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 pr otonated cyclopropane whereas cracking is suggested to progress throug h the classical beta-scission route. Catalyst loading in the NMR exper iments seems to be a very important factor for both initiation of the reaction and final product distribution. The initial source of i-butan e, which is observed as primary product, is the isomer 2,2-dimethylpen tane, which cracks to i-butane and propene. Propene, which has never b een observed, is suggested to be involved in polymerisation reactions producing long hydrocarbon chains which can rearrange and finally crac k to a number of C-5-C-7 isomers. At later stages, the reaction become s 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 paraff ins by H-transfer while the olefinic fraction is involved in polymeris ation reactions. At the final stages of the reaction, protolytic crack ing starts contributing to this process, producing methane, ethane, an d propane. Of great interest is the catalyst loading effect on this mo nomolecular reaction process which defines the final product distribut ion. (C) 1996 Academic Press, Inc.