Conversion of Methane to Methanol on Copper Mordenite: Redox

Subscriber access provided by Kaohsiung Medical University

Kinetics, Catalysis, and Reaction Engineering

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 Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01183 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

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

‡

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 due to 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, like in the high temperature activation case.

* Corresponding author email: [email protected]

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 14

TEXT By selectively oxidizing methane, one could harness the methane that is currently flared and directly convert it to a high value and easily transportable product like 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 high temperature activation in oxygen is not a pre-requisite 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°C and subsequent


2

Page 3 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 high temperature 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 thermal gravimetric 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 pressure increases (1-15 bar). Specifically, we aim 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 0.01M copper nitrate aqueous solution with 1 gram of Na-Mor per 100ml of copper nitrate solution. The sample was stirred for 24 hours and exchanged three times, resulting in a copper


3


Page 4 of 14

content of 4.7 wt.% as determined by AAS. This preparation routine is the same as the one 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 Supplemental Information (SI) Figure 1 and 2. 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 hour 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 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 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.


4

Page 5 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (Top) Cu K-edge XANES for the activation in oxygen for isothermal low temperature and high activation temperature procedures. (Bottom) 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. 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 about 350°C. The end point for the isothermal procedure and the conventional procedure are designated at 200°C and 450°C, respectively, in Figure 1. For the isothermal activation, approximately 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


5


Page 6 of 14

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 high temperature 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 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 low temperature 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 minutes prior to the

Figure 2. XANES region during reaction with increased pressure of methane ACS Paragon Plus Environment

6

Page 7 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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-Kedge 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 fraction of Cu(II) (

Conversion of Methane to Methanol on Copper Mordenite: Redox

Recommend Documents