IR study of alkene allylic activation on magnesium ferrite and alumina catalysts

Abstract

The interaction of propene and butenes with a butene oxydehydrogenation catalyst, MgFe₂O₄, and with an isomerization catalyst, γ-Al₂O₃, have been studied by FTIR spectroscopy. Allyloxy species (prop-2-en-1-oxides from propene and but-3-en-2-oxide from but-1-ene) were observed over MgFe₂O₄, while allyl species (prop-2-en-1-yl from propene, but-3-en-2-yl from but-1-ene and 2-methylprop-2-en-1-yl from isobutene), thought to be σ-bonded to Al³⁺ ions, were observed over γ-Al₂O₃. It is proposed that in all cases the allylic C—H bond is heterolytically broken at cation–anion couples (Mⁿ⁺O^2–) to give rise to anionic allyls. However, when the cation is reducible, as on the Fe³⁺ centres of magnesium ferrite, the allyl anion is further rapidly oxidized to allyloxy species that, at high temperature, can act as cationic allyls which interact weakly with oxide anions. From propene, the cationic allyls can act as symmetric species, as is expected for acrolein synthesis.