Enhanced Catalytic Performance of Fe-containing HZSM-5 for Ethane Non-Oxidative Dehydrogenation via Hydrothermal Post-Treatment

Article abstract of DOI:10.1002/cctc.202100752

A facile strategy is applied to construct Fe supported ZSM-5 (Fe/HZ5-HTS) via hydrothermal post-treatment and applied to ethane non-oxidative dehydrogenation. Compared with Fe/HZ5-IWI prepared by incipient wetness impregnation, Fe/HZ5-HTS exhibits superior catalytic activity and long catalyst stability with 6000 minutes time-on-stream. An obvious volcanic curve is observed between the ethylene generation rate and Fe content, and 1.0Fe/HZ5-HTS exhibits the highest ethylene generation rate with 0.166 mmol C₂H₄ s^?1 g_Fe^?1 over different Fe loading, which is twice as much as that of 1.0Fe/HZ5-IWI. According to various characterizations, isolated Fe³⁺ species and carburized Fe species are active sites, and the better catalytic performance over 1.0Fe/HZ5-HTS is ascribed to more disperse Fe species and exposing more Fe species in the surface. Besides, the lower ethylene desorption temperature and higher ethane desorption temperature over Fe/HZ5-HTS could suppress the overreaction of the ethylene to generate coke and increase ethane residence reaction time, resulting in less coke deposition and facilitating the catalytic performance.

Full text of DOI:10.1002/cctc.202100752

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Enhanced Catalytic Performance of Fe-containing HZSM-5
for Ethane Non-Oxidative Dehydrogenation via
Hydrothermal Post-Treatment
Lizhi Wu,*[a] Zhiyuan Fu,[a] Zhuangzhuang Ren,[a] Jinhe Wei,[a] Xinhua Gao,[b] Li Tan,[a] and
Yu Tang[a]
A facile strategy is applied to construct Fe supported ZSM-5
(Fe/HZ5-HTS) via hydrothermal post-treatment and applied to
ethane non-oxidative dehydrogenation. Compared with Fe/
HZ5-IWI prepared by incipient wetness impregnation, Fe/HZ5-
HTS exhibits superior catalytic activity and long catalyst stability
with 6000 minutes time-on-stream. An obvious volcanic curve is
observed between the ethylene generation rate and Fe content,
and 1.0Fe/HZ5-HTS exhibits the highest ethylene generation
various characterizations, isolated Fe3+ species and carburized
Fe species are active sites, and the better catalytic performance
over 1.0Fe/HZ5-HTS is ascribed to more disperse Fe species and
exposing more Fe species in the surface. Besides, the lower
ethylene desorption temperature and higher ethane desorption
temperature over Fe/HZ5-HTS could suppress the overreaction
of the ethylene to generate coke and increase ethane residence
reaction time, resulting in less coke deposition and facilitating
the catalytic performance.
À 1
rate with 0.166 mmolC2H4 sÀ 1 gFe over different Fe loading,
which is twice as much as that of 1.0Fe/HZ5-IWI. According to
Introduction
internal heat input.[3e,4] However, oxidative dehydrogenation
suffers great challenge on the low selectivity of ethylene due to
over-oxidation of ethane to carbon dioxide, as well as the heat
recovery and process safety control owing to the highly
exothermic nature of the oxidative dehydrogenation
process.[4a,f,5] On the other hand, non-oxidative dehydrogenation
of ethane (EDH) could avoid most of the disadvantages along
with high selectivity of ethylene compared with ODH. The
drawbacks of non-oxidative dehydrogenation are relatively high
energy demand, low single-pass conversion limited by the
thermodynamic equilibrium, and rapid deactivation of catalyst
from coking.[5–6]
Currently, non-oxidative dehydrogenation catalyst of light
alkanes, such as ethane and propane, mainly contains PtÀ M
(M=Sn, Zn, Ga, In)[6d,l,7] and CrOx[6a–c,g–j,m] supported catalyst, as
the most extensively studied systems owing to its high activity.
Nevertheless, owing to environmentally unfriendly CrOx-based
catalyst, and high-cost as well as easily agglomeration of PtÀ M
catalyst, it is urgent to develop highly active, selective as well as
coke-resistant catalysts.
HZSM-5, a typical MFI-type zeolite, has been widely applied
in the chemical industry, such as alkylation, isomerization and
aromatization catalyst of light alkanes.[8] On the other hand,
owing to the tri-dimensional micropore structure, large specific
surface area, appropriate acidity and thermally stable, HZSM-5
is considered to be a good catalyst support. It is extensively
reported that Fe supported HZSM-5 displayed high reactivity on
oxidative dehydrogenation of propane to propene with N2O as
oxides, which is ascribed to the radical-type character of the
Fe(III)-OÀ species when Fe species contacting N2O.[9] Recently,
Fe/HZ5 is found to be active in non-oxidative dehydrogenation
of ethane, carburized Fe species and metallic Fe species are
Ethylene is one of the most important chemical intermediates
in the petrochemical industry.[1] Currently, the industrial-scale
production of ethylene is mainly based on steam cracking of
hydrocarbon feedstocks, including gaseous alkanes and liquid
petroleum products, which is highly energy intensive and
requiring high temperature.[2] Recently, with the development
of shale gas exploration technology, the production of ethylene
through ethane cracking route has attracted extensive attention
with great advantages, such as lower production cost, high
ethylene yield and less equipment investment compared with
conventional naphtha pyrolysis, as an environmentally benign
chemical process.[1a,3]
Ethane dehydrogenation includes oxidative dehydrogen-
ation (ODH) and non-oxidative dehydrogenation. Oxidative
dehydrogenation of ethane with different oxidizing agents
(such as O2, CO2 and N2O) has attracted great attention,
coupling the endothermic dehydration of ethane with the
strongly exothermic oxidation of hydrogen, which has higher
thermodynamic conversion and avoids the need for excessive
[a] Dr. L. Wu, Z. Fu, Z. Ren, J. Wei, Prof. L. Tan, Prof. Y. Tang
Institute of Molecular Catalysis and In-situ/operando Studies
College of Chemistry
Fuzhou University
350108 Fuzhou (P. R. China)
E-mail: wulz@fzu.edu.cn
[b] Prof. X. Gao
State Key Laboratory of High-Efficiency Utilization of
Coal and Green Chemical Engineering
College of Chemistry and Chemical Engineering
Ningxia University
750021Yinchuan (P. R. China)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202100752
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Table 1. The textural properties of as-prepared Fe catalysts.
