The selective hydrogenation of ethyl stearate to stearyl alcohol over Cu/Fe bimetallic catalysts

Article abstract of DOI:10.1016/j.molcata.2014.05.009

Bimetallic and monometallic catalysts including Cu and/or Fe species were prepared by a co-precipitation method and their catalytic performance was tested for the selective hydrogenation of ethyl stearate to stearyl alcohol. The bimetallic catalysts were observed to be even more active for this selective hydrogenation compared to the monometallic catalysts and their physical mixtures. With a bimetallic catalyst of Cu/Fe (4/1 in mole ratio) reduced at 200 °C, a selectivity to the alcohol reached to above 99% at a conversion of 97% in reaction for 4 h at 230°C, 3.0 MPa. Effects of composition and reduction temperature on the catalytic performance were studied and the properties of catalysts prepared under different conditions were examined by XRD, TPR, N₂ physisorption, and SEM. The relationship of the performance with the properties of the catalysts was discussed, along with the conditions under which synergistic effects of Cu and Fe species appeared and caused the enhancement of the catalytic performance.

Full text of DOI:10.1016/j.molcata.2014.05.009

Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The selective hydrogenation of ethyl stearate to stearyl alcohol over
Cu/Fe bimetallic catalysts
Limin Hea,b,c, Xiaoru Lia,b,c, Weiwei Lina,b,c, Wei Lia,b,c, Haiyang Chenga,b, Yancun Yua,b
,
Shin-ichiro Fujitad, Masahiko Araid, Fengyu Zhaoa,b,∗
a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
c University of Chinese Academy of Sciences, Beijing 100049, PR China
d Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic and monometallic catalysts including Cu and/or Fe species were prepared by a co-precipitation
method and their catalytic performance was tested for the selective hydrogenation of ethyl stearate to
stearyl alcohol. The bimetallic catalysts were observed to be even more active for this selective hydro-
genation compared to the monometallic catalysts and their physical mixtures. With a bimetallic catalyst
of Cu/Fe (4/1 in mole ratio) reduced at 200 ◦C, a selectivity to the alcohol reached to above 99% at a conver-
sion of 97% in reaction for 4 h at 230 ◦C, 3.0 MPa. Effects of composition and reduction temperature on the
catalytic performance were studied and the properties of catalysts prepared under different conditions
were examined by XRD, TPR, N2 physisorption, and SEM. The relationship of the performance with the
properties of the catalysts was discussed, along with the conditions under which synergistic effects of Cu
and Fe species appeared and caused the enhancement of the catalytic performance.
Received 8 December 2013
Received in revised form 12 May 2014
Accepted 13 May 2014
Available online 23 May 2014
Keywords:
Cu/Fe catalyst
Synergistic effect
Hydrogenation
Ethyl stearate
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
activation of hydrogen molecule and fatty esters are of importance
[10–12]. Recently, it was reported that iron could act as an elec-
Fatty alcohols and their derivatives, especially higher alco-
hols, are industrially important intermediates in the synthesis
of surfactants and plasticizers [1,2]. Higher alcohols are long
chain fatty alcohols with a carbon number of 12–22, such as lauryl
alcohol, cetyl alcohol and stearyl alcohol. The most commonly used
method to produce fatty alcohols is the selective hydrogenation
of fatty acids or fatty acid esters over heterogeneous catalysts
[3,4]. Copper catalyst is a good candidate for the selective hydro-
genation of C O bond to alcohol without further hydrogenolysis
of C O bond formed [5,6]. Considering that copper itself is low
active for this reaction, one should use supports or promoters to
improve the activity and selectivity. Currently, copper catalysts,
such as Cu/SiO2, Cu/ZnO, Cu–Zn/Al2O3, have been widely studied
for the selective hydrogenation of fatty esters to corresponding
alcohols [7–9]. The influence of dispersion of Cu particles and the
tronic/chemical promoter to modify metal catalysts and improve
their catalytic activity and selectivity because it increased surface
area and dispersion of host metal elements [13–15]. For example,
the electronegative metal of Fe could enhance the selectivity of
hydrogenation of unsaturated aldehydes to unsaturated alcohols
over Pt catalyst [16,17]. An Fe modified CoB amorphous alloy
catalyst increased the selectivity to crotyl alcohol during the
selective hydrogenation of crotonaldehyde [18]. Therefore, the Fe
modified catalysts may produce preferential adsorption sites for
the C O bond, thus enhancing the selective hydrogenation of C
O
to C OH. This would be due to the changes in structural and/or
electronic properties of Cu species caused by the addition of Fe.
