Efficient Palladium(0) supported on reduced graphene oxide for selective oxidation of olefins using graphene oxide as a ‘solid weak acid’

Article abstract of DOI:10.1016/j.catcom.2019.01.020

Selective oxidation of olefin derivatives to ketones has made innovative development over palladium(0) supported on reduced graphene oxide. Compared to traditional Wacker oxidation, the novel method offers an economical and environment-friendly option by using graphene oxide (GO) as a ‘solid weak acid’ instead of classical homogeneous catalysts like H₂SO₄ and CF₃COOH. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope and transmission electron microscopy images of Pd⁰/RGO showed that the nanoscaled Pd particles generated at the flake structure of reduced graphene oxide. Under optimized condition, up to 44 kinds of ketones with different structures can be prepared with excellent yields.

Full text of DOI:10.1016/j.catcom.2019.01.020

Accepted Manuscript
Efficient Palladium(0) supported on reduced graphene oxide for
selective oxidation of olefins using graphene oxide as a ‘solid
weak acid’
Xi Gao, Jianhao Zhou, Xinhua Peng
PII:
S1566-7367(19)30023-8
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.01.020
CATCOM 5604
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
27 November 2018
23 January 2019
27 January 2019
Please cite this article as: X. Gao, J. Zhou and X. Peng, Efficient Palladium(0) supported
on reduced graphene oxide for selective oxidation of olefins using graphene oxide as
a
‘solid weak acid’, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.01.020
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ACCEPTED MANUSCRIPT
Efficient Palladium(0) supported on reduced graphene oxide for
selective oxidation of olefins using graphene oxide as a ‘solid weak
acid'
Xi Gao, Jianhao Zhou, and Xinhua Peng*
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, 210094, China
* Email: xhpeng@njust.edu.cn

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Abstract
Selective oxidation of olefin derivatives to ketones has made innovative
development over palladium(0) supported on reduced graphene oxide. Compared to
traditional Wacker oxidation, the novel method offers an economical and
environment-friendly option by using graphene oxide (GO) as a ‘solid weak acid’
instead of classical homogeneous catalysts like H2SO4 and CF3COOH. X-ray
diffraction, X-ray photoelectron spectroscopy, scanning electron microscope and
transmission electron microscopy images of Pd0/RGO showed that the nanoscaled Pd
particles generated at the flake structure of reduced graphene oxide. Under optimized
condition, up to 44 kinds of ketones with different structures can be prepared with
excellent yields.
Keywords: solid weak acid; graphene oxide; palladium catalyst; Wacker oxidation;
olefins
1. Introduction
Owing to the special feature of graphene, its functionalized forms have emerged as
promising materials for diverse applications, including heterogeneous catalysis,
sensors, supercapacitors, aircrafts and many other fields [1-4]. Graphene oxide (GO),
the simplest derivative of graphene, has been discovered and synthesized long before
the study of graphene. Because of the large surface area, GO should be a distinctive
two-dimensional material for supporting metallic and bimetallic nanoparticles [5-7].
Supporting nanoscaled metal particles on reduced graphene oxide layers may provide
a huge specific surface area and thermal stability for potential applications in

