High Silica Zeolites as an Alternative to Weak Base Adsorbents in

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Ind. Eng. Chem. Res. 2010, 49, 1837–1843

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High Silica Zeolites as an Alternative to Weak Base Adsorbents in Succinic Acid Recovery C ¸ ag˘ri Efe, Luuk A. M. van der Wielen, and Adrie J. J. Straathof* Department of Biotechnology, Delft UniVersity of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands

Initial studies were performed on succinic acid adsorption from aqueous solutions by zeolite powders. CVB28014 (high-silica ZSM-5) showed higher equilibrium loadings (up to 0.16 g/g) than CBV-901 and CP811C300, and was used for follow up studies. In the presence of Na+ counterions, the succinic acid adsorption decreased in parallel with the succinic acid dissociation, but the adsorbent also showed some affinity toward sodium hydrogensuccinate with selectivities in the range 10-20 toward succinic acid. The presence of acetic acid resulted in lower succinic acid loadings but the capacities remained sufficient for efficient recovery. The selectivity between succinic acid and acetic acid ranged from 1 to 6. Increasing the temperature to 70 °C reduced the equilibrium loadings, but in ethanol the succinic acid loadings showed a more significant drop. Therefore, regeneration might be achieved by using an adsorption-competitive solvent like ethanol. The current results suggest that this may lead to an attractive option for the recovery of succinic acid from fermentation media. Hydrophobic rather than ionic interactions are used, thus avoiding regeneration involving acid and base and the associated waste salt production. 1. Introduction We are strongly dependent on fossil carbon sources for energy purposes as well as for the production of a wide range of bulk chemicals and structural materials like polyesters. The latter are used in the production of textiles, packaging materials, films, construction, and interior design materials for houses and various daily usage products. To reduce the dominance of fossil carbon sources as feedstock and to reduce carbon dioxide emission, building blocks originating from fossil carbon sources should be replaced by biobased counterparts. Succinic acid, which is also known as amber acid or butanedioic acid, is one of the best candidates for C4 building blocks.1,2 It is an intermediate of the citrate cycle and produced at high yield as major fermentation end-product by microorganisms like Actinobacillus succinogenes, Anaerobiospirillum succiniciproducens, Mannheimia succiniciproducens, and some recombinant Escherichia coli and Corynebacterium glutamicum strains.1 Succinic acid can be used as an intermediate for plant growth stimulants, food ingredients, feed additives, green solvents, detergents and surfactants, health agents, corrosion inhibitors, and as precursor for the production of gamma-butyrolactone, 1,4-butanediol, tetrahydrofuran, 4-aminobutyrate, polyesters, and other carboxylic acids like maleic, fumaric, itaconic and aspartic acids.2-4 The projected market size for succinic acid is estimated to be in the order of 3 × 105 ton/year.4 However, the production cost of succinic acid should be low. The current succinic acid fermentations achieve yields that are attractive for future economical large scale production. The succinate titers and yields of these strains vary between different species. For C. glutamicum mutants a succinate titer of 146 g/L (at 0.92 mol/ mol glucose) has been achieved.5 Downstream processing may account for a large proportion of the total production costs in fermentation processes.1 Therefore, there is a major interest in cost reduction of the separation and purification of succinic acid from fermentation broth. * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +31-15-2782330. Fax: +31-15-2782355.

A straightforward manner for the recovery of succinic acid from fermentation media involves the precipitation of calcium succinate using Ca(OH)2 or CaO and subsequent reacidification using sulfuric acid.6 However, due to the costs of these acids and bases and the large amount of waste salt generated during the process, many researchers concentrate on alternative methods to purify succinic acid from fermentation broth. Being a weak acid, succinic acid in fermentation media is a fraction of the total concentration of succinate species. Here we define “total succinate” as the sum of dissociated and undissociated species. The succinic acid fraction is a function of the pH. Sustainable and cost-effective recovery of succinic acid is easier to achieve for higher concentrations of the undissociated succinic acid species in fermentation media. Therefore, a microorganism producing succinic acid at low pH is favorable for efficient downstream processing. It can be expected that future research on fermentation of succinic acid will focus on low pH. Therefore, our focus is the recovery of undissociated succinic acid from low pH fermentation medium (pH ≈ 4) to reduce the amount of acid and base consumption and salt generation. At this pH undissociated succinic acid is ∼60% of the total succinate mass. If the undissociated succinic acid is captured, the remaining succinate salts can be recycled to the fermenter for pH control. The capturing process should show a high selectivity between undissociated and dissociated species. The operations proposed in the literature to recover succinic acid from fermentation media are reactive extraction using organophosphates (trioctylphosphine oxide and tri-nbutylphosphate), aliphatic amines (tri-n-octylamine; alamine 336 (C8H17)2N(C10H21) or Aliquat 336 ([(C8H17)2NH(C10H21)]+Cl-),7-11 adsorption,11-15 and electrodialysis.16 All these separation processes proved to be effective in recovering the succinic acid from the fermentation medium, but have disadvantages. At the target fermentation pH of ∼4, the use of electrodialysis is a much less useful recovery option than at pH 7, where succinic acid is completely dissociated. Moreover, the use of reactive extractants and amine-functionalized adsorbents present problems in sustainable regeneration of the separation agent.

