Oxidation of methane and ethylene in water at ambient conditions

Article abstract of DOI:10.1016/j.cattod.2010.02.007

Available spectroscopic, labelling and reactivity data show that stable μ-nitrido diiron phthalocyanine activates H₂O₂ to form a high-valent diiron oxo species. This species is a very powerful oxidant which oxidizes methane in pure water at 25-60 °C to methanol, formaldehyde and formic acid. The catalytic activity can significantly be increased in the presence of a diluted acid solution. Thus, a high turnover number of 209 was attained in 0.075 M H₂SO₄. Oxidation of ethylene resulted in the formation of formic acid as a major product and formaldehyde with high turnover numbers. Under optimal conditions, 426 mol HCOOH and 37 mol CH ₂O per mole of catalyst were obtained in pure water. The practical and green features of this novel approach (H₂O₂ as the clean oxidant, water as the clean reaction medium, easily accessible solid catalyst) as well as the relevance to biological oxidation (binuclear structure of bio-inspired complex) are of great importance both from practical and fundamental points of view.

Full text of DOI:10.1016/j.cattod.2010.02.007

Catalysis Today 157 (2010) 149–154
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Catalysis Today
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Oxidation of methane and ethylene in water at ambient conditions
A.B. Sorokina,∗, E.V. Kudrika, L.X. Alvareza, P. Afanasieva, J.M.M. Milleta, D. Bouchub
a Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS-Université Lyon 1, 2, av. A. Einstein, 69626 Villeurbanne, France
b Université Lyon 1, UMR 5246, Centre Commun de Spectrométrie de Masse, 43 bd du 11 novembre 1918, 69622 Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 March 2010
Available spectroscopic, labelling and reactivity data show that stable ␮-nitrido diiron phthalocyanine
activates H2O2 to form a high-valent diiron oxo species. This species is a very powerful oxidant which
oxidizes methane in pure water at 25–60 ◦C to methanol, formaldehyde and formic acid. The catalytic
activity can significantly be increased in the presence of a diluted acid solution. Thus, a high turnover
number of 209 was attained in 0.075 M H2SO4. Oxidation of ethylene resulted in the formation of formic
acid as a major product and formaldehyde with high turnover numbers. Under optimal conditions,
426 mol HCOOH and 37 mol CH2O per mole of catalyst were obtained in pure water. The practical and
green features of this novel approach (H2O2 as the clean oxidant, water as the clean reaction medium,
easily accessible solid catalyst) as well as the relevance to biological oxidation (binuclear structure of
bio-inspired complex) are of great importance both from practical and fundamental points of view.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Methane
Ethylene
Oxidation
Phthalocyanine
Iron
Catalysis
Hydrogen peroxide
1. Introduction
within a triazine-based framework containing bipyridyl fragments.
Heterogeneous catalysts showed catalytic activity comparable to
Transformation of methane, the most abundant and the least
reactive component of natural gas is one of the most difficult
chemical challenges of a great practical importance and of contin-
uing interest [1–4]. Conversion of methane is particularly difficult
compared to other alkanes because of very high dissociation
energy of methane C–H bond (435 kJ mol−1). Not surprisingly,
current approaches to utilization of methane involve mainly high-
temperature processes. For example, the use of CH4 in the industry
is based on the initial high-temperature conversion of methane to
syngas (H2 + CO) which further can be transformed into methanol
or fuels [5–7]. During several decades a direct low temperature
conversion of CH4 to valuable products continues to be a major
challenge in catalysis. This field has largely been inspired by the
work by Shilov and co-workers [3,4]. They discovered CH4 acti-
vation on platinum complexes via Pt–CH3 intermediate followed
by its oxidation by Pt(IV) complex. However, inactivation of the
system occurred owing to irreversible formation of metal. This non-
catalytic chemistry based on Pt compounds was transformed into
a catalytic process by stabilization of species using appropriate lig-
ands and strong acids [8–14]. The most efficient oxidation of CH4 in
oleum (terminal oxidant) containing Pt(II) bipyrimidine complex at
220 ◦C provided ∼90% methane conversion and selective formation
of methyl bisulfate [8]. Another interesting approach was proposed
by Schüth and co-workers [14]. They covalently immobilized Pt(II)
the Periana’s system at 215 ◦C in 30% oleum and were stable over
at least five recycling steps. Activation of CH4 at other transition
metal centres (Pt, Pd, Hg, Au, Tl, and Ir) has also been published
[15–20]. As can be seen from aforementioned examples, harsh
conditions (strong concentrated acids and high temperatures) are
usually applied in these processes. Perspectives and challenges in
C–H bond activation on transition metal sites have been reviewed
[21]. Several other systems involving V, Fe, Os and Rh compounds
have also been reported [22–26]. An interesting approach of acti-
vation of methane in the gas phase using metal-free radical-cation
oxides was published by groups of Schwartz [27] and de Petris et al.
