A Kinetic Study of Methane Partial Oxidation over Fe-ZSM-5 Using N2O as an Oxidant

Article abstract of DOI:10.1002/cphc.201701202

Catalytic methane oxidation using N₂O was investigated at 300 °C over Fe-ZSM-5. This reaction rapidly produces coke (retained organic species), and causes catalyst fouling. The introduction of water into the feed-stream resulted in a significant decrease in the coke selectivity and an increase in the selectivity to the desired product, methanol, from ca. 1 % up to 16 %. A detailed investigation was carried out to determine the fundamental effect of water on the reaction pathway and catalyst stability. The delplot technique was utilised to identify primary and secondary reaction products. This kinetic study suggests that observed gas phase products (CO, CO₂, CH₃OH, C₂H₄ and C₂H₆) form as primary products whilst coke is a secondary product. Dimethyl ether was not detected, however we consider that the formation of C₂ products are likely to be due to an initial condensation of methanol within the pores of the zeolite and hence considered pseudo-primary products. According to a second order delplot analysis, coke is considered a secondary product and its formation correlates with CH₃OH formation. Control experiments in the absence of methane revealed that the rate of N₂O decomposition is similar to that of the full reaction mixture, indicating that the loss of active alpha-oxygen sites is the likely cause of the decrease in activity observed and water does not inhibit this process.

Full text of DOI:10.1002/cphc.201701202

Accepted Article
Title: A kinetic study of methane partial oxidation over FeZSM-5 using
N2O as an oxidant
Authors: Ying Kit Chow, Nicholas Dummer, James Carter, Randall
Meyer, Robert Armstrong, Christopher Williams, Greg Shaw,
Sara Jacobs, Madan Bhasin, David Willock, Stuart Taylor,
and Graham John Hutchings
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.201701202
Link to VoR: http://dx.doi.org/10.1002/cphc.201701202
A Journal of
www.chemphyschem.org

ChemPhysChem
10.1002/cphc.201701202
2
A kinetic study of methane partial oxidation over FeZSM-5 using N O as an oxidant
a
a
a
b
Ying Kit Chow , Nicholas F. Dummer *, James H. Carter , Randall J. Meyer , Robert D.
a
a
a
b
c
Armstrong , Christopher Williams , Greg Shaw , Sara Yacob , Madan M. Bhasin , David J.
a
a
a
Willock , Stuart H. Taylor , and Graham J. Hutchings *
a
Cardiff Catalysis Institute, School of chemistry, Cardiff University, Main Building, Park Place,
Cardiff, CF10 3AT, UK.
b
ExxonMobil Research and Engineering, Corporate Strategic Research, Annandale, NJ 08801,
USA
c
Innovative Catalytic Solutions, LLC, Charleston, WV, 25314, USA.
*
Corresponding author emails: dummernf@cardiff.ac.uk and hutch@cardiff.ac.uk
1
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Abstract
2
Catalytic methane oxidation using N O was investigated at 300 °C over Fe-ZSM-5. This reaction rapidly
produces coke (retained organic species), and causes catalyst fouling. The introduction of water into
the feed-stream resulted in a significant decrease in the coke selectivity and an increase in the
selectivity to the desired product, methanol, from ca. 1 % up to 16 %. A detailed investigation was
carried out to determine the fundamental effect of water on the reaction pathway and catalyst
stability. The delplot technique was utilised to identify primary and secondary reaction products. This
kinetic study suggests that observed gas phase products; CO, CO
primary products whilst coke is a secondary product. Dimethyl ether was not detected, however we
consider that the formation of C products are likely to be due to an initial condensation of methanol
within the pores of the zeolite and hence considered pseudo-primary products. According to a second
order delplot analysis, coke is considered a secondary product and its formation correlates with CH OH
formation. Control experiments in the absence of methane revealed that the rate of N
2 3
, CH OH, C
H
2 4
and C
H
2 6
form as
2
3
2
O
decomposition is similar to that of the full reaction mixture, indicating that the loss of active alpha-
oxygen sites is the likely cause of the decrease in activity observed and water does not inhibit this
process.
Keywords; MFI zeolite, methane oxidation, delplot, methanol, Fe-ZSM-5.
2
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Introduction
Methanol is currently produced from methane industrially by an energy-intensive two-step process
[
1-4]
involving high-temperature reforming, followed by methanol synthesis.
The direct oxidation of
methane to methanol could provide economic benefits for industries and a decentralised process
[
5]
would offer an environmentally conscious alternative to methane flaring. However, this reaction
remains a scientific and technical challenge due to the low reactivity of methane and limited methanol
[
6]
selectivity observed when using oxides or supported metal catalysts.
Recently, a number of zeolite-based systems have been reported which provide an alternative
[
7-10]
approach to partial methane oxidation.
Tomkins et al. reported an isothermal catalytic cycle,
[
11]
converting methane into methanol over Cu-mordenite. In this process the catalyst is first activated
using molecular oxygen, followed by reaction with methane. Methanol was subsequently extracted
using steam, to clear the active site for further oxidation cycles. Roman-Leshkov and co-workers have
shown that continuous catalytic production of methanol from oxygen, water and methane can be
[
7]
achieved with copper zeolites if the process conditions are carefully chosen. Cu- containing zeolites
were also utilised by Sushkevich et al., who reported direct, partial oxidation to methanol using water
[
12]
as both solvent and oxygen source. Use of water in this anaerobic methane oxidation system
indicated that water could act as a cheap oxidant in methane oxidation. Furthermore, water served
as an agent in regenerating the active site through facilitating desorption of partially oxygenated
[
12]
[13]
[14]
products. In contrast to the above systems, it has been shown that using H
primary oxidant, higher conversions in a continuous process can be achieved. Panov and co-workers
reported gas-phase methane oxidation under a catalytic regime using an α-oxygen species generated
when Fe-ZSM-5 was treated with N O at 300 °C, and methanol was formed under a continuous flow
regime. The α-oxygen was generated by decomposing N O over the reversible redox α-Fe sites, which
The exact structure of the oxygen transition state remains debatable
2
O
2
2
or N O
as the
[
14]
2
2
2+
3+ [14, 15]
switch from Fe to Fe .
4+
3+
-
[15, 16]
however, and is generally thought to comprise a mononuclear Fe =O or (Fe -O )
α
site.
Catalyst
deactivation was observed and this was attributed to appreciable coke formation, which led to catalyst
fouling. The same group later reported that addition of water into the feed-stream significantly
improved both the carbon balance and methanol selectivity, with the latter increasing from 1.9 % to
[
14]
1
9 % when 20 % v/v steam was added. Co-feeding of water with the N
2
O and methane also
[
14, 17]
increased the selectivity to CO.
