US2563820A - Preparation of aryl dicarboxylic acids - Google Patents

Description

J. L. DARRAGH ETAL PREPARATION OF ARYI.. DICARBOXYLIC ACIDS Aug. 14, 1951 2 Sheets-Sheet l Filed.. Oct. 4 1947 A TTORNEYS Aug. 14, 1951 J. l.. DARRAGH ETAL 2,563,820

PREPARATION OF ARYL DICARBOXYLIC ACIDS Filed oct. 4, 1947 z sheets-sheet 2 h air, s a H n r m w H m m WM. N D J mmntumzw zocbm w. L. ,n oom ooh oom o?I m n 6 3 .n w m w H L ...u $32515 w oom oom. oov oom W o nm mm 1 d 7. 3 H .A n mw a .l N G l. 9 E E. a A N Z M o .3 l I D 9 mm V H l w G am TTORNEYS Patented Aug. 14, 1951 PREPARATION 0F ARYL DICARBOXYLIC ACIDS John L. umani and mmm J. Miner, Berkeley,

Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application October 4, 1947, Serial No. 777,970

9 Claims. l

This invention relates to the preparation o! aryl carboxylic acids in which a carboxyl group preferably is directly attached to the aromatic nucleus and involves the production of such aryl carboxylic acids from alkyl aromatic hydrocarbons and preferably from polymethyl-substituted aromatic hydrocarbons. More particularly the invention relates to a process for producing mono nuclear polycarboxylic acids, and in its preferred embodiment comprises a process for preparing isomeric phthalic acids, such as the terephthalic isomer.

Known methods for producing isomeric phthalic acids, such as terephthalic, have utilized indirect chemical syntheses which result in costs so high as to be prohibitive for large scale production. Certain more direct methods such as oxidation of para xylene with potassium permanganate or chromic acid require consumption of relatively expensive chemical reagents `and likewise involve excessive costs.

An object of this invention is to provide a new and improved process for the production of aryl carboxylic acids and one which is of special advantage for preparing isomeric phthalic acids. Another object is to furnish an improved method for the synthesis of terephthalic acid.

Additionally, an object is to synthesize mono nuclear aryl polycarboxylic acids with low cost chemical reagents.

Another object is to produce terephthalic acid by a process which involves oxidation with caustic Ialkali. Further, an object is to provide a process capable of enhancing yields of carboxylic acids in the caustic oxidation of aryl methyl chlorides.

Other objects and advantages of the invention will be apparent from the drawing and the following detailed disclosure.

In accordance with this invention aryl carboxylic acids, particularly mono nuclear aryl polycarboxylic acids, are obtained by formation of polyalkyl 4aromatic chlorides in which a chlorine atom is attached to a primary alkyl carbon atom, and conversion of such chlorides to carboxylic acids by reactions involving oxidation with aqueous caustic alkali. Preferably, the chlorinated primary alkyl carbon atom is attached directly to a benzene nucleus, as in chlorides of para xylene:

omc] calci cHcl. ouch Hi glzcl gli: H101 -X -l l PX l Iene p-Xylal p-Chlcromethyl ghloxie dichyloide chloride benul chloride However, the process is applicable to compounds such as CHzCHaCl CHiCHzCl CHxCHsCl ;H3 Hz Cl Hr-CHCI CHiCl CHzOl CHzCl and their homologs. Corresponding derivatives of naphthalene and anthracene also may be utilized.

Briefly described a process embodying the present invention utilizes a combination of process steps involving: I

(l) Formation of an aryl substituted primary alkyl chloride such as xylyl chloride xylyl chloride (CHaCeHiCHzClz), xylylene dichloride (CICHzCaHiCHzCl) or mixtures thereof, either by chlorination of xylene or by chloromethylation of toluene or both;

(2) Separation of at least a xylylene dichloride fraction from the chlorinated reaction mixture;

(3) Conversion of the separated xylene chlorides to corresponding carboxylic acids by hydrolysis and oxidation with aqueous caustic alkali.

(i)4 chlorination of at least a part of the resulting toluic acids to corresponding chloromethy1 benzoic acids, and

(5) Conversion of chloromethyl benzoic acids to the corresponding dicarboxylic acids by hydrolysis and oxidation with caustic alkali.

It should be noted that the chemicals consumed in the foregoing synthesis are chlorine and aqueous caustic alkali, each of which is a low cost bulk chemical. In chloromethylation formaldehyde also is consumed, and this likewise is a low cost bulk product. No expensive oxidizing agents are required. Also, it is important that chlorination to a single chlorinated product is not necessary to substantially complete conversion to phthalic acids.

Since the process of this invention finds its principal present application in the manufacture of terephthalic and/or isophthalic acids. the invention and process will be illustrated hereinafter by reference to'the production of these isomeric phthalic acids from the xylenes and/or toluene. Special emphasis is herein given to the production of terephthalic acid from paraxylene.

In the drawing, Fig. 1 is a flow sheet illustrating in block diagram a process for converting para xylene to terephthalic acid or for obtaining a mixture vof phthalic acids from toluene or for producing mixed phthalic with or without benzoic acid from xylene and toluene.

Fig. 2 is a graphical showing variation with temperature (in F.) of yield of terephthalic acid and terephthalic content of acids from caustic oxidation of chloromethyl benzoic acid.

Fig. 3 is a similar graph showing effect of temperature (in F.) on yield of benzoic acid from benzyl chloride.

