Process for the oxidative purification of dimethyl phthalates
United States Patent [191 Fujita et al.  
 1 1 PROCESS FOR THE OXIDATIVE PURIFICATION OF DIMETIIYL PHTI&#39;IALATES [75] Inventors: Yasuhiro Fujita, Hiroshima;  
 Takenori Nagaoka, Yamaguchi; Yuji Simokawa, Hiroshima, all of Japan [73] Assignee: Mitsui Petrochemical Ind., Ltd., Tokyo, Japan [22] Filed: Apr. 13, 1973 [211 App]. No.: 350,874  
 [301 Foreign Application Priority Data Apr. 13, 1972 Japan 47-36488 [52] US. Cl 260/475 B [51] Int. Cl. C07c 69/80 [58] Field of Search 260/475 B [56] References Cited UNITED STATES PATENTS 3.461.153 8/1969 Tholstrup et a1 260/475 B 3,576,842 4/1971 Kimura et al. 260/475 B [451 Apr. 22, 1975 FOREIGN PATENTS OR APPLICATIONS 1,043,289 9/1966 United Kingdom 260/475 B 1.032.730 6/1966 United Kingdom 260/475 B Primary Examiner-Lorraine A. Weinberger Assistant E.raminer-E. Jane Skelly Attorney, Agent, or Firm--Sughrue, Rothwell, Mion, Zinn &amp; Macpeak [57] ABSTRACT A dimethyl phthalate prepared from a phthalic acid which has been obtained by the oxidation of a xylene is treated with a gas containing molecular oxygen in the presence of a compound selected from the group consisting of an alkali metal compound and/or an alkaline earth metal compound under liquid phase conditions, and then the dimethyl phthalate is separated from the resultant oxidized product, whereby the separated dimethyl phthalate is oxidatively purified. The purification can be improved when a heavy metal compound is used together with the alkali and/or alkaline earth metal compound.  
 18 Claims, No Drawings PROCESS FOR THE OXIDATIVE PURIFICATION 7 OF DIMETHYL PHTI-IALATES BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the oxidative purification of dimethyl phthalates prepared from phthalic acids which have been obtained by the oxidization of xylenes.  
 2. Description of the Prior Art Dimethyl phthalates are useful as plasticizers for a variety of rubbers and plastics and as raw materials for polyesters. In particular, dimethyl terephthalate and dimethyl isophthalate have been widely utilized as intermediates in preparing polyesters. It is well known that, in such use as mentioned above, the existence of even a slight or trace amount of impurities in the intermediate phthalates cause a decrease in the quality of the resultant polyesters. and therefore, it is necessary that the dimethyl phthalates have high purity and high quality. In particular. when dimethyl terephthalate or dimethyl isophthalate is to be conveyed to a remote place while being in a molten state, it is necessary that these phthalates be stable and not discolor, even after being at high temperature and in the molten state for a long period of time, that is, that they have excellent color stability.  
  Phthalic acids which have been obtained by oxidizing xylenes with oxygen in the presence of an oxidation catalyst or by means of an oxidizing agent, such as nitric acid, contain various impurities such as oxidized intermediates, oxidized by-products, etc., although the content of the impurities is small. Accordingly, dimethyl phthalates which are prepared by esterifying these phthalic acids with methanol naturally contain various impurities such as oxidized intermediates, oxidized by-products, esters of oxidized intermediates, esters ofoxidized by-products, and the like, although the content of these impurities is also small.  
  It is well known that among these impurities methyl formylbenzoate greatly affects the preparation of polyesters when the dimethyl terephthalate or dimethyl isophthalate is subjected to transesterification with gly cols, that is, the pressure of methyl formylbenzoate during preparation retards the polycondensation speed, reduces the polymerization degree and harmfully effects the coloration of the polyesters.  
