Methane oxidation using silica-supported N-bridged di-iron phthalocyanine catalyst

Article abstract of DOI:10.1016/j.jcat.2012.03.013

Methane oxidation in water using hydrogen peroxide with a μ-nitrido iron phthalocyanine complex grafted on silica has been investigated in detail. Methyl hydroperoxide is identified as the main primary reaction product from methane oxidation. The catalyst is unstable under reaction conditions and this is discussed. However, the unmodified silica support is also found to be active for this reaction, and in particular, a Fe/SiO₂ catalyst prepared by wet impregnation is also found to be as effective as the μ-nitrido iron phthalocyanine complex grafted on silica, with higher selectivity to useful oxygenates (>80%) and displays minimal leaching or instability. A method for the reliable quantification of CO₂ in both the gas and aqueous phase is reported as this presents a key experimental difficulty in the oxidation of methane in aqueous media.

Full text of DOI:10.1016/j.jcat.2012.03.013

Journal of Catalysis 290 (2012) 177–185
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Methane oxidation using silica-supported N-bridged di-iron phthalocyanine catalyst
Michael M. Forde, Bezzu C. Grazia, Robert Armstrong, Robert L. Jenkins, Mohammed Hasbi Ab Rahim,
Albert F. Carley, Nikolaos Dimitratos, Jose Antonio Lopez-Sanchez, Stuart H. Taylor, Neil B. McKeown,
⇑
Graham John Hutchings
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building Park Place, Cardiff CF103AT, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Methane oxidation in water using hydrogen peroxide with a
l-nitrido iron phthalocyanine complex
Received 12 May 2011
Revised 16 February 2012
Accepted 17 March 2012
Available online 20 April 2012
grafted on silica has been investigated in detail. Methyl hydroperoxide is identified as the main primary
reaction product from methane oxidation. The catalyst is unstable under reaction conditions and this is
discussed. However, the unmodified silica support is also found to be active for this reaction, and in par-
ticular, a Fe/SiO2 catalyst prepared by wet impregnation is also found to be as effective as the l-nitrido
iron phthalocyanine complex grafted on silica, with higher selectivity to useful oxygenates (>80%) and
displays minimal leaching or instability. A method for the reliable quantification of CO2 in both the gas
and aqueous phase is reported as this presents a key experimental difficulty in the oxidation of methane
in aqueous media.
Keywords:
Methane oxidation
di-iron phthalocyanine catalyst
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
advantages if high selectivity can be achieved at a sufficient level
of conversion with the appropriate terminal oxidant, which for
Recent work by Sorokin and co-workers has raised the interest-
ing possibility that a -nitrido iron phthalocyanine complex grafted
commercial purposes has to be molecular oxygen.
l
A number of materials have been proposed for the low-
temperature activation of methane [3–8]. Most of these are
homogenous catalysts and use strong acidic conditions for the
transformation. For example, Periana and co-workers [8] have
utilised a bipyrimidyl Pt complex in oleum, which shows both high
selectivity and methane conversion (ꢀ90%). From an industrial
standpoint, the associated issues of waste disposal, difficulties with
the recyclability of the acidic media, capital outlay to build
large scale plants with acid stable materials and the price volatility
of energy resources means this sort of chemistry cannot be
implemented.
Many studies have sought to emulate natural enzymes in the
design approach for catalysts capable of methane oxidation using
bio-inspired or bio-mimetic approaches. However, to be classified
as effective, these bio-inspired catalysts have to show that they
can compete with the real bench mark which is that displayed
by methane monooxygenase (MMO) [9,10], that is, the catalysts
should be stable under mild reaction conditions and display high
selectivity to methanol, although it is recognised that the catalysts
will necessarily operate by a different mechanism. It is known that
the soluble form of MMO (sMMO) could oxidise C1–C8 hydrocar-
bons efficiently to the corresponding alcohol [11]. It is important
to note that in vivo this enzyme does not produce carbon dioxide
or other over-oxidation products and is therefore highly specific.
Detailed in vitro studies by Dalton and co-workers [11] have shown
that the activity for methanol synthesis is 5 mol (CH3OH) kg
on silica can be an effective bio-inspired catalyst for the selective
oxidation of methane using hydrogen peroxide as the terminal oxi-
dant under mild reaction [1,2]. The demonstration of a truly heter-
ogeneous catalyst for the selective low-temperature oxidation of
methane to methanol can still be considered one of the remaining
grand challenges of catalysis, and consequently, the work presented
by Sorokin and co-workers represents a potentially interesting new
development. To date the activation of methane has been a very ac-
tive field, but over a century of research has not yet led to a process
with commercial potential which can be classified as ‘green cataly-
sis’ for the direct conversion of methane in the liquid phase with a
heterogeneous catalyst. The relative strength of the C–H bond in
methane (435 kJ molÀ1) as compared to bonds in its oxygenated
products is one of the reasons often considered as responsible for
the low selectivity achieved in this oxidation reaction. Whilst the
direct conversion of methane to methanol has not yet been
achieved in a viable manner, the current indirect method involving
the production of syngas and its conversion to methanol has been
fine tuned over many decades of operation and gives a high selec-
tivity to methanol. The indirect process, therefore, represents a dif-
ficult process to improve upon. A process based on the direct
conversion of methane to methanol in a single step might provide
⇑
Corresponding author. Fax: +44 2090874030.
E-mail address: hutch@Cardiff.ac.uk (G.J. Hutchings).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.03.013

178
M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
(catalyst)À1 hÀ1 for sMMO (C.bath) as a complete enzyme with
NADH present (5 mol NADH, 45 °C, 12 min, 2 mg protein, pH 7,
supported Fe catalyst prepared by impregnation of ferric nitrate or
iron(III) acetylacetonate followed by calcination.
l
CH4 6 ml at atmospheric pressure) and this represents the bench-
mark by which bio-inspired or bio-mimetic catalysts should be
judged. However, no data on CO2 production in vitro have been
given. sMMO cannot use hydrogen peroxide in its native form
and molecular oxygen is activated to the reactive peroxy species
by NADH. However, when the NADH cofactor is removed, H2O2
can be used as the terminal oxidant with the enzyme but the cat-
alytic activity decreases to 0.076 mol (CH3OH) hÀ1 kg(MMOH)À1
2. Experimental
2.1. Synthesis and characterisation of
phthalocyaninatoiron on silica
l-nitrido-bis[tetra(tert-butyl)
The
l-nitrido-bis[tetra(tert-butyl)phthalocyaninatoiron] was
synthesised in two steps according to the literature [1,34].
(120 lmol sMMO hydroxylase, 100 mmol H2O2, 15 min, 45 °C)
[12]. It is this observation that often leads researchers to use
H2O2 as an oxidant for methane oxidation. However, it should be
noted that the cost of H2O2 precludes is use in any commercial di-
rect methane oxidation process. Indeed, H2 is the expensive re-
agent in the production of H2O2 and this is derived from
methane commercially.
