Aqueous phase semihydrogenation of alkynes over Ni-Fe bimetallic catalysts

Article abstract of DOI:10.1039/d0cy01153c

Bimetallic Ni-Fe catalysts (Ni/Fe, 1?:?1, 1?:?3, and 3?:?1) are synthesized and explored for their catalytic activity in semihydrogenation of internal alkynes using H2 gas in water-ethanol solution. Our findings revealed that over the Ni1Fe3 catalyst a high diastereoselectivity for Z-alkenes with a high conversion for a wide range of internal alkynes can be achieved at moderate reaction temperature (40 °C). Notably, the selectivity for the Z-alkenes is enhanced in the presence of n-butyl amine as an additive. Deuterium labeling experiments evidenced that H2 gas becomes dissociated homolytically over the catalyst surface to hydrogenate alkynes to alkenes. Synthesized catalysts were successfully characterized by HR-TEM, SEM, XPS, EDS, P-XRD and H2-TPD.

Full text of DOI:10.1039/d0cy01153c

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
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Aqueous Phase Semihydrogenation of Alkynes over Ni-Fe Bimetallic Catalysts
Rohit K. Raia,c Mahendra K. Awasthia, Vipin K. Singhb, Sudipta Roy Barmanb Silke Behrensd,
,
,
and Sanjay K. Singh*a
aCatalysis Group, Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore
453552, Madhya Pradesh, India
bUGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452001, Madhya
Pradesh, India
cKAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah
University of Science and Technology (KAUST), Thuwal, 23955-6900, Kingdom of Saudi
Arabia.
dInstitut für Katalyseforschung und –technologie (IKFT), Karlsruher Institut für Technologie
(KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
E-mail: sksingh@iiti.ac.in
Abstract
Bimetallic Ni-Fe catalysts (Ni/Fe, 1:1, 1:3, and 3:1) are synthesized and explored for their
catalytic activity in semihydrogenation of internal alkynes using H2 gas in water-ethanol
solution. Our findings revealed that over Ni1Fe3 catalyst high diastereoselectivity for Z-alkenes
with a high conversion for a wide range of internal alkynes can be achieved at moderate reaction
temperature (40 ºC). Notably, the selectivity for the Z-alkenes is enhanced in the presence of n-
butyl amine as an additive. Deuterium labeling experiments evidenced that H2 gas gets
dissociated homolytically over the catalyst surface to hydrogenate alkynes to alkene. Synthesized
catalysts were successfully characterized by the HR-TEM, SEM, XPS, EDS, P-XRD and H2-
TPD.
1

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Introduction
Selective semihydrogenation of alkynes to alkenes employing heterogeneous catalysts remains
one of the most studied reactions for both in industry and in academia.1-4 Well-defined
configured olefins (E or Z) are the key building blocks for the synthesis of various fine
chemicals, such as vitamins and various natural products.1-4 In industries, Lindlar catalyst,
consisting of Pd-CaCO3 partially poisoned with Pb(OAc)2 and quinoline, is still the first choice
for the selective semihydrogentaion of alkynes to Z-alkenes.5,6 However, due to various
disadvantages of Lindlar catalysts (use of excess ligand and harmful Pb as a promoter), over the
past decades, many catalysts have been developed based on monometallic Pd,7-10 Au,11-13 Ru,14
Rh,15 Ni,16-25 Co,26 Fe,27 Cu28,29 as well as bimetallic Pd-Ag,30 Pd-Au,31 Pd-Ru,32 Au-Pt,33 Pd-
Cu,34 Pd-Ga,35 Rh-Co,36 Rh-Ni,37 Co-Pt,38 Ni-Cu,39 Ni-Fe,40,41 Cu-Fe,41 Ni-Ga42,43 and trimetallic
Cu-Ni-Fe41 catalysts for the selective semihydrogenation of alkynes to alkenes. In this direction,
particularly, exploration of bimetallic nanoparticles have gained tremendous attention for the
selective hydrogenation of alkynes as compared to monometallic catalyst, because they exhibit
superior chemical and physical properties.30-45 For example, very recently Li et al. have reported
Pd@Au-Pd alloy surface nanocubes for semihydrogenation of alkynes to alkenes (alkene/alkane;
91/9) using hydrogen balloon in toluene at room temperature.31 Similarly, Lu et al. also reported
bimetallic Pd-Pb alloy octahedral nanocrystals based catalyst for the semihydrogenation of
alkynes.45 Lyutakov et al. also explored bimetallic Au/Pt heterostructures for the selective
hydrogenation of alkyne bonds, of 4-ethynylphenyl groups covalently grafted on the catalyst
surface, to alkenyl or alkyl moieties using cyclohexene as a hydrogen source.33 The Au
supported surface plasmon polariton (SPP) activates the hydrogenation of alkyne, while the
thickness of Pt layer tunes the alkenyl vs alkyl selectivity.33 RhNi@MOF-74 alloy catalyst also
2

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exhibited appreciably good activity for the semihydrogenation of alkynes to Z-alkenes.37
Complete selectivity for 4-octene was also reported for the hydrogenation of 4-octyne over Pt
and CoPt3 catalysts using 1-octylamine as an additive, where the amine additive played a crucial
role in tuning the selectivity towards the alkene product over alkane.38 However, considering the
high cost and scarcity of noble metals, it is highly desirable to explore low-cost high natural
abundance non-noble metals based catalysts for semihydrogenation reaction. In this direction,
various non-precious metal catalysts based on Ni16-25, Co26, Fe27, Cu28,29 have been employed for
selective semihydrogenation reactions. Among them, development of Ni-based catalysts has
received tremendous interest, because of their low cost and considerably high catalytic activity.
