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

• Published in: Journal of the American Chemical Society Vol. 125; no. 8; pp. 2136 - 2141
• Main Authors: Clark, Louis A, Sierka, Marek, Sauer, Joachim
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 26.02.2003
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Catalytic reaction; Disproportionation; Reaction path; Molecular dynamics method; Bimolecular reaction; Theoretical study; Supported catalyst; Molecular sieve; m-Xylene; Carbenium compounds; Transition state; Van der Waals interaction; Density functional method; Potential energy surfaces; Benzenic compound; Catalyst activity
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0283302

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.