Selective hydrogenation of unsaturated carbonyls by Ni-Fe-based alloy catalysts

Article abstract of DOI:10.1039/c7cy00945c

Ni-Fe alloy catalysts prepared by a simple hydrothermal method and subsequent H₂ treatment exhibited the greatest activity and selectivity for the hydrogenation of biomass-derived furfural to furfuryl alcohol among the examined second metals, such as Al, Ga, In, Co, and Ti. This work reveals that the alloying of Ni and Fe is a key factor in achieving highly selective hydrogenation of the CO moiety in unsaturated carbonyl substrates. We found that decreasing the temperature of H₂ treatment (i.e. decreasing the crystallite size), e.g. Ni-Fe(2)HT-573 K (TOF = 952 h^-1), increased the activity compared to that over Ni-Fe(2)HT-673 (TOF = 375 h^-1) for furfural hydrogenation. This result suggests that a low-coordinated Ni-Fe alloy was imperative for the catalytic cycle. Moreover, the effect of the metal/support interface was critical; despite the high catalytic performance of Ni-Fe/TiO₂, Ni-Fe/Al₂O₃, and Ni-Fe/CeO₂, Ni-Fe supported on SiO₂, taeniolite, and hydrotalcite catalysts were ineffective. Vibrational studies using FT-IR measurement confirmed that furfural was physically adsorbed on the surface of the Ni-Fe alloy catalyst via an η¹(O) configuration. The synthetic scope of the Ni-Fe catalytic system was very broad; various types of unsaturated carbonyls, such as unsaturated aromatics, unconjugated aliphatics, and a large substituent, were selectively converted into the corresponding unsaturated alcohols.

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Selective hydrogenation of unsaturated carbonyls by Ni-Fe based
alloy catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Wahyu S. Putro, Takashi Kojima, Takayoshi Hara, Nobuyuki Ichikuni, and Shogo Shimazu*
Ni-Fe alloy catalysts prepared by a simple hydrothermal method and subsequent H2 treatment exhibited the greatest
activity and selectivity for the hydrogenation of biomass-derived furfural to furfuryl alcohol among the examined second
metals, such as Al, Ga, In, Co, and Ti. This work reveals that the alloying of Ni and Fe is a key factor in achieving highly
selective hydrogenation of C=O moiety in unsaturated carbonyl substrates. We found that decreasing temperature of H2
treatment (i.e decreasing crystallite size); e.g Ni-Fe(2)HT-573 K (TOF = 952 h-1) increased the activity compared to that over
Ni-Fe(2)HT-673 (TOF = 375 h-1) for furfural hydrogenation. This result suggests that low-coordinated Ni-Fe alloy was
imperative for catalytic cycle. Moreover, the effect of metal/supports interface was critical; despite the high catalytic
performance of Ni–Fe/TiO2, Ni–Fe/Al2O3, and Ni–Fe/CeO2, Ni–Fe supported on SiO2, taeniolite, and hydrotalcite catalysts
were ineffective. Vibrational studies using FT-IR measurement informed that furfural was physically adsorbed on the
surface of Ni-Fe alloy catalyst via η1(O) configuration. The synthetic scope of the Ni–Fe catalyzed system was very broad;
various types of unsaturated carbonyls, such as unsaturated aromatics, unconjugated aliphatic, and a large substituent
DOI: 10.1039/x0xx00000x
www.rsc.org/
were
selectively
converted
into
the
corresponding
unsaturated
alcohols.
unsaturated aldehydes and ketones.9 However, heterogeneous
catalysts are much more desirable from an environmental and
industrial point of view.
1. Introduction
Selective hydrogenation of unsaturated carbonyls containing
C=C and C=O bonds (Scheme 1) has been extensively studied
for fundamental research in catalysis due to it being an
important step in the synthesis of various fine chemicals.1
Although a large number of studies have been performed, the
development of highly active and selective catalysts for the
hydrogenation of C=O bonds remains challenging because
hydrogenation of C=C bonds is thermodynamically and
kinetically favorable compared to that of the C=O bond.2 In
some cases, unexpected isomerization of unsaturated alcohols
to saturated carbonyls may also occur, leading to decreases
catalytic selectivity.3 Conventionally, highly selective
hydrogenation of C=O bonds can be accomplished by
stoichiometric reductive agents, such as LiAlH4 and NaBH4.4
Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction using
aluminum alkoxide catalysts is also highly specific for C=O
hydrogenation.5 However, these methods suffer from low
Scheme 1 Reaction pathways for the hydrogenation of
unsaturated carbonyls.
Heterogeneous monometallic catalysts, such as Pt, Rh, Ru,
Cu and Ni, preferentially hydrogenate C=C bonds, giving the
corresponding saturated ketones.10–12 Improved C=O
hydrogenation can be achieved by using additives or
promoters, tuning particle sizes, varying the nature of the
oxide supports and modifying the active metal with second
metals.13–17 Modifying the active metal with a second metal to
form a bimetallic catalyst is a significant factor in regulating
the catalytic selectivity for C=O bonds. The nature of bimetallic
catalysts, such as the electronic effect, synergistic effect, and
alloy formation, markedly affect the catalytic selectivity.18–20
Bimetallic catalysts based on Rh, Pt, Ru, Ir, Pd and Ni have
been extensively studied to achieve higher selectivity for
hydrogenation of C=O bonds.21–26 In Pd-based catalysts, the
addition of a second metal, such as Co, Sn or Cu, increases the
selectivity for the hydrogenation of various unsaturated
carbonyls.27–29 Catalysts based on Ni, which have similar
catalytic behavior to catalysts based on Pd, are the best
activity,
a limited substrate scope, and production of
equivalent salts. Therefore, the development of active,
selective, and reusable catalysts in a wide range of substrates
would be ideal.
