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DOI: 10.1002/cplu.201400007
Direct Synthesis of Phenol from Benzene by Pd VOx
Nanoparticles Using Molecular Oxygen
Sensen Shang,[a, b] Hua Yang,[a, b] Jun Li,[a] Bo Chen,[a, b] Ying Lv,[a] and Shuang Gao*[a]
Novel Pd VOx nanoparticles (NPs) prepared by a one-pot hydrogenation method are reported. X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and
UV/Vis spectroscopy were performed to characterize the Pd
VOx NPs. The NPs catalyzed benzene directly to phenol in 5.2 %
yield with more than 99 % selectivity using O2 as the sole oxidant.
Direct functionalization of C H bonds, which removes the
need for preparing a functionalized intermediate, has been
one of the most important and promising routes to the economical and straightforward formation of new chemical
bonds.[1] However, it is also difficult to achieve, especially for
the aromatic Csp2 H bonds with a high bond energy (113 kcal
mol 1).[2] Among them, the selective direct hydroxylation of
benzene, which bears great academic and commercial importance, still remains a very challenging task despite significant
developments in the past few decades.[3] To date, many metal
oxide-based catalysts and heteropolyacids have been engaged
to try and surmount the difficulties of selective production of
phenol from benzene without further oxygenation of phenol
to hydroquinone and COx, which is usually unavoidable because the oxidation of phenol is much easier than that of benzene.[4]
Recently, breakthroughs in nanocatalysis provided a new
chance for the aerobic oxidation process. Metal clusters on the
nanoscale were endowed with numerous different features
(high specific surface area, metastable structures, unique catalytic properties, etc.) from those of the traditional materials
and attracted a wide range of research interest.[5] Among
them, bimetallic nanoparticles (NPs) even exhibited special
synergistic catalytic performance. For example, Au Pd NPs exhibited exceptional activities in the oxidation of toluene and
alcohol,[5a,b] Pt Co NPs were used for high-performance Fischer–Tropsch synthesis in the liquid phase,[5c] and Pd Pt bimetallic catalyst could even be used for oxygen reduction with extraordinary activity.[5d] To the best of our knowledge, there is
[a] S. Shang, Dr. H. Yang, Dr. J. Li, B. Chen, Y. Lv, Prof. S. Gao
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023 (P. R. China)
E-mail: [email protected]
[b] S. Shang, Dr. H. Yang, B. Chen
University of Chinese Academy of Sciences
Beijing 10049 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201400007.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
no report about the catalytic performance of bimetallic NPs in
the direct hydroxylation of benzene to phenol. We prepared
the novel Pd VOx NPs to accomplish this challenging process
with a phenol yield of 5.2 % and more than 99 % selectivity
using O2 as the sole oxidant.
The typical procedure for the preparation of Pd VOx bimetallic NPs was a facile hydrogen reduction process. Interaction
between Pd and V can be elucidated by straightforward comparison of the solution color of the unreduced and reduced
samples (Figures S1 and S2 in the Supporting Information).
Also, the interaction between Pd and V can be further confirmed by UV/Vis spectroscopy. The absorption peak above
300 nm for an unreduced solution of vanadyl acetylacetonate
([VO(acac)2]) and palladium acetate (Pd(OAc)2) disappeared
after the reducing procedure (see Figure S3). However, the
maximum absorption peak of 10 % Pd VOx NPs occurs at
274 nm, which did not even exist in the unreduced solution of
Pd(OAc)2 or [VO(acac)2]. Meanwhile, another absorption peak
around 242 nm is also observed in the 10 % Pd VOx NPs.
For the Pd Pt NPs, it was reported that hydrogen was absorbed and Pd H was generated, which can be identified by
1
H NMR spectroscopy.[6] Proton exchanges occurred between
the hydrogen proton of Pd H and the protons of the solution
in 50 % acetic acid. That is, Pd H was not generated in the
Pd VOx NP solution (Figure S4).
