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Impurity Formation in the Beckmann Rearrangement of Cyclohexanone Oxime to Yield #-Caprolactam Chencan Du, Jisong Zhang, Liantang Li, Kai Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03824 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017
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Impurity Formation in the Beckmann Rearrangement of Cyclohexanone Oxime to Yield ε-Caprolactam Chencan Du, Jisong Zhang, Liantang Li, Kai Wang, Guangsheng Luo∗ The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
Abstract 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, SO3 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 SO3 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.
Keywords: Beckmann rearrangement; ε-Caprolactam; Octahydrophenazine; Impurity; Reaction pathway
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1. Introduction ε-Caprolactam (CPL), the monomer of nylon-6, is used extensively in the manufacture of high quality nylon-6 fibers and resins.1 Recently, extensive research has been undertaken to develop novel synthetic procedures towards CPL, with the vapor-phase Beckmann rearrangement over solid catalysts2-4 and the liquid-phase Beckmann rearrangement over solid5-7 or organic catalysts8-10 appearing the most promising due to no by-production of ammonium sulfate. However, the vapor-phase process tends to exhibit relatively low selectivity in addition to catalyst deactivation,5-8 while a poor catalytic efficiency and catalyst circulation are issues in the liquid-phase process.6-8 In addition, for the industrial production of CPL, ~90% of CPL is produced using the conventional cyclohexanone (CYC) process, in which the Beckmann rearrangement of cyclohexanone oxime (CHO) is catalyzed by concentrated sulfuric acid or oleum.11 As such, further investigations into the conventional CPL production process are both practical and meaningful in the context of further process development and optimization. In the current industrial route to CPL, a relatively high ratio of acid to oxime (i.e., 1.2–1.7) is employed to decrease by-product formation.12,13 Although the selectivity of the Beckmann rearrangement of CHO can reach >98%, a long downstream purification process involving extraction, evaporation, ion exchange, and distillation is usually required to remove a number of impurities,12 which results in significantly enhanced investment and operating costs. As such, studies into controlling impurity formation in the CPL production process are of particular interest. As previously reported, the presence of organic impurities in CPL can have a detrimental effect on the polymer quality,14 with even trace amounts of some impurities causing quality issues. In this context, Romero et al.15 examined the influence of various impurities on the permanganate number (PN) of CPL, and found that the presence of 1 ppm aniline resulted in a 50% reduction of the PN value. In addition, Jodra et al.16 studied the influence of several impurities on the product PN, volatile base content, and absorbance at 290 nm. To date, several methods have been
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adopted to study the impurities present in CPL, including gas chromatography (GC),16–18
high
performance
liquid
chromatography
(HPLC),19
and
gas
chromatography-mass spectrometry (GC-MS),20,21 with the main impurities detected including
CYC,
cyclohexylamine,
2-cyclohexen-1-one
aniline,
toluidine,
(2-CYO),
CHO,
cyclohexanol,
methyl-δ-valerolactam,
1,2,3,4,6,7,8,9-octahydrophenazine (OHP), ε-caprolactone, and adipic imide.16–21 However, the majority of work carried out to date has focused on analyzing the impurities present in commercial CPL, despite these impurities potentially originating from the raw materials, the various reaction steps, and the separation processes.9 Thus, with the exception of a single study into the dehydrogenation of cyclohexanol to cyclohexanone,22 few reports exist into the impurities originating from the above individual processes. Indeed, although a number of impurities are known to originate from the Beckmann rearrangement stage of synthesis, no reports appear to exist in this area. As such, studies into the impurities formed in the Beckmann rearrangement of CHO would be of particular interest. More specifically, the relationship between impurity formation and the operating parameters has received little attention, although the presence of some impurities has been examined.18,22 However, these studies focused on the CPL production process in general, with little attention being paid specifically to the Beckmann rearrangement process. For example, although it has been verified that 2-CYO is derived from the dehydration of CYC during the CYC production process,22,23 this impurity can also be formed during the Beckmann rearrangement, and so this process will be examined herein. Indeed, the current reported pathway is based mainly on general chemical knowledge rather than on experimental evidence, and so we herein aim to identify the impurities formed during the Beckmann rearrangement of CHO and investigate the influence of stirrer speed, acid/oxime (A/O) ratio, SO3 concentration, temperature, and CHO concentration on impurity formation. A potential pathway for impurity formation is also proposed based on the experimental results.