Recent breakthroughs in the selective hydrogenation of
unsaturated carbonyls have been focused on the development
of homogeneous and heterogeneous catalysts. Homogeneous
catalysts were intensively studied using Ir, Ru, and W with
ligands.6–8 J. G. de Vries and coworkers recently reported an
active Ru complex catalyst for the hydrogenation of α,β-
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alternative to avoid the use of expensive noble metal.
Moreover, Ni has an exceptional reactivity compared with Pd
providing an advantage for the catalysis, but it gives rise a
problem for the selective catalysis.30 Therefore, the
improvement of selectivity for hydrogenation requires high
effort.
TiCl3 (Aldrich), TiO2 (WAKO), CeO2 (WAKO), Al2O3 (WAKO),
Hydrotalcite (WAKO), Na-Taeniolite (TOPY Industry, LOT 45-
974A), Carriact SiO2 P-10 (TOPY Industry, UH-2890), and trans-
decahydronaphthalene (WAKO) were purchased and used as
received. Organic chemical compounds, such as 2-propanol
(WAKO, 99.7%) and furfural (TCl, >98%), were purified using a
standard procedure recorded at the Purification of Laboratory
Chemicals.
Our group previously found that Sn alloyed with Ni-based
catalyst exhibited highly efficient and selective hydrogenation
of various unsaturated carbonyl compounds.25,26 Li et al. also
reported that Ni-In intermetallic nanocrystals with tunable
particle size via the use of layered double hydroxide (LDH)
precursors enhanced catalytic activity and selectivity for the
hydrogenation of unsaturated carbonyls.31 For bimetallic Ni-Fe
catalyst, Halilu et al. reported that magnetic Fe(NiFe)O4-SiO2
nanocatalyst selectively hydrogenated furfural to furfuryl
alcohol with 94% conversion and 100% selectivity.32 Bimetallic
Ni-Fe–based catalysts have also been used for the
hydrogenation of various nitro compounds.33,34 However,
alloyed Ni-Fe catalysts have not been extensively studied for
the selective hydrogenation of various unsaturated carbonyls.
Furfural was initially used as a model for the present
studies of selective hydrogenation due to the presence of C=O
and C=C groups in the chemical structure. As one of the most
important biomass-derived platform molecules, furfural,
obtained through the hydrolysis/isomerization/dehydration of
hemicellulose, is widely converted to a large number of useful
chemicals through a series of complex reaction networks.35
Furfural may also undergo diverse reactions, such as
2.2 Synthesis of bulk Ni-Fe alloy
The Ni-Fe alloy catalyst was prepared by
a simple
hydrothermal method and subsequent H2 treatment with NiCl2
and FeCl3 as precursors, NaOH as the precipitator, and water
as the green solvent.37 At room temperature, NiCl2 (4 mmol)
was dissolved in 10 mL deionized water. Using different
glassware, FeCl3 (2 mmol) was dissolved in 10 mL deionized
water. Aqueous NiCl2 was added dropwise to aqueous FeCl3.
The solution was stirred for 30 minutes at 600 rpm. NaOH (3
M) was added dropwise to the aqueous solution to reach
pH∼12. All operations were performed at ambient conditions.
The suspension was then placed in a sealed Teflon autoclave
for the hydrothermal process at 423 K for 24 h. The black fluffy
solid was filtered, washed using deionized water and acetone
several times, and dried under vacuum overnight. The black
powder was treated with H2 at prescribed temperature (523-
723 K) prior to the catalytic reaction. Ni-M(1)HT-673 (M = Al,
Ga, In, Co, and Ti) catalysts were prepared by the same
decarbonylation,
hydrogenolysis,
oxidation,
and
hydrogenation into valuable chemicals and fuels, such as
furfuryl alcohol, 2-methylfuran, 2-methyltetrahydrofurfuryl
alcohol, tetrahydrofurfuryl alcohol, cyclopentanone, furfuryl
amine, and 1,5-pentanediol.32 Furfuryl alcohol, which can be
used to produce resins, synthetic fibers, and chemical
products, such as vitamin C, lysine, and tetrahydrofurfuryl
alcohol, is considered one of the most interesting and valuable
furfural derivatives.36 Therefore, selective hydrogenation of
the C=O moiety of furfural is necessary to obtain furfuryl
alcohol.
After determining the optimum furfural hydrogenation
conditions, bimetallic Ni-Fe–based alloy catalysts were applied
for selective hydrogenation of a broad substrate scope of
unsaturated carbonyl compounds. We expanded our study to
the preparation of Ni-M–based catalysts using other
electropositive metals, such as Al, Ga, In, Co, and Ti (Fig. S1(a)),
to compare their catalytic performances in the hydrogenation
of furfural. Moreover, supported catalysts based on metal
oxide, such as SiO2, CeO2, Al2O3 and TiO2, were also assessed in
this study (Fig. S1(b)). Both bulk and supported Ni-Fe catalysts
procedure using AlCl3, GaCl3, InCl3
precursor, respectively.
⋅4H2O, CoCl2, and TiCl3 as a
2.3 Synthesis of supported Ni-Fe/MxOy catalysts
Ni-Fe(2)(wt. x)/TiO2 catalysts (x= 5%, 10%, 25%, and 50%) were
similarly prepared by the hydrothermal and subsequent H2
treatment method, in which the TiO2 was added prior to pH
adjustment using NaOH 3M. Ni-Fe(2)(wt. 25%)/TiO2 catalyst
was used as a standard of support catalysts. Ni-Fe(2)(wt.
25%)/MxOy catalysts (MxOy = SiO2, taeniolite, hydrotalcite,
CeO2, and Al2O3) were prepared by the same procedure as Ni-
Fe(2)(wt. 25%)/TiO2 catalyst.
