Article pubs.acs.org/IECR
Purification of Terpenyl Amine by Reactive Extraction Robin Schulz, Rebecca van den Bongard, Jessika Islam, and Tim Zeiner* Laboratory of Fluid Separations, TU Dortmund University, Emil-Figge-Str. 70, 44227 Dortmund, Germany ABSTRACT: This work analyzed the separation and purification of terpenyl amine by a reactive extraction in which an acid was used as complexing agent. Several different acids were investigated to form a water-soluble complex, but only carboxylic acids were found to be suitable for a reactive extraction of terpenyl amine. The best results according to yield and selectivity were found for acetic acid. Moreover, there was no formation of an emulsion using the acetic acid. The reaction mechanism and reaction location were determined to characterize this complexation reaction of acetic acid with terpenyl amine. The main mechanism was the Hbond formation at the interface between the organic and aqueous phases. Finally, the pseudomole-fraction-based equilibrium constant of 4.26 was estimated with a complex factor of 1 to 4.26 with selectivity above 98%.
the reaction consisted of only β-myrcene, terpenyl amine, and byproducts (dimers, isomers, and telomeres). Morpholine was distributed completely in the aqueous phase. Therefore, the focus of this work was the purification of terpenyl amine from the organic phase. The organic phase after the reaction consisted of approximately 7.3 wt % terpenyl amine, 0.1 wt % byproducts, and 92.6 wt % β-myrcene.6 A purification via distillation was economically not suitable because of the small concentrations of terpenyl amine in the organic phase (maximum concentration of terpenyl amine was 10 wt %6). To capture products in small concentrations, a physical extraction could be used. However, a suitable solvent could not be identified because it is quite challenging to separate a nonpolar product from a nonpolar solvent by extraction. As a result, reactive extraction was chosen for this purification task. Reactive extraction has been used in mining and environmental technologies to separate metals by ion exchange since the medieval ages.7,8 Recently, the approach has also been applied to purify biotechnological fermentation broths. Using this separation method, an in situ product removal was possible and the conversion and selectivity could be increased. One reported application is the separation of carboxylic acids from aqueous solutions.9−17 Thereby, an organic amine formed a complex with a carboxylic acid. This complex mainly partitions in the organic phase.9 This approach was transferred to the investigated separation task. Here, the product is an amine which is present in the organic phase. This amine forms a complex with a water-soluble acid, whereas the resulting complex mainly partitions in the aqueous phase.
1. INTRODUCTION Amines based on renewable resources are a focus of the chemical industry because there is an expected long-term increase in fossil fuel raw material prices.1,2 In particular, consider the growing global market of alkyl amines: USD 4.09 billion in 2014 with an increase of 6.5% to 2015. This fits well with the predicted increase of the global market of amines, from USD 13.35 billion in 2015 to USD 19.90 billion in 2020.3 One possibile method of producing alkyl amines is the hydroamination of simple and not highly functionalized renewable resources, e.g., terpenes. Terpenes are hydrocarbons consisting of a different number of isoprene units, (C5)n2, which are not highly functionalized and do not compete with the food industry. One industrially available terpene is β-myrcene,4 which is produced via the pyrolysis of β-pinene. This is a waste product of paper production.5 β-Myrcene reacts with morpholine (Figure 1) in a homogeneous catalyzed hydroamination to terpenyl amine.6 However, by the application of homogeneous reactions, the expensive catalyst must be recycled. In this study, the recycling was conducted by a liquid−liquid system consisting of β-myrcene and water. The organic phase after
Received: Revised: Accepted: Published:
Figure 1. Reaction of β-myrcene and morpholine to terpenyl amine. © 2016 American Chemical Society