2.1.1. Synthesis of tetra(tert-butyl)phthalocyaninatoiron (FePctBu4)
Following the procedure of Hanack and co-workers [34],
Fe(CO)5 (0.675 g, 3.45 mmol) was added in small portions to a
solution of 4-tert-butylphthalonitrile (2.5 g, 13.8 mmol) in ethyl-
ene glycol (18 ml) maintained at 200 °C. After 1 h, the reaction
mixture was cooled and water added (30 ml). The resulting blue
solid was collected by filtration, washed with water and methanol,
dried under vacuum and finally heated at 300 °C under N2 atmo-
sphere (2.05 g, 75%). LRMS, (ES+), m/z: 792.31 (Mꢀ+, 100%).
The main approach utilised in bio-mimetic alkane oxidation
catalysts uses organometallic complexes to activate oxidants such
as H2O2 [13]. Usually, V, Fe, Ru, Co, Cu and Mn complexes of por-
phyrins, phthalocyanines and Schiff bases [13–26] have been em-
ployed for the oxidation of many alkanes but they have not been
extensively used for methane activation. It is apparent from the re-
cent literature that efforts to produce a bio-mimetic methane oxi-
dation catalyst have been focussed on using phthalocyanine-based
complexes either as homogeneous or as heterogeneous catalysts.
This is based, in part, on the observation that in cytochrome
P-450 enzymes an iron porphyrin complex inserts an O atom into
the C–H bond via an abstraction-radical rebound mechanism to
give the hydroxylated product using an oxy-ferryl porphyrin radi-
cal intermediate generated from the activation of molecular oxy-
gen [27,28]. It is well known that in most cases anchoring
organometallic complexes onto supports can remove issues of cat-
alyst recyclability, loss of catalytic activity with time due to com-
plex auto-oxidation or polymerisation [29,30] and may increase
selectivity to desired products. Encapsulation into microporous
and mesoporous materials, as well as grafting onto inorganic or
polymer supports is used for this purpose. Two reported studies
are of note for methane oxidation and both use heterogenised
phthalocyanine catalysts anchored to supports in different ways
[1,2,31]. The system reported by Raja and Ratnasamy [31] utilised
Cu, Co and Fe complexes encapsulated in a zeolite to achieve par-
tial oxidation of methane with selectivity of 15.1%, 52.9% and 19.5%
to methanol, formic acid and CO2 respectively, when using tertiary
butyl hydroperoxide (TBHP) as oxidant and water as solvent. How-
ever, isotopic labelling studies were not performed for validation of
the results and it should be noted that TBHP can decompose to give
methanol under these reaction conditions. Chloro-substituted
phthalocyanines, as opposed to alkyl-substituted phthalocyanines,
were employed to increase catalytic activity and stability [32,33].
The recent reports by Sorokin and co-workers [1,2] also show that
a phthalocyanine complex can be effective in the oxidation of
methane under mild reaction conditions. In view of the continued
interest in these catalysts, we have undertaken to study the
2.1.2. Synthesis of
(FePctBu4)2N
l-nitrido-bis[tetra(tert-butyl)phthalocyaninatoiron]
Following the procedure of Sorokin et al. [1], FePctBu4 (0.8 g,
1 mmol), and NaN3 (3 g, 46 mmol) were suspended in 70 ml of
oxygen-free xylene under N2. The mixture was heated for 6 h at
150 °C with stirring. On cooling, the reaction mixture was filtered
to remove solid impurities and the solvent was removed by distil-
lation under reduced pressure. The crude product was added to a
column of neutral alumina with CH2Cl2 and then a blue fraction
was eluted using a mixture of CH2Cl2 and EtOH (starting with
100:1 ratio and increasing the amount of EtOH). The product still
contained impurities and further purification was achieved by
chromatography using silica as substrate and a mixture of CH2Cl2
and EtOH as eluent (starting with 10:1 ratio and increasing the
amount of EtOH). The solvent was evaporated and the solid dried
in a vacuum oven to afford pure (FePctBu4)2N (0.31 g, 39%). MS
(MALDI-TOF): cluster centred at m/z 1599 (M+) and cluster centred
at m/z 1615 (M + O). UV/vis, 1HNMR and IR spectra all identical to
those reported by Sorokin et al. [1].
2.1.3. Synthesis of ((FePctBu4)2N)@SiO2
The SiO2-supported complex (FePctBu4)2N@SiO2, was prepared
according to the published method of Sorokin and co-workers
[1]. The Fe content was measured by dissolving the catalyst first
in hydrogen peroxide (50% in water) and then in concentrated
HNO3 to afford complete dissolution of the silica. Metal loading
was 0.2 wt.% Fe which corresponds to a loading of 18.5 lmol/g of
the supported complex. Note regular dissolution in aqua regia
was ineffective.
IR spectrometry was performed by a Jasco FTIR 660 Plus instru-
ment using discs (10 mg) prepared from catalyst (5 mg) in KBr
(50 mg) after extensive drying and stored in a humidity controlled
dessicator. The spectra were taken at ambient temperature in air.
All UV–Vis spectra of solutions were performed on a JascoV-570
UVVis–NIR instrument at ambient temperature in air. Solid state
diffuse reflectance spectra were recorded on a Perkin Elmer
Lambad900 instrument. MS-ESI was measured on a Waters LCT
Premier Xe instrument and Maldi-TOF spectra were acquired on
a Waters Maldi Micro MX instrument.
catalyst used in the Sorokin study, namely
l-nitrido diiron tetra-
tert-butylphthalocyanine grafted onto high surface area silica
((FePctBu4)2N@SiO2) for the aqueous phase oxidation methane
with hydrogen peroxide. We were primarily concerned with the
stability of the catalyst, due to reports of possible catalyst degrada-
tion [1], and the overall selectivity of the process, since it is indi-
cated that CO2 is not expected to be a product using this catalyst
[1]. To carry out this work, we have established a full analytical
protocol to determine the nature of all the products formed when
(FePctBu4)2N@SiO2 is used as a catalyst for methane oxidation with
H2O2 in water. In this study, we present our detailed findings and
we compare the activity and selectivity of the catalyst with a silica-
2.2. Synthesis of Fe/SiO2 by wet impregnation
A 0.2 wt.% Fe on silica was made by stirring silica (1.996 g,
Degussa, which contains 20 ppm Fe as an impurity) in Fe(NO3)3(aq)

M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
179
(Sigma, 10 ml, 0.072 mol) for 1 h and then the water evaporated at
110 °C by drying in an oven for 16 h. The sample obtained was
calcined at 400 °C for 3 h in static air. A second sample with 2.5%
Fe loading was prepared in the same way but changing the
Fe(NO3)3(aq) concentration. This method was also used to prepare
Fe/SiO2 using iron(III) acetylacetonate (Sigma, purity > 99.5%) as
the precursor and a 60–40 mixture of acetone–water as the sol-
vent. The dried sample was calcined at 400 °C before use.
against known standards. The detection limit of this method was
ca. 0.1 mol of oxygenate products and the accuracy was ca.
0.1 mol of oxygenate product. CH2O was also quantified by the
l
l
Nash method [35] immediately after filtering the reaction mixture.
3. Results and discussion
3.1. Product identification for methane oxidation using
(FePctBu4)2N@SiO2
2.3. Oxidation of methane
The catalytic oxidation of methane was carried out using a
stainless steel autoclave (Parr reactor) containing a Teflon liner
vessel with total volume of 50 ml (working volume of 35 ml). In
a typical experiment, catalyst (50 mg) was added to pure water
(10 ml) containing a measured amount of H2O2 (50 wt.% H2O2).
