AU731622B2 - Process for production of aromatic tricarboxylic acids - Google Patents

Description

WO 98/55441 PCT/EP98/03291 PROCESS FOR PRODUCTION OF AROMATIC TRICARBOXYLIC ACIDS BACKGROUND OF THE INVENTION The process described herein relates to the production of aromatic tricarboxyiic acids having only two vicinal carboxylic acid groups and more particularly it pertains to an improved technique for catalytic liquid phase oxidation of a trimethyl substituted aromatic hydrocarbon having only two methyl substituents on vicinal ring carbons, such as pseudocumene to an aromatic tricarboxylic acid having only two carboxyilic acid group substituents on vicinal ring carbons and having the third carboxyl group as a substituent on a non-vicinal ring carbon, such as trimellitic acid.

The conversion of trimethyl substituted aromatic hydrocarbons by catalytic liquid phase oxidation in the presence of heavy metal oxidation catalysts and side chain oxidation initiators or promoters to aromatic tricarboxylic acids is described in the technical literature. In general, the uses of different catalytic systems are proposed. All employ heavy metals of the class of those having atomic weight from about 50 to about 200, desirably those in this class which are of variable valence or transition metals, and show a preference for using cobalt, alone or in combination with manganese. These oxidation metal catalysts are usually introduced in a form which is soluble in the hydrocarbon to be oxidized and/or an oxidation solvent medium is which preferably acetic WO 98/55441 PCT/EP98/03291 acid. The catalyst systems are provided by the use in combination with said heavy metals of one promoter or initiator of side chain oxidation, which is typically a compound containing bromine.

Any form of bromine supplying ionic bromine in the rection system, i.e. hydrogen bromide or combined bromine as in organic bromides, can be used. The discovery of the system of catalysis provided by heavy metal oxidation catalysts and bromine for the rapid, high conversion of di-, tri- and other polysubstituted aromatic compounds with air in a liquid system on a once through basis is described in U.S. Patent No.

2,833,816. Later patents teach applications of said unique system of catalysis to various means for exploiting that oxidation method, for the commercial production of benzene tricarboxylic acids. As described in U. S. Patent No 3,920,735, it has been found that zirconium is unique among the Group IV B metals to substantially enhance the activity of the bromine-cobalt or the bromine-cobalt-manganese systems of catalysis.

The use of cerium in association with cobalt or cobalt-manganese as transition metal catalyst is described in US Pat. No 3,491,144 and US Pat. No 3,683,016.

In general the aforementioned catalytic liquid phase oxidations using air as a source of molecular oxygen are conducted at 150 0 C to 250 0 C *and at a pressure adequate to maintain a liquid phase of alkyl substituted aromatic hydrocarbons. Commercial WO 98/55441 PCT/EP98/03291 developments utilizing the foregoing systems of catalysis employ controlled reaction temperature within a narrow range; staged reaction temperatures such as those starting at a low or initiation temperatures, increasing reaction temperature, to obtain maximum oxidation or substantial completion of the oxidation to oxidize small amounts of partial oxidation by-products such as for example methylol benzoic acids, formylbenzoic acid. Staged oxidations have been applied to time staged intermittent batchwise or semicontinuous mode of operation.

It has been found that certain polymethyl substituted aromatic compounds, when oxidized in the foregoing catalytic liquid oxidation systems, appear to produce oxidation by-products which provide undesired autoinhibitions of oxidation. That is, there are formed partial oxidation products which prevent substantial completion of the oxidation of the polymethyl substituted aromatic hydrocarbon feeds to the desired aromatic polycarboxylic acids. This autoinhibition is most pronounced in the oxidation of aromatic compounds having two methyl substituents on vicinal ring carbons, like 1, 2, 4 trimethylbenzene (pseudocumene). In the catalytic liquid phase oxidation of pseudocumene the autoinhibition has the effect of limiting trimellitic acid yields to the range of 65% to 75% mol. The effect of autoinhibition appears to be that of preventing the oxidation of methyl substituted phthalic acids to trimellitic acid and the oxidation of reducibie partial oxidation WO 98/55441 PCT/EP98/03291 products such as formyl phthalic acids and methylol phthalic acids to trimellitic acid. Trimellitic acid appears to have an autoinhibiting effect on the oxidation of pseudocumene rather than an auto-oxidative effect. Some free radical mechanisms are believed to adversely affect the oxidation of methyl phthalic acids and the reducible partial oxidation products. The same or a similar autoinhibition occurs in the catalytic liquid phase oxidation of other trimethyl substituted aromatic compounds having only two methyls on vicinal ring carbons.

