DE1015428B - Process for the continuous production of aromatic isocyanates - Google Patents

Description

Process for the continuous production of aromatic isocyanates The invention relates to a process for the preparation of isocyanates from aromatic compounds Amines and phosgene in the presence of an inert liquid.

One of the common methods of making aromatic isocyanates consists in the direct reaction between phosgene and a certain aromatic Amine, the -N C O group being formed and hydrogen chloride being split off. These Method has certain difficulties; one of which is that the -N C 0 group is very reactive and easily reacts with the amino groups. That The aromatic isocyanate formed can thus directly with one in the reaction mass contained free amine react further. In part, this can cause this difficulty be avoided that one uses the same hydrochloride instead of the free amine. -In any case, however, the reaction must be batchwise at a relatively lower rate Temperature are carried out, including a fairly long "reaction time required will. If higher temperatures are used, the reaction time is shortened, it forms however, undesirable by-products and tar materials.

Processes in the vapor phase have also already been proposed however, these give low yields and tar products are also formed.

US Pat. No. 2,480088 describes a process relating to the reaction of primary amines with phosgene to form a carbamyl chloride, which is then decomposed into the corresponding isocyanate in a separate process step. The yields in this process are relatively low.

Another continuous process involves a cascade system, in which an amine or its hydrochloride at relatively high temperatures treated with phosgene in a solvent in successive reaction vessels will. However, this method has the disadvantage that for the individual vessels with accessories together a relatively large amount of space is required @. Also are the yields with this procedure also small, and about the same as one obtained in the usual batch process.

It is also known from German patent specification 801993 to react an amine hydrochloride suspended in a boiling solvent with a separately prepared saturated phosgene solution and, after all of the amine hydrochloride has reacted, to interrupt the supply of phosgene and to separate off the isocyanate formed.

This process takes place via the additional process stage of Amine hydrochloride formation and cannot be carried out continuously: it is further from the German patent $ 70 847 a method is known in which phosgene and amine are continuously reacted in vapor form and that Isocyanate formed is dissolved out with a solvent, discharged and then out the solvent is separated off by fractional distillation °.

In this process, a separate distillation stage for recovery of the isocyanate requires, it is inevitable that the isocyanate formed its residence in the reaction medium with further amine, with the formation of ureas and a corresponding reduction in the yield reacts.

The present invention therefore relates to a method of manufacture aromatic isocyanates, which can be carried out continuously and better Yields than previous methods. There is only a minimum amount unwanted by-products and tar materials. According to the invention, the continuous Preparation of the isocyanates using a single reaction vessel. the The invention will become apparent from the following description in detail.

The objects of the invention are achieved in that one aromatic amine with phosgene dissolved or suspended in an inert liquid at one above the decomposition point of the als Intermediate product formed Carbamylchlorids lying temperature brings about the reaction by the phosgene introduces into the solution or suspension and the desired aromatic isocyanate then separates.

The boiling point of the inert liquid is above the boiling point of the aromatic isocyanate to be formed. The reaction with phosgene is at a Temperature above the boiling point of the aromatic isocyanate, but below the boiling point of the inert liquid, and the aromatic isocyanate is distilled off as it is formed.

The process is relatively simple and can be done in one reaction vessel be carried out, which is heated to the desired temperature and with a agitator, e.g. B. a flat blade stirrer, is provided to ensure careful distribution to ensure the gaseous phosgene. The reaction vessel should also have an under inlet pipe reaching the surface of the liquid for the introduction of the gas and have a liquid delivery approach. The reflux apparatus and the Discharge lines for the vapors from the reaction vessel can be of the usual type.

According to the invention there is provided a solvent with a boiling range from Z. B. introduced about 350 to 400 ° into the reaction vessel and to several degrees heated above the boiling point of the aromatic isocyanate to be produced. One a Solvent for the template containing aromatic isocyanate is provided, which on one to prevent the formation of the corresponding carbamyl chloride sufficiently high temperature is maintained. At this temperature is the solubility of phosgene and hydrogen chloride in the solvent are minimal.

The amine to be reacted is chlorinated in a lower boiling point aromatic solvent, which is suitably the same as that in the template, dissolved and placed in the reaction vessel below the level of the liquid contained therein introduced. At the same time, a phosgene stream is under the surface of the reaction mass initiated. The aromatic isocyanate is formed with the formation of hydrogen chloride. Since the temperature of the reaction mass is above the boiling point of the isocyanate, it evaporates the latter immediately and arrives with the hydrogen chloride and the excess Phosgene in the template. The isocyanate condenses in the vapor and liquid of the solvent in the template, and hydrogen chloride and phosgene from the Exit reflux condenser are passed into a washing device.

