3,071,614 United States Patent O?fice Patented Jan. 1, 1963

1 2 Examples of aromatic hydrocarbon carboxylic acids, 3,071,614 , PURIFICATION OF AROMATIC ACIDS and of typical aliphatic substituted aromatic feedstocks James 0. Knobloch, Hobart, lind., assignoi- to Standard which may be oxidized with molecular oxygen in the Oil Company, Qhicago, HL, a corporation of Indiana presence of a metal oxidation catalyst (preferably in the No Drawing. Filed Oct. 25, 1957, Ser. No. 692,272 conjoint presence of an oxidation catalyst and bromine) 7 Claims. (Cl. 260-525) to produce them, are: TABLE I This invention relates to aromatic carboxylic acids. More particularly it relates to the puri?cation of aromatic Group Acid Feedstocks hydrocarbon carboxylic acids (“aromatic acids") derived 10 from the metal-catalyzed liquid phase oxidation of ali Benzene Carboxylic ______Benzoic ______Toluene, ethylben phatic-substituted aromatic compounds with molecular zene, stryene. o-Phthalic- _. _____ o~Xylene, indene, oxygen. Such aromatic acids are often contaminated o-toluic acid. with traces of a tar-like yellowish or tan-colored oxida Isophthalic ______m-Cyrnene, m xylene, rn-diacetyl tion byproduct which creates a troublesome puri?cation 15 benzene. Terephthalic ______p-Xylene, p-tolual problem in the recovery of pure aromatic acids. These dehyde, hydroxy byproducts moreover inhibit crystallization when it is at cumic acid. Pseudocumene. tempted to recrystallize aromatic acids from aqueous Mesitylene. solutions to such an extent that a byproduct-contaminated Prehnitene. solution containing ?ve times the saturation concentra 20 Hexamethylbenzene. Durene. tion of aromatic acids remains stable for Weeks despite Alkyl-benzene Carboxylic-.- m-Xylene. Hemiinellitene. shock-chilling and seeding. phthalic. An object of the present invention is to provide an im t-Butyl-o-phthalic- t-Butyl-orthoxylene. p-Cumic ______p-Cymene. proved process for purifying aromatic hydrocarbon car m-Ethyl henzoic_. rn-- Iethylethyl boxylic acids derived from the metal-catalyzed liquid 25 benzene. Durylie___ Durene. phase molecular-oxygen oxidation of aliphatic-substituted Polynuclear Aromatic _____ ._ Diphenic ______Phenanthrene. aromatic compounds. A further object is to purify aque Naphthalene 2, 2,6-Dimethyl naph dicarboxylic. thalene. ous solutions of aromatic acids. Yet another object is to Naphthoic ______Methyl naph remove tar-like colored oxidation byproducts which in thalenes. hibit crystallization of aromatic acids from aqueous solu 30 Naphthalic _____ -_ Acenaphthene. tions. An additional object is to provide an aromatic acid puri?cation process in which the treating agent may be Liquid phase molecular-‘oxygen oxidation processes are regenerated. Other objects will become more apparent conducted according to known procedures at a tempera ‘as the description of this invention proceeds. ture in the range of 60 to 275° C., and preferably 170 ‘ In accordance with the objects above, it has now been 35 225° C. The pressure may be from atmospheric to about discovered that aqueous solutions of aromatic hydrocar 100 atmosphere or more, and is desirably about 28 atmos bon carboxylic acids which are contaminated by colored pheres. Air, air containing a diluting proportion of an byproducts of the metal-catalyzed liquid phase molecular inert gas, commercially pure oxygen and ozone are com oxygen oxidation of aliphatic-substituted aromatic com mon sources of moleculartoxygen. Oxidation catalysts 4-0 pounds may be puri?ed by treating the solution with are soluble forms of one or more metals, preferably salts adsorbent alumina. A major portion of the byproducts of the known heavy metal oxidation catalysts such as are thus adsorbed on the alumina, and the aromatic acids cerium, cobalt, manganese, lead, chromium, nickel, may be recovered in a pure state from the treated solu molybdenum, and tungsten. The metal catalyst may be tion by such convenient means as crystallization and/or ‘added to the reaction in elemental form, or as an ionic 45 evaporation. Alumina treating may if desired be con compound such as cobalt acetate or ammonium molyb— ducted continuously, and may also be integrated with date, or in combined form such as tetraethyllead or cobalt other aromatic acid separation and puri?cation steps. ‘versene. Similarly bromine may be elemental, in an ionic Liquid phase oxidations employing molecular oxygen compound such as HBr or ammonium bromide, or as and a metal oxidation catalyst have recently become ex 50 tetrabromoethane or benzyl bromide. The oxidation re tremely important in the commercial preparation of aro~ action is advantageously conducted in the presence of an matic acids. In these processes an aliphatic hydrocarbon ‘ inent solvent for the feedstock and catalyst; the solvent substituent on an aromatic ring is oxidized to a nuclear— preferably being a saturated monocarboxylic acid having ly-attached carboxylic acid ‘group. The aliphatic sub ‘from 2 to about 8 carbon atoms in the molecule such as stituent may be methyl, normal, secondary, tertiary or 55 acetic acid, but may be such diverse liquids as Water, alicyclic and may be either saturated or unsaturated. An benzophenone, benzonitrile, octyl alcohol, mineral oil, or aromatic having more than one aliphatic substituent may chlorinated hydrocarbons. require the conjoint presence of a metal oxidation catalyst Aromatic acids are separated in crude form from a and bromine as the catalyst to effect the production of reaction mixture by any one or more of a variety of high yields of aromatic polycarboxylic acids. Since poly 60 physical or chemical techniques. Insoluble aromatic acids carboxylic acids are more di?icult to produce than mono such as isophthalic or terephthalic may be ?ltered, centri carboxylic ‘acids, by control over the reaction conditions fuged, or decanted from the mixture at elevated tempera~ it is possible to favor the oxidation of less than all of tures, and even the more soluble acids such as benzoic the aliphatic substituents in order to produce alkyl-sub and phthalic may be removed by these techniques from stituted aromatic mono- or polycarboxylic acids. Thus a cooled reaction mixture. Aromatic acids may also be the characterizations “aromatic hydrocarbon carboxylic obtained in very impure form by merely evaporating Water acids” or “aromatic acids” as employed in the speci?ca and the solvent, or by extraction with selective solvents. tion and in the claims relate to mononuclear ‘and poly Since it is often more convenient to resolve mixtures of nuclear aromatic compounds which have at least one isomeric aromatic acids than to separate isomers of the nuclearly-attached carboxylic acid group and which may feedstock, quite commonly an isomeric mixture of ali in addition have one or more hydrocarbon substituent phatic-substituted aromatic compounds is oxidized at one such as an alkyl or alkenyl group. time, and a combination of one or more physical and/ or 3,071,614 3 4 chemical separation techniques employed to resolve the aqueous aromatic acid solutions are preferably either of mixed aromatic acids in the reaction mixture. two types, the so-called “activated alumina” and “ac— In all of the foregoing separations—whether the aro tivated bauxite.” Both are forms of aluminum oxide matic acid is recovered as a ?ltered solid, a distillation or which have ‘been heated to remove most of the bound evaporation bottoms, or as an extract—the aromatic acids water and to provide an adsorbent having a surface area are contaminated by deep yellow or tan-colored oxidation between about 50 and 500 square meters per gram. “Ac byproducts. When concentrated, these byproducts have a tivated alumina” is obtained by heating the product of tar~like consistency. They boil within a very wide range the Bayer process for the preparation of alumina. The of temperature and hence‘ cannot be removed by distil Bayer process involves extracting bauxite (aluminum ore) lation. Extensive investigation has shown that tar-like with hot caustic, cooling and diluting the extract with cold byproducts from the oxidation of a single alkylbenzene water to hydrolyze and precipitate the alumina, and cal may have phenolic, acidic, ester, carbonyl, and ole?nic cining the precipitated alumina. “Activated bauxite” is groups, and as a consequence are soluble to some extent merely heat-activated natural bauxite. Both products are in most common solvents for aromatic acids. stable crystalline materials which are supplied commer Attempts to remove these byproducts from aromatic cially in hard grains, lumps, balls, and tablets of various acids by prior-art methods have failed to suggest a proc— mesh sizes. Other heat or chemically activated alumina .ess suitable for commercial adaptation. For example, containing materials which