Oxidation of limonene over carbon anchored transition metal Schiff base complexes: Effect of the linking agent

The oxidation of limonene over carbon anchored cobalt acetylacetonate catalysts is reported. The reaction is carried out in batch reactor, at 60 °C, in the solvent system acetone:t-butanol, with t-butyl hydroperoxide as oxygen supplier. Complex anchoring was achieved in four consecutive steps: (i) oxidation of activated carbon with nitric acid, (ii) treatment with thionyl chloride that converts the free carboxylic acid surface groups into acyl chloride functionalities, (iii) reaction between the carbon surface acyl chloride functionalities and linear diamines used as linking agents and (iv) Schiff condensation between the remaining free amino group of the linking agent and the acetylacetonate complex. The resulting carbon-based materials were characterised by X-ray photoelectron spectroscopy (XPS), DRIFT, nitrogen adsorption isotherms and temperature-programmed desorption. Ethylenediamine, tetramethylenediamine, hexamethylenediamine and dodecamethylenediamine are used as linking agents. Two different carbon supports are prepared by oxidising the parent activated carbon with 1 M HNO3 or 13 M HNO3 in order to obtain support surfaces with different oxygen contents. The effects of the chain length of the linking agent as well as the support's oxygen content on the orientation of the reaction towards epoxidation or autoxidation is discussed.