Potential Approach for Fractionating Oxygenated Aromatic

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Ind. Eng. Chem. Res. 2000, 39, 3347-3355

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SEPARATIONS Potential Approach for Fractionating Oxygenated Aromatic Compounds from Renewable Resources Jeffrey A. Koehler,† Brian J. Brune,† Tianhong Chen,†,§ Amy Jo Glemza,† Prashanth Vishwanath,† Paul J. Smith,‡ and Gregory F. Payne*,†,§ Departments of Chemical and Biochemical Engineering and Chemistry and Biochemistry, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, and Center for Agricultural Biotechnology, 5115 Plant Sciences Building, University of Maryland, College Park, Maryland 20742-4450

Various oxygenated aromatic compounds (OACs) could be obtained from renewable resources if separations were available to fractionate the complex mixtures. We examined adsorption for the fractionation of OACs into (i) acids, (ii) phenols, (iii) alcohols, and (iv) compounds lacking hydrogen-bond donating ability. Specifically, we studied adsorption from a nonpolar solvent onto a neutral acrylic ester sorbent. The acid, 3-phenylpropionic acid, exists in hexane as monomers and self-associated (e.g., dimeric) species. Phenomenological evidence indicates that monomers can adsorb but that adsorption competes with solution-phase dimerization. Previous studies indicate that phenols and alcohols adsorb through a hydrogen-bonding mechanism, while OACs lacking hydrogen-bond donating ability (i.e. aldehydes, ketones, esters, and ethers) adsorb weakly. Isotherms show that these four classes of OACs adsorb with significantly different affinities. Desorption is achieved by exploiting deprotonation reactions in an aqueous desorption phase and that the acids and phenols can be selectively desorbed based on differences in their pKa’s. A potential approach for fractionating renewable OACs is considered. Introduction There is increasing interest in developing biomass as an alternative feedstock for chemical manufacture,1,2 and the recovery of oxygenated aromatic compounds (OACs) from plant biomass has gained some attention over the years.3 Currently, a variety of low molecular weight OACs are directly recovered from the plant biomass and marketed as specialty “natural” ingredients (e.g., flavors, fragrances, and antioxidants). Considerable effort has also been devoted to obtaining OACs from the plant phenolic polymer lignin.4-6 Much of this effort is motivated by the availability of abundant lignin resources. For instance, the pulp and paper industry in the United States is reported to generate 50 million metric tons of waste lignin annually.6 Although there has been some progress in converting lignins into adhesives7 and the chemical flavoring agent vanillin,8,9 the majority of the lignin generated in the pulp and paper industry is simply burnt as a fuel.6 We believe that new opportunities for obtaining OACs from renewable resources may evolve for two general reasons: (i) continued and increasing environmental concerns and * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (301) 405-8389. Fax: (301) 3149075. † Department of Chemical and Biochemical Engineering, University of Maryland Baltimore County. ‡ Department of Chemistry and Biochemistry, University of Maryland Baltimore County. § Center for Agricultural Biotechnology, University of Maryland, College Park.

(ii) technical advances in understanding and manipulating plant biomass. Safety, health, and environmental concerns are challenging traditional chemical manufacturing approaches. Currently, OACs are synthesized from petroleumderived aromatic hydrocarbons (benzene, toluene, and xylene). To overcome the inherent difficulty of partially oxidizing hydrocarbons, various synthetic approaches have been used. One approach is to employ complex reaction schemes. For instance, the conversion of benzene to phenol involves propene alkylation, partial oxidation to a peroxide, and cleavage with the formation of equimolar amounts of acetone. A second approach to producing partially oxidized OACs is to chlorinate the aromatic hydrocarbons and use water to displace the chlorine. Chlorinated intermediates are used for the synthesis of benzyl alcohol and benzaldehyde. A final approach for introducing oxygen into aromatic hydrocarbons is to use a reactive reagent. For instance, ethylene oxide is added to benzene to produce 2phenylethanol. Environmental concerns are also mandating changes in how biomass is delignified and these changes will likely change the nature of the lignin available for OAC recovery. Concerns for chlorinated effluents (e.g., dioxins) is leading the pulp and paper industry away from the use of chlorine toward the adoption of oxidative methods for delignifying and bleaching pulp.10-12 Also, concerns of global climate change will likely lead to additional sources of byproduct lignin. Efforts to reduce greenhouse gas emissions have stimulated interest in

