Superfine CoNi alloy embedded in Al2O3 nanosheets for efficient tandem catalytic reduction of nitroaromatic compounds by ammonia borane

Article abstract of DOI:10.1039/c9dt03838h

Aromatic amino compounds are important and universally used chemical intermediates in a wide range of industrial fields. Thus, their production with high efficiency and selectivity under ambient conditions is expected and demanded in modern industry. Herein, a series of superfine CoNi alloy nanoparticles embedded in Al₂O₃ nanosheet (Co_xNi_1-x/Al₂O₃, where x represents the content of Co in the precursor) catalysts was fabricated from CoNiAl-LDH and used to catalyze the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of nitroaromatics to the corresponding amines. Systematic experiments indicate that the composition, size, morphology and catalytic performance of the Co_xNi_1-x/Al₂O₃ catalysts can be easily controlled by changing the content of Ni in the CoNiAl-LDH precursor. Particularly, Co_0.67Ni_0.33/Al₂O₃ exhibited the best tandem catalytic performance among the six samples. This as-prepared catalyst not only showed a moderate turn-over-frequency value (TOF: 34.5 mol_H2 molCo_0.67Ni_0.33^-1 min^-1 at 298 K without base or additives) and relatively low activation energy (32.4 kJ mol^-1) for the dehydrogenation of AB, but also superior catalytic activity (conversion yield reaching up to 100%) and selectivity (>99%) for the tandem reductive transformation of in excess of sixteen types of nitroaromatics to aromatic amines. Density functional theory (DFT) calculations suggest that the construction of the CoNi alloy optimized the electronic structure with respect to the pure component, promoting its activity for AB hydrolysis and nitroaromatics hydrogenation. Finally, the catalyst could be easily recycled using a magnet due to the magnetic properties of the Co_0.67Ni_0.33 alloy.

Full text of DOI:10.1039/c9dt03838h

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Superfine CoNi alloy embedded in Al O
2
3
nanosheets for efficient tandem catalytic
reduction of nitroaromatic compounds by
ammonia borane†
Cite this: Dalton Trans., 2019, 48,
17499
Sihang Cheng, Yanchun Liu, Yingnan Zhao, Xinyu Zhao, Zhongling Lang,*
Huaqiao Tan, * Tianyu Qiu and Yonghui Wang*
Aromatic amino compounds are important and universally used chemical intermediates in a wide range
of industrial fields. Thus, their production with high efficiency and selectivity under ambient conditions is
expected and demanded in modern industry. Herein, a series of superfine CoNi alloy nanoparticles
2 3 x 2 3
embedded in Al O nanosheet (Co Ni1−x/Al O , where x represents the content of Co in the precursor)
catalysts was fabricated from CoNiAl-LDH and used to catalyze the tandem dehydrogenation of ammonia
borane (AB) and hydrogenation of nitroaromatics to the corresponding amines. Systematic experiments
indicate that the composition, size, morphology and catalytic performance of the Co
can be easily controlled by changing the content of Ni in the CoNiAl-LDH precursor. Particularly,
exhibited the best tandem catalytic performance among the six samples. This as-prepared
x 2 3
Ni1−x/Al O catalysts
Co0.67Ni0.33/Al
catalyst not only showed a moderate turn-over-frequency value (TOF: 34.5 molH2 molCo0.67Ni0.33 min−1 at
98 K without base or additives) and relatively low activation energy (32.4 kJ mol−1) for the dehydrogenation
2 3
O
−1
2
of AB, but also superior catalytic activity (conversion yield reaching up to 100%) and selectivity (>99%) for the
tandem reductive transformation of in excess of sixteen types of nitroaromatics to aromatic amines. Density
functional theory (DFT) calculations suggest that the construction of the CoNi alloy optimized the electronic
structure with respect to the pure component, promoting its activity for AB hydrolysis and nitroaromatics
hydrogenation. Finally, the catalyst could be easily recycled using a magnet due to the magnetic properties of
the Co0.67Ni0.33 alloy.
Received 28th September 2019,
Accepted 5th November 2019
DOI: 10.1039/c9dt03838h
rsc.li/dalton
the hydrogenation reaction occurring under high pressure and
high temperature with relatively low reaction efficiency.
