Process for producing methyl acetate from methyl formate

Abstract

The object of the invention is a process for the preparation of methyl acetate starting from the methyl formate, characterized in that methyl formate is converted at a temperature of 170.degree. to 220.degree. C. in the presence of:a) a catalyst based on rhodium,b) an iodine-containing promoter selected from among the alkaline, alkaline earth, phosphonium or ammonium iodides and the mixtures of covalent compounds of iodine supplemented with a phosphine or a tertiary amine.c) a solvent selected from a cyclic N-alkyl amide.

Description

The present invention relates to a process for the selective preparation of methyl acetate starting from methyl formate.

The isomerization of methyl formate to acetic acid according to the following scheme is a well-known reaction. ##STR1## In the liquid phase, it is catalysed by numerous transition metals of group VIII, among which iridium, rhodium, cobalt, ruthenium and palladium are the most often cited in the literature. The reaction is usually carried out in the presence of an iodine-containing promoter (CH.sub.3 I, HI), at a temperature varying between 150.degree. and 200.degree. C. and most often under a pressure of carbon monoxide, the role of which consists uniquely in stabilizing the catalytic species.

The selectivity towards acetic acid, which may be good (99%), depends on the experimental conditions: catalysts, solvent, promoter, co-catalyst . . . .

The influence of these various parameters has been studied by Roper et al. (Erdol und Kohle, Erdgas Petrochem., 38 (1), 1985), the principal by-product always being methyl acetate which is formed in appreciable proportions essentially when the activity of the catalytic system is low (rate of the cycle less than 20 h.sup.-1).

The present invention aims to provide a process for the synthesis of methyl acetate having a good selectivity starting from methyl formate. The reactions which may be involved are the following: ##STR2##

Thus, under the conditions or the reaction, the formic acid initially co-produced is decomposed to give CO, hydrogen, CO.sub.2 and water.

The invention is characterized in that methyl formate is made to react at a temperature of 170.degree. to 220.degree. C. in the presence of:

a) a catalyst based on rhodium, b) an iodine-containing promoter selected from among the alkaline, alkaline earth, phosphonium and ammonium iodides and the covalent compounds of iodine supplemented with a phosphine or a tertiary amine, this promoter being present in a molar concentration of 0.05 to 1 mole/liter, c) a solvent selected from a cyclic N-alkyl amide.

As an example of a catalyst based on rhodium mention may be made of RhCl.sub.3, 3H.sub.2 O--RhCl.sub.2 (CO).sub.4 or Rh(CO)Cl(PPh.sub.3).sub.2.

The iodine-containing promoter is preferably an alkaline or alkaline earth iodide such as LiI, NaI, KI, CaI.sub.2 . . . .

It may also possibly be covalent: I.sub.2, CH.sub.3 I, HI . . . . In this case, the addition of a phosphine PR.sub.3 or a tertiary amine NR.sub.3 (R being an alkyl or aryl group, in particular PPh.sub.3) in a ratio of 1 to 2 with the promoter proves to be beneficial.

The solvent is advantageously used in a molar proportion of 10 to 50% with respect to the formate. As an example of a cyclic N-(C.sub.1 -C.sub.6 alkyl) amide mention may be made of N-methylpyrrolidone, N-ethylpyrrolidone and N,N-dimethylimidazolidinone.

The reaction may be carried out without an initial pressure of CO. It is, however, advantageous to work under a pressure of CO included between 1.10.sup.5 and 20.10.sup.5 Pa, in particular in order to avoid the decomposition of the formate into CO and methanol and to improve the selectivity towards methyl acetate. Higher pressures are also possible but most often result in a fall in activity.

The catalyst is advantageously used in molar proportion included between 10.sup.-3 and 4.10.sup.-4.

