Comparative performance of Cu-zeolites in the isothermal conversion of methane to methanol

Article abstract of DOI:10.1039/c9cc05659a

The isothermal, low-temperature stepwise conversion of methane to methanol over copper-exchanged zeolites eliminates the time-consuming heating and cooling steps of the conventional high temperature activation approach. To better understand differences between the two approaches, a series of zeolites were screened, of which omega zeolite (MAZ) showed superior performance in both the isothermal and conventional approaches.

Full text of DOI:10.1039/c9cc05659a

View Article Online
View Journal
ChemComm
Chemical Communications
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Knorpp, M. A.
Newton, S. C. M. Mizuno, J. Zhu, H. Mebrate, A. B. Pinar Prieto and J. van Bokhoven, Chem. Commun.,
2019, DOI: 10.1039/C9CC05659A.
Volume 54
Number
January 2018
Pages 1-112
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
1
4
ChemComm
Chemical Communications
rsc.li/chemcomm
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
rsc.li/chemcomm

Page 1 of 5
Please d oC hn eo mt Ca do mj u ms t margins
View Article Online
DOI: 10.1039/C9CC05659A
COMMUNICATION
Comparative performance of Cu-zeolites in the isothermal
conversion of methane to methanol
a
a
b
a,b
b
Amy J. Knorpp, Mark A. Newton, Stefanie C. M. Mizuno, Jie Zhu, Hiwote Mebrate, Ana B.
Received 00th January 20xx,
Accepted 00th January 20xx
b
a,b
Pinar,* Jeroen A. van Bokhoven*
DOI: 10.1039/x0xx00000x
The isothermal, low-temperature stepwise conversion of methane line with steam or off-line with water).4,10–15 This conventional
to methanol over copper-exchanged zeolites eliminates the time- high temperature activation procedure has been extensively
consuming heating and cooling steps of the conventional high used to evaluate methanol yields achievable for a wide range of
4
,13,16,17
temperature activation approach. To better understand differences zeolite structures.
Within this stepwise procedure, much
between the two approaches, a series of zeolites were screened, of focus has been given to the nature of the active site with
4
,14,18–23
which omega zeolite (MAZ) showed superior performance in both multiple different motifs proposed.
In spite of multiple
the isothermal and conventional approaches.
proposed active sites, these works agree that high activation
temperature (>623K) in oxygen is required to form the active
sites which store the activated oxygen that will selectively
During the formation of fossil fuels over millions of years, a
mixture of crude oil and methane often evolved together.
Ultimately this leads to their co-extraction at petroleum
extraction sites. For remote oil extraction sites, this methane
can be more expensive to transport than its market value, and
depending on local environmental legislation, the methane is
4,14,24
convert methane to methanol.
It was assumed that a high
temperature activation was required until a low-temperature,
isothermal, procedure was introduced where the activation and
reaction are conducted at 473K but with an elevated pressure
25,26
of methane to boost the methanol yield.
The isothermal
1
reinjected, flared, or vented into the atmosphere. Converting
procedure suffers some penalty in regards to methanol yield in
this stranded methane to different products (i.e. methanol)
would not only mitigate waste but also allow for a nimbler
25
the case of Cu-mordenite (MOR), but it has a clear advantage
in terms of scale-up and ultimately, industrial implementation,
because the isothermal procedure eliminates time-consuming
heating and cooling between steps and cycles. Recent
economic analysis of the direct conversion of methane to
methanol has shown that (at least) a 50-fold improvement is
needed for this process to be considered as industrially
promising, and it highlighted research areas (methanol
productivity, cost of material, and cycling time) to close this
2
hydrocarbon economy. Methane is industrially converted to
methanol using a catalytic steam reforming process to produce
syngas in the first stage, followed by subsequent catalytic
3
reaction to form methanol. This route to methanol is too costly
for implementation at dispersed methane sources.
