Efficient oxidation of alkane with O2 and H2 by Eu-Ti-Pt catalytic system

Article abstract of DOI:10.1246/cl.2005.1486

Alkane was efficiently oxidized to alcohol and ketone with O₂ and H₂ by EuCl₃-TiO(acac)₂-Pt/SiO₂ catalytic system with very high H₂-utilization efficiencies, 75% for adamantane and 60% for cyclohexane oxidations. ESR studies suggested that electrons originated in H₂ effectively conducted to O₂ through Pt, Eu³⁺/Eu²⁺, and Ti⁴⁺/Ti ³⁺. Eu²⁺ and Ti³⁺ species reductively activated O₂, generating active oxygen species. Copyright

Full text of DOI:10.1246/cl.2005.1486

1
486
Chemistry Letters Vol.34, No.11 (2005)
Efficient Oxidation of Alkane with O and H by Eu–Ti–Pt Catalytic System
2
2
ꢀ
Ichiro Yamanaka, Toshikazu Gomi, Takashi Nabeta, and Kiyoshi Otsuka
Department of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552
(Received August 8, 2005; CL-051020)
Alkane was efficiently oxidized to alcohol and ketone with
O2 and H2 by EuCl3–TiO(acac)2–Pt/SiO2 catalytic system with
very high H2-utilization efficiencies, 75% for adamantane and
ates was fairly good about 29.1%, but the H2-eff of 6% was lower
than that of the benzene hydroxylation (10%).
3þ
It was found that counter anion species of Eu strongly
affected the catalytic activity for the oxidation. EuCl3 and
Eu(ClO4)3 shows the good oxidation activities, 35.3 and
34.9% yields, and the higher H2-eff of 22.3 and 22.1%, respec-
tively. Eu(NO3)3, Eu(OAc)3, and Eu(acac)3 were inactive.4
EuCl3 catalyst is more favor for the oxidation because of the
higher selectivity to the useful secondary oxygenate (2 and 3),
49% (EuCl3) > 44% (Eu2(ClO4)3) > 36% (Eu(OTf)3).
After detailed studies in influences of various reaction con-
ditions on the adamantane oxidation, it was found that lower
concentrations of EuCl3 and TiO(acac)2 and higher P(H2) were
favor for the efficient oxidation. Figure 1 shows the effects of
P(H2) on the oxidation. The formation rates of 1, 2, 3, and the
yield of H2O increased with increasing P(H2). The formation
rates were maximum around P(H2) ¼ 80 kPa. It is to be noted
that the H2O yield sharply decreased with increasing P(H2) >
75 kPa rather than decreasing in the formation rates of 1, 2,
and 3. Therefore, the H2-eff drastically increased with P(H2) >
60% for cyclohexane oxidations. ESR studies suggested that
electrons originated in H2 effectively conducted to O2 through
3þ
2þ
4þ
3þ
2þ
3þ
Pt, Eu /Eu , and Ti /Ti . Eu and Ti species reductive-
ly activated O2, generating active oxygen species.
Oxidation of alkane remains an attractive topic in organic
synthesis and catalysis under mild conditions. An essential point
is how to catalytically generate active oxygen species. H2O2 is a
1
useful and effective oxidant for the oxidation. H2O2, however,
is an expensive oxidant for production of basic chemicals. A
candidate of an efficient and economic oxidant is a gas mixture
of O2 and H2. Monooxygenase activates O2 with NADH corre-
sponding to H2 and oxygenates hydrocarbons. Since an artificial
model of monooxygenase P-450 (Mn-TPP, Pt-colloid, O2, and
2
H2) has been reported by Tabushi, several catalytic oxidation
3
systems have been studied (Eq 1).
ꢁ
8
0 kPa. The highest H2-eff was 75% with 14% yield at
þ
R{H þ O2 þ H2ð2e ; 2H Þ ! R{OH þ H2O:
ð1Þ
P(H2) ¼ 96 kPa (P(O2) ¼ 5 kPa) in 1 h. This 75% H2-eff is
excellent to compare with the values of other oxidation system.3
A ratio of secondary (2 and 3) and tertiary (1) oxygenate yields
A serious problem is a large amount of H2O formation
through deep reduction of O2 (2H2 þ O2 ! 2H2O). When H2-
utilization efficiency (H2-eff) was defined by Eq 2, the H2-eff
ꢃ
ꢃ
per a number of C–H bond (2 :3 ) characterizes a nature of
ꢃ
3
ꢃ
values were lower than 20% in previous works. In other words,
active oxygen species. The 2 :3 values were constant (1:2.8)
at all P(H ). This fact suggests that the drastic increase in the
80% H2 was uselessly consumed to H2O formation.
