Chapter 7
Nature of the Co-Mn-Br Catalyst in the Methylaromatic Compounds Process Kinetic and Thermodynamic Studies Downloaded by NORTH CAROLINA STATE UNIV on October 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch007
1
2
W. Partenheimer and R. K. Gipe 1
Amoco Chemical Corporation, P.O. Box 3011, Naperville, IL 60566-7011 Consultant, 72 Evans Road, Brookline, MA 02146 2
We have used the reaction of m-chloroperbenzoic acid with Co/Mn/Br as a model system to attempt to understand the nature of this important autoxidation catalyst. Using stopped-flow and UV-VIS kinetic techniques, we have determined the step-wise order in which the catalyst components react with each other. The cobalt(II) is initially oxidized to Co(III) by the peracid, the cobalt(III) then oxidizes the manganese to Μn(IIΙ), which then oxidizes the bromide. The order of these redox reactions is the opposite to that expected from thermodynamics. Suggestions will be made of the relationship of this model to the known characteristics of autoxidation processes.
A highly efficient, general method to produce aromatic acids is via the liquid phase reaction of methylaromatic compounds with dioxygen: Co(OAc) ,Mn(OAc) , HBr > Q H ( C O O H ) + η H 0 (1) HOAc 25-250°C 1-15 atms 2
C H^ (CH ) + 0 6
n)
3
n
2
2
(M
n
2
Hundreds of different carboxylic acids have been produced via this method (1). The industrial process, dubbed the Amoco MC process, produces billions of pounds of terephthalic acid, isophthalic acid and trimellitic acid annually. Industrial processes using just cobalt as the catalyst rather than Co/Mn/Br have also been developed (2-3). The characteristics of the reaction suggest that it is, at least partially, a free radical chain mechanism involving peroxy
0097-6156/93/0523-0081$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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82
CATALYTIC SELECTIVE OXIDATION
and alkoxy radicals, peroxides and peracids (2-5). The easily observable intermediates are alcohols, acetates of the alcohol (since the solvent is acetic acid) and aldehydes. We are greatly indebted to the pioneering work of Jones who characterized much of the chemistry and reported the initial kinetic datafo-ity. He has shown that reaction of m-chloroperbenzoic acid (MCPBA) with Co(II) acetate initially generates an active form of cobalt, labeled Co(III)a, which slowly re-arranges to a less active form Co(m)s. The difference in rates of these forms with bromide and manganese(II) are 30,000 and 6,000 respectively. There have been numerous studies of solid and solution forms of Co(DI) acetate which suggest various different polynuclear compounds form (10). Peracids form as transient species from the oxidation of benzaldehyde during autoxidation. For convenience we have chosen m-chloroperbenzoic acid (MCPBA) as our oxidant since this would be similar to the peracid formed from the very important intermediate 4-carboxybenzaldehyde formed during the oxidation of p-xylene (2). MCPBA would be formed in very low concentrations during oxidation hence we normally study the reaction of MCPBA with an excess of catalyst components i.e. MCPBA <
13.MCPBA + Co(n) 14.MCPBA + (Co+Mn) 15.Co(in)a +Mn(II) 16.MCPBA + (Co + Mn+NaBr) 17.Co(III)a + Mn(II) 18.Μη(ΙΠ) + NaBr 19.Co(ffl)ox + (Co+Mn) 20.Co(ffl)a 21.Co(ni)s + p-xylene h
a
M C B A + Co(UI) M C B A + KBr M C B A + Mn(III) MCBA Co(ni)s 3
d
6. Co(m)a + Mn(II) 7. Co(ni)a + NaBr 8. M C P B A + (Co+Mn) * 9. Co(in)a + Mn(II) 10.MCPBA + (Co+Mn+NaBr) ll.Co(in)a + Μ η ( Π ) 12.Μη(ΙΠ) + H B r
Temp,C
Products
g
Co(II) Co(II) MCBA Co(II) MCBA Co(II) Mn(U)
+ Mn(UI) + NaBr + Co(III)a + Mn(III) + Co(III)a + Μη(ΙΠ) + HBr, e
3
M C B A + Co(in)a M C B A + Co(IU)a Co(II) + Mn(III) M C B A + Co(in)a Co(II) + Μη(ΙΠ) Mn(II) + NaBr Co(II) + . Μ η ( Ι Π ) Co(III)s Co(II) 3
1
66(2) 0.08 0.017 2x10* 0.0070 (0.002) 6.6(.l) 0.59(.01) 43(.2) 7.1(.l) 36(1) 7.3(.l) 0.0066 (0.0004) 307(12) 232(18) 123(16) 330(25) 82(2) O.ll(.Ol) 1.19 0.24(.04). 0.00012 J
30 23 23 25 25 23 23 25 25 25 25 25 60 60 60 60 60 60 60 60 80
Comments
autocatalytic rxn 0.1% H 0
0
2
1st of 2 rxns 2nd of 2 rxns 1st of 2 rxns 2nd of 3 rxns 3rd of 3 rxns
1st of 2 rxns 2nd of 2 rxns 1st of 3 rxns 2nd of 3 rxns 3rd of 3 rxns
[MCPBA]o=0.0005 M and all others 0.0100M unless otherwise stated. All data have been measured in our labs using traditional and stopped flow apparatus unless otherwise stated. Initial compounds are Cobalt(II) and manganese (Π) acetate tetrahydrates. Standard deviation based on at least three independent measurements. Standard deviation in parenthesis (). Autocatalytic reaction, rate refers to fast part of S curve. Rate of thermal decomposition reported in ref (11) for perbenzoic acid. A number of other peracids give approximately the same rates. [NaBr]o=0.01M, [Co(II)]o=0.005M, [MCPBA]o=0.0003M. Refers to MCPBA being added to a mixture of Co(II) + Μη(Π) acetates. Refers to MCPBA being added to a mixture of Co(II) + Μη(Π) acetates and NaBr. A sample of Co(ffl) prepared via ozone. [Co(II)]o=0.01M, [Co(m)s]o=0.001M, [Μη(Π)]ο=0.01 M. [Co(II)]o=0.01M, [Co(m)]o=0.001. [Co(ffl)]o=0.001M, [p-xylene]o = 1.0 M, under nitrogen.
