MXPA99009703A - Method for producing cyclic lactams - Google Patents

Abstract

The invention relates to a method for producing cyclic lactams of formula (I), in which R1 stands for a hydrogen atom, an alkyl, a cycloalkyl or an aryl;A stands for a C3-C12-alkylene radical, which can be substituted by 1, 2, 3, 4, 5 or 6 substituents, selected independently from each other from the alkyl, cycloalkyl or aryl groups. In this method, a&ohgr;-amino carbonic acid nitrile of formula (II) HR1N - A - CN, in which R1 and A have the meaning given above, is reacted in the presence of at least one catalyst. The method is characterized in that nitrile II is converted into an oligomer mixture and then treated with overheated water vapour.

Description

METHOD FOR PREPARING CYCLIC LACTAMS Description The present invention relates to a method for the preparation of cyclic lactams by conversion of a? -aminocarboxylic nitrile in the presence of at least one catalyst. Cyclic lactams have great application as starting materials for the preparation of polyamides (nylons) by polymerization that opens the ring. The technically most important lactam is e-caprolactam, in its character as a cyclic amide of e-aminocaproic acid, which is used mainly for the manufacture of nylon 6 (Perlon®). The most important procedure for obtaining e-caprolactam is that of cyclohexanone-oxime, in which it first converts cyclohexanone with hydroxylamine into oxime and then undergoes a Beckmann transposition to obtain e-caprolactam. This classical obtaining route requires improvements, since on the one hand, it has several stages and on the other, sulfates or other byproducts are produced. Hence, the most modern methods for obtaining cyclic lactams use nitriles? -aminocarboxylic acids in the character of educts. For example, the preparation of 6-aminocaproic nitrile is carried out by unilateral selective hydroperoxidation of adiponitrile. In US-A-4,629,085 the conversion of 6-aminocaproic nitrile with water, in gas phase, onto a special acidic silica gel at 300 ° C is described. By diluting the substrate with water, ammonia and hydrogen / nitrogen, caprolactam can be obtained with a quantitative conversion of a selectivity of more than 95%, but after 150 hours there is a marked decrease in the selectivity and the conversion by deactivation of the gel. silica.

A similar gas phase process is also described in US-A-4,625,023. In this case a very dilute gaseous stream of 6-aminocaproic nitrile, adiponitrile, ammonia, water and carrier gas is passed over silica gel and a catalyst bed of titanium-copper / chromium / barium oxide. The selectivity of caprolactam is 91% with 95% conversion. In this case too, the drawback of deactivation of the catalyst remains. In US-A-2,245,129 the preparation of linear polyamides in a two-step process is described. In a first stage a low molecular weight intermediate is formed by heating a 50% aqueous solution of 6-aminocaproic nitrile at 220 ° C for 20 h, which is re-polymerized in a second stage, after removing ammonia and the excess of water. US-A-2,301,964 describes the uncatalyzed conversion at 185 ° C of aminocaproic nitrile in the form of an aqueous solution to give caprolactam. Clearly less than 90% is obtained and a residue is also obtained that has not been described in detail. FR-A-2,029,640 discloses a process for cyclizing 6-aminocapronitrile to caprolactam, using as catalysts Zn metal or powder or Cu oxides, hydroxides, halides, rubidium cyanides, lead, mercury or the elements with number of order from 21 to 30 or from 30 to 48. The described catalysts are used in autoclaves with discontinuous drive agitator as suspension catalysts. In this procedure up to 83% of caprolactam is obtained. However, complete separation of the caprolactam catalysts presents difficulties, since caprolactam can form compounds with the soluble components of the metal used or very fine particles can be formed by mechanical agitation.

