Transition metal substituted polyoxometalates and their application in the direct hydroxylation of benzene to phenol with hydrogen peroxide

Article abstract of DOI:10.1007/s11164-010-0208-4

A series of transition metal substituted polyoxometalates with a Keggin structure were prepared and utilized for the hydroxylation of benzene to phenol. Among the compounds tested, [(CH₃)₄N]₄PMo ₁₁VO₄₀

Full text of DOI:10.1007/s11164-010-0208-4

Res Chem Intermed (2010) 36:959–968
DOI 10.1007/s11164-010-0208-4
Transition metal substituted polyoxometalates
and their application in the direct hydroxylation
of benzene to phenol with hydrogen peroxide
•
Jiaqi Chen Jun Li Yi Zhang Shuang Gao
•
•
Received: 11 May 2010 / Accepted: 27 October 2010 / Published online: 20 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A series of transition metal substituted polyoxometalates with a Keggin
structure were prepared and utilized for the hydroxylation of benzene to phenol.
Among the compounds tested, [(CH3)4N]4PMo11VO40 exhibits the highest phenol
yield (13.0%) and selectivity (90.6%) in acetic acid/acetonitrile. Vanadium peroxo
is the active site of the reaction, and ammonium also plays an important role. The
influence of various reaction parameters, such as solvent, reaction time, reaction
temperature, and amount of hydrogen peroxide used were investigated to obtain the
optimal reaction conditions.
Keywords Hydroxylation Á Benzene Á Phenol Á Transition metal substituted
polyoxometalates
Introduction
Phenol is one of the most important intermediates in the chemical industry,
especially for the manufacture of petrochemicals, agrochemicals, and plastics, e.g.,
phenol resins, aniline, caprolactam, bisphenol A, epoxy resins, or polycarbonates
[1–3]. Nearly all its current production is based upon the cumene process, which is
also the most important route to produce acetone. In this technique, acetone is co-
produced with phenol in a 1:1 M ratio. Due to varying expected market growth for
phenol and acetone, as well as the promotion of atom economy and green chemistry,
much effort has been devoted to developing new processes for a one-step synthesis
J. Chen Á J. Li Á Y. Zhang Á S. Gao (&)
Dalian Institute of Chemical Physics, The Chinese Academy of Sciences,
Group 208, 457 Zhongshan Road, 116023 Dalian, People’s Republic of China
e-mail: sgao@dicp.ac.cn
J. Chen
Graduate School of the Chinese Academy of Sciences, 100049 Beijing, People’s Republic of China
123