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heterogeneous catalysis, fuel cells, chemo-sensors, hydrogen transport and so on
[8-10]. Especially, the electronic defects in the structure of RGO can be useful for
heterogeneous catalysis in chemical reaction, as they make it possible to design the
local features to acquire new surface functions which can promote the interactions
with the anchored metal nanoparticles [2].
Since the pioneering studies of Smidt and co-workers in the 1950s, Pd(II) has been
generally used as catalyst to oxidize olefins into methyl ketones, for manufacturing
fine chemicals, agrichemicals and organic intermediates [11]. Especially, a
well-known Tsuji-Wacker oxidation efficiently conducts on more complex substrates
with PdCl2, CuCl and O2 in a DMF-H2O media [12]. However, there are still some
issues of catalyst recovery, chlorinated by-products and polymerization of double
bonds with this method. What’s more, pollutant by Cu2+ during this process has been
considered a primary restriction of the reaction [12]. In order to substitute the Cu
co-catalyst, other researchers have utilized new catalysts or solvents such as
palladium-containing compounds [13], ionic liquids [14, 15], and ethylene carbonate
[16]. Moreover, several novel oxidation systems, such as Fe2(SO4)3 [17], NO [18],
1,4-benzoquinone [19, 20], CrO3 [21], Sc(OTf)3 [22], hypervalent iodine compounds
[23] and an atmosphere of O2 [24], have been reported. Clearly, a number of these
improvements are significant contributions and supply a Wacker-type oxidation
without copper. Anyway, there are some needs to establish a mild, economical and
eco-friendly Wacker oxidation system by the use of clean aqueous hydrogen peroxide
[14, 15, 25]. In 1980, Mimoun and co-workers firstly develop an original oxidation

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method of H2O2 with Pd(OAc)2 [26]. There is a limitation that this system is not
efficient for olefin derivatives. Lately, Cao’s group has utilized palladium catalyst,
which is producted by a PBO ligand and AgOTf, to accomplish the reaction from
styrene derivatives to corresponding ketones [27]. Thus, with challenges, it is of great
interest to find an applied catalyst combining with H2O2 as oxidizing agent to
constitute a reaction intermediate that will efficiently implement the Wacker oxidation
of styrenes. In 2013, Kulkarni’s group employs Pd0/C and KBrO3 to execute Wacker
oxidation of styrenes [28]. Afterwards, the Wacker oxidation becomes app licable and
economical using this kind of heterogeneous recyclable catalyst. On this basis, we
have reported a novel copper-free method with aqueous hydrogen peroxide as oxidant
in the presence of Pd0/C catalyst [29].
Here we present a remarkable Wacker oxidation using nanoscaled Pd particles
supported on RGO layers (Pd0/RGO, Fig. 1). Specially, GO serves as a ‘solid weak
acid’to displace liquid strong acids such as H2SO4 and CF3COOH.
2. Experimental
2.1 Materials and methods
All chemical raw materials, including original reagents and solvents (analytically
pure) etc. were sold without further processing. Column chromatography was
occurred on silica gel (300-400 mesh) using the mixture of hexyl hydride and ethyl
acetate as a developing solvent.
The methods of GC, NMR, SEM, TEM, XRD, XPS, Raman, NH3-TPD and ICP
were shown in the supplementary material.

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2.2 General procedure for Synthesis of GO and Pd0/RGO composite
Graphene oxide (GO) was prepared from graphite powder by a modified Hummer’s
method in our group [30]. The Pd0/RGO composite was prepared by a facile chemical
reduction reaction. Briefly, 0.02 g GO was dispersed in 20 mL deionized water to
yield uniform dispersion by ultrasound. Subsequently, the homogeneous GO
dispersion was mixed with 10 mL PdCl2 solution (0.08 mol/L in water). Then 10 mL
NaBH4 solution (0.12 mol/Lin water) was added in a dropwise manner and stirred for
o
another 24 h at 40 C. Afterwards, the precipitation was centrifuged, washed with
deionized water and absolute ethanol for three times each. The final products were
dried at 70 oC for 6 h in a vacuum oven.
2.3 General procedure for Synthesis of nitrostillbenes
This kind of reaction was occurred in a 50mL round-bottom flask under magnetic
stirring [31]. At first, 2,4,6-trinitrotoluene (2.0 mmol), 2,4-dinitrotoluene (2.0 mmol)
or 2,4-dimethyl-1,3,5-trinitrobenzene (1.0 mmol) reacted with benzaldehyde (2.0
mmol) in dry toluene (40 mL) using piperidine (0.4 mmol) as base catalyst. The
whole process was refluxed and monitored by thin-layer chromatography. After 24 h,
the toluene in the reaction system was evaporated for reuse. Then, absolute ethanol
was added dropwise until little sediments appeared. The temperature was reduced to
room temperature and placed in a freezer overnight. The sediments were leached with
washing and drying. In the end, the crude solid was crystallised with appropriate
solvents such as the mixed solution of acetone and n-hexane.
2.4 General procedure for Wacker oxidation of olefins

