Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-04-02
2004-07-27
Parsa, J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S939000
Reexamination Certificate
active
06768032
ABSTRACT:
BACKGROUND
The present invention relates to a continuous isothermal process for preparing mononitrotoluenes using mixtures of sulfuric acid and phosphoric acid. The resultant waste acid is subjected to a concentration integrated into the process and is then recycled to the process.
Mononitrotoluenes are important intermediates for preparing optical brighteners, crop protection agents and pharmaceutical products. They can be prepared on an industrial scale, for example, by isothermal nitration of toluene. In this case, toluene is reacted with a mixture of sulfuric acid and nitric acid (mixed acid, nitrating acid) (Kirk-Othmer, Encyclopedia of Chemical Technology Vol. 17, 4th Edition 1996, “Nitration” and “Nitrobenzenes and Nitrotoluene”).
When nitrations are performed in pure sulfuric acid, customarily a ratio of o-nitrotoluene to para-nitrotoluene of approximately 1.65 is obtained (Albright & Hanson, Industrial and Laboratory Nitrations, 1976, pp. 300-312). Replacing sulfuric acid by pure phosphoric acid leads to a shifting of this ratio towards a greater fraction of the para isomer (Olah et al., Methods and Mechanisms, 1989, pp. 15-18). However, owing to the reduced acid strength of phosphoric acid, compared with sulfuric acid, when it is used there is customarily greater retardation of the reaction. To achieve comparable reaction rates, therefore, the use of highly concentrated polyphosphoric acids is recommended, which can be prepared from the addition of P
2
O
5
to orthophosphoric acid.
DE 164 36 00 A assesses this procedure, in that reference is made to the problems associated with the use of polyphosphoric acids with respect to their corrosivity towards materials customary in industry.
DE 164 36 00 A discloses in this respect process parameters for nitrating toluene in phosphoric acid, where the use of a 100.8% strength phosphoric acid leads to an o/p-nitrotoluene ratio of 0.90. However, the high excesses of mixed acid used in the examples and the long reaction times given indicate a low space-time yield to those skilled in the art, which makes economic utilization of the proposed process appear doubtful.
Typically in DE 164 36 00 A, an approximately 97% strength phosphoric acid is obtained at the reactor outlet. The concentration step required for recycling this waste acid requires evaporating the reaction water at temperatures of approximately 180° C. at a pressure of approximately 20 mbar, as may be taken from the boiling diagram of the H
2
O—H
3
PO
4
system. These parameters may only be achieved with considerable expenditure both with respect to the materials used and with respect to equipment requirements and thus obstruct economically advantageous use of pure phosphoric acid as a mixed acid component. Thus, for example, the isocorrosion curves of enamel under these process conditions mean that no resistance to a phosphoric acid of this concentration may be expected.
DE 164 36 00 A additionally develops a process alternative in which sulfuric acid is added to the mixed acid in an amount of up to 80% of the phosphoric acid used. This addition of sulfuric acid, compared with the procedure in pure phosphoric acid, does not lead to a significant shift of the isomer ratio towards the ortho isomer. Nevertheless, it does not indicate to those skilled in the art a solution of the problems occurring with the use of pure phosphoric acid, since the drastic process conditions to concentrate waste acid, the increased corrosivity of the acid towards customary materials, and the low space-time yield further counteract industrial implementation of a process to be used with economic advantage.
Industrial processes for the mononitration of toluene using mixed acid are associated with the production of waste acids which are contaminated with considerable proportions of organic compounds, for example dinitrotoluenes, or nitrated cresols, and must be worked up in a process- and cost-intensive manner.
Thus, for example, in the case of nitration with the use of pure sulfuric acid, processes have been developed which comprise concentrating sulfuric acid, with the concentrated sulfuric acid being freed from water and organic compounds, then recycled back to the nitration reaction in a circulation process, and thus avoiding the production of waste acid.
DE 195 39 205 A discloses process parameters for the mononitration of aromatics, with the mixed acids being matched to the properties of the aromatics to be nitrated in such a manner as to produce an approximately 70% strength waste sulfuric acid. In addition, the use of partially concentrated waste acids having a sulfuric acid concentration between 85% and 92% is described.
U.S. Pat. No. 4,772,757 describes a process for preparing nitrobenzene in which the waste acid produced is concentrated to 75 to 92% and is recirculated back to the nitration process. Since toluene is markedly more sensitive to oxidation compared with benzene, owing to the methyl group, and has a tendency in nitration to form by-products, when reaction conditions for nitrating benzene are applied to the nitration of toluene, an increase in the amount of unwanted by-products may be expected.
Owing to the solubility of organic compounds in inorganic acids, organic by-products such as oxalic acid or benzoic acid accumulate in concentrated waste acids, which are recirculated to the nitration reaction. In addition, enrichment in nitrosylsulfuric acid can occur. The decomposition of these by-products and the heat of decomposition released in the course of this can lead to unwanted decomposition of the reaction product of nitrotoluene.
Furthermore, the space-time yield is impaired, since the organic by-products can react with the nitric acid used in oxidative degradation reactions and thus some of the nitric acid is no longer available for the actual nitration.
There was therefore a requirement for a continuous isothermal process for preparing mononitrotoluene having an increased yield of para-nitrotoluene by using mixtures of sulfuric acid and phosphoric acid, in which the process permits inexpensive concentration of the waste acid with subsequent recycling to the nitration reaction within the meaning of a cyclic process, without organic by-products accumulating in the concentrated acid. In addition, the use of dilute nitric acid of 60-70% by weight is desirable, since this leads to marked cost reduction compared with the use of highly concentrated nitric acid.
SUMMARY
The invention relates to a continuous process for preparing mononitrotoluenes comprising feeding toulene, nitric acid, and a mixed acid component into a reactor and reacting, under isothermal reaction conditions, the toluene with the nitric acid and the mixed acid component, wherein the mixed acid component comprises from about 45 to about 80% sulfuric acid, from about 9 to about 45% phosphoric acid and from about 5 to about 15% water. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
DESCRIPTION
A continuous process has been found for preparing mononitrotoluenes by reacting toluene with nitric acid, sulfuric acid and phosphoric acid under isothermal reaction conditions, which is characterized in that mixtures of sulfuric acid and phosphoric acid are used as mixed acid component, which mixtures comprise from about 45 to about 80% sulfuric acid, from about 9 to about 45% phosphoric acid and from about 5 to about 15% water.
A particularly preferred embodiment of the inventive process is characterized in that mixtures of sulfuric acid and phosphoric acid are used as mixed acid component, which mixtures are composed of from about 64 to about 78% sulfuric acid, from about 10 to about 27% phosphoric acid and from about 8 to about 13% water.
A particular embodiment of the inventive process is characterized in that, in a subsequent step, the mixed acid component and from about 60 to about 70% strength nitric acid and toluene are fed into the reactor. The inventive process is characterized in that mix
Demuth Ralf
Gotta Matthias
Ronge Georg
Weber Hans-Martin
Zirngiebl Eberhard
Akorli Godffied R.
Bayer Chemicals AG
Eyl Diderico van
Parsa J.
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