Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 629-634
629
cyclo[2.2.2]octane,1449-91-8; trans-2-chloro-5-(chloromethyl)-5- 36912-42-2; cis-2-thiophenoxy-5-(chloromethyl)-5-methyl-2-oxo21071-81-8; cis-2-meth- 1,3,2-dioxaphosphorinane, methyl-2-oxo-l,3,2-dioxaphosphorinane, 36912-43-3; trans-2-thiophenoxy-5(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, (chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, oxy-528097-12-3; trans-2-methoxy-5-(chloromethyl)-5-methyl-2-oxo- 36912-44-4; cis-2-(pentylamino)-5-(chloromethyl)-5-methyl-21,3,2-dioxaphosphorinane, 36912-27-3; cis-2-hydroxy-5-(chlorooxo-1,3,2-dioxaphosphorinane,92366-30-8;trans-2-(pentylmethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-29-5; amino)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphoricis-2-phenoxy-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphos-nane,92366-31-9; trans-2-(4-acetylphenoxy)-5-(chloromethyl)-5phorinane, 36912-30-8; trans-2-phenoxy-5-(chloromethyl)-5- methyl-2-oxo-1,3,2-dioxaphosphorinane, 92366-32-0. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36895-18-8; cis-2-(4methoxyphenoxy)-5-(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxa- Literature Cited Bauman, M.; Wadsworth, W. S.,Jr. J . Am. Chem. SOC. 1978, 100, 6380. phosphorinane, 36912-31-9;trans-2-(4-methoxyphenoxy)-5P.; Ramirez, F.; Ugi, 1.; Marquarding, D. Angew. Chem., Int. Ed. (chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, Gillespie, Engl. 1973, 12, 91. 36912-32-0;cis-2-(4-methylphenoxy)-5-(chloromethyl)-5- Rajan, S.; Kang, S.;Gutowsky, H.; Oidfield, E. J . 8/01, Chem. 1981, 256, 1160. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-33-1; trans-2-(4methylphenoxy)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxa- Skarjune, R.; Oidfieid, E. Bbchemlstry 1979, 78, 5903. Stec, W. J. Acc. Chem. Res. 1983, 16, 411. phosphorinane,36912-34-2; cis-2-(4-bromophenoxy)-5-(chloro- Tsai, M.; Jlang, R.; Bruzik, K. J . Am. Chem. SOC.1983, 105, 2478. methyl)-5-methyl-2-oxo-1,3,2-dioxaphosporinane, 36912-35-3; Wadsworth, W. S.,Jr.; Larsen, S.;Horten, J. L. J . Org. Chem. 1983, 38, 256. trans-2-(4-bromophenoxy)-5-(chloromethyl)-5-methyl-2-oxoW. S.,Jr.; Tsay, Y. G. J . Org. Chem. 1974, 39, 984. 1,3,2-dioxaphosphorinane,36912-36-4; cis-2-(4-nitrophenoxy)- Wadsworth. W. G.; Wadsworth, W. S., Jr. J . Am. Chem. SOC. 1983, 105, 5-(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, Wadsworth, 1031. 36912-37-5;trans-2-(4-nitrophenoxy)-5-(chloromethyl)-5- Wadsworth, W. S., Jr.; Wllde, R. L. Chem. Commun. 1978, 93. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-38-6; cis-2-(2,4- Wadsworth, W. S.,Jr.; Wilde, R. L. J . Org. Chem. 1976, 41, 1264. W. S., Jr. J . Org. Chem. 1973, 38, 2921. dinitrophenoxy)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxa- Wadsworth, Wadsworth, W. S.,Jr. "Phosphorus Chemistry"; ACS Symp. Ser. 1981, No. phosphorinane, 36912-39-7;trans-2-(2,4-dinitrophenoxy)-5171, 548. (chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, Westheimer, F. H. Acc. Chem. Res. 1968, 1 , 70. 36912-40-0; cis-2-benzoyloxy-5-(chloromethyl)-5-methyl-2-oxo1,3,2-dioxaphosphosphorinane, 36912-41-1; trans-2-benzoyloxyReceiued for reuiew February 8, 1984 5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, Accepted August 3,1984
Byproduct Identification in the Terephthalic Acid Production Process and Possible Mechanisms of Their Formation Paolo Roffla," Plerangelo Callnl, and Lulgl Motta Montedipe S.p.A., Research Center of Boliate, Via S.Pletro 50, 20021 Boliate MI, Italy
Serglo Tontl Montedlpe S.p.A., Research Center of Port0 Marghera, Via dell'ElettricltiS 4 1, 30 175 Port0 Marghera VE, Italy
Many byproducts are formed in the p-xylene oxidation process to terephthalic acid and they are present in the mother liquor solution. Their identification has been carried out by HPLC and GLC-MS. W e have found that most of these by products are derivativesof benzoic acid,phenol,terephthalic acid,diphenyl,fluorenone,benzophenone, anthraquinone,and aromatic esters. The formation of these compounds is due to radical or ionic reactions occurring in the oxidation process among p -xylene(and its impurities),oxidation intermediates,reaction solvent,and the catalytic system. Some possible mechanisms assumed for these side reactions are given.
