Interactions of l-Alanine with Alumina as Studied by Vibrational

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Interactions of L-Alanine with Alumina as Studied by Vibrational Spectroscopy Ana R. Garcia,†,‡ Ricardo Brito de Barros,† Alexandra Fidalgo,† and Laura M. Ilharco*,† Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´ cnico, AV. RoVisco Pais, 1, 1049-001 Lisboa, Portugal, and Departamento de Quı´mica, Bioquı´mica e Farma´ cia, FCT, UniVersidade do AlgarVe, Campus de Gambelas, 8000 Faro, Portugal ReceiVed May 18, 2007. In Final Form: June 27, 2007 The interactions of L-alanine with γ- and R-alumina have been investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). L-Alanine/alumina samples were dried from aqueous suspensions, at 36.5 °C, with two amino acid concentrations (0.4 and 0.8 mmol g-1) and at different pH values (1, 6, and 13). The vibrational spectra proved that the nature of L-alanine interactions with both aluminas is the same (hydrogen bonding), although the groups involved depend on the L-alanine form and on alumina surface groups, both controlled by the pH. For samples prepared at pH 1, cationic L-alanine [CH3CH(NH3+)COOH] displaces physisorbed water from alumina, and strong hydrogen bonds are established between the carbonyl groups of alanine, as electron donors, and the surface Al-OH2+ groups of alumina. This occurs at the expense of alanine dimer dissociation and breaking of intramolecular bonds. When samples are prepared at pH 6, the interacting groups are Al-OH2+ and the carboxylate groups of zwitterionic L-alanine [CH3CH(NH3+)COO-]. The affinity of L-alanine toward alumina decreases, as the strong NH3+‚‚‚-OOC intermolecular hydrogen bonds prevail over the interactions with alumina. Thus, for a load of 0.8 mmol g-1, phase segregation is observed. On R-alumina, crystal deposition is even observed for a load of 0.4 mmol g-1. At pH 13, the carboxylate groups of anionic L-alanine [CH3CH(NH2)COO-] are not affected by alumina. Instead, hydrogen bond interactions occur between NH2 and the Al-OH surface groups of the substrate. Complementary N2 adsorption-desorption isotherms showed that adsorption of L-alanine occurs onto the alumina pore network for samples prepared at pH 1 and 13, whereas at pH 6 the amino acid/alumina interactions are not strong enough to promote adsorption. The mesoporous structure and the high specific surface area of γ-alumina make it a more efficient substrate for adsorption of L-alanine. For each alumina, however, it is the nature of the specific interactions and not the porosity of the substrate that determines the adsorption process.

1. Introduction The interactions of amino acids with inorganic compounds, such as metal oxides, are believed to have played a relevant role in the formation of the first peptides, since the condensation reactions between amino acids are known to occur only in the presence of catalysts, under the conditions presumed on primitive earth.1 In particular, activated alumina is a highly efficient catalyst for peptide bond formation.2 Even amino acids that are nonreactive on silica or clay minerals surfaces react efficiently on activated alumina, under mild conditions (≈400 K).3 The amino acid/ alumina interactions are also of interest in the fields of soil chemistry (in the study of groundwater systems contamination)4 and prosthetic medicine (in the rejection processes of artificial shoulder and hip implants).5 On alumina-supported metal catalysts, when it is important to understand the amino acidmetal reaction mechanism, the amino acid/alumina interactions are a perturbation to be avoided, and so deserve also a great deal of attention. Alumina (Al2O3) is the common designation for quite different materials that may be produced by thermal treatment of Al * Corresponding author. Tel: +351-218419220. Fax: +351-218464455. E-mail: [email protected]. † Instituto Superior Te ´ cnico. ‡ Universidade do Algarve. (1) Lahav, N. Heterogen. Chem. Ver. 1994, 1, 159. (2) Bujda´k, J.; Rode, B. M. J. Therm. Anal. Calorim. 2003, 73, 797. (3) Bujda´k, J.; Rode, B. M. Catal. Lett. 2003, 91, 149. (4) Fitts, J. P.; Persson, P.; Brown, G. E., Jr.; Parks, G. A. J. Colloid Interface Sci. 1999, 220, 133. (5) Sargeant, A.; Goswami, T. Mater. Des. 2006, 27, 287.

hydroxides [Al(OH)3, gibbsite, bayerite, and norstrandite].6 The thermodynamically most stable phase of alumina is corundum or R-Al2O3, which crystallizes in the hexagonal system and is the ultimate product of the thermal treatment (above 1400 K).7,8 Aluminum monohydrates (AlOOH, boehmite, and pseudoboehmite) are obtained at relatively low temperatures (353-723 K). At intermediate temperatures (750-1370 K), several transition alumina phases may result: the so-called “low temperature” (η and γ-Al2O3) and “high temperature” (δ and θ-Al2O3) phases.9 All of these metastable aluminas crystallize in the cubic system, with defective spinel-type structure (where some of the lattice hollow sites in the cubic close packed array of oxygens are vacant, for sake of electrical neutrality, since the cations are exclusively Al3+).10 They are the only catalytically active aluminas, showing a wide scope of reactivities due to the presence of Lewis acid and basic sites (Al3+ and O2- ions, respectively). Catalytic reactions or adsorption on these materials may be understood as acid-base reactions or interactions, the surface behaving as a solid acid or base. In aqueous suspensions, hydroxylated alumina is formed, and the acid-base properties may be interpreted in terms of proton adsorption and desorption at the aluminol functional surface (6) Pecharromań, C.; Sobrados, I.; Iglesias, J. E.; Gonza´lez-Carrenõ, T.; Sanz, J. J. Phys. Chem. B 1999, 103, 6160. (7) Busca, G.; Lorenzelli, V.; Ramis, G.; Willey, R. J. Langmuir 1993, 9, 1492. (8) Cava, S.; Tebcherani, S. M.; Pianaro, S. A.; Paskocimas, C. A.; Longo, E.; Varela, J. A. Mater. Chem. Phys. 2006, 97, 102. (9) Nortier, P.; Fourre, P.; Mohammed Saad, A. B.; Saur, O.; Lavalley, J. C. Appl. Catal. 1990, 61, 141. (10) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497.

