A study of stereoselective adsorption in smectites using thin-layer chromatography

Abstract

The stereoselective center in a group of smectite-layered clay minerals was investigated to determine chiral discrimination at either the crystal edge of the smectite or in the interlayer space. In this study, thin-layer chromatography (TLC) was used to probe the stereoselective adsorption site in the smectites. Because uniform-sized particles are crucial for facile, homogeneous, and reproducible production of a TLC plate, core-shell type particles were used, in which a hectorite-like layered silicate was directly grown on monodisperse silica spheres with a 2.6 μm diameter. The core-shell microspheres prepared by the hydrothermal reaction of lithium fluoride, magnesium chloride, and silica in the presence of urea at 373 K for two days were deposited on a glass plate using a splendor. The formation of the resulting layered silicate on the TLC plate was achieved via ion-exchange reactions with enantiomeric Δ-tris(1,10-phenanthroline)nickel(II). The intercalated Ni complex formed pores at the molecular level in the interlayer spaces. The optical resolution of tris(acetylacetonato)ruthenium(III) using methanol as a mobile phase was found to be influenced by the size of the pores and could be adjusted by changing the negative-layer charge density that was varied according to the molar ratio of the Li/Mg added into the hydrothermal reactions. Two split TLC spots attributed to the Δ- and Λ-isomers of the acetylacetonato Ru complex were observed with no tailing when the TLC plate included the layered silicate with a greater density of charged layers. These findings indicated that an increase in the spatial density of the interlayer Ni complex (a narrower pore) enhanced interactions between the Ru complex and the intercalation compound, leading to optical resolution. The TLC experiments showed that the interlayer void space plays an important role when stereoselective adsorption sites are located in the interlayer space.