Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2002-08-05
2003-11-11
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S218000, C560S203000, C560S205000
Reexamination Certificate
active
06646155
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of esters of an &agr;,&bgr;-unsaturated carboxylic acid by reacting the carboxylic acid with an alkanol in the presence of an acidic esterification catalyst and in the presence of an entraining agent with which the water of reaction formed during the esterification is removed from the esterification process.
2. Description of the Background
The preparation of alkyl esters of &agr;,&bgr;-unsaturated carboxylic acids is generally known. It is usually carried out by esterifying the acids with the corresponding alcohol at elevated temperatures in the liquid phase with or without a solvent and in the presence of a strong acid as a catalyst (cf. for example DE-A-23 39 519). In order to avoid polymerization, polymerization inhibitors are generally added. Of particular industrial importance is the esterification of (meth)acrylic acid. The term (meth)acrylic acid denotes acrylic or methacrylic acid in the known manner.
The disadvantage of the esterification processes of the prior art is that, under the esterification conditions, unconverted starting alcohol and unconverted carboxylic acid undergo addition at the double bond of already formed alkyl carboxylate as secondary reactions (Michael addition). Multiple addition is also possible. Furthermore, mixed types may occur. These adducts are alkoxy esters and acyloxy esters and they are referred to briefly as oxyesters. Typical of the oxyesters is the fact that their boiling point is above the boiling points of starting acid, starting alcohol, desired ester formed and any organic solvent present. In the production of the desired ester, they are therefore obtained as a residue and result in considerable reductions in yield. There have therefore been numerous attempts to recover at least a part of the compounds used or the desired ester from the oxyesters (cf. for example DE-A-195 36 191 and the prior art mentioned therein).
In the cleavage process of DE 195 36 191, a considerable amount of olefins is formed, which cannot be further used in the esterification process and therefore has to be separated off and removed.
A further disadvantage of the conventional esterification processes is a consequence of the fact that the ester formation is based on an equilibrium reaction. In order to obtain economical conversions, as a rule a starting material is used in excess and/or the water of esterification formed and/or the desired ester are removed from the equilibrium. In order to shift the equilibrium in the direction of ester formation, an organic entraining agent which forms an azeotropic mixture with water is frequently added. In particular the esterification with higher alkanols is carried out in the presence of an entraining agent for the water of reaction (Kirk-Othmer, “Encyclopedia of Chemical Technology”, Vol. 1, page 347, Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1985, Vol. A1, 168; U.S. Pat. Nos. 2,917,538, 5,386,052). Preferably, inert hydrocarbons, e.g. cyclohexane, hexane, benzene and toluene, are used.
The addition of a further “foreign” solvent is however disadvantageous since it has to be separated off separately and, if necessary, purified before being recycled.
DE 2 548 561 therefore proposes separating off the water of reaction formed in the reaction of acrylic acid with ethylhexanol as an azeotropic mixture with ethylhexanol. According to the example, ethylhexanol is used in excess relative to acrylic acid (1:1.42). The disadvantage here is that the esterification has to be carried out under reduced pressure and a residue of about 8% by weight, based on the desired ester, is obtained and has to be disposed of. The process thus causes environmental pollution and is uneconomical.
Influencing the esterification equilibrium by using a larger excess of a starting material (alkanol or carboxylic acid) leads to the formation of byproducts, e.g. ethers and olefins from the alkanol used and acyloxyesters and alkoxyesters by Michael addition of the alkanol or of the carboxylic acid. This is described, for example for acrylic acid, in U.S. Pat. No. 4,280,010 and in DE-A-2 339 519. These byproducts have to be separated off and disposed of in an expensive manner, which is uneconomical and pollutes the environment.
Even without the use of a relatively large excess of alcohol, some of the esters and the alcohol are cleaved under the strongly acidic esterification conditions and olefins are formed, as described, for example, in DE-A-195 36 191 and Houben-Weyl, Methoden der Organischen Chemie, Volume VIII/3, 1952, page 534.
Most processes of the prior art therefore have in common the disadvantage that undesired byproducts have to be separated off and that an additional solvent is required as an entraining agent for removing the water of esterification.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an esterification process which is simple to carry out and in which the formation of undesired byproducts is reduced.
We have found, surprisingly, that this object is achieved if the olefin which is formed by the elimination of water from the alkanol used for the esterification is used as an entraining agent.
The present invention therefore relates to a process for the preparation of an ester of an &agr;,&bgr;-unsaturated carboxylic acid by reacting the carboxylic acid with an alcohol which is selected from C
6
-C
12
-alkanols, cyclopentanol and cyclohexanol, in the presence of an acidic esterification catalyst and of an entraining agent for removing the water of reaction formed in the esterification, wherein the entraining agent used is the olefin on which the alcohol is based, i.e. an olefin which corresponds to an olefin obtainable by eliminating water from the alcohol employed is used.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With the exception of the entraining agent used, the novel esterification process is carried out in a conventional manner. Suitable processes are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1985, Vol. A1, 168, 169; Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 1, pages 341-348; U.S. Pat. Nos. 2,917,538; 5,385,052. The processes are described using acrylic acid as an example, but the novel process is not restricted to acrylic acid. The esters of other &agr;,&bgr;-unsaturated carboxylic acids can be prepared analogously.
The esterifications are typically carried out at from about 80 to 160° C., preferably from 90 to 130° C., and in the presence of an acidic esterification catalyst, for example a mineral acid, a sulfonic acid or phosphoric acid. Sulfuric acid and sulfonic acids are particularly suitable, especially p-toluenesulfonic, benzenesulfonic, dodecylbenzenesulfonic and methanesulfonic acid. The amount of catalyst is from about 0.1 to 10, preferably from 0.5 to 5, % by weight, based on the other starting materials. The esterification is usually also carried out in the presence of an inhibitor which inhibits the polymerization of the carboxylic acid and/or of the ester. Particularly suitable inhibitors are hydroquinone, hydroquinone monomethyl ether, p-methoxyphenol, p-benzoquinone, phenothiazine, 4-hydroxy-2,2,6,6-tetramethyl-1-oxylpiperidine and methylene blue, which are used for stabilization in amounts of from about 200 to 2,000 ppm, based on the weight of the starting materials. The novel process is however not restricted to specific catalysts or inhibitors, and the acid:alcohol ratios, the pressure prevailing during the esterification and the reaction time also play just as minor a role. A typical molar alcohol:acid ratio is from about 1:0.7 to 1:1.2, and typical reaction times are from about 1 to 10, preferably from about 1 to 6, hours. The esterification can be carried out at atmospheric, superatmospheric or reduced pressure and both continuously and batchwise.
Suitable apparatuses for carrying out the esterification and isolating the desired ester from the reaction mixture are conventi
Herzog Reinhard
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Reyes Hector M
Rotman Alan L.
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