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J. Chen et al.
of phenol with fewer by-products. Hence, the direct hydroxylation of benzene to
phenol has become one of the most challenging and exciting areas of study from an
economic and environmental viewpoint.
In recent years, the direct hydroxylation of benzene with various oxidants has
been extensively investigated. Benzene hydroxylation is done through three main
pathways. The first route involves passing gas over zeolite catalysts, such as Fe-
ZSM-5, Fe-TS-1, and Fe/MFI, with N2O as an oxidant at temperatures of over
600 K. In this method, the selectivity is 90%, and the benzene conversion is 20%
[4–8]. However, the availability of N2O as an oxidant is limited because of the high
temperature in the reaction process and the deactivation of the catalysts by heavy
coke formation. The second route involves hydroxylation by molecular oxygen both
in the gas and liquid phases. Examples are the well-known in situ hydroxylation
over Pd/O2/H2 with 77% phenol selectivity and 20% yield [9, 10], the use of ion-
exchange resins [11], and catalysis by polyoxometalates [12]. The cheapest oxidant,
molecular oxygen, can be used by introducing a reductive agent, such as H2, but the
lower yields of the reaction and the selectivity control limit its industrial
application. The third route involves hydroxylation using H2O2 as an oxidant.
Various heterogeneous catalysts, such as supported VOx, V-MCM-41 [13], and iron-
based catalysts [14], have been studied. In the presence of V-MCM-41, phenol
selectivity is at least 40%, and benzene conversion is 10%. In the presence of H2O2,
TS-1 or modified TS molecular sieves also show good activity, resulting in
selectivity that is greater than 90% and benzene conversion that is greater than 35%
[15, 16]. Keggin-type HPAs have recently been found to be good homogeneous
catalysts for this reaction in the presence of H2O2 [17–19]. This catalyst exhibits
high activity, resulting in phenol selectivity that is over 90% and phenol yield that is
over 17% [20]. The recovery and reuse of the catalyst are important problems in
homogeneous systems [21–24]. In our previous work, [(CH3)4N]4PMo11VO40 was
used as a new solid catalyst to hydroxylate benzene to phenol in the presence of
H2O2. The catalyst showed high stability and exhibited a steady performance even
when it was recycled for oxidation reaction. The phenol yield was 12.4%, and the
phenol selectivity was 85.7% [25].
Herein, a series of transition metal substituted polyoxometalate compounds were
synthesized and their catalytic activity in benzene hydroxylation were compared and
the reaction factors on the hydroxylation of benzene with [(CH3)4N]4PMo11VO40 as
the catalyst, such as reaction times, reaction temperatures, solvents and the amount
of H2O2 used were investigated in detail.
Experimental
Catalyst preparation
All chemicals were of analytical grade and used without any further purification.
The heteropoly acid, H4PMo11VO40, was prepared by a method described by
Tsigdions and Hallada [26]. Briefly, 1.42 g Na2HPO4Á12H2O was mixed with a
solution of 1.22 g sodium metavanadate, which had been dissolved by boiling in
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Transition metal substituted polyoxometalates
961
20 mL water. The mixture was cooled and acidified to a red color with 1 mL
concentrated sulfuric acid. To this mixture, a solution of 26.6 g Na2MoO4Á2H2O
dissolved in 40 mL water was added. Finally, 17 mL of concentrated sulfuric acid
was slowly added under vigorous stirring to form a lighter red solution. After
cooling the solution, the heteropoly acid was then extracted with 100 mL ethyl
ether. After extraction, heteropoly etherate appeared as a middle oil-like red layer.
This layer was collected, and ether was removed. The product was obtained after
adding a little water. It was kept in a vacuum desiccator for 1 day. The orange
powder was recrystallized for further use. The catalyst was synthesized by adding
H4PMo11VO40 into a solution of tetra-methyl ammonium hydroxide (4 equiv) at
room temperature for 2 h while stirring. Furthermore, different ammonium 11-
molybdo-1-vanadophosphates were obtained when various ammonium compounds,
such as cetyl trimethyl ammonium 11-molybdo-1-vanadophosphate and octodecyl
dimethyl benzyl ammonium 11-molybdo-1-vanadophosphate, replaced tetra-methyl
ammonium hydroxide. The other phosphomolybdates substituted by different
metals and similar catalysts were synthesized by procedures reported in [27–31].
Catalyst characterization
Elemental analysis of the catalyst was performed with ICP-plasma flame emission
spectroscopy (Thermo Elemental). The infrared (IR) spectra of the catalyst were
obtained with a resolution of 4 cm-1 in KBr disks at room temperature by an FT-IR
spectrophotometer (NEXUS470; Bruker). Powder X-ray diffraction (XRD) mea-
surement was performed using a diffractometer (Miniflex; Rigaku) with a CuKa
radiation at 0.02 steps per second in the angle range of 2h = 5-90°. 31P MAS NMR
was performed on a Varian Infinity plus-400 spectrometer and 51V NMR was done
by a Bruker Avance 300 MHz spectrometer.
General procedure for benzene hydroxylation
Benzene hydroxylation was performed for 2 h in a 50-mL flask placed in a water
bath set at 60 °C. Typically, 0.78 g (10 mmol) benzene and 0.207 g (0.1 mmol)
catalyst were added to 6 mL acetonitrile. To this solution, 2.2 g H2O2 (30%) was
added dropwise. A magnetic stirrer was used during the process. Quantification of
the products was done by (HPLC) instrument (Agilent-1100 series) equipped with
an Agilent ZORBAX Eclipse XDB-C8 column (4.6 mm 9 150 mm, 5 lm), which
was calibrated by an external standard method, and by a gas chromatography (GC)
instrument (Agilent 4890 series) equipped with an SE-30 capillary column.
Results and discussion
Hydroxylation of benzene to phenol by the prepared catalysts
The quaternary ammonium cations of vanadium-containing phosphomolydbates
catalyst could influence their catalytic activity significantly [25]. In this paper,
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attempts were made to synthesize some vanadium-containing phosphomolybdates
catalysts with the metal cations Cs?, K?, and Ca2? to determine whether metal
cations have the same effects as an ammonium one. Only CsxP4–xMo11VO40 catalyst
was obtained, but catalysts with K? and Ca2? cations were difficult to obtain
because of their good solubility in water. The CsxP4–xMo11VO40 catalyst was
efficient for benzene hydroxylation and resulted in a 12.2% phenol yield (Fig. 1a).
As a control, the activities of H-PMo11 and H-PW11 without metal substituted
were checked in hydroxylation of benzene to phenol (Fig. 1i, j). The results showed
their activities were very poor. Also, the activities of tetra-methyl ammonium
polyoxometalates substituted by transition metal V, Cu and Co were compared in
benzene hydroxylation. From the results of Fig. 1b, c, and d, the V-containing
polyoxometalates catalyst showed the best catalytic activity. The similar catalytic
activity order (V [ Cu [ Co) exhibited in TBA-PW11X (X = Cu, V, Co) series
(Fig. 1f, h, g). And for the V-containing phosphoxometalates catalysts with the
same cations, the catalytic activity of phosphomolydbates was better than that of
phosphotungstates.
Among the catalysts, tetramethyl ammonium 11-molybdo-1-vanadophosphate
was still the best one. The mechanism of action of the [(CH3)4N]4PMo11VO40
catalyst was discussed in a previous communication through designed experiments
[25]. Herein, the reaction factors on the hydroxylation of benzene with
[(CH3)4N]4PMo11VO40 as the catalyst, such as reaction times, reaction tempera-
tures, solvents and the amount of H2O2 used, were investigated in detail.
Fig. 1 Comparison of phenol yield over different catalysts. Reaction conditions: 10 mmol benzene,
0.1 mmol catalyst, 20 mmol H2O2 (30%), 6 mL acetonitrile, 60 °C water bath, 2 h. a CsxP4-xMo11VO40
,
b [(CH3)4N]4PMo11VO40, c [(CH3)4N]5PMo11CuO40, d [(CH3)4N]5PMo11CoO40, e [(C4H9)4N]4PMo11
VO40, f [(C4H9)4N]5PW11CuO40, g [(C4H9)4N]5PW11CoO40, h [(C4H9)4N]4PW11VO40, i H3PMo12O40,
j H3PW12O40, k H4PW11VO40
123

