DE1178082B - Process for the production of trioxane - Google Patents

Description

Process for the production of trioxane It is known that trioxane is made from concentrated aqueous formaldehyde solutions in the presence of strongly acidic catalysts arises (Journal of the Chemical Society [London], 121 [1922], p. 2738).

The trioxane is expediently in the form of the reaction mixture an azeotrope with water that boils at 91.3 ° C and 700/0 trioxane and removed Contains 30 ovo of water (U.S. Patent 2,347,447). Wins from the distillate you then remove the trioxane by extracting it with a solvent that cannot be used with water is miscible, and by subsequent fractional distillation of the organic Phase. A disadvantage of this process is that when distilling off the Trioxane-water azeotrope, depending on the composition of the reaction mixture, too more or less large amounts of formaldehyde pass over (cf. the USA. patent 2 347 447, p. 2, left column, table). This formaldehyde now beats easily settles in the colder parts of the device as a polymer and thus leads to blockages of pipelines and similar parts of small cross-section. The company must then interrupted and the device, expediently by the action of steam, are freed from the formaldehyde polymer.

Apart from the undesired business interruption, the procedure is also associated with considerable losses of formaldehyde. The yields of trioxane, based on converted formaldehyde, are accordingly only 800/0 of theory.

It has now been found that you can trioxane by heating an aqueous concentrated formaldehyde solution in the presence of an acidic catalyst and distillation of the trioxane formed is advantageously obtained if the reaction mixture is before or during the distillation methanol or the semi-formal of methanol in such a Amount adds to the deposition of solid polymeric formaldehyde in the device is prevented, the distillate obtained in a known manner with a Water immiscible organic solvent extracted and after evaporation of the solvent distilled the residue obtained.

In the case of the method according to the invention, they even take months There is virtually no deposition of polymeric formaldehyde in the device for working up the reaction mixture. This may be because the Methanol, which distills over together with the trioxane and water, also does so with the pass over- the formaldehyde forms a semi-formal and thus the polymerization of the formaldehyde prevented. The yields are considerable compared to the known procedures increased, they are generally above 990/0 of theory. Surprisingly the formation of trioxane from formaldehyde by the addition of methanol does not impaired, although one would have expected according to US Pat. No. 2,465,489 should that the addition of methanol prevents the formation of trioxane.

The formaldehyde is used in the usual concentrations. Man works advantageously with 30 to 700% aqueous formaldehyde solutions. Particularly favorable results are obtained with about 600% solutions. if the formaldehyde concentration as a result of the formation of trioxane and depletion, which arises from the fact that formaldehyde distills off together with the trioxane, under 30 to 400/0 has fallen, it is expedient to apply it again the initial value that either solid paraformaldehyde is added to the solution or that one subtracts part of the impoverished aqueous solution and through fresh highly concentrated Replaced formaldehyde.

The withdrawn, formaldehyde-depleted solution can be returned to a Formaldehyde content of around 600/0.

As acidic catalysts are advantageously the strong or medium-strength mineral acids, especially sulfuric acid, are used. The concentration the sulfuric acid should be between 0.1 and 7 percent by weight, based on the amount used Reaction mixture, preferably about 2 percent by weight. At lower The rate of formation of the trioxane drops sharply at values, while at too high a rate Concentration- some of the formaldehyde does not react to trioxane, but rather through Side reactions, for example with disproportionation to methanol and formic acid in a Cannizzarro reaction.

The amounts of methanol or its semi-formal to be used depend on the concentration of formaldehyde in the reaction mixture. Advantageously is the amount of methanol or its semi-formal to be added during the distillation 0.5 to 5 parts by weight per part of trioxane distilled off. With the help of Sight glasses check whether there is solid polymeric formaldehyde in the device separates, and increase the supplied amount of methanol or its semi-formal. if that's the case. The amount of methanol used to prevent deposition of solid formaldehyde polymers is required is less than the amount of semi-formal, which is needed to achieve the same effect.

The trioxane distills off in the form of a ternary azeotrope, the boils at 92 ° C. under atmospheric pressure.

The methanol or its semi-formal is before the reaction mixture or added during the removal of the trioxane by distillation. For example you can use the concentrated aqueous formaldehyde solution with the addition of the mineral acid Heat for some time, then add an appropriate amount of methanol to the reaction mixture or add its semi-formal and distill off the trioxane formed.