Introduction
Aromatic amino compounds are important chemical raw Aiming to improve the efficiency and soften the reaction con-
materials and intermediates, which are widely used for the ditions, tandem in situ hydrogen evolution and catalytic hydro-
production of medicines, pesticides, dyes and other fine genation of nitroaromatics will be an ideal and promising
1
–6
14–25
chemicals.
They are generally prepared via the hydrogen- solution.
Based on this consideration, recently, hydrogen
7
–10
26,27
28,29
ation reduction of nitroaromatics,
metal- and hydrogen-based reduction routes are most com- NaBH4,
where particularly the storage materials (such as NH BH ,
NH NH ·H O,
2 2 2
3
3
3
0,31
32,33
and HCOOH
) were found to decompose under
monly used. However, these two approaches generate masses mild conditions to produce active hydrogen, and then directly
2
+
of waste residue and heavy metal pollutants (such as Fe or hydrogenate the substrates in situ in a homogeneous reaction
2
+
Zn ), and rely on expensive noble-metal catalysts (Pt, Pd, etc.) system. This not only provides a safe and green strategy for the
1
1–13
for direct hydrogenation with hydrogen gas.
In addition, reduction of nitroaromatics, but also boost the reaction
the poor solubility of H in various solvents always results in efficiency. However, the main challenge is to choose a suitable
2
hydrogen source and construct efficient tandem catalysts.
3 3
Ammonia borane (AB, NH BH ), as a desirable hydrogen
Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of
Chemistry, Northeast Normal University, Changchun, 130024, China.
storage material, is non-toxic and possesses excellent hydrogen
capacity (19.6 wt%), stability and solubility, making it a good
E-mail: langzl554@nenu.edu.cn, tanhq870@nenu.edu.cn, wangyh319@nenu.edu.cn
34–37
candidate as a hydrogen source.
However, tandem cata-
†
Electronic supplementary information (ESI) available: Additional experimental
lysts, which can catalyze both the dehydrogenation of AB and
hydrogenation of nitroaromatics, have been largely unex-
and theoretical section, additional figures and tables. See DOI: 10.1039/
c9dt03838h
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plored. Although a few noble metal-based catalysts have been avoid the waste of AB. Also, it displayed superior catalytic
1
5,19,20,38
reported,
their scarce resources and expensive cost activity and high selectivity for the tandem reductive trans-
extremely restrict their large-scale applications. Transition formation of nitroaromatic compounds (NACs) to aromatic
metal-based (such as Co, Ni and Cu) nanocatalysts, especially amines with conversion yields close to 100% and selectivity
Co and Ni and their multicomponent complexes, have demon- >99% within 30 min. In addition, owing to the magnetic pro-
strated good performances as tandem catalysis. Furthermore, perties of the Co0.67Ni0.33 alloy, the catalyst could be easily
they possess magnetic characteristics and can be easily separ- recycled using a magnet. The recycle experiments reveal that it
2
6,39–43
ated during the catalytic reaction.
Nevertheless, these also possesses good stability and durability for the tandem cat-
TMs nanoparticles (NPs) inevitably aggregate and become oxi- alysis, and its composition, morphology and catalytic activity
4
4
dized due to their high surface energy,
making them showed negligible change after 10 runs. The DFT calculation
unstable and rapidly deactivated. To boost the catalytic activity results suggest that the alloy CoNi structure can optimize the
and durability, fabricating low-cost multimetallic nanocatalysts electronic structure of pure Ni and Co surface, further promot-
with special catalytic reactivity and optimized electronic struc- ing the dehydrogenation of AB and selective reduction of nitro-
4
5–48
tures by confining them on suitable supports
is a feasible benzene to amine. The superior catalytic activity and excellent
demonstrate that it is a promising
method. Layered double hydroxides (LDHs) have been estab- stability of Co0.67Ni0.33/Al
2 3
O
lished as excellent precursors to construct metal-based cata- candidate for the highly efficient and selective catalytic
lysts with many advantages, such as tunable chemical com- preparation of aromatic amino compounds.
ponents, ordered and uniform metal cation dispersion within
4
9–51
their layers, controllable particle size, and morphology.
