The effect of oxidant species on direct, non-syngas conversion of methane to methanol over an FePO4 catalyst material

Article abstract of DOI:10.1039/c9ra02327e

The effect of the phase transformation of a FePO₄ catalyst material from the tridymite-like (tdm) FePO₄ to the α-domain (α-Fe₃(P₂O₇)₂) during the direct selective oxidation of methane to methanol was studied using oxidant species O₂, H₂O and N₂O. The main reaction products were CH₃OH, carbon dioxide and carbon monoxide, whereas formaldehyde was produced in rather minute amounts. Results showed that the single-step non-syngas activation of CH₄ to oxygenate(s) on a solid FePO₄ phase-specific catalyst was influenced by the nature of the oxidizer used for the CH₄ turnover. Fresh and activated FePO₄ powder samples and their modified physicochemical surface and bulk properties, which affected the conversion and selectivity in the partial oxidation (POX) mechanism of CH₄, were investigated. Temperature-programmed re-oxidation (TPRO) profiles indicated that the type of moieties utilised in the procedures, determined the re-oxidizing pathway of the reduced multiphase FePO₄ system. M?ssbauer spectroscopy measurements along with X-ray diffraction (XRD) examination of neat, hydrogenated and spent catalytic compounds, demonstrated a variation of the phosphate into a mixture of crystallites, which depended on operating process conditions (for example time-on-stream). The M?ssbauer spectra revealed the change of the initial ferric orthophosphate, FePO₄ (tdm), to the divalent metal form, iron(ii) pyrophosphate (Fe₂P₂O₇); thereafter, reactivity was governed by the interaction (strength) with individual oxidizing agents. The Fe³⁺ ? Fe²⁺ chemical redox cycle can play a key mechanistic role in tailored multistep design, while the advantage of iron-based heterogeneous catalysis primarily lies in being inexpensive and comprising non-critical raw resources. When compared to the other catalysts reported in the literature, the FePO₄-tdm phase catalysts showed in this work exhibited a high activity towards methanol i.e., 12.3 × 10^-3 μmol_MeOH g_cat h^-1 using N₂O as an oxidant. This catalyst also showed a high activity with O₂ as an oxidant (5.3 × 10^-3 μmol_MeOH g_cat h^-1). Further investigations will include continuous reactor unit engineering optimisation.

Full text of DOI:10.1039/c9ra02327e

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The effect of oxidant species on direct, non-syngas
conversion of methane to methanol over an FePO4
catalyst material†
Cite this: RSC Adv., 2019, 9, 30989
a
b
cd
Venkata D. B. C. Dasireddy, * Darko Hanzel, Krish Bharuth-Ram
a
and Blaz Likozar
ˇ
The effect of the phase transformation of a FePO
4
4
catalyst material from the tridymite-like (tdm) FePO to
the a-domain (a-Fe
using oxidant species O
carbon monoxide, whereas formaldehyde was produced in rather minute amounts. Results showed that
the single-step non-syngas activation of CH to oxygenate(s) on a solid FePO phase-specific catalyst
was influenced by the nature of the oxidizer used for the CH turnover. Fresh and activated FePO
powder samples and their modified physicochemical surface and bulk properties, which affected the
conversion and selectivity in the partial oxidation (POX) mechanism of CH , were investigated.
Temperature-programmed re-oxidation (TPRO) profiles indicated that the type of moieties utilised in the
procedures, determined the re-oxidizing pathway of the reduced multiphase FePO system. M o¨ ssbauer
3 2
(P O
7
2
) ) during the direct selective oxidation of methane to methanol was studied
2
, H O and N O. The main reaction products were CH OH, carbon dioxide and
2
2
3
4
4
4
4
4
4
spectroscopy measurements along with X-ray diffraction (XRD) examination of neat, hydrogenated and
spent catalytic compounds, demonstrated a variation of the phosphate into a mixture of crystallites,
which depended on operating process conditions (for example time-on-stream). The M o¨ ssbauer spectra
4
revealed the change of the initial ferric orthophosphate, FePO (tdm), to the divalent metal form, iron(II)
pyrophosphate (Fe ); thereafter, reactivity was governed by the interaction (strength) with individual
2 2 7
P O
3+
2+
oxidizing agents. The Fe 4 Fe chemical redox cycle can play a key mechanistic role in tailored
multistep design, while the advantage of iron-based heterogeneous catalysis primarily lies in being
inexpensive and comprising non-critical raw resources. When compared to the other catalysts reported
Received 27th March 2019
Accepted 7th September 2019
in the literature, the FePO
4
-tdm phase catalysts showed in this work exhibited a high activity towards
ꢁ3
ꢁ1
methanol i.e., 12.3 ꢀ 10 mmolMeOH
activity with O
continuous reactor unit engineering optimisation.
g
cat
h
2
using N O as an oxidant. This catalyst also showed a high
DOI: 10.1039/c9ra02327e
ꢁ3
ꢁ1
2
as an oxidant (5.3 ꢀ 10
mmolMeOH
g
cat
h
). Further investigations will include
rsc.li/rsc-advances
natural gas into products such as methanol, which under
ambient temperature and pressure is a liquid. However, over
1
Introduction
2
The direct conversion of methane to methanol has been the past few decades, the conversion of methane to methanol
attracting considerable attention because of its great potential has remained as one of the major unrequited challenges in
application in the efficient utilization of abundant natural gas chemistry. To activate methane, usually high temperatures are
reserves. In the last decade a number of interesting approaches required. At these temperatures, formed methanol undergoes
3
,4
was suggested for the effective implementation of this difficult further oxidation to CO
and H O, as illustrated below.