The following examples illustrate the process of the invention:

EXAMPLE 1

1 g of LiI in solution in 30 cm.sup.3 of methyl formate and 10 cm.sup.3 of N-methylpyrrolidone (NMP) are introduced into a 100 cm.sup.3 autoclave. The autoclave is purged by a current of CO, then pressurised to 5.10.sup.5 Pa. Then it is heated to 180.degree. C. with stirring; when the temperature has stabilized, 0.06 g of RhCl.sub.3, 3H.sub.2 O dissolved in 10 cm.sup.3 of NMP is injected rapidly (2 minutes) with a dosing pump. The conversion of the methyl formate is monitored by the gas chromatographic analysis of periodic samples, the end of the injection of the catalyst being taken as time zero. At the end of 4 hours of reaction the stirring is stopped, the autoclave is cooled, the gas and liquid phases are analysed by chromatography.

______________________________________Results:Concentration in the Concentration in theliquid phase (molar %) gas phase (molar %)______________________________________MeOMe = 1.2 H.sub.2 = 2MeOH = 1.7 CO = 48HCOOCH.sub.3 = 78 CO.sub.2 = 34CH.sub.3 COOCH.sub.3 = 18.3 CH.sub.4 = 16______________________________________

The kinetic results obtained in Example 1 are presented in Table I, as well as other results obtained by utilizing an experimental protocol similar to that described in Example 1.

TABLE 1______________________________________ % molar % selectivities total in terms cycleExpt. Exptl. t con- of methyl rateNo. conditions (h) version acetate (h.sup.-1)______________________________________1 RhCl.sub.3.3H.sub.2 O = 0.1 8.7 53 0.25 mmole LiI = 1 g 0.25 21 94.2 1680 FOMe = 500 mmoles 1 24.8 94.4 496 NMP = 20 cm.sup.3 3 28.4 91.5 189 T = 180.degree. C. PCO = 1 MPa2 RhCl.sub.3.3H.sub.2 O = 1.25 4.8 39.6 0.25 mmole ICH.sub.3 = 1 g 2.5 11.3 83.2 90 PPh.sub.3 = 1 g 4.5 16.8 73.7 75 FOMe = 500 mmoles NMP = 20 cm.sup.3 T = 180.degree. C. PCO = 1 MPa3 RhCl.sub.3.3H.sub.2 O = 0.5 17.9 75 716 0.25 mmole LiI = 0.15 g 1 26 85.8 520 ICH.sub.3 = 1 g 2 33.5 92.5 325 FOMe = 500 mmoles 3 36.8 92.7 245 NMP = 20 cm.sup.3 T = 180.degree. C. PCO = 1.5 MPa4 RhCl.sub.3.3H.sub.2 O = 0.5 15.6 79.5 414 0.125 mmole LiI = 1.5 g 1 34.8 81.6 462 NMP = 20 cm.sup.3 2.5 39.1 84.6 207 FOMe = 166 mmoles T = 180.degree. C. PCO = 5 MPa5 RhCl.sub.3.3H.sub.2 O = 1.5 34 93.3 373 0.15 mmole LiI = 2 g 2.5 71.1 80.4 468 FOMe = 245 mmoles 3.5 92 50.1 432 NMP = 15 cm.sup.3 T = 196.degree. C. PCO = 10 MPa6 RhCl.sub.3.3H.sub.2 O = 3.5 34.5 81.7 162 0.15 mmole LiI = 1 g 4.5 52 80.3 190 FOMe = 247 mmoles 5.5 72.6 71.9 217 NMP = 15 cm.sup.3 T = 196.degree. C. PCO = 30 MPa7 RhCl.sub.3.3H.sub.2 O = 3 71.7 92.5 393 0.15 mmole LiI = 0.25 g 4 81.9 90.6 337 FOMe = 247 mmoles 5 88.5 81.4 291 NMP = 15 cm.sup.3 T = 196.degree. C. PCO = 30 MPa______________________________________ FOMe = methyl formate NMP = Nmethylpyrrolidone

EXAMPLE 8

One proceeded as in Example 1 but in utilizing 0.065 mmole of catalyst RhCl.sub.3, 3H.sub.2 O, 180 mmole of methyl formate, 50 cm.sup.3 of solvent NMP at a temperature of 184.degree. C. under a CO pressure of 5 MPa. The amount of iodine-containing promoter LiI was made to vary and the percentage conversion into methyl acetate and the selectivity were determined at the end of 2 h.