2.4 Characterization
Substrate conversion was determined using a GC (Shimadzu
14B) equipped with a flame ionization detector and an
InertCap-624 capillary column (30 m, DF=1.4 μm, 0.25 mm) to
analyze various unsaturated carbonyl substrates with trans-
decahydronaphtalene as an internal standard solution. The GC
was operated under the following conditions: injection
temperature (493 K), detection temperature (523 K), initial
temperature (343 K), initial time (0 min), progress rate (10 K
min-1), final temperature (523 K), and final time (5 min) via
split mode (1/50). The system used nitrogen gas as the carrier
gas at a pressure of 50 kPa for carrier P1 and 100 kPa for
carrier P2. XRD measurements were performed on a Rigaku
MiniFlex 600 using monochromatic Cu Kα radiation (λ =
0.15418 nm). The XRD was operated at 40 kV and 15 mA with
divergence and scattering slit of 1°, a step width of 0.02° and a
scan speed of 10° min-1. The N2 adsorption-desorption
were prepared by
a simple hydrothermal method and
subsequent H2 treatment to reduce the metal catalyst in the
range of 523-723 K. This method is substantial for the
formation of Ni-Fe alloy and the catalytic performance of
selective furfural hydrogenation in liquid-phase reaction using
2-propanol as a solvent under H2 atmosphere (Fig. S2 and S3).
2. Experimental
2.1 Materials
NiCl2 (WAKO), FeCl3 (WAKO), NaOH (WAKO), InCl3⋅4H2O
(WAKO), GaCl3 (WAKO), CoCl2 (Kanto Chemical), AlCl3 (WAKO),
2 | J. Name., 2012, 00, 1-3
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isotherms were measured at 77 K on a Belsorp Max (BEL 3.1 Catalyst Characterization
Japan). The samples were degassed at 423 K for 2 h to remove
physisorbed gases prior to the measurements. The specific
surface areas were calculated using the BET equation, and the
average pore volume was estimated based on the adsorbed N2
volume at a relative pressure of approximately 0.995 using BJH
approach based on the desorption branch. TEM analyses were
performed on a Hitachi High-tech H-7650 microscope with a
field-emissive gun, operated at 150 kV. Samples for TEM
investigation were prepared by dispersing the catalyst
powders with water and placing several drops of the
suspension onto a Cu grid coated with carbon film.
The FTIR study of furfural adsorption was conducted in the
gas phase equipped by U-glass using a HORIBA FT-720
instrument and KBr to pelletize the Ni-Fe catalyst. The sample
was reduced using H2 at 573 K for 1 h prior to the adsorption
of furfural. Liquid furfural was vaporized and circulated onto
the sample for 2 h at 423 K. After evacuating the circulated
vapor furfural, the IR spectra were recorded at ambient
conditions with a resolution of 4 cm-1 for 32 scans.
For H2 uptake, the catalyst was first treated with H2 at 673
Fig. 1 XRD profile of (a) Fe HT-673; (b) Ni HT-673; (c) Ni-Fe(2)HT-
K
for 30 min. The H2 measurement was subsequently
523K; (d) Ni-Fe(2)HT-573K; (e) Ni-Fe(2)HT-623K; (f) Ni-Fe(2)HT-
conducted twice at 273 K due to the possibility of physical
adsorption on the tested catalysts. The latter adsorption, The phase composition of the as-synthesized Fe, Ni, and Ni-
representing the total physical adsorption, was subtracted Fe(2) alloy catalysts was characterized by XRD, and the typical
from the former adsorption to obtain the chemical adsorption XRD patterns of each sample are shown in Fig. 1 for 2θ angle
of H2. Each adsorption step was performed for 2 h until values ranging from 20 to 80°. The XRD pattern of as-prepared
equilibrium. The CO chemical adsorption proceeded using a Fe HT-673 catalyst (Fig. 1(a)) exhibits reflection attributable to
similar procedure to the H2 uptake measurement. Considering Fe3O4 phase with cubic structure (JCPDS card no. 85-1436). The
the formation of metal CO complexes, such as Ni(CO)4 and reflections of as-synthesized Ni catalysts (Fig. 1(b)) shows
Fe(CO)4, the chemisorption was executed at 273 K.
intensive peaks at 2θ = 44.49°, 51.9°, and 76.42°, which
correspond to face-centered cubic (fcc) Ni with a = 0.3523 nm
2.4 Activity test
and Fm3m space group based on standard JCPDS card no. 04-
The hydrogenation reaction was performed in a stainless steel 0850. Three distinct diffraction peaks at 2θ of 44.15°, 51.4°,
autoclave with an inserted glass vessel containing 0.05 g 75.6° displayed in Fig. 1(d)-(g) are indexed as (111), (200), and
catalyst, 1.1 mmol substrate (substrate/Ni ratio = 2), 3 mL 2- (220) of fcc γ-phase nickel-iron alloy, respectively, consistent
propanol as solvent, and 0.08 g trans-decahydronaphtalene as with the standard card (JCPDS card no. 38-0419, with a =
an internal standard. After the reactor was sealed, the air 0.3545 nm and Pm3m space group). No iron or nickel oxides or
content was quickly purged by flushing three times with 10 bar hydroxides phases were detected, which indicated that the as-
hydrogen gas. The temperature was then increased to 423 K synthesized Ni-Fe(2) alloy catalysts had a high degree of
for a certain reaction time. The bulk Ni-Fe alloy catalysts were crystallinity. Ni3Fe alloy formation was also indicated by the
easily removed using an external magnet or centrifugation for shift of the Ni-Fe alloy peaks to lower angle (ca.
