Carboxylic acid-functionalized phosphovanadomolybdate-paired ionic polymer as a green heterogeneous catalyst for hydroxylation of benzene

Article abstract of DOI:10.1016/j.catcom.2013.06.004

A novel PMoV-paired ionic polymer was prepared by anion-exchange of a newly task-specific designed carboxylic acid-functionalized ionic polymer with a Keggin V-containing heteropolyacid, and characterized by FT-IR, SEM, TG, XRD, ¹H NMR, and CNH

Full text of DOI:10.1016/j.catcom.2013.06.004

Carboxylic acid-functionalized phosphovanadomolybdate-paired ionic poly-
mer as a green heterogeneous catalyst for hydroxylation of benzene
Yan Leng, Jian Liu, Pingping Jiang, Jun Wang
PII:
DOI:
Reference:
S1566-7367(13)00209-4
doi: 10.1016/j.catcom.2013.06.004
CATCOM 3525
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
7 April 2013
9 May 2013
7 June 2013
Please cite this article as: Yan Leng, Jian Liu, Pingping Jiang, Jun Wang, Carboxylic
acid-functionalized phosphovanadomolybdate-paired ionic polymer as a green hetero-
geneous catalyst for hydroxylation of benzene, Catalysis Communications (2013), doi:
1
0.1016/j.catcom.2013.06.004
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ACCEPTED MANUSCRIPT
Carboxylic acid-functionalized phosphovanadomolybdate-paired ionic polymer
as a green heterogeneous catalyst for hydroxylation of benzene
a
a
a
b
Yan Leng, * Jian Liu, Pingping Jiang, and Jun Wang
a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi 214122, China
b
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical
Engineering, Nanjing University of Technology, Nanjing 210009, China;
*
Corresponding author, Tel: +86-510-85917090, Fax: +86-510-85917763, E-mail: lengyan1114@126.com
Abstract: A novel PMoV-paired ionic polymer was prepared by anion-exchange of a newly
task-specific designed carboxylic acid-functionalized ionic polymer with a Keggin V-containing
1
heteropolyacid, and characterized by FT-IR, SEM, TG, XRD, H NMR, and CNH elemental
analysis. Its catalytic performance for hydroxylation of benzene with H O was studied using
2
2
CH CN as solvent. The obtained polymer is revealed to be a highly efficient heterogeneous
3
catalyst for hydroxylation of benzene, showing a high yield to phenol 26.9% with 100% selectivity.
The solid catalyst has a convenient recoverability with a considerable catalyst recovery rate of
above 90%, and the reused catalyst in the fourth run showed a phenol yield of 17.8%.
Keywords: Poly(ionic liquid); Heteropolyacids; Hydroxylation of benzene; Heterogeneous
catalysis; Phenol
1
Introduction
Phenol is one of the most important basic chemicals. Industrially, it is produced by the
three-step cumene process with high energy consumption and environmental pollution [1].
Therefore, many potential attempts have been made for the one-step hydroxylation of benzene to
phenol using different oxidants, such as N O [2], O [3], H O [4], and the H -O system with the
2
2
2
2
2
2
Pd-based composite membrane reactors [5]. H O has a clear advantage because of its high
2
2
availability, nontoxic by-product and mild reaction conditions for hydroxylation of benzene,
where metal oxides [4,6,7], molecular sieves (e.g. TS-1 and V-MCM-41) [8], schiff base metal
[9,10], and polyoxometalates (POMs) [11] have been extensively investigated as the catalysts.
Significant advance in this area is the use of periodic mesoporous organosilica-grafted vanadyl
1

