GB2161810A - Process for the production of acetic and other anhydrides - Google Patents

Abstract

A process for the production of acetic anhydride or other alkanoic (eg propionic) anhydride from a C2-6 alkene, carbon monoxide and either methanol or acetic acid comprises a carbonylation step at elevated temperature and pressure in the presence of a Group VIII noble metal and an iodide promoter. Acetic anhydride and the other anhydride are then obtained by distilling the products of the carbonylation step in the presence of a catalyst which is able to decompose any mixed anhydride formed.

Description

SPECIFICATION Process for the production of carboxylic acid anhydride The present invention relates to a process for the production of a carboxylic acid anhydride.

Acetic anhydride has been known as an industrial chemical for many years. It constitutes the second largest end use for acetic acid and is extensively employed in the production of cellulose acetate and other esters. Smaller quantities are used in the temperature bleach activators. Uses have been proposed for other carboxylic acid anhydrides, such as propionic anhydride.

On an industrial scale acetic anhydride is currently produced either by reaction of ketene and acetic acid, the ketene being obtained either by dehydration of acetic acid or by decomposition of acetone, or by the oxidation of acetaldehyde, which also yields acetic acid. Each of these conventional routes has well-known disadvantages which knowledge, together with the increasing attention being paid to chemical syntheses involving carbon monoxide and carbon monoxide/hydrogen mixtures, has led to invetigations into the production of acetic anhydride utilising these materials.One approach to the preparation of an anhydride of a monocarboxylic acid is described in the complete specification of GB 1468940, in which a carboxylate ester satisfying the formula RCOOR or an ether satisfying the formula ROR is reacted with an acyl halide satisfying the formula RCOX, formed in situ or in a separate stage, under substantially anhydrous conditions, wherein Xis iodide or bromide, the Rs may be the same or different and each R is a monovalent hydrocarbon radical or a substituted monovalent hydrocarbon radical wherein the or each substituent is inert. The acyl halide may be produced by carbonylation of a halide satisfying the formula RX at superatmospheric pressure in the presence as catalyst of a Group VIII noble metal.Another approach to the production of carboxylic acid anhydrides is described in the complete specifications of GB Patents Nos 1398530, 1367607 and 1448010 (all to Monsanto Co) in which an ethylenically unsaturated compound is reacted with carbon monoxide in the presence of a carboxylic acid and a catalyst which is either a Group VIII metal or metal compound and a phenol, thiophenol, a substituted phenol orthiophenol, a Cm 30 alkanethiocarboxylic acid, a C1.30 fluoroalkanecarboxylic acid, a Cm 30 fluoroalkanethiocarboxylic acid or a C1 30 fluoroalkanesulphonic acid as promoter (GB 1398530), or an iridium complex comprising at least one ligand derived from (a) a compound which is a tertiary organoderivative of phosphorus, arsenic or antimony in which the total number of carbon atoms in the organic groups is from 3 to 90, (b) an olefinic compound having from 2 to 40 carbon atoms, or (c) a diolefinic compound having from 4 to 40 carbon atoms, in the substantial absence of added halide coponents (GB 1367607) or (a) an unchelated rhodium (I), rhodium (III), iridium (I) or iridium (III) component, (b) an iodide component, and (c) a catalyst preserver or regenerator component which comprises hydrogen or a compound which yields hydrogen under the reaction conditions (GB 1448010).Replacement of the carboxylic acid by water at a quantitative level at least stoichiometric with the olefin reactant results in the formation of carboxylic acids rather than their anhydrides.

Our copending European Patent Application No 83302846.7 BP Case 5365 describes the production of a carboxylic acid anhydride or a mixture of carboxylic acid anhydrides by reacting an olefinically unsaturated compound and an alcohol with carbon monoxide under substantially anhydrous conditions in the presence of a rhodium or iridium compound as catalyst and of an iodide promoter, and in particular the reaction of ethylene with methanol and carbon monoxide to form a mixture of acetic anhydride, propionic anhydride and acetopropionic anhydride. The latter can be eliminated by disproportionation. This process has now been further improved to increase the proportion of acetic anhydride in the final product.

