Cobalt aluminum silicate complexes prepared by the non-hydrolytic sol–gel route and their catalytic activity in hydrocarbon oxidation

Abstract

This work describes the optimized conditions for the preparation of cobalt aluminum silicate complexes through a simplified methodology. These materials, designated CoAlSi-NHG, were obtained by a non-hydrolytic sol–gel route involving the condensation of aluminum chloride with diisopropylether in the presence of cobalt chloride, followed by reaction with tetraethoxysilane. The obtained solids were heat-treated at various temperatures, and the resulting materials were characterized by ultraviolet-visible spectroscopy, X-ray diffraction, 29Si and 27Al NMR, transmission electron microscopy, surface area, thermogravimetric analysis, and differential thermal analysis. All the materials consisted of an aluminum silicate matrix, and they all remained amorphous throughout heat treatment up to 750 °C. The onset of crystallization took place at 1000 °C, and the formation of mullite was observed thereafter. All the CoAlSi-NHGs obtained in this work via different heat treatments were used as catalysts in the oxidation of (Z)-cyclooctene, cyclohexane, and n-heptane by iodozylbenzene. The non-hydrolytic sol–gel route led to the obtention of extremely interesting catalytic systems capable of selectively oxidizing various types of reactions. The intermediate species is the oxidizing species oxo-cobalt, which is similar to the active species obtained in the case of enzymes such as cytochrome P-450.