10.1021/la701467y CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

Interactions of L-Alanine with Alumina

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groups, as follows:11

≡AlOH(aq) + H+(aq) a ≡AlOH2+(aq) log10 K1 ) 7.51 (γ-Al2O3) /6.55 (R-Al2O3) 12

(1) 13

≡AlOH(aq) a ≡AlO-(aq) + H+(aq) (2) 12 13 log10 K2 ) -8.87 (γ-Al2O3) /-12.25 (R-Al2O3) Therefore, the surface chemical properties of alumina in aqueous suspensions (species and charge) are a function of pH: AlOH2+ may be the unique species present for γ and R-Al2O3 (for pH below ∼3.5), it may be predominant but with some fraction of the neutral form AlOH (for pH ∼7), or AlO- may be largely predominant (for pH ∼13). A detailed surface speciation has been obtained for mesoporous and particulate γ-aluminas by pH titration.14 The species involved in the surface protonation reactions [AlOH2+, AlOH, and AlO-] are assumed based on Pauling bond valence principles, as a simplification of the surface chargingspecies: [≡Al(OH2)2]+,[≡AlOH(OH2)],and[≡Al(OH)2]-, respectively.13 In the absence of specific interactions with the electrolyte ions, the point of zero charge (PZC) of alumina is well determined [0.5×(pKa1 + pKa2)], and the surface is positively or negatively charged depending on whether pH is lower or higher than the PZC.15 However, specific interactions with electrolyte cations shift the PZC toward lower pH values, whereas those with anions shift it to higher pH values. In addition to the acid-base chemistry, dissolution of alumina, without surface charge effects, may occur according to eqs 3 and 4 for low pH values and pH above 6, respectively16

0.5 Al2O3(s) + 3H+ a Al3+ + 1.5 H2O

(3)

0.5 Al2O3(s) + 1.5 H2O + OH- a Al(OH)4-

(4)

L-Alanine is the simplest chiral amino acid present in nature and is typically chosen as a good model for studying biological systems that exhibit peptidic bonding. The crystalline structure of L-alanine was determined by X-ray diffraction17 and by neutron diffraction18 as orthorhombic, with four molecules per unit cell (space group P212121 or D2). Like other amino acids, in the solid-state L-alanine is zwitterionic [CH3CH(NH3+)COO-], with intermolecular hydrogen bonds between the NH3+ and COO- groups.19,20 It was shown that the three N-H bonds are not equivalent, due to the involvement in hydrogen bonds of unequal strength. In the gas phase and in inert matrix, L-alanine is neutral [CH3CH(NH2)COOH] and may occur as different rotamers, some of them stabilized by intramolecular H bonds.21,22 The geometries of the three most stable conformers were assessed by density

(11) Dyer, C.; Hendra, P.; Forsling, W.; Ranheimer, M. Spectrochim. Acta A 1993, 49, 691. (12) Laiti, E.; O ¨ hman, L-O.; Nordin, J.; Sjo¨berg, S. J. Colloid Interface Sci. 1995, 175, 230. (13) Johnson, S. B.; Yoon, T. H.; Kocar, B. D.; Brown, G. E., Jr. Langmuir 2004, 20, 4996. (14) Wang, Y.; Bryan, C.; Xu, H.; Pohl, P.; Yang, Y.; Brinker, C. J. J. Colloid Interface Sci. 2002, 254, 23. (15) Kosmulski, M. J. Colloid Interface Sci. 2006, 298, 730. (16) Lindsay, W. L. Chemical Equilibria in Solids; J. Wiley & Sons: New York, 1979. (17) Destro, R.; Marsh, R. E. J. Phys. Chem. 1988, 92, 966. (18) Lehman, M. S.; Koetzle, T. F.; Hamilton, W. C. J. Am. Chem. Soc. 1972, 94, 2657. (19) Wang, C. H.; Storms, R. D. J. Chem. Phys. 1971, 55, 3291. (20) Cao, X.; Fischer, G. Chem. Phys. 2000, 255, 195. (21) Iijima, K.; Nakano, M. J. Mol. Struct. 1999, 485-486, 255. (22) Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Adamowicz, L. J. Phys. Chem. A 1998, 102, 4623.

functional theory (DFT) calculations.23 In aqueous solution, L-alanine has two acid-base equilibria, with pKa 2.35 and 9.87, at 25 °C.24 Overlooking the buffer effect, below pH 2, more than 70% of the carboxyl group is protonated and L-alanine is cationic [CH3CH(NH3+)COOH], whereas above pH 11 more than 90% of the amino groups are deprotonated and L-alanine becomes anionic [CH3CH(NH2)COO-]. At intermediate pH values (58), more than 99% of L-alanine is stable in the zwitterionic form. The interaction of L-alanine with a solid matrix, such as alumina, is determined not only by the stable amino acid form but also by the chemical nature (active sites) and structure of the matrix surface. This has been well supported by an adsorption study of L-alanine on bohemite at room temperature, at different pH values (2.5, 6.0, and 12.2).25 It is the aim of the present work to characterize the interactions between L-alanine and two aluminas by diffuse reflectance infrared spectroscopy (DRIFTS). The infrared spectrum of alumina has been well discussed in the literature.10,26 The skeletal region (600-900 cm-1) allows distinguishing between different Al2O3 phases: the strong Al-O-Al stretching mode of condensed AlO6 octahedra occurs in the range 600-750 cm-1, whereas the stretching modes of a lattice of condensed or interlinked AlO4 tetrahedra appear between 750 and 900 cm-1. The presence of surface hydroxyl groups and their hydrogen bonding to adsorbed water has also been discussed through the analysis of the position and shape of the OH stretching band (above 2600 cm-1).27,28 The vibrational characterization of L-alanine in the neutral and zwitterionic forms has been the object of theoretical and experimental approaches.19,20,22,23 The frequencies calculated by DFT for the different conformers of neutral L-alanine correlated successfully with the experimental values obtained in lowtemperature Ar matrix.23 The vibrational modes of zwitterionic L-alanine are affected by the solvent, shifting with respect to the neutral gas and solid phases. This effect is stronger for groups involved in specific interactions, such as NH3+. Namely, the N-H stretching modes undergo a downshift of ∼500 cm-1 with respect to those of the free monomer, and theoretical data point to one N-H bond involved in intramolecular hydrogen bonding, whereas the other two interact with water molecules.29 To fulfill the purpose of the present work, the amino acid/ alumina interactions were maximized by drying aqueous suspensions of L-alanine/Al2O3 prepared at pH 1, 6, and 13, at 36.5 °C, and containing different loads of amino acid. These pH values were meant to isolate the different forms of L-alanine (cationic, zwitterionic, and anionic, respectively), and to favor predominant alumina surface species (AlOH2+, at pH 1 and 6, and AlO- at pH 13), clearly apart from the PZC of alumina. From the DRIFT results and the complementary information obtained from nitrogen adsorption-desorption isotherms, it was proved that, depending on the pH, the L-alanine/alumina interactions may range from strong intermolecular hydrogen bonds, involving specific groups of L-alanine and of alumina, to extremely weak, resulting in phase segregation. (23) Lambie, B.; Ramaekers, R.; Maes, G. Spectrochim. Acta A 2003, 59, 1387. (24) Smith, P. K.; Taylor, A. C.; Smith, E. R. B. J. Biol. Chem. 1937/38, 122, 109. (25) El Shafei, G. M. S.; Philip, C. A. J. Colloid Interface Sci. 1997, 185, 140. (26) Tarte, P. Spectrochim. Acta A 1967, 23, 2127. (27) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: New York, 1986; Vols. I and II. (28) Socrates, G. Infrared and Raman Characteristic Group Frequencies, 3rd ed.; J. Wiley & Sons: New York, 2001. (29) Gontrani, L.; Mennucci, B.; Tomasi, J. J. Mol. Struct. (THEOCHEM) 2000, 500, 113.