Furthermore, LDHs have also been extensively used in cataly-
sis, and deemed to be good supports for loading and stabiliz-
ing various metal catalysts. These benefits inspired us to use
Results and discussion
Preparation and characterization
an LDH as a precursor to construct multifunctional catalysts The preparation of the Co Ni /Al O composites involved a
x
1−x
2 3
for coupling the dehydrogenation of AB and the hydrogenation two-step process, as displayed in Fig. 1. Firstly, Co Ni1−xAl-
x
5
2,53
of nitroaromatics.
LDH sheets were prepared by assembling three transition
Herein, we report a series of CoNi alloy NPs embedded on metal (Co, Ni and Al) nitrates via urea-assisted co-
5
4–56
2+
2+
Al
2
O
3
nanosheet (Co
x
Ni1−x/Al
2
O
3
, where x represents the precipitation.
Ni and Co ions could be simultaneously
content of Co in the precursor) catalysts via the thermal hydrolyzed in an alkaline solution caused by urea decompo-
3
+
reduction of the corresponding CoNiAl-LDH precursors sition, and co-precipitated with Al to form LDH precursors
denoted as Co Ni1−xAl-LDH) under an H /Ar mixed atmo- due to the similar Ksp of their hydroxides. As the Ni content in
(
x
2
sphere. The active metal components (Co and Ni) could be pre- the precursor increased, the color changed from pink to light
cisely tuned because of the similar Ksp values of hydroxide Co green (Fig. S1†). Powder X-ray diffraction (PXRD) revealed that
and Ni. Besides composition, the size, morphology and cata- all the precursors exhibit similar typical LDH structures
lytic performance of these Co
x 2 3
Ni1−x/Al O catalysts could also (Fig. S2†). The peaks at 11.5°, 23.1° and 34.6° can be assigned
be easily controlled by changing the content of Ni in precur- to the (0 0 3), (0 0 6) and (0 1 2) planes of the LDHs, respect-
5
7
sors. Among the studied samples, the Co0.67Ni0.33/Al
2
O
3
ively. With an increase in the Ni content in the structure, the
species exhibited the best catalytic performance. It possesses a peaks at 11.5° and 23.1° slightly shifted toward a lower angle,
−
1
−1
moderate TOF value (34.54 molH2 molCo0.67Ni0.33
2
min
at and two new peaks of NiAl-LDH emerged at about 60.8° and
−
1
98 K) and relatively low activation energy (32.4 kJ mol ) for 62.1° (Fig. S2†), implying that Ni was successfully incorporated
the dehydrogenation of ammonia borane (AB), which is advan- into the LDH precursors. Furthermore, scanning electron
tageous for the subsequent tandem hydrogenation reaction to microscopy (SEM) and transmission electron microscopy
make full use of the active hydrogen generated in situ and (TEM) indicate that the diameter and thickness of these
Fig. 1 Schematic illustration of the fabrication route of Co
x
Ni1−x/Al
2
O
3
nanosheet difunctional catalysts (x represents the content of Co).
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Co
x
Ni1−xAl-LDH sheets decreased as the content of Ni resolution TEM revealed that the lattice spacing of the diffrac-
increased (Fig. S3 and S4,† respectively). Specifically, the com- tion fringes is about 0.21 nm (Fig. 2c), which can be assigned
5
7–59
position and size of the LDH precursors could be finely tuned to the (1 1 1) plane of Co0.67Ni0.33
.
The TEM elemental
2
+
2+
by adjusting the ratio of Co /Ni . Subsequently, the LDH pre- mapping (Fig. 2d and e) further displays that Co, Ni, Al and O
cursors were further reduced at 650 °C in an H /Ar atmosphere are distributed in the sheets, suggesting the composition of
2
to produce the corresponding alumina-loaded CoNi alloy cata- the Co0.67Ni0.33/Al
lysts (Co Ni1−x/Al ). The catalytic ammonia borane mappings of the Co0.67Ni0.33 nanoparticles also demonstrate
decomposition and nitrobenzene hydrogenation experiments the mixed distribution of Co and Ni elements, further verifying
demonstrate that the optimized Co0.67Ni0.33/Al possesses the structure of the Co0.67Ni0.33 alloy. The accurate elemental
the best catalytic performance, and therefore the characteriz- composition of Co0.67Ni0.33/Al was determined by EDX and
ation of Co0.67Ni0.33/Al O is discussed as a representative inductively coupled plasma atomic emission spectroscopy
2 3
O composite. Meanwhile, the elemental
x
2 3
O
2 3
O
2 3
O
2
3
example.
