Copper-Exchanged Omega (MAZ) Zeolite: Copper-concentration Dependent Active Sites and its Unprecedented Methane to Methanol Conversion

Article abstract of DOI:10.1002/cctc.201801809

The direct conversion of methane to methanol may provide a way to utilize methane that is otherwise wasted. Here we have explored one of the most promising copper zeolite materials to date, copper-exchanged omega (MAZ) zeolite. With highly crystalline and uniform zeolite omega, we can produce 150 umol-methanol/gram-zeolite under 1 bar methane and as high as 200 umol-methanol/gram-zeolite under 30 bar methane, the highest yield ever reported. Furthermore, zeolite omega's ability to convert methane to methanol has a distinct copper-concentration dependent behavior. At lower copper loadings, Cu–MAZ is inactive, and once a minimum copper loading is reached, Cu–MAZ converts methane to methanol more selective than the widely studied Cu-mordenite. With the unprecedented high conversion and selectivity, Cu–MAZ provides insight into the active copper phase and its mechanism which was quantified as two electrons per molecule of methanol produced.

Full text of DOI:10.1002/cctc.201801809

Accepted Article
Title: Copper-exchanged omega (MAZ) zeolite: copper-concentration
dependent active sites and its unprecedented methane to
methanol conversion
Authors: Amy Jenelle Knorpp, Ana Belen Pinar, Mark Newton, Vitaly
Sushkevich, and Jeroen A. van Bokhoven
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To be cited as: ChemCatChem 10.1002/cctc.201801809
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Copper-exchanged omega (MAZ) zeolite: copper-concentration
dependent active sites and its unprecedented methane to
methanol conversion
Amy J. Knorpp[a], Ana B. Pinar[b]*, Mark Newton[b], Vitaly Sushkevich[b], and Jeroen A. van Bokhoven[a,b]*
Abstract: The direct conversion of methane to methanol may provide
a way to utilize methane that is otherwise wasted. Here we have
explored one of the most promising copper zeolite materials to date,
copper-exchanged omega (MAZ) zeolite. With highly crystalline and
uniform zeolite omega, we can produce 150 umol-methanol/gram-
zeolite under 1 bar methane and as high as 200 umol-methanol/gram-
zeolite under 30 bar methane, the highest yield ever reported.
Furthermore, zeolite omega’s ability to convert methane to methanol
has a distinct copper-concentration dependent behavior. At lower
copper loadings, Cu-MAZ is inactive, and once a minimum copper
loading is reached, Cu-MAZ converts methane to methanol more
selective than the widely studied Cu-mordenite. With the
unprecedented high conversion and selectivity, Cu-MAZ provides
insight into the active copper phase and its mechanism which was
quantified as two electrons per molecule of methanol produced.
other zeolites and non-zeolite materials in an attempt to achieve
higher yields of methanol or uniform active sites.[14,17,28,29] In one
such survey of zeolite types, zeolite omega (MAZ)[30] was shown
to be comparable to mordenite (MOR) in the conversion of
methane to methanol.[28] However, zeolite omega was not pure
and contained
a common co-phase sodalite (SOD). By
synthesizing a pure, highly crystalline and uniform zeolite omega,
we report here an unprecedented conversion of methane to
methanol for copper zeolites which allows for better
understanding of the active sites and its mechanism.
A series of Cu-omega (Cu-MAZ) samples were prepared
through cation exchange with varying amounts of copper ranging
from 1.5 wt.% to 4.7 wt.% The details on the sample synthesis
and its characterization are given in the Supporting Information.