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2. Experimental Section Aqueous solutions (∼0.1 M) of L-alanine (Aldrich, 99%) were prepared at pH 1, 6, and 13, using as solvent a 0.15 M solution of HCl (Aldrich Fixanal 0.5 M), bidistilled water, and a 0.15 M solution of NaOH (Aldrich, 97+%), respectively. The Al2O3 powders were Alfa Aesar (activated, neutral, gamma, 99.9%) and Metrohm (electrode polishing alumina, 99.99%). The specific surface areas of the two aluminas were determined from adsorption-desorption isotherms of N2 at 77 K, as 126 and 25 m2 g-1, respectively. The L-alanine/alumina suspensions were prepared by stirring ∼0.1 g of alumina with the appropriate volume of each amino acid solution (125 g alumina dm-3), in order to obtain concentrations of 0.4 and 0.8 mmol of L-alanine per g of substrate. These suspensions were kept at 36.5 °C, in an incubator with continuous orbital stirring (100 rpm), until complete solvent evaporation (∼96 h). The residual water was subsequently removed, at the same temperature, in a vacuum oven (at ∼10-4 mbar), for 24 h. Following the same procedure at each pH, pure alumina was dried from a reference suspension and pure L-alanine was reprecipitated from solution. The dried L-alanine samples were visually different, ranging from needle-like crystals (pH 1) to a fine powder (pH 6) and a gel (pH 13). The structure of the alumina, L-alanine, and L-alanine/alumina dried samples was analyzed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The sample preparation consisted in grinding a mixture of KBr (Sigma-Aldrich, FTIR grade) and each sample powder, in appropriate weight proportions to obtain spectral absorbance in the range of applicability of the Kubelka-Munk transformation.30 The amount of sample was sufficient to be considered of infinite thickness. The DRIFT spectra were collected at 4 cm-1 resolution, with a Mattson Research Series 1 FTIR spectrometer, using a wide-band MCT detector (4000-400 cm-1) and a Graseby/Specac Selector (with specular reflection blocker). The spectra were the ratio of 500 single beam scans of the sample to the same number of background scans for pure KBr. No baseline corrections were made. The diffuse reflectance spectra were transformed to Kubelka-Munk units using the FIRST software. The specific surface area and pore structure of all the samples were examined by N2 adsorption-desorption isotherms at 77 K, with 5 s equilibration times, using an ASAP 2000, Micromeritics. The samples were weighed and outgassed (5 µm Hg) for 12 h at 36 °C prior to analysis. Higher temperatures were avoided to preserve the adsorbate. The specific surface areas were estimated by Brunnaer-Emmett-Teller (BET)31 analysis of the adsorption isotherms in the relative pressure, p/p0, range of 0.05 to 0.3, using a N2 cross-sectional area of 16.2 Å.2 The specific pore volumes were retrieved from the adsorption isotherms at a single condensation point (p/p0 ) 0.995). The pore size distributions were calculated from the desorption branch of the isotherms using the BarrettJoyner-Halenda (BJH) algorithm, assuming cylindrical pore shapes.32

3. Results and Discussion 3.1. L-Alanine Vibrational Analysis. The DRIFT spectra of crystalline L-alanine, as purchased, and after precipitation from an aqueous solution at pH 6 (Figure 1) are very similar, and characteristic of the zwitterionic form [CH3CH(NH3+)COO-].20,33 This is clearly evidenced by the absence of the νOH and νCdO modes (expected at 3560-3590 and 1715-1790 cm-1, respectively, for the neutral form22,34) and by the observation of the νasOCO-, at 1621 cm-1, and the νsOCO-, at 1360 cm-1, in good agreement with previous experimental results and Hartree-Fock (30) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (31) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (32) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (33) Rozenberg, M.; Shoham, G.; Reva, I.; Fausto, R. Spectrochim. Acta A 2003, 58, 3253. (34) Cao, X.; Fischer, G. Spectrochim. Acta A 1999, 55, 2329.

Figure 1. DRIFT spectra of L-alanine as purchased (crystals) and after precipitation from aqueous solution at the indicated pH. The spectra were normalized to the maximum.

(HF/SCRF) calculations.34 Because of the low symmetry of L-alanine and of the strong intermolecular hydrogen bonding between the amino (-NH3+) and the carboxylate (-COO-) groups, the number of observable modes is rather large. In the 4000-2000 cm-1 region, despite the complexity due to intermode and intermolecular couplings, there are bands associated with the νNH3+ fundamentals (3080 and 2988 cm-1) and the νCH3 antisymmetric (2940 cm-1) and symmetric (∼2888 cm-1) modes. In the C-H deformation region, the δasCH3 (1456 cm-1) and the δCH (1307 cm-1) bands are well defined. The absence of NH3+ stretching modes in the range 3140-3314 cm-1 is an indication that there are no L-alanine monomers.20 The downward shift and broadness of the νNH3+ band suggest significant hydrogen bonding involving amino and carboxylate groups, with formation of a three-dimensional network.18 The strong νCdO band at 1723 cm-1, and the νOH mode at 3203 cm-1, observed for L-alanine obtained from a pH 1 solution, are characteristic of the cationic form [CH3CH(NH3+)COOH] (expected to exceed 96% in solution at this pH). The downward shifts of these two bands, with respect to neutral monomeric alanine,34,35 point to the predominance of L-alanine dimers. The strong νC-O mode, at 1197 cm-1, is an additional indication for cationic L-alanine (this mode appears at 1192 cm-1 in the neutral form34). The spectrum of L-alanine precipitated from the solution at pH 13 shows a broad band at 3322 cm-1, assignable to the νasNH2 mode, and two strong νOCO- modes, at 1581 and 1368 cm-1, confirming the large predominance of the anionic form [CH3CH(NH2)COO-]. In solution at this pH, more than 99% of L-alanine is anionic. No bands are detected between 1600 and 2900 cm-1, and the frequencies of the νsNH2 and νasCH3 modes, at 2975 and 2938 cm-1, respectively, correlate with theoretical and experimental data on neutral L-alanine.29 This spectrum is poorly resolved in the low wavenumber region, due to hydrogen bonding and/or a low degree of crystallinity, in accordance with the vitreous gel texture of the L-alanine obtained after drying. The above vibrational analysis clearly shows that, upon precipitation from aqueous solutions at pH 1, 6, and 13, L-alanine retains its largely predominant form, i.e., cationic, zwitterionic, and anionic, respectively, probably stabilized by the counterions (Cl- at pH 1 and Na+ at pH 13), for electric neutrality of the molecule. (35) Rosado, M. T. S.; Duarte, M. L. R. S.; Fausto, R. J. Mol. Struct. 1997, 410-411, 343.


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Figure 2. DRIFT spectra of the aluminas from Alfa Aesar (A) and Metrohm (B): untreated (a) and dried from an aqueous suspension at pH 1 (b), pH 6 (c), and pH 13 (d). The spectra were normalized to the maximum. Insets: stacked spectra, excluding the Al-O-Al stretching region.

Figure 3. N2 adsorption-desorption isotherms, at 77 K, of γ-alumina (A) and R-alumina (B), untreated and treated at different pH, as indicated. The values of specific surface areas, ABET, and pore volumes, Vp, are included.

