Process for the production of acetic acid by the carbonylation of dimethyl ether

A process for the production of acetic acid which comprises reacting carbon monoxide with a carbonylatable reactant comprising greater than 10%, typically from 30 to 100%, by weight dimethyl ether introduced to a reactor in which there is maintained at elevated temperature a liquid reaction composition comprising a Group VIII noble metal catalyst, for example rhodium or iridium, methyl iodide promoter, an optional co-promoter and water at a concentration in the liquid reaction composition of from 1.0 to 10% by weight.

The present invention relates to a process for the production of acetic 
acid by the carbonylation of a carbonylatable reactant comprising dimethyl 
ether. 
Hydrocarbonylation involving the reaction of dimethyl ether, acetic acid, 
hydrogen and carbon monoxide to form ethylidene diacetate is described in 
European patent publications EP 0566370-A2 and EP 0566371-A2. According to 
these patent applications the catalyst system consists essentially of a 
Group VIII metal, methyl iodide, lithium iodide and optionally lithium 
acetate. The reaction is said to be preferably run using a 1:1 to 4:1 
molar ratio of carbon monoxide to hydrogen. Although water may be added to 
the reactor feed, the final reaction conditions are essentially anhydrous. 
The carbonylation of dimethyl ether/methanol mixtures prepared from 
synthesis gas is described in U.S. Pat. Nos. 5,189,203 and 5,286,900. The 
products of the carbonylation process are said to be acetic acid, methyl 
acetate and/or acetic anhydride depending upon whether water is also fed 
to the reactor. Homogeneous or heterogeneous catalysts are said to be 
usable; however, in the experimental examples only heterogeneous rhodium 
on activated carbon is used. No indication of the benefits to liquid-phase 
carbonylation rate of using dimethyl ether are given. 
U.S. Pat. No. 3,769,329 relates to a process for the reaction of alcohols 
and the ester, ether and halide derivatives thereof, with carbon monoxide 
in the presence of catalyst systems containing as active constituents a 
rhodium component and a halogen component to yield carboxylic acids and/or 
esters selectively and efficiently. U.S. Pat. No. 3,772,380 relates to a 
similar process in which the active constituents of the catalyst system 
are an iridium component and a halogen component. In both U.S. Pat. No. 
3,769,329 and U.S. Pat. No. 3,772,380 dimethyl ether is listed as one of a 
number of a suitable feed materials for the carbonylation reaction. The 
use of methanol feedstock containing 10 weight percent dimethyl ether is 
described in Example 19 of U.S. Pat. No. 3,769,329 and Example 19 of U.S. 
Pat. No. 3,772,380; however, the use of such a mixed feedstock is merely 
said to have no deleterious effect on the reaction. 
GB 1234641 relates to a process for the treatment of a reactant selected 
from an alcohol, halide, ester, ether or phenol with carbon monoxide to 
give organic acids and/or esters in the presence of a catalyst comprising 
a noble metal component selected from iridium, platinum, palladium, osmium 
and ruthenium and their compounds, and a promoter substance which is said 
to be a halogen or a halogen compound. It is stated in GB 1234641 that 
dimethyl ether, as a by-product, is undesirable because it suppresses the 
carbon monoxide partial pressure and ultimately causes a decrease in the 
desired carbonylation reaction rate. Example 7 of GB 1234641 relates to an 
iridium catalysed reaction in which a methanol feedstock containing 10% by 
weight dimethyl ether is carbonylated at a carbon monoxide partial 
pressure of about 700 psig and at a reaction temperature of 175.degree. C. 
in the presence of methyl iodide promoter. This example is said to 
demonstrate that an impure alcohol feedstock having ether in the alcohol 
has no deleterious effect on the reaction. 
None of the above patents teach that using dimethyl ether as feed to a 
liquid-phase carbonylation reaction has any beneficial effect on the 
carbonylation rate. 
It has now been unexpectedly found that in the production of acetic acid by 
liquid phase carbonylation in the presence of a Group VIII noble metal 
catalyst, methyl iodide promoter and a finite concentration of water the 
reaction rate of dimethyl ether carbonylation can be greater than that of 
methyl acetate and/or methanol. 
Thus, according to the present invention there is provided a process for 
the production of acetic acid which process comprises reacting carbon 
monoxide with a carbonylatable reactant introduced to a reactor in which 
there is maintained at elevated temperature a liquid reaction composition 
comprising a Group VIII noble metal catalyst, methyl iodide promoter, an 
optional co-promoter, and at least a finite concentration of water 
characterised in that the carbonylatable reactant comprises greater than 
10% by weight dimethyl ether and the concentration of water in the liquid 
reaction composition is from 0.1 to 10% by weight. 
