GB2394472A - Production of acetic acid by iridium-catalysed carbonylation of methanol and/or derivative thereof using a bis-phosphonate promoter - Google Patents

Abstract

A process for the production of acetic acid by the iridium catalysed carbonylation of methanol and/or a reactive derivative thereof in the substantial absence of a conventional metal promoter such as ruthenium, osmium and rhenium wherein the water concentration in the liquid reaction composition is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and wherein the liquid reaction composition also comprises as promoter a bis-phosphonate compound having the formula I :- <EMI ID=1.1 HE=28 WI=58 LX=298 LY=1487 TI=CF> <PC>wherein R<1>, R<2>, R<3>, R<4> are independently hydrogen or an organic functional group; Y is an optionally substituted C1-C10 alkylene or C6-C10 aryl group. In particular, the reaction is carbonylation of methyl acetate and the bis-phosphonate compound is bis-1,2-dimethoxyphosphoryl benzene (dmpb).

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, methyl iodide cocatalyst and a catalyst promoter.

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+ I) 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 forth 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 I and 6%. The process of WO-A-95/31426 is otherwise 20 unprompted. El>-A-0643034 describes a process for the carbonylation of methanol and/or a reactive derivative thereof in the presence of acetic acid, an iridium catalyst, methyl iodide, at least a finite concentration of water, methyl acetate and a promoter selected

from ruthenium and osmium. Batch and continuous experiments are described therein.

In the continuous experiments the water concentration is as low as 6.8 TO by weight.

EP-A- 0752406 discloses a process for the production of acetic acid comprising (1) continuously feeding methanol anchor a reactive derivative thereof and carbon 5 monoxide to a carbonylation reactor which contains a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and at least one promoter; (2) contacting the methanol and/or reactive derivative thereof with the carbon monoxide in the liquid reaction composition to produce acetic acid; and (3) recovering acetic acid 10 from the liquid reaction composition characterized in that there is continuously maintained in the liquid reaction composition throughout the course of the reaction (a) water at a concentration of no greater than 6.5 % by weight, (b) methyl acetate at a concentration in the range l to 35 % by weight and (c) methyl iodide at a concentration in the range 4 to 20 % by weight.

15 There remains a need for an improved iridium-catalysed promoted carbonylation process in which conventional metal promoters such as ruthenium, osmium, rhenium, tungsten, zinc, gallium, cadmium, mercury and indium are not employed.

The technical problem is solved by the use in an iridium-catalysed promoted carbonylation process of a liquid reaction composition defined in terms of water 20 composition and which contains a bis-phosphonatc compound as promoter.

Accordingly the present invention provides a process for the production of acetic acid by carbonylating with carbon monoxide methanol andVor 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, 25 tungsten, rhenium, zinc, gallium, cadmium, mercury and indium, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and a promoter wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and wherein the promoter is a bis 30 phosphonate compound having the formula I:

R -0\ ANY\ O-R

11 11 O O (1)

wherein R', R2, R3, R4 are independently hydrogen or an organic functional group; Y is 5 an optionally substituted C-CO alkylene or C6-CO aryl group.

The present invention also provides the use of a bis-phosphonate compound as promoter in a process for the production of acetic acid by carbonylating with carbon monoxide methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of a promoter selected 10 from the group consisting of ruthenium, osmium, tungsten, rhenium, zinc, gallium, cadmium, mercury and indium, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and a promoter wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration 15 occurs and wherein said promoter is a bis-phosphonate compound of formula (I) as hereindefined above.

The present invention provides several technical advantages. The increased carbonylation rate at the low water concentration of the present invention may allow operation at a reduced iridium catalyst concentration whilst maintaining the rate of 20 carbonylation. lithe rate of production of by-products propionic acid, methane, hydrogen and carbon dioxide may be reduced.

The process of the present invention is carried out in the absence of conventional promoters used in iridium-catalysed carbonylation processes such as ruthenium, 25 osmium, rhenium, zinc, gallium, tungsten, cadmium, mercury and indium thereby allowing reduced costs.

The process of the present invention does not require the presence of a co promotcr such as alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating I-, salts capable of generating I- or mixtures thereof.

30 Water may be formed in situ in the liquid reaction composition, for example, by

the esteriEcation reaction between methanol reactant and acetic acid product. Small 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 5 separated Prom other components of reaction composition withdrawn from the reactor and may be recycled in controlled amounts to maintain the required concentration of water in the liquid reaction composition.

