GB2393437A

GB2393437A - Production of acetic acid by iridium-catalysed carbonylation using one or more metal iodides or iodide-generating salts or complexes

Info

Publication number: GB2393437A
Application number: GB0316681A
Authority: GB
Inventors: John Glenn Sunley
Original assignee: BP Chemicals Ltd
Current assignee: BP Chemicals Ltd
Priority date: 2002-09-30
Filing date: 2003-07-16
Publication date: 2004-03-31
Anticipated expiration: 2023-07-16
Also published as: GB0316681D0; GB2393437B; GB0222618D0; FR2845086B1; FR2845086A1

Abstract

A process for the production of acetic acid by the iridium-catalysed carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in the substantial absence of a conventional metal promoter such as ruthenium, osmium and rhenium, but wherein the liquid reaction composition comprises a salt soluble in the composition selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt and manganese, salts capable of generating I-, lanthanide metal complexes capable of generating I- and mixtures of two or more thereof. The water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration and the molar ratio of iodide : iridium is in the range [greater than 0 to 0.5] :1. The reaction composition further comprises methyl iodide co-catalyst, acetic acid and methyl acetate. The salt is preferably lithium or sodium iodide.

Description

PROCESS FOR THE PRODUCTION OF ACETIC ACID

The present invention relates to a process for the production of acetic acid and in particular to a process for the production of acetic acid by carbonylation in the presence of an iridium catalyst and a methyl iodide co-catalyst.

Preparation of carboxylic acids by iridium-catalysed carbonylation processes is 5 known and is described, for example in GB-A-1234121, US-A3772380, DE-A 1767150, EP-A-0616997, EP-A-0618184, EP-A-0618183,

EP-A-0657386 and WO-A-95/31426 WO-A-95/31426 discloses a process for the production of carboxylic acids or their esters having (n+1) carbon atoms by the liquid phase reaction of carbon monoxide 10 with at least one alcohol having (n) carbon atoms in the presence of a catalytic system based on a compound of iridium and a halogen co-catalyst. The process is characterized by maintaining in the reaction medium water in a volume between greater than 0 and 10%, typically between 0.5 and 8%, preferably between 2 and 8%; the ester corresponding to the carboxylic acid and the alcohol in a volume varying between 2 and 15 40%; and iodides in soluble form of such a nature that the atomic ratio of the iodides to iridium is between greater than 0 and 10, typically between greater than 0 and 3, preferably between greater than 0 and 1.5. The volume of halogen cocatalyst in the reaction medium is between greater than 0 and 10%; typically between 0.5 and 8%, and preferably between 1 and 6%. Suitable iodides include alkaline earth metal and alkali 20 metal iodides, and specifically lithium iodide. The process of WO-A-95/31426 is otherwise unprompted.

In the promoted iridium-catalysed carbonylation processes of EP-A-0 643 034 and EP-A-0 752 406 it is said that ionic contaminants such as, for example, (a)

corrosion metals, particularly nickel, iron and chromium and (b) phosphines or nitrogen-containing compounds or ligands which may quaternise in situ should be kept to a minimum in the liquid reaction composition as these will have an adverse effect on the reaction by generating I- in the liquid reaction composition which has an adverse S effect on the reaction rate.

In WO-A-96/237757 which is directed to the preparation of iridium carboxylates and their use in inter alla carbonylation reactions, the use of promoters not being mentioned, it is stated in contrast to WO-A95/314326 that alkaline or alkaline earth ions are preferably eliminated, since their presence may have a harmful influence on the 10 kinetics and selectivity of subsequent reactions in which the iridium carboxylate will be used as catalyst.

EP-A- 0 849 248 is directed to a carbonylation process of a liquid composition containing a promoter, such as ruthenium, rhenium and osmium and a co-promoter wherein the co-promoter is selected from alkali metal iodides, alkaline earth metals, 15 metal complexes and salts capable of generating I-. The use of a both promoter and a co-promoter provides a process with relatively high yields and selectivity. In Comparative Experiment G. data is provided showing that the use of 1 equivalent of lithium iodide to an unpromoted iridium catalysed carbonylation process reduces the carbonylation rate by approx. 50%.