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
84
CATALYTIC SELECTIVE OXIDATION
reduced by bromide, example 8. This sequence is shown in Figure 2. To confirm this sequence we have added MCPBA to mixtures of Co(n)/Mn(n) (examples 8,9;14,15) and to Co(II)/Mn(II)/NaBr mixtures (examples 10-12;16-18) and observe the two and three consecutive reactions respectively. We have repeated the latter two experiments at 35, 45, and 60°C in which we obtain the same type of absorbance changes and similar relative rates. The data at 60°C are given on Table I.
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The Importance of the Conversion of Co(III)a to Co(III)s As pointed out in the introduction, Co(III)s is much less reactive than Co(m)a by several orders of magnitude. CLEARLY, WE DO NOT WANT Co(ffl)s TO F O R M DURING C A T A L Y Z E D REACTIONS BECAUSE OF ITS G R E A T L Y R E D U C E D REACTIVITY. Table I illustrates that the rate of re-arrangement of Co(III)a to Co(III)s is much slower than the reaction of Co(III)a with Mn(II). Hence Co(III)s does not play an important role in Co/Mn/Br mixtures. This is confirmed in example 19 where a sample of Co(III) acetate, prepared by ozonolysis and having a uv-vis spectrum nearly identical to Co(m)s, is reacted with Mn(II). The rate of reaction is 80 times slower at 60°C than with Co(DI)a. Jones concludes (6) that in processes using only cobalt as the catalyst (no manganese or bromide) the rate of re-arrangement of Co(m)a to Co(III)s is now faster than the rate of Co(III) with p-xylene. The p-xylene is forced to react with the less active form of cobalt(m). This is illustrated on Figure 2. The Thermodynamics of the Sequence of Reactions when MCPBA is Reacted with a Co(II)/Mn(II)/Br- Mixture. The free energies of the appropriate reactions are given on Table II. As can be seen, the most easily oxidizable substance is bromide ion followed by Μη(Π) and finally Co(II). We are assuming MCPBA to be the strongest oxidant since it completely reacts with Co(II) to give Co(III). When MCPBA reacts with a mixture of Co/Mn/Br however, it does not react with the most easily oxidizable substance - the bromide ion- but rather the hardest to oxidize substance-Co(II). Similarly Co(III) does not choose the most spontaneous choice available to it (the bromide ion) but rather reacts with Μη(Π). The peroxides and peracids formed in autocatalytic systems are highly energetic molecules. We now see that the Co/Mn/Br catalyst serves to rapidly relax this energy in increasingly lower steps winding up with a highly selective bromide(O) radical (probably as a complex with the metal). The bromide(O) transient species quickly reacts with methylaromatic compounds to form PhCH radicals and hence continues to propagate the chain sequence. #
2
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
PARTENHEIMER AND GIPE
Co-Mn-Br Catalyst
υ 03
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PC Ο
0.00, WAVELENGTH, NM
Figure 1. UV-VIS Spectra of Co(III)a (—), Co(III)s (-.-), Co(II) (--..), and Mn(III)(...). Co(III)a = Co(III)s = 0.00100M, Co(II) = 0.0200M, Mn(III) = 0.00200M a. Thermal Decomposition MCPBA
> MCBA + OH*
CIPhCH, > ClPhCH * + H 0 2
very slow
2
+0
2
+o, free radical chain sequence to aldehyde < b. Cobalt Catalyzed Co(n)
2
MCPBA
> Co(in)a -MCBA
>
Co(III)s
CIPhCH, > CIPhCH ' -Co(B) 2
+o
+o
2
2
free radical chain sequence to aldehyde < c. Cobalt/Manganese/Bromide Catalyzed Co(II) Mn(II) BrCIPhCH, MCPBA >Co(in)a > Μη(ΙΠ) >Mn(H)-Br >ClPhCH * 4 -MCBA -Co(II) -Μη(Π) -Co(n) e
2
+o
2
+o
2
free radical chain sequence to aldehyde
m-chlorobenzoic Acid Co(m) —- ->Co(H) Μη(ΠΙ)— ->Mn(II) Br->Br 2
ClPh(=0)OH
+ Co(III)
2
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
(3)
7. PARTENHEIMER AND GIPE
87
Co-Mn-Br Catalyst
Table III. Comparison of the Reaction of M-Chloroperbenzoic Acid with and without Cobalt at 60°C
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Thermal Decomposition Half-life, sec Ea,Kcal/mol m-chlorobenzoic acid, mol% yld. Chlorobenzene, mol% yJd. Carbon dioxide, mmol Methylacetate, mmol
900 27 89 4.8 0.70 0.034
a a a
With Cobalt(II) Acetate 0.0023 9.5 100
