GB2333773A - Production of acetic anhydride - Google Patents

Abstract

A process for the production of acetic anhydride by carbonylation of methyl acetate and/or dimethyl ether in the presence of iridium catalyst, alkyl halide co-catalyst, at least one promoter selected from ruthenium, cadmium, osmium, rhenium, zinc, mercury, gallium, indium and tungsten and an iodide salt co-promoter in which to initiate the reaction, hydrogen is present at least initially in an amount of at least 0.5 vol% based on the carbon monoxide feed.

Description

PROCESS FOR TUE PRODUCTION OF ACETIC ANHYDRIDE The present invention relates generally to the production of acetic anhydride and in particular to a process for the production of acetic anhydride by the carbonylation in a substantially anhydrous liquid phase of methyl acetate in the presence of an iridium catalyst.

It is known from, for example EP-A-0643034 (BP Case No. 8363) and EP A-0749948 (BP Case No. 8633), to produce acetic acid by carbonylating methanol and/or a reactive derivative thereof in a liquid containing a finite concentration of water in the presence of an iridium catalyst, a halide co-catalyst, and a metallic promoter, for example ruthenium and/or osmium. It is stated in EP-A-0643034, for example, that ionic contaminants, such as, for example (a) corrosion metals, particularly nickel, iron and chromium and (b) phosphine 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 r in the liquid reaction composition which has a deleterious effect on the reaction rate.

It is known from, for example EP-A-0728727 (BP Case No. 8548), to produce acetic anhydride by carbonylating methyl acetate in a substantially anhydrous liquid phase in the presence of a rhodium catalyst, a halide co-catalyst and ruthenium and/or osmium as a promoter and optionally also as co-promoter a Group IA iodide, e.g. lithium iodide, a quaternary ammonium iodide or a phosphonium iodide. In this process it is said that ruthenium (III) trichloride and osmium (m) trichloride were not suitable sources of promoter unless hydrogen is also present, though it is demonstrated in Example 17 that acetic anhydride is nevertheless produced in the absence of hydrogen. In view of the statement regarding the adverse effect of I. in the process ofEP-A-0643034 it would not be expected that iridium could be used in place of rhodium as catalyst in the process of EP-A-0728727 (8548).

Nevertheless it would be desirable to use a promoted iridium catalyst for the production of acetic anhydride by the carbonylation of methyl acetate under substantially anhydrous conditions. We have found however that using substantially pure carbon monoxide as the gaseous feedstock there is no detectable reaction. The solution to this problem we have found is to initiate the reaction in the presence of hydrogen.

Accordingly, the present invention provides a process for the production of acetic anhydride by the carbonylation of methyl acetate and/or dimethyl ether which process comprises contacting in a carbonylation reactor carbon monoxide with a substantially anhydrous liquid composition comprising methyl acetate and/or dimethyl ether, acetic anhydride, an iridium catalyst, an alkyl halide co-catalyst, at least one promoter selected from ruthenium, cadmium, osmium, rhenium, zinc, mercury, gallium, indium and tungsten, and a co-promoter selected from alkali metal jodides, alkaline earth metal iodides, metal complexes capable of generating r, salts capable of generating F, and mixtures of two-or more thereof wherein at least initially hydrogen in an amount greater than 0.5 vol % based on the carbon monoxide feed is contacted with the liquid composition.

By the term "substantially anhydrous liquid" is meant a liquid in which water is completely absent or is present in an amount less than 0.1% by weight.

As the feedstock there is used either methyl acetate, dimethyl ether or a mixture of methyl acetate and dimethyl ether.

The carbon monoxide employed in the process may be essentially pure or may contain inert impurities such as carbon dioxide, methane, nitrogen, noble gases and C1 to C4 hydrocarbons. It is preferred that the carbon monoxide be substantilly anhydrous. The carbon monoxide partial pressure may suitably be in the range from greater than zero to 40 bar, typically from 4 to 30 bar.

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 400 to 5000 ppm measured as iridium, preferably in the range 700 to 5000 measured as iridium.