10.1021/ie901110b  2010 American Chemical Society Published on Web 01/05/2010

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Table 1. Product Specifications for the Zeolites adsorbent type

zeolite type

CBV-901 FAU CP811-C-300 BEA CBV 28014 ZSM-5 (MFI)

SiO2/Al2O3 nominal pore surface size23 (nm) mole ratio cation form area m2/g ∼0.66 ∼0.74 ∼0.55

80 360 280

hydrogen hydrogen ammonium

700 620 400

Their regeneration implies the addition of bases to back-extract or desorp succinate, which later requires acidification.12 However, adsorbent types like high silica zeolite function on the basis of the hydrophobicity of the sorbate and their regeneration might not require pH shifting agents. The SiO2/Al2O3 ratio of these zeolites should approach infinity like in silicalite-1 to achieve the highest hydrophobicity. Some earlier work suggested the potential of these zeolites. Jun et al.14 prepared tertiary amine-functionalized mesoporous silica and obtained a succinic acid loading of 0.49 mol/kg adsorbent at 13.63 mmol/L equilibrium concentration at room temperature. Davison et al.12 reported loading of 0.593 mol/kg adsorbent at 31.19 mmol/L equilibrium concentration using silicalite powders at 25 °C, but they did not select these for further research because they focused on adsorption of total succinate rather than undissociated succinic acid. During ethanol pervaporation studies Bowen et al.17 observed a drop in pervaporation performance of high silica zeolite membranes caused mainly by succinic acid adsorption. Their study showed that succinic acid can be adsorbed up to 0.7 mol/kg adsorbent in liquid mixtures initially containing 1.77 mol/kg ethanol and 0.4 mol/kg succinic acid at 21 °C. The high silica zeolite that they used was CBV-28014 ZSM-5, but they did not consider this for succinic acid adsorption. On the basis of these findings, we decided to study the potential of high silica zeolite adsorbents to remove succinic acid from fermentation media. In this manuscript, we focus on the adsorption isotherms. Considering the large number of species present in a typical fermentation broth, fermentation media are mimicked using aqueous solutions of succinic acid with two major succinic acid fermentation by products, namely succinate and acetic acid. Sodium is used as succinate counterion in this study, because in actual processes NaOH is suitable for controlling the fermentation pH. Calcium salts are usually much less soluble, potassium salts are more expensive, and ammonium salts would be consumed by microbial growth. 2. Materials and Methods The zeolite adsorbents were purchased from Zeolyst International (Valley Forge, PA). Three different zeolite powders were used: CBV-901, CP811C-300, and CBV 28014 (See Table 1). The zeolites were calcined for 6 h at 600 °C before use. After calcination, the sorbents were stored in an oven at 75 °C to prevent hydration. Succinic acid (99%) and disodium succinate (Na2C4H4O4, 99%) were purchased from SigmaAldrich Chemicals (Steinheim, Germany), acetic acid (99%) was purchased from J. T. Baker (Deventer, Netherlands), and absolute ethanol was purchased from Merck (Darmstadt, Germany). 2.1. Adsorption Measurements. Stock solutions were prepared of known concentrations of disodium succinate, succinic acid, and acetic acid in demineralized water or ethanol. The target mass fractions were obtained by mixing these stock solutions in calculated ratios. Total succinate (Suc) is expressed as C4H6O4 + C4H5O4- + C4H4O42-. Sorption isotherms were generated by contacting known masses of sorbent (typically 0.5-2 g) and solution (typically

