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Euro Petrochemistry 2019: Conversion of CH4 into Value-added Chemicals by Plasma-Catalysis-Yanhui Yi-Dalian University of Technology

Yanhui Yi

Methane (CH₄), the main component of natural gas and shale gas, has a large reservation and wide distribution in the world, and thus it has been considered as an alternative energy source for oil. However, due to high stability (439 kJ/mol C-H bond energy), negligible electron affinity and low polarizability of CH₄ molecule, catalytic conversion of CH₄ into value-added chemicals is considered the “holy grail” of catalytic chemistry, and thus effective utilization of CH₄ has attracted much attention. Herein, we report a CH₄/NH₃ plasma reaction promoted by Pt and Cu catalysts for synthesis of hydrocyanic acid (HCN) at low temperature (400 ^oC).  HCN, an important chemical in organic chemistry, is widely used in pesticide, medicine, metallurgy, fuel and polymer, but it is currently produced through Andruddow process (1000-1100 ^oC,  Pt-Rh alloy gauze catalyst), the reaction of CH₄, NH₃, and O₂, or BMA process (1300 ^oC, Pt mesh catalyst), the reaction of CH₄ and NH₃ at atmospheric pressure. That is, the plasma catalysis technology has dramatically lowered the reaction temperature for HCN synthesis. We also report a CH₄/O₂ plasma reaction promoted by Ni/Al₂O₃ catalysts for production of CH₃OH. Under the conditions of 85℃, 2:1 CH₄/O₂ molar ratio, 0.393 s residence time and 30 W discharge power, 66.6 % methanol selectivity is achieved with 6.4 % methane conversion. The Ni/Al₂O₃ catalysts were characterized by TPR, XRD, XPS and HRTEM, and the results show that the production of CH₃OH is mainly attributed to the highly dispersed NiO phase which has a strong interaction with Al₂O₃ support. In addition, 0D modelling (ZD-Plaskin) results show that CH₃OH is mainly produced through the radical reactions CH₄ + O(1D) → CH₃O + H，CH₃O + H → CH₃OH and CH₃O + HCO → CH₃OH + CO

Chemical transformations of CO₂ into value‐added chemicals and fuels have been regarded as a key element for creating a sustainable low‐carbon economy in the chemical and energy industry. A particularly significant route that is currently being developed for CO₂ utilization is catalytic CO₂ hydrogenation. This process can produce a range of fuels and chemicals, including CO, formic acid, methanol, hydrocarbons, and alcohols; however, high H₂ consumptions (CO₂+3 H₂→CH₃OH+H₂O) and high operating pressures (ca. 30–300 bar) are major challenges associated with this process.

Instead of using H₂, the direct conversion of CO₂ with CH₄ (dry reforming of methane, DRM) into liquid fuels and chemicals (e.g., acetic acid) represents another promising route for both CO₂ valorization and CH₄ activation. CH₄ is an ideal H supplier to replace H₂ in CO₂ hydrogenation as CH₄ has a high H density and is available from a range of sources (e.g., natural gas, shale gas, biogas, and flared gas). Moreover, it is an inexpensive carbon source that can increase the atom utilization of CO₂ hydrogenation owing to the stoichiometric ratio of C and O atoms, as well as reduce the formation of water.

Recently, Ge and co‐workers investigated the direct C−C coupling of CO₂ and CH₄ to form acetic acid on a Zn‐doped ceria catalyst by density functional theory (DFT) modeling;¹ this is an attractive route as the direct conversion of CO₂ and CH₄ into acetic acid is a reaction with 100 % atom economy [Equation 1]. However, this reaction is thermodynamically unfavorable under practical conditions. The conventional indirect catalytic process often proceeds through two steps (Scheme 1): 1) DRM to produce syngas (CO and H₂) at high temperatures (>700 °C), and 2) conversion of syngas into liquid fuels and chemicals at high pressures. This indirect route for CO₂ valorization and CH₄ activation is inefficient as the DRM process for syngas production is highly endothermic and requires high temperatures and energy input [Equation 2]. Catalyst deactivation due to carbon deposition is another challenge impacting the use of this reaction on a commercial scale. It is almost impossible to directly convert two stable and inert molecules (CO₂ and CH₄) into liquid fuels or chemicals in a one‐step catalytic process bypassing the production of syngas. A stepwise method was proposed to convert CO₂ and CH₄ into acetic acid over Cu/Co‐based catalysts,² Pd/C, Pt/Al₂O₃,³ Pd/SiO₂, and Rh/SiO₂⁴ by heterogeneous catalysis. The catalyst was first exposed to CH₄, forming CH_x species on the catalyst surface. Subsequently, the feed gas was changed from CH₄ to CO₂, and acetic acid was formed through the reaction of CO₂ with CH_x over the catalyst. This indirect process is complicated by the periodic change of reactants and the product collection.⁵

In conclusion, the one‐step room‐temperature synthesis of liquid fuels and chemicals from the direct reforming of CO₂ with CH₄ has been achieved by using a novel atmospheric‐pressure DBD reactor. The total selectivity for liquid chemicals was approximately 50–60 %, with acetic acid as the major product. The CH₄/CO₂ molar ratio and the type of catalyst can be used to manipulate the production of different oxygenates. These results clearly show that non‐thermal plasmas can be used to overcome the thermodynamic barrier for the direct transformation of CH₄ and CO₂ into a range of strategically important platform chemicals, especially for the production of acetic acid with 100 % atom economy. Additionally, combining the DBD with noble‐metal catalysts produced formaldehyde, which cannot be generated in the same plasma reaction without a catalyst. This finding suggests that new research should be directed at designing a catalyst with high selectivity towards a desirable product.

Note: This work is partly presented at 8^th International Conference on Petro Chemistry and Oil-Gas Marketing July 15-16, 2019 Amsterdam, Netherlands