Oxidation of alkanes with H2O on Ir(acac)3 supported on a carbon fiber-anode

Article abstract of DOI:10.1039/b006107g

Ir(acac)₃ supported on a carbon fiber-anode catalyzed the oxidations of alkanes (cyclohexane, n-hexane, adamantane) by generating strong electrophilic oxygen species from H₂O at 1.3 V (AglAgCl) with high current efficiency, for example, 45% for cyclohexanone formation.

Full text of DOI:10.1039/b006107g

Oxidation of alkanes with H O on Ir(acac) supported on a carbon fiber-anode
2
3
Ichiro Yamanaka,* Taishi Furukawa and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail: yamanaka@o.cc.titech.ac.jp
Received (in Cambridge, UK) 27th July 2000, Accepted 6th October 2000
First published as an Advance Article on the web 31st October 2000
Ir(acac)
oxidations of alkanes (cyclohexane, n-hexane, adamantane)
by generating strong electrophilic oxygen species from H
at 1.3 V (Ag|AgCl) with high current efficiency, for example,
5% for cyclohexanone formation.
3
supported on a carbon fiber-anode catalyzed the
2
O
4
Direct oxidation of alkanes is one of the key oxidation processes
expected in the 21st century. On the other hand, the electro-
organic synthesis of chemicals is also expected to be a key
technology in green chemistry because the electrolysis can be
operated under mild conditions. However, it is not so easy to
realize direct oxidation of alkanes by electrolysis because their
redox potentials [1.85 V (vs. NHE) for cyclohexane to 2.25 V
1
for methane in aqueous solutions at pH = 0 ] are higher than
those of solvents and electrolytes. Some work on the anodic
oxidation of alkanes in aqueous media > 358 K has been
2
2
2
2
Fig. 1 Electrocatalytic oxidation of cyclohexane with H O over various
anodes at 1.8 V (Ag|AgCl) or 1.5 V(*) at 298 K.
Therefore, attention was paid to the electrocatalytic function of
the most active Ir(acac) /CF anode.
3
the cathode, indirect electrochemical oxidation, though the
current efficiency of the produced oxygenates were low at
10O + 4 H + 4 e–
+
(1)
cy-C O ? cy-C
6
H
12 + H
2
6
H
<
10%.4
To increase electrocatalytic activity for the oxidation of
cyclohexane, (i) preparation conditions of the Ir-anode [types of
We have recently reported an indirect anodic epoxidation of
propene to propene oxide on PtO
benzene to benzoquinone on a carbon fiber-anode at 293 K.5
membrane electrolysis cell was assembled as [reactant|porous
anode|silica wool membrane impregnated with H PO aq. |por-
ous Pt-graphite-cathode|H O(g), He]. An advantage of this
2
/Pt-anode and oxidation of
carbon material, amounts of Ir(acac)
amounts of PTFE (1–50 mg) and impregnation methods] were
tested. The best components of the Ir-anode were [Ir(acac)
(0.025 mol%)/CF] (50 mg) and PTFE (5 mg). (ii) The study on
the effects of various electrolytes (H PO , HCl, HClO , H SO
HNO , Na SO , NaOH and KOH) in the membrane and their
concentrations on the oxidation of cyclohexane indicated that
3
loading (0–2 mol%),
,6
A
3
3
4
2
3
4
4
2
4
,
system compared with conventional ones is that the separation
procedure of the products from electrolyte is quite easy. We
proposed that a nascent oxygen species generated on the anode
