Hydrotalcite is a type of layered double hydroxide (LDH), in which magnesium hydroxide (Mg(OH)₂) is one of the main components of the layered structure. The properties of Mg(OH)₂ directly affect the structural stability, thermal stability and chemical stability of hydrotalcite. The following is a detailed analysis of the effect of magnesium hydroxide on the stability of hydrotalcite:
1. Structural stability
(1) Contribution of magnesium ions to the layered structure
The basic structure of hydrotalcite consists of Mg²⁺ and Al³⁺ (or Fe³⁺, Cr³⁺, etc.) forming octahedral hydroxide layers to form positively charged layers.
Mg²⁺ has a large hydration radius (about 0.86 Å) and can coordinate with OH⁻ to form stable Mg(OH)₆ octahedrons to maintain the regular arrangement of the layers.
Changes in the Mg/Al ratio (or Mg/M³⁺ ratio) will affect the interlayer spacing and electrostatic interactions between the layers of hydrotalcite, thereby affecting the structural stability.
(2) Effect of Mg(OH)₂ content on hydrotalcite crystal morphology
When the proportion of Mg(OH)₂ is high, the crystal growth of hydrotalcite will tend to a regular hexagonal sheet structure, making it more stable.
When the Mg/Al ratio is too low (such as Mg/Al < 2), the Al³⁺ content is high, which will lead to enhanced interlayer electrostatic interaction, which may cause structural collapse or increase defects, reducing the stability of hydrotalcite.
When the Mg/Al ratio is too high (such as Mg/Al > 4), free Mg(OH)₂ may precipitate, resulting in an uneven structure of hydrotalcite, affecting stability.
2. Thermal stability
Hydrotalcite undergoes a series of thermal decomposition processes when heated. The presence of Mg(OH)₂ plays a key role in the thermal stability of hydrotalcite.
(1) Thermal decomposition behavior
Hydrotalcite loses interlayer water at about 200-250°C and can still maintain a stable layered structure.
At about 300-500°C, Mg(OH)₂ begins to dehydrate and decompose, causing the layered structure to gradually collapse and form an amorphous oxide (MgO-Al₂O₃ composite oxide).
At about 600-800°C, the layered structure of hydrotalcite completely disintegrates, forming a porous MgO-Al₂O₃ mixed oxide with a high specific surface area.
(2) Buffering effect of Mg(OH)₂
The MgO generated after the decomposition of Mg(OH)₂ has a certain self-healing property, which can partially restore the structure of hydrotalcite in water or humid environment and improve its thermal stability.
By adjusting the Mg/Al ratio or introducing Zn²⁺, Fe³⁺ and other ions, the thermal stability of hydrotalcite can be regulated to make it more stable at high temperatures.
3. Chemical stability
The presence of Mg(OH)₂ will affect the chemical stability of hydrotalcite under different pH conditions.
(1) Stability in alkaline environment
Mg(OH)₂ itself is relatively stable in alkaline environment (pH > 9), so hydrotalcite can maintain a complete layered structure under alkaline conditions.
When the pH value increases further, Mg(OH)₂ may dissolve, causing the layered structure of hydrotalcite to change.
(2) Stability in acidic environment
Under acidic conditions (pH < 4), Mg(OH)₂ easily dissolves, causing the hydrotalcite structure to collapse, Mg²⁺ in the layer to be lost, and the layered structure to be destroyed.
The stability of hydrotalcite in acidic environment can be improved by doping with metals such as Zn²⁺ and Fe³⁺.
4. Effect on adsorption performance
Since hydrotalcite has interlayer anion exchange capacity, its adsorption performance is closely related to the content of Mg(OH)₂.
Hydrotalcite with high magnesium content: The layer has a strong ability to carry positive charge, and can more effectively adsorb anions such as Cl⁻, NO₃⁻, SO₄²⁻, etc., improving adsorption stability.
Hydrotalcite with low magnesium content: Due to the high charge density of the layer, there may be a strong electrostatic effect between the layers, which affects the exchange capacity of adsorbed anions.
The presence of Mg(OH)₂: It can improve the adsorption capacity of hydrotalcite for organic pollutants and heavy metal ions, and is suitable for the field of environmental remediation.
5. Impact in application
The effect of magnesium hydroxide on the stability of hydrotalcite makes it have different application values in multiple fields:
Catalyst carrier: The thermal decomposition product (MgO) of Mg(OH)₂ can form a porous structure with high specific surface area, enhancing the dispersion and stability of the catalyst.
Flame retardant material: The endothermic effect of thermal decomposition of Mg(OH)₂ can improve the flame retardant properties of hydrotalcite, making it suitable for plastics, rubber and other materials.
Drug sustained-release carrier: Mg(OH)₂ can provide a stable layered structure, control drug release, and improve drug stability and biocompatibility.
Heavy metal adsorbent: The layered structure of Mg(OH)₂ can adsorb heavy metal ions such as Pb²⁺ and Cd²⁺, improving the pollution control ability of hydrotalcite.