期刊
CHEMISTRY-A EUROPEAN JOURNAL
卷 12, 期 2, 页码 457-465出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200500382
关键词
butane; GC-MS analysis; NMR spectroscopy; reaction mechanisms; zeolites
By using C-13 MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively C-13-labeled n-butane on zeolite H-ZSM-5 at 430-470K has been demonstrated to proceed through two pathways: 1) scrambling of the selective C-13-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ C-13 MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the C-13-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to C-13 MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E-a=75 kJmol(-1) for the scrambling and 71 kJmol(-1) for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n-butane as being the rate-determining stage of the n-butane conversion on zeolite H-ZSM-5.
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