But it is also possible to use the methanol or its semi-formal to be added to the reaction mixture from the beginning. The implementation can also be continuous be carried out by allowing the reaction in the presence of an appropriate amount of methanol or its semi-formal executes, the resulting trioxane in the form of his ternary azeotrope with water and methanol is continuously removed and the concentration of formaldehyde and methanol or its semi-formal in the reaction mixture by running Supply of formaldehyde and a corresponding amount of methanol or its semi-formal maintains.

The distillate in which trioxane, water, methanol and formaldehyde (the both of the latter substances are at least partially contained in the form of the semi-formal), is not in the usual way by extracting the trioxane by means of a with water Miscible solvent freed from the trioxane. Suitable solvents are, for example Methylene chloride, chloroform, cyclohexane, benzene, chlorobenzene or diethyl ether. The remaining aqueous mixture has a boiling point of 96 to 970 C and mainly contains the semi-formal of methanol.

This mixture is used continuously or batchwise Preparation of trioxane in the acidic reaction solution before or during the distillation of the ternary trioxane azeotrope added.

Fresh methanol or its semi-formal is only used at the beginning of the process, as long as the mentioned mixture is not yet available.

The extract obtained is converted into by evaporation of the extractant and subsequent distillation of the residue obtained to obtain the trioxane. However, it is difficult to use relatively small amounts of monomeric formaldehyde, which stubbornly follow the trioxane and sometimes considerably if it is used further bother to remove. It has already been recommended that the contaminated trioxane be mixed with ammonia to treat, being the formaldehyde is converted into hexamethylenetetramine. That However, trioxane is contaminated by hexamethylenetetramine, which is difficult to separate. According to another known method, the formaldehyde is obtained by washing the trioxane crystals removed with cold methanol or ethanol.

However, this cleaning must be repeated several times and is because of the good solubility of trioxane in alcohols with considerable losses tied together. In addition, the trioxane treated in this way also contains alcohols, which are in a must be removed again by distillation. The small amounts of formaldehyde, that accompany the trioxane can be reliably and considerably easier than after remove the previously known methods when looking at the residue left after removal of the extractant remains, before or during its distillation with a Alkali or alkaline earth hydroxide or an alkali or alkaline earth metal in the case of increased Temperature treated. This treatment makes it about 0.5 percent by weight Formaldehyde dissolved in the trioxane turns into a dark brown resin in a short time implemented. Surprisingly, the trioxane is not attacked.

One uses, for example, sodium hydroxide, or potassium hydroxide Barium hydroxide or, in its place, the corresponding free metals. One turns the basic substances concerned are expediently in amounts of 0.1 to 0.5 percent by weight, based on the residue to be distilled. Treatment of the residue is carried out before or during the distillation, expediently at a Temperature from 80 to 1100 C. In general, the residue is heated with the appropriate Base for a short time, e.g. B. 5 minutes to 1100 C. There is formaldehyde in the distilled trioxane neither by the smell nor analytically detectable. The product shows the pure chloroform-like odor of the trioxane.

The pure trioxane melts at 640 C. It serves as the starting material for the production of high molecular weight polyoxymethylene which is used as plastic is used.

The parts mentioned in the following examples are parts by weight. They relate to the roughness -.- parts as g to ccm.

Example 1 1170 parts of 600 / o aqueous formaldehyde solution and 25 Parts of concentrated sulfuric acid are heated to the boil (internal temperature 1000 C). 237 parts of methanol are introduced into the reaction mixture over the course of 5 hours a. The escaping vapors rise through a packed column up to a reflux condenser, which is charged with 920 C warm water. the higher-boiling fractions are condensed there and act on the column a return of 150 parts per hour. The one with a temperature of 920 C Passing vapors are completely suppressed by means of a cooler.

The condensate obtained in this way (800 parts) is countercurrent twice extracted with 208 parts of methylene chloride each time. The first solution is then in fractionated and distilled in a packed column. First the methylene chloride is stripped off at 41.50 ° C., the sump temperature gradually increasing up to 1000 C. at the further distillation go after a small forerun, consisting of methylene chloride and trioxane, at 114.5 C 235 Parts of a trioxane contaminated by small amounts of formaldehyde. That The product obtained is kept liquid by heating the drain lines. Of the Forerun consisting of trioxane and methylene chloride is used with the second extract combined, which is used for the first extraction of a subsequent batch.

