Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2019, 58, 11878−11890
Linear Amides in Caprolactam from Linear Ketone Impurities in Cyclohexanone Obtained from Cyclohexane: Kinetics and Identification D. Lorenzo,* A. Romero, L. Del-Arco, and A. Santos Chemical Engineering and Materials Department, Universidad Complutense de Madrid, Madrid 28040, Spain
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S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION
After oximation, the Beckmann rearrangement (BR) of cyclohexanone oxime in oleum media produces CPL.7,18,19 However, besides CPL, other byproducts are formed in the Beckmann rearrangement process. These byproducts can be attributed to the evolution of the impurities originally present in ONE stream19−21 as well as those formed from cyclohexanone oxime or CPL through the oximation,19,20,22 rearrangement, purification, or storage steps.7,23,24 The quality of the nylon-6 fibers is closely related with the impurities present in CPL. The type and amount of impurities have marked negative effects on the quality of the nylon-6.19,22 Quantification and identification of impurities gives valuable information about the quality of CPL, but it is a complex task. The main difficulties are the great variety and low concentration.7,25,26 The most common methods are based on global parameters to determine CPL quality can be classified as physical, in which a physical property is measured, such as
Nylon-6 is produced by ring-opening polymerization of εcaprolactam (CPL). More than 98% of the CPL is produced using cyclohexanone (ONE) as an intermediate. ONE is obtained either by hydrogenation of phenol or by catalytic and noncatalytic oxidation of cyclohexane with air or by hydration of cyclohexene to cyclohexanol followed by dehydrogenation.1,2 The most common ONE production process is promoted by oxidation of cyclohexane in the presence of catalytic metal salts to obtain a reaction mixture known as KA-oil, which contains ONE, cyclohexanol, and other impurities.3−5 ONE, after being purified from KA-oil, reacts with hydroxylamine (HA) (usually added as hydroxylamine sulfate (HAS)) to produce cyclohexanone oxime.1,6,7 In this reaction, the sulfuric acid formed is neutralized using ammonia, and ammonium sulfate is obtained as a byproduct.1,6,8−10 To avoid the formation of ammonium sulfate (AS), the ammoximation of cyclohexanone has been recently proposed. In the ammoximation of cyclohexanone, aqueous H2O2 and ammonia react with ONE by means of solid titanosilicate as a catalyst (known as TS1).1,6,11−17 © 2019 American Chemical Society
Received: Revised: Accepted: Published: 11878
April 15, 2019 June 4, 2019 June 6, 2019 June 7, 2019 DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
Article
Industrial & Engineering Chemistry Research
2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemical used as reactants in the oximation reactions were hexanal (6AL), 2-heptanone (2− 7ONE), 3-heptanone (3−7ONE), 4-heptanone (4−7ONE), 2octanone (2−8ONE), 3-octanone (3−8ONE), and hydroxylamine sulfate (HAS). Caprolactam (CPL) and ammonium sulfate (AS) were also used. The reaction samples were prepared using 1-butanol as solvent and 1−4-benzodioxane N-ethylaniline as standard internal compounds (ISTDs) to quantify. In Table S1, provided as Supporting Information, the chemical formula, the suppliers, the purity, and the molecular weights of the chemicals used are summarized. 2.2. Experimental Setups and Procedure. 2.2.1. Synthesis and Purification of Pure Oximes from Linear Ketones and Hexanal. The oximes were synthesized from the pure linear ketones or 6AL in a semibatch stirred reactor, shown in Figure 1.