2
2
1
transformation. A most attractive approach is to convert the
1
2
1
2
þ
O
2
þ
O
2
CH
4
ƒƒ! CH
3
OH ƒƒ! HCHO
a
Department of Catalysis and Chemical Reaction Engineering, National Institute of
Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia. E-mail: dasireddy@ki.si; Fax:
1
1
+
386 1 4760300; Tel: +386 1 4760504
þ
O
2
þ
O
2
b
2
2
Department of Low and Medium Energy Physics, “Jozef Stefan” Institute, Jamova cesta
ƒ
ƒ! HCOOH ƒƒ! CO
2
þ H
2
O
(1)
3
9, 1000 Ljubljana, Slovenia
c
Physics Department, Durban University of Technology, Durban 4000, South Africa
d
School of Chemistry and Physcis, University of KwaZulu Natal, Durban 4000, South
Africa
Methanol is formed via methane oxidation by a-oxygen,
CH + (M –O* ) , migrated from a-oxygen sites. It is generally
accepted that a-sites perform the oxidation via the reversible
n
ꢁ
4
a
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra02327e
1
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n
n+1.3
redox transition M 4 M
. Much of the work on the variety
C
3
H
6
over iron-containing catalysts. Iron is peculiar for
of catalysts that have been investigated for partial oxidation of obtaining high selectivity to propylene oxide, and the modi-
methane to methanol has been summarized in recent litera- cation of the iron sites with an alkali metal salt can promote the
5
–7
19
ture reports. The majority of the studies involved supported C H epoxidation. Christos et al. have shown that the reactivity
3
6
metal oxide catalysts, primarily vanadium and molybdenum of commercial zeolite-based catalysts containing Fe and/or Cu
1
,2
oxide. Higher activity and selectivity to the desired products cations for the partial oxidation of methane is inuenced by the
over mixed metal oxide catalysts can be attributed to the acid sites strength and concentration in the catalyst which
formation of easily reducible metal oxide species caused by depends on the Si/Al molar ratio and type of zeolite. It was
interactions between the metals. Loading of supported phase shown that the Fe cations are responsible for the superior
below the monolayer coverage has been shown to be preferable oxygenates productivity, while the crucial role of Cu is to
for the high production of formaldehyde and methanol from maintain high MeOH selectivity by suppressing the production
8
,9
1,10
methane. Otsuka et al.
reported that the conversion of of the deeper oxidation product, HCOOH. Partial oxidation of
methane is accelerated by co-feeding hydrogen with oxygen methane over iron phosphate supported on silica produced
1
7,20
18,21
over several iron containing catalysts. The co-feeding of high formaldehyde yields.
hydrogen induces the formation of methanol over FePO shown that the nature of the oxidant, however, was observed to
FeAsO
In the literature,
it has been
4
,
4
and FAPO-5 (Fe : Al : P ¼ 0.1 : 0.9 : 1.0) catalysts at play a vital role in favoring yield towards methanol. In line with
atmospheric pressure and in the temperature range of 350– the state of the art direct conversion of methane to methanol,
ꢂ
5
00 C. Thus, in order to design a better catalyst, it is quite we have undertaken this work with the aim to investigate which
important to understand which the effective and/or ineffective of the oxidant species, O , H O or N O, would lead to achieving
2
2
2
iron sites are in the selective oxidation of methane to a high yield of methanol over FePO catalysts. As an attempt to
4
methanol.
A low temperature (150 C), isothermal, gas-phase recy- synthesize FePO
clable process was described for the partial oxidation of then revealed by different characterisation techniques and the
achieve this objective, an ammonia gel method was employed to
ꢂ
4
catalyst, the resulting catalyst structures were
1
1
methane to methanol over Cu/ZSM-5 by Sheppard et al.,
inuence of oxidants on the structure and phase of FePO
4
which showed a stable formation of methanol for a long period during the reaction was examined.
of time. Depending on the iron content and activation condi-
tions, a variety of Fe species may be available in the zeolite,
ranging from isolated Fe(II) and Fe(III) cations and oligo-
2
. Experiment
nuclear Fe complexes up to large agglomerates of iron oxide. 2.1 Synthesis of the catalyst
FeZSM-5 zeolites have a long application history as catalysts
The catalysts were synthesized by the ammonia gel method
3
,4,12
for oxidations by N
2
O.
Methanol and dimethyl ether (DME)
22–24
described by Friedrich et al.
This catalytic preparation
were the products extracted from the catalytic surface. Co-
feeding water strongly increased methanol selectivity, which
methodology was chosen because of its simplicity and to ach-
ieve a better control of particle size and morphology of the
active phase. In essence, an appropriate amount of ferric
ꢂ
attained a fractional concentration of 62% at 275 C. The
location, dispersion and environment (acidic or alkaline) of
iron sites and the nature of oxidant are key factors in deter-
mining the catalytic performances of iron-containing meso-
nitrate, Fe(NO
water and then a dilute ammonium solution (25% NH
3
)
3
$9H
2
O (99%, Sigma Aldrich) was dissolved in
in H O,
3
2
Sigma Aldrich) was added. This led to precipitation and
formation of iron(III) hydroxide (brown gel). Orthophosphoric
acid (85% H PO , Sigma Aldrich) was added while stirring the
3 4
1
3,14
porous materials for selective oxidation reactions.
Shiota
1
5
and Yoshizawa have computed and analysed the reactions of
+
the rst row MO complexes (M ¼ Sc, Ti, V, Cr, Mn, Fe, Co, Ni
gel, followed by a 40 wt% silica solution. The stirred mixture
and Cu) and methane, which can competitively form methanol
ꢂ
was then heated to 60 C and kept at this temperature for 2 h.
1
6
and methyl radical. Anderson et al. carried out a systematic
study of the conversion of methane using a number of metal
oxide catalysts. They demonstrated that cobalt oxide is the
most active single component catalyst which resulted in a high
conversion, but with a very low selectivity towards methanol
synthesis.
ꢂ
The obtained gel was heated at 90 C for 12 h, and the dried
ꢂ
solid achieved was then calcined at a temperature 500 C for 4 h
in a 2 bar ow of air.
2.2 Characterization of catalysts
ˇ
´
17,18
Stolcova and co-workers
examined the inuence of The characterizations of fresh and used catalysts are described
structure and reactivity of copper iron pyrophosphate catalysts below. Nitrogen physisorption analyses were carried out by
ꢂ
for the selective oxidation of methane using O2 and N O as degassing the catalysts under N ow for 4 h at 200 C. The
2
2
oxidizing agents. These oxidants showed appreciable impact on degassed samples were analysed in the Micromeritics ASAP
the onset of both methane conversion and the primary oxida- 2020 multi-point surface area and porosity analyser. Powder X-
tion products. The catalytic results showed that the lattice ray diffraction (XRD) studies were conducted using the PAN-
ꢂ
oxygen of the catalyst could react with methane molecules alytical X'Pert Pro instrument. Scanning from 10 to 90 was
producing methanol and that replenishment of the lattice carried out using the CuKa-radiation source with the wave-
oxygen by N
2
O takes places rather readily and rapidly. Wang length of 0.15406 nm. Temperature programmed reduction-
13
et al. showed the use of N O oxidant for the epoxidation of oxidation (TPRO) was performed using the Micromeritics 2920
2
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À
Á
Autochem II Chemisorption Analyser. Initially, the reduction of
½
CH
4
ꢅ ꢁ ½CH
4
ꢅout
in
CH
4
conversionðdCH4 mol %Þ ¼
ꢀ 100
the catalyst was done using 4.9 mol% H in Ar as a reducing
2
½CH ꢅ
4
in
25,26
agent as in the method described in.
