Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2018, 57, 12036−12039
Conversion of Methane to Methanol on Copper Mordenite: Redox Mechanism of Isothermal and High-Temperature-Activation Procedures Amy J. Knorpp,† Mark A. Newton,† Ana B. Pinar,*,‡ and Jeroen A. van Bokhoven†,‡ †
ETH Zurich, Wolfgang Paulistrasse 10, Zurich, CH-8093, Switzerland Paul Scherrer Institute, Villigen, CH-5232, Switzerland
‡
Ind. Eng. Chem. Res. 2018.57:12036-12039. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/17/18. For personal use only.
S Supporting Information *
ABSTRACT: In the search for a commercially viable process to convert methane directly to methanol, the low-temperature isothermal process represents a promising new reaction pathway, because of its easier operation at lower temperatures than previously proposed processes with a high activation temperature. Isothermal methane to methanol conversion on copper-exchanged mordenite was examined by in situ XAS and compared to the conventional high-temperature activation procedure. Despite the copper remaining partially hydrated during the isothermal procedure, methanol can form when elevated pressures are applied. By observing the reduction of copper during reaction at elevated pressures, the mechanism was found to be a two-electron reduction−oxidation reaction for the isothermal procedure, as in the high-temperature activation case. °C and subsequent reaction with pressurized methane. With 1 bar of methane and an activation temperature of 200 °C, the yield of methanol was only 0.3 μmol g−1, but with increasing pressures of methane, the yield increased to 56.2 μmol g−1 at 36 bar.5,15 These studies demonstrate that high-temperature activation is not a necessity. The nature of the active sites in this case is entirely open to question in the sense that classically assumed dimers and oligomer copper oxo sites should not be formed at such low temperatures. Little is known regarding the local environment of the copper or the conversion mechanism for this isothermal stepwise procedure. For copper zeolites, in situ X-ray absorption spectroscopy (XAS) at the Cu K-edge has been one of the primary characterization techniques used to probe the local electronic and geometric structure of the copper21,22 and the hightemperature activation procedure has been widely studied with this method.8,11,16,17 Furthermore, it is one of the primary tools that have led to the proposition that Cu trimer species can exist in mordenite.11 However, this technique has not been previously applied to the lesser known isothermal procedure at increased pressures. In this work, we compare the high-temperature activation with the isothermal procedure using in situ Cu K-edge XAS and monitoring the extent of dehydration observed by thermogravimetric analysis (TGA). The goal is to track the changes in copper through in situ XAS for the low-temperature activation and for the methane reaction as the pressure
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y selectively oxidizing methane, one could harness the methane that is currently flared and directly convert it to a high-value and easily transportable product, such as methanol.1,2 Not only can methane be converted directly to methanol through natural biological systems,3,4 but researchers have also observed conversion in inorganic systems such as copper-exchanged zeolites.5−20 However, the conversion of methane to methanol on copper zeolites requires a stepwise procedure, and only a fraction of the copper present participates actively in this conversion, resulting in reduced yields of methanol.10,16,18 To complicate the issue, there is still much debate regarding the precise copper nature of the active site species,7−13 and there are very few experiments that address the reaction mechanism. Copper in the form of μ-oxo dicopper has been widely reported as an active species in the conversion of methane to methanol in copper zeolites.7−10 Most likely, if copper dimers are active, species of higher nuclearity, including trimers, are also likely to react to methane.11−13 Regardless of the nuclearity, these proposed active sites require high activation temperatures (>350 °C) in oxygen and the complete removal of water from the zeolite structure.8−11 These active sites are highly hydrophilic and are not stable in trace amounts of water.7 As a result, a “standard” procedure involving a high temperature (450 °C) activation of the copper in oxygen followed by reaction with methane at 150−200 °C has evolved and dominated the study of this important chemistry.7,8,11,14,16 However, recently, it has been demonstrated that this hightemperature activation in oxygen is not a prerequisite for the conversion of methane to methanol in Cu-mordenite. Methane-to-methanol conversion has been achieved on copper mordenite using isothermal low-temperature conditions.5,15 This isothermal procedure utilizes activation in oxygen at 200 © 2018 American Chemical Society
Received: Revised: Accepted: Published: 12036
March 17, 2018 August 5, 2018 August 13, 2018 August 13, 2018 DOI: 10.1021/acs.iecr.8b01183 Ind. Eng. Chem. Res. 2018, 57, 12036−12039
Article
Industrial & Engineering Chemistry Research
