Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.53 n.4 Concepción dic. 2008
J. Chil. Chem. Soc, 53, N° 4 (2008) págs: 1682-1683
SYNTHESIS AND ABSORPTION PROPERTIES OF NOVEL NA SPECIFIC ADSORBENT Li 1+xLaxZr2-x(Po4)3
Department of Chemistry, Dezhou University, shandong Dezhou 253023, P. R. China;
A novel kind of adsorbent Li 1+xLaxZr2-x(Po4)3 was synthesized by solid state reaction method. The influence of the content of doping lanthanum on the adsorbent Li 1+xLaxZr2-x(Po4)3 was investigated by XRD and FTIR spectra while the morphology of powders was observed by SEM. The investigation of the adsorption properties showed that the adsorbent can selectively adsorb sodium with the adsorption capacity of 49.83 mg/g. The optimum conditions of adsorption are at pH 10.0-11.0 in LiCl solution.
Keywords: LiCl, Li 1+xLaxZr2-x(Po4)3 , adsorbent, separation.
Lithium chloride is an industrial raw material, from which lithium compounds and in particular metallic lithium are produced. To make further processing of the lithium chloride more economic and efficient, it is very necessary to provide the raw material as puré as possible. The presence of very small quantities of sodium in the lithium metal will make it highly reactive and much different in properties than purity lithium metal. So as the raw material of LiCl it is required in low content of Na.
The ordinary separation method is to extract sodium with isopropanol1, which not only consumes substantive organic solvent, but also is serious harm to environment. The adsorption method is briefly and feasible in theory, but the synthesis of an applicable adsorbent is a big problem. In the decades, antimonic and polyantimonic acid have been studied in the field2, but it failed to practice because of the high cost. At present, to meet the rapidly increasing demand on lithium chloride, especially the high puré lithium chloride, it is urgent to remove Na+ in the produce of the high puré lithium chloride.
NASICON(Acronym of natrium superionic conductor) materials present interesting sensitive and selective properties against alkaline cations due to their structure. Li 1+xLaxZr2-x(Po4)3 possesses the NASICON-type structure, is especially good candidates to determine alkaline ions concentrations in solution or to sepárate monovalent cations from a mixture of multivalent ions3,4.
Li 1+xLaxZr2-x(Po4)3 composed of both MO6 octahedra and P04 tetrahedra which are linked by their corners to form a opened-three-dimension (3D) network structure. The resulted structure consists of Type I sites(octahedral O-coordination) and Type II sites(10-fold O-coordination) forthe mobile Li ions to occupy. Li+ ions move from one site to another passing through bottle-necks defined by the anionic skeleton [AlxTi2x(P04)3](1+x)-.
Zr(4d series) and Ti(3d series) which belongs to same group of elements in the periodic table, have similar chemical properties. Lanthanide series are widely used in the field of catalysts, functional materials and new material5. The valence of La and Al mainly is +3, Then, the attempt was done in this paper to improve the adsorption performance by substitute La and Zr for Al and Ti.
The results showed that Li 1+xLaxZr2-x(Po4)3 show higher absorption capacities towards Na+. The exchange capacity of Li1.4La0 4Zrt 6(P04)3 was 49.83 mg/g, which is four times of Li1.4La0 4Zrt 6(P04)3. Compared with other methods, the novel Na specific adsorbent of Li 1+xLaxZr2-x(Po4)3, described here is a simpler and more convenient way to remove Na+ from LiCl solution which suggest the promising application.
1 Preparation of Li 1+xLaxZr2-x(Po4)3 with Different amounts of Dopant
Li 1+xLaxZr2-x(Po4)3 (x=0.0~1.0) was prepared by solid state reaction of Li2C03(A. R.),Zr02(A.R.), La203(A.R.), NH4H?P04(A.R.), C6H14(A.R.). The starting materials were weighed in stoichiometric amounts and homogenized using a mixer. The mixture was put in a tubular furnace and had been heated for 6 h at 600 °C to decompose the oxalate and the phosphate. The powder was cooled down to room temperature and then pressed into Ф 10 mm pellets under 20 MPa. After grinding and homogenization, the mixture was transferred to the furnace and annealed at 1000 °C for 20 h.
