Ind. Eng. Chem. Res. 2006, 45, 9075-9079
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SEPARATIONS Dependence of Extraction Equilibrium on Apparent Basicity of Extractant Xinchang Shan, Wei Qin,* Shuo Wang, and Youyuan Dai State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China
The extraction method based on reversible chemical complexation is highly effective and selective for separating polar organic solutes from dilute solutions. In this paper, trioctylamine (TOA) in 1-octanol, trialkyl phosphine oxide (TRPO), and tri-butyl-phosphate (TBP) in kerosene were selected as typical extractants. The apparent basicity of these extractants to HCl was measured. Dependences of the extraction equilibrium behavior on the apparent basicity of the extractant were investigated. Results showed that the apparent basicity of the extractant depended on extractant type and concentration. A new mathematic model was proposed, which can predict the extraction equilibrium of Lewis acid including the association driving force (∆pKa) and the hydrophobicity of the complex. The apparent extraction equilibrium constant for various extractant systems can be predicted well by using different coefficients in the equation. The correlative equation can be used as the basic style to correlate other extraction equilibria. Introduction Liquid-liquid extraction is an economical, efficient, and environmentally benign method for the separation of dilute solutions, which include carboxylic acids, amino acids, amines, phenols, and so on. Extractive recovery of solutes from dilute aqueous solutions has received increasing attention.1-5 In previous studies, the optimal mixture solvents for various types of extractants were found. For long-chain aliphatic tertiary amines, for example, trioctylamine (TOA) or Alamine 336, the optimal mixture solvent was extractant-protonic diluent,6,7 because a polar diluent could increase the extracting power of nonpolar amines by providing additional solvating power that allows a higher fraction of polar Lewis acid-amine complexes to stay in the organic phase, and the solvation effect of diluent caused by formation of H-bonds between the complex and protonic polar diluent improved the extraction power more efficiently than that of the non-protonic polar diluent. Therefore, the protonic polar diluent, for example, 1-octanol, 2-octanol, or CHCl3, was more suitable as a diluent to TOA or Alamine 336. For phosphorus-bonded oxygen-containing extractant, for example, trialkyl phosphine oxide (TRPO) and tri-butyl-phosphate (TBP), the optimal mixture solvent was extractant-inert diluent.8,9 The inert diluent, which only had a diluting effect, did not affect the solvating power of complex in the mixture solvent apparently. Three important factors had been found to influence the equilibrium characteristics of amine extraction of carboxylic acids from aqueous solutions, that is, the nature of the acid, the dissociated equilibrium constant of acid (pKa)10 and the hydrophobicity of the acid (log P),11 the nature of the solvent, and the apparent basicity of extractant to HCl (pKa,B).6-8 Qin et al.7 studied the extraction equilibrium behavior of monocarboxylic * To whom correspondence should be addressed. Tel.: +86 10 62782748. Fax: +86 10 62782748. E-mail:
[email protected].