[28]. The most representative catalytic systems for transformation
of methane are listed in Table 1.
Two catalytic systems are distinguished by their high catalytic
activity. Homogeneous [8] and heterogeneous [14] (bipym)PtIICl2
complex in 9–30% oleum at 215–220 ◦C transforms CH4 to
CH3OSO3H with TON ∼ 200–500. Vanadium complexes with N,O-
or O,O-ligands in trifluoroacetic acid in the presence of K2S2O8 as
oxidant initiate a radical oxidation of CH4 into CH3COOH with very
high TONs up to 5600 [22]. The catalytic activity of other systems
is significantly lower. Inspection of Table 1 shows that harsh reac-
tion conditions (relatively high temperatures, strong concentrated
acids) are generally needed to perform the oxidation of methane.
Only few systems were reported on oxidation of methane in aque-
ous medium, but their efficiency was not very high.
In nature, methane monooxygenase (MMO) enzymes transform
CH4 to CH3OH in water under mild physiological conditions [29].
A chemical system, capable of oxidizing CH4 at ambient condi-
∗
Corresponding author. Tel.: +33 472 44 53 37; fax: +33 472 44 53 99.
E-mail address: alexander.sorokin@ircelyon.univ-lyon1.fr (A.B. Sorokin).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.007

150
A.B. Sorokin et al. / Catalysis Today 157 (2010) 149–154
Table 1
Catalytic systems for low temperature transformation of methane.
Catalyst
Conditions
Products (yield, %)
TON
Ref.
(Bipym)PtIICl2
oleum, 220 ◦
C
CH3OSO3H (∼90)a
∼500
33
0.5
[8]
[9]
[10]
[(Mebipym)PtIICl2]+ [H4PV2Mo10O40] /SiO2
water, acid, O2, 50 ◦
C
CH3OH, HCHO, CH3CHO (0.51)b
CH3OH (0.02–0.17)b
-
(Bipym)PtIICl2
PtCl2
H2SO4 (96%), ionic liquid, 220 ◦
30% oleum, 215 ◦
CF3COOH, (CF3CO)2O, 80–100 ◦
C
C
3.5
(Bipym)PtIICl2
K2[PtCl4]-CTF
N-heterocyclic carbene
Pd complex, K2S2O8
PdSO4
C
CH3OH (35–39)b
CH3OH (72)b
158–355
201–246
5–30
[14]
[16]
[17]
CF3COOCH3 (0.93–3.67)b
96% H2SO4, 180 ◦
C
C
CH3OH, CH3COOH
5–18
CO2 (1–12)a
Pd(OAc)2, NaNO2, O2, benzoquinone
PdSO4
CF3COOH, 80 ◦
96% H2SO4, 180 ◦
CF3COOD, 180 ◦
CF3COOH, 80 ◦
C
CF3COOCH3 (0.04–0.09)b
CH3COOH, COx CH3OSO3H (1.4)b
CF3COOCH3 (<5)a
3–7
14.2
6.3
19.7–5600
19–24
12
[18]
[19]
[20]
[22]
[23]
[24]
[25]
Ir(6-phenyl-2,2ꢀ-bipyridine), NaIO4
V complexes with N,O or O,O-ligands, K2S2O8
[Fe4(N3O2-L)4(␮-O)2]4+, H2O2
OsCl3, H2O2
C
C
CH3COOH (43–8.8)a
H2O, 50–80 ◦
C
CH3OH, CH3OOH, HCOOH (no data)
CH3OH, CH3OOH, HCHO, CO2 (0.14)b
CH3OH, HCHO, CO2 (0.07)b
H2O, 90 ◦
D2O, 50 ◦
C
C
OsO4, NaIO4
0.006
a
Yield on the initial amount of methane indicated in paper.
