Structural and textural evolution of nanocrystalline Mg–Al layered double hydroxides during mechanical treatment

Well-crystallized nanocrystalline Mg–Al layered double hydroxides (LDHs) were prepared by hydrothermal method and subjected to mechanical treatment to evaluate the structural and textural changes upon grinding. The powder samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field scanning electron microscopy (FESEM), and N2 adsorption analysis. Grinding caused progressive reduction of crystallite size and structural degradation of the Mg–Al LDHs, resulting in a decrease in its crystallinity. The grinding products at various treatment times were mainly with micropores, resulting from the transformation of the original mesoporous structure, whereas the total pore volume of the milling samples gradually decreased with prolonged treatments. At shorter milling time (< 48 min), marked increases in surface area coupled with a severe reduction in crystallite size were observed. Delamination and fragmentation of the crystallites mainly occurred at this stage. At intermediate milling time (48 min to 72 min), the crystallite size approached a saturation value (ca.4.4 nm in the c axis direction), and the surface area reached a maximum value (about 117.0 m2g− 1). Aggregations of the crystallites were clearly observed at this period. During the last milling stage (from 72 min to 240 min), heavy agglomeration contributed to the reduction in surface area, and the final grinding products were amorphous materials due to the complete destruction of the layer structure of the solids. The distribution, orientation, and aggregation degree of the Mg–Al LDH platelets during the mechanical process may be responsible for the textural evolution.