FT-IR Studies on Light Olefin Skeletal Isomerization Catalysis: I. The Interaction of C4 Olefins and Alcohols with Pure γ-Alumina

Abstract

The skeletal isomerization ofn-butene to isobutene and the interaction of the four butene isomers and ofsec-butanol andtert-butanol with a pure γ-alumina sample were investigated. γ-Al2O3is quite active (>34% conversion) and a selective catalyst forn-butene to isobutene isomerization at 753 K (>80% selectivity). FT-IR spectroscopic experiments show that the C4 olefins are involved in two different interactions on the alumina surface at room temperature: (i) H-bonding over surface hydroxy groups and (ii) reactive adsorption giving rise to allyl species. Both species have been fully characterized spectroscopically for all four butene isomers. At higher temperatures, carboxylates appear, some of which are aromatic. Alcohol adsorption shows that 2-butoxy andtert-butoxy groups decompose to give gaseousn-butene and isobutene, respectively, near 473 K. Moreover, traces oftert-butoxy species are observed starting from 2-butoxides. H-bonded olefin species are assumed to be precursors for carbenium ions that give rise to double-bond and skeletal isomerization at high temperature. Butoxy groups are proposed to act as the unstable intermediate adsorbed forms for carbenium ions. Allyl species are identified as intermediates in double-bond isomerization at low temperature and in the formation of by-products and coking agents (aromatics and carboxylates) at high temperature.