The morphology of Co0.67Ni0.33/Al
(ICP-AES). The ratio of Co/Ni is 0.669/0.331 (Fig. 2f), which is
was characterized in consistent with the initial dosage ratio of precursor used in
2
O
3
detail by SEM and TEM. As shown in Fig. 2a and S3c,† the two- the synthesis. Similar results were achieved in the preparation
dimensional (2D) sheet architecture of the precursors was well of another Co Ni1−x/Al , as displayed in Table S1.† The ratio
maintained in Co0.67Ni0.33/Al after H /Ar reduction under of Co/Ni could be finely regulated by the feed ratio owing to
high temperature. The sheets are several hundred nanometers the similar Ksp of their hydroxides.
x
2 3
O
2
O
3
2
in size, and about 5.5 nm in thickness. The TEM images
The structural change in the precursor was validated by
exhibit a more detailed structure of the sheets. As displayed in XRD (Fig. 3a and Fig. S2c†). The peaks at 11.5°, 23.1° and
Fig. 2b, many irregular CoNi alloy nanoparticles are well 34.6° are assigned to the (0 0 3), (0 0 6) and (0 1 2) planes of
embedded in the amorphous Al
diameter of these particles is approximately 6.2 ± 1 nm (Fig. 2b hydroxide structure. After H
and inset). In comparison, the size of monometallic Co in Co/ appeared. Meanwhile, some new diffraction peaks emerged at
Al and Ni in Ni/Al are 27.5 ± 4 nm and 9 ± 2 nm, 44.35°, 51.46° and 76.04°, which can be indexed to the (1 1 1),
respectively, which is obviously larger than the alloy particles (2 0 0), (2 2 0) crystal planes of the CoNi alloy, respectively.
Fig. S5†). This means that the alloy of Co0.67Ni0.33 has more Furthermore, the main peak at 44.35° is located between the
2
O
3
nanosheets. The average the precursor Co0.67Ni0.33Al-LDH, indicating the layered
2
reduction, the above peaks dis-
2
O
3
2 3
O
(
0
0
active sites than the monometallic Co or Ni particles. High- characteristic peaks of metallic Co and Ni , further verifying
2 3
Fig. 2 (a) SEM image, (b) TEM image and inset image of the particles size distribution of Co0.67Ni0.33/Al O . (c) HRTEM image confirming formation
of a discrete CoNi alloy NPs. (d–e) TEM image of Co0.67Ni0.33/Al
of Co0.67Ni0.33/Al
2
O
3
and the corresponding elemental mappings of Co, Ni, Al, and O. (f) EDX spectra
O .
2 3
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6
2–65
inevitable surface oxidation upon exposure to the air.
In
the high-resolution Al 2p XPS (Fig. S10b†), the peak at 74.6 eV
3
+
can be attributed to Al . Also, according to the literature, the
preparation conditions and some related results of XRD and
3
+
XPS, it can be determined that Al mainly exists in the form
4
9,52,56,66
of low crystalline or amorphous Al
2
O
3
.
Considering
the above results, the Co0.67Ni0.33 alloy NPs embedded in thin
Al O nanosheets were successfully prepared and further
2
3
employed in the following catalytic reactions.
Catalytic performance of the catalysts towards NH
decomposition
3 3
BH
The experiment for the catalytic decomposition of ammonia
borane (AB) to produce hydrogen was carried out in a self-
made reactor with a gas collector. As displayed in Fig. S11 and
S12a,† the self-decomposition of ammonia–borane in aqueous
solution was negligible in the absence of catalysts. With the
Fig. 3 (a) XRD pattern of the Co0.67Ni0.33/Al
2
O
3
sample (the positions of addition of Co
Ni1−x/Al
O
, the decomposition of ammonia–
and Ni/Al
10 mg) catalysts were able to decompose AB (0.12 M, 10 mL)
x
2
3
the diamonds and squares represent the diffraction peaks of alumina
and Co0.67Ni0.33 alloy, respectively). (b) adsorption isotherm of
Co0.67Ni0.33/Al at 77 K (filled and open symbols represent adsorption
and desorption branches, respectively). (c–d) High-resolution XPS
spectra of Co0.67Ni0.33/Al : (c) Co and (d) Ni.
