Effect of pH on Dicarboxylic Acids Extraction by Amine-Based


Riki Canari and Aharon M. Eyal*. Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel. The work presente...
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Ind. Eng. Chem. Res. 2003, 42, 1293-1300

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Effect of pH on Dicarboxylic Acids Extraction by Amine-Based Extractants Riki Canari and Aharon M. Eyal* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

The work presented here studies the effect of pH on the extraction of malic, maleic, and glutaric acids by Primene JMT, tris(2-ethylhexyl)amine , and tri-n-octylamine and the IR and NMR spectra of the formed organic phases. Three categories were found: (1) In the first case, the extractant is a weaker base than both anions of the acid, i.e., pHhn < pKa1 < pKa2. The undissociated acid is the dominant species in the organic phase. Extraction is mainly due to H-bonding or solvation interactions and shows a drop-off at pH ≈ pKa1. (2) For systems in which pKa1 < pHhn < pKa2, at acid-to-amine molar ratios greater than 1, undissociated acid is abovestoichiometrically extracted, R3NH+‚‚‚AH-‚‚‚HAH, in addition to ion-paired acid. As the pH is increased, the undissociated acid is neutralized first. Thus, the extraction curve shows a dropoff at pH ≈ pKa1. At stoichiometric loading, the monovalent ion pair, R3NH+‚‚‚AH-, becomes the dominant species. Upon further pH elevation, the protonated amine R3NH+ is the neutralized acid. (3) In systems where pKa1 < pKa2 < pHhn, the divalent ion pair R3NH+‚‚‚A2-‚‚‚+HNR3 is formed. Thus, the extraction curve shows an additional drop-off at pH ≈ pKa2. 1. Introduction Fermentation processes have recently become of great importance in manufacturing industrial, feed, and food products (e.g., carboxylic acid). These processes use lowcost renewable resources (mainly cereals) as raw material. Fermentation broths from carboxylic acid production are characterized by relatively low product concentrations and, in some cases, by relatively high pH’s, as most carboxylic acid producing microorganisms are sensitive to low pH. To make the fermentative route more attractive, separation science is faced with the need to generate economical technologies to separate carboxylic acid products from fermentation liquors of relatively high pH.1,2 In some cases, a strong acid is added, and the free carboxylic acid is separated from the acidulated broth (by extraction, distillation, etc.). Yet, in these cases, a byproduct salt is produced. In others processes, CO2 is introduced and partially acidulates the broth. In these cases, the pH of the broth is a function of the CO2 pressure. Thus, in all cases, it is important to understand the separation efficiency as a function of the pH. Depending on the pH of the solution, dicarboxylic acids can be found in three forms: (1) undissociated acid, HAH (where A represents the anion of the acid); (2) monovalent ion, AH-; and (3) divalent ion, A2-. These species can be found in aqueous phases as well as in organic phases, particularly in those containing organic bases, such as long-chain amines. From infrared spectra of such solutions, Gusakova et al.3 concluded that malonic acid forms divalent ions when introduced into solutions of dibutylamine in various diluents or triethylamine in methanol. This, however, is not the case upon extraction of malonic acid by a mixture of triethylamine and dioxane, acetonitrile, or chloroform. They concluded that the alkanol hydroxyl group inter* To whom correspondence should be addressed. Tel.: 9722-6585843. Fax: 972-2-6584533. E-mail: [email protected]

feres with the intramolecular hydrogen bond of the dicarboxylic acid, allowing for the formation of a divalent ion pair with the amine, i.e., R3NH+‚‚‚A2-‚‚‚+HNR3 (where R3N represents the amine). Tamada and King4-7 studied the extraction of carboxylic acids. The formation of a divalent ion pair [“(1,2)complex”] in dicarboxylic acids extraction by aminebased extractants was analyzed by the law of mass action and IR spectroscopy.6 They concluded that succinic acid forms a divalent ion pair upon extraction by tricaprylylamine (Alamine 336) in 1-octanol but does not form such an ion pair upon extraction by the same amine in methylene chloride, chloroform, nitrobenzene, or methyl isobutyl ketone (MIBK). Fumaric acid forms a divalent ion pair when extracted by Alamine 336 in chloroform or in MIBK. However, no evidence of divalent ion-pair formation was found for maleic acid extraction by Alamine 336 in chloroform. Tamada and King’s main conclusion was that the ability to form intramolecular hydrogen bonds inhibits the formation of divalent ion pairs. Kuwano et al.8 concluded that oxalic acid forms a divalent ion pair when extracted by a secondary amine in hexane. Frolov et al.9 concluded that the formation of a sulfuric acid divalent ion pair is more favored in extraction by a more basic amine, according to the following sequence: tri-n-octylamine > benzyldinonylamine > tribenzylamine > N-octylaniline. (It is not clear from the article how they determined the relative basicities of the amines.) Kirsch et al.10 examined the extraction of oxalic acid by tri-n-octylamine of various concentrations in chloroform, toluene, or MIBK. They determined the acid species in the organic phase by using IR spectroscopy and by calculating the nonprotonated amine concentration. They concluded that divalent ions are formed with the amine in systems where the diluent is either chloroform or MIBK, but not when the diluent is toluene. Tung and King11 studied the effects of pH on succinic acid extraction by Alamine 336 and by Amberlite LA-2

