Aerobic oxidation of benzene to phenol over polyoxometalate-paired PdII-coordinated hybrid: Reductant-free heterogeneous catalysis

Article abstract of DOI:10.1016/j.catcom.2014.09.031

Aerobic oxidation of benzene was carried out over the organic-inorganic hybrid catalyst of [(C₃CNpy)₂Pd(OAc)₂]₂HPMoV₂, a solid catalyst containing the Keggin polyoxometalate PMoV₂ and noble

Full text of DOI:10.1016/j.catcom.2014.09.031

Catalysis Communications 59 (2015) 1–4
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Aerobic oxidation of benzene to phenol over polyoxometalate-paired
II
Pd -coordinated hybrid: Reductant-free heterogeneous catalysis
a
a
b
a
a
a,
Zhouyang Long , Yangqing Liu , Pingping Zhao , Qian Wang , Yu Zhou , Jun Wang ⁎
a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, PR China
School of Chemical Engineering, Shandong University of Technology, Zibo, Shandong 255049, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2014
Received in revised form 27 August 2014
Accepted 17 September 2014
Available online 28 September 2014
2 2
Aerobic oxidation of benzene was carried out over the organic–inorganic hybrid catalyst of [(C CNpy) Pd(OAc) ]
3
2
2 2 2
HPMoV , a solid catalyst containing the Keggin polyoxometalate PMoV and noble metal Pd(OAc) in its ionic struc-
ture. The features of this heterogeneous reaction system were studied by tuning various reaction parameters includ-
ing solvent selection, function of buffer agent, and effects of reaction time, reaction temperature and catalyst
amount. The results show the high phenol yield of 9.8% without aided by any sacrificial reducing reagents. The
conjugation of Pd(OAc)
for its high activity is preliminarily discussed.
2
and PMoV
2
within the ionic structure of [(C
3
CNpy)
2 2 2 2
Pd(OAc) ] HPMoV that accounts
Keywords:
Heterogeneous catalysis
Polyoxometalate
Hydroxylation of benzene
Phenol
© 2014 Elsevier B.V. All rights reserved.
Palladium acetate
1
. Introduction
catalyst is preferred due to the convenience in catalyst isolation [13].
However, very few heterogeneous catalysts for reductant-free aerobic
oxidation of benzene to phenol have been reported so far. For example,
Energy shortage and environmental pollution are two serious prob-
lems in modern society, which are being greatly aggravated by highly
energy-consuming and polluting chemical processes. Therefore, energy-
efficient and green processes are urgently demanded to realize sustain-
able chemical production. Phenol is one of the most important raw
chemicals, which is currently produced by the three-step cumene process
with the disadvantageous of high energy consumption, low phenol yield
and serious pollution [1,2]. To replace the cumene process, one-step
hydroxylation of benzene to phenol has been extensively investigated
for decades [3]. Among the oxidants for this reaction, such as molecular
2 2
to achieve a heterogeneous catalyst, Pd(OAc) and PMoV were simul-
taneously immobilized on the porous support, but leading to a much
lowered phenol yield of 0.7% [11]. Further, the nanoparticle catalyst
Pd-VO synthesized from Pd(OAc) and VO(acac) showed a consider-
x 2 2
able phenol yield of 5.2% [14], while another advance is obtaining of the
high phenol yield of 13.6% over the dual-catalysis system combined by
2 2
graphitic carbon nitride with PMoV , but PMoV is still a homogeneous
catalyst in that system [12].
Herein we report a reductant-free heterogeneous system for aerobic
oxygen (O
2
), hydrogen peroxide, nitrous oxide, and so on, O
2
is ideal
oxidation of benzene to phenol catalyzed by a polyoxometalate-paired
II
due to its wide availability, low cost and environmental benignity [4].
