Reduced graphene oxide supported nickel-palladium alloy nanoparticles as a superior catalyst for the hydrogenation of alkenes and alkynes under ambient conditions

Article abstract of DOI:10.1039/c5ra25376d

Addressed herein is the superior catalytic performance of reduced graphene oxide supported Ni₃₀Pd₇₀ alloy nanoparticles (rGO-Ni₃₀Pd₇₀) for the direct hydrogenation of alkenes and alkynes to alkanes, which surpasses the commercial Pd/C catalyst both in activity and stability. A variety of cyclic or aromatic alkenes and alkynes (a total of 17 examples) were rapidly reduced to the corresponding alkanes with high yields (>99%) via the presented direct hydrogenation protocol under ambient conditions. Compared to the commercially available Pd/C (10 wt%) catalyst, the rGO-Ni₃₀Pd₇₀ catalyst provided higher yields in shorter reaction times under the optimized conditions. Moreover, the rGO-Ni₃₀Pd₇₀ catalysts were more stable and durable than the commercial Pd/C catalysts by preserving their initial activity after five consecutive runs in the hydrogenation reactions.

Full text of DOI:10.1039/c5ra25376d

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ARTICLE
Reduced Graphene Oxide Supported Nickel-Palladium Alloy
Nanoparticles as a Superior Catalyst for the Hydrogenation of
Alkenes and Alkynes under Ambient Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Yasin Cetinkaya,*a,b Önder Metin,c Metin Balci*a
DOI: 10.1039/x0xx00000x
Adressed herein is the superior catalytic performance of reduced graphene oxide supported Ni30Pd70 alloy nanoparticles
(rGO-Ni30Pd70) for the direct hydrogenation of alkenes and alkynes to alkanes, which surpasses the commercial Pd/C catalyst
both in activity and stability. A variety of cyclic or aromatic alkenes and alkynes (total 17 examples) were rapidly reduced to
the corresponding alkanes with high yields (>99%) via the presented direct hydrogenation protocol under ambient
conditions. Compared to the commercially available Pd/C (10 wt%) catalyst, the rGO-Ni30Pd70 catalyst provided higher yields
in shorter reaction times under the optimized conditions. Moreover, the rGO- Ni30Pd70 catalysts were more stable and
durable than the commercial Pd/C catalysts by preserving its initial activity after five consecutive runs in the hydrogenation
reactions.
www.rsc.org/
their high surface to volume ratio and more active/selective
surface atoms compared to those of bulk Pd catalysts.7-11
However, Pd is a noble metal and their use as catalysts is not
Introduction
Hydrogenation of unsaturated hydrocarbons such as alkenes
and alkynes is a versatile tool in synthetic organic chemistry for
the synthesis of various organic compounds that are important
building blocks used in the total synthesis and industrial
products.1-3 In the classical hydrogenation reactions that have
been known since the 19th century and widely used in various
industries,4 unsaturated organic compounds react with
molecular hydrogen in the presence of a transition metal
catalyst to yield the corresponding alkanes under high pressure
and temperature using a special hydrogenation equipment. The
commercially available Pd/C is still a popular choice as catalyst
for the direct hydrogenation reactions although many other Pd-
based or other metal catalyst systems have been developed.5,6
However, Pd/C is an expensive catalyst and there are other
drawbacks for its use in the hydrogenation reactions such as
having very limited reusability and being flammable upon the
exposure to air after the hydrogenation reactions. Therefore, a
more economical, safer and reusable Pd-based catalysts system
that has a potential to be commercialized is still required for the
direct hydrogenation reactions.
