Rhodium-cobalt bimetallic nanoparticles: A catalyst for selective hydrogenation of unsaturated carbon-carbon bonds with hydrous hydrazine

Article abstract of DOI:10.1002/adsc.201200576

Our studies on the effect of metal compositions on the catalytic activity of the rhodium-based bimetallic nanocatalysts revealed that the nanoparticles with a 4:1 ratio of rhodium to cobalt, were more active than the rhodium monometallic nanoparticles in the selective hydrogenation of unsaturated carbon-carbon bonds with hydrous hydrazine as a hydrogen source. The nanocatalysts effected this hydrogenation process in good-to-excellent yields with high functional group tolerance, and could be reused 10 times without the loss of catalytic activity. The catalyst characterization by X-ray photoelectron spectroscopy, transmission electron microscopy and line-scanning analysis suggested that the coexistence of metallic rhodium and cobalt plays an important role in the enhancement of catalytic activity. Copyright

Full text of DOI:10.1002/adsc.201200576

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DOI: 10.1002/adsc.201200576
Rhodium-Cobalt Bimetallic Nanoparticles: A Catalyst for
Selective Hydrogenation of Unsaturated Carbon-Carbon Bonds
with Hydrous Hydrazine
a
a
a,
Jin Lin, Jing Chen, and Weiping Su *
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy
of Sciences, Yangqiao west road 155#, Fuzhou, Fujian 350002, Peopleꢀs Republic of China
Fax : (+86)-591-83771575; phone: (+86)-591-83771575; e-mail: wpsu@fjirsm.ac.cn
Received: July 2, 2012; Revised: September 3, 2012; Published online: December 7, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200576.
Abstract: Our studies on the effect of metal compo-
sitions on the catalytic activity of the rhodium-
based bimetallic nanocatalysts revealed that the
nanoparticles with a 4:1 ratio of rhodium to cobalt,
were more active than the rhodium monometallic
nanoparticles in the selective hydrogenation of un-
saturated carbon-carbon bonds with hydrous hydra-
zine as a hydrogen source. The nanocatalysts effect-
ed this hydrogenation process in good-to-excellent
yields with high functional group tolerance, and
could be reused 10 times without the loss of catalyt-
ic activity. The catalyst characterization by X-ray
photoelectron spectroscopy, transmission electron
microscopy and line-scanning analysis suggested
that the coexistence of metallic rhodium and cobalt
plays an important role in the enhancement of cata-
lytic activity.
struct bimetallic nanoparticles enable us to control
the structures, sizes and compositions of bimetallic
nanoparticles, which are all important factors for de-
[
1,2]
termining their catalytic properties.
studies in this field mainly focus on electrocatalysis,
The intense
[3]
[
4]
[5]
aerobic oxidation of CO, and so on. Very recently,
the applications of bimetallic nanoparticle catalysts to
[
6]
organic transformations have emerged.
Hydrogenation of unsaturated carbon-carbon bonds
is a key reaction in organic chemistry, which has been
widely used in the synthesis of natural products, phar-
maceuticals and for chemical engineering. Various hy-
drogenation catalysts have been developed such as
[
7]
Pd/C, Raney Ni, Rh/C, Pt and so on. However, the
common hydrogen source, hydrogen, is difficult and
dangerous to handle in the laboratory due to its haz-
ardous properties such as flammability and explosibil-
ity. Hydrous hydrazine (formerly hydrazine hydrate)
is considered as a promising candidate for a hydrogen
source since it is much easier in handling, safer and
Keywords: alloys; bimetallic nanoparticles; unsatu-
rated carbon-carbon bonds; hydrous hydrazine (hy-
drazine hydrate); hydrogenation
[
9]
more stable than hydrogen. Consequently, highly ef-
ficient methods for the catalytic hydrogenation of un-
saturated hydrocarbons with hydrous hydrazine have
been sought for more than twenty years both in in-
[
8,9]
dustrial and academic laboratories.
In their elegant studies, Xu and co-workers
[5b]
Noble metal catalysts are used on a large scale in cur-
dis-
rent chemical industries. The limited resources of closed that Rh Ni bimetallic nanocatalysts can cata-
4
noble metals and the requirement for sustainable de- lyze the complete decomposition of hydrous hydra-
velopment have both prompted chemists to develop zine into hydrogen and nitrogen under mild condi-
new-generation catalysts in which noble metal con- tions with high selectivity. Since the metal-catalyzed
tents are effectively reduced with optimum catalytic decomposition of hydrous hydrazine generally occurs
reactivities being maintained. In this regard, the bi- via metal hydride intermediates that are common to
metallic nanoparticles often exhibit an enhancement those of the hydrogenation of unsaturated bonds with
[
9e]
of catalytic activity and selectivity compared with hydrogen, the successful capture of these metal hy-
their monometallic counterparts, and hence possess dride intermediates by alkenes or alkynes would
a great potential to be new-generation catalysts by enable the hydrogenation of unsaturated hydrocar-
partially or even completely replacing noble metals bons with hydrous hydrazine as a hydrogen source,
with non-noble metals. Importantly, significant advan- and the complete conversion of hydrous hydrazine
ces in the development of synthetic strategies to con- into nitrogen and hydrogen would also allow full use
Adv. Synth. Catal. 2013, 355, 41 – 46
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41

