Disproportionation of cis-2-pentene over Mo Al₂O₃ and Co&z.sbnd;Mo Al₂O₃. Kinetic study and nature of active site

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 C_4v, is proposed. New considerations must be advanced for the mechanism of disproportionation taking into account the C_4v, 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.