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2.3. Analytical Methods. The density was measured by an oscillating U-tube density meter (DMA 46 chempro PAAR, 2%), and the weight fractions of the organic phases were estimated by gas chromatography (Shimadzu GC-14B with a flame ionization detector and a 30 m Agilent HP-5 column with a film thickness of 0.25 μm, a diameter of 0.32 cm, and a prepressure of 65 kPa). The measurement inaccuracy was below 2%. Because the complex in the aqueous phase could not be directly determined, it was broken by distillation of the aqueous phase in a distillation apparatus. Then, the concentrations of organic compounds were determined by the same gas chromatography. The weight fractions in the aqueous phase could be calculated by a mass balance, and the acetic acid concentration in the aqueous phase was estimated by an equivalence point titration via a sodium hydroxide titrator (Mettler Toledo DL58). The pH of the aqueous phase was measured by a pH meter (inoLab WTW pH720). The infrared spectroscopy measurements were conducted by a Fourier transformation infrared spectroscope (Mettler Toledo React IR 15). Thereby, no sample preparation was necessary. 2.4. Performance Parameters. For the reactive extraction, the extract phase is the aqueous phase and the raffinate phase is the organic phase with terpenyl amine as solute. The efficiency of a single-stage extraction was described by distribution coefficient Di of component i: xi ,aq Di = xi ,org (1)
To optimize this separation process, the complexation mechanism is required. This mechanism allows us to determine the conditions needed to achieve high yields.18,19 Different reaction locations and mechanisms have been suggested in the literature.20−25 According to Canari and Eyal,23 the complexation would take place in the organic phase; however, Kurzrock and Weuster-Botz26 indicated the interface between aqueous and organic phases as the complexation location. In addition to the different locations, there were four possibilities of the complexation mechanism: anion exchange, ion-pair formation, H-bond formation, and solvation.9,20−24 In this study, a screening of solvents for the extraction and then a systematic acid group screening to find an appropriate acid for the reactive extraction was conducted. After the selection of an acid, the mechanism of the complexation was clarified. Based on this investigation, the pseudo-mole-fractionbased equilibrium constant of the complexation could be calculated.
2. METHODS 2.1. Chemicals. The terpenyl amine was produced via homogeneous catalyzed hydroamination of β-myrcene (SigmaAldrich, 90%) and morpholine (Alfa Aesar GmbH & Co KG, ≥ 99.0%). The catalyst system was palladium(II) trifluoroacetate (ABCR, 97%) with 1,4-bis(diphenylphosphino)butane (Acros Organics, 98%). The reaction took place in toluene (Prolabo, 99.5%) at 390 K and 5 bar. The reaction mixture was purified in a distillation apparatus (Heidolph Laborota 4003) with a terpenyl amine purity of 98%. The solvents for the extraction were 2-ethylphenol (Alfa Aesar GmbH & Co KG, 98%), chloroform (Prolabo, 99.8%), cyclopentanol (Alfa Aesar, 99%), diethyl carbonate (Merck, ≥99.0%), diethyl ethanolamine (Sigma-Aldrich, 98%), dimethyl carbonate (Merck, ≥99.0%), phenol (Merck, ≥99.0%), triethyl amine (Alfa Aesar, 99%), 1,2butanediol (Fluka, ≥98.0%), 1,3-propanediol (Sigma-Aldrich, ≥99.6%), 1,5-pentanediol (Alfa Aesar, 97%), butylene carbonate (Acros Organics, 97%), ethylene glycol (VWR, 99.7%), and propylene carbonate (Merck, ≥99.0%). The acids for reactive extraction were hydrochloric acid (VWR, 2 mol·L−1), phosphoric acid (Merck, 85%), glutaric acid (Merck, 99%), dichloroacetic acid (Merck, 98%), formic acid (VWR, 99%), acetic acid (Merck, 99%), propionic acid (Merck, 99%), and butyric acid (Ridel-de-Haen, 98%). Other used chemicals were sodium hydroxide solution (Bernd Kraft GmbH, 0.1 mol·L−1), dibutyl ether (Merck, 99%), and deionized water. 2.2. Experimental Setup. Experiments were conducted in extraction vessels. The mixing time was 1 h at a stirrer speed of 300 rpm. Afterward, the mixture was settled for 15 min to ensure a complete phase separation and thermodynamic equilibrium, which was clarified by preliminary experiments. Before the phases were separated, the volume of each phase was estimated by a volume scale at the extraction vessels. After phase separation, the phases were analyzed to determine the concentration of each component. All experiments were conducted at 20 °C and 1 bar. Screening experiments for the reactive extraction experiments had an acid concentration in the aqueous phase of 1 mol·L−1 and a terpenyl amine weight fraction in the organic phase of 10 wt %. The mass phase ratio between organic and aqueous phase was set to one. Measurements of the equilibrium constant were conducted with an acid concentration between 0.5 and 4 mol·L−1 and a terpenyl amine weight fraction range from 3 to 50 wt %.