US-A-3,485,821 discloses the cyclization of 6-aminocaproic acid dissolved in water at temperatures of 150 to 350 ° C to give caprolactam. From DE-A-952,442 a method is known by which caprolactam and aminocaproic acid ester is obtained in two stages by hydrogenated amination of 5-formylvaleric acid esters. In US-A-3,988,319 a method is described for cyclizing 6-aminocaproic acid in a solvent such as methanol or ethanol. To avoid secondary reactions of 6-aminocaproic acid, the amino acid must be dissolved with such slowness that it is not present as a solid. For this, temperatures of approx. 170 ° C. On the other hand, the water content of the solution should not exceed 40%, since open chain polymers are formed from the opposite. The water that remains free in the reaction must be removed if alcohol is used again. In Ind. Eng. Chem. Process Des.Dav. 17 (1978) 9-16 it is described that the cyclization of 6-aminocaproic acid in water to obtain caprolactam produces significant amounts of oligomers - if one does not work with concentrations lower than 13% and temperatures of approx. 300 ° C. A.BIade-Font describes in Tetrahedron-Lett., 21 (1980) 2443-2446, the cyclization of 6-aminocaronic acid in suspension form in toluene, in the presence of aluminum oxide or silica gel, with elimination of the water of reaction. For the complete resorption of the caprolactam, the catalyst must be washed with methylene chloride / methanol and the polymer precipitated with diethyl ether. After 20 hours of reaction, 82% of caprolactam is obtained in aluminum oxide or 75% in silica gel. EP-A-271,815 discloses the cyclization of 6-aminocaproic acid esters to obtain caprolactam, by cyclizing the ester dissolved in an aromatic hydrocarbon at 100-320 ° C and simultaneously separating the dissociated alcohol. EP-A-376,122 describes the cyclization of esters of 6-aminocaproic acid to obtain caprolactam, by cyclizing the ester dissolved in aromatic hydrocarbon and adding water, at temperatures of 230-35 ° C. It is known to retro-fold polyamide-6 in caprolactam. The splitting is produced by the action of acidic or basic catalysts, with increased temperature, often with the intervention of water vapor, that is, in the low pressure range. In Chem. Ing. Techn. 45 (1973) 1510 describes the technical procedure of a splitting method for nylon-6 waste with superheated steam, which requires the concentration of a caprolactam-water solution. In EP-A-209021 the unfolding is carried out in a turbulent bed of aluminum oxide. For the cleavage of polyamide-6, in EP-A-529,470 potassium carbonate is added as catalyst and the reaction is carried out at 250-320 ° C, simultaneously separating the caprolactam by vacuum distillation. All these methods for splitting polyamide-6 and obtaining caprolactam have the drawback of being very energy consumers for separating large quantities of water and for separating catalysts such as phosphoric acids and their salts, potassium carbonate or alkalimetal oxides. In the gas phase reactions, the polymer is heated to temperatures between 270 and 400CC and unfolded together with water in a fluidized bed reactor. The consequences are the formation of by-products and the deactivation by incrustation of the landfills of the catalyst. US-A-4,568,736 discloses a process for obtaining polyamides, in which α -aminonitriles are converted with water in the presence of a phosphorous catalyst such as, for example, phosphoric acid, phosphorous acid, hydrophosphorous acid, etc. The conversion is carried out in a two-stage process, forming in the first stage, at temperatures of 200 to 300 ° C with increased pressure of between approx. 14 and 66 bar, an intermediate product of low molecular weight polyamide, which is repolymerized in a second stage with reduction of pressure at atmospheric pressure or at pressures below atmospheric and simultaneous temperature increase, to give high weight polyamides molecular. In general, this second stage is carried out under inert gas. The products thus obtained, in general, still contain phosphorus. Its quality does not equal that of the products obtained by polymerization of cyclic lactams. WO 95/14665 describes a method for preparing cyclic lactams by conversion of aminocarboxylic nitriles with water in aqueous phase, in a solid bed reactor in the presence of heterogeneous catalysts which do not have soluble components, under reaction conditions. The conversion is carried out in water or in mixtures of solvents containing water. The reaction temperature is, in general, between 140 and 320 ° C with increased pressure in the range up to 250 bar. The drawback of this method is the formation of undesirable byproducts, such as non-double-oligomers under reaction conditions and 6-aminocaproic acid amide. When mixtures of alcoholic solvents are used, undesirable esters are always produced, such as, for example, 6-aminocaproic acid ethyl ester. DE-A-44 43 125 describes a process for obtaining caprolactam by heating 6-aminocaproic nitrile in the presence of heterogeneous catalysts and water, with increased pressure. First, a mixture of nitrile, water and an alcohol is converted in the presence of the catalyst into a mixture