Yield on the initial amount of methane calculated from experimental data available in paper.
b
in 50 mL of CH2Cl2 and 1 g of silica (Degussa aerogel, 200 m2/g)
was added to the solution. The mixture was stirred at 25 ◦C for 2 h,
then the solvent was removed under reduced pressure. The solid
materials were dried in vacuum at room temperature for 1 h, then
at 50 ◦C for 6 h. The complex loadings of solids were 17.5–20, 30
and 40 ␮mol/g.
tions like MMO does, would be a significant achievement. However,
chemical analogues for such a process are unknown despite con-
siderable efforts [30]. An essential feature of MMO is an active site
containing two iron centers in a non-heme environment. On the
other hand, cytochrome P-450 enzymes, which efficiently catalyze
a variety of reactions including oxidation of C–H bonds of alka-
nes, contain a mononuclear iron porphyrin active site [31]. We
have proposed to use binuclear iron complexes with macrocyclic
porphyrin-like ligands as catalysts for oxidation [32]. Although
these and similar complexes have often been considered as inert
in catalysis, we have shown that ␮-nitrido diiron phthalocyanine
complexes (Fig. 1) possess remarkable catalytic properties [32–36].
More data on preparation and characterization of N-bridged
binuclear complexes can be found in selected original papers
[37–40] and review [41]. Importantly, our available data indicate
that ␮-nitrido diiron tetra-tert-butylphthalocyanine (FePctBu4)2N
operates via oxo-transfer chemistry involving high-valent diiron
oxo species. In this paper we resume the spectroscopic, labelling
and reactivity data previously obtained under homogeneous con-
ditions. We report on the preparation, characterization and use
of heterogeneous (FePctBu4)2N–SiO2 catalysts for the oxidation of
methane and ethylene by H2O2 in aqueous media.
Oxidation of methane and ethylene was performed in
a
37 mL stainless steel autoclave with Teflon liner inside. The reac-
tor was charged with 50 mg of (FePctBu4)2N-SiO2 containing
0.875–0.925 ␮mol of complex, 2 mL of water, 60 ␮L of 35% hydro-
gen peroxide solution in water (678 ␮mol of oxidant) and 32 bars
of CH4 or C2H4. The reactor was heated at desired temperature
(25–60 ◦C) with magnetic stirring for 20 h. Hydrogen peroxide was
consumed after reaction. The yield of formic acid was determined
by GC–MS using isobutyric acid as an external standard and by
HPLC. The HPLC determination of formic acid was made using
an Agilent 1100 Liquid Chromatograph equipped with a 20 ␮L
injection loop and multiple wavelength detector at 210 nm. Chro-
matographic analyses were carried out at room temperature using
an ion exclusion Coregel-87H3 column with 0.004 M H2SO4 as
eluent (flow rate of 0.8 mL/min). The yield of formaldehyde was
determined by Nash method [42]. The labelling experiments were
performed using approximately 1:1 mixture of 13CH4 (99 atom%
13C, Sigma–Aldrich) and CH4. Isotopic composition of products was
determined by GC–MS.
2. Experimental
Nitrido-bis[tetra(tert-butyl)phthalocyaninatoiron] was pre-
pared as previously described [33]. Supported catalysts were
prepared as follows. (FePctBu4)2N (28, 42 or 56 mg) was dissolved
3. Results and discussion
3.1. Spectroscopic characterization of intermediates generated
from (FePctBu4)2N and H2O2
The distinctive feature of (FePctBu4)2N is its high stability which
is an important requirement for the development of an oxidation
catalyst. Phthalocyanine core is thermally and chemically stable.
