FR2517297A1 - Liquid phase carbonylation of methyl acetate - in presence of cobalt, at least one of titanium, zirconium and hafnium and anionic iodide - Google Patents

Abstract

Liquid phase carbonylation of Me acetate is effected in the presence of (a) a Co source, (b) a source of Ti, Zr and/or Hf with an oxidn. degree of at most 3; (c) an ionic iodide A(+) I(-), where A(+) is a quaternary onium cation derived from the elements of the N gp. or an alkai metal cation, and (d) opt., a carboxylate of formula A'(n+)(OCOR)n(-) (where n is 1 or 2, A'(n+) is as above, and A'(n+) and A(+) can be the same or different, and A'(n+) can also be an alkaline-earth cation; R is 1-8C alkyl, aralkyl or aryl). The total amt. of halogenated cpds. present in the reaction medium (expressed in g-atom of halogen and denoted by XT) is such that the atomic ratio XT/(A(+)+nA'(n+)) is at most 1. Addition of a cocatalyst of the type (b) contg. ingredients (a), (c) and opt. (d) gives acetic anhydride in high hourly yield. The Co can be used at low concn.

Description

PROCESS FOR CARBONYLATION OF METHYL ACETATE
The subject of the present invention is a process for the carbonylation of methyl acetate.

 The present invention more particularly relates to an improvement in the carbonylation process of methyl acetate, using a cobalt catalyst.

U.S. Patent No. 2,730,546 describes the carbonylation of methyl acetate to acetic anhydride in the presence of a catalyst selected from cobalt complexes of the general formula
[R4A2 CoX4 wherein X represents a bromine or iodine atom, A nitrogen or phosphorus atom and R represents a lower alkyl radical, for example.

These complexes can be formed in situ by loading on the one hand a cobalt halide (CoX'2) and on the other hand a quaternary ammonium (or phosphonium) halide (R4AX). The formation of the complexes in question can then be represented by the following reaction

 However, the effectiveness of these cobalt catalysts seems relatively low. This type of process, the interest of principle is not disputed, could lead to an industrial achievement.

- Etape 2

French patent 2,242,362 (corresponding to US applications Nos. 394,220 and 467,977, respectively, of 4 September 1973 and 8 May 1974) describes a process for the preparation of acetic anhydride in two stages, in which, during a first step, the bromide, or preferably the iodide, of methyl is carbonylated to obtain the corresponding acetyl halide, this acetyl halide being in turn reacted with methyl acetate to conduct in a second step, to acetic anhydride, which corresponds to the following reaction scheme, in the case where methyl iodide is the starting material
- Step 1

- 2nd step

 As this diagram makes clear, methyl iodide, the starting material of step 1, is "regenerated" in step 2.

Step 1 is advantageously carried out in the presence of a rhodium-based catalyst; Step 2 would be favored by the presence of lithium and / or chromium. The two steps can be carried out in the same reaction zone, which will then contain methyl iodide, methyl acetate, rhodium and, if appropriate, lithium and / or chromium, or even acetyl iodide. , zone in which one will also introduce carbon monoxide.

 US Pat. No. 4,115,444 proposes an improvement of the technique described in the aforementioned French patent, which improvement consists in adding to the reaction medium an organic compound of phosphorus or nitrogen, in which the phosphorus or nitrogen is trivalent. This patent confirms the potential interest of catalytic systems based on rhodium, or even palladium or iridium, and chromium in this reaction.

 French Patent No. 2,303,788 (corresponding to US applications Nos. 556,750 and 654,661 respectively of March 10, 1975 and February 5, 1976) shows that the presence of a significant amount of hydrogen in the reaction medium defined above. significantly influences the orientation of the reaction. Indeed, under these conditions, a mixture containing a predominant proportion of acetic acid, varying amounts of ethylidene diacetate, acetic anhydride and acetaldehyde are obtained.

 The main interest of these processes using catalysts based on rhodium, or even palladium or iridium, systems based on the couple (rhodium-chromium) appearing to be the most active, lies mainly in the possibility that they provide access to acetic anhydride from methyl acetate using lower carbon monoxide partial pressures than those required in the prior processes.

 Nevertheless, the attempts to develop this type of process, even on a simple pre-industrial scale, come up against non-negligible difficulties.

