Transition metal-substituted polyoxometalates supported on MCM-41 as catalysts in the oxidation of cyclohexane and cyclooctane with H2O2

Article abstract of DOI:10.1016/j.mencom.2012.05.014

The Keggin type polyoxometalates (Bu4N)4HPW11CoO39, (Bu4N)4PW11FeO39, (Bu4N)4HPW11CuO39 and (Bu4N)4PW11VO40 have been synthesized and supported on MCM-41 to be used as catalysts for the solvent-free oxidation of cyclohexane and cyclooctane with H2O2.

Full text of DOI:10.1016/j.mencom.2012.05.014

Mendeleev
Communications
Mendeleev Commun., 2012, 22, 152–153
Transition metal-substituted polyoxometalates supported on MCM-41
as catalysts in the oxidation of cyclohexane and cyclooctane with H2O2
Jirarot Jatupisarnpong and Wimonrat Trakarnpruk*
Department of Chemistry, Faculty of Science and Center of Excellence for Petroleum, Petrochemicals, and
Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand. E-mail: Wimonrat.T@chula.ac.th
DOI: 10.1016/j.mencom.2012.05.014
The Keggin type polyoxometalates (Bu4N)4HPW11CoO39, (Bu4N)4PW11FeO39, (Bu4N)4HPW11CuO39 and (Bu4N)4PW11VO40 have been
synthesized and supported on MCM-41 to be used as catalysts for the solvent-free oxidation of cyclohexane and cyclooctane with H2O2.
Keggin type polyoxometalates (POMs) are efficient catalysts
for the oxidation of organic substrates.1,2 Transition metal mono-
substituted heteropoly anions have been used for the catalytic
oxidation of alkenes with oxygen.3 The catalytic homogeneous
oxidation of cyclohexane and cyclooctane was performed in an
acetonitrile solvent with H2O2: in the presence of the tetrabutyl-
ammonium salts of Keggin-type polyoxotungstates, [XW11O39]n−
and [XW11MO39](n−m)− (X = P, Si, B and Miii = Fe, Mn) afforded
substantial amounts of alkyl hydroperoxides.4 The tetrabutyl-
ammonium salts of [Fe4(H2O)2(PW9O34)2]6−, which are soluble
in acetonitrile, showed higher cyclohexane conversion than
[M4(H2O)2(PW9O34)2]10− (Mii = Co, Mn) but the main product
was cyclohexyl hydroperoxide. The Fe and Co catalysts yielded
96 and 85% conversion of cyclooctane, respectively.5 For the
solvent-free reactions, surfactant-type polyoxoperoxometalates
were prepared and shown to be effective for the epoxidation and
oxidative cleavage of olefins to carboxylic acids with H2O2,6
the synthesis of adipic acid7 and the oxidation of hexanol and
octanol.8 The long-chain lipophilic cation in these catalysts acts
as a phase transfer agent.
The IR spectra of PW11M/MCM-41 revealed the principal
bands (W=O and W–O–W at 962–965 and 886–890 cm–1, respec-
tively) of PW11M, confirmed the retention of the PW11M struc-
ture after immobilization. However, the band at 1082–1085 cm–1
attributed to nas(P–O) vibration was obscured by an intense and
broad band of MCM-41. In the XRD diffraction patterns of
PW11M/MCM-41 samples (Figure 1), the absence of peaks due
to crystalline PW11M phases indicates that PW11M is highly
dispersed. The N2 adsorption–desorption on PW11M/MCM-41
exhibited a reversible type IV isotherm typical of mesoporous
materials. The loadings of PW11M caused a decrease in the surface
area (SBET), pore volume and pore diameter of the catalysts
(Table 1).
The main products of the catalyzed oxidation of cycloalkanes
with H2O2 were cycloalkanol, cycloalkanone and cycloalkyl hydro-
peroxide15 (Scheme 1)‡ with trace amounts of acids. Table 1 sum-
m
arizes the results of the oxidation of cyclohexane and cyclooctane
.
Blank reactions without catalysts or H2O2 and in the presence
of MCM-41 were performed and no oxidation products were
detected. The activity of the catalysts depended on the transition
The POMs supported on porous solids have high surface
areas and the numbers of accessible active catalytic sites. They
combine the advantages of molecular complexes and reusable
solids, as well as new activities and selectivities.9,10 The selective
oxidation of cyclohexane yields cyclohexanol and cyclohexanone,
important intermediates in the production of adipic acid and capro-
lactam and in the manufacture of nylon-6 and nylon-66 polymers.
MCM-41 was prepared according to a published procedure11
with SiO2 :CTABr:NH4OH:H2O = 1.0:0.12:8:114. The tetra-
butylammonium salts of transition metal-substituted POMs:
(Bu4N)4HPW11CoO39, (Bu4N)4PW11FeO39, (Bu4N)4HPW11CuO39
and (Bu4N)4PW11VO40 (PW11M; M = Coii, Feiii, Cuii or Vv)
were synthesized and characterized as described elsewhere.4,12,13
PW11M was incorporated into MCM-41 by incipient wetness
impregnation (in MeCN) with 16 wt% loading14 [this is a
maximum loading at which the leaching of PW11M from the
support was not observed when stirring PW11M/MCM-41 in
hot cyclohexane and H2O2 for 8 h, we also attempted to load
PW11M higher than 16 wt% but found that leaching (>2 wt%)
occurred]. The PW11M/MCM-41 catalysts were studied by BET,
XRD, FT-IR and ICP.†
(100)
(110)
(200)
(210)
1
2
3
4
5
1.5
3.5
5.5
7.5
9.5
2q/°
1
2
3
4
5
10
20
30
40
50
2q/°
Figure 1 XRD patterns of (1) MCM-41 and (2)–(5) PW11M/MCM-41
[M = (2) Coii, (3) Feiii, (4) Cuii and (5) Vv].
†
Specific surface areas were measured using the BET method on a
‡
BELSORP-mini. XRD measurements were performed on a Rigaku
DMAX 2002/Ultima Plus powder X-ray diffractometer. The IR spectra
were obtained on a Nicolet FT-IR Impact 410 spectrophotometer with a
pressed KBr pellet. The amount of PW11M was analyzed by inductive
coupled plasma emission (ICP, Perkin Elmer model PLASMA-1000).
A substrate (92 mmol), 5 wt% catalyst and aqueous H2O2 (30%) were
added to a 60 ml Parr reactor. The reactor was heated to 80°C and stirred
for 8 h. The catalyst was separated, and the products were extracted with
diethyl ether and dried. They were analyzed by GC [Varian CP-3800 GC
chromatograph; CP-Sil (30 m´0.25 mm) column].
© 2012 Mendeleev Communications. All rights reserved.
– 152 –