3.2. Characterization of Reference Alumina Samples. DRIFT spectroscopy and N2 adsorption-desorption data at 77 K were used to characterize the reference samples of both aluminas. The infrared spectra (Figure 2A,B) were normalized to the band of maximum intensity. The stacked spectra in the insets allow a better exam between 1100 and 4000 cm-1. In Figure 2A, the fingerprints of γ-Al2O3 are clear: the broad band with maximum at 540 cm-1 and a shoulder at ∼850 cm-1 are characteristic of the stretching modes of interlinked AlO6 octahedra and AlO4 tetrahedra, respectively.26 On the other hand, the spectra in Figure 2B exhibit a pattern characteristic of R-Al2O3 or corundum: the strong bands at 641, 594, and 447 cm-1 are assigned to the stretching modes of condensed AlO6 octahedra, split in several components due to lowering of the local symmetry and/or resolution of degenerate modes. The splitting in longitudinal and transverse optical components of the ν(metal-O-metal) modes is characteristic of crystalline metal

oxides.36 The shoulder at ∼800 cm-1 does not allow excluding the presence of Al ions bearing a “quasi-tetrahedral” coordination.10 Adsorbed water is evidenced in untreated γ-alumina (where it is required for stabilization11) by the δHOH band, at 1640 cm-1 (spectrum 2A-a). On the contrary, in untreated R-alumina, water is apparently absent (spectrum 2B-a). The N2 adsorption-desorption isotherms of the reference alumina samples are shown in Figure 3. All of the γ-alumina samples (Figure 3A) exhibit type IV isotherms, with hysteresis loops characteristic of capillary condensation in mesopores. Type H1 hysteresis reveals a well-defined open-ended cylindrical pore network.31,37 The isotherms of R-alumina (Figure 3B) are type II, typical of nonporous or macroporous networks. The small (36) Decius, J. C.; Hexter, R. M.; Molecular Vibrations in Crystals; McGrawHill: New York, 1977. (37) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. 2001, 105, 6817.

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high-pressure hysteresis results from interparticle capillary condensation. Untreated γ-alumina is much more porous than R-alumina, as shown by the specific surface areas (126 versus 25 m2 g-1) and total pore volumes (0.25 versus 0.08 cm3 g-1). For γ-Al2O3, the effects of dispersing in solutions with different pH values are well apparent from the DRIFTS, as well as from the sorption data. From the analysis of the N2 isotherms (Figure 3A), it is clear that, at pH 1, some dissolution of the smaller alumina particles has occurred, followed by reprecipitation with coarsening, resulting in smaller surface area and pore volume but slightly larger average pore diameters (which increase from 5.71 to 6.03 nm, as determined by the BJH method).38 The treatment at pH 6 and 13 apparently consisted only in a washing process of the alumina surface, with the consequent surface area increase. From Figure 2A, it is clear that γ-alumina retains adsorbed water after treatment at pH 1 and 6, since the δHOH band remains at 1640 and 1632 cm-1, respectively. The shift to 1660 cm-1 for pH 13 is too large to allow assigning the band exclusively to that water mode. The broad shape of the νOH band in all of the spectra suggests different types of hydroxyl groups, namely structural and from water (either adsorbed or resulting from interparticle capillary condensation), involved in diverse hydrogen bonding.27,28 At pH 1, this band is unstructured, with maximum at ∼3144 cm-1, consistent with strongly bonded OH groups. At pH 6 and 13, there is an increase in the relative intensity of the higher wavenumber components, evidencing a larger proportion of less hydrogen-bonded OH groups. Along with the most common models on the OH stretching modes of catalytic aluminas, the components in the range 3600-3740 cm-1 may be primarily assigned to OH groups bonded to two or three Al cations.7,10,39 Besides, for the sample prepared at pH 13, the weak sharp feature at 3550 cm-1 is consistent with the presence of hydrated alumina phases, such as Al(OH)3, that may form by a dissolution/precipitation process.11 Nevertheless, the modifications resulting from this process do not alter significantly the porous structure of γ-Al2O3. The major differences induced by pH variation on the spectra of this alumina are in the range 1300-1600 cm-1 and may be related with carbon dioxide uptake.10,40 In fact, CO2 may adsorb on uncoordinated cations (Lewis acid sites), loosing symmetry but keeping the linear geometry (changing from point group D∞h to C∞V), or adsorb at surface OH groups (Lewis base sites) as mono or bidentate species, in a bent geometry.41 In the first case, strong bands in the 23002400 cm-1 region are expected, whereas in the latter, the characteristic modes of carbonate species may be observed around 1500 cm-1. In the spectrum of γ-alumina prepared at pH 1 (Figure 2A-b), no bands are observed in any of these regions, whereas at pH 6 (Figure 2A-c) carbonates, whose formation is favored by a small fraction of surface AlOH groups, may be identified by the bands at 1531 and 1371 cm-1. These are assigned to the νasOCO and νsOCO modes of (Al-O)-CO2, respectively, in a monodentate configuration (only the carbon atom bonds to a surface oxygen), as indicated by the small frequency difference between the two modes (160 cm-1).10,42 In the bidentate species (the carbon atom bonds to a surface oxygen and one oxygen to a surface Al), this shift would be higher and a νCdO mode above 1700 cm-1 would appear.43 The νC-O(Al) mode is not (38) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press, Inc.: San Diego, CA, 1990. (39) Morterra, C.; Bolis, V.; Magnacca, G. Langmuir 1994, 10, 1812. (40) Rege, S. U.; Yang, R. T. Chem. Eng. Sci. 2001, 56, 3781. (41) Morterra, C.; Zecchina, A.; Coluccia, S.; Chiorino, A. J. Chem. Soc. Faraday Trans. 1977, 73, 1544. (42) Baltrusaitis, J.; Jensen, J. H.; Grassian, V. H. J. Phys. Chem. B 2006, 110, 12005. (43) Wijnja, H.; Schulthess, C. P. Spectrochim. Acta A 1999, 55, 861.

