Ind. Eng. Chem. Res. 1992,31,453-462
453
KINETICS AND CATALYSIS Liquid-Phase Catalytic Oxidation of p -Xylene+ Pavagada Raghavendrachar and Subramania Ramachandran* Research Centre, Indian Petrochemicals Corporation Limited, PO: Petrochemicals, Vadodara 391 346,India
This paper reviews the technological aspects of p-xylene oxidation to terephthalic acid and dimethyl terephthalate highlighting the industrial processes, reaction mechanism, and kinetics. Sixty-nine references are cited.
Introduction The direct oxidation of hydrocarbons with air or oxygen is a commercially important reaction for the production of oxygenated compounds from relatively cheap petroleum and natural gas feedstock (Prengle and Barona, 1970). While both the vapor- and liquid-phase processes are in commercial operation, recent trends indicate a shift over to liquid-phase processes. Apart from the economic advantages such as higher yield, improved selectivity, and milder reaction conditions normally associated with liquid-phase oxidation, advent of novel coordination complex catalysts has also acted in favor of liquid-phase processes. In addition to a large number of conventional applications, homogeneous liquid-phase oxidations are also employed for the production of intermediates for condensation polymers. In this article developments in liquid-phase oxidation of p-xylene to terephthalic acid, an important raw material for polyester fibers, are reviewed. A search of the literature shows that the production of terephthalic acid from homogeneous liquid-phase oxidation of p-xylene is mostly dealt with by a large number of patents. In the open literature we find several reports on subjects such as establishingoptimum conditions for higher conversions (Chervinskii et al., 1962; Chervinskii and Zherebtsova, 1965), use of heterogeneous catalysts (Caloyannis and Graydon, 1971), use of paraldehyde as promotor (Nakaoka et al., 1973), autoxidation of p-xylene in the presence of different promoters (Patton and Seppi, 1970), and kinetics of the reaction (Digurov et al., 1970). Ichikawa et al. (1970) outline a process for high-purity terephthalic acid. However, it can be said that the available literature is not complete when one looks for mechanism, kinetics, or engineering aspects of this reaction, There exists a scope and a need to study the kinetics of this reaction catalyzed by soluble catalysts such as naphthenates, stearates, and transition-metal salts. This paper is an attempt to sum up developments in some of the important aspects. Processes for Terephthalic Acid The commercially important polyesters are produced by polymerizing a dibasic acid with a glycol. In particular, the polymers of terephthalic acid with ethylene glycol belong to a very important class of man-made fibers. Terephthalic acid (TPA) is generally produced by the
* Author to whom correspondence should be addressed. IPCL Communication No. 183.
oxidation of p-xylene with air or oxygen. However, the technology has to overcome a number of problems to produce fiber grade TPA. The catalyst system is to be such that the oxidation leads to the highest yield of TPA under mild operating conditions. Often the product has to be purified from oxidation by-products and catalyst species to obtain fiber grade TPA. One of the well-established techniques is to esterify TPA to dimethyl terephthalate (DMT) to aid in this purification. DMT can be converted to TPA before polymerization, or it can also be directly used. Several technologies have emerged to produce fiber grade TPA in an economical operation. A brief description of the existing process for the production of TPA and DMT is given below.
Processes for Technical Grade TPA HN03 Oxidation of p -Xylene. This process involves liquid-phase oxidation of p-xylene in dilute HNO, of about 30-40 w t % at temperatures ranging from 160 to 200 "C and pressures 8.5-13.5 bar (Burrows et al., 1951, 1953; Boffa et al., 1963). TPA precipitates from the reaction mixture and is separated and purified in subsequent steps. This process has several disadvantages like the higher consumption of HN03, possibilities of explosions, and low purity of the product. Though earlier employed by several manufacturers (Du Pont, ICI, BSAF, Montecatini Edison, etc.), this process is more or less obsolete today. Catalytic Liquid-Phase Air Oxidation. Currently in commercial use by AMOCO, ICI, Montecatini, Maruzen, and Kuraray-Yuka, this process consists of homogeneous liquid-phase oxidation of p-xylene in a solvent by air in the presence of a heavy metal catalyst. There are different versions of this process depending on the catalysts used and operating conditions. Mid-CenturyProcess. Based on an original discovery by Mid-Century Corporation (Saffer and Barker, 1958), this process employs cobalt and manganese acetates or amrnoqium molybdate as catalysts. Acetic acid is used as reaction solvent and sodium bromide acts as catalyst promoter reducing the induction period. Qpical operating conditions are 195-205 "C and about 28bar pressure. The product TPA precipitates out as an insoluble solid whereas the undesirable reaction products remain in solution. The product on further purification has an assay of 97-99% and p-xylene conversion exceeds 95% with a 90% selectivity to TPA. A simplified picture of this process is shown in Figure 1. The main advantage of this process is its versatility. With slightly different operating conditions this process can be used to oxidize toluene to benzoic acid,
OSSS-5SS5/92/2631-0453$03.00/0 0 1992 American Chemical Society
454 Ind. Eng. Chem. Res., Vol. 31, No. 2,1992 REACTOR
__
GAS LIQUID SEPARAlOR
SURGE
CENTRIFUGE RESIOUE STILL
PROF I E R SYNTHESIZER
SCCVENT OEHYORATION TOWER
OXIOATION MIXING REACTOR TANK SEPARATOR
,
0
PROWCT
METHANOL
ESTERIFICATION REACTOR
GEHYORATION
,a
START START
sc LV GI MET HY 1TEREPHTALATE
START
Figure 1. Manufacture of TPA by Mid-Century process. Reprinted with permission from ref 66. Copyright 1964 Gulf Publishing Co. START
I
+
OEHYORATION COLUMN
' $~~~~&'"
TPA PURIFICATION
Figure 4. Manufacture of TPA and DMT by Toray process. Reprinted with permiasion from ref 34. Copyright 1977 Gulf Publishing co.
ACTIVAlOR TPA PROOU(1
CATALYST
FUEPURIFI[ATIDI P-XYLENE [AlALYSl
OXIDIZER
v+
COOLER
ACID RECOVERY SYSTEM
Figure 2. Manufacture of TPA by Eastman-Kodak process. Reprinted with permission from ref 31. Copyright 1973 Gulf Publishing co.
