Efficient, scalable, closed-loop synthesis of highly crystalline pure phase MgAl-layered double hydroxides intercalated with hydroxyl anions

Abstract

Layered double hydroxides (LDHs) can play an important role in various areas, but conventional LDHs synthesis often causes product agglomeration and generates plenty of high-salt wastewater, and requires a time-consuming aging process to reach the desired purity and crystalline state. Herein, we report the synthesis of MgAl-LDH, a representative of these kinds of ionic lamellar inorganic solids, with a novel method involving the reaction of magnesium oxide (MgO) with aluminate ions (Al(OH)₄⁻) in a strongly alkaline environment. The formation of MgAl-LDH follows a mechanism of interfacial dissolution–reprecipitation (IDR), i.e., Mg²⁺ ions released at the interface of dissolved MgO react immediately with Al(OH)₄⁻ ions to reprecipitate as MgAl-LDH. The obtained MgAl-LDH has no impurity phases and shows high crystallinity, high specific surface area, and a narrow particle size distribution. Moreover, MgAl-LDH is intercalated with OH⁻ anions, so it can be directly used as a Brønsted base catalyst and ion exchanger. The novel method requires no time-consuming aging process and is highly scalable. It is also shown that a closed-loop synthesis of MgAl-LDH without waste discharge can be achieved with an appropriate Al source, e.g., Al(OH)₃, and a recycled NaOH solution.