MXPA97003955A - Caprolact preparation - Google Patents

Abstract

The present invention relates to a process for the preparation of caprolactam by heating 6-aminocapronitrile in the presence of a heterogeneous catalyst and water under super atmospheric pressure, wherein (a) 6-aminocapronitrile, or a mixture containing essentially 6-aminocapronitrile and water and a low-boiling or high-boiling alcohol are heated in the presence of a heterogeneous catalyst in a reactor A to give a mixture I, (b) the mixture I is then distilled to give a higher fraction, caprolactam and a lower product, ammonia which is removed before distillation if present in mixture I, and then, (c1) the upper fraction is fed into reactor a of step (a), if desired, the upper fraction is mixed with the alcohol or water or 6-aminocapronitrile used in step (a) before being fed into reactor A, or (c2) the upper fraction, if desired, with the lower product of stage (b), is fed into the re actor B, the upper fraction is mixed, if desired, with the alcohol or water or the 6-aminocapronitrile used in step (a) before being fed into reactor B, and then caprolactam is obtained in the same manner as in step (b) by distillation, and any of the following (d1) the lower product of step (b) is fed into reactor A of step (a) or (d2) if desired, water and if desired, low-boiling or high-boiling alcohol is added to the lower product and the latter is then heated in the same manner as in step (a) in an additional reactor c to give a reaction mixture, starting from from which caprolactam is obtained by distillation, or (d3) water is added to the lower product, which is then heated in a reactor D without the addition of a catalyst to give a reaction mixture and caprolactam is obtained from the mixture of reaction by distillation, or (d4) the product below which water and a base have been added, it is heated in a reactor E to give a reaction mixture, from which caprolactam is obtained by distillation

Description

PREPARATION OF CAPROLACTAMA The present invention relates to an improved process for the preparation of caprolactam by heating 6-aminocapronitrile in the presence of a heterogeneous catalyst and water at super atmospheric pressure. Various patents and literature publications describe the preparation of 6-aminocapronitrile by hydrogenation of one of the nitrile groups of adiponitrile. The use of nickel-Raney is described, for example, in DE 836 938, DE 848 654 (both by BASF) and U.S. Patent 5 151 543 (DuPont). Kinetic investigations are described in C. Mathieu et al., chem. Eng. Sci. 47 (1992), 2289-2224.

U.S. Patent 4 628 085 (Allied) describes the reaction of 6-aminocapronitrile with water in gas phase on a special acidic silica gel (Porasil A) at 300 ° C. By diluting 1.9% of substrate with water (14%), ammonia, (6.3%) and hydrogen / nitrogen, caprolactam can be obtained in a quantitative conversion with a selectivity above 95%, but a notable decrease in the conversion and Selectivity of at least 5% of each is carried out only during the 150 hours as a result of deactivation. Also in US Pat. No. 4,625,023 (Allied) a similar gas-phase process is described. In this patent, a highly diluted gas stream of 6-aminocapronitrile, adiponitrile, ammonia, water and carrier gas are passed over a silica gel and a copper / chromium / titanium-barium oxide catalyst bed. The selectivity of caprolactam is 91% to 85% conversion. The problem of deactivation of the catalyst is described and measures are taken to reduce this situation but no information is given regarding the results. These processes have the disadvantage of the rapid deactivation of the catalyst. This problem is not resolved.

U.S. Patent 2 245 129 (DuPont) describes the preparation of linear polyamides by heating a 50% strength aqueous solution of 6-a-napronitrile at 220 ° C for 20 hours. There is no information about the formation of caprolactam. On the other hand, U.S. Patent 2 301 964 describes the uncatalyzed conversion of aminocapronityl (as an aqueous solution) to caprolactam at 285 ° C; The yield is substantially less than 80%, and a residue is obtained which is no longer spoken.

