Study On Thermal Decomposition And Reconstruction Of Hydrotalcite Like Compounds

Hydrotalcite-like-compounds(simplified as HTLcs)also called anionic clay can make various technical applications because of its special properties with exchangeable anion in the interlayer and adjustable adsorption and catalysis performance of aperture.The complex metal oxides derived from HTLcs calcination have extensive use in the field of catalysis and adsorption etc.Reconstruction effect is a important one in these property of complex oxides .Using reconstruction effect,the derived complex oxides can regain the anion and structural hydroxy,and then recover parent HTLcs layer structure at a given condition.The effect has the potential application foreground in these aspects such as xatalyst modifying, interlayer chemistry, water treatment, etc.The experiment study on thermal decomposition and reconstruction progress, and synthesize some kinds of HTLcs (different M²⁺/M^2-,different interlaminar anions,different lamellar ion) Through researching calcination process and comparing different structural reconstruction factors and combining a series of characterization, we make a full seminar about the thermal decomposition progress and the mechanics of reconstruction and the application of reconstruction effect.we synthesize a series of HTLcs with the same structure and different composition by coprecipitation method. Chiefly we adopt the constant PH method to synthesize CO₃-HTLcs because the anion CO₃^2- is easy to converse CO₂ escaping, and adopt the variable PH method to synthesize other anionic HTLcs. Through the characterization to the synthesized samples with FT-IR and XRD, The result indicates that the degree of isomorphous substitution about lamellar positive ion and the interlayer structural close degree after replacing determines the sample crystallinity of positive-ion HTLcs with different ratio and different pattems.Further the anion dimension of anionic HTLcs and the quantity of negative charge is related to the crystallinity of HTLcs.we study the thermal decomposition progress in detail. Through TG-DTG and XRD tracing analysis, we find that the thermostability of ZnAl-CO₃-HTLcs is weak. At about 120℃, the interlayer water begin to pull out ,and only has a remarkable weightless course. At about 280℃, it turns up the maximum agravity. Likewise the other type of ZnAl-HTLcs also has the similar course. The different lamellar metal ions have larger fluence for thermal decomposition progress mostly because the different radiuses and the bond of M-O of these metal ions don't make the sheets of HTLcs combine close. Through calcining HTLcs with temperature increment, the diffraction peaks of HTLcs vanish gently and the diffraction peaks of oxidates gradually strengthen. When the temperature is above 800℃, the diffraction peaks of spinel appear and become sharp.Studying on the various influence conditions of structural reconstruction effect: Combining a series of means of characterization, we analyse various influence factors of structural reconstruction effect, including the different M²⁺/M³⁺, calcination temperature, water-thermal treatment temperature, PH of reconstruction media, times of structural reconstruction, the different positive ion and interlayer anions etc. The researchful results indicate that the choices of calcination temperature of HTLcs and the constituent of metallic element are the determinant impact to structural reconstruction effect. When the calcination temperature is too high, the sample of calcination is difficultly reconstructed because a large quantity of spinal exist. And the constituent of metallic element of HTLcs is a another important factor whether HTLcs can reconstruct. Following the TG-DTG curve and calcining under the decomposed temperature, Zn₂Al-CO₃-CHTLcs and Mg₂Al-CO₃-CHTLcs can all reconstruct after they are treated at 80℃in the de-ionized water. But A1(OH)₃ is formed after Ni₂Al-CO₃-CHTLcs is treated in the same condition. When other factors affecting reconstruction variate in a given extension, they will affect chiefly some physical parameters of HTLcs , such as crystallinity and purity. TO discussthe factors affecting reconstruction are mostly ready to the subsequent experiment of application and can probe into the mechanice of structural reconstruction by researchful conclusion.Through dehydrogenization and dehydration of ethanol , we can characterize the catalytic activity of reconstruction, and the result shows that the reconstruction makes basicity of compound oxides become stronger. At the same time , the stronger basicity of the complex oxides can improve the conversion of the reactant and the selectivity of the products in the reaction of the dehydrogenization.