3
2
4
2
1
2 4
the best electrolyte was H SO (aq) (1 mol l ).
from H
2
O would oxidize propene and benzene, i.e. indirect
Fig. 2(a) shows the formation rates of the products as a
function of the anode potential by using the best anode and
electrolyte described above. In the upper part (b), the current
density and the current efficiency (CE) were plotted. When the
anode potential was increased above 1.3 V (Ag|AgCl), the
5,6
electrochemical oxidation at the anode. However, the two
anodes mentioned above were not active at all for the oxidation
of alkanes. Therefore, the purpose of this work was to design a
new active anode for the oxidation of alkanes with H
2
O by using
the membrane electrolysis cell.
2
current density and the formation rates of CyO and CO rapidly
Fig. 1 shows the catalytic activities of various anodes for the
oxidation of cyclohexane during electrolysis of H O in H PO
aq) (0.7 mol l ) at 298 K. The anodes were prepared by the
increased. The potential of 1.3 V for the onset of oxidation is far
lower than the theoretical oxidation potential of 1.65 V (1.85 V
2
3
4
2
1
1
(
vs. NHE) at pH = 0. This result suggest that an indirect
hot-press method from various electrocatalyst powders [metal
blacks and metal compounds (0.5 mol%) supported on carbon
electrochemical oxidation of cyclohexane proceeds on the Ir-
anode. The maximum formation rate of CyO was obtained at ca.
1.5 V. Here, the turnover number of Ir for CyO formation
4
fibers (CF)] and poly(tetrafluoroethylene) (PTFE) powder with
2
a geometric surface of ca. 2 cm . The membrane electrolysis
exceeded 110 in 1 h. In contrast, the formation rates of CO
2
and
cell attached to various anodes was assembled and the anode
compartment was filled with pure cyclohexane (40 ml). The
electrocatalytic oxidation of cyclohexane with H O was carried
2
out by applying the anode potential; 1.5 V (Ag|AgCl) at Pd-
black, Pt-black, Ir-powder, Ir-black anodes and 1.8 V at
O continuously increased when increasing the anode potential
2
further. The decrease in the formation rate of CyO above 1.5 V
may be due to the desorption of active oxygen species on the Ir-
anode to the gas phase or to the consumption of the active
oxygen in the successive oxidation of CyO to adipic acid or
FeCl(TPP)/CF, Co
IrCl /CF anodes. Products were cyclohexanone (CyO), cyclo-
hexanol (CyOH), adipic acid (AA), CO and O . The results in
Fig. 1 indicate that Ir compounds, especially Ir(acac) supported
2
O
3
/CF, MnO
2
/CF, Ir/CF, IrO
2
, IrCl
3
/CF,
CO
8% upon increasing the anode potential.
Effects of solvents (CH Cl , CCl , Et
the oxidation were studied to optimize the oxidation. When a
mixture of CH Cl and cyclohexane (50+50 vol%) was used for
the oxidation at 1.5 V, the CE for CyO formation dramatically
2
. Thus, the CE for the CyO formation decreased from 20 to
4
2
2
2
2
4
2
O, DMF and THF) on
3
on carbon fiber (CF), are active for the CyO formation, though
current efficiencies (CE) estimated from eqn. (1) were not good.
2
2
DOI: 10.1039/b006107g
Chem. Commun., 2000, 2209–2210
This journal is © The Royal Society of Chemistry 2000
2209

