Ammonia borane dehydrogenation and selective hydrogenation of functionalized nitroarene over a porous nickel-cobalt bimetallic catalyst

Article abstract of DOI:10.1039/c9ra01551e

The hydrolysis of ammonia borane is a promising strategy for hydrogen energy exploration and exploitation. The in situ produced hydrogen could be directly utilized in hydrogenation reactions. In this work, a bimetallic nickel-cobalt material with porous structure was developed through the pyrolysis of ZIF-67 incorporated with Ni ions. Through the introduction of Ni(NO₃)₂ as an etching agent, the ZIF-67 polyhedrons were transformed into hollow nanospheres, and further evolved into irregular nanosheets. The bimetallic NiCo phase was formed after pyrolysis in a nitrogen atmosphere at high temperature, with the decomposition and release of organic ligands as gaseous molecules under flowing nitrogen. The obtained bimetallic NiCo porous materials show superior catalytic performance towards hydrolytic dehydrogenation of ammonia borane, thereby nitrobenzene with reducible functional groups can be reduced with high selectivity to the corresponding aniline.

Full text of DOI:10.1039/c9ra01551e

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Ammonia borane dehydrogenation and selective
hydrogenation of functionalized nitroarene over
a porous nickel–cobalt bimetallic catalyst†
Cite this: RSC Adv., 2019, 9, 14580
*
Hui Miao,
Kelong Ma, Huiru Zhu, Kun Yin, Ying Zhang and Yumin Cui
The hydrolysis of ammonia borane is a promising strategy for hydrogen energy exploration and exploitation.
The in situ produced hydrogen could be directly utilized in hydrogenation reactions. In this work,
a bimetallic nickel–cobalt material with porous structure was developed through the pyrolysis of ZIF-67
incorporated with Ni ions. Through the introduction of Ni(NO3)2 as an etching agent, the ZIF-67
polyhedrons were transformed into hollow nanospheres, and further evolved into irregular nanosheets.
The bimetallic NiCo phase was formed after pyrolysis in a nitrogen atmosphere at high temperature, with
the decomposition and release of organic ligands as gaseous molecules under flowing nitrogen. The
obtained bimetallic NiCo porous materials show superior catalytic performance towards hydrolytic
dehydrogenation of ammonia borane, thereby nitrobenzene with reducible functional groups can be
reduced with high selectivity to the corresponding aniline.
Received 1st March 2019
Accepted 18th April 2019
DOI: 10.1039/c9ra01551e
rsc.li/rsc-advances
Co-based materials derived from might be promising alterna-
tive non-noble metal catalyst in heterogeneous catalysis.
1. Introduction
Functional anilines are widely existed as key intermediates in
dyes, pigments, herbicides and pharmaceuticals.18 The catalytic
hydrogenation of nitroarenes using hydrogen as a reduce agent is
the main protocol for the high efficiency and atom economic.19
Noble metal catalysts such as Pd and Pt have shown excellent
catalytic activity in nitrobenzene hydrogenation.20–23 However,
precious metals are rare, expensive and difficult to control the
selectivity of hydrogenation. It is imperative to nd non-precious
metal catalysts that can replace precious metals. It has been re-
ported that non-noble metal catalysts such as Fe and Co have
been used in nitrobenzene hydrogenation.24–26 Actually, the direct
use of hydrogen is uneconomical and increases the risk of reac-
tion. Catalytic hydrogenation of non-noble metals generally
requires higher reaction temperature and pressure.27–30 It is
highly desirable to realize selective hydrogenation of nitroben-
zene over non-noble metal catalysts under mild conditions.