Introduction The degree of purity of the terephthalic acid used in polyester fiber production must be very high and comparable to that of dimethyl terephthalate,which, unlike terephthalic acid,is easily purified by crystallizationwith commercial solvents and by distillation under specified conditions. Terephthalic acid is usually produced by oxidation of p-xylenein acetic acid solution at temperatures of 180-230 OC using Co,Mn, and Br as componentsof the catalytic system (Mid-CenturyProcess) (Burney et al., 1959; Vora et al.,1977; Nippon Chem.Consul.Inc.,1980). The impurities present in the terephthalic acid are formed through side reactions in the p-xyleneoxidation; they CM coprecipitate during the reaction or be embodied in the product in the final step of the mother liquor separation. The nature of these byproduds and their possible 0196-4321/04/1223-0629$01.50/0
presence as impurities depend both on the operating conditions adopted for the oxidation (i.e.,temperature, reaction time,catalytic system, etc.) and the process technology selected. The impurities present in the terephthalic acid used in polyester fiber synthesis slow down the polymerization rate or decrease the average molecular weight of the polymer. This is mainly due to the presence of monofunctional compounds,such as benzoic acid or p-toluicacid,but some other impurities can also cause coloring of the polymer as a consequence of their thermal instability during polyesterification (Nippon Chem. Consul. Inc.,1975). These by-productsare always present in the liquid reaction phase. Being highboiling products, they tend to build up in the solventwhen the latter is recycled without purification. The result is that the terephthalic acid can 0 1984 American Chemical Society
630 Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 4, 1984
be highly polluted. The identification problem of the impurities produced in different p-xylene oxidation processes has already been faced (Ichikawa and Tacheuchi, 1972; Ichikawa et al., 1970; Matsuzawa, 1976). This paper describes our work for the recovery and the identification of the byproducts present in the mother liquor of a Mid-Century type oxidation process. Results The compounds present in the residue obtained after distillation of the mother liquor can be classified as p xylene oxidation intermediates and byproducts. Besides the intermediates reported in the literature (p-tolualdehyde, p-toluic acid, 4-carboxybenzaldehyde (Burney et al., 1959; Ichikawa and Takeuchi, 1972; Ichikawa et al., 1970; Bawn and Wright, 1968; Bryant et al., 1971; Allen, 1981), we also detected the presence of other intermediates resulting from the partial oxidation of one or both methyl groups of p-xylene and containing an alcoholic group, either free or esterified with acetic or terephthalic acid, or two aldehydic groups, or an aldehydic group and an alcoholic group, or a -CH2Br group. However, it was more interesting for us, from an industrial point of view, to find out the nature of the byproducts. We succeeded in identifying most of them, which can be grouped, according to their structure, in the following classes: benzoic acid and its derivatives, phenol derivatives, terephthalic acid derivatives, diphenyl derivatives, fluorenone derivatives, benzophenone derivatives, anthraquinone derivatives, and aromatic esters. The byproduct formation is due to the different radical or ionic reactions occurring in the oxidation process among p xylene (and its impurities), oxidation intermediates, reaction solvent, and catalytic system. Steady-State Concentration of the Oxidation Intermediates and Byproducts The mother liquor recycle does not appreciably affect the concentration of the p-xylene oxidation intermediates, provided that the activity of the catalytic system remains unvaried. The byproducts more resistant to a further oxidative degradation tend, on the contrary, to enrich in the liquid phase of the recycle; a purge of this phase allows one to maintain a proper concentration even for the most undesired byproducts. Among the various intermediates, it would be worthwhile considering the oxidation behavior of esters, the presence of which has been evidenced in the terephthalic acid process. These compounds are oxidation-resistant and their conversion is normally bound to their hydrolysis rate, although a direct oxidative attack to the benzyl CH2 group should not be excluded (Starnes, 1966). In an industrial process involving the recycle of the mother liquor, the esters, namely 4-carboxybenzyl acetate, tend to stabilize at higher steady concentrations. This is especially true when the oxidation is carried out at very low water concentrations. A similar trend is observed for an analogous compound: methyl acetate, which should be considered to be a byproduct of this process. As is already known, methyl acetate can be easily recovered from the process mother liquor and possibly exploited as such, but if it is recycled to the oxidation in order to recover part of the equivalent acetic acid, its steady-state concentration will be at higher value and in such a way that its hydrolysis and oxidation rates are equal to its formation rate; there is the same amount of methyl acetate in the feed and in the outlet stream [(A methyl acetate)/(1000 g p xylene) = 0 in Figure 11 (Roffia and Tonti, 1983). The data reported in Figure 1have been obtained from oxidation runs carried out feeding different amounts of
0
25 F E ~ D
ESTER
100
75
50
i
4 methyl a c e t e t e 1000 g p-xylene
Figure 1. Relationship between the A methyl acetate and the methyl acetate feed (e),where A methyl acetate = methyl acetate out - methyl acetate in, and the steady-state concentration of the ester as a function of the methyl acetate feed (w). Oxidation conditions: 220 "C, 2.4 MPa, Co-Mn-Br components of the catalytic system, p-xylenelacetic acid ratio 1:4,air as oxidant, water concentration in the mother liquor 11%.
methyl acetate to the oxidation. By recycling only part of the methyl acetate present in the outlet stream the value of (A methyl acetate)/(1000 g of p-xylene) is positive. Its value becomes negative if additional ester is added to a full recycle. Table I shows the concentration values for most of the oxidation intermediates and some byproducts present in solution at the outlet of an industrial reactor operating under steady-state conditions.
By products Benzoic Acid and Its Derivatives. This class of compounds is defined by the common presence of a single carboxylic group (compounds I-VII). The amount of
I1
111
IV
V
VI
vi,
benzoic acid produced is higher than the ethylbenzene present as impurity in the p-xylene feed. Therefore, it appears reasonable to assume that the formation of part of compounds I, 11, V, VI, and VI1 is preceded by the formation of a phenyl radical through decarbonylation or decarboxylation reactions (Dermietzel et al., 1983) of an aldehydic or carboxylic intermediate (eq 1-3). The decarbonylationreaction rate can be accelerated by a catalytic system not suitable to promoting the oxidation of the aldehydic intermediates (Allen and Aguilo, 1968), but also by a slow oxygen dissolution and distribution rate in the reaction medium. The metal-promoted decarboxylation reaction of the carboxylic acids (Lande and Kochi, 1968; Anderson and Kochi, 1970; Serguchev and Beletskaya, 1980) is normally favored by severe oxidation conditions (temperature and catalyst concentration) and by very low concentrations
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 831
Table I. Steady-State Concentrations of Intermediates and Some Byproducts Present in the Mother Liquor Solutions of Three Different Runs Performed by Changing Only the Water Concentrationn
(1)
(21
(31
X' M
r radd
= Co.Mn
(Hendriks et al., 1978) of the substrate to be oxidized. The phenyl radicals thus forming either cause hydrogen abstraction and give rise to the monofunctional compound (I) or react with bromine and form compound 11; or, as last chance, they can be oxidized into phenols (V, VI, and VII) by the oxygen or into phenyl esters by the transition metal catalyst. The phenolic compounds can easily give rise to brominated derivatives by reacting with the bromine liberated from the catalytic system. Compound I11 is a product of the termination reaction between benzoyl radical and a methyl radical in conformity with acetone formation in the acetaldehyde oxidation process (Twigg, 1966). Compound IV is a bromination product of a p-xylene oxidation intermediate. The mechanism of its formation is likely due to an electrophilic substitution catalyzed by corrosion metals (eq 4). COOH
COOH
intermediates and byproducts (ppm)* in mother liquor 1,4-benzenedimethanol diacetate benzaldehyde, 4-(hydroxymethyl)terephthaldehyde benzoic acid, 4-(hydroxymethy1)benzoic acid, 4-(hydroxymethyl acetahe)tlenzoic acid, 4-formylp- tolualdehyde p-toluic acid benzoic acid, 4-(bromomethyl)benzene, l-(bromomethyl)-4-methylo-phthalic acid behzoic acid trimellitic acid methyl acetate
OOxidation conditions: 220 "C, 2.4 MPa, Co-Mn-Br components of the catalytic system, p-xylenelacetic acid ratio 1:4, air as oxidant. bHPLC has been used for the analysis. The concentration of the aldehydic compounds was also checked by a polarographic determination. The duplicate analysis on the same sample was always reproducible to &1-2%; the analysis on different samples taken from the reactor operating at the steady-state conditions was reproducible to * 5 % .