The carbonylatable reactant comprises greater than 10% by weight dimethyl 
ether, typically between 30 and 100% by weight dimethyl ether, for example 
from 50 to 100% by weight dimethyl ether. 
Preferably, the carbonylatable reactant comprises dimethyl ether together 
with methanol and/or methyl acetate. Preferably, the carbonylation 
reactant comprises dimethyl ether and methanol. 
In the production of acetic acid by the liquid phase carbonylation of 
dimethyl ether in the presence of a Group VIII noble metal catalyst, 
methyl iodide promoter and a finite concentration of water, reaction might 
be expected to proceed by strong acid catalysed hydrolysis (for example by 
HI formed in situ) of dimethyl ether. The methanol formed in situ by 
strong acid catalysed hydrolysis of dimethyl ether together with any 
methanol co-reactant would be predominantly converted to methyl acetate in 
the liquid reaction composition by reaction with acetic acid product or 
solvent. The equilibrium between an ester and an alcohol is described in 
Organic Chemistry John McMurry, p 777, Brooks/Cole, 1984, 1st Ed where the 
ester is said to be favoured at high alcohol or low water concentrations. 
A carbonylatable reactant comprising dimethyl ether and methanol may 
suitably be obtained by reacting a mixture of carbon monoxide and hydrogen 
in the presence of a methanol synthesis catalyst and a methanol 
dehydration catalyst. Alternatively the reactant may be obtained by 
reacting carbon monoxide with hydrogen in the presence of a methanol 
synthesis catalyst in a first step and thereafter reacting a part of the 
methanol formed in the first step with a methanol dehydration catalyst in 
a second step. Preferably, the carbonylatable reactant comprising dimethyl 
ether and methanol is obtained from synthesis gas (a 1:1 molar mixture of 
carbon monoxide and hydrogen). Preferably, the methanol synthesis catalyst 
is a conventional catalyst comprising copper oxide and zinc oxide 
supported on alumina. Preferably, the methanol dehydration catalyst is an 
acid catalyst, more preferably a zeolite catalyst such as ZSM-5. Suitable 
processes for the production of a carbonylatable reactant comprising 
dimethyl ether are described for example in U.S. Pat. No. 5,286,900, U.S. 
Pat. No. 5,189,203 and U.S. Pat. No. 4,417,000. 
Water may be formed in situ in the liquid reaction composition, for 
example, by the esterification reaction between methanol formed in situ by 
hydrolysis of dimethyl ether/methanol co-reactant and acetic acid 
product/acetic acid solvent. However, water is also consumed in situ in 
the liquid reaction composition when dimethyl ether is hydrolysed to 
methanol. Water may also be introduced to the carbonylation reactor either 
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 water in the liquid 
reaction composition. Preferably, the concentration of water in the liquid 
reaction composition is from 1 to 10%, more preferably from 1 to 8%, by 
weight. 
The process of the present invention has been found to be particularly 
beneficial for the production of acetic acid at relatively low water 
concentrations. Under these conditions the process of the present 
invention has the advantage of providing increased rate of carbonylation 
and/or increased catalyst stability over processes which do not employ 
dimethyl ether as reactant. For both rhodium- and iridium-catalysed liquid 
phase carbonylation as hereinbefore mentioned the water concentration is 
preferably from 1 to 10% by weight, more preferably from 1 to 8% by 
weight. It has been found that for carbonylation of reactants comprising 
greater than 10% by weight dimethyl ether with rhodium catalysts such low 
water concentrations can be achieved without the need to use a co-promoter 
such as a group IA or IIA metal iodide, a quaternary ammonium iodide or a 
phosphonium iodide. 
Preferably, the Group VIII noble metal catalyst in the liquid reaction 
composition comprises a rhodium- or an iridium-containing compound which 
is soluble in the liquid reaction composition. The rhodium- or 
iridium-containing compound 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 iridium-containing compounds which may be added to the 
liquid reaction composition include IrCl.sub.3, IrI.sub.3, IrBr.sub.3, 
Ir(CO).sub.2 I!.sub.2, Ir(CO).sub.2 Cl!.sub.2, Ir(CO).sub.2 Br!.sub.2, 
Ir(CO).sub.2 I.sub.2 !.sup.-, Ir(CO).sub.2 Br.sub.2 !.sup.-, 
Ir(CO).sub.2 I!.sub.2, Ir(CH.sub.3)I.sub.3 (CO).sub.2 !.sup.-, Ir.sub.4 
(CO).sub.12, IrCl.sub.3.4H.sub.2 O, IrBr.sub.3.4H.sub.2 O, Ir.sub.3 
(CO).sub.12, iridium metal, Ir.sub.2 O.sub.3, IrO.sub.2, 
Ir(acac)(CO).sub.2, Ir(acac).sub.3, iridium acetate, Ir.sub.3 
O(OAc).sub.6 (H.sub.2 O).sub.3 !OAc!, and hexachloroiridic acid H.sub.2 
IrCl.sub.6 !. Preferred are chloride-free complexes of iridium such as 
acetates, oxalates and acetoacetates. 
