2-Methylhexane Cracking on Y Zeolites: Catalytic Cycles and Reaction Selectivity

Abstract

We have formulated a quantitative kinetic model for analyzing the catalytic cracking of 2-methylhexane over USY-zeolite-based catalysts. The model is based on carbocation chemistry which includes carbenium ion initiation, isomerization, olefin desorption, β-scission, oligomerization, and hydride ion transfer reactions. It describes the complex product distribution at different reaction conditions and for catalysts with different Brønsted acid strengths. Three catalytic cycles dominate this reaction and determine activity and selectivity: the initiation/desorption, initiation/β-scission, and hydride ion transfer/β-scission cycles. The rates of these cycles decrease with increasing steaming severity, which reduces catalyst acid strength. The overall site time yield for 2-methylhexane cracking decreases as severity of steaming increases. Because the cycles do not produce excess paraffins, the paraffin to olefin ratio is always lower than 1. β-Scission reactions follow initiation and hydride ion transfer reactions and are important reactions of 2-methylhexane cracking. Olefin adsorption–desorption reactions determine surface coverage of carbenium ions, and although these reactions are in quasi-equilibrium, they play a crucial role in influencing the rates of other surface processes.