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Supported MoO x /Al 2 O 3 catalysts were synthesized by incipient-wetness impregnation of aqueous ammonium heptamolybdate, dried at room temperature and 110 °C, and finally calcined at 500 °C in air. The catalysts were spectroscopically characterized with in situ Raman, UV−vis, DRIFTS, and TPSR, both after calcination and during propylene metathesis reaction conditions. Three distinct MoO x species on the Al 2 O 3 support were identified: isolated surface dioxo (O) 2 MoO 2 , anchored to the basic HO-μ 1 -Al IV sites (<1 Mo atom/ nm 2 ), oligomeric surface mono-oxo OMoO 4/5 anchored to more acidic HO-μ 1 -Al V/VI sites (1−4.6 Mo atoms/nm 2 ), and crystalline MoO 3 nanoparticles also present above monolayer coverage (>4.6 Mo atoms/nm 2 ). During propylene metathesis, activation proceeds by removal of oxo MoO bonds and insertion of CH 2 and CHCH 3 alkyls, which maintain the surface MoO x species in the Mo 6+ oxidation state. The surface oligomeric mono-oxo OMoO 4/5 species easily activate at mild temperatures of 25−200 °C, while the isolated surface dioxo (O) 2 MoO 2 species require very high temperatures for activation (>400 °C). The crystalline MoO 3 nanoparticles decrease the number of accessible activated surface MoO x sites by their physical blocking. This study establishes the structure−reactivity relationship for olefin metathesis by supported MoO x /Al 2 O 3 catalysts and demonstrates the significant role that the anchoring surface hydroxyl sites on alumina have on the reactivity of surface MoO x species.

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This perspective article critically reviews the catalysis literature on the nature of the surface CrO x sites present on SiO 2 in different environments. The recent application of in situ spectroscopic techniques that directly monitor the surface chromia sites on silica in different environments has significantly improved our fundamental understanding of supported CrO x /SiO 2 catalysts.

The activation of supported MoO x /Al 2 O 3 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 C 3 D 6 −C 3 H 6 isotopically labeled switching, C 2 H 4 −C 4 H 8 titration, and temperature-programmed reaction experiments. The activation of the surface MoO x sites on Al 2 O 3 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 Mo 6+ sites to Mo 4+ sites and creates a vacancy for the coordination of the next CH 2 CHCH 3 molecule. Oxidative addition from subsequent adsorption of propylene on the surface Mo 4+ sites results in the formation of surface MoCH 2 and MoCHCH 3 reactive intermediates and oxidizes the reduced surface molybdena sites back to Mo 6+ . This study establishes the activation mechanism and surface intermediates during olefin metathesis by supported MoO x /Al 2 O 3 catalysts under reaction conditions.