DE1125912B - Process for the preparation of acrylic acid esters and ª ‡ -alkylacrylic acid esters by splitting off alcohol from ª ‰ -alkoxymonocarboxylic acid esters - Google Patents

Description

Process for the preparation of acrylic acid and α-alkyl acrylic acid esters by splitting off alcohol from ß-Alkoxymonocarbonsäureestern It is known that one Acrylic acid esters are obtained by converting ß-alkoxypropionic acid esters in liquid or gaseous form Condition treated with dehydrating agents such as sulfuric acid (cf. the German Patent 573,724). Volatile, acidic, organic sulfur compounds are formed, which increase the shelf life of the formed Reduce acrylic acid esters In addition, the yields fluctuate due to polymerization losses and are 68.5 to 80% in the case of methyl acrylate.

In U.S. Patent 2,393,737, the representation of a, B-unsaturated Esters by splitting off alcohol from ß-alkoxy-substituted propionic acid or. Isobutyric acid esters in the liquid state in the presence of alkaline catalysts, namely alkali metal alcoholates described. To increase the reaction rate and achieve better yields it has been proposed that the sodium methylate used as a catalyst in methanol to dissolve and in the heated solution the ß-methoxyisobutyrate in the To be added dropwise as the methyl methacrylate formed is distilled off. on in this way, yields of up to 92.50 / 0, based on converted P-methoxyisobutter acid methyl ester. The separation of the large excess of methanol from the methyl methacrylate however, extractive or azeotropic distillation is associated with great costs.

The main disadvantage of this process is that the catalyst after a short time, our own tests showed 12 hours, encrusted. In contrast, show some of the catalysts of the invention after 50 hours, others after 100 hours Their catalytic effect has not yet diminished for hours. Also have trials show that without adding alcohol to the reaction mixture of ß-alkoxycarboxylic acid ester and alkali metal alcoholate, no significant cleavage of the B-alkoxycarboxylic acid esters entry.

Another shortcoming of this process is that its catalyst cannot be revived or is very difficult to revive. In contrast, the Catalysts according to the process of the invention by burning off with oxygen-containing Gases can easily be revived at elevated temperatures without the Catalyst must be removed from the reaction vessel.

In order to increase the conversion rate, the alkaline one was also used Cleavage of IB-alkoxy-substituted esters in the gas state to form α-unsaturated esters carried out according to the method of USA patent specification 2457225, with alkali and Alkaline earth compounds are used as catalysts.

The cleavage products are essentially free of acid, but occurs a considerable thermal degradation of the formed ester, which results in a strong yellow-brown coloration of the cleavage product and the immediate deposition of coal on the catalyst and in one with it associated reduction in effect expresses.

According to the method of British patent specification 584 607 are used for Cleavage of a-alkoxyisobutyric acid esters dehydrating agents, e.g. B. phosphorus pentoxide, active aluminum oxide or anhydrous oxalic acid is used. When using the split off methanol is almost completely destroyed by phosphorus pentoxide. aside from that phosphorus pentoxide is converted into phosphoric acid, which is highly corrosive the vessel walls acts. Working with phosphorus pentoxide is also dangerous.

It should also not be overlooked that in this process large amounts of catalyst must be used.

The method of U.S. Patent 2571212 is to manufacture limited by unsaturated monocarboxylic acid ester, which is an alkoxy group in the ß-position contain.

The ß, ß-dialkoxy-substituted carboxylic acid esters contain an acetal-like one Atomic group as opposed to the ethereal group of the method of the invention. In addition, unlike ether, acetal-like atomic groups are known to be light fissile.

According to the method of German patent specification 729342, although acidic catalysts used.

As a result, the best yield is only 84.8% of methyl acrylate obtain.

The situation is similar in the process of the German patent specification 847443, in which the yields of unsaturated esters between 6Q6 and 670 / o be.

According to the method of German patent specification 885542 is only Acrylic acid methyl ester prepared while according to the method of the invention both the homologous esters of acrylic acid as well as those of the a-alkyl-substituted acrylic acids can be won.

It has now been found that acrylic acid and α-alkyl acrylic acid ester are obtained which do not have an alkoxy group in the-position, by elimination of alcohol from ß-alkoxymonocarboxylic acid esters in the presence of basic catalysts, which contain alkali or alkaline earth compounds.

The process consists in that one of gaseous fl-Alkoxymonocarbonsäureestem at a temperature of 330 to 4000 C, preferably 370 to 3900 C, splits off alcohol in the presence of catalysts consisting of silica and 5 to 30%, preferably 5 to 15 0 / G, alkali and / or alkaline earth silicates, based on the total weight of the catalyst.