Furthermore, the effect of water has been shown to effect the
3+
-
3+
-
decomposition of N
2
O through hydroxylation of the (Fe –O )
α
sites to form (Fe –OH ) which requires
α
[
15]
temperatures above 400 °C to reactivate the α-Fe sites. Similarly, Bulushev et al. demonstrated the
3
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
effect of co-fed water on N
2
O decomposition over Fe-ZSM-5 and indicated that N O reversible
2
[
18]
adsorption, (O)Fe loading from N
2
O and O desorption were affected.
2
The reaction mechanism of CH
4
oxidation using N O over Fe-ZSM-5 is complex and involves parallel
2
reaction pathways, which are not fully understood. The delplot technique, first developed by Bhore et
al., is a powerful tool for determining the rank of reaction products, where multiple pathways occur
[
19]
simultaneously. The method uses a plot of molar yield/conversion versus conversion and aids the
identification of primary and non-primary reaction products. For example, Wei et al. employed the
delplot method in their analysis of the selective hydrogenation of acrolein on supported silver
[
20]
catalysts and found that while the desired product allyl alcohol was a primary product, propanal
was both a primary product and a secondary product, via isomerisation of allyl alcohol. Rajkhowa et
[
21]
al. applied the technique in a kinetic study of Cu- catalysed liquid phase glycerol hydrogenolysis
.
Using delplot analysis, the authors were able to identify that acetol was the primary product and
[
21]
propylene glycol was a secondary product.
In the present work, we investigate the effect of water addition to the feed on the oxidation of
methane using N O over Fe-ZSM-5. Specifically, we apply the delplot technique to investigate the way
2
in which water addition affects reaction pathways. We then study the mechanism by which catalyst
deactivation occurs, through characterisation of catalysts both prior to and following catalytic testing.
Comparison of the pre- and post-reaction catalyst samples indicate that H
along with reduced coking, although N O conversion was found to decrease at a similar rate in both
systems. These findings highlight the complex nature of the methane oxidation reaction over Fe-ZSM-
catalysts.
2
O can prevent Fe migration
2
5
4
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Experimental
Preparation of H-ZSM-5
NH -ZSM-5 (Zeolyst, SiO
4
2 2
:Al O
3
molar ratio = 30, 3 g) was calcined under flowing air at 550 °C, with a
-
1
temperature ramp of 20 °C min for 3h to yield H-ZSM-5 (30). The sample was then allowed to cool to
room temperature under flowing air.
Preparation of Fe-ZSM-5
All catalysts were prepared by chemical vapour impregnation (CVI). H-ZSM-5 (30) (1.98 g) and Fe (III)
acetylacetonate (Fe(acac) ) (Sigma Aldrich, 99.9% purity, 2.53 g) were physically mixed and then
3
-
3
heated to 150 °C under continuous vacuum (ca 10 mbar) for 2 h. The resulting material was allowed
to cool to room temperature prior to calcination in static air for 3 h (550 °C, temperature ramp at 20 °C
-
1
min ). The Fe-ZSM-5 was pelleted to a mesh size of 20-40 prior to testing.
Catalyst Testing
Methane oxidation reactions were performed in a fixed bed continuous flow reactor. The desired mass
of catalyst (typically 0.44 g and from 0.12 to 1.2 g to construct the delplot) was placed in a stainless
steel tube supported by quartz wool. In a typical methane oxidation reaction the feed mixture
comprised of 20 % CH
4
(Air Products) + 2 % N
2
O (BOC, AA Grade) with Ar (BOC Pureshield) balance
-
1
-1
(
(
total flow rate = 55 ml min , typically GHSV = 3600 h ) at 1 atm pressure. The reaction temperature
typically 300 °C) was controlled by an isothermal oven (± 1 °C) with a thermocouple located directly
above the catalyst bed.
The addition of water to the reaction stream was carried out using a syringe pump (Sono-Tek, Syringe
-
1
Pump TI). The total flow rate of the feed mixture in the presence of water was kept at 55 ml min ,
containing 20 % CH + 20 % H O + 2% N O (Ar balance). Post-reaction catalyst samples will be referred
4
2
2
to as (i) Fe-ZSM-5-0% (no water added to feed stream) and (ii) Fe-ZSM-5-20% (20 % v/v water added
to the feed-stream).
Reaction products were analysed with online gas chromatography (GC) at 30 min intervals using an
Agilent 7890B GC equipped with a methaniser. CH
4
, CO, CO
2
, C
H
2 4
and C
2
H were detected and
6
quantified by flame ionisation detector (FID) using a Hayesep packed column. Methanol, dimethyl
ether and aromatic products were detected by FID following separation on a capillary column (Agilent,
PoraBOND U). N
2
O and N were detected by a thermal conductivity detector (TCD).
2
CH
4
2
and N O conversions were calculated according to Equations 1 and 2.
5
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
ꢁ
ꢂꢃꢄꢁꢅꢆꢇꢂꢃꢄꢁꢈꢉꢊ
CH ꢁconversionꢁ(%)ꢁ=ꢁ
ꢁxꢁ100
Equation 1
ꢀ
ꢂꢃꢄꢁꢅꢆ
ꢁ
where, CH4 in and CH4 out represent the molar fraction of CH
4
at the inlet and outlet, respectively.
Equation 2
ꢁ
ꢌ ꢎ ꢇꢌ ꢎ
ꢍ
ꢏꢐ
ꢍ
ꢑꢒꢓ
N Oꢁconversionꢁ(%)ꢁ=ꢁ
ꢋ
ꢁxꢁ100
ꢌ ꢎ
ꢍ
ꢏꢐ
2
Where, N O
2
in and N O
2
out represent the molar fraction of N O at the inlet and outlet, respectively.
i
The selectivity for product (S ) was calculated as follows, coke assumed to be the remainder:
ꢖꢗꢘꢙꢚꢛꢁꢘꢜꢁꢝꢞꢘꢟꢙꢠꢛꢁ(ꢡ)ꢁꢝꢞꢘꢟꢙꢠꢢꢟꢁꢁ(ꢗꢘꢣꢁꢠꢖꢞꢤꢘꢚ)ꢁ
ꢔꢕꢁ(%) =ꢁ
ꢁ× ꢁ100
Equation 3
ꢂꢃ ꢁꢠꢘꢚꢥꢢꢞꢛꢢꢟꢁ(ꢗꢘꢣꢁꢠꢖꢞꢤꢘꢚ)
ꢄ
Catalyst characterisation
Diffuse-reflectance UV-Vis spectra were collected using an Agilent Cary 4000 UV-vis
-
1
spectrophotometer. Samples were scanned between 200 and 800 nm (400 nm min ). Thermal
gravimetric analysis (TGA) was performed using a PerkinElmer TGA 4000. Samples (ca. 20 – 30 mg)
-
1
were loaded into ceramic crucibles and then heated to 900 °C (5 °C min ) under a flow of air (50 ml
-
1
min ). Nitrogen adsorption isotherms were collected on a Micromeritics 3Flex. Samples (ca. 0.020 g)
were degassed (150 °C, 6 h) prior to analysis. Analyses were carried out at -196 °C with P measured
continuously. Free space was measured post- analysis with He. Pore size analysis was carried out using
Micromeritics 3Flex software, N -Cylindrical Pores- Oxide Surface DFT Model.