Alternatively, methane can be converted to methanol directly
4
over copper-exchanged zeolites; however, the development of
this approach has not reached a level of industrial applicability,
though it is advancing rapidly. Direct methane to methanol
conversion is difficult, because the higher reactivity of methanol
9
gap. Given this, and the possible cycling time improvement, the
isothermal procedure needs just as much or more attention
paid to it as the conventional procedure, with the ultimate goal
of rationally designing a zeolite that not only produces high
methanol yields in the laboratory but can also be more easily
implemented beyond the laboratory.
Unlike the conventional procedure, the isothermal route
has not been as extensively studied. The first reports of the
isothermal procedure evaluated three zeolite (Cu-Y, Cu-MOR,
5
–8
than methane sets a limit to the yield that can be obtained.
Currently, research in this area is at a transition point where
scientific refocusing towards processes with more industrial
9
feasibility are needed.
One promising system using copper-exchanged zeolites is a
stoichiometric stepwise (“chemical looping”) procedure. The
most widely studied stepwise procedure requires a high
25,26
and Cu-ZSM-5).
yield of 55 µmol/gram-zeolite at 30 bar methane pressure.
This value is below regularly observed methanol yields (~160
Cu-MOR achieved the highest methanol
2
temperature activation step (723K in O ), a methane reaction
25
step (473K in CH ), and finally a methanol extraction step (on-
4
2
7
µmol/gram-zeolite) for the conventional procedure.
Conversely for Cu-Y, more methanol was produced in the
isothermal procedure than in the conventional procedure
where it is virtually inactive; and with further optimization of
the reaction temperature, its methanol yield can be further
a. Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1,
093 Zurich, Switzerland. E-mail: Jeroen.vanbokhoven@chem.ethz.ch
Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, 5232
Villigen, Switzerland
8
2
5
b.
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please d oC hn eo mt Ca do mj u ms t margins
Page 2 of 5
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C9CC05659A
Figure 1: Methanol yields of various zeolites under isothermal conditions (6 bar methane) and conventional, high temperature activation conditions. Chemical analysis
of exchanged zeolites tested for isothermal conversion of methane to methanol. Typical structure features, pore and channels sizes are included.31
increased.28 This observation for Cu-Y is important because it minutes. For comparison, the same samples were also tested
shows that the structure-property relationships and active sites under conventional high temperature activation conditions
for the low-temperature, isothermal procedure are different with activation for 1 hour in oxygen at 723 K and reaction at 1
than for the well-studied conventional procedure. This opens bar methane for 30 minutes at 473K. Methanol was extracted
several questions for copper-exchanged zeolites and the off-line and quantified by GC-FID. Figure 1 shows the methanol
isothermal procedure, that including: what is the active site?; yields for the screened zeolites under both the isothermal and
are there patterns to structure-property relationships?; and, conventional high temperature activation approaches.
can higher methanol yields be achieved?
A very wide variation in performance according to the
To better understand the similarity and differences between applied method and the zeolite type employed results from this
the low-temperature isothermal approach and the comparison. The zeolites (FER, CHA, MOR6.5,MOR10, and MAZ)
4
,13,14,27
conventional high temperature activation approach, a series of that are active in the conventional procedure
all show
zeolite frameworks were screened in both procedures. Zeolite conversion for the isothermal procedure, and the conventional
samples were selected based on their varying performance in procedure consistently outperforms the isothermal one. The
the conventional high temperature activation procedure. First highest methanol yield was observed for MAZ-B with 76 µmol-
zeolites with high activity in the conventional procedure were MeOH/gram-zeolite; however, whether omega zeolite (MAZ) is
selected (CHA,1
3,29
FER,
17
and MAZ
16,30
). Secondly the active is highly dependent on its synthesis and resulting
isothermal procedure allows an opportunity to examine more morphology. For MAZ-A, the methanol yield was severely
zeolite frameworks than the conventional procedure, due to reduced with only 14 µmol-MeOH/gram-zeolite for the
the fact some zeolites are not stable at high activation isothermal method. For low-copper containing MAZ-B, both the
temperatures. Without having to consider this limitation, we conventional and isothermal approaches yielded no methanol.
were able to select zeolites (MER, GIS, LTA, and OFF) based on
the quantity of 8-membered rings in a unit cell.