Jin Lin et al.
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of the hydrogen content of hydrous hydrazine for hy- salts. The resultant nanocatalysts were all isolated for
drogenation process. More importantly, the incorpora- the subsequent use in the catalysis experiments.
tion of a second metal into the Rh-based bimetallic
As expected, the Rh monometallic nanoparticle,
nanocatalyst would change the electron density on which proved to be an active catalyst for the decom-
[
5f]
the Rh center due to the difference of electronegativi- position of hydrous hydrazine, effected the hydro-
ty between the two metals, which could have a pro- genation of styrene (Table 1, entry 1). However, the
found effect on the activity of the resulting nanocata- bimetallic nanocatalyst Rh Ni, which works well for
4
lyst.
the selective decomposition of hydrous hydrazine, was
In this communication, we report for the first time significantly less effective than the Rh nanoparticles
an Rh/Co (Rh Co) bimetallic nanoparticle catalyst (Table 1, entry 11). Next, we turned our attention to
4
that efficiently promotes hydrogenation of unsaturat- the Rh-based bimetallic nanocatalysts incorporating
ed hydrocarbons with hydrous hydrazine as a hydro- other metal components. Gratifyingly, The Rh/Co bi-
gen source. The Rh Co nanocatalyst exhibits higher metallic nanocatalyst with the 4:1 ratio of Rh to Co,
4
activity than the monometallic Rh nanocatalyst as il- namely Rh Co, exhibited higher activity than the Rh
4
lustrated by the fact that the turnover frequency monometallic nanocatalyst, affording 89% yield for
(
TOF) of the Rh Co nanocatalyst observed over the the hydrogenation of styrene (Table 1, entry 12). Fur-
4
initial 30 min is 2.4 times higher than that of the Rh ther increasing the Co content in Rh/Co bimetallic
monometallic nanocatalyst, and was reused 10 times nanoparticles led to a decrease in catalysis activity
without loss of catalytic activity (see Figures in the (Table 1, entries 13–15). To identify whether the incor-
Supporting Information).
poration of Co into the Rh-based nanoparticle was
Initially, we used the hydrogenation of styrene with the origin of the enhanced catalysis activity, we exam-
hydrous hydrazine as a model system to screen ined the performance of a physical mixture of Rh
a series of monometallic nanoparticles and bimetallic nanoparticles and Co nanoparticles. This physical
nanoparticles with varying compositions (Table 1). mixture behaved similarly to the Rh monometallic
The bimetallic nanocatalysts were prepared by surfac- nanoparticles, implying that the enhancement of cata-
tant-assisted co-reduction of the aqueous solutions lytic activity exhibited by Rh Co stems from the syn-
4
containing two metal salts with various molar ratios ergic effect of Rh and Co in the bimetallic nanoparti-
in the presence of CTAB (hexadecyltrimethylammo- cles.
nium bromide) using an aqueous solution of NaBH4
We further established the variation of the product
as a reductant, and monometallic nanocatalysts by yield with time under a given catalyst (Figure S1 in
CTAB-assisted reduction of the corresponding metal Supporting Information), from which the turnover
frequency can be figured out. The turnover frequency
determined over the initial 30 min for Rh Co nanoca-
4
talyst (based on Rh) is 2.4 times higher than that for
Table 1. Optimization of catalysts for the hydrogenation of the Rh monometallic nanocatalyst, clearly showing
[a]
styrene.
that the Rh Co nanocatalyst is more active than the
4
Rh monometallic nanocatalyst.
[b]
Entry
Catalyst (mol%)
Time [h]
Yield [%]
Next, various solvents and surfactants were
screened (Table S1, in the Supporting Information). A
survey of solvents showed that toluene provided the
best outcome. For the cationic surfactants, the lengths
of their organic chains and counter ions are two im-
portant factors influencing the activity of the catalyst:
generally, long organic chains provided more active
catalysts and chloride and fluoride anions were much
better than bromide and iodide.
With the optimized protocol established, we investi-
gated the generality of the reaction (Table 2). In some
cases, slight adjustments were required to the reaction
conditions due to the distinct reactivity of substrates.
As shown in Table 2, a variety of unsaturated carbon-
carbon bonds can be hydrogenated in good yields. In
addition, the nanocatalysts still provided excellent
performance after ten cycles (Table 2, entry 2). The
selective hydrogenation of olefins was successfully
achieved even in the presence of reducible substitu-
ents, such as bromo, cyano, carboxyl and ester group
1
2
3
4
5
6
7
8
9
Rh (1.3)
Pd (1.3)
Pt (1.3)
Cu (2)
Fe (2)
1.5
1.5
1.5
3
3
3
3
3
3
3
73
57
16
6
14
1
2
3
2
6
20
89
33
12
8
Ni (2)
Co (2)
Ru (2)
Au (2)
Ir (2)
10
11
12
13
14
15
16
Rh Ni (1.3/0.325)
3
4
Rh Co (1.3/0.325)
1.5
1.5
1.5
1.5
1.5
4
RhCo (1.3/1.3)
RhCo (1.3/5.2)
4
RhCo16 (1.3/20.8)[
c]
Rh Co(1.3/0.325)
53
4
[
a]
.
4
Reaction conditions: styrene (0.2 mmol), N H H O
5 equiv.), CTAB (5 mol%), toluene (2 mL), 608C.
GC yields using n-dodecane as the internal standard.
Physical mixture.
2
2
(
[
[
b]
c]
42
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Adv. Synth. Catal. 2013, 355, 41 – 46