Preferably, the concentration of the iridium-containing compound in the 
liquid reaction composition is in the range 100 to 6000 ppm by weight of 
iridium. 
Examples of suitable rhodium-containing compounds which may be added to the 
liquid reaction composition include Rh(CO).sub.2 Cl!.sub.2, Rh(CO).sub.2 
I!.sub.2, Rh(Cod)Cl!.sub.2, rhodium III chloride, rhodium III chloride 
trihydrate, rhodium III bromide, rhodium III iodide, rhodium III acetate, 
rhodium dicarbonylacetylacetonate, RhCl.sub.3 (PPh.sub.3).sub.3 and 
RhCl(CO)(PPh.sub.3).sub.2. 
Preferably, the concentration of the rhodium-containing compound in the 
liquid reaction composition is in the range 10 to 1500 ppm by weight of 
rhodium. 
When the Group VIII noble metal catalyst is iridium, the optional 
co-promoter may be selected from the group consisting of ruthenium, 
osmium, rhenium, cadmium, mercury, zinc, gallium, indium and tungsten. The 
optional co-promoter may comprise any ruthenium-, osmium-, rhenium-, 
cadmium-, mercury-, zinc-, gallium-, indium- or tungsten-containing 
compound which is soluble in the liquid reaction composition. The optional 
co-promoter may be added to the liquid reaction composition of the 
carbonylation reaction in any suitable form which dissolves in the liquid 
reaction composition or is convertible to soluble form. 
Examples or suitable ruthenium-containing compounds which may be used as 
optional co-promoter include ruthenium (III) chloride, ruthenium (III) 
chloride trihydrate, ruthenium (IV) chloride, ruthenium (III) bromide, 
ruthenium metal, ruthenium oxides, ruthenium (III) formate, Ru(CO).sub.3 
I.sub.3 !.sup.- H.sup.+, RuI.sub.2 (CO).sub.4, tetra(aceto) 
chlororuthenium(II,III), ruthenium (III) acetate, ruthenium (III) 
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(4-cymene)diruthenium(II), 
tetrachlorobis(benzene)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 
optional co-promoter include osmium (III) chloride hydrate and anhydrous, 
osmium metal, osmium tetraoxide, triosmiumdodecacarbonyl, 
pentachloror-.mu.-nitrododiosmium and mixed osmium halocarbonyls such as 
OsI.sub.2 (CO).sub.4, tricarbonyldichloroosmium (II) dimer and other 
organoosmium complexes. 
Examples of suitable rhenium-containing compounds which may be used as 
optional co-promoter include Re.sub.2 (CO).sub.10, Re(CO).sub.5 Cl, 
Re(CO).sub.5 Br, Re(CO).sub.5 I, ReCl.sub.3.xH.sub.2 O and 
ReC.sub.5.yH.sub.2 O. 
Examples of suitable cadmium-containing compounds which may be used as 
optional co-promoter include Cd(OAc).sub.2, CdI.sub.2, CdBr.sub.2, 
CdCl.sub.2, Cd(OH).sub.2 and cadmium acetylacetonate. 
Examples of suitable mercury-containing compounds which may be used as 
optional co-promoter include Hg(OAc).sub.2, HgI.sub.2, HgBr.sub.2, 
Hg.sub.2 I.sub.2, and HgCI.sub.2. 
Examples of suitable zinc-containing compounds which may be used as 
optional co-promoter include Zn(OAc).sub.2, Zn(OH).sub.2, ZnI.sub.2, 
ZnBr.sub.2, ZnCl.sub.2, and zinc acetylacetonate. 
Examples of suitable gallium-containing compounds which may be used as 
optional co-promoter include gallium acetylacetonate, gallium acetate, 
GaCl.sub.3, GaBr.sub.3, GaI.sub.3, Ga.sub.2 Cl.sub.4 and Ga(OH).sub.3. 
Examples of suitable indium-containing compounds which may be used as 
optional co-promoter include indium acetylacetonate, indium acetate, 
InCl.sub.3, InBr.sub.3, InI.sub.3, InI and In(OH).sub.3. 