According to the present invention a process for the production of a carboxylic acid anhydride comprises reacting ethylene, carbon monoxide and either methanol or acetic acid at elevated temperature in the presence of a Group VIII noble metal catalyst and a promoter comprising iodine or bromine in free or combined form to produce a mixture of acetic anhydride, propionic anhydride and acetopropionic anhydride and distilling the mixture in the presence of a catalyst for converting the last mentioned an hydride into the first two and separating the acetic anhydride from the propionic anhydride.

The process can therefore yield acetic an hydride and propionic anhydride as products. Alternatively the latter can be converted to propionic acid, or methyl propionate. The latter can be recycled to the carbonylation reaction.

The carbonylation to form the anhydrides can be effected as described in (i) our copending European Patent Application No 83302846.7 BP Case 5365 in which methanol is reacted with carbon monoxide and ethylene to form a product containing acetopropionic anhydride CH3OH + 2CO + C2H4 C2H5CO2COCH3 Reaction A (ii) in UK Patent Nos 1,398,530; 1,367,067 and 1,448,010 in which ethylene is carbonylated in a carboxylic acid solvent C2H4+CO+CH3COOH C2H5CO2COCH3 Reaction B orby (iii) heterogeneous carbonylation of ethylene in the presence of acetic acid using a supported catalyst as described for example in our copending UK Patent Application No 8314139 (BP Case 5582) Reaction B In order to reduce the formation of substantial amounts of carboxylic acids all the reactants should preferably be substantially anhydrous, that is they should contain in total less than 3%, more preferably less than 1%w/wwater.

The carbon monoxide is preferably substantially pure or it may contain minor impurities such as hydrogen, carbon dioxide, nitrogen or methane.

The catalyst preferably comprises a rhodium or an iridium component or a mixture thereof. Any convenient form of the metal may be used in the zero valent state or in any higher valent form. Thus the rhodium or iridium may be added as the elemental metal in finely divided for, as a simple salt such as the halide, as an organometallic compound such as a carboxylate salt or as a coordination compound with ligands such as carbon monoxide, halides, phosphines, arsines and trivalent nitrogen compounds. Suitable forms of rhodium or iridium are described in the complete specifications of GB 1253758 and the aforesaid GB 1398530, 1367607 and 1448010 to which reference may be made for further details.Examples of compounds which may be added include [rh(CO)2CI]2; RhC13.3H20; Rh Br3; Rhl3; IrBr3; Girl3; RhBr3.3H2O; IrBr3.3H2O; Rh2(CO)4Br2; lr2(CO)4l2; Rh[(C6H5)3P]2(CO)l and lrBr[(CeH5)3P13.

The promoter comprises iodine in free or combined form. Suitably iodine may be added in the form of elementary iodine, hydrogen iodide, an inorganic halide salt, such as for example sodium or potasium iodide, or a quaternary ammonium or phosphonium halide, such as for example a tetraalkylammonium halide. Particularly preferred are organo-iodides, such as alkyl iodides, eg methyl iodide.

A copromoter may also be employed with advantage. Suitably the copromotor may be a quaternary nitrogen, phosphorus or arsenic compound, preferably a heterocyclic aromatic compound in which at least one of the hetero atoms is a quaternary nitrogen atom, either added as such or in a form capable of forming the quaternary compound under the reaction conditions. Suitable heterocyclic aromatic quaternary nitrogen compounds which may be added as such include N-methyl pyridinium iodide; N,N-dimethylimidazolinium iodide; N-methyl-3-picolinium iodide; N-methyl-3, 4-lutidinium iodide and N-methyl-quinolinium iodide.

Suitable quaternary phosphorous compounds include tributylmethylphosphonium iodide, triphenylmethylphosphonium iodide, trilaurylmethylphosphonium iodide and trioctylmethylphosphonium iodide. It is preferred, however, to add a heterocyclic aromatic compound of trivalent nitrogen which is capable of forming a quaternary compound under the reaction conditons. Suitable trivalent forms of nitrogen are heterocyclic amines such as pyridine, picoline, quinoline, methylquinoine, hydroxyquinoline pyrrole, pyrrolidine, pyrrolidone and the like or an imidazole, such as imidazole, methyl imidazole and the like.

