A new route to control product selectivity in the oxidative dehydrogenation of cyclohexane and cyclohexene

Article abstract of DOI:10.1246/cl.2004.822

The product selectivity can be controlled by adding acetic acid in feed over vanadium phosphate (VPO) in gas phase oxidative dehydrogenation (ODH), in which cyclohexane and cyclohexene are oxidized to cyclohexene and 1,3-cyclohexadiene (1,3-CHD), respectively, at almost 100% selectivity. This approach is also an efficient method to capture the very unstable intermediates in the mechanism study.

Full text of DOI:10.1246/cl.2004.822

822
Chemistry Letters Vol.33, No.7 (2004)
A New Route to Control Product Selectivity in the Oxidative Dehydrogenation
of Cyclohexane and Cyclohexene
Yujun Zhu, Jing Li, Xiangguang Yang,Ã and Yue Wu
Changchun institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China
(Received March 18, 2004; CL-040308)
The product selectivity can be controlled by adding acetic
acid in feed over vanadium phosphate (VPO) in gas phase oxida-
tive dehydrogenation (ODH), in which cyclohexane and cyclo-
hexene are oxidized to cyclohexene and 1,3-cyclohexadiene
(1,3-CHD), respectively, at almost 100% selectivity. This ap-
proach is also an efficient method to capture the very unstable
intermediates in the mechanism study.
to cyclohexane is up to 12.9:1 at 723 and 773 K, the conversion
of cyclohexane is at 6.9% and at 11.9%, respectively. When the
mole ratio of acetic acid to cyclohexane is from 5.3:1 to 7.7:1,
cyclohexene, 1,3-CHD (trace 1,4-CHD), benzene, maleic anhy-
dride and COx can appear simultaneously at 723 K. These results
indicate that both cyclohexene and 1,3-CHD are intermediates in
the oxidative dehydrogenation of cyclohexane to benzene and
then benzene is oxidized further to MA.
Table 1. The results of cyclohexane ODH to cyclohexene add-
ing different quantity of HOAc at 723 K8
The selective catalytic oxidative dehydrogenation has been
the subject of many studies due to the importance of refining
and reforming process in the petroleum industry, but there are
very few practical processes for converting hydrocarbons direct-
ly to more valuable products.1 This is attributed to the relatively
inert C–H bond in the hydrocarbons and the desired products are
relatively active, which will result in very low selectivity of the
desired products or must keep at very low conversion.2 Here we
show a new route to control product selectivity in the oxidative
dehydrogenation by adding acetic acid (HOAc) in feed over
phosphates catalysts.
In recent years, it has been the focus topics to exploit Earth’s
resources more efficiently and cleanly.2 A new process to pro-
duce adipic acid (the precursor for Nylon-6,6) by oxidation of
cyclohexene with H2O2 has been reported.3 It is a green process
and is completely different from the traditional process to pro-
duce adipic acid from cyclohexene as raw material. In the prac-
tical process of the hydrogenation of benzene to cyclohexene, it
is not only the low yield of cyclohexene (ꢀ30%), but also the
low selectivity of cyclohexene (ꢀ60%)4 because the complete
hydrogenation of benzene to cyclohexane is much more favora-
ble in thermodynamics.5 Considering the oxidation of cyclohex-
ene to adipic acid with H2O2, it might need another process to
obtain cyclohexene by oxidative dehydrogenation of cyclohex-
ane in gas phase. But only a mixture of maleic anhydride
(MA) and benzene generally is obtained over vanadium phos-
phate (VPO) under conventional conditions.6 It is well known
that vanadium phosphorous oxides have been widely investigat-
ed for partial oxidation and oxidative dehydrogenation of hydro-
carbons.6
HOAc/C6H12 Conversion
Selectivity/mole %
/mole ratio
/mole % C6H10 1,3-CHD C6H6 MA, COx
0
24.7
16.9
14.5
11.8
7.5
0
0.70
0
63.0
68.1
23.7
13.7
4.0
37.0
31.2
18.2
0.9
0
2.4
5.3
7.7
10.3
12.9
0
50.0
8.1
2
83.4
95.5
0.5
0
6.9
100
0
0
Reaction condition: catalyst, 0.20 g; cyclohexane, 9.25 mmolÁhÀ1
;
air, 40 mLÁhÀ1
.
In the meanwhile, the disadvantage in the new route is that
about 2–3% acetic acid has been oxidized into COx at 723 K.
Considering very low stability of 1,3-cyclohexadiene in
thermodynamics as an intermediate in the oxidative dehydrogen-
ation of cyclohexane,9 the above approach is also applied to the