In the preferred embodiment of the invention, that boils for that Amine used solvents also below the temperature of the reaction vessel, so that it also distilled over into the template and not in the reaction vessel accumulates. This method has the obvious advantage that the isocyanate is immediately removed the reaction zone is removed so that it has no opportunity to use it as the starting material amine used to react. The conversion takes place when simple amines are used in the corresponding isocyanate practically quantitatively. Even with the diisocyanates the yields are very high, namely 20 to 25% higher than those mentioned above previous procedure.

The appropriate solvent for the reaction is. a high boiling one cyclic hydrocarbon fraction or a chlorinated aromatic hydrocarbon fraction used. Suitable are e.g. B. higher boiling naphthenic or aromatic hydrocarbons. The solvent used must be sensitive to the reactants and the reaction products be inert. The boiling point of the solvent depends on the boiling point of the to be formed Isocyanate and the temperature used. The boiling point of the solvent only needs to be about 20 to 50 ° higher than the reaction temperature. He can of course also be higher. In general, the boiling point will not be below about 180 ° lie. Certain fractions of hydrocarbons from petroleum or other sources, z. B. naphthenic fractions are suitable. Directly or indirectly from petroleum or hydrocarbons obtained from coal tar, e.g. B. p-diphenylbenzene, diphenylmethane and triphenylmethane, are also suitable typical aromatic hydrocarbons. The aromatic hydrocarbons can be chlorinated to increase their boiling points will. Such chlorinated aromatic hydrocarbons are very useful. Of the The degree of chlorination is not important. There is also one major condition the boiling range in that the solvent with phosgene, hydrogen chloride, the Amine and the isocyanate essentially. does not react and also at the reaction temperature is thermally stable. Preferred chlorinated hydrocarbons are the chlorinated ones Diphenyls and chlorinated benzenes. Other suitable hydrocarbons are higher-boiling naphthenic fractions from asphaltic crude oils. at the production of low-boiling isocyanates, e.g. B. of phenyl isocyanate, simple chlorinated hydrocarbons such as trichlorobenzene can be used as solvents. As a solvent for the amine and for taking up the aromatic isocyanate formed can also be a chlorinated aromatic hydrocarbon, but usually with lower chlorine content, e.g. B. monochlorobenzene or o-dichlorobenzene is used will. This solvent should expediently boil in such a range that it distilled over with the isocyanate.

The temperature limits are not important, they are only required is that the temperature of the solvent in the reaction mass is a few degrees is higher than the boiling point of the isocyanate to be produced. Higher temperatures are not useful because they can give rise to undesirable side reactions. A practical one The range is between 1 and 20 ° above the boiling point of the isocyanates. if of course the temperature comes too close to the boiling point of the solvent, distilled too much of the solvent in the reaction medium is transferred into the template. Preferred Temperatures for carrying out the reaction are between about 100 and 300 ° expediently between 130 and 300 °. Higher temperatures are not practical, there then some decomposition of the isocyanate can take place. Generally determined the decomposition point of the isocyanate formed the upper temperature limit, beyond which the yields decrease sharply.

The solvent in the template is generally based on such Maintained temperature that a slight reflux takes place. It seems that the The vapor of the solvent dissolves the vapor of the aromatic isocyanate and the same then condensed in the solvent. Hence, this should have dissolving effects as well The boiling point of the solvent must be carefully observed to prevent the formation of an aromatic Carbamyl chloride by reaction of the isocyanate with hydrogen chloride. After this all the amine has been added to the reaction solution will be a short time further phosgene introduced to the reaction with any unreacted amine to complete. Dry nitrogen is then used to remove the whole dissolved hydrogen chloride and phosgene passed through the system. On it will the isocyanate by distilling off the solvent and fractionating the aromatic Isocyanate obtained. The reaction is usually carried out at atmospheric pressure. However, the distillation is intended to lower the boiling points and to reduce the decomposition Tending Isocy anate can be carried out under reduced pressure.

The following examples illustrate the invention. Example I In a with a stirrer, a thermometer, a gas feed, a liquid feed and a heated reaction vessel provided with a condenser became 450 parts of a introduced chlorinated polyphenyls with a boiling range of 385 to 420 °. the Temperature was set at 260 °. Due to the under the surface of the chlorinated Polyphenyls reaching liquid feed then became a solution of 16 parts 2, 4-Tolylenediamine introduced in 260 parts of o-dichlorobenzene. This lasted 50 minutes and the reaction mass was stirred, during which time the temperature rose 255 to 265 ° was held. At the same time one passed through during the 50 minutes the gas inlet pipe, which also reached below the surface of the liquid, 60 parts of phosgene into the reaction mass at a constant rate.