Up to date, the liquid phase catalytic reactions of aerobic oxidation of
benzene with O
Pd -coordinated ionic hybrid. Recently, our group prepared a PMoV
2
-
II
based Pd -coordinated ionic hybrid catalyst [(C
HPMoV specifically for oxidative dehydrogenation of benzene to
biphenyl [15]. Based on the known result that the combination of
Pd(OAc) with PMoV can effectively catalyze the reductant-free
aerobic oxidation of benzene to phenol [11] and the fact that
3 2 2 2
CNpy) Pd(OAc) ]
2
to phenol are mostly reported in the presence of sacri-
, CO, ascorbic acid), exhibiting relatively
2
ficial reducing agents (i.e. H
2
high phenol yields [4–7]. It is however very desirable that aerobic oxida-
tion of benzene to phenol could effectively take place in the absence of a
sacrificial reducing agent due to more atomic economy, but the early re-
2
2
II
3 2 2 2 2 2
[(C CNpy) Pd(OAc) ] HPMoV involves both Pd and PMoV sites
ports are still rare [8–12]. In this context, the combination of Pd(OAc)
x
with V-containing polyoxometalate H3+ PMo12 (PMoV ,
x = 1–3) is the eminent homogeneous catalyst giving a phenol yield
2
within the cation and anion moieties of its ionic structure (Fig. S1), it
is rational to expect that the ionic hybrid may be catalytically very active
for aerobic oxidation of benzene to phenol. Catalytic tests of this work
x
−
x
V
x
of 9.4% [11]. From the point of view of greener catalysis, a heterogeneous
3 2 2 2 2
reveal that [(C CNpy) Pd(OAc) ] HPMoV exhibits a high yield of
phenol for this reaction as a heterogeneous catalyst without adding
any sacrificial reducing reagents after optimization of reaction condi-
tions. Catalytic mechanism is preliminarily discussed according to
reaction results, catalyst structure, and previous findings.
⁎
Corresponding author. Tel.: +86 25 83172264.
E-mail address: junwang@njtech.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.catcom.2014.09.031
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

2
Z. Long et al. / Catalysis Communications 59 (2015) 1–4
2
2
. Experimental
Table 1
3 2 2 2 2
Aerobic oxidation of benzene to phenol catalyzed by [(C CNpy) Pd(OAc) ] HPMoV using
different solvents.
a
.1. Materials and methods
All solvents and reagents were purchased commercially and used
without further purification. Elemental analyses (C, H and N) were per-
formed on a CHN elemental analyzer (Vario EL cube). Fourier transform
infrared (FT-IR) spectra were recorded on a Nicolet iS10 FT-IR instru-
−
1
ment (KBr disks) in the 4000–400 cm region. The amount of leached
palladium species in the filtrate after a reaction was measured using a
Jarrell-Ash 1100 ICP-AES spectrometer.
Entry
Solvent
Volume (mL)a
LiOAc (g)
Phenol yield (%)b
2
.2. Catalyst preparation
(C CNpy) Pd(OAc)
1
2
3
4
5
6
7
8
9
Acetonitrile-H
2
O
2/4
2/4
2/4
2/4
2/4
0/6
3/3
4/2
2/4
2/4
2/4
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0
0
0
Ethanol-H
2
O
[
3
2
2 2
] HPMoV
2
has been prepared and characterized
DMF-H O
2
by CHN elemental analysis, TG, FT-IR, UV–vis, ESR, SEM and XRD in our
previous paper [15]. CHN elemental analysis for the presently synthe-
sized [(C
Acetone-H
2
O
2.3
5.6
0
3.1
1.4
2.5
4.3
4.0
Acetic acid-H
Acetic acid-H
Acetic acid-H
2
2
2
O
O
O
3 2 2 2 2
CNpy) Pd(OAc) ] HPMoV , found: C 19.11 wt.%, N 4.08 wt.%,
H 2.01 wt.%; calcd: C 19.07 wt.%, N 4.04 wt.%, H 2.06 wt.%.
Acetic acid-H O
2
Acetic acid-H
Acetic acid-H
Acetic acid-H
2
2
2
O
O
O
Without
0.1
0.3
1
1
0
1
2
.3. Catalytic test
a
Reaction conditions: benzene 2 mL (22.5 mmol), catalyst 0.05 g (0.08 mol% based on
2.0 MPa, 110 °C, 4 h.
Phenol yield (%) = mmol phenol / mmol initial benzene.
