Selective liquid phase oxidation of ethyl benzene to acetophenone by palladium nanoparticles immobilized on a g-C3N4-rGO composite as a recyclable catalyst

Article abstract of DOI:10.1039/c8nj06469e

In this study, synthesis of acetophenone as a widely used industrial compound was investigated by liquid phase oxidation of ethylbenzene with TBHP as an oxidant in acetonitrile over Pd/g-C₃N₄-rGO as a recyclable catalyst. The hybrid

Full text of DOI:10.1039/c8nj06469e

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Selective Liquid Phase Oxidation of Ethyl Benzene to Acetophenone by Palladium
Nanoparticles Immobilized on g-C N -rGO Composite as Recyclable Catalyst
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a
a,*
a
b
Sheyda Nilforoushan , Mehran Ghiaci , Sayyed Mahdi Hosseini , Sophie Laurent ,
b
Robert N. Muller
aIsfahan University of Technology, Department of Chemistry, Isfahan 8415683111, Iran
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bUniversity Mons, NMR & Mol Imaging Lab, Department Gen Organic& Biomedical
Chemistry, B-7000 Mons (Belgium)
*
Corresponding author: Mehran Ghiaci
Email address: mghiaci@cc.iut.ac.ir, Tel: +983133913254
Abstract
In this study, synthesis of acetophenone as a widely used industrial compound was
investigated by liquid phase oxidation of ethylbenzene with TBHP as an oxidant in acetonitrile
over Pd/g-C N -rGO as a recyclable catalyst. The hybrid composite of graphitic carbon nitride
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(g-C N ) and reduced graphene oxide (rGO) was synthesized by a simple wet impregnation
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method and was used as support for palladium nanoparticles. The characterization of the Pd/g-
C N -rGO composite was done by different techniques such as XPS, FT-IR, X-ray Diffraction,
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FESEM-EDS and Mapping, TEM, TGA, ICP-OES, and BET-Surface area. The Pd nanoparticles
were homogeneously distributed on the hybrid support with reasonable surface area. Moreover,
this system revealed a good performance in the production of acetophenone as the main product
of oxidation of ethylbenzene in mild conditions with 67% and 97% conversion and selectivity,
respectively. In addition, the proposed catalyst also retained its activity and selectivity for
acetophenone after 5 runs.
Keywords: Acetophenone; Heterogeneous catalysis; Oxidation; Palladium nanoparticles.

