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Langmuir 2006, 22, 9350-9356
Phenylalanine and Arginine Adsorption in Zeolites X, Y, and β John E. Krohn and Michael Tsapatsis* Department of Chemical Engineering and Material Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455-0132 ReceiVed June 17, 2006. In Final Form: August 10, 2006 This paper documents a continuation of work published in Langmuir 2005, 21, 8743-8750. We report new aspects of this amino acid adsorption study, including the effect of changing zeolite framework type and results from adsorption of mixed amino acids. In single-amino-acid adsorption experiments, zeolite Y (Si/Al ) 2.5) was found to adsorb no phenylalanine while admitting arginine to a similar extent as observed with zeolite β. Using this zeolite Y, we measured mixed-amino-acid separation selectivities for arginine/phenylalanine as large as 25 000 or 2 orders of magnitude larger than the corresponding selectivities measured with zeolite β.
Introduction business1
Amino acid production is a 1.5 billion dollar that is driven primarily by the need for animal feed supplements and flavoring agents.2 However, specialty uses of amino acids are a rapidly growing business,1 and research involving a variety of areas such as pharmaceutical3 and biomedical4 applications is increasing. Commercial production of amino acids typically involves extraction from natural products, chemical or enzymatic synthesis, or microbiological fermentation.5 In all of these processes, a product separation mechanism is required. Amino acids can then be used in the production of oligopeptides and larger biopolymers, but a challenging separation step is again required to remove unreacted amino acids from dipeptides.6 Zeolites have been investigated as an alternative adsorption medium to the resins commonly used for amino acid separations.2,5,7 We previously investigated amino acid adsorption on zeolite β as a function of solution conditions and zeolite aluminum content.7 To continue this amino acid study, we measured the adsorption behavior of zeolites X and Y to gain some information regarding the influence of zeolite structure. Like zeolite β, zeolites X and Y have a three-dimensional, interconnected, 12-memberring pore system. This feature makes them particularly attractive in industrial applications of microporous media. Another similarity is that faujasite-type materials have pores large enough to host adsorption of amino acids. Still, there are sufficient differences between these two large-pore zeolites to merit comparison of their adsorption behavior. While our focus is on separations, it is our intent to add to the general understanding of interactions between amino acids and silicate materials in order to advance related fields such as the templating of new porous materials8-10 and the study of reactions in confined spaces.11
Background Amino Acids. Amino acids are composed of basic amino and acidic carboxyl functional groups as well as a characteristic side chain. Except at low pH, the carboxyl group tends to be (1) Mirasol, F. Chem. Market Rep. 2000, 258, 5-12. (2) Munsch, S.; Hartmann, M.; Ernst, S. Chem. Commun. 2001, 1978-1979. (3) Loffet, A. J. Pept. Sci. 2002, 8, 1-7. (4) Panitch, A.; Yamaoka, T.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Macromolecules 1999, 32, 1701-1703. (5) Yonsel, S.; Schaefer-Treffenfeldt, W.; Kiss, A.; Sextl, E.; Naujok, H. DE Patent 4217203, 1993. (6) Stockhammer, S.; Schaefer-Treffenfeldt, W.; Knaup, G.; Drauz, K.; Sextl, E. DE Patent 19535751, 1997. (7) Krohn, J. E.; Tsapatsis, M. Langmuir 2005, 21, 8743-8750.
