The hydroxylation of benzene to phenol over heteropolyacid encapsulated in silica

Article abstract of DOI:10.1016/j.catcom.2014.06.014

The vanadium-substituted heteropoly acid (H₅PMo ₁₀V₂O₄₀ nH₂O) (PMoV₂) was encapsulated in silica as an immobilized catalyst (PMoV₂@SiO ₂) for the hydroxylation of benz

Full text of DOI:10.1016/j.catcom.2014.06.014

The Hydroxylation of Benzene to Phenol over Heteropolyacid Encapsulated
in Silica
Yan Li, Zhi Wang, Rizhi Chen, Yong Wang, Weihong Xing, Jun Wang,
Jun Huang
PII:
DOI:
Reference:
S1566-7367(14)00232-5
doi: 10.1016/j.catcom.2014.06.014
CATCOM 3939
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 March 2014
2 June 2014
16 June 2014
Please cite this article as: Yan Li, Zhi Wang, Rizhi Chen, Yong Wang, Weihong Xing,
Jun Wang, Jun Huang, The Hydroxylation of Benzene to Phenol over Heteropolyacid En-
capsulated in Silica, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.06.014
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ACCEPTED MANUSCRIPT
The Hydroxylation of Benzene to Phenol over Heteropolyacid
Encapsulated in Silica
Yan Li, Zhi Wang, Rizhi Chen, Yong Wang, Weihong Xing, Jun Wang and Jun Huang*
*
State Key Laboratory of Materials-Oriented Chemical Engineering，College of Chemistry
and Chemical Engineering，Nanjing Tech University, Nanjing 210009(China)，Fax: (+86)
25-83172261; E-mail: junhuang@njtech.edu.cn
1

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Abstract
The vanadium-substituted heteropoly acid (H PMo V O nH O) (PMoV ) was
5
10
2
40
2
2
encapsulated in silica as an immobilized catalyst (PMoV @SiO ) for the
2
2
hydroxylation of benzene to phenol with H O . The PMoV @SiO was
2
2
2
2
prepared by the one-pot hydrolysis of tetraethyl orthosilicate (TEOS) with
PMoV and Pluronic P123, and then the P123 was removed by pyrolysis at
2
o
5
00 C to give PMoV @SiO . The PMoV @SiO was found to be highly
2
2
2
2
porous with micropores, mesopores and macropores, and the PMoV @SiO
2
2
was highly active and selective for the hydroxylation of benzene under mild
conditions. Moreover, the catalyst can be reused at least 5 times without
obvious deactivation.
Keywords: Hydroxylation; Phenol; Vanadium; Heteropolyacid; Heterogeneous
2

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1
Introduction
As an important intermediate for synthesizing phenol resins, caprolactam,
fibers, and many other chemicals [1], phenol is industrially mainly produced by
the three-step cumene process with high energy consumption, low yields
(ca.5% based on the initial benzene), and environment pollution [2,3]. From
the viewpoints of economy and the environment, the one-step hydroxylation of
benzene to phenol is highly attractive [4] compared to the three-step cumene
process and other methods for the produce of phenol, such as nucleophilic
substitution of aryl halides and chlorination/sulfonation processes of benzene
[
5,6]. Recently, the oxidants for the direct hydroxylation of benzene to phenol
have been investigated extensively, including nitrous oxide [7,8], hydrogen
peroxide [4,9], molecular oxygen [2,10], as well as a mixture of oxygen and
hydrogen [1]. Among these oxidants, hydrogen peroxide is a wise choice since
it is cheap and efficient with nontoxic water as the only by-product.
Although the direct hydroxylation of benzene to phenol with hydrogen
peroxide in liquid phase has been explored extensively, the process is
associated with a poor selectivity, namely, a substantial overoxidation of
phenol occurs, forming various by-products, such as catechol, hydroquinone,
and benzoquinone [11]. The one-step hydroxylation of benzene to phenol with
H O is still an interesting topic, and efficient catalysts are the main research
2
2
objectives. Transition metal catalysts have been developed for the oxidation of
benzene with H O , including Ti, Fe, Cu and V [12-15], but the catalysts were
2
2
3

