Simple vanadium(V) catalyst for oxidation of alkane with O2 under mild conditions

Article abstract of DOI:10.1246/cl.2007.114

Selective oxidation of cyclohexane and adamantane was performed by simple V(5+) catalyst with 1 atm O₂ in acetic acid at 338-393 K. UV-vis and electron spin resonance studies suggested that peroxide species on V(5+) functioned during the oxidation. The oxidation activity was enhanced three times for cyclohexane and 6.5 times for adamantane by addition of a small amount of CF₃SO₃H. Copyright

Full text of DOI:10.1246/cl.2007.114

114
Chemistry Letters Vol.36, No.1 (2007)
Simple Vanadium(V) Catalyst for Oxidation of Alkane
with O2 under Mild Conditions
Hirokazu Kobayashi and Ichiro Yamanakaꢀ
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-S1-43 Ookayama, Meguro-ku, Tokyo 152-8552
(Received September 6, 2006; CL-061032; E-mail: yamanaka@apc.titech.ac.jp)
Selective oxidation of cyclohexane and adamantane was
activities were 9 TON(Co) > 3 TON(Mn) > 1 TON(Fe) in
6 h. Other acetylacetonato metal complexes were not active
under the reaction conditions in this work.
performed by simple V(5+) catalyst with 1 atm O2 in acetic acid
at 338–393 K. UV–vis and electron spin resonance studies
suggested that peroxide species on V(5+) functioned during
the oxidation. The oxidation activity was enhanced three times
for cyclohexane and 6.5 times for adamantane by addition of a
small amount of CF3SO3H.
Various vanadium compounds were tested for oxidation
of cyclohexane: VO(acac)2 (44 TON) > V(acac)3 (36) >
NH4VO3 (35) > VO(C2O4) (33) > VCl3 (15) > VOSO4
(13) > VO-TPP (7) > VBr3 (5) > H4PVMo11O40 (0.1) in 6 h.
Product selectivities were very similar. VO(acac)2, V(acac)3,
NH4VO3, and VO(C2O4) had similar catalytic activities but their
oxidation states were different, V4þ, V3þ, V5þ, and V4þ, respec-
tively. These results indicate that common active vanadium
species should form during the oxidation. We chose VO(acac)2
as catalyst hereafter because it gave the highest products
yield and good reproducibility.
Selective partial oxidation of hydrocarbons under mild con-
ditions is an attractive subject for the green and sustainable
chemical processes. Many catalytic oxidation systems using
various oxidants (TBHP, H2O2, O2/H2, O2/CO mixture, etc.)
have been reported under mild conditions1,2 but the simplest
and most economical oxidant is O2. The ultimate catalytic oxi-
dation system should work with O2 without any reducing
agents.3,4 In the industrial oxidation process, auto-oxidation of
cyclohexane to cyclohexanol and cyclohexanone operates under
severe conditions, P (air) > 10 atm, T > 423 K, with promoters.
It would be a very attractive prospect to develop a new catalytic
system working with O2 (ꢁ1 atm) and at ꢁ423 K.
Influences of reaction temperature, P(O2), concentrations of
cyclohexane and VO(acac)2, on the formation rates of products
were studied to obtain information on kinetics and optimum
reaction conditions. The formation rate of the sum of the oxy-
genates depended on first order of the cyclohexane concentra-
tions (0–20 vol %), on first order of the VO(acac)2 concentration
(0–0.7 mM), and on a half order of P(O2) (0–1 atm). The forma-
tion rate increased exponentially from 338 to 365 K. The appa-
The aim of this work is to find an effective and simple cata-
lytic system to oxidize alkanes with O2 and to obtain information
on the oxidation mechanism. We chose cyclohexane and ada-
mantane as alkane substrates in this study. Cyclohexanone is es-
sential material in 6,6-nylon. Adamantane oxygenates are essen-
tial materials in photoresist resins with excimer ArF laser light
and medicines for Parkinson’s disease. In addition, adamantane
oxygenates are mainly manufactured with co-production of pitch
by the sulfuric acid oxidation.
rent activation energy was 90 kJ molꢂ1
.