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clear in this spectrum, as only a weak shoulder is observed at ∼1000 cm-1. The formation of carbonate species is more remarkable at pH 13 (Figure 2A-d): the intensity of the corresponding broad spectral region grows, with maxima at 1402 and 1528 cm-1, assignable to monodentate (Al-O)-CO2; bicarbonate is also present, identified by the bands at 1267 (δCOH) and 1660 cm-1 (νCO),10,42 the latter having increased intensity by some contribution from the water deformation mode. For R-Al2O3, the N2 adsorption-desorption isotherms (Figure 3B) remain type II upon dispersion, and show that, at pH 1 and 13, there are essentially dissolution/reprecipitation processes, accompanied by a decrease in the specific surface area and total pore volume. At pH 6, these processes are not evident. Besides, it seems that there were not adsorbed impurities responsible for decreasing the alumina surface area, since washing did not result in an increased specific area. From the DRIFTS data there is no evidence of adsorbed water for R-Al2O3 at pH 6 and 13, similarly to untreated alumina, since the δHOH band is absent in spectra 2B-c and d. At pH 1 (Figure 2B-b), however, the deformation mode at 1640 cm-1 and the νOH broad band, with maximum at ∼3200 cm-1, correlate with physically adsorbed water. At pH 13 (Figure 2B-d), the sharp components of the νOH band at 3460 and 3550 cm-1 (without correspondence in the water deformation band) evidence the presence of hydrated alumina forms. Monodentate surface carbonate species are clear at this pH, as a sharp band develops at 1451 cm-1, with a shoulder at 1408 cm-1.44 From the relative intensities of the carbonate versus alumina bands at pH 13, there is apparently less CO2 uptake on R than on γ-Al2O3. However, taking into account the difference in specific surface areas (about five times higher for γ-Al2O3), the CO2 uptake per alumina surface area is probably higher on R-alumina, as reported by Morterra et al. for the adsorption of gas CO2 by alumina surfaces.10 3.3. L-Alanine/Alumina Systems. (i) Samples Prepared at pH 1. The presence of L-alanine does not induce changes in the isotherm type and hysteresis loop of γ- and R-Al2O3: they remain type IV with H1 hysteresis and type II, respectively (Figure 4A,B). However, there is a clear decrease in the total pore volume with increasing amino acid load (visible from the global lowering of the isotherms), more pronounced for γ-Al2O3, which shows that L-alanine adsorbs onto alumina pore surfaces. For γ-Al2O3 samples, there is a drastic decrease of the specific surface area and total pore volume for an L-alanine load of 0.4 mmol g-1. This variation is not proportional to the amino acid load, since for 0.8 mmol g-1 a smaller decrease is induced. On R-Al2O3, the trend is the same, although the values of the specific surface areas and total pore volumes are much lower; the percent variation of the pore volume from 0.4 to 0.8 mmol g-1 (∼14%) suggests that partial saturation of the active sites may occur between these two loads. The pore size distribution by BJH analysis of the desorption isotherms for L-alanine/γ-Al2O3 samples (inset in Figure 4A) narrows with increasing amino acid load while keeping the most probable diameter. This indicates that adsorption does not occur at preferential pores. For R-Al2O3, the BJH analysis is not valid, since this alumina does not contain mesopores. Nonetheless, the observed decrease in surface area and pore volumes with the L-alanine load implies some degree of porosity. The DRIFT spectra in Figure 5 refer to L-alanine adsorbed onto γ- and R-Al2O3 at pH 1 (A and B, respectively), for loads of 0.4 and 0.8 mmol g-1. Taking into account the specific surface (44) Casarin, M.; Falcomer, D.; Glisenti, A.; Vittadini, A. Inorg. Chem. 2003, 42, 436.


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Figure 4. N2 adsorption-desorption isotherms of γ-alumina (A) and R-alumina (B) at pH 1: (a) pure alumina; (b) with 0.4 mmol g-1 of L-alanine; (c) with 0.8 mmol g-1 of L-alanine. Also included are the specific surface areas (ABET) and pore volumes (VP). Inset in (A): BJH pore size distribution from the desorption branch of the γ-alumina isotherms.

Figure 5. DRIFT spectra (normalized to the maximum) of L-alanine adsorbed onto γ-alumina (A) and R-alumina (B), at pH 1. Insets: stacked spectra excluding the Al-O-Al stretching region: (a) pure L-alanine; (b) pure alumina; (c) 0.4 mmol g-1; (d) 0.8 mmol g-1.

areas of the two aluminas upon treatment, these loads are equivalent to 4.5 and 9.0 µmol m-2 (on γ-Al2O3) and 25 and 50 µmol m-2 (on R-Al2O3). The reference spectra of the corresponding alumina and L-alanine are included. The strong νAlO6/AlO4 bands below 1000 cm-1 are broad on both aluminas and, for the higher amino acid load, modifications in the band shape are observed, suggesting slight structural changes that lead to different relative intensities of the optical components of the νAl-O-Al modes.36 In this spectral region, any changes that might give information on the L-alanine structure are masked. The broad νOH band is also mostly due to alumina. Nevertheless, its maximum shifts to lower wavenumbers with increasing L-alanine content on both aluminas, which may result from the increase of the νNH and νOH bands of the amino acid and/or indicate a higher involvement of the surface alumina OH2+ groups in hydrogen bonding. Concurrently, for 0.4 mmol g-1 on γ-alumina (Figure 5A-c), the L-alanine νCdO (1723 cm-1) and νC-O (1228 cm-1) bands apparently disappear as a consequence

of adsorption. For the higher concentration (0.8 mmol g-1), a band at 1203 cm-1 indicates that some nonadsorbed L-alanine may be deposited. These results confirm those from N2 adsorption. On R-alumina, the shoulder that remains at 1723 cm-1 for 0.4 mmol g-1 (Figure 5B-c) shows that some L-alanine crystals are already deposited as such, which is consistent with the lower specific surface area of this alumina. The perturbations induced on the δsNH3+ band of L-alanine (at 1493 cm-1) upon adsorption appear to be much more minor than on the νCdO, both in intensity and in frequency. Since in the spectral region between 1400 and 1800 cm-1 the carbonyl stretching and amine deformation modes are overlapped with the deformation band of adsorbed water, the interactions of L-alanine with γ- and R-alumina could only be clarified by decomposing this spectral region. A nonlinear least-squares fitting method was used, assuming Gaussian band profiles for all of the components. No baseline corrections were made and no restrictions were imposed on the band positions and widths. The band

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Table 1. Spectral Decomposition in the Range 1400-1800 cm-1 of Samples Prepared at pH 1a L-ala

γ-Al2O3

0.4 L-ala (γ-Al2O3)


0.4 L-ala (R-Al2O3)

0.8 L-ala (R-Al2O3)

1750 (0.9%) 1726 (9.0%)

1724 (23.5%)

1673 (14.0%)

1677 (6.4%)

1640 (22.2%) 1605 (5.2%)

1647/1640 (3.8%/20.5%) 1607 (0.8%)

1584 (13.0%) 1542 (0.2%)

1588 (12.7%) 1528 (1.7%)

1478 (4.1%)

1493 (27.3%) 1478 (2.7%)

1424 (2.1%)

1447 (3.5%) 1419 (1.9%)

1495 (17.0%) 1478 (1.9%) 1468 (4.3%) 1447 (4.2%) 1419 (3.2%)

R-Al2O3

1747 (2.3%) 1724 (44.2%)

1731 (2.8%) 1701 (1.2%)

1706 (7.6%)

1666 (2.4%)

1672 (10.3%)

1639 (48.8%)

1647 (16.5%)

1596 (0.4%)

1598 (25.8%)

1578 (9.7%) 1521 (10.7%)

1562 (2.0%) 1541 (0.2%)

1680 (3.3%)

1647 (5.2%)

1641 (95.2%)

1603 (7.2%) 1580 (7.7%) 1519 (4.8%) 1504 (8.7%) 1491 (13.5%)

1641 (98.0%)

1528 (2.0%)

1503 (25.2%) 1491 (17.2%) 1478 (2.1%)

1466 (4.1%) 1417 (6.0%) a

1421 (0.5%)

assigns.10,33,35,43,44 νCdO νCdO (dimers) νCdO (H bonded) νCdO (intram. H bonded) νCdO (H bonded) δHOH (H2O phys.) δasNH3+ (intram. H bonded) δasNH3+ δHOH (H2O chem.) δsNH3+ (intram. H bonded) δsNH3+ δasCH3

The fitted areas (in %) are indicated between brackets. The amino acid loads are expressed in mmol of L-alanine per g of Al2O3.

