Structural and Textural Evolution during Folding of Layers of Layered

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Langmuir 2008, 24, 8904-8911

Structural and Textural Evolution during Folding of Layers of Layered Double Hydroxides María de Jesu´s Martínez-Ortiz,† Enrique Lima,*,‡ Víctor Lara,‡ and Juan Meńdez Vivar‡ Instituto Politećnico NacionalsESIQIE, UPALM Edif. 7, Zacatenco, 07738 Me´xico D.F., Mexico, and UniVersidad Autońoma Metropolitana, Iztapalapa, A. P. 55-532, AV. San Rafael Atlixco No. 186 Col. Vicentina, 09340 Me´xico D.F., Mexico ReceiVed January 19, 2008. ReVised Manuscript ReceiVed May 28, 2008 Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.

Introduction Mesoporous materials have attracted the attention of many research teams because of their physicochemical and porosity properties. The utility of these materials is manifested in their microstructures, which allow molecules access to large internal surfaces and cavities that enhance catalytic activity and adsorptive capacity.1 Recently, some oxides such as magnesium oxide and alumina have been successfully mesostructured.2,3 Siliceous MCM-41,4 however, remains the most studied and applied mesoporous molecular sieve. In 1992, Beck et al.5 reported the synthesis of a new classification of silicate/aluminosilicate materials, including MCM-41. A templating mechanism, in which surfactant liquid crystal structures serve as organic templates, is proposed for the formation of these materials.6,7 The surfactants used in this synthesis can be neutral, cationic, or anionic.8–10 They drive the forces to form the crystalline phase. In this context, the preparation of mesoporous materials by interaction of layered silicates with different surfactants has been reported.11,12 Following this approach, it could be possible to use other layered compounds to generate mesoporous-like materials with a very different wall composition than siliceous materials. Indeed, the * Corresponding author: phone (525) 55804 4667; fax (525) 55804 4666; e-mail [email protected]. † Instituto Politećnico NacionalsESIQIE. ‡ Universidad Autońoma Metropolitana. (1) Soller-Illia, G.; Sańchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (2) Zhang, Z.; Pinnavaia, T. J. Am. Chem. Soc. 2002, 124, 12294. (3) Kimura, T. Chem. Mater. 2005, 15, 3742. (4) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (5) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.; Olson, D. H.; Sheppard, E. W.; Mccullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (6) Huo, Q.; Margolese, D.; Ciesla, U.; Demuth, D.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (7) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874. (8) Olivier, S.; Kupermann, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47. (9) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 1685. (10) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (11) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem Soc., Chem. Commun. 1993, 680. ´ lvarez, C.; Pe´rez-Pariente, J. Chem. Mater. 2006, (12) García, I. D.; Ma´rquez-A 18, 2283.