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limited in low selectivity or bad reusability. Tanev et al. [4] Investigated the
catalytic hydroxylation of benzene over Ti-MCM-41, which showed 68%
conversion and 98% selectivity towards phenol under mild reaction conditions.
Contrastingly, the conversion of benzene was about 31% with 95% selectivity
to phenol over titanium silicalite-1 (TS-1) for the hydroxylation of benzene with
hydrogen peroxide. Cu on MCM-41 was reported to be active for the
hydroxylation of benzene, which gave 21% conversion of benzene and 94%
selectivity to phenol [16]. Fe-MCM-41 was also found to be good catalyst for
the hydroxylation of benzene to phenol [17]. In addition, V-containing catalysts
were studied extensively since V was an active center for the hydroxylation of
benzene to phenol under mild conditions. Vanadium oxide on clay was
reported as catalyst for the hydroxylation of benzene to phenol with hydrogen
peroxide by Xu et al. [18], and 14% conversion and 94% phenol selectivity
were obtained. Vanadyl acetylacetonate complex grafted to periodic
mesoporous organosilica was reported as a catalyst for the hydroxylation of
benzene, which showed high selectivity towards the phenol and good
recyclability [19]. Keggin type phosphovanadomolybdic acid supported on
amine functionalized SBA-15 was developed and applied for the hydroxylation
of benzene with 20% conversion of benzene and 95% selectivity towards
phenol [20]. The drawback of the transition metal catalysts is the low selectivity,
and the heteropolyacid catalysts was limited in the reusability. Thus the
development of efficient and reusable catalysts is highly desirable for the
4

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hydroxylation of benzene to phenol.
Recently, immobilized heteropolyacid catalysts were developed and
applied for related reactions, and thus the catalysts were recycled easily [21,
2
2]. We had developed the polyoxometalate(POM)-ionic hybrid catalysts and
used as efficient and reusable catalysts for the hydroxylation of benzene,
which gave phenol in 31.4% with 95.8% selectivity [23]. Furthermore, the
encapsulated heteropolyacid (HPA) H PW O was reported to be highly
3
12 40
active for the esterification [22]. And here, we developed a new strategy for the
synthesizing the V-containing catalyst PMoV @SiO by hydrolyzation of
2
2
tetraethyl orthosilicate (TEOS) with Keggin type PMoV and nonionic (Pluronic
2
o
P123). And then the precatalyst was pyrolyzed at 500 C under Ar to give
PMoV @SiO catalyst. The catalyst PMoV @SiO was characterized by
2
2
2
2
Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), and N2
ad-desorption, and the PMoV @SiO was found to be highly active, selective
2
2
and reusable for the hydroxylation of benzene to phenol with hydrogen
peroxide.
2
2
Results and disscussions
.1 Catalyst characterization
PMoV @SiO was characterized by TF-IR, XRD and N ad-desorption. As
2
2
2
-1
shown in Fig. S1(a), the FT-IR bands at 1060, 960, 860, and 780 cm were
assigned to vas(P-O), vas(Mo=O) (terminal oxygen), v (Mo-O -Mo)
as
b
5

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(
corner-sharping oxygen), and v (Mo-O -Mo) (edge-sharping oxygen) for the
as
c
Keggin structures of neat PMoV . The four Keggin-structured FT-IR bands
2
were preserved for PMoV @SiO with broader FT-IR peaks, and shifted to
2
2
higher vibration frequencies with respect to PMoV . This comparison implies
2
that Keggin structures PMoV in PMoV @SiO was preserved and the local
2
2
2
environment of the Keggin unit is relatively affected by silica (siloxane and
silanol groups) [22]. In order to find more insights into the catalyst, solid-state
31
P-NMR was performed for both PMoV @SiO and PMoV (see Fig. S2). A
2
2
2
single peak at -0.5 ppm was observed for both PMoV @SiO and PMoV ,
2
2
2
which implied that the keggin structure of the PMoV was remained well in
2
PMoV @SiO . The N adsorption-desorption isotherm is shown in Fig. S3. The
2
2
2
2
-1
catalyst PMoV @SiO had a BET surface area of 177 m g . As seen in the Fig.
2
2
o
S3(a), the steep rise at P/P =0 indicated the existence of micropores, and the
o
hysteresis loop in the P/P range from 0.45 to 1.0 reflects plenty of mesopores
/
macropores. The pore distribution was presented in Fig. S3(b), which showed
irregular porous distribution with micro, meso and macropores. XRD patterns
of PMoV and PMoV @SiO are shown in Fig. S4. Pure PMoV presented a
2
2
2
2
set of diffraction peaks for the triclinic crystal structure of the Keggin-type HPA.
But no obvious peak was found in the catalyst PMoV @SiO , which indicates
2
2
PMoV was homogeneously disappeared in amorphous silica. Elemental
2
analysis (EA) was used to analyse the composition of PMoV @SiO and
2
2
PMoV @SiO -Air, and the results are as followed (PMoV @SiO : found C
2
2
2
2
6

