Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-05-11
2003-09-09
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S216000, C562S598000, C562S599000, C568S715000, C568S835000, C568S876000
Reexamination Certificate
active
06617470
ABSTRACT:
The present invention relates to a process for esterifying (meth)acrylic acid with an alkanol in the presence of an esterification catalyst, in which unconverted starting compounds and (meth)acrylic ester formed are removed from the reaction mixture by distillation, leaving behind an oxyester-containing bottom product, the bottom product is separated off and the oxyesters contained therein are thereafter cleaved in the presence of at least one acid catalyst by the action of elevated temperatures.
In this-application, the term “(meth)acrylic acid” is an abbreviation for acrylic or methacrylic acid.
Furthermore, the term “oligomeric (meth)acrylic acid” used in the further course of this application means the Michael adducts of (meth)acrylic acid with itself and with the resulting secondary products. Such Michael adducts may be characterized by the formula (III),
where z is an integer from 1 to 5 and R′ is H or CH
3
,
and are to be distinguished here from (monomeric) (meth)acrylic acid and from (meth)acrylic acid polymers (which are obtainable by free radical polymerization of (meth)acrylic acid). The essential feature is that the Michael addition reaction of (meth)acrylic acid with itself and its resulting secondary products is reversible. Oligomeric (meth)acrylic acid is obtained, for example, in the distillative treatment of (for example crude) (meth)acrylic acid (the term “crude” indicates a small amount of, in particular, aldehydic impurities still present) in the bottom product (cf. for example DE-A 22 35 326).
Usually, the preparation of alkyl esters of (meth)acrylic acid by esterifying (meth)acrylic acid with alkanols at elevated temperature is carried out in the liquid phase with or without solvents and in the presence of, as a catalyst, acids other than (meth)acrylic acid (cf. for example DE-A 23 39 519). The disadvantage of this method of preparation is that, as secondary reactions under the abovementioned esterification conditions, still unconverted starting alcohol undergoes addition at the ethylenically unsaturated double bond of already formed alkyl (meth)acrylate (Michael addition reaction) with formation of a compound of the formula I below and still unconverted (meth)acrylic acid undergoes addition at said double bond with formation of a compound of the formula II.
Successive multiple addition is also possible. Furthermore, mixed types can occur. These adducts (alkoxyesters and acyloxyesters) are referred to as oxyesters for short:
where x and y are integers from 1 to 5,
R is alkyl and
R′ is R or CH
3
.
The problem of oxyester formation is particularly pronounced in the preparation of esters of acrylic acid, the oxyesters mainly formed being the alkoxypropionic esters and the acyloxypropionic esters where x and y are 1. In the preparation of esters of methacrylic acid, the oxyester formation takes place to a lesser extent. The formation of oxyesters is described, inter alia, in DE-A 23 39 529 and in U.S. Pat. No. 5,734,075. The above publications confirm that the formation of oxyesters takes place essentially independently of the special esterification conditions. Of very particular importance is the oxyester formation in the preparation of acrylic esters of C
1
- to C
8
-alkanols, in particular of C
4
- to C
8
-alkanols, very particularly in the preparation of n-butyl acrylate and 2-ethylhexyl acrylate (for which reason the present invention is used in particular in connection with these esterifications).
Typical of the oxyesters is that their boiling point is above the boiling points of starting acid, starting alcohol, desired esters formed and any organic solvent present.
The working up of any desired esterification reaction mixture is usually carried out by separating unconverted starting compounds and resulting desired esters from the reaction mixture by distillation, the acid catalyst used for the esterification being separated off beforehand, if required, by extraction by means of water and/or aqueous alkali (cf. for example Ullmann's Encyclopedia of Industrial Chemistry, Vol. A1, 5th Ed., VCH, page 167 et seq.). The bottom product remaining in such working up by distillation contains the oxyesters, which result in considerably lower yields.
The prior art (e.g. DE-A 19 701 737, DE-A 19 536 191, DE-A 19 536 184, DE-A 19 547 485, DE-A 19 547 459 and CN-A 1063678) therefore contains various processes which cleave the oxyesters contained in the bottom product separated off, in the presence of at least one acid catalyst by the action of elevated temperatures, and separate off the resulting cleavage products, preferably by evaporation. Recommended suitable acidic cleavage catalysts are protic acids other than monomeric and oligomeric (meth)acrylic acid (acid strength>that of (meth)acrylic acid), for example mineral acids, such as sulfuric acid or phosphoric acid, and organic acids, such as methanesulfonic acid or p-toluenesulfonic acid. These definitions also apply to this application.
The disadvantage of the cleavage processes of the prior art is however that the resulting cleavage residue is generally highly viscous and as a rule contains solids.
Consequently, the cleavage residue is scarcely pumpable and can therefore be disposed of only with difficulty. Moreover, the generally tar-like solid is deposited on the wall surfaces in the course of time (fouling), which, for example, reduces the passage of heat or can lead to blockages, making it necessary to clean the wall surfaces from time to time.
U.S. Pat. No. 5,734,075 recommends carrying out the cleavage process in the absence of acidic cleavage catalysts and instead in the presence of oligomeric (meth)acrylic acid to reduce the abovementioned problems. However, the disadvantage of this procedure is that the cleavage, in particular in the case of bottom products which originate from esterifications with relatively long-chain alkanols, takes place comparatively slowly and gives only comparatively low conversions.
DE-A 19 536 184 recommends separating the oxyesters from the bottom product by distillation before cleaving in order to reduce said problems. The disadvantage of this procedure is the necessity of an additional distillation step.
The addition of a solvent as a diluent for reducing the viscosity problems has also been proposed. However, the requirement of an additional component is likewise disadvantageous.
It is an object of the present invention to provide a process for esterifying (meth)acrylic acid with an alkanol in the presence of an esterification catalyst, in which unconverted starting compounds and (meth)acrylic ester formed are removed from the reaction mixture by distillation, leaving behind an oxyester-containing bottom product, the bottom product is separated off and the oxyesters contained therein are thereafter cleaved in the presence of at least one acid catalyst by the action of elevated temperature, which process does not have the stated disadvantages of the processes of the prior art.
We have found that this object is achieved by a process for esterifying (meth)acrylic acid with an alkanol in the presence of an esterification catalyst, in which unconverted starting compounds and (meth)acrylic ester formed are removed from the reaction mixture by distillation, leaving behind an oxyester-containing bottom product, the bottom product is separated off and the oxyesters contained therein are thereafter cleaved in the presence of at least one acid catalyst by the action of elevated temperatures, wherein at least one aromatic sulfonic acid of the formula IV,
where
R″ independently of one another are each alkyl of six to twenty carbon atoms,
u is an integer from 1 to 3, and
v is 1 or 2,
is present as the at least one acid catalyst for cleaving the oxyesters.
The abovementioned sulfonic acids of the formula (IV) are disclosed, for example, in EP-A 521 488. u may be 1 or 2 or 3 and v may be 1 or 2. Frequently, the radicals R″ are alkyl radicals of 8 to 16 or 10 to 14 carbon atoms. Suitable typical compounds (IV) are accordingly, for example, octylbenzenesu
Aichinger Heinrich
Beckmann Stefan
Herbst Holger
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Oh Taylor V.
Rotman Alan L.
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