Propylene metathesis catalysts prepared by interaction of Re 2(CO) 10 with γ-Al 2O 3

Abstract

In this work new data on the state of Re and on the nature of active sites in Re 2(CO) 10 derived catalysts are discussed. Active catalysts for propylene metathesis can be obtained by several treatments of Re 2(CO) 10/Al 2O 3: (1)O 2 treatment at 393 K of partially decarbonylated samples of Re 2(CO) 10/Al 2O 3. In this case, part of the oxygen reversibly binds to the catalyst, probably in molecular form. The activity of this catalyst increases linearly up to an O:Re ratio close to 0.03. Subsequent addition of oxygen has no effect on activity. (2)O 2 treatment at 300 K of Re 2(CO) 10/Al 2O 3 samples completely decarbonylated at 773 K. According to the EXAFS and TEM data, completely decarbonylated samples contain associated Re(II) ions, with Re—Re distances different from those in metallic rhenium. O 2 treatment leads to rearrangement in the clusters. Part of the oxygen is adsorbed in the molecular form as O 2 − on Al ions of the support (TPD and ESR data). The adsorption occurs reversibly and without exchange with the oxygen of the Al 2O 3. The activity of the catalysts correlates with their molecular oxygen content. (3) Partial reduction at 473–573 K of Re 2(CO) 10/Al 2O 3 preoxidized at 573 K. The concentration of catalytically active Re atoms is assumed to be less than 2–3% of the rhenium content in the catalysts. Metal-carbene complexes active in metathesis are formed, via the oxidation of propylene, by rhenium ions in a high oxidation state.