use of layered precursors should lead to mesophases with some structural characteristics of the parent layer precursor. For instance, hydrotalcite-like compounds, hereafter HT, are anionic exchangers built up of brucite-like layers separated by anion and water. HT have the general formula [M2+1-xM3+x(OH)2]x+ [Az-x/z · nH2O]x-, where M2+ is a divalent cation (Mg, Ni, Zn, Co, Fe,...), M3+ is a trivalent cation (Al, Fe, Cr, Mn), and Azis an anion (CO32-, SO42-, OH-, Cl-,...).13–15 HT materials are widely applied as adsorbents, catalysts, and catalyst precursors,16–18 among others. HT calcination, at temperatures between 400 and 600 °C, causes the formation of mixed oxides, (M2+, M3+)-O, with a periclase-like structure, able to recover the layered structure after contact with water or anionic solutions. This conversion of the periclase-like mixed oxides into hydrotalcite-like structure is commonly called a “memory effect”.18–21 Preparation of mesoporous mixed oxides from pillared oxovanadate layered double hydroxide (LDH) has been previously reported.22,23 However, HT has not been used as precursor, in combination with surfactants, to prepare mesostructured materials. The anionic exchange capacity and the memory effect of hydrotalcites can be conveniently used to incorporate surfactant anions, and then the template effect can be used to generate a mesostructured HT. Among the hydrotalcite-like compounds, the most used is the HT containing magnesium, aluminum, and carbonate as the divalent and trivalent metal and anion, respectively. Therefore, we have chosen this HT to obtain the analogous mesoporous material. Sodium dodecyl sulfate (SDS; Figure 1) was selected as anionic surfactant. (13) Miyata, S. Clays Clay Miner. 1980, 28, 50. (14) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (15) Hibino, T.; Kosuge, K.; Tsunashima, A. Clays Clay Miner. 1996, 44, 151. (16) Laguna, H.; Loera, S.; Ibarra, I. A.; Lima, E.; Vera, M. A.; Lara, V. Microporous Mesoporous Mater. 2007, 98, 234. (17) Lima, E.; de Meńorval, L. C.; Tichit, D.; Laspe´ras, M.; Graffin, P.; Fajula, F. J. Phys. Chem. B 2003, 107, 4070. (18) Lima, E.; Laspe´ras, M.; de Meńorval, L. C.; Tichit, D.; Fajula, F. J. Catal. 2004, 223, 28. (19) Hibino, T.; Tsunashima, A. Chem. Mater. 1998, 10, 4055. (20) Stanimirova, T. S.; Kirov, G.; Donolova, E. J. Mater. Sci. Lett. 2001, 20, 453. (21) Mackenzie, K. J.; Meinhald, R. H.; Sherriff, B. L.; Xu, Z. J. J. Mater. Chem. 1993, 3, 1263. (22) Carja, G.; Delahay, G. Appl. Catal., B 2004, 47, 59. (23) Carja, G.; Nakamura, R.; Aida, T.; Niiyama, H. Microporous Mesoporous Mater. 2001, 47, 275.

10.1021/la801442n CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

Folding of Layers of LDHs

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Figure 1. Chemical structure of sodium dodecyl sulfate surfactant.

Materials and Methods Preparation of Hydrotalcite-like Compound and Its Memory Effect. A layered double hydroxide (LDH) with Mg/Al atomic ratio of 3 was prepared by coprecipitation at pH 10, by dropwise simultaneous addition of a 1 M solution of magnesium and aluminum nitrates and 2 M NaOH to a 0.1 M solution of Na2CO3. The resulting solid was washed repeatedly with distilled water and then dried at 80 °C for 8 h. Hydrotalcite-like structure was confirmed by X-ray diffraction; this sample is labeled as HTCO3. Thermal treatment of HT-CO3, at 550 °C in air, led to the formation of the corresponding mixed oxide with periclase-like structure. By hydration of this mixed oxide, the lamellar structure was recovered. At the end of three destruction-regeneration cycles, the solid was called HT-CO3-3ME. Surfactant Incorporation into Hydrotalcite. Surfactant containing HT was obtained when 2.3 g of mixed oxide, (Mg,Al)-O, emerging from calcination at 550 °C of HT-CO33ME, was put in contact with 150 mL of 0.9 M SDS surfactant (Sigma-Aldrich, 99%). Then the suspension was refluxed for 2 weeks. On the eighth day of reflux, an amount of nitric acid (1 M) was added, and then the reflux was maintained for 7 days more. The structural evolution of the material was monitored continuously by sampling every 24 h. Dried samples at 100 °C were analyzed by X-ray diffraction and IR and solid-state NMR spectroscopy. Dried samples were labeled as HT-SDS-X, where X corresponds to the reflux time (in days). The surfactant was removed in one of two ways: by calcination in air at 400 °C or by treating the HT-SDS sample three successive times with a 1 N Na2CO3 solution. After removal of surfactant, samples were labeled by adding the prefix TH or TS, for samples thermally treated or treated with carbonated solution, respectively. Characterization. Materials were characterized by transmission electronic microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), 27Al, 1H f 27Al and 1H f 13C NMR spectroscopy, thermogravimetric analysis, and N2 adsorption. FTIR spectra of samples (CsI waffles), at 2 cm-1 resolution and room temperature, were obtained on a Perkin-Elmer FTIR 2000 IR spectrometer. A detector DTGS Mid-IR (CsI) was used. Powder XRD patterns were obtained on a Siemens D-5000 diffractometer, where a Cu target KR ray (λ ) 0.154 nm) was used as an X-ray source. Conventional identification of crystalline compounds was performed by comparing the diffractograms with JCPDS (Joint Committee on Powder Diffraction Standards) files. Small-angle X-ray scattering experiments were performed with a Kratky camera coupled to a copper anode X-ray tube whose KR radiation was selected with a nickel filter. The SAXS intensity data, I(h), were collected with a linear proportional counter. Then they were processed with the ITP program24–28 where the scattering vector is defined as h ) 4π sin θ/λ, where θ and λ are the scattering angle and the X-ray wavelength, respectively. The shape of the scattering objects was estimated from the Kratky plot, that is, h2I(h) versus h. The shape is determined depending (24) Glatter, (25) Glatter, (26) Glatter, (27) Glatter, (28) Glatter,