CATALYST AND I N l E R r n T E S
DISTILLAlloN
mwm
CATPLYST PURIFICATION SYSlEM RfACTOR
CRYSTAllKER SOLID
DfHmRAlOR
SEWRAlOR ---OFF GAS
SlARl
Figure 3. Manufacture of TPA by Maruzen process. Reprinted with permission from ref 35. Copyright 1979 Gulf Publishing Co.
pseudocumene to trimellitic acid, or mesitylene to trimesic acid. Eastman-Kodak Process. This technology utilizes a cobalt catalyst activated by acetaldehyde. The liquidphase oxidation is carried out in acetic acid solvent at 120-175 "C and a t moderate pressures of 7.5-15 bar. Acetic acid is a coproduct in this process and is about 0.55-1.1 kg/kg of TPA. Use of acetaldehyde in place of bromide also facilitates the use of conventional construction material. A simplified flow diagram is presented in Figure 2. Maruzen Process. This is a single-step process for the fiber grade TPA. The catalytic oxidation is carried out in acetic acid solution at such conditions which minimize formation of impurities. The slurry from the reactor is conditioned in a crystallizer, and the product is subjected to repeated washings to obtain fiber grade. Figure 3 illustrates the main process steps. Toray Process. In this process, shown in Figure 4, p-xylene is oxidized in acetic acid solution using a cobalt catalyst and paraldehyde as promoter (Nakaoka et al., 1973;Hydrocarbon Process., 1977c. Compared with other promoters in use, paraldehyde is more effective and higher yields of TPA are obtained at relatively mild reaction conditions. Besides, exclusion of bromine salts from the catalyst system allows use of stainless steel equipment.
-
Figure 5. Manufacture of high-purity TPA by Teijin process. Reprinted with permission from ref 69. Copyright 1969 McGraw-Hill Inc.
This catalyst system together with mild operating conditions helps in reducing formation of colored impurities. Typically the process operates a t 110-140 "C and 30 bar and the product purity is about 99%, with major impurities being p-toluic acid and cobalt salts. Part of the TPA can also be converted to DMT through an esterification step. Teijin Process. This process is based on single-step air oxidation of p-xylene (Ichikawa et aL, 1970;Yoshimura, 1969) and is simpler than other competing processes for TPA. The oxidation is carried out at 100-130 "C and 10 bar using acetic acid as solvent and a cobalt salt as catalyst with no promoters. The yield of TPA is 9590,taking into account oxidation of intermediates in the recirculation system. The main feature of this process is that the crude product does not contain coloring impurities such as fluorenone and biphenyl ketones and can be easily purified by washing and crystallization to fiber grade. Also, moat of the mother liquor can be recycled after a simple purification step since impurities that hinder the reaction are absent (Figure 5). However, this process requires relatively large amounts of catalyst.
Processes for Polymer Grade TPA The liquid-phase-oxidation processes described above do not usually produce material of fiber grade. To produce this grade, the processing conditions have to be very stringent and the product has to be recrystallized (Mitaui Petrochemical Industries Limited (Japan), 1964). Technical grade TPA can be converted to fiber grade by purification methods such as sublimation, or solubilization of TPA by forming its disodium, dipotassium, or diammonium salts. Various purification schemes such as adsorption and recrystallization have been described in the literature (Maclean, 1960; Standard Oil Co. (Indiana),
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 455 EVAPORATED WATER
t
cp2
W A S H WATER
P -XYLENE
DEIONIZED
START P-XYLENE
FEED
HUTHFR LIOUOR
REGENERATOR REACTOR
t
PRIMARY HYDROLYSI5 REACTOR
TPA PRODUCT
Figure 6. Manufacture of fiber grade TPA by Lummus process. Reprinted with permission from ref 17. Copyright 1973 Gulf Publishing co.
1965). Park and Mickle Wright (1977) described a process in which p-xylene is oxidized with air in the presence of cobalt and manganese acetates and sodium bromide in acetic acid solution at 225 "C and 15 bar with a residence time in reactor of 90 min. The TPA crystals formed are of 99.95% purity and on recrystallization from aqueous acetic acid solution can reach 99.96% purity. Some important processes for fiber grade TPA are outlined below. Henkel Rearrangement Route. This is based on the conversion of benzenecarboxylic acids to their potassium salts and subsequent rearrangement of the salts in the presence of carbon dioxide and catalysts such as cadmium or zinc oxides to give dipotassium terephthalate, which can be converted to pure TPA. The process of Teijin, Hercules, and Kawasaki-Kasei is called the Henkel-I process and involves use of phthalic anhydride obtained from oxidation of naphthalene. (Schenk and Schiller, 1959; Raecke et al., 1959). In the Teijin process the phthalic anhydride is converted to dipotassium o-phthalate and isomerized at 350-450 "C in the presence of carbon dioxide at 10-100 bar to yield dipotassium terephthalate. Mitsubishi employs the Henkel-I1 process (Raecke et al., 1957; Schirp, 1958) using as feedstock benzoic acid obtained from toluene. Benzoic acid is converted to its potassium salt which undergoes disproportionation to yield dipotassium terephthalate as product and benzene as coproduct. The reaction conditions of both Henkel-I and -11 processes are similar. The dipotassium terephthalate is recrystallized, and its colored impurities are removed by adsorption. Pure crystals of TPA are precipitated by addition of sulfuric acid to the salt solution. Lummus Route. This is a new route based on the ammoxidation of p-xylene to terephthalonitrile followed by hydrolysis to TPA (Gelbein et al., 1973). For ammonolysis a novel metal oxide catalyst is used which also provides oxygen required for the reaction. The reduced catalyst is reoxidized in a separate step. Selectivity in ammonolysis is 90 mol % to terephthalonitrile and ptolunitrile with major by-products being oxides of carbon. Benzonitrile and HCN are also produced in minor amounts, but no aldehydes are formed. Terephthalonitrile
is converted to fiber grade TPA in three stages. In the first stage it is hydrolyzed and simultaneously steam stripped to give monoammonium terephthalate (MAT) which in the second stage undergoes thermal decomposition to give TPA. Hydrolysis of residual terephthalamic acid coming with product is carried out in the third stage, thus minimizing the nitrogen-containing impurities. A simplified flow sheet is presented in Figure 6. Processes for DMT Though polyesterification can be carried out using diacids and diok by direct condensation, the reaction usually results in low molecular weight polyesters. Special techniques are required to obtain high molecular weight polymer because complete removal of water from the reaction mixtdre without loss of monomers is difficult. Though such special techniques are available now, a practical method to obtain high molecular weight polymer involves ester interchange reactions. Hence the technical grade TPA is usually converted to DMT for use in polyesterification. DMT is also easy to purify by distillation and recrystallization. A few important processes for DMT are mentioned below. Dynamit Nobel Process. This involves oxidation of a mixture of p-xylene and p-methyl toluate in the presence of a cobalt catalyst at 140-170 "C and 4-8 bar with air, followed by esterification with methanol at 250-280 "C and 20-25 bar. p-Methyl toluate is an intermediate in this process as can be seen from the reaction scheme given below. oxidation CH3-CGH4-CH3 + 1 7 2 0 2 CH3-CeH4-COOH + H2O
-
(1)
CH~OOC-CGH~-CH~ + 17202 CH300C-CeH4-COOH + H20 (2) esterification CH3-CeH4-COOH + CH30H CH3-C6Hd-COOCH3 + H2O (3) CH3OOC-CGH4-COOH + CH30H CH~OOC-CGH~-COOCH~ + HzO (4)
-
456 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 OXIDIZER YPARAlOR ESTERI- METHANOL ESTER CRYSlA- PETWNOL FlER OlSllN DlSllN LL'ZER OiSlLN 1 2
OM1 OISlllAllOH
MECHANISM I CH;
CH ?