It has been found that a higher thermal driving force, higher reaction temperature or a selected stage of use of higher reaction temperature in batch operation, effectively results in higher trimellitic acid yields.

However, reaction temperatures above 230 0 -240 0

C

induce decarboxylation of trimellitic acid to phthalic acids and the ultimate result is a lower rather than a higher trimellitic acid yield.

The preparation of trimellitic acid by oxidation of pseudocumene in the presence of lower alkanoic acid reaction solvents presents a problem of its own.

Trimellitic acid is substantially soluble in the reaction solvent media to make recovery of more than about 65% to 70% of trimellitic acid commercially not feasible by the crystallization thereof from the liquid reaction mixture. Thus the lower the oxidation yield of trimellitic acid the lower will be the recovery of the desired product from a crystallization technique.

WO 98/55441 PCT/EP98/03291 Trimellitic acid recovery can be increased by removing a substantial portion or all of the acidic reaction solvent. However, when there are also present large amounts of such by-products as benzoic acid (two -COOH groups being lost by decarboxylation), the three phthalic acid isomers, methylphthalic acids, reducible partial oxidation products such as formyl phthalic acids and methylol phthalic acids and the like, there are too many closely related acid impurities in admixture with trimellitic acid to make a commercially feasible recovery of it in a suitably pure form. A recovery system wherein the total liquid reaction mixture is distilied, trimellitic acid is dehydrated to its intramolecular anhydride and this anhydride is distilled off, recovered and becomes a feasible commercially recovery system, provided a high yield of trimellitic acid and a lower yield of methylphthalic acids and reducible partial oxidation products is obtainable.

It has been discovered, and described in the literature, that the prior oxidation problems which came from the autoinhibitions during pseudocumene oxidation in a catalytic liquid phase system was provided, in general, by having too active a catalyst system in the beginning and during about 2/3 of the oxidation and a system not sufficiently active in the last of the oxidations. By oxidation rate studies applied to the oxidation of the second and the third methyl groups it has been shown how the catalytic liquid phase oxidation of pseudocumene could be WO 98/55441 PCT/EP98/03291 conducted in order to achieve higher yields of conversion.

The oxidation rate studies have shown that the yields of liquid phase oxidations of trimethyl substituted aromatic hydrocarbons, such as pseudocumene, can be improved using, during the initial stage of the oxidation of pseudocumene, a combination of side-chain initiator bromine with heavy metal catalysts having the oxidation potential at least equal to that of cobalt and manganese.

In the following stages of reaction, the temperature is increased while additional catalyst consisting of manganese, alone or in association with zirconium and/or cerium, is added with additional bromine promoter.

The staged batch mode of operation of the reaction, as described in U.S. Pat. No 3,920,715 and in other patents, although providing a more efficient system of reaction compared with the previous status of the technology of oxidation of trimethyl substituted aromatic hydrocarbons, still presents several disadvantages.

The consumption of metal catalysts substantially contributes to the cost of production of trimellitic anhydride.

Recycle of catalyst is mentioned in the literature, but requires rather complex and expensive procedures for freeing the metals from the contaminants.

Furthermore, batch oxidations have disadvantages because the. concentration of the hydrocarbon to be oxidized is high at the start of the reaction and its rate of oxidation is difficult to control. This leads to a low concentration of dissolved oxygen and to an increased amount of radical reaction producing high boiling point by-products which reduce the yield.

Thermally induced destruction of methyl groups occurs, leading to the formation of xylenes, which become oxidized to dicarboxylic acids, contributing to yield losses.

Finally the batch mode of operation requires additional operating costs and presents higher safety hazards due to the risk of forming' explosive mixtures particularly at the transient conditions of the reaction start and end of the batch).

It is an object of the present invention to o overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

20 DESCRIPTION OF THE INVENTION It is an object of the present invention to provide an improved process for production of tricarboxylic acids having only two carboxylic acid o*oo group substituents on vicinal ring carbons, such as 25 trimellitic acid, by means of catalytic liquid phase oxidation of trimethyl substituted aromatic hydrocarbons having only two methyl substituents on vicinal ring carbons, such as pseudocumene.

As described in the background, the liquid phase oxidation reaction, as applied to pseudocumene, is very difficult and has been industrially practiced as a batch process because the reaction product, trimellitic acid, is a poison for the catalyst.

According to a first aspect the invention provides a process for the production of aromatic tricarboxylic acids, by continuous liquid phase oxidation of a trimethyl aromatic feedstock, comprising: a)providing at least three stages of reaction in series consisting of one initial reactor, ond or more than one intermediate reactor, one final reactor; b)introducing into the initial reactor an oxygen containing gas, the aromatic feedstock, a solvent, and a primary catalyst provided by transition or variable valence metals; the catalysis being 15 promoted by the use of a ketone or an aldehyde; 'the temperature at the initial reactor being oee -:prtuea between 90 0 C and 140°C; c) introducing into the intermediate reactor(s) an oxygen containing gas, the effluent from the first reactor and a secondary catalyst consisting of a e heavy metal oxidation catalyst; in the eeee intermediate reactor(s) the catalysts being provided by the addition of bromine in the form of ego• organic or inorganic compounds; the temperature at the intermediate reactor(s) ranging between 1300 to 190 0

C;

d)introducing into the final reactor an oxygen containing gas, the effluent from the intermediate reactor(s) and a stream of mother liquor 4 containing the catalyst recovered from the product separation section and reactivated by the addition of bromine; the temperature at the final reactor being from 170 0 C to 220 0

C;

e)operating the reaction system at a pressure not lower than the minimum pressure necessary to maintain the solvent at the liquid phase; f)using as oxidizing medium oxygen being dissolved in a recycling stream of reaction gas effluents; g)adjusting the oxygen content in each stage of reaction in order to assure an adequate oxidation rate, assuring at same time that for safety purposes, the oxygen concentration in the exhaust gas does not exceed 8% by volume.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

20 The use of a ketone, such as methylethylketone, or of an aldehyde, such as acetaldehyde as promoter of the catalysis provided by transition or variable valence metals in liquid phase oxidations for the preparation of benzene carboxylic acids, has been described in US **gig 25 Pat. No 2,245,528 and practiced in the industry for the production of terephthalic acid and isophthalic acid.

However, such an aldehyde or ketone promoted catalysis was found not suitable in the oxidation of trimethylbenzene being only capable of converting the trimethylbenzene to its benzene mono- and dicarboxylic derivatives.

RIt has now been found that the use of a ketone or of an aldehyde, preferably acetaldehyde, in the initial stage of a reaction system comprising at least 3 stages of reaction in series, eliminates the risk of autoinhibition, reduces the amount of metal catalysts and of bromine activator and allows to recycle to the final stage of reaction a fraction of the mother liquor, containing metal catalyst, recovered after crystallization and filtration of the reactor effluent, allowing a substantial reduction of the global consumption of metal catalysts.

The performances of this novel process of catalytic oxidation of pseudocumene to trimellitic acid *o o *o* WO 98/55441 PCT/EP98/03291 are enhanced by the use of oxygen as the oxidizing agent.

Optimum overall performances can be achieved by optimising, at each stage of reaction, the major operating parameters such as the temperature, the partial pressure of oxygen in the oxidizing medium, the concentration of a primary and secondary metal catalysts, the concentration of the promoter (aldehyde or ketone in the initial stage, bromine in the intermediate and final stages), the amount of mother liquor containing catalyst being recycled to the final stage of reaction.

The embodiments of the process object of the present invention can be illustrated by the following examples: EXAMPLE (STANDARD) STANDARD experiments were carried out in three liter fully equipped autoclaves connected in series.

Oxygen, dissolved in a recycling off gas stream, was used as the oxidizing agent.

A feedstock mixture was continuously introduced in the first autoclave consisting of 480 gr of pseudocumene, 960 gr of acetic acid containing 4% of water, 50 g of acetaldehyde and with a primary catalyst consisting of 360 mg of cobalt, supplied in the form of cobalt acetate, and of 240 mg manganese, supplied in the form of manganese acetate.

The mixture was oxidized at a temperature of about 125 0 C. The oxidizing medium was oxygen with 16% vol concentration in a gaseous stream consisting of carbon WO 98/55441 PCT/EP98/03291 dioxide, carbon monoxide, water and acetic acid vapours plus minor amounts of inerts.

The effluent from the first autoclave was continuously transfered to the intermediate autoclave where an additional stream of catalytic component (secondary catalyst) was added, consisting of 5 mg of zirconium, supplied in the form of zirconium octanoate, of 20 mg of cerium, supplied in the form of cerium chloride, and of 400 mg of bromine supplied in the form of hydrogen bromide.

In the intermediate autoclave the oxidation was continued at a temperature of about 170 0 C. The oxidizing medium was oxygen with 19% by vol.

concentration in the gaseous stream of the abovementioned composition.

The effluent from the intermediate autoclave was continuously transferred to the third autoclave where an additional stream was added consisting of recycling mother liquor containing about 150 mg of metals (Co-Mn- Ce-Zr) catalysts, added with 250 mg of fresh hydrogen bromide.

In the third autoclave the oxidation was completed at a temperature of about 195 0

C.

The oxidizing medium was oxygen with 21% by vol.

concentration in a gaseous stream of the abovementioned composition.

The overall performances of the reaction resulted as follows: yield in trimellitic acid: 92,8% mol yield in by-product: 3,4% mol WO 98/55441 PCT/EP98/03291 yield in CO+CO 2 3,8% mol Following crystallization, a crude solid stream of trimellitic acid was separated by filtration and a fraction of mother liquor, containing about 150 mg of metals (Co, Mn, Ce, Zr) was recycled to the third autoclave.

The pressure in the reaction system was about 24 Bar.

COMPARATIVE EXAMPLE A The STANDARD experiment was repeated, replacing the acetaldehyde fed to the first autoclave with 250 mg of bromine and operating the first autoclave at a temperature of 1600C instead of 1250C.

The performance of the reaction resulted as follows: yield in trimellitic acid 82,7% mol yield in by-products: 12,2% mol yield in CO+CO 2 5,1% mol COMPARATIVE EXAMPLE B The experiment described in COMPARATIVE EXAMPLE A was repeated without recycling mother liquor containing metal catalysts to the third autoclave.

The performances of the reaction resulted as follows: trimellitic acid product yield: 84,2% mol by-product yield: 10,9% mol CO+C02 yield: 4,9% mol WO 98/55441 PCT/EP98/03291 COMPARATIVE EXAMPLE C The experiment described in COMPARATIVE EXAMPLE B was repeated doubling the amount of metal catalyst and of bromine fed to each reactor.

The performances of the reaction resulted as follows: trimellitic acid product yield: by-product yield: CO+CO, yield: 89,9% mol 5,2% mol 4,9% mol COMPARATIVE EXAMPLE D The STANDARD experiment was repeated in identical conditions using air instead of oxygen.

The performances of the reaction resulted as follows: trimellitic acid product yield: by-product yield: CO+COz yield: 90,2% mol 4,9% mol 4,9% mol

Claims (30)

1. A process for the production of aromatic tricarboxylic acids, by continuous liquid phase oxidation of a trimethyl aromatic feedstock, comprising: a)providing at least three stages of reaction in series consisting of one initial reactor, one or more than one intermediate reactor, one final reactor; b)introducing into the initial reactor an oxygen containing gas, the aromatic feedstock, a solvent, and a primary catalyst provided by transition or variable valence metals; the catalysis being promoted by the use of a ketone or an aldehyde; "the temperature at the initial reactor being between 900C and 1400C; l c)introducing into the intermediate reactor(s) an oxygen containing gas, the effluent from the first .reactor and a secondary catalyst consisting of a g heavy metal oxidation catalyst; in the intermediate reactor(s) the catalysts being provided by the addition of bromine in the form of organic or inorganic compounds; the temperature at the intermediate reactor(s) ranging between 1300 to 1900C; d)introducing into the final reactor an oxygen containing gas, the effluent from the intermediate reactor(s) and a stream of mother liquor -containing the catalyst recovered from the product separation section and reactivated by the addition of bromine; the temperature at the final reactor being from 170°C to 2200C; e)operating the reaction system at a pressure not lower than the minimum pressure necessary to maintain the solvent at the liquid phase; f)using as oxidizing medium oxygen being dissolved in a recycling stream of reaction gas effluents; g)adjusting the oxygen content in each stage of reaction in order to assure an adequate oxidation rate, assuring at same time that for safety purposes, the oxygen concentration in the exhaust gas does not exceed 8% by volume. 15

