ACS Catalysis
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results allowed us to assign the GCꢀMS peak 7a in Figure 4 to
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2ꢀipNPTH. Similarly, peak 10c can be tentatively assigned to
the most slim 2ꢀipꢀ7ꢀmNPTH isomer. If such is the case, peak
7b could then be attributed to the fatter 1ꢀipNPTH isomer.
Therefore, we think that that the type of active aromatic hyꢀ
drocarbon pool species formed during ETP over cageꢀ
containing zeolite catalysts can differ notably according to the
size and shape of zeolite cages.
As expected from the low propene selectivity, the GCꢀMS
signals of naphthaleneꢀbased derivatives are completely missꢀ
ing in the chromatogram from HꢀZSMꢀ5 with two intersecting
10ꢀring channels. However, they are clearly resolved in the
chromatogram from HꢀUZMꢀ35 containing mseꢀ2 cages. Of
particular interest is that the type and concentration of such
bicyclic compounds found in this largeꢀpore zeolite is signifiꢀ
cantly different from those in HꢀSSZꢀ13 with the slightly more
rotund chaꢀcages. This indicates that the formation of active
aromatic hydrocarbon pool species during ETP can be greatly
affected not only by the composition of zeolitic catalysts, but
also by the size and shape of their cages, if present.
In summary, we have demonstrated that HꢀUZMꢀ35 with
the MSE topology outperforms any of the earlier zeolitic cataꢀ
lysts tested in the ETP reaction. A combination of GCꢀMS and
DFT calculation results reveals that the superior ETP perforꢀ
mance of this largeꢀpore zeolite comes from its unique cylinꢀ
drical cages in which ethene can be effectively converted to
isopropylnaphthaleneꢀbased hydrocarbon species, playing a
central role as reaction centers in propene formation. Apart
from the relatively lower coke forming tendency compared to
HꢀSSZꢀ13 with small 8ꢀring windows, the ETP stability of Hꢀ
UZMꢀ35 can be further improved by mild dealumination.
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To clarify the role of isopropylnaphthalene (ipNPTH) and
isopropylmethylnaphthalene (ipmNPTH) derivatives during
ETP, we reacted ethene over HꢀUZMꢀ35 at 673 K for 90 min
on stream, when the maximum propene yield is obtained (Figꢀ
ure 2), divided the resulting catalyst into a series of batches,
and then flushed them in a pure N2 stream (30 mL minꢀ1) at the
same temperature for times up to 90 min. As shown in Figure
5, the two peaks 7a and 10c, which represent the ipNPTH and
ipmNPTH species, respectively, increase in intensity at the
beginning of the flushing time, level off, and then decrease
after 4 min. We also monitored their generation as a function
of TOS in HꢀUZMꢀ35 during ETP at the same reaction temꢀ
perature (Figure 5). Both peaks were found to continuously
increase until 90 min on stream. The same trend can be obꢀ
served for peaks 3 and 4a due to naphthalene and methylnaphꢀ
thalenes that are the products of ipNPTH and ipmNPTH deꢀ
compositions, respectively, leading to propene formation.
Therefore, it is most likely that ipNPTH and ipmNPTH speꢀ
cies are the most important active intermediates of the ETP
reaction over HꢀUZMꢀ35. This matches well with the rapid
increase in organic content deposit on this largeꢀpore zeolite at
early TOS (Figure S1), as well as in propene yield (Figure 2).
ASSOCIATED CONTENT
Supporting Information. Experimental section, characterization
data, and additional results. This material is available free of
AUTHOR INFORMATION
Corresponding Author
*S.B.H: eꢀmail, sbhong@postech.ac.kr
Notes
The authors have applied for a patent based on the reaction reportꢀ
ed in this paper.
ACKNOWLEDGMENT
This work was supported the National Creative Research Initiaꢀ
tive Program (2012R1A3A2048833) through the National Reꢀ
search Foundation of Korea.
Other major groups of aromatic hydrocarbon compounds
clearly observed in Figure 5 include the polymethylnaphthaꢀ
lenes with two, three, and four methyl groups represented by
peaks 6aꢀc, 8b, and 12, respectively. We should note here that
while the generation and consumption patterns of these aroꢀ
matic species are essentially the same as those of ipNPTH and
ipmNPTH, such naphthaleneꢀbased derivatives are known as
active organic reaction centers for MTO catalyis.24 Therefore,
we cannot rule out the possibility that polymethylnaphthalenes
could also play such a role during ETP. To elucidate this, furꢀ
ther study is currently underway in our laboratory.
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Finally, we calculated the strain energies of two ipNPTH
isomers (i.e., 1ꢀisopropylnaphthalene (1ꢀipNPTH) and 2ꢀ
isopropylnaphthalene (2ꢀipNPTH)) and two ipmNPTH isoꢀ
mers in SSZꢀ13 and UZMꢀ35 to gain further insights into the
intrazeolitic formation of naphthaleneꢀbased intermediates
during ETP (Figure S10). Here we selected 1ꢀisopropylꢀ3ꢀ
methylnaphthalene (1ꢀipꢀ3ꢀmNPTH) and 2ꢀisopropylꢀ7ꢀ
methylnaphthalene (2ꢀipꢀ7ꢀmNPTH), fattest and most slim
among the 14 possible ipmNPTH isomers, respectively (Table
S2 and Figure S9), as two representative isomers to save the
computational cost. Zero or small differences (0 and 7 kJ molꢀ1,
respectively) in the strain energy are observed for these two
pairs of naphthalene derivatives when embedded in SSZꢀ13.
As shown in Figure S10, however, their strain energy differꢀ
ences become much larger (24 and 145 kJ molꢀ1, respectively)
upon encapsulation within the mseꢀ2 cages in UZMꢀ35. These
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