where x is the mole fraction in the aqueous (aq) phase or the organic (org) phase, respectively. On the basis of the distribution coefficients, the terpenyl amine (TA) selectivity of a physical extraction (PE) SPE,TA is defined as follows:
SPE,TA =
DTA DMYR
(2)
MYR is the abbreviation for β-myrcene. This definition is in a way different from reactive extraction because selectivities less than 1 mean that less solute is extracted compared to other components like β-myrcene. Therefore, a high selectivity is desired, and purification takes place only by selectivities greater than 1, and an extraction is also suitable only with selectivities greater than 1.27 In the complexation reaction of terpenyl amine, the product is the complex including terpenyl amine and acid. The terpenyl amine yield, YTA is calculated as follows: n TA,aq YTA = n TA,0,org (3) The abbreviation for the mole number of TA in the different phases is n. Before the reactive extraction, the initial mole number of terpenyl amine in the organic phase is nTA,0,org. This is the maximum mole number that can be extracted. Thereby, nTA,aq is the mole number of terpenyl amine in the aqueous phase after the reactive extraction. If the whole terpenyl amine would be extracted, nTA,aq is equal to nTA,0,org. This would result in a yield of 100%. The selectivity of a reactive extraction (RE), SRE,TA, is defined as n TA,aq SRE,TA = nOverall extracted,aq (4) 5764
DOI: 10.1021/acs.iecr.6b00739 Ind. Eng. Chem. Res. 2016, 55, 5763−5769
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experimentally that reactive extractions with an ion-pair formation are favored by strong acids. Furthermore, Canari and Eyal23−25,35 give the necessary conditions for H-bonds. If nondissociated acid mainly exists in the organic phase based on a higher basicity of the acid anion compared to the amine, the reactive extraction is dominated by H-bonds. An extraction with more than one acid molecule for each amine molecule by H-bonds between the acid and the free electron pair of the nitrogen of the amine takes place if the amount of acid in the aqueous phase and hence the concentration of nondissociated acid is high enough. Therefore, the H-bonds of nondissociated molecules are the dominant mechanisms of the extraction, if the basicity of the acid anion is greater than that of the amine, pKa,A > pKa,B. According to Canari and Eyal,23 the basicity of the amine in the organic phase is the main aspect for the identification of the reaction mechanism. Higher basicity of the amine results in a favored bond to a proton of the acid. However, the measurement of such basicities is a challenge when the amine is insoluble in water. Canari and Eyal23 propose an experimental procedure to determine the pKa,B of an amine by the use of the halfneutralization pHhn(HCl). This procedure is based on the formation of water-soluble ammonium hydrochloride in a reactive extraction of hydrochloric acid in an aqueous phase with an amine. Hereby, the amine is water-insoluble and forms with water, and the reaction takes place at the interface. After a subsequent half-neutralization with a sodium hydroxide solution is performed, the pH of the aqueous phase is measured. The pKa,B of the amine can then be calculated using the measured pH of the aqueous phase. The difference between the pHhn(HCl), the basicity of the amine in the organic phase, and the basicity of the acid pKa,A indicates the degree of dissociation of the acid in the organic phase. A comparison of this difference with the results of infrared spectroscopy for the investigation of the acid form shows three possibilities: pHhn(HCl) − pKa,A = 0, dissociated and nondissociated acid; pHhn(HCl) − pKa,A < 0, main form of the acid is nondissociated-acid; and pHhn(HCl) − pKa,A > 0, acid form in the organic phase is mainly dissociated acid.
