Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5 catalysts in a flow reactor

Article abstract of DOI:10.1016/j.cattod.2015.09.011

The selective oxidation of methane to methanol is a key challenge in catalysis. Iron and copper modified ZSM-5 catalysts are shown to be effective for this reaction using H₂O₂ as the oxidant under continuous flow operation. Co-impregnation of ZSM-5 with Fe and Cu by chemical vapour impregnation yielded catalysts that showed high selectivity to methanol (>92% selectivity, 0.5% conversion), as the only product in the liquid phase. The catalysts investigated did not deactivate during continuous reaction, and methanol selectivity remained high. The effect of reaction pressure, temperature, hydrogen peroxide concentration and catalyst mass were investigated. An increase in any of these led to increased methane conversion, with high methanol selectivity (≥73%) maintained throughout. Catalysts were characterised using DR-FTIR, DR-UV-Vis and ²⁷Al MAS-NMR spectroscopy.

Full text of DOI:10.1016/j.cattod.2015.09.011

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Continuous selective oxidation of methane to methanol over Cu- and
Fe-modified ZSM-5 catalysts in a flow reactor
Jun Xua,b, Robert D. Armstrong , Greg Shaw , Nicholas F. Dummer , Simon J. Freakley ,
a
a
a
a
a
a,∗
Stuart H. Taylor , Graham J. Hutchings
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom
National Centre for Magnetic Resonance in Wuhan, State Key Laboratory Magnetic Resonance and Atomic and Molecular Physics,
a
b
Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2015
Received in revised form 24 August 2015
Accepted 2 September 2015
Available online xxx
The selective oxidation of methane to methanol is a key challenge in catalysis. Iron and copper modified
ZSM-5 catalysts are shown to be effective for this reaction using H2O2 as the oxidant under continuous
flow operation. Co-impregnation of ZSM-5 with Fe and Cu by chemical vapour impregnation yielded
catalysts that showed high selectivity to methanol (>92% selectivity, 0.5% conversion), as the only prod-
uct in the liquid phase. The catalysts investigated did not deactivate during continuous reaction, and
methanol selectivity remained high. The effect of reaction pressure, temperature, hydrogen peroxide
concentration and catalyst mass were investigated. An increase in any of these led to increased methane
conversion, with high methanol selectivity (≥73%) maintained throughout. Catalysts were characterised
Keywords:
Methane
Methanol
Selective oxidation
ZSM-5
2
7
using DR-FTIR, DR-UV-Vis and Al MAS-NMR spectroscopy.
© 2015 Elsevier B.V. All rights reserved.
Hydrogen peroxide
Chemical vapour impregnation
1
. Introduction
phase oxidation of methane over Cu-modified zeolites activated in
O2 [8–14] or NO/N O [15] has also been studied, with proposed
2
Natural gas has been hailed as a bridging feedstock for society’s
active sites for the O treated catalysts not unlike the binuclear Cu
2
transition away from a petroleum-dependent economy [1]. How-
ever, the direct catalytic upgrading of its principal components;
methane and ethane, to oxygenated products has yet to be realised
under mild, environmentally benign conditions. Whilst processes
for conversion of methane to higher value products have been
site found in methane monooxygenase enzymes. Meanwhile non-
catalytic activation using N O/O2 treated Fe/ZSM-5 has also been
2
reported (with ␣-oxygen as the in situ generated oxidant) [16–18].
Periana and co-workers have extensively studied the oxidation
of methane in acidic media-yielding methyl esters at temper-
−
1
◦
commercialised, high substrate stability (ꢀHC–H = 439.57 kJ mol
)
atures of <200 C [19–21]. These indirect, methyl ester-yielding
means that harsh conditions are often employed for activation [2,3].
Direct upgrading of methane via oxidation to a more energy dense
product, such as methanol, is therefore an attractive prospect. Such
a process must operate under mild reaction conditions to pre-
vent further oxidation of methanol to products like formic acid
processes typically require homogeneous metal complexes which
activate methane through electrophilic attack of the C H bond,
affording high reaction selectivity yet incurring additional hydrol-
ysis steps to yield methanol. Such homogeneous processes have
been extensively reviewed by Periana et al. and Shilov et al.
[22,23] and heterogenised by Schüth and co-workers [24,25]. A
number of homogeneously [26–28] and heterogeneously [29–32]
catalysed aqueous processes for the direct oxidation of methane
and CO . Several approaches to methane oxidation have been
2
reported. The enzymatic direct oxidation of methane to methanol
has been studied, with metalloenzymes shown to perform this
reaction selectively under mild conditions with molecular oxygen
as the terminal oxidant [4,5]. This has stimulated the search for syn-
thetic analogues of these enzymes’ active sites [6,7]. Indeed, the gas
with H O2 have recently been reported. These benefit from the
2
clean decomposition of the oxidant to H O as an environmentally
2
benign byproduct. The importance of efficient, selective utilisation
of methane as a feedstock for the synthesis of bulk chemicals is
explored in recent reviews of the catalytic upgrading of methane
[33–37]. Unfortunately despite extensive research in the field, no
approach has yet been deemed commercially viable, with methanol
∗ Corresponding author.
E-mail address: Hutch@cardiff.ac.uk (G.J. Hutchings).
http://dx.doi.org/10.1016/j.cattod.2015.09.011
0
920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Xu, et al., Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5
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Scheme 1. Reaction scheme for methane oxidation over ZSM-5 (30) catalysts as proposed by previous studies [31,41].
still produced through an energy intensive two-step process which
proceeds via synthesis gas.