Nickel metal is known for its high catalytic activity for hydrogenation of unsaturated C-C bonds,
but the presence of a second metal can effectively tune the properties of the Ni catalyst.46 Liu et
al. recently reported NiCu nanoalloy catalyst for semihydrogenation of alkyne to achieve 100%
conversion at 160 ºC in toluene with improved selectivity for alkene (>70%).39 Earlier,
hydrogenation of 4-octyne was also explored using various monometallic (Fe, Ni, Co, and Cu)
and bimetallic (NiFe, FeCo and FeCu) catalysts at room temperature under 5 bar H2 pressure in
THF.40 Among these, FeCu and Cu nanoparticles were found to be inactive, while other
bimetallic NiFe and FeCo exhibited similar activity (with intermediate selectivity for the alkene
product) as the monometallic Ni, Co and Fe.40 Ternary Cu3-xNixFe catalysts were reported to
exhibit high selectivity for propene and ethene in a gas-phase hydrogenation reaction of propyne
and ethyne, respectively, at 1 bar pressure (H2/alkyne = 1.5) and 250 ºC.41 Results inferred that
the optimum Ni content and intimate contact of Cu and Ni in the Cu3-xNixFe catalysts was found
to be essential to achieve high alkene selectivity. Literature also revealed that Fe, being of low
cost and abundant in nature, could be a promising metal partner to fine-tune the performance of
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nickel in the bimetallic Ni-Fe catalyst.46,47 The presence of Fe in the bimetallic Ni-Fe catalyst
may influence the catalytic activity of Ni-Fe catalyst in several ways, which includes Fe to Ni
electronic charge transfer, tuning of the adsorption behavior of reactants and the availability of
hydrogen atoms on the catalyst surface, and hence may tune the product selectivity in
hydrogenation reactions.46 However, the development of bimetallic Ni-Fe catalysts and the
investigation of their catalytic performance for the semihydrogenation of internal alkynes to the
corresponding alkenes are yet to be explored in detail. Further, few of the recent reports show
that the semihydrogenation of alkynes to alkenes over Ni catalyst can be achieved using NaBH4
as a reducing agent.17 However, this methodology demands an excess of NaBH4, because only a
part of NaBH4 was used for the reduction of alkyne and the rest of NaBH4 generated hydrogen
gas, which escaped from the reaction medium. Notably, the reported Ni catalyst could not
catalyze this reaction in the presence of H2 gas.17 Very recently, Huang et al. reported CuO
rhombic dodecahedra crystals for selective semihydrogenation of alkyne in the presence of
ammonia borane.28 Therefore, these reports inferred that the development of highly active
catalysts for the selective semihydrogenation of alkyne to alkene required both facile activation
of H2 and suppressed overhydrogenation reaction. Further, most of these reactions require high
temperature, pressure and the use of excess inorganic hydrides (NaBH4 and NH3BH3), which
needs to be overcome.
Hence, on the basis of available reports, it is highly desirable to develop a low-cost and
more environmentally benign catalytic system for semihydrogenation of alkynes. Further, using
molecular H2 gas a hydrogen source is more economical and environment friendly. Over the past
few years, we developed several cost-effective and high-performance non-noble metal-based
catalysts particularly bimetallic catalyst for various organic transformations.47-49 Herein, we have
4

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synthesized bimetallic Ni-Fe (Ni/Fe, 1:1, 1:3, and 3:1) alloy nanoparticles along with
monometallic Ni and Fe nanoparticles and explored their catalytic activity for the
semihydrogenation of alkynes to Z-alkenes using molecular H2 as a hydrogen source at
atmospheric pressure in water-ethanol solution at 40 ºC. We probed several characterization tools
to establish the alloy composition of Ni and Fe in the synthesized bimetallic Ni-Fe catalysts.
Moreover, we also investigated the effect of Ni-to-Fe content and amine additives in tuning the
selectivity towards the Z-alkene products. Further, the selective semihydrogenation of a wide
range of alkynes was also explored over the best performing catalyst under the optimized
reaction condition.
Results and discussions
Synthesis and characterization of bimetallic Ni-Fe catalysts
Our bimetallic Ni-Fe catalysts were prepared by co-reduction of an aqueous solution of nickel(II)
chloride and ferrous(II) sulfate using an aqueous solution of sodium borohydride as a reducing
agent in the presence of poly(vinylpyrrolidone) (PVP).47 Different compositions of the bimetallic
Ni-Fe catalysts were prepared by varying the Ni/Fe molar ratios (Ni/Fe, 3:1, 1:1 and 1:3).
Analogously, the monometallic Ni and Fe catalysts were also synthesized. The molar
composition of Ni/Fe in the bimetallic Ni3Fe1, Ni1Fe1 and Ni1Fe3 catalysts were further
authenticated by inductively coupled plasma optical emission spectrometry (ICP-OES) analyses,
which matches well with the expected Ni/Fe composition (Table S1). Scanning Electron
Microscopy (SEM) images along with SEM-energy dispersive X-ray spectroscopy (EDS) and
elemental mapping evidenced the presence of both Ni and Fe in the bimetallic Ni3Fe1, Ni1Fe1 and
Ni1Fe3 catalysts (Fig. S1-S3). Morphology and chemical composition of all the bimetallic
Ni3Fe1, Ni1Fe1 and Ni1Fe3 catalysts were also authenticated by transmission electron microscopy
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(TEM), TEM-EDS point and line scan analyses (Fig. 1 and Fig. S4-S5). TEM images exhibited
the presence of irregular shaped nanoparticles for the bimetallic Ni-Fe catalysts (Fig. S4). In
particular, TEM images of Ni1Fe3 catalyst inferred the presence of nanoparticles with an average
size of ca. 6.5 nm (Fig. 1 and Fig. S4). Moreover, the TEM-EDS analysis and high-angle annular
dark field EDS (HAADF-EDS) elemental mapping of Ni and Fe exhibited a very similar
distribution, which further manifested the co-existence of both Ni and Fe metal in Ni1Fe3
catalyst. Consistent with elemental mapping, the EDS line scan profile of Ni1Fe3 catalyst also
endorsed the co-existence of both Ni and Fe metal in the proposed composition (Fig. S5). It can
be also seen that elemental mapping of Ni1Fe3 catalyst also manifested the presence of C, N and
O atoms, suggesting the presence of PVP polymer around the nanoparticles (Fig. S6).50 The
selected-area electron diffraction (SAED) exhibited diffused ring patterns for the Ni1Fe3 catalyst
(Fig. 1d). Moreover, the lattice spacing of the fringes in the HR-TEM image of Ni1Fe3
nanoparticles was measured to be 0.21 nm, which is within the expected value of Ni (d = 2.06
nm) and Fe (d = 2.08 nm), supporting the alloy nature of Ni1Fe3 catalyst (Fig. 1c). Moreover, the
alloy composition of bimetallic Ni3Fe1 and Ni1Fe1 catalysts was also confirmed by the HAADF-
STEM (scanning transmission electron microscopy) and EDS line scanning analyses. The EDS
line scanning profiles of Ni and Fe of the randomly chosen nanoparticles of Ni3Fe1 and Ni1Fe1
catalysts corresponding to their respective HAADF-STEM images, clearly supported the uniform
alloy composition of the bimetallic Ni-Fe catalysts with no substantial segregation for either of
the elements (Fig. 1). Moreover, these results also eliminate the formation of core-shell structure
or other phases for the bimetallic Ni-Fe catalysts. A thin oxide coating over the nanoparticles
was, however, observed in TEM images, but this could due to the partial oxidation of the
nanoparticle surface upon exposure to air during sample preparation. Hence, the above EDS
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point and line scanning and elemental mapping results clearly manifested the homogeneous
distribution of both the metals with the proposed Ni/Fe compositions in the bimetallic, Ni1Fe3,
Ni1Fe1 and Ni3Fe1 catalysts (Fig. 1 and Fig. S5, S7-S8). Powder X-ray diffraction (P-XRD)
pattern of the bimetallic Ni3Fe1, Ni1Fe1 and Ni1Fe3 catalysts illustrated a broad diffraction peak
centered at 2θ value of 40–50° which lies between the fcc peak of Ni (JCPDS 65-0380) and Fe
(JCPDS 52-0513), further supporting the alloy composition of the metallic Ni and Fe for the
bimetallic Ni-Fe catalysts (Fig. 1m).47 The observed broad diffraction peak in the P-XRD of the
bimetallic Ni-Fe catalysts could be due to the smaller nanoparticles as these catalysts were
synthesized in the presence of PVP stabilizer.50
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Fig. 1 Characterization of the bimetallic Ni-Fe catalysts. (a) TEM, (b) particle size distribution
(c) HR-TEM, (d) SEAD pattern, (e-g) TEM image and the corresponding elemental mapping for
Ni and Fe, and (h) TEM-EDS for Ni1Fe3 catalyst. (i-j) HAADF-STEM image and corresponding
EDS line scan for Ni1Fe1 and (k-l) Ni3Fe1 catalyst, respectively, (m) P-XRD pattern of
monometallic (Ni and Fe) and bimetallic Ni1Fe3, Ni1Fe1, Ni3Fe1 catalysts.