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1
. Introduction
Recently, carbon materials with nano porous structures are mostly considered as a support
in heterogeneous catalysts due to superior thermal and chemical stabilities [1, 2]. Among carbon
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allotropes, graphene, which includes thick carbon planes with sp hybrid, has been used in
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different fields because of its prominent optical [3], electrical [4] and mechanical [5] properties.
Graphene has different applications in super capacitors [6], Li-ion batteries [7], catalysts [8, 9],
and biological fields [10]. Nowadays, because of the simplicity of synthesis and economic
benefits, different derivatives of graphene-like graphene oxide (GO) and reduced graphene oxide
(rGO) have attracted more attention in comparison with pure graphene [11, 12]. Catalytic
systems which contain metal on nitrogen-doped carbon attracted the attention of researchers
because of their chemical, electrical and structural properties for manufacturing of chemicals
with high activity and selectivity in catalytic oxidation, reduction processes and glucose sensing
[13-18]. The use of inexpensive materials as well as reproducible resources for producing
nitrogen-doped carbon materials have been favored in recent years [13, 14]. In general, pyrolysis
of rich nitrogen and carbon-containing materials with a metal salt leads to the formation of metal
carbon-nitrogen doped structures, which can be used as a general route for producing
nanoparticles-based carbon nitride catalysts [14, 19].
Graphitic carbon nitride (g-C N ) is a carbonaceous material with high nitrogen content
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that can be used as N-substituted graphite [20, 21]. In addition, the presence of structural and
surface defects, as well as high content of nitrogen atoms with their coordination capability, has
converted it to a good candidate for application in catalytic applications. In other words, it is the
active support that could increase the catalytic performance and has the capability for use in
surface modification, photocatalysis, and optical sensors [22-28]. In order to synthesize bulky g-
C N using thermal treatment of nitrogen-rich precursors by a pyrolysis technique, various
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compounds such as cyanamide, dicyandiamide, urea, thiourea as well as melamine are used [22,
9-33]. All of these materials are carcinogenic and expensive except melamine. Thermal
synthesis of g-C N based on melamine as the precursor, depending on the reaction conditions,
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leads to crystalline structures with different condensation, and is generally done through heating
melamine at high pressures or low-vacuum systems [22, 34-36]. In addition, achieving a
honeycomb arrangement of the g-C N structure with s-triazine rings is more probable when
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starting with melamine as the precursor and gradually increasing temperature with low rate [30,
31, 37]. During the process of carbon nitride condensation, when using urea or thiourea;
ammonia and H X gases will be produced. Therefore, on a large scale process for g-C N
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synthesis using these materials as the precursor is highly toxic and inappropriate environmentally
and economically. However, melamine has a high tendency to sublimate above 290 °C [38]. To
solve this problem, melamine condensation is carried out in a closed or a semi-closed system
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[
36-39]. G-C N samples prepared through thermal annealing of melamine show irregular big
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particles with very low specific surface area (less than 10 m .g ) [39, 40]. Hence, many efforts
have been done to modify g-C N synthesis for achieving more active sites and increasing the
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surface area. One way of modifying the process is to protonate the melamine by strong mineral
acids [41-43].
The coupling of g-C N with carbon materials such as graphene and its derivatives has
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been practiced for producing compatible composites because both materials have two-
dimensional carbon networks [22, 44, 45, 47]. In a study by Quan et al. GO/g-C N composite
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prepared by sonication technique, showed acceptable performance in the degradation of
rhodamine B and 2, 4-dichlorophenol under visible light irradiation conditions [22]. Recently, a
hybrid composite of g-C N -rGO or in combination with various metals has attracted more
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attention by improving the catalytic performance of g-C N [45-50]. Wang et al. used g-C N -
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rGO composite to support Pd nanoparticles for the investigation of electro-oxidation of formic
acid and methanol [51].
Acetophenone is a valuable intermediate in pharmaceuticals and resins industry. It can be
used as the solvent, tear gas, and can also be used as flavors. Furthermore, hydrogenation of
acetophenone to l-phenyl ethanol is carried out with the purpose of producing aromatic ester
fragrances [52, 53]. Industrially, acetophenone is obtained as a byproduct of the Hock process
which is performed for the production of phenol. Also, selective decomposition of cummen
hydroperoxide is performed in the presence of copper catalysts at 50-150 °C to obtain
acetophenone [54]. Another process for producing acetophenone is Friedel-Crafts acylation of
benzene with acetyl chloride in the presence of stoichiometric amounts of AlCl [55], which is
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not an eco-friendly process. Interest in the development of chemical material synthesis through
green and economic processes has led to extensive research to achieve fine products like phenyl

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ketones from selective oxidation of alkyl arenes. Oxidation of ethylbenzene using heterogeneous
catalytic systems and environmentally-friendly oxidizing reagents such as O , H O , and tert-
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butyl hydroperoxide (TBHP) is considered an appropriate method for synthesis of acetophenone,
which has none of the above problems and limitations [56-58]. This process has byproducts such
as l-phenyl ethanol, 2-phenyl ethanol, benzaldehyde, styrene and benzoic acid [56, 59, 60].
Therefore, designing a catalytic system to achieve acetophenone with high selectivity is
important because of the purification process as one of the most important and expensive
processes for obtaining pure products among industrial processes. In this case, catalytic systems
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such as M-MCM-41 (M=Ti, V, Cr) [61], Ag/SiO [52], Co(II) porphyrin complex [62], Mn-
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Ti/SBA-15 [56], Ni-Al/hydrotalcite [63], Flower-like Co-Cu microspheres [64], VO /HZSM-22
x
[57], Mn/MCM-41 [65], and Zeolite-Y encapsulating Zn(II), Cu(II) and Ru(III) complexes of
imidazole [60] have been reported. Although acetophenone with high conversion and selectivity
is obtained in industrial processes, they are not desirable due to the use of hazardous starting
materials and reagents in large scales as well as harsh reaction conditions. In addition, many of
these systems significantly lose their activity after several runs. In this respect, for manufacturing
acetophenone by catalytic oxidation, Pd-based catalysts mainly focus on Wacker oxidation,
oxidative coupling, alcohol oxidation, reduction of nitroarenes and C-C cross-coupling reactions
[
66-71]. One of the related works in this area is our previous work which focused on the
modification of bentonite surface with a designed surfactant, i.e. 3,3'-(dodecyl-azanediyl)bis(N-
2-(2-aminoethyl amino)ethyl) propanamide (DAEP), which provided the possibility of
(
homogeneous and nano metric distribution of palladium particles. Although this process led to
very high conversion and selectivity of acetophenone, there was no possibility for this process to
be carried out in industrial scales due to the complexity of the process, time-consuming synthesis
of catalyst, and the use of expensive materials [72].
In the present study, after the modification of the synthetic route of melamine-based
graphitized carbon nitride using H SO , a hybrid composite of g-C N and rGO was formed
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using a thermal annealing process that finally led to the g-C N -rGO structure using a simple,
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short time, and cast-effecting and green synthetic method. In fact, by modifying the GO surface
with g-C N through increasing π-π interactions, the restacking of GO plates was prevented. In
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the optimized conditions, g-C N -rGO composite with a ratio of 1:2 is chosen as support for
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immobilizing Pd nanoparticles. This combination played an important role in changing the