dissociated, giving the compound a more negative charge. At low pH, the amino group is protonated and the molecule has a positive charge. At an interim pH known as the isoelectric point, the amino acid in solution has, on average, a net charge of zero. At this point, a solution contains positively and negatively charged amino acid ions in equal quantities. A solution at the isoelectric point also has a large concentration of amino acid that has both its amino and carboxyl groups charged. This dually charged species, referred to as a zwitterion, can be considered net-neutral with respect to overall charge. The interconversion of these charged species is detailed in Figure 1. The side chain, designated above with an R, can be a dissociating functional group. In the case of arginine (see Figure 2) the side chain is an amino group allowing the molecule to carry a +2 charge at low pH. Phenylalanine’s nondissociating, low-polarity side chain makes this amino acid more hydrophobic than most of the 20 common amino acids.12 In light of the equilibria given in Figure 1 and the assumption that R is nondissociating, the effect of pH on the charged species concentrations can be determined by use of dissociation equilibrium relations:
K1 )
mA(mH+ mA+
K2 )
mA-mH+ mA(
(1)
Here mA+, mA-, mA(, and mH+ are the cationic, anionic, and zwitterionic amino acid concentrations, respectively, followed by the free proton concentration. Figure 3 shows amino acid mole fractions, for example, xA) mA- /mAT with mAT ≡ mA+ + mA- + mA(, obtained from published dissociation constants.14 While the phenylalanine plot is generated as given above, production of the arginine plot requires consideration of a fourth species of +2 charge. Zeolites. Faujasite-type structures are constructed from sodalite cages connected by six-member-ring pores.15 Spherical super(8) Che, S.; Garcia-Bennett, A. E.; Yokoi, T.; Sakamoto, K.; Kunieda, H.; Terasaki, O.; Tatsumi, T. Nat. Mater. 2003, 2, 801-805. (9) Belton, D.; Paine, G.; Patwardhan, S. V.; Perry, C. C. J. Mater. Chem. 2004, 14, 2231-2241. (10) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756-768. (11) Zamaraev, K. I.; Salganik, R. I.; Rommannikov, V. N.; Vlasov, V. A.; Khramtsov, V. V. Dokl. Chem. 1995, 340, 56-58. (12) Radzicka, A.; Wolfenden, R. Biochemistry 1988, 27, 1664-1670. (13) Ching, C. B.; Hidajat, K.; Uddin, M. S. Sep. Sci. Technol. 1989, 24, 581-597. (14) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 85th ed.;CRC Press: Boca Raton, FL, 2004/2005; Section Biochemistry.
10.1021/la061743m CCC: $33.50 © 2006 American Chemical Society Published on Web 09/23/2006
Adsorption Zeolites X, Y, and β
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Figure 1. Amino acid dissociation equilibrium.
Figure 2. Pertinent amino acids. Cylindrical size approximations are provided in parentheses.13
Figure 3. Relative concentrations of ionic species. Data are presented for phenylalanine (Phe) and arginine (Arg). xA+, xA2+, xA-, and xA( represent fractions of amino acid in solution existing as cationic monovalent, cationic divalent, anionic, and zwitterionic species.
cages, 11.18 Å in size, reside between the sodalite cages and are connected by 12-MR pores of size 7.4 Å × 7.4 Å.16 Like faujasites, zeolite β has a three-dimensional, interconnected 12-MR pore system.17 In contrast, zeolite β’s framework features no cage, and its largest pores have dimensions of 6.6 Å × 6.7 Å.16 Adsorption. In previous work, we reported on the adsorption of phenylalanine and arginine on zeolite β.7 Several observations from this previous work are of particular use in analyzing adsorption results for zeolites X and Y. These observations are as follows: (1) As the pH rises above 9, adsorption of phenylalanine becomes insensitive to framework Si/Al. This implies, for pH (15) Breck, D. W. In Zeolite Molecular SieVes: Structure, Chemistry, and Use; Wiley: New York, 1974; p 752. (16) Baerlocher, C., Meier, W. M., Olson, D. H., Eds. Atlas of Zeolite Framework Types; Elsevier: Amsterdam, 2002; p 308. (17) Newsam, J. M.; Treacy, M. M. J.; Koetsier, W. T.; De Gruyter, C. B. Proc. R. Soc. London, Ser. A: Math., Phys. Eng. Sci. 1988, 420, 375-405.