Reference to the drawing will reveal that a xylene fraction such as para xylene is introduced by way of feed line I to a chlorination zone Il where chlorine is introduced into the methyl side chains. The chlorination reaction is conducted under conditions which favor side chain chlorination and may result in the following reactions:

CH; CHzCl heat i Ch HC1+ (p-xylyl chloride) light Hx Ha CHgCl CHsCl heat Cl; HC1+ (p-xylylene dichloride) light Hs BaC1 CHzCl CHOI:

heat C11 HC1+ (p-xylal chloride) light Ha Ha Triand polychlorinated compounds also may be formed. If minor amounts of ring chlorination occurs, the subsequent process steps still yield the desired compounds. The mixture of chlorinated products then passes as indicated by line I2 to a separation zone i3 and, as here shown, three fractions of chlorinated xylenes are obtained. Any suitable method of separation. such as distillation under relatively high vacuum (to minimize decomposition of the chlorides), may be used. When the hydrocarbon feed is para xylene, a para-xylyl chloride fraction, a para-xylal chloride fraction and a para-xylylene dichloride fraction are conducted as shown by lines i4, I6 and II respectively to further processing units. If benzyl chloride is formed from any toluene present in the xylene feed, this chloride is removed by way of line 29. Lower boiling fractions, such as unchlorinated hydrocarbons, may be removed through outlet 30 and higher boiling compounds are withdrawn as indiacted byline 35.

The para xylyl chloride fraction from line' Il is converted to para toluic acid by reactions involving hydrolysis and oxidation of the hydrolyzed product to toluic acid.

lo CHxCl CH|0H Nuon Naci Hg HI onion cooN Nuon 2N,

Hg Hl These reactions may be eilected either substan- H CHCh =0 COON:

CHO

l Hl Ha After being converted from salt to the free 55 acid the para toluic acid from zones Il and 2i next is subjected to side chain chlorination to form chloromethyl benzoic acids by the reactions:

CHI Q Cl: 00H

(Molten) CHgG] Q HC1 00H Side reactions also may occur, for example:

CHClz Q 21101 oon CH: C Ch son, anon oon H Minor chlorination in the ring is not precluded.

Although the para toluic acid may be chlorinated while dissolved in solvents such as carbon tetrachloride, better results have been obtained by direct chlorination of the molten acid, for example. at temperatures of lfrom 190 C. to 270 C.

The foregoing conversion to chloromethyl benzoic acid is effected in reaction zone 22 from which the chlorinated products are conveyed as shown by line 23 to reaction zone 2l for conversion to terephthalic acid. In this reaction zone the chloromethyl groups are hydrolyzed and then oxidized by aqueous caustic alkali to a carboxyl group. thereby yielding terephthalic acid by the following reactions:

H501 mon C|}OONa CIJOONa H2OH OONa Corresponding reactions occur with dichloro methyl toluic and trichloro methyl toluic acids.

The reaction mixture containing terephthalic acid passes to product recovery zone 26. Any suitable method of recovery and purification may be used, for example, acidication, crystallization and washing with solvents.

When and if a mixture of xylenes is fed to chlorination zone Ii, a corresponding mixture of ortho, meta and para xylene chloride isomers will be obtained. The product will therefore be mixed isomeric phthalic acids; but, if desired, a separation between para toluic acid on the one hand and meta and ortho toluic acids on the other hand may be elected, as by distillation. Two different phthalic acid products, namely a terephthalic acid and a mixture of ortho and meta phthalic acid, may then be produced from the two separated toluic acid fractions. After such a separation (not shown in flow sheet), the mixed ortho and meta toluic acid fraction may be withdrawn as indicated by line 21 from caustic oxidiation zone I9. The resulting para toluic passes to zone 22l for chlorination as shown. The xylylene dichlorides may be separated in suitable fashion. Thus, because of its high melting point the para xylylene dichloride may be separated by crystallization. The ortho and meta isomers may be separated from each other by fractional distillation, preferably under vacuum to avoid decomposition.

Toluene which may be contained in the mixed xylene feed or separately introduced by way of valve-controlled line 28 will be chlorinated in zone Il to form benzyl chloride which is conducted to separation zone I3 and withdrawn 6 therefrom as a benzyl chloride fraction indicated by line 29. The benzyl chloride fraction is converted to a mixture of xylylene dichlorides by chloromethylatlon in zone 3|. The xylylene dichloride product is hydrolyzed to the corres' reaction zone 36. The hydrogen chloride formedin xylene chlorination zone Il together with formaldehyde is utilized as shown to effect the chloromethylation reaction. A mixture of ortho, meta and para xylyl chlorides from reaction zone 36 is passed by way of line 31 to separation zone 38. If desired the xylyl chlorides may be fractionated and a puried para xylyl chloride fraction removed from separation zone 38 by way of line 40.' When a mixed xylene hydrocarbon feed is used for zone Il, only mixed chlorides need be separated in zone 36. Hydrolysis to methyl benzyl alcohol in reaction zone I3 and conversion to a mixture of toluic acids in reaction zone IS is effected as previously disclosed. If and when it is desirable to synthesize a relatively pure terephthallc acid, the mixed toluic acids are fractionated to separate a para toluic product and leave a mixed ortho and meta toluic acid fraction as previously disclosed. The para toluic acid fraction will then be passed to chlorination zone 22 for conversion to para chloromethyl benzoic acid. The chloromethyl benzoic acid then passes as shown by line '.'3 to caustic oxidation zone 24 where it is converted to terephthalic acid in the manner hereinbefore disclosed.

The process of this invention and suitable modes of operation will be readily apparent in the light of the following illustrative data and examples.