10.1021/ie010579p CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003

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in various diluents, as well as the effects of pH on succinic acid absorption by a number of solid sorbents. They concluded that the extraction capacities of the amines are maintained high at relatively high pH values because of the stabilization effect of the diluents on the acid-base complex. They also concluded that secondary amines provide higher capacities than do tertiary amines when the additional proton of the secondary amine interacts with the solvent. The authors did not focus on the question of the effect of the pH on the divalent acid species in the organic phase. The present article is one of a series of articles in which we analyze the effect of pH on the extraction of carboxylic acids.12-14 The first ones consider the extraction of momocarboxyic acids,12,15 where we divide the extraction mechanisms into two main categories: (1) H-bonding and solvation and (2) ion-pair formation. For relatively weak bases, extraction is conducted either by H-bonding or by solvation and is determined mainly by the concentration of the undissociated acid. Thus, the extraction is strongly dependent on the pKa of the acid. On the other hand, when the amine extractant is more basic than the anion of the extracted acid, ion-pair formation is the dominant mechanism. In these cases, the extraction strongly depends on the pKhn (pH of halfneutralization) of the extractant, determined according to the method of Grinstead and Davis.16,17 The results described above show that the formation of divalent ion pairs of a diprotic acid in an amine-based extractant depends on the properties of the diluent, as well as on those of the amine and of the extracted acid. Can we predict the systems in which such species will form? What are the effects of the extractant’s basicity, the extracted acid’s pKa’s, and the pH on the acid species in the organic phase? In the study presented here, we attempt to answer these questions. The organic phases were loaded with the acids, and the pH was increased gradually by adding small portions of a mineral base. As a result, the acidic species in the organic phases were neutralized according to the their acidities, similarly to the process observed in titration curves. These experiments were conducted using the following systems: (1) systems in which the extractant is a weaker base than both anions of the extracted acid, i.e., pHhn < pKa1 < pKa2; (2) systems in which the extractant is a stronger base than the first anion of the extracted acid but a weaker base than the second anion, i.e., pKa1 < pHhn < pKa2; and (3) systems in which the extractant is a stronger base than both anions of the extracted acid, i.e., pKa1 < pKa2 < pHhn. 2. Experimental Section Materials. The amines used were a primary amine of the structure RR′R′′CNH2 with a total of 18-22 carbon atoms, Primene JMT (Rohm & Haas, technical grade), tri-n-octylamine (TOA; Hoechst, technical grade), and the branched tertiary amine tris(2-ethylhexyl)amine (TEHA; Hoechst, technical grade). The solvents used were 1-octanol (Merck, 99%) and the low-aromatics kerosene Parasol (Paz). The acids used were maleic acid [HOOC-CHdCH-COOH (cis); BDH, 99.5-105%], glutaric acid [HOOC-(CH2)3-COOH; Fluka, 98.5%], and D,L-malic acid [HOOC-CH2CH(OH)-COOH; BDH, 99%]. Table 1 presents the pKa values of the extracted acids and the pHhn (pH of half-neutralization of loaded amine in the given diluent) values of the protonated amine. Experimental Methods. Three extractants were used: 0.5 mol/kg TEHA in 1-octanol, 0.5 mol/kg TOA

Table 1. pKa1, pKa2, and pKa3 Values of the Acids or pHhn Value of the Extractant acid

pKa or pHhna

glutaric malic maleic protonated TEHA in 1-octanol protonated TOA in kerosene + 20% 1-octanol protonated JMT in 1-octanol

4.4, 5.4 3.4, 5.1 1.8, 6.1 1.8 4.5, 5.5 7

a

The first pHhn value was determined for the amine when loaded with hydrochloric acid, and the second when loaded with dichloroacetic acid.

in kerosene + 20% 1-octanol, and 0.5 mol/kg JMT in 1-octanol. The extractants were equilibrated with small amounts of highly concentrated aqueous acidic solutions to give clear loaded organic phases. In each test, samples from these loaded organic phases were equilibrated with similar volumes of aqueous solutions containing increasing amounts of NaOH. Table 2 presents the parameters of the experiments: the organic-to-aqueous (w/w) phase ratios (O/A), the initial acid and amine concentrations in the loaded organic phase, and the added amounts of NaOH in each set of experiments. The acid concentration in the organic phase was determined by titration with 0.1 N NaOH using phenolphthalein as the indicator and 2-propanol as the cosolvent. The pH was determined using a Cole-Parmer pH meter (8350-95,-97) with a combined pH glass electrode. IR spectra of the organic phases were obtained with Nicolet 510 FTIR spectrometer, and 1H NMR spectra of the organic phases were recorded on Bruker AMX-400 and -300 spectometers (operating at 400 and 300 MHz, respectively) according to the method outlined in the Poulad-Meyer work.18 The pH values at which the extractions were most strongly affected by changes in pH (denoted here as “drop-offs”) were calculated by finding the torsion points of the extraction vs pH curves (the points where the first derivative curves reach minima). 3. Results and Discussion Glutaric Acid Extraction by TEHA in 1-Octanol. This system represents a case in which the extractant is a weaker base than both anions of the extracted acid, i.e., pHhn < pKa1 < pKa2. Figure 1, plot A (b), shows the effect of pH on the extraction of glutaric acid by 0.5 mol/kg TEHA in 1-octanol. It shows a sharp drop-off in the extraction as the pH increases. The pH value of this drop-off is about 4.5. TEHA is a bulky branched tertiary amine. Steric hindrance interferes with the stabilization of the amineacid ion pair through aggregation (self-solvation). As a result, the apparent basicity of TEHA is very low, pHhn(HCl) ) 1.8, even though a protic enhancer (1octanol) is added. The pKa1 (4.4) and pKa2 (5.4) of glutaric acid are both higher than the pHhn(HCl) of the extractant, i.e., the two anions of glutaric acid act as stronger bases than the extractant. Hence, the formation of a protonated amine ion pair is energetically unfavorable. Therefore, the acid species in the organic phase is expected to be the undissociated form of the acid H-bonded to the amine (R3N‚‚‚HAH), or solvated. As the pH is increased, the strongest acid in both phases is neutralized first, that is, the first acidic group of glutaric acid. Therefore, we can attribute the sharp

