Coking, aging and regeneration of zeolites: XVI. Influence of the composition of HY zeolites on the removal of coke through oxidative treatment Original Research Article

Coke removal by oxidative treatment was studied for a number of HY zeolites with framework Si/Al ratios in the range of 4 to 100, coked to different levels during propene transformation at 450°C. Except for the zeolites with high Si/Al ratios there was practically no effect of the coke content, hence of the nature and of the location of coke molecules, on coke oxidation. Oxidation of pure pyrene trapped in zeolite supercages showed furthermore the same tendency as coke oxidation: The density of framework aluminium atoms, hence of zeolite acid sites, was the determining parameter. Neither extraframework aluminium species nor small quantities of sodium (<0.15 wt.-%) in the zeolites influenced coke removal. However the CO to CO2 ratio in the oxidation product diminished with increasing sodium content. Oxidation began by the hydrogen atoms with formation of oxygenated compounds and water, and a mechanism involving radical cations as intermediates was proposed to explain this first step of coke oxidation. The subsequent steps of coke removal, comprising decomposition of the oxygenated compounds, would also be catalyzed by acid sites.