Evans, D., Osborn, J. A., Wilkinson, G., J . Chem. SOC. A1968, 3133. Graham, W. A. G., Inorg. Chem. 7, 315 (1968). Grim, S. O., Wheatland, D. A,, Inorg. Chem. 8, 1716 (1969). Halpern, J., “Proceedings of 3rd Congress on Catalysis.” pp. 147-62, North Holland Publishing Co., Amsterdam, 1965. Henderson, W. A., Jr., Streuli, C. A., J . Amer. Chem. SOC.82, 5791 (1960). Montelatici, S., van der Ent, A., Osborn, J . A., Wilkinson, G., J . Chem. SOC.A1968, 1054. Osborn, J. A., Jardin, F. H., Young, J. F., Wilkinson, G., J . Chem. SOC.A1966, 1711. Pidcock, A., Richards, R . T., Venanzi, L. M., J . Chem. SOC.A1966, 1707. Slaugh, L. H., Mullineaux, R . D., J . Organometal. Chem. 13, 469 (1968). Stewart, R. P., Treichel, P. M., Inorg. Chem. 7, 1942 (1968).
Tucci, E. R., ING. ENG. CHEM. PROD.RES. DEVELOP. 7, 32 (1968a). Tucci, E. R., IND. ENG. CHEM. PROD.RES. DEVELOP. 7, 125 (196813). Tucci, E. R., IND. ENG. CHEM. PROD.RES. DEVELOP. 8, 215 (1969a). Tucci, E . R.,IND. ENG. CHEM. PROD.RES. DEVELOP. 8, 286 (1969b). Tucci, E . R., Deffner. J. F., Ward, J. V. (to Gulf Research & Development Co.), U.S.Patent 3,510,524 (May 5, 1970). Wender, I., Metlin, S.,Ergun, S., Sternberg. H. W., Greenfield, H., J . Amer. Chem. SOC.78, 5401 (1956).
RECEIVED for review January 22, 1970 ACCEPTED July 16, 1970 9th Spring Symposium of the Catalysis Society, Pittsburgh, Pa., 1970.
Terephthalic Acid from p-Xylene James W. Patton’ and Ned F. Seppi Marathon Oil Co., Littleton, Colo. 80120 The cobalt acetate-catalyzed autoxidation of p-xylene to terephthalic acid i s promoted by certain carboxylic acids, alcohols, a n d epoxides. The reactions were run under pressure, from 50 to 150 psig, a t temperatures from 100” t o 150°C. Of the carboxylic acids studied, 2-methylbutyric acid stands out in augmenting the p-xylene autoxidation. For secondary alcohols, 2-butunol was most effective. For tertiary alcohols, the requirement for a successful promoter seems to be the presence of t w o alkyl groups which are larger than ethyl. Of the epoxides studied, only 2,3-epoxybutane proved to be beneficial. The success of these promoters can in general be rationalized on the basis of the oxidation products expected from cobaltic ion oxidations.
