Ni2P Nanoalloy as an Air-Stable and Versatile Hydrogenation Catalyst in Water: P-Alloying Strategy for Designing Smart Catalysts

Article abstract of DOI:10.1002/chem.202005037

Non-noble metal-based hydrogenation catalysts have limited practical applications because they exhibit low activity, require harsh reaction conditions, and are unstable in air. To overcome these limitations, herein we propose the alloying of non-noble metal nanoparticles with phosphorus as a promising strategy for developing smart catalysts that exhibit both excellent activity and air stability. We synthesized a novel nickel phosphide nanoalloy (nano-Ni₂P) with coordinatively unsaturated Ni active sites. Unlike conventional air-unstable non-noble metal catalysts, nano-Ni₂P retained its metallic nature in air, and exhibited a high activity for the hydrogenation of various substrates with polar functional groups, such as aldehydes, ketones, nitriles, and nitroarenes to the desired products in excellent yields in water. Furthermore, the used nano-Ni₂P catalyst was easy to handle in air and could be reused without pretreatment, providing a simple and clean catalyst system for general hydrogenation reactions.

Full text of DOI:10.1002/chem.202005037

Accepted Article
Accepted Article
Title: Ni2P Nanoalloy as an Air-Stable and Versatile Hydrogenation
Catalyst in Water: P-Alloying Strategy for Designing Smart
Catalysts
Authors: Shu Fujita, Sho Yamaguchi, Jun Yamasaki, Kiyotaka
Nakajima, Seiji Yamazoe, Tomoo Mizugaki, and Takato
Mitsudome
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Chemistry - A European Journal
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Ni P Nanoalloy as an Air-Stable and Versatile Hydrogenation
2
Catalyst in Water: P-Alloying Strategy for Designing Smart
Catalysts
Shu Fujita, [a] Sho Yamaguchi, [a] Jun Yamasaki, [b] Kiyotaka Nakajima, [c] Seiji Yamazoe, [d]
[
a]
[a]
Tomoo Mizugaki, and Takato Mitsudome*
[
a]
S. Fujita, Dr. S. Yamaguchi, Prof. T. Mizugaki, Dr. T. Mitsudome
Department of Materials Engineering Science, Graduate School of Engineering Science
Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
E-mail: mitsudom@chemh.es.osaka-u.ac.jp
Dr. J. Yamasaki
Research Center for Ultra-High Voltage Electron Microscopy
Osaka University
[
[
[
b]
c]
d]
7-1, Mihogaoka, Ibaraki, Osaka 567-0047 (Japan)
Dr. K. Nakajima
Institute for Catalysis
Hokkaido University
Kita 21 Nishi 10, Kita-ku, Sapporo 001-0021 (Japan)
Prof. S. Yamazoe
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan University
1-1 Minami Osawa, Hachioji, Tokyo 192-0397 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: Non-noble metal-based hydrogenation catalysts have
limited practical applications because they exhibit low activity, require
harsh reaction conditions, and are unstable in air. To overcome these
limitations, herein we propose the alloying of non-noble metal
nanoparticles with phosphorus as a promising strategy for developing
smart catalysts that exhibit both excellent activity and air stability. We
conventional nickel and/or cobalt sponge catalysts, e.g., Raney
catalysts, are often employed for hydrogenation.[3,4] Recently,
these catalyst systems have attracted much attention for
application in biorefineries and fuel cells.[
5,6]
However, despite
their high efficiency, they also present a critical limitation of
pyrophoricity.[7] Hence, they should be prepared under strictly
inert conditions with excess NaOH while dissolving Al in the metal
synthesized a novel nickel phosphide nanoalloy (nano-Ni
coordinatively unsaturated Ni active sites. Unlike conventional air-
unstable non-noble metal catalysts, nano-Ni P retained its metallic
2
P) with
alloy, which necessitates
a delicate control of conditions;
2
furthermore, their preparation is accompanied by the production
of a large quantity of metal wastes. Because of their pyrophoricity,
it is difficult to handle activated sponge-metal catalysts during
operation, which makes chemical processes complicated; in
addition, they are prone to deactivation during long-time
storage.[ Sponge-metal catalysts also suffer from inadequate
nature in air, and exhibited a high activity for the hydrogenation of
various substrates with polar functional groups, such as aldehydes,
ketones, nitriles, and nitroarenes to the desired products in excellent
2
yields in water. Furthermore, the used nano-Ni P catalyst was easy to
8]
handle in air and could be reused without pretreatment, providing a
simple and clean catalyst system for general hydrogenation reactions.