10.1021/ie000235j CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000

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Scheme 1

converting cellulosic biomass into renewable transportation fuel (i.e., fermentation ethanol). The initial step in cellulose-to-ethanol operations typically use acids to hydrolyze the cellulose polymer, and these conditions also hydrolyze key linkages in lignin.13 Thus, both low molecular weight sugars and OACs are released during acid hydrolysis. An added incentive for recovering OACs in cellulose-to-ethanol operations is that some of the low molecular weight phenols released during acid hydrolysis are inhibitory to subsequent fermentation microorganisms.14,15 In addition to environmental pressures, technical advances may also facilitate the recovery of OACs from renewable resources. First, there have been considerable recent advances in genetically altering plants to modify the types and levels of their phenols.16-21 Although most of this effort has focused on the development of biomass that can be more readily delignified, it is reasonable to speculate that analogous approaches could be utilized to facilitate the recovery of OACs. For instance, it seems plausible that transgenic plants could be engineered to be enriched in desirable low molecular weight OACs or to produce a lignin fraction that can be readily digested to recover desirable OACs. Complementing the advances in biotechnology are advances in chemistry that enable a fuller understanding of lignin biosynthesis and potential chemical transformations.6,22-24 An additional technical challenge for recovering OACs from renewable resources is separations. The oxygen heteroatom confers polarity to OACs and this leads to their low volatilities and thermodynamic nonidealities. These characteristics as well as the complexity of typical OAC mixtures make distillation-based separations a less desirable option. As an alternative to distillation, we are studying an adsorption-based approach. Specifically, we are considering whether OACs could be fractionated as illustrated in Scheme 1. In Scheme 1 we envision that the biomass would first be subjected to procedures to release low molecular weight OACs. For compounds that are readily released from the biomass (e.g., components of essential oils and oleoresins), no reactions would be required. For the case of lignins and tannins, depolymerization reactions would be required and such reactions could be directed toward desired products. For instance, thermal treatment under hydrogen-donating conditions preferentially yields omethoxyphenols (guaiacols) and alkylphenols,5 while oxidative conditions generally yield aromatic aldehydes, ketones, and acids.25 Further, depolymerization reactions may be coupled with additional reactions to generate substituted OACs or esters (e.g., parabens or gallate esters). The first step in recovery would be to extract the reacted biomass using an appropriate solvent after

which the OAC-containing solvent would be transferred for adsorptive separations. As illustrated in Scheme 1, we are studying an approach to fractionate OACs into four general classes based on their acidities and hydrogen-bonding abilities. As suggested in Scheme 1, fractionation would be achieved not only based on selectivities conferred in the adsorption step but also by selectivities conferred in the desorption step. Previously, we studied the adsorption of various OACs from a nonpolar solvent onto a neutral acrylic ester sorbent. As will be briefly discussed, those studies focused on the adsorption of three classes of OACs: phenols, alcohols, and compounds that lack hydrogenbond acidity (i.e., aldehydes, ketones, esters, and ethers). In this study, we examined adsorption of the carboxylic acid 3-phenylpropionic acid (hydrocinnamic acid) from hexane onto the neutral acrylic ester sorbent. We report phenomenological evidence that acid adsorption involves the monomeric species and that adsorption competes with solution-phase dimerization. Additionally, we show that OACs adsorb with substantially different affinities, with acids and phenols adsorbing most favorably followed by alcohols and then OACs that lack hydrogenbond acidities. Finally, we show that acidic and phenolic OACs can be desorbed using an aqueous desorption phase and that selective desorption can be achieved by controlling the pH of the aqueous desorption phase. Materials and Methods Experimental procedures are described briefly since further details have been provided elsewhere.26-28 The acrylic ester (XAD-7, Rohm and Haas) and styrenic (XAD-16, Rohm and Haas) sorbents used in this study were obtained from Sigma Chemical Co. and reported to have surface areas of 450 and 800 m2/g, respectively. The sorbents were cleaned extensively and dried prior to use. Batch adsorption studies were conducted by adding dried sorbent to hexane solutions containing known concentrations of the solute. The sorbent and solutions were contacted for 1-2 days to ensure equilibration. We occasionally tested samples over time to ensure adequate time was allowed for equilibration. However, we did not examine the kinetics of adsorption to determine the minimum time required for equilibration. After equilibration, the hexane-phase solute concentration was measured by UV-visible spectrophotometry. Absorbances were measured at 258, 278, and 260 nm for 3-phenylpropionic acid, p-cresol, and 3-phenyl-1propanol, respectively. The amount of solute adsorbed (q; mmol/g) was calculated from 0 (Corg,total - Corg,total)Vorg q) A