FR-A 2 029 540 describes a process for the cyclization of 6-aminocapronitrile to caprolactam by means of catalysts, the catalysts used are metal Zn or Cu powders or oxides, hydroxides, halides or cyanides of rubidium, lead, mercury or other elements having atomic numbers from 21 to 30 or from 39 to 48. The described catalysts are used as suspension catalysts in stirred autoclaves, in batch form. Caprolactam is obtained in yields up to 83%. However, complete separation of the catalysts from the desired caprolactam product presents problems since the caprolactam can form compounds with the soluble components that are used, or very fine particles can be formed as a result of mechanical agitation. It is known that 6-aminocaproic acid dissolved in water (US Patent 3 485 821) can be cyclized to caprolactam 350-350 ° C.

DE-C 952 442 discloses a process in which caprolactam is obtained in addition to the aminocaproates by amination of the 5-formylvalerates under hydrogenation conditions in two stages. U.S. Patent 3,988,329 (see also DE 2 535 689) describes a process for the cyclization of 6-aminocaproic acid in methanol or ethanol as solvents. To avoid side reactions of 6-aminocaproic acid, however, the amino acid must dissolve so slowly that it does not accumulate as a solid. Temperatures of around 170 ° C are required for this purpose. In addition, the water content of the solution should not exceed 40%, since otherwise the open chain polymers are formed. The water released from the reaction must be separated when the alcohol is reused.

However, the authors of U.S. Patent 3,988,319 also establish in Ind. Eng. Chem. Process Des. Dev. 17 (1978), 9-16, that the cyclization of 6-aminocaproic acid in water to obtain caprolactam gives rise to significant amounts of oligomers unless they use concentrations below 13% and temperatures around 300 ° C. In A. Balde-Font Tetrahedron Lett. 21 (1989), 2443-2446, the cyclization of 6-aminocaproic acid is described as a suspension in toluene in presence in alumina or silica gel with removal of water from the reaction. For complete desorption of the caprolactam, the catalyst should be washed with methylene chloride / methanol and the polymers precipitated with diethyl ether. The yield of the caprolactam after a reaction time of 20 hours is 82% on alumina and 75% on silica gel. Patent EP 271 815 describes the cyclization of the 6-aminocaproates to caprolactam by dissolving the ester in an aromatic hydrocarbon, carrying out the cyclization from 100 to 320 ° C and at the same time separating the removed alcohol. EP-A 376 122 describes the cyclization of 6-aminocaproates to caprolactam by dissolving the ester in an aromatic hydrocarbon and carrying out the cyclization with the additional use of water of 230 to 350 ° C, in particular from 260 to 340 ° C . It is known that polyamide 6 can be split to obtain caprolactam. The splitting is carried out under the action of acidic or basic catalysts at elevated temperatures, often in the presence of steam, that is, at reduced pressure. In Chem Ing. Techn. 45 (1973), 1510 describes the technical procedure for a process of splitting with superheated steam, it being necessary to concentrate a solution of caprolactam / water for the process. In EP 209021, the splitting is carried out in a bed of fluidized alumina. In EP 529 470 the potassium carbonate is used as a catalyst for the cleavage of polyamide 6 and the reaction is carried out from 250 to 320 ° C with simultaneous elimination of caprolactam by distillation under reduced pressure.

The disadvantages of all processes to date for the splitting of polyamide 6 to caprolactam are the separation, with a high energy consumption, of large amounts of water and the elimination of catalysts, such as phosphoric acids and salts of these , potassium carbonate or alkali metal oxides. In the case of reactions in the gas phase, the polymer is heated to, as a rule, 270-400 ° C and is split together with water in a fluidized bed reactor. The formation of the by-products and the deactivation due to the agglomeration of the catalyst bed are the result.

An object of the present invention is to provide an improved process for the preparation of caprolactam starting from 6-aminocapronitrile, which can be carried out in a technically feasible manner with high selectivity and without problems of rapid deactivation of the catalysts that are used, furthermore, the process must be capable of being carried out without significant production of low boiling point fractions and / or high boiling point.

