Synthesis of immobilized heteropolyanion-based ionic liquids on mesoporous silica SBA-15 as a heterogeneous catalyst for alkylation

Article abstract of DOI:10.1039/c4ra03531c

Phosphotungstic acid (H₃PW₁₂O₄₀) has been successfully loaded onto sulfonate-functionalized ionic liquid-modified mesoporous silica SBA-15 by total anion-exchange. The immobilized catalysts were characterized by XRD, N₂ adsorption, TEM, and FTIR spectroscopy. Characterization results show that the mesopore structure of SBA-15 was maintained well even after surface modification and the subsequent anion-exchange step of [PW₁₂O₄₀]^3- (PW). In comparison with the task-specific basic ionic liquid (1-(propyl-3-sulfonate) 3-methyl-imidazolium phosphotungstate), the obtained catalyst showed much higher efficiency in alkylation of o-xylene with styrene. More importantly, such an immobilized task-specific basic ionic liquid could be reused without significant loss of catalytic activity even after recycling six times. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03531c

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Graphical Abstract
Phosphotungstic acid (H PW O ) has been successfully loaded onto
3
12 40
sulfonate-functionalized ionic liquid modified mesoporous silica SBA-15 by totally
anion-exchange. The obtained catalysts were highly effective for alkylation of
o-xylene with styrene in terms of yield and stability. Compared with the
corresponding pure ionic liquid, both the catalytic stability and reusability were
greatly enhanced.

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Synthesis of immobilized heteropolyanionꢀbased ionic liquids on
mesoporous silica SBAꢀ15 as heterogeneous catalyst for alkylation
a
a,
a
a
a
a
a
Xiaoli Sheng , Yuming Zhou *, Yongle Yang , Yiwei Zhang , Zewu Zhang , Shijian Zhou , Xiaoqin Fu ,
a
Shuo Zhao
a
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P.R.China
Abstract Phosphotungstic acid (H PW O ) has been successfully loaded onto
3
12 40
sulfonateꢀfunctionalized ionic liquid modified mesoporous silica SBAꢀ15 by totally
anionꢀexchange. The immobilized catalysts were characterized by XRD, N adsorption, TEM,
2
and FTIR spectroscopy. Characterization results show that the mesopore structure of SBAꢀ15 was
maintained well even after the surface modification and the subsequent anionꢀexchange step of
3
ꢀ
[
PW O ] (PW). In comparison with the taskꢀspecific basic ionic liquid (1ꢀ(propylꢀ3ꢀsulfonate)
12 40
3ꢀmethylꢀimidazolium phosphotungstate), the obtained catalyst showed much higher efficiency in
alkylation of oꢀxylene with styrene. More importantly, such immobilized taskꢀspecific basic ionic
liquid could be reused without significant loss of catalytic activity after 6 times recycling.
*To whom correspondence should be addressed. Tel: +8625ꢀ52090617. Fax: +8625ꢀ52090617.
E-mail address: ymzhou@seu.edu.cn (Y. Zhou)
1

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1. Introduction
The alkylation of oꢀxylene with styrene to give 1ꢀphenylꢀlꢀarylethane is an industrially
1
important reaction . Phenylxylylethane(PXE) is a colorless synthetic liquid with many excellent
properties suitable for various applications, e.g., solvent for pressureꢀsensitive record materials,
1
plasticizer, heating medium, electricꢀinsulating oil and highꢀboiling solvent . Nevertheless, one
of the main problems in this reaction is the formation of styrene oligomers generated as
byproducts, especially when homogeneous Brønsted or Lewis acid catalysts (such as H SO , BF ,
2
4
3
1
HF, AlCl or FeCl ) are used . Such the aforementioned traditional acids are generally related to
3
3
problems of high corrosiveness, poor shapeꢀselectivity, low hydrothermal stability and difficulties
in waste acid treatment. The hastily demand for environmentally friendly catalytic technology
urges development of heterogeneous catalysts thanks to their advantages such as high activity and
selectivity, ease of separation and reusability. For PXE synthesis, a monumental series of solid
2
3
4, 5
acid catalysts such as silicaꢀalumina , cationꢀexchange resins , sulfated zirconia/titania
,
6
7, 8
heteropolyacid salts and modified mesoporous silicas
are adopted as catalysts, all of which
have promoted the catalytic activity. However, these catalysts have disadvantages as well, for
example, the compatibility of reusability, operation loss, and high mass transfer resistance, which
limit their practical application in alkylation.