T h e autoxidation of p-xylene to terephthalic acid, using various promoters and cobalt acetate as catalyst, was investigated. Experimental
The autoxidations were conducted in Fischer & Porter aerosol compatibility tubes, which have working pressures of 500 psig a t ordinary temperatures. A number of these tubes were deliberately exploded t o verify the manufacturer’s operating pressure claims. In no case did the glass tube fail under 700 psig. This was relatively safe under our oxidizing conditions, which never exceeded 150 psig. Nevertheless, adequate safety shielding was employed. The glass tubes were heated in a constant temperature silicone oil bath. Although the oil baths were maintained a t a constant temperature, the reactor contents routinely exhibited exotherms of 40” to 60°C during periods of maximum oxidation rate. The tubes were stirred magnetically by an apparatus described previously (Patton and Johnson, 1966). A manostat (Patton, 1965, 1966) was used to maintain the reactors under constant pressure. The pressure of the oxygen reservoir was monitored by a pressure transducer whose output was fed to a recorder. T o whom correspondence should be sent
The oxygen reservoir was not thermostated, but the laboratory room temperature was relatively constant, so that the oxygen consumption data are probably accurate to within 10%. Quarter-inch stainless steel tubing was fitted to the top of the aerosol compatibility tubes by means of Swagelok or Gyrolok compression fittings. I t was possible to run a thermocouple into the reactor through these fittings to monitor the internal temperature of the reactor a t all times. Two types of reactors were used. The first (Patton and Johnson, 1967) used a tee mounted in the head of the reactor so that a gas inlet and outlet tube could be employed. A magnetic gas circulation pump enabled the gases to be circulated through Mallcosorb to remove carbon dioxide, so the reactions would not be smothered. The second type coupled a graduated heavy gage glass tube through a needle valve and tee to the reactor. This tube was maintained under the same pressure as the reactor. Known amounts of materials could be added to the reactor while reactions under pressure were in progress. In both reactor types, the incoming stainless steel tubing was water-jacketed t o keep the volatile materials in the reactors. The materials used were: p-xylene, Phillips research grade, 99.9t mole csc pure; cobaltous acetate tetrahydrate, Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
521
Matheson Coleman & Bell, reagent grade; and glacial acetic acid, Mallinckrodt, analytical reagent. The aliphatic carboxylic acids tested as activators were all from Eastman Organic Chemicals; of the alcohols tested as activators, 2-methyl-2-butanol and 2-pentanol were obtained from K & K Laboratories, the rest from Eastman Organic Chemicals. All materials tested as activators were used as supplied, without further purification. The oxygen used was from the Air Reduction Co., U.S.P. The precipitated terephthalic acid was determined by conversion to the trimethylsilyl esters (Klebe et a1 , 1966). p-Toluic acid and p-carboxybenzaldehyde were found as impurities in the terephthalic acid. The trimethylsilyl esters were determined on an F & M Model 720-4 gas chromatograph, using a 5% Apiezon L on Chromosorb W, 2-foot. 4-inch stainless steel column. An ether solution was injected a t 50°C with a program rate of 10" per minute, using helium as a carrier gas a t 25 psig and a flow rate of 41 ml per minute. Further autoxidation of the mother liquors with methyl ethyl ketone afforded additional terephthalic acid. Similar gas chromatographic analysis of this material consistently afforded a 95% material balance based on the p-xylene charged. The consistent 5% loss has not been rationalized, but may be due to aromatic carboxylic acid decarboxylation by cobaltic ions (Starnes, 1966).
Table I. Effect of Cobalt Concentration on Autoxidation of p-Xylene
3.0 ml of p-xylene, 25.0 ml of cobaltous acetate in acetic acid a t 100" C and 50 psig oxygen" CO(OAC)? .4H.O Concentration, M
Oxygen Absorbed, Mmoles
Mole O h Yield Terephthalic Acid
0.10 0.20 0.40
59 67 70
58.6 80.4 85.7
' Reaction time 20 hr.
Table II. Effect of Pressure and Temperature on Autoxidation of p-Xylene
3.0 ml of p-xylene, 25.0 ml of 0.20M Co(OAc)?.4H20in acetic acid Reaction Time, Hr
Temp., C
Oxygen Absorbed, Mmoles
20 20 20 20 20 20 20 1
100 100 100 100 125 125 125 150
35 67 68 68 70 70 71 18
Mole % Yield Terephthalic Acid
0 psig
50 psig
100 psig
150 psig
6.70 80.4 76.9 79.1 84.5 84.0 85.0 3.7
Results