Hydroxylation of benzene was carried out in a customer-designed
benzene substrate), O
2
temperature controllable pressured titanium reactor (25 mL) equipped
with a mechanical stirrer. In a typical run, 0.05 g (0.08 mol%) [(C CNpy)
Pd(OAc) HPMoV , 0.2 g LiOAc, 2 mL (22.5 mmol) benzene, and
aqueous solution of acetic acid (2 mL acetic acid and 4 mL H O) were
added into the reactor successively. After the reactor was charged
with 2.0 MPa O at room temperature, the reaction was conducted at
10 °C for 4 h with vigorous stirring. After reaction, 1,4-dioxane was
b
3
2
2
]
2
2
2
which largely accounts for the much enhanced phenol yield (entry 5).
Besides, previous reports [8,10] have suggested that acetic acid partici-
2
1
pates in the formation of the reactive intermediates C
Pd(OAc) , which is crucial for the catalysis (also see this in detail
later). However, increasing the acetic acid concentration in the mixed
solvent acetic acid-H O from 2/4 (entry 5) to 3/3 (entry 7) and further
6 5
H PdOAc and
added into the product mixture as an internal standard. The mixture
was analyzed by gas chromatograph (GC) with a FID and a capillary
column (SE-54; SE-54; 30 m × 0.32 mm × 0.25 μm). Because only the
phenol GC peak was detected as the product when the reaction was
performed around the above typical conditions, the GC-measured selec-
tivity for phenol is reasonably estimated to be above 99%. For recovering
the solid catalyst, it was filtrated from the reacted mixture, washed with
acetic acid and dichloromethane, and dried in vacuum oven at 80 °C for
C
6
H
5
3
2
to 4/2 (entry 8) gives phenol yields of 5.6%, 3.1% and 1.4%, respectively.
This substantial decrease in phenol yields may associate to excessive
oxidation caused by radical processes in the highly concentrated acetic
acid aqueous solutions [8].
It is noted that the solvent of aqueous acetic acid used above was
buffered with LiOAc. Without LiOAc (entry 9), the phenol yield dropped
to 2.5%, with the GC peak of biphenyl clearly detected (though in trace
amount) and darkened color of the reacted mixture observed. As the
over-oxidation byproducts, catechol, hydroquinone or benzoquinone
are well detectable with the present GC analysis, but they were unde-
tectable in this work. Consequently, the observed dark color implies
the occurrence of tar as the deep-oxidation byproduct. When the
amount of LiOAc increased to 0.1 g and 0.2 g, the phenol yield increased
to 4.3% and 5.6% (entries 10 and 5) with complete disappearance of
biphenyl, while the dark color of tar could not be visualized any more.
The disappearance of biphenyl and enhanced phenol production consist
with the previous proposal that the addition of the buffer agent LiOAc
favors the hydroxylation at the expense of coupling reaction [10]. In ad-
dition, LiOAc buffers the aqueous acetic acid, reduces the overpowering
oxygen radicals, and thus enhances the selective hydroxylation process
[8,10,17]. However, when excessive amount of LiOAc (0.3 g) was used,
the generation of reactive oxygen species may be depressed, and the
phenol yield decreased to 4.0% (entry 11).
For an aerobic oxidation process, the oxygen pressure should be
one of the most important parameters to influence the reaction result.
As shown in Fig. 1(A), at the relatively low O
only a small amount of phenol (1.2% yield) was obtained with explicit
detection of biphenyl (0.4% yield). When the O pressure increased to
2.0 MPa, the phenol yield was pronouncedly enhanced to 5.6% with
diminishing of biphenyl to zero. Stoichiometrically, two benzene mole-
cules react with a half O
while the reaction of two benzene molecules with one O
leads to the formation of two phenol molecules. The high O
8
h. The average values of three parallel reaction tests were given for
phenol yields.