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1295 Table 2. Experimental Conditions: Organic-to-Aqueous Phase Weight Ratios (O/A), Initial Acid and Amine Concentrations in the Loaded Organic Phase, and Amounts of NaOH Added organic-phase composition

initial O/A (g/g)

initial acid conc [mol/kg (mequiv)]

initial amine conc [mol/kg (mmol)]

amount NaOH added (mmol)

glutaric acid and TEHA in 1-octanol

pHhn < pKa1 < pKa2 4.5/5 0.5 (4.5)

0.44 (1.98)

0-4.2

maleic acid and TEHA in 1-octanol maleic acid and TOA in kerosene + 20% 1-octanol

pKa1 < pHhn < pKa2 4.1/5 0.33 (2.7) 5/6 0.58 (5.8)

0.45 (1.84) 0.46 (2.3)

0-3.6 0-6

maleic acid and JMT in 1-octanol glutaric acid and JMT in 1-octanol malic acid and JMT in 1-octanol citric acid and JMT in 1-octanol

pKa1 < pKa2 < pHhn 4/8.7 0.47a (3.8) 5/6 0.59 (5.9) 5/9.5 0.85 (8.5) 5/8 0.5 (7.5)

0.46 (1.84) 0.42 (2.1) 0.4 (2) 0.4 (2)

0-4.7 0-6 0-9.5 0-8

a

In this experiment, the acid loadings in some contacts were higher than 0.47 mol of acid/kg of organic phase.

decrease in the extraction at pH ≈ 4.5 to the neutralization of this acidic group (pKa1 ) 4.4), as demonstrated in eq 1

R3N‚‚‚HAHorg + NaOH S R3Norg + AH-aq + Na+ + H2O (1) Infrared spectra of the organic phases were recorded to examine the dominant acid species in the organic phase. These spectra are analyzed later in this article. Maleic Acid Extraction by TEHA in 1-Octanol. This system represents the case in which the extractant is a stronger base than the first anion of the extracted acid (or of similar basicity) but a weaker base than the second anion of the acid, i.e., pKa1 e pHhn < pKa2. Figure 1, plot B (9), shows the effect of pH on maleic acid extraction by 0.5 mol/kg TEHA in 1-octanol. The plot shows a drop-off at a pH of ∼3. The pHhn(HCl) value of TEHA in 1-octanol is 1.8, whereas the pKa1 and pKa2 values of maleic acid are 1.8 and 6.1, respectively. According to these values, the extractant’s basicity is similar to that of the anion of maleic acid’s first acidic group. Therefore, despite the bulkiness of this amine, ion-pair formation with the first acidic group of maleic acid takes place, to some extent, to give R3NH+‚‚‚AH-. As the pH is increased, undissociated maleic acid is neutralized according to eq 1, whereas the protonated amine is neutralized according to eq 2

R3NH+‚‚‚AH-org + NaOH S R3Norg + AH-aq + Na+aq + H2O (2) One might ask: Why does the extraction decrease at about pH ) 3 and not at pH ) 1.8, which is the pHhn(HCl) of the extractant, and the pKa1 of the acid? Our previous article1 showed that the pHhn values of amines are very sensitive to the composition of the organic phase, especially to the properties of the extracted acid. Thus, the pHhn value of 0.5 mol/kg TEHA in 1-octanol, when loaded with hydrochloric acid, is 1.8, but when loaded with trichloroacetic acid, the value is 4.68! Thus, should one conclude that the drop-off in the extraction of maleic acid is associated with the neutralization of the undissociated acid or with the neutralization of the protonated amine? (An analysis below of the infrared spectra of the organic phases examines this question.) Maleic Acid Extraction by TOA in 20% 1-Octanol and Kerosene. Figure 1, plot C (2), shows the effect of pH on maleic acid extraction by 0.5 mol/kg TOA in

Figure 1. Extraction of dicarboxylic acids.

20% 1-octanol in kerosene. As the pH goes up, the extraction shows a small decrease at the lower pH range (pH ≈ 1.1), a plateau where the acid-to-amine molar ratio (Z) is about 1, and a drop-off at about pH ) 5.5. TOA is less bulky than TEHA, as it is a straight-chain tertiary amine. Hence, the formation of ion-pair aggregates in the case of TOA is less sterically hindered, and TOA’s pHhn is much higher than TEHA’s (pHhn(HCl) ) 3 and 1.8 for TOA and TEHA, respectively). In the extraction considered here, pKa1,maleic acid (1.8) < pHhn (3) < pKa2 (6.1), i.e., the extractant acts as a stronger base than the anion of the first acidic group but as a weaker base than the anion of the second acidic group. Therefore, ion-pair formation between the amine and the first acidic group of maleic acid, R3NH+‚‚‚AH-org, is energetically favorable, but ion-pair formation with the second anion is not. We suggest that, at higher acid concentration (lower pH range), undissociated maleic acid is also extracted, either through H-bonding to the monovalent ion pair, R3NH+‚‚‚AH-‚‚‚HAH, or through a solvation mechanism. With increasing the pH, the strongest acidic group is neutralized (by the added NaOH), which is the first carboxylic group of the undissociated maleic acid, that is above-stoichiometrically extracted, R3NH+AH-‚‚‚HAH, with a pKa of 1.8, according to eq 3

NaOHaq + R3NH+‚‚‚AH-‚‚‚HAHorg T R3NH+‚‚‚AH-org + H2Oaq + HA-aq + Naaq (3) According to plot C in Figure 1, this reaction takes place at a pH of about 1.1, which is close to the pKa1 of the

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kind of a “double salt”, as shown in the following equation

2R3NH+‚‚‚AH-org+ 2NaOH T 2R3NH+‚‚‚A2-‚‚‚+Naorg + 2H2O (4) Because of the low stability of this double salt in the organic phase, an ion-exchange reaction of two double salt molecules takes place to form a divalent ion pair, where the second carboxylic group is paired with the protonated amine, as follows

