Catalytic oxidation of cyclohexene by molecular oxygen over isopolyoxometalates

Article abstract of DOI:10.1246/cl.2004.200

Isopolyoxomolybdates exhibited high selectivities for the industrially useful products (cyclohexene oxide and 2-cyclohexen-1-ol) catalyzing effectively the reaction of initial product cyclohexenyl hydroperoxide with cyclohexene in the oxidation of cyclohexene by molecular oxygen.

Full text of DOI:10.1246/cl.2004.200

200
Chemistry Letters Vol.33, No.2 (2004)
Catalytic Oxidation of Cyclohexene by Molecular Oxygen over Isopolyoxometalates
Yanyong Liu,ꢀ Kazuhisa Murata, Megumu Inaba, Hitoshi Nakajima,y Masahiko Koyay, and Keizou Tomokuniy
Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology,
AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
yCorporate R&G Laboratories, Asahi Kasei Corporation, 3-13-1 Ushiodori, Kurashiki, Okayama 712-8633
(Received October 24, 2003; CL-031011)
Isopolyoxomolybdates exhibited high selectivities for the
COOH
OH
industrially useful products (cyclohexene oxide and 2-cyclohex-
en-1-ol) catalyzing effectively the reaction of initial product cy-
clohexenyl hydroperoxide with cyclohexene in the oxidation of
cyclohexene by molecular oxygen.
O
COOH
OH
OH
Polyoxometalates are polymeric oxoanions formed from
one kind of polyanion (so-called isopolyoxometalates) or formed
by condensation of more than two different mononuclear oxoan-
ions (so-called heteropolyoxometalates).1 Polyoxometalate cata-
lysts are very attractive since they can effectively catalyze both
acid and oxidation reactions in heterogeneous and homogeneous
reaction systems.2
Scheme 1.
troscopy and element analysis.
In a dry box, the catalyst (1.5 mmol) was added to a glass vial
containing 1.5 mL of 1,2-dichloroethane, 0.1 mL of acetonitrile
and a magnetic bar. Then freshly distilled cyclohexene
(0.5 mL, 4.94 mmol) was added to the solution. The vial was
sealed and attached to a vacuum line, cooled to 77 K (liquid
N2 bath), and 1 atm of O2 was introduced to the system. After
reacted under magnetic agitation, the reaction solution was ana-
lyzed by gas chromatography with a capillary column.
Because cyclohexene has been manufactured by the hydro-
genation of benzene for the industrial preparation by Asahi Ka-
sei Corporation,3 the oxidation of cyclohexene to useful chemi-
cals attracted a wide interest in industry.4 Cyclohexene could be
oxidized to cyclohexene oxide (denoted by Ch-O) or adipic acid
using hydrogen peroxide as the oxidant.5 Unfortunately, hydro-
gen peroxide is currently too expensive to allow an economically
viable process. Molecular oxygen is the best oxidant due to low
cost and significant advantages for environment. The main prod-
uct of the cyclohexene oxidation by molecular oxygen without
any reduction reagents reported in the literatures is 2-cyclohex-
en-1-one (denoted by Ch-one).6 The only possible usage of Ch-
one in industry is the hydrogenation of Ch-one to produce cyclo-
hexanone. However, compared to the direct oxidation of cyclo-
hexane to cyclohexanone, there is no economical benefit to pro-
duce cyclohexanone through oxidation of cyclohexene to Ch-
one followed by hydrogenation of Ch-one. In this study, we wish
to report that isopolyoxomolybdates exhibited high selectivities
for the industrially useful products (Ch-O, 1,2-cyclohexanediol
(denoted by Ch-diol) and 2-cyclohexen-1-ol (denoted by Ch-
ol)) in the oxidation of cyclohexene by molecular oxygen.
Adipic acid is a necessary raw material for the manufacture
of nylon-6,6. HNO3 is used as the oxidant for the worldwide in-
dustrial adipic acid production and about 400000 metric tons of
N2O are emitted each year.7 1,3-Cyclohexadiene is widely ap-
plied in resin and perfumery industry.8 We suggest a new route
to manufacture adipic acid and 1,3-cyclohexadiene from the ox-
idation of cyclohexene as described in Scheme 1.
The conversion and product distribution over the various
catalysts are shown in Table 1. The conversion of cyclohexene
was 3.3% and the main product was cyclohexene hydroperoxide
(denoted by CHHP) in the absence of a catalyst at 323 K for 24 h,
which indicates that CHHP is the initial product in the oxidation
of cyclohexene with molecular oxygen. The selectivities for IUP
were 80.6, 90.1, and 82.3% over (Bu4N)2Mo2O7, (Bu4N)2Mo6-
O19, and (Bu4N)4Mo8O26 at the conversions of 31.8, 36.9, and
28.1%, respectively. These results indicated that isopolyoxomo-
lybdates, especially (Bu4N)2Mo6O19, are suitable catalysts for
the new process as described in Scheme 1. Isopolyoxotungstate
(Bu4N)2W6O19 showed a high conversion of 52.7% but the se-
lectivity for IUP was only 42.1%. The cyclohexene conversion
and the selectivity for IUP were 30.5 and 63.6% over
isopolyoxovanadate (Bu4N)6V10O28. As for the heteropolyoxo-
metalates, (Bu4N)3PMo12O40, and (Bu4N)3PW12O40 showed
