IL24531A - Method for purifying terephthalic acid - Google Patents

Description

24531/2 Method for purifying terephthalic acid ALLIED CHEMICAL CORPORATION C. 23479 The present invention relates to a method for the purification of terephthalic acid* More particularly it relates to a method of obtaining terephthalic acid in a purified form from crude terephthalic acid containing aromatic aldehyde impurities, and especially from crude terephthalic acid obtained by the liquid phase oxidation of para-bis( lower alkyl)benzenes* Terephthalic acid is used in large quantities as the starting material for the preparation of fibre-forming polyesters of the linear polyalkylene terephthalate class* Recent improvements in the preparation of the latter, e.g. via direct condensation of ethylene glycol with terephthalic acid, require terephthalic acid of exceptionally high purity* A number of processes are known for the manufacture of terephthalic acid* One general process, which is of interest commercially because of the relatively low cost and availability of the raw material, involves liquid phase oxidation of p_-xylene or other p_~bis( lower alkyl )benzenes with oxygen or an oxygen-containing gas in the presence of a metallic oxidation catalyst, such as a cobalt salt, usually with the assistance of a promoter or initiator such as an aldehyde or a ketone* Processes of this type are disclosed, for example, in United States Patents 2, 552, 268 (Filed 22nd March, 19 9 ) J 2, 788, 367 (Filed 5th March, 1953 ) 5 2, 853, 514 (Filed 22nd July, 1955 ) I 2, 952, 704 (Filed l4th November, 1957 ) and 2,959 , 613 (Filed 20th January, 1958 ); British Patent 825,975 (Filed 5th June, 1957 ) ; and Belgian Patent 621 , 323 (Filed 10th August, 1962 ) .

Terephthalic acid which has been obtained by such processes is usually contaminated with oxidation by-products such as p-toluic acid and 4-carboxybenzaldehyde, as well as with other amounts of the metallic catalysts employed, which result in the formation of badly discoloured polyalkylene terephthalate fibres of poor quality when the terephthalic acid is employed for their manufacture* Owing to the nature of terephthalic acid, ordinary methods of purification cannot be employed to obtain it from such contaminated terephthalic acid in a sufficiently pure fora to be useful in the manufacture of fibres of the linear polyalkylene terephthalate class* Terephthalic acid is a solid which does not melt and which cannot be distilled; it sublimes directly from the solid to the vapour phase at about 400°C. It is practically insoluble in water and in the usual organic solvents employed for purification of organic compounds under the conditions normally employed* Thus, only a portion of the aforementioned contaminantSj and particularly -carboxybenzaldehyde and cobalt acetate, can be removed by conventional washing of the crude acid, e*g» with hot glacial acetic acid and hot water* The impurities which are locked within the crystal masses of crude terephthalic acid are not removed* As noted in United States Patent No. 3,080,421 (Filed 18th November, 1958)1 when impure terephthalic acid is dissolved in hot acid concentrated sulphuric/ degradation of the impurities results in a darkening of the solution, whereas solutions of pure terephthalic acid in concentrated sulphuric acid are stable and remain substan~ tially colourless even when heated for several hours as high as 230°C* This provides a convenient test of purity and generally indicates that a terephthalic acid showing little or no darkening in hot concentrated sulphuric acid can be reacted with ethylene glycol to produce a polymer of excellent colour and melting point* directed to a purification process which includes dissolving impure terephthalic acid in hot concentrated sulphuric acid, cooling to precipitate terephthalic acid, converting the terephthalic acid thus obtained to an ammonium phthalate solution, treating the solution with activated carbon and regenerating terephthalic acid by acidification. To produce terephthalic acid of the desired purity, the process involves a special, tedious sequence of operations and requires use of concentrated sulphuric acid, which is a serious disadvantage* The present invention provides a method of purifying or terephthalic acid in which such disadvantages are reduced /eliaiftated According to the present invention there is provided a process for purifying terephthalic acid contaminated by impurities that cause formation of off-colour polyethylene terephthalate of inferior quality when the terephthalic acid is reacted with ethylene glycol, which comprises heating the contaminated terephthalic or cycloaliphatic 0r a mixture thereo: acid with an aliphatic/ketone containing 3 to 8 carbon atoms/ t a temperature of at least 150°C. , preferably at least 200°C« , and above the boiling point at atmospheric pressure of the mixture, under a pressure sufficient to maintain a substantial portion of ketone in the liquid phase to produce a solution of terephthalic acid in the liquid ketone, cooling the solution and separating the crystalline terephthalic acid which separates out* The aliphatic ketone may be open-chain or cyclic and mixtures of such ketones having 3 to 8 carbon atoms in the molecule may be used* The pressure required to maintain the ketone or autogenous, resulting vapourisation of the ketone and achieved by carrying out the heating in a closed vessel, or may be achieved by introdu6ing into with the terephthalic acid under the conditions employed, e.g. nitrogen* The fact that the impure terephthalic acid will dissolve in the ketones under the stated conditions is surprising because the ketones are normally at best poor solvents for terephthalic acid* They are liquids which even at their boiling points at atmospheric pressure dissolve practically no terephthalic acid (less than 1% of their weight). The substantial insolubility of terephthalic acid in such ketones as acetone is employed in the processes of United States Patents 2,857,429 (Filed 11th April,1956) and 2,952,704 (Filed l4th November, 1957) as a means for separating it from impurities soluble therein* It is a feature of the present invention that the impurities of the type referred to above are sufficiently soluble in the ketones employed to permit repeated use of the ketones in a cyclic process involving reuse of the ketones in purifying subsequent quantities of contaminated terephthalic acid, thereby constituting an additional advantage over the use of sulphuric acid as a purifying agent. This also permits the purification process of the present invention to be combined with continuous processes for the manufacture of terephthalic acid of high purity from j «xylene and related p_^bis( lower alkyl)benzenes as, for example, a process of the type described in Belgian Patent 621,323 (Filed 10th August, 1962).

