Polyoxometalate-based Gemini ionic catalysts for selective oxidation of benzyl alcohol with hydrogen peroxide in water

Article abstract of DOI:10.1039/c8cy01191e

Keggin-type phosphotungstic acid (H₃PW₁₂O₄₀, HPW)-based di-imidazolium ionic liquid (IL) hybrids were prepared by exchanging the protons of HPW with PEG-bridged di-imidazolium cations in water. Characterization results sug

Full text of DOI:10.1039/c8cy01191e

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DOI: 10.1039/C8CY01191E
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Polyoxometalate-based Gemini ionic catalysts for selective oxidation of
benzyl alcohol with hydrogen peroxide in water
Pengbo Hao,† Mingjie Zhang,† Wei Zhang, Zhiyang Tang, Ni Luo, Rong Tan,* and Donghong Yin
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Kegginꢀtype phosphotungstic acid (H3PW12O40, HPW)ꢀbased diꢀimidazolium ionic liquid (IL) hybrids
were prepared by exchanging the protons of HPW with PEGꢀbridged diꢀimidazolium cations in water.
Characterization results suggested that intact Kegginꢀtype [PW12O40]3ꢀ anion was incorporated with
various PEGꢀmodified diꢀimidazolium cation through ionic interaction, giving PWꢀbased ionic catalysts
10 with Gemini structure. PEGꢀbridged diꢀimidazolium cations could not only fineꢀtune the redox properties
of PW anions but also enhanced compatibility of catalyst in the aqueous system, which in turns improved
the catalytic performance. Furthermore, combination of double catalytic sites into a single molecule
enforced an intramolecular, cooperative reaction pathway resulting in further enhanced reaction rate and
higher selectivity in selective oxidation of benzyl alcohol with H2O2 in water. More importantly, the diꢀ
15 imidazolium IL cations imparted reaction controlled phaseꢀtransfer function to the PW hybrids. The
catalysts were thus precipitated from the aqueous system upon consumption of H2O2 and could be reused
several times without loss of activity and selectivity.
the environment,13ꢀ15 was the ideal oxidant for this reaction
undoubtedly from the viewpoint of green chemistry.
Introduction
Polyoxometalates (POMs), a large family of bulky clusters of
40 transition metal oxide with structural diversity, have received
much attention in liquidꢀphase catalytic oxidation due to their
controllable redox and acidic properties.16ꢀ24 In particular,
Kegginꢀtype phosphotungstic acid (H3PW12O40, HPW) have been
revealed to be very effective in the selective oxidation of alcohols
45 with aqueous H2O2.25 However, pure POMs utilized in their bulk
form had some drawbacks such as lower activity due to their very
low surface area (<10 m2 gꢀ1) and difficulty for catalyst recovery
due to their waterꢀsolubility.26 Immobilizing POMs onto supports
with a large surface area ensured their enlarged surface area and
50 facile recovery, but obtained heterogeneous HPW catalysts are
often plagued by massꢀtransfer limitations during reactions.27
Modification of POMs with organic units has been also applied to
improve catalytic activity and reusability of POM catalysts,28
since it could fineꢀtune the solubility and oxidizing capability of
Benzaldehyde was an important chemical intermediate and raw
20 material widely used in the manufacture of perfumes, agricultural
chemicals, spices and other fine chemicals.1ꢀ4 The conventional
process for producing benzaldehyde is the hydrolysis of benzyl
chloride.5 While, a trace amount of chlorine inevitably existed in
the product, which didn’t meet practical requirements for
25 perfumery and the pharmaceutical industry. Liquidꢀphase
oxidation of toluene and vaporꢀphase oxidation of benzyl alcohol
were thus developed to produce chlorineꢀfree benzaldehyde.6ꢀ9
But in the former process, the selectivity for benzaldehyde was
rather low,10, 11 and the latter process often requires high reaction
30 temperatures, which resulted in high energy consumption, low
selectivity for benzaldehyde, and deactivation of catalysts.12 In
view of these drawbacks of traditional synthesis methods, the
liquidꢀphase oxidation of benzyl alcohol to benzaldehyde was
more preferable, if it could be of low energy consumption, high
35 selectivity for the desired product, and preventing catalyst
deactivation. Hydrogen peroxide, which was cheap and mild for
55 POM species. For example, Zhu’ group reported
a
H4PMo11VO40ꢀbased amphiphilic catalyst via functionalization of
H4PMo11VO40 by long alkyl chainꢀcontaining cationic
surfactants.29 They found that the amphiphilic characteristics of
catalysts facilitated the access of organic benzyl alcohol to
60 catalytic center, resulting in the enhanced catalytic activity for the
selective oxidation of benzyl alcohol with aqueous H2O2.
Unfortunately, the hydrophobic long alkyl chains in such hybrid
hindered the interaction of H2O2 with POM to form active perꢀ
POM species. As a result, only moderate efficiency (60.6% of
65 conversion) was observed over the amphiphilic catalyst in
selective oxidation of benzyl alcohol with H2O2.
* Key Laboratory of Chemical Biology and Traditional Chinese
Medicine Research (Ministry of Education); National & Local Joint
Engineering Laboratory for New Petroꢀchemical Materials and Fine
Utilization of Resources, Hunan Normal University, Changsha
410081 (P. R. China)
Fax:
+86ꢀ731ꢀ8872531;
Tel:
+86ꢀ731ꢀ8872576;
Eꢀmail:
yiyangtanrong@126.com
†
These authors contributed equally to this work and should be
considered coꢀfirst authors.
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Room temperature IL with fascinating compatibility was an
attractive organic modifier for POM.30ꢀ35 The inherent features,
such as low volatility, large liquid ranges, and tunable polarity
were heated from room temperature up to 800 °C was 10 K minꢀ
1under flowing air using alumina sample holders. Elemental
analyses of C, H and N were carried out on a Vario EL III
often promote the formation of hybrids with novel structures. 60 elemental analyser made in Germany. NMR spectra of samples
5
Especially, the hybridization of ILs often transfers the ILs’
properties to hybrids and fineꢀtunes their compatibility in
heterogeneous system, which was very promising for their
were recorded on a BRUKER AVANCE ꢀ500 spectrometer.
