EP0941213A1

EP0941213A1 - Method for producing (meth)acrylic acid esters

Info

Publication number: EP0941213A1
Application number: EP97951937A
Authority: EP
Inventors: Heinrich Aichinger; Michael Fried; Gerhard Nestler
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1996-11-25
Filing date: 1997-11-21
Publication date: 1999-09-15
Also published as: WO1998023578A1; CZ173099A3; JP2001504506A; DE19648743A1; US6187947B1; AU5554298A

Abstract

The invention relates to a method for producing (meth)acrylic acid esters by making (meth)acrylic acid react with C4-C6 alkanols in the presence of a strong inorganic acid or of C4-C6 monoalkyl ester of a polyvalent strong acid as catalyzer. Said method is characterized in that the non-reacted alkanol is removed through distillation from the reaction mixture obtained and subsequently the catalyzer is eliminated from the mixture obtained.

Description

Process for the preparation of (meth) acrylic acid esters

description

The invention relates to a process for the preparation of esters of acrylic acid or methacrylic acid [(meth) acrylic acid].

(Meth) acrylic acid esters are usually produced on an industrial scale by esterification of (meth) acrylic acid with alkanols in the presence of strong acids as esterification catalysts (e.g. sulfuric acid, phosphoric acid, alkanesulfonic acids, arylsulfonic acids or cation exchangers with sulfonic acid groups). Such methods are e.g. from Kirk Othmer, "Encyclopedia of Chemical Technology", Vol. 1, pp. 347-348.

In addition to the desired ester and optionally an entrainer, the esterification mixture contains the unreacted starting materials (alkanol, (meth) acrylic acid) and the catalyst as well as the corresponding alkyl acetate and dialkyl ether as low-boiling by-products. The acetic acid alkyl ester is formed from acetic acid, which is always formed as a by-product in the synthesis of acrylic acid by propylene oxidation or from acetylene according to Reppe and can only be separated from the acrylic acid with great difficulty, e.g. is known from EP-A 551 111 and GB-B-1 120 284. As is well known, ethers are formed from alkanols in the presence of strong acids.

The esterification mixture is usually worked up in several stages:

1. Extraction of the catalyst with water (known from EP-A-618 187) and / or neutralization with an aqueous alkali (known from EP-A-566 047 and EP-A-609 127).

2. Rinsing with water and removal of alkanol (known from EP-A-566 047, column 1, lines 19-21)

3. Distillative separation of the low boilers (e.g. acetate, ether, unreacted alkanol) and any entrainer used (known from US Pat. No. 2,917,538, column 1,

Z. 61-68).

4.Distillative separation of the (meth) acrylate from the high boilers (e.g. oligomers, polymers, Michael addition products,

Polymerization inhibitors). In particular, the methods listed under 1 and 2, however, have the following disadvantages:

The acids used as catalysts and their esters which may have formed must be removed from the reaction mixture before further processing. As a rule, this is achieved by washing out and neutralizing the reaction mixture with alkali and alkaline earth metal solution or carbonate solutions. This results in waste water, the elimination of which is complex and polluting. If sulfuric acid is used as the catalyst, the monoester of sulfuric acid with the alkanol in question is predominantly formed, as mentioned. The salts of the sulfuric acid monoesters, in particular the esters with higher alkanols, are surface-active and, when disposed of, would significantly impair the quality of the waste water from the process and cause a not inconsiderable loss of valuable product.

Furthermore, in the case of direct neutralization, a sludge layer occurs which severely impairs the separation between the ester phase and the water phase (EP-A-566 047, column 1, lines 49ff). To solve this problem, neutralization with bases of different strengths is proposed, but this is very complex (ibidem).

For ecological and economic reasons it is therefore desirable to recover the catalyst. The prior art already describes corresponding methods. For example, CZ-B-179 808 discloses a process for recovering mineral acids from esterification mixtures by extracting the esterification mixture with water, concentrating the aqueous phase by distillation and recycling the concentrated aqueous catalyst solution thus obtained into the esterification reaction. However, this process is energy-intensive.

