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.