In this work, we prepared two kinds of Cu/Fe bimetallic cata-
lysts by co-precipitation method and their catalytic performances
were discussed for the hydrogenation of ethyl stearate to stearyl
alcohol as a model reaction. The composition of Cu/Fe catalysts
was found to greatly influence their catalytic behavior. The cor-
relation of their catalytic performance with the properties of Cu/Fe
catalysts, such as phase composition, morphology, specific sur-
face area, pore diameter, and pore volume was discussed in detail.
Strong electronic interactions of Cu and Cu2O with Fe3O4 and Lewis
acid sites should play crucial roles for high yield of fatty alcohol
∗
Corresponding author at: State Key Laboratory of Electroanalytical Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, PR China. Tel.: +86 431 85262 410; fax: +86 431 85262410.
E-mail address: zhaofy@ciac.ac.cn (F. Zhao).
http://dx.doi.org/10.1016/j.molcata.2014.05.009
1381-1169/© 2014 Elsevier B.V. All rights reserved.

144
L. He et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
observed in the selective hydrogenation of ethyl stearate to stearyl
alcohol.
2. Experimental
2.1. Synthesis and characterization of catalysts
All chemicals used in the present work are analytical grade and
used without further purification. The Cu/Fe catalysts were pre-
pared by a simple co-precipitation method. Typically, an aqueous
solution of cupric nitrate (Cu(NO3)3·3H2O, 0.25 M) was mixed with
an aqueous solution of ferric nitrate (Fe(NO3)3·9H2O, 0.25 M) with
a Cu/Fe molar ratio of either 1/4 or 4/1. After the mixture solu-
tion was stirred at 80 ◦C for 30 min, an aqueous solution of sodium
hydroxide (NaOH, 1 M) was added dropwise to the salt solution
under vigorously stirring until the pH reached to ca. 9–10. The mix-
ture obtained was further stirred at 80 ◦C for 4 h and then aged for
another 1 h at room temperature. The precipitate formed was fil-
trated, washed with distilled water for several times to remove the
impurities, and then dried at 110 ◦C. The samples so prepared will
be denoted as Cu/4Fe-pre and 4Cu/Fe-pre in which Cu/Fe ratio is 1/4
and 4/1, respectively. These samples were calcined at 300 ◦C for 4 h
in air; the samples will be denoted as Cu/4Fe-C300 and 4Cu/Fe-
C300, respectively. For comparison, monometallic oxide materials
were also prepared by similar procedures. The samples prepared
before and after the calcination will be denoted as Cu-pre (Fe-pre)
and Cu-C300 (Fe-C300), respectively. The samples were further
reduced in hydrogen atmosphere at different temperatures (150,
200 and 500 ◦C) for 90 min, which will be denoted, for example, as
Cu/4Fe-R150.
The structure and crystal phase composition of the samples
were determined by powder X-ray diffraction patterns with a
˚
Brucker D8 GADDS diffractometer using Co K radiation (1.79 A)
and the crystallite size was calculated with the Scherrer’s equa-
tion. The samples for XRD examination were reduced at a certain
temperature and reserved in ethanol for avoiding exposure to
air before XRD tests. The temperature-programmed reduction
(TPR) analysis was performed under a flow of 5% H2/N2 mix-
ture (30 ml/min), with a heating rate of 10 K/min. Surface acidity
was studied by NH3 temperature-programmed desorption (TPD)
on a TP-5080 Multi-functional Automatic Adsorption Instrument
(Tianjin Xianquan Industry and Trade Development Co. Ltd, China)
equipped with a TCD. Prior to the TPD experiments, 100 mg of
the catalyst sample was pre-reduced in situ at 200 ◦C for 90 min
in a N2/H2 gaseous mixture. The morphology of the samples
was recorded by scanning electron microscope (SEM Hitachi S-
4800). N2 adsorption/desorption isotherms were measured using
a Micromeritics ASAP 2020 Analzer (USA). The surface areas were
calculated using the Brunauer–Emmett–Teller (BET) equation. Pore
volume (Vp) was estimated using the adsorption branch of the N2
isotherm curve. Pore size distribution curves were calculated by the
Barrett–Joyner–Halenda (BJH) method using adsorption branch of
the N2 isotherms.