present as an impurity in the p-xylene. This is further evidence of the formation of trialkylbenzene during the oxidation step. A possible pathway leading to trialkylbenzene involves the addition reaction to the p-xylene and to the intermediate oxidation compounds of radicals forming in the metal-promoted acetic acid decomposition. It has been shown that cobalt(II1) decomposes the acetic acid yielding methyl radical and carbon dioxide (Hendriks, 1979). (CH3C00)3Co
Phenol Derivatives. The compounds detected are compounds VIII-XI. Decarbonylationor decarboxylation
IX
VII,
XI
X
followed by oxidation of the phenyl radicals represents the intermediate stage of the phenol synthesis; the phenol is then rapidly brominated with consequent formation of compounds IX and X. Compound XI can be either the product of the solvolysis of a brominated derivative which can take place through a nucleophilic substitution catalyzed by an oxidant (Eberson, 1983) or the oxidation product of a trialkylbenzene. The trialkylbenzenes may be present as impurities in the p-xylene feed, but also form in more or less significant amounts in the mid-century type oxidation process. Terephthalic Acid Derivatives. This class of compounds consists of compounds XII-XVI. The amounts
GIH3 &, COOH
XI1
COOH
Xlll
COOH
XIV
a@arCOOH
xv
COOH OH
*
XVI
of dibasic methyl acid (XII) and trimellitic acid (XIII) are higher than expected from the oxidation of pseudocumene,
water wt 70 6 14 20 1279 854 800 113 68 102 10 10 10 413 271 250 2024 1570 1285 45 240 65 130 82 100 690 40 218 77 18 34 10 10 10 10 35 35 8964 10020 12430 376 518 387 3900 2700 2200
-
0
I1
(CH3C0),Co
+
CO,
+
CH,.
(5)
The methyl radical formation in the p-xylene oxidation process is evidenced by the presence of CHI in the off-gases and of methanol and methyl acetate in the mother liquor. This radical has much chance of adding to an aromatic nucleus yielding trialkylbenzenes, since its addition to the benzene ring is only 3 or 4 times slower than hydrogen abstraction from a CH3 group of an aromatic compound (Buckley et al., 1956; Heiba et al., 1968). The acetic acid decomposition caused by manganese(III), the other component of the catalytic system, has been found to produce the .CH,COOH radical (Finkbeiner and Bush, 1968; Van der Ploeg et al., 1968). Now we can confirm that this radical forms also in the p-xylene oxidation process, as we succeeded in identifying the succinic acid in the reaction solvent, whose formation is clearly due to the coupling of two radicals. CH2COOH
- il
HO-C-CH~-CHZ-COH
il
(6)
The formation of such a radical is favored by a low steady-state concentration of p-xylene and likely represents one of the stages leading to the degradation of the reaction solvent to C02 and HzO. Due to its electrophilicity, this radical can easily add to the p-xylene through the mechanism shown in reaction 7. The decomposition of the xylyl
@
+'CH2COOH
HOOCCH2
.(
832 Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
acetic acid is easily promoted by the catalytic system itself, and it yields dimethylbenzyl acetate (Heiba et al., 19691, which can be further oxidized. Another pathway for the formation of trialkylbenzene is the methylation of p-xylene with methyl bromide. We have detected the presence of methyl bromide in the process off-gases. It seems reasonable to state that its formation, as that of the methyl acetate, is due to the decomposition of acetic acid and to the oxidation of the methyl radical through a ligand transfer mechanism from the catalytic system CH,. + MX,Br [CH3.MX,Br] CH,Br + MX2 (8)
-
various aromatic derivatives (Starnes, 1966). Compound XIX could therefore be considered as a product of the p-xylene phenylation reaction either preceded or followed by methylation reaction, oxidative degradation of intermediates, or solvolysis of bromine derivatives. Benzophenone Derivatives. Bi- and polyfunctional compounds (XX-XXIV) were identified as benzophenone
-
It is well-known that the composition of the oxidative decarboxylation products depends significantly on the oxidant, the nature of the anions, and the ligands associated with the metal cations (Serguchev and Beletskaya, 1980). Since methyl bromide is unstable under the oxidation conditions, it can decompose, but it can also react with p-xylene yielding alkylation products (eq 9). On the F
i i
YY
xx '
YY
YXI
I
derivatives. The different number of substituents for these byproducts is consistent with the hypothesis that at least three different reactions types are involved in their formation. A termination reaction between an aroyl and a phenyl radical could be suggested for the formation of compounds (XX) and (XXI) (reaction 11). On the other
Q