Prior to testing, the catalyst was pressed into wafers at a pres-
sure of 20 t/in . Wafers were then sieved to form pellets of uniform
2
It has previously been reported that ZSM-5 materials contain-
ing trace amounts of iron (as dimeric ␮-oxo-hydroxo iron species)
can catalyse the direct conversion of methane and ethane to
oxygenated products, utilising hydrogen peroxide as the oxidant
dimension (20/40 mesh, 800–400 ␮m).
2.2. Catalyst characterisation
[
31,32,38–40]. The oxidation of methane to methanol was shown
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) was performed using a Bruker Tensor 27 spectrometer fit-
to proceed via formation of methylhydroperoxide (CH OOH), and
deep oxidation to formic acid and CO2 was observed [31]. Appre-
ciable methane conversion (10%) and high oxygenate selectivity
(
Furthermore, incorporation of Cu into the reaction as either a
homogeneous additive or heterogeneous component of the zeolite
catalyst allows tuning of reaction selectivity to favour methanol
3
ted with a liquid N – cooled MCT detector. Samples were housed
2
in a Praying Mantis high temperature diffuse reflection environ-
mental reaction chamber (HVC-DRP-4) fitted with calcium fluoride
windows. Samples were pre-treated prior to spectra acquisition by
◦
>90%) have been reported at temperatures as low as of 50 C.
2+
◦
◦
−1
−1
heating at 200 C (10 C min ) in a flow of N2 (10 ml min ) for
−1
−1
,
1
4
h. Scans were collected over the range 4000 cm to 1500 cm
cm resolution, 64 scans against a KBr background.
(
>85%) as the major product. Catalytic reaction pathways deter-
−1
27
mined for the oxidation of methane with H O2 are shown in
2
Al solid-state NMR experiments were carried out at 7.05 T on
Scheme 1 [31]. Previous studies have suggested that the intrinsic
activity of ZSM-5 is derived from the presence of octahedral (extra
framework) Fe species, formed during high temperature activation
of the zeolite. The role of Cu2+ in effecting high primary product
selectivity has been studied, and is attributed to catalytic termina-
tion of hydroxyl radicals [41].
In this paper we aim to translate the catalyst system from oper-
ation in a batch autoclave to a continuous flow reactor in order
to further study catalyst deactivation and determine whether high
selectivity to methanol might be achieved under mild reaction con-
ditions.
a Varian Infinityplus 300 spectrometer. The resonance frequencies
were 299.78 MHz and 78.11 MHz for 1H and 27Al respectively. A
27
4
mm double resonance probe was employed to acquire NMR
27
spectra. The Al MAS spectra were acquired using a one-pulse
sequence with a short radio frequency (rf) pulse of 0.25 ␮s (cor-
responding to a ␲/18 flip angle) and a pulse delay of 0.8 s. The
magic angle spinning rate was set to 10 kHz. The chemical shift
was referenced to a solution of 1 M Al(NO ) .
3
3
UV–vis spectra were collected on a Varian 4000 UV–vis spec-
trophotometer. Scans were collected over a wavelength range
−1
2
00–850 nm, at a scan rate of 150 nm min . Background scans
were taken using a high purity PTFE disc.
2
. Experimental
2.3. Catalyst performance evaluation
2.1. Catalyst preparation
Catalyst performance was measured in a continuous flow fixed
Fe and Cu were impregnated onto ZSM-5 (Zeolyst,
bed stainless steel reactor. A reactor schematic is shown in Fig. S1.
An aqueous feed containing hydrogen peroxide (Sigma–Aldrich,
typically 0.123 M) was controlled by an HPLC pump (Waters) and
methane (Air Products, 99.9%) flow was controlled by a mass flow
controller (Brooks). Both were fed down through the catalyst bed
(VBed = 3.6 ml) which was composed of layers of pelleted catalyst
and SiC according to the method reported by Al-Dahhan et al. [43].
The stainless steel reactor had a total length of 13 cm and internal
diameter of 1.6 cm. Liquid and gaseous products were separated in
a high pressure liquid gas separator (VTotal = 18 ml) and collected
periodically for analysis over a 5 h period. Reactor pressure was
maintained using a back pressure regulator.
SiO /Al O = 23, 30 or 80) via chemical vapour impregnation
2
2
3
(
CVI) according to the procedure previously reported [39,42].
◦
◦
−1
NH -ZSM-5 was calcined in a flow of air (550 C, 20 C min ,
4
3
h) to yield H-ZSM-5. This was then either (i) activated in static
◦
air (3 h, 550 C) and tested without further modification or (ii)
modified through chemical vapour impregnation. The procedure
for simultaneous impregnation of ZSM-5 with 1.5 wt% Fe and
1
.5 wt% Cu follows;
H-ZSM-5 (3.5 g) was dried at 150 C for 2 h under contin-
◦
uous vacuum. Once dried, H-ZSM-5 (1.95 g) was added to a
Schlenk flask followed by Cu(acac)2 (Sigma–Aldrich, 99.9% purity,
0
0
.103 g, 0.393 mmol) and Fe(acac) (Sigma–Aldrich, 99.9%, 0.158 g,
CO2 was quantified by GC using a Varian 450-GC instrument
fitted with a methaniser-FID and TCD and a CP-Sil 5CB capil-
lary column (50 m × 0.33 mm). Liquid products were quantified
by solvent suppressed 1H NMR on a Bruker Ultrashield 500 MHz
spectrometer. A sealed glass capillary containing TMS in CDCl3,
calibrated against commercial standards, was added to the NMR
3
.448 mmol). Following physical mixing of the metal precursors
◦
and zeolite, the dry mixture was heated to 150 C under continu-
ous vacuum (ca. 10 mbar) for 2 h (1 h at 140 C for monometallic
Cu catalysts). The sample was then allowed to cool to ambient
−3
◦
◦
◦
−1
temperature and calcined in static air (550 C, 20 C min , 3 h).