Chemical states of Ni and Fe elements in the bimetallic Ni-Fe catalysts were established
by high-resolution X-ray photoelectron spectroscopy (XPS). Fig. 2 displayed the Ni 2p and Fe 2p
spectra of the bimetallic Ni-Fe catalyst. In the Ni 2p3/2 spectrum of Ni1Fe3 catalyst, two major
fitted bands at 852.5 eV and 853.1 eV are respectively associated with the core levels of metallic
Ni0 and Ni2+ states.47,51,52 Analogously, Ni1Fe1 and Ni3Fe1 catalysts also displayed XPS peaks for
Ni 2p3/2 with two fitted bands corresponding to the metallic Ni0 (major) and Ni2+ (minor) states
(Fig. 2). Notably, the fitted peak for the metallic Ni0 is prominent and intense as compared to the
Ni2+ band. The observed minor Ni2+ peaks could be due to the easy re-oxidation of Ni0 at the
surface to Ni2+ upon exposure to air during the sample preparation for XPS analysis. On the
other hand, Fe 2p peaks were fitted with three bands for all the bimetallic Ni3Fe1, Ni1Fe1 and
Ni1Fe3 and the monometallic Fe catalysts. The prominent peak at 706.7 eV is corresponding to
the metallic Fe0, while peaks at 711.4 eV and 708.7 eV are associated with Fe3+ and Fe2+ species,
respectively. 47,51,52 Therefore, the presence of peaks corresponding to Ni0 and Fe0 in the XPS
spectra endorsed the co-existence of both Ni and Fe in the bimetallic Ni3Fe1, Ni1Fe1 and Ni1Fe3
catalysts. It is evident from XPS pattern that the metallic Fe0 species are in abundance in all the
bimetallic Ni3Fe1, Ni1Fe1 and Ni1Fe3 catalysts, while easy re-oxidation of Fe0 species in the air
probably resulted in the formation of Fe2+/Fe3+ species over the catalyst surface,51,52 as also
observed as a thin oxide coating in the TEM images of these bimetallic Ni-Fe catalysts.
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Fig. 2 XPS spectra of the bimetallic Ni3Fe1, Ni1Fe1 and Ni1Fe3 catalysts along with the
monometallic (Ni and Fe) catalysts.
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Optimization of catalytic reaction conditions
At an outset, catalytic efficacy of the synthesized bimetallic Ni-Fe (Ni/Fe, 1:1, 1:3, 3:1) catalysts
along with the monometallic Ni and Fe catalysts were investigated for semihydrogenation of
alkynes using diphenylacetylene (1a) as model a substrate in the presence of hydrogen gas in
water-ethanol solution. Results summarized in Table 1 inferred that the monometallic Ni catalyst
completely hydrogenated the diphenylacetylene (1a) to dibenzyl (3a), whereas Fe catalyst was
found to be inactive under analogous reaction condition (Table 1, entries 1-2). Analogous to Ni
catalyst, bimetallic Ni1Fe1 and Ni3Fe1 catalysts were also active for the complete hydrogenation
of diphenylacetylene (1a) to dibenzyl (3a) (Table 1, entries 3-4). Even at lower conversion of 1a
(75% in 3h), the selectivity for the overhydrogenated product (3a) was as high as 44% over the
Ni1Fe1 catalyst (Table 1, entry 4). On the other hand, Ni1Fe3 catalyst exhibited high selectivity
(70%) for the semihydrogenated products (cis-/trans-stilbene 2a, Z/E; 93/7) with 91%
conversion of 1a, under analogous reaction condition (Table 1, entry 5, Fig. 3a). Since it is
clearly seen by TEM and XPS data that the bimetallic Ni-Fe catalysts have Ni and Fe in alloy
composition, the observed catalytic performance of Ni1Fe3 catalyst clearly evidenced the crucial
role of the molar composition of Ni/Fe in achieving higher selectivity for alkenes.
Table 1 Optimization of reaction condition for semihydrogenation of diphenylacetylene (1a)a
Ph
Ph
Catalysts
Ph
Ph
Ph
Ph
Ph
Ph
Solvent, additives
H2 balloon, Temp.
1a
3a
2a
Entry
Catalyst
Temp. Time Additives EtOH:H2O
Conv.
Sel.
Sel.
(ºC)
(h)
(%)b
(2a/3a)
(Z/E)
(%)b
0/0
0/0
0/0
(%)b
1.
2.
3.
4.
Ni
Fe
Ni3Fe1
Ni1Fe1
50
50
50
50
5
5
5
5
--
--
--
--
1:1
1:1
1:1
1:1
>99
0
0/>99
0/0
0/>99
0/>99
99
>99(75)c
0/0
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5.
6.
7.
8.
9.
10.
Ni1Fe3
Ni1Fe3
Ni1Fe3
Ni1Fe3
Ni1Fe3
Ni1Fe3
50
50
50
25
40
50
5
5
5
5
5
5
--
--
--
--
--
1:1
1:0
0:1
1:1
1:1
1:1
91
0
12
57
83
98
70/30
0/0
23/77
58/42
77/23
84/16
93/7
0/0
34/66
91/9
84/16
96/4
n-propyl
amine
n-propyl
amine
n-butyl
amine
iso-butyl
amine
n-pentyl
amine
PPh3
11.
