Liquid-phase oxidation of toluene to benzaldehyde with molecular oxygen catalyzed by copper nanoparticles supported on graphene

Article abstract of DOI:10.1007/s11164-018-3404-2

Highly-dispersed copper nanoparticles (Cu NPs) were fabricated on the surface of reduced graphene oxide via direct hydrazine hydrate reduction of Cu²⁺ in aqueous solution. Scanning electron microscope and transmission electron microscope images

Full text of DOI:10.1007/s11164-018-3404-2

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3404-2
Liquid-phase oxidation of toluene to benzaldehyde
with molecular oxygen catalyzed by copper
nanoparticles supported on graphene
1
1
1
1
Guangliang Song
•
Liang Feng
•
Jie Xu
•
Hongjun Zhu
Received: 3 October 2017 / Accepted: 19 March 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Highly-dispersed copper nanoparticles (Cu NPs) were fabricated on the
2
?
surface of reduced graphene oxide via direct hydrazine hydrate reduction of Cu in
aqueous solution. Scanning electron microscope and transmission electron micro-
scope images show that the Cu NPs are distributed on the surface of graphene
nanosheets, and the average particle size was about 40 nm. The Cu NPs supported
on graphene have high reaction activity for the oxidation of toluene to corre-
sponding benzaldehyde. It was found that the selectivity reached 66.5% and the
conversion of toluene reached 11.5%.
Keywords Copper nanoparticles Á Graphene Á Toluene oxidation
Introduction
2
Graphene, a kind of two-dimensional single layer material with sp hybridized
carbons, exhibits excellent properties in various areas, which span from energy
storage, diode and nanophotonic devices to supporters of nanocatalysts [1–6].
Particularly in catalytic research, graphene, which has a large specific surface, has
been extensively studied as a support for the dispersion of other materials [7–13].
Recently, extensive researches have reported that graphene–inorganic nanoma-
terial-based hybrid materials have been applied in different catalytic systems
&
&
1
Guangliang Song
songgl@njtech.edu.cn
Hongjun Zhu
zhuhj@njtech.edu.cn
Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing
Tech University, Nanjing 211816, China
123

