I. Yamanaka et al. / Catalysis Today 157 (2010) 286–290
289
Table 1
2 3 2 2
Reactivity of active oxygen species on the Pt/Eu O /TiO /SiO catalyst.
Entry
Reactant
cy-C , cy-C
Remarks
a
ꢄ
1
6
D
6
6
H
6
KIE = 2.2 (HO , KIE = 1.2)
b
2
cis-1,2-dimethyl-cyclohexane
trans-1,2-dimethyl-cyclohexane
cis:trans = 54:46 (1-ol), cis:trans = 100:0 (3-ol, 4-ol)
cis:trans = 49:51 (1-ol), cis:trans = 0:100 (3-ol, 4-ol)
b
3
c
4
Adamantane, CCl
4
Selectivity (%): 1-AdCl(59), 2-AdCl(30), 1-AdOH(1), 2-AdOH(2), 2-AdO(8)
T = 313 K, reaction time 10 min, Pt/Eu
2
O
3
/TiO /SiO catalyst 0.1 g; loadings: Eu
2
2
2
O
3
2.0 wt%, TiO
2
2.4 wt%, Pt 1.0 wt%, reactant/acetic acid (20 mL). P(O ) 5 kPa, P(H ) 96 kPa, total
2
2
ꢀ1
flow rate 20 mL min
.
a
cy-C
6
D
6
2 mL, cy-C
cis- or trans-1,2-dimethyl-cyclohexane 2 mL, 1-ol: 1,2-dimethylcyclohexan-1-ol, 3-ol: 1,2-dimethylcyclohexan-3-ol and -3-one, 4-ol: 1,2-dimethylcyclohexan-4-ol and -
-one.
6 6
H 2 mL.
b
4
c
4
Adamantane 50 mM/CCl 1 mL + AcOH 19 mL.
contrast, we observe large particles of Pt and domains of Eu and Ti
on SiO at the Eu /TiO /Pt/SiO inactive catalyst (Fig. 4b). We
confirmed formation of Pt on the used catalyst by XRD. Broad and
weak X ray diffraction at 2 = 408 and 478 was observed. Very
similar un-uniform TEM images were observed on other two
inactive catalysts. These indicated highly dispersion of well
mixture of Pt, Eu and Ti elements was essential for catalysis in
the oxidation.
In the case of oxidation of cis-1,2-dimethyl-cyclohexane, we
obtained cis-compounds (1,2-dimethyl-cyclohexan-3-ol, -4-ol, -3-
one, -4-one) and cis- and trans-1,2-dimethyl-cyclohexan-1-ol
(Entry 2). Retention of configuration of cis-1,2-dimethyl cyclohex-
ane was perfect in the former products (oxygenated at 3- or 4-
positions, cis:trans = 100:0), but the configuration did not retained
in 1,2-dimethyl-cyclohexan-1-ol (cis:trans = 54:46). Very similar
reactivity was observed in the oxidation of trans-1,2-dimethyl-
cyclohexane (Entry 3). The cis:trans was 0:100 for the products
oxygenated at 3- or 4-positions, and that was 51:49 for the product
at 1-position. These facts indicate that a cleavage of C–H bonds
occurs and may form an alkyl radical intermediate during the
oxidation. Therefore, the configuration of the four carbon bonds
disappeared during the oxidation.