Among the recent developments in catalysis, ionic liquids(ILs) have been attracted much
attention as being environmentalꢀfriendly reaction media for their unique properties of high
9
, 10
thermal stability, negligible vapor pressure, tunable acidity and selective dissolvability
.
Though ionic liquids possessed such promising advantages, their widespread practical application
was still hampered by several drawbacks such as unendurable viscosity, high cost, tedious
1
1
purification procedure of the product and requirements of long reaction time . Therefore, in
order to overcome the above shortcomings, immobilized ionic liquid catalyst combining the
1
2,
advantageous characteristics of ionic liquids, inorganic acids and solid acids had been proposed
1
3
.
Indeed, rapidly increased reports have become available describing the use of the ILs
1
4
15
immobilized on various supports in the FriedelꢀCrafts reactions . Ho¨ lderich et al. developed
a process for the alkylation of benzene, toluene, naphthalene, and phenol with dodecene in the
presence of immobilized 1ꢀbutylꢀ3ꢀmethylꢀimidazolium chloride with AlCl (AlꢀIIL). The
3
immobilized IL played an important role in significant reduction of overall cost of the process. A
2

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comparison of different supports showed that carriers based on SiO and Al O retained higher
2
2
3
amounts of chloroaluminate IL than ZrO and TiO carriers after the extraction, and only the
2
2
1
6
silicaꢀbased supports were active in the alkylation reactions. In 2009, Yin and coꢀworkers
reported the preparation and catalytic activity of novel periodic mesoporous organosilica (PMO)
materials incorporating Lewis acidic chloroindate(III) ionic liquid moieties. It was observed that
the immobilization of the Lewis acidic ionic liquids with increasing amount of InCl led to a
3
drastic increase in the catalytic activity ranging from 14 to 100 conversions with constant
selectivity of 100%.
It is well known that Lewis acidic chloroindate(III) ionic liquid moieties is limited to anions
with a high affinity to oxygen. Therefore, Brønsted acidic ILs used in some acidꢀcatalyzed
processes have aroused considerable interest since they show higher stability towards air and
1
7
water . With those points in mind, we designed a new heteropolyanionꢀbased ILs containing the
sulfonic acid functional group as “taskꢀspecific” catalysts immobilized on mesoporous silica
SBAꢀ15. The immobilized catalysts were fully characterized by XRD, N adsorption, TEM, and
2
FTIR spectroscopy. Using the FriedelꢀCrafts alkylation as test reaction, the immobilized TSILs
showed high efficiency and stability.
2. Experimental
2.1 Catalysts preparation
Fig.
1
shows the schematic procedures for the synthesis of immobilized
HPWꢀPMIMPSꢀSBAꢀ15 materials. Nꢀtriethoxysilylpropylimidazole(compound 1) was
synthesized by refluxing 53.00 mmol of imidazole with 50.00 mmol of
3ꢀtriethoxysilylpropylchloride in toluene (50 mL) at 70 ℃for 24 h under a nitrogen atmosphere.
1,3ꢀpropane sulfon e（ 10.50 mmol）was added to 2.72 g of the compound 1, followed by stirring at
50 ℃for 8 h under a nitrogen atmosphere. A white precipitate was filtered, washed with diethyl
ether
three
times,
and
then
dried
in
vacuum.