Suitable ketones for use in the practice of the present invention include, for example: Boiling Point at Atmospheric Pressure Ketone °C« acetone 56 2-butanone(methylethylketone) 82 cyclopentanone 131 2-methylcyclopentanone 139 4-heptanone 144 3—heptanone 148 cyclohexanone 156 2-methylcyclohexanone l66 2, 2-dimethylcyclohexanone 170 2«octanone 173 cycloheptanone 182 cyclo-octanone 195_197 Cycloaliphatic ketones containing 5 to 6 ring carbon atoms, and especially cyclohexanone are preferred* While it is preferred that the ketones are anhydrous, ketones containing up to about 10% by weight of water can be used* The amount of ketone employed in the practice of the present invention will vary with the individual ketone and the conditions employed. Since the process is essentially a dissolution of the crude terephthalic acid in the liquid ketone under sufficiently elevated temperature and pressure conditions to maintain the ketone in the liquid phase, followed by a crystallisation of pure terephthalic acid from the ketone, at least sufficient ketone is employed to accomplish dissolution of the terephthalic acid without necessitating the use of extremely high temperatures and pressures.

Preferably, about 5 to 25 parts of ketone (or mixture of ketones), per part by weight of crude terephthalic acid, are employed* Preferred temperatures range from about 200 to about 300°C* at pressures of about 100-200 p.s*i«g. (7-14 kg. /cm2 gauge). Although temperatures above 325°C* and pressures in excess of 200 p.s»i*g* (14 kg»/cm gauge) can be used, they add unnecessarily to the cost of the apparatus and fuel required* Preferably, the crystallization process of the present invention is carried out in an inert atmosphere (that is, an atmosphere non-reactive with terephthalic acid under the conditions employed), such as nitrogen, to avoid possible, adverse oxidation by air under the elevated temperature and pressure conditions* If desired, the pressure can be increased by maintaining said gas under pressure greater than the autogenous pressure developed* The period of time during which the mixture is maintained at said temperature and pressure also will vary* The mixture is maintained at least for a sufficient period of time to ensure dissolution of the terephathlic acid in the hot liquid ketonic solvent* Thus, under the preferred conditions set out above, the mixture is heated for a period from at least a quarter hour to about an hour* The purification of terephthalic acid containing polynierisable impurities such as 4-carboxybenzaldehyde would appear to be assisted by the heating under pressure for at least a quarter of an hour* At the conclusion of the heating period, the mixture is cooled to cause crystallisation of the terephthalic acid from the ketonic solvent* For example, the mixture is cooled to a temperature between the ambient temperature and 150°C. (or below the boiling point at atmospheric pressure of the ketonic solvent, in the case of these ketones boiling below 150°C* ) to retain the ketonic solvent in the liquid phase when the mixture is returned to atmospheric pressure* In general, the mixture is cooled to between 0° d 1 0°C.

