The present invention relates to improvements in carbonylation processes. More particularly, the invention relates to improvements in carbonylation processes which arise from stabilising the compound or compounds which act as catalysts for such processes. Carbonylation processes in which small organic molecules, for example nonpolar molecules such as alkenes or alkynes or polar molecules such as alcohols, esters, or ethers, are reacted with carbon monoxide in the liquid phase and in the presence of a transition metal catalyst are known and in some cases used commercially.

In particular, one successful class of carbonylation processes is that in which the active catalyst is derived from a compound of the Group VIII metal rhodium, used in conjunction with a halide promoter. When compared with other transition metal complexes, such as cobalt carbonyl, nickel carbonyl or iron carbonyl, which are often regarded as 'classical' carbonylation catalysts, rhodium catalysts can be seen to have two advantages. First the rhodium catalysts are more reactive which means that for a given amount of feedstock the rate of reaction is much greater. This leads to two possible desirable options; either the throughput of feedstock can be increased or, as rate of reaction is related to temperature, the process can be operated at lower temperature. Either option can lead to substantial reductions in overall process costs. A second desirable attribute which rhodium catalysts possess is

Chat, relative to 'classical' carbonylation catalysts, they exhibit superior thermal stability. This is particularly important from a process viewpoint since, in general, at a given temperature the only way to overcome problems of catalyst deactivation is to work at elevated carbon monoxide pressure. Thus, carbonylation processes which formerly used classical catalysts always involved working at high te pertures, frequently in excess of 150 bars. Rhodium catalysts however allow similar reactions to be carried out, with equivalent or superior productivities, at pressures of less than 75 bars. By working at lower pressures many of the problems and costs associated with the design and use of high pressure processing equipment are obviated.

Rhodium catalysts have only one major drawback over the classical carbonylation catalysts. Because rhodium is a relatively rare metal which is difficult to purify, its cost per unit weight is substantially in excess of more common metals such as cobalt, nickel and iron. As a consequence, rhodium carbonylation catalysts are extremely expensive and it is desirable that quantitative catalyst recovery from the reactor and product stream is, as far as possible, effected.

The most effective method of ensuring that catalyst losses are reduced is to add a stabiliser which prevents precipitation of the catalyst from the liquid streams. This is particularly important in the product stream since, at some point after reaction the carbon monoxide overpressure is reduced and hence the probability of small amounts of catalyst precipitating from solution is increased. However, it will be appreciated by the skilled chemist that the size of a commercial plant is such that small individual catalyst losses in, for example, the reactor, transfer lines and recycle lines can amount to substantial total losses.

Additives which stabilise rhodium catalysts during liquid phase carbonylation of nonpolar olefinic feedstocks have been disclosed. For example, from US 3,818,060 it is known that organic derivatives of pentavalent phoshporus, arsenic, antimony, nitrogen or bismouth may be used as stabilisers during the liquid phase carbonylation of

ethylenically unsaturated compounds. Furthermore, from US 3,579,552 it is known that inter alia phosphines, amine and trihaloβtannates can also be used to produce soluble rhodium coordination complexes for the same reaction. With regard to carbonylation reactions which involve the use of polar feedstocks, e.g. alcohols, esters and ethers, and a rhodium/halide catalyst European patent application 0055618A discloses four classes of stabiliser (1) N,N,N',N*- tetramethyl-o-phenylendiamine and 2,3'dipyridyl. (2) a substituted diphosphine having the formula

Rl R4

\ /

P - R ₃ - P

/ \

R ₂ R ₅ wherein R _j , R ₂ , 4 and R5 are alkyl or substituted alkyl groups and R3 a polymethylene group

(3) a dibasic or polybasic acid having the formula

0 0

// ^/ /

HO - C - Y _x - COH or

wherein Y _j is of the formula (CXιX ₂ ) _m in which m is 2 to 10 and wherein Y ₂ , Y3, and Y5 are independently selected from hydrogen, a halogen, lower alkyl, aryl, hydroxyl, carboxyl amino, hydroxy-subβtituted alkyl and carboxysubstituted alkyl or

(4) a compound of germanium antimony, tin or an alkali metal all of which reduce the loss of rhodium catalyst from solution in those parts of the carbonylation plant where the catalyst is exposed to conditions which are deficient in carbon monoxide.

It has now been discovered that when polar feedstocks such as those disclosed in European patent application 0 055 618A are used, two further classes of compound may be used to stabilise the

rhodium Satalyst system and prevent catalyst loss by precipitation. Thus, in this invention, precipitation of rhodium in rhodium/halide catalyst systems generally under carbon monoxide deficient conditions is prevented or retarded by the addition to the system of a stabilising compound selected from a thiol or an imidazole.

Accordingly, the present invention comprises a process for the liquid phase carbonylation of an alcohol, ester or ether by reaction with carbon monoxide in the presence of a rhodium catalyst system comprising a rhodium component, an odide or bromide component and a stabilising compound characterised in that the stabilising compound is selected from a thiol or an imidazole.

In another embodiment, a process is provided for reducing the loss of rhodium through precipitation from catalyst systems containing a rhodium component characterised in that a stabilising compound selected from a thiol or an imidazole is added to the catalyst system.

The feedstocks which may be carbonylated by this process are alcohols, esters or ethers. Any member of these three families may be used but lower members of each type (having 1 to 6 carbons atoms) are preferred. Examples of, particularly preferred feedstocks are methanol, ethanol, methyl acetate, ethyl acetate, and dimethyl ether.