The stability of the binuclear structure is provided by the bridging
nitrogen atom. The 1s N XPS spectrum showed a high-energy signal
of bridging nitrogen of a low intensity at 402.4 eV compared with
the signal at 398.7 eV due to 16 nitrogen atoms of two phthalo-
cyanine ligands [32]. A strong molecular peak at m/z = 1599.8 was
observed in the ESI-MS spectrum with no signals of monomer units
also indicating a high stability of (FePctBu4)2N [32]. UV–vis and
EPR data indicated a slow reaction between (FePctBu4)2N and H2O2
to form hydroperoxo (PctBu4)FeIV N–FeIII(PctBu4)–OOH complex
[32,33]. It should be noted that the interaction of peroxides with
iron complexes is an important subject of numerous studies and
Fig. 1. Structure of ␮-nitrido bridged diiron tetra-tert-butylphthalocyanine.

A.B. Sorokin et al. / Catalysis Today 157 (2010) 149–154
151
Fig. 2. Proposed mechanism for activation of H2O2 by binuclear (FePctBu4)2N complex.
debates in literature. Iron mononuclear complexes can initiate the
formation of free radicals via homolytic cleavage of O–O perox-
ide bond resulting in free-radical oxidation. Otherwise, high-valent
iron oxo species, e.g. LFeIV O and L•+FeIV O (L – ligand) are formed
via heterolytic cleavage of O–O bond. In the latter case, these inter-
mediates are oxidizing species. Activation of peroxides by binuclear
porphyrin-like complexes is not yet well studied. Possible reac-
tion pathways in the (FePctBu4)2N–H2O2 system are summarized
in Fig. 2.
Fig. 3. Oxidation of methane in CD3CN as a test on the origin of oxidation products.
GC-MS analysis of the isotopic composition of formic acid using
intensities of m/z = 29 (HCO+) and m/z = 30 (DCO+) signals showed
that formic acid obtained in oxidation of CH4 in CD3CN contained
68% of HCOOH and 32% of DCOOH. Thus, one can conclude that
oxidation of CH4 does occur, but the reaction is masked by the oxi-
dation of the MeCN solvent. In order to circumvent the oxidation of
organic solvent the best option would be to perform the oxidation of
CH4 in water which is a clean and inert solvent. Since (FePctBu4)2N
is not soluble in water we supported this complex onto silica in
order to carry out heterogeneous oxidation.
Hydroperoxo complex 1 formed from (FePctBu4)2N and H2O2
can undergo homolytic or heterolytic O–O cleavage to generate
complex 2 and hydroxyl radical or complex 3, respectively. The
complex 1 is negatively charged that should favour a formation
of 3 with release of OH−. Indeed, a high stability of phthalo-
cyanine moiety, 18O labelling data and analysis of kinetics were
not consistent with participation of free radicals [33]. Under the
electrospray ionization mass-spectrometry conditions complex 1
formed a high-valent diiron oxo complex 3 as evidenced by appear-
ance of m/z = 1615.9 signal in ESI-MS spectrum. When H218O2
was used instead of H216O2 the spectrum exhibited a signal at
m/z = 1617.9 corresponding to 18O-containing complex 3 [33]. Iso-
topic distribution patterns of the molecular peak in both cases,
with H216O2 and H218O2, were identical to the theoretical dis-
tribution patterns. Importantly, 3 retained the diiron structure.
The complex 3 is formally Fe(IV)Fe(V) oxo species which can also
be described as Fe(IV)Fe(IV) phthalocyanine cation-radical oxo
form (Fig. 2). Such electrophilic species coordinated with electron-
deficient phthalocyanine ligands should be a powerful oxidant.
To prove this hypothesis we have investigated the oxidation of
methane and ethylene using (FePctBu4)2N–H2O2 system.
3.3. Preparation and characterization of (FePctBu4)2N–SiO2
supported catalysts
(FePctBu4)2N was supported onto silica (Degussa aerogel, spe-
cific surface – 200 m2/g) by physical absorption from the solution in
CH2Cl2. Three supported catalysts were prepared with the complex
contents of 20, 30 and 40 ␮mol/g. Specific surfaces of supported
catalysts were 180–185 m2/g. The diffuse reflectance UV–vis spec-
trum of (FePctBu4)2N–SiO2 in comparison with UV–vis spectrum
of (FePctBu4)2N in CH2Cl2 solution is shown in Fig. 4.
