CONTINUOUS PROCESS FOR THE SYNTHESIS OF AROMATIC URETHANES

Abstract: Process for the preparation of aromatic urethanes consisting in reacting organic carbonates with aromatic amines in reactors operating in continuous, feeding a total quantity of aromatic amines in a percentage, with respect to the weight of the total mixture, ranging from 4 to 30, and maintening a low concentration of amine in the reactor.

Description: CONTINUOUS PROCESS FOR THE SYNTHESIS OF AROMATIC URETHANES The present invention relates to a process for the preparation of aromatic urethanes which consists in reacting organic carbonates with aromatic amines, in reactors operating in continuous, capa- ble of maintaining a suitable concentration of a ine in the reaction mixture. Aromatic urethanes are valuable intermediates used for the production of phyto-drugs, dyes, pharmaceutical compounds, and also aromatic isocyanates used in the synthesis of polyurethanes: among aro ¬ matic urethanes, those of greatest interest from an industrial point of view are 4,4'- methylenediphenyldiurethane (MDU) and toluenediure- thane (TDU) used for the preparation of methylene- diphenyl diisocyanate (MDI) and toluene diisocya- nate ( TDI ) . Processes are known for the production of urethanes based on the functionalization of amines with an organic carbonate, preferably dimethylcar- bonate (DMC) , in the presence of suitable catalysts according to the following reaction scheme: cat R-NH 2 + (CH 3 0) 2 CO R-NH COOCH3+CH 3 OH For example, Italian patent 1,141,960 de- scribes a process for the preparation of aromatic urethanes by means of the reaction between aromatic amines and alkyl carbonates, said reaction taking place in the presence of a catalyst consisting of an alcoholate of an alkaline or earth-alkaline metal. The reaction is carried out in liquid phase in the presence of a molar ratio carbonate/amine varying from 10/1 to 1/1. Italian patent 1,229,144 describes a process for the preparation of carbamates which comprises the initial reaction between alkyl or cyclo-alkyl carbonate, in a quantity equal to or higher than the stoichiometric value, and an amine, causing the formation of a mixture of carbamate and urea, and the subsequent reaction of the urea thus formed with carbonate: carbamation catalysts consisting of Lewis acids such as bivalent tin and zinc chlorides, or salts of said metals with mono-or bi- carboxylic organic acids, or again trivalent fluorine compounds such as iron chloride and acetylace- tonate, are used for the purpose. The processes for the preparation of aromatic urethanes according to the two patents mentioned above, are effected so as to minimize the formation of by-products typical of the reaction between or- ganic carbonates and aromatic amines: ureas and N- methylated compounds. It is known in fact that ureas are formed by the reaction of an amine group, which has not yet reacted, with a urethane group of another molecule and which may however be converted to urethanes in a subsequent finishing phase, which in fact reduces the damage. N-methylated products on the other hand, are extremely harmful as, in addition to reducing the yield to urethane in the synthesis, they signifi- cantly lower the yield in the subsequent thermal decomposition reaction of urethane to the corre ¬ sponding isocyanate: in fact the N-methylate group is capable of reacting with the isocyanate already formed producing compounds of a pitchy nature. According to the known art, of which the two above patents represent an example, it is therefore possible to limit the formation of these byproducts by resorting to the use of particular catalytic systems, or by carrying out the process in different reactive steps. The Applicant has now found that the formation of the above by-products, in particular N- methylated compounds, can be further reduced by effecting the synthesis of aromatic urethanes in con- tinuous reactors allowing low concentrations of aromatic amine to be used. Without any intention of explaining the process mechanisms, it has in fact been found that the formation of N-methylated products is influenced by the concentration of amine present in the reaction environment: when operating in batch reactors, into which all of the amine is charged in the initial phase, its concentration is high and this leads to the formation of significant quantities of N- methylated by-products; when