Selective mild oxidation of methane to methanol or formic acid on Fe-MOR catalysts

Article abstract of DOI:10.1039/c9cy01640f

Controllable methane oxidation directly into value-added products under mild conditions remains a challenge. Herein, an active Fe/MOR catalyst was synthesized via simple solid-state ion exchange, and its activity in the selective oxidation of methane with H₂O₂ in the aqueous phase was intensively investigated. The octahedral dimeric Fe³⁺ species [Fe₂(μ-O)₂] in the extra framework was confirmed as the initial active site by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and extended X-ray absorption fine structure, UV-vis diffuse-reflectance spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy in combination with DFT calculations. The DFT calculations indicated that methanol formation via methyl peroxide (CH₃OOH?) on [Fe₂(μ-OH)₂O₂] is the most favorable pathway compared to the direct formation of methanol via CH₃O?. The formed CH₃OH is easily further oxidized by hydroxyl radicals (OH) resulting in non-selective methane oxidation. In contrast, the Fe/MOR catalyst could lead to a high methanol selectivity of 71.3% in the presence of homogeneous Cu²⁺ precursor, which efficiently suppressed the over-oxidation of methanol, and a high formic acid selectivity up to 81-82% at a slightly higher reaction temperature by mildly shifting the oxidation of methanol and formaldehyde to the target product.

Full text of DOI:10.1039/c9cy01640f

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Selective mild oxidation of methane to methanol
or formic acid on Fe–MOR catalysts†
Cite this: Catal. Sci. Technol., 2019,
9, 6946
Zhihao Fang,‡a Haruno Murayama,‡b Qi Zhao,‡a Bing Liu, a Feng Jiang,a
a
Yuebing Xu, a Makoto Tokunagab and Xiaohao Liu
*
Controllable methane oxidation directly into value-added products under mild conditions remains a
challenge. Herein, an active Fe/MOR catalyst was synthesized via simple solid-state ion exchange, and its
activity in the selective oxidation of methane with H2O2 in the aqueous phase was intensively investigated.
The octahedral dimeric Fe3+ species [Fe2IJμ-O)2] in the extra framework was confirmed as the initial active
site by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and extended X-ray
absorption fine structure, UV-vis diffuse-reflectance spectroscopy, and high-angle annular dark-field
scanning transmission electron microscopy in combination with DFT calculations. The DFT calculations
indicated that methanol formation via methyl peroxide (CH3OOH*) on [Fe2IJμ-OH)2O2] is the most
favorable pathway compared to the direct formation of methanol via CH3O*. The formed CH3OH is easily
further oxidized by hydroxyl radicals (˙OH) resulting in non-selective methane oxidation. In contrast, the Fe/
MOR catalyst could lead to a high methanol selectivity of 71.3% in the presence of homogeneous Cu2+
precursor, which efficiently suppressed the over-oxidation of methanol, and a high formic acid selectivity
up to 81–82% at a slightly higher reaction temperature by mildly shifting the oxidation of methanol and
formaldehyde to the target product.
Received 16th August 2019,
Accepted 4th November 2019
DOI: 10.1039/c9cy01640f
rsc.li/catalysis
Recently, the direct conversion of methane to chemicals
using much milder reaction conditions has inspired
1. Introduction
Methane (CH4), the major component of natural gas, shale
gas, methane hydrates, and coal-bed gas, is one of the most
abundantly available carbon-based feedstocks.1,2 There is a
strong demand for transforming it into value-added
chemicals, such as methanol and formic acid, as it is an
economical way to generate these products from the currently
depleting petroleum reserves. Indeed, methanol is a very
desirable clean fuel source, an alternative source for the
generation of electrical energy via fuel cell (FC) technology
applications,3 and an excellent building block in the
production of many chemicals.4–9 The current strategy for the
conversion of methane into chemicals mainly involves an
energy-intensive indirect pathway, in which a synthesis gas
(H2 + CO) is first formed, followed by the catalytic conversion
of the syngas.10–19
researchers as a potential route that would drastically reduce
the processing cost.20–25 A persisting challenge in methane
functionalization is its intrinsic low reactivity as it is
relatively inert owing to the high activation energy of the first
C–H bond,26–28 and the desired products, such as methanol,
are more prone to oxidation than methane.29 Thus, the key
problem in the direct conversion of methane to other
chemicals is the nonselective activation of methane, which
makes it difficult to achieve high selectivity toward the
desired products because of the hardly controllable over-
oxidation.