CHZOO'
START
START
MECHANISM
II
S'A4 START
Figure 7. Manufacture of DMT by Dynamit Nobel process. Reprinted with permission from ref 32. Copyright 1977 Gulf Publishing co. RECC-62
--
CEHYCQAWN CaLJPl? -__
C I G ~
5EPAaATOR
0
CErlYCIAAl3N REAUOR
r(CDU[T
COLUklN --__
3
Ii'FICAT,ON
WX'Ncl ;FIIM
YETHA':?.
A
I
START
STAR1
Figure 8. Manufacture of TPA and DMT by Uni-HUs process. Reprinted with permission from ref 33. Copyright 1977 Gulf Publishing Co.
No reaction solvent is used. The crude DMT is distilled to remove p-methyl toluate and further purified by recrystallization from methanol. This process is depicted in Figure 7. Uni-Hulls Process. In this process p-xylene is oxidized to TPA in the liquid phase at moderate temperature and pressure conditions to obtain technical grade. In the second stage, TPA is esterified with methanol in the presence of a catalyst to DMT which is purified in subsequent steps (Figure 8). Because of moderate reaction conditions, DMT yields are claimed to be higher. Toray Process. As mentioned earlier, in this process part of the TPA can be converted to DMT (Figure 4), which can be purified to fiber grade. A process somewhat similar to the Dynamit Nobel process has been described by Hoffman et al. (1971) which uses cobalt 2-ethylhexanoate catalyst promoted by manganese 2-ethylhexanoate. Reaction Mechanism Elucidation of the mechanism of reaction helps in selecting more efficient catalysts, optimizing the reaction intermediates, selecting reaction conditions, and designing of a commercial reactor. This process also generates ideas for further research and development. The oxidation of hydrocarbons is usually explained in terms of the classical free radical chain mechanism involving the initiation, propogation, and termination steps (Rice and Herzfeld, 1934). An excellent bibliography is available on the mechanisms of both catalyzed and uncatalyzed hydrocarbon oxidations (Shigeyasu, 1964, Shtern, 1964; Emanuel et al., 1967; Aleksandrov et al., 1968; Yasuhiro and Keiji, 1975). Hanotier and Hanotier-Bridoux (1981) have presented a review on the mechanism of liquid-phase oxidation of alkyl aromatics by molecular oxygen with special reference to the oxidation of durene catalyzed by cobalt(II1) and promoted by bromides, aldehydes, and ketones. Generally the reaction starts with the formation of the hydrocarbon free radical which reacts with oxygen, forming
Figure 9. Combination of electron-transfer and hydrogen-abstraction mechanisms in cooxidation of p-xylene and p-methyl toluate. Reprinted with permission from ref 48. Copyright 1980 John Wiley & Sons, Inc.
hydroperoxides. Depending on the reaction conditions the oxidation proceeds to give alcohols, aldehydes/ketones, acids, and finally oxides of carbon and water. However, oxidation of monocarboxylic acids to dicarboxylic acids, typical of the production of terephthalic acid, is a complicated process (Burney et al., 1959; Fortuin et al., 1959; Ravens, 1959; Towle and Baldwin, 1964). Initiation of the oxidation of alkyl aromatic hydrocarbons is explained in terms of two mechanisms: (1)the electron-transfer mechanism in which electron transfer occurs from arene to a cobalt(II1) complex producing an arene radical cation which in turn forms a benzyl radical by proton loss; (2) abstraction mechanism in which benzylic hydrogen is abstracted by bromine atoms, RO' radicals, or ROO' radicals. This abstraction is much easier for methylbenzenes than alkanes because benzyl radical is relatively more stable. Mechanism 1is more applicable to the first step of the Dynamit Nobel process to a considerable extent, and mechanism 2 is important in the second step as also in the Mid-Century process. Catalysis of xylene oxidation by cobalt salts is characterized by an induction period in which cobalt(I1) ions are oxidized to Co(II1). Monomeric Co(II1) ion is a powerful oxidizing agent when it is surrounded by 0-donor ligands such as water, OH- ions, or RCOO- ions. Co(II1) oxidizes xylenes to radical cations by electron transfer. Thus in the Dynamit Nobel process, p-methylbenzyl radicals are produced which react with oxygen, giving alkylperoxy radicals. These radicals can attack the p-toluate to abstract hydrogen to generate a new radical which initiates oxidation of the second methyl group. The p-methylbenzyl hydroperoxide forms alcohol, aldehyde, and f i i y p-toluic acid (Parshall, 1980). The cooxidation of p-xylene and p-methyl toluate is illustrated in Figure 9. It appears that Co(1II) cannot effectively oxidize p-toluic acid by mechanism 1because the electron-withdrawing carboxylic acid group raises the oxidation potential of p-toluic acid. However, mechanism 2 is less sensitive to arene (11)electron density than is mechanism 1, and hence p-toluic acid oxidation is initiated predominantly by hydrogen abstraction. However, the results of Kashima and Kamiya (1974), who studied the kinetics of oxidation of p-toluic acid in acetic acid solutions with cobalt acetate indicated that a electron-transfer mechanism between p-toluic acid and cobaltic acetate initiates the oxidation. In the Mid-Century process a mixture of cobaltic, manganic, and bromide salts are used as catalyst species. Under the reaction conditions employed, Mn(II1) and Co(1II) are not powerful enough to initiate the free-radical reaction by electron-transfer mechanism. The combined effect of Co and Mn ions is only to decompose hydroperoxides to yield free radicals. The initiation mechanism is predominantly hydrogen abstraction from methyl groups
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 457 highest yield of TPA. On the basis of results of their experiments on oxidation of p-toluic acid, a free-radical chain mechanism is proposed in which the first step is reaction of paraldehyde (PA) with oxygen to form peroxidic compounds which promote oxidation of cobaltous ion: PA + O2 + Co2+ X* + Co3+ (13)
-+ + + + -
C6H5-CH3 + Co3+
-+
C6H5-CHz'
C ~ H ~ - C H SX' C6H5-CHz'
LCOMPLEX~ Figure 10. Bromine cycle in a bromide-promotedoxidation of a hydrocarbon. Reprinted with permission from ref 48. Copyright 1980 John Wiley & Sons, Inc.