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behavior of the nucleation growth of Pd particles without any agglomeration in accordance with
our characterization analysis, and finally the formation of a very ordered structure with
honeycomb units. In the end, the catalytic performance of ternary composite Pd/g-C N -rGO was
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investigated through the oxidation reaction of ethylbenzene in the presence of TBHP as an
oxidizing agent and acetonitrile as solvent. Probably, high activity of the hybrid g-C N -rGO
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composite and the lack of agglomeration and proper distribution of Pd nanoparticles played an
important role in achieving the acetophenone product with high selectivity and minimum related
by-products.
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. Experimental
.1 Materials
All chemical materials and solvents were purchased from Merck and Aldrich companies
and were used without any purification.
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.2 Preparation of GO
GO was synthesized by a modified Hummer’s method from graphite powder [73]. In the
first step, 2 g of graphite powder, 2 g of sodium nitrate and 92 mL concentrated sulfuric acid
were added to a three neck round bottom flask equipped with a mechanical stirrer at 0 °C. In the
next step, 12 g of KMnO was slowly added to the flask and stirred vigorously at 35 °C for 1 h.
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Then, the reaction flask was placed into an ice bath and 100 mL of DI water was slowly added to
it. In this step, the above composite was put in an oil bath with a temperature of 98 °C for 15
minutes. After that, the flask was put into an ice bath while 100 mL of DI water and 20 mL of
H O (30%) were added to it. Subsequently, it was put in an ultrasound bath for 3 h under
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stirring. In the last step, the prepared samples were centrifuged and washed with 5% HCl
solution and DI water to neutral pH. The obtained brown powder was dried under vacuum at 80
°
C for 24 h, which resulted in the GO powder.
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.3 Preparation of g-C N
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g melamine and 1 g Pluronic P123 co-polymer were dissolved in 100 mL of DI water and
the solution was refluxed for 1 h. After cooling the mixture to room temperature, 3 mL
H O/H SO (1.5 mL H O and 1.5 mL H SO ) was added. The white precipitate was separated and
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washed thoroughly with water and then dried at 80 °C. Finally, the dried precipitate was
pyrolyzed at 600 °C with a rate of 50 °C/h for 4 h under argon atmosphere.
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.4 Preparation of g-C N -rGO composite
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In the first stage, 1 g of g-C N and 2 g of GO were dispersed in 50 mL ethanol and the
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mixture was put under continuous ultra-sonication. The prepared mixture was maintained at room
temperature for 24 h under stirring, and then after 2x15 min ultra-sonication, the solvent was
removed and the solid was pyrolyzed under argon atmosphere at 700 °C for 1 h.
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.5 Synthesis of Pd (4 wt%)/g-C N -rGO catalyst
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At first, 1 g of the g-C N -rGO composite was distributed in 25 mL ethanol, and then a
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solution of 40 mg of palladium acetate in 5 mL THF was added to it during 2 h under stirring.
The mixture was put under continuous ultra-sonication (4x15 min). The solvent was removed
using rotary evaporator. In the last stage, the obtained powder was washed with DI water and
dried in an oven at room temperature; the prepared catalyst was designated as Pd (4 wt%)/g-
C N -rGO. For comparison, Pd nanoparticles supported on rGO (Pd/rGO) and g-C N (Pd/g-
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C N ) were also prepared using the same procedure.
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.6 Catalyst characterization
The structure and morphology of the catalysts were characterized as follows: FT-IR spectra
of the samples were studied by JASCO, FT-IR (680 plus) spectrophotometer. X-Ray diffraction
analyses were performed using a Philips 87, Netherland with Cu anode (40 kV, 30 mA)
connected to a DACO-MP microprocessor using the Diffract-AT software. X-ray photoelectron
spectroscopy (XPS) measurement was performed by a VG ESCALAB 3 MK II. The analysis
chamber pressure was <10-9 Torr. The sample was excited using Mg Kα (1253.6 eV) radiation
(operated at 300 W), having an instrument resolution of 0.7 eV. The component peaks were
separated by the VG Avantage software. Field emission scanning electron microscopy (FESEM)
with energy dispersive X-ray spectrometer and mapping were obtained using the TESCAN
MIRA3. BET surface area was used through N adsorption/desorption isotherms at 77.3 K using
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a micro metrics PHS-1020 (PHSCHINA). TEM images were taken using a Philips CM-120
microscopy with an accelerating voltage of 150 kV. TGA analysis was obtained using a STA503
TA (Germany) under argon flow with a scan range from 25 to 800 °C under the constant heating