The 2,4-tolylene diisocyanate formed distilled in proportion to it Formation from the reaction mass together with the o-dichlorobenzene and was in condensed and collected a template, the temperature of which was kept at 180 to 185 ° to prevent the formation of the corresponding carbamyl chloride. The during The hydrogen chloride formed in the reaction went with the diisocyanate and the o-dichlorobenzene and could escape through a reflux condenser located on the template. The temperature of 180 to 185 ° in the template was sufficient for a slight reflux to maintain, thereby increasing the solubility of the hydrogen chloride in the solution was kept to a minimum.

After all of the 2,4-tolylenediamine had been added, a further 20 Minutes phosgene introduced, during which time the temperature of the reaction mass was held at 255 to 265 °. When the introduction of phosgene was complete, it became nitrogen passed through the reflux solution in the template to about any still contained therein Remove hydrogen chloride and phosgene. Then the o-dichlorobenzene from the Original distilled off, and the 2, 4-tolylene diisocyanate obtained was at 4 mm Hg fractionated at 80 to 85 °. The yield was 20.5 parts or 88.5% of theory.

Example II Using the equipment of Example I, 200 parts of trichlorobenzene and 50 parts of monochlorobenzene were introduced into the reaction flask. 200 parts of monochlorobenzene were added to the template. The system was made by heating to a little over 100 ° while passing through a stream of dry nitrogen and expelling dried by traces of water. Then the reaction vessel was heated to 180 °, and phosgene was passed for 15 minutes at a rate of 50 parts per hour through. Then one passed through the under the surface of the reaction mass sufficient liquid feed for 2.75 hours a solution of 45 parts of aniline in 290 parts of dry monochlorobenzene at a steady rate. Phosgene was further introduced and the temperature was kept at 180 to 190 °. It made up phenyl isocyanate and distilled together with monochlorobenzene and a little trichlorobenzene into the template held at 130 °. After all the aniline has been added, the temperature was increased to 212 ° over 40 minutes.

The phosgene flow was interrupted and 30 minutes were started Remove hydrogen chloride and phosgene nitrogen by gently refluxing liquid in the template. 56.5 parts of phenyl isocyanate were obtained from the initial charge, which corresponds to a yield of 98% of theory.

In the above example, p-chloroaniline can be used in place of the aniline can be used to form p-chlorophenyl isocyanate. Example III It became the Setup from Example I used. The reaction flask was filled with 450 parts of a commercial chlorinated polyphenyls with a boiling range of 400 to 430 ° loaded. 100 parts of o-dichlorobenzene were added to the template. The reaction solution was on while passing in 50 parts of phosgene per hour for about 30 minutes Heated to 250 °. A solution of 22 was then passed through the liquid feed line Parts of m-phenylenediamine in 258 parts of o-dichlorobenzene over 2 hours. Of the Phosgene flow was at a rate of 100 parts per hour and the reaction temperature held at 245 to 250 °. The template was at a temperature of 180 to 185 ° held to achieve gentle reflux of the solvent. The m-phenylene diisocyanate distilled over into the receiver together with some o-dichlorobenzene.

During the addition of the amine solution, it was noticed that the liquid inlet pipe a little reddish black granular precipitate settled. After the addition the solution was therefore a second solution of 22 parts of m-phenylenediamine in 200 Parts of o-dichlorobenzene with the addition of 22 parts of the above-mentioned chlorinated Polyphenyls prepared and kept in the reaction flask for 1.5 hours initiated at a temperature of 240 to 245 °. The speed of phosgene discharge stayed at 100 parts per hour. Formed at the end of the liquid inlet pipe no precipitate.