are insoluble in water may be aromatic acids produced by the Willgerodt oxidation of employed with somewhat lesser e?ectiveness. an alkylbenzene with ammonium sul?de, ammonium sul In the practice of the present invention, the initial step fate, and water are decolorized by passing the reaction 20 is obtaining an equeous solution of the aromatic acid mixture thru activated charcoal, but When it was at or the mixture of aromatic acids. This may be done tempted to purify byproduct-contaminated ammonium either by dissolving the acid in water, extracting a soluble phthalate solutions with charcoal or with alumina, it was acid from a less-soluble one with Water, extracting a found that the phthalic acid could not be recovered from distillation bottoms with water, or inherently by conduct the treated solution in an acceptable yield. Adsorbent ing the oxidation in Water or a water-containing inert alumina will not purify aqueous solutions of alkali metal solvent. The aqueous solvent used for dissolving the salts or aromatic acids, and of course only the very vola aromatic acid may be Water alone or may be water with tile aromatic acids may be puri?ed by distillation. Treat minor amounts of solubilizing agents such as the lower ing aqueous solutions of alkali metal salts of liquid-phase alcohols, e.g. methanol, or the lower saturated aliphatic catalytic aromatic acids with charcoal is extremely ef acids as acetic, but these are not essential. fective, but requires the use of stoichiometric amounts The necessary quantity and temperature of the dissolv of alkalies to dissolve the aromatic acids and then equal ing water depends upon the solubility of the aromatic acid quantities of mineral acids to “spring” the treated aro and also on whether a single aromatic acid or a mixture matic acids. of acids is to be dissolved. As will be shown hereinafter, In contrast to the methods described above, the process aromatic acids are more soluble in water containing other of this invention, i.e., purifying aqueous aromatic acid aromatic acids than in pure water. Since most aromatic solutions by treating with adsorbent alumina, is simple, acids are comparatively insoluble it is desirable to conduct ?exible, highly effective, and very economical. It is also the dissolving and alumina treating at an elevated tem the only process which permits regeneration of the ad perature, ie from about 20 to 300° C. or higher, and sorbent. No extraneous reagents are necessary, except 40 preferably from 50 to 100° C. At temperature substan for a small amount of an alkali used for adsorbent re tially above 100° C. pressure containing equipment is re generation, which affords appreciable cost savings and quired. Table II below presents the solubility data at reduces sources of possible contamination. Moreover, various temperatures for some of the more common aro the process may be integrated with aromatic acid separa matic acids, together with the vapor pressure (in pounds tion processes such as water extraction of orthophthalic 45 per square inch absolute) of pure water at the respective acid from orthophthalic-isophthalic-terephthalic mixtures, temperatures shown. In the table benzoic acid is desig or the extraction of an isophthalic acid concentrate from nated BA, ortho-phthalic acid PA, phthalic anhydride isophthalic-terephthalic mixtures with hot water wherein PAN, isophthalic acid IA, terephthalic acid TA, and the extract solutions may be puri?ed by alumina treat trimellitic acid TMLA. TABLE II

Solubility in grams per 100 grams Water

Pressure, IA + TA Temp, ‘’ C. p.s.i.a. BA PA PAN IA TA _ TMLA IA TA

0. 011 0. 0014 1. 75 0.037 0.0046 ______0.32 . 3.3 51 >100

ment. Further, equipment corrosion is negligible in con After dissolving the aromatic acid (together with con trast to acid-springing techniques. Process control is sim taminating oxidation byproducts and any oxidation cat pli?ed by the use of White adsorbent alumina crystals alyst) in water, the resulting solution is advantageously since adsorbent eifectiveness ‘and capacity may be moni ?ltered to remove insoluble polymers and any undissolved tored visually by observing the color of the alumina. 