GB2121794A - Process for preparing acetic acid - Google Patents

Abstract

Acetic acid is produced by reacting methanol with carbon monoxide in the presence of a catalyst comprising a molybdenum-nickel or a tungsten- nickel co-catalyst component, in the presence of an iodide, in the presence of a promoter comprising an organo- phosphorus compound or an organo- nitrogen compound and in the presence of 2 to 8% of water, based on the reaction mixture.

Description

SPECIFICATION Process for preparing acetic acid This invention relates to the prepartion of acetic acid from methanol by carbonylation.

Acetic acid has been known as an industrial chemical for many years and large amounts are used in the manufacture of various products. Proposals for producing carboxylic acids by the action of carbon monoxide upon alcohols (carbonylation) have been described, for example, in Reppe et al. U.S. 2,729,651 and in Holmes U.S. 4,133,963 and 4,218,340. However, such prior proposals involving carbonylation reactions have required the use of very high pressures. Carbonylation proesses effective at lower pressures have also been proposed. French Patent 1,573,130, for example, describes the carbonylation of methanol and mixtures of methanol with methyl acetate in the presence of compounds of Group VIII noble metals such is iridium, platinum, palladium, osmium and ruthenium and in the presence of bromine or iodine under more moderate pressures than those contemplated by Reppe et al. and Holmes.U.S. 3,769,329 and 3,772,380 produce acetic acid from the same reactants using an iridium or rhodium component with bromine or iodine. Schulz (U.S.

Patents 3,689,533 and 3,717,670) has disclosed a vapor-phase process for acetic acid production employing various catalysts comprising a rhodium component dispersed on a carrier. These lower-pressure carbonylation disclosures, however, require the use of expensive noble metals. More recently, Belgian Patent 860,557 has proposed the preparation of carboxylic acids by carbonylation of alcohols in the presence of a nickel catalyst promoted by a trivalent phosphorus compound and in the presence of an iodide. In this process low pressurecarbonylation is made possible without the use of a noble metal. This process is effective but there is room for improvement in terms of yields of the desired acid.

An improved process is described in our co-pending application 8138936, Publication No. GB-2089803-A.

That application discloses the preparation of acetic acid by the carbonylation of methanol in the presence of a catalyst comprising a molybdenum-nickei or a tungsten-nickel co-catalyst component in the presence of an iodide and in the presence of a promoter comprising an organo-phosphorus compound or an organo-nitrogen compound.

It is an object of the present invention to provide a further improved process embodying the carbonylation of methanol in the presence of a catalyst of the character just described.

In accordance with the invention, the surprising discovery has been made that the rate of the carbonylation reaction wherein methanol is converted to acetic acid can be increased significantly by incorporating in the reaction system a small but critical amount of water, the water also helping to keep the catalyst in a soluble form. Thus, in accordance with the invention, methanol is carbonylated in the presence of a catalyst comprising a molybdenum-nickel or a tungsten-nickel co-catalyst component, in the presence of an iodide, in the presence of a promoter comprising an organo-phosphorus compound or an organonitrogen compound and in the presence of water in the range of 2 to 8%, preferably 4 to 6%, based on the weight of the reaction mixture.

In the process to which the invention relates, a wide range of temperatures, e.g., to 350"C. are suitable but temperatures of 100 to 2500C. are preferably employed and the more peferred temperatures generally lie in the range of 125 to 2250C. Temperatures lower than those mentioned can be used but they tend to lead to reduced reaction rates, and higher temperatures may also be employed but there is no particular advantage in their use. The time of reaction is also not a parameter of the process and depends largely upon the temperature employed, but typical residence times, by way of example, will generally fall in the range of 0.1 to 20 hours.The reaction is carried out under superatmospheric pressure but, excessively high pressures, which require special high-pressure equipment are not necessary. In general, the reaction is effectively carried out by employing a carbon monoxide partial pressure which is preferably at least 15 but less than 2,000 psi, most preferably 15 to 1,000 psi., although CO partial pressures of 1 to 5,000 or even up to 10,000 psi can also be employed. By establishing the partial pressure of carbon monoxide at the values specified, adequate amounts of this reactant are always present. The total pressure is, of course, that which will provide the desired carbon monoxide partial pressure and preferably it is that required to maintain the liquid phase and in this case the reaction can be advantageously carried out in an autoclave or similar apparatus.

