Solid-state NMR investigation of n-heptane cracking over zeolite beta

n-Heptane cracking over zeolite H-Beta has been studied by solid-state NMR spectroscopy. It is shown that isomerisation and cracking are two related processes at the early stages of the reaction. Isomerisation is proposed to proceed faster than cracking via the intermediacy of protonated cyclopropane whereas cracking is suggested to progress through the classical beta-scission route. Catalyst loading in the NMR experiments seems to be a very important factor for both initiation of the reaction and final product distribution. The initial source of i-butane, which is observed as primary product, is the isomer 2,2-dimethylpentane, which cracks to i-butane and propene. Propene, which has never been observed, is suggested to be involved in polymerisation reactions producing long hydrocarbon chains which can rearrange and finally crack to a number of C-5-C-7 isomers. At later stages, the reaction becomes very complex. More of the heptane isomers crack by beta-scission to a number of C-4 and C-3 fractions. The carbenium ions desorb as paraffins by H-transfer while the olefinic fraction is involved in polymerisation reactions. At the final stages of the reaction, protolytic cracking starts contributing to this process, producing methane, ethane, and propane. Of great interest is the catalyst loading effect on this monomolecular reaction process which defines the final product distribution. (C) 1996 Academic Press, Inc.