We have found that this goal is achieved by a process for the preparation of caprolactam by heating 6-aminocapronitrile in the presence of a heterogeneous catalyst and water under superatmospheric pressure, where (a) the 6-aminocapronitrile or a mixture containing essentially 6-aminocapronitrile and water and a low-boiling or high-boiling alcohol are heated in the presence of a heterogeneous catalyst in a reactor A to give a mixture I , (b) the mixture I is then distilled to give an upper fraction, caprolactam and a lower product, ammonia which is removed before distillation if present in the mixture I, and then (cl) the upper fraction is fed to the reactor A of step (a), if desired, the upper fraction is mixed with the alcohol and / or water and / or 6-aminocapronitrile which was used in step (a) before being fed to reactor A, or (c2) ) the upper fraction, if desired, with the lower product of step (b) is fed into a reactor D, the upper fraction can be mixed, if desired, with alcohol and / or water and / or 6-aminocapronitrile used in stage (a) before being fed into reactor B and then s caprolactam is obtained in the same manner as in step (d) by distillation, and (d) the lower product of step (d) is fed into reactor A of stage (a), or (d2) if desired, water is added and if desired an alcohol with low boiling point or one with At high boiling point the lower product is added and then the latter is heated in the same manner as in step (a) in another reactor C to give a reaction mixture, from which caprolactam is obtained by distillation, or ( d3) water is added to the lower product, which is then heated in a reactor D without the addition of a catalyst to give a reaction mixture, and caprolactam is obtained from the reaction mixture by distillation, or (d4) lower product in which water is added and a base is heated in a reactor E to give a reaction mixture, from which caprolactam is obtained by distillation. The 6-aminocapronitrile which is used according to the invention as starting material is generally obtained by hydrogenation of adiponitrile by a known process, for example, that described in DE-A 836 938, DE-A 848 654 or QS 5,151,543. It is also possible to introduce into the reactor A mixtures containing essentially 6-aminocapronitrile and hexamethylenediamine, adiponitrile and / or caprolactam, as well as fractions with a high boiling point (high boilers), such as 6-aminocaproamide, 6-aminocaproic acid, polycaprolactam and oligomers of caprolactam and other 6-aminocaproates that are obtained when working with caprolactam prepared according to the invention, and the 6-aminocaproates can be low boiling or high boiling depending on the ester radical. According to the invention, 6-aminocapronitrile is reacted with water, preferably from 0.01 to 35, particularly preferably from 1 to 10 moles of water which are used per mole of 6-amylocapronitrile. In accordance with the invention, the low-boiling or high-boiling alcohol is further used in a dilution of from 1 to 50% by weight of 6-aminocapronitrile which is preferably chosen. The low-boiling alcohols are those, which at 5 mbar have a boiling point not higher than 10 ° C below the boiling point of the caprolactam, for example, the alkanes of C? -C? 0, in particular methanol , ethanol, n-propanol, isopropanol, n-butanol, isobutanol or sec-butanol are particularly preferred methanol or ethanol. The high-boiling alcohols are those which, at 5 mbar, have a boiling point of at least 10 ° C above the boiling point of the caprolactam, for example, the polyetherols, such as tetraethylene glycol. In another embodiment, from 0 to 5, preferably from 0.1 to 2% by weight of ammonia and / or hydrogen and / nitrogen may be added to the reaction mixture. The heterogeneous catalytic examples that can be used are: the oxides, acids, basic or amphoteric of the elements of the second, third or fourth main groups of the periodic table, such as calcium oxide, magnesium oxide, boron oxide, alumina, oxide tin, or silica such as pyrogenic silica, silica gel, kieselguhr, quartz or mixtures thereof, and metal oxides of the second to sixth subgroups of the Periodic Table, with titanium oxide, in amorphous form or as anatase or rutile, oxide of zirconium, zinc oxide, manganese oxide or mixtures of these. Oxides of lanthanides and actinides, such as rust, thorium oxide, praseodymium oxide, samarium oxide, oxides mixed with rare earths or mixtures of these with the aforementioned oxides can also be used. Other catalysts can be, for example: vanadium oxide, niobium oxide, iron oxide, chromium oxide, molybdenum oxide, tungsten oxide or mixtures thereof. Mixtures of the aforementioned oxides with other oxides are also possible. Some sulfides, selenides, and tellurides such as zinc teluriide, tin selenide, molybdenum sulfide, tungsten sulfide, nickel, zinc and chromium sulphides can also be used. The aforementioned compounds may be doped with main compounds 1 to 7 of the Periodic Table or may contain these. Zeolites, phosphates and heteropolyacids and acidic and alkaline ion exchangers such as Naphion® are suitable catalysts and may also be used. If required, these catalysts can contain up to 50% by weight in each case of copper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium, palladium, platinum, silver or rhodium. Depending on the composition of the catalyst, the latter can be used as a supported or unsupported catalyst. For example, titanium dioxide can be used as titanium dioxide extrudates or as titanium dioxide applied in a thin layer on a support. All the methods described in the literature can be used by applying TiO? on a support such as silica, alumina, or zirconium dioxide. In this way, a thin layer of Ti02 can be applied by hydrolysis of the titanium organyls, such as titanium isopropylate or titanium butylate or by hydrolysis of TiCl4 or other compounds containing inorganic titanium. The sols containing titanium oxide can also be used, according to the invention, the reaction of the stage (a) is made from 100 to 320 ° C, preferably from 160 to 280 ° C, particularly preferred from 180 to 260 ° C. Usually, the reaction in step (a) is carried out under superatmospheric pressure, the pressure that is chosen must be from 0.1 to 50, preferably from 0.5 to 25 MPa.

