Novel piperaziniumpolyoxometalate-based acidic ionic liquids as highly efficient catalysts for the selective oxidation of alcohols

Article abstract of DOI:10.1016/j.catcom.2019.105748

Two novel heteropolyanion-based Br?nsted Acidic Ionic Liquids (BAILs), butane mono and bis sulfoacid-functionalized 1,4-dimethylpiperazinium salts of phosphotungstate catalyst ([PipBs₁]₃-PW and [PipBs₂]₃-(PW)₂) were synthesized and well characterized with FTIR, ¹H and ¹³C NMR, Electro-Spray Ionization Mass Spectrometry (ESI-MS), Elemental Analysis, EDX, BET and TGA techniques. Selective oxidation of benzyl alcohol with 35% aqueous hydrogen peroxide under solvent-free conditions was carried out using these catalysts without adding any phase transfer catalyst. The introduced catalysts presented a self-separation performance after reaction, which can be easily recovered and quite steadily reused as confirmed by six-run recycling tests. Moreover, bis sulfoacid-functionalized 1,4-dimethylpiperazinium salt of phosphortungstate showed the best catalytic performance among the prepared catalysts for the oxidation reaction.

Full text of DOI:10.1016/j.catcom.2019.105748

Accepted Manuscript
Novel piperaziniumpolyoxometalate-based acidic ionic liquids as
highly efficient catalysts for the selective oxidation of alcohols
Mosadegh Keshavarz, Masoumeh Tabatabaee, Maryam Shahabi
PII:
S1566-7367(19)30210-9
https://doi.org/10.1016/j.catcom.2019.105748
105748
DOI:
Article Number:
Reference:
CATCOM 105748
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
21 February 2019
28 June 2019
28 June 2019
Please cite this article as: M. Keshavarz, M. Tabatabaee and M. Shahabi, Novel
piperaziniumpolyoxometalate-based acidic ionic liquids as highly efficient catalysts for
the selective oxidation of alcohols, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.105748
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ACCEPTED MANUSCRIPT
Novel piperaziniumpolyoxometalate-based acidic ionic liquids as highly
efficient catalysts for the selective oxidation of alcohols
1

2
2
Mosadegh Keshavarz , Masoumeh Tabatabaee , Maryam Shahabi
1
2
Department of Chemistry, Faculty Gas and Petroleum, Yasouj University, Gachsaran, Iran
Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran
Abstract
Two novel heteropolyanion-based Brønsted Acidic Ionic Liquids (BAILs), butane mono and bis
sulfoacid-functionalized 1,4-dimethylpiperazinium salts of phosphotungstate catalyst ([PipBs
1
]
3
-PW and
1
13
[
PipBs ] -(PW) ) were synthesized and well characterized with FTIR, H and C NMR, Electro-Spray
2
3
2
Ionization Mass Spectrometry (ESI-MS), Elemental Analysis, EDX, BET and TGA techniques. Selective
oxidation of benzyl alcohol with 35% aqueous hydrogen peroxide under solvent-free conditions was
carried out using these catalysts without adding any phase transfer catalyst. The introduced catalysts
presented a self-separation performance after reaction, which can be easily recovered and quite steadily
reused as confirmed by six-run recycling tests. Moreover, bis sulfoacid-functionalized 1,4-
dimethylpiperazinium salt of phosphortungstate showed the best catalytic performance among the
prepared catalysts for the oxidation reaction.
Keywords: 1,4-dimethylpiperazine; 1,4-Butane Sultone; Heteropoly Acid; Ionic Liquid; Catalyst;
Reusable Catalyst; Oxidation of Alcohols.
1
. Introduction

