Enhanced Hydrogen Bonding for the Adsorptive Recovery and

Aug 31, 2002 - Chemicals that are available from renewable resources typically contain ... Specifically, we examined the adsorption of compounds that ...
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Enhanced Hydrogen Bonding for the Adsorptive Recovery and Separations of Oxygenated Aromatic Compounds from Renewable Resources Jaclyn L. Brown,†,‡ Tianhong Chen,†,‡ Heather D. Embree,†,‡ and Gregory F. Payne*,†,‡ Center for Agricultural Biotechnology, 5115 Plant Sciences Building, University of Maryland Biotechnology Institute, College Park, Maryland 20742-4450, and Department of Chemical and Biochemical Engineering, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250

Chemicals that are available from renewable resources typically contain heteroatoms that make hydrogen bonding a prominent intermolecular interaction. We are examining how hydrogen bonding can be exploited as an adsorption mechanism to facilitate the recovery and separations of oxygenated aromatic compounds from renewable resources. Specifically, we examined the adsorption of compounds that have weak or attenuated hydrogen-bonding abilities. The solvent in our study was hexane, which is commonly used in the food industry to extract chemicals from plant material (e.g., for vegetable oils and flavor extracts). The low dielectric constant of hexane also facilitates the hydrogen-bonding adsorption mechanism. Three polymeric adsorbents of differing basicities were studied, an acrylic ester sorbent (XAD-7, Rohm and Haas), a pyridine sorbent (Reillex 425, Reilly Industries), and a tertiary amine sorbent (IRA-93, Rohm and Haas). Adsorption affinities (related to the adsorption equilibrium constant, Keq) and adsorption enthalpies (-∆H°) were observed to increase with increasing basicity of the sorbent’s binding site. Infrared spectroscopy was used to study the interaction mechanism between the solutes and small-molecule analogues of the binding sites for the three sorbents. These studies support the conclusion that the observed increase in adsorption results from increased hydrogen-bond strengths. These results indicate that, by adjusting the hydrogen-bond accepting abilities of the adsorbent, it is possible to alter adsorption affinities to balance the dual objectives of recovery and selectivity. Introduction Renewable resources could provide an attractive feedstock for chemicals.1,2 Unfortunately, renewable feedstocks (e.g., plant extracts or fermentation broths) are typically complex mixtures in which individual chemical components exist at low concentrations. To obtain pure chemicals from such feedstocks, it is desirable for the separation operations to offer both high concentrating abilities for efficient recovery and sufficient selectivities to yield separation factors greater than about 3. We are studying hydrogen bonding with the goal of exploiting this mechanism for adsorptive separations. Our hope is that improvements in adsorptive separations will enable the practical recovery of oxygenated aromatic compounds from renewable feedstocks (e.g., from plant extracts or mixtures generated from lignin pyrolysis).3 In previous studies, we used an acrylic ester sorbent (Rohm and Haas, XAD-7) in which the sorbent’s carbonyl group provides a hydrogen-bond-accepting site that can “complement” the hydrogen-bond-donating groups of alcohol, phenol, and carboxylic acid adsorbates.3 These studies have demonstrated the potential for exploiting hydrogen bonding to enhance adsorptive selectivities (i.e., separation factors). Specifically, ac* Corresponding author: Gregory F. Payne. E-mail: payne@ umbi.umd.edu. Tel.: (301) 405-8389. Fax: (301) 314-9075. † University of Maryland Biotechnology Institute. ‡ University of Maryland Baltimore County.

