobalt and the rate was ob tained for n-dodecane. The oxidation rate of n-dodecane decreases as cobalt concentration is increased to more than 0.05M, probably owing to chain termination by cobalt as reported for Tetralin (8). The oxidation rate of toluene is nearly second order with respect to cobalt (Figure 3). Hydrocarbon Concentration. The steady rate of hydrocarbon oxi dation is exactly first order with respect to hydrocarbon concentration, but it tends to be independent of this concentration below 1.0M ( Figure 4). The cobalt-catalyzed autoxidation of Tetralin (6) and ethylbenzene at 0.05M cobalt in the absence of bromide is exactly second order with respect to hydrocarbon concentration. In reactions catalyzed by cobalt and bromide the oxidation rates of ethylbenzene, cumene, and Tetralin start to decrease after several percent conversion and are roughly proportional to the hydrocarbon concentration during the oxidation.



196

OXIDATION

Cobalt,

O F ORGANIC COMPOUNDS

Π

mole I.

Figure 2. Steady rate of hydrocarbon oxidation as a function of cobalt concentration at NaBr/Co = 2/1 at 80°C. A: 4.07M ethylbenzene in acetic acid B: 2.20 n-dodecane in η-butyric acid Broken line: oxidation rate of ethylbenzene in absence of NaBr

However, for toluene and p-xylene, the rates are constant up to 7 0 % conversion as the NaBr/cobalt ratio increases (Figure 5 ) . The rate seems independent of hydrocarbon concentration during the oxidation, although the steady oxidation rate is exactly proportional to initial hydrocarbon concentration. Separate experiments showed that the rate increases if benzaldehyde is added but decreases as hydrocarbon and bromide ion are consumed or water is added. According to visible spectra ( Figure 6 ) and potentiometric titration, the concentration of bromide ion decreases gradually as toluene oxidation proceeds, but the total amount of bromide ion after treating the solution with alkali remains constant (Figure 7 ) . The bromide ion converted to organic bromides (benzyl bromide) during the oxidation can be hardly


40.

KAMiYA

197



regenerated at temperatures below 80°C. About 25 toluene molecules were oxidized per bromine atom at 80°C. Ratio of Bromide Ion to Cobalt. As the NaBr/cobalt molar ratio is increased, the oxidation rate increases and becomes constant at N a B r / cobalt = 1/1 or 2/1. A N a B r effect similar to that i n Figure 8 was observed independent of cobalt and hydrocarbon concentrations at tem peratures from 35° to 80°C.

2 ioο

0> Φ

3

Λ t

"7

Ε

2

$ ΙΟ"

4

CM

Ο

7

6 4 2 IO'

5

5

ΙΟ" 2 4 Cobalt, mole I"' 2

6

Figure 3. Steady rate of oxidation of 4.7M toluene in acetic acid as a func tion of cobalt concentration at NaBr/Co = 2/1 A: 80°C. B: 65°C. C: 50°C.

However, when a strong bromide such as calcium bromide is used, the oxidation rate reaches a maximum (Figure 8) and decreases as the ratio is increased, probably owing to the formation of inactive cobalt dibromide. Effect of Acid Strength. W h e n a higher fatty acid is used instead of acetic acid, the oxidation rate is lower as the carbon number of the


198

OXIDATION OF ORGANIC COMPOUNDS II

fatty acid is increased and is of the order of acid strength given i n Table II. The equilibrium constant may be aifected by the anion of cobalt salts since the oxidation rates of ethylbenzene and n-dodecane i n propionic or butyric acids increase 1 5 % by using cobalt acetate instead of cobalt decanoate. In a stronger acidic solution decanoate anion w i l l be substi tuted by solvent acid anion. A weaker acid anion w i l l result i n a smaller equilibrium constant—i.e., smaller cobalt bromide concentration. C o ( O O C R ) + N a B r ^ C o B r ( O O C R ) + RCOONa Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch040

2

The solvent acid also affects the oxidation rate since the rate with cobalt acetate (Table I I ) is reduced i n propionic or butyric acids i n contrast to the increase i n the hydroperoxide decomposition rate.

Ι0~ Γ 3

Figure 4. Steady rate of hydrocar bon oxidation as a function of hy drocarbon concentration with 5 X 10~ M CoAc and 1 X lO'M NaBr in acetic acid 2

A: Ethylbenzene at 80°C. B: p-Xylene at 80 C. C: Tetralinat35°C. e

Oxidation Products. Although the ratio of hydroxyl to carbonyl products is 1/1 or nearly so i n the ordinary metal salt-catalyzed autoxidation of hydrocarbons, higher proportions of carbonyl compounds are obtained i n autoxidations catalyzed b y cobalt and bromide ion—e.g., In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

40.

KAMiYA

199


acetophenone from ethylbenzene, dodecanone from dodecane, and benzaldehyde from toluene. This change in alcohol-to-carbonyl ratio estab lishes the presence of different chain carriers in the presence of bromide.


'-•51-

I Ο

-

I

'

1

1

Ο

I

2

3

Τ ime

ι -

4

hr

}

Figure 5. Consumption of toluene during the oxidation catalyzed by 2 X 10~ M CoAc and NaBr in acetic acid 2

NaBr concentrations: A: 2 X 10->M B: 4 X 10~*Μ C: 8 X 10-'M D: 1.6 X lO-'M

Partial Pressure of Oxygen. It was reported that the oxidation rate of p-toluic acid catalyzed by cobalt and bromide ion at 130 °C. is half order with respect to the partial pressure of oxygen, and the initiating reaction is suggested to be the following. HBr + 0 « = * H 0 2

+Br-

2

Therefore, the effect of partial pressure on the oxidation rate of ethylbenzene, Tetralin and p-xylene was examined carefully. However, at temperatures below 80°C. the rate was independent of the partial pressure of oxygen at pressures of 400-760 mm. H g . The dependence of the rate on the oxygen pressure (11) at higher temperature may be ascribed to the rate at which oxygen dissolves in the solution or to the following equilibrium ( 1 ). R- +

0 ^R0 2

2


200

OXIDATION OF ORGANIC COMPOUNDS

II

c ω Ο σ ο α


Ο

26

24

22

20 ΚΚ

18

16

14

Figure 6. Visible spectra of 4.7M toluene in acetic acid with 5 X 10 M cobalt and 1 X 10~ M NaBr at 80°C. during oxidation S

2

Minutes:

(5) : 105 (6) : 120

Synergistic Effect of Metal Salts. W h e n 20% of the cobalt is re placed by manganese, approximately a sixfold increase in the oxidation rate of p-xylene (Figure 9) was observed, showing that the metals are strongly synergistic. Similar synergistic results are obtained i n the oxida tion of ethylbenzene and cumene (Figure 9). In contrast to the effect of manganese, cupric salt strongly retards the oxidation, reducing the rate of p-xylene by a factor of more than 10 when 20% of cobalt is replaced by cupric acetate. Effect on the Oxidation of Polymers. In the cobalt bromide catalysis, the steric hindrance to the intramolecular hydrogen abstraction in the autoxidation of polymers is expected to be reduced remarkably. The oxidation rate of 0.5 gram atactic polypropylene with 0.02M cobalt acetate in 10 ml. of a 1/1 mixture by volume of chlorobenzene and acetic acid increases from 2.7 Χ 10~ to 2.76 Χ 10" mole kg. sec." in the presence of 0.04M sodium bromide. The rate of powdered isotactic polypropylene under the same conditions increases only from 2.05 Χ 10~ to 2.45 Χ 10" mole kg." sec." i n the presence of sodium bromide. 4

3

_ 1

1

3

3

1

1


40.

KAMiYA


201


The effect of bromide ion was more pronounced i n polystyrene oxi dation. Although polystyrene i n a 1/1 mixture by volume of chlorobenzene and acetic acid is barely autoxidized at 100 °C. i n the presence of cobalt salt or initiators, the oxidation catalyzed b y cobalt is so strongly accelerated by bromide ion that it proceeds rapidly even at temperatures as low as 45°C. (Figure 10).

ω Time, hours

Figure 7. Concentration of bromide ion during the oxidation of 4.7M toluene in acetic acid with 2 X 10~ M cobalt and 4 X 10~ M NaBr at 80°C. 2

2

A: Before alkali treatment B: After alkali treatment

Figure 8. Steady rate of oxidation of 4.07M ethylbenzene catalyzed by CoAc and bromide ion in acetic acid as a function of the Br/Co molar ratio A: CoAc, 2 X I0"*M, NaBr at 65°C. B: CoAc, 1X 10~'M, CaBu at 80°C.


202

OXIDATION

O F ORGANIC

COMPOUNDS

Π

Table II. Effect of Cobalt Anion and of Solvent Acid on Oxidation Rate of 4.07M Ethylbenzene with 2 X 10" M Cobalt and 4 X 10" M NaBr in Acetic Acid at 65°C. 2

2

-dQ /dt 2

Cobalt Salt

Solvent

Χ 10 , 4

mole/liter/sec.

