Biomass-derived Fe-NC hybrid for hydrogenation with formic acid: Control of Fe-based nanoparticle distribution

Article abstract of DOI:10.1039/d0ra01356k

A series of Fe-NC catalysts were synthesized by pyrolyzing an Fe complex and wheat flour at 500 °C. All of them were characterized and applied in the catalytic transfer hydrogenation of nitroarenes with formic acid. It was found that the catalytic activity was significantly affected by the size and distribution of Fe-based nanoparticles (NPs), which could be easily regulated by altering the Fe source. Meanwhile, more basic nitrogen sites were preserved on the catalyst so that the reaction ran smoothly without base additives. Among all catalysts, Fe-NC-FeCl₂ exhibited the best catalytic performance due to smaller Fe₃O₄ NPs and greater N doping. Moreover, it showed excellent applicability for diverse nitroarenes. Obviously, this work demonstrates the importance of the metallic NPs' size and distribution, providing a new insight into the design of M-NC catalysts. The catalyst is economical and eco-friendly, and shows potential application value in industry.

Full text of DOI:10.1039/d0ra01356k

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Biomass-derived Fe-NC hybrid for hydrogenation
with formic acid: control of Fe-based nanoparticle
Cite this: RSC Adv., 2020, 10, 10689
distribution†
abc
Lu Liu,‡a Bowei Wang,
‡
Ruixiao Gao,a Dan Zhang,a Wensheng Xu,a
abc
abc
abc
*
*
Ligong Chen,
Xilong Yan
and Yang Li
A series of Fe-NC catalysts were synthesized by pyrolyzing an Fe complex and wheat flour at 500 ꢀC. All of
them were characterized and applied in the catalytic transfer hydrogenation of nitroarenes with formic acid.
It was found that the catalytic activity was significantly affected by the size and distribution of Fe-based
nanoparticles (NPs), which could be easily regulated by altering the Fe source. Meanwhile, more basic
nitrogen sites were preserved on the catalyst so that the reaction ran smoothly without base additives.
Among all catalysts, Fe-NC-FeCl2 exhibited the best catalytic performance due to smaller Fe3O4 NPs and
greater N doping. Moreover, it showed excellent applicability for diverse nitroarenes. Obviously, this work
demonstrates the importance of the metallic NPs' size and distribution, providing a new insight into the
design of M-NC catalysts. The catalyst is economical and eco-friendly, and shows potential application
value in industry.
Received 12th February 2020
Accepted 6th March 2020
DOI: 10.1039/d0ra01356k
rsc.li/rsc-advances
In the CTH process, the catalyst is of vital importance. In the
past decade, noble metal catalysts had been extensively studied,
showing high activity.14–16 However, high price and scarcity of
Introduction
Aniline and its derivatives are important building blocks and
intermediates in bulk and ne chemicals, and mainly ob-
tained by selective reduction of nitroarenes.1–4 Traditional
industrial reduction methods with iron powder or sodium
sulde are deprecated in industry due to serious pollution.5,6
Instead, catalytic hydrogenation and catalytic transfer hydro-
genation (CTH) are regarded as ideal candidates for nitroarene
reduction. Catalytic hydrogenation undoubtedly has the
advantages of high atom utilization but also inevitably brings
dangers of high temperature and pressure as well.7,8 Fortu-
nately, the CTH method, in which the hydrogen supply
reagent, such as, NH3$BH3,9 HCOOH (FA),10,11 and isopropyl
alcohol,12 is used as a reductant, could overcome the disad-
vantages of catalytic hydrogenation in view of the mild reac-
tion conditions, safe and simple process, etc. Compared with
other reductants, FA, with great application prospects in
hydrogen generation, is nontoxic, renewable and easily
available.13
precious metal seriously restrict the large-scale use and go against
the idea of sustainable development. Therefore, transition metal-
based (e.g., Fe, Co and Ni) heterogeneous catalysts are actively
explored to replace the precious metal catalysts. It is worth
mentioning that Fe is an ideal choice in virtue of the lowest price
and eco-friendliness among all non-noble metals.2,17 Unfortu-
nately, the performance of Fe-based catalysts is far from being
satisfactory in comparison to other non-noble metal catalysts.
