Introduction of sodium to layered double hydroxides LiAI2(OH)7�2H2O

• DOI: 10.1007/bf00291439
• Publication Date: 2004-10-05T10:46:31Z
• Authors: Z. Y. Wen; Z. X. Lin; K. G. Chen
• Keywords: Layered double hydroxides

Layered double hydroxides (LDHs) with the general formula [MZ+lxM3+x(OH)2]A+XA/m • n H20, where A = x f o r z = l a n d A = 2 x 1 f o r z = 2 , consist of positively charged metal hydroxide sheets between which anions X mand water molecules are located [1]. They form a complementary class to the traditional cationic clays such as smectite and zirconium phosphates. To date, there have been many publications pertinent to the synthesis and properties of M2+M 3+ LDHs, and many divalent cations, e.g. Mg 2+, Mn 2+, Fe 2+, Ni 2+, Cu 2+, Zn 2+ and Co 2÷ could easily form the structure [14] . A particular layered lithium aluminium hydroxide [LiA12(OH)6]+X-'H2 O, a typical example of LDHs for z = 1, has also been reported [1, 5-7], which to the best of our knowledge was the only one obtained for the M+M 2+ system and was less widely investigated in general than the M2--M 3+ analogues. It was pointed out that the lithium ion in the hydrotalcite-like compound was difficult to replace by the other alkaline ions or any other monovalent ions with the exception of H ÷ [5], which was interpreted as a cation-size effect. We present here a novel LDHs compound containing mixed alkaline ions of Li + and Na + synthesized by a hydrothermal process. A mixture of CO2-free NaA102, L iOH'H20 and NaOH with molar ratio of Li20:Na20:A1203 = 1:40:20 was placed in a 24 ml autoclave with a Teflon liner. The reaction vessel was filled to 85% capacity with deionized water, sealed and maintained at 250 °C for 20 h. At the end of the hydrothermal reaction, the slurry was removed, filtered and washed several times with distilled water and with ethanol under CO2-free conditions and the product was dried at ambient temperature over silica gel. The chemical composition of the white fine powder thus obtained was detected by atomic absorption technique as Li20:Na20:A1203=4:l:10 and the CO2 content analysed by anhydrous titration was less than 0.8 wt%. The powder as prepared was identified by X-ray diffraction (XRD) with a Rigaku RAX-10 diffractometer using CuK~ radiation (A = 0.15418 nm) and a scanning rate of 1 ° rain -1. The pattern was over the range 10 ° < 20 < 70 °, as shown in Fig. 1, which was compared with that of LiA12(OH)7.2H20 prepared in the same way [8]. It was found that the sample was well crystallized and the diffraction patterns of both cases were similar, with only small shifts of diffraction angles. No peak from any other crystalline material was observed. By using a least-squares