Determination of impurities in industrial caprolactam produced from toluene by SPME and GC–MS

Caprolactam obtained after oximation and Beckman Rearrangement of cyclohexanone is the monomer of nylon-6. As the quality of the nylon-6 fibers is affected by the impurities present in ε-caprolactam, the type and amount of impurities in the cyclohexanone production process should be studied. Characterization of the purity of ε-caprolactam is accomplished using different parameters. Among them, volatile bases evaluate the presence of linear amides as impurities. They act like chain terminations and have a strong impact on average molecular weight and the size of nylon fibers. In ε-caprolactam from cyclohexane oxidation, these linear amides are frequently found, and N-pentylacetamide is one of the most abundant. In this work, it has been proposed and proven that ketones or aldehydes, such as n-heptanones, n-octanones, and hexanal present in cyclohexanone from cyclohexane oxidation, are the precursors of the most linear amides found in CPL. Special attention has been paid to the reactivity of 2-heptanone and 3heptanone in the oximation and Beckmann rearrangement steps since they have a boiling point similar to that of cyclohexanone and are difficult to remove by distillation. To do this, oximation (T = 80 °C and pH = 5) and further rearrangement (in sulfuric media at 100 C) of the aforementioned ketones or aldehydes has been carried out. Oximes and amides produced have been identified by using NMR and/or GC/MSD techniques. The kinetic model for oximation of 2-heptanone and 3-heptanone has been obtained at two temperatures (80 and 85 °C) within the pH range 3.5−6, simulating industrial conditions. In addition, the relative rearrangement rates of the linear oximes have been analyzed, showing the reaction rate to N-pentylacetamide for the production of the linear amides.

Section: Methodsmentioning

confidence: 99%

“…The ramp temperature of the GC oven was programmed to start at 40 °C for 6 min and then ramp up at a rate of 5 K/min to 220 °C, where it was held for 30 min. The carrier gas used was helium, which was set at a constant flow-rate of 1 mL/min . Nitrocyclohexane was used as the internal standard (ISTD).…”

Section: Methodsmentioning

confidence: 99%

Linear Amides in Caprolactam from Linear Ketone Impurities in Cyclohexanone Obtained from Cyclohexane: Kinetics and Identification

Lorenzo

Salvador

Del-Arco

et al. 2019

“…These samples were analyzed by direct injection of the liquid organic samples diluted in BuOH with a CTC CombyPAL (GC sampler 80) and a gas chromatograph (HP6890) coupled with a mass selective detector (MSD HP5973). A capillary INNOWAX column (60 m × 0.32 mm ID × 0.25 μm) as a stationary phase and helium as a carrier gas were used . Nitrocyclohexane was used as the internal standard compound (ISTD).…”

Section: Methodsmentioning

confidence: 99%

“…A capillary INNOWAX column (60 m × 0.32 mm ID × 0.25 μm) as a stationary phase and helium as a carrier gas were used. 34 Nitrocyclohexane was used as the internal standard compound (ISTD).…”

Section: Industrial and Engineering Chemistry Researchmentioning

confidence: 99%

Transformation of Cyclic Ketones as Impurities in Cyclohexanone in the Caprolactam Production Process

Lorenzo

Salvador

Del-Arco

et al. 2019

Nylon 6 fibers are produced by polymerization of caprolactam obtained after oximation and Beckmann rearrangement of cyclohexanone. For this reason, the purity of cyclohexanone used is an important parameter to control the quality of final nylon 6 products. The poor quality of cyclohexanone is related to the presence of some cyclic ketone impurities, such as 2-methylcyclopentanone, n-methylcyclohexanones (n = 2, 3, and 4), and 2-cyclohexen-1-one, which can remain after the purification process of cyclohexanone. In this study, the first step was to study the products promoted during the oximation of these cyclic impurities. The oximation of each impurity was carried out at T = 80 °C and pH = 5, and the reaction products were identified using gas chromatography−mass spectrometry (GC/MS) and NMR. Each cyclic oxime promoted a mixture of cis and trans stereoisomers, and the relative production ratio was determined. Additionally, the operating conditions that can affect the oximation step were studied. To do that, cyclohexanone was spiked with the mentioned cyclic impurities, and the oximation reaction was carried out using an aqueous solution of hydroxylammonium sulfate. The effects of experimental conditions, such as temperature (353.15−358.15 K) and pH (3.5−6), were studied. It was found that the higher the temperature, the higher the impurity conversion. However, when the pH was studied, the conversion of impurities presented a maximum due to the oximation mechanism. Additionally, a kinetic model of oximation reaction of cyclohexanone spiked with the cyclic impurities, based on an apparent constant to take into account that the reaction was carried out in two liquid phases, was proposed to explain the experimental results. The apparent kinetic constants obtained for 2-methylcyclohexanone and 2-cyclohexen-1-one were similar but lower than that for 2methylcyclopentanone. The study was completed promoting the Beckmann rearrangement of the oxime mixtures obtained from the oximation of pure cyclic impurities under mild conditions (sulfuric media at 100 °C). The amides produced were identified by gas chromatograph/mass selective detector (GC/MSD). Cis/trans oxime stereoisomers from n-methylcyclohexanones (n = 2 and 3) and 2-methylcyclopentanone presented the same reactivity to the corresponding amides, but only (Z)-2-cyclohexen-1one oxime reacts in the Beckmann rearrangement to produce its cyclic amide.

“…In addition, Jodra et al studied the influence of several impurities on the product PN, volatile base content, and absorbance at 290 nm. To date, several methods have been employed to study the impurities present in CPL, including gas chromatography (GC), − high-performance liquid chromatography (HPLC), and gas chromatography–mass spectrometry (GC–MS), , with the main impurities detected including CYC, cyclohexylamine, aniline, toluidine, CHO, cyclohexanol, 2-cyclohexen-1-one (2-CYO), methyl-δ-valerolactam, 1,2,3,4,6,7,8,9-octahydrophenazine (OHP), ε-caprolactone, and adipic imide. − However, the majority of the work carried out to date has focused on analyzing the impurities present in commercial CPL, even though these impurities could potentially originate from the raw materials, the various reaction steps, and the separation processes . Thus, with the exception of a single study of the dehydrogenation of cyclohexanol to cyclohexanone, few reports exist on the impurities arising from the above individual processes.…”

Section: Introductionmentioning

confidence: 99%

Impurity Formation in the Beckmann Rearrangement of Cyclohexanone Oxime to Yield ε-Caprolactam

Zhang

Wang

et al. 2017

The main impurities produced in the Beckmann rearrangement of cyclohexanone oxime (CHO) to yield ε-caprolactam (CPL) in oleum were identified as cyclohexanone, 2-cyclohexen-1-one, 2-hydroxycyclohexan-1-one, 1,2-cyclohexanedione, and 1,2,3,4,6,7,8,9-octahydrophenazine. The effects of several operating parameters on impurity formation were also investigated, including stirrer speed, acid/oxime ratio, SO 3 concentration, temperature, and CHO concentration. Although impurity formation is unavoidable in this process, the results indicated that impurity formation was generally reduced with an enhanced mixing performance, a higher acid/oxime ratio, and a higher SO 3 concentration, thereby indicating that these three factors play a key role in determining the impurities resulting from this transformation. Potential formation pathways for the impurities were also proposed and discussed based on the experimental results. These results are therefore expected to contribute to the development of guidelines for process design and reaction-condition optimization for the industrial production of CPL.