With rel'erence to the aforesaid European Application No. 0752406 the rate of the carbonylation reaction is said to increase as the water concentration in the liquid 10 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 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 15 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 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 20 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 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 cncrgy-intensive 25 part of the recovery process and reduced water concentration results in reduced processing difficulty and/or costs.

In the present invention, suitable reactive derivatives of methanol include methyl acctatc, 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.

30 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 of methyl acetate in the liquid reaction composition is suitably in the range I to 70% by weight, preferably 2 to 50 X, by weight, more preferably S to 40% by weight.

In the present invention, the concentration of methyl iodide co-catalyst in the 5 liquid reaction composition is suitably in the range 1rom I to 30% by weight, preferably in the range from I to 20% by weight A method of increasing the carbonylation rate is to increase the methyl iodide concentration. I lowcver this method can cause increased corrosion levels. An advantage of the present invention is that high carbonylation rates at low methyl iodide and water 10 concentrations can be achieved whilst maintaining or even reducing corrosion.

In the present invention, the iridium carbonylation catalyst is suitably present in the liquid reaction composition at a concentration in the range 400 to 5000 ppm measured as iridium, preferably in the range 500 to 3000 ppm measured as iridium, more preferably in the range 700 to 3000 ppm measured as iridium. In the process of 15 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 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 20 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 IrCI3, TrI3, IrBr3, [Ir(CO)2l]2, [Ir(CO)2CI]2, [Ir(CO)2Br]2, [Ir(CO)2I2]H, [Tr(CO)2Br2]H, [Ir(CO)24] H. [Ir(CH3)I3(CO)2]H, Tr4(CO)2, IrCI3.3H2O, IrBr3.3H2O, Ir4(CO) 2, iridium metal, Ir2O3, 25 IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [lr3O(OAc)6(H2O)3][OAc], and hexachloroiridic acid [H2IrCI6], preferably, chloride-free complexes of iridium such as acetates, oxalates and acetoacetates which are soluble in one or more of the carbonylation reaction components such as water, alcohol and/or carboxylic acid.

Particularly preferred is green iridium acetate which may be used in an acetic acid or 30 aqueous acetic acid solution.

In the present invention one or more bis-phosphonate compounds of formula I:

R -O-p/ Cop/ o 4 O O (1)

wherein Rim R2, R3, R4 are independently hydrogen or an organic functional group; Y is an optionally substituted C-CO alkylene or C6-CO aryl group are present in the liquid 5 reaction composition.

The organic functional group (Ri, R2, R3, R4) is suitably an unsubstituted or substituted hydrocarbon group, such as an unsubstituted or substituted alkyl, aryl, or cycloalkyl group.

Suitably, the hydrocarbon group is an alkyl group such as C, to C,5 alkyl or an aryl 10 group such as C6 to CIO aryl.

The hydrocarbon group may be substituted by one or more substituents.

Suitably the substituted hydrocarbon group may be a C, to C,O alkyl or a C6 to C,O aryl group substituted by one or more of -NE [2, -NO2, -Sl 1, halogen, -CO2R5, -COR6, oR7, -COX, -CSX', -CN, -NCS or -NCO wherein R5, R6 and R7 are independently 15 hydrogen, an unsubstituted or substituted hydrocarbon group such as a C'-CO alkyl or a C6 to C,o aryl, optionally substituted with one or more heteroatoms sueh as oxygen, nitrogen or sulphur; X and X'are independently selected from -NH2, -NO2, -SI-l, hydrogen, -C113, -(CH2)n X2, halogen, -CO2R8, -CoR9, -eox3, -CSX4, -CN, NCS or NCO wherein R and R9 are as defined for R5 or R6 or R7 and X2, X3 and X4 are defined 20 as for X or X'; n is 1-10.

Suitably, the substituted hydrocarbon group may be -(CH2)nX5 where n is 110, CHX6X7, CXX9Xt0 where X5, X6, X7, X8, X9 and X are as defined for X or X'.

The organic functional group may comprise nitrogen, oxygen, sulphur atoms or mixtures thereof. Suitably the organic functional group may be -CO2Ri , -SO2Rt i, 25 CORi2, -COX", -CSX'2, -CN, -NCS or-NCO where R' , R", Ri2, are as defined for R5 or R6 or R7; X" and x'2 are as defined for X or X'.