> 9, that cationic amino acid is not adsorbing, because cations adsorb by balancing the negative framework charge produced by the presence of aluminum. (2) At low solution amino acid concentration, isotherms generated at pH > 9 have lower slopes than those generated at pH < 9. This implies that the adsorbate/adsorbent bond is, on average, weaker when pH > 9. (3) When pH > 9, NaCl addition causes an increase of amino acid adsorption (see Figure 8 in the Supporting Information). Furthermore, adsorption increases by an amount that keeps the post-salt-addition result on the curve generated prior to salt addition. When pH < 9, NaCl addition does not cause a significant rise in amino acid adsorption and produces results that are off the no-salt curve. The first two of the above observations for zeolite β suggest, for pH > 9, that the concentration of cationic amino acid is depleted and that adsorption primarily involves zwitterionic species. This proposed scenario is consistent with the effect of NaCl addition when pH > 9. Figure 9 in Supporting Information depicts the proposed mechanism that leads to an increase in adsorption. With this mechanism, sodium cations and amino acid do not compete for adsorption sites. Instead, it is a change in pH that directly alters amino acid adsorption, and this is the reason that the post-salt-addition result remains on the no-salt curve. The above three zeolite β observations also suggest that, for pH < 9, cationic phenylalanine adsorption appears to play a significant role. With regard to the effect of salt, any departure from the no-salt adsorption curve, as seen in Figure 8 of Supporting Information with pH < 9, is an indication of adsorption being influenced by a condition other than pH. Given the combined experimental observations, this other condition is assumed to be the direct competition between added sodium cations and cationic phenylalanine. A thermodynamic equilibrium model was applied to the above adsorption study. This model describes amino acid adsorption as occurring by two mechanisms, ion exchange of cationic amino acid and adsorption of zwitterions. Fitting of the model’s parameters was simplified by taking advantage of the fact that when pH > 9, only zwitterionic adsorption is occurring. This provides a means to decouple fitting of model parameters for the two adsorption mechanisms. This model will be reviewed and reapplied below. In what follows, we present (1) an experimental description, (2) experimental findings for adsorption of single amino acids in faujasite-type materials, (3) a review of our model followed by its application to phenylalanine adsorption in zeolite Y, and (4) experimental findings for adsorption in the presence of mixed amino acids. Experimental Section Three faujasite-type materials, labeled 13X, Y-54, and HISIV1000, were obtained from UOP. These materials will hereafter be referred to as zeolites X, Y:2.5, and Y:8. Elemental analysis for the first two provided silicon-to-aluminum ratios of 1.2 and 2.5, respectively. Y:8 has a framework Si/Al ∼ 8, however, the as-received sample
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Table 1. Zeolite Composition Na-Y:8 H/Na-Y:8 H/Na-Y:2.5 Na-X H/Na-X H-β:12a
Si/Al
Na/Al
8 8 2.5 1.2 1.2 12
0.2 0.01 0.4 1 0.5 0.01
a
Sodium content for H-β:12 is based on nominal product specification provided by Zeolyst International.
also contained considerable extraframework aluminum. Comparison of Y:8’s nominal framework Si/Al ratio with that obtained by elemental analysis of the as-received sample suggests that approximately 17 wt %, on a water-free basis, of sample is to be attributed to this extraframework alumina material. This approximation is obtained with the assumption that the extraframework material, produced during dealumination of the zeolite, is an alumina. However, there have been arguments for this extraframework material to be an amorphous silica-alumina phase.18 The zeolites were received in their sodium cation form. Protonated forms were produced as follows: Each zeolite was placed in twice its weight of 1 M ammonium nitrate and stirred for more than 12 h. The zeolite X material was water-cooled during this ion exchange. The slurries were then filtered, and the solids were dried at 90 °C. This process of ion exchange was carried out two more times for each zeolite. The ion-exchanged zeolites were then rinsed with 4 times their weight in deionized water and dried at 90 °C. The NH4 zeolites as well as Na-Y:8 were then heated at 1 °C/min and calcined for 5 h at 500 °C. Air circulation was maintained throughout the heating, and X-ray diffraction (XRD) (Siemens D5005) was used after calcining to confirm that no drastic structural change occurred. Table 1 provides sodium content for each zeolite tested, based on inductively coupled plasma spectrometry measurements provided by Galbraith Laboratories Inc. Zeolite β samples, of Si/Al mole ratio ) 12 and 100, have been obtained from Zeolyst International. When necessary, protonation was achieved with a previously reported procedure7 similar to that described for the above faujasite-type materials. A notable difference is that calcination was carried out at 450 °C for 5 h. Particle sizes for zeolite β and faujasite-type materials were significantly different (see Figure 4). All faujasite-type materials tested were of similar size. The method of conducting single-amino-acid adsorption experiments also has been provided previously.7 Briefly, a typical experiment would involve a mixture of zeolite powder, deionized water, a single amino acid, and hydrochloric acid or sodium hydroxide. Amino acids tested were l-arginine (BioChemika Ultra g99.5%) and L-phenylalanine (Aldrich 99%). The mixture would be stirred for 8-12 h at a temperature of 21-23 °C. High-performance liquid chromatography would then provide the equilibrated solutionphase amino acid concentration. Adsorption was determined by the change in solution-phase amino acid concentration as a result of zeolite contact. Unless otherwise noted, adsorption is reported in units of millimoles of amino acid per gram of zeolite. Note that zeolite mass, used in reporting adsorption values, does not include the mass of amorphous alumina present in zeolite Y:8. Also, this amorphous alumina is assumed to adsorb no significant amount of amino acid. The standard deviations of amino acid adsorption, solution concentration, and pH values, throughout this text, are less than 0.01 mmol/g, 0.001mol/L, and 0.05, respectively. Mixtures of the above two amino acids also have been used in adsorption experiments. With the exception of the additional amino acid, these experiments were carried out with the same procedure used for single-amino-acid experiments. In all binary-amino-acid experiments, arginine was added immediately after the addition of phenylalanine. (18) Omegna, A.; Prins, R.; van Bokhoven, J. A. J. Phys. Chem. B 2005, 109, 9280-9283.