The catalytic activity of transition metal ions, such as cobalt, has long been recognized in autoxidation reactions. Hydroperoxide intermediates can be oxidized or reduced by transition metal ions. However, the direct oxidation of organic substrates other than hydroperoxides or radical intermediates has until recently not been stressed in autoxidation reactions (Heiba et al., 1969). Such direct oxidations are not generally important for low catalyst concentrations, but when the catalyst concentration is large they can play an important role. I n studying the autoxidation of alkyl aromatic hydrocarbons, such as p-xylene, in acetic acid, catalyzed by cobalt acetate, we found a remarkable dependence of the rate and extent of oxidation upon the cobalt concentration. Data illustrating this are summarized in Table I , for the autoxidation of p-xylene. After a brief induction period, the autoxidation proceeds very rapidly until the p-xylene is all consumed. At this point some terephthalic acid precipitates. The principal product at this stage is p-toluic acid, and now the autoxidation proceeds very slowly. If high cobalt concentrations are employed, relatively high yields of terephthalic acid are obtained when reaction times are long. The effects of temperature and pressure are shown in Table 11. Pressures of a t least 50 psig are needed a t 100"C, and at higher oxidation temperatures even higher pressures are needed. The rate of oxidation of p-toluic acid is the limiting factor. If more p-xylene is added, its rate of oxidation is similar to that of the first p-xylene charge, but it does not appreciably help the oxidation of the p-toluic acid present. The induction time of the additional p-xylene charge is dependent upon the time of addition. p-Xylene added after the oxidation rate has leveled off has an induction almost as long as the original charge, whereas material added during the rapid oxidation rate has a much shorter induction period, as is to be expected, since a 522
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
much higher radical concentration should be present a t this time (Figure 1). The terephthalic acid process used by Mobil Chemical is based on the work of Brill (19601, and uses methyl ethyl ketone as an activator. The ketone is co-oxidized to acetic acid. Since much better yields of terephthalic acid are obtained with high cobalt concentrations, it seemed possible that high concentrations of cobaltic ions may have been instrumental in oxidizing certain intermediates such as p-carboxybenzyl alcohol, p-carboxybenzaldehyde, or even p-toluic acid itself. An excellent review of the oxidizing capabilities of cobaltic ions has recently been Table Ill. Autoxidation of p-Xylene
3.0 ml of p-xylene, 25.0 ml of 0.20M Co(OAc)y.4H?O in acetic acid a t 125"C and 100 psig oxygen plus carboxylic acid promoter" Promoter,' M I
Oxygen Absorbed, Mmoles
Mole Oc'i Yield Terephthalic Acid
None Propionic acid 1 Propionic acid 2 Propionic acid 5 Isovaleric acid 1 Isovaleric acid 2 Isovaleric acid 5 2-Methylbutyric acid 1 2-Methylbutyric acid 2 2-Methyibutyric acid 5 2-Methylbutyric acid 5 Pivaiic acid 1 Pivaiic acid 2 Pivalic acid 5
60 68 80 91 80 88 108 96 122 188 11.5 74 76 90
50.8 61.2 59.3 64.2 56.7 55.7 56.7 74.9 82.3 84.5
... 51.5 42.0 26.2
" Reaction time 2.5 hr. * Gas circulation pump started at oxygen absorption plateau. Circulated through Mallcosorb. ' No p-xylene added.
fl
i 4 0 1
0
o n c d d i t i o n c l I O ml p - x y l e n e added
'0
i
ecch
reocfior ct the poirts indicated by X
''I ;I 20
, 3
I 5
4
6
T m e (hours)
Figure 1.
Effect of adding p-xylene during autoxidation
30 ml of p-xylene plus 2 5 . 0 ml of 0.20M Co(OAc)2.4H?O in ocetic acid at looc C and 50-psig oxygen
published (Waters and Littler, 1965). Accordingly, then, it should be possible to add another substrate to be co-oxidized, and this need not be restricted to those which are oxidized only by radical processes, but can also include substrates resistant to radical attack but subject to direct oxidation by cobaltic ions. The products of such direct oxidation by cobaltic ions would determine whether the material is a satisfactory activator. The importance of very high cobalt concentrations as an alternative approach should not be overlooked (Ichikawa, 1967; Yoshimura, 1969). The promotion of the cobaltic ion oxidation of toluene by aliphatic acids has been recently described (Cooper et al., 1966). Aliphatic carboxylic acids, other than acetic acid, are oxidatively decarboxylated during cobaltcatalyzed autoxidations.
RCO,-Co"'
-
Re
+ COZ + CO"
(1)
However, most acids do not substantially promote the further oxidation of p-toluic acid during a p-xylene autoxidation.
RI I11
RI
R?-C-O--Co
I
R,
R?-C-0.