  It has hitherto been known to purify dimethyl phthalates by treating them with a gas containing molecular oxygen to remove the impurities contained therein. For example, British Pat. Nos. 1,032,730 and 1,043,289 describe such a method. In British Pat. No. 1,032,730, a process for purifying dimethyl phthalates is proposed wherein the dimethyl phthalates are brought into contact with molecular oxygen in the presence of manganese or copper compounds which are soluble in the reaction system. In British Pat. No. 1,043,289 there is proposed a process for purifying dimethyl phthalates wherein the phthalates are brought into contact with molecular oxygen in the presence of chromium or nickel compounds.  
 SUMMARY OF THE INVENTION The inventors have studied the process for the oxidative purification of dimethyl phthalates using molecular oxygen and found the following facts: methyl formylbenzoate may fairly effectively be removed by means LII of the conventional purifying methods as above, but these conventional methods have other faults, that is, upon the removal of the methyl formylbenzoate to purify the dimethyl phthalates by means of these conventional&#39;methods, a slight amount of other new impurities are produced, the structures of which are as yet unknown. These new impurities are themselves colorless, but when dimethyl terephthalate or dimethyl isophthalate has been kept in the molten state at a high temperature for a long period of time, these impurities harmfully effect the color stability. Accordingly, the inventors continued to zealously study the problem of how to avoid the formation of these new impurities formed during the purification of the dimethyl phthalates, in order to develop excellent products which do not discolor even after being maintained in the molten state at high temperature for a long period of time. As a result they found that the purification is surprisingly attained very effectively when the dimethyl phthalates are brought into contact with oxygen or an oxygen containing gas in the presence of an alkali metal compound and/or an alkaline earth metal compound. The inventors have also found that the effect of purification is further improved when a heavy metal compound is used together with the alkali metal and/0r alkaline earth metal compound. Of course, the existence of the alkali metal compound and/or the alkaline earth metal compound does not interfere with the oxidation or oxidative decomposition of the methyl formylbenzoate or other impurities derived from the raw materials. phthalic acids.  
  The object of the present invention is to oxidatively purify dimethyl phthalates prepared from phthalic acids which have been obtained by the oxidization of xylenes.  
  Another object of the present invention is to provide an improved process for the oxidative purification of dimethyl phthalates characterized in that a dimethyl phthalate prepared from a phthalic acid which has been obtained by the oxidization of a xylene is treated with a gas containing molecular oxygen, in the presence of a compound selected from the group consisting of an alkali metal compound and/or an alkaline earth metal compound and under a liquid phase condition, and  
 then the dimethyl phthalate is separated from the resultant oxidized product.  
  The oxidative purification of the present invention can be carried out in the presence of a heavy metal compound together with the alkali metal and/or alkaline earth metal compound.  
 DETAILED DESCRIPTION OF THE INVENTION The dimethyl phthalates which are used in the present invention are prepared by esterifying phthalic acids with methanol, the phthalic acids being ones obtained by oxidizing xylenes, and more specifically are dimethyl orthophthalate, dimethyl terephthalate and dimethyl isophthalate. In particular, the present invention is remarkably effective in the oxidative purifica tion of dimethyl terephthalate.  
  The phthalic acids are usually prepared by oxidizing xylenes in the liquid phase with either molecular oxygen or a molecular oxygen-containing gas in a monocarboxylic acid medium having 2 to 8 carbon atoms, the weight ratio of the monocarboxylic acid medium to the xylenes being in the range from 2 to I up to 12 to l, at a temperature of 90 to 280C and at a pressure of from normal atmosphereic pressure to 50 kg/cm&#34; gauge in the presence of acobalt-containing catalyst.  
  Phthalic acid obtained by reacting xylenes with dilute nitric acid at superatmospheric pressure at a temperature of 150 to 250C can also be used. since a common type of impurity causing the problem which the present invention solves is also encountered with such systems.  
  These processes are not limitative, of course. and the present invention finds application with any dimethyl phthalate system containing impurities as described in this application. the harmful effect of which can be removed in accordance with the present invention.  