 A first set of difficulties lies in the fact that the catalysts based on rhodium or palladium, or iridium, extremely rare and expensive metals, are deactivated, especially during the treatments required to recover the product (s) ) of the reaction. Because of the cost of these catalysts it is essential to regenerate them. Moreover rhodium losses which appear inevitable in the various points of an industrial unit heavily burden the economy of such a process.

 A second series of difficulties has its source in the presence, necessary for the good progress of the reaction and the stabilization of the rhodium, large quantities of methyl iodide (or acetyl) which involves significant risks of corrosion in the various points of an industrial installation. Furthermore, methyl iodide and / or some of its derivatives formed in the reaction medium are responsible for an intolerable contamination of the reaction product (s) which requires the implementation of additional steps for eliminate iodides whose presence is undesirable in the reaction products.These iodinated derivatives present in large quantities not only in the products but also in various effluents from the reaction zone must, for obvious economic reasons, be recovered which involves additional stages of treatment.

 The various problems which are difficult to solve and which are associated with this type of process will appear more clearly on reading French patents Nos. 2 438 023 and 2 438 024 (corresponding respectively to US applications Nos. 949 344 and 949 345 filed October 6, 1978. ) and US Pat. No. 4,241,219.

Furthermore, it is well known that methyl acetate can be obtained by reaction of acetic acid and methanol, the acetic acid can be produced by carbonylation of methanol and methanol can be in turn manufactured by hydrogenation of carbon monoxide. The reactions at issue are likely to be represented as follows

The carbonylation of methyl acetate in a substantially anhydrous medium makes it possible to obtain acetic anhydride according to the following reaction.

 Thus, the interest of a carbonylation process of methyl acetate to acetic anhydride (1) appears clearly when we also consider the reactions (a) to (c) above, since overall this set amounts to a process by which acetic anhydride is produced from carbon monoxide and hydrogen.

 On the other hand, since cobalt is a common metal, it is easy to see the potential value of its use in a carbonylation process of methyl acetate. It is therefore surprising to note that all the earlier solutions proposed recently are based on the recommendation of rhodium-based catalytic systems.

 It has now been found, quite surprisingly, that it is possible to carbonylate methyl acetate efficiently by using a cobalt-based catalyst system.

The subject of the present invention is therefore a process for the carbonylation of methyl acetate in the liquid phase in the presence of
- (a) a source of cobalt,
(b) a source of titanium, zirconium and / or hafnium with an oxidation degree of less than or equal to 3,
(c) an ionic iodide of formula AI in which A + is a cation selected from the group consisting of onium quaternary cations derived from nitrogen group elements and alkali metal cations, and - (d) the where appropriate, a carboxylate of formula
A'n + (OCOt in which n is equal to 1 or 2 and A'n ± has the meaning given above for A +, A'n + and A + may also be identical or different, A'n + may furthermore represent an alkaline-earth cation, and R is an alkyl, aryl or aryl radical having at most 8 carbon atoms, the total amount of halogenated compounds present in the reaction medium (expressed in atom-gram of halogen and designated by XT oans the following) being such that the atomic ratio XT / (A + + n.A'n +) is less than or equal to 1.

 The present method of carbonylation of methyl acetate is remarkable in various puints. Indeed, it has been found quite unexpectedly that the addition of a co-catalyst of the above-mentioned type (b) to the reaction medium containing ingredients of type (a), type (c) and, if appropriate, type (a) mentioned above, makes it possible to obtain acetic anhydride, with a particularly high norane productivity, whereas under the same reaction conditions cobalt alone has only a poor carbonylating activity in the reaction envisaged and that the cocatalyst alone, in the same conditions, does not have practically any carbonylating activity.

 The Applicant, without being bound by any explanation (or reaction mechanism), is led by these surprising facts to think that the carbonylating activity should be attributed, in the present process, the source of cobalt which, under the conditions reaction, would be transformed into a catalytically active species, the precise nature of which is not fully understood.

 The secona constituting metal in the system, co-catalyst of the type (o), mentioned above would only serve to increase the activity and / or the concentration of active species of cobalt. The reason for this remarkable influence is unknown but could however reside in a change in the degree of oxidation and / or the disposition, or even the nature, of the co-ordinates of the central cobalt atom. In other words, the cocatalyst would have the function of facilitating the appearance in the reaction medium of the strong active cooalt.

 Moreover, it has been found quite surprisingly that, contrary to the teachings of the prior art, the effectiveness of the present process is not related to the presence of methyl iodide amounts. On the contrary, when methyl iodide is added during the practice of the present process, a significant degree of effectiveness is observed.