(1)

0 and Corg,total are the initial and equiliwhere Corg,total brated solute concentrations (mmol/L) in the hexane (organic) phase, Vorg is the volume of the hexane phase (L), and A is the mass of the sorbent (g). Desorption studies were conducted by first contacting a known amount of dried sorbent with 20 mL of a solutecontaining hexane solution. This procedure allowed hexane to enter the sorbent pores. After this initial contacting, 20 mL of an aqueous phosphate buffer was added and the three phases (hexane, aqueous, and sorbent) were equilibrated. After equilibration, the

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aqueous-phase pH was measured, the aqueous- and hexane-phase concentrations were determined spectrophotometrically, and the adsorbed-phase concentration was calculated from a material balance. Results and Discussion Effect of Sorbent and Solvent on Adsorption. The choice of sorbent and solvent are integral to any adsorptive separation. To exploit hydrogen bonding for OAC fractionation, it is necessary for the sorbent to have appropriate hydrogen-bonding surface sites. Also, to enhance selectivity, it is desirable to limit competing adsorptive interactions that lead to nonspecific adsorption. Thus, it is desirable to have a sorbent that has a homogeneous surface chemistry and a single type of interaction site. We selected polymeric sorbents in the belief that these sorbents would offer a more homogeneous surface chemistry compared to common alternatives (e.g., activated carbon and silica gels). To demonstrate the importance of the sorbent surface chemistry, we compared adsorption onto two commercially available sorbentssa styrenic and an acrylic ester. Specifically, we generated the batch adsorption isotherms for three OACs: an acid (3-phenylpropionic acid or hydrocinnamic acid), a phenol (p-cresol), and an alcohol (3-phenyl-1-propanol). In these studies, hexane

was used as the solvent, low solute concentrations were used, and adsorption was corrected for differences in surface area between the styrenic and acrylic ester sorbents. Figure 1 shows that all three of the OACs adsorb onto the acrylic ester sorbent with a higher affinity than that for adsorption onto the styrenic sorbent. The higher adsorption affinity for the acrylic ester sorbent has been observed before for various compounds with phenolic and alcoholic hydroxyl groups.26,28,29 As will be discussed, hydrogen bonding is a major mechanism for the adsorption of phenols and alcohols from a nonpolar solvent onto the acrylic ester sorbent. Specifically, a hydrogen bond is established between the hydroxyl proton of the alcohol or phenol and the carbonyl site of the acrylic ester sorbent. The lower affinity for adsorption onto the styrenic sorbent is believed to be due to the weaker hydrogen-bond accepting ability of the styrenic π electrons.28 It should be noted, however, that even OACs with minimal hydrogen-bond donating ability (e.g., benzaldehyde) have been observed to adsorb more favorably onto the acrylic ester than the styrenic sorbent.30 Phenomenological evidence suggests that such compounds adsorb to the acrylic ester sorbent through polar interactions.30 In summary, the acrylic ester sorbent provides appropriate surface sites for the adsorption of OACs from hexane. In addition to the sorbent, the solvent also substantially impacts adsorption. Not only does the solvent affect the adsorption affinity, but also the binding mechanism can be altered by the solvent. Solutes with hydrogen-bond donating ability adsorb from hexane onto

Figure 1. Comparison of acrylic ester and styrenic sorbents for the adsorption of oxygenated aromatic compounds: (a) 3-phenylpropionic acid, (b) p-cresol, and (c) 3-phenyl-1-propanol. Adsorption studies were performed at 30 °C and the specific surface areas for the acrylic and styrenic sorbents were reported by the supplier to be 450 and 800 m2/g, respectively.

