Reactive Extraction of Saturated Aliphatic Dicarboxylic Acids with

Jul 20, 2013 - An equilibrium model is presented that employs the mass action law and is used to determine model parameters and apparent extraction ...
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Reactive Extraction of Saturated Aliphatic Dicarboxylic Acids with Trioctylamine in 1‑Octanol: Equilibria, Model, and Correlation of Apparent Reactive Equilibrium Constants Zhiyong Zhou, Zhenyu Li, and Wei Qin* State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: Extraction equilibria for saturated aliphatic dicarboxylic acids, namely oxalic, malonic, succinic, and adipic acids, with trioctylamine (TOA) in 1-octanol were determined at various TOA concentrations. Using quantitative FT-IR spectra, we determined that the formation of 1:2 acid−amine complexes depends on the pKa2 value, and wavenumbers of specific peaks for the COO− of the acid−amine ion-pair complexes depend on the pKa1 value. An equilibrium model is presented that employs the mass action law and is used to determine model parameters and apparent extraction equilibrium constants (K11, K12, and K21). The extraction abilities for dicarboxylic acids depend on the pKa1 value. The typical overloading curves of TOA/1-octanol for dicarboxylic acids are given. The loadings of TOA calculated using the equilibrium model parameters and apparent extraction equilibrium constants agree with the experimental data. The apparent extraction equilibrium constants depend on the acidity of the dicarboxylic acid and the specific basicity of TOA. The quantitative correlation of log K11 (or log K12) is obtained using pKa1 (or pKa2) and pKa,B ′ .

1. INTRODUCTION Carboxylic acid is an important organic chemical product for both electrochemical and biochemical syntheses. Liquid−liquid extraction is an efficient, economical, and environmentally benign method for the separation of carboxylic acids from diluent solutions, and it receives increasing attention.1−8 Longchain aliphatic amines such as trioctylamine (TOA) are effective extractants for the separation of carboxylic acids from dilute aqueous solutions.9−17 The extractant and acid molecules form a complex in the organic phase, thereby allowing for more acid to be extracted from the aqueous phase.13 Generally, the amine extractants are dissolved in a diluent such as an alcohol, which dilutes the extractant to the desired concentration and controls the viscosity and density of the solvent phase. Many factors have been found to influence the equilibrium extraction characteristics of these systems. Three important variables are the following: the nature of the acid extracted, the concentration of the extractant, and the type of diluent.18,19 The important properties of the acids are the strength of the acid (pKa)19,20 and the hydrophobicity of the acid (log P).21,22 In addition, the specific basicity of the extractant (pKa,B ′ ), defined as follows, is suggested to express the nature of the extractant.

concentration and diluent type, representing the structure of the amine, multiplied by coefficients for diluent solvation properties and for the properties of the anion of the extracted acid (hydrophobic properties and steric hindrance). The equilibria of the extraction of 11 monocarboxylic acids with TOA and correlation of the apparent reactive equilibrium constants have been studied.23 Dicarboxylic acid is an important organic chemical product applied in various fields. Research on equilibria or modeling for some individual dicarboxylic acids can be found: oxalic, succinic, DL-malic, and D-tartaric acids with a tertiary amine in single and binary diluents;24 succinic acid with tertiary amines in 1-octanol/nheptane;25,26 oxalic, malonic, succinic, and adipic acids with Amberlite LA-2 in butyl acetate;27 citric, acetic, and oxalic acids with tri-n-octylamine in the pure solvents toluene, chloroform, and methyl isobutyl ketone.28 However, few studies have focused on calculating and correlating the apparent reactive equilibrium constants, especially for a class of dicarboxylic acids with similar characteristics. The correlation of the apparent reactive equilibrium constants can be used to predict the extraction equilibria for other similar dicarboxylic acids extracted with the same extractant and diluent. The following equilibria have been studied in our lab: extraction of succinic, malic, maleic, and fumaric acids with trioctylamine in chloroform, methyl isobutyl ketone, and 1-octanol;29 the modeling of the extraction of oxalic acid by trioctylamine in n-octanol as well as in the mixture of n-octanol and kerosene.30 For a saturated aliphatic dicarboxylic acid, rare systematic

′ K a,B

R3NH+ ←→ R3N + H+

′ = K a,B

[R3N][H+] [R3NH+]

(1)

Received: Revised: Accepted: Published:

where R3N represents TOA. The species in the organic phase are marked with an overbar, and all of the concentrations are expressed in molar terms. pKa,B ′ depends on the amine © 2013 American Chemical Society