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tube as an external standard. Hydrogen peroxide was quantified
through titration against an acidified solution of Ce(SO ) of known
4
2
concentration, with Ferroin indicator.
3
. Results and discussion
3
.1. Characterisation of catalysts
(
a)
b)
It has previously been reported that trace iron in ZSM-5, present
(
as cationic ␮-oxo-hydroxo species located at exchange sites, can
catalyse the oxidation of methane to methanol by H O2 [31,41].
Increased catalyst productivity can also be achieved through post-
synthesis addition of Fe to ZSM-5 (30) through various techniques
(c)
2
(d)
(e)
[
31,42]. One post-deposition method which has been success-
fully applied is CVI, a solvent and halide-free vapour deposition
technique, which can be applied in the preparation of a range of
supported catalysts [39,42,44]. Deposition of Fe onto ZSM-5 (30) is
reported to lead to formation of a porous FeOy film on the zeolite
surface and within the micropores, whilst Cu deposits as discreet
nanoparticles [39]. Both metals are also present as ion exchanged
species at Brønsted acid sites [39].
120
100
80
60
40
20
0
-20
ppm
Fig. 2. 27Al MAS-NMR spectra showing (a) H-ZSM-5 (30), (b) 1.5% Cu/ZSM-5 (30),
c) 1.5% Fe/ZSM-5 (30), (d) 0.4% Fe 0.4% Cu/ZSM-5 (30) and (e) 1.5% Fe 1.5% Cu/ZSM-5
30).
(
(
Diffuse reflectance FTIR spectra for a series of Fe-, Cu- and FeCu-
ZSM-5 catalysts prepared by CVI are shown in Fig. 1. Following
deposition of metals, the intensity of the band attributed to OH
from paramagnetic Fe species and framework nuclei (27Al) occurs.
This leads to the decrease in the Td Al signal. This is also the case
groups coordinated to Td Al3+ in framework positions (3607 cm
−1
)
27
decreased relative to the bare ZSM-5 [45,46]. This is consistent with
cation exchange at Brønsted sites within the zeolite. In particu-
lar, comparison of spectra 1(d) and 1(e) shows increased exchange
with increased bimetallic loading. A small increase in intensity of
the band attributed to OH groups coordinated to extra-framework
when Cu is deposited onto ZSM-5 (30), where a loss in the inten-
sity of the resonance at 55 ppm suggests speciation of copper as
exchanged Cu2+. Evolution of extra framework Oh Al sites (0 ppm)
in spectra (Fig. 2c–e) is consistent with FTIR spectroscopy studies
and suggests a low degree of Al migration from framework (Td) to
extra framework (Oh) sites.
Fe speciation on ZSM-5 is known to give rise to four UV-active
species. These absorb at; 200–250 nm (isolated Fe3+ in framework
sites), 250–350 nm (isolated or oligomeric extra framework Fe
species in zeolite channels), 350–450 nm (iron oxide clusters) and
>450 nm (large surface oxide species) [52,53]. Monometallic Fe and
bimetallic FeCu catalysts (Fig. 3 spectra c–e) show absorbance over
the range of wavelengths. Such broad speciation of deposited iron
as FeOy on the zeolite surface and within its micropores is consis-
tent with previous studies on CVI catalysts [39,42].
−1
T-atom (3660 cm ) is also observed, and indicates migration of
Fe or Al to extra framework sites when metal impregnated ZSM-5
catalysts are heat treated [47,48].
27Al MAS NMR spectra for the same ZSM-5 catalysts (Fig. 2)
are in agreement with DRIFTS characterisation. Addition of Fe and
Cu has no effect upon the MFI framework. However, a decrease in
27
intensity of the resonance for Td Al in framework sites (55 ppm)
was apparent [49]. This suggests the presence of paramagnetic Fe
species sited at ion exchange positions [50,51], which is consis-
tent with the proposed dimeric ␮-oxo-hydroxo iron active site. The
bridging hydroxyl group (SiOHAl) is the ion exchange position on
which the pseudocontact interaction between unpaired electrons
4000
3500
3000
(
e)
d)
(
e)
(
(d)
(
c)
b)
(
(
c)
(
a)
(b)
a)
(
4000
3500
3000
200
350
500
650
800
-
1
Wavenumber / cm
Wavelength / nm
Fig. 1. DRIFTS spectra of in the O–H vibration region showing (a) H-ZSM-5 (30), (b)
1
1
.5% Cu/ZSM-5 (30), (c) 1.5% Fe/ZSM-5 (30), (d) 0.4% Fe 0.4% Cu/ZSM-5 (30) and (e)
.5% Fe 1.5% Cu/ZSM-5 (30).
Fig. 3. UV–vis spectra showing (a) H-ZSM-5 (30), (b) 1.5% Cu/ZSM-5 (30), (c) 1.5%
Fe/ZSM-5 (30), (d) 0.4% Fe 0.4% Cu/ZSM-5 (30) and (e) 1.5% Fe 1.5% Cu/ZSM-5 (30).
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A gradual decrease in both surface area and micropore volume
was observed for catalysts with increasing metal loading relative
to ZSM-5 (30) (Table 1). This was consistent with the decreasing
ZSM-5 (30) content per gram of catalyst following deposition of
metals.
