3114
Chem. Mater. 2006, 18, 3114-3121
Structural Characterization and Delamination of Lactate-Intercalated Zn,Al-Layered Double Hydroxides C. Jaubertie, M. J. Holgado, M. S. San Roma´n, and V. Rives* Departamento de Quı´mica Inorga´ nica, UniVersidad de Salamanca, 37008 Salamanca, Spain ReceiVed March 2, 2006. ReVised Manuscript ReceiVed April 12, 2006
Organic-inorganic hybrid hydrotalcite-like compounds containing Zn2+ and Al3+ in the brucite-like layers (nominal Zn:Al molar ratios of 2:1, 3:1, and 4:1) and lactate (Lc) as counteranion were synthesized by coprecipitation using 0:1 and 3:1 lactic acid:Al molar ratios to prepare the starting solutions. Their physicochemical properties were studied by element chemical analysis, powder X-ray diffraction, infrared spectroscopy, and thermal analyses. Powder X-ray diffraction showed a pattern characteristic of a hydrotalcite-like structure. FT-IR spectroscopy confirmed the retention of the lactate anions in the interlayer without further contamination. Thermal analyses showed four stages of weight loss and five of heat change when recorded under oxidizing conditions. Dispersion of the dried samples in decarbonated water by ultrasound-assisted treatment at room temperature led to delamination of all wet fractions. Upon solvent evaporation, dried fractions restacked with a higher crystallinity and an increase in basal spacing.
1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays, are twodimensional materials. The best known of these materials is hydrotalcite, Mg6Al2(OH)16(CO3)‚4H2O, whose structure is derived from the brucite Mg(OH)2 (cadmium iodide structure). It consists of hexagonal closest-packing layers of anions with 100% of octahedral sites occupied by magnesium cations every two hydroxyl layers. This is termed a layer structure, as it is composed of repeating OH-Mg-OH‚‚‚ OH-Mg-OH‚‚‚OH-Mg-OH units, where the OH‚‚‚OH interaction is mainly of the van der Waals type.1 If some cations of higher charge but similar radius are isomorphically substituted for the Mg2+ cations, the brucite-like layers become positively charged. This charge excess is balanced by locating anions in the layers that are not occupied by metallic atoms, along with water molecules. In the case of the natural hydrotalcite, for each set of eight Mg2+ cations, two are substituted by Al3+. Hydrotalcite-like compounds are thus materials with positively charged, mixed-metal hydroxide sheets and negatively charged interlayer anions with water molecules. The normalized formula for all LDHs can be written as [M1-xM′x(OH)2](An-)x/n‚mH2O, where x ranges from 0.2 to 0.4 and M and M′ usually stand for divalent and trivalent metallic cations, respectively.2,3 Be-
cause of the high charge density of the layers and the high content of anionic species and water molecules, these compounds exhibit a high anisotropy in their chemical bondings, which are strong within species of a same brucitelike layer and weak between layers. This feature allows us to intercalate a wide diversity of organic or inorganic anions4-6 that modify the properties and reactivity of the material. Because of such versatile properties and the broad scope of applications that emerge from these and derived materials, LDHs have deserved outstanding interest in recent years: catalysis (as catalysts, catalyst supports, or catalyst precursors), adsorption, water purification, anionic exchange, medicine, etc. The properties and applications of these materials have been recently reviewed.7 LDHs are also able to form nanocomposites, by intercalating polymers in the interlayer. A way to facilitate this intercalation is to delaminate LDHs previously synthesized by direct synthesis or hydrothermal treatment,4 making the total surface of the LDH accessible. Nevertheless, because of the strong interlayer electrostatic interactions between the sheets, exfoliation of the sheets in water or other nonaqueous solvents remains difficult.8,9 To overcome this lack of accessibility to the interlayer space, researchers have made several attempts to weaken the stacking of the layers by exchanging the inorganic anions with organophilic ones, such as surfactants.9,10 Exfoliation of LDHs must be related to the miscibility of the solvent with the interlayer anions; they can form
* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 34-923-294-489.
(1) Adams, D. M. Inorganic Solids, An Introduction to Concepts in SolidState Structural Chemistry; John Wiley & Sons Ltd.: London, 1974; p 57. (2) de Roy, A.; Forano, C.; Besse, J. P. Layered Double Hydroxides: Synthesis and Postsynthesis Modification. In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2001; Chapter 1, p 1. (3) Kanezaki, E. Preparation of Layered Double Hydroxides. In Clay Surfaces: Fundamentals and Applications; Wypych, F., Satyanarayana, K. G., Eds.; Interface Science and Technology Series; Elsevier: London, 2004; Vol. 1, Chapter 12, p 345.
(4) (5) (6) (7)
Miyata, S. Clays Clay Miner. 1983, 31, 305. Chibwe, K.; Jones, W. J. Chem. Soc., Chem. Commun. 1989, 926. Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201. Rives, V. Layered Double Hydroxides: Present and Future; Nova Science Publishers: New York, 2001. (8) Albiston, L.; Franklin, K. R.; Lee, E.; Smeulders, J. B. A. F. J. Mater. Chem. 1996, 6, 871. (9) Adachi-Pagano, M.; Forano, C.; Besse, J. P. Chem. Commun. 2000, 91. (10) Leroux, F.; Adachi-Pagano, M.; Intissar, M.; Chauvie`re, S.; Forano, C.; Besse, J. P. J. Mater. Chem. 2001, 11, 105.
