Process for producing caprolactam

Details Process for producing caprolactam Process for producing caprolactam

1997-11-06

2004-01-27

Kifle, Bruck (Department: 1624)

Organic compounds -- part of the class 532-570 series

Organic compounds

Unsubstituted hydrocarbyl chain between the ring and the -c-...

C540S539000

Reexamination Certificate

active

06683177

ABSTRACT:

The present invention relates to an improved process for preparing caprolactam by reacting 6-aminocapronitrile with water in the presence of catalysts.
On heating or storage at room temperature, 6-aminocapronitrile forms a brown tetrahydroazepine derivative (THA derivative I) of the formula
THA derivative I shall also encompass its tautomeric form
EP-A-497,333 describes the direct polymerization of polycaprolactam starting from 6-aminocapronitrile. The problem to be solved in the process mentioned was the removal of tetrahydroazepine (“THA”) before the polymerization step, since tetrahydroazepine leads to discoloration of the polymer obtained on polymerizing caprolactam in the presence of tetrahydroazepine. EP-A-497,333 proposes solving the problem by means of a treatment with a basic compound such as an alkali metal hydroxide or an alkali metal alkoxide. Following the treatment, 6-aminocapronitrile can be conveniently separated from the reaction mixture by distillation, which is not possible without such a treatment. EP-A 502,439 solves the problem of removing THA in the presence of 6-aminocapronitrile by treatment with sodium borohydride. Here too 6-aminocapronitrile can be readily separated from the reaction mixture by distillation after the treatment.
DE-B-25 42 396 and DE-B-25 42 397 describe the conversion of gamma-aminobutyronitrile into a mixture comprising 2-(N-gamma-cyanopropyl)amino-deltal-pyrroline (“CAP”) and 2-amino-deltal-pyrroline (“AP”), and also the further hydrolysis of the isolated CAP to 2-pyrrolidone in the absence of catalysts. Neither reference indicates whether the corresponding THA derivative I can be converted into caprolactam in a similar manner in liquid phase in the presence of heterogeneous catalysts. Furthermore, in the cited DE references CAP is first isolated as a pure substance before it is hydrolyzed. It might therefore be expected that the use of mixtures comprising THA derivative I would promote the formation of undesirable by-products. It is also known that five-membered rings are easier to form than seven-membered rings (see Römpp Chemie Lexikon, 9th edition, editors Falbe and Regitz, Georg Thieme verlag, New York). Altogether and on the basis of experience with THA it might therefore be expected that THA derivative I would lead to discolored caprolactam in the cyclization of 6-aminocapronitrile and to discolored polycaprolactam in the direct conversion of 6-aminocapronitrile into polycaprolactam, unless separated off before the cyclization and before the polymerization step.
It might further be expected that THA derivative I would reduce the lifetime of the catalyst used in the polymerization, since it was known from U.S. Pat. No. 5,162,567 that heating THA produces high boilers, ie. compounds or mixtures with a higher boiling point than 6-aminocapronitrile (accordingly making it easy to remove the 6-aminocapronitrile). High boilers, however, tend to form polymeric or oligomeric decomposition products which can form deposits on catalyst surfaces and so reduce not only the lifetime but also the activity of the catalysts.
It is an object of the present invention to provide a process for cyclizing 6-aminocapronitrile to caprolactam wherein THA derivative I reduces neither the lifetime nor the activity of the cyclization catalyst, nor leads to a caprolactam-containing reaction mixture whose UV number is equal to or higher than that prior to the cyclization step. Preferably the post-cyclization UV number should be smaller than pre-cyclization as a function of the pre-cyclization THA derivative I content. Furthermore, any THA derivative I present in the reaction mixture for the direct polymerization of 6-aminocapronitrile shall be easy to remove or it shall be possible to conduct the reaction in such a way that THA derivative I is eliminated.
We have found that this object is achieved by a process for preparing caprolactam by reacting 6-aminocapronitrile with water in the presence of catalysts, which comprises using a starting mixture of 6-aminocapronitrile and the tetrahydroazepine derivative of the formula
and conducting the reaction in liquid phase in the presence of a heterogeneous catalyst.
The present invention also provides tetrahydroazepine derivative I, a process for its preparation, and the use of THA derivative I for preparing caprolactam.
The reaction of the present invention is carried out in liquid phase in the presence of heterogeneous catalysts at temperatures from generally 140 to 320 ° C., preferably from 160 to 280° C.; the pressure is generally within the range from 41 to 250 bar, preferably from 5 to 150 bar, care having to be taken to ensure that, under the conditions employed, the reaction mixture is predominantly (ie. without the catalyst, which is present in solid phase) liquid. The residence times are generally within the range from 1 to 120, preferably from 1 to 90, in particular from 1 to 60, min. In some cases residence times from 1 to 10 min will prove completely adequate.
The amount of water used is generally at least 0.01 mol, preferably from 0.1 to 20 mol, in particular from 1 to 5 mol, per mole of THA derivative I.
Advantageously THA derivative I is used in the form of a from 1 to 50% strength by weight, in particular from 5 to 50% strength by weight, particularly preferably from 5 to 30% strength by weight, solution in water (in which case the solvent is then also the reactant) or in water-solvent mixtures. Examples of suitable solvents are alkanols such as methanol, ethanol, n- and i-propanol, n-, i- and t-butanol and polyols such as diethylene glycol and tetraethylene glycol, hydrocarbons such as petroleum ether, benzene, toluene, xylene, lactams such as pyrrolidone or caprolactam or alkyl-substituted lactams such as N-methylpyrrolidone, N-methylcaprolactam or N-ethylcaprolactam and also carboxylic esters, preferably of carboxylic acids having from 1 to 8 carbon atoms. Ammonia too can be present in the reaction. It is of course also possible to use mixtures of organic solvents. Mixtures of water and alkanols in a water:alkanol weight ratio of 1-75:25-99, preferably 1-50:50-99, have been determined to be particularly advantageous in some cases.
The THA derivative I content in the 6-aminocapronitrile of the starting mixture can be within the range from 0.01 to 95% by weight, in particular from 0.1 to 50% by weight, particularly preferably from 0.5 to 20% by weight.
The starting mixture customarily has, depending on the level of THA derivative I, a UV number (sum of all absorbances of a 10% by weight solution in ethanol at wavelengths from 280 to 400 nm, based on a path length of 5 cm) within the range from 5 to 40,000.
The starting mixture is obtainable by heating 6-aminocapronitrile with or without solvent. From experience to date, the temperature can be within the range from 20 to 280° C., in particular within the range from 50 to 250° C., particularly preferably within the range from 100 to 230° C. The reaction times are customarily within the range from 10 minutes to 20 hours. As expected, shorter reaction times are possible at higher temperatures. The reaction can be carried out at pressures within the range from 100 kPa to 25 MPa, preferably from 500 kPa to 20 MPa. It can further be advantageous to carry out the reaction in the presence of acidic homogeneous or heterogeneous catalysts such as mineral acid, carboxylic acids, sulfonic acids, titanium dioxide, aluminum oxide, acid ion exchangers or Lewis acids.
If desired, pure THA derivative I can be obtained for example by distillation of unconverted 6-aminocapronitrile, solvents and any by-products.
Examples of suitable heterogeneous catalysts include: acidic, basic or amphoteric oxides of the elements of the second, third or fourth main group of the periodic table, such as calcium oxide, magnesium oxide, boron oxide, aluminum oxide, tin oxide or silicon dioxide as pyrogenic silica, as silica gel, diatomaceous earth, quartz or mixtures thereof, also oxides of metals of secondary groups two to six of the periodic table, such as titanium oxide, amorp

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