operating, on the contrary, in suitable continuous reactors, it is possible to keep a low concentration of the amine in the reaction environment and reduce the formation of N-methylated compounds to well below 1% of the total reacted amine groups. An object of the present invention according to the present patent application therefore relates to a process for the preparation of aromatic urethanes which consists in reacting, in continuous, organic carbonates with aromatic amines maintaining low concentrations of the latter. Reactors suitable for operating in continuous can be advantageously used for the purpose, such as for example reactors of the CSTR type, single or in series, or of the reactive column type. The aromatic amines which can be used are represented by formula (I) : R-(NH 2 ) n (I) wherein n is an integer from 1 to 2, R represents an aryl radical, such as monovalent, bivalent radicals of benzene, toluene, naphthalene, diphenyl, methylene-diphenyl . The aryl radical may contain, as substituents, atoms or radicals non-reactive with the isocyanate function, such as halogen atoms, alkoxy, nitro, cyano, acyl, acyloxy, isocyanate groups. Non-limiting examples of aromatic amines having formula (I) are: 2, 4-diaminotoluene, 2,6- diaminotoluene or mixtures of the two isomers, ani- line, toluidine, 3, 5-dichloroaniline, 4,4'- methylenedianiline, 2, 4 ' -methyle-nedianiline, 2,2'- methylenedianiline or mixtures of isomers. Catalysts suitable for the purposes of the present invention generally consist of Lewis acids such as salts of tin, copper and zinc, or salts of said metals with mono- or bi-carboxylic organic acids. These salts have a pka equal to or higher than 2.8. Among zinc compounds, anhydrous acetate or di- hydrate is preferably used. The reactions comprise the use of organic carbonate as reagent and solvent at the same time and the removal of the alcohol by-product as it forms; the total quantity of aromatic amine which is fed in continuous to the reaction ranges from 4 to 30% by weight with respect to the total mixture. According to the present invention, the reaction is carried out in suitable reactors, operating in continuous in order to maintain the concentration of the aromatic amine preferably within a value rang- ing from 0.1 to 5% by weight with respect to the total mixture, and, in particular, from 0.3 to 2.5% by weight, so as to minimize the formation of byproducts of an N-methylated nature. The excess carbonate used in the reaction is separated from the raw product, according to methods well known to ex- perts in the field. The process according to the present invention is now described in greater detail with reference to the synthesis of toluenediurethane (TDU) , owing to the great industrial interest in this compound: this example is naturally only provided for illustrative purposes and in no way limits the scope of the present invention. The following indications will provide experts in the field with the necessary disclosures for the preparation of a wide range of urethanes. EXAMPLE 1: CSTR SYNTHESIS IN SERIES With respect to the continuous synthesis of TDU in CSTR-type reactors, the transformation reac- tion of toluenedia ine (TDA 80/20, mixture of 2,4/2,6 isomers in a proportion of 80/20) into the corresponding mixture of urethanes can be effected in a continuous plant, for example consisting of 4 CSTR-type reactors in series, each of which having a useful volume of 0.5 liters, in which the amine is transformed into a mixture of TDU (toluenediurethane) /TMU (toluenemonourethane) /ureas and a final 5-liter reactor in which the complete conversion to TDU is effected. Each CSTR is equipped with a heating jacket, differential pressure level control, removal system of the methanol/DMC vapor phase and a reintegration system of the vaporized DMC and the reactors are connected to each other by means of a line equipped with a level control regulated valve. The CSTRs are initially pressurized with nitrogen, by means of a suitable pressure regulation valve system. During the reaction, the pressure of the vapor phase at the head of each single CSTR is kept constant by means of automatic regulation flow valves . The plant is completed by automatic sampling systems at the outlet of each CSTR, a collection tank of the evaporated phase after condensation and a feeding tank of the reagent mixture to the first CSTR. All the process