I, which comprises, in addition to the desired caprolactam, also water, alcohol, 6-aminocaproic ester, ammonia and high-boiling compounds such as 6-aminocaproic acid amide and caprolactam oligomers. This mixture is then subjected to a distillative preparation, obtaining a head reaction, caprolactam and a residue. In order to continue the elaboration, the head reaction in the first stage can be fed back. Optionally you can meet with the waste and. feed in another reactor, if necessary, again. mixed with alcohol and / or water and / or nitrile 6-aminocaprónico to convert it also into capcolactama and submit it to the distillation process. Optionally, the residue obtained in the distillation, if necessary, mixed with water and / or with alcohol can also be fed back to the first reactor or to another, heating again and again obtaining caprolactam. The residue can also be mixed only with water and heated - in another reactor, without adding catalyst, obtaining caprolactam. The residue mixed with water and a base can also be heated in another reactor and caprolactam obtained. To obtain good conversion and yield rates with this method, it is necessary to recycle the overhead reaction that occurs in the distillation to obtain caprolactam and the residue, or if necessary, treat them separately. This means longer periods of reactor occupancy to obtain high performance and secondly, high additional investments for the necessary reactors. As a result, this procedure becomes more expensive than others. In addition, as in the methods already described, undesired 6-aminocaproic acid esters are formed by using alcohols as a solvent. The object of the present invention is to offer an improved method for preparing cyclic lactams starting from nitriles? -amino carboxylic which must overcome, at least partially, the disadvantages described that make the process more expensive. The new improvements prevent, as much as possible, the prolonged occupations of the reactor by recycling an important part of the reaction preparation and the high investment costs because the treatment of it is carried out in separate reactors. Surprisingly it has now been found that the object is reached if the nitriles? -amino carboxylic acids are first catalytically converted into oligomers, which are then split into cyclic lactams using steam, superheated. Hence, the object of the present invention is a method for preparing cyclic lactams of Formula I: wherein R1 is a hydrogen, alkyl, cycloalkyl or aryl atom; A is a C3-C-i2 alkyl radical which may be substituted by 1,2,3,4,5 or 6 substituents independently selected from alkyl, cycloalkyl or aryl; by conversion of a nitrile? -aminocarboxylic of Formula II HR N-A-CN (II) in which R1 and A have the meaning indicated above, in the presence of a catalyst, characterized in that a) nitrile II is converting to a mixture of oligomers b) a K1 catalyst is added and the oligomer mixture containing K1 is treated with superheated steam. In the context of the present invention, the term "alkyl" comprises straight and branched chain alkyl groups. Preference is given to linear or branched C 1 -C 2 -alkyl and C 1 -C 2 -alkyl groups. Examples of alkyl groups include, in particular, methyl, ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, n-pentyl, n-hexyl, n-heptyl, octyl, nonyl. , decilo and dodecilo. Cycloalkyl preferably represents Cs-Cβ-cycloalkyl, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cyclooctyl or cyclopentyimethyl, cyclopentylethyl and cyclohexymethyl and cyclohexylethyl. Aryl preferably represents phenyl, tolyl or naphthyl. If 6-aminocaproic nitrile is used as the nitrile? -amino carboxylic acid of Formula II, the following are understood as oligomers, eg the compounds of the formula: H2N- (CH2) 5 -CO [NH (CH2) 5 -CO]? ONH- (CH2) 5 -X in which m is an integer from 0 to 20 X is CN, COOR, CONH2 or COOH R is a Ci-Cs-alkyl radical or compounds of the formula (C = - = N H2 * H- N (caprolactam iminopentyl nitrile) In addition, the mixture of oligomers can also contain 6-aminocaproic nitrile residues and / or other pre-steps of caprolactam, such as, for example, 6-aminocaproic acid and its esters and amides. The preparation of cyclic lactam I according to the method of the present invention is preferably carried out in a continuous manner, it being possible to carry out the splitting in batches or semilots. The person skilled in the art knows the appropriate reactors, which generally comprise tube reactors, heatable stirred reactors which for step b) are provided with a device for introducing superheated steam and, if necessary, a distillation column. . Suitable reactors are described in Ullmann's Enzykiopádia der technischen Chemie, 3. Ed., Volume 1, p.743 et seq. Appropriate reaction tanks for high pressure work are described ibidem, p. 769 and next. The appropriate distillation columns are described in ibidem, p. 429 and next To avoid condensation at the head of the column during the preparation of high-boiling cyclic lactam, when a distillative separation is required, a column head which can function as a thermostat is preferably used. Optionally, steps a) and / or b) and / or a subsequent separation of the water can also be carried out in separate reactors. Step a) According to a first embodiment of the method of the invention, the conversion of nitrile II into a mixture of oligomers is carried out in liquid phase, in water. For this, the temperatures are, in general, of the range of approx. 