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 reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to a soluble form. Examples of suitable iridiumcontaining compounds which may be added to the liquid reaction composition include IrCl3, Irl3, IrBr3, [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, IrCI3 . 3H2O, IrBr3.3H2O, Ir4(CO)12, iridium metal, Ir203, IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [Ir3O(OAc)6(H2O)3][OAc], and hexachloroiridic acid [H2IrCl6], 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 alcohol and/or carboxylic acid. Particularly preferred is green iridium acetate which may be used in an acetic acid or aqueous acetic acid solution.

In the process of the present invention one or more promoters are present in the reaction composition. Suitable promoters are selected from ruthenium, osmium, tungsten, rhenium, zinc, cadmium, indium, gallium and mercury, and are more preferably selected from ruthenium and osmium. Ruthenium is the most preferred promoter. Preferably, the promoter is present in an effective amount up to the limit of its solubility in the liquid reaction compositioon and/or any liquid process streams recycled to the carbonylation reactor from the acetic acid recovery stage. The promoter is suitably present in the liquid reaction composition at a molar ratio of promoter:iridium of [0.5 to 15]: 1. A suitable promoter concentration is 400 to 20,000 ppm, preferably from 700 to 10,000 ppm.

The promoter may comprise any suitable promoter metal-containing compound which is soluble in the liquid reaction composition. The promoter 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 soluble form. Examples of suitable ruthenium-containing compounds which may be used as sources of promoter include ruthenium (III) chloride, ruthenium (III) chloride trihydrate, ruthenium (IV) chloride, ruthenium (m) bromide, ruthenium metal, ruthenium oxides, ruthenium (III) formate, [Ru(C0)3I3]H, [Ru(CO)2I2]n, [Ru(CO)4I2], [Ru(CO)3I2]2, tetra (aceto)chlororuthenium(II,III), ruthenium (Ill) acetate, ruthenium (m) propionate, ruthenium (III) butyrate, ruthenium pentacarbonyl, trirutheniumdodecacarbonyl and mixed ruthenium halocarbonyls such as dichlorotricarbonylruthenium (II) dimer, dibromotricarbonylruthenium (II) dimer, and other organoruthenium complexes such as tetrachlorobis (4cymene)diruthenium(II), tetrachlorobis(berizene)diruthenium(II), dichloro(cycloocta- 1,5 diene) ruthenium (II) polymer and tris(acetylacetonate)ruthenium (III).

Examples of suitable osmium-containing compounds which may be used as sources or promoter include osmium (m) chloride hydrate, osmium metal, osmium tetraoxide, triosmiumdodecacarbonyl, [Os(CO)4I2], [Os(CO)3I2]2, [Os(CO)3I3]Y, pentachloro-ll-nitrodiosmium and mixed osmium halocarbonyls such as tricarbonyldichloroosmium (11) dimer and other organoosmium complexes.

Examples of suitable tungsten-containing compounds which may be used as sources of promoter include W(CO)6, WCl4, WCl6, WBr5, WI2, or C9H12W(CO)3 and any tungsten chloro-, bromo- or iodo-carbonyl compound.

Examples of suitable rhenium-containing compounds which may be used as sources of promoter include Re2(CO)l0. Re(CO)5Cl, Re(CO)5Br, Re(CO)5I, ReCI3.xH20, [Re(CO)4I]2, [Re(CO)4I2]H: and ReCI5 . yH2O.

Examples of suitable cadmium-containing compounds which may be used as sources of promoter include Cd(OAc)2, CdI2, CdBr2, CdCl2, Cd(OH)2, and cadmium acetylacetonate.

Examples of suitable mercury-containing compounds which may be used as sources of promoter include Hg(OAc)2, HgI2, HgBr2, HgCl2, Hg2I2, and Hg2C12.

Examples of suitable zinc-containing compounds which may be used as sources of promoter include Zn(OAc)2, Zn(OH)2, ZnI2, ZnBr2, ZnCl2 and zinc acetylacetonate.

Examples of suitable gallium-containing compounds which may be used as sources of promoter include gallium acetylacetonate, gallium acetate, GaCl3, GaBr3, GaI3, Ga2CI4 and Ga(OH)3.

Examples of suitable indium-containing compounds which may be used as sources of promoter include indium acetylacetonate, indium acetate, InCI3, InBr3, InI3, InI and In(OH)3.