In small scale operations, the chlorination reaction of zone Il was carried out in glass reactor equipped with a sintered glass plate at the bottom through which gaseous chlorine: was fed. The reactor when used for continuous chlorination was provided with an inlet line at the bottom, through which the hydrocarbon feed entered the reactor and an outlet line near the top for removal of reaction mixture at the same rate as hydrocarbon entered the reaction zone. Reaction temperatures were taken by a thermocouple extending into the reaction liquid. The reactor was topped by a water-cooled condenser through which exit gases were removed and absorbed in aqueous caustic. Vacuum dstillations utilized for separating the xylene chlorides were carried out in distillation columns packed with glass helices and equipped with an electrically heated jacket. Liquid constituents of the reaction mixtures were removed through an ordinary water-cooled head. Para xylylene dichloride and other high melting constituents were taken off the fractionating column through a special head surrounded by a glass jacket containing boiling toluene to prevent solidication in the lines prior to reaching the storage receiver.

Chiorination in batch operation can be effected in the presence of light at approximately C. This temperature may be varied from 4') to C., for example. When theoretical quanaseaseo titles of chlorine necessary to form a xylylene dichlorlde were passed into the reaction mixture, some dehydrohalogenation occurred both during chlorination and upon distillation of the chlorinated product under vacuum at 3-8 mm. mercury pressure. Continuous chlorination was carried out at about 95 C. in the presence of light with greatly improved results. Para xylene and chlorine were fed to the reactor at constant rates and excellent chlorine utilization obtained. Steady state compositions were obtained for each feed ratio in continuous chlorination by taking samples from the continuously withdrawn chlorinated product and analyzing for chlorine content. When chlorine content became constant.

the reactor was assumed to be under steady state TABLE i Distillation analysis of chlorinated p-zylene Distillation No 1 3 4 Results (Per Cent by We Loss l.

From the results of the foregoing distillations, it will be seen that as the chlorine to para xylene feed ratios are increased the yield of .para xylylene di'chloride increases.

A large batch of the reaction mixture of chlorinated compounds was treated for separation as follows: The chlorinated mixture was cooled in an ice bath and the crystals formed were filtered off. The nitrate was then charged to a still and unreacted para xylene and para xylyl chloride removed under vacuum. The still bottoms were dissolvedvin about half their volume of chloroform and the resulting solution cooled in anice bath where additional crystals were recovered and combined with those rst separated. These crystals, after purification by recrystallization from' chloroform, were identied as para xylylene dichloride by chlorine content, melting point and hydrolysis to the para xylylene glycol derivative.

The mother liquor from the foregoing chloroform crystallization was concentrated to 50% of its volume and again cooled in ar ice bath. A second fraction of crystals more soluble in chloroform than para xylylene dichloride was obtained and recrystallized from a mixture of petroleum ether and benzene. These crystals were identied as para xylal chloride by their chlorine content and melting point and by the fact that hydrolysis yielded para talualdehyde rather than the glycol.

After separation of the chlorinated reaction mixture to yield a xylyl chloride fraction, a xylal chloride fraction and a para, xylylene dichloride fraction, these respective intermediates are treated for conversion to the corresponding carboxylic acids or sodium salts thereof.

ooNvERsroN 0F xYLYL CBLORIDE A preferred mode of operation involves simultaneous hydrolysis and oxidation of xylyl chloride to the corresponding toluic acid at 300 C. to 400 C. (S75-750 FJ. The overall reaction may be written:

Beat zNaoH o,

COONa Naci 211,0

. CH: However, it may be effected in two stages:

CHlCl CHloH N 0H Heat N ci mo a CE: CH:

cHioH cooNa NaOH( Heat B o B H5 e .e o, q o, 2

CH3 CH;

A single stage operationpresently'is regarded as most desirable. The reaction may be effected by agitation in a rocker type autoclave under the following conditions:

Time, hours 1 Temperature, C 371 Caustic concentration, percent 8 NaOH, moles 2 Xylyl chloride, moles 1 Yield of toluic acid (moles per mole of xylyl chloride) .85

XYLAL CHLORIDE CONVERSION Xylal chloride is converted to the corresponding toluic acid by hydrolysis and oxidation preferably at 300 C. to 400 C. (S75-'750 F.). These reactions may be effected by agitation in rocker type autoclave as follows:

CONVERSION 0F XYIJYLENE DICHLOR'IDE In small scale runs for this stage of the process reactions were carried out in a rocking autoclave capable of withstanding pressures up to 16,000 pounds per square inch gauge and constructed entirely of Monel metal. The head of the vessel was tted with a thermocouple, a safety valve. vent, a pressure gauge and asample valve. Heat was supplied through strip heaters located in the autoclave casing. The whole assembly was agitated by rocking in a vertical plane with'a maximum deviation from the horizontal of 30 and at a rate of 78 cycles per minute.

,The desired material was charged to the autoclave together with a, predetermined amount of caustic soda, water and the oxidizing agent when one was used. The vessel was lock-closed, placed in a shaker casing. and if a gaseous oxidizing agent auch as air or oxygen was to be used. it wel charged at this time through the sample valve. CHiCl CHioH The amount of oxidizing agent was calculated from the total pressure and known free-space in zNBOH 2mm the vessel. A

Heat was turned on and shaking started. After 5 HIC] H203 reaching reaction temperature, the mixture was allowed to4 react for one hour while controlling D'Xylylelmde p'll 31ml the temperature to maintain it substantially constant. At the end of one hour reaction time (ex elusive of heating and cooling periods), shaking lo 2N0H 4H: was stopped, heat turned oil and the apparatus allowed to cool overnight. Upon removal of the HioH N., cooled reaction mixture from the autoclave, it Terephthalicacid was ltered to remove dirt and traces of insoluble In any event it has been found that terephby-products and next acidied to a pH of 3.0 with l5 thalic acid can be prepared from para xylylene HCl to convert the salts back to free acid whichl forms a precipitate. The precipitated acid was recovered by filtration, washed free of chloride ion, dried and weighed. A sample of the dried product was dissolved in a standard base solution, back-titrated potentiometrically with standard acid and the acid number obtained. Yield was calculated from these data.