Figure 4. Electron microscopic images: SEM images of (a) H-β: 12, (b) Na-Y:2.5, and (c) Na-Y:8 and TEM images of (d) H-β:12 and (e) Na-Y:8.
Figure 5. Arginine’s adsorption on faujasite-type materials with varying pH: (0) Na-Y:8, (b) H/Na-Y:8, ([) H/Na-Y:2.5, (2) H/NaX, and (9) H-β:127. Relevant solution Arg concentrations (moles per liter) are as follows: for H/Na-Y:8, initial 10.9 × 10-2, final (6.1-9.6) × 10-2; for H/Na-Y:2.5, initial 10.9 × 10-2, final (2.45.9) × 10-2; for H/Na-X, initial 10.9 × 10-2, final (6.6-9.1) × 10-2; and for H-β:12, initial 14.7 × 10-2, final (8.0-11.8) × 10-2.
Results and Discussion Arginine Adsorption. All three faujasite-type materials adsorbed arginine with a pH dependency similar to that previously reported for zeolite β7 (see Figure 5). Of the three faujasite-type materials tested, the one with the intermediate Si/Al, zeolite Y:2.5, displays the highest adsorption of arginine. X-ray diffraction results indicate that adsorption of arginine on Na-Y:8 (1 mmol/g) causes the zeolite’s crystallographic unit cell to contract by 0.75% (0.10 Å). No change in unit cell was detected following adsorption of a similar concentration of phenylalanine. Equilibrated pH is different for these two experiments: pH ) 10.0 with arginine and pH ) 5.6 with phenylalanine.
Adsorption Zeolites X, Y, and β
Figure 6. Adsorption of phenylalanine on zeolites X, Y, and β. (b) Na-Y:8, pH ) 5.3-6.4; (0) H/Na-Y:8, pH ) 5.0-5.9; (+) H/NaY:2.5, (1) 17 h, 23 °C, pH 6.6, or (2) 14 days, 23 °C, pH 6.1; (]) H/Na-X, (4) 17 h, 23 °C, pH 8.1, or (5) 14 days, 23 °C, pH 7.6, or (6) 29 h, 85 °C, pH 7.4 (4) H-β:12,7 pH ) 3.4-4.5; (-O-) H-β:9,7 pH ) 4.3-4.5. Nonstandard experimental conditions were used with some samples; all such samples resulted in less than 0.04 mmol/g adsorption.
