1. Field of the Invention
This invention relates to the preparation of organic isocyanates. The organic isocyanates are prepared by thermally cleaving urethanes at temperatures of 175.degree. C. to 600.degree. C. in the presence of carbon.
2. Description of the Prior Art
It is known tha N-substituted urethanes can be thermally cleaved into isocyanates in the gas phase or in the liquid phase. The thermal cleaving is frequently accompanied by various undesirable secondary reactions. Examples of such reactions include the decarboxylation reaction of urethanes which may be accompanied by the formation of primary and secondary amines as well as of olefins. Reactions between the formed isocyanate and urethane result in allophanates and reactions with an amine result in ureas; moreover, polymerization of the isocyanates results in isocyanurates.
According to data in German published application No. 19 44 719 (British Pat. No. 1,247,451), the pyrolysis of urethanes in the vapor phase is carried out at temperatures of 400.degree. C. to 600.degree. C. in the presence of Lewis acids as catalysts with the isocyanate and the alcohol being separated by fractional condensation. Toluene diisocyanate is produced, for example, by pyrolysis of toluene-2,4-diethylurethane in the presence of iron (III)-chloride. Drawbacks of the reaction include low yields combined with considerable amounts of a polymeric by-product, decomposition of the catalyst, and corrosion of the reaction equipment. German published application No. 24 10 505 (U.S. Pat. No. 3,870,739) describes a process wherein the urethane is cleaved at a temperature of 350.degree. C. to 550.degree. C. at a pressure of less than (m+1) times that of the isocyanate vapor pressure in a catalyst-free pyrolysis zone within 15 seconds. One of the drawbacks of this method is that a large amount of heat required for the endothermal cleaving must be made available to the powdered urethane within a very short period of time. Moreover, a solid polymer is incurred as a by-product and the separation of this by-product renders the implementation of a continuous process more difficult.
The thermal cleaving of urethanes in the liquid phase is described, for example, in German application No. 24 21 503 (U.S. Pat. No. 3,962,302) and German application No. 25 30 001 (U.S. Pat. No. 3,919,280). According to data in German application No. 24 21 503, the urethanes are dissolved in an inert solvent such as alkylbenzenes, linear and cyclic hydrocarbons and/or phthalates and are cleaved at temperatures of 175.degree. C. to 350.degree. C. under normal or increased pressure. The resultant isocyanate and alcohol are isolated and separated with the aid of the solvent as a carrier and/or by using an inert gas as carrier. According to German Application No. 25 30 001, higher molecular optionally substituted aliphatic, cycloaliphatic or aromatic hydrocarbons, ethers, esters or keltones are used as reaction media. Distillation is utilized for separating the cleaving products with isocyanate alcohol and carrier material being removed by distillation overhead whereas the reaction medium remains as bottom fraction.
For the preparation of aromatic isocyanates, the urethanes, according to German published application No. 26 35 490 (U.S. Pat. No. 4,081, 472) , are brought in contact with a solution of at least one metal ion such as ions of copper, zinc, aluminum, tin, titanium, vanadium, iron, cobalt and nickel as catalyst, dissolved in a solvent having a boiling point of 200.degree. C. in a metal concentration of at least 0.001 percent by weight based on the solvent at temperatures of 150.degree. C. to 300.degree. C. under reduced pressure. The resultant cleaved products are separated by fractional condensation. However, this results in small amounts of non-distillable polymerization products formed during the reaction remaining in the solvent residue containing the catalyst. After a certain amount of time, this necessitates additional cleaning operations of the solvents containing the catalytic metal ions.
According to German Published Application No.29 42 543, very good cleaving results are obtained if the urethanes are cleaved on catalytically effective, high surface metals which are present in the heterogeneous phase. The drawback of this process is that the metals used as catalysts lose their catalytic activity in time as a result of being coated, thereby, also necessitating additional cleaning operations.