0
2
6
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Results and discussion
Catalytic reactions
Methane oxidation reactions were conducted over Fe-ZSM-5 catalysts in the presence of N
2
O and
[
22]
where reported, co-fed H
an effort to deconvolute the influence of the Fe location and framework acidity on CH
O. Figure 1a shows the product distribution of 2 % Fe-ZSM-5 catalysed methane oxidation with
increasing time-on-stream and Figure 1b shows methane conversion, N O conversion and carbon
balance values for the same reaction, which was carried out in the absence of added water vapour.
2
O. This catalyst was extensively characterised in our previous report in
4
oxidation with
N
2
2
[
14, 23-25]
Catalytic oxidation of methane with N
2
O is observed over Fe-ZSM-5,
with low selectivity
[
14]
towards methanol of ca. 1 %. Parfenov et al. reported that addition of water to the feed-stream can
improve methanol selectivity whilst also inhibiting the accumulation of coke and retained organics.
Trends are consistent with those observed by Parfenov et al. who reported decreasing rates of
methane and N
decrease in detected carbonaceous products observed in Figure 1b at longer times-on-stream.
Similarly, CO selectivity decreases from ca. 60 % to 16 %, whilst selectivity towards C products
increases from 0.6 % to 4.4 % over the same period. Selectivities towards methanol and CO (1.4 %
2
O conversion over the initial 100 min on stream. This is consistent with the gradual
2
2
and 12 % selectivity respectively) are constant over the 2.5 h testing period. Interestingly, no DME or
acetaldehyde were detected in the reactor effluent.
With an aim to suppress catalyst fouling and be consistent with the approach taken by Parfenov et al.
water vapour (20 % v/v) was introduced into the feed stream and results are shown in Figure 2. A
significant increase in methanol selectivity is observed relative to the water- free system, from 1.4 %
to 16 %. A corresponding increase in mass balance is also observed in Figure 2, with less than 30 % of
the methane conversion lost to coke. It has previously been reported that water promotes
[26]
hydroxylation and facilitates desorption of methanol from the catalyst surface. The formation of
[
27-29]
methanol is associated with alpha-oxygen, which is generated by decomposition of N
However, due to the structure of the ZSM-5 zeolite, CH OH has been observed to react at Brønsted
This may be alleviated by removing or blocking
OH selectivity and carbon mass balance. Furthermore,
these findings strongly imply that Brønsted acidity facilitates the conversion of DME into ethene and
2
O on Fe.
3
[
22, 30]
acidic sites to yield DME and higher hydrocarbons.
Brønsted acid sites and thereby improve CH
3
[
22, 30, 31]
eventually, coke.
It is clear that the addition of water leads to a significant improvement in methanol selectivity and the
yield at 1.5 h time-on-stream is 0.16 % (without H O the yield is 0.02 %). This might be assigned to
2
7
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
[
15, 32]
water facilitating methanol desorption and competitively adsorbing at Brønsted acid sites.
further benefit is the reduction in the concentration of C products, specifically C , which is absent
from the reaction effluent. We consider that C is formed from C , probably via methyl benzenes,
Accordingly, water has decreased the effective
Brønsted acidity of the zeolite surface and thereby limiting C formation. This observation is
A
2
H
2 6
H
2 6
H
2 4
[
22, 33, 34]
through the hydrocarbon pool mechanism.
H
2 6
[
32]
consistent with the recent reports of White and co-workers who studied the effect of water on
Brønsted acid sites by measuring C-H activation over H-ZSM-5. They reported that at higher water
concentrations (> 2-3 molecules of water per Brønsted acid site) water inhibited the rate of isobutene
activation. We consider that 20 % water in the feed stream is sufficiently high to induce the previously
observed inhibition of Brønsted acidity. Interestingly, the CO
independent of feed composition (i.e. water/ no water). This implies that CO
methane at Fe sites, as we previously reported low selectivity towards CO (< 1 %) when methanol
O was passed over H-ZSM-5, Fe-ZSM-5, Fe-TS-1 or Fe-Silicalite. Therefore, it is reasonable to
postulate that methane can be oxidised to CO via a second pathway and not exclusively via oxidation
2
selectivity remains at ca. 12 %
2
forms directly from
2
[
22]
and N
2
2
[
14]
of CO (Scheme 1). Indeed, Parfenov et al. reported that the majority of CO is formed through the
decomposition of formic acid, itself formed through the disproportionation of formaldehyde, a short-
lived reaction intermediate not via combustion of CH .
4
Figure 2b shows that following an initial stabilization period, methane conversion reaches a steady
state at 1.1 %. Across the same period, a more pronounced decrease in N O conversion is observed.
Indeed, decreasing N O conversion is observed in both Figs. 1 and 2. To compare the systems
represented in Figures 1 and 2, both methane and N O conversion were normalised to their initial
2
2
2
value, and plotted as a function of time (Fig. 3). A clear decrease in conversion is observed over the 3
h testing period. Notably, this deactivation is insensitive to the presence/ absence of water in the
substrate feed (Fig. 3). This implies that the deactivation mechanism is not solely related to the
blocking of active sites through coke formation (Fig. 1 and 2). This is in contrast to studies by Panov
and co-workers, who observed that the addition of water stabilised conversion over 2 h on line.
To further study the effect of water addition upon reaction pathways during Fe-ZSM-5 catalysed
methane oxidation, the delplot technique was applied. The delplot technique, first developed by
Bhore et al., assists in resolving the rank of products where multiple, simultaneous reaction pathways
[
19]
occur. This is achieved firstly through plotting of selectivity versus conversion and then by plotting
2
selectivity/conversion versus conversion or selectivity/conversion versus conversion and allows
primary, secondary and higher rank products respectively to be identified independent of reaction
8
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
order. Through extrapolation to zero conversion, primary products can be identified where the
intercepts are finite in the first plot and then asymptotic for the second and third plots.