1ꢀsulfonic
group
propylꢀ3ꢀtriethoxysilylpropylꢀimidazolium(compound 2) was obtained. 1.00 g of dry SBAꢀ15
1
8
prepared according to the literature method and 0.40 g of the compound 2 were dispersed in 50
mL of anhydrous toluene, and then refluxed for 24 h under nitrogen protection. After being
3

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filtrated and washed with anhydrous ethanol, the obtained precipitate was Soxhlet extracted over
ethanol for 48 h. The product was filtrated, washed with anhydrous ethanol, and dried at room
temperature under vacuum overnight. The immobilized basic sulfonic ꢀcontaining TSILs is
designated as PMIMPSꢀSBAꢀ15(compound 3). 1.00 g of the obtained compound 3 was dispersed
in 100 mL of deionized water, followed by adding various amount of 12ꢀTungstophosphoric Acid
(HPW) according to different weight ratio of HPW to the compound 3 (n, n = 0.10, 0.20, 0.30, or
0.40), then the resultant suspension was kept stirring at room temperature for 12 h. The resulting
precipitate was filtered, washed with deionized water several times, dried overnight in vacuum,
and Soxhlet extracted over ethanol for 12 h. The obtained sample was denoted as
xHPWꢀPMIMPSꢀSBAꢀ15(compound 4), where x stands for the weight ratio of HPW to the
compound 3.
For comparison purposes, the taskꢀspecific basic ionic liquid 1ꢀ(propylꢀ3ꢀsulfonate)
3
ꢀmethylꢀimidazolium phosphotungstate ([MIMPS] PW O ) was synthesized by the following
3
12 40
1
9
procedure . Methylimidazole (0.11 mol) and 1,3ꢀpropane sulfone (0.10 mol) were dissolved in
toluene (20 mL) and stirred for 24 h at 50 °C under a nitrogen atmosphere. A white precipitate
(
MIMPS)was filtered, washed with diethyl ether three times, and then dried in vacuum. MIMPS
0.06 mol) was added to an aqueous solution of H PW O (0.02 mol), and then the mixture was
(
3
12 40
stirred at room temperature for 24 h. Water was removed in vacuum to give the product as a solid.
2.2 Catalysts Characterization
Powder Xꢀray diffraction (XRD) patterns were obtained with a Rigaku D/maxꢀrC Siemens
diffractometer using nickelꢀfiltered Cu Kα as monochromatic Xꢀray radiation. The scattering
intensities were measured over an angle range of 0.58 < 2θ < 40 with a step size D(2θ) = 0.028
and a step time of 8 s. The nitrogen adsorption and desorption isotherms were measured at
ꢀ196 ℃ on an ASAP 2020 system. Prior to the measurements, all samples were degassed under
vacuum at 80℃ for 4 h. The specific surface area, ABET, was determined from the linear part of
the BET equation (P/P = 0.05ꢀ0.30). The pore size distribution was derived from the adsorption
0
branch of the N isotherm using the BarrettꢀJoynerꢀHalenda (BJH) method. The total pore volume
2
was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P ) of ca. 0.995.
0
Pore structures of the samples were examined by TEM (Jeol, JEMꢀ2000EXII). Fourier
transformation infrared (FTꢀIR) spectra were obtained on a Bruker Tensor 27, scanned from 4000
4

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ꢀ1
ꢀ1
cm to 400 cm . The sample was ground with KBr and pressed into a thin wafer.
Fig. 1. Schematic procedures for the synthesis of immobilized HPWꢀPMIMPSꢀSBAꢀ15 materials
2.3 Catalytic Tests
The alkylation reactions were carried out in a continuously stirred batch reactor under reflux
conditions using a threeꢀneck 100ꢀml roundꢀbottom flask equipped with a condenser. Preliminary
runs were conducted with 7.50 g (0.0721mol) of styrene, 57.35 g (0.5402 mol) of oꢀxylene (mole
ratio of oꢀxylene to styrene, 7.5︰1) and 1.50 g of catalyst (20% w/w of styrene) at 120 ℃ for
180 minutes. The required amount of oꢀxylene was initially added to the reactor at the reaction
temperature, followed by the desired amount of catalyst, a known amount of styrene was then
added to the reaction mixture at the same temperature. After the reaction, unreacted oꢀxylene was
distilled out under atmospheric pressure and then a collected part was called as crude product.