The duration of the reaction in reactor A depends essentially on the chosen process parameters, and in the continuous process, it is generally from 1 to 300, preferably from 5 to 120 minutes. The conversion usually decreases in the case of shorter reaction times, while observations to date have shown that longer reaction times give rise to an increased formation of the oligomers, so that more oligomers would have to be recycled for the unfolding. The cyclization (step (a)) is preferably carried out continuously in a reactor A, preferably in a reactor tube, in a stirred kettle or in a combination thereof. The cyclization (step (a)) can also be carried out in batch form. The reaction time in this case is usually 30 to 300 minutes. According to the invention, the discharge of reactor A is a mixture I consisting essentially of water, alcohol, 6-aminocaproates, obtained by reaction of the 6-aminocaproic acid formed during the reaction and the alcohol used, caprolactam, ammonia and compounds with high boiling point (high boilers) as it should be 6-aminocaproamide and oligomers of caprolactam. According to the invention, in step (b) the mixture I is distilled by a conventional method to give an upper fraction, caprolactam and a lower product. If the mixture I of step (a) contains ammonia, the latter, according to the invention, is removed before distillation. The ammonia can be removed by conventional methods, for example, by distillation or passing a stream of inert gas through the mixture I. The treatment of the mixture 1 can be carried out gradually or simultaneously, for example, in a preferred embodiment eliminating first the water and any low boiling alcohol distilling in azeotropic form with it by distillation and subjecting the resulting residue to one or more distillations, or distilling the mixture I in a single distillation column. In a preferred embodiment, first distillation is carried out under reduced pressure from 10 to 500, preferably from 50 to 350 rabar to give water and possibly alcohol and a distillation residue, which is subjected to another distillation from 90 to 220 ° C. , preferably from 100 to 160 ° C, and from 0.01 to 1, preferably from 0.5 to 300 mbar to give an upper fraction, caprolactam and a lower product. The upper fraction consists, as a rule, of essentially low-boiling alkyl 6-aminocaproates, unconverted 6-aminocapronitrile and, unless previously separated, water and the alcohol used, if it is of low boiling point. As a rule, the lower product is essentially composed of high-boiling components, such as oligomers of caprolactam, 6-aminocaproamide, 6-aminocaproic acid and, depending on the alcohol used, 6-aminocaproates with a high boiling point and, if it is used, high boiling alcohol. According to the invention, the upper fraction, if desired, together with any alcohol and water separated before hand are recycled in reactor A (stage (cl)), and the upper fraction can be mixed with alcohol and / or water and / or 6-aminocapronitrile used in step (a) before being fed into reactor A. According to the invention, the upper fraction obtained in step (b) can optionally be fed into reactor B (step (c2)). ), if desired with the lower product of step (b), and if desired, together with any alcohol and water separated in advance, and the upper fraction can be mixed with the alcohol and / or water and / or 6- aminocapronitrile used in step (a) before being fed into reactor B. The reaction conditions in reactor B are chosen to correspond to those of reactor A. The reaction mixture of reactor B is treated the same as way that stage (b), being caprolactam which is obtained in one or more stages of distillation. In addition to the aforementioned variants, the lower product obtained in step (d) can be treated in step (c2), according to the invention by four other methods, any of in step (di) the lower product of the step (d) is fed into reactor A of stage (a), or step (d2) if desired, water and, if desired, low-boiling or high-boiling