Cooling can be carried out in any desirable way e»g» under elevated pressure, with or without intermittent release of pressure during cooling* It can be advantageously carried out by continuously passing a stream of the hot solution to a vessel at atmospheric pressure, thereby effecting rapid and total release of pressure ( "flashing" )· Cooling is advantageously effected at a rapid rate, for example at a rate of up to 10 C», preferably at , o o about ½ to 5 C., per minute.

A particularly advantageous purification of terephthalic acid is achieved by heating the crude terephthalic acid with about times its weight of cyclohexanone in a closed vessel at a temperature of about 250°C. and a pressure of about 150 p.s.i.g. ( 10. kg./ctn gauge) for about a half an hour, then cooling to a temperature of 110° to 150°C. to crystallise the terephthalic acid, followed by further cooling to ambient temperature and separation of the crystallised terephthalic acid from the remaining cyclohexanone solution of impurities.

The precipitated terephthalic acid is recovered by conventional means; for example, by filtration, decantation, or centrifugation. It is advantageously separated from the ketonic solution of impurities by filtration, and the last traces of mother liquor are removed from the filter cake of purified terephthalic acid by washing them for example, with a suitable solvent, which may be a solvent miscible with the ketone (e.g. the ketone itself, another ketone, an alcohol or an ether) or a liquid having a boiling point below 150°C. or having appreciable solubility in water, or both of said proporties. Removal of the residual traces of the latter solvent can be accomplished by drying the purified terephthalic acid at moderate temperatures, preferably under sub~atmospheric ' crystals with hot water, recovering the crystals separate from the wash water, and drying the recovered terephthalic acid crystals with hot air* No special equipment is required for carrying outthe process of the invention. A conventional pressure reaction vessel, advantageously equipped with an efficient agitator and heating and cooling means, constitutes suitable apparatus* The purification process of the present invention can be utilized in purifying terephthalic acid obtained in various ways* It is advantageously employed in removing organic oxidation products and residual catalysts present in crude terephthalic acids obtained by liquid phase oxidation of p_-bis( lower alkyl )benzenes, that is benzenes substituted by alkyl groups of 1 to 4 carbon atoms* It is especially useful in the purification of terephthalic acid contam-inated by impurities that cause formation of dark coloured solutions when said terephthalic acid is dissolved in concentrated sulphuric acid at about 230°C» , and especially terephthalic acid which is contaminated by 4-carboxybenzaldehyde and metallic impurities (especially cobalt impurities) and which has been obtained by the liquid phase oxidation of a p_-bis( lower alkyl )benzene, and especially xylene, in a lower alkonoic acid reaction medium in the presence of a metallic catalyst and an aliphatic oxo compound (aldehyde or ketone) as an activator* Thus, the process is of particular utility in the purification of crude terephthalic acids obtained by processes of the type disclosed in the patents referred to above, and, especially the processes of United States Patent 2, 853* 514 (Filed 22nd July, 1955 ) and Belgian Patent 621,323 (Filed 10th August, 1962 ) and similar processes.

