Carbocation-Mediated Cyclization of Trienes in Acid Zeolites

The
mechanism by which acid zeolites catalyze the formation of
aromatic species is not fully understood and is important in an array
of industrial processes such as the methanol to gasoline reaction.
The so-called “carbon pool” mechanism is generally agreed
to be the main channel for the formation of hydrocarbons from methanol.
There is, however, no agreed sequence of elementary steps that explains
how linear intermediates transform to cyclic intermediates, let alone
aromatic rings. Recent work suggests the formation of conjugated trienes
during zeolite-catalyzed aromatization, but mechanisms involving triene-derived
carbocations have never been investigated using modern computational
tools. In this work, we propose a new mechanism for cyclization of
hexatriene over the Brønsted acid site of faujasite zeolite.
Microkinetic models (MKM) using the results of Density Functional
Theory (DFT) calculations predict selectivity for neutral 5-membered-ring
intermediates over 6-membered-ring intermediates, as suggested by
infrared and UV–vis spectroscopic results reported by others.
Given that the products of aromatization are 6-membered rings, this
result suggests that triene cyclization can only explain how linear
hydrocarbons become cyclic intermediates but not the mechanisms that
ultimately lead to the aromatic rings seen in industrial zeolite-catalyzed
hydrocarbon processes.