EP-A-0 618 187 (US US-A-5, 386, 052) describes a process for the preparation of (meth) acrylic esters, the catalyst being extracted with water and the extract being returned to the esterification reaction, if appropriate after concentration by distillation . However, it is particularly pointed out that sulfuric acid because of the poor extractability of the

Sulfuric acid monoalkyl ester is unsuitable as a catalyst, because the large amount of water which would be necessary for the adequate extraction of the sulfuric acid monoalkyl ester would adversely affect the esterification reaction. Alkyl or arylsulfonic acids are therefore used as the catalyst (column 2, lines 55ff), which are, however, considerably more expensive than sulfuric acid. After the catalyst has been separated off or the acids have been neutralized, the esterification mixture is saturated with water (1 to 2% by weight, based on the esterification mixture). Before the extraction or neutralization, the esterification mixture contains only about 0.1% by weight of water, based on the reaction mixture. However, the isolation of the alkanol, which is desirable for economic reasons, for recycling into the esterification process is not possible due to the formation of binary and ternary azeotropes by a simple distillative removal of the alkanol from this water-saturated mixture. Such azeotropes are summarized in Table 1 below using the example of butanol.

Table 1

The direct recycling of the butanol fraction would therefore always also mean the recycling of water, which would have an adverse effect on the esterification equilibrium. In addition, the alkyl acetate or dialkyl ether formed would also be recycled, which would lead to an undesired continuous accumulation of this ester or ether in the esterification mixture.

A complex separation by distillation of water, alkanol, acetic acid ester, ether and target ester is therefore necessary to prevent loss of valuable products.

In addition, due to the partially relatively good water solubility of the alkanols (for example butanol approx. 7% at 20 ° C.), considerable amounts of the unreacted alkanols are always extracted with water in the course of neutralizing the residual acids or after-washing. An additional work-up of the water phase (eg by stripping) for the recovery of the alkanols is necessary. The invention is therefore based on the object of developing a technically simple and economical process for the preparation of (meth) acrylic esters which allows the unreacted alkanol to be obtained from the reaction mixture obtained during the esterification by simple distillative isolation. In addition, it should be possible to recycle the alkanol fraction directly into the esterification.

Surprisingly, it has been found that the alkanol can be distilled off from the reaction mixture without acid-catalyzed side reactions such as, for example, the presence of the strongly acidic esterification catalyst during the distillation. Ether or olefin formation or addition of the alkanol to the double bond of the (meth) acrylate (Michael addition) could be observed.

The invention therefore relates to a process for the preparation of (meth) acrylic acid esters by reacting (meth) acrylic acid with C ₄ -C ₆ alkanols in the presence of a strong inorganic acid or a monoester of a multivalent strong inorganic acid as a catalyst, which is characterized in that that the unreacted alkanol is removed by distillation from the reaction mixture, and then the catalyst is removed from the mixture obtained.

The esterification is carried out essentially in a customary manner, ie by reacting (meth) acrylic acid with a C ₆ -C ₆ -alkanol in the presence of a catalyst and at elevated temperature. The molar ratio of alkanol: acrylic acid or methacrylic acid is generally 1: 0.7-1.2. C ₆ -C ₆ -alkanols are, for example, pentanol, hexanol and preferably butanols, in particular n-butanol. The catalyst concentration in the reaction mixture is generally 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the total batch. Sulfuric acid or the corresponding sulfuric acid mono-C ₄ -C ₆ alkyl ester is preferably used as the catalyst.

As polymerization inhibitors, for example phenothiazine, hydroquinone, hydroquinone monomethyl ether or mixtures thereof and optionally air (0.1 to 10 1 / hx 1) in an amount of 100 to 5000 ppm, based on the reaction mixture, are used. Saturated hydrocarbons (for example cyclohexane) or aromatics (for example toluene) can be used in the process according to the invention as entraining agents for removing water from the reaction mixture; however, the reaction is preferably carried out without an additional entrainer. The reaction temperature is generally about 70 to 160 ° C, preferably 90 to 130 ° C.

The reaction time is generally about 1 to 10 hours, preferably 1 to 6 hours.

The reaction can be carried out under normal, negative or positive pressure. The pressure is preferably set so that the water formed distills off during the esterification, for example in the form of a mixture of water, C ₆ -C ₆ -alkanol and ester (the organic components are fed back to the esterification). The esterification can be carried out continuously or batchwise, with the continuous reaction being preferred.

The esterification is carried out in conventional equipment, e.g. in an esterification unit from one or more heatable stirred reactors (cascade), which are optionally equipped with columns, condensers and phase separation vessels. The contents of the reactor are mixed by stirring or other customary and suitable measures.