Suitably, where the organic functional group is an aryl group, such as C6C,o aryl, it may be substituted by one or more heteroatoms, such as sulphur, oxygen, nitrogen or combinations thereof.

30 Preferably, R'-R4 are each hydrogen, or a C,-C'5 alkyl group such as CI13 or a

Cl l(Ct13)2 group.

Y is an optionally substituted Cal- CIO alkylene group such as a C3-Cio cycloalkyl group or Y is an optionally substituted C6-CO aryl group. The (cyclo)alkyl or aryl group may be substituted by one or more substituents such as those defined for X or X 5 above. Y is preferably an unsubstihtcd C-C> alkylene group such as --CH2, -(C112)2-, C3-Co cycloalkyl or a substituted C6-CO aryl, such as a substituted benzene. Most preferably Y is a 1,2 disubstituted benzene or a C3-C, cycloalkyl group.

Suitably, R'-R4 are each hydrogen, or are each a C-Cs alkyl group such as 10 Cl13 or a -CH(CH3)2 group and Y is an unsubstitutcd C'-C' alkylene group such as CH2, -(CH2)2-, or an optionally substituted C3-C' cycloalkyl group or an optionally substituted C6-CO aryl such as a substituted benzene, for example, a 1,2 disubstitutcd benzene ring.

Specific examples of suitable bis-phosphonate compounds include tetraisopropyl 15 1,2 ethylene diphosphonate, mcthylene diphosphonic acid and his 1,2 dimethoxyphosphoryl benzene.

The bis-phosphonate promoter is suitably present in the liquid reaction composition at a molar ratio of'promoter to iridium of [greater than O to 10]:1, preferably Lo.2 to 2]:1, more preferably [0.5 to 1.5]:1.

20 The bis-phosphonate compound employed in the present invention may be added as such to the liquid reaction composition or may be formed in-situ in the liquid reaction composition, t'or example by the addition of the conjugate bis-phosphinic acid to the liquid reaction composition.

The carbon monoxide reactant may be essentially pure or may contain inert 25 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 I mol /0, more preferably less than 30 0.5 mol % and yet more preferably less than 0.3 mol % and/or the partial pressure of hydrogen in the carbonylation reactor is preferably less than I bar partial pressure, more preferably less than 0.5 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 barg, preferably 15 to 100 barg, more preferably IS to 50 barg. The temperature of the carbonylation reaction is suitably in the range 100 to 300 C, preferably in the range 150 5 to 220 C.

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

T he acetic acid product may be recovered from the liquid reaction composition by withdrawing vapour and/or liquid from the carbonylation reactor and recovering 10 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 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 15 composition such as iridium catalyst, methyl iodide co-catalyst, 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 liquid reaction composition. To maintain stability of the iridium catalyst during the acetic acid product recovery stage, water in process streams containing iridium 20 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 to the following examples.

(general Description of the Carbonylation Experiments

25 The experiments were performed in a 300cm3 Zirconium autoclave, equipped with a stirrer and liquid injection facility. The autoclave was pressure tested to 4X106 N/m2 with nitrogen, then flushed three times with carbon monoxide up to IxlO6N/m2. If a promoter or additive was used, this was placed in the autoclave and covered with a portion of the acid (approx. log) prior to the pressure test. An initial charge consisting 30 of methyl acetate (approx 60.0g) acetic acid (approx 58.0g) methyl iodide (approx 14g) and water (approx 0.7g) was placed into the autoclave, which was then repurgcd with carbon monoxide and vented slowly to prevent loss of volatiles.

Carbon monoxide (approx 6-7x 105 N/m2) was placed in the autoclave which

was then hcatcd, with stirring (1500 rpm) to 190 C. The catalyst injection system was primed with approx 1.35g of dihydrogenhexachloroiridate (IV), acetic acid (approx 1 O.Og) and water (approx 5.0g) and injected with an overpressure of carbon monoxide to the hot autoclave, to bring the autoclave pressure to 2.8x 1 o6 N/m2. The reaction rate S was monitored by drop in carbon monoxide pressure from a ballast vessel, typically pressured to 7x l o6 N/m2. The autoclave temperature and pressure were maintained at a constant l 90"C and 2.8x l o6 N/m2 throughout the reaction by pressure and coolant control valves. The reaction was terminated when the drop in ballast pressure became less than 1x104 N/m2 per 5 minutes.

