On the Mechanism Underlying the Direct Conversion of Methane to Methanol by Copper Hosted in Zeolites; Braiding Cu K-Edge XANES and Reactivity Studies

Article abstract of DOI:10.1021/jacs.8b05139

The application and quantification of in situ copper K-edge X-ray absorption near-edge structure (XANES), when linked to independently made reactor-based studies of methanol production, result in a majority relation between the production of Cu^I and methanol from methane that complies with the expectations of a two-electron mechanism founded upon Cu^II/Cu^I redox couples.

Full text of DOI:10.1021/jacs.8b05139

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Communication
On the mechanism underlying the direct conversion
of methane to methanol by copper hosted in zeolites;
braiding Cu K-edge XANES and reactivity studies.
Mark A. Newton, Amy J. Knorpp, Ana B. Pinar, Vitaly. L.
Sushkevich, Dennis Palagin, and Jeroen A. van Bokhoven
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05139 • Publication Date (Web): 02 Aug 2018
Downloaded from http://pubs.acs.org on August 3, 2018
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On the mechanism underlying the direct conversion of methane to
methanol by copper hosted in zeolites; braiding Cu K-edge XANES
and reactivity studies.
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Mark A. Newton1* Amy J. Knorpp,1 Ana B. Pinar,1 Vitaly L. Sushkevich,2 Dennis Palagin,2 Jeroen A.
van Bokhoven.1,2*
1 Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering , ETH, VladimirꢀPrelogꢀ
Weg 1ꢀ5/10, 8093 Zürich, Switzerland.
2 Laboratory for catalysis and sustainable chemistry, Paul Scherrer Institute (PSI), Villigen 5232 Switzerland
Supporting Information Placeholder
The conversion of methane to methanol is a twoꢀelectron proꢀ
cess. Other products observed to be formed by these materials are
higher order in the electrons they require to be handled for their
production; e.g. six electrons for formates and carbon monoxide
and eight for carbon dioxide. In principle this understanding can
provide a framework for deciding whether this conversion is
founded upon a CuI/CuII redox couple or whether another mechaꢀ
nism is at work. If the CuI/CuII mechanism predominates then we
should find that the ratio of CuI to methanol formed should be no
less than two and that, for progressively less selective materials, it
should tend toward infinity. If another mechanism, for instance
that based upon a CuII/CuIII couple, or one that proceeds via a
CuIIꢀO● radical species, is at work then for 100% selectivity to
methanol, CuI should not be observed at all.
ABSTRACT: The application and quantification of in situ copꢀ
per Kꢀedge XANES, when linked to independently made reactorꢀ
based studies of methanol production, result in a majority relation
between the production of CuI and methanol from methane that
complies with the expectations of a two electron mechanism
founded upon CuII/CuI redox couples.
A viable process for direct conversion of methane to chemicals
of higher value and utility, such as methanol, has become the
target of much research.1 Since the demonstration that copper
hosted in ZSMꢀ52ꢀ5 is active for this conversion, a range of zeolite
systems have been shown to be active (e.g. mordenite (MOR).6ꢀ24
,
31
chabasite (CHA),25ꢀ30 and mazzite (MAZ))27,
and numerous
Cu Kꢀedge XAS permits a quantitative insight into the oxidaꢀ
tion states of copper present under reaction conditions.35,36 Indeed,
Cu Kꢀedge XAS has been much used to gain insight into the
species active for this conversion.7, 9, 10, 12, 14, 23, 28, 34 However, the
nature of the zeolites makes obtaining quantitative reactivity and
selectivity data at the same time a difficult proposition that has yet
to be achieved. In these microporous materials, methanol, along
with other products, must be either extracted,6, 7, 9, 10, 18, 23, 27, 28 or
desorbed using steam in a post factum manner:12, 13, 23
works have tried to understand how this conversion is facilitated.
What has evolved, in a situation that mirrors debate concerning
copper containing enzymes that also convert methane to methaꢀ
nol,32ꢀ34 is a diversity of opinion as to the active copper species
15, 19, 28ꢀ31
with monoꢀ and bis µꢀoxo copper dimers,3ꢀ11,
copper
trimers,12 13, 15, 19, 20, 24, 31 and monomers16, 17 all being proposed.