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complex as the heterogeneous catalyst for hydroxylation of benzene with H O [12]. Thus far,
2
2
developing highly efficient and recyclable catalysts for green hydroxylation of benzene with H O2
2
remains an interesting and challenging task.
As well-known, heteropolyacids (HPAs) are a class of well-defined transition metal oxygen
clusters with structural diversity. Because of the rich redox chemistry, HPAs have been widely
used as the catalysts for organic oxidations [13], among which V-containing HPAs have been
proved to be very effective for the hydroxylation of benzene with H O [14,15]. However, the
2
2
catalyst’s isolation is very difficult due to HPAs’ good dissolvability in the polar solvents. For
convenient catalyst recovery, the immobilization of HPAs onto porous carriers becomes the
commonly used strategy to heterogenize the HPAs catalysts [16]. However, this approach always
suffers from the slow reaction rate, and/or leaching of active sites. Another strategy for
heterogeneous HPA catalysts is the combination of HPA anions with organic groups, through
which, HPAs’ polarity, solubility, and redox property could be modified [17,18]. In this context,
some HPA-based organic-inorganic hybrids have been prepared and showed very interesting
catalytic properties in hydroxylation of benzene [19,20]. Actually, IL-cations with versatile
structures can be used as organic blocks to modify HPA for task-specific ionic hybrid catalysts
[21,22]. For example, nitrile-functionalized pyridinium phosphovanadomolybdate was revealed to
be a reaction-controlled phase-transfer catalyst for hydroxylation of benzene with H O [23].
2
2
Recently, we have prepared a HPA-based ionic hybrid by combining dicationic IL cations with
HPA anions for heterogeneous hydroxylation of benzene [24]. Although these catalytic systems
give an improved performance, the reactions still should be carried out in acidic solvent arming to
get high conversion and selectivity, which can easily cause corrosion and environmental pollution.
It is worthwhile to note that Poly(ionic liquids) (PILs) have been used as novel catalysts for
more and more organic syntheses with advantageous of enhanced stability and improved
processability [25-27]. Moreover, the acidity of the catalytic system is demonstrated to be a key
influential factor for enhancing the catalytic activity of HPA in hydroxylation of benzene
[12,28,29]. Accordingly, it is rational to design and prepare an efficient acid-functionalized
HPA-paired ionic polymer catalyst for heterogeneous hydroxylation of benzene with H O in the
2
2
absence of acidic solvent. In this work, we synthesize a new HPA-paired ionic polymer catalyst
2

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Poly(VMCA)-PMoV by pairing the carboxylic acid-functionalized IL polymer with V-containing
Keggin HPA (H PMo V O ) (Scheme 1). The thus obtained solid catalyst is revealed to be a
5
10
2
40
green solid catalyst for the heterogeneous hydroxylation of benzene with H O in CH CN solvent,
2
2
3
showing high catalytic activity and good recyclability.
*
*
*
n
*
COOH
5
Br
N
N
N
N
methanol AIBN
70 oC 24h
5 10 2 40
H PMo V O
5-
PMo10V O
40
ethanol
oC 48 h
0
2
N
N
N
N
8
-
Br
-
Br
HOOC
HOOC
HOOC
VMCA
Poly(VMCA)
Poly(VMCA)-PMoV
Scheme 1. Synthesis of the acidic polymer catalyst Poly(VMCA)-PMoV.
. Experimental
2
2.1 Catalyst preparation
Materials and characterization techniques are shown in Supplementary Material (SM).
H PMo V O (PMoV) was prepared according to our previous literature [20], and the
5
10
2
40
synthetic steps for PMoV-based polymer Poly(VMCA)-PMoV (VMCA: [1-vinyl-3-propane
1
carboxylation imidazolium]Br) are shown in SM. H NMR (300 MHz, D O, TMS) for VMCA δ
2
(ppm) = 3.01 (t, 2H, -CH ), 4.72 (t, 2H, -CH ), 5.45 (d, 1H, -CH), 5.81 (d, 1H, -CH), 7.17 (m, 1H,
2 2
-CH), 7.63 (s, 1H, -CH), 7.79 (s, 1H, -CH), 9.12 (s, 1H, -CH) (see Fig S1 in SM).
2.2 Catalytic test
Benzene (10 mmol), acetonitrile (6 mL), and Poly(VMCA)-PMoV (0.1 g) were added to a 25
mL flask. The reaction started after the addition of aqueous H O (30%, 30 mmol) within 30 min
2
2
at 343 K with vigorous stirring, and then, the solution was stirred for another 4 h at 343 K
Scheme 2). After reaction, the reaction mixture was analyzed by the gas chromatography (GC,
SP-6890A) equipped with a FID detector and a capillary column (SE-54 30m×0.32mm×0.25µm).
(
-13
The precision of column temperature is about：±0.1℃, the detection sensitivity M ≤ 1×10
g/sec(benzene), and the minimum detection limit of catechol and hydroquinone is about: ≥ 10
mg/L. 1,4-dioxane was used as the internal standard to calculate the yield of phenol. Yield of
phenol (based on benzene) = mmol phenol/mmol initial benzene. Selective conversion of H O =
2
2
mmol phenol/mmol initial H O . Under the employed conditions, only phenol was detected as the
2
2
3