In this work, a pseudo-mole-fraction-based equilibrium constant, Kx, is used to describe the equilibrium. This pseudo-mole-fraction-based equilibrium constant does not have to be constant. Moreover, eq 5 does not indicate in which phase the reactants are available because the location of the complexation has not been clarified yet. In this work, three different reaction locations are possible: the organic phase, the aqueous phase, or the interphase.23,26 If the reaction takes place in a bulk phase then the mole fractions of each reactant, which are available in this phase, are used in eq 5. In the case of a reaction at the interphase, the mole fractions of the reactants in different phases have to be used.28−34 This kind of reaction can occur only in two-phase systems. For this reason, it is important to know the reaction location. Furthermore, the number of molecules of acid, which forms a complex with one molecule of terpenyl amine, is unknown. Therefore, the complex factor (cf) is used, which is equal to the stoichiometric coefficient of the acid: Kx =
xComplex cf x TA ·xAcid
(5)
2.5. Complex Mechanism Theory. The mechanisms are divided into four categories. If the acid or the salt of the acid in an aqueous solution reacts with an extracting agent, then anion exchange takes places.22 The nondissociated acid HAaq exchanges its anion A−aq by the anion of the amino salt Xorg. The amino salt binds the anion of the acid A−aq. The mechanism of the anion exchange is determined by the polarity and pKa of HX and HA, the pH of the aqueous phase, and the composition of the organic phase. According to the literature,20−22 the reactive extraction of carboxylic acids via amines, which mainly forms ion-pairs, is performed in a subsequent reaction. The first step of this reaction is the dissociation of carboxylic acid in water. Eyal and Canari22 mentioned that the mechanism of ion-pair formation occurs in the presence of amines. The resulting complex has two possible structural forms: carboxylate and carbonyl form.26 The amine forms an H-bond with the nondissociated acid if a protonation of the amine is not possible because of its low basicity, and the acid is mainly nondissociated.22 However, a complex based on the H-bond formation is shown in the literature.26 The last possibility of the complexation mechanism is solvation. It is a replacement in the solvation sphere of water molecules with nondissociated acid molecules.22 Therefore, solvation is a purely physical interaction which takes place only by a lower basicity of the amine as the acid anion. The complexation has often more than one main mechanism if no strong and no weak acid participate in the reaction.22 According to Canari and Eyal,24 the main mechanisms of the reactive extraction of mono carboxylic acids with amines are ion-pair and H-bond formation, which can overlap. Therefore, the requirements for the identification of the complexation mechanism will be given below. An ion-pair bond between the ammonium cation and the acid anion is possible if the acid in the organic phase is dissociated. According to Canari and Eyal,23−25 a complete protonation of the amine to enable stoichiometric extraction by ion-pair formation with dissociated acid molecules can be found in complexations with amines. The requirements are a higher basicity of the amine as the acid anion or equilibrium with an aqueous phase with a higher pH. Canari and Eyal23 showed
3. RESULTS AND DISCUSSION The separation of terpenyl amine from the organic phase was firstly performed as a physical extraction. For a suitable extraction, high capacities and selectivities are required. The results of the investigated solvents are shown in Figure 2. None of the investigated solvents shows the necessary preconditions. Either the distribution coefficients were less than 1, which would result in large solvent streams, or the selectivity was less than 1, which means that a purification of terpenyl amine is not possible. Only 1,5-pentanediol had a selectivity above 1, but the distribution coefficient of 0.16 would lead to solvent streams that are too large for an economic process. Therefore, the physical extraction could not be used for the purification of terpenyl amine. For this reason, a reactive extraction with an acid was investigated. First, a screening for suitable acids was performed focusing on acid groups of halogen acids, mineral acids, carboxylic and dicarboxylic acids, and substituted carboxylic acids. Hydrochloric acid (halogen acid), phosphoric acid (mineral acid), and glutaric acid (dicarboxylic acid) were not suitable for extracting terpenyl amine. Gas chromatography analysis exhibited no terpenyl amine in the organic phase after the experiment, but there was 5765
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which leads to a leaching to the organic phase. This results in a decreasing yield. Therefore, fewer complexes were formed and the terpenyl amine yield decreased. The results showed that formic acid is the best performing acid for the reactive extraction of terpenyl amine. However, formic acid formed emulsions during the experiments, which are undesirable in an extraction process. This formation of the emulsion can be the result of both lipophilic and hydrophilic structure of the complex formed, which has both a nonpolar tail and a polar head. A decrease of this behavior with longer chain acids was highly probable, and acetic acid formed no emulsions in the investigated temperature range. Therefore, acetic acid was considered as the appropriate acid for further investigations. Formation of any byproducts in the organic phase was excluded by analyzing the organic phase by gas chromatography. There was no detection of dimers or other products in the organic phase. Three locations were possible for the complexation location: the interface, the organic phase, or the aqueous phase. First, the distributions of the components were estimated. Terpenyl amine was practically insoluble in water.6 Therefore, the complexation in the aqueous phase is not possible. Complexation location in the organic phase could be excluded by the distribution coefficients of acetic acid between aqueous and organic phase. Figure 4 shows the results for these distribution coefficients for different initial concentrations of acetic acid in the aqueous phase and different mass phase ratios.