Pure para xylylene dchloride when reacted with four or more mols of caustic soda per mol of the dichloride at 371 C. (700 F.) for one hour yields a terephthalic acid product containing impurities. Yields and impurity contents are illustrated by the following table of data:

TABLE II Caustic oxidation of :cylylene ichloride glycol by subjecting the glycol to caustic oxidation at 371 C. (700 F.) for one hour as in the conversion of para xylylene dichloride. The product from the caustic oxidation of para xylylene glycol contained a neutral product. paratoluic acid, and terephthalic acid substantially as in the previous preparation from para xylylene dichloride. The xylylene glycol was obtained for this reaction by hydrolysis in a dilute water solution at reflux temperature followed by extraction with ether to recover the glycol from the aqueous medium.

Important variables were investigated to determine their effects upon the foregoing caustic Wt. Per Wt. Yield Mols Zy- Conc. Conv. Acid No. lylene Di- Nblsn NaOH Based on Bggltmt ggstn] of Product chloride Used Solution, Chloride, Based on Charge mg. KOH Charged Per Cent Per Cent Charge Per Cent per Gram 0. 50 2.0 8. l 100 l. 0 65 452 0. 50 2.0 8. 1 1(1) 53 410 D. 50 2. 0 8. l 100 2. 3 66 423 0. 50 2.0 8. 7 100 1. 7 64 427 The by-product on which approximate peroxidation reactions. Among the factors invescentages are given above is a neutral, pasty material insoluble in aqueous caustic. The acid .numbers shown in the foregoing tables were obtained on the crude chloride-free acidic reaction products. Acid numbers can be raised to from about 500-512 in all cases by recrystallization from water. The significance of acid numbers is illustrated by theoretical acid numbers as follows:

Toluic acid -412 Terephthalic acid -672 Pure terephthalic and pure toluic acids were isolated from the reaction mixtures and identified by means of acid number and methyl ester; In addition a water insoluble neutral material was found to be present and is removed byk filtration of aqueous solutions during recrystallization. The foregoing reaction is believed to occur step-wise as follows:

tigated were: effect of` concentration of the xylene chloride in the caustic, effect of concentration of caustic and of excess proportions of the caustic solution on the product, effect oi' time and temperature of reaction, and effect of oxidising reagents. Each of these factors is discussed hereinbelow. EFFECT or CONCENTRATION oN CONVERSION 0F XYLYLENE DiCHLoRIDE A decrease in concentration of the xylylene dichloride increases the acid number of the product and reduces the amount of neutral by-product. The weight yield of the product remains substantially constant and the concentration as well as the amount of excess caustic over theoretical appeared to have no effect on the product within the limits investigated. The following series of data -in which the reactions were coni ducted at 700 F. with one hour reaction time are illustrative:

TABLE III dichloride to terephthalic acid Wt. Per Wt. Yield Mols Xy- Mols Conc. Per Cent Cent of of Acids Acid No. lyiene'Di- NaOH NaQH Conv. Bypmdm,t Based on of Product chloride Used Solution, Based on Based on Cham@ mg. KOH Charged Per Cent Chloride Charge Per Cnt per Gram 0. 10 2.0 8. l 100 truce 91 520 0. 02 2. 0 8. l 100 trace 69 577 0. 02 2. 0 8. l 100 trace 67 570 0.02 0.08 0.4 trace 63 572 a,ees,sao

Eirect of temperature, time, and pressure on turbo-mixer equipped with a mercury-sealed .r1/Igiene dichloride conversion-A temperature stirrer. Chlorine was introduced at the bottom between 315-371 C. (60o-700 F.) yields a prodof the reactor which was immersed in an elecuct of a fairly constant acid number. As temtrically heated oil bath. The chlorine was preperatures increased above 371 C. (700 F.) or 5 heated in a coiled feed line immersed in the oil reaction times are prolonged, or both.the yield of bath and leading to the bottom of the reactor. total acid drops on. Thus, it appears that about Exit gases from the reaction mixture 'pass 315 C.` (600 F.) is as high a temperature as is through a side-arm to a condenser. The toluic necessary and that reaction times in excess of acids tend to sublime and plug air condensers.

one hour achieve little or no benefit. However. To alleviate this tendency an oil heated conthe process is operative at temperatures of from denser was attached to the reactor and an air- 250 C. to 400 C. M80-750 F.). Illustrative data cooled condenser in turn attached to the outlet are givenin the following table: of the oil heated condenser. Exit gases. thus.

TABLE IV Oxidation ol xylylene dichloridevariable time and temperature WLPBI Wt. Yield M01" Mols. Conc' Cent of By- Acids Acid No X l iene NaOH Time Temp., Product Dicyh orlde Ngs? Solution Bours F. gragfdugfl lageaggg? mg. KOH Charged Per Cent Charge Per Cent per Gram 002 1.0 4.2 1.0 550 14 a7 5m 0. 02 1.0 4.2 1. 0 600 None 77 584 0. 02 l. 0 4. 2 1. 0 650 None 75 570 0. 02 l. 0 4. 2 4. 0 650 None 75 579 0. 02 2. 8 8. l 4. 75 700 None 43 616 0. 02 l. 0 4. 2 1. 0 750 N one 23 573 0. 02 1. 0 4. 2 4. 25 750 None None Pressure is not critical except that it should be first passed through the oil condenser where a suillcient to maintain the aqueous caustic in major proportion ofA any sublimed acids were liquid phase. condensed. melted and returned to the reactor.