For another approach, 250 parts of paraformaldehyde are added to the reaction vessel (920 / oig) added, reducing the concentration of formaldehyde in the reaction mixture is brought back to about 60%. Then it is done in the manner described above worked further, but instead of pure methanol within 5 hours the aqueous phase that remains after the trioxane has been extracted with methylene chloride, is added to the mixture. The further work-up then takes place after method described above.

When stationary conditions have set in, one gets out each batch 239 parts of trioxane from 63 to 640 ° C., the 228 parts of pure trioxane contain. The yield, based on pure paraformaldehyde, is thus 99% the theory.

Example 2 According to the method described in example 1, paragraph 1 1170 parts of 600 / above aqueous formaldehyde solution, 25 parts are concentrated Sulfuric acid and 237 parts of methanol reacted, and the resulting condensate (800 parts) is extracted twice with 208 parts of methylene chloride each time.

Fractional distillation then turns the first solution into the Removed methylene chloride. 5 parts of sodium hydroxide are added to the residue obtained in this way added, and the mixture is 5 minutes at a temperature of 100 to 1100 C held. Subsequent distillation of the mixture gives 228 parts Trioxane with a boiling point of 1140 C, which solidify to form crystals with a temperature of 640 C. The stabbing one The smell of formaldehyde has completely disappeared and also with ammoniacal No more formaldehyde can be detected in silver nitrate solution (Tollens reagent). Using the so-called Lebbins reagent, an alkaline resorcinol solution, lets prove that the formaldehyde content is below 0.1 ppm.

The same result is obtained if instead of sodium hydroxide an equivalent amount of lithium hydroxide, potassium hydroxide, barium hydroxide, metallic Sodium or potassium is used.

Example 3 1170 parts of 600% strength aqueous formaldehyde solution, 25 Parts of concentrated sulfuric acid and 237 parts of methanol are used in the example 1 apparatus used heated to boiling. The sump temperature is 950 C. Rising vapors are sent over a packed column and at one at 920 C held cooler deposited all higher boiling components as reflux. the with a Vapors passing over at a temperature of 920 C are passed on to a second cooler completely condensed.

As soon as 800 parts of condensate have accumulated, it becomes trioxane extracted with methylene chloride. After the methylene chloride has been distilled off, this is obtained 231 parts of trioxane.

For a further approach, 250 parts of 920% are added to the reaction vessel Paraformaldehyde and then all at once the entire aqueous phase, which gives way after the trioxane had been stripped off with methylene chloride, returned to the mixture. Proceed as described above while boiling and extracting and you get if stationary conditions have been established, 236 parts from each batch Trioxane.

Example 4 950 parts of 500 / above formaldehyde solution, 25 parts concentrated Sulfuric acid and 460 parts of the semi-formal of methanol are used in one of the examples 1 corresponding apparatus brought to the boil. The sump temperature arises to 950 C.

The rising vapors are filled with Raschig rings Column with dephlegmator separated in such a way that only the fractions boiling up to 920 ° C. pass over and then be condensed. 800 parts of this condensate will be extracted with methylene chloride. One receives after distilling off the solvent 230 parts of trioxane.

For another batch, another 250 is added to the reaction vessel Allocate 920 / above paraformaldehyde and at the same time lead all of the aqueous, residue obtained during the extraction. When boiling again and working up of the reaction mixture in the manner described above one receives again 230 parts Trioxane.

Claims (2)

Claims: 1. Process for the production of trioxane by heating an aqueous concentrated formaldehyde solution in the presence of an acidic catalyst and distillation of the trioxane formed, characterized in that the reaction mixture before or during the distillation of methanol or the semi-formal of methanol in a solitary amount adds that in the device the deposition of solid polymer Formaldehyde is prevented, the distillate obtained in a manner known per se extracted with a water-immiscible organic solvent and after after evaporation of the solvent, the residue obtained is distilled. 2. The method according to claim 1, characterized in that the residue obtained after evaporation of the solvent before or during the distillation with an alkali or alkaline earth hydroxide or an alkali or alkaline earth metal treated at elevated temperature. References considered: U.S. Patent No. 2304080.