activity, requiring harsh reaction conditions such as high H
2
pressures for activation.[ To overcome these drawbacks,
supported non-noble metal nanoparticle (NP) systems with good
catalytic activity have been proposed.[10,11] However, these
9]
Introduction
2
catalysts suffer from air-instability and require H -pretreatment at
high temperatures to generate active metallic states (zero
valence) in the NPs. Consequently, from the perspective of
practical concerns and green sustainable chemistry, the
development of air-stable, highly active, and reusable
hydrogenation catalysts that can replace sponge-metal catalysts
is highly desirable. In this regard, a successful strategy, “N-
doping,” was recently reported for the design of air-stable non-
noble metal-based NP catalysts by Beller et al.[12–16] The simple
pyrolysis of metal complexes with nitrogen-based ligands results
The reduction of polar functional groups, including carbonyl, nitro,
and nitrile groups, represents a fundamental and important class
of reactions for producing valuable chemicals in niche research
and industrial processes.[1,2] In this regard, non-noble metal-
based catalysts, such as those based on Ni and Co, play a key
role in hydrogenation reactions because of their abundance, low
cost compared to platinum group metals (Ru, Rh, Ir, Pd, and Pt),
and good activity. Among the non-noble metal catalysts,
1
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Chemistry - A European Journal
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in the production of non-noble metal NPs encapsulated by a
nitrogen-enriched graphene layer. The resulting N-ligated metal
NPs with a core-shell structure are air-stable and active non-noble
metal catalysts that can be used in several hydrogenation
reactions. However, the encapsulation of the non-noble NPs
shields their active surface sites, and thus, the air stability of the
NPs is achieved at the expense of their activity. Therefore, there
is considerable interest in eliminating the trade-off between the air
stability and activity of non-noble metal NPs.
collected by centrifugation. The obtained solids were washed with
a chloroform–acetone solution (1:1, v/v) and dried at room
temperature in vacuo to yield black nano-Ni
protocol, nano-Ni Co P, Cu P, and Fe
synthesized (see Supporting Information (SI) for details). The
crystal structure of the nano-Ni P was evaluated by X-ray
2
P. Using a similar
5
P
4
,
2
3
2
P
were also
2
diffraction (XRD) and the corresponding pattern is shown in
Figure 1a. The observed diffraction peaks can be indexed to the
hexagonal Ni
(TEM) image of the nano-Ni
2
P phase.[34] The transmission electron microscopy
P indicates that the nanoalloys had
Combining a metal with another element in NPs is a key
strategy for designing novel catalysts. Generally, the second
element is also a metal; such metal–metal nanoalloy catalysts
2
worm-like regular 25 nm-long and 3 nm-wide structures (Figure
1b). Selected area electron diffraction (SAED) patterns (inset in
Figure 1b) indicate that the major diffraction rings match well with
have attracted significant attention in
a wide range of
applications.[17,18] In contrast, the use of metal–non-metal
nanoalloys, such as metal phosphide nanoalloy catalysts, has not
been studied comprehensively. Few innovative metal phosphide
catalysts have been reported in organic synthesis[19–24] despite
their high catalytic potential as hydrotreating catalysts, as well as
electrocatalysts and photocatalysts for energy conversion.[25–31] In
this context, our research group has recently focused on exploring
novel catalysis using metal phosphide nanoalloys. We found that
non-noble metal phosphide nanoalloys, such as nickel and cobalt
phosphides, retain their metal-like states in air and exhibit high
catalytic activity for the hydrogenation of biofuranic aldehydes to
diketones[32] and of nitriles to amines.[33] On the basis of our
studies, we envisaged that alloying non-noble metals with
phosphorus (P) would have several merits in organic
transformations. The first is the stabilization of the metallic state
of the metal species, as an X-ray absorption fine structure (XAFS)
study showed that metal phosphide nanoalloys have a metallic
state in air (stabilization effect). The second is the modulation of
the electronic state of the metal species (ligand effect). Density
functional theory calculations revealed that P-alloying can
increase the d-electron density of metals near the Fermi level,
facilitating hydrogenation.[33] The third is the precise creation of a
well-defined catalytic active species in the metal phosphide
nanocrystal, which favors selective reactions. In contrast,
conventional heterogeneous catalysts have multiple ill-defined
active sites, resulting in inferior catalytic performance. These
unique properties of metal phosphide nanoalloys strongly
motivated us to further investigate their catalytic potential in
various organic syntheses.