lines are electrically heated and insulated to avoid any possible precipitations of the mixtures of products. The internal temperature of the reagent mix- ture is kept constant at 160°C, and the pressure at 6 ata. The temperature, preselected at a certain value, is kept constant for the whole duration of the test. The mixture of TDU (toluenediurethane) /TMU (toluenemonourethane) /ureas leaving the CSTR is fi- nally sent to a stirred reactor (finisher) having a volume of 5 1, thus capable of allowing residence times of about 2-3 hours, and is transformed to TDU. The reactor is kept at a temperature ranging from 160 to 180°C, preferably about 170°C. The pressure is maintained at values not exceeding 9 ata. Using the system described above, a test was effected, continuously removing the methanol/DMC phase in each single CSTR. An 80/20 mixture of TDA (equal to 88 g/L in DMC) and 4.7 g/L of zinc acetate dihydrate (equal to 3% moles of catalyst per mole of TDA) in DMC, is fed to the first CSTR. The residence time in each CSTR is 15 minutes. A sample is taken from the 1 st CSTR, under regime conditions, determining from HPLC analysis a 51.8% conversion of the amine groups to products such as TDU, monourethanes and ureas. The concentration of non-reacted TDA proved to be equal to 11.4 g/L or equal to 12.9% of the initial TDA. The N-methylated products present in the reaction are equal to 1.5% of the converted amine groups. This value corresponds to a ratio of N- methylated compounds with respect to the total con- verted compounds equal to 0.9. The reaction mixture, leaving the 4 th CSTR is subsequently sent to the finisher and maintained at 170°C for 3 hours to give an 80/20 TDU product hav- ing a chemical purity equal to 95%. The percentage of N-methylated products proved to be equal to 0.7% of the converted amine groups. EXAMPLE 2: SYNTHESIS IN CSTR IN SERIES Using the same system described above, a test was effected operating in the presence of a removal of the methanol/DMC phase in each single CSTR. An 80/20 mixture of TDA (equal to 60 g/L in DMC) and 3.24 g/L of zinc acetate dihydrate (equal to 3% moles of catalyst per mole of TDA) in DMC, is fed to the first CSTR. The residence time in each CSTR is 15 minutes. A sample is taken from the 1 st CSTR under regime conditions, determining from HPLC analysis a 61.4% conversion of the amine groups to products such as TDU, onourethanes and ureas. The concentration of non-reacted TDA proved to be equal to 5.12 g/L or equal to 8.53% of the initial TDA. This value corresponds to a ratio of N- methylated compounds with respect to the total con- verted compounds equal to 0.65. The reaction mixture is subsequently sent to the finisher and maintained at 170°C for 3 hours to give an 80/20 TDU product having a chemical purity equal to 95.5%. The percentage of N-methylated products proved to be equal to 0.55% of the converted amine groups. EXAMPLE 3: PLANT DESCRIPTION FOR THE CONTINUOUS SYNTHESIS OF TDU IN A REACTIVE COLUMN. The reaction for the transformation of TDA 80/20 into the corresponding mixture of urethanes is carried out in a continuous plant. The plant consists of a column into which the reaction mixture is fed from above and is put in contact in countercurrent with the DMC vapors ris- ing from below, in order to create an equilibrium which allows the methanol to be removed from the reaction. The liquid leaving the bottom of the column enters a reboiler in which DMC vapors are gener- ated, by means of an external heat source, which are fed from the bottom of the column. The amine is transformed in the liquid leaving the reboiler into a mixture of TDU (toluenediure- thane) /TMU (toluenemonourethane) /ureas. The complete conversion to TDU is effected in a final reactor situated after the reboiler. The removal of the DMC/methanol vapor phase is effected at the head of the column by means of an automatic regulation flow valve which allows the reaction system to be maintained at the desired pressure . The reboiler at the bottom of the column is equipped with a heat flow control system to check the quantity of DMC vapor produced. The same reboiler, which also acts as reaction step, is equipped with automatic level control which activates the removal valve of the liquid phase . The plant is completed by automatic sampling systems at the outlet of the reboiler, a collection tank of the evaporated phase after