Examples of suitable tungsten-containing compounds which may be used as 
optional co-promoter include W(CO).sub.6, WCl.sub.4, WCl.sub.6, WBr.sub.5, 
WI.sub.2, C.sub.9 H.sub.12 W(CO).sub.3 or any tungsten chloro-, bromo, or 
iodo-carbonyl compound. 
Preferably, both the iridium- and optional co-promoter compounds are sodium 
free. 
The molar ratio of each optional co-promoter:iridium catalyst is in the 
range (0.1 to 20):1. 
When the Group VIII noble metal catalyst is rhodium, the optional 
co-promoter may be selected from ruthenium, osmium, rhenium and manganese. 
Examples of suitable ruthenium-, osmium-, or rhenium-containing compounds 
are as described above. Examples of suitable maganese-containing compounds 
which may be used include Mn.sub.2 (CO).sub.10, manganese (II) acetate, 
manganese (II) bromide, manganese (II) bromide tetrahydrate, manganese 
(II) chloride, manganese (II) chloride hydrate, manganese (II) iodide, 
manganese (II) oxide, manganese (III) oxide, manganese (IV) oxide, 
Mn(CO).sub.5 Br and Mn(CO).sub.5 I. 
The molar ratio of each optional co-promoter:rhodium catalyst is suitably 
in the range (0.1 to 20):1, except for manganese:rhodium which is in the 
range (0.2 to 20:1). 
When the Group VIII noble metal catalyst is rhodium, the optional 
co-promoter may also be selected from the group consisting of Group IA and 
Group IIA metal iodides, quaternary ammonium iodides and phosphonium 
iodides. The concentration of the optional co-promoter in the liquid 
reaction composition is preferably equivalent, up to 20% by weight of 
lithium iodide. 
The promoter is methyl iodide. When the group VIII noble metal catalyst is 
iridium, the concentration of methyl iodide in the liquid reaction 
composition is preferably in the range of 1 to 20% by weight, preferably 2 
to 15% by weight. When the Group VIII noble metal catalyst is rhodium, the 
concentration of methyl iodide in the liquid reaction composition is 
preferably in the range 1 to 30% by weight, preferably 1 to 20% by weight, 
more preferably 5 to 20% by weight. 
The carbon monoxide reactant may be essentially pure or may contain inert 
impurities such as carbon dioxide, methane, nitrogen, noble gases, water 
and C.sub.1 to C.sub.4 paraffinic hydrocarbons. The presence of hydrogen 
in the carbon monoxide 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. 
When the Group VIII noble metal catalyst is rhodium the pressure of the 
carbonylation reaction is suitably in the range 1 to 100 barg, preferably 
20 to 50 barg. The temperature of the carbonylation reaction is suitably 
in the range 130.degree. to 250.degree. C., preferably in the range 
170.degree. to 200.degree. C. 
When the Group VIII noble metal catalyst is iridium the pressure of the 
carbonylation reaction is suitably in the range 10 to 200 barg, preferably 
10 to 100 barg, most preferably 15 to 50 barg. The temperature of the 
carbonylation reaction is suitably in the range 100.degree. to 300.degree. 
C., preferably in the range 150.degree. to 220.degree. C. 
Acetic acid may be used as a solvent for the reaction. 
The process of the present invention may be performed as a batch or a 
continuous process, preferably as a continuous process. 
The acetic acid product may be removed from the reactor by withdrawing 
liquid reaction composition and separating the acetic acid product by one 
or more flash and/or fractional distillation stages from the other 
components of the liquid reaction composition such as iridium or rhodium 
catalyst, optional co-promoter, methyl iodide, water and unconsumed 
reactants which may be recycled to the reactor to maintain their 
concentrations in the liquid reaction composition. The acetic acid product 
may also be removed as a vapour from the reactor. 
The invention will now be illustrated by way of example only be reference 
to the following examples. In Examples 1 and 2 and Experiments A-D the 
following method and apparatus were employed: 
A 150 ml Hastelloy B2 (Trade Mark) autoclave equipped with a Magnedrive 
(Trade Mark) stirrer, an injection port and cooling coils was used for a 
series of batch carbonylation experiments employing methyl acetate or 
dimethyl ether as feed. For each batch carbonylation experiment employing 
methyl acetate as feed, a liquid injection facility was connected to the 
injection port of the autoclave. For each batch carbonylation experiment 
employing dimethyl ether as feed, a Whitey (Trade Mark) sample bomb was 
connected to the injection port of the autoclave. A gas supply to the 
autoclave was provided from a gas ballast vessel, feed gas being provided 
to maintain the autoclave at a constant pressure and the rate of gas 
uptake being calculated (with an accuracy believed to be +/-1%) from the 
rate at which the pressure falls in the gas ballast vessel. The pressures 
used in batch autoclave experiments for dimethyl ether carbonylation may 
be generally higher than might be expected to be used in a continuous 
process because of the need to have sufficient carbon monoxide partial 
pressure, particularly for the iridium catalysed system. 
