n-Heptane cracking on H- and Ni-containing zeolites

The cracking of n-heptane over zeolites, HZSM5 and H-mordenite (HMOR), at 533 K in a He or H₂ atmosphere proceeds preferentially via a bimolecular chain mechanism. The apparent activation energy was found to be 70 kJ mol^-1 under both reaction conditions indicating the same rate-determining step. The order of the reaction in hydrogen was 0.17 for HZSM5 and 0.27 for HMOR. Competition between a greater availability of active sites and a shorter chain reaction by hydrogenation of intermediate carbenium ions is discussed. An order of -0.5 in hydrogen and an apparent activation energy of 129 kJ mol^-1 was found over NiHZSM5, indicative of a bifunctional mechanism. At a hydrogen pressure of 20 bar the reaction rates over HZSM5 and NiHZSM5 were practically the same.