Due to the grand challenges associated with the direct
conversion of methane to methanol, the development of a
suitable heterogeneous catalyst and environmentally benign
oxidants might practically give a chance to produce valuable
chemicals from methane by selective activation. Hutchings
and co-workers have made great contributions to the mild
oxidation of methane using H2O2 at low temperatures over
various kinds of heterogeneous catalysts, such as Fe-ZSM-5,30
Cu–Fe-ZSM-5,30 AuPd/TiO2,31 AuPdCu/TiO2,32 and AuPd
colloids.33 Tao and co-workers reported Pd1O4 single-sites on
ZSM-5 for the direct conversion of methane to methanol
under mild conditions in the aqueous phase with H2O2,
which exhibited high selectivity for methanol at 86.4%.34
a Department of Chemical Engineering, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, P. R. China.
E-mail: liuxh@jiangnan.edu.cn
b Department of Chemistry, Graduate School of Science, Kyushu University, 744
Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy01640f
‡ These authors contributed equally to this work.
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Moreover, thermodynamic calculations indicated that the
reaction toward direct methanol formation was highly
favorable compared to that of methyl peroxide. Chadwick
and co-workers found that hydrated formaldehyde and
methanediol CH2IJOH)2 in the liquid phase and H2 in the gas
phase could be generated during the selective oxidation of
methane using Fe-ZSM-5 and H2O2.35 Tao and co-workers
reported that mononuclear rhodium species anchored on a
zeolite (Rh-ZSM-5) suspended in an aqueous solution could
directly convert methane to methanol and acetic acid using
O2 and CO under mild conditions.36
2.2 Catalyst characterization
After preparation, the 0.5 wt% Fe/MOR catalysts were
characterized by X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), X-ray absorption near-edge structure
(XANES) and extended X-ray absorption fine structure
(EXAFS), UV-vis diffuse-reflectance spectroscopy (UV-vis DRS),
and high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM).
Powder XRD patterns were collected on a Bruker AXS D2
Advance instrument using a Cu-Kα radiation source (λ =
0.15418 nm). The scans were recorded at 30 kV and 40 mA
and in a 2θ range from 5° to 90° with a step size of 0.02°.
XPS data were collected on a Thermo Fisher ESCALAB 250xi
X-ray photoelectron spectrometer using Al-Kα (1486.8 eV) as
the excitation source at 15 kV and 10 mA. The binding
energies were calibrated using the adventitious C 1s peak at
284.8 eV as the reference. XANES and EXAFS of Fe K-edge
were performed using the BL06 beamline of SAGA-LS (SAGA,
Japan). Synchrotron radiation from the storage ring was
Metal-exchanged zeolites form well-defined metal-oxo
sites37–40 that have shown activity towards the direct
conversion of methane to methanol, as previously
summarized in the case of ZSM-5. However, at present,
only a few closed catalytic systems have been reported for
the direct conversion of methane under mild conditions
by using H2O2 as the oxidant.41 Mordenite (MOR),
a
natural zeolite, is attractive because of its ion exchange
properties, thermal stability, availability and low cost.42 To
monochromatized by
a
Si (111) double-crystal. The
the best of our knowledge,
a metal-exchanged MOR
experiments were conducted in the transmission mode at
room temperature. The samples, catalysts and reference
compounds were pressed into pellets. XANES and EXAFS data
were processed using the REX2000 program (Rigaku Co.). UV-
vis DRS was performed in the range of 200–600 nm on a
Shimadzu UV-3600 Plus Spectrometer with BaSO4 as the
reference. HAADF-STEM images were obtained with an FEI
Talos F200X electron microscope operating at 200 kV. STEM-
EDX spectra were obtained using a super XG1 analysis
system. Prior to measurements, the catalyst samples were
ultrasonicated in high purity ethanol, followed by adding a
drop of the suspension onto a 300 mesh TEM copper grid.
catalyst for mild methane oxidation using H2O2 has not
been reported earlier. In the previous studies, CH2IJOH)2
and HCOOH were always formed as the main byproducts
from the mild oxidation of methane to methanol.30,35
Since formic acid might be formed mainly via the over-
oxidation of methanol and formaldehyde, it is also
expected to achieve high selectivity towards formic acid by
shifting the degree of the oxidation reaction.