Conversely, samples LTA and OFF were inactive in the
conventional high temperature activation procedure, but
3
1
Additional samples of omega zeolite (MAZ) with varying showed activity in the isothermal approach. OFF in particular
performance in the conventional high temperature activation yielded 21.8 µmol-methanol/gram-zeolite, which is comparable
approach were selected. For the conventional procedure, the to the methanol yields observed for MOR10. Unlike the MOR
methanol yield for omega zeolite shows a dependence on the framework, the OFF framework does not have 8-membered
zeolite’s morphology. The highest methanol yield was produced channels and yet produces methanol with yields equivalent to
for the parent zeolite with large 1-2µm bundle rods, and for a MOR6.5 under low-temperature, isothermal conditions. This
morphology of small spheres composed of 300 µm particles, the indicates that such channels are not absolutely necessary for
3
2
methanol yield was severely reduced. The small spheres are the isothermal procedure. What may be more important are the
denoted as MAZ-A and the larger bundled rods as MAZ-B. 8-membered side pockets or the larger 12-membered ring that
2
0
Additionally MAZ-B is inactive for methane conversion at low are observed in both structures.
loadings of copper for the conventional procedure, and this
3
0
No methanol was observed for MER, even though it has high
inactive MAZ-B sample was also tested for the isothermal density of 8-membered rings per unit cell, a structural feature
conversion. Figure 1 shows the zeolites analyzed and the previously concluded to be important for selective oxidation of
1
6
chemical analysis as determined by atomic absorption methane. For samples GIS and LTA degradation of the
spectroscopy as well as structural features of each zeolite. framework was observed in the reacted material despite the
There are no established “standard” conditions to screen low-temperature approach (Figure S3), which explains these
different zeolites in the isothermal procedure. As such, we zeolite poor performance.
selected a scheme that used activation for 1 hour at 473K in
Figure 1 gives additional evidence that the structure-
oxygen followed by a reaction in methane at 6 bar for 30 property relationships of this system are more complex than
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
Please d oC hn eo mt Ca do mj u ms t margins
Journal Name
COMMUNICATION
would be expected from considerations based purely on the
pore and channel size. However, it appears to be generally true
that if a structure is active in the conventional procedure, it is
active when isothermally done, and factors such as the zeolite
synthesis (and resulting morphology) will impact the two
reactive procedures in similar manners. Conversely, if a
structure is inactive in the conventional procedure, it may or
may not be active in the isothermal procedure, and therefore
screening of these frameworks is required.
View Article Online
DOI: 10.1039/C9CC05659A
MAZ-B showed the most promising conversion under
isothermal conditions, and it was further investigated to better
understand the isothermal procedure. To improve the
methanol yields, the reaction was conducted under different
pressures of methane (Figure 2). For each pressure, the
methanol was extracted offline, and multiple extractions were
conducted until no methanol was observed by GC. Figure 2
shows that MAZ-B and MOR10 follow a similar trend, with a
sharp increase of the methanol yield at pressures between 1
and 6 bar, followed by levelling off of methanol yield at higher
pressures. At 20 bar methane, a high methanol yield of 141
µmol-MeOH/gram-zeolite is achieved, which is nearly
equivalent to the highest reported yields obtained from the
conventional high temperature activation procedure for MAZ-B
Figure 2: Methanol yield as pressure increases for MAZB and MOR10. By extending the
activation and reaction time, the methanol yield can be further increased if the reaction
is conducted at low pressure.
To understand the mechanism, the copper oxidation states
were tracked by in-situ Cu K-edge XANES throughout the oxygen
activation and methane reaction. The high copper loaded MAZ-
B has Cu(I) increasing rapidly between 1 and 6 bar and then
subsequently levels off (Figure 3). This pressure dependent
trend is similar to methanol yield obtained in the reaction
studies in Figure 2 as well as previous in-situ Cu K-edge XANES
3
3
for isothermal Cu-MOR. For the inactive low-copper loaded
MAZ-B, the pressure has little effect on the formation of Cu(I)
up to 6 bar with Cu(I) only being observed to form above 9 bar.