basis of the mechanisms suggested for the formation of polysubstituted benzene compounds, one can deduce that various factors can favor these side reactions, i.e., excessive solvent degradation, a not very effective gas-liquid oxygen transfer, enrichment in the reaction medium of corrosion metals acting as catalysts for Friedel-Crafts type reactions. The formation of bromophthalic compounds (XIV and XV) is likely due to bromination of oxidation intermediates. This hypothesis is supported by the identification of compound IV among the byproducts, wherefrom compound XIV would form by oxidation. Compound XVI is again a bromination product; it contains also a phenol group deriving from a solvolysis of a bromine derivative or from oxidation of a polysubstituted phenyl radical. Diphenyl Derivatives. The coupling of the mono- and polysubstituted benzene rings represents the basic structure for the most significant identified compounds. Except for compound XVII, the position of the carboxylic groups is still undefined (compounds XVII-XIX). The identi-
hand, the formation of polysubstituted benzophenone compounds could be determined by the oxidation of diphenylmethane derivatives resulting from the oxidative coupling reactions of alkylaromatic compounds promoted by the catalytic system. It has been shown that the oxidative coupling can be actually caused by cobalt and manganese (Nyberg and Wistrand, 1974). Moreover, the bromoalkyl compounds identified by us can yield diphenylmethane derivatives through an alkylation reaction of p-xylene or its oxidation intermediates (reaction 12).
The reaction could be easily catalyzed by Friedel-Crafts type catalyts present as impurities in the recycle solvent. Fluorenone Derivatives. The identification of the fluorenone derivatives (compounds XXV-XXVIII) would
xxv
Ab
r:
I-
x IX
fication of the diphenyl dicarboxylic acid supports the formation of phenyl radicals through decarbonylation or decarboxylation and their coupling in termination reactions (reaction 10). A similar mechanism was suggested to
XXVll
XXVIll
suggest that their formation can be determined by at least two possible reactions involving other byproducts. One path starts from a preformed diphenyl with a methyl in the ortho position, which is oxidized to a carboxylic group and then converted to ketone through cyclization (reaction 13). The other path may start from a benzophenone with
- R@-QR-R
@@R
E131
c q3
explain the presence of diphenyl or polyphenyl compounds among the byproducts identified in the p-xylene oxidation via the Witten process (Bunger, 1978; Dziwinski et al., 1980), but the formation of polysubstituted diphenyls could also be better explained by the addition reaction of a phenyl radical to the p-xylene or to its oxidation products. It was demonstrated that this reaction occurs in the oxidation of alkylaromatic compounds catalyzed by CoBr2 (Fields and Meyerson, 1968) and in the decomposition of aromatic acids catalyzed by cobalt(II1) in the presence of
XXVI
COOH
6
formation of a radical in the ortho position through a decarbonylation or decarboxylation reaction. The orthobenzophenone radical gives intramolecular cyclization under the oxidizing effect of the catalytic system (Russell and Thomson, 1962). The oxidation of the various substituents may either precede or follow the formation of the fluorenonic structure. Anthraquinone Derivatives. The anthraquinone derivatives (compounds XXIX-XXXII) are likely byproducts resulting from a further reaction of the benzophenone derivatives present in the mother liquor. It
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 633
XXIX
xxx
XXXI
XXXll
should not be excluded either that the anthraquinone compounds may be the oxidation product of dihydroanthracene derivatives, whose formation has been detected in the oxidation of polyalkylbenzenes with manganese acetate in acetic acid (Nyberg and Wistrand, 1974). We did not find either anthracene or dihydroanthracene derivatives among the byproducts of the p-xylene oxidation process. The oxidizing cyclization of benzophenone derivatives would therefore seem the most likely path for anthraquinone formation. It would be favored by the presence of polysubstituted aromatic hydrocarbons as starting compounds. Esters. The esterification products of the p-toluic and the terephthalic acid were identified among the compounds present in the oxidation residue (compounds XXXIIIXXXV). Compound XXXIII should form by partial
XXXlll
XXXN
W V
degradation of the p-xylene oxidation products. Though various assumptions could be made for the reaction giving rise to this compound, it is reasonably presumed that it forms by thermal or catalyzed rearrangement of substituted benzoylperoxide produced by a termination reaction (Yablokov, 1980). I t might also form through oxidation, by cobalt or manganese(II1) of a phenyl radical, whose formation has been proposed as an intermediate stage in the synthesis of other byproducts. The formation of compound XXXIII through a termination reaction between phenyl and aroyloxy radicals would not seem as likely. Direct oxidation of the CH3 group by cobalt and manganese(II1) through an electron-transfer mechanism is assumed to be responsible for the formation of compounds XXXIV and XXXV (Sheldon & Kochi, 1973). Since the p-xylene oxidation is carried out in acetic solution, the formation of benzyl acetate type esters (Table I) will prevail on that of benzyl-p-toluate or terephthalate type esters, and the formation of these compounds through transesterification reactions should not be rejected.
