Synthesis of a stable and porous Co-B nanoparticle catalyst for selective hydrogenation of cinnamaldehyde to cinnamic alcohol

Article abstract of DOI:10.1016/j.catcom.2010.04.018

The Co-B nanoparticle catalyst supported on TiO2 for the selective hydrogenation of cinnamaldehyde to cinnamic alcohol was prepared by a modified electroless plating method. The as-prepared catalyst was characterized by X-ray diffraction and transmission electron microscope. The results showed that porous Co- B nanoparticles were distributed narrowly (40-45 nm) over the support. Compared with the Co-B nanoparticles prepared by chemical reduction of Co ²⁺ ions with borohydride, the as-resulted Co-B nanoparticles were stable to be stored in air, and exhibited higher activity and selectivity of cinnamic alcohol in cinnamaldehyde hydrogenation.

Full text of DOI:10.1016/j.catcom.2010.04.018

Catalysis Communications 11 (2010) 973–976
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Synthesis of a stable and porous Co–B nanoparticle catalyst for selective
hydrogenation of cinnamaldehyde to cinnamic alcohol
⁎
Zhijie Wu, Jingshi Zhao, Minghui Zhang , Wei Li, Keyi Tao
Institute of New Catalytic Materials Science, Department of Material Chemistry, College of Chemistry, and Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin,
300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 March 2010
Received in revised form 20 April 2010
Accepted 22 April 2010
Available online 12 May 2010
The Co–B nanoparticle catalyst supported on TiO2 for the selective hydrogenation of cinnamaldehyde to
cinnamic alcohol was prepared by a modified electroless plating method. The as-prepared catalyst was
characterized by X-ray diffraction and transmission electron microscope. The results showed that porous Co–
B
nanoparticles were distributed narrowly (40–45 nm) over the support. Compared with the Co–B
nanoparticles prepared by chemical reduction of Co2+ ions with borohydride, the as-resulted Co–B
nanoparticles were stable to be stored in air, and exhibited higher activity and selectivity of cinnamic alcohol
in cinnamaldehyde hydrogenation.
Keywords:
Co–B
Cinnamaldehyde
Hydrogenation
Nanoparticles
Electroless plating
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
it is interesting to find that such amorphous metal-metalloid
materials are also widely used in corrosion-resistant science, just
Selective hydrogenation of α, β-unsaturated aldehydes to unsat-
urated alcohols is a critical step in the industrial production of
chemicals, such as perfumes, flavorings and pharmaceuticals [1].
Because the thermodynamics favor the hydrogenation of the C=C
bonds in α, β-unsaturated aldehydes, more attention has been
focused on promoting the selectivity of unsaturated alcohol [2]. The
selective hydrogenation of cinnamaldehyde (CAL) to cinnamic alcohol
(CMO) is frequently reported as a probe reaction to sieve catalysts [3].
Besides the noble metals (Pt, Ir, and Os), transition metal Co exhibits
high hydrogenation activity and CMO selectivity. So far, Co-contained
catalysts are mostly used for the liquid hydrogenation. Recently, two
groups reported that the amorphous Co–B nanoparticles prepared by
chemical reduction of Co2+ ions with borohydride exhibit high
activity and selectivity as noble metals in the CMA hydrogenation [4–
7]. The Co–B nanoparticles are regarded as a kind of metal-metalloid
amorphous alloy particles, in which the coordination-unsaturated and
electron-rich Co element benefits to activate the C=O bond prior to
C=C bond [1–8].
only turning the preparation method to electroless plating technique
[11]. For the electroless plating, the metalloid elements (B and P) are
selectively located at the defect of the plated film to protect the
corrosion of metal crystals [11–14]. The films are composed by various
metal clusters and metal-metalloid clusters, resulting in amorphous
structure with short-range order and long-range disorder. In our
previous work, we have developed the electroless plating method to
synthesize supported amorphous Ni–B nanoparticles for hydrogena-
tion [15–18]. The Ni–B nanoparticles showed high activity and
selectivity owing to the modification of structure and electronic
properties by B element, which is the same as those particles prepared
by chemical reduction, and exhibited higher stability for storing and
recycling [15,16,19,20]. Here, to get a stable, active and selective Co–B
metal catalyst for CMA hydrogenation, a modified electroless plating
method was developed to synthesize amorphous Co–B catalyst. Its
structure and catalytic performance were studied in comparison with
Co–B nanoparticles prepared by impregnation–reduction method.
The Co–B nanoparticles prepared by the chemical reduction
method are sensitive to air or water, and their activities decrease
after storing and reaction because its surface is easily covered by
boron oxides or formed into metal oxides or metal hydroxides [9,10].
This inhibits the further industrial application of such catalysts. In fact,
2. Experimental
2.1. Catalyst preparation
The Co–B nanoparticle catalyst supported on TiO2 was prepared by
a modified electroless plating method, and the silver metal was used
to activate the plating solution [15–18]. The Ag/TiO2 was prepared in
the light of ref. [15]. For the synthesis of Co–B/TiO2 catalyst, the Ag/
TiO2 was added to a Co–B plating solution containing 9 g/L
⁎
Corresponding author. Tel.: +86 22 2350 7730; fax: +86 22 2350 7730.
E-mail address: zhangmh@nankai.edu.cn (M. Zhang).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.04.018