So there are catalysts used that hardly any side reactions cause. In addition, higher sales or higher yields of unsaturated Monocarboxylic acid esters achieved. The alcohol is split off from the vaporous Starting materials in a continuous or intermittent process. They evaporated Starting materials are expediently in an inert atmosphere at normal pressure or reduced pressure, z. B. in the presence of nitrogen, through a filled with the catalyst, heated Pipe led. The heating of the tube can be electrical or by a heated gas done, which is guided by a fan around the pipe, creating local Overheating can be avoided.

After splitting and liquefying, the vapors are in a template cooled to about OC collected. The resulting unsaturated monocarboxylic acid ester is of the unconverted starting ester and the by-products by simple or separated azeotropic distillation. The reaction mixture can, however, also by extractive Distillation, optionally with the addition of polymerization retarders, e.g. B. Nitrogen oxides, are broken down into its components in a known manner.

The splitting off of the alcohol from the ether esters is in the process the process on the reaction temperature and the amount of catalyst present addicted. The load on the catalyst per unit of time can be within wide limits fluctuate and be adapted to the respective conditions.

As alkali or alkaline earth silicates are sodium, magnesium and Calcium silicate suitable. However, other alkali or alkaline earth silicates can also be used with good success be used, e.g. B. lithium, potassium, strontium and barium silicate.

Example 1 methyl p-methoxypropionate is used in vapor form a rate of 0.37 g, B-meth- methyl oxypropionate per cubic centimeter Catalysts passed per hour over a catalyst which consists of 95.10 / o silica and 4.90 / 0 sodium silicate. - The reaction temperature is 3800 C. The Conversion is 89.4%, based on the methyl p-methoxypropionate used, the yield of methyl acrylate 90.10 / 0. In addition, 968 / o methanol are obtained, based on converted P-methoxypropionic acid methyl ester.

The stated conversions and yields are after a period of use of the catalyst unchanged from about 105 hours.

The resulting reaction mixture is worked up in a known manner Way. By fractional distillation at a vacuum of 270mm mercury an azeotropic mixture of 54 ° l methanol and 46% methyl acrylate is obtained first, which all of the methanol split off from the methyl methoxypropionate and contains part of the resulting methyl acrylate. The further separation of methanol-free methyl acrylate and methyl p-methoxypropionate Ordinary distillation is easily possible, since both are not azeotropic with one another Form mixture.

The azeotropic mixture obtained of 46% methyl acrylate and 54 O / o methanol is separated as follows: A 3 m long distillation column Made of V2A steel with an inner diameter of 6 cm with jacket heating and wire mesh filling is in the lower third of the above azeotropic mixture in vapor form with a Feed rate of 200 parts per hour. The jacket heating is set to about 440 C set and the distillation pot heated to about 65 ° C. In the upper part 1200 parts of water heated to about 440 ° C. are fed to the column every hour. The pressure at the top of the column is 250 mm of mercury. Goes at the head of the column at 440 ° C. an azeotropic mixture of 92.8% methyl acrylate and 7.2% Water, which breaks down into two layers when it cools. The lower, watery one Layer is returned to the top of the column. The top layer is peeled off and consists of acrylic acid methyl ester with less than 0.5 D / o methanol.

In a second column, in which the bottom product is continuously the first column is passed, the methanol is distilled off at an elevated temperature.

The catalyst is revitalized by oxidation of the carbon-containing ones Deposits by using a preheated oxygen-containing gas, which is made up of air and an inert gas such as nitrogen at a temperature of 5200 ° C brings into contact with the catalyst. The catalyst revived in this way shows no differences in its effectiveness compared to a freshly prepared one. After being resuscitated five times, the results obtained are unchanged again obtain.

Example 2, n-butyl B-n-butoxyisobutyrate is used at a Temperature of 3800 C in vapor form passed over a catalyst, which from 90% silica and 10% calcium silicate. The speed is 0.7 g of jB-n-butoxyisobutyric acid butyl ester per cubic centimeter of catalyst per hour.

The conversion is 81.0%, based on the n-butyl p-n-butoxyisobutyrate used, with a yield of n-butyl methacrylate of 89%. In addition, 96 O / o n-butanol, based on the converted n-butoxyisobutyric acid n-butyl ester, obtain.