Methane and N O were passed over 2 wt.% Fe-ZSM-5 in a series of experiments using various catalyst
2
masses, whilst the total bed volume was kept constant through addition of SiC fines. This afforded a
conversion profile and enabled assessment of product orders through a delplot (Fig. 4, for minor
product plots see supplemental information Fig. S1). The methane conversion used in this analysis
ranged from ca. 0.4 to 3.1 %. In general, at low conversion conditions the loss of carbon is 30 % of the
total converted carbon and the products detected in the GC are ranked according to their selectivity
in the following order: CO > CO
2
> C
H
2 4
> C
H
2 6
≥ CH OH. As the conversion of methane increases, the
3
mass balance decreases significantly, i.e. the selectivity to coke or retained organics increases and
selectivity towards gas phase products decreases. The delplot analysis would therefore suggest that
coke is a secondary product and the remaining products are primary (Fig. 4 b). Interestingly, analysis
of the data presented in the third-rank delplot in Figure 4c suggests that the missing carbon or coke is
not a higher or tertiary ranked product as the line of best fit is asymptotic. That is, extrapolation of
the line of best fit to the origin indicates that first rank plots for CH
not pass through the origin (see Fig. S1). Clearly, the presence of a C
implies coupling of C products and thereby C products could be classed as secondary products.
Furthermore, C selectivity increases over the initial 100 min on-line, both in the presence and
absence of co-fed H O (Fig. 1 and 2). However, the delplot in Figure 4 is constructed from data at times
of > 100 min, once steady state of CH conversion is observed and therefore this is not fully captured.
According to a reaction mechanism proposed by Panov and co-workers
3
OH, CO, CO
, C H
2 2 4
2
and C H
6
would
2
product in the reaction effluent
1
2
H
2 6
2
4
[
14, 25, 35]
methanol and/or
formaldehyde are the primary product(s) of methane oxidation. Methanol might then desorb into the
gas phase or transfer to other surface sites on Fe-ZSM-5 to eventually form coke via a surface bound
ether (DME). We consider that C
2
H desorbs from these surface bound ether species in a comparable
6
[
36]
mechanism to that found in MTO chemistry. The high concentration of coke in this reaction appears
to support the conclusions expressed by Panov and co-workers with respect to the beneficial effect of
water displacing CH OH more efficiently. The delplot data obtained with Fe-ZSM-5-0% indicates that,
3
as methane conversion increases, coke formation comes to dominate the product distribution. The
formation of coke as opposed to combustion products in this case implies that the reaction is either
oxidant limited or that the strength of product adsorption is higher than that of methane.
Figure 5a-c illustrates the pathway analysis of the reaction of methane and N O over 2 % Fe-ZSM-5
2
where 20 % v/v water is present in the feed-stream (for minor product plots see Fig. S2). The reaction
was studied over a conversion range of 0.7 to 2.3 %. Clearly, the formation of coke is greatly reduced
9
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
even at higher conversion values, as noted in the analysis of the time-on-line data (Fig. 2). Both CH
3
OH
and CO selectivities are enhanced across the conversion range, which further indicates the importance
[
14]
of water in the formation of CO and CH
coke are less when combined at ca. 35 % than the selectivity to CO at 55 % (Fig. 5a). We consider that
the coke formation is most likely due to retention of CH OH or a methoxy species. Indeed, if all the
CH OH produced under these conditions could be desorbed then the CH OH : CO ratio would be less
3
OH. At low conversion (< 1 %) the selectivity to CH OH and
3
3
3
3
[
14]
than 1:1, as was observed by Panov and co-workers. Therefore, the formation of CO cannot solely
be due to the decomposition of HCOOH formed from the disproportionation of HCHO. With increasing
conversion, this ratio is disrupted significantly; the selectivity of CO at a conversion of 2.5 %, is 30 %
3
and that of CH OH is 10 %, whereas coke is 55 %. Under these reaction conditions, water is less able
3
to promote CH OH desorption and so sufficient methanol builds up to allow coke formation. Figure 5b
illustrates the second-rank delplots from this data set which suggests that coke is a secondary product
as the intercept is no longer close to the origin. Had the line of best fit extrapolated to the origin in
the second-rank delplot, coke could be considered a higher order product. In contrast to Fig. 4c, that
shows coke to exponentially increase as the conversion decreases (confirming it as a secondary
product), the presence of water results in a more linear regression to the y axis intercept in Fig. 5c,
suggesting that the kinetic pathway has been altered. Specifically, the formation of coke in the
presence of water appears to also be a tertiary product. This can be interpreted as the result of the
reduction in the rate of coke formation. Interestingly, the selectivity of CO
2
(ca. 10 %) (Fig. 5a) does
not alter substantially across the conversion range studied, implying an independent pathway to that
of CO and CH
3
OH. Furthermore, C
H
2 6
is not detected in the reaction effluent, which suggests that
water can prevent readsorption of C
2 4
H .
Based on these data, we consider that water has a beneficial effect at low conversion conditions (<1 %),
however, at higher conversion (> 1%) it appears unable to competitively adsorb and displace CH OH
as effectively. Indeed, Hunger et al. observed that methanol has a stronger affinity for the ZSM-5
3
[
37]
surface than water, citing the presence of the CH
3
group as a possible reason for this. The CH OH
3
that is retained forms coke more efficiently and lowers the mass balance of the reaction. Under higher
conversion conditions the contact time with the catalyst is increased as, despite operating with a fixed
bed volume, the catalyst mass is higher. Potentially, CH
can re-adsorb further down, reacting with Brønsted acid sites to form coke. From analysis of the
delplot data we propose a reaction network (Scheme 1) whereby CO formation is largely independent
of CH OH and CO concentration, however, CO oxidation could occur. However, the stability of the CO
selectivity contrasts strongly against that of the CO selectivity particularly in Figure 1. The action of
water to promote both CH OH desorption and the disproportionation of HCOOH to CO and H O is
3
OH desorbed at the top of the catalyst bed
2
3
2
3
2
1
0
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
[
14]
captured, as proposed by Panov and co-workers. Adsorption of CH
site is captured to highlight that the formation of CO and CH
determination of CO and C as primary products in the delplot analysis suggests that cooperativity
exists between the active Fe sites for methanol formation and Brønsted acid sites as has been
3
OH at an adjacent Brønsted acid
3
OH is unlikely to remain equivalent. The
H
2 4
[
22]
suggested previously. The formation of C
2
H is thought to occur through hydrogen transfer under
6
[
34]
the umbrella of the Methanol-to-Hydrocarbons chemistry, however, we did not detect any of the
expected dienes, trienes or polymethylbenzenes that would form post-transfer. We consider that this
reaction would take place in the pores of the zeolite and that these products contribute to the
formation of coke. Therefore, for the sake of simplicity they have been omitted from Scheme 1.
Characterisation of pre- and post-reaction samples
The delplot analysis revealed that the major reaction pathways are largely unchanged by the addition
of water to the system, clarifying that the role of water is to displace methanol before it reacts further,
rather than offer a different pathway. When H O is present, analysis of the delplot data suggests that
2
coke can be a secondary or higher rank product (Figs. 5b and c). In addition, the deactivation rate of
the catalyst was unchanged by the presence of water (Fig. 3). This was an unexpected observation as
reaction data (Figs. 1 and 2) showed that the accumulation of coke/organics in the catalyst is
significantly reduced when water is present, so it would be expected that this catalyst is more stable.