Powder Xꢀray diffraction (PXRD) pattern was scanned over a 2θ
angle of 5o to 80o at an interval of 0.02 o. Xꢀray Photoelectron
Spectroscopy (XPS) data were obtained with a KꢀAlpha+ electron
applications
in
heterogeneous catalysis.36
Diꢀcationic
imidazolium IL were known to allow the greater variety and 65 spectrometer from Thermo fisher Scientific using 300 W Al Kα
10 control of ILs’ virtual properties, including solubility and
meltability.37, 38 Based on the fascinating features, we reasoned
that the diꢀcationic IL could be used as the modifier for PW
species and allowed for highly efficient catalyst for efficient
radiation. Surface wettability of the samples was investigated by
the analysis of the contact angles. The particle samples were
directly pelletized without the aid of binders. A drop of DI water
was placed on the pellet and imaged using a CCD camera
selective oxidation of alcohols with aqueous H2O2. It was well 70 (Fainstec, STCꢀGEC83A, Korea). Then, the contact angle was
15 known that the PW species had high negative charge and ability
of forming salts, we thus decided to combine PW anions with IL
cations through ionic interaction. Polyethylene glycol (PEG) with
phaseꢀtransfer capability was employed to modify the IL cation,
obtained by drawing a tangent. At least three measurements were
carried out, and an averaged contact angle was taken. The content
of tungsten in the samples was determined by inductively coupled
plasma mass spectrometry (ICPꢀMS) on a NexION 300X
so as to further enhance the compatibility of catalyst and organic 75 analyzer (PerkinꢀElmer Corp.). The reaction products were
20 substrates for the aqueous oxidation. As expected, the Kegginꢀ
type [PW12O40]3ꢀ anions were successfully coupled with the PEGꢀ
modified diꢀcationic imidazolium IL, giving PWꢀbased ionic
catalysts with Gemini structure. Combination of double catalytic
analyzed by a gas chromatograph (Agilent Technologies 6890N,
HPꢀ624 capillary column, 30 m × 0.25 mm) with flame ionization
detector (FID) using nitrogen as the carrier gas. The products
were identified by GCꢀMS using an HP 5790 series massꢀ
sites into
a single molecule enforced an intramolecular, 80 selective detector.
25 cooperative reaction pathway resulting in enhanced reaction rates
and higher selectivities in selective oxidation of benzyl alcohol
with H2O2 in water. Furthermore, PEGꢀbridged diꢀimidazolium
cations could not only fineꢀtune the redox properties of PW
anions but also enhanced compatibility of catalyst in the aqueous
30 system, further improving the catalytic performance of the PWꢀ
based ionic catalysts. More interestingly, the PWꢀbased IL
hybrids exhibited inverse dissolution–precipitation in water upon
treatment with H2O2, which allowed for the homogeneous
catalysis coupled with heterogeneous separation in the H2O2ꢀ
35 based aqueous oxidation system. Activity switching was
Preparation of polyoxometalate-based Gemini ionic catalysts
(PIPA-n, n = 0, 4, 8, 13, where n represented the repeated units
number of ethylene oxide units in various PEG chains chains)
The Synthesis of PIPA-n (n = 0, 4, 8, 13) was outlined in
85 Scheme 1.
N
N
SOCl2
N
N
N
N
Cl
Cl
HO
OH
O
n
O
n
O
n
Clꢀ
Clꢀ
CH2Cl2, RT
toulene, N2, 110oC
PDILꢀn (n=4, 8, 13)
Kegginꢀtype H3PW12O40
H2O, RT
N
3ꢀ
N
N
N
PW12O40
O
2
n
3
PIPA-n (n=0, 4, 8, 13)
repeatable for seven cycles, which typically realized
homogeneous reaction coupled with heterogeneous separation in
aqueous reactions.
a
Scheme 1 Synthesis of PIPA-n (n = 0, 4, 8, 13).
Preparation of PIPA-n (n = 4, 8, 13)
90
Synthesis of PEGꢀbridged diꢀimidazolium IL: polyethylene
oxideꢀn (50 mmol, n= 4, 8, 13, which represented the
degree of polymerization of polyethylene oxide) was mixed with
pyridine (8 mL) in dichloromethane at room temperature. Thionyl
chloride (110 mmol, 13.09 g) was then dropwise added into the
Experimental
40 Materials and reagents
Kegginꢀtype HPW was purchased from Macklin and used as
received. Nꢀmethylimidazole was purchased from Alfa Aesar.
Benzyl alcohol, 1ꢀphenylethanol, cyclohexanol, 2ꢀphenylethanol,
diphenylmethanol and nꢀhexanol were pursed from Aladdin and
45 used as received. Other commercially available chemicals were
obtained from local suppliers. All of the solvents were purified by
standard procedures before use. Other commercially available
chemicals were laboratoryꢀgrade reagents obtained from local
suppliers.
95 solutions. The reaction mixtures were stirred at room temperature
for 12 h under argon, and then concentrated in vacuum. Gummy
residue was dissolved in ethyl acetate (50 mL) and filtrated to
remove the insoluble salt of pyridinium chloride. Filtrate was
concentrated in vacuum and further dried in vacuum at 40 oC
100 overnight, giving viscous yellow liquid of chlorideꢀterminated
PEGꢀn (n= 4, 8, 13). The obtained chlorideꢀterminated PEGꢀn (n=
4, 8, 13) (3 mmol) was mixed with 1ꢀmethyl imidazole (6.2 mmol,
50 Methods
o
0.51 g) in toluene (40 mL), and was refluxed at 110 C for 48 h
under argon protection. After removal of solvent, gummy residue
105 was thoroughly rinsed with tetrahydrofuran and dried in vacuum
to give a pale yellow viscous liquid of PEGꢀbridged diꢀ
imidazolium ILs (denoted as PDILꢀn, n= 4, 8, 13). PDILꢀ4：FTꢀ
IR (KBr): γmax/cmꢀ1 2921, 2866, 1736, 1650, 1538, 1459, 1354,
1297, 1249, 1109, 946, 845, 756, 665, 525. PDILꢀ8：FTꢀIR
Fourier transform infrared (FTꢀIR) spectra were obtained from
potassium bromide pellets with a resolution of 4 cmꢀ1 and 64
scans in the range of 400–4000 cmꢀ1 using an AVATAR 370
Thermo Nicolet spectrophotometer. The thermogravimetric and
55 differential thermogravimetric (TGꢀDTG) curves were obtained
on a NETZSCH STA 449C thermal analyzer. Samples (ca. 10 mg)
2
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DOI: 10.1039/C8CY01191E
(KBr): γmax/cmꢀ1 2920, 2867, 1736, 1649, 1539, 1458, 1354, 1296,
1249, 1110, 947, 843, 756, 663, 524. PDILꢀ13：FTꢀIR (KBr):
γmax/cmꢀ1 2919, 2868, 1735, 1649, 1540, 1459, 1353, 1297, 1250,
1111, 947, 844, 757, 663, 525.
