JPH10221325A - Method of analyzing epsilon-caprolactam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simpler method of analyzing ε-caprolactam. SOLUTION: In this method of analyzing ε-caprolactam, ε-caprolactam is analyzed by liquid chromatography using an electrochemically detecting method. Or ε-caprolactam obtained by the Beckmann arrangement reaction of cyclohexanone oxime is purified, and impurities in it is monitored. In this case, the above-mentioned monitoring of impurities in ε-caprolactam is performed by liquid chromatography using an electrochemically detecting method, and the indicator of purified ε-caprolactam is obtained from the changes of impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for analyzing ε-caprolactam (hereinafter sometimes referred to as “CL”).

[0002]

2. Description of the Related Art ε-Caprolactam is important as a raw material for polyamide resins. [epsilon] -caprolactam can be obtained industrially, for example, by oximeizing cyclohexanone to cyclohexanone oxime and subjecting it to Beckmann rearrangement. An industrially produced ε-caprolactam product inevitably contains impurities. As a quality evaluation item of such ε-caprolactam, there is a so-called Pz value. This Pz value is calculated according to U.S. Pat.
As described in US Pat. No. 3,438, the obtained ε-caprolactam is dissolved in a predetermined amount of water, and a certain concentration
An amount of an aqueous solution of potassium permanganate is added, and expressed as an absorbance at a specific wavelength (a wavelength of about 410 nm is usually adopted) after a predetermined time. In this case, the smaller the amount of impurities consumed by potassium permanganate, the smaller the Pz value.

[0003] In addition, the Pz value is determined by adding a potassium permanganate aqueous solution in the same manner, and then stirring, while stirring, a comparative standard solution (cobalt chloride (CoCl ₂ .6H ₂ O) and copper sulfate (C
uSO ₄ .5H ₂ O) diluted with a predetermined amount of water) until the color becomes the same. In this case, the smaller the amount of impurities consumed by potassium permanganate, the larger the Pz value. There is a substantially proportional relationship (at least a good correlation) between the two (hereinafter, the Pz value by the former measurement method is referred to as “Pz value by absorbance measurement”, and the Pz value by the latter measurement method is referred to as “Pz value by time measurement. Pz value ”). The Pz value has been used as a representative evaluation standard of the quality of ε-caprolactam so far, and the magnitude of this value determines the quality of the product ε-caprolactam.

[0004]

However, there are the following problems with the CL quality evaluation based on the Pz value. First, there is a measurement problem. For example, when measuring the Pz value by time measurement, the Pz value is generally obtained in the unit of several thousands to tens of thousands of seconds, and the measurement takes time, and during this time, it is necessary to continuously observe the ε-caprolactam solution. Briefly, observations are made intermittently at specific time intervals and Pz
It is not impossible to express the value as more than or less than seconds, but in this case a detailed evaluation of the ε-caprolactam quality becomes difficult. Such a time problem in the measurement is the same in the measurement of the Pz value by the absorbance measurement, and the measurement usually requires one hour or more. Also, Pz
Since the value is based on the reaction of impurities by potassium permanganate for measurement, it is necessary to keep the temperature constant during the above-mentioned long-time measurement. Furthermore, it is necessary to use water of the highest purity that does not contain a Pz component as water that is a solvent of the aqueous solution of ε-caprolactam.

[0005] Second, there is little information obtained from the Pz value. That is, since the Pz value expresses the amount of impurities in ε-caprolactam as a single value, it cannot be determined which impurity in ε-caprolactam originates in the change in Pz value. For example, when the Pz value in the product ε-caprolactam fluctuates in an industrial process, if it is not clear which impurity the fluctuation originates from, the condition of which step in the process should be corrected, etc. However, more precise control may be difficult.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and its object is to provide a simpler ε-element which can be used as an evaluation item substantially similar to the Pz value. An object of the present invention is to provide a method for analyzing caprolactam. Another object is to contribute to quality evaluation of purified ε-caprolactam by providing a method for analyzing ε-caprolactam having a greater amount of information on impurities than measuring the Pz value. The present inventors have conducted various studies to achieve the above object, and as a result, have found that the above object can be achieved by adopting a specific analysis method using a chromatogram, and have reached the present invention.

That is, the gist of the present invention is to provide a method for converting ε-caprolactam to liquid chromatography using electrochemical detection.
And, in particular, purifying ε-caprolactam obtained by the Beckmann rearrangement reaction of cyclohexanone oxime, and monitoring the amount of impurities in ε-caprolactam.
-A method for producing caprolactam, wherein the monitoring is performed on the impurities in ε-caprolactam by liquid chromatography using an electrochemical detection method, and an index of purified ε-caprolactam is obtained from the fluctuation of the impurities. Applied. Hereinafter, the present invention will be described in detail.

