Nitrogen-Doped Graphene-Supported Iron Catalyst for Highly Chemoselective Hydrogenation of Nitroarenes

Article abstract of DOI:10.1002/cctc.201701949

A nitrogen-doped graphene-supported iron catalyst was used for the first time in the hydrogenation of a series of nitroarenes to give the corresponding amines with excellent activity and chemoselectivity under mild reaction conditions. Physicochemical characterization of the catalyst by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and M?ssbauer spectroscopy revealed the formation of iron particles with an iron oxide core and a metallic iron shell that were coated by a few layers of nitrogen-doped graphene. The unique structure of FeN_x/C in the catalyst was proven to contribute to the hydrogenation activity.

Full text of DOI:10.1002/cctc.201701949

DOI: 10.1002/cctc.201701949
Communications
Nitrogen-Doped Graphene-Supported Iron Catalyst for
Highly Chemoselective Hydrogenation of Nitroarenes
Zuojun Wei,*[a] Yaxin Hou,[a] Xinmiao Zhu,[a] Liangyu Guo,[b] Yingxin Liu,[b] and
Anyun Zhang[a]
A nitrogen-doped graphene-supported iron catalyst was used
for the first time in the hydrogenation of a series of nitroarenes
to give the corresponding amines with excellent activity and
chemoselectivity under mild reaction conditions. Physicochem-
ical characterization of the catalyst by transmission electron
microscopy, X-ray diffraction, X-ray photoelectron spectrosco-
py, and Mçssbauer spectroscopy revealed the formation of
iron particles with an iron oxide core and a metallic iron shell
that were coated by a few layers of nitrogen-doped graphene.
The unique structure of FeNx/C in the catalyst was proven to
contribute to the hydrogenation activity.
metal ions chelated with organic ligands are also active for hy-
drogenation and are frequently applied for homogenous catal-
ysis on both laboratory and industrial scales.[5] In such cases, it
is unnecessary to reduce the metal ion because the ligand can
act as an electron donor or accepter to adjust the electronic
dispersion of the core metal ion, so that scission of H2 and the
adsorption of H atoms can occur effectively on the metal
ions.[6]
Both heterogeneous and homogeneous catalytic hydrogena-
tions have their inherent advantages and disadvantages. Very
interestingly, they work for the same catalytic hydrogenation
purpose but always appear as two independent research
areas. Recently, it seems that this estrangement has started to
disappear owing to a breakthrough from Beller’s research
group, who developed an efficient low-loading Fe2O3-N/CB cat-
alyst by pyrolysis of a ferrous acetate–phenanthroline complex
supported on carbon black (CB) at 8008C under an inert gas
atmosphere.[7] A special FeNx microstructure with Fe2O3 parti-
cles surrounded by three to five layers of N-doped graphene in
the catalyst was considered as the unique catalytic active sites,
which performed very well in the hydrogenation of a lot of un-
saturated functional groups.[8] Very recently, a similar FeNx
structure was also found in the pyrolyzed complexes of Co–
phthalocyanine[9] and Co–thioporphyrazine,[10] which were also
proven to exhibit hydrogenation activity.
On the basis of our experience in graphene catalysis,[11] what
interested us most was whether the combination of iron with
N-doped graphene would be active for hydrogenation reac-
tions. Graphene is a new-generation carbon material with a
special two-dimensional planar microstructure, huge theoreti-
cal specific surface area (2630 m2·gÀ1), good chemical stability,
and flexible surface modifiability.[12] Recent studies revealed
that excellent performance and improvement could be ach-
ieved by using graphene materials as catalyst supports or
metal-free catalysts in various reactions.[13] With an atomic size
similar to carbon and three unpaired electrons binding with
carbon, nitrogen is the most frequently used heteroatom that
is doped in the graphene framework to attain additional cata-
lytic properties.
Heterogeneous catalytic hydrogenation plays an important
role in achieving more environmentally benign and atom-eco-
nomic processes in the chemical industry.[1] Noble metals such
as Ru, Rh, Pd, Pt, Ir, and Au and base metals such as Co, Ni, Fe,
and Cu have both been widely utilized as active sites in sup-
ported heterogeneous catalysts for the hydrogenation of vari-
ous unsaturated functional groups.[2] In terms of metal-in-
volved heterogeneous hydrogenation reactions, the Horuiti–
Polanyi mechanism, in which the metallic atoms first catalyze
scission of the HÀH bond in H2 and adsorb the dissociated hy-
drogen atoms, is generally accepted and has been proven by
modern physicochemical characterization techniques and first-
principles calculations.[3] Consequently, a metal that maintains
its metallic state instead of adopting an oxidized state is re-
quired for HÀH scission and the adsorption of H atoms. To
reach this goal, a well-known process in the heterogeneous
catalysis field has been established, that is, H2-TPR (tempera-
ture-programmed reduction), by which the minimum tempera-
ture for complete reduction of the metal oxides into metallic
atoms is determined to prepare metal catalysts.[4] By contrast,
[a] Dr. Z. Wei, Y. Hou, X. Zhu, Prof. A. Zhang
Key Laboratory of Biomass Chemical Engineering of the
Ministry of Education
College of Chemical and Biological Engineering
Zhejiang University
38 Zheda Road, Xihu District, Hangzhou 310027 (P.R. China)
E-mail: weizuojun@zju.edu.cn
Herein, for the first time we report the application of N-
doped graphene-supported iron (Fe/rGO-N) as a highly active
catalyst for the chemoselective hydrogenation of nitroarenes.
Fe/rGO-N was simply prepared by pyrolysis of a mixture of
Fe(OAc)2 and N-doped reduced graphene oxide (rGO-N) at
8008C for 2 h under an inert atmosphere, wherein rGO-N was
prepared in advance through hydrothermal treatment of a
mixture of graphene oxide (GO) from modified Hummers’
[b] L. Guo, Prof. Y. Liu
Research and Development Base of Catalytic Hydrogenation
College of Pharmaceutical Science
Zhejiang University of Technology
18 Chaowang Road, Xiacheng District, Hangzhou 310014 (P.R. China)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cctc.201701949.
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method[11] and urea as the nitrogen source at 1608C for 3 h.
For comparison, the same procedure was applied to a mixture
of Fe(OAc)2 complexes with N-containing ligands (Figure 1)
and carbon black to prepare Fe-ligand/CB catalysts.
entry 11), respectively. In contrast, pure CB and activated
carbon (AC) were inactive (Table 1, entries 12–14). This is in
agreement with the reports that many nitrogen- or boron-
doped carbon materials including graphene show hydrogena-
tion activity.[13c–e,14] As our Fe/rGO-N catalyst showed even
higher hydrogenation activity, we deduced that a new active
site involving a Fe complex might have been formed after
loading with Fe. The possibility that Fe or FeÀC was the hydro-
genation active site could be ruled out, as Fe/CB, Fe/AC, and
Fe powder all gave poor aniline yields (Table 1, entries 15–17).
Furthermore, the possibility that residual Mn, originating from
the preparation of rGO-N through modified Hummers’
method, was the active site could also be ruled out,[11] as both
the FeMn/AC and Mn-L1/CB catalysts were inactive in the hy-
drogenation of nitrobenzene (Table 1, entries 18 and 19). We
therefore speculated that the formation of a FeÀNÀC structure
may have resulted in active sites for hydrogenation.
Figure 1. Structure of the nitrogen-containing ligands.
The catalysts were first applied for the hydrogenation of ni-
trobenzene to aniline. As shown in Table 1, after optimization
(Table 1, entries 1–6), a 100% yield of aniline was obtained
over Fe/rGO-N at 1008C for 2 h (Table 1, entry 4). With respect
to Fe-L1/CB, the yield of aniline was only 89.3% under more
rigorous reaction conditions of 1208C and 15 h (Table 1,
entry 7), which were similar to the yield reported by Beller.[7]
Two other Fe catalysts with new nitrogen-containing ligands,
that is, Fe-L2/CB and Fe-L3/CB, gave even lower yields of ani-
line (Table 1, entries 8 and 9). Notably, pure rGO-N exhibited
hydrogenation activity that was comparable to that of Fe-L1/
CB: after 15 h of reaction, aniline was obtained in yields of 76.5
and 100% at 1008C (Table 1, entry 10) and 1208C (Table 1,
Various physicochemical measurements were performed to
explore the possible active sites of Fe/rGO-N further. Its N2
sorption isotherm is characteristic of a mesoporous structure
of slit holes that are generated from the gaps between the
rGO-N sheets, and its average pore diameter is 38 nm (Fig-
ure S1, Supporting Information).[15] The BET specific surface
area of Fe/rGO-N is 358 m2 gÀ1, which is larger than that of Fe-
L1/CB (100 m2·gÀ1) (Figure S1b). This might be one of the rea-
sons that Fe/rGO-N exhibited higher hydrogenation activity
than Fe-L1/CB (Table 1, entries 4 and 7).
The scanning transmission electron microscopy (STEM)
image reveals that Fe/rGO-N has a nonuniform metal-particle
distribution and a large average size of 50 nm (Figure S2a,b),
which suggests strong agglomeration of the iron particles
during high-temperature pyrolysis. The Fe/O atomic ratio from
the energy-dispersive X-ray spectroscopy (EDS) line scan (Fig-
ure S2a, inset) is much greater than that derived by X-ray pho-
toelectron spectroscopy (XPS) analysis (Table S1), which indi-
cates that the surface species of the metal particles mostly
exists in the metallic atom state. It also shows that the C and
N elements are uniformly distributed on the graphene surface.
On the basis of elemental analysis (EA), the N content in the
original N-doped graphene was 7.1 wt%, whereas the N con-
tent in Fe/rGO-N dropped to 5.4 wt% (or 4.40 wt% by XPS);
this implied rearrangement and leakage of N during pyrolysis
with Fe(OAc)2. The transmission electron microscopy (TEM)
image of Fe/rGO-N shows that the iron particles are surround-
ed by a few layers of rGO-N (marked by arrows, Figure 2a). Its
high-resolution TEM (HRTEM) image (Figure 2b) further con-
firms that two kinds of lattice planes exist in the main body of
the iron particle with interplanar spacings of 0.252 and
0.208 nm, corresponding to the Fe (311) crystalline planes of
g-Fe2O3 and the Fe (111) planes of metallic iron particles, re-
spectively. The lattice fringes of Fe0 are mainly distributed at
the edge, whereas the lattice fringes of g-Fe2O3 can be ob-
served over the whole region, which indicates that the main
component of the metal particles is g-Fe2O3 with the surface
partially covered by Fe0. Moreover, there are also some small g-
Fe2O3 particles embedded in the surface intersect of the metal
particle and rGO-N.
Table 1. Hydrogenation of nitrobenzene to aniline over iron-based
catalysts.[a]
Entry
Catalyst
T [8C]
t [h]
Yield [%]
1
2
3
4
5
6
7
8
Fe/rGO-N
Fe/rGO-N
Fe/rGO-N
Fe/rGO-N
Fe/rGO-N
Fe/rGO-N
Fe-L1/CB
Fe-L2/CB
Fe-L3/CB
rGO-N[b]
rGO-N[b]
CB[b]
AC
–
Fe/CB[c]
Fe/AC[c]
Fe[d]
80
100
100
100
120
120
120
120
120
100
120
120
120
120
120
120
120
120
120
15
1
1.5
2
1
2
15
15
15
15
15
15
15
15
15
15
15
15
15
67.1
77.8
92.2
>99
88.6
>99
89.3
6.9
30.5
76.5
>99
N.D.
N.D.
N.D.
27.8
8.7
9
10
11
12
13
14
15
16
17
18
19
5.2
N.D.
N.D.
FeMn/AC
Mn-L1/CB
[a] Reaction conditions unless otherwise noted: nitrobenzene
(0.125 mmol), catalyst (10.5 mg), THF/H2O (1:1, v/v, 1 mL), H2 (3.0 MPa);
N.D.: not detected. [b] Catalyst (10.0 mg). [c] Fe catalyst was reduced at
5008C for 4 h prior to use. [d] Catalyst (3.0 mg).
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served as the C2 sextet.[21] The other two sextets, C3 and C4,
can be attributed to ferromagnetic body-centered cubic iron
(a-Fe)[22] and g-Fe2O3,[7] which comprise approximately 21.1 and
27.3%, respectively.
The identical analysis results obtained from TEM, XRD, XPS,
and Mçssbauer spectroscopy showed that the main phases of
iron in Fe/rGO-N comprised metallic Fe, Fe2O3, and FeNx spe-
cies. We therefore propose that the microstructure of the
metal particles in Fe/rGO-N can be inferred to have an iron
oxide core partially covered by a Fe0 shell, wherein particles
are coated by a few layers of rGO-N to form the unique FeNx/C
species. Fe2O3 and Fe0 are generated from the disproportiona-
tion of the Fe(OAc)2 precursor during the pyrolysis process.[23]
Moreover, the formation of metallic iron can also be caused by
carbothermal reduction in the presence of the graphene-based
support.[24] Further exposure to the ambient atmosphere
would lead to the oxidation of metallic iron to Fe2O3.[17b] How-
ever, metallic Fe was experimentally proven to be inactive for
the hydrogenation of nitrobenzene (Table 1, entry 14). Neither
a-Fe2O3 nor g-Fe2O3 exhibited clear catalytic performance to-
wards the reaction under the demanding conditions of 1508C
and 10 h.[25] Therefore, it is believed that the formed FeNx
structure mainly contributes to the excellent hydrogenation ac-
tivity of Fe/rGO-N. We postulate that the electron-deficient FeIII
center and the electron-rich N center in FeNx may facilitate het-
erolytic cleavage of H2, which acts as a hydride acceptor and a
proton acceptor, respectively.[26] Moreover, the basicity of rGO-
N originating from doping of N atoms can increase the catalyt-
ic activity of Fe/rGO-N.[27] Furthermore, rGO-N with a special
two-dimensional planar microstructure, large specific surface
area, and other good physicochemical properties also plays a
part in the reaction process. For example, nitroarenes would
be adsorbed onto the rGO-N surface through p–p interaction
between their aromatic rings and the giant p bond of rGO-N,
which is beneficial to activate the reaction.[11b,13c] Actually, tran-
sition metals supported on nitrogen-doped carbon materials
(Fe/CoÀNÀC) have been widely applied in the oxygen reduc-
tion reaction (ORR) since the 1960s, and electron-deficient Fe/
Co is generally regarded as the active sites for the adsorption
and activation of oxygen.[28] For the hydrogenation reaction,
however, further effort is still required to reveal the active
structure and mechanism of Fe/rGO-N through a combination
of computational and experimental studies.
Figure 2. a) TEM image, b) HRTEM image, c) XRD pattern, d,) X-ray photo-
electron Fe2p and N1s spectra, and f) room-temperature 57Fe Mçssbauer
spectrum of Fe/rGO-N.
As shown in the X-ray diffraction (XRD) pattern (Figure 2c),
the diffraction peak with 2qꢀ268 is indexed to the C (002)
plane of the graphite sheets in rGO-N. Besides, peaks of Fe2O3,
FeNx, and Fe0 can be observed at 2q=36.0, 43.5, and 44.58, re-
spectively, in Fe/rGO-N.[16] In the X-ray photoelectron Fe2p
spectrum (Figure 2d), the binding energies (BEs) of 707.0 and
719.5 eV are assigned to the 2p3/2 and 2p1/2 peaks of Fe0,[17] and
the peaks at BE=711.4 and 723.9 eV, along with the presence
of a characteristic satellite peak at BE=d715.3 eV, suggest the
existence of Fe2O3.[18] The N1s peaks in Figure 2e can be divid-
ed into four species: pyridinic N at a BE of 398.5 eV, FeÀN at a
BE of 399.6 eV, pyrrolic N at a BE of 400.5 eV, and graphitic N
at a BE of 402.0 eV, which indicates the successful doping of ni-
trogen into the graphene framework and the formation of
FeNx sites.[7,8,17c,19] The 57Fe Mçssbauer spectrum of Fe/rGO-N
recorded at room temperature (Figure 2 f) consists of one
quadrupole doublet (C1) and three magnetic sextets (C2, C3,
and C4), and their corresponding hyperfine parameters are
summarized in Table S2. The C1 doublet accounts for the larg-
est proportion (42.7%) of the iron species in Fe/rGO-N and can
be assigned to the ferric FeNx center embedded in the gra-
phene plane.[20] A small contribution of a-Fe2O3 (8.9%) is ob-
Figure 3. Recycling test of Fe/rGO-N for the hydrogenation of nitrobenzene.
Reaction conditions: nitrobenzene (0.125 mmol), catalyst (10.5 mg), THF/H2O
(1:1, v/v, 1 mL), H2 (3.0 MPa), 1008C, 2 h. From the sixth run: t=15 h.
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The reusability of Fe/rGO-N was further evaluated in the hy-
drogenation of nitrobenzene through simple centrifugation
and washing with THF. A gradual decrease in the aniline yield
was observed over five successive runs, as shown in Figure 3.
It is deduced that the main reason for the deactivation of Fe/
rGO-N was the destruction of FeNx species with a decrease in
the iron particle size from 50 to 36 nm after eight reuses (Fig-
ure S2).[16] However, the activity of Fe/rGO-N still remained if
the reaction time was extended from 2 to 15 h.
Furthermore, Fe/rGO-N was utilized in the catalytic hydroge-
nation of a series of nitroarenes to explore its general applica-
bility. The results, summarized in Table 2, show that most of
the substrates were converted with satisfactory conversions (>
89%) and selectivities (>93.5%) under relatively mild reaction
conditions, which proved that Fe/rGO-N possessed excellent
catalytic activity and chemoselectivity for the hydrogenation of
structurally diverse nitroarenes. Notably, Fe/rGO-N also exhibit-
ed certain activity towards the hydrogenation of the carbonyl
group (Table 2, entry 14) and dechlorination
(Table 2, entry 15) if the reaction time was extend-
Table 2. Hydrogenation of various nitroarenes over Fe/rGO-N.[a]
ed.
In conclusion, the Fe/rGO-N catalyst was pre-
pared by pyrolysis of a Fe(OAc)2 and rGO-N mix-
ture. The as-prepared catalyst exhibited excellent
activity in the hydrogenation of a series of struc-
turally diverse nitroarenes with good chemoselec-
tivity. Analysis of the experimental results and
characterization data suggested that a unique
FeNx/C structure was formed on the exterior iron
particles embedded within a layer of graphene
sheets and that it was this structure that served as
the catalytic active sites.
Entry Substrate
1
Product
t [h] Conversion[c] [%] Selectivity[c] [%]
2
4
>99
>99
>99
>99
2
3
4
2
2
4
2
>99
>99
>99
>99
>99
>99
4
92.4
5
32.3
Acknowledgements
6[b]
>99
This work was supported by the National Natural
Science Foundation of China (21476211) and the
Zhejiang Provincial Natural Science Foundation of
China (LY14B060003 and LY16B060004).
7
89.0
8[b]
4
>99
>99
>99
Conflict of interest
The authors declare no conflict of interest.
9[b]
10
4
2
>99
>99
Keywords: chemoselectivity
·
doping
·
hydrogenation · iron · graphene
99.1
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Manuscript received: December 6, 2017
Revised manuscript received: January 26, 2018
Version of record online: && &&, 0000
ChemCatChem 2018, 10, 1 – 6
www.chemcatchem.org
5
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

COMMUNICATIONS
Z. Wei,* Y. Hou, X. Zhu, L. Guo, Y. Liu,
A. Zhang
&& – &&
Nitrogen-Doped Graphene-Supported
Iron Catalyst for Highly
Chemoselective Hydrogenation of
Nitroarenes
Iron clad: A nitrogen-doped graphene-
supported iron catalyst is used for the
first time in the hydrogenation of nitro-
arenes to give the corresponding
moselectivity under mild reaction condi-
tions. The unique structure of FeNx/C in
the catalyst is proven to contribute to
the hydrogenation activity.
amines with excellent activity and che-
&
ChemCatChem 2018, 10, 1 – 6
www.chemcatchem.org
6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!