Phosgene was then passed in for a further 45 minutes. By in easy Refluxing liquid in the receiver was passed for 30 minutes for removal of hydrogen chloride and phosgene nitrogen through it. The solvent was at 50 mm Ilg was distilled off, and the crude m-phenylene diisocyanate was obtained fractionated by 46 parts of m-phenylene diisocyanate or a yield of 71% of theory. Its boiling point at 8 mm was 95 to 100 °. Example IV The reaction flask of Apparatus of Example I was washed with 300 parts of a strongly acidic naphthenic Hydrocarbon fraction with a Saybolt universal viscosity of 353 seconds at. 38 °, a specific gravity of 0.8799 at 16 ° and a Distillation range from 151 to 280 ° charged at a pressure of 5 mm. In the Submission was brought to 200 parts of o-dichlorobenzene. The system was started with dry sticksta-ff cleaned, whereupon the reaction flask gradually to 260 ° while passing a slow phosgene was heated to saturation of the system. The template was held at 180 to 185 ° during this time. Then the phosgene flow was increased and kept at 100 parts per hour. At the same time, one passed into the one in the reaction flask Oil contained a solution of 50 parts of m-tolylenediamine and 30 parts of the above mentioned naphthenic oil, dissolved in 500 parts of o-dichlorobenzene, with a uniform Speed for 2.25 hours. The temperature of the reaction flask was held at 260 to 275 ° during this time. After all the amine had been added another 20 minutes phosgene passed through the reaction flask and the temperature held at 280 ° to remove the remaining isocyanate. Then nitrogen was passed in through the template to remove dissolved phosgene and hydrogen chloride. The o-dichlorobenzene was distilled off from the product contained in the template. 65 parts were obtained 2,4-tolylene diisocyanate, which corresponds to a yield of 89% of theory.

While in the preceding examples aromatic amines such as 2, 4-tolylenediamine, Aniline, m-phenylenediamine and chloroaniline can of course be any primary aromatic amine, e.g. B. o- or p = phenylenediamine, o-, m- or p-aminotoluene or other tolylenediamines can be used. In general, amines are which Isocyanates boiling below about 300 ° at atmospheric pressure are suitable. she may contain alkyl or halogen substituents in the aromatic ring. With the isocyanate group reactive groups should be avoided, however. B. hydroxyl, mercapto, Carboxyl, carbonamide or sulfonamide groups, etc. When producing the higher-boiling Isocyanates can be operated under reduced pressure, in which case of course then a high-boiling solvent "is required.

The solution of the amine introduced should contain 5 to 30% of the amine. If the concentration of the amine greatly exceeds 30%, it is difficult enough Get phosgene in solution to be sure that the aniine is with the phosgene and does not react with the isocyanate. When the amine concentration in the solution increases is high, an insoluble compound of unknown composition is also formed. This compound contains a large percentage of non-hydrolyzable chlorine and does not decompose to form an isocyanate. So it is neither that Amine hydrochloride nor a carbamyl chloride. At concentrations below 51 / o this is Process can also be carried out, but economically large amounts of solvent are required. In general, amine solutions are expediently used which are 5 to 20 percent by weight of the amine, the preferred range being 8 to 12 percent by weight. The amine solution can be preheated prior to being placed in the reaction vessel. That is particularly advantageous if the volume of solution added in a certain time a substantial part of the volume of the boiling solution in the reaction vessel matters.

The phosgene must be in a stoichiometric excess of at least 50% with respect to that theoretically necessary for a complete reaction with the amine Amount to be used. Part of the amine reacts with smaller amounts of phosgene with the isocyanate to form tarry precipitates, as a result of which the yield decreases. The phosgene is preferably in an 80 to 100% stoichiometric excess available. The use of larger amounts has no harmful effect other than additional costs and the need to recycle the surplus. The phosgene excess can be recovered and recycled.

It is completely surprising that when using the invention high temperatures in the liquid phase high yields can be obtained because one it has hitherto been considered necessary to prevent the reaction from forming by-products to start at a low temperature. It is also surprising that an implementation between the amine and the isocyanate already formed and present in the solution now takes place on a relatively small scale. That seems to be special solvent used, the temperature at which the reaction was carried out and to be due to the considerable excess of phosgene.

Claims (3)

PATENT CLAIMS: 1. Process for the continuous production of aromatic Isocyanates, characterized in that one is in an inert liquid with higher boiling point than the isocyanate to be formed, dissolved or suspended aromatic Brings amine with phosgene to the reaction, at a temperature above the Boiling point of the isocyanate formed and the decomposition point of the intermediate product formed carbamyl chloride, but below the boiling point of the inert liquid, and that the aromatic isocyanate is distilled off as it is formed. 2. Procedure according to claim 1, characterized in that the reaction is carried out at a temperature between 130 and 300 °. 3. The method according to claim 1, characterized in that that the phosgene is fed in at the same time as a solution of the amine, namely in an excess of at least 50% based on the amount equivalent to the amine. Considered publications: German Patent Specifications No. 848 810, 870 847.