 The generally used conditions for esterifing phthalic acids with methanol are: temperature. 150 300C: pressure. 5 200 kg/cm gauge; reacting 5 30 moles of methanol with 1 mole of phthalic acid in .the liquid phase in the presence of a catalyst or in the absence of a catalyst. typical catalysts being sulfuric acid. copper metal. zinc metal. tungsten metal. oxide of copper. zinc. manganese or molybdenum. salts of organic acids with copper, zinc. manganese or molybdenum. silicotungstate. silicomolybdate. etc. Theimpurities formed during the oxidation of xylenes remain mixed in the resultant dimethyl phthalates. either as such or in the form of esters after being esterified. or in any other forms after being further changed. In particular. formylbenzoic acid is converted into methyl formylbenzoate. which is the primary cause of a decrease in purity and quality of dimethyl phthalates. The present invention is to oxidize or oxidativcly decompose the impurities predominantly consisting of methyl formylbenzoate by means of oxygen or an oxygen containing .gas..-whereupon the formation of any other impurities with .an unknown structure which reduce color stability is avoided. thereby improving-the purity and the quality of the dimethyl phthalates after theyare rendered molten and conveyed.  
  The alkali metal compounds or alkaline earth metal compounds used in the present invention are compounds containing at least one metal of lithium. so-  
 dium. potassium. mmagnesium. and calcium. and in particular. a-xlithium compound, a sodium compound and a potassiumcompound are preferred. The alkali metal compounds or alkaline earth metal compounds are preferably soluble in the reaction system.  
 As the alkali metal compounds and/or alkaline earth ing l to 20 carbon atoms carbonates. bicarbonates.  
 phosphates. borates. hydroxides. alcoholates having 1 to 5 carbon atoms or phenolateshaving 6 to 8 carbon atoms of the above mentioned alkali metals or alkaline earth metals are most preferred. Particularly preferred are for example. salts of aliphatic carboxylic acids having l-to l8 carbon atoms such as formate. acetate. propionate. butyrate. valerate. caproate, caprate, octylate. decanoate. laurate. palmitate. oleate. stearate. oxalate. succinate. adipate. sebacate. etc.; salts of cycloaliphatic carboxylic acid such as naphthenate, hexahydrobenzoate, etc.; salts of aromatic carboxylic acid such as benzoate. terephthalate, isophthalate, orthophthalate. a salt of the monomethylester of terephthalic acid, a salt of the monomethyl ester of isophthalic acid. a salt of the monomethyl ester of orthophthalic acid.; carbonates; bicarbonates; hydroxides; alcoholates such ously, more than one alkali metal compound can be used 11 combination with more than one alkaline earth metal compound. I l  
  The amount of these alkali metaland/or alkaline earth metal compound(s) used is in the range of about l gram-atoms to about gram-atoms. preferably from l0 gram-atoms to 10 gram-atoms. in terms of metal atoms per 1 g of dimethyl phthalate.  
  lf the amount of these compounds exceeds about 5 X l 0&#34; gram-atoms. further improvement on the purification can not be attained, rather. on the contrary, too great an amount of the compounds can cause the blocking or stoppage of the apparatus in the oxidization step. or the distillation step.  
  The heavy metal compound of compounds which can be used in the present invention together with the above mentioned alkali metal and/or alkaline earth metal compounds are compounds of heavy metals as defined in the Periodic chart of the Elements or pages 56 and 57 ofHandbook of Chemistry&#34; 8th Edi- &#39;tion. 1952, Hondbook Publishers. Sandusky. Ohio.  
 as the methylate, ethylate and propionate and pheno- U.S.A.. as disclosed in US. Pat. No. 2.833.816 Saffer et al. (though for different process). The preferred compounds are of at least one of chromium. manganese. cobalt, nickel. copper, molybdenum palladium. antimony and the lanthanides. Particularly preferred are compounds of chromium. manganese. nickel or copper. The heavy metal compounds are preferably soluble in the reaction system. and for this reason. organic acid salts having 1 to 18 carbon atoms inorganic acid salts such as carbonates, phosphates halides. nitrates. bonates. and organic complexs such as acetyl acetonate. phthaloxyanide. hyroxy quinolinate. ethylene diamine tetraacetate. and of the above mentioned heavy metals are preferred. In particular. aliphatic carboxylates such as formate. acetate. propionate. oxalate.  
 valerate. caproate. caprylate. oleate. etc.; cycloaliphatic carboxylates such as naphthenate. hexahydrobenzoate. etc.; aromatic carboxylates such as benzoate. terephthalate monoalkyl ester, etcx, metal acetyl acetonate. metal phthalocyanine, silicafluoride. and like compounds of the above-mentioned heavy metals are preferred.  
