Spectroscopic study of the cokefaction of butene and butane on alumina

Abstract

Ultraviolet–visible reflectance spectroscopy has been used to study the interaction of butane and butene with γ-alumina and to determine the nature of intermediate species occurring during the coking process. With pure alumina, butane or butene give keto-like oxygenated structures at medium temperatures (200–300 °C) which become light aromatics and are ionized and adsorbed on the surface. With chlorinated alumina, a π-allyl cation appears, even at room temperature: with further heating condensation and dehydrogenation occur and polyaromatic cations are formed. Both hydrocarbons give the same results, suggesting that butane is dehydrogenated to butene, which then reacts with the acidic sites on the surface to form organic cations which are strongly adsorbed onto the alumina surface. These species act as a poison for the acidic centres as they cannot be easily desorbed. The initial support, which possesses Lewis–acid centres, becomes Brønsted acidic: the carbonium ions strongly adsorbed on the surface act as a proton source for oligomerization, condensation, cyclization and dehydrogenation.