H. Shi et al. / Catalysis Communications 11 (2010) 588–591
589
N
SO H
X -
CH
X
3
3
X -
N
N
HN
N
-
N
N
SO H
3
X -
MIMPS] X
H C
9
4
[
[Bmim] X
[Hmim] X
[
PSPy] X
I X - = HSO4-
III X - = HSO -
IV X - = HSO -
-
-
4
4
VI X = HSO
4
II X - = H PO4
-
-
-
V X = H PO
4
2
2
Fig. 1. Ionic liquids used in this paper.
UV–vis spectra were recorded on a UV-2450 spectrophotometer
Shimadzu Corporation, Japan) in H O. NMR spectra were recorded
on an AV-400 spectrometer (Bruker Corporation, Germany) in D O.
FT-IR spectra were performed on a Nicolet FT-IR spectrophotome-
ter (Nexus 470, Thermo Electron Corporation) using KBr pellets at
room temperature. TG/DSC was done on STA-449C Jupiter (NET-
ZSCH Corporation, Germany).
2.4. UV–vis acidity determination
(
2
2
According to previous work [17], the Brönsted acidity was eval-
uated from the determination of the Hammett acidity function,
using UV–vis spectroscopy. In the present case, ILs and the indica-
tor 4-nitroaniline were dissolved in H
2
O at concentrations of
ꢀ2
ꢀ3
3.2 ꢁ 10 mol/L and 2.9 ꢁ 10 mol/L, respectively.
2.2. General preparation of Brönsted acidic ionic liquids
3
. Results and discussion
The ionic liquids (ILs) used in this paper (Fig. 1) were synthe-
3
.1. The effect of conventional and microwave heating on the synthesis
sized according to previous literatures [12,16,21]. The ILs were
in ILs
1
13
analyzed by H, C NMR spectroscopies, and the spectral data
agreed with their structures.
For the purpose of comparison, methyl salicylate was synthe-
1
Spectral data for [PSPy][HSO
4
]: H NMR (400 MHz, D
2
O): d 2.28
sized under conventional conditions and under microwave irradia-
tion. Some data under conventional conditions (entries 5–7) are
not given here by reason that it cannot be determined for its low
catalytic activity. Other results are listed in Table 1.
(
1
1
m, 2H), 2.80 (t, 2H), 4.59 (t, 2H), 7.90 (t, 2H), 8.38 (t, 1H), 8.70 (s,
H); C NMR (100 MHz, D
44.32, 145.89.
1
3
2
O): d 26.06, 47.00, 59.84, 128.37,
According to the obtained data under microwave irradiation,
2
.3. Esterification under microwave irradiation
the catalytic activity of SO
MIMPS][HSO ] (Table 1, entries 1 and 2) was better than that of
non-functionalized ILs (entries 3, 4 and 6). The yields can reach
3 4
H-functionalized ILs [PSPy][HSO ] and
[
4
The reactions were carried out in Microwave Synthesis System
(
MAS-I, Sineo Microwave Chemical Technology Co. Ltd., Shanghai,
9
2
1.9–93.6%. Under conventional conditions, the yields ranged from
.2% to 36.6%, respectively, thereby, highlighting the role of micro-
wave irradiation in promoting the esterification. Additionally,
blank experiment (without catalyst) under the same microwave
China) equipped with a magnetic stirrer and a water-cooled con-
denser. Temperature was controlled by automatic adjusting of an
infrared temperature sensor. In a typical procedure, a solution of
the substrates consisting of 0.02 mol salicylic acid, 0.05–0.08 mol
methanol, and 4–12 mmol ILs were prepared and irradiated for
irradiation manifested that the application of SO
ILs could effectively promote the reaction.
3
H-functionalized
1
0–30 min. After the reaction, the mixture became biphasic, and
diethyl ether was added to dissolve the unreacted salicylic acid
SA). The upper layer consisting of the produced ester and some
3
.2. Optimization of reaction conditions
(
unreacted SA was isolated by simple decantation, while the lower
layer, viscous ILs, could be used in next reaction after removal of
water at 90 °C for 6 h. Produced methyl salicylate was analyzed
[
PSPy][HSO
parameters of the synthesis under microwave irradiation. The ef-
fect of varying the concentration of the [PSPy][HSO ] was explored
4
] was used as catalyst to define the optimal reaction
1
1
by H NMR ( H NMR (400 MHz, acetone-d 6): d 3.95 (s, 3H), 6.91
4
(
Table 2, entries 2, 8 and 9). The maximum yield was obtained
when 10 mmol of ILs was added. An excess of ILs resulted in a de-
crease of yield. The optimal molar ratio of methanol to salicylic
(
t, 1H), 6.97 (d, 1H), 7.50 (t, 1H), 7.81 (d, 1H), 10.79 (s, 1H)).
The composition of the products was analyzed by GC-FID (Agi-
lent 7890A, HP-5 column, 30 m ꢁ 0.32 mm i.d. ꢁ 0.25
lm film
thickness). The GC process started at 70 °C and the temperature
was raised to 170 °C at 15 °C/min. The products were identified
by comparing with the standards, and GC results showed that
the major product was methyl salicylate and the minor side prod-
uct was phenol. The conversion and selectivity were calculated
according to the area of chromatograph peak [4].
Table 1
The effect of conventional and microwave heating on the synthesis in ILs.
Entry
Ionic liquid
Yield (%)
MHc
Selectivity(%)b
CHd
MHc
CHd
1
2
[PSPy][HSO
4
]
93.6
91.9
17.6
7.4
4.2
1.7
36.6
33.1
4.5
2.2
–
99.9
99
99.9
99.9
99.9
99.9
–
Conversion of salicylic acid ðSAÞ=%
a
[MIMPS][HSO
[Bmim][HSO
[Hmim][HSO
[MIMPS][H PO
4
]
]
]
½
salicylic acidꢂ
3
4
5
6
7
4
99.9
99.9
99.9
99.9
99.9
¼
100 ꢀ
ꢃ 100
ð1Þ
4
½
salicylic acidꢂ þ ½methyl salicylateꢂ þ ½phenolꢂ
2
4
]
2
[Bmim][H PO
Blank
4
]
–
–
–
–
1.0
Selectivity for methyl salicylate ðMSÞ=%
methyl salicylateꢂ
methyl salicylateꢂ þ ½phenolꢂ
Yield of methyl salicylate ðMSÞ=%
conversion of SA ꢃ selectivity for MS
Reaction conditions: ratio of methanol to salicylic acid = 3:1; ILs 10 mmol; refluxed
for 20 min at 105 °C.
½
¼
ꢃ 100
ð2Þ
ð3Þ
½
a
Refluxed at 95 °C.
Selectivity for methyl salicylate (base on salicylic acid).
MH: microwave heating.
CH: conventional heating.
b
c
¼
d