Hydrogenation Activity of Colloidal Cobalt Boride Particles Synthesized in the CTAB-1-Hexanol-Water Reversed Micellar Systems

Abstract

Colloidal cobalt boride particles are prepared by reducing with NaBH4 the CoCl2 dissolved in the inner water cores of reversed micelles composed of CTAB-Hexanol-Water at near 0°C. These particles show rather high activity in hydrogenation reactions. The solubilization sites and the interaction of Co (II) ions either with CTAB or 1-hexanol molecules were studied by U.V.-visible absorption spectroscopy. It is concluded that Co(II) ions are essentially solvated in the inner water cores and one 1-hexanol molecule is included in their first solvation shells. The size of the inner water core is measured by an indirect method (19F-NMR), based on the distribution of 6-fluorohexanol between the interface and the 1-hexanol organic medium. The size of cobalt boride particles depends upon the micellar composition (especially on the water concentration) and on the Co(II) ion concentration. Larger particles are obtained at higher water content, while the curve of particle size as a function of Co(II) concentration showed a clear-cut minimum. The latter behaviour can be explained, provided a critical number of Co(II) ions is assumed for the initial nuclei formation. By monitoring the physico-chemical properties of the reversed micelles, it is thus possible to prepare colloidal cobalt boride particles of high hydrogenation activity. The hydrogenation of 1-heptene is studied in an ethanol (34 wt %)-micellar system (66 wt %). It is observed that no aggregation of the colloidal particles occurs during the reaction and that CTAB modifies drastically the hydrogenation activity of these particles.