ceptable selectivities can be achieved if adsorbates differ in their hydrogen-bond-donating abilities4 or if hydrogen bonding is differentially attenuated by intramolecular5-7 or steric8 effects. In our previous work, we also observed that adsorbates that have weak hydrogen-bond-donating abilities (e.g., alcohols) adsorb onto the acrylic ester sorbent with low affinities (i.e., low binding constants). Low affinities are not a problem with respect to separations because separation factors are determined by the relative binding affinities, not their magnitudes. Low affinities are, however, problematic with respect to recovery, where the goal is to concentrate the desired compound from the fluid onto the sorbent surface. It would be desirable if the sorbent could offer both high affinities for recovery and high selectivities for separations. To enhance adsorption affinities, we investigated sorbents that offer stronger hydrogen-bond-accepting sites. As illustrated in Scheme 1, we compared the adsorption of weakly hydrogen-bonding solutes onto three commercially available sorbents that have differing basicities. In analogous studies, King and others9-13 have extensively studied the binding of various acidic solutes from aqueous solution onto such sorbents. In aqueous solutions, proton transfer readily occurs, and binding generally results from ionic interactions between the anionic solute and the cationic sites of basic ion-exchange resins. Our focus is on more hydrophobic aromatic solutes that can be processed in low-dielectric environments. Specifically, we study solute adsorption

10.1021/ie020122v CCC: $22.00 © 2002 American Chemical Society Published on Web 08/31/2002

Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 5059 Scheme 1

from hexane because this solvent is commonly used for the extraction of compounds from renewable resources (e.g., for producing vegetable oils and spice extracts). Proton transfer is considerably less favorable in lowdielectric environments,14-22 and we anticipate that hydrogen bonding will be an important mechanism for the binding of weakly acidic solutes onto the basic site of the sorbent.23-26 The goal of this study was to determine whether sorbents with more basic binding sites have enhanced abilities to adsorb solutes through a hydrogen-bonding mechanism. Materials and Methods The polymeric adsorbents used in this study are commercially available acrylic esters (Amberlite XAD7, Rohm and Haas), 4-vinylpyridines (Reillex 425, Reilly Industries), and tertiary-amine-functionalized styrenedivinylbenzenes (Amberlite IRA-93, Rohm and Haas). The surface area of the acrylic ester sorbent is reported by the manufacturer to be 450 m2/g. The total exchange capacities of the pyridine and tertiary amine sorbents were reported to be 6.05 and 4.2 mequiv/g, respectively. Prior to use, the sorbents were washed successively with ethanol, acetone, and hexanes until all UV-absorbing material had been removed from the polymer. After washing, the sorbents were vacuum-dried at 70 °C. All solvents were obtained from Fisher Scientific (Pittsburgh, PA), and all chemicals were purchased from either Sigma Chemicals (St. Louis, MO) or Aldrich Chemicals (Milwaukee, WI). Adsorption studies were carried out by contacting known amounts of dry sorbent with hexane solutions containing a single solute. After equilibration, the hexane-phase solute concentration was determined using a UV-visible spectrophotometer (Spectronic Genesys 2). The adsorbed-phase solute concentration, q, was calculated according to

(Co - C)V q) M

(1)

where Co and C are the initial and equilibrium solute concentrations, respectively; V is the volume of the hexanes solution; and M is the mass of dry sorbent. Infrared (IR) spectroscopy was used to examine hydrogen bonding between an alcoholic solute (3-phenyl1-propanol) and small-molecule analogues of the sorbent binding sites. The small-molecule analogues of the acrylic ester, pyridine, and tertiary amine sorbents were ethylpropionate (EP), pyridine, and triethylamine (TEA), respectively. IR spectra were collected using a FTIR spectrometer (Nicolet Avatar 320) with a 4 cm-1 resolution at 32 scans per sample. The liquid cell was composed of an NaCl window and a Teflon spacer with an optical path length of 0.5 mm. Results and Discusssion Comparison of Phenylpropanol Adsorption on the Three Sorbents. To compare adsorption onto the

Figure 1. Isotherms for 3-phenyl-1-propanol adsorption from hexane onto sorbents of (a) acrylic ester, (b) pyridine, and (c) tertiary amine. Isotherms were measured using batch adsorption studies. The lines in parts b and c are fits to the Langmuir isotherm model.

