Journal de recherche et développement
Libre accès
ISSN: 2311-3278
ISSN: 2311-3278
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 85â, 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 + Hï¼CH3O + H → CH3OH and CH3O + HCO → CH3OH + CO
Chemical transformations of CO2 into valueâadded chemicals and fuels have been regarded as a key element for creating a sustainable lowâcarbon 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. 30–300â 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 Hâ supplier to replace H2 in CO2 hydrogenation as CH4 has a high Hâ 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 coâworkers investigated the direct C−C coupling of CO2 and CH4 to form acetic acid on a Znâdoped 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 [Equationâ 1]. However, this reaction is thermodynamically unfavorable under practical conditions. The conventional indirect catalytic process often proceeds through two steps (Schemeâ 1): 1)â DRM to produce syngas (CO and H2) at high temperatures (>700 °C), and 2)â 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 [Equationâ 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 oneâstep catalytic process bypassing the production of syngas. A stepwise method was proposed to convert CO2 and CH4 into acetic acid over Cu/Coâbased catalysts,2 Pd/C, Pt/Al2O3,3 Pd/SiO2, and Rh/SiO24 by heterogeneous catalysis. The catalyst was first exposed to CH4, forming CHx 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 CHx over the catalyst. This indirect process is complicated by the periodic change of reactants and the product collection.5
In conclusion, the oneâstep roomâtemperature synthesis of liquid fuels and chemicals from the direct reforming of CO2 with CH4 has been achieved by using a novel atmosphericâpressure 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 nonâthermal 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 nobleâmetal 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 8th International Conference on Petro Chemistry and Oil-Gas Marketing July 15-16, 2019 Amsterdam, Netherlands