Ind. Eng. Chem. Res. 2004, 43, 1557-1560
1557
APPLIED CHEMISTRY Effect of Methyl-δ-valerolactams on the Quality of E-Caprolactam Arturo Romero,* Aurora Santos, and Pedro Yustos Departamento Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad Complutense, 28040 Madrid, Spain
Impurities obtained in -caprolactam depend either on the raw material used to obtain cyclohexanone (cyclohexane, phenol, or cyclohexanol) or on the procedure. The usual impurities are cyclohexanone oxime (CHN-ox), aniline, o-toluidine, octahydrophenazine, phenol, -caprolactone, cyclohexylamine, etc. Moreover, methyl-δ-valerolactams (Me-VLMs) are the typical impurities obtained in the -caprolactam production when the latter is prepared from oximation and further Beckmann rearrangement of a cyclohexanone, previously obtained from cyclohexanol dehydrogenation. On the other hand, the effect of these impurities on the PZ parameter, characteristic of the quality of -caprolactam, has been experimentally determined. The value of the PZ parameter gives an idea of the amount and strength of the oxidizable impurities on -caprolactam. To assess the effect of the impurities, these were added in different concentrations to a commercial -caprolactam. It was found that traces of aniline, o-toluidine CHN-ox, phenol, and octahydrophenazine caused a significant reduction in the PZ number while the effect of Me-VLM and cyclohexylamine was less important. Introduction -Caprolactam is mainly used to manufacture nylon 6 and it can be obtained by different processes.1 The designated procedure “conventional route” is the most used and is based on the reaction of cyclohexanone with hydroxylamine and subsequent Beckmann rearrangement of the oxime formed.2,3 Cyclohexanone is obtained both by hydrogenation of phenol4 or from cyclohexane oxidation and further dehydrogenation of the cyclohexanol formed as coproduct in both processes.5 The partial benzene hydrogenation6 leads to a new raw material to obtain cyclohexanone, the cyclohexene, which can be transformed into cyclohexanol for hydration7 and this into cyclohexanone by dehydrogenation. The major drawback of the conventional technology is that the process coproduces large quantities of ammonium sulfate, amounting to 1.6-4.0 tons per ton of caprolactam. As a result, additional facilities are required to produce sulfuric acid as a catalyst as well to recover the coproduced ammonium sulfate, adding to the manufacture of caprolactam. Technological alternatives are developed by Sumitomo, direct ammoximation of cyclohexanone from ammonia and hydrogen peroxide,7 and by BASF, caprolactam is obtained from butadiene.8 However, most of the caprolactam production is currently carried out by the conventional process. The properties of the polyamide obtained in the polymerization process depend on the impurities that contain the -caprolactam.10 The commercial monomer must have a high purity so that the polymer fulfills the requirements imposes by current applications, mainly by fiber specifications. * To whom correspondence should be addressed. Tel./Fax: 34-91-3944171. E-mail:
[email protected].
Because of the great quantity of impurities that can be contained in the -caprolactam, general parameters are employed to characterize its purity level, such as the determination of the permanganate number (PZ),11 the volatile bases content (VB),12 and the absorption to a given wavelength (UV),13 through standards methods. These indirect methods are employed because of the difficulty in identifying and quantifying every single impurity that affect the quality of the -caprolactam within the nylon 6 production process. Furthermore, these parameters supply good information to predict the characteristics that the polymerized -caprolactam will have. However, they have the drawback of not providing information about the cause of the decrease in the quality of the -caprolactam. Because of the great number of chemical transformations and separation operations required to transform the raw materials into -caprolactam, the impurities can be formed somewhere in the process steps. They can be produced from the reactants used in the process, from by-reactions in the different transformations that take place, from the lack of efficiency of the separation processes and even from the degradation and ring opening of the -caprolactam. A great effort has been made to know, to separate, and to identify the impurities in -caprolactams from several raw materials,14-19 separating the impurities by gas and/or liquid chromatography and employing different procedures to identify the isolated peaks. The main impurities found are cyclohexylamine, aniline, toluidine, cyclohexanone oxime, octahydrophenazine, methyl-δ-valerolactams, -caprolactone, phenol, nitrobencene, heptylamine, cyclohexanol, and hexylamine. Although some of these impurities are presented in concentrations lower than 10 ppm, they can produce, depending on their reactivity, signifi-
10.1021/ie0306527 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/28/2004
1558 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004
cant variations in the properties on the polyamide formed within the polymerization reaction.15 Few papers have studied the influence that these impurities have on the quality and properties of the polyamide18,20 and the results depend on the conditions in which the polymerization was carried out. To avoid the dependency of these conditions, a relationship between the quality of the -caprolactam measured by the conventional parameters cited above and the quantification of their impurities is required. Methyl piperidones, also named methyl-δ-valerolactams, are the major impurities when -caprolactam is obtained from cyclohexanol produced by partial hydrogenation of benzene. These valerolactams are formed from the methylcyclopentanols impurities on the cyclohexanol. In the literature some efforts have been made to develop a process to eliminate the reactants that can produce the methyl-δ-valerolactams21 but the effect of the impurities on the -caprolactam quality has not been published but remains mainly as a company know-how. Cyclohexanone oxime results as an impurity in any classic process of -caprolactam synthesis because the oxime is the reactant employed in the rearrangement stage. In this work the effects of impurities mentioned before on the PZ number of a -caprolactam obtained from cyclohexanone is quantified by adding these impurities at different concentrations to a solution of -caprolactam. The relative influence of each impurity will be analyzed. Experimental Section The -caprolactam was obtained by oximation of cyclohexanone with hydroxylamine sulfate to obtain the cyclohexanone oxime (CHN-ox).