supported catalysts. The product solution was analyzed using relative to the (111) peak of Ni (Fig. 1(b)). This result was also
GC. reported by L. Liu and coworkers, who observed a shift of the
The product yield, selectivity, and TOF for each product was Ni3Fe alloy (ca. 0.1 degree) compared to the standard Ni
∼ 0.3 degree)
calculated as follows:
pattern (JCPDS 04-0850).38 The average lattice constant, a,
calculated from the Ni3Fe of the (111) peak for Ni-Fe(2)HT-573-
723 is approximately 3.548 Å, which is slightly larger than that
observed for the Ni catalyst (a = 3.523 Å) because Fe atoms are
incorporated into the lattice site of Ni, leading to enlargement
of the interplanar spacing due to the larger atomic radii of iron
(rFe = 1.26 Å) than that of Ni (rNi = 1.24 Å).39
Yield(%) = mol of product produced ×100%
(1)
(2)
(3)
mol of substrate fed
mol of product produced
Selectivity(%) = mol of substrate consumed ×100%
mol of product (below 50% conversion level)
TOF(h-1) = mol of Ni on surface (
2×H2 chemisorption data)×react. time (h)
Table 1 Physicochemical properties of Ni-Fe(2) alloy catalysts.
3. Results and discussion
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[b]
[c]
[d]
D[e] Composition[f]
(a) (b)
Catalyst
Ni-Fe(2)[a]
SBET
108.6 24.9
97.0 22.3 12.9 14
Vm
H2
Ni-Fe Alloy
(22 nm)



Ni-Fe(2)HT-523

Ni3Fe (66.7%)
Ni (33.3%)
Ni3Fe (66.9%)
Ni (33.1%)
Ni3Fe (84.5%)
Ni (15.5%)
Ni3Fe (68.4%)
Ni (31.6%)
Ni-Fe(2)HT-573
Ni-Fe(2)HT-623
Ni-Fe(2)HT-673
Ni-Fe(2)HT-723
18.5
13.2
9.5
4.3 24.3 16
3.0 24.8 22
2.2 22.4 19
1.5 26.9 29
TiO anatase
2
200 nm D = 123.4 ± 22.1 nm
100 nm
Length = 569.5 ± 64.07 nm
TiO2
Fig. 2 Representative transmission electron microscopy (TEM)
images of (a) Ni-Fe(2)HT-573 and (b) Ni-Fe(2)/TiO2 HT-573.
6.4
Fig. 2(a) shows the overall morphology of the bulk Ni-Fe
sample composed of rod-like structures that are connected to
[a] Ni-Fe(2) without H2 treatment catalyst. [b] BET specific
surface areas determined by N2 physisorption at 77 K (m2 g-1).
[c] Pore volume (cm3 g-1). [d] H2 uptake after corrected for
each other to form
a disordered-net architecture. The
connection of the rod structure can be seen in the high-
magnification image of the bulk Ni-Fe alloy sample (Fig. 2(a)).
The average length of the rod Ni-Fe alloy structure was
estimated at 569 ± 64 nm. Commercial TiO2 anatase purchased
from Wako Pure Chemical Industry, Japan has a spherical
structure with average size of approximately 123 ± 22 nm (Fig.
2(b)). The presence of loaded Ni-Fe alloy nanoparticles was
confirmed by a small spot on the TiO2 matrix with a particle
diameter in the range of 18-22 nm (Fig. 2(b)).
physical and chemical adsorption at 273 K (
crystallite size of Ni-Fe derived from the XRD peak of Ni3Fe at
θ ∼ 44° using Scherrer’s equation (nm). [f] Composition (%)
ꢀ
mol g-1). [e] The
2
was estimated using the Multi-Rietveld Analysis Program on
Rietica software.
The physical properties, such as the BET surface area, pore
volume, H2 uptake, crystallite size of the Ni-Fe alloy, and
composition of the major components of Ni-Fe(2), with various
temperatures of H2 treatment catalysts are summarized in
Table 1. Type II adsorption isotherms with typical H2 hysteresis
loops and a P/Po at approximately 0.4-0.8 were obtained for
the H2-untreated Ni-Fe(2) catalyst (Fig. S4(a)). Such a typical
3.2 Catalytic Reactions
Table 2 Selective hydrogenation of furfural over various bulk
(Ni-M) and supported Ni-Fe catalysts.
isotherm, according to IUPAC classifications, indicates
a
disordered mesoporous system on the Ni-Fe(2) catalyst.40 The
typical isotherm gradually changed to a type III adsorption
isotherm without hysteresis loops after H2 treatment at 523-
723 K (Fig. S4(a)). Increasing the H2 treatment temperature
notably decreased the Ni-Fe catalyst porosity, i.e., the surface
area and pore volume, compared with the Ni-Fe(2) without H2
treatment (Table 1, 2nd and 3rd columns) due to the
aggregation of particles. The pore volumes (BET) of the Ni-
Fe(2)HT-523, HT-573, HT-623, HT-673, and HT-723 catalysts
were 22.3 cm3 g-1, 4.3 cm3 g-1, 3.0 cm3 g-1, 2.2 cm3 g-1, and 1.5
cm3 g-1, respectively, which were comparable to the previous
trend.37 The H2 uptake of various investigated Ni-Fe(2)
Time Conv. Selectivity [%]
Entry
Catalyst
Ni HT-673
(min)
[%]
1
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
61
43
24
0
1
2
3
4
5
6
7
8
9
120 100
120 100
7
Ni-Al(1)HT-673
Ni-Ga(1)HT-673
Ni-In(1)HT-673
Ni-Co(1)HT-673
Ni-Ti(1)HT-673
Ni-Fe(1)HT-673
Ni-Fe(2)HT-673
Ni-Fe(2)HT-673[a]
47
48
95
89
72
97
95
13
96
92
95
77
90
93
97
91
120
120
120
120
120
120
99
20
85
64
32
74
9
20
3
5
120 100
75
4
catalysts was in the range of 12.9-26.9
ꢀ
mol g-1, relatively
10 Ni-Fe(2)HT-673[b]
11 Ni-Fe(2)HT-573[c]
180
30
30
30
30
30
30
99
90
21
22
42
99
95
higher than that of our previous results in Ni-Sn alloy
catalysts.26
8
12 Ni-Fe(2)/SiO2HT-573[c]
13 Ni-Fe(2)/Tnl HT-573[c]
14 Ni-Fe(2)/Htc HT-573[c]
15 Ni-Fe(2)/CeO2HT-573[c]
16 Ni-Fe(2)/Al2O3HT-573[c]
17 Ni-Fe(2)/TiO2HT-573[c]
5
In the present report, a large particle size was represented
by estimating the crystallite size of the Ni3Fe alloy from the
XRD pattern (Fig. 1). Based on our calculation, the crystallite
size of the Ni-Fe(2) catalysts relatively increased with
increasing H2 treatment temperature.