This difference of opinion has fundamental ramifications for
the reaction mechanism. Mediation of methanol conversion by
dimers or pairs of Cu atoms3ꢀ11, 16, 17, 19, 28ꢀ31 favours a mechanism
founded upon a CuII/CuI couple. Conversely, the invocation of a
single [Cu3O3]2+ species12, 13, 15, 19, 20, 24, 31 prefers the presence CuII
and CuIII centres.33 In this mechanism, that has also been derived
for dimers,15 the appearance of CuI in is associated with over
oxidation of methanol (to yield, for instance, carbon monoxide,
carbon dioxide, and formates) and not selectivity for methanol.
Indeed, the formal reduction of copper centres is not required,
with charge localization occurring via the formation of a CuIIꢀO●
radical species.15
We have taken the following approach: copper systems, based
upon mordenite (MOR), mazzite (MAZ), chabasite, (CHA) and
ZSMꢀ5 (MFI) have been measured for reactivity toward methane
using in situ Cu Kꢀedge XANES, for both high temperature actiꢀ
vation (723 K in flowing oxygen (15 mlminꢀ1)) and isothermal 4, 22
(activation at 473 K in oxygen followed by pressurization in
methane to between 6 and 15 bar) routes. Methanol yields for
each sample were then established using either by extraction and
gas chromatography, (GC) or online Mass spectrometry, (MS).
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Figure 1 shows examples of copper Kꢀedge XANES obtained
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from different Cu/MOR samples activated at 723 K in oxygen; an
example of the Cu Kꢀedge XANES obtained from a fresh sample
((B);blue) is also given along that for a bulk CuII (CuO) reference
(red). A complete list of sample preparation, characterization, and
performance is given as SI.
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Figure 2. Cu Kꢀedge XANES for each of samples shown in Figure
1 (AꢀE; see SI. table 5a) after exposure to flowing methane at 473
K, along with: (red) sample B after heating in methane at 723 K
(B@723 K/CH4); and, (blue) a bulk Cu2O reference.
In each case, exposure to methane at 473 K, results in a portion
of the CuII being transformed to CuI. The degree to which this
happens varies very considerably; a situation that reflects the
literature where, for highꢀtemperature activation, levels of CuI
observed after nominally the same treatment, also vary greatly.7, 9,
10, 12, 14, 23 More often than not7, 9, 14, 23, 28 quantification of CuI has
previously been made through reference to a standard material
(Cu2O). From Figure 2 we see that both the intensity and the
position of the CuI preꢀedge feature in bulk Cu2O differs signifiꢀ
cantly to that derived from heating a Cu/MOR sample to 723 K in
methane; an observation first made, by Alayon et al.10 Therefore,
the use of bulk Cu2O to quantify CuI levels in these cases is sigꢀ
nificantly flawed. In our Linear Combination analysis (LCA)
therefore, we consider only internal standards derived from each
sample through treatment in CH4 to 723 K.39 Examples of the
LCA, including fits and residuals, are given as SI (Figure s3) . In
addition we have also used and Principle component analysis
(PCA) and EXAFS to verify validity of the assumption intrinsic to
the LCA in terms of the use of such an internal standard. A full
description of these methods is given as SI.
Figure 1. Cu Kꢀedge XANES for five ((AꢀE, see SI. table 5a)
Cu/MOR samples at 723 K in flowing oxygen along with (blue) a
spectrum for (B) measured prior to the experiment (B@RT) and
(red) a bulk, room temperature, CuO standard.
The spectrum measured under flowing argon (B@RT) gives an
example of a fresh sample (B) that, having been stored in air is
significantly hydrated. Heating to 723 K under oxygen causes the
XANES in each case to change to that which is considered as
indicative of CuII that is no longer hydrated; the variation in the
edge energy of ≤ 0.5 eV is commensurate with the CuII standard.
We further note clear variations existing in these spectra of the
Cu/MOR samples (8995ꢀ9005 eV), and that a single sample (D),
show signs of low level CuI formation due to autoreduction; a
phenomenon that has been observed, if not explained, previousꢀ
ly.38
Studies made concerning the spectral character of inorganic
copper trimers35 and copper containing perovskites (where Cu
exists in the CuIII state),39 have demonstrated that CuIII trimers
show a shift in the Cu Kꢀedge energy to higher energies of 1 to
2.7 eV. However, the situation for mixed valence trimers is more
subtle and dependent upon additional factors (particularly metal to
ligand charge transfer).35, 40 On the basis of Figure 1 the simplest
conclusion (based on edge position) would be that the activated
state is comprised solely of CuII. That said, given the complex
situation for mixed valence trimers,35, 39,40 we should more properꢀ
ly conclude that these spectra cannot unequivocally determine
whether the copper is present only as CuII or as mixed valence
(CuII/CuIII) species.