Conclusions The major objective of this work has been the identification of most of the byproducts from the p-xylene oxidation process. Both the oxidation stage and the quality of the terephthalic acid are affected by these compounds. Our findings prompted us to investigate, in an experimental way, the relationship between the process variables and some secondary reactions. As is known, the main process variables affecting the specific consumption of p-xylene, acetic acid, and catalyst as well as the quality of the terephthalic acid produced are temperature-pressure, composition of the catalytic system, promoters (type and concentration), solventlp-xylene ratio, water concentration, p-xylene conversion degree, oxygen mass-transfer conditions, and amount of mother liquor recycle. In a preliminary work we have found, in good agreement with the possible mechanisms discussed herein, that the process variables can favor or hinder the kinetics of the secondary reactions; the formation of byproducts is promoted by: (a) an excessive degradation of the reaction solvent and of the oxidation intermediates, determined by
an unsuitable composition of the catalytic system, severe experimentalconditions (too high temperature with respect to catalyst type and concentration), per pass conversion of p-xylene, high water concentration in the reaction medium; (b) insufficient oxygen mass transfer from the gas to the liquid phase over the whole reactor solution. Since the dissolved oxygen quickly reacts with all the radicals, it can effectively block them before the occurrence of addition reactions to the oxidation intermediates, coupling reactions, and decarbonylation or ester formation through metal electron transfer oxidation; and ( c ) enrichment of the corrosion metals in the reaction medium due to mother liquor recycle. Such metals catalyze Friedel-Crafts type reactions yielding polysubstituted aromatic compounds and brominated compounds. The formation of the latter modifies the composition and the activity of the catalytic system. The byproducts and intermediates identification is to be considered the first necessary step in order to improve the terephthalic acid synthesis. Any further work aimed at the determination of correlations among the various process parameters and the formation rate of the main product and the byproducts would give an essential contribution to a deeper knowledge of the p-xylene oxidation process. On this basis one could arrange plant operating conditions in such a way as to reduce the formation of organic impurities. It could be consequently possible: (a) to recycle almost all the reaction solvent after simple drying; (b) to produce terephthalic acid free from organic byproducts, that is, easier to purify (e.g., to obtain a transmittance at 340 nm 1 98%); ( c ) to reduce acetic acid losses through the combustion to CO, and the formation of methyl acetate; (d) to reuse the catalytic system after purification from corrosion metals; (e) to recycle the methyl acetate to the oxidation to recover the equivalent acetic acid, unless the producer has planned a better exploitation of the ester. These improvements will drastically cut the manufacturing costs of the terephthalic acid.