borane was boosted rapidly. The Co/Al
(
completely, yielding about 87 mL of H at 313 K in 5.75 and
2
5.05 min, respectively. Under the same conditions, the cata-
lytic activity of Co Ni /Al O increased first and then
2
O
3
2 3
O
N
2
2
O
3
2
O
3
x
1−x
2 3
decreased with an increase in nickel content, which may be
related to the optimization of the electronic structure and mor-
4
7
the alloy composition (Fig. S8†). These results are consistent phology changes of the alloy. Among them, Co0.67Ni0.33
with the HRTEM and elemental mapping results. Except for Al O exhibited the best catalytic performance for the
/
2
3
the alloy peaks, some weak peaks at 36.5° and 45.9° can be decomposition of AB. AB could be completely decomposed
assigned to low crystallinity Al O (JCPDF: 46-1215). The nitro- within 0.98 min. Its initial turnover frequency (TOF) was up to
2
3
−
1
−1
gen adsorption–desorption isotherms (Fig. 3b and Fig. S9†) 93.9 molH2 molCo0.67Ni0.33 min without any external protec-
give the BET surface areas of Co/Al , Ni/ tion or additives. Then, the decomposition kinetics and the
Al O , which are 116.3, 126.6 and 149.6 m g , respectively. recycle experiments of NH BH catalyzed by Co0.67Ni0.33/Al O
2 3 2 3
O , Co0.67Ni0.33/Al O
2
−1
2
3
3
3
2 3
With an increase in the Ni content, the specific surface area of were also systematically studied (see AB dehydrogenation
the corresponding samples increased, which is in good agree- section in ESI†).
ment with the SEM results. Then, X-ray photoelectron spec-
troscopy (XPS) was employed to further determine the compo-
sitions and element chemical states of Co0.67Ni0.33/Al O . The
2 3
Catalytic performance evaluation for the hydrogenation of
nitroaromatic compounds
full spectrum reveals that Co, Ni, Al and O coexist, confirming Owing to its opportune and durable catalytic activity for the
the chemical composition of Co0.67Ni0.33/Al (Fig. S10†). As dehydrogenation of AB, Co0.67Ni0.33/Al O was used as a good
2 3
O
2
3
shown in Fig. 3c, the high-resolution Co 2p XPS exhibits two candidate for the subsequent hydrogenation of nitroaromatic
peaks at 780.5 eV and 796.6 eV, which are ascribed to Co 2p3/2 compounds (NACs). The tandem reaction was performed gen-
and 2p1/2, respectively.
erally in 10 mL mixed solvent (vmethanol/vwater: 2 : 3 mL/mL)
These peaks can be split into three sets of peaks at 778.4 containing 3 mg of Co0.67Ni0.33/Al O and 0.1 mmol of nitro-
2
3
eV/794.1 eV, 780.4 eV/796.3 eV and 782.1 eV/797.7 eV, which benzene (NB) (cat/NB was set at 0.163 : 1 mol/mol) at 323 K.
0
2+
3+
correspond to Co , Co and Co , respectively. In addition, the The control experiments displayed that NB hardly reacted with
peaks at 785.6 eV and 802.4 eV can be assigned to the satellite AB in the absence of the Co Ni /Al O catalysts. No aniline
x
1−x
2 3
peak of Co. Similarly, in the Ni 2p XPS, the two peaks at 852.7 (AN) was detected after 9 h (Table S4,† entry 1). When AB was
and 869.3 eV (Fig. 3d) can be attributed to Ni (0) 2p3/2 and replaced by H as the hydrogen source, the hydrogenation of
2
2p1/2, while the peaks at 855.5 eV and 872.8 eV correspond to NB was extremely slow. Its conversion was only about 6%
2
+
57,59–61
Ni 2p3/2 and 2p1/2, respectively.
