Understanding the window effect in zeolite catalysis

Cracking mechanisms in the zeolite literature have traditionally assumed full adsorption. Simulations indicate abnormally low adsorption in ERI-, AFX-, CHA-, RHO-, and KFI-type zeolites for chain lengths close to or longer than the cage size. Very long molecules adsorb partially into a cage near the outer surface. After scission the nonadsorbed part can end up in the product or re-adsorb and undergo another scission. This mechanism is characteristic for cage/window-type zeolites with small windows close to the diameter of the adsorbate (see picture).