2
H2-eff is due to the deceleration of the deep reduction of O2 to
H2-eff ¼ (oxygenates yield)=(H2O yield) ꢂ 100(%):
ð2Þ
Recently, we have reported the Eu(OTf)3–TiO(acac)2–Pt/
SiO2 catalytic system which was active for the hydroxylation
of benzene to phenol with O2 and H2. The H2-eff increased with
0
0
0
0
.4
.3
.2
.1
100
80
H2O x 0.1
4
increasing P(H2) for the formation of phenol but hydrogenation
of benzene to cyclohexane became dominant above 50 kPa. The
positive dependence of the H2-eff on P(H2) suggested that higher
P(H2) would be favor for activation of O2 by the Eu–Ti–Pt-
catalytic system. Therefore, we focused in oxidation of alkanes,
especially adamantane to 1-adamantanol 1, 2-adamanatanol 2,
and 2-adamantanone 3. Adamantane oxygenates are essential
compounds for medicines of Perkinson disease, anticancer and
anti-HIV. 2 and 3 are essential for the synthesis of photoresist
6
0
CO2
40
2
0
0
0
0
0
0
.3
total
5
materials with ArF laser light. Therefore, we are able to expect
.2
.1
a large demand of adamantane oxygenates, especially 2 and 3.
Adamantane oxygenates are now manufactured by the sulfuric
acid-oxidation method which is not environmental friendly
process.5
1
3
2
We applied the Eu(OTf)3 (6 mM)–TiO(acac)2 (1 mM)–
wt % Pt/SiO2 (0.1 g) which was active for the hydroxylation
of benzene, to the oxidation of adamantane (50 mM in
MeCO2H) with O2 (33.8 kPa) and H2 (67.5 kPa). Major products
were 1, 2, 3, and minor ones were 1,3-adamantadiol and 5-hy-
droxy-2-adamantanone. The sum yield of adamantane oxygen-
0
1
0
20
40
60
80
100
4
2
P(H ) / kPa
Figure 1. Effect of P(H2) on the adamantane oxidation with O2
catalyzed by EuCl3 1 mM, TiO(acac)2 0.167 mM, and Pt/SiO2
ꢃ
0
.1 g at 40 C.
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.11 (2005)
1487
H2O and not due to changing of the reactivity of active oxygen
species.
We studied kinetic curves of the adamantane oxidation at
P(H2) ¼ 96 and P(O2) ¼ 5 kPa. Yields of adamantane oxygen-
ates linearly increased with reaction times but slightly decelerat-
ed after 2 h. The H2-eff was extrapolated to ca. 100% at early
stage of the oxidation but decreased with reaction time, 46%
at 3 h. When H2O of 1 mmol corresponding to a H2O yield at 3 h
was added in the initial reaction mixture, the formation rate of
oxygenates decelerated 0.6 times as slow as the standard one.
This fact suggests that H2O accumulated by equiv. 1 inhibits
the oxidation activity. Dehydration treatment should be essential
for the efficient continuous oxidation of adamantane.
In order to know that the significant high H2-eff is peculiar to
the adamantane oxidation or universal to alkane oxidation, we
ꢃ
studied the oxidation of cyclohexane which C–H bond (2 ) ener-
ꢁ
1
gy was 94 kcal mol as strong as that for adamantane. We ob-
served a very similar dependence of the cyclohexane oxidation
on P(H2). Formation rates of cy-C6H11OH, cy-C6H10O, CO2,
and H2O were maximum at P(H2) ¼ 80 kPa. The H2-eff increas-
ed above P(H2) ¼ 80 kPa and showed the maximum of 60% at
P(H2) ¼ 96 kPa. The H2-eff was extrapolated to 90% at the early
stage of the oxidation. These facts suggest that the EuCl3–
TiO(acac)2–Pt/SiO2 catalytic system can selectively activate
O2 and oxygenate alkanes.
Figure 2. ESR spectra of (1) Eu(OTf) 6 mM, (2) TiO(acac)
3
2
1 mM, and (3) the mixture in MeCO H after reduction with
2
To obtain information for the oxidation potential of the
active oxygen species, oxidation of stable gaseous alkanes
H2 (1 atm) by Pt/SiO2 for 30 min at 298 K and quenched by
liquid N2.
(
propane, ethane, and methane) was carried out. A gas mixture
mediately react with O2. A life of reduced oxygen species may
be very short or silent for the ESR absorption.
of alkane (34 kPa), H2 (62), and O2 (5) was bubbled through
the EuCl3–TiO(acac)2–Pt/SiO2/MeCO2H mixture at 298 K. In
propane oxidation (1 C–H bond: 97 kcal mol and 2 C–H
bond: 94), significant yields of 2-propanol, acetone and 1-prop-
anol, propionaldehyde were obtained. The sum formation rates
of oxygenates was 150 mmol h with 10% H2-eff. This value
is fairly high though the concentration of propane is low. A ratio
of 1 :2 was 1:7. Ethane (C–H bond: 98 kcal mol ) was also
oxygenated to ethanol and acetaldehyde but the formation rate
We are able to propose a model of Eu–Ti–Pt catalysis for the
0
ꢃ
ꢁ1
ꢃ
activation of O2. Step 1: Pt dissociatively activates H2 to H
ꢄ
.
