Solid-State NMR Characterization of the Structure of Intermediates Formed from Olefins on Metal Oxides (Al2O3 and Ga2O3)

Abstract

The interaction of olefins with metal oxides affords surface species which are considered to be the intermediates in olefin isomerization reactions. The nature and the structure of these intermediates after earlier characterization by IR spectroscopy still remain debatable. In this paper by using 13C solid-state NMR spectroscopy we have characterized the surface intermediates formed from propene, n-butene, and isobutene on γ-Al2O3 and α-Ga2O3, based on analysis of specific chemical shifts expected for similar organometallic or oxygenated compounds. NMR clearly shows that both allylic and alkoxy intermediates are simultaneously formed on two studied metal oxides. Propene affords isopropoxy and allylic intermediates on both oxides. Allyl formed on alumina is bound to the Al3+ cations of metal oxide surface in a η1,η2-like fashion, whereas allyl on α-Ga2O3 is bound to the Ga3+ cation exclusively in a η1-like fashion. n-Butene gives 2-butoxy species for both metal oxides, whereas allylic species (σ-allyl) was identified for this olefin only for γ-Al2O3. Adsorption of isobutene results to the formation of tert-butoxy and allylic species on both metal oxides. π-Allyl with η3-like fashion of allyl bonding to the oxide surface is formed in the case of γ-Al2O3, whereas σ-allyl is formed in the case α-Ga2O3. Both allylic and alkoxy species can be involved as intermediates in a double bond shift reaction of olefins on metal oxide surfaces.