Also, its corresponding within 9 h (Table S4,† entry 2), which may be related to the
satellite peaks are located at 861.1 eV and 878.5 eV. The pres- poor solubility of H2 limiting the full contact between reac-
ence of metallic Co and Ni confirms the that Co0.67Ni0.33 alloy tants and catalysts. In contrast, when AB was used as the
was successfully formed under an H /Ar atmosphere at 650 °C, hydrogen source, the reductive transformation of NB to AN
2
which is consistent with the corresponding H
programmed reduction (H -TPR) results (Fig. S6 and S7†). The than 99% in 15 min (Table S4,† entry 3 and Table 1, entry 1).
oxidation states such as Co , Co and Ni result from the This is significantly superior to the traditional hydrogenation
2
temperature- was sharply boosted. The conversion and selectivity was higher
2
2
+
3+
2+
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Table 1 Co0.67Ni0.33/Al
2
O
3
-catalyzed tandem reaction of various strates with two methyl groups had a similar conversion
efficiency as that with one methyl group. Conversely, when the
a
2
R–NO compounds
Co0:67Ni0:33=Al2O3
substituents were electron-withdrawing groups, the tandem
hydrogenation of NACs was relatively slower.
R ꢀ NO2 ꢀꢀꢀꢀꢀꢀꢀꢀꢀ! R ꢀ NH2
MeOH=H2O;AB;323 K
c
Entry
Substrate
Product
Yield (%)
>99
Time (min)
As shown in entries 13–16, when halogen (F-, Cl- and Br-)
and nitro was the substituent, the corresponding hydrogen-
ation reaction took about 20–30 min, which is clearly slower
than that of NB. Specifically, Co0.67Ni0.33/Al O has excellent
1
2
15
5
>99
2
3
and universal catalytic activity for the tandem in situ dehydro-
genation of AB and hydrogenation of various NACs. In
addition, after the tandem catalysis, Co0.67Ni0.33/Al O could
3
4
5
>99
>99
>99
6
6
6
2
3
also be easily separated using a magnet (Fig. S11†). As shown
in Fig. S18,† the stability and reusability of the Co0.67Ni0.33
Al O catalyst was investigated taking the tandem hydrogen-
/
2
3
6
7
8
9
>99
>99
>99
>99
6
10
10
10
ation of 4-methoxynitrobenzene as an example. After 10 cycles,
the catalytic activity and selectivity of Co0.67Ni0.33/Al for the
2 3
O
tandem dehydrogenation of AB and hydrogenation of NACs
exhibited a negligible decrease. Meanwhile, TEM and XRD after
cycling also indicate that the morphology and composition of
Co0.67Ni0.33/Al O were also well maintained (Fig. S19c and
2
3
1
1
0
1
>99
>99
10
10
S19d,† respectively), demonstrating the outstanding stability of
. The excellent catalytic activity, good stability,
2 3
Co0.67Ni0.33/Al O
mild reaction conditions and simple recyclability of the catalyst,
as confirmed by the above experiments, endow this catalyst
promising application in the clean, efficient and selective prepa-
ration of aromatic amino compounds.
1
2
>99
15
1
1
1
1
3
4
5
6
>99
>99
>97
>97
20
25
30
30
Theoretical investigation
A theoretical simulation based on the density functional
theory (DFT) method was performed to give deep insight into
the promotion effect of the CoNi alloy NPs on the decompo-
sition of AB and hydrogenation of nitrobenzene to aniline. A
CoNi mixed slab with a Co : Ni ratio of 2 : 1 based on the (111)
facet was applied to approximately model the Co0.67Ni0.33 com-
posite in experiment. For comparison, results on Ni(111) and
Co(111) were also considered. The computational details are
supplied in the ESI.† For metal NP-mediated AB dehydrogena-
tion, three fundamental steps are generally involved: (i) co-
b
a
General reaction conditions: 0.1 mmol NACs, 0.4 mmol NH
mg Co0.67Ni0.33/Al (32%, 0.0163 mmol cat), 10 mL of methanol/
3 3
BH ,
3
2 3
O
b
water (v/v = 2/3), 323 K. Reaction conditions: 0.1 mmol 1, 3-dinitro-
benzene, 0.6 mmol NH
determined via GC.
c
3
BH
3
.