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Z. Wu et al. / Catalysis Communications 11 (2010) 973–976
CoCl2·6H2O, 50 g/L Na2C4H4O6·2H2O, 2 g/L Na2B4O7·10H2O, 2 g/L
NH4Cl, 6 g/L KBH4, and NaOH for adjusting the pH=13. The plating
occurred at 45 °C for 30 min. The resulted black product (denoted as
Co–B(EP)/TiO2) was washed with distilled water by centrifugation
thoroughly to obtain pH=7, then washed with ethanol to remove the
water. The unsupported Co–B catalyst was also prepared by adding
2.0 mL 0.1 mol/L AgNO3 solution to 200 mL Co–B plating solution. The
resulted supported and unsupported catalysts were dried at 80 °C in
oven, and stored in air at ambient conditions. For comparison, a
conventional Co–B(IR)/TiO2 catalyst containing 15 wt.% Co was
prepared by impregnation–reduction method with the same mole
ratios of borohydride/Co=3, and was stored in ethanol [21]. The Co/
TiO2 catalyst was also prepared by wetness impregnation of TiO2 with
Co(NO3)2 solution and reduced with hydrogen gas at 400 °C.
2.2. Characterization
The chemical composition of samples was analyzed by inductively
coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS
Intrepid Spectrometer. X-ray diffraction patterns were collected on
the Rigaku D/max-2500 diffractometer by using Ni-filtered Cu Kα
radiation (λ=1.5418 Å) at a scan rate of 2°/min. Transmission
electron microscopy (TEM) was carried out on a FEI Tecnai G2 high
resolution transmission electron microscope. The active surface area
(SCo) was measured by hydrogen chemisorptions on a Quantachrome
CHEMBET 3000 TPR/TPD unit by assuming H/Co(s)=1 [22].
Fig. 1. XRD patterns of unsupported and supported Co–B catalysts.
diffraction spots or rings corresponding to crystalline Co or cobalt
boride are observed.
The supported Co-based catalysts with similar Co loading were
prepared by electroless plating and impregnation–reduction meth-
ods, and their detail compositions and properties are listed in Table 1.
The unsupported Co–B nanoparticles prepared from chemical reduc-
tion with borohydride usually show in solid sphere shape [6,7]. The
corresponding supported Co–B nanoparticles have smaller particle
size and high dispersion. But it is inevitable that some parts of Co–B
nanoparticles are aggregated in an unsupported form [15], and some
parts of Co metals are present in oxidation states [23], which will
reduce the metallic active sites on the Co–B(IR)/TiO2 catalysts. We
note that the Co–B films prepared by electroless plating show
excellent anti-corrosion to water and air [11]. Although the Co–B
(EP)/TiO2 has lower Co loading than Co–B(IR)/TiO2, the Co–B(EP)/
TiO2 catalyst exhibits a higher active metal surface area (SCo).
2.3. Catalyst testing
The hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol
(CMO) was performed in 250 mL stainless steel autoclave which
contained 0.1 g metal catalyst, 20.0 mL CAL and 100.0 mL ethanol. The
experiment was carried out at 100 °C and 1.5 MPa of hydrogen
pressure with stirring at 800 rpm for 3 h The possibility of diffusion
limitation during the catalytic tests was investigated as the following
procedures described elsewhere [16]. Experiments were carried out at
different stirring velocities in the range of 100–1000 rpm. The
constancy of the activity and selectivity above 600 rpm ensured that
external diffusion limitation was absent at the rotation speed selected.
The catalytic activity of the inner wall of the reactor could be
neglected, as found by experiments. Blank experiment with Ag/TiO2
catalyst was carried, and no conversion of CAL was detected. The
products were analyzed by a gas chromatograph equipped with a
flame ionization detector.
3. Results and discussion
3.1. Characterization of catalysts
Fig. 1 shows the XRD patterns of unsupported and supported Co–B
samples. Two broad and featureless peaks at 2θ=22° and 45° are
observed in the XRD pattern of unsupported Co–B sample. The peak at
22° corresponds to the presence of amorphous boron oxides, and the
peak around 2θ=45° indicates the amorphous structure of Co–B
nanoparticles. The supported Co–B(EP)/TiO2 catalyst exhibits only
diffraction lines corresponding to anatase TiO2. It is the result of high
dispersion of Co–B particles on the support and the weak diffraction
signal of amorphous Co–B [15]. A homogeneous distribution of Co–B
nanoparticles is observed in the TEM image of the Co–B(EP)/TiO2
sample (Fig. 2). Co–B nanoparticles of 40–45 nm are present in a
flower-like shape, suggesting the porous structure of Ni–B particles, in
which the pores occur from the diffusion of hydrogen during the
precipitation of Co clusters on active metal nuclei [17,18]. The
amorphous structure of Co–B nanoparticles is also confirmed by the
selected area electron diffraction (SAED) (inset of Fig. 2), as no
Fig. 2. TEM image of Co–B(EP)/TiO2 catalyst.