Eect of oxidizing agents on conversion of The remaining gases next flowed to the air con- .rylylene dichiarate- It has been found that use denser and were led to a caustic absorber for of an oxidizing agent, such as air, achieves a extracting HC1 and residual acidic products carmarked improvement in yields and purity of the ried thereby. y terephthalic acid product. In fact, substantially 4 Although para toluic acid, for example. may theoretical yields of nearly pure terephthalic be chlorinated in boiling carbon tetrachloride in acid have been obtained by intimately contacting the presence of light, direct chlorination of the reaction mixture of aqueous caustic and molten toluic acids yields better results. The xylene chlorides with air. The data in the table toluic acid is charged to the reactor, melted, below exemplify the excellent results obtainable. heated to the desired temperature and there- TABLE V Caustic oxidation of .'rglylene dichloride in the presence of added oxidizing agents Mols Mols Conc. Mols. of Per Cent of Mol Acid No. Tereph- Xylylene NaOH NaOH oxidizing Available Theoretical Yield Acids thalic Dichloride Used Solution, Agent Used Oxygen Oxygen of mg. KOH in Prod., Charged Per Cent Charged Charged Acids `per Gram Per Cent 0.02 0. 015 3.0 NaoGl 0.01 ss 023 s1 0.02 0.00 3.3 Naocl 0.02 100 04 673 100 0.02 1.0 4.2 Air 0.08 400 as 674 100 0.25 1. a7 1.0 o, 0.40 200 00 ses 9a All of the above runs were made at 315 C. after contacted with gaseous chlorine under con- (600 F.) and one hour reaction time at this temstant agitation. Temperatures of from 190 C. perature. to 270 C. are suitable conditions. While chlo- CONVERSION OF TOLUIC ACIDS rination of para tolulc acid to form para methyl benzoic acid is a preferred aspect of the inven- Returning now to the intermediate toluic acids, tion since a main objective is to produce terephit will be recalled that, in addition to use as thalic acid, it was found that all three isomers such. these compounds may be converted to their of toluic acid will undergo direct side-chain chlocorresponding phthalic acids by chlorination in rinationrin the molten state. Ortho toluic acid the methyl group and caustic alkali oxidation gave a 13.7% chlorine-containing product, for of the resulting chloromethyl benzoic acid example; and two runs with meta toluic acid (a chloro para toluic acid). Each of these stages yielded products containing 16.8 and 19.1% chlowill be described. s rine respectively. Various runs made with para chlorination of toluic acida-The direct chlotoluic acid and data on results obtained are given rination of toluic acids was carried out in a glass in Table VI:

TABLE VI chlorination of p-toluic acid Per Cent Gm. of Per Cent Acid Acid No. Gm. Per Cent Tereph- Cla used Per Cent of Reacted No. slgle Descripcion or Charge "f 221' thune T'p" per glp. 0101. o1, which Prfgl-n Chlgfme mg. KOH/m Acid m Tulum Beaded Dehydro- 0151.01@ weiyht Kon Charge Acid halogenated g Der gm i p-Toluic Acid (C. P.) 412 1. 19 375 2 p-Toluic Acid Laboratory Prep. 414 .8 246-252 1.07 14.2 450 M. P. 178-180.5 C. 3 p-Toluic Acid, Laboratory Prep. 414 0.8 246-252 1.07 13.0 447 M. P. 178l80.5 C. 4 p-Toluic Acid, Laboratory Prep. 422 3.8 246-252 .668 68 34 1.11 13 2 446 M. P. 177 220 C. 5 p-Toluic Acid, Laboratory Prep. 414 .8 252 .884 73 44 1. l0 16.4 480 M. P. 178-180.5 C. 8 p-Toluic Acid, Laboratory Prep... 426 5. 3 246-252 .922 66 .56 96 14. 4 461 7 p-Tolulc Acid, from Pilot Plant, 445 12.5 246-252 .838 79 55 1.08 13.7 520 Caustic purified. 3 d0 Y 447 13. 3 232-241 982 82 52 1.09 17. 7 562 9 p-Tolulo Acid, from Pilot Plant, 447 13.3 2552-241 .932 73 55 1.11 14.1 560 l Caustic puriiled, air and chlorine used. 10 p-Toluic Acid, from Pilot Plant, 439 13.0 232-241 .882 80 66 .98 12.4 616 Steam stripped, containing l about 2% polymer.

,I u' Attention isV directed to the fact that in Table The parachloromethyl toluic acid desirably is converted to terephthalic acid in a single stage caustic oxidation process by contacting with aqueous caustic at 15G-400 C., for example at 315 C. (600 F.). The following examples are illustrative and show that chlorine content of the feed within the ranges tested has no profound effect upon the acid number of the product. A large excess of oxygen diminished the weight yield of product.

TABLE VII Oxidation. of chlorinated p-toluic acid in caustic soda at high-temperature Product chlorinated Chlorine Moles Moles NaOH Yield of Tolmc Acid m Charge, Oxygen Rm NO- charged, Per cem Nbsgg ruggrlt' charred la? Per Cent frffegf Acid No. Grams of Theory (as air) Per Cr'n p-Toluic tehalric mgs. KOH by wt of Acid Acid per Gram Charge tions. While some of the para toluic acid charge stocks contained terephthalic acid, the amount 'therein is insufficient to account for the high acid number of the products. Extraction of the chlorinated products with proof alcohol (in which terephthalic acid is insoluble) yielded an alcohol soluble portion and an alcohol insoluble portion having acid numbers of 366 and 522, respectively. Accordingly, it is believed that the chlorine serves as an ,oxidizing agent to convert a portion of the para toluic acid to terephthalic acid by an oxidation reaction which is presently not understood.