10 After cooling, a gas analysis sample was taken, and the autoclave vented. The liquid components were discharged, and analysed for liquid byproducts by known established gas chromatography methods. Detected components are quantified by integration of the component peaks relative to an external standard and expressed as parts per million (ppm) by weight.

15 In the batch reactions, 'Total' propanoic acid was defined as the sum of propanoic acid and its precursors (ethyl acetate and ethyl iodide) converted to ppm propanoic acid) detected in the quenched liquid products of the batch reaction expressed in ppm.

The rate of gas uptake at a certain point in a reaction run was used to calculate 20 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) The methyl acetate concentration was calculated during the course of the reaction from starting composition, assuming that one mole of methyl acetate was 25 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 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 30 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 experiment the term 'reaction composition' refers to the total

composition of the components in the autoclave in the cold degassed state.

Experiments A-1) and Examples 1-5 The general procedure described hereinabove was employed. The charge compositions are given in Table 1.

5 Experiments A to D are not according to the present invention for the reason that no bis-phosphonate compound was present in the liquid reaction composition.

Examples I to 5 demonstrate the effect on carbonylation rate of adding a bis phosphonate compound according to the present invention (1,2 bis(dimethoxyphosphoryl)benzene (dmpb), in the absence of a conventional promoter, 10 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 Charge compositions for iridium catalysed reactions in a 300 cm3 15 zirconium batch autoclave Experiment/ MeOAe Acetic Mel Water E 12IrCI6 Promoter Amount Example (g) Acid (g) (g) (8a) of (g) promoter 61. 08 59.19 13.99 0.80 1.349

B 60.52 58.94 13.97 0.70 1.360

C 60.02 68.99 13.96 6.46 0.642

D 60.02 69.00 13.96 6.40 0.636

61.13 59.31 13.97 0.72 1.37 dmpb 0.142 2 60.25 59.39 13.98 0.71 1. 39 dmpb 0.233 3 60.04 58.54 14.05 0.71 1.37 dmpb 0.347 60.11 58.01 13. 99 0.70 1.39 dmpb 0.455 5 60.04 58.12 13.96 0.69 I.359 dmpb 0.542 a) Weight expressed as pure 112IrCI6

Table 2 Carbonylation rate data for iridium catalysed reactions in a 300 cm3 zirconium autoclave l xamplc/ Promotcr Molar Watcr Ratc/mol/l/h Propanoic H2 CO2 CH4 1xpt Ratio o/o wlw (p 30 /O aciJ (mmol) (mmol) (mmol) Promotcr: McOAc (ppl') Iridium A 2. 0 14 719 0.1 1.6 2.3

B 2.0 13.1 379 0.7 1.1 1.5

C 2.1 n/a n/a n a n a D 2.0 13.3 n/a n/a n/a n/a 1 dmpb 0.3: 1 2.0 17. 6 737 0.1 1.1 1.5 2 dmpb 0.5: 1 2.0 15.8 501 i.3 1.4 1.3 3 dmpb 0.75: 1 2. 0 16.6 373 0.8 2.2 1.3 4 dmpb 1: 1 2.0 18.8 566 O.R 1.2 1 6 5 dmpb 1.25: 1 2.0 17.5 397 0.7 2.0 1.3 s From Table 2 a comparison of Examples I -5 with Experiments A-D demonstrates that adding a bisphosphonate compound according to the invention provides a significant increase in carbonylation rate over that achieved by iridium-catalysed carbonylation in 10 the absence of a bis-phosphonate promoter under the same conditions.

Claims (1)

1. Claims:

   1. A process for the production of acetic acid by carbonylating with carbon monoxide 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, tungsten, rhenium, zinc, gallium, 5 cadmium and indium, said liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and a promoter wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and wherein the promoter is a bis-phosphonate compound having the formula I: 1 \ i\ / 4 R -O P P O-R

   11 11 O O (I)

   l 5 wherein Rt, R2, R3, R4 are independently hydrogen or an organic functional group; Y is an optionally substituted C-Cro alkylene or C6-CO aryl group.

   2. A process according to claim I wherein R', R2, R3, R4 are independently an organic functional group selected from an unsubstituted hydrocarbon group and a substituted hydrocarbon group.