10.1021/cm060512y CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006
Lactate-Intercalated Zn,Al-Layered Double Hydroxides
Chem. Mater., Vol. 18, No. 13, 2006 3115
Table 1. Nomenclature, Amounts Reacted, and Elemental Composition of the Synthesized Samples systematic name ZAL210 ZAL310 ZAL410 ZAL213 ZAL313 ZAL413
theoretical molar ratio
Zn-Lc (g/L of soln)
Al-Lc (g/L of soln)
HLc (mL/100 mL of H20)
wt % Zn
wt % Al
wt % C
Zn:Al 2:1 HLc:Al 0:1 Zn:Al 3:1 HLc:Al 0:1 Zn:Al 4:1 HLc:Al 0:1 Zn:Al 2:1 HLc:Al 3:1 Zn:Al 3:1 HLc:Al 3:1 Zn:Al 4:1 HLc:Al 3:1
58.7 88.0 117.5 58.7 88.0 117.5
31.5 31.5 31.5 31.5 31.5 31.5
0 0 0 27.7 27.7 27.7
25.60 36.36 40.95 31.76 35.88 34.88
5.39 5.59 4.74 6.65 4.78 5.88
7.20 7.47 6.33 8.88 6.38 7.85
strong hydrogen bondings, which can lead to large volumes of solvent having access to the interlayer space, thus accelerating the delamination process and increasing the amount of delaminated material.11 In addition to dodecyl sulfate-intercalated LDHs,9,10,12-15 compounds with glycine,11,16,17 amino acids,18 lactate,19 or more recently, nitrate20,21 have also been successfully delaminated. Over and above the intercalation of large anions in the interlayer, this process should, as noted above, allow the synthesis of LDH/ polymer nanocomposites, which are of interest because the electrical, mechanical, optical, and other physicochemical properties of these materials are often favorably modified by the nanometer level of interphasic interactions.2 For example, earlier syntheses of such materials have conduced to nanocomposites with enhanced thermal12,22-28 and mechanical29,30 properties. These materials have also been claimed to have catalytic activity.16,31,32 The high dependence of the catalytic activity on the nature of the layer cations (e.g., the influence of their surface acidity or basicity) makes it worthwhile to study systems containing different metal cations. On the other hand, delamination of LDHs in water instead of in organic solvents should also be pursued, mainly because of the less-expensive and less-contaminating character of water. Consequently, although delamination of a Mg,Al-lactate system has been reported,19 in this paper, we report the delamination in water of lactate-intercalated Zn,Al-hydrotalcite-like compounds by ultrasound-assisted treat(11) Hibino, T.; Jones, W. J. Mater. Chem. 2001, 11, 1321. (12) O’Leary, S.; O’Hare, D.; Seeley, G. Chem. Commun. 2002, 1506. (13) Jobba´gy, M.; Regazzoni, A. E. J. Colloid Interface Sci. 2004, 275, 345. (14) Guo, Y.; Zang, H.; Zhao, L.; Li, G. D.; Chen, J. S.; Xu, L. J. Solid State Chem. 2005, 178, 1830. (15) Venugopal, B. R.; Shivakumara, C.; Rajamathi, M. J. Colloid Interface Sci. 2006, 294, 234. (16) Wypych, F.; Bubniak, G. A.; Halma, M.; Nakagaki, S. J. Colloid Interface Sci. 2003, 264, 203. (17) Kottegoda, N. S.; Jones, W. Macromol. Symp. 2005, 222, 65. (18) Hibino, T. Chem. Mater. 2004, 16, 5482. (19) Hibino, T.; Kobayashi, M. J. Mater. Chem. 2005, 15, 653. (20) Wu, Q.; Olafsen, A.; Vistad, Ø.B.; Roots, J.; Norby, P. J. Mater. Chem. 2005, 15, 4695. (21) Li, L.; Ma, R.; Ebina, Y.; Iyi, N.; Sasaki, T. Chem. Mater. 2005, 17, 4386. (22) Li, B.; Hu, Y.; Liu, J.; Chen, Z.; Fan, W. Colloid Polym. Sci. 2003, 281, 998. (23) Li, B.; Hu, Y.; Zhang, R.; Chen, Z.; Fan, W. Mater. Res. Bull. 2003, 38, 1567. (24) Chen, W.; Qu, B. Chem. Mater. 2003, 15, 3208. (25) Chen, W.; Feng, L.; Qu, B. Chem. Mater. 2004, 16, 368. (26) Qiu, L.; Chen, W.; Qu, B. Colloid Polym. Sci. 2005, 283, 1241. (27) Qiu, L.; Chen, W.; Qu, B. Polym. Degrad. Stabil. 2005, 87, 433. (28) Ding, P.; Qu, B. J. Colloid Interface Sci. 2005, 291, 13. (29) Hsueh, H. B.; Chen, C. Y. Polymer 2003, 44, 5275. (30) Du, L.; Qu, B.; Meng, Y.; Zhu, Q. Compos. Sci. Technol. 2006, 66, 913. (31) Wypych, F.; Bail, A.; Halma, M.; Nakagaki, S. J. Catal. 2005, 234, 431. (32) Nakagaki, S.; Halma, M.; Bail, A.; Carbajal, G. G.; Wypych, F. J. Colloid Interface Sci. 2005, 281, 417.