Cyclohexanone reagent was obtained from cyclohexanol dehydrogenation after separation of nonreacted cyclohexanol as described in a previous work.22 Cyclohexanone was further purified through several steps to eliminate the main impurities (methyl cyclopentanols and methyl cyclopentanones). The oximation was carried out in a semibatch operation. Two hundred grams of an aqueous solution 45% in weight of hydroxylamine sulfate was thermostated at 85 °C in a 500-mL three-necked glass flask. The pH of this solution was set to 4.5 by adding a 20% aqueous solution of NH3. Then, 100 mL of cyclohexanone was fed to the flask at a liquid flow rate of about 0.8 mL/ min and the pH was set between 3.5 and 4.5 by adding the aqueous 20% NH3 solution when necessary. Reaction temperature was set at 85 ( 1 °C. After the addition of the 100 mL of cyclohexanone, an amount of 10 g of the HAS solution was added, and the reaction media was maintained for 45 min at 85 °C for cyclohexanone quantitative consumption. Organic and aqueous phases are separated in a preheated glass funnel and the organic phase (cyclohexanone oxime and organic impurities) is recovered for the next reaction step. A sample of this organic phase is analyzed by GC/MS. Afterward, the Beckmann rearrangement of cyclohexanone oxime obtained was carried out in concentrated sulfuric acid media to yield -caprolactam.
This reaction was done in a semibatch manner by adding slowly 50 g of CHN-ox over 250 mL of H2SO4 (96%) and thermostated at 75 °C in a 500-mL glass flask. After addition of cyclohexanone oxime, the reaction medium was maintained for 120 min at 75 °C in the flask until the oxime was quantitatively reacted, which was confirmed by GC/MS analysis. The flask was closed during the reaction to increase the SO3 content. Then the flask was cooled to room temperature and the reaction media was neutralized with NH3 (30% v/v) at a temperature of 15 °C. The organic phase was extracted by three fractions of 100 mL of benzene. Benzene was removed in a vacuum and the organic phase containing -caprolactam was distillated in a short-path distillation apparatus (T ) 72 °C at 266 Pa). Distillated -caprolactam was analyzed to quantify the impurities content. All the reactants were purchased from SigmaAldrich. Impurities added to this distillated -caprolactam are cyclohexylamine, aniline, toluidine, cyclohexanone oxime, octahydrophenazine, methyl-δ-valerolactams, -caprolactone, phenol, nitrobencene, 2-heptylamine, cyclohexanol, and hexylamine. Because of the purity of cyclohexanone employed and the quantitative consumption of reactants on both oximation and Beckmann rearrangement steps, the content of the impurities identified was lower than the detection limit (≈1 ppm). The cyclohexanone oxime synthesis procedure was previously mentioned. The 2-methyl-δ-valerolactam is not a commercial product and it was synthesized from 2-methylcyclopentanone (98%, Aldrich) by a similar procedure as previously followed for -caprolactam from cyclohexanone: 2-methylcyclopentanone oxime was obtained by oximation of 2-methylcyclopentanone and methyl-δ-valerolactam was synthesized by Beckmann rearrangement of 2-methylcyclopentanone oxime. The 1,2,3,4,6,7,8,9-octahydrophenazine was synthesized from cyclohexylamine in three reaction steps. In the first phase of the syntheses, N,N-dichlorocyclohexylamine was prepared in 80% yields from the cyclohexylamine by acidifying a suspension of the amine in potassium hypochlorite solution with acetic acid in a rapid procedure. Next, the clear yellow oil of crude N,N-dichlorocyclohexylamine was added dropwise to a methanolic sodium methoxide solution (vigorous stirring was used to maintain a gentle reflux), to