appearance and solubility, the solidification point, the color number that is expressed in APHA (American Public Health Association) units and the absorbance at 290 nm of a 50% aqueous solution of CPL. In the second group of tests, a chemical reaction is promoted to quantify the amount of impurities. These techniques include permanganate number (PZ), volatile bases content (VB), absorption to a given wavelength (UV), free bases or acids, and residual cyclohexanone oxime.10 These quality tests for CPL are based on the properties of impurities as lumping compounds. However, the behavior of CPL in the polymerization process and the properties of the fibers (as color, strength, etc.) will depend on the specific impurities present in CPL. In this sense, a “chromatographic quality test” would be a more consistent criterion for the caprolactam quality assessment. The impurities in commercial caprolactam has been widely studied applying different analysis techniques, mainly by GC/MSD18,19,21,25−27 HPLC.23 As mentioned before, one of the key parameters used to evaluate CPL quality is the volatile base (VB) content, measured by the ISO 8661 method. This method is based on the reaction of a CPL sample with sodium hydroxide producing the hydrolysis of the amide impurities and generating amines (volatile bases) that are distilled and collected in a HCl solution. The amount of amines produced is determined by titration of the remaining HCl. The content of VB is expressed either in mequiv/kg or in mg of NH3/kg.7 Therefore, the VB value is directly related to the presence of linear amides in CPL. Linear amides reduce the chain length in CPL polymerization, producing lower quality nylon. Linear amides could be related to the presence of linear ketones or aldehydes in cyclohexanone.19,20,25,28 On the other hand, the most probable linear ketones in distilled cyclohexanone will be those with similar boiling points of ONE (155 °C) as 2-heptanone (2−7ONE, 151 °C) and 3heptanone (3−7ONE, 147 °C). Moreover, impurities like 4 heptanone (4−7ONE 143 °C), 2-octanone (2−8ONE 173 °C), and 3-octanone (3−8ONE, 170 °C) could be present in ONE as the purification process is not good enough, although only in small amounts. On the basis of our knowledge, oximation and BR of these linear ketones has not been previously studied in literature. However, oximation and BR of these ketones could explain the formation of the amides commonly found in CPL, Npentylacetamide and N-butylacetamide.19,21,23,25 Therefore, in this work, the oximation and BR of the linear ketones mentioned above is studied. Furthermore, while hexanamide has been found as an impurity in CPL,23,27 oximation and BR of hexanal (6AL) have also been achieved. The oximation reaction will be tested with HA, and BR will be studied in acid. First, the synthesis of oximes from pure linear ketones will be carried out in order to identify the oximes produced. The main product and the distribution of stereoisomers will be identified by mass spectra and NMR techniques. The BR of these oximes in CPL will be studied to identify the amides formed. In addition, oximation of cyclohexanone doped with the most probable linear ketones (2−7ONE and 3−7ONE) will be carried out to determine the kinetic oximation model of these impurities.
Figure 1. Reaction setup used for oximation and Beckmann rearrangement reactions.
A thermostatic glycerine bath was used to maintain a constant temperature. The temperature was fixed using a thermocouple (IKA C-MG HS 7) and a PID controller. The syntheses were carried out at 82 °C and pH = 5. The oximation reaction takes place in two liquid phases, one of which is an organic phase compounded by the ketone or aldehyde and the oxime produced. The other one is an aqueous phase compounded by ammonium and hydroxylamine salts in water. The reactor was initially loaded with an aqueous solution of 45% w/w of HAS and heated until the reaction temperature was reached. The pH of this aqueous solution was about 3.2, so ammonium solution (25% w/w) was added to reach pH = 5. The initial organic phase (pure ketone or aldehyde) was separately heated until the reaction temperature and added to the reactor. The runs were carried out with an HAS excess, stc CHAS,0/(CHAS, = 1.1, where CHAS,0 was the initial HAS concentration and Cstc HAS, the stoichiometric HAS concentration 11879
DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
Article
Industrial & Engineering Chemistry Research
Table 1. Oximation Runs Carried out with Cyclohexanone Spiked with 300 mg/kg of 2- Heptanone (2-7ONE) and 3-Heptanone (3-7ONE)a run
T, K
pH
CONE,0, mmol·(kg−1 org)
C3−7ONE,0, mmol·(kg−1 org)
C2−7ONE,0, mmol·(kg−1 org)
CONEOX,0, mmol·(kg−1 org)
CHAS,0/(Cstc HAS,)
1 2 3 4 5 6
353 353 353 358 358 358
3.5 4.5 6 3.5 4.5 6
2038 2035 2030 2030 2032 2035
2.63 2.65 2.64 2.64 2.63 2.64
2.64 2.63 2.64 2.63 2.64 2.64
7071 7074 7078 7078 7077 7074
4.67 4.66 4.67 4.67 4.66 4.66
Aw = 100 rpm. Waq = 0.130 kg. Worg = 0.075 kg. S = 30.9 cm2. CHASo = 2471 mmol. kg aq.−1
a
required for oximation of the ketone or aldehyde added. During the oximation, free acid was formed, and the pH was controlled by adding an ammonium solution (25% w/w). To do this, a dosing device (800 Dosino Metrohm) and a pH glass electrode with temperature compensation (Methohm) were used. It was considered that the reaction ended when the required addition of NH3 to keep a constant pH was almost negligible. Both phases were well mixed using a magnetic plate (IKA C-MG HS 7) to avoid mass-transfer limitations. The organic phase was separated by decantation from the reaction mixture. The organic phase was rinsed three times with water (milli-Q purity) to remove the dissolved salts. The washed organic phase was mainly compounded by pure oxime. However, it was distilled under vacuum conditions (T = 80 K P = 7 mbar) using a glass oven (Büchi B-585) along with a vacuum pump (Büchi V-300) to ensure high purity of oxime. In this step, the unreacted ketone or aldehyde was removed from the final product. 2.2.2. Oximation of a Mixture of Cyclohexanone, 2Heptanone (2−7ONE), and 3-Heptanone (3−7ONE). To determine the kinetic model of the most probable linear ketones in cyclohexanone (ONE), some reactions were tested by spiking a mixture of ONE (20% w/w), 2−7ONE (C2−7ONE), and 3− 7ONE (C3−7ONE) (300 mg·kg−1 each heptanone) in cyclohexanone oxime (balance). The reaction took place in the batch reactor shown in Figure 1. Temperature and pH used are summarized in Table 1. The temperature should be high enough to ensure that the oxime keeps in the liquid phase (the melting point of cyclohexanone oxime is 74.6 °C with a content of 3 wt % of water7,29). However, if the temperature increases above 85 °C the losses of the free HA are significant while this is a volatile compound. Therefore, the temperature range selected was 80− 85 °C, which is the industrial operation range of temperature.7,29 The oximation reaction involves two liquid phases. The organic phase (Worg= 75 g) has the composition mentioned above and the aqueous phase (Waq= 130g) has a 45 wt % in HAS. Temperature and pH were kept constant in each reaction. As used elsewhere,30 an IKA laboratory agitator (RW 20 digital) was used for mixing in these runs in order to obtain a low agitation speed (Aw = 100 rpm) that produces a defined interface between both phases. This area corresponds to the transversal section of the batch reactor (S). The run 2 was carried out at agitation speed of 50, 100, and 150 rpm. Small differences were found in the organic composition with time. Besides, the phases were well separated at visual aspect with a defined interphase. Therefore, the agitation value of 100 rpm was selected to carry out the kinetic runs. The progress of the reaction was monitored taking samples of about 1g of both the organic and aqueous phases at different reaction times. The organic phase was diluted in 4 mL BuOH and the aqueous phase in 4 mL of pure water in order to avoid
