Hydroxylation of Benzene to Phenol by Molecular Oxygen
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In particular, Keggin-structured heteropolyacids have been
widely used as catalysts in oxidation reactions due to their
tailorable redox properties [15]. Various organic units such
as aliphatic amines [16], metalloporphyrins [17], and
bipyrimidinylplatinum [18] were doped into Keggin het-
eropolyacids to modify their catalytic performances in
oxidation reactions. Recently, we observed the promotion
effects of cyclodextrin [19] and pyridine [20, 21] in the
hydroxylation of benzene to phenol by molecular oxygen
and/or hydrogen peroxide when anchoring them to Keggin
heteropolyacids.
(40 mL) solution of salicylaldehyde (21 g, 86.20 mmol)
was added to the ethanol (20 mL) solution of 1, 6-hex-
anediamine (10 g, 86.20 mmol) under stirring, and the
mixture was refluxed at 353 K for 6 h. After cooled to
room temperature, the resulting precipitate was filtered off,
re-crystallized in ethanol, and dried in a vacuum oven at
353 K for 12 h to obtain the yellow crystals, the L ligand.
Afterwards, the solution of MnCl2ꢀ4H2O (1.90 g,
9.57 mmol) in ethanol (40 mL) was added to the solution
of L (3.10 g, 9.57 mmol) in hot ethanol (50 mL). The
obtained mixture was stirred further at room temperature
for 3 h; the dark green precipitate, L-Mn, was collected by
filtration, washing with ethanol, and drying in a vacuum
oven at 353 K for 12 h.
On the other hand, Schiff base metal complexes are
well-known catalysts for many oxidation reactions [22],
thus, the combination of Schiff base metal compounds with
heteropolyacids should lead to a kind of novel organic–
inorganic hybrid materials, the catalytic property of which
may be reinforced by the two moieties [17]. Nevertheless,
they are largely overlooked in catalysis context [23]. Until
recently, Mirkhani et al. [24] prepared a series of hybrid
compounds (M-salen-POM, M = Fe, Co, Ni, Mn),
revealing their good catalytic activities in the oxidation of
alkane and alkene.
For preparing L-Mn-PMoV, a molar ratio of L-Mn to
PMoV of 2:1 was used. In detail, the solution of L-Mn
(0.25 g, 0.56 mmol) in methanol (40 mL) was added to the
solution of H4PMo11VO40 (0.05 g, 0.28 mmol) in metha-
nol (10 mL) under vigorous stirring, and the mixture was
stirred at room temperature for 12 h. The resulting yellow
precipitate was collected by filtration, washing with
methanol, and drying in a vacuum oven at 333 K for 12 h
to give the solid catalyst, L-Mn-PMoV. Another sample L-
Mn-PMoV# was obtained by the same way as described
above, except that the molar ratio of L-Mn to PMoV was
1:1 rather than 2:1.
In this work, we prepare a new organic–inorganic hybrid
catalyst by attaching a manganese Schiff base complex to
Keggin-structured molybdovanadophosphoric heteropoly-
acid (PMoV), and use it as the catalyst in the hydroxylation
of benzene to phenol with molecular oxygen as the oxidant,
observing a remarkably enhanced yield of phenol due to
the synergy effect between the Schiff base manganese
complex and PMoV.
2.2 Characterizations of Catalysts
C and N elemental analyses were obtained on a Vario EL
III elemental analyzer. Thermal gravimetric analysis
(TGA) was conducted on a TA Instrument (Netzsch, TG/
209/F3) operated under the air atmosphere. The tempera-
ture program was a simple linear ramp from 303 to 873 K
with the ramp rate of 10 K min-1. Infrared spectra (IR)
were collected using KBr pellets on a Nexus 870 Fourier
transform infrared spectrophotometer. To make the KBr
pellet, 1 mg of sample was mixed with approximately
80 mg of KBr. The spectrum of air was subtracted from the
spectrum for the tested sample. UV–Vis absorption spectra
were recorded on a PE Lambda 35 UV–Vis spectrometer
using dimethyl sulphoxide (DMSO) as the solvent.
2 Experimental
2.1 Preparation of the Catalyst L-Mn-PMoV
All solvents and reagents (analytical grade) were purchased
commercially and were used as it is. H4PMo11VO40
(PMoV) was prepared according to the procedure described
in our previous report [20]. MoO3 (17.77 g, 120 mmol) and
V2O5 (1.02 g, 5.61 mmol) were added to 250 mL of
deionized water. The mixture was heated up to the reflux
temperature, when the aqueous solution of H3PO4 (85
wt%) (1.29 g, 13.16 mmol) was added drop-wise to the
reaction mixture. After a clear orange-red solution
appeared, the solution was cooled to the room temperature.
The orange-red powder PMoV product was obtained by
evaporation of the solution to dryness, re-crystallizing for
purification, and further drying in a vacuum oven at 373 K
for 12 h.
2.3 Catalytic Tests
The hydroxylation of benzene was carried out in a cus-
tomer-designed temperature controllable pressured titanic
reactor (100 mL) equipped with a mechanical stirrer. In a
typical experiment, 0.1 g (0.038 mmol) catalyst, 0.60 g
ascorbic acid, and 2 mL benzene were added into 25 mL of
the aqueous solution of acetonitrile (50 vol%) successively.
After the system was charged with 2.0 MPa O2 at room
temperature, the hydroxylation reaction was conducted at
The manganese Schiff base complex that is designated
as L-Mn (L: N,N0-disalicylidene-1, 6-hexanediamine) was
prepared following the previous literature [25]. The ethanol
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