  The amount of the heavy metal compound(s) used is sufficient if in the range of about 10 gram-atoms to about 10&#39; gram-atoms of metal atoms, per 1 g of the dimethyl phthalates treated. Mixtures of heavy metal compounds can, of course, be used to exhibit the effects of the individual components.  
  In the process of the present invention, the contact of the dimethyl phthalates with the oxygen or oxygen containing gas is carried out in the liquid phase. that is, in such state that the dimethyl phthalates are molten or are dissolved in a solvent which is inert to the oxidization. As the solvent. hydrocarbons,halogenated hydrocarbons. organic acids. organic acid esters. etc., can be used. Specific examples of the solvent used are hydrocarbons such as hexane. heptane, benzene. toluene. xylene. etc.. halogenated hydrocarbons such as chloroform. trichloroethane, chlorobenzene. etc., and organic acid esters such as ethylacetate, butylacetate. methyl benzoate. etc. Organic acids should not be used as the solvent. It is preferred to bring the dimethyl phthalates into contact with a gas containing molecular oxygen while the phthalates are in the molten state. without using the solvent 7 In the process of the present invention, pure oxygen. air admixed with oxygen gas or a mixture of oxygen with an inert gas such as N or CO can be used forthe oxidization. The amount of the oxygen containing gas used is sufficient if it is more than the amount metrically necessary for the oxidization of impurities. and thus the amount is not specifically limited, however, an amount of about 3! to about 1 K1 of oxygen per 1 kg of dimethyl phthalate calculated as oxygen at a normal temperature and pressure is industrially prefer able. In order to prevent the danger of explosion, steam, inert gas, etc., may be introduced in the gaseous part of the reaction system.  
  Although the temperature of the treatment&#39;of dimethyl phthalates with a gas containing molecular oxygen can be varied in a wide range, a temperature of about 140 to about 300C, particularly 150 to 280C, is generally preferred. The treatment is normally carried out under atmospheric pressure, but can be carried out under elevated pressure or under reduced pressure. In particular, the pressure of the treatment is in the ranges of atmospheric pressure to a guage pressure of about, 30 kg/cm The time for the oxidization reaction in the present invention is not particularly limited, but on an industrial scale will usually be from about to about 240 minutes, more often to 120 minutes being sufficient.  
  The process of the present invention can be performed according to any batch or continuous manner. For the contact of dimethyl phthalate with molecular oxygen, any well-known gas-liquid contact method can be used in the present invention. Bubbling the oxygen gas through the&#39;dimethyl phthalate, or contacting the dimethyl phthalate with the&#39;oxygen gas using a gasliquid contact apparatus&#34; with a sieve tray is&#39; especially preferable; While a method where the oxgen is bubbled through the di&#39;methyl phthalate&#39;and the mixture is violently stirredcan also be used, the purification is not as good as with the methods recited above. If necessary, crude dimethyl phthalates obtained from phthalic acids can previously bedis&#39;tilled for the purpose of removing substances which interfere with the oxidization reaction such as iron compounds, and afterwards the present method be applied to the resultant fraction.  
 The separation of high&#39; purity dimethyl phthalates from the oxidized reaction productfwhich is obtained from the contact with a gas containing molecular oxygen in accordance with this invention is carried ina known manner. Distillation of&#39;oxidized reaction prod-.&#39;  
 uct is the method most commonly employed for separation of high purity dimethyl phthalates. The distillation of the final product can be conducted under reduced pressure, atmospheric pressure or elevated pressure.  
 tion after cooling the solution containing oxidized reaction product. a  
  The present invention will now be illustrated in more detail by the following examples of several preferred embodiments of the present invention.  
