On the Mechanism Underlying the Direct Conversion of Methane to

Communication Cite This: J. Am. Chem. Soc. 2018, 140, 10090−10093

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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 Mark A. Newton,*,† Amy J. Knorpp,† Ana B. Pinar,† Vitaly L. Sushkevich,‡ Dennis Palagin,‡ and Jeroen A. van Bokhoven*,†,‡

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†

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland ‡ Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, Villigen 5232, Switzerland S Supporting Information *

whether this conversion is founded upon a CuI/CuII redox couple or another mechanism 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 2 and that, for progressively less selective materials, it should tend toward infinity. If another mechanism is at work, e.g., that based upon a CuII/CuIII couple or one that proceeds via a CuII−O• radical species, then for 100% selectivity for methanol, CuI should not be observed at all. Cu K-edge X-ray absorption spectroscopy (XAS) provides quantitative insight into the oxidation states of copper present under reaction conditions.35,36 Indeed, Cu K-edge XAS has often been used to gain insight into the active species 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 extracted6,7,9,10,18,23,27,28 or desorbed using steam in a post factum manner.12,13,23 We have taken the following approach: copper systems based upon MOR, MAZ, CHA, and MFI have been measured for reactivity toward methane using in situ Cu K-edge X-ray absorption near-edge structure (XANES)37 for both the hightemperature activation route (723 K in flowing oxygen (15 mL min−1)) and the isothermal route4,22 (activation at 473 K in oxygen followed by pressurization in methane to between 6 and 15 bar). Methanol yields for each sample were then established using either extraction and gas chromatography (GC) or online mass spectrometry (MS). Figure 1 shows examples of Cu K-edge XANES obtained from different Cu/MOR samples activated at 723 K in oxygen; the Cu K-edge XANES obtained from a fresh sample (B@RT, blue) is also given, along with that for a bulk CuII reference (CuO, red). A complete list of sample preparation, characterization, and performance data is given in section 7 in the Supporting Information (SI). The spectrum measured under flowing argon (B@RT) is an example of the spectrum 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

ABSTRACT: The application and quantification of in situ copper K-edge X-ray absorption near-edge structure (XANES), when linked to independently made reactorbased 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 ZSM-5 (MFI)2−5 is active for this conversion, a range of zeolite systems have been shown to be active (e.g., mordenite (MOR),6−24 chabasite (CHA),25−30 and mazzite (MAZ)),27,31 and numerous 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 methanol,32−34 is a diversity of opinion as to the active copper species, with mono- and bis(μ-oxo) copper dimers,3−11,15,19,28−31 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 atoms3−11,16,17,19,28−31 favors 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 centers.33 In this mechanism, which has also been derived for dimers,15 the appearance of CuI is associated with overoxidation of methanol (to yield, for instance, carbon monoxide, carbon dioxide, and formates) and not selectivity for methanol. Indeed, the formal reduction of copper centers is not required, with charge localization occurring via the formation of a CuII−O• radical species.15 The conversion of methane to methanol is a two-electron process. Other products observed to be formed by these materials require higher numbers of electrons to be handled for their production, e.g., six electrons for formates and carbon monoxide and eight electrons for carbon dioxide. In principle, this understanding can provide a framework for deciding © 2018 American Chemical Society

Received: May 18, 2018 Published: August 2, 2018 10090

DOI: 10.1021/jacs.8b05139 J. Am. Chem. Soc. 2018, 140, 10090−10093

Communication

Journal of the American Chemical Society

Figure 2. Cu K-edge XANES for each of samples shown in Figure 1 (A−E; see SI section 7a) after exposure to flowing methane at 473 K along with spectra for (red) sample B after heating in methane at 723 K (B@723 K/CH4) and (blue) a bulk Cu2O reference.

Figure 1. Cu K-edge XANES for five Cu/MOR samples (A−E; see SI section 7a) at 723 K in flowing oxygen along with spectra for (blue) B measured prior to the experiment (B@RT) and (red) a roomtemperature bulk CuO standard.

including fits and residuals, are given in Figure S3) . In addition, we have also used principal component analysis (PCA) and extended X-ray absorption fine structure (EXAFS) to verify the validity of the assumption intrinsic to the LCA in terms of the use of such internal standards. A full description of these methods is given in the SI. Figure 3 shows CuI/CH3OH ratios obtained from our XAS and reactivity measurements. Also included are data from previous studies12,14,28 where reactivity information and Cu Kedge XANES data are given and estimation of the CuI/ CH3OH ratio is possible. What emerges are CuI/CH3OH