12.
13.
14.
Ni1Fe3
Ni1Fe3
Ni1Fe3
Ni1Fe3
40
40
40
40
5
4
5
3
1:1
1:1
1:1
1:1
93
99
82
60
91/9
91/9
91/9
96/4
89/11
94/6
97/3
87/13
15.
16.
Ni1Fe3
Ni1Fe3
50
40
5
5
1:1
1:1
5
>99/--/-- >99/0
triethyl
amine
NaOH
99
30/70
80/20
17.
18.
Ni1Fe3
40
40
5
4
1:1
1:1
43
38
78/22
93/7
86/14
88/12
Ni/Fe3O4
(1:3)d
n-butyl
amine
aReaction conditions: 1a (1.0 mmol), catalyst (10 mol%), additive (0.55 mmol) under H2
b
atmosphere. Conversion of 1a and selectivity of (Z) cis-2a, (E) trans-2a and 3a was determined
by 1H NMR. c2a/3a; 56/44, 3h. dNi/Fe molar ratio.
At this stage, we also investigated the effect of ethanol-water ratio on the conversion and
selectivity of the products for the semihydrogenation of 1a over the best performing Ni1Fe3
catalyst. In ethanol, reaction could not proceed, and we recovered only the reactant 1a after the
reaction (Table 1, entry 6), whereas in pure water, 12% conversion of 1a was observed with only
lower selectivity for alkene (2a/3a; 23/77, Z/E; 34/66) (Table 1, entry 7). In contrary to the
above, we achieved good conversion with moderate selectivity for 2a in water-ethanol (v/v, 1:1)
mixture, suggesting the important role of water in this reaction (Table 1, entry 5). In the
literature, we found that water helps in the diffusion of hydrogen atoms on the surface of iron
oxide.53 Recently, similar findings were also reported by Yang et al. for the hydrogenation of α,β
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unsaturated aldehydes.54 Therefore, these precedent reports and our experimental results
evidenced that presumably water is assisting in the diffusion of hydrogen atoms on the catalyst
surface during the catalytic semihydrogenation reactions. Moreover, ethanol is completely
miscible in water without any phase separation and the presence of ethanol also facilitates the
solubility of diphenylacetylene in the ethanol-water solvent medium during the catalytic
hydrogenation of alkyne to Z-alkene over NiFe catalyst at 40 ℃ – 50 ℃. Furthermore, we have
also studied the effect of temperature, where we observed that performing reaction at room
temperature the activity and selectivity of the catalyst drastically decreased (Table 1, entry 8),
whereas at 40 °C, 83% conversion of 1a was achieved with 77% selectivity (Z/E; 84/16) for 2a
(Table 1, entry 9).
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Fig. 3 (a) Catalytic activity of the monometallic (Ni and Fe) and bimetallic Ni-Fe catalysts for
the hydrogenation of diphenylacetylene (1a) (ain the presence of n-butyl amine as an additive).
(b) Time dependent catalytic activity of Ni1Fe3 catalyst with n-butyl amine as an additive.
Screening of additives
A judicious selection of additives with appropriate adsorption properties with the catalyst surface
can lead to a remarkable enhancement in the selectivity for the desired product. In this regard,
amine-based additives are generally known to accelerate catalytic activity for the
semihydrogenation of alkyne to alkene.38,55 Therefore, we made further attempts to increase the
selectivity of the desired alkene product with high Z/E ratio by employing various amine-based
additives on the semihydrogenation of 1a catalyze over best performing Ni1Fe3 catalyst at 40 °C
in water-ethanol (1:1 v/v) solution. Firstly, we used n-propyl amine as an additive and observed a
substantial increase in the selectivity for the desired alkene product (2a) to 84% (Z/E 96/4) as
compared to 70% (Z/E 93/7) selectivity obtained for Ni1Fe3 catalyst in the absence of the
additive (Table 1, entries 5, 10). Notably, we observed that the apparent conversion of
diphenylacetylene (1a) was analogous for both the cases (with or without the additive) over the
Ni1Fe3 catalyst. Earlier reports also inferred that introducing primary amines as additives may
significantly tune the selectivity towards alkene product for the hydrogenation of 4-octyne over
CoPt3 and Pt catalysts.38 Analogously, high selectivity for alkene products were also reported
using amine additives based on diethylenetriamine (DETA) tridentate ligand for the
hydrogenation of alkynes over Pd nanoparticle supported on boron nitride (Pd/BN).55
Noticeably, higher selectivity for alkene products (2a, 91%, (Z/E; 91/9) with appreciably good
conversion (93%) was achieved even at 40 °C (Table 1, entry 11). Reports revealed that
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selectivity towards the alkene products can also be enhanced by tuning the steric crowding over
the catalyst surface.38,55 Therefore, to get a better understanding of the role of amine additives on
the selective semihydrogenation of 1a to the cis-/trans-alkene products (2a), we further
employed other aliphatic amines additives having branched and long carbon chain (n = 4 and 5).
Results showed that with the increase in the carbon chain of the linear amine additives,
conversion of 1a was also increased while maintaining the high selectivity for the Z-stilbene (2a)
(Table 1, entries 11-12). The observed higher activity of Ni1Fe3 catalyst in the presence of n-
butyl amine as compared to n-propyl amine was presumably due to the higher basicity of
former.38 Further increase in carbon chain to n-pentyl amine does not significantly change the
conversion and selectivity for the studied reaction (Table 1, entry 14). On the other hand,
relatively lower activity was observed in the presence of branched aliphatic amines (iso-butyl
amine, conv. 82%) as additives as compared to the corresponding linear amines (n-butyl amine,
conv. 99%), presumably due to steric effect (Table 1, entries 12-13). 38,55 Further the content of
the additive also significantly influenced the catalytic activity, where higher selectivity for the
alkene product (2a) was achieved with 0.55 equiv. of the n-butyl amine additive (Table S2).