Alternatively, there may be added a compound of formula: R5 X - R6 (II) R7 wherein Xis nitrogen, phosphorus or arsenic and R5, R6 and R7 which may be the same or different are alkyl groups having up to 20, preferably from 1 to 8, carbon atoms or monocyic aryl groups, or when Xis phosphorus or arsenic R7 may be the group:

wherein R7 and R3 are each a monocyclic aryl group or an alkyl group and n is zero or an integer in the range 1 to 20. Suitable compounds of formula (II) include trimethylamine, triethylamine, triphenylamine, triphenylarsine, methyldiphenylarsine, trimethylphosphine, tripropylphosphine and triphenylphosphine.

With regard to the amounts of the various reactants employed, the molar ratio of ethylene to methanol to carbon monoxide may suitably be 1:1:1, though different ratios may be employed if so desired. Although any ratio of promoter to rhodium or iridium may be employed, suitable ratios expressed as atoms of iodine in the promoter to atoms of rhodium or iridium in the catalyst component may suitably be in the range from 1:1 to 2500:1, preferably from 3:1 to 300:1. Using a promoter his is formed 'in situ' it will be necessary to make adjustments to the foregoing ranges to make allowance for the fact tht iodine is utilised in forming the quaternary compound.The molar ratio of rhodium or iridium compound in the catalyst component to the quaternary compound employed as copromoter may suitably be in the range from 1:1 to 1:1200, pfirabiy from 1:10 to 1:300.

The elevated temperature may suitably be in the range from 50 to 3000C, preferably from 125 to 250"C. The partial pressure of carbon monoxide may suitably be in the range from 1.0 to 1000 bar, preferably from 10 to 100 bar. Higher pressures may be used if so desired under appropriate conditions.

The process may be carried out in the liquid phase or in the vapour phase. In the case of both liquid and vapour phase operation, the catalyst and optionally also the promoter or copromoter may be supported, ie they may be dispersed on a conventional support material, such as for example alumina, silica, silica/alumina, zeolites, clays, titania or zirconia. The catalyst and optionally the promoter and/or copromoter may be applied to the support in conventional manner, eg by impregnation from solution. The catalyst concentration on the support may suitably be in the range from 0.01 to 10% by weight. Alternatively, the metal catalyst components may be attached to an ion exchange resin orto a functionalised inorganic oxide such as those described in the complete specification of GB 1503315 or European patent publication No 18102 (BP Case No 4752).

The process of the invention may be carried out batchwise or continuously.

Reaction of mthanol and ethylene with carbon monoxide is believed to proceed according to the following equations:

A mixture of three acid anhydrides (1), (2) and (3) is thus produced. The mixture of products (1), (2) and (3) is distilled in the presence of a catalyst for accelerating the interconversion of (1) into (2) and (3) and acetic anhydride (2) distilled out of the mixture, thereby driving the equilibrium in the direction of (2) and (3).

The invention is illustrated by the accompanying drawing which shows the various stages of reaction, equilibration, distillation and esterification.

Ethylene, carbon monoxide and a liquid reactant which is either methanol or acetic acid (methanol if reaction A is employed, or acetic acid if B is employed) are fed continuously via lines 21,22 and 23 respectively into a stirred reactor 1. The reactor is also continuously supplied with recycled materials via lines 25, 26 and 27 and recycled catalyst via line 24 from a flash tank 2.

Solution is removed from reactor 1 via line 28 to the flash tank 2, from which the majority of the volatile products are taken overhead via line 29 to a distillation column 3. The function of the latter is to remove as overheads methyl iodide and ethyl iodide together with any methyl acetate and ethyl acetate which may be present for return to the reactor 1. The bottoms from colun 3 containing acetic and propionic acids, and acetic, propionic and acetopropionic anhydrides are mixed with an acid catalyst and fed to column 4.

The purpose of the acid catalyst is to ensure that equilibria (13) and (14) are rapid, so that the lightest boiling component, acetic acid may be removed from the head of column 4 and returned to the reactor.

CH3CO2H + (C2H5CO)2O CH3COOCOC2H5 + C2H5CO2H (13) CH3COOCOC2H5 + CH3CO2H (CH3CO)2O + C2H5CO2H (14) The bottoms from column 4, containing a mixture of the acid catalyst and acetic, propionic and acetopropionic anhydrides are removed to column 5. Since equilibrium (15) is also rapid in the presence of an acid catalyst, acetic an hydride may be preferentially removed overhead from column 5 and retained as a product.