three sorbents, we performed batch adsorption studies using the alcohol 3-phenyl-1-propanol. Figure 1 shows that this alcohol adsorbs with low affinities onto the acrylic ester sorbent (note difference in scales in Figure 1). For the concentration range studied, adsorption onto the acrylic ester sorbent is confined to the linear region of the isotherm. In comparison, parts b and c of Figure 1 show that adsorption of this alcohol onto the pyridine and tertiary amine sorbents is considerably more favorable. For these latter two sorbents, adsorption can be fit to the Langmuir model

q)

qmaxC 1 +C Keq

(2)

where qmax is the saturation adsorption capacity and Keq is the equilibrium binding constant (M-1). Because qmax should depend only on the accessible number of binding sites, the Langmuir isotherms in Figure 1 were fit assuming qmax to be independent of temperature. The

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fitted values of qmax for the pyridine and tertiary amine sorbents are 1.80 and 1.05 mmol/g, respectively. These values are lower than the capacities reported by the manufacturers. Specifically, the pyridine sorbent is reported to have a capacity of 6 mequiv/g (vs the fitted qmax of 1.80 mmol/g). The tertiary amine sorbent is reported to have a capacity of 4 mequiv/g (vs 1.05 mmol/ g). Presumably, these discrepancies are due to differences in the accessible number of binding sites when the sorbents are contacted in different solvents. Reduced capacities have been previously observed when hydrophilic sorbents (i.e., ion-exchange resins) are contacted in nonaqueous solutions, with the reductions generally being attributed to differences in polymer swelling.23,25,27 The fitted binding constants (Keq) for phenylpropanol adsorption at 25 °C were 1.2 × 102 and 4.4 × 102 M-1 for the pyridine and tertiary amine sorbents, respectively. Although we do not have an experimentally measured value for the acrylic ester sorbent, Mardis et al.28 computed a binding constant of 2 M-1 for the binding of phenylpropanol to a model acrylic ester site. Thus, the trends in the binding constants are consistent with expectations that the alcohol would bind to the acrylic ester sorbent less favorably and to the tertiary amine sorbent more favorably. Adsorption binding constants from Langmuir fits have been reported to vary considerably between 100 and 104 M-1,29-35 with the higher values typical for the adsorption of contaminants from fluids (e.g., for wastewater treatment). A few recent studies have reported binding constants for adsorption due to hydrogen bonding from nonaqueous solvents. One study reported that ethanol adsorbed from n-heptane onto a strong acid exchanger/catalyst (Amberlyst 15), with equilibrium binding constants reported to be in the range 101-102 M-1.36 A second set of studies reported that phenols adsorbed from heptane and toluene onto tertiary amine sorbents via a hydrogenbonding mechanism. Binding constants for these studies were reported to be 100-102 M-1 depending on the phenol.23-25 The adsorption enthalpies were determined from a van’t Hoff analysis. Specifically, the binding constant varies with temperature according to

∂ ln Keq ∂(1/T)

)-

∆H° R

(3)

where -∆H° is the adsorption enthalpy. The adsorption enthalpies for the pyridine and tertiary amine sorbents could be determined directly from the above equation using the Keq values fitted in Figure 1. However, Keq values for adsorption onto the acrylic ester sorbent could not be determined from the linear isotherm of Figure 1a. If the Langmuir model is assumed to be valid for the acrylic ester sorbent, then the product (Keqqmax) can be determined from this isotherm. Specifically, in the low-concentration limit, the Langmuir model simplifies to

lim Cf0

(Cq ) ) lim Cf0

( )

qmax ) Keqqmax 1 +C Keq

(4)

where the left-hand side of eq 4 can be determined experimentally from the slope of the isotherm that passes through the origin. Because qmax is independent of temperature, the van’t Hoff relationship can be

Figure 2. van’t Hoff plots for 3-phenyl-1-propanol adsorption from hexane onto the acrylic ester, pyridine, and tertiary amine sorbents. Error bars (95% confidence interval) are too small relative to the symbols to be observed in the figure.

evaluated in the form

∂ ln(Keqqmax) ∂(1/T)

)-

∆H° R

(5)