Figure 2. Distribution coefficients (light gray) and selectivities (dark gray) of the estimated solvents for the physical extraction of terpenyl amine with a mass phase ratio of one and a weight fraction of terpenyl amine of 10 wt %.
also no terpenyl amine in the aqueous phase. Therefore, these acids react with terpenyl amine and form no complex. In the group of substituted carboxylic acids, dichloroacetic acid was selected. During the experiments with dichloroacetic acid, a third phase was formed which was colored yellow. Gas chromatography analysis and the yellow color showed the presence of morpholine, one reactant in the production of terpenyl amine. Furthermore, no terpenyl amine was detected in the organic, aqueous, and new third phase. From the group of carboxylic acids, formic acid was analyzed. Thereby, a yield of 80% with a selectivity of 98% was achieved (Figure 3).
Figure 4. Distribution coefficient of acetic acid between 0.5 and 8.2 mol L−1 acetic acid start concentration in the aqueous phase for mass phase ratios of 0.5 (white squares), 1 (gray triangles), and 2 (black circles).
All values of the distribution coefficients were higher than 9000; the concentrations of acetic acid in the organic phase had a very high error based on the small values, which resulted in the large error bars of the distribution coefficients. Therefore, almost no acetic acid was present in the organic phase. Thus, the reaction could not take place in the organic phase. The only possibility for the complexation location was the interface, because it was the only place where all reactants could meet. Regarding eq 5, the remaining unknown parameter was the complex factor, cf, which could be estimated by the measurement of the pseudo-mole-fraction-based equilibrium constant after the determination of the reaction stoichiometry and the mechanisms. The identification of the reaction mechanism was based on the acid terpenyl amine dissociation constant, pKa,B, and on the infrared spectroscopy analysis of the aqueous phase. At first, the
Figure 3. Dependency of terpenyl amine yield, YTA (light gray), and selectivity, SRE,TA (dark gray), of carbon chain length of the carboxylic acids.
Therefore, all water-soluble carboxylic acids were studied: formic acid, acetic acid, propionic acid, and butyric acid. Figure 3 shows the terpenyl amine yield and selectivity as a function of the carboxylic acid carbon chain length. Terpenyl amine selectivity for formic, acetic, and propionic acid was approximately 98%, and for butyric acid, it was approximately 95%. However, the terpenyl amine yield decreased sharply with increasing carboxylic acid carbon chain length. This decrease from formic acid to acetic acid was due to the additional increase in the pKa value. Moreover, carboxylic acids with longer carbon chain length are less polar, 5766
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1713, 1770, and 1785 cm−1. The results can be explained by the use of different solvents: Kirsch et al.14 used toluene, whereas we used water. The different solvents led to a peak shift. A peak shift could be very significant, and the assignment of peaks could be arbitrary.37 However, the small difference in wave numbers allows a peak identification. Therefore, the peaks of Figure 5 could be identified for the cyclic dimer of acetic acid at 1713 cm−1, for the monomer at 1770 cm−1, and for the linear dimer at 1785 cm−1. By using the Beer−Lambert law, the concentration of the different acetic acid forms could be calculated, because the absorbance is independent of the substance concentration. Therefore, the absorbance coefficient as a function of the overall acetic acid concentration was required. The literature provides absorbance coefficients only with smaller acetic acid concentrations. However, the exact concentrations were not important; only the difference between the concentrations was important. Therefore, the absorbance coefficients were applied, which are known from the literature.14 Thus, the concentrations of acetic acid monomer and cyclic and linear dimers could be calculated. The results and the absorbance are shown in Table 1. The amount of cyclic dimers and the amount of monomers and linear dimers were approximately equal. However, it is not excluded that H-bonds between water and acetic acid molecules overlapped the shown peaks. However, by the infrared spectroscopy it was shown that an H-bond formation between terpenyl amine and acetic acid was possible. The last step was to analyze the aqueous phase after the reaction to determine the complex. In the aqueous phase were acetic acid and the complex with terpenyl amine. Figure 6 shows the infrared spectrum of the aqueous phase after the reaction. Peak identification could be accomplished using the peaks reported in the literature.14