100 to 350 ° C, preferably 120 to 250 ° C. The reaction time is, in general, of the range of approx. 1 to 48 hours, preferably of approx. 2 to 24 hours. The molar ratio of water to the nitrile? -amino carboxylic acid is, in general, approx. 0.01: 1 to 20: 1, preferably of approx. 0.5: 1 to 10: '1. Advantageously, in this variant of the method, the nitrile? -amino carboxylic acid of Formula 11 is used without solvents, in water. In this way, the formation of the corresponding '? -amino carboxylic esters customary in the current state of the art is prevented. However, mixtures of - solvents formed by water and inert solvents. Among the appropriate solvents we will mention, for example, aliphatic hydrocarbons as a petroleum ether, aromatic hydrocarbons such as benzol, toluol and xylol, lactams such as pyrrolidone, alkyl substituted lactams such as N-methylpyrrolidone, N-methylcaprolactam or N-ethylcaprolactam, as well as carboxylic esters, preferably organic acids with 1 to 8 carbon atoms. Preferably the nitriles? -amino carboxylics serve at the same time as reagent and solvent. In the first variant of the reaction step a) and in the conversion of the nitrile II into oligomer mixture, a K1 catalyst is added. Next, suitable catalysts K1 will be described, which catalyze both the formation and the cleavage of oligomers. If a K1 catalyst is used in step a), it is generally unnecessary to add more in step b). In general, the oligomer mixtures obtained in this first variant of the method of the invention can be isolated or treated in reaction step b) without further treatment. The ammonia released in the oligomerization can be separated during the reaction or remain in the system. In a second variant of the method mixtures of oligomers can be prepared according to that described in US-A-2,245,129, to which reference is made herein. In this case, the repolimerization described therein of the oligomers obtained is omitted. Instead, they are used in step b) of the method of the present invention for their splitting. In a third variant of step a) of the method of this invention, nitrile II in an inert solvent and in the absence of water is converted into a mixture of oligomers. The reaction temperature is, in general, of the range of .aprox. 100 to 250 ° C, preferably of approx. 120 to 230 ° C. The reaction time is, in general, of the range of approx. 1 to 80 hours, preferably of approx. 2 to 60 hours. In a special embodiment of this third variant of the reaction step a) of the method of the present invention, the conversion of the nitrile II into the oligomer mixture is carried out in the presence of a K2 catalyst. This is, in general, different from the catalyst K1 used in step b). Next, the appropriate K2 catalysts will be described. At the end of the conversion, catalyst K2 is separated from the resulting oligomer mixture. If a heterogeneous catalyst is used, this separation can be carried out by customary methods such as, for example, sedimentation, filtration or centrifugation. The methods and devices suitable for their realization are described in Ullmanns Enzykiopádia der technischen Chemie, 3. Ed., Volume 1, p. 470 and next. In a fourth variant of reaction a) of the method of this invention, mixtures of oligomers are obtained with low valency ruthenium complexes, as described by S.-I. Murahashi, in Chemtracts, Inorg. Chem. 8 (1996), p. 89-105, or by copper catalysis, according to customary methods, known to the person skilled in the art. ~ Before the reprocessing of the mixture of oligomers in step b) of the process, the inert solvent added and / or the unconverted nitrile ll and / or other volatile by-products are removed from the mixture in general. This can be carried out, for example, by distillation, preferably under reduced pressure, for example, ca. 1 to 100 mbar, at temperatures of the range of the above-mentioned reaction temperatures and, for example, with the aid of the distillation column required in step b) of the method of this invention, Inert solvents suitable for preparing the mixture of oligomers according to "the second variant of a) are the inert solvents mentioned in the first variant of a). After separating catalyst K2 and / or other volatile components, the mixture of oligomers is used analogously to the mixture of oligomers obtained in the first variant of step a) of the method of the invention for its splitting in stage b) Step b) To obtain cyclic lactams of Formula I according to the method of the present invention, the mixtures of oligomers are treated with steam from superheated water in the presence of a K1 catalyst, what unbundles them and, if necessary, simultaneously breaks them up. For this, a K1 catalyst is added to the oligomer mixture, if it is not already present because it was added in step a). For the treatment, superheated steam is used, which is introduced, in general, into the reaction vessel where the mixture of oligomers is located. The introduction into the reaction vessel is carried out, for example, through a dip tube, below the level of the liquid mixture. The temperature of the reaction mixture will be, in general, in the range of approx. 