 The precise reason for the negative influence of methyl iodide, a phenomenon in total contradiction with the teaching of the prior art, is unknown. The Applicant, without being bound by any explanation, assumes that methyl iodide, when it is present in significant amounts, breaks the complex equilibria which would be particularly at the origin of the very high reactivity of cobalt in the conditions described.

 On the other hand, the carbonylating activity of the previously defined system is not related to the precise nature of the initially loaded cobalt compound (s). Any source of cobalt capable of reacting with the carbon monoxide in the reaction medium to give carbonyl complexes of cobalt can be used in the context of the present process. Typical sources of cobalt are, for example, finely divided metallic cobalt, inorganic salts of cobalt (nitrate, carbonate, halides, etc.) or organic salts, in particular carboxylates. Cobalt carbonyls or hydrocarbonyls can also be used. Among the cobalt derivatives suitable for carrying out the process according to the invention, mention may be made of formate, acetate and more particularly dicobaltoctacarbonyl.

 The precise amount of cobalt involved in the reaction is not fundamental. In general, operation is carried out with a quantity of cobalt such that the concentration in the reaction medium expressed in milliatoms-grams per liter (mAt-g / l) is between 0.1 and 500 and preferably between 0.5 and 100 mAt -g / l.

 An advantage of the present process is that it is possible to obtain good results with a low cobalt concentration.

To carry out the present process, a source of titanium, zirconium and / or hafnium with a degree of oxidation of less than or equal to 3 is also used. Examples of sources of titanium, zirconium or of hafnium at the oxidation state of less than or equal to 3 likely to be suitable for the implementation of the present process include titanium, zirconium and hafnium metal and alloys containing at least 70% by weight of these metals and the compounds of formulas I to III below
- (I) MOX
- (II) (Tr - C5H5) x MXy
- (III) (IT-C5H5) 2 M (CO) 2 in which M represents a titanium, zirconium or hafnium atom, X represents a chlorine, bromine or iodine atom and x and y are integers, which may be null separately, less than or equal to 3 and such that their sum is equal to 2 or 3.

 Titanium, zirconium, metal hafnium or an alloy containing at least 70% by weight of these metals are advantageously used.

 If the metals themselves are perfectly suitable for carrying out the present process, it is also possible to use titanium, zirconium and / or hafnium alloys which will contain at most 30% of other alloyed elements.

 Among the common elements likely to be alloyed with titanium include aluminum, vanadium, molybdenum, tin, chromium and silicon. Common elements that can be combined with zirconium include tin, iron, chromium and nickel.

 Of course, mixtures of titanium, zirconium and / or hafnium or alloys containing two or three of these metals can be used.

By way of example of titanium alloys, mention may be made of
- the Ti - 6A1 - 4 V
- Ti - 5A1 - 2.5 Sn
Ti - 8Al - 1 Mo - 1 V and - Ti - 13V - 11 Cr - 3 Al.

the figures indicating the weight percentage of the element in question; examples of alloys of zirconium include
- Zr - 1.5 Sn - 0.12 Fe - 0.10 Cr - 0.05 Ni and - Zr - 1.5 Sn - 0.12 Fe - 0.10 Cr
The metals and alloys just mentioned can be used in any convenient form, for example in the form of powder, son or plate.

 The walls of the reactor can be constructed, for example, of titanium or be jacketed titanium and make a more or less significant contribution, as cocatalyst.

 Metal titanium is preferably used.

 The amount of cocatalyst to be used in the present process is not fundamental. In general, this amount will be such that the M / Co atomic ratio, M representing a titanium, zirconium and / or hafnium atom, is between 0.1 and 1000 and preferably between 0.5 and 0.5. and 500.

 For a good implementation of the invention, this ratio will be greater than or equal to 10.

 The process according to the present invention also requires the presence of an ionic iodide of formula A + I in which A + is a cation selected from the group consisting of quaternary onium cations derived from elements of the nitrogen group and alkali metal cations. .