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2.3. FT-IR Spectra Analysis. The solutes were extracted into the extractant to prepare organic solutions with different amine−acid ratios. The organic solutions were injected into a sample cell with a thickness of 0.05 mm between two CaF2 plates. IR spectra were taken on a Shimadzu 8201PC FT-IR spectrometer. 2.4. Sample Analysis. The aqueous samples were analyzed for solute concentration (Caq) by using titration with NaOH as the standard solution and phenolphthalein as the indicator. Each measurement was performed in duplicate. The dicarboxylic acid concentration in the organic phase (Corg) was calculated by material balance based on the volumes of the two phases and Caq at equilibrium. However, the solute concentration in the organic phase was also determined by stripping the organic phase with a small amount of NaOH solution (0.02 mol·L−1). The alkaline solution containing the organic salt was analyzed using a high-performance liquid chromatography system (Waters, Milford, Massachusetts, USA). The results from these two methods agreed within a deviation of 3%, which resulted from the volume changing for each phase and the error of the analysis method. The pH value of the aqueous phase was determined with a pH meter (Italy Hanna pH 201 model) with deviation of ±0.02.

research for equilibria, model, and correlation of apparent reactive equilibrium constants can be found. In this work, the extraction of several saturated aliphatic dicarboxylic acids by a tertiary amine extractant in 1-octanol was carried out. Batch extraction experiments were performed with oxalic, malonic, succinic, and adipic acids, which are raw materials of the pharmaceutical,31 dyestuff, cosmetic, and food industries.32 The extractant used was TOA. 1-Octanol was selected as a typical diluent to improve the extraction properties of TOA. An equilibrium model was used to determine the model parameters and apparent extraction equilibrium constants. Finally, the apparent extraction equilibrium constants were correlated with the nature of the saturated aliphatic dicarboxylic acids and extractants that were used.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the saturated aliphatic dicarboxylic acids were analytical reagents and were used to prepare organic acid solutions of various concentrations. The characteristics of the dicarboxylic acids are presented in Table 1. TOA with a purity of >99 mass % (Fluka) was used as the extractant. 1Octanol (analytical reagent) was used as the protic diluent in this work. Table 1. Physical Properties of Carboxylic Acids

a

acid

pKa133

pKa233

log Pa

oxalic malonic succinic adipic

1.23 2.83 4.16 4.42

4.19 5.69 5.61 5.41

−0.854 −0.714 −0.580 0.439

3. RESULTS AND DISCUSSION 3.1. Determination of Complex Structures of Dicarboxylic Acid−Amine by Quantitative FT-IR Spectra. The quantitative FT-IR spectra have been applied to the characterization of the bonding mechanisms of TOA/1-octanol. Four dicarboxylic acidsoxalic, malonic, succinic, and adipicwere selected in different RTOA/acid (R represents the molar ratio of TOA and acid), and the results are shown in Figure 1 and Table 2. A specific peak for the carboxylate of the dicarboxylic acid−TOA complexes exists in the range 1550−1650 cm−1 for all FT-IR spectra of the organic phases of acid−amine. The intensity of the peaks is gradually enhanced with increasing the concentration of dicarboxylic acid, which indicates that 1:1 ionpair complexes of acid−amine could be formed with TOA and these four dicarboxylic acids. No significant specific peaks for acid dimers have been found in the range 1200−1300 cm−1 for any FT-IR spectrum of the organic phase of acid−amine, which indicates that 2:1 ion-pair complexes of acid−amine are difficult to form. Because the hydrophilicity of dicarboxylic acid is proportional to the number of carboxyls, dicarboxylic acid with two carboxyls can obtain higher hydrophilicity than monocarboxylic acid. As shown in Figure 1a, specific peaks for COO− and no specific peaks for CO have been found when RTOA/acid > 5, which indicates that the 1:2 ion-pair complexes of acid−amine for oxalic acid were formed. Specific peaks for CO could be found with increasing concentration of the acids. The results indicate that both 1:1 and 1:2 acid−amine ion-pair complexes for oxalic acid and TOA can be formed. The FT-IR spectra for the malonic, succinic, and adipic dicarboxylic acids show that specific peaks for CO and COO− have been found when RTOA/acid > 10 ≫ 1. The intensity of both peaks is gradually enhanced with increasing concentrations of dicarboxylic acid, as shown in Figure 1b−d. Therefore, only 1:1 acid−amine ionpair complexes can be formed for these three dicarboxylic acids and TOA. In addition, 1:2 acid−amine hydrogen-bond complexes may be formed for these three dicarboxylic acids and TOA.

Hydrophobicity.