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 that clear variations exist in these spectra of the Cu/MOR samples (8995−9005 eV) and that a single sample, D, shows signs of low-level CuI formation due to autoreduction, a phenomenon that has been observed, if not explained, previously.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 for CuIII trimers the Cu K edge shows a shift of 1 to 2.7 eV toward higher energy. 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 composed solely of CuII. That said, given the complex situation for mixed-valence trimers,35,39,40 we should more properly conclude that these spectra cannot unequivocally determine whether the copper is present only as CuII or as mixed-valence (CuII/CuIII) species. Figure 2 shows Cu K-edge XANES obtained in each case after exposure to methane at 473 K. Spectra derived from a bulk Cu2O reference (blue) and a sample after reaction with methane at 723 K (B@723 K/CH4, red) are also given, the latter serving as the internal reference for CuI (also see the SI). In each case, exposure to methane at 473 K results in the transformation of a portion of the CuII into CuI. The degree to which this happens varies very considerably, a situation that reflects the literature, where for high-temperature activation the levels of CuI observed after nominally the same treatment also vary greatly.7,9,10,12,14,23 More often than not,7,9,14,23,28 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 pre-edge feature in bulk Cu2O differ significantly from those 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 significantly flawed. In our linear combination analysis (LCA), we therefore consider only internal standards derived from each sample through treatment at 723 K in CH4.39 Examples of the LCA,

Figure 3. Relation of methanol yields to CuI/CH3OH ratios for Cu/ MOR systems using both high-temperature activation (solid circles: A, red; B, blue; C, green; D, dark green; E, black) and isothermal (○, 473 K/high pressure (6 to 15 bar)) methods. A further three systems (MAZ (light gray), CHA (dark gray), and ZSM-5 (violet with blueshaded ellipse) are also included for high-temperature activation. Eight results are derived from the literature (square symbols: red, ref 7; blue, ref 12; black, ref 14; green, ref 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 (pink-shaded region). 10091

DOI: 10.1021/jacs.8b05139 J. Am. Chem. Soc. 2018, 140, 10090−10093

Communication

Journal of the American Chemical Society ratios that in considerable majority fall within the range of 2 to 4, which is precisely the range expected if the reduction of CuII to CuI is the foundation of methane conversion to methanol and these materials are highly, if not to 100%, selective for methanol. However, there are a number of data points that yield high CuI/CH3OH ratios (arising from the extant literature7,14 and both the Cu/ZSM-5 and Cu/CHA samples). Equally, there are two results that are anomalously low (one from this study and one from previously published data12) and on their own might favor a mechanism that is not based on a CuI/CuII redox couple. The most likely source of very high CuI/CH3OH ratios is a decidedly lower selectivity toward methanol. However, in most of the previous studies, the total selectivity was not assessed,7,10,14 and therefore, we cannot be certain whether this is the origin or 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. However, for the ZSM-5 sample (highlighted in Figure 3 in the blue-shaded ellipse), we have measured the level of carbon dioxide production alongside the methanol using mass spectrometry. When we account for the carbon dioxide produced, (see SI section 7b notes) we find that the paradigm of a CuI/CuII redox couple can account for 80−95% of the electrons required to explain the product distribution. This result is therefore also found to be consistent with the basic expectations of a CuII/CuI redox mechanism. The two other outliers that lead to anomalously low CuI/ CH3OH ratios cannot be explained in the same terms. However, in view of what is known regarding the extreme sensitivity of CuI species to the presence of oxidants, either leaks, net gas purity, or non-isothermal operation of the entire sample could result in reduced 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, subsequently masking the true level 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 in refs 12 and 13 and the number of electrons required to be handled for each product quantified, ca. 389 μmol of electrons per gram (see SI section 7d notes) is required to account for the products reported. The XANES presented in ref 12, however, can only account for ca. 112.5 μmol of electrons per gram according to our estimate of the level of CuI (at most 25%). The same situation arises for one of our Cu/MOR samples (sample E). This sample yields 60 μmol of methanol per gram, and therefore, 120 μmol of electrons per gram is required to explain this yield. In the XAS experiment, only 5% CuI was observed, equating to only 36 μmol of reduced CuI per gram. 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 experimental factors considered above and therefore are not representative of the true nature of these materials. To summarize, 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 approaches to the direct conversion of methane to methanol. We have then correlated the production of CuI

with independent assessments of methanol production in these systems. Cu K-edge XANES is unable to yield any unequivocal evidence 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 produced in situ using XANES with independently established methanol yields, we obtain results that overwhelmingly favor the expectations of a mechanism based on the CuI/CuII redox couple rather than any other mechanism, be it be based upon CuII/CuIII redox or the involvement of CuII−O• radical species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05139. Sample synthesis/characterization; reactivity testing and product quantification; X-ray measurements and data processing; procedures and checks 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 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected]. ORCID

Mark A. Newton: 0000-0002-6389-2144 Dennis Palagin: 0000-0001-5251-3471 Jeroen A. van Bokhoven: 0000-0002-4166-2284 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out at the SuperXAS, Swiss Norwegian (BM31), and Dutch Belgian (BM26) beamlines. We thank both the ESRF and the SLS for access to these facilities and 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. D.P. acknowledges the use of the computing facilities of the Swiss National Supercomputing Center (CSCS). M.A.N. thanks Shell and A.B.P. and V.L.S. acknowledge the Energy System Integration (ESI) platform at the Paul Scherrer Institute for funding.

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