Figure 7. Phenylalanine (b) and benzene (]) adsorption on Na+form faujasite-type materials. Benzene data adapted from Barthomeuf.19
Phenylalanine Adsorption. Neither the sodium cation nor the protonated forms of zeolites X and Y:2.5 exhibit significant phenylalanine adsorption (see Figure 6). Raising the mixing time and temperature fails to produce adsorption for these materials (see caption for Figure 6). In contrast, both Na-Y:8 and H/NaY:8 exhibit a phenylalanine capacity similar to that previously reported for zeolite β7 with comparable Si/Al. However, the Y:8 materials show a less steep isotherm relative to that of zeolite β. This lower slope suggests that adsorbate/adsorbent interaction is weaker in the case of Y:8, relative to the β materials. In our previous publication,7 a reduction of isotherm slope was presented as evidence that adsorption of cationic phenylalanine was no longer occurring. In what follows, we will show that this is not the case for zeolite Y:8. Instead, adsorption of cations is contributing significantly to the Y:8 isotherm shown in Figure 6. Also, we will suggest that the difference in slopes observed in Figure 6 is a result of differences in the strengths of adsorption sites in the various zeolites. At saturation of Y:8 (Al/unit cell ∼ 22), there are ∼12 phenylalanine molecules per unit cell. This level of loading is similar to that expected for benzene adsorption, based on previously reported results from single-component vapor-phase adsorption in zeolite Y19 (see Figure 7). However, this analogy does not hold for all zeolites tested. Rising aluminum content produces an increase in benzene adsorption and a decrease in phenylalanine adsorption. The exclusion of phenylalanine at high aluminum concentrations suggests competition with water molecules and will be shown later to enable highly selective separations of mixtures of phenylalanine and arginine. As the zeolite Na-Y:8 sample, of Figure 6, was loaded with (19) Barthomeuf, D. Spectroscopic inVestigations of zeolite properties; Derouane, E. G., Lemos, F., Naccache, C., Ribeiro, F. R., Eds.; Zeolite Microporous Solids: Synthesis, Structure, and Reactivity; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. 352, pp 193-223.
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Figure 8. Phenylalanine’s adsorption on zeolite Y:8 with varying pH: (b) Na-Y:8, (4) H/Na-Y:8, and (O) H/Na-Y:8 with 1 M NaCl. Initial solution phenylalanine concentration was 11 × 10-2 mol/L. Equilibrated solution phenylalanine concentration ranged from 6.6 × 10-2 to 10 × 10-2 mol/L. While triangles represent measured quantities, the dashed line is a rough estimate for expected phenylalanine adsorption on H/Na-Y:8. Arrows connect beforeand-after cases of experiments involving addition of NaCl.
phenylalanine, the solution pH dropped from 6.4 to 5.3. As shown in Figure 8, this decrease in pH did little to the total phenylalanine adsorbed. Figure 8 presents adsorption of phenylalanine as a function of pH. Zeolite Y:8 displays relative insensitivity to pH under mildly acidic conditions. Adsorption then decreases with increasing alkalinity. This behavior is similar to that observed with zeolite β.7 However, the decrease in adsorption appears steeper and appears to initiate at pH ∼ 7.5 versus zeolite β’s pH ∼ 6.5. Partial protonation of the zeolite does not appear to change its performance with respect to pH variation. Also shown in Figure 8 is phenylalanine’s adsorption response to salt addition. Upon addition of sodium chloride to a sample containing H/Na-Y:8 and phenylalanine, the pH is reduced. For pH < 9, addition of NaCl produces no significant change in phenylalanine adsorption. This behavior is similar to that observed with zeolite β, as reviewed in the Introduction. The unresponsiveness of phenylalanine adsorption to NaCl addition, when pH < 9, signifies the presence of significant adsorbed cationic phenylalanine on Y:8, as is the case for zeolite β. This conclusion can be independently reached by use of a high-pH isotherm that will be presented later in the modeling section. For pH > 9, salt addition produces an increase in phenylalanine adsorption on H/Na-Y:8. A similar response to salt addition was observed with zeolite β. However, with zeolite β and pH > 9, salt addition produced results that remained on the pre-salt adsorption curve (see Figure 8 of Supporting Information). This response contributed to our conclusion that phenylalanine was adsorbing on zeolite β, at pH > 9, exclusively in its zwitterionic state (see Figure 9 of Supporting Information). Unlike the results for zeolite β, however, salt addition produces a result that is off the pre-salt curve, despite having pH > 9. This suggests that ion exchange may still be contributing to phenylalanine’s adsorption. This uncertainty will be considered in the next section. To summarize, H/Na-Y:2.5 and H/Na-X adsorb arginine while excluding phenylalanine. H/Na-Y:8 and H-β:12 have similar adsorption capacities for both amino acids, and their adsorption of phenylalanine changes similarly with respect to pH change and salt addition. While phenylalanine adsorption increases with salt addition for both H/Na-Y:8 and H-β:12, the charged state of the amino acid is unclear in the case of H/Na-Y:8 at pH > 9. Modeling of Phenylalanine Adsorption on Dealuminated Zeolite Y. A thermodynamic equilibrium model for the adsorption
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Figure 9. Phenylalanine isotherms on H-β and H/Na-Y:8. (4) H/NaY:8, pH > 9; (2) H-β:127, pH > 9; (O) H-β:1007, pH ) 6-6.5; (b) H-β:100, pH > 9. Modeling results are presented as lines. Note that total phenylalanine adsorption is plotted versus the concentration of zwitterions in solution.
