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Mechanism Studies of the Conversion of 13 C‐Labeled n‐ Butane on Zeolite H‐ZSM‐5 by Using 13 C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis
Author(s) -
Luzgin Mikhail V.,
Stepanov Alexander G.,
Arzumanov Sergei S.,
Rogov Vladimir A.,
Parmon Valentin N.,
Wang Wei,
Hunger Michael,
Freude Dieter
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200500382
Subject(s) - isobutane , butane , chemistry , magic angle spinning , propane , nuclear magnetic resonance spectroscopy , carbenium ion , zeolite , carbon 13 nmr , catalysis , stereochemistry , organic chemistry
Abstract By using 13 C MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively 13 C‐labeled n‐ butane on zeolite H‐ZSM‐5 at 430–470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13 C‐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 13 C MAS NMR and complementary ex situ GC–MS data provided evidence for a monomolecular mechanism of the 13 C‐label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13 C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies ( E a =75 kJ mol −1 for the scrambling and 71 kJ mol −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.