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Use of Entropy-Stabilized Oxides (ESOs) to Control Partial Oxidation of Methane

Investigators Arun Majumdar, Professor, Department of Mechanical Engineering William C. Chueh, Assistant Professor, Department of Materials Science & Engineering Chenlu Xie, Postdoctoral Scholar Jimmy Rojas, Graduate Student Eddie Sun, Graduate Student Abstract

The abundance and low cost of methane have offered the prospects of widespread distribution and use as a low-carbon energy source. Direct conversion of methane to liquid fuel such as methanol is becoming increasingly economically and environmentally attractive, although it remains chemically challenging due to the intrinsic inertness of C-H bond in methane. Current industrial processes require the intermediate step of reforming methane to syngas, which is costly and energy intensive. Despite decades of research, current direct conversion process still suffer from low catalyst activity, poor methanol selectivity, high severity of operating conditions, and rapid catalyst deactivation.

The goal of this project is to identify materials and cost-effective processes for direct conversion of methane to methanol with high yield and selectivity. We first investigate a series of poly-cation oxides material, which contain four or more different metals cations and offer a wide range of metal-oxygen binding energies on the surface. This heterogeneity of the metal-oxide binding energy on the surface enable us to control kinetics of methane oxidation. We have demonstrated that the heterogeneity of the metal-oxygen binding energy could give tunable onset temperature for CO2 production, however, CO2 is formed as the only products. We hypothesize that the elevated temperature is crucial in methane activation but will inevitably favor the deep oxidation of methane to CO2 and/or CO.

Therefore, we switched to investigate the direct conversion of methane to methanol at room temperature in aqueous solution. Liquid water could stabilize methanol due the favorable solvation free energy compared to the gas phase. We have successfully demonstrated methanol production and now in the process of improving the methanol yield.

Introduction

In the U.S., 8.15 million tons of methane were emitted from natural gas and petroleum systems in 2016. IEA estimates that methane emissions from the natural gas supply chain are approximately 1.7% of total natural gas production. It is estimated that $200 million of methane is wasted in flaring and venting. Direct conversion of methane to liquid fuel suitable for transportation and storage would be a cost-effective way to utilize these flaring/venting operations. The simplest liquid that can be produced from methane is methanol.1-3 The current industrial process is via an

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indirect pathway where methane is first reformed to syngas under high temperature (>800°C) and high pressure and then syngas is converted to methanol. This indirect pathway is costly and energy intensive.

Direct methane partial oxidation to methanol with oxygen have been studied for decades, and a fundamental selectivity-conversion trade-off is found for all catalysts, where the methanol selectivity is always lower to get a higher methane conversion.4-6 The intrinsic inertness of C-H bonds in methane has brought extreme challenges for the direct conversion process, not only in the methane activation step, but also in controlling chemo-selectivity to avoid overoxidation of methanol under harsh reaction conditions (high temperature, strong oxidants etc.)

We are aiming at identify materials and cost-effective process for direct conversion of methane to methanol with high yield and selectivity.

Background

The reactivity of oxygen is key to the direct conversion of methane to methanol. The adsorbed oxygen facilitate the C-H activation in methane, however, the overabundance of oxygen would lead to further oxidation to CO2 or CO. By controlling the energetics of the adsorbed oxygen via the binding energy of O with the neighboring metal cations in oxides, but not via gas phase oxygen, would help control the kinetics of methane oxidation. Our approach is to (a) divide the catalytic process into a two-step chemical looping process (Figure 1); (b) leverage the heterogeneity of the metal-oxygen binding energy in the entropy-stabilized oxides (ESOs) or poly-cation oxides (PCOs). In these PCOs which contain four or more metal cations, the enthalpy of the metal-oxygen bond can the entropy of O can be tuned in a unprecedented way, which offer new opportunities for redox reactions. We have recently discovered that a PCO (FeMgCoNi)Ox can split water with unprecedented low temperature TH ~1000°C , far lower than those achieved by the state-of-the-art materials.7 In two-step process, methane is activated on the oxides surface and the formed CH3 group would bond with an oxygen atom or OH group on the catalyst surface. The dissociated methane molecules could thus lead to various methane oxidation product and the oxidation process would need to stop at methanol to prevent over oxidation to undesirable products (i.e. CO2). In these PCOs, the multiple metal cations in a matrix of oxygen atoms could lead to cooperative effects in catalysis, i.e. binding and stabilizing intermediates.

Figure 1. Two-step process of partial oxidation of methane to form methanol using PCOs.

Results

We successfully synthesized a series of poly-cation oxides using sol-gel method.7 For example, the typical morphology of the as-synthesized (MgFeCoNi)Ox in shown in Figure 2(a), and the particle size is about 0.5 um in diameter. The oxides contains two phases, spinel and rocksalt phase, as shown in the XRD (Figure 1(b)).

