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Amino acids, precursors for cationic and anionic intercalationsynthesis and characterization of amino acid pillared materialsA. Fudalaa, I. Pa linko b,*, I. KiricsiaaApplied Chemistry Department, Jo zsef Attila University, Rerrich B. te r 1, Szeged, H-6720 HungarybDepartment of Organic Chemistry, Jo zsef Attila University, Do m te r 8, Szeged, H-6720 HungaryReceived 24 August 1998; received in revised form 16 November 1998; accepted 16 November 1998AbstractThe preparation and characterization of amino acid pillared materials are reported in this contribution. Host substances wereNa-montmorillonite for cationic and hydrotalcite for anionic pillaring. Guest molecules were L-phenylalanine and L-tyrosine.The pillared materials were characterized by powder X-ray diffraction, BET measurements and FTIR spectroscopy. Pillaringwas successful: the layers propped open and the basal distances increased significantly. For hydrotalcite this increase wasalways significantly larger than for montmorillonite. This fact indicated that the spatial arrangement of the amino acid moietiesis widely different. A model for this arrangement is given. q1999 Elsevier Science B.V. All rights reserved.Keywords: Intercalation; L-tyrosine; L-phenylalanine; Na-montmorillonite; Zn-Al hydrotalcite; Synthesis; Characterization; Model for spatialarrangement1. IntroductionPillared layer materials find application in separa-tion science as well as in fine chemical industry 1.They are of two main types depending on the methodof intercalation. Certain bulky organic or inorganiccations and anions may be used as pillaring agentsvia cation and anionexchange, respectively. Inorganicpolyhydroxy cations are used for preparing pillaredlayer materials, most frequently pillared layer clays(PILCs) 2. Ion exchange is straightforward here,however, direct pillaring of layered double hydro-xides (LDHs) with bulky inorganic anions is rarelypossible 3. Generally, bulky organic anions usedfirst for propping the layers open, then follows theexchange with inorganic anions 4. Of course, thissecond step need not be carried out, LDHs pillaredwith organic anions may be of interest just as PILCswhen the organic cation is the intercalated agent. It isespecially so, if the pillars are chiral, as these quasitwo-dimensional channel systems are not only welldefined structures with huge pore openings, but theoptically active pillars create a chiral environment.Materials like these may easily find use in chiralseparation just as in enantioselective catalysis. Inview of fundamental research it is an added bonus ifthe same compound can be used for cationic as well asanionicintercalation.Aminoacidscanbethecompounds of choice as, depending on the preparationof pH, they can be cations or anions. Moreover, theirL enantiomers are easily accessible.In this article we report on the preparation andcharacterization of amino acid pillared materials.The host substances are Na-montmorillonite forcationic and ZnAl hydrotalcite for anionic pillaring.Journal of Molecular Structure 482483 (1999) 33370022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00835-7* Corresponding author. Fax: 0036 62 425 523.E-mail address: palinkochem.u-szeged.hu (I. Pa linko )TheguestmoleculesareL-phenylalanineandL-tyrosine.2. ExperimentalGuest materials, L-phenylalanine (pI ? 5.5) andL-tyrosine (pI ? 5.7), were obtained from Aldrich andwere used as received.One of the host substances, Na-montmorillonite(Na-mont), was a commercial product:Bentolit H, SCP Laport, ion-exchange capacity:80 meq/100 g. The other, ZnAl hydrotalcite (HT)was prepared on the basis of Taylors method 5.The pH of 250 cm30.1 mol/dm3Al(NO3)3solutionwas adjusted to 7.0 with the addition of the appro-priate amount of a solution containing 0.8 molNa2CO3and 1.6 mol NaOH in 1 dm3water. After stir-ring for 1 h, 250 cm30.3 mol/dm3Zn(NO3)2solutionwas added dropwise. The pH of the slurry was kept at6.0 6.5 with adding appropriate amount of the alka-line solution described above. After the completedelivery of Zn21solution, the slurry was boiled forone week under reflux. The gel formed was auto-claved at 458 K for 18 h in a teflon-coated container.Double-distilled, chloride-free water and inert atmo-sphere (Ar) were used throughout the synthesis.Cationic pillaring was carried out at pH ? 4, whilefor anionic intercalation the pH was set to 8. Na-montwas preswollen in double-distilled water for 24 h andthe pH of the slurry was set to 4 by 0.1 mol/dm3HClsolution. Then, the aqueous solution of the amino acidwas mixed to it under vigorous stirring at roomtemperature. The suspension was stirred for an addi-tional day. Then, the intercalated material was filteredoff and dried in air. A HT aqueous suspension wasprepared and its pH was set to 8 by 0.1 mol/dm3NH4OH solution. Then, the aqueous solution of theamino acid was mixed to it under vigorous stirring atroom temperature. The temperature was raised to333 K and the suspension was stirred for an additionalday. Finally, the intercalated material was filtered offand dried in air.XRD measurements were performed on a DRON 2powder X-ray diffractometer using the Cu-Ka radia-tion. Diffractograms of the air-dried samples wereregistered. Basal