Mosadegh Keshavarz; mail: chem.mosadegh@gmail.com; Zip-code: 75918-74831
1

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Selective oxidation of alcohols to their corresponding aldehyde compounds is one of the most
important functional group transformations in organic chemistry and is commonly used in academic and
industrial procedures. Recently, several catalysts have been reported for this transformation [1-5].
Heteropolyacids (HPAs) are complex protic acids with self-specific structural and physicochemical
properties [6–8]. HPAs are highly soluble in polar solvents, and their ultra-strong Brønsted acidity
classifies them as solid superacids [9]. HPAs have versatile properties such as strong acidity, good
thermal stability, and excellent oxidizing capability. These properties together with the stable and
relatively nontoxic characteristics rendering HPA widely appreciated as environmentally benign catalysts
for potential practical replacement of hazardous liquid acids conventionally used in industrial processes.
HPAs have revealed high capability for the catalytic oxidation of alcohols using H
2
2
O as an oxidant.
However, the unmodified HPAs as catalysts suffer from several disadvantages such as poor solubility,
low selectivity and difficulty in separation and non-recycling procedures. Moreover, if used in non-polar
medium, the HPAs are marginally active and not soluble due to the low accessibility of their active sites.
However, bulk HPAs are soluble in polar media that make their recovery a difficult task. In order to avoid
these drawbacks, different approaches were attempted to heterogenize HPAs, namely: impregnation into
porous supports [10,11], entrapment in solid matrices via sol–gel [12], grafting on functionalized surfaces
[
13], and so forth. Unfortunately, the use of supported HPA catalysts is not recommended in polar
solvents (e.g., H O or HOAc) because of their high solubility, which in turn leads to leaching of HPA
2
from the solid support and compromises the above mentioned advantages of supported HPA catalyst [14].
Thus, to facilitate easy separation of products or recovery and recycling of the HPA catalyst, an
innovative design of a homogeneous catalyst that does not leach in polar media, while being well
dispersed in non-polar media, still remains a challenge and would be desirable.
Ion-pair immobilization is a versatile way to keep the catalyst and support tightly together and modify
the catalytic activity [15,16]. One of the best approaches to improve its catalytic performance and
recovery efficiency is the modification of HPAs by organic molecules to produce different structures and
functions. HPAs have multi-electron structures and can be modified by ion-exchange electrostatic
interactions [17-19]. However, these POM hybrid materials still exhibit some drawbacks, including low
2
-1
BET specific surface area (m g ) and small pore diameter of the catalysts [20]. Both factors result in
insufficient contact between catalyst and reactants, reducing the catalytic efficiency. Therefore, it is
necessary to develop new strategies to design POM hybrid catalysts with high BET specific surface area,
large pore diameter and high recyclability.
2

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To reach this goal, a new family of Brønsted acidic ionic liquids (BAILs), which is composed of
polyoxometalate (POM) anions and appropriate organic cations, has attracted intense interest. Until today,
a series of HPA-IL catalysts have been prepared and their catalytic activities have been investigated in
various organic transformations [21-26].
Herein, we report the preparation, characterization and investigation of the catalytic activity of a new
series of ILs-cation phosphotungstate based on 1,4-dimethyl piperazine as catalyst, for the selective
oxidation of alcohols with hydrogen peroxide under solvent-free conditions.
2
. Experimental
2
.1.1. Preparation of 1,4-dimethylPiperazinium-Butane sulfonate mono-linkage (PipBs )
1
A round-bottom flask containing a magnet was charged with 1,4-dimethylpiperazine (10 mmol) and
,4-butane sultone (15 mmol) and stirred for 4 h under nitrogen atmosphere at room temperature. The
resulting white precipitate was filtered and washed with diethyl ether and then dried in vacuum, and the
1
resulting solid was named 1,4-dimethylPiperazinium-Butane sulfonate mono-linkage (PipBs
yield of white solid product was obtained.
1
). A 95%
1
13
The spectral H NMR, C NMR and elemental analysis data for the 1,4-dimethylPiperazinium-
1
Butane sulfonate mono-linkage (PipBs
1
) were as follows: H NMR (400 MHz, D
2
O, TMS): δ 1.65-1.80
(
m, 2H), 1.81-1.97 (m, 2H), 2.28 (s, 3H), 2.66-2.86 (m, 4H), 2.88 (t, 3H, J= 6.8 Hz), 3.02 (s, 3H), 3.20-
1
3
3
.50 (m, 6 H). C NMR (400 MHz, D
2
O): δ 20.3, 22.1, 31.20, 46.7, 50.5, 54.0, 56.5, 58.1. Anal. Calcd
for C10
H
22
N
2
O
3
S:C, 47.98; H, 8.86; N, 11.19; S, 12.81. Found: 47.95; H, 8.90; N, 11.21; S, 12.83.
2
.1.2. Preparation of 1,4-dimethylPiperazinium-Butane sulfonate double-linkage (PipBs
A round-bottom flask containing a magnet was charged with 1,4-dimethylpiperazine (10 mmol) and
,4-butane sultone (30 mmol) and stirred for 2 h under nitrogen atmosphere at ꢀꢁ ꢃ. The resulting white
precipitate was filtered and washed with diethyl ether and then dried in vacuum, and the resulting solid
2
)
1
ꢂ
was named 1,4-dimethylPiperazinium-Butane sulfonate double-linkage (PipBs
solid product was obtained.
2
). A 90% yield of white
1
13
The spectral H NMR, C NMR and elemental analysis data for the 1,4-DimethylPipirazinium-
1
Butane sulfonate double-linkage (PipBs
2
) were as follows: H NMR (400 MHz, D
2
O, TMS): δ 1.ꢀ5-1.82
(m, 4H), 1.82-2.35 (m, 4H), 2.90 (t, 4H, J= 8 Hz), 3.22 (s, 3H), 2.24 (s, 3H), 3.30-4.50 (m, 12H). (ESI-
MS) m/z Calcd for PipBs
2
: 386.15; Found: 306.12. Anal. Calcd for C14
H
30
N
2
O
6
2
S : C, 43.50; H, 7.82; N,
7.25; S, 16.59%. Found: C, 43.40; H, 7.88; N, 7.23; S, 16.63%
3