alcohol can be added to the product lower, preferably from 0.1 to 25 times, particularly preferred from 0.15 to 15 times the weight of water and preferably from 1 to 25 times particularly preferred from 3 to 15 times the weight of the alcohol and the heating is then carried out from the same way in step (a) in another reactor C to give a reaction mixture from which caprolactam is obtained by distillation, preferably in the same manner as in step (b), or (d3) water is added to the lower product, preferably 5 to 25 times, particularly preferably 7 to 15 times the weight of water and the heating is carried out without the addition of a catalyst in a reactor D, where the reaction conditions are preferably chosen to be similar to those of reactor AI [sic] except that the temperature chosen is from 200 to 350 ° C, preferably from 280 to 320 ° C and the residence time from 5 to 240 minutes, to give a reaction mixture from which caprolactam is obtained by distillation preferably in the same manner as in step (b), or d4) the lower product to which water is added and a base is heated in a reactor E to give a reaction mixture, from which caprolactam is obtained by distillation, preferably in the same manner as in step (b), the heating of the lower product is preferably carried out at reduced pressure in general from 0.1 to 50, preferably from 1 to 10 mbar, in the presence of the base, as Rule from 1 to 10, preferably from 1 to 3% by weight from it, in the reactor E preferably to a reactor tube, from 200 to 400 ° C, preferably from 280 to 320 ° C. In principle, the lower product can, of course, also be treated by the methods of the prior art, for example by subjecting it to one of the conventional gas phase processes or to one of the common treatment methods with acid catalysts. However, due to the disadvantages of the processes of the prior art, the novel modalities mentioned above (c2) and (di) a (d4) are preferred. Preferred bases are alkali metal hydroxides and alkali metal carbonates such as sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate or mixtures thereof, particularly preferably sodium hydroxide and / or potassium hydroxide. The advantage of the novel process is that the caprolactam is obtained in a technically feasible way with high selectivity and in a high yield starting from 6-to inocapronitrile without the problems of rapid deactivation of the catalysts being used, and the production of significant quantities of fractions of low boiling point and high boiling point. EXAMPLES Example 1 (a) At 70 ml / h a 10% solution of 6-aminocapronitrile, 3.2% by weight of water and ethanol (residue) were from [sic] to 100 bar (10 Pa) in a heated reactor tube 230 ° C, with a capacity of 20 ml, a diameter of 6 mm and a length of 710 mm and filled with titanium oxide (anatase) in the form of 1.5 mm extrudates. Quantitative evaluation by gas chromatography of the reaction mixture gave the following yields (zinc ethanol and water): 91% caprolactam, 4% 6-ethylnocaproate and 1% 6-aminocapronitrile. A stream of the product collected for 200 hours was liberated from ethanol and water, and the resulting impure lactam was distilled. 56 g of the low-boiling components (low boilers) and 126 g of high-boiling components (high boilers) and 1232 g of caprolactam were obtained. The low boiling fractions essentially contained 6-aminocaproate of ethyl and 6-aminocapronitrile not converted and the high-boiling fractions contained essentially oligomers. (b) 445 g of water were added to 126 g of oligomers, 56 g of low-boiling fractions (from (a)) and 1200 g of 6-aminocapronitrile, and the mixture was diluted with ethanol to give a concentration of 10% by weight of the solution. This solution was pumped again at a rate of 70 ml / h through the reactor 230 ° C and 100 bar. The yields of the product discharged were determined by gas chromatographic analysis (calculated without ethanol or water) and were: 87% caprolactam, 3% ethyl 6-aminocaproate and 0.5% 