United States Patent Office E. 2 by the oxidation of 1,2,4-trimethyl benzene (pseudocu mene), or other 1,2,4-trialkyl benzenes. Pseudocumene, the principal raw material, may be readily separated 2,729,674 by fractional distillation from the C-9 aromatic frac DECARBOXYLATION OF TRIMELLTIC ASCD tion obtained for example from the catalytic reform 5 ing or hydroforming of cracked and/or straight-run Art C. McKinnis, Long Beach, Calif., assignor to Union gasoline fractions. The use of 1,2,4-trialkyl benzenes Gil Corinpany of California, Los Angeles, Calif., a cor provides a much more economical, plentiful, and ver poration of California satile Source of raw materials than the Xylene fractions No Drawing. Application February 8, 1954, provide. Seria No. 408,997 0. The primary object of this invention is therefore to provide more economical sources of raw material for 11 Claims. (C. 260-522) the production of isophthalic acid. A more specific ob ject is to provide economical methods for the mono-de This invention relates to methods for effecting mono 5 carboxylation of trimellitic acid to form selectively decarboxylation of trimelitic acid (benzene 1,2,4-tricar isophthalic acid in preference to the other isomeric acids. boxylic acid) to form corresponding dibasic acids. More A still further object is to provide economical raw ma specifically, the invention concerns methods which selec terials for isophthalic acid which may be readily sepa tively remove the 1-carboxyl group whereby a product rated from other materials with which they occur. Still is obtained which is almost wholly isophthalic acid (bein 20 further objects include the provision of techniques for the Zene 1,3-dicarboxylic acid). The methods employed com process which will reduce corrosivity, heat requirements, prise essentially the heating of trimellitic acid in the pres and reaction time to practical minimum values. Other ence of water and certain critical, less-than-stoichiometric objects and advantages will be apparent to those skilled proportions of alkalies. This procedure is found to yield in the art from the more detailed description which foll a dibasic acid product which is e.g. about 95% isophthalic 25 lows. acid, with only minor proportions of terephthalic acid, Very little is apparently known about the decarboxyla and substantially no ortho-phthalic acid. tion of trimellitic acid. It is known generally that poly The aromatic dicarboxylic acids are highly important carboxylic aromatic acids which contain ortho-carboxyl industrial raw materials by virtue of their use in the groups may be decarboxylated under the influence of heat manufacture of polymeric ester resins, monomeric esters 30 and/or catalysts to remove one of the ortho-carboxyl for plasticizers, and similar materials. Ortho-phthalic groups. Ortho-phthalic acid and hemi-mellitic acid (1,2,3- acid is most used for such purposes, but isophthalic acid benzene tricarboxylic acid) for example may be mono is equal or superior to ortho-phthalic for most applica decarboxylated to produce respectively benzoic acid and tions. Isophthalic acid may be manufactured by the con isophthalic acid. Insofar as I am aware however, no trolled oxidation of pure meta-xylene. This oxidation 35 decarboxylation methods are now known for producing may be performed in two separate stages, the first stage Substantially pure isophthalic acid from higher carboxyl involving low temperature catalytic oxidation to meta ated acids, nor for mono-decarboxylating trimellitic acid toluic acid, and the second stage involving a higher tem in the 1- or 2-position. perature, liquid phase, non-catalytic oxidation of the Considering trimelitic acid as a raw material, it would toluic acid to isophthalic acid. This process is economi 40 appear that the mono-decarboxylation thereof would cally undesirable in that it requires as the starting ma probably yield isophthalic acid, terephthalic acid, or a terial substantially pure meta-xylene, which is very diffi mixture thereof, and possibly some ortho-phthalic acid cult to separate from p-Xylene. if water is not present to prevent the formation of the an Alternatively, a mixture of isomeric xylenes may be hydride between the two ortho-carboxyl groups. After similarly oxidized to produce a mixture of the corre 45 extensive experimentation and the consideration of many sponding dibasic acids, i. e. Ortho-phthalic, isophthalic methods, some of which actually gave predominantly and terephthalic acids. Usually the product is also con terephthalic acid, or mixtures containing terephthalic taminated with benzoic acid and/or toluic acids. Benzoic, acid, the present method was discovered which common toluic and ortho-phthalic acids are easily separated by sol ly results in the production of a product which is at least vent extraction, or by other methods. However the sepa 50 about 95 mole percent isophthalic acid, and not more