GB1567377A - Preparation of oxalic acid and its esters - Google Patents

Description

(54) PREPARATION OF OXALIC ACID AND ITS ESTERS (71) We, MONTEDISON S.p.A., of Largo Donegani 1/2, 20121 Milano, Italy, an Italian Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the preparation of oxalic acid dialkyl esters and optionally, from these, of oxalic acid and salts thereof.

Oxalic acid and its esters are important compounds of industrial interest. Thus, the acid finds use in the textile industry as an auxiliary stripping agent in the dyeing of wool and as a bleaching agent for natural fibres. It is also used as a pickling agent for metal surfaces, especially for copper. In industry it is also known for use as a dehydrogenating agent in condensation reactions. The esters are known solvents, such as for instance the diethylester for cellulose.

It is known to prepare oxalic esters by oxidative reaction of carbon monoxide and monobasic alcohols using oxygen and also using quinones, preferably in a substantially anhydrous medium, and catalyzed by Redoxsystems. These latter in general consist of the finely subdivided metal or of soluble salts or complexes (citrates, chelates) of a noble metal of the Pt group, such as for instance Pd, Os, in conjunction with a salt and/or a complex of another metal more electro-positive than the noble metal, such as Fe, Co, Ni, Cu, Mn, etc., possessing more than one oxidative state. The reaction is preferably conducted in the presence of co-catalysts and/or complexing agents consisting of soluble salts of alkali metals, such as LiCI or KC1.

Nevertheless, processes of this type, because of the simultaneous occurrence of secondary reactions leading to the formation of carbonates, CO2, and unwanted esters such as acetates and formates cannot be considered fully satisfactory from the industrial point of view having regard for the low yields and for the relatively burdensome separation and purification operations involved.

Moreover, the use of gaseous CO/O2 mixtures with the corresponding risk of explosions constitutes a further serious obstacle to industrial application.

At the same time the other known processes, for instance dehydrogenation of sodium formate and conversion to calcium oxalate followed by acidification, or oxidation of propylene with HNO3 catalized by e.g. Fe or Cr, do not lead to satisfactory results because of the considerable technological and operational difficulties involved, especially for mass production.

It is an object of the invention to provide a simple and economical process for the preparation of esters of oxalic acid (and indirectly of oxalic acid itself) relatively free from the drawbacks of the prior processes indicated above, and in particular affording high yields and purity of the product.

The invention consists in a process for preparing an oxalic di(CI -C8) alkyl ester by reacting a cupric (C g -C8) alkoxy halide with carbon monoxide in the presence of a catalyst promoting the formation of the oxalic dialkyl ester.

The preferred alkyl group, corresponding to the alkoxy group of the cupric alkoxy halide, is one containing up to 4 carbon atoms, especially methyl or ethyl. The preferred halide is the chloride or bromide, i.e. the preferred reagents are cupric methoxy or ethoxy chloride or bromide.

The discovery of this reaction is the more surprising, considering that it is known to react copper compounds of the above-indicated type type with CO. The products have, however, been exclusively the corresponding diester of carbonic acid, formed quantitatively from two molecules of the cupric alkoxy halide and one of carbon monoxide, these giving one molecule of the carbonic diester and two of the cuprous halide. There was no indication in the prior art that catalysts might be found to direct the reaction along any other route.

The new reaction is preferably conducted in a substantially anhydrous organic liquid medium, suitably in an inert solvent. It is preferably conducted at atmospheric or superatmospheric pressure.

As a catalyst there may be used Pd salts soluble in the reaction medium or mixtures thereof such as halides, sulphate, nitrate, acetylacetonate or acetate, preferably Pd (acetylacetonate)2. It is also possible to use metallic Pd including e.g. Pd on carbon, or complexes of zero-valent Pd which are in general well known such as Pd complexes with binders including phosphines or dibenzylidene-acetone.