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Fig. 2 Effects of anode potentials on (a) the formation rates of products and
b) current and current efficiency for the oxidation of cyclohexane on the
Ir(acac) /CF anode with H O at 289 K.
(
3
2
increased from 15 to 45% while retaining a high formation rate
of CyO. The increase in the CE was due to the decrease in both
Fig. 3 Cyclic voltammograms of Ir(acac)
(
3 2 4
/CF and CF electrodes in H SO
aq) (1 mol l21) at atmospheric pressure of Ar. Scan speed 0.5 V s
21
.
current and formation rates of CO
are formed through eqns. (2)–(4), their CE are estimated to be
2 2
and O . If these by-products
anode potential of 1.5 V (Ag|AgCl). CyOH was efficiently
oxidized to CyO with a high CE of 73% (assuming 2e
2
1
8, 0.1, 10 and 15% for adipic acid, CyOH, CO
2
and O
2
oxidation). On the other hand, CyO was oxidized to adipic acid
respectively, giving a sum of CE of 88% at 1.5 V.
2
with a low CE of 17% (6e oxidation). These results suggest
+
2
cy-C
cy-C
cy-C
6
H
12 + 4 H
12 + 12 H
12 + H O ? cy-C
2
O ? C
4
H
8
(CO
2
H)
2
+ 10 H + 10 e (2)
that active species generated on the Ir-anode oxidizes cyclohex-
ane to CyO through CyOH. Deep oxidations of CyO to adipic
+
2
6
H
2
O ? 6 CO
2
+ 36 H + 36 e
(3)
(4)
+
2
2
acid and CO were not fast. In oxidation of hexane, hexan-2-one
6
H
2
6
H
11OH + 2 H + 2 e
2
(
4
CE 14.0%, assuming 4e oxidation), hexan-3-one (13.9%,
e
On the other hand, the CE for the formation of H
2
at the cathode
2
2
oxidation) and hexanoic acid (0.4%, 6e oxidation) were
was almost 100% within experimental error of ±2%. These
results suggest the formation of undetectable products or
oxidation of the anode itself.
A conventional H-type cell with the same [Ir(acac)
mol%)/CF] anode was applied for the oxidation of cyclohexane
dissolved in MeCN or dispersed as micelles in aqueous H SO
However, the oxidation of cyclohexane did not proceed at all.
Our membrane electrolysis method is unique for the oxidation.
After the oxidation, no Ir compounds were present in the
produced. The relative reactivities of secondary and primary C–
H bonds (2°+1°) in hexane were estimated from the quantities of
the oxygenated products and found to 250+1 when normalized
per C–H bond. Thus the secondary C–H bond was selectively
2
(0.025
oxygenated on the Ir-anode. In oxidation of adamantane,
2
4
.
2
1
-adamantanol (CE 4.9%, assuming 2e oxidation) and 2-ada-
2
mantanone (0.7%, 4e oxidation) were produced. The low CE
observed here was due to the low solubility of adamantane in
2 2
CH Cl solvent. The relative reactivities of tertiary and
2 4
reaction mixture or H SO (aq) in the membrane according to
ICP analysis. This result strongly suggests that Ir species are
fixed on the CF.
secondary C–H bonds (3°+2°) was 42+1. These results suggest
that the active species has very strong electrophilicity for the
oxidation of alkanes. This strong electrophilicity must be
caused by the high oxidation state of Ir species suggested by CV
experiments (Fig. 3).
Fig. 3 shows cyclic voltammograms (CV) of (a) the [0.025
mol% Ir(acac)
aqueous H SO
2
/CF] electrode and (b) the CF electrode in
under atmospheric pressure of Ar (geometric
2
4
2
areas of both electrodes was 0.02 cm ). Several redox couples
were observed in the CV (a) of Ir(acac) /CF not observed in the
3
Notes and references
1 A. E. Shilov, Activation of Saturated Hydrocarbons by Transition Metal
Complexes, D. Reidel, Dordrecht, 1984.
CV (b) of CF, except for the redox couple due to quinone/
hydroquinone groups present on carbon surfaces. A redox
couple at 0.8–0.9 V (Ag|AgCl) (Ox-1/Red-1) can be assigned
2
J. O’M. Bockris, E. Gileadi and G. E. Stoner, J. Electrochem. Soc., 1969,
3, 427.
7
3+
0 7
to Ir /Ir . The ratio of peak areas of Ox-1, Red-1, Ox-2 and
Red-2 in Fig. 3(a) were roughly estimated to be 1+1+1+1. These
facts suggested that the couple at higher potential (Ox-2/Red-2)
can be ascribed to the transfer of three electrons, probably the
3
G. J. Edwards, S. R. Jones and J. M. Mellor, J. Chem. Soc., Chem.
Commun., 1975, 816; G. J. Edwards, S. R. Jones and J. M. Mellor,
Tetrahedron Lett., 1976, 8, 631.
4
K. Otsuka and I. Yamanaka, Catal. Today, 2000, 57, 71; E. R. Savinova,
A. O. Kuzmin, F. Frusteri, A. Parmaliana and V. N. Parmon, Stud. Surf.
Sci. Catal., 1998, 119, 429; R. L. Cook and A. F. Sammels,
J. Electrochem. Soc., 1990, 137, 2007.
6+
3+
redox couple Ir /Ir according to eqn. (5) or (6).
Ir3+ ? Ir6+ + 3 e2
(5)
(6)
Ir3+ + H O ? [IrO]4+ + 2 H + 3 e
+
2
2
5 I. Yamanaka, K. Satio and K. Otsuka, Electrochem. Solid State Lett.,
999, 2, 131; K. Otsuka, T. Ushiyama, I. Yamanaka and K. Ebitani,
1
We believe that the formation of such high oxidation state Ir
J. Catal., 1995, 157, 450.
8
species on CF would catalyse the oxidation of cyclohexane at
6
7
I. Yamanaka, T. Furukawa and K. Otsuka, Chem. Lett., 1998, 1059.
Ir /Ir = 1.15 V (NHE), Lange’s Handbook of Chemistry, ed. J. A.
>
1.3 V(Ag|AgCl).
To gain information about the reactivity of active species,
3+
0
Dean, McGraw-Hill, New York, 1985.
8 L. A. Arasmaskova, A. V. Romanemko and Y. I. Yemakov, React. Kinet.
Catal. Lett., 1980, 13, 391.
2
1
oxidations of CyOH (4.8 mol l ), CyO (4.8), hexane (3.8) and
adamantane (0.25) dissolved in CH Cl were studied at an
2
2
2210
Chem. Commun., 2000, 2209–2210