Sonochemical fabrication of Pd/TiO2-nanotubes/Ti plate as a green catalyst for oxidation of alkylarenes and benzyl alcohols

Article abstract of DOI:10.1007/s13738-019-01620-7

A facile and fast strategy has been employed to fabricate Pd nanoparticles supported on TiO₂ nanotubes/Ti plate via sonochemical deposition. Microstructure studies showed the homogeneous deposition of Pd nanoparticles on the walls of TiO₂ nanotubes/Ti plate. The synthesized plate was applied as a novel catalyst for the oxidation of benzyl alcohol and ethylbenzene derivatives. The results of catalytic experiments demonstrated that the modified plate was an efficient green catalyst for the oxidation of benzyl alcohols to benzoic acid derivatives in H₂O. The oxidation of alkylarenes was carried out in EtOH:H₂O (1:1) ended up with the formation of the corresponding ketone as the sole product. High yields and excellent selectivities were obtained for the oxidation reactions in green solvents using green oxidant. Superior catalytic activity, easy catalyst recovery, and reusability of the catalyst are some advantages of the modified PdNPs/TiO₂ nanotubes/Ti plate, indicating a potential application of the catalyst in the industrial oxidation reactions.

Full text of DOI:10.1007/s13738-019-01620-7

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01620-7
ORIGINAL PAPER
Sonochemical fabrication of Pd/TiO -nanotubes/Ti plate as a green
2
catalyst for oxidation of alkylarenes and benzyl alcohols
1
2
1
Sajjad Keshipour · Masoud Faraji · Parisa Aboozari Asl
Received: 29 January 2017 / Accepted: 23 January 2019
©
Iranian Chemical Society 2019
Abstract
A facile and fast strategy has been employed to fabricate Pd nanoparticles supported on TiO nanotubes/Ti plate via sono-
2
chemical deposition. Microstructure studies showed the homogeneous deposition of Pd nanoparticles on the walls of TiO
2
nanotubes/Ti plate. The synthesized plate was applied as a novel catalyst for the oxidation of benzyl alcohol and ethylbenzene
derivatives. The results of catalytic experiments demonstrated that the modified plate was an efficient green catalyst for the
oxidation of benzyl alcohols to benzoic acid derivatives in H O. The oxidation of alkylarenes was carried out in EtOH:H O
2
2
(
1:1) ended up with the formation of the corresponding ketone as the sole product. High yields and excellent selectivities
were obtained for the oxidation reactions in green solvents using green oxidant. Superior catalytic activity, easy catalyst
recovery, and reusability of the catalyst are some advantages of the modified PdNPs/TiO nanotubes/Ti plate, indicating a
2
potential application of the catalyst in the industrial oxidation reactions.
Keywords Titania · Nanotubes · Catalyst · Oxidation · Heterogeneous catalyst
Introduction
with catalytic amounts of Pd using O or H O as the oxidant
2 2 2
[9]. These reactions are performable with homogeneous and
The oxidation of organic substrates into a valuable organic
products is one of the most important reactions in the
organic synthesis both for laboratory and industry proposes
heterogeneous Pd species such as Pd(0), Pd(II), and Pd(IV)
[10]. It has been known that homogeneous Pd catalysts due
to the existence of catalyst in the same phase with starting
material can give high yields [11–13]. Today, heterogeneous
catalytic systems have been widely employed for the vari-
ous catalytic transformations due to significant advantages
of heterogeneous catalysts compared to the homogeneous
systems [14, 15]. Powdery catalysts containing Pd nano-
particles (PdNPs) have been found to be effective catalysts
for the oxidation reactions, but the oxidation reactions with
these catalysts have some drawbacks such as stirring dur-
ing the reaction, separation of the catalyst from the reaction
solvent after each run, and the particle aggregation. It seems
the immobilizing Pd nanoparticles on a suitable plate as a
support can be considered as an excellent strategy for inhib-
iting from some extra processes for recovery of the catalyst
like sedimentation, centrifugation and filtration. Moreover,
the mesoporous properties of the support can significantly
affect microstructures of palladium, leading to the low size
and, therefore, high active catalyst which can improve the
performance of the catalyst. Among various types of plates,
[
1, 2]. Hundreds of different catalysts and reagents have been
introduced for the oxidation of organic chemicals [3]. Many
industrial oxidation reactions were reported with stoichio-
metric amounts of reagents that gave a big amount of hazard-
ous chemical wastes [4–6]. In recent years, transition metal
catalysts have attracted a great deal of attention for the oxi-