The crude product was analyzed with GCꢀ9890A gas chromatograph equipped with OVꢀ1
5

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capillary column and a flame ionization detector (FID). The yield of PXE was defined as follows:
actual product weight
yield of PXE(%) =
×100
all theoretical product weight
actual product weight = crude product weight ×PXE(chromatography)%
3
. Results and discussion
100
B
A
110
200
f
c
e
b
a
d
5
10
15
20
25
30
35
40
1
2
3
4
5
2
Theta(degrees)
2Theta(degrees)
Fig. 2
Powder lowꢀangle Xꢀray diffraction patterns of the samples siliceous SBAꢀ15(Aꢀa),
PMIMPSꢀSBAꢀ15(Aꢀb) and 30%HPWꢀPMIMPSꢀSBAꢀ15(Aꢀc) and powder highꢀangle Xꢀray diffraction
patterns of the HPW(Bꢀd), 30%HPWꢀPMIMPSꢀSBAꢀ15(Bꢀe) and 40%HPWꢀPMIMPSꢀSBAꢀ15(Bꢀf).
The lowꢀangle XRD patterns of the siliceous SBAꢀ15, PMIMPSꢀSBAꢀ15 and
3
3
0%HPWꢀPMIMPSꢀSBAꢀ15, as well as the highꢀangle XRD patterns of the Keggin HPW,
0%HPWꢀPMIMPSꢀSBAꢀ15 and 40%HPWꢀPMIMPSꢀSBAꢀ15 are shown in Fig. 2. The siliceous
SBAꢀ15 showed a pattern with a very strong reflection at 2θ=0.86
weaker reflections at 2θ=1.46 and 2θ=1.68 for d110 and d200, respectively, associated with the
quasiꢀregular arrangement of mesopores with hexagonal symmetry [17]. In contrast, three
reflections of d100 d110, and d200 could clearly be observed for PMIMPSꢀSBAꢀ15 and
0%HPWꢀPMIMPSꢀSBAꢀ15, although the intensities of d100 d110, and d200 reflections decreased
°for d100 and two other
°
°
,
3
,
slightly and the three diffraction peaks shifted to higher angle in comparison with the siliceous
SBAꢀ15. This can be due to decreased local order, and/or the reduction of scattering contrast
20
induced by the incorporated organic moieties and PW anions . Moreover, the highꢀangle XRD
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pattern of the 30%HPWꢀPMIMPSꢀSBAꢀ15 does not show the distinct diffraction peaks
corresponding to the crystalline HPW phase (Fig. 2Bꢀe vs Fig. 2Bꢀd), which suggests the high
2
1
dispersion of PW in the channels of the 30%HPWꢀPMIMPSꢀSBAꢀ15 . However, at higher
HPW loading(40%HPWꢀPMIMPSꢀSBAꢀ15), the highꢀangle XRD pattern showed some distinct
diffraction peaks corresponding to the crystalline HPW phase (Fig. 2Bꢀf), demonstrating the
agglomeration of the excessive HPW in the mesopore channels of PMIMPSꢀSBAꢀ15.