Ammonia borane (NH3BH3, AB) is non-toxic, safe, stable and
environmentally friendly compounds with high quality
hydrogen storage (mass fraction of 19.6%).31,32 Based on these
characteristics, it is the most potential new type of hydrogen
storage.33–35 To combine the dehydrogenation of ammonia
borane and hydrogenation of nitrobenzene would be an ideal
strategy to realize selective hydrogenation of nitrobenzene with
reducible functional groups under mild condition.36 To nd
cheap and efficient catalysts to reduce the cost of hydrogen
production would be highly desirable but challenging. The non-
precious metal catalysts, nickel and cobalt based nanoparticles
or alloy catalysts have shown superior catalytic effect in
hydrogen production by ammonia borane hydrolysis.37,38
Metal–organic frameworks (MOFs) are crystalline porous
materials prepared by the self-assembly of metal ions and
organic linkers, which have been used as templates or precur-
sors for nanostructured porous materials.1–4 The morphology of
MOF materials shows diversity because of the exible and
controllable composition, structure and pore size.5 They are
attractive as ideal precursors for the preparation of porous
nano-materials with various morphologies or structures.6 MOF-
derived metal composites oen show large surface area and
hierarchical pore structures. Metal nanocrystals,7 oxides,8
suldes,9 nitrides10 and phosphides11 have been successfully
prepared through the evolution of MOFs, and have shown
extraordinary performance in catalysis, sensing, energy storage
and conversion, separation and other elds.12 ZIF-67 is a kind of
zeolite imidazole skeleton material with Co2+ ions as metal sites
and 2-methylimidazole as organic ligands.13 Through pyrolysis
at high temperature, ZIF-67 converted into nitrogen doped
carbon encapsulated Co/CoOx nanocrystals (denoted as Co–N–
C). The Co–N–C species has shown superior catalytic perfor-
mance in both oxidation and reduction reactions.14–16 Through
further phosphorization or sulfuration, the corresponding CoP–
N–C or CoS–N–C can be used as an electrocatalytic material for
the oxygen reduction reaction (ORR) or water splitting.9,17 The
School of Chemistry and Materials Engineering, Fuyang Normal University, Anhui
Provincial Key Laboratory for Degradation and Monitoring of Pollution of the
Environment, Fuyang, 236037, China. E-mail: huimiao@mail.ahnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01551e
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The convenient and effective preparation strategy of non-
noble catalysts is still a challenge due to the lower electroneg-
ativity.39 Thermal annealing with high temperature is always
need, which oen results in serious agglomeration and losing
of surface area or active sites. To construct porous structure
materials is propitious to expose more active site and mass
transfer of substances in catalysis.40 The developed porous
catalysts such as Ni, Au, Cu, etc. have shown improved or
enhanced performance in many catalytic processes.41–43 In
general, the synthetic methods of porous catalysts are based on
template, corrosion or assembling of smaller particles.44,45 To
develop novel, more convenient and effective synthetic method
is crucial to higher requirements in catalysis. Here, we present
a convenient and effective method to synthesize porous NiCo
bimetallic catalyst with controllable composition through
pyrolysis of Ni-ZIF-67. The obtained NiCo bimetallic catalysts
show excellent activity towards hydrogen production by hydro-
lysis of ammonia borane.
2.5 Characterization
X-ray diffraction (XRD) was performed on a Philips X'pert
diffractometer equipped with a Ni-ltered Cu Ka radiation
source. The particle size and morphology of as-synthesized
samples were determined by using Hitachi model H-800
transmission electron microscope. High-resolution trans-
mission electron microscopy (HRTEM) images were taken using
a JEM-2100 electron microscope. X-ray photoelectron spectros-
copy (XPS) experiments were performed on a ULVAC PHI
Quantera microprobe. Binding energies (BE) were calibrated by
setting the measured binding energies of C 1s to 284.6 eV.
2.6 Dehydrogenation of ammonia borane
The catalysts were suspended in water (5.0 mL) of a two-necked
round-bottomed ask. A gas cylinder lled with water was
connected to one neck of the reaction ask (the other neck was
sealed) to measure the volume of hydrogen. The reaction was
ꢀ
initiated in a water bath at 25 C under ambient atmosphere.
1 mmol of ammonia borane was added into the reaction ask
with stirring. By measuring the displaced water, the produced
volume of hydrogen could be recorded.
2. Experimental
2.1 Materials
Co(NO3)2$6H2O (99%) and Ni(NO3)2$6H2O (99%) were
purchased from Aladdin. Ammonia borane (90%) was
purchased from Aldrich. Methanol and 2-methylimidazole were
of analytical grade from the Sinopharm Chemical Reagent. All
kinds of nitrobenzenes were of analytical grade from the Energy
Chemical. All the reagents in this work were used without
further purication.
