Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-09-22
2003-04-29
Shippen, Michael L. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C560S208000, C562S546000, C562S547000, C562S600000
Reexamination Certificate
active
06555707
ABSTRACT:
The present invention relates to a process for preparing acrylic acid that involves using an inert high-boiling solvent to cool a gaseous reaction mixture which comprises acrylic acid and is obtained in the gas-phase oxidation to prepare acrylic acid and that produces a gaseous mixture comprising acrylic acid. Moreover it relates very generally to the use of an inert high-boiling solvent to cool a gaseous reaction mixture produced in the gas-phase oxidation to prepare acrylic acid, and to a process for preparing acrylates.
Because of its highly reactive double bond and the acid function, acrylic acid is a valuable monomer for preparing addition polymers, for example, for aqueous polymer dispersions suitable as adhesives.
One route to acrylic acid is the gas-phase oxidation of propylene and/or acrolein with oxygen or gases comprising oxygen 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 oxidation are generally multicomponent oxide-type systems based, for example, on oxides of molybdenum, of chromium, of vanadium or of tellurium.
However, this process leads not to pure acrylic acid but rather to a gas mixture which in addition to acrylic acid comprises secondary components—mainly unreacted acrolein and/or propylene, steam, oxides of carbon, nitrogen, oxygen, acetic acid, formaldehyde, benzaldehyde, furfurals, and maleic anhydride. The acrylic acid has to be separated from this gas mixture subsequently.
Isolation of the acrylic acid from the gaseous reaction mixture is generally carried out by countercurrent absorption using, for example, a high-boiling solvent or solvent mixture and a plurality of subsequent distillative processing steps, as is described, for example, in DE-A 21 36 396 and DE-A 43 08 087. In EP-B 0 009 545, U.S. Pat. No. 5,154,800, DE-A 34 29 391 and DE-A 21 21 123 absorption takes place first with water/aqueous acrylic acid in countercurrent and is followed by extractive or azeotropic distillation.
A disadvantage with these processes is that the organic solvent/solvent mixture used for the absorption and, where appropriate, extraction must be separated off again in a separate distillation step and possibly purified before being used again.
A further disadvantage of these processes is that the acetic acid produced in the acrylic acid preparation (purity: 0.5 to 10% by weight relative to the amount of acrylic acid) has to be separated off in a complex distillation stage. Because of the small differences in boiling point and the great tendency of acrylic acid to polymerize, this stage generally involves a number of distillation steps. as can be seen, inter alia, from U.S. Pat. No. 3,844,903, and is the cause of considerable losses of acrylic acid (cf. EP-A 398 226).
In view of the known fact that acrylic compounds have a high tendency to undergo addition polymerization, processes operating with multistage distillative workup are disadvantageous in very general terms since they heighten the polymerization tendency of the acrylic acid.
Also known from the prior art is the preparation of acrylates by acid-catalyzed esterification of acrylic acid with one or more alkanols. In very general terms it is known of such esterification reactions that they are equilibrium reactions and thus that the presence of water in the reaction equilibrium prevents economic conversion rates. Accordingly, the acrylic acid employed in general is substantially anhydrous and the water of reaction formed during the esterification is removed by distillation with the aid, if desired, of an entrainer.
As already mentioned earlier, the oxidative preparation of acrylic acid starting from the corresponding C
3
precursors also produces considerable amounts of acetic acid (0.5 to 10% by weight). Because of the in some instances small differences in boiling point and high polymerization tendency of acrylic acid when exposed to heat the distillative separation of the abovementioned byproducts is difficult and laborious (U.S. Pat. No. 3,844,903, DE-A 2 164 767).
In the esterification of acrylic acid, containing acetic acid, with alkanols the acetic acid too is esterified; the formation of the acetic ester entails additional separation effort and a loss of alkanol. A further point to note is that the distillative separation of the alkyl acetate from the esterification mixture, and especially the separation of unreacted alkanol, is hindered by the formation of binary azeotropes.
In the case of, say, butanol, the butanol/butyl acetate azeotrope boils at 115.8° C. (57% butanol); butanol boils at 117.4° C. and butyl acetate at 125.6° C.
Since the acetic esters are quite highly volatile and are not polymerizable, high-purity acrylates are generally required for the preparation of polymers; that is, acrylates which are as far as possible free—i.e., essentially free—from acetic esters. Indeed, one of the consequences of the residual acetic ester in a coating dispersion or in an adhesive, for example, would be severe odor nuisance. Laborious removal (deodorization) of the acetic ester would be necessary.
As emerges from the above there is a fundamental problem in acrylate preparation of excessive consumption of alkanols, which is disadvantageous from both an economic and an environmental standpoint.
Various attempts have therefore been made in the past to solve the problems caused in esterification with alkanols by the byproducts, such as acetic acid and water, that are produced during the synthesis of acrylic acid by gas-phase oxidation.
For instance, DE-A 20 35 228 describes the esterification of aqueous acrylic acid (water content at least 30%) in the presence of acidic catalysts, such as sulfuric, sulfonic and/or phosphoric acid, and an organic solvent mixture.
This process is particularly disadvantageous in that, owing to the large amount of water and the consequent reduction in catalyst concentration, a large amount of catalyst has to be used. In order to obtain useful conversion rates and rates of esterification, moreover, said document requires the use of specific solvent mixtures consisting of an aromatic and an aliphatic hydrocarbon. A further condition for successful implementation of this process is that the solvent mixture must also have a significantly higher boiling point than the acrylate.
EP-A 0 398 226 proposes partial condensation of the reaction gases from the propylene oxidation and direct esterification of the resultant “enriched” acrylic acid, with the acrylic acid remaining in the reaction gas being conventionally extracted by scrubbing with water and separated off by distillation. This process of two-stage acrylic acid condensation is highly laborious and does not produce acrylic acid which is free from, or even low in, acetic acid. As shown by the examples reproduced therein, the esterification mixtures that are obtained still contain from 1.8 to 2.5% by weight of acetic ester.
In DE-A 16 68 362 the acrylic acid-containing reaction gas from the oxidation of propylene is treated with the mixture of high boilers obtained in the course of the esterification, consisting essentially of maleic esters, polyacrylic acid and polyacrylic esters, and the acrylic acid solution obtained in this treatment is freed-from the low boilers by distillation. The solution thus obtained at the bottom of the distillation column, which includes acrylic acid, is esterified with alkanols in the presence of acidic cation exchangers.
A disadvantage of this process is that the workup of acrylic acid is coupled to the ester preparation, and the option of preparing different esters is lost. Furthermore, the formation of high-boiling esters of polyacrylic acid, the excess of which has to be removed, entails considerable losses of alkanol.
JA 7 014 529-R describes the preparation of butyl acrylate from aqueous acrylic acid. According to the process described in said document acrylic acid is extracted from the aqueous solution with a butyl acrylate/butanol mixture and then the ext
Bolz Gerhard
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Shippen Michael L.
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