dation of various saturated and unsaturated organic chemi-
cals in the presence of cheap and green oxidant such as O or
2
H O [7, 8]. The oxidation of alkylarenes and alcohols to the
2
2
corresponding carbonyl compounds have been carried out
*
Sajjad Keshipour
S.keshipour@urmia.ac.ir
Masoud Faraji
Ma.faraji@urmia.ac.ir
1
2
Department of Nanochemistry, Nanotechnology Research
Center, Urmia University, Urmia, Iran
Electrochemistry Research Laboratory, Department
of Physical Chemistry, Chemistry Faculty, Urmia University,
Urmia, Iran
TiO nanotubes/Ti fabricated by anodizing of titanium plate
2
can be considered as a suitable substrate because of its high
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
specific surface area, inexpensiveness, good mechanical and
chemical stability, controlled pore structure and non-toxicity
HCl). It has been believed that the reduction of metal ions
can be carried out by sonolysis of water to primary reduc-
ing radicals, i.e., H atoms [21, 22]. The major outcomes for
nanomaterial synthesis via sonochemical procedure are high
catalytic performance due to an increase in specific surface
area and narrow size distribution [23, 24]. Sonochemical
deposition is a more effective route towards the synthesis of
PdNPS with much smaller size, homogeneously Pd disper-
sion, and higher surface area than those prepared by tradi-
tional methods [25, 26].
[
16].
In continue of the our research group efforts to introduce
new efficient heterogeneous catalytic systems for the oxida-
tion reactions [17–20], herein, we investigated the catalytic
applications of TiO nanotubes/Ti plate modified with Pd
2
NPs as a new heterogeneous catalyst in the oxidation of
organic substrates. The catalyst was prepared via a simple
surface sonochemical modification of TiO nanotubes/Ti
2
plate with Pd NPs. To the best of our knowledge, this work
is the first report about use of sonochemically modified TiO
Characterization and electrochemical studies
2
nanotubes/Ti plate with Pd NPs as a catalyst for the oxida-
tion of benzyl alcohols and alkylarenes (Scheme 1).
The surface morphology of the plate was characterized with
a scanning electron microscope (Philips, Model XL30). In
addition, elemental mappings were detected by an energy
dispersive X-ray spectrometer (EDS) attached to the SEM.
Electrochemical impedance spectroscopy (EIS) measure-
ments were carried by Autolab PGSTAT302N potentiostat.
Experimental
Catalyst preparation
Highly oriented TiO nanotubes/Ti plates were obtained via
Typical procedure for the oxidation of benzyl
alcohol
2
electrochemical anodizing of Ti metal plates with a geomet-
2
ric area of 3 cm using a DC power source in a two-electrode
electrochemical cell. Briefly, prior to each anodizing, Ti
plates were polished with emery papers, cleaned in deion-
ized water in an ultrasonic bath, and subsequently were dried
in air at room temperature. Electrochemical anodizing of the
cleaned plates was conducted at 20 V for 2 h in an electrolyte
Benzyl alcohol (0.10 g, 1.00 mmol) was added to a round-
bottomed flask containing of H O (5 mL) and PdNPs/TiO
2
2
nanotubes/Ti plate. The mixture was heated to 90 °C, and
H O (3 mmol) was added dropwise to the reaction vessel
2
2
during 0.5 h. After 24 h, the plate was removed, the solvent
was evaporated under vacuum, and the product was analyzed
by GC. The conversions were calculated in the presence of
internal standard using integrals of the GC analyses.
containing of glycerol/water (75:25, vol%)+0.5 wt% NH F
4
at room temperature using a conventional two-electrode con-
2
figuration with platinum foil with a geometric area of 9 cm
as a counter electrode. The distance between the electrodes
was maintained at 2 cm. After the anodizing process, the
anodized plates were rinsed by deionized water, dried in
the air at room temperature, and finally were annealed at
Typical procedure for the oxidation of ethylbenzene
Ethylbenzene (0.10 g, 1.00 mmol) was added to a round-bot-
4
50 °C for 2 h to crystallize the anodized plates from an
tomed flask containing of H O:EtOH (1:1) (5 mL), PdNPs/
2
amorphous phase to anatase phase. The loading of PdNPs on
TiO nanotubes/Ti plate, and KOH (0.03 g, 0.5 mmol). The
2
the TiO nanotubes plate was carried out by electroless-plat-
mixture was heated to 90 °C, and H O (3 mmol) was added
2
2
2
ing under sonication condition for 30 s in a bath containing
dropwise to the reaction vessel during 0.5 h. After 24 h,
the plate was removed, the solvent was evaporated under
vacuum, and the product was analyzed by GC. The conve-
sions were calculated in the presence of internal standard
using integrals of the GC analyses.