2R3NH+‚‚‚A2-‚‚‚+Naorg T Figure 2. Extraction of dicarboxylic acids by 0.5 mol/kg JMT in octanol.

acid. Then, the curve shows a plateau at Z ) 1, suggesting that the monovalent ion pair, R3NH+AH(org), is the dominant species. The plateau is followed by a drop-off with a torsion point at pH ≈ 5.5. This drop-off, we suggest, is correlated with the neutralization reaction of the protonated amine, as demonstrated in eq 2. Infrared spectra of the organic phases of this system are analyzed in a subsequent section. Malic Acid and Glutaric Acid Extraction by JMT. This system is an example of a case in which the extractant is a stronger base than both anions of the extracted acid, i.e., pKa1 < pKa2 < pHhn. Figure 2, plot A (9), shows the effect of pH on the extraction of malic acid by an extractant composed of 0.5 mol/kg JMT in 1-octanol. The plot shows, in order of increasing pH, a plateau at about Z ) 0.9, a drop-off with a torsion point at about pH ) 4, a plateau at about Z ) 0.5, and another drop-off with a torsion point at about pH ) 7.5. Figure 2, plot B (b), shows the effect of pH on glutaric acid extraction by the same extractant. It shows a similar dependence: a plateau at about Z ) 1, a dropoff at about pH ) 4.7, a small plateau at a about Z ) 0.5, and another drop-off with a torsion point at about pH ) 7.5. JMT is a primary amine, which is less bulky than the two tertiary amines described above. Grinstead et al.17 noted that ion-pair aggregation of primary amines takes place even in very dilute amine hydrochloride solutions of 10-5 M, (compared with 10-2 M in the case of tertiary amine hydrochlorides). They explained this result by the lower hindrance with the formation of aggregates. In addition, the two hydrogens on the nitrogen atom stabilize the ion-pair aggregates of the primary amine salts by forming some sort of a hydrogen-bonded structure. As a result, the pHhn of this system is very high (pHhn(HCl) ) 7), higher than the both the first and the second pKa’s of both malic acid (pKa1 ) 3.4, pKa2 ) 5.1) and glutaric acid (pKa1 ) 4.4, pKa2 ) 5.4). The analysis of plots A and B in Figure 2 is as follows: At lower pH, the molar ratio between the acid and the amine in the organic phase is about 1. Thus, it is suggested that the dominant species is the monovalent ion pair, R3NH+‚‚‚AH-. Going to higher pH values, two main types of acidic groups can be neutralized: (1) the second carboxylic group of glutaric/malic acid in its monovalent ion pair, R3NH+‚‚‚AH(pKa ) 5.5), and (2) the protonated amine, R3NH+‚‚‚AH(pHhn(HCl) ) 7.0). Because the strongest acid in the system is preferably neutralized, the second carboxylic group of the dicarboxylic acid is neutralized to form a

R3NH+‚‚‚A2-‚‚‚+HNR3org + 2Na+aq + A2-aq (5) We suggest that the divalent ion pair, R3NH+‚‚‚A2-‚‚‚+HNR3, is the dominant species where the distribution curves show a plateau, Z ≈ 0.5. The pH at which the second acidic group is neutralized, 0.5 < Z < 1, depends on the pKa2 of the acid in the organic phase. This value is close to the value given in the literature for aqueous media. The results show that the torsion point upon extraction of malic acid is about pH ) 4, whereas pKa2,malic acid ) 5.1, and that upon extraction of glutaric acid is about pH ) 4.7, whereas pKa2,glutaric acid ) 5.4. A similar phenomenon was observed in Tung and King’s11 results, where succinic acid was extracted by A-336 and by Amberlite-LA-2 (tertiary and secondary amines, respectively), both in 1-octanol. Their results showed that the pKa2(org) of succinic acid is about 4 whereas its pKa2 in aqueous medium is 5.6. The deviations might result from the difference between the proton activities in the two phases. The next drop-off is at about pH ) 7.5, which corresponds to the neutralization reaction of the protonated amine. The free amine re-forms in the organic phase, according to the equation

2NaOHaq + R3NH+‚‚‚A2-‚‚‚+HNR3(org) T R3Norg + 2H2Oaq+ A2-aq + 2Na+aq (6) Maleic Acid Extraction by JMT. Figure 2, plot C (2), shows the effect of pH on the extraction of maleic acid by an extractant composed of 0.5 mol/kg JMT in 1-octanol. At low pH, the acid/amine molar ratio is about 1.8. On moving to higher pH values, the plot shows a drop-off at pH ∼1.1. Then, the curve shows a plateau at about Z ) 1, followed by a single drop-off where the pH range is 6-8, rather than two drop offs for glutaric acid and for malic acid extractions. At the lower pH range, undissociated maleic acid is extracted, either through a solvation mechanism or through H-bonding to the monovalent ion pair to form R3NH+‚‚‚AH-‚‚‚HAH. Starting at that point and moving to higher pH values, three main types of acidic groups can be neutralized (as in the case of maleic acid extraction by a TOA-based extractant): (1) the first carboxylic group of undissociated maleic acid, bonded to the monovalent ion pair, R3NH+‚‚‚AH-‚‚‚HAH (pKa ) 1.8), (2) the second carboxylic group of the maleic acid (pKa ) 6.1) (in its monovalent ion pair or in the undissociated form, R3NH+‚‚‚AH-‚‚‚HAH), and (3) the protonated amine, R3NH+‚‚‚AH-‚‚‚HAH (pHhn(HCl) ) 7.0). The first carboxylic acid group is preferably neutralized according to eq 3.