the hexagonal structure of Ni
structure of Ni
P.[35] Other metal phosphide nanoalloys were also
characterized by XRD analysis and TEM observations (Figure S1
and S2, respectively). The elemental distribution of Ni and P in
the nano-Ni P was determined by high-angle annular dark-field
2
scanning transmission electron microscopy (HAADF-STEM)
coupled with energy-dispersive X-ray spectroscopy (EDS) (Figure
1 c–f). Elemental mapping of the nano-Ni P confirmed the
2
homogeneous distribution of Ni and P; the molar ratio of Ni and P
was calculated to be ~2:1 (Figure S3). These results demonstrate
2
P, further confirming the crystal
2
the successful synthesis of uniform worm-like nano-Ni
2
P, unlike
previously reported sheet-[36] and rod-like[37,38] nano-Ni
P.
2
Figure 1. Characterization of nano-Ni
pattern of nano-Ni P. The reference pattern of Ni
is also included in the XRD plot. (b) TEM image of nano-Ni
like morphology (the inset shows the corresponding SAED pattern). (c) HAADF-
STEM image of nano-Ni P. Elemental mapping of (d) Ni and (e) P. (f) Composite
2
P with worm-like morphology. (a) XRD
P (JCPDS card no. 03-0953)
P showing a worm-
2
2
Herein, we demonstrate that “P-alloying” is a powerful and
promising strategy for designing smart catalysts exhibiting high
activity and good air stability for efficient and environmentally
benign hydrogenation reactions. We synthesized a novel nickel
2
2
overlay image of (d) and (e).
2
phosphide nanoalloy (nano-Ni P) to act as an air-stable and
highly active catalyst for the hydrogenation of a variety of
carbonyls, nitroarenes, and nitriles using water as the solvent.
2
The synthesized nano-Ni P was also analyzed using X-ray
photoelectron spectroscopy (XPS) to evaluate the electronic state
of Ni in the nanoalloy. The obtained Ni 2p spectrum includes
peaks at 853.0 and 870.3 eV corresponding to Ni 2p3/2 and Ni
Results and Discussion
2
(
p
1/2, respectively, which are close to those of metallic Ni 2p3/2
852.8 eV) and Ni 2p1/2 (870.0 eV) (Figure S4). Subsequently,
XAFS analysis was carried out on the nano-Ni P in ambient
conditions. The X-ray absorption near-edge structure (XANES)
spectra of Ni foil, NiO, nano-Ni P, and bulk Ni P are shown in
Figure 2a. The absorption edge energy of the nano-Ni P is similar
to that of the Ni foil, suggesting that the Ni species in nano-Ni
Preparation and characterization of nano-Ni
nano-Ni P was synthesized via a solvothermal reaction between
nickel(II) acetylacetonate (Ni(acac) ), hexadecylamine, and
triphenylphosphite. The reaction mixture was stirred at 120 °C for
h in vacuo before increasing the temperature to 315 °C; while
2
P
2
2
2
2
2
2
1
2
P
stirring, the system was equilibrated at this temperature in an
argon atmosphere for 2 h to produce a black colloidal solution.