G. Song et al.
[14–16]. Incorporating metal nanoparticles into graphene has recently attracted
significant interest.
For example, noble metal nanoparticles (NPs) such as Pd and Pt supported on
graphene have been used as s catalyst for oxidation reduction reaction (ORR)
[
17, 18]. However, Cu NPs synthesized by chemical reduction of copper salts with
hydrazine hydrate in solution show prominent advantages such as simplicity and
feasibility [19–21].
On the other hand, the oxidation of toluene is one of the most important
functional group transformations in organic synthesis because of its oxidation
products, benzaldehyde (BzH), benzyl alcohol (BzOH) and benzoic acid (BzA).
Generally, gas-phase and liquid-phase oxidation can achieve oxidation of toluene. It
is well known that gas-phase oxidation need high temperatures and high pressure
[
22]. Compared with gas-phase oxidation, liquid-phase oxidation provides catalytic
oxidation processing under relatively mild conditions [23, 24]. BzH, a typical
product of toluene oxidation, is a very important material for the synthesis of spices
and pharmaceutical intermediates, dyestuffs and the agrochemical industries.
Generally, in industry, BzH is produced by the hydrolysis of benzylidene chloride
[
25]. However, chlorine unavoidably exists in the product, limiting the use of the
BzH. Therefore, overcoming this problem has prompted research on the liquid-
phase oxidation of toluene to BzH [26]. In the process of toluene oxidation, it is
harder to obtain BzH than BzOH and BzA. So, many researchers have tried many
different catalysts in the oxidation of toluene including MnO , porphyrin, Ag/WO
x
3
and so on [24, 27, 28]. Copper-containing carbon nitride polymer (Cu–C N ) was
3
4
synthesized by Zhang [29], and used as a catalyst in the liquid-phase oxidation of
toluene with 18.3% conversation. However, the total selectivity of BzOH, BzH and
BzA was only 79.4% under optimum conditions. Han [30] reported a novel Cu- and
boron-doped graphitic carbon nitride catalyst (Cu–CNB) for the selective oxidation
of toluene to BzH, but used tert-butyl hydroperoxide as the oxidant. However, there
has so far been no report on toluene oxidation catalyzed over Cu/graphene. Also, Cu
NPs exhibit excellent catalytic performance in ORR as already stated. Therefore,
they is very significant for the study of toluene oxidation over Cu/graphene.
Here, we have tried toluene catalytic oxidation by Cu NPs supported on
graphene. Cu NPs supported on graphene with good catalytic activity and high
selectivity for toluene oxidation to BzH were achieved.
Experimental
Instruments and materials
Chemicals used in this study including toluene (A.R.), graphite powder and cupric
sulfate were provided by Shanghai Linfeng Chemical. H O (30%) and hydrazine
2
2
hydrate were obtained from Nanjing Chemical Reagent. The standard benzyl
alcohol, benzaldehyde and benzoic acid were purchased from Wuxi Yasheng
Chemical. HPLC analysis (H O:CH CN = 65:35, v = 1 mL/min, k = 254 nm) was
2
3
performed on a DIONEX HPLC-2004 equipped with a C18 column and a UV–VIS
1
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Liquid-phase oxidation of toluene to benzaldehyde with…
detector. The supported Cu NPs were characterized by transmission electron
microscopy (TEM) (JEOL 2010 LaB6) and scanning electron microscopy (SEM)
(
Hitachi S-3400 N). The XRD of the samples was characterized on a Bruker-D8
Advance X-ray diffractometer with Cu Ka radiation.
Synthesis of graphene oxide (GO)
A modified Hummer’s method [31] was used to prepare graphene oxide (GO) and
reduced graphene (RGO). A total of 2.0 g of graphite powder and 46 mL of H SO
2
4
(
98%) were placed in a flask and stirred for 2 h. Then, KMnO (10.0 g) was added
4
slowly into the flask while the temperature was maintained at 273 K. The mixture
was then transferred into a water bath, which was maintained at 308 K for 30 min.
Finally, deionized (DI) water was added continuously into the flask under vigorous
stirring. The temperature was increased to 363 K and maintained for another
3
0 min. Next, 6.0 mL H O (30%) was added into the mixture to end the reaction,
2 2
and the color of solution turned light yellow immediately. The mixture was filtered
with a PP Millipore filter (pore size = 0.22 lm), and then washed with DI water
three times and ethanol three times. The filter was then dissolved in 2 L DI water to
acquire 1 mg/mL GO solution.
Synthesis of Cu/graphene
The GO solution (1.0 mg/mL) and the CuSO solution (0.1 mol/L) were mixed at a
4
volume ratio (200:4 mL). After stirring for 10 h, 30 mL of KOH solution (1 mol/L)
and 4 mL of hydrazine hydrate (80 wt%) solution were added into the mixture.
Then, 100 mg cetyl trimethyl ammonium bromide (CTAB) was slowly added into
the mixture to prevent the agglomeration of the nanoparticles. The mixture were
placed in a water bath at 353 K for 2 h to accomplish the reduction of both GO and
2
?
Cu . Finally, the system was filtered to remove the reducing agents and washed
with DI water and ethanol several times. The filter was dried in a vacuum at 338 K.
Synthesis of Cu NPs
Amounts of 4 mL CuSO solution (0.1 mol/L) and 30 mL KOH solution (1 mol/L)
4
were placed in a flask, then 4 mL of hydrazine hydrate (80 wt%) solution and
1
00 mg CTAB were slowly added into the mixture. The mixture was placed in a
2
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water bath at 353 K for 2 h to accomplish the reduction of Cu . Finally, the system
was filtered to remove the reducing agents and washed with DI water and ethanol
several times. The filter was dried in a vacuum at 338 K.
Toluene oxidation over Cu/graphene
Amounts of 12.0 g toluene, 0.6 g catalyst, 6 mL 30% H O and solvent 150 mL
2
2
methanol were added to a 500-mL autoclave with a mechanical stirrer which was
charged with oxygen to maintain a pressure of 2.0 MPa. The system was stirred at
the reaction temperature for 11 h. At the end of the reaction, the mixture solution
123