Temperature pro-
grammed desorption (TPD) was carried out using the Micro-
meritics 2920 Autochem II Chemisorption Analyser as well.
Aer reduction, the catalysts were pre-treated at 350 C under
½
moles of oxidantꢅ
out
ꢂ
Oxidant conversionðdoxi mol%Þ ¼
ꢀ 100
½
moles of oxidantꢅ
in
the stream of helium for 60 min. The temperature was conse-
ꢂ
quently decreased to 80 C. Appropriate pre-chosen gas was
passed over the catalysts (10 mol% CO
Ar) at the ow rate of 30 mL min for 60 min. The excess gas
was removed by purging with helium for 30 min. The temper-
2
in He or 4.9 mol% H
2
in
À
Á
½CH OHꢅ
3
out
ꢁ
1
CH
3
OH selectivity
S
CH3OH mol% ¼
ꢀ 100
½CH ꢅ ꢁ ½CH ꢅ
4
4 out
in
ꢂ
ature was thereaer gradually raised to 900 C by ramping at
ꢂ
ꢁ1
1
0 C min under the ow of helium, wherein the desorption
data of CO or H was recorded. The desorption data of O , H
and N O were also recorded in the same procedure. Metal
dispersion is calculated using CO chemisorption which is The effect of reaction temperature on methane conversion over
2
2
2
2
O
3
Results and discussion
2
24,26
illustrated in literature
4
FePO catalyst with different oxidative environments was
The structural morphology of the prepared catalysts was investigated as shown in Fig. 1. Methane conversion with N O
2
studied using eld-emission scanning electron microscope and O increases with temperature; however when H O is used
2
2
(Carl Zeiss, FE-SEM SUPRA 35VP), equipped with energy- as an oxidant, methane conversion increases at much lower
dispersive X-ray spectroscopy hardware (Oxford Instruments, linear rate with temperature. Furthermore, the maximum
model INCA 400). Particle size, morphology and elemental methane conversion (17%) was obtained when oxygen is used as
ꢂ
mapping performed by EDXS analyses were investigated using an oxidant at a temperature of 500 C, the highest reaction
Cs corrected scanning transmission electron microscope JEOL temperature in this study. The differences in conversion rates
2
ARM 200 CF equipped with JEOL Centurio 100 mm EDXS clearly show that selective oxidation of methane on FePO
4
is
5
7
system.
Fe-M ¨o ssbauer spectroscopy measurements were inuenced by the nature of the oxidant. These facts strongly
made at room temperature (RT) in conventional transmission suggest that the reaction mechanisms are different. This
29
5
7
geometry with a Co source embedded in Rh matrix.
conclusion is strengthened by the greater conversions with O2
than N O, which is in agreement with the reaction thermody-
2
namics and the activation energy differences between these two
oxidants. This means that there exists a lower kinetic barrier
catalysts for O than N O in catalytic selective oxidation of methane.
2 2
2
.3 Partial oxidation of methane
The catalytic partial oxidation runs using the FePO
were carried out in a horizontal xed-bed U-shaped quartz
1
4
When considering the effect of temperature on the methane
27,28
reactor as described in.
the middle of the reactor and a ow of N
The catalyst (ꢃ0.4 g) was placed in conversion, two factors need to be considered. Firstly, methane
ꢁ1
2
(30 mL min ) was conversion will depend on the oxidant feed concentration,
ꢂ
introduced into the reactor at a temperature of 200 C in order secondly, even if total oxidant consumption occurs, methane
to remove the physisorbed gases from the surface of the cata- conversion can change with a change in selectivity. Most of the
2,3,11,18,30,31
lyst. The catalytic runs were carried out under atmospheric studies
reported in literature have examined the effect
ꢂ
pressure at the temperature range of 200–500 C using undi- of temperature over a range varying from approximately 300 to
ꢂ
luted high purity CH
O or H
4
(99.95%) and the appropriate oxidant (O
2
,
500 C. It has been found that very low conversion occurs until
ꢁ
1
N
2
2
O) at ow rate of 60 mL min , corresponding to the a critical temperature is reached, aer which a very rapid rise in
gas hour space velocity (GHSV) of 3600 h with a methane to conversion is observed (Fig. 1). This usually corresponds to total
oxidant ratio of 1 : 1. The gas products were analysed using an oxidant consumption. Typically the products obtained in the
ꢁ1
Agilent 490 Micro GC TCD equipped with CP-Molsieve and partial oxidation of reaction consist of CH
3
2
OH, CO, CO , HCHO
PoraPolt U columns. Reported values are given aer 5 h of the and H
2
O. CO
2
and H
2
ꢂ
O are formed initially at temperatures
ꢂ
reaction under steady-state conditions. The product mixture below 300 C. At 400 C both selectivity and yield of methanol
including methane, methanol, carbon oxides and nitrogen were are found to pass through a maximum before decreasing as the
analysed by online quadrupole mass spectrometry (MS). The temperature is increased further (Fig. 2). Methanol formation is
2 2
signals in the MS are calibrated with different mole fractions of accompanied by the production of CO along with CO and H O.
methane, methanol, carbon oxides and nitrogen in order to It was generally found that increasing the temperature well
determine the mole composition of gases in the outow. No above the self-ignition temperature of methanol favors the
other gaseous products were detected during the reaction. All production of CO, CO
the data points were recorded in duplicate with a standard methanol.
2
2
and H O at the expense of
1,10,18
deviation of ꢄ 2%. The carbon mass balances are in the range of
To make a better comparison of catalytic performance
among the oxidants, we further carried out the reactions with
98–99%.
ꢁ1
The methane conversion (d) was calculated using the various ow rates between a GHSV of 2000 and 7000 h . The
following equation:
conversion of methane increased and the selectivity to
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catalyst with varying temperature (GHSV ¼ 3600 hꢁ and methane to oxidant
1
Fig. 1 Influence of oxidant on methane conversion over FePO
ratio of 1 : 1).