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1297 Table 3. Pertinent Peaks and Their Assignments in the IR Spectra of Glutaric Acid in JMT and TEHA in 1-Octanol JMT in 1-octanol peak

(cm-1)

1710 1640-1650 1550

TEHA in 1-octanol

0.5 > Z

0.5 < Z < 1

Z>1

0.5 > Z

0.5 < Z < 1

Z>1

assignment

v V

v V V

V V V

v v v

V V v

V V v

COOH COOH, H2O COO-

The curve then shows a single, large drop-off in the pH range 6-8. As suggested above, a divalent ion pair can be found in the organic phase only when the amine is a stronger base than the second anion of the dicarboxylic acid. Therefore, it can be formed in this particular case, because pKa2,maleic acid (6.1) < pKhn (7.5). If so, the single drop-off is a combination of two reactions: one relating to the neutralization of the second carboxylic group and the second to the neutralization of the protonated amine. An alternative suggestion is that the decrease in the extraction is a result of a single reaction, namely, the neutralization of the protonated amine bonded to the monovalant acid species (as in the case of maleic acid extraction by TOA), indicating no divalent ion-pair formation. To investigate the maleic acid species in the organic phases, IR and NMR spectra of the loaded JMT extractant, as well as of loaded TOA and TEHA extractants, were recorded. Infrared Spectroscopic Experiments. Typically, in the spectra of carboxylic acid in organic phases, a peak appearing in the range of about 1710-1650 cm-1 belongs to the nonsymmetric carbonylic bond, indicating an undissociated carboxylic group. A peak at about 1550 cm-1 corresponds to the symmetric carbonylic bond, indicating a carboxylic group in its dissociated form. Other peaks that also serve as useful tools for investigating the acid species are those in the “fingerprint” area (wavenumber < 1400 cm-1) corresponding to the effects that related bonds have on each other. Unfortunately, the peaks of symmetric and asymmetric carbonylic bonds are not always at the same wavenumber, i.e., they could change from one carboxylic acid to the other. A peak at about 1640 cm-1 indicates the presence of water in the organic phase. IR Spectra of JMT in 1-Octanol Loaded with Glutaric Acid. The data from the IR spectra of JMT in 1-octanol loaded with glutaric acid are presented in Table 3 (a strong peak is indicated by V, a weak one by v, and no peak by -). The spectrum of the abovestoichiometrically loaded organic phase (Z > 1) shows a peak at 1710 cm-1, which diminishes as the concentration of the acid decreases. It is suggested that this peak arises from the undissociated acid molecules H-bonded to the monovalent ion pair, that is, R3NH+‚‚‚AH-‚‚‚HOOC-(CH2)3-COOH. When the acid concentration is low (0.5 < Z < 1), the spectrum shows two strong peaks in the relevant range: one at 1550 cm-1 and another at 1640 cm-1. The first one is related to the dissociated carboxylic group ion-paired to the amine, R3NH+‚‚‚-OOC-(CH2)3-COOH. The other, at 1640 cm-1, is related to the undissociated carboxylic group of the acid in the ion pair R3NH+‚‚‚-OOC-(CH2)3-COOH. As the acid concentration in the organic phase decreases, the peak at 1640 cm-1 becomes smaller, whereas that at 1550 cm-1 increases. We suggest that this effect is related to the increased number of dissociated carboxylic groups, because the number of acid molecules that form the divalent ion-pair species R3NH+‚‚‚-OOC-(CH2)3-COO-‚‚‚+HNR3 increases.

Theoretically, we can expect that no peak would be observed at 1640 cm-1 (or only a small one) below an acid-to-amine molar ratio of 0.5. This is so because the divalent ion pair is the dominant species and the concentration of the undissociated acid group in the organic phase is negligible. However, because this peak is also related to the interaction with water, it can be observed even at very low loadings. These infrared spectroscopy results support the analysis of the extraction curve based on our theory. Our analysis suggests that the dominant species is the monovalent ion pair at about Z ) 1, R3NH+‚‚‚AH-, where the extraction curve shows a plateau (Figure 2, plot B) and the IR spectrum shows the two strong peaks at 1550 and 1640 cm-1. Moreover, our analysis suggests that the divalent ion pair is the dominant species at about Z ) 0.5, R3NH+‚‚‚A2-‚‚‚+HNR3, where the curve shows another plateau and the IR spectrum shows a very large peak at 1550 cm-1. IR Spectra of TEHA in 1-Octanol Loaded with Glutaric Acid. The infrared spectra of glutaric acid in TEHA (Table 3) show the same three peaks in the relevant area as in the spectrum of the above-stoichiometrically (Z > 1) loaded JMT (1710, 1640, and 1550 cm-1), although their proportions are different. The spectra show a very strong peak at 1710 cm-1, belonging to the undissociated carboxylic group, and a very small peak at about 1550 cm-1, belonging to the dissociated group. Therefore, these infrared spectra confirm that the main acid species in this very weak base extractant is the undissociated acid, as was concluded before. (The peak at 1640 cm-1 from the interaction with water in the organic phase can also be observed.) IR Spectra of Maleic Acid in Extractants. Table 4 lists the pertinent peaks in the IR spectra of maleic acid in the three extractants (TEHA in 1-octanol, TOA, and JMT) and their assignments (based on the literature and as concluded from our results). A spectrum of the acid in 1-octanol is also included to confirm the assignments of the peaks of the acid in its undissociated form. The conclusions drawn from this table are as follows: (1) The spectra of the loaded TEHA- and TOA-based extractants show that, for all of the acid-to-amine molar ratios examined, the same pertinent peaks are found and that they become weaker as the molar ratio is reduced. Therefore, the same mechanism is suggested for the extraction of maleic acid by these two extractants. These spectra show peaks for the carboxylate form (1590-1520 cm-1) and for the undissociated form (17701710, 1670-1680, and 1620-1630 cm-1). These observations support our view that the main acid species in the loaded TEHA and TOA extractants in the range of 0 < Z < 1 is the monovalent ion pair, which contains dissociated and undissociated groups, R3NH+‚‚‚OOC-CHdCH-COOH. This conclusion is also supported by similar peaks in the fingerprint area (2) The spectra of the loaded JMT extractant for 0.5 < Z < 1 are similar to those of the loaded TEHA- and TOA-based extractants. Therefore, it is suggested that the extraction mechanism of JMT in that range is the