[b]
[b]
[b]
No.
Samples
Fe loading[a]
[wt%]
Si/Al ratio[a]
SBET
Vmicro
Vmeso
Total acidity[c]
[mmolNH3 gÀ 1
]
[m2 gÀ 1
]
[cm3 gÀ 1
]
[cm3 gÀ 1
]
1
2
3
4
5
6
7
8
9
HZSM-5
/
29.0
28.9
27.8
28.2
28.5
28.8
28.5
28.1
/
331
322
324
314
364
369
376
367
358
0.144
0.135
0.138
0.139
0.088
0.078
0.059
0.097
0.113
0.055
0.077
0.064
0.057
0.188
0.217
0.179
0.176
0.141
0.517
0.449
0.426
0.432
0.514
0.443
0.438
0.418
/
0.5Fe/HZ5-IWI
1.0Fe/HZ5-IWI
2.0Fe/HZ5-IWI
0.5Fe/HZ5-HTS
1.0Fe/HZ5-HTS
2.0Fe/HZ5-HTS
1.0Fe/HZ5-meso-IWI[d]
1.0Fe/Silicalite-1-IWI
0.43
0.89
1.83
0.51
0.83
1.72
0.90
0.93
[a] Detected by ICP-OES analysis, [b] Calculated by BET method. SBET, Vmicro and Vmeso stand for specific surface area, microporous volume and mesoporous
volume, respectively, [c] Determined from NH3-TPD, [d] Catalyst was prepared via incipient wetness impregnation of Fe precursor to mesoporous ZSM-5.
supposed to be the active sites for EDH reaction, which exhibits Results and Discussion
both high activity and excellent stability.[10]
In this present work, we propose a facile strategy to
construct Fe supported ZSM-5 (Fe/HZ5-HTS) via hydrothermal
post-treatment of the conventional HZSM-5 with the introduc-
tion of Fe precursor. Compared with the conventional incipient
wetness impregnation method (Fe/HZ5-IWI), it is found that the
new Fe/HZ5-HTS demonstrates to be highly active and long
catalyst stability with less coke deposition on non-oxidative
dehydrogenation of ethane. According to the systematic
characterizations including TEM, UV/Vis, NH3-TPD, H2-TPR, XPS
and TG results, isolated Fe3+ species and carburized Fe species
are supposed to be the active sites over EDH, and the high-
performance of Fe/HZ5-HTS could be ascribed to the more
dispersive Fe species over HZSM-5 and exposing more Fe active
sites including carburized Fe species and metallic Fe species in
the surface. And it is found that the relatively lower ethylene
desorption temperature and higher ethane desorption temper-
ature over Fe/HZ5-HTS are observed, which could suppress the
overreaction of the ethylene to generate coke and increase
corresponding ethane residence reaction time, respectively, and
facilitate the catalytic performance correspondingly.
The XRD patterns (Figure 1) of all the prepared samples present
°
°
the representative peaks at 7–9 and 23–24 , indicating all the
samples of framework structure of MFI host are well preserved
after introducing Fe species. Besides, no diffraction peaks of
[10a,11]
°
°
°
Fe2O3 (33.2 , 35.7 , and 53.6 ) or other metallic Fe species
are observed over prepared Fe/HZ5 samples, indicating that the
Fe species are highly dispersed over HZSM-5 support.
The detailed textural properties of catalysts based on the N2
adsorption at 77 K, ICP, NH3-TPD have been concluded in
Table 1. The Fe content of each Fe/HZ5 sample is close to
nominal value. As for Fe/HZ5-IWI catalysts, it is found that the
BET surface area, microporous and mesoporous volume keep
almost unchanged compared with parent HZSM-5 sample
regardless of the different Fe loading, and the corresponding N2
physisorption shows an adsorption branch of type I isotherm
(Figure 2a–d), typical for microporous materials. Nevertheless,
when Fe is introduced into HZSM-5 via alkali hydrothermal
post-treatment, the BET surface area and mesoporous volume
of Fe/HZ5-HTS samples with different Fe content increase 33–
45 m2 gÀ 1 and 0.124–0.162 cm3 gÀ 1, respectively, and the corre-
Figure 2. N2 physisorption isotherms for HZSM-5, Fe/HZ5-IWI and Fe/HZ5-
HTS. (a) HZSM-5; (b) 0.5Fe/HZ5-IWI; (c) 1.0Fe/HZ5-IWI; (d) 2.0Fe/HZ5-IWI; (e)
0.5Fe/HZ5-HTS; (f) 1.0Fe/HZ5-HTS; (g) 2.0Fe/HZ5-HTS; (h) 1.0Fe/HZ5-meso-
IWI; (i) 1.0Fe/Silicalite-1.