70 aromatic acid. This operation may be conducted in a And ?nally the product may be recovered merely by conventional pressure ?lter as for example a Shriver plate cooling the puri?ed solution to crystallize the aromatic and-frame ?lter press employing canvas ?lter cloths. carboxylic acids, or by evaporating the water, or by a The adsorbent alumina may be disposed either in one combination employing both. or more ?xed “percolation process” beds or it may be Aluminas suitable for treating byproduct-contaminated 75 slurried with the solution either ‘batchwise, continuously, 3,071,614 5 or intermittently in a “contact process.” In a percolation none of the sensible heat content of the solution can be operation the adsorbent preferably has a mesh size recovered by heat exchange, as may be accomplished between about 5 and 90 US. standard screen size, and when aromatic acids are recovered by cooling or evap the solution is passed through the bed either up?ow or orating the solution. downflow. Since adsorbent alumina is normally a glassy As a. preferred embodiment of the aromatic acid re white solid, if the adsorption vessel is provided with covery step, the puri?ed solution is cooled by indirect portholes or other viewing means it is possible to visually heat exchange with the water used for initially dissolv monitor the saturation of the alumina by following the ing the crude aromatic acid. The temperature of the progression of color through the bed, and discontinuing cooled puri?ed solution may be any temperature at the flow of solution when the bed becomes completely which the aromatic acid has a solubility less than its colored. 'Flow rates through the bed are regulated to saturation concentration in the aqueous solvent. Cooling give between about 5 minutes and 1 hour or more con may be quite rapid in which event the aromatic acid tact time, preferably a time in excess of 10 minutes. The crystallizes in the form of tiny crystals, or may be gradual, necessary quantity of alumina depends on the amount of e.g. over several hours, to “grow" the aromatic acid byproducts present and the degree of byproducts re 15 crystals. The cooling rate and/or temperature may be moval desired; it may range from 1A0, to 10 parts by regulated if it is desired to separate more than one dis weight per part of dissolved acid, ‘but is preferably from solved‘ aromatic acid by selective crystallization. Since 1/2 to 2 parts. The percolation temperature may range crystallization from a treated solution may commence from the temperature used for dissolving the aromatic almost immediately upon reaching the saturation temper acid to as low as the saturation temperature; although, as 20 ature, it is desirable to effect cooling and crystallization in will be shown by the examples hereinafter presented, per a scraped-surface jacketed tank or scraped-wall tube colation can be employed with supersaturated solutions, type heat exchanger to prevent crystal accumulation on this procedure is undesirable in a percolation process as the vessel walls. Filtration, centrifugation, decantation, crystallization of aromatic acids from supersaturated solu or hydrocyclones may be used to separate the aromatic tions inevitably occurs in the bed. Percolation adsorption 25 acid crystals from the mother liquor. The mother liquor is very advantageously used when operating at pressures may be evaporated entirely or in part to recover addi substantially in excess of atmospheric. tional aromatic acids or, and preferably, is recycled to As an alternate to percolation, the contact process may the aromatic acid dissolving step. be employed wherein ?nes (approximately l00-200 The recovered aromatic acid crystals are pure white mesh) or larger particles of the adsorbent are held in 30 or only slightly tinged with yellow and may be air dried suspension in a fluid stream from which they are sep or dried under vacuum to obtain pure aromatic acids of arated by ?ltration after a sufficient time of contact. The commerce. contact process may be conducted either by slurrying The adsorbent alumina employed for purifying the ?nes or larger adsorbent particles with the solution in an aqueous aromatic acid solution may be regenerated and open tank or in a pressure vessel or by injecting ?nes into 35 reused by washing with a solvent for the colored oxida a pipeline carrying the solution, which pipeline is of suf tion byproducts. Preferably the solvent comprises an ficient length to provide a suitably long contact time. aqueous solution of an alkali metal hydroxide or other base Again the contact time is preferably within 5 minutes compound such as a