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G. Chatel et al. / Applied Catalysis A: General 478 (2014) 157–164
ionic liquid ([C8 mim]Cl) on the selective oxidation of cyclohex-
anol to cyclohexanone in the presence of WO3 catalyst (0.6 mol%)
[23]. Based on COSMO-RS (COnductor-like Screening MOdel for
polar ILs, especially those with a long alkyl chain (e.g., octyl chain).
In recent years, the use of room-temperature ionic liquids
(RTILs) as reaction medium has become increasingly attractive
[24–26]. The reason is that these ionic solvents display interest-
to replace volatile organic solvents (low vapor pressure, air and
moisture stabilities, high polarity, etc.). Moreover, ILs display very
considerable potential for the improvement of chemical processes,
and for the enhancement of reaction rates in organic chemistry
[27,28].
([NTf2]−) in the epoxidation of cyclic olefins by H2O2 in the pres-
ence of a Mn-porphyrin as catalyst [29,30]. It is their low viscosity
and chemical stability under oxidation conditions which prompted
us to choose them [29–33].
In the present study, we reported the application of these
hydrophobic ILs in the oxidation of cyclohexanol to cyclohexanone
by H2O2, in the presence of metal salts as catalysts. We paid par-
ticular attention to the study of the role of each reagent, to better
understand the reaction mechanism and to optimise the reaction in
terms of efficiency and eco-friendliness. In this context, we investi-
gated the use of ultrasound and microwave as activation methods
having major consequences, such as a reduction in reaction time
and in energy consumption. The novelty of our contribution lies
in the unique combination of the metal salt with a quaternary
ammonium-based co-catalyst, under non-conventional activation
conditions, to assist an oxidation reaction.
tapered microtip probe operating at a frequency of 20 kHz was
used and its acoustic power in water (Pacous,1 = 0.787 W mL−1 and
Pacous,2 = 1.26 W mL−1) was determined by calorimetry according to
previous work [31].
Microwave irradiations were performed by means of
a
MicroSynth reactor (35 cm × 35 cm × 35 cm cavity) from Milestone.
Temperature was measured and controlled with an optic fiber ther-
mometer (ATC-FO).
2.2. Ionic liquids syntheses and characterizations
Methyloctylpyrrolidinium
ethyloctylpiperidinium ([C8epip][NTf2]),
([C8mpyr]Br,
[C8mpyr][NTf2]),
octylpyridinium
([C8mim]Cl,
[C8mim]Br, [C8mim][NTf2]) based ILs (formula given in Table 1)
were synthesized, purified and characterized (1H and 13C NMR,
FTIR, UV–visible, mass spectrometry, cyclic voltammetry, water
content, viscosity and density measurements) as described
previously [29,30].
Synthesis of [Aliquat][NTf2] from Aliquat 336 (Fig. 1): Aliquat
336 (2.27 g, 5.6 mmol, 1 equiv.) was added to a solution of LiNTf2
(1.69 g, 5.9 mmol, 1.1 equiv.) in distilled water (50 mL). The mix-
ture was stirred at room temperature under argon for 5 h and
extracted with CH2Cl2 (3 × 20 mL). The combined organic phases
were then washed with water (2 × 10 mL) and with a saturated
sodium chloride solution (2 × 10 mL), and then dried over MgSO4.
After filtration, the resultant mixture was stirred for 2 h with
activated charcoal. Filtration on celite to remove charcoal and evap-
oration under reduced pressure (rotary evaporator, 2 × 10−3 bar)
afforded the colourlessness desired [Aliquat][NTf2] (Fig. 1b, 2.74 g,
76%).
1H NMR (C27H54O4N2F6S2, ı ppm, CDCl3, 300 MHz): ı 0.82 (9H,
m, 3 CH3), 1.28 (30H, m, 15 CH2), 1.58 (6H, m, 3 CH2), 2.91 (3H, m,
1 CH3), 3.12 (6H, m, 3 CH2); 13C NMR (ı ppm, CDCl3, 75.5 MHz): ı
15.0 (2 C), 15.1, 23.3, 23.6, 23.7, 26.9, 27.2 (2 C), 30.0–30.55 (9 C),
32.7, 32.9, 33.9, 49.4, 62.9 (2 C), 63.9, 121.0 (2 C, JC–F = 321.2 Hz).