Each sample was tested for methane to methanol conversion
under the conventional stepwise procedure[9,10,13-15,17,18,21]
(activation at 723K in oxygen and reaction at 473K with methane
at 1 bar) as well as under a high-pressure procedure using
methane at 30 bar and at 473K during the methane reaction
step.[20] For comparison, a commercial mordenite (Zeoflair 800,
Zeochem AG, Si/Al = 10) was also tested. Copper-exchanged
mordenite (Cu-MOR) is known to begin converting methane to
methanol at low copper concentrations,[15] and we observed a
similar trend (Figure 1). However, a different trend is observed for
Cu-MAZ. There is a minimum copper concentration required
before any methane is converted to methanol. This is reminiscent
of the copper-dependent behavior of Cu-ZSM-5[9,10] where at low
copper concentrations no methanol is formed, and inactive
copper resides in the 6-membered rings of ZSM-5.[31] As copper
concentration increases in Cu-ZSM-5, the active site was
determined spectroscopically as a Cu dimer (i.e. CuOCu).[18]
However, for ZSM-5 only a small fraction (5-10%) of the additional
copper is active with the majority remaining inactive, thus resulting
During petroleum extraction, methane often accompanies the
crude oil as it is carried to the surface. Utilizing this methane is
not always economically feasible due to the low price of methane
and the high costs of contaminant removal and transportation.[1]
Depending on local regulations, this methane is exhausted as
waste or converted to carbon dioxide by flaring.[2] Ultimately this
results in a waste of fossil fuels with an estimated 143 billion m3-
/year.[3] To avoid this waste, converting this methane to more
valuable molecules like methanol could be a win-win situation for
economics and the environment.[4] Directly converting methane to
methanol has its share of scientific challenges that have given it
a reputation as the “dream reaction.”[5–7] The methane must be
stabilized in a partially oxidized state rather than undergo much
more favorable complete oxidation to carbon dioxide.[7,8] Copper
exchanged zeolites can do just that when a stepwise looping
procedure is applied.[9–17] The most widely studied copper zeolites
are ZSM-5 and mordenite, and the overall methanol yield has
improved over the last 15 years as we have gained better
understanding of the procedure, extraction and favorable zeolite
characteristics.[9-23] However, the yield is still below any
industrially viable levels, and there is no consensus on the active
site or mechanism responsible for the methane to methanol
conversion.[9,10,13,15,18,21,23-27] This has led researchers to explore
[a]
[b]
Amy Knorpp, Dr. Mark Newton, Prof. Dr. Jeroen A. van Bokhoven
Institute for Chemical and Bioengineering
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jeroen.vanbokhoven@chem.ethz.ch
Dr. Ana B. Pinar, Dr. Vitaly Sushkevich, Prof. Dr. Jeroen A. van
Bokhoven
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institute
5232 Villigen (Switzerland)
Figure 1: Methanol yield as a function of copper loading for Cu-omega and
Cu-mordenite. Only active copper loadings were used to establish the
trendline. For zeolite omega, the x-intercept indicates the minimum copper
loading required for conversion.
E-mail: ana.pinar@psi.ch
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in low methanol yields (10-20 µmol-MEOH/g-zeolite) at higher
copper contents.[9,10] Cu-MOR shows no minimum Cu threshold
activity behavior, but Cu-MOR has been shown to contain mixed
co-existing inactive and/or multiple active copper sites.[24]
Generating single sites within zeolites has been one way to make
a step forward in identifying the active site, as seen for the
identification of trimers in mordenite.[15]
By contrast to ZSM-5, in MAZ nearly all additional copper is
active after the minimum copper requirement is reached. As the
copper concentration is increased, the methanol yield increases
at a higher rate than for mordenite with a methanol yield as high
as 150 µmol-MeOH/g-zeolite for 1 bar of methane. Methanol yield
can further be increased to 200 µmol-MeOH/g-zeolite by reacting
with methane at 30 bar.
For the lowest copper concentration (1.5 wt%. Cu),
methanol was not produced during the reaction at 1 bar. By
extrapolating the x-intercept in Figure 1 for the active samples, we
can quantify the inactive copper and the minimum concentration
of copper needed in zeolite omega for methane to methanol
conversion (297 µmol-Cu/g-zeolite for 1 bar). By increasing the
methane pressure to 30 bar, the methanol yield increases across
all copper concentrations and shifts the x-intercept by 71 µmol-
Cu/g-zeolite to 226 µmol-Cu/g-zeolite. We speculate that the
pressure is activating previously inactive sites, as was seen in
mordenite.[20] By noting the slope (Figure 1), the amount of
coppers participating per methanol for the 1 bar procedure is 2.8
mol-Cu/mol-MeOH and for the 30 bar procedure it is 2.6 mol-
Cu/mol-MeOH, suggesting that at least two copper atoms are
required for methanol formation, and proposed sites with three or
more participating coppers can be excluded for this system.