 Quaternary onium cations derived from the elements of the group of nitrogen are understood to mean cations formed from nitrogen, phosphorus, arsenic or antimony and from four identical or different monovalent hydrocarbon groups, the free valence of which is carried by a carbon atom, each group being connected to the aforementioned element by said free valence, any two of these groups may also form together a single divalent radical.

dans-laquelle Q représente un atome d'azote ou de phosphore, R1, R2,
R3 et R4, pouvant être identiques ou différents représentent des radicaux organiques dont la valence libre est portée par un atome de carbone, deux quelconques de ces divers radicaux pouvant éventuellement former ensemble un radical unique divalent.Among these compounds, the Applicant recommends the use of quaternary phosphonium (or ammonium) iodides. The cations of these iodides can be conveniently represented by formulas (IV) to (VI) below

in which Q represents a nitrogen or phosphorus atom, R1, R2,
R3 and R4, which may be identical or different, represent organic radicals whose free valency is borne by a carbon atom, any two of these various radicals possibly possibly forming a single divalent radical.

More specifically, R 1, R 2, R 3 and R 4 may represent linear or branched alkyl radicals, cycloalkyl, aralkyl (for example benzyl) or monocyclic aryl radicals, having at most 16 carbon atoms, which may optionally be substituted with 1 to 3 carbon atoms. alkyl radicals having 1 to 4 carbon atoms; two of the radicals
R1 to R4 may optionally form together a single divalent radical - alkylene or alkenylene - having 3 to 6 carbon atoms (for example a tetramethylene or hexamethylene radical) and optionally 1 or 2 ethylenic double bonds, said radical being able to carry 1 to 3 alkyl substituents having 1 to 4 carbon atoms.

in which R5, R6, R7 and R8, which may be identical or different, represent alkyl radicals having from 1 to 4 carbon atoms, one of the radicals R7 or R8 may also represent hydrogen, R7 and R8 may optionally together form a radical single divalent alkylene having from 3 to 6 carbon atoms, for example tetramethylene or hexamethylene; R6 and R7 or R8 may together form a single radical divalent - alkylene or alkenylene - having 4 carbon atoms and optionally 1 or 2 ethylenic double bonds, the nitrogen atom then being included in a heterocycle, to constitute, by for example, a pyridinium cation.

in which R5 and Q + have the meaning given above, Rg being identical to R5 represents an alkyl radical having 1 to 4 carbon atoms or a phenyl radical, and y is an integer between 1 and 10 inclusive and preferably between 1 and 6 included. By way of example of quaternary ammonium iodides suitable for carrying out the present process, mention may be made of iodides
of tetramethylammonium
triethylmethylammonium
tributylmethylammonium,
trimethyl (n-propyl) ammonium,
tetraethylammonium,
tetrabutylammonium,
dodecyltrimethylammonium,
benzyltrimethylammonium,
benzyldimethylprodylammonium,
benzyldimethyloctylammonium,
dimethyldiphenylammonium,
methyltriphenylammonium,
N, N-dimethyltrimethylammonium,
N, N-diethyl trimethylammonium,
N, N-dimethyl tetramethylammonium,
N, N-diethyl tetramethylammonium,
N-methylpyridinium
N-ethylpyridinium
N-methylpicolinium
By way of example of quaternary phosphonium iodides which are also suitable for carrying out the present process, mention may be made of iodides
tetramethylphosphonium,
of ethyltrimethylphosphonium,
trimethylpentylphosphonium,
octyltrimethylphosphonium,
dodecyltrimethylphosphonium,
trimethylphenylphosphonium,
of diethyldimethylphosphonium,
of dicyclohexyldimethylphosphonium,
dimethylidiphenylphosphonium,
cyclohexyltrimethylphosphonium,
triethylmethylphosphonium,
methyl-tri (isopropyl) phosphonium,
methyl-tri (n-propyl) phosphonium,
methyl-tri (n-butyl) phosphonium,
methyl-tri (2-methylpropyl) phosphonium,
methyltricyclohexylphosphonium,
methyltriphenylphosphonium,
methyltribenzylphosphonium,
methyl-tri (4-methylphenylphosphonium,
methyltrixylylphosphonium,
diethylmethylphenylphosphonium,
dibenzylmethylphenylphosphonium,
ethyltriphenylphosphonium,
tetraethylphosphonium,
ethyl-tri (n-propyl) phosphonium,
triethylpentylphosphonium,
ethyltriphenylphosphonium,
n-butyl-tri (n-propyl) phosphonium,
butyltriphenylphosphonium,
benzyltriphenylphosphonium,
(ss-phenylethyl) dimethylphenylphosphonium,
tetraphenylphosphonium,
triphenyl (methyl-4-phenyl) phosphonium
The precise nature of the quaternary ammonium or phosphonium cation is not fundamental to the present process. The choice among these compounds is more oriented by practical considerations, such as solubility in the reaction medium, availability and convenience of use.