2. A process according to claim 1 wherein the aromatic tricarboxylic acid is trimellitic acid.

3. A process according to claim 1 or 2 wherein the trimethylaromatic feedstock is pseudocumene.

4. A process according to any one of the preceding claims wherein the primary catalyst is cobalt and/or manganese.

5. A process according to any one of the preceding claims wherein the catalysis is promoted by methylethylketone.

6. A process according to any one of the Tpreceding claims wherein the secondary catalyst is cerium and/or zirconium.

7. A process according to any one of the preceding claims wherein the bromine is in the form of hydrogen bromide.

8. A process according to any one of the preceding claims wherein the temperature at the initial reactor is between 1200C and 1300C.

9. A process according to any one of the preceding claims at the initial reactor the solvent is acetic acid and the catalysis is promoted by acetaldehyde. *o

10. A process according to any one of the o "..preceding claims the temperature at the intermediate reactor is between 1600 and 1800C. 20

11. A process according to any one of the ee preceding claims the temperature at the final reactor is between 1800 and 2100C.

.12. A process according to any one of the preceding claims wherein the ketone or aldehyde promoter fed to the first reactor varies from 5% to wt, relatively to trimethylbenzene.

13. A process according to any one of the preceding claims wherein the ketone or aldehyde promoter, fed to the first reactor varies from 8% to 12%, wt relatively to trimethylbenzene.

14. A process according to any one of the preceding claims wherein the concentration of primary catalyst varies between 0,1% and 0,3% by weight, relatively to the trimethylbenzene feed.

A process according to any one of the preceding claims wherein the concentration of primary catalyst varies between 0.1% and 0.2% wt with respect to the trimethylbenzene feed.

16. A process according to any one of claims 4-15 wherein the concentration of manganese in the primary oo catalyst is between 25% to 50% wt, referred to the total weight of primary catalyst. o

17. A process according to any one of claims 4-16 wherein the concentration of manganese in the primary eoe catalyst is from 35% to 45% wt, referred to the total weight of primary catalyst.

18. A process according to any one of the preceding claims wherein the concentration of the secondary catalyst varies between 0,002% and 0,01% wt with respect to the trimethylbenzene feed.

19. A process according to any one of the 0 preceding claims wherein the concentration of the secondary catalyst varies between from 0,004% to 0,006% by weight, with respect to the trimethylbenzene feed.

A process according to any one of claims 6-19, wherein the concentration of zirconium in the secondary catalyst is between 10% to 40%, referred to the total amount of secondary catalyst.

21. A process according to any one of claims 6-20, wherein the concentration of zirconium in the secondary catalyst is from 15% to 25% wt of the total amount of secondary catalyst.

22. A process according to any one of the 5 preceding claims wherein the concentration of bromine "fed to the intermediate reactor(s) varies between 0,06% and 0,15%, relatively to the trimethylbenzene feed.

23. A process according to any one of the preceding claims wherein the concentration of bromine fed to the intermediate reactor(s) varies between 0,08% eeoc to 0,12% wt, relative to the trimethylbenzene feed.

24. A process as defined in any one of the preceding claims wherein a fraction of the mother liquor stream recovered after the separation of the tricarboxylic acid, is recycled to the final reactor and contains from 10% to 40% of the total fresh metal catalysts fed to the reaction system.

A process as defined in claim 24 wherein a fraction of the mother liquor stream recovered after the separation of the tricarboxylic acid, is recycled to the final reactor and contains from 20% to 30% wt of the total fresh metal catalysts fed to the reaction system.

26. A process according to any one of -the preceding claims wherein the concentration of bromine fed to the final reactor ranges between 0,02% to 0,1% wt, with respect to the trimethyl benzene feed.

27. A process according to any one of the preceding claims wherein the concentration of bromine 15 fed to the final reactor varies between 0,04 to 0,08% wt, with respect to the trimethyl benzene feed.

28. A process according to any one of the preceding claims wherein the product is trimellitic 20 anhydride.

29. An aromatic tricarboxylic acid when prepared 0 by a process according to any one of the preceding claims. A process for the production of an aromatic tricarboxylic acid substantially as herein described with reference to any one of the examples. R-A

30 DATED this 5th Day of July 2000 SCHEMICAL TECHNOLOGIES AND KNOW-HOW LIMITED