2CH3COOCOC2H5 (CH3CO)2O + (C2H5CO)2O (15) The remaining propionic anhydride is fed to column 6 and acid catalyst is recycled to the feed stream to column 4. If reaction A is employed, the propionic anhydride may be reacted in column 6 with part of the methanol feed to give methyl propionate (which is recycled to the reactor for carbonylation) and propionic acid (as product). In this case, the overall reaction stoichiometry is: 2CH3OH + 3CO + C2H4 (CH3CO)2 + C2H5CO2H s (16) Otherwise, the propionic anhydride is reacted with water to give only propionic acid.In this case (reaction B) the voerall stoichiometry is as follows: 2CH3CO2H + 2C2H4 + 2CO + Fi20 2C2H5CO2H + (CH3CO)2O (17) Experiment 1 The following mixture, which is a typical product mixture from the co-carbonylation of methanol and ethylene, was charged to a flask equipped with a 20-plate Oldershaw Column, reflux head, and nitrogen purge.

Methyl acetate 19.6 g Methyl propionate 10.5 g Acetic anhydride 73.5 g Acetic acid 60.2 g Propionic acid 37.8 g Propionic acid 39.9 g Methyl iodide 16.8 g Ethyl iodide 23.8 9 The mixture was then distilled and fractions removed at a reflux ratio 4:1. Analysis by gas chromatography (GC) showed that fractions collected at temperatures up to 800C contained practically all the alkyl iodides and methyl esters, and that fractions collected up to 138"C contained in total 88.1 g acetic acid. This constitutes 97% by weight of the acetic acid theoretically recoverable. The residue was found to contain a mixture of acetic, propionic and acetopropionic anhydride, together with small amounts of propionic acid.

This experiment demonstrates the feasibility of operating columns 3 and 4 of the above described process scheme.

Experiment2 A mixture comprising 49.1% (by weight) of acetic anhydride and 50.9% propionic acid (molar ratio 0.7:1) was fed continuously at a rate of 203 ml/hrto plate 15 of a 40 plate 2" galss Oldershaw Column equipped with a stainless steel reboiler. The head temperature was controlled at 119 C and distiliate was removed continuously at a reflux ratio of 8:1. GC analysis showed that under these conditions, the head sample contained 94.3% by wieght acetic acid (removal rate 72.0 g/h) whereas the base contained only 3.2% acetic acid and 8.5% propionic acid, together with 15.7% acetic anhydride, 22.7% propionic anhydride, and 49.4% acetopropionic anhydride.

This experiment demonstrates the feasibility of column 4 of the above described process scheme.

The advantage of the above described process is that the sole products are acetic anhydride and the carboxylic acid or its an hydride derived from the olefin, for example propionic acid in the case of ethylene.

Claims (4)

1. A process for the production of acetic anhydride which comprises reacting ethylene, carbon monoxide and either methanol or acetic acid at elevated temperature in the presence of a Group VIII noble metal catalyst and a promoter comprising iodine or bromine in free or combined form to produce a mixture of acetic anhydride, propionic anhydride and acetopropionic anhydride and distilling the mixture in the presence of a catalyst for converting the last mentioned an hydride into the first two and separating the acetic anhydride from the propionic anhydride.

2. A process as claimed in Claim 1 wherein the mixture, optionally, from which low boiling materials such as methyl iodide and ethyl iodide together with any methyl acetate and ethyl acetate have been removed, is passed to a distillation column from which acetic acid is removed and then to a further column where conversion of acetopropionic anhydride to acetic anhydride and propionic anhydrides is effected and the former removed as an overhead fraction to thereby yield acetic anhydride and propionic anhydride as products.

3. A process as claimed in Claim 2 wherein the propionic anhydride is either converted to propionic acid or to methyl propionate and the latter passed to the carbonylation reactor.

4. A process for the production of acetic anhydride and an acid or anhydride derived from an olefin which process comprises reacting the olefin containing from 2 to 6 carbon atoms, carbon monoxide and either methanol or acetic acid at elevated temperature in the presence of a Group VIII noble metal catalyst and a promoter comprising iodine or bromine in free or combined form to produce a mixture of anhdrides containing acetic anhydride and a mixed anhydride, distilling the mixture in the presence of a catalyst for converting the mixed anhydride by disproportionation to acetic anhydride and another anhydride and separating the acetic anhydride from the other coproduced anhydride and optionally converting the latter to its corresponding acid.