23
10
1
3
30 100
0.367 M; WCAT
(substrate/Ni= 2); Viso-PrOH= 3 mL; P H2= 30 bar; and T= 423 K.
[a] Physically mixed Ni-Fe(2) catalyst; [b] Reaction conditions
H2= 10 bar; [c] Reaction conditions: C SUBS= 0.367 M; WCAT
H2= 10 bar; and T=
9
The proportion of crystallite phase contained in the various
Ni-Fe(2) catalysts was estimated using the multi-Rietveld
analysis program LH-Riet in the Rietica software package for
each XRD profile of the Ni-Fe(2) catalysts, as shown in Fig.
1.26,37 The major phases were detected as zerometallic Ni and
Ni3Fe alloy, in which the Ni3Fe alloy phase was notably larger
than that of the Ni phase with half composed of the Ni3Fe alloy
phase.
Reaction conditions:
C
°
=
= 50 mg
SUBS
°
:
P
°
°
=
50 mg (substrate/Ni= 2); Viso-PrOH= 3 mL; P
°
423 K. Tnl= taeniolite; Htc= hydrotalcite.
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The influence of various second metals (M = Al, Ga, In, Co,
The physically mixed Ni-Fe catalyst (Table 2, entry 9)
Ti, and Fe) in bimetallic Ni-M catalysts for the selective identically behaved to the catalytic activity exhibited over Ni
hydrogenation of furfural to produce furfuryl alcohol is HT-673 (Table 2, entry 1). It confirmed that the synergistic
summarized in Table 2. The reactions were initially performed effect between Ni and Fe in the physically mixed Ni-Fe catalyst
under 30 bar H2 at 423 K for 2 h in the screening series of was not achieved as expected, leading to the low selectivity for
various second metals. Ni metal exhibited high activity for the furfural hydrogenation. Our previous work reported that the
hydrogenation of furfural with low selectivity of FFalc optimized conditions of selective hydrogenation for furfural
(approximately 7%), providing various by-products, such as using Ni-Fe(2)HT-673 was observed under 10 bar of H2 for 3 h
(b)
tetrahydrofurfuryl
alcohol,
methyl
furan,
methyl reaction time to give 96% selectivity for product 1 (Table 2,
tetrahydrofuran, tetrahydrofuran, and trace amount of 1,5- entry 10).37 In the present report, we found that decreasing
pentanediol (Table 2, entry 1). Modifying the Ni catalyst with a temperature of H2 treatment at 573 K (TOF = 952 h-1, entry 11)
variety of second metals significantly enhanced the selectivity markedly increased the activity compared to that over Ni-
for furfuryl alcohol (Table 2, entries 2-7). These results Fe(2)HT-673 (TOF = 375 h-1, entry 10) under 10 bar of H2.
confirmed that second metals play an important role in the Therefore, Ni-Fe(2)HT-573 catalysts under 10 bar H2 of
modification of furfural adsorption, leading to the proximity of reaction condition was applied for further experiment (Table 2,
the C=O site to the active metal (Ni) and activation of the entries 12-17).
aldehyde group, resulting in high selectivity for the
corresponding unsaturated alcohol product.
The effect of the substrate/Ni ratio on the reaction was
evaluated by varying the catalyst ratio from 2 to 10 prior to
These results suggest that Ni-In, Ni-Co, and Ni-Fe catalysts study the effect of oxide support (Fig. S6). The percent
are effective for the selective hydrogenation of furfural (Table conversion of furfural to furfuryl alcohol increased with
2, entries 4, 5, 7, respectively). Because the Ni-Fe catalyst increasing catalyst loading, which was due to the proportional
showed the highest selectivity, the Ni-Fe alloy catalyst was increase of active sites (Ni) in the catalyst. The selectivity for
adopted as a model catalyst for the present study of selective furfuryl alcohol was not affected by catalyst loading since the
hydrogenation (Table 2, entry 7). We previously reported value was consistently greater than 90% in all cases. The
preliminary results of the selective hydrogenation for furfural, hydrogenation with ratio of substrate/Ni = 2 over the Ni-
where the selectivity of Ni-Fe catalysts increased as the atomic Fe(2)HT-573 catalysts was preferable and was employed for
ratio of Ni/Fe decreased, and Ni-Fe(2) HT-673 catalysts the further investigations.
displayed the highest activity and selectivity for product 1
The effect of Ni-Fe loading amount on TiO2 was
(Table 2, entry 8).37 The Ni-Fe(2) catalysts also exhibited high subsequently investigated. A series of Ni-Fe(2)/TiO2 catalysts
recyclability for the chemoselective hydrogenation of furfural with Ni-Fe loading of 5, 10, 20, 25, and 50 wt. % were
in a liquid-phase reaction system. In contrast, hydrogenation prepared. The XRD patterns of Ni-Fe alloy were not apparent
did not proceed without catalyst or with Fe metal catalyst for 5%-25% Ni-Fe loadings (Fig. S7). A small broad Ni-Fe peak
under identical conditions to entry 8, demonstrating that the was detected in the Ni-Fe(50 wt. %)/TiO2 catalyst, confirming
catalysts were required for the reaction and that iron alone that the Ni-Fe was well dispersed on the TiO2 support. The
could not hydrogenate furfural (results not shown). Indication catalytic performance was studied by maintaining mole
of transfer hydrogenation due to the use of 2-propanol as substrate/Ni ratio at 2 for each prepared catalyst (Fig. S8). The
solvent was not proven since the hydrogenation reaction did Ni-Fe(wt. 25%)/TiO2 catalyst exhibited the best performance in
not proceed at the same condition to entry 8 under N2 gas, the liquid-phase reaction system. The selectivity was greater
which is probably caused by the kinetic limitation arising from than 80% for the corresponding furfuryl alcohol. Based on
the slow rate of transfer hydrogenation compared with the these results, the support effect was investigated using a fixed
rate of hydrogenation using molecular hydrogen.