2.2. Liquid−Liquid Extraction Experiments. The solvents used were amine−diluent mixtures. All extraction experiments were conducted in 100 mL flasks at 25 ± 0.5 °C. A total of 20 mL of the mixture solvent and 20 mL of the dicarboxylic acid solution were added to a flask without adjustment of the solution pH. The flask containing the mixture was shaken for 6 h in a shaker bath with a vibration rate of 200 rpm and then left to equilibrate for 1−2 h, during which the two phases separated. The upper layer (organic phase) was removed and its volume measured. The bottom layer (aqueous phase) was taken for pH and solute concentration analyses. It was noted that after extraction slight changes in the phase volume were observed for all extractions performed with 1octanol as the diluent. The organic phase volume increased by about 5%, with a corresponding decrease in the aqueous-phase volume, which could be due to the water transfer into the organic phase to solvate the complex.34 For extractions with the pure TOA system, a third emulsion phase was observed at the interface between the aqueous and organic phases. According to FT-IR analyses, this had a high initial concentration of dicarboxylic acid, which consists of a complex between TOA and dicarboxylic acid. The equilibrium pH is different from the initial pH because of the removal of acid and the change in the amount of amine dissolved in the aqueous phase. The solution pH tended to increase in this work, and the difference in pH was malonic acid > succinic acid > adipic acid, which agrees with the sequence of forming 1:2 acid−amine ion-pair complexes. The value of pKa2 follows the sequence oxalic acid (4.19) > adipic acid (5.41) > succinic acid (5.61) > malonic acid (5.69), which agrees with the sequence of forming 1:2 acid−amine ion-pair complexes. Since the value of pKa2 is the main factor, the 1:2 acid−amine ion-pair complexes can not be formed when pKa2 ≥ 5.41 (adipic acid). As shown in Figure 2, the wavenumbers of specific peaks for COO− of the dicarboxylic acid−TOA ion-pair complexes are proportional to the strengths of oxalic, malonic, succinic, and adipic acids: the stronger the acid, the larger the wavenumber. There is a linear relationship between the wavenumbers of specific peaks and the values of pKa1, but there is no relationship between the numbers and the values of pKa2.

Figure 2. Dependence of the wavenumbers of specific peaks for dicarboxylic acid−TOA complexes in FT-IR spectra on pKa1.

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3.2. Model and Fitting of Apparent Reaction Equilibrium Constants. The reactive extraction equilibrium for polar dilute solutions of organic solutes can be described by the mass action law, in which the equilibrium behavior is modeled by postulating the formation of various stoichiometric complexes of acid and amine.18 In addition, we assumed that the dicarboxylic acid is extracted into the organic phase through the physical solubility of the solute in the diluent of the organic phase and the formation of the 1:1, 2:1, and 1:2 acid−amine complexes, where the 2:1 complexes result from the dimer of the acid in the organic phase35 and the 1:2 complex results from ion-pair or hydrogen-bond association between the 1:1 complex and the amine. The physical extraction with diluent and the reactive extraction with TOA fit the simple additive model. The complexes are formed by stepwise reactions as follows:

[H 2A] =

1 + 10

+ 102pH − pKa1− pKa2

(10)

K11[R3N][H 2A](1 + 2K12[R3N] + K 21[H 2A]) + [R3N] − S0 = 0

(11)

where ϕ is the volume fraction of diluent in the organic phase, S0 is the initial concentration of TOA in the organic phase, and Corg is the total concentration of dicarboxylic acid in the organic phase. The loading of TOA, Zcal, is a function of the pKa of the dicarboxylic acid, ϕ, m, S0, pH, K11, K12, and K21. Using eqs 8−11, the value of Zcal can be predicted for any system if the constants K11, K12, and K21 are known. The apparent extraction constants K11, K12, and K21 were determined by fitting the experimental data for Z to eqs 8−11 using a least-squares regression method. The results are listed in Table 3. It should be noted that the obtained values of K12 for malonic, succinic, and adipic acids represent forming the 1:2 acid−amine hydrogen-bound complexes with TOA.