3
. Results and discussion
This work is to reveal the reductant-free heterogeneous catalysis
for aerobic oxidation of benzene to phenol over the hybrid catalyst
(C CNpy) Pd(OAc) HPMoV . However, in our recent paper [15], the
[
3
2
]
2 2
2
same hybrid catalyst has been demonstrated to be highly efficient for
oxidative dehydrogenation of benzene with molecular oxygen to biphe-
nyl. Therefore, tuning of reaction conditions should be crucial in this
work. Owing to extreme importance of the nature of solvent used in a
liquid-phase reaction [16], we started with solvent selection in investi-
gating the effects of reaction parameters, with the results shown in
Table 1. It is seen that no phenol was yielded with solvents of aqueous
solutions of acetonitrile, ethanol or DMF (dimethylformamide) (entries
1
–3), and only low phenol yield of 2.3% was observed with aqueous
solution of acetone (entry 4). The highest phenol yield of 5.6% was
obtained in aqueous solution of acetic acid (entry 5), indicating that
aqueous acetic acid solution is the preferred solvent for this reaction.
Then the effect of volume ratio of acetic acid to water on the phenol
yield was investigated with the certain volume for the mixed solvent
of 6 mL. Entry 6 of Table 1 showed no phenol product with pure water
as the solvent. Benzene is immiscible with water; therefore, the solid
catalyst well dispersed in water is difficult to contact benzene substrate,
which hinders the play of its catalytic role in the reaction. Adding acetic
acid into water increases the organic character of the aqueous solvent
and thus benefits the solubilization of benzene in the reaction medium,
2
pressure of 1.0 MPa,
2
2
molecule to produce one biphenyl molecule;
molecule
pressure
2
2

Z. Long et al. / Catalysis Communications 59 (2015) 1–4
3
Fig. 1. Effects of reaction conditions on phenol yield in [(C
catalyst (0.08 mol%), 0.2 g LiOAc, 2 mL HOAc with 4 mL H O (solvent), 110 °C, 4 h. (B) Time variation with other conditions: 0.05 g catalyst (0.08 mol%), 2 mL HOAc with 4 mL H
.2 g LiOAc, 2.0 MPa O , 110 °C. (C) Temperature variation with other conditions: 0.05 g catalyst (0.08 mol%), 2 mL HOAc with 4 mL H O (solvent), 2.0 MPa O , 0.2 g LiOAc, 4 h. (D) Catalyst
amount variation with other conditions: 2 mL HOAc with 4 mL H O (solvent), 2.0 MPa O , 0.2 g LiOAc, 120 °C, 4 h; for combined homogeneous catalyst Pd(OAc) + PMoV : Pd(OAc)
.32 mol%, PMoV 0.16 mol%.
3 2 2 2 2
CNpy) Pd(OAc) ] HPMoV -catalyzed aerobic oxidation of benzene. (A) Oxygen pressure variation with other conditions: 0.05 g
2
2
O (solvent),
0
2
2
2
2
2
2
2
2
0
2
means more O
2
molecules dissolved in the liquid phase of the reaction,
heterogeneous catalyst in this work are much higher than those over
the heterogeneous catalysts reported previously (i.e., Pd(OAc) and
PMoV simultaneously immobilized on mesoporous silica with phenol
yield 0.7%, TON 3.68 and TOF 0.368 h [11]; and nanoparticle Pd-VO
which therefore favors the formation of phenol rather than biphenyl.