O.; Hainisch, B. J. Appl. Crystallogr. 1984, 17, 435. O.; Gruber, K. J. Appl. Crystallogr. 1993, 26, 512. O. J. Appl. Crystallogr. 1981, 14, 101. O. J. Appl. Crystallogr. 1988, 21, 886. O. Prog. Colloid Polym. Sci. 1991, 84, 46.

Figure 2. X-ray diffraction patterns of samples (a) HT-CO3, (b) HTCO3 thermally treated, (c) HT-CO3 thermally treated and rehydrated, and (d) HT-CO3 sample after three memory effects (sample HT-CO33ME).

on the Kratky curve shape: for instance, if the curve presents a peak, the particles are globular.29 Fractal dimension values of the scattering objects were calculated from the slope of the curve log I(h) versus log (h).30 The one-pulse solid-state 27Al magic-angle spinning (MAS) NMR spectra were acquired at a frequency of 78.15 MHz on an Avance II Fourier transform spectrometer by use of a 4 mm MAS probe with a spinning rate of 10 kHz. Short single pulses (π/12) were used with a repetition time of 0.5 s. Chemical shifts were referenced to aqueous [Al(OH2)6]3+. 27Al cross-polarization (CP) MAS NMR experiments were performed setting the pulse power for both channels, heteronuclear and 1H so that the 90° times are equal, Hartmann-Hahn matching condition. In separate experiments, 27Al were used as sources of the magnetization which was transferred by cross-polarization to the other spin species.31,32 After the contact time where both radiofrequencies fields are on, free induction decays were accumulated. Porosity and surface area measurements were performed on a volumetric adsorption ASAP 2000 Micromeritics apparatus. Before adsorption, the samples were outgassed for 12 h at 180 °C. Nitrogen was used as adsorbate at liquid nitrogen temperature (77 K). Thermogravimetric analyses were performed on TA Instruments equipment. Samples were heat-treated at a heating rate of 5 °C min-1 from room temperature to 900 °C in nitrogen flow. TEM analysis was performed in a JEOL JEM-1200EX transmission electron microscope to obtain images at 200 kV. The powder samples were prepared by standard methods.

Results Starting Layered Double Hydroxide and Its Memory Effect. Figure 2 exhibits the XRD patterns of as-synthesized HT-CO3, the corresponding mixed periclase-like oxide after thermal treatment, and the regenerated LDH after one and three memory effects. No significant changes are observed between starting and final LDHs, HT-CO3 and HT-CO3-3ME. (29) Kataoka, M.; Flanagan, J. M.; Tokunaga, F.; Engelman, D. M. Use of X-ray solution scattering for protein folding study. In Synchrotron Radiation in the Biosciences; Chanse, B., Deisenhofer, J., Ebashi, S., Goodhead, D. T., Huxley, H. E., Eds.; Clarendon Press: Oxford, U.K., 1994; Vol. 4, p 87. (30) Harrison, A. Fractals in Chemistry; Oxford University Press Inc.: New York, 1995. (31) Fyfe, C. A.; Mueller, K. T.; Crondey, H.; Wong-Moon, K. C. J. Phys. Chem. 1993, 97, 13484. (32) Amoureux, J. P.; Pruski, M. Mol. Phys. 2002, 100, 1595.

