ACS Catalysis
Letter
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the latter band in H-HPM-1 suggests that, unlike the case of H-
ITQ-12, Al is preferentially located at D4Rs in H-HPM-1. The
higher resistance to coke formation on zeolites with higher Si/
Al ratios (Figures 3d) may be due to their low acid site density,
rather than to their acid strength.
ion (M), the so-called C scrambling. Extensive C scrambling
has been repeatedly reported during 1-butene skeletal isomer-
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ization over fresh H-FER, but not over the aged catalyst.
However, in those reports, scrambling data on fresh H-FER
were obtained under static conditions (batch reaction), in
which bimolecular rereaction between products can occur in
the absence of a continuous supply of reactant molecules. To
avoid this, we carried out the 13C scrambling experiments over
H-FER(8.9) and H-HPM-1 in a flow reactor under the same
dynamic conditions as those (673 K and 7.5 h− WHSV) used
to obtain the catalytic results in Figure 2a−c. In contrast to the
There are at least three possible mechanisms for the acid-
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b,c
catalyzed isomerization of 1-butenes to isobutene:
mono-
molecular, bimolecular, and pseudomonomolecular. The true
monomolecular mechanism is based on the formation of a
protonated cyclopropene moiety that may be broken to give a
secondary carbenium ion (that produces a 1-butene molecule
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by losing H ) or a primary carbenium ion, which will be able to
results previously reported, C scrambling in isobutene, as well
rearrange to give isobutene. This mechanism is fundamentally
selective to isobutene, because when the primary cation does
not yield isobutene, it gives back n-butenes. However, the
monomolecular pathway is generally considered not favored
due to the formation of an unstable primary carbenium ion.
The bimolecular mechanism, which requires strong acid sites,
consists of a dimerization followed by cracking and is
unselective, always yielding a fraction of propene and pentenes
as in the other three butene isomers, is not largely significant in
fresh H-FER(8.9). Additionally, it is hardly perceptible in fresh
H-HPM-1, aged H-FER, and aged H-HPM-1 (Figures 4 and
(and even hexenes and heptenes), in addition to n-butenes.
Finally, in the pseudomonomolecular pathway, the active site is
proposed to be a tertiary carbenium ion trapped close to the
pore mouth in a coke-like environment. This cation would react
with 1-butene, producing a secondary carbenium ion that can
rearrange to a more stable tertiary carbenium ion which, after
desorption, would produce isobutene. Although significant
controversy still exists, the performance of, for instance, H-FER
has been explained as follows: initially, the most favorable but
unselective bimolecular reaction would take place with high
activity. As coke is formed by side-reactions and pores start
getting blocked, the formation of tertiary carbenium species in
the pore mouth would be able to catalyze the selective
formation of isobutene. This pore mouth catalysis would
explain the very high activity of H-FER nanoneedles and of
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micro/mesoporous H-FER zeolite.
This explanation, however, does not hold for H-HPM-1: high
degrees of its 1-butene reactivity and isobutene selectivity from
the onset of the reaction require a different explanation, further
supported by the behavior at a higher WHSV (Figure 2d).
Additionally, H-HPM-1 is made of heavily interpenetrated
isobutene yield cannot be due to any pore mouth effect
enhanced in a nanocrystalline catalyst, which is corroborated by
Figure 4. Online GC-MS analyses of isobutene and 2-methyl-2-butene
produced after 1-butene skeletal isomerization using unlabeled 1-
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butene over (a) fresh H-HPM-1 and using 1-[1- C]butene over (b)
fresh H-HPM-1, (c) aged H-HPM-1, (d) fresh H-FER(8.9), and (e)
−1
aged H-FER(8.9) at 673 K and 7.5 h WHSV for 5 min. The
isobutene and 2-methyl-2-butene peaks in gas chromatograms are
marked in dark cyan, and the M-1, M, and M+1 peaks in mass spectra
are marked in red for the clear observation of 13C scrambling.
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its small external surface (70 m g ).
Gas chromatography−mass spectroscopy (GC-MS) analyses
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of the products obtained using C-labeled 1-butene (i.e., 1-
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[
1- C]butene) as a reactant in 1-butene skeletal isomerization
characterized by extensive 13C scrambling (Figures 4 and S8),
as expected from their necessary formation by the bimolecular
pathway. We also note that the distribution of pentene isomers
have long been recognized as a reliable method to check
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whether the bimolecular pathway operates predominantly. If
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isobutene is formed from 1-[1- C]butene via the mono-
molecular or pseudomonomolecular pathway, the 13C atom
could not migrate to a different molecule, and thus, the
isobutene produced should possess only one 1 C atom. In the
bimolecular reaction, however, the octyl carbenium ion isomers
containing two 13C atoms, produced by dimerization, would
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is essentially identical to the thermodynamic distribution,
suggesting that cracking of dimers over these two medium-pore
zeolites is not shape-selective.
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Therefore, both mono- and bimolecular reactions, which are
the main sources of isobutene and pentenes, respectively, occur
from the onset of the reaction over H-FER(8.9), as well as over
H-HPM-1, despite notable differences in their catalytic
performance. The bimolecular path has a much larger
contribution over fresh H-FER(8.9) than over fresh HPM-1,
but this contribution is gradually reduced with time, leading to
large changes in the performance of the former zeolite over
time. Thus, the prevailing mechanism for the selective
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continue to isomerize among them. Because the two
C
atoms should thus spread out randomly in these carbenium
ions during the reaction, the isobutene molecules produced via
a bimolecular pathway may contain zero, one, or two 13
C
atoms. This can be eventually evidenced in the mass spectrum
of isobutene by the intensity increase of the fragments one mass
atomic unit smaller (M-1) or larger (M+1) than the molecular
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ACS Catal. 2015, 5, 2270−2274