4
methanol decreased with decrease in the ow rate as expected methanol was the highest when N O is used as oxidant. In
2
(ESI, Fig. S1†). From these data, the plot (Fig. 3) of methanol addition to that, the selectivity of methanol at low conversions
selectivity versus methane conversion was constructed. The (<2%) of methane was almost identical (see rst two data points
selectivity towards methanol was very high at lower methane at nearly 100% selectivity in Fig. 3). By elevating the reaction
conversion at higher ow rate. However, when the comparison temperature at these conditions also, methanol selectivity
was made at higher methane conversion, the selectivity to gradually dropped and raised the CO production. This was
Fig. 2 Influence of oxidant on CH
3
OH selectivity in methane partial oxidation reaction over FePO
4
catalyst with varying temperature (GHSV ¼
ꢁ1
3
600 h and methane to oxidant ratio of 1 : 1).
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ꢂ
ꢂ
Fig. 3 Selectivity towards methanol varying with methane conversion over FePO
(
4
catalyst at the temperatures of (a) 300 C and (b) 400
C
GHSV ¼ 2000–7000 hꢁ1, methane to oxidant ratio of 1 : 1).
a consequence of the low stability of methanol at higher product distribution depend on the nature of the oxidant and
16,29
temperatures and thereby it over oxidised to carbon oxides. It the reaction conditions.
has been well recognized that the selective oxidation of
methane proceeds via redox mechanism, but the pathways and heterolytic mechanisms.
Activation of methane may occur by both hemolytic and
2,27
Thus the investigations of used
catalysts of FePO
4
might give an insight into the modied
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physicochemical properties of the catalysts which further in a reaction such as oxidative dehydrogenation gives formal-
inuenced the conversion and selectivity in the partial oxida- dehyde which is easily converted to CO and CO . Hydrogenation
2
1
tion of methane. The rate determining step would be the of the methoxy radical yields methanol. Stabilization of the
rupture of the C–H bond and the formation of CH and HO methoxy radical by hydrogenation is a key step to achieve a high
3
1,29
18
radicals were postulated as the initial step.
For methane methanol yield. Fig. 4 shows the selectivity towards products
oxidation to methanol, the most strongly supported mechanism in the partial oxidation of methane at iso-conversions of 10%
consists of consecutive conversion scheme as shown in eqn (1). and 5%. At both iso-conversion conditions (similar conversions
The methoxy radical is an important intermediate in the reac- at the same temperature), methanol was formed in high
19
2 2
tion pathway. Elimination of hydrogen from methoxy radical quantity when N O used as an oxidant. CO is the major product
Fig. 4 Selectivity towards products in the partial oxidation of methane at an iso-conversion of (a) 5% and (b) 10% over FePO
oxidants (temperatures of 400 C and methane to oxidant ratio of 1 : 1).
4
catalyst with various
ꢂ
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with both O
2
and H
2
O. This could be either due to the further with a homogeneous dispersion of the particles. Aer reduc-
oxidation of methoxy radical to CO or the direct combustion of tion, the agglomeration of these particles was observed. Aer
2
methane to CO . On the other hand, when N O was used, these oxidation with oxygen the crystalline nature is retained with
2
2
effects were less systematic; the production rate of methanol a separate agglomerated bulk particles. Aer oxidation with
was approximately doubled (Fig. 4). Using N O, methanol O and H O, the catalysts showed a relatively poor crystalline
selectivity was higher, with decreased CO selectivity. The structure and an amorphous like morphology (Fig. 5d and e).
catalytic differences have been ascribed to the differing The BET surface areas, measured by the physical nitrogen
oxidizing power of O , N O and H O. In accordance with the adsorption for all of the samples, are presented in Table 1. The
Mars–van Krevelen mechanism,
2
N
2
2
2
2
2
2
,2,29
1
2
ꢁ1
4
which has been proposed, specic surface area of fresh FePO was found to be 19 m g .
the re-oxidation of the catalyst will be less effective with N O However, BET surface area decreased aer reduction and
2
and H O compared to O . At all conditions, formaldehyde is oxidation treatments. Aer reduction, the surface area of the
2
2
2
ꢁ1
formed in very minor quantities, which could also be due to catalyst reduced drastically to 9 m g , due to the blocking of
a secondary oxidation of formaldehyde to CO or CO the pores of the FePO by the amorphous carbon which formed
Fig. 5 shows the SEM images of FePO catalysts treated under large crystallites, as evidenced by XRD and pore-size distribu-
2
.
4
4
various conditions. The fresh catalyst possesses rough crystal- tion measurements.
line morphology with a particle sizes ranging from 50–80 nm
Fig. 5 Scanning electron micrographs (SEM) of FePO
d) oxidized with N O and (e) oxidized with H O.
4 4 2
catalysts under various conditions (a) fresh, (b) reduced under CH , (c) oxidized with O ,
(
2
2
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4
Table 1 Particulate properties of fresh, reduced and oxidized FePO nanocomposite catalysts
Pore volume
2
ꢁ1
3
ꢁ1
a
b
Catalyst condition
Surface area (m g
)
(cm g
)
Metal dispersion (%)
Crystallite size (nm)
Fresh
19
9
15
12
13
0.021
0.015
0.020
0.019
0.018
27.8
10.3
22.3
17.3
15.8
28
41
32
35
35
4
Reduced (with CH ) catalyst
Oxidized (with O
Oxidized (with N
2
) catalyst
O) catalyst
2
2
Oxidized (with H O) catalyst
a
32
b
Calculated from CO chemisorption . Average crystallite size calculated by Scherrer equation.
The N
reduced and oxidised catalysts (ESI, Fig. S2†) can be categorised reduced sample was obtained using reducing the fresh FePO
as the type IV isotherms, with a distinct hysteresis loop, catalyst under 10% CH in Ar. In the TPR prole, the catalyst
2
adsorption–desorption isotherms of the fresh, of the reduced and oxidized samples were carried out. The
4
4
3
3
ꢂ
observed in the relative pressure (P/P
pore-size distribution, calculated from the desorption counter- (Fig. 6). These peaks represent reduction of FePO to Fe P O as
o
) range of 0.47–0.79. The showed three peaks at the temperatures of 492, 625 and 815 C
4
2 2 7
22,34
part using Barrett–Joyner–Halenda (BJH) method, showed suggested in:
a dominant peak in the mesoporous range (ESI, Fig. S3†). The
ꢂ
C
4
00ꢁ650
FePO
4
ðtridymiteÞ ƒƒƒ ƒƒ ! b-Fe ðP
2
O
7
Þ2
3
metal dispersion of Fe species was calculated from CO chemi-
sorption. Metal dispersion showed the similar trend to surface
area, as the fresh catalyst showed a high metal dispersion
compared to the reduced and oxidised catalysts. Among the
oxidised catalysts, the catalyst oxidised with oxygen showed
a higher metal dispersion compared to the catalysts oxidised
with N O or H O. This could be due to the amount of available
ꢂ
7
00ꢁ850
C
ƒƒƒ ƒƒ ! Fe
2
P
2
O
7
ðpyrophosphateÞ
3
5,36
As reported in the literature,
the rst step in the reduc-
ꢂ
tion occurs above the temperatures of 500 C. The use of CH
4
as
2
2
reductant in this study accelerated the reduction, probably due
to the activation and spill over of hydrogen from the metal
centers to the iron phosphate.
oxygen present in N O or H O. This shows that the nitrous oxide
2
2
or water provides adequate oxidation of reduced sites of the
catalyst. This may explain the fact that the number of active
sites is altered and the nature of the active sites of the oxidised
catalyst remained unchanged.