The system was pressurised with methane to a fixed pressure
(32 bar, 0.033 mol) after air in the reactor was removed by purging
three times with methane. The autoclave was heated to the desired
reaction temperature of 50 °C. Once the reaction temperature was
attained, the solution was vigorously stirred at 1500 rpm and
maintained at the reaction temperature for a fixed period (0.5–
20 h). At the end of the reaction, the autoclave was cooled in ice
to a temperature below 10 °C to minimise loss of volatile products
and the reaction gas removed for analysis in a gas sampling bag.
The reaction mixture was filtered and analysed by 1HNMR using
D2O as a solvent and a calibrated TMS in CDCl3 insert (see Supple-
mentary information).
Some reactions were also performed in a smaller stainless steel
reactor with internal volume of 15 ml. For these reactions catalyst
(50 mg) was used in water (2 ml) charged with an appropriate
amount of H2O2 (50% in water). Methane (44 bar, 0.023 mol) was
used for these reactions in an effort to closely match the published
conditions for this catalyst [1]. The analysis of products was the
same as for the 10 ml reactions. All reactions with FeCl3(aq), SiO2
or Fe/SiO2 were performed as for the 10 ml reaction volume previ-
ously described. Fe analysis of both liquid and solid samples was
carried out using ICP and AAS analyses.
The (FePctBu4)2N@SiO2 catalyst was synthesised and analysed
by MS, IR, UV–VIS and XPS (see Supplementary information Figs.
S1–S9). An attempt to study the single crystal structure of the orga-
nometallic complex was unsuccessful due to an inability to obtain
suitable crystals for the diffraction study. However, all other tech-
niques show the material to be identical to that reported by Soro-
kin and co-workers [1]. The catalyst was then used for the
oxidation of methane under the standard reaction conditions for
20 h. Our initial studies focussed on ensuring complete analysis
of the reaction products. In particular, it is necessary to determine
the levels of CO2 in both the liquid and the gas phases as the reac-
tion is conducted in a three phase system (solid catalyst, solvent
with H2O2 and methane gas). Analysis was, therefore, carried out
using a combination of NMR and GC methods to achieve the neces-
sary sensitivity. 1HNMR was used for the identification and quan-
tification of liquid phase oxygenated products. This method has the
advantage of being unambiguous when compared to GC methods
of analysis which typically have low sensitivity to C1–C2 acids
and formaldehyde. We observed no overlapping signals in the
1HNMR spectra and we identified methyl hydroperoxide as a reac-
tion product (Fig. 1) along with hydrated CH2O, formic acid, meth-
anol, acetic acid and acetone. No ethane is observed in the reaction
products. The assignment of methyl hydroperoxide was based on
the work of Suss-Fink and co-workers [36]. As noted by Shul’pin
[37], alkyl hydroperoxy species can be reduced using suitable
reducing agents, and in the case of methyl hydroperoxide, metha-
nol is the only product. Methyl hydroperoxy cannot be directly
measured by GC analysis without reduction and these values can
be compared with those obtained before reduction. However,
methyl hydroperoxide is sufficiently stable to be quantified by
1HNMR for up to 2–3 days after the reaction if stored below 5 °C.
We consider this species to be the initial product of methane oxi-
dation in this catalytic system. Interestingly, methyl hydroperox-
ide has not been directly observed in the previous studies with
this catalyst [1,2].
Formaldehyde was not observed in the 2 ml reaction (entry 2
Table 2) by either 1HNMR spectroscopy or GC analysis and was
therefore quantified by the Nash method [35]. However, this
method was complicated by interference with leached Fe from
(FePctBu4)2N@SiO2 present in the solution that can react with the
Nash reagent giving a false-positive result for this analysis (see
Supplementary Table S1). We were able to correct for the contribu-
tion of the leached Fe in small scale reactions (2 ml) reactions after
dilution of an aliquot of the reaction mixture to ensure negligible
Fe in solution. This was not possible for most of the larger scale
(10 ml) reactions where the combination of low concentration of
formaldehyde and high concentration of leached Fe precluded this
correction being feasible. However, it should be noted that low
concentrations of hydrated formaldehyde was observed by 1HNMR
using (FePctBu4)2N@SiO2 as a catalyst for methane oxidation in our
10 ml reactions.
2.4. Analysis of products
Gas phase CO2 was determined by GC analysis on Varian 450-
GC equipped with aCP-SiL5CB column (50 m, 0.33 mm diameter,
He carrier gas) using a methaniser and an FID. Liquid products
could be analysed by liquid GC injection using the same equipment
but it was not sensitive enough to detect the low levels of HCOOH
and CH3COOH produced in some of these reactions. For this reason
1HNMR was the method of choice for liquid analysis. However, CO2
in the liquid phase was suitably analysed using our GC-FID meth-
od. For this, the initial water solvent was degassed with N2 until a
steady peak for CO2 was observed upon GC analysis of the gas
phase. An aliquot (10 ml) was used as detailed previously for the
reaction. The reaction mixture was analysed immediately after
depressurisation once enough filtrate had been collected for anal-
ysis (usually within 1 min). For this procedure, the gas and liquid
injection ports of the instrument were first flushed with N2 and
kept under low N2 flow to prevent injection of ambient air which
would contribute significantly to the CO2 in the analysis. A stan-
dard saturated solution of CO2 in water was prepared by stirring
water in a pressurised atmosphere of CO2/N2 to confirm the CO2
in water assignment.
1HNMR (500 MHz Bruker spectrometer) unambiguously identi-
fied all major liquid phase reaction products including CH2O that is
observed as hydrated CH2O (depending on the concentration). An
internal standard containing 1% TMS in CDCl3 was placed in a sealed
tube and used to quantify the product amount after calibration
We developed two methods for the analysis of CO2 as this can
be present in both the gas and the liquid phases following reaction.
For the gas analysis of CO2, an aliquot of the gas following reaction
was analysed directly by GC (see Supporting Fig. S10). This was

180
M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
Fig. 1. 1HNMR spectra of the reaction mixture obtained from the oxidation of methane using (FePctBu4)2N@SiO2 and H2O2 following reaction for 20 h. The signals at 0, 1.55
and 7.26 are due to the TMS standard and solvent contained in the calibration insert. In this example hydrated CH2O cannot be observed but it would be observed at
d = 5.1 ppm in the spectrum.
compared with a similar analysis prior to reaction as the high-pur-
ity methane we utilised contained a very minor CO2 impurity
which required correction. For quantification of CO2 in the liquid
phase, we developed a method of GC analysis where CO2 was
quantified reliably after flushing the gas in the reactor with N2
and then analysing the liquid by GC. It is essential that this proce-
dure is carried out immediately after the reaction mixture is re-
moved from the reactor. To validate this method, we also
performed an outgassing experiment in which the reactor was ini-
tially vented to remove the gas, which was analysed for CO2 as de-
scribed earlier, and then, the reactor was refilled with CH4 to the
reaction pressure (30 bar total pressure) and vented again with
analysis of the gas mixture for CO2. This procedure was repeated,
usually 5–6 times, until the CO2 level observed was identical to
Table 1
Results of using the two protocols for total CO2 analysis.