Figure 1. Representative XRD patterns of ZSM-5, Fe/HZ5-IWI and Fe/HZ5-HTS
with different Fe content.
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sponding N2 physisorption shows an adsorption branch of type
IV isotherm (Figure 2e–g), indicating the existence of meso-
pores, which is ascribed to the hydrothermal alkali treatment
environment resulting in the Si dissolution to generate
mesoporous. To further eliminate that the existence of meso-
pore may possibly influence the EDH performance, mesopore
included sample 1.0Fe/HZ5-meso-IWI (Table 1 No. 8, Figure 2 h)
is prepared according to incipient wetness impregnation of Fe
precursor to mesoporous HZSM-5 as described in experimental
Section.
other hand, it is found the acid sites in 1.0Fe/Silicalite-1-IWI is
very weak since no existence of Al sites.
Figure 4 and Figure 5 show the TEM and STEM-EDS
elemental mapping of 1.0Fe/HZ5-IWI and 1.0Fe/HZ5-HTS, which
could be used to evaluate the dispersion of the Fe species. It is
obvious that some small, aggregated Fe oxides nanoparticles
could be observed over 1.0Fe/HZ5-IWI, while it is hardly to see
the Fe oxides nanoparticles over 1.0Fe/HZ5-HTS from the TEM
images in Figure 4, which could be concluded that Fe species
over 1.0Fe/HZ5-HTS is highly dispersed compared with that of
1.0Fe/HZ5-IWI. In addition, EDS mapping is further used to
illustrate the corresponding element distribution (Figure 5), it is
found the element of Si, O, Al distribute homogeneously over
1.0Fe/HZ5-IWI and 1.0Fe/HZ5-HTS samples. Nevertheless, Fe
element distributes non-homogeneously obviously over 1.0Fe/
HZ5-IWI with aggregated Fe oxides clusters on both edges,
while the Fe element distributes homogeneously over 1.0Fe/
HZ5-HTS. The TEM and further EDS mapping results demon-
strate that the Fe species are highly dispersed over support
HZSM-5 via hydrothermal alkali post-treatment compared with
conventional incipient wetness impregnation.
The catalyst acidity is further characterized by NH3-TPD, as
shown in Figure 3. The NH3-TPD profiles for HZSM-5, Fe/HZ5
°
samples all show two major desorption peaks at around 210 C
°
and 420 C, which could be assigned to that NH3 strongly
adsorbed on the weak acid sites and strong acid sites of HZSM-
5.[12] It could be found that with the incremental Fe content
°
over Fe/HZ5-IWI and Fe/HZ5-HTS, the peak intensity at 420 C
decreases gradually, which indicates the strong acid sites of
HZSM-5 is preferred to be neutralized. Correspondingly, the
total amount of acid sites after introducing Fe species decreases
based on the further quantitative analysis of desorbed NH3
(Figure 3 and Table 1). The decreased acidity over Fe/HZ5
samples could be ascribed to that the introduction of Fe species
exchanges the Brønsted acid protons of SiÀ O(H)-Al. On the
To further distinguish different Fe species, H2-TPR is
conducted to understand the corresponding Fe species struc-
ture over different catalysts, as shown in Figure 6. Three major
°
peaks at 350, 450 and 550 C appear over 2.0Fe/HZ5-IWI and
2.0Fe/HZ5-HTS samples (Figure 6A), which could be ascribed to
the reduction peaks of Fe2O3, Fe3O4, and FeO species,
respectively.[13] This suggests that the Fe species are similar
when Fe loading is 2.0 wt% regardless of Fe introduction
manner, and exists mainly in the manner of iron oxides.
Nevertheless, when Fe content is further decreased to 1.0 wt%
or less, it could be found that Fe introduction manner affects Fe
states greatly. As for 1.0Fe/HZ5-IWI and 1.0Fe/Silicalite-1-IWI via
incipient wetness impregnation (Figure 6B), the obvious peaks
°
at 350, 450 and 550 C representing Fe2O3, Fe3O4 and FeO
species are still observed, indicating that mostly Fe species still
exist in the manner of iron oxides. On the other hand, it is
°
found that only slight peaks at 350 and 550 C corresponding to
Fe2O3 and FeO species appear over 1.0Fe/HZ5-HTS, and the
corresponding peak intensity is obviously lower than that of
1.0Fe/HZ5-IWI. When the Fe loading is further decreased to
0.5 wt%, the corresponding Fe reduction peaks intensity over
0.5Fe/HZ5-HTS is also lower than that of 0.5Fe/HZ5-IWI (Fig-
ure 6C). Above H2-TPR experimental results demonstrate that
the Fe species over 0.5Fe/HZ5-HTS and 1.0Fe/HZ5-HTS is
difficult to be reduced compared with that of Fe/HZ5-IWI, which
could be ascribed to that a more highly dispersed Fe species in
HZSM-5 and a stronger interaction between Fe sites and HZSM-
5 support, which is in accordance with previous TEM and EDS
mapping results.