To investigate the phase transformations occurred during
reduction and oxidation, powder XRD and M ¨o ssbauer analysis
Aer the reduction, the oxidation was conducted using O
O and H O as oxidants (10% of oxidant in Ar), separately. The
TPRO prole under H
2
,
N
2
2
2
O exhibited a very wide range of oxida-
ꢂ
tion prole in the temperature ranging from 200–370 C (Fig. 7).
When N O is used an oxidant, only one peak was exhibited and
2
4 4
Fig. 6 Temperature programmed reduction of FePO catalyst under 10% CH in Ar.
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4
Fig. 7 Temperature programmed reduction–oxidation (TPRO) profiles of FePO catalyst under various oxidant environments.
ꢂ
two peaks in the prole occurred with O
2
as oxidant. There were absence of the b-phase which would give a peak at 2q ¼ 36
ꢂ
no peaks observed above 500 C, in the TPRO proles. Various range (ESI, Fig. S4†). Oxidation of the catalyst in H O showed
2
37,38
literature reports
stated that the oxidative transformation of a similar pattern to the catalyst oxidized with oxygen, but with
iron pyrophosphate phase to quartz phase occurs in two steps
as below:
ꢂ
\
500
C
Fe
2
P
2
O
7
ðpyrophosphateÞ ƒƒƒ! a-Fe
3
ðP
2
O
7
Þ2
ꢂ
.
500
C
ƒ
ƒ ƒƒ ! FePO
4
ðquartzÞ
The formation of the quartz type phase has been reported in
the literature; however, it was formed at temperatures above
ꢂ
39
5
00 C. However, it was also observed that the transformation
between the a-phase and Fe P O is reversible. Thus, from
2
2 7
TPRO proles, it is evident that the type of oxidant used in the
re-oxidation inuenced the re-oxidation path way of reduced
FePO catalyst. The powder XRD patterns of the fresh, reduced
4
and oxidized catalysts are shown in Fig. 8.
The fresh catalyst showed the presence of the FePO
4
trydimite-like (tdm) phase by exhibiting a main peak at a 2q ¼
ꢂ
ꢂ
3
4 and minor peaks in the range of 24–30 . The XRD pattern of
the reduced catalyst (with methane) shows two distinct peaks at
ꢂ
ꢂ
2
q values of 24 and 30 , conrming the formation of the
Fe phase range (ESI, Fig. S4†). The formation of this phase
is also observed in literature
2 2 7
P O
22,37
4
when FePO is reduced under
various hydrocarbon reduction atmospheres. When the reduced
catalyst is oxidised in the presence of oxygen, the formation of
the a-phase (a-Fe (P O ) ) is observed. The XRD prole of this
3 2 7 2
catalyst showed two characteristic major peaks in the region of
ꢂ
2
q ¼ 33–36 indicating the presence of a-phase (a-Fe (P O ) )
3
2 7 2
along with some minor peaks. Some of the XRD peaks in this
region also coincide with peaks characteristic of the b-phase.
22,37
The XRD prole of the catalyst oxidized with N
2
O also showed
phase, but (with N
Fig. 8 M o¨ ssbauer spectra of fresh, reduced (with CH
O, O and H O) FePO catalyst.
4
) and oxidized
the presence of the a-phase along with Fe
2
P
2
O
7
2
2
2
4
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Table 2 M o¨ ssbauer parameters, isomer shift (IS), electric quadrupole splitting (QS), G (HWHM), and the attributed phases, determined from the
a
spectra of the fresh catalyst after calcination, reduction and oxidation
G
ꢁ1
ꢁ1
ꢁ1
Sample
IS (mm s
)
QS (mm s
)
(HWHM) (mm s
)
Fe species
Area (%)
Attributed phase
3+
3+
3+
2+
2+
2+
3+
3+
2+
2+
3+
3+
3+
2+
2+
3+
3+
3+
2+
2+
Fresh catalyst
Reduced with CH
0.42(1)
0.37(1)
0.34(1)
1.00(2)
0.67(8)
2.53(8)
2.29(4)
1.35(7)
1.01(6)
0.60(3)
2.23(9)
2.71(9)
0.24(5)
0.78(1)
0.81(2)
2.00(5)
2.74(6)
0.22(3)
1.03(1)
0.56(1)
2.75(5)
2.16(6)
0.17
0.16
0.15
0.17
0.17
0.17
0.23
0.22
0.21
0.22
0.20
0.20
0.23
0.20
0.20
0.16
0.20
0.23
0.22
0.20
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
100
FePO
Fe (PO
FePO -low quartz
Fe
4
-tdm
)
4 6
4
25(2)
17(2)
27(4)
19(1)
3(1)
28(4)
46(4)
6(1)
7
0.35(8)
1.28(7)
1.29(1)
1.25(3)
4
2 2 7
P O
Oxidized with O
2
2
0.41(6)
a-Fe
3
(P
2
O
7
)
2
2
2
0.40(1)
1.02(8)
1.15(4)
0.31(1)
Fe
2
P
2
O
7
7(1)
13(2)
34(3)
29(3)
6(1)
FePO
4
-tdm
Oxidized with N
O
0.31(1)
a-Fe (P O )
3
2 7
0.51(2)
1.15(2)
1.08(2)
0.30(1)
2 2 7
Fe P O
7(1)
24(1)
33(1)
56(2)
6(1)
FePO
4
-tdm
Oxidized with H
2
O
0.39(1)
a-Fe (P O )
3
2 7
0.39(1)
1.14(2)
1.07(3)
2 2 7
Fe P O
5(1)
a
The isomer shis are expressed relative to a-Fe at room temperature.