The process of the present invention effects a marked crude terephthalic acid which forms a very dark coloured solution when heated with concentrated sulphuric acid at about 23Ο°C» can be purified in accordance with the present invention to provide a terephthalic acid which} when similarly treated with concentrated sulphuric acid, forms a solution coloured only a pale yellow, and which when polymerised with ethylene glycol provides substantially colourless polyethylene terephthalate of high quality and eminently suitable for use as textile fibre.

If it is desired to effect a more complete purification of the terephthalic acid, this can be readily accomplished in accordance with the present invention merely by repeating the crystallisation process* In this way it is possible to produce a terephthalic acid of such purity that only a very pale yellow solution is formed upon treatment with concentrated sulphuric acid as above.

In the case of terephthalic acid contaminated by substantial amounts of 4-carboxybenzaldehyde, it is possible in accordance with the present invention to remove 90-95 by weight or more of the 4-carboxybenzaldehyde from the terephthalic acid.

The following Examples further illustrate the invention. Parts and percentages are by weight and temperatures are in degree centigrade.

EXAMPLE 1 Crude terephthalic acid (25 parts, dry weight), containing 97% of terephthalic acid, 1.96% of 4-carboxybenzaldehyde and about 100 parts per million of cobalt, and which had been obtained by liquid phase air oxidation of j -xylene in acetic acid containing cobalt acetate and acetaldehyde, followed by washing of the resulting terephthalic acid with glacial acetic acid at Cyclohexanone ( 2875 parts) w s added, the autoclave was closed* nitrogen was introduced to displace the air over the mixture, and the mixture was heated to 250° with efficient agitation, the pressure rising to 150 p.s.i.g. (IO.5 kg./cm gauge). The resulting solution was maintained at 250° for a half hour. The mass was then cooled to 110° over a half hour period, with accompanying fall in pressure, and the resulting slurry was filtered* The filter cake of terephthalic acid was washed with 1000 parts of cyclohexanone and then slurried with 3OOO parts of water at 95° for 15 minutes. The terephthalic acid was recovered by filtration, washed with 1000 parts of water at 95° and dried at 95° in vacuo. The resulting terephthalic acid, which was recovered in substantially quantitative yield, contained 0·03¾ of 4-carboxybenzaldehyde and 12 p.p.ra. of cobalt. When reacted with ethylene glycol, a substantially colourless polyethylene terephthalate forming fibres of good quality was produced.

EXAMPLE 2 The purified terephthalic acid ( 25Ο parts) obtained in Example 1 was again subjected to the process of Example 1. The resulting terephthalic acid contained 0.005$ of 4-C rboxybenzaldehyde and 6 p.p.m. of cobalt. When reacted with ethylene glycol, polyethylene terephthalate was obtained having even less colour than that of Example 1 and producing fibres of excellent quality* Sample of the crude terephthalic acid used as starting material in above Example 1 and the purified products of Examples 1 and 2 were tested for purity according to the following procedure: The sample of terephthalic acid (l part) was added to concentrated sulphuric acid ( 9 parts of 95· 5-96· 5% sulphuric acid); the mixture was heated to 235°j and the colour of the resulting solution was observed.

TABLE I Colour of l i solution in concentrated Sample sulphuric acid Crude terephthalic acid black Purified terephthalic acid of Example 1 pale yellow Purified terephthalic acid of Example 2 very pale yellow EXAMPLE 3 Part A. The procedure of Example 1 was repeated substantially as described except that 250 parts of another crude terephthalic acid which had been obtained similarly, but which contained 1*31% of 4-carboxybenzaldehyde, was dissolved in 3000 parts of cyclohexanone containing 8% of water by heating at a o 2 temperature of 200 and a pressure of 150 p.s.i.g. ( 10« 5 kg»/cra gauge) the terephthalic acid was precipitated by cooling the nass to 60° ; and the purified terephthalic acid was recovered by filtration and was washed with 1000 parts of cyclohexanone. The resulting . product was divided into two equal portions.