by bromine atoms. The Mn and Co ions oxidize the bromine ions formed to bromine atoms thus ensuring the availability of bromine atoms for initiation. Figure 10 illustrates this bromine cycle. The coordination of bromide ion to Co(I1) may facilitate electron transfer to oxygen or peroxy species to form Co(II1)-Br complex. Co(II1) being a strong oxidizing agent can abstract an electron from a bromide ligand to yield bromine atom. The bromine atom abstracts hydrogen from methyl group thus completing the cycle. When this reaction is conducted in acetic acid medium with Mn(OA&, the carboxylmethyl radicals ('CH,COOH) obtained from decomposition of the manganic salt play a significant role. These radicals can abstract protons from methylbenzenes initiating the desired oxidation or can also add on to aromatic rings or benzyl radicals. It is also reported that these radicals provide a pathway for complete oxidation of acetic acid (Heiba et al., 1969). In the Teijin process mentioned earlier, p-xylene is oxidized using excess amounts of cobalt catalyst (0.4-0.5 mol of Co/mol of p-xylene) in the absence of promoters. Ichikawa et al. (1970) have investigated this system and propose the following reaction scheme. C&-CH3 + COS+ C&,-CH2' 4- c02+ H+ (5)
-+ + c02++ -+ + + + -
C6Hb-CHz' C&,-CHz02'
0 2
C6H5-CH202'
C,&-CHO
C~HS-CHO Co3' C6&-C'O
C6H5-CO3' C,&-CO3H
+
CO''
C6H5-CO2'
C,&-C'O
02
+ CO3+ OH+ co2++ H+
C6H5-C03*
+ R' C&&-C02' c03++ OHRH C6H5-COOH + R' RH
C6H5-CO3H
(6) (7) (8)
(9)
(10) (11)
(12)
Here the initial step is thought to be hydrogen abstraction by cobaltic ion. The propagation step consists of oxidation of the substrates by cobaltic ion and reduction of radicals and hydroperoxides by cobaltous ion. These authors report that hydrogen abstraction by peroxy radicals is insignificant compared to that by cobaltic ions. Their experimental results show also that promoters play a significant role only when catalyst concentrations are low. On the basis of structure analysis, the catalyst is proposed to exist as a binuclear complex of cobaltic and cobaltous ions. The Toray process, as described already, oxidizes pxylenes with air using cobalt catalyst and paraldehyde as promoter in acetic acid solutions. Nakaoka et al. (1973) have studied this reaction experimentally. It is reported that salts of cobalt (particularly its acetate) are the best catalysts and of all promoters examined paraldehyde gives
4
02
H+ + Co2+
(14)
+ XH
(15)
C6Hb-CHz'
C6H5-CH200'
(16) C&,-CH200' CO'' C~HS-CHO+ OH- + Co3+ (17) CeH5-CHO + 0 2 C,H,-COOH (18) This scheme is based on the observation that we of Co(II1) in place of Co(I1) reduces inhibition period. The active free radicals from PA can also abstract hydrogen from methylbenzenes besides maintaining a high Co(II1) level in the reaction mixture. Several other authors have also reported a free-radical mechanism for the oxidation. Russell (1956) determined the rates of oxidation of arylalkyl hydrocarbons in the liquid phase with oxygen in the presence of tert-butyl perbenzoate and proposed a free-radical mechanism. Onopchenko et al. (1972) conducted Co(II1)-catalyzed oxidations of some alkyltoluenes with oxygen under mild conditions and proposed a reaction mechanism which involves interaction of Co(II1) with alkyltoluenes to form free-radical cations which lose hydrogen to form free radicals and subsequent trapping of these radicals by oxygen to form peroxy radicals. Hronec and Ilavsky (1982) studied the oxidation of a mixture of p-xylene and p-toluic acid in aqueous medium using Co and Mn salts at 183-190 "C. They observe that for p-xylene to be oxidized,p-toluic acid must be present and the catalyst concentration should exceed a certain critical value. The free-radical mechanism proposed involves an initial hydrogen abstraction from alkyl aromatic substrates by radical species. Czytko and Bub (1981) have reported the influence of water on cobalt(II1)-promoted oxidation of toluene in acetic acid solutions. While no effect of water was detected under anaerobic conditions, water enhances the oxidation a t lower concentrations and inhibits it a t higher concentrations. A reaction scheme involving water-free radical interaction is presented and the kinetic constants are experimentally determined. Chester et al. (1978) have experimentally investigated the dramatic effects of very small amounts of zirconium on the oxidation rate and the possible role of zirconium in the autoxidation. A general discussion on the catalytic route of hydrocarbon oxidation in the presence of transition-metal ions in protic media is presented by Geletii and Zakharov (1984). Recent studies (Sheldon, 1981) show that doped cobalt salts give a superior performance in oxidation of methyl aromatics. Small amounts of ZrO(OAc)zand H ~ O ( O A C ) ~ cause rate enhancement of Co(OAc)z-catalyzedoxidation of p-xylene. This is attributed to the ability of Zr(IV) and Hf(IV) to attain higher than six-coordination in solution and the resistance of Zr(IV) and Hf(IV) to reduction. Also, monomer-dimer equilibrium of cobalt acetate is redistributed favorably by forming a weak complex of Co(1II)-monomer by Zr or Hf. Besides, the oxidation seems also to be influenced by the presence of strong acids, showing a rate enhancement. It can be concluded that although excellent studies to elucidate the mechanism were conducted by several authors, with oxidation of methylbenzenes being a complex
458 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
2 1
i ii
0.4
' } -. 2 0.3
2=
I
i
i
L 8 CONC. C o A q x1j2ml/l
0
Figure 13. Rate of formation of p-toluic acid from p-xylene as a function of catalyst concentration. Reprinted with permission from ref 14. Copyright 1974 Elsevier Science Publishers Ltd.
TIME, minutes
Figure 11. Kinetic curves for oxidation of p-xylene. Rates of formation of p-toluenealdehyde (1);p-toluic acid (2);terephthalic acid (3). T = 105 OC. Concentrations in mol/L p-xylene = 0.56, water = 6.26 x IO-,, CoAc, = 5.43 X lo-,. Reprinted with permission from ref 14. Copyright 1974 Elsevier Science Publishers Ltd.
2.0r
I/ 1 - 0
0.25
0.75
P-XYLENE mol / i
Figure 12. Rate of formation of p-toluic acid from p-xylene vs concentration of p-xylene. T = 70 OC. Concentrations in mol/L CoAc, = 7.3 X lo-,. Reprinted with permission NaI3r = 6.3 X from ref 14. Copyright 1974 Elsevier Science Publishers Ltd.
process involving dozens of free-radical reactions, there seems to be a large scope for further studies. For each case involving a new catalyst, promoter, solvent, or reaction conditions, the mechanism has to be studied afresh and to the degree of sophistication required by the intended application.
Kinetics of p-Xylene Oxidation In order to gain a better insight into the reaction mechanism and to identify the effects of different parameters on the progress of reaction, it is essential to study the kinetics. Though several workers have studied the kinetics of p-xylene oxidation under various conditions, no definite rate expressions are given, nor are the values of all rate constants reported. This is primarily because these studies were aimed at testing new catalyst recipes, optimizing reaction conditions, or understanding the mechanism of reaction. Some of the important investigations are briefly covered below. Bromide-Promoted Oxidation. Digurov et al. (1970) have studied experimentally the liquid-phase oxidation of p-xylene catalyzed by cobalt acetate and sodium bromide. The concentration-time curve (Figure 11) shows that the formation of TPA commences only after p-tolualdehyde has been completely converted to p-toluic acid. The rate of oxidation of p-xylene to p-toluic acid (primary oxidation) is directly proportional to the concentrations of pxylene (Figure 12), catalyst (Figure 13), and activator (Figure 14). It is also reported that the reaction is independent of oxygen partial pressure in the range 0.2-1.0 bar. The rate of oxidation of p-toluic acid to TPA (secondary oxidation) is also first order with respect to catalyst concentration (Figure 15) but, unlike the primary oxidation, is proportional to the square root of oxygen partial pressure. However, the secondary oxidation rate appears to be independent of bromine ion concentration, though it is zero in the total absence of bromine. It may be inter-
3
CONC. NaBr x1O2 mol/\ Figure 14. Rate of formation of p-toluic acid from p-xylene as a function of bromide activator concentration. T = 70 OC. Concentrations in mol/L CoAc, = 7.3 X lo-,, p-xylene = 0.515. Reprinted with permission from ref 14. Copyright 1974 Elsevier Science Publishers Ltd.