15-25 g) in 30-mL sealed vessels. The sorbent mass was increased up to 2 g per vessel for high concentrations starting from 0.5 g for low concentrations. The vessels were magnetically stirred at 600 rpm at room temperature for at least 48 h. Control studies showed that equilibrium was reached in less than 20 min. One milliliter samples were collected and centrifuged for 15 min to remove the powder. Aqueous phase solute concentrations were determined by HPLC. The detection errors were up to 3%. Triplicate experiments were conducted. Error bars are shown in the graphs unless reproducibility was high. All the concentrations mentioned in isotherm graphs are equilibrium concentrations of the specified component according to control experiments. Unless otherwise specified the experiments were conducted at room temperature (20 °C ( 2). In temperature dependent experiments, the time between the sampling and centrifugation was kept as low as possible to prevent equilibrium shifts due to temperature drop or rise. Before sampling, the equilibrium experiments were left at 70 °C for one more day without stirring to let the adsorbents settle. By doing so the amount of adsorbent in contact with sampled supernatant was minimized. The pipet tips and eppendorf tubes used for the sampling were preheated to the temperature of interest to reduce the temperature drop during the sampling; 1 mL of sample was collected and centrifuged at 40 °C for 5 min and subsequently the supernatant was rapidly removed for analysis. Instead of directly measuring the effect of pH on sorbent capacity, stock solutions of succinic acid disodium salt and succinic acid were mixed in various molar ratios (typically 0.5, 1.0, and 1.5 initial Na:Suc ratio) without any pH adjustment. Initial and final pH of the solution were recorded. The equilibrium experiments were conducted as described before. The final solution was also analyzed for Na+ concentration. 2.2. Analytical Methods. HPLC analysis of the liquid phase Suc concentration was done using a Waters Alliance liquid chromatograph equipped with a BioRad HPX-87H 300 mm × 7.8 mm column (Cat. no.: 1250140) at 59 °C. The mobile phase was 1.5 mmol/L phosphoric acid in milli-Q water at 0.6 mL/ min and 70 °C. The injection volume was 10 µL with a Waters 717 autosampler; detection was at 210 nm (Waters 484 tunable adsorbance detector). The retention times for succinic and acetic acid were 11.43 and 14.87 min, respectively. For sodium analysis, the inductively coupled plasma-optical emission spectrometry (ICP-OES) technique was used for the sample solutions in 0.5 M nitric acid in the range of 0-10 mg/L Na. 2.3. Calculation of Sorbent Loadings. The zeolite particles (∼1 µm diameter) that were used have been produced by crystallization and have a uniform pore structure without any pore size distribution. Therefore, due to the pore sizes that are just sufficient to accommodate succinic acid, and the hydrophobicity of the zeolites, the sorbents are assumed to adsorb solute and solvent, but do not contain liquid phase in the pores. The initial concentrations of the samples were known and the final equilibrium concentrations were obtained using HPLC. Inserting the initial and final solution concentrations and adsorbent and solution masses in eq 1 yields the loading q)

aq Maq 0 CSuc,0 - MeqCSuc,eq

Mads

(1)

where q is the adsorption loading (mol Suc/kg adsorbent), Maq is the mass of the solution (g), CSuc is the molar concentration of Suc (mol/kg solution), and Mads is the mass of solute free

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

dry adsorbent (g). Subscripts 0 and eq represent the initial and equilibrium values, respectively. The adsorption of succinic acid changes the final mass of the solution phase, and this mass was not measured. However, this mass and the correct loadings could be calculated by iterating the mass balances until convergence. In the first iteration, it was assumed that the change in total solution mass was negligible to obtain an initial guess of the adsorption loading. Water loading might generate additional errors if not taken into account. However, Bowen and Vane18 showed for CBV 28014 a maximum water loading of 0.039 g/g adsorbent and with the increase in the concentration of a second adsorbate the water loading dropped to values as low as 0.02 g/g adsorbent. These values do not lead to significant errors in our calculations. Therefore, only the adsorption of succinic acid was taken into account in the loading calculations, and the water adsorption was neglected for all types of zeolites. 2.4. Adsorption Isotherms. Adsorption on zeolites is usually modeled using a Langmuir adsorption isotherm. Depending on the branching of the adsorbed molecules, a single site or dualsite Langmuir adsorption isotherm can be used. There are different descriptions for the dual site adsorption behavior. According to one theory linear molecules are adsorbed on the channel walls. However, when the molecules are branched they can also adsorb on the channel crossings due to their molecular orientation, generating a second site of adsorption.19 According to another description, during normal loading the molecules are located in the main channels while during high loading the molecules also diffuse into narrower channels.20 Two other possible descriptions for the dual site model are based on the presence of polar acidic sites and the two different pore diameters in the zeolite framework. The dual site adsorption isotherm equation is qsat,A,ikA,iCi

qtot,i )