ꢂ
the notable absence of a peak at a 2q ¼ 28 , indicating the
M ¨o ssbauer spectra of fresh, reduced (with CH
4
) and oxidized
absence of the b-phase under H
2
O oxidation also. This could be (with N
2
O, O and H O) FePO catalysts are shown in Fig. 8. The
2
2
4
due to a high oxidizing atmosphere being required for the spectra were corrected for thickness effects and then tted with
5
1
formation of b-phase and the low oxidizing strength of H O the analysis code RECOIL using Lorentzian line shapes for the
compared to O2.
2
22,37
spectral components. The spectral t parameters (isomer shi
4 2 2 2 4
Fig. 9 Phase quantification of fresh, reduced (with CH ) and oxidized (with N O, O and H O) FePO catalyst determined from the M o¨ ssbauer
spectra shown in Fig. 8.
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Table 3 M o¨ ssbauer parameters, isomer shift (IS), electric quadrupole splitting (QS), line width G (HWHM) and the attributed phases of the spent
a
catalyst after oxidation in O
2
, N
2
O and H
2
O atmospheres
G
ꢁ1
ꢁ1
ꢁ1
Oxidizing atmosphere
IS (mm s
)
QS (mm s
)
(HWHM) (mm s
)
Fe species
Area(%)
Attributed phase
a-Fe (P
Fe
3+
2+
2+
2+
3+
3+
3+
2+
2+
3+
3+
3+
2+
2+
2+
O
2
2
0.40(1)
0.61(2)
1.04(2)
2.20(5)
2.74(4)
0.21(2)
0.68(3)
1.09(3)
2.00(5)
2.78(4)
0.22(8)
0.65(5)
0.63(1)
1.20(2)
2.75(5)
2.54(8)
0.23
0.20
0.25
0.18
0.14
0.23
0.20
0.15
0.16
0.17
0.22
0.17
0.18
0.22
0.20
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
51(2)
23(2)
7(1)
3
2 7 2
O )
0.41(2)
1.03(8)
1.11(4)
0.29(1)
2 2 7
P O
5(1)
12(2)
44(3)
18(3)
6(1)
FePO
Fe (PO
FePO -low quartz
Fe
4
-tdm
N
O
0.38(1)
7
4 6
)
0.40(2)
1.19(4)
1.13(2)
0.30(1)
4
2 2 7
P O
6(1)
26(1)
45(2)
26(1)
17(2)
6(1)
4
FePO -tdm
H
2
O
0.30(1)
a-Fe
a-Fe
3
(P
(P
2
O
O
7
)
)
2
2
0.50(1)
0.39(1)
1.14(2)
1.25(3)
3
2
7
2 2 7
Fe P O
5(1)
a
The isomer shis are expressed relative to a-Fe at room temperature.
3
+
(
IS), electric quadrupole splitting (QS), line width (HWHM), area are in agreement with the Fe species in the various iron
20,22,23
fractions (f)) and phase assignments are collected in Table 2 phosphate type phases, namely the a-phase.
where the isomer shis are given relative to a-Fe at room
In the Powder XRD diffractogram of the used catalyst aer
temperature. The phase assignments were made on the basis of the reaction with O
parameters reported in ref. 36, 37 and 40–42.
2
atmosphere, two major peaks are evident at
The phase parameters of the fresh catalyst conrm that only
the FePO -tdm phase is present in the catalyst, in agreement
4
23,24,37
2+
with literature and previous work.
Two Fe species which
are observed aer the reduction of the fresh catalyst under
ꢂ
22
methane at 500 C, are attributable to the Fe
7% contribution from a FePO low quartz phase is evident,
most likely the result of the unreduced phase present in the
2
P
2
O
7
phase.
A
1
4
37
catalyst. The effect of oxygen atmosphere on the phase
formation is reected by the M ¨o ssbauer spectrum for the O2
oxidized catalyst (Fig. 8), which shows a 74% spectral area due
to a Fe iron phase and a weaker Fe component with IS of
.15 mm s and QS of 2.71 mm s . These values are in
accordance with the values of Fe and Fe components in the
a-phase of the catalyst, a-Fe (P . The XRD prole of the
catalyst also supports this assignment. In addition, a trydimite
like FePO phase is observed with a spectral area of 13%.
3
+
2+
ꢁ
1
ꢁ1
1
3
+
2+
3
2 7 2
O )
4
The M ¨o ssbauer spectrum obtained for the oxidized catalyst
with N O (Fig. 8), showed a 12% contribution from components
2
2
+
with IS and QS values characteristic of Fe (Fig. 9 and Table 2)
which can be assigned to the Fe phase. In addition, the
spectrum showed a 18% Fe component with parameters cor-
2 2 7
P O
3
+
2
+
responding to the a-phase together with a mixture of 44% Fe
3
+
in the Fe
which were not characteristic of a typical FePO
the oxidized catalyst, with a 26% site fraction is observed.
The M ¨o ssbauer spectrum of the catalyst oxidized with H O,
2
P
2
O
7
phase. A Fe species, with IS and QS values
4
-tdm phase in
2
2,37
2
2
+
showed Fe components with a combined 11% intensity, which
can be assigned to the Fe
the presence of the Fe species with a 88% site fraction
2
3
P
2
O
7
phase. The spectrum also shows
+
(Fig. 10). The respective IS and QS values of this ferric species
Fig. 10 M o¨ ssbauer spectra of used catalysts after reaction with (a) O
b) N O and (c) H O.
2
,
(
2
2
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2 2 2
Fig. 11 Phase quantification of used catalysts after reaction with O , N O and H O, determined from the M o¨ ssbauer spectra.
ꢂ
ꢂ
3
4.2 and 35.1 which can be attributed to the Fe
2 2 7
P O phase, intensities of 11% and 71% (Fig. 11), which can be assigned to
20,22,23
but this region also coincides with an overlap of the most the a-Fe
3
(P
2
O
7
)
2
phase.
Similar to the catalyst obtained
20,37
intense peak of the a-phase.
used catalysts (Fig. 10), the a-Fe
spectral component, was the dominant phase (74%). The reported that formation of the b-phase is dependent on the
In the M ¨o ssbauer data of the aer reaction under N
2
O atmosphere, no evidence of b-phase
3
+
17,18,24
3
(P
2
O
7
)
2
phase, with a Fe
was observed aer the reaction with H
2
O also.
It has been
43,44
appearance of the Fe P O phase was also observed, with ferric catalyst structure and redox atmosphere.