Figure 3. NH3-TPD profiles of HZSM-5, Fe/HZ5 and Fe/Silicatlite-1.
UV/Vis spectra is further used to investigate the nature of Fe
species in Fe-zeolite samples, the corresponding UV/Vis spectra
of the Fe/HZ5-IWI and Fe/HZ5-HTS are compared in Figure 7.
According to literature report, bands below 300 nm are
assigned to isolated Fe3+ sites with two ligand-to-metal charge-
transfer transitions, t1!t2 and t1!e, whereby their specific
position depends on the number of ligands.[14] From the UV/Vis
Figure 4. TEM images of Fe/HZ5-IWI and Fe/HZ5-HTS.
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Figure 5. HAADF-STEM images and the corresponding elemental mapping of 1.0Fe/HZ5-IWI (a) and 1.0Fe/HZ5-HTS (b).
Figure 6. H2-TPR profiles of Fe/HZ5-IWI and Fe/HZ5-HTS.
Figure 7. UV/Vis spectra of Fe/HZ5-IWI (A) and Fe/HZ5-HTS (B) with different Fe content.
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spectra of the Fe/HZ5-IWI and Fe/HZ5-HTS in Figure 7, two
bands appear at 210 nm and 251 nm over Fe/HZ5-HTS samples
(Figure 7B), corresponding to isolated tetrahedral and high
coordination Fe3+ sites. On the other hand, corresponding
isolated tetrahedral Fe3+ sites absorption band over Fe/HZ5-IWI
appears at 210 nm and high coordination Fe3+ sites absorption
band is red-shifted to 271 nm compared with Fe/HZ5-HTS,
indicating the higher coordination of Fe3+ sites over Fe/HZ5-
IWI.[14a] In addition, bands at 300–400 nm and >400 nm are
observed over both Fe/HZ5-IWI and Fe/HZ5-HTS catalysts,
which are assigned to octahedral Fe3+ in small oligomeric FexOy
clusters and large Fe2O3 particles, respectively.[14] To further
estimate the Fe species distribution with Fe content and Fe
introduction manner, the UV/Vis spectrum of Fe/HZ5-IWI and
Fe/HZ5-HTS catalysts with different Fe content is deconvoluted
with r2 >0.999, and the results are summarized in Table 2.
Correspondingly, the percentage of isolated Fe3+ sites, oligo-
meric FexOy clusters and Fe2O3 particles could be confirmed
semi-quantitative over Fe/HZ5-IWI and Fe/HZ5-HTS with the
analysis of deconvoluted data (Table S1). And it is surprised to
find that Fe introduction manner would result in different Fe
species distribution. As for Fe/HZ5-IWI with different Fe content,
it could be found that the content of isolated Fe3+ sites is about
25–32%, while corresponding oligomeric FexOy clusters and
Fe2O3 particles are 36–43% and 28–39%, respectively, which
indicating that major Fe species over Fe/HZ5-IWI exist in the
manner of oligomeric FexOy clusters and large Fe2O3 particles.
On the other hand, when Fe is introduced via hydrothermal
post-treatment over Fe/HZ5-HTS catalysts, it could be found
that the content of isolated Fe3+ sites is about 41–74%, which
is obviously higher than that of Fe/HZ5-IWI. While correspond-
ing oligomeric FexOy clusters and Fe2O3 particles are 26–40%
and 0–29%, lower than that of Fe/HZ5-IWI. And it is worth
noting that when Fe loading is rather high (2.0%), correspond-
ing large Fe2O3 particles are highest, which indicated that high
Fe content loading is not beneficial for Fe species homoge-
neous distribution.
To further identify the surface state of iron ions on incipient
wetness impregnation and hydrothermal alkali post-treatment
catalysts, the samples are characterized by XPS spectra. The
corresponding Fe 2p XPS spectra of prepared fresh catalysts are
shown in Figure 8A, which contains two components at BE of
~711 eV (Fe 2p3/2) and ~724 eV (Fe 2p1/2), respectively. It is
noted that the corresponding peaks intensity of 1.0Fe/HZ5-HTS
is obviously higher than that of 1.0Fe/HZ5-IWI, the further
surface atomic concentration analysis show that the surface Fe
atomic concentration over 1.0Fe/HZ5-HTS is 1.75 at.%, which is
much higher than that of 1.0Fe/HZ5-IWI (0.52 at.%). The higher
Fe content in the surface over 1.0Fe/HZ5-HTS could facilitate
the catalytic reaction correspondingly. In addition, to further
confirm the corresponding Fe state, the further curve fitting
result of Fe2p spectrum over 1.0FeHZ5-HTS is shown in
Figure 8A and (Table S2) (the signal intensity of 1.0Fe/HZ5-IWI is
too low and could not fit properly), it could be found that peaks
appear with main peaks at 711.0 eV, 712.9 eV representing
octahedral and tetrahedral Fe3+ species, and along with
shoulder peaks at 709.6 eV corresponding to octahedral Fe2+
species,[10a] which indicates Fe species mainly exists in the
manner of isolated Fe3+ over Fe/HZ5-HTS, in accordance with
previous UV/Vis characterization result.
Table 2. Percentage of the area of the sub-bands (I1 at λ<300 nm, I2 at
300 <λ<400 nm, and I3 at λ>400 nm) derived by deconvolution of the
UV/VIS DRS spectra.