condensation, and a feeding tank of the reagent mixture. All the process lines are electrically heated and insulated to avoid any possible precipitations of the mix- tures of products. The internal temperature of the reagent mixture is set at 160°C, and the pressure at 6 ata. The column consists of about 20 theoretical plates, allowing an overall residence time of the liquid phase of 1-3 hours. The reboiler has a volume corresponding to a residence time of 30 minutes. The mixture of TDU/TMU/ureas leaving the reboiler is finally sent to a reactor capable of al- lowing residence times of about 2-3 hours, and is transformed into TDU. The reactor is kept at a temperature ranging from 160 to 180°C, preferably 170°C. The pressure is maintained at values not exceeding 9 ata. Using the system described above, a test was carried out feeding the column from above with a solution, preheated to 85°C, of TDA 80/20 (equal to 40 g/L in DMC) and 3.1 g/L of zinc acetate dihydrate (equal to 3% moles of catalyst per mole of TDA) in DMC. The residence time for each single step is 6 minutes equal to a total of 2 hours whereas the mixture is kept for 30 minutes in the reboiler. The distillation degree of the vapors in coun- tercurrent inside the column is equal to 0.6 V dis- tillate/V fed. The mixture leaving the bottom of the column, consisting of TDU (toluenediurethane) /TMU (toluene- monourethane) / ureas (about 190 g/L) is kept for 30 minutes in the boiler. A sample is taken at the outlet of the reboiler under regime conditions, determining from HPLC analysis a 96.5% conversion of the amine groups to products such as TDU, monourethanes and ureas . The TDA has completely disappeared. The N- methylated products present in the reactions are equal to 0.93% of the converted amine groups. This value corresponds to a ratio of N-methylated com- pounds with respect to the total converted compounds equal to 0.96%. The reaction mixture is subsequently sent to the finisher and maintained at 170°C for 3 hours to give an 80/20 TDU product having a chemical purity equal to 96.5%. The percentage of N-methylated products proved to be equal to 0.9% of the converted amine groups. COMPARATIVE EXAMPLE 4 165 g of TDA 80:20, 1600 g of DMC and 8.9 g of zinc acetate dihydrate (3% in moles with respect to the TDA) were charged under a nitrogen atmosphere into a cylindrical autoclave having a useful volume of 3 liters. The autoclave was then heated to 160°C for 1.5 hours under stirring (300 rpm) , continu- ously removing the methanol/DMC mixture by distil- lation. After 1.5 hours of reaction, the distillation was interrupted and the temperature inside the reactor was brought to 170°C for 3 hours (finishing phase) . Upon HPLC analysis of the reaction, it was found that the conversion of the initial TDA was 99% with a selectivity to TDU equal to 94%. The N- ethylated products corresponded to 3.2% with respect to the converted amine groups . COMPARATIVE EXAMPLE 5 400 ml of DMC and 75 g of TDA 80:20 were charged into a steel autoclave having a volume of 1000 ml. The autoclave was heated to 160°C under stirring (300 rp ) in a nitrogen atmosphere. When the operating temperature had been reached, zinc acetate hydrate (3.94 g, equal to 3% in moles with respect to the TDA) in 100 ml of DMC was charged into the autoclave under pressure and heat. After 15 minutes a sample of the reaction mixture was taken. HPLC analysis showed that the non-reacted TDA was equal to 75% of the initial quantity, corresponding to a concentration of 87 g/L. The N-methylated products present in the reaction proved to be 12.5% with respect to the con- verted amine groups. COMPARATIVE EXAMPLE 6 400 ml of anhydrous DMC and 75 g of TDA 80:20 were charged into a steel autoclave having a volume of 1000 ml. After stabilization of the operating conditions (temperature 160°C, stirring at 300 rpm in an nitrogen atmosphere), 3.94 g of zinc acetate hydrate (3% in moles with respect to the TDA) in 100 ml of DMC were charged into the autoclave under pressure and heat. A sample of the reaction mixture was taken after 30 minutes. An analysis of the re ¬ action products was carried out by means of HPLC. It was found that 88% of the TDA had reacted (remaining TDA equal to 12% of the initial TDA, corre ¬ sponding to a concentration of 17.5 g/L) . The quan- tity of N-methylated derivatives with respect to the converted amine groups was 5.2%.