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1
2.47 wt%, H 1.496 wt%; PMoV @SiO -Air: found C 0.13 wt%, H 0.697 wt%).
2
2
From the EA results, we found the chemical difference (12.47 wt% carbon
contained in PMoV @SiO ) between PMoV @SiO -Air with PMoV @SiO ,
2
2
2
2
2
2
and we can image that the PMoV @SiO was covered with a thin C film from
2
2
pyrolysis of P123, which was consistent with the literature results [24]. When
o
the PMoV @SiO was calcined again at 500 C under air, the carbon film was
2
2
combusted and the black PMoV @SiO turn to light yellow solid
2
2
PMoV @SiO -Air (Fig. S5). The content of vanadium in PMoV @SiO is
2
2
2
2
0
.0364 g V O /g (0.34 g PMoV /g) determined by ICP-AES.
2 5 2
2
.2 The screening of the catalysts
Catalysts were tested for the hydroxylation of benzene with H O , and the
2
2
results were showed in Table 1. We initially conducted the reaction without
catalyst or with the V-free phosphomolybdic acid (H PMo O ), and no phenol
3
12 40
was obtained (entries 1, 2). The hydroxylation of benzene with homogeneous
with single V-containing PMoV and double V-containing PMoV gave phenol in
2
1
2.5% and 25.0% yield respectively (entries 3,4), which proved that the V is
crucial and the double V-containing POM was more active for the hydroxylation
of benzene to phenol [25]. Similar yield (10.9% to 12.5%) of phenol was
obtained by heterogeneous PMoV@SiO catalyst compare to homogeneous
2
PMoV (entry 5). In addition, the hydroxylation of benzene with PMoV @SiO2
2
afforded phenol in high yield (21.6%) (entry 7). Interestingly, the catalyst
7

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PMoV @SiO -Air, which was calcined under air was not as active as
2
2
PMoV @SiO , and phenol was obtained in 17.6% (entry 6). As PMoV @SiO
2
2
2
2
was covered with a thin carbon film, which may be helpful for the hydroxylation
of benzene with H O . Additionally, activated carbon treated by H O was
2
2
2
2
reported to be active for the hydroxylation of benzene, which was consistent
with the results [26].
2
.2 Solvent effects
Solvents were tested for the hydroxylation of benzene, and the mixture of
acetonitrile and acetic acid was suitable solvent. The effects of volume ratios of
acetonitrile to acetic acid are summarized in Table 2. When pure acetonitrile
and pure acetic acid were used as the solvent respectively, the yields of the
phenol were low (16.7 % and 16.1 % respectively, Table 2, entries 1, 7). When
mixture of acetonitrile and acetic acid was used as the solvent (volume ratio
5
.5:0.5), the yield of phenol increased to 17.2 % (Table 2, entry 2). The
maximum yield of phenol was achieved in 21.6 % with volume ratio of
acetonitrile to acetic acid as 5 : 1 (Table 2, entry 3). The addition of more acetic
acid leaded to decreasing yield of phenol (Table 2, entries 4-6).The addition
of acidic acid can mix the two liquid phases (H O/CH CN) into one liquid phase,
2
3
and then enhance the contact frequency of the reactants (benzene and H O ).
2
2
On the other hand, the yield of phenol is affected by the decomposition of H O .
2
2
The weak acidic environment is beneficial for the hydroxylation of benzene [27].
However, the strong acidic environment can accelerate the decomposition of
8

ACCEPTED MANUSCRIPT
H O [28], which leads to the decrease of phenol yield.
2
2
2
.3 Catalyst amount
Catalyst amount was optimized for the PMoV @SiO catalyzed
2
2
hydroxylation of benzene, and the results are summarized in Fig. 1. As shown
in the Fig. 1, when the catalyst amount increased from 20 mg (0.008 mmol V)
to 150 mg (0.06 mmol V), the yields of phenol increased gradually from 3.7 %
to 21.6 %. However, when more catalyst added for the transformation, the
yield of phenol was decreased gradually, which may be related to the
decomposition of H O in the presence of more vanadium catalyst. Therefore,
2
2
1
50 mg (0.06 mmol V) was the optimized catalyst amount for the hydroxylation
of benzene to phenol with H O .
2
2
2
.4 Reaction temperature
Reaction temperature was studied for the hydroxylation of benzene with
H O on PMoV @SiO , and the result are presented in Fig. 2. As shown in Fig.
2
2
2
2
2
, the yield of phenol increased rapidly with the increase of temperature and
o
the yield reached to the maximum at 60 C. However, the yield of phenol
4+
decreased gradually with further increased temperature. The oxidation of V
to active species with H O is enhanced when the reaction temperature was
2
2
o
increased to about 60 C [29], and thus the yield of phenol was increased. But
when the reaction temperature was increased further, H O2 was
2
decomposition dramatically, and then the yield of phenol decreased.
2
.5 Hydrogen peroxide amount
9