20 Thus, hitherto, the art in general has been directed to iridiumcatalysed carbonylation processes which employ a metal promoter such as ruthenium, rhenium or osmium. There remains a need for an iridiumcatalysed carbonylation process in which a conventional metal promoter such as ruthenium, osmium or rhenium is not used.

25 It has now been found that an iridium-catalysed carbonylation process can be carried out in the absence of a conventional metal promoter by using an iodide as promoter. Accordingly, the present invention provides a process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive 30 derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of a promoter selected from the group consisting of ruthenium, osmium, rhenium, tungsten, zinc, cadmium, iridium, gallium and mercury, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co

catalyst, a finite concentration of water, acetic acid, methyl acetate and a salt soluble in the liquid reaction composition wherein the salt is selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt and manganese, salts capable of generating I-, 5 lanthanide metal complexes capable of generating I- and mixtures of two or more thereof wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs, the salt is present in the liquid reaction composition in an amount effective as a promoter and such that the salt provides in the liquid reaction composition a molar ratio of iodide: iridium in the 10 range [greater than 0 to 0.5]: 1.

The present invention also provides the use of a salt selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt and manganese, salts capable of generating I-, lanthanide metal complexes capable of generating I- and mixtures of two or more 15 thereof in a process for the production of acetic acid by carbonylation with carbon monoxide of methanol andlor a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and said salt wherein the water concentration is at or 20 below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the salt is present in the liquid reaction composition in an amount effective as a promoter and such that the salt provides in the liquid reaction composition a molar ratio of iodide: iridium in the range [greater than O to 0.5]: 1 The present invention utilises a liquid reaction composition comprising as 25 promoter an iodide. The composition does not require the presence of conventional metal promoters used in the carbonylation process. By conventional metal promoters is meant ruthenium, osmium, tungsten, rhenium, zinc, cadmium, indium, gallium, and mercury. The process of the present invention unexpectedly provides a benefit in 30 carbonylation rate at low water concentration which may allow operation at a reduced iridium catalyst concentration whilst maintaining the rate of carbonylation.

The rate of production of the by-products propionic acid, methane, hydrogen and carbon dioxide may be reduced.

Additional benefits may be derived by avoiding the need to use expensive metal promoters such as ruthenium, osmium and rhenium.

Water may be formed in situ in the liquid reaction composition, for example, by the esterification reaction between methanol reactant and acetic acid product. Small 5 amounts of water may also be produced by hydrogenation of methanol to produce methane and water. Water may be introduced to the carbonylation reactor together with or separately from other components of the liquid reaction composition. Water may be separated from other components of reaction composition withdrawn from the reactor and may be recycled in controlled amounts to maintain the required concentration of 10 water in the liquid reaction composition.

With reference to the aforesaid published European Application No. 0 752 406 the rate of the carbonylation reaction is said to increase as the water concentration in the liquid reaction composition is reduced from a concentration of greater than 6.5% by weight, passes through a maximum at a water concentration of no greater than 6.5% by 15 weight and then declines as very low water concentrations are approached. In Figure 8 of the aforesaid application there is a plot of reaction rate versus water concentration which clearly shows a maximum. The water concentration at which the carbonylation rate is a maximum is said to increase as the concentration of methyl acetate in the liquid reaction composition is increased. It is believed that the water concentration at which 20 the carbonylation rate is a maximum decreases as the concentration of methyl iodide in the liquid reaction composition is increased. For the purpose of the present invention the water concentration in the liquid reaction composition is preferably maintained below 6%, more preferably below 4.5% by weight. Operating at such a low water concentration according to the present invention gives rise to the advantage that 25 recovery of acetic acid from the reaction composition withdrawn from the carbonylation reactor is facilitated because the amount of water which has to be separated from the acetic acid is reduced; separation of water from the acetic acid is an energy-intensive part of the recovery process and reduced water concentration results in reduced processing difficulty and/or costs.

30 In the process of the present invention, suitable reactive derivatives of methanol include methyl acetate, dimethyl ether and methyl iodide. A mixture of methanol and reactive derivatives thereof may be used as reactants in the process of the present invention. Preferably, methanol and/or methyl acetate are used as reactants. If methyl

acetate or dimethyl ether are used, water co-reactant is required to produce acetic acid.