3.2. Flow reactor studies
For initial studies we tested H-ZSM-5 (30) and modified vari-
ants thereof, as these showed high intrinsic activity for methane
activation in batch reactor studies [31,41,42]. Productivities of
−1
−1
7
.4 molMethane converted kgcat
h were previously reported for H-
ZSM-5 (30) in a batch reactor under the following test conditions;
◦
2
7 mg catalyst, 0.5 M H O /5000 ␮mol, 30 bar CH , 50 C, 0.5 h
2
2
4
[
31]. When tested under continuous operation the same cata-
−
1
−1
lyst shows a productivity of 0.08 molMethane converted kgcat
h
(Table 1). Decreased catalyst productivity implies diffusion lim-
itation under the continuous flow regime. Under stirred batch
conditions a far lower catalyst mass (27 mg) was used, at compa-
rable oxidant and substrate concentrations. Under flow conditions
due to the high catalyst mass (1.5 g) the reaction is operating out-
side of the kinetic regime. Total oxygenate selectivity (combined
CH OOH, CH OH and HCOOH selectivity) is comparable between
3
3
reactor regimes (92% under flow vs 95% in batch [31]) though
secondary oxidation products are favoured under continuous con-
ditions, with CH OOH, CH OH and HCOOH selectivities of 1.6,
3
3
8
1
.7 and 81.6% respectively. These selectivities are compared with
7.7, 15.4 and 44.0% reported in batch studies [31]. High formic
acid selectivity is consistent with the catalytic nature of the oxi-
dation of methanol to formic acid coupled with the relatively
high catalyst: product ratio observed in the fixed bed reactor. An
H O used/products ratio of 10:1 suggests efficient oxidant utilisa-
2
2
tion under flow conditions, as this compares with 24:1 previously
reported under batch conditions [39]. The effect of increased sub-
strate and oxidant concentration within the catalyst bed is further
explored later in this paper.
2
+
Addition of Cu
shifts the product distribution to favour
−
1
methanol (89.1% selectivity) with comparable TOF (h ) (Table 1)
to the proton form zeolite. We therefore suggest that the catalysis
is consistent with previous studies [31,38,41]. Post synthesis depo-
sition of active metal (1.5 wt% Fe) to ZSM-5 (30) leads to increased
H O decomposition and effects a decrease in reaction rate. This
2
2
is in poor agreement with batch studies [31]. With 98% oxidant
decomposition it is probable that the reaction is operating under
oxidant lean conditions towards the end of the catalyst bed, and this
is effectively slowing the rate of reaction. Through co-deposition
of Fe and Cu (1:1 wt ratio, 3 wt% total loading) the productivity
−1
−1
) can be main-
of H-ZSM-5 (30) (0.08 mol Methane converted
tained whilst also affording high methanol selectivity (92.2%). In
this case high H O decomposition (92.9%) is favourable, as it pre-
h
2
2
vents deep oxidation to CO farther through the catalyst bed. Once
2
more the molar ratio of H O decomposed: products formed were
2
2
comparable to previous studies at 26:1. Surprisingly, no primary
product (CH OOH) was observed in the product stream of Fe- and
3
FeCu/ZSM-5 catalysed reactions. This is likely to be due to the high
Fe: product ratio, as methylhydroperoxide is known to undergo
facile catalytic conversion to methanol over Fe/ZSM-5 [41].
Although monometallic Cu/ZSM-5 (30) showed comparable
rates of methane conversion, methanol selectivity and higher TOF
−
1
(
h
) values per molFe (Table 1), previous batch studies have
shown bimetallic FeCu/ZSM-5 (30) catalysts to be more produc-
tive when this reaction is performed under non-diffusion limited
conditions [31]. The bimetallic catalyst 1.5% Fe 1.5% Cu/ZSM-5
(
30) was assessed for on-line stability over 10 h of continu-
ous operation (Fig. 4). For this, and further studies, a lower
bed loading (0.25 g) was used to minimise diffusion limitations.
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1
00
0.5
0.4
0.3
0.2
0.1
0.0
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
2
4
6
8
10
Time / h
˜
CH
3
OH, p CO
2
, £ Methane Conversion
Fig. 4. Temporal analysis of methane conversion and reaction selectivity over a
0 h period of assessment. Test conditions; 0.25 g catalyst, P(CH4) = 20 bar, Flow
1
−
1
−
1
◦
(
CH4) = 10 ml min , 0.25 ml min of 0.123 M H2O2/H2O, 50 C.
Following a 1 h stabilisation period, a steady state (0.27% con-
−
1
−1
version, 0.26 molMethane converted kgcat
h
, ± 0.022) was attained.
Once more, two reaction products were detected; methanol and
CO2 at selectivities of 80% and 20% respectively. At this lower
catalyst loading 38% H O2 remained in the reactor liquid exit
2
stream. The decreased oxygenate selectivity when compared with
the higher 1.5 g bed loading (80% vs 92%) is attributed to greater
availability of oxidant later in the catalyst bed, which increases the
propensity towards deep oxidation. Catalysts were stable to deac-
tivation and poisoning by reaction products over the 10 h testing
period.
3.3. Reaction parameter studies
An important consideration in the design of a catalytic process
is the influence of reaction conditions on reaction rate and prod-
uct selectivity. Selecting 1.5% Fe 1.5% Cu/ZSM-5 (30), the effect of
varying; methane pressure, hydrogen peroxide concentration, tem-
perature and catalyst mass were studied, with the results reported
in Table 2.
Previous studies have reported that the rate of methane
oxidation with ZSM-5 (30) and H O2 has a 1st order depend-
2
ency upon both substrate and oxidant concentration [41].
The dependence on substrate concentration became nonlin-
ear at higher pressures of methane (>5 bar, [CH ] = 0.0058 M,
4
[
H O ] = 0.5 M) with the reaction becoming oxidant limited
2 2
as RateH2O2 Conversion ꢀ Rate
[41]. The rate of
Methane conversion
methane oxidation shows a strong dependence on methane pres-
sure/concentration under flow conditions (Table 2, Entries 1–3). By
increasing reactor pressure from 10 to 30 bar, catalyst productivity
−
1
−1
increased by 76%, from 0.17 to 0.30 molMethane Converted kgcat
h .