Therefore, characterisation of the fresh and used samples from reactions in the presence and absence
of water was carried out to better understand the cause of catalyst deactivation.
TGA was carried out to calculate the quantity of carbon retained after the 3 h reaction. Figure 6
illustrates the mass loss when fresh Fe-ZSM-5, Fe-ZSM-5-0% and Fe-ZSM-5-20% were heated in
flowing air over the temperature range of 30 to 900 °C. The fresh catalyst and the one used with 20 %
H
2
O exhibit almost identical TGA profiles, with weight losses occurring below 200 °C. This is consistent
with the loss of physisorbed water and carbonates. In contrast, Fe-ZSM-5-0% exhibits a broad weight
loss centred at 400 °C. This is ascribed to CO production, formed as coke is combusted. These
2
observations are consistent with the poor mass balance and high selectivity to coke seen in the
reaction data presented in Figure 1 and provides further evidence of the role of water in suppressing
coke formation. The broad thermal event represented by a bi-modal weight loss (Fig. 6b), is consistent
[
38-40]
with observations made by Weckhuysen and co-workers
sites within ZSM-5.
concerning coke build-up on different
The mass loss over the temperature range of 220 – 600 °C can be ascribed to coke formed in the
reaction being removed in the form of carbon oxides, as described above. This corresponds to 795
1
1
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
-
1
-1
µmol of carbon per gram of catalyst per hour (μmol
C
gcat h ) for the Fe-ZSM-5-0% sample (Table 1,
-
1
-
Entry 4). In contrast, when water was present the rate of coke accumulation was just 59 μmol
C
g
cat
h
1
-1 -1
.
The potential coking rate expressed in μmol
C
gcat h was also calculated to compare the moles of
carbon derived from TGA measurements with the expected value based on the reaction data
-
1
-1
presented in Figs. 1 and 2. Respectively, carbon deposition rates of 587 and 166 μmol gcat h were
C
calculated for Fe-ZSM-5-0% and Fe-ZSM-5-20%, which does not match with the amount of carbon
removed during the TGA analysis. A similar discrepancy was observed in the recent work of Panov and
[
14]
-1 -1
co-workers, where a rate of 1400 μmol
C
g
cat h was measured by TPO, however, based on their
-
1
-1
reaction data only 795 μmol gcat h would be expected. We consider that the discrepancy observed
C
may be due to the uneven distribution of coke along the bed profile.
The significant coking observed with Fe-ZSM-5-0% did not greatly impact the reactivity, as evidenced
in Fig. 1b. The reduction of methane conversion is ca. 20 % over the 3 h reaction and ca. 40 % for N
Fig. 3). The selectivity to methanol and C products remains low during the time-on-line reaction,
although the fall and subsequent plateau of the CO selectivity suggests that the Fe based active sites
are not all deactivated by coke. The influence of H O in the feed-stream was further investigated
2
O
(
2
3+
2
through nitrogen adsorption isotherms; these were collected for the samples and compared to the
parent H-ZSM-5 material (Fig. S3). The BET surface area and micro-pore volume of the materials are
shown in Table 1. As expected the addition of Fe (2 wt. %) reduced the total surface area of the parent
2
-1
H-ZSM-5 material by ca. 75 m g , although the micro-pore volume was not greatly affected. Analysis
of the post-reaction samples reveals that the surface area and micro-pore volume of Fe-ZSM-5-20% is
comparable to that of unused, Fe-ZSM-5. This is consistent with TGA results in Fig. 6 and the relatively
low carbon loss observed in Figure 2 with water in the reaction feed-stream preventing significant
coke accumulation on the catalyst surface. In the case of Fe-ZSM-5-0%, the absence of water in the
feed-stream has clearly affected the nitrogen adsorption measurements. The BET surface area and
micro-pore volume decreased by ca. 40 % when compared to the unused Fe-ZSM-5, from 359 to 210
2
-1
3 -1
m g and 0.142 to 0.087 cm g respectively. These significant surface area and porosity reductions
are clearly visible in the N isotherm measurements displayed in Figure S3.
2
The formation of CO has been suggested by Panov and co-workers to originate from adsorbed CH
3
OH
[
14]
or HCHO reacting with H
that CH OH can still form via the (Fe -O )
potentially through surface diffusion
2
O to HCOOH and subsequently CO. Therefore, it is reasonable to expect
3+
-
3
α
site although can be retained within the zeolite pore system,
[
14, 24, 41]
where water is not present. This suggests that the
decreases in activity observed in Figure 1 and 2 are not strictly related to increased Fe site blocking by
coke in the case of Fe-ZSM-5-0% and surface hydration in Fe-ZSM-5-20%.
1
2
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
UV-Vis was carried out to observe the structure of the Fe species before and after the reaction to
account for the comparable N O decomposition rate despite differing coking rates. Figure 7 shows an
overlay of spectra for Fe-ZSM-5, Fe-ZSM-5-0% and Fe-ZSM-5-20%. Bands at 210 and 240 nm
correspond to the t and t e transitions of the FeO which are isomorphously substituted into
the tetrahedral sites of the zeolite framework , while bands at 250 nm correspond to isolated
2
1
t
2
1
4
[
42]
3+
[43]
octahedral Fe species in extra-framework Al
2
O
3
.
Iron oxide clusters are indicated by bands at 300
[
44]
to 450 nm and bulk iron oxide species are shown at λ > 450 nm. Fe-ZSM-5 and Fe-ZSM-5-20% exhibit
comparable spectra, whilst Fe-ZSM-5-0% exhibits more resolved bands at 210, 240, 410 and 550 - 800
nm. The latter band indicates that iron oxide clusters and bulk iron oxide on the external surface were
formed during the reaction without water, and the addition of water appears to suppress this process.
[
42]
The formation of Fe
x
O clusters is common under high temperature calcination conditions or
y
[
45]
through steam treatment. However, the reaction temperature of 300 °C is not considered sufficient
to promote Fe migration, as this typically occurs >600 °C and is favoured by high heating rate. The
formation of Fe
x
O clusters is also more likely to occur when moisture remained in the zeolite as
y
[
42]
observed by Kumar et al. Therefore, the formation of the distinct iron oxide clusters observed in
the Fe-ZSM-5-0% sample is intriguing. The rapid accumulation of coke that occurs may be linked to
3+
3+
the instability of the Fe active sites in the absence of water. Reductive treatment of Fe in FeZSM-5
is known to result in an irreversible agglomeration process forming Fe . This Fe magnetite phase
can readily undergo further transformation to α-Fe particles by subsequent oxidation , which
may be represented by the broad band observed from 550 to 800 nm. Furthermore, this broad band
signifies the irreversible redistribution of Fe ion species following treatment with CH and N O. The
redox properties of the isolated Fe ions may also be different when coexisting with a large
3
O
4
3
O
4
[
46]
2
O
3
4
2
3+
[
42]
concentration of Fe
x
O clusters, as observed with NO reduction in a previous report. We consider
y
3+
that weakly bound Fe may migrate when water is not present to yield iron oxide species, however,
as the conversion of methane and N O remain stable after 100 min time-on-line (Fig. 1). Such
migration appears to be unrelated to the activity of the catalyst. According to the UV/Vis spectra, Fe-
ZSM-5-0% has retained a sufficient quantity of FeO tetrahedra and extra-framework Fe to complete
2
4
the methane oxidation reaction via the ɑ-oxygen species.
