SK71793A3 - Process for production of isocyanic acid - Google Patents

Description

Process for preparing isocyanic acid

Technique

The invention relates to a process for the preparation of isocyanic acid by decomposition of N, N-disubstituted urea derivatives.

BACKGROUND OF THE INVENTION

Due to its high reactivity, isocyanic acid is a valuable building block containing 1 carbon atom for the synthesis of a large number of compounds.

According to EP-A-0 124 704, this substance can be obtained by heating molten urea in a fluidized bed. This gives a mixture of isocyanic acid and ammonia. However, the isolation of isocyanic acid from this mixture presents difficulties because the cooling of the mixture produces ammonium isocyanate, which is very easily isomerized to urea again.

From Chemical Abstracts, Vol. 81, 1974, p. 172444m and Volume 82, 1975, p. 173135u is a known thermal decomposition of urea with simultaneous separation of ammonia, which is then formed which is then possible to convert to isocyanic acid. and incomplete. It then describes improved isocyanic acids and ammonia in Europe with the addition of tertiary amines or ethers. SUMMARY OF THE INVENTION It is an object of the present invention to provide a new, simple process for the preparation of isocyanic acid which, in carrying out the process, can be obtained in high yield and purity without the need for subsequent isolation steps.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION The present invention provides a process for the preparation of isocyanic acid by decomposing an N, N-disubstituted solid product, cyanuric acid, at a temperature of 330 to 600

However, only EP-A-0416 ammonia is slowly decomposed

236 with patent i 1 ’·

the urea decomposes at higher temperature into a hardly volatile secondary amine and isocyanic acid, which is discharged through the top of the column.

In principle, all N, N-disubstituted urea derivatives which decompose into isocyanic acid and a hardly volatile secondary amine are suitable as starting materials for the production of isocyanic acid according to the invention.

Particularly preferred are N, N-disubstituted urea derivatives of formula (I)

R

N

(I) where

9

R and R are the same or different and are C1 -C4 alkyl radicals, straight, branched or cyclic, unsubstituted or substituted by C1 -C6 alkoxy or a phenol radical or a phenol radical unsubstituted or substituted by C1 -C6 alkyl radicals C 1 -C 6 alkoxy, hydroxy, halogen, such as chlorine or bromine, nitro or amino.

Alkyl radicals are, for example, methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, hexyl, octyl, decyl, dodecyl, hexadecyl, octadecyl, cyclohexyl or cycloheptyl. C 1 -C 6 alkoxy groups include methoxy, ethoxy, propoxy, butoxy or hexyloxy. Substituted phenol residues include, for example, tollyl, aminophenyl or chlorophenyl.

Particularly preferred starting materials are, for example, N, N-dibutylurea, N, N-dihexylurea, N, N-dioctylurea. N-dioctadecylurea, N, N-dicyclohexylurea and dibenzylurea. These substituted urea derivatives can be obtained, for example, by N-alkylation of urea as described in EP-A-0 471 983.

The substituted urea derivatives can also be used in the absence of a diluent in liquid or melt form.

However, the decomposition can also be carried out in a diluent which is inert under the reaction conditions. Among these diluents, for example, aliphatic or aromatic hydrocarbons, such as dodecane, hexadecane, octadecane, toluene and xylenes, or else ethers, such as diethylene glycodibutyl ether or paraffins or mixtures thereof, are suitable. Hexadecane and diethylene glycol dibutyl ether are preferably used.

However, it is also possible to use the amine formed in the decomposition of urea as a diluent.

It is also possible to add to the reaction. a solvent for the isocyanic acid which prevents the polymerization of the isocyanic acid or the isocyanic acid to stabilize by forming complexes. This solvent can then be condensed in the refrigerator together with the isocyanic acid after the reaction to give a clear, well workable solution of pure isocyanic acid. Suitable isocyanic acid solvents are, for example, aliphatic or aromatic hydrocarbons, optionally still halogenated, for example chloroform, methylene chloride, toluene and xylenes, and also ethers such as tetrahydrofuran, dioxane, diisopropyl ether, diethyl ether, tert-butyl methyl ether, diethoxyethyl ether, diethylene ether, triethylene ether provided that the boiling point of the solvent used is not higher than the reaction temperature used. A preferred solvent is toluene or an ether such as diethylene glycol dibutyl ether, diethyl ether, diethoxyethane or dimethoxyethane. It is preferred to add an amount of solvent such that a maximum of 10% isocyanic acid solution is obtained from the condenser.

The reaction temperature is between 90 and 400, preferably between 150 and 400, depending on the urea derivative used

300 and especially 180 to 260 ° C.

The isolation of the resulting isocyanic acid can be accomplished in various ways.

For example, it is possible to proceed by catalysing the isocyanic acid or a mixture of isocyanic acid and solvent by distilling off and condensing in a liquid nitrogen-cooled condenser or absorbing it into any of the above-mentioned suitable solvents, the boiling point of which may be higher than the reaction temperature.

A fractionation column can also be used to improve the separation of the secondary amine and the isocyanic acid, whereby the amine, optionally entrained with the product, can be washed back with the condensing solvent back into the reaction mixture, or the isocyanic acid from the reaction mixture can be removed. or carbon dioxide.

Said isocyanic acid isolation procedures can also be combined. The process can be carried out batchwise or also continuously, for example in a thin-bed evaporator or in a packed column.

In this way it is possible to obtain isocyanic acid in high yield and in high purity. The resulting isocyanic acid can then be stored as such or as a solution at -80 to -20 ° C for several weeks or can be further processed immediately after absorption in a solvent by the addition of the corresponding reactant.