carbonate. A caustic solution con to 1 hour, optimally more than 10 minutes, and quan taining from about 1 to 10% sodium potassium or lith tities of alumina similar to those in percolation are ium hydroxide is very effective for this purpose. After required. Separation of the adsorbent particles may be regeneration, the adsorbent bed is washed with de accomplished by ?ltration, centrifugation, or settling, or mineralized water to obviate the possibility of contaminat by the use of hydrocyclones. An interesting phenomenon ing the aromatic acids with the hydroxide. The regen observed in the contact process is that it may be em erated bed is washed with water to a neutral pH prior to ployed with cold supersaturated solutions yet, because returning the bed on stream. Organic solvents such as colored by-products removal is not quite complete,‘ 45 methanol, pyridine, chloroform, benzene, or hexane are crystallization does not commerce instantaneously. not effective for desorbing colored byproducts. The treated solution may contain dissolved metal Various embodiments of the present invention are oxidation catalyst, traces of dissolved alumina, and/or further illustrated by the examples below. bromides. These may be removed by passing the solu tion through a strong acid-acting cation exchange resin 50 Example I to remove the metal catalyst and alumina and by passing the solution through a weakly basic anion exchange resin Adsorbent alumina was employed in a contact process to remove the bromide. With ion exchange resins (par to purify a byproduct-contaminated supersaturated ortho ticularly of the anion type) temperatures substantially phthalic acid solution. The solution was obtained by heat in excess of about 150° C. are undesirable because of ing 70 g. of o-phthalic acid that was contaminated by some solubility of the resin in hot water. colored oxidation byproducts of the bromine~promoted The alumina treated and preferably deionized solu metal-catalyzed air oxidation of orthoxy-lene in an acetic tion may then be treated for recovery of the aromatic acid medium in 1 liter of water to boiling, cooling to 25 ° acid. While a variety of chemical and physical separa C. and ?ltering off solids. tion means are available, two physical methods are out 60 By titration with a standard base, the solution, having standing with respect to economy and ef?ciency of opera a deep yellow color, was found to contain 3.69 grams of tion. In the ?rst, the solution is cooled in order to ortho-phthalic acid per 100 ml. of solution at 25° C. crystallize the aromatic acid therefrom, while in the Since saturation at this temperature is a concentration of second, a part or all of the water is evaporated to effect only 0.74 gram per 100 ml., the solution had 400% more crystallization. Both methods may be employed simul aromatic acid than at saturation, yet was quite stable. taneously or concurrently; for example the solution may Evaporating an aliquot portion‘ con?rmed the ortho be cooled to crystallize aromatic acids which are then phthalic acid concentration. The solids obtained on ?ltered off, and the mother liquor evaporated to recover evaporation had an acid number of 667; the theoretical additional aromatic acids. By concurrent use of crystal acid number of phthalic acid is 675. lization and evaporation ,the solution is ?ashed into a 70 A 100 ml. sample of the original solution was added to a lower pressure region where part or all of the water is beaker containing 1.4 grams of adsorbent alumina manu evaporated, leaving crystals or a crystal-containing con factured by the Fischer Scienti?c Company and designated centrated slurry of cold water and aromatic acids. Flash grade A—54~l/ 2, 80-200 mesh. The suspension was stirred ing has the advantage of reducing equipment costs but is for 20 minutes at room temperature and then ?ltered somewhat expensive in terms of heat requirements as 75 through a tared M~porosity fritted glass crucible. The re 3,071,814 covered alumina weighed 1.497 grams after air drying and of glass wool. The alumina ?lled the column to a depth was light yellow in color. of 15 inches above the supporting glass wool. The col - The ?ltrate was almost colorless. Standing overnight umn was heated to 94° C. by circulating oil from a con at room temperature caused phthalic acid to precipitate. stant temperature bath through the jacket. A total of The solid material was ?ltered off and found to weigh 400 cc. of a deep yellow aqueous solution of ortho-phthalic 2.566 grams after air drying. This represents a 69% acid containing 6.10 grams of acid per 100 ml. (24.40 recovery of phthalic acid. The solids had an acid num grams ortho-phthalic acid in all) was eluted through the ber of 673 (theoretical is ‘675) and contained only a trace column in 5i0—55 minutes at 94° C. with 2 p.s.i.g. nitrogen of ‘the original yellow color. The ?ltrate was evaporated pressure. At this temperature, the solution was not to dryness and 0.965 gram (26% of the original ortho 10 saturated with respect to phthalic acid. phthalic acid) of very white ortho-phthalic acid was re After the yellow solution had been eluted through the covered. ' column, the color band extended about 11/: inches down Thus a 96% recovery of white almost pure ortho from the top of the column, suggesting an ultimate ef phthalic acid was obtained. :Eective adsorbent life of 160 cc. of solution per gram of 15 alumina. The solution left the column having only a Example [I trace of a very light green color. On cooling to 20° 0., An alumina percolation operation at room temperature 10.143 grams (41.5%) of ortho-phthalic acid having a was employed to purify the supersaturated solution of Ex faint trace of yellow color was deposited. The cooled ample I. solution was ?ltered to separate the solid ortho-phthalic The bed was prepared by slurrying 25 grams of Fischer 20 acid, and the water-White ?ltrate evaporated to yield A-541/2 alumina with 98 cc. of water saturated with an additional 9.69 grams (37%) quantity of ortho reagent grade ortho-phthalic acid at 25° C., and the sus phthalic acid that showed no trace of any color. pension poured into a glass column to form a bed 13% The adsorbent bed was washed with three 100 ml. por inches high by % inch I.D. which was full of liquid. To tions of water at 94° C. The wash water had only a this column was added 100 ml. of the deep-yellow-colored 25 trace of a very light green color. The color band on byproduct - contaminated supersaturated ortho - phthalic the alumina appeared unaffected by the hot wash water. solution of Example I. With nitrogen pressure (4 p.s.i.g.) The wash water eluted 3.05 grams (12.5%) of ortho this yellow solution was eluted down through the column phthalic ‘acid. . in 10 minutes, leaving a color band at the very top of Thus a total of 22.88 grams of almost colorless ortho the column. An additional 100 ml. of the same contami 30 phthalic acid was recovered. This represents 914% of the nated solution was added to the column. After 17 hours vortho-phthalic acid charged. ' under the same nitrogen pressure only 88 grams of solu Example V tion had been eluted ‘from the second 10 ml. charge. This indicated that the supersaturated solution was depositing A byproduobcontaminated aqueous trimellitic acid solu solid ortho-phthalic acid within the column which was 35 tion was alumina treated in a percolation operation at plugging the adsorbent bed. 26° C. The solution was unsaturated with respect to The eluted solutions were water White and were evap trimellitic acid at the treating temperature. orated to recover 3.22 grams of perfectly White ortho The trimellitic acid was prepared by oxidizing pseudo phthalic acid crystals. A total of 7.82 grams of ortho cumene with air in an acetic acid solvent and in the pres phthalic acid had been added to the column. 40 ence of a cobalt-manganese~bromine catalyst. The tri mellitic acid was ?ltered from the oxidation reaction mix Example III ture and given an initial puri?cation by recrystallization A deep yellow supersaturated aqueous ortho-phthalic from water. Sixteen grams of the recrystallized acid acid solution was alumina treated in a contact operation (acid number 798; theoretical acid number is 801) having at 70° C. 45 a APHA color in excess of 500 (solution color in di The original solution was prepared as in Example I methylformamide) was dissolved in one liter of water and found by titration to contain 3.64 grams of ortho at 26° C. phthalic acid per 100 ml. of solution, representing super Fischer alumina (A-541/2, 80-200 mesh, 25.0 grams) saturation at room temperature to the extent of about was prepared by slurrying 25.0 grams of the alumina with 2.94 grams per 1100 ml. solution. It was very deep yellow 50 5 successive portions of 100 ml. of water, allowing the in color. coarser material to settle, and decanting the suspension A 100 ml. sample of this solution was slurried with of ?nes. The coarse alumina particles were then slurried 1.3660 grams of Fischer A-54-1/2 adsorbent alumina. with 250 ml. of water and poured into a 25 inch high by The suspension was heated in a beaker to 70° C. and held % inch diameter glass tube which had a glass wool plug between 70 and 80° C. for 10 minutes with stirring. The 55 sealing the bottom thereof. alumina was ?ltered o?