5
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Z. Yang et al.
alcohols and can be easily separated and recovered by
precipitation and centrifugalization.
mixture was stirred at 65 °C for 72 h. After the solvent was
evaporated under vacuum, the residue was purified by
precipitation in diethyl ether and 4.83 g of QMPEG was
obtained.
2
Experimental
2.4 Preparation of QMPEG-Supported
2
.1 Materials
Phosphotungstic Acid Catalyst (QMPEG-PTA)
All chemicals were of analytical grade, which were pur-
chased from commercial sources and used as received, and
methoxypolyethylene glycol (MPEG) was afforded with a
MW of 5,000.
QMPEG-PTA catalyst was prepared in a typical procedure
as follows to a solution of 4.7 g QMPEG 4(4.7 g) in 40 ml
deionized water, 5.73 g of PTA was added. After the
solution was stirred for 24 h at room temperature, the
solvent was removed and the residue was dried under
vacuum at 40 °C. The crude product was washed alter-
nately by diethyl ether and ethanol, and then dried under
vacuum [3–5].
2
.2 Characterization
The fourier transform infrared spectra (FT-IR) of samples
were performed on an AVATAR360-IR spectrometer
(
Nicolet, USA) using KBr pellets. X-ray diffraction (XRD)
of different samples were recorded on a Rigaku D/Max-2400
Rigaku, Japan) using Ni-filtered Cu Ka (k = 0.15406 nm)
2.5 Typical Procedure for the Esterification
of Carboxylic Acids with Alcohols Employing
QMPEG-PTA Catalyst
(
radiation. Thermal stability of QMPEG support and
QMPEG-PTA catalyst was confirmed by thermogravimetric
analysis (TGA) (Perk in Elmer). Analysis of the conversion
and selectivity was performed on Shimadzu GC-2010A gas
chromatography (Shimadzu, Japan).
To a 10 ml round-bottomed flask with a reflux condenser,
acetic acid (6 mmol), 1,4-butanediol (2 mmol), and
QMPEG-PTA catalyst (116 mg, 0.009 mmol PTA) were
added. The mixture was intensely stirred under air at 50 °C
for 3 h. After the reaction, the mixture was poured into
diethyl ether and the catalyst was precipitated as a yellow
solid. This precipitation was collected, washed with diethyl
ether and acetone alternately, dried under vacuum and
reused in the next cycle. The combined organic phase was
analyzed by the gas chromatography. Further more, the
effects of the catalyst amount, reaction time and reaction
temperature on the catalytic activity and selectivity over
the QMPEG-PTA catalyst were investigated.
2
.3 Preparation of Quaternary Ammonium Salt
Functionalized MPEG Support (QMPEG)
As shown in Scheme 1, the QMPEG support was prepared
in five-step procedure by modification of the method pre-
viously reported [16–18]. In a typical synthesis of QMPEG,
firstly, 6.0 g MPEG was dissolved in 20 ml anhydrous
CH Cl , and then 0.55 ml of mesyl chloride and 8 ml
2
2
n-trioctylamine were added under stirring. After the mix-
ture was stirred for 12 h at room temperature, the solvent
was evaporated under vacuum and the crude product was
purified by precipitation in diethyl ether to afford 5.85 g
intermediate 1. Second, to a solution of intermediate 1
3 Results and Discussion
3.1 Characterization of QMPEG-PTA Catalyst
(
5.7 g) in 40 ml DMF, 1.0 g Cs CO and 0.42 g 2-(4-
2 3
hydroxyphenyl) ethanol were added. After being stirred for
8 h at room temperature, the mixture was concentrated
Figure 1 showed the FT-IR spectra of (a) QMPEG, (b)
PTA and (c) QMPEG-PTA. In Fig. 1b, four characteristic
1
-
1
and the solvent was removed. Then the crude product was
purified by precipitation in diethyl ether and 5.5 g of
intermediate 2 was obtained. Third, to a solution of inter-
mediate 2 (5.3 g) in CH Cl , 0.5 ml of PBr was added at
bands located at 1,080, 983, 891 and 792 cm were cor-
responded to the vibration of P–O , W=O , W–O –W and
a
d
b
W–O –W for the Keggin structure of PTA, respectively
c
[19]. For the QMPEG-PTA catalyst (Fig. 1c), the above
mentioned four bands weakened distinctively, and the
2
2
3
0
°C and the solution was vigorous stirring for 5 h. After
-
1
being warmed up to room temperature, the mixture was
stirred for another 10 h, concentrated and the solvent was
removed. Then the crude product was purified by precipi-
tation in diethyl ether and 5.15 g of intermediate 3 was
afforded. Last, to a solution of 5.0 g intermediate 3 in
single band at 983 cm
was split into two bands
-
1
(corresponding to 980 and 960 cm ), which may be
attributed to the construction of hydrogen bonding among
the terminal oxygen atoms of PTA and amino groups of
organic cations and the intramolecular charge transfer
between amino groups and W species [20]. All the
2
0 ml toluene, 2.5 ml of (n-Bu) N was added and then the
3
1
23