The reaction mixture obtained after the esterification is largely freed of any alkanol still present in a conventional distillation apparatus (for example a column with sieve trays, Raschig rings, aligned packings, etc.). The acetic acid CC ₆ -alkyl ester and the di-C ₆ -C ₆ -alkyl ether formed during the esterification are also partially distilled off.

The distillation conditions depend on the type of alkanol used and are preferably chosen so that a maximum of 50% ether or acetate can be found in the distillate. Typical distillation conditions are e.g. in the case of n-butanol, a bottom temperature of 80 to 150 ° C, a pressure of 200 to 1000 mbar and a reflux ratio of 2 to 8.

The distillate (5 to 15% of the feed) generally contains 60 to 80% n-butanol, 2 to 4% n-butyl acetate, 1 to 3% di-n-butyl ether and 2 to 4% water.

Only 10 to 50% of the acetate and ether formed during the esterification can be found in the distillate, which can thus be fed back into the esterification without problems.

The alkanol is distilled off preferably up to a content of residual alkanol in the reaction mixture, which causes the extraction of the catalyst (sulfuric acid or the corresponding monoalkyl esters) with water. In particular, the residual alkanol content is <5% by weight, preferably <3% by weight, particularly preferably 1% by weight, based on the reaction mixture.

In a particularly preferred embodiment, the recovered alkanol is returned directly to the esterification.

The catalyst is preferably removed from the reaction mixture by extraction with water.

When using sulfuric acid as a catalyst, it has surprisingly been found that the alkanol content has a great influence on the extractability of the monoalkyl sulfuric acid formed from sulfuric acid and alkanol, which also acts as an esterification catalyst (see Table 1). After distilling off the alkanol according to the invention, the catalyst can be extracted with small amounts of water so that the extract can be returned to the esterification. The conditions for the extraction of the catalyst from the esterification mixture are preferably such that the catalyst concentration in the aqueous phase is at least 20% by weight, in particular at least 25% by weight, based on the aqueous extract, and the degree of extraction is at least 70 %, in particular at least 80%.

To achieve this, use about 5 to for extraction

20% by weight of water, in particular about 6 to 15% by weight of water, based on the total weight of the mixture to be extracted. The extract obtained can be returned directly to the esterification without concentrating it.

The extraction can be carried out in a manner known per se. Preferably extracted in countercurrent, e.g. in columns without energy input, pulsed columns, columns with rotating inserts, mixer-settler equipment or in static mixers.

The extraction can be carried out at ambient temperature or higher, but expediently at a temperature in the range from about 15 to 40 ° C.

If an extraction / neutralization of the residual acids (catalyst and (meth) acrylic acid) with the aid of an aqueous base is then necessary, this can be done in a conventional extraction apparatus (see above), the base requirement being low due to the high degree of extraction of the catalyst and the Extraction proceeds without the phase separation problems described in EP-A-0 566 074.

The ester is isolated from the reaction mixture freed from catalyst and, if appropriate, residual carboxylic acid and low boilers, in a customary manner, in particular by distillation, e.g. by distillation in a sieve tray column.

The following example explains the invention, but without restricting it. The percentages are percentages by weight. "Butanol" and "butyl" mean n-butanol and n-butyl.

A cascade of stirred tanks consisting of 3 stirred reactors with 1 1 reaction volume each, which are equipped with a column, condenser and phase separation vessel, is mixed with 500 g of acrylic acid, 547 g of n-butanol,

Feed 12.3 g of sulfuric acid and 1 g of phenothiazine per hour.

The reaction temperature in the reactors is 105 ° C, 118 ° C and

121 ° C, the pressure 700 mbar. At the top of the column, a mixture of water, n-butanol and n-butyl acrylate is obtained, which breaks down into an aqueous and an organic phase, the organic phase being fed to the column as reflux.

According to the gas chromatographic analysis, the reactor discharge contains:

87.6% butyl acrylate 4.5% butanol 0.3% butyl acetate 0.5% dibutyl ether The water content is 0.1%.

This esterification mixture is continuously distilled in a sieve tray column with 55 trays at a bottom temperature of 121 ° C (440 mbar) or a top temperature of 85 ° C (300 mbar), with the addition to the middle tray, the reflux ratio 3, 5 and the distillate decrease accounts for 6% of the feed.