Synthesis of PIPA-n (n = 4, 8, 13): PEGꢀbridged diꢀ
imidazolium ILs (3 mmol) were mixed with Kegginꢀtype HPW
(2.1 mmol, 6.05 g) in water (50 mL). The mixture was stirred at
room temperature for 48 h, resulting in white precipitate. After
filtration, the resulting precipitate was collected, washed with
8.59; H, 1.58; N, 2.37%. Tungsten content: 3.47 mmol/g
(theoretical value: 3.63 mmol/g); FTꢀIR (KBr): γmax/cmꢀ1 3150,
3108, 2962, 1565, 1460, 1160, 1079, 979, 895, 805, 625, 518.
3ꢀ
N
3ꢀ
PW12O40
N
N
N
PW12O40
5
N
2
N
3
3
PIPA-0
ILꢀPW
60
Chart 1 The structures of PIPA-0 and IL-PW.
Catalyst testing
The PW–based catalyst (1.3 mol% of substrate, based on the
10 deionized water several times, and dried in vacuum overnight at
room temperature, yielding PIPA-n (n = 4, 8, 13) as a white
powder. PIPA-4: Calc. for C54H96N12O12P2W24O80: C, 9.46; H,
1.41; N, 2.45%. Found: C, 9.35; H, 1.37; N, 2.57%. Tungsten
65 PW content) was mixed with benzyl alcohol (10 mmol) in
deionized water (1.0 mL) under stirring. H2O2 (30 wt.%, 15 mmol)
was diluted to the concentration of 10 wt.%, and then added
dropwise to the above mixture within 1.5 h. The resulting mixture
was stirred at 95 °C until the reaction was judged to be complete
70 based on GC analysis. After reaction, ethyl acetate (3 × 10 mL)
was used to extract the products. Combined organic layers were
dried with anhydrous Na2SO4. The products were analyzed by a
gas chromatograph and further identified by GCꢀMS. PWꢀbased
catalyst was precipitated from the reaction system and separated
75 from the upper liquid phase by centrifugation. Being further
washed with ethyl acetate and dried under vacuum, the recovered
catalyst was used for the recycling experiment.
1
content: 3.61 mmol/g (theoretical value: 3.50 mmol/g); H NMR
15 (DMSOꢀd6, 500 MHz) δH, ppm: 9.04 (s, 2 H, ring NꢀCH2ꢀN),
7.68 (t, 4 H, ring ꢀNꢀCH2ꢀCH2), 7.65 (t, 4 H, ring ꢀNꢀCH2ꢀCH2),
3.52ꢀ3.57 (m, 16 H, CH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀ), 1.24 (s,
6 H, ring ꢀNꢀCH3); FTꢀIR (KBr): γmax/cmꢀ1 3149, 2971, 2883,
1079, 1050, 979, 895, 804, 625, 521. PIPA-8: Calc. for
20 C78H144N12O24P2W24O80: C, 12.68; H, 1.96; N, 2.28%. Found: C,
12.71; H, 1.83; N, 2.19%. Tungsten content: 3.36 mmol/g
(theoretical value: 3.25 mmol/g); 1H NMR (DMSOꢀd6, 500 MHz)
δH, ppm: 9.03 (s, 2 H, ring NꢀCH2ꢀN), 7.69 (t, 4 H, ring ꢀNꢀCH2ꢀ
CH2), 7.66 (t, 4 H, ring ꢀNꢀCH2ꢀCH2), 3.52–3.53 (m, 32 H, CH2ꢀ
25 CH2ꢀOꢀCH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀ), 1.24 (s, 6 H, ring ꢀNꢀCH3); FTꢀ
IR (KBr): γmax/cmꢀ1 3150, 2969, 2884, 1079, 1049, 979, 895, 805,
625, 521. PIPA-13: Calc. for C108H204N12O40P2W24O80: C, 16.08;
H, 2.55; N, 2.08%. Found: C, 16.01; H, 2.38; N, 1.97%. Tungsten
Results and discussion
Preparation of catalysts
1
content: 2.69 mmol/g (theoretical value: 2.98 mmol/g); H NMR
80 Hybridization of POMs with IL units was an effective strategy
to achieve POMꢀbased hybrid catalysts with improved catalytic
activity and convenient recovery, since the solubility and
oxidizing capability of the obtained POMꢀbased hybrids could be
adjusted by the introduction of the organic moieties.39 Different
85 from the common organic units (ꢀNH2 etc), IL as countercation
will transfer the ILs’ properties to hybrids and fineꢀtune their
compatible properties, which are very promising for the
30 (DMSOꢀd6, 500 MHz) δH, ppm: 9.03 (s, 2 H, ring NꢀCH2ꢀN),
7.69 (t, 4 H, ring ꢀNꢀCH2ꢀCH2), 7.66ꢀ7.67 (t, 4 H, ring ꢀNꢀCH2ꢀ
CH2), 3.52–3.54 (m, 52 H, CH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀ),
3.52 (m, 48 H, CH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀOꢀCH2ꢀCH2ꢀ), 1.24 (s, 6 H,
ring ꢀNꢀCH3); FTꢀIR (KBr): γmax/cmꢀ1 3151, 2970, 2883, 1080,
35 1049, 979, 896, 805, 625, 522.
Preparation of PIPA-0
39
applications in the field of heterogeneous catalysis.36,
To evaluate the function of PEG moiety, a Gemini analog of
PEGꢀfree counterpart of PIPA-0 in which ethyl group was
employed as the linker, was prepared as a control catalyst. The
40 preparation procedure was similar to that of PIPA-n (n = 4, 8, 13),
except for using 1,2ꢀdichloroethane instead of chlorideꢀ
terminated PEG to bridge 1ꢀmethylimidazole, as shown in
Scheme 1. PIPA-0. Calc. for C30H72N12P2W24O80: C, 5.67; H,
1.14; N, 2.64%. Found: C, 5.84; H, 1.38; N, 2.39%. Tungsten
45 content: 3.64 mmol/g (theoretical value: 3.78 mmol/g); FTꢀIR
(KBr): γmax/cmꢀ1 3334, 3162, 2962, 1438, 1079, 1049, 979, 895,
805, 742, 625, 521.
Especially, diꢀcationic imidazolium IL are known to allow the
90 greater variety and control of the ILs’ vital properties, including
solubility and meltability.40 The fascinating advantages
encouraged us to incorporate the Kegginꢀtype HPW with diꢀ
imidazolium IL cation to achieve efficient and recoverable PW/IL
hybrids for the selective oxidation of alcohols with H2O2 in water.