This form allows data for all three sorbents to be compared on the same plot. Figure 2 compares the van’t Hoff behavior for phenylpropanol adsorption onto the three sorbents. For all temperatures studied, the product Keqqmax was greatest for adsorption onto the tertiary amine sorbent and least for adsorption onto the acrylic ester sorbent. Consistent with this trend, the enthalpies for phenylpropanol adsorption onto the tertiary amine, pyridine, and acrylic ester sorbents were determined to be -43 ( 1, -38 ( 3, and -30 ( 0.3 kJ/mol, respectively. In summary, the adsorption data are consistent with the expectation that adsorption results from hydrogenbond formation and that affinities and enthalpies for adsorption are enhanced with increasing basicity of the sorbent’s binding site. Spectroscopic Evidence for Hydrogen Bonding. There are two general approaches to the study of adsorption mechanisms. The first is to prepare welldefined surfaces and employ advanced spectroscopic methods.37-39 The second approach is to use a smallmolecule analogue of the binding site and apply spectroscopic and/or molecular modeling methods to study interactions between solutes and the analogue.40,41 In previous studies, we used ethylpropionate (EP) as a small-molecule analogue of the acrylic ester’s binding site.6,7,28,42 Experimentally, we used infrared (IR) spectroscopy to measure shifts in the O-H stretching frequency that result from hydrogen bonding. Often, the frequency shift (∆νOH) correlates with the strength of the hydrogen bond.43 Here, we continue using EP as our analogue for the acrylic ester sorbent, and we use pyridine and triethylamine (TEA) as analogues for the pyridine and tertiary amine sorbents, respectively. The IR spectra for phenylpropanol in hexane is shown in Figure 3. The peak at 3646 cm-1 and shoulder at 3633 cm-1 are assigned to the OH-stretching of the nonhydrogen-bond conformations of phenylpropanol.28 A small peak at 3612 cm-1 is also observed in the alcohol’s IR spectra. This lower-frequency peak is believed to be due to a phenylpropanol conformer in which the alcoholic OH is intramolecularly hydrogen bonded to the π-electrons of the aromatic ring.28,44 When EP, the small-molecule analogue of the acrylic ester sorbent, is added to the hexane solution containing phenylpropanol,

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Figure 4. Correlation between adsorption and hydrogen bonding to the small-molecule analogues. The thermodynamics for 3-phenyl-1-propanol adsorption are quantified by the adsorption enthalpy (-∆H°), while hydrogen bonding of 3-phenyl-1-propanol to the small-molecule analogues is quantified by the IR frequency shift (∆νOH). Figure 3. FTIR spectra for hexane solutions containing 10 mM 3-phenyl-1-propanol and 50 mM of ethylproprionate (EP), pyridine, or triethylamine (TEA). EP, pyridine, and TEA are small-molecule analogues of the acrylic ester, pyridine, and tertiary amine sorbents, respectively.

Figure 3 shows that a broad peak appears at 3568 cm-1. This lower-frequency peak has been observed before and is assigned to the OH-stretching frequency for the intermolecularly hydrogen-bonded complex between phenylpropanol and EP.28 (It should be noted that the peak at 3474 cm-1 in the phenylpropanol/EP spectrum is due to water that is present in EP.) Figure 3 also shows IR spectra for hexane solutions containing phenylpropanol and small-molecule analogues of the pyridine and tertiary amine sorbents. Broad peaks were observed at 3383 and 3234 cm-1 when phenylpropanol was mixed with pyridine and TEA, respectively. These broad, lower-frequency peaks are attributed to the OH-stretching of the phenypropanol/ pyridine and phenylpropanol/TEA complexes. Thus, the observed frequency shifts with the three small-molecule analogues are consistent with the formation of hydrogen bonds with phenylpropanol. Further, the relative frequency shifts (∆νOH ) 78, 263, and 412 cm-1 for EP, pyridine, and TEA, respectively) observed in Figure 3 are consistent with the expectation that the weakest binding would occur with the ester while the strongest binding would occur with the tertiary amine. Although the spectra in Figure 3 do not provide definitive proof, it seems likely that interactions are confined to hydrogenbonding interactions and that proton transfer does not occur. Figure 3 provides direct evidence for hydrogen bonding between phenylpropanol and the small-molecule analogues. Next, we examined whether phenylpropanol adsorption correlates with hydrogen bonding to these analogues. In previous studies with EP, we examined a series of solutes and observed a correlation between the adsorption affinity (q/C ≈ Keqqmax) and the frequency shift (∆νOH) for hydrogen bonding to EP.42 That correlation provided evidence that EP was a reasonable analogue of the acrylic ester’s binding site. In this study, we could not use the binding affinity as a measure of adsorption because the three sorbents have different binding capacities (i.e., different qmax values) and an experimentally observed Keq is not available for adsorption onto the acrylic ester sorbent. Rather, we used the adsorption enthalpy and examined whether this ther-