4
4
.5×10− M of PdCl at pH 1 s (pH value was adjusted with
2
Results and discussion
Structural and morphological characteristics
The SEM images in Fig. 1 show significant differences in
the morphologies of the fabricated plates. The SEM images
in Fig. 1a show an array of highly ordered TiO nanotubes
Scheme 1 Oxidation of benzyl alcohol and ethylbenzene in the pres-
2
ence of the catalyst
formed after anodic oxidation and calcination at 450 °C,
1
3

Journal of the Iranian Chemical Society
Fig. 1 SEM images of bare TiO nanotubes/Ti plate (a), PdNPs/TiO nanotubes/Ti plate (b)
2
2
where the nanotubes are 140–210 nm and 20–30 nm in the
diameter and wall thickness, respectively. Figure 1b shows
byproduct such as benzaldehyde. The oxidation reaction of
benzyl alcohol needs Pd and gave lower yields in the absence
of Pd (Table 1, entry 5). The reaction needs 3 mmol H O
SEM micrograph of the PdNPs-loaded TiO nanotubes,
2
2
2
where NPs of Pd grown on the walls of TiO nanotubes and
for obtaining high yields, and the yields decreased in low
amounts of H O (Table 1, entries 3 and 4). The reaction
2
especially on the top section are observed. The wall thick-
2
2
ness of the Pd/TiO nanotubes’ plate is 30–40 nm. To probe
temperature is an important factor for the reaction and the
yield was decreased in low temperatures especially at room
temperature (Table 1, entries 5 and 6). After screening a
2
the chemical composition, the plate material was further
characterized by EDX-MAP analysis. Figure 2 shows the
EDX spectrum of the PdNPs-TiO nanotubes/Ti plate that
variety of solvents, H O was determined to be the best sol-
2
2
confirms the presence of palladium on TiO nanotubes/Ti. In
vent (Table 1, entries 7–12). The reaction in H O, CH CN,
2
2
3
addition, Pd is uniformly distributed on the TiO nanotubes/
and CH Cl gave benzoic acid selectively. Low yields of
2
2 2
Ti plate, suggesting that the current synthesis strategy is
the benzoic acid were obtained in solvent free conditions,
o-xylene, MeOH, and EtOH, also benzaldehyde produced
as the by-product. The reaction was examined for excess
of benzyl alcohol for obtaining the quantitative conversion
ability of the plate which 12 mmol of benzoic acid was pro-
duced during 24 h. Excellent selectivity of the reaction in
green solvent, high yield, easy work up, green oxidant, and
stirring-free oxidation conditions are important features for
this approach.
effective to modify the surface of TiO nanotubes/Ti plate.
2
The EIS measurements were carried out in aqueous solu-
tion containing 10 mM K [Fe(CN) ]:K [Fe(CN) ] (1:1) and
3
6
4
6
0
.1 M KCl plates at open-circuit potential (OCP) in the fre-
quency range of 100 kHz–10 mHz to evaluate the charge
transfer characteristics of the prepared plates. As can be seen
from Fig. 3, the Nyquist plots consist of a depressed semicir-
cle, corresponding the charge-transfer resistance (R ) in the
ct
interface of the plates with electrolyte [27]. The PdNPs/TiO
The catalyst application was extended to the oxidation
of various benzyl alcohols, including 4-nitrobenzyl alco-
hol, 2-chlorobenzyl alcohol, and 4-methylbenzyl alcohol
(Table 2). High conversions were obtained for the oxidation
of benzyl alcohols derivatives.
2
nanotubes/Ti plate possesses lower R than TiO nanotubes/
ct
2
Ti plate, indicating the highly network structure formed by
PdNPs.
Catalytic activity
The catalytic activity of PdNPs/TiO nanotubes/Ti plate
2
was evaluated also in the oxidation reaction of ethylbenzene.
The reaction conditions were optimized for the oxidation of
ethylbenzene (3) with KOH as a base and H O as an oxi-
The catalytic activity of PdNPs/TiO nanotubes/Ti plate
2
was evaluated in the oxidation reaction of benzyl alcohol.