.-c E
/
2k CONC. CoAc2 x 1l ?t 3 mol/(
Figure 15. Rate of formation of TPA from p-toluic acid as a function of catalyst concentration. T = 90 OC. Concentrations in mol/L p-toluic acid = 0.46. Reprinted with permission NaI3r = 7.2 X from ref 14. Copyright 1974 Elsevier Science Publishers Ltd.
esting to compare these findings with those of Bang and Chandalia (1974), who studied the oxidation of methyl p-toluate by air in the presence of cobalt or manganese naphthenates at atmospheric pressures and 120-180 OC. It is reported that the order of reaction is 2.5 with respect to concentration of methyl p-toluate and 0.5 with respect to partial pressure of oxygen, with an activation energy of 16.5 kcal/mol. Kamiya (1974) has studied experimentally the relative activities of several hydrocarbons in the autoxidations catalyzed by cobalt and bromide ions, using a competitive oxidation method. The results show that cobaltic acetate monobromide is the true catalyst and chain carrier and that the bromide ions slowly convert to an inactive organic bromide by the reaction of Co(II1)-Br with alkyl radicals. Partenheimer (1990) showed by thermodynamic and kinetic arguments that there is a reaction of cobalt (and only cobalt) with acetic acid which can generate radicals of alkyl aromatic8 (p-xylene). When the first methyl group on p-xylene is oxidized, the rate of reaction is diminished greatly due to the electronic deactivation of the ring. Oxygenation is terminated due to various deactivation mechanisms. The decarboxylation of acetic acid by cobalt at higher temperature is overcome by using cooxidants p-xylene in the Witten process and acetaldehyde in the
Ind. Eng. Chem. Res., Vol. 31, No. 2,1992 459 Table I. Influence of Solvents on Conversion and Yield of Terephthalic acid from p -Xylene (Cobalt Stearate Concentration = 1 X mol/L; T = 138 "C) (Reprinted with Permission from Ref. 37. Copyright 1974 Elsevier Science Publishers Ltd.) conv reaction p-xylene, yield of solvent time, h % TPA, mol % none 2 41.4 1.7 n-butyric acid 2.5 -37 5.2 n-valeric acid 3.5 69.5 15 isovaleric acid 2.0 60 7 caprionic acid 2.5 64.5 8 bromobenzene 2.5 75 7
-
-
a
+ Q
Y
8 % 0.G
sz
8bar
-
0.2-
1
p,"
TEMPERATURE I°C I Figure 18. Effect of temperature on rate (42).
0.1-
n% x e 0.08' I
&a
I
140°C 2 hr.
u 0.6-
g9
1
02
0.8-
no
z 0
PX 1s w t Yo I A c O l 2 CO 0.5 W t Yo
1.0-
h
OXYGEN PRESSURE (bar ) Figure 17. Effect of oxygen pressure on rate (42).
0.06
t
Table 11. Effect of the Ratio of Cobaltic Ion to Total Cobalt Ion (42)" concn, g-ion x lo-' Co3+ Co2+ Co3+/(t0tal Co), % yield of TPA, % 3
G
6 8 10 PA ( ~%I t
15
20
Figure 16. Effect of paraldehyde (PA) in the continuous oxidation of p-xylene (42).
Eastman-Kodak process. Bromide is added to overcome the problem of acetic acid and aromatic acid decarboxylation by rapid electron transfer from cobalt to bromide. Manganese operates much the same way as bromide since another rapid electron-transfer reaction occurs between cobalt and manganese which thus decreases the decarboxylation reactions further. The effect of solvents is studied by Ivanov et al. (1969), who conclude that inhibition period reduces and rate increases as the molecular weight of the acid solvent is increased (Table I). They also studied the influence of different bromine derivatives as reaction promoters. Paraldehyde-Promoted Oxidation. Nakaoka et al. (1973)have studied the oxidation of p-xylene (or PTA) in acetic acid solutions with air using cobalt acetate catalyst promoted by paraldehyde (PA). The amount of PA added to the system has an effect on the progress of oxidation to TPA. Figure 16 shows that the fraction of unreacted matter in the oxidation products from p-xylene in the continuous process is reduced according to approximately the second power of the amount of PA added. The reaction rate is also influenced by oxygen pressure (Figure 17) and reaction temperature (Figure 18). The optimum temperature for the paraldehyde-promoted oxidation is around 120-140 "C. Another important controlling factor for the reaction is the ratio of cobaltic ion concentration to total cobalt ion concentration. The rate decreases with a decrease in this ratio even under constant cobaltic ion concentration, as shown in Table 11. Also, since cobaltic ion is the main agent for hydrogen abstraction, it is necessary to maintain a high cobaltic ion concentration. In acetic acid solutions cobaltic ion reduces to cobaltous ion, which can be described as a first-order irreversible
8.4 8.4 8.4 8.4
3.6 15.6 27.6 43.7
70.0 35.5 23.3 16.1
73.6 73.0 61.0 42.7
OConditions: PA = 0.02 g; TPA = 1 g; AcOH = 1 mL; O2= 10 bar; T = 100 "C; time = 2 h. Table 111. Reduction Rate of Cobaltic Ion in Acetic Acid Solution (42) rate const, h-' in 5% in 10% HzO-g0% H&95% temp,"C inAdH" AcOHb AdHC
90 100 110 120 130 OEd,,,
0.00806 0.0242 0.374 1.27 4.90
0.0307 0.0887 0.238 0.726 2.74
0.0443 0.0968
0.300 0.866 2.29
= 42.3 kcal/mol. bEa&va* = 33.4 kcal/mol. cEdwb.on
= 27.8 kcal/mol.
reaction. The rate constant values shown in Table I11 indicate an exceedingly high reduction rate above 130 "C, and this explains the difficulty in oxidizing methylbenzenes above 140 OC. The authors have also studied the mechanism of oxidation of PA under the reaction conditions. Acetaldehyde-Promoted Oxidation. Hirai et al. (1967)have derived a simple rate expression for the reaction in a semibatch bubble tower reactor. -d(p-xylene)/dt = k,,(p-xylene)"[l + ( ~ ~ ~ ~ ~ ] ~ ( c a t a l y s t(19) )~(p01)~ where a is an effectiveness factor and PACH is the partial pressure of acetaldehyde. The oxidation was also carried out in bubble tower reactors with rotary gas distributors (Nishiguchi et al., 1974),and here again it is observed that diffusion is the slowest step. The reaction rate increases with increase in gas-liquid contact area. No rate expres-
460 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
r
1
I
I
IL
6 4
'
ds
64 ' ' 1' 2 RATIO OF ColOAc)~.Lti$ 10 pXYLENE (mole mole)
'
0
20
40
80
60
9 H20 Figure 21. Effect of water on TPA and C02yields (26). (0,m) Mn 0 ) Co catalyzed. catalyzed; (0,
Figure 19. Yield dependence on amount of catalyst (36).