Any soluble rhodium containing catalyst useful in the carbonylation of acohols, esters or ethers may be used herein. The source of rhodium may be, for example, a simple inorganic salt such as rhodium chloride, bromide, iodide, or nitrate; a carbonyl or organometallic complex of rhodium, or a coordination complex. Finely divided rhodium metal which becomes solubilised in the reaction medium may also be used. When the iodide or bromide component used in conjunction with the catalyst is an iodide, it can be added as elemental iodine, hydrogen iodide, an iodide salt, for example sodium iodide, or an organic source of iodide such as an alkyl or aryl iodide. A preferred source of the iodide component is methyl iodide. It is possible to supply part of the iodide with the rhodium by using, for

example, a compound such as rhodium triiodide. The concentration of iodide is such as to produce a rhodium to iodide ratio of at least 1:4 preferably between 1:10 and 1:1000. When a bromide component is used, it may be added in a corresponding way and combinations of iodide and bromide can be employed.

The thiols which can act a stabilisers in this invention may be any alkyl or aryl thiol although Ci to Cio alkyls thiols and benzenes thiols are preferred. The molecule may also contain one or more thiol groups. Preferred thiols include but are not limited to 1,2-ethanedithiol, ethanethiol propanethiol, benzenethiol and substituted derivatives thereof.

When an imidazole is used as the stabiliser it will have the general formula

in which R _lf R ₂ , R3 and R4 are each independently hydrogen, alkyl, aryl, cycloalkyl or alkaryl hydrocarbyl radicals. A preferred imidazole is N-methylimidazole.

The stabilising compound is present in amounts such that the molar ratio of stabilising compound to rhodium component is at least 0.5:1 and preferably in the range 0.5:1 to 10 ⁵ :1. The carbonylation reaction as described herein is carried out by contacting a solution of the catalyst system in the polar feedstock with carbon monoxide in an appropriate reactor. The concentration of the soluble rhodium component will in general be such as to constitute between 10 ppm and 3000 ppm of the reaction mixture.

As mentioned previously, the reaction is carried out under superatmospheric pressure and at elevated temperature. Although the optimum conditions will depend on the particular feedstock used, the reaction is generally carried out at a pressure of greater than 10 bars, preferably 10 to 100 bars and at a temperature in the range

of 100 £δ ^~ 250 ^β C. For the preferred feedstocks mentioned herein, the optimum temperature and pressure ranges will vary somewhat. However, the ranges of such optimum temperatures and pressure for a given feedstock will be familiar to those skilled in the art of ^* carbonylation.

It is preferable that the carbon monoxide used in this invention is as pure as possible. However, a certain amount of inert diluent gases such as nitrogen or gases which are often co-produced with carbon monoxide, such as hydrogen, may be present. If hydrogen is present is should be at level less than 50 mole Z. of the total gas stream.

The stabilising influence of the thiols and lmidazoles described herein will now be Illustrated by the following Examples. However, these Examples should not be construed as limiting the scope of this invention which includes equivalent embodiments, modifications and variations. Example 1

An autoclave was charged with methanol (78.9 g), methyl Iodide (49.3 g), acetic acid (178.5 g) and water (51.3 g). Rhodium trichloride was added at a level such that the initial rhodium concentration in solution was 340 ppm. Enough 1,2-ethanedithiol was added to give a 1,2-ethanedithiol to rhodium molar ratio of 2.5:1. The autoclave was then sealed and the temperature and carbon monoxide overpressure adjusted to 185°C and 400 psig (ca 27 bars) respectively. Under these conditions the liquid phase carbonylation of methanol to acetic acid took place. After 45 minutes the rhodium content of the product solution was determined by atomic absorption spectroβcopy. The results showed a concentration of 308 ppm indicating that 93% of the rhodium still remained in solution. The weight of the product solution was 368.7 g. Comparative Test A

An autoclave was charged with methanol (78.3 g), methyl iodide (47.7 g), acetic acid (177.1 g) and water (50.4 g). Rhodium trichloride was added at a level such that the initial rhodium concentration in solution was 340 ppm, but no stabiliser was added.

After 45~minutes under conditions identical to those used in Example 1, analysis of the product solution by atomic absorption spectroscopy showed a rhodium concentration of 315 ppm indicating that in this case only 87% of the rhodium still remained in solution. The weight of the product solution was 333 g. Example 2

Methanol (625 g/hr) was fed to a reactor, at 175°C and 27 bars pressure, which contained a rhodium/iodide catalyst and as stabiliser N-methylimidazole. The molar ratio of rhodium to iodide was 1:185 and the molar ratio of stabiliser to rhodium was 80:1. The catalyst and stabiliser were separated from the acetic acid, containing product stream, by distillation, downstream of the reactor and subsequently recycled. Loss of rhodium from the liquid phase was followed by analysing samples of the product stream for rhodium at various times. The change in rhodium in solution over

50 hours, as measured by atomic absorption, is shown in the Table.

TIME ON STREAM (HRS) CONCENTRATION OF RHODIUM

IN LIQUID PHASE (PPM) 0 540

8 542

18.7 547

49.5 491

Example 2 shows that the addition of N-methylimidazole as stabiliser reduces the loss of rhodium from the solution. Under equivalent conditions, in the absence of stabilisers, rhodium losses from solution approach 100 ppm/2 hours.