3
0
2
250
000
750
500
250
000
50
00
50
(b)
2
25
4
0%HPW-PMIMPS-SBA-15
40%HPW-PMIMPS-SBA-15
1
20
1
30%HPW-PMIMPS-SBA-15
30%HPW-PMIMPS-SBA-15
1
15
20%HPW-PMIMPS-SBA-15
20%HPW-PMIMPS-SBA-15
10%HPW-PMIMPS-SBA-15
1
10
7
10%HPW-PMIMPS-SBA-15
5
5
0
2
PMIMPS-SBA-15
0.2
PMIMPS-SBA-15
0
0.0
0.4
0.6
0.8
1.0
0
5
10
15
20
25
30
35
40
45
50
P/P0
RelativePressure( )
Pore Diameter(nm)
Fig. 3 Nitrogen adsorptionꢀdesorption isotherms (left) and pore size distribution (right) of the samples
PMIMPSꢀSBAꢀ15 and xHPWꢀPMIMPSꢀSBAꢀ15(x=10%, 20%, 30% and 40%)
Table 1.Textural parameters of the PMIMPSꢀSBAꢀ15 and xHPWꢀPMIMPSꢀSBAꢀ15
a
b
Sample
HPW
wt%)
ꢀ
Vtotal
SBET
Pore size
3
ꢀ1
2
ꢀ1
(
(cm g )
(m g )
618
(nm)
PMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
BET method
0.85
0.70
0.66
0.61
0.58
5.90
5.60
5.50
5.48
5.42
1
2
3
4
10
446
20
376
30
363
40
330
a
b
BJH model applied to the desorption branch of the isotherms
N adsorptionꢀdesorption isotherms record the textural properties of the PMIMPSꢀSBAꢀ15 and
2
xHPWꢀPMIMPSꢀSBAꢀ15, as shown in Fig. 3. As can be seen, all samples display typical typeꢀIV
7

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isotherms with a clear adsorptionꢀdesorption hysteresis loops at the relative pressure of 0.65 <
P/P < 0.85 (Fig. 3 left), characteristic of mesoporous material with uniform cylindrical channel
0
and narrow pore size distribution [17]. The shape of the isotherms varies with the ionic liquid
moieties and PW anions. On the other hand, the BJH pore size distribution (calculated from the
analysis of the desorption branch of the isotherms) is presented in Fig. 3 right. It can be seen that
all samples exhibit a fairly uniform pore size distribution (5ꢀ7 nm). Effect of the ionic liquid
moieties and PW anions loadings on the corresponding surface area, pore diameter and pore
volume of HPWꢀPMIMPSꢀSBAꢀ15 catalysts are presented in Table 1. As expected,
HPWꢀPMIMPSꢀSBAꢀ15 present decreased pore volume compared to the parent
PMIMPSꢀSBAꢀ15. Increased PW anions loading leads to a gradually decrease in pore volume.
Especially, some pore blockage have taken place when the PW anions content increased over
4
0%, which suggests that the PW anions are located inside the pore indeed. These details are also
reflected in the surface area and pore size of HPWꢀPMIMPSꢀSBAꢀ15 catalysts. However, the
2
surface area of the samples up to 30% HPW loaded was more than 300 m /g and the pore volume
3
was in the range of 0.61ꢀ0.70 cm /g (Table 1), which is sufficient for catalytic performance.
Fig. 4. Typical TEM images of PMIMPSꢀSBAꢀ15 (a and b) and 30%HPWꢀPMIMPSꢀSBAꢀ15 (c and d). The
bars in the photos are 20 nm.
TEM
micrographs
of
the
representative
sample
PMIMPSꢀSBAꢀ15
and
3
0%HPWꢀPMIMPSꢀSBAꢀ15 (Fig. 4) provide a direct visualization of wellꢀordered hexagonal
8

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arrays of 2D mesoporous channels particularly in the direction perpendicular to the pore axis (Fig.
a and c) or along the direction of the pore axis (Fig. 4b and d). The TEM images confirm the
4
intact mesostructures of all the samples even though the PW and ionic liquid moieties were
loaded in the channel of SBAꢀ15.
