Process for the preparation of isocyanates

Organic compounds -- part of the class 532-570 series – Organic compounds – Isocyanate esters

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C07C26300

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active

059257830

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BRIEF SUMMARY
The present invention relates to a process for the preparation of organic isocyanates by reacting the appropriate amines with phosgene.
During the preparation of organic isocyanates, the appropriate amine is reacted with phosgene, the organic isocyanate being formed with hydrogen chloride as by-product. As a rule, an excess of phosgene is used, with the result that the gas obtained as a by-product is a mixture of hydrogen chloride and phosgene.
Various methods for carrying out said process are already described in the literature. Even slight economic improvements in such an important industrial-scale process are evidently of great economic interest.
Many of the known processes have disadvantages such that, for example, they achieve high yields only if the concentrations of the reactants are kept at a low level, or they require long reaction and residence times.
It is well known to prepare organic isocyanates in two stages by reacting primary amines with an excess of phosgene at temperatures up to approx. 80.degree. C. and then subjecting the product containing carbamoyl chloride to a further treatment with phosgene at higher temperatures to form the corresponding isocyanate. On an industrial scale, the reaction of amines with phosgene is usually carried out in phosgenation towers (at atmospheric pressure or under a moderate pressure).
It is also well known (U.S. Pat. No. 28 22 373) to prepare organic isocyanates continuously by mixing a phosgene solution in a turbulent flow with a solution of an organic amine in a reactor which is operated in a closed circuit. In said process, the solution of the isocyanate in the organic solvent may be returned to the reactor in a closed circuit in order to increase the concentration of isocyanate in the solution. A disadvantage of said process is that the maximum concentration of the isocyanate in the circuit should not be substantially above 15% and the concentration of the organic amine in the solvent should be only 5 to 30%.
In DE-A 18 11 609, solutions of isocyanate in an organic solvent are recycled during phosgenation in the presence of an at least 400% excess of phosgene.
Other phosgenation reactions in a closed-circuit reactor are described, e.g. in JP-A 60/10774, in which a solution containing isocyanate is pumped round, but high yields are obtained only with amine concentrations of 5 to 10%.
In DE-A 32 12 510, a phosgenation reaction takes place in two stages in a closed-circuit reactor in an organic solvent, wherein the first stage is carried out at normal pressure or at pressures up to 10 bar (10.sup.6 Pa) above atmospheric and at temperatures of 60 to 100.degree. C. with residence times of .gtoreq.20 min, and the second stage leading to the isocyanate end product is carried out at the same pressure and temperatures of 120 to 160.degree. C. with residence times of .gtoreq.10 min.
DE-B 11 92 641 describes phosgenation reactions in the presence of the isocyanate to be prepared as solvent. When the process is carried out continuously, carbamoyl chloride is formed in the first stage of the reaction with the constant addition of phosgene, and the carbamoyl chloride is split off in a second stage. A disadvantage of said two-stage process is mainly the long reaction time until complete dissolution of the carbamic acid suspension. In continuous operation, the desired isocyanate is obtained as a distillate, i.e. only distillable isocyanates may be produced in this way. Even in batchwise operation, only distillable isocyanates are prepared in the examples.
Phosgenation reactions in the presence of an excess of organic isocyanate are also described in DE-A 22 52 068, wherein carbamoyl chloride is formed in a two-stage process from pre-heated phosgene and pre-heated amine (without solvent) at a pressure above atmospheric (approx. 100 bar) (10.sup.7 Pa). In the second stage at a lower pressure (approx. 20 bar) (2.10.sup.6 Pa) and with a further addition of phosgene/isocyanate, carbamoyl chloride undergoes thermal dissociation to the isocyanate. Further work-up takes place

REFERENCES:
patent: 2822373 (1958-02-01), Beck
patent: 3287387 (1966-11-01), Denton et al.
patent: 3781320 (1973-12-01), Irwin
patent: 3978105 (1976-08-01), Fuchs et al.
patent: 4422976 (1983-12-01), Yamamoto et al.

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