Experimental Section The mother liquor of an industrial plant for the terephthalic acid production by oxidation of the p-xylene in acetic acid solution at 220 "C, 2.4 MPa, and in the presence of Co, Mn, and Br as components of the catalytic system, was evaporated under reduced pressure. The residue obtained from the distillation was worked up for catalyst removal by extraction with water or with ionic exchange resins. After conversion of the carboxylic compounds into the corresponding methyl esters by reaction with trimethyl phosphate in pyridine the residue was analyzed by a gas chromatograph linked to a mass spectrometer. The characterization of a part of the residue components was done by comparison with spectra of the authentic compounds. The HPLC analysis performed on the original crude residue has confirmed, by comparison with the retention time of the authentic compounds, the presence of the same byproducts identified by GLC-MS. For the byproducts marked with the numbers V, VII, XI, x v , XVI, XVIII, XIX, XXII, XXIII, XXIV, XXVII, XXVIII, XXIX, XXXI, and XXXII, the structure was inferred only from the mass spectral cracking patterns because authentic samples were not available. For some byproducts present in small amounts in the residue, the preparative reverse HPLC was used for the separation of samples which were esterified with diazomethane before GLC-MS analysis if no reactive group, except the carboxylic one, was present.
Ind. Eng. Chem. Prod. Res. Dev. 1984,23, 634-637
634
The aromatic aldehydic compounds were also characterized by polarographic determination. Their identification was determined by the reduction potentials. The GLC analyses were performed by a Fractovap 2300 Carlo Erba using a 2-m 10% Silicone Gum Rubber UCCW-982 glass column on Chromosorb W AW-DMCS 80/100 mesh, a flame ionization detector, and oven temperature programmed from 130 to 180 O C a t a rate of 5 OC/min. The mass spectra were recorded by using a Varian CH7 mass spectrometer. The HPLC analyses were performed with an H P 1084 B chromatograph equipped with an Autosampler, H P detector at fixed wavelength 250 nm, and data system terminal. Oven temperature was 40 "C, and a 25-cm column was filled with Lichrosorb RB-18 eluted with a solution of demineralized water, acetonitrile, and acetic acid. The polarographic determination of aromatic aldehydes was performed by a Brucker E 310 instrument. The sample was dissolved in diluted alkali solution at pH 9. The solution was transferred into an electrolysis vessel and purged with nitrogen. Registry No. 11, 586-76-5; 111, 586-89-0;IV, 7697-26-9; V, 14348-41-5;VI, 3337-62-0; VII, 92126-56-2; VIII, 106-44-5;IX, 615-58-7;X, 118-79-6;XI, 698-27-1; XII, 5156-01-4;XIV, 586-35-6; XV, 92126-59-5; XVI, 92126-60-8; XVII, 787-70-2; XVIII, 73589-72-7;XIX, 92126-61-9; XX, 30861-74-6; XXI, 964-68-1; XXII, 92126-62-0;XXIII, 92126-63-1;XXIV, 92126-64-2;XXV, 92126-65-3; XXVI, 92126-66-4; XXVII, 92126-67-5; XXVIII, 92126-68-6; XXIX, 92126-69-7; XXX, 92216-94-9; XXXI, 92126-70-0;XXXII, 92126-71-1;XXXIII, 81460-41-5;XXXIV, 92126-58-4;XXXV, 92126-57-3;TPA, 100-21-0;p-xylene, 106-42-3; 1,4-benezenedimethanol diacetate, 14720-70-8; 4-(hydroxymethyl)benzaldehyde, 52010-97-6;terephthaldehyde, 623-27-8; 4-(hydroxymethy1)benzoic acid, 3006-96-0;4-(hydroxymethyl)benzoic acid acetate, 2345-34-8;4-formylbenzoicacid, 619-66-9; p-tolualdehyde, 104-87-0; p-toluic acid, 99-94-5; 4-bromomethylbenzoic acid, 6232-888; l-(bromomethyl)-4-methylbenzene,