G. Song et al.
was filtered to remove the catalyst. The yield was analyzed by HPLC [32] and
nitrobenzene as the internal standard.
Characterization
SEM (INSPECT-F; FEI) was used to analyze the metal nanoparticles supported on
grapheme film. The crystalline phases of the prepared films were measured by thin-
film X-ray diffraction (XRD; X’pert pro-MPD). The microstructure of the prepared
films was measured using TEM (JEM-2100F; JEOL, Japan). The contents of BzH,
BzOH and BzA were characterized by HPLC.
Results and discussion
Characterizations of the catalyst
Figure 1 illustrates the XRD patterns of Cu/graphene as compared with that of
graphene. The XRD spectrum of Cu/graphene (Cu loading 9.6%) showed
characteristic peaks at 43.1°, 50.5°, and 74.9° [33] with very high intensity. These
peaks were assigned to the (110), (200) and (220) crystallographic planes of the
cubic copper crystal in the XRD patterns of Cu/graphene, respectively. The
crystallite sizes of the Cu/graphene and graphene were 36.1 nm and 35.9 nm,
respectively, according to the Scherrer equation. Figure 2 shows the TEM and SEM
images of the Cu/graphene nanostructure. The SEM micrograph shows the surface
morphology of the Cu/graphene films and reveals that the Cu NPs were either
distributed on the surface of graphene sheets or covered by the graphene sheets,
which was confirmed by the XRD conclusions. Meanwhile, the TEM image reveals
Fig. 1 XRD spectrum of Cu/graphene and graphene films
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Liquid-phase oxidation of toluene to benzaldehyde with…
Fig. 2 SEM of Cu/graphene films (left) and TEM of Cu/graphene films (right)
that Cu NPs were wrapped in graphene sheets and the average size of the Cu NPs
was about 40 nm.
Conversion and selectivity of toluene oxidation by Cu/graphene
The toluene oxidation conversion and selectivity catalyzed by Cu NPs,
Cu/graphene, graphene and blank tests at 8 h, 338 K and 2.0 MPa O , respectively,
2
are listed in Table 1. Graphene itself had little activity in toluene oxidation [34]
which was even below the blank tests in toluene catalytic oxidation. And Cu NPs
Table 1 Catalytic performance over various catalysts in toluene oxidation
Entries
Catalyst
Conv (%)
BzOH (%)
BzH (%)
BzA (%)
1
2
3
4
None
0.21
4.1
32.6
32.3
23.4
11.0
62.7
52.4
71.0
66.5
4.6
15.2
5.5
Copper
Graphene
Cu/graphene
0.2
11.5
22.5
Reaction conditions: toluene 12.0 g, catalyst 0.6 g, methanol 150 mL, reaction temperature 338 K,
reaction time 8 h and pressure 2.0 MPa
123

G. Song et al.
had higher conversion (4.1%). Interestingly, the toluene conversion was signifi-
cantly improved to 11.5% by Cu/graphene. The high reaction activity could be
contributing to the highly-dispersed distribution of the Cu NPs on the graphene
surface. Furthermore, the selectivity of BzH reached 66.5% in the toluene oxidation
which was much higher than BzA (22.5%) and BzOH (11.0%).
The change of toluene oxidation conversion and selectivity with the reaction
process are shown in Fig. 3a. As the reaction proceeded from 1 to 11 h, the
selectivity of the BzH decreased continuously with the selectivity of BzA increasing
and the selectivity of BzOH remaining almost unchanged. These results indicated
that the generating rate of BzH was slower than its conversion to BzA from 1 to
1
1 h. The conversion of toluene increased continuously from 1 to 11 h. Addition-
ally, there was an increase in the yield of BzH from 1 to 8 h, and a decrease in the
yield of BzH after 8 h. The highest yield reached 7.25% at 8 h.
Fig. 3 a The conversion and selectivity of toluene with reaction time. Reaction conditions: toluene
12.0 g, 9.6 wt% Cu/graphene 0.6 g, methanol 150 mL, reaction temperature 338 K, pressure 2.0 MPa.
b The conversion and selectivity at various reaction temperatures. Reaction conditions: toluene 12.0 g,
reaction time 8 h, methanol 150 mL, catalyst 0.6 g and pressure 2.0 MPa. c The conversion of toluene
with different weight of catalyst. Reaction conditions: toluene 12.0 g, reaction temperature 338 K,
reaction time 8 h, methanol 150 mL, pressure 2.0 MPa. d The reusability of the catalyst. Reaction
conditions: substrate/metal mass ratio 20, reaction temperature 338 K, reaction time 8 h and pressure
2.0 MPa
1
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Liquid-phase oxidation of toluene to benzaldehyde with…
The effect of reaction temperature on the catalytic performance is shown in
Fig. 3b. The conversion of toluene increased from 1.2 to 11.5% with an increase of
the reaction temperature from 298 to 348 K at 8 h, but the selectivity of BzH
decreased, and meanwhile the selectivity of BzOH and BzA increased as the
temperature increased. This implies that, as the reaction temperature rose, the
generating rate of BzH was slower than that of its conversion into BzA. Both
conversion and selectivity were considered, and st 338 K, the optimum temperature,
the conversion of toluene reached 11.5% and the selectivity of BzH reached 66.5%.
Furthermore, the effect of catalyst weight on the catalytic performance is shown
in Fig. 3c. The results show that the conversion of toluene increased from 4.2 to
11.5% as the catalyst weight increased from 150 mg to 600 mg, whereas, when the
catalyst weight exceeded 600 mg, the conversion decreased continuously.
Effect of O pressures and solvents
2
The effect of O pressures and solvents are shown in Tables 2 and 3. As shown in
2
Table 2, the conversation of toluene increased from 0.5 to 12.2% as the O pressure
2
was increased from 0.5 to 2.5 Mpa. And when the O pressure was increased from
2
0.5 to 2.0 Mpa, the relative contents of BzA increased from 4.7 to 22.5% and BzH
decreased from 82.7 to 66.5%, while the BzOH showed almost no change. But when
the O pressure increased from 2.0 to 2.5 Mpa, the relative contents of BzH rapidly
2
reduced from 66.5 to 52.2% while BzA increased from 22.5 to 38.6%. This fact can
be explained partly because the BzH converts easily to BzA at high pressure.
As shown in Table 3, the conversation of toluene was increased from 5.6% in a
weak polar solvent such as CH CN to 8.2% in a strong polar solvent such as
3
CH COOH, and then to 11.2% in a medium polar solvent such as CH OH. And in
3
3
CH COOH, the main oxidation product of toluene was BzOH, which was confirmed
3
by previous research [35]. In weak polar solvents such as CH Cl and CH CN, the
2
2
3
BzH was 42.6 and 50.4%, respectively, while in a medium polar solvent such as
CH OH BzH it was 66.5%.
3
2
Table 2 Effect of O reaction pressures
Entries
O
2
(Mpa)
Conv (%)
BzOH (%)
BzH (%)
BzA (%)
1
2
3
4
5
0.5
1.0
1.5
2.0
2.5
0.5
2.3
12.6
12.3
11.9
11.0
11.2
82.7
74.0
68.8
66.5
50.2
4.7
15.7
19.3
22.5
38.6
5.6
11.5
12.2
Reaction conditions: toluene 12.0 g, catalyst 0.6 g, methanol 150 mL, reaction temperature 338 K,
reaction time 8 h
123