 EXAMPLES l27 Paraxylene was oxidizedin acetic acid solvent inthe presence of a cobalt compound, a manganese compound and a bromine compound to obtain terephthalic acid. The oxidation reactor was fed ,(hqurly rate) 300g terephthalic acid 300g.  
 methanol 350g.  
 catalyst none.  
 reaction temperature 260C. reaction pressure l()() kg/cm&#34; g&#39;uage and reaction time 40 minutes.  
 Methanol and water were separated distillation from the resultant product mixture to obtain dimethyl terephthalate. The resultantester contained 0.4 percent (by weight) of methyl paraformylbenzoate. The molten color of the dimethyl terephthalate obtained at C was 500 APHA.  
  To 300g of the above dimethyl terephthalate there was added an alkali metal compound or an alkaline earth metal compound as shown in Table l hereunder. and oxygen gas was fed thereto at a rate of 20l/hour. whereby the contact of the esterand the oxygen gas was performed for &#39;30 minutes at 235C and at almospheric pressure whereafter the resultant reaction mixture was distilled at a pressure of 40 mm and at a temperature &#39;of l83C (at the top of &#39;the towerlf&#39;The content of methyl paraformyl&#39;benzoate (&#39;4-CBA-M) in the dimethyl terephthalate (DMT) obtained after the distillation the initial molten color and the color stability of the dimethyl terephthalate are shown in Table l.  
  After the dimethyl terephthalate was heated for 5 minutes&#39;at&#39;200C, the initial molten color of the dimethyl terephthalate was measured by comparison with the APHA (American Public Health Association) standard solution (See.U.:S. Pat. N o, 3,288,843 Fried- COMPARATIVE EXAMPLE 2 To 300g of the dimethyl terephthalate of Example I I there was&#39;added sodium acetate in an amount of 5 X l0 gram-atoms/g. and. the.mixture was heated at 235C for 30 at atmospheric pressure minutes without feeding oxygen thereto. and then distilled as in Example l. The results of analysisof the resultant dimethyl terphthalate obtained after the distillation are also shown in Table l, forlcornparison.  
 &#39; Table 1 Alkali metal compound or Purified DMT Alkaline earth metal Initial compound r 4-CBA-Mc molten Color Example (X l&#34;? gram-atoms/g-DMT) (ppm) Color (APHA) stability 1 I sodium&#39;acetate 35 I0 50 2 lithium propionate MK) 35 I0 50 3 potassium butyrate 500 40 55 4 sodium octylate l I0 60 5 calcium laurylate l0. 45 I0 70 6 magnesium caprate I000 45 I0 70 7 potassium palmitate l I0 60 8 sodium formate 50 30 10 55 9 sodium oxalate l0000 10 50 10 calcium succinate 300 I0 75 l 1 magnesium adipate 800 40 I0 80 12 sodium naphthenate 30 I0 I3 potassium hexahydro l00 35 I0 50 henzoate l4 disodium terephthalatc 10 30 I0 I5 potassium monomethyl 500 40 I0 55 terephthalate to sodium henzoate I00 35 I0 60 17 lithium henzoate I00 30 I0 60 18 sodium carbonate [0 30 I0 50 I9 potassium bicarbonate 5 35 I0 55 20 sodium phosphate 300 30 I0 2| sodium hydroxide I0 35 I0 50 22 lithium hydroxide 3 35 I0 50 23 potassium hydroxide &#39;l 35 I0 60 24 sodium methylate I0 30 I0 50 25 potassium cthylate 50 35 10 60 26 lithium isopropionate 50 40 I0 60 27 sodium horate I00 35 I0 80 Comparative none 40 35 l 50 Example I Comparative Example 2 sodium acetate 5 3000 60 )00 without oxygen. only under heat.  