For each batch carbonylation experiment in which dimethyl ether was used as 
feed and iridium as catalyst, the autoclave was charged with optional 
co-promoter, the iridium catalyst and the liquid components of the liquid 
reaction composition excluding the dimethyl ether feed. 
Dimethyl ether was pre-charged to the Whitey (Trade Mark) bomb by 
transferring an amount of dimethyl ether which exceeded the required 
weight of feed from a cylinder (supplied by Aldrich) to the bomb which was 
pre-weighed and chilled in cardice. The bomb was slowly vented until the 
desired weight of dimethyl ether feed was retained in the bomb and was 
then connected to the injection port of the autoclave. 
The autoclave was flushed twice with nitrogen and once with carbon monoxide 
and the autoclave sealed. The contents of the autoclave were then heated 
with stirring (1000 rpm) to the desired reaction temperature. After 
allowing the system to stabilise for about 30 minutes, the dimethyl ether 
feed was transferred to the autoclave by over-pressurising the bomb with 
carbon monoxide and then opening the injection port of the autoclave. The 
pressure in the autoclave was subsequently maintained at the desired 
reaction pressure with carbon monoxide fed on demand from the gas ballast 
vessel. 
For each batch carbonylation experiment in which dimethyl ether was used as 
feed and rhodium as catalyst the above procedure was employed except that 
the catalyst was not charged to the autoclave with the liquid components 
and promoter of the liquid reaction composition excluding the dimethyl 
ether feed. Instead, the rhodium catalyst in aqueous acetic acid was 
introduced to the autoclave by means of a Gilson (Trade Mark) HPLC pump 
connected to an inlet valve on the autoclave immediately prior to 
introducing the dimethyl ether feed to the autoclave. 
For each batch carbonylation experiment in which methyl acetate was used as 
feed the above procedure was employed except that methyl acetate was 
charged to the autoclave together with optional co-promoter, and the 
components of the liquid reaction composition excluding part of the acetic 
acid and/or water charge in which the rhodium and iridium catalyst was 
dissolved. 
After allowing the system to stabilise for about 30 minutes, the rhodium or 
iridium catalyst solution was injected into the autoclave through the 
liquid injection facility under pressure of carbon monoxide. 
Reactions employing dimethyl ether and methyl acetate as feed were compared 
under conditions such that the amount of carbon monoxide consumed, if the 
reactions proceeded to completion, would be the same. Furthermore, the 
final liquid reaction compositions would be expected to be the same. The 
starting compositions for the batch carbonylation experiments were 
calculated by considering the following equilibria: 
______________________________________ 
2 Methyl acetate + 2 H.sub.2 O 
= 2 Methanol + 2 Acetic acid 
(1) 
2 Methanol = Dimethyl ether + H.sub.2 O 
(2) 
2 Methyl Acetate + H.sub.2 O 
= Dimethyl ether + 2 Acetic acid 
(3) 
______________________________________ 
Thus, the molar amount of dimethyl ether feed required to replace an amount 
of methyl acetate feed can readily be calculated by considering Equation 
(3). For example, 2 moles of methyl acetate and 1 mole of water in a 
liquid reaction composition should be replaced with 1 mole of dimethyl 
ether and 2 moles of acetic acid. 
Gas uptake from the ballast vessel was measured every 30 seconds and from 
this was calculated the rate of carbonylation, expressed as immoles of 
carbon monoxide per hour (mmol/hr). After uptake of carbon monoxide from 
the ballast vessel had ceased or the reaction had proceeded for 40 
minutes, whichever was sooner, the autoclave was isolated from the gas 
supply. The contents of the autoclave were cooled to room temperature and 
the gases were cautiously vented from the autoclave, sampled and analysed 
by gas chromatography. The liquid reaction composition was discharged from 
the autoclave, sampled and was analysed for liquid products and 
by-products by gas chromatography. 
To obtain a reliable baseline a number of identical baseline runs may have 
to be performed to condition the autoclave such that consistent rates are 
achieved. This conditioning period is often different from autoclave to 
autoclave. 