complexes). XANES spectra are sensitive to changes in the local geometry, and as the temperature increases and water is driven away, the coordination of the first shell reduces, and the geometry around the copper is transformed from an octahedral shape to a more planar structure.17 This explains the observed decrease in the maximum absorption while an 8986 eV shoulder develops. This observation is consistent with earlier works for the high-temperature activation procedure.8,11,16,17 The isothermal final activated state differs from the conventional activation in that the maximum absorption and the magnitude of the shoulder at 8986 eV are less pronounced. This can be attributed to the fact that the copper is not fully dehydrated by the end of the isothermal low-temperature activation step. Figure 1 compares the linear combination fitting (LCF) for the XANES spectra with the TGA mass loss. By tracking the progression of the dehydration, a large portion of the copper can be observed to be dehydrated before 200 °C; however, it does not fully dehydrate until ∼350 °C. The end point for the isothermal procedure and the conventional procedure are designated at 200 and 450 °C, respectively, in Figure 1. For the isothermal activation, ∼19% (± 2%) of the copper remains in a hydrated state, as determined by XANES spectra. A TGA experiment was conducted to analyze the weight change during the activation. The weight loss from dehydration of the copper zeolite continues well beyond 200 °C. At the end of the 200 °C activation step, 2.5% of the total weight of the zeolite remains as water. In a unit cell of mordenite, this corresponds to 1.6 water molecules per copper atom. This system is likely in dynamic equilibrium, and the exact number of water molecules associating with each copper will fluctuate. For activation at 200 °C in the isothermal low-temperature procedure, we can conclude that the copper is not able to form the same active species as the dimer or oligomeric copper, which has been proposed as the active site in the hightemperature activation procedure, because the moisture content and temperature do not satisfy the needs for formation and stability of the dimer or trimer. An alternative water stable active species may be responsible for the conversion of methane to methanol for this isothermal low-temperature procedure.17 A water-stable species has been hypothesized previously as a secondary active site for the conventional high-temperature activation procedure, experimentally17,19 and by density functional theory (DFT) calculations.20 By introducing saturated water after high-temperature activation (450 °C) but before methane introduction, a trace amount of methanol was still detected.17 This water stable species is likely the same or very similar to the active species in the isothermal lowtemperature procedure. The low yield of methanol from this water-stable active site is significantly improved by reacting with higher pressures of methane. After the activation step, the sample was purged with helium for 15 min prior to the introduction of methane. The pressure was then systematically increased using a back-pressure regulator. The pressure was increased stepwise to 1, 6, 10, and 15 bar. At each pressure after equilibration, Cu K-edge XANES and EXAFS were collected. When methane was introduced into the system at 1 bar, only a minute change was observed in the XANES. Figure 2 shows that, as the pressure increases, a shoulder at 8983 eV becomes more pronounced. This shoulder corresponds to the 1s → 4p transition that is characteristic of the spectra for Cu(I). At 1 bar, only a small
increases (1−15 bar). Specifically, we intend to shed light upon what may be the structural-reactive differences that exist between the high activation temperature and low-temperature isothermal routes and further establish how this new route may work. A commercial zeolite sample (Zeoflair 800, ZeoChem, Si/Al = 10) was cation-exchanged with 0.01 M copper nitrate aqueous solution with 1 g of Na-Mor per 100 mL of copper nitrate solution. The sample was stirred for 24 h and exchanged three times, resulting in a copper content of 4.7 wt %, as determined by AAS. This preparation routine is the same as that used in the first demonstration of the isothermal methane-to-methanol conversion procedure.5,15 For in situ XAS, the copper mordenite sample was positioned between quartz wool in a 1 mm OD capillary with 0.1 mm wall thickness. XAS measurements were performed in transmission mode at ESRF BM 31 (Swiss Norwegian Beamline) and SuperXAS at SLS (Swiss Light Source). The treatment schemes for the high-temperature activation and isothermal stepwise treatments are described in the Supporting Information (Figures S1 and S2). For activation, oxygen was passed through the capillary, and the sample was heated by a hot air blower to 200 °C at 10 °C/ min and held for 1 h in oxygen. X-ray absorption near edge structure (XANES) spectra were measured throughout the activation process. Figure 1 shows the Cu K-edge XANES spectra recorded during the temperature increase. For comparison, the conventional high-temperature activation (450 °C) procedure was conducted on a fresh sample. Prior to activation, the copper is in a hydrated Cu(II) state (hexaqua
Figure 1. (Top) Cu K-edge XANES for the activation in oxygen for isothermal low-temperature and high-activation-temperature procedures. (Bottom) Linear combination fitting (LCF) of the XANES region for hydrated and dehydrated copper during activation. The accompanying weight loss by TGA is shown for the activation temperature range. 12037
DOI: 10.1021/acs.iecr.8b01183 Ind. Eng. Chem. Res. 2018, 57, 12036−12039
Article
Industrial & Engineering Chemistry Research
in XANES11 or unexplained high Cu(I) conversion.17 In addition, the in situ XAS studies of high-temperature activation have often only been generated on one data point, which makes correlation and quantification difficult.11,16,17 The isothermal procedure studied in this work provides an opportunity to examine the pressure-dependent changes in the XANES spectra with methanol yield and ultimately quantify the amount of electrons used per methanol formed. To examine the mechanism, the Cu(I) formation from XANES is divided by the methanol yield and is plotted against the methane pressure in Figure 3. As the pressure increases, the
Figure 2. XANES region during reaction with increased pressure of methane.
fraction of Cu(II) (
