Coupling of ethylene dimerization and olefin metathesis reactions on a bifunctional catalyst: active sites, surface intermediates, reaction mechanism and kinetics

Conversion of ethylene to propylene proceeds on a single bi-functional catalyst through the coupling of ethylene dimerization and olefin metathesis catalytic reactions. In this study, a supported bi-functional 8%NiSO₄-8%ReO₄/γ-Al₂O₃ catalyst was synthesized for the conversion of ethylene to propylene, in which NiSO₄ performs ethylene dimerization to butenes (primarily 2-butene) and ReO₄ performs olefin metathesis of ethylene and 2-butene to propylene. This bi-functional catalyst was extensively physically characterized at the molecular-level (in situ Raman, IR spectroscopy, UV-vis spectroscopy, etc.) and chemically probed (C₂=-TPSR, C₃=-TPSR, C₂⁼/C₄⁼ titration, and steady state ethylene dimerization and olefin metathesis reactions) providing new insights about the fundamental molecular structure-function relationships. A synergy between surface SO₄ and NiO_x allows for ethylene dimerization while ReO₄ surface performs olefin metathesis, with metathesis proceeding via surface Re=CH₂ and Re=CHCH₃ intermediates. The addition of NiSO₄ to the supported ReO₄/Al₂O₃ catalyst significantly enhances the rate of olefin metathesis. The promotional activity changes are related to the competitive adsorption of these oxides on specific surface hydroxyls of the alumina support. These results will be discussed in detail during the presentation.