Preparation and Characteraction of New Magnetic Co–Al HTLc/Fe3O4Solid Base
© to the authors 2008
Received: 9 May 2008
Accepted: 18 August 2008
Published: 5 September 2008
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.
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 Kα radiation (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
Crystal structure parameters of magnetic Co–Al HTLcs with different Fe/Co ratios
Elemental analysis results for the synthesized compounds
x m a
TEM and SEM
Magnetic property of magnetic Co–Al HTLcs with different Fe/Co ratios
Saturation magnetization (emu/g)
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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