FT-IR studies on light olefin skeletal isomerization catalysis .1. The interaction of C4 olefins and alcohols with pure gamma-alumina

The skeletal isomerization of n-butene to isobutene and the interaction of the four butene isomers and of sec-butanol and tert-butanol with a pure gamma-alumina sample were investigated. gamma-Al2O3 is quite active (> 34% conversion) and a selective catalyst for n-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 and tert-butoxy groups decompose to give gaseous n-butene and isobutene, respectively, near 473 K. Moreover, traces of tert-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. (C) 1997 Academic Press