Synthesis of layered double hydroxides through continuous flow processes: A review

Continuous production processes allow scaling up of layered double hydroxides (LDHs) and avoid the drawbacks induced by conventional coprecipitation. These drawbacks result from variable supersaturation rate due to non constant pH and concentration of the solutions and from long residence times hindering a fine control of the size and morphology of the particles. Continuous flow processes allow reducing the residence time and maintaining almost constant supersaturation producing LDHs in large amounts with constant quality. We report here the different continuous flow methods for the production of LDHs particles with controlled size and morphology or individual nanosheets, and of LDH-based hybrids and nanocomposites. The paper will focus on the design of the reactors showing a decrease of their volume and an improvement of the mixing and heat and mass transfers. Cylindrical tank under steady-state conditions lead to particles with a narrower size distribution than in batch reactor. Then processes with vigorously stirred microreactors in the so-called in-line dispersion-precipitation method were developed. Counter-current flow reactors with particles formed at the interface of solutions flowing up and down were further used to obtain ultra-fine LDH nanoplates and efficient surface modification with surfactants. Hydrothermal continuous or co-flow reactors exhibit great versatility allowing the preparation of exfoliated or functional LDHs, LDH nanoplates on alumina-coated substrates, and reduced graphene oxide/LDH nanocomposite films. The microfluidic technology is very promising for preparing LDHs of different compositions and functionalities. The reaction conditions as well as the structural and morphological properties of the materials are discussed and applications are reported.