DE885542C - Process for the production of acrylic acid methyl ester - Google Patents

Description

Process for the preparation of acyl-yl acid methyl ester The invention relates to the production of Acr is acid methyl ester, especially in continuous operation. y In the American. Patent 2464768 is a method of generation of acrylic acid esters have been proposed, which is the hydracrylic acid as Ausgaligsmaterial served. The acid is concentrated in heated along with alcohol Acid, e.g. B. phosphoric acid or sulfuric acid, introduced, the amount of alcohol three to four times the amount theoretically required for esterification is required, and the procedure is such that the mineral acid is the larger Forms part of the reaction mixture. Although the alcohol used in large excess was used, 2, contained the resulting reaction product distilled off free acrylic acid. The used kata-insator had to be removed and put through all the time fresh acid, which is supplied with the acrylic acid and the alcohol, is replaced will. The methyl acrylate was calculated in a yield of 62114 the applied hydracrylic acid obtained.

It has also been proposed to produce methyl acrylate from methoxypropionic acid by splitting off alcohol under heat in the presence of acidic catalysts. If the reaction is carried out in a still, acrylic acid will distill over as it is formed. The proposal has also been made to carry out the reaction under reflux cooling and to work up the reaction product later by distillation. In this case, the split off methyl alcohol esterifies the carboxyl group to a limited extent, especially if the duration of the reflux condenser treatment is extended. Concentrated sulfuric acid has been used in relatively large quantities as the acid catalyst for carrying out the conversion. However, this acid was diluted in the course of time by the water released during the esterification. In addition, water was added at various times during the course of the procedure to facilitate the reaction. From this it can be seen that the proposed process can only be carried out in batches and that the addition of water, with the resulting increasing dilution of the acid, prevents it from being reused for further approaches. In addition, the high proportion of acid in the reaction mixture causes significant carbonization, so that the contamination and dilution of the acid requires purification and concentration for its reuse after each batch. This would be very costly.

It was also found that the split off from the methoxypropionic acid Methyl alcohol is insufficient to completely esterify the acrylic acid produced, because methyl alcohol forms an azeotropic mixture with the methyl acrylate, so that methyl alcohol from the! Mixture is removed. To remedy this deficiency, it has been proposed to decompose methoxypropionic acid in the presence of methyl alcohol to effect in excess. Despite this excess methyl alcohol, however, escaped a certain amount of the free acrylic acid of the esterification and was in the. distillate found.

The fact that free acrylic acid is negotiated in the distillate makes theirs, however Separation from the ester and water, and on top of that their conversion into the corresponding Ester required. But since the free acid is more readily polymerizable than the ester is, their formation is a major disadvantage of these known processes, so that the benefits of using hydracrylyl acid or metboxypropionic acid grown as a starting material in place of methoxypropionic acid ester, thereby nullifying it be made. Furthermore, the presence of free acrylic acid has the consequence that the selection of the building materials which are used for the distillation columns let is limited. Also goes through when cleaning the ester, for example Wash-out distillation with water, most, if not all, of the acid lost.

It is an object of the invention to provide a method which does the Production of acrylic acid methyl ester from methoxypropionic acid with particular advantage allowed in continuous operation in high yield and practically free of free acid, whereby the above-mentioned disadvantages are avoided.

The invention is based on the discovery that methyl α-methoxypropionate is a very effective entrainer for water, with which it forms a mixture with a constant boiling point of 95 ' which contains water and ethyl α-methoxypropionate in almost equal amounts.

The process for the preparation of methyl acrylate is therefore characterized in that fl-methoxypropionic acid is introduced into a heated mixture which consists of an acidic catalyst, substantial amounts of methyl β-methoxypropionate and methyl alcohol and the methyl acrylate as it is formed by distillation with the water of reaction formed, methyl alcohol and .beta.- methoxypropionic acid methyl ester are removed as entrainers from the heated mixture and the acrylic acid methyl ester is separated off from the distillate.

A particularly advantageous embodiment of the method of the invention consists in the implementation in continuous operation, whereby for a sufficient amount of methyl alcohol and β-methoxypropionic acid in the heated mixture is to be provided for its content of methyl alcohol and methoxypropionic acid, methyl ester essentially constant keep.

As acidic catalysts for the cleavage of the methoxy group in shape of methyl alcohol can include such acidic compounds as sulfuric acid, phosphoric acid and sulfonic acids such as tolu. olsulfonic acid, can be used. Particularly useful The catalysts are sulfuric acid and its acidic esters. A relatively small one The amount of catalyst in the heated mixture is sufficient to achieve the desired elimination of alcohol and to effect esterification. The quantities to be used can be within considerable limits vary. It was found that amounts of 150 / G and less of the total amount the mixture produced good results.

