W.-L. Wong et al. / Applied Catalysis A: General 453 (2013) 244–249
245
Scheme 1. Mechanism of bicarbonate-activated oxidation of alcohols.
standard in the quantitative GC–MS measurements. ESI-MS anal-
ysis was carried out by a Waters Acquity SQD Mass Spectrometer.
Ionic liquids 2–4 were prepared according to literature reported
procedures [21,22].
2.2. Preparation of di-cationic ionic liquid 1
Fig. 1. A schematic diagram showing the use of biphasic ionic liquid/n-heptane in
the selective oxidation of benzyl alcohol to benzaldehyde.
A solution of 1,6-dibromohexane (20 mL, 0.13 mol) was care-
fully added to 1-methylpyrrolidine (29 mL, 2.1 equiv.) in 100 mL
acetonitrile. The mixture was gently heated to reflux under argon
for 24 h. After cooling to room temperature, white solids of the
bromide salt precipitated out. The solvent was decanted and the
product was washed thoroughly with diethyl ether and dried
under vacuum. The white solids (12.4 g, 0.03 mol) were fur-
ther reacted with silver (I) oxide (16.7 g, 1.2 equiv.) in water
to yield the ionic liquid with hydroxide as the counter anions.
The silver bromide precipitates were removed by filtration and
collected for recycling. After removal of solvent, the hydrox-
ide product was obtained and re-dissolved in methanol. CO2
gas was then purged directly for 30 min into the solution to
period of 40 min using a peristaltic pump. Caution! The reaction
can be very exothermic if the hydrogen peroxide is added too quickly.
Benzaldehyde was obtained as a colourless liquid in 83% yield
by decantation of the n-heptane layer and additional extraction
(n-heptane, 2× 60 mL) from the ionic liquid medium. The crude
product was characterized by GC–MS and 1H NMR. The heptane
was collected and recycled in the catalysis and extraction process.
2.5. Procedures for recycling study
−
afford the desired ionic liquid 1 with bicarbonate (HCO3 ) as
To a 25 mL round-bottom flask, 1 (5.6 mmol, 2.1 g), de-ionized
H2O (500 L), Mn(OAc)2 (0.025 mol%), benzyl alcohol (310 L,
3 mmol), and n-heptane (6 mL) were added. The mixture was then
stirred vigorously. Aqueous hydrogen peroxide (6× 60 L, 35 wt.%
in water) was injected into the reaction mixture over a period of
20 min. Benzaldehyde was obtained by decanting the n-heptane
layer and further extraction from the ionic liquid with n-heptane
the counter anions. An overall yield of 95% was obtained in the
2
+
+
preparation. HR-MS: [1] : Found 127.1630, calc. 127.1361; [1] :
Found 315.2659, calc. 315.2648; 1H NMR (D O): ı 1.40–1.50 (m,
2
4
3
6
H), 1.78–1.92 (m, 4H), 2.21 (s, 8H), 3.04 (s, 6H), 3.30–3.38 (m, 4H),
1
3
.45–3.60 (m, 8H); C NMR (D O): ı 24.10, 25.80, 28.13, 50.85,
2
6.90, 67.10, 163.03.
(
2× 5 mL). The solution was dried over anhydrous sodium sulfate
2.3. General procedures for the oxidation of alcohols
and n-heptane was removed under vacuum. The conversion of ben-
zyl alcohol and the yield of benzaldehyde were determined with
GC–MS. The recovered ionic liquid 1 was reused for the next cycle
after removal of water by freeze-drying under vacuum.
To
a 25 mL round-bottom flask, ionic liquid (5.6 mmol),
Mn(OAc)2 (0.025 mol%), alcohol (3 mmol), and n-heptane (6 mL)
were added. De-ionized water (0.5 mL) was added to lower the vis-
cosity of the mixture. The resulting solution was then kept stirring
vigorously. Under ambient conditions, aqueous hydrogen peroxide
3. Results and discussion
(
6× 60 L, 35 wt.% in water) was injected into the reaction mixture
The cation of ionic liquid 1 consists of two N-methyl-
in 3 min intervals over 20 min. After the reaction had been com-
pleted, the n-heptane solution (upper layer) was carefully decanted
from the ionic liquid medium. In order to completely extract the
aldehyde from the ionic liquid with residual n-heptane, additional
n-heptane (2× 5 mL) was added and the combined organic solution
was then dried over anhydrous sodium sulfate. After the removal
of solvent under vacuum, the aldehyde was isolated as a colourless
liquid and characterized by GC–MS and H NMR. The n-heptane
collected was reused in catalysis and extraction process. It was
found that the use of n-heptane, n-hexane, n-pentane, or petroleum
ether (40–60 C) as the extraction phase in the catalysis showed
pyrrolidinium groups, which are linked through a hexyl chain
(Scheme 2). Bicarbonate was chosen to be the anion to provide a
−
−
high concentration of HCO3 in the medium so that the active per-
oxymonocarbonate (HCO4 ) species can be generated effectively
through the reaction with H O2 (Scheme 1). Ionic liquid 1 was
2
synthesized by our previous reported procedures [23] with slight
modifications and was characterized by 1H and C NMR and high
resolution MS. The highly polar nature of the 1 makes it immiscible
with most non-polar organic solvents but allows certain miscibility
with alcohols. We thus take advantage of this property to design a
biphasic catalytic system in which the oxidation of alcohol occurs
in the ionic liquid phase and the aldehyde produced is readily
extracted into the upper n-heptane layer. A schematic diagram of
the catalytic oxidation process is depicted in Fig. 1. The instanta-
neous separation of product from the reaction medium not only
prevents the aldehyde from over-oxidation to carboxylic acid dur-
ing the reaction, but also makes the separation simple and allows
easy recovery of the ionic liquid and catalyst by decantation.
Benzyl alcohol was used as a model substrate to examine the cat-
alytic ability and aldehyde selectivity of the biphasic system. After
optimization of the catalytic system under various reaction condi-
tions (Table 1 and Fig. 2), in a 3-mmol reaction where 0.025 mol %
13
1
◦
no observable difference as these solvents are immiscible with the
ionic liquid. However, heptane was chosen because it is considered
as a greener alternative solvent for chemical processes [20].
2
.4. Procedures for larger scale catalytic oxidation of benzyl
alcohol
To a 500 mL round-bottom flask equipped with magnetic stir-
rer, 1 (70 g, 186 mmol) in de-ionized H O (18 mL), Mn(OAc)2
2
(
0.025 mol%), benzyl alcohol (10.3 mL, 100 mmol), and n-heptane
(
200 mL) were added. The flask was placed in a water bath main-
◦
tained at 20 C. Aqueous hydrogen peroxide (12 mL, 35 wt.% in
water) was then added dropwise to the reaction mixture over a
of Mn(OAc) catalyst and 1.4 equiv. of H O were used, the conver-
2
2
2
sion to benzaldehyde was almost completed (98 ± 1%) in 20 min at