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product, without finding any GC signals for catechol, hydroquinone and benzoquinone, i.e., the
selectivity of phenol over this catalyst is 100%.
OH
Cat: Poly(VMCA)-PMoV
+
H O
+ H O
2
2
2
CH CN 70°C 4 h
3
Scheme 2. Hydroxylation of benzene catalyzed by Poly(VMCA)-PMoV.
3. Results and discussion
3.1 Characterizations of Poly(VMCA)-PMoV
The XRD pattern (see Fig S2 in SM) for neat H PMo V O displayed the featured diffraction
5
10
2
40
peaks for a HPA crystal, indicating an excellent long range order of the Keggin anions. However,
for the polymer Poly(VMCA)-PMoV, most of these peaks disappeared, this phenomenon
demonstrates the loss of the long-range crystal order for the hybrid, mostly due to the
rearrangement of the Keggin anions during the self-assembly of heteropolyanions and polymeric
cations. Moreover, the profile of the Poly(VMCA)-PMoV in the SEM image (see Fig S3 in SM)
also confirms an amorphous structure.
The thermal stabilities of Poly(VMCA) and Poly(VMCA)-PMoV were investigated by TG
analyses in Fig 1. The decomposition temperature of Poly(VMCA)-PMoV was about 300 C, that
is remarkably higher than 220 C for Poly(VMCA). This fact indicates that the combination of
Poly(VMCA) with PMoV led to an ionic polymer, but not a physical mixture. The drastic weight
loss of nearly 28% in the temperature range 300-580C corresponds to the thermal decomposition
of the organic moiety Poly(VMCA) in the catalyst. The elemental analysis for
Poly(VMCA)-PMoV found C 18.13%, N 5.01%, and H 2.17% by weight percentage, which is in
well agreement with the calculated value C 18.71%, N 5.45%, and H 2.16%.
Fig. 2 compares the FT-IR spectrum of Poly(VMCA)-PMoV with those of Poly(VMCA) and
neat H PMo V O . The four characteristic bands for the Keggin structure of H PMo V O
5
10
2
40
5
10
2
40
−
1
appeared at 1061 (P-O ), 960 (Mo=O ), 864 (Mo-O -Mo) and 783 (Mo-O -Mo) cm , respectively.
a
d
b
c
For the Poly(VMCA)-PMoV, the four peaks appeared distinctively, with the simultaneous
-1
occurrence of the new characteristic bands at 1569, 1169, and 1715 cm that are attributed to C=N
4

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and C-N stretching, and C=O bending vibrations, respectively, in the polymeric cations. These
features allow to draw that both organic and inorganic moieties of Poly(VMCA)-PMoV retain the
original structures of their parents. The slight shifts of the Keggin band for Poly(VMCA)-PMoV
compared with H PMo V O are resulted from the strong ionic interaction between polymer
5
10
2
40
cations and heteropolyanions. All the above results verify the rationality of the chemical
composition of Poly(VMCA)-PMoV illustrated in Scheme 1.
3
.2 Hydroxylation of benzene with H O over Poly(VMCA)-PMoV and various control
2 2
catalysts
It is known that the solvent can significantly affect the catalytic activity of the hydroxylation of
benzene, and a mixed solvent of acetonitrile and acetic acid might help for the phenol formation
[27-29]. Accordingly, we first carried out the hydroxylation of benzene in the presence of
CH CN/CH COOH (volume ratio 1:1) solvent over various catalysts, with results listed in Table 1.
3
3
In all cases phenol was detected as the only product without detecting diphenols and
benzoquinone. The V-free H PMo O catalyst was inactive (entry 1). H PMo V O exhibited a
3
12 40
5
10
2
40
high yield of phenol 27.5% (entry 2), which consistent with the proposal that the V ions in
HPA-anions are catalytically active centers for hydroxylation of benzene to phenol [11,30].
However, H PMo V O caused a homogeneous reaction system that the catalyst recovery and
5
10
2
40
reuse is difficult. The ionic hybrid [Bmim] PMoV (Bmim: 1-methyl-3-butyl imidazole) gave
5
27.7% yield, but also cause a homogeneous catalysis (entry 3). In contrast, the
COOH-functionalized polymer Poly(VMCA)-PMoV led to a liquid-solid heterogeneous reaction
system and presented a similar phenol yield 27.3% (entry 4). This result indicates that it is the
featured structure of the polymeric IL cation that endows the Poly(VMCA)-PMoV as an active
insoluble catalyst for hydroxylation of benzene with H O .
2
2
As Poly(VMCA)-PMoV is a good heterogeneous catalyst for hydroxylation of benzene with
H O using mixed CH CN/CH COOH as a solvent. However, there is already a COOH
2
2
3
3
component in Poly(VMCA)-PMoV, and it is thus reasonable to consider that the
COOH-containing organic moiety therein might function as an acidic solvent. Accordingly, we
further attempted the hydroxylation of benzene over Poly(VMCA)-PMoV in the absence of
CH COOH. As expected, Poly(VMCA)-PMoV still behaved as heterogeneous catalyst and gave a
3
5