By using only small amounts of the acidic catalyst charring is either completely avoided or at least greatly reduced while Large amounts of the strong acids tend to retain water, its removal higher temperatures are required, which in turn cause charring and thus losses cause valuable material. By using the acidic catalyst in relatively small amount it acts only as a catalyst and serves not as a means of dehydrating the mixture, thereby shifting the ester equilibrium would occur. However, the small amount has the disadvantage that the The effectiveness of the splitting off of alcohol is considerably reduced by the dilution that occurs or even stop altogether if allowed the water collected in the mixture. Although part of the esterification water formed by distillation and abortion using the methyl acrylate produced is withdrawn from the mixture, it is completely removed according to the invention through abortion with the help of ß-methoxypropionic acid methyl ester as an entrainer. This complete removal of the Water from the mixture the elimination of alcohol and esterification in continuous operation.

According to the invention, the mixture containing the methyl methoxypropionate, into which the P-methoxypropionic acid is continuously introduced for the elimination of alcohol and simultaneous esterification, contains considerable, preferably predominant, amounts of methyl methoxypropionate, so that the partial vapor pressure of the same in the mixture is sufficiently high to ensure complete Removal of the water, as long as it has not already been removed by the methyl acrylate produced, can be made possible by means of the methyl methoxypropionate vapors. The suitable amount of this ester in the mixture is 20% or more, preferably even more than 40%, calculated on the total weight of the mixture.

The temperature in the reaction mixture is kept so high that it is sufficient to cause the alcohol to be split off. Generally it lies above i2o '. Temperatures which are above the boiling point of methyl methoxypropionate, are 14:21, but should be avoided appropriately. Suitable Temperatures are between i2o and 140 ", for example.

The presence of methyl alcohol in the reaction mixture can be brought about by continuously feeding methyl alcohol into the reaction mixture either separately from the methoxypropionic acid or at the same time with the same. However, it can also be achieved by ensuring that sufficient methyl methoxypropionate is present, which, under the influence of the heat and the catalyst, yields methyl alcohol through., # Llzoliolal) cleavage. The amount of methyl alcohol to be added to the reaction mixture depends partly on the amount of alcohol which leaves the reaction mixture with the distillate and has to be replaced, but also on the amount of methyl methoxypropionate which is withdrawn from the system as an entrainer for water. This last-mentioned amount can be reduced by subjecting the steam mixture which has been driven off to a fractionation and recycling as much of the methyl methoxypropionate as possible into the mixture. Finally, you can also separate the methoxypropionic acid methyl ester from the distillate, remove the water from it and then return it to the reaction vessel. When methyl alcohol is added to the mixture, it is appropriate to use 1 mole of methyl alcohol or less for every 1 mole of methoxypropionic acid fed to the mixture. More than 1 mole can be added; however, it is advisable not to exceed 2 moles of methyl alcohol per 1 mole of methoxypropionic acid, because this gives rise to unnecessarily large amounts of methyl methoxypropionate.

Those from the reaction mixture in association with the methyl acrylate produced or the methyl methoxypropionate as a mixture with a constant boiling point The amount of water to be removed can be easily removed from that converted into esters in the system Calculate acidity. This enables the amount of methyl methoxypropionate, which is needed to remove this amount of water when the distillation has started by adjusting the fractionation of the distillate accordingly and to regulate the temperature in the Fraktioniersysteni so that only that amount Methoxypropionic acid ester, which is necessary to avoid this with the aid of the methyl acrylate to remove expelled water, the mixture is removed while the excess is returned to methoxypropionic acid ester in the reaction mixture.

It was also found that the use of a proportionate small amount of acidic catalyst has the additional advantage that the formation of dimethyl ether from part of the methyl alcohol either completely avoided or is reduced to a negligible amount. This could with the previously known processes in which large amounts of acid were used, can be observed to a considerable extent. This avoidance is important because the formation of dimethyl ether is not just an irretrievable loss Methyl alcohol, but also a special cleaning of the end product, in which the dimethyl ether is found as an impurity, makes necessary.

23 el The process according to the invention can be carried out in a simple reaction vessel be made, in which the escaping vapors by having a high Concentration of methoxypropionic acid methyl ester in the reaction mixture upright receives all the water formed with a certain amount of methoxypropionic acid methyl ester in addition to the methyl acrylate and methyl alcohol. These fumes can then passed into a rectification column, where they are converted into a methyl acrylate, a little methyl alcohol and water containing distillate and a higher boiling one from Methoxypropionic acid methyl ester and water existing fraction are separated.

