MXPA99006690A - Method for separating 6-aminocapronitrile from mixtures containing 6-aminocapronitrile and an imine - Google Patents

Abstract

The invention relates to a method for separating 6-aminocapronitrile from mixtures (I) containing 6-aminocapronitrile and an imine (II) by distillation. Said method is characterized in that distillation occurs in the presence of carbon dioxide.

Description

SEPARATION OF 6-AMINOCAPRONITRILO FROM MIXTURES CONTAINING 6-AMINOCAPRONITRILO AND AN IMINE Description The present invention relates to a process for the distillative separation of 6-aminocapronitrile from mixtures (I) containing 6-aminocapronitrile and an imine (II) • The partial hydrogenation of adiponitrile to 6-aminocapronitrile in the presence of a Metal-based catalyst such as nickel, cobalt, iron, rhodium or ruthenium is generally known, for example from EP-A-161 419, EP-A-77 911, US-A-4,389,348, US-A-4, 601, 859, WO 93/1207, DE-A 42 35 466, DE-A 19 500 222 and the German application 19 548 289.1. By-products include imines, especially tetrahydroazepine (THA) of the formula ß-aminocapronitrile is mainly used for the production of fibers through caprolactam as an intermediate or by direct polymerization in nylon 6. For this, 6-aminocapronitrile has to be very pure, in which context it is known that the separation of THA presents problems . US-A-5, 162, 567 describes the reaction of a mixture containing 6-aminocapronitrile and THA with an organic carbonyl compound, for example a ketone or an aldehyde, at elevated temperature and then the separation of 6-aminocapronitrile from mix. US-A-5, 153, 351 describes the reaction of a mixture containing 6-aminocapronitrile and THA with an active organic CH-methylene compound, for example alonitrile, cyclopentadiene, nitromethane or nitroethane, and then the separation of 6-aminocapronitrile from mix. The disadvantage of this process is that the addition of another organic compound to the mixture makes it more difficult to prepare the pure 6-aminocapronitrile. In US-A-5, 133, 838, a mixture containing 6-aminocapronityl and THA is reacted with an inorganic hydride such as lithium borohydride. As a disadvantage of this process the hydride has to be used in a multiple excess of the stoichiometric amount necessary. In addition, care should be taken in the subsequent distillation so as not to hydrogenate the valuable product, 6-aminocapronitrile. EP-A-97 333 describes a process by which a mixture containing 6-aminocapronitrile and THA is reacted with an alkaline compound. The advantage is that the alkaline compound has to be used in excess of the stoichiometrically necessary amount and the 6-aminocapronitrile has to be distilled from the resulting reaction mixture under greatly reduced pressure. An object of the present invention is to provide a technically simple and economical process for separating 6-aminocapronitrile from a mixture containing mainly 6-aminocapronitrile and THA solving the aforementioned disadvantages. We have found that this objective is achieved by a process for the distillative separation of 6-aminocapronitrile from mixtures (I) containing 6-aminocapronitrile and an imine (II), which consists in carrying out the distillation in the presence of dioxide. carbon. Mixtures (I) can be obtained in a conventional manner by partial hydrogenation of DNA, for example according to a process as described in EP-A-161 419, EP-A-77 922, US-A-4, 389 , 348, US-A-4, 601, 859, WO 93/1207, DE-A 43 35 466, DE-A 19 500 222 and the German application 19 548 289.1, in general, performing the hydrogenation in the presence of catalysts that contain nickel, cobalt, iron, rhodium or ruthenium. The catalysts can be used as supported or unsupported catalysts. Catalyst supports include, for example, aluminum oxide, silicon dioxide, titanium dioxide, magnesium oxide, activated carbons and spinels. Examples of unsupported catalysts are Raney nickel and Raney cobalt. The hydrogenation produces a mixture containing 6-aminocapronitrile, HMD and an imine (II) with or without DNA. From this mixture it is possible to obtain a mixture (I) containing mainly 6-aminocapronitrile and an imine (II), for example, by distillation. An imine (II) is suitably selected from aromatic imines, preferably aliphatic, such as acyclic or especially cyclic, and also mixtures thereof, particularly preferably THA. The imines (II) can be present in mixture (I) as individual compounds or as addition products, for example with amines such as 6-aminocapronitrile, in which case these addition products are hereby likewise included by imines ( II). The carbon dioxide can be added to the distillation mixture before or preferably during distillation in the form of a compound that releases carbon dioxide under the conditions of distillation, such as ammonium carbonate, ammonium carbamate or urea. or mixtures thereof, in which case these compounds can be added in pure form or in a liquid diluent, such as in one or more constituents of the mixture (I), or in the form of solid, liquid or preferably carbon dioxide. gaseous, for example in the form of a gas containing carbon dioxide, or especially in the form of pure gaseous carbon dioxide containing only the customary impurities. The carbon dioxide content of the distillation mixture advantageously should be within the range of 0.1 to 1 mole of carbon dioxide per mole of the imine function of the imine (II). In general, from 0.022 to 0.22 m3 per standard kilogram of imine are contemplated for this purpose. The apparatus for suitable distillation includes any customary apparatus, as described for example in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Vol. 7, John Wiley &; Sons, New York, 1979, pages 870-881, as may be valve plate columns, sieve plate columns or columns packed with cushioned or arranged packing. The distillation is advantageously carried out within the range of 10 to 100 mbar, preferably within the range of 20 to 50 mbar.

Examples 6-aminocapronitrile with the content of THA (&wt., Based on the mixture) as seen in the table was subjected to a fractional distillation in batches in a distillation column (diameter 43 mm, height 2.4 m , packaging of Sulzer CY fabric, 22 theoretical plates) at a reflux ratio of 5: 1. The purified 6-aminocapronitrile was obtained as distillate. The results are reported in Table 1.

Claims (7)

1. A process for the distillative separation of 6-aminocapronitrile from the mixtures (I) containing 6-aminocapronitrile and one imine (II), which consists in carrying out the distillation in the presence of carbon dioxide.

2. The process as recited in claim 1, wherein the imine (II) is a cyclic imine.

3. The process as recited in claim 1 or 2, wherein the imine (II) is tetrahydroazepine.

4. The process as recited in any of claims 1 to 3, wherein the carbon dioxide content of the distillation mixture is within the range of 0.1 to 1 mole of carbon dioxide per mole of the imine function of the imine (II). The process as mentioned in any of claims 1 to 4, wherein the gas consisting of carbon dioxide is introduced into the lower region of a distillation column. 6. The process, as mentioned in any of claims 1 to 5, wherein the distillation is carried out within the range of 10 to 100 mbar, preferably within the range of 20 to 50 mbar. The process as recited in any of claims 1 to 7, wherein the distillation mixture has added thereto a compound (III) that releases carbon dioxide under the conditions of distillation. The process, as mentioned in any of claims 1 to 7, wherein the compound (III) is urea, ammonium carbonate, ammonium carbamate or a mixture thereof.