The final reaction mixture will normally contain volatile components such as hydrocarbyl iodide, unreacted alcohol and may contain the corresponding ester and/or ether along with the product acid and these volatile components, after separation from the acid, can be recycled to the reaction. At the end of the desired residence time the reaction mixture is separated into its several constituents, as by distillation. Preferably, the reaction product is introduced into a distillation zone which may be a fractional distillation column, or a series of columns, effective to separate the volatile components from the product acid and to separate the product acid from the less volatile catalyst and promoter components of the reaction mixture.The boiling points of the volatile components are sufficiently far apart that their separation by conventional distillation presents no particular problem. Likewise, the higher-boiling organic components can be readily distilled away from the metal catalyst components and any organic promoter which may be in the form of a relatively non-volatile complex. The thus recovered co-catalyst as well as promoter, including the iodide component can then be combined with fresh amounts of alcohol and carbon monoxide and reacted to produce additional quantities of carboxylic acid.

Although not necessary, the process can be carried out in the presence of an organic solvent or diluent. Since methanol has a relatively low boiling point, the presence of a higher-boiling solvent or diluent, preferably acetic acid, or the corresponding ester, e.g., methyl acetate, will make it possible to employ more moderate total pressures. Alternatively, the solvent or diluent may be any organic solvent which is inert in the environment of the process such as hydrocarbons, e.g., octane, benzene, toluene, xylene and tetralin, or halogenated hydrocarbons such as the chlorobenzenes, e.g., trichlorobenzene, or carboxylic acids, or esters such as cellosolve acetate, and the like. Mixtures of solvents can also be used, such as mixtures of methyl acetate and acetic acid.The carboxylic acid, when used, should preferably be acetic acid since the preferred solvent is one that is indigenous to the system, e.g., acetic acid and/or methyl acetate. A solvent or diluent, when not an indigenous component is suitably selected which has a boiling point sufficiently different from the components of the reaction mixture so that it can be readily separated, as will be apparent to persons skilled in the art.

The carbon monoxide is suitably employed in substantially pure form, as available commercially, but inert diluents such as carbon dioxide, nitrogen, methane, and noble gases can be present if desired. The presence of inert diluents does not affect the carbonylation reaction but their presence makes it necessary to increase the total pressure in order to maintin the desired CO partial pressure. Hydrogen may be present and may tend to stabilize the catalyst. Indeed, in order to obtain low CO partial pressures the CO fed may be diluted with hydrogen or any inert gas such as those mentioned above. It has been surprisingly found that the presence of hydrogen does not lead to the formation of reduction products. The diluent gas, e.g., hydrogen, may generally be used in amounts up to about 95%, if desired.Particularly desirable is the use of hydrogen in an amount such that the ratio of the partial pressure of hydrogen in the system to the partial pressure of carbon monoxide is in the range of 0.05 to 0.4, preferably 0.15 to 0.25, as disclosed in our copending application entitled Process for the Preparation of Acetic Acid and filedj on even date herewith. Such use of hydrogen has been found to have a highly favorable effect upon reaction rate in a system of the character with which this invention is concerned.

The co-catalyst components can be employed in any convenient form, viz., in the zero valent state or in any highervalentform. For example, the nickel and the molybdenum ortungsten can bethe metals themselves in findely divided form, or a compound both organic or inorganic, which is effective to introduce the co-catalyst components into the reaction system. Thus, typical compounds include the carbonate, oxide, hydroxide, bromide, iodide, chloride, oxyhalide, hydride, lower alkoxide (methoxide), phenoxide, or Mo, W or Ni carboxylates wherein the carboxylate ion is derived from an alkanoic acid of 1 to 20 carbon atoms such as acetates, butyrates, decanoates, laurates, benzoates, and the like. Similarly, complexes of any of the co-catalyst components can be employed, e.g., carbonyls and metal alkyls as well as chelates, association compounds and enol salts.Examples of other complexes include bis-(triphenylphosphine) nickel dicarbonyl, tricyclopentadienyl trinickel dicarbonyl, tetrakis (triphenyiphosphite) nickel, and corresponding complexes of the other components, such as molydenum hexacarbonyl and tungsten hexacarbonyl. Included among the catalyst components listed above are complexes of the metal co-catalyst components with organic promoter ligands derived from the organic promoters hereinafter described.

Particularly preferred are the elemental forms, compounds which are halides, especially iodides, and organic salts, e.g., salts of the monocarboxylic acid corresponding to the acid being produced. It will be understood that the foregoing compounds and complexes are merely illustrative of suitable forms of the several co-catalyst components and are not intended to be limiting.