Zn:Al molar ratio solution solid 2 3 4 2 3 4
1.96 2.68 3.57 1.97 3.10 2.45
ment at room temperature. These samples have been previously characterized by powder X-ray diffraction, FTIR spectroscopy, and thermal analyses. Our main interest was to evaluate the influence of different treatments applied to these samples and to correlate them to delamination likeness. 2. Experimental Section The samples were prepared by the coprecipitation method,4 using zinc and aluminum lactates as precursors. Zinc L-lactate was kindly provided by Purac. Aluminum L-lactate and L-lactic acid (HLc) were both supplied by Aldrich. Sodium hydroxide pellets were from Panreac. All chemicals were used without further purification. All solutions were prepared using water previously decarbonated by boiling and bubbling N2. Two series of three samples each have been synthesized; in each series the nominal Zn:Al molar ratios are 2:1, 3:1, and 4:1, with the two series differing in the Al:lactic acid ratio. In one series, the Zn and Al lactates were reacted with NaOH without added lactic acid (i.e., 1:0 Al:lactic acid molar ratio), whereas in the other series, lactic acid was added to the basic reaction medium (1:3 Al:lactic acid molar ratio). In the case of the series synthesized without added lactic acid, a 1000 mL starting aqueous solution of Zn-lactate and Al-lactate was prepared using decarbonated water, with the Zn:Al molar ratios given above and heating at 50 °C to favor dissolution. Details on the amounts of reagents used are given in Table 1. This solution was dropwise added under flowing N2 gas during 5 h to 100 mL of decarbonated water previously adjusted to pH 10 by the addition of an 8 M NaOH solution. The pH was adjusted and controlled with a Crison pHBurette 24 instrument. The whitish suspension was vigorously stirred magnetically in an inert atmosphere and aged for 24 h at room temperature. The solid was then separated from the suspension by centrifugation (4000 rpm, 8 min) and was washed several times with decarbonated water until the supernatant water became only slightly cloudy. The wet Lc-LDHs were finally dried under vacuum. As said above, in the synthesis using an excess of lactic acid, the molar amount of lactic acid was three times that of Al-lactate. The lactate anion is provided by lactic acid, which becomes deprotonated when dissolved in a basic medium; a 100 mL aqueous solution of L-lactic acid was adjusted to pH 10 by adding 8 M NaOH, and then the as-above mixed solution of zinc and aluminum lactates was added dropwise. The consequent treatment and steps of the process were the same as in the method without lactate anion described above. The samples are named ZALx1y, where x stands for the nominal Zn:Al molar ratio (2, 3, 4) in the starting solution and y stands for the initial Al:HLc molar ratio (0, 3). When describing the results obtained, unless stated, we will group the samples in two series: those synthesized without an excess of lactate anion (ZAL0 series) and those with an excess of lactate anion (ZAL3 series). Delamination under reflux conditions in water or butanol was unsuccessful; however, it was attained by ultrasonic treatment in a Fungisonics monobarrel 2.8 L apparatus with an operating power of 100 W. A portion of 0.4 g of dried LDH was mixed with 2 mL
3116 Chem. Mater., Vol. 18, No. 13, 2006 of decarbonated water in a Petri capsule; the resulting paste, rapidly covered with a thin Parafilm tape to avoid any contamination by atmospheric CO2, was ultrasonicated for 30 min at room temperature. The paste was placed on a sample holder, and the XRD pattern was recorded. The tape was then removed and, after total evaporation of the solvent in air, the resulting solid was ground and its XRD pattern again recorded. Element chemical analyses for Zn and Al were carried out by atomic absorption in a Mark 2 ELL-240 apparatus, in Servicio General de Ana´lisis Quı´mico Aplicado (University of Salamanca, Spain), after previous dissolution of the samples in nitric acid. The carbon content was determined on a Leco CHNS-932 apparatus. Powder X-ray diffraction (PXRD) patterns were recorded on a Siemens D-500 instrument, using graphite-filtered Cu KR radiation (λ ) 1.54 Å). The operating voltage and current were 40 kV and 30 mA, respectively (power 1200 W). The 2-70° range (2θ scale) was investigated with a step size of 0.05° and a step time of 1.5 s. The diffractometer was interfaced to a DACO-MP data acquisition microprocessor provided with Diffract AT software. Identification of the crystalline phases was made by comparison with the JCPDS files33 and literature data. FT-IR spectra in the 4000-400 cm-1 region were recorded on a Perkin-Elmer FTIR 1600 instrument using the KBr pellet technique (ca. 1 mg of sample/300 mg of KBr). One hundred spectra, recorded with a nominal resolution of 4 cm-1, were averaged to improve the signal-to-noise ratio. Thermogravimetric (TG) and differential thermal (DTA) analyses were carried out in Instituto de Recursos Naturales y Agrobiologı´a (CSIC, Salamanca, Spain) using a SDT600 apparatus from TA Instruments in flowing (100 mL/min) nitrogen or synthetic air (from L’Air Liquide) at a heating rate of 10°/min. Alumina (Merck) calcined at 1200 °C was used as a reference for the DTA measurements.
3. Results and Discussion 3.1. Characterization of Lactate-Intercalated Zn,AlLayered Double Hydroxides. Element chemical analysis data for metals and carbon content for all six samples are given in Table 1. The results for the ZAL0 series samples showed a reasonable agreement (90-98%), within experimental error, between the Zn:Al molar ratios in the solids isolated and those of these cations in the parent solutions, suggesting that precipitation was almost complete. In some cases (samples ZAL310 and ZAL410), the solid is somewhat enriched in aluminum when compared with the composition of the corresponding starting solutions. However, this lack of coincidence between ratios is rather common in the literature and has been ascribed to a preferential precipitation of one or another cation as hydroxide during the stirring process.34 Although the Zn:Al molar ratios in samples ZAL310 and ZAL410 are really close (90%) to that existing in the starting solution, it is worth noticing that these samples are not chemically pure but contain zinc oxide (as shown by PXRD, see below). The Zn:Al molar ratios in the solids (33) JCPDS PDF: Joint Committee on Powder Diffraction Standards Powder Diffraction Files; International Centre for Diffraction Data: Newtown Square, PA, 1977. (34) de Roy, A.; Forano, C.; El Malki, K.; Besse, J. P. Anionic Clays: Trends in Pillaring Chemistry. In Expanded Clays and Other Microporous Solids, Synthesis of Microporous Materials; Occelli, M. L., Robson, H., Eds.; Van Nostrand Reinhold: New York, 1997; Chapter 7, p 108.