Acetic acid Propionic acid η-Butyric acid

1.55 1.37 1.18

Cobalt decanoate

Acetic acid Propionic acid η-Butyric acid

1.55 1.21 0.99


Cobalt acetate

2h

3μ 2 I 0

1

1

1

1

0.2

0.4

0.6

0.8

,

mole/mole

Co/(Co + Mn)

ι 1.0

Figure 9. Mixing effect of cobalt with man ganese on the oxidation of hydrocarbons at NaBr/metal = 2/1 in acetic acid A: Ethylbenzene, 4.07M at 80°C; total metal con centration, 3 X 10-'M B: p-Xylene, 4.06M at 80°C; total metal, 2 X 10-*M C. Cumene, 3.58M at 50°C; total metal, 2 X 10-'Μ

Discussion Ravens (11) proposed that hydrogen bromide initiates chains by reacting with molecular oxygen to produce bromine atom which initiates


40.

KAMiYA


203


the peroxide chain. H a y and Blanchard (4) concluded that hydrogen bromide does not initiate chains but accelerates chain propagation by reacting with peroxy radicals so that hydrocarbon is attacked by bromine atom rather than peroxide radicals, and hydrogen bromide is regenerated from organic bromide by cobalt.

Figure 10. Steady rate of polystyrene oxidation as a function of hydrocarbon concentration catalyzed by2X 10~ M Co Ac in 1 X 10~ M NaBr in a 1:1 mixture by volume of chlorobenzene and acetic acid 2

2

A: B:

at45°C. at60°C.

However, our fast oxidations were obtained under conditions where the concentration of free hydrogen bromide was extremely low. The following observations suggest that free hydrogen bromide is probably not responsible for chain propagation. (a) The steady concentration of Tetralin hydroperoxide (10) is only slightly affected b y adding sodium bromide. (b) Even at temperatures where organic bromide is stable, the catalyst retains its activity for a long time. F o r example, the number of hydrocarbon molecules oxidized per bromine atom was only 1-2 i n the oxidation by hydrogen bromide and A I B N but 25 i n the oxidation by cobalt and sodium bromide. ( c ) Carbonyl compounds are formed i n greater yields than alcohols. (d) The rate of oxidation is first order i n hydrocarbon concentra tions at high cobalt concentrations.


204

OXIDATION OF ORGANIC COMPOUNDS II

To explain the above results, we suggest that a cobalt bromide rather than hydrogen bromide complex is responsible for the propagation step. A hydrogen-bonded cobalt bromide complex may be considered. Co (Ac)Br(HAc)„ + R 0 - -> Co Br(Ac) (HAc)„ _ + products 2+

3+

2

2

x

(1)

Κ Co Br-(Ac) (HAc)„. Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch040

3+

2

+ R H - * Co (Ac)Br(HAc)„ + R-

(2)

2+

1

It is known that olefins are oxidized easily b y cobaltic sulfate (2) and toluene b y C o ( H 0 ) (3). W e found that various hydrocarbons (12) such as toluene, ethylbenzene, and cumene can be oxidized rapidly by cobaltic acetate i n the absence of oxygen. Assuming that the propagation reaction proceeds by Reactions 1 and 2 and that fc > k/, the steady-state treatment, including well-established elementary reactions (3 to 6 ) , gives Reactions 7 and 8 when the rate of hydroperoxide decomposition equals its rate of formation. 3 +

2

6

4

R O O H + (Co),, -> radicals

(3)

R 0 - 4- R H - » R O O H + R-

(4)

R +0 -*R0 -

(5)

2

2

2

R 0 - + R 0 - - » inactive products + 0 2

2

(6)

2

(ROOH) =fc (RH) /2fc' fc (Co)^ 3

(-d0 /dt) 2

Br

- (-d0 /dt) 2

0

2

2

1

(7)

6

= Jfc f ( R H ) (cobalt complex)/2& 88

6

(8)