acids by trioctylamine (TOA). The apparent basicity of TOA to HCl, pKa,B, was measured in various diluents. A correlated equation of the apparent extraction equilibrium constant of the 1:1 complex between acid and TOA, K11, was obtained with log P, pKa, and pKa,B as follows:
log K11 ) 1.3164[(pKa,B - pKa) + log(93.47P)]
(1)
The empirical correlated eq 1 obtained from the watercarboxylic acid-TOA/diluent is limited for explaining the extraction mechanism and predicting the extraction equilibrium behavior for other extractant systems. In this work, three types of typical mixed extractants, TOA/ 1-octanol, TRPO/kerosene, and TBP/kerosene, were selected as the solvent. The apparent basicity of the extractant to the HCl, pKa,B, was measured at various extractant systems. The apparent extraction equilibrium constants for carboxylic acid extracted by TOA and TRPO and phenols extracted by TBP were correlated with the nature of the solutes and extractant used. Definition of Apparent Basicity. Generally, the basicity of a compound represents the power of releasing hydrogen ion in the aqueous solution. In this paper, the apparent basicity of the basic or neutral extractant (TOA, TRPO, TBP) to HCl (pKa,B) is defined as follows: Ka,B
E‚H(org)+ y\zE(org) + H(aq)+ Ka,B )
[E](org)[H +](aq) [E‚H+](org)
(2) (3)
where E stands for the extractant. Materials and Methods Chemicals. All of the solutes used analytical reagents. The physical properties of the solute are listed in Table 1. TOA with
10.1021/ie060814+ CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2006
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Table 1. Physical Properties of Solute pKa12
solute
formic acid acetic acid propionic acid butyric acid valeric acid caproic acid o-cresol m-cresol p-cresol phloroglucinol o-dihydroxybenzene
log P13
solute
Carboxylic Acid-TOA/1-Octanol and Carboxylic Acid-TRPO/Kerosene Systems 3.75 -0.538 lactic acid 4.75 -0.313 chloroacetic acid 4.87 0.290 dichloroacetic acid 4.81 0.802 trichloroacetic acid 4.82 1.352 glycolic acid 4.84 1.920 glyoxylic acid 10.26 10.00 10.27 8.45 9.36
Phenol-TBP/Kerosene Systems 1.95 m-dihydroxybenzene 1.96 p-dihydroxybenzene 1.94 phenol 0.04 1,2,3-trihydroxybenzene 0.88
pKa12
log P13
3.86 2.87 1.26 0.52 3.83 3.34
-0.780 0.139 0.705 0.713 -1.097 -0.413
9.44 9.91 9.99 9.03
0.80 0.59 1.48 0.09
Table 2. Apparent Basicity of Various Extractants S0 (mol‚L-1)
φ
pKa,B
0.200 0.400 0.500 0.600 1.000 1.101 1.650 1.762 1.980 2.203
TOA/1-Octanol System 0.09 0.18 0.23 0.27 0.45 0.50 0.75 0.80 0.90 1.00
4.82 4.74 4.66 4.64 4.52 4.45 3.99 3.86 3.62 3.32
0.1257 0.2514 0.6285 0.7542 1.0056 1.2570 1.7598 2.2626 2.5140
TRPO/Kerosene System 0.05 0.10 0.20 0.30 0.40 0.50 0.70 0.90 1.00
0.01 0.01 0.04 0.06 0.06 0.06 0.07 0.16 0.24
0.180 0.361 0.722 1.083 1.805 2.888 3.610
TBP/Kerosene System 0.05 0.10 0.20 0.30 0.50 0.80 1.00
-0.49 -0.48 -0.47 -0.45 -0.45 -0.44 -0.44
purity >99 wt %, TBP with purity >98.5%, and TRPO, kindly supplied by CYTEC Canada Incorporation free of charge without further purification, were used as the extractants. The protonic diluent, 1-octanol, and nonpolar diluent, kerosene, were used in this work. Kerosene was supplied by Petro China with the distillation range from 170 to 310 °C, and 1-octanol was an analytical purity reagent. Apparent Basicity Determination. For the experiment operation, conveniently, pKa,B is almost equal to the solution pH according to eq 3, if [E](org) ≈ [E‚H +](org) and [E](org) . [H+]. All of the experiments for pKa,B determination were conducted in 100 mL flasks at 25 ( 0.5 °C. Organic phases were prepared containing different extractant concentrations in a diluent. 10 mL of organic phase was mixed with 10 mL of HCl solution with one-half molar concentration of the extractant to make the complex concentration approach the concentration of free extractant at equilibrium state. The flask containing the organic phase and HCl solution was shaken for about 6 h in a shaker bath with a vibration rate of 200 rpm and then left to settle for 1-2 h, during which the two phases separated. The upper layer (extractant phase) was removed, and the aqueous phase was sampled for pH and HCl concentration analyses. It should be noted that the HCl concentration extracted into the organic phase at equilibrium state was dependent on the
Figure 1. Effect of extractant volume fraction on the apparent basicity.