At least some of the methanol and/or reactive derivative thereof will be converted to, and hence present as, methyl acetate in the liquid reaction composition by reaction with acetic acid product or solvent. In the process of the present invention the concentration 5 of methyl acetate in the liquid reaction composition is suitably in the range 1 to 70% by weight, preferably 2 to 50 % by weight, more preferably 5 to 40% by weight.

In the process of the present invention, the concentration of methyl iodide co-

catalyst in the liquid reaction composition is suitably in the range from I to 30% by weight, preferably in the range from I to 20% by weight.

10 In the process of the present invention, the iridium carbonylation catalyst is suitably present in the liquid reaction composition at a concentration in the range 100 to 6000 ppm measured as iridium. In the process of the present invention, the rate of the carbonylation reaction increases as the concentration of iridium is increased.

The iridium catalyst in the liquid reaction composition may comprise any 15 iridium-containing compound which is soluble in the liquid reaction composition. The iridium catalyst may be added to the liquid reaction composition for the carbonylation reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to a soluble form. Examples of suitable iridium-containing compounds which may be added to the liquid reaction composition include IrC13, IrI3, IrBr3, 20 [Ir(CO)2I]2, [Ir(CO)2CI]2, [Ir(co)2Br]2, [Ir(Co)2I2]-H+, [Ir(Co)2Br2]- H+, [Ir(CO)2I4]-H+, [Ir(CH3)I3(co)2]-H+, Ir4(CO)12, IrC13.3H2O, IrBr3. 3H2O, Ir4(CO)12, iridium metal, Ir2O3, IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [Ir3O(OAc)6(H2O)3][OAc], and hexachloroiridic acid [H2IrC16], preferably, chloride free complexes of iridium such as acetates, oxalates and acetoacetates which are soluble 25 in one or more of the carbonylation reaction components such as water, alcohol and/or carboxylic acid.

In the process of the present invention the iodide ion may be derived from any of the salts selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt, and 30 manganese, salts capable of generating I-, lanthanide metal complexes capable of generating I- and mixtures of two or more thereof. Any of these salts may be used provided that the salt is sufficiently soluble in the liquid reaction composition to

provide the desired level of iodide ion.

The iodide ion may be derived from an iodide salt. Alternatively, the iodide ion may be generated in-situ, for example, by the use of a salt capable of generating iodide ion in the liquid reaction composition. Typically, such salts include, organic salts, such 5 as quaternary ammonium or phosphonium salts, such as quaternary ammonium iodides or quaternary phosphonium iodides which may be added as such. Inorganic salts capable of generating iodide ion in-situ may also be used, such as salts of the alkali metals and alkaline earth metals capable of generating iodide ions and salts of the elements nickel, chromium, iron, molybdenum, cobalt and manganese capable of 10 generating iodide ions.

Salts capable of generating iodide ion in-situ include acetate salts, such as the acetates of the alkali metals, for example, lithium acetate, sodium acetate and potassium acetate, acetates of the alkaline earth metals, such as the acetates of magnesium, calcium, strontium and barium and acetates of the elements nickel, chromium, iron, 15 molybdenum, cobalt and manganese Suitable alkali metal salts iodides are, for example, lithium iodide, sodium iodide, potassium iodide, Suitably, the alkaline earth metal iodides are the iodides of magnesium, calcium, strontium and barium.

20 Suitably, the iodides of the elements nickel, manganese, iron, cobalt, molybdenum and chromium may be NiI2, CrI3, FeI2, CoI2 and MnI2.

Suitable, lanthanide metal complexes capable of generating I-, include complexes of samarium, gadolinium and cerium.

Mixtures of two or more salts may be used. Preferably, the salt is an alkali metal 25 iodide or an alkali metal acetate.

The amount of salt present in the liquid reaction composition is such that the amount of iodide ion generated by the salt provides a molar ratio of iodide to iridium in the range [greater than O to 0.5]: 1.

Suitably, the amount of alkali metal iodide or acetate used may be in the range 5 30 - 5000 ppm, for example 5-1000 ppm.