The molar ratio of palladium with respect to the cupric compound is preferably from 0.0001 to 0.1 mols of Pd per 1 mol of the copper compound. Ratios different from these are acceptable but do not appear to confer any advantage.

The inert reaction medium may consist of monofunctional alkanols of up to 8 carbon atoms, and preferably up to 4 carbon atoms; methyl and ethyl alcohols are particularly suitable. Other suitable solvents, which may be used with the alkanol if desired, include aliphatic aromatic hydrocarbons which are inert under the reaction conditions, and mixtures thereof.

Thus, for example, mixtures containing up to 75 - 959 of benzene and 25 - 5% of alcohol have been found very suitable. Other inert solvents which may be used include benzene, acetone, ethylacetate and tetrahydrofurane.

The carbon monoxide may be used in the presence or absence of H2, so that it is possible to use synthesis gas. The carbon monoxide partial pressure may suitably be between 1 and 100 atmospheres absolute.

The most useful reaction temperature range is from 20 to 2000C, with from 50 to 1200C being preferred.

The reaction times may vary, depending on the temperature and the pressure employed, within wide limits.

Whenever desired, the use of CO in admixture with inert gases is admissible.

The yields of oxalic ester are generally practically quantitative with respect to the CO and the cupric compound. In the case of palladium this acts exclusively as a catalyst, and is not consumed.

The separation of the reaction product from the reaction medium solvent and from the catalyst may be easily achieved e.g. by distillation. From the ester the acid is easily obtainable by hydrolysis using conventional methods.

The distillation residue, containing the corresponding cuprous salt and the catalyst, may be used for further reaction after regeneration of the cupric compound, which may be effected by known methods, for instance by oxidation with air and/or oxygen in a suitable alkanolic medium.

The cupric alkoxy halide may also be used in its known form as a complex with a basic organic binder, such as for instance pyridine or picoline.

Because of the relatively mild operational conditions, the process of the invention is particularly convenient. Other advantages consist in the selectivity for the desired products and in the reduction of the operational risks of explosivity, in the absence of CO/O2 mixtures.

Also, of particularly merit is the possibility of using mixtures of CO and hydrogen as such synthesis gas, without detriment to the process.

The following examples illustrate how the invention may be carried into effect.

EXAMPLE 1 Into a stainless steel autoclave of 1 litre capacity, fitted with a glass vial, there were charged 30 ml of methanol, 0.15 g of Pd (acetylacetonate)2 and 3.06 g of Cu(OCH3)CI.

Thereupon CO was fed in up to 100 atm. and the temperature was brought up to 450C. The reaction mass was then held for 6 hours whilst stirring at the same temperature. The reaction mass was then distilled and there were obtained 1.39 grams of methyl oxalate having a boiling point of 65" - 67"C/12 mm Hg and a melting point of 530C.

The yield based on the reacted CU(OCH3)CI was 100%.

EXAMPLE 2 Proceeding in the same way as in example I, 25 ml of benzene, 5 ml of methanol, 3.43 grams of Cu(OCH3)C1, and 0.15 g of Pd (acetylacetonate)2 were charged into the autoclave.

Under the same operational conditions as in example 1 there were obtained 1.56 grams of methyl oxalate in quantitative yield, based on the reacted Cu(OCH3 )Cl.

EXAMPLE 3 Proceeding as in example 1, but using 3.15 g of Cu(OCH3)CI and holding the autoclave for 5 hours at room temperature, 1.43 g of methyl oxalate were obtained, again representing a yield, on the reacted Cu(OCH3)Cl, of 100%.

EXAMPLE 4 Proceeding as in example 1, but using only 2.91 g of Cu(OCH3)C1, and a reaction time of only 4 hours, 1.08 g of methyl oxalate was obtained. This amounts to a yield on the reacted Cu(OCH3)Cl of 82%.

EXAMPLE 5 An autoclave as in example 1 was charged with 30 ml of methanol, 0.15 g of Pd (acetyl acetonate)2,and 3.12 g of Cu(OCH3)C1. Then, CO was fed in up to 12 atm. and the autoclave left at room temperature for 11 hours.