Mendeleev Commun., 2012, 22, 152–153
OH
O
OOH
4
(100)
3
2
H2O2
+
+
+
+
(110)
3.5
catalyst
(200)
1
OH
O
OOH
0
1.5
5.5
7.5
9.5
H2O2
2q/°
catalyst
Figure 2 XRD pattern of PW11V/MCM after use.
Scheme 1
500
Table 1 Oxidation of cyclohexane and cyclooctane (catalyst, 0.4 g; substrate,
92 mmol; H2O2/substrate molar ratio, 4:1; reaction time, 8 h; temperature,
80°C).
400
300
200
100
0
Pore Con-
Pore
Selectivity (%)b
-one -ol -OOH
Sub-
strate
SBET
/
dia-
ver-
Catalyst
volume/
m2 g–1
meter/ sion
cm3 g–1
nm
(%)a
Cyclo- MCM
980
0.82
0.41
0.41
0.42
0.38
0.40
2.97
0
—
73 27
72 28
35 55 10
76 24
70 30
—
—
0
0
hexane PW11Co/MCM 760
PW11Co/MCMc 760
PW11Fe/MCM 790
PW11V/MCM 754
PW11Cu/MCM 765
2.75 15
2.75 17
2.68 13
2.66 19
2.72 18
0
0.25
0.50
0.75
1.00
0
0
p/p0
Cyclo- PW11Co/MCM
octane PW11Fe/MCM
PW11V/MCM
19
16
21
20
5
16
11
74 26
33 53 14
74 26
72 28
40 60
0
Figure 3 Nitrogen adsorption–desorption isotherm of PW11V/MCM after use.
0
0
0
81
0
shows the N2 adsorption–desorption isotherm of PW11V/MCM,
which still reveals mesoporous structure.
The oxidation on these PW11M/MCM catalysts occurs via a
radical pathway since no products were detected upon the addition
of a radical scavenger (2,6-di-tert-butyl-4-methylphenol).17
PW11Cu/MCM
Cyclo- PW11Cod
hexane PW11Fee
PW11Cud
13
8
36 64
a Based on the gas chromatographic peak areas. b Expressed as a percentage
of the total products; -ol = cycloalkanol; -one = cycloalkanone; -OOH =
cycloalkylhydroperoxide.c H2O2/aceticacidmolarratio,1:1. d Cyclohexane,
18.5 mmol; catalyst, 0.04 mmol; H2O2 /cyclohexane molar ratio, 2:1;
MeCN, 10 ml; 80°C, 12 h; e Same as d but at 60°C.4
This work was supported by a research grant ‘The 90th
Anniversary of Chulalongkorn University Fund’.
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metal introduced. The activity increased in the order V > Cu >
> Co > Fe. Compared between the supported and the unsupported
catalysts, PW11M/MCM showed higher selectivity for cyclo-
hexanone, except for PW11Fe/MCM, revealing the ability of
solid matrices, MCM-41 to control surface reactions modifying
the adsorption–desorption equilibria of reagents, reaction inter-
mediates and products. The polar surface of the siliceous support
is expected to favor the accumulation of produced alcohols at the
interfaces and, consequently, oxidation to ketones. These results
are in agreement with published data.10 No cycloalkyl hydro-
peroxide was detected (except for the case of PW11Fe/MCM), this
is similar to the result over (Bu4N)7H3[Co4(H2O)2(PW9O34)2].16
The addition of acetic acid led to a further increase in the cyclo-
hexane conversion in the presence of PW11Co/MCM from 15 to
17%. This can be attributed to the stabilization of H2O2 as peroxy
acetic acid species. PW11Fe/MCM showed 10 and 14% selectivity
for cycloalkyl hydroperoxide in cyclohexane and cyclooctane,
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respectively. The hydroperoxide was presumably formed by Feiii-
16
·
initiated generation of HO . However, the cycloalkyl hydro-
peroxide formation is much less than that with the unsupported
catalyst. We performed oxidation in the absence of a solvent.
Cyclooctane can also be oxidized by these catalysts with higher
conversions. The most active catalyst PW11V/MCM gave a higher
conversion (21%) of cyclooctane than that reported (16%).10 The
solid catalysts could be filtered off and reused, the activities
dropped ~1–2% after the third run without change in product
selectivity. This isduetosomeleaching(13–14%PW11Mremained,
analyzed by ICP) or surface coverage of the catalysts after triple
use. However, the structure of the MCM-41 support was not col-
lapsed, as shown in Figure 2. Diffraction peaks from MCM-41 are
still observed, except with reduction in peak intensities. Figure 3
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Received: 27th September 2011; Com. 11/3804
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