200 to 350 ° C, preferably 220 to 300 ° C. The temperature of the superheated steam will be, generally in the range of approx. 240 to 320 ° C, preferably 260 to 300 ° C. To prevent solid products from being deposited in the head of the column in the case of high-boiling lactams, it can be thermostated, as already mentioned. The water vapor flow is, in general, from 200 to 800 g / l of preparation per hour, preferably from 400 to 600 g / l of preparation h. An aqueous mixture or aqueous fractions of Formula! Lactams are obtained, reducing the content of I as the conversion or fractionation time increases. The completion of the splitting is recognized by a reduction in the temperature of the distillation product at the head of the column. Preferably, only fractions containing product with a lactam content of more than 5% by weight, preferably more than 10% by weight, are used for the subsequent separation of the water. The residue obtained in step b) during the fractionation can preferably be used for a new splitting. Following step b) the water and, if necessary, the low-boiling products remaining in the lactam-containing fractions can be separated. The separation of the water from the lactam-containing fractions is carried out in a customary manner, by methods known to the artisan. Among these methods is the distillation at normal or reduced pressure. If, in the separation of low-boiling products, components which are suitable as monomer modules are obtained, such as, for example, 6-aminocaproic nitrile or 4-aminocaproic ester, they can be recycled to step a) for oligomerization. Cyclic lactams 5 which are not N-substituted are preferably prepared with the method of the invention. In Formulas I and II, R2 then represents hydrogen. On the other hand, the cyclic lactams of the Formula I whose alkylene residue is not substituted are preferably prepared by the method of the invention. In Formula I and II, A then represents an unsubstituted C3-C-i2-alkylene residue With particular preference, A represents a C3, C5 or Cn-alkylene residue. The corresponding cyclic lactams of Formula I are β-butyrolactam, e- • - caprolactam and laurinlactam. . The cyclic lactams of Formula I are especially, e-caprolactam. In a preferred embodiment of the method of the invention, the catalyst K1 is a homogeneous catalyst. K1 catalysts which comprise a phosphorus compound are preferred. Suitable catalysts include, for example, the catalysts described in US Pat. No. 4,568,736 for the preparation of polyamides, which at the same time also catalyze the cleavage of oligomeric amides. These include, for example, phosphoric acid, diphosphoric acid, metaphosphoric acids and polyphosphoric acids. Also suitable are the salts and esters of the phosphorous acids, such as trialkyl phosphites, for example tri methyphosphite and triethylphosphite, and triaryl phosphites, such as, for example, triphenyl phosphite. Other suitable K-1 catalysts are phosphonic acid, its organic derivatives with a PC-link such as, for example, alkylphosphonic and arylphosphonic acids, as well as esters and salts of phosphonic acid, phosphonates and esters and salts of organic derivatives. of phosphonic acid, the 5 alkylphosphonates and arylphosphonates. Other suitable K1 catalysts are the esters and salts of the phosphorous acid, the phosphonites. Also suitable are phosphinic acid and its esters and salts, the phosphinates. The K1 catalysts mentioned can be used alone or in mixtures. According to a preferred embodiment of the method of this invention the catalyst K1 used is orthophosphoric acid or a polyphosphoric acid. The amount of the catalyst K1 is, in general, approx. 0.01 to wt% by weight, preferably 0.1 to 3% by weight with respect to the amount of nitrile? -aminocarboxylic acid of Formula II. Preferably, a heterogeneous catalyst is used as catalyst K2. K2 catalysts can be used, for example: acidic, basic or amphoteric oxides of the elements of the second, third or fourth main group of the periodic system, such as calcium, magnesium, boron, aluminum, tin or silicon oxide, such as pyrogenic silicon oxide, silica gel, kieselguhr, quartz or mixtures thereof; also, metal oxides from the second to the sixth subgroup of the periodic system, such as zirconium, zinc or manganese oxide or mixtures thereof. Lantanide and actinide oxides, such as cerium oxide, thorium oxide, praseodymium oxide, samarium oxide, mixed oxide of Seltenerd, or mixtures thereof with oxides mentioned above, can also be used. Other catalysts may be, for example: vanadium, niobium, iron, chromium, molybdenum, wolfram oxides or mixtures thereof. Other aforementioned oxides may also be mixed together. "Some sulphides, selenides, tellurides, such as zinc telluride, tin selenide, molybdenum sulphide, tungsten sulphide, nickel sulphide, zinc and chromium may also be used. endowed with compounds of the first and seventh main group of the periodic system or contain them.