modynamic measure of adsorption correlates with spectroscopic measurements of binding to the small-molecule analogue. Figure 4 shows a good correlation between the frequency shift for small-molecule binding and adsorption onto the various sorbents. This correlation supports the conclusions that (i) adsorption results from the same mechanism as small-molecule binding (i.e., hydrogen bonding) and (ii) differences in the adsorption affinity are due to differences in the hydrogen-bonding strength. We believe that this correlation is important because it provides justification for the use of smallmolecule analogues as a simple tool for probing adsorption mechanisms. Adsorption of Compounds That Have Attenuated Hydrogen-Bonding Abilities. A few instances can be found in which adsorptive hydrogen bonding is preferentially attenuated and this attenuation leads to a reduction in adsorption affinities. Because of its preferential nature, the attenuation of hydrogen bonding can be exploited to enhance separation factors.5,8,23,25 However, the reduction in affinity can be a disadvantage in terms of recovery. We examined the tertiary amine sorbent to determine whether the more basic sorption site would lead to increased adsorption affinities for systems with attenuated hydrogen-bonding abilities. Specifically, we considered two cases relevant to the recovery of chemicals from renewable resources and compared adsorption onto the tertiary amine sorbent with adsorption onto the acrylic ester sorbent. Ortho-substituted phenols are common in nature, and in some cases, the ortho substituents can form intramolecular hydrogen bonds with the phenolic hydroxyl group. These intramolecular hydrogen bonds compete with, and attenuate, intermolecular (i.e., adsorptive) hydrogen bonding.5-7,25 The separation advantage is that intramolecular hydrogen bonding differentially attenuates adsorption of the ortho-substituted compounds and provides a means to separate them from other phenols, including the para isomers. For instance, p-methoxyphenol was observed to be preferentially adsorbed relative to its ortho isomer, and separation factors of 15 were observed.5 Although beneficial for separations, the attenuation of hydrogen bonding limits the ability to recover o-methoxyphenols (i.e., guaiacols) from process streams. Compounds with structures related to guaiacol are common in lignin feedstocks and could be used as a source of intermediate and specialty chemicals. Examples of compounds that have structures related to guaiacol are the food chemical vanillin and

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Figure 5. Comparison of o-methoxyphenol (guaiacol) adsorption onto the tertiary amine and acrylic ester sorbents. Isotherms were measured using batch adsorption studies at 25 °C. The isotherm for adsorption onto the acrylic ester sorbent was reported elsewhere,5 and the dotted line indicates interpolation of experimental data (affinities for this sorbent were so low that high solute concentrations were required to obtain measurable adsorption).