This pressure range equates to an increase in methane solubility
from 0.012 to 0.035 M, as calculated using Henry’s Law. In con-
trast to this, an increase in hydrogen peroxide concentration from
0
.06 to 0.25 M (Table 2, Entries 4, 2 and 5) effects a 33% increase
−
1
−1
in productivity from 0.21 to 0.28 molMethane Converted kgcat
h .
This is a strong indication that the reaction is operating under dif-
fusion limited conditions. Two additional approaches were used
to further enhance methane conversion; increasing the reac-
tor temperature (Table 2, Entries 6, 2 and 7) and increasing
the bed loading (Table 2, Entries 8, 9, 10 and 2). Unsurpris-
ingly the rate of methane oxidation increased upon increasing
◦
the reaction temperature from 25 to 75 C, with productivity
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100
(
a)
90
80
70
60
50
40
30
20
10
0
23
30
80
CH OH
100
0.40
3
0.35
0.30
0.25
0.20
80
60
40
0.15
0
.10
.05
CO2
20
0
0
0.00
0.0
0.1
0.2
0.3
0.4
0.5
20
30
40
50
60
70
80
Methane Conversion / %
SiO / Al O
2
2
3
Fig. 5. Conversion vs selectivity plots for methane oxidation reactions catalysed by
.5% Fe 1.5% Cu/ZSM-5 (30). Symbols represent studies into the effect of; methane
(
b)
1
23
30
80
pressure (᭹ꢁ), hydrogen peroxide concentration (᭿)ᮀ, reactor temperature (ꢀꢂ),
6
5
4
3
2
1
0
catalyst mass (ꢁ♦) and standard conditions (ଝꢁ).
0
.35
−
1
−1
h , despite
0.30
increasing from 0.22 to 0.34 molMethane Converted kgcat
decreasing methane solubility. However, hydrogen peroxide was
less efficiently incorporated into products as the reactor tem-
perature increased. This suggests that competing decomposition
pathways become more favourable with increasing temperature.
Methane conversion was also enhanced through increasing the cat-
alyst loading within the fixed bed. Indeed, conversion increased
from 0.26 to 0.5% when the catalyst mass was increased from
0
.25
.20
0
0.15
0.10
0
.25 to 1.5 g. Due to the non-linear relationship between mass
0
.05
.00
and conversion, this equates to a decrease in productivity from
−1
−1
h . Decreasing productiv-
0.25 to 0.08 molMethane Converted kgcat
0
ity is another clear indication that the reaction is substrate/oxidant
limited. Future studies into this catalytic system should therefore
address facilitating transport to the active site through use of more
concentrated hydrogen peroxide feeds and increased methane
pressures. Interestingly, reaction selectivity was relatively insen-
sitive to the variable test conditions studied. Fig. 5 correlates
conversion and selectivity data from Table 2. High methanol selec-
tivity (>73%) was maintained despite an increase of conversion.
This suggests that methanol is efficiently removed from the catalyst
bed into the liquid gas separator, thereby preventing deep oxida-
tion. This is an important result, and a clear indication that greater
methanol yields might be achieved without sacrificing reaction
selectivity.
20
30
40
50
60
70
80
SiO /Al O ratio
2
2
3
Fig. 6. Methane oxidation over 1.5% Fe 1.5% Cu/ZSM-5 catalysts where
SiO2/Al2O3 = 23, 30 and 80. Showing (a) reaction selectivity and methane conversion
(b) Catalyst productivity and efficiency of H2O2 utilisation. Test conditions; 0.25 g cat-
alyst, P(CH4) = 20 bar, Flow (CH4) = 10 ml min , 0.25 ml min of 0.123 M H2O2/H2O,
0 C. (a) CH3OH (᭹), CO2 (ꢀ) Methane Conversion (ᮀ). (b) RateMethane Conversion (ꢁ)
−
1
−1
◦
5
efficiency of H2O2 utilisation in products (♦).
compared with 0.25 and 0.09 for ZSM-5 (30) and (80) respectively.
Added to this, methanol selectivity was highest with the most acidic
zeolite (84.7%), decreasing to 49.5% when H-ZSM-5 (80) was used
as the support. Despite increasing conversion, hydrogen peroxide
usage decreased with increasing alumina content (64–57%). 4.4%
3.4. Effect of varying the zeolite acidity
of the oxygen in decomposed H O was retained in products when
2
2
ZSM-5 (23) was used as the support, which falls to 1.1% for ZSM-5
An additional route to increasing methanol productivity is
(
[
80). Acid-stabilisation of H O in Fenton’s type systems is known
2 2
through increasing the active site density of ZSM-5. Given that
the dimeric ␮-oxo-hydroxo iron active site is proposed to bridge
ion exchange sites [31], this might be achieved through post-
synthesis deposition of active metal (Fe). However, as shown in
Fig. 3 Fe speciation is a largely unselective and limited by the
zeolite’s exchange capacity. Alternatively, an increased density of
active sites might be achieved by varying of the parent zeolite’s
SiO /Al O ratio. Varying the Brønsted acidity of H-ZSM-5 has pre-
54], and the decrease in H O usage is clearly due to such an effect.
2
2
Meanwhile, the increase in catalyst productivity is attributed to an
increase in the population density of catalytically active Fe sites.
A future publication will explore the speciation of active metals
on ZSM-5 as a function of exchange capacity and relate this with
catalytic performance in the oxidation of short chain alkanes.