Transition metal substituted polyoxometalates
963
Table 1 Recycling reaction of the catalyst
Recycle time
Phenol yield (%)
Recovery efficiency (%)
0a
1b
2b
3b
4b
5b
a
12.4
12.2
12.4
12.1
12.1
12.1
86.4
86.3
84.4
85.4
82.8
83.5
Fresh reaction: 0.78 g (10 mmol) benzene, 0.208 g (0.1 mmol) catalyst, 2.26 g (20 mmol) H2O2
(30%), 6 mL acetonitrile at 60 °C water bath for 2 h
b
Recycle reaction: the amount of substrate, oxidant and solvent were calculated according to the ratio of
recovery efficiency
Recycling of the catalyst
Given that [(CH3)4N]4PMo11VO40 is a solid catalyst, it was recycled after its
recovery from a typical reaction. The recycling reaction was done five times to
observe the possibility of reusing the solid catalyst. This was based on a minimal
decrease of recovery efficiency after the fourth recycle in our previous work [28]. In
the fifth recycle, the yield was nearly the same, and the catalyst recovery efficiency
was 83.5%, without any obvious decrease (Table 1). The phenol yield reached about
12.0%, and about 85% of the catalyst (relative to the amount used in every recycle)
could be recovered. The recovered catalyst was also compared with the fresh
catalyst through IR spectroscopy, 31P MAS NMR spectroscopy, and XRD (Fig. 2).
The infrared spectra over the range of 700–1,100 cm-1 for the recovered and fresh
catalysts showed that both can retain four typical skeletal vibrations of the Keggin
oxoanion [32]. The 31P MAS NMR spectra of the catalysts exhibited a single peak at
-4.1 ppm, and the XRD pattern of the recovered catalyst was similar to that of the
fresh catalyst. These indicate that tetramethyl ammonium 11-molybdo-1-vanado-
phosphate stable and its activity was the same as that of the fresh catalyst.
Effect of reaction conditions
Effect of H2O2 amount
The effect of the H2O2 amount on phenol yield over [(CH3)4N]4PMo11VO40 is
shown in Fig. 3. Different concentrations of H2O2 were used. The phenol yield
increased considerably when the amount of H2O2 added was increased. The same
tendency resulted from reactions with 30% and 50% H2O2. When the ratio was
more than 2:1 (H2O2:benzene), the yield increased slowly, and a slight decrease was
observed at a ratio of 3:1 (H2O2:benzene). These results might be due to further
oxidation of phenol. According to the results, 30% H2O2 was the optimum amount
of oxidant for this reaction.
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J. Chen et al.
Fig. 2 Characterization of the catalyst with 31P MAS NMR spectroscopy, XRD, and FT-IR
spectroscopy. a Fresh [(CH3)4N]4PMo11VO40, b recovered catalyst A from the reaction
Fig. 3 Effect of H2O2 amount. Reaction conditions: 0.78 g (10 mmol) benzene, 0.208 g (0.1 mmol)
catalyst [(CH3)4N]4PMo11VO40, H2O2 (30 or 50%), 6 mL acetonitrile, 50 °C water bath, 2 h
Effect of reaction time
Figure 4 shows the behavior of the phenol yield with increasing reaction time. The
yield clearly increased at the beginning of the reaction, and it reached a maximum
123