95 PEGs with phaseꢀtransfer capability were employed as the bridge
of diꢀimidazolium IL cation, as so to further circumvent the mass
transfer limitation of the aqueous oxidation.
The synthesis route for the PW/IL hybrids of PIPA-n (n = 4, 8,
13) was outlined in Scheme 1. PEGꢀbridged diꢀimidazolium
100 chlorine IL with various length of polyether linker was obtained
by nucleophilic substitution of chlorideꢀterminated PEGꢀn (n= 4,
8, 13) with Nꢀmethylimidazole under alkaline condition.
Successive anionꢀexchange with Kegginꢀtyped HPW allowed for
the incorporation of diꢀimidazolium cation with PW anion
105 through ion interaction. The approach gave PWꢀbased IL hybrids
of PIPA-n (n = 4, 8, 13) with Gemini structure. For comparison,
a PEGꢀfree counterpart of PIPA-0, in which an ethyl group was
acted as the bridge, was also prepared according to a similar
procedure, except for the use of 1,2ꢀdichloroethane instead of
Preparation of monomeric PW-based IL hybrids (denoted as
ILꢀPW hybrid)
50
To elucidate the intramolecular, cooperative effect of Gemini
structure of PIPA-n, a monoꢀcationic counterpart of 1ꢀmethylꢀ3ꢀ
butyl imidazolium phosphotungstate (denoted as ILꢀPW) was
also prepared as a control catalyst by direct anionꢀexchange of 1ꢀ
methylꢀ3ꢀbutylimidazolium chlorine (denoted as [BMIm]Cl) with
55 HPW. The structure of ILꢀPW hybrid was shown in Chart 1. Calc.
for C24H54N6PW12O40: C, 8.71; H, 1.63; N, 2.54%. Found: C,
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chlorideꢀterminated PEG, as shown in Scheme 1.
55 catalytic activity of the PWꢀbased catalyst.
Unlike the pristine HPW that is soluble in water, the obtained
PIPA-n (n =0, 4, 8, 13) were insoluble in water, as shown by the
typical PIPA-8 (Fig. 1A). The change in solubility indicated
successful incorporation of PW ion with PEGꢀbased IL.
Interestingly, it formed waterꢀsoluble active species by the action
of H2O2 during oxidation (Fig. 1B); and returned to insoluble
when H2O2 was used up (Fig. 1C). The H2O2ꢀinduced inverse
waterꢀsolubility made PIPA-n behave as reaction controlled
a
895
805
5
1079
b
990
c
10 phaseꢀtransfer catalysts for the selective oxidation of alcohols
with aqueous H2O2 in water.
,
c
625
947 895
805
1079
1049
979
2000
1500
1000
500
Wavenumbers (cmꢀ1)
Fig. 1 Photographs of PIPA-8ꢀmediated oxidation of benzyl alcohol with
H2O2 in water (Before reaction (A); during reaction (B); after
15 reaction(C)). Reaction conditions: Catalyst (1.3 mol% substrate, based on
the PW loading), benzyl alcohol (10 mmol), water (1.5 ml), H2O2 (30
wt.%, 15 mmol), 95 °C, 6 h.
Fig. 2 FTꢀIR spectra of Kegginꢀtype HPW (a), PDILꢀ8 (b), PIPA-8 (c),
and PIPA-8 reused for seven times (c’).
60 XPS
XPS provided further evidence for adjusting redox property of
W species by diꢀimidazolium cation in PIPA-8. Fig. 3 showed
the XPS of pure HPW and PIPA-8. As expected, pure HPW
exhibited only O1s (associated with physisorbed and structural
65 water in HPW), P2p, and W4f peaks in the XPS survey spectrum
(Fig. 3A). New C1s (284.6 ev) and N1s (400.2 eV) signals
appeared in XPS survey spectrum of PIPA-8. It suggested the
corporation of organic diꢀimidazolium cation with Keggin PW
anion. Notably, the combination resulted in an increase in the
Characterization of Samples
20 FT-IR
FTꢀIR spectrum was used to identify structural and bonding
changes in PIPA-n hybrids. PIPA-8 with a middling length of
PEG spacer was chose as the typical hybrid for the structural
identification. Fig. 2 showed the FTꢀIR spectra of pristine
25 Kegginꢀtype HPW, PEGꢀbridged diꢀimidazolium chlorinum IL of
PDILꢀ8, PIPA-8 and PIPA-8 (reused for seven times) hybrids.
Clearly, Kegginꢀtype HPW showed characteristic PꢀOa stretching
(1079 cmꢀ1), W=Od terminal stretching (990 cmꢀ1), stretching of
W–Oc–W inter bridges between cornerꢀsharing WO6 octahedra
30 (895 cmꢀ1), and stretching of W–Oe–W intra bridges between
edgeꢀsharing WO6 octahedra (805 cmꢀ1) in FTꢀIR spectrum (Fig.
2a).41 Notably, these characteristic bands remained in FTꢀIR
spectrum of PIPA-8, in spite of a slight shift (Fig. 2c vs. 2a ). It
suggested the incorporation of intact Kegginꢀtyped PW anion in
35 PIPA-8 hybrid. Red shift of W=Od band from 990 to 979 cmꢀ1
confirmed the presence of ionic interaction between imidazolium
IL cation and PW anion upon the hybridization.42, 43 Furthermore,
for the hybrid, the single PꢀOa at 1079 cmꢀ1 split into three bands
at 1079 and 1049 cmꢀ1 (Fig. 2c). It might attribute to the
40 formation of hydrogen bonding between terminal oxygen atoms
6＋
70 binding energy of W (W4f7/2 and W4f5/2) from 36.17 and 38.33
ev to 36.44 and 38.59 ev, respectively (Fig. 3B). Increased
binding energy verified electronic interactions between the diꢀ
imidazolium cations and PW anion. Redox property of W species
could thus be fineꢀtuned by the combined imidazolium IL cation.
75 Indeed, low valent W5+ species was coexisted with W6+ species in
PIPA-8, as evident from the appearance of new peak at ca. 34.9
ev in W4f XPS spectrum (Fig. 3B). These results strongly
indicated the partial reduction of W6+ species in the PW moiety of
PIPA-8, which arisen from the intermolecular electronic
80 interactions between the PEGꢀbridged diꢀimidazolium cations and
the Keggin PW anion.
