(515a) Toward Improved Heterogeneous Catalysts for Olefin Metathesis

Propylene (C₃⁼) is the second largest feedstock, after ethylene, for the petrochemical industry and a shortage is developing that will increase over the next decade.  To meet this demand, industry is turning to heterogeneous olefin metathesis catalysis via C₂⁼ + C₄⁼ → 2 C₃⁼.  This process still employs inexpensive, simply prepared, and ill-defined, multicomponent heterogeneous catalytic systems.  A key challenge is to improve performance of catalysts for this reaction.  Unlike the well-defined, single-site homogeneous olefin metathesis catalysts, a lack of fundamental information about the active sites of commercial heterogeneous olefin metathesis catalysts based on supported metal oxides has hindered further development.  Heterogeneous catalysts consist of transition metal oxides (WO₃, MoO₃, and Re₂O₇) supported on high surface area oxide supports (typically SiO₂ and Al₂O₃).  The catalytic active sites, electronic and molecular structures, and surface reaction intermediates of these catalysts are poorly understood. In this talk, we will elucidate molecular/electronic-catalytic activity/selectivity relationships by forming designed ReO_x surface oxide species on a planar Al₂O₃ single-crystal film support. The surface is characterized by using high resolution X-ray photoelectron spectroscopy (HR-XPS), low energy ion scattering (LEIS), ultraviolet photoelectron spectroscopy (UPS), high resolution electron-energy loss spectroscopy (HREELS), and infrared-absorption spectroscopy (IRAS). ReO_x species on planar Al₂O₃ films were synthesized and characterized with atomic and nanoscale control of the reactive sites present. By controlling the O₂ partial pressure in the background gas and the substrate temperature, we control the oxidation states of the evaporated Re and Re-oxide nanostructures formed.  The structures associated with each oxidation state and the initial reactivity of each structure were then determined. The adsorption, desorption, reaction pathways, and reaction activation energies of ethylene at various reactive sites was probed using temperature programmed desorption (TPD).  Increased fundamental understanding of the nature and reactivity of active sites will provide a firm basis for the development of improved heterogeneous metathesis catalysts.