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Impurity Content and Quality Definition of Commercial E-Caprolactam L. 0. Jodra, A. Romero, F. Garda-Ochoa,' and J. Aracll Depatiment of Physical Chemistry of Industrial Processes. Faculty of Chemistry, Complutense University, Madrid, Spain
The relation between the conventional analysis for quality of a commercial E-caprolactam and the gas chromatographic analysis has been studied. Impurities affecting the quality are, among others, aniline, 0- and p-toluidine, pentylacetamide, methylvalerolactam and octahydrophenazine. Correlations between the relevant impurity concentrations and the parameters defined as permanganate number, volatile basis, and ultraviolet are proposed. Color and alkalinity do not seem to have any relation with the aforementioned impurities.
Introduction 6-Caprolactam is mostly used for the production of POlyamidic synthetic fibers. In the polymerization process, the quality of the raw material is of paramount importance. €-Caprolactam is manufactured by several processes, most of which are based upon Beckmann's rearrangement of cyclohexanone oxime (Wiest and Hopff, 1943; Elmendord, 1971), previously obtained from cyclohexanone. Other processes such as photolytic rearrangement of nitrosocyclohexane (Taylor, 1962; Ito, 1963) and reduction of nitrocyclohexane by reaction with sodium salts (Donaruma, 1956) are also used. Another possible process (Donati et al., 1968; Sioli and Guiffe, 1974) is the reaction between hexahydrobenzoic and nitrosylsulfuric acids. The impurities present in t-caprolactam depend upon the manufacturing process. The studies carried out here usualIy refer to the impurities common to all manufacturing processes, with special emphasis on those using cyclohexanone as raw material. Basically, the impurities in t-caprolactam can be formed from those impurities present in the raw materials, from intermediate products in the manufacturing steps or in the auxiliary operations or from interactions among all these compounds. In regard to an c-caprolactam manufactured by the oxidation of cyclohexane, the starting raw materials are cyclohexane, oleum, and hydroxylamine. The more common impurities in cyclohexane are n-pentane, n-hexane, methylcyclohexane, n-heptane, and methylcyclopentane. These yield pentylamine, hexylamine, heptylamine, 0-and p-toluidine, and methylvalerolactam. Also n-pentane and n-heptane produce, after the formation of pentanoic and heptanoic acids, n-pentylacetamide and heptamide. The different steps in the above process are the oxidation of cyclohexane, dehydrogenation of cyclohexanol, oximation of cyclohexanone, and rearrangement of cyclohexanone oxime. These impurities are therefore oxidized to the corresponding alcohols,which, in the oximation step, yield the different amines. If the corresponding carboxylic acid is obtained, the final impurity is the amide. Of course, intermediate products, such as cyclohexanol, cyclohexanone, and cyclohexanone oxime, may be present in the E-caprolactam, but also can react and form other impurities, such as aniline and e-caprolactone, both from cyclohexanone in the oxidation and rearrangement steps. Finally, an example of impurity introduced by an auxiliary operation is nitrobenzene, incorporated to the w a prolactam in the extraction with toluene, from which it is an impurity.
Different authors have studied the influence of several impurites in the process of polymerization. Stresinka and Mokry (1974) have studied the effect of cyclohexylamine, cyclohexanol, aniline, cyclohexanone oxime, t-caprolactone, methylcaprolactam,and octahydrophenazine,and Kralicek et al. (1974) have studied isobutylacetamine, n-methylvaleramide, and methylvalerolactam. Fisiuk et al. (1978) studied the influence of, among others, cyclohexanol, aniline, cyclohexanone, and cyclohexanone oxime on an industrial c-caprolactam produced from aniline. Some impurities hinder the polymerization (Kralicek et al. 1974; Fisiuk et al., 1978) because they react with the amidic, carboxylic, or both groups from t-caprolactam to produce a polymer of low molecular weight. The impurities causing this effect are mainly (Stresinka and Mokry, 1974; Kralicek et al., 1974), cyclohexanone, hydroxylamine, cyclohexanone oxime, amides, and amines (cyclohexylamine by a greater order than aniline (Stresinka and Mokry, 1974)). A decrease in viscosity of the polyamide has been reported (Kralicek et al., 1974) when the cyclohexanone oxime, cyclohexanone, and aniline contents increase. However, cyclohexanone reacts with the amidic group, whereas hydroxylamine and aniline react with the carboxylic group. t-Caprolactone, which seems to be hydrolyzed (Stresinka and Mokry, 1974) to t-hydroxycaproic acid, also decreases the viscosity of the product. Other impurities produce different effects. Thus, important quantities of nitrobenzene (Stresinka and Mokry, 1974) give a dark brown polymer instead of the normal light or white color. Alcohols, methylcaprolactams, and octahydrophenazine do not influence the molecular weight of the polymer (Stresinka and Mokry, 1974). However, octahydrophenazine, aniline, and methylvalerolactam, like nitrobenzene, change the color of the polymer. On an industrial basis, the quality of the monomer is determined using conventional quality levels defined according to Standard Technical Gost 7850-74 as: (1) permanganate number, (2) content of volatile bases, (3) ultraviolet, (4) color, and ( 5 ) alkalinity or acidity. These magnitudes, although having a certain physical sense, do not provide concrete information on the existing impurities, such as their nature or quantity. On the other hand, due to the great variety of impurities present in the t-caprolactam, the influence of each impurity in the parameters is not known nowadays. The impurities determining the permanganate number are those which oxidize more easily (Allinger and Cava, 0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 563
Figure 1. Typical chromatogram obtained.