ration of the remaining isophthalic acid from the tereph about 5 mole percent terephthalic and ortho-phthalic thalic acid is more difficult and expensive. Solvent ex aC1CS traction involves the inherent disadvantage that the acids The actual process employed for achieving the above are only slightly soluble in most solvents. Fractional results is extremely simple. Trimellitic acid, either in crystallization is also impractical because the acids tend 55 pure form or admixed with other oxidation products, is to sublime. The chemical properties of the two acids admixed with water and sufficient of, e. g. a monobasic are almost identical so that selective chemical action is alkali to provide between about 0.1 and 2 moles thereof generally uneconomical. per mole of the portion of trimellitic acid which is in so Isophthalic acid has therefore suffered an economic lution at the reaction temperature. The mixture is then handicap in requiring, incident to its manufacture, either 60 heated under pressure at e. g. 150-400° C., and pref the difficult separation of meta- and para-xylene, or the erably between about 220 and 350° C., until the evolu difficult separation of meta- and para-phthalic acids. This tion of CO2 ceases. This usually requires from about 15 factor has been largely responsible for the widespread minutes to three hours, depending upon the temperature. use of ortho-phthalic acid, which may be cheaply pro Trimelitic acid is considerably more soluble in water duced by the oxidation of naphthalene. According to the 65 than is isophthalic acid, and hence the latter may con present invention isophthalic acid may be produced from tinuously precipitate out as formed if low reaction tem inexpensive, easily isolable, raw materials, and no diffi peratures are employed. The trimellitic acid may be cult separations are involved, thereby rendering isoph wholly or partly in solution at the inception of reaction. thalic acid competitive with orth-ophthalic acid. As the dissolved trimelitic acid is converted to isophthalic The present invention takes an entirely new approach. 70 acid, more trimellitic dissolves until the reaction has gone to the production of isophthalic acid. The trimellitic acid to substantial completion. . employed as starting material may be readily obtained Ordinarily, at the preferred temperature range of 220 2,729,674 3 4. 350° C., the trimellitic acid is miscible in all proportions The crude isophthalic acid which is initially formed with water. The isophthalic acid formed, which melts at is ordinarily sufficiently pure after a simple water wash about 330 C., will either form a solid phase, or may re for most uses. However, it may be further purified if main dissolved in the reaction mixture. The phase rela desired, as by washing with hot water or hot methanol tionships obtaining during reaction depend largely upon and the like. The product may be recrystallized if de temperature and the proportion of water employed. If sired from other solvents. Alternatively, the acids may a concentrated solution of trimellitic acid is employed, the be separated by selective esterification, the isophthalic isophthalic acid may largely remain dissolved if the orig acid being more readily esterifiable. inal solution is dilute it is probable that most of the isoph In one modification of the process, a crude mixture of thalic acid formed is in a second solid phase. it is O carboxylic acids obtained by the oxidation of mixtures found that the decarboxylation proceeds more rapidly in of isomeric C-9 and/or C-10 aromatic hydrocarbons dilute solutions, which may be a result either of more may be decarboxylated to give a final product which is favorable ionic equilibria, or the elimination of isoph 80-90% isophthalic acid. The mixed aromatic hydro thalic acid from the aqueous phase. It is therefore carbons may be obtained by known methods from gaso preferable to maintain in the reaction vessel at least about lines which have been subjected to catalytic reforming or 500 ml. of water per mole of dissolved trimelitic acid. hydroforming operations. The isomer distribution in A critical aspect of the invention resides in the pro the C-9 and C-10 aromatic fractions obtained from such portion of alkali employed. It is found that if there is gasolines is such that the oxidation-decarboxylation prod sufficient alkali to combine with all the trimelitic acid uct, after removal of water-soluble acids, is usually 80 carboxyl groups in solution, i. e. three or more moles of 20 90% isophthalic acid, the remainder being almost ex monobasic alkali or 1.5 moles of dibasic alkali, per mole clusively terephthalic acid. The principal hydrocarbons of trimellitic acid, substantially no decarboxylation will in such fractions, and their corresponding oxidation and occur. On the other hand, if no alkali is employed, a decarboxylation products are shown in the following mixture of isophthalic and terephthalic acids is produced, table: typically comprising about 60% of the former and 40% 25 of the latter. TABLE I The alkali employed is preferably monobasic, and should preferably form water-soluble dihydrogen salts Principal decar with trimellitic acid. This preferred class includes pri Big Acid obtained toxylation prod marily the alkali metal hydroxides, carbonates, bicar 30 Hydrocarbon Pgy, by oxidation uct, and water bonates, phosphates and the like, as well as annonium solubilitythereof. hydroxide. Organic bases may also be employed, e. g. C-9 Aromatics: ethylamine, dimethylamine, trimethylamine, ethylamine, isopropylbenzene.