Study On The Structure And Dissolution Behavior Of Layered Double Hydroxide

Layered double hydroxides（LDHs）are anionic layered materials.Due to the unique structure,the metal elements,charge density and types of interlayer guests of the main layer can be designed and precisely regulated.LDHs have been widely used in many fields such as catalysis,electrode materials,functional additives,and magnetic materials.However,there are few studies on the solubility and stability of LDHs in aqueous solution,which limits their application performance research and large-scale industrial production.In this paper,four types of LDHs of Mg Al,Ca Al,Zn Al and Mg Fe were synthesized by nucleation/crystallization isolation method.The structure and morphology of the samples were characterized by XRD,SEM and FT-IR.Its dissolution thermodynamics and kinetics at different temperatures were studied,and its structural stability in aqueous solution was discussed.The main research contents of this paper are as follows:（1）A dissolution equilibrium experiment was designed to study different temperatures（25℃,40℃and 60℃）,different cation molar ratios（Mg/Al were 2,3 and 4）and different interlayer anions(CO₃^2–,Cl^–)on the solubility of LDHs,and the solubility products(K_sp)of various LDHs were calculated.The results show that the increase of temperature and the increase of Mg/Al value will lead to the decrease of K_sp,Since that CO₃^2–can form hydrogen bonds with the hydroxyl groups of the layers in the interlayer,LDHs with CO₃^2–in the interlayer have higher structural stability and smaller K_sp than LDHs with Cl^–in the interlayer;for the same anion intercalation The K_sp order of layer LDHs is Mg₂Fe<Zn₂Al<Mg₂Al<Ca₂Al.The stability of LDHs is consistent with the stability of the corresponding metal hydroxides of the main body of the layer.（2）The reaction enthalpy(ΔH_r^○,_298.15K),reaction entropy(ΔS_r^○,_298.15K)and reaction Gibbs free energy(ΔG_r^○,_298.15K)of various LDHs dissolution reactions were calculated.The enthalpy of formation(ΔH^○_f,298.15K),entropy of formation(ΔS^○_f,298.15K),and Gibbs free energy of formation(ΔG^○_f,298.15K)were calculated for various LDHs.The results show that the dissolution of LDHs is an exothermic process.The dissolution reaction exotherm is the reason why the K_sp of LDHs decreases with increasing temperature.（3）The dissolution kinetics were studied by monitoring the concentration of each ion slowly released during the dissolution equilibrium of LDHs,and it was found that the dissolution reaction rate of LDHs was controlled by the diffusion of the ash layer or the product layer.The dissolution activation energies（Ea）of various LDHs were calculated at the same time,and the results showed that the increase of Mg/Al would lead to the increase of Ea of LDHs,indicating that the higher the ratio of Mg/Al,the less easily dissociated LDHs in aqueous solution and the better the stability;the interlayer is The LDHs of CO₃^2–have a larger Ea than the LDHs with Cl^–in the interlayer,indicating that the former has higher stability;for the same anion intercalation LDHs,the Ea order of the LDHs is Ca₂Al<Mg₂Al<Zn₂Al<Mg₂Fe,and its stability is in order improve.