The carbon monoxide reactant may be essentially pure or may contain inert impurities such as carbon dioxide, methane, nitrogen, noble gases, water and Cl to C4 paraffinic hydrocarbons. The presence of hydrogen in the carbon monoxide feed and

generated in situ by the water gas shift reaction is preferably kept low as its presence may result in the formation of hydrogenation products. Thus, the amount of hydrogen in the carbon monoxide reactant is preferably less than 1 mol %, more preferably less than O.S mol % and yet more preferably less than 0.3 mol % and/or the partial pressure of 5 hydrogen in the carbonylation reactor is preferably less than 1 bar partial pressure, more preferably less than O.S bar and yet more preferably less than 0.3 bar. The partial pressure of carbon monoxide in the reactor is in the range greater than O to 40 bar, typically from 4 to 30 bar.

The total pressure of the carbonylation reaction is suitably in the range 10 to 200 10barg, preferably 15 to lOO barg, more preferably 15 to 50 barg. The temperature of the carbonylation reaction is suitably in the range 100 to 300 C, preferably in the range 150 to 220 C.

The process of the present invention may be performed as a batch or as a continuous process, preferably as a continuous process.

1 SThe acetic acid product may be recovered from the liquid reaction composition by withdrawing vapour and/or liquid from the carbonylation reactor and recovering acetic acid from the withdrawn material. Preferably, acetic acid is recovered from the liquid reaction composition by continuously withdrawing liquid reaction composition from the carbonylation reactor and recovering acetic acid from the withdrawn liquid 20 reaction composition by one or more flash and/or fractional distillation stages in which the acetic acid is separated from the other components of the liquid reaction composition such as iridium catalyst, methyl iodide co-catalyst, iodide promoter, methyl acetate, unreacted methanol and/or reactive derivative thereof, water and acetic acid solvent which may be recycled to the reactor to maintain their concentrations in the 25 liquid reaction composition. To maintain stability of the iridium catalyst during the acetic acid product recovery stage, water in process streams containing iridium carbonylation catalyst for recycle to the carbonylation reactor should be maintained at a concentration of at least 0.5 % by weight.

The invention will now be illustrated by way of example only and with reference 30 to the following Figure and Examples.

Figure l is a graph showing the effect of Lit addition and water concentration on-

carbonylation rate for iridium catalysed carbonylation of methanol at 30% w/w methyl acetate.

General Description of the Carbonvlation Experiments

All experiments were performed using a 300 cm3 zirconium autoclave equipped with stirrer and a liquid catalyst injection facility.

The autoclave was pressure tested to 4 x lOfi N/m2 with nitrogen, vented via a 5 gas sampling system, and purged with carbon monoxide several times (I x 106 N/m2).

If a promoter was used this was placed in the autoclave and covered with a portion of the acetic acid charge (ca. leg) prior to the pressure test. An initial charge of methyl acetate (ca beg), acetic acid (ca. 58g), methyl iodide (ca 14g) and water (ca. 0.7g) was placed in the autoclave which was then repurged with carbon monoxide and vented 10 slowly to prevent loss of volatiles.

Carbon monoxide (ca 6-7 x 1Os N/m2) was introduced into the autoclave which was then heated with stirring (1500 rpm) to a temperature of 190 C. Approximately 0.6g of dihydrogenhexachloroiridate (IV), acetic acid (approx l O.Og) and water (approx S.Og) was injected with an overpressure of carbon monoxide to the hot autoclave, to 15 bring the autoclave pressure to 2.8x106 N/m2 The reaction rate was monitored by drop in carbon monoxide pressure from a ballast vessel, typically pressured to 7X106 N/m2 The autoclave temperature and pressure were maintained at a constant 1 90 C and 2.8x 1 o6 N/m2 throughout the reaction by pressure and coolant control valves. The reaction was terminated when the drop in 20 ballast pressure became less than 1x104 N/m2 per 5 minutes.

The rate of gas uptake at a certain point in a reaction run was used to calculate the carbonylation rate, as number of moles of reactant consumed per litre of cold degassed reactor composition per hour (mol/l/h) at a particular reactor composition (total reactor composition based on a cold degassed volume).

25 The methyl acetate concentration was calculated during the course of the reaction from starting composition, assuming that one mole of methyl acetate was consumed for every mole of carbon monoxide that was consumed. No allowance was made for organic components in the autoclave headspace.

By monitoring the rate of carbonylation reaction and calculating the 30 concentration of the reaction components during the experiment, it is possible to determine the rate of carbonylation reaction which would be expected if a carbonylation process were to be operated continuously whilst maintaining under steady state, a liquid reaction composition which is the same as the total reaction composition calculated at

any particular point in the batch experiment.