Figure 6. Spectral decomposition in the region 1400-1800 cm-1 of pure L-alanine (A), pure γ-Al2O3 (B), and pure R-Al2O3 (C) prepared at pH 1.

positions were confirmed by the second derivative of the spectra. The best fits were obtained with χ2 ≈ 10-7 and correlation coefficient of 0.999. The decomposition results (position and percent area for each component) are summarized in Table 1. The spectral decompositions for pure L-alanine and the two aluminas are shown in Figure 6. The decomposition of the L-alanine spectrum confirmed that the main bands, νCdO (1723 cm-1) and δsNH3+ (1493 cm-1), are in fact the result of two overlapped components. Intramolecular hydrogen bonds between the carbonyl and amine groups are possibly responsible for the minor components at 1680 and 1504 cm-1, respectively. In the spectrum of γ-alumina (Figure 6B), the two components of the δHOH mode, at 1641 and 1519 cm-1, are assigned to physically and chemically adsorbed water, respectively.44 Their relative intensities (taken as the ratio between the corresponding % areas) are ∼20:1. For a load of 0.4 mmol g-1 of L-alanine, this ratio changes to ∼5:1. Although these ratios are not intended to yield speciation, this evolution clearly points to a displacement of physisorbed water by adsorbed L-alanine. For the same sample, the changes in the L-alanine νCdO and δsNH3+ components provide complementary information: the disappearance of the 1504 cm-1 component associated with the appearance of a small one at 1478 cm-1 suggests that the hydrogen bonds involving the NH3+ groups were partially broken.27,28 A significant fraction of the L-alanine dimers was dissociated, because the component of the carbonyl

stretch at 1731 cm-1 has a relative intensity of 0.44 (taken as its % area over the total % area of the νCdO components), instead of 0.93 for pure L-alanine. The disappearance of the νC-O doublet at 1228/1197 cm-1, characteristic of dimers, is a confirmation.27 Besides, the new small component at 1701 cm-1 may be assigned to carbonyl groups previously involved in intramolecular bonds that were weakened. On the other hand, the presence of strongly hydrogen-bonded CdO groups upon adsorption is revealed by the new component at 1666 cm-1. The shift of the νOH band of alumina to lower wavenumbers, irrespectively of water displacement, is consistent with more interactive hydroxyl groups.28 These effects taken together prove that the L-alanine/γ-alumina interactions involve the carbonyl groups of L-alanine and the surface OH2+ groups of alumina through strong hydrogen bonds, at the expense of dimer dissociation and breaking of intramolecular bonds. These interactions explain the apparent disappearance in the spectrum of the νCdO band. Since there is surface dissolution/reprecipitation of alumina at this pH, the complexation of Al3+ ions in solution could be contemplated. However, the reference spectrum of L-alanine at this pH proves that there are no available carboxylate ligands. The component at 1666 cm-1, obtained by band decomposition, could be erroneously assigned to the νasOCO carboxylate mode, but there is no evidence of a new band assignable to the corresponding symmetric mode, which would appear at ∼1360 cm-1.28 For a load of 0.8 mmol g-1, the


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Figure 7. DRIFT spectra (normalized to the maximum) of L-alanine adsorbed onto γ-alumina (A) and R-alumina (B), at pH 6. Insets: stacked spectra excluding the Al-O-Al stretching region: (a) pure L-alanine; (b) pure alumina; (c) 0.4 mmol g-1; (d) 0.8 mmol g-1. L-alanine components overlapped with the water deformation bands increase, not allowing a clear identification of the nature of adsorbed water. However, the conclusions on the adsorbate/ substrate interactions drawn from the sample with 0.4 mmol g-1 are confirmed, since the main component of the νCdO band (1672 cm-1) corresponds to the highly interacting carbonyl groups and the one regarding dimers is absent. The spectral decomposition for R-alumina shows that the ratio of physisorbed to chemisorbed water is approximately 50:1, as a result of the treatment at this pH. For the L-alanine load of 0.4 mmol g-1, the frequencies and relative intensities of the νCdO and δsNH3+ components that were assigned to adsorbed molecules are similar to those on γ-alumina, pointing to the same type of alanine/alumina interactions. When the amino acid load is increased to 0.8 mmol g-1, the relative intensity of the νCdO component at 1724 cm-1 (% area over the total % area of the νCdO components, as above) increases to 0.79. As this ratio approaches the value obtained for pure L-alanine (0.93), it seems that the saturation of alumina active sites observed by N2 adsorption results in the deposition of L-alanine. Therefore, for a sufficiently high load, a fraction of the amino acid does not bond preferentially to alumina and precipitates as dimers. However, contrarily to what happened on γ-alumina, the decomposition of the δsNH3+ band shows that the intramolecular hydrogen bonds existing in pure L-alanine are not favored, since the component at ∼1504 cm-1 is not detected. In summary, the DRIFT spectra show that the interactions of L-alanine with γ- and R-alumina at pH 1 are of the same nature: strong hydrogen bonds between the carbonyl groups of the amino acid and the AlOH2+ groups of alumina, at the expense of L-alanine dimers and/or intramolecular interactions. The amino acid/Al2O3 interactions account for L-alanine adsorption within the alumina pore network, as proved by N2 adsorption. The L-alanine adsorption causes water displacement from alumina, and higher loads result in partial amino acid segregation, due to an active site saturation effect, particularly in R-alumina. (ii) Samples Prepared at pH 6. From the DRIFT spectra of the samples prepared at pH 6 (Figure 7), it is clear that the amount of adsorbed L-alanine on R-alumina is exceedingly low, since even the amino acid strong bands are of extremely low relative intensity. This could reflect the experimental observation

that, at this pH, L-alanine tends to crystallize at the reservoir walls as the solvent evaporates from the dispersions, showing a certain propensity for segregation (even for low loads). The changes in the νOH band of γ-alumina with increasing amino acid content (Figure 7A) are quite different from those observed at pH 1. It does not shift downward but rather broadens toward lower wavenumbers, due to overlapping with the complex νNH and νCH region of zwitterionic L-alanine. Therefore, this spectral region is not a good source of information on the L-alanine/γ-Al2O3 interactions. For R-alumina (Figure 7B), no apparent shifts of the νOH band are either observed. However, the relative intensity of the higher wavenumber component (at ∼3470 cm-1) increases with increasing amino acid content, as if a higher fraction of less bonded OH groups were present. Therefore, any modification induced by the presence of L-alanine is in the sense of reducing H bonds in which these groups of R-alumina are involved. The most informative region of the spectra, between 1200 and 1800 cm-1, contains information on both the substrate and the amino acid. For R-alumina, the relative intensities of the substrate bands are quite low, and the main features observed are due to the amino acid, increasing continuously with its load. These bands are not shifted with respect to pure L-alanine, suggesting that the strong interactions between amino acid molecules prevail over those with R-Al2O3. For γ-alumina, the relative intensity of this region increases upon L-alanine adsorption, but the positions and relative intensities of the bands are not sensitive to the amino acid load. The only band that shows noticeable modifications is the strong and broad one at ∼1594 cm-1 (Figure 7A-c and d) that results from the overlapping of the νasOCOand δNH3+ modes of L-alanine with the δHOH and νCO modes of alumina (physisorbed water and carbonate species, respectively). Further insight was obtained by decomposition of this spectral region, summarized in Table 2. The decomposition of the pure L-alanine spectrum allowed resolving two components of the strong δasNH3+ band, at 1550 and 1593 cm-1, and also of the νasOCO- band, at 1620 and 1641 cm-1. The existence of two components for each of these modes is not surprising, since zwitterionic L-alanine forms threedimensional structures by hydrogen bonding between these two