There is employed in the liquid reaction composition a co-promoter selected from alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating F, salts capable of generating r and mixtures of two or more thereof. Suitable alkali metal iodides include lithium iodide. Metal complexes capable of generating I@ include complexes of the lanthanide metals, for example lanthanum and cerium, and nickel, iron and chromium, suitably in ionic form, preferably in the form of their iodides. Salts capable of generating I are, for example, aluminium and quaternary ammonium and phosphonium iodides.

A preferred co-promoter salt capable of generating r in situ is lithium iodide or A1(OAc)20H.

The co-promoter is suitably present in amounts such that it is effective in increasing the carbonylation rate. Typically when the iridium catalyst to ruthenium promoter molar ratio is about 1 :[2 to 5] the molar ratio of co-promoter (expressed as F): iridium, is suitably in the range [1 to 10]: 1. The amount of such Co- promoter introduced to the liquid reaction composition should be selected to take account of the presence of F from other sources because it is believed that an excessive amount of F in the liquid reaction composition may be detrimental.

It is possible to include in the liquid composition an additional catalyst. A preferred additional catalyst is rhodium, which may be added to the liquid reaction composition in any suitable form which dissolves in the liquid reaction composition or is convertible to a soluble form. Examples of suitable rhodium-containing compounds which may be added to the liquid reaction composition include [Rh(CO)2Cl]2, [Rh(CO2)2I]2, [Rh(Cod)Cl]2, rhodium (III) chloride, rhodium (III) chloride trihydrate, rhodium (III) bromide, rhodium (III) iodide, rhodium (III) acetate, rhodium dicarbonylacetylacetonate, RhCl3(PPh3)3and RhCl(CO)(PPh3)2.

The molar ratio of rhodium to iridium catalyst may suitabl;be be in the range [0.1 to 0.5]:1, typically [0.2 to 0.4]:1.

As regards the alkyl halide co-catalyst the halide moiety is suitably iodide and the alkyl moiety is suitably a C1 to C10, preferably a C1 to C4 alkyl group. A preferred co-catalyst is methyl iodide. Suitably the concentration of alkyl halide in the liquid reaction composition is in the range from 1 to 30% by weight, preferably from 1 to 20% by weight.

The liquid reaction composition will also contain acetic anhydride as the principal product of the carbonylation process.

Whilst the process may be operated in the absence of added solvent it is preferred to incorporate in the liquid reaction composition a solvent, which may suitably be a carboxylic acid and is preferably acetic acid.

The process may be operated batchwise or continuously, preferably continuously. Whether the process is operated batchwise or continuously it is necessary to initiate the carbonylation process by the addition of hydrogen to the liquid reaction composition. Thereafter it is preferred to add further hydrogen.

This may be accomplished in continuous operation for example by feeding hydrogen either continuously or intermittently either separate from or together with the carbon monoxide. The amount of hydrogen added should be at least sufficient to initiate the carbonylation reaction and thereafter maintain the continuation of the carbonylation process. Typically the amount of hydrogen may be in the range from greater than 0.5 to 10% by volume based on the carbon monoxide feed.

The process may suitably be operated at a temperature in the range from 130 to 2500C, preferably from 170 to 200"C and a total pressure in the range from 1 to 100 barg, preferably from 20 to 50 barg.

In a preferred embodiment the present invention provides a continuous process for the production of acetic anhydride by the carbonylation of methyl acetate which process comprises feeding methyl acetate and carbon monoxide continuously and hydrogen in an amount greater than 0.5 vol % based on the carbon monoxide feed either continuously or intermittently to a carbonylation reactor containing a substantially anhydrous liquid composition comprising methyl acetate in an amount in the range from 25 to 35% by weight, acetic anhydride, a catalyst system comprising methyl iodide in an amount from 10 to 15% by weight and iridium, ruthenium lithium and rhodium in a molar ratio of [1]:[2 to 5]:[1 to 10]:t0.1 to 0.25] and in an amount from 5000 to 12000 ppm and, comprising the remainder of the liquid composition, acetic acid.