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With the dissolution of substrate (0.4 mmol) in CH3CN (2 mL), Pd0/RGO (0.01 g),
H2O (0.5 mL), and GO (0.01 g) were orderly added into a pressure bottle (35 mL).
o
The mixture was dispersed by ultrasound for about 30 minutes at 25 C. Then H2O2
(30 wt%, 4 mmol) was cautiously added dropwise. Immediately, the reaction system
was heated to 55 °C with lid closed until the process was fully completed (detected by
TLC). Subsequently, Pd0/RGO and GO were removed by centrifuge. The mixture was
extracted by deionized water and ethyl acetate. After the layers were separated, the
organic part was washed with deionized water, dried with anhydrous NaSO4, filtered
and evaporated by reduced pressure distillation. Finally, purification of the crude
product was carried out by column chromatography. For 14, 18, 24, 40 (Table 2),
excess hydrogen peroxide was added after half of the total reaction time.
2.5 General procedure for the hot-filtration test
With the dissolution of substrate (0.4 mmol) in CH3CN (2 mL), Pd0/RGO (0.01 g),
H2O (0.5 mL), and GO (0.01 g) were orderly added into a pressure bottle (35 mL).
The mixture was dispersed by ultrasound for about 30 min at room temperature. Then
H2O2 (30 wt%, 4.0 mmol) was added in a dropwise manner. After this, the mixture
was heated to 55 °C with lid closed. The catalyst was immediately filtered out after
two hours, and the concentration of acetophenone was determined by GC every two
hours.
After hot-filtration text, the solvent was eliminated by rotary evaporator. And the
residue was resolved in aqua regia for ICP analysis. According to the result of ICP
analysis, none of Pd existed in the solution phase after 30 minutes of reaction with hot

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filtration.
3. Results and discussion
3.1 Characterization of catalysts
The structural features of Pd0/RGO and GO were characterized by using XRD,
SEM, TEM, Raman, NH3-TPD, XPS and ICP analyses.
The XRD patterns of Pd0/RGO and GO were displayed in Fig. S1. The Pd peaks
appeared at 40.1o, 46.6o, and 68.1o, which were attributed to the (1 1 1), (2 0 0) and (2
2 0) planes of face-centered cubic structure of Pd (JCPDS no. 46-1043). Compared to
graphene oxide, the XRD pattern of Pd0/RGO showed the formation of Pd particles
and RGO as the characteristic peaks of Pd were emerged and the ones of GO were
disappeared.
Through the observation of SEM images, as shown in Fig. S2, the surface
morphologies of GO (a-c) and Pd0/RGO (d-f) were adequately manifested. Pure GO
was obvious flake structure with smooth surface while Pd0/RGO exhibited stacked-up
flakes with rough surface.
Further research by transmission electron microscopy (TEM) analysis showed that
solid nanoscaled Pd particles were supported on the RGO layers densely (Fig. S3).
Besides, two-dimensional graphene oxide (g-k) and Pd nanoparticles (o and p) were
observed very clearly.
Meanwhile, the chemical groups and structure defects on RGO sheets could serve
as the nucleating cores of metal nanoparticles. According to Raman analyses,
nanoscaled Pd particles had a powerful interaction with the oxygen-containing groups