The reaction vessel can also come from a rectifying column exist in which the vapors leaving the boiler to a certain extent be fractionated so that at the top of the rectifying column, although the kettle Methoxypropionic acid methyl ester in the same concentration level as that just described Contains configuration, sufficient methyl methoxypropionate is present, uni complete and permanent removal of all water during the reaction to ensure. By proceeding in this way, the person leaving the system Amount of methyl methoxypropionate to that required to remove the water Minimum dimensions are restricted.

For the purpose of separating the resulting from the reaction and methyl alcohol, Acrylic acid methyl ester, Water and methoxypropionate ethyl ester containing steamer you can use an additional entrainer, which the separation of the water from its stripping agent methoxypropionic acid methyl ester causes. As such a stripping agent, methyl acrylate itself can be used which is then returned to a suitable part of the rectification column is, the amount should be sufficient to include everything in the reaction mixture Remove water as distillate.

One. simplified modification of the process is that methoxypropionic acid and methyl alcohol are introduced into the kettle of a fractionation column, in which are the methoxypropionic acid methyl ester and the acidic catalyst. In the lower part of the column there is enough ethyl acrylate as either Liquid or vapor introduced in such a way that a methyl alcohol, Acrylic acid methyl ester and all the water formed in the reaction mixture containing results. Originally used together with the methyl acrylate as an entrainer Serving methyl methoxypropionate is used as reflux in the fractionating column retained and fed back into the reaction mixture. It is surprising that under the specified conditions even the recycling of the methyl acrylate in the cycle when used as an additional drag agent for the water does not lead to significant losses through polymerization from the vapor mixture, which is perhaps due to the absence of the unsaturated acid.

Instead of acrylic acid methyl ester, any one can of course also be used other, low-boiling expelling agent for water, which easily becomes whole or partially separate from the ethyl acrylate during the cleaning process lets, e.g. B. Benzene, use in the same equipment.

Instead of introducing the vapors leaving the reaction vessel at the foot of a column, where the excess of methyl methoxypropionate is returned to the boiler as reflux, the steam mixture can also advantageously be introduced in the middle of a fractionating column. By addition of a suitable stripping agent such as methyl acrylate, may be all the water are here together with methyl alcohol and methyl acrylate formed removed as distillate, while anhydrous Methoxypro # pionsäuremethylester at the bottom of the fractionating column - withdrawn and, if so, can be recycled to the -Reaktionsmedium desired. If this column is operated without the addition of a drag agent, hydrous methoxypropionic acid methyl ester is obtained at the foot of the column, which can be freed from water in a further, separate column with the aid of another stripping agent. The anhydrous methyl methoxypropionate can then be fed back into the reaction mixture. The methyl acrylate, which is distilled over with methyl alcohol and water and condensed, can be passed into a column in which the vapor mixture is washed in countercurrent with water. As a result, the methyl alcohol is washed out, while the methyl acrylate distills over as an azeotropic mixture. When condensing, the distillate separates into two layers, the topmost layer containing almost all of the acrylic acid ester, while the lower layer consists of water in which a small amount of methyl acrylate is dissolved.

It was found that the mixture with a minimum boiling point of methyl methoxypropionate and water consists of 53.7 percent by weight of ester and 46.3 percent by weight of water. The mixture separates into two layers as it cools. The upper layer contains at: 251 15.3 percent by weight water and the lower 46.5 percent by weight ester. In this way, it is possible to use the methyl methoxypropionate itself as a means of separating the mixture of water and methyl methoxypropionate, as it is when working up the distillate which is obtained during the cleavage of alcohol and the esterification process and contains methyl acrylate, methyl alcohol, methyl methoxypropionate and water. The vapors distilling over from the reaction mixture are passed into a heated column in which they are washed in countercurrent with water. The azeotropic mixture of methyl acrylate and water, boiling at 71 ", distills from the top of the column, while a mixture of methyl alcohol, methyl methoxypropionate and water is drawn off at its base. This mixture is then passed into a fractionation column, from which methyl alcohol is distilled off at the top and another A mixture of water and methyl methoxyproptonic acid is removed. This last mixture is fed to a customary two-column system. An azeotropic mixture of methyl methoxypropionate and water distills over from the first column. The condensate is fed to a separating device in which two layers are formed. In some cases this separation of the azeotropic mixture into two layers does not occur readily, apparently due to small amounts of methyl alcohol which had remained in the higher-boiling fraction taken from the bottom of the fractionating column To promote the phase separation, a small amount of methyl acrylate is added to the liquid fed into the column. Approximately 2 1 / o methyl acrylate, calculated on the amount of methoxypropionic acid methyl ester fed to the column, has generally been found to be sufficient for achieving the desired effect of rapid separation into two layers. The upper phase, consisting of methoxypropionic acid methyl ester and a small amount of water, is withdrawn and fed to a further 1, fractionating column, where it is separated into anhydrous ester, which leaves the column at the bottom, while an azeotropic water ester Mixture is distilled over and fed back to the separation device. The lower phase, consisting mainly of water and a small amount of methyl methoxypropionate, can be passed back into the first or, more appropriately, into the third column.

