Graphene-supported Pd nanoparticles: Microwave-assisted synthesis and as microwave-active selective hydrogenation catalysts

Article abstract of DOI:10.1039/c3ra41340c

Facile synthesis of graphene-supported Pd nanoparticles (NPs) with an average size of 5 nm (Pd/G) was achieved by a microwave-assisted reduction approach; the obtained Pd/G can be very effectively coupled to the microwave field, making it a high-performance catalyst (with turnover frequency (TOF) of 158 465 h^-1) for microwave-assisted selective hydrogenation of isophorone at low temperatures. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41340c

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Graphene-supported Pd nanoparticles: microwave-
assisted synthesis and as microwave-active selective
hydrogenation catalysts3
Cite this: RSC Advances, 2013, 3, 10131
Received 20th March 2013,
Accepted 8th May 2013
ab
a
Jing-He Yang and Ding Ma*
DOI: 10.1039/c3ra41340c
www.rsc.org/advances
Facile synthesis of graphene-supported Pd nanoparticles (NPs)
with an average size of 5 nm (Pd/G) was achieved by a
microwave-assisted reduction approach; the obtained Pd/G can
be very effectively coupled to the microwave field, making it a
high-performance catalyst (with turnover frequency (TOF) of
Graphene (G) is a huge open p-electron system with a unique
electronic structure, which ignites diverse applications such as
electronics, photovoltaics, batteries, supercapacitors and so
1
3–15
on.
Very recently, it has been shown that graphene can be
used as the catalyst or the advantageous carrier for the catalytic
active components for oxidation, hydrogenation and carbon-
2
1
1
58 465 h ) for microwave-assisted selective hydrogenation of
1
6–24
isophorone at low temperatures.
carbon coupling reactions.
However, graphene/layered
carbons are very sensitive to microwave irradiation. Herein,
we demonstrate the potential of a graphene-type support in the
microwave-assisted synthesis of metal/graphene nano-compo-
sites. Significantly, because of the good ability to adsorb
microwave of the obtained Pd NPs/graphene nano-composites,
it is microwave-active in the low temperature selective hydro-
genation reaction, leading to superior catalytic performance as
compared to the conventional counterparts.
Dihydroisophorone (3,3,5-trimethylcyclohexanone, DHIPO) is a
solvent used for vinyl resins, varnishes, lacquers, paints and other
applications, and can be produced through the selective hydro-
genation of isophorone (3, 5, 5-trimethyl-2-cyclohexen-1-one) as
1–3
shown in Scheme 1. In addition to nickel and nickel alloys, Pd/
4–7
C is the most commonly used catalyst for this reaction.
However, it is difficult to get a good selectivity at the same time as
8
obtaining good activity in such a system. While the boiling point
Graphene oxide (GO) was prepared by exfoliating graphite with
strong oxidizing agents. GO was reduced by hydrazine hydrate in
ammonia solution to generate chemically converted graphene
of DHIPO (462 K) is close to that of trans-3, 3, 5-trimethylcyclohex-
anol (467 K), a by-product, excellent selectivity to the designated
product is particularly important to avoid the complicated and
time-consuming product separation and purification processes.
Therefore, the development of a green and efficient process with
the high conversion and selectivity is the urgent need for this
25
(CCG). 250 mg CCG and 25 mg palladium acetate were mixed
with 30 ml 0.1 M sodium dodecyl sulfate solution in 80 ml quartz
tube. The mixture was sonicated at room temperature for 1 h.
Microwave-assisted treatment of the mixture under 1.2 MPa
hydrogen at 373 K for 30 min in a CEM Discover high pressure
microwave apparatus. After filtration, washing and drying at 333 K
for 12 h, Pd/G catalysts were obtained (Pd content of 3.2 wt.%,
determined by ICP). The same method was applied for the
synthesis of control catalysts, such as Pd/active carbon (Pd/AC), Pd/
8
important hydrogenation reaction. Indeed, there are attempts to
develop new methodologies for this reaction. For instance,
supercritical carbon dioxide has proven to be a suitable green
solvent system which leads to high reaction rates and easy
separation of solid catalysts and products. Hence a commercia-
8–12
lization of the process was realized.
However, as the combina-
graphite (Pd/Grp) and Pd/SiO
wt.% and 4.4 wt.%, respectively, as determined by ICP) (Fig. S4,
ESI ).
2
(with Pd content of 3.7 wt.%, 5.0
tion of high activity/selectivity is rarely reported, there are room for
developing new catalytic systems and therefore further improving
the reaction efficiency.
3
a
Beijing National Laboratory for Molecular Sciences, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China.
E-mail: dma@pku.edu.cn; Fax: (+86)10 62758603
b
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian 116012, China
3
Electronic supplementary information (ESI) available: synthesis, characterization,
reaction conditions and other experimental details can be found in the ESI. See DOI:
0.1039/c3ra41340c
1
Scheme 1 The reaction of hydrogenation of isophorone.
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Fig. 1 TEM images (A) and size distribution (B) of Pd/G catalyst.