Z. Wu et al. / Catalysis Communications 11 (2010) 973–976
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Table 1
Chemical compositions and catalytic properties of catalysts.
Samples
Co loading
(wt.%)
SBET
S
Composition
(%)
TOF
Conversion
(%)
Selectivitiesc
Co
(m2/g)
(m2/gCo
)
(mol mol−1 s−1
)
a
b
CMO
HCAL
HCMO
CEE
Co–B(EP)/TiO2
Co–B(IR)/TiO2
Co–B(IR)/TiO2
Co/TiO2
Ag/TiO2
Raney Ni
13.9
15.0
15.0
15.0
0.2
42.5
37.2
36.7
–
15.7
7.5
0.5
2.8
0
Co52.1B47.9
Co68.7B31.3
Co68.7B31.3
–
–
–
0.27
0.24
0.017
0.054
–
98.0
40.9
0.2
3.5
b0.1
100
99.8
68.2
72.1
58.8
–
0
0.2
8.9
5.2
7.5
–
0
20.1
22.1
31.4
–
2.8
0.6
2.3
–
d
24.3
–
–
54.1e
0.079
23.2
65.7
11.1
0
a
The SCo was measured by the H2 chemisorptions, which were performed by using a dynamic pulse method. The catalyst was treated by an Ar stream at 200 °C for 1.0 h firstly,
which was well below its crystallization temperature.
b
Reaction turnover frequency which is defined as mole of reactant converted per mol of surface metal atom per second.
Hydrocinnamaldehyde: HCAL; cinnamyl alcohol: CMO; hydrocinnamyl alcohol: HCMO; and cinnamyl ethyl ether (C6H5CH CH–CH2–O–C2H5): CEE.
The catalyst was stored in air for 24 h.
The active surface area of nickel.
c
d
e
3.2. Activity and stability of catalysts
amorphous Ni–B nanoparticles prepared by chemical reduction
were sensitive to the oxygen, air and solvent (especially for water),
and the surface spices of nickel oxide or hydroxide occurred during
the storing and reactions [15–19], which agreed with other group's
work [10]. In fact, the review paper (ref. [10]) also pointed the similar
problem on the Co–B nanoparticles as Ni–B nanoparticles occurred.
So, we concluded that the deactivation of Co–B(IR)/TiO2 catalyst
maybe due to the surface changes, such as surface oxidation [10,15]. In
short, the Co–B(EP)/TiO2 catalyst exhibits higher activity and better
stability for selective hydrogenation of CAL than Co–B(IR)/TiO2.
Alternatively, the electroless plating provides a simple route to
synthesize high active and selective supported Co–B catalyst. In
addition, for the electroless plating, some Co2+ should be left in the
plating solution after plating due to the complexation of Co2+ and
Na2C4H4O6·2H2O, and the residual Co2+ can be recycled for a further
electroless plating, which also reduce the cost of preparation.
Table 1 summarizes the physical and catalytic properties of Co
catalysts. Blank experiment indicated that Ag/TiO2 precursor has a
negligible effect on the reaction. After the deposition of Co–B
nanoparticles on Ag/TiO2 supports, higher surface area was detected.
It is obvious that the higher the surface area of active metal (SCo), the
higher the turnover frequency (TOF) and conversion in CAL
hydrogenation, suggesting that the activity of Co catalyst is dependent
on the SCo. The low SCo value of Co/TiO2 should be ascribed to the
sintering of Co during H2 reduction because of the small surface area
of TiO2 and high metal loading. The porous Raney Ni catalyst presents
high SNi value, and exhibits the highest conversion when the same
amount of metal was used in hydrogenation. However, the amor-
phous Co–B exhibits higher intrinsic activity than Raney Ni catalyst
and crystallite Co catalysts. Deng et al. point that the unique short-