the acrylic ester sorbent through a hydrogen-bonding mechanism.30-32 However, adsorption of the same compounds from an aqueous solvent does not appear to involve hydrogen-bond formation between the solute and the sorbent.32-34 Apparently, water out-competes the solutes for hydrogen bonding, and adsorption from water results from van der Waals and hydrophobic interactions. To avoid such competing hydrogen-bonding interactions, a low dielectric solvent must be chosen. We have worked with hexane, although it seems reasonable that analogous results would be achieved for other hydrocarbon-based solvents or possibly even for gases and supercritical fluids. Obviously, solvent selection must balance the desire to exploit a polar adsorption mechanism with the difficulty of dissolving polar OACs in low dielectric solvents. From a separations standpoint, the effect of solvent on the adsorption mechanism is significant since hydrogen bonding can be exploited to confer selectivity to binding while van der Waals and hydrophobic interactions are much less specific. The effect of solvent on both

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Figure 2. Adsorption isotherms for various oxygenated aromatic compounds (OACs) at 30 °C. The acids and phenols adsorb with the highest affinities while the alcohols adsorb with an order-ofmagnitude lower affinity. Compounds that lack hydrogen-bond acidity adsorb with low affinities. Data for each OAC were clustered in the region in which it could be most accurately measured by spectrophotometry.

Figure 3. Phenylpropionic acid (hydrocinnamic acid) adsorption from hexane onto the acrylic ester sorbent at various temperatures.

the adsorption mechanism and binding selectivity was demonstrated both experimentally and by molecular mechanics calculations.32 Potential for Fractionating Oxygenated Aromatic Compounds: Adsorption. To illustrate the potential for adsorptive fractionation, we compared the batch adsorption isotherms for various OACs from hexane onto the acrylic ester sorbent. Figure 2 shows a few generalizations in the adsorption affinities for these compounds. First, the acid (3-phenylpropionic acid) and the phenols (phenol, p-cresol, and p-methoxyphenol) adsorb with the highest affinities. The alcohols (3phenyl-1-propanol and 2-phenylethanol) adsorb with about a 5- to 10-fold lower affinity than the phenols. Finally, Figure 2 shows that compounds that lack hydrogen-bond donating ability (benzaldehyde, acetophenone, methyl benzoate, ethyl benzoate, and anisole) have the lowest affinities for adsorption from hexane onto the acrylic ester sorbent. These differences in affinity suggest the potential for using adsorption to partially fractionate the OACs into classes such as acids, phenols, alcohols, and poor adsorbers. (a) Adsorption of Phenylpropionic Acid. Figure 3 shows several isotherms for phenylpropionic acid adsorption over a broad concentration range. At low concentrations, phenylpropionic acid has a high affinity for adsorption and Figure 3 suggests that the isotherms may even have non-zero intercepts. At higher concentrations, the isotherms have considerable curvature.

Figure 4. Partitioning of phenylpropionic acid between hexane and aqueous phases. The pH of the aqueous phase was adjusted to 2 to limit aqueous-phase dissociation reactions. The nonlinear behavior is attributed to acid dimerization in the hexane phase. The solid line is a fit to eq 5. For clarity, results from only one temperature are shown.

Since attempts to fit both the low- and high-concentration regions to a Langmuir model were not successful, we do not show a fit of the data. Typically, curvature in the isotherm is attributed to an approach to saturation of the sorbent’s binding sites. However, for the case of acid adsorption, it seems possible that isotherm curvature could result from interactions occurring in the solution phase. Specifically, acids are known to selfassociate in an organic solvent and it seems possible that self-association could alter adsorption. The self-association of acids in nonpolar solvents primarily results in the formation of dimers.35 A common approach to estimate dimerization equilibrium constants is to measure how partitioning varies as a function of acid concentration.36 For this, we measured the partitioning of phenylpropionic acid between a hexane phase and an aqueous phase. To ensure that the acid was protonated in both phases, we adjusted the aqueous phase pH to 2 with HCl. Figure 4 shows an increasing slope for a plot of the acid concentration in the hexane phase (Corg,total) versus aqueous-phase acid concentration (Caqu,total). This increasing slope is characteristic of acid partitioning and is explained in terms of acid dimerization in the organic phase.36 Assuming acid self-association is confined to dimer formation, the observed partition coefficient (Dobs) can be expressed in terms of the individual species