In order to further investigate the origin of Fe restructuring and catalyst deactivation, control
experiments with 20% CH
4
/Ar or 2% N O/Ar were fed over the catalyst at 300 °C for 3 h and UV-Vis
2
spectroscopy was carried out on the used catalysts (Fig. 8). It can be seen that the spectra are very
similar after the control experiments were carried out, indicating that the Fe speciation did not greatly
2 4
change as a result of exposure to N O or CH . However, in both samples exposed to CH
4
or N O there
2
1
3
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
were modest shifts to higher wavelengths. This implies that a modest restructuring of Fe can occur
with CH or N O present, however, the cause of the significant shifts seen in Fe-ZSM-5-0% could be
due to an intermediate or reaction product that was not present in these control experiments.
4
2
Additionally, the stability of N
catalyst deactivation observed was related to poisoning of the active site induced by N
control experiment was carried out which involved passing 2 % N O in Ar over the Fe-ZSM-5 catalyst.
The conversion of N O was normalised to the initial value (after 5 min on-stream) and compared in
Figure 9. This is plotted with N O conversion rates from the methane oxidation reactions which were
presented in Figure 3. The N O conversion rate dropped rapidly in the control experiment, but was
slightly more stable than the full reaction feed. In the absence of a reductant, N O decomposition is
2
O conversion in the absence of CH
4
was examined to determine if the
2
O (Fig. 9). A
2
2
2
2
2
[
47]
rate-limited at lower temperatures, such as 300 °C, by oxygen recombination. Specifically, the
dissociation of N O on Fe sites is facile, but the reaction does not turn over because O must be formed
from two oxygen atoms to close the catalytic cycle. We consider that the rapid drop in N O conversion
O and that the rate of CH activation
O turn-over appears to be a limiting factor
conversion and appears unaffected by Fe restructuring or coke accumulation.
2
2
2
2+
observed in Figure 9 could be due to saturation of -Fe sites by N
2
4
is lower than that of N
2
O dissociation. The decrease in N
2
to efficient CH
4
Overall, the characterisation of the fresh and used catalyst showed that the introduction of water
suppressed coke formation, by promoting the desorption of methanol from the surface. Additionally,
it was found that in the absence of water, the Fe species restructured during the reaction. However,
the overall rate of catalyst deactivation was the same with and without water present, suggesting that
these processes were not affecting the catalytic turnover of CH
4
. Thermal-gravimetric analysis and N
2
porosimetry revealed coke formation and surface area loss during reactions carried out in the absence
of water. Therefore, it is possible that the deactivation of the catalyst in the absence of water is caused
3+
-
by coke formation near (Fe -O )
α
, loss of porosity and Fe restructuring. Additionally, the initial drop in
2
N O conversion rates results from slow regeneration of these species. Interestingly, a similar rate of
[
48]
deactivation occurs when water, a known inhibitor of N O decomposition, is present.
2
1
4
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Conclusion
The influence of water on the reaction of CH
4
2
and N O over Fe-ZSM-5 catalysts was probed with delplot
3
+
-
analysis. The yield of methanol on alpha-oxygen sites (Fe -O )
conditions studied, as the rate of CH OH desorption is low on Fe-ZSM-5 catalysts. In the absence of
water, CH OH can migrate to nearby Brønsted acid sites where it is converted to coke via processes
exemplified in the MTO reaction.
α
is low, particularly under the reaction
[14]
3
3
[
30, 31]
Indeed, delplot analysis showed that coke is a secondary
reaction product when water is not present. Conversely, in the presence of water, coke becomes a
secondary or tertiary product, indicative of a reduction in the rate of formation of coke. Other reaction
products including CO and C
desorption of CH OH or a methoxy species from the catalyst surface. Interestingly, with increasing
methane conversion, CO selectivity was found to be largely independent of variations in the yield of
H
2 4
were found to be primary products, which further implies slow
3
2
coke or CO. This suggested that total oxidation of methane or other oxygenates operates via a
separate pathway and is not influenced by the presence of water. Furthermore, characterisation
studies show that water can prevent loss of catalyst porosity through inhibiting coke formation, whilst
also preventing migration/ aggregation of Fe species. Therefore, the complex reaction mechanism of
selective methane oxidation appears to be greatly affected by the presence of water and the
decomposition rate of N O. It is clear that catalyst deactivation occurs at a rate which is unaffected by
2
water addition, suggesting that coking and changes in iron speciation do not have a significant
detrimental effect upon catalyst performance at the short reaction times on stream studied. In the
absence of CH
of deactivation which is independent of catalyst fouling. This investigation clarifies the reaction
pathway of methane activation using N O over Fe-ZSM-5 and underlines the complex nature of the
4 2
, the rate of N O conversion also decreased rapidly, suggesting a common mechanism
2
reaction, where multiple reaction and deactivation pathways take place.
1
5
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Acknowledgments
The authors wish to thank ExxonMobil for financial support.
References
[1] A. I. Olivos-Suarez, A. Szecsenyi, E. J. M. Hensen, J. Ruiz-Martinez, E. A. Pidko, J. Gascon Acs Catalysis.
2
016, 6, 2965-2981.
[
[
[
[
2] M. J. da Silva Fuel Processing Technology. 2016, 145, 42-61.
3] G. A. Olah Angewandte Chemie-International Edition. 2005, 44, 2636-2639.
4] T. Mokrani, M. Scurrell Catalysis Reviews-Science and Engineering. 2009, 51, 1-145.
5] 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 Angewandte Chemie-International Edition. 2012, 51, 5129-
5
133.
[
[
[
[
6] J. A. Labinger Journal of Molecular Catalysis a-Chemical. 2004, 220, 27-35.
7] K. Narsimhan, K. Iyoki, K. Dinh, Y. Roman-Leshkov Acs Central Science. 2016, 2, 424-429.
8] V. S. Arutyunov Journal of Natural Gas Chemistry. 2004, 13, 10-22.
9] M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs, R. A. Schoonheydt Journal of the American
Chemical Society. 2005, 127, 1394-1395.