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Figure 3. 27Al MAS NMR spectra of (a) HT-CO3, (b) HT-CO3 thermally treated, and (c) HT-CO3 sample after three memory effects (sample HT-CO3-3ME).

Figure 3 shows a 27Al MAS NMR spectrum for the starting HT-CO3 composed of a single peak close to 10 ppm, supporting that this sample contains only octahedral aluminum in agreement with the coordination into brucite-like layers.16,21,33 When the HT-CO3 sample is calcined, the mixed oxide is formed and a part of the octahedral aluminum turns out to be tetrahedrally coordinated (signal close to 60 ppm was clearly resolved) in accord with the periclase-like structure. However, after the memory effect, the layered structure was recovered, but the total amount of tetrahedral aluminum did not recover the 6-fold coordination. Indeed, the spectrum of sample HT-CO3-3ME shows that, after three memory effects performed on the sample, a minor amount of tetrahedral aluminum was extracted from the LDH network. The mechanism of regeneration of LDH structure from mixed oxide remains an open subject. Some authors believe that regeneration occurs topotactically without the dissolution of the sample or the memory effect proceeds through the dissolution of the mixed oxide and subsequent hydrotalcite crystallization.19–21,34 It is worth mentioning that (Mg,Al)-O could be visualized as a highly homogeneous and very reactive system of alumina and magnesia that, in the presence of a carbonatecontaining solution, forms the carbonated hydrotalcite owing to the chemical reaction between the basic MgO and the amphoteric alumina. Confirmation of this reaction was reported by an analogous synthesis of magnesium-aluminum hydrotalcite performed from mechanical mixtures of alumina xerogel and microcrystalline MgO, from the decomposition of Mg basic carbonate, in hot water at temperature as low as 80 °C.35 The opposite reaction, synthesis of oxides from the HT, was also previously reported,36 with the Wagner reactions set where suggested. Actually, the presence of Al(IV) in the sample after consecutive memory effects is clear but it is difficult to understand the location. Indeed two reasonable hypotheses can be formulated: (1) Al(IV) could be part of alumina segregated (not detected by XRD) or (2) Al(IV) is related to defects induced at the surface or external layers of a particle of LDH. In order to discriminate one of these (33) Lippmaa, E.; Samoson, A.; Magi, M. J. Am. Chem. Soc. 1986, 108, 1730. (34) Hibino, T.; Tsunashima, A. Clays Clay Miner. 1997, 45, 842. (35) Mascolo, G.; Marino, O. Miner. Mag. 1980, 43, 619. (36) Martıńez-Gallegos, S.; Pfeiffer, H.; Lima, E.; Espinosa, M.; Bosch, P.; Bulbulian, S. Microporous Mesoporous Mater. 2006, 94, 234.

Martıńez-Ortiz et al.

Figure 4. (a) 27Al MAS NMR and (b) 27Al CP MAS NMR spectra of sample HT-CO3-3ME calcined at 400 °C.

possibilities, a cross-polarization 1H f 27Al NMR experiment was performed on sample HT-CO3-3ME (calcined at 400 °C). The indirect excitation of aluminum at the surface is preferential because of the polarization transfer from the excited protons of water remaining at the surface to the neighboring aluminum atoms throughout heteronuclear dipolar interaction. It is worth remembered that two nuclei, 27Al and 1H, can interact through dipolar coupling according to the Hamiltonian operator37 as follows:

H)-

γHγAlp 3 rHAl

(3 cos2 θ - 1)IAlIH

(1)

where γH and γAl are the gyromagnetic constants of Al and H nuclei, p is Planck’s constant, I is the nuclear spin, and θ is the angle between the internuclear vector and the external magnetic field. Note that dipolar interaction depends on the distance (rHAl) between the Al and H nuclei. Thus, in the acquisition of the spectrum in Figure 4b the aluminum at the solid surface (those in defects such as corners, edges, etc.) are preferentially excited as they are close to hydrogen atoms of remaining water. One can observe that the signal attributed to Al(IV) is more intense in the CP spectrum than in the one-pulse spectrum, suggesting that in effect these aluminum atoms could be present mainly at the surface. In alumina, the Al(IV) could be at the surface, of course, but a large amount should be in the bulk as they form part of a spinel structure. Furthermore, in a typical alumina the ratio of NMR signals, Al(IV)/Al(VI), is close to 1/3, which is not the case in Figure 4b. LDH in the Presence of Surfactant. Figure 5 displays the XRD patterns of HT-SDS-X series. Diffractograms did change significantly with time in the surfactant/mixed oxide suspension. It is clear that after 1 day the memory effect, characteristic of mixed oxide (Mg,Al)-O, was achieved. The interlayer distance, measured from the position of the weak 003 peak, was 7.9 Å for this reconstructed compound. Such layer distance corresponds (37) Abragman, A. Les principes du magne´tisme nucleaire; Presses Universitaire de France: Paris, 1961.


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Figure 5. X-ray diffraction patterns of samples (a) HT-SDS-1, (b) HT-SDS-2, (c) HT-SDS-3, (d) HT-SDS-1, (e) (HT-SDS-3), and (f) HT-SDS-14. Peaks labeled with asterisks in spectra a and b correspond to hydrotalcite compound. Table 1. Assignment of FTIR Bands band 3487 2919 2841 1628 1550 1467 1364 1202 1112 1047 976 570

Figure 6. FTIR spectra of samples (a) HT-SDS-1, (b) HT-SDS-2, (c) HT-SDS-3, and (d) HT-SDS-14.

to carbonate anions hosted into the interlayer space. Nevertheless, this structure was not stable in the surfactant medium. On the second day some narrow peaks were observed; they were not identified as an inorganic crystalline phase and should then be attributed to surfactant adsorbed on the reconstructed LDH. For periods as long as 3 days, the diffractograms are of poor quality, revealing that crystalline structure was partially lost. Indeed, on the third day, the X-ray diffraction peak at small diffraction angles (2θ ) 3.2°) was observed for the first time. This peak was found in all the samples treated at 3 days and longer. Even the samples took in the second week presented this X-ray diffraction peak (Figure 5f). FTIR spectra displayed in Figure 6 are composed of absorption bands due to inorganic matrix and the surfactant; of course some bands of one compound are eclipsed by the bands due to the other one. For instance, the absorption bands of carbonate anions are found between 1200 and 1700 cm-1. In this spectral window the surfactant also presents several absorptions, including the strong absorption at 1210 cm-1 due to asymmetric S-O stretching mode. Band positions are marked with dashed lines on the spectra; assignment of bands is summarized in Table 1 according to

(cm-1

)

assignment

source

νs (OH, H2O) νs (CH) νs (CH) bending (H2O) νs (Al-O) νs (CO3) νas (CO3) νas (SO4) bending (Al-O-H) νas (C-O) νs (Al-O) νs (Al-O)

HT-like compound surfactant SDS surfactant SDS HT-like compound HT-like compound HT-like compound HT-like compound surfactant SDS HT-like compound surfactant SDS HT-like compound HT-like compound