2
2
3
viously been explored [31], with a SiO /Al O ratio of 30 shown
4. Conclusions
2
2
3
to be optimal. Fig. 6 shows a clear benefit of increasing the con-
−
centration of AlO4 (at constant metal loading). For 1.5% Fe 1.5%
It has been demonstrated that bimetallic FeCu/ZSM-5 effec-
tively catalyses the direct oxidation of methane to methanol
under continuous flow conditions. Mild reaction conditions and
a green oxidant are used without additional promoters or pH
Cu/ZSM-5, a decrease in SiO /Al O enhanced catalyst productivity,
whilst also increasing reaction selectivity. Catalyst productivity for
FeCu/ZSM-5 (23) was 0.34 molMethane Converted kgcat
2
2
3
−
1
−1
h which is
Please cite this article in press as: J. Xu, et al., Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5
catalysts in a flow reactor, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.011

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J. Xu et al. / Catalysis Today xxx (2015) xxx–xxx
7
control. Reaction selectivity is high towards the primary oxidation
products with limited over oxidation (>73% methanol selectiv-
ity), showing low sensitivity to experimental variables. It has been
shown that maximising the ion exchange capacity of ZSM-5 bene-
fits reaction rate, selectivity and efficiency. Increasing the efficiency
with which the active oxygen is retained in products is a key con-
sideration for future catalyst design studies.
[20] R.A. Periana, D.J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, Platinum
catalysts for the high-yield oxidation of methane to a methanol derivative,
Science 280 (1998) 560–564.
[
21] B.G. Hashiguchi, M.M. Konnick, S.M. Bischof, S.J. Gustafson, D. Devarajan, N.
Gunsalus, D.H. Ess, R.A. Periana, Main-group compounds selectively oxidize
mixtures of methane, ethane, and propane to alcohol esters, Science 343
(
2014) 1232–1237.
[
22] R.A. Periana, G. Bhalla, W.J. Tenn Iii, K.J.H. Young, X.Y. Liu, O. Mironov, C.J.
Jones, V.R. Ziatdinov, Perspectives on some challenges and approaches for
developing the next generation of selective, low temperature, oxidation
catalysts for alkane hydroxylation based on the CH activation reaction, J. Mol.
Catal. A: Chem. 220 (2004) 7–25.
23] A.E. Shilov, G.B. Shul’pin, Activation of C–H bonds by metal complexes, Chem.
Rev. 97 (1997) 2879–2932.
24] R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas, F. Schüth, Solid catalysts for
the selective low-temperature oxidation of methane to methanol, Angew.
Chem. Int. Ed. 48 (2009) 6909–6912.
Acknowledgements
[
[
We thank Cardiff University and The Chinese Academy of Sci-
ence for their financial support.
[
25] M. Soorholtz, R.J. White, T. Zimmermann, M.M. Titirici, M. Antonietti,
R. Palkovits, F. Schüth, Direct methane oxidation over Pt-modified
nitrogen-doped carbons, Chem. Commun. 49 (2013)
Appendix A. Supplementary data
2
40–242.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.09.
[26] A.B. Sorokin, E.V. Kudrik, L.X. Alvarez, P. Afanasiev, J.M.M. Millet, D. Bouchu,
Oxidation of methane and ethylene in water at ambient conditions, Catal.
Today 157 (2010) 149–154.
27] A.B. Sorokin, E.V. Kudrik, D. Bouchu, Bio-inspired oxidation of methane in
water catalyzed by N-bridged diiron phthalocyanine complex, Chem.
Commun. (2008) 2562–2564.
[28] G.B. Shul’pin, G.V. Nizova, Y.N. Kozlov, L. Gonzalez Cuervo, G. Süss-Fink,
Hydrogen peroxide oxygenation of alkanes including methane and ethane
catalyzed by iron complexes in acetonitrile, Adv. Synth. Catal. 346 (2004)
317–332.
[29] G.B. Shul’pin, T. Sooknoi, V.B. Romakh, G. Süss-Fink, L.S. Shul’pina,
Regioselective alkane oxygenation with H2O2 catalyzed by titanosilicalite
TS-1, Tetrahedron Lett. 47 (2006) 3071–3075.
[30] M.M. Forde, B.C. Grazia, R. Armstrong, R.L. Jenkins, M.H.A. Rahim, A.F. Carley,
N. Dimitratos, J.A. Lopez-Sanchez, S.H. Taylor, N.B. McKeown, G.J. Hutchings,
Methane oxidation using silica-supported N-bridged di-iron phthalocyanine
catalyst, J. Catal. 290 (2012) 177–185.
[31] C. Hammond, M.M. Forde, M.H. Ab Rahim, A. Thetford, Q. He, R.L. Jenkins, N.
Dimitratos, J.A. Lopez-Sanchez, N.F. Dummer, D.M. Murphy, A.F. Carley, S.H.
Taylor, D.J. Willock, E.E. Stangland, J. Kang, H. Hagen, C.J. Kiely, G.J. Hutchings,
Direct catalytic conversion of methane to methanol in an aqueous medium by
using copper-promoted Fe-ZSM-5, Angew. Chem. Int. Ed. 51 (2012)
5129–5133.
[32] A.K.M.L. Rahman, M. Kumashiro, T. Ishihara, Direct synthesis of formic acid by
partial oxidation of methane on H-ZSM-5 solid acid catalyst, Catal. Commun.
12 (2011) 1198–1200.
[33] K. Otsuka, Y. Wang, Direct conversion of methane into oxygenates, Appl.
Catal. A: Gen. 222 (2001) 145–161.
011.
[
References
[
[
[
[
1] G.A. Olah, Beyond oil and gas: the methanol economy, Angew. Chem. Int. Ed.
4 (2005) 2636–2639.
2] S.J. Blanksby, G.B. Ellison, Bond dissociation energies of organic molecules,
Acc. Chem. Res. 36 (2003) 255–263.