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by the appearance of a characteristic peak at 635 cm−1 (Fig. S4) [32]. This strong
covalent interaction made the loading process easier to form Pd0/RGO.
NH3-TPD was carried out to explore the total acidity and acid strength distribution
of graphene oxide, as shown in Fig. S5. According to the results of NH3-TPD, it was
indicated that there were only weak acid sites (the characteristic peak) in the surface
of graphene oxide. And the total acid density was 0.12 mmol/g which showed that the
mass percentage of -COOH residue contained in 0.01 g of GO was 0.54% and it was
equivalent to a catalyst amount of 0.3 mol% comparing with 0.4 mmol of each
substrate. The high acid sites were sufficient enough for the oxidation reaction of
olefins and made graphene oxide a promising acid catalyst for valence change of
palladium as described in results and discussion.
To further investigate Pd0/RGO, X-ray photoelectron spectroscopy (XPS) was
conducted to explore the surface compositions and chemical states. As shown in Fig.
S6, the experimental data indicated that the zero valence palladium, which catalysed
the reaction cycle, occupied the overwhelming majority. And C1s spectra was
depicted which could be separated into four peaks, ascribed to C-C, C-OH, C-O-C
and π-π bonds in reduced graphene oxide, respectively. Combining with Raman
spectra, it had been shown that most of oxygen groups were reduced while part of
them were still remaining on the reduced graphene oxide to support palladium
nanoparticles.
3.2 Catalytic results

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Initial probed experiments were carried out with sensitive styrene as the model
substrate. Styrene was oxidized with H2O2 (30 wt%, 10 equiv.) over Pd0/RGO (0.01 g)
catalyst and GO (0.01 g) dissolved in mixed solvent including H2O (0.5 mL) and
CH3CN (2 mL) at room temperature firstly. As a result, acetophenone of 68% yield
was obtained after 48 hours (Table 1, entry 1). Comparing the optimized results of
reactions, the effect of changing amount of catalyst was not obvious (entries 2-4, 12
and 13). Over 12 hours, the reaction process was finished (entries 2, 5-7). Raising the
temperature could improve the yield of oxidation product, whereas higher temperature
led to a decline in the yields because of the increased by-products (entries 1, 2, 8 and
9). On strong reductive conditions, most oxygenic groups of grahene oxide were
destroyed. As expected, no product was detected (entries 10, 11) in the absence of
catalyst. Instead of Pd0/RGO, some other catalysts were used (entries 14 and 15) and
the result showed that Pd0/RGO was the most efficient catalyst. For further research
of catalyst activity, we used inductive coupled plasma emission spectrometer (ICP) to
analyze the content of Pd(0) in 0.01g of Pd(0)/RGO with aqua regia decomposition ,
as shown in Table S1. The mass percentage of Pd(0) was 32.7%, and it was equivalent
to a catalyst amount of 7.7 mol% comparing with 0.4mmol of substrates. From TEM
images of Pd0/RGO, the mean grain size was 12.51nm with the dispersion of 3%.
Based on this value, the TON of Pd0/RGO was 412. Meanwhile, the dispersion of 5%
Pd/C was about 20%. So the TON of Pd/C was 366 (entry 14). And the TON of
Pd(OAc)2 was 18.4 (entry 15). In consideration of TONs, Pd0/RGO was more
effective than Pd0/C and Pd(OAc)2 under the same other reaction conditions. In order