Dobs )

Corg,total Corg,m + 2Corg,d ) Caqu,total Caqu,m

(2)

where Corg,m and Corg,d are the organic-phase concentrations of the monomer and dimer, respectively. At a pH of 2, the total concentration in the aqueous phase (Caqu,total) is assumed to be equal to the concentration of the protonated (i.e., neutral) monomer (Caqu,m). The concentrations of the organic-phase species are related by the dimerization equilibrium constant Kd:

Kd )

Corg,d Corg,m2

(3)

It is further assumed that the partitioning of the neutral monomeric species is characterized by a partition coefficient, Kp:

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Kp )

Corg,m Caqu,m

(4)

Ultimately, the observed partitioning can be related to Kd, Kp, and Caqu,m by

Dobs )

Corg,total ) Kp(1 + 2KdKpCaqu,m) Caqu,total

(5)

As shown by the solid line in Figure 4, the experimental data for phenylpropionic acid partitioning are well fit by eq 5. Values of Kp and Kd at 25 °C were determined from the best fits to be 0.09 and 9.3 L/mmol, respectively. Although we are unaware of other reports with phenylpropionic acid, the Kd values observed here are an order of magnitude lower than those reported by Fuji et al.35 for acetic acid dimerization (Kd ) 0.85 L/mmol for acetic acid based on partitioning into hexane at 25 °C). Using the dimerization constants estimated from partitioning studies, we tested the hypothesis in Scheme 2 that only the monomeric acid species adsorbs from hexane onto the acrylic ester sorbent while adsorption of the dimer is negligible. Specifically, we used Kd values determined from the partitioning study to re-evaluate the isotherms in Figure 3. Figure 5 shows that when the adsorbed concentrations are plotted as functions of the hexane-phase monomer concentrations, nearly linear isotherms are observed. The linearity of these isotherms provides phenomenological support that acid adsorption involves primarily the monomeric species. Further, the linearity in Figure 5 suggests that the curvature observed in Figure 3 is due to solution-phase interactions (i.e., dimerization) and is not due to an approach to saturation of adsorption sites. Although the above analysis provides evidence supporting Scheme 2, three additional points must be considered. First, we cannot provide a satisfying molecular-level explanation of the non-zero intercept in the isotherms of Figure 5. A typical explanation for a non-zero intercept is that the sorbent may have a small population of high-affinity binding sites. Second, the above analysis assumes that hexane-phase acid selfassociation is similar in the partitioning and adsorption studies. Specifically, this assumes that water introduced into the hexane phase during partitioning does not affect dimerization equilibrium. Christian et al.37 reported that water can lower dimerization constants, as compared to Kd’s obtained in anhydrous solvents. Finally, the above analysis only provides phenomenological evidence that acid adsorption is due to hydrogen-bond formation as suggested in Scheme 2. We are currently examining spectroscopic methods to provide direct evidence for hydrogen bonding. (b) Adsorption of Phenols. The adsorption of phenols onto the acrylic ester sorbent has been studied in greater detail than any of the other classes of compounds. Initial studies provided phenomenological evidence that adsorption from hexane resulted from the formation of a hydrogen bond between the phenolic hydroxyl and the acrylic ester sorbent.33 The adsorption enthalpies for phenols were measured to be -25 to -35 kJ/molsconsistent with enthalpies expected for phenol hydrogen bonding.27,31 More direct evidence for a hydrogen-bonding adsorption mechanism was obtained using ethyl propionate (EP) as a small molecule analogue of the sorbent

Figure 5. Phenylpropionic acid adsorption expressed in terms of the acid’s monomer concentration in the hexane phase.