1+

qsat,B,ikB,iCi

+

n

∑k

A,iCi

(2)

n

1+

i)1

∑k

B,iCi

i)1

where qsat is the saturation loading and k is the adsorption affinity constant. A and B represent two different adsorption sites, and i is the component of interest. The equation is given for multicomponent mixtures.18 Concentrations of species like succinic acid and succinate are not only related by eq 2 but also by the dissociation equilibrium. Experiments in the presence of Na+ were modeled using dissociation constants of succinic acid at different ionic strengths that were obtained using eq 3. pKa,j ) pKa,j0 - ∆z2 ∆z2 ) |

∑ (V z

2 k k )conjugated bases

I)

Ka,1 )

A√I + ∆εiI I + b√I

CC4H5O4-CH+ CC4H6O40

1 2

- (Vlzl2)acids |

(3) (4)

n

∑Cz

2

(5)

l l

l)1

and

Ka,2 )

CC4H4O4)CH+ CC4H5O4-

(6) where pKa,j is the negative logarithm of the dissociation constant (Ka,j) of the jth dissociation state and the superscript 0 refers to the values at zero ionic charge and 20 °C, which were obtained as 4.22 and 5.64 for the first and second dissociation, respec-

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Figure 1. The succinic acid isotherm for zeolite CBV-28014, CP811-C300, and CBV-901 at 20 °C in water with equilibrium concentrations for both solid and liquid phase. Parameters for the Langmuir model are given in the text. Error bars that merge with the symbols have been left out.

tively,21 z is the ion charge, and A and b are Debye-Hu¨ckel constants with values 0.5046 and 1.5, respectively.22 I is the ionic strength and V is the stoichiometric coefficient in a dissociation reaction. ∆εi accounts for short-range nonelectrostatic interactions with other species in the system and was taken as 0.08 and 0.13 for the first and second dissociations, respectively, based on oxalic acid.22 3. Results and Discussion 3.1. Selecting the Most Efficient High Silica Zeolite. In experiments with aqueous succinic acid solutions, three zeolites were compared for their loadings at room temperature (Figure 1). Among these three adsorbents, CBV-28014 showed the highest equilibrium loading at all concentrations with saturation loading of (1.35 mol/kg adsorbent, which is 0.16 g/g) at room temperature. Davison et al.12 reported loadings up to 0.59 mol/ kg adsorbent for silicalite powders at 0.03 mol/kg succinic acid concentration but in the current study this loading was achieved at 0.003 mol/kg succinic acid concentration. CP811-C-300 and CBV-901 had similar isotherms and performed significantly less than CBV-28014 with lower equilibrium loadings (up to ∼0.8 mol/kg adsorbent). Preliminary experiments showed that the water loadings for CP811-C-300 and CBV-901 are much higher than for CBV-28014, which may be related to the low succinic acid adsorption by CP811-C-300 and CBV-901. The dual site Langmuir isotherm parameters for succinic acid that were obtained are 0.93, 732.8, 0.49, and 15.3 mol/kg for qsat,A, kA, qsat,B, and kB, respectively. The subsequent experiments were carried out using the CBV-28014 type zeolite. Since CP811C-300 and CBV-901 is not of interest for the rest of the study the model parameters have only been estimated for CBV-28014. 3.2. Effect of Counterions on Adsorption. One of the major obstacles in fermentative succinic acid production is the separation of undissociated succinic acid from the sodium salts of this compound. To see the effect of the presence of disodium succinate and sodium hydrogensuccinate on succinic acid adsorption behavior, adsorption isotherms at different Na+ concentrations were generated. The adsorption isotherms for initial Na+:Suc molar ratios of 0.5, 1, and 1.5 are given in Figure 2. The increasing initial Na+ concentration resulted in a drop in Suc loadings which might be explained by assuming that only the undissociated succinic acid species is adsorbed. Calculating the concentrations of undissociated succinic acid species using the equilibrium liquid phase concentrations of Suc

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Figure 2. Total succinate (Suc) isotherm of zeolite CBV-28014 at 20 °C for pure succinic acid (b) and Na+:Suc ratios of 0.5 (+), 1 ([), and 1.5 (9).

Figure 3. Data from Figure 2 replotted using undissociated succinic acid content. Legends are Na+:Suc ratios of 0.5 (+), 1 ([), and 1.5 (9). Line is dual site Langmuir isotherm for undissociated succinic acid.