These observations can be associated with the tuning of surface adsorption sites by the amine
additives, where larger coverage of additives over the catalyst surface competes with the alkene
product and hence hindered the over hydrogenation of alkene to alkane. 38,55 In contrary to the
amine-based additives, other additives, such as phosphine, trimethylamine and the inorganic base
(NaOH) exhibited either poor conversion of 1a or poor selectivity for the alkene (2a) products
(Table 1, entries 15-17). Therefore, among several additives screened, n-butyl amine exhibited
the significantly tuned and high selectivity for Z-alkene (2a) with higher conversion of alkyne
(1a) under the optimized reaction condition (Table 1 and Table S2). Furthermore, to rule out the
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role of polyvinylpyrrolidone (PVP) in controlling the reaction selectivity,50 we synthesized
Ni1Fe3 nanoparticles without using PVP and investigated their activity for semihydrogenation of
1a, with/without the n-butyl amine additive, under the optimized reaction condition. Though the
results (as summarized in Table S2), inferred the loss in the catalytic activity for Ni1Fe3 catalyst
(synthesized without using PVP), as compared to Ni1Fe3 catalyst (synthesized using PVP), the
selectivity of the Z-stilbene was not affected significantly. These findings suggest that PVP has
no significant role in tuning the selectivity of the semihydrogenated product (2a). However, the
absence of PVP may induce the agglomeration of nanoparticles, which presumably resulted in
the observed loss in the catalytic activity for Ni1Fe3 catalyst synthesized in the absence of PVP.
Consistent with the above results, TEM images of Ni1Fe3 catalyst, synthesized without using
PVP, also displayed agglomeration of nanoparticles (Fig. S9). Further, dynamic light scattering
(DLS) data of the aqueous-ethanol suspension of Ni1Fe3 nanoparticles also showed the high rate
of agglomeration of Ni1Fe3 nanoparticles synthesized in the absence of PVP, as indicated from
the distribution curves (Fig. S10). Moreover, P-XRD of Ni3Fe1 catalyst synthesized in the
absence of PVP, also displayed intense diffraction peaks at 2θ value of 40–50°, as compared to
those synthesized in the presence of PVP (Fig. S11).
The time-course profile for the hydrogenation of 1a over Ni1Fe3 catalyst in the presence
of n-butyl amine additive depicted that with the increase in reaction time conversion of 1a also
increased with no significant loss in the selectivity for Z-stilbene (2a) (Fig. 3b). Further, the
catalytic activity of the high performing Ni1Fe3 catalyst was also compared with several other
catalysts (Ni, Ni1Fe1, Ni/Fe3O4) under the optimized reaction condition (1a (1.0 mmol), catalyst
(10 mol%), n-butyl amine (0.55 mmol), H2 atmosphere, 40 ℃). Results clearly inferred that the
yield of the desired Z-stilbene (2a) for Ni and Ni1Fe1 catalysts was less as compared to the Ni1Fe3
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catalyst (Fig. S12). The observed trend is analogous to that observed for the semihydrogenation
reaction performed over various NiFe catalysts in the absence of n-butyl amine additive (Fig. 3a
and Table 1). The observed lower selectivity of Z-alkene over these catalysts, was presumably
due to their higher Ni content which is favorable for the formation of overhydrogenated
product.41 These results are consistent with the earlier findings of Pérez-Ramírez et al., where
with the increase in Ni content in the Cu3-xNixFe catalyst undesired formation of
overhydrogenation alkane product was observed.41 Further, the poor conversion (38%) of 1a
over Ni/Fe3O4 (Ni/Fe; 1/3 similar to Ni1Fe3) evidenced that the atomic level interaction between
Ni and Fe is crucial to achieve enhanced catalytic activity for the semihydrogenation of alkyne
(Table 1, entry 18). Moreover, performing the catalytic reaction by replacing H2 gas with the
other reducing agents (NaBH4 and N2H4) exhibited only a moderate conversion of 1a as
compared to H2 gas (Table S2). Hence, higher catalytic activity for the semihydrogenation
reaction of alkyne (1a) over Ni1Fe3 catalyst can be attributed to the high stability of adsorbed
alkynes over the catalyst surface, while selectivity for alkene (2a, Z/E) products was governed by
the low stability of the adsorbed alkenes on the catalyst surface influenced by the alkyne and the
amine additive.56,57 Furthermore, the observed traces of unwanted dibenzyl (3a) product was
formed via the second hydrogenation of alkenes, whereas trans-alkene was formed by the cis-
trans isomerization.56,57 Notably, Ni1Fe3 catalyst can be magnetically recovered using an external
magnet (Fig. S13). The recovered Ni1Fe3 nanoparticle catalyst was reused for another catalytic
run for the semihydrogenation of 1a and achieved high selectivity (89%) for 2a with appreciably
high diastereoselectivity ratio (Z/E 94:6) (Table S2, entry 5). These findings suggest that indeed
the Ni1Fe3 catalyst is catalytically active for the semihydrogenation of alkynes. The recovered
catalyst was characterized by TEM and EDS, which manifested the co-existence of both the Ni
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and Fe metals in the Ni1Fe3 catalyst with no significant segregation for either of the metals (Fig.
S14). However, the activity decreased in the further catalytic runs, presumably due to the
oxidation of catalysts during the catalytic reaction, as also suggested by the XPS analysis of the
spent catalyst (Fig. S15).
Heterogeneity test of Ni1Fe3 catalyst
We performed heterogeneity experiments as shown in Scheme 1, where under the optimized
reaction condition, semihydrogenation of diphenylacetylene (1a) was performed over the Ni1Fe3
catalyst. After 2 h of the reaction, reaction mixture was separated into two parts – first part was
1
analyzed by H NMR, while from the second part Ni1Fe3 catalyst was removed and the reaction
was allowed to run without catalyst for longer reaction time (3 h) under H2 balloon.
Ni1Fe3 catalyst
removed from the
reaction mixture
Ni1Fe3 catalyst
Ph
Ph
Ph
Ph
No reaction
C2H5OH/H2O
n-butylamine
H2 Balloon
C2H5OH/H2O
n-butylamine
H2 Balloon
47% Conv.
88% slec.
40 °C, 2 h
40 °C, 3 h
Scheme 1 Heterogeneity test of Ni1Fe3 catalyst for the semihydrogenation reaction of 1a
Result inferred that reaction could not proceed in the absence of Ni1Fe3 catalyst, hence
supporting the heterogeneous nature of Ni1Fe3 catalyst for the semihydrogenation of alkynes.
Moreover, the ICP-OES was used to identify traces of metals leached during the catalytic
reaction. Results are summarized in table S1. As expected, there was no change in the molar
composition of Ni and Fe after the catalytic reaction, suggesting that the studied Ni1Fe3 catalyst
is stable in water-ethanol solution. Furthermore, catalyst poisoning tests were also performed
using varying content of CS2 (Fig. S16), where complete poisoning of Ni1Fe3 catalyst was
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achieved with 0.5 equiv. of CS2 (as per Ni content in Ni1Fe3 catalyst), further supporting the
heterogeneous nature of the studied catalyst.