The yields of aromatic amines were
adsorption of H
2 3 3
O and NH BH on the catalyst; (ii) adsorbed
route using H
2
as the hydrogen source, indicating the advan- H O decomposes into OH* and H*; (iii) the formed OH*
2
tage of the tandem in situ dehydrogenation of AB and hydro- attacks the B–N bond in the AB molecule to produce the
genation of NACs. To verify the universality of this tandem cat- BH OH* intermediate. Our DFT results in Table S5† indicate
3
alysis, the hydrogenation of a series of NACs with different the co-adsorption of H O and AB molecules on all the selected
2
substituents was further studied. As illustrated in Table 1, the surfaces is exothermic by releasing energy of 0.95 eV on
reductive transformation of NACs was clearly accelerated due Ni(111), 1.30 eV on Co(111), and 1.23 eV on Co Ni(111). The
2
to the introduction of electron-donating groups (Table 1, two B–H bonds in AB adsorbed on Co Ni(111) through two Co
2
entries 2–11). For example, 4-methoxynitrobenzene was quanti- atoms are 0.16 eV more stable than that on Ni (Fig. 4a). This
tatively converted to 4-methoxyaniline within 5 min (Table 1, favorable process provided sufficient reaction precursors for
entry 2) using 3 mg of Co0.67Ni0.33/Al
2 3
O as the catalyst. As the following steps. We then focused on the water activation
shown in entries 6–8, the steric hindrance effect of substitu- process, which has been established as the rate-determining
6
7
ents also had some influence on the above hydrogenation reac- step (RDS) in previous reports. As illustrated in Fig. 4b, the
tions. When the methyl was located at the ortho or meta-posi- kinetic barrier for H–OH bond splitting decreased to 0.86 eV
tion, it took about 10 min for the transformation of NACs to on Co Ni(111), which is 0.11 and 0.07 eV lower than that on
2
the corresponding anilines, which is obviously lower than that pure Ni(111) and Co(111), respectively. This difference may be
of its para-isomer. However, as displayed in entries 9–11, the attributed to the optimized electronic property in the alloy. We
effect of this steric hindrance was relatively limited. The sub- examined the d-band center for the three clean surfaces, and
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Conclusions
In summary, a series of CoNi alloy NPs embedded in Al
nanosheets (Co Ni1−x/Al ) was prepared via the simple
reduction of CoNiAl-LDH precursors. Co Ni /Al O showed
2 3
O
x
2 3
O
x
1−x
2 3
an efficient catalytic performance for the dehydrogenation of
ammonia borane (AB, NH BH ) and the tandem hydrogen-
ation of nitroaromatics. The composition, size, morphology
and catalytic performance of these Co Ni1−x/Al catalysts
could be well controlled by adjusting the content of Ni in the
precursors. Among the samples, the optimized Co0.67Ni0.33
Al exhibited the best tandem catalytic performance. It not
only possessed moderate TOF value (34.54 molH2
3
3
x
2 3
O
/
2 3
O
a
−
1
−1
molCo
min at 298 K) and relatively low activation
0
.67Ni
0.33
−
1
energy (32.4 kJ mol ) for the dehydrogenation of AB, but also
exhibited superior catalytic activity and high selectivity for the
tandem reductive transformation of in excess of sixteen types
of nitroaromatics to the corresponding aromatic amines with
conversions close to 100% and selectivity >99% within 30 min.
In addition, these catalysts could be easily recycled using
magnets. The recycle experiments further demonstrated the
excellent stability and durability of the catalysts for the tandem
catalysis, which makes them promising candidates for the
2
Fig. 4 (a) Top view of the optimized geometry for AB and H O co-
adsorbed on Co
transition state representation for the RDS of AB hydrolysis (H
2
Ni (111). (b) DFT-computed energy potential profile and highly efficient and selective catalytic preparation of aromatic
2
O dis-
amino compounds. Finally, theoretical simulations helped to
prove that the alloy CoNi structure not only promotes the RDS
step of AB hydrolysis, but also optimizes the adsorption prop-
erty for the involved intermediates and further promotes the
selective reduction of nitroaromatics toward amines. This work
sociation) and (c) energy profile for the stepwise hydrogenation of
nitrobenzene.
our computed d-band center of Ni(111) and Co(111) are −1.42 provides an important reference for the rational design and
and −1.40 eV, respectively, while it is −1.45 eV for the alloy construction of efficient and stable tandem catalysts, and also
system. This downshift caused the difference in both the develops a feasible way to achieve the efficient and selective
adsorption and activation barrier. Consequently, the CoNi conversion of nitroaromatics to amines under mild
alloy is more beneficial for overcoming the barrier of the RDS conditions.
toward AB hydrolysis, which is consistent with our experi-
mental hydrogen evolution results, as mentioned above.