In addition, the further situ Raman and Drifts-CO character-
ization results are shown in Figure S1 and Figure S2. As for situ
Raman results, besides the bands at 380 and 800 cmÀ 1
corresponding to characteristic of MFI structure of ZSM-5,[15]
bands at 600 and 433 cmÀ 1 ascribed to bulky Fe2O3 aggregates
also appear over both Fe/HZ5-IWI and Fe/HZ5-HTS. And it is
noted that band at 1063 cmÀ 1 corresponding to asymmetric
-Fe-OÀ Si- stretching vibrational appears on both catalysts,[15]
and the corresponding bands intensity over Fe/HZ5-HTS at
No.
Sample
I1[a][%]
I2[b][%]
I3[c][%]
1
2
3
4
5
6
0.5Fe/HZ5-IWI
1.0Fe/HZ5-IWI
2.0Fe/HZ5-IWI
0.5Fe-HZ5-HTS
1.0Fe-HZ5-HTS
2.0Fe-HZ5-HTS
25
32
25
74
41
41
43
40
36
26
40
30
32
28
39
0
19
29
[a] Isolated Fe3+ in tetrahedral and higher coordination, [b] Small
oligomeric FexOy clusters, [c] Large Fe2O3 particles.
Figure 8. XPS spectra in the Fe 2p region of 1.0Fe/HZ5-IWI and 1.0Fe/HZ5-HTS catalysts before and after EDH reaction. (oct, octahedral; tet, tetrahedral).
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1063 cmÀ 1 is a little higher that of Fe/HZ5-IWI, indicating that
more isolated Fe sites is over Fe/HZ5-HTS. On the other hand,
from the drift-CO characterization results in Figure S2, the
bands at 1740–1750 cmÀ 1 and 1940–1960 cmÀ 1 appear over
both Fe/HZ5-IWI and Fe/HZ5-HTS, which could be ascribed to
the bridged CO on Fe atoms with the modes of Fe3(CO) and
Fe2(CO),[15a] respectively. And it could be found that the
corresponding band intensity at 1738 cmÀ 1 over Fe/HZ5-HTS is
higher than that of Fe/HZ5-IWI, which indicating more isolated
Fe3+ is over Fe/HZ5-HTS, in accordance with previous UV/Vis
result.
The catalytic performance of various catalysts as a function
of time on stream are shown in Figure 9. It is found that HZSM-
5 is nearly inactive with merely 6.0% ethane conversation over
EDH, which indicates the introduction of Fe active sites is
essential. Besides, it is observed that the selectivity of ethylene
over HZSM-5 is rather low with merely 43.3%, which may come
from the strong Brønsted acid sites of HZSM-5, as characterized
in Figure 3, resulting in overreaction of product ethylene to
generate coke.
ethylene yield is 8.2%, 20.3% and 29.2% at time 120 minutes
for 0.5, 1.0 and 2.0Fe/HZ5-IWI, respectively (Table 3). And it is
found that the corresponding specific reaction yield rate of
ethylene normalized by Fe content over 0.5Fe/HZ5-IWI, 1.0Fe/
À 1
HZ5-IWI and 2.0Fe/HZ5-IWI are 0.065 mmolC2H4 sÀ 1 gFe
,
0.078 mmolC2H4 sÀ 1 gFe and 0.054 mmol C2H4 sÀ 1 gFeÀ 1, respec-
tively, which indicates that the ethylene generation rate and Fe
content presents an obvious volcanic curve over Fe/HZ5-IWI.
On the other hand, when Fe is introduced via hydrothermal
post-treatment, the ethylene yield is 20.0%, 40.4% and 26.1%
at time 120 minutes for 0.5, 1.0 and 2.0Fe/HZ5-HTS, and the
corresponding specific reaction yield rate of ethylene normal-
ized by Fe content presents in the order of 1.0Fe/HZ5-HTS
À 1
(0.166 mmol
C2H4 sÀ 1 gFeÀ 1)>0.5Fe/HZ5-HTS
(0.052 mmolC2H4
(0.133 mmolC2H4 sÀ 1 gFeÀ 1)@2.0Fe/HZ5-HTS
sÀ 1 gFeÀ 1), in which similar volcanic curve is observed between
the ethylene generation rate and Fe content compared with
that of Fe/HZ5-IWI. When Fe loading is high (2.0 wt%), the
corresponding ethylene yield rate normalized by Fe content
decreases obviously, which could be ascribed to formation of
large inactive Fe2O3 particles, which is demonstrated by
previous UV/Vis characterization result (Figure 7 and Table 2). In
addition, it is found the corresponding specific reaction yield
rate of ethylene normalized by Fe content over 1.0Fe/HZ5-HTS
(0.166 mmol C2H4 sÀ 1 gFeÀ 1) is twice as much as 1.0Fe/HZ5-IWI
(0.078 mmolC2H4 sÀ 1 gFeÀ 1), which demonstrates that the Fe/
As for the Fe/HZ5 samples, it is observed that the ethylene
selectivity all reaches to 75–90% and then keeps steady for
about 1200 minutes regardless of Fe introduction way, while
the corresponding C2H4 yield increases rapidly initially and
reaches a maximum in about 30 minutes, then followed by a
smooth decay to different extents. As for Fe/HZ5-IWI, the
Figure 9. EDH performance of HZSM-5, Fe/HZ5-IWI and Fe/HZ5-HTS. Reaction conditions: 0.100 g catalyst and 0.500 g quartz sand (40-60 mesh), 10 mL/min
°
5%C2H6/Ar, 650 C.
