Novel magnetic hydrotalcite-like compounds (HTLcs) were synthesized through introducing magnetic substrates (Fe3O4) into the Co–Al HTLcs materials by hydrothermal method. The magnetic Co–Al HTLcs with different Fe3O4contents were characterized in detail by XRD, FT-IR, SEM, TEM, DSC, and VSM techniques. It has been found that the magnetic substrates were incorporated with HTLcs successfully, although the addition of Fe3O4might hinder the growth rate of the crystal nucleus. The morphology of the samples showed the relatively uniform hexagonal platelet-like sheets. The grain boundaries were well defined with narrow size distribution. Moreover, the Co–Al HTLcs doped with magnetic substrates presented the paramagnetic property.
Keywords:Magnetic Co–Al hydrotalcite; Hydrothermal method; Paramagnetism; Nanoparticles; X-ray techniques
Hydrotalcite-like compounds (HTLcs) are a family of two-dimensional nanostructured lamellar ionic compounds, which contains positively charged layers and exchangeable anion in the interlayer [1-3]. Recently, HTLcs have received considerable attention in view of their potential usefulness as catalysts and catalyst precursors, as well as for applications in areas as diverse as medicine science, ion exchangers, oil field exploration, or sorption processes [4-7]. However, the separation and recovery of these solid base mixed oxides from the reaction products are still difficult. So a large amount of separation energy and cost are consumed for the extra equipment and treatments for separation and recovery. Therefore, it is essential to synthesize a novel solid base catalyst to extend the utility of catalysts and develop green routes. To date, most attention is put on investigating the magnetic properties of brucite-type hydroxides of general formula M2+n(OH)m(A)p (A is generally a carboxylate or dicarboxylate anion), as these layered materials present ferro-, ferri-, antiferro-, or unusual metamagnetic behaviors. For example, Pérez-Ramírez et al  reported the magnetic behavior of Co–Al, Ni–Al, and Mg–Al hydrotalcites, as well as the mixed oxides obtained after calcination. Trujillano et al.  reported the magnetic properties of a series of layered Cu–Al hydroxides intercalated with alkylsulfonates. Carja et al.  reported new magnetic layered structures which can be used as precursors for new hybrid nanostructures, such as aspirin-hydrotalcite-like anionic clays.
In the present work, we designed and synthesized magnetic Co–Al HTLcs through introducing magnetic Fe3O4nanoparticles using the hydrothermal method in autoclaves under autogenous water vapor pressure at 180 °C for 6 h. However, up to our best knowledge, such a study has not been carried out on hydrotalcite. The magnetic HTLcs materials with super-paramagnetism make them possible to achieve the ease of recovery, waste generation, environmental friendliness, and recycling of HTLcs through the external rotating magnetic field. This novel magnetic HTLcs are expected to act as green catalyst and hence solve the above mentioned disadvantages.
Magnetic nanoparticles were prepared by dissolving 0.01 mol of FeSO4and 0.01 mol of Fe2(SO4)3in water solution under stirring at 45 °C, and 20 wt% of NH3 · H2O were added dropwise together at a constant pH value of 10–11. The obtained material (Fe3O4) was recovered, washed several times with deionized water until the pH was neutral. The obtained Fe3O4was preserved as suspension.
Magnetic Co–Al HTLcs was prepared by the hydrothermal process. An aqueous solution containing 0.40 M Co(NO3)2 · 6H2O and 0.13 M Al(NO3)3 · 9H2O was added dropwise to Fe3O4solution with Fe/Co molar ratio equal to 0.01, 0.02, 0.05, and 0.2, respectively, under vigorous stirring. During the synthesis, the temperature was maintained at 60 °C and pH at about 11 by the simultaneous addition of NaOH and Na2CO3solution. Then the mixture was transferred to an autoclave pressure vessel and hydrothermally treated at 180 °C for 6 h. The autoclave was then cooled down to room temperature. The resulting solid products were separated by filtration, washed with distilled water, and dried at 80 °C for 24 h.
Powder X-ray diffraction (XRD) data were collected in the 2θ range of 5–75° on a Rigaku D/max-IIIB diffractometer using Cu Kαradiation (λ = 1.5406 Å). FT-IR spectrum was recorded on a Nicolet 5DX spectrophotometer using KBr pellet technique. Transmission electron microscopy (TEM) experiment was performed on a PHILIPS CM 200 FEG electron microscope with an acceleration voltage of 200 kV. The samples were dispersed in ethanol, and carbon-coated copper grids were used as the sample holder. Scanning electron microscopy (SEM) was performed on a Japan JEOL JSM-6480A instrument at an acceleration voltage of 20 kV and a working distance of 10 mm. Thermogravimetry-differential scanning calorimetry (DSC) was performed on a NEZSCH STA 409PC thermoanalyzer in the temperature range of 40–600 °C with a heating rate of 10 °C/min. Magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM, JDAW-2000). The nickel, aluminum, and iron contents were determined by inductively coupled plasma mass spectrometry (ICP-MS) emission spectroscopy.
