Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-11-08
2004-09-07
Gitomer, Ralph (Department: 1651)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06787669
ABSTRACT:
The present invention relates to the treatment of the reaction mixtures resulting from an oxidation reaction of cyclohexane to adipic acid and more particularly to the separation of the various constituents of the said mixtures and to the purification of the adipic acid.
The direct oxidation of cyclohexane to adipic acid is a process which has been operated for a long time, in particular because of the obvious advantages which there would be in converting cyclohexane to adipic acid in a single stage and without employing an oxidizing agent, such as nitric acid, this compound generating nitrogen oxides which would subsequently have to be treated in order to prevent any pollution.
Patent WO-A-94/07834 discloses the oxidation of cyclic hydrocarbons to corresponding diacids, in the liquid phase comprising a solvent, using a gas comprising oxygen in the presence of an oxidation catalyst, such as a cobalt compound, the said solvent comprising an organic acid having only primary or secondary hydrogen atoms. This patent enlarges more particularly on the stages of treatment of the final reaction mixture. This treatment consists in separating the diacid formed, by cooling the mixture in order to bring about the precipitation of the said diacid, and in separating, by filtration, the diacid from two liquid phases, a non-polar phase, which is recycled, and a polar phase, which is also recycled after optional hydrolysis and separation of an additional amount of diacid.
This patent provides a solution for oxidizing cyclohexane to adipic acid in one stage with an industrially acceptable selectivity but it does not furnish an industrially applicable solution to the treatment of the reaction mixture resulting from the oxidation, taking into account the separation of the various reaction products and byproducts, unconverted materials and catalyst.
In addition, it turns out in practice that such a perfunctory treatment process does not result in an adipic acid exhibiting the purity required in a great many applications of this highly important starting material.
This is because, whether for the production of polyamide 6,6 or for other applications, such as the production of some polyurethanes, the purity of the adipic acid employed must be extremely high, both for contents of organic byproducts, which can result in undesiable colourations, and for contents of metal residues, in particular traces of the catalyst used.
The present invention thus relates to an improved process for the treatment of the reaction mixture resulting from the direct oxidation of cyclohexane to adipic acid by molecular oxygen in an organic solvent in the presence of a catalyst. characterized in that the said process comprises:
a separation into two liquid phases by settling: an upper phase, which essentially cyclohexane, and a lower phase, essentially comprising the solvent, the diacids formed, the catalyst and a portion of the other reaction products and of the unconverted cyclohexane;
a distillation of the said lower phase, making it possible to separate, on the one hand, a distillate comprising at least a portion of the most volatile compounds, such as the organic solvent and water, as well as unconverted cyclohexane, cyclohexanone, cyclohexanol, cyclohexyl esters and lactones possibly present, and, on the other hand, the distillation bottoms comprising the diacids formed and the catalyst;
a separation of the catalyst from the distillation bottoms obtained above, either by crystallization from water, by electrodialysis or by passing over an ion-exchange resin, after dissolution of the said distillation bottoms in water, or alternatively by washing with water or by liquid-liquid extraction;
a reducing and/or oxidizing purification treatment ot the adipic acid in aqueous solution;
a crystallization, preceding or following the purification treatment, when the crystallization has not been carried out in order to separate the catalyst;
a recrystallization of the adipic acid from water.
The cyclohexane phase obtained in the stage of separation by settling is generally reintroduced into a cyclohexane oxidation operation.
The organic solvent employed in the oxidartion of cyclohexane is more particularly chosen from aliphatic carboxylic acids. It is generally acetic acid.
The catalyst preferably comprises cobalt, manganese or a mixture of cobalt with one or more other metals, such as manganese, chromium, iron, zirconium, hafnium or copper. Among cobalt-based mixtures, catalysts comprising either cobalt and chromium, or cobalt, chromium and zirconium, or cobalt and iron, or cobalt and manganese, or cobalt andzirconium and/or hafnium are more particularly well suited. This catalyst is used for the oxidation of cyclohexane in the form of compounds of these metals which are soluble in the reaction mixture.
The stage of distillation of the lower phase is carried out so that most, insofar as possible virtually all, of the unconverted cyclohexane which may still be present in this lower phase and of the solvent, in particular of the carboxylic acid preferably used, is separated from the adipic acid. This stage makes it possible co separate the light organic compounds (more volatile than the diacids), which it is advantageous to recycle in the stage of oxidation of cyclohexane, optionally after a treatment intended to remove water from them.
Mention may be made, as examples of such light organic compounds, of adipogenic compounds (capable of being converted into adipic acid), such as cyclohexanol, cyclohexanone or cyclohexyl acetate, and other compounds, such as lactones (essentially butyrolactone and valerolactone).
The distillation stage is generally carried out at a temperature of 25° C. to 250° C. and under an absolute pressure of between 10 Pa and atmospheric pressure. The temperature of the mixture during the distillation will preferably be maintained between 70° C. and 150° C.
The distillation can, if necessary, be carried out in several successive stages, in particular in the preferred form, where it is desired to remove the largest portion, for example more than 90% and even more than 99%, of the solvent, such as an aliphatic carboxylic acid.
To perfect the separation of the light organic compounds mentioned above, use may be made, in the distillation, of an inert entrainer which can either be water in the form of steam or also be an inert gas, such as nitrogen.
The distillation stage can optionally be completed by an extraction of the distillation bottoms using a water-immiscible organic solvent. This extraction can be used to separate the esters, in particular cyclohexyl esters, which can be found in the distillation bottoms. Use may be made, as non-limiting example of such organic solvents, of aliphatic, cycloaliphatic or aromatic hydrocarbons, aliphatic, cycloaliphatic or aromatic carboxylic acid esters, or ketones. As it is preferable to avoid as far as possible the introduction of new compounds into the process of the invention, cyclohexane will advantageously be used as extraction solvent. The extract can be recycled in a new oxidation reaction, either directly or after hydrolysis of the esters.
The distillate obtained in the distillation operation described above comprises the various volatile compounds and water. These volatile compounds are of economic value and are thus recycled in a new oxidation reaction of cyclohexane, after an at least partial removal of the water by any known means, in particular by azeotropic distillation.
The distillation bottoms obtained at the end of the distillation, which have, if appropriate, been subjected to the extraction operation, are treated in order to separate the catalyst which they comprise.
This separation can be carried cut, in a first alternative form, by a crystallization process which consists essentially in dissolving the said distillation bottoms in the minimum amount of water, generally while hot, and then crystallizing mainly the adipic acid. Furthermore, the aqueous solution comprising the catalyst can be treated in ortder to isolate the said catalyst, which can be re
Costantini Michel
Fache Eric
Leconte Philippe
Burns Doane Swecker & Mathis L.L.P.
Gitomer Ralph
Rhodia Fiber & Resin Intermediates
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