Experiment A 
A baseline experiment was performed with a rhodium catalyst without 
promoter at a high water concentration (decreasing from an initial charge 
of 17.0% by weight to a calculated value of 11.6% by weight assuming 100% 
conversion of substrate). The rate of carbon monoxide uptake from the 
ballast vessel was calculated to be 628 mmol/hr and this rate remained 
constant throughout the course of the reaction until all the methyl 
acetate substrate was consumed. This experiment is not an example 
according to the present invention because dimethyl ether was not used as 
feed to the carbonylation reaction. 
Experiment B 
Experiment A was repeated (water concentration decreasing from 14.4 to 
11.6% by weight as above) except that dimethyl ether was used as feed, the 
amount of dimethyl ether employed being calculated using Equation (3) 
above. The rate of carbon monoxide uptake from the ballast vessel was 
calculated to be 610 mmol/hr. The rate of carbon monoxide uptake remained 
constant throughout the course of the reaction. This experiment is not an 
example according to the present invention because greater than 10% by 
weight water in the reaction composition was employed. It demonstrates 
when compared with Experiment A that at high water concentrations (i.e. 
greater than 10% by weight) in the reaction mixture improvement in the 
carbonylation rate is not obtained when dimethyl ether is substituted for 
methyl acetate as feed in an amount greater than 10% by weight of the 
feed. 
Experiment C 
A baseline experiment was performed at a lower (decreasing from 5.1 to 0.5% 
by weight as above) water concentration than employed in Experiment A. The 
rate of carbon monoxide uptake from the ballast vessel measured after 5 
minutes was found to be 594 mmol/hr. The rate of gas uptake was found to 
constantly decrease during the course of the reaction as the water 
concentration steadily reduced, this was believed to be a consequence of 
progressive catalyst deactivation at low water concentrations. This 
experiment is not an example according to the present invention because no 
dimethyl ether was used as feed to the carbonylation reaction.

EXAMPLE 1 
Experiment C was repeated (water concentration decreasing from 2.8 to 0.4% 
by weight as above) except that dimethyl ether was used as feed, the 
amount of dimethyl ether employed being calculated using Equation (3). The 
rate of carbon monoxide uptake from the ballast vessel after 5 minutes was 
found to be 350 mmol/hr. In contrast to Experiment C, no reduction in the 
rate of carbon monoxide uptake was observed during the reaction. This 
example is according to the present invention and shows that the use of 
dimethyl ether at low water concentrations has a beneficial effect on the 
stability of a rhodium catalyst without the need to use a co-promoter such 
as an iodide salt, for example lithium iodide. 
Experiment D 
A baseline experiment was performed (water concentration decreasing from 
10.8 to 2.7% by weight as above) using an iridium catalyst and a ruthenium 
co-promoter with methyl acetate as feed to the carbonylation reaction. The 
rate of gas uptake from the ballast vessel after 5 minutes was found to be 
1615 mmol/hr. This is not an example according to the present invention 
because no dimethyl ether was used as feed to the carbonylation reaction. 
EXAMPLE 2 
Experiment D was repeated (water concentration decreasing from 7.0 to 2.7% 
by weight as above) except that dimethyl ether was used as feed, the 
amount of dimethyl ether being calculated using Equation (3). The rate of 
uptake of carbon monoxide from the ballast vessel after 5 minutes was 
found to be 1969 mmol/hr. This example is according to the present 
invention and demonstrates that increased carbonylation rates can be 
achieved by employing dimethyl ether as feed to a carbonylation reaction. 
The autoclave charges, reaction temperatures and pressures for Experiments 
A-D and Examples 1 and 2 are given in Table 1. Analyses of the 
non-condensable gases vented from the autoclave at the end of the 
experiment are given in Table 2. Analyses of the liquid reaction 
compositions at the end of the experiments revealed that acetic acid was 
the major product (greater than 99% by weight) for all experiments. 
TABLE 1 
__________________________________________________________________________ 
Autoclave charge and Reaction Conditions. 
Run 
Temp 
Pressure 
Time 
MeOAC 
DME MeI Water 
AcOH 
Ir Ru Rh 
Experiment 
Feed (.degree.C.) 