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The decomposition of hydrogen peroxide is unavoidable in the
hydroxylation of benzene to phenol. In order to improve the utilization of
hydrogen peroxide, the amount of hydrogen peroxide was investigated for the
hydroxylation of benzene to phenol and the results are summarized in Fig. 3
The yield of phenol increased quickly from 6.1 % to 21.6 % as the molar ratio
of hydrogen peroxide to benzene was increased from 1 to 2.7, and decreased
slightly when more hydrogen peroxide was added. The addition of more
hydrogen peroxide resulted in overoxidation of phenol, and phenol was
obtained in decreased yield with some byproducts.
2
.6 Catalyst reusability
The PMoV @SiO catalyst is a heterogeneous catalyst and can be
2
2
recovered easily by filtration, the recyclability of PMoV @SiO was studied for
2
2
the hydroxylation of benzene with H O . The PMoV @SiO catalyst was used
2
2
2
2
and recycled for 5 times for the hydroxylation of benzene with with H O , and
2
2
the yields of phenol were 21.6 %, 21 %, 20.9 %, 20.6 % and 19.9 %,
respectively, as shown in Fig. 4. The selectivity toward phenol remained 100%
st
th
although the conversion of benzene was decreased slowly from 1 to the 5
time reaction. In order to test the leak of the catalyst (vanadium species), we
measured the content of the catalyst (vanadium species) recovered from the
th
5
time reaction by ICP-AES. The vanadium was leaked about 4.7% after 5
reaction cycles, and the catalytic performance was consistent with the Fig. 4.
The FT-IR spectra of the recovered catalyst after 5 times use were presented
10

ACCEPTED MANUSCRIPT
(
Fig. S1(c)), and the structure of PMoV @SiO did not changed, which meant
2
2
that maybe 5% of PMoV leaked. As PMoV is strongly acidic catalyst for the
2
2
hydrolysis of TEOS with P123, the PMoV was firstly encapsulated with silica
2
by hydrolysis of TEOS around PMoV . And then the encapsulated PMoV was
2
2
coated with a thin layer of carbon by the carbonization of the P123 to give the
catalyst PMoV @SiO . Although the silica is not stable enough in hydrothermal
2
2
condition, the PMoV @SiO catalyst was covered with a thin film of carbon,
2
2
which is beneficial for the reusability.
3
Conclusions
In conclusion, the vanadium-substituted heteropoly acid PMoV was
2
encapsulated in silica as an immobilized catalyst (PMoV @SiO ) for the
2
2
hydroxylation of benzene to phenol with H O . The Keggin-structured was
2
2
preserved in PMoV @SiO and the PMoV was dispersed homogeneously in
2
2
2
the silica with carbon thin film coated outside. Therefore, PMoV @SiO was
2
2
found to be a heterogeneous and reusable catalyst for the hydroxylation of
benzene. In addition, the PMoV @SiO catalyst was highly efficient for the
2
2
hydroxylation of benzene to phenol. Comparing to homogeneous PMoV2
catalyst, the PMoV @SiO catalyst exhibited similar activity (21.6%) and
2
2
higher selectivity (100%). The PMoV @SiO catalyst can be reused at least 5
2
2
times without obvious deactivation.
11

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Acknowledgements ((optional))
This work was supported by the National Natural Science of Foundation of
China (grant No. 21136005), the National High Technology Research and
Development Program of China (No. 2012AA03A606) and the Project of
Priority Academic Program Development of Jiangsu Higher Education
Institutions (PAPD).
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a
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Captions:
Fig. 1 The yield of phenol as a function of amount of catalyst used for the
hydroxylation of benzene with H O . Reaction conditions: benzene (1 mL,
2
2
11.22 mmol), hydrogen peroxide (30 wt%, 3 mL, 29.5 mmol), acetonitrile (5
o
mL), acetic acid (1 mL), 60 C, 6 h.
17