2
2
Therefore, the oxidation of benzyl alcohol (1) with H O was
dant in the presence of PdNPs/TiO nanotubes/Ti plate. It
2
2
2
investigated for the optimization of the reaction conditions in
was found that H O:EtOH (1:1) as the solvent, 3 equivalents
2
the presence of synthesized PdNPs/TiO nanotubes/Ti plate.
of H O as the oxidant, and 0.5 mmol KOH as the base at
2
2
2
It was found that H O as the solvent and three equivalents of
90 °C is the best reaction conditions for the oxidation of
ethylbenzene (Table 3). Oxidation of ethylbenzene (1 mmol)
proceeded to give acetophenone (2) as the sole product with
>99 conversion in 24 h without any byproduct. The oxida-
tion reaction of ethylbenzene needs Pd and did not proceed
2
H O as the oxidant at 90 °C is the best reaction conditions
2
2
for the oxidation of benzyl alcohol (Table 1). Oxidation of
benzyl alcohol (1 mmol) proceeded to give benzoic acid (2)
as the sole product with >99 conversion in 24 h without any
1
3

Journal of the Iranian Chemical Society
Fig. 2 EDX spectra obtained of PdNPs/TiO nanotubes/Ti plate as well as EDX mapping
2
in the absence of Pd plate (Table 3, entry 5). The reaction
was performed with 3 mmol H O , and the yield decreased
2
2
in low amounts of H O (Table 3, entries 3 and 4). The reac-
6
5
4
3
2
1
00
00
00
00
00
00
0
2
2
tion gave high yield at 90 °C and the yield was decreased in
low temperatures, especially at room temperature (Table 3,
entries 5 and 6). After screening a variety of solvents, EtOH
2
TiO nanotubes/Ti Plate
and H O:EtOH (1:1) were recognized to be the best solvents
2
which H O:EtOH (1:1) was selected due to more accordance
2
with green chemistry principles (Table 3, entries 7–12). The
reaction of excess ethylbenzene with the plate gave 9 mmol
of acetophenone during 24 h. The reaction has advantages
that observed in the oxidation of benzyl alcohol, including
excellent selectivity, green solvent, high yield, easy work-up,
green oxidant, and stirring free.
2
Pd/TiO nanotubes/Ti Plate
The catalyst application was extended to the oxidation
of various alkylarenes (Table 4). High conversions were
obtained for the oxidation of alkylarenes.
0
200
400
600
800
1000
1200
1400
Z'/ohm
Potential Pd leaching into the reaction mixture was stud-
ied for oxidation of benzyl alcohol and ethylbenzene with
Fig. 3 Impedance Nyquist plots for the PdNPs/TiO nanotubes/Ti
2
plate and TiO nanotubes/Ti plate
2
1
3

Journal of the Iranian Chemical Society
Table 1 Optimization of the reaction conditions for oxidation of ben-
Table 3 Optimization of the reaction conditions for oxidation of eth-
zyl alcohol
ylbenzene
Entry
Solvent
Temp. (°C)
Conversion (%)
Benzaldehyde
Entry
Solvent
Temp. °C
Conversion (%)
Benzoic acid
1a
2
EtOH:H O (1:1)
EtOH:H O (1:1)
EtOH:H O (1:1)
EtOH:H O (1:1)
EtOH:H O (1:1)
EtOH:H O (1:1)
90
90
90
90
80
r.t.
90
90
90
90
90
90
0
>99
86
39
96
43
46
64
29
2
1a
2
H O
90
90
90
90
80
r.t.
90
90
90
90
90
90
0
0
0
0
0
19
>99
72
31
95
39
31
68
>99
52
2
b
3
4
2
2
H O
2
c
b
2
3
H O
2
c
5
6
7
8
9
2
4
H O
2
2
H O
2
H O
2
H O
0
2
o-xylene
MeCN
MeOH
CH Cl
–
22
11
0
21
0
o-Xylene
MeCN
MeOH
10
11
12
83
61
>99
2
2
EtOH
CH Cl2
57
74
2
EtOH
5
Reaction conditions: ethylbenzene (1 mmol), catalyst, H O
2
2
(
3 mmol), KOH (0.5 mmol), solvent (5 mL), 24 h
Without catalyst
Reaction conditions: benzyl alcohol (1 mmol), catalyst, H O
2
2
a
(
3 mmol), solvent (5 mL), 24 h
Without catalyst
b
c
a
H O (2 mmol)
2 2
b
c
Without H O
2 2
H O (2 mmol)
2
2
Without H O2
2
Table 4 Oxidation of various alkylarenes using PdNPs/TiO2 nano-
tubes/Ti plate
Table 2 Oxidation of various benzyl alcohol derivatives using
PdNPs/TiO nanotubes/Ti plate
Entry Alcohol
Product
Conversion
%)
2
(
Entry Alcohol
Product
Conversion
1
O
> 99
(
%)
1
OH
OH
> 99
O
O N
2
2
3
O
O
94
O N
2
2
OH
OH
95
92
O
F
O
91
F
O
3
OH
OH
O
Reaction conditions: alkylarene (1 mmol), catalyst, H O (3 mmol),
2
2
KOH (0.5 mmol), EtOH:H O (5 mL), 24 h
2
Reaction conditions: alcohol (1 mmol), catalyst, H O (3 mmol), H O
2
2
2
(
5 mL), 24 h
FAAS analysis. For this purpose, after removing the plate,
the solvents were evaporated, and the residues were dis-
Table 5 Successive trials
Trial
Conversion (%)
solved in HNO . The analysis of these samples with FAAS
using recoverable PdNPs/TiO2
3
nanotubes/Ti plate for oxidation
1
showed that the Pd concentrations of the reaction solutions
were less than the detection limit. These results indicate that
virtually no Pd leach from the surface into the solutions.