range ordering but long-range disordering structure of the amor-
phous alloy is responsible for its excellent activity and selectivity in
many hydrogenation reactions [21]. For Co–B nanoparticles, the Co
active site has a stronger synergistic effect between each other and is
more highly unsaturated than metal Co, which increases the activity
of Co atoms [3]. Thus, the amorphous Co–B shows high activity for
hydrogenation of CAL. Table 1 showed that the main products were
hydrocinnamaldehyde (HCAL), cinnamyl alcohol (CMO), and hydro-
cinnamyl alcohol (HCMO), and a little amount of cinnamyl ethyl ether
(CEE, C6H5CH CH–CH2–O–C2H5). The HCMO comes from the further
hydrogenation of HCAL and CMO, and the CEE is ascribed to the
reaction of CMO with ethanol or the direct reaction of CAL and ethanol
with H2 [24–26]. The Raney Ni catalyst shows the high selectivity
toward C=C bond hydrogenation to hydrocinnamaldehyde (HCMA),
but Co-based catalysts exhibit much higher selectivity of CMO [2]. The
amorphous Co–B nanoparticle catalyst shows a higher selectivity of
CMO than crystalline Co catalyst. For the amorphous Co–B nanopar-
ticles, some electrons partially transfer from B to Co, making Co
electron-enriched and B electron-deficient in the Co–B nanoparticles
[3–6]. The electron-deficient B enhances the total adsorption strength
of C=O group on Co–B, and the adsorption of hydrogen increases on
the electron-enriched Co active sites [3]. Both Co–B/TiO2 catalysts
show higher activity and selectivity than Co/TiO2. For the Co–B(EP)/
TiO2, the high SCo and B content result in a high TOF and selectivity of
CMO (99.8%). Unlike the Co–B(EP)/TiO2 catalyst stored in air, the Co–B
(IR)/TiO2 exhibits little hydrogenation activity after storing in air for
24 h.
4. Conclusions
The Co–B nanoparticles supported on TiO2 were prepared by the
electroless plating method. The porous Co–B nanoparticles were
Fig. 3 shows the stability of Co–B/TiO2 catalyst for the CAL
hydrogenation. The Co–B(EP)/TiO2 catalyst shows stable conversion
and selectivity of CMO during five runs. The conversion on Co–B(IR)/
TiO2 catalyst decreases with hydrogenation times, although the
selectivity is also stable. In our previous work, we found that
Fig. 3. Catalytic activities and selectivities of Co–B/TiO2 catalysts in the hydrogenation
of CAL.

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narrowly distributed with 40–45 nm. The TEM and XRD confirmed the
amorphous structure and high dispersion of Co–B nanoparticles. In
comparison with Co–B nanoparticles prepared by chemical reduction,
the resulted amorphous Co–B(EP)/TiO2 catalyst showed higher
activity and better stability in hydrogenation of CAL. In conclusion,
with a high conversion, CAL is highly selectively hydrogenated to CMO
over supported Co–B (EP)/TiO2 catalyst. The present strategy would
provide a versatile route of uniform and stable Co nanoparticle
catalysts, which would be promising for the design of high activity
and selectivity hydrogenation catalysts with high chemical stability.
Acknowledgement
The present work was supported by the Natural Science
Foundation of Tianjin (10JCYBJC04400).
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