----- 152.4 benzoic------benzoic (S.). ethanolamine, pyridine, etc. However, any alkaline ma propylbenzene.------159.2 ----- do------D9. terial may be employed, including the alkaline earth hy n-ethyltoluene- - 161.3 m-phthalic- m-phthalic (ins.). droxides, magnesium hydroxide and the like. The pre 35 p-ethyltoluene.------162.0 p-phthalic p-phthalic (ins.). mesitylenemethylbenzene). (1,3,5-tri- 64, 7 trimesic--- trimesic (v. S.). ferred alkalis are lithium, sodium, potassium and ann O-ethyltoluene.------165. o-phthalic----- o-phthalic (S.). rthonium hydroxides. pseudocumene (1,2,4- 169.1 trinellitic.----- m-phthalic (ins.). The preferred operative proportions of alkali may be trimethylbenzene). defined in terms of relative normality; i. e. the trirnellitic hemimellitenetrimethylbenzene). (1,2,3- 176. heminellitic m-phthalic (ins.). 40 C-10 Aromatics: acid hydrogen normality of the reactant solution should Im-diethylbenzene.---- 8. m-phthalic. --- m-phthalic (ins.). be maintained at between about 1.5 and 30 times the nor o-diethylbenzene.----- 183. o-phthalic----- o-phthalic (s.). mality of the alkali, and preferably between about 3 and p-diethylbenzene.----, 184 p-phthalic----- p-phthalic (ins.).

2-ethyl-p-xylene------185 trimellitic.----- m-phthalic (ins.). 15 times its normality. If the trimelitic acid is initially 5-ethyl-in-xylene.... -- 186 trimesic------trimesic (v.s.). all in solution, it will be apparent that as the reaction 45 4-ethyl-o-xylene.-----. 86 trimelitic.--- m-phthalic (ins.). proceeds and isophthalic acid is formed, the trimelitic durenemethylbenzene). (1,2,4,5-tetra- 194 pyromellitic.--- Do. acid hydrogen normality of the solution will decrease isodurene (1,2,3,5-tet- 197 prehnitic.----- trimesic (v. S.). while the alkali normality will remain fairly constant. ramethylbenzene). In this case therefore, it will be necessary to add, con tinuously or intermittently, sufficient trinellitic acid to 50 It will be seen therefore that the only acids in the final maintain the operative ratio of acid to base. mixture which are water-insoluble are isophthalic and It should be noted that the acid hydrogen of the 1-car terephthalic. The water-soluble acids usually comprise boxyl group of trimelitic acid is more strongly ionized only a minor proportion of the total decarboxylation than the acid hydrogens on the 2 and 4 positions, or the product, acid hydrogens of isophthalic acid. It is therefore prob 55 The invention may perhaps be more readily understood ably the specific effect of the base on the 1-carboxyl group from the following examples which are, however, illustra which is responsible for the peculiar resuits observed. tive only: Also, as a corollary thereto, since isophthalic is a much weaker acid than trimellitic, it is not critical as to whether Example I the former remains dissolved in the reaction medium or 60 This example illustrates the use of various proportions forms a separate phase; in either case the base remains of alkali, and shows the critical relationship existing in largely associated with the 1-carboxyl group of trimelitic the acid-base ratios. acid. One-half mole (105 gms.) of trimelitic acid is placed A very convenient method for carrying out the process in each of three stainless steel pressure- vessels equipped consists in heating the reaction mixture with agitation, 65 with agitators and pressure relief valves. About 500 ml. continuously withdrawing a slip-stream, cooling if it nec of water is then added to each vessel. To vessel No. 1 essary to precipitate isophthalic acid, separating the isoph is added 60 gms. (1.5 moles) of sodium hydroxide. To thalic acid therefronn, adding to the remaining slip vessel No. 2 is added 4 gms. (0.1 mole) of sodium hy stream liquor an amount of trimellitic acid approximately droxide. No alkali is added to the third vessel. Each equivalent to the isophthalic acid removed, and return vessel is then heated with stirring at 250-290° C. for ing the augmented stream to the reaction vessel. It may 1.5 hours, after which time the evolution of CO2 sub be necessary to add Small amounts of alkali and water stantially ceases. The contents of each vessel is then occasionally to make up for slight losses. In this man cooled to about 80° C. and filtered. The solid precipitate ner the desired ratio of alkali to trimellitic acid may be in each case is washed with 400 ml. of hot water, and continuously maintained in the reaction vessel. 