Factors affecting the crystal size of the MgAl-LDH (layered double hydroxide) prepared by using ammonia-releasing reagents

For the so-called “homogeneous precipitation” of MgAl-LDH (layered double hydroxides), factors influencing the size of the LDH crystals were investigated by using two kinds of starting metal salts (Cl− and NO3− salts) and two types of ammonia-releasing reagents (ARRs), urea and HMT(hexamethylenetetramine). The reaction was conducted under a hydrothermal condition at 140 °C for 1 day. In these systems, the amounts of the ARRs were varied while fixing the Mg/Al ratio = 2.0 and the total metal concentration [Mg + Al] = 0.15 M. It was revealed that the crystal size range was the widest for the NO3−/HMT combination. In this system, the amounts of the HMT and the anion species were the factors controlling the size and state of the resulting CO32− LDH. For a higher HMT concentration, the LDH size was about 1 μm, and, by reducing its concentration, the crystal size increased to 5–6 μm. With a further reduction in the HMT concentration, rosette-like LDH aggregates as large as 20 μm appeared. The influences of other factors, such as the total metal concentration and reaction time, were also investigated. The growth of LDH crystals in a circle on Al hydroxide precursors was assumed to explain the aggregation formation and various morphologies of the LDH crystals.