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Table 4. Pertinent Peaks and Their Assignment in the IR Spectra of Maleic Acid in 1-Octanol in TEHA, TOA, and JMT in 1-Octanol TEHA in 1-octanol

TOA in 1-octanol

JMT in 1-octanol

peak (cm-1)

1-octanol

0.5 > Z

0.5 < Z < 1

0.5 > Z

0.5 < Z < 1

Z>1

0.5 > Z

0.5 < Z < 1

Z>1

assignment

1770-1710 1670-1680 1650 1620-1630 1590-1520 1440 1300 1220 890

V V -

v v v v v

V V ? V V V

v v ? v v v V

V V ? V V V V

V V ? V V V V

v V v v -

V V ? V V V V

V V ? V V V V

CdO COOH H2O COOH COOdivalent divalent monovalent monovalent

same (monovalent ion-pair formation) as was determined for the other two. (3) In all of the spectra, the peak at about 1650 cm-1 (indicating the interaction with water in the organic phase) is covered by peaks at 1670-1680 and at 16701680 cm-1 (which indicate undissociated carboxylic groups). In cases where these peaks are very weak or nonexistent, the water-related peak at 1650 cm-1 can be observed. (4) The spectra of the loaded JMT extractant for Z < 0.5 differ from those for Z > 0.5. The spectra in the first case show the water-related peak at about 1650 cm-1. This observation indicates that the concentration of undissociated carboxylic group is very low in the loaded JMT extractant at Z < 0.5. A strong peak is found at 1590-1520 cm-1 as compared to the weak peak found in the Z > 0.5 case. This shows that, although the acid concentration in the organic phase is low, the concentration of the carboxylic group in its dissociated form is high (as in the case of the divalent ion pair R3NH+‚‚‚-OOC-CHdCH-COO-‚‚‚+HNR3). In addition, the peaks in the fingerprint area in the Z < 0.5 case are different from those for Z > 0.5. These observations support our assumption that the divalent ion pair is the dominant species when pKa2 < pHhn and Z < 0.5. NMR Spectra. Maleic Acid in JMT and in TOA. NMR spectroscopy was used to study maleic acid interactions in the JMT-based extractant and to compare them with those in the TOA-based extractant. The NMR spectra of the maleic acid loaded extractant show four groups of peaks: a large group upfield (1.1-1.9 ppm) that is related to the hydrogens of the alkyl chains of the amine and of the 1-octanol molecules [CH3(CH2)CH2-OH]; a triplet further downfield (3.9-3.8 ppm) related to the methyl hydrogens in the position R to the hydroxyl group of the 1-octanol molecules [CH3(CH2)CH2-OH]; a wide peak in the 5.7-6.2 ppm range related to the hydrogen of the hydroxyl group of the 1-octanol molecules, to the proton bound to the amine, and to the hydrogens of the water molecules in the organic phase; and a singlet at about 6.2 ppm related to the vinylic hydrogens of maleic acid (HOOC-CHdCH-COOH). Two interesting phenomena occur with increasing pH. First, the peak in the range of 5.7-6.2 ppm becomes sharper. Second, there is an upfield shift of the singlet peak that corresponds to the vinylic hydrogens of the acid. This upfield shift indicates that the hydrogens are in more negatively charged surroundings. Figure 3 presents the results of an analysis of the NMR spectra of this extraction system, along with those for maleic acid extraction by a TOA-based extractant. The x axis refers to the maleic acid concentration in the organic phase, and the y axis refers to the peak location

Figure 3. Location (ppm) of the NMR radiation absorption of the vinylic hydrogens of maleic acid extracted by JMT- and TOA-based extractants versus the maleic concentration in the organic phase.

of the vinylic hydrogens. This figure shows a sharp upfield shift of this peak in the case of JMT, starting at a pH that is close to the pKa2 value of the acid. At this pH, the acid-to-amine molar ratio is 0.33/0.5 ()2/3), indicating similar concentrations of monovalent ion pairs, R3NH+‚‚‚-OOC-CHdCH-COOH, and divalent ion pairs, R3NH+‚‚‚-OOC-CHdCH-COO-‚‚‚+HNR3. In contrast, in the case where the acid is extracted by a TOA-based extractant, the location of this peak does not change. In summary, we expected that the formation of divalent ion pairs is energetically favorable for extraction by JMT but not for extraction by tertiary aminebased extractant. This occurs because the second carboxylic anion is a stronger base than the tertiary amine (TOA) but a weaker base than the primary amine (JMT). The NMR and IR spectra confirm our expectations. This is so even though the extraction curve does not show the two drop-offs in the extraction typical for neutralization of the second carboxylic acid and the protonated amine. The reason for this is that the torsion points of these two drop-offs are too close to be separated from each other by a plateau (pKa2 ) 6.1 and pHhn ) 7.5). Thus, only one large decrease in the extraction can be observed. Application of Our Analysis to Other Data from the Literature. As mentioned in the Introduction, the formation of divalent ion pairs of carboxylic acids in organic phases has been studied in the literature using spectroscopy and other methods such as mathematical modeling of the extraction loading. In Table 5, the conclusions reached by the previous authors are compared with those from our analysis. Our analysis suggests a simple tool for predicting the formation of divalent ion pairs. In cases where the difference between the pHhn of the extractant and the pKa2 of the acid is positive (pKa2 < pHhn), it predicts that a divalent ion

Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1299 Table 4. Application of Our Analysis to Other Data from the Literature divalent ion-pair formation acid

extractant

pKa2

pHhn(HCl)

according to our analysis (pHhn - pKa2)

according to the article (method used)

succinic11

A-336 in 1-octanol A-336 chloroform A-336 in 1-octanol A-336 chloroform A-336 in 1-octanol A-336 chloroform triethylamine in methanol dibutylamine in all diluents triethylamine in chloroform, acetonitrile, or dioxane long-chain alkyl secondary amine in hexane tri-n-octylamine in chloroform or MIBK tri-n-octylamine in toluene

5.5 5.5 4.4 4.4 6.1 6.1 5.7 5.7 5.7 4.2 4.2 4.2

∼6a ∼5b ∼6a ∼5b ∼6a ∼5b ∼6a ∼7c ∼5b ∼7c ∼5b ∼4d

yes not clear yes yes not clear no yes yes not clear yes yes not clear

yes (IR, extraction curve) no (IR, extraction curve) yes (IR, extraction curve) yes (IR, extraction curve) no (IR, extraction curve) no (IR, extraction curve) yes (IR) yes (IR) no (IR) yes (extraction curve) yes (IR, extraction curve) no (IR, extraction curve)

fumaric6 maleic2 malonic3 oxalic8 oxalic10 oxalic10 d

a Tested12 for 0.5 mol/kg A-304 in 1-octanol. b Tested12 for 0.5 mol/kg A-304 in kerosene. c Tested16 for 0.1M didecylamine in toluene. Tested16 for 0.1M trioctylamine in toluene.

pair, R3NH+‚‚‚A2-‚‚‚+HNR3, will be formed. Such is the case in the extractions of succinic acid11 by A-336 in 1-octanol, fumaric acid6 by A-336 in 1-octanol or in chloroform, and malonic acid3 by dibutylamine in all diluents. In addition, divalent ion pairs were formed in the extraction of oxalic acid8 by a long-chain alkyl secondary amine in hexane and in the extraction of oxalic10 acid by tri-n-octylamine in chloroform or in MIBK. In all of these cases pKa2 < pHhn, in agreement with the analysis. On the other hand, the theory predicts that, in cases where pKa2 > pHhn, divalent ion pairs will not be formed, as in the case of succinic11 and maleic2 acid extraction by A-336 in chloroform; melonic3 acid extraction by triethylamine in chloroform, acetonitrile, dioxane; and oxalic acid10 extraction by tri-n-octylamine in toluene. The table shows that, in most cases, our simple method successfully predicts the formation of the divalent species. 4. Conclusions The extraction of dicarboxylic acid by amine-based extractants as a function of the pH was found to be dependent mainly on the acid-base properties of the acid and the extractant. The conclusions from our study are as follows: (1) In systems in which the extractant is a weaker base than the anions of the extracted acid, i.e., pHhn < pKa1 < pKa2, the dominant acid species in the organic phase is the undissociated acid, for all acid-to-amine molar ratios. This species is formed by H-bonding or by solvation interactions in the organic phase. The added mineral base, which raises the pH, neutralizes the strongest acid in the system, which is the first acidic group of the carboxylic acid, to form the monovalent salt. This salt spontaneously leaves the organic phase, and as a result, the extraction is reduced significantly at a pH close to the pKa1 of the acid down to zero loading. (2) In systems in which the extractant is a stronger base than the first anion of the extracted acid but a weaker base than the second anion, i.e., pKa1 < pHhn < pKa2, the dominant acid species in the organic phase is a function of the pH and of the acid-to-amine molar ratio. The extraction can be divided into three main regions: (a) At acid-to-amine molar ratios greater than 1, the undissociated acid is above-stoichiometrically extracted, in addition to the ion-paired extracted acid, to form R3NH+‚‚‚AH-‚‚‚HAH. At higher pH, the added

mineral base neutralizes the strongest acid in the system, which is the first acidic group of the undissociated carboxylic acid. The species left in the organic phase is the monovalent ion pair R3NH+‚‚‚AH-. As a result, the extraction curve shows a decrease around the pKa1 of the acid, but only until stoichiometric loading is reached. (b) At stoichiometric loading, which is in the range of pKa1 < pH < pHhn, the monovalent ion pair R3NH+‚‚‚AH- is the dominant species. The extraction curve shows a plateau. (c) The additional mineral base above this point neutralizes the strongest acid in the system, which is the protonated amine, R3NH+. The acid, in its monovalent mineral salt form, leaves the organic phase, which now contains the amine in its free form. As a result, the extraction is reduced significantly at a pH about equal to the pHhn down to zero loading. (3) The extraction curves of systems in which the extractant is a stronger base than the two anions of the extracted acid, i.e., pKa1 < pKa2 < pHhn, can be divided into the following categories: (a) At acid-to-amine molar ratios greater than 1, the neutralized species is the first acidic group of the above-stoichiometrically extracted acid, and the curve shows a drop-off close to pKa1, until stoichiometric loading is reached. Thus, at pH < pKa1, the dominant species is R3NH+‚‚‚AH-‚‚‚HAH. (b) At stoichiometric loading, which is in the range pKa2 > pH > pKa1, the monovalent ion pair (R3NH+‚‚‚AH-) is the dominant species, and the curve shows a plateau. (c) The additional mineral base above this point neutralizes the strongest acid in the system, which is the second carboxylic acid, R3NH+‚‚‚AH-, forming a kind of double salt R3NH+‚‚‚A2-‚‚‚+Na. This salt is stable in neither the organic phase nor the aqueous phase. However, because the extractant acts as a stronger base than the second anion of the extracted carboxylic acid, two double salt units ion-exchange to form the divalent ion pair R3NH+‚‚‚A2-‚‚‚+HNR3 in the organic phase. As a result, the extraction curve shows a second drop-off at the pKa2 of the extracted acid, followed by a plateau at an acidto-amine molar ratio of 0.5 and in the pH range of pHhn > pH > pKa2, where the divalent ion pair R3NH+‚‚‚A2-‚‚‚+HNR3, is the dominant species. (d) The addition of mineral base above this point neutralizes the next strongest acid in the system, which is the protonated amine R3NH+. As a result, the free amine remains in the organic phase, and the acid, in its divalent mineral salt form, leaves into the aqueous phase. Thus, the extraction curve shows another dropoff at the pHhn of the extractant.