ACCEPTED MANUSCRIPT
+
3-
2+
3-
2
.2. Preparation of [PipBs
PipBs (18 mmol) was dissolved in 25 ml of distillated water and added to a round-bottom flask
containing a magnet and 25 ml of an aqueous solution of H 40 (6 mmol). Then, the mixture was
stirred at room temperature for 24 h. Finally, water was removed in vacuum to give [PipBs
1
]
3
PW12
O
40 and [PipBs
2
]
3
(PW12
O
40
)
2
catalysts
1
3
PW12O
+
3-
40
1
] PW12O
3
2
+
3
3-
40
as solid. For the preparation of [PipBs
2
]
(PW12O
)
2
catalyst, 12 mmol of H PW12O40 and 18 mmol of
3
PipBs
2
were used with the same procedure. To simplify the catalyst names, we used [PipBs
1
3
] -PW and
1
13
[
PipBs
2
]
3
-(PW)
2
in the text. The prepared catalysts were characterized with FT-IR, H NMR, C NMR,
TGA, BET, ESI-MS and EDX analysis. Some characterization details of the prepared catalysts are shown
in the Electronic Supporting Information (ESI).
2
.3. Typical procedure forthe [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW)
2
catalyzed oxidation of alcohols
-PW or [PipBs
The liquid phase catalytic oxidation of alcohols in the presence of [PipBs
was performed in a two-necked round-bottom flask (25 mL) connected with a reflux condenser and a
thermometer. In a typical reaction, a mixture of [PipBs -PW (0.05 mmol) or [PipBs -(PW) (0.03
mmol) and alcohol (60 mmol) were added to the glass flask. The reaction was started by the drop-wise
addition of H solution (50 mmol) under vigorous stirring ꢀꢁ ꢂꢃ C. After 5 h, the reaction mixture was
1
]
3
2
]
3
-(PW)
2
1
]
3
2
]
3
2
2
O
2
ꢄ
extracted with diethyl ether (3 × 15 mL), and the solvent was evaporated in vacuum. The residual liquid
was analyzed by gas chromatography (Agilent 6820) equipped with a flame ionization detector and a
capillary column (DB-5, 3ꢃ m × ꢃ.45 mm × ꢃ.42 μm). Pure oxidation products were obtained by column
chromatography (petroleum ether-ethyl acetate 9:1 (V/V) and identified by the comparison of their IR and
1
H NMR spectra with those of standard samples. The aqueous phase was subjected to rotary evaporation
and then dried at 80 °C under vacuum for 4 h to give regenerated ionic liquids. The same set of
experiments on the catalytic oxidation of an alcohol with hydrogen peroxide using recycled ionic liquids
was repeated at least six times. All the products were known and their spectra were compared with those
of reported compounds [1-5, 28].
3
. Results and Discussion
3
.1. Preparation of [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW) catalysts
2
The representative route followed to prepare the novel introduced Brønsted acidic ionic liquids with
phosphotungstate anion (BAILs-PW) is depicted in Scheme 1. At the first, in a clean simple two-step
procedure, 1,4-dimethylpiperazine and 1,4-butane sultone were reacted in 1:1.5 molar ratio at room
temperature for 4 h to achieve the mono linker structure 4-[1,4-dimethylpiperazin-1-ium-1-yl]butane-1-
sulfonate (PipBs
1
), while for the preparation of 4,4'-[1,4-dimethylpiperazine-1,4-diium-1,4-
diyl)bis(butane-1-sulfonate) (PipBs
2
), 1:3 molar ratio of 1,4-dimethylpiperazine and 1,4-butane sultone
4