Part B» One portion was treated with hot water and dried substantially as described in Example 1· The resulting purified terephthalic acid contained 0» 12¾ of 4-«carboxybenzaldehyde« Part C» The other portion was again subjected to the crystallisation process of Part A of this Example, employing cyclohexanone containing 8% water* The resulting terephthalic acid contained 0.056% of 4~carboxybenzaldehyde» EXAMPLE 4 A crude terephthalic acid ( 250 parts) of the type employed in Example 1, but containing 1 * 3% 4-*carboxybenzaldehyde, was 4750 parts (instead of 2875 parts) of cyclohexanone* The 4-carboxybenzaldehyde content of the terephthalic acid obtained EXAMPLE 5 The product of above Example 4 was again subjected to the process of Example 4· The yield of purified terephthalic acid obtained was 242 parts, which corresponds to 96·77¾ of theory-based on the crude terephthalic acid employed in Example 4· The 4-carboxybenzaldehyde content of the resulting purified terephthalic acid was 0«004%.

EXAMPLE 6 For purposes of comparison, 250 parts of crude terephthalic acid of the type employed in Example 1 (but containing 1·3½ of 4-carboxybenzaldehyde) was slurried with 2875 parts of cyclohexanone and the slurry was refluxed at atmospheric pressure for 4 hours, then cooled to* 60° and filtered. The recovered terephthalic acid was washed with cyclohexanone, then further treated and dried, as described in Example 1· It contained 1* 119. of 4-carboxybenz-aldehyde*

Claims (11)

HAOTG MOW particularly described and ascertained the nature of our said invention and in what manner the same is to he performed, we declare tha t what we claim is

1. · Process for purifying terephthalic acid contaminated by impurities that cause formation of off-colour polyethylene terephthalate of inferior quality when the terephthalic acid is reacted with ethylene glycol, which comprises heating the or cycloaliphatic ^ ■' contaminated terephthalic acid with an aliphatic/ketone containing or a mixture thereof 0 ^>l* 3 to 8 carbon atoms /it a temperature of at least 150 C« , and above the boiling point at atmospheric pressure of the mixture, under a pressure sufficient to maintain a substantial portion of the ketone in the liquid phase to produce a solution of terephthalic acid in the liquid ketone, cooling the solution and separating the crystalline terephthalic acid.

2. » Process according to claim 1 in which the terephthalic acid has been obtained by liquid phase oxidation of a p_«bis( lower alkyl )benzene and is contaminated by impurities that cause formation of dark coloured solutions when the said terephthalic acid is dissolved in concentrated sulphuric acid at about 230°C.

3. · Process according to claim 2 in which the terephthalic acid is contaminated by 4-carboxybenzaldehyde and metallic impurities and has been obtained by the liquid phase oxidation of a p_-«bis( lower alkyl )benzene in a lower alkanoic acid reaction medium in the presence of a metallic catalyst and an aliphatic oxo compound activator*

4. * Process according to claim 3 in which the metallic impurities comprise cobalt*

5. · Process according to any one of claims 2-4 in which the _-bis( lower alkyl )benzene is p-xylene*

6. · Process according to any one of claims 1-5 n which the terephthalic acid and ketone are heated in the liquid phase at a temperature of at least 200°C* under a pressure of at least 100 p*s*i«g* (7 kg*/cra gauge)*

7. · Process according to any of claims 1 to 6 in which the ketone is a cycloaliphatic ketone having 5 to 6 ring carbon atoms*

8. * Process according to claim 7 in which the ketone is cyclohexanone*

9. · Process according to any one of claims 1-8 in which the terephthalic acid and ketone are heated at 200° to 300°C* for at least a quarter of an hour*

10. * Process according to claim 9 in which the terephthalic acid is heated with about 5-25 times its weight of cyclohexanone at a pressure of 100 to 200 p*s*i*g* (7*"l kg*/cm gauge) for l/4 - 1 hour, and the resulting solution is cooled at a rate of up to 10°C* per minute*

11. * Process according to claim 1 substantially as hereinbefore described* 12· Process for the purification of terephthalic acid substantially as described in any one of the foregoing Examples 1-5· 13· Terephthalic acid obtained by a process claimed in any one of claims 1-12· Bated this 26th day of October, 1965 For the/Applicants