   3. A process according to claim I or claim 2 wherein Ret, R2, R3, R4 are each hydrogen or are each an unsubstituted hydrocarbon group.

   4. A process according to claim 3 wherein each unsubstituted hydrocarbon group is a- CHIN- group or a CH(CI-13)2- group.

   5 5. A process according to any one of claims I to 4 wherein Y is selected from the group consisting of an unsubstituted C-CO alkylene group, an optionally substituted C6-Co aryl and an optionally unsubstituted C3-C' cycloalkyl.

   6. A process according to claim 5 wherein the unsubstituted C-C'alkylene group is selected from the group consisting of-CH2- and (CH2)2-.

   10 7. A process according to claim 5 wherein the optionally substituted C6-CO aryl is a substituted benzene.

   8. A process according to claim I wherein Y is selected from an unsubstituted C'-

   Co alkylene group, an optionally substituted C3-Co cycloalkyl group and an optionally substituted C6-Co aryl and R', R2, R3, R4 are each hydrogen or are each an 15 unsubstituted hydrocarbon group.

   9. A process according to claim 8 wherein each unsubstituted hydrocarbon group is a methyl group.

   10. A process according to claim I wherein the bis-phosphonate is selected from the group consisting of tetraisopropyl 1,2 ethylene diphosphonate, methylene diphosphonic 20 acid and his 1,2 dimethoxyphosphoryl benzene.

   I 1. A process according to any one of claims 1 to 10 wherein the bisphosphonate is formed in-situ in the liquid reaction composition.

   12. A process according to claim 11 wherein the bis-phosphonate is formed from the conjugate bis-phosphinic acid.

   25 13. A process according to any one of claims I to 10 wherein the bisphosphonate promoter is added to the liquid reaction composition as a bisphosphonate compound.

   14. A process according to any one of claims I to 13 wherein the bisphosphonate compound is present in the liquid reaction composition at a molar ratio of bis-

   phosphonate compound: iridium in the range [greater than O to 10]: 1.

   30 15. A process according to claim 14 wherein the molar ratio of bisphosphonate compound: iridium is in the range [0.2: 2]: 1 16. A process according to claim 14 or claim 15 wherein the molar ratio of bis-

   phosphonate compound: iridium is in the range [0.5 to 1.5]: 1.

   17. A process according to any one of claims I to I 6 in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium.

   18. A process according to any one ol claims I to 17 in which the methyl acetate 5 concentration in the liquid reaction composition is in the range I to 70 OX, by weight.

   19. A process according to any one of claims I to 18 in which the methyl iodide concentration in the liquid reaction composition is in the range I to 30% by weight.

   20. A process according to any one of claims I to 19 wherein methanol, methyl acetate or mixtures thereof are carbonylated.

   10 21. A process according to any one of claims I to 20 in which the water concentration in the liquid reaction composition is maintained below 6% by weight.

   22. A process according to claim 21 in which the water concentration in the liquid reaction composition is maintained below 4.5% by weight.

   23. A process according to any one of claims I to 22 in which the carbonylation 15 reaction temperature is in the range 100 to 300 C and the total pressure is in the range 10 to 200 barg.

   24. A process according to any one of claims I to 23 in which the liquid reaction composition is substantially devoid of a co-promoter selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, metal complexes capable of 20 generating 1-, salts capable of generating 1- and mixtures thereof.

   25. A process according to claim I in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium, the methyl acetate concentration in the liquid reaction composition is in the range I to 70% by weight, the methyl iodide concentration in the liquid 25 reaction composition is in the range I to 30% by weight, the bis-phosphonate compound is present in the liquid reaction composition at a molar ratio of bis-phosphonate compound: iridium in the range [greater than O to I 0]: 1 and the water concentration in the liquid reaction composition is maintained below 6.0% by weight.

   Amendments to the claims have been filed as follows Claims: 1. A process for the production of acetic acid by carbonylating with carbon monoxide methanol and/or a reactive derivative thereof in a carbonylation reactor containing a liquid reaction composition substantially devoid of a promoter selected from the grroup consisting of ruthenium, osmium, tungsten, rhenium, zinc, gallium, cadmium and indium, said liquid reaction composition comprising an iridian1 carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and a promoter wherein the water concentration is at or below that at which the maximum in the graph of carbonylation rate versus water concentration occurs and whcrch1 the promoter is a bis-phosphonate compound having the Iormula I: R -0\ /Y\ /O-R

   R -O P P O-R

   11 11 O O (I)

   wherein Ri, R2, R3, R4 are independently hydrogen or an organic functional group; Y is an optionally substituted C-CO alkylene or C6-CO aryl group.

