On the question of carbonium ions as intermediates over silica-alumina and acidic zeolites

Abstract

Activation energies for the isomerization of an equilibrated mixture of 2-methylpent-1-ene and 2-methylpent-2-ene to several isomers with an ultrastable Y catalyst have been found to correspond closely with the barriers to the rearrangement of the same “postulated” carbonium ions in super-acids. This provides evidence for relatively stable carbonium ions as reaction intermediates on this solid acid. The same rearrangements with a conventional amorphous SiO 2Al 2O 3 catalyst involve distinctly different barriers and are more indicative of the presence of surface alkoxy compounds as intermediates. With these catalysts it is proposed that rearrangements may occur through polar (cationic-like) transition states.