Deuterium labeling experiments
Deuterium labeling experiments were carried out to determine the source of hydrogen atom in
the alkene product, and to eliminate the direct role of the solvent molecules in the observed
catalytic activity (Scheme 2).
Ni1Fe3 (10 mol %)
D
D
D D Ph
D
Ph
D
Ph
Ph
D
C2H5OH/H2O
n-butyl amine
Ph
Ph
Ph
D
Ph
D2 balloon, 40 °C, 4 h
Selc.
90 %
0%
Conv. 96%
10%
Scheme 2 Deuterium labeling experiments for semihydrogenation of diphenylacetylene (1a)
over Ni1Fe3 catalyst.
Initially, the catalytic semihydrogenation of diphenylacetylene (1a) was performed over
Ni1Fe3 catalyst in (a) D2O-ethanol (1:1 v/v) and (b) D2O, under the optimized reaction condition,
where deuterated-labeled products were not observed (Fig. S17-S18). However, deuterium
2
incorporation in the alkene/alkane products (as authenticated by D NMR and GC-MS analysis)
was only observed, when the reaction was performed using D2 gas over the Ni1Fe3 catalyst in
water-ethanol under the optimized reaction condition (Fig. S19-S20). Therefore, the above
findings evidenced that the source of hydrogen in the hydrogenated products of alkynes was H2
gas and not water or ethanol.58
Catalyst structure and reaction pathway
It is evident from our experimental findings that the Ni surface is active for the hydrogenation of
alkynes, as complete hydrogenation of diphenylacetylene (1a) to dibenzyl (3a) was achieved
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over the monometallic Ni catalyst, whereas Fe was found to be inactive. On the other hand,
alloying Fe with Ni can significantly tune the alkene selectivity, while maintaining the high
catalytic activity for all the bimetallic Ni-Fe catalysts. P-XRD, TEM-EDS line scan, and XPS
endorsed the formation of the bimetallic Ni-Fe alloy catalysts, where ICP results authenticated
the desired Ni-to-Fe ratio in the synthesized bimetallic Ni-Fe catalysts. Therefore, the observed
tuned catalytic activity of Ni1Fe3 catalyst can be associated with the effect of alloying Fe with Ni
in Ni/Fe = 1:3, which significantly tuned the properties of the catalyst surface for interaction with
alkynes and controlled activation of H2 molecules. To further investigate the interaction behavior
of catalyst surface with H2 molecule, and to correlate the observed trend with the composition of
the bimetallic Ni-Fe catalysts, H2-Temperature programmed desorption (TPD) experiments over
monometallic (Ni and Fe) and bimetallic (Ni3Fe1, Ni1Fe1 and Ni1Fe3) were performed and the
corresponding H2-TPD traces are shown in Figures S21 and S22.59,60 H2-TPD of monometallic
Fe catalyst exhibited a small desorption peak, while the monometallic Ni displayed a prominent
hydrogen desorption peak at 371 °C. Therefore, it is clear that the Ni nanoparticle catalyst desorb
greater amount of H-species, indicating a greater capacity of Ni surface to activate H2 compared
to Fe catalyst.41 Further, it is known that H2 dissociation over Ni(111) surface is exothermic (–
0.75 eV), and hence the availability of abundance of hydrogen atoms over the catalyst surface
may result in overhydrogenation of alkyne to alkane, which is in good agreement to our
experimental results and most of the literature reports.41 On the other hand, alloying Fe with Ni
resulted in a significant tuning in the adsorption behavior of catalyst surfaces. As compared to
the monometallic Ni and Fe catalysts, the H2-TPD measurements of the bimetallic Ni-Fe
catalysts inferred a remarkable shift in the H2 desorption peak maxima towards higher
temperature region (379–381 °C), suggesting the strong interaction of the bimetallic Ni-Fe with
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H2 molecules. As it is clearly seen from TEM-EDS and XPS results that the bimetallic Ni-Fe
alloy catalysts contain Ni and Fe in their alloy composition, the H2-TPD results also endorsed
that in the bimetallic Ni-Fe alloy catalysts interaction between Ni and Fe significantly tuned the
properties of the catalyst active sites, leading to the enhanced interaction of H2 molecules with
the catalyst surface. These observations are also consistent with the trend in catalytic
performance of the bimetallic Ni-Fe catalysts, where all these catalysts having varying Ni to Fe
ratio exhibited higher catalytic activity for the hydrogenation of diphenylacetylene (1a) (Table
1). Notably, in contrast to the bimetallic Ni1Fe1 and Ni3Fe1 catalysts, the Ni1Fe3 catalyst
desorbed less hydrogen, but with a higher desorption maxima (380 °C). These observations
inferred that alloying Fe with Ni not only induced the facile activation of H2 molecules over the
catalyst surface, the higher content of Fe in Ni1Fe3 catalyst also significantly control the
adsorption sites on the catalyst surface. Further tuning of catalyst surface of the bimetallic Ni-Fe
catalyst by alloying Fe with Ni in an appropriate composition may greatly influence the
interaction of alkene over the catalyst surface and thus disfavoring overhydrogenation.46,47
Hence, the observed higher catalytic activity of Ni1Fe3 catalyst is not only related to the higher
intrinsic activity of Ni, but also to Fe which plays an important role in tuning the adsorption sites
of the catalyst surface, as also evidenced by the characterization data and H2-TPD experiments,
which is considered to be a key factor for the observed higher selectivity for alkene over the
Ni1Fe3 catalyst.
Compared to the literature reported non-noble metal based bimetallic Ni-Cu, Ni-Fe and
Ni-Co catalysts,40,41 the Ni1Fe3 catalyst explored in this work displayed high catalytic activity
with excellent selectivity for the desired Z-alkene product using H2 balloon at 40 ℃. Liu et al.
employed NiCu catalysts for semihydrogenation of internal and terminal alkynes to
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corresponding alkenes at 65 ℃ – 100 ℃ in toluene under 4 bar – 10 bar H2 pressure.39 They
achieved 81.3% selectivity for stilbene (2a) from diphenylacetylene (1a) over NiCu catalyst
(pre-NiCu/MMO, MMO = mixed metal oxide) at 80 ℃ in toluene under 10 bar H2 pressure. The
observed high alkene selectivity over NiCu catalyst was attributed to easy desorption of alkene
over the electron rich Ni surface resulted due to the charge transfer from Cu to Ni. de Vries et al.
also explored NiFe catalyst along with several bimetallic NiCo and NiCu catalysts and
monometallic Ni, Co, Fe and Cu catalysts for the semihydrogenation reaction of 1-octyne in THF
under 5 bar H2 at room temperature.40 Their findings inferred that the monometallic Ni and Co
nanoparticles were more active towards 1-octyne hydrogenation with 86% and 76% alkene
selectivity for the semihydrogenated product as compared to Fe nanoparticles (70% alkene sel.).
