PROCESS FOR PREPARING METHYL ISOCYANATE

Abstract: A process for preparing methyl isocyanate by heating phenyl N-methylcarbamate in a decomposition zone to produce a vapor stream containing phenol and methyl isocyanate, and thereafter rapidly separating the methyl isocyanate from the phenol to inhibit formation of the starting compound phenyl N-methylcarbamate.

Description: PROCESS FOR PREPARING METHYL ISOCYANATE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing methyl isocyanate. Description of the Prior Art The production of alkyl isocyanates by the pyrolysis of alkyl or aryl carbamates is well known (see for example Ann 562 205 (1949) and Methoden der Orgeneschem Chemie, Vol. 8, 126 (1952)). In general, the conventional processes involve decomposing an alkyl or aryl carbamate at elevated temperatures to produce the isocyanate together with either an alcohol or phenol as a by-product, as illustrated in the following equation: 0 II ROCNHR' » R'NCO + ROH Δ wherein R and R 1 are alkyl or aryl. Although the conventional methods are effective, they usually afford only mediocre yields of isocyanates. Aside from this drawback, some disclosed processes also require particular operating conditions, or the use of a particular reactant or inert substance during the pyrolysis operation for the production of the desired isocyanate in acceptable quantities. For example, as- described in U.S. Patent 4,514,339 issued to Ugo Bonand et.al. on April 30, 1985, the thermal decomposition of phenyl N-alkylcarbamates (also known as phenyl N-alkyl urethanes) must be conducted in the presence of an added amount of phenol or using a reaction mixture having a molar ratio of phenol to phenyl N-methyl urethane not lower than 1:1 throughout the thermal decomposition period. As another example, the use of high-boiling inert solvent for the thermal decomposition of aryl N-alkylcarbamates is disclosed in U.S. Patent 3,919,278 issued on November 11, 1975 to Rudolph Rosenthal. According to the patent disclosure, isocyanates are produced by thermally decomposing esters of carbamic acids (i.e., urethanes or alkyl carbamates) dissolved in a suitable inert solvent and the corresponding isocyanate and alcohol are recovered separately. A further process for preparing isocyanate is described in U.S. Patent 4»123,450 issued to Harry W. Weber, Jr., on October 31, 1978. According to the disclosure, alk lisocyanates are prepared by reacting a phenol or substituted phenol and phosgene in dichloromethane with aqueous caustic solution to produce a chloroformate derivative. The chloroformate is then reacted with aqueous alkylamine to yield an N-alkylcarbamate compound which, after the solvent is stripped, is pyrolized to yield the alkyl isocyanate. More recently, U.S Patent 4,195,031 issued March 25, 1980 discloses a process for the continuous production of monoisocyanate. The patent discloses the use of a packed column in the process for decomposing aryl methylcarbamate in the generation of methyl isocyanate. The process however utilizes a high-boiling organic solvent to aid the decomposition process. Unfortunately however, these prior art processes are not entirely satisfactory from a practical -commercial standpoint. In the first place, the use of an added amount of phenol or inert solvent reduces the yields of the desired isocyanate. Additionally, the disclosed processes require extra raw materials and material-handling steps. The present invention is particularly suitable for the production of methyl isocyanate. As is known, methyl isocyanate (MIC) is an important carbamoylating agent for the manufacture of a variety of methyl carbamate pesticides. Although this compound can be synthesized from phosgene and methylamine-in specially designed equipment under certain conditions, shipping MIC from one place to another is undesirable because of its hazardous properties. According to the present invention, MIC can be generated easily in high yields (greater than 85%) by thermal decomposition of phenyl N-methylcarbamate, which is a relatively non-toxic compound. By employing the method of the present invention, MIC can be produced on-site as needed thus eliminating the handling and shipping of this highly toxic material. SUMMARY OF THE INVENTION Broadly contemplated, the present invention provides a process for the preparation of methyl isocyanate which comprises heating a phenyl N-methylcarbamate compound in a decomposition zone said phenyl N-methylcarbamate being represented by the formula: wherein n is an integer of 0 to 5 and each R is individually an alkyl of 1 to 5 carbon atoms, to form methyl isocyanate and a phenol corresponding to the phenolic moiety of the said phenyl N-methylcarbamate as a by-product and simultaneously or immediately thereafter rapidly separating said methyl isocyanate from said phenol to inhibit reformation of said phenyl N-methylcarbamate compounds. Examples of useful phenolic moieties in the above formula are 4-methylphenyl, 2-isopropylphenyl, 2-(l-methylpropyl)ρhenyl, 3-ethylphenyl and the like. The preferred starting compound is phenyl N-methylcarbamate which decomposes to form phenol and methyl isocyanate. Brief Description of the Drawings Fig. 1 is diagram of the apparatus employed for the cracking operation. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to an embodiment of the process of the present invention, the decomposition of phenyl N-methylcarbamate is conducted in the absence of any inert solvent or any substance which adversely affects the decomposition or the recovery of reaction products. In general the temperature and pressure conditions are such as to remove by vaporization, from the reaction environment both the phenol and the methyl isocyanate as they are being formed. An important aspect of the present invention is that the vaporized phenol and methyl isocyanate must be rapidly separated in order to inhibit formation of the starting material. In addition, a sufficient residence time must be also provided for the decomposition of the phenyl N-methylcarbamate. It has been found that the use of a packed column directly above the reaction vessel will provide the necessary time for the decomposition reaction. The decomposition process can be carried out at atmospheric, sub-atmospheric, and super-atmospheric pressure. However, in general, slightly super-atmospheric pressures are preferred in order to provide a forward flow of the vaporized products. The decomposition reaction can be conducted at a temperature range of from about 170°C to about 270°C, preferably at a temperature range of about 190°C to 230°C. As mentioned previously, methyl isocyanate must be rapidly separated from the phenolic by-product so that little or none of the phenol remains in the decomposition zone to react with formed methyl isocyanate. Figure 1 illustrates the equipment useful for conducting the decomposition reaction and rapidly separating the phenol from the methyl isocyanate. Referring to Figure 1, there is illustrated a reactor vessel 10 preferably of glass construction and which can be equipped with agitation means and as provided by magnetic stirrer 1 . The vessel 10 can be provided with temperature indication means such as thermometer 12, and with a gas inlet 16 for introduction of an inert gas such as nitrogen. Reactor vessel 10 can be heated by heater 17 which can be any available conventional type heater adapted to heat reactor vessel 10 to the required temperature. Reactor vessel 10 is further provided with outlet 18 to which is mounted column 20 which is an insulated column packed with Raschig rings. Column 20 is wired for electrical heating (not shown) for maintaining temperatures in the column at the required ranges. Column 20 is topped with a distillation device 22 including a condenser 24 equipped with cooling means for condensing phenol which collects in collector 26 and passes through line 28 into phenol receiver 30. Vapors containing methyl isocyanate passes from condenser 24 through line 32 into cold trap 34 which is equipped with cooling means (not shown) for collecting methyl isocyanate which is directed through line 36 into receiver 38. In a typical mode of operation, a quantity of a phenyl-N-methylcarbamate compound is charged to reactor 10 and heated to temperatures of about 170°C to about 270°C under atmospheric pressure accompanied with stirring by stirrer 14 and a nitrogen gas charge which is introduced through line 16. As the thermal decomposition occurs, vapors of phenol by-product and methyl isocyanate are evolved which must be rapidly separated and recovered. The evolved vapors pass through outlet 18 through column 20 which is maintained at 90°C to about 230°C, and through distillation device 22 containing condenser 24 which is maintained at a temperature of about 60°C to about 70°C. At these temperatures the phenol is condensed and is collected in collector 26 thereafter passing through line 28 into phenol receiver 30. The vapor stream containing methyl isocyanate leaves condenser 24 through line and enters a second condenser or cold trap 34 which is maintained at temperatures sufficient to condense the methyl isocyanate from the vapor stream. Generally temperatures of about -30°C to about 0°C are suitable for this purpose. Methyl isocyanate leaves cold trap 34 through line 36 and is collected in receiver 38. According to the process of the present invention, after a period of about 2-4 hours, excellent recovery of both methyl isocyanate and phenol is achieved. The process can be carried out either as a batch process or as a.continuous process by continuously feeding of molten phenyl N-methylcarbamate into the reaction zone while maintaining decomposition conditions. The following examples will further illustrate the invention. Example I A quantity of phenyl N-methylcarbamate (151.3 g, 99% pure) was charged to a 500 ml glass vessel fitted with a thermostat-controlled heating mantle. The flask was equipped with a magnet stirrer, thermometer, nitrogen inlet and topped with an 18 inch (I.D. 1") insulated column packed with Raschig rings. The column was wired for electrical heating and was kept at 90-170°C during the decomposition reaction. The column was topped by a distillation device with a condenser kept at 60°C for collecting phenol; at the top of the condenser was a cold