After cooling to room temperature, the colloidal particles were
existed in a metal-like state, which is consistent with the XPS
results; this unique feature distinguishes the nanoalloy
2
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synthesized in this study from conventional Ni NPs and pyrophoric
sponge Ni catalysts. Figure 2b shows the Fourier transform of the
which can cooperatively activate reactants with metal
nanoalloys.[ For this purpose, nano-Ni
39]
P was initially dispersed
2
extended XAFS (FT-EXAFS) spectra of Ni foil, NiO, nano-Ni
2
P,
in hexane and then loaded on different metal oxide supports. The
hydrotalcite (HT, Mg Al (OH)16CO ·4H O) support was found to
significantly enhance the catalytic performance of nano-Ni P,
and bulk Ni P. The transform function of the nano-Ni P exhibits
2
2
6
2
3
2
Ni‒P and Ni‒Ni bonds at 1.7–2.0 and 2.0–2.5 Å, respectively. The
absence of peaks corresponding to Ni‒O indicates that the nano-
2
resulting in a 93% yield for 2a, which is a significant improvement
Ni
2
P do not undergo oxidation in air. The curve-fitting analysis of
compared to that for entry 1 under the same conditions (entry 7).
the FT-EXAFS spectra is shown in Figure S5 and the results are
summarized in Table S1. The curve-fitting analysis suggests that
Other oxide supports such as Y
were also effective in producing 2a at moderate to high yields
except for SiO (entries 9–14). It has been reported that carbonyl
2 3 2 2 2 3 2 5
O , ZrO , TiO , Al O , and Nb O
the Ni‒Ni bond of the nano-Ni
2.58 Å v/s 2.48 Å) because Ni
tetrahedra that share vertices. In addition, the CNNi-Ni/CNNi-P ratio
1.8) of nano-Ni P was lower than that of bulk Ni P (2.8) (CN is
the coordination number; the ideal value of CNNi-Ni/CNNi-P in Ni
crystals is 3.0). The low CNNi-Ni/CNNi-P ratio of nano-Ni P can be
explained by the exposure of coordinatively unsaturated Ni atoms
on the surface. The crystal structure of Ni P is shown in Figure
c; it included crystal planes, such as the (300) plane, in which
the Ni(2) content was high. Figure 2d shows the proposed
structure of nano-Ni P with a (300) plane; the exposure of Ni(2) at
2
P is longer than that of the Ni foil
2
(
2
P consists of a network of NiP
4
moieties are activated at the oxygen vacancy sites of HT, which
then act as Lewis acid sites.[ This was confirmed by the Fourier
transform infrared (FT-IR) analysis of acetophenone adsorbed on
HT; the intense band assigned to C=O stretching vibrations red-
shifted from 1708 cm−1 to 1691 cm , while no such shift was
40]
(
2
2
2
P
−1
2
observed after the adsorption of acetophenone on nano-Ni
2
P
2
(Figure S6). Thus, the activated C=O moieties in 1a could then be
2
hydrogenated by the adjacent nano-Ni
interface.
2
P at the nanoalloy/HT
2
the surface is responsible for their low CNNi-Ni value. In addition, in
these nanoalloys, the Ni‒P bonds (Ni(1)‒P in Figure 2c) were
more rigid than the Ni‒Ni bonds (Ni(1)‒Ni(2)) because the latter
exhibited a large Debye Waller (DW) value (DW > 0.01, Table S1).
Therefore, coordinatively unsaturated Ni(2) sites are formed on
Table 1. Hydrogenation of acetophenone using metal phosphide catalysts.[a]
2
the Ni P nanoalloy surface.
Entry
1
Catalyst
Yield[b] [%]
nano-Ni
nano-Ni
2
P
16
97
3
2[c]
3
P
2
2
nano-Co P
4
nano-Ni
5
P
4
<1
0
5
3
nano-Cu P
6
nano-Fe
2
P
0
Figure 2. Electronic state and structure of nano-Ni
spectra and (b) Fourier transform k -weighted EXAFS of Ni foil, NiO, nano-Ni
2
P. (a) Ni K-edge XANES
P,
P. (d) Proposed surface structure of
P with a (300) plane.