Entry Experiment
Products (
l
mol)
Ratio CO2
liquid/gas
CO2 in
liquid
CO2 in
gasa
1
2
GC analysis of reaction
liquid + gas
5.31b
55.7
63.7
1:10.5
1:9.6
Out gassing of reaction mixture. 6.61c
Gas only analysis
Catalyst: acidified Fe(NO3)3(aq), 10 ml water, 0.5 M H2O2, 0.5 h, 30 bar CH4,
1500 rpm.
a
Measured by GC-FID method using first reaction gas extracted.
Measured by modified liquid analysis under a N2 atmosphere.
Estimated by the sum total of CO2 in the reaction gas mixture after four
b
c
degassing steps. Note these data do not include the analysis of oxygenated liquid
phase products entries 1 and 2 were performed separately using the same experi-
mental as described. pH of the final reaction mixture was 3–4.
that observed at the start of the reaction (approximately 0.5 lmol
in the gas mixture). This method gave results in agreement with
the combined gas and liquid phase analysis method (Table 1). It
Table 2
Results of aqueous phase methane oxidation using the (FePctBu4)2N2@SiO2 and H2O2.
Entry
Products/mMg
CH3COOH
Products/
CO2
l
molh
%Fe leachedi
TONk
CH3OH
CH2Oj
HCOOH
CH3COOH
(CH3)2CO
CO2 (aq)
1a
2b
3c
4d
5e
6f
n.r.
n.r.
4.7 (9.4)
4.73 (9.46)
1.27 (12.7)
0
0.36 (3.6)
0
9.2 (18.4)
1.55 (3.09)
1.39 (13.9)
Trace
0.77 (7.7)
4.4 (44.0)
2.76 (27.6)
n.r.
n.r.
n.r
0
n.r
7.8
29
59
24
37
55
48
0.84 (1.67)
1.77 (17.7)
1.03 (10.3)
1.24 (12.4)
0.84 (8.4)
1.29 (12.9)
0.65 (1.29)
1.16 (11.6)
0.75 (7.50)
0.36 (3.6)
0.2 (2.0)
0.32 (3.2)
2.70 (5.39)
0.90 (9.02)
0.88 (8.8)
0.47 (4.7)
0.47 (4.7)
0.13 (1.3)
4.98 (9.96)
0.44 (4.42)
0.90 (9.0)
0.39 (3.9)
0.23 (2.3)
0.03 (0.3)
19.55
31.50
7.4
28.9
35.1
13.0
0.85
2.65
n.d
n.d
n.d
n.d.
40.2
94.2
19.4
n.d
25
n.d
7f*
0.59 (5.9)
Entries 1–4 ran for 20 h at 50 °C using 50 mg of catalyst.
a
Data calculated from the literature.
2 ml water, 44 bar CH4 (0.023 mol), 0.35 M H2O2.
10 ml water, 32 bar CH4 (0.033 mol), 0.5 M H2O2.
10 ml water, 32 bar CH4 (0.033 mol), 0.07 M H2O2.
b
c
d
e
10 ml water, 23 bar CH4 (0.023 mol), 0.35 M H2O2, 60 °C.
f
10 ml water, 23 bar CH4 (0.023 mol), 0.35 M H2O2, 0.075 M H2SO4, 60 °C.
Reuse of catalyst from 6f under the same conditions.
Analysis using 1HNMR except for CH2O.
f*
g
h
i
Analysis using GC-FID. Trace products taken was less than 0.1 l
mol as detected using 1HNMR.
Quantified using AAS based on detected Fe in reaction filtrate relative to Fe in catalyst at start of experiment.
j
Analysed as hydrated CH2O using 1HNMR and the Nash method.
k
Turnover number calculated on total Fe including the support. () = product amount in lmol for that specific reaction volume; ‘n.r’ = not reported, ‘n.d’ = not determined.

M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
181
should be noted that most of the CO2 is observed in the gas phase
(Table 1). As these methods of CO2 analysis require extensive ana-
lytical time (ca. 4 h per analysis), we report the full quantification
for key reactions to demonstrate the full selectivity data.
the absence of acid, entry 5 Table 2, and gave 49% oxygenate selec-
tivity together with C2 decomposition products. When an aqueous
0.075 M H2SO4 solution was used as the solvent, entry 6 Table 2,
the activity improved dramatically as a twofold increase in C1 oxy-
genates was observed as compared to the test performed in the ab-
sence of acid. Additionally, the oxygenate selectivity was improved
from 49% to 62% in the presence of acid though there was total con-
sumption of the hydrogen peroxide. Visually the loss of blue colour
in the recovered material was not as severe which prompted us to
perform a re-use test under acidic conditions. The data presented
in entry 7 Table 2 show product amounts similar to that observed
in entry 6 but with different product distribution and higher oxy-
genate selectivity. However, in this case, the recovered catalyst
was completely decolourised and XPS analysis of the used material
showed no Fe was present on the surface of the used catalyst (see
Supporting information Fig. S11). Thus, we proposed that the result
of the reuse test was probably due to leached Fe which under
acidic conditions has appreciable activity with good selectivity
for methane oxidation (refer to entry 3 Table 3 for an example of
this). Thus, it is apparent that (FePctBu4)2N@SiO2 is unstable under
reaction conditions and that catalyst decomposition or homoge-
neous Fe based methane oxidation may be responsible for a major
proportion of the products observed due to its reaction with H2O2
over the extended reaction time.
3.2. Methane oxidation using (FePctBu4)2N@SiO2
Table 2 shows the results for the oxidation of methane using
(FePctBu4)2N@SiO2 as catalyst carried out under different condi-
tions. When the reaction conditions are identical (entry 2) to those
reported previously in the literature (entry 1), we observed the
same values of formaldehyde as reported. However, after repeated
attempts and checking the possibility of H/D exchange issues with
the 1HNMR procedure, we could not obtain the value of HCOOH
previously reported [1,2] and we observed significantly lower con-
centrations of HCOOH (9.2 mM for entry 1 vs. 1.55 mM for entry 2
Table 2). Low amounts of methanol and methyl hydroperoxide
were detected for the 20 h reaction as shown in entry 2, Table 2.
Using these reaction conditions that are identical to those used
by Sorokin and co-workers [1], substantial amounts of CO2 were
also produced (20.4 lmol, entry 2 Table 2) giving a total oxygenate
selectivity of only 43%. Coupled with the low levels of products
(total methane conversion of 0.16% or oxygenate productivity of
0.016 mol (C1 oxygenates) kg (catalyst)À1 hÀ1) these results dem-
onstrate that the (FePctBu4)2N@SiO2 catalyst gives a performance
in methane oxidation that is very far from the activity of the
benchmark provided by sMMO. Furthermore, products resulting
from the decomposition of (FePctBu4)2N@SiO2, namely the C2 com-
pounds of acetone and acetic acid, are observed even at very low
methane conversions.