trap (0°C) for the collection of methyl isocyanate. The phenyl N-methylcarbamate in the reaction flask was initially heated to about 220°C and was being stirred. The vaporized methyl isocyanate and phenol were removed separately and simultaneously as they were formed while kept at 200-217°C. The decomposition reaction was maintained in this manner until no more methyl isocyanate and phenol were distilled over, and a flask temperature of 260°C was reached. A total of 49.6 g of methyl isocyanate was collected in the cold trap. The phenol receiver recovered 71.8 g of material consisting of 95.9 percent of phenol and 4.1 percent of phenyl N-methylcarbamate. The residue left in the reaction flask was 15.1 g containing 82.1 percent of phenol, 13.8 percent of phenyl N-methylcarbamate, and 4.1 percent of unknown materials. The yield of methyl isocyanate was 88 percent based on the amount of material recovered. Example II The apparatus and procedure were as illustrated and described in U.S. Patent 4,514,339. A mixture of 56.48 g (0.37 mole) of phenyl N-methylcarbamate and 35.17 g (0.37 mole) of phenol was charged to a glass flask, which was fitted with a thermostat-controlled heating mantle and was equipped with a magnetic stirrer, thermometer, and a nitrogen inlet-tube. The flask was topped by a distillation device with a condenser maintained at 70°C for the collection of phenol and a subsequent cold trap (0°C) for collecting the methyl isocyanate. The mixture in the reaction flask was heated to boiling at 198-215°C under a nitrogen atmosphere and was being stirred. The vaporized methyl isocyanate and phenol were collected separately while keeping the flask temperature at 198-205°C. The reaction was maintained for 6 hours until no more methyl isocyanate and phenol were distilled over. 13.4 g of methyl isocyanate was collected in the cold trap. The phenol trap recovered 71 g of a mixture consisting of 92.8 percent of phenol and 7.2 percent of phenyl N-methylcarbamate. The residue left in the reaction flask was 7.1 g which included '5.1 g of condenser drainback material, and contained 72.6 percent of phenol, 3.3 percent of phenyl N-methylcarbamate, and other unidentified materials. The methyl isocyanate yield was 63.5 percent based on the amount of product recovered. Example III A quantity of 2-(l-methylpropyl)phenyl N-methylcarbamate (171.7 g, 0.82 mole) was charged to the same equipment as described in Example I. During the thermal decomposition period, the column was kept at 90-222°C while the reaction kettle was kept at 210°C to 270°C. The vaporized methyl isocyanate and 2-(l-methylpropyl)phenol were removed separately and simultaneously as they were formed. The decomposition reaction was maintained in this manner until no more methyl isocyanate and 2-(l-methylpropyl)phenol were distilled over and a kettle temperature of 270°C was reached. A total of 44.9 g of methyl isocyanate was collected in the cold trap. In the phenol receiver, 102.1 g of 2-(l-methylpropyl)phenol was recovered which consisted of 98.1 percent of residue left in the kettle was 12.5 g containing 87.5 percent of 2-(l-methylpropyl)phenol, 5.2 percent of 2-(l-methylpropyl)phenyl N-methylcarbamate and other unknown materials. The yield of methyl isocyanate was 96 percent based on the amount of material recovered.

Claims: CT/US87/00537 - 11 - WHAT IS CLAIMED IS: 1. A process for the preparation of methyl isocyanate which comprises heating phenyl N-methylcarbamate in a decomposition zone, said phenyl N-methylcarbamate being represented by the formula: wherein n is an integer of 0 to 5, and each R is individually an alkyl of 1 to 5 carbon atoms, to form methyl isocyanate and a phenol corresponding to the phenolic moiety of the said phenyl N-methylcarbamate compound as a by-product and simultaneously or immediately thereafter rapidly separating said methyl isocyanate from said phenol to inhibit reformation of said phenyl N-methylcarbamate. 2. A process according to claim 1 wherein said phenyl N-methylcarbamate compound is phenyl N-methylcarbamate. 3. A process according to claim 1 wherein the product to be thermally composed is 2-(l-methylpropyl)phenyl N-methylcarbamate. 4. A process according to claim 1 wherein vapors evolved in the decomposition zone are directed through a packed column and thence through distillation means for separately condensing phenol and methyl isocyanate vapors. 5. A process according to claim 1 wherein said phenyl N-methylcarbamate compound is heated in said decomposition zone at a temperature of from about 170°C to about 270°C. 6. A process according to claim 1 wherein said phenyl-N-methylcarbamate compound is heated in said decomposition zone at a temperature of from about 190°C to about 230°C. 7. A process according to claim 4 wherein said phenol is condensed at a temperature of from about 9Q°C to about 220°C. 8. A process according to claim 4 wherein said methyl isocyanate is condensed at a temperature of from about -30°C to about 0°C.

Inventors: CHAN JOHN KAI-FAI, CHAN, JOHN, KAI-FAI
Applicants: RHONE POULENC BV
Application year: 1987
Main classification: C07C119
IPC codes: C07C67, C07C241, C07C263, C07C265