2
2
O
3
2
0
2
u
ESR studies using various samples, the Pt/Eu
active catalyst, the Eu /TiO /Pt/SiO inactive catalyst, Pt/Eu
SiO , Pt/TiO /SiO samples, were done in the gas mixture of O
, and in H . Then, we obtained a few significant ESR signals of the
samples in H
2 3 2 2
O /TiO /SiO
2
O
3
2
2
2 3
O /
2
4
2
2
and
H
2
2
2
+
2
. First, ESR signals assigned to mixture of Eu and
Ti signals were observed the sample of Pt/Eu /TiO /SiO and
AcOH/CH Cl solutions with introduction of H . The ESR signal of
was also observed on the Pt/Eu /SiO sample with
but the ESR signals of Ti were not at the Pt/
3
+
2
O
3
2
2
2
2
2
4
We added a radical trap reagent of CCl (1 mL) to the standard
2
+
Eu
introduction of H
TiO /SiO sample. These observations suggest that Eu produces
by reduction of Eu
reduction of TiO with Eu . The ESR signals of Eu and Ti on the
active catalyst immediately disappeared when O was introduced
to the ESR sample. These indicate that electron transfer reactions
proceed through and active oxygen
! Pt ! Eu ! Ti ! O
species finally generate on the active catalysts [9]; therefore, the
loading order of Pt, Eu , and TiO should be essential for the
2
O
3
2
mixture of adamantane oxidation (20 mL) and obtained adaman-
tan-1-Cl (59% selectivity) and adamantan-2-Cl (30%) as the major
products. Adamantan-1-Cl and adamantan-2-Cl should be pro-
3+
2
2
+
4
2
3
+
2
O
3
with H
2
on Pt and Ti should produce by
4
duced through the trapping adamantyl radical with CCl . This
2+
2+
3+
2
indicates formation of alkyl radical intermediate during the
oxidation.
2
We can propose reaction mechanisms from the experimental
H
2
2
facts described above, as illustrated in Fig. 5. The Pt/Eu
SiO catalyst has very uniform surface and very small Pt particles
(<2 nm) well dispersed on amorphous oxides of Eu and TiO
on the SiO support because we could not observe clear particles
and XRD diffraction. PtO easily reduce to Pt with the reaction gas
mixture of O and H because we observed the color change of
the catalyst to dark gray and no influence of the pre-reduction of
the catalyst on the oxidation. H activates on Pt and supplies
electrons and protons. Electron reduces Eu
2 3 2
O /TiO /
2
2
O
3
2
2
O
3
2
oxidation catalysis. Any ESR signals were not observed on the
inactive catalyst. We tried to quench the active oxygen species
2
during the oxidation with O
not be detected ESR active species.
To obtain information for nature of active oxygen species of the
Pt/Eu /TiO /SiO catalyst, several oxidations of hydrocarbons
2
and H
2
using liquid N
2
but we could
2
2
2
2
+
2
O
3
2
2
2 3
O to Eu form; then,
2
+
3+
2+
were conducted, as summarized in Table 1. Reaction time was
reduced 10 min to obtain initial products selectivity.
Eu reduces TiO
2
to Ti form because of ESR studies. Eu and
3
+
Ti concertedly activate O and the activate oxygen species
2
A kinetic isotope effect (KIE) was 2.2 calculated from a ratio of
should form on the catalyst; however, formula of the active
oxygen species cannot be clarified. The active oxygen species
attach and cleave a C–H bond of alkane generating alkyl radical
intermediate; then oxygen species recombine with alkyl radical
intermediate forming oxygenated products because we observed
the KIE and radical trapping products. The rate-determining step
is cleavage of C–H bond. The active oxygen species has radical
character and configuration of C–H bonds does not retain during
the oxidation.
6 6
initial conversion rates of cy-C D12 and cy-C H12 during the
oxidation of the mixture (Entry 1). The KIE of 2.2 is larger than
ꢄ
values of 1.2–1.4 observed in HO . This fairly large KIE value
indicates the rate-determining step is a cleavage C–H bond of
cyclohexane.
4. Conclusions
We developed the Pt/Eu
2
O
3
/TiO
and H
and Pt on the SiO
affects the oxidation catalysis because electron transfer reac-
2
/SiO
in acetic acid at 313 K.
support strongly
2
solid catalyst for
oxidation of alkane with O
Loading order of Eu , TiO
2
2
2
O
3
2
2
3
+/2+
4+/3+
tions from H
2
through Pt, Eu
2
O
3
(Eu
), TiO
2
(Ti
2
) to O are
essential for the formation of active oxygen species. Active
oxygen species cannot be identified but has radical character for
2 3 2 2
Fig. 5. Reaction mechanism model of oxidation on Pt/Eu O /TiO /SiO catalyst.