2.7 Hydrogenation of nitrobenzene
In a typical reaction, nitrobenzene (0.5 mmol) was charged into
a round bottom ask with ethanol (0.5 mL) and deionized water (5
mL). Then porous NiCo bimetallic catalyst (0.02 mmol) was
introduced. Aer ammonia borane (1 mmol) was added, the round
bottom ask was sealed immediately and the reaction mixture was
stirred at 25 ꢀC. The conversion and selectivity were determined by
GC using n-hexadecane (100 mL) internal standard.
2.2 Synthesis of ZIF-67
In a typical synthesis, 0.5 g Co(NO3)2 was dissolved in 15 mL of
methanol; then 0.5 g 2-methylimidazole was dissolved in 15 mL 3. Results and discussion
of methanol. Then both solutions were mixed and stirred for
3.1 Characterization of bimetallic NiCo porous materials
24 h at room temperature, then the resulting ZIF-67 solids were
separated by centrifuging and washed with methanol for 3
The synthetic route of NiCo bimetallic catalyst is schematically
shown in Scheme 1. The corresponding structure model for the
ZIF-67-Co rhombic dodecahedron is schematically illustrated.
ZIF-67 was chosen for its high content of cobalt, which could be
promising precursor for the synthesis of Co nanoparticles or
metal oxide. Ni(NO3)2 was introduced as etching agents to
transform the ZIF-67 polyhedrons into hollow nanospheres,
and irregular nanosheets were subsequently generated with
further increase the Ni salt. Aer pyrolysis in nitrogen atmo-
sphere at high temperature, the Ni2+ and Co2+ were reduced in
situ into bimetallic NiCo phase. Meanwhile, the organic ligand
decomposed into gaseous molecule and released at high
temperature under owing nitrogen.
ꢀ
times, and nally dried at 50 C at vacuum for 24 h.
2.3 Synthesis of Ni-ZIF-67
The obtained ZIF-67 (0.1 g) was dispersed in 30 mL methanol
and 0.1 g Ni(NO3)2 with 10 mL methanol was introduced, then
stirred for 24 h at room temperature. The Ni-ZIF-67 was ob-
tained aer centrifuging and washing with methanol for 3
times. The resulting Ni-ZIF-67 solids were nally dried at 50 ꢀC
at vacuum for 24 h.
2.4 Synthesis of bimetallic NiCo porous materials
100 mg of Ni-ZIF-67 solid was placed into a quartz boat, which
was charged into the middle of a quartz tube in a furnace with
a continuous nitrogen ow of 50 mL minÀ1. The furnace was
heated from room temperature to the targeted temperature
À1
ꢀ
ꢀ
(600–900 C) with a programmed heating rate of 2.5 C min
.
Aer the pyrolysis for 2 hours, the sample was naturally cooled
down to room temperature. The resultant samples were deno-
ted as NiCo-T (T denotes as pyrolysis temperature).
Scheme 1 Schematic illustration of the synthetic route to porous
NiCo bimetallic catalyst.
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Through the characterization by Transmission Electron NiCo porous material. Almost no carbon elements and nitrogen
Microscopy (TEM), ZIF-67 with polyhedrons shape and smooth elements can be detected, which is consistence with the
surface which have a diameter of 700 nm and an edge length of previous inference about the disappearance of organic moiety.
about 500 mm could be obtained according to reported The above characterization on the NiCo architecture reveals
methods (Fig. 1a).15 Fig. 1a shows that the architectures are that pores in the architecture are actually constructed by the
constructed from many wrinkled nanosheets. Based on the highly packed NiCo nanoparticles.