3
2
7
nano-Ni
nano-Ni
nano-Ni
nano-Ni
nano-Ni
nano-Ni
nano-Ni
2
P/HT
93
88
79
73
63
54
30
7
and bulk Ni
2 2
P. (c) Crystal structure of Ni
the worm-like nano-Ni
2
8[d]
9
P/HT
2
2
2
2
2
2
P/Y
P/ZrO
P/TiO
P/Al
P/Nb
2
O
3
2
Hydrogenation of valuable chemicals using nano-Ni P
catalysts.
Acetophenone
After studying the morphological, structural, and elemental
characteristics of the synthesized metal phosphide nanoalloys,
we investigated their catalytic potential for the hydrogenation of
acetophenone (1a) as a model substrate in water without any
1
0
1
2
1
2
12
2 3
O
13
2 5
O
additives at 100 °C and 20 bar H
2
; the results are shown in Table
14
2
nano-Ni P/SiO2
1
. Among the studied non-noble metal phosphides, nano-Ni P
2
showed the highest activity and provided a 16% yield of 1-
phenylethanol (2a) for 1 h (entry 1). Upon increasing the reaction
time to 12 h, 2a was obtained as the sole product at 97% yield
[a] Reaction conditions: catalyst (5 mol% metal), acetophenone (0.5 mmol),
water (3 mL), (20 bar), 100 °C, and h. [b] Determined by gas
chromatography-mass spectrometry (GC-MS) using dimethyl sulfone as an
H
2
1
2
internal standard. [c] 12 h. [d] H (1 bar), 150 °C, 12 h.
(
entry 2). nano-Co
hydrogenation reaction, but provided a low yield (3%) of 2a (entry
), while other metal phosphide nanoalloys (Ni , Cu P, and
Fe P) were almost inactive (entries 4–6). Subsequently, we
studied the supporting effect of nano-Ni P, which exhibited the
2
P also functioned as catalysts for the
Aldehydes and ketones
Non-noble metal catalysts often require harsh reaction conditions
to effectively catalyze the hydrogenation of carbonyl compounds
3
5
P
4
3
2
2
(Table S3). In contrast, the nano-Ni P/HT was effective even
2
best catalytic activity among the studied metal phosphides, for the
under relatively mild conditions. For example, a high yield of 2a
hydrogenation of 1a in water using different metal oxide supports,
could be obtained at just 1 bar H2 (entry 8). Furthermore, the
3
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2
nano-Ni P/HT system could work effectively on a wide range of
substrates under 1 bar H
the catalytic activity of Ni
2
. Although there are a few reports on
P for the hydrogenation of aldehydes
2
and ketones,[
41–45]
these compounds require organic solvents
and/or suffer from a limited substrate scope (only one or two
carbonyls were tested). In sharp contrast, using the nano-Ni P/HT
system described herein, various aldehydes and ketones,
including functionalized aromatic, heteroaromatic, alicyclic, and
aliphatic groups, could be efficiently hydrogenated to the
2
Figure 4. Hydrogenation of aqueous HMF (20 wt%) using nano-Ni P/HT as the
catalyst.
2
Recycling experiments (example: acetophenone reaction)
The reusability of heterogeneous catalysts is one of their
significant advantages over homogeneous catalysts. In our study,
the used nano-Ni P/HT could be easily recovered from the
2
reaction mixture by centrifugation and could then be reused
without any loss of catalytic activity even after five recycling runs
corresponding alcohols at excellent yields even at atmospheric H
pressure (Figure 3). These facts support our claim that nano-
Ni P/HT is an air-stable and active catalyst for the hydrogenation
2
2
of aldehydes and ketones in water.
(
Figure 5a). We further assessed the reaction rate at 30 min, i.e.,
before the reaction reached completion, and found that 2a was
obtained in similar yields (♢; Figure 5a) using the used and fresh
nano-Ni
Ni P/HT after the reaction revealed spherical nano-Ni
with a mean diameter of 5.9 nm, indicating that the worm-like
nano-Ni P transformed into spherical nano-Ni P during the
2
P catalysts. Interestingly, the TEM analysis of nano-
2
2
P particles
2
2
hydrogenation reaction. This spherical morphology was
maintained over five recycling runs (Figure S7). The XRD pattern
of nano-Ni
change before and after the reaction (Figure S8 and Table S2).
