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 interactio n of the four butene isomers and of sec-butanol and tert-butanol with a pure gamma-alumina sample were investigated. gamma-Al2O3 is quite ac tive (> 34% conversion) and a selective catalyst for n-butene to isobu tene isomerization at 753 K (> 80% selectivity). FT-IR spectroscopic e xperiments show that the C4 olefins are involved in two different inte ractions on the alumina surface at room temperature: (i) H-bonding ove r surface hydroxy groups and (ii) reactive adsorption giving rise to a llyl species, Both species have been fully characterized spectroscopic ally for all four butene isomers. At higher temperatures, carboxylates appear, some of which are aromatic. Alcohol adsorption shows that 2-b utoxy and tert-butoxy groups decompose to give gaseous n-butene and is obutene, respectively, near 473 K. Moreover, traces of tert-butoxy spe cies are observed starting from 2-butoxides. H-bonded olefin species a re assumed to be precursors for carbenium ions that give rise to doubl e-bond and skeletal isomerization at high temperature. Butoxy groups a re proposed to act as the unstable intermediate adsorbed forms for car benium ions. Allyl species are identified as intermediates in double-b ond isomerization at low temperature and in the formation of by-produc ts and coking agents (aromatics and carboxylates) at high temperature. (C) 1997 Academic Press.