1.41 grams of methyl oxalate were obtained a yield on the reacted Cu(OCH3)C1 of 100%.

EXAMPLE 6 Proceeding as in example 1, the autoclave was charged with 3 ml of methanol, 30 ml of C6H6 ,0.15 g of Pd (acetylacetonate)2 and 2.88 g of Cu(OCH3)CI. Then CO and H2 were fed in up to a partial pressure of 50 atm. each, and the autoclave was heated to 450C. The autoclave was then left at this temperature for 6 hours. 1.10 g of methyl oxalate were obtained, representing a yield on the reacted Cu(OCH3)C1 of 85%.

EXAMPLE 7 Proceeding according to example 1, the autoclave was charged with: 2 ml of methanol, 30 ml of benzene, 0.15 g of Pd (acetylacetonateh, and 3.28 g of Cu(OCH3)C1. Then the autoclave was pressurised with CO and H2 upto 50 atm.

each and the whole was brought up to a temperature of 60"C.

The autoclave was then left at this temperature for 2 hours. 1.488 g of methyl oxalate were produced, representing a yield, on the reacted Cu(OCH3)C1, of 100%.

EXAMPLE 8 Proceeding as in example 1, the autoclave was charged with: 30 ml of benzene, 0.15 g of Pd (acetylacetonate)2 and 2.88 g of Cu(OCH3) Cl. Then CO was fed in up to a pressure of 50 atm. and the temperature was brought to 450C.

The autoclave was left at this temperature for 6 hours. There were obtained 0.466 grams of methyl oxalate.

EXAMPLE 9 The procedure of example 8 was followed, except that the temperature in the autoclave was 700C. There were obtained 1.174 g of methyl oxalate.

EXAMPLE 10 The procedure of example 8 was followed, except that the reaction time was 2 hours. There were obtained 1.02 g of methyl oxalate.

EXAMPLE 11 The procedure of example 8 was followed, except that instead of benzene there was- used acetone. There were obtained 0.646 g of methyl oxalate.

EXAMPLE 12 The procedure of example 8 was followed, except that instead of benzene there was used ethylacetate. There were obtained 0.656 g of methyl oxalate.

EXAMPLE 13 The procedure of example 8 was followed, except that instead of benzene there was used tetrahydrofurane. There were obtained 0.692 grams of methyl oxalate.

EXAMPLE 14 Proceeding as in example 1,30 ml of benzene, 2 ml of methanol, 0.15 g of Pd (acetylacetonate)2 and 2.94 g of Cu(OCH3)C1 were charged into the autoclave. Then CO was fed in upto a pressure of 50 atm. and the temperature was brought to 600C. The autoclave was then left at this temperature for 2 hours.

There were obtained 1.39 g of methyl oxalate.

The yield on the reacted Cu(OCH3)Cl was 100%.

EXAMPLE 15 Proceeding as in example 1, but in the absence of solvents, the autoclave was charged with 0.6 g of Pd (acetylacetonate)2 and 2.3 g of Cu(OCH3 )Cl. The autoclave was then pressurised with CO to 100 atm. and the temperature was brought to 600C. The autoclave was then left at this temperature for 6 hours. There were obtained 0.33 g of methyl oxalate.

EXAMPLE 16 (comparison example in the absence of catalyst) Following the procedure of example 1, the autoclave was charged with 40 ml of methanol and 2.88 g of Cu(OCH3)Cl. Then CO was fed in upto a pressure of 100 atm. and the temperature was brought to 500C. The autoclave was then left at this temperature for 4 hours.

No trace of oxalate could be found.

EXAMPLE 17 3.8 g of Cu(OCH3)Br were prepared in situ in an autoclave as described in example 1, from CuBr2 (5 grams) and CH3ONa (1.2 grams) in 30 ml of methanol.