Ni-Fe amount (wt. 25%) (Table 2, entries 11- 17). The catalytic
100
100
(a)
(b)
1400
HT-573
HT-523
80
80
(b)
1200
HT-523
HT-673
HT-573
1000
800
600
400
200
60
40
20
0
60
40
20
0
HT-623
HT-623
HT-673
HT-723
80
HT-723
12
17
22
Crystallite size (nm)
27
0
20
40 60
Progress of time (min)
100
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results confirmed that several supports, including Al2O3, CeO2, Fe(2)/TiO2 gave 100% conversion for both substrates with 91%
and TiO2 (Table 2, entries 15-17), improved the selective and 87% selectivity at 423 K. The high selectivity observed in
hydrogenation of furfural compared to bulk catalyst (Table 2, the hydrogenation of aromatic carbonyls was led by the high
entry 11), which can be attributed to the relatively high stability of the C=C bonds in the aromatic ring.
dispersion of the Ni–Fe species on the supports giving rise to
active sites with a significantly higher catalytic activity. Layered Table 3 Selective hydrogenation of aromatic C=O compounds
structures, such as taeniolite and hydrotalcite, are over bulk and supported Ni-Fe(2)HT-573 catalysts.
inappropriate for this reaction system (entries 13 and 14).
Variation temperature of H2 treatment had a considerable
effect on the catalytic activity, as illustrated in Fig. 3(a).
Decreasing temperature of H2 treatment notably promoted
catalytic activity. For example, 96% conversion was achieved in
45 minutes with a high selectivity (95%) using Ni-Fe(2)HT-573
catalysts. However, high conversion (>95%) could be achieved
after a longer reaction time of 3 h using the Ni-Fe(2)HT-673
catalysts. In other words, the increase of activity may be
noticed from the calculation of TOF for each catalyst (Ni-
t
[min]
45
Conv.
(Sel.) [%]
96(95)
Entry
1
Catalyst
Substrate
Product
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
30
100(91)
2
Ni-Fe(2)HT-573
Ni-Fe(2)HT-573
60
100(99)
93(99)
3
4
5
375
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
60
30
95(85)
100(87)
Fe(2)HT-723
ꢀ
216 h-1; HT-673
ꢀ
ꢀ
ꢀ
375 h-1; HT-623 384 h-1;
Ni-Fe(2)HT-573
Ni-Fe(2)HT-573
60
92(89)
48(98)
HT-573ꢀ
952 h-1; and HT-523
1166 h-1.
The effects of crystallite size on the catalytic activity (in
term of TOF value) and selectivity are shown in Fig. 3(b),
indicating that the TOF decreased with increasing Ni-Fe alloy
crystallite size. As it is well known that smaller particle
possesses more coordinatively unsaturated particle sites,
which are responsible for the catalytic activity of the
hydrogenation of C=O bonds. The TOF results represent that
6
180
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
30
30
60
15
30
15
60
60
30
30
93(76)
98(98)
94(77)
92(95)
100(67)
95(92)
37(77)
56(87)
23(92)
50(95)
7
8
9
the Ni-Fe(2)HT-523 showed the most active catalyst (TOF
∼
O
1166 h-1). However, the formation rate of Ni-Fe(2)HT-523
catalyst was relatively lower than that of the Ni-Fe(2)HT-573
catalyst. For instance, the Ni-Fe(2)HT-573 catalyst gave 83%
yield of furfuryl alcohol in 30 minute reaction, whereas the Ni-
Fe(2)HT-523 catalyst resulted 55% yield at the same reaction
time. Furthermore, Ni-Fe alloy was not completely produced in
Ni-Fe(2)HT-523 with Fe3O4 as one of the side products, based
on XRD pattern of Fig. 1(c). Therefore, the bulk Ni-Fe(2)HT-573
and supported Ni-Fe(2)(wt. 25%)/TiO2 HT-573 was considered
as standard catalysts for further hydrogenation of various
substrate scopes under the present reaction system.
10
11
12
13
14
Ni-Fe(2)HT-573
45
100(0)
Ni-Fe(2)HT-573
Ni-Fe/TiO2
15
15
67(0)
4(44)
Ni-Fe(2)HT-573
45
11(11)
15
16
Ni-Fe(2)HT-573
45
23(25)
Ni-Fe(2)HT-573
Ni-Fe/TiO2 HT
30
30
86(81)
99(95)
3.3 Substrate Scope
Reaction conditions: °
(substrate/Ni= 2); Viso-PrOH= 3 mL; P
C
=
0.367 M; WCAT
=
50 mg
SUBS
The catalytic performance of bulk and TiO2-supported Ni-Fe(2)
catalysts in the hydrogenation of various types of unsaturated
carbonyl compounds is listed in Table 3. In this study, the
substrates are divided into three main groups of unsaturated
carbonyls: aromatics (Table 3, entries 1-6), unconjugated
aliphatic (Table 3, entries 7-11), and conjugated C=O
compounds (Table 3, entries 12-16). High selectivity (≥85%) for
hydrogenation of aromatic C=O bonds to corresponding
unsaturated alcohols was achieved over bulk Ni-Fe and
supported Ni-Fe(2)/TiO2 catalysts (Table 3, entries 1-6). The
activity of the catalysts varied for the respective substrate. The
catalytic activity over the Ni-Fe(2)/TiO2 catalysts was higher
than that over bulk Ni-Fe(2) catalysts for the hydrogenation of
furfural and 3-vinylbenzaldehyde (Table 3, entries 1 and 4).