Instead of completely removing water from the upper phase, which contains approximately 95010 ester and 1511 / o water, in the separate column, it is preferably returned directly to the reaction vessel. If a not inconsiderable amount of water is added to the reaction medium as a result, it has been found that this does not impair the effectiveness of the process. On the other hand, this procedure makes it possible to do without a third fractionation column and the removal of the admixed water effected therein.

As another embodiment of the process, the vapors flowing out of the reaction vessel can be converted into a distillate containing methyl alcohol, methyl 1 -, methyl acrylate and perhaps some water, from which methyl acrylate is separated off by washing distillation with water, and a remaining methyl methoxypropionate and water Separate product. This latter mixture is separated into two phases, the upper layer of which consists largely of methyl methoxypropionate and a relatively small amount of water either directly recirculated into the reaction vessel or first dewatered and then returned. The ester can be removed as an azeotropic mixture with water from the lower water-rich phase, but less in methoxypropionic acid methyl ester, the distillate being fed to the same separation device which serves to separate the discharge at the foot of the first fractionation column into two phases. 1 0 il various ways can "around this forward in the context of the present invention to be modified. For example, one can first fractionation column, is separated by means of which the low-boiling products from the water-holding ilgen methoxypropionate, at a point below the feed point, where the methyl alcohol concentration negligible The lower layer is returned to the column at a suitable point, while the layer which forms the upper phase in the separator and contains the 851% ester goes into the reaction Vessel is returned, either directly or after previous dehydration in a special z # column, as has been described above. Ester-free water is drained off at the foot of this column. The process according to the invention has made it possible to use methyl acrylate continuously hey The acidic catalyst is retained for a very long time and the contamination of the methyl acrylate produced by free acrylic acid is practically completely avoided. There is practically no polymerization in the reaction vessel and also practically. no carbonization takes place in the reaction mixture. The addition of polymerization inhibitors to the reaction mixture in the reaction vessel, which was regarded as indispensable in the previously known process, has become practically unnecessary.

The following examples serve to illustrate the invention Procedure.

t> Example i In a reaction vessel equipped with a stirrer, in which a mixture of 47.6% methyl methoxypropionate, 4o #, 211/9 methoxypropionic acid, ia "/ & sulfuric acid and small amounts of methyl acrylate, acrylic acid, methyl alcohol and water at 130 ' was kept boiling, at atmospheric pressure 104 parts of methoxypropionic acid and 32 parts of methyl alcohol per hour were passed in. The vapors leaving the reaction vessel were passed through a short reflux condenser, the top of which was 740, and then passed into a fractionation column at the top a mixture of 77.4 parts of methyl acrylate and 28.8 parts of methyl alcohol passed over and at the base of which a mixture of 1.8 parts methyl methoxypropionate and 18 parts water was removed per hour. The methyl acrylate was obtained from the distillate, for example, by washing distillation of the mixture with water. The mixture withdrawn at the foot of the column was in egg Another column passed into which benzene was introduced as an entrainer. At the foot of this second column, anhydrous methyl methoxypropionate was taken off, which was returned to the reaction vessel.

This continuous process can be carried out for a long time without any appreciable deterioration in the reaction. The amount of methyl acrylate produced was 218 g / hour per liter of reaction volume. The yield of Acrvlsäuremethylester in the reaction vessel was virtually quantitative, Example 2 In a reaction vessel containing a boiling mixture of the same type as described in Example i, was at the same temperature a mixture of 93,6Teilen methoxypropionic acid, 28.8 . Parts of methyl alcohol and 11.8 parts of methoxypropionic acid methyl ester initiated per hour. The distillate in the reaction vessel consisted of 77.4 parts of methyl acrylate, 28.8 parts of methyl alcohol, 1 parts of methyl methoxypropionate and 16: 2 parts of water. It was separated into its components in the manner described in Example i. 27: 2 g of methyl acrylate were obtained per hour per liter of reaction volume; the yield was also practically quantitative here.