TEM images show that palladium nanoparticles with an
average size of 5 nm were uniformly distributed over the carbon
support (Fig. 1). The chemisorption experiments indicate that the
metal dispersion is 18.8%, corresponding to the mean crystallite
size of 5.9 nm, which agrees well with the TEM observation. As
Fig. 2 The Pd 3d XPS spectra of different catalysts (A); The recycling of the Pd/G
shown in O 1s XPS (Fig. S6, ESI
3
), the 532 eV and 533.5 eV is
2 2
catalyst for the hydrogenation of isophorone (B) (1.2 MPa H , 333 K, 0.5 ml H O, 5
min, 2 mg 3.2 wt.% Pd/G, 3 mmol substrate, microwave-assisted); TEM images of
used Pd/G catalyst (C); The effect of reaction temperature on catalytic behavior of
corresponding to CLO and C–O bond, respectively. There are more
C–O species on active carbon but CLO species on graphite.
Instead, graphene prepared by current method contains not only
C–O species but also abundant CLO species. The synthesis process
of Pd/G was followed by UV-vis spectroscopy. As shown in Fig. S1,
2 2
different catalyst systems (1.2 MPa H , 0.5 ml H O, 0.5 mmol substrate, 5 min, 2 mg
catalyst) (D).
3
ESI , the peak corresponding to palladium acetate starts to
decrease in 30 s under microwave irritation, indicating that the
microwave-assisted reduction/nucleation processed very fast. After
observation clearly indicates that Pd/G catalyst can be effectively
coupled with microwave field, leading to the fast and highly
efficient selective hydrogenation of isophorone. As a control, GO
1
min, it seems that the reduction process completed, demon-
with more oxygen-containing groups (Fig. S7, ESI
catalyst (Fig. S8, ESI ) shows a relatively poor catalytic performance
3
Table S1, ESI ). Even if 2 mg pure Pd nanoparticles (Fig. S9, ESI )
3
) supported Pd
strating the efficiency of microwave-assisted method. Following
the same method, Pd/active carbon (Pd/AC), Pd/graphite (Pd/Grp),
and Pd/SiO catalysts were prepared. The chemical states of those
2
Pd-based catalysts were studied by XPS (Fig. 2A). Clearly, the Pd/AC
catalyst was dominated with the metallic Pd, while a peak at 336.9
3
(
3
were used as catalysts, its activity is still lower than that of Pd/G
catalysts. For Pd/G catalyzed reaction, even if we increase the
amount of substrate in the reaction, the catalytic performance
remains almost the same, showing the Pd/G catalyzed hydrogena-
tion is highly microwave-active (Table 1, entry 6 and 10). Indeed, a
2
eV for Pd/SiO sample suggests that there are strong interaction
between Pd and silica, leading to the formation of Pd–Si bond or
26,27
PdO.
The graphite and graphene supported Pd NPs catalysts
21
remarkable turnover frequency (TOF) of 158 465 h was recorded
exhibit a peak at 335.3 eV with a shoulder at around 337.6 eV. This
result implies that the valence state of the Pd/Grp and Pd/G
catalysts were a mixture of metallic Pd and Pd .
for the 3.2 wt% Pd/G catalyst, which surpassed the conventional
catalysts. Although both the components of Pd/G nano-composite
can adsorb microwave, the integrity of composite is pivotal for the
construction of a successive microwave-active catalytic system. As
2+
Microwave-assisted hydrogenation of isophorone was con-
ducted at 333 K in a quartz pressure vessel, with water as the
solvent and hydrogen (1.2 MPa) as the reducing agent. In a blank
experiment without using catalyst, the results showed that no
noticeable activity was observed by GC over a 5 min run (Table 1,
entry 1). When the pristine graphene was used as catalyst for the
isophorone hydrogenation reaction, no activity was presented
either (Table 1, entry 2). When the 0.1 wt.% Pd/G was used as
catalyst, a 17.4% isophorone conversion and 54% selectivity
towards the desired product were observed (Table 1, entry 3). The
catalytic performance increased with higher Pd loading (Table 1,
2
shown in Table 1, the Pd/AC, Pd/Grp and Pd/SiO catalysts
exhibited activity in the selective hydrogenation of isophorone.
However, the concomitant good activity/selectivity under micro-
wave irradiation condition can only be achieved over Pd/G nano-
composite catalyst. It is worth noting that both graphene and
graphite possesses a giant p-system, which may promote reactant/
catalysts interaction (especially for reactant with a p-system as in
28,29
the current study) at the same time of adsorbing microwave.