We applied this V catalytic system for the oxidation of
adamantane. Figure 1 shows time courses of the oxidation of
adamantane under suitable reaction conditions (O2 1 atm,
adamantane 0.47 M, VO(acac)2 0.47 mM, 393 K) based on the
above cyclohexane oxidation conditions. The V catalytic system
was very active for the oxidation of adamantane. The main prod-
Oxidation of adamantane was carried out in a Pyrex glass
flask (100 mL) with a condenser, a gas-introduction tube and a
thermocouple. Catalyst (VO(acac)2, 0.5 mM) was dissolved in
a mixture (20 mL) of alkane and acetic acid solvent in the flask.
The oxidation was started by stirring with a magnetic spin bar
and bubbling O2 (10 mL minꢂ1) into the solution, and continuing
for 6 h at 338–393 K. The products were analyzed by GC, GC-
MS, and HPLC.
1-AdOX
5
4
3
2
1
0
500
400
300
200
100
0
2-AdOX
2-AdO
1,3-AdOX
total
CO2
First, a screening test for various acetylacetonato metal
complexes (TiO2þ, VO2þ, Mn2þ, Fe3þ, Co3þ, Ni2þ, Cu2þ
,
Zr4þ, MoO22þ, Pd2þ, La3þ, Eu3þ, and Ir3þ) that were expected
to activate O2 was carried out for oxidation of cyclohexane
(1.85 M, 4 mL) with 1 atm O2 at 365 K. We have found that
VO(acac)2 was active for the oxidation of cyclohexane. Products
were cyclohexanol (CyOH, 60% selectivity), cyclohexanone
(CyO, 30%), cyclohexyl acetate (CyOAc, 10%), and CO2. The
turnover number based on V atom (TON(V)) for the sum yield
of CyOH, CyO, and CyOAc was over 44 in 6 h. CO2 was mainly
produced by oxidation of AcOH. Among the active promoters of
Mn, Fe, and Co under the auto-oxidation conditions, oxidation
0
120
240
360
480
600
Reaction time/min
Figure 1. Kinetic curves of the oxidation of adamantane
(0.47 M) with O2 (1 atm) in AcOH (20 mL) by VO(acac)2
(0.47 mM) at 393 K. AdOX: AdOH + AdOAc.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.1 (2007)
115
uct was 1-adamantanol. 1-Adamantanol (2%) was detected as
acetic acid ester, therefore the sum of 1-adamantanol and its es-
ter was plotted as 1-AdOX in Figure 1. Other major products
were 2-adamantanol (2-AdOX) and 2-adamantanone (2-AdO).
The time courses of the sum of adamantane oxygenates and
CO2 showed weak sigmoidal curves but the oxidation proceeded
smoothly and the TON(V) was over 440 (44% yield) at 10 h.
Products distributions were almost constant for 10 h. The selec-
tivities of 2ꢃ-oxygenated products, defined as the ratio of sum
yield of secondary oxygenates to total yield, were about 25%.
The conversion of adamantane increased after 10 h but the
distribution of di-oxygenated products, such as 1,3-adamantane-
diol, 5-hydroxy-2-adamantanone, and their esters increased
significantly. When air was used for the oxidation, selective
oxygenation of adamantane proceeded with a good oxidation
activity, 12% yield and 120 TON(V) at 6 h.
2
1.5
1
(a)
(b)
(c)
0.5
0
250
300
350
400
450
500
Wavelength/nm
To clarify the reaction mechanisms, detailed studies were
carried out. First, to obtain information on the rate-determining
step, kinetic-isotope-effect (KIE) studies were carried out for ox-
idation of a mixture of cy-C6D12 and cy-C6H12 at 365 K. The
KIE value was 2.8, which suggested that C–H bond cleavage
should be involved in the rate-determining step of the oxidation.
The activation energy (90 kJ molꢂ1) mentioned earlier was lower
than the dissociation energy of the C–H bond (391 kJ molꢂ1),
which reflects the oxidation ability of the V active species.
Second, to obtain information on the character of active oxygen,
the retention of C–H bond configuration was studied during
oxidations of cis-, or trans-1,2-dimethylcyclohexane. Mixtures
of cis- and trans-1,2-dimethylcyclohexan-1-ol were obtained.