The distillate contains according to the gas chromatographic analysis

64.7% butanol, 28.1% butyl acrylate, 1.9% butyl acetate, 1.9% dibutyl ether and is returned directly to the esterification. The recycle rate is about 40% for the acetate and about 25% for the ether, and the recovery rate for butanol is about 85%.

The distillation bottoms, which, according to the analysis, contain butyl acrylate and 0.7% butanol,

Contains 0.2% butyl acetate, 0.4% dibutyl ether, 2.11% sulfuric acid monobutyl ester, is cooled to 30 ° C. and treated with 6% water in a mixer-settler apparatus. The water phase obtained contains 24.6% monobutyl sulfuric acid and is returned directly to the esterification. The degree of extraction is 87%.

The organic phase, which still contains 0.8% acrylic acid and 0.25% butyl sulfurate, is treated in a mixer-settler apparatus with a 6% aqueous sodium hydroxide solution (5% with respect to the organic phase).

The resulting aqueous phase contains, in addition to the corresponding sodium salts, only 0.3% butanol and is disposed of.

The low boilers (mainly acetate and ether) are separated from the largely acid-free organic phase (acrylic acid content <100 ppm) obtained in a sieve plate column (60 plates). The bottom temperature is 110 ° C., the top temperature 88 ° C. at 160 mbar and the reflux ratio 7, 10% of the feed being separated off as distillate.

The butyl acrylate is obtained by distillation in a further sieve plate column (30 plates) (bottom temperature 108 ° C., top temperature 80 ° C. at 100 mbar, reflux ratio 0.6).

The dependence of the degree of catalyst extraction on the butanol content of the reaction mixture fed to the extraction was determined by extracting esterification mixtures, which had different butanol contents, by esterification of acrylic acid with butanol in the presence of sulfuric acid, in a separatory funnel, once with 10% water at 25 ° C. The results of these tests are shown in Table 2.

Table 2

It can be seen that the degree of extraction increases with decreasing butanol content.

Claims

claims

1. Process for the preparation of (meth) acrylic acid esters

5 implementation of (meth) acrylic acid with C -C ₆ alkanols in the presence of a strong inorganic acid or a C ₄ -C ₆ monoalkyl ester of a polyvalent strong inorganic acid as a catalyst, characterized in that the unreacted alkanol is distilled out removed the reaction mixture obtained, and then removed the catalyst from the mixture obtained.

2. The method according to claim 1, characterized in that the distillative removal of the unreacted alkanol 5 to a content of residual alkanol in the reaction mixture of less than 5 wt .-%, in particular less than 3 wt .-%, based on the reaction mixture , performs.

3. The method according to claim 1 or 2, characterized in that 0 the alkanol fraction removed from the reaction mixture is returned to the esterification.

4. The method according to any one of the preceding claims, characterized in that n-butanol is used as the alkanol. 5

5. The method according to any one of the preceding claims, characterized in that sulfuric acid or a sulfuric acid monoalkyl ester is used as the catalyst.

6. The method according to any one of the preceding claims, characterized in that the catalyst is regenerated from the reaction mixture by extraction with water.

7. The method according to claim 6, characterized in that 35 the aqueous solution of the by extraction with water from the

Reaction mixture regenerated catalyst returns to the esterification.

8. The method according to claim 6 or 7, characterized in that 40 one carries out the extraction of the catalyst so that the

At least catalyst concentration in the water phase

20% by weight, in particular at least 25% by weight, based on the aqueous extract.

45 170/96 O 98/23578 H PCT / EP97 / 06512 ^" = -

9. The method according to any one of claims 6 to 8, characterized in that the extraction of the catalyst is carried out so that the degree of extraction is at least 80 wt .-%, based on the amount of catalyst in the reaction mixture.

10. The method according to any one of claims 6 to 9, characterized in that one carries out the extraction at 15 to 40 ° C.

11. The method according to any one of claims 6 to 10, characterized in that the extraction is carried out in countercurrent or in a static mixer.

12. The method according to any one of the preceding claims, characterized in that the reaction temperature is 70 to 160 ° C, in particular 90 to 130 ° C.

13. The method according to any one of the preceding claims, characterized in that the reaction time is 1 to 10, in particular 1 to 6 hours.

14. The method according to any one of the preceding claims, characterized in that it is carried out continuously.