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high phenol yield of 26.9% (entry 6), which is at a similarly level to that with CH CN/CH COOH
3
3
as the solvent. The selective conversion of H O for the hydroxylation of benzene over catalyst
2
2
Poly(VMCA)-PMoV is 8.97%, and the benzene remained in the reaction mixture was about
9.5%. Influences of reaction conditions on the phenol yields in hydroxylation of benzene with
H O over the catalyst Poly(VMCA)-PMoV are shown in SM. However, the homogeneous
6
2
2
H PMo V O and [Bmim] PMoV caused much lower catalytic activities (entries 7 and 8).
5
10
2
40
5
Additionally, our previous report [24] revealed that a HPA-based dicationic ionic hybrid
[
Dmim] PMoV (Dmim: 1,1’-(butane-1,4-diyl)-bis(3-methylimidazolium)) exhibit high yield
2.5
26.5% of phenol for hydroxylation of benzene in a heterogeneous catalysis system with
CH CN/CH COOH solvent (entry 5). Nevertheless, when using CH CN as the solvent, it only
3
3
3
showed a low phenol yield 17.2% (entry 9).
The above comparisons seem to suggest a “solvent role” of the COOH-containing organic PIL
in the absence of CH COOH solvent, which may account for the high activity and selectivity of
3
Poly(VMCA)-PMoV catalyst for hydroxylation of benzene. On the other hand, it has been
reported that the appropriate Brønsted acidic sites in the catalyst are favorable for hydroxylation of
benzene to phenol through the electrostatic attraction of weakly basic benzene molecular onto the
catalyst [12]. The framework of the present polymer catalyst is Brønsted acidic on account of the -
COOH groups, we thus rationally assume that weakly basic benzene can be easily adsorbed onto
the acidic framework of the polymer catalyst Poly(VMCA)-PMoV. Once benzene is converted
into phenol, an acidic molecule, within the framework, acidic phenol is desorbed immediately
from the acidic polymeric framework, and thus preventing further formation of over oxidized
products.
3.3 Reusability of Poly(VMCA)-PMoV for hydroxylation of benzene
After reaction, the solid catalyst could be easily recovered by filtration with a high recovery
ration of above 90%, and directly reused for the next run (Figure 3). Poly(VMCA)-PMoV presents
a consistent 100% selectivity over four catalytic cycles, however, the yield of phenol decreased
slowly over the reused catalysts. The reused catalyst in the fourth run showed a yield of 17.8%.
The profile of IR spectra for the recovered catalyst after the fourth run (Fig 2d) is similar to that of
6

