A common industrial process for producing an amide compound involves Beckmann rearrangement of a corresponding oxime compound. For example, ε-caprolactam which is industrially useful is produced by Beckmann rearrangement of cyclohexanone oxime. Rearrangement catalysts used are generally concentrated sulfuric acid and oleum. Since these strong acids must be used in the stoichiometric amounts or more, they form a large amount of ammonium sulfate as a byproduct during neutralization. Although laurolactam, which is a starting material for Nylon 12, is also produced in a similar manner, the process is more complex because cyclododecanone oxime as an intermediate product has a high melting point. In producing ε-caprolactam, both cyclohexanone oxime and ε-caprolactam have relatively low melting points, so that oxime formation or rearrangement reaction can be conducted in a solvent-free system, but production of laurolactam requires a reaction solvent. This reaction solvent must be able to substantially dissolve cyclododecanone oxime and be inert to concentrated sulfuric acid or oleum, and therefore the selection of the solvent is considerably restricted.
The following references have described a known process for industrially producing laurolactam from cyclododecanone and an aqueous solution of hydroxylamine.
Patent Reference 1 has described the following process. Cyclododecanone is converted into an oxime using isopropylcyclohexane as a solvent, and after separating layers, a resulting solution of cyclododecanone oxime in isopropylcyclohexane is slowly added to concentrated sulfuric acid at a low temperature to prepare a solution of a cyclododecanone oxime sulfate adduct in sulfuric acid. After separating and recovering isopropylcyclohexane, the residual solution of cyclododecanone oxime sulfate adduct in sulfuric acid is heated to carry out Beckmann rearrangement of the oxime. After the rearrangement reaction, water is added to the system to dilute sulfuric acid and from the mixture, the laurolactam produced is extracted with an organic solvent. Here, the extraction solvent may be isopropylcyclohexane or cyclododecanone. The extraction solvent is recovered by distillation from the resulting extraction solution and then laurolactam in the residue is purified by distillation.
This process does not generate ammonium sulfate as a byproduct in the rearrangement reaction step, but requires enormously large facilities and energy for treating a large amount of waste diluted sulfuric acid. Furthermore, since cyclododecanone reacts with concentrated sulfuric acid to form a byproduct, the oxime-forming reaction must be completed for eliminating residual cyclododecanone, but due to hydrophobicity of isopropylcyclohexane, a mass transfer rate is low in an oil-water interface, leading to a longer oxime-forming reaction. As a whole, the process involves many steps of separation, recovery and recycling of solvents and, therefore, requires considerably large equipment expenses and energy.
Patent Reference 2 has described the following process. A mixture of cyclododecanone and cyclohexanone is blended with an aqueous solution of hydroxylamine to produce oximes. Cyclohexanone oxime produced has a low melting point and is a good solvent for cyclododecanone oxime, so that the reaction can be conducted at 100° C. or lower and at an ambient pressure. Furthermore, cyclohexanone oxime is adequately hydrophilic for the oxime-forming reaction to quickly proceed, and the mixture is transferred to the rearrangement step without residual cyclohexanone or cyclododecanone. A rearrangement catalyst used is concentrated sulfuric acid or oleum. Whereas laurolactam produced has a high melting point, it is highly soluble in caprolactam having a low melting point which is simultaneously produced. Therefore the reaction can be carried out even at a temperature of 100° C. or lower. The resulting rearrangement reaction solution is neutralized with ammonia water and then extracted with an organic solvent. Caprolactam can be dissolved in water to some extent, but is extracted into an organic solvent due to salting-out effect of ammonium sulfate formed by neutralization. Next, a large amount of water is added to the solution containing extracted laurolactam and caprolactam, and caprolactam is extracted into the aqueous phase. From the separated organic phase, the organic solvent is recovered and laurolactam is purified by distillation. On the other hand, the aqueous phase is concentrated and after removing impurities, caprolactam is purified.
This process is excellent in that laurolactam and caprolactam can be produced together. However, as a process for producing laurolactam, it has the following problems; (1) separation and purification of caprolactam requires large amounts of equipment expenses, resulting in low investment efficiency and the process involves operations of low energy efficiency such as concentration of an aqueous solution of caprolactam; (2) there is a restriction to a production ratio of laurolactam/caprolactam; and (3) caprolactam is a low-value-added product in comparison with laurolactam and an use efficiency of hydroxylamine is low.