Example 3, 8 - (2 - ethyl) - butoxypropionic acid - (2 - ethyl) butyl ester is in vapor form at a rate of 0.5 g ß- (2-ethyl) -butoxypropionic acid (2-ethyl) butyl ester per cubic centimeter of catalyst per hour passed over a catalyst, which consists of 95 01o silicic acid and 5 O / o lithium silicate. The reaction temperature is 380C C. With a conversion of 84 O / o, based on the fl- (2-ethyl) -butoxypropionic acid (2-ethyl) -butyl ester used, acrylic acid (2-ethyl) butyl ester is obtained in a yield of 90.0% and 2% Ethylbutanol in a yield of 96%, based on the converted starting ester.

Example 4 ß-Methoxyisobutyric acid n-hexyl ester is in vapor form at a temperature of 3700 C at a rate of 0.6 g of ß-methoxyisobutyric acid n-hexyl ester per cubic centimeter of catalyst per hour passed over a catalyst, which consists of 90% silica and 100% barium silicate.

83 0 / o of the nhexyl p-methoxyisobutyrate used implemented. The yield of n-hexyl methacrylate is 90.3%, the methanol yield 95%, based in each case on the converted ß-methoxyisobutyric acid n-hexyl ester.

Example 5 a -isopropyl -, B - methoxypropionic acid methyl ester in vapor form at a temperature of 380 ° C at a rate of 1.3 g methyl α-isopropyl-β-methoxypropionate per cubic centimeter of catalyst per hour passed over a catalyst which consists of 95% silica and 5% sodium silicate consists. At a conversion of 79%, based on the methyl α-I sopropyl £ -methoxypropionate used, a.-Isopropylacrylic acid methyl ester is obtained in a yield of 93.1% and Methanol in a yield of 92.0%, based in each case on the converted starting ester.

Example 6 The catalyst is prepared as follows: To one Solution of 4446 g (15.7 mol) of sodium metasilicate, Na2Si O39 H2 0, in 7800 cm3 of water a solution of 305 g (1.5 Mol) magnesium chloride hexahydrate in 570 cm3 of water, some of which are silica hydrogel and completely precipitates magnesia hydrogel. Then add 5700 to the mixture cm3 of 5N hydrochloric acid to precipitate most of the silica hydrogel. Of the slurried Precipitate is filtered off and washed with aqueous ammonium chloride solution washed. Further washing with water will then remove all water-soluble salts removed. The filter cake is dried at 1500 C, then into pieces of suitable Size broken, sifted and calcined at a temperature of 6000 C. The finished one The catalyst consists of 85 ovo silica and 15% magnesium silicate, Mg Si 03. This catalyst is ß-n-butoxypropionic acid n-butyl ester in vapor form a temperature of 3800 C at a rate of 0.6 g of ßn-butoxypropionic acid n-butyl ester per cubic centimeter of catalyst passed per hour. The conversion is 82.0%, based on the ß-n-butoxypropionic acid n-butyl ester used, with a yield of acrylic acid n-butyl ester of 90.0 O / o. In addition, 97 ° lo butanol are related on converted ß-n-butoxypropionic acid n-butyl ester obtained.

Example 7 α-ethyl-ß-methoxypropionic acid n-butyl ester in vapor form at a temperature of 3700 C at a rate of 1.0 g a-ethyl-fl-methoxypropionic acid n-butyl ester per cubic centimeter of catalyst per hour passed over a catalyst which consists of 92 ovo silica and 8 D / o magnesium silicate consists. With a conversion of 83.1%, based on the a-ethyl-ß-methoxypropionic acid butyl ester used, n-butyl α-ethylacrylate is obtained in a yield of 96.1% and methanol in a yield of 98%, based in each case on the converted starting ester.

Claims (2)

PATENT CLAIMS 1. Process for the production of acrylic acid and α-alkyl acrylic acid esters, which do not have an alkoxy group in the ß-position, by splitting off alcohol from ß-alkoxymonocarboxylic acid esters in the presence of basic catalysts, which alkali or alkaline earth compounds contain, characterized in that from vaporous ß-alkoxymonocarboxylic acid esters at a temperature of 330 to 4000 C, preferably 370 to 3900 C, splits off alcohol, in the presence of catalysts consisting of silica and 5 to 30 O / o, preferably 5 to 15 D / o, alkali and / or alkaline earth silicates, based on the total weight of the catalyst. 2. The method according to claim 1, characterized in that the Carries out alcohol elimination in the presence of a catalyst consisting of silica and sodium silicate. Considered publications: German Patent Specifications No. 729,342, 847443, 855542; U.S. Patents Nos. 2341 663, 2393 737, 2457225, 2571212, 2816921; British Patent No. 584 607.