(barg) 
(min) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
__________________________________________________________________________ 
Experiment A 
MeOAc 
185 27.5 27.5 
244 -- 101 772 744 -- -- 0.40.sup.a 
Experiment B 
DME 185 27.8 27.5 
-- 122 102 622 981 -- -- 0.40.sup.b 
Experiment C 
MeOAc 
185 27.8 40 244 -- 101 272 894 -- -- 0.20.sup.c 
Example 1 
DME 185 27.3 40 -- 124 101 7 1164 
-- -- 0.40.sup.d 
Experiment D 
MeOAc 
190 38.0 23 389 -- 41 261 739 0.94.sup.e 
0.62.sup.f 
-- 
Example 2 
DME 190 38.2 20.5 
-- 195 41 346 477 0.94.sup.g 
0.63.sup.f 
-- 
__________________________________________________________________________ 
.sup.a RhCl.sup.3.3H.sup.2 O dissolved in 139 mmol water and 42 mmol 
acetic acid. 
.sup.b RhCl.sup.3.3H.sup.2 O dissoved in 167 mmol water and 50 mmol aceti 
acid. 
.sup.c Rh.sub.2 (CO).sub.4 Cl.sub.2 dissolved in 83 mmol acetic acid. 
.sup.d RhCl.sup.3.3H.sub.2 O dissolved in 139 mmol water and 58 mmol 
acetic acid. 
.sup.e IrCl.sub.3.3H.sub.2 O dissolved in 278 mmol water. 
.sup.f Ru.sub.3 (CO).sub.12. 
.sup.g IrCl.sub.3.3H.sub.2 O. 
TABLE 2 
______________________________________ 
Analyses of Non-Condensable Gases 
Experiment 
Methane (% v/v) 
CO.sub.2 (% v/v) 
DME (% v/v) 
______________________________________ 
Experiment A 
0.2 2.6 -- 
Experiment B 
0.3 2.2 1.5 
Experiment C 
trace trace -- 
Example 1 2.0 -- -- 
Experiment D 
4.4 2.9 -- 
Example 2 4.3 1.9 5.1 
______________________________________ 
Balance comprises hydrogen (not measured), nitrogen and carbon monoxide. 
High Pressure Infrard Cell Experiments 
The following experiments were performed with a high pressure infrared 
cell. In these experiments rates were based on total gas uptake and no 
attempt was made to compensate for partitioning of dimethyl ether between 
gas and liquid phases. 
Experiment E--Carbonylation of methyl acetate with rhodium catalyst 
The following solution was charged to high pressure infrared cell. 
______________________________________ 
Cell charge 
Methyl acetate 4.70 g 
Methyl iodide 3.60 g 
Water 1.28 g 
Acetic acid 13.29 g 
Injector charge 
Acetic acid 2.00 g 
Rh(CO).sub.2 Cl!.sub.2 
0.025 g 
______________________________________ 
The solution was flushed and pressured with carbon monoxide and heated to 
185.degree. C. where the catalyst was injected with carbon monoxide so 
that the total pressure in the cell was 30 barg. The pressure was 
maintained by feeding carbon monoxide from a ballast vessel and the 
reaction was monitored by measuring the pressure drop in the ballast 
vessel. Infrared spectra of the rhodium species present was recorded 
throughout the reaction. When gas uptake had stopped the product solution 
was analysed by gas chromatography. 
The carbonylation rate was initially linear followed by a steady decrease 
in rate corresponding to a decrease in the active catalytic species 
Rh(CO).sub.2 I.sub.2 !.sup.- and an increase of the inactive catalytic 
species Rh(CO).sub.2 I.sub.4 !.sup.-. 
______________________________________ 
Calculated methyl acetate 
Carbonylation rate 
Rh as Rh(CO).sub.2 I.sub.2 
concentration (%) 
(mol/hr) (%) 
______________________________________ 
18.4 0.161 100 
15 0.151 88 
10 0.126 70 
5 0.092 50 
1 0.023 17 
______________________________________ 
This is not an example according to the present invention because no 
dimethyl ether was used as feed to the carbonylation reaction. 
EXAMPLE 3 
Carbonylation of Dimethyl Ether With Rhodium Catalyst 
Experiment D was repeated as above using dimethyl ether in place of methyl 
acetate. 
______________________________________ 
Cell charge 
Dimethyl ether 1.47 g 
Water 0.69 g 
Acetic acid 18.14 g 
Injector charge 
Methyl iodide 3.75 g 
Rh(CO).sub.2 Cl!.sub.2 
0.025 g 
______________________________________ 
The carbonylation rate was 0.169 mol/hr and this was linear until close to 
the end of the reaction. The rhodium catalyst was present totally as 
Rh(CO).sub.2 I.sub.2 !.sup.-. 
In contrast with Experiment E this is an example according to the present 
invention because dimethyl ether was present in the feed. It demonstrates 
that the rate of carbon monoxide uptake is accelerated by the presence of 
dimethyl ether at low water levels and that the catalyst is stabilised. 
