MO Calculations Involving μ-Hydrido Cation Intermediates Relevant to the Heptane to Toluene Dehydrocyclization Reaction

Abstract

The catalytic dehydrocyclization of alkanes at 400~500°C into aromatics and dihydrogen is a very important petrochemical reaction. Many catalysts have been investigated, but the socalled "dual-function" type catalyst, having both acidic (Al 2 O 3 ) and dehydrogenation (Pt) sites, appear to be superior to those having only a dehydrogenation capacity. The present studies originated from a suggestion by Davis that carbocations may be reactive intermediates in the cyclization part of the overall reaction, with the resulting cycloalkane then dehydrogenated to aromatics by the Pt site. The actual dehydrocyclization reaction of an observable μ-Η-bridged cyclodecyl cation is assumed as a reasonable model for how simple linear and branched alkyl cation intermediates could also cyclize. In this work, molecular orbital studies have been performed on both the cyclodecyl cation dehydrocyclization reaction, and on simple n-heptyl and n-octyl cation systems, with the calculations yielding similar ground-state and transition-state structures. An important aspect of commercial dehydrocyclization reactions may involve an alkane isomerization equilibrium which is faster than the dehydrocyclization reaction, and this possibility has also been explored computationally. 268