1971; Roberts et al., 1971; Finar, 1961): aliphatic and aromatic amines, oximes, unsaturated compounds, and products formed by oxidation of e-caprolactam. The content of volatile basis depends upon those impurities producing ammonia by heating in a strong alkaline medium (Cotton and Wilkinson, 1972): aliphatic amides and aliphatic and aromatic amines. Ultraviolet absorption is due to impurities containing chromophore groups, which absorb in the region of 290 pm: aromatic amines, heterocyclic compounds, azo compounds, and products formed by oxidation of t-caprolactam (Rao, 1975). The color determining impurities are those absorbing in the visible region: aromatic amines, oximes and azoic colorants (Rao, 1975). Alkalinity or acidity is determined by those im. purities presenting alkaline or acid character. Apart from the above mentioned impurities, there is one more, adipimide, formed by the oxidation during storage of the ecaproladam, which affects permanganate number, ultraviolet, color, and alkalinity (Dsneladze et al., 1976). A relationship between these quality parameters and the abnormal polymerization effects has been proposed. A decrease in the permanganate number produces a slow and irregular polymerization, with a sensible decrease of viscosity. An increase in the volatile base content yields a lower molecular weight polymer. High values in the ultraviolet absorption and color units involve a colored polymer. Alkalinity or acidity, produced by e-caprolactam oxidation, also produce colored compounds, with color maintained until the final product. Many studies have been carried out to relate the conventional quality parameters and the different impurities present in the e-caprolactam. Iliescu (1966) correlated the permanganate number with several impurities detected by gas-liquid chromatography by means of an exponential equation. A similar function was found by Ioseliani and Ru’inskii (1975) for the permanganate number and the volatile basis of a pure 6-caprolactam, to which a mixture of impurities-cyclohexanol, cyclohexanone, aniline, cyclohexanone oxime, and octahydrophenazine-was added. Dsneladze et al. (1976) also correlated by an exponential function the permanganate number. In fact, some authors give their results in a graphic way. The present work also intends to establish a relationship between the quality of waprolactam as given by gas-liquid chromatography and the conventional quality parameters. The studies have been carried out with an industrial t-
Table 1. Material Used compound pentylamine hexylamine heptylamine cyclohexylamine hexylamine cyclohexanone cyclohexanol aniline nitrobenzene o-toluidine p-toluidine cyclohexanone oxime heptamide pentylacetamide meth ylhexamide E-caprolactone methylvalerolactam octahydrophenazine
supplier Merck Merck Merck, Ferosa
Merck Merck Merck Merck
quality
purity
chromatographic chromatographic chromatographic chromatographic
99% 99% 99% 99%
industrial industrial chromatographic chromatographic chromatographic chromatographic industrial
99.99% 99.99% 99% 99% 99% 99% 99.99%
synthesized 98% -_ synthesized 98% -synthesized 98% chromatographic 99% Merck -_ synthesized 98% synthesized
98%
Table 11. Nature and Retention Time of the Different Peaks peak
1 2 3 4 5 6 7 8
9 10 11 12 13
14 15 16 17 18 19
compound pentylamine hexylamine hepty lamine cyclohexylamine cyclohexanone cyclohexanol unknown unknown aniline nitrobenzene 0-, p-toluidine cyclohexanone oxime pentylacetamide unknown E -caprolactone heptamide unknown methylvalerolactam e-caprolactam octahydrophenazine
tR,
22 24 28 33 43 53 75 90 120 160 176 224 27 5
340 420 560 653 7 60 1710
caprolactam measuring the quality by both methods, before and after purifying the product by catalytic hydrogenation with Raney nickel.
