Oscillations and Chaos in Some O2 Oxidations - Advances in

6 Oscillations and Chaos in Some O Oxidations Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch006

2

J. D. Druliner , L. D. Greller , M. G. Roelofs , and 1

2

1

E.

Wasserman

1

Central Research and Development, Ε. I. d u Pont de Nemours and C o m p a n y , Experimental Station, P . O . Box 80328, Wilmington, DE 19880-0328 SmithKline Beecham, P . O . Box 1539, K i n g of Prussia, P A 19406-0939 1

2

Oscillatory or chaotic dynamics were found for the metal-catalyzed air oxidations of benzaldehyde, cyclohexanone, toluene, and p-xylene. Hydrocarbon autoxidations are suggested as an appropriate area in which to look for new examples of oscillatory behavior.

W ^ E L L - D O C U M E N T E D O S C I L L A T I O N S of chemical species in homogeneous reactions were reported in 1921. Bray observed the unusual behavior in the reaction of H 0 with I (I). Since then a variety of dynamic behavior has been reported for numerous chemical systems, particularly in the oxidation and reduction of halogens and oxyhalogens. Reactions that are sufficiently far from equilibrium can give rise to a range of dynamic phenomena, in cluding multiple stationary states and simple, two-period, and aperiodic (chaotic) oscillations (2-5). In this chapter, we summarize chemical observations and dynamics calculations involving our own studies of several o r g a n i c - 0 oxidations. These oxidations include the catalyzed reaction of 0 with benzaldehyde, cyclohexanone, toluene, and p-xylene. Given the highly exothermic nature of these reactions and the explosive potential of organic vapor-oxygen gas mixtures, appropriate safety precautions must be taken. Thus, all experi ments described here were performed with proper barricades. 2

2

2

2

2

Some rather general broad-ranging features associated with hydrocarbon autoxidations in solution suggested that this was an appropriate area in which to look for new examples of oscillating behavior. These autoxidations have 0065-2393/92/0230-0095$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

96

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

features that are generally associated with oscillating behavior. Such a system should

• be far from equilibrium;

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch006

• have sufficient mechanistic complexity; and • exhibit autocatalytic behavior.

When far from equilibrium, a reaction may be highly exothermic and exhibit nonlinear kinetics. The mechanistic complexity of a system can be expressed in terms of independent first-order differential equations. At least two such equations, for two intermediates, are required for periodic oscil lations; at least three are required for aperiodic, chaotic behavior. Finally, feedback is necessary; that is, the product of a reaction step influences the rate of formation or destruction of that product. Autoxidation reactions involve feedback in the autocatalytic buildup of alkyl radicals. These intermediates arise from alkyl hydroperoxides and their hydroxyl and alkoxyl radical homolysis products. 0 RH R' — U R O O " > ROOH + R 2

ι

RO

e

RH e

> W + ROH RH

+ HO'

>W + HOH

Ο 2 Oxidation of Benzaldehyde While searching for examples of oscillating oxidations at D u Pont, we learned of the seminal discovery by J. R. Jensen (6). H e examined an often-studied Ο oxidation reaction, the Co-Br-catalyzed air oxidation of benzaldehyde to benzoic acid in acetic acid-water solutions. His experimental setup differed from those usually used by incorporating an electrode, a detector with a short response time. Sustained oscillations can be observed with it (Figure 1). The period varies with conditions between about 15 s and 7 min. 2

At the lowest potentials, these solutions are the characteristic pink of C o . Near the maxima they are the dark green of C o in acetic acid. Simultaneous measurement of the redox potential and the visible absorbance maximum of C o at 610 nm (Figure 2) reveals that these quantities exhibit parallel behavior (7, 8). 2 +

3 +

3 +

Although Br~ was present in the reaction, measurements taken with a Br" ion-selective electrode showed that its concentration varies only slightly


D R U LINER ET AL.

