New Oxidation Process for duction of Terephthalic Acid from p-Xylene YATARO ICHIKAWA GENTARQ YAMASHITA MICH IY U KI TO KASH I KI TEIZO YAMAJI
As polyester fiber production increases, raw materials TPA and DMT increase in importance
erephthalic acid (TPA) and dimethyl terephthalate T ( D M T ) are the raw materials for polyester which are increasing in importance in accordance with the increasing production of polyester fiber throughout the world (Table I). Recent Japanese production of and drmand for polyester in 1968 and 1971 are shown in Table 11. I n 1962, before we began investigating the present process, the methods listed in Table I11 were being used to produce terephthalic acid from p-xylene by oxidation with niolecular oxygen. At the same time, studies were begun on the direct mterification method using a high-purity terephthalic acid to produce polyester, replacing the usual dimethyl terephthalate esterexchange method. I n 1963 the methods were commercialized by both Teijin Ltd. and Toyo Rayon Co., Ltd. Interest in the research on production of terephthalic acid, therefore, was naturally focused on the production of high-purity terephthalic acid by the air oxidation of p-xylene. I n contrast with the two well-established methods in 38
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table 111, the methods which belong to the third group were thought to be practical for production of highpurity terephthalic acid because their reaction could be carried out under mild conditions (Table IV). The cobalt catalyst, on further investigation, had a profound effect on the reaction temperature, partial pressure of oxygen, and the organic solvents used. This fact suggested these co-oxidation agents are not absolutely essential for the progress of this oxidation reaction. Many experiments were carried out which finally culminated in a process capable of producing terephthalic acid from p-xylene on a commercial scale by using considerably larger amounts of cobalt catalyst than usually thought necessary. The process is outlined below.
oxidation Reaction
The catalyst, especially its amount, has an important role in the oxidation of this process. The relatioiiship between the one-pass yield of terephthalic acid and the
amount of catalyst used is shown in Figure 1. Improvement in the yield was observed by using a larger amount of catalyst than usual. Maximum yield was obtained with the amount of the catalyst between 0.4 and 0.5 mol Co/mol p-xylene, but no improvement in the yield was found when a higher amount of the catalyst was added. T o find the effect of the cobalt catalyst in this reaction, the reaction mechanism of p-xylene to terephthalic acid was investigated. It was assumed from the analyses of the samples in the intermediate steps that the reaction in this process proceeds by steps shown in Figure 2, involving chain reactions. T h e p-xylene oxidation mechanism, estimated by analyzing the experimental results with a relatively high content of cobalt catalyst, is thought to be as shown in Figure 3. I n this process the initiation step involves abstraction of hydrogen by cobaltic ion, and the propagation step consists of oxidation of the substrates by cobaltic ion and reduction of radicals and hydroperoxides by cobaltous ion. This means that cobalt ions play important roles as chain carriers in this reaction. Of course it is estimated that the hydrogen abstraction by peroxy radicals (Figure 4) occurs as observed in the usual air oxidation, but it is thought the resulting production is much less than that achieved by cobaltic ions as shown in Schemes 1 and 4 in Figure 3. Comparison of the experimental results and the values calculated using the mechanism suggested by Figure 3 shows good agreement (Figure 5), which gives good support for use of this reaction mechanism. Examinations were also made on the structure of the catalyst. From the results of chemical analyses (total amount of Co, Co3+, RCO-, HtO, and C and H elemental analyses), X-ray diffraction, and infrared and ultraviolet absorption spectra (p-dihydroxy structure), the structure of the catalyst was estimated to be a binuclear complex of cobaltic and cobaltous ions as shown in Figure 6. Peshanski isolated the compound with a structure shown in Figure 6 by oxidizing cobaltous acetate with persulfate. T h e structure of this compound resembles the catalyst structure in our process, but there is a definite difference in the point that our catalyst structure contains Co(I1) and Co(II1) species in the binuclear complex, while only the Co(II1) species is found in the complex isolated by Peshanski. Table V shows the results obtained from the experiments carried out under our reaction conditions where the co-oxidizing agents used in the low-temperature air oxidation cited before were added. These additives are efficient in the region of low catalyst content and do not show any effectiveness in high catalyst content. T o show the characteristics of the oxidation in this process, the isolated oxidation by-products are listed in Table VI. These compounds were isolated from a large amount of concentrated mother liquor of oxidation reaction from a pilot operation. The amount of these compounds decreases in the order of $-toluic acid, 4-carboxybenzaldehyde (4-
TABLE I .
POLYESTER FIBER PRODUCTION (1968)
Country U.S.A. West Europe Japan
TABLE II.