>99
>99
>99
>99
>99
>99
of benzyl alcohol
2
3
4
5
6
Recyclability of the PdNPs/TiO nanotubes/Ti plate was
2
examined in the oxidation of benzyl alcohol. After carrying
out the reaction, the catalyst was separated, washed with
acetone (2×5 mL), and reused. Any decreases in the reac-
tion yield were observed after six repetitive cycles for this
reaction (Table 5).
Reaction conditions: benzyl
alcohol (1 mmol), catalyst,
H O (3 mmol), 90 °C, 24 h
2
2
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Journal of the Iranian Chemical Society
Although the mechanisms of these reactions have not
been established experimentally, the formation of species
suggest that the introduced approach can be an interesting
route for the production of benzoic acid from benzyl alcohol.
5
and 6 is conceivable in the oxidation reactions of benzyl
alcohol and ethylbenzene in the presence of H O and Pd
2
2
(
Scheme 2). These active spaces can perform the oxida-
Conclusion
tion reactions as the procedure determined with Roman
numbers.
Synthesis and characterization of PdNPs/TiO nanotubes/Ti
2
The results for our catalyst are compared with some
recent reports of Pd-catalysed oxidation of benzyl alco-
hol with respect to solvent, reaction duration, temperature,
yield, and selectivity (Table 6). Performing the reaction in
solvent-free conditions, and in short reaction times are the
advantages for some of the previous reports. The oxidation
plate were performed with a homogeneous distribution of
Pd NPs on the TiO nanotubes. Synthesis was conducted by
2
sonochemical deposition of PdNPs onto TiO nanotubes/Ti
2
plate at room temperature. The plate showed good catalytic
activity in the oxidation reactions of benzyl alcohol and eth-
ylbenzene derivatives using H O at 90 °C. The oxidation
2
2
reaction of benzyl alcohol with PdNPs/TiO nanotubes/Ti
of benzyl alcohols in H O as a green solvent gave benzoic
2
2
plate has some advantages such as greener solvent compared
acid derivatives with high yield. The oxidation of alkylar-
to entry 1, and low reaction temperature compared to entries
enes in EtOH/H O led to the formation of corresponding
2
2
and 3. However, the method is a good oxidation reaction
ketones selectively in high yield. Use of a plate as a cata-
lyst facilitated the oxidation reactions procedure with very
easy separation of the catalyst from the reaction mixture and
stirring-free conditions. The catalyst has a high stability and
recyclable for several times.
regarding high yield and excellent selectivity which are very
important factors for oxidation reactions. The heterogeneity
of the catalyst is also another advantage of this work which
makes catalyst recovery easy. Regarding these results, we
Scheme 2 Proposed mechanism
for the oxidation of ethylben-
zene and benzyl alcohol
Table 6 Comparison of the
Entry Catalyst
Solvent Time (h) Temp. (°C) Selectivity (%) Yield (%)
benzyl alcohol oxidation results
with the previous report
1
2
3
4
Pd(II)-DPA@chitosan [28]
Pd@CeO [29]
Pd@NMC [30]
PdNPs/TiO nanotubes/Ti plate H O
EtOH
24
1
80
160
160
90
100
88.9
91.2
100
96
–
–
20.8
59.8
>99
2
1
24
2
2
1
3

Journal of the Iranian Chemical Society
Acknowledgements The authors wish to express thanks to the office
of vice chancellor of research of Urmia University for the financial
support.
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