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Cracking of an all-transn-alkane, via idealized Lewis acid and Bronsted acid catalysis, was examined using density functional theory. Optimized geometries and transitions states were determined for catalyst -re ctant complexes, using AlCl 3 and HCl ‚AlCl3 as the Lewis and Bronsted acids. For the Lewis acid cycle, hydridetransfer steps are seen to have large barriers in both forward and reverse directions, and an unstable physisorbed carbenium ion (lying 20 kcal mol -1 above the chemisorbed intermediate) is the launching point for the â-scission that leads to products. For the Bronsted acid cycle, proton-transfer steps have smaller barriers in both forward and reverse directions, and a semistable physisorbed alkanium ion is the launching point for the alkanium R-scission that leads to products. In the idealized Lewis cycle, formation of HCl units (and hence Bronsted acids) was found to be a common side reaction. A recent ionic-liquid catalysis study is mentioned as motivation, although our study is not a computational modeling study; we are more interested in the fundamental differences between Brosnted and Lewis mechanisms rather than merely mimicking a particular system. However, results of exploratory optimizations of various intermediates with Al 2Cl7 as the catalyst are presented to provide the first step for future modeling studies on the ionic liquid system.

A Theoretical Comparison of Lewis Acid vs Bronsted Acid Catalysis for n-Hexane f Propane +