I
180
20 0
OC
REACTION TIME
Figure 20. Change of reaction mass composition with time (36).
sions are derived here, but concentration-time plots are presented. MEK-Promoted Oxidation. Liquid-phase oxidation of p-xylene using cobalt acetate catalyst promoted by methyl ethyl ketone (MEK) has been studied by Brill (1960). The presence of water is reported to increase the induction period and to reduce the reaction rate. Increasing the reaction temperature reduces the induction period but the overall rate decreases due to decreased oxygen solubility. However, it is observed that reduction in induction period and increase in reaction rate can simultaneously be achieved by increasing oxygen partial pressure. In this system, it is also observed that relatively large amounts of catalyst are required since below a critical amount the catalyst is ineffective. Oxidation i n t h e Absence of Promoters. Ichikawa et al. (1970) have studied p-xylene oxidation in acetic acid solutions with cobalt catalyst without using any promoters or accelerators such as bromides, ketones, and aldehydes. This study reveals that high yields of TPA can be obtained even in the absence of promoters, provided the catalyst concentration is sufficiently high (Figure 19). A reaction mechanism is proposed based on which the concentrations of intermediates and products are calculated. A good agreement is seen between these calculated values and the experimental points (Figure 20). However, the rate equations and values of constants are not reported by the authors. Oxidation i n Aqueous Systems. Hronec (1980) has studied the technological aspects of oxidation of polyalkyl aromatics. Using cobalt (and manganese) salts as catalysts, a mixture of p-xylene and p-toluic acid is oxidized in water in the temperature range 180-190 "C. The concentration of water influences the yield and selectivity of oxidation (Figure 21), and the rate is profoundly influenced by temperature (Figure 22). It is also reported that at elevated temperatures the oxidation proceeds without an induction period and a free-radical mechanism is applicable (Hronec et al., 1985). I t is reported that in many
Figure 22. Effect of temperature on TPA production catalyzed by Mn and Co Catalysts (26). (m) 0.81 mmol of manganese(I1)acetate; (0) 1.5 mmol of cobalt(II)alkanoate, ( 0 )Mn(II) Co(II) mixed (0.60 + 1.5).
+
solvents including water, pyridine enhances the activity of cobalt bromide catalysts giving high yields of TPA (Hronec and Ilavsky, 1983; Cvengrosova et al., 1987). Hronec and co-workershave also investigated the oxidation of p-xylene in aqueous systems using metal oxide catalysts (Hronec and Hrabe, 19861, supported metal oxide catalysts (Hronec and Majling, 1987), and transition-metal catalysts (Hronec and Ilavsky, 1987b). In the case of cobalt oxide catalyzed reaction the rate is second order with respect to p-xylene concentration and first order with respect to catalyst concentration but is only 0.1 order with respect to partial pressure of oxygen (Hronec and Ilavsky, 1987a). However, the oxidation of p-xylene proceeds only if ptoluic acid is present in the reaction mixture. Novotny et al. (1989a) have studied the reaction of cobalt(II1) acetate with p-toluic acid at 65-90 OC in aqueous systems. It is observed that the reaction proceeds by intermolecular oxidation of the toluate ligand and the corresponding cation radical is the intermediate product. The equilibrium between active monomeric and less active dimeric Co(III) species is influenced by p-toluic acid, Co(II) ions, and a strong acid, which in turn influences reaction rates. These authors have also studied the kinetics of oxidation of p-toluic acid and a mixture of p-toluic acid and p-xylene by Co(II1) in chlorobenzene under inert atmospheres (Novotny et al., 1989b). It is concluded that above a critical concentration p-toluic acid forms Co(1II) complexes soluble in aromatic hydrocarbons containing at least one toluate ligand and at higher concentrations ptoluic acid is oxidized by Co(II1) to the corresponding cation radical which on decomposition produces aryloxy radicals which in turn oxidize p-xylene. The direct oxidation of hydrocarbons by the monomeric Co(II1) species in the presence of p-toluic acid is therefore considered to be less important. Basic Types of Reactors In order to study the kinetics of liquid-phase oxidation, it is important that the reaction should take place in the
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 461 pure kinetic regime. To ensure the rapid dissolution of oxygen in the liquid phase, a sufficiently large interface between the gas and the liquid must be created and the whole mass of liquid must be vigorously stirred. The simplest and most widely used method of stirring the liquid satisfactorily is to bubble oxygen or air through a layer of the hydrocarbon undergoing oxidation. The smaller the bubbles of gas passing through the liquid and the deeper the layer of liquid, the better are the conditions for the saturation of the liquid with oxygen. Consequently, reactors of the bubbling type must have a cylindrical shape with a sufficiently high ratio of the height to diameter (kd = 5-15:l). Standard commercial reactors in the liquid-phase-oxidation processes are (i) sparged column reactor, (ii) continuous stirred tank reactor (CSTR), (iii) tubular reactor, and (iv) multistaged continuous reactors (Prengle and Barona, 1970b). In sparged column reactors the agitation is created by the motion of gas bubbling through liquid which causes small back-mixing. Coalescence is important due to the bubble flow regime. There exist quiescent and turbulent flow regimes at superficial gas velocities below 0.045-0.06 m/s and above 0.075 m/s, respectively. In a CSTR back-mixing is very large and coalescence is small but the solid particle size may reduce due to the shear action of the impeller. Each system needs a typical agitation level which is required to be determined from experiments. For effective dispersion of gas phase, it is required to maintain a minimum agitation rate (Westerterp et al., 1963). Higher agitation rates increase the interfacial area but without altering the heat- and mass-transfer characteristics of the system. Power required for agitation of a given system can be calculated from correlations available in the literature (Rushton, 1960). In sparged column reactor and CSTR, the dispersing phase (usually air or 0,)is continuously sparged through the liquid phase which is fed to the reactor in batches or continuously. In some cases when side reactions are minimized at low conversions of the reactant, the unconverted liquid stream is continuously recycled to the reactor. In spite of the kinetic advantages, a tubular reactor is difficult to use in liquid-phase reactions, especially with gas-liquid systems. The main drawbacks are keeping the gas phase in the liquid and designing for a sufficiently low pressure drop. In multistage reaction columns, the gas-liquid mixture is redispersed at various points along the reactor by the use of impellers and the annular baffles. The efficiency of such columns is higher because of the mixing within the stage and also between the stages. Tray columns operate better with high tray efficiencies than do baffle columns. A batch reactor is advantageous as far as the scale-up to a commercial size is concerned, but the heat-transfer area per unit volume decreases as the reactor size increases.