O1s
36.19
HPW
A
HPW
B
38.33
W4f
of PW anions and the hydrogen atoms (ꢀNꢀC(H)2ꢀNꢀ) in
43
imidazolium ring.42,
Indeed, the C–H stretch modes in
P2p
imidazole ring (at 625 cmꢀ1) also exhibited a split upon
hybridization due to the participation of hydrogen bond (Fig. 2c
45 vs. 2b). The results demonstrated that Kegginꢀtype PW anion was
coupled with PEGꢀbridged diꢀimidazolium IL cations in PIPA-8
through ionic interaction together with hydrogen bond. In
addition, we noticed that a new band at 947 cmꢀ1 appeared as a
branch of the band at 979 cmꢀ1 (W=Od) in FTꢀIR spectrum of
O1s
PIPA-8
36.44
PIPA-8
C1s
38.59
W4f
P2p
N1s
34.9
44
50 PIPA-8, It was reported to be the reduced W5+ species.43,
Therefore, W6+ species coexisted with low valent W5+ species in
the typical hybrid of PIPA-8. The observation provided direct
evidence that the combined imidazolium IL cation adjusted the
redox property of W species in PIPA-8, which in turn fineꢀtuned
1200
800
400
0
30 32 34 36 38 40
Bonding Energy (ev)
Bonding Energy (ev)
Fig. 3 XPS survey spectra (A) and W4f XPS spectra (B) of HPW and
PIPA-8.
4
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UV-vis
UVꢀvis DR spectra provides an evidence for the intermolecular
crystallization water, which were hydrogenꢀbonded to the acidic
protons to form the [H2O. . .H+. . .OH2] ions. And the third
weight loss resulted from the removal of all acidic protons from
electronic interactions between the PEGꢀbridged diꢀimidazolium
cations and the Keggin PW anion. As shown in Fig. 4, pristine
Kegginꢀtype HPW exhibited three absorption peaks at 216, 258
and 322 nm (Fig. 4a). They were assigned to Oa2ꢀ→P5+ chargeꢀ
transfer transition, O2ꢀ→W6+ charge transfer where W atoms were
located in W–Oe–W intra bridges between edgeꢀsharing WO6
octahedra, and O2ꢀ→W6+ charge transfer where W atoms were
10 located in W–Oc–W inter bridges between cornerꢀsharing WO6
octahedra, respectively.41 For PW/IL hybrid of PIPA-8, the O2ꢀ
→W6+ charge transfer peak at 322 nm blueꢀshifted to 296 nm
(Fig. 4b vs. 4a), which could be attributed to the presence of ionic
and hydrogen bond interactions between PW anion and
15 imidazolium cations.41, 45 The other two absorptions remain at the
same energy upon the hybridization (Fig. 4b vs. 4a). We could
thus deduce that the only O2ꢀ atoms which were located in W–Oc–
the anhydrous H3PW12O40. It corresponded to loss of
46
45 constitutional water to form an anhydride phase PW12O38.5
.
5
Notably, such loss of physical adsorbed and structural water was
o
not observed in the case of PIPA-8 hybrid below 200 C (Fig.
5B). It was speculated that the hydrophobicity of PIPA-8 reduced
the adsorbed water in the composite material, and the
50 crystallization water should be also nonexistent due to the
absence of acidic protons in the hybrid. The observations
suggested the removal of structural water and three protons
during the assembly of HPW with PEGꢀbridged imidazolium IL.
Similar results were discussed in the previous reports.45 Above
55 200 oC, the organic part (PEGꢀbridged imidazolium cation) in
PIPA-8 begin to decompose, and the sharp loss in weight (ca. 13
wt.%) was appeared at 226 oC. Notably, the initial decomposition
temperature of organic part significantly decreased as compared
W
inter bridges between cornerꢀsharing WO6 octahedra
o
with that in PDILꢀ8 which exhibited the decomposition at 322 C
participated in the electronic interaction. Notably, the typical
20 PIPA-8 exhibited absorption band associated with reduced W5+
species at 505 nm (see Fig. 4 inset curve a). And the band was
undetectable for pristine Kegginꢀtype HPW (see Fig. 4 inset
curve b). The observation further confirmed the existence of
strong interaction between cations and anions, which finely
25 adjusted the redox properties of W species.
60 (Fig. 5B vs. 5C). The decrease in decomposition temperature
should be probably due to that bulky PW cations increased the
distance between cationic PEGꢀbridged imidazolium moieties in
the hybrids, which thus reduced thermal stability of the organic
o
part. Further weight loss in the temperature range of 407–649 C
65 (ca. 8.0 wt.%) could be assigned to the decomposition of the
remaining organic part, and /or the destroyed of the Keggin
258
o
structure. The decomposition was complete at about 649 C to
322
216
form the constituent oxides P2O5 and WO3. TGA analysis
revealed that the content of organic moiety in PIPA-8 was ca. 21
70 wt.% (0.40 mmol gꢀ1). PW content could thus be estimated to be
ca. 0.27 mmol gꢀ1 in the PIPA-8 hybrid, which approximate to
a
the content of the PW12O40 moiety (0.263 mmol gꢀ1) calculated
3ꢀ
505
from ICPꢀMS result (3.36 mmol/g of tungsten content).
b
296
100
100
A
B
100
99
98
97
96
95
C
b
400 500 600 700 800
b
95
90
85
80
75
70
b
80
60
40
20
0
a
b
b'
a
a
200
300
400
500
600
700
800
a
Wavelength (nm)
0
200 400 600 800
Temperature (oC)
0
200 400 600 800
Temperature (oC)
0
200 400 600 800
Fig. 4 UV–Vis DR spectra of Kegginꢀtype HPW (a), PIPA-8 (b) and
PIPA-8 reused for seven times (b’). Inset shows the enlarged UV–Vis DR
spectra of Kegginꢀtype HPW (a) and PIPA-8 (b) in the range of 400ꢀ800
nm.
Temperature (oC)
75 Fig. 5 TGꢀDTG curves of pristine Kegginꢀtype HPW (A), PIPA-8 hybrid
(B) and PDILꢀ8 (C) ((a) thermogravimetric curves; (b) differential
thermogravimetric curves).
30
TG-DTG
Thermogravimetric analysis (TGA) further verified the
incoporation of diꢀimidazolium IL cation with PW anion through
electronic interaction, as shown in Fig. 5. Pristine Kegginꢀtype
SEM
80 SEM images gave direct information of the hybridization of diꢀ
imidazolium cations with PW anions, as well as the morphologies
a
b
35 HPW showed three distinct steps of weight loss at 74, 178 and
of corresponding hybrids, as shown in Fig. 6. Obviously, pristine
Kegginꢀtype HPW exhibited bulk form with very low surface
area (Fig. 6a). Combining the bulk HPW with diꢀimidazolium IL
85 resulted in the formation of uniform particle (ca. 650 nm) with a
regular cubic shape (Fig. 6b). The great difference in morphology
confirmed the hybridization of bulk HPW with diꢀimidazolium IL.
o
421 C in the combined TGꢀDTG curves, when it was heated
o
from room temperature to 800 C under airflow (Fig. 5A). The
first weight loss was due to the removal of physisorbed water (a
variable amount depending on the number of hydration waters in
40 the sample). The second weight loss accounted for the removal of
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Notably, introducing various PEG moieties in the imidazolium IL
cations didn’t significantly affect the hybridization behavior of
PIPA-n, since PIPA-n (n= 4, 8, 13) gave a similar morphology
(0.154184 nm) and β was the line broadening in radian obtained
from the full width at half maximum, θ was the Bragg angle.