practical in industrial applications because of its high price and
limited source. In recent years, both to reduce the amount of
Pd present in the catalyst system12 and to further enhance Pd
catalyst activity, Pd is alloyed with a first-row transition
metal.13,14 These bimetallic Pd alloy NPs showed much
enhanced catalytic performance than their monometallic
counterparts owing to the synergistic effects resulted from two
metal interactions.15-17 In our recent studies, we have
successfully demonstrated that monodisperse NiPd,18 CoPd19
and CuPd20 alloy NPs supported on reduced graphene oxide
(rGO) were highly active catalysts in various organic reactions
compared to their monometallic counterparts. Those studies
demonstrated that the alloying of a first row transition metal
with Pd has a great effect on its activity and selectivity in organic
synthesis besides the preparation of more economical Pd
catalysts. Moreover, the unique role of rGO on the high activity
of the MPd alloy NPs that either stems from the well-dispersion
NPs over the rGO nanosheets or the presence of a 2D graphitic
plane near each rGO-MPd catalysts helps to pull organic
reactants in close range contact with the NPs has been
demonstrated by our recent studies as well.21,22
In recent years, the use of Pd nanoparticles (NPs) has
attracted great attention in the organic synthesis because of
On the other hand, alkenes and alkynes are widely found in
petroleum, synthetic and natural products that led the
researchers to develop more efficient methods for their
reduction.23 Catalytic reduction of a double bond or a triple
bond can be accomplished under either homogeneous24,25 or
heterogeneous26 catalytic conditions. In this regard, we recently
reported an efficient method for homogeneous reduction of
C=C bonds and C≡C bonds with diimide that is generated in situ
from hydrazine hydrate by oxidation with oxygen.27 In our
a.Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey,
E-mail: yasin.cetinkaya@atauni.edu.tr; mbalci@metu.edu.tr
b.Department of Food Technology, Oltu Vocational School, Atatürk University, 25400
Erzurum, Turkey
c. Department of Chemistry, Faculty of Science, Atatürk University, 25240 Erzurum,
Turkey
† Electronic Supplementary Information (ESI) available: GC-MS spectra of all
starting and products compounds and TEM image of Pd/C
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General procedure for the direct hydrogenation of alkenes and
another study, we have reported the monodisperse NiPd alloy
nanoparticles (NPs) assembled on reduced graphene oxide
(rGO) as catalysts for the transfer hydrogenation of aromatic
nitro or nitriles to primary amines by using ammonia borane
(NH3-BH3) as a hydrogen reservoir in aqueous methanol
solutions at room temperature.22 Although the Ar-NO2 or Ar-CN
compounds were successfully reduced by the rGO-Ni3oPd70
catalysed transfer hydrogenation reactions, the protocol did
not work over the reduction of alkenes and alkynes. This was
most probably due to the hydridic character of the catalytic
transfer hydrogenation protocol.28 Therefore; we then decide
to test the rGO-Ni30Pd70 catalyst for the direct hydrogenation of
alkenes and alkynes to alkanes. Herein, we report the superior
performance of the rGO-Ni30Pd70 catalyst overshadowing the
commercial Pd/C catalyst for the direct hydrogenation of
alkenes and alkynes under 1 atm H2 gas in methanol at room
temperature.
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DOI: 10.1039/C5RA25376D
alkynes
All catalytic hydrogenation reactions either on alkene or alkyne
substrates were performed in a 50 mL round bottom flask under
1 atm hydrogen gas at room temperature. In the typical
catalytic hydrogenation experiment, 1 mmol of substrate and 2
mg of catalysts (G-Ni30Pd70 or Pd/C) were mixed in 5 mL of
methanol in the flask, and following purged with H2 gas for
three times and then proceeded under 1 atm of H2 gas at room
temperature. The progression of hydrogenation reactions was
followed by the GC-MS at certain time intervals.
Reusability of rGO-Ni30Pd70 catalysts in the hydrogenation reactions
The reusability of rGO-Ni30Pd70 catalysts was investigated by
performing the hydrogenation on styrene. For a typical
reusability experiment, the hydrogenation reaction on styrene
was initiated by using the recovered rGO-Ni30Pd70 catalysts and
analyzing the conversion for each run. After each catalytic run,
the solvent was evaporated and the residue was dissolved in 5
mL ethanol. The mixture was centrifuged in 9000 rpm by 10 min
and the catalyst was recovered. After the top phase was
decanted, the recovered catalysts were washed with ethanol
and dried under vacuum. The catalyst isolated was reused for
the next run under the same reaction conditions described as in
the section entitled “General procedure for the direct
hydrogenation of alkenes and alkynes”.
