Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-09-28
2002-10-01
Shah, Mukund J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C562S600000
Reexamination Certificate
active
06458989
ABSTRACT:
The present invention relates to a process for preparing pure (meth)acrylic acid from a crude (meth)acrylic acid, in which a crude (meth)acrylic acid which is of low water content and in addition to (meth)acrylic acid essentially comprises acetic acid and possibly aldehydes is treated with a compound which is able to remove aldehydes, and the aldehyde-free crude (meth)acrylic acid thus obtained is subjected to imprecise distillation, and then the (meth)acrylic acid obtained as the bottom product, which is substantially free from acetic acid, is separated from the other high boilers to give pure (meth)acrylic acid. Preferably, the low-boiling fraction obtained in the imprecise distillation, which consists predominantly of (meth)acrylic acid and acetic acid, is esterified with alkanols to give (meth)acrylates.
By the term “crude (meth)acrylic acid” used in accordance with the invention is meant a mixture comprising (meth)acrylic acid that is prepared by catalytic gas-phase oxidation of C
3
and/or C
4
precursors, absorption in a high-boiling solvent, desorption of the low boilers, and distillative separation from the solvent, comprises at least 90% by weight of (meth)acrylic acid, and is substantially free from water.
By “pure (meth)acrylic acid” is meant an aldehyde-free (meth)acrylic acid have a purity of at least 99.7% by weight, which is suitable for preparing high molecular mass addition polymers.
The term (meth)acrylic acid or (meth)acrylate used in the context of the present invention stands for acrylic and methacrylic acid or, respectively, their esters.
By virtue of its highly reactive double bond and the acid function, (meth)acrylic acid forms a valuable monomer for preparing addition polymers—for example, for aqueous polymer dispersions suitable as adhesives.
One way to obtain acrylic acid is by gas-phase oxidation of propylene and/or acrolein with oxygen, or oxygen-containing gases, in the presence of catalysts at elevated temperature, preferably with dilution of the reactants with inert gases and/or steam owing to the high heat of reaction.
Catalysts employed in this reaction are generally multicomponent systems of oxide type, based, for example, on oxides of molybdenum, of chromium, of vanadium or of tellurium.
However, this process produces not pure acrylic acid but rather a gas mixture which in addition to acrylic acid comprises secondary components—essentially unreacted acrolein and/or propylene, steam, oxides of carbon, nitrogen, oxygen, acetic acid, formaldehyde, benzaldehyde, furfurals, and maleic anhydride—from which the acrylic acid must subsequently be separated off.
Methacrylic acid can be prepared analogously starting from the corresponding C
4
compounds.
(Meth)acrylic contaminated in this way (crude (meth)acrylic acid) cannot be used further directly for, say, the preparation of high molecular mass polymers but instead must be subjected to a laborious purification procedure. In general, this requires a plurality of distillation steps in order to attain the necessary purity of at least approximately 99.7% (pure (meth)acrylic acid) (see Kirk-Othmer, Encycl. of Techn. Chem., 4th Ed., pp. 299-300).
Aldehydes, which have an adverse effect on the polymerisation behavior of the (meth)acrylic acid, are removed generally by treatment with primary amines, hydrazines or aminoguanidine and subsequent distillation. In this case, said compounds which are able to remove aldehydes are added prior to the distillation.
Owing to the small differences in boiling point and the generally high tendency of (meth)acrylic acid to undergo polymerization on thermal exposure, the distillative separation of the acetic acid from the (meth)acrylic acid constitutes an extremely difficult purification step, which entails a high level of technical effort, and losses in yield.
According to DE-A 19 50 750 (see col. 2, lines 31-53), a distillation column having at least 55 trays and a reflux ratio of 15 is required to separate acrylic acid from acetic acid to some extent.
In order to solve this problem, a variety of proposals have been made to date:
The separation of acrylic acid from, for example, the reaction mixture from the oxidation of propylene, which consists essentially of acrylic acid, acetic acid, aldehydes and water, by means of an azeotropic distillation, in which the acetic acid and the water are distilled off with an entrainer; entrainers employed are, for example, esters, alcohols, ketones, aromatics, alkanes, or mixtures thereof (see DE-A 19 50 750, GB-B 1 120 284 and EP-A 0 551 111).
JA 71-06 886 proposes solving this problem by an extractive distillation with formamides and/or acetamides.
The processes described above are technically complex. Furthermore, according to these documents, the low-boiling fraction obtained in the above processes, which consists predominantly of acetic acid, acrylic acid, aldehydes, water and possibly solvents, is generally discarded.
EP-A 0 727 408 proposes the use of the low-boiling fraction obtained in the distillative recovery of pure acrylic acid for preparing alkyl acetates. A prerequisite for this is extremely precise separation of acetic from acrylic acid, which entails great problems and a high level of technical effort, and requires laborious purification of the ester. The process described therein is, therefore, comparatively uneconomic.
DE-A 21 64 767 proposes a distillative separation in which an acrylic acid content from 10 to 70% is established in the low-boiling fraction. In this way, it is said, the customary polymerization problems are largely avoided. The mixture of acetic and acrylic acid is passed to the workup stage of the aqueous crude acrylic acid. This process can only be carried out when an independent crude acrylic acid preparation stage, or an apparatus for working up aqueous acrylic acid solutions, is present, which is only seldom the case.
The preparation of (meth)acrylates by acid-catalyzed esterification of (meth)acrylic acid with one or more alkanols is also known from the prior art. Of such esterification reactions it is known, very generally, that they are equilibrium reactions and, consequently, that the presence of water in the reaction equilibrium prevents economic conversion rates. Accordingly, the (meth)acrylic acid employed is generally substantially free from water, and the water of reaction produced in the esterification is removed by distillation, with or without the aid of an entrainer.
As already mentioned at the outset, the preparation of (meth)acrylic acid by oxidation starting from the corresponding C
3
/C
4
precursors also produces considerable amounts of acetic acid (from about 0.5 to 10% by weight). Owing to the small differences in boiling point and the high tendency of the (meth)acrylic acid to undergo polymerization on thermal exposure, distillative separation of the abovementioned byproducts is difficult and expensive (U.S. Pat. No. 3,844,903, DE-A 2 164 767).
When (meth)acrylic acid containing acetic acid is esterified with alkanols the acetic acid too is esterified, and the formation of acetic esters entails additional separation effort and a loss of alkanol. Another point to note here is that the distillative separation of the acetic ester from the esterification mixture, especially its separation from unreacted alkanol, is hindered by the formation of binary azeotropes. In the case of butanol, for example, the butanol/butyl acetate azeotrope boils at 115.8° C. (57% butanol), with butanol boiling at 117.4° C. and butyl acetate at 125.6° C.
Since the acetic esters are of relatively high volatility and are not polymerizable, there is a general need when preparing polymers for (meth)acrylic esters of high purity, i.e., being free as far as possible—that is substantially free—from acetic esters. The acetic ester remaining, say, in a coating dispersion or in an adhesive would in fact be the origin, inter alia, of a severe odor nuisance. Laborious removal (deodorization) of the acetic ester would be required.
As can be perceived from the above text, a fundamental problem in (meth)acrylate
Aichinger Heinrich
Martan Hans
Nestler Gerhard
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shah Mukund J.
Tucker Zachary
LandOfFree
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