Fig. 1. XRD patterns of precursor and calcined samples.
autoclave was cooled to room temperature. The products were col-
lected and then analyzed by gas chromatography with FID detector
with a DB-1 capillary column and GC–MS.
3. Results and discussion
3.1. Textual properties and surface acidity
Fig. 1 shows the XRD patterns of the precursor and calcined
Cu/Fe catalysts. The monometallic oxides, iron and copper oxides,
were also analyzed for comparison. The Fe-pre sample gave a rel-
ative weak characteristic diffraction of Fe(OH)3 (JSCPDS 29-0713),
and after calcination at 300 ◦C it was transferred to ␣-Fe2O3 (JCPDS
33-0664). Both Cu-pre and Cu-C300 sample presented CuO phase.
This is because that copper hydroxide Cu(OH)2 was metastable and
thermodynamically easy to transfer into more stable compound of
CuO [19]. In this system, the aging temperature was 80 ◦C so that
the precipitate Cu(OH)2 formed was directly decomposed to CuO
in the hot alkali solution [20]. For Cu/4Fe samples, no any diffrac-
tion peaks could be detected before and after the calcination at
300 ◦C, indicating that both the Cu and Fe existed in an amorphous
state. Zhang et al. reported that a certain amount of Cu could pre-
vent the crystallization of ␣-Fe2O3, and more importantly it could
increase the temperature of phase transformation from Fe(OH)x
2.2. Activity test
Reaction experiments were carried out in a 50 ml stainless steel
autoclave. All the catalysts were pre-reduced in hydrogen at tem-
perature (150, 200, 500 ◦C) for 90 min before the reaction. After
reduction, the catalyst was transferred into the autoclave, in which
5.0 ml hexane was added as solvent, under the protection of hydro-
gen. After that, a certain amount of ethyl stearate was added and
the autoclave was sealed. The reaction was operated under a stir-
ring rate of 1300 rpm (without diffusion limitation) at 230 ◦C and a
H2 pressure of 3.0 MPa H2. After the reaction for several hours, the

L. He et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
145
Fig. 2. TPR profiles of monometallic and bimetallic catalysts calcined at 300 ◦C.
For comparison, physical mixtures of Cu-C300 + 4Fe-C300 and 4Cu-C300 + Fe-C300
(mixed in equal amounts).
to ␣-Fe2O3 [21]. For 4Cu/Fe samples, strong diffraction peaks of
CuO (JCPDS 48-1548) were observed, due to the high amount of Cu
loaded. However, there was no any diffraction peak of iron species
in precursor and calcined samples, indicating iron species existed
in amorphous state in the 4Cu/Fe sample as well.
The hydrogen pre-reduction is a necessary and important pro-
cess to activate the catalyst before its use. Fig. 2 shows the H2-TPR
results of the calcined monometallic and bimetallic catalysts. A
sharp and intensive reduction peak was observed for Cu-C300 at
about 204 ◦C, which was assigned to the reduction of CuO to metal-
lic Cu, but some Cu2O species still remained in the sample reduced
at 200 ◦C (Fig. 3). After reduction at 600 ◦C, CuO was almost reduced
to metallic Cu with a very little amount of Cu2O phase. For Fe-C300
sample, a weak peak at around 360 ◦C was assigned to the reduction
of Fe2O3 to Fe3O4, and a wide peak at around 518 ◦C (372–580 ◦C)
to the further reduction of Fe3O4 to Fe. This reduction behavior is in
accordance with the literature that the reduction of Fe2O3 occurs
through two steps of Fe2O3 to Fe3O4 and then Fe3O4 to Fe [22].
Figs. 2 and 3 showed that Fe2O3 cannot be reduced at 200 ◦C but
fully reduced to metallic Fe at 600 ◦C.