In this work, we prepared
a catalyst consisting of
binuclear Fe sites on the extra framework of MOR by using
the relatively simple procedure of solid-state ion exchange,
which could activate methane selectively with H2O2. One of
our targets was to produce the desired products, methanol or
formic acid, selectively. Another goal was to gain the
fundamental understanding of the catalytic mechanism
clearly through the catalytic experiments and catalyst
characterization combined with density functional theory
(DFT) calculations.
2.3 Computational details
All the spin-polarized density functional theory (DFT)
calculations were performed using the Vienna ab initio
simulation package (VASP).43,44 The D3 correction method
(DFT-D3) was employed in order to include the van der Waals
(vdW) interactions,45 which cannot be ignored in zeolites due
to their porous structure. The projector-augmented wave
(PAW) method was used to represent the core–valence
interactions.46 The valence electrons were described based on
a plane wave with an energy cutoff of 400 eV. Generalized
gradient approximation with the Perdew–Burke–Ernzerhof
(GGA-PBE) functional was used to model the electronic
exchange and correlations.47 Electron smearing was
2. Experimental methods
2.1 Catalyst preparation
The Fe species was incorporated in MOR zeolite (also
other supports, such as α-Al2O3, SBA-15 and SiO2) by
solid-state ion exchange. The typical procedure for the
preparation of 0.5 wt% Fe/MOR (H-MOR, with a SiO2/
Al2O3 molar ratio of 25) catalyst is described below. First,
FeIJacac)3 (0.0316 g, 0.09 mmol) was mixed with the
desired amount of MOR (0.995 g) and ground with a
mortar and pestle for 40 minutes. In the second step, the
samples were calcined at 550 °C with a heating rate of 2
°C min−1 in static air or a flow of H2 (100 mL min−1) for
3 h. The prepared catalysts were labelled as ‘Fe/MOR-air’
and ‘Fe/MOR-H2’. In a similar manner, the Cu/MOR, Ni/
MOR and Ru/MOR catalysts were also prepared.
employed via the Gaussian smearing method with
a
consistent smearing width of 0.05 eV. The conjugate gradient
algorithm was used in geometry optimization calculations.
Optimized structures were obtained by minimizing the force
on each ion until it was less than 0.03 eV Å−1. The transition
states (TSs) of the elementary reactions were located using
the climbing-image nudged elastic band (CI-NEB)
method48,49 and were confirmed to have a single imaginary
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frequency. The activation barrier was calculated as the energy
difference between the transition and initial states.
The Gibbs free energy change (ΔG) is defined as:50
majorly HCHO and HCOOH, were detected in the liquid
phase (Fig. S2†). Furthermore, various kinds of metals, such
as Cu, Fe, Ni and Ru, were screened for the oxidation
reaction of methane, of which Fe/MOR-air showed the
highest activity (Fig. 1b). This result indicated that the
aqueous phase oxidation of methane using H2O2 strongly
depended upon the nature of the metal and the support as
the presence of Fe species in MOR was crucial for substantial
catalytic activity. Fe/MOR-air catalysts with higher iron
loadings of 1.0 wt% or 2.0 wt% showed slightly higher
productivity with a similar oxygenate selectivity (Fig. S3†). In
contrast, Fe loading at 0.5 wt% exhibited a markedly higher
yield, which was calculated as moles (products)/moles (metal
iron)/time. Thus, we focused on the 0.5 wt% Fe/MOR catalyst
for subsequent systematic studies. In the case of Fe/MOR-H2,
the result showed that the rate of oxidation of methane was
obviously enhanced compared to that of Fe/MOR-air (Fig.
S4†).