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rate of 10 °C/min. The palladium content in samples was determined by inductively-coupled
plasma optical emission spectroscopy (ICP-OES) using a Shimadzu ARL 34000 instrument. The
reaction products were analyzed by GC-Ms (Fisons Instruments 8060, USA) and an Agilent gas
chromatography equipped with HP5 5% phenyl methyl siloxane capillary column and FID
detector was used for determination of the types and amounts of the products, respectively
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(bromobenzene as internal standard).
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.7 Typical oxidation procedure
Liquid phase oxidation of ethylbenzene to acetophenone was done in a round bottom flask
in reflux conditions. Therefore, 10 mg catalyst, 1 mmol ethylbenzene (122 μl) and 480 μl of
TBHP (80%) were put into a 10 mL flask including 5 mL acetonitrile as solvent, and the system
was refluxed at 80 °C for 24 h. After the completion of the reaction, the reaction mixture was
centrifuged to separate the catalyst and after preparing standard solutions containing an internal
standard (bromobenzene), the amount and type of products were analyzed by GC and GC-MS,
respectively.
3
. Results and discussion
3.1 Characterization of catalysts
.1.1 XPS analysis
Surface chemical composition and oxidation states of Pd in the Pd/g-C N -rGO were
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studied using X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum revealed the
presence of C, N, O and Pd elements in the sample (Fig. 1a). The precise amount of C, N, O and
Pd elements (at %) of the sample according to XPS analysis, is equal to 63.1, 17.4, 17.4 and
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.2%, respectively. Fig. S1 shows the spectra of carbon (C 1s), nitrogen (N 1s) and oxygen (O
s) separately and as can be seen, the peak at about 400.1 eV is attributed to pyrrolic-type
nitrogen [74, 75]. The Pd XPS spectrum always appears in a doublet (3d and 3d ) due to spin-
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orbit coupling and is separated by 5.26 eV [76]. Fig. 1b shows the Pd XPS spectrum of the Pd/g-
C N -rGO fresh catalyst. The peaks at 335.0 and 337.3 eV were attributed to 3d and 3d of
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Pd° and Pd (palladium oxide), respectively. Also, the peak at 339.6 eV is representative of the
Pd (IV) (Pd 3d ) that indicates the presence of PdO [77-80]. By calculating the surface area of
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the peaks of different palladium species, clearly, Pd was distinguished as the major phase on
the surface of the catalyst (about 45%). According to previously reported data [81], the amine
groups on the carbon support increases Pd° stability and makes them resistant to re-oxidation in
environmental conditions. It seems that the presence of 19.4% of palladium in the form of
metallic palladium is due to the presence of nitrogen groups of the g-C N -rGO composite.
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Herein, the nitrogen atoms as metal-anchoring sites lead to a homogeneous, fine dispersion of
metal nanoparticles and increase their stability on the support surface, and as we will see later, a
strong coupling between Pd NPs and g-C N -rGO leads to creating a stable catalyst with high
3
4
reusability.
Figure 1. XPS scan survey for Pd/g-C N -rGO (a), Deconvoluted high resolution spectrum of Pd 3d (b)
3
4
3.1.2 FT-IR spectra
Fig. 2 shows the FTIR spectra of g-C N , GO, rGO, g-C N -rGO and Pd (4 wt%)/g-C N -
3
4
3
4
3
4
-1
rGO. The stretching vibrations of C-N bonds in the g-C N structure appear at 1200-1650 cm
3
4
-1
(
Fig. 2a). The wide peak at 3428 cm is ascribed to stretching vibration of the N-H bond and the
-1
peak at 806 cm is related to tri-s-triazine unit in g-C N [29]. In the GO spectrum (Fig. 2b), a
3
4
-1
wide peak at 3416 cm shows stretching vibrations of the hydroxyl groups and the peaks at
-1
1721, 1621 and 1056 cm were due to stretching vibrations of C=O, C=C and C-O bonds,
respectively [82]. Because of decrease in oxygen functional groups at the surface of GO through
pyrolysis, almost all peaks attributed to oxygen-containing functional groups in rGO (Fig. 2c)