G. Song et al.
BzA (%)
Table 3 Affection of solvents
Entries
Solvents
Conv (%)
BzOH (%)
BzH (%)
1
2
3
4
CH
CH
CH
CH
3
2
3
3
COOH
Cl
8.2
6.7
78.2
35.8
35.2
11.0
12.7
42.6
50.4
66.5
9.1
21.6
14.4
22.5
2
CN
OH
5.6
11.5
Reaction conditions: toluene 12.0 g, catalyst 0.6 g, solvent 150 mL, reaction temperature 338 K, reaction
time 8 h and pressure 2.0 MPa
Reusability of the catalyst
After the reaction finished, the reused catalyst was obtained by centrifuging,
washing several times by ethanol, and drying at 338 K in a vacuum for 4 h. This
catalyst was used in the next run under the same conditions and the results are
shown in Fig. 3d. In the first three times, there is no significant decrease in the
conversion from 11.5 to 9.3% and selectivity from 66.5 to 63.1%, and the activity of
the catalyst was almost unchanged. Unfortunately, the toluene conversion declined
to 3.7% at the fourth time likely attributed to the decrease in the amount of Cu NPs.
Moreover, the XRD patterns of fresh and reused Cu/graphene (Fig. 4) were almost
unchanged, which suggested that the structure and the composition of the reused
catalyst were also unchanged. These results imply that the catalyst could be
separated and recycled.
Fig. 4 XRD spectrum of the reused Cu/graphene films after 4 times
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Liquid-phase oxidation of toluene to benzaldehyde with…
Conclusions
In this study, we have found that highly-dispersed Cu NPs could be supported on the
2
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surface of RGO via a direct hydrazine hydrate reduction of Cu in aqueous
solution. The complex catalyst Cu/graphene (9.6%) used in toluene oxidation with
oxygen as oxidant exhibited excellent conversion of toluene (11.5%) and selectivity
to BzH (66.5%). Furthermore, this catalyst shows excellent conversion and
selectivity without changing its catalytic activity even after 3 reuses.
Acknowledgements This work is financially supported by the National key R&D Program of China
(
(
2017YFB0307202), the Industry-Academy-Research Prospective joint project of Jiangsu Province
BY2016005-06) and Jiangsu Planned Projects for Postdoctoral Research Funds (1402213C).
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