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γ-Al2O3



0.4 L-ala (R-Al2O3)

0.8 L-ala (R-Al2O3)

1659 (22.6%)

1640 (32.7%)

1643 (21.4%)

1642 (17.2%)

1626 (6.9%) 1609 (6.7%) 1593 (12.1%)

1618 (4.5%)

1623 (14.8%)

1622 (22.2%)

1594 (4.6%)

1593 (25.1%)

1591 (23.6%)

δasNH3+ (intram. H bonded) νasOCO [(Al-O)-CO2, monodentate] δasNH3+ νasOCO [(Al-O)-CO2, monodentate]

1551 (13.6%) 1522 (0.6%)

1542 (16.9%) 1518 (0.3%)

1506 (4.6%)

1503 (0.4%)

1461 (2.9%)

1454 (3.8%)

1411 (8.7%)

1411 (7.6%)

δasNH3+ δHOH (H2O chem.) or νasOCO [(Al-O)-CO2, monodentate] δsNH3+ (intram. H bonded) δsNH3+ νsOCO [(Al-O)-CO2, monodentate] δasCH3+ νsOCO [(Al-O)-CO2, monodentate] δsCH3 νsOCO [(Al-O)-CO2, monodentate]

R-Al2O3

1641 (10.9%) 1639 (54.2%) 1620 (10.5%) 1593 (33.7%)

1577 (49.1%) 1574 (14.1%) 1563 (4.2%)

1569 (8.4%)

1521 (24.5%)

1546 (7.7%) 1518 (8.0%)

1551 (21.9%) 1550 (10.2%)

1545 (0.2%) 1519 (21.8%)

1507 (8.7%) 1488 (8.4%) 1457 (5.8%) 1456 (9.2%)

1463 (2.0%)

1459 (4.1%) 1420 (23.1%)

1413 (9.3%) 1406 (7.1%)

1410 (10.0%)

1413 (0.8%) 1410 (9.1%)

1369 (5.3%)

1365 (6.3%)

1349 (1.8%)

1348 (1.8%)

assigns.10,33,35,43,44 νasOCOνasOCO- /δHOH (H2O phys.) δHOH (H2O phys.) νasOCO-

1366 (9.9%)

1360 (7.5%) a

1363 (7.2%) 1344 (1.2%)

1362 (8.1%)

νsOCO [(Al-O)-CO2, monodentate]/ νsOCO- (L-alanine) νsOCO-

The relative areas (in %) are indicated between brackets. The amino acid loads are expressed in mmol of L-alanine per g of Al2O3.

groups, and some conformers are able to establish intramolecular hydrogen bonds. For L-alanine/γ-alumina samples, two minor components are retrieved at 1349 and 1609 cm-1, assignable to the symmetric and antisymmetric νOCO- modes of carboxylate groups more strongly bonded than in pure alanine. Additionally, the component at 1488 cm-1 may be assigned to NH3+ groups that lost hydrogen bonding upon adsorption. These results show that a fraction of L-alanine molecules are in fact interacting with γ-alumina, by hydrogen bonding between the carboxylate groups and the Al-OH2+ groups at the alumina surface (still predominant at this pH). However, the affinity of L-alanine toward γ-alumina seems to be lower, when compared to pH 1, because a 0.8 mmol g-1 load results in amino acid deposition. The nature of the proposed interactions between L-alanine and γ-Al2O3 is in good agreement with that found for Cu(II)-glutamate complexes with γ-alumina in acidic suspensions.4 For L-alanine/R-alumina samples, the shifts observed on the main components of the amino acid are within spectral resolution, confirming that there are no preferential interactions with the substrate. Even for the load of 0.4 mmol g-1, the tendency of

L-alanine to crystallize becomes clear from the spectrum, which is consistent with strong NH3+‚‚‚-OOC intermolecular interactions. The weaker interactions observed at this pH, which account for L-alanine segregation, are also reflected on the N2 adsorption isotherms (Figure 8). The small effect caused by an amino acid load of 0.4 mmol g-1 is consistent with poor adsorption onto the pores of both aluminas at pH 6. For a 0.8 mmol g-1 load, there is an abrupt decrease in the specific surface area and pore volume for both aluminas. Such variation cannot be attributed to adsorption onto the porous network (since the spectral data do not support this hypothesis) but must rather result from a pore blocking effect, by the three-dimensional crystalline array formed by zwitterionic L-alanine. (iii) Samples Prepared at pH 13. On L-alanine/γ-alumina samples (spectra in Figure 9A), the νNH/νOH region is dominated by the νOH band of alumina, but a clear downward shift (of ∼30 cm-1) occurs upon adsorption of L-alanine, indicating that the alumina OH groups are hydrogen bonded to the amino acid.


Langmuir, Vol. 23, No. 20, 2007 10173

Figure 8. N2 adsorption-desorption isotherms of γ-alumina (A) and R-alumina (B) at pH 6: (a) pure alumina; (b) with 0.4 mmol g-1 of L-alanine; (c) with 0.8 mmol g-1 of L-alanine. Inset in (A): BJH pore size distribution from the desorption branch of the γ-alumina isotherms. The values of specific surface areas, ABET, and pore volumes, Vp, are included.

Figure 9. DRIFT spectra (normalized to the maximum) of L-alanine adsorbed onto γ-alumina (A) and R-alumina (B), at pH 13. Insets: stacked spectra excluding the Al-O-Al stretching region: (a) pure L-alanine; (b) pure alumina; (c) 0.4 mmol g-1; (d) 0.8 mmol g-1.

On L-alanine/R-alumina samples (spectra in Figure 9B), as the OH groups are in very low concentration, the 2800-3700 cm-1 region is mainly influenced by the νNH mode of L-alanine. The ∼20 cm-1 shift to lower wavenumbers with respect to the reference amino acid spectrum, observed for a 0.4 mmol g-1 load (Figure 9B-c), is indicative of hydrogen bonding of the NH2 group. Increasing the amino acid load to 0.8 mmol g-1 (Figure 9B-d), the spectrum resembles that of pure L-alanine. In the range between 1800 and 1200 cm-1, the spectra are dominated by the carbonate modes and/or any adsorbed water of alumina, plus the carboxylate modes of L-alanine. On R-Al2O3, the carbonate related νCO modes are well resolved from those of the carboxylate group, and their band shapes and positions are similar to the reference spectra. This is a good indication that the carboxylate groups of the amino acid are not perturbed, and thus the L-alanine/alumina interactions involve only the NH2 groups. On γ-Al2O3, the broadness and intensity of the carbonate

modes and the presence of adsorbed water prevent a clear assignment of the amino acid bands, and only by spectral decomposition some insight on the possible interactions could be achieved. The fitting results are summarized in Table 3. The decomposition of the L-alanine spectrum allows resolving the two components of the very strong band at ∼1581 cm-1: νasOCO-, at 1580 cm-1, and δNH2 mode, at 1631 cm-1. The latter is shifted to lower wavenumbers with respect to dimeric neutral L-alanine,22 which is consistent with no aggregation by hydrogen bonds involving the NH2 groups. In the broad band of γ-Al2O3, it was possible to identify the water deformation mode (at 1639 cm-1) and those related to carbonate species, at 1374, 1484, 1577, and 1673 cm-1, plus a minor one at 1260 cm-1. The components at 1577 and 1484 cm-1 are tentatively assigned to the νasOCO mode and the one at 1374 cm-1 to the νsOCO mode of monodentate carbonates,