According to the Debye–Scherrer equation, we could calculate
to PIPA-0 (Fig. 6c–e vs.6b). We could thus deduce that the 40 the average size of PIPA-8 was 685 nm. This result was in
5
hybridization process was only triggered by the electronic
interaction between diꢀimidazolium cations and PW anions,
independent of the PEG linkers.
agreement with the SEM image for PIPA-8 shown in Fig. 6.
Water contact angle (WCA) measurement
The wettability of PIPA-n (n= 0, 4, 8, 13) was studied by means
45 of static WCA measurements (Fig. 8). The WCA value of PIPA-
0 was ca. 131°, implying the hybrid was hydrophobic (Fig. 8A).
After PEG modification, WCA value became smaller (56ꢀ80°),
implying the PIPA-n (n= 4, 8, 13) became hydrophilic (Fig. 8Bꢀ
8D). Considering the amphiphilic of PEG modifier, it was
50 concluded that the wettability transformation of PIPA-n (n= 0, 4,
8, 13) was essentially originated from PEG linker of
polyoxometalateꢀbased Gemini ionic catalysts. Indeed, the WCA
value of PIPA-n (n= 4, 8, 13) increased as the length of polyether
chain increased (Fig. 8B vs. 8C vs. 8D). The various wettability
55 would directly affect the inherent phase transfer capability of
PIPA-n (n= 0, 4, 8, 13), and further their catalytic efficiency in
oxidation of benzyl alcohol in water.
a
b
e
d
c
10 Fig. 6 SEM images of pristine Kegginꢀtype HPW (a), PIPA-0 (b), PIPA-
(c), PIPA-8 (d), and PIPA-13 (e)
B
4
A
PXRD
PXRD was used to further study the change in structure of PW
15 during the hybridization process. The wideꢀangle PXRD patterns
of pristine Kegginꢀtype HPW and the typical PIPA-8 were shown
in Fig. 7.
D
C
a
b
60 Fig. 8 Contact angle measurements of PIPA-0 (A), PIPA-4 (B), PIPA-8
(C) and PIPA-13 (D).
b'
Catalytic performances
5
10
15
20
25
30
35
40
Since benzaldehyde was a very important carbonyl compound
65 for widespread applications, catalytic performances of the PIPA-
n (n=0, 4, 8, 13) were evaluate in selective oxidation of benzyl
alcohol to benzaldehyde in water using aqueous H2O2 (30 wt.%)
as an oxidant. The results were summarized in Table 1. To
demonstrate the positive effect of diꢀcationic imidazolium IL on
70 catalytic performance, the monoꢀcationic counterpart of ILꢀPW
hybrid was prepared as the control catalyst by electrostatic
coupling of PW anion with 1ꢀmethylꢀ3ꢀbutyl imidazolium cation.
The Kegginꢀtyped HPW was also used for comparison.
Obviously, pure HPW showed low catalytic efficiency in the
75 aqueous selective oxidation of benzyl alcohol with H2O2 (30
wt.%) due to poor accessibility of hydrophobic substrate to waterꢀ
soluble active site in water. Only 48% conversion of the benzyl
alcohol with moderate selectivity (67%) was observed in spite of
the waterꢀsolubility of HPW (Table 1, entry 1). Monoꢀcationic
80 ILꢀPW hybrid offered higher conversion of 59% (Table 1, entry 3)
and selectivity (80%) than the neat HPW under identical reaction
conditions, although the countered IL of [BMIm]Cl was inactive
2 Theta (degrees)
Fig. 7 PXRD patterns of pristine Kegginꢀtype HPW (a), PIPA-8 (b) and
PIPA-8 reused for seven times (b’).
20
Obviously, a set of sharp Bragg peaks, characteristic of Kegginꢀ
type HPW, were obvious in the PXRD pattern of neat HPW (Fig.
7a). While, all these peaks disappeared in the case of PIPA-8,
25 despite the preservation of Keggin structure of PW species upon
the hybridization (Fig. 7a vs. 7b). Concomitantly, a novel set of
wellꢀresolved diffraction peaks appeared in the PXRD pattern of
PIPA-8 (Fig. 7a vs. 7b). The observations suggested that Kegginꢀ
type HPW was successfully incorporated with the diꢀimidazolium
30 IL through ionic interactions, as well as hydrogen bonding,
giving rise to a different crystal structure. The particle size of
PIPA-8 could be quantitatively evaluated from the XRD data
using the Debye–Scherrer equation, D = kλ/βcosθ, which gave a
relationship between peak broadening in XRD and particle size.
35 In this equation D was the thickness of the crystal, k was the
Debye–Scherrer constant (0.89), λ was the Xꢀray wavelength
6
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(Table 1, entry 2). It indicated the efficient tuning of the redox
OH
20
21
22
HPW
ILꢀPW
5
9
57
75
78
O
properties of W species by the imidazolium IL cation. Moreover,
it couldn’t be excluded that the compatible imidazoleꢀbased IL
cation may favor the adsorption of the benzyl alcohol molecule
and thus enhanced the activity. However, the monoꢀcationic ILꢀ
PW hybrid was far less active than the diꢀcationic ILꢀbased
PIPA-n (n=0, 4, 8, 13) (Table 1, entry 3 vs. entries 4–7). Using
aqueous H2O2 as oxygen source, the diꢀcation ILꢀbased PIPA-0
gave higher conversions (71%) and selectivity (82%) in the
PIPA-13
17
5
a) Catalyst (1.3 mol% substrate, based on the PW loading), substrates (10
mmol), water (1.5 ml), H2O2 (30 wt.%, 15 mmol), 95 °C, 6 h.
30 b) Conversion was based on alcohols.
c) Selectivity was based on the corresponding aldehydes or ketones.