Jaubertie et al.
Figure 1. PXRD patterns of (a) ZAL210, (b) ZAL310, (c) ZAL410, (d) ZAL213, (e) ZAL313, and (f) ZAL413 (* ) zincite, ZnO).
of series ZAL3 are close to the values of the starting solutions (98-103%) for samples ZAL213 and ZAL313. However, the experimental Zn:Al molar ratio for sample ZAL413 is rather unexpected and very much lower than the value in the solution (60%); the large excess of zinc and lactate used during the synthesis may have lead to the formation of a zinc lactate phase (which is eliminated by washing) or a zinc-lactate complex, leaving an aluminum-rich solid. In the ZAL3 series, zinc lactate precipitates and zincite is not formed in sample ZAL413. The discrepancy between the Zn: Al ratios in the solutions and in the solid phases in the ZAL0 series is probably due to the too high pH used, and then a better agreement is found when lactic acid is added. Powder X-ray diffraction patterns for both sets of samples are shown in Figure 1. All six samples patterns indicate formation of mostly one well-crystallized hydrotalcite-like phase (JCPDS: 38-0486),33 although with impurities in some cases, as discussed below, with harmonics due to the basal planes (00l), which confirms a layered structure. Samples have not been contaminated by atmospheric carbon dioxide, as no peak corresponding to a secondary carbonateintercalated hydrotalcite-like phase is recorded.33 Weak and strong peaks due to zincite (JCPDS: 36-1451)33 are also recorded for samples ZAL310 and ZAL410, respectively. Crystallinity is not extremely high in any case, but it should be noted that the ZAL0 series samples (Figure 1, patterns a-c) exhibit sharper and more-symmetric reflections at a lower diffraction angle than the ZAL3 series samples (Figure 1, patterns d-f), which indicates a more-organized stacking arrangement in the first series.35,36 In all samples, the intercalation of the organic anion leads to the expansion of the interlayer space in comparison with the values reported in the literature for the Zn,Al-carbonate hydrotalcite (Zn: Al molar ratio ) 3).33 So, sample ZAL210 shows main diffraction maxima at 14.64, 7.13, 4.78, and 3.61 Å, corresponding to diffraction by basal planes (003), (006), (009), and (00.12), respectively, of the hydrotalcite-like structure with a rhombohedral packing of the layers. The diffractogram also exhibits a double set close to 61° 2θ where the maximum at 1.53 Å corresponds to diffraction by planes (110).37 Diffraction lines due to planes (0kl) are not clearly recorded, as they are extremely broad and tailing toward (35) The´venot, F.; Szymanski, R.; Chaumette, P. Clays Clay Miner. 1989, 37, 396. (36) Hickey, L.; Kloprogge, J. T.; Frost, R. L. J. Mater. Sci. 2000, 35, 4347. (37) Cavani, F.; Trifiro´, F.; Vaccari, A. Catal. Today 1991, 11, 173.
Lactate-Intercalated Zn,Al-Layered Double Hydroxides
Chem. Mater., Vol. 18, No. 13, 2006 3117
Table 2. Formula and Lattice Parameters (Å) of the Synthesized Samples sample
formula
c
a
d003
ZAL210 ZAL310 ZAL410 ZAL213 ZAL313 ZAL413
[Zn0.66Al0.34(OH)2](Lc)0.34‚1.82H2O
43.36 41.39 40.89 40.39 40.65 40.54
3.06 3.07 3.06 3.05 3.05 3.06
14.45 13.80 13.63 13.46 13.55 13.51
a
a a
[Zn0.66Al0.34(OH)2](Lc)0.34‚0.99H2O [Zn0.76Al0.24(OH)2](Lc)0.24‚0.77H2O [Zn0.71Al0.29(OH)2](Lc)0.29‚0.63H2O
Not determined because of the presence of ZnO (see text).
