Activation Mechanism and Surface Intermediates during Olefin Metathesis by Supported MoOx/Al2O3 Catalysts

The activation of supported MoOx/Al2O3 catalysts and the resulting surface intermediates was examined with in situ diffuse reflectance infrared Fourier transform spectroscopy during olefin metathesis reaction conditions. The studies were aided with C3D6–C3H6 isotopically labeled switching, C2H4–C4H8 titration, and temperature-programmed reaction experiments. The activation of the surface MoOx sites on Al2O3 by propylene initiates by forming surface isopropoxide species that subsequently dehydrogenate to acetone and surface Mo–OH. The desorption of acetone from the catalyst surface reduces the Mo6+ sites to Mo4+ sites and creates a vacancy for the coordination of the next CH2═CHCH3 molecule. Oxidative addition from subsequent adsorption of propylene on the surface Mo4+ sites results in the formation of surface Mo═CH2 and Mo═CHCH3 reactive intermediates and oxidizes the reduced surface molybdena sites back to Mo6+. This study establishes the activation mechanism and surface intermediates during olefin metathesis by supported MoOx/Al2O3 catalysts under reaction conditions.