Conclusion A brief review of the technological aspects of liquidphase oxidation of p-xylene to terephthalic acid highlighting the industrial processes, reaction mechanisms, and kinetic studies presented here indicates a large scope for further experimental and theoretical investigations on all aspects of this important reaction. Acknowledgment Documentation assistance from Mr. N. M. Nyshadham is gratefully acknowledged. We thank the management of Indian Petrochemicals Corporation Limited for permitting us to publish this paper and Dr. I. S. Bhardwaj,
Director (R&D), and Dr. T. S. R. Prasada Rao, Director (IIP, Dehradun), for their kind encouragement. Registry No. TPA, 10021-0;DMT, 120-61-6;MeCa4-p-Me, 106-42-3.
Literature Cited (1) Aleksandrov, V. N.; Kaminskii, A. Ya.; Gitis, S. S.; Pankova, N. A; Kaminskaya, E. G. Mechanism of Formation of Phthalic Acids during the Liquid Phase Catalytic Oxidation of Xylene Isomers. Zh. Org. Khim. 1968,4, 1808. (2) Bang, S.R.; Chandalia, S. B. Liquid Phase Oxidation of Methyl-p-toluate to Monomethyl Terephthalate. Indian Chem. Eng. 1974,16,T92. (3) Boffa, G.; Costabello, D.; Maiorano, G. Terephthalic Acid. Ger. Offen. 1146047,1963. (4) Brill, W. F.Terephthalic Acid by Single Stage Oxidation. Ind. Eng. Chem. 1960,52,837. (5) Burney, D. E.; Weisemann, G. H.; Nathan, F. Dibasic Acids by Direct Oxidation. Pet. Refin. 1959,38 (6),186. (6) Burrows, L. A.; Cavanaugh, R. M.; Nagle, W. M. Improved Preparation of Terephthalic Acid. Br. Pat. 655074,1951. (7) Burrows, L. A,; Cavanaugh, R. M.; Nagle, W. M. Terephthalic Acid. U.S.Pat. 2636899,1953. (8) Caloyannis, A. G.; Graydon, W. F. Heterogeneous Catalysis in the Oxidation of p-Xylene in the Liquid Phase. J. Catal. 1971, 22, 287. (9) Chervinskii, K. A.; Ivanov, A. M.; Nikitina, L. A. Liquid Phase Oxidation of p-Xylene. Khim. Prom. 1962,10,742. (10) Cheninskii, K. A.; Zherebtaova, L. P. Nature of Limiting Yield in the Liquid Phase Oxidation of p-Xylene. Kinet. Katal. 1965, 6,792. (11) Chester, A. W.; Landis, P. S.; Scott, E. J. Y. Oxidize Aromatics over Doped Cobalt. CHEMTECH 1978,June, 366. (12) Cvengrosova, Z.; Hronec, M.; Kizlink, L.; Malik, L.; Ilavsky, J. Study of p-Xylene Reaction with Co-Br-Pyridine Catalyst. 2. Reaction in Aqueous System. J. Mol. Catal. 1987,40,235. (13) Czytko, M. P.; Bub, G. K. Oxidation of Toluene by Cobalt(II1) Acetate in Acetic Acid Solutions-Influence of Water. Ind. Eng. Chem. Rod. Res. Deu. 1981,20,481. (14) Digurov, N. G.; Dyakonov, J. A.; Lebedev, N. N.; Suchkov, V. V.;Yoshkova, L. A. Kinetics of the Liquid Phase Oxidation of p-Xylene into Terephthalic Acid with a Cobalt Bromide Catalyst. Izv. Vyssh. Uchebn. Zaved. Khim., Khim. Tekhnol. 1970,13,407. (15) Emanuel, N. M.; Denisov, E. T.; Maizus, Z.K. Liquid Phase Oxidation of Hydrocarbons; Plenum Press: New York, 1967. (16) Fortuin, J. P.; Wade, M. J.; Van Oosten, R. P. New Route for Dicarboxylic Acids. Pet. Refin. 1959,38,189. (17) Gelbein, A. P.; Sze, M. C.; Whitehead, R. T. New Route to Terephthalic. Hydrocarbon Process. 1973,52 (9),209. (18) Geletii, Yu. V.;Zakharov, I. V. Catalytic Route of Hydrocarbon Oxidation by Dioxygen in the Presence of Transition Metal Ions in Protic Media. Oxid. Commun. 1984,6,23. (19) Hanotier, J.; Hanotier-Bridoux, M. Mechanism of the Liquid Phase Homogeneous Oxidation of Alkylaromatic Hydrocarbons by Cobalt Salta. J. Mol. Catal. 1981,12,133. (20) Heiba, E.I.; Dessau, R. M.; Koehl, Jr., W. J. Oxidation of Metal Salts. (3). The Reaction of Manganic Acetate with Aromatic Hydrocarbons and the Reactivity of Carboxy Methyl Radical. J. Am. Chem. SOC.1969,91,138. (21) Hirai, T.; Nishino, H.; Yano, M.; Harano, Y.; Imoto, T. Low Temperature Oxidation of p-Xylene in Liquid Phase using Acetaldehyde as Accelerator (2). Process with a Bubble Tower. Mem. Fac. Eng., Osaka City Univ. 1967,9,73. (22) Hoffman, G.; Irlweek, K.; Cordes, R. Dimethyl Terephthalate. Ger. Offen. 2010137, 1971. (23)Hronec, M. Oxidation of Polyalkylated Aromatic Hydrocarbons. XI. Study of Structure and Activity of Cobalt Bromide Complexes in the Oxidation of Alkyl Aromatic Hydrocarbons. Collect. Czech. Chem. Commun. 1980,45,1555. (24) Hronec, M.; Cvengrosova, Z.; Ilavsky, J. Kinetics and Mechanism of Cobalt Catalyzed Oxidation of p-Xylene in the Presence of Water. Ind. Eng. Chem. Process Des. Deu. 1985,24,787. (25) Hronec, M.; Hrabe, Z. Liquid Phase Oxidation of p-Xylene Catalyzed by Metal Oxides. Znd. Eng. Chem. Rod. Res. Dev. 1986,25,257. (26) Hronec, M.; Ilavsky, J. Oxidation of Polyalkylaromatic Hydrocarbons. XII. Technological Aapecta of p-Xylene Oxidation