The presence of acetic acid and acetone, previously reported as
not being derived from CH4 oxidation in labelling studies by Soro-
kin and co-workers [1,2] demonstrates that decomposition of the
silica-supported organometallic complex is occurring as the catal-
ysis proceeds. This indicates that Fe could be leached into the solu-
tion as the reaction proceeds, and therefore, we analysed reaction
mixtures for solubilised Fe. It should be noted that (FePctBu4)2N@-
SiO2 is highly insoluble in water, the solvent for the reaction, but is
decomposed by reaction with H2O2. In entry 2, the leached Fe ac-
counts for ca. 40% of the initial Fe present in the catalyst, and the
sum total of acetone and acetic acid accounts for about 46% of
the starting carbon present in the catalyst. These data indicate that
the (FePctBu4)2N@SiO2 catalyst is unstable under these reaction
conditions and clearly cannot be recovered and re-used.
At a higher oxidant level (entry 3, 500 mM H2O2), a higher level
of gas phase CO2 is detected. Even for a very low oxidant concen-
tration (entry 4, 70 mM H2O2), the catalyst still shows substantial
decomposition products (acetone and acetic acid) and partial
selectivity to oxygenates of 71%. By comparing the data in entry
3–4, it is apparent that decreasing the oxidant concentration effec-
tively prevents over-oxidation of the primary products to HCOOH
and CO2. Due to similarities in the CO2 results between the 2 ml
and 10 ml reactions, entries 2 and 3 in Table 2, we were prompted
to consider that the catalyst decomposition was responsible for the
formation of some of the CO2 and/or the useful oxygenates rather
than being derived from CH4. This aspect of the stability of the
(FePctBu4)2N@SiO2 catalyst is explored further in a subsequent
section. It is important to note that the catalyst showed total loss
of its original blue–green colour after 20 h reaction under the con-
ditions in entry 3 and major loss of colour for the data reported in
entries 2,4 (see Supporting information Fig. S14).
3.3. Methane oxidation using components in the (FePctBu4)2N2@SiO2
catalyst
The results of Section 3.2 demonstrate that the catalyst is unsta-
ble under reaction conditions. It is, therefore, useful to know the
activity of a number of the components of the catalyst for this reac-
tion. The blank reaction can be considered to be the reaction cata-
lysed by the H2O2 (auto-oxidation) or the silica used as a support
for the complex. CH4 oxidation by H2O2 in the absence of any
added catalyst was not observed under the reaction conditions.
However, we observed that the silica support was active for meth-
ane oxidation (entry 1, Table 3) and was also active for the decom-
position of H2O2 as very little remained at the end of the reaction.
However, it should be noted that the C1 oxygenate selectivity ob-
served with the SiO2 support alone is high (80.4%) compared with
the (FePctBu4)2N@SiO2 catalyst (entry 3, Table 2). Under these reac-
tion conditions, the phthalocyanine-based catalyst leached a total
concentration of 10 ppm Fe into the reaction mixture after 20 h.
The leached Fe could be in the form of intact (FePctBu4)2N (i.e.
the organometallic complex being wholly leached off the silica
support) or as un-complexed aqueous Fe which would then act
as a homogeneous catalyst. However, we did not observe the intact
complex in solution after the reaction and in view of its high affin-
ity for silica we do not consider that the complex leaches in this
way.
We therefore investigated the reaction of this concentration of
Fe using FeCl3 and the results (entry 2, Table 3) show that soluble
Fe can catalyse the reaction and CH3OOH and CH3OH are both ob-
served. However, large amounts of CO2 are produced. Of course the
catalyst could leach this amount of Fe as a maximum amount over
the course of the 20 h reaction and so this experiment possibly
overestimates the contribution of the homogeneously catalysed
component of the reaction; but the experiment demonstrates that
the reactions with (FePctBu4)2N@SiO2 are not wholly heteroge-
neously catalysed. Furthermore, under acidic conditions (entry 3
Table 3), homogeneous Fe based oxidation shows higher catalytic
activity and oxygenate selectivity but in all reactions high hydro-
gen peroxide usage occurs. It is well known that Fenton chemistry
[37–39] principally proceeds through hydroxyl and hydroperoxy
radicals, and thus, we postulate the possible involvement of these
species not only in methane activation, in a similar way to benzene
Finally, we studied the catalyst under (0.075 M H2SO4) since it
was reported that the catalyst had higher activity and stability un-
der acidic conditions [1,2]. We modified the reaction conditions so
that similar levels of oxidant, catalyst and substrate as in the ‘2 ml’
reaction were employed. The freshly prepared material was used in

182
M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
Table 3
Comparison of the oxidation activity of components of (FePctBu4)2N@SiO2 for methane oxidationa.
Entry Catalyst
Products (
l
mol)
Oxygenate
Oxygenate
selectivityf
Unused H2O2
(
l
mol)g TONh
productivitye
CH3OOHb CH3OHb CH2Oc HCOOHb CO2 in gas
d
1
2
3
SiO2
4.14
3.08
6.81
4.4
0
0
7.3
0
27.5
40.3
1.76
66.4
18.5
0.007
18.3
413
80.4
35.6
77
<200
<50
1875
503
58
15
10 ppm Fe(aq)[i] 2.35
30 ppm Fe(aq)[i] 10.0
a
Conditions entries 1, 2 – reaction time 20 h; temperature 50 °C; [H2O2] = 0.5 M, P (CH4) = 32 bar, mass of solid catalysts used was 50 mg. Conditions entry 3 – reaction
time 0.5 h; temperature 50 °C; [H2O2] = 0.5 M, P (CH4) = 32 bar, pH3.
b
Analysis using 1HNMR.
c
Analysed as hydrated CH2O by 1HNMR.
d
Analysis using GC-FID with a methaniser.
Oxygenates productivity = mol of oxygenates (MeOOH + CH3OH + CH2O HCOOH)/kgcat/reaction time (h).
e
f
Total C1 oxygenates/total C1 oxygenates + CO2 in gas.
g
Assayed by Ce+4(aq) titration.
h
Turnover number calculated on total Fe including the support.
Using FeCl3, For entry 1 [Fe] in filtrate was < 1.25 ppm.
i
oxidation with Fenton’s reagent [40], but also in catalyst degrada-
tion. Though it was reported that the initial interaction of H2O2
with (FePctBu4)2N does not produce hydroxyl radicals [1,2], if lea-
ched iron results in hydroxyl radicals being formed then these
could play a major role in the observed chemistry. Hence, we can
conclude that leached Fe can contribute significantly to the oxida-
tion activity observed when using (FePctBu4)2N@SiO2 as a catalyst.
It is also possible that the action of silica in the presence of hydro-
gen peroxide initiates catalyst degradation since silica has been
shown to be active for the methane oxidation.
and acetaldehyde. These products result from the oxidation of the
tert-butyl groups of the ligand and the colour of the complex was
retained after reaction (see Supplementary Fig. S12). It is known
that organic compounds with t-butyl groups or other OH groups
are also either autoxidised by H2O2 or oxidised by homogeneous
metal catalysed reactions. For example, use of tertiary butyl hydro-
peroxide as oxidant can give products that are easily misidentified
as coming from methane oxidation (see Supplementary Fig. S13).
