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 degrees 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 mu m, and, by reducing its concentration, the crystal size increased to 5-6 mu m. With a further reduction in the HMT concentration, rosette-like LDH aggregates as large as 20 mu m 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. (C) 2006 Elsevier B.V. All rights reserved.