A Novel Method For The Synthesis Of Hydrotalcites And A Theoretical Study Of Their Structure And Properties

In this Thesis, a novel synthetic method for hydrotalcite-like materials is employed in which nucleation and aging steps are performed separately. This overcomes the disadvantage of conventional coprecipitation methods. The conditions required for the synthesis of hydrotalcite-like materials and other factors which influence the formation of hydrotalcite-like materials have been investigated systematically. A variety of properties of hydrotalcite-like materials have been discussed. Based on the experimental results, the relations between structure and properties have been studied using quantum chemical calculation methods. I. A novel synthetic method for hydrotalcite-like materialsThe disadvantage of traditional coprecipitation methods for the preparation of hydrotalcite-like materials is that nucleation and aging steps are performed at the same time so that the particle size distributionof products is relatively wide. A novel synthetic method has been developed in our laboratories in which nucleation and aging steps are performed separately. The crystallinity of the product prepared by this new method is higher than that of the product produced using traditional routes. Furthermore the particle size can be readily controlled and the crystallite size distribution is relatively narrow. The mean particle size and size distribution of hydrotalcite-like materials can be readily controlled by variation of the aging time.II. Physicochemical properties of nano-sized hydrotalcite-like materials1. The stability and rheological properties of aqueous nano-sized Mg-Al-C03 LDHs dispersions have been studied. It has been found that the stability of aqueous dispersions containing nano-sized Mg-Al-CO3 LDHs depends on both the solid content and the crystallinity of the LDHs and that stable aqueous LDHs dispersions can exist without any dispersants. The rheological properties of aqueous LDHs dispersions were found to be closely related to the layered structure of LDHs. The thinning of aqueous LDHs dispersions was found to be reversible and dependent on time. XRD,SEM and TEM were used to probe the structure of LDHs and their dispersed states. It was shown that rod-shaped aggregates were present in the stable aqueous LDHs dispersions.2. Nano-sized Mg-Al-CO3 LDHs were treated with stearic acid and atitanium ester using a wet method. The modified nano-sized Mg-Al-CO3 LDHs showed good dispersion and floatation properties with their layer structure remaining almost unchanged.3. The structures of Mg-Al-CO3 and Zn-Al-CO3 hydrotalcites have been studied by means of XRD, IR and TG-DTA. Although both have the same crystal structures, their thermal stabilities are quite different. The results of a theoretical study indicate that the OH-- CO3--OH and OH--H2O--CO3桹H bond strengths in Mg-Al-CO3 hydrotalcite are stronger than those in Zn-Al-CO3 hydrotalcite. It has been shown that the thermal stability of Mg-Al-CO3 hydrotalcite is higher than that of Zn-Al-CO3 "hydrotalcite.4. Nano-sized Zn-Al composite oxides were synthesized by the novel synthetic method. The influence of Zn/Al molar ratio, the structure of Zn-Al composite oxides before calcination, calcination temperature and heating rates on UV absorbing properties was investigated. It has been found that Zn-Al composite oxides are very effective in absorbing ultraviolet radiation and that this property is closely connected with their crystal structure. The results show that higher Zn/Al molar ratios and higher crystallinity of the Zn-Al composite oxides before calcination are advantageous in terms of UV absorbing characteristics. It seems that the heating rate during calcination is a less important factor in determining its ultraviolet absorption properties.5. Using the 'memory effect' of hydrotalcite-like materials, new products can be obtained by reaction of calcined Zn/Al LDHs with organic ultraviolet absorbents. Sodium salicylate and 2-hydroxy-4-methoxy-benzophenone-5-sulfonic-acid were chosen as ultrav...