| Popis: |
The determination of the active site for the olefin disproportionation catalyst Mo Al 2 O 3 is the object of the present ESR, uv and kinetic investigation. Comparison of the rate of disproportionation with that of reduction Mo 6+ → Mo 5+ shows that Mo 6+ is the active site, the aging of the catalyst being the result of the reduction of Mo 6+ to Mo 5+ 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 Mo 6+ . The highest activity is obtained for a pretreatment temperature of 580 °C under O 2 , which is also in favor of Mo 6+ as the active species. Ultraviolet spectroscopy shows that two types of symmetry occur in the case of Mo Al 2 O 3 catalyst: tetrahedral and octahedral. However, by comparing the ESR and uv spectrum of Mo 5+ , it seems that only Mo 6+ in octahedral coordination is the active species. A model of the active Mo 6+ site with a symmetry C 4 v , is proposed. New considerations must be advanced for the mechanism of disproportionation taking into account the C 4 v , 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. |