In 2013, Beller and co-workers developed a novel Fe-NC
catalyst with Fe2O3 particles embedded in nitrogen-doped
carbon layers by pyrolysis of iron–phenanthroline complex on
carbon (Vulcan XC72R).18 It was successfully applied in the CTH
of nitrobenzene by FA in the presence of triethylamine.19
Henceforth, the M-NC (M ¼ Fe, Co, Ni or their oxide) catalysts
have emerged as a new class of transition metal-based catalysts
due to high activity and selectivity,20 low metal loading, as well as
the excellent stability. Nevertheless, there are still some limita-
tions about M-NC catalysts. For instance, high temperature
pyrolysis would result in the agglomeration of metal nano-
particles (NPs). Consequently, the as-synthesized catalysts
generally suffer from nonuniform dispersion of metal NPs owing
to uncontrollable pyrolysis process,21 which dramatically
restrains the enhancement of catalytic activity. In some cases,
acid etching is employed to remove large metal NPs to promote
catalytic activity,22 but this strategy is not eco-friendly. Thus,
exploring effective strategies to control the NPs' size and distri-
bution of M-NC catalyst is still a challenge. In our recent study, it
aSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300350,
P. R. China. E-mail: lgchen@tju.edu.cn; liyang777@tju.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, P. R. China
cTianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, P. R.
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01356k
‡ These authors contributed equally to this work.
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was found that Co salts could regulate the NPs size of Co-NC 5–80ꢀ with a scan rate of 4ꢀ minꢁ1. Raman spectra were per-
catalyst and further impact their activity.23 However, as for Fe- formed with DXR Microscope at room temperature. Trans-
NC catalyst, some previous works reported that the change of mission electron microscopy (TEM) observations were
the Fe source could signicantly deactivate the catalyst despite performed on Tecnai G2 F20 high resolution analytical electron
the change of the catalyst morphology.24 In addition, high microscope operating at an acceleration voltage of 200 kV.
ꢀ
pyrolysis temperature about 800 C is usually required to opti- Scanning electron microscopy (SEM) images were obtained by
mize catalytic activity in most reported literature.25–27
Hitachi S-4800 microscope. X-ray photoelectron spectroscopy
Harsh preparation condition limits the development of these (XPS) measurements were carried out on a PHI 1600 spec-
novel catalysts, because the size and distribution of metal-based trometer with a Mg Ka X-ray source for excitation. The induc-
NPs are uncontrollable. In our previous work, Fe-NC catalyst tively coupled plasma optical emission spectrometer (ICP-OES)
was successfully synthesized at 500 ꢀC, which exhibited good analysis were performed on VISTA-MPX. Elemental analysis
activity for catalytic hydrogenation. Although many small Fe- (EA) was conducted on VarioEL cube elemental analyzer. N2
based NPs with the size of 2–10 nm were found, there were adsorption–desorption experiments were performed on NOVA
still many large NPs (100–180 nm).28
2000e analyzer (Quantachrome, U.S.A.) at a liquid nitrogen
In this work, we want to establish a novel strategy to construct temperature.
highly-dispersed Fe-NC catalyst by pyrolyzing Fe complex and
biomass carbon source at low temperature. On the one hand, the
low pyrolysis temperature is in favor of the size and distribution of
Fe-based NPs. Meanwhile, the NPs size was tried to regulate by
changing the Fe salts. Moreover, the low pyrolysis temperature is
also benecial to doping N in the carbon matrix as pyridinic N
species,29 which can serve as co-catalyst to promote the decom-
position of O–H bond in FA due to the Lewis basicity and avoid the
addition of basic assistant.30,31 The as-synthesized catalysts were
characterized by XRD, Raman, SEM, TEM, N2 adsorption–desorp-
tion measurement, ICP-OES, elemental analysis, and XPS. Their
activity was evaluated by the CTH of nitrobenzene using FA as
reductant. The structure–activity relationship was discussed in
detail. Furthermore, the general applicability of the catalyst was
also explored.
Catalyst evaluation
Transfer hydrogenation of nitrobenzene with FA. 0.25 mmol
of nitrobenzene, 0.75 mmol of formic acid, 30 mg of catalyst,
1 mL of THF, and 1 mL of H2O were added to 25 mL autoclave
with a Teon liner, ushing 3 times with nitrogen to remove air
and ꢀpressurizing to 1 MPa. The reaction was performed at
120 C for 6 h with magnetically stirred. Aer cooling down to
room temperature, reaction liquid was ltered and analyzed by
GC. Aer the reaction, Fe-NC-FeCl2 was collected by simple
magnetic separation and washed with ethanol several times for
next cycle.
Results and discussion
As described above, a series of Fe-NC-X (X ¼ FeCl2, FeCl3, FeSO4,
Fe(NO3)3, Fe(acac)3) catalysts were obtained by pyrolysis of iron–
phenanthroline complexes together with biomass carbon
source. Their compositions and structures were characterized
systematically.
As shown in Fig. 1a, XRD patterns of all Fe-NC-X catalysts
were similar, indicating similar phase composition. Diffraction
peaks at 29.9, 35.3, 42.8, 53.2, 56.7, 62.2ꢀ demonstrate the
presence of Fe3O4. The broad peak at ꢂ20ꢀ is ascribed to the
amorphous carbon induced by relatively low pyrolysis temper-
ature. It is markedly different from many previously reported
examples, in which a sharp peak appeared corresponding to
graphitic carbon, as a result of higher pyrolysis temperature.32,33
The Raman spectra of all catalysts are represented in Fig. 1b,
in which D band (ꢂ1350 cmꢁ1) indicates the defect and vacancy
Experimental
Catalysts preparation
The following iron salts were utilized respectively in the prep-
aration of the Fe-based catalysts: FeCl2$4H2O, FeCl3$6H2O,
FeSO4$7H2O, Fe(NO3)3$9H2O and Fe(acac)3. Take Fe-NC-FeCl2
as an example, FeCl2 was dissolved in 40 mL distilled water, and
then 10 mL ethanol solution of 1,10-phenanthroline was added
dropwise (Fe : phenanthroline ¼ 1 : 3 molar ratio), the reaction
mixture was stirred at 60 ꢀC for 2 h. Subsequent operations were
similar to our previous work.28 The obtained precursors were
pyrolyzed at a heating raꢀte of 5 ꢀC minꢁ1 in Ar atmosphere and
held for 2 hours at 500 C. Cooling naturally to room temper-
ature, the catalysts were washed by distilled water. Herein, ve
iron salts are FeCl2$4H2O, FeCl3$6H2O, FeSO4$7H2O,
Fe(NO3)3$9H2O and Fe(acac)3, and the corresponding catalysts
were denoted as Fe-NC-X (X ¼ FeCl2, FeCl3, FeSO4, Fe(NO3)3 and
Fe(acac)3), respectively. By comparison, the catalyst Fe–C was
prepared by direct pyrolysis of a mixture of iron salts, baking
soda and wheatmeal. Similar procedure was applied to prepare
the sole NC without metal salts.
Characterization
The X-ray diffraction (XRD) patterns were gained by Mini Flex
600 using a Cu Ka X-ray source (40 kV, 100 mA) in the range of Fig. 1 (a) X-ray diffraction patterns and (b) Raman spectra of Fe-NC-X.
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of material edge and G band (ꢂ1580 cmꢁ1) spawned by in-plane ꢂ5 nm dispersed rarely in the porous carbon matrix. The largest
vibration of the sp2 carbon atoms.34 There are slight differences particle size in Fe-NC-FeSO4 may be due to the stronger elec-
among the relative intensity ratio of D to G band (ID/IG) for all trostatic interaction between SO42ꢁ and iron than other anions,
catalysts, demonstrating iron salts have no obvious effect on which may cause that the decomposition of iron species during
degree of graphitization. Moreover, the hump-like spectra of all pyrolysis become more difficult and higher temperature is
catalysts suggests the presence of amorphous carbon, which is required.23 Firstly, iron sulfate agglomerates at lower tempera-
agreed well with XRD results.
ture, and then decomposes to form larger nanoparticles at
Scanning electronic microscopy (SEM) and transmission higher temperature, eventually lead to larger particle size of the
electron microscopy (TEM) technique were employed to inves- NPs in Fe-NC-FeSO4 catalyst. Meanwhile, no graphitic shells
tigate the morphology of catalysts. We can found the particular were found around Fe3O4 NPs for all samples. This can be due
sponge-like porous architecture for all samples (Fig. S1†), sug- to low graphitization degree of carbon matrix at low pyrolysis
gesting the important pore-creating role of NaHCO3 in con- temperature. In this respect, TEM results agree well with XRD
structing porous morphology during pyrolysis process.28
The TEM images (Fig. 2) reveal that Fe3O4 nanoparticles
and Raman characterizations.
The element compositions of all Fe-NC-X catalysts were
dispersed on the carbon matrix in all Fe-NC-X catalysts. Never- detected by EA and ICP-OES analyses. According to Table 1, the
theless, it is worth noting that the particle sizes of Fe3O4 are element compositions of all catalysts are similar. Among them,
extremely different from each other. The average particle sizes Fe content of Fe-NC-FeCl2 was determined as the highest
of Fe3O4 in Fe-NC-Fe(NO3)3, Fe-NC-FeCl2, Fe-NC-FeCl3 and Fe- (6.70%), while the Fe loading of Fe-NC-FeCl3 was the lowest. No
NC-Fe(acac)3 are 38.10 nm, 5.54 nm, 4.58 nm and 6.22 nm, signicant differences in N content were observed because of
respectively. In Fe-NC-FeSO4, it can be found a broad diameter identical nitrogen source and pyrolysis temperature. Hence, the
distribution of the Fe3O4 NPs, apart from a large proportion of Fe salts cannot obviously inuence element composition of the
relatively larger Fe3O4 particles up to ꢂ130 nm, small particles pyrolyzed catalyst. The N2 adsorption/desorption isotherm of
Fe-NC-X was measured to study the surface area, pore size and
pore volume, according to Table S1,† the specic surface area
and pore property of catalysts are not decisive to their catalytic
activity which was presented in reported literature about carbon
materials.35,36
The chemical state of nitrogen was investigated by XPS.
The N 1s spectra shown in Fig. 3 exhibit four peaks with binding
energy of 398, 399.4, 400.2, and 402 eV, assigned to pyridinic N,
Fe-N, pyrrolic N, and graphitic N, respectively.37 As shown in
Table S2,† the dominating species in all catalysts is pyridinic N,
which exhibits the strongest basicity among the several types
of N.
At this point, the transfer hydrogenation of nitrobenzene with
formic acid was chosen as a model reaction to explore the cata-
lytic performance of the as-prepared Fe-NC-X catalysts. In the
reaction, except the target product aniline, a small amount of
formanilide was detected, which is produced by the reaction of
aniline and FA. As shown in Table 2, conversion of nitrobenzene
does not occur without catalyst. When the reaction was con-
ducted over NC, only 6.1% nitrobenzene conversion was ach-
ieved, indicating that Fe3O4 NPs serve as active sites for transfer
hydrogenation. Furthermore, the catalyst without N doped
(donated as Fe-C) gives 23.5% nitrobenzene conversion, the
Table 1 The elemental contents of Fe-NC-X
ICP
EA
Samples
C (wt%)
H (wt%)
N (wt%)
Fe (wt%)
Fe-NC-FeSO4
Fe-NC-FeCl2
Fe-NC-FeCl3
Fe-NC-Fe(acac)3
Fe-NC-Fe(NO3)3
64.30
67.65
68.10
66.69
68.53
2.05
2.33
2.19
2.33
2.11
5.37
5.81
5.79
5.63
5.73
6.29
6.70
5.78
5.96
6.38
Fig. 2 TEM images of (a) Fe-NC-FeSO4, (b) Fe-NC-FeCl2, (c) Fe-NC-
FeCl3, (d) Fe-NC-Fe(acac)3 and (e) Fe-NC-Fe(NO3)3.
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Fe(acac)3 are slightly lower than that of Fe-NC-FeCl2. Fe-NC-FeSO4
and Fe-NC-Fe(NO3)3 exhibited inferior behavior to the aforesaid
three catalysts under identical reaction condition. Combined
with the results of characterizations, the displayed catalytic
activity follows the opposite order of particle size. Hence, we
speculate that the major factor that affects CTH activity of Fe-NC-
X catalysts is the size and dispersity of Fe3O4 NPs. According to
TEM images, in Fe-NC-FeSO4 the size and dispersity of Fe3O4 NPs
is worst, so it exhibits the worst catalytic performance. Further-
more, the slight differences among the CTH activities of Fe-NC-
FeCl2, Fe-NC-FeCl3, and Fe-NC-Fe(acac)3 are probably caused by
the Fe loadings. In Table S4,† we compare the reported catalysts
for transfer hydrogenation of nitrobenzene, noble metal catalysts
or homogeneous catalysts are usually employed together with
basic additive. Compared with reported similar catalysts, our
catalysts can exhibit excellent performance without any basic
additive. This is attributed to doped pyridinic N species, which
shows stronger basicity than other doped N species and endows
the catalyst some basic properties.38 It is believed to be benecial
to dissociate the O–H bond in formic acid,30 hence leading to
improved catalytic activity.
We have investigated the reduction abilities of Fe-NC-X using
molecular hydrogen and the catalytic mechanism have been
illustrated in our previous work.28 The compared capacity of Fe-
NC-X for catalytic hydrogenation is in accordance with CTH of
nitrobenzene (Table S3†), but catalytic hydrogenation takes
longer reaction time than CTH, implying similar active sites but
different hydrogenation path. It could be concluded that
nitrobenzene is reduced by in situ generated metal hydrides
rather than molecular hydrogen decomposed by FA (Scheme 1).
On the catalyst surface, abundant nitrogenous functional
groups, especially for pyridinic N species, could function as
Lewis-base sites,39–42 behaving a similar role as that of base
additives. The formic acid (hydrogen-donor) is absorbed on the
basic sites, and then is removed proton by electron-rich
nitrogen atoms to form HCOOꢁ, thus accelerating the transfer
of hydrogen to metal NPs and the formation of metal hydrides.
Hydrogen is transferred as metal hydrides intermediate and
reacts with substrate absorbed on NPs. To sum up, the syner-
gistic effect of doping N and Fe3O4 NPs is key factor for the
activity of CTH.
Fig. 3 N 1s XPS spectra of (a) Fe-NC-FeSO4, (b) Fe-NC-FeCl2, (c) Fe-
NC-FeCl3, (d) Fe-NC-Fe(acac)3 and (e) Fe-NC-Fe(NO3)3.
sharp decrease in activity implies the inuential role of nitrogen
modication. Obviously, all Fe-NC-X catalysts exhibit CTH activ-
ities for nitrobenzene without extra base, meanwhile, metal
species and N dopant are strictly required to afford the
outstanding catalyst. However, the activities of Fe-NC-X are
different. As presented comparison of nitrobenzene reduction in
Table 2, Fe-NC-FeCl2 gives best nitrobenzene conversion and
aniline selectivity. The activities of Fe-NC-FeCl3 and Fe-NC-
Table 2 CTH results of nitrobenzene over different catalystsa
Entry
Catalyst
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
No catalyst
Fe-NC-FeSO4
Fe-NC-FeCl2
Fe-NC-FeCl3
Fe-NC-Fe(acac)3
Fe-NC-Fe(NO3)3
NC
—
—
62.7
98.0
96.7
94.5
82.7
6.1
90.3
97.1
84.9
91.9
94.1
100
67.0
Fe-C
23.5
a
Reaction conditions: nitrobenzene (0.25 mmol), FA (0.75 mmol),
30 mg of catalyst, solvent: 1 mL THF + 1 mL H2O, 1 MPa N2, 120 ꢀC
and 6 h.