Production, lo3 metric tons
Rate of increase over previous year,
%
490 310
53 44
188
18
POLYESTER PRODUCTION I N JAPAN
Production 103 metric tons
Rate of increase over previous year, %
1968
188
18
1969 1970 1971
218 251 286
16 15 14
Year
TABLE 111.
p-XYLENE OXIDATION METHODS
Oxidation methods Bromine-promoted oxidation Oxidation via functional derivatives Oxidation with co-oxidation agent
Company Amoco Chemicals, Mitsui Petrochemical, Maruzen Oil, I.C.I. Hercules, Chemische Werke Witten Mobil Oil, Tennessee Eastman
TABLE IV. CHARACTERISTICS O F OXIDATION . W I T H CO-OXIDATION AGENT Reaction temperature. . . . . . . . . . . . . . . . .Below 15OOC Heavy metal catalyst. . . . . . . . . . . . . . . . ..Limited to cobalt Solvent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Lower fatty acid Co-oxidation agents. . . . . . . . . . . . . . . . . . .Used
CBA), p-acetoxymethyl benzoic acid, isophthalic acid, benzoic acid, and o-toluic acid, and others are present only in trace. These compounds can be converted into terephthalic acid by circulation of the mother liquor to the oxidation reactor with the exception of compounds derived from the impurities of p-xylene-Le., isophthalic acid, benzoic acid, o-toluic acid, and acetophenone. For example, p-acetoxymethyl benzoic acid can be oxidized in the same way as p-xylene in the presence of cobaltic ion to give terephthalic acid in a high yield with a small amount of 4-carboxybenzaldehyde. As the oxidation in this process is carried out under a mild condition, the by-products (Table VII) found in the VOL. 6 2
NO. 4 A P R I L 1 9 7 0
39
90
-
0 -1
a
Figure 3. Reaction scheme
I 80
t-
r a W a W -I
LL
0
70
n -I
w >-
6o0
0.2 0.4 0.6 0.8
I:O
Figure 4. Hydrogen abstraction by peroxy radical
1.2
RATIO O F C O ( O A ~ ) ~ * ~TO H~O p-XYLENE (MOLE/MOLE) Figure 7 .
Yield dependence on amount of catalyst
L
a
a
I-
REACTION TIME Figure 2. Reaction steps i n this process 40
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Figure 5 . Periodical variation of each component
oxidation under relatively drastic conditions are not formed. I n this process, therefore, circulation can become possible only by removing water from most of the mother liquor. Process Description
Figure 7 shows that this process consists of three main sections, oxidation, purification, and recovery. Oxidation section. Oxidation is carried out continuously in one step a t a temperature lower than 1 5 O O C with the pressure u p to several tens of atmosphere. Acetic acid is used as solvent plus a relatively large amount of cobalt catalyst, and oxygen-containing gases, such as air as the oxidizing gas. The terephthalic acid produced in the oxidation section is transferred to the purification section after separation from the oxidation mother liquor. The mother liquor, containing the catalyst, oxidation intermediates, solvent, and water produced by the oxidation reaction, is transferred to the purification section. Over 9 5y0 yield of terephthalic acid by oxidation is attained when oxidation of the intermediate products in the recirculation system is included. Purification section. The crude terephthalic acid transferred from the oxidation section contains a small amount of oxidation intermediates, catalyst, and solvent, but is used without drying in the preliminary purification section. It is purified in the liquid phase using the same solvent, acetic acid, as in the oxidation. Most of the impurities are eliminated by the preliminary purification, and the fiber grade terephthalic acid is produced by crystallization following a preliminary purification. Recovery section. Recovery section consists of the recovery of the solvent and the catalyst. I n the solvent recovery section, water produced in the oxidation reaction is eliminated from the oxidation mother liquor which is separated in the oxidation section from the top of the distillation column. The water-eliminated oxidated mother liquor is recycled to the oxidation section without further treatment and is used again. A part of the mother liquor is treated, and impurities produced by the oxidation are separated. The catalyst is almost completely recovered and recycled to the oxidation section. T h e impurities separated in this section are the products from the impurities in the raw material, p-xylene, and the organic and inorganic impurities produced in all sections. T h e ratio of mother liquor recycled to the portion treated in this section was determined from the results AUTHORS Yataro Ichikawa, Chief of the 70th Research
Laboratory, Gentaro Yamashita, Chief of the 3rd Research Laboratory, and Michiyuki Tokashiki, Chemical Engineer of 70th Research Laboratory, are associated with the Products Development Institute, Teijin Ltd., Iwakuni, Japan; Teizo Yamaji formerly Chemist of 70th Research Laboratory, is now a graduate student at Clarkson College of Technology, Potsdam, N . Y. This is one of the papers from the Symposium on Novel Processes and Technology of the European and Japanese Chemical Industry, presented at the 758th ACS Meeting in New York, N . Y., September 7-72, 7969.