The acetic anhydride product may be recovered from the liquid reaction composition by withdrawing vapour and/or liquid from the carbonylation reactor and recovering acetic anhydride from the withdrawn material. Preferably acetic anhydride is recovered from the liquid reaction composition by continuously withdrawing liquid reaction composition from the carbonylation reactor and recovering acetic anhydride from the withdrawn liquid reaction composition by one or more flash and/or fractional distillation stages in which the acetic anhydride is separated from the other components of the liquid reaction composition such as iridium catalyst, methyl iodide co-catalyst, promoter, co-promoter, methyl acetate and acetic acid solvent which may be recycled to the reactor to maintain their concentrations in the liquid reaction composition.

The invention will now be illustrated by reference to the following Examples.

Examples 1-12 and Comparison Test 1 ExPerimental Method A 300 ml Hastelloy B2 autoclave equipped with a dispersimax stirrer, a ballast vessel and a liquid catalyst injection system was used for a series of batch anhydrous carbonylation experiments. A gas supply to the autoclave, provided from a gas ballast vessel, fed gas to the autoclave to maintain a constant pressure.

The rate of gas uptake was calculated from the rate at which the pressure fell in the ballast vessel.

At the end of the experiment liquid and gas samples from the autoclave were analysed by gas chromatography.

For each batch carbonylation experiment the autoclave was charged with any solid promoters required (Ru(CO)4I2 and LiOAc) and the main liquid charge containing acetic acid, methyl acetate, acetic anhydride and methyl iodide. The autoclave was then sealed and flushed twice with nitrogen and once with hydrogen, leaving the required partial pressure of hydrogen in the autoclave at this stage. The autoclave was then additionally charged with ca. 5 barg of the feed gas and heated with stirring (950 r.p.m.) to 185"C. Once stable at this temperature the catalyst solution, consisting of ca. 2.04g of a water solution of H2IrCl6 dissolved in acetic acid, was then added from the injection vessel using an overpressure of the feed gas. After catalyst injection the autoclave pressure was kept constant at 28 barg (i0.5 barg) using the gas feed from the ballast vessel.

After gas uptake from the ballast vessel had ceased, or after a set reaction period, the autoclave was isolated from the gas supply and the reactor contents cooled to room temperature. The autoclave was vented and the vent gases sampled and analysed. The liquid reaction composition was discharged from the autoclave, sampled and analysed for liquid products and by-products. Component charges are given in Table 1, reaction conditions in Table 2, and reaction rates at appropriate %wt levels of MeOAc in Table 3.

Table 1. Reaction Compositions (weights in Grams)

Autoclave Charge Catalyst Injection Example Ru(CO)4I2 LiOAc** AcOH Ac2O MeI H2IrCl6 AcOH solution* 1 2.438 0.266 34.352 60.030 21.150 20.010 2.048 9.994 2 2.439 0.270 34.312 60.072 21.105 20.050 2.045 9.998 3 2.434 0.267 34.998 60.031 21.121 20.037 2.045 9.919 CT1 2.437 - 34.327 60.226 21.108 20.100 2.035 9.945 4 2.460 0.538 33.631 59.823 21.102 20.456 2.042 10.026 5 2.439 2.698 29.880 58.435 21.161 23.400 2.039 9.997 6 6.084 1.326 28.682 59.245 21.114 21.590 2.036 9.994 7 2.457 0.537 34.052 59.809 21.117 20.466 2.050 9.459 8 6.087 1.325 28.693 59.256 21.110 21.640 2.041 9.986 9 2.424 0.537 33.573 60.190 21.127 20.462 2.045 9.897 10 2.438 0.542 48.122 59.821 6.593 20.460 2.044 10.008 11 2.432 0.538 17.984 75.012 21.159 20.603 2.054 9.996 12 2.440 0.536 63.150 30.300 21.106 20.463 2.041 9.998 * H2IrCl6 solution in water contains 24.39% Ir ** LiOAc used in dihydrate form Table 2. Reaction Conditions for Experiments @ 28 barg

Example Gaseous ppH2 %wt. Ratio %wt.