At elevated pressures of 20 bar methane, this sample became
2
7
(150 µmol-MeOH/gram-zeolite) and Cu-MOR. However, this
yield is lower than reported for MAZ-B when both high
temperature activation (723 K) and high pressures (36 bar) (200
3
active with a small amount of methanol (11 µmol-CH OH/gram-
3
0
zeo.) extracted. Under high methane pressure, previously
inactive copper in MAZ-B can become active. This was also
observed for MAZ-B in the high activation temperature
µmol-MeOH/g-zeolite) are applied. MAZ-B shows superior
selectivity to MOR10, which results in MAZ-B’s higher methanol
yield. In-situ FTIR (Figure S6) shows trace by-products of
formate and carbon monoxide for MOR10, while for MAZ-B only
trace carbon monoxide was observed. In general, the selectivity
for the isothermal and the conventional procedures for both
procedures are very high; however, the selectivity-conversion
3
0
procedure. The reason for this can only be speculated, but it
3
6
is possible that improved diffusion, copper mobililty and the
higher reduction potential may be responsible for this effect.
The observed reduction of copper in Figure 3 is consistent
with a mechanism that involves a reduction-oxidation reaction
with Cu(I) formation as fundamental to methanol formation.
Increasing evidence has recently been derived to suggest that
this is the dominant mechanism for conversion of methane to
methanol under high activation temperature conditions,
7
limit applies, and the conversion is unquantifiably low.
When the reaction and activation steps are extended by
four-fold in time, the methanol yield increases for the lower
pressure of methane case (3 bar methane), but the methanol
yield remains similar for the high pressure of 20 bar (Figure 2).
This may result from diffusion limitations at the lower
temperature of operation that are overcome at higher methane
pressures.
2
7,30,34,35
and this mechanistic motif can be expanded to the
isothermal process for Cu-MOR.33 Figure 3 shows the Cu(I)
formed per methanol averaged over two separate in-situ Cu K-
edge XAS experiments. As the pressure is increased, the results
Figure 3: (Left) In-situ XAS at the Cu k-edge of low and high copper loaded MAZ-B as methane pressure increases during the reaction for the low temperature, isothermal protocol.
Right) As the methane pressure increases, the higher copper loaded MAZ increases the formation of Cu(I) and levels off after 10 bar. Meanwhile, the low copper loaded sample,
Cu(I) formation only occurs after 6 bar methane. Table shows the Cu(I) formed per methanol which indicates that it is a two-electron redox process at higher pressures.
(
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please d oC hn eo mt Ca do mj u ms t margins
Page 4 of 5
COMMUNICATION
Journal Name
2 E. M. Alayon, M. Nachtegaal, M. Ranocchiari and J. A. van
1
1
1
converge to approximately 2 Cu(I) formed per methanol. This
indicates, at least mechanistically, the conversion of methane
to methanol under the isothermal and conventional procedures
View Article Online
Bokhoven, Chem. Commun., 2012, 48, 404–406.
DOI: 10.1039/C9CC05659A
3 M. J. Wulfers, S. Teketel, B. Ipek and R. F. Lobo, Chem.
Commun., 2015, 51, 4447–4450.
4 S. Grundner, M. A. C. Markovits, G. Li, M. Tromp, E. A. Pidko,
E. J. M. Hensen, A. Jentys, M. Sanchez-Sanchez and J. A.
Lercher, Nat. Commun., 2015, 6, 7546.
3
4
are similar. One may speculate that a precursor species to the
generally assumed active site formed during the high-
temperature activation is active in the case of isothermal and
low temperature operation. However, water is not completely
removed during activation of isothermal procedure, leaving
partially water-poisoned active sites (Figure S7).