10 selective oxidation of benzyl alcohol with aqueous H2O2 (30
wt.%) in water (Table 1, entry 4). The reason for higher
efficiency of PIPA-n might be the diꢀcationic structure of
imidazolium IL. It could couple with trivalent PW anions at a
molar ratio of 3: 2. Multiple catalytic PW sites were thus
15 combined into the formed hybrid through the ionic selfꢀassembly
route,47 enforcing an intramolecular, cooperative reaction
pathway. Incorporating a phaseꢀtransfer PEG group into the diꢀ
imidazoleꢀbased IL linker further improved the catalytic
efficiency (Table 1, entries 5–7). The higher activity of PIPA-n
20 (n= 4, 8, 13) should be related with the builtꢀin phase transfer
capability of PIPA-n arisen from PEG midifier. Actually,
amphiphilic characteristics of PEG moiety not only improved the
solubility of organic substrate in the aqueous reaction, but also
enhanced interaction of H2O2 with PW anion to form PW4 species
25 that was the real active catalyst in the selective oxidation.48
Table 1 Results of the selective oxidation of alcohols to aldehydes with
To further understand the advantages of enhanced cooperative
catalysis, as well as builtꢀin phase transfer capability, for PIPA-n
35 (n =0, 4, 8, 13) in the aqueous oxidation, kinetics was thus used
to study the reaction rates of aqueous selective oxidation of
benzyl alcohol over PIPA-0, PIPA-4, PIPA-8, PIPA-13, and
monoꢀcationic ILꢀPW hybrid in detail. The corresponding kinetic
curves and rate curves are shown in Fig. 9. Clearly, benefiting
40 from the Gemini structure, PIPA-n (n =0, 4, 8, 13) afforded
higher efficiency than the monoꢀcounterpart of ILꢀPW (Fig. 9aꢀd
vs.9e). Although it also has a diꢀcationic structure, PIPA-0 was
far less active than PIPA-n (n =4, 8, 13) as evidenced by the
lower conversion of benzyl alcohol (Fig. 9Aꢀd vs. 9A(aꢀc)) and
45 reduced Kobs (Fig. 9Bꢀd vs. 9B(aꢀc)). This observation
demonstrated that inherent phase transfer capability of PIPA-n (n
=4, 8, 13) arisen from PEG linker was favored in the catalysis.
Indeed, the length of PEG spacer was found to influence reaction
rates of the aqueous selective oxidation of benzyl alcohol. PIPA-
50 13 showed the highest catalytic efficiency, giving almost
quantitative conversion (96%) of benzyl alcohol within 6 h (Fig.
9a). Further shortening the bridged polyether chain led to a
decreased activity in the reaction accordingly (Fig. 9b and 9c). A
possible explanation might be that the length of polyether chain
55 directly affected not only the inherent phase transfer capability of
catalyst, but also the cooperative interaction of the bridged active
sites. Although increasing the total length of PEG spacer reduced
the phase transfer capability of catalysts, it might enhance the
flexibility of the bulky PW species, and thus improved the
60 intramolecular cooperation resulting in enhanced reaction rates.
Therefore, PIPA-13 with the longest length of PEG spacer
presented here gave highest overall activity in the selective
oxidation of benzyl alcohol in water.
H2O2 in water over various PWꢀbased catalysts a
Product
Entry
1
Catalyst
HPW
Substrates
Conv. (%) b
Sel. (%) c
67
s
48
Trace
59
2
[BMIm]Cl
ILꢀPW
PIPA-0
PIPA-4
PIPA-8
PIPA-13
HPW
/
3
80
CH2OH
CHO
4
71
82
5
88
79
6
91
84
7
96
86
8
48
>99
>99
>99
>99
>99
>99
>99
>99
O
OH
100
80
60
40
20
0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
A
a
B
9
ILꢀPW
PIPA-13
HPW
70
10
11
12
13
14
15
95
b
a
43
b
c
d
e
OH
O
ILꢀPW
PIPA-13
HPW
82
c
95
37
d
e
O
ILꢀPW
56
OH
16
PIPA-13
74
>99
0
50 100 150 200 250 300 350
Time (min)
50 100 150 200 250 300 350
Time (min)
CHO
17
18
19
HPW
ILꢀPW
4
7
61
90
89
CH2OH
65
Fig. 9 Kinetic curves (A) and rate curves (B) of selective oxidation of
benzyl alcohol over PIPA-13 (a), PIPA-8 (b), PIPA-4 (c), PIPA-0 (d),
and ILꢀPW (e) in water.
PIPA-13
12
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Remarkable enhancement of reaction rates with the 50 investigate the stability of typical PIPA-13 hybrid during the
employment of the PIPA-13 was also observed in the case of 1ꢀ
phenylethanol, cyclohexanol, diphenylmethanol, 2ꢀphenylethanol,
and nꢀhexanol, as shown by their conversions in Table 1 (Table 1,
entry 10 vs. entries 8 and 9, entry 13 vs. entries 11 and 12, entry
reaction (Fig. 10). PIPA-13 hybrid exhibited distinct P signal of
[PW12O40]3ꢀ at ꢀ15.1 ppm in 31P NMR spectrum (Fig. 10a). The P
signal gradually disappeared as reaction progress accompanied by
the appearance of signal associated with [PW4O8(O2)8]3ꢀ species
5
16 vs. entries 14 and 15, entry 19 vs. entries 17 and 18, entry 22 55 at ꢀ0.9 ppm (Fig. 10bꢀe). The distinguishing change suggested the
vs. entries 20 and 21). Conversions of the corresponding alcohols
were dependent on the nature of substrates used in this work.
Secondary alcohols, such as 1ꢀphenylethanol and cyclohexanol,
partial degradation of [PW12O40]3ꢀ into peroxoꢀbridged
[PW4O8(O2)8]3ꢀ species by the reaction with H2O2. The resulting
[PW4O8(O2)8]3ꢀ species was waterꢀsoluble and acted as the real
active species for catalyzing alcohols oxidation. With the
10 were readily oxidized in water using aqueous H2O2 as oxidant,
giving corresponding ketones with high yields (95%) (Table 1, 60 consumption of H2O2, the soluble [PW4O8(O2)8]3ꢀ species
entries 10 and 13). Diphenylmethanol underwent a slower
oxidation over PIPA-13 due to unfavorable accessibility of bulky
substrate to active sites (Table 1, entry 16). Primary aliphatic
15 alcohols, such as 2ꢀphenylethanol and 1ꢀhexanol (Table 1, entries
transformed into the insoluble Kegginꢀtype [PW12O40]3ꢀ species,
as evident from the reappeared [PW12O40]3ꢀ signal at ꢀ15.1 ppm
(Fig. 10e). Notably, the [PW4O8(O2)8]3ꢀ signal in 31P NMR almost
disappeared, and only the signal associated with [PW12O40]3ꢀ
19 and 22), were far less reactive than the secondary alcohols 65 existed after the complete consumption of H2O2 at 6 h (Fig. 10f).