462 Ind. Eng. Chem. Res., Vol. 31, No. 2,1992 to Terephthalic Acid in Water. Ind.Eng. Chem. Prod. Res. Dev. 1982,21, 455. (27) Hronec, M.; Ilavsky, J. Effect of Pyridine on Cobalt Bromide Catalyzed Oxidation of p-Xylene in Various Solvents. Oxid. Commun. 1983,3,303. (28) Hronec, M.; Ilavsky, J. Kinetics of p-Xylene Oxidation Catalyzed by Cobalt Oxide. React. K i w t . Catal. Lett. 1987a, 33,299. (29) Hronec, M.; Ilavsky, J. Role of Transition Metala during pXylene Oxidation in Aqueous System. React. Kinet. Catal. Lett. 1987b, 33,323. (30) Hronec, M.; Majling, J. Liquid Phase Oxidation of p-Xylene Catalyzed by Supported Metal Oxides. Appl. Catal. 1987,29,67. (31) Hydrocarbon Process. 1973,52 ( l l ) , 184. (32) Hydrocarbon Process. 1977a, 56 ( l l ) , 147. (33) Hydrocarbon Process. 1977b, 56 ( l l ) , 229. (34) Hydrocarbon Process. 1977c, 56 ( l l ) , 230. (35) Hydrocarbon Process. 1979,59 ( l l ) , 246. (36) Ichikawa, Y.; Yamashita, G.; Tokashiki, M.; Yamaji, T. New Oxidation Process for Production of Terephthalic Acid from pXylene. Ind. Eng. Chem. 1970, 62, 38. (37) Ivanov, A. M.; Chervinskii, K. A,; Baranova, E. I. Role of the Solvent during the Catalytic Oxidation of p-Xylene. Neftekhimiya 1969, 9,892. (38) Kamiya, Y. Catalysis by Cobalt and Bromide Ions in the Autooxidation of Alkyl Benzenes in Acetic acid. J. Catal. 1974,33, 480. (39) Kashima, M.; Kamiya, Y. Autooxidation of Aromatic Hydrocarbons Catalyzed with Cobaltic Acetate in Acetic Acid Solutions. 111. Oxidation of p-Toluic acid. Bull. Chem. SOC.Jpn. 1974,47, 481. (40)Maclean, D. Purification of Terephthalic Acid. Br. Pat. 835721, 1960. (41) Mitsui Petrochemical Industries Limited (Japan). Terephthalic Acid. Fr. Pat. 1355274, 1964. (42) Nakaoka, K.; Miyama, Y.; Matauhim, S.; Wakamatsu, S. Preparation of Terephthalic Acid using Paraldehyde Promoter. Ind. Eng. Chem. Prod. Res. Dev. 1973,12,150. (43) Niehiguchi, K.; Yano, M.; Harano, Y. Low Temperature Liquid Phase Oxidation of p-Xylene in Bubble Tower Reactors with Rotary Gas Distributors. Int. Chem. Eng. 1974,14,571. (44) Novotny, J.; Hronec, M.; Ilavsky, J. Kinetics of Cobalt(II1) Reduction. 1. Reaction in Aqueous Solution of p-Toluic Acid. Ind. Eng. Chem. Res. 1989a, 28,1467. (45) Novotny, J.; Hronec, M.; Ilavsky, J. Kinetics of Cobalt(II1) Reduction. 2. Reaction in p-Xylene Solution of p-Toluic Acid. Znd. Eng. Chem. Res. 1989b, 28,1471. (46) Onopchenko, A.; Schultz, J. G. D.; Sukischer, R. Non Classical Oxidation of Aromatics. (1) Cobaltic Ion Catalyzed Oxidations of D-Cvmene. D-Ethvl Toluene, and Secondan - - Butvl - Toluenes. J . Org. Chek.-1972,-37, 1414. . (47) Park, C. W.; Mickle Wright, D. G. Fibre Grade Terephthalic Acid. U.S.Pat. 4053506. 1977. (48) Parshall, G. W. Homogeneous Catalysis; Wiley: New York, 1980. (49) Partenheimer, W. A Chemical Model for the Ammo "MC" Oxygenation Process to Produce Terephthalic Acid. In Catalysis
of Organic Reactions; Blackburn, D. W., Ed.; Marcel Dekker: New York, 1990, p 321. (50) Patton, J. W.; Seppi, N. F. Terephthalic Acid from p-Xylene. Ind. Eng. Chem. Prod. Res. Deu. 1970,9,521. (51) Prengle, Jr., H. W.; Barona, N. Make petrochemicals by Liquid Phase Oxidation. Hydrocarbon Process. 1970a, 49 (3), 106. (52) Prengle, Jr., H. W.; Barona, N. Make Petrochemicals by Liquid Phase Oxidation (2). Kinetics, Mass Transfer and Reactor Design. Hydrocarbon Process. 1970b, 49 ( l l ) , 150. (53) Raecke, B.; Blaser, B.; Stein, W.; Schirp, H. Rearrangements of Salts of Aromatic or Heterocyclic Carboxylic Acids. U.S.Pat. 2891992, 1959. (54) Raecke, B.; Stein, W.; Schirp, H. Terephthalic Acid. U.S. Pat. 2794830,1957. (55) Ravens, D. A. S. The Kinetics and Mechanism of the Autooxidation of p-Toluic Acid in Acetic Acid Solution Catalyzed by Cobalt and Manganese Bromides. Trans. Faraday SOC.1959,55, 1768. (56) Rice, F. 0.;Herzfeld, K. F. Thermal Decomposition of Organic Compounds from the Stand Point of Free Radicals. VI. Mechanism of Some Chain Reactions. J. Am. Chem. SOC.1934,56,284. (57) Rushton, J. H.; Costich, E. W.; Everett, H. J. Power Characteristics of Mixing Impellers. Chem. Eng. h o g . 1950,46,395,467. (58) Russell, G. A. The Rates of Oxidation of Aralkyl Hydrocarbons. Polar Effects in Free Radical Reactions. J. Am. Chem. Soc. 1956, 78, 1047. (59) Saffer, A.; Barker, R. S. Aromatic Polycarboxylic Acids. U.S. Pat. 2833816,1968. (60)Schenk, W.; Schiller, G. Terephthalic Acid. US.Pat. 2905709, 1959. (61) Schirp, H. Aromatic Carboxylic Acids. US. Pat. 2866815,1958. (62) Sheldon, R. Metal Catalyzed Oxidations of Organic Compounds. Academic Press: New York, 1981. (63) Shigeyasu, M. Oxidaton of Alkyl Benzene. I. Reaction Producta in the Liquid Phase Air Oxidation of p-Xylene Catalyzed by Bromine Compounds Heavy Metal Salts. Kogyo Kagaku Zasshi 1964,67, 1396. (64) Shtern, V. Ya. The Gas Phase Oxidation of Hydrocarbons; MacMillan: New York, 1964. (65) Standard Oil Co. (Indiana). Purification of Terephthalic Acid. Br. Pat. 994769, 1965. (66) Towle, P. H.; Baldwin, R. H. Make Most Aromatic Acids Using Mid-Century Oxidation Process. Hydrocarbon Process. 1964,43 ( l l ) , 149. (67) Westerterp, K. R.; Van Dierendowck, L. L.; De Kraa, J. A. Interfacial Areas in Agitated Gas-Liquid Contactors. Chem. Eng. Sei. 1963, 18, 157. (68) Yasuhiro, F.; Keiji, K. Reaction Mechanism of Metal Ions in the Liquid Phase Oxidation of Petroleum Chemicals. Sekiyu Gakkakhi 1975,18, 139. (69) Yoshimura, T. New Process Can Reduce Terephthalic Acid Costs. Chem. Eng. 1969, 76 (lo), 78.
Received for review August 5, 1991 Accepted August 23, 1991