The specified co-catalyst components employed may contain impurities normally associated with the commercially available metal or metal compounds and need not be purified further.

The organo-phosphorus promoter is preferably a phosphine, e.g. of the formula

wherein R1, R2 and R3 may be the same or different, and are alkyl, cycloalkyl, aryl groups, amide groups, e.g., hexamethyl phosphorus triamide, or halogen atoms, preferably containing 1 to 20 carbon atoms in the case of alkyl and cycloalkyl groups and 6 to 18 carbon atoms in the case of aryl groups. Typical hydrocarbyl phosphines include trimethylphosphine, tripropylphosphine, tricyclohexylphosphine and triphenylphosphine. Preferably the organo-nitrogen promoter is a tertiary amine or a polyfunctional nitrogen-containing compound, such as an amide, a hydroxy amine, a keto amine, a di-, tri and other polyamine or a nitrogen-containing compound which comprises two or more other functional groups. Typicai organo- nitrogen promoters include 2-hydroxypyridine,8-quinolinol,1-methylpyrrolidinone,2-imidazoiidone, N,Ndimethylacetamide, dicyclohexylacetamide, dicyclohexylmethylamine, 2,6-diaminopyridine, 2-quinolinol, N,N-diethyltoluamide, and imidazole.

Although generally the organic promoter is added separately to the catalyst system, it is also possible to add it as a complex with any of the co-catalyst metals, such as bis(triphenylphosphine) nickel dicarbonyl and tetrakis (triphenyl phosphite) nickel. Both free organic promoters and complexed promoters can also be used. When a complex of the organic promoter can also be added.

The amount of each co-catalyst component employed is in no way critical and is not a parameter of the process of the invention and can vary over a wide range. As is well known to persons skilied in the art, the amount of catalyst used is that which will provide the desired suitable and reasonable reaction rate since reaction rate is influenced by the amount of catalyst. However, essentially any amount of catalyst will facilitate the basic reaction and can be considered a catalytically-effective quantity. Typically, however, each catalyst component is employed in the amount of 1 mol per 10 to 10,000 mols of alcohol, preferably 1 mol per 100 to 5,000 mols of alcohol and most preferably 1 moi per 300 to 1,000 mols of alcohol.

The ratio of nickel to the second co-catalyst component can vary. Typically, it is one mol of the nickel per 0.01 to 100 mols of the second co-catalyst component, preferably the nickel component is used in the amount of 1 mol per 0.1 to 20 mols, most preferably 1 mol per 1 to 10 mols of the second co-catalyst component.

The quantity of organic promoter can also vary widely but typically it is used in the amount of 1 mol per 0.1 to 10 mols of the co-catalyst components.

The amount of iodide component may also vary widely but, in general, it should be present in an amount of at least 10 mols (expressed as I) per hundred mols of alcohol. Typically, there are used 10 to 50 mols of the iodide per 100 mols of alcohol, preferably 17 to 35 mols per 100 mols. Ordinarily, more than 200 mols of iodide per 100 mols of alcohol are not used. It will be understood, however, that the iodide component does not have to be added to the system as a hydrocarbyl iodide but may be supplied as another organic iodide or as the hydroiodide or other inorganic iodide, e.g., a salt, such as the alkali metal or other metal salt, or even as elemental iodine.

A particular embodiment of the catalyst comprising the molybdenum-nickel or tungsten-nickel co-catalyst component, the organic promoter component and the iodide component can be represented by the following formula X:T:Z:Q, wherein Xis molybdenum or tungsten, T is nickel, X and T being in zero valent form or in the form of a halide, an oxide, a carboxylate of 1 to 20 carbon atoms, a carbonyl or an hydride; Z is an iodide source which is hydrogen iodide, iodine, an alkyl iodide wherein the alkyl group contains 1 to 20 carbon atoms or an alkali metal iodide, and 0 is an organo-phosphorus compound or an organo-nitrogen compound wherein the phosphorus and the nitrogen are trivalent.Preferred are the nitrogen and phosphorus compounds previously indicated as being preferably used and in themost preferred form 0 is a phosphine of the formula

as hereinbefore defined, especially hydrocarbyl phosphines, the molar ratio of X to T being 0.1 - 10:1, the molar ratio of X + Tto 0 being 0.05 - 20:1 and the molar ratio of to X + T being 1 - 1,000:1.