______________________________________promoter % conversion into % selectivity incatalyst ratio methyl acetate terms of methyl acetate______________________________________ 40 5 14100 30 70200 30 84______________________________________

EXAMPLE 9

By way of comparison, one proceeded as in Example 1 but in utilizing 1.3 mmole of RhCl.sub.3, 3H.sub.2 O; 820 mmoles of methyl formate without solvent at 180.degree. C. and 20 MPa of CO. Practically only acetic acid is obtained, which shows clearly the influence of the solvent.

______________________________________ % % molar % molar total selectivity in selectivityExperimental t con- terms methyl in terms ofconditions (h) version acetate acetic acid______________________________________RhCl.sub.3.3H.sub.2 O = 1 72 2.2 97.91.30 mmolesLiI = 2.8 gFCMe = 820 mmoles 2 96.5 0.5 99.5T = 180.degree. C.P = 20 MPa 3 99.3 0.3 99.7______________________________________

Claims

1. A process for the preparation of methyl acetate starting from methyl formate, comprising the step of converting methyl formate to methyl acetate with good selectivity at a temperature of 170.degree. to 220.degree. C. in the presence of

a) a catalytically effective amount of a catalyst based on rhodium,

b) an iodine-containing promoter selected from alkaline iodides, alkaline earth iodides, phosphonium iodides, ammonium iodides, and the covalent compounds of iodine supplemented with a phosphine or a tertiary amine in a ratio of 1 to 2 with said promoter, this promoter being present in a molar concentration of 0.05 to 1 mole/liter, and

c) a solvent selected from cyclic N-alkyl amides.

2. A process for the preparation of methyl acetate starting from methyl formate, comprising the step of converting methyl formate to methyl acetate with good selectivity at a temperature of 170.degree. to 20.degree. C. in the presence of a catalyst based on rhodium; a promoter comprising an alkaline iodide or an alkaline earth iodide, this promoter being present in a molar concentration of 0.05 to 1 mole/liter; and a solvent selected from a cyclic N-alkyl amide.

3. A process according to claim 1, characterized in that the promoter is a covalent compound of iodine in a mixture with a tertiary amine.

4. A process according to claim 1, characterized in that the promoter is a covalent compound of iodine in a mixture with a phosphine.

5. A process according to claim 1, characterized in that the solvent is utilized in a molar proportion of 10 to 50% with respect to the methyl formate.

6. A process according to claim 1, characterized in that one works under a CO pressure of 1.50.sup.5 to 20.10.sup.5 Pa.

7. A process for the catalytic preparation of methyl acetate from methyl formate, comprising:

contacting in a cyclic N-alkyl amide solvent and at a temperature of from 170.degree. C. to 220.degree. C., methyl formate, a rhodium-based catalyst present in a catalyst effective amount, and iodide ions present in a promoter effective amount; and

obtaining methyl acetate with good selectivity.

8. The process of claim 7, wherein said iodide ions are present in an amount of from 0.05 to 1 mole/liter.

9. The process of claim 7, wherein said rhodium-based catalyst is present in an amount of from 10.sup.-3 to 4.times.10.sup.-4 M.

10. The process of claim 7, wherein said cyclic N-alkyl amide is a cyclic N-(C.sub.1-6 alkyl) amide.

11. The process of claim 7, wherein said cyclic N-alkyl amide is N-methylpyrrolidone, N-ethylpyrrolidone or N,N-dimethylimidazolidinone.