Figure 4. Na+ loadings of zeolite CBV-28014 at different Na+:Suc ratios (Figure 2). Legends are Na+:Suc ratios of 0.5 (+), 1 ([), and 1.5 (9).

and Na+ and the dissociation constants led to reasonable correspondence between the data sets (Figure 3). However, this explanation turns out to be too simple. The analysis of the equilibrated solutions showed dissolved Na+ concentrations lower than the initial concentrations, which suggested adsorption of Na+ (Figure 4). It should be noted that the dissolved sodium determinations showed a standard deviation of up to 7% from calibrated values, which led to a very large error for adsorbed sodium calculations even for triplicate experiments. It is wellknown that the high silica zeolite adsorbents also behave as ion exchange adsorbent due to the remaining Al2O3. The Al

Figure 5. pH profile of final equilibrium solutions. Legends are Na+:Suc ratios of 0.5 (+), 1 ([), and 1.5 (9). Initial pH values are ∼ 4.03, 4.7, and 5.4, respectively, for Na+:Suc ratios of 0.5, 1, and 1.5.

sites in the framework of zeolites are negatively charged and these charges are balanced by cations. CVB-28014 has ammonium as cation when it is shipped. However, ammonium is evaporated during the calcination and an H+ remains as the counterion. When Na+ is present in aqueous solutions the H+ and Na+ can exchange, and the sodium content of the solution decreases. The general molecular formula of ZSM-5 type zeolites is given as Na+n(H2O)16[AlnSi96-nO192].23 Using this formula and the SiO2/Al2O3 ratio of 280 (n ) 0.681), the maximum Na+ capacity is ∼0.118 mol Na+/kg adsorbent. This capacity is lower than most of the sodium loadings obtained from the mass balances (Figure 4). The sodium adsorption patterns suggest some additional adsorption of sodium as succinate salts, together with undissociated succinic acid. An increasing Na+:Suc molar ratio resulted in an increasing Na+ adsorption at the same Na+ concentrations (Figure 4) and in a higher equilibrium pH (Figure 5) with a corresponding increase in concentration of succinate species, which might lead to competition between adsorption of dissociated succinate species and undissociated succinic acid. While the concentration of dissociated succinate increased, the adsorption of sodium as counterion also increased. Experiments using Na2C4H4O4 (not presented) showed no significant adsorption, which suggests that the monodissociated hydrogensuccinate (C4H5O4-) is the only dissociated succinate species adsorbed by the zeolite particles. This might be due to the high polarity of C4H4O4). Moreover, a closer look at Figure 3 shows that, when based on undissociated succinic acid, the adsorption isotherms coincided when the Na+:Suc ratios were low, but led to a slight underestimation of the loading when the sodium content increased (Figure 3). The drop in succinic acid loading in presence of Na+ ion could be due to the reduced succinic acid adsorption capacities by the Na+ form of ZSM-5 as compared to H+ form, due to the steric hindrance. Modeling studies based on eq 2 (not presented) also overestimated the adsorption capacities in the presence of Na+, which suggests lower adsorption by the sodium form of ZSM-5. On the basis of this information the calculated selectivity of adsorbents between undissociated succinic acid and sodium hydrogensuccinate ([CH2Suc/CNaHSuc]adsorbed/[CH2Suc/ CNaHSuc]aqueous) was scattering between 10 and 20 when Na+ ion exchange was subtracted from the total Na+ adsorption. The effect of solution pH on total succinate adsorption cannot be studied independently from the effect of Na+ concentration, due to the charge balances. In the absence of Na+, the pH is low (pH < 3) and the adsorption is strong, whereas at higher

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Figure 6. The acetic acid loading of zeolite CBV-28014 at 20 °C in pure acetic acid solution (0) and at a 1:4 initial acetic acid to succinic acid mass ratio (9). Data from Bowen and Vane18 (4) have been included (21 °C). Lines are the model fit.

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Figure 8. The succinic acid loading in the presence of initially 10 (4) and 20 g/kg (2) acetic acid and no acetic acid (b) at 20 °C. Lines are model fits.