ΔG = ΔEDFT + ΔEZPE − TΔS
where ΔEDFT is the total energy difference calculated by DFT,
ΔEZPE is the zero-point energy correction, and TΔS is the
entropy contribution derived from the standard partition
functions51 or the experimental values.52
The structure of MOR zeolite was modeled according to
the study by Yoshizawa et al.53 The MOR zeolite framework
had a two-dimensional channel system with 12- and 8-MR
channels along the [001] direction and 8-MR side pockets
along the [010] direction.53 The calculated lattice parameters
for MOR unit cell were a = 18.094 Å, b = 20.516 Å and c =
7.514 Å. A previous study has proved that a dimeric Fe
species is the active site for the conversion of CH4 using
H2O2 in zeolites.30 Accordingly, in this study, two Si atoms of
the MOR zeolite framework were replaced by two Al atoms,
resulting in a negative charge of −2, which was then
compensated by the [Fe2IJμ-O)2]2+ dimer species to form a
neutral system. The optimized structure of the MOR-
supported [Fe2IJμ-O)2]2+ dimer species is shown in Fig. S1.†
Although Fe/MOR catalysts were uniquely active in the
direct mild oxidation of methane, the selectivity towards
methanol or formic acid was still relatively low. We,
therefore, attempted at improving the selectivity towards a
single target product. In spite of the copper catalyst
showing no direct effect on methane activation (Fig. 1b),
the addition of the homogeneous Cu2+ species (copperIJII)
nitrate) (Fig. 1c) and the heterogeneous Cu/MOR (Fig. S5†)
to Fe/MOR-H2 shifted the selectivity to methanol by
suppressing its over-oxidation to form HCHO and
2.4 Catalyst testing
The direct oxidation of methane to methanol or formic acid
was carried out using a 100 mL stainless-steel autoclave
reactor. In a typical experiment, the reactor was charged with
20 mL of 0.5 M H2O2 aqueous solution and the desired
amount of catalyst (30 mg). After purging with methane gas
(95% methane and 5% nitrogen) several times to remove the
residual air in the reactor, the autoclave was pressurized to
30 bar using methane gas. The autoclave was then heated to
80 °C and maintained for 60 min with stirring at 800 rpm.
After the reaction, the autoclave was cooled to a temperature
below 5 °C in an ice bath in order to minimize the loss of
volatile products. The resultant solution was centrifuged and
the supernatant was quantified by solvent suppressed 1H
NMR (AVANCE III HD400 MHz, Bruker) using TMSP/D2O as
the internal standard. The gas-phase products were analyzed
by using two integrated online GCs (GC 7820A, Agilent),
which acted as a thermal conductivity detector coupled with
a flame ionization detector. CO2 formation was measured by
using N2 as the internal standard.
HCOOH.30 As
a result, high selectivity for methanol
(71.3%) was obtained when the weight ratio of CuIJNO3)2
to Fe/MOR-H2 was 1.04 : 1.0 (Fig. 1c). Clearly, the presence
of Cu2+ species lowered the rate of oxidation of methane
with H2O2 in the aqueous phase. Despite that, a higher
initial concentration of H2O2 had
a clear effect on
increasing the overall productivity, though the methanol
selectivity was slightly lower (Fig. S6†).
Encouraged by the remarkable enhancement of
methanol selectivity with suppressed over-oxidation, we
investigated the effect of a wide-range temperature on the
productivity and selectivity towards the other desired
product, formic acid. The reaction temperatures from 40
to 90 °C not only resulted in a higher yield of total liquid
products but also markedly increased selectivity for formic
acid, as shown in Fig. 1d. Interestingly, a further increase
in temperature to 100 and 110 °C enhanced the selectivity
for formic acid to as high as 81–82%, accompanied by
the disappearance of CH3OOH and HCHO, which usually
result from the successive oxidation reactions. However,
the yield of the total liquid products obviously decreased
at 120 °C, which was not mainly owing to the further
oxidation of HCOOH to CO2. It is possible that H2O2
decomposition into H2O and O2 was enhanced at higher
reaction temperatures, which decreased the formation of
˙OH, leading to suppressed methane oxidation. This result
also indicated that CH3OOH and HCHO were relatively
easier to oxidize compared to HCOOH.
3. Results and discussion
3.1 Direct partial oxidation of methane
First, methane was partially oxidized with H2O2 in the
aqueous phase using 0.5 wt% Fe catalysts on a series of
different support materials, such as H-MOR, Al2O3, SBA-15
and SiO2, which were prepared by solid-state ion exchange. It
was found that only Fe/MOR-air displayed a significant
activity in the mild oxidation of methane at 80 °C (Fig. 1a).
The oxygenated products, including CH3OOH, CH3OH, and
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Fig. 1 Comparison of the activity and selectivity for partial methane oxidation on the various as-prepared catalysts: (a) 0.5 wt% Fe loaded on
different supports, catalyst calcination in static air; (b) different metals (with the same loading as that of Fe) loaded on H-MOR, calcination
treatment in air; (c) effect of the copperIJII) nitrate trihydrate on the methanol selectivity of the oxidation reaction using 0.5 wt% Fe/MOR-H2; (d)
effect of the reaction temperature on the formic acid selectivity of the oxidation reaction using 0.5 wt% Fe/MOR-H2. Reaction condition: H2O2
solution 20 mL, [H2O2] = 0.5 M, 30 bar of 95% CH4/N2, 30 mg catalyst, and 80 °C for a–c.