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R-Al2O3


1673 (12.2%)

1729 (1.7%) 1687 (5.5%) 1651 (21.3%)

1651 (6.7%)

νCdO [(Al-O)-CO2, bidentate] νCO (bicarbonate) δNH2 (H bonded) δHOH (H2O phys) δNH2

1605 (3.7%) 1585 (13.3%) 1523 (1.7%) 1488 (17.9%)

1602 (10.2%) 1580 (11.6%) 1540 (3.1%) 1476 (33.3%)

νasOCO- (L-alanine) νasOCO [(Al-O)-CO2, monodentate]

0.4 L-ala (R-Al2O3)

1639 (1.1%) 1631 (18.0%) 1580 (28.4%) 1577 (17.4%) 1484 (38.2%)

1562 (6.3%) 1472 (56.3%)

δasCH3

1469 (25.9%) 1450 (0.7%)

1453 (18.9%)

1414 (5.6%)

1413 (0.3%)

1444 (11.8%) 1408 (5.5%) 1396 (20.1%)

1374 (30.3%) 1371 (11.7%) 1304 (9.6%) 1260 (0.7%) a

assigns.10,33,35,43,44

γ-Al2O3

1371 (10.0%) 1302 (4.8%) 1268 (0.9%)

1455 (2.0%) 1442 (14.2%) 1411 (4.3%) 1395 (0.8%) 1375 (9.7%) 1304 (4.2%)

νsOCO [(Al-O)-CO2, monodentate] δsCH3 νsOCO [(Al-O)-CO2, weakly bound] νsOCO- (L-alanine) δCH δCOH (bicarbonate)

The relative areas (in %) are indicated between brackets. The amino acid loads are expressed in mmol of L-alanine per g of Al2O3.

Figure 10. N2 adsorption-desorption isotherms of γ-alumina (A) and R-alumina (B) at pH 13: (a) pure alumina; (b) 0.4 mmol g-1; (c) 0.8 mmol g-1. Inset in (A): BJH pore size distribution from the desorption branch of the γ-alumina isotherms.

whereas those at 1673 and 1260 cm-1 may be related with νCO and δCOH modes of bicarbonate species, respectively.42,43 The most relevant information regarding L-alanine/γ-alumina interactions may be retrieved from the evolution of the δNH2 mode upon adsorption. For a load of 0.4 mmol g-1, this component is shifted upward (from 1631 to 1651 cm-1), indicating a strong involvement of the NH2 groups in hydrogen bonding with the OH surface groups of γ-alumina (predominant at this pH), as suggested above. These interactions do not involve the amino acid COO- group, as can be inferred from the steadiness of its fitted components (νasOCO- and νsOCO-, at 1371 and 1585 cm-1, respectively). The appearance of the band at 1729 cm-1 suggests the formation of a small fraction of bidentate carbonates. However, the absence of adsorbed water and the reduction in carbonate uptake for the samples containing L-alanine (the bands at 1639 and 1374 cm-1 disappear, and that at 1577 cm-1 decreases and shifts) suggest that the amino acid displaces adsorbed water and competes with carbonate species in solution for the alumina active sites. On R-alumina, the νsOCO- and νasOCO- modes of L-alanine also remain unperturbed by adsorption, whereas the δNH2 mode shifts to 1651 cm-1, indicating that hydrogen bonding involves

the same groups as on γ-alumina. Accordingly, the carbonate uptake is lowered by the presence of the amino acid (its related component at 1408 cm-1 is absent and those at 1540 and 1395 cm-1 are very weak). The NH2‚‚‚HO-Al interactions detected by the DRIFT spectra account for L-alanine adsorption onto the porous network of alumina, as confirmed by the N2 isotherms in Figure 10. There is a decrease in the total pore volume and specific surface areas upon amino acid adsorption, as observed for pH 1. On γ-Al2O3, these values decrease continuously with increasing L-alanine load, showing that L-alanine adsorbs onto the mesopores, with no evidence of site saturation. On R-Al2O3, the variation is much attenuated, indicating less efficiency in adsorption; the total pore volume actually remains constant when increasing the load to 0.8 mmol g-1, confirming the site saturation effect suggested by the spectra.

4. Conclusions From the detailed analysis of vibrational spectra (DRIFT) of samples dried from suspensions prepared at 36.5 °C and at different pH values, relevant conclusions on the L-alanine/alumina


nature of the amino acid/alumina interactions were drawn. As references, pure γ- and R-alumina prepared under the same conditions and pure L-alanine precipitated from aqueous solutions were used. It was shown that cationic L-alanine establishes hydrogen bonds with the Al-OH2+ groups of both aluminas through the CdO group (at pH 1). In the zwitterionic form (pH 6), the same alumina groups are involved with the amino acid carboxylate, although the NH3+‚‚‚-OOC intermolecular interactions are prevalent. For anionic L-alanine (pH 13), the amino (NH2) groups are rather involved in hydrogen bonds with the Al-OH groups of alumina. These interactions account for L-alanine adsorption onto the alumina pore network at pH 1 and 13 and its deposition at the surface at pH 6. Depending on the suspension pH, adsorbed L-alanine displaces physisorbed water and/or competes with the adsorption of carbonates. For the same sample preparation conditions, γ-alumina is a more active substrate toward the adsorption of this amino acid, which may be justified by its mesoporous structure and a much higher specific surface area. However, the L-alanine/alumina interactions at specific sites are more determining on the adsorption process than the available surface area of the substrate, since both aluminas treated at pH 6 have the largest ABET values and adsorb less L-alanine.

Langmuir, Vol. 23, No. 20, 2007 10175

In conclusion, for eventual catalytic applications, appropriate experimental conditions must be set, depending on whether alumina is intended to act as a catalyst for amino acid reactions or just as an inert support (in this case, the presence of the zwitterionic form, i.e., near neutral pH must be favored). Acknowledgment. This work was supported by Fundaç aõ para a Cieˆncia e a Tecnologia (FCT), Project POCI/QUI/60918/ 2004. R.B.B. and A.F. acknowledge FCT for post-doc Grants SFRH/BPD/14912/2004 and SFRH/BPD/20234/2004, respectively. The authors acknowledge Prof. Rui Almeida (I.S.T.) for the availability of the ASAP 2000 equipment. Supporting Information Available: Table containing the frequencies and band assignments of the DRIFT spectra of L-alanine as purchased, and reprecipitated from aqueous solutions at pH 1, 6, and 13. Spectral decomposition, in the region 1400-1800 cm-1, of pure L-alanine and γ- and R-alumina prepared at pH 13. This information is available free of charge via the Internet at http://pubs.acs.org. LA701467Y

Interactions of l-Alanine with Alumina as Studied by Vibrational

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