Figure 9. Acetic acid loadings as a function of succinic acid concentration. Conditions of Figure 8 apply. Markers are initially 10 (4) and 20 g/kg (2) acetic acid. Lines are model fits. Figure 7. The succinic acid loading for conditions of Figure 6. In the absence of acetic acid (b) and at a 1:4 initial acetic acid to succinic acid mass ratio (0) at 20 °C. Lines are model fits.

pH (at higher Na+ and lower undissociated succinic acid concentrations) the total succinate adsorption decreases (Figures 2 and 5). 3.2. Effect of Acetic Acid on Adsorption Capacity. Acetic acid is known to be the major byproduct of the succinic acid fermentations. Its concentration can vary from one bacterial strain to another. Song and Lee3 and Lee and co-workers24 reported succinic acid to acetic acid mass ratios of 1:25.8 and 1:4.04, respectively, for A. succiniciproducens. Okino et al.5 reported a ratio of 1:9.1 for C. glutamicum. Therefore, the acetic acid adsorption can be an important factor in adsorption efficiencies. To observe the effect of acetic acid on succinic acid adsorption, three different experimental conditions were prepared. The initial mass ratio of acetic acid to succinic acid was set to 1:4 for the first case (Figures 6 and 7) and for the second case the isotherms were generated with fixed initial acetic acid concentrations of 5 and 10 g/kg (Figures 8 and 9). The acetic acid isotherms are in line with the data reported by Bowen and Vane.18 The deviations at high concentration can be explained by errors generated by the HPLC, which were up to 3%. The acetic acid loading decreased in the presence of succinic acid (Figure 6), whereas the maximum succinic acid loadings in the experimental concentration range dropped from 1.35 to 1.05, 1.15, and 0.95 mol/kg adsorbent (for initially 0.34 mol/kg solution), respectively, in the initial presence of 1:4 mass ratio, 10 g/kg, and 20 g/kg acetic acid (Figures 7-9). The selectivity between succinic acid and acetic acid ([CH2Suc/ Cacetic acid]adsorbed/[CH2Suc/Cacetic acid]aqueous), on mole fraction basis)

was toward succinic acid at low succinic acid concentrations (∼6) and decreased with increasing succinic acid concentration to ∼1. Therefore, the separation of succinic acid and acetic acid relies on the concentration ratio of these two compounds. The dual site Langmuir adsorption model gave reasonable fits to the data. The fitted parameters for acetic acid are 0.85 mol/kg, 53.7 kg/mol, 1.63 mol/kg, and 13 kg/mol for qsat,A, kA, qsat,B2 and kB, respectively. 3.4. Effect of Ethanol on Succinic Acid Adsorption. A major obstacle in adsorption processes is desorption of the adsorbate. Carboxylic acids can be desorbed by utilizing bases to dissociate the acids that are released in the liquid phase. However, for sustainable operation and to minimize the number of downstream unit operations the use of bases should be avoided in carboxylic acid recovery. Regeneration of adsorbents can also be achieved by displacement, involving replacing the adsorbed molecules by using concentration swing with a reusable competitive adsorbate. Ethanol might serve this purpose. One well-known property of silicalite membranes is their high ethanol loadings. Bowen and Vane18 reported a maximum ethanol loading of 0.126 g/g adsorbent for CVB28014 type zeolites. On the basis of this information succinic acid adsorption isotherms in absolute ethanol were determined to establish the effect of ethanol on adsorption of succinic acid (Figure 10). For succinic acid in ethanol solution, the maximum observed succinic acid loading in the experimental concentration range was approximately 0.34 mol/kg adsorbent, which is lower by a factor of 3 than in aqueous solutions. It is also known that succinate salts have a low solubility in ethanol,25 which might increase the H2Suc/NaHSuc selectivity during the regeneration.

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Figure 10. Succinic acid loadings of CBV-28014 in aqueous solution (b), in ethanol (O), and in aqueous solutions containing initially 100 (9) and 200 g/kg (0) ethanol at 20 °C. Lines are model fits.