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Table IV. Conventional Levels of Quality Results ~
run
PN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
1 5 600 1 9 200 1 3 200 1 9 000 1 5 600 1 3 400 1 4 400 18 000 36 000 21 000 30 000 1 6 600 28 300 28 400 28 800 1 9 000 18 000 1 6 800 19 500 12 000 1 2 000 1 6 000 18 000 22 800 12 800 23 200 1 6 500 18 000 21 000 1 9 500 109 000 109 000 109 000 109 000 94 500 1200
VB 15 19 12 16 13 16 12 11 12 14 12 16 10 10 10 11 14 15 17 17 16 15 14 13 15 15 16 16 12 10 6 6 6 6 7 14
UV
CO
AL
5.10 4.30 4.20 4.70 4.15 5,30 4.65 5.40 3.70 4.40 4.00 5.10 4.25 5.20 3.90 5.70 4.30 5.00 3.70 6.00 7.10 5.80 6.60 5.60 5.70 5.40 7.20 5.00 4.30 4.35 1.85 1.80 1.95 2.00 2.20 6.10
5 10 10 10 10 15 10 5 5 20 10 10 10 10 5 5 10 10 15 10 10 10 10 15 15 10 10 10 10 15 10 10 10 10 15 80
18.6 20.0 22.0 17.6 23.4 21.0 25.0 20.0 9.0 8.0 7.1 6.1 8.2 7.2 8.7 5.7 17.0 19.0 18.0 17.50 12.80 12.80 9.30 4.90 5.40 4.00 6.20 9.20 10.30 10.90 0.10 0.09 0.10 0.26 3.00 -0.26
The t-caprolactam was manufactured by the Inventa process: oxidation of cyclohexane yielding cyclohexanone, which is oximated, and then the cyclohexanone oxime rearranged to a e-caprolactam. Equipment. Purification by hydrogenation of the ecaprolactam was carried out in a slurry reactor. Analytical Methods. The e-caprolactam, as indicated above, was analyzed before and after the hydrogenation by two different methods: conventional and gas-liquid chromatography. The conventional analysis was carried out according to the Standard Technical Gost 7850-74 except the water content analysis, for which the Karl Fischer method was used. The gas-liquid chromatographic analysis was carried out under the following conditions. Chromatograph Hewlett-Packard, 5710 A. Column: material, mild steel; stationary phase, Carbowax 20 M, 20%; solid support, Chromosorb W AW,DMCS 80/100; length, 6 ft; outside diameter, 1/8 in. Temperatures: injection port, 250 "C; flame ionization detectors, 250 "C; oven, programmed, 140
d
"C for 8 min and then heating a t 2 OC min to reach 180 "C. Carrier gas: nitrogen; flow, 35 cm /min. Sample: 6 X L; solvent, methylene chloride. Speed chart: 0.25 in. /min. The c-caprolactam is dissolved in methylene chloride before the analysis (50% weight). Materials. The product to be analyzed is a mixture of e-caprolactam and water (between 3 and 6% of water). The hydrogen used as reactant was supplied by the Sociedad Espaiiola del Oxigeno Co., with a 99.998% purity. The compounds used in the identification of the possible impurities in the c-caprolactam were acquired or synthesized for this purpose with a high purity. The suppliers, quality, and purity are detailed in Table I. Results Impurity Identification, Characterization of the impurities of e-caprolactam is a problem still to be solved, due probably to the number of substances which may accompany commercial e-caprolactam. These substances, apart from their structural complexity, vary with the raw material and the method of production used. Figure 1 shows a typical chromatogram of the ecaprolactam studied. Eighteen peaks can be observed besides that corresponding to the t-caprolactam. Identification of the nature of the different peaks has been undertaken by the methods usually used in gas-liquid chromatography. The results obtained are shown in Table 11. The retention time of the 11th peak corresponds to those of the compounds 0- and p-toluidine and cyclohexanone oxime. It has been identified as corresponding to 0-and p-toluidine because the cyclohexanone oxime has never exceeded 4 ppm as other specific analytical methods have shown. Relationship between Both Methods of Analysis. The quantitative analysis of the samples by both analytical methods before and after hydrogenation are shown in Tables I11 and IV. Table I11 shows the results obtained by chromatography-using the absolute area quantitative method-assuming that all impurities give the same chromatographic response. Table IV shows the conventional parameters for the same samples. Discussion The experimental results indicate that, in the range of concentrations studied, of all the impurities previously mentioned, the ones which seem to have definite influence on the conventional parameters are: aniline, 0- and ptoluidine, pentylacetamide, methylvalerolactam, and octahydrophenazine. The conventional parameters and the concentration of the mentioned impurities have been correlated by linear regression. Table V gives the equations found. A correlation has been also established among the contents of all the relevant impurities and each conventional parameter. Because of the complexity of the calculations, a Fortran IV program was prepared. The equations found are