However, Pd/Grp and Pd/G have a huge difference in reaction
performance, albeit Pd/Grp is much better than Pd/AC and Pd/
3
entry 4 and Fig. S10, ESI ). When Pd loading reached 3.2 wt.%, an
SiO . A possible explanation is that as CCG was prepared from the
2
unexpected high isophorone conversion of 99.8% was observed
with selectivity to DHIPO dramatically increased to 97%,
demonstrating the possibility of coexistence of high activity and
high selectivity (Table 1, entry 5). Significantly, the reaction time is
just 5 min while the amount of catalyst used was 2 mg. This
reduction of graphene oxide, the edge was decorated with
carboxyls and other oxygen-containing groups, which make the
catalyst well dispersed in the multi-phase reaction mixture (with
water as solvent, isophorone as the organic phase, and the catalyst
3
as the solid phase, see Fig. 3 and Fig. S2, ESI ). Most possibly, the
1
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a
Table 1 Microwave-assisted hydrogenation of isophorone under various conditions
d
Entry
Catalysts
Loading amount (wt.%)
Time (min)
Conv. (%)
DHIPO Sel. (%)
TOF(/h)
1
2
3
4
5
6
7
8
blank
graphene
Pd/G
Pd/G
Pd/G
Pd/G
Pd/Grp
Pd/AC
—
—
5
5
5
5
5
5
5
5
5
10
—
—
—
—
—
—
0.1
0.6
3.2
3.2
3.7
5.0
4.4
3.2
17.4
29.2
99.8
98.4
35.0
9.0
54.1
85.5
97.4
98.9
83.9
.99.0
.99.0
93.1
290 236
85 363
52 818
104 997
16 149
3073
b
9
Pd/SiO
Pd/G
2
7.4
99.0
2871
158 465
c
1
0
a
2
Hydrogen pressure: 1.2 MPa, reaction temperature: 333 K, isophorone: 0.5 mmol, solvent: 0.5 ml H O, 2 mg catalyst, microwave-assisted
b
c
d
reaction. isophorone: 1 mmol. isophorone: 3 mmol. the metal dispersion was all unified regarded as 18.8%.
reaction occurred at the interfaces of various phases in the
complicated reaction mixture while the good dispersion of the
catalysts between the oil/water phases make itself very efficient in
the adsorption/reaction. The application scope of this catalytic
system was explored by using a variety of a, b-unsaturated ketone
under similar reaction conditions. As summarized in Table 2, the
Pd/G catalyst exhibited a good activity and selectivity for
hydrogenation of all the reactants. Moreover, the catalytic
performance of Pd/G catalyst remained at a very high level even
after five reaction cycles, while there is no obvious particle
aggregation observed after the reaction (Fig. 2B and C). This
indicates clearly that Pd/G catalyst is a very stable catalyst for
microwave-assisted hydrogenation reaction.
In order to verify the function of microwave field in this
reaction, the catalytic performance of these Pd/G and Pd/SiO
catalysts in microwave-assisted and conventional oil bath heating
conditions was investigated in the hydrogenation of isophorone
under different temperatures (Fig. 2D). Under conventional oil
bath heating conditions at 303 K, the conversion of the reactions
2
2
with Pd/G as catalyst was more than twice that of the Pd/SiO (only
20%) while the selectivity was only about 70% of the latter. When
the temperature was raised to 313 K, for Pd/SiO it was surprising
2
to observe that the selectivity dropped rapidly to about 40% while
the conversion did not show a significant increase, which means
raising temperature was beneficial to the formation of byproducts
at this range. Further increase in the reaction temperature
(through 323 K to 333 K), the conversion and selectivity improved
modestly (Pd/SiO ) with conversion less than 20% and selectivity
2
about 60%. Instead, for Pd/G, only at a relatively high reaction
temperature range (323 K to 333 K), a good conversion (60%) and
selectivity (99%) was observed. This result reveals that in
conventional oil bath heating conditions, Pd/SiO was the catalyst
2
with typical low conversion and higher selectivity at low
Table 2 Microwave-assisted hydrogenation of a, b-unsaturated ketone under
a
the same conditions
b
b
Entry
1
Substrate
Conversion (%)
98.0
Ketone Sel. (%)
TOF(/h)
93.8
52 285
2
97.1
99.6
51 751
3
4
83.7
99.1
98.6
99.6
44 282
52 818
Fig. 3 Possible reaction scheme of microwave-assisted hydrogenation of isophor-
one: the edge of graphene was decorated with oxygen-containing groups which
confers it surfactant-like capability, emulsifying the organic phase, isophorone, and
water during the reaction. Moreover, water, especially graphene are able to couple
with microwave field, making the transformation of the adsorbed a, b-unsaturated
ketone very efficient.
5
.99.0
98.7
52 818
a
1
.2 MPa H , 333 K, 0.5 ml H O, 5 min, 2 mg 3.2%Pd/G,
2
2
b
microwave-assisted reaction. the metal dispersion was 18.8%.
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4
In conclusion, we have developed an efficient method to
synthesize Pd/graphene nano-composite catalysts by microwave-
induced hydrogen reduction method. The composite can be
effectively coupled with microwave field, making it an excellent
catalyst for selective hydrogenation reactions. In the microwave-
assisted hydrogenation of isophorone reaction, an excellent
catalytic behavior (with activity/selectivity higher than 95%, and
a TOF of 158 465 h ) can be realized at ambient temperature in
less than 5 min, surpassing its conventional counterparts
operating both in thermal and microwave modes. Although the
potential of this strategy is not yet fully demonstrated, it can
already be seen to provide a simple, fast, and efficient approach
for microwave-active graphene-based catalytic reactions.
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Acknowledgements
This work received financial support from the Natural Science
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