Table 3. Catalytic performance of various catalysts.
No.
Samples
Conv.(C2H6)[a]
[%]
Yield(C2H4)[a]
[%]
Sel.(C2H4)[a]
[%]
Rate[b]
À 1
[mmol C2H4 SÀ 1 gFe
]
1
2
3
4
5
6
7
8
9
HZSM-5
6.0
8.6
2.6
8.2
43.3
95.1
82.6
79.0
89.6
75.7
84.0
75.2
93.5
–
0.5Fe/HZ5-IWI
1.0Fe/HZ5-IWI
2.0Fe/HZ5-IWI
0.5Fe/HZ5-HTS
1.0Fe/HZ5-HTS
2.0Fe/HZ5-HTS
1.0Fe/HZ5-meso-HTS
1.0Fe/Silicalite-1-HTS
0.065
0.078
0.054
0.133
0.166
0.052
0.088
0.021
24.6
36.9
22.4
53.4
31.0
31.1
6.1
20.3
29.2
20.0
40.4
26.1
23.4
5.7
°
[a] C2H6 conversion, C2H6 yield, C2H4 selectivity, CH4 selectivity were collected under the 650 C reaction condition after 120 mins, [b] Reaction rate was
°
calculated based on the yield of C2H4 under 650 C reaction condition.
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Figure 10. C2H6-TPD (A) and C2H4-TPD (B) profiles of 1.0Fe/HZ5-IWI and 1.0Fe/HZ5-HTS.
HZ5-HTS presents better catalytic performance compared with
that of Fe/HZ5-IWI at the same Fe content, which could be
ascribed to more isolated Fe3+ sites and exposing more Fe
species in the surface over Fe/HZ5-HTS compared with Fe/HZ5-
IWI (Figure 7 and Figure 8).
which would increase its residence time on the surface
correspondingly thus resulting in the higher conversion. On the
other hand, as for C2H4-TPD (Figure 10B), two C2H4 desorption
°
peaks around 195 and 265 C are all observed over the 1.0Fe/
HZ5-IWI and 1.0Fe/HZ5-HTS, while a higher C2H4-TPD desorp-
°
Besides, it is worth noting that the mesoporous volume
over Fe/HZ5-HTS samples is obviously higher than that of Fe/
HZ5-IWI samples based on BET characterization results (Table 1),
which is mainly generated by alkaline dissolution of Si species
in the hydrothermal treatment process. To further validate the
influence of hierarchical pore, 1.0Fe/HZ5-meso-IWI with meso-
porous is prepared (Table 1, No.8), it is found that the yield of
ethylene is 23.4% and corresponding specific reaction yield rate
of ethylene normalized by Fe content is 0.088 mmol
tion peak at 366 C is observed over 1.0Fe/HZ5-IWI, which may
further facilitate the overreaction of the ethylene to generate
coke deposition in the presence of Brønsted acid sites.
The amount of deposited carbon on the 1.0Fe/HZ5-IWI and
1.0Fe/HZ5-HTS after 1260 mins time on stream is further
investigated by TG, as shown in Figure 11. The TG loss weight
curve could be divided into two parts, the physical adsorbed
°
H2O below 100 C and the coke deposition between 200 to
°
800 C. It could be found that the coke deposition (2.77%) and
À 1
C2H4 sÀ 1 gFe (Table 3, No.8), which is a little higher than 1.0Fe/
weight loss temperature peak (558 C) of 1.0Fe/HZ5-HTS is
°
°
HZ5-IWI, but still much lower than 1.0Fe/HZ5-HTS, which
indicates the promotion effect via improving diffusion confine-
ment with hierarchical pore is limited, since the microporous
structure of HZSM-5 (0.51×0.55 nm and 0.53×0.56 nm) is
theoretically enough for ethane and ethylene diffusion.
obviously lower than that of 1.0Fe/HZ5-IWI (4.42% and 664 C,
respectively) after the same reaction time. The less coke
deposition would facilitate the life stability of Fe/HZ5-HTS.
Figure 8B shows the Fe 2p XPS results of used samples, it
could be found that the corresponding Fe 2p signal intensity is
obviously lower than that of fresh samples, which may be
ascribed to that the adsorption of organic substance or coke
In addition, 1.0Fe/Silicalite-1-IWI without introduction Al
sites is also prepared as contrast. The yield of ethylene is only
5.7% and the corresponding specific reaction yield rate of
ethylene normalized by Fe content is only 0.021 mmol
À 1
C2H4 sÀ 1 gFe (Table 3, No.9), which is much lower than that of
Fe/HZ5, indicating the existence of Al to generate Brønsted acid
sites is essential, since the implantation of Fe species is around
the SiÀ O(H)-Al Brønsted acid sites. The merely Silicalite-1 as
support would result in the aggregation of Fe species.
On the other hand, the catalytic performance including
conversion and selectivity could be influenced by correspond-
ing adsorption/desorption behavior of the ethane and ethylene
on the surface. It is generally accepted that the adsorption of
ethane would increase the residence time further facilitating
the reaction, and coke deposition is formed by overreaction of
the ethylene.[10b] Correspondingly, C2H6-TPD and C2H4-TPD
experiments over the 1.0Fe/HZ5-IWI and 1.0Fe/HZ5-HTS cata-
lysts are investigated. As shown in Figure 10A, it could be found
that the desorption temperature of ethane over 1.0Fe/HZ5-HTS
Figure 11. TG and DTG curves of Fe/HZ5-IWI and Fe/HZ5-HTS.