2
Instead of generating H from the H* intermediate on the
Experimental
Materials
surface, the adsorbed H is used for the hydrogenation reaction
in the presence of nitrobenzene. Therefore, a moderate H
adsorption ability is essential, where either too strong or too
weak adsorption can hinder its combination with nitro-
Ni(NO ) ·6H O, Co(NO ) ·6H O, Al(NO ) ·9H O, urea and
3 2
2
3 2
2
3 3
2
3 3
ammonia borane complex (NH BH , AB, 97%) were from
Aladdin Industrial Co., Ltd. Methanol and the nitroaromatic
compounds (e.g., nitrobenzene (NB), 4-nitrophenol, 4-methyl-
nitrobenzene, 4-fluoronitrobenzene, 4-chloronitrobenzene,
and 1,3-dinitrobenzene) used in the tandem reactions were
purchased from Sigma-Aldrich. All reagents were used as
received. Ultrapure water (18.2 MΩ cm resistivity) was
employed throughout all experiments, which was purified via
a Millipore system.
benzene. Here, it should be noted that ΔEH* on Co
found to be more promising (−0.43 eV) than the cases of Ni
111) (−0.67 eV) and Co(111) (−0.53 eV). This advantageous
but less strong interaction promotes the easier desorption of H
from the surface and boost the hydrogenation. As shown in
Fig. 4c, the DFT calculations demonstrated that nitrobenzene
can be stepwise hydrogenated to yield nitrosobenzene,
N-phenylhydroxylamine and aniline, with a reaction energy of
2
Ni(111) is
(
−
1.12, 0.15 and −1.63 eV on Co Ni(111), respectively. Specifically,
2
Catalyst synthesis
the formation of nitrosobenzene and N-phenylhydroxylamine is
largely promoted by the integration of Co and Ni metal due to
Co Ni1−XAl-LDH precursors. Co0.67Ni0.33Al-LDH nanosheets,
X
the different interactions between each intermediate and Co as a representative precursor, were synthesized via a typical
or Ni) (Table S5†). By calculation, we these three model systems urea assisted co-precipitation reaction procedure as
toward nitrobenzene reduction follow the catalytic activity order follows. Briefly, 2.5 mmol of Ni(NO ) ·6H O, 5 mmol of
(
3
3
2
of Co Ni(111) > Co(111) > Ni(111).
Co(NO ) ·6H O, 2.5 mmol of Al(NO ) ·9H O and 50 mmol of
3 2 2 3 3 2
2
17504 | Dalton Trans., 2019, 48, 17499–17506
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urea were dissolved in 100 mL of water. The resulting homo-
geneous solution was refluxed at 110 °C for 12 h under mag-
netic stirring. After cooling to room temperature naturally, the
resulting precipitant was collected by centrifugation, washed
several times with water and ethanol, and finally vacuum dried
at 60 °C for 12 h. Co Ni1−xAl-LDH nanosheets with different
x
atomic ratios of Co/Ni were synthesized using the same
approach, tuning the amount of metal nitrate precursors
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2
+
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Co Ni1−x/Al catalysts. Typically, the Co
cursors were calcined and reduced in an H /Ar (1/9, v/v) stream
at a temperature of 650 °C for 4 h, with a heating rate of 5 °C
x
2
O
3
x
Ni1−xAl-LDH pre-
2
−
1
min . When the temperature dropped to 25 °C, nitrogen gas
was bubbled through for 30 min, and then a black powder
sample was obtained (denoted as Co Ni1−x/Al O ). In the
process of precursor synthesis, except for the absence of
Co and Ni salts, all other conditions were consistent and as-
prepared Al O samples were further obtained.
x
2 3
9 J. Feng, S. Handa, F. Gallou and B. H. Lipshutz, Angew.
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2
3
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2 K. Wilson, Appl. Organomet. Chem., 2007, 21, 1002–1002.
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This work was financially supported by the National Natural
Science Foundation of China (grant no. 21771033, 21671036),
the Fundamental Research Funds for the Central Universities
1777–1782.
1
6 Ö. Metin, A. Mendoza-Garcia, D. Dalmızrak, M. S. Gültekin
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(
grant no. 2412018BJ001, 2412018ZD007 and 2412018QD005),
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1
(
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Development Project of Jilin Province (grant no.
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