Example 3 In a reaction vessel with a reaction mixture, which like the one described in the first two examples was similar in composition, were added at the same temperature for an hour 5 1.7 parts methoxypropionic acid, 15,9- parts of methyl alcohol and 7.66 parts Water of saturated methyl methoxypropionate (85 percent by weight) was introduced. The distillate from the reaction vessel contained 12.6 parts of methyl methoxypropionate and 36.7 parts of methyl acrylate along with water and methyl alcohol. The result was 184 g of methyl acrylate per hour per liter of reaction volume. The yield was again practically quantitative.

The distillate was fed to a fractionation column in which 38o parts of water trickled down from above every hour. The distillate separated into two layers, with the top oil layer of water-saturated methyl acrylate being removed. The fractionation residues were fed into a second distillation device, from which methyl alcohol as the distillate and methoxypropionic acid methyl ester as the residue were obtained. This was placed in a further distillation device, from which 8511 / Giger, water-saturated methyl methoxypropionate was obtained as a distillate as an upper layer and returned to the reaction vessel. A small addition of methyl acrylate to the - distillation facilitated the phase separation in the separation device because this phase separation is disturbed by small traces of methyl alcohol in the fractionation. Example 4 A 76.5 percent by weight was added continuously to p-toluenesulfonic acid in a reaction vessel. Given a solution of methoxypropionic acid in methyl alcohol. The vessel was kept at 14d ". A distillate was distilled off through a short reflux condenser at such a rate that the reaction volume remained essentially constant. After equilibrium had occurred, the vessel contained approximately 15 percent by weight of p-toluenesulfonic acid and 67 percent by weight of methyl methoxypropionate - Distillate contained 45.4% methyl acrylate and 10% methyl methoxypropionate, in addition to methyl alcohol and water.

The conversion of the methoxypropionic acid fed to the reaction vessel amounted to 891 / G. Similar results were obtained when those in the examples used acidic catalysts by equivalent amounts of phosphoric acid or acidic Sulfuric acid esters were replaced.

Example 5 In a container of concentrated sulfuric acid was continuously introduced containing an 46.9 weight percent and 53.1 methoxypropionic wichtsprozentMethoxypropionsäuremethylesterenthaltende overall mixture. The mixture in the container was held at 135 "". A distillate was removed through a short reflux condenser in such an amount and replaced by the above mixture of methoxypropionic acid and methoxypropionic acid methyl ester that the reaction volume was kept practically constant. After equilibrium had been reached, the container x contained about 15% sulfuric acid and 73 % by weight of methyl methoxypropionate. The distillate contained 671 / o methyl acrylate and 6.10 / o. Methoxypropionic acid methyl ester in addition to methyl alcohol and water. The implementation was gi. 6%.

The distillate was worked up to obtain the various constituents as described in Examples i or 3 .

Claims (2)

PATENT CLAIMS. i. Process for the production of acrylic acid methyl ester from methoxypropionic acid by splitting off alcohol, characterized in that one Fl-Methoxypropionäureeiiier abf i2o to 14o1 heated, an acidic catalyst and at least 20 percent by weight of methyl methoxypropionate and methyl alcohol containing reactant mixture supplied from the hot reaction mixture by distillation the acrylic acid methyl ester according to its formation, together with methyl alcohol, Water and ß-methoxypropionic acid methyl ester using the acrylic acid methyl ester and the ß-methoxypropionic acid methyl ester removed as an entrainer for water and separating off the methyl acrylate from the resulting distillate. 2. The method according to claim i, characterized in that a strong mineral acid whose acidic alkyl ester or p-toluenesulfonic acid is used as the acidic catalyst. 3. The method according to claim i and 2-, characterized in that the acidic catalyst is used in an amount not significantly exceeding 15 percent by weight of the total mixture. 4. The method according to claim i to 3, characterized in that the amount of f-methoxypropionic acid methyl ester present in the reaction mixture is more than 4o percent by weight of the reaction mixture. 5. The method according to claim 1 to 4, characterized in that during the continuous operation of the process, the concentration of the methyl methoxypropionate and the methyl alcohol in the reaction mixture z3 is kept practically constant by adding methyl methoxypropionate and / or methyl alcohol. 6. The method according to claim 1 to 5, characterized in that the amount of methyl metlioxypropionate to be added is adapted to the amount of water to be removed from the reaction mixture. 7. The method according to claim i to 6, characterized in that the vapors expelled from the reaction mixture are separated in a fractionating column into a methyl acrylate, methyl alcohol and water-containing distillate and a residue containing water and methyl methoxypropionate. 8. The method according to claim 7, characterized in that the distillate containing methyl acrylate, methyl alcohol and water is freed from methyl alcohol in a second fractionating column by washing distillation with water, while methyl acrylate and water distill over.