Experiments F and G and Examples 4 and 5 were carried out in an analogous 
manner to those described in Experiments A-D and Examples 1 and 2 with the 
exception that a 300 mL Hastelloy B2 (Trade Mark) autoclave was used. Also 
a dual liquid injection facility connected to the injection port of the 
autoclave allowed introduction of either Rh or Ir catalyst followed by DME 
substrate by use of an over pressure of carbon monoxide gas as described 
in previous examples. Furthermore, gas uptake from the ballast vessel was 
measured every 2 seconds rather than every 30 seconds as described in 
previous examples. 
Experiment F 
A baseline experiment was performed (water concentration decreasing from 
9.7 to 1.6% by weight during the course of the reaction assuming 100% 
conversion of substrate) using an iridium catalyst with methyl acetate as 
a feed to the carbonylation reaction. The rate of gas uptake from the 
ballast vessel after 5 minutes was found to be 2226 mmol/hr. This is not 
an example according to the present invention because no dimethyl ether 
was employed. 
EXAMPLE 4 
Experiment F was repeated (water concentration decreasing from 5.7 to 1.6% 
by weight as above) except that dimethyl ether was used as a feed, the 
amount used being calculated using Equation (3). The rate of carbon 
monoxide uptake after 5 minutes was found to be 2722 mmol/hr. This 
example, which is according to the present invention, demonstrates faster 
carbonylation rates can be achieved by using dimethyl ether as a feed to a 
carbonylation reaction, as opposed to methyl acetate as used in Experiment 
F. 
Experiment G 
An experiment was performed with lithium iodide and hydrogen (both 
precharged to the autoclave prior to heating to reaction temperature) and 
a rhodium catalyst. Methyl acetate was used as a carbonylation feed and 
the water concentration decreased from 5.1 to 0.5% by weight as above. The 
rate of carbon monoxide uptake from the ballast vessel after 5 minutes was 
found to be 1773 mmol/hr. This is not according to the present invention 
as no dimethyl ether was used. 
EXAMPLE 5 
Experiment G was repeated (water concentration decreasing from 2.8 to 0.5% 
by weight as above) except that dimethyl ether was used as a feed. The 
rate of carbon monoxide uptake after 5 minutes was found to be 2100 
mmol/hr. This is according to the present invention. The use of dimethyl 
ether enhances the carbonylation rate when compared with Experiment G. 
The autoclave charges, reaction temperatures and pressures for Experiments 
F and G and Examples 4 and 5 are given in Table 3 and the analyses of the 
non-condensable gases vented from the autoclave at the end of the 
experiments are in Table 4. 
TABLE 3 
__________________________________________________________________________ 
Run 
Temp 
Pressure 
Time 
MeOAc 
DME Mel Water 
AcOH 
Ir Rh Hydrogen 
Li 
Experiment 
Feed (.degree.C.) 
(BarG) 
(mins) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(mmol) 
(BarG) 
(mmol) 
__________________________________________________________________________ 
Experiment F 
MeOAc 
185 34.8 34.5 
777 -- 82 583 1496 
1.89.sup.a 
-- -- -- 
Example 4 
DME 185 34.8 25.8 
-- 386 85 200 2267 
1.89.sup.b 
-- -- 
Experiment G 
MeOAc 
185 50 19.1 
475 -- 203 542 1505 
-- 0.40.sup.c 
1.5 149 
Example 5 
DME 185 50 16.5 
-- 240 202 298 1979 
-- 0.40.sup.d 
1.6 148 
__________________________________________________________________________ 
.sup.a H.sub.2 IrCl.sub.6 dissolved in 370 mmol H.sub.2 O 
.sup.b H.sub.2 IrCl.sub.6 dissolved in 364 mmol H.sub.2 O 
.sup.c Rh.sub.2 (CO).sub.4 Cl.sub.2 dissolved in 117 mmol AcOH; 13 mmol 
MeOAc 
.sup.d Rh.sub.2 (CO).sub.4 Cl.sub.2 dissolved in 133 mmol AcOH 
TABLE 4 
______________________________________ 
Methane DME 
Experiment 
(% v/v) CO.sub.2 (% v/v) 
H.sub.2 (% v/v) 
(% v/v) 
______________________________________ 
Experiment F 
6.03 3.0 4.0 -- 
Example 4 
0.9 4.0 1.77 .sup.a 
Experiment G 
.sup.b .sup.b .sup.b .sup.b 
Example 5 
.sup.b .sup.b .sup.b .sup.b 
______________________________________ 
Balance: Nitrogen and carbon monoxide. 
.sup.a Dimethyl ether could not be accurately measured 
.sup.b Not recorded.