Coke Formation on Protonic Zeolites : Rate and Selectivity

Abstract

The coking and the aging rates of four protonic zeolites (HY, H mordenite, H offretite and HZSM-5) were determined during n-heptane cracking at 450°C. On each of the zeolites, the coke was characterized by conventional analysis (GC, HPLC, NMR, MS) of the solvent extracts obtained after dissolution of the zeolite framework by hydrofluoric acid. Coking, aging rates and coke composition are definitely dependent on the zeolite porous structure, which confirms that coke formation is a shape-selective reaction. Thus the coking rate, which is high when the available space near the active centers is significant (e.g. larger cavities of HY) is slow due to steric constraints in the formation of reactional intermediates when this space is reduced (e.g. ZSM-5). Deactivation is faster when molecule circulation in the zeolite is unidimensional (activity highly dependent on pore blockage) than when it is tridimensional. Coke is polyaromatic and very heavy if the reactions which lead to its formation are but slightly limited by steric constraints (e.g. HY, mordenite); it will be less aromatic and lighter in the opposite case (gmelinite cages of offretite and channels of ZSM-5 zeolite).