Hydrothermal Synthesis and Characterization of 3R Polytypes of Mg-Al Layered Double Hydroxides

Abstract

Layered Double Hydroxides (LDH) is a unique group of clays that have an anionic exchange capability. This research explored the hydrothermal method as an alternative method to synthesize Mg-Al LDH. It is a simple and more environmentally friendly compared to the conventional method of co-precipitation. Furthermore, depending on the synthesis condition, two different polytypes, namely 3R1 and 3R2 can be synthesized. The first part of the research was focused on the optimization of the hydrothermal synthesis. Various pre-treatment techniques of the reactants were investigated. The use of a microwave system as an alternative energy source resulted in the formation of a unique donut-shaped crystal which provides enlargement of the specific surface area of the {hk0} faces, needed for adsorption application. The growth mechanism of such donut-like crystals is studied by AFM as well as by STEM-EDX. The interrelation of polytype 3R1 and 3R2 along with the chemical composition and structure of polytype 3R2 is addressed in the second part. The transition temperature is approximately at 110 ºC with 3R1 being stabile at lower temperatures and 3R2 at higher temperatures. Polytype 3R2 was also found to have more aluminum content compared to 3R1. The excess aluminium is the presence as tetrahedrally coordinated aluminate ion located in the interlayer as charge compensation. The apical oxygen of the aluminate is grafted onto the octahedral metal layer, inducing the formation of 3R2 stacking. This grafted structure might explain the reluctance of polytype 3R2 to be ion exchanged compared to 3R1.