Structure, Properties, Synthesis and Application of Hydrotalcite
2022-08-02
Hydrotalcite materials, also known as anionic clays, are a type of layered columnar compounds with a special structure, which are composed of positively charged metal hydroxide layers and exchangeable anions filled between layers, so they are also called layered bimetallic composites. hydroxide. Hydrotalcite was first discovered by Crica in Sweden in 1842. The typical chemical composition of hydrotalcite is Mg6Al2(OH)16CO3·4H2O, Similar to the structure of brucite (Mg(OH)2, Brucite). It is composed of positively charged host laminates and intercalated guest anions and water molecules.
Hydrotalcite Structure
Hydrotalcite has a structure and composition similar to magnesium hydroxide and aluminum hydroxide. When it is heated and decomposed, it releases a large amount of water and carbon dioxide, absorbs a large amount of heat, and can reduce the temperature of the combustion system; the water algae vapor and carbon dioxide gas released by decomposition It can dilute and block flammable gases; the magnesium aluminum oxide produced by thermal decomposition and the carbide formed when the polymer material is burned form a protective film of picoline on the surface of the material, thereby blocking the further intrusion of oxygen and also acting as a flame retardant Effect.
The solid product after the decomposition of hydrotalcite particles has a large specific surface area and strong alkalinity, which can absorb the acid gas and smoke released during the thermal decomposition of the material in time and convert them into corresponding compounds, thereby playing the role of suppressing and eliminating smoke. effect. Therefore, hydrotalcite is a new type of non-toxic and halogen-free flame retardant for smoke suppression. Its flame retardant performance is obviously better than that of aluminum hydroxide and magnesium hydroxide, and it has the advantages of both.
Hydrotalcite is a non-toxic, thermally stable material. It can be used as an efficient, non-toxic and cheap heat stabilizer for polyvinyl chloride. It can effectively absorb the HCI produced by the decomposition of PVC during processing and use, improve the processing conditions of PVC and heat stabilizer. It can be used together with organotin or lead-zinc as heat stabilizer, or used together with other additives to further improve the heat stability of PVC.
Synthesis Method Of Hydrotalcite
Coprecipitation Method
The soluble metal salt solution is mixed with the alkali solution, and a co-precipitation reaction occurs to form a precipitate. The solution containing the precipitate is crystallized through crystallization, and the hydrotalcite product is obtained after washing, suction filtration, drying, and grinding.
Features: The technological process is very simple and the application range is very wide. During the synthesis process, the nucleation process of crystals and the crystallization process proceed simultaneously, which may cause uneven distribution of hydrotalcite particles and a large range of particle sizes. At present, co-precipitation is the most commonly used and basic method for synthesizing hydrotalcite.
Hydrothermal Synthesis
Quickly mix the mixture of divalent and trivalent metal ions and lye or slowly drop them together, there will be precipitation, and immediately add the emulsion to the autoclave after filtration, usually at about 100 ° C Under aging for a certain period of time, the product can be obtained after filtering, washing, drying and grinding.
Features: The operation method is very simple, the obtained hydrotalcite crystal structure is more complete and the particles are relatively small, the shape will be more regular, and the particle size distribution of the sample is narrower.
Ion Exchange Method
The anion exchange method refers to the method of obtaining the hydrotalcite product by utilizing the unique property of the anion exchangeability between the layers, and the anions between the hydrotalcite layers are replaced by the anions to be inserted.
Features: Appropriate solvent and swelling conditions are conducive to the expansion of the hydrotalcite precursor laminate, which is convenient for ion exchange; the smaller the particle size and the higher the charge, the stronger the anion exchange capacity; the lower the pH value is conducive to reducing the water The interaction of anions between talc layers is beneficial to ion exchange, but when the pH is too low, the internal structure will be destroyed, which is not conducive to the formation of hydrotalcite.
Hydrotalcite Use
Hydrotalcite has many uses, and plays a very important role in chemical production. It can be used as a chemical raw material, and can also be used as a catalyst to catalyze the catalysis that occurs independently. In the biological field, hydrotalcite can also be used to kill bacteria and inhibit the growth of many microorganisms. Researchers in the fields of chemistry and biology are still researching and exploring the role of hydrotalcite, hoping that hydrotalcite can be excavated for more functions.
In the application of treating organic pollutants, due to the special layered structure of hydrotalcite materials, no secondary pollution, easy regeneration, and low cost, hydrotalcite materials have attracted extensive attention as photocatalysts in environmental governance. Applications in medicine. Alkaline drugs made from the alkalinity of LDHs can be used to treat some common gastric diseases such as gastritis, gastric ulcer, and duodenal ulcer by adjusting the pH value of gastric juice through neutralization reaction.
It has applications in flame retardants and PVC heat stabilizers, as well as in the synthesis of new materials. As a kind of green and pollution-free three-dimensional structure nanomaterials, hydrotalcite compounds have great research potential and application value.
