Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-05-07
2002-03-05
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S205000
Reexamination Certificate
active
06353130
ABSTRACT:
The present invention relates to a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 1 to 8 carbon atoms in a solvent-free phase at elevated temperatures and in the presence of an acidic esterification catalyst, in which the (meth)acrylic acid, the alkanol and the acid catalyst are fed to a reaction zone, the resulting water as part of a mixture comprising starting alkanol is removed from the reaction mixture by rectification, during the residence time in the reaction zone, in a rectification unit I mounted on the reaction zone, the distillate obtained is separated into an organic phase containing starting alkanol and into a water-containing aqueous phase, the organic phase and, if required, a part of the aqueous phase are recycled to the rectification unit I, the reaction mixture containing the desired ester is passed from the reaction zone into a separation zone comprising further rectification units and the resulting alkyl ester of (meth)acrylic acid is isolated in said separation zone.
The term “(meth)acrylic” is used herein as abbreviated notation for “acrylic or methacrylic”.
Alkyl esters of (meth)acrylic acid are generally known and are important, for example, as reactive monoethylenically unsaturated monomers for the preparation of aqueous polymer dispersions by the free radical aqueous emulsion polymerization method, which dispersions are used, for example, as adhesives.
Usually, the preparation of the alkyl (meth)acrylates is carried out by direct, acid-catalyzed reaction (esterification) of (meth)acrylic acid with the corresponding alkanols.
One route for the industrial production of (meth)acrylic acid is the catalytic gas-phase oxidation of suitable C
3
-/C
4
-precursors (e.g. propylene, acrolein, isobutene or methacrolein) with molecular oxygen. However, this procedure gives not pure (meth)acrylic acid but a gas mixture which, in relation to (meth)acrylic acid, contains, inter alia, acetic acid as a byproduct, the separation of which from (meth)acrylic acid, in particular by rectification, is expensive (cf. for example DE-A 19814449 and DE-A 19814421).
On the other hand, direct esterification of (meth)acrylic acid with alkanols is carried out predominantly by a method in which, with continuous removal of the water of reaction by distillation, the product ester too is separated from the reaction mixture. The distillate is then separated into an aqueous phase and into an organic phase which contains the desired ester and from which the desired ester must be isolated. The latter is usually carried out by separation steps involving rectification (cf. for example DE-A 19536178). The presence of an organic solvent as an azeotropic water entrainer does not require such an esterification procedure.
With the use of (meth)acrylic acid containing acetic acid (known as crude (meth)acrylic acid), however, the corresponding alkyl ester of acetic acid is unavoidably formed as a byproduct and as part of the abovementioned organic phase in the course of a direct esterification.
However, alkyl acetates as impurities in alkyl (meth)acrylates prove to be troublesome in many fields of use of alkyl (meth)acrylates since they are on the one hand not susceptible to free radical polymerization and, on the other hand, are relatively highly volatile.
In the preparation of aqueous polymer dispersions containing alkyl (meth)acrylates as polymerized units, for example, aqueous polymer dispersions containing free alkyl acetate would be obtained. Owing to the generally good solubility of the alkyl acetate in the polymer particles present in dispersed form in the aqueous polymer dispersion, subsequent removal of alkyl acetate, for example by stripping with air or steam, is possible only with difficulty and by expensive procedures. On the other hand, the alkyl acetate partial pressure of aqueous polymer dispersions containing alkyl acetate is sufficiently high to result in alkyl acetate workplace concentrations which are not completely safe in the atmosphere surrounding the place of processing of the polymer dispersion, and it is for this reason that polymerizers generally demand essentially alkyl acetate-free alkyl (meth)acrylate as starting material for their polymerizations.
On the route to such alkyl acetate-free alkyl (meth)acrylates, the high separation efficiency (i.e. a high reflux ratio and/or a large number of theoretical plates), which promotes both separation of alkyl (meth)acrylates and alkyl acetates by rectification, in particular in the presence of alkanol and water, and separation of the acetic acid from (meth)acrylic acid by rectification, can if necessary be circumvented in a manner known per se by carrying out the separation by rectification with reduced separation efficiency in a singly unsharp manner.
The term “singly unsharp” rectification refers to a rectification in which a mixture which contains two or more components is rectified under conditions (e.g. low reflux ratio and/or a small number of theoretical plates) such that only “one” component of the mixture is obtained in high purity. In the simplest case, this means that, instead of separating a mixture consisting of A (e.g. n-butyl acrylate) and B (e.g. n-butyl acetate) by rectification into essentially pure A and into essentially pure B, for example, only a separation into essentially pure A (n-butyl acrylate) and into a mixture of B (n-butyl acetate) and A (n-butyl acrylate) is performed.
The disadvantage of such an unsharp separation by rectification is evident: a part of the product desired in essentially pure form is lost as a component of the mixture, i.e. the yield of desired product is reduced.
At the same time, a shift in the acetic acid separation from the (meth)acrylic acid level to the alkyl (meth)acrylate level would be desirable insofar as (meth)acrylic acid is a substantially more readily polymerizable monomer than the corresponding alkyl ester, which is why the high temperatures required have a particularly disadvantageous effect on a separation by rectification with a high separation efficiency at the (meth)acrylic acid level. Separation of acetic acid by crystallization at the (meth)acrylic acid level is therefore often thought to be necessary for solving the problem. The disadvantage of this solution is however that it requires investment in a crystallization unit.
It is an object of the present invention to provide a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 1 to 8 carbon atoms in a solvent-free phase, in which, on the one hand, acetic acid-containing crude (meth)acrylic acid can still be used as a starting material and, on the other hand, a substantially alkyl acetate-free alkyl (meth)acrylate is obtained with a limited separation efficiency and without substantial loss of desired product.
We have found that this object is achieved by a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 1 to 8 carbon atoms in a solvent-free phase at elevated temperatures and in the presence of an acidic esterification catalyst, in which the (meth)acrylic acid, the alkanol and the acid catalyst are fed to a reaction zone, the resulting water as part of a mixture comprising starting alkanol is removed from the reaction mixture by rectification, during the residence time in the reaction zone, in a rectification unit I mounted on the reaction zone, the distillate obtained is separated into an organic phase containing starting alkanol and into a water-containing aqueous phase, the organic phase is recycled to the rectification unit I, the reaction mixture containing the desired ester is passed from the reaction zone into a separation zone comprising further rectification units and the resulting alkyl ester of (meth)acrylic acid is isolated in said separation zone, wherein
the (meth)acrylic acid used is an acetic acid-containing crude (meth)acrylic acid,
the acidic esterification catalyst is
Aichinger Heinrich
Herbst Holger
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Forohar Farhard
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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