pHhn(HCl) was determined with hydrochloric acid to 4.00. The pKa,B of terpenyl amine could then be calculated by using the Cl− ion concentration of 0.21 mol·L−1 to 4.63. The comparison with the acidity of acetic acid with a pKa,A of 4.7636 showed a lower pKa,B of terpenyl amine. Therefore, terpenyl amine was the weaker base. The acid anion was more willing to bind protons of the dissociated acetic acid in the aqueous phase because of the higher basicity of acetic acid compared to terpenyl amine. Furthermore, a deprotonation of more acetic acid molecules was not favored. Protonation of terpenyl amine and resulting formation of the complex by an ion-pair bond were trivial. The mechanism based on H-bonds is completely independent of the basicity of terpenyl amine. Thereby, the Hbond formation could be identified as main mechanism. The next step was to verify this assumption by infrared spectroscopy analysis of the aqueous phase to identify the acetic acid form, dissociated or nondissociated, and the ratio between the carboxylate and carbonyl forms. The aqueous phase was analyzed by infrared spectroscopy to determine the acetic acid form. Acetic acid could be dissociated or nondissociated, and the nondissociated acetic acid could form linear or cyclic dimers. Dimers of the acid could exist as two nondissociated acetic acid molecules, whereas linear dimers were able to form H-bonds with terpenyl amine and cyclic dimers were not able to form H-bonds with terpenyl amine. Figure 5 shows the infrared spectrum of the aqueous phase.
Figure 5. Infrared spectrum of water with acetic acid (infrared spectrum of pure water was subtracted) as monomer (1762 cm−1), cyclic dimer (1713 cm−1), and linear dimer (1781 cm−1).
The spectrum of water was subtracted to prevent an overlay of the spectrum of acetic acid with water. This resulted in the peaks for the monomer and the two dimers of acetic acid. A measured pH range from 3.9 to 4.0 underlined the presence of nondissociated acetic acid with an acidity of 4.76.36 The identification of the peaks used spectra data which are present in the literature.14,17 The monomer of acetic acid was at a wavenumber of 1762 cm−1, cyclic dimer at a wavenumber of 1713 cm−1, and linear dimer at a wavenumber of 1781 cm−1. Compared to Figure 5, small differences in the wave numbers were obtained. The wave numbers of acetic acid in water were
Figure 6. Infrared spectrum of the aqueous phase after the reaction (infrared spectrum of water with acetic acid was subtracted) with nondissociated acetic acid (1715 cm−1), dissociated acetic acid (1596 cm−1), complex in carbonyl form (1275 cm−1), and complex in carboxylate form (1555 cm−1).
Kirsch et al.14 indicated the peaks for the complexation reaction of tri-n-octylamine with acetic acid in toluene as 1097
Table 1. Absorbance Coefficients and Concentrations for Acetic Acid Monomer and Cyclic and Linear Dimers in Water acetic acid form
absorbance (%)
absorbance coefficient (L·μm−1·mol−1)
concentration (mol·L−1)
monomer cyclic dimer linear dimer
14.3 95.7 8.6
0.0622 0.0371 0.2300
0.0239 0.0241 0.0433
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Industrial & Engineering Chemistry Research cm−1 for free amine, 1256 cm−1 for H-bond with a carbonyl form, and 1575 cm−1 for the complex with a carboxylate form. Furthermore, Canari and Eyal23 indicated the peaks for the complexation of tri-n-octylamine with acetic acid in kerosene for dissociated acetic acid as 1573 cm−1 and for nondissociated acetic acid as 1715 cm−1. By using this assignment and the knowledge about a peak shift, the peak identification led to 1715 cm−1 for nondissociated acetic acid, 1596 cm−1 for dissociated acetic acid, 1275 cm−1 for the complex with a carbonyl form, and 1555 cm−1 for the complex with a carboxylate form. A comparison of the peaks of nondissociated and dissociated acetic acid showed a higher amount of nondissociated acetic acid. This proved the assumption of identification of the mechanism based on the knowledge of acetic acid. Furthermore, the absorbance of the complex with a carbonyl form was much higher than the complex absorbance with a carboxylate form. Therefore, the main mechanism could be identified as H-bond formation. However, the H-bond formation was not the only complexation mechanism; there was always an overlay between the H-bond formation and the ion-pair formation. Pseudo-mole-fraction-based equilibrium constant and complex factor were estimated with the knowledge of the equilibrium concentrations, which are based on the reaction mechanism and location. This pseudo-mole-fraction-based equilibrium constant is similar to the determination of the equilibrium constants of interfacial reactions that are wellknown from the literature.28−34 A parameter fit, minimizing the sum of all deviations between the fitted reaction equilibrium constant and the experimental data for the reaction equilibrium constant and the complex factor for all experiments showed constant reaction equilibrium constant of 4.26 with a complex factor of 1 (Figure 7). Therefore, the variation of the terpenyl
By the use of the pseudo-mole-fraction-based equilibrium constant, a theoretical design of a mixer−settler unit can be proposed. Therefore, we used a mass-based phase ratio of 1 and a weight fraction of terpenyl amine of 7 wt % in the organic phase and an acetic acid concentration of 1 mol % in the aqueous phase. These initial conditions, the estimated pseudomole-fraction-based equilibrium constant, and a target yield of terpenyl amine of 99% resulted in four theoretical equilibrium stages.