As previously mentioned, the invention is based on the discovery that, in a system of the character described above, when a small but critical amount of water is added to the reaction mixture to maintain in the reaction mixture an amount of water in the range of 2 to 8% based on the weight of the reaction mixture, there is a surprising and unexpected increase in reaction rate, and the catalyst is maintained in a soluble form.

It will be apparent that the above-described reaction lends itself readily to continuous operation in which the reactants, water and catalyst are continuously supplied to the appropriate reaction zone and the reaction mixture continuously distilled to separate the volatile organic constituents and to provide a net product consisting essentially of carboxylic acid with the other organic components being recycled and, in a liquid-phase reaction a residual catalyst containing fraction also being recycled.

The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that they are given for illustrative purposes only and are not to be construed as limitative of the invention.

Example 1 The apparatus used in this example was a one-liter autoclave provided with an electrically-heated jacket, a magnetically driven agitator, gas and liquid feed lines, and a gas-liquid take-off line at the vapor-liquid interface. The apparatus was operated at a temperature of 200"C. and at a total pressure of 1250 psig, with a carbon monoxide partial pressure of 940 psi. The carbon monoxide partial pressure was maintained by supplying this gas continuously in the amount required.

The liquid feed stream, which was supplied at the rate of 250 grams/hr., consisted of a mixture of 38 wt. % methyl iodide, 33 wt. % methyl alcohol, 7 wt. % acetic acid, 6 wt. % methyl acetate, and 5 wt. % water, plus 0.2 wt. % nickel (added as nickel iodide), 0.3 wt. % molybdenum (added as molybdenum carbonyl)'and 4 wt.

% triphenyl phosphine.

After steady-state operation has been reached, the reaction was carried out on a continuous basis for approximately 16 hours. The collected effluent was found to contain about 0.1% by weight of solids, and analysis by gas chromatography (G.C.) of the liquid showed that acetic acid has been produced at the rate of 3.2 gram-mols per hour per liter.

Comparative E The process and apparatus described in Example 1 were used in this experiment under the conditions specified except that the water was omitted from the feed. The effluent was found to contain about 0.1% by weight of solids and the reaction rate was found to have fallen to 2.6 gram-mols per hour per liter.

Example 2 The apparatus described in Example 1 was used and was operated at a temperature of 200"C. and at a total pressure of 1250 psig, with a hydrogen-carbon monoxide mixture such that there was a carbon monoxide partial pressure of 870 psi and a hydrogen partial pressure of 57 psi. The carbon monoxide and hydrogen partial pressures were maintained by supplying these two gases continuously in the amounts required.

The liquid feed stream described in Example 1 was used and it was supplied at the rate of 250 grams/hr., and the reaction was carried out in the manner specified in Example 1. The collected effluent was found to contain about 0.1% by weight of solids, and analysis by gas chromatography (G.C.) of the liquid showed that acetic acid had been produced at the rate of 5.6 gram-mols per hour per liter.

Comparative Example B The apparatus described in Example 1 was again used and was operated at a temperature of 200"C. and at a total pressure of 1250 psig, with a hydrogen-carbon monoxide mixture such that there was a carbon monoxide partial pressure of 890 psi and a hydrogen partial pressure of 74 psi. The carbon monoxide and hydrogen partial pressures were maintained by supplying these two gases continuously in the amounts required.

The liquid feed stream, which was supplied at the rate of 250 grams/hr., was that described in Example 1, except that the water was omitted, and the reaction was carried out in the manner specified in Example 1.

The collected effluent was found to contain about 4.5% by weight of solids, and analysis by gas chromatography (G.C.) of the liquid showed that the rate of acetic acid formation had fallen to 4.5 gram-mols per hour per liter.

Claims (5)

1. A process for the preparation of acetic acid which comprises reacting methanol with carbon monoxide in the presence of a catalyst comprising a molybdenum-nickel or a tungsten-nickel co-catalyst component, in the presence of an iodide, in the presence of a promoter comprising an organo-phosphorus compound or an organo-nitrogen compound and in the presence of 2 to 8% of water based on the weight of the reaction mixture.

2. A process as claimed in Claim 1, wherein the amount of water present in the feed is 4 to 6%.

3. A process as claimed in Claim 1, substantially as hereinbefore described with particular reference to the Examples.

4. A process as claimed in Claim 1, substantially as illustrated in any one of the Examples.

5. Acetic acid when prepared by the process claimed in any one of the preceding claims.