preparation of 1,3-CHD by the oxidative dehydrogenation of cy-
clohexene. It is well known that 1,3-CHD-based polymers are a
very interesting class of materials due to their good heat, weath-
er, impact, abrasion, and chemical resistances, low water absorp-
tion, birefringence, and excellent transparency and rigidity.10
Nowadays, many methods for preparation of 1,3-CHD have been
reported in the literature.11 These preparation methods are not
suitable to produce 1,3-CHD in large scale, and they are lower
efficient and non-environmental-friendly. In the catalytic meth-
ods, Amano reported that cyclohexene is dehydrogenated with
only 3–32% selectivity to 1,3-CHD over C/Al2O3 under non-ox-
idative conditions.12 Iwasawa reported only 16% selectivity to
cyclohexadiene under 2.55% conversion to cyclohexene over
polynaphthoquinone catalyst in the presence of oxygen.13
In the absence of HOAc, no 1,3-CHD can be detected in
products even if both conversion and reaction temperatures keep
lower.
It is found that VPO catalysts7 also show very strong ability
to abstract H-atom in partial oxidation of cyclohexane in gas
phase in our experiments as shown in Table 1. In the absence
of acetic acid, the main products in cyclohexane oxidation are
benzene, maleic anhydride (MA), COx and no cyclohexene at
various temperatures.6 In this study, it is found that the selectiv-
ity of products appears a huge change when acetic acid is added
in the feed. The selectivity of the main product changes from
MA to benzene, 1,3-cyclohexadiene, and then to cyclohexene
as shown in Table 1. Besides, almost ꢀ100% selectivity to cy-
clohexene can be obtained when the mole ratio of acetic acid
Table 2 gives the result of the oxidative dehydrogenation of
cyclohexene with the various amount of HOAc over VPO cata-
lyst7 at 723 K. When the molar ratio of HOAc to cyclohexene in
feed varies from 0 to 1:1, a small amount of 1,3-CHD and a large
amount of benzene appear in the products with a decrease of
conversion. With an increase of HOAc amount, the selectivity
to 1,3-CHD also gradually increases. The high selectivity to
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.7 (2004)
823
1,3-CHD (almost 100%) appears in the molar ratio of HOAc to
cyclohexene above 12.1:1 at 3.8% conversion. These results in-
dicate that addition of HOAc in feed can adjust very efficiently
the oxidation of 1,3-CHD to benzene. Like the oxidation of cy-
clohxane, ꢀ2:5% HOAc is also oxidized to COx in this reaction.
on the described process. If a trace amount of primary species,
olefin and/or oxygenated intermediate, can be captured by add-
ing acetic acid in the feed over different catalysts, thus, it pro-
vides the information on the reaction mechanism and distin-
guishes the function of the catalyst in catalytic selective
oxidation. This work will be described in elsewhere.
The new route shows that HOAc in the feed can efficiently
adjust the oxidation ability of catalysts and can be applied into
the production of very unstable intermediates with high selectiv-
ity. Besides, the process could also be thought to be the new way
to investigate the mechanism in oxidative dehydrogenation be-
cause it can capture efficiently the unstable intermediates, which
also prove the powerful information to understand the mecha-
nism in oxidation dehydrogenation. It takes a new inspiration
how to control selective oxidation of alkane.
Table 2. The results of cyclohexene ODH to 1,3-CHD adding
different quantity of HOAc at 723 K8
HOAc/C6H10 Conversion
Selectivity/mole %
/mole ratio
/mole %
1,3-CHD C6H6 MA, COx
0
99.9
95.0
85.0
14.8
10.0
8.10
5.00
3.80
0
0
55.8
68.2
76.8
74.0
80.0
47.2
22.5
0
44.2
31.8
22.2
25.0
0.40
1.0
2.2
4.7
7.2
9.6
12.1
<1:00
1.00
11.0
47.8
77.5
100
9.00
5.00
0
This work is supported by the National Natural Science
Foundation (29973041) and the 973 Programme of the Ministry
of Science and Technology (2003CB615800) of China.
0
Reaction condition: catalyst, 0.20 g; cyclohexane, 9.86 mmolÁ
h
À1; air, 40 mLÁhÀ1
In general, these results show that the above process can be
.
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acetic acid in the oxidative dehydrogenation of cyclohexane and
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Published on the web (Advance View) June 7, 2004; DOI 10.1246/cl.2004.822