Experimental
Materials
Palladium(II) acetylacetonate (Pd(acac)2, 99%), Nickel(II)
acetate tetrahydrate (Ni(ac)2.4H2O, 98%), , oleylamine (OAm,
>70%), borane-tert-butylamine (BTB, 97%), 1-octadecene (tech.
grade, 90%), hexanes (99%), ethanol (99%), all the chemicals
used for the graphene synthesis except natural graphite flakes
and all alkenes and alkynes used in the hydrogenation reactions
were purchased from Sigma-Aldrich and used as received.
Natural graphite flakes (325 mesh) were purchased from Alfa-
Aesar. Pd/C catalyst (10 wt. % Pd) was purchased from ACROS
Organics.
Results and discussion
Monodisperse Ni30Pd70 alloy NPs were synthesized by using our
well-established protocol reported earlier.22 According to a
typical synthesis procedure, coreduction of palladium(II)
acetylacetonate (0.2 mmol) and nickel(II) acetate tetrahydrate
(0.2 mmol) in oleylamine solution was achieved with borane-t-
butylamine at 100 °C to yield monodisperse Ni30Pd70 alloy NPs.
The colloidal Ni30Pd70 NPs were deposited on rGO via liquid
phase self-assembly method to use them as catalysts for the
direct hydrogenation reactions. As we reported full
characterization of Ni30Pd70 alloy NPs and rGO-Ni30Pd70 catalyst,
we herein only report a limited characterization data which are
Instrumentation
Transmission electron microscope (TEM) images were obtained
by using FEI Tecnai G2 Spirit BioTwin High-Contrast microscope
operating at 120 kV. The X-ray diffraction pattern (XRD) was
recorded on a Rigaku Miniflex diffractometer with CuKα (30 kV,
15 mA, λ= 1.54051 Å). The metal content of the NiPd alloy NPs
and G-Ni30Pd70 catalysts was determined by using Leiman series
inductively coupled plasma-mass spectroscopy (ICP-MS) after
each sample was completely dissolved in aqua-regia
(HNO3/HCl: 1/3 v/v ratio). The monitoring of the hydrogenation recorded by transmission electron microscope (TEM) and
reactions was followed by an Agilent GC-MS instrument
equipped with electron impact (EI) technique using helium as
carrying gas.
inductively-couple plasma mass spectrometer (ICP-MS). Figure
shows representative TEM images of colloidal Ni30Pd70 NPs
1
and rGO-Ni30Pd70 catalysts revealing the uniform particle
distribution of the alloy NPs over entire TEM grid with an
average particle size of 3.5 nm (Figure 1A) and preservation of
their size and morphology on rGO support (Figure 1B). To show
the crystal structure of the rGO-Ni30Pd70 catalysts, the XRD
pattern was presented in SI (Figure S3). Besides the composition
Ni30Pd70 alloy NPs, the metal content of rGO-Ni30Pd70 was also
determined to be 14 wt% (Ni+Pd) by ICP-MS analyses. According
to the ICP-MS results, the Pd content of rGO-Ni30Pd70 catalysts
was calculated to be 9.8 wt%, which makes the rGO-Ni30Pd70
catalyst suitable to compare its catalytic performance with the
commercial 10 wt% Pd/C catalyst in the same reaction. The rGO-
Ni30Pd70 catalyst was used without any further purification in
Synthesis of monodisperse Ni30Pd70 alloy nanoparticles
The monodisperse Ni30Pd70 alloy NPs were synthesized by using
our well-established protocol published elsewhere.22
Synthesis of reduced graphene oxide (rGO) and supporting Ni30Pd70
alloy nanoparticles on rGO
The reduced graphene oxide used in this study was prepared by
using well-established two-step chemical route and the Ni30Pd70
NPs were supported on rGO via liquid-phase self-assembly
method which are published elsewhere.19-22
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the direct hydrogenation reactions of alkenes and alkynes
under 1 atm hydrogen gas pressure in methanol at room investigated the scope of the rGO-Ni30Pd70 catalyzed