Figure 3 shows CuI/CH3OH ratios obtained from our XAS and
reactivity measurements. Also included are data from previous
14, 28
studies12,
where reactivity information and Cu Kꢀedge
XANES data is given and estimation of the CuI/CH3OH ratio
possible.
What emerges are CuI/CH3OH ratios that in considerable maꢀ
jority fall within the range 2 to 4; precisely the range expected if
the reduction of CuII to CuI is the foundation methane conversion
to methanol and these materials are highly, if not to 100%, selecꢀ
tive for methanol.
There are, however, a number of data points that yield high
CuI/CH3OH ratios (arising from the extant literature,7,14 and both
the Cu/ZSMꢀ5 and Cu/CHA sample). Equally, there are two
results that are anomalously low (one from this study and one
from previously published data12) that on their own might favour
a mechanism that is not based on a CuI/CuII redox couple.
The most likely source very high CuI/CH3OH ratios is a decidꢀ
edly lower selectivity toward methanol. However, in most previꢀ
ous studies the total selectivity was not assessed;7, 10, 14As such,
we cannot be certain of this or whether these high ratios have
other sources e.g. the use of bulk Cu2O for quantification of the
CuI, or simply that the sample temperature was higher than
thought.
Figure 2 shows Cu Kꢀedge XANES obtained in each case after
exposure to methane at 473 K. Spectra derived from a bulk Cu2O
reference (blue), along with that derived from sample (Bꢀred) post
reaction with methane at 723 K for are also given; the latter servꢀ
ing as the internal reference for CuI. (see also SI)
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This sample yields 60 µmolgꢀ1 methanol; 120 µmolgꢀ1 electrons
are therefore required to explain this yield. In the XAS experiment
only 5% CuI is observed, equating to only 36 µmolgꢀ1 reduced
CuI.
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As these two results are very much an outlying minority we
conclude that they are indicative of XAFS experiments that have
been adversely affected by one or more of the experimentally
factors considered above and not, therefore, representative of the
true nature of these materials.
To summarise, we have measured CuI formation resulting from
the reaction of a range of Cu/zeolite samples with methane at 473
K, for both high temperature and isothermal/high pressure apꢀ
proaches to the direct conversion of methane to methanol. We
have then correlated the production of CuI with independently
made assessment of methanol production in these systems.
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Cu Kꢀedge XANES is unable to yield any unequivocal eviꢀ
dence for the presence of CuIII, as would be required by models
involving trimeric species. Equally, however, the numerous and
subtle factors that may contribute to the spectral character of
mixed valence (CuII/CuIII) species preclude a definitive conclusion
being drawn.
However, when we correlate the observed levels of CuI proꢀ
duced in situ using XANES with independently established methꢀ
anol yields we obtain results that overwhelmingly favour the
expectations of a mechanism based upon the CuI/CuII redox couꢀ
ple rather than any other mechanism, be it be based upon
CuII/CuIII redox or the involvement of CuIIꢀO● radical species.
Figure 3. Relation of methanol yield to CuI/CH3OH ratios for
Cu/MOR systems using both high temperature activation (filled
circles: (A) = ●; (B) = ●; (C) = ●; (D) = ●; (E) = ●) and isotherꢀ
mal (○, 473 K/high pressure (6 to 15 bar)) methods. A further
three systems (MAZ (●), CHA (●) and ZSMꢀ5 (●ꢀblue ellipse) are
also included for high temperature activation. Eight results
ASSOCIATED CONTENT
Supporting Information
14,
28
(square symbols) are derived from the literature (■7, ■12,
■
■ )
The green line shows the CuI/CH3OH ratio expected for 100%
selectivity within a CuII/CuI redox scenario. The inset shows an
expanded region of the plot highlighting the predominance of
results in the 2 < CuI/CH3OH < 3 range and the two experiments
that returned anomalously low CuI/CH3OH ratios.