incremental amount of Ni salt, ZIF-67 polyhedrons were rstly
To further study the conversion of ZIF-67 to Ni-ZIF-67 aer
etched into hollow nanospheres and then destroyed by more Ni adding Ni(NO3)2, the phase structure of the samples was
salt into nanosheets (Fig. S1†). Aer pyrolysis of the eventually analyzed by X-ray diffraction (XRD). The X-ray diffraction (XRD)
2+
obtained Ni-ZIF-67 at 800 C, the Ni and Co2+ were simulta- patterns conrmed high crystallinity and zeolite type structure
ꢀ
neously reduced into NiCo crystalline, which exhibit the of ZIF-67 (Fig. S1†), and the diffraction peaks are identical to its
synthesized architectures are composed of sheet-like nano- corresponding simulated pattern. Aer the introduction of Ni
structures. Interestingly, different from the reported work about salt, the XRD pattern of product changed notably with gradually
the organic ligand of ZIF-67 convert into nitrogen doped carbon increasing the amount (Fig. S2†). The characteristic peaks of
materials, we found that no carbon was formed and only ZIF-67 disappears, and the appearance of a new narrow
metallic NiCo was produced. The magnied TEM image diffraction peaks at 10ꢀ was found. Aer calcined at 800 ꢀC, all
(Fig. 1b) conrms the nanoporous structures of these nano- of the detectable diffraction peaks agree well with standard data
sheets. The NiCo bimetallic material displays the honeycomb- of cubic phase NiCo, and no other phases can be detected,
like microstructures with pores, in which small NiCo nano- indicating that the as-obtained calcined product is pure crys-
particles interconnect with each other, forming a large number talline (Fig. 2a). Moreover, with increasing calcination temper-
of pores, and interstitial structures on their exterior surfaces. ature, the shape of these diffraction peaks become sharper and
Based on the characterization results, it is reasonably supported narrower. It is conrmed that the successful conversion of Ni-
that the pyrolysis of the Ni-ZIF-67 would induce thermal ZIF-67 to cubic phase NiCo aer thermal treatment based on
decomposition of organic ligands and the release of a mass of above HRTEM and XRD results. To evaluate the specic surface
gaseous molecule from the interior of the nanosheets, thus areas and porosity of the obtained porous NiCo bimetallic
leading to the formation of porosity le inside the particles. The catalysts, the N2 adsorption–desorption isotherms and pore
High Resolution Transmission Electron Microscopy (HRTEM) sizes were shown in Fig. 2b. The BET surface areas is 184.2 m2
image (Fig. 1f) shows the lattice fringes with interfringe gÀ1, and micropore size centered at 1.2 nm.
distances of 0.259 nm and 0.224 nm, which is between the
The chemical and electronic states of the NiCo architecture
characteristics of Ni and Co crystal phases in the (200) plane were further determined by X-ray photoelectron spectroscopy
and (220) plane respectively. The energy-dispersive X-ray spec- (XPS). All of the binding energies in the XPS analysis were cor-
troscopy (EDS) mapping prole provides evidence of a homo- rected for specimen charging by referencing them to the C 1s
geneous distribution of nickel and cobalt (Fig. 1g) in bimetallic peak (284.6 eV). Fig. 2c shows the high-resolution Ni 2p spec-
trum. It presents two prominent peaks at 852.4 and 867.5 eV,
assigned to the Ni 2p3/2 and Ni 2p1/2 peaks respectively. Further
deconvolution of the Ni 2p spectra yields two tting peaks at
Fig. 1 (a) TEM image of representative ZIF-67 nanocrystals; (b) TEM
image of Ni-ZIF-67 with Ni/Co ratio 1 : 1; (c) TEM image of NiCo Fig. 2 (a) The XRD patterns of the samples resulting from annealing
bimetallic materials obtained from pyrolysis of Ni-ZIF-67 at 800 ꢀC; (d Ni-ZIF-67 at various temperatures; (b) N2 adsorption/desorption
and e) the magnified TEM images of NiCo bimetallic materials; (f) isotherms and pore distribution of porous NiCo bimetallic catalyst
HRTEM image of NiCo bimetallic materials; (g) EDS-mapping of NiCo obtained at 800 ꢀC; XPS spectra of Ni 2p peaks (c) and Co 2p peaks (d)
bimetallic materials.
from Ni1Co1 bimetallic porous materials annealing at 800 ꢀC.