higher 2θ values, as observed in other hydrotalcite-like materials synthesized by the coprecipitation method.38 These spacing values are not fully in agreement with those previously reported by Hibino et al.19 for Mg,Al-hydrotalcite-like compounds intercalated with lactate anion, for which the diffraction maximum corresponding to basal plane (003) was recorded at 7.6 Å. Lattice parameters c and a and the basal spacing d have been calculated for all samples, assuming a rhombohedral symmetry and an hexagonal cell (polytype 3R1);39 the values are included in Table 2. The c parameter, which corresponds to three times the distance between the brucite-like layers, has been calculated from the first two harmonics, ascribed to planes (003) and (006), respectively, according to c ) 3/2(d003 + 2 d006);40,41 the a parameter, the average cation-cation distance within the layers, has been calculated from the position of the first peak in the doublet close to 2θ ) 61°, due to planes (110), according to a ) 2d110.37 The c lattice parameter mainly depends on the thickness of the interlayer, as it is a measure of the Coulomb forces between the layer and the interlayer; the calculated values of c for ZAL0 and ZAL3 series being very close to each other (c = 41-43 Å) suggests a similar extent of electrostatic interaction regardless of whether lactic acid is used during the synthesis. The average values of d003 for each set of samples are 13.96 and 13.48 Å, respectively. If the width of the brucite-like layer (4.8 Å)42 is subtracted, the gallery height is 9.16 Å for the ZAL0 series and 8.68 Å for the ZAL3 series. The size of the lactate anion, as calculated with the program ChemLab (Chem. Office Ultra 9.0, 2005) and taking into account the van der Waals radii,43 is 7.23-7.48 Å, somewhat smaller than the gallery height. This means there is room enough to locate the organic anion with its hydrocarbonated chain perpendicular to the layers in the interlayer, along with water molecules. With regards to the a parameter, it is directly related to the distance between two neighboring octahedra in the same brucite-like layer; its value exclusively depends on the size of the layer cations, i.e., on their ionic radii. The a value in all samples almost corresponds to the brucite one (= 3.05 Å),37 despite
the difference existing between the radii of the divalent and trivalent cations here studied. FT-IR spectra were recorded for all six samples. The FTIR spectra of the lactate-intercalated LDH samples are very similar to each other; for this reason, only the FT-IR spectrum for sample ZAL210 is shown in Figure 2, along with that for the ultrasonicated wet fraction derived from sample ZAL210 and for commercial zinc lactate. The spectrum for sample ZAL210 is composed of absorption bands due to normal vibration modes corresponding to layer hydroxyl groups, water molecules, M-O and M-O-M′ stretching vibrations (M and M′ being the layer cations), whose ascription has been thoroughly discussed in the literature,44,45 and interlayer lactate anion. Bearing in mind the chemical formula of lactate anion, CH3-CHOH-COO-, its intercalation in the interlayer should give rise to absorption bands due to the carboxylate group, the hydroxyl group, and the carbon-hydrogen bonds (methyl group and C-H linked to the hydroxyl group). Moreover, comparison of the spectra of the intercalated samples with that of zinc lactate used as a precursor in the synthesis might confirm the intercalation of the lactate anion in the interlayer. Intense bands are recorded at 1580 cm-1 (overlapped with the band generated by the deformation mode of water) and 1390 cm-1, assigned to the antisymmetric and symmetric stretching modes, respectively, of the carboxylate group.46 The presence of this group is also responsible for the shoulder at 2870 cm-1 on the broad band centered in 3450 cm-1; this shoulder corresponds to stretching modes of C-H.47 Deformation vibrations are also responsible for the peak at 1390 cm-1 (overlapped with the symmetric stretching mode of the carboxylate group) and for very weak peaks at 1310 and
(38) Barriga, C.; Ferna´ndez, J. M.; Ulibarri, M. A.; Labajos, F. M.; Rives, V. J. Solid State Chem. 1996, 124, 205. (39) Bookin, A. S.; Cherkashin, V. I.; Drits, V. A. Clays Clay Miner. 1991, 41, 631. (40) Ulibarri, M. A.; Labajos, F. M.; Rives, V.; Trujillano, R.; Kagunya, W.; Jones, W. Inorg. Chem. 1994, 33, 2592. (41) Ulibarri, M. A.; Labajos, F. M.; Rives, V.; Trujillano, R.; Kagunya, W.; Jones, W. Mol. Cryst. Liq. Cryst. 1994, 244, 167. (42) Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. (43) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, Principles of Structure and ReactiVity, 4th ed.; Harper Collins: New York, 1993; p 114.
(44) Kloprogge, J. T.; Frost, R. L. Infrared and Raman Spectroscopic Studies of Layered Double Hydroxides (LDHs). In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2001; Chapter 5, p 139. (45) Kloprogge, J. T. Infrared and Raman Spectroscopy of Naturally Occurring Hydrotalcites and Their Synthetic Equivalent. In The Application of Vibrational Spectroscopy to Clay Minerals and Layered Double Hydroxides; Kloprogge, J. T., Ed.; CMS Workshop Lectures, The Clay Mineral Society: Aurora, CO, 2005; Vol. 13, p 203. (46) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; J. Wiley & Sons Inc.: New York, 1997. (47) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: London, 1975.
Figure 2. FT-IR spectra of (a) ZAL210, (b) the dried fraction obtained from the ultrasound-assisted treatment (power 100 W) in decarbonated water of sample ZAL210, (c) commercial zinc lactate in the 400-4000 cm-1 range (* ) peaks registered in ZAL210 and commercial zinc lactate).
3118 Chem. Mater., Vol. 18, No. 13, 2006
Jaubertie et al. Table 3. Thermogravimetric Parameters of the As-Synthesized Samples sample ZAL210 ZAL310 ZAL410 ZAL213 ZAL313 ZAL413
T1 (°C) T3 (°C) T4 (°C) T5 (°C) W1+2+3a W4b 99 104 102 98 103 108
241 226 216 249 212 223
408 404 392 415 397 404
788 800 800 785 801 794
18 17 15 17 17 20
19 18 15 28 22 20
total weight loss (%) 37 35 30 45 39 40
a Sum of first, second, and third weight losses, in percent. b Amount of the fourth weight loss, in percent.
Figure 3. TG and DTA traces of ZAL210 recorded in (solid lines) oxidizing and (dotted lines) inert atmospheres.
