The determination of the active site for the olefin disproportionation catalyst Mo Al2O3 is the object of the present ESR, uv and kinetic investigation. Comparison of the rate of disproportionation with that of reduction Mo6+ → Mo5+ shows that Mo6+ is the active site, the aging of the catalyst being the result of the reduction of Mo6+ to Mo5+ by the reactant. The variation of catalytic activity with Mo content shows a maximum at 7% Mo, which is also the case for the number of oxidizing sites Mo6+. The highest activity is obtained for a pretreatment temperature of 580 °C under O2, which is also in favor of Mo6+ as the active species. Ultraviolet spectroscopy shows that two types of symmetry occur in the case of Mo Al2O3 catalyst: tetrahedral and octahedral. However, by comparing the ESR and uv spectrum of Mo5+, it seems that only Mo6+ in octahedral coordination is the active species. A model of the active Mo6+ site with a symmetry C4v, is proposed. New considerations must be advanced for the mechanism of disproportionation taking into account the C4v, symmetry and the high oxidation state of the active site. Kinetic studies have also shown a high selectivity in favor of the cis isomer of butene at low temperature.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry