Y. Cao, et al.
AppliedCatalysisA,General592(2020)117434
chloronitrobenzene (AR, 99 %), 3-chloronitrobenzene (AR, 98 %), 4-
chloronitrobenzene (AR, 99 %), 3-bromonitrobenzene (AR, 99 %), 4-
bromonitrobenzene (AR, 99 %) and 3-iodonitrobenzene (AR, 99 %)
were purchased from Aladdin Chemistry Co., Ltd. 2-bromonitrobenzene
(AR, 99 %), 2-iodonitrobenzene (AR, 98 %) and 4-iodonitrobenzene
(AR, 98 %) were purchased from McLean (Shanghai) Biochemical
Technology Co., Ltd. Co(NO3)2·6H2O (AR, 99 %), ethanol (AR, 99.8 %)
and cobalt acetate tetrahydrate (AR, 99 %) were purchased from
Guanghua (Guangdong) Technology Co., Ltd. Cobalt (II) acet-
ylacetonate (AR, 98 %) was obtained from Civic (Shanghai) Chemical
Technology Co., Ltd. All the reagents were used without further pur-
ification.
Scheme 1. Possible reaction pathways for the hydrogenation of halogenated
nitrobenzenes.
catalysts, series of non-noble metal catalysts were developed succes-
sively [9,28–34]. For example, Raney nickel catalyst has been reported
that it can be successfully applied in the selective hydrogenation of
halogenated nitrobenzenes, but inhibitors are needed to inhibit the
dehalogenation phenomenon [9]. Fe-based catalysts have also been
found to be selective for this reaction. But only 97 % selectivity for the
5-chloro-2-methoxyaniline was obtained, and relatively harsh reaction
conditions (120 °C, 5 MPa H2) were needed [28]. Additionally, Beller
et al. reported that the halogenated nitrobenzenes could be transformed
into the corresponding anilines over cobalt-phenanthroline complexes
derived Co oxide-N/C catalyst under 5 MPa H2 at 110 °C [29]. How-
ever, for the 3-chloroaniline and 4-chloroaniline, only 95 % selectivity
was achieved. Wang et al. also demonstrated that the Co°/Co3O4@
NCNTs catalyst exhibited excellent catalytic performance for the hy-
drogenation of substituted nitroarenes with a wide scope under 3 MPa
H2 at 110 °C [30]. Similar problem, slight dehalogenation happened
and only 96 % selectivity was obtained for the 3-iodoaniline. It can be
seen that these non-noble metal catalysts also suffer from some lim-
itations such as the occurrence of slight dehalogenation, relatively
costly nitrogen-containing precursors, high reaction temperature
(> 100 °C) and/or H2 pressure (> 2 MPa). Consequently, the devel-
opment of a cost-effective non-noble metal catalyst system with both
high selectivity and activity for halogenated nitrobenzenes hydro-
genation is highly desirable.
Tannic acid (TA), extracted from plant tissue, contains abundant
galloyl or catechol groups, making it possess a strong binding affinity
for various metal ions. More importantly, low price, the abundance, and
environmental sustainability of TA grant it highly practical for many
applications [35–37]. For example, the metal-TA coordination poly-
mers have been widely used as a versatile platform for the functional
surface engineering [38–40]. Moreover, TA also exhibits great potential
as carbon precursors for the fabricating metal/carbon composites using
its strong chelating ability to metal ions.
Herein, we demonstrate the successful synthesis of N-doped carbon
supported Co (Co@CN) catalysts through one-pot pyrolysis of a Co
(Ⅱ)-TA coordination polymers with melamine. Co (Ⅱ)-TA coordination
polymers play an important role in preventing the Co species from
aggregation during pyrolysis process. Melamine, a common industrial
chemical, can function as a soft template for the formation of sheet-like
N-doped carbon and the dispersion of Co NPs (nanoparticles). The
Co@CN catalyst displayed excellent catalytic performance for the li-
quid-phase hydrogenation of halogenated nitrobenzenes under 1 MPa
2.2. Catalyst preparation
The preparation of Co@CN catalysts involved one-pot pyrolysis of a
mixture of Co(NO3)2·6H2O, TA and melamine, similar with previous
reports [41,42]. In a typical experiment, the aqueous solution con-
taining 500 mg of Co(NO3)2·6H2O and a certain amount (300, 400, 500,
600 mg) of TA was stirred for 1 h, and then, 5 g of melamine was added
into the above solution to form a slurry. After that, the slurry was he-
ated at 80 °C until completely drying to form a powder. The dried
powder was transformed into a furnace and underwent thermal treat-
ment at 350 °C for 2 h and then at 550 °C for 2 h, finally at 700 °C for 2
h under nitrogen flow of 200 mL/min. The pyrolyzed samples were
denoted as Co@CN-X, where X represents the amount of TA used in the
catalyst preparation. For comparison, Co@C-TA, and Co@CN-Mel were
obtained through the same procedure. Additionally, the mixture of TA
(400 mg), Co(NO3)2·6H2O (500 mg) and melamine (5 g) was also
pyrolyzed at 350 ℃ and 550 ℃, and the samples were named as
Co@CN-400-350 and Co@CN-400-550, respectively.
2.3. Characterizations
The crystallographic phase of the as-synthesized materials was
characterized by X-ray powder diffractometer (XRD, Shimadzu XRD-
600, Cu Kα source, λ =1.5406 Å). The specific surface areas and pore
size distribution were evaluated by nitrogen adsorption-desorption
isotherms on an 3H-2000PS2 nitrogen adsorption apparatus (Beishide
Instrument, China) at 77 K. Specific surface area and pore structure
were determined by the standard BET method based on the relative
pressure between 0.04 and 0.32. Inductively coupled plasma mass
spectrometer (ICP-MS) was employed to analyze the Co content in the
obtained Co@CN samples on a USA Perkin Elmer NexION 350D. X-ray
photoelectron spectroscopy (XPS) characterization were carried out
with an Axis Ultra DLD X-ray photoelectron spectroscopy(Al Kα ra-
diation) produced by Kratos Company in the United Kingdom to ana-
lyze the surface chemical structure of the prepared catalysts. The
morphology of catalysts was analyzed by scanning electron microscopy
(SEM, Verios G4, FEI). The metal nanoparticles in the catalysts were
observed using transmission electron microscopy (TEM) with a JEM-
2100. The mean particle size and size distribution were calculated
based on over 200 individual particles. The CO chemisorption test was
carried out in the chemisorption analyzer (Micromeritics Auto Chem
2920, USA) equipped with a thermal conductivity detector (TCD) using
a pulse of 10 % CO/90 % He at 40 °C. Supposing the CO and Co stoi-
chiometry was 1:1, the adsorption capacity of carbon monoxide and
surface area of cobalt metal were calculated by following equations.:
H
2 at 60 °C, which is better than that of the most previous reported Co-
based catalysts (Table S4). For the chloroanilines, bromoanilines, and
iodoanilines, including all regioisomers, 99 % selectivity could be
achieved at almost complete conversion of the substrates.
nCO
mCo
CO uptake=
(1)
2. Experimental section
SCo × R× nCO
metal area=
mCo
(2)
2.1. Materials
where nCO is the adsorption capacity of carbon monoxide(mol), mCo is
the true content of cobalt in the prepared catalyst(g), R is Avogadro's
Melamine (AR, 99 %), Tannic acid (AR, 99 %), 2-
2