For comparison, physical mixtures of monometallic Cu and Fe
samples were also examined, in which the Cu/Fe ratio was 1/4 or
4/1. The TPR patterns obtained were very similar to the sum of
those monometallic samples. However, bimetallic Cu/Fe catalysts
showed very different reduction behavior. The reduction peak of
Cu/4Fe-C300 shifted to low temperature with a relatively broad
peak at 144 ◦C compared to Cu-C300 sample. As for 4Cu/Fe catalyst,
the reduction peak split into two peaks at low temperatures of 155
and 183 ◦C (Fig. 2). It is therefore suggested that there exist strong
interactions between Cu and Fe species in the bimetallic catalysts.
The XRD patterns of bimetallic Cu/Fe catalysts reduced at dif-
ferent temperatures are shown in Fig. 4. For Cu/4Fe-R150 and
Cu/4Fe-R200, only Fe3O4 (JCPDS 65-3107) was detected without
any diffraction of copper species. As seen in Fig. 3, Fe2O3 cannot be
reduced at 200 ◦C in the monometallic sample but it was reduced
into Fe3O4 even at 150 ◦C in the bimetallic sample. These results
indicated that the reduction temperature of Fe2O3 decreased in the
presence of copper. After reduced at 500 ◦C, it showed characteris-
tic diffraction peaks of metal Cu (JCPDS 04-0836) due to Cu (1 1 1)
at 43.3◦, Cu (2 0 0) at 50.4◦, and Cu (2 20 ) at 74.1◦ and also metal Fe
(JCPDS 06-0696) due to Fe (1 1 0) at 44.7◦, Fe (0 0 4) at 65.0◦, and Fe
(2 1 1) at 82.3◦.
Fig. 3. XRD patterns of monometallic Cu and Fe samples after hydrogen reduction
at different temperatures of 200 ◦C and 600 ◦C.
The 4Cu/Fe catalyst showed the diffraction peaks of Fe3O4
(JCPDS 65-3107) after reduced at 150 ◦C as shown in Fig. 4. The main
metallic Cu (1 1 1) diffraction peak appears at 43.3◦ that is close
to Fe3O4 (4 0 0) at 43.1◦, so it is difficult to discriminate these two
species. However, one can see clear metallic Cu (2 0 0) and Cu (2 2 0)
crystal plane diffraction peaks at 50.4◦ and 74.1◦. Therefore, Fe3O4
and Cu should co-exist in this sample. In addition, after reduced at
150 ◦C, the 4Cu/Fe catalyst still exhibited the characteristic diffrac-
tion peaks of CuO phase at 35.6◦ and 38.8◦ (JCPDS 65-2309). The
diffraction peaks of CuO disappeared after it was reduced at 200 ◦C
and the intensity of the diffraction peaks of Cu2O (JCPDS 65-3288)
at 36.5◦ and the metallic Cu (JCPDS 04-0836) at 43.3 ◦C increased
when the reduction temperature increased from 150 ◦C to 200 ◦C.
These results suggested that CuO species were gradually reduced to
Cu2O and Cu with increasing reduction temperature. After reduced
at 200 ◦C, the 4Cu/Fe catalyst presented three kinds of species,
Cu2O, Cu and Fe3O4. At high reduction temperature of 500 ◦C, it
showed the strong diffraction peaks characteristic of metallic Cu
(JCPDS 04-0836) and Fe (JCPDS 06-0696), indicating the complete
reduction of CuO and Fe2O3 to their metallic states.
The SEM images of Cu/Fe catalysts collected before and after
H2 reduction at 150, 200, 500 ◦C are shown in Figure S1 (in
the supporting information). The two kinds of the bimetallic

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L. He et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
Table 1
Surface areas and pore parameters of the calcined and reduced catalysts.
Catalyst
SBET (m2 g−1
)
Vp (cm3 g−1
)
Dp (nm)
Cu/4Fe-C300
Cu/4Fe-R200
4Cu/Fe-C300
4Cu/Fe-R200
4.8
227.2
7.9
0.16
0.34
0.31
0.43
3.76
3.75
9.46
3.42
127.5
The surface areas were calculated using the BET equation. Pore volume (Vp) was esti-
mated using the adsorption branch of the N2 isotherm curve. Average pore diameter
(Dp) was estimated from BJH desorption determination.
Fig. 5. NH3-TPD profiles of monometallic and bimetallic samples.
before hydrogen reduction. After Cu/Fe catalysts were reduced at
200 ◦C, the larger pores of 9.5 nm and 7.6 nm disappeared and these
showed narrow pore size distributions in the range of 2–6.4 nm.