degree of extraction of HCl. In this paper, TOA/1-octanol has a large degree of HCl extraction; therefore, almost all of the HCl in the aqueous phase can be extracted into the organic phase. The conditions [E](org) ≈ [E‚H +](org) and [E](org) . [H+] can be achieved. When TBP/kerosene and TRPO/kerosene were used as the solvent, the degree of HCl extraction is less, part of the HCl entered into the organic phase, and [E](org) > [E‚H +](org); then, the pH value of the aqueous solution is less than the pKa,B. In this case, the initial concentration of HCl in the aqueous solution that was much more than one-half the molar concentration of the extractant was used to ensure that [E](org) ≈ [E‚H+](org). The pKa,B value can be evaluated by eq 3, which shows the material balances of extractant and HCl, pH value, and HCl concentration in the aqueous solution at equilibrium state. According to the definition of pKa,B, pKa,B represents the dissociation power of water to the association compound between the extractant and HCl. The less is the dissociation power of water, the less is the residual concentration of HCl in the aqueous phase under the condition of pKa,B determination. Therefore, the larger is the pKa,B, the stronger is the basicity of extractant. To verify the pKa determination method proposed in this paper, the pKa,B values for two amines were measured using this method; for example, pKa,B values of octylamine and din-butylamine were 9.95 and 10.87, respectively. These data are very close to those (10.65 and 10.64) in Lange’s Handbook of Chemistry.12 It can be proved that the pKa,B determination method is feasible and accurate. Analysis Methods. Aqueous samples were analyzed for HCl concentration by using titration with NaOH as the standard solution and phenolphthalein as the indicator. The pH of the aqueous phase was determined by a pH meter (Hanna pH 201 model) with a deviation of (0.02. The measurements were duplicated. The measured pKa,B values are listed in Table 2. Results and Discussion Effect of Extractant Concentration on the pKa,B. Figure 1 shows the effect of extractant concentration in various solvent systems on the pKa,B. It was found that the pKa,B value decreases with increasing TOA concentration in the TOA/1-octanol system, while the pKa,B value increases with increasing extractant concentration slightly in the TRPO/kerosene and TBP/ kerosene systems. The pKa,B varying with the extraction concentration for TRPO or TBP solvent was the same as that for the TOA/CCl4 system.7
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An aliphatic amine (TOA), by itself, is a relatively poor solvation medium for polar complexes; thus, in an active diluent (1-octanol), the solvation occurs in a less favorable solvating medium. Therefore, the solvation power and pKa,B increase with increasing diluent concentration (decreasing TOA concentration) in 1-octanol. When TOA was used as extractant, low extractant concentration could provide a large association power. As TRPO or TBP itself is a relatively good solvation medium for polar complexes, the inert diluent does not affect the dissolvation of complexes apparently. Thus, pKa,B increases with increasing TRPO or TBP concentration in kerosene slightly. An optimal phosphorus extractant system should be considered with high extractant concentration. Comparison with Various Extractants. As indicated in Figure 1, all pKa,B values fall in the order of TOA > TRPO > TBP. TOA is a tertiary amine with a moderately strong Lewis basic extractant. Generally, TOA associates with the Lewis acid molecule by forming an acid-amine ion-pair complex.14 Both TRPO and TBP are neutral organophosphorus extractants. They associate with the Lewis acid molecule by forming an acidamine hydrogen-bond complex.9 Obviously, the basicity of TOA is larger than that of neutral organophosphorus extractant according to the type of association bond between extractant and solute. It was known that, for a neutral organophosphorus extractant, the basicity of the extractant depends on the number and type of alkyl chains bound to the phosphorus atom, and the basicity of organophosphorus compound increases with increasing alkyl chains.15 Because the electronegativity of TBP with ester group bound to PdO is less than that of TRPO with alkyl group, the acidity of TBP is stronger than that of TRPO; that is, pKa,B of TRPO is stronger than that of TBP. Mathematic Models of Extraction Equilibrium. Generally, liquid-liquid extraction was a very complicated chemical process. Many factors have been found to influence the equilibrium