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Figure 2. (a) Representative SEM image of as-synthesized PCO (MgFeCoNi)Ox; (b) X-ray diffraction (XRD) of as-synthesized PCO (MgFeCoNi)Ox.7

We first examined the performances of the PCO (MgNiZnCuCo)Ox in a home-built high temperature and high pressure reactor. A mass spectrometer is used to identify the product distribution. The ramping rate of the temperature is carefully controlled to avoid the overoxidation. No product is observed below 350°C, CO2 starts to form at 350°C and the activity is significantly improved with the increase of temperature. However, no methanol is formed among the test temperature from 25-500°C and CO2 is the only product. This is likely due to the high activity of oxygen in the (MgNiZnCuCo)Ox, thus the oxidation process does not stop and finally lead to CO2. We then use Ti to substitute Ni in the oxides to get less “active” oxygen, CO2 is still the only product under all temperatures, while the reaction onset temperature for CO2 formation is increased to 400°C. We also tried a variety of PCOs with different M-O bond energy, including (AlFeCrGaTi)Ox, (MgFeCoNi)Ox, (FeCoMgNiMn)Ox, (MgCaNiCoFeMn)Ox, however, only CO2 and trace amount of CO are observed as products.

We hypothesize that the heterogeneity of metal-oxygen bond could tune the activation of methane molecules, however, the right intermediate (i.e. CH3O*) for methanol formation might not form on the oxides. Supported copper nanoparticles are known to selectively produce methanol in 150-300°C with surface adsorbed CO, CO2 and H intermediates.8 Therefore, we deposit Cu nanoparticles on these PCOs and investigated their reaction performance. However, no methanol was observed and CO2 is still the main product with onset temperature as low as 250°C.

These results indicate that the heterogeneity of M-O bond could tune the methane activation rate but cannot stop the cascading oxidation to form CO2. In these processes, elevated temperature is crucial to activate methane, but it would favor the deep oxidation of methane to CO2 and/or CO.

Thus, a strategy that allows for methane activation at milder condition and stabilizing methanol simultaneously is favorable to achieve improved methanol yields.

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Room-temperature conversion of methane to methanol

We switched to investigate a room-temperature approach on direct methane to methanol conversion in water, emulating the process in mono-methane-oxygenase (MMO). This approach benefits from the mild reaction conditions and favorable solvation free energy of an aqueous reaction media that stabilizes the methanol product to achieve both high methanol selectivity and high methanol production rates.

We have built a reactor system to test direct conversion of methane to methanol in our laboratory, and gas and liquids products are detected using gas chromatography (GC) and nuclear magnetic resonance (NMR), respectively (Figure 3(a)). A series of control experiment are done to validate the results. There is no methanol formation under the same reaction condition but without catalysts. This indicates that the catalysts are crucial to the methanol formation. We are investigating the product distribution dependence on a variety of experimental factors. We are also developing new reactor design to improve methanol production for potential commercial application.

Figure 3. (a) Home built atalytic reactor system for direct CH4 conversion to methanol and ethane/ethylene. (b) 1H-NMR spectra of solution from CH4 conversion from a series of control experiments.

Progress and future work

We have demonstrated that the methane activation onset temperature are tuned by the metal-oxygen binding energy in oxides, however, only CO2 is formed. We speculate that the elevated temperature is critical to activate methane but favors the further oxidation to CO2 or CO. Therefore, we switched to a room-temperature approach to direct convert methane to methanol. We have successfully demonstrated that this approach is a promising way to produce methanol. Future work will focus on improving the methanol yields by tuning reaction conditions and catalysts and developing new reactor design for potential commercial application.

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Reference

1. Tomkins, P.; Ranocchiari, M.; van Bokhoven, J. A., Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond. Accounts of Chemical Research 2017, 50 (2), 418-425. 2. Ravi, M.; Ranocchiari, M.; van Bokhoven, J. A., The Direct Catalytic Oxidation of Methane to Methanol—A Critical Assessment. Angewandte Chemie International Edition 2017, 56 (52), 16464-16483. 3. Malakoff, D., The gas surge. Science 2014, 344 (6191), 1464. 4. Latimer, A. A.; Kakekhani, A.; Kulkarni, A. R.; Nørskov, J. K., Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies. ACS Catalysis 2018, 8 (8), 6894-6907. 5. Sushkevich, V. L.; Palagin, D.; Ranocchiari, M.; van Bokhoven, J. A., Selective anaerobic oxidation of methane enables direct synthesis of methanol. Science 2017, 356 (6337), 523. 6. Olivos-Suarez, A. I.; Szécsényi, À.; Hensen, E. J. M.; Ruiz-Martinez, J.; Pidko, E. A.; Gascon, J., Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities. ACS Catalysis 2016, 6 (5), 2965-2981. 7. Zhai, S.; Rojas, J.; Ahlborg, N.; Lim, K.; Toney, M. F.; Jin, H.; Chueh, W. C.; Majumdar, A., The use of poly-cation oxides to lower the temperature of two-step thermochemical water splitting. Energy & Environmental Science 2018, 11 (8), 2172-2178. 8. Chinchen, G. C.; Waugh, K. C.; Whan, D. A., The activity and state of the copper surface in methanol synthesis catalysts. Applied Catalysis 1986, 25 (1), 101-107. 10. Xie, J.; Jin, R.; Li, A.; Bi, Y.; Ruan, Q.; Deng, Y.; Zhang, Y.; Yao, S.; Sankar, G.; Ma, D.; Tang, J., Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. Nature Catalysis 2018, 1 (11), 889-896.

Contacts Arun Majumdar: arunava@stanford.edu William C. Chueh: wchueh@stanford.edu Chenlu Xie: clxie@stanford.edu Jimmy Rojas: jimmy06@stanford.edu Eddie Sun: eddiesun@stanford.edu