obtain R-aminocyclohexanone hydrochloride aqueous solution in 50% yields by hydrolysis with ice and addition of concentrated hydrochloric acid. A potassium hydroxide solution was added to the R-aminocyclohexanone hydrochloride aqueous solution, followed by a hydrogen peroxide addition. A crude product of 1,2,3,4,6,7,8,9-octahydrophenazine was collected, air-dried, and recrystallized from petroleum ether, givinq a 40% yield of purity product.23 The rest of the impurity compounds in Table 1 were purchased from Sigma-Aldrich. Analytical Methods Impurities of Asahi cyclohexanone have been analyzed by GC/MS (HP6890 GC-MS, detector MSD 5973 by using a DB1 J&SCIENTIC (polydimethyl siloxane) 30 m × 0.25 mm × 1 µm column at the following
Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1559 Table 1. Impurities Added to the Distillated E-Caprolactam; PZo ) 17000 s impurity
acronym
Cimpurity (mg/kg -caprolactam)
aniline -caprolactone cyclohexanol cyclohexanone oxime cyclohexylamine heptylamine hexylamine methyl-δ-valerolactam nitrobencene octahydrophenazine phenol p-toluidine
ANL CLN CHL CHN-ox CHA HPA HXA Me-VLM NBZ OHP PhOH TOL
2-10 100-2000 1000-10000 10-1000 500-10000 1000-12000 500-5000 100-3000 500-10000 10-1000 10-500 1-10
Figure 2. Effect of octahydrophenazine, caprolactone, cyclohexylamine, and nitrobencene on the PZ value.
Figure 3. Effect of methyl-δ-valeloractam, hexylamine, cyclohexanol, and heptylamine on the PZ value. Figure 1. Effect of aniline, o-toluidine, cyclohexanone oxime, and phenol on the PZ value.
conditions (method 1): carrier gas, helium, 1 mL/min, Tinjector ) 230 °C, Tdetector ) 250 °C. Split 25:1, oven temperature 50 °C for 30 min. The injection volume was 1 µL. Products and impurities of cyclohexanone oximation and Beckmann rearrangement were analyzed by using the DB1 J&SCIENTIC column described above. Analysis conditions were as follows (method 2): carrier gas, helium, 1 mL/min, Tinjector ) 280 °C, Tdetector ) 230 °C. Split 25,1:1. Oven temperature To ) 100 °C, rate 10 °C/ min, T1 ) 280 °C, 1 min. The injection volume was 1 µL. This sample was obtained by dissolving 1 g of solid (cyclohexanone oxime or -caprolactam) in 88 g of benzene. The protocol followed for the PZ determination was as follows: 3 g of -caprolactam was dissolved in 100 mL of ultrapure distillated water by adding 1 mL of an aqueous solution of potassium permanganate, 0.002 M, with agitation. The time required (in seconds) to reach the same color as a reference solution (3 g of (NO3)2Co + 12 mg of K2Cr2O7 in 1 L of water) was measured. Therefore, the PZ number is the time required by the oxidizable impurities in -caprolactam to reduce a fixed amount of KMnO4. The procedure was described in the literature.24 Influence of Impurities on PZ Number The PZ number of the distillated -caprolactam was measured by the procedure described above and a value of 17000 s was obtained (PZo). The amounts of impurities added are summarized in Table 1. The ratios PZ/ PZo for each sample obtained as a function of impurity contents are shown in Figures 1, 2, and 3. From these figures it can be seen that a significant difference is found in PZ reduction depending on the impurity. For example, 1 ppm of aniline produces the same PZ decrease as 1000 ppm of heptylamine. To make an easy assessment of the results, the required amounts of each
Table 2. Comparison of PZ Reduction for the Impurity Tested: mg of Impurity/kg of E-Caprolactam Required To Produce a X Reduction on the Permanganate Number impurity
X ) 10%
X ) 50%
aniline o-toluidine cyclohexanone oxime phenol octahydrophenazine -caprolactone methyl-δ-valerolactam cyclohexylamine nitrobencene hexylamine cyclohexanol 2-heptylamine