[10] P. J. Smeets, M. H. Groothaert, R. A. Schoonheydt Catalysis Today. 2005, 110, 303-309.
[11] P. Tomkins, A. Mansouri, S. E. Bozbag, F. Krumeich, M. B. Park, E. M. C. Alayon, M. Ranocchiari, J.
A. van Bokhoven Angewandte Chemie-International Edition. 2016, 55, 5467-5471.
[
[
12] V. L. Sushkevich, D. Palagin, M. Ranocchiari, J. A. van Bokhoven Science. 2017, 356, 523-+.
13] J. Xu, R. D. Armstrong, G. Shaw, N. F. Dummer, S. J. Freakley, S. H. Taylor, G. J. Hutchings Catalysis
Today. 2016, 270, 93-100.
[
[
14] M. V. Parfenov, E. V. Starokon, L. V. Pirutko, G. I. Panov Journal of Catalysis. 2014, 318, 14-21.
15] G. I. Panov, E. V. Starokon, L. V. Pirutko, E. A. Paukshtis, V. N. Parmon Journal of Catalysis. 2008,
2
[
3
54, 110-120.
16] B. R. Wood, J. A. Reimer, A. T. Bell, M. T. Janicke, K. C. Ott Journal of Catalysis. 2004, 225, 300-
06.
[17] M. C. Kung, S. S. Y. Lin, H. H. Kung Topics in Catalysis. 2012, 55, 108-115.
[18] D. A. Bulushev, P. M. Prechtl, A. Renken, L. Kiwi-Minsker Industrial & Engineering Chemistry
Research. 2007, 46, 4178-4185.
19] N. A. Bhore, M. T. Klein, K. B. Bischoff Industrial & Engineering Chemistry Research. 1990, 29, 313-
16.
20] H. J. Wei, C. Gomez, J. J. Liu, N. Guo, T. P. Wu, R. Lobo-Lapidus, C. L. Marshall, J. T. Miller, R. J.
Meyer Journal of Catalysis. 2013, 298, 18-26.
[
3
[
[21] T. Rajkhowa, G. B. Marin, J. W. Thybaut Applied Catalysis B: Environmental. 2017, 205, 469-480.
[22] K. Chow, N. F. Dummer, J. Carter, C. Williams, G. Shaw, D. J. Willock, S. H. Taylor, S. Yacob, R. J.
Meyer, M. Bhasin, G. J. Hutchings Catalysis Science & Technology. 2017.
[23] G. I. Panov, K. A. Dubkov, E. V. Starokon Catalysis Today. 2006, 117, 148-155.
[24] E. V. Starokon, M. V. Parfenov, S. S. Arzumanov, L. V. Pirutko, A. G. Stepanov, G. I. Panov Journal
of Catalysis. 2013, 300, 47-54.
25] E. V. Starokon, M. V. Parfenov, L. V. Pirutko, S. I. Abornev, G. I. Panov Journal of Physical Chemistry
C. 2011, 115, 2155-2161.
26] G. N. Li, E. A. Pidko, R. A. van Santen, Z. C. Feng, C. Li, E. J. M. Hensen Journal of Catalysis. 2011,
84, 194-206.
[
[
2
1
6
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
[27] K. A. Dubkov, N. S. Ovanesyan, A. A. Shteinman, E. V. Starokon, G. I. Panov Journal of Catalysis.
2
002, 207, 341-352.
[28] M. F. Fellah, I. Onal Journal of Physical Chemistry C. 2010, 114, 3042-3051.
[29] B. E. R. Snyder, P. Vanelderen, M. L. Bols, S. D. Hallaert, L. H. Bottger, L. Ungur, K. Pierloot, R. A.
Schoonheydt, B. F. Sels, E. I. Solomon Nature (London, U. K.). 2016, 536, 317-321.
[30] P. Tian, Y. X. Wei, M. Ye, Z. M. Liu Acs Catalysis. 2015, 5, 1922-1938.
[31] M. Z. Zhang, S. T. Xu, Y. X. Wei, J. Z. Li, J. R. Chen, J. B. Wang, W. N. Zhang, S. S. Gao, X. J. Li, C. X.
Wang, Z. M. Liu Rsc Advances. 2016, 6, 95855-95864.
[32] K. Chen, J. Damron, C. Pearson, D. Resasco, L. Zhang, J. L. White ACS Catalysis. 2014, 4, 3039-3044.
[33] M. Bjorgen, S. Svelle, F. Joensen, J. Nerlov, S. Kolboe, F. Bonino, L. Palumbo, S. Bordiga, U. Olsbye
Journal of Catalysis. 2007, 249, 195-207.
[34] S. Ilias, A. Bhan ACS Catalysis. 2013, 3, 18-31.
[35] G. I. Panov, K. A. Dubkov, Y. A. Paukshtis Catalysis by Unique Metal Ion Structures in Solid Matrices:
from Science to Application. 2001, 13, 149-163.
36] S. Svelle, F. Joensen, J. Nerlov, U. Olsbye, K. P. Lillerud, S. Kolboe, M. Bjorgen Journal of the
American Chemical Society. 2006, 128, 14770-14771.
[
[37] B. Hunger, S. Matysik, M. Heuchel, W.-D. Einicke Langmuir. 1997, 13, 6249-6254.
[38] D. Mores, E. Stavitski, M. H. F. Kox, J. Kornatowski, U. Olsbye, B. M. Weckhuysen Chemistry-a
European Journal. 2008, 14, 11320-11327.
39] D. Mores, J. Kornatowski, U. Olsbye, B. M. Weckhuysen Chemistry-a European Journal. 2011, 17,
874-2884.
40] L. R. Aramburo, S. Teketel, S. Svelle, S. R. Bare, B. Arstad, H. W. Zandbergen, U. Olsbye, F. M. F. de
Groot, B. M. Weckhuysen Journal of Catalysis. 2013, 307, 185-193.
41] G. K. Boreskov in Catalytic Activation of Dioxygen, Vol., Springer Berlin Heidelberg, Berlin,
Heidelberg, 1982, pp.39-137.
[
2
[
[
[
[
[
[
[
[
[
42] M. S. Kumar, M. Schwidder, W. Grünert, A. Brückner. 2004, 227, 384-397.
43] M. Schwidder, W. Grunert, U. Bentrup, A. Bruckner Journal of Catalysis. 2006, 239, 173-186.
44] J. Perez-Ramirez, M. S. Kumar, A. Bruckner Journal of Catalysis. 2004, 223, 13-27.
45] H. T. Lee, H. K. Rhee Korean Journal of Chemical Engineering. 2002, 19, 574-579.
46] A. V. Kucherov, C. N. Montreuil, T. N. Kucherova, M. Shelef Catalysis Letters. 1998, 56, 173-181.