previous works.38,39 Two points should be emphasized: (1) Spectra of sample treated with surfactant for 3 days (HT-SDS-3) and longer exhibit a band at 1112 cm-1 that can be attributed to an Al-O-H bending vibration. This band was not observed in the spectra of the samples HT-SDS-1 and HT-SDS-2. (2) The band around 1370 cm-1, due to carbonate anions, diminishes in samples treated with surfactant for 3 days or longer. 27Al MAS NMR spectra in Figure 7 reveal that a minor amount of tetrahedral aluminum remains in HT-SDS-1, which, it should be emphasized, has been restored to a layered structure. However, tetrahedral aluminum progressively acquires the octahedral coordination, and after 2 weeks all aluminum recovered is octahedrally coordinated. Note that the supposition that Al(IV) is present in external layers of LDHs is consistent with their conversion to Al(VI). Alternatively, if it is assumed that Al(IV) is in a alumina segregated phase, all Al(IV) is not expected to turn to Al(VI) because in such a case aluminum is in a stable spinel structure. In Figure 8 are presented the cross polarization (CP) 1H f 13C MAS NMR spectra of HT samples, treated with surfactant for different times. The assignment of peaks is included in Table 2. A major point should be stated: after 3 days of treatment, the resonances of carbons 5, 6, and 4 turn out to be broader and a slight shift toward stronger field is observed. The attempt to take out the surfactant in sample HT-SDS-7, by calcination in air at 400 °C, promotes the loss of ordered (38) Xu, Z. P.; Zeng, H. C. Chem. Mater. 2001, 13, 4564. (39) Kannan, S.; Venkov, T.; Hadjiivanov, K.; Knozinger, H. Langmuir 2004, 20, 730.

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Figure 7. 27Al MAS NMR spectra of (a) HT-CO3-3ME, (b) HT-SDS-3, (c) HT-SDS-6, and (d) HT-SDS-14. Inset: Region where the tetrahedral aluminum signal appears.

Figure 8. 13C CP MAS NMR spectra of samples (a) HT-SDS-1, (b) HT-SDS-2, (c) HT-SDS-3, and (d) HT-SDS-14. 13

Table 2. Attribution of C NMR Peaks for Spectra Exhibited in Figure 5 carbona

chemical shift, δ (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 a

69.06 26.34 23.01 27.01 31.02 31.02 32.68 32.68 32.68 34.34 24.36 14.01

Numbering of carbon atoms from Figure 1.

mesoporous material as detected by XRD (diffractograms not shown). From this observation, it has to be concluded that the framework of layers is not chemically joined to maintain the mesoporous structure. In accordance with this result, the reflux was then prolonged 7 days more, but at the eighth day an amount

Figure 9. X-ray diffraction patterns of HT-SDS-14 sample under different conditions: (a) HT after the treatment with surfactant; (b) sample a, calcined in air at 400 °C; (c) sample a, after surfactant removal by treatment in aqueous Na2CO3.

Figure 10. Thermogravimetric profiles of (-) HT-SDS-14 and (---) HT-SDS-14-TS.

of nitric acid was added in order to promote the hydrolysis of the edge aluminums to generate continuous Al-O-Al-O chains in the mesoporous material. The surfactant removal in HT-SDS14 sample, by calcination in air at 400 °C results, again, in the destruction of the mesoporous-like material. Then the surfactant was removed under gentler conditions: by treating the sample three times with a 1 N Na2CO3 solution at 80 °C, followed by washing the sample several times with distilled water. Thus, the ordered mesoporous structure was maintained (Figure 9). In Figure 10 are compared the weight loss curves of the HTSDS-14 and HT-SDS-14-TS samples. The thermal decomposition of the surfactant is expected in the range 300-500 °C. In such a range, the sample treated in a carbonated solution lost a minor amount of weight, if compared with the HT-SDS-14 sample. It should be concluded that HT-SDS-14-TS contains a smaller amount of surfactant than HT-DS-14. The loss of water was, of course, also observed between 60 and 230 °C. Both samples lost similar amounts of water. Note, however, that in sample free of surfactant the water is easily lost at lower temperatures than the


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Figure 11. TEM images of HT-SDS-14 sample (a, b) before and (c, d) after removal of surfactant with a treatment in aqueous Na2CO3.