Claims: CLAIMS 1. A process for the preparation of aromatic urethanes consisting in reacting organic carbonates with aromatic amines in reactors operat- ing in continuous and feeding a total quantity of aromatic amines in a percentage with respect to the weight of the total mixture ranging from 4 to 30. 2. The process for the preparation of aromatic urethanes according to the previous claim, wherein the reaction between organic carbonates and aromatic amines is carried out in reactors in continuous, the concentration of the aromatic amine being maintained at 0.1 to 5% by weight with respect to the total mixture. 3. The process for the preparation of aromatic urethanes according to the previous claim, wherein the concentration of aromatic amine is preferably maintained at 0.1 to 2.5% with re- spect to the total mixture. 4. The process for the preparation of aromatic urethanes consisting in reacting organic car ¬ bonates with aromatic amines according to each of the previous claims, characterized in that the reaction is carried out in reactors se- lected from those of the CSTR or reactive column type . 5. The process for the preparation of aromatic urethanes consisting in reacting organic car- bonates with aromatic amines according to one or more of the previous claims, characterized in that the reaction is carried out in the presence of a catalyst selected from Lewis acids with a pka equal to or higher than 2.8%. 6. The process for the preparation of aromatic urethanes according to the previous claim, characterized in that the reaction between organic carbonate and aromatic amine is prefera ¬ bly carried out in the presence of anhydrous zinc acetate or zinc acetate dihydrate. 7. The process for the preparation of aromatic urethanes consisting in reacting organic carbonates with aromatic amines according to one or more of the previous claims, characterized in that the reaction is carried out in the presence of organic carbonate alone acting as solvent . 8. The process for the preparation of aromatic urethanes consisting in reacting organic car- bonates with aromatic amines according to one - IS or more of the previous claims, wherein the reaction is accompanied by the removal of the alcohol, as it is formed. 9. The process for the preparation of aromatic urethanes consisting in reacting organic carbonates with aromatic amines according to one or more of the previous claims, characterized in that the reaction is carried out starting from an amine having the formula R-(NH 2 )n wherein n is 1 or 2, R represents an aryl radical selected from monovalent and bivalent radicals of benzene, toluene, naphthalene, diphenyl, methylene-diphenyl . 10. The process for the preparation of aromatic urethanes consisting in reacting organic carbonates with aromatic amines according to one or more of the previous claims, characterized in that the reaction is carried out starting from an amine preferably selected from 2,4- diaminotoluene, 2, 6-diaminotoluene or their mixtures, aniline, toluidine, 3,5- diaminotoluene, 4, -methylenedianiline, 2,4- ethylenedianiline, 2, 2-methylenedianiline or mixtures of these isomers.

Inventors: BATTISTEL EZIO, BIGNAZZI RENZO, BOSETTI ALDO, CARLETTI VITTORIO, CESTI PIETRO, BATTISTEL, EZIO, BIGNAZZI, RENZO, BOSETTI, ALDO, CARLETTI, VITTORIO, CESTI, PIETRO
Applicants: BATTISTEL EZIO, BIGNAZZI RENZO, BOSETTI ALDO, CARLETTI VITTORIO, CESTI PIETRO, ENI SPA, ENICHEM SPA, BATTISTEL, EZIO, BIGNAZZI, RENZO, BOSETTI, ALDO, CARLETTI, VITTORIO, CESTI, PIETRO, ENI S.P.A., ENICHEM S.P.A.
Application year: 2001
Main classification: C07C269
IPC codes: C07B61, C07C269, C07C271