salt precipitation at room temperature. The organic phase was filtered through a 40 μm filter and analyzed. Triplicate tests were performed, and the average values were used for figures with an experimental error below 5%. 2.2.3. Beckmann Rearrangement (BR) Runs. The linear oximes synthesized from the pure ketones and hexanal were used to spike pure CPL (0.5%w impurity: w CPL). The mixture was heated to 313.15 K to melt the CPL obtaining a homogeneous liquid solution. About 15 g of this mixture was dissolved in 15 g of sulfuric acid, in a reactor similar to the schematic in Figure 1. The BR runs were carried out at 100 °C for 240 min, taking samples at different reaction times. The reaction media is known as sulfate lactam oil (SLO), which is a mixture of lactam sulfate salts and free sulfuric acid, which must be neutralized. SLO was neutralized at 80 °C using an ammonium solution (25% w/w) with a dosing device to ensure a final pH of 8. Under these conditions, three phases were formed in the reactor: (1) an organic phase comprising a mixture of lactams and water, known as lactam oil (LO); (2) an aqueous phase made up of a saturated solution of ammonium sulfate; and (3) a solid phase formed by precipitated ammonium sulfate. LO was separated by decantation of the solid and aqueous phases. The lactam was obtained by distillation of the free water within the LO mixture (T = 110 °C, P = 8 mbar in a Büchi V300). The solid formed was used to characterize the reaction products during the Beckmann rearrangement. 2.3. Analytical Methods. The organic phase samples from the oximation runs were analyzed by injecting the liquid organic samples into a HP6890 gas chromatograph, along with a HP5973 mass spectrometric detector (MSD) and a CTC CombyPAL (GC sampler 80). A capillary INNOWAX column (60 m × 0.32 mm i.d. × 0.25 μm) was used as a stationary phase. 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.31 Nitrocyclohexane was used as the internal standard (ISTD). 1 H NMR and 13C MNR spectra of pure synthesized oximes were recorded on a Bruker AVANCE300 MHz spectrometer (CDCl3 was used as an internal standard). The aqueous phase was used to determine the concentrations of free sulfuric acid, SA, and HAS by titration with a solution of NaOH with a known concentration using a Metrohm pH glass electrode. The solid samples of CPL, spiked with the impurities generated in the Beckmann rearrangement, were analyzed by headspace solid-phase microextraction (SPME) using a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/ PDMS) fiber. One gram of sample was added to a 10 mL vial, and the SPME fiber was exposed to the headspace of the vial 11880
DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
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Industrial & Engineering Chemistry Research during 3600 s at 40 °C. After achieving equilibrium, the analytes absorbed were analyzed promoting the desorption in the GC/ MSD injector conducted at 270 °C (180 s) in splitless mode. The GC oven was programmed with the same temperature ramp as that used in the direct liquid injection. Software MassHunter B08 (Agilent Technologies) was used to obtain the mass spectra of each peak. The identification of the compound related to each peak in the GC/MS chromatogram was carried out by comparing the results obtained with the NIST library records32 (version NIST011) and/or using the Wiley Databases provided by Scifinder Database (Academic access). Assignation of compounds to chromatographic peaks which mass spectra could not be identified with the databases before cited was accomplished taking into account the isotopic masses predicted by the tool provided by Loos et al.,33 the elution time and the area distribution among the stereoisomers (the area ratios of the amides formed in BR should be in agreement with area ratios of the corresponding oxime stereoisomers). In addition, it when available, it was taken into account the information found in the literature for the spectra of some linear amides.
Table 2. Synthesized Oximes from Pure Hexanal and Linear Ketones Presented in the ONE Are Obtained from Cyclohexane Oxidation To Produce CPLa Xjb
purityc
target
QF1
QF2
29.26
94
>99.9
73
114
101
30.02
>99.9
59
72
86
29.87
97
>99.9
87
100
55
98.7
>99.9
73
86
55
32.08/32.21
97.3
98
87
55
72
32.76
98.2
99
73
86
55
tR (min)
compd 4-heptanone oxime (Z)-1-hexanal oxime (E)-1-hexanal oxime (Z)-3-heptanone oxime (E)-3-heptanone oxime (Z)-2-heptanone oxime (E)-2-heptanone oxime (Z)-3-octanone oxime (E)-3-octanone oxime (Z)-2-octanone oxime (E)-Z-octanone oxime
3. RESULTS AND DISCUSSION 3.1. Oximation of Pure Ketones and Hexanal. The oximation reactions were tested with pure ketones or hexanal
29.98 30.50 30.67
32.93
a
Retention time (tR), target ion, and quualifiers (QF1 and QF2) are calculated from the results obtained by GC/MSD. bConversion of pure ketone or hexanal. cPurity calculated as the purity of the racemic mixture.