That is, the hydrogen reduction largely altered the morphology,
surface area, pore size and pore volume of the Cu/Fe catalysts.
The distribution and strength of acidic sites of Cu/Fe catalysts
were studied by NH3-TPD. Before the NH3-TPD tests, all the samples
were pre-reduced in hydrogen at 200 ◦C for 90 min. Fig. 5 shows the
NH3 desorption patterns obtained. For CuO sample (Cu-C300, pre-
pared in our experiment), a single weak desorption peak appeared
at about 100 ◦C. CuO, as a common p-type semiconductor, typi-
cally has little Lewis acidic sites. The bimetallic Cu/4Fe and 4Cu/Fe
catalysts possessed a broadly distributed desorption peaks around
100 ◦C which was similar to Fe-C300 sample. Lewis acid sites favor
the activation of C O through interactions with the electron pair of
oxygen atom, which should result in the enhancement of the reac-
tivity of carbonyl compounds, ethyl stearate in the present work.
It was reported that Lewis acid sites of Cu/SiO2 catalyst facilitated
the conversion of dimethyl oxalate to ethanol [23].
Fig. 4. XRD patterns of bimetallic Cu/Fe catalysts after reduction at different tem-
peratures of 150 ◦C, 200 ◦C, and 500 ◦C.
catalysts different in the composition showed different mor-
phologies. Before reduction, the Cu/4Fe catalyst showed spherical
particles of ca. 60–100 nm in diameter but the 4Cu/Fe catalyst gave
a shuttle or nanoplate structure of about 100 nm. After reduced at
150 ◦C and 200 ◦C, the spheres and nanoplates were assembled to
big microspheres for both Cu/4Fe and 4Cu/Fe catalysts. Although
exceptionally large particles were observed, most of the particles
were well dispersed with particle size less than 120 nm. When the
catalysts were reduced at a higher temperature of 500 ◦C, the par-
ticles aggregated significantly for Cu/4Fe catalyst as seen in Figure
S1(d), while 4Cu/Fe catalyst showed a bright metal surface in Figure
S1(h).
3.2. Catalytic performance
The Cu/Fe catalysts were further characterized by N2 isotherms.
The surface areas and pore size distributions are shown in Table 1
and Figure S2. The BET surface areas of unreduced Cu/Fe catalysts
were extremely small, 4.8 m2 g−1 for Cu/4Fe catalyst and 7.9 m2 g−1
for 4Cu/Fe catalyst. After hydrogen reduction at 200 ◦C, the surface
area of Cu/Fe catalysts considerably increased to 227.2 m2 g−1 for
the former catalyst and 127.5 m2 g−1 for the latter. In addition, the
pore volume slightly increased after hydrogen. The Cu/Fe catalysts
are in micro- and mesoporous structures, which include mainly
pores in the range of 2–9.5 nm as shown in Figure S2. The pore
size distribution was rather wide for Cu/4Fe and 4Cu/Fe catalysts
The catalytic performance of monometallic and bimetallic sam-
ples was tested for the selective hydrogenation of ethyl stearate
to stearyl alcohol (Scheme 1). The reaction results obtained are
given in Tables 2 and 3. The present bimetallic Cu/Fe cata-
lysts are very active and selective to the formation of stearyl
Catalyst
CH3(CH2)16COOC2H5
2H2
+
CH3(CH2)16CH2OH
C2H5OH
+
Scheme 1. Selective hydrogenation of ethyl stearate to stearyl alcohol.

L. He et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
147
Table 2
The influence of reduction temperature was examined for the
two bimetallic catalysts. The reduction temperature had a signif-
icant effect on the catalytic activity as shown in Table 3. When
the reduction temperature was raised from 150 ◦C to 200 ◦C, the
catalytic activity increased for both Cu/4Fe and 4Cu/Fe catalysts.