Figure 10. Modeling isotherm for phenylalanine on zeolite NaY:8. (2) Measured mPheT, (s) calculated mPheT, and (---) calculated mPhe(.
Table 2. Parameter Estimates for Zwitterionic Phenylalanine Adsorption on Zeolites Y and β mB°
K( zeolite type
est (mol/L)-1
H/Na-Y:8 H-β:127 H-β:1007
8 100 150
1
/2(95% CI) (mol/L)-1
est (mmol/g)
7 40 40
0.98 0.45 0.59
1/
2(95% CI) (mmol/g)
0.62 0.07 0.05
of amino acids was presented previously.7 This model describes amino acid adsorption as occurring by two mechanisms, ion exchange of cationic amino acid and adsorption of zwitterions. Terminology used below matches that used in the model’s derivation. For example, ion exchange between cationic phenylalanine and protons occurs as H+ + S h + Phe+ T H+ + Phe+. Here an overbar is used to designate an adsorbed species, while corresponding symbols lacking the overbar refer to bulk solutionphase species. ms° is the initial concentration of unoccupied sites, S h , that can host adsorption of cationic phenylalanine. These sites are considered a subset of the sites available to smaller cations such as protons and sodium cations. Also, zwitterion h T Phe(, where adsorption is considered to occur as Phe( + B B h is a site available exclusively for adsorption of a zwitterion, Phe(. Finally, the concentration of total phenylalanine adsorbed is mPheT ≡ mPhe+ + mPhe(. This model is routinely first applied to isotherms (see Figure 9) where, because of either high pH or high Si/Al conditions, adsorption of cationic phenylalanine is not occurring, that is, mPheT ) mPhe(. Modeling results shown in Figure 9 were generated from parameter values given in Table 2. As provided by eqs 13 and 14 of Supporting Information, K( is the zwitterion adsorption equilibrium coefficient and mB° is the initial concentration of sites available for adsorption of zwitterions. Also included in Table 2 is the half-width of the 95% confidence interval associated with each parameter estimate. Figures 10 and 11 present modeling results for H/Na-Y:8’s low-pH isotherm and saturation adsorption with varying pH. The fact that measured mPheT is greater than any predicted mPhe( offers support, independent of the NaCl study, for the argument that cationic phenylalanine adsorption is occurring for pH < 9. Values for the ion-exchange parameters are provided in Table 3. As provided by eqs 6 and 7 of Supporting Information, KPhe+ and KNa+ are selectivity coefficients describing the ion exchanges H+ + S h + Phe+ T H+ + Phe+ and H+ + Na+ T H+ + Na+. The above results indicate that both cationic and zwitterionic phenylalanine are more strongly adsorbed by zeolite β than zeolite Y.
Figure 11. Modeling saturation adsorption with varying pH for phenylalanine on zeolite Na-Y:8. (2) Measured mPheT, (s) calculated mPheT, and (---) calculated mPhe(.
The difference in selectivity coefficients remains if we consider the possibility that cationic adsorption occurs at pH > 9. Even in the extreme case that adsorption at pH > 9 is entirely cationic in nature, the resulting selectivity coefficients (not shown) are still at least 2 orders of magnitude smaller for zeolite Y:8 than for zeolite β. Competitive Adsorption. The above-described exclusion of phenylalanine from zeolites X and Y:2.5 suggests that these materials may provide a highly selective separation of arginine from phenylalanine. Experiments have been carried out with a mixture of these amino acids to gain some understanding of competitive adsorption behavior. A model has not been developed for describing these results, however. In what follows, adsorption selectivity is defined as (mArgT/mArgT)/(mPheT/mPheT). Results are presented first for separations using zeolite β (see Table 4). The data for H-β:12 demonstrate dependence of selectivity on both solution amino acid concentration and pH. Reducing the amount of initially added arginine decreases mArgT by 2 orders of magnitude (see samples 9 and 14 of Table 4), but mArgT decreases by less than half. This change is primarily responsible for the increase in selectivity by 2 orders of magnitude. Addition of sodium hydroxide to increase pH (see samples 8 and 9 of Table 4) affects mPheT/mPheT more than mArgT/mArgT, producing an increase in selectivity. This effect of pH on selectivity is suggested, qualitatively, by single-amino-acid adsorption results where the ideal selectivity is maximized at high pH (see Figure 12). Samples 1, 3, 5, and 15 of Table 4 demonstrate the increase in amino acid adsorption selectivity, for H-β, with increasing aluminum content (see Figure 13). These samples have similar equilibrated solution amino acid concentrations, have a narrow distribution of pH conditions, and have not been pH-adjusted with acids or bases. This relationship between selectivity and aluminum content suggests one contributing factor behind the extreme selectivities observed with high-aluminum-content faujasite-type materials (see Figure 13 and Table 5). That zeolite β’s selectivity does not depend on aluminum content, between Si/Al ) 38 and 100, suggests that aluminum concentration is only affecting the selectivity of the cation adsorption mechanism.