About 31–37% of the configuration was retained during the
oxidation of tertiary C–H bonds of 1,2-dimethylcyclohexane.
Regioselectivity of 2ꢃ:3ꢃ per number of C–H bonds was 1:9 in
the oxidation of adamantane and cis-1,2-dimethylcyclohexane.
These data suggest electrophilicity of active oxygen species.
To obtain more information on V species and active oxygen
species, we compared the solutions before and after the oxida-
tion using UV–vis (Figure 2) and electron spin resonance
(ESR) spectrometers. When AcOH was added to VO(acac)2/
CH2Cl2 solutions, the UV absorption at 300 nm assigned to
VO(acac)2 disappeared; in contrast, an absorption at 270 nm as-
signed to acetylacetone (acacH) appeared. This observation
proved that the acacꢂ ligand of VO(acac)2 easily exchanged
with AcOꢂ. VO(OAc)2 is the precursor of active V species.
The VO(OAc)2/AcOH solutions (sample 1) showed typical
ESR spectra of V4þ (g ¼ 1:984: 8 signals). A broad UV absorp-
tion was observed from 300 to 400 nm for the reaction mixture
under the oxidation conditions (Figure 2b, sample 2) compared
with the mixture before the oxidation (Figure 2a). This broad
UV absorption disappeared by passing Ar (Figure 2c, sample
3). These observations indicate that species giving the UV broad
absorption are reversible ones. ESR studies were completed for
sample 2 quenched with liquid N2 (ꢂ196 ꢃC) but no ESR signal
was observed. The V4þ (VO(OAc)2) species is oxidized to V5þ
under the oxidation conditions. As in sample 2, no ESR signal
was observed in sample 3. The oxidation state of V (V5þ) did
not change by passing Ar but the broad UV absorption species
disappeared (Figures 2b and 2c). These observations indicate
that the V5þ catalyst activates O2 and oxidizes cyclohexane
retaining its oxidation state (5+). It has been reported that
Figure 2. UV–vis absorption spectra of the reaction mixture (a)
before and (b) after oxidation for 3 h at 365 K, and (c) after purg-
ing with Ar for 0.5 h at 365 K. Reaction mixture: VO(acac)2
0.5 mM, cyclohexane 1.8 M, AcOH 16 mL, O2 or Ar 1 atm.
vanadium peroxide species show a broad UV absorption from
300 to 400 nm.5 The broad UV absorption species may be due
to V5þ(OO) species.
According to this model, an increase in the oxidation
activity of V catalyst would be expected by exchange of the
AcOꢂ ligand for a more electron-withdrawing one. Therefore,
additions of stronger acids (0.02 vol %) were examined for the
oxidation of cyclohexane with O2 by VO(acac)2/AcOH at
365 K. Their TON rates for the sum of products were CF3SO3H
(22 TON(V) hꢂ1) > CH3SO3H (10.6) > CF3CO2H (7.8) >
standard (7.3) > H2SO4 (5.5). The addition of CF3SO3H was
very effective for the oxidation. Product selectivities were
CyOAc (77%), CyOH (18), and CyO (5). The esterification of
CyOH and AcOH was very quickly promoted by 0.02 vol %
CF3SO3H addition. This oxidation system was also effective
for the oxidation of adamantane. The TON rate for the sum of
products increased from 2.0 to 13.0 TON hꢂ1 with the addition
of 0.02 vol % CF3SO3H at 365 K. 2ꢃ-Selectivity of 32% increas-
ed slightly, but the KIE value was the same with the addition of
CF3SO3H. These data indicate that differences in the nature of
the active oxygen species are small.
Detailed O2 activation mechanisms and the real form of
the active oxygen species are still to be clarified. However, this
simple catalytic system could be one of the candidates for the
partial oxidation of alkanes with O2 under mild conditions.
This study is supported by the Grant-in-Aid Scientific
Research, JSPS Research Fellow, No. 18ꢄ5782.
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Published on the web (Advance View) December 2, 2006; doi:10.1246/cl.2007.114