spacings were determined from theposition of the d(0 0 1) reflection (Na-mont andL-amino acidNa-mont) or the d(0 0 3) reflection (HTand the L-amino acid-HT). Data are in the secondcolumn of Table 1.BET measurements were performed in a conven-tional volumetric adsorption apparatus at the tempera-ture of liquid nitrogen (77 K). Before measurementsthe samples were evacuated for 3 h, generally at393 K. For L-Phe-montmorillonitevarious othertemperatureswerealsoapplied(473,573and673 K). Data are in the third column of Table 1.The KBr pellettechnique (1 mgofsample in 200 mgof KBr) was applied for monitoring changes in the IRspectra of the samples. The FTIR spectra of the hostandguest materialsandthehostguestcomplexesweretaken and compared. The 4000 500 cm21range wasinvestigated. Spectra were recorded by a MattsonGenesis I spectrophotometer with 1 cm21resolution.The 4000 cm21500 cm21range was investigated.A schematic model for visualizing the possiblespatial arrangement of the amino acid (protonated ordeprotonated) was built with the help of the MM 1force field method included in the Hyperchem 4.5package 6. The protonated and deprotonated deriva-tives of L-phenylalanine were optimized with thismethod and the size of the ions were measured andused in constructing the model.3. Results and discussionXRD measurements made clear that pillaring wasA. Fudala et al. / Journal of Molecular Structure 482483 (1999) 333734Table 1Characteristic data on the host materials and the intercalated struc-turesSamplesBasal spacinga/nmBET area /m2g21Na-montmorillonite1.2058.6 (393 K)bL-Tyr-montmorillonite1.4671.1 (393 K)L-Phe-montmorillonite1.4993.5 (393 K)40.6 (473 K)42.1 (573 K)29.8 (673 K)Hydrotalcite0.8983.0 (393 K)L-Tyr-hydrotalcite1.75101.2 (393 K)L-Phe-hydrotalcite1.8096.4 (393 K)aCalculated from the d(0 0 1) reflection for the montmorilloniteand the d(0 0 3) reflection for the hydrotalcite derivatives.bTemperature of evacuation for 3 h.successful. The layers propped open and the basaldistances increased significantly. For hydrotalcitethis increase was always significantly larger than formontmorillonite. This fact is an obvious indicationthat the spatial arrangement of the amino acidmoieties is very different.The FTIR spectra of the guest materials the parentamino acids from which the pillaring ions are derivedand the pillared substances are collected in Figs. 1and 2.When the spectra of L-Phe, L-Phe-mont and Na-mont are compared only slight differences can beobserved. In the OH stretching region the secondband at 3410 cm21became more pronounced in theA. Fudala et al. / Journal of Molecular Structure 482483 (1999) 333735Fig. 1. FTIR spectra of the air-dried montmorillonite and hydrotalcite intercalated L-phenylalanine, protonated and deprotonated, respec-tively, the guest substances and the pure amino acid (KBr technique).intercalated material than it was in the host Na-mont,just as the band at 1643 cm21. This latter one coin-cides with an intense band of L-Phe-HT and the pureL-Phe sample and does not appear in the amino acidfree HT spectrum. The appearance of this band maybe taken as the sign of successful intercalation. Thereare no significant differences in the OH region of thespectrum for L-Phe-HT and HT indicating that thedeprotonated form takes part in the pillaring process.However, another sharp band at 1007 cm21is seen inthe spectrum of L-Phe-HT and it is not in that of HT.This band again coincides with a band in the spectrumof pure Phe.Very similar observations may be made when theA. Fudala et al. / Journal of Molecular Structure 482483 (1999) 333736Fig. 2. FTIR spectra of the air-dried montmorillonite and hydrotalcite intercalated L-tyrosine, protonated and deprotonated, respectively, theguest substances and the pure amino acid (KBr technique).spectra of LT-yr-mont, Na-mont, L-Tyr-HT, HT andL-Tyr are compared (Fig. 2). Here, again, the band at3410 cm21indicating the presence of the carboxylicgroup becomes intense in L-Tyr-mont. Moreover, thefine structure of the band in the 16501100 cm21range closely resembles that portion in the spectrumof L-Tyr. In the OH region the spectra of L-Tyr-HTand HT are also very similar: obviously the carbox-ylate form was intercalated. The intense peaks in thespectrum of L-Tyr-HT at higher wavenumbers than1400 cm21are signs of successful intercalation aswell. They are not observed in the spectrum of HT.Differences in basal distances and the significantalterations in the spectra of cation pillared and anionintercalated structures clearly show that protonatedand deprotonated forms are arranged differentlybetween the layers of the guest materials. Force field(MM1) calculations gave numerical data as far asionicdimensionsoftheaminoacidionsareconcerned. Approximate data for the minimum struc-tures of protonated and deprotonated L-Phe (theywere used for model building) are 0.53 nm (width),0.79 nm (length) and 0.54 nm (width), 0.79 nm(length), respectively. Interlayer spacings for L-Phe-HT and L-Phe-mont are 1.32 nm and 0.53 nm, respec-tively. From these data a first approximation modelfor the spatial arrangement of the anionic or cationicpillars may be given in Scheme 1.4. ConclusionsAmino acids in their protonated and deprotonatedforms can be intercalated between the layers