0.15 g of Pd (acetylacetonate)2 were then added. The autoclave was then pressurized with CO upto 50 atm. and was held at 450C for 6 bours, whilst stirring. There were obtained 0.558 g of methyl oxalate.

EXAMPLE 18 Proceeding as in example 1, the autoclave was charged with 2.9 g of Cu(OCH3)Cl, 30 ml of benzene, 2 ml of methanol, and 0.15 g of Pd (acetylacetonate)2. The mixture was then kept stirred at 600C for 3 hours under a pressure of 5 atm. of CO. There were obtained 0.93 g of methyl oxalate.

EXAMPLE 19 Into a glass flask, fitted with a magnetic stirrer, there were charged 2.9 g of Cu(OCH3 )Cl, 20 ml of benzene, 10 ml of methanol and 0.15 g of Pd (acetylacetonate)2. The flask was then degassed with a flow of CO the atmosphere being maintained by a CO supply. Stirring was continued for 10 hours whilst maintaining the temperature at 500 to 600C. There were obtained 0.54 g of methyl oxalate.

EXAMPLE 20 3.82 g of Cu(OC4H9)Cl were prepared in situ in an autoclave as described in example 1, from 3 g of Curl2 and 2.1 g of C4H9ONa in 50 ml of butanol. The mass was then diluted with 30 ml of benzene and treated with 0.15 grams of Pd (acetylacetonate)2. The autoclave was then pressurised to 50 atm. with CO and held for 3 hours at 600C. There were thus obtained 1.65 g of butyl oxalate.

Claims (21)

WHAT WE CLAIM IS:

1. A process for preparing an oxalic di (C1 -C8) alkyl ester by reacting a cupric (C1 -C8) alkoxy halide with carbon monoxide in the presence of a catalyst promoting the formation of the oxalic dialkyl ester.

2. A process according to claim 1 in which the catalyst comprises a palladium salt the reaction being conducted in an anhydrous organic liquid medium in which said salt is soluble.

3. A process according to claim 1 in which the catalyst is metallic palladium.

4. A process according to claim 1 in which the catalyst is a complex of zero-valent palladium.

5. A process according to claim 4 in which the palladium is complexed with phosphine or with dibenzylidine-acetone.

6. A process according to any of claims 2-5 in which an amount of catalyst is used such as to afford a molar ratio of palladium to copper of0.0001 -0.1:1.

7. A process according to any of the foregoing claims in which the halide used is the chloride or the bromide.

8. A process according to any of the foregoing claims, conducted in a substantially anhydrous organic liquid medium which does not hinder the reaction.

9. A process according to claim 8 in which the liquid is a monofunctionaI alkanol of up to 8 carbon atoms, benzene, acetone, ethyl acetate, tetrahydrofurane and/or a mixture thereof.

10. A process according to claim 9 in which the catalyst is palladium di-(acetylacetonate).

11. A process according to any of the foregoing claims, conducted at 20 - 200"C.

12. A process according to claim 11, conducted at 50 -- 1200C.

13. A process according to any of the foregoing claims, conducted at atmospheric or superatmospheric pressure.

14. A process according to any of the foregoing claims, in which the carbon monoxide is used in admixture and hydrogen.

15. A process according to any of the foregoing claims, in which the cupric alkoxy halide used is a cupric alkoxy chloride or bromide wherein the alkoxy group has 1 - 4 carbon atoms.

16. A process according to claim 15 wherein said halide is cupric methoxy chloride.

17. A process according to any of the foregoing claims in which the reaction product, after separation of the oxalic ester, is treated to obtain cupric alkoxy halide which is then reused for the production of further oxalic ester.

18. A process according to any of the foregoing claims in which the cupric alkoxy halide is used as a complex with an organic base.

19. A process according to claim 1, substantially as hereinbefore exemplified.

20. Oxalic dialkyl esters when obtained by a process as set forth in any of the foregoing claims.

21. Oxalic acid or a salt thereof when obtained from an ester according to claim 20.