 EXAMPLES 28 4l To 300g of the dimethyl terephthalate of Example I was added an alkali metal compound or an alkaline earth metal compound and a heavy metal compound as shown inthe Table 2. and oxygen gas was fed thereto at a rate of 20 I/hour. whereby the contact of the ester Table 2 Alkali metal compound Heavy metal Initial Color or Alakline earth compound molten stametal compound (X l0 4-CBA-Me color bility Example (X l0&#39; gram-atoms/g) gram-atoms/g) (ppm) 1 (APHA) (APHA) 2s lithium acetate s nickel 1 l0 ll) 30 V naphthenate 29 sodium acetate 0.5 chromium l0 l0 I0 30 benzoate 30 sodium bicarbonate 50 chromium 20 I0 5 l5 naphthena&#39;te 3i potassium benzoate l00 chromium l0 l0 5 25 hexahydrobenzoate 3 2 potassium 10,000 chromium 50 5 5 l5 monomethyl monomethyl I terephthalate terephthalate 33 potassium carbonate 5 manganese 1 l0 l0 2S naphthenate 34 potassium I00 chromium 20 l0 I0 20 bicarbonate caproate 35 magnesium acetate 50 copper t &#34;1 l0 I0 30 naphthenate I I 36 sodium monomethyl chromium w isophthalate 5,000 acetate l0 5 5 I5 37 dipotassiu&#39;m&#39; 500 palladium so 0 I0 40 ortht&#39;lphthalate ucetate- Table 2 -Continued A i Alkali metal compound Heavy metal lnitial Color or.Alakline earth compound molten stat-l metal compound (X l&#34; 4-CBA-Me color bility Example (X 10&#39;&#34; gram-atoms/g) granvatoms/g) (ppm) (APHA) (APHA) 38 disodium sebacate 100 chromium l0 5 5 l5 acetyl acetonate 39 calcium stearate nickel l0 l0 10 25 oxalate 40 sodium succinate 5 antimony 100 l0 I0 40 acetate 41 sodium carbonate 100 molybdenum l0 10 naphthenate Comparative chromium 20 10 7 10 100 Example 3 none naphthenate i EXAMPLE 42 Metaxylene was oxidized in acetic acid solvent and in the presence of a cobalt compound. a manganese compound and a bromine compound following the routine of Example 1 to obtain isophthalic acid. The isophthalic acid obtained was esterified with methanol in the absence of catalyst following the routine of Example 1. and methanol and water were separated by distillation from the resultant product mixture as in Example 1 to obtain dimethyl isophthalate. The thus obtained ester contained 0.5 percent (by weight) methyl metaformylbenzoate. The molten color of the obtained dimethyl isophthalate at 150c was 700 APHA.  
  To 300g of the above dimethyl isophthalate was added sodium acetate in an amount of 5 X 10 gramatoms in terms of sodium metal. and air was introduced into the resultant mixture at 180C at a rate of ZOI/h&#39;our whereby the mixture was contacted with air for minutes at 180C. whereafter the resultant reaction mixture was distilled. The content of methyl metaformylbenzoate in the dimethyl isophthalate obtained after the distillation was 50 ppm (by weight), and the initial molten color thereof was 15 APHA and the color statility thereof was 40 APHA.  
  While the invention has been described in detail and with reference to specific embodiments thereof. it will be apparent to one skilled in the art that various changes and modifications can be made therein departing from the spirit and scope thereof.  
 we claim:  
  1. A process for the oxidative purification of dimethyl phthalates wherein the formation of impurities which reduce color stability is avoided characterized in that:  
 a dimethyl phthalate prepared from a phthalic acid which has been obtained by the oxidation of a xylene is treated with a gas containing molecular oxygen in the presence of a compound selected from the group consisting of an alkali metal compound, an alkaline earth metal compound and a mixture thereof in combination with a heavy metal compound. said purification being under liquid phase conditions, and then the dimethyl phthalate is separated from the resultant oxidized product; wherein the amount of the compound selected from the group consisting of the alkali metal compound, alkaline earth metal compound and mixture thereof added is in the range of about 10 gram-atoms to about 5 X 10 gram-atoms, in terms of metal atoms. per 1 g of dimethyl phthalate;  
 the alkali metal and/or alkaline earth metal compounds are at least one member selected from the group consisting of organic acid salts having 1- to 20 carbon atoms, carbonates, bicarbonates. phosphates. borates. hydroxidesQalcoholates having 1 to 5 carbon atoms and phenolates having6 to &#39;8 carbon atoms of alkali metals and alkalineearth metals; and I. i i  
 the heavy metal compound is selected from the group consisting of a compound of chromium. manganese nickel and copper.  