In the batch experiments the term 'reaction composition' refers to the total composition of the components in the autoclave in the cold degassed state EXAMPLES 1 to 5 and EXPERIMENTS A to E. 5 The general procedure described hereinabove was employed. The charge compositions are given in Table 1.

Experiments A to D are not according to the present invention for the reason that either no iodide salt or no salt capable of generating iodide ion was present in the liquid reaction composition.

10 Experiment E is not according to the invention for the reason that, although an iodide salt was present in the liquid reaction composition, the molar ratio iodide: iridium was not in the range [greater than 0 to 0. 5]: 1.

Examples 1 to 5 demonstrate the effect on carbonylation rate of adding an iodide salt in a molar ratio of iodide: iridium in the range [greater than 0 to 0.5]: 1, in the 15 absence of a conventional promoter, using an iridium catalyst at 190 C and 28 berg total pressure. Rate data at a methyl acetate (MeOAc) concentration of cat 30% and a water concentration of ca 2% w/w are given in Table 2.

Table 1

Experiment/ MeOAc AcOH MeI Water H2IrCl6 iodide Amount Example I(g) I(g) /(g) /(g) /(ga) I(g) 61.08 69.18 13.99 6.60 0.636

B 60.52 68.97 13.97 6.47 0.640

C 60.02 68.99 13.96 6.46 0.642

D 60.02 69.00 13.96 6.40 0.636

E 60.00 68.80 13.96 6 41 0.637 Lil 0.209 1 61.13 69.03 13.96 6.42 0. 647 LiI 0.063 60.01 68.94 13.97 6.42 0.635 LiI 0.111 3 60.18 68.06 13. 99 6.44 0.647 NaI 0.035 61.01 68.07 13.91 6.43 0.642 NaI 0.060 61.27 68. 27 13.31 6.42 0.642 NaI 0.122 a) Weight expressed as pure H2IrCI6 Table 2

Experiment/ Iodide Molar Ratio Water Rate/mol/l/h Example Iodide: Iridium % w/w (A 30% MeOAc 2.0 14

B 2.0 13.1

C 2.1 12.1

D 2.0 13.3

LiI 2.0 6.3 1 LiI 0.3: 1 1.9 17.6 LiI 0.5: 1 2.0 15.7 Nat 0.15:1 2.0 19.3 NaI 0.3:1 1.9 18.3 NaI 0.5:1 1.9 14.6

From Table 2 it can clearly be seen from a comparison of Experiments A-D ( no promoter salt present in the liquid reaction composition) with Experiment E (iodide is present at a molar ratio iodide: iridium of 1: 1), that the addition of lithium iodide to 5 an unpromoted iridium catalysed carbonylation reaction has a detrimental effect on the carbonylation rate under the same conditions.

Comparison of Examples 1 to 5 with Experiments A-E demonstrates the significant promotional effect on carbonylation rate of adding Lit or Nat at a molar ratio iodide: iridium in the range [greater than O to 0.5]: 1 10 Figure 1 shows the effect of adding lithium iodide at water concentrations where the carbonylation rate is increasing with decreasing water concentrations - that is at water concentrations which are at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs. Under these conditions, addition of the lithium iodide caused a significant increase in the carbonylation rate.

15 Figure 1 also shows that under the same conditions, the use of high levels of lithium iodide causes a significant reduction in the carbonylation rate.

Claims

Claims:

1. A process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of a promoter selected from the group consisting of ruthenium, osmium, rhenium, tungsten, zinc, cadmium, 5 indium, gallium and mercury, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and a salt soluble in the liquid reaction composition wherein the salt is selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt and 10 manganese, salts capable of generating I-, lanthanide metal complexes capable of generating I- and mixtures of two or more thereof wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs, the salt is present in the liquid reaction composition in an amount effective as a promoter and such that the salt provides in the liquid reaction composition 15 a molar ratio of iodide: iridium in the range [greater than 0 to 0.5]: 1.

2. A process according to claim I wherein the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 100 to 6000 ppm measured as iridium, the methyl acetate concentration in the liquid reaction composition is in the range I to 70% by weight and the methyl iodide concentration in the liquid 20 reaction composition is in the range I to 30% by weight.