However, one drawback of using an alcohol to desorp succinic acid from zeolites is the risk of a zeolite catalyzed esterification reaction involving succinic acid and ethanol. To check if a reaction occurs we performed long-term experiments (>4 days) at high temperatures. The dissolved succinic acid concentrations dropped with increasing time and temperature only for ethanol as solvent. Therefore, the isotherms generated with pure ethanol in the absence of water might overestimate the succinic acid loadings due to an additional drop in concentrations caused by a reaction. To minimize the reaction, aqueous solutions of succinic acid and ethanol were generated using initial ethanol concentrations of 100 and 200 g/kg (Figure 10). Equilibrium experiments were conducted at room temperature for 5 h (Figure 10) and 3 days (not presented). The differences in isotherms were within the error range suggesting the absence of a reaction at these conditions. Owing to the high ethanol concentrations, the HLPC errors (up to 3%) magnified, leading to deviations of up to ∼200% in ethanol loading calculations. Therefore, these ethanol loadings are not presented and are excluded from the parameter estimation. The model parameters for ethanol were obtained using all data reported in the current study (Figure 10) and the ethanol adsorption data reported by Bowen and Vane.18 Using eq 2, and the aforementioned succinic acid adsorption parameters, the fitted parameters for ethanol are 0.96 mol/kg, 19.7 kg/mol, 1.78 mol/kg, and 11 kg/mol for qsat,A, kA, qsat,B, and kB, respectively. This leads to a good model fit in Figure 10. 3.5. Effect of Temperature on Adsorption Capacity. Desorption strategies include pressure swing and temperature swing in addition to concentration swing. Since the current system is aqueous, pressure swing is not a valid desorption option. To establish the effect of temperature swing, the adsorption isotherm at 70 °C was determined (Figure 11). The temperature increase resulted in a drop in succinic acid loading. However, the change was rather modest. The maximum loading in the concentration interval decreased to 1.16 mol/kg adsorbent (at 0.25 mol/kg solution at equilibrium) but at low aqueous equilibrium concentrations (i.e., 0.025-0.1 mol/kg) the drop in loading was larger.

Figure 11. Succinic acid loadings of zeolite CBV-28014 at 70 (O) and at 20 °C (b).

this dissociation. High errors in Na+ analysis precluded accurate characterization of the system, but the current findings suggest competitive adsorption of noncharged succinic acid and sodium hydrogensuccinate, with a selectivity in the range of 10-20. The final pH shows that the zeolite performance around pH 4.5 is still acceptable. However, adsorption would be much more efficient for a pH in the range of 3-4. Therefore, for efficient adsorption operation the fermentation pH should be maintained at pH values not higher than 4. The presence of acetic acid decreased the succinic acid loading, but not severely for acetic acid to succinic acid ratios expected from fermentations. The dual site Langmuir adsorption model described the isotherms with only slight deviations. The effect of a temperature swing to 70 °C was insufficient for selecting this as an option for succinic acid desorption. Preliminary experiments showed that a competitive solvent such as ethanol may be a good option for displacement regeneration. However, the subsequent regeneration of the zeolite should be investigated. The regeneration conditions and the solvent should be selected after a detailed analysis considering possible undesired mechanisms like an esterification reaction in ethanol. Since real fermentation media will contain other impurities in addition to sodium succinate, acetic acid, and ethanol, the succinic acid loadings will be different. However, the competitive effect of such impurities may be modest due to their expected lower concentrations. The interest of our batch studies is to observe the effect of impurities on the adsorption behavior. Therefore, follow up studies will have to be done with real fermentation media once these fermentations have been optimized. The current results indicate that the use of CBV-28014 in succinic acid recovery might be an efficient option, but additional studies will also be required especially with respect to desorption and the overall process. Column experiments will give better understanding of the adsorption process and will be performed in our future studies. Acknowledgment

4. Conclusion and Recommendations Three different powder zeolites were screened for succinic acid removal from aqueous solutions. CBV-28014 type zeolite showed the highest succinic acid loadings in the studied concentration range (0.01-0.34 mol succinic acid/kg solution at equilibrium). The loadings dropped when succinic acid was more dissociated, and some Na+ adsorption was observed upon

We would like to thank Mr. Joop Padmos from DelftChemTech, Delft University of Technology for the sodium analysis. This project is financially supported by The Netherlands Ministry of Economic Affairs and the B-Basic partner organizations (www.b-basic.nl) through B-Basic, a public-private NWOACTS programme (ACTS ) Advanced Chemical Technologies for Sustainability).

Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010

Literature Cited (1) Bechthold, I.; Bretz, K.; Kabasci, S.; Kopitzky, R.; Springer, A. Succinic Acid: A New Platform Chemical for Biobased Polymers from Renewable Resources. Chem. Eng. Technol. 2008, 31, 647. (2) McKinlay, J. B.; Vieille, C.; Zeikus, J. G. Prospects for a Bio-Based Succinate Industry. Appl. Microbiol. Biotechnol. 2007, 76, 727. (3) Song, H.; Lee, S. Y. Production of Succinic Acid by Bacterial Fermentation. Enzyme Microb. Technol. 2006, 39, 352. (4) Zeikus, J. G.; Jain, M. K.; Elankovan, M. K. Biotechnology of Succinic Acid Production and Markets for Derived Industrial Products. Appl. Microbiol. Biotechnol. 1999, 51, 545. (5) Okino, S.; Noburyu, R.; Suda, M.; Jojima, T.; Inui, M.; Yukawa, H. An Efficient Succinic Acid Production Process in a Metabolically Engineered Corynebacterium glutamicum Strain. Appl. Microbiol. Biotechnol. 2008, 81, 459. (6) Datta, R.; Glassner, D. A.; Jain, M. K.; Vick Roy J. R. Fermentation and Purification Process for Succinic Acid. U.S. Patent 5,168,055, 1992. (7) Hong, Y. K.; Hong, W. H. Equilibrium Studies on Reactive Extraction of Succinic Acid from Aqueous Solutions with Tertiary Amines. Bioprocess Eng. 2000, 22, 477. (8) Huh, Y. S.; Jun, Y. S.; Hong, Y. K.; Song, H.; Lee, S. Y.; Hong, W. H. Effective Purification of Succinic Acid from Fermentation Broth Produced by Mannheimia succiniciproducens. Process Biochem. 2006, 41, 1461. (9) 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. (10) Vieux, A. S.; Rutagengwa, N.; Rulinda, J. B.; Balikungeri, A. Extraction of Some Dicarboxylic Acids by Tri-isooctylamine. Anal. Chim. Acta 1974, 68, 415. (11) Tung, L. A.; King, C. J. Sorption and Extraction of Lactic and Succinic Acids at pH-Greater-than-pKA1: 1. Factors Governing Equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (12) Davison, B. H.; Nghiem, N. P.; Richardson, G. L. Succinic Acid Adsorption from Fermentation Broth and Regeneration. Appl. Biochem. Biotechnol. 2004, 113-16, 653. (13) Garcia, A. A. Strategies for the Recovery of Chemicals from Fermentation: A Review of the Use of Polymeric Adsorbents. Biotechnol. Prog. 1991, 7, 33.

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(14) Jun, Y. S.; Huh, Y. S.; Park, H. S.; Thomas, A.; Jeon, S. J.; Lee, E. Z.; Won, H. J.; Hong, W. H.; Lee, S. Y.; Hong, Y. K. Adsorption of Pyruvic and Succinic Acid by Amine-Functionalized SBA-15 for the Purification of Succinic Acid from Fermentation Broth. J. Phys. Chem. C. 2007, 111, 13076. (15) Lee, C. Y.; Pedram, E. O.; Hines, A. L. Adsorption of Oxalic, Malonic, and Succinic Acids on Activated Carbon. J. Chem. Eng. Data 1986, 31, 133. (16) Glassner, D. A.; Datta, R. Process for the Production and Purification of Succinic Acid. U.S. Patent 5,143,834, 1992. (17) Bowen, T. C.; Meier, R. G.; Vane, L. M. Stability of MFI ZeoliteFilled PDMS Membranes During Pervaporative Ethanol Recovery from Aqueous Mixtures Containing Acetic Acid. J. Membr. Sci. 2007, 298, 117. (18) Bowen, T. C.; Vane, L. M. Ethanol, Acetic Acid, and Water Adsorption from Binary and Ternary Liquid Mixtures on High-Silica Zeolites. Langmuir 2006, 22, 3721. (19) Bowen, T. C.; Noble, R. D.; Falconer, J. L. Fundamentals and Applications of Pervaporation Through Zeolite Membranes. J. Membr. Sci. 2004, 24, 5–1. (20) van Koningsveld, H. Localization of Nonframework Species in MFI. J. Mol. Catal. A 1998, 134, 89. (21) Alberty, R. A. Thermodynamic Properties of Weak Acids Involved in Enzyme-Catalyzed Reactions. J. Phys. Chem. B 2006, 110, 5012. (22) Reed, D. T.; Clark, S. B.; Rao, L. Actinide Speciation In High Ionic Strength Media; Springer Verlag: New York, 1999. (23) Xu, R.; Pang, W.; Yu, J.; Huo, Q.; Chen, J. Structural Chemistry of Microporous Materials, Chemistry of Zeolites and Related Porous Materials; John Wiley & Sons (Asia) Pte Ltd.: Singapore, 2007; p 20-85. (24) Lee, P. C.; Lee, W. G.; Kwon, S.; Lee, S. Y.; Chang, H. N. Succinic Acid Production by Anaerobiospirillum succiniciproducens: effects of the H2/CO2 supply and glucose concentration. Enzyme Microbiol. Technol. 1999, 24, 549. (25) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press, LCC.: Boca Raton, FL, 2006; http://www.hbcpnetbase.com.

ReceiVed for reView July 9, 2009 ReVised manuscript receiVed December 16, 2009 Accepted December 16, 2009 IE901110B