Vol. 27, No. 4 (2015)
New Brønsted-Lewis Acidic Quaternary Ammonium Ionic Liquids 1261
[PSPip][SO4]Zn: 1H NMR (500 MHz, D2O): δ 1.36-1.61
(m, 10H), 2.56-2.63 (m, 4H), 2.76 (t, 2H, J = 9.0 Hz), 3.17 (d,
2H, J = 9.0 Hz). IR (KBr, νmax, cm-1): 948, 1051, 1180, 1465,
1664, 3410 cm-1.
[PSMor][SO4]Cu, X = SO4, M = Cu
O
1/2M2+
X2–
[PSMor][SO4]Zn, X = SO4, M = Zn
+
NH
[PSMor][HPO4]Cu, X = HPO4, M = Cu
[PSMor][HPO4]Zn, X = HPO4, M = Zn
SO3H
Acetalization of aromatic aldehydes with diols: Aromatic
aldehydes (0.1 mol), cyclohexane (10 mL), diols (0.15 mol)
and the catalyst were mixed together in a three necked round
bottomed flask equipped with a magnetic stirrer and Dean-
Stark apparatus. The mixture was refluxed for 2 h. On comple-
tion, the catalyst was collected, washing with ether and drying
in an oven at 80 °C for about 2 h.
Acidity evaluation by Hammett method with UV-visible
spectroscopy: Water solutions of the ionic liquids were prepared
from deionized water and the ionic liquids. All spectra were
recorded with a UV757CRT spectrophotometer.
IR spectroscopy: Fourier transform infrared spectroscopy
(FT-IR) was carried out on a VERTEX70 FT-IR spectrometer
in the range of 4500-400 cm-1. IR samples were prepared by
mixing probe liquids and ionic liquids in a volume ratio of
2:5.
[PSPip][SO4]Cu, X = SO4, M = Cu
[PSPip][SO4]Zn, X = SO4, M = Zn
[PSPip][HPO4]Cu, X = HPO4, M = Cu
[PSPip][HPO4]Zn, X = HPO4, M = Zn
1/2M2+
X2–
+
NH
SO3 H
HSO4–
+
NH
[PSPip][HSO4]
SO3H
SO3 H
Cl–
ZnCl2
[PSPip][Cl]ZnCl2
+
NH
Fig. 1. Structures of Brønsted-Lewis acidic ionic liquids and Brønsted/
Lewis acidic ionic liquids used in this paper
Brønsted-Lewis acidic ionic liquids exhibited the remarkably
better catalytic activity than Brønsted/Lewis acidic ionic liquids
(Table-1, entries 9 and 12). The synergy of both Brønsted and
Lewis acid sites might enhance the activity8. All the Brønsted-
Lewis acidic ionic liquids afforded good to excellent yields of
products. [PSPip][SO4]Zn was found to be the best catalyst for
the acetalization, leading to 96 % yield of the desired product.
Compared with the traditional acidic catalysts, easy
recycling is an attractive property of the acidic ionic liquid.
Consequently, the catalytic activity of recycled [PSPip][SO4]Zn
was examined. [PSPip][SO4]Zn could be reused for 6 times at
least and there was no obvious decrease in yield of the product,
which indicating that the new Brønsted-Lewis acidic ionic
liquid was high efficient and recyclable catalyst for the
acetalization. To ultimately discriminate the catalytic activities
of various ionic liquids, their loadings should be decreased to
real "catalytic amounts". The reactions were conducted with
different catalyst loadings which ranged from 0.1 to 1.0 mol %.
The yield of the product increased with the increase of the
catalyst amount. Nearly quantitative yield was achieved when
only 0.7 mol % of [PSPip][SO4]Zn was used. Further increase
of the catalyst loading led to no significant improvement in
yield. Evidently it was enough to catalyze the acetalization
efficiently at very low loading for [PSPip][SO4]Zn.
RESULTS AND DISCUSSION
Synthesis of Brønsted-Lewis acidic ionic liquids: Brønsted
Lewis acidic ionic liquids were synthesized by three step
synthesis route (Scheme-I). For this study, Brønsted-Lewis
acidic ionic liquids having [PSMor] or [PSPip] cation, [SO4]
or [HPO4] anion, together with Cu2+ or Zn2+ Lewis acid site were
prepared. In the first step, the condensation of morpholine
(Mor) or piperidine (Pip) and 1,4-butane sulfonate afforded
intermediate 1a or 2a. Then, a stoichiometric amount of sulfuric
acid or phosphoric acid was mixed with the intermediate to
afford the homogeneous liquid phase 1b or 2b. In the third
step, Lewis acid site was introduced to give the corresponding
ionic liquids as shown in Fig. 1.
Catalytic properties of Brønsted-Lewis acidic ionic
liquids:A comparative catalytic activity of different catalysts
including concentrated sulfuric acid, p-toluenesulfonic acid,
Brønsted-Lewis acidic ionic liquids as well as Brønsted/Lewis
acidic ionic liquids was summarized in Table-1. The results
demonstrated that the catalytic performances of Brønsted-
Lewis acidic ionic liquids with the conjugate bases of SO42-
were better than those of ionic liquids with the conjugate bases
2-
of HPO4 , even concentrated sulfuric acid and PTSA under
the same reaction conditions. Furthermore, it was noted that the
O
N
2+
O
-
-
1/2M
O
O
HSO /H PO
4
2
4
2-
2-
SO / HPO
4
4
OH
OH
S
OH
NH
N
H
NH
S
O
O
S
O
O
S
O
MO
H SO or H PO
4
O
O
O
2
4
3
1a
2a
O
1b
o
60 C, 3 h
o
80 C, 5 h
2+
-
-
1/2M
HSO /H PO
4
4
2
2-
2-
OH
SO / HPO
4
4
N
H
N
OH
OH
S
S
NH
NH
O
S
O
O
O
O
O
2b
MO = CuO or ZnO
Scheme-I: Synthesis route of Brønsted-Lewis acidic ionic liquids