DE1124481B - Process for the preparation of acrylic acid and ª ‡ -alkyl acrylic acid esters by splitting off alcohol from ª ‡ or 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 a- or, B-alkoxymonocarboxylic acid esters It is known that acrylic acid esters are obtained by adding ß-alkoxypropionic acid esters in liquid or in the gaseous state with dehydrating agents such as sulfuric acid (see German patent specification 573 724). In doing so, reduction reactions volatile acidic organic sulfur compounds are formed which reduce the shelf life reduce the acrylic acid ester formed. In addition, the yields vary Polymerization losses and amount to 68.5 to 80% in the case of methyl acrylate.

In U.S. Patent 2,393,737, the representation of o;, ß-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 92.5%, based on converted ß-methoxyisobutyrate methyl ester, obtain. 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, attempts by the inventor resulted in 12 hours, encrusted. On the other hand show the catalysts of the invention partly after 50 hours, still others after 100 hours, their catalytic effect has not yet diminished. Also have Experiments show that without adding alcohol to the reaction mixture of β-alkoxycarboxylic acid ester and alkali metal alcoholate, no significant cleavage of the ß-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 ß-alkoxy-substituted esters in the gas state to form os, ß-unsaturated esters according to the method of the USA patent specification 2,457,225, 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 584607 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 Phosphorus pentoxide, the split off methanol is almost completely destroyed. 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. not The fact that large amounts of catalyst are used in this process must also be overlooked must be used.

The method of U.S. Patent 2,571,212 is upon manufacture limited by unsaturated monocarboxylic acid esters, which are 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, in contrast to ethers, acetal-like atomic groups are known to be light fissile.

According to the method of German patent specification 729 342, 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 847 443, in which the yields of unsaturated esters between 60.6 and 670 / o be.

According to the method of German patent specification 885 542 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 o; -alkyl-substituted acrylic acids can be won.

It has been found that acrylic and α-alkyl acrylic esters, which do not have an alkoxy group in the ß-position, by splitting off alcohol from a- or ß-Alkoxymonocarbonsäureestern in the presence of acidic catalysts thereby produces that from vaporous - or fl-Alkoxymonocarbonsäureestern at a Temperature from 200 to 400 "C, especially 270 to 3500 C, alcohol splits off in the presence of catalysts made from silica and from either 0.1 to 1001o of oxides the metals of the IV. or V subgroup of the periodic table or 0.4% titanium dioxide, 14.7 0 / o aluminum oxide, 2.7 0 / o ferric oxide, 0.7 0 / o calcium oxide and 15.4 0 / o chemical bound water exist.

As metal oxides of subgroups IV and V of the periodic table the oxides of titanium, zirconium, thorium and tantalum can be used.

So there are catalysts used that hardly any side reactions cause. In addition, there are higher conversions or higher yields of acrylic and o; -Alkylacrylic acid esters achieved.

The alcohol is split off from the vaporous starting materials in an ongoing or intermittent proceeding. The vaporized raw materials are at normal pressure or negative pressure, expediently in an inert atmosphere, for. B. in the presence of nitrogen, heated by a filled with the catalyst 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.

The vapors are released after the alcohol has been split off and liquefied collected in a template cooled to about 0 ° C. The resulting unsaturated Monocarboxylic acid ester is derived from the unconverted starting ester and the by-products separated by simple or azeotropic distillation. The reaction mixture can but also by extractive distillation, optionally with the addition of polymerization retarders, z. 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.

Example 1 The catalyst is prepared in a known manner Way by mixing the solutions of the chlorides of titanium or tantalum in low molecular weight Alcohols, e.g. B. in methyl or ethyl alcohol, with one to precipitate the silica sufficient amount of aqueous Acid, such as hydrochloric acid. Then an aqueous Entered alkali silicate solution in this mixture and washed with water Catalyst at 110 "C, expediently dried and in a steam-containing atmosphere then calcined at about 350 ° C.

ß-Methoxypropionsäuremethylester is at a temperature of 270 "C passed in vapor form over a catalyst which consists of 99 ° / 0 silica and 10 / o finely divided titanium dioxide. The throughput speed is 0.37 g of ß-methoxypropionic acid methyl ester per cubic centimeter of catalyst per hour. The conversion is 75.9%, based on the ß-methoxypropionic acid methyl ester used, with a yield of methyl acrylate of 95.5%.

In addition, 72.2% of the methanol split off, based on the reacted ß-Methoxypropionic acid methyl ester obtained. The proportion of acrylic acid in the total resulting reaction mixture of acrylic acid and acrylic acid methyl ester is 2.4 O / o. The crude resulting reaction mixture is completely colorless.

Silica alone does not show any at a cleavage temperature of 270.degree Splitting effect. At a temperature of 300 ° C., the reaction conditions are otherwise identical a conversion of 90.9 ° / 0 and a yield of methyl acrylate of 91.30 / ,, based on the converted ß-methoxypropionic acid methyl ester.