extraction characteristics of extraction systems. The factors comprehend the nature of the acid, the concentration of extractant, the type of diluent, temperature,16 water coextraction,16-18 etc. In this paper, the experimental temperature remains constant. Actually, water enters the organic phase with the extractant, complex, and diluent. Water coextraction decreases in the order 1-octanol > kerosene, which is the same order as the solubility of water in the diluent alone.16 Apparently, in inert diluent, for example, kerosene, water exists in the organic phase with extractant and complex, and in active diluents, especially, 1-octanol, H-bond association between extractant and diluent formed, which has disturbed the association between extractant and water; then, water coextraction in the extraction system depended on the extractant concentration. Additionally, a polar dilute solution was studied in this paper; then, the stoichiometric ratio of extractant to solute is much greater than 1. On the basis of the above discussion, three parameters, the nature of the acid, the concentration of extractant, and the type of diluent, were considered in describing the extraction equilibrium. The reactive extraction equilibrium for 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 extractant. In this paper, the extraction equilibrium was described by a set of equilibrium expressions involving the association of TOA or TRPO with carboxylic acid in a 1:1 molecule ratio, the dissociation equilibrium of carboxylic acid in water, and physical extraction of the complex by diluent as follows:
[E‚HA](org)
K11
E(org) + HA(aq) y\z E‚HA(org) K11 ) Ka
+
-
HA(aq) y\z H(aq) + A(aq)
(4)
[E](org)[HA](aq)
Ka )
[H+](aq)[A-](aq)
(5)
[HA](aq)
K11a
EH(org)+ + A(aq)- {\} E‚HA(aq) K11a ) MC
E‚HA(aq) y\z E‚HA(org)
[E‚HA](aq) [E‚H +](org)[A-](aq) MC )
E‚HA(org) E‚HA(aq)
(6)
(7)
then,
Ka K11 ) MC‚K11a Ka,B
(8)
where Ka is the dissociated constant of the acid. MC refers to the physics partition coefficient of complex. K11 and K11a are the apparent extraction equilibrium constants of carboxylic acid at the interface and in the aqueous phase, respectively. According to the definition of MC, it relates to the hydrophobicity of the complex. On the basis of the author’s obtained results of prediction organics hydrophobicity with a quantitative structure properties relationship (QSPR),19 MC could be expressed by the sum of hydrophobicity for the individual compound in the complex,
MC ) a1PSa2PEa3
(9)
where PS and PE are the hydrophobicities of the solute and extractant, respectively. Because K11a is similar to the one for the neutral reaction between weak acid and weak base in the aqueous solution, it must be related to the acidity of solute and the basicity of extractant, that is, K11a ) f(pKa/pKa,B), and K11a can be assumed:
K11a ) a4
( ) Ka Ka,B
a5-1
Next, eq 8 becomes,
K11 ) a1a4PSa2PEa3‚
(10)
( ) Ka Ka,B
a5
(11)
log K11 ) a5(pKa,B - pKa) + log(a1a4PEa3) + a2 log P (12) Equation 12 can be divided into two items, (pKa,B - pKa) and log(a1a4PEa3) + a2 log P. The former, association driving force (∆pKa) as the normal neutral reaction of acid and base, interprets the ability of forming a complex with extractant and solute; the larger is the ∆pKa, the more the complex formed. The latter represents the hydrophobicity of the complex, and the log(a1a4PEa3) item becomes the constant for a fixed extractant system; then, the larger is the log P, the more the complex dissolved in the organic phase. Therefore, eq 12 eventually annotates the reactive extraction mechanism. By regression with 44, 18 sets of experimental data for carboxylic acid extracted by TOA and TRPO, respectively, in which the apparent extraction equilibrium constants, K11, can be obtained from refs 20 and 21, the coefficients a2, a5, and
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the hydrophobicity of the complex, hold the concrete explanation. Thus, eq 12 can be used as the basic style to correlate other extraction equilibria. Conclusion In the present work, the apparent basicity of extractant to HCl was measured in the TOA/1-octanol, TRPO/kerosene, and TBP/kerosene systems. A correlative equation among the apparent extraction equilibrium constant, K11, the association driving force (∆pKa), and the hydrophobicity of the solute was obtained on the basis of extraction mechanism, and the apparent extraction equilibrium constant for various extractant systems can be predicted very well by using the different coefficients in the equation. Acknowledgment Figure 2. Plot of log K11 versus log K11,cal.