(Table 1, entries 10, 13, and 16) and also the benzyl alcohol
(Table 1, entry 7). The conversions were less than 17% when 1.5ꢀ
fold molar excess of H2O2 was used (Table 1, entries 19 and 22).
Superiority of PIPA-13 over other reported PWꢀbased hybrids,
such as selfꢀassembled ILꢀPW hybrid ([PyHA]3PW and
[TMGOH]2.2H0.8PW),49 ILꢀmodified SBAꢀ15 supported PW
catalyst (PWꢀNH2ꢀILꢀSBAꢀ15),50 and MCMꢀ41 supported PW
catalyst ((PW)3/MCMꢀ41)51 was seen in Table 2. Obviously,
We thus propose that the [PW12O40]3ꢀ species was recovered after
the reaction, and the waterꢀinsolubility of catalyst ensured it to be
facilely separated from the aqueous system for reuse.
20
a
b
c
25 PIPA-13 was more efficient than these reported PWꢀbased
hybrids either homogeneous or heterogeneous in oxidation of
benzyl alcohol with H2O2 in water (Table 2, entry 1 vs. entries 2ꢀ
d
e
5). Notably, although PWꢀNH2ꢀILꢀSBAꢀ15 had
a similar
countered imidazolium IL cation, it required 3ꢀfold molar excess
30 of H2O2 to afford the yield of benzaldehyde (83%) equal to that
over PIPA-13 (Table 2, entry 4 vs. 1). Furthermore, the ILꢀfree
hybrid of (PW)3/MCMꢀ41 gave only 20% yield of benzaldehyde
even with 3ꢀfold molar excess of H2O2 (Table R1, entry 5).
Higher efficiency of PIPA-13 demonstrated the advantages of
35 builtꢀin phase transfer capability, as well as enhanced cooperative
action, for the PWꢀbased Gemini ionic catalyst in the aqueous
f
30
20
10
0
ꢀ10
ꢀ20
ꢀ30
ꢀ40
ꢀ50
ppm
70 Fig. 10 Changes in 31P NMR spectra of PIPA-13 in selective oxidation of
benzyl alcohol with H2O2 in water: before reaction (a), the reaction after 1
h (b), 2 h (c), 3 h (d), 5 h (e), and 6 h (f).
oxidation.
Table 2 Comparison of results obtained over different catalysts in the
a
selective oxidation of benzyl alcohol with H2O2
En
try
Reaction
time (h)
Mol ration
(H2O2/alcohol)
Yield b
(%)
Indeed, the PIPA-n (n= 4, 8, 13) could be almost
75 quantitatively (>99%) recovered from the reaction mixture by
centrifugation. Fig. 11 showed the reusability of PIPA-n (n = 4,
8, 13) hybrids in selective oxidation of benzyl alcohol to
benzaldehyde with H2O2 in water. As expected, all the hybrids
could be reused for seven times with no appreciable decrease in
80 activity and selectivity of benzaldehyde, which demonstrated
their excellent stability. ICPꢀMS analysis of recovered typical
PIPA-8 gave an almost identical content of tungsten element
(3.19 mmol/g) to that of the fresh one (3.36 mmol/g).
Furthermore, a negligible amount of soluble active species could
85 be detected in the filtrate in terms of tungsten percentage
according to ICPꢀMS analysis. FTꢀIR (Fig. 2c vs. 2c’) and UVꢀ
vis spectra (Fig. 4b vs. 4b’) gave direct evidence of the stability
of PIPA-n (n= 4, 8, 13) based on the fact that no significant
change in typical PIPA-8 occurred even after reusing seven
90 times. Furthermore, PXRD analysis indicated that the structural
integrity of PIPA-8 was wellꢀpreserved during the reaction, since
crystallinity of PIPA-8 was well retained after the recyclability
test (Fig. 7b vs. 7b’). The observations demonstrated the perfect
Catalyst
Ref
This
work
1
2
3
4
5
PIPA-13
6
6
1.5
1.5
1.5
3.0
3.0
83
81
70
83
20
[TMGOH]2.2
0.8PW
H
Ref. c
Ref. c
Ref. d
Ref. e
[PyHA]3PW
6
PWꢀNH2ꢀILꢀ
SBAꢀ15
6
(PW)3/MCMꢀ
41
18
40 a) Catalyst (1.3 mol% substrate, based on the PW loading), substrates (10
mmol), water (1.5 ml), H2O2 (30 wt.%, 15 mmol), 95 °C, 6 h.
b) Determined by GC.
c) Data from the Ref.49
d) Data from the Ref. 50
45 e) Data from the Ref. 51
It should be more attractive if the activity and structural
integrity of PIPA-n (n =4, 8, 13) was preserved during the
aqueous oxidation at 95 oC. 31P NMR was employed to
8
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C8CY01191E
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100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
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20
0
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Run times
Run times
Run times
Fig. 11 Reusability of PIPA-4 (A), PIPA-8 (B) and PIPA-13 (C) in the
selective oxidation of benzyl alcohol with H2O2 as an oxidant (a:
conversion; b: selectivity).
5
65
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10 Keggin PWꢀanions. The diꢀimidazolium IL cations endowed the
PW catalysts with some distinct characteristics as follows: (a)
dense active sites for cooperative catalysis; (b) high compatibility
in water; (c) builtꢀin phase transfer capability for aqueous
oxidation. The new PWꢀbased catalysts thus exhibited excellent
15 catalytic efficiency, convenient recovery and steady reuse in the
liquidꢀphase oxidation of alcohols to corresponding aldehydes
with aqueous H2O2 (30 wt.%) in water. Salient advantages of the
PWꢀbased ionic hybrids inspired us to develop more versatile
POMꢀbased catalysts for heterogeneous H2O2ꢀbased organic
20 oxidations in water.
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Graphical Abstract
Polyoxometalate-based Gemini ionic catalysts for selective
oxidation of benzyl alcohol with hydrogen peroxide in water
Pengbo Hao, Rong Tan*, Mingjie Zhang, Wei Zhang, Zhiyang Tang, Ni Luo and Donghong Yin
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of
Resources, Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry
of Education), Hunan Normal University, Changsha 410081, P. R. China.
Polyoxometalate-based Gemini ionic hybrids with inherent phase-transfer capability are the highly
efficient and recyclable catalysts in the selective oxidation of alcohols with H2O2 in water.
* Corresponding authors. Fax: +86-731-8872531. Tel: +86-731-8872576
E-mail: yiyangtanrong@126.com