N-HEPTANE CRACKING ON H-CONTAINING AND NI-CONTAINING ZEOLITES

The cracking of n-heptane over zeolites, HZSM5 and H-mordenite (HMOR), at 533 K in a He or H-2 atmosphere proceeds preferentially via a bimo lecular chain mechanism. The apparent activation energy was found to b e 70 kJ mol(-1) under both reaction conditions indicating the same rat e-determining step. The order of the reaction in hydrogen was 0.17 for HZSM5 and 0.27 for HMOR. Competition between a greater availability o f active sites and a shorter chain reaction by hydrogenation of interm ediate carbenium ions is discussed. An order of -0.5 in hydrogen and a n 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 sam e.