3] M.A. Pena, J.P. Gomez, J.L.G. Fierro, New catalytic routes for syngas and
hydrogen production, Appl. Catal., A 144 (1996) 7–57.
4] J. Colby, D.I. Stirling, H. Dalton, Soluble methane mono-oxygenase of
methylococcus-capsulatus-(bath) – ability to oxygenate normal-alkanes,
normal-alkenes, ethers, and alicyclic, aromatic and heterocyclic-compounds,
Biochem. J. 165 (1977) 395–402.
4
[
[
[
5] L. Que, W.B. Tolman, Biologically inspired oxidation catalysis, Nature 455
(
2008) 333–340.
6] A.M. Khenkin, A.E. Shilov, Biomimetic alkane oxidation in the presence of iron
complexes, New J. Chem. 13 (1989) 659–667.
7] E.V. Kudrik, P. Afanasiev, L.X. Alvarez, P. Dubourdeaux, M. Clemancey, J.M.
Latour, G. Blondin, D. Bouchu, F. Albrieux, S.E. Nefedov, A.B. Sorokin, An
N-bridged high-valent diiron-oxo species on a porphyrin platform that can
oxidize methane, Nat. Chem. 4 (2012) 1024–1029.
[
[
8] E.M. Alayon, M. Nachtegaal, M. Ranocchiari, J.A. van Bokhoven, Catalytic
conversion of methane to methanol over Cu-mordenite, Chem. Commun. 48
(
2012) 404–406.
[34] C. Hammond, S. Conrad, I. Hermans, Oxidative methane upgrading,
ChemSusChem 5 (2012) 1668–1686.
[35] R. Horn, R. Schlögl, Methane activation by heterogeneous catalysis, Catal. Lett.
145 (2015) 23–39.
[36] A. Caballero, P.J. Perez, Methane as raw material in synthetic chemistry: the
final frontier, Chem. Soc. Rev. 42 (2013) 8809–8820.
[37] P. Tang, Q. Zhu, Z. Wu, D. Ma, Methane activation: the past and future, Energy
Environ. Sci. 7 (2014) 2580–2591.
9] E.M.C. Alayon, M. Nachtegaal, A. Bodi, M. Ranocchiari, J.A. van Bokhoven,
Bis([small mu]-oxo) versus mono([small mu]-oxo)dicopper cores in a zeolite
for converting methane to methanol: an in situ XAS and DFT investigation,
Phys. Chem. Chem. Phys. 17 (2015) 7681–7693.
[
[
[
[
10] P.J. Smeets, R.G. Hadt, J.S. Woertink, P. Vanelderen, R.A. Schoonheydt, B.F. Sels,
E.I. Solomon, Oxygen precursor to the reactive intermediate in methanol
synthesis by Cu-ZSM-5, J. Am. Chem. Soc. 132 (2010) 14736–14738.
11] N. Beznis, B. Weckhuysen, J. Bitter, Cu-ZSM-5 zeolites for the formation of
methanol from methane and oxygen: probing the active sites and spectator
species, Catal. Lett. 138 (2010) 14–22.
12] M.H. Groothaert, P.J. Smeets, B.F. Sels, P.A. Jacobs, R.A. Schoonheydt, Selective
oxidation of methane by the bis(␮-oxo)dicopper core stabilized on ZSM-5 and
mordenite zeolites, J. Am. Chem. Soc. 127 (2005) 1394–1395.
13] S. Grundner, M.A.C. Markovits, G. Li, M. Tromp, E.A. Pidko, E.J.M. Hensen, A.
Jentys, M. Sanchez-Sanchez, J.A. Lercher, Single-site trinuclear copper oxygen
clusters in mordenite for selective conversion of methane to methanol, Nat.
Commun. 6 (2015).
[38] C. Hammond, N. Dimitratos, R.L. Jenkins, J.A. Lopez-Sanchez, S.A. Kondrat, M.
Hasbi ab Rahim, M.M. Forde, A. Thetford, S.H. Taylor, H. Hagen, E.E. Stangland,
J.H. Kang, J.M. Moulijn, D.J. Willock, G.J. Hutchings, Elucidation and evolution
of the active component within Cu/Fe/ZSM-5 for catalytic methane oxidation:
from synthesis to catalysis, ACS Catal. 3 (2013) 689–699.
[39] M.M. Forde, R.D. Armstrong, C. Hammond, Q. He, R.L. Jenkins, S.A. Kondrat, N.
Dimitratos, J.A. Lopez-Sanchez, S.H. Taylor, D. Willock, C.J. Kiely, G.J.
Hutchings, Partial oxidation of ethane to oxygenates using Fe- and
Cu-containing ZSM-5, J. Am. Chem. Soc. 135 (2013) 11087–11099.
[40] A.K.M.L. Rahman, R. Indo, H. Hagiwara, T. Ishihara, Direct conversion of
ethane to acetic acid over H-ZSM-5 using H2O2 in aqueous phase, Appl. Catal.
A: Gen. 456 (2013) 82–87.
[41] C. Hammond, R.L. Jenkins, N. Dimitratos, J.A. Lopez-Sanchez, M.H. ab Rahim,
M.M. Forde, A. Thetford, D.M. Murphy, H. Hagen, E.E. Stangland, J.M. Moulijn,
S.H. Taylor, D.J. Willock, G.J. Hutchings, Catalytic and mechanistic insights of
the low-temperature selective oxidation of methane over Cu-promoted
Fe-ZSM-5, Chem. – A Eur. J. 18 (2012) 15735–15745.
[42] M.M. Forde, R.D. Armstrong, R. McVicker, P.P. Wells, N. Dimitratos, Q. He, L.