On the other hand, Cu catalyst was found to be inactive for the hydrogenation of 1-octyne.
Notably, they observed that the bimetallic NiFe (80% alkene sel.) and NiCo (75% alkene sel.)
catalysts could not performed well and exhibited catalytic activity analogous to the
corresponding monometallic catalysts.40 Ternary Cu-Ni-Fe and Cu3Fe catalysts were also
explored for gas-phase hydrogenation of propyne at 250 ℃ with high alkene selectivity, where
the presence of Ni greatly improved the alkene selectivity by disfavoring oligomerization.41
Compared to these reports, our investigations showed that the monometallic Fe catalyst was
inactive, while Ni catalyst exhibited high activity for the hydrogenation of 1a but with no alkene
(2a) selectivity at 50 ℃ using H2 balloon in water-ethanol solution (Table 1). On the other hand,
bimetallic Ni-Fe catalysts exhibited tuned catalytic performance, where the Ni to Fe ratio in the
bimetallic Ni-Fe catalysts significantly influenced the catalytic activity and selectivity towards
the semihydrogenated alkene product (2a). We observed that Ni1Fe3 outperformed the other
bimetallic Ni-Fe catalysts and exhibited appreciably good conversion (91%) of 1a with high
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selectivity (70%; Z/E 93/7) for the semihydrogenated product Z-stilbene (2a) at 50 ℃ using H2
balloon in water-ethanol solution. Moreover, amine additives further improved the selectivity to
91% (Z/E 96/4) for Z-stilbene (2a) even 40 ℃ with 99% conversion of (1a), presumably by
disfavoring alkene adsorption and overhydrogenation due to relatively high adsorption behavior
of amine additive as compared to the alkene product. Hence, based on our experimental findings
and the available precedent reports,58 it is evident that presumably the homolytic dissociation of
H2 gas initially occurred over the catalyst surface, which was followed by the concerted transfer
of hydrogen atoms from the catalyst surface to the adsorbed alkynes leading to the formation of
the Z-alkene products. A schematic representation of reaction pathway for alkene
dehydrogenation is displayed in Fig. 4.
H2
R
H
R'
H
H2
R
R'
H
R
R'
H
H
H
Ni
Fe
NH2
=
Fig. 4 Schematic representation of the reaction pathway for semihydrogenation of alkynes over
Ni-Fe catalysts.
Substrate generality for semihydrogenation of alkynes
To expand the scope and generality of the highly active Ni1Fe3 catalyst, we further investigated
catalytic semihydrogenation of a wide range of internal alkynes (1a-1n) under the optimized
reaction condition in water-ethanol solution at 40 ºC, where high selectivity towards the
respective Z-alkenes products was achieved (Table 2, entries 1-12). When para-substituted diaryl
alkynes (1b and 1c) were used, reaction proceeded well to yield the respective alkenes with high
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selectivity (Z/E 100/0 (2b) and 97/3 (2c)) (Table 2, entries 2-3). Notably, the sterically hindered
meta-dimethyl substituted diphenylacetylene (1d) was also selectively hydrogenated to alkene
(2d) (2d/3d 91/9, Z/E: 100/0) with good conversion (95%) (Table 2, entry 4). On the other hand,
with the diphenylacetylene substrate (1e) having amide group at para position poor conversion
was observed, attributed to the weak adsorption of triple bond over the catalyst surface (Table 2,
entry 5).61 Analogous to diphenylacetylene, aryl-alkyl alkynes (1f–1j) were also effectively
hydrogenated to their corresponding alkene products (2f–2j) excellent selectivity for the Z-
alkenes (Z/E 93/7 – 100/0). The hydroxyl substituted internal alkyne (1f) was hydrogenated to
the corresponding Z-alkene (2f) good conversion (90%) and high selectivity for the
corresponding Z-alkene (Z/E 100/0), but reaction was little sluggish (Table 2, entry 6).62
Moreover, the more sterically hindered hydroxyl substituted internal alkynes 1g and 1h were also
successfully hydrogenated to Z-alkenes (Z/E 93/7 (2g) and 96/4 (2h) with high conversion (99%)
(Table 2, entries 7-8). The observed high selectivity for Z-alkenes with sterically bulky hydroxyl
substituted internal alkynes was probably due to the weak interaction of alcohol group with the
catalyst surface which retarded the over reduction of alkene (2g and 2h) to alkane (3g and 3h).
Moreover, semihydrogenation of alkyl-arylalkynes with substituted (1i) and unsubstituted (1j)
aliphatic chain also proceeded well to their corresponding Z-alkene products with good
selectivity (Z/E 97/3 (2i) and 100/0 (2j)) and high conversion (99%) (Table 2, entries 9-10).
Furthermore, we also explored Ni1Fe3 catalyst for the hydrogenation of bi(hetero)aryl alkynes
(1k and 1l), albeit lower conversion of alkynes to their corresponding alkene (2k and 2l)
products was observed, which can be associated with the strong interactions of the heteroatoms
(S and N) with the catalyst surface (Table 2, entries 11-12). Nevertheless, higher selectivity of
83% (2k) and 100% (2l) with complete selectivity for Z-alkene products was achieved over the
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Ni1Fe3 catalyst. Moreover, aliphatic internal alkynes, 3-hexyn-2-ol (1m) and 5-decyne (1n) can
also be successfully hydrogenated to Z-alkenes with very good selectivity (Table 2, entries 13
and 14). Apart from the internal alkynes, Ni1Fe3 catalyst was also subjected for the
hydrogenation of terminal alkynes, where only totally hydrogenated products were observed
(Scheme S1). Moreover, for practical application, we synthesized the catalyst on a higher scale
(10 times higher than the small-scale synthesis) and investigated its catalytic activity under the
optimized reaction condition. Results inferred that the activity and selectivity of the synthesized
Ni1Fe3 catalyst for the semihydrogenation of 1a to Z-stilbene (2a) do not change (Table S2).
Furthermore, the TEM image of the synthesized catalysts at higher scale shows the presence of
nanoparticles with an average particle size of ca. ~6.0 nm (Fig. S23), which is consistent with the
particle size (ca. ~6.5 nm) for the Ni1Fe3 catalyst synthesized in small scale (Fig. 1 and Fig. S4).
Table 2 Substrate scope of the Ni1Fe3 catalyst for semihydrogenation of internal alkynesa
R'
R'
Ni1Fe3 catalysts
R
R'
R
R'
R
R
C2H5OH/H2O
n-butyl amine
H2 balloon, 40 C
1a-1n
3a-3n
2/3 (%)c
2a-2n
Entry
Reactants
Products
Time
(h)
4
Conv.