log(a1a4PEa3) in eq 12 were determined. The equation becomes, for the TOA/1-octanol system,
log K11 ) 1.6169(pKa,B - pKa) + 1.3370 log P + 2.1615 (n ) 44, r ) 0.9930) (13)
This work was supported by the National Natural Science Foundation of China, Grant No. 29836130. Note Added after ASAP Publication. This article was released ASAP on November 22, 2006 with four wrong values in Table 2, column 1. The correct version was posted on November 28, 2006.
and for the TRPO/kerosene system,
log K11 ) 0.6676(pKa,B - pKa) + 0.5276 log P + 3.6976 (n ) 18, r ) 0.9616) (14) Plots of log K11 versus log K11,cal for carboxylic acid extracted by TOA and TRPO systems are shown in Figure 2. A good correlative coefficient (r) and a small deviation between the calculated log K11 and the measured data were obtained. In general, the degree of extraction for a carboxylic acid is directly proportional to both extractant concentration and K11, and less than 50% (in volume) diluent was used with TOA to improve the physical properties of the solvents, viscosity and surface tension, and made the mixture much easier to handle than the pure amine. At the TOA concentration range of 5-50%, K11 only had a little decrease because the pKa,B of TOA value had a little increase from 4.50 to 4.75. Therefore, a larger TOA concentration could provide a larger degree of extraction. Because eq 12 was derived on the basis of the reactive extraction mechanism, it may be used to predict other extractant systems. Phenol extracted by TBP/kerosene was selected to predict its K11, and the apparent extraction equilibrium constants, K11, were available from ref 22; the coefficients in eq 12 were determined by fitting 18 data to eq 12, and eq 12 was correlated as,
log K11 ) 0.0664(pKa,B - pKa) + 1.0268‚log P + 1.2427 (n ) 18, r ) 0.9923) (15) Plots of log K11 versus log K11,cal for phenol extracted by TBP/kerosene are shown in Figure 2. A small deviation (within (5%) between the calculated log K11 and the measured data was obtained. It has been shown that the calculated log K11 values fit the measured data very well. Comparing eq 12 to 14, it can be found that the coefficient (a5) falls in the order of TOA > TRPO > TBP, which is the same as the order of their pKa,B value. The coefficient log(a1a4PEa3) showed the order of TRPO > TOA > TBP, which is the same as the order of their hydrophobicity. Obviously, the two items in eq 12, association driving force (∆pKa) and
Nomenclature K11 ) apparent extraction equilibrium constant of the 1:1 complex at interface (L‚mol-1) K11a ) apparent extraction equilibrium constant of the 1:1 complex in aqueous phase (L‚mol-1) log P ) hydrophobicity parameter of the acid MC ) physics partition coefficient of the complex PE ) hydrophobicity of the extractant PS ) hydrophobicity of the solute pKa ) dissociated equilibrium constant of the Lewis acid (mol L-1) pKa,B ) the apparent basicity of extractant to HCl (mol‚L-1) r ) correlative coefficient S0 ) initial concentration of extractant in the organic phase (mol‚ L-1) φ ) volume fraction of extractant Subscript org ) species in the organic phase aq ) species in the aqueous phase Literature Cited (1) Kertes, A. S.; King, C. J. Extraction Chemistry of Fermentation Product Carboxylic Acids. Biotechnol. Bioeng. 1986, 28, 269-282. (2) Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335-1342. (3) King, C. J. Amine-Based Systems for Carboxylic Acid Recovery: Tertiary Amines and the Proper Choice of Diluent Allow Extraction and Recovery from Water. CHEMTECH 1992, 5, 285-291. (4) Hartl, J.; Marr, R. Extraction Processes for Bioproduct Separation. Sep. Sci. Technol. 1993, 28, 805-819. (5) Juang, R. S.; Huang, R. H. Comparison of Extraction Equilibria of Succinic and Tartaric Acids from Aqueous Solutions with Tri-n-octylamine. Ind. Eng. Chem. Res. 1996, 35, 1944-1950. (6) Shan, X. C.; Qin, W.; Dai, Y. Y. Dependence of Extraction Equilibrium of Monocarboxylic Acid from Aqueous Solutions on the Relative Basicity of Extractant. Chem. Eng. Sci. 2006, 61, 2574-2581.