By in-situ Fourier-transform infrared spectroscopy (FTIR),
Cu-MAZ shows superior selectivity to Cu-MOR, and this explains
in part the better performance of Cu-MAZ over Cu-MOR. Across
all copper loadings of Cu-MAZ, no by-products were observed,
but for Cu-MOR both carbon monoxide (Figure 2) and formate
(Figure S7) were detected. The framework structure of omega
provides a favorable environment to stabilize the methoxy species
rather than over-oxidation. The asymmetric and symmetric
vibrations of the C-H methyl group for these methoxy species
were observed in the FTIR and follow a similar concentration-
Figure 3: In-situ Cu K-edge XANES spectra of Cu-MAZ with varying copper
loadings at the end of reaction with methane at 473K. The activated form is
after treatment in oxygen at 723K for 1 hour, and the reduced state was
formed by heating to 723K in methane.
dependent behavior as observed in reactor studies (Figure 1). For
the lowest loaded zeolite omega, no methoxy species were
detected, and this further confirms the reactivity studies that show
that this sample is inactive. As the copper content increases, the
methoxy species become more prominent as expected from the
reaction studies and surpasses mordenite (Figure S7).
To better understand the role of copper in the reaction, we
examined this series of Cu-MAZ samples by in-situ XAS at the
DUBBLE and SNBL beamlines at the European Syncrotron
Radiation Facility. Changes in the Cu K-edge XANES spectra
were monitored throughout the activation and reaction steps. This
technique allows for in-situ probing of the oxidation states and is
sensitive to local electronic and geometric structure of the
copper.[32,33] Similar to what has been observed in other Cu-
zeolites[12,13,15,17], during activation the maximum absorption
decreases while a shoulder at 8986 eV becomes pronounced due
to the dehydration and the lowering of the first coordination shell
(Figure S5).
The XANES spectra were similar for all loadings of copper
for the activation phase (Figure S5). However, the samples varied
greatly for the conversion of Cu(II) to Cu(I) during the reaction
step (Figure 3). The lowest loaded/inactive sample only showed
a small amount of Cu(II) converted to Cu(I). As the copper content
increased, the conversion of Cu(II) to Cu(I) increases.
An accurate correlation and quantification of the reduction
of Cu(II) to Cu(I) due to methanol formation can be complicated
by the formation of by-products like carbon monoxide, which will
also convert Cu(II) to Cu(I). However, given the distinct regions of
active and inactive copper in Cu-MAZ, the correlation between
Cu(I) and methanol is apparent, and the high selectivity of zeolite
omega means that quantification of the mechanism will be more
accurate than for any other previous Cu-zeolite.[34] By comparing
Cu(I) to the methanol production, we are able to quantify the
electrons involved in the conversion of methane to methanol on
Cu-omega at each loading (Figure 4) as well as for additional
synthesized Cu-MAZ samples (Table S2). With this method,
about two electrons are participating in the conversion of each
methane to methanol, pointing toward a two-electron redox
mechanism.
Figure 2: In-situ FTIR spectra of surface species formed during the reaction
with methane at 473K for 30 minutes. (Left) Methoxy is the surface
intermediate species for methanol. (Right) Carbon monoxide is formed due
to unselective conversion of methane to methanol in Cu-MOR.
In Figure 4, we summarize two methods to quantify the
copper and electrons involved for the conversion of methane to
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Calbry-Muzyka for proofreading. Dr. Pinar and Dr. Sushkevich
acknowledges the Energy System integration (ESI) platform at
the Paul Scherrer Institute for funding.
Keywords: copper zeolites • heterogeneous catalysis • mazzite•
methane • methanol
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The authors would like to thank Dr. Hermann Emerich at the
Swiss Norwegian beamline (BM 31) as well as Dipanjan Banerjee
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One of the most promising copper
zeolite materials to date is explored
for the direct conversion of methane to
methanol, copper-exchanged omega
(MAZ) zeolite. At lower copper
loadings, Cu-MAZ is inactive, and
once a minimum copper loading is
reached, Cu-MAZ converts methane
to methanol with remarkable
Amy J. Knorpp, Ana B. Pinar*, Mark
Newton, Vitaly Sushkevich, Jeroen A.
van Bokhoven*
Copper-exchanged omega (MAZ):
copper-concentration dependent
active sites for methane to methanol
conversion
selectivity and an unprecedented
methanol yield which allowed for the
determination of the mechanism as a
two-electron process.
This article is protected by copyright. All rights reserved.