Sample synthesis/characterisation; reactivity testing and product
quantification; Xꢀray measurements, data processing; procedures
and check and balances; details of LCA and PCA analyses and
validation of quantification using XANES; examples of EXAFS
derived from activated and methane reduced samples; full listing
of samples and results used to construct figure 3 with notes.
However, for the ZSMꢀ5 sample (highlighted in Figure 3 in the
blue ellipse), we have measured the level of carbon dioxide proꢀ
duction alongside the methanol using mass spectrometry. When
we account for the carbon dioxide produced, (see SI, notes to
table 7 b) we find that for 80 – 95% of the electrons required to
explain the product distribution within the paradigm of a CuI/CuII
redox couple can be accounted for. This result is, therefore, also
found to consistent with the basic expectations of a CuII/CuI redox
mechanism.
Author Contributions
Notes
The authors declare no competing financial interests.
AUTHOR INFORMATION
Corresponding Author
The two other outliers, that lead to anomalously low
CuI/CH3OH ratios, cannot be accounted for in the same terms.
However, given what is known regarding the extreme sensitivity
of CuI species to the presence of oxidants, then either leaks, net
gas purity, or a nonꢀisothermal operation of the entire sample
could result in a reduced of CuI formation. Even if there are no
leaks, and the intrinsic gas purity is sufficient, any portion of the
zeolite that does not reach the temperature required for complete
dehydration could act as a reservoir for water can subsequently
mask the true levels of CuI formation intrinsic to the material;
there is also the possibility that the temperature of the sample was
significantly lower than thought.
From the reactivity data presented in12, 13, and the number of
electrons required to be handled for each product quantified, ca.
389 µmolgꢀ1 of electrons (see SI, notes to table 7d), are required to
account for the products reported. The XANES presented in12 can
only account, however, for ca. 112.5 µmolgꢀ1 of electrons accordꢀ
ing to our estimate of the levels of CuI (at most 25%). The same
situation arises for one of our Cu/MOR samples (sample (E)).
jeroen.vanbokhoven@chem.ethz.ch, mark.newton@chem.ethz.ch
.
ACKNOWLEDGMENT
This work was carried out at the SuperXAS and at the Swiss
Norwegian (BM31) and the Dutch Belgian (BM26) beamlines.
We thank both the ESRF and the SLS for access to these facilities
along with Hermann Emerich, Dragos Stoian, Alessandro Longo,
Dipanjan Banerjee, Maarten Nachtegaal and Olga Safonova for
local contacting. Petr Sot, Jordan Meyet, and Leonid Bloch are
thanked for experimental assistance. DP acknowledges the use of
the computing facilities of the Swiss National Supercomputing
Center (CSCS). MAN thanks Shell, whilst ABP and VLS,
acknowledge the Energy System integration (ESI) platform at the
Paul Scherrer Institute, for funding.
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The combination of quantitative in situ Cu Kꢀedge XAS and reactivity studies strongly favours a CuII/CuI redox mechanism
as the foundation of the selective oxidation of methane to methanol by copper exchanged zeolites.
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Figure 1. Cu K-edge XANES for five ((A-E, see SI. table 5a) Cu/MOR samples at 723 K in flowing oxygen
along with (blue) a spectrum for (B) measured prior to the experiment (B@RT) and (red) a bulk, room
temperature, CuO standard.
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Figure 2. Cu K-edge XANES for each of samples shown in Figure 1 (A-E; see SI. table 5a) after exposure to
flowing methane at 473 K, along with: (red) sample B after heating in methane at 723 K (B@723 K/CH4);
and, (blue) a bulk Cu2O reference.
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Figure 3. Relation of methanol yield to CuI/CH3OH ratios for Cu/MOR systems using both high temperature
activation (filled circles: (A) = ●; (B) = ●; (C) = ●; (D) = ●; (E) = ●) and isothermal (○, 473 K/high pressure
(6 to 15 bar)) methods. A further three systems (MAZ (●), CHA (●) and ZSMꢀ5 (●ꢀblue ellipse) are also
included for high temperature activation. Eight results (square symbols) are derived from the literature (■7,
■12, ■ 14, ■ 28) The green line shows the CuI/CH3OH ratio expected for 100% selectivity within a CuII/CuI
redox scenario. The inset shows an expanded region of the plot highlighting the predominance of results in
the 2 < CuI/CH3OH < 3 range and the two experiments that returned anomalously low CuI/CH3OH ratios.
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