On comparing sample 10 with samples 8 anri'9, it appears that removal of impurities from the crude toluic acid by caustic purification or distillation achieves substantial improvements in chlorine utilization, and in yields and purity of product.

Conversion of para chloromethyl benzoc acid.-

The effect of different variables on the single stage conversion of para chloromethyl benzoic acid was investigated.

Eect of temperature on conversion of chloromethyl benzoic acida- Effect of temperature is decided in altering yields of total acids as well as per cent terephthalic acid in the reaction product. The effect of changes in temperature on total weight of acids and on the percentage of TABLE VIII Elect of temperature on caustic oxidation of a-chioro-p-toluic acid Product Omgimllimd M 1 Neon M0 0 C 05 0' Charged, Used Per Cent (as air) Per Cent Tonne Tere hmgs.

Grams by Wt o' d thal c KOH per charia .4cm Gram 5.0 1.o 4.2 0.04 300 te a1 sa 51s 5.o= 1.0 4.2 0.04 400 ss 22 1s 017 5.0 1.0 4.2 0.04 500 04 n s0 644 5.0 1.0 4.2 0.04 000 es a 07 ses Eect of concentrations on conversion of chloromethyl benzoic acid.-The foregoing runs were performed with the chlorinated toluic acids present in dilute solution and with a high molar excess of caustic soda. Additional tests revealed that the reaction can be eiected with good yields and relatively high etilciency' at high concentrations of the chlorinated acids and without large excess of caustic soda. given in Table 1X:

Illustrative data are 25 product.

their` corresponding phthalic acids by caustic oxidation at temperatures within the range of 150- 400 C., e. g. at 315 C. (600 F.), for a reaction period of one hour in a manner similar to the foregoing examples with para chloromethyl toluic acid. 'I'he invention and condition for reaction as herein disclosed is applicable to the ortho and meta isomers as well as to the para Efect of caustic concentration-Relatively di- TABLE 1X Caustic oxidation of n-chloro-p-toluic acid Product ghloiio- Chlicrine C o u c Mols one' Mols Yield of Sample Acid Charge NaOH NaOH Oxy en Total Per Cent No. Charged, Per Cent Solution, g Per Cent Acid No.

Grams of Used Per Cent Charged ggf pTolulo Ttflxigl ms. KOH Theory wt of Acid Acid per Gram Charge 57. 9 65 l. 5 5.6 0. 35 95 14 86 638 69. 5 66 l. 5 5. 6 0. 37 93 8 92 654 103 85 2. 0 9. 9 0. 60 95 4 96 662 80.0 68 l. 5 8.1 0.50 98 3 97 666 86. 6 59 l. 6 8.4 0.53 87 19 81 l 625 l This experiment was made on a chlorinated crude toluic acid.

In the foregoing tests, oxygen was substituted for air so that lower pressures could be utilized.

Eect of oxidizing agents- Except where otherwise noted, oxygen in the form of air was used for all of the reported examples on the conversion of para chloromethyl benzoic acid. It has been found that quantities of oxygen less than one mol per mol of para chloromethyl benzoic. acid, although operative, result in loss in conversion of terephthalic acid and yield reaction products having lower acid numbers. A slight excess of oxygen is preferred as will be apparent from the following data:

TABLE X CONVERSION BY CHLOROMETHYLATION Utilization of by-product HC1 by chloromethylation of toluene or of benzyl chloride (produced by chlorination of toluene contained in or added to the xylene feed) has been shown in the ded Use of oxygen in caustic oxidation of a-chlorop-toluic acid to terephthalic acid Product Chloro- Chlorine Mols Toluic Mols NaOH Yield of Samp] Acid i Charge NaOH Solution, Oxygen Total Per Cent No' Char ed Per Cent Used Per Cent Charged Acids Per Per C'-lt Tere l1 Amd No Gmls 0l' Theory (as air) Ceut'by pT0lu1c thac mgs. KOB WL .Acid Acid per Gram Charge Ortho chloromethyl toluic acid and meta. scription of the iiow sheet of Fig. 1. This type of chloromethyl toluic acid have been converted to reaction will be illustrated in connection with the 17 chloromethylation of benzyl chloride to form para xylylene dichloride. The reaction CHN, CHIC] Reaction mix allowed to stand overnight, washed,

H dried, and ltered. Reaction produc-ts sepa- Hzo HC1 H20 5 alilsid by fractional distillation at reduced pres- Cmcl The/resulting product may be converted to toluic acids as hereinbefore disclosed. is carried out in the presence of a condensation From the foregoing it will become apparent catalyst such as zinc chloride, hydrogen fluoride, that the combination of process steps herein disor aluminum chloride. The tendency of benzyl closed affords a method for obtaining substanchloride to react with itself may be reduced by tially complete conversion of a xylene, toluene or feeding the benzyl chloride to an excess of the analogous methyl-substituted aromatic hydrocarreaction mixture and by such other expedients bon to corresponding carboxylic acids with high as will occur to those skilled in the art. yields while avoiding the necessity for complete The process is illustrated by small scale examchlcrination to theoretical dichloride content. It ples in which benzyl chloride, formaldehyde, catis unnecessary to obtain maximum yield of the alyst, and solvent were mixed and contacted with i xylylene dichloride, for example, in order to pro- HCl for a period of several hours. The reaction duce dicarboxylic acids thereby permitting undermixture was allowed to stand overnight, washed, chlorination with resulting higher selectivity for dried and ltered. Solvent and unreacted benzyl side-chain chlorination as against ring chlorinachloride were removed by distillation and the dition while simultaneously minimizing overchlo chloride was recovered by crystallization. Data rination in the side chain with attendant higher and results of these runs are given in Table XI: overall chlorine and caustic consumption. The