Scheme 2

surface. Because EP is soluble in hexane, it was possible to use IR spectroscopy to experimentally study hydrogen bonding between various phenols and EP. From shifts in the OH stretching frequency of the IR spectra, it was possible to show that phenols hydrogen bond with EP. Further, the shift in frequency for the OH stretching peaks for the phenol-EP complexes could be correlated to the affinity for the phenols to adsorb onto the acrylic ester sorbent.27 This correlation between small molecule hydrogen bonding and adsorption supports the use of EP as an experimental tool to study adsorption. Molecular modeling provided additional support for the hydrogen-bonding adsorption mechanism. Using EP as an analogue of the sorbent surface, calculations indicated that phenols bind to EP through the formation of a hydrogen bond between the phenolic hydroxyl and EP’s carbonyl oxygen.32 Substituents can alter the adsorption of individual phenols. Electron-withdrawing/-donating substituents can increase/decrease adsorption indirectly by altering the hydrogen-bond donating ability of the phenolic hydroxyl group. As a result, phenols with differing substituents adsorb with different affinities.27 However, these substituent effects typically alter adsorption affinities by less than a factor of 2-3 for OACs commonly encountered in renewable resources. Thus, it may not be possible to separate individual phenolic compounds based solely on differences in the substituent’s ability to strengthen/weaken the hydrogen bond formed between the phenolic hydroxyl and the carbonyl oxygen. There are two instances in which ortho substituents disrupt the adsorption of phenols. First, adsorption from hexane is disrupted if an ortho substituent can form an

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intramolecular hydrogen bond with the phenolic hydroxyl group. Experiments and modeling indicate that intramolecular hydrogen bonding competes with and disrupts intermolecular (i.e., adsorptive) hydrogen bonding.26,32,38 This behavior has been observed for methoxyphenol, hydroxyacetophenone, and methyl hydroxybenzoate. In each of these cases, the affinity for adsorp-

tion of the ortho isomer is more than an order of magnitude less than that of the corresponding para isomer. A second instance in which ortho substituents can disrupt adsorption of a phenolic is when the substituent sterically hinders hydrogen bonding. This affect was observed to be important for ortho disubstitution with bulky substituents.28 (c) Adsorption of Alcohols. Early studies with a series of aromatic alcohols (Ar-(CH2)n-OH) provided phenomenological evidence that adsorption from an aqueous solvent results from van der Waals and hydrophobic interactions, while adsorption from hexane involves hydrogen bonding.34 Consistent with a hydrogenbonding mechanism, the enthalpies for adsorption of these alcohols from hexane were measured to be about -25 kJ/mol and were largely insensitive to the number of methylene groups (n). The use of IR spectroscopy to confirm the role of a hydrogen-bonding mechanism for alcohol adsorption is less straightforward than that for phenols. Because aromatic alcohols have considerable conformational flexibility, various low-energy conformations coexist and multiple OH stretching peaks appear in their IR spectra. For instance, several groups have reported multiple low-energy conformations for 2-phenylethanol and some of these low-energy conformations have a weak intramolecular hydrogen bond between the alcoholic OH and the aromatic π electrons.39-41 Nevertheless, when the soluble sorbent analogue, EP, was added to hexane solutions containing 2-phenylethanol, we observed a broad, lower frequency peak in the IR spectra. This observation indicates that a hydrogen-bonded complex is formed between the alcohol and EPsand by analogy between the alcohol and the acrylic ester sorbent. Additional modeling studies support the conclusion that hydrogen bonding is an important contributing mechanism for alcohol-EP binding. However, modeling also suggests that, in some cases, alternative interactions (e.g., van der Waals interactions) may be important in the binding of alcohols from hexane.42 With respect to separations, Figure 2 shows that the alcohols adsorb from hexane onto the acrylic ester sorbent with considerably lower affinities than typical phenols. This difference in adsorption affinities suggests the potential for adsorption to separate phenols from alcohols. Experimental results showed that p-cresol was preferentially adsorbed from a hexane solution containing both p-cresol and benzyl alcohol and a separation factor of 4 was observed.31 In contrast, it may be difficult to separate alcohols from other alcohols using adsorption onto the acrylic ester sorbent. Alcohols form weak hydrogen bonds and there do not appear to be significant attenuating effects (e.g., substituent or

