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´
,
Fig. 2 GC-MS total ion chromatograms of CH Cl
2
2
extracts from
2
À1
H-SAPO-34 after MTO at 623 K and 3.6 h WHSV for (a) 15,
Z.-P. Liu, J. Phys. Chem. C, 2009, 113, 4584; (f) M. Bjørgen,
F. Joensen, K.-P. Lillerud, U. Olsbye and S. Svelle, Catal. Today,
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b) 30, (c) 60, (d) 180 and (e) 540 min on stream, showing the
identification of each of the observed peaks. The asterisk represents
the mass signal of C Cl used as an internal standard.
2
6
from all catalysts studied here, however, it appears that they
5
ꢀ
+
may be much less stable than HMB , probably due to their
lower molecular symmetry, which deserves further investigations.
Fig. 2 also shows that the hydrocarbon species formed on
H-SAPO-34 after 540 min on stream become very diverse.
The dominant products include naphthalene, monomethyl-
naphthalenes and polymethylnaphthalenes with 2–4 methyl
(
d) A. V. Kucherov, A. A. Slinkin, D. A. Kondratyev,
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8
,14
groups, as already reported,
together with phenanthrene.
Given the highly symmetric nature of the intense ESR signal
from this catalyst after MTO for the same period of TOS
(
before their neutral ones, like the HMB case.
Fig. 1), the PAH radical cations may be the species generated
+
6
M. Che and E. Bozon-Verduraz, in Handbook of Heterogeneous
ꢀ
Catalysis, G. Ertl, H. Kno
Weinheim, 1997, vol. 2, p. 641.
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zinger and J. Weitkamp, Wiley-VCH,
In summary, our work provides clear experimental evidence
ꢀ
+
that the HMB radical cation, whose existence during MTO
over zeolitic catalysts was until now pure speculation, is one of
the true transition states of this heterogeneously very important
reaction. Because the acidic microporous framework contributes
to stabilise organic guest cations, spatial conditions could play
a crucial role in persisting other aromatic radical cations like
the heptamethylbenzenium ion suggested as important MTO
2
8
J. H. Lee, M. B. Park, J. K. Lee, H. K. Min, M. K. Song and
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0 J. A. Weil and J. R. Bolton, Electron Paramagnetic Resonance:
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1
5
reaction intermediates. In that case, the optimisation of the
framework topology of crystalline, microporous materials
along with the control of their acidic properties will be one
possible approach to directly identify major cationic hydro-
1
1
2 A. F. Bedilo, A. M. Volodin, G. A. Zenkovets and
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carbon species other than HMB
solid catalysts using ESR spectroscopy.
during MTO over such
1
8, 8386.
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1
This work was supported by the National Research Foun-
dation of Korea (2010-0029626 and R0A-2007-000-20050-9).
2
9
500 Chem. Commun., 2011, 47, 9498–9500
This journal is c The Royal Society of Chemistry 2011