They can also be suitable catalysts, zeolite, phosphates and heteropoly acids, as well as acidic and alkaline ion exchangers such as Naphion®. These catalysts can contain up to 50% by weight of copper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium, palladium, platinum, silver or rhodium. The K2 catalysts can be used, according to their composition, as total contact catalysts or as carrier catalysts. The titanium dioxide can be used, eg in the form of bands or as a thin layer on a carrier. To apply TiO2 on a carrier such as silicon, aluminum or zirconium dioxide, any of the methods described in the literature can be used. This is possible • - apply a thin layer of TiO? by hydrolysis of titanium organelles, such as titanium isopropylate or titanium butylate or by hydrolysis of TiCU or other titanium-containing inorganic compounds. Brines containing titanium oxide can also be used. The amounts of catalyst K2 is, in general, approx. 0.01 to 5% by weight, preferably approx. 0.1 to 5% by weight with respect to the amount of α-aminocarboxylic acid. The invention is explained below by means of the following non-limiting examples. EXAMPLES Example 1 In a three-necked container, 500 ml, which is provided with a vacuum column consisting of a 15 cm glass tube without charge, an intake tube for steam and an electric thermostat dissolve 250 g (2.2 mol) of 6-aminocaproic nitrile and 5 g of polyphosphoric acid (density 2.6 g / ml, 2% by weight with respect to nitrile) dissolved in 20 g (1.1 mol) of water. The flask is heated by means of a 900 W burner. On the column there is a thermostatted head at 80 ° C, with water cooling. The cloudy solution is heated for 18 h with reflux, whereby the temperature increases from 133 to 155 ° C. The preparation is then heated with the burner to 250 ° C, with 5 g of aminocaproic acid nitrile being removed as the distillation product. Then, at a temperature of 250 ° C, 125 g / h of steam heated to 275 ° C is added. The steam is heated to the desired temperature in an oil-heated coil (1500 mm long, 6 mm diameter) and introduced into the reaction flask. Water vapor containing caprolactam, which has passed through the reaction medium at 270-275 ° C, condenses at the head of the column at 80 ° C. The distillation product is received in alternating fractions. After 1 h, 201 g of an aqueous solution of caprolactam 33.1% are obtained. This fraction also contains 30.4 g (0.27 mol) of caprolactam-iminopentylnitrile (caprolactim- (6-aminocaproic nitrile) and some unconverted aminocaproic nitrile.After another 2 h, 315 g of a solution are removed. of caprolactam 18.7%, after another 2 h, 277 g of a solution of caprolactam 14.4% and after another 45 minutes, 94 g of a solution of caprolactam 8.7% and after of another 1.5 h, 211 g of a caprolactam solution now only 2.7%, In total 1099 g of a solution of caprolactam at 15.8% in 7.3 h (174 g caprolactam) are obtained. end of the split is recognized by the drop in head temperature, the residue of 21.5 g is composed of residual oligomer and catalyst and can be used again for another splitting.The yield was 76% with a conversion of 87%. The selectivity was 87%, thanks to the reuse of the residue, the selectivity of caprolactam increased.