The kinetic results on the oxidation of secondary hydrogen ( Figures 2 and 4) show good agreement with Reaction 8. The induction period i n the oxidation of ethylbenzene catalyzed by cobalt and sodium bromide i n the presence of 2,6-di-tert-butyl-p-cresol indicates that the direct initiation is negligible compared with the rate of initiation by the cobalt-catalyzed decomposition of hydroperoxide. Literature Cited (1) Benson, S. W., J. Am. Chem. Soc. 87, 972 (1965). (2) Bawn, C. Ε.H.,Sharp, J. Α., J. Chem. Soc. 1957, 1854. (3) Cooper, Τ. Α., Clifford, Α. Α., Mills, D. J., Waters, W. Α., J. Chem. Soc. 1966 (B) 793. (4) Hay, A. S., Blanchard, H. S., Can. J. Chem. 43, 1306 (1965). (5) Howard, J. Α., Ingold, K. U., Can. J. Chem. 42, 1044 (1964). (β) Kamiya, Y., Beaton, S., Lafortune, Α., Ingold, K. U., Can. J. Chem. 41, 2020 (1963). (7) Kamiya, Y., Beaton, S., Lafortune, Α., Ingold, K. U., Can. J. Chem. 41, 2034 (1963). (8) Kamiya,Y.,Ingold, K. U., Can. J. Chem. 42, 2424 (1964). (9) Kamiya,Y.,Bull. Chem. Soc. Japan 38, 2156 (1965).


40. (10) (11) (12) (13)

KAMIYA


205

Kamiya, Y., Tetrahedron 22, 2029 (1966). Ravens, D. A. S., Trans. Faraday Soc. 55, 1768 (1959). Sakota, K., Kamiya, Y., Ohta, N., unpublished work. Woodward, A. E., Mesrobian, R. B., J. Am. Chem. Soc. 75, 6189 (1953).


RECEIVED October 9, 1967.

Discussion P. de Radzitzky: D r . Kamiya has observed a synergistic effect when manganese is added to the catalytic system Br/Co during the oxidations of p-xylene, cumene, and ethylbenzene, but no effect was observed for Tetralin or dodecane. Although the mechanism advanced by the author has possibly some validity, I think that the explanation is much simpler. When using the catalytic system Br/Co the oxidation of ethylbenzene stops at acetophenone and that of p-xylene at an aldehyde. Oxidizing ethylbenzene i n acetic acid and using only cobalt as catalyst we have noticed that a reaction which has stopped when a good part of ethyl benzene has been transformed chiefly into acetophenone can be restarted with extreme vigor by adding manganese, which oxidizes acetophenone almost quantitatively and quickly to benzoic acid. Similarly cumene is oxidized to acetophenone and then to benzoic acid as shown by a study of the synthesis of terephthalic acid from p-diisopropylbenzene [Van Helden, R., Kooyman, E . C , Rec. Trav. Chim. 8 0 , 57 (1961)]. Undoubt edly p-tolualdehyde w i l l also be oxidized to p-toluic acid i n the presence of manganese. O n the other hand, Tetralin which does not contain acetyl groups and dodecanone which contains only a minor amount of unactivated acetyl groups do not oxidize further under the same conditions when one adds manganese. Therefore, I think that the higher rate of oxygen absorption observed when manganese is added to the Br/Co system is well explained for p-xylene, cumene, and ethylbenzene by the simultaneous oxidation of the acetyl or aldehyde groups originally formed. The fact that dodecane and Tetralin are not oxidized at a higher rate when manganese is added also supports this hypothesis. Allan S. Hay: It has been amply and repeatedly demonstrated i n this conference that many of the steps i n autoxidation reactions are not adequately understood. When one adds a catalyst to an already complex


206

OXIDATION O F

II

reaction (in this case one consisting of cobalt and bromide ions), the number of discrete steps or reaction intermediates that one can write is overwhelming. D r . Kamiya has attempted to explain the role of the catalyst accord ing to Reactions 13-15, and he has attempted to differentiate between these proposed reaction steps and the simplified Reactions 3 and 4. It is not clear to me what types of structures he is trying to portray by using the empirical formulae C o B r H and Co Br~. There does not seem to be any evidence for any unusual complexes in these solutions, and there does not seem to be any need to postulate them. It really becomes a matter of semantics because nobody believes that in solution B r - , for example, exists as such, but it must be solvated by or coordinated with other species. D r . Kamiya also implies that the initiation step is a direct reaction of the hydrocarbon with C o (III) ion. To my knowledge, a reaction such as this in acetic acid solution has never been demonstrated. W e have shown that the reaction between cobalt (III) acetate in acetic acid and toluene is negligibly slow. It would be more likely to consider the reaction 2 +


ORGANIC COMPOUNDS

3+

[CoBr] -»Co 2 +

2 +

+ Br

and the Br- formed could act as the initiator. In considering the equilibrium depicted i n Reaction 2 the author states essentially that at higher and higher dilution since the equilibrium would be displaced more to the right, the rate of the reaction should increase if this rate depends on H B r concentration. It is true that the equilibrium would be displaced to the right, but the absolute concen tration of H B r in solution obviously decreases as the concentration of cobalt acetate bromide decreases; hence, the rate should, of course, decrease as C o A c B r concentration decreases.


Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions

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