Furthermore, the XAFS results proved that the Ni in nano-Ni P/HT
2 2
P and the Ni loading amounts in nano-Ni P/HT did not
2
retained its metallic nature even after catalysis (Figure S9). To
investigate metal leaching, we conducted hot-filtration tests on
nano-Ni
2
P/HT, as shown in Figure 5b. After filtering out nano-
2
Ni P/HT at 50% conversion of 1a, the filtrate was again treated
under similar reaction conditions. However, the new reaction did
not yield any products. Inductively coupled plasma-atomic
emission spectrometry (ICP-AES) of the filtrate confirmed no
leaching of Ni species from the catalyst (the concentration of Ni
ions was below the detection limit). These results are highly
consistent with the results of recycling experiments and confirm
2
Figure 3. Hydrogenation of aldehydes and ketones catalyzed by nano-Ni P/HT.
Reaction conditions: nano-Ni P/HT (167 mg; 5 mol% Ni), substrate (0.5 mmol),
2
water (3 mL), 150 °C, and 6 h. Yield was determined by GC-MS using dimethyl
sulfone as an internal standard. Values in parentheses are isolated yield. [a]
Catalyst (333 mg; 10 mol% Ni) and 12 h. [b] Catalyst (333 mg; 10 mol% Ni) and
2
the high durability of nano-Ni P/HT catalyst systems.
24 h. [c] 60 °C and 10 h. [d] 100 °C.
Biomass transformation (example: HMF)
We further studied the catalytic activity of nano-Ni P/HT in a
2
biorefinery for the transformation of biomass into fuels and
valuable chemicals. The hydrogenation of a biomass-derived
platform molecule, 5-hydroxymethylfurfural (HMF), to 2,5-
bis(hydroxymethyl)furan (BHMF), which is a promising monomer
for polyesters and self-healing polymers, is one of the most
important biorefinery reactions.[46–48] From a practical standpoint,
the transformation of highly concentrated HMF to BHMF in water
is challenging because HMF is thermally unstable and is easily
oligomerized.[49] It is to be noted that nano-Ni
P/HT functioned
2
well at an HMF concentration of 20 wt% in water at 60 °C,
resulting in a high BHMF yield (Figure 4). Thus, this is the first
time that the selective production of BHMF from a concentrated
HMF solution could be successfully achieved using a non-noble
metal catalyst without base additives or the pre-acetalization of
HMF.[50]
Figure 5. Recycling and hot-filtration experiments. (a) Reuse of nano-Ni
for the hydrogenation of acetophenone to 1-phenylethanol. Reaction time: 1 h
blue bars), 30 min (♢). (b) Time course for the hydrogenation of acetophenone
using nano-Ni P/HT. (●) Without catalyst filtration and (♦) with catalyst removal
by hot-filtration after 30 min. Reaction conditions: nano-Ni P/HT (167 mg; 5
2
P/HT
(
2
2
2
mol% Ni), acetophenone (0.5 mmol), water (3 mL), H (20 bar), 100 °C.
4
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Nitriles
hydrogenation of nitroarenes. The reduction of aromatic nitro
compounds is the most straightforward method for the synthesis
of the corresponding aniline derivatives, which are valuable
2
The high performance of the nano-Ni P in water encouraged us
to evaluate their catalytic effect for the hydrogenation of nitriles,
which is of high significance in the synthesis of primary
amines.[51,52] The hydrogenation of valeronitrile was carried out in
intermediates
in
the
production
[58,59]
of
agrochemicals,
pharmaceuticals, and dyes.
Although the catalytic activity of
water in the presence of NH
nano-Ni P at 130 °C and 40 bar H
summarized in Table S4. Unsupported nano-Ni
3
using supported and unsupported
, and the observed yields are
P showed the
nickel phosphide for the hydrogenation of nitroarenes has been
reported,[60–62] there is no information available on it in water. The
hydrogenation of nitrobenzene in water using supported and
2
2
2
highest activity to afford the corresponding amylamine at a yield
of ~89%. Meanwhile, the studied metal oxide supports were
ineffective for hydrogenation; instead, they promoted a side-
unsupported nano-Ni
shown in Table S5. Interestingly, unsupported nano-Ni
the highest activity to afford a 94% yield of aniline, whereas nano-
Ni P immobilized on different supports afforded only low yields.