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Fig. 2 The optimization of reaction temperature for the hydroxylation of
benzene with H O . Reaction conditions: benzene (1 mL, 11.22 mmol),
2
2
hydrogen peroxide (30 wt%, 3 mL, 29.5 mmol), acetonitrile (5 mL), acetic acid
1 mL), catalyst (150 mg, 0.06 mmol V), 6 h.
(
18

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Fig. 3 The influence of molar ratio of H O to benzne on the yield of phenol in
2
2
the hydroxylation of benzene. Reaction conditions: benzene (1 mL, 11.22
mmol), acetonitrile (5 mL), acetic acid (1 mL), catalyst (150 mg, 0.06 mmol V),
o
6
h, 60 C.
19

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Fig. 4 Catalytic reusability of PMoV @SiO for hydroxylation of benzene to
2
2
phenol with H O . Reaction conditions: benzene (1 mL, 11.22 mmol), hydrogen
2
2
peroxide (30 wt%, 3mL, 29.5 mmol), acetonitrile (5 mL), acetic acid (1 mL),
o
catalyst (150 mg, 0.06 mmol V), 6 h, 60 C
Table 1: The screening of catalysts for the hydroxylation of benzene to phenol
a
2
with H O
2
b
c
d
Entry Catalysts
Con. (%)
Y. (%)
Sel. (%)
1
2
3
none
0
0
-
-
H PMo O
3
0
0
12 40
e
d
PMoV
13.9
12.5
90
20

ACCEPTED MANUSCRIPT
f
d
d
4
5
6
PMoV2
26.3
11.8
25.0
10.9
17.6
21.6
95
92
PMoV@SiO2
PMoV @SiO -Air 17.6
100
2
2
7
PMoV @SiO
21.6
100
2
2
a
Reaction conditions: catalyst (150 mg, 0.06mmol V), benzene (1 mL, 11.22
mmol), acetonitrile (5 mL), acetic acid (1 mL), H O (30 wt%, 3 mL, 29.5 mmol),
2
2
o
b
c
d
6
0 C, 6 h; Conversion of benzene; Yield to phenol; Selectivity for phenol,
e
hydroquinone and benzoquinone as the by-products; PMoV (100 mg, 0.06
f
mmol V); PMoV (50 mg, 0.06 mmol V).
2
Table 2:. The optimization of the solvent mixture (volume ratio of acetonitrile to
a
2
acetic acid) for the hydroxylation of benzene to phenol with H O
2
b
c
Entry Acetonitrile / acetic acid
Con. (%)
Y. (%)
Sel. (%)
100
1
2
3
4
5
6
6/0
16.7
17.2
21.6
14.3
14.7
15.1
16.1
16.7
17.2
21.6
14.3
14.7
14.8
15.3
5.5/0.5
5/1
100
100
4.5/1.5
3/3
100
100
d
2:4
98
d
7
0/6
95
a
Reaction conditions: benzene (1 mL, 11.22 mmol), solvent (6 mL), hydrogen
21

ACCEPTED MANUSCRIPT
o
peroxide (30 wt%, 3 mL, 29.5 mmol), catalyst (150 mg, 0.06 mmol V), 60 C, 6
b
c
d
h.
Conversion of benzene;
Yield to phenol;
Selectivity to phenol,
hydroquinone and benzoquinone as the by-products.
22

ACCEPTED MANUSCRIPT
Graphical Abstract:
The Hydroxylation of Benzene to Phenol over Heteropolyacid
Encapsulated in Silica
Yan Li, Zhi Wang, Rizhi Chen, Yong Wang, Weihong Xing, Jun Wang and Jun Huang*
*
State Key Laboratory of Materials-Oriented Chemical Engineering，College of Chemistry
and Chemical Engineering，Nanjing Tech University, Nanjing 210009(China)，Fax: (+86)
5-83172261; E-mail: junhuang@njtech.edu.cn
2
Graphical Abstract
23

ACCEPTED MANUSCRIPT
Research Highlights

The vanadium-substituted heteropoly acid (H PMoV O nH O) PMoV
5 2 40 2 2
was encapsulated in silica with preserved Keggin-structured
PMoV @SiO ).
(
2
2



The PMoV found to be dispersed homogeneously in the silica coated with
2
thin carbon film in PMoV @SiO .
2
2
The PMoV @SiO was highly active and selective for the hydroxylation of
2
2
benzene.
The PMoV @SiO was heterogeneous and reusable.
2
2
24