TABLE XI Data from chlormnethylation experiments Sample No 22 23 24 25 Reactants:

Benzyl Chloride 1 m01 2 111018 1 mol l mol. Formaldehyde.- .54 eq. of formalin.-. 2.1 eq. of (CHi0)x.. l eq. of (CH2O)z. 1.33 eq. of (CHO x. Source oi' HC1 anhydrous HC1 anhydrous HC1. anhydrous HC1-.." 125 ml. conc. HCl. Catalyst,... 69 gm.l 138 gmJ 68 gm. Zl'iClz 60 ml. 85% H31 Solvent 350 mi. Ethylene 500 mi. Ethylene 300 ml. Ethylene 110 gm. Glacial Ace Chloride. Chloride. Chloride. tic Acid. Temperature, C 50 50 50 100. Time, Hours 8 9.. 8 4%, Analysis of Product: 2 n Urreagted Benzyl Chloride, Per 1l.. 92, xy/Iieiie Diehloridarercent 30.. se. 3s. 2. Polymers and Di-chloromethylated 27 25 42 Nil.

Products, Per Cent.

1 Catalyst prepared by fusing 210 gm. of ZnClz and stirring in 10 gm. of A101; as it cooled.

2 Yields are based on the amount of formaldehyde used.

The xylylene dichloride may be converted to corresponding phthalic acid by either a single stage caustic oxidation or by first hydrolyzing to form the xylylene glycol and then oxidizing to the dicarboxylic acid. Although a single stage conversion usually is preferred, conditions for a twostage process will he illustrated. In the rst stage xylylene dichloride is subjected to hydrolysis in dilute water solution at refiuxing temperature. The xylylene glycol is recovered by ether extraction of the reaction solution. In the second stage the recovered glycol is subjected to caustic oxidation by intimately contacting the glycol with air or oxygen and an aqueous caustic solution at, for example, 371 C. (700 F.) for one hour as disclosed in the various processes for converting the chlorides to the acids. The resulting product will be a mixture of para toluic acid and terephthalic acid which can be separated in any suitable manner as by distillation. If desired, the para toluic acid may be passed to a chlorination stage for conversion to para chloromethyl benzoic acid after which a caustic oxidation will serve to form terephthalic acid therefrom.

Xylyl chloride is obtained from toluene by chloromethylation in a manner analogous to the chloromethylation of benzyl chloride. Exemplary conditions are:

Ethylene dichloride, ml 500 Toluene, moles 1 Zinc chloride, gr 68 invention also furnishes a process for completely converting a mixture of chlorinated methyl benzenes, such as xylenes, to carboxylic acids notwithstanding that the degree of chlorination or the number of chlorine atoms per molecule of xylene varies widely. Despite this variation in chlorine content, production of corresponding carboxylic acid salts may be effected in single stage conversions. The process also is adapted to produce polycarboxylic acids from the underchlorinated compounds by further chlorination and conversion of any monocarboxylic acids from the foregoing single stage conversions. The process enables economical utilization of by-product HCl produced in the chlorination reactions. It is important that terephthalic and the like acids are obtained by this invention with relatively cheap bulk chemicals, such as caustic soda, chlorine and, when desired, formaldehyde.

The benefits of various features of this invention are applicable to lthe caustic oxidation of benzyl chloride, benzyl alcohol or benzaldehyde to benzoic acid. This is especially true concerning the use of air or gaseous oxygen to increase the yield of acid produced. For example, in comparing runs made at 371 C. (700 F'.), 3800 Pounds per square inch, and a reaction time of one hour, it was found that by utilizing two mols of caustic in 8% aqueous solution per mol of benzyl chloride without addition of air Yor oxygen, about 88% yield of benzoic acid was obtained with about 8% by-product. In a second run at 19 371 C. (700 F.) at the same pressure and reaction time, the results were as follows:

Benzyl chloride reacted, mols 0.107

NaOH used, mols 0.225 Solution strength, percent 2 Mols of O2 (as air) 0.12 Mol yield of acids, percent 96 Mol yield of by-product, percent 4 Intimate contact oi the gaseous oxygen with the liquid reactants was obtained by vigorous agitation. It will be observed that the presence of this oxidizing agent reduced the amount of byproduct formed from about 8 to 4%, a factor of approximately one-half.

The eiiect of temperature on caustic oxidation of benzyl chloride to benzoic acid is illustrated by the following discussion and data.

Fig. 3 reveals that reduction of temperature below 340 C. (650 F.) decreases yield of acid but does not render the process inoperative. Optimum reaction temperature for conversion of benzyl chloride to benzoic acid is in the range of from S70-400 C. (700-750o F.).

When either benzyl alcohol or benzaldehyde was substituted for the benzyl chloride under exactly the same reaction conditions as were used for the benzyl chloride conversion, somewhat higher yields oi the acid were obtained.

Suitable temperatures for conversion of benzaldehyde and benzyl alcohol to benzoic acid by this reaction are 300 to 400 C.