Scheme 1. Oximation Reaction of (a) Ketones and (b) Aldehydes
Table 3. Ratio of Cis (Z)/Trans (E) Oxime Stereoisomers Formed from the Linear Ketones and Aldehyde promoter compd 4−7ONE 3−7ONE 2−7ONE 2−8ONE/3−8ONE 6AL
Sj =
using hydroxylammonium sulfate as a reagent. In Scheme 1, the general reaction path is summarized, where a stereoisomer mixture compounded by cis/trans isomers is expected. The organic compounds resulting from the reaction was identified by GC/MSD. The mass spectrum of the chromatogram peaks was obtained, and the nonreacted ketone was quantified. After distillation of the oximation mixture, the nonreacted ketone or aldehyde were removed by distillation obtaining the isomer mixture of oxime. In Table 2, the conversion of pure ketones or hexanal (Xj), the purity after distillation step, the retention time (tR) in the chromatogram and the main qualifiers in the mass spectra are summarized. The conversions were calculated from the GC/MSD results. These values were also confirmed using the amount of NH3 consumed in the oximation reaction and with the remaining HAS quantified by titration with NaOH, as summarized in eq 1. The selectivity was calculated with eq 2, considering the amount of oxime produced Xj =
n j − nj 0
nj
0
=
VNH3 ρNH C NH3 3
t
nj
0
=
noxime nj ,0 − nj
(Z) oxime isomer, %
(E) oxime isomer, %
non-stereoisomers promoted 50 50 27 73 not identified 44 56
(2)
where nj and nj,0 (j = ketones, hexanal) are the mol of compound j and the initial mol used, respectively, VNH3 is the total volume of NH3 wasted during the oximation reaction, and ρNH3 and CNH3 are the density at 25 °C and the concentration of the ammonia aqueous solution used in mol/L. The required volume of sodium hydroxide and the concentration of the alkali ion the titration method are represented by VNaOH and CNaOH. Selectivity values obtained were higher than 98% in all cases The oxime stereoisomers obtained in the oximation of linear ketones are shown in Figure S1, where a mixture of 10000 mg· kg−1 of pure oximes of n-heptanone (n = 2−4) and n-octanone (n = 2 and 3) diluted in 1-butanol were analyzed. In Table 2, the main qualifiers m/z (target, QF1 and QF2) were selected as the most abundant ions detected in the mass spectra shown in Figures S2−S7 of the Supporting Information. As expected, the 4−7ONE oxime produced only one stereoisomer. On the contrary, in the chromatograms of the final samples of the oximation reaction of pure 2−7ONE, pure 3−7ONE, 2−8ONE, 3−8ONE, and 6-AL, two peaks were detected, which correspond to the stereoisomers of the oximes.
0.5(nHAS0 − VNaOHC NaOH) nj
0
(1) 11881
DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
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Figure 2. Influence of pH on the remaining ONE (1-XONE) profile vs time, runs 1−6 in Table 1: (a) T = 80 °C, (b) T = 85 °C. Symbols depict experimental results and lines depict the values predicted by the kinetic model using the model and parameters reported in a previous study.30
Figure 3. Influence of pH on the remaining 3-heptanone (1-X3−7ONE) profile vs time, runs 1−6 in Table 1: (a) T = 80 °C, (b) T = 85 °C. Symbols depict experimental results and lines depict the values predicted by the kinetic model using eqs 11−15.
The tR of the oxime stereoisomers are summarized in Table S2 of the Supporting Information. The mass spectra of each peak in the GC/MS chromatogram was compared with the NIST011 library records.32 Of all the oximes obtained, only the mass spectra of 4−7ONE oxime was included in this library (Figure S2 of the Supporting Information). Using the GC/MSD method, both stereoisomers produced in the oximation reaction of each ketone or hexanal were well separated. However, the mass spectra of the isomers of the same oxime were very similar (given in Figures S2−S7 of the Supporting Information), which meant that assigning the peaks was not possible. Assigning the GC peaks to the cis or trans configuration of the oxime stereoisomers was carried out by analyzing the stereoisomer mixture, which was produced in the oximation reaction of the pure ketones or hexanal, by NMR. The spectra of the pure oximes are summarized in Figures S8−S11 of the Supporting Information.