 In this respect, the ammonium or quaternary phosphonium iodides represented either by the formula (IV) in which any one of the radicals R1 to R4 is chosen from linear alkyl radicals having from 1 to 4 carbon atoms, or by Formulas (V) or (VI) in which R 5 (or R 6) is also an alkyl radical having 1 to 4 carbon atoms, are particularly suitable.

In addition, among the ammonium iodides are preferred those whose cations correspond to the formula (IV) in which all the radicals
R 1 to R 4 are chosen from linear alkyl radicals having from 1 to 4 carbon atoms and at least three of which are identical.

 Likewise, among the quaternary phosphonium iodides, those whose cations correspond to formula (IV) in which any one of the radicals R 1 to R 4 represents a linear alkyl radical having from 1 to 4 carbon atoms, the three are preferred. other radicals being identical and chosen from phenyl, tolyl or xylyl radicals.

 The iodides of the alkali metals are also suitable for carrying out the present invention. However, in the case where such iodides are used, it is also necessary to introduce into the reaction medium a solvent chosen from tetramethylenesulfone, tetramethylurea, N-methylpyrrolidone and monocarboxylic acid amides, which are derived from acids having at most 8 carbon atoms, and the nitrogen atom of which has two alkyl substituents having a maximum of 4 carbon atoms. This type of solvent is used in a proportion of 10 to 50% (by volume) of the reaction medium, although higher or lower proportions can be used.

 The amount of ionic iodide to be used in the context of the present process is generally such that the molar ratio I- / Co is greater than or equal to 5 and preferably greater than or equal to 10. It is not necessary to It is advantageous if this ratio exceeds the value of 200. The molar ratio I- / Co will advantageously be set at a value of between 15 and 100.

 As indicated at the beginning of this specification, the present invention can also be implemented in the presence of a carboxylate of formula A, n + (OCOR) n- in which n is equal to 1 or 2 and A'n + has the meaning given. above for A +, A'n + and A + may also be identical or different, A'n + may, in addition, represent an alkaline earth cation, and R is an alkyl, aryl or aryl radical having at most 8 carbon atoms. carbon.

This embodiment is an advantageous variant of the present process when using an alkaline iodide. Indeed, the carboxylates in question seem to limit the in-situ appearance of methyl iodide (the adverse effect of which has already been reported in the present process) which can be formed by reaction of an alkaline iodide with the acetate of methyl (starting material) according to the reaction scheme below, the formation of methyl iodide being greater starting from lithium iodide than from an equivalent amount of sodium iodide or potassium iodide

 It is not necessary that the carboxylate A'n = + (OCOR) be derived from the same cation as the ionic iodide used. The carboxylate is advantageously an acetate.

It will be noted that certain acetates may be considered as the adduct of methyl acetate and a phosphine, amine, arsine or stibine according to the scheme given in the particular case of triphenylphosphine to simplify the presentation

This is the reason why each mole of phosphine, amine, arsine or stibine, possibly charged or fed will be assimilated to a gram equivalent of cation A '+ in the condition connecting the total amount of halogen ( XT) to that of the A + cations and
A'n + present in the reaction medium.

 When a carboxylate of the type defined above is used, its amount is in general such that the molar ratio A, n + / A + is between 0.01 and 20. Preferably, this ratio will be advantageously between 0.05 and 10. between 0.1 and 5.

 According to an advantageous variant, the present process is also carried out in the presence of hydrogen. In the context of this variant, the hydrogen partial pressure determined at 25 ° C. will be less than 50 bars.

 As practitioners know, the nature of the product obtained is a function of the water content of the reaction medium. When operating in a reaction medium which, given the various industrial constraints, contains the least possible water (substantially anhydrous medium), acetic anhydride is selectively obtained. On the other hand, when the medium contains a significant proportion of water, it is possible to obtain acetic acid in a proportion corresponding to that of the acetic anhydride obtained in a virtually anhydrous medium. Inasmuch as it is economically more advantageous to produce acetic anhydride, the present process will be conveniently conducted in a substantially anhydrous medium.