1
5 S. E. Bozbag, E. M. C. Alayon, J. Pecháček, M. Nachtegaal, M.
Ranocchiari and J. A. van Bokhoven, Catal. Sci. Technol.,
2
016, 6, 5011–5022.
1
1
6 M. B. Park, S. H. Ahn, A. Mansouri, M. Ranocchiari and J. A.
van Bokhoven, ChemCatChem, 2017, 9, 3705–3713.
By expanding the database of zeolites tested for the
conversion of methane to methanol under isothermal
conditions, there is a significant overlap between zeolites (MAZ-
B, MOR10, MOR6.5, CHA, and FER) that are active for both the
conventional, high temperature and isothermal stepwise
approaches to the selective activation of methane. Conversely,
there are also zeolites (OFF) that are active in the isothermal but
not the conventional procedure, showing that the structure-
activity relationships observed in the conventional procedure
cannot be absolutely applied to the isothermal procedure
across all zeolite types. However, there is similarity between the
two methods in respect to the mechanism, which is a 2-electron
reduction process, and the selection of the parent zeolites
morphology can affect the final yield.
7 D. Pappas, E. Borfecchia, M. Dyballa, K. A. Lomachenko, A.
Martini, G. Berlier, B. Arstad, C. Lamberti, S. Bordiga, U.
Olsbye, S. Svelle and P. Beato, ChemCatChem, 2019, 10, 621–
6
27.
1
1
8 J. S. Woertink, P. J. Smeets, M. H. Groothaert, M. A. Vance, B.
F. Sels, R. A. Schoonheydt and E. I. Solomon, Proc. Natl. Acad.
Sci., 2009, 106, 18908–18913.
9 P. Vanelderen, J. Vancauwenbergh, M. L. Tsai, R. G. Hadt, E. I.
Solomon, R. A. Schoonheydt and B. F. Sels, ChemPhysChem,
2
014, 15, 91–99.
2
2
2
0 B. E. R. Snyder, P. Vanelderen, R. A. Schoonheydt, B. F. Sels
and E. I. Solomon, J. Am. Chem. Soc., 2018, 140, 9236–9243.
1 V. L. Sushkevich, D. Palagin, M. Ranocchiari, J. A. van
Bokhoven, Science, 2017, 356, 523.
2 J. Meyet, K. Searles, M. A. Newton, M. Wörle, A. P. van Bavel,
A. D. Horton, J. A. van Bokhoven and C. Coperet, Angew.
Chem. Int. Ed., 2019, 58, 9841-9845.
Of the screened zeolites, MAZ-B showed superior
performance, and by further investigating MAZ-B under low- 23 D. Palagin, A. J. Knorpp, A. B. Pinar, M. Ranocchiari and J. A.
van Bokhoven, Nanoscale, 2017, 9, 1144–1153.
temperature isothermal conditions, conversions of methane to
methanol at levels that are commensurate with the
conventional high temperature activation have been achieved.
Furthermore, these levels come without the significant penalty
of very large swings in temperature required by the latter
method.
2
4 E. M. C. Alayon, M. Nachtegaal, A. Bodi, M. Ranocchiari and
J. A. van Bokhoven, Phys. Chem. Chem. Phys., 2015, 17,
7
681–7693.
2
5 P. Tomkins, A. Mansouri, S. E. Bozbag, F. Krumeich, M. B.
Park, E. M. C. Alayon, M. Ranocchiari and J. A. van Bokhoven,
Angew. Chemie - Int. Ed., 2016, 55, 5557–5561.
2
2
6 P. Tomkins, M. Ranocchiari and J. A. van Bokhoven, Acc.
Chem. Res., 2017, 50, 418–425.
7 D. K. Pappas, A. Martini, M. Dyballa, K. Kvande, S. Teketel, K.
A. Lomachenko, R. Baran, P. Glatzel, B. Arstad, G. Berlier, C.
Lamberti, S. Bordiga, U. Olsbye, S. Svelle, P. Beato and E.
Borfecchia, J. Am. Chem. Soc., 2018, 140, 15270–15278.