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(4) In systems where the differences between two consecutive drop-offs are less than about 2 pH units, it is difficult to see the expected “plateau, decrease, plateau, decrease” shape in the extraction curve. An example is the case of maleic acid extraction by JMT, where pKa2,maleic acid ) 6.1 and pHhn ) 7.5. The plateau, which indicates divalent ion-pair formation at Z ) 0.5, becomes unified with the two decreases related to the neutralization reactions of the second carboxylic group and of the protonated amine. Spectroscopic results confirm the formation of divalent malate in the organic phase. (5) In systems in which divalent ion pairs are formed, the value of the torsion point at 0.5 < Z < 1 is related to the pKa2 of the acid in the organic phase. This value is not the same as the pKa2 of the acid in the aqueous phase. For example, for the extraction of malic acid by JMT, pKa2(org) is about pH ) 4, whereas pKa2,malic acid ) 5.1. For the extraction of glutaric acid, the pKa2(org) is about ) 4.7, whereas pKa2,glutaric acid ) 5.4. A similar phenomenon can be observed in the work of Tung and King.11 Our explanation is that the deviations are a function of the difference in the proton activities in the two phases. (6) The results presented in the literature3,6,8,10,18 are well explained by our analysis. Literature Cited (1) Reisinger, H.; King, C. J. Extraction and Sorption of Acetic Acid at pH above pKa To Form Calcium Magnesium Acetate. Ind. Eng. Chem. Res. 1995, 34, 845. (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. (3) Gusakova, G. V.; Denisov, G. S.; Smolyanskii, A. L.; Shraiber, V. M. Hydrogen Bonding and Proton Transfer in Carboxylic Acid-Amine Systsems. Spectroscopic Investigation of the Equilibrium between a Molecular Complex and Ion Pair. J. Gen. Chem. USSR 1989, 56, 531 (English Translation). (4) Tamada, J. A. Extraction of Carboxylic Acids by Amine Extractants. Ph.D. Thesis, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, 1989. (5) Tamada, J. A.; Kertes, A. S.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling. Ind. Eng. Chem. Res. 1990, 29, 1319. (6) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids

with Amine Extractants. 2. Chemical Interaction and Interpretation of Data. Ind. Eng. Chem. Res. 1990, 29, 1327. (7) Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 3. Effect of Temperature, Water Coextraction, and Process Considerations. Ind. Eng. Chem. Res. 1990, 29, 1333. (8) Kuwano, Y.; Kusano, K.; Takahashi, T.; Kondo, K.; Nakashio, F. Extraction Equilibria of Lower Carboxylic Acids with Long-Chain Alkylamine. Soc. Chem. Eng. Jpn. 1982, 4, 404. (9) Frolov, Y. G.; Sergiegievskii, V. V.; Nasonova, G. I. Effect of the Basicity of Amines on the Formation of Their Sulfate Salts during Extraction. Russ. J. Inorg. Chem. 1973, 18 (6), 546. (10) Kirsch, T.; Maurer, G. Distribution of Oxalic Acid between Water and Organic Solution of Tri-n-octylamine. Ind. Eng. Chem. Res. 1996, 35, 1722. (11) Tung, A. T.; King, C. J. Sorption and Extraction of Lactic and Succinic Acid at pH > pKa1. 1. Factors Governing Equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (12) Canari, R.; Eyal, A. M. Extraction of Carboxylic Acids by Amine-Based Extractants: Apparent Extractant Basicity According to the pH of Half-Neutralization. Ind. Eng. Chem. Res. 2003, 42, 1285-1292. (13) Canari, R.; Eyal, A. M. Selectivity in Monocarboxylic Acids Extraction from Their Mixture Solutions Using an Amine-Based Extractant: Effect of pH. Ind. Eng. Chem. Res. 2003, 42, 13011307. (14) Canari, R.; Eyal. A. M. Acids Extraction by Amine-Based Extractants: An Analysis of the Effect of Anion Concentration in the Aqueous Phase Using the Donnan Model. Ind. Eng. Chem. Res. 2003, 42, 1315-1320. (15) Eyal, A. M.; Canari, R. pH Dependence of Carboxylic and Mineral Acid Extraction by Amine-Based Extractants: Effect of pKa, Amine Basicity, and Diluent Properties. Ind. Eng. Chem. Res. 1995, 34, 4, 1789. (16) Grinstead, R. R. Base Strengths of Amine in Liquid-Liquid Extraction Systems. In Proceedings of the International Solvent Extraction Conference (ISEC); Dyrssen, D., Liljenzin, J. O., Rydberg, J., Eds.; North Holland Publishing Company: Amsterdam, 1966; p 427. (17) Grinstead, R. R.; Davis, J. C.; Base Strengths of AmineAmine Hydrochloride Systems in Toluene. J. Phys. Chem. 1968, 72 (5), 1630. (18) Poulad-Meyer, L. A Spectral Study of the Transfer of Acidic or Basic Compounds from Aqueous Phases into Multicomponent Organic Extracting Media. Ph.D. Thesis, Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel, 2001.

Received for review July 5, 2001 Revised manuscript received October 31, 2002 Accepted November 3, 2002 IE010579P