2. Materials
2.1. Materials
2.3. Optimized cyclohexanol oxidation with Aliquat 336
1-Methylpyrrolidine, 1-ethylpiperidine, 1-methylimidazole,
pyridine, 1-octylchloride, 1-octylbromide, activated charcoal
NORIT®, Celite® 545, hydrogen peroxide (30%: wt% solution in
water), cyclohexanol (98%), sodium tungstate dehydrate (99%)
were purchased from Acros, magnesium sulfate from Chimie-
Plus Laboratoires, tungstic acid (99%), cyclopentanol (99%),
cycloheptanol (99%), cyclooctanol (99%) and Aliquat® 336 from
Sigma-Aldrich, bismuth(III) sulfate (99%) from Alfa Aesar, iron(II)
sulfate (99%) and iron(III) nitrate nonahydrate (99%) from Prolabo,
dichloromethane and ethyl acetate from Carlo Erba Reagents,
LiNTf2 from Solvionic. Chemicals were used without further
purification.
Cyclohexanol (1.04 mL, 10 mmol, 1 equiv.), tungstic acid
(58.9 mg, 0.24 mmol, 2.4 mol%), Aliquat 336 (275 mg, 0.68 mmol,
6.8 mol%) and 30% hydrogen peroxide (2.04 mL, 20 mmol, 2 equiv.)
were introduced into a glass tube. The mixture was stirred at
90 ◦C for 30 min. Then, the organic phase was extracted with
ethyl acetate (3 × 2 mL) and cyclohexane (2 × 2 mL). The com-
bined organic phases were dried with MgSO4 and analysed by
gas chromatography (GC). The same procedure was used from the
other substrates: cyclopentanol (0.91 mL, 10 mmol, 1 equiv.), cyclo-
heptanol (1.20 mL, 10 mmol, 1 equiv.) and cyclooctanol (1.32 mL,
10 mmol, 1 equiv.).
Cyclohexanone: C6H10O, ı ppm, CDCl3, 300 MHz): ı 1.73 (2H,
m, 1 CH2), 1.86 (4H, m, 2 CH2), 2.36 (4H, m, 2 CH2); Cyclopen-
tanone (C5H8O, ı ppm, CDCl3, 300 MHz): ı 1.97 (4H, m, 2 CH2), 2.16
(4H, m, 2 CH2); Cycloheptanone (C7H12O, ı ppm, CDCl3, 300 MHz):
ı 1.49–1.81 (8H, m, 4 CH2), 2.49 (4H, m, 2 CH2); Cyclooctanone
(C8H14O, ı ppm, CDCl3, 300 MHz): ı 1.28–1.72 (6H, m, 3 CH2), 1.88
(4H, m, 2 CH2), 2.41 (4H, m, 2 CH2).
1H NMR spectra were recorded in CDCl3 (Euriso-Top, Saint
Aubin, France) at 23 ◦C using a Bruker DRX300 spectrometer,
at 300 MHz and 75.5 MHz for 1H and 13C, respectively. Chemi-
cal shifts (ı) are reported in ppm relative to tetra-methylsilane
(TMS).
Gas chromatography was performed on a GC8000series gas
chromatograph from Fisons Instruments using a flame-ionization
detector (250 ◦C) and equipped with an HP1 capillary column
(dimethylpolysiloxane, 50 m × 0.32 mm × 0.52 m) from Agilent
technologies. The program used an isothermal temperature of 45 ◦C
for 14 min, and then a 30 ◦C min−1 ramp for 2 min and at last, an
isothermal temperature of 100 ◦C for 2 min. The 4 L samples were
injected at 275 ◦C for GC analyses.
2.4. Microwave assisted cyclohexanol oxidation with Aliquat 336
Cyclohexanol (1.04 mL, 10 mmol,
1 equiv.), tungstic acid
(58.9 mg, 0.24 mmol, 2.4 mol%), Aliquat 336 (275 mg, 0.68 mmol,
6.8 mol%) and 30% hydrogen peroxide (2.04 mL, 20 mmol, 2 equiv.)
were introduced into a 50 mL quartz reactor equipped with a 15 bar
pressure cap. The mixture was stirred, under microwave irradia-
tion at 90 ◦C for 2.5 min (P = 80 W). After stopping the microwave
Ultrasound was generated by a digital Sonifier® S-250D from
Branson (Pelec,1 = 11.5 W and Pelec,2 = 22.9 W). A 3 mm diameter