Journal of the American Chemical Society
Communication
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products is suppressed. The selectivity of CH4 is relatively higher
(16%) as compared to most Fe-based catalysts. In terms of Fe-
based catalysts, Fe carbides are the active species as CO
dissociation occurring on these sites,18,24 even though Fe3O4, Fe
metal, or surface iron phases are reported to be the active species
for FTS.25 So prior to reaching the best catalytic activities, it is
first activated in syngas. For comparison, selectivity for C5−C12 is
∼36% over the physical mixed catalyst of Fe3O4 NPs and ZSM-5
(Table S1). Compared with other Fe-zeolite-based FTS catalysts,
our Fe3O4@ZSM-5 microsphere catalysts also show a better C5−
C12 selectivity as a promoter-free catalyst. Usually, some
promoters are added into Fe-based catalysts to decrease the
CH4 selectivity.26 However in our work, no promoters are added,
resulting in relatively high methane selectivity. The effect of
zeolites on the selectivity to different hydrocarbons has been
reported in the literature.27,28 The zeolite acid sites play an
important role in cracking of heavier hydrocarbons, leading to
good selectivity of gasoline products in FTS.
In conclusion, we demonstrate a reproducible in situ
crystallization route to synthesize uniform Fe3O4@ZSM-5
microspheres with aggregated-ZSM-5 NRs and well-dispersed
uniform Fe3O4 NPs. The obtained samples possess a relatively
uniform size (6−9 μm), high surface area, well-designed NR-
aggregated shape, ultrafine Fe3O4 NPs, and high magnetization
(∼ 8.6 emu/g). We propose a NP-assisted recrystallization
process from surface to core for the formation of such unique
Fe3O4@ZSM-5 microspheres. Moreover, the obtained micro-
spheres show excellent catalytic performance in FTS. The CO
conversion on the Fe3O4@ZSM-5 (Si/Fe = 11) can reach 87%
and remain at least 110 h, and selectivity toward C5−C12 is as
high as 44.6%. This in situ crystallization route could be extended
to synthesize other metal oxide-zeolite composites with unique
nanostructure, e.g., NiO@zeolite or Co3O4@zeolite. Most
importantly, in possessing magnetic property, the Fe3O4@
ZSM-5 composite material allows for practical handling, e.g., in
magnetically stabilized beds to reduce catalyst losses.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
■
S
AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
(21) Yao, J. F.; Li, D.; Zhang, X. Y.; Kong, C. H.; Yue, W. B.; Zhou, W.
Z.; Wang, H. T. Angew. Chem., Int. Ed. 2008, 47, 8397.
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ACKNOWLEDGMENTS
■
This work was supported by NSF of China (20890123,
21073043, 21210004), State Key Basic Research Program of
the PRC (2009AA033701, 2009CB930400), Science and
Technology Commission of Shanghai Municipality
(08DZ2270500, 10JC1401800), and Shanghai Leading Academ-
ic Discipline Project (B108). Partial support from Australian
Research Council Discovery Project (DP0986192) and Austral-
ian Endeavour Executive Award for M.D.
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