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the fresh one without found any bands assigned to the hydroxylated organic groups, which
demonstrates a quite stable structure of this hybrid catalyst. However, the intensities of the four IR
bands for the Keggin structure of PMoV decreased to some extent. Moreover, the result of
ICP-AES analysis for the reacted filtrate showed that about 7.6 wt.% Mo of Poly(VMCA)-PMoV
has been leached into the reaction media. These observations indicate that the decrease in catalytic
activity over the recycled catalyst is possibly due to the leaching of the PMoV anions during the
reaction. To increase the catalyst’s reusability, further research is still required.
4.
Conclusion
In summary, we have developed a novel HPA-paired –COOH-containing ionic polymer
catalyst Poly(VMCA)-PMoV by pairing –COOH-functionalized polymeric IL cations with
Keggin V-containing HPA-anions. The hydroxylation of benzene with H O2 by the
2
Poly(VMCA)-PMoV is an environmentally friendly catalytic process, as the incorporation of the
homogeneous PMoV anions and –COOH groups onto the ionic polymeric framework not only
affords high efficiency and recyclability of the catalyst, but also avoids the use of acidic solvents.
Acknowledgements
The authors thank the financial support from the Programme of National Natural Science
Foundation of China (Nos. 21206052 and 21136005).
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a
Table 1. Catalytic performance of various catalysts for the hydroxylation of benzene with H O
2
2
Yield of
Entry
Catalyst
Solvent
Solubility in reaction
phenol (%)
—
1
2
3
4
5
6
7
8
9
H PMo O
CH CN/CH COOH
Soluble
Soluble
3
12 40
3
3
H PMo V O
CH CN/CH COOH
27.5
5
10
2
40
3
3
[Bmim] PMoV
CH CN/CH COOH
Soluble
27.7
5
3
3
PVAM-PMoV
CH CN/CH COOH
Insoluble
Insoluble
Soluble
27.3
3
3
[Dmim] PMoV
CH CN/CH COOH
26.5
2.5
3
3
H PMo V O
CH CN
18.1
5
10
2
40
3
[Bmim] PMoV
CH CN
Soluble
18.3
5
3
PVAM-PMoV
CH CN
Insoluble
Insoluble
26.9
3
[Dmim] PMoV
CH CN
17.2
2.5
3
a
Reaction conditions: catalyst (0.1 g), benzene (10 mmol), solvent (6 mL, CH CN/CH COOH:
3
3
o
volume ratio 1:1), 30% H O (30 mmol), 70 C, 4 h.
2
2
9

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Figure captions
Fig. 1. TG curve of (a) Poly(VMCA)-PMoV and (b) Poly(VMCA).
Fig. 2. FT-IR spectra of (a) Poly(VMCA), (b) H PMo V O , (c) fresh Poly(VMCA)-PMoV, and
5
10
2
40
(d) recovered Poly(VMCA)-PMoV.
Fig. 3. Catalytic reusability of Poly(VMCA)-PMoV for the hydroxylation of benzene with H O .
2
2
10

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1
00
8
6
4
2
0
0
0
0
(a)
(b)
0
1
00
200
300
400
500
600
700
o
Temperature ( C)
Fig. 1. TG curve of (a) Poly(VMCA)-PMoV and (b) Poly(VMCA).
11

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(
a)
b)
(
1
402
960
1
061
8
64
1
715
7
83
(c)
1055
9
47
8
72
3
480
7
93
(d)
4
000 3500 3000
1500
1000
500
-
1
Wavenumbers (cm )
Fig. 2. FT-IR spectra of (a) Poly(VMCA), (b) H PMo V O , (c) fresh Poly(VMCA)-PMoV, and
5
10
2
40
(d) recovered Poly(VMCA)-PMoV.
12

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2
2
1
1
5
0
5
0
5
0
1
2
3
4
Reaction run
Fig. 3. Catalytic reusability of Poly(VMCA)-PMoV for the hydroxylation of benzene with H O .
2
2
13

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Graphical Abstract
A novel PMoV-paired ionic polymer was prepared by anion-exchange of a newly task-specific
designed carboxylic acid-functionalized ionic polymer with a Keggin V-containing heteropolyacid.
The polymer was revealed to be a highly active and selective solid catalyst for hydroxylation of
benzene with H O in the absence of acidic solvent, and could be easily recovered and reused.
2
2
14

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Highlights
>
>
A new polymer is prepared with acidic poly(ionic liquid) and heteropolyacid.
The polymer catalyst leads to heterogeneous hydroxylation of benzene with
H O .
2
2
>
>
The catalyst exhibits high conversion and selectivity in the absence of acidic
solvent.
The catalyst could be simply recovered by filtration and reused.
15