H. Lü et al. / Applied Catalysis A: General 441–442 (2012) 136–141
137
structure as catalyst for the oxidation of cyclohexane and this sys-
tem proven to own significant industrial potential due to its high
performance. The study on oxidative mechanism of cyclohexane
was also conducted.
and analyzed by high-prerformance liquid chromatography (HPLC)
using the Agilent 1200 Series HPLC equipped with an Agilent XDB-
C18 column (150 × 4.6 × 5) and a VWD ultraviolet detector set at
212 nm, the heptane diacid was used as the internal standard.
2
. Experimental
3. Results and discussion
2.1. Chemicals and materials
3.1. The characterization of catalysts
(
NH ) Mo7O24,
Al(NO ) ,
HIO4,
Na MoO4
and
4
6
3
3
2
[
(
(C18H37) N(CH ) ]Cl were purchased from Sinopharm Chemical
Shanghai, China) and Robiot Co., Ltd. (Nanjing, China). Cyclo-
2
3 2
The IR spectra of three catalysts exhibit similar features as
shown in Fig. 1a. The characteristic peaks at 938, 920, 720, 662,
hexane, chloroform, chlorobenzene and ethanol were obtained
from Tianjin Chemical (Tianjin, China). All the chemicals were
analytical grade reagents and were used as received without
further purification. Double distilled water was used in the present
work.
−
21 cm for [(C18H37) N(CH ) ] Mo7O (1), 941, 914, 721, 655,
2 3 2 6 24
1
4
4
7
−
1
21 cm for [(C H37) N(CH ) ] H AlMo O (2), and 944, 917,
18
2
3
2
6
3
6
24
−
1
20, 635, 418 cm for [(C18H37) N(CH ) ]5IMo O (3) could be
2
3
2
6
24
ascribed to the Anderson polyoxoanions [24,25]. The high wave
−
1
number characteristic peaks at 2955, 2918, and 2851 cm
attributed to the quaternary ammonium cation.
are
2
.2. Methods
UV–vis spectra (Fig. 1b) of the three Anderson type catalysts
reveal very intense absorption in the range 200–330 nm spectral
2
.2.1. Preparation of the Anderson-type catalysts
Na H AlMo O
and
Na5IMo O
were
prepared
region, which is the characteristic of polymer Mo O Mo structures
3
6
6
24
6
24
2
−
6+
according to the procedures described elsewhere [19,23].
originated by the charge transfer processes from O to Mo in
octahedral coordination [26,27].
[
(C18H37) N(CH ) ] Mo7O
(1) was prepared as following:
0 ml ethanolic solution with 6 mmol quaternary ammonium
2
3
2
6
24
2
inside was added dropwise into a 40 ml aqueous solution of
NH ) Mo7O (1 mmol) under stirring at room temperature.
(
4
6
24
A snow-white precipitate was immediately formed. After con-
tinuous stirring for 4 h, the resulted mixture was filtered and
◦
dried at 60 C in vacuum for 24 h to produce catalysts. There
(1)
are three kinds of Anderson-type polyoxometalates involved,
including (NH ) Mo7O , Na H AlMo O and Na5IMo O24. The
4
6
24
3
6
6
24
6
other two catalysts, [(C18H37) N(CH ) ] H AlMo O (2) and
2
3
2
6
3
6
24
[
(C18H37) N(CH ) ]5IMo O (3), were synthesized with the same
2 3 2 6 24
(2)
procedure.
2.2.2. Characterization of the Anderson-type catalysts
The infrared spectrum (IR) of the catalyst, diluted with KBr and
(3)
pressed into a pellet, was recorded on a Nicolet 470 FTIR spectrom-
eter. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was
recorded on a TU-1901 (Beijing General Analytical Instrument Ltd
Co., China) with BaSO4 as the internal standard. The scanning pat-
terns were recorded at 200–800 nm in a step-scan mode with a step
of 5 nm.
a
3
000
2800 1000
Wavenumber /cm
500
-1
2.2.3. Oxidation experiment and analytical methods
The liquid-phase oxidation of cyclohexane was performed in a
00 mL autoclave reactor. Typically, 10 ml cyclohexane and a cer-
1
tain amounts of catalyst were added into the reactor. The reactor
was heated to the reaction temperature after sealing, while agita-
tion was conducted by means of a magnetic stirrer. Upon heating
to the reaction temperature, the reactor was charged with a spec-
ified pressure of O . During the reaction, intermittent oxygen was
2
fed to maintain the reactor in constant pressure. When the reaction
was stopped, the temperature was cooled to room temperature and
the pressure was reduced to atmospheric pressure. Acid products
were recovered by the filtration of the obtained oxidation resul-
tants (a mixture of solid and liquid) and drying of the separated
(
1)
2)
3)
(
(
◦
solid at 50 C in vacuum for 24 h. The liquid obtained after the
b
filtration was diluted and dissolved with 10 g of chloroform. The
reaction products were identified by their comparison with authen-
tic products and by GC–MS analysis. The quantitative analyses of
cyclohexane, cyclohexanol and cyclohexanone were carried out by
an gas chromatography, which was equipped with an PEG-20 M
column (30 m × 0.25 mm × 0.3 mm), using chlorobenzene as the
internal standard. The acid products were dissolved in methanol
200
300
400
500
600
700
800
Wavelength /nm
Fig. 1. (a) FTIR spectrum of three Anderson catalysts. (b) UV–vis DR spec-
tra of three Anderson catalysts. Note. (1) [(C18H37)2N(CH3)2]6Mo7O24, (2)
[(C18H37)2N(CH3)2]6H3AlMo6O24, and (3) [(C18H37)2N(CH3)2]5IMo6O24)].