the expectorant guaifenesin. Figure 5 shows that the affinity for guaiacol adsorption onto the tertiary amine sorbent is considerably enhanced compared to adsorption onto the acrylic ester sorbent. Tocopherols are a family of antioxidant compounds found in oil seed crops. Because R-tocopherol has been reported to have the greatest antioxidant activity,45 there has been considerable recent interest in separating R-tocopherol from other compounds in the family. Previous studies indicate that tocopherols adsorb onto the acrylic ester sorbent through the formation of a hydrogen bond between the phenolic hydroxyl of the tocopherol and the carbonyl group of the sorbent. Hydrogen bonding of R-tocopherol was observed to be attenuated relative to that of δ-tocopherol because R-tocopherol’s phenolic hydroxyl is flanked by two methyl substituents that sterically impede hydrogen bonding. This attenuation selectively reduces the affinity for R-tocopherol adsorption relative to that of δ-tocopherol, and adsorptive separation factors of 3.3 were observed.8 Figure 6 shows that the affinities for both R- and δ-tocopherol adsorption are enhanced with the tertiary amine sorbent compared to adsorption onto the acrylic ester sorbent. Importantly, Figure 6 shows that δ-tocopherol adsorbs with a considerably higher affinity than R-tocopherol (note the different scales in parts a and b of Figure 6). Thus, the enhanced hydrogenbonding ability of the tertiary amine sorbent leads to increased adsorption affinities but does not eliminate the beneficial selectivity that results from differential attenuation of R-tocopherol’s hydrogen bonding. Conclusions Currently, few generic approaches are available that provide inexpensive and flexible means for recovering/ separating chemicals from renewable resources. Distillation, which provides a generic approach for separating petrochemicals, is typically not applicable for biochemicals because of their reduced volatilities. The heteroatoms (oxygen and nitrogen) that reduce volatilities also make hydrogen bonding a major intermolecular interaction mechanism for biochemicals. Hydrogen bonding could also be useful for adsorptive separations because this mechanism offers appropriate binding affinities, is reversible, and can confer selectivity. The key to exploiting hydrogen bonding for adsorptive separations is

Figure 6. Comparison of adsorption onto the tertiary amine and acrylic ester sorbents for (a) R-tocopherol and (b) δ-tocopherol. Isotherms were measured using batch adsorption studies at 24 °C. Isotherms for adsorption onto the acrylic ester sorbent were reported elsewhere.8

controlling this mechanism and limiting other, nonspecific interactions. Here, we report that inexpensive, commercially available sorbents with increased basicities can enhance the adsorption strengths for compounds with weak or attenuated hydrogen-bonding abilities. Additionally, results for tocopherol adsorption indicate that the enhanced binding does not lead to reductions in selectivity. In the longer term, we believe these results suggest the potential for fractionating renewable chemicals using a series of adsorption columns of differing hydrogen-bonding strengths. This approach would be analogous to the use of a series of distillation columns operating at different temperatures for fractionating petrochemicals. The approach of enhancing adsorption by strengthening hydrogen-bonding interactions is straightforward. Unfortunately, such straightforward approaches often fail for adsorption because adsorption mechanisms are difficult to control and poorly characterized. It appears that hydrogen bonding is the “controlling” adsorption mechanism in our systems because the polymeric sorbents have relatively homogeneous surface chemistries while the low-dielectric environment limits nonspecific interactions among the solvent, solute, and sorbent. The thermodynamic data for adsorption (Keq and ∆H°) are consistent with the contention that hydrogen bonding controls adsorption. To augment this thermodynamic data with mechanistic information, we used smallmolecule analogues of the sorbent binding sites and applied spectroscopic techniques (FTIR spectroscopy). The correlation in Figure 4 indicates that smallmolecule analogues can provide a valuable tool for characterizing adsorption mechanisms.