Scheme 3

intramolecular hydrogen-bonding effects) that can be exploited to confer selectivity. (d) Adsorption of Non-Hydrogen-Bond Donating Compounds. Figure 2 shows that compounds that have limited hydrogen-bond donating abilities adsorb poorly to the acrylic ester sorbent. Nevertheless, measurable affinities are observed in some cases (e.g., benzaldehyde) and this affinity appears to correlate to the dipolarity/ polarizability of the compound.30 Because of their poor adsorption, non-hydrogen-bonding compounds should be readily separable from acids, phenols, and alcohols. Potential for Fractionating Oxygenated Aromatic Compounds: Desorption. It is important to recognize that, after an adsorption step, the pores of the sorbent will be filled with the nonpolar hydrocarbon solventseven if the sorbent bed is drained. It may be possible to remove the pore phase; however, it may be more convenient to use a desorption approach that does not require removal of the pore phase. In our studies, we considered an aqueous desorption phase as illustrated in Scheme 3 and examined if control of the pH of this desorption phase could be exploited to preferentially desorb OACs based on differences in their acid/base properties. Scheme 3 shows that desorption of OACs could be driven by dissociation in the aqueous phase. Differences in the dissociation constants (i.e., pKa’s) between the carboxylic acids, phenols, and alcohols should allow desorption to be selective depending on the pH of the aqueous desorption phase. Desorption can be quantified using a mass balance of the various species in the three phases

mtotal ) maqu + morg + mads

(6)

where the subscripts on the right side refer to the aqueous phase, organic pore phase, and adsorbed phase, respectively. In the aqueous phase, acidic OACs can exist in neutral or anionic forms. As indicated in Scheme 3, we only consider species with one dissociable group and for this case

maqu ) (Caqu,m + Caqu,a)Vaqu ) Caqu,m(1 + 10(pH-pKa))Vaqu (7) where Caqu,a is the aqueous-phase concentration of the anionic species. Using eq 4 to relate the partitioning of the neutral monomeric species,

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maqu )

Corg,m (1 + 10(pH-pKa))Vaqu Kp

(8)

In the organic pore phase, the acid can exist as a monomer or as an associated species. In our analysis we only considered association of the carboxylic acids since the organic-phase concentrations for the alcohols and phenols were too low for self-association reactions (i.e., Kd for the phenol and alcohol were assumed to be zero). Also, on the basis of the agreement in previously described partitioning studies, we only considered association that leads to dimers. With these assumptions, the mass in the organic pore phase is

morg ) (Corg,m + 2KdCorg,m2)Vorg

(9)

Finally, if adsorption is confined to the linear region of the isotherm, then the adsorbed-phase concentration is proportional to the organic pore phase concentration of the adsorbing species. For all compounds studied (including the carboxylic acid), the adsorbing species is considered to be the monomer:

q ) KadsCorg,m

(10)

It should be noted that Kads is a pseudo adsorption equilibrium constant that is equivalent to the slope of the isotherm in the linear region. With this assumption,

mads ) KadsACorg,m

(11)

where A is the mass of the sorbent. Using eqs 6-11 and appropriate equilibrium values, it was possible to predict how individual OACs would distribute among the three phases and how this distribution would change as a function of the aqueous-phase pH. Table 1 lists the various equilibrium values used in this study. Equilibrium values for acid dimerization and partitioning were obtained at 25 °C as described above. We did not attempt to account for the non-zero intercept in the adsorption isotherm for phenylpropionic acid but simply estimated Kads from Figure 5 to be 1 L/g. Equilibrium partition coefficients for the phenol, p-cresol, and the alcohol, 3-phenyl-1-propanol, were obtained from Payne and Maity.43 The Kads values listed in Table 1 for p-cresol and phenylpropanol were measured independently for this study and were somewhat higher than previously reported43 presumably because a different batch of sorbent was used. To experimentally test predictions of OAC desorption, we performed an experiment in which individual OACs were first adsorbed from 20 mL of hexane onto the acrylic ester sorbent. After equilibration in this batch adsorption, 20 mL of an aqueous desorption phase of a controlled pH was added to the hexane-sorbent system. In practice, it would be desirable to remove excess hexane and retain only the hexane that was residing in the pores. Experimentally, it was convenient to retain the entire 20 mL of hexane because it allowed the hexane phase to be sampled. Thus, we could measure the OAC concentrations in both the aqueous and hexane phases and calculate the amount of OAC adsorbed onto the sorbent from a material balance. Figure 6a shows how phenylpropionic acid distributes among the sorbent, hexane, and water phases. The lines are predicted behavior, while the symbols represent experimental results. As can be seen from Figure 6a,