temperature. In the initial study, the direct hydrogenation of hydrogenation reaction over various alkenes and alkynes and
styrene (1) was selected as a test reaction to determine the the results were depicted in Table 1. The product analysis was
optimum reaction conditions for the rGO-Ni30Pd70 catalyzed studied by GC-MS after certain time intervals. The reduction of
ARTICLE
The optimized reaction conditions in hand, we then
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DOI: 10.1039/C5RA25376D
direct hydrogenation reactions. In the course of optimization alkenes
experiments, the effect of several reaction parameters were were completed in 10-20 min (Table 1, entries 1-4). The
studied including solvent, temperature, catalyst loading and reduction of 1,3-cyclooctadiene ( ) and 1,5-cyclooctadiene (11
reaction time. It is found that 1 mmol styrene was quantitavely were completed in 2 h and 7 h with yields of >99%, respectively
converted to ethylbenzene (2) within 10 min when the reaction Table 1, entries 5 and 6). The conjugated diene unit in was
1, 3, 5, and 7 to corresponding alkanes 2, 4, 6, and 8
9
)
(
9
proceed by using 2 mg of G-Ni30Pd70 catalysts in 5 mL of reduced faster than the isolated diene unit in 11. Furthermore,
methanol under 1 atm H2 gas at room temperature Under the the reduction of cycloheptatriene (13) to cycloheptane (14) was
optimized reaction conditions, we then examine the completed in 4 h (Table 1, entry 7). p-benzoquinone (15) and 5-
performance of commercial Pd/C catalyst in the direct vinyl-2-norbornene (21) were reduced in yields of >99% within
hydrogenation of styrene. The Pd/C catalyst was also efficient 100 and 50 min, respectively (Table 1, entries 8 and 11). Both,
in catalyzing the hydrogenation reaction, but the quantitave bicyclic compound 1H-indene (17) and tricyclic 1,4-dihydro-1,4-
conversion of styrene to ethylbenzene was completed in 50 epoxynaphthalene (19) were converted to corresponding
min. This simple test clearly reveals that rGO-Ni30Pd70 catalyst is alkanes in 12 min in yields of >99% (Table 1, entries 9 and 10)
.
much more active than the commercial Pd/C catalyst in the We assume that the strained double bonds incorporated in a
direct hydrogenation reaction.
ring are reduced much rapidly. (1Z,5E,9E)-cyclododeca-1,5,9-
triene (23) was reduced to cyclododecane (24) in 8 h in high
yield (>99%) (Table 1, entry 12). Finally, the hydrogenation of α-
pinene (25) to 2,6,6-trimethylbicyclo-[3.1.1]heptane (26) was
completed in 48 h in a yield of 84% (Table 1, entry 13). The
decreased reactivity of 25 was attributed to the steric hindrance
around double bond caused by bridge methyl groups.
Furthermore, we examined the scope of the reaction by
submitting a range of alkynes (conjugated and nonconjugated)
to reduction under the same reaction conditions.
Phenylacetylene (27) was reduced to ethylbenzene (2) in 18 min
(Table 1, entry 14). However, diphenylacetylene (28) was less
reactive than the phenylacetylene, which is reduced to 1,2-
diphenylethane in 20 min. Strong electron withdrawing groups
in 30 did not have much effect on the reduction kinetics. Pyrrole
derivative having a nonconjugated alkyne functionality (32) was
also smoothly reduced to N-propyl pyrrole derivative (33
)
within 30 min almost in quantitative yield.
Table 1. Scope of rGO-Ni30Pd70 catalyzed direct hydrogenation
Entry
Substrate
Product
Time
Yield
(%)[a]
10
min
1
>99%
1
3
5
2
4
6
12
min
2
3
>99%
>99%
Figure 1. Representative TEM images (A) of colloidal Ni30Pd70 alloy
NPs and (B) rGO-Ni30Pd70 catalysts.
12
min
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20
min
>99%
30
min
17
>99%
7
9
8
32
[a]Determined by GC-MS analysis.