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and g-C N -rGO (Fig. 2d) were declined [50, 51, 83]. In addition, the peaks at 808 and 1245 cm
3
4
in g-C N -rGO were ascribed to the tri-s-triazine units and the stretching vibration of the C-N
3
4
bond in the g-C N structure.
3
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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0
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. FTIR spectra of g-C N (a), GO (b), rGO (c), g-C N -rGO (d) and Pd/g-C N -rGO
3
4
3
4
3
4
3.1.3 XRD and TGA analysis
X-ray Diffraction patterns of g-C N , GO, g-C N -rGO composite and Pd (4 wt%)/g-C N -
3
4
3
4
3
4
°
rGO catalyst are shown in Fig. 3. In the XRD pattern of g-C N , the strongest peak at 2θ = 27.4
3
4
°
is related to (002) plane and the peak at 2θ = 13.0 is assigned to (100) planes of tri-s-triazine
unit based on JCPDS-87-1526 pattern [84]. Furthermore, in the GO structure the peak related to
°
°
(
001) planes has appeared at 2θ = 12.3 [85]. The wide peak observed at 2θ = 26.1 in the

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structure of g-C N -rGO is possibly related to covalent interactions between rGO planes and g-
3
4
C N , where the graphene planes are separated because of in situ growth of g-C N planes [86].
3
4
3
4
Furthermore, the diffraction peaks corresponding to Pd nanoparticles are not observed in the
Pd/g-C N -rGO catalyst because of its good dispersion and very small amounts based on ICP
3
4
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
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9
0
1
2
3
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
analysis.
Figure 3. XRD patterns of g-C N , GO, g-C N -rGO and Pd/g-C N -rGO
3
4
3
4
3
4
To investigate the thermal stability of the Pd (4 wt%)/g-C N -rGO catalyst, thermal
3
4
gravimetric analysis (TGA) was performed in the scanning range of 25 to 800 °C with a rate of
0 °C/min where it shows about 40% weight loss up to 800 °C (Fig. 4). The gentle slope
1
observed until 400 °C is probably related to the loss of surface water, CO, CO , and other
2
oxygen functional groups of the catalyst that resulted in 10.4% loss of the mass [87]. The 25%
loss of weight in a range of 400-600 °C arises from the destruction of the g-C N -rGO planes.
3
4
The minor mass-loss between 600-800 °C could be related to the loss of oxygen from palladium
oxide present in the char yield. Finally, the overall weight loss of 40% from the fresh catalyst
was observed from 25 to 800 °C.

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0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. TGA curve for Pd/g-C N -rGO nanocatalyst
3
4
3
.1.4 FESEM-EDS and TEM images
Fig. 5 shows the FESEM images of the g-C N , g-C N -rGO and Pd (4 wt%)/g-C N -rGO
3
4
3
4
3
4
samples. The lamellar structure of g-C N can clearly be seen in Figs 5a, and 5b where after
3
4
pyrolysis and formation of the g-C N -rGO sample, the surface morphology of the composite
3
4
had completely changed (Figs 5c, and 5d), and the sample has the two-dimensional sheet-like
structure with many wrinkles. In addition, the images of (e, and f) in this figure show the planar
structure of the Pd (4 wt%)/g-C N -rGO catalyst.
3
4

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0
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0
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0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. FESEM images of g-C N (a, b), g-C N -rGO (c, d), and Pd/g-C N -rGO (e, f)
3
4
3
4
3
4
Energy dispersive X-ray Mapping analysis was also performed to study whether palladium
nanoparticles were homogeneously distributed in the support (Fig. 6a-h). The EDS data showed
the amount of 1.87 wt% of palladium on the surface of the g-C N -rGO composite. In addition,
3
4
in mapping images of this catalyst, different elements such as C, O, N, and Pd are shown under
different colors in which Pd NPs are dispersed quite homogeneously on the surface of the g-
C N -rGO support.
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4

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3
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. EDS mapping analysis result for the Pd/g-C N -rGO
3
4
TEM images of the Pd (4 wt%)/g-C N -rGO were obtained to observe particle size
3
4
distributions of palladium nanoparticles (Fig. 7). The particle size distributions were obtained
using the software that counted approximately 100-150 particles. The Pd nanoparticles were
uniformly dispersed in the g-C N -rGO network, and the statistical analysis showed that the
3
4
mean diameters of the spherical shape palladium nanoparticles were 9 nm.
Figure 7. TEM images of Pd/g-C N -rGO and histogram of the size distribution of the Pd NPs.
3
4