3. A process according to claim I or claim 2 wherein the methyl acetate concentration in the reaction composition is in the range 2 to 50% by weight.

4. A process according to claim3 wherein the methyl acetate concentration in the reaction composition is in the range 5 to 40% by weight.

5. A process according to any one of claims 1 to 4 wherein the methyl iodide concentation in the reaction composition is in the range 1 to 20% by weight.

5 6. A process according to any one of claims I to 5 in which the water concentration in the reaction composition is maintained at below 6% by weight.

7. A process according to claim 6 in which the water concentration in the reaction composition is maintained at below 4.5% by weight.

8. A process according to any one of claims 1 to 7 wherein the alkali metal iodide 10 salt is selected from lithium iodide, sodium iodide, potassium iodide and mixtures thereof, and the alkaline earth metal iodide salt is selected from magnesium iodide, calcium iodide, strontium iodide, barium iodide and mixtures thereof 9. A process according to claim 8 wherein the alkali metal iodide salt is selected from lithium iodide, sodium iodide and mixtures thereof.

15 10. A process according to any one of claims I to 7 wherein the salt capable of generating I- is selected from the group consisting of quaternary ammonium salts, quaternary phosphonium salts and mixtures thereof.

11. A process according to claim 10 wherein the quaternary ammonium salt is a quaternary ammonium iodide and the quaternary phosphonium salt is a quaternary 20 phosphonium iodide.

12. A process according to any one of claims 1 to 7 wherein the salt capable of generating I- is selected from salts of the elements nickel, chromium, iron, molybdenum, cobalt, manganese and mixtures thereof.

13. A process according to any one of claims 1 to 7 wherein the salt capable of 25 generating I- is an acetate salt.

14. A process according to claim 13 wherein the acetate salt is selected from the group consisting of acetates of the elements nickel, manganese, iron, cobalt, molybdenum and chromium, alkali metal acetates, alkaline earth metal acetates and mixtures thereof.

30 15. A process according to claim 14 wherein the alkali metal acetate is selected from the group consisting of lithium acetate, sodium acetate, potassium acetate and mixtures thereof and the alkaline earth metal acetate is selected from the group consisting of

magnesium acetate, calcium acetate, strontium acetate, barium acetate and mixtures thereof 16. A process according to claim 15 wherein the alkali metal acetate is selected from lithium acetate, sodium acetate and mixtures thereof.

S 17. A process according to any one of claims 1 to 7 wherein the iodide of the elements of nickel, chromium, iron, cobalt and managanese is selected from the group consisting of NiI2, CrI3, Fel, CoI2, and Mnl2.

18. A process according to any one of claims 1 to 7 wherein the lanthanide metal complex capable of generating 1- is selected from complexes of samarium, gadolinium, 10 cerium and mixtures thereof.

19. A process according to any one of claims 1 to 9 and 13 to 16 wherein the amount of alkali metal iodide or alkali metal acetate added to the reaction composition is in the range 5 to 5000 ppm.

20. A process according to any one of claims 1 to 19 in which the carbonylation 15 reaction temperature is in the range lOO to 300 C and the total pressure is in the range lO to 200 barg.

21. A process according to any one of claims l to 20 wherein the amount of hydrogen present in the carbon monoxide reactant is less than I mol% and/or the partial pressure of hydrogen in the carbonylation reactor is less than 1 bar.

20 22. A process according to any one of claims 1 to 21 wherein the partial pressure of carbon monoxide reactant is in the range greater than O to 40 bar.

23. A process substantially as hereindescribed and with reference to Examples 1 to 5 and Figure 1.

24. Use of a salt selected from the group consisting of alkali metal iodides, alkaline 25 earth metal iodides, iodides of the elements iron, nickel, chromium, molybdenum, cobalt and manganese, salts capable of generating I-, lanthanide metal complexes capable of generating I- and mixtures of two or more thereof in a process for the production of acetic acid by carbonylation with carbon monoxide of methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction 30 composition, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finiec concentration of water, acetic acid, methyl acetate and said salt wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and the

salt is present in the liquid reaction composition in an amount effective as a promoter and such that the salt provides in the liquid reaction composition a molar ratio of iodide: iridium in the range [greater than O to 0.5]: 1