I
RI
R1
I1
R,-C=O
3.0 ml of p-xylene, 25.0 ml of 0.20M C O ( O A C ) ~ . ~ H ~ O
in acetic acid a t 125" C and 150 psig oxygen plus 1.0 ml of promoter added initially" Oxygen Absorbed, Mmoles
Mole Yo Yield Terephthalic Acid
60 84 86 68 79 76 103 105 82 79 100 100 104 114
31.3 50.0 50.1 36.8 40.5 32.2 79.6 86.0 48.6 50.0 71.2 80.5 80.2 83.7
3.0 ml ofp-xylene, 26.0 ml of 0.20M C O ( O A C ) ~ . ~ H ~ O in acetic acid at 125" C and 150 psig oxygen plus 1.0 ml promoter added a t oxygen absorption plateau Promoter
Methanol Ethanol 1-Propanol 2-Propanol 1-Butanol Isobutyl alcohol 2-Butanol Methyl ethyl ketone 2-Methyl-2-butanol 3-Methyl-1-butanol 2-Pentanol 3-Pentanol 3-Ethyl-3-pentanol 3-Methyl-3-pentanol
' Reaction time 2.5 hr.
+ CO + Rj* ( 2 )
Table V. A u t o x i d a t i o n of p - X y l e n e
Table IV. A u t o x i d a t i o n of p - X y l e n e
Promoter
The nature of the resulting radical in the cobaltic ion oxidation of the carboxylic acid is all-important in determining whether the rate of oxidation of p-xylene to terephthalic acid is increased (Table 111). The only acid that significantly improved the yield of p-xylene was 2-methylbutyric acid. The product of the oxidation of 2-methylbutyric acid by cobaltic ions is the sec-butyl radical. Reaction of this radical with oxygen produces a peroxy radical that ultimately yields 2-butanol, methyl ethyl ketone, or oxidation products of these. Interestingly, pivalic acid, which yields tert-butyl radicals upon oxidation by cobaltic ions, impeded the autoxidation of p-xylene when present in large amounts. Other examples of the autoxidative decarboxylation of aliphatic carboxylic acids by cobalt catalysis have appeared in the patent literature (Melchiore, 1962; Pasky, 1966). The co-oxidation of alcohols can also promote the autoxidation of p-xylene to terephthalic acid (Chibnik. 1966). Primary and secondary alcohols can be oxidized either by radical attack or by direct oxidation with cobaltic ions. However. tertiary alcohols can be oxidized only by cobaltic ions. A variety of alcohols was examined for their effectiveness as activators in the autoxidation of p-xylene (Tables IV and VI. For the experiments in Table IV, the alcohols were added initially to the reaction mixture, whereas in Table V, the alcohols were added while the reaction was in progress just after the initial rapid rate had subsided. I n the case of the tertiary alcohols. the oxidation by cobaltic ions can lead either to an alkoxy radical or directly t o a ketone and an alkyl radical (Waters and Littler, 1965).
Methanol Ethanol 1-Propanol 2-Propanol 1-Butanol Isobutyl alcohol 2-Butanol 2-Butanol Methyl ethyl ketone 2-Methyl-2-butanol 3-Methyl-1-butanol 2-Pentanol 3-Pentanol 3-Ethyl-3-pentanol 3-Methyl-3-pentanol
Reaction Time, Hr
2.5 2.5 2.5 2.5 2.5 2.5 2.5 5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
Oxygen Absorbed, Mmoles
Mole O h Yield Terephthalic Acid
48 70 65 48 63
24.8 53.1 46.4 24.3 44.3 31.1 54.2 €45.6 90.8 46.8 35.9 85.3 85.3 71.3 63.1
_-
an
66 89 92
58 60 85 85 82 -r I3
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
523
Table VI. Autoxidation of p-Xylene
3.0 ml of p-xylene, 25 ml of 0.20M Co(OAc)?.4 H 2 0in acetic acid Promoter added a t plateau
'
Promoter 1 MI
Temp., ' C
Pressure, PSlG
Reaction Time, Hr
Propylene oxide 12-Epoxybutane 1.2-Epoxyisobutane 2.S-Epoxybutane ;3-Acetoxy-2-hutanol cis-2,3-epoxybutane trans-2,3-epoxybutane cis-2,3-epoxybutane~ trans-2.3-epoxyhutane'
100 100 100 100 100 125 125 125 125
100 100 100 100 150 150 150 150 150
4.4 4.4 4.4 4.0 4.0 2.5 2.5 2.5 2.5
Max. Temp.,
O
135 132 153 134(130)" 149 165 153 164 173
C
Oxygen Absorbed, Mmoles
Mole Oh Yield Terephthalic Acid
55 56 54 88 71 86 87 90 92
39.3 44.6 33.3 92.5 82.4 88.0 89.8 59.1 55.5
Second exotherm. "Activator added initially rather than a t plateau.