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to reveal the superiority of GO, graphene, H2SO4 and Amberlite®IR-120 were used as
co-catalysts instead of GO (entries 16-18). Compared with acidic cation exchange
resin catalysts, GO embodied the same catalytic effect. In addition, extra water was
needed to disperse the catalysts (entry 19). Increasing the amount of H2O2 to 20
equivalents had less impact on the reaction, but a lower yield in acetophenone
formation was observed while decreasing the amount of H2O2 to 5 equivalents (entries
20-22). Only 35 % of product was obtained under solvent-free conditions (entry 23).
Instead of CH3CN, DMSO was used as a solvent and the result was unsatisfactory
(entry 24). To summarize, optimization was realized from the following conditions:
Pd0/RGO (0.01 g), GO (0.01 g), aqueous hydrogen peroxide (10 equiv.) in H2O (0.5
mL) and acetonitrile (2 mL) at 55 ˚C for 12h.
Followed by the investigation from above optimization, various substrates were
chosen to do further research and the results were revealed in Table 2. To our
satisfaction, the researched objects could obtain good to decent yields and epoxides
were not detected in all reactions. Comparing the yields of aromatic ketones between
same groups at different sites (3, 15 and 19; 7, 17 and 20; 4 and 16; 5 and 21), a
substituent in the ortho-position or meta-position obviously reduced the yield because
of the steric and electronic effects. Moreover, styrenes with electron-donating
para-substituents could smoothly transform into the corresponding arones (6-10, 13
and 23), while electron-withdrawing substituents brought about lower yields except
for fluoro-substituent (2-5, 11, 12, 15, 16, 19, 21 and 22). Likewise, the conditions
were applied to other aromatic olefins and provided expected products (25-30). Then,

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the polyenes involving dienes and trienes substrates (14, 18, 24 and 40) at improved
conditions were oxidized in favourable yields. On account of simple structures, both
conversions and selectivities were acceptable for cycloolefins and chain olefins
(31-44). Notably, not only the terminal olefins but also the general ones could react
smoothly to give expected products (26 and 37). On account of the reduced electron
cloud density and increased steric hindrance, some polynitrostillbenes couldn’t react
with H2O2 under the optimized conditions (45-47). A gram-scale application of the
new method for trans-stilbene resulted in an excellent separated yield of 81% (26).
3.3 Recycle of catalysts
The Pd0/RGO and GO could be reused up to 5 times (Fig. 2) after regenerated from
the reaction mixture using centrifuge, followed by washing with acetonitrile and
deionized water. After 5 times of recycling, the yield declined by only 10% at the
same reaction time.
Then a hot-filtration test was conducted and the result confirmed that few
palladium catalyst stayed in the solvent (Fig. S7). This process presented a better
recyclability of catalytic system than our previous work [29]. Overall, the novel
catalytic system was suitable for different kinds of olefins over graphene oxide as a
'solid weak acid'.
3.4 Plausible catalytic mechanism
Another NH3-TPD was carried out to explore the total acidity of Pd0/RGO.
However, from the results of NH3-TPD, there wasn’t any -COOH in the surface of
Pd0/RGO because the total acidity was 0 mmol/g. This indicated that only GO played

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the role of acid in this kind of Wacker oxidation. Based on the progress in the field
[15, 26, 33-35], a plausible mechanism for the clean oxidation with aqueous hydrogen
peroxide over Pd0/RGO was proposed to elucidate the olefins transformation
illustrated in Fig. S1. The palladium hydroperoxidic intermediate (a) was probably
transited by an interaction between GO and H2O2 (step 1). Moreover, the similar
intermediate like R-Pd-OOtBu had been separated out and detected [26]. Then, the
intermediate transferred oxygen to the double bond through a pseudo cyclic
hydroperoxypalladation middle state of the synergetic olefin to eventually form target
product and palladium hydroxyl intermediate (c) (step 2-3). Further theoretical
calculation presumed the penta cyclic structure (b), a plausible intermediate [33, 34].
Finally, this palladium hydroxyl intermediate (c) was regenerated by hydrogen
peroxide to the palladium hydroperoxidic intermediate accomplishing the catalytic
cycle (step 4). Meanwhile, the palladium hydroxyl intermediate might undergo a
hydroxypalladation of the olefin to afford ketones and Pd0 by hydrogen peroxide (step
5). After the end of reaction, the catalysts were restored.
4. Conclusion
To summarize, a recyclable, universal and efficient catalytic system for the
Wacker oxidation of olefins, in order to prepare corresponding ketones, had
been developed successfully. We believed that Pd0/RGO as a heterogeneous
catalyst was of considerable potential in Wacker oxidation using H2O2 as the
mild oxidant or in other catalysis. Up to 44 kinds of ketones with different
structures and functions could be prepared with excellent yields and the highest

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yield of these products reached 97%. Not only that, but the palladium catalyst
could be easily recovered with the help of centrifugalization and recycled at
least five times without obvious deactivation. For the first time, GO served as a
‘solid weak acid’ to displace liquid strong acids such as H2SO4 and CF3COOH.
Acknowledgement
TEM and SEM were performed at the testing center of Yangzhou University. This
research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
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from methyl nitrite and CO over activated carbon supported Wacker-type
catalysts: The surface chemistry of activated carbon, Catal. Commun. 72 (2015)
43-48.