600 cm-1. Stretching C-O vibrations from the hydroxyl group give rise to absorptions in the 1200-1100 cm-1 region; in our case, these vibrations result in bands of medium and low intensity at 1120 and 1045 cm-1, respectively.47 Finally, C-H bonds give rise to weak peaks; antisymmetric and symmetric stretching modes of the CH3 group are responsible for the weak peak at 2980 cm-1 and for an even weaker peak at 2930 cm-1, respectively.48 The low-intensity peak at 1440 cm-1 is due to the antisymmetric deformation mode of the methyl group, whereas the symmetric deformation mode of the same group results in a peak at 1390 cm-1, superimposed with symmetric stretching mode νCO and hydroxyl deformation, and in two weak peaks at 1360 and 850 cm-1.47 The rocking mode of the methyl group is superimposed to the stretching modes of C-O and C-C bonds in the peak of medium intensity at 1120 cm-1.47 The interpretation of the spectrum of sample ZAL210 and its comparison with that for commercial zinc lactate confirms the intercalation of the lactate anion in the interlayer of the as-synthesized hydrotalcite-like compound. The same conclusion can be reached for the other five samples. Thermogravimetric and differential thermal analyses of all six samples were first carried out in nitrogen and then in synthetic air to detect any combustion process occurring during calcination. The curves obtained were very similar for all samples using a given reaction atmosphere. The TG curves together with the corresponding DTA curves for sample ZAL210 are shown in Figure 3, and the thermogravimetric parameters for the six samples studied in air are summarized in Table 3. The formulas of all six samples are given in Table 2. Sample ZAL210 undergoes a progressive weight loss in four steps when the temperature increases, whatever the atmosphere used. Associated with this loss, up to five stages can be observed in its DTA curve recorded in an oxidizing atmosphere. According to previously reported data,49 the first two broad, weak, ill-defined endothermic DTA effects recorded around 99 and 160 °C correspond to the removal of physisorbed and interlayer, weakly bonded water molecules, respectively. These two effects are im(48) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: London, 1990. (49) Rives, V. Study of Layered Double Hydroxides by Thermal Methods. In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers: New York, 2001; Chapter 4, p 115.
mediately followed by a third endothermic effect, centered at 240 °C and extending up to ca. 300 °C, that is attributed to dehydroxylation of the brucite-like layers. These three processes lead to the collapse of the layered structure.50 The corresponding TG profile shows a multiple, overlapped weight loss in the temperature range 90-300 °C. Ill-defined inflection points can hardly be distinguished in the TG curve, making uncertain any calculation leading to the determination of the water content from the first two weight-loss steps, as the weight loss due to removal of interlayer molecular water cannot be distinguished from that due to dehydroxylation. The first sharp endothermic DTA peak is coincident when the curves are recorded in nitrogen or synthetic air, thus confirming that it corresponds to a nonoxidizing process, i.e., dehydration via the removal of physisorbed and interlayer water molecules. The DTA diagram recorded in nitrogen shows an endothermic effect between 240 and 510 °C that must correspond to dehydroxylation, whereas in an oxidizing atmosphere, the sharp, intense, exothermic effect centered at 408 °C after destruction of the layered structure should be associated with the combustion of the organic interlayer anion. Comparison of the DTA curves recorded in nitrogen and oxygen atmospheres in the 400-500 °C range suggests that, under oxidizing conditions, dehydroxylation and lactate combustion take place simultaneously, with dehydroxylation being endothermic and canceled by the exothermic peak corresponding to lactate combustion. The noticeable asymmetry of this exothermic peak indicates that this process takes place in more than one single step, probably if some lactate anions or other (unidentified) intermediate moieties formed at the beginning of the lactate combustion remain strongly retained to the layers and are finally removed at a somewhat higher temperature. The exothermic effect at 788 °C has no counterpart in the TG curve, suggesting that it corresponds to a phase-change process. The DTA results in the 600-900 °C range have been related to the PXRD study of the solids obtained after calcination at increasing temperatures in air. The PXRD patterns of the calcined sample are included in Figure 4. After calcination at 600 °C (i.e., after removal of hydroxyl groups and lactate anions), the layered structure collapses and crystallization of zinc oxide (zincite, JCPDS: 36-1451),33 begins. Although thermal decomposition of the Zn,Allactate LDH should produce some aluminum-containing phase, it could not be detected by PXRD, suggesting that it exists as an amorphous phase or as a solid solution of (50) Rives, V. Inorg. Chem. 1999, 38, 406.
Lactate-Intercalated Zn,Al-Layered Double Hydroxides
Figure 4. PXRD patterns of ZAL210 (a) at room temperature and calcined at (b) 600, (c) 700, and (d) 900 °C (* ) zincite, ZnO; I ) gahnite, ZnAl2O4).
