5
8
K. Raabová et al. / Catalysis Today 204 (2013) 54–59
to extraframework positions. In our case, we did not observe
such clustering, most probably due to the low concentration of
iron in our samples. Higher temperature of the calcination at
60
55
50
45
40
35
30
25
20
15
10
5
X propane
S propylene
S ACN
Y
= 11 %
Y
= 11 %
ACN
ACN
S COx
◦
7
00 C in air led to extensive extraction of iron ions from the
zeolite lattice and clustering of iron in zeolite channels and to
formation of larger Fe O3 particles [25]. Even higher tempera-
2
◦
ture of the calcination (900 C) led to the similar result as was
reported by Wacław et al. [28]. On the other hand, treatment of
the sample in the water vapor led to extensive clustering already
◦
at the temperature of 600 C, as was detected by Perez-Ramirez
et al. [29], where they found out that about 15% of iron was
present as a clustered oxide phase. Worth of note is that they
used higher concentration, 0.68 wt.% Fe. In our case, hydrother-
mal pretreatment did not lead to creation of larger oxide cluster,
but the nitridation at higher temperature did. This activation at
such a high temperature resulted in the change of the color of
0
N600
GRN
Fig. 4. Comparison of the catalytic performance of Fe-silicalite activated by GRN for
5 h and by nitridation at 600 ◦C in the direct ammoxidation of propane. Reaction
conditions: T = 540 C, mcat = 80 mg, F = 100 cm /min, C3H8/NH3/O2 = 2.5/5/5 vol.%.
−
1
◦
3
the sample causing absorption band below 35,000 cm
which
did not permit us to infer upon the presence of oxide cluster of
iron.
4
. Conclusion
Our spectroscopic results can be summed up as follows. Low
temperature of the nitridation leads to creation of Si NH2 group,
however in very low concentration. Upon this activation a small
amount of iron is extracted to extraframework positions, creating
Our study was focused on the catalytic performance of Fe-
silicalite in the direct ammoxidation of propane, its activation by
nitridation and subsequent characterization. Our study led to some
very interesting conclusions:
◦
highly coordinated iron ions. Higher temperature, 600 C, leads to
creation of bridged Si NH Si group, the coordination of iron being
◦
◦
very similar to the one observed at 540 C. Nitridation at 700 C
leads to creation of isolated NH2 groups, bridged Si NH Si groups
i) it is possible to insert nitrogen in the zeolite structure at the
◦
temperature not excessively high – already 600 C led to the well
distinguishable absorption band in FTIR spectrum at 3408 cm
and moreover to the exchange of Brönsted OH group by NH group.
2
−1
Most probably this activation also leads to formation of iron nitride
and iron oxide particles; however we were not able to prove this
hypothesis.
resulting from the presence of Si NH Si,
ii) the structure of nitrided material (the coordination of nitrogen
in the zeolite structure) highly depends on the temperature of
the nitridation,
Correlating our spectroscopic results with the results from cat-
alytic study, we can hypothesize that the increased activity is
caused by the presence of bridged Si NH Si group in the zeo-
lite structure. Regarding the role of the iron ion, we cannot say
unequivocally what kind of coordination of iron led to the increased
activity (if any) because of the very similar UV–Vis spectra of
◦
iii) nitridation at 600 C leads to the material with very good cat-
alytic properties, which does not tend to loose its activity during
the TOS and by contact with oxygen,
iv) from correlation of the spectroscopic results with the results
from catalytic study, we suppose that the presence of
amide/imide species in the structure of silicalite is advantageous
for the catalytic activity of the sample,
v) regarding the tendency of the material to formation of iron
clusters, which negatively influences the catalyst performance,
ammonia is better extracting agent with respect to water vapor.
◦
◦
the samples pretreated by nitridation at 540 C, 600 C and by
hydrothermal pretreatment. The poor catalytic results reached
◦
over the sample activated at 700 C could be explained by the
formation of larger oxide clusters of iron ions – not visible in
UV–Vis spectrum because of the broad overlapping band detected
−1
at the wavelengths below 35,000 cm . Although from FTIR spec-
troscopy we found out that this sample contained different kinds
of amide, imide species, the presence of oxide clusters of iron neg-
atively influenced both the selectivity to ACN and conversion of
propane.
Acknowledgements
A financial support of the Grant Agency of the Czech Republic
under the project Nos. P106/12/P083 (KR) and P106/12/G015 (RB)
and the UniCRE project (CZ.1.05/2.1.00/03.0071) (EB) are highly
acknowledged.
Earlier described activation of Fe-silicalite – by gas reduction
nitridation was advantageous because of the low temperature
◦
of the activation, being 540 C. Such a low temperature did not
cause any extensive clustering of iron ions, as was confirmed
by UV–Vis spectroscopy [30]. The result obtained on the sam-
ple activated by GRN and by nitridation at 600 C is reported
References
◦
[
[
1] C. Zhang, Z. Xu, K. Wan, Q. Liu, Applied Catalysis A 258 (2004) 55–61.
2] T. Hasegawa, C.K. Krishnan, M. Ogura, Microporous and Mesoporous Materials
in Fig. 4. Interestingly, both samples reached the same yield of
ACN, however they had different distribution of products and
conversion of propane. In the case of GRN the conversion of
propane was a little bit lower (25.4%) compared to the sam-
ple N600. Another important feature is the high selectivity to
COx of sample N600 compared to GRN. This correlates with
the result from the sample N700 where we detected very high
selectivity to COx at the expense of selectivity to ACN. From
this point of view it would be interesting to further study
the nitridation at various temperatures and to find the opti-
mum temperature for the nitridation of Fe-silicalite where we
would detect high selectivity to ACN at high conversion of
propane.
132 (2010) 290–295.
[3] K. Narasimharao, M. Hartmann, H.H. Thiel, S. Ernst, Microporous and Meso-
porous Materials 90 (2006) 377–383.
[
4] G. Wu, X. Wang, Y. Yang, L. Li, G. Wang, N. Guan, Microporous and Mesoporous
Materials 127 (2010) 25–31.
[5] H. Li, Q. Lei, X. Zhang, J. Suo, ChemCatChem 3 (2011) 143–145.
[6] S. Ernst, M. Hartmann, S. Sauerbeck, T. Bongers, Applied Catalysis A 200 (2000)
117–123.
[
7] M. Srasra, G. Poncelet, P. Grange, S. Delsarte, Studies in Surface Science and
Catalysis 158B (2005) 1811–1818.
[8] F. Hayashi, K. Ishizu, M. Iwamoto, European Journal of Inorganic Chemistry
010 (2010) 2235–2243.
2
[
9] G. Wu, S. Jiang, L. Li, F. Zhang, Y. Yang, N. Guan, M. Mihaylov, H. Knözinger,
Microporous and Mesoporous Materials 135 (2010) 2–8.
[10] N. Chino, T. Okubo, Microporous and Mesoporous Materials 87 (2005) 15–22.