Olefin metathesis over well-defined active fixed molybdenum catalysts. Structure and oxidation state of the active site and reaction mechanism

Abstract

Fixed Mo catalysts (supported molecular catalysts), synthesised using the facile reaction between Mo (π-C₃H₅)₄ and the surface acidic OH groups of alumina, silica and silica–alumina, were found to be very active for propene and but-1-ene metathesis in the temperature range 268–308 K. The isolated and coordinatively unsaturated Mo⁴⁺ species which are chemically bonded to the support surface are the active species. The nature of the ligands and supports coordinated to the Mo⁴⁺ ions produce different catalytic activities and selectivities. The support also works as a reservoir/supplier of reactants. Metathesis on fixed Mo catalysts is explained by a one-site chain mechanism involving carbene intermediates.