Scheme
nitroarenes.
1
Proposed mechanism for transfer hydrogenation of
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In order to demonstrate the general applicability of Fe-NC based catalyst undergoes 5 times reuse without a signicant
catalyst, reduction of various substituted nitroarenes were drop of activity (Fig. S4†), revealing the good mechanical
conducted under optimal reaction conditions. The performance stability and acid-resistance of the catalyst, which provides
of Fe-NC-FeCl2 to convert nitroarenes to corresponding anilines possibility for the application in actual production.
is summarized in Table 3. It shows excellent performance for
halogenated nitrobenzene, for example, uoro-, chloro- and
bromonitrobenzene are transformed into corresponding halo-
Conclusions
genated anilines with high yields. It is noteworthy that no
dehalogenation occurring, which is of great signicance in
carbon catalysts via pyrolysis at 500 C. The chemical compo-
industrial production. For the substituted position, o- and m-
nitrotoluene require longer time and more FA than p-nitro-
toluene, it accounts for steric hindrance effect of the substitu-
ents. Besides, nitroanisole achieves complete conversion within
10 hours. To our delight, many reducible groups, such as
to the results of characterizations and experiments, Fe-NC-X
aldehyde and nitrile, are well tolerated in the catalyst system,
the multi-functional nitro-compounds can be selectively
reduced to corresponding anilines in good yield with prolonged
time. In addition, 6-nitroquinoline, which is poisonous for
metal catalysts,43 was efficiently converted to the corresponding
aniline. In summary, the transfer hydrogenation of diverse
In conclusion, we developed a seriesꢀof Fe-based, N-decorated
sition and structural characterizations were measured by XRD,
Raman Spectra, SEM, TEM, XPS, ICP and EA. The sponge-like
porous catalysts were successfully applied to the transfer
hydrogenation of nitro compounds with formic acid. According
catalysts exhibited different catalytic activity for transfer
hydrogenation depending on the size and distribution of Fe-
based NPs. Moreover, the reaction carried on efficiently
without assistant of base additives owing to large amount of
basic nitrogen sites of the catalysts. Among them, the catalyst
Fe-NC-FeCl2 with highest activity possesses small Fe3O4 NPs,
nitroarenes to anilines occurred with excellent conversion and
more N doped, boosting the catalytic activity collectively.
high selectivity, and no extra base was required in all cases
above, indicating that the Fe-NC-FeCl2 possesses good substrate
tolerance for the nitro group reduction in the presence of
Particularly, the controllability of NPs' size and dispersion can
be easily achieved by altering types of iron salts. Different iron
salts lead to a variety of active particle size, thereby resulting in
diverse catalytic activity. The versatile catalyst also affords
diverse functional groups.
Finally, the stability and reusability of Fe-NC-FeCl2 was
investigated. The catalyst can be recovered by magnetic sepa-
taining multiple reducible groups. Moreover, the above catalyst
ration and washed with ethanol, and then it was reused for CTH
excellent chemoselectivity towards various nitroaromatic con-
was used for successive 5 runs, no obvious deactivation was
of nitrobenzene in next cycle. Gratifyingly, our robust iron-
observed.
Conflicts of interest
Table 3 Catalytic transfer hydrogenation of substituted nitroarenesa
There are no conicts to declare.
Entry Substrate
1
Product
t (h) Conv. (%) Sel. (%)
6
100
97.1
Acknowledgements
2
3
6
6
95.9
100
92.5
100
Financial support was provided by the National Natural Science
Foundation of China (Grant No. 21808161).
4
10
12
100
100
100
Notes and references
5b
97.1
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Reaction conditions: nitro compounds (0.25 mmol), 30 mg of Fe-NC-
FeCl2, formic acid (0.75 mmol), 1 mL of H2O and 1 mL of THF, 120 ꢀ
and 1 MPa N2. b 3.5 eq. FA was added. c 130 ꢀC.
C
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