The nano-Ni P also exhibited high activity toward various
nitroarenes, resulting in anilines at high yields (Figure 8). Notably,
a wide range of functional groups, including halogen (6c and 6d),
methoxy (6e), amino (6f), ester (6g), amide (6h), and hydroxy (6i–
6k) groups, were tolerated under the reaction conditions
employed; thus, the highly selective synthesis of functionalized
2
P catalysts was studied and the results are
2
P showed
reaction to form alcohols. In addition, the nano-Ni
2
P catalysts
2
could work on a broad range of substrates; aromatic, aliphatic,
and alicyclic nitriles could be converted to the corresponding
primary amines at high yields (Figure 6). To the best of our
knowledge, thus far, only sponge Ni[53] and Co(II)/Zn(0)[54]
catalysts can function in water for nitrile hydrogenation, but their
2
2
substrate scope is limited. Hence, nano-Ni P represents the first
example of non-noble metal catalysts that can work on a broad
range of substrates for the hydrogenation of nitriles in water.
2
anilines could be achieved. In particular, nano-Ni P could catalyze
the hydrogenation of methyl 2-nitrobenzoate (5g) to methyl 2-
aminobenzoate (6g), which is used as a bird repellent for food
protection and for flavoring medicines, candies, and soft drinks;[
thus, the green synthesis of 6g in water could be achieved.
Notably, nano-Ni
2
P efficiently promoted the hydrogenation of
63]
benzonitrile (3a) to benzylamine (4a) under just 1 bar of H
2
(Figure
7
), whereas previously reported Ni catalysts inevitably required
[
55–57]
pressurized H
2
.
2
This result demonstrates that nano-Ni P also
functions as a highly active catalyst for the hydrogenation of
nitriles under mild conditions.
Figure 8. Hydrogenation of nitroarenes catalyzed by nano-Ni
conditions: nano-Ni P (5 mol% Ni), substrate (0.5 mmol), water (3 mL), H
bar), 120 °C, and 3 h. Yield was determined by GC-MS using dimethyl sulfone
2
P. Reaction
2
2
(20
Figure 6. Hydrogenation of nitriles catalyzed by nano-Ni
nano-Ni P (5 mol% Ni), substrate (0.5 mmol), 12.5% NH
30 °C, and 3 h. Yield was determined by GC-MS using dimethyl sulfone as an
internal standard. Values in parentheses are isolated yield as a hydrochloride
salt. [a] 25% NH aq. (3 mL) and 6 h. [b] 6 h.
2
P. Reaction conditions:
2
3
aq. (3 mL), H (40 bar),
2
as an internal standard. Values in parentheses are isolated yields. [a] 6 h. [b] H
40 bar), 150 °C, and 6 h. [c] H (40 bar) and 150 °C.
2
1
(
2
3
Comparison of catalytic activities of nano-Ni
catalyst for HMF hydrogenation
2
P and Raney Ni
Finally, to demonstrate the advantages of nano-Ni
over conventional catalysts, we compared the catalytic activities
of nano-Ni P/HT and sponge Ni for the aqueous-phase
hydrogenation of HMF as a model compound. Based on the
turnover frequency (TOF), it was observed that nano-Ni P/HT
2
P catalysts
2
2 2
Figure 7. Hydrogenation of benzonitrile using nano-Ni P under 1 bar H .
2
exhibited a catalytic activity 22 times higher than that of sponge
Ni (Figure 9a). Furthermore, the activation energies required for
HMF hydrogenation using nano-Ni P/HT and sponge Ni were
2
Nitroarenes
To demonstrate the generality of the hydrogenation catalysis of
nano-Ni P in water, they were once again used to catalyze the
calculated to 37.9 and 51.4 kJ/mol, respectively, from the slopes
of their Arrhenius plots (Figure 9b). These results clearly
2
5
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Chemistry - A European Journal
10.1002/chem.202005037
FULL PAPER
demonstrate that nano-Ni
activity and air stability than sponge Ni.