75 then dried in an air stream at 70 C. Analysis of the 2,729,874 The oxidation mixture, which comprises mainly trime products from the three runs gives the following com litic, trimesic, hemimelitic, pyromellitic, prehnitic, o-, parative data: m- and p-phthalic, and benzonic acids is then subjected TABLE II to decarboxylation under the conditions outlined in Ex ample II, employing 0.5 N sodium hydroxide, and main Isophthalic Terephthalie Total Con- 5 taining the reaction mixture about 3.5 N with respect to Product Wt, gms. acid,Percent Wt. acid,Percent Wt. Dibasicversion acids to the intial acid mixture. The product is composed ap proximately as follows: Wessel No. 1.------2.------() 67 ( 'o. ()1. 3 () 80. 7 Component: Weight percent Wessel No. 3.----- 58 60,2 39 700 O Benzoic acid ------1.5 ophthalic acid ------4. 1. No solid product obtained. trimesic acid ------14 Further analysis of the soluble product from vessel No. piphthalic acid ------9 1 showed it to consist of trisodium trimellitate almost mphthalic acid ------69 exclusively. The aqueous mother liquor from vessel No. 5 This mixture is then water-washed to remove the trimesic, 2 is found to contain about 18 gms. of trimelitic acid benzoic and o-phthalic acids, leaving a final product in the form of sodium salts, and the total yield of dibasic which is approximately 89% isophthalic acid and 11% acids from this run, based on trimelitic acid converted, terephthalic acid. This example shows that the bulk of is therefore 97.5%. The aqueous mother liquor from the higher hydrocarbons such as durene, 4-ethyl-o-xylene, vessel No. 3 is found to contain about 30 grams of tri- 20 2-ethyl-p-Xylene, as well as substantially all of the pseudo mellitic acid, and the yield from this run is therefore cumene and hemimellitene are ultimately converted to 98.5%. isophthalic acid. The 1,3,5-hydrocarbons are converted This example shows, by the improved conversion in to trimesic acid which is readily separated by water wash vessel No. 2, as compared to vessel No. 3, that the alkali ing. Nearly all of the terephthalic acid which is produced actually accelerates the decarboxylation until the ratio of 25 comes from the original para-dialkyl benzenes. alkali to trimellitic acid becomes too large. Substantially the same results are obtained in all the Example II above examples by substituting equivalent amounts of potassium, lithium or ammonium hydroxide for the This example illustrates a suitable continuous method sodium hydroxide. Other alkalis such as calcium hy for effecting the decarboxylation. Sufficient trimellitic 30 droxide or dimethylamine give commensurate results. acid is added to 2 liters of water in a stainless steel pres The foregoing disclosure of this invention is not to be sure vessel equipped with agitator, gas relief valve, and considered as limiting since many variations may be a liquid withdrawal port in the lower section, to form a made by those skilled in the art without departing from saturated solution at 60° C. At this temperature the sol the scope or spirit of the following claims: ubility of trimellitic acid is approximately 190 gms. per 35 I claim: liter. Forty grams of sodium hydroxide is then added, 1. A method for decarboxylating trimellitic acid to thereby forming a solution which is about 2.71 N with form isophthalic acid which comprises heating trimellitic respect to trimellitic acid and 0.5 N with respect acid at a temperature between about 150 and 400 to sodium hydroxide. The mixture is heated at 300 C. in the presence of water and a significant proportion 320 C. with good agitation for an initial period of 30 40 of alkali which is less than stoichiometric but sufficient to minutes. While continuing heating and agitation, the increase materially the ratio of isophthalic acid/tereph liquid withdrawal port is then opened sufficiently to with thalic acid formed over that formed in the absence of draw continuously a slip-stream of liquid product at the alkali, and recovering the resulting isophthalic acid. rate of about 15 ml./minute. The slip-stream is cooled 2. A method as defined in claim 1 wherein the said in a water-jacketed tube to 60° C., forced under pressure 45 alkali is a water-soluble monobasic alkali which forms through a filter press, and then percolated through a bed water-soluble dihydrogen salts of trimelitic acid. of solid trimelitic acid at 60° C. in order to form a 3. A method as defined in claim 1 wherein said alkali saturated solution. The resulting solution is then pumped is selected from the class consisting of the hydroxides and back into the reaction vessel at a second inlet port in alkaline salts of the alkali metals and ammonium. the mid-section thereof. Gaseous CO2 is continuously 50 4. A method as defined in claim 1 wherein the trimel exhausted through the gas relief valve. Solid isophthalic litic acid hydrogen normality of the reactant solution is acid is periodically removed from the filter press and maintained at between about 1.5 and 30 times the nor washed with hot water. A small amount of 0.5 N sodium mality of said alkali. hydroxide solution is added to the slip-stream occasion 5. A method as defined in claim 1 wherein the trimel ally to make for small losses in the solid product. 