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Figures
Fig. 1 Nanoscaled Pd particles supported on reduced graphene oxide layers
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Recycle time
Fig. 2 Reusability of catalysts for Wacker oxidation reaction. Reaction conditions:
styrene (0.4 mmol), Pd0/RGO (0.01 g), GO (0.01 g), H2O2 (30 wt %, 10 equiv.), H2O
(0.5 mL), and CH3CN (2.0 mL), 55 ˚C, GC yields based on the external standard.

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Fig. 3 Plausible mechanism of Wacker oxidation reaction.

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Tables
Table 1 Optimization of reaction conditionsa
H2O2
(equiv.)
Temp.
(˚C)
Yield
(%)b
Entry
1
Cat.
GO (g)
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
-
Time (h)
48
12
12
12
6
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
-
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
r.t.
55
55
55
55
55
55
65
75
55
55
55
55
55
55
55
55
68
95
92c
91d
74
91
95
90
66e
0
2
3
4
5
6
9
7
15
12
12
12
12
12
12
12
12
12
12
8
9
10
11
12
13
14
15
16
17
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/C
0
0.005
0.02
0.01
0.01
83
93
86f
92g
0
Pd(OAc)2
Pd0/RGO
Pd0/RGO
h
-
i
-
92

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j
18
19
20
21
22
23
24
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
Pd0/RGO
-
10
10
-
55
55
55
55
55
55
55
12
12
12
12
12
12
12
94
87k
0
0.01
0.01
0.01
0.01
0.01
0.01
5
81
88
35l
77m
20
10
10
a styrene (0.4 mmol), Pd0/RGO (0.01 g), GO (0.01 g), H2O2 (30 wt %, 10 equiv.),
b
H2O (0.5 mL), CH3CN (2.0 mL), except for specified conditions. GC yields based
c
d
e
on the external standard.
Pd0/RGO (0.005 g).
Pd0/RGO (0.02g).
27 %
f
g
h
benzaldehyde was formed. Pd0/C (5 % Pd, 0.01 g). Pd(OAc)2 (5 mol %).
i
j
k
graphene (0.01 g). H2SO4 (70 wt %, 0.4 equiv.). Amberlite®R-120 (0.01g).
l
without H2O. without CH3CN and 42 % benzaldehyde was formed. m DMSO (2.0
mL).
Table 2 Reactions scope of Wacker oxidation reactiona

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a olefins (0.4 mmol), Pd0/RGO (0.01 g), GO (0.01 g), H2O2 (30 wt %, 10 equiv.), H2O
b
(0.5 mL) and CH3CN (2.0 mL), 55˚C, 12 h, isolated yield. H2O2 (30 wt %, 20
d
e
equiv.), 24 h; c 65˚C. H2O2 (30 wt %, 30 equiv.), 24 h. trans-stilbene (5.56 mmol),
Pd0/RGO (0.14 g), GO (0.14 g), H2O2 (30 wt %, 10 equiv.), H2O (7.0 mL) and
CH3CN (28.0 mL), 55˚C.

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Highlights

Our work provides an innovative method of selective Wacker oxidation over
Pd0/RGO.



GO serves as a ‘solid weak acid’ to replace liquid strong acids for the first time.
Up to 44 kinds of ketones are prepared with excellent yields.
The catalyst can be recovered and reused.

Figure 1

Figure 2

Figure 3