3.2 Catalyst characterization
geometry).59 This kind of distortion suggested that there was
a mixture of 3d and 4p states of the metal, in view of a direct
dipole transition of the 3d band into 4p character.60 As
shown in Fig. 2c (the inset is the corresponding enlarged pre-
edge peak; blue line), the Fe/MOR-air sample showed a much
more intense pre-edge peak than that of α-Fe2O3 but was
slightly weaker compared to that of γ-Fe2O3. It is known that
the crystal structure of α-compounds is oriented in a way that
all of the cations are octahedrally coordinated.61 However, in
the γ-compounds, three-quarters of the Fe3+ cations are
octahedrally coordinated, whereas the other quarter of
cations are tetrahedrally coordinated.62 The pre-edge feather
of the Fe/MOR-air sample indicated the major octahedral
coordination of the Fe species in MOR mixed with a small
amount of tetrahedrally coordinated Fe species, which is well
in agreement with the fact that Fe primarily exists in
octahedral coordination in phyllosilicates. In contrast, the
substitution of Fe for Si in the framework of MOR results in
tetrahedral coordination, but is minimal.63 Fig. 2c displays
the comparison of the XANES spectra of the Fe/MOR-air
sample and the reference materials (α-Fe2O3 and γ-Fe2O3),
indicating the predominance of the Fe3+ species in the
sample, based on the edge position at ∼7134 eV. In the case
of the Fe/MOR-H2 sample, several obviously different energy
positions were observed, such as 1) a clear shift of the edge
position to a lower energy value at ∼7127 eV,64 2) an increase
in the intensity of the pre-edge peak at ∼7113 eV,65 and 3) a
In order to establish the relationship between the catalyst
structure and reaction performance, the nature of the Fe
species in the samples labelled as 0.5 wt% Fe/MOR-air and
0.5 wt% Fe/MOR-H2 was systematically identified using
various characterization techniques.
As shown in Fig. 2a, the diffraction peaks assigned to iron
oxides or metallic Fe were not found in the XRD spectra,54–56
suggesting that the Fe species were highly dispersed in MOR
zeolite. Indeed, it was difficult to observe the diffraction
peaks at a low iron loading despite the iron nanoparticle size
of 6–8 nm. XPS measurements confirmed the presence of
Fe3+ as the major species in the Fe/MOR-air catalyst as the
binding energies of Fe 2p3/2 and Fe 2p1/2 were obtained at
712.0 eV and 725.5 eV, respectively (Fig. 2b).57 For the Fe/
MOR-H2 catalyst heat-treated in hydrogen, the remarkably
low intensity of the binding energy for Fe 2p3/2 and its shift
to a slightly lower value at 711.7 eV indicated the reduction
of Fe3+ species to a lower-valence Fe species.58
The XANES spectrum of the Fe K-edge was used to
determine detailed information on both the oxidation state
and coordination geometry of the Fe species in MOR zeolite.
The pre-edge peak at 7116 eV could be assigned to the
forbidden 1s → 3d transition, and its intensity in the XANES
spectra was caused by the distortion degree from the
octahedral symmetry (or a shift to tetrahedral coordination
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Fig. 2 The structural characterization of H-MOR and the as-prepared 0.5 wt% Fe/MOR-air and 0.5 wt% Fe/MOR-H2 catalysts: (a) XRD patterns of
the above catalysts; (b) XPS spectra of the (a) 0.5 wt% Fe/MOR-air and (b) 0.5 wt% Fe/MOR-H2 catalysts in the Fe 2p region; (c) XANES spectra of
α-Fe2O3, γ-Fe2O3, 0.5 wt% Fe/MOR-air and 0.5 wt% Fe/MOR-H2 at the K-edge. The inset is the magnified image of pre-edge XANES spectra; (d)
Fourier transform magnitudes of the k3-weighted EXAFS data of 0.5 wt% Fe/MOR-air and the reference samples, including Fe foil, FeO and
γ-Fe2O3; (e) the k3-weighted EXAFS oscillations for the 0.5 wt% Fe/MOR-air catalyst and the γ-Fe2O3 reference; (f) DR UV-vis absorbance spectra
of the as-prepared samples.
decrease in the intensity of the pre-edge peak at ∼7116 eV.