Transition metal substituted polyoxometalates
965
Fig. 4 Influence of reaction time. Reaction conditions: 0.78 g (10 mmol) benzene, 0.208 g (0.1 mmol)
catalyst [(CH3)4N]4PMo11VO40, 2.26 g (20 mmol) H2O2 (30%), 6 mL acetonitrile, 60 °C water bath
Table 2 Comparison of
different solvents
Solvent
Phenol yield (%)
Ethyl acetate
Butyl acetate
1,2-dichloroethane
n-hexane
\1.0
\1.0
\1.0
\1.0
\1.0
\1.0
4.5
Methanol
Ethanol
Reaction conditions: 0.78 g
(10 mmol) benzene, 2.26 g
(20 mmol) H2O2 (30%), 0.208 g
(0.1 mmol)
Formic acid
Acetic acid
1,4-dioxane
Tetrahydrofuran
Acetonitrile
5.7
\1.0
3.4
[(CH3)4N]4PMo11VO40, 6 mL
solvent at 60 °C water bath for
2 h
12.4
value of 12.4% after 120 min. Then, it decreased—a behavior that might be due to
the deep oxidation of phenol.
Effect of solvents
To optimize the reaction catalyzed by [(CH3)4N]4PMo11VO40, the effect of reaction
temperature was investigated in our previous work [25]. The effect of different
solvents is discussed in this paper (Table 2). Acetonitrile was the best solvent,
resulting in a 12.4% phenol yield. Although acetic acid had been reported to be a
good solvent for oxidation reaction with H2O2, the use of this solvent for our
reaction was not satisfactory. Thus, a mixture of both acetonitrile and acetic acid
was tested. The reaction results are shown in Table 3. The catalytic activity slightly
improved with the use of the acetonitrile and acetic acid mixture, resulting in 13.0%
phenol yield and 90.6% selectivity. The recovery efficiency of the fresh catalyst also
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J. Chen et al.
Table 3 Reaction in a mixed solution of acetonitrile and acetic acid
Acetic acid:acetonitrile (g:g)
Yield (%)
Recovery efficiency (%)
1:3a
1:2
1:1
2:1
3:1
1:3b
13.0
12.5
12.1
11.7
10.9
14.7
90.6
88.5
89.0
85.6
86.0
79.3
Reaction conditions: 0.78 g (10 mmol) benzene, 2.26 g (20 mmol) H2O2 (30%), 0.208 g (0.1 mmol)
[(CH3)4N]4PMo11VO40, 4.8 g solvent at 60 °C water bath for 2 h
a
The selectivity of this reaction was 90.6%
b
0.2 g ascorbic acid was added
increased to 90.6%. To prevent phenol from forming deep oxidation product,
ascorbic acid was added to this reaction. The phenol yield improved to 14.7%, but
the recovery efficiency was much lower than the previous level. With the addition of
ascorbic acid, the system became more complicated and the reaction cost also
increased.
Conclusions
A series of polymetalates substituted with transition metals was prepared for the
direct hydroxylation of benzene to phenol. Compared with various catalysts, the
vanadium species was very crucial during the reaction. The cations of the catalysts
were also important. The catalytic behavior was further investigated through the
influence of various reaction parameters, such as solvents, reaction times, and so on.
Among the catalysts tested, [(CH3)4N]4PMo11VO40 gave the best results. It was
confirmed to have good stability after five recycling reactions. The highest phenol
yield was 13.0%, and the selectivity was 90.6% in acetic acid/acetonitrile (volume
ratio, 1:3).
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