Obviously, water
2
2 7
and ferrous species each contributing towards an 11% total site plays a role in avoiding high reduction atmospheres. Similar
4
3
22,37
fraction. The formation of the a-phase results from the observation was reported in literature,
transformation between the FePO -tdm and Fe phases, the water co-feed in the reaction, blocks the formation of the
which takes place reversibly, depending on strength of the less selective b-phase. In summary, the XRD and M ¨o ssbauer
that the addition of
4
2 2 7
P O
20,23
redox atmosphere
a mixed ferric and ferrous pyrophosphate consisting of both of N
and the fact that the a-Fe
3
(P
2
O
7
)
2
phase is spectra of the fresh and used catalysts oxidized in the presence
O and H O showed that the a-phase was formed in high
quantity when the formation of b-phase was suppressed.
2
2
37,43
22
Fe
4
(P
The diffractogram obtained for the used catalyst aer the
reaction with N O (ESI, Fig. S5†), showed an intense, sharp peak nitrous oxide in the processes in which the a-oxygen sites were
2 7 3 2 2 7
O ) and Fe P O .
3,12
Panov et al.
assessed the methane partial oxidation by
2
ꢂ
ꢂ
at 2q ¼ 34.5 . The M ¨o ssbauer data (Fig. 10) for the used catalyst created at 160 C temperature over FeZSM-5 zeolite. They
3
+
aer the reaction with N
2
O showed typical of Fe species (88%) discovered that the reactions occur through a hydrogen
and a Fe species (12%) with parameters which correlate with abstraction mechanism, making methoxy or hydroxy groups
the FePO -low quartz and Fe (PO phases. In addition, there is bounded to the a-sites. When the same reaction is performed
a component (22%) with IS and QS values corresponding to Fe
which we identied as Fe phase. These results showed an ratio equal to 1 : 1, the reactions directly provide methanol.
2
+
4
7
4 6
)
2
+
ꢂ
4 2
with heating to 160 C, they veried that at CH : N O molar
2 2 7
P O
enhancement towards the a-phase aer reaction with N O Wood et al. studied the mechanism of “catalytic” oxidation
2
atmosphere. The absence of the b-phase during the oxidation reactions of methane by nitrous oxide over Fe-ZSM-5 zeolite,
reactions has been observed in previous studies, especially and concluded that the primary products of methane oxidation
during the oxidative dehydrogenation of isobutyric acid with are methoxy groups bounded to active iron (i.e., Fe–OCH
3
). As
22,23,43
2+
a water co-feed over FePO
been reported that an FePO
4
catalyst.
In literature, it has can be seen, the Fe cations of a-sites are activated by nitrous
catalyst consisting of the quartz oxide generating adsorbed oxidant species (i.e., Fe –Ocꢁ)
3
+
4
a
-
type phase as the precursor, undergoes phase transformation sites, which convert methane to adsorbed methanol. Aer-
during a catalytic reaction involving the oxidative dehydroge- wards, adsorbed methanol can be converted to dimethyl ether
4
5
nation of isobutyric acid to form Fe (PO ) and the a-phase and water. Beznis et al. assessed the activity of Co–ZSM-5 solid
7
4 6
18,23
(Table 3).
catalysts on reactions of partial oxidation of methane. They
The diffractogram of the used catalyst using H O as an found that methanol production proportionally increased in
2
oxidant showed the presence of Fe
2
P
2
O
7
phase along with a- relation to surface area of catalyst, which can be increased
O as an oxidant treating the catalyst with NaOH. These authors discovered that
Fig. 10), showed Fe and Fe components with a relative the active sites (i.e., cobalt oxidic species, such as Co and
phase. The M ¨o ssbauer spectrum, aer using H
2
2
+
3+
(
3 4
O
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Table 4 Comparison of catalytic performance of Fe based catalysts for methane activation reaction reported in literature
Temperature
( C)
ꢂ
ꢁ1
S. no
Catalyst
Oxidant
Methanol TOF (mmolMeOH
g
cat
h
)
Reference
ꢁ
ꢁ3
4
1
2
3
4
5
6
7
8
9
2% Fe–ZSM5a
2% Fe–ZSM5
2% Fe–ZSM5
0.5% Fe–SIL-1
O
N
N
N
O
O
O
O
O
N
2
2
2
2
2
2
2
2
2
2
300
200
550
550
400
500
250
500
300
300
6.3 ꢀ 10
5.5 ꢀ 10
7.1 ꢀ 10
8 ꢀ 10
4
3
19
2
14
48
O
O
O
ꢁ3
ꢁ3
ꢁ4
ꢁ4
ꢁ4
ꢁ4
ꢁ3
ꢁ3
FePO
FePO
4
/MCM-41
/SBA-15
7.5 ꢀ 10
1.2 ꢀ 10
1.5 ꢀ 10
2.1 ꢀ 10
5.3 ꢀ 10
12.3 ꢀ 10
4
2% Co–ZSM5
0.5% Fe–SiO
45 and 47
49 and 50
This work
This work
2
FePO
FePO
4
-tdm
-tdm
1
0
4
O
a
Batch reactor, under a pressure of 2 bar.
CoO, present on catalyst external surface) were also propor- MeOH selectivity than the unsupported and the MCM-41-
14,48,50
tionally formed in relation to surface area of solid catalyst. In supported ones in the partial oxidation of CH
4 2
with O .
the presence of N O, CH would be oxidized by peroxo species The catalyst with a loading amount of 5 wt% shows the highest
2
4
MO formed by the reaction of N O with a Fe]O centre. Fe]O MeOH selectivity at a given CH4 conversion and the highest
2
2
3
,17,46
centres may be the active sites for methanol formation.
kinetic results obtained on the oxidation of CH by N O showed catalyst. It is likely that the improved catalytic performances of
that carbon oxides are probably primary products and may also the SBA-15-supported samples are related to the enhanced
stem from methanol or formaldehyde as secondary products. redox properties of FePO species, the large porous diameter
The direct pathway to carbon oxides could be ascribed to the and the high inertness of SBA-15. With compared to the other
existence of surface sites where the intermediate oxygenates catalysts reported in literature, the FePO -tdm phase catalysts
expected are strongly attached and not allowed to desorb in the showed in this work exhibited a high activity towards methanol
4
The MeOH formation rate based on the amount of FePO in the
4
2
4
48
4
ꢁ
3
ꢁ1
gas phase. In contrast, formation of methanol or formaldehyde i.e., 12.3 ꢀ 10 mmolMeOH gcat
h
using N O as an oxidant.
