Hebei Messi Biology Co., Ltd. states that in the vast family of materials chemistry, there is a class of functional materials known as “Layered Double Hydroxides” (LDHs), among which the most prestigious is magnesium-aluminum hydrotalcite, with the chemical formula Mg₆Al₂(OH)₁₆CO₃·4H₂O. Its structure resembles a multi-layer pastry—between positively charged magnesium-aluminum hydroxide octahedral layers are sandwiched exchangeable anions such as carbonate. This unique “layer + interlayer” structure endows it with multiple capabilities such as adsorption, intercalation, ion exchange, and basic catalysis. It is widely used as a PVC heat stabilizer, halogen-free flame retardant, dye adsorbent, and catalyst support. To produce magnesium-aluminum hydrotalcite with high purity and regular lamellae, magnesium oxide serves as the most crucial “cornerstone.”
Why magnesium oxide instead of other magnesium sources? Traditional co-precipitation methods mostly use magnesium nitrate or magnesium chloride as magnesium salts, but anions (NO₃⁻, Cl⁻) remain in the system and require repeated washing to remove, which consumes water and time while potentially introducing impurity interference. Switching to magnesium oxide is clean and decisive: MgO undergoes hydrolysis upon encountering water, generating Mg(OH)₂ while releasing Mg²⁺ and OH⁻. It simultaneously provides the magnesium source and “conveniently” creates the alkaline environment required for synthesis, eliminating the need for large additional drops of NaOH and avoiding sodium ion residues. It is truly a “two birds with one stone” approach.
The reaction mechanism can be summarized as a three-step process of “hydrolysis-precipitation-crystallization.” In a closed hydrothermal autoclave, MgO first slowly hydrates to form Mg(OH)₂ and dissociates Mg²⁺; the aluminum source (commonly Al(OH)₃ or soluble aluminum salts) exists in the form of Al(OH)₄⁻ under alkaline conditions. The two meet at the solid-liquid interface, depositing into “precursor layers” containing Mg and Al. Through sustained heating, metal ions diffuse and rearrange within the crystal lattice, ultimately growing into well-crystallized magnesium-aluminum hydrotalcite flake crystals. Studies confirm that when n(MgO)∶n(Al(OH)₃)∶n(Na₂CO₃)=2∶1∶1 under hydrothermal reaction at 140–160°C for 4 to 14 hours, products with pure phases and complete lamellae can be obtained. If the crystallinity of MgO is higher, the crystal form of the resulting hydrotalcite will also be more regular.
From a process perspective, the magnesium oxide route also holds several “economic advantages.” First, it eliminates tedious multiple water washing and purification steps, reducing both energy consumption and working hours. Second, MgO is inexpensive and readily available; industry can directly utilize medium-purity raw materials such as light-burned magnesium powder, and the tail liquid can be recycled and reused with zero “three-wastes” (wastewater, waste gas, and waste residue) emissions, evaluated as an environmentally friendly clean synthesis route. Third, by regulating MgO activity (specific surface area, calcination temperature) and feeding molar ratios (usually controlling the Mg/Al feeding ratio between 1 and 3), the Mg²⁺/Al³⁺
ratio and grain size in the product can be “customized,” providing powders of various specifications for downstream applications.
The careful selection of magnesium oxide often determines the upper quality limit of hydrotalcite. Light magnesium oxide with excessively high activity hydrolyzes too quickly, easily generating Mg(OH)₂ impurity phases; if the activity is too low, the reaction is incomplete, and residual MgO affects purity. Therefore, the industry tends to select heavy or light-burned magnesium oxide with medium activity and uniform particle size, combined with closed hydrothermal or CO₂ carbonation processes, to match the release rate of Mg²⁺ with the growth rate of the lamellae. Fine-tuning this step is the core challenge in scaling up from laboratory small samples to industrial production.
Today, magnesium-aluminum hydrotalcite synthesized using magnesium oxide as the magnesium source has emerged in applications such as polyolefin flame retardancy, acid gas capture, and heavy metal and phosphate wastewater treatment—for instance, achieving a removal rate of over 97% for 20 mg/L phosphate-containing wastewater. It can be said that this unassuming white powder is uplifting a new world of layered functional materials in the capacity of a “raw material,” while also writing a vivid footnote for green synthetic chemistry.