  2. A process as claimed in claim 1 wherein the alkali metal compound is a compound of a metal selected from the group consisting of lithium, sodium and potas-- sium.  
  3. A process as claimed in claim 1 wherein the amount of the heavy metal compound is in the range of about 10 gram-atoms to about 10&#34; gram-atoms as heavy metal per gram of dimethyl phthalate.  
  4. A process as claimed in claim 1 wherein the treatment of the dimethyl phthalate with the gas containing molecular oxygen is carried out while the dimethyl phthalate is molten and at a temperature in the range of about C to about 300C.  
  5. A process as claimed in claim 1 wherein dimethyl phthalate is dimethyl terephthalate.  
  6. A process as claimed in claim 1 wherein the dimethyl phthalate is separated by the distillation under a reduced pressure from the resultant oxidized product.  
  7. A process as claimed in claim 1 wherein said purification is conducted in the presence of an alkali metal compound.  
  8. A process as claimed in claim 1 wherein said purification is conducted in the presence of an alkaline earth metal compound.  
  9. A process as claimed in claim 1 wherein said puriflcation is conducted in the presence of a mixture of an alkali metal compound and an alkaline earth metal compound.  
  10. A process as claimed in claim 1 wherein said alkali metal compound or alkaline earth metal compound is an organic acid salt having 1 to 20 carbon atoms.  
  ll. A process as claimed in claim 1 wherein said alkali metal or alkaline earth metal compound is selected from the group consisting of a carbonate and bicarbonate.  
  12. A process as claimed in claim 1 wherein said alkali metal or alkaline earth metal compound is a borate.  
  13. A process as claimed in claim 1 wherein said alkali metal or alkaline earth metal compound is a hydroxide. 7  
  14. A process as claimed in claim 1 wherein said akali metal or alkaline earth metal compound is a phosphate.  
  15. A process as claimed in claim 1 wherein said alkali metal or alkaline earth metal compound is an alcoholate having 1 to 5 carbon atoms.  
  16. A process as claimed in claim 1 wherein said alkali metal or alkaline earth metal compound is a phenolate having 6 to 8 carbon atoms.  
  17. A process as claimed in claim 1 wherein the major impurity separated is methyl formyl benzoate, and the alkali metal and/or alkaline earth metal compound serve to prevent the formation of impurities from methyl formyl benzoate during the oxidative purification; which oxidative purification is conducted under the following conditions:-  
 the amount of&#39;the compound selected from the group consisting of the alkali metal compound alkaline earth metal compound and mixture thereof added is in the .range of about 10 gram-atoms to about 5 X 10 gram-atoms. in terms of metal atoms. per  
  12 l g-of dimethyl phthalate the treatment of the dimethyl phthalate. with the gas containing molecular oxygen is carried out while the dimethyl phthalate ,is molten and at a temperature in the range of about C to about 300C. the pressure is atmospheric. &#39;to about 30 kg/cm gauge; the amount of oxygen is more than-the stoichimetric amount necessary for the oxidation of impurities and is about 31 to about 1 K1 per 1 kg of dimethyl terephthalate (calculated at normal temperature and pressure); the time of oxidative purification is about 10 to about 240 minutes and wherein the dimethyl phthalate is separated by the distillation under a reduced pressure from the resultant oxidized product. l8.-A process as claimed in claim 17 where the alkali metal and/or alkaline earth metal compound are at least one member selected from the group consisting of organic acid salts having 1 to 20 carbon atoms, carbonates. bicarbonates. phosphates, borates, hydroxides. alcoholates having 1 to 5 carbon atoms and phenolates having 6 to 8 carbon atoms of alkali metals and alkaline earth metals, the amount of alkali metal compound and/or alkaline earth metal compound is 10 gramatoms to l0&#34; gram atoms (same basis) and temperature is to 280C and the time is 20 minutes to 120