Figure 6 . Catalyst structure
TABLE V. p-XYLENE OXIDATION I N T H I S PROCESS USING CO-OXIDATION AGENTS
Co-oxidation agent MEKa
Condition HzO
Low Co content
+
High Co content Low 0 2 pressure and low Co content High Co content and high 0 2 pressure
0 8
Low Co content
AcH
N o improvement
Effective N o improvement
High Co content a
Results Effective No improvement Effective
Methyl ethyl ketone.
TABLE VI. Substance
OXIDATION BY-PRODUCTS (THIS PROCESS)
Structural formula
Remarks
p-Toluic acid 4-Carboxybenzaldehyde p-iice toxymethylbenzoic acid
HOW,
Impurity frompxylene
Isophthalic acid
&COOH
Benzoic acid
@COOH
Impurity from pxylene
o-Toluic acid
&COO.
Impurity from 16xylene
p-Tolualdeh yde p-Methylbenzyl acetate Impurity frompxylene
Acetophenone
VOL. 6 2
NO. 4 A P R I L 1 9 7 0
41
Figure 7.
TABLE V I I .
Teijin process
OXIDATION BY-PRODUCTS (OTHER PROCESS)
By-$roduct
Influence Color
H~~C--@[+CC'OH
Literature Shigeyasu, Ozaki,
J . Chem. Sac. Japan
0
(Ind. Chem. Sec.), 68, 304 (1965) Shigeyasu, Ozaki, J . Chem. Sac. Japan (Ind. Chem. Sec.), 68, 304 (1965)
II
0
Hinder Shigeyasu, Ozaki, oxidation J . Chem. Sac. Japan (Ind. Chem. Sec.), 67,1396 (1964)
TABLE V I I I . CONSUMPTION O F RAW MATERIAL§
Material
Consumption 65 kg/100 kg TPX
$-Xylene Solvent Catalyst
Less than 10 kg/100 kg T P A Less than 0.10 kg/100 kg TPA
TABLE I X .
SCALE-UP FACTORS
(50-300 metric tons/day) Section Total Oxidation Purification Recovery
42
Factor 0.65 0.76 0.62 0.50
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
of the pilot plant operation and from that computed from the accumulation of impurities in the total system. A continuous pilot plant s>-stem consisting of three main sections mentioned above was constructed and has been in operation to collect process and engineering data for industrial and commercial plants. For this process, three tentative flows were selected to supply the best quality product economically, computed on the basis of results obtained from the laboratory-scale experiment and the pilot plant. Each of these three cases was optimized by using IBM 7040 and IBM 360 Model 50. The reaction conditions on an industrial scale were decided from optimization results and operative effectiveness. The characteristics of this process are summarized below. In this process, oxidation is carried out under a mild condition. Therefore, (a) terephthalic acid yield based (b) crude upon $-xylene is high, being over 95%; terephthalic acid produced by this oxidation process does not contain coloring impurities such as fluorenone and biphenyl ketone compounds, so that they are easily purified to terephthalic acid of good color tone; (c) impurities that hinder the reaction are not produced in the oxidation section, so that most of the oxidation mother liquor can be recycled. This is the important reason why this process is economical in spite of the fact that a relatively large amount of catalyst is used; (d) cobalt catalyst alone is used, without any accelerators or promoters such as bromides, aldehydes, and ketones, so that the process system is simplified; ( e ) the mild oxidation condition makes it possible to use stainless steel for the apparatus, with no need for expensive materials like titanium. In the purification section the same solvent as the one for the oxidation section is used. This saves the trouble of drying or separating the solvent between the sections and makes the total process simple. I n the recovery section, most of the oxidation mother liquor containing the catalyst can be recycled by eliminating water from it, and only a part of it is treated so efficiently that it is possible to avoid recovery loss in spite of the use of a relatively large amount of the catalyst. Economic data. Consumption of main raw materials is shown in Table VIII. These values were fully confirmed by using the pilot plant. The scale-up factors for this process from 50 metric tonsiday to 300 metric tons/day are shown in Table IX. Other economic data are satisfactory. These data indicated that this process can fully compete with the conventional processes now in operation. Conclusion
A process for production of high-purity terephthalic acid by air oxidation of @-xylene developed by Teijin Ltd. has been outlined, and is completely covered by patents submitted by the company. Terephthalic acid produced by this process can also be reacted before the stage of fiber-grade purification with methanol to form dimethyl terephthalate or with ethylene oxide to form his- (P-hydroxyethyl)terephthalate.