Oscillations

and Chaos

in Some 0

2

97

Oxidations

r

ι


POTENTIAL OF Pt ELECTRODE

I

I

L

Ο

5

10 TIME (min)

Figure

1. Oscillations in redox potential accompanying zaldehyde at 70 °C in acetic acid-water

40 TIME (S)

Ο2 oxidation (90:10).

of ben

80

Figure 2. Oscillations in redox potential and visible absorbance accompanying O2 oxidation of benzaldehyde at 70 °C in acetic acid-water (90:10). (Repro duced from reference 7. Copyright 1983 American Chemical Society.)


98


within a period. This lack of variation in [ B r ] is in marked contrast to that of the Belousov-Zhabotinskii and Briggs-Rauscher reactions, both of which are characterized by orders of magnitude changes in concentrations of Br (or I ) and oxyhalogen intermediates, such as H B r O (I, 2, 9). Two different reaction stages, I and II, can be distinguished in the z

benzaldehyde system (Figure 3). During stage I the concentration of dissolved 0 increases to a maximum of ~25% of saturation and then decreases to near zero. During stage II the dissolved 0 remains undetectable, while the redox potential quickly drops from its highest to its lowest values. The results of O labeling were compatible with two different contributions to the generation of benzoic acid, the final oxidation product. During stage I benzoyl radicals react primarily with dissolved 0 , forming benzoyloxy radicals. During stage II, with its absence of dissolved 0 , oxygen from H 0 combines with benzoyl radicals as they are oxidized by C o . Increased concentrations of radicals during step II could be detected by electron paramagnetic resonance (EPR) spectroscopy (10). This evidence of two different stages in the oxygen incorporation indicates how the richness of oscillating 2


2

l s

2

2

3 +

2

POTENTIAL OF Pt ELECTRODE

DISSOLVED OXYGEN CONCENTRATION Figure 3. Comparison of potential of the Pt electrode with dissolved O2 concentration in the oxidation of benzaldehyde at 70 ° C in acetic acid-water (90:10). (Reproduced from reference 7. Copyright 1983 American Chemical Society.)


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DRULINER ET AL.

99

Oscillations and Chaos in Some 0 Oxidations 2


behavior, encountered in a reaction often studied under steady-state con ditions, may yield additional mechanistic insight. Other groups working with this system have described oscillations dur ing oxidations of acetaldehyde or propionaldehyde (11), an alternative de tailed mechanism (12, 13), and a simplified mechanism for the oscillations (13). Despite some differences, both proposed detailed mechanisms (8, 13) are fundamentally the same and possess some unusual mechanistic features. Both involve reactions whereby benzoyl radicals are autocatalytically pro duced via reactions in which two radicals are the products of one step. Both assign a key role to the oxidation of benzoyl radicals in a reaction involving a complex containing C o and Br". 3 +

Ο Oxidation of Cyclohexanone 2

The Ο oxidation of cyclohexanone was examined for oscillating behavior in an otherwise well-established aliphatic reaction (14). Unlike the high yield (>98%) obtained in the oxidation of benzaldehyde to benzoic acid, the re action of Ο with cyclohexanone to form adipic acid is accompanied by at least 100 other products detectable by capillary gas chromatographic (GC) analysis. The same concentrations of Co(OAc) and NaBr in acetic acid-water were used for the cyclohexanone system as previously employed with benz aldehyde. Modestly higher concentrations of O were used with tempera tures of about 100 °C versus 70 °C with benzaldehyde. 2

2

2

z

In a comparison of the two systems, the cyclohexanone system exhibits greater changes in both redox potentials and dissolved 0 concentrations (Figure 4). The qualitative behavior of both variables also differed for the two systems. With cyclohexanone the redox potential rises slowly during the greater part of each cycle, followed by a rapid rise to a maximum value 2

1

3

%

*

1

5

Time, min Figure 4. Comparison of potential of the Pt electrode with dissolved 0 con centration in the oxidation of cyclohexanone at 105 °C in acetic acid-water (90:10). (Reproduced from reference 14. Copyright 1988 American Chemical Society.) 2