The conversion increased with the reaction time while keeping the
high selectivity to stearyl alcohol. In addition, the 4Cu/Fe catalyst is
more active than the Cu/4Fe. For example, when the catalysts were
reduced at 150 ◦C, the conversion values of Cu/4Fe and 4Cu/Fe cat-
alysts were 68.0% and 98.5% in 6 h, respectively. However, when
Cu/Fe catalysts were reduced at a high temperature of 500 ◦C, the
activity decreased sharply and the 4Cu/Fe became less active than
Cu/4Fe. Which could be explained by (1) the seriously aggregation
of the metal particles at high activation temperatures; (2) the Cu2O
disappeared at high reduction temperature, thus its function as the
Lewis acid sites to accelerate the activation of C O lost [24]; and
(3) the Cu/4Fe with higher Fe content possesses more Lewis acid
sites than 4Cu/Fe according to the results of NH3-TPD in Fig. 5.
In summary, the reduction temperature has strong influence on
the activity of the Cu/Fe bimetallic catalysts, the 4Cu/Fe is more
active than Cu/4Fe at the lower reduction temperature, while it is
reverse at the higher reaction temperature due to the disappear-
ance of advantages like the amount of active sites, Lewis acid sites
Catalytic performance of Cu/Fe catalysts in ester hydrogenation.
Entry
Catalyst
Conv. (%)
Select. (%)
1
2
3
4
5
6
CuO
Fe2O3
Cu-C300 + 4Fe-C300
4Cu-C300 + Fe-C300
Cu/4Fe-C300
4Cu/Fe-C300
5.0
–
3.8
10.0
70.0
97.3
98.5
–
98.6
99.0
99.2
99.4
Reaction conditions: ethyl stearate 0.5 mmol, hexane 5.0 ml, catalyst 50 mg, H2
3.0 MPa, 230 ◦C, 4 h. All the catalysts were reduced in hydrogen at 200 ◦C for 90 min
before reaction.
alcohol (entries 5,6), in which the selectivity to stearyl alcohol was
above 99%. The monometallic catalyst and the physical mixtures of
monometallic catalysts were also selective to the alcohol but even
less active, the conversion being <10.0% (entries 1, 3, 4) as com-
pared to 70.0% and 97.3% obtained with the bimetallic catalysts. The
monometallic Fe sample has no activity, indicating that Cu species
was the main active center during the selective hydrogenation of
ethyl stearate to stearyl alcohol. These above results suggested the
existence of interactions between Cu and Fe species in the Cu/Fe
catalysts.
Fig. 6. XPS spectra of Cu 2p, Cu LMM, Fe 2p and O 1 s of (a) Cu-C300, (b) Cu/4Fe and (c) 4Cu/Fe reduced at 200 ◦C.

148
L. He et al. / Journal of Molecular Catalysis A: Chemical 392 (2014) 143–149
Table 3
it mainly corresponds to the oxygen species of Fe3O4 due to the
high content Fe, indicating that the Cu species mainly existed as
Cu0 on the surface of Cu/4Fe, in agreement with the analysis from
Cu 2p. Thus, it concluded that Cu+, Cu0 and Fe3O4 were the main
species on the surface of Cu/Fe catalysts reduced at 200 ◦C based on
both the results of XPS and XRD, and a stronger electronic interac-
tion existed between the Cu and Fe as confirmed by the shift of the
binding energy of Cu 2p. More importantly, the activity of Cu/Fe
catalysts is consistent with the nature of active species on their
surfaces.
Catalytic performances of Cu/Fe catalysts reduced at different temperatures.
Catalyst
Cu/4Fe
T (◦C)
t (h)
Conv. (%)
Select. (%)
150
1
3
6
1
4
6
8
12
21.6
33.2
68.0
27.8
70.0
92.2
10.0
28.0
98.3
98.6
98.4
98.7
99.2
99.0
99.1
98.9
200
500
150
The synergistic effects of Cu0/Cu+ species and Fe3O4 should be
responsible for the high catalytic performance of 4Cu/Fe catalyst in
selective hydrogenation of ethyl stearate to stearyl alcohol.
4Cu/Fe
1
3
6
1
3
4
6
12
30.0
45.1
98.5
51.9
78.5
97.3
6.5
98.5
98.3
98.2
98.4
98.7
99.4
98.8
99.0
200
500
4. Conclusions
10.5
The bimetallic Cu/Fe catalysts, prepared by a co-precipitation
method, show even higher performance in the selective hydro-
genation of ethyl stearyl to stearyl alcohol with respect to the
reaction rate and the selectivity to the alcohol, as compared to
the monometallic catalysts and their physical mixtures. The activ-
ity depends on the composition of the catalysts; 4Cu/Fe catalyst is
more active than the Cu/4Fe one while the selectivity to the alco-
hol is almost the same between the two catalysts. With the former
catalyst reduced at 200 ◦C, a conversion of >97% and a selectivity of
>99% can be achieved within 4 h at a reaction temperature of 230 ◦C.