Patent Reference 3 has described Beckmann rearrangement of an oxime compound in a polar solvent, wherein a rearrangement catalyst used is an aromatic compound (1) containing, as an aromatic-ring member, at least one carbon atom having a leaving group, (2) containing at least three aromatic-ring members, which are either or both of heteroatoms or/and carbon atoms having an electron-withdrawing group, and (3) wherein two of the heteroatoms and/or carbon atoms having an electron-withdrawing group are at the ortho- or para-position to the carbon atom having an electron-withdrawing group. Non-Patent Reference 1 has described in detail a rearrangement reaction using the rearrangement catalyst disclosed in Patent Reference 3. Focusing attention on a low yield of rearrangement when a non-polar solvent is used in Non-Patent Reference 1, Patent Reference 4, 5 and 6 have improved the yield and thus extended the range of solvents which can be used, to non-polar solvents. Generally, a non-polar solvent is more thermally and chemically stable, has a lower boiling point and has a smaller evaporative latent heat than a polar solvent, and thus, can be easily recovered and recycled, while it little dissolves a polar organic material or an inorganic material.
We have selected trichlorotriazine among the catalysts disclosed in Patent Reference 3, and have investigated its reaction with cyclododecanone oxime.
Patent Reference 3 and Non-Patent Reference 1 have explained a mechanism of the catalytic action of trichlorotriazine (R3—Cl wherein R3 represents dichlorotriazinyl) in Beckmann rearrangement of a ketoxime (R1—C(—R2)═N—OH wherein R1 and R2 represent alkyl or these may together form cycloalkane) as follows.
First, hydrogen chloride is eliminated from trichlorotriazine and the ketoxime to form R1—C(—R2)═N—O—R3 (ether). By the rearrangement reaction, this ether is converted into R1—N═C(—R2)—O—R3, to which the ketoxime is then added, and via a Meisenheimer complex (R1—N═C(—R2)—O—R3−—O+—(H)—N═C(R2)—R1), an amide (R1—NH—C(═O)—R2) is formed by elimination while R1—C(—R2)═N—O—R3 is regenerated.
In accordance with the above mechanism, trichlorotriazine is rapidly consumed in the initial stage of the reaction to form cyclododecylideneaminoxydichlorotriazine (corresponding to R1—C(—R2)═N—O—R3; hereinafter, referred to as “MOCT”) and the reaction proceeds through the above reaction cycle, and therefore, this reaction cycle must be completed only by adding a catalytic amount of trichlorotriazine at the beginning of the reaction. It has been, however, found that when the reaction of cyclododecanone oxime is conducted using a catalytic amount of trichlorotriazine, the reaction stops at a certain conversion.
It is known that trichlorotriazine is hydrolyzed by water to give trioxytriazine. Trioxytriazine is catalytically inactive to the rearrangement. Furthermore, trichlorotriazine has three eliminable chlorine atoms and it is unclear how many chlorines should be hydrolyzed for making it inactive as a rearrangement catalyst. It can be, therefore, supposed that the rearrangement reaction can be completed by adding a small amount of trichlorotriazine if water is completely removed in a solution of cyclododecanone oxime to be rearranged. However, cyclododecanone oxime, which has a hydrophilic oxime group, is hygroscopic and it is, therefore, very difficult to make the cyclododecanone oxime absolutely dried.
On the other hand, in accordance with the above reaction mechanism, formation of MOCT is competitive to hydrolysis of trichlorotriazine, and when a rate of MOCT formation is larger than a rate of trichlorotriazine hydrolysis, a solution of cyclododecanone oxime is not necessarily absolutely dried.
We have first sampled and analyzed a reaction solution with the lapse of time for elucidating the mechanism of termination of the rearrangement reaction. As a result, it has been found that as the rearrangement reaction of cyclododecanone oxime proceeds, there is regenerated trichlorotriazine, which is gradually converted into trioxytriazine.
In other words, it has been supposed that in the initial reaction stage where the starting cyclododecanone oxime exists in an adequate amount, the reaction proceeds according to the reaction cycle, but in the final stage of the reaction, a cyclododecanone oxime concentration is so reduced that Meisenheimer-complex formation becomes slower, resulting in that 2-azacyclotridecanoxydichlorotriazine (corresponding to R1—N═C(—R2)—O—R3) reacts with hydrogen chloride generated during formation of MOCT from trichlorotriazine and cyclododecanone oxime to generate laurolactam and to regenerate trichlorotriazine which is further hydrolyzed to give trioxytriazine, leading to termination of the rearrangement reaction.
Furthermore, since trioxytriazine generated as a result of hydrolysis of trichlorotriazine and its precursor are very insoluble in a non-polar solvent, they are precipitated in the reaction vessel and finally attach the vessel wall. Disadvantageously, precipitation of solids in the reaction vessel reduces a thermal conductivity coefficient of the reaction vessel, which makes operation unstable in a commercial apparatus. There are no literatures disclosing means for avoiding precipitation of such a catalyst residue within a reaction vessel.
Herein, an inactive precipitate which is formed by the change of the chemical structure of trichlorotriazine as a rearrangement catalyst added in the rearrangement step and lost rearrangement activity and precipitated as a solid, is distinguished from a residual catalyst whose chemical structure is unchanged and is dissolved in the reaction solution. Furthermore, a material whose chemical structure is changed but still retains its catalyst activity is called an active intermediate. A catalyst is a general name including the above three categories.