°
°
(185 C) is a little higher than that of 1.0Fe/HZ5-IWI (164 C),
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cover in the surface. Besides, both 1.0Fe/HZ5-HTS and 1.0Fe/
HZ5-IWI catalysts show major peak at 711.4 eV corresponding
to isolated Fe3+, indicating that isolated Fe3+ species as main
active sites keep stable after reaction. In addition, it is reported
that carburized iron (FexC) is suggested to be catalytically active
sites on the dehydrogenation of ethane[10a] and dehydro-
aromatization of CH4.[16] And it could be found a minor peak
appears at 706.9 eV over 1.0Fe/HZ5-HTS, which could be
surface over Fe/HZ5-HTS are responsible for better EDH catalytic
performance in comparison with Fe/HZ5-IWI. In addition, the
lower ethylene desorption temperature and higher ethane
desorption temperature over Fe/HZ5-HTS could suppress the
overreaction of the ethylene to generate coke and increase
corresponding residence reaction time, respectively.
ascribed to FexC species generated during EDH, and the Experimental Section
corresponding FexC species peak intensity is higher than that of
1.0Fe/HZ5-IWI, which could result in the better EDH catalytic
performance of Fe/HZ5-HTS.
To further investigate the time-on-stream stability of Fe/
HZ5-HTS, 0.5Fe/HZ-HTS is evaluated, and corresponding cata-
lytic result is shown in Figure 12. It is surprised to find that the
catalytic performance of 0.5Fe/HZ-HTS keeps stable and is not
deactivated during 6000 minutes time-on-stream, which in-
dicates Fe/HZ5-HTS is a high-performance catalyst.
HZSM-5 (Si/Al=29) was purchased from FUYU (Zhangjiang) New
Materials Technology Co., Ltd. Two sets of Fe/HZ5 catalysts were
prepared via hydrothermal treatment process and conventional
incipient wetness impregnation, respectively.
In the first catalyst set, the Fe/HZ5-HTS catalysts with different Fe
loading (0.5, 1.0 and 2.0 wt%) were synthesized through hydro-
thermal post-treatment of the conventional sample HZSM-5
according to the literature.[17] HZSM-5 was suspended in an
aqueous solution that contained TPABr, ethylamine (EA) and
Fe(NO3)3·9H2O in a PTFE lining, the typical aqueous solution has the
following molar composition: x Fe: 1.0 SiO2: 0.04 TPABr: 0.07 EA:10
H2O, x=0.0054, 0.0107 and 0.0214. Then the mixture was stirred for
half an hour. The PTFE lining was put into a stainless-steel autoclave
Conclusion
°
and crystallized at 170 C for 24 h. Then the product was recovered
°
by filtration, drying, and calcinations (550 C) for 6 h in air. The
In summary, a non-noble metal Fe supported HZSM-5 catalysts,
via conventional incipient wetness impregnation and hydro-
thermal post-treatment method, are systematic investigated in
the non-oxidative dehydrogenation of ethane to ethylene. In
comparison with Fe/HZ5-IWI via incipient wetness impregna-
tion, Fe/HZ5-HTS with low Fe content via hydrothermal post-
treatment exhibits superior catalytic activity and a long catalyst
stability with 6000 minutes time-on-stream. And 1.0Fe/HZ5-HTS
shows the highest C2H4 yield with 0.166 mmolC2H4 sÀ 1 gFeÀ 1 over
calcined sample was denoted as x Fe/HZ5-HTS (x represents the Fe
weight loading with 0.5, 1.0 and 2.0 wt%).
In the second catalyst set, the Fe/HZ5-IWI catalysts with different Fe
loadings (0.5, 1.0 and 2.0 wt%) were prepared by conventional
incipient wetness impregnation method using aqueous solutions of
°
Fe(NO3)3 9H2O. After impregnation, the catalysts were dried at 80 C
°
in air overnight and further calcinations at 550 C for 6 h in air. The
calcined sample was denoted as x Fe/HZ5-IWI (x represents the Fe
weight loading with 0.5, 1.0 and 2.0 wt%).
various Fe loading, which is twice as much as that of 1.0Fe/HZ5-
As contrast, Fe/Silicalite-1-IWI with 1.0 wt% Fe loading was also
prepared via incipient wetness impregnation procedure the same
as Fe/HZ5-IWI without introduction Al species. In addition,
hierarchical pore Fe/HZ5-meso with 1.0 wt% Fe loading was
prepared as follows: firstly, HZSM-5-meso was prepared according
the procedure described in the above first catalyst preparation with
TPABr, ethylamine, but without introducing the Fe precursor.[17]
Then the product was recovered by filtration, drying, and calcina-
À 1
IWI with 0.078 mmol C2H4 sÀ 1 gFe
. Based on the various
characterizations methods including XPS, UV/Vis, H2-TPR, TEM,
EDS and XPS characterizations, it is found that isolated Fe3+
species and carburized Fe species are active sites over EDH. And
more disperse Fe species and exposing more Fe species in the
°
tions (550 C), finally to obtain the hierarchical pore HZSM-5-meso.