Table V. Relationship between Compound Contents and Conventional Parameters compound aniline
PN
PN = 117223e-0.07a r = 0.93
0-, p-toluidine
pentylacetamide methylvalerolactam octahydrophenazine
__ PN = 195092e-0*0mu r = 0.94 PN = 1249644e-0*0'f r = 0.88
uv
VB VB = 6.59 r = 0.92
+ 0.25a
VB = 5.66 + 0.88t r = 0.74 VB = 4.88 + 0 . 6 3 ~ r = 0.94 VB = 4.70e-0-08u r = 0.92
__
UV = 1.71 + 0.13a r = 0.96 UV = 1.78 + 0.24t r = 0.77
__
UV = 1.46e0*m'v r = 0.97 UV = 0.52e0.wf r = 0.88
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PN = 57860 - 0.05 exp(-O.OOla) + 0.80 exp(-O,OOlu) + 0.26 exp(--O.OOlf) (1) IrI = 0.99
+
VI3 = 18.47 - 0 . 3 9 ~ 0.28t + 0 . 4 5 ~- exp(-O.OOlu) ( 2 ) Irl = 0.93
UV = 2.17
+ 0 . 6 0 ~- 0.31t - 0.31 exp(-O.OOlu) + 0.07exp(-O.O01f) (3)
Irl = 0.94
As every one of the terms in eq 1 is exponential, small variations in aniline, methylvalerolactam, and octahydrophenazine concentrations result in great variations of the permanganate number. The other two conventional parameters depend on the impurity concentration in a different way. There are linear terms-aniline, o- and p-toluidine, and pentylacetamide for volatile bases and only the former two for the ultraviolet. There are exponential terms-methylvalerolactam for volatile bases and the same compound and octahydrophenazine for the ultraviolet. The impurities whose concentrations have a linear influence have narrow variation ranges, while those with an exponential influence are precisely those with a wider range. No relationship has been found within the limits considered between the concentrations of the different impurities and the conventional parameters of color and alkalinity. It is important to observe the coincidence between the results obtained and those found previously (Dsneladze et al., 1976; Ioseliani and Ru'inskii, 1975), although a different experimental method was used. We also want to point out the possibilities opened by the method described here as it enables one to deduce, using good correlations, which impurities cause the decrease in quality of the t-caprolactam. On an industrial scale, this could contribute toward improving the ment,ioned quality by modifying the process variables and/or
imparting stricter quality standards to the raw material used. Nomenclature a = aniline concentration, ppm AL = alkalinity, mequiv/kg CO = color apha units f = octohydrophenazine concentration, ppm PN = permanganate number, s p = pentylacetamide concentration, ppm r = correlation coefficient t = 0- and p-toluidine concentration, ppm t R = retention time, s UV = ultraviolet absorption u = methylvalerolactam concentration, ppm VB = volatile bases contents, ppm Literature Cited Allinger, N. L.; Cava, M. "Organic Chemistry", 1st ed.;Worth: New Yo&, 1971. Cotton, F. A,; Wilkinson, G. "Advanced Inorganic Chemlstry", 3rd ed.;Wiley: New York, 1972. Donaruma, L. G. US. Patent 2763644, Sept 18, 1956. Donati, I.; Sioli, G.; Taverna, M. Chim. Id.(MUan) 1988, 50(9), 997. Dsneladze, N.; Padiurashvill, V.; Micadre, G.; Kakebadze, N. Tr. Gruz. Polk. Inst. Lenin 1978, 53. Elmendord, J. Gsrman Patent 2 106385, Aug 19, 1971. Flnar, I. L. "Organic Chemlstry", Vol. I, 4th ed.; Longmans, Green 8 Co.: New York, 1963. Flsiuk, L. T.; Lezhina, L. A.; Butkin, V. T. Khim. VoMm 1978, 7 , 5. Iliescu, D. V.; Obreja, G.; Sandescu,'F. Chlm. Ami. (Bucharest) 1968, 4 , 202. Ioseliani, E. G.; Ru'inskii, V. P. Tr-N-i Proek. In-ta. Arot. Prom-stl plodu&tov. Organ. Sinteza. 1975, 34, 49. Ito, Y. US. Patent 3 090 739, 1963. Kralicek, J.; Kondelikova, J.; Kubaner, V.; Zamostny, 2.; Anh, L. T. Chem. Rum. 1874, 24/79(12), 620. Rao, C. N. "Ukravlolet and Visible Spectroscopy Chemlcal Applications", 3rd ed.;Butterworths: London, 1975. Roberts, J. D.; Stewart, R.; Caserb, M. C. "Organic Chemlstry", 1st ed.; W. A. Benjamin; New York, 1971. Sioli, G.; Guiffre, L. wdrocarbon Process. 1874. 54(7), 124. Stresinka, J.; Mokry, J. Chem. plum. 1974, 24(6), 299. Taylor, R. P. U S . Patent 3062612, May 22, 1962. Wiest, G.; Hopff, H. U S . Patent 2297520, Sept 29, 1943.
Received for review September 11, 1980 Revised Manuscript Received April 21, 1981 Accepted May 6,1981