We then added soluble Fe3+ and observed that the decomposition
of the ligand was enhanced by the addition of this non-complexed
Fe3+ (entries 2–4, Table 4) but the phthalocyanine ring remained
intact as evidenced by the retention of colour following reaction
(see Supplementary Fig. S12). We then reacted the supported com-
plex (FePctBu4)2N@SiO2 under the same reaction conditions (entry
5, Table 4) and now we observe that the phthalocyanine breaks
down and CO2 is observed in significant amounts. The C2 products
are derived from the reaction of the tert-butyl groups of the phtha-
locyanine and the additional CO2 observed originates from the deg-
radation of the ligand as evidenced by the loss of colour after the
reaction (see Supplementary Fig. S12). The observation of small
amounts of C1 products in these experiments could mean that
the selectivity observed for the C1 products during the reaction
of the catalyst with methane (Table 2) may be overestimated.
The observation of enhanced phthalocyanine decomposition when
3.4. Catalyst decomposition
As we have observed catalyst decomposition during the reac-
tion, it follows that we have only partially answered the question
concerning catalyst selectivity as to whether or not all the CO2 or
C1 products observed in the reaction originate from methane oxi-
dation or from the methyl groups in the catalyst? We therefore
studied the decomposition of both the ligand and the catalyst by
stirring under reaction conditions in the absence of methane. In
this case, methane was replaced with He or N2. First, we investi-
gated the supported ligand in the absence of complexed Fe (entry
1, Table 4) and this was found to produce a range of products
including methanol and formic acid as well as acetone, acetic acid
Table 4
Reactions of SiO2-supported tBu4Pc and (FePctBu4)2N with aqueous H2O2
.
a
Entry Setup
Aqueous phase products (
l
mol)
Gas Phase
product mol
Total Aqueous
Carbon
balance%
l
Products (lmol)
HCOOHb CH3OHb (CH3)2COb CH3COOHb CH3CHOc CH2Od CO2
e
1
2
tBu4Pc – SiO2 + H2O2
0.36
0.71
0.29
0.43
2.86
2.57
1.90
2.00
1.57
1.00
n.d
n.d
2.63
6.39
6.98
6.71
21.2
23.6
tBu4Pc –
SiO2 + H2O2 + 0.2 ppm Fe
(aq)
3
4
tBu4Pc –
0.71
0.43
0.29
0.29
2.58
2.00
2.14
1.29
3.00
5.57
n.d
n.d
5.00
4.18
8.72
9.58
26.9
26.6
SiO2 + H2O2 + 5 ppm Fe (aq)
tBu4Pc –
SiO2 + H2O2 + 10 ppm Fe
(aq)
5
6
(FePctBu4)2N – SiO2 + H2O2
(FePctBu4)2N – SiO2 + H2O2
n.d
1.09
(0.23)
n.d
0.89
4.57
(1.33)
5.29
(1.9)
n.d
(1.71)
n.d
(1.28)
20.14
11.1
8.86
53.7
6.45*
45.7*
a
Conditions entries 1–5; water (10 ml), 50 mg of supported materials, [H2O2] = 0.5 M, 50 °C, 20 h, 1500 rpm, P (N2) = 32 bar; conditions entry 6; water (10 ml), catalyst
(17.8 mg), [H2O2] = 0.16 M, 60 °C, 20 h, 1500 rpm, P (13CH4) = 8 bar; Fe(aq) from Fe(NO3)3.
b
Analysed using 1HNMR.
c
Analysed as hydrated CH3CHO by 1HNMR.
d
Analysed as hydrated CH2O by 1HNMR.
e
Analysed using GC-FID. () refer to 12C products.
*
Based solely upon 12C products. n.d not detected.

M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
183
the Fe3+ is complexed is an important observation and, as dis-
cussed subsequently, indicates the iron phthalocyanine complex
is a catalyst for its own oxidative decomposition.
the catalyst. Although the ratio of 13CO2 to 12CO2 formed could
not be determined by our analytical methods, there is clear
evidence for the formation of CO2 from the catalyst since in the
absence of methane (entry 1, 2 Table 4) CO2 is produced.
In these studies, we have shown that the Fe in the catalyst can be
leached and it is known that Fe3+ in the presence of H2O2 can gen-
erate OH radicals by Fenton’s reaction [37–39] which can then react
with the phthalocyanine leading to its degradation. Hence, we have
investigated the effect of adding a scavenger for OH radicals. In the
presence of a hydroxyl radical scavenger, sodium sulphite, we ob-
served that the catalyst decomposition decreased by approximately
30% with H2O2: sulphite ratio of 2:1 (see Supplementary Fig. S14).
Hence, the catalyst becomes more stable in the presence of a hydro-
xyl radical scavenger. Previous studies have proposed that the reac-
tion mechanism does not include hydroxyl radical formation by
homolysis of the O–O bond from the iron hydroperoxy species ini-
tially formed by interaction of H2O2 with the iron centre [1,2,41,42].
Rather a high valent iron oxo species was put forward as the active
oxidising species, but this result does indicate the possible involve-
ment of hydroxyl radicals in the overall chemistry observed, and we
consider that this is potentially important for the mechanism of the
catalyst decomposition. Considering the role of OH radicals in the
decomposition of the phthalocyanine, two pathways might be pos-
sible: (i) generation of OH radicals by the iron phthalocyanine (this
would lead to inherent instability) and (ii) generation of OH radicals
in the close vicinity of the adsorbed complex leading to its decom-
position which, in turn, provides more iron sites leading to autocat-
alytic decomposition. The SiO2 support alone, which contains
20 ppm Fe as an impurity, (entry 1, Table 3) shows significant
H2O2 decomposition which will lead to OH radical formation. It is
clear therefore that there is probably a significant contribution from
pathway (ii). However, the observation that complexed Fe is more
reactive in the decomposition of the phthalocyanine clearly also
shows that pathway (i) is also operating. Hence, at this stage, it is
considered that both pathways are operating under the reaction
conditions we have used.
Finally, the effect of the reaction time on product formation was
investigated (Fig. 2). It is apparent that even at low reaction times,
when the amount of methane oxidation products is low, apprecia-
ble amounts of the C2 catalyst decomposition products are ob-
served. As time proceeds, the activity of the catalyst levels off
then shows increased activity after 6 h. We propose that the contri-
bution from homogeneous iron catalysed reactions is significant
after this time as about 6 ppm of Fe was detected in the reaction
filtrate for the 6 h reaction. We observed that CH3OOH was the ma-
jor product throughout the entire reaction as one would expect
from the interaction of a metal bound hydroperoxy species and
methane under suitable conditions. As noted before, this species
can be generated by homogeneous Fe reactions (entry 3, Table 3)
and hence it is not unique to the (FePctBu4)2N@SiO2 catalyst.
Colour loss is always observed with this catalyst and this is pro-
nounced using 0.5 M H2O2 in our 10 ml reaction (see Supplemen-
tary Fig. S15).
3.5. Reactions using Fe/SiO2
In Section 3.3, we noted that the silica support showed some
activity for the oxidation of methane possibly originating from
transition metal impurities. Therefore, we tested a 0.2% Fe/SiO2
catalyst prepared by an impregnation procedure. The metal load-
ing matches the metal content of the phthalocyaninato-based cat-
alyst though the metal sites in both systems will be significantly
different. Interestingly, this catalyst was very selective to oxygen-
ates and when compared to the phthalocyanine-based system it
has superior selectivity and high hydrogen peroxide utilisation,
though the productivity of C1 oxygenates is lower (entry 1 Table 5).