 As indicated above, it is necessary that the total amount of halogenated compounds present in the reaction medium (XT> expressed in atom-gram of halogen) is such that the ratio (atomic) XT / (A + + n. 'n +) is less than or equal to 1.

 This condition does not exclude the possibility of using halogens (X2), halogenated acids (HX), alkyl halides (RX), cobalt halides (CoX2), halogenated compounds of titanium, zirconium or hafnium corresponding to formulas (I) and (II) previously indicated, but it implies that if we load or feed these types of compounds will necessarily load or feed either a carboxylate [Ain + (OCOR ) n3 defined above, either a phosphine, an amine, an arsine or a stibine, in an amount at least equivalent to the amount of gram atoms of halogen (X) introduced, if necessary, by means of the halogenated compounds mentioned ci It is recognized that under the reaction conditions the halogenated compounds X2, HX, and CoX2 will react with methyl acetate to give rise to a methyl halide, which will in turn react (as well as the alkyl halides). RX) on phosphine, amine, arsine or stibine to form the corresponding quaternary onium halide.

 That is why, in the case where the halogenated compounds of types X2, HX, RX, and CoX2 are used, each mole of phosphine, amine, arsine or stibine introduced to neutralize somehow these sources of halogen will be considered as a gram equivalent of cation A + in the condition connecting the total amount of halogen (XT) to that of the A + and A'n + cations present in the reaction medium.

 The methyl halide CH3X, as well as the alkyl halides (RX) may also react on the carboxylates [Ain + (OCOR)] to give rise to the corresponding ionic halides (A'n + X).

dans laquelle Q' représente un atome de phosphore, d'azote, d'arsenic ou d'antimoine, et R1 à R3 ont la même signification que précédemment.The phosphines, amines, arsines and stibines which may be used, may be represented by formula (VII) below.

wherein Q 'represents a phosphorus, nitrogen, arsenic or antimony atom, and R1 to R3 have the same meaning as above.

 It is advantageous to use a phosphine, and more particularly to triphenylphosphine.

According to an advantageous variant, the reaction medium contains in addition to methyl acetate and the various constituents of the catalytic system, defined herein a solvent selected from the group consisting of tetramethylenesulphone, tetramethylurea,
N-methylpyrrolidone, the amides of monocarboxylic acids, which are derived from acids having at most 8 carbon atoms, and the nitrogen atom of which has two alkyl substituents having at most 4 carbon atoms, acetic acid and acetic anhydride.

 The partial pressure of the carbon monoxide measured at 25 ° C. is generally greater than 10 bars and, preferably, greater than 30 bars.

 The reaction temperature which can vary within wide limits is generally between 160 and 300 OC. The operation is preferably carried out at a temperature of between 180 and 240 ° C., in which range the catalytic system develops optimum efficiency.

 An advantage of the present process lies in the possibility of effectively carbonylating methyl acetate even with a low concentration of cobalt, cocatalyst and ionic iodide, under a total temperature pressure of the order of 80 to 300 bar, which is therefore much lower than that usually recommended for cobalt-based catalyst systems.

 Another advantage of the present process resides in the disappearance of the various constraints associated with the use of the catalytic systems recently proposed to carry out this carbonylation.

 As is well known to those skilled in the art, methyl acetate, a starting material in the present process, can be formed in situ from dimethyl ether, so it is quite possible within the scope of this invention. charge or feed dimethyl ether or a mixture of dimethyl ether and methyl acetate.

 At the end of the operation, the products obtained can be easily separated by fractional distillation of the resulting mixture, for example.

 The following examples illustrate the invention without limiting its field or spirit.

In the examples as well as in the control tests, the procedure used is as follows
In a Hatelloy 82 autoclave of 125 ml capacity, methyl acetate, one or more solvents, if any, and the various components of the catalyst system are charged. After closing the autoclave, a carbon monoxide pressure and a hydrogen pressure, respectively designated by P (CO) and P (H2) (values measured at 25 ° C.) are admitted.

 The agitation, by a system of back and forth, is started and the autoclave is then brought to the chosen temperature in about 25 minutes. The total temperature pressure, designated P (T), is maintained substantially at the value indicated by successive refills of carbon monoxide containing at most 1% (by volume) of hydrogen.

After a fixed reaction time, the autoclave is cooled and degassed. The reaction mixture is then analyzed by chromatography and potentiometry.