4. CONCLUSION In this study, a suitable system for the reactive extraction of terpenyl amine was explored. Terpenyl amine, an organic amine, was converted with an aqueous acid solution to a watersoluble complex. To find an appropriate acid, an acid screening was performed; the screening showed the best results for carboxylic acids and especially for acetic acid. Subsequently, the complexation of terpenyl amine with acetic acid to a water-soluble complex was described by the complexation mechanism, location, and the determination of the complex factor and the pseudo-mole-fraction-based equilibrium constant. The result is that one acetic acid molecule forms with one terpenyl amine molecule a watersoluble complex, and the reaction takes place at the interface between the aqueous and organic phase via an H-bond formation. The theoretical description was conducted by the estimation of the pseudo-mole-fraction-based equilibrium constant as 4.26 and selectivities above 98%. The constant mole-fraction-based equilibrium constant showed that a constant complex factor of 1 is reasonable. The results of this study demonstrate the ability to extract amines with acids. A scale-up can be made based on the determined complexation mechanism and pseudo-mole-fraction-based equilibrium constant.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The research leading to these results received funding from the project “Sustainable Chemical Synthesis (SusChemSys)”. The project Sustainable Chemical Synthesis (SusChemSys) is cofinanced by the European Regional Development Fund (ERDF) and the state of North Rhine - Westphalia, Germany, under the Operational Programme “Regional Competitiveness and Employment” 2007−2013.
Figure 7. Pseudo-mole-fraction-based equilibrium constant with a complex factor of 1 of the complexation reaction with acetic acid in a range of 0.5−4 mol·L−1 acetic acid in the aqueous phase and 3−50 wt % terpenyl amine in the organic phase.
■
NOTATION
Latin Letters
c = Concentration cf = Complex factor D = Distribution coefficient n = Number of moles S = Selectivity x = Mole fraction Y = Yield
amine weight fraction in the organic phase and the reproducibility experiments were conducted by an acetic acid concentration in the aqueous phase of 1 mol·L−1. A constant reaction equilibrium constant was reached in the measured concentration range of acetic acid and terpenyl amine with a deviation of 20%. Only one experiment including the error bars was not in this range. Based on the weight fractions of terpenyl amine in the organic phase after the production and to minimize the required acetic acid, this deviation was negligible.
Superscripts
0 = Initial aq = Aqueous phase 5768
DOI: 10.1021/acs.iecr.6b00739 Ind. Eng. Chem. Res. 2016, 55, 5763−5769
Article
Industrial & Engineering Chemistry Research
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org = Organic phase PE = Physical extraction RE = Reactive extraction Abbreviations
AA = Acetic acid HCl = Chloric acid Kx = Pseudo-mole-fraction-based equilibrium constant MYR = β-Myrcene pKa,A = Acid dissociation constant of an acid pKa,B = Acid dissociation constant of a base pHhn(HCl) = pH of half-neutralization with HCl TA = Terpenyl amine
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REFERENCES
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DOI: 10.1021/acs.iecr.6b00739 Ind. Eng. Chem. Res. 2016, 55, 5763−5769
