Mechanism studies of the conversion of C-13-labeled n-butane on zeolite H-ZSM-5 by using C-13 magic angle spinning NMR spectroscopy and GC-MS analysis

摘要

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.