These results indicated that some Fe2+ was present in the
sample, which is consistent with the XPS data (Fig. 2b, case
b).
bonds, respectively. Compared to γ-Fe2O3, Fe/MOR-air
exhibited a much smaller peak at 2.66 Å for the Fe–O–Fe
bonds, originating from two very close Fe atoms bridged by an
oxygen atom. This result indicated that iron oxide was highly
dispersed on MOR zeolite, which was reflected by the XRD
and XPS data (Fig. 2a and b). However, the peak intensity for
the Fe–O bonds of Fe/MOR-air was similar to that in γ-Fe2O3.
We noted that there were almost no Fe–Fe bonds in Fe/MOR-
air. The Fe K-edge EXAFS oscillations for Fe/MOR-air and the
reference materials (γ-Fe2O3 and Fe) are shown in Fig. 2e and
S7.† Different features were observed compared to the
references because of the dampening of oscillation in the
catalyst. This is consistent with the results reported by Corrias
et al.66 The EXAFS oscillation of the nano-sized γ-Fe2O3 was
more dampened than that of the γ-Fe2O3 micro-crystal. Given
The EXAFS experiments were performed to identify the
coordination environment of the Fe atoms in MOR zeolite. As
shown in Fig. 2d, the Fourier transform magnitudes of the k3-
weighted EXAFS data of the Fe/MOR-air sample displayed a
coordination environment, clearly different from the reference
samples Fe foil, FeO and γ-Fe2O3. For the metallic Fe foil, the
main peak at 2.18 Å was assigned to the Fe–Fe bonds. In
contrast to the Fe foil, two distinct peaks at 1.62 and 2.65 Å
were found for FeO due to the presence of Fe–O and Fe–O–Fe
coordination, respectively. Similarly, the two main peaks at
1.45 and 2.77 Å in γ-Fe2O3 represented the Fe–O and Fe–O–Fe
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that the EXAFS oscillation in the high k region mostly
originates from heavy atoms,67 the dampening of oscillation in
Fe/MOR-air suggested that the iron oxide molecules were
highly dispersed on MOR zeolite. Table S1† shows the
structural parameters for Fe/MOR-air, which suggests that the
Fe–Fe coordination number was low (1.155 0.368) with an
average Fe–Fe distance of 3.328 0.021 Å and an average Fe–O
separation of 1.910 0.025 Å. These values show that dimeric
Fe species exist in the sample, according to previously reported
data.68
The UV-vis DR spectra were used to determine the nature
of the Fe sites in Fe/MOR-air and Fe/MOR-H2. As shown in
Fig. 2f, the Fe species in MOR zeolite resulted in four
different UV-active species.69,70 They are the tetrahedral Fe3+
in the framework (230 nm), the singly dispersed Fe3+ in the
extra framework octahedral positions (275 nm), the
octahedral Fe3+ in the oligomeric clusters inside the zeolite
channels (355 nm) and the larger iron oxide clusters located
on the external surface of the zeolite crystals (>450 nm).71,72
For the Fe/MOR-H2 catalyst, it was clear that the UV-vis
absorption bands at 275 nm and 355 nm had notably
increased, indicating the presence of more extra framework
octahedral Fe3+ species. Accordingly, the amount of the large
iron oxide clusters on the surface (>450 nm) obviously
decreased, which might be due to the migration of the
surface Fe species into the zeolite channels.
HADDF-STEM images were acquired to visualize the
clustered Fe species on MOR zeolite (Fig. 3). It was
interesting that the calcination treatment in static air led to
two kinds of particle size distribution at 1.5 0.5 nm and 6.9
1.9 nm, respectively (Fig. 3a). It suggested that the Fe
species was not homogeneously dispersed in MOR zeolite. In
contrast, the calcination treatment in H2 caused
a
homogeneous dispersion of Fe species as the average particle
size of iron oxide was at 2.0 0.5 nm (Fig. 3b and S8†). The
above results were also consistent with the UV-vis DR spectra,
indicating the presence of large clusters of iron oxide over
the Fe/MOR-air catalyst (Fig. 2f).
3.3 Reaction mechanism
Based on these characterization results, we concluded that the
Fe species in MOR zeolite existed mainly as the octahedral
dimeric Fe3+ species and/or the singly dispersed octahedral
Fe3+ in the extra framework positions. According to the
experimental results and the UV-vis DR spectra, it seems that
the oxidation reaction of methane occurs on the extra
framework octahedral dimeric Fe3+ species (Scheme 1, 1). To
Fig. 3 HAADF-STEM images, STEM-EDX elemental mappings and particle size distributions of the (a) 0.5 wt% Fe/MOR-air and (b) 0.5 wt% Fe/
MOR-H2 catalysts.