aluminum in zincite (Zn(Al)O). Upon calcination at 700 °C, the enhanced zinc oxide peaks are recorded, together with additional weak peaks due to a spinel phase (gahnite, JCPDS: 5-0669).33 Formation of Zn(Al)O could also account for the formation of the gahnite phase above 700 °C because of structural similarities. Calcination at 900 °C leads to crystallization of the well-defined simple oxide (ZnO) and spinel (ZnAl2O4) phases, without formation of new phases. Overall, the decomposition follows a pattern similar to that previously reported for other layered materials containing organic moieties in the interlayer.38,51 With respect to the DTA curves of all other samples, they all show the same behavior. Some differences in the temperatures at which the effects are recorded on the DTA curves of ZAL3 series samples can be noticed (Table 3). Variations for ZAL0 series cannot be accounted for, because of the simultaneous presence of zincite and the LDH phase. It should be noted that the endothermic effect corresponding to dehydroxylation slightly shifts toward lower temperature when the Zn content is increased. Actually, taking into account that the Zn:Al molar ratio for sample ZAL413 lies between those of samples ZAL213 and ZAL313, it is observed that from a 1.97 Zn:Al molar ratio (sample ZAL213) to a 3.10 Zn:Al molar ratio (sample ZAL313), the temperature for removal of hydroxyl groups decreases from 249 to 212 °C (Tables 2 and 3). A shift of the peak corresponding to dehydration toward lower temperature when the charge density decreases has already been reported in the literature for carbonate-intercalated hydrotalcite-like compounds.52,53 Relationships between the dehydroxylation temperature and the Zn:Al molar ratio have been less-studied. Nevertheless, once physisorbed and interlayer water molecules have been removed, the remaining material is composed of a mixed zinc and aluminum hydroxide intercalated with lactate. As dehydration of zinc hydroxide occurs at 140 °C54 and aluminum hydroxide dehydration is observed (51) Kannan, S.; Rives, V.; Kno¨zinger, H. J. Solid. State Chem. 2004, 177, 319. (52) Lo´pez-Salinas, E.; Torres-Garcı´a, E.; Garcı´a-Sa´nchez, M. J. Phys. Chem. Solids 1997, 58, 919. (53) Kun, S. K.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 348. (54) Oswald, H. R.; Asper, R. In Preparation and Crystal Growth of Materials with Layered Structures; Lieth, R. M. A., Ed.; Reidel Publishing: Dordrecht, The Netherlands, 1977.
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around 270 °C,55 it is expected that dehydroxylation shifts toward lower temperature when the zinc content increases. Because the PXRD analyses of the residue of sample ZAL210 after calcination indicate the presence of a mixture of ZnO (or Zn(Al)O) and the Zn,Al spinel, the total weight loss may be used to estimate the number of interlayer water molecules; consequently, the same calculation can be made for all the samples. The total weight loss after calcination at 900 °C of sample ZAL210 amounts to ca. 37% of the initial sample weight, of which dehydration accounts for 18%, whereas dehydroxylation and combustion of the organic interlayer anion contribute around 19%. From the total weight loss values summarized in Table 3 and the chemical composition of the samples (see Table 1), we can determine the number of water molecules existing in the interlayer space, and hence, the formulas of the compounds, ; the results are given in Table 2. Lactate content has been calculated from carbon content and from the formula of the lactate anion. No value is given for samples ZAL310 and ZAL410 because of the simultaneous presence of ZnO. 3.2. Delamination Process and Reconstruction. Delamination was first studied under reflux conditions on a dried sample of ZAL210, using butanol or decarbonated water as the dispersant. Butanol (330 mL) was added to 0.5 g of sample ZAL210. This suspension, covered with Parafilm tape to avoid any carbon dioxide contamination, was dispersed by means of ultrasound (power 100 W) for 15 min at room temperature. The as-prepared dispersion underwent a reflux treatment for 16 h in an inert atmosphere, maintaining a temperature of 110 °C with a glycerine bath and vigorous magnetic stirring. After this period, the product was left to settle and three fractions were isolated; the slightly cloudy and dispersed suspension, the solid decanted at the bottom of the flask, and some solid particles obtained after centrifuging of the suspension. All three fractions were characterized by XRD. Oriented aggregates were used for the suspension and the solid; for the third fraction, the centrifuged solid particles were dried and their XRD diagram recorded by the conventional method. The same experimental procedure was followed, using decarbonated water as solvent. In the case of butanol, the diffractogram of the suspension showed only a broad effect between 20 and 40° (2θ scale), characteristic of the glass sample holder. The diagram is flat, without any peak around 2θ = 61°, which could characterize a brucite-like layer. XRD diagrams of the settled solid and the solid particles are similar to that for sample ZAL210, leading to the conclusion that no delamination process occurred. In the case of decarbonated water, the formation of zinc oxide is observed in the case of the solid and the solid particles, whereas the presence of a hydrotalcite-like phase is evidenced from the diffractogram of the suspension, with intense and well-defined harmonic peaks at low 2θ angles. The formation of zincite can be due to the crystal(55) (a) Wu, C.-P.; Lee, J.-S.; Liao, Y.-J. Thermal Analysis of Aluminum Trihydroxide. Presented in part at the 32nd Annual Conference on Thermal Analysis and Applications, North American Thermal Analysis Society (NATAS), Williamsburg, VA, Oct 2004. (b) Earnest, C. M. Thermal Analysis in the Alumina Industry. Presented in part at the 2nd Annual Thermal Analysis Technical Presentations, Smyrna, GA, Nov 2004.
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Jaubertie et al.