The hydrogenation of furfural and 3-vinylbenzaldehyde over
bulk Ni-Fe(2) gave 96% and 95% conversion with 95% and 85%
selectivity, respectively, whereas the same reaction over Ni-
°
H2= 10 bar; T= 423 K.
For the hydrogenation of the unsaturated-unconjugated
carbonyls listed in entries 7-11, the Ni-Fe alloy catalysts
provided moderately high activity and selectivity. Bulk Ni-Fe(2)
catalysts resulted moderate selectivity (≥67%) for the
corresponding unsaturated alcohols, which was relatively
lower than supported Ni-Fe(2)/TiO2 catalysts (selectivity
≥87%). For instance, the hydrogenation of citronellal over bulk
Ni-Fe(2) gave 77% selectivity (94% conversion) in an hour,
whereas the same reaction over Ni-Fe(2)/TiO2 gave 95%
selectivity (92% conversion) in 15 minutes (Table 3, entry 8).
The hydrogenation of 3-cyclohexene-1-carboxaldehyde over
bulk Ni-Fe(2) and Ni-Fe(2)/TiO2 catalysts gave notable
conversions of 37% and 56% with 77% and 87% selectivity,
respectively, to corresponding alcohol in an hour reaction
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(Table 3, entry 10). In case of 5-norbornene-2-carboxaldehyde, listed in Table 4. Relatively high selectivity (52%) for the
bulk Ni-Fe(2) gave 23% conversion with 92% selectivity, and Ni- hydrogenation of 2-cyclohexene-1-one to produce the
Fe(2)/TiO2 gave 50% conversion with 95% selectivity (Table 3, corresponding unsaturated alcohol was achieved over Ni-
entry 11). Similar trend results were also obtained in the Fe(2)/Al2O3 catalyst (Table 4, entry 3). However, due to the low
hydrogenation of 10-undecenal (Table 3, entry 7) and 2- conversion (9%), the reaction was prolonged for 60 minutes.
cyclohexene-1-carboxaldehyde (Table 3, entry 9) over bulk and The conversion increased to 20%, while the selectivity
supported catalysts. Although no aromatic ring is present in unexpectedly decreased to 26% (Table 4, entry 4), indicating a
the chemical structure of unsaturated-unconjugated likely isomerization process. The desired unsaturated alcohol
carbonyls, high selectivity was produced due to the long- was initially formed, and upon further reaction, the product
separated chain of C-C single bonds between the C=C and C=O was partially isomerized to
a saturated carbonyl and
bonds. It can be speculated that the dispersion of active sites subsequently hydrogenated the C=O bond to produce
on the support could enhance the catalytic activity due to the saturated alcohol. Musolino et al. proved that the
active sites-support interaction.26
isomerization of
Among the previously examined substrates, the Pd/TiO2 catalysts, consequently suppressing the selectivity.3
hydrogenation of -unsaturated carbonyls, i.e. conjugated Interestingly, the opposite result was observed by Ni-Fe/CeO2
α,β-unsaturated alcohols proceeded over
α,β
C=O compounds, is considered the greatest challenge because catalyst, because the selectivity for C=C bonds was gradually
of the low stability of the C=C groups due to the lack of space- enhanced with decreasing reaction temperature (Table 4,
filling substituents at the C=C bonds. The expected entry 5-8). Approximately 98% selectivity at 100% conversion
unsaturated alcohol was not obtained for the hydrogenation for C=C bonds was obtained at 403 K (Table 4, entry 7). All
of conjugated C=O compounds over bulk and TiO2-supported these results exhibited that supported Ni-Fe/MxOy catalysts
Ni-Fe(2) catalysts (Table 3, entries 12-15). Moreover, the bulk accelerated and/or altered the catalytic hydrogenation on
Ni-Fe(2) catalysts could not hydrogenate the C=O groups of various unsaturated carbonyls, compared to the corresponding
cinnamaldehyde
and
2-cyclohexene-1-one
to
the bulk Ni-Fe catalysts. Furthermore, the Ni-Fe-based alloy
corresponding unsaturated alcohols even at 1% yield (Table 3, catalysts are highly suitable for hydrogenation on C=O group of
entries 12 and 13). On the other hand, the presence of a various unsaturated carbonyls, such as aromatic,
methyl substituent on the C=C (C3-C4) of 3-methyl-3-pentene- unconjugated, and large-substituent substrate system.
2-on (Table 3, entry 16) significantly increased the selectivity (>
80%) for the corresponding unsaturated alcohol since the 3.4 Proposed mechanism study
larger substituent prevented adsorption of C=C moiety via the
(e)
methyl functional group by Pauli repulsion and subsequent
hydrogenation of C=O bond.41
(d)
(c)
Table 4. Selective hydrogenation of 2-cyclohexene-1-one over
(b)
bulk and various supported Ni-Fe(2) catalysts.