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[Fe2(μ-O)2] + H2O2 → [Fe2(μ-O)2O2]
(5)
3.3.2 CH3OH formation mechanism. We considered two
different CH3OH formation mechanisms, namely direct
pathway and CH3OOH intermediate pathway, as shown in
Fig. 5 and S10.† The Gibbs free energy change (ΔG) at the
experimental temperature (80 °C) for every elementary
reaction step was calculated. In the direct pathway, CH4
adsorbs weakly with an adsorption energy of −0.14 eV and
then, reacts with the terminal O species in [Fe2IJμ-OH)2O2] to
form CH3O* with a barrier of 0.87 eV. Subsequently, an
adjacent H* combines with CH3O* to form CH3OH*.
However, this CH3OH* species strongly binds to the surface.
The desorption of CH3OH* requires a high energy of 1.61 eV,
indicating that CH3OH* desorption is rather difficult in the
direct pathway. Therefore, CH3OH production via the direct
pathway is unfavorable. In the CH3OOH intermediate
pathway, after the formation of the CH3O* species (Scheme 1
, 3), H2O2 is adsorbed at the active site with an adsorption
energy of −0.24 eV. Then, CH3O* reacts with H2O2* to form
the CH3OOH* intermediate (Scheme 1, 4). This step has an
activation barrier of 0.80 eV. The removal of one O in CH3-
OOH* intermediate has a low barrier of 0.35 eV, leading to
the formation of the CH3OH* species with the generation of
5 in Scheme 1. The desorption energy for CH3OH* was 0.91
eV, which is much lower than that in the direct pathway,
indicating that CH3OH production via the CH3OOH
intermediate pathway is preferred. H2O2 is beneficial for the
formation of the CH3OOH intermediate and thus, promotes
the production of CH3OH. These results also suggest that
CH3OH* desorption is the rate-limiting step in methanol
production.
HCHO and HCOOH can also be formed as products
through the CH3OOH* intermediate. CH3OOH* undergoes
dehydrogenation to form the CH2OOH* species with an
activation barrier of 0.66 eV (Fig. 5, red line). Then, the CH2-
OOH* species continues to undergo dehydrogenation to form
HCOOH (Fig. 5, black line). The cleavage of the O–OH bond
in CH2OOH* leads to HCHO formation with the generation
of Fe–OH*. Compared with the CH3OH formation process
through the CH3OOH* intermediate, the activation barrier
for the dehydrogenation of CH3OOH* to form CH2OOH* is
almost twice as high, indicating that the CH3OOH*
intermediate would prefer to form CH3OH rather than HCHO
and HCOOH. Another reaction pathway is shown for the
formation of HCOOH. CH3O* can react with H2O2* to form
CHO*, and then, CHO* reacts with an adjacent OH* to form
HCOOH (Fig. 5, green line).
Scheme 1 Proposed catalytic cycle for the liquid-phase oxidation of
methane to methanol using H2O2 over the octahedral dimeric Fe3+
species in H-MOR.
gain an understanding of the nature of the active site, DFT
calculations were used to confirm the reaction mechanism.
3.3.1 Identifying the reasonable structure of the active site
under the reaction conditions. H2O2 might affect the
structure of the active site in the reaction. In order to obtain
the most reasonable structure of the active site under the
reaction conditions, based on the structure of [Fe2IJμ-O)2]
(Fig. 4(a)), five alternative model structures (Fig. 4(b)–(f))
were evaluated and optimized, which are denoted as [Fe2IJμ-
O)2O], [Fe2IJμ-O)2O2], [Fe2IJμ-O)2IJOH)2], [Fe2IJμ-OH)2] and [Fe2IJμ-
OH)2O2], respectively. These five structures were formed via
the chemical eqn (1)–(5), respectively. We calculated the
Gibbs free energy changes (ΔG) of these five chemical
equations, and the results are shown in Fig. S9.† It could be
seen that [Fe2IJμ-OH)2O2] was the most thermodynamically
stable structure under the reaction conditions. Furthermore,
in the [Fe2IJμ-OH)2O2] structure, the Fe center was five-
coordinated, which is in good agreement with the EXAFS
results (Table S1†), indicating that the [Fe2IJμ-OH)2O2] model
(Scheme 1, 2) is reasonable. We thus used this model to
represent the active site in the following DFT calculations.