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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
.1.5 N sorption studies and ICP-OES measurements
2
The specific surface area (SBET), Barrett-Joyner-Halenda desorption average pore diameter
dBJH), total pore volume (V ), average pore size (daverage) and Langmuir surface area are
(
tot
summarized in Table 1. According to the IUPAC definitions (Fig. 8b), the Pd (4 wt%)/g-C N -
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
rGO catalyst should be categorized as a Type III solid with macro-mesoporous pores (Fig. 8a)
88].
[
Table 1. The pore parameters of the Pd (4 wt.%)/g-C N -rGO nanocatalyst
3
4
S
Langmuir
-1
2
-1 a
BJH (nm)b
3
-1 c
d
average (nm)d
Sample
S
BET (m g )
d
V
tot (cm g )
2
(m g )
Pd/g-C
3
N
4
-rGO
77.3
1.2
0.3
2.2
120.5
a
b
Brunauer–Emmet–Teller (BET) surface area. Pore diameter calculated by the Barrett–Joyner–Halenda (BJH) method utilizing
c
the adsorption branches. Total pore volume calculated as the amount of nitrogen adsorbed at relative pressure.
d
e
Average pore diameter. Langmuir surface area.
Figure 8. N adsorption-desorption isotherm of Pd/g-C N -rGO catalyst (a), IUPAC isotherms for
2
3
4
gas physisorption
The precise amount of loaded Pd metal in the Pd (4 wt%)/g-C N -rGO catalyst was
3
4
-1
measured using ICP analysis, where the fresh catalyst had 0.11 mmol.g , and the catalyst after 5
-1
runs had 0.09 mmol.g . As will be discussed, this catalyst has retained its activity without
significant loss of its activity after five runs.

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3
.2 Catalytic performance
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1. Oxidation reaction of ethyl benzene catalyzed by Pd/g-C N -rGO in the presence of
3
4
TBHP
Catalyst activity of the Pd/g-C N -rGO catalyst was investigated through ethylbenzene
3
4
oxidation in acetonitrile in the presence of TBHP as the oxidizing reagent. The oxidation of the
benzylic C-H bond in ethylbenzene using TBHP through a radical chain resulted in
acetophenone as the main product and benzaldehyde as a byproduct (Scheme1). Effect of the
amount of catalyst on reaction progress is depicted in Fig. S2. As displayed, the amount of 10 mg
of the Pd (4 wt%)/g-C N -rGO catalyst leads to maximum conversion and selectivity to
3
4
acetophenone. By increasing the amount of catalyst to larger amounts, the selectivity and
conversion did not change appreciably. Therefore, in all subsequent experiments, 10 mg of the
catalyst was used.
As shown in Table 2, the time of the reaction has a considerable influence on the reactivity
and selectivity of the reaction. By enhancing the time from 6 to 24 h, not only the conversion of
ethylbenzene to acetophenone was increased, but also the selectivity was improved to some
extent. Based on the data reported in this work, we cannot calculate the TOF [89, 90], and
only the TON was reported which might be sufficient from an economic point of view.
Therefore, 24 h was chosen as the optimal time.

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8
9
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. The effect of reaction time on the conversion and selectivity of EB oxidation
Selectivity
(%)
TONa
Entry
Time(h)
Conversion
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
(%)
H
1
2
3
4
6
30
41
67
67
4545
3106
2538
1692
90
95
97
97
10
5
12
24
36
3
3
Reaction conditions: EB: TBHP (mol ratio) = 1:4, temperature = reflux, 5 ml CH
Turnover Number = [moles of ethyl benzene consumed]/(total mole metal)
3
CN, 10 mg Pd (4 wt.%)/g-C
3
N
4
-rGO
a
The effect of temperature on the oxidation of ethylbenzene over the Pd (4 wt%)/g-C N -
3
4
rGO catalyst was also studied in the temperature range of 40 to 80 °C (CH CN as solvent) (Table
3
3
). The results show that by increasing the temperature, a high conversion of 67% with a
selectivity of 97% for acetophenone was achieved at 80 °C.
Table 3. The influence of reaction temperature on the conversion and selectivity of EB oxidation
Selectivity
TONa
Entry Temperature Conversion
(%)
O
O
OH
O
(°C)
(%)
H
OH
1
2
3
40
60
80
22
36
67
833
1364
2538
64
89
97
19
5
4
6
13
-
-
3
-
Reaction conditions: EB: TBHP (mol ratio) = 1:4, 24h, 5 mL CH
3
CN, 10 mg Pd (4 wt.%)/g-C
3
N
4
-rGO
a
Turnover number = [moles of ethyl benzene consumed]/(total mole metal).
After optimizing the amount of catalyst, reaction time and temperature, the amount of Pd
loaded on the g-C N -rGO composite was also optimized (Figure S3). By increasing the amount
3
4