ACCEPTED MANUSCRIPT
were reacted ꢄꢅ ꢀꢁ
ꢂ
1
or PipBs was dissolved in distilled
2
water and an aqueous solution of H
3
PW12O
+
3
3-
2+
3
3-
Finally, water solvent was removed in vacuum to give [PipBs
solids.
1
]
PW12
O
40 and [PipBs
2
]
(PW12
O
40
2
) as
Scheme 1. Preparation of [PipBs1]3-PW and [PipBs2]3-(PW)2.
3
3
.2 Characterization of catalysts
.2.1 FTIR spectra
The FT-IR technique is quite useful to identify structural and bonding changes in the Keggin unit
present in the prepared BAILs-PW compared to that of the neat H 40. FT-IR spectra of pure 1,4-
dimethylpiperazine, [PipBs -PW, [PipBs -(PW) and neat H 40 are illustrated in Fig. 1. The FT-
IR spectrum of PWA shows specific absorption IR bands corresponding to the Keggin structure at 802,
3
PW12O
1
]
3
2
]
3
2
3
PW12
O
-
1
8
91, 979 and 1079 cm , which are assigned to the stretching vibrations ʋas(W–Oe–W), ʋas (W–Oc–W),
as(W–Ot) and ʋas(P–O*), respectively (Fig. 1d). From the FT-IR spectrum of [PipBs -PW (Fig. 1b) and
PipBs ] -(PW) (Fig. 1c), in spite of the slight shift, four featured band positions appeared distinctively
ʋ
1 3
]
[
(
2
3
2
-
1
800, 894, 977 , 1078 cm ), which indicates that the Keggin structure of H
40 were substituted by the organic cations. Finally, the characteristic IR
bands of pure 1,4-dimethylpiperazine are obviously presented in the FT-IR spectra of [PipBs -PW and
PipBs -(PW) catalysts.These observations are clear proofs for the combination of organic cations
3
PW12O40 was well reserved
after the protons in the H PW12O
3
]
1 3
[
2
]
3
2
5

ACCEPTED MANUSCRIPT
(
PipBs
1
and PipBs
2
) with H PW12O40 via ionic interactions to produce two new catalysts of PWA salts
3
using 1,4-dimethylpiperazine as organic cationic support.
Fig. 1. FT-IR spectrumof pure 1,4-dimethylpiperazine (a), [PipBs1]3-PW (b) [PipBs2]3-(PW)2 (c) H3PW12O40 (d).
3
.2.2 NMR spectra of PipBs
1
and PipBs
2
The identification of PipBs
chemical shifts of N-CH
1
from PipBs
2
1
structure was simply confirmed with the comparsion of the
1
3
groups from their H NMR spectra. In the H NMR spectrum of PipBs
1
, more
group is
shows two singlet peaks for
groups at 3.22 and 3.24 ppm, which indicates that the chemical environment
around these groups is not completly identical (Fig. 2). Alternatively, in the structure of PipBs , two
+
deshielded N -CH
3
group is appeared as a singlet peak at 3.02 ppm, while less deshilded N-CH
3
1
appeared as a singlet peak at 2.28 ppm. The H NMR spectrum of PipBs
2
+
both deshielded N -CH
3
2
1
methyl groups ocuppy almost different steric poistions. On the basis of H NMR principles, one of them
has axial conformation affected by the electron density of axial H3 and H5 to be more shielded (3.22
ppm), while another methyl group has equiterial conformation and is far from this shielding enviroment to
1
3
sense shielding effects (3.24 ppm) [27]. From the C-NMR spectrum of PipBs
1
, the chemical shift of
+
deshielded N -CH
3
group was observed at 54 ppm, while N-CH
3
was seen at 46.7 ppm (Fig. S1).
6