The high performance of this catalyst may result from synergistic
effects of Cu and Fe species, for which the presence of Cu2O, Cu and
Fe3O4 should be requisite. Large surface area and Lewis acid sites
present on the bimetallic catalysts may also be important factors.
The Lewis acid sites are effective for the activation of the carbonyl
group of the substrate.
Reaction conditions: ethyl stearate 0.5 mmol, hexane 5.0 ml, catalyst 50 mg, H2
3.0 MPa, 230 ◦C. The catalysts were reduced at T (◦C) for 90 min before reaction.
of Cu2O for the 4Cu/Fe catalyst, but the Lewis acid sites of Fe species
become dominant factor to accelerate the reactivity of C O bond
for the Cu/4Fe catalyst at the higher reduction temperature.
3.3. The catalyst structure and activity relationship
The present results show that monometallic CuO and Fe2O3 cat-
alysts and their physical mixtures are less active; however, the
bimetallic Cu/Fe catalysts indicate much higher performance in the
activity and selectivity. These strongly suggests that interactions
exist between Cu and Fe species in these bimetallic catalysts, which
was also suggested by H2-TPR results that the reducibility of Cu and
Fe species changed as compared to the monometallic catalysts (as
shown in Fig. 2).
Acknowledgements
Further evidence of interaction between Cu and Fe was obtained
from Ex situ XPS measurements. The XPS spectra of pure CuO,
Cu/4Fe and 4Cu/Fe samples are shown in Fig. 6. The XPS spectra
of Cu 2p appeared at the binding energies around 932.2–932.7 eV
(Cu 2p3/2) and 952.4 eV (Cu 2p1/2) was ascribed to Cu0 and Cu+ [24],
respectively, it is clearly that the copper species were reduced to
Cu+ and Cu0 for all the samples reduced at 200 ◦C. However, the
peak of Cu 2p3/2 for Cu/Fe samples shifted to the lower binding
energies (932.3 eV and 932.5 eV) compared with that (932.7 eV)
of pure CuO. It confirmed that an electronic interaction existed
between Cu and Fe in the Cu/Fe bimetallic catalysts with an elec-
tron transferring from Fe to Cu. Moreover, we have further analyzed
the Cu LMM Auger signals of Cu+ and Cu0, the peak at 918.4 eV
is attributed to metallic copper Cu0, and the peak at 916.5 eV is
assigned to Cu+ [25,26]. In addition, a plasmon satellite structure
was resolved at binding energy of 920–922 eV, it is clear that the
Cu0/Cu+ value on the surface of 4Cu/Fe is higher than that of Cu/4Fe
catalyst, and thus the former presented higher activity. For the
Cu/4Fe and 4Cu/Fe samples, they presented similar Fe 2p spectra,
Fe 2p3/2 at about 711.2 eV and Fe 2p1/2 at about 724.5 eV, suggesting
the Fe species in Cu/4Fe and 4Cu/Fe samples existed mainly as mag-
netite Fe3O4 [27,28], which is also consistent with the XRD results
(in Fig. 4). In addition, the signal intensity of Fe 2p in 4Cu/Fe is much
weaker than that in Cu/4Fe due to the lower Fe content. As for pure
CuO, the peak of O 1 s at 530.7 eV was the characteristic location
of O2−, and peak at 532.2 eV was the chemisorbed oxygen [29].
While, for 4Cu/Fe, the peak of O 1 s was broadened, indicating the
presence of multi-component copper species. It was divided into
two peaks, one at 530.2 eV attributes to O atoms bond to metals (Fe
and Cu), and another at 531.5 eV associates with chemisorbed oxy-
gen. But for the Cu/4Fe, only a single peak presented at 530.3 eV,
The authors gratefully acknowledge the financial supports from
NSFC 21273222, the JSPS-NSFC Bilateral Research Program, and
from Jilin Province, China 20100562.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.05.009.
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