Alkane dehydrocyclization mechanism

A review of results impacting the dehydrocyclization mechanisms for monofunctional catalysts proposed to date has been made. The data indicate that, while alkane conversions at low temperatures (about 300 degrees C or less) may involve reversible adsorption, irreversible adsorption of the alkane is the rate determining step at higher temperatures representative of naphtha reforming. The extent of irreversible adsorption depends upon hydrogen partial pressure, with alkane adsorption being irreversible near atmospheric pressure and gradually changing to reversible adsorption at naphtha reforming pressure. At all pressures, primary aromatic products are formed by a mechanism involving direct six-carbon ring formation. At low pressures, the cyclopentane cyclization product yields carbon on catalyst and cracked products. At higher pressures, the cyclopentane structures undergo nonselective hydrogenolysis to produce isomers of the alkane feed by a monofunctional metal catalyzed pathway. A common cyclization pathway is proposed for the formation of C-5- and C-6-ring structures. With bifunctional catalysts, the acid catalyzed cyclization is more rapid than the monofunctional metal cyclization. (C) 1999 Elsevier Science B.V. All rights reserved.