As seen in Figure S8, the NMR analysis of the purified oxime confirms that oximation of 4−7ONE produces only one compound, since 4−7ONE is a symmetric ketone. On the contrary, the oximation of 3−7ONE yields two stereoisomer compounds. From the NMR analysis (Figure S9 of the Supporting Information), both isomers of 3−7ONE oxime can be identified using the triplet signals at chemical shift δ 0.95 and 0.85 ppm. These signals are generated by the protons of the CH3 group (position 1 in the carbon chain). The highest chemical shift is due to the trans (E) isomer. These protons are less deshielding by the oxygen effect. The ratio between both isomers can be determined using these signals and set as 50% cis and 50% trans, in line with the similar chromatographic areas of both isomers in the GC chromatogram in Figure S1 of the Supporting Information. The 2−7ONE oximation produces two stereoisomers. The 2−7ONE oxime NMR spectrum is shown in Figure S10 of the Supporting Information. The stereoisomers can be identified using the signal produced by the CH3, position 1 in the carbon 11882
DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
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Figure 4. Influence of pH on the remaining 2-heptanone (1-X2−7ONE) profile vs time, runs 1−6 in Table 1: (a) T = 80 K, (b) T = 85°C. Symbols depict experimental results and lines depict the values predicted by the kinetic model using eqs 11−15.
Scheme 2. Overall Reaction Path of the Oximation of Ketones
Scheme 3. Beckmann Rearrangement of Linear Oxime Stereoisomers
Table 4. Apparent Kinetic Constant, kap,j, Estimated by Experimenting with 2-7ONE and 3-7ONE Conversion vs Time in Runs 1−6 (Table 1) eqs 11−15
Scheme 4. Beckman Rearrangement of Aldehyde Oxime Stereoisomers
kap,i·1011 (SQR) kg1.5·mmol−0.5·min−1·cm−2 pH j ONE30 3−7ONE 2−7ONE
T, °C
3.5
4.5
6.0
80 85 80 85 80 85
2.14 2.56 1.75 (0.04) 2.94 (0.15) 1.82 (0.06) 3.53 (0.21)
2.68 3.23 1.90 (0.21) 3.30 (0.21) 3.00 (0.17) 4.14 (0.10)
3.75 4.66 1.03 (0.06) 2.38 (0.14) 1.21 (0.09) 2.24 (0.15)
(δ 2.20 ppm). The ratio between the CH3 singlet and CH2 triplet of both isomers can be used to set the isomer ratio at 27% cis and 73% trans in line with chromatographic areas in Figure S1 of the Supporting Information. Although the same analytical procedure was carried out for 2− 8ONE oxime and 3−8ONE oxime, the chemical shifts promoted by the longest carbon chain (6 or 5 carbon) did not allow the stereoisomer ratio to be distinguished. The 6AL also promotes two isomers. The NMR spectrum is represented in Figure S11 of the Supporting Information. The
chain, close to the CN-OH group. The oxime functional group is the only neighbor of this CH3, meaning that CH3 is a singlet. The singlet of the trans (E) isomer should have a higher chemical shift (δ 1.87 ppm) due to the oxygen being closer to the protons of the singlet and a decrease in electron density. The same conclusion can be reached using the CH2 group, position 3 of the chain. In this group, the electron density is more affected by the oxygen of the oxime functional group of the cis (Z) isomer, and the triplet signal of this proton has a higher chemical shift (δ 2.48 ppm) than the same group of the trans (E) isomer 11883
DOI: 10.1021/acs.iecr.9b01997 Ind. Eng. Chem. Res. 2019, 58, 11878−11890
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triplet due to the CH2 neighbor. The triplet signal of the isomer cis (δ 6.74 ppm) is less affected by the oxygen of the oxime group than the signal of the trans (E) isomer (δ 7.44). The same ratio can be studied using the quadruplets of the CH2 group, position 2 of the carbon chain, and the protons of the isomer cis are now more affected by the oxygen and its chemical shift (δ 2.40 ppm) is higher than the trans isomer (δ 2.21 ppm). The main conclusions obtained are summarized in Table 3, where the mass ratios obtained for the stereoisomer promoted from each linear ketone and hexanal are gathered. 