   2. A process according to claim 1 wherein Ret, R2, R3, R4 are independently an organic functional group selected fron1 an unsubstituted hydrocarbon group and a substituted hydrocarbon group.

   3. A process according to claim 1 or claim 2 wherein Ri, R2, R3, R4 are each hydrogen or are each an unsubstituted hydrocarbon group.

   4. A process according to claim 3 wherein each unsubstituted hydrocarbon group is a -CH3- group or a CH(CH3)2- group.

   5. A process according to any one of claims l to 4 wherein Y is selected from the group consisting of an unsubstituted C-CO alkylene group, an optionally substituted C6-CO aryl and an optionally unsubstituted C3-Co cycloalkyl.

   6. A process according to claim 5 wherein the unsubstituted C-C, alkylene group is selected from the group consisting of---CH2- and (CH2)2 7. A process according to claim 5 wherein the optionally substituted CO-C lo aryl is a substituted benzene.

   8. A process according to claim l wherein Y is selected From an unsubstituted C-

   Co alkylene group, an optionally substituted C3-Co eyeloalkyl group and an optionally substituted C6-Co aryl and Rat, R2, R3, R4 are each hydrogen or are each an unsubstituted hydrocarbon group.

   9. A process according to claim 8 wherein each unsubstitutcd hydrocarbon group is a methyl group.

   l O. A process according to claim l wherein the bis-phosphonate is selected fi-om the group consisting of tetraisopropyl 1,2 ethylene diphosphonate, methylene diphosphonic acid and his 1,2 dimethoxyphosphoryl benzene.

   11. A process according to any one of claims 1 to 10 wherein the bisphosphonate is formed in-situ in the liquid reaction composition.

   12. A process according to claim 11 wherein the bis-phospllonate is formed from the conjugate bis-phosphinic acid.

   13. A process according to any one of claims 1 to 10 wherein the bisphosphonate promoter is added to the liquid reaction composition as a bisphosphonate compound.

   14. A process according to any one of claims 1 to l 3 wherein the bisphosphonate compound is present in the liquid reaction composition at a molar ratio of bis phospholate compound: iridium in the range [greater than O to 10]: 1.

   15. A process according to claim 14 wherein the molar ratio of bisphosphonate compound: iridium is in the range [0.2 to 2]: 1 16. A process according to claim 14 or claim 15 wherein the molar ratio of bis phosphonate compound: iridium is in the range [0.5 to 1.5]: 1.

   Ill 1(

   l 7. A process according to any one of claims 1 to 16 in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium.

   18. A process according to any one of claims l to 17 in which the methyl acetate concentration in the liquid reaction composition is in the range l to 70 TO by weight.

   l 9. A process according to any one of claims I to l 8 in which the methyl iodide concentration in the liquid reaction composition is in the range 1 to 30 /O by weight.

   20. A process according to any one of claims 1 to 19 wherein methanol, methyl acetate or mixtures thereof arc carbonylated.

   21. A process according to any one of claims 1 to 20 in which the water concentration in the liquid reaction composition is maintained below 6% by weight.

   22. A process according to claim 21 in which the water concentration in the liquid reaction composition is maintained below 4.5% by weight.

   23. A process according to any one of claims l to 22 in which the carbonylation reaction temperature is in the range l GO to 300 C and the tote] pressure is in the range lOto200barg. 24. A process according to any one of claims l to 23 in which the liquid reaction composition is substantially devoid of a co-promoter selected from the group consisting of alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating 1-, salts capable of generating I- and mixtures thereof.

   25. A process according to claim 1 in which the iridium carbonylation catalyst concentration in the liquid reaction composition is in the range 400 to 5000 ppm measured as iridium, the methyl acetate concentration in the liquid reaction composition is in the range l to 70% by weight, the methyl iodide concentration in the liquid reaction composition is in the range I to 30 /O by weight, the bis-phosphonatc compound is present in the liquid reaction composition at a molar ratio of bis-phosphonate compound: iridium in the range [greater than O to l O]: 1 and the water concentration in the liquid reaction composition is maintained below 6.0% by weight.

   1?