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 9079 (7) Qin, W.; Li, Z. Y.; Dai, Y. Y. Extraction of Monocarboxylic Acids with Trioctylamine: Equilibria and Correlation of Apparent Reactive Equilibrium Constant. Ind. Eng. Chem. Res. 2003, 42, 6196-6204. (8) Shan, X. C.; Qin, W.; Dai, Y. Y. Dependence of Extraction Equilibrium of Monocarboxylic Acid on the Relative Basicity of Extractant (TRPO). Proc. ISEC2005 2005, 220-226. (9) Li, Y. X.; Wang, Y. D.; Dai, Y. Y. Effect of Diluents on the Extraction of Oxalic Acid by Trialkylphosphine Oxide. Chin. J. Chem. Eng. 2004, 12, 143-148. (10) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretation of Data. Ind. Eng. Chem. Res. 1990, 29, 1327-1333. (11) Hano, T.; Matsumoto, M.; Ohtake, T.; Sasaki, K.; Kawano, Y. Extraction Equilibria of Organic Acids with Tri-n-octylphosphineoxide. J. Chem. Eng. Jpn. 1990, 23, 734-738. (12) Lange’s Handbook of Chemistry, XXth ed.; John, A. D., Ed.; McGraw-Hill Book Co. Inc.: New York, 1972. (13) Leo, A.; Hansch, C.; Elkins, S. Partition Coefficients and Their Uses. Chem. ReV. 1971, 71, 525-616. (14) Wen, M.; Yang, Y. Y.; Dai, Y. Y.; Wang, J. D. Mechanism of Extraction for Carboxylic Acid with Tri-n-octylamine (1) Acetic Acid-TOA System. J. Chem. Ind. Eng. (in Chinese) 1998, 49, 303-309. (15) Danesi, P. R.; Reichley-Yinger, L.; Mason, G.; Kaplan, L.; Horwitz, E. P.; Diamond, H. Selectivity-Structure Trends in the Extraction of Co(II) and Ni(II) by Dialkyl Phosphoric, Alky Alkylphosphonic, and Dialkyphosphinic Acids. SolVent Extr. Ion Exch. 1985, 3, 435-452.
(16) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 3. Effect of Temperature, Water Coxetraction, and Process Considerations. Ind. Eng. Chem. Res. 1990, 29, 1333-1338. (17) Choi, K.; Tedder, D. W. Molecular Interactions in ChloroformDiluent Systems. AIChE J. 1996, 43, 196-211. (18) Blaylock, C. R.; Tedder, D. W. Competitive Equilibria in the System: Water, Nitric, Acid, Tri-n-Butyl Phosphate, and Amsco 125-82. Mixtures. SolVent Extr. Ion Exch. 1989, 7, 249-271. (19) Wang, M.; Qin, W.; Dai, Y. Y. Extraction Equilibria Behavior of Monocarboxylic Acids by Trioctylamine(III) Quantitative Structure-Property Relationship of Apparent Extraction Equilibrium Constant. J. Chem. Ind. Eng. (in Chinese) 2004, 55, 65-68. (20) Shan, X. C.; Qin, W.; Dai, Y. Y. Reactive Extraction Equilibria Behavior of Monocarboxylic Acids by Trialkylphosphine Oxide. J. Chem. Ind. Eng. (in Chinese) 2005, 56, 2346-2350. (21) Wang, Y. D.; Li, Y. X.; Li, Y.; Wang, J. Y.; Li, Z. Y.; Dai, Y. Y. Extraction Equilibria of Monocarboxylic Acids with Trialkylphosphine Oxide. J. Chem. Eng. Data 2001, 46, 831-837. (22) Yang, Y. Y.; Guo, J. H.; Dai, Y. Y. Extraction of Phenols Based on Chemical Complexation in a Wide Range of pH. J. Chem. Ind. Eng. (in Chinese) 1997, 6, 706-712.
ReceiVed for reView June 27, 2006 ReVised manuscript receiVed October 7, 2006 Accepted October 17, 2006 IE060814+