104-81-4;o-phthalic acid, 8899-3;benzoic acid, 65-85-0;trimellitic acid, 528-44-9; methyl acetate, 79-20-9. L i t e r a t u r e Cited Allen, G. C.; Aguilo, A. Adv. Chem. Ser. 1888, No. 76, 363. Allen, J. K. (Standard Oil Co.) U S . Patent 4 266 084, 1981. Anderson, J. M.; Kochi, J. K. J . Am. Chem. SOC. 1970, 92,2450. Bawn, C. E. H.; Wright, T. K. Discuss. Faraday SOC. 1888, 46, 164. Bryant, H. S.; Duvol, C.; McMakln, L.; Savoca, J. Chem. Eng. frog. 1871, 67, 69. Buckley, R. P.; Leavitt, F.; Szwarc, M. J . Am. Chem. SOC.1956, 78, 5557. Biinger, H. Compend. Dtsch , Ges . Mineralmlwiss . Kohl8 Chem. 1878, 78-79, 417. Burney, D. E.; Weisemann, G.; Fragen, N. Pet. Refiner 1958, 38, 186. Dermietzel, J.; WienhoM, C.; Grundmann, H.; Staschok, A,; Koch, J.; Bordes, E. Zfl-Mltteilungen (Lelprig) 1983, 71, 85. Dzlwinski, E.; Hepter, J.; Pokorska, 2 . Chem. Anal. (Warsaw) 1980, 25, 1021. Eberson, L. J . Mol. Catal. 1883, 20, 27, and references cited therein. Fields, E. K.; Meyerson, S . Adv. Chem. Ser. 1988, No. 76, 395. Finkbeiner, H.; Bush, J. B. Discuss. Faraday SOC. 1988, 46, 150. Heiba. E. I.; Dessau, R. M.; Koehl, W. J. J . Am. Chem. SOC. 1988, 9 0 , 1082. Heiba, E. I.; Dessau, R. M.; Koehl, W. J. J . Am. Chem. SOC.1988, 91,138. Hendrks, C. F.; Van Beek, H. C. A.: Heertjes, P. M. Ind. Eng. Chem. Rod. Res. Dev. 1978, 17, 256. Hendriks, C. F.; Van Beek. H. C. A.; Heertjes, P. M. Ind. Eng. Chem. Rod. Res. Dev. 1878, 18, 43. Ichikawa, Y.; Takeuchl, Y. Hydrocarbon Process. 1872, 105. Ichikawa, Y.; Yamashlta, G.; Tokashikl, M.; Yamaji, T. fnd. Eng. Chem. 1970, 62,38. Lande, S. S . ; Kochi, J. K. J . Am. Chem. SOC. 1988, 90,5196. Matsuzawa, K. C . E . E . R . 1976, 8, 25. Nlppon Chem. Consul. Inc., Report No. 1, Nov 1975. Nippon Chem. Consul. Inc., Report No. 15, Jan 1980. Nyberg, K.; Wistrand, L. G. Chem. Scripta 1974, 6 , 234. Roffia, P.; Tonti, S . Montedipe, internal report. Russell, J.; Thomson, R. J . Chem. SOC. 1882, 3 3 , 79. Serguchev, Y. A.; Beletskaya, I.P. RUSS.Chem. Rev. 1980, 49, 1119. Sheldon, R. A.; Kochi, J. K. Oxhl. Combust. Rev. 1973, 5 , 135. Starnes, W. H. J . Org. Chem. 1988, 31, 1436. Twlgg, G. H. Chem. Ind. (London) 1968, 12,476. Van der Ploeg, R. E.; de Korte, R. W.; Kooyman, E. C. J . Catal. lg88s IO, 52. Vora, B. V.; Pujado, P. R.; Persak, R. A. Chem. Ing. frog. 1977, 74. Yablokov, V. A. Russ. Chem. Rev. 1980, 49, 833.
Received for review December 21, 1983 Revised manuscript received May 29, 1984 Accepted July 24, 1984
Separation of n-Paraffins from Petroleum Fractions by Adsorption on 5A Molecular Sieves Rasheed S. At-Ameert" Department of Chemical Engineering, Kuwait UniversivsKuwait
Fathl A. Owaysl Petroleum Technology Department, Kuwait Institute for Scientific Research, Kuwait
Separation of n-paraffins from Kuwait petroleum fractions was accomplished by selective adsorption on molecular sieves type 5A. Straight-run kerosene and naphtha-kerosene blend were used as feedstocks due to their relatively high n-paraffin content. The average purity of the recovered n-paraffins was 96.6%; it was found to be independent of the composition of the feedstock used. The efficiency of the adsorption process was compared with the efficiency of urea adduct as an alternative separation technique, and the results indicated that the former was more efficient. The separation of n-alkanes from the hydrocarbon feedstocks resulted in relatively large improvements in the pour points of these fuels which would extend their application to low-temperature climates.
Introduction
Normal paraffins, the straight-chain organic compounds with the general formula CnH2n+2, have many industrial applications. For instance, single cell protein is produced from n-paraffins by fermentation. Large volumes of n0196-432118411223-0634$01.50/0
paraffins are used as feedstocks for the production of chlorinated paraffins, which in turn are used in several industries such as lubricating oil, additives, plasticizers, flame retardants, coating materials, paints, surfactants, etc. A potential outlet for the consumption of n-paraffins is 0 1984 American Chemical Society