K a1

H 2A ← → H+ + HA−

K a1 = [H+][HA−]/[H 2A]

Caq pH − pK a1

(2)

K a2

HA− ← → H+ + A2 − K a2 = [H+][A2 −]/[HA−]

Table 3. Model Parameters for TOA−Carboxylic Acid Systems

(3)

m

H 2A ↔ H 2A

diluent

m = [H 2A]/[H 2A]

(4)

K11

R3N + H 2A ↔ R3N · H 2A

1-octanol

(5) 1-octanol

K12

R3N· H 2A + R3N ← → (R3N)2 · H 2A

(6)

K 21

R3N·H 2A + H 2A ← → R3N·(H 2A)2

100% TOA

(7)

1-octanol

where H2A represents dicarboxylic acid, Ka is the dissociation constant of acid in the aqueous phase, m is the physical extraction constant of the acid for the pure diluent obtained from the partition coefficient with the same initial concentration of dicarboxylic acid in the TOA-diluent system, Kpq is the apparent equilibrium constant (p = 1 or 2, q = 1 or 2), the square brackets indicate concentrations (all of the concentrations are expressed in molar terms), and the overbar signifies species in the organic phase. The loading of TOA, Z, is defined as the total concentration of acid (all forms) bonded to TOA in the organic phase divided by the total concentration of TOA (all forms) in the organic phase. With the appropriate material balance, Z is determined for a given set of stoichiometries as Zexp =

Zcal = =

(Corg − [H 2A]) S0

=

100% TOA 1-octanol

a

m

log K11 (L·mol−1)

log K12 (L·mol−1)

log K21 (L·mol−1)

Oxalic Acid, pKa1 = 1.23, pKa2 = 4.19 0.500 0.140 5.477 2.602 1.101 0.140 5.301 2.079 Malonic Acid, pKa1 = 2.83, pKa2 = 5.69 0.500 0.193 3.813 0.602 1.101 0.193 3.699 a 2.203 a 2.699 a

−0.097 −0.086 −0.086

Succinic Acid, pKa1 = 4.16, pKa2 0.200 0.263 2.301 0.500 0.263 2.079 1.101 0.263 1.813 1.762 0.263 1.544 2.203 a 1.041

= 5.61 1.301 0.699 0.301 a a

−0.187 −0.301 0.114 0.188 0.415

= 4.42, pKa2 = 5.41 1.785 1.792 1.556 1.230 1.462 a

a a 0.230

Adipic Acid, pKa1 0.200 2.745 0.500 2.745 1.101 2.745

0.079 0

Kmn = 0 (m = 1−3, n = 1, 2).

As shown in Figure 3, the calculated values of Z agree with the experimental values of Z when the dicarboxylic acid concentration is low (Z < 1), which is the typical concentration range for the extraction method,36 and most points are within ±10% deviation. Thus, the model is valid in representing the equilibrium behavior of saturated aliphatic dicarboxylic acid− trioctylamine systems at Z < 1. 3.3. Equilibria of Dicarboxylic Acids in TOA/1-Octanol and Overloading of TOA (Z > 1). The equilibrium curves of oxalic, malonic, succinic, and adipic acids in 0.50 mol·L−1 TOA/1-octanol systems are shown in Figure 4. The extraction abilities follow the sequence oxalic acid > malonic acid > succinic acid > adipic acid at Z < 1, which agrees with the sequence of the values of pKa1 in Table 1: the stronger the acid, the greater the extraction ability. This result agrees with the dependence of the wavenumbers of specific peaks for COO− of

(Corg − ϕm[H 2A]) S0

CTOA (mol·L−1)

(8)

([R3N ·H 2A] + 2[(R3N)2 ·H 2A] + [R3N ·(H 2A)2 ]) S0 K11[R3N][H 2A](1 + K12[R3N] + 2K 21[H 2A]) S0 (9)

In addition, [H2A] and [R3N] can be obtained from the dissociated equilibrium of dicarboxylic acid (eq 10) and the mass balance of TOA in the organic phase (eq 11), respectively 10798

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concentration of dicarboxylic acids increases in the aqueous phase. The concentration curve of dicarboxylic acids in the organic phase has an almost linear relationship with that in the aqueous phase at Z > 1. The calculated data of Z agree with the experimental data of Z at Z > 1. 3.4. Correlation of Apparent Reactive Equilibrium Constants. Most of the complexes exist in the 1:1 and 1:2 forms when the dicarboxylic acid concentration is low (Z < 1), which is the typical concentration range for the extraction method.36 Thus, the apparent equilibrium constants, K11 and K12, can be used to represent the degree of extraction. The correlations of these constants were investigated. Generally, the degree of extraction depends on two major factors: (1) the association ability between the solute and TOA, pKa,B ′ − pKa, and (2) the solvation ability of the polar complex in the diluent, log P. As dicarboxylic acid with two carboxyls can obtain higher hydrophilicity and greater pKa1 than monocarboxylic acid, dicarboxylic acid shows extraction equilibrium characteristics different than those of monocarboxylic acid. Since the structures and physical properties of dicarboxylic acids are responsible for their extraction equilibrium characteristics, oxalic, malonic, succinic, and adipic acids with similar structures were treated as one group for fitting. As was the case in our previous work,23 the extraction equilibrium parameter, log K11 or log K12, mainly depends on the properties of the organic acid and TOA, log P, pKa1 or pKa2, and pK′a,B.23 A correlation equation can be expressed as follows:

Figure 3. Comparison of experimental and calculated data for the loading of TOA.