‘ at 70° C. and was tan in color. All of the byproduct-contaminated trimellitic acid solu On cooling the ?ltrate to 40° C., crystallization com tion was eluted downward from the bed over a 11/: hour menced. The solution was permitted ‘to remain at room period. No attempt was made to wash the bed free of temperature for about 15 hours and produced coarse occluded solution. crystalline ortho-phthalic acid having a lemon~yellow 60 After elutriation, a color band extended about 4 inches color. 2.213 grams (61%) of ortho-phthalic acid was re down from the top of the column and was most intense in covered by ?ltration, and an additional 1.24 grams (34%) the ?rst 1/2 inch, thereafter tapering in intensity. by evaporation of the water-white ?ltrate. The eluted solution was ?ltered from a small amount Example I V of suspended alumina and was evaporated under vacuum 65 at 45° C. A deep yellow solution of ortho-phthalic acid (derived The trimellitic acid recovered by vacuum evaporation [from the air oxidation of orthoxylene in acetic acid in the weighed 12.9 grams and represented 80.7% of the tri presence of a cobalt bromide catalyst) was treated by mellitic acid charged. It had an APHA color of approxi~ percolation through an adsorbent alumina bed at 94° C. mately 15. The adsorbent bed was prepared by slurrying a mixture From the discussion and examples above, it is seen that of 25.0 grams of Fischer A-54l/2 adsorbent alumina in adsorbent alumina is extremely effective for removing 92 cc. of room-temperature-saturated aqueous ortho colored tar-like byproducts from aqueous aromatic acid phthalic acid (reagent grade) solution into a jacketed solutions. By either percolating or slurrying the solution glass tube. The jacketed column was 24 inches long by with alumina it is possible to adsorb practically all of the % inch ID. and was plugged at the bottom with a wad 75 tar-like materials from solution and permit recovery of 8,071,614 10 high purity aromatic acids. The process of the invention tion and crystallizing aromatic hydrocarbon carboxylic furthermore allows the use of relatively low cost ad acid therefrom. sorbents and also permits of their regeneration merely by 6. Process of claim 1 wherein the temperature of said Washing with a basic solution. contacting step is from about 20 to 300° C. Having described the invention, I claim: 5 7. Process of claim 6 wherein the temperature of said 1. In a process for purifying an aromatic hydrocarbon contacting step is from 50—-100° C. carboxylic acid which is contaminated with traces of tar like byproducts of the heavy-metal-catalyzed liquid-phase References Cited in the ?le of this patent molecular-oxygen oxidation of an aliphatic substituted UNITED STATES PATENTS aromatic compound, the improvement of contacting an 10 aqueous solution containing aromatic hydrocarbon car 2,154,626 Koch ______Apr. 18, 1939 boXylic acid and said byproducts with adsorbent alumina, 2,726,262 Grosskinsky et a1 ______Dec. 6, 1955 whereby said byproducts are selectively adsorbed onto the 2,744,938 Urban ______.__ May 8, 1956 alumina. 2,862,958 Goreau ______Dec. 2, 1958 2. Process of claim 1 wherein said aromatic hydrocar 15 OTHER REFERENCES bon carboxylic acid is a benzene polycarboxylic acid. 3. Process of claim 2 wherein said benzene polycar Weissberger, Technique of Organic Chemistry, vol. V, boxylic acid is ortho-phthalic acid. Adsorption and Chromatograph, pages 189-90 and 206 4. Process of claim 2 wherein said benzene polycar (1951). (Copy in Library.) boxylic acid is trimellitic acid. National Bureau of Standards, Bibliography of Solid 5. Process of claim 1 wherein aromatic hydrocarbon 20 Adsorbents, 1943 to 1953, article Nos. 3232, 919, 4498, carboxylic acid is recovered from said aqueous solution 4611, 4667, 8977, 9038, 9081, 7920, 12747, 4835, 2689 subsequent to said contacting step by cooling said solu and 3785. (Circular 566.) (Copy in Division 31.)