6-aminocapronitrile, After distillation obtained from the second discharge 1182 g of caprolactam, 36 g fractions with low boiling point and 150 g of fractions with high boiling point. All of these, 36 g of recyclable low-boiling fractions, 150 g of high-boiling fractions and 2432 g of caprolactam were obtained from 2600 g of 6-aminocapronitrils. The total yield was 93% and the selectivity 98%. Example 2 (a) At 100 ml / h, a 10% solution of 6-aminocapronitrile, 16.0% by weight of water and ethanol (residue) were from [sic] at 200 bar (10 Pa) in a reactor tube heated to 260 ° C, with a capacity of 20 ml, a diameter of 6 mm and a length of 710 mm and filled with titanium oxide (anatase) in the 1.5 mm extrudate shape. The quantitative evaluation by gas chromatography of the reaction mixture gave the following yields (without ethanol or water: 93% caprolactam, 2% ethyl 6-aminocaproate, A stream of the product collected during 200 hours was released from ethanol and water and the resulting impure lactam was distilled, resulting in 55 g of the low-boiling components (low boilers) and 140 g of high-boiling components (high boilers) and 1820 g of caprolactam. boiling essentially contained ethyl 6-aminocaproate and the high-boiling components contained essentially oligomers (b) 3830 g of water were added to 140 g of oligomers, 55 g of low-boiling components (from Example 2a) and 2200 g of 6-aminocapronitrile, and the mixture was diluted with ethanol to give a 10% solution by weight, this solution was pumped again at a rate of 100 ml / ha. after the reactor at 260 ° C and 200 bar (20 Pa). The yields of the product discharged were determined by gas chromatography analysis (calculated without ethanol or water) and were: 91% caprolactam, and 2% ethyl 6-aminocaproate. After distillation, 2129 g of caprolactam, 57 g of low-boiling components and 196 g of components with high boiling point were obtained from the second discharge. All these compounds, 57 g of low-boiling, recyclable compounds, 196 g of high-boiling components and 3945 g of caprolactam were obtained from 4200 g of 6-aminocapronitrile. The total yield was 94% and the selectivity was 99%. Example 3 (a) At 15 ml / h, a solution of 10% by weight of 6-aminocapronitrile, 3.2% by weight of water and ethanol (residue) were from [sic] to 100 bar (10 Pa) in a reactor tube heated at 200 ° C, with a capacity of 20 ml, diameter of 6 mm and length of 710 mm and filled with titanium oxide (anatase) in the form of 1.5 mm extrudates. Quantitative evaluation by gas chromatography of the reaction mixture gave the following yields (without ethanol or water: 88% caprolactam, 4% ethyl 6-aminocaproate and 4% 6-aminocapronitrile) A stream of the product collected during 200 hours was released from ethanol and water, and the resulting impure lactam was distilled, 29 g of the low-boiling components (low boilers) and 12 g of high-boiling components (high boilers) and 260 g of caprolactam were The low-boiling components essentially contained 6-aminocaproate of ethyl and 6-aminocapronitrile not converted, and the components with high boiling point contained essentially oligomers (b) 94g of water were added to 12g of oligomers, 29 g of low-boiling components (from Example 3a) and 260 g of 6-aminocapronitrile, and the mixture was diluted with ethanol to give a 10% solution by weight. The solution was pumped again at a rate of 100 ml / h through the reactor at 250 ° C and 200 bar (20 Pa). The yields of the product discharged were determined by gas chromatography analysis (calculated without ethanol or water) and were: 91% caprolactam, and 2% ethyl 6-aminocaproate. After distillation, 265 g of caprolactam, 83 g of low-boiling components and 25 g of components with a high boiling point were obtained from the second discharge. All of these, 83 g of low-boiling, recyclable compounds, 25 g of high-boiling components and 525 g of caprolactam were obtained from 560 g of 6-aminocapronitrile. The total yield is 94% and the selectivity is 99%.