ACCEPTED MANUSCRIPT
1
Fig. 2. HNMR (400 MHz, D2O) spectrum of PipBs1 (blue) and PipBs2 (green).
3
.2.3 EDX analysis
The EDX spectra of the fresh and reused catalysts show that the [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW)
2
have organometallic composition. The EDX spectra of the fresh catalysts confirm the existence of butane
sulfonate and phosphotangestate in the structure of the catalysts, which is another proof for the successful
preparation of target BAILs-PW catalysts (see Fig. S2). The comparison of the elements and their weight
percent in the fresh and reused catalyst are almost the same, indicating that the structure, composition and
catalytic activity of [PhBs
1
3
] -PW solid remained with negligible changes after 6 runs (in agreement with
negligible product yield differences obtained during the 6 runs, Fig. 3).
3
.2.4 TG Analysis
The TG analysis of two BAILs-PW catalysts indicated that these catalysts are thermally stable up to 
300 °C. For example, [PipBs
2
]
3
-(PW) catalyst shows an endothermic peak in the DSC curve emerged at
2
1
60 °C, which refers to the loss of water (mass loss 1%). After this step, three step-by-step exothermic
+
peaks above 300 °C (320, 475 and 600 °C) are arisen. Most probably, [PipBs
2
] cation is decomposed in
the first and second steps at 325 °C and 475 °C, respectively, then in continue, [PW O ] anion is
1
2
40
decomposed at  600 °C (see Fig. S3).
3
.3
Selective oxidation reaction catalyzed by [PipBs
The novel ionic liquids presented in this work have very high surface activity and solubility in water
and solve the problem of immiscibility for the hydrophobic alcohol in aqueous H . [PipBs -PW and
PipBs -(PW) catalysts are immiscible with diethyl ether, n-hexane, toluene, and ethyl acetate, but are
1
]
3
-PW and [PipBs
2
]
3
-(PW)
2
using aqueous H
2
O
2
2
O
2
1 3
]
[
2
]
3
2
soluble in methanol, ethanol and water. Their water content is 1.2 wt% and 2.3 wt%, respectively (Karl-
Fischer titration). The oxidation reaction was carried out under homogenous conditions without adding
any phase transfer catalyst or co-solvent. The acidity of catalysts was measured using a UV–visible
7

ACCEPTED MANUSCRIPT
spectrophotometer with 4-nitroaniline as a basic indicator. The results revealed higher acidity than
traditional H
2
SO
4
; the obtained results for [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW) were calculated to be 0.81
2
and 1.45 mmol/g, respectively.
At first, the oxidation of benzyl alcohol was selected as a model reaction. The effects of reaction
temperature, reaction time, amount of catalyst and H on benzyl alcohol oxidation were examined. The
2
O
2
conversion of benzyl alcohol was improved with increasing the reaction temperature, but the selectivity
was decreased. When the reaction time was increased, an increase in the conversion of benzyl alcohol was
observed, while the selectivity to benzaldehyde decreased slightly for the deep oxidation. The yield of
benzaldehyde had straight relationship with the amount of H
alcohol/H from 2.0 to 1.0, the conversion largely increased from 50 to 85% but the selectivity
decreased from 95 to 90% due to the deep oxidation of benzaldehyde. The optimized molar ratio of
BzOH/ H was 1.2:1. When the catalyst amount was increased, an increase in the conversion was
2
O
2
. By increasing the molar ratio of benzyl
2
O
2
2
O
2
observed, while the selectivity was slightly decreased.
After optimizing the reaction conditions, oxidations were performed under these optimized conditions.
After using [PipBs
of the product was 91% when [PipBs
PipBs -(PW) was selected as catalyst of choice, considering the beter catalytic activity obtained after
using a lower amount of catalyst. The generality of the oxidation reaction by using [PipBs -(PW) as
catalyst was examined for a diversity of alcohol substrates (in some cases [PipBs -PW was also
2
]
3
-(PW)
2
as catalyst, benzaldehyde was obtained with a yield of 96%, while the yield
]
1 3
-PW was used as catalyst. After this success, more efficient
[
2
]
3
2
2
]
3
2
]
1 3
examined). The oxidation of substituted alcohols was examined, and it was found that all substituted
benzyl alcohols can be oxidized smoothly to the targeted benzaldehydes with good to excellent yields.
However, considering the rate and yields of the reaction, the results were higher for alcohols with
electron-donating groups. As can be seen in Table 1, non- benzylic alcohols such as cyclohexanol and
cyclopentanol were oxidized to the desired ketones with yields of 80 and 89%, respectively. For
hydrophilic alcohols, the oxidation was carried out under homogeneous conditions and good results were
obtained, which could be attributed to the formation of micro-emulsion.
8