3.2. Homogeneous oxidation of methane in organic solvents
Initial experiments of CH4 oxidation were performed in CH3CN
containing 0.1 mM (FePctBu4)2N and 67 mM H2O2. GC–MS analysis
showed presence of formic acid and traces of methanol. When per-
forming oxidation of CH4 in organic solvents, a particular attention
should be paid concerning the origin of products. Indeed, if the cat-
alytic system is strong enough to oxidize CH4, it could also be able
to oxidize the organic solvent which concentration is much higher
than that of methane. Furthermore, the products of oxidation of
organic solvents (such as MeCN, acetone, etc.) might be the same
as products of oxidation of methane. This problem has not usually
been discussed in literature. Consequently, a closed control of the
experiments should be performed in order to determine the origin
of oxidation products and to confirm the occurrence of methane
oxidation. In fact, such a control experiment is easy to perform by
using of a deuterated solvent. If the oxidation of CH4 is performed
in CD3CN, the products issued from CH4 will contain H atoms and
the products of solvent oxidation will be deuterated (Fig. 3).
Fig. 4. The diffuse reflectance UV–vis spectrum of solid (FePctBu4)2N–SiO2 (thin
line) and UV–vis spectrum of (FePctBu4)2N in CH2Cl2 solution (bold line).

152
A.B. Sorokin et al. / Catalysis Today 157 (2010) 149–154
Table 2
Oxidation of CH4 by H2O2 catalyzed by supported (FePctBu4)2N.a.
Run
T (◦C)
[CH2O] (mM)
[HCOOH] (mM)
CH2O (mol/catalyst,mol)
HCOOH (mol/catalyst,mol)
TONb
Ref.
1
2
25
40
60
60
60
0
6.0
8.6
10.5
15.0
22.0
0
13.0
18.6
22.8
19.6
21.7
13
29
26
20
22
[33]
[33]
[33]
This work
This work
4.8
1.5
n.d.
n.d.
10.4
3.2
n.d.
n.d.
3
4c
5d
a
Conditions: 2 mL of water, methane, 32 bars, 0.925 ␮mol of supported catalyst (18.5 ␮mol/g), 678 ␮mol of H2O2, reaction time – 20 h.
TON – turnover number, number of CH4 molecules transformed into the products by one molecule of catalyst.
With (FePctBu4)2N–SiO2 (30 ␮mol/g).
b
c
d
With (FePctBu4)2N–SiO2 (40 ␮mol/g).
UV–vis spectrum of (FePctBu4)2N in CH2Cl2 solution exhibits
Soret and Q bands at 341 and 642 nm, respectively. Upon absorption
of (FePctBu4)2N onto silica the broader bands were observed at 348
and 639 nm. A novel absorption band of low intensity was detected
at 453 nm. The origin of this band is not yet understood. When
(FePctBu4)2N was removed from the supported (FePctBu4)2N–SiO2
catalyst by the treatment with CH2Cl2, the solution showed the
same UV–vis spectrum as was observed before absorption, indi-
cating that (FePctBu4)2N retained its binuclear structure during
supporting onto silica.
3.4. Heterogeneous oxidation of methane in water
The initial experiments on heterogeneous oxidation of CH4 were
performed in pure water. The degradation of organic ligand of
the catalyst under oxidation conditions is potentially a source of
methanol, formaldehyde or formic acid that should be checked
using labelled 13CH4. The oxidation of CH4 was unambiguously evi-
Fig. 6. Dependence of the efficiency of oxidation of methane on the H2SO4 concen-
tration, other experimental conditions being the same as in Table 2, run 3.
denced using labeled 13CH4 in about 1:1 mixture with CH4 [33]. 13
C
higher temperatures. However, the catalytic activity was kept
almost the same in the range 40–80 ◦C (TON ∼ 26–29). At 40 ◦C the
HCOOH/CH2O ratio was about 2:1 while at higher temperatures
the amount of CH2O decreased [33]. When the supported catalysts
more charged in (FePctBu4)2N (30 and 40 ␮mol/g) were used, the
turnover numbers were practically the same as those for supported
catalyst with 18.5 ␮mol/g charge. By using more charged catalysts
more concentrated solutions of formic acid can be obtained.
