COKING, AGING AND REGENERATION OF ZEOLITES .16. INFLUENCE OF THE COMPOSITION OF HY ZEOLITES ON THE REMOVAL OF COKE THROUGH OXIDATIVE TREATMENT

Coke removal by oxidative treatment was studied for a number of HY zeo lites with framework Si/Al ratios in the range of 4 to 100, coked to d ifferent levels during propene transformation at 450 degrees C. Except for the zeolites with high Si/Al ratios there was practically no effe ct of the coke content, hence of the nature and of the location of cok e molecules, on coke oxidation. Oxidation of pure pyrene trapped in ze olite supercages showed furthermore the same tendency as coke oxidatio n: The density of framework aluminium atoms, hence of zeolite acid sit es, was the determining parameter. Neither extraframework aluminium sp ecies nor small quantities of sodium (<0.15 wt.-%) in the zeolites inf luenced coke removal. However the CO to CO2 ratio in the oxidation pro duct 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 co ke removal, comprising decomposition of the oxygenated compounds, woul d also be catalyzed by acid sites.