Lu, R.L. Jenkins, C. Hammond, J.A. Lopez-Sanchez, C.J. Kiely, G.J. Hutchings,
Light alkane oxidation using catalysts prepared by chemical vapour
impregnation: tuning alcohol selectivity through catalyst pre-treatment,
Chem. Sci. 5 (2014) 3603–3616.
[
[
[
14] P. Vanelderen, R.G. Hadt, P.J. Smeets, E.I. Solomon, R.A. Schoonheydt, B.F. Sels,
Cu-ZSM-5: a biomimetic inorganic model for methane oxidation, J. Catal. 284
(
2011) 157–164.
15] T. Sheppard, C.D. Hamill, A. Goguet, D.W. Rooney, J.M. Thompson, A low
temperature, isothermal gas-phase system for conversion of methane to
methanol over Cu-ZSM-5, Chem. Commun. 50 (2014) 11053–11055.
16] K.A. Dubkov, V.I. Sobolev, E.P. Talsi, M.A. Rodkin, N.H. Watkins, A.A.
Shteinman, G.I. Panov, Kinetic isotope effects and mechanism of biomimetic
oxidation of methane and benzene on FeZSM-5 zeolite, J. Mol. Catal. A: Chem.
1
23 (1997) 155–161.
[
[
[
17] M.V. Parfenov, E.V. Starokon, L.V. Pirutko, G.I. Panov, Quasicatalytic and
catalytic oxidation of methane to methanol by nitrous oxide over FeZSM-5
zeolite, J. Catal. 318 (2014) 14–21.
18] E.V. Starokon, M.V. Parfenov, S.S. Arzumanov, L.V. Pirutko, A.G. Stepanov, G.I.
Panov, Oxidation of methane to methanol on the surface of FeZSM-5 zeolite, J.
Catal. 300 (2013) 47–54.
19] R.A. Periana, D.J. Taube, E.R. Evitt, D.G. Löffler, P.R. Wentrcek, G. Voss, T.
Masuda, A mercury-catalyzed, high-yield system for the oxidation of
methane to methanol, Science 259 (1993) 340–343.
[43] M.H. Al-Dahhan, Y. Wu, M.P. Dudukovic, Reproducible technique for packing
laboratory-scale trickle-bed reactors with a mixture of catalyst and fines, Ind.
Eng. Chem. Res. 34 (1995) 741–747.
[44] M.M. Forde, L. Kesavan, M.I. bin Saiman, Q. He, N. Dimitratos, J.A.
Lopez-Sanchez, R.L. Jenkins, S.H. Taylor, C.J. Kiely, G.J. Hutchings, high activity
redox catalysts synthesized by chemical vapor impregnation, ACS Nano 8
(2014) 957–969.
Please cite this article in press as: J. Xu, et al., Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5
catalysts in a flow reactor, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.011

G Model
CATTOD-9797; No. of Pages8
ARTICLE IN PRESS
J. Xu et al. / Catalysis Today xxx (2015) xxx–xxx
8
[
[
45] F.W. Schuetze, F. Roessner, J. Meusinger, H. Papp, Hydrogen/deuterium
exchange on dealuminated H-ZSM-5 zeolites studied by time resolved FTIR
spectroscopy, Stud. Surf. Sci. Catal. 112 (1997) 127–134.
[50] P. Marturano, L. Drozdová, A. Kogelbauer, R. Prins, Fe/ZSM-5 prepared by
sublimation of FeCl3: the structure of the Fe species as determined by IR, 27Al
MAS NMR, and EXAFS spectroscopy, J. Catal. 192 (2000) 236–247.
[51] G.A. Webb, The Effects of Paramagnetism on NMR Spectra of Nuclei Other
than Protons, Wiley, 1974, pp. 127–142.
[52] M.S. Kumar, J. Perez-Ramirez, M.N. Debbagh, B. Smarsly, U. Bentrup, A.
Brueckner, Evidence of the vital role of the pore network on various catalytic
conversions of N2O over Fe-silicalite and Fe-SBA-15 with the same iron
constitution, Appl. Catal., B 62 (2006) 244–254.
[53] J. Perez-Ramirez, J.C. Groen, A. Brueckner, M.S. Kumar, U. Bentrup, M.N.
Debbagh, L.A. Villaescusa, Evolution of isomorphously substituted iron
zeolites during activation: comparison of Fe-␤ and Fe-ZSM-5, J. Catal. 232
(2005) 318–334.
[54] Y. Jung, J.Y. Park, S.O. Ko, Y.H. Kim, Stabilization of hydrogen peroxide using
phthalic acids in the Fenton and Fenton-like oxidation, Chemosphere 90
(2013) 812–819.
46] A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M.
Padovan, C. Otero Arean, Low-temperature Fourier-transform infrared
investigation of the interaction of carbon monoxide with nanosized ZSM 5
and silicalite, J. Chem. Soc. Faraday Trans. 88 (1992) 2959–2969.
47] L.M. Kustov, V.B. Kazansky, S. Beran, L. Kubelkova, P. Jiru, Adsorption of
carbon-monoxide on ZSM-5 zeolites – infrared spectroscopic study and
quantum-chemical calculations, J. Phys. Chem. 91 (1987) 5247–5251.
48] S. Bordiga, E. Escalona Platero, C. Otero Arean, C. Lamberti, A. Zecchina, Low
temperature carbon monoxide adsorption on Na-ZSM-5 zeolites: an FTIR
investigation, J. Catal. 137 (1992) 179–185.
[
[
[
49] C.A. Fyfe, J.M. Thomas, J. Klinowski, G.C. Gobbi, Magic-angle-spinning NMR
(
2
MAS-NMR) spectroscopy and the structure of zeolites, Angew. Chem. Int. Ed.
2 (1983) 259–275.
Please cite this article in press as: J. Xu, et al., Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5
catalysts in a flow reactor, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.011