sample containing surfactant; this result suggests that surfactant makes the diffusion of water molecules more difficult. Finally, in the region from 600 to 800 °C, dehydroxylation and decarbonation occur; both sample lost close to 10% weight. Note that at the end of analysis the difference in total weight loss between HT-SDS-14 and HT-SDS-14-TS is close to 7%. This difference should be attributed to the amount of surfactant incorporated in HT-SDS-14, revealing that the preparation of mesostructured material was only partially achieved. 13C CP MAS NMR spectra (not shown) of samples after surfactant removal presented a very low signal/noise ratio, confirming the elimination of surfactant. In this sense, the FTIR spectrum of HT-SDS-14-TS sample shows that the intensity of bands assigned to S-O and C-H diminishes considerably in comparison with HT-SDS-14. We turn now to TEM images in Figure 11 corresponding to samples HT-DS-14 and HT-SDS-14-TS. In both samples are observed zones where disordered ducts were formed. After removal of surfactant the solid appears to be lesser dense. Indeed, the resulting material presents zones where fibrous material is identified: the material is not completely ordered, as generally occurs in typical MCM materials. Figure 12 compares the Kratky plots of samples HT-CO33ME and HT-SDS-14. The scattering profiles of sample free of surfactant exhibit a region at moderate angles where the intensity is related to h-2, and at higher scattering vector the scattering intensity tends to be proportional to h-1, clearly corresponding to bidimensional objects; that is, the scattered heterogeneities are the platelets of LDHs. Nevertheless, in surfactant-containing samples, the profile is characteristic of fibrous materials. These results support strongly textural modification as a consequence of surfactant incorporation. After removal of surfactant, the profile of the fibrous material is maintained. Table 3 compares the fractal dimension values, as determined from SAXS data, for layered

Figure 12. Kratky plots of (a) HT-CO3-3ME and (b) HT-SDS-14-TS. Table 3 sample

fractal dimensiona

specific surface areab (m2/g)

HT-CO3-3ME HT-SDS-14-TS

2.2 2.7

85 132

a Determined from SAXS data, under the Porod law. b Determined from nitrogen isotherms by application of the BET equation.

sample (HT-CO3-3ME) and folded sample after removal of surfactant (HT-SDS-14). The fractal dimension of 2.2 for HTCO3-3ME indicates a relatively smooth surface, with a rougher surface for the folded sample, fractal dimension of 2.7. Adsorption-desorption isotherms of N2 also evolve when the transformation from HT-CO3-3ME to HT-SDS-14-TS is performed (Figure 13). While both isotherms can be considered as type IV according to IUPAC classification, they disagree in the form of hysteresis loops. On one hand the type H3 loop, observed

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Figure 13. Nitrogen adsorption (b)-desorption (O) isotherms of (a) HT-CO3-3ME and (b) HT-SDS-14-TS.

Figure 14. Possible mechanism for the folding of layers of LDH to give fibrous hydrotalcites.

in the isotherm of sample HT-CO3-3ME, is often given by the aggregates of platelike particles or by adsorbents exhibiting slitshaped pores. On the other hand, the isotherm of sample HTSDS-14-TS presents a type H2 loop characteristic of materials with a complex pore structure. Another significant difference in isotherms of Figure 13 is the volume adsorbed: HT-SDS-14-TS sample adsorbs a higher amount of sorbate than HT-CO3-3ME. By application of the Brunauer-Emmett-Teller (BET) equation, surface specific areas of the lamellar and folded LDH samples was estimated to be 85 and 132 m2/g, respectively (Table 3).

Discussion The results cited above have shown that mixed oxide (Mg,Al)-O, emerged from the calcination of a regenerated layered double hydroxide, recovered the layered structure because of the memory effect of such materials when in contact with a surfactant anionic containing aqueous solution. However, after 1 day under hydrothermal treatment, the reconstructed layered structure did not incorporate the surfactant anions in the interlayer space of material. Because of the great affinity of LDHs for CO32- species, these anions occupy in the first instance the space between the brucite-like layers as revealed by the interlayer distance (7.9 Å). Furthermore, the XRD results have shown that by the second day the surfactant has been adsorbed on hydrotalcite; their incorporation was, then, only kinetically limited for times as short as 1 day. The interaction between the surfactant and brucite-like layers is enhanced as time goes on, and by the third day a mesostructured material was obtained as shown by the XRD peak at low angle. Indeed, two diffraction peaks are observed