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binding energies of 852.2 and 867.2 eV are ascribed to Ni0, while
another two tting peaks at 854.0 and 869.2 eV are attributed to
Ni2+. Fig. 2d shows the deconvolution of high-resolution Co 2p
spectrum, and the Co 2p spectra yields two tting peaks at
binding energies of 779.9 and 794.9 eV are ascribed to Co0,
while another two tting peaks at 781.9 and 796.9 eV are
attributed to Co2+. Therefore, the main valence state of NiCo
bimetallic porous materials is zero with partial oxidation on the
surface when exposing to air.
3.3 Catalytic activity in hydrogenation of nitrobenzene
To demonstrate the general applicability of nitroarenes reduction
of the porous NiCo bimetallic catalyst, we explored the hydro-
genation of various nitroarenes with reducible functional groups
through AB dehydrogenation (Table 1). Cyano group substituted
nitrobenzene at para-position converted into aniline by nearly
100% conversion and selectivity (entry 1). The reduction of ester
or amide substituted nitrobenzene produced corresponding
anilines with 88% or 85% conversion (entries 2 and 3). Particu-
larly, the most-challenging nitrobenzene with ethynyl or vinyl
substituted could also be reduced into ethynyl or vinyl aniline
with 97% or 99% conversion and near 100% selectivity (entries 4
and 5). Halogenated nitrobenzenes, such as chloro-, bromo-
substituted, were converted into halogenated anilines in excel-
lent conversion (89–95%) and selectivity (95–98%) (entries 6 and
7). Moreover, 3-nitropyridine compound could also be reduced
into the 3-aminopyridine which is an important building blocks
in pharmaceuticals and agrochemicals with 98% conversion and
100% selectivity (entry 8). The above results show that the porous
3.2 Catalytic activity in dehydrogenation of ammonia borane
The as-prepared porous NiCo bimetallic catalysts were tested for
the hydrolytic dehydrogenation of AB. For the Ni1Co4 catalysts,
a rapid (20 minutes) and nearly linear hydrogen evolution curves
was obtained without observable induction period (Fig. 3a). As
the catalyst obtained from pyrolysis of pure ZIF-67 (denoted as
Co–N–C, Fig. S3 and S4†) was employed in the hydrogen gener-
ation of aqueous AB, moderate activities for the hydrolysis was
found (60 minutes). The precursors were also studied as catalyst
for the hydrolytic dehydrogenation of AB. The pure Ni(NO3)2 and
ZIF-67 show little activity and negligible activities respectively.
The difference in the activity may be ascribed to the introduction
of Ni in Co–N–C structure and the electronic effects between Ni
and Co. Further reasonable modication of the metal ratios
showed that the reaction time decreased with the increasing of Ni
amount, and the Ni1Co1 could complete the reaction during just
7 minutes (Fig. 3b). The temperature dependence of the catalytic
Table 1 The hydrogenation of functionalized nitrobenzene through
ammonia borane dehydrogenation with porous NiCo bimetallic
catalyst
ꢀ
reaction was studied in the range 25–35 C, with initial AB and
Entrya
1
Substrate
Product
Con.b (%)
99
Sel.b (%)
100
Ni1Co1 concentrations of 1 and 0.05 mmol, respectively; the
results are shown in Fig. 3c. When the hydrogen generation of
aqueous AB was initiated at 30 ꢀC and 35 ꢀC, the reaction could
be completed at three and two minutes.
2
88
100
3
4
85
97
100
100
5
6
99
89
99
95
7
95
98
98
Fig. 3 (a) Volume of hydrogen evolved versus reaction time for AB
dehydrogenation reaction (1 mmol) catalyzed by the different cata-
lysts; (b) volume of hydrogen evolved versus reaction time for AB
dehydrogenation reaction catalyzed by the as-prepared bimetallic
NiCo porous catalysts with different Ni/Co ratio; (c) volume of
hydrogen evolved versus reaction time for AB dehydrogenation
reaction catalyzed by the as-prepared bimetallic Ni1Co1 porous cata-
lysts with different temperatures.
8
100
a
Reaction condition: 0.5 mmol nitroarene, 0.02 mmol catalyst,
ammonia borane (1 mmol), 25 ꢀC, 2 hours, 10 : 1 water–ethanol (5.5
mL). Conversion and selectivity were determined by GC using n-
hexadecane (100 mL) internal standard.
b
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