Feed (barg) Ac2O Ir:Ru.Li MeOAc 1 CO 1.4 14 1:2:1 40 2 CO 1.4 14 1:2:1 40 3 CO - 3.5 14 1:2:1 40 CT1 CO 1.4 14 1:2:0 40 4 CO 1.4 14 1:2:2 40 5 CO 1.4 14 1:2:10 - 40 6 CO 1.4 14 1:5:5 40 7 2%H2/CO 1.4 14 1:2:2 40 8 2%H2/CO 14 14 1:5:5 40 9 10%H2/CO 1.4 14 1:2:2 40 10 2%H2CO/CO 1.4 0 1:2:2 40 11 CO 1.4 14 1:2:2 50 12 2%H2/CO 1.4 14 1:2:2 20 * initial ppH2 fed to the reactor at room temperature Table 3. Reaction Rates

Exampl Rate @ 50% Rate @ 40% Rate @ 30% Rate @ 20% e MeOAc MeOAc MeOAc MeOAc mol/l/hr) (molar) (molar) (moWhr) 1 4.S 2 4.4 3 5.3 CT1 ~0.8 4 ~4.2 ~0.6 5 1.4 0.9 6 | | ~4.3 | ~0.9 7 5.7 2.5 8 6.0 2.8 9 6.1 2.4 10 16.8* 3.4 11 ~4.2 ~0.8 12 4.5 * High rate observed due to reaction starting in aqueous conditions.

Comparision Test 1 (CT1) is not an example according to the present invention because no co-promoter was present.

Comparision Tests 2 to 5 Examples 1 and 2 and CT 1 were repeated in the absence of hydrogen at 18 barg as opposed to 28 barg and no measurable conversion to acetic anhydride was detected. Increasing the hydrogen to 0.5 vol % in the carbon monoxide feed and increasing the total pressure over the range 18 to 60 barg also resulted in no measurable conversion to acetic anhydride.

These are not examples according to the present invention because hydrogen was absent throughout. They are included only for the purpose of comparision.

Example 13 Anhedrous Carbonylation of Methyl Acetate with Ir/Cd/Li (1:5:2) Experimental Method As previously described for Examples 1-12 except: For each batch carbonylation experiment the autoclave was charged with any solid promoters required (Cd(OAc)2 and LiOAc) and the main liquid charge containing acetic acid, methyl acetate, acetic anhydride and methyl iodide. The autoclave was then sealed and flushed twice with nitrogen and once with hydrogen, leaving 1.4 barg partial pressure of hydrogen in the autoclave at this stage. The autoclave was then additionally charged with ca. 5 barg of the feed gas and heated with stirring (950 r.p.m.) to 185"C. Once stable at this temperature the catalyst solution, consisting of ca. 2.04g of a water solution of H2IrCl6 dissolved in acetic acid, was then added from the injection vessel using an overpressure of the feed gas. After catalyst injection the autoclave pressure was kept constant at 28 barg (+0.5 barg) using the gas feed from the ballast vessel.

Component charges are given in Table 4, and reaction rates at appropriate %wt levels of MeOAc in Table 6.

Batch reaction run @ ( 28 28 barg CO, 1.4 barg pp 112 14%wt. acetic anhydride, 40%wt. methyl acetate.

Table 4. Reaction ComDosition (weiehts in Grams)

Autoclave Charge Catalyst Injection Example Cd(OAc)2** LiOAc** AcOH MeOAc Ac20 Mel H21rC16 AcOH solution* 13 3.476 0.538 30.789 57.874 21.140 24.220 2.034 9.798 *H2lrC16 solution in water contains 24.39% Ir ** LiOAc and Cd(OAc)2 both used in dihydrate form Example 14 and ComParison Test 6 Anhvdrous Carbonvlation of Methyl Acetate with Rhodium Rh/Ru/Li (0.4:2:2) and Rh/Ir/Ru/Li (0.4:1:2:2) Experimental Method As previously described for Examples 1-12 except: For each batch carbonylation experiment the autoclave was charged with any solid promoters required (Ru(CO)42 and LiOAc) and the main liquid charge containing acetic acid, methyl acetate, acetic anhydride and methyl iodide. The autoclave was then sealed and flushed twice with nitrogen and once with hydrogen, leaving 1.4 barg partial pressure of hydrogen in the autoclave at this stage. The autoclave was then additionally charged with ca. 5 barg of the feed gas and heated with stirring (950 r.p.m.) to 1850C. Once stable at this temperature the catalyst solution, consisting of ca. 0.200g [Rh(CO)2Cl]2 (and 2.04g of a water solution of H2IrCl6 if required) dissolved in acetic acid, was then added from the injection vessel using an overpressure of the feed gas. After catalyst injection the autoclave pressure was kept constant at 28 barg (+0.5 barg) using the gas feed from the ballast vessel.