However, too much oxygen usually causes the excessive oxidation of
the already produced phenol, and correspondingly, the phenol yield
decreased to 4.9% at the higher O
report [15] has shown that at the much lower O
the hybrid catalyst [(C CNpy) Pd(OAc) HPMoV
high biphenyl yield (18.3%) through aerobic oxidative dehydrogenation
of benzene, reasonably in agreement with the present observation that
low oxygen pressure facilitates the generation of biphenyl. Additionally,
that domination of biphenyl product was achieved in the absence of the
buffer LiOAc [15], also in accordance with the result in Table 1. The
effects of reaction time, reaction temperature and catalyst amount
shown in Figs. 1(B)–(D) indicate that 4 h, 120 °C and 0.1 g (0.16 mol%)
2
2
−
1
x
−
1
2
pressure of 2.5 MPa. Our previous
pressure of 0.3 MPa,
could give a very
with phenol yield 5.2%, TON 26 and TOF 5.2 h [14]). The comparison
implies that connecting Pd(OAc) with PMoV into the ionic structure of
, rather than immobilizing Pd(OAc)
onto a support, must have led to a more remarkable syn-
2
2
2
3
2
2
]
2
2
[(C
3
CNpy)
2
Pd(OAc)
2
]
2
HPMoV
2
2
and PMoV
2
ergistic effect between the two catalytically active moieties. For the
homogeneous catalytic system of Pd(OAc) with PMoV , it has been
established (Fig. S2) that Pd(OAc) first electrophilically attacks the
substrate benzene to form the palladium acetate intermediate
PdOAc. Then PMoV oxidizes C PdOAc to phenylpalladium(IV)
triacetate and becomes the reduced state PMoV
the phenylpalladium(IV) triacetate decomposes to produce Pd(OAc)
and phenyl acetate, with the latter being finally hydrolyzed to phenol.
2
2
2
C H
6 5
2
6 5
H
[
red]
2
itself. Afterwards
[
(C
Under more severe conditions (i.e., t N 4 h, T N 120 °C, catalyst amount N
.1 g), the phenol yield decreased reversely due to the continuous over-
3
CNpy)
2
Pd(OAc)
2
]
2
HPMoV
2
were suitable reaction parameters.
2
On the other hand, the PMoV[
PMoV to complete the catalytic cycle [8,10]. For the present catalyst
[(C CNpy) Pd(OAc) HPMoV , on the one hand, Pd(OAc) and PMoV
are confined in one ionic structure with the shortened distance of the
two moieties within molecular level, which may enable PMoV to
oxidize the intermediate C PdOAc from the neighboring Pd(II) site
red]
is oxidized to its original form
0
2
oxidation of phenol into the deep-oxidation byproduct tar, because the
product phenol is more reactive than the substrate benzene in this strong
oxidation environment [18].
2
3
2
]
2 2
2
2
2
From all the above results, the high phenol yield of 9.8% can be
2
achieved over the hybrid catalyst [(C
out being aided by any sacrificial reducing reagents under the optimal
conditions: 0.1 g (0.16 mol%) catalyst, 2 mL HOAc with 4 mL H
as the solvent, 2.0 MPa O , 0.2 g LiOAc, 120 °C, 4 h. At this moment,
TON (turnover number: mmol phenol / mmol Pd) and TOF (turnover
frequency: mmol phenol / (mmol Pd × h reaction time)) for
3
CNpy)
2
Pd(OAc)
2
]
2
HPMoV
2
with-
6 5
H
more efficiently. On the other hand, higher oxidation states of coordina-
tion compounds might be attainable in the presence of a conjugate
polyoxometalate, proposed by Neumann [19]. Accordingly, the Pd(IV)
species may be achieved more easily from Pd(II) when it is conjugated
2
O
2
by PMoV
Pd(II). It is thus suggested that the above two factors mostly account
for the high activity of [(C CNpy) Pd(OAc) HPMoV
2
, which benefits the valence change between Pd(IV) and
−
1
[
(C
3 2 2 2 2
CNpy) Pd(OAc) ] HPMoV are 30.6 and 7.65 h , respectively.
Therefore, both the phenol yield and TON/TOF values given by the
3
2
]
2 2
2
.

4
Z. Long et al. / Catalysis Communications 59 (2015) 1–4
Fig. 2. (A) Catalytic reusability of [(C
3
CNpy)
2
Pd(OAc)
2
]
2
HPMoV
2
2
for aerobic oxidation of benzene to phenol; reaction conditions: 0.1 g catalyst (0.16% mol%), 2 mL HOAc with 4 mL H O
2 3 2 2 2 2
(solvent), 2.0 MPa O , 0.2 g LiOAc, 120 °C, 4 h. (B) IR spectra of [(C CNpy) Pd(OAc) ] HPMoV : (a) fresh sample, and (b) recycled sample.
As a heterogeneous catalyst, [(C
3
CNpy)
2
Pd(OAc)
2
]
2
HPMoV
2
was
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