NMR analysis of the reaction mixture showed the signals at 81.6 and
165.5 ppm attributed to hydrated formaldehyde 13CH2(OH)2 and
formic acid H13COOH, respectively. Although no signal at 49 ppm
due to 13CH3OH was seen in 13C NMR spectrum, a small amount
of methanol-13C was detected by GC–MS. Noteworthy, the iso-
topic composition of methanol and formic acid corresponded to the
isotopic composition of methane [32,33]. This experiment clearly
demonstrates that these products originate from the oxidation of
CH4 but not from degradation of the catalyst. 13C NMR analy-
sis of the reaction mixture did not show the signal at 125 ppm
due to possible 13CO2 formation during oxidation of 13CH4. The
stability of HCOOH (major final product) was studied in the sepa-
rate experiments. HCOOH was completely stable under reaction
conditions (60 ◦C, water or 0.2 M H2SO4) during standard reac-
tion time. These findings strongly suggest that CO2 should not
be formed in detectable amounts during oxidation of methane by
(FePctBu4)2N–H2O2 system.
The heterogeneous oxidation of CH4 in water was stud-
ied at different temperatures (Table 2). Significantly, the
(FePctBu4)2N–SiO2–H2O2 system was capable of oxidizing
methane even at 25 ◦C in pure water, although with a moderate
turnover number of 13. The catalytic system was more active at
Fig. 7. Time course for oxidation of methane by (FePctBu4)2N–H2O2 system in
0.075 M H2SO4 at 60 ◦C.
Fig. 5. Proposed mechanism of formation of complex 3 in the presence of acid.

A.B. Sorokin et al. / Catalysis Today 157 (2010) 149–154
153
Table 3
Oxidation of ethylene by H2O2 catalyzed by supported (FePctBu4)2N complex.a.
Run
T (◦C)
[HCOOH] (mM)
[CH2O] (mM)
HCOOH (mol/catalyst, mol)
CH2O (mol/catalyst,mol)
TONb
Yieldc on H2O2 (%)
Oxidation in water
1d
25
40
60
60
70
Traces
50.3
86.3
43.5
93.3
n.d.
23.1
14.3
7.9
n.d.
n.d.
n.d
83.9
115.0
117.4
231.8
n.d.
36
51
28
57
2
3
115.0
197.3
198.7
426.5
52.8
32.6
36.1
37.0
4e
5e
8.1
Oxidation in 0.1 M H2SO4
6f
25
60
60
40.7
107.6
66.0
8.3
17.9
12.6
92.9
246.1
301.7
19.0
41.0
28.7
56.0
143.5
165.3
26
69
60
7
8e
a
Conditions: 2 mL of water or 0.1 M H2SO4, ethylene – 32 bars, 0.875 ␮mol of supported catalyst, 678 ␮mol of H2O2, reaction time – 20 h.
TON – turnover number, number of C2H4 molecules transformed into the products by one molecule of catalyst; calculated as [(mol of HCOOH)/2 + (mol of CH2O)/2]/mol
b
of catalyst.
c
Calculated supposing that four or two molecules of H2O2 are necessary to convert one C2H4 molecule to two HCOOH or CH2O molecules.
Reaction time: 48 h.
Two times less of catalyst.
Reaction time: 60 h.
d
e
f
We have proposed that the catalytic activity of
231.8 at 70 ◦C (Table 3, run 5). The HCOOH/CH2O ratio increases
from 2.2 at 40 ◦C (Table 3, run 2) to 11.5 at 70 ◦C (Table 3, run 5).
Using a half amount of the catalyst ∼0.1 M HCOOH aqueous solution
was obtained in the latter case. Interestingly, there was no such a
strong influence of acid as was observed for oxidation of methane.