for HT-SDS-3 which, because of the positions, do not correspond to the 003 and 006 harmonics. The intercalation of surfactant into HDL could be ruled out from this result. Instead of intercalation, it seems that a part of the material was mesostructured. 13C CP MAS NMR results demonstrated that the main interactions of surfactant/hydrotalcite layers take place between CH2 of the surfactant chain and oxygens of the brucite-like layers. This is explained because of the mobility of the surfactant chain. 27Al MAS NMR results support that the layers could be joined by tetrahedral aluminum present at the surface. After joining, tetrahedral aluminum turns out to be octahedral because of the formation of Al-O-Al groups, as confirmed by FTIR spectroscopy. Assuming this mechanism to join the layers of hydrotalcites, we have attempted to promote the hydrolysis of tetrahedral Al to give octahedral Al, catalyzed by adding HNO3. However, the acid was immediately neutralized by the LDH, a solid with predominantly basic character. Furthermore, the formation of Al-O-Al is originated by the presence of Al(IV). This was the reason to submit the HT-CO3 sample to three memory effects before the interaction with the surfactant. The creation of Al(IV) as memory effects are performed was previously reported in other works.19,20,40 It is generally agree that after a first destruction-to regeneration cycle of LDH, Al(IV) sites are not created. A first attempt to synthesize a mesostructured HT was carried out with a sample submitted to only one memory effect and, after 3 days, the obtainment of mesoporous phase (40) Da´vila, V.; Lima, E.; Bulbulian, S.; Bosch, P. Microporous Mesoporous Mater. 2008, 107, 240.


failed. Of course, the memory effect cycles could be repeated more than three times; however, in such a case most likely a stable alumina segregates. Thus, we have chosen to work with our sample HT-CO3-3ME. We return now to the mesostructured material prepared with LDH submitted to three memory effects. At the end of 14 days with surfactant, all Al(IV) turned out to be Al(VI), and the created joint remained discrete to maintain the union of layers in the mesostructured material. Note that the distribution of Al(IV) in the edges is random; thus, the layers did not join homogeneously and distortions of mesoporosity occurred. Then, the hexagonal arrangement of MCM41-like materials is not achieved. Instead, as shown by electronic microscopy, mesoporous-like hydrotalcites, randomly ordered, are obtained, with carbonate anions incorporated. From the discussion of the results the following model emerges: mixed oxide (Mg,Al)-O, in the presence of sodium dodecyl sulfate aqueous solutions, recover the layered structure typical of hydrotalcites, and then the most external layers in a particle of LDH are folded to give a mesostructured material by the formation of small amounts of Al-OH-Al groups (Figure 14). This local disturbance in the layered structure is visualized indeed as an evolution from platelike HTs to fibrous HTs. A feature to be emphasized is that mesostructured hydrotalcites result in a gain of specific surface area (SSA). Indeed, typically, the hydrotalcites outgassed at temperatures as low as 200 °C exhibit SSA of 50-90

Langmuir, Vol. 24, No. 16, 2008 8911

m2/g, but in the mesostructured form they reach 132 m2/g. This fact, apparently of little relevance, is remarkable due to interest in some applications of hydrotalcites when the Bro¨nsted basicity is present. The gain of surface area is probably due to rearrangement or folding of brucite-like layers, which produced a material built of interconnected networks of pores of different sizesandshapesassuggestedbythenitrogenadsorption-desorption isotherm and strongly supported by the fractal dimension value.

Conclusion Disordered fibrous hydrotalcites can be obtained from layered hydrotalcites and anionic surfactants. Some of the brucite-like layers of hydrotalcites are folded and they can be united through weak interactions to form Al-O-Al during the template mechanism. To form Al-O-Al bonds the presence of the Al 4-fold is required, they can be formed by submitting the LDH to repeated memory effects. The texture of the mesoporous hydrotalcites is very different, if compared with layered hydrotalcites. Acknowledgment. Thanks are due to C. Flores (IIM-UNAM) and Eng. Esteban Fregoso (IIM-UNAM) for TEM and thermal analysis, respectively. We especially acknowledge student Arlett Xoxhiquetzal Saúz Gress (ESIQIE-IPN) for technical help. LA801442N

Structural and Textural Evolution during Folding of Layers of Layered

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