(%)c
99
Z/E (%)c
1
91/9
96/4
1a
1b
2
3
6
7
84
99
92/8
100/0
97/3
87/13
OMe
1c
OMe
4
7
95
91/9
100/0
1d
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O
5
10
7
74/26
100/0
NH2
1e
O
H2N
6
7
11
8
90
99
96/4
94/6
100/0
93/7
OH
OH
1f
OH
OH
1g
8
12
99
93/7
96/4
MeO
OH
OH
OH
1h
1i
MeO
9
5
5
99
99
98/2
95/5
97/3
OH
10
100/0
1j
1k
1l
11b
12b
13d
14 d
10
9
57
19
99
99
82/18
100/0
100/0
95/5
100/0
100/0
100/0
100/0
N
S
N
S
6.5
4
OH
OH
1m
1n
aReaction conditions: Alkyne (1.0 mmol), catalyst (10 mol%), n-butyl amine (0.55 mmol), water-
b
ethanol (20 mL, 1:1 v/v), 40 ºC. Alkyne (0.5 mmol), catalyst (20 mol%), n-butyl amine (0.55
mmol), water-ethanol (20 mL, 1:1 v/v), 40 ºC. cConversion of the substrate and selectivity of (Z) cis-
1
d
alkene, (E) trans-alkene and alkane was determined by H NMR. Conversion of the substrate and
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selectivity of (Z) cis-alkene, (E) trans-alkene and alkane was determined by GC.
Experimental
Syntheses of Ni1Fe1, Ni3Fe1 and Ni1Fe3 catalysts
Bimetallic Ni-Fe catalysts with varying Ni/Fe molar ratio were synthesized using
polyvinylpyrrolidone (PVP) stabilized aqueous phase co-reduction process.47 Typically, 0.012 g
of NiCl2·6H2O and 0.042 g of FeSO4·7H2O were dissolved in 5 mL of distilled water containing
0.100 g of PVP and sonicated for 5 min. An aqueous solution of sodium borohydride (5 mL,
0.050 g) was added dropwise to the above mixture and sonicated again for 5 min to obtain a
black suspension of Ni1Fe3 catalyst, which was magnetically separated and washed with a
copious amount of deionized water and ethanol under inert gas protection in a Schlenk line.
Analogously, Ni1Fe1 and Ni3Fe1 catalysts were synthesized using the appropriate amount of the
Ni+2 and Fe+2 precursors in the specified molar ratio. Moreover, the monometallic Ni and Fe
catalysts were also synthesized following an analogous synthetic procedure. It should be noted
that all the catalysts were magnetically separated except the Ni catalyst, which was separated by
centrifugation (5000 rpm for 10 min). The XPS survey analyses of the synthesized catalysts (Fig.
S24-S28) exhibit the presence of prominent peaks of the desired elements, suggesting that the
synthesized catalysts are free from impurities such as metal salts or other byproducts.
Catalytic semihydrogenation of alkynes
Reaction flask was charged with the freshly prepared catalyst (10 mol%) dispersed in
water/ethanol (20 mL, 1:1 v/v), and was purged with N2 gas. After that alkynes (1.0 mmol) and
n-butyl amine (0.55 mmol) were added in the solution and the reaction mixture was stirred at 40
°C equipped with a H2 balloon. After completion of the reaction, catalyst was magnetically
separated from the reaction mixture. The reaction mixture was extracted with dichloromethane (3
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× 5 mL), and the combined organic layer was dried over anhydrous Na2SO4, filtered, and
1
13
concentrated under reduced pressure. Products were characterized by H NMR, C NMR and
GC-MS.
Conclusion
We synthesized bimetallic Ni-Fe catalysts and explored these catalysts for the catalytic
semihydrogenation of internal alkynes to Z-alkenes under moderate reaction conditions. All the
monometallic (Ni and Fe) and bimetallic Ni-Fe catalysts were extensively characterized to
establish their morphology, chemical nature and adsorption sites using HR-TEM, SEM, XPS,
EDS point and line scan, PXRD and H2-TPD measurements. Results inferred that the catalytic
activity of the studied bimetallic Ni-Fe catalysts was significantly governed by the Ni-to-Fe ratio,
where Ni1Fe3 catalyst exhibited the superior catalytic activity for the semihydrogenation of
alkene with high diastereoselectivity for Z-alkenes. Notably, significantly distinct catalytic
performance of the bimetallic Ni-Fe catalysts as compared to the monometallic Ni and Fe
catalysts was observed suggesting the crucial role of the alloying Fe with Ni in achieving
superior catalytic performance of the bimetallic Ni-Fe catalyst. H2-TPD results inferred that
alloying Fe with Ni not only induced the facile activation of H2 molecules over the catalyst
surface, but the higher content of Fe in Ni1Fe3 catalyst also significantly controlled the
adsorption sites on the catalyst surface. Moreover, we emphasized the role of amine-based
additives to substantially enhance the alkene selectivity, and found that n-butyl amine
accelerated alkene selectivity by disfavoring overhydrogenation of alkyne to Z-alkane. Hence,
the generality and scope of the high performing Ni1Fe3 catalyst were explored for the catalytic
hydrogenation of a wide range of internal alkynes to respective alkenes wherein significantly
high selectivity for Z-alkene was achieved. Advantageously, the bimetallic Ni-Fe catalysts can be
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easily recovered using an external magnet after the catalytic reaction. Furthermore, deuterium
labeling experiments using D2 and D2O revealed that the source of hydrogen in alkenes is H2 gas,
and not water or ethanol. Worthy to mention here, the studied Ni1Fe3 catalyst exhibited excellent
stability in water-ethanol solvent, as confirmed by ICP, TEM and XPS. We believe that the
synthesized bimetallic Ni-Fe catalysts represent a class of magnetically recoverable low-cost
high-performing non-noble metal-based catalysts which can open new ways to the scientific
community for exploring such catalysts for various other industrially important reactions.
Acknowledgements
This work is financially supported by the Science and Engineering Research Board, New Delhi
(EMR/2016/005783) and Council for Scientific and Industrial Research, New Delhi
(01(2885)/17/EMR-II) and the Indian Institute of Technology Indore, India. SIC, IIT Indore,
UGC-DAE, Indore and IKFT-KIT are acknowledged for the instrumentation facilities. R.K.R.
thanks IIT Indore for his research associateship. M.K.A. thanks INSPIRE for senior research
fellowship. We would like to acknowledge Martin Hähsler for support with TEM analysis.
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