The secondary amine resulting from this process as a second product can be purified, for example, by distillation under reduced pressure and used as a starting material for various other reactions, or it is possible to make a maxial from the bottom of the column containing small unreacted urea derivative and possibly a diluent. further directly used without further processing to produce a substituted urea derivative according to the method of EP-A-0 410 168.

The following examples illustrate the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Batch production of isocyanic acid

In a suitable apparatus, consisting of a reaction vessel with an inlet for inert gas and for an isocyanic acid solvent, with a thermometer fitted with a heating device, 20 cm long

A 2% solution of dibenzylurea in hexadecane (1 g / 49 g solvent) was used as a template using a Vigreux column and a condenser and then heated to 200 ° C. simultaneous feeding of a nitrogen stream of 17 liters per hour. In addition, diethyl ether was fed to the bottom of the column at 80 mL per hour.

The resulting isocyanic acid and diethyl ether are absorbed in a gas purged vessel, cooled with a mixture of water and ice and containing diethyl ether.

To determine the yield, the organic phase is extracted from this vessel with water and the isocyanic acid content is titrated potentiometrically with silver nitrate or the organic phase is extracted with aqueous sodium hydroxide solution and the excess of this hydroxide is then determined by back titration with hydrochloric acid.

Yield: 86%.

In a similar manner to Example 1, the compounds of the Examples below can be obtained together with the reaction conditions.

pR.com ;. derivative urea thinner concentration of solution i mass. temperature ° C rod in ₂ solvent for isocyanic acid yield Ä 2 dibenzylurea hexadecane 5 250 n l 1 / h diethyl ether 40 ml / h 57 3 dibenzylurea hexadecane 10 200 17 l'h diethyl ether 80 ml / h 50 4 dibenzylurea hexadecane 5 200 17 1 / h diethyl ether 40 ml / h 69 5 dibenzylurea hexadecane 5 200 34 1 / h - 69 6 dibenzylurea hexadecane 5 200 17 1 / h diethoxyethane 80 ml / h 80 7 dioctylurea hexadecane 5 200 17 1 / h diethyl ether

80inl / h 65

Example 8

Continuous operation in a thin-film evaporator

A solution of dioctylurea in hexadecane containing 5% by weight dioctylurea is fed to the thin-evaporating apparatus over a period of one hour at a reaction temperature of 230 ° C. A stream of nitrogen was fed to the apparatus at a rate of 17 liters per hour. The resulting isocyanic acid is absorbed into a vessel containing as a template 50 ml of toluene and cooled with water at 15 ° C.

To determine the yield, the toluene solution is extracted with sodium hydroxide and the isocyanic acid content is determined by back titration with hydrochloric acid.

Yield: 50%.

Example 9

Continuous production in a packed column

The column, packed with Raschig rings and equipped with a condenser, is heated to 250 ° C and a 10% / wt solution of dioctylurea in hexadecane at a concentration of 5% by weight is fed through the top of the column. 17 liters of nitrogen per hour are fed to the bottom. The stripping gas and the resulting isocyanic acid are cooled to room temperature in the condenser and absorbed into a purging gas flask, a cooled mixture of water and ice chippings and filled with diethyl ether. _;

The yields were determined in a manner similar to Example 8.

Yield: 86%.

Example 10

The procedure was carried out in a similar manner to Example 9, except that a 5% by weight solution of dioctylurea in diethylene glycol dibutyl ether was used, the solution was fed at 10 ml / h, nitrogen was fed at 5 liters per hour, reaction temperature was 200 ° C.

Yield: 75%.

Example 11

The procedure is carried out in a similar manner to Example 9 except that a 5% by weight solution of dibenzylurea in diethylene glycol dibutyl ether is used, the solution is fed at 20 ml / h, 80 ml of diethyl ether per hour is used as the solvent for isocyanic acid. 17 l / h and the reaction temperature is 200 ° C.

• • • • • • • • • •

Yield: 75%.

Example 12

The procedure is carried out in a similar manner to Example 9 except that a solution of dioctylurea in 5% by weight of diethylene glycol dibutyl ether is used, the solution is fed at 10 ml / h, 80 ml of diethylene glycol dibutyl ether are used per hour as solvents for isocyanic acid. 17 l / h and the reaction temperature is 240 ° C.

Yield: 80%.

Example 13

The procedure was carried out in a similar manner to Example 9 except that a solution of dioctylurea in diethylene glycol dibutyl ether at a concentration of 10% by weight was used, the solution was fed at 20 ml / h, 80 ml of diethylene glycol dibutyl ether was used as the solvent for 1 / h and the reaction temperature is 240 ° C.

Yield: 68%.

Claims (5)

PATENT CLAIM Λ ^? M Process for producing isocyanic acid, characterized in that the N, N-disubstituted urea is decomposed at higher temperature into a hardly volatile secondary amine and isocyanic acid which is discharged through the top of the column. 2. The method according to claim 1, characterized in characterized in that as starting material instead of N, N-di-substituted urea of the formula I, wherein and _R2 are the same or different and are an alkyl radical having 1 to 24 carbon atoms of a straight, branched, or a cyclic chain, unsubstituted or substituted by C1 -C6 alkoxy or a phenyl radical, unsubstituted or substituted by C1 -C6 alkyl, C1 -C6 alkoxy, hydroxy, halogen, such as chlorine or bromine, nitro or amino. Process according to claim 1, characterized in that N, N-disubstituted urea of the formula I is used as starting material, in which R 1 and R ₂ represent a straight, branched or cyclic alkyl radical having 4 to 20 carbon atoms or a benzole residue. Process according to Claim 1, characterized in that the decomposition of the substituted urea is carried out without diluent or also in a diluent inert under the reaction conditions, optionally in combination with a solvent for isocyanic acid and / or with an inert gas stream. Process according to claim 1, characterized in that the reaction temperature is in the range of 150 to 300, preferably 180 to 260 ° C.