Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
1998-09-23
2001-05-15
Geist, Gary (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C562S523000
Reexamination Certificate
active
06232490
ABSTRACT:
BACKGROUND OF THE INVENTION
The object of the present invention is the preparation of acetic acid and/or methyl acetate, by using an isomerisation reaction of methyl formate.
Various means of access to acetic acid are known and used industrially. Amongst these are the carbonylation reactions of methanol. This carbonylation reaction may notably be carried out in the liquid phase, under the pressure of carbon monoxide, which is one of the reagents, in the presence of a homogeneous catalytic system comprising a compound based on rhodium and/or iridium and an iodinated promoter.
Another means of access is constituted by the isomerisation reaction of methyl formate in the presence of a catalyst based on rhodium or iridium.
In the isomerisation methods catalysed by iridium, the reaction is carried out with a solvent which is selected from the carboxylic acids, and more particularly, the acid produced. The reaction is moreover carried out under an atmosphere comprising nitrogen. It has in fact been noted that the carbon monoxide did not bring about any particular advantage during the reaction and could even be the cause of a certain inhibition of the isomerisation reaction, it favouring side reactions. It is to be noted that such a behaviour is entirely different from that which is observed when the catalytic system is rhodium-based, in which case the presence of carbon monoxide is essential for keeping the metal in the homogeneous phase. This type of method, whose interest is not questioned, does not have any real interest industrially since the reaction which is described therein is not sufficiently efficient. In fact, the reaction rates are only in the order of 2 mol/h/l of acid and/or ester produced.
It has been proposed, in order to improve the results of the above-mentioned method, to use the isomerisation reaction in the presence of a strong acid of the sulphonic acid type, such as paratoluenesulphonic acid for example. Under the conditions of this method, the reaction is carried out using significant amounts of methyl formate to be isomerised, which is consequently also used as solvent for the reaction. If this improvement contributes to the improvement of the activity of the reaction, it does nevertheless have the disadvantage of necessitating the use of a further compound, which does not simplify the method.
Further, it is possible that this acid degrades under the conditions of the reaction medium.
SUMMARY OF THE INVENTION
An aim of the present invention is to propose a method of isomerisation of methyl formate, into acetic acid and/or methyl acetate, whose productivity is improved with respect to the two isomerisation variants described above, this without it being necessary to employ a further compound to this end.
Furthermore, the method according to the invention is very selective.
These and other goals are met by the present invention which has therefore for object the preparation of acetic acid and/or methyl acetate, by reaction of methyl formate, in the presence of water, a solvent and a catalytic system comprising at least one halogenated promoter and at least one iridium-based compound. The reaction according to the present invention is characterised by the fact that it is carried out in maintaining a partial carbon monoxide pressure between 0.1.10
5
Pa and 25.10
5
Pa, and in maintaining a methyl formate amount of below 20% by weight of the reaction mixture.
It has in fact been noted, on the contrary to what was asserted in the prior art, that the presence of carbon monoxide was essential for the isomerisation reaction of the ester in the presence of iridium.
Furthermore, another important characteristic of the reaction is that the amount of methyl formate which reacts must be at the most 20% in order to obtain the best productivities.
Thus, the combination of these two characteristics has enabled multiplying the productivity of the first method described by 10, simply by maintaining the conditions of partial carbon monoxide pressure and ester concentration during the reaction within the intervals mentioned. Furthermore, the productivity of the method according to the invention is better than that of the variant which comprises strong acid since reaction rates of the same, even greater order of magnitude, have been obtained at lower temperatures.
DETAILED DESCRIPTION OF THE INVENTION
For greater clarity, the nature of the reagents shall first of all be described.
Thus, the reaction is carried out with methyl formate.
The method of the invention is carried out in the presence of a catalytic system which comprises at least one halogenated promoter and at least one iridium-based compound.
The halogenated promoter, which represents one of the constituents of the catalytic system, is selected preferably from iodinated compounds.
The halogenated promoter may be in the form of iodine, alone or in combination with other elements such as for example hydrogen, a C
1
-C
10
alkyl radical, a C
1
-C
10
acyl radical, a C
6
-C
10
aryl radical, or even alkali metal iodides or metal iodides, such as transition metal iodides, or iodides of metals of column II A of the periodic classification of the elements.
It is to be noted that the halogenated promoter may be constituted of a mixture of several of the above-mentioned promoters.
The context of the present invention shall not be left in preparing said halogenated promoters in situ with the aid of appropriate precursors.
As examples of promoters which are appropriate to the present invention, iodine, hydroiodic acid, methyl iodide, ethyl iodide, 1,1-diiodoethane, acetyl iodide, aluminium iodide, chromium iodide, lithium iodide, potassium iodide may be notably cited, without intending to be limiting.
According to a particular embodiment of the invention, the promoter used comprises hydrogen or a C
1
-C
10
alkyl radical. Preferably, the halogenated promoter comprises iodine and a radical of the methyl type.
The second element of the catalytic system used in the method according to the invention is constituted by at least one iridium-based compound.
First of all, the reaction according to the invention is more particularly carried out in the presence of a homogeneous catalyst. In other words, this signifies that the iridium-based compound is notably in a form which is soluble in the reaction mixture. It is to be noted that the presence of a part of said iridium-based compound in a non-dissolved form, does not have any major difficulty for the implementation of the reaction.
All the iridium compounds which are soluble or able to be dissolved in the reaction medium, under the conditions of implementation of the reaction, may be used. As examples, and without intending to be limiting, metallic iridium, its simple salts, its oxides or even its co-ordination complexes may notably be appropriate in the implementation of the invention.
As simple iridium salts, the iridium halides are conventionally used. The halogen is most particularly selected from chlorine, bromine and iodine, the latter being preferred. Thus, compounds such as IrI
3
, IrBr
3
, IrCl
3
, IrI
3
.4H
2
O, IrI
4
, IrBr
3
.4H
2
O may be used in the method according to the invention.
Oxides selected from IrO
2
, Ir
2
O
3
.xH
2
O may equally be conveniently used in the method according to the invention.
Regarding the soluble co-ordination complexes of iridium, the compounds which are most commonly used are those having ligands selected from carbon monoxide, or a carbon monoxide/halogen combination, the halogen being selected from chlorine, bromine or more particularly iodine. It is not nevertheless excluded to use soluble iridium complexes whose ligands are selected from the organo-phosphorus compounds and organo-nitrogen compounds for example.
As co-ordination complexes known to the person skilled in the art which are particularly convenient in the implementation of the invention, Ir
4
(CO)
12
, Ir(CO)
2
I
2
−
Q
+
, Ir(CO)
2
Br
2
−
Q
+
, Ir(CO)
2
Cl
2
−
Q
+
, may be cited without intention to limit; in which formulae Q may be notably hydro
Patois Carl
Perron Robert
Thiebaut Daniel
Acetex Chimie
Deemie Robert W.
Dennison, Scheiner Schultz & Wakeman
Geist Gary
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