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of palladium loaded on the support to 6 wt%, a decrease in the conversion of ethylbenzene was
observed that could be related to more aggregation of palladium nanoparticles and decreasing the
active sites due to the constant amount of the g-C N -rGO support.
3
4
Tables 4 and 5 show catalytic oxidation of ethylbenzene over g-C N , g-C N -rGO,
3
4
3
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Pd/rGO, and Pd/g-C N as catalysts. These catalysts have shown a lower conversion in
3
4
comparison with Pd (4 wt%)/g-C N -rGO. Reactivity of g-C N without metal could be related to
3
4
3
4
electron-deficient defects existing on its surface, and there should be much fewer defects on the
surface of the g-C N -rGO sample. It seems that the presence of nitrogen atoms in the
3
4
graphitized carbon nitride structure have an important role in improving the stability of
palladium nanoparticles with a high disparity in the Pd (4 wt%)/g-C N -rGO catalyst [91].
3
4
Hence, Pd/g-C N -rGO nanostructure with the highest efficiency of ethylbenzene conversion and
3
4
selectivity toward acetophenone was selected as the best catalyst for the reaction.
Table 4. Results of EB oxidation catalyzed by different catalysts
Selectivity
TONa
Entry
Catalysts
Conversion
(%)
O
O
(%)
H
1
2
3
4
g-C N
58
30
67
39
-
96
92
97
93
4
8
3
7
3
4
g-C N -rGO
-
60912
-
3
4
Pd/g-C N -rGO
3
4
Pd/g-C N
3
4
3
Reaction conditions: EB: TBHP (mol ratio) = 1:4, 24 h, temperature = reflux, 5 mL CH CN, 10 mg cat
a
Turnover number = [moles of ethyl benzene consumed]/[total mole metal]

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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 5. Nitrogen doping effect on the conversion and selectivity of EB oxidation
Selectivity
TONa
Entry
Nitrogen
doping effect
Conversion
(%)
(%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
OH
H
1
2
Pd/rGO
40
67
-
88
97
5
3
7
-
Pd/g-C N rGO
60912
3
4
Reaction conditions: EB: TBHP (mol ratio) = 1:4, 24 h, temperature = reflux, 5 mL CH
Turnover number = [moles of ethyl benzene consumed]/[total mole metal]
3
CN, 10 mg cat
a
Based on the results shown in Table 6, it seems that TBHP 80% as oxidizing reagent
should be chosen as the best oxidant compared to H O and O because of its more intrinsic
2
2
2
activity and solubility in the mentioned reactions. Furthermore, according to literature, TBHP
with the generation of active oxygen species on redox sites prevents removal of O from the
2
reaction mixture, and as a result, high conversion of ethylbenzene is obtained [56]. On the other
hand, when O is used as the oxidant, the reaction conversion is very low because of its easy
2
expulsion from the reaction mixture, and more importantly activation energy of breaking the C-H
bond by molecular oxygen. Decomposition of H O in the reaction conditions explains its low
2
2
reactivity. Therefore, TBHP was selected as the best oxidant in a molar ratio of 1:4 for
ethylbenzene oxidation to acetophenone (Table 7). Moreover, the reaction was conducted in
different solvents and in agreement with previously reported data, acetonitrile was applied as the
medium for the ethylbenzene oxidation reaction. Probably, the polar nature of this solvent
resulted in partial decomposition of TBHP at the beginning of the reaction.
Moreover, it is clearly documented that oxidation of ethyl benzene using TBHP as oxidant
follows the free-radical mechanism and goes through formation of ethyl benzene hydroperoxide
as intermediate. This intermediate could be activated by the catalyst in two different routes for
producing acetophenone or benzaldehyde where benzoic acid is obtained from oxidation of
benzaldehyde. If we accept that oxidation of ethyl benzene hydroperoxide was occurred on the
active sites of the catalyst, then for producing benzaldehyde or acetophenone, a C-C or a C-H
bond must be broken, respectively (Scheme 2). Therefore, it might be reasonable to say that at

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lower temperatures, the conformational arrangement shown in route a, has more chance to be
formed.
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9
0
Scheme 2. Proposed mechanism for conversion of ethyl benzene hydroperoxide for producing
benzaldehyde (route a), and acetophenone (route b).