55 litic acid hydrogen normality of the reactant solution is After operating in this manner for a period of 12 maintained at between about 1.5 and 30 times the nor hours, 1920 gms. of solid product is obtained which ana mality of said alkali, and wherein sufficient water is em lyzes 96.4% pure isophthalic acid. The over-all yield of ployed to provide at least about 500 ml. thereof per mole pure isophthalic acid, based on the trimelitic acid con of dissolved trimellitic acid. sumed, is 94.3%. 60 6. A continuous method for decarboxylating trimel Example III litic acid to form isophthalic acid which comprises heat This example illustrates the results obtainable by de ing trimetilitic acid at a temperature between about 150 carboxylating the mixed acids obtained by oxidation of and 400° C. in the presence of water and a significant a mixture of C-9 and C-10 aromatic hydrocarbons. proportion of alkali which is less than stoichiometric but The mixed acids are obtained by oxidizing a C-9, 65 sufficient to increase materially the ratio of isophthalic C-10 fraction (B. P. 150-200 C.) of the aromatic hydro acid/terephthalic acid formed over that formed in the carbons extracted from a full-range naphthenic gasoline absence of alkali, continuously removing a slip-stream of which had been subjected to hydroforming-aromatization the reaction mixture, removing isophthalic acid from said in the presence of a platinum-alumina catalyst at 950 F., slip-stream, replacing said isophthalic acid removed with 1.0 L. H. S. V., 10,000 S. C. F. of hydrogen/bbl. of 70 approximately an equivalent mole-proportion of trimel feed, and at a pressure of 200 p.s. i. g. litic acid, and then returning said slip-stream to the re The C-9, C-10 hydrocarbon fraction was oxidized first action mixture. with air in the presence of a cobalt naphthenate catalyst 7. A method as defined in claim 6 wherein the trimei at 150° C. for 3 hours, and then with 30% nitric acid litic acid hydrogen normality of said reaction mixture at 200 C. for 1 hour in the absence of catalyst. 75 2,729,674. 7 8 is maintained at between about 1.5 and 30 times the 10. A process as defined in claim 9 wherein the said normality of said alkali. initial acid mixture is obtained by the oxidation of a mix 8. A method as defined in claim 6 wherein the trimel ture of isomeric C-9 and C-10 alkyl aromatic hydro litic acid hydrogen normality of said reaction mixture is carbons, said hydrocarbon mixture boiling between about maintained at between about 1.5 and 30 times the normal 150 and 200 C. and having been isolated from a gasol ity of said alkali by cooling said slip-stream to a given line obtained by the catalytic hydroforming of a naph temperature level at which the trimellitic acid solubility thenic gasoline fraction. is just sufficient to provide the desired ratio of alkali to 11. A method for obtaining isophthalic acid from a trimelitic acid, and saturating said slip-stream at said mixture consisting predominantly of trimellitic acid, tri temperature with trimellitic acid. O 9. A method for obtaining isophthalic acid from a mesic acid, and hemimelitic acid, which comprises heat mixture consisting predominantly of trimelitic acid, tri ing said acid mixture at a temperature between about 150 mesic acid, hemimellitic acid, pyromellitic acid and prehn and 400° C. in the presence of water and an aqueous re itic acid which comprises heating said acid mixture at action mixture containing an alkali wherein the trimellitic a temperature between about 150 and 400° C. in the acid hydrogen normality of said reaction mixture is main presence of water and an aqueous reaction mixture con 5 tained at between about 1.5 and 30 times the normality of taining an alkali wherein the trimelitic acid hydrogen said alkali, removing water-soluble acids from the decar normality of said reaction mixture is maintained at be boxylated mixture, and recovering therefrom substan tween about 1.5 and 30 times the normality of said alkali, tially water-insoluble acid mixture which consists mainly removing water-soluble acids from the decarboxylated of a major proportion of isophthalic acid and a minor mixture, and recovering therefrom substantially water 20 proportion of terephthalic acid. insoluble acid mixture which consists mainly of a major No references cited. proportion of isophthalic acid and a minor proportion of terephthalic acid.