Adsorption Zeolites X, Y, and β
Langmuir, Vol. 22, No. 22, 2006 9355 Table 3. Parameter Estimates for Cationic Phenylalanine Adsorption ms°
KPhe+ zeolite type
est (mmol/g)-1
H/Na-Y:8 H-β:127 H-β:1007
1.4 200
1
/2(95% CI) (mmol/g)-1
est (mmol/g)
1.2 100
0.64 0.47 0
KNa+ 1
/2(95% CI) (mmol/g)
est
0.12 0.03
2 × 10-6 3 × 10-3
1
/2(95% CI) 1 × 10-6 1 × 10-3
Table 4. Competitive Adsorption of Mixed Amino Acids on Zeolite β: Phenylalanine and Arginine sample name 1 2 3 4 5 6 7 8b 8-Rb 8-Rb 8-Rb 9 10 11 12 13 14 15
zeolite type:Si/Al
final Phe soln concn (mol/L)
Phe adsorbed (mmol/g)
final Arg soln concn (mol/L)
Arg adsorbed (mmol/g)
H-β:9 H-β:9 H-β:37.5 H-β:37.5 H-β:100 H-β:100 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12 H-β:12
8.29 × 8.31 × 10-3 8.93 × 10-2 1.36 × 10-2 8.24 × 10-2 1.05 × 10-2 5.45 × 10-3 7.94 × 10-3 7.26 × 10-3 7.10 × 10-3 7.30 × 10-3 1.65 × 10-2 5.52 × 10-2 6.09 × 10-2 6.27 × 10-2 5.92 × 10-2 5.79 × 10-2 8.21 × 10-2
5.35 × 2.50 × 10-1 4.11 × 10-1 1.60 × 10-1 5.45 × 10-1 2.12 × 10-1 3.03 × 10-1 2.55 × 10-1 2.67 × 10-1 2.70 × 10-1 2.67 × 10-1 1.01 × 10-1 4.05 × 10-1 3.03 × 10-1 2.72 × 10-1 3.35 × 10-1 3.60 × 10-1 5.49 × 10-1
9.07 × 1.70 × 10-4 7.44 × 10-3 3.57 × 10-3 5.04 × 10-3 1.22 × 10-3 7.71 × 10-3 3.20 × 10-4 2.59 × 10-4 2.35 × 10-4 2.70 × 10-4 5.52 × 10-4 1.25 × 10-1 1.12 × 10-1 1.03 × 10-1 9.76 × 10-2 9.71 × 10-2 1.15 × 10-3
4.66 × 4.79 × 10-1 3.42 × 10-1 4.18 × 10-1 3.89 × 10-1 4.63 × 10-1 3.41 × 10-1 4.77 × 10-1 4.78 × 10-1 4.78 × 10-1 4.78 × 10-1 4.72 × 10-1 4.08 × 10-1 6.46 × 10-1 8.01 × 10-1 8.95 × 10-1 9.07 × 10-1 4.59 × 10-1
10-2
10-1
10-4
10-1
pHa
selectivity
6.48 6.18 7.95 8.16 8.02 8.22 (4.37) 6.65
79.6 94.0 10.0 10.0 11.7 18.7 0.8 46.3 50.1 53.4 48.4 139.9 0.4 1.2 1.8 1.6 1.5 59.6
(9.36) (3.95) (5.38) (6.70) (7.99) 9.37 6.69
a Parentheses around pH values indicates pH was obtained by addition of HCl or NaOH, all other samples are free of acid/base adjustments. Samples labeled 8-R are reproductions of sample 8. The standard deviation of the four selectivities is 3.0. The pH was not measured for these reproductions.