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6
Table 6. The results of EB oxidation with different oxidant
Selectivity
(%)
TONe
Entry
Oxidant effect
Conversion
O
O
H
(%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
TBHP a
67
5
2538
189
38
97
90
3
10
-
a
H O
2
2
O2 b
1
100
95
c
O /TBHP
2
31
21
1174
795
5
TBHP d
87
13
a
Reaction conditions: EB:TBHP (mol ratio) = 1:4, 24 h, temperature = reflux, 5 mL CH
3
CN, 10 mg Pd (4 wt.%)/g-C
3 4
N -rGO
b
1
mmol EB, 24 h, 80 °C, 5 mL CH
3
CN, 20 bar O
2
, The reaction was carried out inside the reactor, 10 mg Pd (4 wt.%)/g-C N -
3 4
rGO
c
1 mmol EB, 19 μl TBHP, 24 h, 110 °C, 5 mL CH
3
CN, 20 bar O
2
, The reaction was carried out in an auto pressure reactor, 10
mg of Pd (4 wt.%)/g-C
1 mmol EB, 19 μl TBHP, 24 h, 110 °C, 5 mL CH
3
N
4
-rGO
d
3
CN, The reaction was carried out in an auto pressure reactor, 10 mg Pd (4
wt.%)/g-C
3
N
4
-rGO
e
Turnover number = [moles of ethyl benzene consumed]/(total mole metal)
Table 7. The effect of molar ratio of EB: TBHP on conversion and selectivity to products
Selectivity
TONa
Entry
EB:TBHP
(mol)
Conversion
(%)
(%)
O
O
H
1
2
3
4
5
1:1
1:2
1:3
1:4
1:5
45
47
60
67
67
1704
1780
2273
2538
2538
96
93
97
97
97
4
7
3
3
3
Reaction conditions: 1 mmol EB, 24 h, temperature = reflux, 5 mL CH
3
3 4
CN, 10 mg Pd (4 wt.%)/g-C N -rGO
a
Turnover number = [moles of ethyl benzene consumed]/(total mole metal)

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.2.1 Reusability of catalyst
After the optimization of parameters affecting the activity of Pd (4 wt%)/g-C N -rGO
3
4
catalyst in selective ethylbenzene oxidation to acetophenone, reusability of the prepared catalyst
was examined under obtained optimized conditions. To this end, the catalyst was simply
separated by filtration after the first run, washed with acetone, dried at 80 °C, and used again in
the sequential runs. Reusability of catalyst was examined for 5 runs in a similar manner. The
results of Table S1 indicate high stability of Pd (4 wt%)/g-C N -rGO catalyst during the reaction
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
4
without considerable loss of its activity. In addition, the results of ICP analysis of the catalyst
after 5 runs showed that only low amount of palladium (from 0.11 mmol in fresh catalyst
reached to 0.09 mmol) was leached from g-C N -rGO support. Based on the observed data, we
3
4
can conclude that g-C N -rGO is a good and effective support for dispersing and immobilizing
3
4
Pd nanoparticles.
3
.2.2 Comparison with the other researches on oxidation of ethylbenzene
Table S2 shows the results obtained by catalytic oxidation of ethylbenzene with TBHP as
the oxidant in the presence of Pd/g-C N -rGO catalyst as compared to other catalytic systems
3
4
previously reported in the literature. In our previous work [92], we used supercritical CO for
2
performing this reaction where we reached excellent conversion and selectivity. But comparing
the operational difficulties for performing such reaction in supercritical conditions makes this
work acceptable. This is quite obvious that the Pd/g-C N -rGO catalyst used in the present study
3
4
has a reasonable catalytic activity and performance selectivity, and also has more appropriate and
milder conditions in comparison to other catalysts.
4
. Conclusion
Since selective oxidation of ethylbenzene is hardly performed because of the lack of
electron active groups in aromatic rings, more active, economic and green catalysts with
reusability properties are of importance. Therefore, in this study, the hybrid structure with
honeycomb units was formed for immobilizing Pd nanoparticles through the modification of GO
surface with g-C N by simple wet impregnation method that led to more active sites or in other
3
4
words, increasing the number of defects which enhancing the reactivity of the catalyst. The
catalyst was characterized by TEM, XPS, FESEM-EDS & Mapping, X-ray Diffraction, BET–

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Surface area, FT-IR, TGA, and ICP-OES techniques and tested in the ethylbenzene oxidation
reaction by TBHP as oxidant reagent, and the result showed 67% conversion and 97% selectivity
toward acetophenone under optimized conditions. Moreover, this catalyst retained its activity
and selectivity to the desired product and remained approximately constant after 5 runs.
0
1
2
3
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0
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Acknowledgments
We gratefully acknowledge financial supports from the Isfahan University of Technology
and technical support from Mons University.
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DOI: 10.1039/C8NJ06469E
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A hybrid structure g-C N -rGO with honeycomb units was prepared for immobilizing Pd
nanoparticles by simple wet impregnation method.
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New Journal of Chemistry
Page 28 of 28
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DOI: 10.1039/C8NJ06469E
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