(c)
+
+
+
+
#
#
#
#
∗
∗
#
(
b)
•
∗
∗
•
•
•
•
•
•
#
(
a)
•
•
• •
•
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber cm
Fig. 5 FTꢀIR spectra of pure HPW(a), PMIMPSꢀSBAꢀ15 (b) and 30%HPWꢀPMIMPSꢀSBAꢀ15 (c)
Fig. 5 compares the FTꢀIR spectrum of 30%HPWꢀPMIMPSꢀSBAꢀ15 with those of
PMIMPSꢀSBAꢀ15 and neat Kegginꢀtype HPW. In the cases of the samples incorporated with the
terminal HPWꢀbased ionic complexes, 30%HPWꢀPMIMPSꢀSBAꢀ15 represented an almost
identical FTꢀIR spectrum to PMIMPSꢀSBAꢀ15 (Fig. 5b vs. Fig. 5c). More specifically, both the
ꢀ1
ꢀ1
samples showed vibration bands at 3160, 3118 cm (+), 1556,1451,1170 cm (#), and 1242,738
ꢀ1
cm (*), which can be assigned, respectively, to the aromatic CꢀH stretching in imidazolium
2
2
cations, the feature vibrations of imidazolium moieties, the CꢀSi stretching vibrations . Such
observations suggest that the grafting process presented here do not change the structure of the
ionic liquid. As shown in Fig. 5, four characteristic bands at around 1080, 980, 890 and 800
ꢀ
1
cm (
·) appears distinctively in the FTꢀIR spectrum of the Kegginꢀtype HPW (Fig. 5a), which are
assigned to asymmetric stretching vibration of PꢀO , W=O , WꢀO ꢀW in corner shared octahedral
a
d
b
2
3
and WꢀO ꢀW in edge shared octahedral, respectively . The four bands also appear in the FTꢀIR
c
spectrum of the supported 30%HPWꢀPMIMPSꢀSBAꢀ15, which indicates that the Keggin
structure of PW anions is well reserved (Fig. 5c vs Fig. 5a). Therefore, combined Fig 4 and Fig 5,
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it can be indicative of covalent attachment of HPW and linker with SBAꢀ15 surface.
The catalytic activity of different catalysts is showed in Table 2. The detailed reaction scheme
is shown in scheme 1, reaction (1) is the PXE formation reaction whereas reaction (2) and (3)
represent the formation of styrene oligomers and more substitutes, respectively.
Cat
+
OꢀXylene
Styrene
Phenylxylyl ethane(PXE)
(1)
n
Cat
Styrene
Styrene Oligomer
(2)
Cat
OꢀXylene
Styrene
more substitutes
(3)
Scheme 1 Reaction scheme of alkylation of oꢀxylene with styrene over a heterogeneous catalyst
As shown in Table 2, the homogeneous HPW show very high catalytic performances for the
reaction, however, it is difficult to separate the HPW from the product mixture. Furthermore, the
SBAꢀ15
support
itself
shows
no
activity
performances.
Furthermore,
3
0%HPWꢀPMIMPSꢀSBAꢀ15 sample exhibits the highest product yield and good selectivity,
meaning that suitable concentration of PW anions is advantageous to the reaction. In this case a
significant difference is found among these catalysts as a function of loading the PW anions. The
PXE yield decreases in the following order: 30%HPWꢀPMIMPSꢀSBAꢀ15 (93.9%) >
4
0%HPWꢀPMIMPSꢀSBAꢀ15
(91.2%)
>
20%HPWꢀPMIMPSꢀSBAꢀ15
(80.5%)
>
1
0%HPWꢀPMIMPSꢀSBAꢀ15 (71.8%). In the case of 40%HPWꢀPMIMPSꢀSBAꢀ15 the product
yield is found to be less compared to 30%HPWꢀPMIMPSꢀSBAꢀ15 catalyst, which is probably
due to the blocking of active sites by the dense Keggin structure, thereby changing the
1
0

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morphology of the support (mesopore diameter and volume) and giving diffusional constraints.