We prepared and studied wet impregnation catalysts for 4 h
reactions as the reference phthalocyanine-based system has appre-
ciable Fe leaching after this reaction time. XRD characterisation of
the Fe/SiO2 sample prepared from Fe(NO3)3 showed that haematite
was present with an estimated particle size of 16 nm, whilst the
sample prepared from Fe(acac)3 was amorphous to X-rays indicat-
ing that any oxide present must have a particle size of below 5 nm
(see Supplementary Fig. S16). The metal loadings were 2.58 and
2.43 wt.% Fe, respectively, as determined by AAS. Both these sam-
ples were active for methane oxidation with high selectivity to oxy-
genates >85% (entries 1 and 3 Table 5). The leaching observed under
these reaction conditions is minimal indicating catalyst stability is
superior to the (FePctBu4)2N@SiO2 catalyst. These data demonstrate
We then used 13CH4 to indicate the contribution to C1 oxygen-
ates from catalyst degradation since any non-13C products must
originate from carbon in the catalyst under these conditions. As
shown in entry 6 Table 4, methyl hydroperoxide, methanol and
formic acid contain the 13C label. We also detected 12C formic acid,
hydrated formaldehyde as well as acetic acid, acetone and acetalde-
hyde under these conditions. The products containing 12C
accounted for ca. 46% of the initial carbon in the catalyst, whilst
the actual oxygenate productivity of the catalyst based on 13C prod-
ucts was 0.014 mol oxygenates kg (cat)À1 hÀ1. This experiment
shows unambiguously that formic acid, formaldehyde, acetic acid,
acetone and acetaldehyde can be derived from the degradation of
Fig. 2. Product reaction profile for the (FePctBu4)2N@SiO2 catalyst using H2O2 for methane oxidation. All conditions as in Entry 3 Table 2. ꢁ Total C1 oxygenates; N total C2
oxygenates; j CO2 in the gas phase.

184
M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
Table 5
Comparison of activities of various Fe/SiO2 catalysts for CH4 oxidation.
Entry Catalyst Products ( mol)
l
Oxygenate
productivity
Oxygenate
selectivitye
Unused H2O2
mol)f
Leached Fe(aq) TONh
ppm
d
g
(
l
MeOOHa MeOHa CH2Ob HCOOHa CO2 in
gasc
1
2
(FePctBu4)2N@SiO2
17.7
13.5
11.6
1.90
12.7
0
13.9
0.61
26.6
2.59
0.056
0.016
67.7
86.1
2600
3620
9.7
<0.1
47
10
0.2 wt.% Fe/SiO2 (from
Fe(NO3)3)
3
4
(FePctBu4)2N@SiO2
19.3
2.32
4.11
Trace* 2.58
11.3
8.7
4.85
0.121
0.155
73.5
86.5
3660
2220
3.9
0.1
19
1.7
2.43 wt.% Fe/SiO2 (from 13.6
Fe(acac)3)
2.04
5
2.58 wt.% Fe/SiO2 (from 4.94
Fe(NO3)3)
12.4
4.49
2.71
4.03
0.123
85.9
3900
0.28
1.2
Conditions entry 1, 2: 20 h, 50 mg catalyst, 0.5 M H2O2, 32 bar CH4, 10 ml reaction volume. Conditions entries 3–5: 4 h, 50 mg catalyst, 0.5 M H2O2, 32 bar CH4, 10 ml reaction
volume. All catalysts calcined at 400 °C, 3 h in static air except (FePctBu4)2N@SiO2.
a
Analysis using 1HNMR.
b
Observed as hydrated CH2O by 1HNMR.
c
Analysis by GC-FID.
d
moles (MeOOH + MeOH + CH2O + HCOOH) hÀ1 kg(catalyst)À1
Total C1 oxygenates/(total C1 oxygenates + CO2 in gas).
Assayed by Ce+4 (aq) titration.
.
e
f
g
h
Analysed by AAS of the reaction filtrate with 0.1 ppm being the lowest calibration point.
Turnover number calculated on total Fe including the support.
*
Trace taken as <0.1 lmol.
Table 6
Activity of 2.43 wt.% Fe/SiO2 catalyst made by impregnation method from Fe (acac)3 calcined at 400 °C.
Entry Catalyst
mass (mg)
Rxn.
time (h)
Products (
l
mol)
Oxygenate
Oxygenate
selectivitye
Unused H2O2
mol)f
Leached
TONh
productivityd
(
l
Fe(aq) ppmg
MeOOHa MeOHa CH2Ob HCOOHa CO2 in
gasc
1
2
3
22.5
22.5
50
4
20
0.17
9.89
12.0
6.0
2.00
6.04
1.51
0
8.03
0
0.83
8.86
0
2.64
8.59
1.05
0.141
0.078
0.901
82.8
80.3
87.7
3526
1433
4126
<0.1
0.71
<0.1
1.6
4.5
0.4
Conditions: 32 bar CH4, 10 ml reaction volume, 50 °C.
a
Analysis using 1HNMR.
b
Observed as hydrated CH2O by 1HNMR.
c
Analysis by GC-FID.
d
moles (MeOOH + MeOH + CH2O + HCOOH) hÀ1 kg(catalyst)À1
Total C1 oxygenates/total C1 oxygenates + CO2 in gas.
Assayed by Ce+4(aq) titration.
.
e
f
g
h
Analysed by AAS of the reaction filtrate with 0.1 ppm being the lowest calibration point.
Turnover number calculated on total Fe including the support.
0.15
0.1
0.05
0
100
90
80
70
60
50
40
30
20
10
0
1st
2nd
3rd
Catalyst Reuse Number
Fig. 3. Reuse testing for 2.43 wt.% Fe/SiO2 calcined at 400 °C in static air. Conditions P (CH4) = 30 bar, [H2O2] = 0.5 M, T = 50 °C, t = 4 h, 1500 rpm, catalyst mass = 22.5, 22.5,
13 mg for tests 1,2,3 respectively. The recovered catalyst was dried in air at ambient temperature after washing with distilled water in between uses.

M.M. Forde et al. / Journal of Catalysis 290 (2012) 177–185
185
that the relatively simple supported metal oxide catalysts, prepared
References
by wet impregnation, can perform selective methane oxidation
using H2O2 as the oxidant. Also we observe that the catalyst con-
taining the smaller iron oxide particles gives a higher yield of
methyl hydroperoxide vs. methanol (entries 2 and 3 Table 5).
Further study of these catalysts showed that even after 20 h the
selectivity to oxygenates is still 80% with a similar productivity to
the unstable (FePctBu4)2N@SiO2 system (entry 2 table 6 vs. entry
3 Table 2). At longer reaction times or with higher catalyst mass,
we observe the sequential oxidation of methyl hydroperoxide to
formic acid via the alcohol and aldehyde (Table 6). Finally, we com-
pared the Fe/SiO2 catalyst to MMO by carrying out a 10 min reac-
tion (entry 3 Table 6). For Fe/SiO2, the productivity is one-fifth of
that observed with the whole native sMMO enzyme for methane
to methanol conversion. Additionally, the Fe/SiO2 catalyst
does not show an induction period that is a hallmark of the
(FePctBu4)2N@SiO2 catalyst, and also no formaldehyde or formic
acid was detected for this short reaction (entry 3 Table 6). We note
that these Fe/SiO2 catalysts are stable and reusable even after pro-
longed reaction times as demonstrated by almost identical produc-
tivity and selectivity even after three reuse tests (Fig. 3). It should be
noted that without further modification the Fe/SiO2 is not stable
under acidic conditions and shows appreciable leaching in similar
manner as the supported phthalocyanine catalyst. Further tuning
of this catalyst was not performed.