lization of initially amorphous zinc oxide.56In conclusion, delamination under reflux conditions is not suitable in the case of lactate-intercalated Zn,Al-layered double hydroxides. Delamination was also studied, first on samples ZAL210 and ZAL213, by ultrasonic treatment, using butanol or decarbonated water as dispersants and with an ultrasound power of 0 (i.e., no ultrasound), 100, or 200 W. In the case of butanol on sample ZAL210, powers of 0 or 100 W have no effect on the original sample, whereas using a power of 200 W produces the destruction of the hydrotalcite-like phase (the diffractogram of the wet fraction resulting from the treatment of sample ZAL210 corresponds to that of commercial zinc lactate). In the case of the same dispersant on sample ZAL213, the hydrotalcite-like phase is still present, without a loss in the intensities of the diffraction peaks, whatever the ultrasound power is. The same study with decarbonated water on samples ZAL210 and ZAL213 showed that, in both cases, no change was observed without application of ultrasound, with the diffractograms recorded for the wet fractions being similar to those of the samples. The results of the treatment in decarbonated water, with an ultrasound power of 100 W for all samples, are shown in Figure 5. As discussed above, a very broad effect extending between 20 and 33° (2θ scale), characteristic of the glass sample holder, can be recorded in the diffractograms of the wet fractions in all cases (Figure 5, a curves). Compared with the original samples (Figure 5, curves Φ), almost all peaks
corresponding to basal planes disappear in the case of the wet fractions, whereas the intensity of the peak corresponding to diffraction by plane (110) (2θ = 61°) does not change or scarcely decreases. As discussed above, the a parameter corresponds to the average distance between two neighboring octahedra in a given brucite-like layer, whereas the intensity of the diffraction maxima due to diffraction by (00l) planes depends on the number of stacked layers; the higher the number of planes, the more intense the reflected intensity. Consequently, the intensity of the peak recorded is proportional to the number of planes or rather, in our case, to the thickness of the layers or, in other words, to the number of stacked layers in the compound. The decrease in the intensity of the peaks at a low diffraction angle indicates that the brucite-like layers are being separated. During the exfoliation process, decarbonated water overcomes the van der Waals forces that maintain the hydrocarbon chains bonded to the layers; these layers lose their short-range spatial correlation and, for that reason, peaks associated with reflection (00l) are disappearing.13 Delamination is not complete, as some peaks corresponding to reflection by basal planes are still recorded, although with a much lower intensity than in the pristine samples. It is worth noticing that the presence of an impurity such as zinc oxide (samples ZAL310 and ZAL410) does not prevent these samples from delaminating. With regard to the dried fractions (Figure 5, b curves), as the solvent is evaporating, the attractive van der Waals interactions become predominant and all fractions restack, recovering the hydrotalcite-like phase.33 The fractions resulting from samples ZAL210 and ZAL213, which were the more-crystalline pristine materials, restack to solids, giving rise to XRD diagrams with smaller intensity and definition of the peaks at a low diffraction angle, as if these reconstructed fractions were less organized than the original ones.57 The other dried fractions restack, presenting a higher crystallinity. Besides, peaks corresponding to basal planes are much better resolved. Like glycine-intercalated layered double hydroxides delaminated in formamide,11 the ultrasound-assisted treatment in decarbonated water allows us to obtain better-ordered lactate-intercalated hydrotalcite-like phases. Besides, as was already confirmed in previous works, peaks corresponding to basal reflections slightly shift toward lower 2θ values in the dried fractions, i.e., restacked fractions present a higher basal spacing than the parent compounds. This can indicate a new ordering of the intercalated anions or the incorporation of more water molecules in the interlayer.10 It must be highlighted that, although the wet fractions are left to dry in an air atmosphere, they are not contaminated by carbon dioxide. As for the delamination process, zinc oxide, although in appreciable quantities, does not keep samples ZAL310 and ZAL410 from recovering their restacked structure. The same treatment with a power of 200 W leads to the same conclusions, without improving the delamination process. Stable, translucent, colloidal solutions containing 4 g of LDH per liter of decarbonated water were obtained after ultrasonic treatment and 2 days of stirring, which confirms total delamination.9-11
(56) Kooli, F.; Depege, C.; Ennagadi, A.; de Roy, A.; Besse, J. P. Clays Clay Miner. 1997, 45, 92.
(57) Rajamathi, J. T.; Ravishankar, N.; Rajamathi, M. Solid State Sci. 2005, 7, 195.
Figure 5. PXRD patterns of the (a) wet and (b) dried reconstructed fractions resulting from the ultrasonic treatment (power 100 W) in decarbonated water of the (Φ) pristine samples (* ) zincite, ZnO).
Lactate-Intercalated Zn,Al-Layered Double Hydroxides
For a complete characterization of the dried fractions, the FT-IR spectra of all reconstructed samples were registered. Like the pristine materials, all were similar; for that reason, only the FT-IR spectrum of the dried fraction resulting from sample ZAL210 is included in Figure 2 (curve b). This spectrum is almost coincident with that for sample ZAL210. Some peaks (such as the ones at 1440 or 1085 cm-1) are better recorded in the case of the dried fraction, but this might be due to the slightly higher transmittance of this fraction. Delamination does not modify the FT-IR spectra, as all samples recover the hydrotalcite-like phase. 4. Conclusions Lactate-intercalated Zn,Al-layered double hydroxides were prepared by coprecipitation, and the synthesis route was optimized to avoid any contamination by atmospheric carbon dioxide. Nominal cationic molar ratios in the solids were in most of the cases in quite good agreement with the ones in the starting solutions. PXRD indicate for all samples the formation of mostly one well-crystallized hydrotalcite-like
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phase. The intercalation of the lactate anion led to an increase in the basal spacing, compared with carbonate-intercalated Zn,Al-layered double hydroxides. The optimum molar ratio for obtaining crystalline samples was Zn2+/Al3+ ) 2. Element chemical analysis and FT-IR spectroscopy confirmed lactate was the unique anionic moiety in the interlayer. Upon calcination, water molecules and hydroxyl groups were first eliminated, inducing the structure to collapse; the combustion of the interlayer lactate anion then took place. An increase in temperature led to the formation of a mixture of simple zinc oxide and spinel. In the case of the ZAL3 series, when the charge density of the layers was decreased, dehydroxylation occurred at lower temperature as a consequence of the weaker layer-interlayer interactions. The ultrasoundassisted dispersion of samples in decarbonated water allowed for almost complete delamination of all the wet fractions. Acknowledgment. The authors thank ERDF and MCyT for Grant MAT2003-06605-C02-01. CM060512Y