2
P has a significantly higher catalytic
content, redispersion and precipitation cycles were repeated using a
chloroform–acetone (1:1, v/v) mixed solvent. The obtained powder was
dried in vacuo overnight.
2
Preparation of nano-Ni P/Support.
2
Typically, nano-Ni P (30 mg) was dispersed in hexane (100 mL) via
sonication for 1 h and then stirred with HT (1.0 g) for 6 h at room
temperature. The obtained product was dried in vacuo overnight to yield a
gray nano-Ni
other nano-Ni
All nano-Ni P/support catalysts were characterized by ICP-AES analysis
Table S2) and TEM observations (Figure S10).
2
P/HT powder. A similar procedure was followed to prepare
2
P/support (Y , ZrO , TiO , Al , Nb , SiO ) catalysts.
2
O
3
2
2
2
O
3
2
O
5
2
2
(
2
Hydrogenation of acetophenone using nano-Ni P/HT as the catalyst.
2
Typically, nano-Ni P/HT (0.167 g) was placed in a 50 mL stainless steel
autoclave with a Teflon inner cylinder, to which acetophenone (0.5 mmol)
and distilled water (3 mL) were subsequently added. The reaction mixture
2
was stirred vigorously at 100 °C at 20 bar H for 1 h. Following the reaction,
the autoclave was cooled in an ice-water bath and the hydrogen gas was
carefully released. The resulting reaction mixture was diluted with
dimethoxyethane and analyzed by GC-MS.
Acknowledgements
2
Figure 9. Comparison of catalytic activities of nano-Ni P/HT and sponge Ni. (a)
TOF of HMF hydrogenation over nano-Ni
2
P/HT and sponge Ni catalysts at
This work was supported by JSPS KAKENHI Grant Nos.
120 °C. (b) Arrhenius plot of HMF hydrogenation using nano-Ni
2
P/HT and
26105003, 17H03457, and 18H01790. This study was partially
sponge Ni catalysts. TOF was defined as the number of moles of 2,5-
bis(hydroxymethyl)furan produced per mole of Ni per hour.
supported by the Cooperative Research Program of Institute for
Catalysis, Hokkaido University (19A1002). A part of this work was
supported by the “Nanotechnology Platform Program” at
Hokkaido University (A-19-HK-0039 and A-20-HK-0011) and
Nanotechnology Open Facilities in Osaka University (A-19-OS-
Conclusion
0
060), Ministry of Education, Culture, Sports, Science and
2
In summary, for the first time, we demonstrated that nano-Ni P is
Technology (MEXT), Japan. We thank Dr. Toshiaki Ina (SPring-
an air-stable, highly active, and reusable non-noble metal-based
heterogeneous catalyst for the hydrogenation of polar functional
groups in water. Aldehyde, ketone, nitrile, and nitroarene groups
were efficiently hydrogenated to the corresponding alcohols,
8)
for XAFS measurements (2019A1390, 2019A1649,
2019B1560, and 2019B1858) and Ryo Ota of Hokkaido University
for STEM analysis.
amines, and anilines using nano-Ni
XPS results revealed that the nano-Ni
2
P-based catalysts. XAFS and
P surface has numerous
2
Keywords: carbonyl • heterogeneous catalysis • hydrogenation
coordinatively unsaturated Ni active sites and its metallic nature
was retained even after reuse in air, demonstrating its high activity
and durability. Our results show that “P- alloying” of metal NPs is
a powerful strategy to design smart hydrogenation catalysts with
good air stability and high activity that can substitute conventional
air-unstable non-noble metal catalysts with lower activities. This
strategy will be highly useful for developing next-generation green
and sustainable chemical processes.
• nickel phosphide • nitrile
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Entry for the Table of Contents
A novel nickel phosphide nanoalloy (nano-Ni
2
P) retained its metallic nature in air and exhibited a high activity for the hydrogenation of
2
carbonyls, nitriles, and nitroarenes in water. The used nano-Ni P could be easily handled and reused without pretreatment, providing
a simple and clean catalyst system for general hydrogenation reactions.
8
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