It is useful to compare the turn over frequency of these systems
to sMMO hydroxylase using aqueous H2O2 as oxidant. Considering
there are two mol of active Fe per mol of sMMO hydroxylase (i.e. a
di-iron active site) the TOF is 2.38 mol oxygenates per (h  mol
(Fe)) at 0.5 h reaction time. For 2.43 wt% Fe/SiO2 reported in Table
6 the TOF is 2.0 mol oxygenates per (h  mole (Fe)), whilst for
(FePctBu4)2N@SiO2 which had 0.2 wt.% Fe the TOF is 1.03 mol oxy-
genates (h  mole (Fe)) at 0.5 h reaction time. These data show
that the catalyst prepared by impregnation is both stable and is
comparable in activity to the phthalocyanine catalyst for methane
oxidation with H2O2 and also highlights that traditional heteroge-
neous metal oxide catalysts can compete with the enzyme systems
for low temperature methane oxidation.
[1] A.B. Sorokin, E.V. Kudrik, D. Bouchu, Chem. Commun. (2008) 2562.
[2] A.B. Sorokin, E.V. Kudrik, L.X. Alvarez, P. Afansiev, J.-M.M. Millet, D. Bouchu,
Catal. Today 157 (2010) 2562.
[3] P.J. Smeets, J.S. Woertink, B.F. Sels, E.I. Solomon, R.A. Schoonheydt, Inorg. Chem.
49 (2010) 3573.
[4] O.V. Krylov, Catal. Today 18 (1993) 209.
[5] K. Otsuka, Y. Wang, App. Catal. A 222 (2001) 145.
[6] E.D. Park, S. Hee-Choi, J.S. Lee, J. Catal. 194 (2000) 33.
[7] A.E. Shilov, G.B. Shul’pin, Chem. Rev. 97 (1997) 2879.
[8] R.A. Periana, D.J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, Science 280
(1998) 560.
[9] M. Merkx, D.A. Kopp, M.H. Sazinsky, J.L. Blazy, J. Muller, J. Lippard, Angew.
Chem. Int. Ed. 40 (2001) 2782.
[10] R.A. Periana, G. Bhalla, W. Tenn, K. Young, X.Y. Lui, O. Mironov, C. Jones, V.R.
Ziatdinov, J. Mol. Catal. A 220 (2004) 7.
[11] J. Colby, D.I. Stirling, H. Dalton, Biochem. J. 165 (1977) 395.
[12] Y. Jiang, P.C. Wilkins, H. Dalton, Biochim. Biophys. Acta Protein Struct. Mol.
Enzymol. 1163 (1993) 105.
[13] G.B. Shul’pin, G.S. Mishra, L.S. Shul’pina, T.V. Strelkova, A.J.L. Pombeiro, Catal.
Commun. 8 (2007) 1516.
[14] W. Nam, S.E. Park, I.K. Lim, M.H. Lim, J. Hong, J. Kim, J. Am. Chem. Soc 125
(2003) 14674.
[15] V. Mirkhani, M. Moghadam, S. Tangestaninejad, B. Bahramian, A. Mallekpoor-
Shalamzari, App. Catal. A 321 (2007) 49.
[16] A. Kozlov, A. Kozlova, K. Asakura, Y. Iwasawa, J. Mol. Catal. A 137 (1999) 223.
[17] M.D. Khokhar, R.S. Shukla, R.V. Jasra, V. Raksh, J. Mol. Catal. A 299 (2009) 108.
[18] R.F. Parton, G.J. Peere, P.E. Neys, P.A. Jacobs, R. Claessens, G.V. Baron, J. Mol.
Catal. A Chem. 113 (1996) 445.
[19] P.P. Knops-Gerrits, M. L’Abbé, W.H. Leung, A.M. Van Bavel, G. Langouche, I.
Bruynseraede, P.A. Jacobs, Stud. Surf. Sci. Catal. 101 (1996) 811.
[20] K.J. Balkus, M. Eissa, R. Levado, J. Am. Chem. Soc. 117 (1995) 10753.
[21] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt,
C.T.W. Chu, D.H. Olson, E.W. Sheppard, J. Am. Chem. Soc. 114 (1992) 10834.
[22] R.L. Garten, W.N. Delgass, M. Boudart, J. Catal. 18 (1970) 90.
[23] I.W.C.E. Arends, R.A. Sheldon, M. Wallau, U. Schuchart, Angew. Chem. Int. Ed.
36 (1997) 1145.
[24] P.P. Knops-Gerrits, D.E. de Vos, P.A. Jacobs, J. Mol. Catal. A 117 (1997) 57.
[25] A. Corma, Chem. Rev. 97 (1997) 2373.
[26] P.P. Knops-Gerrits, A. Verberckmoes, R. Schoonheydt, M. Ichikawa, P.A. Jacobs,
Microporous Mesporous Mater. 21 (1998) 475.
[27] M. Newcomb, H.P. Toy, Acc. Chem. Res. 33 (2000) 449.
[28] B. Meunier, S.P. de Visser, S. Shaik, Chem. Rev. 104 (2004) 3947.
[29] F. Bedioui, Coord. Chem. Rev. 114 (1995) 39.
[30] R.F. Parton, I.F.J. Vankelecom, M.J.A. Casselman, C.P. Bezoukhanova, J.B.
Utterhoeven, P.A. Jacobs, Nature 370 (1994) 541.
[31] R. Raja, P. Ratnasamy, App. Catal. A 158 (1997) L7.
[32] R. Raja, P. Ratnasamy, Catal. Lett. 48 (1997) 1.
[33] R. Raja, P. Ratnasamy, Catal. Today 141 (2009) 3.
[34] J. Metz, O. Schneider, M. Hanack, Inorg. Chem. 23 (1984) 1065.
[35] T. Nash, Biochem. J. 55 (1953) 416.
Acknowledgment
[36] G. Suss-Fink, G.V. Nizova, S. Stanislas, G.B. Shul’pin, J. Mol. Catal. A 130 (1998)
163.
We wish to thank the Dow Chemical Company for funding
through the Dow Methane Challenge.
[37] G.B. Shul’pin, J. Mol. Catal. A Chem. 189 (2002) 39–66.
[38] F. Haber, J. Weiss, Proc. Roy. Soc. (London) A147 (1934) 332.
[39] Q. Yuan, W.P. Deng, Q.H. Zhang, Y. Wang, Adv. Syn. Catal. 349 (2007) 1199.
[40] J. Smith, R. Lindsay, R.O.C. Norman, J. Chem. Soc (1963) 2897.
[41] A.B. Sorokin, E.V. Kudrik, Chem. Eur. J. 14 (2008) 7123.
[42] P. Afanasiev, E.V. Kudrik, J.-M.M. Millet, D. Bouchu, Alexander, B. Sorokin,
Dalton Trans. 40 (2011) 701.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.03.013.