low activities and low yields of IUP in the oxidation of cyclohex-
ene with molecular oxygen. (Bu4N)4PW11Co(H2O)O39, which is
the most active polyoxometalate for the oxidation of cyclohex-
ene with molecular oxygen reported in the literatures,6 showed
the highest conversion of 58.5% in this study but the selectivity
for IUP was very low (31.1%). (Bu4N)4PMo11Ru(H2O)O39 is
the most selective catalyst for the formation of Ch-O and Ch-
ol from the oxidation of cyclohexene with molecular oxygen re-
ported in the literatures,6 but it only showed a selectivity for IUP
of 54.6% in this study, which was much lower than those of iso-
polyoxomolybdates in the oxidation of cyclohexene with molec-
ular oxygen. As for the solvent effect, polyoxometalates showed
high activities in 1,2-dichloroethane but the solubility of polyox-
ometalates in 1,2-dichloroethane is low. On the other hand, pol-
yoxometalates easily solve in acetonitrile but the activities over
polyoxometalates were low in acetonitrile. Thus we used 1,2-di-
The main task in the new process is how to improve the
yields of the industrially useful products (denoted by IUP, in-
cluded Ch-ol, Ch-O, and Ch-diol) and impress the production
of the industrially needless products in the cyclohexene oxida-
tion using molecular oxygen as an oxidant.
Polyoxometalates were prepared according to the methods
reported in the literatures.6,9 The structures of the polyoxometa-
lates synthesized in this study were confirmed by infrared spec-
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.2 (2004)
Table 1. Oxidation of cyclohexene with molecular oxygen over various catalysts at 323 Ka
201
Catalyst
Conversion
/%
Selectivity/%
OH
OOH
OH
O
OH
OH
IUPb
O
O
Blank
(Bu4N)2[Mo2O7]
2.4
0.5
7.7
8.3
6.2
2.8
3.2
6.5
2.1
9.0
80.6
90.1
82.3
42.1
53.0
31.1
54.6
3.3
31.8
36.9
28.1
52.7
16.6
58.5
32.3
6.1
6.4
13.2
4.7
10.6
38.6
25.2
56.3
36.5
84.5
3.4
3.8
0.2
2.8
1.4
3.7
7.9
4.3
3.9
3.3
35.3
40.2
36.9
20.7
23.4
6.4
37.6
41.6
39.2
18.6
26.4
18.2
33.6
(Bu4N)2[Mo6O19]
(Bu4N)4[Mo8O26]
(Bu4N)2[W6O19]
(Bu4N)3[PMo12O40]
(Bu4N)4[PW11Co(H2O)O39]
(Bu4N)4[PMo11Ru (H2O)O39]
3.3
11.4
17.5
8.7
18.9
5.5
aReaction conditions: catalyst, 1.5 mmol; 1,2-dichloroethane, 1.5 mL; acetonitrile, 0.1 mL; cyclohexene, 0.5 mL; pO2 ¼ 1 atm; reaction
time, 24 h. bIUP: industrially useful products, the sum of cyclohexene oxide, 1, 2-cyclohexenediol and 2-cyclohene-1-ol.
ly catalyzed Step 2 in Scheme 2 and the rate of Step 2 was much
faster than the rates of other steps. Once CHHP formed, it con-
verts to Ch-O and Ch-ol very quickly over isopolyoxomolyb-
dates. The selectivity for Ch-O (including Ch-diol) was lower
than 50% over every catalyst in this study (Table 1), which indi-
cates that Ch-O was produced as described in Step 2 in Scheme 2
with forming the same amount of Ch-ol. The contribution of the
direct cyclohexene epoxidation by molecular oxygen is very
small over isopolyoxomolybdates since the direct epoxidation
of alkene by molecular oxygen would produce epoxide in high
selectivity.
OH
OH
+
O
OH
+
OOH
OH
+
Step 2
+ O2
+
O
Step 1
The used (Bu4N)2Mo6O19 catalyst was obtained by vacuum
distillation of the mixture after reaction. The conversion and the
selectivity for IUP did not decrease after reused for two times.
These results indicate that (Bu4N)2Mo6O19 is reusable in the ox-
idation of cyclohexene by molecular oxygen.
+ H2O
OH
OH
O
+
H2O
References
Scheme 2.
1
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The oxidation of cyclohexene with molecular oxygen ini-
tially forms CHHP as shown in Scheme 2 (Step 1). CHHP is
not stable and can form Ch-O and Ch-ol by epoxidation of cyclo-
hexene (Step 2), form 2,3-epoxycyclohexanol by reacting with
Ch-ol (Step 3), or decompose to Ch-one and water (Step 4).
The conversion of cyclohexene is controlled by the rate of Step
1 in Scheme 2 and the selectivity for IUP is controlled by the rate
ratio of Step 2 to Step 3 and Step 4. (Bu4N)2W6O19 and
(Bu4N)4PW11Co(H2O)O39 showed high conversions in the oxi-
dation of cyclohexene, which indicated that they effectively cat-
alyzed Step 1 in Scheme 2. However, they did not effectively
catalyze Step 2 in Scheme 2. The formed CHHP exists in the sys-
tem for a long time and forms the undesirable products (Step 3
and Step 4 in Scheme 2). (Bu4N)4PMo11Ru(H2O)O39 is a bi-
functional catalyst which simultaneously catalyzes Step 1 and
Step 2 in Scheme 2.6b It showed higher selectivity for IUP than
those of (Bu4N)2W6O19 and (Bu4N)4PW11Co(H2O)O39. Isopo-
lyoxomolybdates catalyzed the Step 1 in Scheme 2 since the cy-
clohexene conversions over isopolyoxomolybdates were much
higher than that of blank (without a catalyst). Moreover, isopo-
lyoxomolybdates showed high selectivities for Ch-O and Ch-ol,
which indicates that isopolyoxomolybdates extremely effective-
3
4
5
6
7
8
9
Published on the web (Advance View) January 25, 2004; DOI 10.1246/cl.2004.200