The as-synthesized Pd VOx NPs and Pd- or V-based compounds were employed to catalyze the direct hydroxylation of
benzene (Table 1). On utilizing 10 % Pd VOx NPs as the catalyst, no phenol was obtained without benzene (Table 1,
entry 1). Besides, Pd(OAc)2 together with [VO(acac)2] showed
no catalytic activity after the mixture was heated at 140 8C for
5 hours (Table 1, entry 2). Pd NPs did not work under the same
conditions, which suggested that Pd seeds are inactive without
vanadium (Table 1, entry 3). Without Pd, oxygen could not be
activated directly by vanadium, even if the reaction temperature was increased to 200 8C (Table 1, entry 4). An acceptable
amount (0.30 mmol) of phenol can be accessed through catalysis by the 10 % Pd VOx NPs, which was prepared under
0.8 MPa H2 at 60 8C for 4 hours (Table 1, entry 5). A higher reducing temperature (Table 1, entry 6) and longer reducing time
(Table 1, entry 7) were employed to investigate the role played
by the reducing procedure. Even though the reducing temperature was increased, the catalytic performance was not improved as shown in Table 1, entry 6. Prolonging the reducing
time was found to be available, and the phenol yield increased
from 0.30 to 0.45 mmol (Table 1, entry 7). The phenol yield can
reach 5.2 % under a reducing pressure of 1.4 MPa (Table 1, entries 7–9). Higher or lower reducing pressure both decreased
the yield of phenol.
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addition, X-ray photoelectron spectroscopy (XPS) was
employed to investigate the oxidation state of pallaEntry Catalyst
Reducing Reducing
Phenol
Phenol
dium species, as shown in Figure 1 b. The weak sigtime [h]
pressure [MPa] yield [%] selectivity [%]
nals observed (number of scans = 200) also suggested the coverage of vanadium oxide NPs on the palla10 % Pd VOx NPs
4
0.8
0
–
1[b]
2[c]
Pd(OAc)2/[VO(acac)2] –
–
0
–
dium surface. Emissions at 334.6 and 339.9 eV were
3
Pd seeds
4
0.8
0
–
ascribed to the 3d5/2 and 3d3/2 regions of Pd0, and
[d]
[VO(acac)2]
–
–
0
–
4
definitely suggested that the Pd(OAc)2 was reduced
4
0.8
3
99
5
10 % Pd VOx NPs
by H2 to Pd0. Also, a slight Pd loss peak was observed
10 % Pd VOx NPs
4
0.8
3
99
6[e]
6
0.8
4.5
99
7
10 % Pd VOx NPs
at 345.5 eV. Thus, it was supposed that Pd seeds
6
2.0
2.6
99
8
10 % Pd VOx NPs
were coated with vanadium oxide NPs and the coor6
1.4
5.2
99
9
10 % Pd VOx NPs
dination environment between benzene and Pd
[a] Reaction conditions: benzene (10 mmol) was added to 50 wt % acetic acid (6.8 mL)
seeds was depressed, which meant that phenol and
containing freshly prepared 10 % Pd VOx NPs (0.02 mmol Pd) as catalyst. The mixture
not biphenyl was the sole product.
was charged with 2 MPa O2 and the reaction was performed at 140 8C for 5 h.
The aqueous-phase hydroxylation of benzene in[b] Without benzene. [c] 0.02 mmol Pd(OAc)2 and 0.2 mmol [VO(acac)2]. [d] The reaction temperature was increased to 200 8C. [e] The reducing temperature was 90 8C.
vestigated in this work provides opportunities to
consider this process under different reaction conditions. Homogeneous catalysts Pd(OAc)2 with [VOMoreover, XRD characterization of Pd VOx NPs and Pd seeds
(acac)2] showed no catalytic activity to the hydroxylation of
was performed to further elucidate the properties of Pd VOx
benzene. Interestingly, detailed studies show that the diameter
NPs. Meanwhile, Pd seeds were prepared by reductive polyviof the Pd VOx NPs had a significant effect on the catalytic acnylpyrrolidone (PVP)-coated Pd(OAc)2 with 1.4 MPa H2 at 60 8C
tivity (see Figure 2 and Figure S5). The catalytic activity showed
for 6 hours. No PdO peak around 2q = 348 was observed from
a slight increase when the particle diameter was reduced from
not only Pd VOx NPs but also Pd seeds (see Figure 1 a), which
10.3 to 8.0 nm, but a dramatic increase when the diameter was
indicated that there was no PdII but Pd0 in the Pd VOx NPs. Inreduced to 7.2 nm. Further reduction of the particle diameter
terestingly, the reflection occurring at 2q = 408, which is asto 6.3 nm resulted in the maximum catalytic activity of
cribed to the small crystalline domains of Pd(111), cannot be
7.8 molphenolmolPd 1 h 1. As the size of the Pd VOx NPs was refound in the Pd VOx NPs. This finding is probably because Pd
duced from 6.3 to 5.4 nm, the catalytic activity decreased dramatically to a minimum value of 1.5 molphenolmolPd 1 h 1. Deis well covered by the vanadium oxide NPs in Pd VOx NPs. In
spite efforts to rationalize this very unusual phenomenon, no
atomic-level explanation is currently available.
