C. Sievers et al. / Journal of Catalysis 246 (2007) 315–324
323
MALDI-TOF MS is attributed to the selectivity of the ioniza-
tion process [43].
and redistributes into smaller entities, which eventually block
most Brønsted acid sites. At this stage, some molecules migrate
toward the pore mouth, leading to a significant reduction in the
micropore volume by blocking. While this process continues,
these molecules are being alkylated, forming larger deposits
at the outer surface and pore mouth, more effectively block-
ing access to the micropores. For a limited time, alkylation is
observed on the acid sites, which are still accessible. Olefin
addition at the outside of the zeolite increases in importance
and leads to rapid accumulation of deposits once the sites for
alkylation are not accessible. Most of the deposit molecules are
bicyclic systems with one or two double bonds and alkyl side
chains. The nature of the deposits does not change significantly
during the catalyst lifetime. Significant amounts of aromatic
compounds are seen only when deactivation is apparent. This
suggests that in addition to these processes, dehydrogenation
can occur either via hydrogen elimination from chemisorbed
molecules or via multiple hydride transfer steps. To maintain
long catalyst life, zeolite catalysts should be regenerated before
redistribution of the hydrocarbon sets in.
The periodicity in the MALDI-TOF mass spectra indicates
that most deposit molecules contain alkyl side chains attached
to the bicyclic ring system. These molecules are so bulky that
they must be located at the pore mouth or at the outer surface of
the zeolite, so that at least parts of them lie outside the microp-
orous network of the zeolite.
The preference toward compounds with carbon numbers that
are multiples of 4 indicates that considerable amounts of the
deposits are formed by oligomerization of butene without be-
ing affected by cracking. This observation is in apparent con-
trast to the fact that the concentration of C5–C7 products was
8–15 wt%, indicating that cracking of carbenium ions is an im-
portant reaction pathway. However, the large deposit molecules
discussed above are chemisorbed via the bicyclic system. In that
scenario, the formation of C5–C7 alkanes involves cracking of
the alkyl side chains, including the formation of a di-cation.
Because cracking of the saturated side chain would require for-
mation of a carbonium ion or a hydride-transfer step involving
very large carbenium ions, we conclude that it is rather unlikely
under the reaction conditions used here. This in turn indicates
that cracking leading to C5–C7 products occurs preferentially
via smaller carbenium ions.
The largest detected masses correspond to molecules with 40
carbon atoms, which contain long and bulky alkyl side chains.
Because these molecules are readily observed in MALDI-TOF
MS measurements, they must be located preferentially near or
on the outer surface of the catalyst. We speculate that during the
second stage, bicyclic molecules migrate to the pore mouths,
where the side chains are added. Because all characterization
methods suggest the absence of major changes in the deposits’
nature while the catalyst remains active, we conclude that mi-
gration of the deposits must play a key role in deactivation by
inducing pore mouth plugging.
Acknowledgments
The authors thank Professor Freude and Mr. Schneider for
the 27Al DOR NMR measurements, and Mr. Krause for the
GC–MS measurements and support for MALDI-TOF MS mea-
surements. They also thank Patrick Magnoux for GC–MS ref-
erence measurements. Financial support from Süd-Chemie AG
and Lurgi GmbH is gratefully acknowledged. The authors thank
Dr. G. Burgfels and Dr. H. Buchold for fruitful discussions.
Partial financial support by the European Union in the frame-
work of NMP3-CT-2005-011730 IDECAT WP5 is gratefully
acknowledged.
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otherwise the cyclic compounds could rapidly dehydrogenate
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5. Conclusion
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