3.2. Kinetic Model of 2-Heptanone (2−7ONE) and 3Heptanone (3−7ONE) Oximation. As explained above, 2− 7ONE and 3−7ONE are the most probable linear ketones in pure cyclohexanone obtained by distillation because their boiling points are similar to that of cyclohexanone. As described in the Experimental Section, a mixture of 20% cyclohexanone in cyclohexanone oxime was spiked with 300 mg·kg−1 of each 7ONE. Runs 1−3 (80 °C) and 4−6 (85 °C) in Table 1 were carried out by changing the pH at each temperature. The experimental profile of remaining ONE fraction over time is shown in Figure 2. The remaining profile of impurities over time, 1 − Xj vs time, are plotted in Figure 3 and Figure 4 for 3−7ONE and 2−7ONE, respectively. It can be considered that the same oximation mechanism applies to both linear ketones and cyclohexanone. The mechanism of oximation reaction should include two stages30,34−36 summarized in Scheme 2. The first one is the production of free hydroxylamine (from the HA cation: HAH+) which reacts with the ketone or the aldehyde to form an adduct intermedia (step S-O1 in Scheme 2). The reaction rate of this first step is favored at higher pH. The second step is the dehydration of this adduct to produce the oxime (S-O2 in Scheme 2). The reaction rate of this second step is favored at low pH. In the literature, Keglevich et al.37,38 have studied the oximation of methyl ethyl ketone, benzaldehyde, and acetone, finding that there is a maximum in the reaction rate with the pH, as was also noticed in our work. They found that at pH = 10, 8, and 2.5 the oximation reaction was completed at 1, 60, and 30 min, respectively. This maximum can be explained if there is a change of the limiting step of the global rate of oximation with the pH modification. If the first step controls the oximation rate, the global rate will be favored by a higher pH whereas when the dehydration is the limiting step the global rate will be favored by the lower pH. The equilibrium constant Ka defined in Eq. 3) is used to calculate dissociated HA:
Table 5. Reaction Path of the Main Linear Impurities Presented in Pure CPL Obtained from Cyclohexanone, Which Is in Turn Obtained by Cyclohexane Oxidation
C HA =
K aC HA+ C H+
(3)
The pH has an effect on the dissociation of HA+, according to eq 3. The higher the pH, the higher free HA. However, once the HA reacts with the ketone or hexanal, a protonated intermedia is formed in stage 2. It must be dehydrated, and this reaction is lower when the pH decreases.34 As can be seen, the pH has an opposite effect on stages 1 and 2 of the oximation mechanism. As found elsewhere in the cyclohexanone oximation reaction,30 pH has a slight positive effect on the ONE conversion, which was experimentally confirmed as shown in Figure 2. As seen above, the higher the pH, the higher the cyclohexanone conversion. This fact can be explained if the pH
ratio between cis and trans oxime isomers can be set at 44:56%, respectively. The proton used to set the ratio is the CH group in position 1 of the carbon chain. The signal of this proton is a 11884
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Industrial & Engineering Chemistry Research
Figure 5. Natural logarithm of remaining (E)-2−7ONE oxime vs natural logarithm of remaining compound j. Symbols depict the experimental results and lines represent the linear regression of the experimental data assumed in eq 19.
increases the rate of stage 1, where HA is produced, against the negative effect of the pH on the rate of stage 2. On the other hand, in the oximation of 3−7ONE and 2− 7ONE, the effect of pH observed is opposite than the one for the cyclohexanone reaction. In Figures 3 and 4, the remaining 3− 7ONE and 2−7ONE profiles vs reaction time are plotted. As seen above, the conversion decreases when pH increases although small differences are found between pH 3.5 and 4.5 (almost within the 7% analytical error). The pH net effect on the n-7ONE conversion can be explained, considering that the negative effect of the pH in the dehydration reactions predominates against its positive effect on the stage 1 rate when the pH reaction media increases. Regarding the effect of temperature, the higher the temperature, the higher the ketone conversion for a given reaction time. However, temperatures above 87 °C are not recommended because loss of hydroxylamine by evaporation from the reaction mixture can be significant.30 3.2.1. Oximation Kinetic Model. The kinetic model proposed here for the oximation reaction is based on the one developed elsewhere for cyclohexanone oximation with HA.30 The following assumptions were made:
was zero in the organic phase. On the other hand, at reaction conditions the initial aqueous phase is saturated in HAS and AS is release with the reaction progress (eq 10). In additional experiments carried out replacing the concentration of HAS used in oximation runs for the same concentration of ammonium sulfate, it was confirmed that the high salt content in the aqueous phase promotes the “salting out” effect and the concentration of organic species in the aqueous phase is low (