100


and a very rapid drop to a minimum. The dissolved 0 concentration rises slowly to near saturation and then decreases very quickly as the redox po tential reaches a maximum. Whereas two distinct stages of behavior were discernible with benzal dehyde (Figure 3), three stages can be assigned in the cyclohexanone system (Figure 5). During stage A the concentration of C o remains relatively constant and the redox potential rises very slowly to a knee marking the beginning of stage B. During Β both redox potential and [ C o ] rise quickly to maximum values. During stage C both redox potential and [ C o ] rapidly drop to their minimum values. The long period of relatively flat potential in stage A , corresponding to slow autoxidation of cyclohexanone, is absent in the benzaldehyde system. 2

3 +

3 +


3 +

0.64

ι

800

ι Absorbance at 610 η m

0.60

3

Potential in mV 700

0.56 600

S 0.52 Ο 500

0.48

0.44 0.0

Stage A ι

250.0

500.0

750.0 Time, sec

1000.0

1250.0

1500.0

Figure 5. Comparison of redox potential and visible absorbance dunng one oscilfotion cycle accompanying 0 oxidation of cyclohexanone at 99 ° C in acetic acid-water (90:10). (Reproduced from reference 14. Copyright 1988 American Chemical Society.) 2

The primary role of cobalt during stage A is to facilitate the gradual increase in the concentrations of 2-hydroperoxycyclohexanone and 2-hydroxycyclohexanone, with little increase in the concentration of C o . Stage Β begins when the concentration of 2-hydroxycyclohexanone builds to mod erate concentrations. Its facile oxidation by C o gives intermediate R C O (radical X), and stage Β begins with the steps outlined in Scheme I. Rapid 3 +

3 +


DRULINER E T A L .

Oscillations

and Chaos

in Some O

z

Oxidations

101


6.

Scheme I. Ring-opening

reactions

of

2-hydroxycyclohexanone.

generation of R C 0 H (species Z , Scheme I) results and effects conversion of C o to C o . C o , in turn, oxidizes 2-hydroxycyclohexanone even more rapidly. 3

2 +

3 +

3 +

When the rather constant rate of transfer of 0 from gas to the liquid phase is finally unable to match its increasing rate of consumption, stage C begins, and C o is converted to C o . The major reaction pathways for conversion of C o to C o at low dissolved 0 concentrations may well involve reactions of the 2-cyclohexanonyl radical with solvent or Br" ion to 2

3 +

2 +

3 +

2 +

2


102



700

mV

200 200 1

1

205

1

210

1

1

215

220

1

225

Seconds Figure 6. Electrochemical potential measured at a pktinum electrode during oxidation of 13 wt % p-xylene in 90:10 wt/wt acetic acid-water solvent in a 7L continuously fed stirred tank reactor containing Co(OAc) , Mn(OAc) , and HBr catalysts at 100, 220, and 210 ppm wt basis metal atom and Br, respectively. Twenty-five seconds of a typical 300-s digitized trace sampled at 30 Hz is shown. The average fundamental period of the aperiodic oscillation is about 4.5 s, as determined from power spectral analysis of a full 300-s trace. 2

2

yield the intermediates 2-hydroxycyclohexanone, 2-acetoxycyclohexanone, and 2-bromocyclohexanone. It would be impractical to try to incorporate the many reactions required to account for formation of the 100 or more byproducts into a kinetic model. However, a 29-step kinetic model that was developed describes the main features of the 0 oxidation of cyclohexanone to adipic acid (14). The model simulates the observed oscillating behavior of C o , C o , dissolved 0 , and the many organic intermediates most likely to be on the pathway between cyclohexanone and acids such as adipic acid. 2

3 +

2 +

2

In contrast to the benzaldehyde system, the cyclohexanone system undergoes sustained oscillations in the absence of Br" ion (15). Although the rates of individual reaction steps involving C o are undoubtedly decreased somewhat in the absence of Br", the system as a whole still contains sufficient complexity and robustness to exhibit oscillating behavior. The Br"-free cy3 +


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DRU LINER ET AL.

Oscillations and Chaos in Some 0 Oxidations 2

103

clohexanone system represents one of the few known examples of liquidphase halogen-free autoxidation oscillators.