In the examples the following conventions are used
- Pr: designates the productivity expressed in gram
acetic anhydride per hour per liter
- RR: denotes the number of moles of acetic anhydride
formed per 100 moles of methyl acetate
loaded
- mat-g: means milliatome-gram
- mmol: means millimole
All pressures [P (CO), P (H2) and P (T)] are expressed in bars.

EXAMPLES 1 to 3
In the autoclave and according to the procedure described above, a series of tests is carried out on a charge consisting of
- 25 ml of methyl acetate
- 5 ml of acetic acid
- 15 μl of N-methylpyrrolidone
1 mg of cobalt in the form of dicobaltoctacarbonyl (Co =
20 mat-g / l)
- titanium powder (Ti / Co = 62.5)
- methyltriphenylphosphonium iodide (I / Co = 15)
XT / (A + + n.A'n +) = 1
The specific conditions as well as the results obtained in two hours of reaction at 210 ° C are given in the table below.

The control tests a and b also appearing in this table were carried out in the absence of titanium (Ti / Co = O). In control test c, reported in this table, 19 mmol of methyl iodide was added to the charge described above; in this test XT / (A + + n.A'n +) = 2.27.

<Tb>

: <SEP> Ex <SEP> nO <SEP>: <SEP> Ti / Co <SEP>: <SEP> P (CO) <SEP>: <SEP> P (H2) <SEP>: <SEP> P ( T) <SEP>: <SEP> RR <SEP>: <SEP> Pr
<tb>: <SEP> a <SEP><SEP>:<SEP> 0 <SEP>: <SEP> 80 <SEP>: <SEP> (+) <SEP>: <SEP> 150 <SEP>: <SEP> 15.5 <SEP>: <SEP> 49
<tb>: <SEP> 1 <SEP>: <SEP> 62.5 <SEP>: <SEP>"<SEP>:<SEP> (+) <SEP>: <SEP>"<SEP>:<SEP> 33.2 <SEP>: <SEP> 104
<tb>: <SEP> 2 <SEP>: <SEP>"<SEP>:<SEP>"<SEP>:<SEP> 10 <SEP>: <SEP>"<SEP>:<SEP> 35.9 <SEP>: <SEP> 113
<tb>: <SEP> b <SEP>: <SE> O <SEP>: <SEP>"<SEP>:<SEP>"<SEP>:<SEP>"<SEP>:<SEP> 29.3 <SEP>: <SEP> 91
<tb>: <SEP> 3 <SEP>: <SEP> 62.5 <SEP>: <SEP> 130 <SEP><SEP>:<SEP>"<SEP>:<SEP> 250 <SEP>: <SEP> 54.9 <SEP>: <SEP> 173
<tb>: <SEP> c <SEP>: <SEP> 62.5 <SEP>: <SEP> 90 <SEP>: <SEP> (+) <SEP>: <SEP> 250 <SEP>: <SEP > 11.6: <SEP> 37
<Tb>
(+): less than 1 bar.

EXAMPLE 4
In the autoclave and according to the procedure described above, a test is carried out on a charge consisting of
- 40 ml of methyl acetate
- 10 ml of acetic acid
- 0.5 mat-g of cobalt in the form of dicobaltoctacarbonyl
(Co = 10 m-g / l)
- titanium powder (Ti / Co = 250)
- methyltriphenylphosphonium iodide (I / Co = 20)
XT / (A + + n.A'n +) = 1 and under the following conditions
- P (CO) = 130
- P (H2) = 10
- Temperature = 210 OC
- P (T) = 250
The results obtained in two hours of reaction at the indicated temperature are as follows
- RR = 16.6
- Pr = 85
EXAMPLE 5
Example 4 is reproduced with the following modifications
- Co = 40 m-g / l
Ti / Co = 15.6
- I- / Co = 12.4
- Temperature = 180 OC
The results obtained in two hours of reaction at the temperature of 180 OC are as follows
RR = 31.8
Pr = 162.

EXAMPLE 6
Example 2 is reproduced by replacing the titanium with zirconium powder (Zr / Co = 33). The results are as follows
- RR = 36
- Pr = 113.

Sample test of
Example 2 is reproduced by replacing the titanium with zirconyl bis (acetylacetonate) (Zr / Co = 10).

The results are as follows
- RR = 6
- Pr = 20.