2 Characterization and Measurements of adsorption capacity
The thermal analysis (DSC-TGA) was carried out by employing a TA SDT Q600. The samples were heated to 1000 °C at a heating rate of 10 °C/min under nitrogen atmosphere.
The X-ray diffraction(XRD) was performed at room temperature on a Rigaku D/max-3B X-Ray diffractometer, the X-ray beam was nickel-filtered Cu Kα (λ=0.15406 nm) radiation operated at 40 kV and 30 mA; and the data were collected from 3o to 80°(20) at a scanning rate of 5 °/min.
The Fourier transform infrared(FTIR) spectra were recorded on a Thermo Nicolet Nexus in the wave number range of 4000-400 cm"1. Care was taken to press all the KBr pellets under the same conditions to minimize any effect of pressure on peak frequencies for the power samples.
2.4 Measurements of adsorption capacity
1.0 g diffraction patterns of Li 1+xLaxZr2-x(Po4)3 samples were added to 100 g LiCl solution contained 0.06% Na+. The concentration of Na+ in solution is measured after stirring the solution for 10 h at pH 11.0. The adsorption capacity of diffraction patterns of Li 1.4La0.4Zr1.6(Po4)3 samples was carried out at different pH value and different content of doping lanthanum.
RESULTS AND DISCUSSION
Fig. 1 shows DSC-TGA curves of the raw material. There are four endothermic peaks at 121 °C 204 °C 310 °C and 341 °C in DSC curves. TG curve revealed the mass loss of 24.95% which occurred at 20 °C to 600 °C, while no change in weight was found from 6001000 °C. The following reaction can be expected to have mass loss of 24.52%:
The phase evolution of diffraction patterns Li 1+xLaxZr2-x(Po4)3 were studied by the XRD analysis (Fig. 2). The substitution of La3+ for Zr4+ was tried synthetically to observe the change of the crystalline structure. The LiZr2(P04)3 structure was retained in the solid solution range at x≤0.4. It was indexed in the rhombohedral system with lattice: rhomb-centered, space group R3c and the cell parameters: a=0.88077 nm, b=0.88077 nm, c=2.2715 nm, α=90°, β=90°, γ=120°. It showed that trifle La3+ dopant did not affect the structure of the material. Some additional diffraction peaks appeared in the XRD patterns when x is above 0.4.
The adsorption capacity of diffraction patterns of Li 1+xLaxZr2-x(Po4)3 samples was shown in Table 2. The adsorption capacity of Li 1+xLaxZr2-x(Po4)3 to Na+ was 5.16 mg/g and the La3+ dopant considerably improved its specific adsorption capacity to Na+. When x=0.4, the adsorption capacity reached the máximum valué of 49.83 mg/g. So the optimum dopant quantity of La3+ was x=0.4.
After adding La in Li 1+xLaxZr2-x(Po4)3 crystal structure, Zr was replaced by La and Li, which produced negative charge among crystal layers and made La and Li becoming two active centers and benefited to adsorb some catión. However, the excessive La would destroy the structure of LiZr2(P04)3 with forming Li3P04 and LaP04, while Na can not be adsorbed. The idiographic mechanism needs to be further studied.
In this study, we synthesized a novel kind of adsorbent Li 1+xLaxZr2-x(Po4)3by solid state reaction method. The trifle La3+ dopant did not affect the structure of the material but considerably improved its specific adsorption capacity towards Na+. It could be used to remove Na+from lithium chloride. The results of adsorbing test showed that its exchange capacity was high with the máximum valué of 49.83 mg/g at x=0.4, pH=10.0~11.0. That method was a simpler and more convenient way to remove Na+ from lithium chloride solution.
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This work was supported by the National Key Technologies R&D Program of China during the lOth Five-Year Plan Period (No.2004BA602B-01) and the Key Technologies R & D Program of Qinghai province(No.2005-G-173).
(Received: December 17, 2008 - Accepted: June 2, 2008)