b
Table 5. Competitive Adsorption of Mixed Amino Acids on Zeolites X and Y: Phenylalanine and Arginine sample name 1 2 3 4 5 6 7
zeolite type:Si/Al
final Phe soln concn (mol/L)
Phe adsorbed (mmol/g)
final Arg soln concn (mol/L)
Arg adsorbed (mmol/g)
pH
selectivity
H/Na-X:1.2 H/Na-Y:2.5 H/Na-Y:2.5 H/Na-Y:2.5 Na-Y:8 Na-Y:8 H/Na-Y:8
2.27 × 2.21 × 10-2 2.21 × 10-2 2.16 × 10-2 1.36 × 10-2 6.97 × 10-2 1.19 × 10-2
3.90 × 8.16 × 10-3 8.79 × 10-3 1.85 × 10-2 1.94 × 10-1 1.79 × 10-1 2.14 × 10-1
3.66 × 1.10 × 10-4 1.54 × 10-4 1.70 × 10-4 3.22 × 10-3 1.02 × 10-1 1.88 × 10-3
4.13 × 1.01 0.99 1.13 5.11 × 10-1 9.69 × 10-1 4.98 × 10-1
8.70 7.17 7.17 7.15 8.21 9.24 7.99
657 24 884 16 208 7718 11.2 3.7 14.8
10-2
10-3
We have previously reported7 that zeolite β adsorbs phenylalanine mainly in its zwitterionic state when the zeolite’s Si/Al ) 100. Of the faujasite-type materials tested, the one with the intermediate Si/Al, H/Na-Y:2.5, produces the highest selectivity (see Table 5). Recall, from the above single-amino-acid experiments, that this intermediate Si/Al material adsorbed the most arginine and largely excluded phenylalanine. As with singleamino-acid adsorption, there is little difference between adsorption
Figure 12. Single-amino-acid adsorption and ideal selectivity for arginine and phenylalanine on H-β:12. (2) Measured mArgT,7 (9) measured mPheT,7 (s) ideal selectivity, and (0) measured mixedamino-acid selectivity. Ideal selectivity is defined as given above, with the individual concentrations obtained from the single-aminoacid adsorption model.
10-3
10-1
performance of H/Na-Y:8 and Na-Y:8 (see samples 5 and 7 of Table 5). These results reinforce the idea that, with water as the solvent, arginine can access adsorption sites on as-synthesized faujasite-type materials that are not available to phenylalanine.
Conclusion Faujasite-type materials share many common features with zeolite β with respect to adsorption of amino acids. With a
Figure 13. Adsorption selectivity, for arginine versus phenylalanine, as a function of aluminum content in zeolites β and Y. Standard deviation for selectivity is 3.
9356 Langmuir, Vol. 22, No. 22, 2006
framework Si/Al value of approximately 8, zeolites Y and β exhibit similar adsorption capacities under a wide range of pH conditions. However, some notable differences exist between the zeolites. For adsorption of phenylalanine, adsorbate/adsorbent interactions appear significantly weaker with H/Na-Y:8 than with zeolite β. At sufficiently high aluminum content, faujasite-type materials show little capacity for adsorption of phenylalanine while the adsorption of arginine remains similar to that on zeolite β. Large selectivities are observed when the two amino acids are mixed. The rapid growth of industrial amino acid production and the variety of prospective uses for amino acid and peptide products justifies continued work toward improvement of production technology. Improving our understanding of interactions between amino acids and silicate materials may also prove useful in related
Krohn and Tsapatsis
studies such as the templating of new porous materials and the study of reactions in confined spaces. Acknowledgment. Funding for this work was provided by NSF (CTS-05 22518). All characterizations were performed at the Minnesota Characterization Facility, which receives support from NSF through the National Nanotechnology Infrastructure Network (NNIN). TEM images in this document were produced by Sandeep Kumar of the University of Minnesota. Supporting Information Available: Relevant figures and equations have been reproduced from Krohn, J. E.; Tsapatsis, M. Langmuir 2005, 21, 8743-8750. This material is available free of charge via the Internet at http://pubs.acs.org. LA061743M