Component charges are given in Table 5, and reaction rates at appropriate %wt levels of MeOAc in Table 6.

Batch reaction run@ 28 barg 2%HCO, 1.4 barg pp H2, (initial pull2 fed to the reactor at room temperature) 14%wt. acetic anhydride, 40% wt. methyl acetate.

Table 5. Recation Compositions (weights in Grams)

Autoclave Charge Catalyst Injection Example Ru(CO)4I2 LiOAc** AcOH MeOAc Ac2O MeI [Rh(CO)2Cl]2 H2IrCl6 AcOH solution* CT6 2.429 0.551 35.414 59.875 21.160 20.455 0.215 - 9.985 14 2.449 0.545 33.402 59.844 21.144 20.451 0.209 2.040 10.001 *H2IrCl6 solution in water contains 24.39% Ir **LiOAc used in dihydrate form Table 6. Reaction Rates for Exeriments run with Cd or Rh

Example Rate @ 40% Rate @ 30% Rate @ 20% MeOAc MeOAc MeOAc mol/l/hr) (mol/l/hr) (mol/l/hr) 13 3.5 CT6 4.5 3.0 1.2 14 10.5 5.9 7.6 Comparison Test (CT)6 is not an example according to the present invention because iridium was not a component ofthe catalyst system. It is included only for the purpose of comparison.

Further experiments (15-17) were performed at 28 barg at conditions set out below:

Autoclave Charge Cat. Inject Example Ru(CO)4I2 LiOAc* AcOH MeOAc Ac2O MeI H2IrCl6 AcOH solution** 15 2.47 0.53 31.26 60.02 21.00 20.483 2.267 12.01 16 - 0.53 27.05 60.09 21.02 20.453 2.299 16.30 17 2.45 0.54 43.37 60.13 21.03 20.48 - 0 *LiOAc added in dihydrate form **H2IrCl6 solution in water contains 22.26% Ir

Example Reaction Gaseous ppH2 %wt. Ratio %wt. pressure feed (barg)* Ac2O Ir:Ru:Li MeOAc (barg) 15 28 2%H2/CO 1.4 14 1:2:2 40 16 28 2%H2/CO 1.4 14 1:0:2 40 17 28 2%H2/CO 1.4 14 1:2:0 40 *initial pp H2 fed into the reactor at room temperature

Example Rate @ 39% MeOAc Rate @ 35% MeOAc (mol/l/hr) (mol/l/hr) 15 2.8 1.6 16 0.5 0.4 17 0.1 Comparison of example 15 with examples 16 and 17 demonstrates that there is a synergy between ruthenium, added as [Ru(CO)J2], and iridium, added [H2IrC16], in the presence of lithium, added as [LiOAc], in the anhydrous carbonylation of methyl acetate to produce acetic anhydride when hydrogen is present.

Further experiments (18-22) were performed under conditions set out below which show that Al(OAc)20H is a suitable iodide generating co-promoter for anhydrous carbonylation in the presence of iridium.

Autoclave Charge Cat. Inject Ex Ru(CO)4I2 Al(OAc)2OH LiOAc* AcOH MeOAc Ac2O MeI H2IrCl2 AcOH solution ** 18 2.45 ~ ~ 27.36 60.00 21.00 20.4 2.294 16.50 8 19 2.45 0.86 ~ 28.67 60.12 21.06 20.4 2.278 14.65 20 2.45 0.43 ~ 30.05 60.00 21.02 20.4 2.296 13.95 8 21 2.45 ~ 0.54 27.76 60.00 21.01 20.4 2.293 15.54 7 22 2.46 ~ 0.26 28.02 60.14 21.00 20.4 2.279 15.41 9 *LiOAc added in dihydrate form **H2IrCl6 solution in water contains 22.26% Ir