Claims (1)

1. CLAIMS A process for the preparation of caprolactam by heating 6-aminocapronitrile in the presence of heterogeneous catalysts and water at super atmospheric pressure, wherein (a) 6-aminocapronitrile, a mixture containing essentially 6-aminocapronitrile and water and a low boiling alcohol or high boiling point are heated in the presence of a heterogeneous catalyst in a reactor A to give a mixture I, (b) the mixture I then distilled to give a higher fraction, caprolactam and a lower product, ammonia which is removed before of the distillation if present in the mixture I, and then, (cl) the upper fraction is fed into the reactor A of stage (a), if desired, the upper fraction is mixed with the alcohol or water or aminocapronitrile used in stage (a) before being fed into reactor A, or (c2) the upper fraction, if desired, with the lower product of stage (d), is fed into reactor B, the upper fraction HE feed, if desired, with the alcohol or water or 6-aminocapronitrile used in step (a) before being fed into reactor B, and then caprolactam is obtained in the same manner as in step (b) by distillation , and any of the following (di) the lower product of step (b) is fed into reactor A of step (a) or (d2) if desired, water is added and, if desired, low alcohol is added. boiling point or high boiling point to the lower product and the latter is then heated in the same manner as in step (a) in another reactor C to give a reaction mixture, from which caprolactam is obtained by distillation , or (d3) water is added to the lower product, which is then heated in a reactor D without the addition of a catalyst to give a reaction mixture, and caprolactam is obtained from the reaction mixture by distillation, or ( d4) the inferior product to which water has been added and a bas e is heated in a reactor E to give a reaction mixture, from which caprolactam is obtained by distillation. The process, according to claim 1, wherein the distillation in steps (d2), (d3) and (d4) is carried out in the same manner as in step (d) to give caprolactam, a fraction with low boiling point and a lower product, with the proviso that the low-boiling fraction is recycled in step (b). . The process, according to claim 1 or 2, wherein a temperature of 100 to 320 ° C is maintained in reactors A to D. The process according to any of claims 1 to 3, wherein a pressure of 0.1 to 50 MPa is maintained in reactors A to D. The process according to any of claims 1 to 4, wherein 0.01 to 35 moles of water are used per mole of 6-aminocapronitrile. . The process, according to any of claims 1 to 5 wherein a dwell time from 1 to 300 minutes is maintained in reactors A to D., The process according to any of claims 1 to 6, wherein an alcohol selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol and tetraethylene glycol is used. SUMMARY OF THE INVENTION The present invention relates to the caprolactam compound which is prepared by heating 6-aminocapronitrile in the presence of a heterogeneous catalyst and water at super atmospheric pressure by a method in which (a) 6-aminocapronitrile, a mixture containing essentially 6-aminocapronitrile and water and a low-boiling or high-boiling alcohol are heated in the presence of a heterogeneous catalyst in reactor A to give a mixture I, (b) the mixture I is then distilled to give an upper fraction, caprolactam and a lower product, ammonia which is removed before distillation if it is present in the mixture I, and then (cl) the upper fraction is fed into reactor A of step (a), if desired, the upper fraction is mixed with the alcohol or with water or 6-aminocapronitrile used in step (a) above if fed into reactor A, or (c2) the upper fraction, if desired, with the lower product of stage (d), are fed into reactor B, the upper fraction is fed, if desired, with the alcohol or water or 6-aminocapronitrile used in step (a) before being fed into reactor B, and then caprolactam is obtained in the same manner as in step (b) by distillation, and any of the following ( di) the lower product of step (b) is fed into reactor A of step (a), or (d2) if desired, water is added, and if desired, low-boiling alcohol is added or high boiling point to the lower product and the latter is then heated in the same manner as in step (a) in another reactor C to give a reaction mixture, from which caprolactam is obtained by distillation, or (d3) water is added to the lower product, which is then heated in a reactor D without the addition of a catalyst to give a reaction mixture, and obtains caprolactam from the reaction mixture by distillation, or (d4) the lower product to which water and a base have been added, is heated in a reactor E to give a reaction mixture, from which caprolactam is obtained by distillation.