EPR STUDY OF THE INTERACTION OF O- IONS WITH CB3OH WITHIN THE COORDINATION SPHERE OF MO IONS GRAFTED ON SILICA - A NEW APPROACH FOR THE STUDY OF THE MECHANISM OF CATALYTIC REACTIONS

In an attempt to explain the mechanism of methanol oxidation, a new approach has been used in which cycles of model reactions between methanol and in situ generated O-, both adsorbed on the surface of grafted Mo/SiO2 catalysts, have been studied by EPR. The model reactions were designed so as to reproduce the most important catalytic steps of methanol oxidation. The finding that those have threshold temperatures of occurrence has permitted us to separate and characterize them. The use of O- has allowed bypassing the rate-determining step, i.e., the abstraction of methyl hydrogen, revealing subsequent steps of this process. The oxidation of methanol involves the following elementary steps, a ligand to ligand hydrogen transfer (LLHT) and proton transfer (LLPT), a ligand to metal electron transfer (LMET), as well as the migration of a radical (CH2OH)-C-. intermediate species. It also confirms the intermediate migration hypothesis proposed earlier on the basis of kinetic results. The sequence of these steps depends on the temperature, the oxidation state, and the dispersion degree of molybdenum ions on the silica support. The study has shown that the reaction takes place within the coordination sphere of molybdenum, revealing the role of isolated coordinatively unsaturated Mo5+/Mo6+ redox couples as the reaction active centers. The reductive migration of (CH2OH)-C-. intermediates provides a reasonable explanation for the dispersion sensitivity of methanol oxidation.