[0008] The method of synthesizing ε-caprolactam to be analyzed in the present invention is not particularly limited, but generally, a method in which cyclohexanone oxime is subjected to Beckmann rearrangement to obtain ε-caprolactam is employed. As such a synthesis method, for example, a method in which Beckmann rearrangement is performed in concentrated sulfuric acid or fuming sulfuric acid to obtain ε-caprolactam sulfate, followed by neutralization with an alkali, cyclohexanone oxime in the presence of a solid acid catalyst in a gas phase or a liquid phase A method of performing Beckmann rearrangement, a method of performing Beckmann rearrangement in a state where the catalyst is uniformly dissolved in a liquid phase, and the like can be given. When cyclohexanone oxime is brought into contact with sulfuric acid or fuming sulfuric acid, the amount of sulfuric acid or fuming sulfuric acid to cyclohexanone oxime is usually 1.0 to 2.0 in molar ratio. Usually, it is preferable to contact with fuming sulfuric acid. The concentration of free sulfur trioxide in fuming sulfuric acid is usually 1 to 30% by weight. The reaction temperature of the Beckmann rearrangement is usually 60 to 130 ° C., preferably 70 to 1 ° C.
Although it is 00 ° C., the yield tends to be better at lower temperatures. This reaction is exothermic and usually removes heat,
This can be done by externally circulating the reaction solution and / or by providing a jacket with cooling water inside. The Beckmann rearrangement can be performed in one or more stages. The reaction solution subjected to Beckmann rearrangement is usually 130 to 20
By performing heat treatment at a temperature of 0 ° C. for about 0.1 to 10 hours, the quality of decomposing impurities can be improved. The ε-caprolactam obtained by any of the above methods is purified to a product by distillation or crystallization.

In the present invention, ε-caprolactam is subjected to analysis by liquid chromatography, and the most significant feature is that an electrochemical detection method is used as a detection method at this time. Electrochemical detection method (Electrochem
Ical detection (ECD) is a detection method in which a voltage is applied to a sample and an oxidation-reduction current flowing by an oxidation reaction of the sample is detected. Since the susceptibility of the detection component to oxidation is substantially determined by the applied voltage, it is extremely easy to selectively quantify the sample component concentration. An electrochemical detector is a detector that measures a current flowing during a constant potential electrode reaction when an electrochemically active substance undergoes oxidation or reduction on an electrode surface. The specific selectivity of an electrochemical detector is obtained by the fact that only the electrode active substance is detected. In addition, since the active substance has a specific oxidation-reduction potential, and the electrode reaction does not occur in a system in which the set potential of the working electrode does not reach that, the specific selectivity is further enhanced.

Until now, most of the analysis of a sample using the electrochemical detection method has a relatively limited oxidation reaction system, such as those containing phenolic hydroxyl groups such as catecholamines, aromatic amines, and the like. It was used for quantitative determination of cyanide and the like. Therefore, application to analysis of a sample containing an oxidizable component such as ε-caprolactam which is difficult to identify and has an unknown structure has not been known so far. in addition,
The component to be oxidized by a general electrochemical detector is often only a part of the component to be oxidized by potassium permanganate, and this is used as an evaluation item similar to the Pz value. There was no thought. In the present invention, a sample in which ε-caprolactam is used as an aqueous solution is prepared, and the sample is analyzed using an apparatus system for liquid chromatography having an electrochemical detector as a detector. The packing material used for chromatography is not particularly limited, and various materials such as silica gel, chemically bonded silica gel bonded with groups such as octadecyl group and cyanopropyl group, porous polymer, and ion exchange resin can be used. However, from the viewpoint of ease of setting the resolution and analysis conditions, a chemically bonded silica gel is preferable.

[0011]

The ε-caprolactam containing a plurality of impurities is subjected to liquid chromatography to separate each component. Each separated component is detected by an electrochemical detector. The amount of the detected component is expressed as a magnitude of an electrode reaction current generated by an oxidation reaction due to an applied voltage, and is quantified according to the amount of the oxidized component. Note that ε
-Caprolactam itself is not detected by the electrochemical detector when eluted by liquid chromatography.

[0012]

【Example】

Example 1 (Synthesis of ε-caprolactam) Cyclohexanone oxime and 25% fuming sulfuric acid were subjected to a Beckmann rearrangement reaction with a residence time of 1 hour in a reactor. At this time, the stirring speed of the reactor was set to 300 rpm or more in order to suppress local heat generation, and cooling water was flowed through the jacket of the reactor to maintain the reaction temperature at 70 to 100 ° C. The acidity (weight ratio of sulfuric acid to the total of sulfuric acid and cyclohexanone oxime) of the Beckmann rearrangement liquid after the reaction was 51% to 61%.
And the free SO ₃ concentration is 1 to 1
The conditions were changed in the range of 5% to obtain ε-caprolactam of various qualities. The resulting Beckmann rearrangement reaction solution was transferred to a jacketed stirring tank, the SO ₃ concentration was maintained at 1 to 15%, and the mixture was treated at a treatment temperature of 90 to 125 ° C. for 0 to 3 hours while stirring at a stirring speed of 100 rpm or more. ₃ treatment liquids were obtained. The obtained SO ₃ treatment liquid was neutralized with aqueous ammonia. For the neutralization reaction, pass warm water through a jacketed stirring tank,
The neutralization was performed at a temperature of 70 ° C. and a pH of 7.0 to 7.5.