8 V. L. Sushkevich and J. A. van Bokhoven, ACS Catal., 2019, 9,
Conflicts of interest
There are no conflicts to declare.
2
2
6
293–6304.
Notes and references
9 D. K. Pappas, E. Borfecchia, M. Dyballa, I. A. Pankin, K. A.
Lomachenko, A. Martini, M. Signorile, S. Teketel, B. Arstad,
G. Berlier, C. Lamberti, S. Bordiga, U. Olsbye, K. P. Lillerud, S.
Svelle and P. Beato, J. Am. Chem. Soc., 2017, 139, 14961.
0 A. J. Knorpp, A. B. Pinar, M. Newton, V. Sushkevich and J. A.
van Bokhoven, ChemCatChem, 2018, 10, 5593–5596.
1 C. Baerlocher and L. B. McCusker, Database of Zeolite
Structures:, http://www.iza-structure.org/databases/.
2 A. J. Knorpp, M. A. Newton, V. L. Sushkevich, P. P.
Zimmermann, A. B. Pinar and J. A. van Bokhoven, Catal. Sci.
Technol., 2019, 9, 2806–2811.
1
T. A. Brzustowski, Prog. Energy Combust. Sci., 1976, 2, 129–
41
1
3
3
3
2
3
G. A. Olah, Angew. Chemie - Int. Ed., 2005, 44, 2636–2639.
G. C. Chinchen, P. J. Denny, J. R. Jennings, K. C. Waugh and P.
Group, Appl. Catal., 1988, 36, 1–65.
4
5
6
7
8
9
1
1
M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs and R.
A. Schoonheydt, J. Am. Chem. Soc., 2005, 127, 1394–1395.
J.-P. Lange, K. P. De Jong, J. Ansorge and P. J. A. Tijm, in
Studies in Surface Science and Catalysis, 1997, 107, 81–86.
M. Ahlquist, R. J. Nielsen, R. A. Periana and W. A. Goddard, J.
Am. Chem. Soc., 2009, 131, 17110–17115.
M. Ravi, M. Ranocchiari and J. A. van Bokhoven, Angew.
Chemie - Int. Ed., 2017, 56, 16464–16483.
A. A. Latimer, A. Kakekhani, A. R. Kulkarni and J. K. Nørskov,
ACS Catal., 2018, 8, 6894–6907.
J.-P. Lange, V. L. Sushkevich, A. J. Knorpp and J. A. van
Bokhoven, Ind. Eng. Chem. Res., 2019, 58, 8674–8680.
0 N. V. Beznis, B. M. Weckhuysen and J. H. Bitter, Catal.
Letters, 2010, 138, 14–22.
1 P. Vanelderen, R. G. Hadt, P. J. Smeets, E. I. Solomon, R. A.
Schoonheydt and B. F. Sels, J. Catal., 2011, 284, 157–154.
3
3
3 A. J. Knorpp, M. A. Newton, A. B. Pinar and J. A. van
Bokhoven, Ind. Eng. Chem. Res., 2018, 57, 12036–12039.
4 M. A. Newton, A. J. Knorpp, A. B. Pinar, V. L. Sushkevich, D.
Palagin and J. A. van Bokhoven, J. Am. Chem. Soc., 2018, 140,
1
0090–10093.
3
3
5 G. Brezicki, J. D. Kammert, T. B. Gunnoe, C. Paolucci and R. J.
Davis, ACS Catal., 2019, 9, 5308–5319.
6 C. Paolucci, I. Khurana, A. A. Parekh, S. Li, A. J. Shih, H. Li, J. R.
Di Iorio, J. D. Albarracin-caballero, A. Yezerets, J. T. Miller, W.
N. Delgass, F. H. Ribeiro, W. F. Schneider and R. Gounder,
Science, 2017, 357, 898–903.
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
ChemComm
View Article Online
DOI: 10.1039/C9CC05659A
A series of zeolites were screened for the direct conversion of methane to methanol under
isothermal low-temperature stepwise conditions, of the screened zeolites, omega zeolite (MAZ)
showed superior performance.