EXAMPLE 2 In an apparatus such as that of Example 1, 1000 g of 6-aminocaproic nitrile in 500 g or xylene, together with 100 g of titanium oxide, were heated at reflux for 40 h. The suspended titanium oxide solution was then removed and the solvent was removed by distillation; the residue was distilled at 1 mbar and 156-158 ° C. 600 g of pure caprolactam-iminopentyl nitrile were obtained. In the background are 75 g of unconverted aminocaproic nitrile and 250 g of polymer substantially capable of unfolding. As in Example 1, 250 g of caprolactam-iminopentyl nitrile were mixed with 5 g of phosphoric acid (density 2.6 g / ml).; 2% by weight of the educt) and the preparation was heated with a mirror burner at 250 ° C. At 250 ° C, with a feed rate of 125 g / h, steam was introduced at 275 °. Water vapor containing caprolactam passing through the reaction medium at 270-275 ° C is condensed at 80 ° C at the head of the column and the distilled material is received in alternating fractions. After 1 hour, 260 g of a 33% aqueous caprolactam solution are obtained. This fraction also contains 70 g (0.7 mol) of caprolactam-iminopentyl nitrile and traces of nitrile- unconverted aminocaproic acid. After another hour, 146 g of a 20.2% caprolactam solution are removed; after another 2 hours, 300 g of a caprolactam solution at 14.4% and after another two hours, 258 g of a caprolactam solution at 7.9%. In total, 964 g of a solution of caprolactam 18.8% (181 g caprolactam) are obtained in the course of 6 hours. The end of the split is recognized by the drop in head temperature. The remaining residue of 15.5% consists of residual catalyst and oligomers and can be reused for splitting. The yield was 67% with a conversion of 71%. The selectivity was 95%. Using the residue again increased the selectivity of caprolactam.

Claims (1)

1. CLAIMS A process for preparing cyclic lactams of the formula I: AC = 0 (I) N R1 wherein: R1 is hydrogen, alkyl, cycloalkyl or aryl, and A is C3-C12 alkylene substituted or unsubstituted by 1, 2, 3, 4 or 6 selected substituents, independent of each other, of the group consisting of alkyl, cycloalkyl and aryl, by conversion of a ts-aminocapronitrile of the formula II: HR ^ -A-CN (II) where R1 and A are each As already defined, in the presence of a catalyst, which consists of a phosphorus compound, the process consists of: a) converting the nitrile II into a mixture of oligomers, b) adding a Kl catalyst and treating the mixture of oligomers that contains _ Kl with superheated steam. The process, as mentioned in claim 1, wherein the conversion of nitrile II in step a) is carried out in the presence of water. The process, as mentioned in claim 2, wherein the conversion in step a) in the same manner is carried out in the presence of the catalyst Kl. The process, as mentioned in claim 1, wherein the conversion of nitrile II in step a) is carried out in an inert solvent. The process, as mentioned in claim 4, wherein the conversion in step a) is carried out in the presence of a heterogeneous catalyst K2, which consists of at least one oxide, sulfide, selenide and / or teluriure of the elements of the second, third or fourth major groups, of the second, third, fourth, fifth or sixth transition group, of the lanthanides, of the actinides, a zeolite, a phosphate, a heteropolyacid, an ion exchanger and mixtures thereof. The process, as mentioned in any of claims 5 or 6, further consists in removing the catalyst Kl and, if appropriate, the unconverted solvent and / or nitrile II and / or other by-products, from the mixture of oligomers, then of the conversion in step a). The process, as mentioned in any of the preceding claims, wherein the conversion in step a) is carried out from 100 ° to 350 ° C, preferably from 120 to 250 ° C. The process, as mentioned in any of the preceding claims, wherein the binding of the mixture of oligomers in step b) is carried out from 220 to 300 ° C. The process, as mentioned in any of the preceding claims, in where, in step b) superheated steam is passed to the mixture of oligomers. The process, as mentioned in any of the preceding claims, wherein the catalyst Kl is used in an amount within the range of 0.01 to 10% by weight, preferably in the range of 0.1 to 3% by weight, based on the w-aminocapronitrile. The process, as mentioned in any of claims 5 to 10, wherein the catalyst K2 further consists of at least one compound of the first and / or second major group (s) of the periodic table. The process, as mentioned in any of claims 5 to 11, wherein the catalyst K2 consists of at least one metal selected from the group consisting of Ti, Cu, Sn, Zn, Mn, Fe, Co, Ni, Ru, Pd, Pt, AG and Rh. The process, as mentioned in any of claims 5 to 12, wherein the catalyst K2 is used in an amount within the range of 0.01 to 5% by weight, preferably within the range of 0.1 to 3% by weight. weight, based on tp-aminocapronitrile. The process, as mentioned in any of the preceding claims, wherein the lactam of formula I is e-caprolactam.