Figure 6. Effect of pH on the distribution of OACs among an aqueous desorption phase, a hexane phase (expected to reside within the sorbent pores), and an adsorbed phase. Results demonstrate that the acid and phenol can be driven into an aqueous desorption phase by the aqueous-phase dissociation reaction and that desorption can be controlled by the pH of the desorption phase. The lines are predicted from eqs 6-11 where the thermodynamic parameters are given in Table 1. Experimental results were obtained using (a) 2.01 mmol/L phenylpropionic acid and 126 mg of sorbent, (b) 0.56 mmol/L p-cresol and 164 mg of sorbent, and (c) 2.06 mmol/L phenylpropanol and 261 mg of sorbent. All studies were conducted at 25 °C and Vaqu ) Vorg ) 20 mL. Table 1. Equilibrium Constants Used To Simulate Desorption of OACs (25 °C) phenylpropionic acid Kp Kd (L/mmol) pKa Kads (L/g) a

0.09a 9.3a 4.37 ∼1

p-cresol

phenylpropanol

0.55b

1.41b

10.17 0.289

∼14 0.084

Determined in this study. b From ref 43.

when the pH is raised above 4, phenylpropionic acid is predicted to be driven from both the sorbent and the hexane phases into the aqueous phase. Experimental

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results agree reasonably well with these predictions. Interestingly, experiment shows that a small amount of acid remains adsorbed, even at high pH, and this value is calculated to be about 0.01 mmol/g, which is similar to the intercept observed in the isotherms of Figure 5. Figure 6b shows results for the desorption of p-cresol. At pHs below about 9, the concentrations in the various phases are unaffected by pH. At a pH of 11, p-cresol is driven from the sorbent and hexane phases by the aqueous-phase dissociation reaction. Because of pcresol’s high pKa, desorption was not complete at the pH values studied. Again, Figure 6b shows reasonably good agreement between experiment and prediction. Figure 6c shows results for phenylpropanol. Because the alcoholic OH is protonated under the conditions studied, the concentrations in the three phases were not expected to vary over the pH range studied. Again, experiment agrees well with these predictions. In summary, the above results indicate that aqueousphase dissociation reactions can be exploited to desorb OACs. Further, these results suggest that selectivity can be conferred during desorption by controlling the pH of the aqueous desorption phase. Conclusions One unique feature of our work is the use of a neutral sorbent to adsorb a carboxylic acid from a nonpolar phase. Our phenomenological results indicate that adsorption involves primarily the monomeric species. Additionally, monomer adsorption appears to compete with solution-phase dimerization and this solutionphase interaction is responsible for the observed curvature in the isotherms of Figure 3. We propose in Scheme 2 that acids adsorb by the formation of a hydrogen bond. Acid desorption is readily accomplished using an aqueous desorption phase at a pH above about 4 as illustrated in Scheme 3. We believe the sequence of extraction into a nonpolar solvent, adsorption onto a neutral sorbent, and desorption into an aqueous phase may offer advantages compared to a more traditional sequence of extraction into an aqueous phase followed by ion-exchange adsorption. Specifically, salts are excluded from the nonpolar solvent phase and mild conditions can be used to accomplish both desorption and regeneration of the neutral sorbent. Nonspecific salt binding and complex regeneration requirements can be problematic for traditional ion-exchange processes. More broadly, advances in separations are required to more fully utilize renewable resources as chemical feedstocks. We propose an adsorption approach to recover oxygenated aromatic compounds (OACs) from biomass and suggest that adsorption could be used to fractionate OACs into four classes: acids, phenols, alcohols, and compounds that cannot hydrogen bond to the acrylic ester sorbent. As illustrated in Scheme 1, the OACs would first be extracted from biomass using a nonpolar solvent that allows a hydrogen-bonding mechanism to be exploited in the subsequent adsorption operation. Our results demonstrate that OACs from these four classes adsorb with substantially different affinities and these differences appear to be due to differences in their hydrogen-bonding abilities. Desorption can be accomplished by exploiting dissociation reactions in an aqueous desorption phase as illustrated by Scheme 3. Our results indicate that by controlling the pH of the desorption phase, the OACs can be

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Received for review February 14, 2000 Revised manuscript received June 13, 2000 Accepted June 19, 2000 IE000235J