In the presence of 0.1 M H2SO4 turnover numbers in HCOOH and
CH2O were 246.1 and 41.0, respectively (Table 3, run 7) as compared
to 197.3 and 32.6 obtained in water (Table 3, run 3). It should be
noted that the yields of products on H2O2 oxidant were quite high
reaching 69%.
(FePctBu4)2N–H2O2 system is associated with the formation
of powerful oxidizing species 3 via heterolytic cleavage of O–O
bond in hydroperoxo complex 1 and release of OH− (Fig. 2). Since
H2O is a better leaving group than OH−, it is reasonable to propose
that the formation of 3 should be favoured by protonation of
peroxide oxygen (Fig. 5).
In order to check this hypothesis we have studied the influence
of concentration of acid on the catalytic activity of oxidation of
methane to formic acid at 60 ◦C. Indeed, a significant improvement
of catalytic activity was observed in the presence of 0.05–0.1 M
H2SO4 (Fig. 6).
4. Conclusions
Even at low acid concentration of 0.05 M the TONHCOOH was
increased by factor 3 to attain 64 cycles. The maximal catalytic
activity was observed in 0.075 M H2SO4. Under these conditions
the catalyst showed a very high performance: 209 molecules CH4
per catalyst molecule were oxidized to formic acid. Using 50 mg
of the supported catalyst containing 1.6 mg of (FePctBu4)2N com-
plex, 2 mL of 0.11 M HCOOH solution was obtained after 20 h. This
activity is far higher than that of the most published systems oper-
ating via CH4 activation (Table 1) approaching to that of Periana’s
system based on Pt(II) bipyrimidine complex in oleum at 220 ◦C.
Further increase of the H2SO4 concentration resulted in a progres-
sive decrease of catalytic activity. In the 0.2 M H2SO4 solution the
catalytic activity was almost the same as that in pure water. It
should be noted that when the oxidation was performed in the
presence of small amount of H2SO4, the catalyst was surprisingly
stable. No appreciate decrease in blue colour of the catalyst was
detected indicating the stability of phthalocyanine chromophore
under reaction conditions.
We have discovered a first bio-inspired catalytic system for the
mild oxidation of CH4 in water. The practical and green features
of this novel approach (H2O2 as the clean oxidant, water as the
clean reaction medium, easily accessible solid catalyst) as well as
the relevance to biological oxidation (binuclear structure of bio-
inspired complex) are of great importance both from the practical
and fundamental points of view. N-Bridged diiron phthalocyanine
complexes show a new unexpected reactivity and provide a novel
promising approach in the field of oxidation. Importantly, avail-
able spectroscopic, labelling and reactivity data are consistent with
involvement of a high-valent diron oxo species rather than with
free-radical chemistry [32–36].
From the practical point of view, formic acid was proposed to
be used as a raw material for hydrogen production [43]. Moreover,
recent research has shown that formic acid has the potential to
power fuel cells for electricity generation and automobiles [44].
The clean heterogeneous oxidation of methane and ethylene under
mild conditions described in this paper presents a basis for a novel
approach for the transformation of difficult-to-oxidize hydrocar-
bons to useful product.
The kinetics of oxidation methane was studied under the opti-
mal conditions and presented in Fig. 7. The kinetic curve shows an
induction period. This induction period can be explained by the ini-
tial formation of formaldehyde followed by its oxidation to formic
acid.
Acknowledgment
3.5. Heterogeneous oxidation of ethylene
We are grateful to Agence National de Recherche (ANR,
France, grant ANR-08-BLANC-0183-01) for financial support of this
research.
The scope of (FePctBu4)2N–H2O2 system was further checked in
oxidation of ethylene in pure water and in 0.1 M H2SO4. The results
are summarized in Table 3.
The main products of ethylene oxidation were formaldehyde
and formic acid indicating the cleavage of C–C bond. Ethylene oxide
or ethylene glycol were not detected in the course of the oxidation
of ethylene. In contrast to oxidation of methane, the system in pure
water was practically inactive at 25 ◦C. However, the oxidation in
water was quite efficient at higher temperatures achieving TON of
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