In this work, we investigated the dosage of Brønsted acidic ILs
(structures shown in Fig. 1) in the esterification of glycerol with
acetic acid. For enhancing the acidity and catalytic performance
of these ILs, SO3H-functionalized ILs which have two alkyl
sulfonic acid groups in the imidazolium cation structure were
applied. The influence of various reaction parameters such as
reaction time, reaction temperature, molar ratio of reactant and
catalyst amount to catalytic performance are also studied.
Representative procedure for the esterification of glycerol.
The esterification of glycerol with acetic acid was conducted
in a 50 ml round bottom flask, equipped with magnetic stirring.
In a typical reaction, glycerol (0.1 mol) with acetic acid (0.6 mol)
and [(HSO3–p)2im][HSO4] (0.2 mmol) were reacted at 100 ◦C for
1 h.
Results and discussion
Determination of the Hammett acidity function of ionic liquids
Experimental
The Brønsted acidity of the investigated ILs was evaluated
from the determination of the Hammett acidity functions
using UV-visible spectroscopy with 4-nitroanline as the in-
dicator in H2O.20,28–31 As shown in Fig. 2, the maximum
absorbance of the unprotonated form of the indicator was
observed at 382 nm, which decreased with addition of acid.
The decreasing order of the absorbance of the ILs was ob-
served as following: [Hmim][HSO4] > [HSO3-pmim][HSO4] >
H2SO4 > [(HSO3-p)2im][tos] > [(HSO3-p)2im][HSO4] ª [(HSO3-
p)2im][NTf2], which indicates that the acidity order of ILs to be
adverse.
General remarks
The 1H-NMR and 13C-NMR spectra were recorded on a
Varian INOVA 400 MHz with TMS as the internal standard.
The MS-spectra was determined by High Performance Liquid
Chromatography/Mass Selective Detector (HP1100LC/MSD).
Chemicals were used without purification as commercially
available. UV-Vis spectra were obtained by UV (SHIMADZU
2500). A qualitative analysis of the product was conducted by
GC (Agilent 6890/5973) with a HP-5MS column; quantitative
analysis was carried out on a GC (Agilent 7890) equipped with
a HP-5 column (30 m¥0.25 mm).
Synthesis procedure of ionic liquids. The ionic liquids used
in this article (Fig. 1) were synthesized according to the
literature.16,20,27 The obtained data are as follows:
[Hmim][HSO4]:1H-NMR (400 MHz, D2O, d ppm): 3.911 (s,
3H), 7.667 (s, H), 7.728 (s, H), 9.080 (s, H); ESI-MS positive
mass peaks (m/z)(+): 83.1 [M]+.
[HSO3–pmim][HSO4]:1H-NMR (400 MHz, D2O, d ppm):
1.883–1.966 (m, 2H), 2.512–2.560 (t, 2H), 3.560 (s, 3H), 3.951–
3.966 (t, 2H), 7.066 (s, H), 7.219 (s, H), 8.349 (s, H); 13C-NMR
(400 MHz, D2O, d ppm): 25.014, 35.664, 47.166, 47.630, 122.070,
123.704, 136.072. ESI-MS positive mass peaks (m/z)(+): 205.0
[M]+, 409.0 [2M+ - H+]+.
1
[(HSO3–p)2im][HSO4]: H-NMR (400 MHz, D2O, d ppm):
Fig. 2 Absorption spectra of 4-nitroaniline for various Brønsted acidic
2.046–2.080 (t, 4H), 2.658–2.692 (t, 4H), 4.096–4.128 (t, 4H),
7.309 (s, 2H), 8.598 (s, 1H); 13C-NMR (400 MHz, D2O, d ppm):
24.417, 46.677, 47.295, 121.991, 135.014. ESI-MS positive mass
peaks (m/z)(+): 313.0 [M]+, 625.0 [2M+ - H+]+.
ILs in H2O.
It is obvious that the Brønsted acidity of the ILs depended
both on the nature of the cation and anion. ILs bearing two alkyl
sulfonic acid groups on the cation have stronger acidity than
those of bearing only one SO3H-group, which showed stronger
acidity than those of bearing no functionalized groups ([(HSO3-
p)2im][HSO4] vs. [HSO3-pmim][HSO4] and [Hmim][HSO4]), sug-
gesting the significant effect of the alkyl sulfonic acid group on
the acidity of the ILs. The acidity of the ILs with the anions
[HSO4]- and [NTf2]- were stronger than those with the [tos]-
anion ([(HSO3-p)2mim][HSO4] and [(HSO3-p)2mim][NTf2] vs.
[(HSO3-p)2mim][tos]). Finally, double SO3H-functionalized ILs
exhibited a stronger Brønsted acidity than all other tested ILs
and even stronger acidity than H2SO4.
[(HSO3–p)2im][NTf2]: 1H-NMR (400 MHz, D2O, d ppm):
2.116–2.150 (t, 4H), 2.723–2.758 (t, 4H), 4.160–4.194 (t, 4H),
7.371 (s, 2H), 8.675 (s, 1H). 13C-NMR (400 MHz, D2O, d
ppm): 24.362, 46.593, 47.233, 116.999, 120.941, 135.014. ESI-
MS positive mass peaks (m/z)(+): 313.0 [M]+, 625.0 [2M+
-
H+]+.
[(HSO3–p)2im][tos]: 1H-NMR (400 MHz, D2O, d ppm):
2.227–2.299 (m,4H), 2.324 (s, 3H), 2.856–2.893 (t, 4H), 4.286–
4.322 (t, 4H), 7.288–7.308 (d, 2H), 7.497 (s, 2H), 7.610–7.630 (d,
2H), 8.801 (s, H). 13C-NMR (400 MHz, D2O, d ppm): 21.525,
26.082, 48.294, 48.927, 123.621, 126.386, 130.494, 136.786,
140.432, 143.536. ESI-MS positive mass peaks (m/z)(+): 313.1
[M]+, 625.1 [2M+ - H+]+.
Catalytic experiments
Determination of the Hammett acidity function of ionic liquids.
The ionic liquids were dissolved in distilled water and 4-
nitroaniline added as indicator. Then their UV-vis spectra were
recorded on a SHIMADZU UV-2550 spectrophotometer at
room temperature.
Comparison of catalytic activity of the different ionic liquids
in the esterification of glycerol. The esterification products of
glycerol in the Brønsted acidic ILs were monoacetin (mono-),
diacetin (di-) and triacetin (tri-) (Scheme 1). In order to choose
better Brønsted acidic ILs as catalysts to optimize the reaction
698 | Green Chem., 2011, 13, 697–701
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