(a)
1716 cm-1
1722 cm-1
Temp. Conv[c] Selectivity[d](%)
Entry
Catalyst
(K)
(%)
69
1
0
2
3
1
2
3
4
5
6
7
8
Ni-Fe(2)[a]
423
423
423
423
423
413
403
393
88
41
40
74
42
68
98
98
12
Ni-Fe/TiO2
Ni-Fe/Al2O3
Ni-Fe/Al2O3
Ni-Fe/CeO2
Ni-Fe/CeO2
Ni-Fe/CeO2
Ni-Fe/CeO2
4
9
44
52
26
0
16
8
1900
1800
1700
1600
1500
1400
Wavenumber (cm-1)
[b]
20
0
Fig. 4 FT-IR spectra of (a) furfural vapor, (b) furfural adsorbed
on Ni HT-573, (c) furfural adsorbed on Fe HT-573, (d) furfural
adsorbed on Ni-Fe(2)HT-573, (e) furfural adsorbed on Ni-
Fe(2)/TiO2 HT-573.
100
100
100
42
58
0
32
0
2
2
0
Reaction conditions
:
Co
=
SUBS
0.367 M; WCAT
=
200 mg
The adsorption of carbonyl groups on metal surfaces is mainly
coordinated via two possible modes: an η1(O) configuration, in
which O atom of carbonyl linearly coordinates with surface
metal atoms, or an η2(C,O) configuration, where both C and O
atoms interact with an intrinsic surface metal. The former type
is considered a weaker carbonyl-surface bond than the latter.
Using DRIFT spectroscopy, S. Sitthisa et al. found that furfural
was weakly adsorbed on pure Cu metal with a mono-
(Substrate/Ni= 2); Viso-PrOH= 3 mL; PH2= 10 bar; and t= 15 min.
[a] WCAT= 50 mg. [b] t = 60 min. [c] Convesion determined by
GC using an internal standard technique. [d] Selectivity to
product 2.
The low selectivity on cyclohexene-1-one prompted us to
examine the catalytic performance using various supported
catalysts for the hydrogenation of 2-cyclohexene-1-one, as
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coordinated η1(O) configuration observed at 1670 cm-1.42 They the Ni atom to the π* C=O group.46 Thus, with decreasing
explained that weakly bonded η1(O) configuration caused Ni/Fe ratio (increasing relative amount of Fe), the
smaller downshift versus η2(C,O) configuration which would hydrogenation of the C=O groups became easier than that of
result in a larger downshift, i.e., below 1450 cm-1. In the the C=C bonds, resulting in higher selectivity for the
present result, we attempted to elucidate the adsorption corresponding unsaturated alcohols.37 In the case of the
properties of furfural on Ni, Fe, Ni-Fe(2)HT, and Ni-Fe/TiO2 conjugated C=O substrate (n=0), low selectivity for
catalysts using FT-IR. As shown in Fig. 4(a), the C=O stretching unsaturated alcohols was observed. The lack of a space-filling
vibration mode of free furfural vapor was observed at substituent at the C=C bonds and the close distance between
approximately 1722 cm-1 with C=C furan ring observed at 1575 the C=O and C=C sites makes it challenging for catalysts to
cm-1 and 1479 cm-1.43 The adsorption states of the C=O bands selectively hydrogenate the C=O bonds. Furthermore, the
were slightly downshifted to 1716 cm-1 for furfural adsorbed isomerization of the unsaturated alcohol to the corresponding
on the Fe HT-573, Ni-Fe(2)HT, and Ni-Fe/TiO2 HT-573 catalysts saturated carbonyl made it increasingly difficult for the Ni-Fe–
(Fig. 4(b)-(e)). The lower frequency can be assigned to weakly based alloy catalyst to produce unsaturated alcohols.
physical adsorption of the C=O bond due to a weakening of the
CO bond resulted from the coordination with catalyst
surface. The η1(O)-aldehyde configuration of furfural was more
likely formed, since the band at below 1450 cm-1 indicating the
η2(C,O) configuration was not observed. On the contrary,
significant downshift was not detected for Ni HT-573 (Fig.
4(b)). Therefore, these results indicate that furfural C=O may
adsorb mainly on Fe species rather than Ni surface via η1(O)
configuration.
Conclusions
Ni-Fe alloy catalysts synthesized using a simple hydrothermal
method and subsequent H2 treatment showed 96% furfural
conversion with 95% selectivity to obtain the corresponding
furfuryl alcohol over Ni-Fe(2)HT-573 catalysts. The dispersion
of Ni-Fe(2) on CeO2, Al2O3, and TiO2 catalysts enhanced its
intrinsic activity and selectivity under the reaction conditions
compared to the bulk catalyst. FT-IR study revealed that the
furfural tends to adsorb through the η1(O) configuration on Ni-
Fe surface. Such catalyst design promotes the selective
hydrogenation of a wide range of organics with multiple
groups, e.g., unsaturated aromatics, unconjugated aliphatic,
and a large-substituent substrate. The low selectivity for the
hydrogenation of conjugated carbonyls might be due to the
isomerization of unsaturated alcohol. These results are
envisioned that the future work with more precise control
over isomerization process would further boost the catalytic
selectivity over Ni-Fe–based catalyst system.
Acknowledgements
This partial work was financially supported by JSPS KAKENHI
Grant Number 15K06565 and JSPS Bilateral Joint Research
Projects (2014-2017).
Scheme
2
Plausible mechanism for the selective
hydrogenation of unsaturated alcohols over Ni-Fe–based alloy.
Notes and references
Based on the results listed in Table 1, Ni-Fe alloy catalysts
remarkably enhanced the selective C=O hydrogenation of
furfural compared to monometallic Ni catalysts. Concerning
the role of metallic Fe, H. Li et al. revealed via XPS that metallic
Fe partially donated electrons to Ni.44 Electron-deficient Fe
could attract the lone pair of oxygen in the carbonyl group via
side-bonding interaction, as shown in Scheme 2, which
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bond. Ni has activity for homolytic-H2 dissociation to create
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site because the more electron-enriched Ni atoms also activate
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826.
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Catalysis Science & Technology
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DOI: 10.1039/C7CY00945C
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Ni-Fe alloy catalysts prepared by simple hydrothermal and subsequent H2 treatment
method showed highly selective hydrogenation for various unsaturated carbonyls.