47] L. Kiwi-Minsker, D. A. Bulushev, A. Renken Catalysis Today. 2005, 110, 191-198.
48] D. A. Bulushev, L. Kiwi-Minsker, A. Renken Journal of Catalysis. 2004, 222, 389-396.
1
7
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
Tables, Schemes and Figures
Table 1. BET surface areas and pore volumes of MFI catalysts pre- and post-reaction.
c
d
Total surface
Coke produced
Missing carbon
b
V
(
micropore
a, b
-1 -1
-1 -1
Entry
Catalyst
area
(μmol
C
g
cat h )
(μmol
C
g
cat h )
3
-1
cm g )
2
-1
(
m g )
434
1
2
3
4
H-ZSM-5
Fe-ZSM-5
Fe-ZSM-5-20%
Fe-ZSM-5-0%
0.169
0.142
0.134
0.087
-
-
59
-
-
359
352
210
166
587
795
a
Surface area determined from nitrogen adsorption measurement using the BET equation at -196
b
°
%
C. Quantitative analysis is per unit mass of sample – in the case of Entry 4 this includes ca. 5 wt.
c
carbonaceous deposits. Coke production calculated from mass loss over 200 – 600 °C from TGA
measeurements as described in experimental section. Expected missing carbon calculated from
yield of missing carbon from 3 h time on-stream tests presented in Figs. 1 and 2.
d
1
8
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
H2O
Fe-ZSM-5
*
HCO H + CH OH
*
CH OH + *HCHO
2
3
CH4
3
N O
2
H2O)
(
CO)
B
(
B
CO + H O
CO2
+
*CH OCH
3
C H
2 4
2
3
B
H O
2
C2H6
*coke
Scheme 1. Proposed reaction network for CH
4
oxidation with N O over Fe-ZSM-5 catalysts according
2
to delplot analysis; B is Brønsted acid site and * indicates adsorbed or intermediate species not
detected in the reactor effluent.
1
9
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
(
a)
60
50
40
30
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
Time-on-stream /h
(
b)
6
0
70
60
50
40
50
30
40
20
30
10
20
2
10
1
0
0
0.0
0.5
1.0
1.5
2.0
2.5
Time-on-stream /h
2
Fig. 1. The oxidation of methane over 2 % Fe/ZSM-5 (30) using N O at 300 °C showing (a) temporal
3
evolution of products ( CO; ꢀ CH OH; ꢁ CO
2 2 6
; ꢂC H ;
2
C H
4
and ꢀ ‘’missing carbon’’) and (b)
carbon mass balance (ꢁ) and conversion of methane (ꢀ)/N
2
O (ꢁ).
2
0
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
(
a)
60
50
40
30
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time-on-stream /h
(
b)
30
100
90
25
80
20
70
15
60
10
50
1
40
30
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Time-on-stream /h
Fig. 2. The oxidation of methane over 2.5 % Fe/ZSM-5 (30) using N
2
O at 300 °C with 20 % v/v H
2
O
added to the substrate feed, showing (a) temporal evolution of products ( CO; ꢀ CH
3
OH; ꢁ CO ;
2
ꢂC
2
H
6
and ꢀ ‘’missing carbon’’) and (b) carbon mass balance (ꢁ) and conversion of methane
O (ꢁ).
(
ꢀ)/N
2
2
1
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
1
0
0
0
0
0
0
0
0
0
0
.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
.0
0
0.5
1
1.5
2
2.5
3
3.5
Time on-stream /h
Fig. 3. The conversion of methane (20 % H
2
O ꢀ and 0 % H
2 2
O ꢁ) and N O (20 % H
2
O ꢃ and 0 % H O
2
ꢄ) normalised to their initial values at t over 2% Fe/ZSM-5 at 300 °C.
5
2
2
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
a)
b)
c)
90
80
70
60
50
40
30
20
10
0
120
200
180
160
140
120
1
00
80
60
100
8
0
0
40
6
40
20
0
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Conversion / %
Conversion / %
Conversion / %
Fig. 4. First rank (a), second rank (b) and third rank (c) delplots taken from data collected over a series of experiments using different masses of 2 % Fe-ZSM-
5
3
at 300 °C; (ꢂ) CH OH, (ꢀ) CO
2 2 6
, (■) CO, (ꢅ) C H , (ꢆ) C
H
2 4
and (●) missing carbon.
2
3
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
a)
b)
c)
70
60
50
40
30
20
10
0
100
160
9
8
0
0
140
120
100
80
70
60
50
40
30
20
10
0
60
40
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Conversion / %
Conversion / %
Conversion / %
Fig. 5. First rank (a), second rank (b) and third rank (c) delplots taken from data collected over a series of experiments using varying masses of 2 % Fe-ZSM-5
at 300 °C with water present in the feed-stream; (ꢂ) CH OH, (ꢀ) CO , (■) CO, (ꢆ) C and (●) missing carbon.
3
2
2 4
H
2
4
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
(
a)
1
00
9
9
8
8
5
0
5
0
2
00
400
600
800
O
Temperature / C
(
b)
0
0
0
.02
.01
.00
-
-
-
-
-
0.01
0.02
0.03
0.04
0.05
2
00
400
600
800
O
Temperature / C
Fig. 6. TGA profiles of fresh and used catalysts showing weight loss profiles (a) and weight loss derivate
profiles (b) for Fe-ZSM-5 (black), Fe-ZSM-5-20% (blue dots) and Fe-ZSM-5-0% (red dashes).
2
5
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
3
2
1
0
200
300
400
500
600
700
800
Wavelength / nm
Fig. 7. UV-vis spectra of Fe-ZSM-5 (black line), Fe-ZSM-5-20% (blue dots) and Fe-ZSM-5-0% (red
dashes).
2
6
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
2
1
1
0
0
.0
.5
.0
.5
.0
2
00
300
400
500
600
700
800
Wavelength / nm
Fig. 8. UV-Vis spectra of fresh 2 % Fe-ZSM-5 (blue dots) and following control experiments under 20%
CH /Ar (black line) and 2% N O/Ar (red dashes).
4
2
2
7
This article is protected by copyright. All rights reserved.

ChemPhysChem
10.1002/cphc.201701202
1
0
0
0
0
0
0
.0
.9
.8
.7
.6
.5
.4
0
0.5
1
1.5
2
2.5
3
3.5
Time on-stream /h
2
Fig. 9. Normalised N O conversion over 2% Fe/ZSM-5 at 300 °C in flows of; CH
4
2
+ 2% N O ( ), 20%
CH + 2% N O + 20% H
4
2
2
O ( ) and 2% N O (ꢂ) at 300 °C over 2 wt.% Fe-ZSM-5 normalised to their
2
-1
initial values. All gas compositions balanced with Ar. Total flow rate = 55 ml min .
2
8
This article is protected by copyright. All rights reserved.