Then 1.0 wt% Fe was supported over above HZSM-5-meso via
incipient wetness impregnation method. Then sample was dried at
°
°
80 C in air overnight and further calcinations at 550 C for 6 h in air.
Finally, the calcined sample was denoted 1.0Fe/HZ5-meso-IWI.
Experimental Details.
The X-ray diffraction (XRD) patterns were measured on a Rigaku
Ultima IV diffractometer using Cu Kα radiation and a nickel filter in
°
°
the 2 θ angle range from 5 to 60 at 35 kV and 25 mA. Inductively
coupled plasma (ICP) atomic emission spectroscopy was performed
on a Thermo IRIS Intrepid II XSP atomic emission spectrometer.
Nitrogen physisorption was carried out on a BEL-MAX instrument at
°
77 K after outgassing the samples for 6 h under vacuum at 300 C.
The UV-Visible diffuse reflectance spectra (UV/Vis) were recorded
on a Shimadzu UV-2400PC spectrophotometer using BaSO4 plate as
a reference. The temperature programmed experiments were
tested on an AutoChem II 2920 instrument equipment equipped
with
a thermal conductivity detector. For NH3-TPD, typically,
100 mg of sample was pre-treated in helium stream (30 mL·minÀ 1
)
°
°
at 550 C for 1 h. The adsorption of NH3 was carried out at 50 C for
Figure 12. The catalytic stability evaluation of 0.5Fe/HZ5-HTS over ethane
dehydrogenation to ethylene.
°
1 h, followed by purging with helium at 100 C for 2 h to remove
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physisorbed NH3 from the catalyst surface. The TPD profile was
Keywords: Ethane dehydrogenation · Fe/ZSM-5 · hydrothermal
post-treatment · highly dispersed Fe
À 1
°
°
recorded at a heating rate of 5 C min from 100 to 550 C. For the
C2H6-TPD and C2H4-TPD, 100 mg of each sample was pretreated at
°
°
650 C for 1 h under 5% H2/Ar and cooled to 50 C, then exposed to
C2H6 or C2H4 flow for 30 min until saturation, followed by purging
with He for 30 min to remove physisorbed C2H6 or C2H4. The
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°
°
temperature was then ramped to 600 C at 10 C/min and held for
1 h until complete desorption of C2H6 or C2H4. As for H2-TPR,
°
typically, 100 mg of sample was pretreated in He at 300 C for
°
60 min followed by cooling down to 50 C, then sample was
reduced in a flow of H2/He mixture (5 vol.% H2) from 50 to 850 C
with a heating rate of 10 C/min. Transmission electron microscopy
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measured by X-ray photoelectron spectroscopy (XPS) on a ESCALAB
250.
°
°
Catalytic performance evaluation was carried out at the atmos-
pheric pressure in a quartz tubular fixed-bed reactor with 7 mm
inner diameter and 63 cm length. A mixture of 100 mg catalysts
and 500 mg quartz sand with 40–60 mesh size distribution was
loaded in the reactor. The catalyst bed temperature was measured
by a thermocouple centered axially inside the reactor. Before
°
reaction, catalyst was pretreated with 30 mL/min 5% H2 at 650 C
for 2 h, then 10 mL/min 5% C2H6/Ar was introduced. The reaction
products were analyzed by an online GC-2014 equipped with an
FID detector to detect hydrocarbons and a TCD detector to detect
hydrogen. The carbon balance calculated from the ratio of sum of
the reaction products to educts was over 95% for all catalysts
tested during reaction process. The ethane conversion
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methane selectivity (Sel.(CH4)) were calculated according to Equa-
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Conv:ðC2H6Þ ð%Þ ¼ ð½C2H6�inÀ ½C2H6�outÞ=½C2H6�in � 100
(1)
Sel:ðC2H4Þ ð%Þ ¼ ½C2H4�=ð½C2H6�inÀ ½C2H6�outÞ � 100
(2)
Sel:ðCH4Þ ð%Þ ¼ ð0:5 � ½CH4�Þ=ð½C2H6�inÀ ½C2H6�outÞ � 100
(3)
YieldðC2H4Þ ð%Þ ¼ ½C2H4�=½C2H6�in � 100
(4)
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Acknowledgements
We gratefully acknowledge the National Natural Science Founda-
tion of China (U19B2003, 21902027 and 21902029), the Founda-
tion of State Key Laboratory of High-Efficiency Utilization of Coal
and Green Chemical Engineering (2019-KF-23), Natural Science
Foundation of Fujian Province (2020 J01443 and 2020 J05121),
Educational Committee of Fujian Province (JAT200042) and
Fuzhou University Testing Fund of precious apparatus (2021T006).
Conflict of Interest
The authors declare no conflict of interest.
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Manuscript received: May 24, 2021
Revised manuscript received: July 6, 2021
Accepted manuscript online: July 7, 2021
Version of record online: ■■■, ■■■■
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Hydrothermal post-treatment: A
high-performance ethane non-
oxidative dehydrogenation is
prepared via hydrothermal post-
treatment. Isolated Fe3+ species and
carburized Fe species are active sites,
and the excellent catalytic perform-
ance is ascribed to more disperse Fe
species and exposing more Fe
species in the surface.
Dr. L. Wu*, Z. Fu, Z. Ren, J. Wei, Prof. X.
Gao, Prof. L. Tan, Prof. Y. Tang
1 – 11
Enhanced Catalytic Performance of
Fe-containing HZSM-5 for Ethane
Non-Oxidative Dehydrogenation via
Hydrothermal Post-Treatment