Mechanism of aromatization of hydrocarbons on platinum. Aromatization of 13C-labelled 3-methylhexanes: evidence for a multistep mechanism

Abstract

The aromatization of 3-methylhexane labelled on all possible positions has been studied in the temperature range 340–380 °C over a series of Pt–Al₂O₃ catalysts with a metal dispersion extending from 0.04 to 1. ¹³C labelling technique using mass spectrometry suggests that the major 1–6 ring-closure mechanism is accompanied by an additional mechanism involving a 1–5 dehydrocyclization as a preliminary step.

Comparison between the relative contributions of the two mechanisms in the aromatization of various labelled 3-methylhexanes, the absence of temperature and hydrogen-pressure effects in addition to the absence of any appreciable particle-size effect indicate that the additional mechanism consists, in the adsorbed phase, of three consecutive steps: 1–5 ring closure, ring opening and 1–6 ring closure, without desorption. The absence of any particle-size effect suggests that 1–5 ring closure and 1–6 ring closure involve the same catalytic sites on the metal surface.