Air or gaseous oxygen is advantageously used as an oxidizing agent in the foregoing reactions. Caustic oxidation of benzyl chloride, benzyl alcohol and benzaldehyde all produce the same by-product, mainly, benzyl ether, and in the case of benzyl chloride some unreacted benzyl alcohol. These by-products may be recycled to vmain reaction zone in order to suppress further formal tion thereof and increase the yield of acid.

Reference has been made to the fact that when and if ring chlorination occurs as a side reaction, the desired carboxylic acid is nevertheless obtained. This factor is believed highly significant to the good yields and superior purity of the products herein obtained. Under the caustic oxidation reaction conditions herein disclosed, the chlorine in the ring is removed and converted to HC1 initially, which isof course neutralized by the caustic yielding a desired carboxylic acid. The removal of such ring chlorine is illustrated in conversion of chloro benzyl chloride to benzoic acid in which the following reaction occurs:

CHxCl CO OH Analogous reactions occur with xylyl, xylal and xylylene chlorides containing chlorine in the ring as well as in the side chain.

20 Although the caustic alkali herein utilized for purposes of illustration has been sodium hydroxide, other strong alkali hydroxides such as Apotassium hydroxide may be substituted therefor. Likewise other modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. and only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for producing benzene carboxylic acids which comprises oxidizing a xylene having at least l and not more than 2 chlorine atoms substituted in each methyl group by contact with aqueous caustic alkali in an oxidation zone at a temperature in the range about 300 to 400 C. under a pressure suiiicient to maintain the aqueous caustic alkali in liquid phase to form a reaction product comprising benzene carboxylic acid salts and hydrogen.

2. The method as defined in claim 1, wherein a free oxygen-containing gas is introduced into the oxidation zone at reaction temperature to consume produced hydrogen.

3. The method as defined in claim 2, wherein the chlorinated xylene consists predominantly of xylene dichloride.

4. The method as defined in claim 1, wherein the chlorinated xylene consists predominantly of xylene dichloride.

5. A process of producing a benzene dicarboxylic acid which comprises mixing an aqueous caustic alkali with a chloromethyl benzoic acid` reducing pressure buildup from hydrogen so formed by intimately contacting said liquid phase mixture with a gas containing free oxygen while at said reaction temperature, and recovering benzene dicarboxylic acid from the reaction mixture ina form selected from the group consisting of the free acid and a salt thereof.

6. A process of producing terephthalic acid which comprises converting para-mono-chloromethyl benzoic acid to terephthalic acid by oxidation with aqueous caustic alkali at a temperature of from to 400 C. and under sufficient pressure to maintain said aqueous caustic in liquid phase.

7. The method as defined in claim 6, wherein a free-oxygen-containing gas is introduced into the mixture of aqueous caustic alkali and paramono-chloromethyl benzoic acid at conversion temperature.

8. A process of producing terephthalic acid which comprises mixing an aqueous caustic alkali with para-xylylene dichloride, oxidizing said xylylene dichloride to terephthalic acid with said aqueous caustic alkali at a pressure sufficient to maintain at least a part of the water in liquid phase and at a reaction temperature of from 300 to 400 C. whereby hydrogen formation occurs, reducing pressure buildup from hydrogen so formed by intimately contacting said liquid phase mixture with a gas containing free oxygen while at said reaction temperature and recovering terephthalic acid from the reaction mixture in a assasno form selected from the group consisting of the free acid and a salt thereof.

9. A process of producing terephthalic acid from a para-di-(chloromethyl) benzene having no more than 2 hydrogen atoms of the methyl 5 groups substituted by chlorine, which comprises converting said di-(chloromethyl) benzene to said acid by oxidation with aqueous caustic alkali in liquid phase at 300 to 400 C. whereby hydrogen formation occurs, and reducing pressure buildup from hydrogen so formed by intimately contacting said liquid phase mixture with free oxygen during said oxidation with caustic alkali and at a temperature of from 300 to 400 C.

JOHN L. DARRAGH. ROBERT J. MILLER.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 22 Number Country Date 364,442 Germany Nov. 25, 1922 639,578 Germany Dec. 8, 1936 OTHER REFERENCES Meyer: Ber. Deut. Chem., vol. 14, p. 2394 (1882).

Colson et al.: Ann. Chim. et Phys. (6th series), vol. 11 (1887), pp. 27-28.

Auwers et al.: Ber. Deut. Chem., vol. 36, p. 3907 (1903).

Simonis: Ber. Deut. Chem., vol. 45fpage 1588 (1912).

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Posner et a1.: Ber. Deut. Chem., vol. 57, p. 1137 (1924).

Meunier et al.: Beilstein (Handbuch, 4th ed.), vol. 7,\p. 176 (1925).

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Shorigin et al.: Chem. Abstracts, vol. 23, pp. 3680-3681 (1929). 4

Gomberg et al.: Beilstein (Handbuch, 4th ed.), vol. 6, 2nd suppl., p. 404 (1944).

Claims (1)

1. A PROCESS FOR PRODUCING BENZENE CARBOXYLIC ACIDS WHICH COMPRISES OXIDIZING A XYLENE HAVING AT LEAST 1 AND NOT MORE THAN 2 CHLORINE ATOMS SUBSTITUTED IN EACH METHYL GROUP BY CONTACT WITH AQUEOUS CAUSTIC ALKALI IN AN OXIDATION ZONE AT A TEMPERATURE IN THE RANGE ABOUT 300 TO 400* C. UNDER A PRESSURE SUFFICIENT TO MAINTAIN THE AQUEOUS CAUSTIC ALKALI IN LIQUID PHASE TO FORM A REACTION PRODUCT COMPRISING BENZENE CARBOXYLIC ACID SALTS AND HYDROGEN.

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