Example Reaction Gaseous ppH2 %wt. Ratio Ratio %wt. pressure feed (barg)* Ac2O Ir:Ru:Al Ir:Ru:Li MeOAc (barg) 18 40 2%H2/CO 1.4 14 1:2:0 1:2:0 40 19 40 2%H2/CO 1.4 14 1:2:2 ~ 40 20 40 2%H2/CO 1.4 14 1:2:1 ~ 40 21 40 2%H2/CO 1.4 14 1:2:2 40 22 40 2%H2/CO 1.4 14 ~ 1:2:1 40 *initial pp H2 fed into the reactor at room temperature

Example Rate @ 39% MeOAc Rate @ 35% MeOAc mol/l/hr) (mol/l/hr) 18 1.2 0.5 19 5.7 3.3 20 3.4 1.4 21 3.8 2.6 22 4.1 2.3 A further example (23) was performed as set out below: Example 23, combined with examples 18, 20 and 21, illustrates the trend on adding more lithium to the reaction system. With iridium and ruthenium present in the molar ratio 1:2 the rate is seen to peak with 2 moles of lithium per-iridium present. With lithium added in excess ofthis (as in example 23) a detrimental effect on reaction rate is observed.

Autoclave Charge Catalyst Inject Ex Ru(CO)4I2 LiOAc* AcOH MeOAc Ac2O Mel H2IrCl2 AcOH solution** 23 2.45 1.07 29.43 60.00 21.00 20.49 2.289 14.34 *LiOAc added in dihydrate form **H2IrCl6 solution in water contains 22.26% Ir

Example Reaction Gaseous ppH2 %wt. Ratio Ratio %wt. pressure feed (barg)* Ac2O Ir:Ru:Al Ir:Ru:Li MeOAc (barg) 23 40 2%H2/CO 1.4 14 1:2:1 40 *initial pp H2 fed into the reactor at room temperature

Example Rate @ 39% MeOAc Rate @ 35% MeOAc (mol/l/hr) (mol/l/hr) 23 1.1 0.8

Claims (10)

1. Claims: 1. A process for the production of acetic anhydride by the carbonylation of methyl acetate and/or dimethyl ether which process comprises contacting in a carbonylation reactor carbon monoxide with a substantially anhydrous liquid composition comprising methyl acetate and/or dimethyl ether, acetic anhydride, an iridium catalyst, an alkyl halide co-catalyst, at least one promoter selected from ruthenium, cadmium, osmium, rhenium, zinc, mercury, gallium, indium and tungsten, and a co-promoter selected from alkali metal iodides, alkaline earth metal iodides, metal complexes capable of generating I, salts capable of generating F, and mixtures of two or more thereof wherein at least initially hydrogen in an amount greater than 0.5 vol % based on the carbon monoxide feed is contacted with the liquid composition.
2. 2. A process as claimed in claim 1 wherein the amount of hydrogen is in the range from greater than 0.5% to 10% by volume based on the carbon monoxide feed.
3. 3. A process as claimed in claim 1 or claim 2 wherein the molar ratio of promoter: iridium is in the range [0.5 to 15]: 1.
4. 4. A process as claimed in any one of the preceding claims wherein the molar ratio of co-promoter (expressed as I: iridium is in the range [1 to 10]:1.
5. 5. A process as claimed in any one of the preceding claims wherein the copromoter salt capable of generating r in situ is lithium iodide or Al(OAc)20H.
6. 6. A process as claimed in any one of the preceding claims wherein there is included in the liquid composition rhodium.
7. 7. A process as claimed in claim 6 wherein the molar ratio of rhodium: iridium catalyst is in the range [0.1 to 0.5]:1.
8. 8. A process as claimed in any one of the preceding claim which process comprises feeding methyl acetate and carbon monoxide continuously and hydrogen in an amount greater than 0.5 vol% based on the carbon monoxide feed either continuously or intermittently to a carbonylation reactor containing a substantially anhydrous liquid composition comprising methyl acetate in an amount in the range from 25 to 35% by weight, acetic anhydride, a catalyst system comprising methyl iodide in an amount from 10 to 15% by weight and iridium, ruthenium, lithium and rhodium in a molar ratio of[1]:[2 to 5]:[1 to 10]:[0.1 to 0.5] and in an amount from 5000 to 12000 ppm and, comprising the remainder of the liquid composition, acetic acid.
9. 9. A process substantially as hereindescribed.
10. 10. Acetic anhydride prepared by any one of the preceding claims.