ꢁ
þ
3þ
2þ
0
Step 2: H
ꢄ
(e and H ) reduces Eu to Eu over Pt . Step 3:
2þ
4þ
3þ
2þ
3þ
Eu reduces Ti to Ti . Step 4: Eu and Ti species should
concertedly activate O2. Step 5: The active oxygen species
selectively and effectively oxygenate C–H bonds (ca. 94 kcal
ꢁ1
ꢃ
ꢃ
ꢁ1
ꢁ1
mol ) of adamantane and cyclohexane. The active oxygen
ꢃ
ꢃ
ꢃ
specie has radical character because of a low 1 :2 :3 value of
ꢁ
ꢁ1
of the sum of the products was only 20 mmol h with a low
H2-eff of 1%. On the other hand, no significant formation of
products was observed in the CH4 oxidation (C–H bond:
1
:7:20. The reduction potential of H2 (2e ) finally conducts to
3þ
2þ
4þ
3þ
O2 through Pt, Eu /Eu , and Ti /Ti . The three different
functions of Eu, Ti, and Pt concert and perform the efficient
and selective oxidation.
ꢁ
1
1
04 kcal mol ). The maximum oxidation potential of the active
ꢃ
ꢁ
1
ꢃ
ꢃ
oxygen species is about 98 kcal mol . A low 1 :2 :3 value of
:7:20 suggests radical character of the active oxygen species
1
rather than electrophilic one.
References
6
1
K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi, and
N. Mizuno, Science, 300, 964 (2003); P. Battioni, J. P. Reaud, J. F.
Bartoli, M. Reina-Artiles, M. Fort, and D. Mansuy, J. Am. Chem.
Soc., 110, 8462 (1988).
We carried out ESR studies to clarify oxidation states of
Eu and Ti reduced with H2 by Pt/SiO2. In Eu(OTf)3–Pt/SiO2
system (Figure 2(1)), a broad ESR signal was observed at g ¼
2
3
I. Tabushi and N. Koga, J. Am. Chem. Soc., 101, 6456 (1979).
T. Hayashi, K. Tanaka, and M. Haruta, J. Catal., 178, 566 (1998);
T. Miyake, M. Hamada, Y. Sasaki, and M. Oguri, Appl. Catal., A,
131, 33 (1995); T. Tatsumi, K. Yuasa, and H. Tominaga, J. Chem.
Soc., Chem. Commun., 1992, 1446; K. Otsuka, I. Yamanaka, and
K. Hosokawa, Nature, 345, 697 (1990); A. Kunai, T. Wani, Y.
Uehara, F. Iwasaki, Y. Kuroda, S. Ito, and K. Sasaki, Bull. Chem.
Soc. Jpn., 67, 2613 (1989); N. Herron and C. A. Tolman, J. Am.
Chem. Soc., 109, 2837 (1987).
:00, which was assigned to Eu2 . Second, a very weak ESR
þ
7
2
signal was observed at g ¼ 1:95 for TiO(acac)2–Pt/SiO2 system
3þ
8
(
2), which was assigned to Ti . This result suggests that
4þ
reduction of Ti with H2 by Pt/SiO2 is very slow. In the case
of Eu(OTf)3–TiO(acac)2–Pt/SiO2 system (3), a broad signal of
2þ
3þ
Eu and a strong signal of Ti (around g ¼ 1:95) were over-
4þ
lapped in the large signals. This result suggests that Ti reduces
3þ
3þ
2þ
to Ti through a mediation of Eu /Eu . In other word, elec-
trons originated H2 conduct to Pt, Eu, and Ti. To obtain direct
information for the active oxygen species, a gas mixture of O2
and H2 was introduced into the Eu–Ti–Pt catalytic system and
quenched by liquid N2. We could not observe any significant
4
I. Yamanaka, T. Nabeta, S. Takenaka, and K. Otsuka, Stud. Surf. Sci.
Catal., 130, 815 (2000).
5
6
7
8
K. Nishimura and S. Suzuki, Aromatikkusu, 53, 102 (2001).
M. J. Nappa and C. A. Tolman, Inorg. Chem., 24, 4711 (1985).
H. Ebendorff-Heidepriem and D. Ehrt, J. Phys., 11, 7627 (1999).
S. A. Sergeev, V. A. Poluboyarov, V. A. Zakharov, V. F. Anufrienko,
and G. D. Bukatov, Makromol. Chem., 187, 243 (1986).
2þ
3þ
ESR signals. This result proposed that Eu and Ti species im-
Published on the web (Advance View) September 28, 2005; DOI 10.1246/cl.2005.1486