Mechanism of n-alkane transformations over a solid superacid of Lewis character, Al2O3/AlCl3

Abstract

In the presence of Al₂O₃/AlCl₃ superacid catalysts light alkanes undergo low-temperature isomerization and decomposition. Isomerization is initiated by attack of a superacid Lewis centre (an Al³⁺ cation with a pronounced deficit of electrons caused by the adsorption of AlCl₃ on an adjacent oxygen anion) on the electrons of an alkane C—H bond. The mechanism of n-alkane decomposition depends on the length of the carbon chain. n-Butane decomposition is catalysed by superacid Lewis centres and proceeds via dimer C₈-transition-state cracking. In n-pentane and n-hexane decomposition dimers are not present in any reaction step and these reactions are catalysed by a different type of active centre. These sites (surface-bonded carbenium cations) are formed as a result of superacid Lewis centre attack on the C—C bonds of the alkane.