33
>99%
>99%
>99%
5
6
7
2 h
10
To understand whether the presented protocol can be scaled-
up, another experiment was performed by increasing the
amount of styrene up to 10 mmol whereas the amount of
catalyst was kept constant at 2 mg. The progress of the reaction
was monitored by doing product analysis with GC-MS at certain
time intervals (10, 30, 50, 60, 70, 80, 90 min) and the results
were depicted in Table 2. As clearly seen from Table 2, only 4%
of yield was obtained within initial 10 min, but the complete
reduction was achieved after 90 min. These results clearly
indicate that there is no major effect of substrate amount (by
keeping the catalyst amount at constant) in the catalytic
hydrogenation protocol and the presented catalytic protocol
has a potential to be used in industry. Moreover, the same
scale-up test was also performed by using the commercial Pd/C
catalyst (2 mg) and the quantitative conversion of styrene to
ethylbenzene completed in 400 min. This is another important
evident for the obvious advantageous of rGO-Ni30Pd70 catalyst
over the commercial Pd/C one.
7 h
4 h
11
13
12
14
100
min
>99%
8
15
17
16
18
12
min
>99%
>99%
>99%
9
Table 2. Hydrogenation of styrene (1) (10 mmol) with rGO-Ni30Pd70
12
min
10
11
catalysts (2 mg).
19
21
20
22
Time (min)
Yield (%)[a]
10
30
50
60
70
80
90
4
21
37
59
77
95
100
50
min
>99%
84%
12
13
8 h
[a]Determined by GC-MS analysis.
23
25
24
26
Another significant aspect was the study of the reusability of the
catalysts (rGO-Ni30Pd70) for industrial applications. Therefore,
we tested the reusability of rGO-Ni30Pd70 catalysts in the
48 h
hydrogenation of styrene (1). The results are shown in Table 3.
As it can be clearly seen from the Table 3, the rGO-Ni30Pd70
catalyst preserved its initial activity after five consecutive runs
indicating its durability in the direct hydrogenation reactions.
We performed same reusability test in the presence of the
commercial Pd/C catalyst and it is observed that the Pd/C
catalyst loss its 20% of its initial activity after the five
consecutive runs indicating that rGO-Ni30Pd70 is more durable
catalyst than the Pd/C in the direct hydrogenation reaction.
18
min
>99%
>99%
>99%
14
15
16
27
28
2
20
min
29
Table 3. Reusability of the rGO-Ni30Pd70 catalyst in the hydrogenation
of styrene
20
min
31
30
Run
1
2
3
4
5
Yield(%)[a]
>99%
>99%
>99%
>99%
>99%
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[a]Determined by GC/MS analysis.
19 H. Goksu, H. Can, K. Sendil, M. S. Gültekin aVinewdAOrti.cMle Oentliinne,
Appl. Catal. A-Gen. 2014, 488, 17D6O-1I:8120..1039/C5RA25376D
20 S. Diyarbakir, H. Can and O. Metin, ACS Appl. Mater.
Interfaces 2015, 7, 3199-3206.
Conclusions
21 B. Nişancı, K .Ganjehyan, Ö. Metin, A. Daştan and B.
Törok, J. Mol. Catal. A 2015, 409, 191-197.
In summary, we have demonstrated a facile catalytic method
for the reduction of alkenes and alkynes. The catalytic reduction
method presented here has distinct advantageous over the
existing one such as being rapid, using a recyclable and more
economical Pd catalysts and being applicable and scale-up for
variety of alkenes or alkynes under very mild conditions.
Moreover, it has been demonstrated by many times that rGO-
Ni30Pd70 catalyst showed much better performance than the
commercial Pd/C one and we believe that rGO-Ni30Pd70 catalyst
has a potential to surpass the commercial Pd/C catalyst in
organic synthesis.
22 H. Goksu, S. F. Ho, O. Metin, K. Korkmaz, A. Mendoza
Garcia, M. S. Gultekin and S. Sun, ACS Catalysis, 2014, 4,
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Acknowledgments
We thank to the Scientific and Technological Research Council
of Turkey (TUBITAK, Grant No. TBAG-112T360), the Middle East
Technical University, the Atatürk University and the Turkish
Academy of Sciences (TUBA) for financial support of this work.
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Graphical Abstract
rGO-Ni30Pd70 catalyst showed a superior catalytic performance surpassing the commercial
Pd/C catalyst both in activity and stability for the hydrogenation of alkenes and alkynes to
alkanes.