For apprehending this catalytic process, some control experiments were performed (Table 2). The process catalyzed by
Pd(OAc)2, H4PMo11VO40, and H4PMo12O40 reported by Ishii
et al.[7] in the biphenyl production process is still prone to produce biphenyl (Table 2, entry 1). Although the reaction catalyzed by Pd VOx NPs proceeded under the reaction conditions
of the Ishii system, no process of phenol production occurred
(Table 2, entry 2). Table 2, entry 3 indicates that the 10 % Pd
VOx NP catalytic system charged with H2 resulted in a very low
phenol yield, which indicates that the catalytic process does
not undergo an in situ H2O2 generation pathway. This can be
further confirmed by adding a small amount of H2O2 at the
preliminary stage of the reaction (Table 2, entry 4), so it seems
that the hydroxylation process was not initiated by the additional H2O2. Moreover, the Pd VOx NP catalyst was also used in
the oxidation of styrene (Scheme S1). It had been reported
that a benzaldehyde yield of 88 % can be achieved catalyzed
by Pd(OAc)2 after 24 hours at 100 8C.[8] In this work, we employed a lower reaction temperature of 80 8C and preferred
a similar reaction pressure as the oxidation condition; then,
a benzaldehyde yield of 86.7 % as well as a benzoic acid yield
of 3.7 % were achieved in just 1 hour.
In conclusion, we have prepared and carefully characterized
the novel Pd VOx NPs. This catalyst was synthesized by reduction of carbonyl groups of [VO(acac)2] catalyzed by rapidly
Figure 1. a) XRD patterns of the Pd seeds and 10 % Pd VOx NPs. b) XPS
formed Pd seeds with hydrogen as the reducing agent. This
spectra of Pd 3d5/2, Pd 3d3/2, and Pdloss.
Table 1. The catalytic performance of different catalysts.[a]
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benzene can be oxidized to
form phenol with high selectivity, which is different from the
traditional Pd- or Pd/V-catalyzed
hydroxylation processes.
Experimental Section
In a typical experiment, [VO(acac)2]
(0.0530 g, 0.2 mmol) and Pd(OAc)2
(0.0045 g, 0.02 mmol) were added
to 50 wt % acetic acid (6.8 mL) containing PVP (0.22 g, 2.0 mmol). The
mixture was reduced under
0.8 MPa hydrogen at 60 8C for 4 h,
and 10 % Pd VOx NP catalysts
were obtained. It was believed
that the rapidly formed Pd seeds
catalyzed the hydrogenation of
carbonyl groups of [VO(acac)2] at
a mild temperature, thus resulting
in the simultaneous formation of
Pd VOx bimetallic NPs.
10 % Pd VOx NPs with a diameter
of 10.39 nm/50 wt % acetic acid
containing 10 % Pd VOx NPs
(0.2 mmol) as catalyst. Firstly, the
reaction was charged with 2 MPa
O2 and performed at 140 8C for
5 h, then charged with 1.4 MPa H2
and performed at 60 8C for 6 h. Finally, benzene (10 mmol) was
added and the mixture was
charged with 2 MPa O2, with reaction at 140 8C.
10 % Pd VOx NPs with a diameter
of 8.09 nm/50 wt % acetic acid
containing 10 % Pd VOx NPs
(0.2 mmol) as catalyst. Firstly, the
reaction was charged with 2 MPa
O2 and performed at 140 8C for
5 h, then charged with 1.4 MPa H2
and PVP (0.2 mmol) and performed
at 60 8C for 6 h. Finally, benzene
(10 mmol) was added and the mixture was charged with 2 MPa O2,
with reaction at 140 8C.
In a typical hydroxylation reaction,
benzene (0.78 g, 10 mmol) was
added to 50 wt % acetic acid
(6.8 mL) containing freshly preNPs
pared
10 %
Pd VOx
(0.02 mmol, calculated by Pd
amount). The reaction mixture
mentioned above was placed in
a 60 mL Teflon-lined stainless-steel
Figure 2. a–e) TEM and diameter distribution of Pd VOx NPs with different diameters; scale bar = 20 nm.
reactor, and the reactor was
charged with 2 MPa O2. The reacmetastable bimetallic nanoscale catalyst exhibited an interesttion was performed with magnetic stirring at 140 8C for 5 h. After
ing catalytic performance. In the process catalyzed by Pd VOx
the reaction, dioxane (0.1 g) was added as an internal standard.
NPs, molecular oxygen is employed as the sole oxidant, and
Then methyl acetate was added to the mixture to make it homo 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: January 7, 2014
Revised: February 18, 2014
Published online on March 11, 2014
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