Olefin metathesis over supported MoOx catalysts: influence of the oxide support

Abstract

A series of supported MoO_x catalysts on different oxide supports (Al₂O₃, TiO₂, ZrO₂, SiO₂) were synthesized and investigated for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR-MS, propylene-TPSR-IR, and ethylene/2-butene titration. Under dehydrated conditions at monolayer coverage or maximum surface dispersion, the surface MoO_x sites are present as a mixture of isolated di-oxo (O)₂Mo(–O–Al)₂ and oligomeric mono-oxo OMo(–O–Al)_4/5 sites on Al₂O₃, primarily oligomeric mono-oxo OMo(–O–Ti)_4/5 on TiO₂, isolated di-oxo (O)₂Mo(–O–Zr)₂ and oligomeric mono-oxo OMo(–O–Zr)_4/5 on ZrO₂, and isolated di-oxo (O)₂Mo(–O–Si)₂ on SiO₂. The bridged (S₂-OH) and tri-coordinated (S₃-OH) anchoring surface hydroxyls of the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO_x sites at low temperatures (<100 °C). The isolated anchoring surface hydroxyls (S-OH) of the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO_x sites at high temperatures (>350 °C). Olefin metathesis by the more redox active supported MoO_x/TiO₂ and MoO_x/ZrO₂ catalysts is retarded by the formation of stable surface acetone and acetate species that block olefin adsorption. The oxide supports are potent ligands that tune the activation and surface chemistry of the surface MoO_x sites for olefin metathesis. This is the first time that the influence of oxide supports on the activation and surface chemistry of supported MoO_x sites has been systematically examined.