Recent advances in layered double hydroxide (LDH)-based materials: fabrication, modification strategies, characterization, promising environmental catalytic applications, and prospective aspects

Abstract

Layered double hydroxides (LDHs) are clay networks with brucite (Mg(OH₂)) layers that are coupled with anions between the produced layers. The building structure of LDHs follows the formula [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n·yH₂O, where M³⁺ and M² are trivalent and divalent cations in the structural units (sheets), respectively; x is the M³⁺ to (M²⁺ + M³⁺) cation ratio of the structure; and Aⁿ is an interlayer anion. LDHs can be created utilizing simple approaches that regulate the layer structure, chemical composition, and shape of the crystals generated by adapting production parameters. The first method of modifying LDH composites is through intercalation, involving the insertion of inorganic or organic precursors into their composition, which can then be employed for a variety of purposes. The next method is a simple physical mixing technique between the created LDHs and advanced materials, such as activated carbon, graphene and its derivatives, and carbon nanotubes, for utilization as base substances in energy storage, supercapacitors, photo- and electrocatalysts, water splitting, and toxic gas removal from the surrounding environment. The final strategy is the synthesis of polymer–LDH composites by inserting effective polymers during the manufacturing process of LDHs to create nano-composites that can be utilized for energy, fire retardant, gas barrier, and wastewater cleaning applications. LDHs are a type of fine chemical that can be designed to have a desired chemical structure and performance for various purposes, such as redox reactions, bromination, ethoxylation, aldol condensation, NO_x and SO_x elimination, and biofuel production. Because LDH substances are not harmful to the environment, their different applications are unique in terms of green chemistry as they are recyclable and eco-friendly catalysts. The present review investigated the various methods used to create LDHs and the improvement of the produced composites via enhanced temperature calcination; intercalation of their structures by small-, medium-, and high-nuclear anions; and support by carbon compounds. The evaluation methods and the best prospective uses, such as biofuel generation, catalysis, water splitting, charge transfer, and wastewater treatment, are comprehensively reported according to the most current studies, and the future directions of LDHs are highlighted.