Rhodium-Cobalt Bimetallic Nanoparticles: A Catalyst for Selective Hydrogenation
Table 2. Selective hydrogenation of unsaturated CÀC (Table 2, entries 6–9). In contrast to cinnamic acid
[a]
bonds.
ethyl ester, chalcone gave a lower yield which may be
attributed to the side reaction between substrate and
hydrazine to generate heterocyclic compounds
[
9c]
(
Table 2, entry 8). Both isolated olefins and conju-
gated olefins afforded the corresponding alkanes in
good yield. Besides, internal alkenes were also stud-
ied. It is noteworthy that cis-1,2-diphenylethylene re-
acted more easily than trans-1,2-diphenylethylene
which required more harsh condition (Table 2,
entry 4). Good results were also obtained with hetero-
cyclic substrates (Table 2, entries 10 and 12).This reac-
tion was applicable to alkynes (Table 2, entries 13–
19). By introducing hydrazine hydrate exactly,
a number of alkynes bearing some reactive functional
groups can be selectively hydrogenated to the corre-
sponding alkenes with moderate to good yields. Nota-
bly, 1-octyne provided a lower yield presumably be-
cause the alkyne lacking conjugation to aromatic ring
was readily overreduced to be relevant alkane
(
Table 2, entry 18).
To illustrate the difference in reactivity between
alkene and alkyne, competition reactions were carried
out (Figure S2 and Figure S3 in the Supporting Infor-
mation). Both alkyl- and phenyl-substituted alkynes
were observed to preferentially undergo hydrogena-
tion over alkenes. Besides, ICP analysis of the super-
natant show that no Rh was detected (the instru-
mentꢀs detection limit is 0.5 ppb), and a negligible
amount of Co was observed (0.011 ppm). These re-
sults indicate that no leaching of catalyst occurred
during the reaction.
As shown in Figure 1 (a), transmission electron mi-
croscopy (TEM) of Rh Co nanocatalyst exhibits
4
a good dispersion of nanoparticles in the solution.
The high resolution transmission electron microscope
(
HRTEM) image [Figure 1, (b)] shows approximately
spherical nanopartcles with an average size of approx-
imate 2.1 nm (Figure S4 in the Supporting Informa-
tion), which was still unchanged after multiple reuses
(
Figure S9 in the Supporting Information). The crys-
talline nature of the Rh Co nanocatalyst confirmed
4
by selective area electron diffraction (SAED) meas-
urements (Figure S8 in supporting information) and
the inset of Figure 1 (b) in which the (111) facets are
[a]
Reaction conditions: Rh Co catalyst (2 mol%/0.5 mol%),
4
substrate (0.2 mmol), CTAB (5 mol%), toluene (2 mL),
8
08C, 3 h.
[
[
b]
c]
GC yields using n-dodecane as the internal standard.
Yield of the tenth run. The amount of Rh Co catalyst is
[d]
4
4
mol%/1 mol%.
[
[
[
[
[
e]
f]
Ethylbenzene was used as a solvent.
Isolated yields.
At 1008C.
The yield of n-octane is 15%.
Ratio cis/trans/alkane=1/0.20/0.35.
g]
h]
i]
Adv. Synth. Catal. 2013, 355, 41 – 46
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asc.wiley-vch.de
43