The group, R i , that cleaves, will be the group that is the most stable radical of the three alkyl groups of the carbinol. Thus, to obtain a methylene ketone from a tertiary alcohol, the choice is in favor of one in which two of the alkyl groups are ethyl or larger. None of the primary alcohols are effective activators. Secondary alcohols that are effective as activators include 2-butanol, 2-pentanol, and 3-pentanol. Of the tertiary alcohols examined only 3-methyl-3-pentanol and 3-ethyl3-pentanol were effective. Thus, the direct oxidation of certain aliphatic carboxylic acids or alcohols by cobaltic ions can produce activators similar to those obtained by the co-oxidation of methyl ethyl ketone with alkyl aromatic hydrocarbons. By the use of these materials, yields of terephthalic acid in excess of 80% can be obtained in 21/L hours. With longer reaction times, larger yields can be obtained. The reaction conditions are relatively mild, 100" to 125" C with total pressures of 50 to 150 psig, using molecular oxygen as the oxidant. Of the epoxides studied, only 2,3-epoxybutane was successful as a promoter. I t is not clear whether 2,3epoxybutane is being oxidized directly by cobaltic ions or whether radical attack is producing a radical that can rearrange t o the same radical expected from methyl ethyl ketone.
0
I
CH 3 C-GHCH
i
(3)
X o work has yet been reported on the oxidation of ethers by cobaltic ions, so this path must remain speculative for the time being. Table VI summarizes the data for epoxides.
These oxidations are facile, and the products readily autoxidize further, producing hydroperoxides and peroxy radicals that in turn are capable of oxidizing several cobaltous ions t o cobaltic ions. The net result is a steady increase in cobaltic ion concentration until it is high enough to sustain a rapid oxidation of the intermediate p-toluic acid. Several promoters, such as 2-methylbutyric acid and the tertiary alcohols, are not susceptible to free radical attack. Their oxidation can be explained by reaction with cobaltic ions (Waters and Littler, 1965). Literature Cited
Brill, W. F., Ind. Eng. Chem. 52, 837 (1960). Chibnik, S., U. S. Patent 3,284,493 (Nov. 8, 1966). Cooper, T. A.! Clifford, A. A . , Mills, D. J., Waters, W. A., J . Chem. SOC.1966B, 793. Heiba, E. I., Dessan, R. M., Koehl, W. J., Jr., Preprint, Div. Petroleum Chem., 14(2),A44, 157th Meeting, ACS, Minneapolis, April 1969. Ichikawa, I., U. S. Patent 3,334,135 (Aug. 1, 1967). Klebe, J. F.. Finkbeiner, H., White, D. M., J . Amer. Chem. Soc. 88. 3390 (1966). Melchiore, J. J., U. S. Patent 3,055,839 (Sept. 25, 1962). Pasky, J. Z., U.S. Patent 3,251,878 (May 17, 1966). Patton, J . W., Chem. Eng. (Neu: York) 72, 226 (June 7, 1965). Patton, J . W., Chem. Eng. (Neu: York) 73, 208 (April 11, 1966). Patton, J. W., Johnson, I. D., J . Chem. Ecluc. 43, 390 (1966). Patton, J. W., Johnson, I.D., J . Chem. Educ. 44, 467 (1967). Starnes, W. H., Jr., J . Org. Chem. 31, 1436 (1966). Waters, W. A., Littler, J. S., "Oxidation in Organic Chemistry," K. B. Wiberg, Ed., Chap. 3, Academic Press, New York, 1965. Yoshimura, T.. Chem. Eng. (New Y o r k ) 76, 78 (May 5, 1969).
Conclusions
The success of promoters tested is due to the nature of products resulting from oxidation by cobaltic ions.
524
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
RECEIVED for review April 13, 1970 ACCEPTED June 25, 1970