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Acknowledgment This work was supported by the United States Department of Agriculture (2001-35504-10667). Literature Cited (1) Dale, B. E. Biobased Industrial Products: Bioprocess Engineering When Cost Really Counts. Biotechnol. Prog. 1999, 15, 775. (2) Lynd, L. R.; Wyman, C. E.; Gerngross, T. U. Biocommodity Engineering. Biotechnol. Prog. 1999, 15, 777. (3) Embree, H. D.; Chen, T.; Payne, G. F. Oxygenated aromatic compounds from renewable resources: Motivation, opportunities, and adsorptive separations. Chem. Eng. J. 2001, 84, 133. (4) Koehler, J. A.; Brune, B. J.; Chen, T.; Glemza, A. J.; Vishwanath, P.; Smith, P. J.; Payne, G. F. Potential approach for fractionating oxygenated aromatic compounds from renewable resources. Ind. Eng. Chem. Res. 2000, 39, 3347. (5) Glemza, A. J.; Koehler, J. A.; Brune, B. J.; Payne, G. F. Selective adsorption of methoxyphenol positional isomers. Ind. Eng. Chem. Res. 1998, 37, 3685. (6) Glemza, A. J.; Mardis, K. L.; Chaudhry, A. A.; Gilson, M. K.; Payne, G. F. Competition between intra- and intermolecular hydrogen bonding: Effect on para/ortho selectivity for substituted phenols. Ind. Eng. Chem. Res. 2000, 39, 463. (7) Mardis, K. L.; Glemza, A. J.; Brune, B. J.; Payne, G. F.; Gilson, M. K. The differential adsorption of phenol derivatives onto a polymeric sorbent: A combined molecular modeling and experimental study. J. Phys. Chem. B 1999, 103, 9879. (8) Chen, T.; Payne, G. F. Separation of R- and δ-tocopherols due to an attenuation of hydrogen bonding. Ind. Eng. Chem. Res. 2001, 40, 3413. (9) Garcia, A. A.; King, C. J. The use of basic polymer sorbents for the recovery of acetic acid from dilute aqueous solution. Ind. Eng. Chem. Res. 1989, 28, 204. (10) Garcia, A. A. Strategies for the recovery of chemicals from fermentation: A review of the use of polymeric adsorbents. Biotechnol. Prog. 1991, 7, 33. (11) Tung, L. A.; King, C. J. Sorption and extraction of lactic and succinic acids at pH > pKa1. 1. Factors governing equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (12) Husson, S. M.; King, C. J. Regeneration of lactic and succinic acid-laden basic sorbents by leaching with a volatile base in an organic solvent. Ind. Eng. Chem. Res. 1998, 37, 2996. (13) Husson, S. M.; King, C. J. Multiple-acid equilibria in adsorption of carboxylic acids from dilute aqueous solution. Ind. Eng. Chem. Res. 1999, 38, 502. (14) Staib, A.; Borgis, D.; Hynes, J. T. Proton transfer in hydrogen-bonded acid-base complexes in polar solvents. J. Chem. Phys. 1995, 102, 2487. (15) Langner, R.; Zundel, G. FTIR investigation of polarizable hydrogen bonds in carboxylic acid-pyridine complexes in the midand far-IR region. J. Chem. Soc., Faraday Trans. 1995, 91, 3831. (16) Ilczyszyn, M.; Ratajczak, H. Proton exchange mechanisms in phenol-tertiary amine aprotic solvent systems. J. Chem. Soc., Faraday Trans. 1995, 91, 3859. (17) Ilczyszyn, M.; Ratajczak, H. Protonation of pyridines by phenol, thiophenol, trifluoroacetic acid and methanesulfonic acid in aprotic solvents. J. Chem. Soc., Faraday Trans. 1995, 91, 1611. (18) Schreiber, V.; Kulbida, A.; Rospenk, M.; Sobczyk, L.; Rabold, A.; Zundel, G. Temperature effect on proton-transfer equilibrium and IR spectra of chlorophenol-tributylamine systems. J. Chem Soc., Faraday Trans. 1996, 92, 2555. (19) Majerz, I.; Malarski, Z.; Sobczyk, L. Proton transfer and correlations between the C-O, O-H, N-H and O‚‚‚N bond lengths in amine phenolates. Chem. Phys. Lett. 1997, 274, 361. (20) Rospenk, M.; Zeegers-Kuyskens, T. FT-IR (7500-1800 cm-1) study of hydrogen-bond complexes between phenols-OH(OD) and pyridine. Evidence of proton transfer in the second vibrational excited state. J. Phys. Chem. A 1997, 101, 8428. (21) Ramos, M.; Alkorta, I.; Elguero, J.; Golubev, N. S.; Denisov, G. S.; Benedict, H.; Limbach, H.-H. Theoretical study of the influence of electric fields on hydrogen bonded acid-base complexes. J. Phys. Chem. A 1997, 101, 9791.

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Received for review February 12, 2002 Revised manuscript received July 24, 2002 Accepted July 29, 2002 IE020122V