EXAMPLE 7
Example 2 is reproduced by replacing the titanium with 3 g of a titanium-zirconium alloy consisting of 70% by weight of titanium and 30% by weight of zirconium (Ti + Zr) / Co = 54, the reaction time being one hour only. The results obtained are as follows
- RR = 29.2
Pr = 182.

EXAMPLE 8
Example 2 is reproduced by replacing the titanium with 2.9 g of a titanium-chromium alloy consisting of 82% by weight of titanium and 18% by weight of chromium (Ti / Co = 50), the reaction time being one hour only. The results obtained are as follows
- RR = 39.6
Pr = 251.

EXAMPLE 9
In the autoclave and according to the procedure described above, a test is carried out on a charge consisting of
- 25 ml of methyl acetate
- 5 ml of acetic acid
- 15 ml of tetramethylenesulphone
- 1 mat-g of cobalt in the form of dicobaltoctacarbonyl
(Co = 20 mat-g / l)
- titanium powder (Ti / Co = 62.5)
sodium iodide (I- / Co = 15)
XT / (A + + n.A'n +) = 1 and under the following conditions
- P (CO) = 120
- P (H2): less than 1 bar
- Temperature = 180 OC
- P (T) = 200
The results obtained in two hours of reaction at the indicated temperature are as follows
- RR = 10.8
- Pr = 34
EXAMPLE 10
Example 9 above is reproduced by adding 15 mmol of sodium acetate to the feedstock.
XT / (A + + n.A'n +) = 0.5
the results obtained are as follows
- RR = 22.3
Pr = 71.

Claims (14)

 1. - Process for the carbonylation of methyl acetate in the liquid phase in the presence  - (a) a source of cobalt,  (b) a source of titanium, zirconium and / or hafnium with a degree of oxidation of less than or equal to 3.  (c) an ionic iodide of formula A + I in which A + is a cation selected from the group consisting of quaternary onium cations derived from elements of the nitrogen group and alkali metal cations, and - (d) if necessary, a carboxylate of formula Atn + (OCOR) in which n is 1 or 2 and A'n + has the meaning given above for A +, A, n + and A + may also be identical or different, A'n + may, in addition, represent an alkaline earth cation, and R is an alkyl, aralkyl or aryl radical having at most 8 carbon atoms, the total amount of halogenated compounds present in the reaction medium (expressed in gram atom) of halogen and designated XT) being such that the atomic ratio XT / (A + + n.A'n +) is less than or equal to 1.  2. - Process according to claim 1, characterized in that the ingredient of type (b) is selected from titanium, zirconium and hafnium metal and alloys containing at least 70% by weight of one or more of these metals.  3. - Process according to claim 1, characterized in that the ingredient of type (b) is titanium metal.  4. - Process according to claim 1, characterized in that the ionic iodide is a quaternary phosphonium iodide.  5. - Process according to any one of the preceding claims, characterized in that the partial pressure of the carbon monoxide measured at 25 OC is greater than or equal to 10 bar and preferably greater than or equal to 30 bar.  6. - Process according to any one of the preceding claims, characterized in that the temperature is between 160 and 300 OC and preferably between 180 and 240 OC.  7. - Process according to any one of the preceding claims, characterized in that the cobalt concentration is between 0.1 and 500 mat-g / l.  8. - Process according to any one of the preceding claims, characterized in that the reaction medium contains a solvent selected from the group consisting of tetramethylenesulphone, tetramethylurea, N-methylpyrrolidone, acid amides.  monocarooxylics, which are derived from acids having at most 8 carbon atoms, and the nitrogen atom has two alkyl substituents having at most 4 carbon atoms, acetic acid and acetic anhydride.  9. - Process according to any one of the preceding claims, characterized in that the molar ratio I- / Co is greater than or equal to 5 and, preferably greater than or equal to 10.  10. - Process according to any one of the preceding claims, characterized in that the molar ratio I- / Co is between 15 and 100.  11. - Process according to any one of the preceding claims, characterized in that the atomic ratio M / Co, M representing a titanium atom, zirconium and / or hafnium, is between 0.1 and 1000.  12. - Process according to any one of the preceding claims, characterized in that the atomic ratio M / Co, M representing a titanium atom, zirconium and / or hafnium, is between 0.5 and 500.  13. - Method according to claim 11 or 12, characterized in that the M / Co ratio is greater than or equal to 10.  14. - Process according to any one of the preceding claims, characterized in that the cobalt concentration is between 0.5 and 100 m-g / l.