Ο 2 Oxidation of Toluene Studies of the 0 oxidation of toluene (16), a prototypical organic oxidation, were carried out under conditions more vigorous than those used for cyclo hexanone. In this case, Mn(OAc) was used with Co(OAc) and NaBr, again in acetic acid solution but at 140 °C and 140 psig of air, a substantial increase over the previous examples. As reaction conditions become more stressful, oscillations may evolve from simple to complex to perhaps aperiodic. In the toluene studies, aperiodic temporal oscillations were observed in both light absorption and electrochemical potential between platinum and silver elec trodes. A complex time behavior is observed with significant noise and variations in periodic potentials. Several different analyses demonstrate that the dynamics are chaotic. The arguments are somewhat complex, and the details will be published elsewhere. 2


2

2

As a further example of an oscillating 0 oxidation system, the oxidation of p-xylene was studied (17). The reaction conditions included concentrations of Co(OAc) , Mn(OAc) , and Η Br decreased about threefold from those used for the oxidation of toluene. Also, a temperature of 200 °C was used, versus 140-150 °C for toluene. Aperiodic temporal oscillations in electrochemical potential at platinum and silver electrodes were again observed (Figure 6). In comparison with the corresponding oscillating behavior in the toluene system, the p-xylene electrochemical oscillations were more complex and covered a somewhat larger range of potential. For both toluene and p-xylene oxidations, the detailed analysis of the time behavior exhibited features characteristics of chaos, as distinct from simple or quasiperiodicity. In the examples of 0 oxidation discussed, an increase in the driving force for reaction can cause the system to progress from the usual steadystate conditions to periodic oscillations and to aperiodic or chaotic behavior. Although oscillations involve considerably more complexity than steady-state behavior, they may yield insights that might not otherwise be available. 2

2

2

2

References

1. Bray, W. C. J. Am. Chem. Soc. 1921, 43, 1262. 2. (a) Field, R. J.; Noyes, R. M. Acc. Chem. Res. 1977, 10, 214. (b) Field, R. J.; Noyes, R. M. Acc. Chem. Res. 1977, 10, 273. 3. Nicolis, G.; Prigogene, I. Self-Organization in Nonequilibrium Systems; Wi New York, 1977. 4. Berge, P.; Pomeau, Y.; Vidal, C. Order Within Chaos; Wiley: New York, 1984. 5. Argoul, F.; Arneodo, Α.; Richetti, P.; Roux, J. C. Acc. Chem. Res. 1987, 20, 436-442. 6. Jensen, J. H. J. Am. Chem. Soc. 1983, 105, 2639.



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7. Roelofs, M. G.; Wasserman, E.; Jensen, J. H.; Nader, A. E. J. Am. Chem. Soc. 1983, 105, 6329. 8. Roelofs, M. G.; Wasserman, E.; Jensen, J. H . J. Am. Chem. Soc. 1987, 109, 4207. 9. Field, R. J.; Burger, M. Oscillations and Traveling Waves in Chemical Systems; Wiley: New York, 1985. 10. Roelofs, M. G.; Jensen, J. H. J. Phys. Chem. 1987, 91, 3380. 11. Rastogi, R. P.; Das, I.; Mishra, S. B. S.; Jaiswal, K. Indian J. Chem., Sect. A 1991, 30A, 1. 12. Colussi, A. J.; Ghibaudi, E.; Yuan, Z.; Noyes, R. M. J. Am. Chem. Soc. 1990, 112, 8660. 13. Guslander, J.; Noyes, R. M.; Colussi, A. J. J. Phys. Chem. 1991, 95, 4387. 14. Druliner, J. D.; Wasserman, E. J. Am. Chem. Soc. 1988, 110, 5270. 15. Druliner, J. D.; Greller, L. D.; Wasserman, E. J. Phys. Chem. 1991, 95, 1519. 16. Greller, L. D.; Roelofs, M. G.; Wasserman, E. to be published. 17. Greller, L. D.; Kegelman, M. R.; Lawson, J. R.; Markham, G. P.; Roelofs, M. G.; Wasserman, E. J. Am. Chem. Soc., submitted for publication. RECEIVED for review November 11, 1990. ACCEPTED revised manuscript October 9, 1991.


Oscillations and Chaos in Some O2 Oxidations - Advances in

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