尿素法合成高结晶度类水滑石

Synthesis of High Crystallinity Hydrotalcite-like Compounds with Urea Method

尿素的高温分解特性被用于控制层状化合物合成过程中的pH值。室温下尿素可与硝酸盐形成均一溶液,当其高温分解时可以保证溶液内部各点的pH值始终一致,因而可获得高结晶度的水滑石样品。利用该法可合成出Mg-Al、Zn-Al及Ni-Al类水滑石,但难以合成出Co-Al,Mn-Al及Co-Cr类水滑石,相信这与不同金属离子发生沉淀时所需的pH值有关。

Three methods, titration coprecipitation, anionic exchange and thermal decomposition-reconstitution methods, have been so far used for the synthesis of the anionic clay, respectively. However, the titration coprecipitation method includes low supersaturation and high supersaturation processes. But the common drawback of the method is that the instantaneous pH value is certainly different in every part of the slurry no matter how to increase the stirring speed. Although it is easier to crystallize in the low supersaturaion condition, which usually gives rise to precipitation, than in the supersaturation condition that the rate of nucleation is higher than that of crystal growth, it is still difficult to obtain the hydrotalcites with high crystallinity. In order to overcome this shortcoming brought by titration coprecipitation process, urea decomposition-homogeneous coprecipitation was investigated in synthesis of hydrotalcite-like compounds. The decomposing property of urea in high temperature has been used to control the pH value in the synthesizing process. Urea, blended with metal nitrate, forms a kind of homogeneous solution in ambient temperature. Furthermore, when it decomposes in high temperature, the pH value is the same in everywhere in the solution. Therefore, hydrotalcite-like compounds with high crystallinity can be achieved. Making use of this method can synthesize the hydrotalcite-like compounds of Mg-Al, Zn-Al and Ni-Al. However, it is difficult to synthesize the Co-Al, Mn-Al and Co-Cr hydrotalcite-like compounds. The cause is certain to correlate the pH value in the process of precipitation.