日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Cation-exchangeable layered silicates including smectite group of layered clay minerals have been investigated in various applications such as molecular recognizable adsorbents. Here, we overview the surface coverage of an amorphous silica with smectite-like layered silicates through the direct crystallization technique. The silica substrates （e.g., monodisperse spherical particles, fibers, hollow microspheres） undergo the heterogeneous nucleation of a hectorite-like silicate after the hydrothermal reactions with lithium fluoride and magnesium dichlorides in the presence of urea. An aluminosilicate microcrystal has also grown on the surfaces by adding aluminum ions to the initial solution. Because of partial dissolution of the silica substrates initiated by the hydrolysis of urea, the shape of the silica substrate has been maintained after the hydrothermal reactions. Cation exchange properties are displayed for the interlayer cations in the formed hectorite crystals to be replaced with metal (a rare-earth ion) and organic (quaternary alkylammonium and methylene blue) cations, as is usually observed for the smectites. We focus in this article on the reaction mechanism (affected by the chemical compositions of the starting mixtures) and applications of the resulting hybrids (e.g., HPLC column packing materials for chiral discrimination, filtering, and magnetic recovery). Such a hierarchical design including the crystal engineering on the surface of defined morphology will be useful for separation of a target molecule especially in a flow system.