ACCEPTED MANUSCRIPT
Table 1. Selective oxidation of alcohols catalyzed by [PipBs
2
]
3
-(PW)
2
and [PipBs
1
]
3
-PWsolids.
Conv.
% )
Yield
(% )
Entry
Substrate
Product
(
a
b
a
b
1
98 /93
96 /91
a
a
2
3
94
88
a
98
a
90
a
a
4
5
93
83
a
b
a
b
94 /85
80 /72
a
b
a
b
6
7
95 /89
80 /74
a
a
93
89
a
Results for [PipBs ] -(PW) catalyst, reaction conditions: 0.03 mmol catalyst, 50 mmol H O , 60 mmol
2
3
2
2
2
alcohol, 90 °C, 3 h.
9

ACCEPTED MANUSCRIPT
b
Results for [PipBs
alcohol, 90 °C, 4 h.
1
]
3
-PW catalyst, reaction conditions: 0.05 mmol catalyst, 50 mmol H
2
2
O , 60 mmol
In order to explain these high-performance oxidation activities, the oxidation of benzyl alcohol using
reported [bmim]HSO , H 40, heteropolyacid ionic hybrid and S4SiIL catalysts were
carried out under the same reaction conditions, and the obtained results are shown in Table 2.
4
4
SiW12
O
40, H PW12O
3
Table 2. The catalytic activity of [PipBs
2
]
3
-(PW)
2
for the oxidation of benzyl alcohol in comparison with
different catalysts.
a,b
Catalyst
Product
Conv.(%)
Yield(%)
[
bmim]HSO
4
[24]
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
5
5
H
3
PW12
O
40 [24]
59
91
92
99
98
46
80
83
94
[DpyAM]PW [28]
[
n-C16
H
33N(CH
3
3
) ]PW12O40 [29]
S4SiIL [24]
[
PipBs
2
]
3
-(PW)
2
[This work]
96
a
b
Reaction conditions: 0.03 mmol catalyst, 50 mmol H
2
O
2
, 60 mmol alcohol, 90 °C, 3 h. Yields refer to
GC yields.
As it can be seen from Table 2, [PipBs
2
]
3
-(PW)
2
catalyst presents superior activity as well as S4SiIL in
is
enhanced under strong acidic conditions (high Brønsted acidity and content), and with the increase of
acidity, the oxidation capacity of H is improved. Second, oxidation and acid catalytic sites co-exist in
the structure of the introduced [PipBs -PWand [PipBs -(PW) catalysts. Thus, the two catalytic sites
the oxidation of BzOH which could be attributed to the following: First, the oxidation capacity of H
2
O
2
2
O
2
1
]
3
2
]
3
2
were combined by the strong ionic interactions between the organic cation and heteropolyanion that led
to an improved oxidation activity for phosphotungstate anion (synergistic effect) [24]. Third, both
catalysts exhibit a larger BET specific surface area than pure H
3
PW12
O40 and have higher solubility in
2
water. While the BET surface area of bulk PWA was 11.0 m /g, [PipBs
1
]
3
-PW and [PipBs -(PW) have
2
]
3
2
1
0