The stated conversions and yields are after a period of use of the catalyst unchanged for about 120 hours.

Is according to Example 2 of US Pat. No. 2,457,225 a by Soaking of silica gel with 10% aqueous borax solution prepared catalyst are used, no measurable conversions and are under the above conditions Yields achieved.

The resulting reaction mixture is worked up in a known manner Way. By fractional distillation under a vacuum of 270 mm of mercury an azeotropic mixture of 540 / o methanol and 46% methyl acrylate is obtained first, which all of the methanol split off from the ß-methoxypropionic acid methyl ester and contains part of the resulting methyl acrylate. The further separation of methanol-free acrylic acid methyl ester and ß-methoxypropionic acid methyl ester Ordinary distillation is easily possible, since both are not azeotropic with one another Form mixture.

The resulting azeotropic mixture of 46% and methyl acrylate 54% methanol is separated as follows.

A 3 m long distillation column made of V2A steel with an internal diameter of 6 cm with jacket heating and wire mesh filling, the above becomes in the lower third azeotropic mixture in vapor form at a rate of 200 parts per hour fed.

The jacket heating is set to about 44 ° C and the still heated to about 6½ "C. In the upper part of the column, 1200 parts per hour are increased Water heated to about 44 ° C is supplied. The pressure at the top of the column is 250 mm of mercury.

An azeotropic mixture of 92.8 ° C. emerges at the top of the column at 44.degree 0 / o acrylic acid methyl ester and 7.2 0 / o water over, which on cooling in separates two layers. The lower, aqueous layer is at the top of the column returned. The top layer is peeled off and consists of methyl acrylate with less than 0.5% 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 550 "C brings the catalyst into contact. The catalyst revived in this way shows no differences in its effectiveness compared to a freshly prepared one Catalyst. After resuscitation five times, the results obtained will be unchanged received again.

Example 2 The catalyst is prepared as in Example 1.

ß-Methoxyisobutyric acid methyl ester is in vapor form at a rate of 1.0 g of methyl β-methoxyisobutyrate per cubic centimeter of catalyst per hour passed over a catalyst which consists of 99.8% silica and 0.2 very finely divided Tantalum oxide. The reaction temperature is 300 ° C. The conversion is, based on on the ß-methoxyisobutyrate used, 82.0% with a yield of methacrylic acid methyl ester of 94.0%, with 74.7% being formed in methanol on converted-methoxyisobutyric acid methyl ester.

The crude, resulting reaction mixture is almost colorless. At 350 "C the conversion is 89.40 / o and the yield of methyl methacrylate is 90.70 / ,, based on converted ß-methoxyisobutyrate.

Example 3 methyl a-methoxyisobutyrate is used in vapor form a temperature of 260 "C with a rate of 0.7 g methyl methoxyisobutyrate per cubic centimeter of catalyst per hour passed over a catalyst, which consists of 990% silica and 1 0in titanium dioxide. The conversion is 81%, based on the methyl methoxyisobutyrate used. The yield at Methyl methacrylate is 950 / o 'based on the converted methyl cc-methoxyisobutyrate.

Example 4 ß-methoxypropionic acid methyl ester is in vapor form in a temperature of 300 "C at a rate of 0.37 g of ß-methoxypropionic acid methyl ester per cubic centimeter of catalyst per hour over a catalyst of the type Montmorillonite, which consists of 66.1% silica, 14.7% aluminum oxide, 2.7% ferric oxide, 0.7% calcium oxide, 0.4% titanium dioxide and 15.4% water. The conversion is 81.5%, based on the methyl β-methoxypropionate used. The yield of methyl acrylate is 90.7% and that of methanol is 72.0% on converted ß-methoxypropionic acid methyl ester. The crude ester obtained is complete colorless. The proportion of acrylic acid in the resulting amount of acrylic acid and acrylic acid methyl ester is 4.120 / o.

Claims (2)

PATENT CLAIMS: 1. Process for the production of acrylic acid and o; -Alkylacrylic acid esters which do not carry an alkoxy group in the ß-position Alcohol elimination from os- or ß-alkoxymonocarboxylic acid esters is acidic in the presence acting catalysts, characterized in that from vaporous oc or ß-Alkoxymonocarbonsäureestern at a temperature of 200 to 400 ° C, especially 270 to 350 "C, alcohol is split off in the presence of catalysts made from silica and from either 0.1 to 100 / o oxides of the metals of the IV. or V. subgroup des Periodic Table or 0.4% titanium dioxide, 14.7% aluminum oxide, 2.7% ferric oxide, 0.7% calcium oxide and 15.4% chemically bound water exist. 2. The method according to claim 1, characterized in that the Carries out alcohol elimination in the presence of a catalyst made of silica gel and oxides of titanium, zirconium, thorium or tantalum. Considered publications: German Patent Specifications No. 729,342, 847,443, 885,542; U.S. Patents Nos. 2,341,663, 2,393,737, 2457225,2571212, 2816921; British Patent No. 584 607.