In addition, it is worth of noting that the [MIMPS] PW O shows good catalytic performances,
3
12
4
because it has sufficient PW anions content.
a
Table 2. Activity of various catalysts in alkylation of oꢀxylene with styrene
c
d
catalyst
Styrene conversion
%)
PXE Yield
(%)
PXE Selectivity
(%)
(
SBAꢀ15
ꢀ
ꢀ
ꢀ
b
HPW
100
74.8
83.9
97.8
95.0
94.5
97.9
71.8
80.5
93.9
91.2
90.8
9:1
9:1
9:1
9:1
9:1
9:1
1
2
3
4
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
0%HPWꢀPMIMPSꢀSBAꢀ15
[
MIMPS] PW O
3 12 4
a
Reaction conditions: oꢀxylene : styrene = 7.5:1,reaction temperature=120℃, reaction time= 3.0h, catalyst loading =20% (w/w
of styrene).
b
Hompgeneous catalyst, 0.30g.
Isolated yield based on the amount of styrene.
Ratio of paraꢀtoꢀortho product.
c
d
100
30% HPW-PMIMPS-SBA-15
MIMPS] PW O
[
3
12
4
80
60
40
20
0
1
2
3
4
5
6
Reaction cycle
Fig. 6 Catalytic Stability of the 30%HPWꢀPMIMPSꢀSBAꢀ15 and [MIMPS] PW O catalysts in the alkylation
3
12
4
of oꢀxylene with styrene(Reaction conditions: oꢀxylene
︰
styrene = 7.5
︰1, reaction temperature=120℃,
reaction time= 3.0h, catalyst loading =20% (w/w of styrene).
Stability and reusability of the catalyst were very important for any catalytic system. The
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DOI: 10.1039/C4RA03531C
catalytic reusability of the 30%HPWꢀPMIMPSꢀSBAꢀ15 and [MIMPS] PW O catalysts was
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4
evaluated by carrying out the reaction with used catalyst under the optimized conditions. After
completion of the reaction, the catalyst was easily recovered from the reaction mixture by
centrifugation and, used for the subsequent catalytic runs without further activation. The data
obtained are shown in Fig. 6. Interestingly, the activity decreased slightly after reuse of the
3
0%HPWꢀPMIMPSꢀSBAꢀ15 for six times. Indeed, the yield of PXE product was as high as
9.5% even after the 30%HPWꢀPMIMPSꢀSBAꢀ15 was recycled for six times. On contrast, the
8
yield of PXE product significantly decreased from 90.8 to 30.3% as pure ionic liquid of
MIMPS] PW O was recycled for six times under the same reaction conditions. These results
[
3
12
4
indicate that the immobilized heteropolyanionꢀbased ionic liquid presented here possesses much
higher catalytic efficiency in comparison with the corresponding pure ionic liquid. Therefore,
immobilized ionic liquid catalyst on porous carriers used as heterogeneous catalysts can
overcome the mass transfer problems in practical application. Indeed, through immobilized ionic
liquid catalyst on various supports can obtain larger specific surface area and more stable. And
after reaction, it can be also easier separation and better reusability.
4. Conclusions
In summary, the work herein presented fabricating HPWꢀbased ionic liquid immobilized on
mesoporous silica SBAꢀ15 by totally anionꢀexchange. The resultant catalysts showed better
catalytic efficiency and reusability than the corresponding taskꢀspecific basic ionic liquid in the
alkylation of oꢀxylene with styrene. It was found that the occasion of adding HPW during the
synthesis of catalysts played a crucial role in PW anions loading and successive catalytic
performance. Moreover, the use of such heterogeneous catalyst can decrease the consumption of
ionic liquid remarkably. As such, the synthesis procedure can be simplified significantly, which
may open an opportunity for wide applications of immobilized heteropolyanionꢀbased ionic
liquid in catalysis.
Acknowledgments
The authors are grateful to the financial supports of National Natural Science Foundation of
China (Grant No. 21306023, 21376051, 21106017), Natural Science Foundation of Jiangsu
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DOI: 10.1039/C4RA03531C
Province of China (Grant No. BK20131288), Fund Project for Transformation of Scientific and
Technological Achievements of Jiangsu Province of China (Grant No. BA2011086), Key
Program for the Scientific Research Guiding Found of Basic Scientific Research Operation
Expenditure of Southeast University (Grant No. 3207043101) and Instrumental Analysis Fund of
Southeast University.
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