Results and Discussion
The XRD patterns of magnetic Co–Al HTLcs with different Fe/Co ratios were shown in Fig. 1. All the XRD patterns of the samples showed the typical reflections of the basal (003), (006), (009), (015), (110), and (113) planes . As seen in the Fig. 1, these diffraction peaks became less narrow and intense with the increase of Fe/Co ratios. The results indicated that the addition of Fe3O4 might hinder the growth rate of the crystal nucleus. For the Fe3O4-containing samples, the third peak corresponding to the basal (009) plane was divided into two distinct peaks. The first one, recorded at lower diffraction angles corresponds to the basal spacing (009), whereas the second one might come from the Fe3O4. Moreover, the unchangeable intersheet spacing (d003) presented that the magnetism (Fe3O4) was highly dispersed in the hydrotalcite structure. Assuming a 3R polytypism for the hydrotalcite, the lattice parameters a and c have been calculated from the positions of the XRD peaks . The lattice parameters of the samples were presented in Table 1. The c and a values were quite similar for all the samples, and the differences found can be within experimental error. The element chemical analysis data for magnetic Co–Al HTLcs with different Fe/Co ratios were given in Table 2. As seen in Table 2, the values of the Co/Al ratio for magnetic hydrotalcite with lower Fe/Co ratio were close to the expected one. The result was consistent with the XRD analysis.
Figure 1. Powder XRD patterns of magnetic Co–Al HTLcs with different Fe/Co ratios. (a) 0, (b) 0.01, (c) 0.02, (d) 0.05, (e) 0.2
Table 1. Crystal structure parameters of magnetic Co–Al HTLcs with different Fe/Co ratios
Table 2. Elemental analysis results for the synthesized compounds
TEM and SEM
The morphology of magnetic Co–Al HTLcs was investigated by SEM and TEM and the images were shown in Fig. 2. As shown in Fig. 2, all the Fe3O4-containing particles consisted of relatively uniform hexagonal platelet-like sheets. The grain boundaries were well-defined with narrow size distribution. No diffraction fringes were observed, although the sample was crystalline, as evidenced by the XRD pattern.
Figure 2. SEM (a) and TEM (b) images of magnetic Co–Al HTLcs
Figure 3 displayed FT-IR spectra of the magnetic hydrotalcites samples with different Fe/Co ratios. A broad absorption band centered at ≈3,470 cm−1 present in all samples is attributed to O–H stretching vibration of hydrogen-bonded hydroxyl groups in the brucite-like sheets and of water in the interlayer space [13,14]. Water-bending vibrations of the interlayer water were observed for all samples at 1,632 cm−1. It was noted that the strong absorption bands at 1,470 cm−1 can be indexed to the v3 mode of CO32− ions. The intensity of this band became weaker when the Fe3O4 intercalate into the HTlcs. The other bands at 1,040 and 864 cm−1were characterized to the v1 and v2 modes of the carbonate ion, respectively. Besides, an absorption band at 662 cm−1 in Fig. 2a was ascribed to Co–O stretching. Such a band was also present in the spectra for the magnetic hydrotalcites samples, but centered in a lower wavenumber region, 617 cm−1. Finally, the bands at wavenumbers 417 cm−1 were attributed to Fe–OH vibrations.
Figure 3. FT-IR spectra of magnetic Co–Al HTLcs with different Fe/Co ratios. (a) 0, (b) 0.01, (c) 0.02, (d) 0.05, (e) 0.2
To examine the thermal stability of as-synthesized sample during calcination, the Thermogravimetry-differential scanning calorimetry analysis was carried out in nitrogen. The DSC curves of hydrotalcites samples with different Fe/Co ratios were shown in Fig. 4. As seen in Fig. 4, all the DSC profiles exhibited two apparent endothermic events which were in agreement with the typical decomposition mechanism of HTLcs . The first endothermal peak at lower temperature can be assigned to the desorption of the loss of interlayer water. The second endothermal peak at higher temperatures (T > 250 °C) was related to the decomposition (dehydroxylation) of the hydroxide layers and the removal of anions (carbonate) in the brucite-like layers [16,17]. It was interesting to note that, when the Fe/Co ratio was higher than 0.05, the two endothermic bands were shifted toward higher combustion temperatures. This observation can be related to the pillared effect of Fe3O4 in the layer of HTlcs.
Figure 4. DSC curves of magnetic Co–Al HTLcs with different Fe/Co ratios. (a) 0.01, (b) 0.02, (c) 0.05, (d) 0.2
The magnetic hysteresis curve of magnetic Co–Al HTLcs (Fe/Co = 0.2) performed at room temperature was depicted in Fig. 5. The magnetic capabilities of Co–Al HTLcs with different Fe/Co ratios were given in Table 3. As displayed in the figure, the magnetic curves increased linearly with increasing field at low magnetization, while the magnetic curves increased slightly with further increasing field, and finally maintained in the fixed value. The hysteresis loop of all the samples exhibited typical characteristic of paramagnetic materials with coercivity (Hc) values of 0 Oe. The emerging of paramagnetism at room temperature may be due to the Fe3O4 particle with very small particle size . As seen in Table 3, the value of saturation magnetization decreased from 15.6 to 0.98 emu/g with the decrease of Fe/Co ratios because of the change of particle size of the magnetic Co–Al HTLcs.
In summary, magnetic Co–Al HTLcs has been successfully synthesized through hydrothermal method. On the basis of XRD investigations, it has been found that the magnetic substrate (Fe3O4) was introduced into the structure of HTLcs. This result was also detected in FT-IR analysis. DSC method revealed that when the Fe/Co ratio was higher than 0.05, the exothermic bands were shifted toward higher combustion temperatures. Moreover, the introduction of Fe3O4endowed the composites with paramagnetism. We expected such novel material has promising applications as the green catalysts or catalyst supporters.
Financial support from the Key Technology R&D program of Heilongjiang Province (no. G202A423) and Science Fund for Young Scholar of Harbin City (no. 2004AFQXJ038) are greatly acknowledged.
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