Synthesis and Characterization of Expanded Smectites Containing Trinuclear Co Complexes

Several smectites were pillared with metal complexes, such as the trimeric cobalt cation [Co3(OC2H4NH2)6]3+ and its divalent analogue [Co3(OC2H4NH2)6]2+, containing clusters of one to three cobalt cations separated by bridging ligands. In pillared interlayered clays (PILC) prepared by reacting Na−montmorillonite with Co3(H2NCH2CH2O)6(ClO4) 3 solutions, pillar orientation is controlled by the trimer concentration in the interlamellar space. Initially the Co trimer (0.72 nm × 0.48 nm in size) lies parallel to the clay surface. Then as more trimer is introduced, it forms an angle of ∼48° relative to the clay surface that increases to 61° as the clay reached its highest Co loadings. FT IR and FT LRS spectra together with chemical analysis have been used to ascertain the presence of pristine trimer ions in all the PILC samples. XPS results indicate that reduction of the trivalent trimer to its divalent analogue [Co3(OC2H4NH2)6]2+ has occurred during PILC preparation. XRD patterns and molecular scale AFM images have revealed the presence of trimer molecules also on the clay external surface. Thermogravimetric analyses of [Co3(OC2H4NH2)6]3+ PILC are all qualitatively similar despite the variance in d spacing seen at different Co trimer loadings. Ligand decomposition occurs in two steps in the 250−450 °C temperature range. At T > 250 °C, ligand losses cause a total structural collapse of the pillared structure. In contrast, prior to 250 °C, the interlamellar expansion decreases and the trimer seems to adopt a position parallel to the clay surface. The d spacing changed very little over the temperature range where initial ligand losses occur. Partial decomposition of the trimeric complex can thus be achieved while a 0.45 nm layer expansion is maintained, indicating that these materials could be considered as cobalt catalysts for low temperature (T < 200 °C) applications.