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2. Experimental 2.1 Materials For the purpose of this study, the following reagents were employed: CHO, CPL, CYC,
2-CYO,
(1,2-CHD),
2-hydroxycyclohexan-1-one
2-chlorocyclohexanone
(2-HCO),
(2-CCH),
1,2-cyclohexanedione
dichloromethane,
ammonia,
concentrated sulfuric acid, and oleum. Further information regarding the purities and commercial sources of these reagents can be found in Table S1 of the Supporting Information. In addition, the purities of the organic reagents were confirmed by conventional methods (i.e., HPLC and GC), while those of the inorganic acids were determined by titration with NaOH. All purchased reagents were used as received without any additional purification. As OHP is not commercially available, it was prepared using a previously reported literature technique,24 and its identity and purity (>98%) were confirmed by GC-MS and H1 NMR spectroscopy. Further details regarding the preparation and analysis of OHP are provided in the Supporting Information.
2.2 Experimental Procedure The Beckmann rearrangement of CHO was carried out in a three-necked, round-bottomed flask (50 mL) placed over an IKA magnetic stirrer (RCT Basic, Germany). To simulate the industrial process, the desired quantity of CPL was added prior to the addition of CHO. This allowed more facile control of the solution temperature due to the larger heat capacity and the lower quantity of heat released compared with the direct addition of CHO to oleum. The operating parameters examined for this process were as follows: A/O ratio (0.8, 1.0, 1.2, 1.4, 1.6), SO3 concentration (0, 3, 6, 9, 12, 15%), CHO concentration (6, 8, 10, 12, 14%), temperature (70, 80, 90, 100 °C), and stirrer speed (90, 200, 360, 500, 800 rpm), where the A/O ratio, SO3 concentration, and CHO concentration were determined using Eqs. 1–3, respectively. The typical reaction procedure employed was as follows. Concentrated H2SO4 (95–98%, 5 g), oleum (20–23%, 5 g), and CPL (>99.5%,
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77.6 mmol) were dissolved in a flask at a stirrer speed of 360 rpm and preheated to 80 °C using an oil bath. CHO (>98%, 8.6 mmol) was then added at a time, and the reaction allowed to proceed for 30 min. After that, the solution was allowed to cool to 25 °C, and the pH was adjusted to 5–6 by the dropwise addition of ammonia. During this process, the pH was monitored on-line using a SevenEasy S20 pH meter (Mettler Toledo, Shanghai). Following treatment with ammonia, a two-phase system was recovered, which consisted of a solution containing the crude CPL (upper phase) and a saturated ammonium sulfate solution (lower phase). The upper phase containing the crude CPL (1 mL) was then extracted with dichloromethane (3 × 2.0 mL) and the resulting organic phase was dried over anhydrous sodium sulfate. The solution was then filtered and analyzed by GC.
A/O ratio =
H2SO4 (mol) + SO3 (mol) CHO (mol) + CPL (mol)
SO3 concentration (%) =
(1)
SO3 (mass) ×100 H2SO4 (mass) + SO3 (mass)
(2)
CHO (mol) ×100 CHO (mol) + CPL (mol)
(3)
CHO concentration (%) = 2.3 Analytical Methods
The collected samples were analyzed using a Shimadzu GC-2014 instrument equipped with a capillary column (30 m length, 0.25 mm i.d., AB-Inowax, Abelbonded) with injector and detector temperatures of 260 and 280 °C, respectively. The temperature of the GC column was increased from 80 to 230 °C at a rate of 20 °C/min and this temperature was maintained for 3 min. Nitrogen was employed as the carrier gas at a constant flow rate of 2 mL/min. For all samples, an injection volume of 1 µL was employed. The external standard method was used to determine the concentrations of CYC, 2-CYO, 2-HCO, 1,2-CHD, CHO, CPL, and OHP within a measurement error of