ACCEPTED MANUSCRIPT
by far higher surface areas. With entering the channels of H
small molecules of PipBs1 and PipBs2, the surface area was increased from 11.0 to 30 and 50 m /g for
PipBs -PW and [PipBs -(PW) , respectively. It is noted that in the case of hydrophilic alcohols,
3
PW12O40 to the flexible chains of organic
2
[
1
]
3
2
]
3
2
oxidation is carried out under homogeneous reaction conditions, whereas in the case of hydrophobic
alcohols, the good oxidation reaction results could be attributed to the formation of micro-emulsion. This
is because organic small molecules with flexible chains can easily enter the channels of H
3
PW12O .
40
These catalysts form a net-like trap configuration, thereby increasing the number density of active sites of
the catalyst, thus allowing the interaction of BzOH.
Based on the above and other reported data in the literature [24,29], a plausible mechanism for alcohol
oxidation could be proposed in Scheme 2. H
2
O
2
is first activated by the multi-SO
3
H functionalized
organic cation, the activated H then reacts with heteropolyanion to generate active peroxo intermediate.
2
O
2
The latter active intermediate is responsible for the oxidation of alcohol to the corresponding aldehyde,
ketone or carboxylic acid. We believe that 1,4-dimethylpiperazine, which has chair-type configuration, is
more flexible and more dynamic organic support than previously reported organic supports, such as
isoquinoline and 4-pheny pyridine [25], allowing it to be more efficient organic support for the
preparation of HPA-ILs.
Scheme 2. A possible mechanism for oxidation of alcohols catalyzed by introduced HPA-ILs.
3
.4. Recoverability and Reusability of catalysts
As mentioned in the experimental section, the aqueous phase was subjected to rotary evaporation, and
then dried at 80 °C under a vacuum for 4 h to give regenerated ionic liquids. The same set of experiments
on the catalytic oxidation of alcohol with hydrogen peroxide by using recycled ionic liquids was repeated
at least six times. To study the recycling and reusability performance of the introduced BAILs-PW, the
oxidation reaction of benzaldehyde was selected as a model reaction. The reaction was checked for six
repeated runs (fresh + five consecutive runs) using [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW) as catalysts. The
2
yield of the desired benzaldehyde product for six repeated runs is presented in Fig. 3. It is obvious that the
1
1

ACCEPTED MANUSCRIPT
yield difference between the first and 6th runs is not considerable for both catalysts by taking into
account the weight loss in the operation of recovering the catalysts. These observations indicate that a
slow deactivation of the catalysts occurred. Additionally, the EDX analysis performed on the [PhBs ] -
1
3
PW after six runs demonstrated a quite stable structure and very low leaching of catalyst elements,
indicating that the catalyst efficiency is almost completely maintained during 6 consecutive runs, in
agreement with the recoverability/reusability studies.
96
94
92
90
88
86
84
PipBs2-PW
PipBs1-PW
Run 1 Run 2 Run 3 Run 4 Run 5 Run 6
Fig. 3. The effect of recycling of [PipBs1]3-PW and [PipBs2]3-(PW)2 for the oxidation reaction of benzyl alcohol on
the benzaldehyde yield. Reaction conditions: Molar ratio of benzyl alcohol/H2O2 = 1.2, loading of [PipBs1]3-
PW=0.05 g and [PipBs ] -(PW) =0.03 g, T=90 ꢂC , t=4 and 3 h, respectively.
2
3
2
4
. Conclusions
A new series of BAILs with the phosphotungstate anion was prepared and well characterized. The
catalytic activity and reusability of the novel introduced acidic catalysts was examined for the selective
oxidation reaction of benzyl alcohol using aqueous H under optimized reaction conditions. It was
found that [PipBs -(PW) is a superior catalyst for the given oxidation reaction. The catalysts showed
2
O
2
2
]
3
2
excellent activity and were simply separable and reusable up to 6 runs tested. The EDX analysis and the
recoverability/reusability study of the introduced catalysts over repeated runs proved their quite stable
structures, with almost completely retained performance. Other appications of these specific BAILs are
under investigation in our laboratory.
Acknowledgements
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The authors gratefully acknowledge the Research Council of Yasouj University and Islamic Azad
University Yazd Branch for their financial support.
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H33N(CH
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Highlights





Novel SO
3
H-functionalized polyoxometalate-based ionic liquids
Polyoxometalate-based ionic liquids catalyzed oxidation ofalcohols
The catalysts were easily separable and reusable
The simple and green procedure
Very lowleaching ofthe polyoxometalate anion
1
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