Subsequently, the neutralized solution was extracted with benzene. For the extraction, the mixture was stirred in a stirring tank for 10 minutes and then allowed to stand for 5 minutes to collect an oil phase. The water phase was extracted again with benzene, and a total of three oil phases were collected. The amount of benzene used was adjusted so that the concentration of the theoretical amount of ε-caprolactam was 18% by weight. From the collected oil phase, benzene was distilled off by a conventional method to obtain crude ε-caprolactam. Finally, the crude ε-caprolactam was purified by distillation. In the distillation, after adding an appropriate amount of a 25% aqueous solution of caustic soda to crude ε-caprolactam, the initial fraction was 10% by weight, the main fraction was 80% by weight, and the bottom was 10% by weight.
And the main fraction was subjected to quality evaluation as purified ε-caprolactam.

(Measurement of Pz value by absorbance measurement) The Pz value (Pz value by absorbance measurement) of the obtained purified ε-caprolactam (sample Nos. 1 to 26) was measured by the following method. 1 g of a purified ε-caprolactam sample was added to 100 parts of purified water.
and 1 ml of a 0.01 N aqueous potassium permanganate solution was added thereto, and the mixture was stirred.
The absorbance for light of nm is measured, and P
The z value was used. (Liquid chromatography using ECD) The purified ε-caprolactam (sample N
o1-26), liquid chromatography using ECD was performed. In each case, a total of eight peaks were obtained, and all these peaks (X1 to X8) were quantified. Fig. 1 shows an example of a chromatogram. Note that X
The peak of 6 was identified as aniline.

[Chromatography conditions] Column: MCI gel ODS1HU (Mitsubishi Chemical trade name,
Octadecyl group-bonded silica gel (average particle size about 5 μm)
Is filled in a 4.6 mmφ * 15 cm tube) Eluent: 0.05 M in a water / methanol = 7/3 mixture
Concentrated solution of NaH ₂ PO ₄ (pH 5.5) Flow rate: 1 ml / min Injection volume: 5 μl Detector: +0.9 V Internal standard: toluidine Working electrode: glassy carbon Reference electrode: Ag / AgCl Quantitative method: Aniline was quantified using a standard substance. Other components were converted to toluidine.

(Regression Analysis) A regression analysis was performed using the analysis values (X1 to X8) of the eight peaks on the chromatogram of the liquid chromatography using the ECD and the Pz value (Y) obtained above. went. Table 1 shows the analysis results of the Pz value.
Table 2 shows the results of the regression analysis. According to Table-2,
It can be seen that an extremely good correlation is established between the conventionally used Pz value and the analysis value of the present invention. In addition, even when the production conditions were changed, there was a difference in the peak intensity and the presence or absence of the peak on the chromatogram. In any case, a very good correlation with the Pz value obtained by the absorbance measurement was obtained.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

According to the present invention, each component (Pz component) affecting the Pz value of ε-caprolactam can be separated and detected. Accordingly, in a method for producing purified ε-caprolactam comprising purifying ε-caprolactam obtained by the Beckmann rearrangement reaction of cyclohexanone oxime and monitoring the purity, the monitor is used to remove impurities in ε-caprolactam by the method of the present invention. By analyzing, the fluctuation of the impurities can provide an index of purified ε-caprolactam. For example, P of ε-caprolactam
Information that could not be determined by previous Pz value measurements, such as which impurities increased or decreased during z changes, or what the breakdown of the impurities were for the same Pz value. Can be given.

Further, the converted Pz value obtained from the concentration of each component of the obtained chromatogram has a very good correlation with the Pz value obtained by the conventional method, and thus can be used instead of the conventional Pz value. Further, the method of the present invention can be said to be a simpler method than the conventional method of measuring the Pz value, since the analysis can be performed automatically in a short time. In addition, by quickly controlling the production conditions from the analysis results, it becomes possible to produce ε-caprolactam with more stable quality.

[Brief description of the drawings]

FIG. 1 shows an example of a chromatogram when purified ε-caprolactam is analyzed by liquid chromatography using an electrochemical detector.

Claims (2)

[Claims] 1. A method for analyzing ε-caprolactam, wherein ε-caprolactam is analyzed by liquid chromatography using an electrochemical detection method. 2. A method for producing purified ε-caprolactam, comprising purifying ε-caprolactam obtained by the Beckmann rearrangement reaction of cyclohexanone oxime and monitoring the amount of the impurities, wherein the monitor monitors the impurities in the ε-caprolactam by converting the impurities into ε-caprolactam. A method characterized by performing liquid chromatography using a chemical detection method, and obtaining an index of purified ε-caprolactam from a change in the impurity.