Catalyst regeneration in the CH3OH, HCHO, and HCOOH
formation processes was calculated, and the results are
shown in Fig. S11–S14.† They indicated that the catalytic cycle
could be completed in all these reaction pathways because
the energy barriers are not high. The structures of the
transition states in the CH3OH, HCHO, and HCOOH
formation processes are presented in Fig. S15.† We also
considered the H transfer process from the bridge OH group
[Fe2(μ-O)2] + H2O2 → [Fe2(μ-O)2O] + H2O
[Fe2(μ-O)2] + H2O2 → [Fe2(μ-O)2O2] + H2
[Fe2(μ-O)2] + H2O2 → [Fe2(μ-O)2(OH)2]
[Fe2(μ-O)2] + H2O2 → [Fe2(μ-OH)2] + O2
(1)
(2)
(3)
(4)
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Fig. 4 Calculated structures of MOR-supported (a) [Fe2IJμ-O)2], (b) [Fe2IJμ-O)2O], (c) [Fe2IJμ-O)2O2], (d) [Fe2IJμ-O)2IJOH)2], (e) [Fe2IJμ-OH)2] and (f)
[Fe2IJμ-OH)2O2]. Red, yellow, purple, grey blue and white balls represent O, Si, Al, Fe and H atoms, respectively.
to one of the terminal bare O atoms, as shown in Fig. S16
and S17.† The relatively high energy barrier indicated that
this process is unfavorable.
Indeed, oxidation of CH3OH was carried out over Fe/MOR-H2
under the same conditions as in Fig. S4,† which confirmed
that CH3OH could be efficiently oxidized into HCHO and
HCOOH as 90.7% of 500 μmol CH3OH was converted into
121.6 μmol HCHO, 267.8 μmol HCOOH and 64 μmol CO2,
respectively. The time-on-stream analysis of the liquid-phase
oxidation of methane clearly showed that methyl
hydroperoxide (CH3OOH) was first formed as the primary
product (Fig. 6). Then, formaldehyde and formic acid were
formed in sequential reactions. Further oxidation of HCOOH
to CO2 is relatively difficult as a longer reaction time resulted
in a slight increase in the selectivity for CO2. This hypothesis
was also demonstrated by experiments using different H2O2
concentrations and varied reaction temperatures (Fig. S6†
3.4 Selectivity towards HCHO and HCOOH
Based on the discussion on the reaction mechanism derived
from DFT calculations, the primary reaction product
methanol (CH3OH) should be obtained at a much higher
quantity than those of formaldehyde (HCHO) and formic acid
(HCOOH), which are directly obtained from the conversion of
the CH3OOH* intermediate. However, it was observed that
formaldehyde and formic acid were mainly formed at 80 °C
and [H2O2] = 0.5 M (Fig. S6†). This result indicated that the
over-oxidation of methanol might be responsible for the
higher selectivity towards formaldehyde and formic acid.
Fig.
6 Time-on-stream profile of the liquid-phase oxidation of
methane with H2O2 and 0.5 wt% Fe/MOR-H2. Key: ■ = methanol
(CH3OH); ● = methyl hydroperoxide (CH3OOH); ▲ = formic acid
(HCOOH); ▼ = formaldehyde (HCHO) ♦ = CO2. Reaction conditions:
H2O2 solution 20 mL, [H2O2] = 0.5 M, 30 bar of 95% CH4/N2, 80 °C,
and 30 mg catalyst.
Fig. 5 Calculated energy profiles for CH3OH formation via a direct
pathway and the CH3OOH intermediate, and HCOOH formation and
HCHO formation using Fe/MOR.
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Scheme 2 Proposed reaction pathway controlling the oxidation of methane into methanol and formic acid using H2O2 over the Fe/MOR catalyst.
and 1d). In specific, higher concentrations of H2O2 resulted Acknowledgements
in lower selectivity towards methanol, and higher reaction
The authors are grateful for the financial support from the
temperatures resulted in higher selectivity for formic acid by
National Natural Science Foundation of China (21576119,
promoting the over-oxidation of reaction intermediates
21878127, 21706098, 21606108, 21802054), the Natural
(Scheme 2). In contrast, to obtain high selectivity for
Science Foundation of Jiangsu Province (BK20180587), and
methanol, the over-oxidation reaction should be minimized
the Fundamental Research Funds for the Central Universities
as much as possible. As shown in Fig. 1c, the addition of
(JUSRP11813, JUSRP11709, JUSRP51720B).
Cu2+ could efficiently increase methanol selectivity, which
might be due to a decrease in the concentration of hydroxyl
radicals (˙OH) (Scheme 2). Indeed, the formed ˙OH radicals
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