2
would rather correspond to oxygenate precursors more easily This catalyst also showed a high activity with O
2
as an oxidant
ꢁ
3
ꢁ1
desorbed because less retained at the catalyst surface.
(5.3 ꢀ 10 mmolMeOH
cat
g h ).
In literature, it is well established that Fe based catalysts
entail different chemical properties considering those of their
other individual monometallic components, e.g. achieving the
4
Conclusion
synergistic effects in methane activation reactions. In general, A high yield of methanol was observed over FePO when using
4
ZSM-5 catalysts show a higher activity for methane activation N O as oxidant. The methane conversion with O and N O
2
2
2
3,4
than other supports. Panov and co-workers showed the increased steeply with temperature but increased much slower
ꢂ
methane oxidation over Fe-ZSM-5 catalysts by N
2
O at 200 C and when H O was applied as an oxidizing agent. Furthermore, the
2
showed that methanol that formed via methane oxidation by a- maximum CH4 consumption (17 mol%) was obtained when
III
ꢁ
ꢂ
oxygen, CH
4
+ (Fe –Oc )a, migrated from a-sites, initiating new oxygen had been used as the oxidant at 500 C, the highest in
ꢂ
reaction cycles. At 200 C, a 4 h run provided a turn over this study. The selectivity towards methanol was very good at
ꢁ3
ꢁ1
frequency (TOF) of 5.5 ꢀ 10 mmolMeOH
g
cat
h
. Beznis and co- lower methane conversions (elevated ow rates). Additionally,
45,47
workers
showed that the selective activation of methane when a comparison was made at all the levels of CH4
towards methanol over Co–ZSM-5 can be inuenced by altering consumption, it was the highest when N O was used as an
2
the micro-and meso-porosity of the zeolite material. They oxidant. The present results clearly show that the selective
showed a linear relationship between the ZSM-5 surface area oxidation of methane over FePO is inuenced by the nature of
4
ꢁ
3
and the amount of methanol produced (5.5 ꢀ 10 mmolMeOH the oxidizing agent. Furthermore, the fresh catalyst possessed
ꢁ
1
ꢂ
g
cat
h ) over Co–ZSM-5 from methane and oxygen at 250 C a rough crystalline morphology with particle sizes ranging from
(
Table 4).
Zhang and co-workers
50 to 80 nm, and with a homogeneous dispersion of crystallites.
showed that the iron species Aer reduction, agglomeration of these nanoparticles was
48–50
introduced into mesoporous silica SBA-15 could catalyze the observed. Aer the oxidation with O , the crystalline nature was
2
selective oxidation of CH4 to methanol by O2 and that the retained, but with separate agglomerated bulk particles present.
catalyst with a Fe content of 0.5 wt% provided the highest From TPRO proles, it was evident that the type of oxidant, used
ꢁ
4
ꢁ1
single-pass yield (2.1 ꢀ 10
cat
mmolMeOH g h ). The TOF in re-oxidation, inuenced the pathway of oxidation for
towards methanol formation decreased with increasing Fe a reduced FePO catalyst.
4
content. They also studied SBA-15-supported iron phosphate
FePO ) for the partial oxidation of CH with O . The SBA-15- diffraction, proved to be a very sensitive tool in providing an
supported FePO catalysts exhibit higher CH
M ¨o ssbauer spectroscopy, complemented with powder X-ray
(
4
4
2
4
4
conversion and understanding of the phase transformations of FePO material
4
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using O
2
, H
2
O and N
2
O as oxidizing agents for the selective
9 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 and
G. J. Hutchings, ACS Catal., 2013, 3, 689–699.
conversion of methane to methanol. The M ¨o ssbauer data
provided evidence that the Fe P O phase was dominant in the
reduced catalyst sample, while its amount decreased ve-fold
aer the oxidation with O due to the formation of a-
2
2 7
2
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+
Fe
dominant phase (74%). The appearance of the Fe
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3
(P
2
O
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2
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phase 11 T. Sheppard, C. D. Hamill, A. Goguet, D. W. Rooney and
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2 2 7
P O
of the a-phase results from the transformation between the
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FePO -tdm and Fe P O phases, which takes place reversibly,
4
2 2 7
depending on strength of the redox atmosphere and the fact
that the a-Fe (P
phosphate consisting of both Fe
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4
2
O
7
)
3
2 2 7
P O
2
2
4
this work exhibited a high activity towards methanol i.e., 12.3 ꢀ
Eng. Chem., 1961, 53, 809–812.
ꢁ3
ꢁ1
ˇ
10
mmolMeOH
g
cat
h
using N
O as an oxidant. This catalyst 17 R. Polni ˇs er, M. Stolcov ´a , M. Hronec and M. Mikula, Appl.
2
ꢁ
3
also showed a high activity with O
2
as an oxidant (5.3 ꢀ 10
Catal., A, 2011, 400, 122–130.
ꢁ1
ˇ
´
mmolMeOH
g
cat
h
).
18 M. Stolcova, C. Litterscheid, M. Hronec and R. Glaum, in
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Conflicts of interest
1
2
2
2
2
9 C. Kalamaras, D. Palomas, R. Bos, A. Horton, M. Crimmin
and K. Hellgardt, Catal. Lett., 2016, 146, 483–492.
0 R. L. McCormick, G. O. Alptekin, D. L. Williamson and
T. R. Ohno, Top. Catal., 2000, 10, 115–122.
1 J. Schumann, A. Tarasov, N. Thomas, R. Schl ¨o gl and
M. Behrens, Appl. Catal., A, 2016, 516, 117–126.
2 F. B. Khan, K. Bharuth-Ram and H. B. Friedrich, Hyperne
Interact., 2010, 197, 317–323.
3 F. B. Khan, V. D. B. C. Dasireddy, K. Bharuth-Ram,
H. Masenda and H. B. Friedrich, Hyperne Interact., 2015,
There are no conicts to declare.
Acknowledgements
The authors acknowledge the nancial support from the
Slovenian Research Agency (research core funding No. P2ꢁ0152
and P1ꢁ0112) within the project Direct Conversion of Methane
to Higher Hydrocarbons using Superacid Catalysts, J2–7319.
KB-R acknowledges support from the National Research Foun-
dation (South Africa).
231, 123–129.
2
4 V. D. B. C. Dasireddy, K. Bharuth-Ram, A. Harilal, S. Singh
and H. B. Friedrich, Hyperne Interact., 2015, 231, 137–142.
5 V. D. B. C. Dasireddy, S. Singh and H. B. Friedrich, Appl.
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