Jin Lin et al.
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Figure 1. (a) TEM image of Rh Co nanocatalyst, (b) HR-TEM image and lattice fringe image (inset, inverse FFT image
4
after filtering) of Rh Co nanocatalyst, (c) XPS spectrum of Rh Co nanocatalyst before and after Ar etching, (d1) HAADF-
4
4
STEM image, (d2) EDS spectrum, (d3) line scanning profile along the red line shown in (d1)
illustrated. In order to investigate the composition of 4:1 approximately, similar to those from ICP analysis.
the nanoparticles, high-angle annular dark-field scan- Notably, in XRD pattern (Figure S8 in the Support-
ning transmission electron microscopy (HAADF- ing Information), the shift of the diffraction peak of
STEM) and energy-dispersive X-ray spectroscopy Rh Co nanoparticles toward lower angles compared
4
(
EDS) line scanning analysis were carried out. As to that of the Rh nanocatalyst, implied a lattice ex-
shown in Figure 1 (d3), the compositional line scan- pansion resulted from substitution of Co atoms for
ning profile proved the coexistence of Rh and Co in Rh atoms.
nanoparticles as well as the bimetallic nature of the
X-ray photoelectron spectroscopy (XPS) of Rh Co
4
nanocatalysts, of which the atomic ratio of Rh:Co was nanocatalyst indicated the coexistence of metallic Rh
44
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Adv. Synth. Catal. 2013, 355, 41 – 46

Rhodium-Cobalt Bimetallic Nanoparticles: A Catalyst for Selective Hydrogenation
and Co [Figure 1 (c)]. A thin oxide film, probably re- this type of catalysts with high activity but low noble
sulted from exposure to air, was observed with bind- metal content.
ing energies for Rh 3d5/2 and 3d3/2 at 309.61 and
3
2
14.05 eV, respectively, and binding energies for Co
p3/2 and 2p1/2 at 781.55 and 797.97 eV, respectively. Experimental Section
This oxide layer can be easily removed by Ar sputter-
ing for 240 seconds. As expected, after Ar etching for General Procedure
2
40 s, the signals of Rh 3d5/2 and 3d3/2 with binding en-
5
mL of an aqueous solution of RhCl ·xH O (0.106 g),
3 2
ergies of 307.25 and 312.20 eV, respectively, were de-
tected, which can be attributed to Rh(0), as well as
the signals of Co 2p3/2 and 2p1/2 with binding energies
CoCl ·6H O (0.024 g) and centyltrimethylammonium bro-
mide (CTAB, 0.210 g) were obtained by sonication and stir-
ring for 10 min. To this a solution of 3 mL aqueous NaBH4
2
2
of 778.31 and 792.95 eV, respectively, which can be at- (0.0040 g) was quickly added, resulting in a black suspen-
[
10]
tributed to Co(0). It is noteworthy that there was sion. After reaction, the suspension was centrifuged
no obvious change in the relative intensities of peaks (5000 rpm, 5 min, 298K) for separating the nanocatalysts
from the solution. And then the separated nanocatalysts
of Rh(0) and Co(0) after Ar sputtering for 960 s, im-
were washed for 3 times with ethanol and 3 times with
water, dried under vacuum at 298K for 10 h.
plying the uniform composition of metallic Rh and
Co in the nanocatalyst. Moreover, it was observed
that the Rh 3d5/2 levels for the Rh Co nanocatalysts
shift to higher binding energies relative to that for
monometallic Rh sample, and the Co 2p3/2 levels shift
to lower binding energies relative to that for mono-
metallic Co sample (Table S2 in the Supporting Infor- ried out. The reaction was performed at 608C for 3 h. The
mation). This observation indicated the alloy com- yield of product was determined by GC using n-dodecane as
position of the nanocatalyst. Besides, no evident var- the internal standard.
In a typical hydrogenation, the nanocatalyst was generat-
4
ed freshly before reaction. In an N atmosphere, 0.0023 g
nanocatalysts and 0.0040 g CTAB, were introduced to
2
a Schlenk tube. Then, introduction of reactants involving
2 mL styrene, 24 mL N H ·H O and 2 mL toluene was car-
2
2
4
2
[11]
iation was observed in the XPS patterns of Rh Co
4
nanocatalysts before and after hydrogenation, consis-
tent with their constant catalytic activity in the recycle
process. (Figure S5 and Figure S6, in the Supporting
Acknowledgements
Information)
Financial support from the 973 Program (2011CB932404,
The above observations point out that the co-pres- 2011CBA00501), NSFC (21103192, 20925102), the Key Proj-
ence of metallic Rh and Co plays an important role in ect from CAS and NSF of Fujian Province (2009HZ0005) is
the significant improvement of catalytic performance. greatly appreciated.
Intermetallic electronic interactions would lead to
a modification of Rh electronic structure and exert
a considerable effect on adjustment of the bonding
pattern between catalyst surface and reactants, as well
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