Stable Mechanistically-Relevant Aromatic-Based Carbenium Ions in Zeolite Catalysts

The existence of carbenium ion species is assumed in many zeolite catalysis mechanisms. Using
computational techniques that include environmental effects, a benzenium-type carbenium ion is identified
in zeolite catalysts for the first time. Localization of nearby transition states indicate that this species may
play an important role as an intermediate in the bimolecular m-xylene disproportionation reaction. The
barrier to back-donation of the proton from the benzenium ion is at least 50 kJ/mol, meaning that this
species may be spectroscopically observable. An additional carbenium ion intermediate, formed by
abstraction of a hydride from m-xylene, is also predicted. The stability of this second new carbenium ion
suggests that aromatic-based carbenium ions are likely to be intermediates in many zeolite-catalyzed
reactions. Two types of fundamentally different fully periodic calculations support the stability predictions.