′ − pK an + log P) + C2 log K1n = C1(pK a,B (n = 1 or 2)

(12)

The constants C1 and C2 in eq 12 can be obtained by fitting data points for log K11 or log K12 in Table 3 to eq 12 using a least-squares regression method. Equation 12 for oxalic, malonic, succinic, and adipic acids then becomes eq 13

Figure 4. Equilibrium curves of dicarboxylic acids in 0.50 mol·L−1 TOA/1-octanol systems.

′ − 1.564) + 0.082 log K1n = − 1.075(pK an − pK a,B

the dicarboxylic acid−TOA ion-pair complexes in FT-IR spectra on the values of pKa1. The 1:1 acid−amine complexes and free dicarboxylic acids are the main components of the organic phase at Z > 1, which means that overloading of TOA occurred. The typical overloading curves of TOA/1-octanol for malonic acid are shown in Figure 5. At Z < 1, the concentration of dicarboxylic acids in the organic phase increases sharply as the

(n = 1 or 2)

R = 0.9832

(13)

According to the correlation coefficient, R = 0.9832, and the deviation (within ±20%) between the calculated log K11 and log K12 values and the experimental data, the calculated log K11 and log K12 values fit the experimental data points very well, as shown in Figure 6. Thus, eq 13 can be used to predict the extraction equilibrium of saturated aliphatic dicarboxylic acids with TOA in 1-octanol.

4. CONCLUSION In this work, liquid−liquid extraction equilibria and complex structure studies for saturated aliphatic dicarboxylic acids (oxalic, malonic, succinic, and adipic acids) with TOA in 1octanol were conducted at various TOA concentrations. Ionpair complexes of acid−amine in the ratio 1:1 could be formed with TOA and these four dicarboxylic acids. The 1:2 acid− amine ion-pair complexes could not be formed when pKa2 ≥ 5.41. The wavenumbers of specific peaks for COO− of the dicarboxylic acid−TOA ion-pair complexes were proportional to the strengths of the acids. When the mass action law and suitable assumptions are used, a model for extraction equilibrium was evaluated and the apparent extraction constants were determined by fitting the experimental data

Figure 5. Typical overloading curve of TOA/1-octanol for malonic acid. 10799

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using a least-squares regression method. Stronger acids can obtain greater extraction abilities in TOA/1-octanol systems. The concentrations of dicarboxylic acids in the organic phase are almost linear with the dicarboxylic acid concentrations in the aqueous phase at Z > 1. The apparent extraction constants depended on the pKa of the dicarboxylic acid and the specific basicity of the extractant. The log K1n values for oxalic, malonic, succinic, and adipic acids are linear with (pKan − pK′a,B − 1.564) with a slope of −1.075. The predicted loadings of TOA agree with the experimental values.

ASSOCIATED CONTENT

S Supporting Information *

Experimental equilibrium data for saturated aliphatic dicarboxylic acids. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Relationship between [(pKa1 or pKa2) − pKa,B ′ − 1.564] and the apparent extraction equilibrium constant log K11 or log K12.

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ϕ = volume fraction of diluent in the organic phase R = correlation coefficient [S] = concentration of free TOA in the organic phase (mol·L−1) S0 = initial concentration of TOA in the organic phase (mol·L−1) Z = loading of TOA

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, Grant 29836130. NOTATION Caq = total concentration of acid in the aqueous phase (mol·L−1) Corg = total concentration of acid in the organic phase (mol·L−1) K11 = apparent equilibrium constant of the 1:1 complex (L·mol−1) K12 = apparent equilibrium constant of the 1:2 complex (L·mol−1) K21 = apparent equilibrium constant of the 2:1 complex (L·mol−1) Ka1 = first dissociation constant of the solute (mol·L−1) Ka2 = second dissociation constant of the solute (mol·L−1) pK′a,B = specific basicity of TOA (mol·L−1) log P = solvation ability of the polar complex in the diluent m = physical extraction constant of the acid for the pure diluents 10800

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dx.doi.org/10.1021/ie4008002 | Ind. Eng. Chem. Res. 2013, 52, 10795−10801